Nonlinear Optical Materials for the Smart Filtering of Optical Radiation

Review pubs.acs.org/CR

Nonlinear Optical Materials for the Smart Filtering of Optical Radiation Danilo Dini,*,† Mário J. F. Calvete,*,‡ and Michael Hanack*,§ †

Department of Chemistry, University of Rome “La Sapienza”, P.le Aldo Moro 5, I-00185 Rome, Italy CQC, Department of Chemistry, Faculty of Science and Technology, University of Coimbra, Rua Larga, P 3004-535 Coimbra, Portugal § Institut für Organische Chemie, Universität Tübingen, Auf der Morgenstelle 18, D-72076 Tübingen, Germany ‡

ABSTRACT: The control of luminous radiation has extremely important implications for modern and future technologies as well as in medicine. In this Review, we detail chemical structures and their relevant photophysical features for various groups of materials, including organic dyes such as metalloporphyrins and metallophthalocyanines (and derivatives), other common organic materials, mixed metal complexes and clusters, fullerenes, dendrimeric nanocomposites, polymeric materials (organic and/or inorganic), inorganic semiconductors, and other nanoscopic materials, utilized or potentially useful for the realization of devices able to filter in a smart way an external radiation. The concept of smart is referred to the characteristic of those materials that are capable to filter the radiation in a dynamic way without the need of an ancillary system for the activation of the required transmission change. In particular, this Review gives emphasis to the nonlinear optical properties of photoactive materials for the function of optical power limiting. All known mechanisms of optical limiting have been analyzed and discussed for the different types of materials.

CONTENTS 1. Introduction 1.1. Functions and Evaluation of Optically Smart Materials 1.2. Optical Limiting Effect as Nonlinear Optical Process 1.2.1. Mechanism Based on Reverse Saturable Absorption 1.3. Other Mechanisms 1.3.1. Nonlinear Scattering/Thermal and Nonlinear Refraction 1.3.2. Multiphoton Absorption 1.4. Experimental Techniques for the Characterization of OL Materials 1.4.1. Intensity-Dependent Transmission Measurements 1.4.2. Z-Scan Technique 1.4.3. Degenerate Four-Wave Mixing for OL Studies 2. Inorganic Materials 2.1. Nanocrystals 2.2. Nanocomposites 2.2.1. ZnO-Based Composites 2.2.2. Other Nanocomposites 2.3. Quantum Dots 2.4. Nanowires 2.5. Metal Clusters 2.5.1. Sulfur-Containing Cluster Compounds 2.5.2. Selenium-Containing Cluster Compounds © 2016 American Chemical Society

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2.5.3. Other Metal Clusters 2.6. Metal Nanoparticles 2.6.1. Gold Nanoparticles 2.6.2. Silver Nanoparticles 2.6.3. Other Metal Nanoparticles 2.6.4. Magnetic Nanoparticles 2.7. Carbon-Based Nanoparticles Hybrid Organic−Inorganic Materials 3.1. Metal−Organic Complexes 3.2. Metal Acetylides and Vinylidenes Organic Materials 4.1. Non Macrocyclic Molecules 4.2. Fluorene- and Carbazole-Based Compounds 4.3. Polymethine Cyanine Dyes 4.4. Dendrimers 4.5. Polymeric Materials Carbon-Based Materials 5.1. Fullerenes 5.2. Carbon Nanotubes 5.3. Graphenes Porphyrins and Related Macrocycles Phthalocyanines and Related Macrocycles 7.1. Advantages of Large π-Electron Macrocycles for OL 7.2. Aggregation Issues 7.3. Effect of the Combination Central AtomAxial Ligand

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Received: January 25, 2016 Published: November 10, 2016

13063 13043

DOI: 10.1021/acs.chemrev.6b00033 Chem. Rev. 2016, 116, 13043−13233

Chemical Reviews 7.4. Effect of Peripheral Substitution in Phthalocyanines 7.5. Dimeric and Oligomeric Phthalocyanines 7.6. Phthalocyanine Hybrids 7.7. Other Types of Phthalocyanine Related Compounds 7.8. Naphthalocyanines 8. Concluding Remarks and Perspectives Author Information Corresponding Authors Notes Biographies Acknowledgments References

Review

properties of transmission, reflectance, and scattering through a mechanism triggered either directly or indirectly by the external radiation itself. For example, the palpebra constitutes an optically dynamic filter, which is activated by the nervous system upon detection of a too intense dangerous radiation by eyesight itself. Dynamic features are commonly displayed by photochromic, nonlinear optical (NLO), and electrochromic materials/devices when modified reversibly by their lightabsorption properties upon irradiation with low-, high-intensity radiations, and slowly changing electric fields, respectively. In contrast to photochromic and NLO materials, the spectral action of an electrochromic material can be evidenced only when the active material is electrically polarized by an additional power source. This implies that the optical dynamic feature of the electrochromic materials is not directly activated by the radiation itself. As a consequence of that, electrochromic materials need an additional ancillary LSE, which drives opportunely the polarization controlling the absorption/ reflectance properties of the chromogenic material. Besides being dynamic, the material/device protecting the LSE should constitute also a smart system; that is, it should change appropriately its optical properties upon modification of the parameters of the external radiation at the minimum cost of maintenance to keep a state of comfort or to preserve the active status of a LSE. This aspect is particularly important when LSEs are represented by optical sensors or eyes, the active state of which requires a minimum of luminous energy. Besides that, an optically smart system should warrant also the powering of the working state of those LSEs that produce energy from solar radiation, for example, photovoltaic cells. The definition of the concepts of optically dynamic and smart systems here given has been referred indistinctly to materials and devices. In the process of engineering and realizing devices with optically dynamic and smart features, the attention is focused mainly toward the recognition of useful feedback mechanisms of control that allow an efficient and rapid communication between an ancillary LSE sensing the external radiation, and the optical elements (e.g., filters, lenses, shutters, polarizers, monochromators), which are indirectly photoswitched by the ancillary LSE with the aim of protecting and/or activating the main LSE. An optically dynamic/smart device would require the presence of at least one ancillary LSE and the successive photoswitched automation of the connected optical elements, but does not necessarily involves the utilization of optically dynamic/smart materials. It is evident that the main competences required by the construction of such devices are those of experts of photonics, of sensors, and of optical, electrical, and mechanical engineers. On the other side, when optically dynamic/smart materials are considered, the main content of the related research is constituted by the design and the synthesis of materials with switchable optical properties. Such an aspect will involve the efforts of material scientists, synthetic chemists, physical chemists, spectroscopists, laser scientists, and physicists that attempt the definition of unambiguous correlations between optical properties and chemical structure, crystalline phase, and dispersion state of the photoactive material. In this Review, we intend to detail the chemical structures and their relevant photophysical features for various groups of photoswitchable materials utilized or potentially useful for the realization of devices able to filter in a smart way an external radiation. In doing so, we will focus also on the description of the mechanisms of photoswitching at a microscopic level and describe the parameters that evaluate the

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1. INTRODUCTION The ever-growing use of optical radiation originated either from natural or from artificial sources in important applicative fields like energy conversion, communications, imaging, machining, surgery, and medicine imposes the development of adequate tools for the control of the radiation features to accomplish the desired function driven by the light in the most efficient, durable, safe, and fast way. This is because the utilization of the photons as chemical reagents in photovoltaics and medicine (e.g., in photodynamic therapy), or data deliverer in the fields of optical communication and imaging, or mechanical energy deliverer in machining and surgery is progressively increasing with time. Therefore, the control of luminous radiation has extremely important implications for modern and future technologies. This can be generally achieved through the synthesis of opportune materials as well as the design and construction of devices operating as monochromators, filters, lenses/mirrors, and shutters to modulate the features of (1) energy, (2) intensity, (3) direction of propagation, and (4) duration of the radiation, respectively. In the actual operative conditions of exploitation of radiation, the four main parameters that characterize it can vary in an extremely wide interval of values and conditions. This requires a correspondingly great variety of devices, configurations, and ample choice of optically active materials for the efficacious control of the radiation. Particularly crucial is the situation in which the light source is external; that is, it emits a radiation not controllable by the light-sensitive element that interacts, either purposely or accidentally, with such a radiation. The external source can have either a natural origin like the light coming from white hot bodies (sun and stars), which is generally exploited for energy conversion scopes, or an artificial origin like the radiation coming from lasers with stray or even hostile features when communication and defense applications are considered. In both cases, the light-controlling systems designed for the correct and efficient use of the light-sensitive element (LSE) interacting with the radiation must fulfill a series of stricter requirements due to a larger number of unknown and irreproducible factors associated with the erratic nature of the external radiation in comparison to the situation in which the radiation source has a fully controlled emission. The LSE could be a solar cell, the eye, a photodetector, a sensor, or a waveguide, that is, a device or a human organ that generally operates in very different conditions. As a consequence of that, the optical device or the photoactive material dealing with uncontrolled radiations should possess dynamic features that consist of the general capability of altering the optical 13044


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given medium, the parameters n0 and α vary with the wavelength of the incident radiation, while dn/dT refers to the thermal variation of the refractive index within the poorly absorbing medium that acts as a thermal lens by virtue of the refractive index gradient at the wavelength of operation. Because, for a Gaussian beam, the power PD detected at the photodetector (D in Figure 1b) is a function of the aperture radius (a) and, more importantly, of the laser beam radius (w) in correspondence of the aperture according to the relationship:

optical performances of the various classes of materials in the act of filtering the optical radiation smartly. Among them, particular attention will be given to the function of optical limiting (OL) or optical power limiting (OPL), which consists of the high transmission of sustainable light intensities and the decreasing transmission of progressively increasing intensities of light beams through special materials. Such an effect of decrease of the optical transmission through an ancillary lightsensing device is required when the light impinging on the main LSE exceeds a predefined value of intensity determined by the nature itself of the main LSE (vide infra). The description of smart optical devices, either OPL or not, will be given eventually in those cases where the configuration of the device is strongly dependent on the chemical-physical features of the photoactive material employed. Those devices, the modus operandi of which is based mainly on the engineering of a feedback mechanism, but not related to the optical properties of photoswitchable materials, will not constitute the content of the present work, with the description of pertinent optical setups, mechano-optical sensors, transducers, actuators, and electrical circuitry being beyond the scope of this Review centered intentionally on materials.

PD = P0(1 − e−2(a

2

/ w 2)

)

(2)

for the evaluation of the OPL effect generated by a thermal lens it is important to relate w, that is, the effect produced, with the focal length F and, through eq 1, with P0. The expansion of the laser beam at the aperture (Figure 2) is given by the equation:4 2a 2 = w2

4πa 2b1/λ

2a 2 2 D D 2 2 (2b12 + 8Dd1 + 8d12) w 2 (4d1 + b1 ) − F 2

F

+ [b12 + 4(D + d1)2 ] =

1.1. Functions and Evaluation of Optically Smart Materials

As outlined in the previous paragraph, one of the most relevant functions studied in applied optics is related with the realization of the OL (or OPL) effect. Historically, the first aware realization of such an effect (Figure 1) was made by means of a

4πa 2b1/λ D2 (4d12 F2

+ b12) −

D (2b12 F

+ [b12 + 4(D + d1)2 ]

+ 8Dd1 + 8d12) (3)

Figure 2. (a) L1 and L2 are the lenses that focus the beam in the middle of the cell containing the nonlinear optical material, and in the aperture at the P plane where the photodetector (D4) is located, respectively. In linear/nonlinear regimes, the beam follows the continuous/dotted line profile. (b) OPL effect generated by CS2 adopting the optical setup displayed in (a). Itrans is the intensity at 1.06 μm passing through the orifice of P plane and measured at the photodetector D4 (a). Pin is the power of the incident radiation (pulse duration: 40 ps).

Figure 1. (a) Optical power transmitted by the nitrobenzene thermal lens at 488 nm upon increase of the incident light power; (b) representation of the intensity distribution of the laser beam through the thermal lens based on nitrobenzene for OPL. A and D stand for aperture and photodetector, respectively. Adapted with permission from ref 2. Copyright 1967 American Institute of Physics.

thermal lens,1 which utilized nitrobenzene as a poorly absorbing medium at the wavelength of 488 nm when the monochromatic radiation was produced by a CW Ar+ laser with maximum power output of 60 mW.2 At the basis of the OPL effect was the defocusing of the laser beam (Figure 1b) due to the displacement of the focal length following the buildup of a gradient of refractive index (n) generated by the residual absorption of the optical medium.3 In the particular case of a laser beam with Gaussian spatial profile, the focal length (F) of the thermal lens has an incident power (P0) dependency of the type:2 Kπn0w0 F= dn 0.24P0lα dT (1)

in which λ, b1, d1, and D are the wavelength of the laser radiation, a confocal parameter of the beam,4 the distance between the Gaussian beam waist and the lens,4 and the distance between the lens and the aperture, respectively.2 The material properties that intervene on the OPL effect via thermal lensing are the absorption coefficient α(λ), and the dependence of the refractive index on the temperature (eqs 1−3). The correlation between refractive index and temperature stems directly from the changes of density produced by the local increase of temperature in the light-absorbing material. The working principle of the thermal lens for the OPL of CW radiation could be exploited for the OPL of pulsed radiations as well, through the utilization of poorly absorbing liquids.5 In the particular case of the OPL of picosecond pulsed radiation at 1.06 μm of wavelength via self-focusing,6 the photoactive nonlinear optical (NLO) material was liquid CS2 (Figure 2a and b). In the case of OPL of a pulsed radiation, beside the nonlinear refractive index (n2) of the material,7 the onset of nonlinear transmission depends also on the f/number of the

where K, n0, w0, l, α, and T are the thermal conductivity (in J m−1 s−1 K−1, SI units), the refractive index of the medium at room temperature, the radius (in m) of the Gaussian beam at the point of incidence with the medium, the thickness of the medium (in m), the absorption coefficient of the medium (in m−1), and the absolute temperature (in K), respectively. For a 13045


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efficaciousness of the mechanism of RSA for the OPL function is dependent on light intensity, that is, the number of photons impinging the material per unit time, and on the photophysical characteristics of the active material.9 Such a mechanism can be considerably different for the various kinds of materials acting as OPL active systems, for example, semiconducting crystals27 or conjugated molecules.28 RSA consists of the involvement of electronic states at superior energies, which possess stronger optical absorption at the wavelength of irradiation with respect to the ground state of the same material. Such highly absorbing excited states are accessible only upon irradiation with intense light that promotes electrons in excited states with variable lifetime. The maximum incident energy (EMAX) and intensity (IMAX) sustainable by the optical sensor before its failure represent system-characteristic thresholds of safety. Specifically for the eye, EMAX is denominated maximum permissible exposure (MPE), which is equal to 0.2 μJ for pulsed light when the incident radiation is shorter than 15 ms.20 In terms of fluence, MPE becomes 0.5 μJ cm−2 in the conditions of continuous wave (CW). The optical density (ODsyst) related to these critical parameters of the systems is defined as

focusing lens (L1 of Figure 2a), the collection efficiency of the second lens (L2 of Figure 2a), and the polarization state (i.e., if linear or circular) of the incident laser radiation.5 For the OPL system of Figure 2a, the relationship between the incident power for the onset of the NLO regime (PNLO in W) and the material property n2 (in m2 W−1) is given by8 PNLO = A1(λ in 2 /n2)

(4)

with λin and A1 being the wavelength of the incident radiation, and an adimensional constant that depends on vacuum permittivity, respectively. The combined effects of thermal lensing and self-focusing presented in the prototypical optical power limiters are based on a change of speed of light in the nonlinear medium upon increase of incident intensity. Such an OPL mechanism operating in CS2, CCl4, nitrobenzene, and chloronaphthalene does not involve any strongly lightabsorbing state (either ground or excited) of these liquids, but fundamentally relies on an optically induced heating of the solvent.1,5 This is different from the case when the OPL effect is achieved mainly through the intervention of optical absorption with nonlinear features.9 Nonlinear optical absorption for OPL can include simultaneous and/or sequential multiphoton absorption in molecular materials10 and crystals,11,12 or intraband free-carrier absorption13 typical of inorganic semiconductors.14,15 Depending on the mechanism, the photophysical parameters of the photoactive materials, which control the extent of OPL mainly via NLO absorption, will be different.16 It must be pointed out that the observation of nonlinear absorption phenomena for the achievement of OPL does not exclude the concurrent intervention of nonlinear refraction and/or scattering that might follow or occur simultaneously with NLO absorption.17,18 The latter phenomenon refers generally to the alteration of the properties of optical transmission in a material subjected to an incident radiation with varying intensity (Iin) or fluence (Fin).19 Optical transmission properties of materials are commonly described through the parameters of (i) absorption coefficient (α in m−1), useful for homogeneous materials in the configuration of thin films, (ii) absorption cross-section (σ in m2), and (iii) molar extinction coefficient (ε in M−1 cm−1) as defined by the law of Lambert−Beer and adopted in the case of photoactive molecular materials. In regimes of NLO absorption, the other material parameters of relevance are the two-photon absorption cross-section (σ2 in m4 s/photon molecule), and the two- and three-photon absorption coefficient (β, in m/W, and γ, in m3/ W2, respectively). Radiation-induced changes can be profitable for OPL purposes20−22 when the nonlinear optical effect consists of the reversible increase of absorption in the irradiated material upon increase of Iin (or Fin) with respect to the unperturbed non irradiated state of the material itself. Such a NLO effect is indicated as reverse saturable absorption (RSA),23 and involves either the formation of excited electronic states through the simultaneous absorption of more than one photon (multiphoton absorption) at high irradiance,10 or the formation of excited states having optical absorption at the wavelength of irradiation (λin) stronger than the ground state in the case of cumulative sequential multiphoton absorption.24 In general, such highly absorbing states need to be populated at a sufficiently high rate with respect to their lifetime to become effective in the process of OPL.25 Moreover, because upper states population takes place upon pumping of the ground state, for the latter is required a minimum of absorption at λin to initiate the cascade process of multiphoton absorption.26 The

ODsyst = −log(E T /EMAX ) = −log(IT/IMAX)

(5)

where ET and IT represent the actual energy and the actual intensity reaching the optical sensor, respectively. To maintain the active status of the optical sensor and avoid the occurrence of its either temporary or permanent blinding, the condition ODsyst ≥ 0 must be constantly fulfilled. In doing so, any smart optical material (or device) that absorbs, scatters, and blocks9 the incoming light in a way that ET is below EMAX and IT not higher than IMAX will accomplish the desired OL function. On the other hand, in many cases optical sensors require also a minimum of luminous energy/intensity for a correct operation within the range of safety (this is particularly evident for the eye that loses operativity when IT < 10−6 cd m−2 in the visible range under a regime of scotopic vision). Therefore, to evaluate the OL action of a smart device/material with optimum features, one has to consider also another parameter depending on the characteristics of the sensor, that is, the upper limit of OD (ODMAX) defined as ODMAX ≤ −log(Emin /EMAX ) = −log(Imin /IMAX)

(6)

where Emin and Imin represent, respectively, the minimum energy and the minimum intensity values below which the optical sensor can no longer operate correctly and efficiently despite its safe status. The combination of eqs 5 and 6 imposes the conditions: Emin ≤ E T ≤ Emax

(7a)

and Imin ≤ IT ≤ Imax

(7b)

for the luminous energies and intensities to be transmitted by the optical limiter before reaching the optical sensor to warrant simultaneously safety and operativity of the sensor. The graph obtained by plotting the output energy ET versus the input energy Ein for a designed limiting material or device is of the type presented in ref 22. Ideally, the trend there presented should be displayed by the OPL material at all wavelengths of operation of the LSE, taking into account the dependence of Emin, EMAX, Imin, and IMAX on the wavelength (λ) of the radiation. 13046


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pilots,105,106 and already gave rise to main developments in meeting the necessary requirements for optical limiters.20,21,107−110 In a broad sense, a NLO material acting as an optical limiter strongly attenuates intense optical beams and performs low transmittance under high-intensity illumination while the optical beam passes through the limiter material linearly at lower light intensities. The development of powerful optical limiters has proven to be a somewhat demanding task because strong optical limiting materials are meager. While many strategies have been used for attenuating light intensity, the use of nonlinear optical materials that exhibit either strong nonlinear absorptive or strong nonlinear scattering responses is apparently the most promising in the limiting of nanosecond (ns) or picosecond (ps) laser pulses. Among powerful nonlinear absorptive materials under active investigations are organic dyes such as metalloporphyrins and metallophthalocyanines, mixed metal complexes and clusters, fullerenes, dendrimeric nanocomposites, polymeric materials (organic and/or inorganic), inorganic semiconductors, and nanoscopic materials, besides common organic materials, which will be further focused on in detail in this Review.

Another aspect related to the usefulness of a smart OPL material for sensor protection is its chemical-physical stability of the material in the environment where the optical sensor (the LSE) operates. It is particular important to consider the issue of the thermal stability of the photoactive OPL material in a temperature interval that might go from about 170 to 350 K on earth surface and the mesosphere. Within such a wide interval, the bulk of the photoactive material or its dispersion within a host structure could undergo phase transformations and/or phase separations, change of the aggregation state, or decompose if the active material is encapsulated in a closed system. In terms of chemical stability, the ambient conditions where LSEs are usually ad operated do not constitute a particularly severe environment with the exception of those photoactive materials for which O2 represents an oxidizing agent altering their photophysical properties, or the humidity of the atmosphere constitutes a factor of material degradation. Moreover, the solar radiation, especially in the UV portion, could pose other stability issues if the photoactive material is a metal complex or an organic molecule with frontier levels in the visible and near UV (vide infra). Historically, the field of experimental nonlinear optics had its advent with the observation of second harmonic generation by Franken et al.,29,30 in which coherent, high-intensity laser light having a center wavelength of 694 nm, irradiated on a sample of crystalline quartz, produced radiation at twice the frequency (or one-half the wavelength, i.e., 347 nm) of the incident radiation. Of course it is not innocent that the birth of experimental nonlinear optics closely followed the experimental realization of the laser.31,32 We have come a long way since those striking findings. Initial studies were based on materials responses,3,6,11,14,33−38 as well as on understanding the theoretical phenomena associated with the nonlinear optical effect.39−57 The own definition of nonlinear optical effect had its upgrading, and nowadays is referred to as the interaction between materials and high-intense electromagnetic field brought about from high-power laser pulse changes in the input optical parameters such as frequency and swing. Nonlinear optical (NLO) materials based on the above optical response have an important application in the manipulation of optical signals in optical communication and other optical signal processes and mechanisms.58−87 The development of modern optical technology demanded the ability to control the intensity of light in a prearranged and expected manner. For the manipulation of optical beams in the passive method,9 nonlinear optical materials whose transmittance decreases significantly with increasing light fluence have received more recent attention.9,88−102 These materials and so-called related devices have been named optical limiters or optical power limiters. There is great current interest in the development of organic and inorganic optical limiting materials for applications scoping from the protection of optical sensors from laser irradiation to all-optical switching. Rapid advances in laser technology have resulted in new superior laser systems that are compact, efficient, and operate at a variety of wavelengths. Undeniably, several common applications are omnipresent in everyday life, such as CD players, scanners, and lecture presentation pointers, to name a few. However, military applications have also remained swift. This increased presence of lasers in “friendly functions” and in potential enemy weapons systems represents significant vulnerabilities to the eyesight of military personnel,103,104 optical sensors, and both military and commercial aircraft

1.2. Optical Limiting Effect as Nonlinear Optical Process

1.2.1. Mechanism Based on Reverse Saturable Absorption. Devices like optical limiters or saturable filters show OL effect when the intensity of a light beam is strongly attenuated once the input intensity exceeds a threshold value (Ilim). The latter is determined by the characteristics of the system interacting with the light beam, and represents a critical parameter for the evaluation of the OL properties of the device. The ideal behavior of an optical limiter is shown in Figure 3, with Iout and Iin, respectively, being the intensity of the light beam transmitted by the optical limiter and the intensity of the incoming light.

Figure 3. (a) Trend of the light intensity Iout transmitted by an ideal optical limiter versus the incoming light intensity Iin. In the abscissae axis, the threshold intensity Ilim at which Iout saturates is indicated. (b) Energy diagram of a four-level system. Full lines indicate absorption from ground [1 → 3] and excited states [2 → 4]; dashed lines represent intersystem crossing [3 → 2] and excited-state fluorescence [4 → 2]; dotted line indicates phosphorescence [2 → 1].

The most remarkable feature of the plot in Figure 3a is the so-called effect of residual absorption,111 consisting of the flattening of the curve at values Iin > Ilim. This would correspond to a reduced transmittance once Iin > Ilim. In fact, the transmittance T defined as (dIout/dIin) and corresponding to the slope of the curve in Figure 3a is not constant for an ideal saturable filter and tends to zero when Iin > Ilim. Therefore, such behavior represents a case of NLO effect because the transmission of the system is not directly proportional to Iin in the operative range of Iin. The response 13047


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Iout versus Iin varies with the frequency ω of the incident light depending on the absorption properties of the optical limiter. As a consequence, the OL produced by a given system is verified only at those ω values, which are system specific. The verification of variable transmission as a function of Iin implies the existence of different processes of light absorption in the system at the various levels of irradiance. An adequate model accounting for the effect of residual absorption is the four-level system112 in which the absorption of an excited state is considered.113 The scheme of the energy levels for a four-level system is presented in Figure 3b. The system absorbs at the frequency of interest ω* through the transitions [1 → 3] and [2 → 4] being (hω132π) = (hω132π). In the latter equation, ωij indicates the frequency of absorption for the transition [i → j], and h is Planck’s constant. Therefore, the four-levels model predicts the occurrence of a two-photons absorption process due to the consecutive electronic transitions114 [1 → 3] and [2 → 4]. In the fourlevel model, the following assumptions are made: Energy levels have no degeneracy; intersystem crossing (ISC) transition [3 → 2] is fast (it is supposed that levels 1,3 belong to singlet states); and fluorescence from the upper excited state [4 → 2] is fast (it is supposed that levels 2,4 belong to triplet states). These assumptions are motivated by the fact that effective OL requires fast ISC to populate the excited state i = 2. Moreover, a high value of the absorption coefficient for the transition [2 → 4] is also required for the efficient absorption of the second photon. In the four-level model, the net absorption coefficient α at ω* is expressed by the formula:112,114,115 α = (α13Ilim + α24Iin)/(Iin + Ilim)

β = 5.3ε13ε24[N1]ΦISC

where [N1] is the concentration of the active entities in the ground state and ΦISC is the ISC efficiency at the working frequency ω*. The units of ΦISC are [mol s J−1]. The expression for α as formulated in eq 11 is more convenient than eq 8 because it can be directly used for the analysis of the NLO properties of the system. In fact, the term β can be directly correlated to the imaginary part of the third-order optical susceptibility χ(3) through the equation:116,117 Im[χ (3) ] = [(2c 2n0 2β )/(πw*)]*10−22 C m V −3

γ = χ (3) /Nf 4

(8)

(9)

(10)

to include the second photon absorption (z is the direction of light propagation in eq 10). Therefore, the four-levels model accounts for the variations of α with Iin (eq 8) and predicts that the saturation threshold Ilim decreases with the increase of the absorption cross-section for the transition from the ground state, and with the lifetime of the decay [2 → 1] (eq 9). In the four-level system, the net absorption coefficient for a sequential two-photon absorption process can be expressed also in the alternative form:114,115 α = α13 + βIin

(11)

with −(dIin /dz) = α13Iin + βIin 2

(15)

where N is the number of active species in the system per volume unit and f is a local electric field correction factor.114 In the calculation of the various elements constituting the fourth-rank tensor χ(3), the transition dipole moments are summed over all of the excited states taking into account all of the resonances of the molecule.120,121 This exhaustive treatment wants to point out the fact that the spectral properties of the system in the various excited states are of paramount importance in evaluating the OL properties of the system if multiple-photons absorption must occur.37 The performance of optical limiters based on the mechanism of excited-state absorption is generally evaluated in terms of several parameters, which are directly proportional to the excited-state cross section σε.88,122−124 These parameters are the ratio of excited-state to ground-state cross section σe/σg where σe can be a weighted average of σ[1→3] and σ[2→4] (see Figure 3b) or the difference (σe − σg).88,122,125 The OL properties of a saturable absorber must be also evaluated in terms of temporal response for a given pulse of incident light. In fact, the response time should be short enough to reduce the possible disruptive effects from the incoming intense light.111 The lifetime of the excited state i = 2 should be longer than the light pulse duration to have an accumulation of the population in this state. Moreover, the system should switch in a transparent state shortly after the light pulse is off.126 When the duration of the incoming light pulses is shorter than the time required to populate the state i = 2, in some cases, it can be verified that the second photon is absorbed by a transition between singlet states having i = 3 as initial state and j = n with n = 5 or higher as final state. Such a process is competitive with the ISC and intervenes in systems possessing more than four levels.127,128 This singlet−singlet transition can result as effective in optical limiting as well, if σ3n is high enough to be comparable with σ24.

In eqs 8 and 9, αij, τij, and σij are, respectively, the absorption coefficient, the lifetime, and the absorption cross-section for the transition between the levels [i → j]. Equation 1 implies a modification of the Lambert−Beer law according to the new formulation: −(dIin /dz) = Iin*[(α13Ilim + α24Iin)/(Iin + Ilim)]

(14)

with c and n0, respectively, as the speed of the light in vacuum and the linear refractive index of the system. In eq 14, χ(3) is expressed with the units of the international system (SI), which correspond to [C4 m J−3]. From eq 14, it descends that the OL effect is related to a third-order NLO property of the system. The latter result has two important consequences: the first is the absence of symmetry restrictions for the system in showing optical limiting unlike second-order NLO effects, which are instead symmetry-dependent.118,119 The second consequence is related to the fact that their verification does not require necessarily the occurrence of resonance at the frequency of excitation ω* or its multiples. At a molecular level, χ(3) is directly correlated with the second hyperpolarizability γ through the relationship:

with Ilim = hω*/(2πσ13τ21)

(13)

(12)

being the alternative modified version of the Lambert−Beer law for a two-photon absorption (TPA) process. In eqs 11 and 12, the term β represents a nonlinear absorption term, which averages the molar extinction coefficients ε13 and ε24 according to the relationship: 13048


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Excited-state absorption (ESA)119,129 sees the promotion of a molecule to the first excited state by absorption of just one photon, followed by the absorption of another photon from this excited before decaying to the ground state. To understand the origin of ESA effect, one has to postulate that a molecule is excited by the absorption of one photon. The effective absorption cross section of this material can be described as σeff = η0σg + η1σexc = σg + η1(σexc − σg)

process, the whole system becomes inhomogeneous at the boundary between the two components.15,150,151 Thermal processes in solution are capable of generating refractive effects, which can also be exploited in optical limiting. As a matter of fact, historically, the first optical limiter was based on thermal refraction.1,2,152 The thermally induced optical nonlinearity occurs when the media absorbs light and dissipates that energy in the form of heat, causing an expansion on the material and modifying then its refractive index. Generally, these optical limiters are not more than absorbing dyes dissolved in a media that possesses a high nonlinear refractive index, behaving as a good thermal conductor. One advantage of these simple systems is their large spectral band of action in the visible region. However, their response time is higher than several nanoseconds, which prevents its utilization for ultra short pulses. In the next sections, these mechanisms will be analyzed, but it can be now anticipated that NLS is a well-known phenomenon leading to optical limits in colloidal suspensions of silica particles, carbon particle suspensions, fullerenes, and nanoparticles. 1.3.2. Multiphoton Absorption. The possibility of simultaneous multiphoton absorption was first hypothesized by Goppert-Mayer in 1931,153 who proposed a mechanism by which photons of the same energies absorbed within a sufficiently short time window, that is, 1 ns) metastable excited states. Other authors500 prepared the organometallic clusters HFeCO 3 (CO) 10 (PMe 3 ), HFeCO 3 (CO) 10 (PPh 3 ), HFeCO3(CO)12, and [Net4]+[FeCO3(CO)12]− and performed OL experiments using 8 ns pulses at 532 nm and CH2Cl2 as solvent. The solutions with 70% of nominal transmission showed limiting throughputs of 350−700 mJ/cm2 without focusing. Figure 21 shows that optical limiting was mainly dependent on ligand substitution, while the counterion appeared to have only a minimal effect.

Scheme 1. Syntheses of 1−3 from Reactions of [MS4]2 (M = Mo, W) and CuCN/Cu[N(CN)2] with 4,4′-pytz Ligandsa

a Reproduced with permission from ref 501. Copyright 2015 John Wiley and Sons.

energy storage,506 water purification,507 in solar cells508 and biological applications, including drug delivery509 and theranostics.510 2.6.1. Gold Nanoparticles. In particular, gold NPs have been attracting much attention in nonlinear optics due to their characteristic optical transitions associated with the surface plasmon resonance.490,493,511−516 For example, some authors prepared Au NPs of different middling sizes (2.5, 9, and 15 nm) by γ-radiolysis and stabilized the nanoparticles in suspension with poly(N-vinyl-2-pyrrolidone).490 It was found that the larger AuNPs were more powerful in limiting of ns radiation at 532 nm. At the basis of the mechanism of this OL effect, there was the vaporization and/or fragmentation of the NPs regarded as light scattering centers.490,493 Other authors studied larger Au NPs (diameter 35 nm) and also compared their OPL properties with those of Au nanorods and composites of Au NPs encapsulated in dendrimers,514 or onto La−Pb borate glasses.515 The effect of thickness on the NLO properties of Au NPs produced via laser ablation of high-purity Au bulk in distilled water was investigated by measuring the transmitted intensity of the laser beam through the samples with increasing laser intensity.517 The third-order nonlinearity of Au NPs-fluid was determined with Z-scan experiments. The authors found that the magnitudes of both real and imaginary parts of third-order susceptibility of samples increased upon film thickness increase. Other authors518 prepared Au NPs for OL via the microwave irradiation method and obtained particles with average size 7.7 nm. They observed RSA at longitudinal surface plasmon resonance of the Au NPs, using Z-scan method and transient absorption spectroscopy with excitation at 532 nm and ns laser pulses. Measurement of the NLO performance in water by the Z-scan technique was also investigated using the Nd:YAG laser at 532 nm.519 The efficiency and the nature of the nonlinear response result depended on NPs size and aggregation state. The third-order optical susceptibility χ(3) assumed the values of

Figure 21. OL effect of the Fe−Co-based clusters. Reproduced with permission from ref 500. Copyright 1990 SPIE Publications.

The 3D-tetrazine chromophore-based metal−organic frameworks (MOFs) {(Et4N)[WS4Cu3(CN)2(4,4′-pytz)0.5]}n (1), {[MoS 4 Cu 4 (CN) 2 (4,4′-py tz) 2 ]·CH 2 Cl 2 } n (2), and {[WS4Cu3(4,4′-pytz)3]·[N(CN)2]}n (3) (4,4′-pytz = 3,6-bis(4-pyridyl)tetrazine) have been synthesized (Scheme 1) for OL.501 The NLO properties of 1−3 were investigated with the Z-scan technique and using 5 ns pulses at 532 nm. The study showed that the π-delocalizable tetrazine-based 4,4′-pytz ligands contributed to the strong third-order NLO properties, displaying third-order susceptibilities (χ(3)) of 1.23 × 10−11, 2.11 × 10−11, and 1.71 × 10−11 esu for 1, 2, and 3, respectively. 2.6. Metal Nanoparticles

Looking up to different sets of nanoparticles (NPs), the subfamily of those constituted by metal has also taken due attention regarding its NLO properties. There are several methods to prepare metal NPs. Some include photoreduction502 and reduction using various reducing agents in association with protective polymers or surfactants.93 The NPs of various single metals including Ag,502,503 Au,504 Cu,505 Pt,505 Pd,505 and Ru505 have been synthesized and characterized. However, the physical properties of metal NPs are still not fully understood. In recent years, awareness of the importance of these materials has led academia to find an array of applications, 13066


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6.34 × 10−6 esu for the smaller NPs and 2.13 × 10−5 esu for the larger NP agglomerations. The protection of gold nanocomposites with ligands such as terpyridine (tpy) and bipyridine (bpy) for OL was also tested in combination with C60.520,521 Both C60/tpy-Au and C60/bpyAu displayed stronger nonlinear refraction due to self-focusing effects than its counterparts C60tpy and C60bpy alone. This is because C60 and its derivatives own a relatively weak excitedstate refraction, although they display a strong ESA in visible and NIR ranges.522 Sun et al. reported that covalently bonded Au NPs/ polylysine hybrid materials show enhanced OL with respect to the single components, which was associated with the interparticle interactions.523 To better understand the possible effect of interparticle interactions, OL of Au NPs aggregates induced by the presence of electrolytes in NP solution was later studied.524 The effects of electrolyte concentration, the individual NP size, and different types of electrolyte on UV− vis absorption and OL have been investigated. It was concluded that aggregated Au NPs exhibit pronounced OL for ns pulses at 532 nm (Figure 22).524

were of positive sign. These studies revealed that the polymers were constituted by polymer/Au micelles of hydrodynamic radii between 25 and 53 nm. Similarly, the OL behavior of other thin polymer films doped with Au NPs was investigated under CW illumination at 488 and 514 nm.527,528 The effectiveness of the OL action was evaluated, by measuring the transmitted irradiance in a 300 ms time interval. This value corresponds to the blinking time of the human eye, and the experiment was carried out to assess the total fluence reaching the retina. Closed aperture Z-scan measurements reveal a negative nonlinear refractive index of around 10−6 W cm−2, comparable with that reported for liquid samples, due to the nonlocal thermal refraction process. Other authors have studied the NLO properties of different nanoparticles in suspensions by using the same excitation conditions and experimental technique (35 ps, 532 nm, and Zscan).529,530 In these studies, colloidal suspensions of Au were prepared and measured as well as the Au NPs encapsulated into hybrid block copolymer micelles with different size of Au NPs. It was found positive nonlinear absorption and negative nonlinear refraction for the Au colloidal suspensions. In particular, it was reported a nonlinear absorption coefficient of 9.4 × 10−12 cm/W, which is in agreement with the value determined by Iliopoulos et al.526 for the Au nanoclusters, and an opposite sign nonlinear refraction. However, because no details were given on the size of Au NPs and/or any datum of UV−vis absorption spectrum, it is difficult to proceed to a more direct comparison. In a parallel study, Zhan et al. have studied the NLO response of 1-dodecanethiol stabilized Au NPs in THF employing the same conditions as the previous studies, except that were carried out at 1064 nm laser pulses.531 They have reported negative nonlinear refraction (Re[χ(3)] = 2.69 × 10−13 esu). However, the size of these Au NPs was about 3.3 nm, which is significantly smaller than the size of the Au nanoclusters studied by Iliopoulos et al.526 This fact can explain the difference of the sign of the nonlinearity among the different reports and the results, as it is known that such size variations can be the origin of dramatic changes in the NLO response of metallic NPs. The nonlinearity properties of Au/Pd NP blends,532 prepared at different concentrations, have been investigated using single beam Z-scan with a CW Ar+ laser beam, at 514 nm. The Au/Pd NPs showed a good NLO response, and the sign of the nonlinear refractive index was found to be negative, in the order of 10−8 cm2/W. A nonstraightforward relationship was obtained for the nonlinear refractive index as a function of Au/ Pd ratio, because the nonlinear refractive index had an improved behavior when the Au/Pd atomic ratio was increased. Au, Ag, and Au−Ag alloy NPs,533 having hydrodynamic diameters from 26 to 30 nm, have also been evaluated for OL of ns laser pulses. The authors used a Q-switched Nd:YAG laser working at both harmonics, that is, at wavelengths of 1064 or 532 nm with pulse width of 3 ns. All samples showed attenuation at high input energies of at least 2 orders of magnitude. The best OL effect was achieved by the Ag NPs at 532 nm, while at 1064 nm there was nearly no difference in the NLO behavior for the three different systems. All three samples presented an attenuation of more than 2 orders of magnitude at high input energies. 2.6.2. Silver Nanoparticles. Also, other materials based on other metal composites were studied, for example, Cu NPs, in which particles’ size variation ranged from 6.6 to 30.5 nm using

Figure 22. OL curves of AuNP aggregates formed from 10 nm AuNPs at 1.00 M KCl with different apertures sizes in front of the output detector. Reproduced with permission from ref 524. Copyright 2006 American Chemical Society.

Free-carrier absorption rather than nonlinear scattering has been found to be the dominant contribution to the OL behavior as was also reported by other authors.525 Higher electrolyte concentration induced a better OL performance, and the aggregates formed from larger Au NPs exhibit improved OL. The types of electrolytes also show a prominent effect on the OL of Au NP aggregates. However, according to the authors, due to the lack of long-term stability (due to the occurrence of flocculation and that the formation of precipitates was observed shortly after OL experiments), the Au NP aggregates could not be good candidates for practical OL applications. Therefore, it remains an open question whether is worth continuing the study of such systems for their improvement or not.525 Later, the NLO response of Au NPs suspensions encapsulated into hybrid block copolymer micelles was investigated by using Z-scan technique, with 35 ps pulse at 532 nm.526 The block copolymer used for the encapsulation of the Au NPs was poly(isoprene-b-acrylic acid). All dispersions were exhibiting strong OL action, and real and imaginary parts of the third-order nonlinear susceptibility χ(3) of the dispersions were determined. Their values were of the order of 10−13 esu for Au NPs concentration ranging between 1 and 20 mM and 13067


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Z-scan technique with 6 ns pulses at 532 nm. The third-order susceptibility χ(3) was measured to be 1.38 × 10−9 esu with Ag NPs exhibiting strong RSA. The corresponding limiting threshold was 1.8 J/cm2. The Ag NPs synthesized by 200 keV Ag ions implantation with a surface dose of 2 × 1017 ions/cm2 have been studied for their third-order NLO properties,539 using the Z-scan technique at 1064 nm. Ag NPs displayed refractive OL effect at 1064 nm, with nonlinear susceptibility χ(3) = 1.8 × 10−7 esu. Similarly, other authors540 investigated the nonlinear response of AgNPs with several particle mean sizes in a low-power regime of electromagnetic field based on nonlocal thermo-optic models. The resulting OL was due to nonlinear refraction of the samples due to thermal lens formation under low-power CW excitation. When the laser power was low, the self-defocusing effect was mainly dominated by surface plasmon resonance effect. Silver colloidal solutions for OL were prepared in situ in the presence of poly(methyl methacrylate) (PMMA). PMMA was polymerized by reversible addition−fragmentation transfer and tested for OL using Z-scan technique with 13 ns pulses at 532 nm.541 The spherical Ag NPs had an average diameter of 10 nm. Enhancement of the optical nonlinearity was observed upon increase of concentration and was ascribed to surface plasmon resonance. The third-order nonlinear susceptibility χ(3) was measured to be 1.05 × 10−11 esu when the mass concentration was 2.13 mg/mL. The authors found also that the switch from saturable absorption to RSA at higher incident laser energy was responsible for the OL effect. Simple silver nanocolloids were prepared by chemical reduction, and their nonlinear absorption properties were investigated using the Zscan method in the open aperture configuration with ns laser pulses at 532 nm.542 The authors observed a switch over from saturable absorption to RSA when the input intensity was increased from 28.1 to 175.8 MW/cm2. This NLO effect was ascribed to the combination of ground-state plasmon band bleaching and excited-state absorption. Dispersions of Ag NPs in poly(vinyl alcohol)/tetraethyl orthosilicate matrix were also tested for OPL using single beam Z-scan technique at 532 nm with a Q-switched Nd:YAG laser.543 These authors obtained χ(3) values in the order of 10−10 esu and confirmed that the action mechanisms responsible for third-order optical nonlinearity of Ag nanocomposite film were RSA and periphery self-defocusing of surface plasmon resonance. Assuming a small volume ratio of the AG NPs in the composite, the authors estimated the thirdorder optical susceptibility of AgNPs to be of χ(3) = 2 × 10−5 esu. The nonlinear absorption in the sample enhanced with increasing input energy (Figure 24). The clamping levels of the Ag nanocomposite film were found to be ∼20, 17, and 14.5 μJ at the linear transmittance of 82.7%, 79.1%, and 73.2% when the Ag concentration was 0.41%, 0.83%, and 1.33%, respectively.543 The role of the surface plasmon resonance on the OL properties of Ag−Cu nanoclusters codoped in SiO2 matrix with a Cu/Ag molar ratio of 1, 2, and 3, respectively,142 was evaluated using the second harmonic of high-power ns and ps Nd:YAG lasers. The excitation near the surface plasmon resonance of Cu resulted in an enhanced OL behavior with increasing Cu concentration, while no dependence was observed when the excitation was near the surface-plasmon resonance of Ag. In any case, the limiting threshold was considerably reduced by approximately 10−15 times.

poly(N-vinyl-2-pyrrolidone) PVP, as stabilizer. These systems with precise stoichiometric ratios were simply achieved by varying the amount of the protective polymer PVP in the solutions. The χ(3)/α0 values for Cu colloids were found to be of the order of magnitude of 10−12−10−11 esu cm.534 It was reported535 that AgBr particles trapped in nanosols exhibit interesting OL characteristics. Other investigations considering Ag NPs, normally stabilized with PVP as polymeric system, have been carried out. Some results suggest that these materials may represent a new class of potent optical limiters. The authors claimed the OL performance of the silvercontaining nanoparticles in stable suspensions was reported among the best of all known OL materials toward ns laser pulses at 532 nm, when compared to C60 and chloroaluminum phthalocyanine in solution, that is, the benchmarks for highperformance optical limiters (Figure 23).535

Figure 23. OL responses of the nanocrystalline Ag metal particles in PVP polymer-stabilized ethanol suspension (○) of 90% linear transmittance at 532 are compared to those of C60 in toluene (□), chloroaluminum phthalocyanine in DMF (▽), and the Ni NPs in DMF suspension (△) of the same linear transmittance. Reproduced with permission from ref 535. Copyright 1999 American Chemical Society.

Ag-containing NPs show relatively weak absorption in much of the visible region. Moreover, their transient absorption spectrum is also broad.536,537 Therefore, the Ag-based materials may be used as broad band optical limiters. Between Ag NPs and Ag2S NPs for OL, the first is chemically rather less stable than the second and more susceptible to oxidation under ambient conditions.535,536 Rao and co-workers538 managed to produce a simple and eco-friendly biosynthetic strategy to obtain Ag NPs, using Coriandrum sativum leaf extract as reducing agent. The aqueous Ag+ cations when exposed to leaf extract were reduced and resulted in AgNPs whose average size was 26 nm. The NLO properties of the Ag NPs thus obtained were then studied using 13068


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Figure 25. Fluence-dependent transmittance of thin-shell hollow spheres of Cu2O and CuS at 1064 nm. Reproduced with permission from ref 546. Copyright 2008 American Chemical Society.

Figure 24. OL behavior of Ag nanocomposite films. Reproduced with permission from ref 543. Copyright 2014 AIP Publishing LLC.

Composed NPs such as the TiO2−CeO2 nanocomposites were also studied for OL.548,549 Open aperture Z-scan technique with Nd:YAG laser (532 nm, 7 ns, 10 Hz) was used. It was found that the bandgap of the nanocomposite could be tuned by varying the Ce/Ti atomic ratio. As the CeO2 amount increased, the bandgap of the nanocomposites decreased, and the authors proposed that this bandgap change was the smearing of localized states of Ce3+ into the forbidden energy gap, thus acting as the intermediate state. Also, the TPA coefficient increased with the increase of CeO2 amount, due to the modification of TiO2 dipole symmetry. The OL thresholds are represented in Figure 26 for several nanocomposites. At 196

544

Kik and co-workers observed a significant increase in the thermally induced NLO response in compositionally heterogeneous plasmonic Ag−Au−Ag trimeric NPs, and compared them to plasmon resonant Au monomeric and Ag dimeric NPs. The nonlinear absorption performance of the trimer exceeded the peak performance of isolated Au NPs by a factor 40, which was attributed to cascaded plasmon resonance. This could occur for the extreme field concentration in the central nanoparticle of the trimer. Nanoparticles constituted of metallic salts are also attractive for NLO and OL applications. For instance, Ag2S NPs were synthesized by ns pulsed laser ablation of a silver plate in dimethyl sulfoxide, 545 and their OL performance was investigated under irradiation with ns pulses at 532 nm. Ag2S NPs samples displayed OL behavior, and the observation of an asymmetrical configuration of the data indicated that nonlinear scattering was occurring in this materials. 2.6.3. Other Metal Nanoparticles. Uniform hollow spheres of Cu2O and CuS were successfully synthesized by chemical transformation of in situ formed sacrificial templates containing Cu(I) in aqueous solutions.546 The shell thickness of these hollow spheres was about 130−180 nm (thick-shell) and about 20−25 nm (thin-shell). The OL properties of the thin-shell hollow spheres of Cu2O and CuS were characterized with ns pulses, and strong OL responses were detected for both the Cu2O and the CuS hollow spheres. The transmittances of the suspensions formed by the CuS hollow spheres are continuously attenuated with increasing incident fluence from very low fluence values, suggesting that the large OL effects occurred at an incident fluence much lower than that for the Cu2O hollow spheres (Figure 25). Here, the linear transmittances for both Cu2O and CuS hollow spheres were determined by the linear transmittance spectra obtained from the normal optical absorption. The thresholds of Cu2O and CuS were measured to be 1.78 and 0.51 J/cm2, respectively.546 Silica-coated core−shell TiO2 nanoparticles were synthesized by hydrothermal method547 and evaluated for their OL performance using the open aperture Z-scan in both pulsed and CW modes at 532 nm. RSA was observed during the open aperture Z-scan using low power CW Nd:YAG laser and was more efficient in the core−shell nanoparticles when compared to individual nanoparticles. NLO transmission measurements at 532 nm using 5 ns laser pulses on the TiO2 as well as TiO2@ SiO2 core−shell nanoparticles showed the existence of effective three-photon absorption at this wavelength.

Figure 26. Optical limiting response of TiO2−CeO2 nanocomposites. Reproduced with permission from ref 548. Copyright 2014 Springer Science and Business Media.

MW/cm2 intensity pulses, when Ti content was 75% (Ce content = 25%), a limiting threshold of 183 MW/cm2 was observed (t75c25), while for a Ce content of 75%, the limiting threshold became 136 MW/cm2 (t25c75). TiO 2 nanoparticles doped with several metals, like platinum550 or zirconium,551 were prepared by sol−gel method. Open aperture Z-scan measurements conducted at 532 nm using 7 ns laser pulses showed nonlinear absorption arising from a TPA process, and the OL performances of nanoparticles were considerably enhanced with increasing concentration of the metals. BaTiO3 nanoparticles552,553 of different sizes were prepared, and their NLO absorption properties were evaluated by the single-beam open aperture Z-scan technique using 5 ns laser pulses at 532 nm. Results showed that all samples exhibited a size-dependent NLO response, with OL efficiency increasing with grain size. 13069


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incidence. Therefore, the NLO process operating in the suspensions of PbS particles is believed to be the nonlinear scattering depending upon the level of incidence and resulting in the loss of transmitted energy. When this phenomenon occurs, the contribution of the bubbles due to the heat transfer from the particles to the surrounding liquid is the main cause of OL occurrence. The characteristic times for laser-induced bubbles growth are short enough to allow an efficient limiting of ns laser pulses, these entities being considered as the scattering centers.559 The nonlinear absorption and scattering of aluminum nanoparticles (Al NPs) in various polar and nonpolar solvents were studied with open aperture Z-scan technique at 532 nm under ns pulse excitation.560 The studies showed that the extent of OL due to nonlinear scattering was comparable to that of TPA. Both TPA and nonlinear scattering coefficients were estimated by the theoretical fit of the open aperture Z-scan curves. Co NPs have been studied,561,562 and showed an OL effect stronger than benchmark C60. The dependence of OL on the solvent used was also evaluated. It was found that solvents like chlorobenzene and chloroform managed to display limiting thresholds of 0.51 and 0.56 J/cm2, respectively. Poly(vinylpyrrolidone) PVP-stabilized Pt NPs563 were investigated with 8 ns pulses at 532 nm. The main mechanism at the basis of their OL was attributed to interband transition of platinum during the excitation of ns pulses, giving near 10 times attenuation of the incident energy. Silicon-based nanoparticles are also attractive nanostructures for use in the field of OL. While not a metal, semimetallic Si can also produce nanoparticles of several sizes. For instance, Mansour564 investigated samples of Si NPs for their nonlinear thermo-optical response using the Z-scan technique. The experimental results showed that the thermo-optical response of the studied solvents (water and DMSO) was enhanced by the presence of Si NPs. Low power OL, with low limiting threshold, was obtained, based on the phenomenon of thermal defocusing. Si NPs dispersed in toluene were also tested with ns laser pulses at 532 nm.565 Their OL properties were found to be better than standard optical limiter fullerene C60 in toluene with a threshold about 3 times smaller. The main mechanism of the resulting OL effect was nonlinear absorption and thermal nonlinear refraction, besides nonlinear scattering. Samples of hydroxyapatite,566 a naturally occurring mineral (Ca10(PO4)6(OH)2, HAP), were coated with silica, producing HAP@SiO2 core−shell nanoparticles. These were evaluated for OL and compared to the pure HAP nanoparticles. The NLO properties of HAP and HAP@SiO2 nanoparticles suspended in ethylene glycol were studied at 532 nm using 5 ns Nd:YAG laser pulses. Results pointed to an effective three-photon absorption, which was enhanced in the case of HAP@SiO2 nanoparticles. 2.6.4. Magnetic Nanoparticles. Magnetic nanoparticles possess superparamagnetic properties and offer great potential in a variety of applications either in their bare form or through surface coating with functional groups chosen for a specific application.567 A report on an optical limiter based on ferrofluids like stable colloidal suspensions of nanomagnetic materials (typically magnetite or cobalt), which are suspended in a suitable base fluid,568 appeared. These systems were impressive in terms of a high shelf life and remarkable thermal stability. Both of these characteristics are important requirements for sustainable use with intense lasers.569 The colloidal suspensions contain

Additionally, ZrO2 nanoparticles were synthesized by the chemical precipitation method and coated with silica.554 The OL performance of these nanostructures was studied using open-aperture Z-scan technique in which ns laser pulses at 532 nm have been used. Both pure and silica-coated ZrO 2 nanoparticles displayed good OL behavior due to the ESA arising from three-photon absorption. In another work,555 birnessite-type manganese oxide (γ-MnO2) nanoparticles having 50 nm size have been synthesized using the gelation routes method. Using closed and open-aperture Z-scan with a CW He−Ne laser at 632.8 nm modulated at different incident powers, nonlinear negative refractive indices in the order of 10−7 cm2 W−1 were obtained. Continuing, other metals nanoparticles such as those of nickel,556 antimony,557 or yttrium558 have also been reported for their NLO properties. While Sb2Se3 nanoparticles presented a TPA coefficient of 5 × 10−10 m/W at 532 nm,557 at the same wavelength NiS NPs displayed a very effective limiting threshold of 0.3 J/cm2 originating from induced nonlinear scattering.556 Yttrium iron garnet (Y3Fe5O12) nanoparticles, on their turn, showed strong RSA for 10 Hz and 7 ns laser pulses at 532 nm and OL intensity threshold of 103 MW/cm2 at input laser pulse intensity of 152 MW/cm2.558 Broadband OL effects have been observed at the wavelengths of 355, 1064, and 430 nm and every 30 nm from 470 to 620 nm for ethanol dispersed lead sulfide (PbS) NPs prepared via a surfactant micelle template-inducing reaction.559 Characterization of the nonlinear light-scattering process in the suspensions of PbS nanoparticles was performed at 532 nm measuring the energy of the scattered light as a function of the angle between the transmitted light and the scattered light. The angular distribution of the scattered energy from 10° to 165° is shown in Figure 27, where the distribution of the scattered

Figure 27. Angular dependence of the light scattered by a suspension of PbS NPs at the incident fluence of 0.49 J/cm2. Reproduced with permission from ref 559. Copyright 2008 Springer Science and Business Media.

energy can be observed. As shown in Figure 28, the rate of the scattered energy with the variation of the incident energy at a fixed angle of 20° was measured. The scattering rate increased linearly at lower incidence and stayed almost constant at high

Figure 28. Scattering rate as a function of the incident fluence at the scattering angle of 20° in PbS nanoparticles suspension. Reproduced with permission from ref 559. Copyright 2008 Springer Science and Business Media. 13070


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W for ns and 10−15 m/W for fs laser pulse energies, respectively. Protection of magnetic nanoparticles was found to be an effective manner to obtain long-standing nanoparticles.574 Carbon-encapsulated Fe, Fe3C, and NdC2 nanoparticles with the mean size below 50 nm were investigated under excitation with 532 and 1064 nm with 7 ns laser pulses. All of these protected magnetic NPs exhibited poor OL with the best limiting threshold of 5.4 J/cm2. Spinel ferrites,575,576 NiFe2O4, Ni0.5Zn0.5Fe2O4, ZnFe2O4, Ni0.5Co0.5Fe2O4, and CoFe2O4 with an average particle grain size of 8 nm were also investigated for their OL performance using the open aperture Z-scan technique. The obtained nonlinearity was fitted with a two-photon like absorption process. The nonlinearity was found to increase with increasing particle size, and upon comparison of the OL properties, ZnFe2O4 was found to be a better candidate for this specific NLO application. The NLO performance of other magnetic nanoparticles, Co3O4577 and V2O5,578 has been also studied. An enhancement of the nonlinear optical performance was obtained with the formation and the strengthening of the photonic band gap of the Co3O4 nanoparticles, under high power illumination.577 Nanocomposites of Ni/NiO, Ni/NiCo2O4, and Co/Co3O4579 have been synthesized by a chemical reduction technique, and their NLO transmissions were measured using the open aperture Z-scan technique, which employed 7 ns laser pulses at 532 nm. Experimental results have shown that the NiO/ NiO−Co3O4/Co3O4 nanocomposites exhibited generally good OL performance, with strong nonlinear absorption caused by TPA and excited-state absorption. Another type of magnetic nanoparticles, Ni−Ag and Fe−Ag bimetallic NPs,580 was prepared for OL and analyzed using the open aperture Z-scan technique (5 ns as well as 100 fs laser pulses). The effective nonlinear absorption in Ni−Ag and Fe− Ag nanoparticles was found to be higher than that of pristine Ag NPs. Some variations have also been introduced, for instance, the use of metal nanowires (MNWs), the application of which have been explored mostly in high-density storage devices.581−583 The first report on the OL properties of MNWs was published by Lin et al.584 using several metal nanowires. The authors found that the limiting threshold at 532 nm is 0.9, 1.2, 1.3, 1.7, 2.5, and 4.2 J/cm2 for Pd, Ni, Pt, Ag, Cu, and Co NWs, respectively (Figure 30). Only Cu and Co show slightly lower OL performances than SWNTs or MWNTs.486,585 Also at 1064 nm, Cu and Co NWs have slightly poorer performance, the limiting thresholds of Pd, Ni, Pt, and Ag NWs being 8, 8, 8, and 10.8 J/cm2, respectively, and larger than 30 J/cm2 for Cu and Co NWs. The observation of OL for all MNWs at 1064 nm suggested that nonlinear scattering was the major mechanism responsible for the nonlinear limiting of the MNWs, because the energy of 1064 nm laser light was too low (1.16 eV) for single-photon excitation. The latter generally requires much higher photon energy (e.g., 2.15 eV for Cu and 3.6 eV for Ag).586 Continuing these studies, the same authors studied Au nanorods (Au NR)587 and Au NWs588,589 solutions to understand better the mechanisms at the basis of their OL effects. In Z-scan measurements, the increase in the laser intensity induced bleaching in the ground-state plasmon absorption of Au NR. This resulted in an increase of transmittance (effect of saturable absorption), when laser irradiance was less than 7 GW/cm2.587 For higher laser

nanosized particles of approximately 80 Å diameter. The NLO transmission of the samples was studied using ns and fs laser pulses. Excited-state absorption phenomena contributed to enhance limiting in the ns excitation regime. Several advantageous features of ferrofluids in terms of device applications are that their optical properties can be controlled by an external magnetic field, besides the good thermal stability and the resistance against the agglomeration.569 The authors studied the samples at two different concentrations, in addition to the pure base fluid (kerosene). Samples having linear transmissions of 0.70 (FF1) and 0.50 (FF2) at the respective wavelengths of analysis were employed in all measurements (Figure 29). When excited with ns pulses at 532 nm, the base fluid kerosene displayed a weak limiting effect, while the ferrofluid suspensions showed a much stronger limiting behavior.

Figure 29. Fluence-dependent transmission of the base fluid (left) and the samples (right) for 532 nm, 7 ns laser pulse excitation. Circles denote experimental data, and solid curves are numerical fits. Reproduced with permission from ref 569. Copyright 2008 American Institute of Physics.

The single beam Z-scan technique has been used to measure the third-order nonlinear refractive index, γ, and the TPA coefficient, β, for quantum-confined Fe2O3 nanoparticles570 coated by an organic molecular layer in toluene and naked particles in hydrosol. For coated Fe2O3 nanoparticles, the ratio γ/β was 5-fold enhanced, when compared to the naked sample. The magnitude of β was ∼1.0 cm/GW for the coated nanoparticles. From the values of γ and β for the coated and bare samples, the authors directly evaluated the figure of merit, Flim, of 0.8 and 4, respectively. Later, the same authors571 investigated the same type of Fe2O3 magnetic nanoparticles using the single-beam Z-scan technique with Ar+ and Ne−He lasers. They reported a large effective nonlinear coefficient, η2 = −8.07 × 10−7 cm2 W−1, thus demonstrating that the NLO response was originated from quantum confinement effects. Iron oxide nanoparticles572 of diameters 31, 44, and 61 nm dispersed in toluene were studied for OL under exposure to ns laser pulses at 532 nm. In the low fluence region, smaller size nanoparticles displayed better OL, when compared to larger size nanoparticles, while in the high fluence region all three samples showed similar limiting performance, with the mechanism governing OL being nonlinear scattering. Other authors573 investigated hexagonal Fe2O3 nanoparticles, using open aperture Z-scan technique with the 5 ns and 100 fs laser pulses at 532 and 800 nm. They observed that at relatively low pulse energies, the samples presented saturable absorption, but at higher energies a switchover from saturable absorption to effective RSA via TPA occurred in both excitation domains, with TPA coefficients calculated to be in the range of 10−10 m/ 13071


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2.7. Carbon-Based Nanoparticles

Systems exhibiting strong transient light scattering due to photoinduced thermal processes, such as carbon black suspensions (CBS), have also been investigated extensively for their strong OL responses over a broad wavelength range. The first of these studies dates back to 1989.133,135,593−595 Normally CBS are studied as suspensions containing carbon nanoparticles with less than 200 nm size.133 Carbon nanoparticles are usually stabilized with a surfactant. Experimental techniques such as the sol−gel method,596 chemical vapor deposition,597 and pulsed laser deposition598,599 can be used to grow carbon nanoparticles. CBS have typical OL responses toward ns laser pulses, and constitute the best examples among the different classes of known optical limiters.93 As a result, CBS are sometimes used as standards in the development of optical limiting materials.600 The understanding of the OL phenomena led some authors to perform a study in which it was concluded that OL response of CBS was dependent on solvent characteristics, as well as on the laser pulse duration. The studies also showed that in the NLO regime determined by ns pulses, the solvent infers less effect on the OL properties.601,602 In particular, the boiling point of the solvent was the most important physical property that played some role in OL properties of CBS.603 Evidence was found that OL threshold was directly dependent on the particle size, but independent of the number of carbon black particles per unit volume in the laser beam path.133 A lower linear transmittance due to a larger number of carbon black particles per unit volume resulted in lower saturated output fluence at the OL level. Moreover, due to the presence of particles in suspension, it was established that OL of CBS suspensions was due to a scattering mechanism.93,133,595 Carbon nanoparticles absorb light and rapidly heat and vaporize the solvent after excitation with a laser pulse. The expanding microplasmas created during the vaporization acted as scattering centers.93,601,604 In addition, the microplasmas rapidly heat the surrounding solvent to form bubbles that strongly enhance the light scattering effect. This can last several microseconds.93 At higher input laser powers, the formation of microplasmas and bubbles is more rapid and more violent, resulting in a significant increase of the phenomenon of nonlinear scattering. The formation of the scattering centers occurs over a period of several nanoseconds, thus making CBS most effective for limiting laser pulses of longer than 10 ns, but ineffective for picosecond laser pulses.605−608 Similar results were obtained by other authors, ascribing their finding to the occurrence of a scattering mechanism.609 When carbon nanoparticle colloids fabricated by laser ablation of a glassy carbon target in water are used instead, OL occurrence is stronger than the benchmark compound, C60, and a quicker decrease in transparency toward the 532 nm laser beam is observed for carbon nanoparticles with respect to C60 when the incident laser fluence is above 0.6 J/cm2. The effects of particle concentration on the OL properties of carbon nanoparticles were also studied in the same work (Figure 31). The higher was the concentration, the stronger was the limiting effect. Yet, as the authors also claim, the improvement of the limiting response at high concentrations is a consequence of an increase in nonlinear scattering sites and not to an improvement of the same scattering effect.610,611 Also, the use of aqueous suspensions of carbon nanoparticles for OL studies at 1064 and 532 nm (first and second harmonic of Nd:YAG laser) was considered. The authors verified the existence of

Figure 30. OL response of several MNWs measured with 7 ns laser pulses at (a) 532 nm and (b) 1064 nm. Reproduced with permission from ref 584. Copyright 2006 American Institute of Physics.

intensities, it was found that the RSA mechanism becomes dominant. This change of mechanism was attributed to freecarrier absorption and/or the formation of strong light scattering centers due to the vaporization of the initial particles induced by the strong laser pulse.490,514 The limiting threshold of the Au NR solution was as high as 24 GW/cm2. The determination of the saturable absorption properties of Au NWs was also carried out realizing Z-scan measurements. Saturation effects were obtained at the low excitation level of 8.0 GW/cm2, which was far below the irradiances where RSA became apparent. Several two-photon absorbers based on NW systems have been studied, and their nonlinear absorption coefficients were determined (Table 3). The magnitude of the different nanostructured materials is quite similar, meaning that few factors have actual importance in the control of the OPL properties, the particle size being the most important feature that one must think of when designing an optical limiter of this type. Table 3. Effective TPA Coefficients Reported in the Literature for Metal and Semiconductor Nanowires and Nanorodsa material

β (×10−11 W/m)

ref

CdO nanowires Te nanowires Ag2Te nanowires Au nanorods Bi nanorods

59.3 3.8 15 5.3 5.3

590 591 591 514 592

a

All values have been obtained with laser excitation at 532 nm, using pulses of 5−10 ns. 13072


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1).614 Also, 5 displayed good OL performance, with a limiting threshold of 0.41 J/cm2.615 Chart 1. Polyhedral Oligomeric Silsesquioxane Compounds Tested for NLO

Figure 31. (a) Comparison of the nonlinear transmittance of carbon nanoparticles in water, of C60 in toluene at 532 nm measured toward a 7 ns laser beam. (b) Nonlinear transmittance of solutions with different concentrations of carbon nanoparticles. The linear transmissions at 532 nm of samples 1−3 are 90%, 80%, and 60%, respectively. The corresponding concentrations are 3, 10, and 36 mM, respectively. Reproduced with permission from ref 610. Copyright 2004 American Institute of Physics.

3.1. Metal−Organic Complexes

In recent years, metal−organic complexes have attracted intense interest for NLO applications, such as OPL. The interest in these materials is based on the fact that the metal to ligand or ligand to metal charge transfer transitions occurring in these complexes can polarize the complex and enhance its nonresonant NLO response. Several authors claim that, when compared to inorganic materials, organic compounds possess a number of advantages, regarding its NLO processes and properties, which include a higher optical damage threshold with respect to inorganic crystals, straightforwardness of synthesis and preparation, structural diversity, and molecular design flexibility. Anyway, organic materials have several drawbacks, like low energy transitions in the UV−vis region (this might improve the NLO efficiency in some range of optical frequencies), but results disadvantageous in terms of compromise between nonlinear efficiency and optical transparency. Moreover, organics present low thermal stability. Hybridization of materials, through the outbreak of organometallic complexes, can combine some virtues from organic molecules, for example, the ownership of large NLO responses, fast response times, and ease of preparation. Nevertheless, they show much greater flexibility of design, for example, by variation in metal and/or ligand environment as well as geometry.616 The optical properties of an erbium two-photon dye salt (Chart 2) were investigated in THF solutions.617 When compared to the monomeric species, the dye salt photoluminescence was considerably quenched, while nonlinear absorption for IR ns excitation was comparatively improved. The fs Z-scan measurements suggested the same TPA coefficient for the dye in both neutral associated and separated ionic forms. The explanation given by the authors was based on the effects generated by the presence of erbium: it enhances the ISC rate; it leads to an efficient triplet state population for the subsequent triplet−triplet ESA when excitation is ns laser pulses in the visible spectrum and the initial NLO process is

nonlinear light scattering in the wide spectral range of 400− 1060 nm (for pulse durations of about 10 ns) and a decrease in efficiency of nonlinear light scattering with the shortening of pulse duration when the energy carried per the single pulse was the same.612 Both luminescent carbon nanoparticles and nanocages were simultaneously synthesized using laser ablation of CBS in poly(ethylene glycol).613 Their OL performance was evaluated using open aperture Z-scan technique at the laser wavelength of 532 nm. The authors observed that the mixture of luminescent carbon NPs and nanocages showed a much stronger OL response than carbon black, or the luminescent carbon NPs and nanocages alone in the PEG500 solution.

3. HYBRID ORGANIC−INORGANIC MATERIALS Organic−inorganic compounds are materials that encompass features of both organic and inorganic nature. The advantage of synthesizing this type of systems arises from the fact that their properties are not simply the sum of both counterparts alone, but rather the consequence of the synergy between the different parts that create a multifunctional system as a whole. Not exactly being inorganic−organic compounds, the silicones, we consider here the necessity to classify polyhedral oligomeric silsesquioxanes (POSS)-based materials as organic− inorganic materials. Some of these materials have been investigated for their OL performance.614,615 Hybrid compounds 4−6 displayed good OL properties, large NLO response, and high thermal stability, with χ(3) values ranging from 5.75 to 9.52 × 10−12 esu for 4 and 6, respectively (Chart 13073


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Chart 2. Transition Metal Complexes Used for NLO

TPA. The ISC rate was found to be 3 orders of magnitude larger for the erbium two-photon dye salt in comparison to that in the monomer form.617 Thiourea-based metal complexes with structure type 8 (Chart 2) have been also scrutinized for OL618−623 and displayed appreciable performance, having limiting thresholds in the range of 5−15 J/cm2 or nonlinear coefficients in the range of 10−11 esu. The uses of dye metal complexes as optical limiters, for their RSA characteristics, have been also considered. The most common strategies for OL investigation relate to their incorporation in polymeric thin films. For instance, naphthol Green B dye (9, Chart 2) embedded in PVA films was fabricated, and their nonlinear absorption properties were investigated using the Z-scan technique under excitation with 5 ns laser light pulses at 532 nm.624 The materials exhibited saturable absorption at the lowest intensity and concentration, switching to RSA on increasing either laser intensity or dye concentration. Nonlinear absorption coefficients (β) were of the order of 10−7 cm/W. Mercurochrome625 (10, Chart 2) and 2-(2-methoxybenzyllideneamino)-5-methylphenylmercuric chloride dyes626 (11, Chart 2) have also been used as dopants for PMMA, with both materials displaying significant nonlinearities. The third-order nonlinear optical properties of metal−organic coordination compound 12 (Chart 2) were investigated by open-aperture Z-scan measurements using Nd:YAG laser (532 nm, 5 ns).627 The third-order nonlinear optical transmission and optical limiting of compound 12 was induced by TPA and RSA mechanisms, with β = 1.5 cm/GW. Several dithioacetate and dithiolate mono- and dianions metal complexes628 (13−15, Chart 3) have been synthesized and characterized through Z-scan measurements, with some showing significant third-order NLO characteristics. Tetraalkylphosphonium cations were utilized in combination with the nonlinear anions to minimize electrostatic interactions within the salt. This led to the attainment of materials representing room-temperature ionic liquids (RTILs). Most of the materials exhibited nonlinear absorption, either through TPA or through RSA, which is desirable for applications as OL. A series of polymeric materials has been investigated with 532 nm laser pulses of 8 ns duration by Z-scan experiments in DMF solutions.629−632 Table 4 gives the χ(3) parameters of the reported metal−organic polymers. It is readily observed that the third-order NLO properties of 1-D polymers are stronger than those of 2-D polymers formed with the same ligand. The

Chart 3. Metal-Based Anions in RTIL Materials: Ni Dithiolate Complexes, [Ni(dmit)2], Metal Thiocyanate Complexes [(MSCN)8], and Cobalt Dicarboliiide [CoCB]

Table 4. List of χ(3) Parameters of Some Metal−Organic Polymersa coordination polymer

χ(3) (10−12 esu)

ref

[Zn(bbbt)(NCS)2]n (1-D) [Co(bbbt)2(NCS)2]n (2-D) [Mn(bbbt)2(NCS)2]n (2-D) [Zn(pbbt)(NCS)2]n (1-D) {[Co(pbbt)2(NCS)2]·H2O}n (1-D) {[Ni(pbbt)2(NCS)2]·H2O}n (1-D) [Pb(bbbm)2(NO3)2]n (2-D) {[Co(bbbm)1.5(NO3)2]CH3OH}n (2-D) {[Zn(NCS)2(bpfp)2]·2H2O}n (2-D) fcz {[Zn(fcz)Cl2]·CH3OH}n (1-D) {[Cd(fcz)2Cl2]·2(CH3OH)(H2O)}n (2-D) {[Co(fcz)2Cl2]·2CH3OH}n (2-D)

16.2 2.05 1.27 10.8 29.4 10.7 16.7 26.7 2.53 0.35 4.48 0.32 0.37

630 631 631 629 629 629 630 629 632 629 629 629 629

fcz = α-(2,4-difluorophenyl)-α-(1H-1,2,4-triazol-ylmethyl)-1H-1,2,4triazole-l-ethanol (common name: fluconazole); bbbt = 1,1-(1,4butanediyl)bis-1H-benzotriazole; pbbt = 1,1-(1,3-propylene)bis-1Hbenzotriazole; bbbm = 1,1-(1,4-butanediyl)bis-1H-benzimidazole; bpfp = N,N-bis(4-pyridylformyl)piperazine. a

χ(3) value of [Zn(bbbt) (NCS)2]n (1-D) is 16.2 × 10−12 esu and is 1 order of magnitude larger than that of [Co(bbbt)2(NCS)2]n 13074


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favorable NLO properties. This led to the conclusion that the heavier atoms Cu or Ag (together with I counteranion in both cases) played a vital role in determining its NLO properties.633,634 RSA of several platinum ter/bipyridyl phenylacetylide complexes (18−20, Chart 5) has been investigated at 532 nm with ns pulses.635−637 All complexes showed RSA, mainly attributed to ESA from triplet states, with limiting thresholds ranging in the broad interval 8−30 mJ/cm2. Sun and coworkers studied binuclear platinum tripyridyl complexes 21 and 22 (Chart 6), carried out at 532 nm using a 4.1 ns (fwhm) Nd:YAG laser.638,639 Compound 21b exhibited the best OL action, with a limiting threshold of 0.075 J/cm2. The transmittance decreased to 17% at an incident fluence of 1.7 J/cm2, that is, a 78% drop in comparison to the linear transmission. Additionally, Zn(II) terpyridine complexes 23 (Chart 7) were also studied for OL purposes.640 The study demonstrated large TPA cross sections, the chlorine bearing complex being the system with superior OPL due to a larger TPA cross-section. The investigations were also extended to other metal-pyridyl complexes using Ru as metal (24−26, Chart 7).641−643 Similar conclusions about the OPL capacity were withdrawn. For instance, the output fluence began to deviate from the 89% linear absorption curve at 1.0 J/cm2 for compound 24 (Figure 32). When the incident fluence was increased to 2.3 J/cm2, the transmittance of complex 24 solution dropped to 80%, indicating RSA. However, they found a difference in the nonlinear absorption of ns and ps laser pulses, probably due to the relative contributions from the excited singlet−singlet and triplet−triplet transitions. It was found strong saturable absorption for ps laser pulses, but slight RSA for ns laser pulses.641 Compound 25 (Chart 7) also exhibited significant NLO properties in the NIR spectral region, displaying a χ(3) value of 1.8 × 10−10 esu.642 This result was consistent with TPA contributing to the observed responses. The substituted 1,10phenanthroline-based Ru(II) coordination complexes 26 (Chart 7) exhibited TPA between 700 and 900 nm, and strong overlap between TPA and ESA spectra. The limiting thresholds were as low as 0.2−0.6 J/cm2 at 750 nm and 0.3−1.5 J/cm2 at 900 nm.643

(2-D), [Mn(bbbt)2(NCS)2]n (2-D), and [Cd(bbbt)2(NCS)2]n (2-D). The χ(3) value 4.48 × 10−12 esu for polymer {[Zn(fcz)Cl2]CH3OH}n (1-D) is about 1 order of magnitude larger than that of polymers {[Cd(fcz)2Cl2]2(CH3OH)(H2O)}n (2-D) and {[Co(fcz)2Cl2]2CH3OH}n (2-D). These results demonstrated the importance of the structure on NLO properties of metal−organic polymers, which could be attributed to delocalization of the π-electron cloud over the whole chain. Furthermore, third-order NLO parameters of 2-D polymers are mainly controlled by organic bridging ligands and because of that could be modulated by coordinating center ions. This means the valence shell structures of central metal ions might have some influence on the strength of NLO properties because the empty orbitals on the metal centers would communicate electronically with the π system of ligands and the axial groups. Some authors also studied the OL effects of these polymers using 40 ps laser pulses at 532 nm. They concluded that fcz and polymer {[Co(fcz)2Cl2]2CH3OH}n (2-D) possess stronger OL effects than polymers {[Zn(fcz)Cl2]CH3OH}n (1-D) and {[Cd(fcz)2Cl2]2(CH3OH)(H2O)}n (2-D). Moreover, the OL threshold was found to be 0.16 and 0.15 J cm−2 for polymer {[Co(fcz)2Cl2]2CH3OH}n (2-D) and fcz, respectively.629−632 Cation-induced coordination polymers also got attention from some authors. Compounds 16633 and 17634 (Chart 4) Chart 4. Monomeric Units as Parts of Coordination Polymers for OPL

were obtained from the reaction of the organic respective molecules with copper and silver iodides, respectively. OL behaviors of these materials were investigated by fluencedependence transmittance measurements using the radiation wavelength of 532 nm generated by a Q-switched Nd:YAG laser (pulse duration: 8 ns). The thresholds found for these materials were 0.980 and 0.920 J/cm2, respectively. Besides the influence of the presence of a π-conjugated system, also the higher nuclearity or heavy atoms were expected to have Chart 5. Platinum Ter/bipyridyl Phenylacetylide Complexes

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Chart 6. Binuclear Platinum Tripyridyl Complexes

Chart 7. Metal Pyridyl Complexes

765 nm. Complexes were also examined at 965 nm, the wavelength at which all complexes behaved as two-photon absorbers. Palladium complexes 29 (Chart 9) showed effective thirdorder NLO properties using ns Z-scan technique at 532 nm, with negative nonlinear refractive index η2 in the order of 10−9 esu, and third-order nonlinear optical susceptibility χ(3) in the order of 10−11 esu.646,647 In the study of the OL characteristics of metal complexes 30 (Chart 9), the effect of the presence of an heavy atom was at the basis of the finding that the Pb complex represented the best compound in the set with a βeff = 4.7 × 10−11 m/W.648 The Z-scan results of tellurium complexes 31 (Chart 9) study revealed that the sample solutions and films exhibited self-defocusing nonlinearity, with negative nonlinear refraction, and switched over from RSA to saturable absorption.649 Several nickel complexes strongly bound to multisulfur ligands containing 1,2-dithiolene ligands (32−36, Chart 10) have been synthesized and characterized with the prospect of measuring their OL properties.650−654 All compounds showed results comparable to those of C60 in toluene solution. The samples were irradiated by a frequency-doubled, Q-switched,

Figure 32. Nonlinear transmission curves at some selected wavelengths comprised in the range 730−990 nm of complex 24 (Chart 7) solution in chloroform (mass concentration: 50 g L−1). Reproduced with permission from ref 643. Copyright 2011 The Royal Society of Chemistry.

Several tris-bipyridyl metalated complexes were analyzed for their cubic nonlinearities (27 and 28, Chart 8).644,645 At 765 nm, all compounds behaved as strong two-photon absorbers, exhibiting strongly negative real parts of the third-order polarizability. The authors discovered the existence of the trend Fe > Ru > Zn > Cu > Ni in the NLO figure of merit at 13076


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Chart 8. Tris-bipyridyl Metalated Complexes

Chart 9. Metal Complexes for NLO Applications

Chart 10. Sulfur-Containing 1,2-Dithiolene Complexes

was slightly larger than those of nickel dithiolenes 33 and 34. For complexes [Ni(medt)2] (36a) (medt = 5,6-dihydro-6methyl-1,4-dithiin-2,3-dithiolate) and [Ni(phdt)2] (36b) (phdt = 5,6-dihydro-5-phenyl-1,4-dithiin-2,3-dithiolate), the optical transmittance began to decrease rapidly as the incident fluence reached about 0.1 J cm−2. The limiting thresholds of complexes 33 and 34 were determined to be 0.3 J cm−2 for the ps measurements and 2.0 J cm−2 for the ns case.652 These results were similar to those obtained by the previous complexes. [(CH3)4N][Ni(dmit)2] (36), dmit2− = 1,3-dithiole-2-thione4,5-dithiolate, was synthesized and studied using the openaperture Z-scan technique with 40 ps pulse width at 1064 nm, and 1 and 15 ns pulse width at 1053 nm. Strong saturable

mode-locked Nd:YAG laser, which produced linearly polarized 35 ps (fwhm) optical pulses at 532 nm (repetition rate: 10 Hz). The maximum incident fluence for all of the samples was set at 4 J cm−2. For [NBu4][Ni(bddt)2] and [Ni(bddt)2] complexes (33 and 34, respectively) with bddt = 4a,5,6,7,8,8a-hexahydro1,4-benzodithiin-2,3-dithiolate, when the incident fluence reached ca. 0.3 J cm−2, the transmittance began to decrease rapidly, and the limiting thresholds of complexes 33 and 34 were found to be ca. 0.6 and 0.7 J cm−2, respectively.650 Similarly, for complex [Ni(ecstmdt)2] (35) (ecstmdt = 2ethoxycarbonylsulfanyl-1-(methylthio)-ethenethiolate), as the incident flux reached 0.2 J cm−2, the transmittance began to decrease with a limiting threshold of 0.8 J cm−2.651 This value 13077


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absorption was found when the sample solution was irradiated by 40 ps and 1 ns laser pulses. On the other hand, when irradiated with a 15 ns laser pulse, a stronger RSA was found, with nonlinear optical absorption coefficients in the order of 10−10 m/W. The reacted copper655,656 (37, Chart 10) and tungsten or molybdenum657 (38a,b, Chart 10) dithiolene complexes have also been tested for third-order optical nonlinearities. Complex 37 was tested by Z-scan technique with ps and ns laser pulses in the NIR region. TPA has been found when the sample solution was irradiated by 40 ps pulse width at 1064 nm and the TPA coefficient was 4 × 10−13 m/W.655,656 Upon excitation with 15 ns laser pulses at 1053 nm, the Z-scan profiles revealed strong RSA. The nonlinear absorption coefficient was found to be 7.07 × 10−11 m/W.655,656 On the other hand, complex 38a, along with complex 38b (Chart 10), exhibited weak OL effect.657 Other metal complexes, (Me4N)2[Cd(dmit)(SPh)2] 39658 (Chart 11), (2-thioxo-1,3-dithiole-4,5-dithiolato)-

Scheme 2. Metal Coordination Compounds for OPL

Chart 11. Thiolate-Based Metal Complexes

triphenylphosphineRu(III) 40659 (Chart 11), and (2-thioxo1,3-dithiole-4,5-dithiolato) triphenylarsenic Ru(III) 41660 (Chart 11), have also been evaluated for NLO properties. All complexes presented good nonlinear parameters with the thirdorder nonlinear optical susceptibility χ(3) having values in the order of 10−10 esu. Heterocyclic nickel(II) complexes (43a and 43b)661 (Scheme 2) exhibited strong TPA and superior OPL properties when compared to their free ligand versions, when these complexes were investigated with open/closed aperture fs Zscan experiments in the range from 680 to 1080 nm. Figure 33 shows the transmitted fluence as a function of the incident fluence. At lower energy, the optical response obeys Beer’s law. When the input energy reaches 0.48 J/cm2 for 43b and 0.39 J/cm2 for 43a, the transmitted energy starts to deviate from linearity, and the onset of the OL action is verified. The damaging threshold is 0.78 and 0.98 J/cm2 for 43a and b, respectively. Similar results were obtained for thiosemicarbazide ligand 44 and its dinuclear complexes M2(dppm)2L(NO3)2 [dppm = bis(diphenylphosphino)methane, M = Cu(I) (45a), Ag(I) (45b)] (Scheme 2).662 The open and closed Z-scan aperture techniques indicated that the complexes 45a and 45b possessed a larger TPA cross section than its ligand L, and that the complex 45b had a much larger third-order nonlinear susceptibility. The OL effect of the metallic complexes 46a−d (Chart 12) was also studied by measuring the nonlinear (energy-dependent) transmission through irradiation by a frequency-doubled, mode-locked, and Q-switched Nd:YAG laser, which produced linearly polarized 35 ps (fwhm) pulses at 532 nm (repetition rate: 10 Hz). It was found that solution transmittance was independent of the incident fluence up to a value of 0.2 J cm−2

Figure 33. OL response of a 1 mm-thick cell of (left) 43a and (right) 43b (see Scheme 2) in THF at 1.0 × 10−3 M. Laser excitation was fs pulses at λ = 690 and 700 nm. The blue circles represent the experimental data, and the red curve is the theoretical data. Reproduced with permission from ref 662. Copyright 2014 Elsevier.

Chart 12. Metal Thiocarbazide Complexes for OPL

(the linear transmittance was about 80%). When the incident fluence exceeded 0.2 J cm−2, the transmittance of 46a solution decreased rapidly having a ps OL threshold of ca. 0.9 J cm−2.663 The limiting thresholds for 46b and 46c measured under the same conditions were around 2 J cm−2. NLO transmittance of 46d did not drop to less than 50% of the linear transmittance due to its closed electronic valence shell (lack of low energy empty orbitals as possible HOMO of the excited complex). 13078


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Chart 13. Acetylide Ruthenium Complexes

Chart 14. Acetylide Ruthenium Complexes for Third-Order NLO

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Chart 15. Metal Acetylene Complexes

Chart 16. Pt:Ethynyl Complexes for OPL

trimetallic octupolar complexes670,676−678 (50, 51, and 52, Chart 13). When nonmetallic dendritic complexes679 (53−57, Chart 14), along with ruthenium-containing analogues, are considered, good NLO properties are achieved without significant loss of optical transparency as required in some optical applications. Attempts to establish structure−property relationships were made. It could be ascertained that an increase of conjugation in the length of the organic chain increases correspondingly the NLO parameters.670 Cubic optical nonlinearities of compounds 53−57 (Chart 14) were determined by Z-scan technique at 820 nm.671 The significant values obtained suggested the dispersion effect of two-photon states contributing to the observed responses. Such a result was noted also in the characerization of other alkynylruthenium bis(diphosphine) complexes.680 For the linear monometallic functionalized alkynyl ruthenium complexes 58−60 (Chart 15),681 the third-order NLO susceptibility was evaluated using Z-scan method at the fundamental wavelength 1064 nm and in the ps regime. Complexes 58−60 reached values of χ(3) in the order of 2 × 10−12 esu. Besides ruthenium complexes, also platinum complexes were considered for NLO studies. For instance, the series of platinum(II) di-imine complexes with different substituents on fluorenyl acetylide ligands (61a−61e, Chart 15) was

3.2. Metal Acetylides and Vinylidenes

Another type of hybrid materials investigated for OL are acetylide complexes, first reported in the early 1960s.664 They have recently attracted significant interest,665,666 because of various applications as functional materials.667−669 Negative real components of the nonlinearities (γreal) were observed in many instances, and significant imaginary components (γimag) were seen for almost all complexes. These findings were consistent with TPA effects contributing to the observed molecular nonlinearities. As a third-order NLO property, the γ values of these hybrids recall the interest for applications in OL. It has been established with organic compounds that the increase of π-electrons delocalization as it occurs in progressing from small molecules to π-conjugated polymers, the introduction of strong donor and acceptor functional groups, the control of chain orientation, packing density, can all result in increased cubic nonlinearity. Where applicable, similar trends have been observed with ruthenium acetylide complexes, although in many instances the regularity of the trend could be lost because many of the small donor−acceptor acetylide complexes have been originally designed for optimizing second-order rather than third-order NLO response. Cubic nonlinearities for some Ru studied acetylide complexes increased considerably in going from monometallic linear complexes670,671 (47 and 49, Chart 13) to bimetallic linear complexes672−675 (48, Chart 13) or to 13080


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synthesized, and the respective OL properties were reported.682 Nonlinear transmission experiments with ns pulses at 532 nm demonstrated that all Pt complexes behaved as strong RSA materials, exhibiting relatively strong singlet and triplet transient absorption from 450 to 800 nm. The trend of OL performance was 61d > 61e > 61c > 61b ≥ 61a, as the result of larger ratios of excited-state absorption cross-section to that of the ground state in the visible to NIR region in going from 61a to 61d. Analytical and numerical simulations, as well as experimental studies of the nonlinear transmission of a Pt-ethynyl complex of type 62 (Chart 16), have been carried out. The authors concluded that 62 was suitable for use as a broadband optical limiter across the visible spectrum.683−690 They also indicated that in the green-blue region the nonlinear response is independent of pulse length for τ < 500 ns (50 ps < τ < 500 ns). In the red region, although they did not find an independent response of pulse length, a significant OL response was seen for τ < 50 ns.683,686 Some authors have studied the OL properties of compounds 63−65 (Chart 16) in THF solutions (c = 0.030 M) and measured them using 5 ns 10 Hz pulses at three wavelengths (532, 550, and 610 nm).686,691−693 A significant OPL effect was found for all three compounds at 532 nm. Nevertheless, the OPL of 64 was smaller than that of 65 and 63. For instance, at Ein = 150 μJ, the values of Eout were 7.6 μJ for 64 and ∼5 μJ for 63 and 65. At 550 nm, the OPL of the three compounds is weaker, and that of 65 is intermediate between 64 and 63. At 610 nm, the OPL is further diminished but less for 63 than for 65 and 64, which have similar values of Eout ≈ 25−26 μJ at Ein = 150 μJ. On the basis of these results, the authors concluded that OPL is very dependent on the wavelengths for 65 and 64 in comparison with 63 and that the introduction of the thiophene ring does not improve the OL of 63, at least under these measurement conditions.691 Some other new platinum(II) acetylide molecules for OPL applications were functionalized with alkoxysilane groups, and incorporated into a glass matrix prepared by a modified sol−gel method.694,695 The modified process allowed the preparation of optically pure organic−inorganic monolithic glass materials, which showed broadband OL with low threshold values at visible wavelengths. From OPL measurements on the hybrid materials, it was found that the properties observed for the chromophore in the solid sol−gel matrix were comparable to those in solution or in PMMA matrixes.692,695 A series of trans-N-heterocyclic carbene platinum(II) acetylide complexes 66a−c (Chart 17) were synthesized via Hagihara reaction of the unprotected aryl-acetylide ligands with

Pt salts.696 These complexes were then studied with ns transient absorption spectroscopy and open aperture ns Zscan. Coupling of the organic chromophores to the platinum center afforded efficient ISC as concluded by the low fluorescence quantum yields of the complexes. Open aperture Z-scan with 10 ns pulses at 606 nm showed comparable optical attenuation relative to a standard sample of 66c. Pulse limiting was achieved via a dual-mechanism of TPA coupled with ESA. Three rod-like Pt(II)-diacetylides (67−69, Chart 18) have been synthesized697 for OL analysis. When subjected to intense

Chart 17. Carbene-Based Platinum(II) Acetylide Complexes

Figure 34. OL response of 0.030 M THF solutions of 67 (■), 68 (red ●), and 69 (green ▲) (see Chart 18) at the laser wavelength 532 nm. Blue ▼ represent the standard compound of reference (62b, see Chart 16). Reproduced with permission from ref 697. Copyright 2013 Elsevier.

Chart 18. Rod-like Pt(II)-Diacetylides

laser light, all three Pt complexes 67−69 showed strong OPL at 532 and 600 nm, with compound 68 being the best performing optical limiter with the stronger RSA. As it can be seen from Figure 34, at relatively low energy of a few J/cm2 of the laser

pulse, most of the light was transmitted through the solutions of 67−69, while a significant leveling of the transmitted energy occurred for pulse fluences of ca. 50−200 J/cm2 at 532 nm. A series of platinum(II) acetylide complexes (70−74, Chart 19) containing p-phenylenevinylene and triphenylamine endcapped moieties have been incorporated into PMMA698 and evaluated for OPL. In the PMMA monoliths, the complexes revealed strong TPA and effective RSA as necessary for OPL 13081


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Chart 19. Platinum(II) Acetylide Complexes Containing p-Phenylenevinylene Moieties

Chart 20. Multibranched Conjugated Platinum(II) Acetylides

μJ with a transmission of 40% at 532 nm. The Zn complexes had a limiting value of 22 μJ (5 mM in pyridine) with a linear transmission of 50%, and 6 μJ (7.5 mM/pyridine), when the linear transmission was ∼40%. The formation of adducts between the thiacalixarenes and a metal salt did not seem to improve the resulting OL effect. Moreover, the authors noticed that similar calixarenes, with CH2 bridges instead of sulfur, did not show any OL behavior at all under the same conditions. These findings evidenced the important role of the sulfur atoms on the optical properties of these macrocycles. On the other hand, clamp levels of ∼4 μJ at 532 nm were observed for both tetra(pentylphenylethynyl)tetrapropoxythiacalix[4]arene (150

purposes, via dual mode mechanism. Other conjugated platinum(II) acetylides with multibranched donor−acceptor structures (75−80, Chart 20) were synthesized, and their NLO properties were evaluated.699 Their transient absorption spectra indicated an effective ISC, with compounds 75−80 displaying ratios of effective nonlinear absorption cross-section to groundstate absorption cross-section σeff/σ0 in the range 4−22. The synthesis of Pt acetylides from ethynylthiacalixarenes was considered to investigate the relative OL properties.700−702 Complex 81a (Chart 21) showed a clamping level at ∼7 μJ but with a very low linear transmission at 532 nm (6%), while the second complex 81b (Chart 21) showed a clamping level at 8 13082


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merit. The cubic nonlinearities of platinum acetylides704 (82b− 85b, Chart 22) have been determined with optical Kerr gate techniques (OKG). Values of γreal were generally low while quite high values of γimag were found (Table 5). Complexes 82−85 (Chart 22) showed NLO data similar in magnitude to the monoruthenium and nickel acetylide complexes (Charts 12−14) discussed previously.704 The NLO properties of several di-, oligo-, and polymetallaynes of some transition metals of groups 10−12 were reported.705−707 Most of these materials were colorless and then with good optical transparencies in the visible spectral region. Such complexes exhibited good OPL for ns laser pulses. On the basis of the results from photophysical studies and theoretical calculations, both the absorption from excited triplet states and that from intramolecular charge-transfer states were found to contribute to the enhancement of the OPL properties in these materials. Electronic influence of groups with strong electronic effects, spatial arrangement, and geometry of metal groups on the optical transparency/nonlinearity optimization was evaluated. The positive contribution of transition metal ions to the OPL of these compounds generally followed the order: Pt > Au > Hg > Pd. The OL thresholds for these polymetallaynes were found to be as low as 0.07 J cm−2 at the linear transmittance of 92%. These were highly transparent materials with excellent figures of merit σex/σ0 ranging in the interval 22−48. Also, acetylide polymers have been the target for OL investigation.708−711 For most of the nickel, palladium, and platinum acetylide polymers, the imaginary part of the nonlinearity is the main provider, meaning significant TPA. Some of the polymers have nonlinearities that are considerably larger than related monometallic acetylide complexes. It was concluded that the platinum polymers are in many cases more efficient than the analogous palladium materials. The nonlinearities of these polymers do not depend strongly on aromatic ring substitution, but on the increasing number of diethynylarenes repeating units.708−711 To weigh the degree of electronic delocalization, the thirdorder NLO responses of compounds 86a−e (Chart 23) were

Chart 21. Tetrahydroxytetrathiacalix[4]arenes for OPL

mM in THF, 99% of linear transmission) and the corresponding platinum complex (30 mM in THF, 83% of linear transmission).700,701 Nickel acetylide complexes (82a−85a, Chart 22) were examined by Z-scan at 800 nm. They displayed negative γreal Chart 22. Nickel and Platinum Acetylide Complexes

Chart 23. Diruthenium Oligomeric Complexes for OPL

and a noteworthy γimag being indicative of two-photon dispersion.703 The γreal values for the nickel complexes are similar to those of their platinum analogues 82b−85b (Chart 22) (Table 5). The induction of a greater electronic delocalization through the addition of triphenylphosphine ligand had no considerable effect on cubic NLO figures of

measured using both nonlinear transmission and DFWM measurements at the frequency-doubled light from Q-switched

Table 5. NLO Parameters of Some Ni and Pt Complexes compd 82a 82b 83a 83b 84a 84b 85a 85b

γreal (10−36 esu) −270 19 −580 30 −420 45 −640 88

± ± ± ± ± ± ± ±

100 5 200 8 100 11 300 22

γimag (10−36 esu) 70 827 300 1134 480 1196 720 2167

± ± ± ± ± ± ± ±

50 207 60 284 150 299 300 542

|γ| (10−36 esu)

λ (nm)

ref

± ± ± ± ± ± ± ±

800 1060/530 800 1060/530 800 1060/530 800 1060/530

703 704 703 704 703 704 703 704

280 827 650 1134 640 1196 960 2167 13083

110 207 210 284 180 299 420 542


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succinic acid 88 single crystal (Chart 24) had a nonlinear refractive index η2 = −2.4189 × 10−8 cm2/W.720 Nonlinear transmission measurements with ns pulses of hybrid organic−inorganic crystal 89 (Chart 24) were also reported.721 The corresponding mechanism of OL action was explained in terms of a three-photon absorption resulting from a two-step process: TPA from ground state followed by ESA. The absorption properties and their polarization dependence have been studied for several crystal orientations in the visible range (450−650 nm). The nonlinear transmission curves represented in Figure 35 emphasize the broadband OL properties of the crystal. On the

Nd:YAG laser with a temporal pulse width of 8 ns and a repetition rate of 10 Hz.712 Shown in Table 6 are the values for Table 6. Third-Order Nonlinear Optical Susceptibilities of Complexes 86a−e (Chart 23) complex 86a 86b 86c 86d 86e

χ(3)/C (10−8 esu M−1) at 532 nm 2.70 3.97 15.0 6.08 10.9

± ± ± ± ±

0.41 0.6 2.3 0.92 1.6

χ(3)/C (10−8 esu M−1) at 800 nm 1.02 1.52 1.02 1.48

± ± ± ±

0.15 0.23 0.15 0.22

the third-order NLO effect at both 532 and 800 nm. At 800 nm, most of the compounds had third-order susceptibilities (imaginary part, Imχ(3))/C around 1 × 10−8 esu M−1. The authors also concluded that the absence of a significant dependence of the NLO susceptibility on the chain length (n) at 800 nm was due to Ru−Ru core localized excitation, not involving a charge transfer event to/from the oligoynyl chain.712 Figure 35. OL performances of a crystal of salt 89 (Chart 24) at different wavelengths ( f/5 geometry). Reproduced with permission from ref 721. Copyright 2002 Elsevier.

4. ORGANIC MATERIALS This section deals with OPL materials of organic origin. Carbon derivatives and the macrocyclic tetrapyrrolic materials, which will be reviewed in successive sections, will be then excluded.

whole visible range, the clamped transmitted energy was fixed at a value below 15 μJ for incident energies as high as 150 μJ. Anyway, the results obtained were strongly dependent on the crystal orientation with respect to the incident laser polarization, a characteristic that represents a serious problem in practical applications.721 The efficiencies of transparent crystals and nanocrystals embedded in sol−gel glass as OL systems in an f/5 configuration of analysis were tested with light pulses of 2.6 ns in the wavelength range 450−650 nm. The authors concluded that OPL was achievable over a large spectral range, but that single crystal devices exhibit a strong anisotropy of NLO properties. For that, the authors proposed the approach of nanocrystallization of organic phases in sol−gel glasses to give optically isotropic materials, which are more suitable for OPL purposes.722 Later, this method was extended to other types of crystallites, for example, organic materials with magnetic properties. Particles of uniform size were achieved through the control of the organic phase nucleation, particle growth, and gelation of the alkoxy matrix in the glass, affording isotropic materials that contained high concentrations of dyes.723,724 For NLO researchers, it is reasonably assumed that πconjugated materials are capable of displaying OL properties. Proper manipulation of the π-conjugation effects by structural design may enhance as well the desired properties of these materials, whether they are homocyclic or heterocyclic aromatic-based compounds.725−728 For instance, RSA, which represents a fundamental and important feature for the design of “smart” passive limiters, was first reported for solutions of anthracene in 1974.729 Before that, nonlinear absorption of light was experimentally observed only in the late 1960s.23,730 The vast majority of examples of RSA were based on the excitation of triplet states, the population of which resulted increased from fast, efficient ISC upon incidence of highly intense pulsed radiation. For example,

4.1. Non Macrocyclic Molecules

Several results in past years have given a cutting edge interest to organic two-photon absorbers of ns pulses.713−717 In fact, at fs and ps time scales, only TPA place, while for ns pulses, stronger OPL can be reached as TPA is combined with ESA.718 In this case, organic molecules have to be optimized for the simultaneous improvement of TPA process and increase of ESA cross sections. Other issues are related to their solubility, aggregation effects, and excited-state lifetime, and the achievement of optical-grade quality in a range of temperatures as wide as possible. Solid samples are generally preferable to concentrated solutions that can give problems of dye nucleation at low temperatures and generate solid-state nanocrystalline materials with different NLO properties with respect to the isolated and dispersed molecules. In fact, several organic salts can give nanocrystalline systems with OPL properties. While compound 87 (Chart 24) displayed nonlinear refractive index η2 = −1.258 × 10−11 esu, nonlinear absorption coefficient β = 2.146 cm/GW, and third-order susceptibility χ(3) = 0.377 × 10−13 esu,719 the crystal bis(2-aminopyridinium)-succinateChart 24. Organic Salts for OPL

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promising results in the intensity range of subpicosecond pulses that generate the NLO process of three-photon absorption even at the incidence frequency of 1 kHz.741 It was also reported the OL effect of trans-4-[p-(N-ethyl-Nhydroxylethylamino)styryl]-N-methylpyridinium salts, like tetraphenylborate (91),742,743 iodides (92),744 or tosylates745 (93) (Chart 25), besides other type of organic molecules.746,747 Compounds 91−93 showed strong TPA. On the basis of this mechanism, an efficient OL performance has been demonstrated in a 2 cm long 91-doped epoxy rod pumped at 1.06 μm from a Q-switched laser with pulses having an intensity in the range 50−250 MW/cm2. The measured nonlinear absorption coefficient reached 6 cm/GW for the tested sample at the dopant concentration of 4 × 10−3 M (Figure 36).742

the heavy atom and/or metal effects in metallophthalocyanines, metalloporphyrins (vide infra), and other materials, like hybrid materials (vide supra), enhance mainly the efficiency and the rate of the ISC step.123,731,732 In the early 1990s, this type of molecule regained due attention, and several works in this field started being reported. Reinhardt and Prasad,733,734 for instance, provided a structure− NLO properties study for a series of benzothiazole, benzoxazole and N-phenylbenzimidazole derivatives. These authors analyzed the effect of nature of the heterocycle, effective conjugation length, electron richness of the nonheterocyclic structural units, and two-dimensional π-conjugation effects on the third-order nonlinear susceptibility χ(3). They have concluded that inclusion of sulfur as heteroatom in the πconjugated system might enhance the nonlinearity, due to either the participation of empty d orbitals or the establishment of steric interactions between each unit. Sulfur-containing aromatic links such as thiophene were found to impart much more positive variations of the NLO properties than those aromatic systems containing nitrogen and/or oxygen atoms.733,734 Also, N,N-diphenylbenzbisimidazole was found to have better NLO performance than benzobisthiazole due to the fact that the first converts the molecular structure from a material having one-dimensional electron conjugation to one having a two-dimensional character. As a result, these materials have shown enhanced third-order optical nonlinearity without reducing the transparency window.733 Some authors have also considered the formation and stabilization of polaron-like radical-cations and bipolaron-like dications by oxidative doping of polyenylic oligomers,735−737 for OPL. These structures could be used in nonlinear optics for the induction of polaronic or bipolaronic charge states in the πconjugation sequences (e.g., polyenes) originating greatly enhanced third-order optical nonlinearity.738 The formation of bipolaron has been confirmed experimentally in both bis(anthracene-)polyenes739 and for α,ω-dithienylpolyenes.740 The nonlinear transmission properties of trans-4-[4(dihexylamino)styryl]-N-(2-2-[2-(2-hydroxy-ethoxy)-ethoxy]ethoxy-ethyl)-pyridinium-tosylate 90 (Chart 25), consisting of a chromophoric unit capable of multiphoton absorption in the NIR range, have been studied with ns pulses at 1.064 μm for the analysis of two-photon excitation, and with subpicoseconds laser pulses at 1.3 μm for the analysis of the process of threephoton excitation.741 A fairly good OPL performance in this new material has been verified at both wavelengths with

Figure 36. Transmitted intensity as a function of the incident intensity at 1.06 μm. The OL system was a 2 cm long rod of 91-doped epoxy resin with dopant concentration of 4 × 10−3 mol/L. The solid curve is the theoretical curve with the best-fitting value of β = 6 cm/GW. The dashed line represents the case for β = 0. Reproduced with permission from ref 743. Copyright 1995 American Institute of Physics.

For 93, the largest value of molecular TPA cross-section was 2.06 × 10−47 cm4·s/photon at 930 nm. At 1064 nm, the cross section decreased to 2.71 × 10−48 cm4·s/photon, which was only one-ninth of that at 930 nm. The upconverted lasing efficiency spectrum has been measured at different wavelengths. The highest efficiency is 5.1% at 1020 nm, whereas it becomes 3.5% at 1064 nm (Figure 37).745 Last, 92 gave a TPA coefficient of 1.54 cm/GW and a corresponding TPA crosssection of 5.2 × 10−20 cm4/GW. Transmission decreased to the

Chart 25. Pyridinium Salts for OL Measurements

Figure 37. OL effect at 930 nm generated by 1 cm thick solution of 93 (Chart 25) in DMF at the concentration 0.05 mol/L. The dashed line represents the linear transmittance. Reproduced with permission from ref 745. Copyright 2001 Springer Science and Business Media. 13085


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inferior limit value of 0.32 as the input intensity increased (Figure 38).746

Figure 39. Nonlinear transmitted intensity versus incident intensity at 1064 nm for the DMSO solutions of 94, 95, 96, 97, and 98 (Chart 26). The dashed line corresponds to the behavior of pure DMSO. Reproduced with permission from ref 749. Copyright 2006 Elsevier. Figure 38. Transmitted intensity versus input intensity of compound 92 in DMF at 5.0 × 10−2 M (excitation with ps pulses). The solid line is a fitting curve based on the assumption of TPA as NLO process. Best-fitting parameter value β = 1.54 cm/GW. The straight dashed line corresponds to liner transmission (at this condition σ2 = 0). Reproduced with permission from ref 746. Copyright 2001 Elsevier.

threshold values were about 7 GW/cm2 for compound 96, 11 GW/cm2 for compounds 97, 98, and 99, and 13 GW/cm2 for compound 95.748 Several other examples based on pyridinium-based compounds (101a−c, 102, Chart 26) were prepared and characterized for OL. Compounds 101 and 102 presented TPA stronger in the NIR than in the visible range.751,752 It was realized that the length of the alkyl chain played an important role in enhancing the TPA cross section through chain length increase.751 Three-photon absorption was observed in compounds 103− 107 (Chart 26).753 Using a mode-locked/Q-switched Nd:YAG pulse with a duration of 35 ps and a repetition rate of 10 Hz at 1064 nm, the molecular three-photon absorption cross sections of 103−107 resulted in the order 10−78−10−76 cm6 s2. It was also observed that the three-photon absorption cross-section as well as the corresponding OL effect could be enhanced by increasing the strength of electron-acceptor units in the stilbazolium derivatives. In addition, other donor−π−acceptor (D−π−A) chromophores,754 with ferrocene as electron donor and pyridinium as electron acceptor moiety (108,109, Chart 27), were synthe-

Other authors have evaluated the OL properties of several stilbazolium derivatives (94−101, Chart 26) with different donor units with a mode-locked/Q-switched Nd:YAG laser, at 1064 nm.748−750 The measured molecular three-photon absorption and two-photon absorption cross sections (α3/α2) were in the order of 10−76 cm6 s2 and 10−47−10−46 cm4·s/ photon, respectively. It was observed that both α3 and α2 values increase linearly with increasing electron-donating ability of the donor units (from N-methylpyrrole, to pyrrole, furan, and thiophene). Figure 39 shows the OL effect of the six molecules 94−99 (Chart 26) studied in DMSO. The OL ability decreased with the trend 94 > 96 > 97 > 98 > 99 > 95.749 Compound 94 had shown the strongest OL effect, with a threshold as low as 4 GW/cm2 and the transmitted intensity limited below 13 GW/ cm2 at the incident intensity of 58 GW/cm2. The other Chart 26. Stilbazolium Derivatives with Different Donor Units

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Chart 27. Donor−Acceptor Compounds Containing Ferrocene for NLO

sized and investigated by the Z-scan technique. Both compounds exhibited RSA and OL effect under ns pulse irradiation. For compound 109, the OL threshold value was about 3.7 mJ, while for compound 1, the OL threshold value was less than 1 mJ when light pulses were in the ns range. Another ferrocene-based chromophore (110, Chart 27) was evaluated for the NLO properties.755,756 Compound 110 showed a value for the nonlinear absorption coefficient and nonlinear refractive index of 43.32 cm/GW and −1.65 × 10−10 esu, respectively. OL effect was attributed to efficient RSA.755 Compounds 111a−l (Chart 27) were studied at 800 nm using fs DFWM. These systems presented third-order NLO susceptibilities between 3.065 and 7.859 × 10−13 esu, and possessed ultrafast response times of less than 70 fs.756 The synthesis and the NLO characterization of the two heteroaromatic-based chromophoric salts 112 and 113 (Chart 28) were reported.757−759 These salt dyes exhibited large TPA Chart 28. Heteroaromatic Organic Salts

Figure 40. Output intensity versus incident intensity for a DMSO solution of chromophore 112 (Chart 28) irradiated with fs pulses. The straight line corresponds to the linear transmission of the solution at low levels of irradiation. Reproduced with permission from ref 757. Copyright 2002 American Chemical Society.

cross-section values in the fs regime (σ2 = 150 × 10−50 cm4 s photon−1 molecule−1 with 150 fs laser pulses), and the corresponding OL effect proved to be relatively strong (Figure 40).757 Pyrilium chromophores 114−118 (Chart 29) were also studied with fs pulses by open Z-scan technique.760 RSA based on TPA and ESA was dominant in most cases, reaching thirdorder nonlinear absorption coefficients of the order of 10−15 cm2. The intensity-dependent refractive index η2 and the

nonlinear susceptibility χ(3) of pyrilium-related dye Hibiscus Sabdariffa 119 (Chart 29) in the ns regime at 532 nm were reported.761 The third-order nonlinearity of dye solution was found to be dominated by nonlinear refraction that led to strong OL. The negative sign of the closed Z-scan profiles was associated with the dominance of a thermooptic effect, and the observed optical nonlinearity was thus of thermal origin. A vast array of azo-based dyes have been investigated for OL (120−130, Chart 30).762−782 For instance, the OL response of dye 120763 as dopant of a polymethylene polymer was reported for 10 ns pulses at 532 nm. The OL effect depended linearly on the increase of dye concentration in the polymer. The higher mass concentration of 0.14 mg of dye per gram of polymer gave a limiting threshold of 0.4 J/cm2, while the lower concentration of 0.02 mg of dye per gram of polymer gave a limiting threshold of 0.7 J/cm2.763 Azo-dyes 121−123 (Chart 30) were exposed to a 532 nm CW laser for the determination of η2 (comprised between −0.54 and −1.09 × 10−8 cm2/W) and of χ(3) (comprised between −2.15 and −4.35 × 10−7 esu).764 Dye 124, also named acid blue 29, was investigated, in different solvents, using a CW 13087


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Chart 29. Pyrilium-Based Chromophores

Chart 30. Azo Dyes Studied for NLO

Chart 31. Hydrazine-Based Compounds for NLO

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OL properties at 532 nm with the nonlinear absorption being attributed to RSA.788 Azomethine derivatives 140 and 141 (Chart 32) were studied using the Z-scan technique with 7 ns pulses at 532

He−Ne laser beam at 632.8 nm wavelength and 35 mW of output power.765−767 The experiments showed that the OL effect of the solutions of dye 124 (Chart 30) becomes stronger for the samples with high polarity solvents, methanol or ethanol being the best solvents. The corresponding limiting thresholds of the solution with different alcoholic solvents were 3 and 4 mW for methanol and ethanol, respectively. Sudan red B (125, Chart 30) OL properties were measured using a CW solid-state laser,768,783 both in liquid solution and in solid-state solution. The limiting efficiency for 125-doped PMMA was found to be better than the dye solution in toluene at 532 nm, with the limiting threshold of 5.8 mW. Chicago sky blue 6B (126, Chart 30) doped PVA films769 were studied at 633 and 532 nm using a CW laser. The corresponding η2 value was in the range of 10−17 cm2/W. Azobenzene dye 127 (Chart 30) in PMMA was analyzed for OL using low-power lasers.772 The values of the limiting thresholds proved to be modulable by changing the intensity of excitation beam. In a similar study,773 the nonlinear absorption coefficient of azobenzene liquid crystal polymer was measured by using open aperture Z-scan technique with 10 ns laser pulses at 532 nm. The azobenzene liquid crystal displayed a β value of 9.02 cm/GW. To further investigate the dependence of the OL properties on intermolecular interactions,774 such as hydrogen bond and dipole−dipole interactions, three polar D−π−A conjugated quinoline-based heterocyclic azo derivatives (128a− c, Chart 30) with different hydrogen-bond recognition abilities were synthesized. The results indicated that the OL properties of these polar D−π−A conjugated optical materials were significantly affected by the molecular structure itself, by the strong intermolecular interactions, and by their synergetic action leading to a general enhancement of the NLO properties. Dyes 129775 and 130776 were also studied for OL using single beam Z-scan technique with pulsed Nd:YAG laser at 532 nm. The nonlinear refractive index η2, nonlinear absorption coefficient β, and the third-order NLO susceptibility χ(3) were dependent from sample concentration. The best concentration value was 0.03 M, which produced η2 = −5.19 × 10−10 cm2/W, β = 7.73 × 10−6 cm/W, and χ(3) = 1.554 × 10−9 esu. Hydrazine-based compounds 131−139 were also tested for NLO applications (Chart 31).784−788 For instance, 2,3butanedione dihydrazone (131) upon irradiation with a Qswitched, frequency doubled Nd:YAG laser784 showed that η2 and β increased linearly with increasing concentration. The ESA cross sections of 131 were found to be larger than the ground-state absorption cross section, thus leading to RSA due to sequential multiphoton absorption. Additionally, large positive third-order optical nonlinearity of the oligoazine derivatives 132 studied with Z-scan technique at 532 nm was found by the same group.785 The corresponding values of χ(3) were in the order of 10−11 esu. Conjugated hydrazine chromophores 133−135 were also evaluated.786 These systems demonstrated efficient third-order nonlinear behavior with energy transmissions of ∼20 mJ for input energies up to 150 mJ. Hydrazones 136−138 were investigated using a single beam Z-scan technique with ns pulses at 532 nm.787 Open aperture data relative to 136−138 indicated TPA at this wavelength, with the nonlinear refractive index η2, nonlinear absorption coefficient β, and effective third-order susceptibility χ(3) being −0.619 × 10−11 esu, 1.42 cm/GW, and 0.22 × 10−13 esu, respectively. Similarly, compounds 139a−f exhibited good

Chart 32. Dyes Studied for OPL

nm.789 The nonlinear refractive index η2 and the third-order nonlinear susceptibility χ(3) were 10−13 cm2/W and 10−13 esu, respectively. Similar to compounds 139 (Chart 31), RSA was identified as the main mechanism responsible for the OL effect. Some authors also tested the organic dye 6-propionyl-2dimethylaminonaphthalene 142 (Chart 32).790 As it can be seen from Figure 41, the performance of dye 142 appeared to be quite poor with respect to the linear optical behavior of the pure solvent.

Figure 41. Nonlinear transmission of a 0.5 M solution of 142 (Chart 32). Solid circles represent the data for the solution, while the hollow circles represent the corresponding data for the pure solvent. The solid line is the fit. Reproduced with permission from ref 790. Copyright 1995 Elsevier.

Another set of dyes for OL (Chart 33) included bromophenol (143),791 ninhydrin (144),792 Leishman dye (145),793 indigo carmine (146),794 safranin O dye (147),795 Azure B dye (148),796 neutral red dye (149),797 rhodamine6G (150),798 PicoGreen dye (151),799 and betanin dye (152),800 among others (153−157). Their NLO properties were determined using the Z-scan technique. The NLO behaviors of this set of dyes differed with regard to the origin of the mechanism at the basis of their NLO properties. For instance, compounds 143, 145, 147, and 148 presented an OL mechanism of thermal origin, with 143 possessing a nonlinear refractive index of 1.7 × 10−8 cm2/W,791 while 145 exhibited a high negative nonlinear index of −2.572 × 10−7 cm2/W and βeff = 2.7 × 10−3 cm/W.793 148 displayed η2, βeff, and χ(3) in the order of 10−5 esu, 10−2 cm/W, and 10−7 esu, respectively.796 Compound 144 has been studied with both 13089


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Chart 33. Dyes for NLO

molecular charge transfer processes that get activated upon increase of the irradiation doses. Some authors presented a model for the nonlinear molecular photonic processes occurring in fiber-core liquid molecules and have analyzed the nonlinear transmission of ns and ps laser pulses through these fibers.804−808 In the ps regime, nonlinear absorptions played a dominant role, while in the ns regime, thermal-density effects were involved for achieving comparable limiting performances. The authors observed that for incident laser energies of >1 mJ, the transmission was clamped to less than 1 mJ. A broad series of donor (D) and/or acceptor (A) substituted derivatives of 1,2-diethynylethenes (DEE) and tetraethynylethenes (TEE) (Chart 34) has been analyzed by third-harmonic generation (THG) experiments. This technique provides a lot of information about routes that might lead to enhanced optical nonlinearities, and allow the establishment of meaningful structure−properties relationships.809,810 A substantial increase of the optical nonlinearities in both DEE- and TEE-based chromophores was observed upon increase of the length of the conjugated framework, irrespective of D/A substitution pattern or degree. For instance, the value of χ(3) increased more than 2 times in passing from 158 to 159 and more than 10 times when 170 was analyzed (which is the system with the highest value for that subseries). Introduction of phenyl substituents also increases the conjugation, therefore inducing an increase of NLO effects as well. More than the extent of substitution, the lack of planarity in the ground-state conformation of the molecules seems to be the most important controlling factor. TEE derivatives show higher χ(3) values than similarly substituted DEE, not only due to higher conjugation, but also

ultrafast and short pulse excitation. The molecule exhibited three-photon absorption in the ultrafast regime, and effective TPA under short pulse excitation.792 Similarly to neutral red dye, which exhibited a limiting threshold of 0.9 J/cm2, also dyes 146, 151, and 152 behaved according to a mechanism of RSA, with 146 displaying optical clamping at about −1 mW,794 and dye 151 showing β = 1.1 m/GW, χ(3) = 9.3 × 10−11 esu, and limiting threshold Ithr = 36.2 MW/cm2.799 The NLO response of several other heterocyclic compounds has been also measured. For instance, N-substituted-5-phenyl1H-pyrazole-4-ethyl carboxylates 153a−f (Chart 33) were studied using open-aperture Z-scan technique at 532 nm801 with 5 ns laser pulses. Within the series of carboxylates 153a−f, compound 153c presented the stronger effect of nonlinear absorption. Compounds 154−156 (Chart 33) of the sydnone type,802 doped into PMMA, were investigated using ns laser pulses at 532 nm with the Z-scan technique. The nonlinear refractive index was found to be of the order of 10−14 cm2/W, while the magnitude of third-order susceptibility was of the order of 10−13 esu. Furthermore, the clamped energy was also measured, and found to be as low as 0.21 mJ. The NLO properties of 1,2,5-thiadiazolo[3,4-g]quinoxalinebased derivatives 157a,b (Chart 33) were tested under the excitation of fs pulses at 532 nm.803 Compound 157b displayed saturable absorption behavior, while no saturation effects were observed with the derivative 157a having a butoxy-biphenyl as donor (Chart 33). Moreover, 157b exhibited much larger TPA cross sections than 157a. The better NLO properties of 157b were ascribed to the introduction of stronger electron-donating group (dithienyl) with the resultant enhancement of intra13090


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Chart 34. Substituted Derivatives of 1,2-Diethynylethenes and Tetraethynylethenes

Chart 35. Thienylenevinylene Derivatives

order susceptibility values χ(3) of several organic molecules, dithienylethylene811−813 (178, Chart 35) and bis-dithiafulvenyl substituted814 (179, Chart 35), and found that the first compounds had lower χ(3) values than for the second. This

due to a less planar conformation. For instance, molecules such as 176 or 177 (Chart 34) show the best results, due to molecule oversizing, which seems to be an effective manner to enhance the χ(3) values. Some authors have compared the third13091


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difference suggested that the fulvenyl core plays a significant role in these properties, also in agreement with the premise of the distortion from planarity for this kind of compounds. Reports on measurements of the third-order susceptibilities and TPA in branched oligothienylenevinylenes derivatives (180−182, Chart 35) using DFWM technique at 532 nm in the ps regime were published.815 All of the studied compounds have shown quite large third-order nonlinear optical properties. The authors have demonstrated the influence of the number of branches on the benzene unit as structural factor determining NLO properties. It was found that oligothienylenevinylenes derivatives possess nonlinear optical properties larger than already known materials such as octupolar molecules and their dipolar subunits.816 It was also found that the value of χ(3) increased with the number of branches of the molecule. Such a trend led to the clear conclusion that branches bring about an increase of the optical limitation effects (Table 7). Such an

OL and NLO stabilization via the mechanism of TPA in organic molecule-doped solid materials like epoxy rod and a composite glass rod doped with the same organic dopant (2,5benzothiazole 3,4-didecyloxy thiophene) was investigated.817 An ultrashort laser source producing 0.5 ps pulses at 602 nm was employed, and the transmissivity of these two materials was measured as a function of the input beam intensity. The measured results were fitted making the reasonable assumption that TPA was the predominant mechanism producing the observed OL effect. Also, the phenomenon of optical stabilization was observed by the same authors: at the input intensity level ∼930 MW/cm2, the fluctuation of the output intensity is 3 times less than the fluctuation of the input stimulus. The OL behavior and TPA properties of organic compounds 183 and 184818,819 (Chart 36) in tetrahydrofuran have been investigated using ultrashort laser source with 0.5 ps pulse width at 602 nm. These authors demonstrated that TPA was the only predominant mechanism causing the observed OL effect. The corresponding β values were in the range 0.25−0.8 cm/GW. The TPA spectra for a series of bis(acceptor)-substituted bis(dibutoxythienyl)ethene and bis(N-hexylpyrrolyl)ethene chromophores 185−191 (Chart 36) have been recorded using the Z-scan technique.820 All of these chromophores showed strong NIR TPA with absorption cross sections in the range of 2400−5900 GM (1 GM = 1 × 10−50 cm4 s/photon). Phosphonato-substituted bithiophenes 192a−c and 193a−c (Chart 37) have been synthesized, and the relative third-order nonlinear absorption measurements at 430 nm with 27 ps laser pulses have been performed.821 These tests demonstrated that all of the compounds here considered showed nonlinear

Table 7. NLO Parameters and Figure of Merit for a Series of Oligothienylenevinylenes Derivatives molecules

β/cm CW−1

χ(3)/10−12 esu

χ(3)/α (10−13 esu cm)

180 181 182

0.34 3.1 5.2

1.0 2.6 4.7

2.6 5.7 6.6

increase within this set of organic compounds was due not only to high level of the π-conjugated charge transfer but also to the increase of the conjugation length. The latter factor clearly plays a determining role in the delocalization of π-electrons under light excitation.815 Chart 36. Thiophene-Containing Organic Molecules for NLO

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investigated with the Z-scan technique. Furane-based chromophores displayed a ratio of the excited-state to ground-state cross-section of nearly 200, an extremely remarkable value for this type of compound mainly due to the very low absorption of the ground state at the wavelength of analysis. The fluence-dependent transmission of the dichloromethane solution of 196 (Chart 38) is presented in Figure 43.823 This

Chart 37. Bithiophene-Based Derivatives for NLO

absorption at this wavelength (Figure 42). For instance, the best performing material, that is, compound 192c, revealed an

Figure 43. ●: Fluence-dependent transmission of diluted 196 (Chart 38) in dichloromethane. Solid line: Theoretical fit. Reproduced with permission from ref 823. Copyright 2001 Elsevier.

result was explained in terms of a scheme of transitions occurring in a five-level model (inset of Figure 43). In this depiction, the relaxation from T1 to S0 was neglected because it was much longer than the 8 ns pulse duration. The solid line in Figure 43 represents the theoretical fit obtained with σ12 = 7.5 × 10−19 cm2, σ34 = 9 × 10−17 cm2, and τisc = 150 ns. The latter value was considered as a very high one. Anthraquinone dyes 198−204 (Chart 39) were also studied for OL. For instance, the NLO properties of 1,4-diamino-9,10Figure 42. Nonlinear transmittance spectra of saturated solutions of (a) 193a, (b) 193b, and (c) 193c (Chart 37) at 430 nm with 27 ps laser pulses. (d) Comparison of nonlinear transmittance profiles of 192c and 193c at the same conditions of irradiation of plots (a)−(c). Reproduced with permission from ref 821. Copyright 2013 The Royal Society of Chemistry.

Chart 39. Anthraquinone-Based Compounds

OL threshold of fluence of 0.9 J cm−2. Furthermore, the nonlinear transmission (NLT) measurements of a solution of radical dimers of tetramethyl-tetrathiafulvalene822 (194, Chart 37), recorded with 9 ns laser pulses at 1064 nm, were reported and interpreted on the basis of a multiphoton absorption process. Radicals of the compound 194 series could be present in solution as isolated molecules or as dimers of radicals as proved by the characteristic band in the NIR spectral region related to a charge transfer process between the two moieties. The authors recorded the T versus Iin plot of a solution of the monomer in acetonitrile at 1064 nm, but could not find any nonlinear response. It was concluded that the observed nonlinear behavior was related to the presence of the dimers. Furane-based chromophores 195−197 (Chart 38), on the other hand,823 exhibited significant RSA processes, when Chart 38. Furane-Based Chromophores

anthraquinone 198 (Chart 39) in solution at variable concentration were tested with single beam Z-scan technique at 532 nm with a low power CW laser.824 The dye displayed self-defocusing and RSA behavior. The authors observed that β, η2, and χ(3) values increased linearly with the concentration, 13093


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being of the order of 10−2 cm/W, 10−6 esu, and 10−5 esu, respectively. Also, sulfonated anthraquinone derivatives 199 and 200 (Chart 39) were evaluated using Z-scan technique with CW He−Ne laser.825,826 The values of βeff, η2, and χ(3) were of the order of 10−3 cm/W, 10−5 esu, and 10−7 esu, respectively. Dye 200 represented the best OL performer, due to increased conjugation in the structure. For lower concentration, the compounds revealed to be reverse saturable absorbers, similarly to what happened when 201 (Chart 39) was investigated for NLO properties.827 Dye 201 presented βeff, η2, and χ(3) of the order of 10−2 cm/W, 10−4 esu, and 10−6 esu, respectively. Indathrones belonging to the family of anthraquinones have also been studied for their NLO properties.92,828 Derivatives of indathrones have strong NIR absorption, and their OPL behavior has been reported for ns pulses, at 532 and 1064 nm. 202−204 (Chart 39) had good OPL properties due to the RSA mechansim. NIR absorbing materials such as the chromophores dipyrromethene boron difluoride (BODIPY) are desirable materials for OPL applications in the range of telecommunication frequencies. These chromophores have an extended aromatic conjugated skeleton, improved by a rigidification with a Lewis acid group such as BF2. For instance, BODIPY 205 (Chart 40) was investigated in the fs regime.829

Figure 44. Nonlinear transmission of 205 (Chart 40) as a function of the input energy at ∼1310 nm carried by laser pulses. Reproduced with permission from ref 829. Copyright 2009 Elsevier.

NIR absorbing aza-BODIPY (aza-borondipyrromethene) dyes 206−208 (Chart 40) have also been tested for OPL.830,831 To increase the NLO properties and further shift the linear absorption into the NIR, electron-donating moieties have been introduced on the periphery of the dye. Z-scan measurements were performed in the spectral range 1200− 1600 nm using a 130 fs-optical parametric amplifier pumped by a Ti/sapphire source laser. At these telecommunication wavelengths, compound 206 did not present any TPA, as opposed to the donor substituted compounds 207 and 208. While molecules 206 and 207 showed a small NLO activity, chromophore 208 presented very broad TPA properties between 1200 and 1450 nm.831 Nonlinear absorbing aza-BODIPY 209 (Chart 40) was incorporated into a sol−gel monolithic matrix at a weight concentration of around 10%.832 Remarkably efficient broadband OPL in the solid state could be observed in the NIR region between 1200 and 1600 nm, with a maximum of efficiency around 1300 nm. The solid system was revealed to be a better OPL system than the solution of the same dye 209. Other molecules combining large TPA and ESA in the NIR region were also investigated (examples 210 and 211 in Chart 41).833 These molecules are of particular interest for optical

Chart 40. BODIPY-Based Chromophores

Chart 41. NIR Two-Photon Absorption Chromophores

limitation in this spectral NIR window (especially in the 700− 800 nm range). They display good transparency in the visible range, high photostability, and excellent solubility in organic solvents. It was noted that the optical limitation efficiency of the TPA chromophores investigated behaved well in the subnanosecond range, but significantly decreased for longer pulses. The authors observed that molecule 210 (Chart 41) displayed the higher optical limitation efficiency at 740 nm, as a result of the best combination of TPA (which decreases above 740 nm) and ESA (which decreases below 740 nm). In addition, a very low optical limitation threshold was maintained in the 700−800 nm range. In contrast, molecule 211 showed a

Chromophore 205 was found to have a broad spectrum of TPA and in the NIR range, with a fairly large cross-section of 101 GM at around 1310 nm. Simultaneously, the authors also demonstrated that the laser fluctuation could be reduced at least 2-fold after passing through the TPA medium containing 205 (Chart 40), which could represent a potentiality for use in the stabilization of light sources utilized in optical communications (Figure 44).829 13094


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The OL behavior of trans-diphenyl-1,3-butadiene (216), 2,2′-3-methyldithienylethylene (217), 2,2′-dithienyl-1,3-butadiene (218), and trans-2,2′-dibenzthienylethylene (219) in chloroform was studied at 532 nm.836 The studied dithienyl polyenes showed OL with high transmittance at low fluence and low nonlinear thresholds in a two-photon process. While 219 and 217 presented limiting thresholds of 23 J/cm2, 218 revealed a lower threshold of 14 J/cm2. The third-order nonlinear optical properties of the two novel styryl dyes, 1,3diethyl-5-(4-methoxybenzylidene)-2-thioxodihydropyrimidine4,6(1H,5H)-dione (220) and 5-(3,4-dimethoxybenzylidene)1,3-diethyl-2-thioxodihydropyrimidine-4,6(1H,5H)-dione (221), were studied using the Z-scan technique with 7 ns laser pulses at 532 nm.837 Figure 45 shows the OPL effect of these two molecules at 532 nm. The limiting thresholds of 220 and 221 samples were

regularly decreasing optical threshold when shifting to lower wavelength, in agreement with a combined increase of TPA and ESA.833 Keeping in mind the same theoretical approach to modulate OL properties, some authors also studied diarylalkynylsubstituted thiophenes 212−214 (Chart 42). It was found Chart 42. Arylalkynyl-Substituted Thiophenes

that NLO absorption at 532 nm of the 0.01 M solutions of the arylalkynyl-substituted thiophenes increased in going from compound 212 to 213 and 214. This trend clearly showed the importance of the extent of π-conjugation in the magnitude of the response.834 The measurements and calculations have shown a greater increase of the OPL effect from compound 212 to 213 than from 213 to 214. For these compounds differing for the conjugation length of the π-electron system, the extent of nonlinear absorption seems to be well correlated with the characteristic of the electronic ground state as estimated by standard ab initio molecular orbital calculations.834 Strong multiphoton absorption and harmonic generation in organic fluorescent chromophores are significant for nonlinear optics applications. Still, most of the fluorescent chromophores (215−231 in Chart 43) cannot combine both of these two physical features efficiently. Dye 215,835 however, could exhibit efficient two- and three-photon absorption under the excitation of fs pulses in solution and third-harmonic generation (THG) when dispersed into PMMA film, thus overcoming the intrinsic limitations of fluorescent chromophores.

Figure 45. OL behavior of styryl dyes 220 and 221. Reproduced with permission from ref 837. Copyright 2009 Elsevier.

70 and 52 mJ, respectively. The difference in the saturation level of the output power was due to the difference in conjugation length and the strength of the acceptor/donor end groups in the two compounds. Because the third-order nonlinear response arises from the extent of delocalization of the electrons, the power limiting was ascribed to TPA

Chart 43. Organic Fluorescent Dyes

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mechanism. The results demonstrated that the molecules possess negative nonlinearity due to the presence of methoxy group in the phenyl ring. Therefore, TPA mechanisms played a major role in the observed nonlinearity with the absorption parameters depending on the strength of the donor group.837 A series of azulenic chromophores (222−231, Chart 43) was studied for their nonlinear properties using Z-scan measurements.838 In general, the dimeric compounds 226−231 presented ratios of excited-state cross section over groundstate cross sections comprised between 2.5 and 12.9. The type of bridge did not play a significant role in the determination of this characteristic figure of merit with compound 231 being the best example within this series. Prasad and co-workers investigated a series of TPA chromophores based on asymmetric triphenylamines (232a− e, Chart 44). The experimental results suggested that when Chart 44. Two-Photon Absorbers Tested for OPL

Figure 46. (a) Nonlinear transmission of a 1 cm thick solution of 232a (Chart 44) in THF (0.04 M) as a function of the input intensity of the laser beam at ∼800 nm. (b) Output intensity as a function of the input intensity based on the same sample of (a). The solid curves are the theoretical curves with the best-fitting parameter β = 3.14 cm GW−1 (β is the effective TPA coefficient). Reproduced with permission from ref 839. Copyright 2004 The Royal Society of Chemistry.

237841 (Chart 45) could be induced by a small change of the linear absorption spectrum upon Zn2+ binding. This effect was ascribed to a sequential TPA process in which one photon was absorbed from the ground state and one photon was absorbed from an excited state. The OL behavior of compound 238 (Chart 45) was experimentally and theoretically studied at 532 nm with ns and ps pulses.842 Energy-dependent transmission measurements were integrated using a three-level molecular model that included TPA as the predominant mechanism of the OL effect. The measured cross-section was as high as α = 10 × 10−18 cm4/ GW. The series of stilbene derivatives 239−243843 (Chart 45) was also synthesized and investigated with open aperture fs Z-scan technique and the measurement of ns NLO transmission. Compound 243 resulted as the best optical limiting performer (Figure 47). The authors compared TPA determined for 239 with those of derivatives 240−243, and found that the substituent groups attached to the 239 periphery had little contribution to TPA enhancement. It was suggested that this large increase was due to the large cross section of ESA. The TPA cross section and OL of 4,4′-bis(dimethylamino) stilbene (244, Chart 45) with ns and fs laser pulses were determined and analyzed.844 Because of the contribution of two-step TPA, the dynamic TPA cross section of 244 for ns pulses was about 3 orders of magnitude larger than that for ultrashort fs pulses. The nonlinear refractive index and nonlinear absorption coefficients of compounds 245a,b (Chart 45) were also investigated using the single beam Zscan technique with a CW argon ion laser at 514.5 nm.845 While both compounds showed significant nonlinear optical parameters, 245a displayed β and χ(3) values of −5.35 × 10−3

either an electron-donor or an electron-acceptor was linked to a trans-stilbene at a para-position, an enhancement in molecular two-photon absorptivity was observed in both cases, in particular, in the range 650−800 nm.839 However, push−pull chromophores (with both donor and acceptor groups) showed larger overall TPA cross sections within the studied spectral region as compared to their monosubstituted analogues. An indication came from the fact that also stronger acceptors produce a more efficient intramolecular charge transfer process upon excitation, leading to increased molecular two-photon responses in this set of model compounds. From Figure 46a, it can be seen that the intensity-dependent nonlinear transmission of the sample dropped from ∼0.94 to ∼0.51 as the input intensity increased from ∼54 to ∼652 MW cm−2. Figure 46b shows the output intensity as a function of the input intensity. A fairly good optical power limiting behavior of solution of 232a can be seen when the input intensity is increased from ∼150 to ∼650 MW cm−2.839 The set of oligophenylenevinylene (OPV) derivatives 233− 245 (Chart 45) was synthesized for the investigation of structural third-order optical nonlinearities.840 The real part and the imaginary part of the molecular nonlinearities of the newly synthesized OPVs were determined by the closed and openaperture Z-scan technique at 800 nm with 100 fs laser pulses. Molecular nonlinearity enhancement due to asymmetric substitution in lower homologues was significant. However, such contribution became less pronounced in the higher homologues, reaching very large molecular nonlinearities values up to 2.5 × 10−32 esu. In another report, a large variation of the nonlinear transmission properties of a cyclen-based stilbene 13096


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Chart 45. Stilbene-Based Compounds for NLO

octupolar compounds 249a−c (Chart 46) were studied using the Z-scan technique at 800 nm with 1 kHz of incidence frequency using 150 fs-laser pulses.847 Their TPA cross sections were obtained from the magnitudes of the transmittance at the bottom of the Z-scan profiles. We found 381, 230, and 85 cm4/ (photon/s) for molecules 249a, 249b, and 249c, respectively. These results were attributed to the increase of TPA cross sections for the octupolar molecules upon increase of the number of the alternating double bonds in the dipolar subunits, or by augmenting the electron-donating strength of the substituent. The three-photon properties of the dye 250 (Chart 46) were studied using fs laser pulses. Three-photon absorption occurred in the wavelength range 1175−1300 nm.848 The three-photon absorption cross section of the dye measured by the nonlinear transmission method reached 5.46 × 10−79 cm6 s2. Other triarylamine derivatives849−851 (251−253, Chart 46) have also been tested for their nonlinear absorption properties. The measurements show a broadband nonlinear absorption range extended between 450 and 650 nm with high efficiency for the planar conjugated system (fluorene, 252) or a hindered donor group (251b). The Z-scan technique has been used to investigate the NLO properties of biphenyl-based compounds 259a,b (Chart 47) with a CW diode laser at 635 nm and 14.5 mW power.852 While both groups influenced the nonlinear optical absorption coefficient β and nonlinear refraction index η2, the cyano group in 259b was the one that most affected these nonlinear

Figure 47. OL response of compounds 239, 242, and 243 (Chart 45) in THF. Excitation source was 8 ns laser pulses at 760 nm. The dotted line corresponds to the linear transmission of the solutions. Reproduced with permission from ref 843. Copyright 2007 Elsevier.

cm/W and 1.2 × 10−4 esu, respectively, at the concentration 10 mM. A theoretical study846 was conducted to investigate the effect of branching on linear and nonlinear optical properties. The study revealed that branching led to cooperative enhancement of two-photon absorption while maintaining high fluorescence quantum yields, thanks to the strong localization of the emitting state. Multipolar molecules 246−253 (Chart 46) have been scrutinized for evaluation in NLO applications. For instance, 13097


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Chart 46. Multipolar Molecules for NLO

Chart 47. Biphenyl-Based Chromophores

The more complex terpyridine-based quadrupolar compounds 263−267 (Chart 48) were also synthesized for OL.854 The influences of electron-donating groups and type of conjugated bridge on the molecular TPA properties have been investigated. Experimental results demonstrated that the chromophores exhibited considerably large TPA properties in moderately polar organic solvents, due to enhanced intramolecular charge transfer. It was also found that the magnitude of the molecular TPA cross section in organic solvents can be effectively enhanced by using strong electron-donating groups or/and rigid bridges. As shown in Figure 48, compounds 263− 267 (Chart 48) showed significant OL behaviors, especially 264. When the input fluence increased from 1 × 10−5 to 4.43 × 10−3 J cm−2, the output fluence changed from 1 × 10−5 to 2.47 × 10−3 J cm−2 for compound 263, while for compound 264

parameters. The responsible mechanism of OL was found to be RSA, and the values of η2 and β for this compound were respectively 21.7 × 10−8 and 9.8 × 10−4 cm/W. Re[χ(3)] and Im[χ(3)] were 2.76 × 10−6 and 1.69 × 10−6 esu, respectively. Some research groups have prepared other types of chromophores, terpyridines-based compounds, to take advantage of the terpyridine moiety as an electron acceptor.853 For instance, TPA cross sections of compounds 260−262 (Chart 48) were determined by open aperture Z-scan technique with fs pulses.853 The maximum value of TPA cross section was determined for 262 with 7938.3 GM in DMF solution. Compounds 260 and 261 instead displayed very low TPA cross sections, thus proving the positive influence of the auxiliary electron-accepting cyanoacetate moieties. 13098


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Chart 48. Terpyridine-Based Chromophores

Another series of linear and multibranched bithiazole-based dyes possessing a terminal triphenylamine group (268−271, Chart 49) were synthesized,855 and the TPA properties were investigated with the open aperture Z-scan technique. TPA absorption cross section was 173, 429, 1132, and 1665 GM for 268, 269, 270, and 271, respectively. This considerable increase in the value of TPA cross section was attributed to the extension of conjugation on the chromophore through the introduction of more triple bonds in the structure. The TPA cross sections of multibranched TPA triazine chromophores 272a−c (Chart 50), and 273a−e (Chart 51) were measured with the open aperture Z-scan technique.856 It was determined to be 447, 854, 1023, 603, and 766 GM for 273a, 273b, 273c, 273d, and 273e, respectively. The observed trend was ascribed to the increase of the electron-donating strength of the end groups and the extension of the conjugation length of the system with the additional insertion of electron-withdrawing perfluoroalkyl as side groups. The authors also determined the OL behavior for these chromophores, studied using a focused 800 nm laser beam with pulses of 140 fs duration. When the incident fluence increased from ∼0.0010 to ∼0.042 J cm−2 (an increase of about 40 times), the transmitted output fluences increased 20, 25, 29, 35, and 35 times, respectively, for compounds 273d, 273a, 273e, 273b, and 273c (Chart 51). The series of large polycyclic aromatic hydrocarbon compounds 274a−c857 and 275−279 (Chart 52) were synthesized and characterized with fs laser spectroscopy. The limiting thresholds of 274a, 274b, and 274c were 22.8, 55.0, and 100.6 GW cm−2, respectively. Some authors reported about the OL properties of a few perylene tetracarboxylic derivatives (275 and 276 in Chart 52), bis(benzimidazo)perylene (275) and bis(benzimidazo)-thio-

Figure 48. (a) Output intensity versus input fluence at 710 nm for chromophores 263, 264, 265, and 267 (Chart 48) in THF at the concentration 5 × 10−3 M; (b) corresponding nonlinear transmittance of the same set of compounds in (a). Solid lines represent the bestfitting curves with TPA coefficient values of 0.32 × 10−10 cm W−1 (compound 263), 4.91 × 10−10 cm W−1 (compound 264), 0.87 × 10−10 cm W−1 (compound 265), and 0.89 × 10−10 cm W−1 (compound 267). Reproduced with permission from ref 854. Copyright 2013 John Wiley and Sons.

from 1 × 10−5 to 6.4 × 10−4 J cm−2, for compound 265 from 1 × 10−5 to 1.83 × 10−3 J cm−2, and for compound 267 from 1 × 10−5 to 1.77 × 10−3 J cm−2. 13099


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perylene (276), which were studied with Z-scan technique using fs laser pulses.858 For instance, the TPA cross section for 275 was δ = 8000 GM (1 GM = 10−50 cm4 s photon−1) at 720 nm, while at 740 nm a value of δ = 3680 GM was found. Figure 49 shows the OL effect for 275 (Chart 52) obtained at 770 nm. The authors concluded that this type of material exhibited useful TPA for OL effect, especially for very short pulses in the wavelength range 730−900 nm.858 Other perylene-pyrene fused derivatives (277−279, Chart 52) were also tested for OPL application.859 The compounds behaved as good optical limiters, with compound 279 as the best term due to its special electronic structure. The nonlinear optical transmittance decreased to 37% of the linear one when the input laser intensity was 108 μJ. Supramolecular self-assembly is also another possibility for achieving materials with good OL properties. Some authors have synthesized supramolecular complexes of Zn 280−282 (Chart 53).860 Changing the spacer bridges between two pyrrol-2-yl-methyleneamine units, self-assembled supramolecules with different shapes such as double-stranded helicates, triangles, and squares could be formed upon control of the number of ligands. Both 280 and 281 showed OPL from 500 to 580 nm, and 282 from 550 to 610 nm. As shown in Figure 50, it was realized that 282 is the best OPL material of the series in Chart 53, with 281 showing a stronger OPL than 280.860 It was also established by the authors that the limiting thresholds and TPA cross sections of these Zn complexes change as the coordination number increases. This suggested that variations in the number of conjugated ligands for self-assembling systems with large conjugation have a significant impact on both the linear absorption and the OPL performance. The authors related this fact to the ability of Zn(II) ions to strengthen ligand-to-ligand interactions, which cause the transfer of π-electrons from one ligand to another. This effect somehow increases the effective electronic conjugation. Furthermore, the more rigid molecular structures with higher symmetries ensured larger TPA cross sections for these Zn complexes.860 Other compounds, like acetone dibenzylidene acetone (DBA) derivatives 283a−e and 285−289 (Chart 54), have been tested as well, using single beam Z-scan technique with ns laser pulses at 532 nm.861 With the exception of 283b, all DBAs showed good OL at 532 nm, exhibiting strong TPA at that wavelength. The nonlinear absorption was found to be higher for 283e. In the case of 283a, for input energies less than 500 μJ/pulse, the output energy increased linearly with incident energy. Yet for energies more than 500 μJ/pulse, the output energy was almost constant clamped at the value of 350 μJ/

Chart 49. Multibranched Dyes for NLO

Chart 50. Tribranched Triazine Cored Compounds

Chart 51. Triazine-Based Chromophores

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Chart 52. Polycyclic Aromatic Compounds

The NLO properties of chalcones of the cyclopentanone type (285−289, Chart 54) were investigated with ns openaperture Z-scan method.863 Their third-order NLO properties were measured. It was found that β = 1 × 10−8 cm/W, while cyclopentanone type malononitrile 289864 (Chart 54) exhibited OL at 1064 nm, due to TPA with involvement of ESA. Compound 289 displayed a NLO cross section of 2.6 × 10−15 cm2. Chalcones 290a−c (Chart 55) were also investigated for OL purposes performing Z-scan and transient transmission measurements with 130 fs laser pulses at 780 nm.865 Figure 51 depicts the intensity-dependent transmission of 290a−c 0.2 mol/L in acetone. Transmittance was constant until the input irradiance of ∼10 GW/cm2 with 130 fs laser pulses at a wavelength of 780 nm. When the input irradiance increases beyond ∼10 GW/cm2, the measured transmittance deviated from the linearity and decreased at ∼100 GW/cm2, suggesting the occurrence of OL. The latter effect was found to decrease in the order 290c > 290b > 290a, according to the strength of donor−acceptor properties of the peripheral substituent.865 Compounds 291−294 (Chart 55) were investigated using Zscan technique866 with a CW Ar+ laser. The corresponding limiting thresholds varied between 3.5 and 10 mW, at 488 nm, while compounds 295a−b867 (Chart 55) in a PMMA film presented nonlinear refractive index, nonlinear absorption coefficient, and magnitude of third-order susceptibility of the order of 10−14 cm2/W, 1 cm/GW, and 10−13 esu, respectively. The NLO parameters of 295a,b wre determined with the Zscan technique utilizing 7 ns laser pulses at 532 nm. Compounds 296a−e868 (Chart 55) were tested using ns Zscan technique at 532 nm. These demonstrated self-defocusing effect with intensity-dependent refractive index (η2) of the order of 10−14 cm2/W and OL threshold in the range 125−181 μJ. Compounds 297−298869 (Chart 55) were dispersed in PMMA film for OL studies. These were analyzed using ns Zscan at 532 nm. 297 and 298 displayed a large negative nonlinear refraction coefficient η2 of the order 10−11 esu and a molecular TPA cross section of 10−46 cm4 s/photon. The nonlinear optical susceptibility was as large as 10−12 esu. Chalcone 299870 (Chart 55) displayed nonlinear refractive index of the order of 10−11 cm2 W−1 and third-order susceptibility of the order of 10−13 esu using ns laser pulses at 532 nm with the Z-scan technique. With the same

Figure 49. Output energy as a function of input energy for compound 275 (Chart 52) at the molecular concentration: 1018 molecules cm−3 in dichloromethane. Solution was placed in a 2 mm thick quartz cuvette and excited at 770 nm. The solid line represents the linear transmittance. Reproduced with permission from ref 858. Copyright 2005 John Wiley and Sons.

pulse. In the case of 283e, for input energies less than 200 μJ/ pulse, the output varied linearly with the input, but for energies more than 200 μJ/pulse, the output energy was constantly blocked at 60 μJ/pulse. For input energies less than 350 μJ/ pulse, in the case of 283d, and 300 μJ/pulse, in the case of 283c, the output energy increases linearly with incident energy. The authors could observe that the limiting threshold decreased from 500 to 60 μJ/pulse and 350 μJ/pulse, respectively, in donor and acceptor type substituted compounds. This was attributed to the variation in delocalized electron density among these compounds.861 Other compounds of the same structural DBA type (284− 289, Chart 54) have been also studied for NLO. For instance, compound 284 (Chart 54) was synthesized and deposited on PMMA to produce a homogeneous film. This system was investigated using open aperture Z-scan method in both ns and fs excitation regimes.862 The mechanism of nonlinear absorption was found to be a TPA process, where the limiting effect of the molecule was originated from the molecule’s peculiar D−π−A−π−D structure, which allowed a higher πelectron delocalization due to the presence of the strong πelectron donor thiophene. 13101


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Chart 53. Supramolecular Self Assemblies of Zn(II) Complexes for OPL

Chart 54. Dibenzylidene Acetone (DBA) Derivatives for NLO

Figure 50. Output intensity of Zn complexes 280−282 (Chart 53) in chloroform as a function of the input intensity when complexes are confined in 1 mm thick quartz cell at different wavelengths. The solid curves are theoretical simulation based on TPA. The dotted lines represent the linear transmittance. Reproduced with permission from ref 860. Copyright 2005 American Institute of Physics.

experimental apparatus,871 the refractive and absorptive nonlinearities of compounds 300−302 (Chart 55) were measured and found to be in the order of 10−11 esu. 300−302 all exhibited TPA. Also, methylsulfanyl substituted chalcones 303,872 304,873 and 305874 (Chart 55) were investigated using Nd:YAG laser at 532 nm. The influence of this functional group consisted of the improvement of the NLO parameters in all three cases. For instance, 304 presented a value for NLO absorption coefficient of 4.5 cm/GW, and ESA cross-section and ground-state absorption cross-section of 5.17 × 10−18 and 5.68 × 10−21 cm2, respectively.873 A series of chalcones containing a terphenyl core differently substituted with various functional groups has been synthesized (306a−d, Chart 55) for OL scopes.875 The Z-scan experiments performed at 532 nm with 7 ns laser pulses from a Nd:YAG

laser in solution revealed that compounds 306a−d exhibited strong nonlinear refraction coefficient of the order 10−11 esu and a molecular TPA cross section of 10−46 cm4 s/photon, with OPL due to TPA. The third-order optical nonlinearities of bis-chalcones 307− 309 (Chart 55) have also been studied either in DMF solution or embedded in solid PMMA matrix,876 by Z-scan technique using ns laser pulses at 532 nm. The compounds exhibited negative nonlinear refractive index as high as 10−11 esu, while the molecular TPA cross sections were of the order 10−46 cm4 s/photon. Bis-chalcone 309 exhibited the lowest optical limiting threshold of 100, 80, and 85 μJ, with the output energy clamped at 50, 45, and 30 μJ, respectively, when concentrations were 1 × 10−2, 1.5 × 10−2, and 3.0 × 10−2 mol/ L. 13102


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Chart 55. Chalcone Derivatives for NLO

Chart 56. Calix[4]arene Derivatives for NLO

Figure 51. Ultrafast OL of the chalcone derivatives 290a−c (Chart 55) in acetone at the concentration 0.2 mol/L. Cuvette was 2 mm thick. NLO transmission is shown as a function of the input intensity at 780 nm with 130 fs pulses excitation. The symbols are the experimental data, while the solid lines are the theoretical fits. Reproduced with permission from ref 865. Copyright 2008 American Institute of Physics.

Calix[4]arene derivatives, more specifically tetrahydroxytetrathiacalix[4]arenes 310a,b and derivatives 311−313 (Chart 56), have been also investigated. 701 Functionalization comprised the use of both phenylazo or ethynylic peripheral substituents. The Z-scan experiments were performed on compound 310a showing nonlinear absorption due to TPA with a corresponding cross section of about 50 × 10−50 cm4 s per photon. OPL measurements on this complex at 532 nm (48% linear transmission) were found to limit the transmitted energy to ∼9 mJ.

The OL effect of the three ferrocenyl substituted calixarenes 311−313877 (Chart 56) was studied with ns pulses at 532 nm, with the OPL performance decreasing with the order 313 > 311 > 312. On the other hand, when fs laser pulses at 800 nm were used, the nonlinearity was in the order 313 > 312 > 311, and the largest value of nonlinear refraction index (η2 = 9.5 × 13103


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10−16 cm2/W) was found for 313. The authors ascribed the OL effect on ns laser pulses as being mainly due to thermal nonlinearity and RSA, whereas the OL effect of fs laser pulses was due mainly to intrinsic electronic nonlinearity contribution.

effect as the nonlinear absorption is negligible. For shorter wavelengths, the nonlinear absorption is strong enough to limit the output energy below 10 mJ. This type of nonlinear absorption was ascribed to three-photon absorption in a twostep process, that is, TPA followed by an excited-state onephoton absorption that occurred during the ns pulse. Bifluorene 314 proved to be a fairly efficient OL molecule in the green spectral range due to its simultaneous large TPA and ESA. Its large TPA cross section is theoretically explained by the enhancement of transition dipole moments that results from the coupling of monomers.878 The OPL in the visible region of a series of two-photon absorbing quadrupoles built from conjugated backbones bearing electro-active peripheral groups was also investigated (315a−f and 316a−c, Chart 57). Structure−property relationships could be derived, emphasizing the role of the conjugated connectors and of the end-groups for improving the OL efficiency.879 From Table 8, it could be evidenced that push−

4.2. Fluorene- and Carbazole-Based Compounds

Fluorenyl-based compounds are also an alternative choice in the quest for good OL materials. These chromophores have good characteristics for this type of application, like strong TPA-based NLO properties. For instance, the 2,2′-(9,9-dihexyl) bifluorene (314, Chart 57) was Chart 57. Two-Photon Absorbing Fluorenyl Molecules for NLO

Table 8. OL Properties of Fluorenyl Molecules

studied in the visible range (450−650 nm), with ns pulses. TPA was obtained by the up-conversion fluorescence method giving resonance at λ = 534 nm with a cross-section σ2 = 60 × 10−50 cm4·s/photon-molecule.878 The authors suggested that through semiempirical quantum chemistry calculations on fluorene and bifluorene, an enhancement of the bifluorene 314 TPA due to coupling effects between monomers would be implicit. The strong nonlinear absorption leads to an efficient OPL in the green and blue parts of the visible spectrum, showing a maximum transmitted energy lower than 10 mJ for an input energy of up to 200 mJ in f/5 optical geometry. Typical OL curves of bifluorene 314 are shown in Figure 52. For wavelengths longer than 600 nm, there is almost no OL

compound

Fth,532a (J cm−2)

Fth,600b (J cm−2)

τexcc (ns)

315a 315b 315c 315d 315e 315f 316a 316b 316c

1.20 0.40 0.14 0.10 0.12 0.05 0.25 0.13 0.03

2.30 0.51 0.20 0.15 0.17 0.08 0.32 0.24

0.79 0.64 0.56 0.74 0.80 0.60 0.87 0.54 0.87

a

Optical limiting threshold at 532 nm. bOptical limiting threshold at 600 nm. cExcited-state lifetime derived calculated for 10−6 M toluene solutions.

push systems were more efficient than pull−pull systems, but at the expense of reduced transparency. Pyridine-N-oxides, that is, terminal moieties that act in synergy as both donor and acceptor end-groups, led to improved efficiency/transparency parameters. Compounds with planarized core and phenylenevinylene linkers were also found to be more efficient than analogous derivatives built from twisted core or phenyleneethynylene linkers. It has been also demonstrated that lengthening the conjugated system does not necessarily lead to improved OL behavior but that the nature of the conjugated connectors plays a significant role. The alternation of phenylene-vinylene and phenylene-ethynylene units allowed significantly the improvement of OL efficiency/transparency parameters in bis-acceptor compounds.879 Other fluorene-based structures (317−320, Chart 58) were later characterized at 775 nm.880 The measured input and output intensity data for these chromophores are plotted in Figure 53. All four chromophores showed OL behavior. The authors stated that when the input energy increased from 0.21 to 4.6 mJ (∼22 times increase), the transmitted energy changed from 0.18 to 2.03 mJ (∼11 times increase) for compound 317, from 0.17 to 1.74 mJ (10 times increase) for compound 318, from 0.17 to 1.5 mJ (∼9 times increase) for compound 319, and from 0.17 to 1.25 mJ (∼7.5 times increase) for compound 320. The OL ability decreased then in the order 320 > 319 > 318 > 317.880 Ferrocene-based fluorene-containing derivatives 321a,b (Chart 59) have been tested for OL activity.881 From the

Figure 52. OPL curves for compound 314 (Chart 57) in chloroform (mass concentration = 400 g/L). Optical geometry was f/5, waist beam 530 nm, and 2.6 ns pulses. Reproduced with permission from ref 878. Copyright 2001 American Institute of Physics. 13104


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were still quite large (more than one-half of those at 1260 nm), which implied that their three-photon absorption halfbandwidths are larger than 340 nm. Figure 54 shows that the output/input characteristic curve starts to deviate from linearity at energy levels of around 0.8 mJ,

Chart 58. Fluorene-Based Structures for OPL

Figure 54. Output energy versus input energy for compounds 321a and 321b (Chart 59) at 1260 nm. The dash-dotted line represents the best fit curve with σ3 coefficient values of 9.0 × 10−6 cm3 GW−2 (321a) and 7.2 × 10−6 cm3 GW−2 (321b). Reproduced with permission from ref 881. Copyright 2005 The Royal Society of Chemistry.

Figure 53. Output intensity versus input intensity of chromophores 317−320 (Chart 58) at 775 nm. The dash-dotted line represents the best-fitting curves with TPA coefficient values of 0.0601 cm GW−1 (317), 0.0787 cm GW−1 (318), 0.114 cm GW−1 (319), and 0.165 cm GW−1 (320). Reproduced with permission from ref 880. Copyright 2008 John Wiley and Sons.

Chart 59. Ferrocene-Containing Fluorene Chromophores

and levels off once the input energy levels are higher than 2.0 mJ. More specifically, when the input energy increased from 0.33 to 5.35 mJ (∼16-fold increase), the transmitted energy only changed from 0.33 to 1.92 mJ (∼5.8-fold increase). This could be seen as a typical OL behavior based on the threephoton absorption mechanism. Similar behavior is seen with 321b (Figure 54b). When the input energy increased from 0.33 to 5.35 mJ (∼16-fold increase), the transmitted energy only changed from 0.33 to 2.14 mJ (∼6.5-fold increase). In this experiment, it was found that three-photon absorption offers a good OPL performance due to its cubic dependence on the incident intensity. The output/input characteristic curves in Figure 54 imply that besides OL, optical stabilization may also be achieved using these three-photon absorbing materials.881 Two-photon OPL of fluorenylperylene diimide-based derivatives 322a,b (Chart 60) was investigated in CH2Cl2 under ps excitation.882 High values of TPA cross sections at λexc = 660 nm, along with ESA, resulted in a relatively strong OPL effect obtained under two-photon excitation. By taking into account the difference in TPA cross sections for 322a and 322b in CH2Cl2 at λexc = 660 nm, the nearly same limiting effect for 322a and 322b could be explained by a more efficient ESA for compound 322a. The figure of merit F = T0/TH (T0 and TH are

three-photon absorption cross-section σ3 data in Table 9, it can be seen that the three-photon absorption peaks for compounds 321a and 321b were close to 1260 nm, although the precise positions could not be determined on the basis of these data. In any case, they can be considered quite large, bearing in mind their modest π-conjugated systems. Three-photon absorption cross-section values for compounds 321a and 321b at 1600 nm Table 9. NLO Absorption Cross Sections for FerroceneBased Fluorene-Containing Derivatives σ3/10−25 cm6 GW−2 compd 321a 321b

σ2/10

−20

4

cm GW

∼0.45 ∼0.40

−1

1260 nm

1314 nm

1460 nm

1600 nm

∼3.75 ∼3.00

∼2.45 ∼2.45

∼2.37 ∼2.54

∼2.12 ∼2.34 13105


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Chart 60. Fluorene-Based Chromophores

Chart 62. Star-Shaped Fluorene Chromophores

the transmittances at low and highest intensity, respectively) of 322a and 322b under ps excitation was about F ≈ 3.5. To evaluate the power limiting performance of other fluorene-based chromophores 323 and 324 (Chart 60), some authors have used fs laser pulses at ∼775 nm to characterize their TPA-based OPL properties.883 The TPA cross-section values of model compounds 323 and 324 were calculated to be ∼455 and ∼735 GM, respectively.883 Trisubstituted fluorene derivatives 325 and 326884 (Chart 61) have been synthesized and their photophysical properties

327a, 327b, and 327c, respectively. This NLO parameter decreased with the increase of the number of bridging fluorene units. Ultrafast dynamics measurements also have shown that there was a fast intramolecular charge transfer process in the time scale of about several ps and a relatively long decay process of the intramolecular charge transfer state, increasing from 327a (1.9 ps), to 327b (3.0 ps) and 327c (6.3 ps), that is, with the increasing number of the fluorene bridges. A triphenylamine-based multipolar dye molecule (328, Chart 62)886 with six branches showed efficient three-photon absorption-based OPL performance in the fs regime, with cross sections σ3 = 21 × 10−24, 22 × 10−24, and 7 × 10−24 cm6 GW−2 at 1217, 1300, and 445 nm, respectively. Several multibranched chromophores containing 2,3-diarylquinoxalinyl units (329−338, Charts 63 and 64) as the electron acceptors have been synthesized for the measurement of their NLO properties.887−890 Most of these compounds possess strong and broad TPA in the NIR region, reaching effective optical-power-attenuation both in fs and in ns time domains. Expansion of the size of the π-conjugated system, through the insertion of different aromatic rings, gave a positive contribution for the enhancement of TPA. The insertion of the electron-rich dialkoxybenzene unit provided a larger increment of molecular TPA,889,890 exhibiting both intense upconverted emission when excited by two-photon process and effective OPL/stabilization properties upon excitation with ns laser pulses.889,890 Compound 339 (Chart 65) was also studied for NLO, under excitation at 800 nm with 8 ns laser pulses at the repetition rate of 10 Hz.891 TPA cross section and two-photon absorptioninduced frequency upconverted emission spectra were measured for 339 solutions in several solvents. The most striking

Chart 61. Trisubstituted Fluorene Derivatives

examined in six different solvents. The results were compared to those of their nonbranched analogue, to establish structure− property relationships. The TPA coefficients were in the order of 10−2 cm GW−1, as determined by a direct nonlinear optical transmission method. When optical systems were pumped by Ti-sapphire laser at 775 nm, the NLO performance was associated with low fluorescence quantum yields. The star-shaped compounds 327a−c885 (Chart 62) are composed of the same central core 1,3,5-triazine, with three arms that consist of specific numbers of bridging fluorene groups and with triphenylamine as electron-donating group at the terminal end of each arm. The measured TPA cross sections were determined to be 1509, 1260, and 789 GM for 13106


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Chart 63. Multibranched Chromophores I

Chart 65. Two-Photon Absorbing Fluorene Derivatives

feature of this chromophore was its remarkably high value of the molecular TPA cross section (∼78 × 10−20 cm2/GW in benzene solution). On the basis of this feature, good OPL and stabilization performance could be demonstrated. The nonlinear transmission of 339 decreased from ∼0.93 to ∼0.3 when the input-beam intensity increased from ∼10 to ∼360 MW/cm2. The relative intensity fluctuation of the output laser pulses was reduced to one-third of that of the input laser beam. Unfortunately, one drawback for this material is its

limited photochemical stability, especially upon exposure to UV radiation in ambient atmosphere, most probably due to the presence of conjugated CC bonds in the molecule, which are easily oxidizable.892 To overcome this issue, other new chromophores have been developed and studied, for example, tris[4-(7-benzothiazol-2-yl9,9-diethylfluoren-2-yl)phenyl]amine (340, Chart 65), with 800 nm laser pulses in both ns and fs regimes.893 This new

Chart 64. Multibranched Chromophores II

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compound exhibited a large TPA cross section and, more importantly from the application point of view, a high photochemical/photothermal stability. The measured effective two-photon absorption cross section was σ2 ≈ 151 × 10−20 cm4/GW for 7 ns laser pulses at 810 nm laser pulses and σ2 ≈ 0.61 × 10−20 cm4/GW for 135 fs light pulses at 796 nm. Figure 55 shows the overall output fluence at 810 nm transmitted by a THF solution of 340 (Chart 65) as a function

Chart 66. TPA Dyes Based on Fluorene Moiety

Compound 351897,898 (Chart 67), which is composed of an electron-accepting center with three arms of conjugated

Figure 55. Output intensity and fluence transmitted by 340 (Chart 65) in THF as a function of input intensity and fluence based on (a) a 1 cm optical path-length. Concentration of 340 was 0.033 M. (b) Fluence-dependent optical transmission of a 1 cm path-length of THF. The solid line is the theoretical curve obtained with a fitting parameter of β = 0.03 cm/MW. Reproduced with permission from ref 893. Copyright 2000 American Chemical Society.

Chart 67. Fluorene-Based Chromophores for NLO

of the input overall fluence. The nonlinear transmittance decreased from ∼0.92 to ∼0.24 when the input beam intensity increased from ∼10 to ∼390 MW/cm2.893 A systematic photophysical study of the nonlinear absorption properties of the TPA absorbing dyes 339−344 (Chart 65) has been carried out.894,895 In this set of phenylamines, the various systems differed from each other for the continuous addition of one phenyl group, thus generating dipolar one-arm (339, 341, and 342), quadrupolar two-arm (343a and 344), and octupolar three-arm (340 and 343b) molecules. From the studies of their NLO properties, it was found that upon increase of branches number there was the increase of the intrinsic TPA cross sections as well as the enhancement of ns nonlinear absorption. Compounds 345−350 (Chart 66) have been synthesized, and their structure−property relationship regarding the nonlinear optical properties has been established.896 By using the two-photon excited fluorescence method and the nonlinear transmission method, the TPA properties of these chromophores were evaluated. The authors found that the process of TPA occurring in the set of Chart 66 was strongly dependent on the electronegativity of the ladder-type core, as well as of the terminal groups. The introduction of strong electron donors (N-alkyl) in both the central core and the terminals led to a chromophore with a high TPA cross-section (2137 GM), with TPA coefficients ranging from 0.131 to 0.256 cm GW−1. OL behaviors of the synthesized chromophores revealed that the chromophore with the highest TPA cross section value, that is, 350, exhibited the best OL performance, with light transmission as low as 19.1% at 770 nm at the incident intensity of 99.6 GW cm−2.

moieties having an electron-donating group at the terminal end of each arm, was evaluated concerning its TPA properties. The analysis was conducted with the realization of a series of different photophysical experiments: steady-state absorption, steady-state and time-resolved emission, fs pump−probe, and ns laser flash photolysis techniques. The authors found that 351 exhibited triplet-based ESA around 800 nm with a high value of effective TPA cross section for ns pulsed radiation. Donor−fluorene−donor type organic dyes 352 and 353 (Chart 67) were synthesized.899 The wavelength of maximum absorption of 352 and 353 in the ground state was 412 and 406 nm, respectively. From OPL experiments (Figure 56), the nonlinear coefficients of 353 and 352 were found to be 8 × 10−21 and 4 × 10−21 cm4/GW, respectively. Furthermore, the series of organic compounds 354−367 (Chart 68) with systematically varied molecular structures exhibited very large effective TPA cross sections as evidenced with the measurement of nonlinear transmission.900 The results 13108


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The multibranched nonlinear chromophore 368902 (Chart 69) containing carbazole moieties presented OPL induced by Chart 69. Multibranched Nonlinear Chromophore

three-photon absorption. It displayed large NLO coefficients, and the corresponding molecular cross section was as high as 10−74 cm2, a value obtained by irradiation with ps laser pulses at 1.06 μm.

Figure 56. OPL of 353 (Chart 67) in chloroform (2 × 10−2 M). Reproduced with permission from ref 899. Copyright 2007 Elsevier.

4.3. Polymethine Cyanine Dyes

of the study indicated that the incorporation of certain combinations of structural elements in the dyes increases effective cross sections as high as 152.5 × 10−48 cm4 s/photon molecule in benzene solutions at 800 nm using 8 ns pulses. For instance, planarity seemed to be crucial for enhancing TPA cross sections. Donors needed to have quite strong donating capacity to maximize the TPA cross section. Pyridine acceptors coupled with diarylamino donors seem to possess the correct strength to allow the localization of te maximum of TPA close to 800 nm. The two molecules 359 and 361 (Chart 68) resulted to have the largest effective TPA cross sections and OPL efficiency. N-Butyl-3,6-diformylcarbazole (367, Chart 68) was also evaluated for OL.901 The occurrence of the three-photon absorption induced the effect of OL of 38 ps pulses at 1064 nm emitted at the repetition rate of 10 Hz. The three-photon absorption coefficient was 6.1 × 10−23 cm6/W2, and the corresponding absorption section was 8.7 × 10−79 cm2.

Cyanines dyes are ionic chromophores, in which the charge is delocalized between two electron-donating groups via a hydrocarbon skeleton with an odd-number of C(sp2) units. In such compounds, a red-shift of the photophysical properties can be achieved by lengthening the π-conjugated backbone, and a ca. 100 nm bathochromic shift is usually obtained with every additional vinylene unit.903 This modulation is generally accompanied by a strong decrease of the photochemical and/ or thermal stability, limiting the number of applicative scopes. Their particular photophysical properties, extremely intense sharp absorption and emission bands located in the NIR,904 confer this class of compounds promising application mainly as two-photon-based optical limiters. The first report on the OPL properties of cyanines was given by Penzkofer’s group,905 which measured the nonlinear energy transmission of concentrated aqueous solutions of green indocyanine 369 (Chart 70) with fs pulses. Experimental

Chart 68. Organic Molecules Studied for NLO

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Chart 70. Polymethine Dyes for NLO

conditions and conclusions on the NLO behavior of 369 were later revisited by Mendonça.906 NLT behavior could be observed from the plot of energy transmission versus cell position, where the NLO transmission was measured over a moving cell sample along the laser beam axis. The intensity-dependent index of refraction and the molecular two-photon absorptivity of 3,3′-dielthylthiacyanine iodide (370, Chart 70)907 have been measured with the fs Zscan technique at 800 nm. The intensity-dependent index change, η2, was large and relatively insensitive to the variation of the refractive index of the solvent used (methanol and DMSO). The TPA cross-section, σ2, was 2.654 × 10−41 and 1.394 × 10−42 cm2 when 370 (Chart 70) was in DMSO and methanol solution, respectively. The nonlinear properties of another set of different polymethine dyes (371−379, Chart 70) were studied and characterized by Van Stryland.76,908,909 Most of these compounds showed NLT in ethanol solution, and in polyurethane acrylate (PUA).908,909 However, some results’ inconsistency, caused by apparent degradation of the compounds, was observed. Nevertheless, the dominant nonlinearity in all of the studied dyes was based on RSA with 371− 379 (Chart 70) possessing large excited-state absorptions and relatively large ground-state absorption cross sections (between 10−17 and 10−15 cm2). The resulting ratios excited/ground-state cross sections reached values up to 200 at 532 nm. The lifetimes of the excited state were of the order of 1 ns in ethanol, increasing to 3 ns in PUA.909 The value of the excitedstate lifetimes in PUA was greater than that in ethanol. This difference reflected the role of the polymeric medium on excited-state relaxation processes of the dye molecules.910 Later, dipolar and symmetrical heptamethine dyes 380−385 (Chart 71) featuring strong linear absorption in the NIR

Chart 71. Symmetrical Heptamethine Dyes

spectral range (700−900 nm) were considered for OL.911−913 The chromophores presented nonlinear absorption properties at 1400−1600 nm, having significant TPA cross sections as measured with fs Z-scan method. The generally high solubility of the heptamethine dyes (Chart 71) allowed the authors to perform a series of numerous OPL experiments at 1500 nm in the ns regime. The results were interpreted in terms of TPA from ground and excited states. A representative result is shown in Figure 57 for the case of a concentrated solution of 380 (Chart 71) using 1500 nm as fundamental incident wavelength. This is the first OL curve at telecommunications wavelength reported in the literature for 13110


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Chart 73. Dendron Decorated Heptamethine Cyanines

Figure 57. Nonlinear transmittance of 380 (Chart 71) at 1500 nm in dichloromethane with different concentrations. c = 223.4 g L−1 (gray ▲), 281.6 g L−1 (□), 320.4 g L−1 (●). Reproduced with permission from ref 911. Copyright 2007 American Chemical Society.

organic molecules. An attenuation of 40% of the incident laser beam (incident energy 3 × 10−4 J) could be attained.911 The role of concentration on nonlinear transmission properties was also studied in the case of 380 at 1500 nm. Upon decrease of concentration from 0.33 to 0.23 mol L−1, a significant flattening of the NLO transmission was observed. Because dicarbocyanine dye 386 (3,3′-diethyloxadicarbocyanine iodide) (Chart 72) is frequently used as a saturable Chart 72. Dicarbocyanines for NLO

tripled Nd:YAG laser source. The dendron decorated chromophore 388 (Chart 73), its precursor 389, and the reference compound 390 showed the characteristic behavior of optical limiters at the telecommunication wavelengths with very similar characteristics. As the linear and NLO properties between 388 and its reference 390 are nearly identical, the authors concluded that the presence of dendrons had no significant effect on the spectroscopic properties and NLO characteristics. The spectral-luminescent and energy absorption characteristics of the indotricarbocyanine dye 391 (Chart 74) as well as the relative ESA spectrum with 15 ns pulses at 532 nm were determined.918 Its nonlinear absorption was investigated using the open aperture Z-scan method. These experiments led the authors to conclude that limitation of laser radiation power by this dye was due to RSA from singlet states. Later, the same authors studied the nonlinear absorption of four photostable polymethine cyanine dyes (392−395, Chart 74) excited by ns second harmonic pulses from a Nd:YAG laser919 and compared it to 391. Compound 391 indeed showed minimal linear absorption in a greater part of the visible spectrum and the strongest nonlinear RSA, resulting in the system with the best OL properties. The effect of optical limitation was was observed in a broad portion of the visible spectrum. Cyanines 396−398920 (Chart 75) in solution were also investigated for NLO applications due to photoinduced intermolecular charge transfer between the cyanine and viologens (as electron acceptors) also present in solution. The resulting NLO behavior was tested with ps and ns laser pulses at 532 nm, and the figure of merit σexc/σg was determined. 396−398 (Chart 75) displayed a clamping effect up to incident fluences of over 2 J/cm2.

absorber of mode-locked dye lasers,914 some authors decided to study the eventual effect of saturable absorption of 386 by ps excitation at 527 nm and by fs excitation in the wavelength region from 581 to 628 nm.915 In the region from 581 to 628 nm in which 386 is commonly employed as mode-locking dye, the ESA cross-section was found to be about 1/5 of the groundstate absorption cross-section. This result confirmed the good saturable mode-locking properties of 386 (Chart 72). Similarly, a hexamethylindotricarbocyanine perchlorate (387, Chart 72) was also characterized for OL. It showed a comparatively better OL effect than benchmark C60. The analysis of the results revealed that the photoexcited triplet state absorption crosssection was higher than the ground-state absorption crosssection, suggesting a RSA behavior in these materials. The limiting threshold of 387 (Chart 72) was 0.45 J/cm2 (at 70% linear transmission).916 Later, some authors considered the synthesis of a dendron decorated heptamethine cyanine (388, Chart 73),830,917 to compare its NLO properties with those of its precursors 389 and 390 (Chart 73). In these latter cases, 389 and 390 have shown significant TPA cross sections in the 1400−1600 nm spectral range with the display of OPL properties. OPL curves were interpreted in terms of TPA followed by ESA phenomena. NLO transmission measurements were carried out at 1420 nm using a ns optical parametric oscillator pumped by a frequency 13111


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Chart 74. Polymethine Cyanine Dyes

Chart 75. Carbocyanines for NLO

4.4. Dendrimers

The TPA and ESA of the two organic cyanine dyes 399 and 400 (Chart 75) and of a ruthenium-based organometallic cyanine921 (401, Chart 75) were compared to rationalize their similar ns OPL efficiency in the telecommunication wavelength range. The TPA contribution to the ns OPL behavior is higher for both organic cyanines, while in the case of the organometallic system, the main process generating the NLO effect is a TPA-induced ESA, in which the ruthenium brings about a broadening of the NIR excited-state absorption band. This effect results in the strong spectral overlap between TPA and ESA spectra. The study of the TPA processes occurring in a series of cyclohexanone/piperidone derivatives922 402 (Chart 76) was conducted. It was found that these compounds possessed TPA cross sections of the order of 3 × 10−47 cm2. The authors ascribed the high cross-section values of the highly stable derivatives of the cyclohexanone (Chart 76) to the long πconjugated electron bridges and to the pending groups with strong electron-releasing characteristics.

Dendrimeric nanostructures have been suggested the past few years as leading candidates for drug delivery systems, quantumconfined structures, and nanoscaled storage units.923,924 First discovered in the early 1980s by D. Tomalia and co-workers,925 these hyperbranched molecules had an obvious growth in the synthetic chemistry of organic materials and also had paved the way to metal-dendrimer nanocomposites (DNCs) production.926,927 Particularly interesting were the poly(amidoamine) (PAMAM) derivatives.927−929 Whereas the fabrication of many new types of DNCs keeps its fruitful exploitation in many directions, the NLO properties of these novel materials are still relatively underdeveloped and not yet optimized. Only recently have DNCs been considered for nonlinear optics, including OL effect at several wavelengths and laser pulse durations.930−932 Later, the mechanism producing the OL effect in these structures514 was identified as focused heating and subsequent bubble formation in the solutions of metalDNCs.930,933 In some cases, the specific OL effect of DNCs had to be evaluated together with those observed in other metal nanocomposites (vide supra).514 Unsurprisingly, the decrease of optical transmission was less pronounced when the concentration of the solution was reduced.930 From open aperture Z-scan profiles at 532 nm, it was concluded that strong optical extinction occurred when concentrated solutions (5.9 × 10−4 mol/dm3) of Ag-DNCs were placed at the laser beam focus for a fluence of 3.3 J/ cm2.930 The probing of excitation and emission dynamics of metal NPs gave a direct correlation between inherent linear and nonlinear optical properties of different metal-based species.

Chart 76. General Structure of the Derivatives of Cyclohexanone/Piperidone

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Chart 77. Electron-Rich Dendrimers

These behaved differently between each other,514,930,931 giving the possibility of varying the dynamics of excitation and emission through the alteration of the particle dimension.934 Some authors performed a study of the OPL effect of some DNCs using a Nd:YAG laser operating at both 532 and 1064 nm with a pulse duration of ∼6.5 ns and at the repetition rates of 2 and 10 Hz.514,930 At the 10 Hz rate, the solution of a metalDNC with concentration of 2.0 × 10−4 mol/L gave a limiting threshold of ∼2.0 J/cm2.930 The OL performance of the AgDNCs is very good if compared to the results obtained with other organic structures such as the AF-380 dye935 (exhibiting a threshold value of ∼2 J/cm2 and a transmission loss of 60% for an input fluence increase up to 13 J/cm2) and single-walled carbon nanotube suspensions936 (having a threshold value of ∼2 J/cm2 and transmission decrease of 70% for fluence increasing up to 6 J/cm2).This comparison proved the Ag dendrimer nanocomposite system as a strong optical limiter at 532 nm. Time-resolved photoluminescence measurements on Ag-DNCs have shown that the excited-state lifetimes are very short for this system.934 RSA processes were seen to develop on a time-scale similar to the excited-state lifetime, which suggested that a small contribution of RSA was coming from Ag-DNCs.931 Also, nonlinear transmission of Au- and AgDNCs was reported for ns pulses at 1064 nm.937 Both of these metal-DNCs showed an appreciable OL effect. The decrease of 12% and 10% in the nonlinear transmission with respect to the linear one was observed for Ag- and Au-DNC, respectively. In fact, the input fluence increased from 1 to 15 J/cm2.937 The authors also compared the pristine dendrimer and the corresponding metal nanocomposites. The dendrimer alone

gave a negative result because no NLO effect was shown. Therefore, it was assumed that the observed NLO effects were mostly due to the metal nanocomposite entities, which acted according to a model of TPA.937 TPA coefficients were 0.7 cm/ GW for the silver and 0.5 cm/GW for the gold nanocomposite. Taking into account the previous work on the mechanisms responsible for large nonlinear transmission coefficients in organic polymers,938 the authors explained the results at 1064 nm in terms of resonantly enhanced TPA when the systems exhibit linear absorption peaks close to exactly one-half the TPA wavelength. Two-photon and three-photon absorption electron-rich dendrimers 403−406 (Chart 77) have also been studied.939−941 Examination of the TPA cross sections revealed the same ordering as for the linear absorption parameter, with complexes 404b, 405a, and 405c having σ2 values between 200 × 10−50 and 300 × 10−50 cm4 s per metal center, and complex 406b having a value of ∼500 × 10−50 cm4 s per metal center. A large system of π-conjugation is recognized to be an important structural factor for the acquisition of good nonlinear properties. Therefore, the NLO characterization of degenerate two- and three-photon absorption properties of a multibranched chromophore with a highly conjugated skeleton was carried out in the fs regime using white-light continuum and nonlinear transmission techniques.942 The relatively strong nonlinear absorption over a wide spectral range and effective OPL behavior of this compound suggested that the combination of large two- and three-photon absorption properties in a single molecule can provide another approach for achieving fast responsive and broadband optical limiters in the fs time domain.942 13113


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The same authors have measured the three-photon absorption spectra for the π-conjugated dendritic chromophore as well as for its one- and three-branched units (407−409, Chart 78).943 Experimental results have shown that the Chart 78. Dendritic Chromophores with Three-Photon Absorption Properties

Figure 58. Output intensity versus input intensity at 1310 nm for chromophores 407−409 (Chart 78). The dash-dotted line represents the best-fitting curve with three-photon absorption coefficient values of 2.5 × 10−6 cm3/GW2 (407), 3.7 × 10−6 cm3/GW2 (408), and 4.3 × 10−6 cm3/GW2 (409). Reproduced with permisssion from ref 943. Copyright 2006 American Chemical Society.

compounds of Chart 78, the order in decreasing performance was 409 > 408 > 407.943 The set of multibranched chromophores 410−413 (Chart 79),944 composed of four analogues with generic skeletons of donor−π−acceptor (D−π−A) derived from functionalized fluorene/oxadiazole moieties, has been synthesized and the NLO properties determined with ns laser spectroscopy. The authors demonstrated the influence of the size of π-framework and the number of electron-donating units in the TPA processes of these compounds. Effective optical powerattenuation behaviors in the ns time domain of the studied chromophores were also demonstrated (Figure 59). It is evidenced that compound 413 (Chart 79) displayed the best performance when compared to the other congeners. Other authors used the strong two-photon absorbing chromophore 4,4′-bis(diphenylamino)stilbene (BDPAS, 414, Chart 80), to design new dendrimers (415 and 416, Chart 80),945 for NLO applications. Enhanced TPA cross section increased proportionally with the increasing number of identical chromophoric units constituting the parent BDPAS. For large dendrimers of G-1 and G-2 generations corresponding to 14 and 30 BDPAS units, respectively, the cooperativity in multiphoton response started to saturate, and increase of BDPAS units did not lead to the hoped improvement. Using three-photon cross sections of large dendrimers, the authors obtained an effective coherent domain size reaching larger values of multiphoton absorption cross sections. In analogous structures, the examination of the TPA cross sections (compounds 417−421,946 Chart 81) revealed very large values of σ2 in the order of 103−104 GM. In fact, the value σ2 = 1.1 × 104 GM of the second generation BDPAS-based dendrimer 421 (Chart 81) is one of the largest intrinsic TPA cross sections ever reported so far for a single molecule. Hyperbranched polyarenes (hb-PAs) 422−424 (Chart 82) were successfully synthesized in high yields by one-pot, singlestep, transition-metal-catalyzed homopolycyclotrimerizations of aromatic diynes and their copolycyclotrimerizations with a monoyne.947 The aromatic polymers were thermally and optically stable. Because of their conjugated electronic structure, the polyarenes emit deep-blue light efficiently. These compounds effectively attenuate also the power of intense 532 nm laser pulses (Figure 60). Figure 60 shows that

dendritic chromophore 409 has a three-photon absorption cross-section peak value of 30.0 × 10−25 cm6/GW2. This value was 10 times larger than that for 407 (2.9 × 10−25 cm6/GW2). The three-photon absorption cross section peak value for 408 was 13.0 × 10−25 cm6/GW2, that is, 4.5 times larger than that for 407. Considering the differences of the structural units in these chromophores, enhancement in the three-photon absorption cross-section value is found in going from the one-branched chromophore to the three-branched one and to the dendritic chromophore. This enhanced three-photon absorption for the dendritic and three-branched chromophores was reasonably attributed to their two-dimensional extensive πdelocalization, to increased intramolecular charge transfer and branches interaction. In this context, there was the manifestation of exhibiting cooperative enhancements between these different mechanisms in the achievement of strong threephoton absorption.943 As shown in Figure 58, all three chromophores 407−409 (Chart 78) showed OL behaviors. When the input intensity increased from 30 to 400 GW/cm2 (13.3 times increase), the transmitted intensity changed from 30 to 225 GW/cm2 (7.5 times increase) for 407, from 30 to 185 GW/cm2 (6.2 times increase) for 408, and from 30 to 175 GW/cm2 (5.8 times increase) for 409. In terms of OL efficiency within the series of 13114


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Chart 79. Multibranched Chromophores for OPL

424 presented the weakest OPL effect as it started to deviate from linearity only at incident fluence above ∼800 mJ/cm2 with a OL fluence threshold Fthr of 2300 mJ/cm2. On the other hand, 422 and 423 behaved similarly regarding its OL performance being Fthr = 1034 and 1050 mJ/cm2, respectively. This indicated that the OL performance of hb-PAs is very sensitive to the change in its molecular structure, offering the opportunity to tune its OL properties through molecular engineering.947 The hyperbrancehd polymer 425 (Chart 83) with a high molecular weight (MW up to 157 800 Da)948 presented high thermal stability with the degradation temperatures as high as 443 and 446 °C in nitrogen atmosphere and air, respectively. While weakly fluorescent in solution, their aggregates and solid powders emitted intense red radiation with fluorescence quantum yields up to 81%, demonstrating a new phenomenon of aggregation-induced or enhanced emission. Moreover, compound 425 exhibited NLO properties and OPL against

Figure 59. Optical power-attenuation curves at 800 nm of chromophores 410−413 (Chart 79) in 0.02 M solutions. Reproduced with permission from ref 944. Copyright 2012 Elsevier.

Chart 80. BDPAS-Based Dendrimers for NLO

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Chart 81. Dendrimers Based on BDPAS

Chart 82. Hyperbranched Polyarenes for OL

Chart 83. Hyperbranched Polymer for OPL

Figure 60. OL responses to 8 ns pulses at 532 nm of dichloromethane solutions of hyperbranched polyarylenes 422−424 (Chart 82). Repetition rate: 10 Hz. Reproduced with permission from ref 947. Copyright 2007 American Chemical Society.

harsh laser pulses. As shown in Figure 61, the THF solution of 425 attenuated the power of intense 532 nm optical pulsesc quite efficaciously with the transmitted fluence initially increasing with the incident fluence but starting to deviate from linearity when the incident fluence exceeded 60 J cm−2. The indenoquinoxaline-based dendritic chromophore 426 (Chart 84)949 was synthesized. It produced strong and 13116


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substrate was formed by an approximately continuous 50 Å thick gold film deposited on surface modified quartz. When peak intensities of the order of 0.1 GW/cm2 were reached, a transmission decrease of ∼8% was detected, while for intensities superior to 0.16 GW/cm2 ablative damage occurred. The dendrimer nanocomposites appeared to have much better optical stability to intense laser pulses and larger NLO responses as well. In fact, nanocomposites showed smooth nonlinear transmission curves and stable open aperture Z-scan curves up to 1 GW/cm2 of entrance intensity (Figure 63).950

Figure 61. OL response to 6 ns pulses at 532 nm of the THF solution of 425 (Chart 83). Mass concentration: 0.02 mg mL−1. Reproduced with permission from ref 948. Copyright 2012 The Royal Society of Chemistry.

Chart 84. Indenoquinoxaline-Based Dendritic Chromophore

Figure 63. Open-aperture Z-scan of the metal dendrimer composite based on 50 Å Au film. Reproduced with permission from ref 950. Copyright 1999 American Institute of Physics.

Some authors have also investigated gold dendrimer−metal nanostructures using ultrafast two-color pump−probe measurements at 800 nm.951 These measurements primarily tested the transient bleaching signals of the metal particles encapsulated in the PAMAM dendrimers (Scheme 3). Systems showed primarily a single rapidly decaying component that recovers to the equilibrium value in less than 5 ps. It was also shown that Scheme 3. Synthesis of Functionalized PAMAM Dendrimer (G2-LRh) and Its Au Complexes

spectrally dispersed TPA across the dynamic tuning range of a Ti:sapphire laser. The maximum TPA coefficient of the studied chromophore was found to reach 31 700 GM. In addition, this dendritic structure was also demonstrated to effectively regulate the optical power of fs pulses at 800 nm (Figure 62). Also, the OL of thin films containing dendrimer−metal nanocomposites have been measured at 532 nm.950 The

Figure 62. Output power versus input power of the ∼30 fs laser pulses at ∼800 nm transmitted by a 0.01 M toluene solution of 426 (Chart 84). Sample has an optical thickness of 1 cm. The solid curve is the theoretical fit with the best-fitting parameter of β = 5.7 cm GW−1. Reproduced with permission from ref 949. Copyright 2014 The Royal Society of Chemistry. 13117


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Chart 85. Dendrimeric Materials Centered on Bis((4-(phenylethynyl)phenyl)ethynyl)bis(tributylphosphine)platinum(II)

Chart 86. Fullerene-Based Dendrimers

incorporation into thin films (Scheme 3).952 The measurements of third-order nonlinear susceptibility χ(3) of these materials were carried out with the DFWM method at 532 nm. In solution, χ(3) values of G2-Au, G2-L-Rh, and G2-L-Rh-Au were found to be 4.5 × 10−12, 5.6 × 10−12, and 14 × 10−12 esu, respectively. The normalized χ(3) values of G2-L-Rh-Au are approximately 1 order of magnitude larger than those of G2-Au

the time constant for the transient bleaching signal increased with pump fluence.951 Many synthetic efforts have been spent to prepare some complex systems, and the preparation of expectedly rewarding DNCs was an important driving force in the quest for OL enhancing tryouts. Some authors synthesized a chromophore functionalized PAMAM dendrimer and performed also its 13118


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the TPA of poly(styrene maleic anhydride)-TiO2 nanocomposites (432, Chart 87) has been studied by Z-scan

and G2-L-Rh. To explain the cooperative NLO effects, the authors considered the local field enhancement mechanism. At the surface plasmon resonance excitation (532 nm), the gold nanoparticles create an enhanced electric field, which is then sensed by the chromophore.952 When analyzing the NLO response of thin films, the authors observed that the larger volume fraction of the metal nanoparticle in G2-Au-PVB film (4.85%) gave a bigger value of χ(3) (177 × 10−12 esu) when compared to the G2-Au-PMMA film (volume fraction: 3.47%, χ(3) = 116 × 10−12 esu). The third-order NLO responses of the nanocomposites in thin films were much larger than those in solutions. For instance, the χ(3) value of the G2-Au-PMMA film (116 × 10−12 esu) was approximately 25 times larger than the corresponding G2-Au in solution (4.5 × 10−12 esu), this fact being ascribed to the large volume fraction of the metal nanoparticles in thin film and the different dielectric constants.952 The effect of dendritic substituents on a NLO chromophore for OPL has been investigated by some authors.691,953 Bis((4(phenylethynyl)phenyl)ethynyl)bis(tributylphosphine)platinum(II) with dendritic end groups (427, Chart 85) was prepared, and its OPL properties were measured at three different wavelengths: 532, 580, and 630 nm. The results have shown that the OPL properties improve with increasing size of the dendritic substituents. It has also been shown that the addition of the dendrons increasead the OPL efficacy as compared to the non dendritic bis((4-(phenylethynyl)phenyl)ethynyl)bis(tributylphosphine) platinum(II).953 Similar dendron-coated 2,5-bis(phenylethynyl) thiophene chromophores (428 and 429, Chart 85)954 were also reported. The dendritic thiophenes exhibited OPL at the laser wavelength of 532 nm. However, the magnitude of OPL for these compounds did not improve with respect to non dendritic thiophene derivatives. Hirsch and Couris955 have reported the NLO response of some water-soluble C70 dendrimers (430 and 431, Chart 86), utilizing ns and ps pulse excitations at 532 nm. Their response was found to be about 7 times smaller than that of bare C70 under ns excitation, while no response was found under ps excitation. These negative results were attributed principally to aggregation phenomena.

Chart 87. Polymeric Materials Tested for NLO

technique.958 The authors attributed the surface state, dielectric confinement effect, and weight percentage/particle concentration of TiO2 in the nanocomposite as mainly responsible for the enhanced nonlinearity of composites. Moreover, the OL effect generated by TPA could be determined under lower CW laser light intensity (∼107 W/m2 limiting threshold). Water-soluble organic polymeric dyes were also found to possess third-order nonlinearity. Thin films of polymers 433 and 434 (Chart 87) showed nonlinear absorption in the IR with a TPA coefficient of 55 and 18 cm/GW, respectively.959,960 Functional polyurethanes 435a,b were synthesized, and their OL properties were evaluated.961,962 These were ascribed to the presence of long π-electron conjugated chromophoric pendants. Polymer 435b displayed better OL properties than 435a at the same value of linear transmittance, although 435a had a larger χ(3) (4.28 × 10−11 esu) than 435b (0.87 × 10−11 esu) from RSA mechanism. This fact was probably due to the presence of a weaker acceptor in 435a than in 435b. Another NLO azo chromophore was used to prepare azobenzene-functionalized poly(aryl ether) 436 (Chart 87).963 The system was tested with the Z-scan technique. The composite displayed high glass transition temperature, good thermal stability, and large third-order nonlinear susceptibility (1.15 × 10−11 esu). The nonlinear absorption and OL properties of chiral polymer 437 (Chart 87) were investigated by Z-scan technique

4.5. Polymeric Materials

Polymers emerged as an important class of third-order nonlinear optical materials back in the 1970s.956 Some studies on transparent nanohybrids of titania-poly(methyl methacrylate) (TiO2-PMMA) thin films possessing enhanced nanocrystallinity were performed with open and closed aperture Zscan techniques using 250 fs laser pulses at 800 nm.282 The highest TPA coefficient (β) and nonlinear refractive index (η2) were observed for the nanohybrid thin films with highest crystallinity of TiO2, as confirmed by open aperture Z-scan techniques using 250 fs laser pulses at 800 nm. It was found that β = 2260 cm GW−1 and η2 = 6.2 × 10−2 cm2 GW−1, respectively. Samoc et al. investigated a ladder polymer obtained by pyrolysis of polyacrylonitrile in the presence of oxygen. Undoped and FeCl3 doped polymers were found to be electrically conducting. Upon doping, the polymer spectra showed changes indicative of the presence of polarons. Subpicosecond DFWM measurements have been performed at 602 nm, yielding effective χ(3) values of the order of 10−11 esu for most samples and indicating the presence of longer-lived excitations contributing to the third-order nonlinearity.957 Also, 13119


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groups,965,968−972 where values as high as 10−5 esu could be obtained. The value of χ(3) for PDAs varied by 5 orders of magnitude depending upon the material, wavelength of analysis, and technique. Polyacetylene [PA, with generic formula (CH)x] itself was also a very interesting example, especially due to the presence of the two isomeric forms cis and trans.973 A comparison of χ(3) in the two isomers indicates that the third-order susceptibility of cis-(CH)x is more than 1 order of magnitude smaller than that of trans-(CH)x. The difference in χ(3) for the two isomers resulted from the fundamental change in polymer symmetry. Moreover, the effects of the modulation of the substitution pattern974−976 or increasing conjugation in the polymer backbone have been studied also in accurate detail for NLO applications.977−980 Later, some authors synthesized a group of functional PA copolymers bearing azobenzene pendant groups. The copolymers exhibited higher thermal stability than unsubstituted PA and large third-order nonlinear susceptibility, as high as 4.62 × 10−11 esu. The χ(3) value of the copolymers was 2 orders of magnitude larger than that of PA, and larger than that of poly(N-carbazoylacetylene) and poly(1-naphthylacetylene).981 The third-order NLO response of the poly(2,5-dialkoxy-pphenyleneethynylene)s (PPE)s 438a−e (Chart 88) was also

in te fs regime.964 RSA was observed in the polymer at 800 nm, and a nonlinear absorption coefficient of 5.97 cm/GW was obtained at the irradiance of 2.75 GW/cm2. In particular, 437 displayed a large ratio (∼251) of excited-state to ground-state absorption cross section and a remarkably low limiting threshold of about 1.60 GW/cm2 at the pump power of 67 mW with a damage threshold of 2.75 GW/cm2 (Figure 64).

Figure 64. OL response of the chiral polymer 437 (Chart 87). Reproduced with permission from ref 964. Copyright 2103 AIP Publishing LLC.

As was already pointed out, the origin of a large third-order nonlinearity, χ(3), resides in the presence of an extended πconjugation system. This structural factor can provide fs or ns response times. These fast NLO properties are sensitive to πconjugation length, and, therefore, a systematic study of the relationship between π-conjugation length of model compounds and their nonlinear optical response was important for a better understanding of the structure−property relationships. To our actual knowledge, polymeric materials were the first to be studied in terms of third-order NLO, to verify which is the primary condition for the existence of efficacious OPL. Polymeric materials in comparison to the monomeric counterparts might offer answers to the question of which factors actually enhance NLO properties, thus providing the suitable hints for structural design. For instance, Stauteret and co-workers965 found large thirdorder optical nonlinearity in polydiacetylene derivatives reaching χ(3) = 8.5 × 10−10 esu. This finding opened new opportunities in the study of organic π-conjugated polymers.966 Furthermore, the NLO properties of linear carbon chains, prepared by laser ablation in water,967 were studied by using ns pulsed excitation with Z-scan technique. A strong OL response was detected at laser fluences above 0.6 J/cm2, the polymer in question displaying a nonlinear absorption coefficient β = 2.1 × 10−10 cm/W, and Im[χ(3)] = 7.0 × 10−8 esu. Organic π-conjugated polymers have then attracted much attention in the field of nonlinear optics. Polydiacetylenes (PDAs), conjugated polymers prepared by polymerization of diacetylene monomers and possessing the generic structural formula, R−CC−CC−R′, where R and R′ refer to the substituent side groups, have been studied for NLO applications. In the polymeric system, the conjugated carbon backbone provides the π-conjugation effect, while the side groups provide structure control and facilitate materials processing. Architectural flexibility thus resulting from the modification of the chemical structures of substituted side groups permitted the attainment of PDAs in the form of single crystals, Langmuir−Blodgett (LB) monolayers, thin films, and highly concentrated solutions. The NLO properties of PDAs have been investigated by a number of research

Chart 88. Conjugated Polymers for NLO

investigated in solution and in the solid state.982 The nonresonant third-order NLO susceptibilities χ(3) were measured by third-harmonic generation experiments (THG), and χ(3) values of up to 5.2 × 10−12 esu in solution and up to 9.6 × 10−13 esu in the solid state were determined. In solution, the NLO susceptibilities were found to be proportional to the content of polymer backbone, which depends on the functionalization of the PPEs. The solid-state NLO susceptibilities were found to be related to the degree of long-range order in the samples, increasing with the increasing order. The same polymeric structure containing the alkyl moiety 2ethylhexyl group (438c)983 was tested for OPL. Optical stabilization based on TPA mechanism was obtained with a polymer solution excited at ∼810 nm with ∼7 ns laser pulses. Optical power stabilization was demonstrated at an average input intensity level of I0 ≈ 400 MW/cm2 with a Δ ≈ ±25% peak-power fluctuation of the laser pulse, displaying σ2 = 101 × 10−20 cm4/GW. Poly(thienyleneethynylene) derivatives 439−441 (Chart 88)984 were also tested for OPL. Polymer 439, measured at 532 nm with ns pulses, presented the best OPL performance, corroborated by the fact that the thiophene rings in 439 13120


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backbone imparted better coplanar character than the other two polymers. This pointed out that a better conjugation structure in 439 is a factor of enhancement of the NLO response. Figure 65 shows the OL behaviors of 439−441

Figure 65. Normalized transmittance of polymers 439−441 (Chart 88) and C60 solution at the same linear transmittance (T ≈ 73%) versus incident fluence. Reproduced with permission from ref 984. Copyright 2002 Elsevier.

solution as compared to that of C60 solution at the same linear transmittance of 73%. It can be seen from the curves that all compounds displayed OL behavior, PTE3 439 representing the best performer of this set of compounds due to the better electronic conjugation. The two linear hyperbranched conjugated polymers 442 and 443 (Chart 89)985 have been synthesized for OPL by

Figure 66. Top: Nonlinear optical transmission of fs pulses at 800 nm for polymers 442 and 443 (Chart 89). Bottom: Output fluence versus input fluence plot corresponding to the data presetend in the top figure. Solid lines are the theoretical fits obtained with TPA mechanism. Reproduced with permission from ref 985. Copyright 2011 John Wiley and Sons.

Chart 89. Linear and Hyperbranched Polymers for OPL Some authors reported the nonlinear absorption and OPL properties of polyfluorenes 444 (Chart 90) induced by TPA.986 Polymer NLO properties were analyzed in chloroform between 450 and 650 nm for ns pulses. The nonlinear absorption was attributed to a three-photon absorption process involving a first step of TPA followed by an ESA process. The corresponding three-photon absorption coefficient presented the highest value for the longest oligomer with n = 60 (α3 ≫ 104 cm3/GW2 at the concentration of 200 g/L). This led to the verification of OPL in the whole visible range (the maximum transmitted energy was lower than 10 μJ for an input energy of up to 350 μJ in a f/15 optical geometry). PAs containing azobenzene-type pendants have been studied.987,988 For instance, polymers 445a and 445b (Chart 90) were studied using 8 ns pulses at 532 nm and displayed similar values of χ(3) = 1.43 × 10−10 esu and of limiting threshold of 0.245 J/cm2.987 PAs 446a and 446b (Chart 90) presented clamped fluences of 0.223 and 0.252 J/cm2, respectively.988 Other azobenzene containing polyacetylenes (447−449, Chart 90)989 gave performances originating from RSA. χ(3) values were comprised between 4.7 × 10−12 esu (449b) and 22.8 × 0−12 esu (447). Polymer 447 was revealed to be the best optical limiter, with limiting thresholds of 0.32 J/ cm2. Tetraphenylethene-functionalized acetylenes were synthesized for OL. Their polymerizations were carried out using WCl6−Ph4Sn as catalyst at elevated temperatures in toluene under nitrogen with the formation of polymers 450a,b (Chart 91).990 Figure 67 shows the OL performance of 450a,b in chloroform at similar (quite low) linear transmittances T of ∼30%. The transmitted fluence of the polymers increased initially with increasing incident fluence and started to deviate

Sonogashira coupling reaction, in which the main chain consisted of bithiazole moieties as electron acceptors (A) and triphenylamino groups as donors (D). The TPA cross sections σ2 determined with the open aperture Z-scan technique using 140 fs pulse at 800 nm were 1014 and 552 GM per repeating unit for 442 and 443, respectively. From the characteristic curves in Figure 66, the incident fluences increased from ∼0.0010 to ∼0.044 J/cm2 (44 times) for both two polymers, whereas the transmitted output fluences only show 20-fold and 29-fold increases for 442 and 443, respectively. The fit of the OL behavior was realized considering the mechanism of TPA.985 13121


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Chart 90. Conjugated Polymers for OL Applications

Chart 91. Several Polymeric Materials Studied

TPA mechanism. The TPA cross section σ2 was not affected by incident intensity, remaining nearly constant across the intensity range 58.5−87.7 GW/cm2, with a value of 736 GM. This is larger than that of the monomer (σ2 = 479 GM). The hyperbranched polyphenylene derivative 452 (Chart 91)992 with silole moieties was synthesized and evaluated for its linear and NLO properties. This polymer was electronically conjugated, as suggested by its strong absorption in the visible spectral region (λmax = 520 nm). Polymer 452 was nonlinear optically active because it strongly attenuated the optical power of intense laser pulses with a low limiting threshold of 180 mJ/ cm2. This polymeric system was also highly emissive upon excitation at low temperatures (cooling-enhanced emission). Branched poly(p-phenylenevinylene) structures 453−456 (Chart 91)993 were also tested to determine their TPA properties, using ns pulses at 800 nm. For comparison, compounds 455 and 456 were equally included in this study. From the nonlinear transmission measurements, the TPA cross sections σ2 were found to be 11.9 × 10−20 cm4/GW for 453, 66.6 × 10−20 cm4/GW for 454, and 44.0 × 10−20 cm4/GW for 456. The nonlinear transmittance of a novel hyperbranched conjugated polymer (457, Chart 92)994 has been measured

Figure 67. OL responses to 35 ps pulses at 532 nm of the chloroform solutions of 450a and 450b (Chart 91). Mass concentration: 0.2 mg/ mL. Reproduced with permission from ref 990. Copyright 2015 American Chemical Society.

from linearity at a value of incident fluence of ∼1 J/cm2. When the incident fluence was further increased, the transmitted fluence reached a plateau and saturated to 0.4 J/cm2.990 A PA containing fluorene groups as pendant (451, Chart 91)991 for OPL was synthesized using a rhodium catalyst. The OL properties at 780 nm were demonstrated to be based on the 13122


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Chart 92. Conjugated Polymers for NLO Studies

using a ns optical parametric oscillator. Polymer 457 showed significant OL performance in the visible region from 490 to 610 nm, based on the combination of TPA and RSA. At longer wavelengths, the mechanism of OL was mainly attributed to TPA, whereas at the shorter wavelengths, the strong OL capabilities resulted from the synergic action of TPA and RSA. Some authors designed fluorene-based triazole functionalized polymers with different linkers (458−460, Chart 92).995 It was found that the rigid conjugated chain between triazole and chromophore effectively enhanced the NLO properties of the resultant polymers. Polymer 458 proved to be the best performing polymer possessing β = 1.7 × 10−10 m/W, η2 = 1.5 × 0−12 cm2/W, and χ(3) = 3.66 × 10−10 esu. On the other end, 460 had the lowest values of nonlinear absorption coefficients, nonlinear refraction, and third-order nonlinear susceptibility, displaying β = 6.7 × 10−11 m/W, η2 = −0.5 × 0−12 cm2/W, and χ(3) = 1.22 × 10−10 esu. NLO studies on hydroxylated pyridyl p-phenylene polymers (461, Chart 92)996 were carried out both in CH2Cl2 solution and in thin films configuration using the Z-scan technique with ns laser pulses comprised in the wavelength range 430−600 nm. The analysis revealed that the nonlinear absorption switched from RSA to saturable absorption at a wavelength of ∼540 nm. It was found that OL of 461 (Chart 92) in solution is superior to a toluene solution of C60 with the same linear transmittance at 532 nm. Figure 68 shows that the fluence-dependent transmission of 1.0 mg/mL polymer 461 in solution is a constant until the input fluence of ∼0.7 J/cm2 at 532 nm for 7 ns laser pulses is reached. When the input energy increased above ∼0.7 J/cm2, the measured transmittance deviated from linearity and decreased dramatically at ∼2 J/ cm2, indicating the occurrence of efficacious OL effect.996 Polythiophene derivatives are another type of conjugated polymer that has created much interest because of its good environmental stability as compared to PAs,997 exhibiting a χ(3) value of about 3 × 10 −11 esu at 1.06 μm. Poly(thienylenevinylene) [χ(3) = 0.32 × 10−10 esu at 1.85 μm],998 polythiophene embedded in PMMA [χ(3) = 0.86 × 10−12 esu at 532 nm],999 polythiophene poly(α-[5,5′-bithiophenediyl]benzylidene-block-α-[5,5′-bithiophenequinodimethanediyl]) [χ(3) = 2.7 × 10−07 esu],1000 and poly(3-methylthiophene-

Figure 68. Nonlinear transmittance of 1 mg/mL CH2Cl2 solution of 461 (Chart 92) at 532 nm with optical thickness of 1 mm (○). ●: Nonlinear transmittance of a toluene solution of C60 under the same experimental conditions of 461. The linear transmittance of the two solutions is ∼90%. Reproduced with permission from ref 996. Copyright 2013 American Chemical Society.

copoly methyl methacrylate) [χ(3) = 7 × 10−12 esu]1001 were also characterized. A new series of conjugated copolymers (462−464, Chart 93)1002 consisting of alternate 3,4-dialkoxythiophene and (1,3,4-oxadiazolyl)pyridine moieties has been reported, and its NLO properties studied by Z-scan technique at 532 nm with 7 ns laser pulses using a Nd:YAG laser in liquid solution. The real part of χ(3) was estimated to be −0.881 × 10−12, −0.901 × 10−12, and −1.030 × 10−12 esu for 462, 463, and 464, respectively, while the imaginary part of χ(3) for the copolymers 462, 463, and 464 was 0.192 × 10−12, 0.253 × 10−12, and 0.272 × 10−12 esu, respectively. Among the copolymers, the best OL behavior was observed with 464 (Chart 93), which exhibited the strongest nonlinear absorption. For incident energies less than 20 μJ/pulse, the output linearly increased with the input. OL was observed for energies above 20 μJ/pulse. Just differing on the copolymer chain, other copolymers (465 and 466,1003,1004 467,1005 468, and 469, Chart 93)1006 were also evaluated for their NLO properties at 532 nm using the single-beam Z-scan technique. All polymers showed strong OL due to effective three-photon absorption. The values of the three-photon absorption coefficients for the polymers 465 and 466 were found to be 9 × 10−24 and 17 × 10−24 m3 13123


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× 10−12 esu.1009 Polymers 477−481 (Chart 94), in turn, displayed electrochemical band gaps between 1.72 and 2.35 eV.1010 Their third-order NLO activities were studied by open aperture Z-scan technique with a Q-switched frequency doubled Nd:YAG laser producing 7 ns laser pulses at 532 nm. Polymers 477−481 (Chart 94) displayed self-defocusing nonlinearity, and their operating mechanism involved RSA reverse with TPA coefficients on the order of 10−9 cm/W. Among the five polymers, 480 was the one showing the best OL performance presenting the highest TPA coefficient of 2.1 × 10−9 cm/W.1010 For polymers 482−485 (Chart 94),1011 based on 3,4-ditetradecyloxythiophene, the relative Z-scan profiles demonstrated that OL and NLO performance were affected by the presence of π-electron conjugation bridges along the polymer chain. Thus, the polymers carrying highly electrondonating 3,4-dialkoxythiophene and diphenylamine (484) displayed larger nonlinear response with TPA coefficients of the order of 10−10 m/W.1011 Polymers 486−488 (Chart 95)1012 also possessed low band gaps of 1.81, 1.76, and 1.63 eV for 486, 487, and 488, respectively. The third-order NLO properties of the copolymers, evaluated by Z-scan method with ns pulses at 532 nm, were based upon a strong refractive effect with nonlinear refraction coefficients of −0.47 × 10−10, −0.95 × 10−10, and −0.64 × 10−10 esu, respectively. The effective third-order nonlinear susceptibilities of 0.82 × 10−11, 1.46 × 10−11, and 1.17 × 10−11 esu were found for 486, 487, and 488, respectively. As shown in Figure 69, the OL thresholds were found to be 0.41, 0.35, and 0.41 GW/cm2 for 486, 487, and 488, respectively.1012 Fluorene-based polymers 489−491 (Chart 95) have also been evaluated for their NLO properties.1013−1016 All polymers 489−491 showed high TPA in the spectral range from 490 to 1100 nm. The TPA cross section at the absorption peaks was as high as 3000 GM.1013,1014 Polymers 492 and 493 (Chart 95) also exhibited high TPA capability and chemical/physical stability upon the action of high-power laser pulses at ∼780 nm and ∼160 fs duration.1015 The authors carried out the nonlinear transmission measurement by varying the input intensity from ∼20 to ∼600 GW/cm2. The corresponding diminution of transmission was from ∼0.8 to ∼0.18 GW/cm2.1015 Polymer 494 (Chart 95) exhibited strong nonlinear absorption, refraction coefficient, and third-order nonlinear susceptibility of 3.19 × 10−8 cm W−1, −0.68 × 10−10 esu, and 1.25 × 10−11 esu, respectively.1016 The polymer also showed a good OL response at 532 nm displaying a limiting threshold of 0.47 GW cm−2.1016 Several carbazole-containing copolymers (495−498, Chart 96) were also tested for NLO applications utilizing DFWM at 1064 nm.1017−1019 The π-conjugated copolymers containing carbazole 495, 496, and 497 presented values of third-order NLO susceptibility of 8.6 × 10−10, 2.7 × 10−13, and 1.6 × 10−13 esu, respectively. The value of the three-photon absorption coefficient for 498 was 2.1 × 10−25 m3 W−2. As shown in Figure 70, polymer 498 showed good OPL behavior.1019 Poly(arylene ethynylenes) containing thiophene ring were synthesized, and the OL properties of these polymers were investigated with a Q-switched Nd:YAG laser producing 10 ns pulses at 532 nm.1020 The experiments showed that the order of the OL response of the polymers is poly(3-hexyl-2,5thienyleneethylene) (P3) > poly(2,5-dihexyloxy-1,4-benzyleneethylene-3-hexyl-2,5-thienyleneethylene) (P2) > poly(9-hexyl3,6-carbazoleethylene-3-hexyl-2,5-thienyleneethylene) (P1). A

Chart 93. Oxadiazole-Based Polymers

W−2,respectively.1003,1004 Copolymer 467 exhibited values for effective three-photon absorption coefficient of 6830 cm−1, third-order nonlinear susceptibility of 1.28 × 10−12 esu, and figure of merit of 1.874 × 10−12 esu.1005 Furthermore, the values of nonlinear absorption coefficients of the polymers 468 and 469 were in the order of 10−8 cm/W.1006 Dithienopyrrole-based polymers 470 (Chart 94)1007 have also shown strong nonlinear absorption. A 17 times suppression (figure-of-merit ∼35) was achieved in a waveguided device, arising from strong and broad spectral range ESA. Other polymers containing the 1,3,4-oxadiazole moiety and thiophene units with different side groups at its 3,4 positions were prepared (471−473, Chart 94).1008 The polymers were found to be thermally stable, and their electrochemical band gaps were determined to be 1.98, 2.14, and 2.18 eV, respectively. Their NLO properties were evaluated by Z-scan method using 532 nm, 7 ns laser pulses. Polymers 471−473 displayed good OL behavior due to effective three-photon absorption with absorption coefficients of 2.5 × 10−24, 1.6 × 10−24, and 1.0 × 10−24 m−3 W2 for 471, 472, and 473, respectively. Several donor−acceptor type copolymers containing 1,3,4oxadiazole and thiophene units were synthesized (474−476, Chart 94), and their NLO properties were investigated.1009−1011 For instance, 474−476 polymers possessed electrochemical band gaps of 2.03, 2.09, and 2.17 eV, respectively.1009 Their NLO properties tested at 532 nm using single beam Z-scan and DFWM techniques with ns laser pulses demonstrated strong OL behavior due to three-photon absorption, which originated values of the effective threephoton absorption coefficients of 71 m−1 for all compounds, third-order nonlinear susceptibilities χ(3) of ∼1.5 × 10−12 esu for all compounds, and figures of merit (F) between 2.0 and 2.2 13124


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Chart 94. Thiophene-Containing Polymers

quality, and the structural variations possible by employing various combinations of ionic polymers and appropriate counterions. The χ(3) values of PMMA grafted by 4dialkylamino-nitrobenzene, 4-dialkylamino-4′-nitrostilbene, 4dialkylamino-4′-nitrodiphenylbutadiene, and 4-dialkylamino-4nitrodiphenylhexatriene were in the range from 0.54 × 10−12 to 7.42 × 10−12 esu at 1.904 μm.1030,1031 The χ(3) values at 1.904 and 1.579 μm have changed by a factor of 15 as the conjugation length increased. For instance, poly(disilanylene-2,2′-bipyridine-5,5′-diyl) ruthenium complex 499 (Chart 97),1032,1033 representing a σ−π photoconductive copolymer, was also studied for nonlinear optics in solution at several concentration. The system produced OL effect at the wavelength of 532 nm. Figure 71 presents the OL performance of 499 at mass concentrations of 0.35, 0.70, and 1.10 g/L. The output laser was clamped at fluence levels of 0.20, 0.10, and 0.04 J/cm2, respectively. Such values varied linearly with solution concentration. Above laser fluences of about 0.5 J/cm2, the samples of 499 (Chart 97) at the concentrations of 0.35 and 0.70 g/L started to behave nonlinearly, while this occurred above 0.2 J/cm2 for the sample with 1.10 g/L concentration. This clearly proved that the OL threshold was concentration dependent.

conjugation length-based structure−properties relationship was found. Moreover, the introduction of the electron-rich aromatic ring into the polymer backbone as donor enhanced the electronic polarizability with consequent improvement of its OL properties.1020 Polymers containing nitrogen heteroatoms such as polyquinoxaline,1021 polyaniline,1022,1023 polypyrrole,1024 polyquinoline,1025 polyazine,1026 and polyphosphazene1027 have also been studied with polyquinoxalines displaying the highest χ(3) values. Dye-grafted polymers are also an object of interst for NLO applications. In particular, two main classes of NLOchromophore functionalized systems have been investigated: side-chain polymers and main-chain polymers. Matsumoto et al. studied symmetrical cyanines having quinoline rings.1028 These polymers were doped at the molar level of 50% by the dye, and transparent films of 0.2−0.3 μm could be casted from the solution by spin coating. The studies on the NLO properties’ dependence on wavelength showed that a polymer film containing 50% cyanine dye has a maximum value of χ(3) 2 × 10−11 esu at around 1.9 μm. Meyer et al.1029 demonstrated that the glasses made from a polyelectrolyte (PEL) could be utilized for NLO applications by adjusting the chemical structures of ionic polymers. The THG efficiency of PEL glasses increased as the dye content increased and reached a maximum with 62% in weight of the dye content. The use of PEL glasses as NLO media has been seriously considered due to the good solution processability, incorporation of high dye concentration, optical 13125


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Chart 95. Miscellaneous Polymers for OPL

Figure 69. Optical limiting curves of compounds 486, 487, and 488 (Chart 95). Reproduced with permission from ref 1012. Copyright 2015 Elsevier.

Figure 70. Normalized nonlinear transmittance versus input fluence for polymer 498 (Chart 96). Inset shows the Z-scan curve. Hollow circles show the experimental data, and the solid line gives the best fit with a three-photon absorption model. Reproduced with permission from ref 1019. Copyright 2010 Springer Science and Business Media.

Chart 96. Carbazole-Containing Polymers

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C60-solutions in CCl4,1056 decalin, 1056 CHCl2,1035 toluene,1055,1057 CS2,1057 or as suspensions in water.1058 The influence of electron-donating solvents, like diethylamine and dimethylamine, was also examined by Sun and Riggs,1055 who proved that these types of solvents with strong electronic effects produce a weaker OL response than toluene, due to the photoexcitation of C60 that in turn photogenerates transient species like C60 anion and diethylamine cation.1055

Chart 97. Polypyridyl−Ruthenium Complex

Table 10. χ(3) Values at Different Wavelengths for Various Fullerene Derivatives

Figure 71. Output versus input fluence of polymer 499 (Chart 97) for 7 ns pulses at 532 nm at three different concentrations. Sample optical thickness: 10 mm. Reproduced with permission from ref 1033. Copyright 1999 Elsevier.

5. CARBON-BASED MATERIALS 5.1. Fullerenes

Fullerenes are carbon clusters with defined closed structure. The determination of the OL effect produced by fullerene materials represents one of the most significant developments in the search for new optical limiters. Since the first reports about NLO properties of fullerenes,1034 it was clear that C60 in toluene solution was an excellent optical limiter toward ns pulses at the wavelength of 532 nm from Nd:YAG laser.1035 It was shown that C60 displayed better OL performance than most of the known OL materials at the time.1035 Since the discovery, the photonic properties of C60 and other members of the fullerene family, like C70, C76, C78, and C84, have been a subject of extensive investigations until recent years. This led to a considerable amount of publications on its OL properties, which were well covered by several reviews.1036−1045 Optical transmission of fullerene solutions decreases by increasing the light intensity. For short pulses (in the ps order), the limiting action is ascribed exclusively to RSA, whereas for longer pulses (in the ns−ms range), additional mechanisms of mainly thermal origin have been invoked.114,486,1046−1052 In this framework, various mechanisms such as simultaneous twophoton from ground and excited states also may play a role.1053 The OPL of fullerenes is associated with RSA.1044,1045 This could be evaluated even with photoacoustic calorimetry.1054 It is no surprise that the performance of C60 solutions, at room temperature, is strongly dependent on the concentration.1055 It is less obvious that OL of fullerenes did not depend much on the type of solvent. This was realized by several research groups that compared the OL results given by

compd

λ/nm

χ(3)/10−12 esu

method/type of material

ref

C60 C60 C60 C60 C60 C60 C60 C60 C60 C70 C70 C70 C70 C70 C70 C70 C70 C70 C70 C70 C76 C78 C84 C86 C90 C94 C96

532 597 633 675 816 1022 1064 1064 1064 532 597 633 675 816 1022 1064 1064 1064 1064 1064 532 532 532 532 532 532 532

0.1 380 200 82 13.2 71.5 7 72 82 0.43 2100 300 64 11.3 29.3 12 5.6 1400 220 0.08 0.28 0.55 0.39 0.49 0.72 0.6 0.67

DFWM/solution DFWM/thin film DFWM/thin film DFWM/thin film THG/thin film THG/thin film DFWM/thin film THG/solution THG/thin film DFWM/solution DFWM/thin film DFWM/thin film DFWM/thin film THG/thin film THG/thin film DFWM/thin film DFWM/solution THG/solution Z-scan/film Z-scan/solution DFWM/solution DFWM/solution DFWM/solution DFWM/solution DFWM/solution DFWM/solution DFWM/solution

1060 1061 1062 1061 1063 1063 1064 1065 1063 1060 1061 1062 1061 1063 1063 1066 1067 1065 1068 1068 1060 1060 1060 1060 1060 1060 1060

It was also reported that the OL threshold for ps laser pulses was higher than that of ns pulses.1059 In a broader sense, RSA from a C60 solution excited with ps pulses is owed to absorption from the first singlet excited state because population in the first triplet excited state is insignificant at this time scale. In the ns regime, OPL effect fins a contribution from the absorption of the triplet excited state (see Table 10). Higher molecular weight fullerenes like C70 have also been studied in detail for OPL. When compared to C60, C70 presents a lower triplet state quantum yield and stronger visible absorption. The triplet state lifetime of C60 is 330 ± 25 ns and increases to 730 ± 50 ns for C70.1069 The possible presence of 3 O2 in solution certainly influences the RSA processes (and therefore the OL behaviors) of C60 and C70 in a different way. Such a fact was proved by Sun et al., who reported that non degassed C60/C70 mixture had a higher threshold than degassed solutions.1070 In general, the higher ground-state absorption cross section for C70 confers a smaller ratio of excited-state to ground-state cross sections, and then a higher threshold for OL in C70 is observed with respect to C60.1035,1071 OL was also analyzed in 13127


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Chart 98. Fullerene Derivatives for NLO

Scheme 4. Synthesis of C60−Ag Nanocomposite

even higher fullerenes (C76 and C84) at λ = 1064 nm for pulses of 10 ns.1056 The saturation threshold of C76 and C84 was higher than that of C60 in the visible region, due to the low quantum yield into the triplet state for these higher fullerenes. Table 10 shows the third-order susceptibility χ(3) values for several fullerene derivatives, which have been measured with several methodologies and under different conditions. Looking at the table as a whole, one can immediately deduce that simple changes in measurement conditions can produce dramatic changes of the χ(3) values. The UV−vis absorption spectrum of C60 is characterized by strong absorption in the UV region and weak absorption in most of the visible region. The OL properties of C60 have stimulated the development of a huge number of derivatives.1072 A documented advantage in the use of fullerenes is, in fact, the possibility of preparing derivatives with adapted properties such as a high solubility in the solvent for sol−gel chemistry, while retaining the NLO properties of the same molecule in solution.1073,1074 Some authors have developed operational models for the definition of C60 derivative-hybrid materials to be employed as photoactive materials in optical limiter.1075−1077,1101,1105,1110 Some authors have tested the OL properties of C60 dimeric structures, along with monomeric of methano-, pyrrolidino-, and amino-C60 derivatives 500−503 (Chart 98).1078 Concerning photophysics, the C60 dimer 500 is basically a pair of linked C60 derivatives with each of them having a [6,6]-closed functionalization pattern (1−2 addition). The main similarities between the C60 dimer 500 and other C60 derivatives 501−503 reside in the absorption and excited-

singlet-state properties rather than in the excited-triplet-state properties. As a result, because the nonlinear absorptive OL responses in the fullerene molecules are due predominantly to the excited-triplet-state absorptions in the ns regime, the C60 dimer 500 (Chart 98) is a weaker optical limiter than C60 and the other C60 derivatives at 532 nm. No substantial intramolecular interactions in both ground and excited states were found, which meant that the intramolecular selfquenching of the excited singlet state was absent, and selfquenching of the excited triplet state was very weak due to the possible formation of an excimer or a charge-transfer state. NLO absorption in solid films of poly(3-octyl thiophene) (P3OT) sensitized with methanofullerene (504, Chart 98)1079 was investigated in the wavelength range 620−960 nm. The large nonlinearity resulted from efficient photoinduced intermolecular charge transfer from P3OT to methanofullerene 504, followed by absorption in the charge separated excited state, with transmission clamping at an average fluence of approximately 0.1 J/cm2. Metallo-fullerene complexes can also be an attractive choice for optical limiters due to their charge transfer nature. Several authors prepared and tested several materials like (η2C70)Mo(CO)2(o-phen) (DBM) (DBM = dibutyl maleate, ophen = 1,10-phenanthroline)1071 or its parent (η2-C60)Mo(CO)2(o-phen) (DBM)1080 and observed enhanced OL effect with respect to sole C60, which was attributed to an increment on the triplet-state absorption due to intramolecular charge transfer. The anchoring of electron-withdrawing groups to C60, such as cyano groups, was also tested. When compared to 13128


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unmodified C60, the OL performance of the first was not improving because of its lower quantum yield, although the excited triplet state absorption was stronger than that of C60.1081 Nanocomposites still represent a quite unexplored area. For instance, C60−Ag nanocomposite 508 and its related composites 505−507 (Scheme 4) were prepared by Sun et al. via the in situ reduction of silver ions encapsulated in a

the sol−gel process offers the possibility of preparing high optical quality bulk systems with long interaction path length. Because of the insolubility of fullerenes in solvents used for sol−gel processing, the past approach was based on entrapping a solution of fullerene in sol−gel-derived films,1084 xerogels,1085 and sonogels.1086 McBranch et al.1087 demonstrated that NLO behavior of fullerene-doped sonogel is similar to that of a liquid solution and not to the bulk, solid forms of fullerenes,1088 with quenching of the optical nonlinearity at high fluences. C60 retains its OL properties after inclusion in solid matrixes such as sol−gel glasses, 1085,1089,1090 poly(methyl methacrylate) (PMMA),1091−1094 and glass−polymer composite samples,1095 among other polymeric matrixes.1096−1098 The effect of the nature of the host matrix was shown by some authors,1094 who incorporated fullerene into PMMA polymer matrix and studied the OL properties of the immobilized fullerene. They found that the OL properties of the resulting composite material were compromised by the degradation/decomposition of the polymer matrix itself. Special procedures have to be employed because normally good solvents for fullerenes are incompatible with the sol−gel process.1046,1099−1102 Incorporation of fullerenes is typically achieved by saturating mesoporous silica glasses with a solution of C60.1085,1089 In a different context, some authors1103,1104 have prepared and incorporated water-soluble C60 derivatives, compatible with the sol−gel process, directly into the solution. Some groups have shown that C60 and C70 derivatives can be incorporated in thin silica matrixes via sol−gel processing.105,106,1105,1106 This methodology allowed the deposition of transparent, optical-quality films due to the higher solubility of the derivatives, allowing one to increase their concentration in silica matrixes. Unlike polymeric substrates, sol−gel materials offer the advantage of good resistance against optical damage. Even better results could be achieved by using functionalized compounds, which chemically link to the inorganic network. High transmittance at low fluences associated with good OL behavior and high damage threshold make these materials promising for possible future OL device development.1101,1107,1108 More recently,1109,1110 it was found that the OPL effect of 1,2-dihydro-1,2 methanofullerene[60]-61-carboxylic acid (509, Chart 99) incorporated in polystyrene derivatives improved with respect to its parent C60. The limiting threshold diminished about 35%. This was ascribed mainly to the contribution of the high concentration of polymer in the composite, originating a stronger absorption of excited triplet state. Prasad and co-workers1111 reported the OL properties of composites formed by a combination of diphenylaminofluorene and C60 with fluorene moieties differing for the length of alkyl chains (510 and 511, Chart 99). These systems offered strong OL effects due to a complex mechanism that significantly enhanced multiphoton absorption. The authors claimed that they found the first examples of C60-containing compounds, 510 and 511, showing high three-photon absorption activities. Compound 511 displayed two-photon absorption and threephoton absorption cross-section values of 82.4 GW and 6.30 × 10−25 cm6/GW2, respectively, with fs pulses. Later, other authors1112−1114 reported the synthesis and applications of pentads 512 and triads 513 (Chart 99). Both compounds exhibited NLO transmittance reduction responses under fs irradiation with a lower transmittance value for pentads 512 in the high laser power region above 80 GW cm−2.

Figure 72. Optical limiting responses of 507 (⬡), 508 (▽), 506 (○), and C60 (□) solutions. Reproduced with permission from ref 1082. Copyright 2001 Elsevier.

monofunctionalized methano-C60 derivative.1082 It has been demonstrated that the OL behavior of the entities alone was inferior to the performance of the nanocomposite at 532 nm using ps and ns pulses. The Ag-nanocomposite 508 was revealed to be quite better than 507 (Figure 72). In any case, a big part of the enhanced OL performance of this system arises from nonlinear scattering. The wavelength-dependent OL and nonlinearity of a fullerene−tungsten complex,1083 C60[W(CO)3diphos] [diphos = 1,2-bis(diphenylphosphino)ethane], were studied in toluene solutions and sol gel films in the range 532−700 nm. This complex gave rise to an important improvement of the OL performance when compared to C60. While a significant increase in χ(3) values was observed for C60[W(CO)3diphos] from 5.8 × 10−12 to 19.0 × 10−12 esu upon increase of the incident laser wavelength from 532 to 700 nm, a corresponding decrease of χ(3) was detected for C60 in the same spectral region ranging from 4.0 × 10−12 to 2.6 × 10−12 esu. Even if these solutions are efficient optical limiters, the use of solid devices is preferred for practical applications due to their superior ease of handling and also due to generally limited solubility of fullerenes. Furthermore, devices such as optical power limiters need a high optical quality in the bulk form. Crystalline films of C60 have been studied,1046 but resulted in inefficient against pulses longer than 100 ps. This was credited to a fast decay of the excited state due to interactions of neighboring C60 molecules in the solid phase. Differently, 13129


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Chart 99. C60 Derivatives for NLO

Chart 100. Fluorene and Carbazole-Containing Fullerene Derivative

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better. Saturation of induced absorption is important in sol−gel glasses and had also to be considered when using the fullerenodendrimers in porous sol−gel glasses for OL.1117 Also, the combination of C60-oligopyridines (C60TPY = 518, Chart 101) with lead nanowires (C60TPY−Pb nano-

This was attributed to the larger TPA cross-section value of 512 than of 513 at the same concentration, and such a variation was correlated to a higher number of fluorene-containing subunits. Two hybrid C60−(antenna) compounds (triad 514 and tetrad 515, Chart 100)1115 with carbazole-based substituents were synthesized and characterized for OPL. Besides enhanced TPA at 780 nm with respect to C60, the authors also demonstrated ultrafast photoresponses at 980 nm revealing TPA cross-section (σ2) values of 995−1100 GM for the hybrid tetrad 515. Other C70 and C60 derivatives (516a−c and 517a−c, Chart 100) were designed for TPA measurements,1116 using fs openaperture Z-scan and frequency-degenerate pump−probe measurements at 780 nm. TPA cross sections of 3.47 ×

Figure 74. Normalized transmission curves versus input fluence for metal nanowires coordinating ligand 518 (Chart 101) in chloroform, ethanol, and DMF, and C60 in toluene. Reproduced with permission from ref 1118. Copyright 2007 Springer Science and Business Media.

wires)1118 or Ag nanowires (C60TPY−Ag nanowires)1119 was tested for OL purposes. A strong solvent dependency was found when chloroform, ethanol, and DMF were used. Because chloroform possesses lower boiling points, both types of C60TPY−M (M = Pb, Ag) nanowires offered stronger OL performances in this solvent, ascribed mainly to absorptioninduced nonlinear scattering (Figure 74). Additionally, other thermodynamic properties such as thermal conductivity and heat capacity, which can cause quick local overheating of the interface solvent/layer, played an important role in determining the OL effect of C60TPY−Pb nanowires. Fullero-pyrrolidine derivatives have been synthesized with silicon alkoxide terminal groups for possible direct binding to the matrix network.1102,1105,1120 Comparison of the OL performances of a solution containing the fullerene derivative and the corresponding solid-state optical limiter was carried out. The OL performances of fulleropyrrolidine in toluene and incorporated in a hybrid material of similar linear transmittance did not diverge noticeably. The authors demonstrated that a very high loading of molecularly dispersed fullerenes (up to 10−2 M) can be achieved through this route. They also indicated that the origins of nonlinear behavior are similar in solution and in the solid state, with RSA with nonlinear scattering and nonlinear refraction being the main contribution at high fluence. The main advantage of using hybrid materials for OL is the higher laser damage threshold, as compared to organic polymers. Values of damage threshold as high as 15−30 J cm −2 have been reported for fulleropyrrolidines in glycidoxypropyltrimethoxysilane−ZrO2 matrixes. These values are significantly larger than those reported for a lower concentration of fullerenes in poly(methyl methacrylate).1102,1105,1120 For instance, the third-order nonlinear optical response of the water-soluble inclusion complex1121 consisting of C60 incorporated in γ-cyclodextrin was investigated employing the Z-scan technique (λexc: 532 nm, 10 ns pulses). Significant OL action was observed, the effect being slightly weaker than that of pure C60 in toluene. By aging the C60-γ-cyclodextrin-water solution, aggregates that improved OL action by a factor of

Figure 73. Open-aperture Z-scans of 1 mm-thick solutions of 10−3 M 516a, 517a, 516b, 517b, 516c, and 517c (Chart 100). Reproduced with permission from ref 1116. Copyright 2009 American Chemical Society.

10−46, 1.64 × 10−46, 1.1 × 10−46, and 0.78 × 10−46 cm4·s/ (photon-molecule) were determined for 516a, 517a, 516c, and 516b, respectively, when toluene was the solvent and the concentrations of 10−4 M were the same for all of these systems. The Z-scan profiles of Figure 73 show that a drop exists in the light transmission of compounds 516−517 with the extent of the drop decreasing in the order of 516a > 517a > 516b > 516c in the concentration range of ∼10−4 M. This trend was also in agreement with the adjusting of the concentrations, where all of the solutions showed a linear transmission of ∼80% recorded at input laser fluences less than 4.5 mJ/cm2.1116 Synthesis and testing of fullereno-dendrimers incorporated in porous SiO2 sol−gel matrixes was also performed. The authors found that the systems with lower dendritic branches were more suitable for OL purposes, where the combination of the absorption cross sections and the quantum yield of ISC are Chart 101. C60-Oligo Pyridine Derivative

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5.2. Carbon Nanotubes

Chart 102. C60 Nanocomposites

Additionally to common fullerenes, the possibility of synthesizing tubular fullerenes or concentric fullerenes gave way to a new field of research: carbon tubules, or more conveniently carbon nanotubes (Figure 76).1124 Typically, there are two main types of carbon nanotubes: single-walled carbon nanotubes (SWNT) that are made from one graphene sheet rolled as a tube, and multiwalled carbon nanotubes (MWNT) that are obtained when several stacked graphitic shells are built to give concentric cylinders or graphene sheets are rolled. The OL properties of carbon nanotubes and graphene have been the target of investigation, and results may be compared to those obtained for C60 and carbon black suspensions (CBS). Goh and co-workers have been the first to report about OL on MWNTs suspended in water1125 and ethanol,1126 or embedded in PMMA films.1125 The analysis of their OL properties covered the spectrum ranging from the visible to IR utilizing 7 ns laser pulses. In ethanol, the limiting threshold was 1 J/cm2 at 532 nm and 6 J/cm2 at 1064 nm. These values are lower than what is obtained for CBS or C60 at 532 nm, but it is lower only with respect to CBS at 1064 nm. In water, a value of 13 J/cm2 at 1064 nm and 1 J/cm2 at 532 nm is the view card of these systems. Also, for MWNT in PMMA, the values are similar, which gives the sureness of being in the presence of a broadband optical limiter working from 532 to 1064 nm. Carbon nanotubes do not show ground-state absorption at 532 and 1064 nm, so the limiting response has to be credited to a mechanism other than RSA. Further experiments with ns pulses at 532 nm confirmed that nonlinear scattering is present in MWNT suspensions. Because the OL properties of MWNTs are similar to those observed with CBS, one can conclude that a similarity exists also in the limiting mechanism.585 However, other phenomena such as TPA have been recently acknowledged.1127 Other authors reported the influence of nonlinear scattering on the OL properties of carbon nanotubes in dependence of their size.1128 It was shown that the OL behavior of carbon nanotubes with shorter tube length and smaller bundle size was much better than for those with a longer length and larger bundle size when the nanotubes were dispersed. Other authors demonstrated that MWNTs in chloroform exhibit better OL performance than in water,1129 and stated that this was due to the existence of a somewhat larger absorption cross section of the studied particles in chloroform than in water. Upon irradiation of chloroform solutions with longer pulses, it was observed faster solvent-bubble growth with consequent activation of strong nonlinear scattering. This was due to the low heat of chloroform vaporization, surface tension, and viscosity.1130,1131 The influence of the solvent properties on the OL of carbon nanotube dispersions was analyzed later.1132,1133 It was concluded that the surface tension of the solvent plays a more important role than its viscosity or boiling point. Size dependency and solubilization properties of MWNTs on the corresponding OL properties were also studied.1134−1137 These results led to the conclusion that long nanotubes show stronger OL properties. The mechanism of nonlinear scattering induces MWNT suspensions to have higher performances than the respective solutions. Also, nanotube diameter is another important factor: the larger is the diameter, the better are the limiting performances.1138,1139 Large rolls or large MWNTs provided that stable suspensions can be prepared and that the sizes are kept small enough to prevent scattering at low fluencies, proved to be efficient optical limiters.1138,1140,1141

Figure 75. Nanocomposite 519 observed by HR-TEM. Reproduced with permission from ref 1122 Copyright 2005 Elsevier.

almost 2 could be obtained. Some authors synthesized 519 (Chart 102, Figure 75)1122 by a laser ablation methodology in a one-step process and analyzed its NLO properties using ns pulses at 532 nm. The composite was demonstrated to be an efficient nonlinear absorber. The mechanism of nonlinear absorption was shown to occur through a light-harvesting step by external metal nanoparticles. This was followed by efficient energy transfer to the fullerene moieties, which absorbed successively in the NLO regime through their triplet states. The C60-incorporated polymers were also used in NLO applications. Polymer 520 (Chart 102) was synthesized by direct fullerenation between a commercial sample of polysulfone and fullerene, using electrophilic aromatic substitution in the presence of AlCl3 as catalyst.1123 C60 was randomly distributed along the polymer, in amounts varying from 0.5 to 3.2 mol %. 520 showed a very high thermal stability with glass transition temperature depending on the C60 content, and low nonlinear thresholds (∼50 mJ/cm2).

Figure 76. Carbon nanotube. 13132


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Composite nanofibers of PMMA with embedded MWNTs were prepared by electrospinning,1142 in such a way that the carbon nanotubes were aligned along the fiber axis and semitransparent films (∼50% transmittance) could be then fabricated using an optically compatible polymeric resin. The films exhibited high limiting capacities, with a limiting threshold of 1.5 J/cm2 at about 50% of linear transmittance at the wavelength to be limited. The OL behavior of carbon nanofibers in polydimethylsiloxane was investigated1143 and compared to that of carbon nanofibers and polyhedral multishell fullerene-like nanostrucChart 103. Carbon-Based Materials for NLO

Figure 77. (a) Open aperture Z-scans and (b) OL responses of C60, 521, 522, 523, and MWNTs/surfactant at 532 nm. Reproduced with permission from ref 1144. Copyright 2008 Elsevier.

nonlinear scattering as acting phenomenon. Also, with SWNTs the solvent effect is quite pronounced, as cleared out by Mishra et al.936 who studied the behavior of SWNT suspensions in water, ethanol, and ethylene glycol. They verified that their OL performance comes mainly from scattering of the suspension, but also that OL in ethanol suspension was the highest among the three tested solvents. The effect of doping of MWNTs with some atoms, for example, Au1151 or B,1152 on OL properties has been analyzed. The metals Au and Ag were deposited on the MWNTs and

tures in water. The absence of degradation of OL characteristics in the carbon nanofibers in polydimethylsiloxane suspension was demonstrated, in contrast to the aqueous suspensions for which considerable degradation was observed. The OL performances1144 of MWNT-ols (521), MWNTsCOOH (522), C60-OH (523) (Chart 103), and MWNTs/ surfactant in water suspensions, along with C60 in toluene, were investigated through both open-aperture and closedaperture Z-scan. Compound 521 showed the best OPL performance, while its related 523 presented unsatisfactory OL due to the disturbance of the conjugated structure of C60. The OL behaviors of carbon nanotube materials were due to nonlinear scattering. From the Z-scan measurements (Figure 77), the limiting thresholds of C60, 521, 522, 523, and MWNTs/surfactant were 235, 88, 150, 253, and 123 mJ/cm2, respectively. NLO transmittance of SWNTs was first reported by Vivien et al.1145,1146 Other studies followed,1129,1130,1147−1150 using either water or chloroform suspensions of SWNTs for wavelengths from 430 to 1064 and several pulse durations (3−100 ns). For instance, at 532 nm (7 ns), in water, the authors found that the limiting threshold of SWNTs was higher than that for CBS, but slightly lower than CBS at 1064 nm under the same conditions. Similar to MWNTs, the analysis of OL properties of SWNTs with variable pulse duration showed

Figure 78. Nonlinear transmittance of MWNT, MWNT-Au05, and MWNT-Ag05 at (a) 532 nm and (b) 1064 nm. Reproduced with permission from ref 1151. Copyright 2005 Elsevier. 13133


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decorated with Cu2S and Cu3SnS4 NPs. When the Cu/Sn ratio was reduced to 1:1, then Cu3SnS4, Cu2S, and SnO2 NPs could

suspended in water. The studies were performed at 532 and 1064 nm with ns laser pulses. It was shown that they possess stronger OL performance as compared to pure carbon nanotubes at 532 nm, but there was no evident OL enhancement at 1064 nm (Figure 78). Improvement was clear, and the authors proposed surface plasma absorption of Au and Ag as phenomena responsible for the enhancement of the OL effect in conjugation with nonlinear scattering.1151 Fluence-dependent transmission and Z-scan measurements with 8 ns pulses showed that the boron-doped MWNT suspensions exhibited a stronger optical nonlinearity with fluence than pure carbon nanotube suspensions in both visible and NIR ranges. The nonlinear response for both materials was dependent on aggregation, and the suspensions displayed a limiting threshold of 8 J/cm2.1152 In the case of metal oxide hybrids of MWNTs dispersions (iron and zirconium oxides),1153 the effective mechanism of nonlinear absorption was three-photon absorption arising due to sequential transitions between the real excited states. Under these circumstances, the effect of nonlinear scattering was minimal. Open aperture Z-scan measurements have been carried out on water dispersed functionalized MWNTs and MWNTs, which have been decorated with iron and zirconium oxide nanoparticles. Limiting thresholds in the range of 0.37− 0.46 J/cm2 were obtained. Similar findings were obtained for TiO2 decorated MWNTs prepared through the sol−gel method.1154 The MWNTs/TiO2 nanocomposite suspensions were investigated with nanosecond pulses at 532 nm with the Z-scan method. The results showed that the MWNTs/TiO2 nanocomposite had a performance superior to that of the MWNTs as suspensions, where the TiO2 nanoparticles induced a limiting threshold reduction and the increasing of nonlinear absorption. When MWNTs and their decorated platinum and palladium nanocomposites were tested,1155 their NLO properties on aqueous dispersions were studied with a high power Nd:YAG laser with 532 nm excitation (10 ns pulses). The metal-MWNTs nanocomposites produced significant nonlinear absorption and scattering with Pt-MWNT and Pd-MWNT nanocomposites exhibiting limiting thresholds of 2 and 7 J/cm2, respectively. The use of Au and Ag particles to decorate MWNTs was also considered.1156,1157 The OL properties of their water suspensions were evaluated at 1064 or 532 nm using a Q-switched Nd:YAG (3 ns pulses). These experiments demonstrated the positive effect of the metal decoration of MWNTs on the respective OL behaviors.1156 On the other hand,1157 the OL properties at 532 nm of Au NPs coated carbon nanotubes and further embedded in silica gel-glass were slightly weaker than those in solution. This reduction was attributed to the synergistic NLO effects of RSA arising from the carbon nanotubes and saturable absorption from the Au NPs embedded in the silica gel-glass. Additionally, MWNTs were covalently functionalized with fourth-generation PAMAM dendrimers possessing a trimesyl core and used as the template for in situ growth of Ag and Cu2O nanoparticles on MWNTs (mean size: 7−8 nm).1158 The OPL measurements of the nanocomposites was carried out with the open-aperture Z-scan technique. Results indicated broadband OPL performance, with enhancement of OL effect in passing from bare MWNTs to their decorated version in water due to the presence of Ag and Cu2O NPs. MWNTs decorated with semiconductor NPs have also been studied with open-aperture Z-scan measurements.1159 With Cu/Sn ratios of 3:1 and 2:1, MWNTs could be simultaneously

Table 11. Detailed Quantity Ratio, Compositions, and Phase Structures of the Nanocomposites Cu/Sn ratios 2:1 1:1 1:2

samples nanocomposite 524 nanocomposite 525 nanocomposite 526

compositions and phase structures tetragonal Cu3SnS4 and cubic Cu2S tetragonal Cu3SnS4, cubic Cu2S, and tetragonal SnO2 triclinic Cu2SnS3 and tetragonal SnO2

be formed on the walls of MWNTs. Further decrease of the atomic ratio resulted in the formation of Cu2SnS3 and SnO2

Figure 79. Normalized nonlinear transmission versus incident fluence calculated from open aperture Z-scan data for pristine MWNTs and nanocomposites 524−526 dispersed in DMF (λ = 532 nm, I0 = 7.98 × 1010 W cm−2 at the focus). Reproduced with permission from ref 1160. Copyright 2010 Elsevier.

instead of Cu3SnS4 and Cu2S. According to our own notation, the nanocomposite denominations are shown in Table 11. All nanocomposites 524−526 presented better OL properties at 532 nm than did pristine MWNTs (Figure 79). The authors ascribed this occurrence to an enhanced nonlinear scattering of the NPs due to the increase of the size of the scattering cross-section over that of the pristine MWNTs.1160 Among the samples, nanocomposite 524 showed the best OL performance at 532 nm, possessing a broad absorption band in the wavelength range of 400−800 nm. In this case, the authors attributed the enhanced OL performance at 532 nm partly due to band gap absorption processes in Cu3SnS4 and Cu2S,1161 and partly to a possible contribution of free-carrier absorption of Cu3SnS4 and Cu2S.600 Stabilization of SWNTs through their suspension in binary solvents was also evaluated in regard to their NLO properties. Polar solvents of the type water-antifreezers (glycerol, polyethylene glycol) were used.1162,1163 The systems presented strong OPL properties with the glycerol-based system showing good resistance to optical bleaching effects. Experiments of fs Z-scan and pump−probe measurements on a carbon nanotube films were also reported.1164,1165 Nanotubes were aligned mainly in the propagation direction of the laser pulses. It could be found that MWNTs film possessed cubic nonlinear absorption and refraction in the laser irradiance range up to 300 GW/cm2. These nonresonant and negative nonlinearities displayed magnitudes of 10−11 esu at 780 nm 13134


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and a relaxation time of ∼2 ps.1164 In addition, SWNT-PVA composite acted as a saturable absorber for pulse generation in Yb-doped fiber lasers.1165 The authors found, for the first time, that mode-locking could be obtained in the dissipative soliton regime at low pumping followed by Q-switching at high pumping, different from conventional pulse dynamic evolutions. Other polymer/MWNT thin films on glass slide substrates have been fabricated by using the spin-coating method.1166 Open Z-scan technique was employed to measure the nonlinear absorption coefficient and OPL effects of these polymer/MWNT thin films, including poly(phenyl ether) (PPE) and poly(fluorenylene ethynylene) (PFE). PPE/ MWNT thin films exhibited a higher OPL effect than PFE/ MWNT, due to the higher π−π interaction between the aromatic groups of the rigid backbone of PPE polymer and the hexagonal carbon atom network existing on the surface of MWNTs. Most of the OL studies on pristine nanotubes or noncovalent nanocomposites concentrate on the physical mechanism and its influencing factors. Although pristine nanotubes possess broadband limiting effects, the nanotubes alone could not satisfy all requirements for laser protection. The development of complex carbon nanotube composites is expected to facilitate the realization of practical OL devices. While a lot of organic dyes exhibit NLO properties at definite wavelength bands, the OL effect in nanotubes covers a broad wavelength range from the visible to the NIR. Nonlinear absorbers, like phthalocyanines (Pcs, vide infra), porphyrins (Pors, vide infra), or other organic materials, have a quick response time in the ps regime, while carbon nanotubes generally respond at best in the ns regime.1167 Merging nonlinear characteristics of nonlinear absorbers and nanotubes

Figure 80. (a) OL responses to 10 ns pulses at 532 nm of 527 (○), 528 (△), and C60 (□) in chloroform solutions. (b) OL responses to 10 ns pulses at 1064 nm of 527 (○) and 528 (△) in chloroform solutions. Reproduced with permission from ref 1168. Copyright 2002 Elsevier.

Chart 104. Amide-Functionalized MWNTs

Figure 81. Nonlinear transmission profiles at 532 nm of PDDAMWNTs (▲), carbocyanine HITCl (blue ■), and blended 529 (red ●) at molar concentrations of (a) 74.5 μM, (b) 37.3 μM, (c) 18.6 μM, and (d) 14.9 μM. Mass concentration of PDDA-MWNT composite is the same being 5 mg/mL. Reproduced with permission from ref 1169. Copyright 2005 John Wiley and Sons.

carbon nanotubes could be candidates for potential broadband OL materials when surface modified with amides. Figure 81 shows OL of 529 (Chart 105),1169 carbocyanine, and PDDA-MWNTs at several concentrations. In all cases, 529 (the blended material) offered better performance than the individual components. Below 0.1 J/cm2, the nonlinear transmittance is dominated by RSA of PDDA-MWNTs at all concentrations. At the highest dye concentration (74.5 μM, Figure 81a), the transmittance decreased to 60%, while at the lowest concentration (14.9 μM, Figure 81d), the transmittance decreased to 80%. At the highest fluences (10 J/cm2), the transmittance decreased to 20%, 42%, 66%, and 74% for PDDA-MWNTs, and 10%, 16%, 24%, and 26% for the composite suspensions, at dye concentrations of 74.5, 37.3, 18.6, and 14.9 μM, respectively.

has resulted in the development of nonlinear absorber−carbon nanotube hybrids kept together by covalent linkages. MWNTs were solubilized in organic solvents by the reaction of amines with the carboxylic acid groups bound to nanotubes (527 and 528, Chart 104).1168 The solubilized MWNTs exhibited good OL performances at both 532 and 1064 nm for ns laser pulses (Figure 80). This indicated that the solubilized 13135


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blends.1167 These blends enhanced the OL performance in the higher energy density region when compared to sole Pcs solutions (vide infra).1174 As compared to C60 and individual nanotubes, the composite solutions show better OL responses for ns laser pulses at 532 nm. The authors attributed the superior performance to the effective combination of the NLO mechanism and the photoinduced electron transfer between the macrocycles and nanotubes. Because the topic of the functionalization of carbon nanotubes with Pors and/or Pcs involves the discussion of compounds not yet treated in this Review, this type of hybrid materials will be reviewed later in porphyrin and phthalocyanine sections. The use of polymeric assemblies of MWNTs in OL studies has been investigated. The pursuit for stable systems led some authors to study several polymer grafted or polymer coated MWNTs,1175 using poly(2-vinylpyridine), poly(4-vinylpyridine), poly(4-vinylphenol) as coating polymers, and poly(ethylene oxide) as grafted onto MWNTs. Both types of systems produced materials with significant OPL performance, having all compounds limiting thresholds of ∼1 J/cm2. Because polymer/DMF solutions did not possess any OL effect, the examined OL behavior was wholly due to the MWNT components. Therefore, the OL effect of MWNTs was not affected by the presence of polymeric assemblies, but the presence of the latter helped to improve the sought stability toward air, moisture, and laser radiation. On the basis of DFWM and Z-scan (open aperture and closed aperture) experiments, it was reported that the OL effect of polymeric systems-MWNTs was based on combined phenomena of nonlinear scattering, nonlinear absorption, and nonlinear refraction.1175

Chart 105. Carbocyanine-MWNTs Blended Composite

The functionalization of SWNTs with a commercial ink of unknown structure was also carried out.1170 The Z-scan studies on the nanocomposite were performed using mode-locked Ti:sapphire fs pulses at 780 nm. The analysis revealed the predominance of TPA displaying σ2 = 5.92 cm/GW at 76 mW of laser power, and a low threshold ultrafast OL of 1.15 × 10−4 J/cm2 in the fs regime. Blau and co-workers1171 demonstrated a strong OL effect from a noncovalently linked tetraphenylporphyrin−nanotube composite held together by van der Waals interactions. The photoinduced electron transfer effects from covalently or noncovalently linked RSA dye−nanotube composites have been widely studied.1172,1173 It is believed that such effects of charge transfer may help to improve the NLO response of such complex material systems. Very recently, some authors reported the linear and NLO properties of a range of Pc-nanotube Chart 106. Carbon Nanotube Nanocomposites for NLO

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0.58 × 10−12 esu for 531c). At the same level of linear transmission, 531a possessed better OL performance in comparison with the other composites, MWNTs and C60 (Figure 82). Light scattering, originating from the thermal induced microplasmas and/or microbubbles, was the mechanism responsible of the OL effect. POSS-polyimidazolium-ionic liquid (POSS = polyhedral oligomeric silsesquioxanes) combined with oxidized MWNTs also produced a significant OL material (532, Chart 106).1181 The NLO behavior of 532 was investigated by the Z-scan technique, with 532 revealing a remarkable susceptibility χ(3) of 2.38 × 10−7 esu, mainly due to the enhanced scattering effect of such an highly polarized system. SWNT composites grafted with conjugated polyacetylenes were also tested for OL.1137 The polyacetylene chains definitively increased the solubility of nanotubes in common organic solvents but had little effect on the OL properties. The OL capability resulted mainly from the carbon nanotubes in the composite. Some authors1182 also prepared polyurethane-urea/MWNT composites by covalent linkage utilizing a sol−gel process. This procedure led to new hybrid materials consisting of homogeneous and transparent systems. There was no observation of OL in the NLO measurements of pure polyurethane-urea, and any OL property of the sample came then exclusively from MWNTs. Coherently, upon decrease of MWNT mass percentage from 0.1% to 0.5%, the OL threshold decreased from 0.4 to 0.2 J cm−2. The nonlinear transmission of poly(ethylene glycol) connected to shortened SWNTs (PEG-sh-SWNTs) in solution was compared to that of a suspension of pure SWNTs in CHCl3 using 9 ns pulses at 532 nm from Nd:YAG laser.1183 Differently from the behavior of non derivatized soluble nanotubes,1065 PEG-sh-SWNTs showed better OL response than that of pure SWNTs. This result suggested that different conditions of cutting, etching, and functionalization may induce important changes of the properties of soluble nanotube samples.1183 Furthermore, polyaniline-MWNT hybrids were considered for OL. These were synthesized via the in situ polymerization of aniline in the presence of phenylenediamine-functionalized MWNTs.1184 The hybrids displayed βeff values between 11 and 23 cm/GW, that is, much higher values than for phenylenediamine-MWCNTs (7.5 cm/GW). In correspondence with that, an enhanced OL behavior was observed due to either a nonlinear RSA and effective TPA from the polyaniline or their combination with nonlinear scattering form the MWNTs.

Noncovalent modification of MWNTs with poly [2methoxy,5-(2′-ethylhexyloxy)-p-phenylenevinylene] (MEHPPV) was also carried out.1176 For the composite MWNT/ MEH-PPV, strong π−π interaction between the MEH-PPV and MWNTs in addition to the wrapping of the polymer was found. The NLO transmittance was measured using a ns optical parametric oscillator pumped with a Nd:YAG system. High OL performance of the composite MWNT/MEH-PPV was observed both in the visible region of 590−680 nm and at the wavelength of 1064 nm. For the sample with the mass concentration ratio of 0.05/0.175 mg/mL in MWNT/MEHPPV, the OL thresholds were 1.0, 1.2, 1.0, 1.0, and 0.25 J/cm2 at wavelengths of 590, 620, 650, 680, and 1064 nm, respectively.1176 The NLO properties of poly{[3-octylthiophene-2,5-diyl]-[p-aminobenzylidenequinomethane]-bonded MWNTs1177 were measured in chloroform solutions using DFWM at 532 nm under ns pulse excitation. High nonlinear absorption coefficient βeff and nonlinear refraction index η2 were obtained as well as significant OL behavior. This positive combination of effects was mainly due to the formation of an intramolecular photoinduced charge-transfer system between the conjugated polymer and the MWNTs. The nanocomposite 530 (Chart 106) for OL studies with 7.3% of MWNTs in mass, and with an average thickness of 10.4 nm, was synthesized via an amidation reaction.1178 The Z-scan experiments on 530 showed a significant reduction in transmission and scattering at both 532 and 1064 nm and indicated a prominent broadband OL effect. Such an effect is promoted by thermally induced nonlinear scattering. It is worth mentioning that 530 displayed βeff = 20.3 cm/GW at 532 nm and 29.1 cm/GW at 1064 nm, while the starting precursor (an aminated MWNT) showed βeff = 0.47 cm/GW at 532 nm, with no signs of nonlinear absorption at 1064 nm. Poly(N-vinylcarbazole)-grafted MWNTs hybrid materials (531, Chart 106)1179,1180 were also synthesized with different percentages of polymer content in the hybrid material (determining the n value). Hybrid 531a is obtained with 79% in weight of polymer, 531b with 90% in weight, and 531c with 96% in weight. The values for the NLO parameters varied with the weight percentage of the polymer. At the same concentration of 1.0 g/ L, 531a had a larger absorption coefficient at 532 nm with respect to 531c, with a correspondingly larger nonlinear extinction coefficient (βeff) (∼4.64 vs ∼1.68 cm/GW). The same trend was observed for the imaginary part the third-order nonlinear susceptibility (Imχ(3) = 1.6 × 10−12 esu for 531a and

Chart 107. Fullerene−Nanotube Hybrid Material

Figure 82. Nonlinear transmission of 531a, 531c, MWNTs, and C60 at the same level of linear transmission (∼82%). Reproduced with permission from ref 1179. Copyright 2010 John Wiley and Sons. 13137


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Nonetheless, other mechanisms may also give rise to optical nonlinearities in graphene derivatives. For instance, some authors1202 demonstrated the efficient use of atomic layer graphene as saturable absorber in a mode-locked fiber laser for the generation of ultrashort soliton pulses (756 fs) at the telecommunication frequencies band. The modulation depth could be tuned in a wide range from 66.5% to 6.2% by varying the graphene thickness. These results suggested that ultrathin graphene films could be potentially useful as optical elements in fiber lasers, as a laser mode locker. Also, nonlinear refraction could be observed,1203 as a parallel mechanism, mainly attributed to transient thermal effect in dispersions of graphene oxide derivatives, when studied in ns, ps, and fs regimes by Zscan technique. In another study,1204 the third-order NLO properties of graphene oxide (GO) dispersed in distilled water were investigated in the fs regime, using a single beam Z-scan technique. Induced by a focused Gaussian beam (λ ≈ 800 nm) with 150 fs pulse duration, GO showed strong nonlinear absorption, which was dominated by the RSA originated from TPA in GO. The influence of organic solvents on the NLO responses of GO solutions has been methodically studied.1205,1206 The OL effect of a few layers of GO dispersed in various organic solvents, such as N,N-dimethylformamide, tetrahydrofuran, Nmethyl-2-pyrrolidone, chlorobenzene, o-dichlorobenzene, and 1-chloronaphthalene, was evaluated. GO dispersion exhibited saturable absorption at low incident intensity and RSA at higher intensity. Nonlinear absorption of GO was found to depend significantly upon the solvent used. The highest values of β were 530 and 410 cm/GW for heavier solvents 1-chloronaphthalene and o-dichlorobenzene, respectively. On the other hand, lighter solvents, such as tetrahydrofuran, produced values of β = 140 cm/GW.1206 The interest of graphene materials in the field of OL is hampered by its typical poor solubility and proneness to agglomerate. Some authors, attempting to overcome these issues, have synthesized GO materials using sol−gel processes to incorporate it in matrixes, such as modified silicates (ormosil)1207−1209 or organic polymers.1210 For instance, high OL effect of GO ormosil hybrid glasses was obtained, with OL thresholds of ∼0.03 J/cm2,1207 and high optical stability and homogeneity.1209 Metal particle decoration of graphene materials is also an efficient manner to obtain more efficient optical power limiters. For instance, Au nanoparticle decoration of graphene1211 resulted in an enhanced OL behavior at a low fluence of ∼0.4 J/cm2, attributed to the photo absorption of Au nanoparticles combined with rapid thermalization of excited carriers by grapheme. The Au decorated graphene displayed OPL threshold of 9 J/cm2. A series of GO/noble metal (Au, Pt, and Pd) NP composites were synthesized via a one-step hydrothermal reaction of HAuCl4, H2PtCl6, and Pd(OAc)2 in the presence of GO.1212 The corresponding NLO and OL properties were investigated using an open-aperture Z-scan technique with ns at 532 nm. Metal NP inclusion on GO surfaces resulted in a significant enhancement of OL because of nonlinear scattering effects (Figure 84). A significant symmetric peak of nonlinear scattering with respect to focus position was evident in the GO/metal nanoparticle composites. However, no NLO scattering peak for pure GO was observed, indicating the absence of this type of contribution for GO alone. This finding

Figure 83. Nonlinear transmission of fullerene (■), nanotubes (gray ●), and nanohybrid 533 (Chart 107) (gray ▲) in CHCl3 at (a) 640 nm, (b) 690 nm, (c) 730 nm, and (d) 760 nm. Reproduced with permission from ref 1185. Copyright 2011 Elsevier.

The nanohybrid 533 (Chart 107)1185 prepared by grafting C60-fullerenes on carbon nanotubes was evaluated for its OL properties at wavelengths where C60 typically does not absorb. The OL performances of nanohybrid 533 and of the two reference compounds (fullerene and nanotubes) are presented in Figure 83. Fullerenes were also tested at the concentration 1 g/L. Their OL performances decreased concurrently with increasing wavelengths (Figure 83a and b), and completely faded above 700 nm (Figure 83c and d). This was due to a lack of linear absorptivity at these wavelengths. In contrast, dispersible carbon nanotubes were highly efficient from visible to NIR, and the same trend, although more pronounced, was observed for the nanohybrid 533. In fact, there was a peak of OL efficiency of 533 at 730 nm and at high fluence (10 J cm−2) as the residual transmittance was only 14%, while that of carbon nanotube derivative was still 25%. The OL threshold of nanohybrid 533 was also significantly lowered (∼150 mJ cm−2) as compared to ∼250 mJ cm−2 for nanotubes.1185 These results showed that there was an overall enhancement of optical limitation due to a synergistic effect between carbon nanotubes and fullerenes in a wavelength range where the fullerene derivative alone would have been ineffective. The authors ascribed the OPL mechanism to recombination of excitons in a carbon nanotube, which created highly energetically excited nanotubes. The latter would transfer its excess energy to C60, which, in turn, becomes activated for OL.1185 5.3. Graphenes

Graphene, a monolayer of graphite, has attracted intense interest in recent years due to its unique two-dimensional (2D) structure and excellent physical and chemical properties, such as high electrical and thermal conductivity, great mechanical strength, and extreme large specific surface area, among others.1186−1191 Several groups have initially studied the NLO properties of dispersions of graphene derivatives,1192−1201 demonstrating that their nonlinear behavior was mainly due to nonlinear scattering. The latter phenomenon would originate from the thermally induced solvent bubbles and microplasmas as in case of its elongated analogues. It was generally observed that the surface tension of the solvents had a strong influence on the OL performance of the graphene dispersions. It was clearly seen that the lower was the surface tension, the smaller was the limiting threshold and the larger was the scattered intensity. 13138


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Such an effect was determined mostly by interband and intraband transitions.1216 The limiting thresholds of CuO-HEG for ns and fs excitations were 4.35 and 1.43 J/cm2, respectively. The limiting threshold of HEG in the fs excitation regime was only 2.7 J/cm2. TiO2-reduced graphene oxide (RGO) nanocomposites were prepared by a hydrothermal method and tested for OL.1217 The intensity-dependent switching between RSA and saturable absorption was observed with the RGO nanocomposites. The principle of graphene decoration with other classes of materials was extended also to QDs as decorating agents. Nanohybrids consisting of graphene derivatives with PbS1218 and CdS1161 QDs were reported, along with their OPL properties. Both RGO−PbS and graphene−CdS nanohybrids exhibited better OL properties when compared to their naked counterparts upon both 532 and 1064 nm excitation in the ns regime. This was attributed by the authors to the synergetic

Figure 84. Typical nonlinear scattering behavior of GO and GO/metal nanoparticle composites at 532 nm. The energy detector was located 45° from the beam axis. Reproduced with permission from ref 1212. Copyright 2014 The Royal Society of Chemistry.

Figure 85. Normalized transmittance of GO-based materials as a function of input fluence. Reproduced with permission from ref 1212 Copyright 2014 The Royal Society of Chemistry.

Figure 86. (a) Z-scan curves at an incident pulse energy of 120 μJ and (b) OL performances of etched and pristine graphene, and C60. Reproduced with permission from ref 1219. Copyright 2013 American Chemical Society.

suggested a high improvement in nonlinear scattering caused by the presence of metal NPs in the composites.1212 The OL curves presented in Figure 85 were determined from the corresponding Z-scan curves. At low fluence, the transmittance of GO/metal NP composites was independent of the input fluence until 0.02 J/cm2. At higher input fluence, OL effect was observed for all GO/metal nanoparticle composites, and transmissions considerably decrease to less than 0.4 at 1.0 J/cm2. The results revealed that the OPL efficiencies of all hybrid GO samples were much higher than that of the GO alone. The OL thresholds of GO, GO/Au-NP, GO/Pt-NP, and GO/Pd-NP composites were 0.56, 0.32, 0.40, and 0.47 J/cm2, respectively.1212 Hydrogen-induced exfoliated graphene (HEG) was decorated with Ag,1213,1214 Pd,1215 Pt,1215 and CuO1216 NPs, and studied for OPL using Z-scan experiments. Enhancements in OPL properties of Ag NPs embedded in graphene have been observed under fs laser pumping at the fundamental and second harmonic wavelengths with kHz repetition rates. Both HEG and Ag-HEG exhibited large nonlinear absorption coefficients at 400 nm displaying χ(3) = 4.4 × 10−10 esu for HEG and 1.4 × 10−10 esu for Ag-HEG.1214 Both Pd-HEG and Pt-HEG nanocomposites showed good OPL behavior. The mechanism of the OPL effect involved TPA and ESA in the ns excitation regime, and saturable absorption in combination with TPA in the (ultrafast) fs excitation regime. The metallo-derivatives of HEG reached limiting thresholds in the range of 1.5−1.8 J/ cm2.1215 In both ultrafast (fs) and short-pulse (ns) laser excitation regimes, CuO-HEG nanocomposites showed an enhancement of the OL properties that arised from the strong nonlinear absorption of semiconducting CuO nanoparticles.

effects arising from charge transfer between the two components. The NLO properties of Fe NP−graphene hybrids,1219 produced by an etching process that employs catalytic steam gasification of carbon by Fe NPs, were studied at 532 nm with ns pulses utilizing the open-aperture Z-scan technique (Figure 86). All scans exhibited a reduction in transmission around the focus of the lens, indicating a typical OL behavior. It was found that Fe-etched graphene exhibited a superior OL effect in comparison to pristine graphene and C60 at the same level of linear transmission (Figure 86b). In contrast to the nonlinear scattering of pristine graphene and the nonlinear absorption of C60, the better OL performance of Fe-etched graphene could be attributed to an effective combination of both effects. A GO hybrid material coordinated with Fe3O4 NPs1220 was studied with the Z-scan technique at 532 nm in the ns and ps regimes. Iron oxide decorated GO exhibited enhanced NLO and OL properties in comparison to pristine GO in the ns regime. However, when compared with fullerene in toluene at different concentrations, the hybrid material showed a weaker OL effect than fullerene at high concentrations, but stronger at lower concentrations. Similarly, ZnO-graphene hybrids were also tested for OPL,1221−1223 and presented better nonlinear absorption properties than their individual counterparts. This finding was probably due to TPA, RSA, and photoinduced electron transfer between ZnO and graphene sheets. On the other hand, graphene−MoS2−PMMA organic glasses exhibited enhanced RSA properties as compared to graphene−PMMA and MoS2−PMMA organic glasses.1224 The photodynamic processes in the graphene−MoS2−PMMA organic glasses were 13139


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optical transmission occurred, but above that energy the transmittance of 534 decreased drastically. This happened only at values above 1.5 J cm−2 for GO and 535. At 532 nm, the limiting threshold value of 534 is 1.55 J cm−2. Figure 87b shows the OL responses of 534, GO, and 535 solutions at 1064 nm. When the incident laser fluence exceeded 2 J cm−2, an abrupt drop in transmittance is observed for 534 solution at a limiting threshold of 8.1 J cm−2.1230 Chemical functionalization of graphene-based materials is probably the best way to induce OPL enhancement. For instance, fluorination of graphene has many advantages due to the unique nature of the C−F bond. Some authors reported the OPL properties of fluorinated graphene oxide (FGO) using the Z-scan technique.1231 The experiments demonstrated that OL of FGO at low fluence was attributed to nonlinear absorption, while that at higher fluence was due to nonlinear scattering. Moreover, its OL threshold was about an order of magnitude better than that of GO. The third-order NLO responses of graphene fluoride dispersed in DMF and also of fluorosurfactant-stabilized graphene fluoride dispersed in water1232 were tested under visible (532 nm) and IR (1064 nm) excitation with ps and ns laser pulses. The experiments demonstrated that the degree of fluorination can importantly modify the NLO responses, thus allowing the effective tailoring of these properties via controlled processes of graphene fluorination. Covalent chemical functionalization is a strategy used very often for solubilizing graphene materials. As a result, it also provides a mean to accumulate effects and phenomena that consist of the combination of multiple nonlinear mechanisms, for example, RSA, TPA nonlinear scattering, as well as photoinduced electron transfer. Such an approach leads to a general improvement of the OL effect for the resulting nanohybrid. For instance, RGO covalently decorated with polyaniline (536, Chart 109) was synthesized and tested for OPL.1233,1234 The hybrid showed improved NLO properties when compared to the GO parent precursor and the pristine polyaniline, due to a combination of nonlinear scattering, TPA, and RSA. The OL threshold was found to be 0.8 GW/cm2 for the hybrid 536 as compared to 3 GW/cm2 for GO and 2 GW/ cm2 for polyaniline. A similar OPL improvement was found for the hybrid 537 (Chart 109),1235,1236 prepared by covalent linkage between thermally-RGO with poly(N-vinyl-carbazole) (PVK), with the wt % of RGO in the resulting polymer being estimated as 11.21%. The resulting hybrid material 537 displayed excellent broadband OL responses at 532 and 1064 nm due to the effective combination of different NLO mechanisms. Hybrid compounds 538 and 539a,b were prepared by Suzuki coupling of polymers 540 and 541 with RGO (Chart 110).1237 These polymer−graphene hybrids were highly soluble in organic solvents, and exhibited an excellent OL performance at 532 nm with ns laser beam. Figure 88 shows the nonlinear transmittance of hybrids 538 and 539a,b, along with polymers 540 and 541.1237 Carbon nanotubes (CNTs) were used as the benchmark material for comparing the OL performance. It was found that the NLO transmittance of 539a and 539b started to fall at 0.1 J cm−2, a smaller value than RGO and CNTs, thus pointing to a comparatively enhanced OL performance. The limiting thresholds were 0.93, 1.2, and 3.6 J cm−2 for 539a, 539b, and CNTs, respectively. These were clearly better than that of the RGO or polymers alone.

analyzed with the energy-level diagram of graphene−MoS2 composites. The analysis further pointed to the larger relaxation time of the graphene−MoS2−PMMA organic glasses, thus leading to enhanced RSA properties. A study on the NLO absorption in the NIR (980 nm) of rare-earth nanomaterials1225,1226 combined with graphene derivatives was conducted. The preparation of the new nanocomposite presented quite interesting OPL activity, with a threshold of 253 mW, while the output power was clamped at 224 mW at high input power density. Noncovalent functionalization of graphene materials was also revealed to be a suitable methodology for obtaining good optical power limiters. Functionalization of graphene sheets with polyacetylene,1227 poly(vinyl alcohol),1228 and carboxChart 108. Graphene Oxide−Dye Complex

ymethyl cellulose1229 represents examples of hybrid systems characterized by noncovalent interaction between graphene and polymers. The hybrids of graphene showed usually enhanced NLO parameters and OL response with increased crystallinity in comparison to the sole organic polymers.

Figure 87. Optical limiting response of the aqueous solutions of 534 (20 mg L−1), GO (34 mg L−1), and 535 (2 × 10−6 M), measured with 7 ns laser pulses at (a) 532 nm and (b) 1064 nm. Reproduced with permission from ref 1230. Copyright 2010 John Wiley and Sons.

Some authors have synthesized a graphene oxide−dye charge-transfer complex (534, Chart 108) by a simple ionexchange process,1230 between a cationized pyrene vinylpiridinium salt (535) and GO. The nonlinear transmittance of the solutions of GO, 534, and 535 in deionized water was studied with 7 ns pulses at 532 and 1064 nm. At 532 nm (Figure 87a), until the input fluence 0.21 J cm−2, no change of 13140


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Chart 109. Polymer Grafted Graphene Derivatives

Chart 110. RGO-Grafted Polymeric Nanocomposites

Chart 111. Sexithiophene Grafted Graphene Derivative

Figure 88. Optical limiting response of carbon nanotubes (CNT), RGO in water, and 538, 539a,b, 540, and 541 in toluene. Excitation was carried out with 7 ns pulses at 532 nm. The linear transmittance of all of the solutions was adjusted to 65%. Reproduced with permission from ref 1237. Copyright 2010 John Wiley and Sons.

A superior OL effect for the GO derivative 542, with respect to the benchmark material C60, was observed. The openaperture Z-scan curves and the OL of 542, GO, and C60 are presented in Figure 89a and b. Hybrid 542 showed strong nonlinear scattering, while GO and C60 did not exhibit nonlinear scattering during the same experimental conditions.

Anchoring of oligothiophenes to GO significantly improved the solubility and the stability of the dispersions of graphenebased hybrid 542 (Chart 111) in organic solvents.1238,1239 13141


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laser (532 nm). The OL thresholds were 52 and 80 mJ cm−2, respectively, for the nano-onions grown with and without 10.532 μm laser excitation. Carbon nanodots, a new class of carbon-based NPs, were evaluated for their nonlinear responses under 4 ns and 35 ps irradiation with visible (532 nm) and IR (1064 nm) pulses utilizing the Z-scan technique.1243,1244 Carbon nanodots exhibited significantly negative nonlinear refractivity (in the order of 10−9 m/W), under all excitation conditions. Other carbon nanodots,1245 synthesized with fs laser ablation of sugar beet bagasse in ethanol, originated strong OL responses at 800 nm when excitation arised from fs laser pulses. An OL threshold of 74 mJ/cm2 was determined. The resulting NLO effect arised from strong TPA. Detonation nanodiamonds are another form of carbon materials, which have been tested for their NLO properties.1246−1251 Nanodiamonds possess features similar to those of carbon nanotubes, because their third-order nonlinear responses originate from nonlinear scattering and nonlinear absorption. They are optically stable materials that increase their OPL properties with the increase of the wavelength. This is why they are mostly tested for OPL at telecommunication wavelengths.

Figure 89. (a) Open-aperture Z-scan curves and (b) OL profiles of 542 (Chart 111), GO, and C60. All sample concentrations were adjusted to have the same linear transmittance of 65% at 532 nm. Reproduced with permission from ref 1239. Copyright 2009 Optical Society of America.

Chart 112. Graphene−Fullerene Hybrid

6. PORPHYRINS AND RELATED MACROCYCLES Porphyrins (Pors) are an all-pervading class of naturally occurring compounds with important biological representatives such as hemes, chlorophyll, and Vitamin B12. A basic structure of a Por macrocycle consists of four pyrrolic subunits linked by four methine bridges, and it is considered a related compound from Pcs.1252−1256 Por synthesis has been extensively improved over the years,1257−1265 and studied for a vast array of

It could be observed that the OL effect of 542 is much better than those of GO and C60. At the highest input fluence of 7.22 J/cm2, the transmitted output fluence was 3.58, 1.68, and 1.01 J/cm2, while the OL thresholds were 1.15, 0.06, and 0.15 J/cm2 for GO, C60, and 542, respectively.

Figure 91. Typical porphyrin’s ground-state electronic absorption spectrum.

Figure 90. Carbon nano-onions. Reproduced with permission from ref 1241. Copyright 1997 AIP Publishing LLC.

applications, including catalysis,1266−1270 photomedicinal applications,1271−1276 energy transfer,1277−1280 and others.1281−1283 The presence of an extended π-electron system in Pors provides the verification of various NLO effects. In Pors, the ground-state electronic absorption is mostly confined to a few narrow regions (Soret and Q bands at about 400 and 600 nm, respectively), thus allowing high transmission in the spectral range comprised between these two characteristic absorption bands (Figure 91). Por’s exhibit large excited-state absorption cross sections (σexc) and long triplet excited-state times (τexc). These properties make Pors interesting materials for the verification of effective OL and have been considered through times as promising materials for OPL.

The graphene−C60 hybrid 543 (Chart 112)1240 has been synthesized and characterized. Its NLO performance was studied using the Z-scan technique. Quite enhanced NLO performance was observed when compared to that of the benchmark material C60 and graphene alone. The ameliorated performance was attributed mainly to photoinduced electron transfer between graphene and C60. Carbon nano-onions (Figure 90),1241 consisting of concentric carbon particles, were also tested for OPL.1242 In this study, the OL characteristics of the nano-onions prepared under different conditions (without laser excitation and with laser excitation at 10.532 μm) were investigated, using an Nd:YAG 13142


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these materials, only 546a and 546e showed a significant decrease of the optical transmission with increasing input fluence. The transmission in other Pors samples remained nearly constant up to the fluence of ∼4 J/cm2, the maximum fluence used in the experiments. The OL effect generated by 546a and 546e was comparable to that of C60 when all of the samples were measured with the same linear transmittance at 527 nm. Additionally, metal-free 5,10,15,20-tetraphenylporphyrin 546g1291 also displayed maxima of TPA cross-section values of 25 and 1−6 GM in laser wavelength ranges of 730− 790 and 1100−1400 nm, respectively. The increasing use of low power coherent CW lasers in various applications stimulated the research and the design of optical limiters with low thresholds. Taking this premise into account, some authors studied the optical nonlinearity exhibited by zinc tetraphenyl porphyrin (546a, Chart 114) at low laser powers.1292 The nonlinear refractive index of the medium was measured using the Z-scan technique, and a value of −1.4 × 10−7 cm2/W at the He−Ne wavelength of 632.8 nm was reported. The origin of nonlinearity was ascribed to a predominant thermo-optical effect, and the NLO behavior was rationalized using the model of the thermal lens. For the systems of Chart 114, the variation in the output intensity was studied as a function of input intensity for two different aperture sizes. Moreover, the influence of the aperture size on the threshold limit and output clamping power was also analyzed. The aperture size of 1 cm was found to correspond to a low threshold value of 1.42 mW and a stable clamped output at 0.07 mW.1292 Whereas limiters based on nonlinear absorption phenomena have been designed for use with high power pulsed laser sources, aperture limited designs based on thermo-optic nonlinearity such as the one studied here can be used as efficient limiters in the low power CW regime.1292 Peripheral halogenation of Pors brings about the modifications of the electronic structure of the substituted molecules due to the high electronegativity of a halogen atom. Moreover, halogenation imparts a strong polar character to the carbon− halogen bond and induces electron deficiency on the halogenated carbon atom. Such features considerably alter the redox, optical, and electronic properties of the halogenated conjugated macrocycles by increasing their oxidation potential, lowering the HOMO level, and shifting the optical absorption to longer wavelengths when compared to analogous unsubstituted systems.1293 The effect of halogen presence on the ISC efficiency of octaethyl porphyrins has been considered.1294 By including one bromine atom at the pyrrole position of 547 (Chart 115), the ISC has increased considerably. On the basis of these experimental results, it is expected that porphyrin bromination leads to new systems with enhanced RSA and with high potential as OL materials.1295 The OL data for octabromotetraphenylporphyrins (548a−f, Chart 115) and some of the other metal complexes have shown the following trend: 548b > 548a > 548c > C60 > 548f in terms of limiting threshold and excited-state absorption cross-section.1295 Octabrominated 548f showed even better performance when compared to the benchmark dye C60 and was comparable to the state-of-the-art soluble phthalocyanine (t-Bu)4PcInCl. Other metals were also inserted into the porphyrin cavity of coordination, and a study demonstrated that the use of Zn as central metal produces the best OL performance,1295,1296 when compared to Cu and Co as central metals. This trend could be explained in terms of the

Pors generate the OL effect through the phenomenon of RSA due to the mechanism of sequential multiphoton absorption. In fact, the first reported work on third-order Chart 113. Chlorophyll A Structure

nonlinearity of Pors was by Blau et al.24 with laser pulses at 532 nm with 80 ps duration. It was found the excited-state absorption cross-section, σexc, was larger than the ground-state absorption cross-section, σg, for zinc, cobalt, and metal-free tetraphenyl Pors.24 Since these results, considerable work has been done to improve the RSA properties of porphyrins and metalloporphyrins, and reports have grown in this field.1284,1285 Mendonça generated RSA from solutions of natural chlorophyll Chart 114. meso-Tetraaryl Porphyrins for NLO

A (544, Chart 113),1286 using the Z-scan technique with picosecond pulses at 532 nm. The experimental data indicated a strong ISC between singlet−triplet excited states. Chlorophyll A was found to be a good candidate as sensitizer in photodynamic therapy; however, due to the low triplet/singlet absorption cross sections, chlorophyll A could not be efficient as an OPL material. Some authors studied a series of tetratolylporphyrin complexes (545a−d, Chart 114),1287−1290 and the results indicated a very high value of nonlinearity for these molecules against ns pulses and reasonably high values against ps pulses. All complexes generally exhibited strong NLO absorption at both 532 and 600 nm.1287,1288 The axially substituted Por 545d (Chart 114) displayed the lowest OL threshold.1289,1290 Tetraphenylporphyrins 546a−g (Chart 114) with M = Zn(II), Cu(II), Ni(II), Co(II), VO(IV), and FeCl(III) were also characterized for OL of 20 ns pulses at 527 nm. Among 13143


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Chart 115. Several Porphyrins Studied for OPL

spin state of the central metal. Both Cu2+ and Co2+ are openshell paramagnetic ions with unfilled d orbitals, involving spin− orbit coupling through Coulombic exchange terms. This results in the decrease of the triplet lifetime and consequently scarce efficiency in the increase of population in the highly absorbing excited state. Zn2+, Cd2+, and Pb2+ ions have closed shells, and the resulting OL performance is improved in these complexes.1297 Nevertheless, the trend of Zn2+ > Cd2+ > Pb2+ was in a reverse order with respect to what was found for the heavy metal phthalocyanine complexes, in which the spin−orbit coupling and nonlinear absorption increases with the atomic number.1298,1299 The OPL of ps pulses at 532 nm was reported for a series of basket handle porphyrins (549a−c, Chart 115). The effect was attributed to RSA.1300 These authors also observed that the presence of bromine, that is, a heavy atom, significantly enhanced the OL properties by increasing the rate of ISC that feeds the triplet state. Compounds 550−552 (Chart 115) were also studied by Zscan technique in both ns and ps regimes.1301 Results showed that both metalation and bromination of diphenylporphyrins 550a−c caused a regular change of magnitude and sign of nonlinear absorption. The transition between saturable absorption and RSA happens as the number of bromines increased (at a small scale) and metal ion changes (much higher scale).1301 The nonlinear absorption properties of the nine porphyrin complexes 550−552 dissolved in CHCl3 are shown in Figure 92,1301 where open aperture Z-scan curves were measured in the case of ns and ps pulses. The solid lines are the theoretical fittings obtained by using a five-level energy model. For free base, zinc, and copper diphenylporphyrins, the change of open aperture Z-scan curves from peak (valley) to

Figure 92. Open aperture Z-scan curves of compounds 550−552 irradiated with ns pulses (a−c) and ps pulses (d−f). The solid lines are the theoretical fittings. Reproduced with permission from ref 1301. Copyright 2008 Elsevier.

valley (peak) indicated that a transition between saturable absorption and RSA occurred due to bromination. Highly fluorinated porphyrins 553a−e (Chart 115)1302 were also tested for their third-order nonlinear optical parameters, using the Z-scan technique at 532 nm. The third-order nonlinear optical susceptibilities χ(3) were of the order 10−12 13144


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Chart 116. Library of trans-A2B2 and A2BC-Porphyrins

esu, and the responsible mechanism was RSA with the effective ESA cross section higher than the ground-state absorption cross-section. Peripheral substitution of Pors with groups that prevent the aggregation caused by π−π stacking was revealed to be a suitable approach of molecular design to obtain efficient optical power limiters. For instance, metalloporphyrins based on 5,10,15,20-tetrakis(p-hexadecyloxyphenyl)-porphyrin1303 were found to be good optical limiters toward ns laser pulses at 532 nm. A particularly interesting system was the lead complex 554 (Chart 115), for which the OL effect was correlated with ground- and excited-state parameters according to a five-level model that generates RSA. A large library of trans-A2B2 and A2BC-porphyrins (555− 571, Chart 116)1304 possessing two arylethynyl substituents at the meso positions has been synthesized and tested for their TPA properties. The compounds showed very high solubility in nonpolar solvents, and did not tend to aggregate. The increase of TPA cross section in the Soret region for porphyrins bearing strong electron-donating groups was attributed to resonance enhancement caused by intensification, red-shift, and broadening of the lowest Q-band. High values of TPA cross sections were found (range: 500−950 GM). Other metalated meso 5,10-A2B2 substituted porphyrins (572a−e, Chart 117) were investigated with the open Z-scan technique at 532 nm in the ns regime.1305,1306 As it can be seen from Figure 93, most of the compounds exhibited a transmission drop with increasing input fluence as expected for OL active materials. All recorded responses were successfully fitted with a four-level model with simultaneous TPA arising from the higher excited states. The trimethylsylyl (TMS) TMS-ethynyl group was found to be a quite efficient meso substituent for the generation of OL effect, where complexes 572a, 572c, and 572d were the strongest positive nonlinear absorbers among the compounds studied. Attachment of acetylene groups to the meso position of Pors shifted bathochromically the absorption bands of 573a,b (Chart 117),1307−1309 due to the resulting variation of its conjugation length. Different metal ions were inserted to evaluate the influence of the central atom on OL effect. The OL experiments were conducted with ps and ns lasers at 532 nm. The results showed that by either extending the conjugation length or by adding heavy metals, the ground-state absorption of the modified meso-alkynyl porphyrins could be red-shifted by ∼200 nm in the visible region when compounds were irradiated in the linear opical regime. Heavier atoms such as Pb helped to enhance the OL performance by increasing the excited-state absorption. At 532 nm for 7 ns pulses, the OL performance of zinc porphyrin was superior to that of the corresponding free base porphyrin.1310 Detailed studies using broadband pump/probe techniques demonstrated that introducing a zinc atom into the porphyrin chromophore had a strong effect on most of the NLO parameters. The major changes were the reduction in the singlet state lifetime, and a broadening and shifting of the excited-state absorption profile. However, the most influential effect for the nonlinear response at 532 nm was the reduction of the ground-state absorption coefficient. Similar results were also obtained for In, Zn, and Pb metal complexes.1311,1312 Pb complex exhibited a large κ-value of ∼45 at 531 nm, as well as RSA from 520 to 720 nm, making it an attractive option as OL material over a large part of the visible window.

Insertion of atoms of the III and IV groups into the coordination cavity of tetrakis(4-triisopropylsilylethynyl) porphyrin was accomplished, and the resulting complexes 574a−h 13145


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Chart 117. Modified meso-Alkynyl Porphyrins

that complexes of In(III), Tl(III), and Pb(II) offered the most sensitive response at 532 nm, and κ-values in the range 45−48 have been recorded. The In(III) and Tl(III) complexes performed NLO absorption across their transmission window from 480 to 620 nm and had relatively long triplet lifetimes of ∼800 ns in air-saturated solution. Other authors1314 prepared the porphyrins 575a−f (Chart 117) characterized by the presence of an extended network of conjugation with respect to unsubstituted Pors. The authors found that the OPL of 5 ns pulses at 532 nm (repetition rate: 20 Hz) could be enhanced via insertion of closed-shell heavy metal ions. In particular, Por 575e exhibited an OL effect stronger than C60. meso-Tetraferrocenyl porphyrin 576a and its metal derivatives 576b−e (Chart 118)1315 were also found to be quite important molecular systems for OL of Q-switched Nd:YAG ns pulses. For instance, compounds 576a, 576b, and 576c exhibited OL performance superior to that of fullerene benchmark with occurrence of RSA phenomenon. Compound 576a also retrieved a ratio σexc/σ0 higher than 7, which attested its capacity to perform as an optical limiter. Figure 94 shows the OL response of 576a, 576b, and 576c along with C60 as a reference term of comparison. For 576a,

Figure 93. Comparison of the nonlinear responses along with fitting curves generated on the basis of the four-level model for 572a−e. Reproduced with permission from ref 1305. Copyright 2013 The Royal Society of Chemistry.

(Chart 117) with Al, Ga, In, Tl, Ge, Sn, and Pb were studied.1313 In these complexes, Ga(III), In(III), Tl(III), and Sn(IV) central metals saturated their valence with chlorine atom(s) as axial ligand(s). The experimental results revealed Chart 118. Functionalized Porphyrins for OPL

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attributed to absorption of diffusive photons that experience longer light pathways than ballistic photons. The investigation of ESA in cationic porphyrin derivative 581 (Chart 119)1319 was conducted with the Z-scan technique with white-light continuum as testing radiation. The transient singlet absorption spectrum showed RSA around the Q-band region (500−650 nm) and a weak saturable absorption around 650 nm, thus demonstrating highly sensitive dependence of the NLO properties on the wavelength of excitation. Cationic and anionic porphyrin metal complex derivatives 582a−c and 583a−c (Chart 119)1320 were experimentally studied using the Z-scan technique and the optical Kerr gate method with ps pulses at 532 nm. Values of nonlinear refractive index values were modest (in the range of −0.1 × 10−14 cm2/ W), except 582b (−0.6 × 10−14 cm2/W) and 582c (−5.4 ×

Figure 94. A comparison of the OL results among 576a, 576b, 576c, and C60 in toluene solutions with 70% of linear transmittance at 532 nm. Reproduced with permission from ref 1315. Copyright 2012 Elsevier.

Chart 120. Water-Soluble Porphyrins for OPL

the deviation from linearity occurs at 20 μJ of incident energy, whereas for 576b, 576c, 576d, and 576e, it occurred at 15, 25, 35, and 40 μJ, respectively.1315 The functionalized β-substituted tetraarylporphyrin 577a,b (Chart 118) displayed TPA,1316 as the introduction of a donor−acceptor moiety with push−pull effect in the β position altered considerably the electronic structure of the Por. The two-photon absorption coefficient in the NIR was increased to ∼56 and ∼187 GM, for 577a and 577b, respectively, which are much higher values than that of meso-tetraphenyl porphyrin with the same central metals (16 GM). Unsymmetrically substituted porphyrin 578 (Chart 118),1317 bearing one D−π−A−π−D pyrimidine chromophore at the periphery, was synthesized (D = donor, A = acceptor) and studied using the Z-scan technique at 532 nm with 4 ns and 21 ps laser pulses. Compound 578 exhibited enhanced NLO absorption, refraction, and OL response when compared to unsymmetrically substituted 579 with a polar, nonconjugated substituent (Chart 118). The enhanced OL properties in passing from 579 to 578 could be ascribed to a combination of ESA with occurrence of RSA, with favorable contributions from nonlinear refraction and fluorescence resonance energy transfer from the D−π−A−π−D pyrimidine chromophore to the porphyrin core. Other authors1318 determined the OL performance of the cationic porphyrins 580 (Chart 119) excited with ns pulses at 532 nm. The OL of 580 could be improved when small-sized (400 nm-diameter) nonabsorbing polystyrene spheres were added to the solution. The enhancement of the OL effect was

10−14 cm2/W). On the other hand, nonlinear absorption coefficient values were much higher for free base porphyrins 582a and 583a displaying 1.2 and 0.7 cm/GW, respectively. A series of porphyrin nanospindles with controlled long axis length distributions of 330, 550, 800 nm, and 4 μm have been successfully fabricated via hierarchical self-assembly of a cationic porphyrin with an anionic surfactant (sodium dodecyl sulfonate, SDS) due to the effective electrostatic interaction (584, Chart 120).1321 With the Z-scan technique (5 ns pulses at 532 nm), the authors observed size-dependent third-order NLO switching properties: the nonlinear absorption changed

Chart 119. Ionic meso-Substituted Porphyrins

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Scheme 5. Reversible Exchange of Protons between Compounds 586 and 587

performance, even better than the benchmark material C60 and MWNTs dispersion. Although 586-Gelatin displayed the largest σeff, its larger linear absorption limits its practical application for OL. On the other hand, 587 had the largest ratio of σeff/σ0, thus showing superior RSA properties and OL performance. At the input fluence of 20 J cm−2, the output fluence of 587 was 1.68 J cm−2, which was much lower than 586 (10.1 J cm−2), meso-tetraphenyl porphyrin (TPP) (6.68 J cm−2), C60 (3.82 J cm−2), and MWNTs (2.87 J cm−2). Also, the limiting threshold for 587 (0.24 J cm−2) was much lower than that for the other compounds: 586/>25 J cm−2, TPP/13.3 J cm−2, C60/5.55 J cm−2, and MWNTs/3.13 J cm−2. Another group of molecules based on highly π-conjugated tetrabenzoporphyrins that strongly absorb the visible light in the range 450−630 nm is the series 588 and 589a,b (Chart 121).710,1324−1327 It was concluded that the NLO behavior of these molecules was due to dominant ESA phenomenon at short wavelengths with evidence of TPA at long wavelengths,1328−1330 displaying limiting thresholds as low as 10 mJ/cm2.1324,1325 A comparative study of the TPA properties of tetrabenzo-, tetraphenyl-, and octaethyl-substituted porphyrins 589b, 590, and 591 (Chart 121)1331 revealed that the TPA efficiency of porphyrins in 710−810 nm region was resonantly enhanced due to nearby Q(0−0) one-photon transition. Because of this effect, σ2 reached values of 4.4, 20, and 130 GM for 591, 590, and 589b. Some authors synthesized several dimeric porphyrinic structures (592 and 593a−c, Chart 122), to study the effect of conjugation extension on the excited-state lifetime of a porphyrin.1332−1334 Picosecond photophysical measurements were carried out, and this study showed that fused porphyrins usually have very short excited-state lifetimes. The authors suggested that these types of dimers have a natural fluorescence lifetime of ∼10 ns, but the singlet excited state is quenched by fast nonradiative decay. Nevertheless, when bromine or iodine is present, the ISC becomes fast enough to compete with nonradiative decay. The triplet lifetime was found to be 225 ns in oxygenated conditions and rises to 280 ns when there is a deoxygenated environment, if bromine is the substituent. It decreases to 52 ns for the iodine containing Por 593b.1335

from saturation absorption to RSA, and the nonlinear refraction from self-defocusing to self-focusing due to the change of the dominant scattering effect. Moreover, switching from Rayleigh scattering (for nanostructures with a smaller size than the wavelength of laser light) to Mie scattering (for nanostructures having a size larger than the laser wavelength) could be also observed. Some authors studied how photoinduced intramolecular charge transfer in porphyrin-viologen dyads (585, Chart 120) could influence the OL performance in the ns range,1322 utilizing ps and ns time-resolved fluorescence and absorption spectroscopy. Nonlinear transmission measurements around 600 nm with ns laser pulses have shown that the dyad solution

Figure 95. Nonlinear transmittance at 532 nm for 586 (in water), 587 (in water), TPP (in toluene), C60 (in toluene), and MWNTs (in DMF), with the same linear transmittance of 75%. Reproduced with permission from ref 1323. Copyright 2013 IOP Publishing.

led to a lower transmission at high incident fluences >1 J/cm2 with respect to the model porphyrin. The NLO properties of water-soluble porphyrins 586, 587 and a film of 586 with gelatin matrix (586-Gelatin) (Scheme 5)1323 were investigated by the open-aperture Z-scan technique in the ns temporal regime. Results showed that 587 exhibited a larger ratio of ESA cross section to that of the ground state, and enhanced RSA properties as compared to 586 and 586-Gelatin. As shown in Figure 95,1323 587 also shows a correspondingly superior OL 13148


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Chart 121. Benzoporphyrin Derivatives

Chart 122. Dimeric Porphyrinic Structures

The enlargement of meso-meso, β−β, β−β triply linked diporphyrin 594 (Chart 122)1336 has been achieved by incorporating dibenzofused structures. Compound 594 showed a significantly disturbed absorption spectrum and a larger TPA cross section as compared to the normal triply linked diporphyrins, with σ2 reaching ca. 15 400 GM at 1260 nm. A wide range of structurally diverse bis-porphyrins and trisporphyrins 595−597 (Chart 123)1337 was investigated for their nonlinear absorption properties, using open Z-scan technique with 6 ns pulsed laser operating at 532 nm. The βeff values of dimers and trimers were in the same range being 2.7 × 10−8 cm W−1 for 595, 1.1 × 10−8 cm W−1 for 596, and 2.6 × 10−8 cm W−1 for 597. A similar trend was observed for the values of Im[χ(3)], which were 9.0 × 10−12 esu for 595, 3.8 × 10−12 esu for 596, and 9.0 × 10−12 esu for 597. Apart from the influence of the particular meso-substituents on the nonlinear absorption, the mutual position of these substituents mainly contributed to determination of the NLO response. This became evident when comparing the L-shaped trimers, which exhibited stronger RSA with respect to linear trimers, probably due to the relatively poorer symmetry of their structures. Other authors prepared a multilayered electrostatically selfassembled material built by alternate deposition of tetrakis(4sulfonatophenyl)porphyrin diacid (H4TPPS2−) and poly(diallyldimethylammonium chloride) (PDDA). This material showed high nonlinear response and good photostability to ultrashort laser pulses. The samples exhibited strong nonlinearities with a large TPA coefficient β of 50 cm/GW. The origin of this large response was attributed to aggregation, which enhances the molecular TPA cross section of H4TPPS−2

from 30 to 1000 GM in water solution due to cooperative effects. In a 20 bilayer film, a further increase by a factor of 1.7 was observed and explained in terms of preferential alignment of J-aggregates in the multilayers.1338 Self-assembled Pors 598−603 (Chart 124) have been also employed to produce high third-order optical nonlinearities.1339 The femtosecond time-resolved optical Kerr effect (OKE) was measured at 800 nm. χ(3) reached values from 10−14 to 10−13 esu for compounds 601−603. Double-strand arrays of porphyrins 604−607 (Chart 125)1340 were also considered for TPA studies. The authors found that the TPA cross-section values increased faster with n for double-strand arrays than for the corresponding singlestrand polymers. The double-strand oligomers showed an extremely large TPA cross section (49 000 GM when n = 4 and 115 000 GM when n = 7). Supramolecular compounds 608−610 based on mesotetraphenylporphyrin (TPP), composed of [H2TPP]2+ cation and vanadium-substituted Keggin-type polyoxometalate (POM) anion [SW12−nVnO40](2+n)− (n = 0−2), were considered for OL studies (Figure 96).1341 Compounds [H2TPP]1.5[SW11VO40]·5CH3CN·4H2O (608), [H2TPP]2[SW10V2O40]·4CH3CN·3H2O (609), and [H2TPP][SW12O40]·4H2O (610) were prepared. The third-order NLO and OL properties of resulting hybrids were studied by using an Nd:YAG laser at 532 nm with pulses of 7 ns. As depicted in Figure 97, the OL profiles of compounds 608−610 represent typical NLO transmittance with a linear increase in output fluence followed by a decrease of the slope at higher fluences. For [H2TPP][ClO4]2, the deviation from 13149


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Chart 123. Examples of Several Dimeric and Trimeric Porphyrins Compounds

Chart 124. Porphyrin Self-Assemblies

(Chart 126)1313,1360 reveals an increase of electronic conjugation in going from 611a, to 611b and 611c (Figure 98, left). The experimental results indicated that complexes were NLO sensitive at 532 nm (Figure 98, right). Their excitedstate/ground-state absorption cross-sections ratio was estimated, and it was determined to be 4.8, 2.5, and 1.4 for 611a, 611b, and 611c, respectively. A lead bis(ethynyl)porphyrin polymer (612, Chart 126)1361 revealed strong nonlinear absorption and effective OL over a

linearity occurred at 0.093 J/cm2, whereas for compounds 608−610, it occurred at 0.069, 0.075, and 0.085 J/cm2, respectively. Moreover, the OL thresholds of compounds 608− 610 followed the trend: 610 (0.54 J/cm2) > 609 (0.37 J/cm2) > 608 (0.28 J/cm2). The effect of conjugation extension on the second-order NLO properties was studied also for monomeric, dimeric, and polymeric porphyrin structures embedded in a matrix.1342−1359 Analysis of the absorption spectra of compounds 611a−c 13150


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Chart 125. Porphyrin Double-Strand Arrays

Chart 126. Metalloporphyrin Systems

Figure 98. Left: Linear absorption spectra of thin films of the monomer (611a), dimer (611b), and polymer (611c) Pors at room temperature. Right: Nonlinear transmission measurements of 611a (●), 611b (■), and 611c (▲). Reproduced with permission from ref 1313. Copyright 1998 Elsevier.

Figure 96. Vanadium-substituted Keggin-type POM anions [SW12−nVnO40](2+n)− (n = 0−2) in polyhedral representation. (A) Color code: W/VO6 octahedra, green; SO4 tetrahedron, yellow; (B) divalent cations of porphyrin [H2TPP]2+ used in this study. Reproduced with permission from ref 1341. Copyright 2014 The Royal Society of Chemistry.

Figure 99. Broadband OL of compound 612 in solution. OL effect for (a) 75 fs and (b) 4 ns pulses. Solutions of the porphyrin polymer had a thickness of 1 mm. Reproduced with permission from ref 1361. Copyright 2009 The Optical Society of America. Figure 97. Optical limiting properties of compounds 608 (black), 609 (red), 610 (blue), and [H2TPP][ClO4]2 (violet) in DMF with the linear transmittance of 0.7 at 532 nm. Reproduced with permission from ref 1341 Copyright 2014 The Royal Society of Chemistry.

low input energies (> ∼50 nJ for fs pulses, > ∼1 μJ for ns pulses). For higher input energies, clamping of the output energies is observed (Figure 99, insets). The optical suppression was significant across the NIR and exceeded 60 times in the fs-pulse regime, and 25 times in the ns-pulse regime. A series of soluble poly(aryl ether ketone)s based on 5,10bis(4-hydroxyphenyl)-15,20-diphenylporphyrin, 4,4′-(hexafluoroisopropylidene) diphenol, and 4,4′-difluorobenzophe-

range ample about 500 nm in the NIR (ca. 1050−1600 nm) and for laser pulse widths spanning from 75 fs to 40 ns. As depicted in Figure 99, the linear transmittances of the 612 samples were quite high (TL ≥ 89% for all wavelengths). The attenuation due to nonlinear absorption began at reasonably 13151


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dimetallic bisporphyrin complexes (615a−c, Chart 128), in which a YbIII porphyrinate moiety was linked to a transitionmetal porphyrinate moiety by a flexible three-carbon chain. To study the OPL properties, intensity-dependent transmittance and Z-scan measurements were performed with a Q-switched Nd:YAG laser at a repetition rate of 10 Hz.1363 The laser frequency was doubled with an output wavelength of 532 nm and a 10 ns pulse width for the Gaussian mode by a frequency doubling crystal. The NLO measurements revealed that the OPL effects of porphyrins can be effectively enhanced by incorporation of lanthanide ions and covalent attachment of the electron-donating diphenylamino moieties to the parent porphyrinate ring. The OL thresholds at 532 nm for these lanthanide monoporphyrinate complexes range from 0.09 to 0.30 J cm−2 at 82% linear transmittance.1363 Upon excitation at 514 nm, the four ytterbium dyads gave NIR emission centered at 998 nm, with the emission intensity increasing in the order of 615a < 615b < 615c. These compounds showed large TPA cross-section values because of the interaction between the porphyrin units, which caused a loss of centrosymmetry.1364 Photophysical studies showed that the transition-metal porphyrinate moiety sensitized NIR emission, and enhanced

Chart 127. Poly(aryl ether ketone)-Based Porphyrin Copolymer

none were synthesized (613a−f, Chart 127).1362 Among these, 613b represents the porphyrin incorporated in the polymer with 5% porphyrin, 613c represents 10% porphyrin, 613d represents 15% porphyrin, 613e represents 20% porphyrin, and Chart 128. Cluster Metal Porphyrinate Complexes

Figure 100. OL response of a toluene solution of 616 irradiated with 5 ns pulses at 532 nm with frequency 20 Hz. The solution was inside a 1 mm cell with a linear transmittance of 75%. Reproduced with permission from ref 1365. Copyright 2003 The Royal Society of Chemistry.

the YbIII emission intensity and lifetime. TPA measurements at 800 nm showed that, except for 615a, the bisporphyrins had quite large TPA cross-section values, σ(2). The OPL measurements at 532 nm showed that the OPL property was enhanced in the order of 615b < 615a < 615c < 614a, and the OPL performance of 615c was comparable to that of C60.1364 The metal−metal bonded indium porphyrin-osmium cluster complex 616 (Chart 128)1365,1366 was synthesized and evaluated for OPL. Its NLO absorption is shown in Figure 100. The transmitted luminous energy deviated from Beer’s law as the input light fluence reached ca. 0.04 J/cm2, and the solution became less transparent upon increase of the incident fluence. The limiting threshold of 616 was found to be 0.4 J/ cm2 in toluene solution when irradiated by a 20 Hz repetitionrate laser pulse at 532 nm. OL has been also studied in different chromophore-doped aluminosilicate gel materials, and the effects of the dynamic processes from different central ions were discussed.1367 The chosen chromophores were metalloporphyrins of tetra-4-

613f represents 25% porphyrin. The materials were tested for OPL. The results of Z-scan and nonlinear transmittance measurements demonstrated that incorporation of the porphyrin chromophore into the main polymer chain improved the OL performance with the increase of percentage of attached porphyrin. The OL threshold followed the trend 613f > 613e > 613d > 613c > 613b, with 1.16 J/cm2 for 613b and 0.5 J/cm2 for 613f. Lanthanide porphyrinates are also complexes that have been recently the focus of some attention (Chart 128). A series of transition metal and lanthanide monoporphyrinate complexes based on 5,10,15,20-tetrakis[p-(diphenylamino)-phenyl]21H,23H-porphine (614a−c, Chart 128) have been synthesized,1363,1364 together with another series of d−f hetero13152


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nonlinear OL properties due to reverse saturable absorption combined with a high refractivity. Also, the combination of porphyrins with conducting polymers such as polyaniline (PANI) have been prepared by mixing the porphyrin derivative 4,4′,4″,4‴-(porphine5,10,15,20- tetraryl)tetrakis(benzoic acid) (PTBA) with the electrochemically formed conducting polymer polyaniline (PANI). Such combinations formed nanoparticles with uniform linear size of 80−100 nm in a DMF/water mixture.1370 The NLO absorption of this nanocomposite was measured at the excitation wavelength of 532 nm (7 ns laser pulses) using the open aperture Z-scan technique. The PTBA−PANI system showed enhanced OL effect as compared to its separated constituents PTBA and PANI. This might have been ascribed to the formation of aggregates of PTBA within the PANI− PTBA nanocomposite, which was also confirmed from AFM analysis. We can say that this tryout is a demonstration of the fact that under favorable conditions of nanoparticle formation, a simple procedure like mixing two media can produce a considerable variation of the OPL properties of a given chemical system toward the desired direction. The combination of porphyrins with carbon materials such as nanotubes is also a quite effective manner to produce OPL materials (Chart 130). For instance, SWNTs were functionalized with porphyrins [free base and chloroiron(III) mesotetraphenylporphyrins-TPP], by π−π interactions, as both SWNTs and porphyrins have π-electron rich structures.1371 Both samples displayed ultrafast nonlinear absorption, determined from open aperture Z-scan studies performed using mode locked Ti:sapphire fs pulses at 780 nm. The authors assumed the mechanisms of optical nonlinearity to be TPA along with nonlinear scattering and a small contribution from saturable absorption. The limiting threshold values of SWNTs-TPP and SWNTsFeTPP composites were 1.23 × 10−4 and 1.38 × 10−4 J/cm2, respectively. In both cases, limiting threshold was better than the available benchmark material such as SWCNTs-TPP, C60, and pure SWCNTs.1126,1372 Covalently functionalized carbon nanotubes with porphyrin derivatives naturally pursued. For instance, some authors prepared composites 618, 619, and 620 (Chart 130).1373−1375 OL was determined via Z-scan experiments with linearly polarized 5 ns pulses at 532 nm generated from a frequency doubled Q-switched Nd:YAG laser. OL thresholds of approximately 70, 100, and 150 mJ cm−2 were found for 618, 619, and 620, respectively. All of these values resulted much smaller than those of C60 (300 mJ cm−2) and SWNTs (250 mJ cm−2) in the same testing conditions of irradiation. Composites 621 and 622 (Chart 130) were synthesized by radical polymerization.1376 The Z-scan studies at 532 nm with 4 ns laser pulses revealed that these nanohybrids exhibited generically good NLO properties. Comparison between the OL effect of compounds 621 and 622 and that of TPP, MWNTs, and mixture of both was effectuated (Figure 101). All compounds showed that the output fluence increased linearly with increasing input fluence at low incident energies, and deviations from linearity with further increase in input fluence pointed to the occurrence of OL effects. The order of the diminution in output fluence was 621 > TPP > 622 > MWNTs, suggesting that the OL performance of 621 was superior to that of the 622 and to that of the individual porphyrin as well as MWNTs.

sulfonatophenylporphyrinato-copper(II) and tetra-4-sulfonatophenylporphyrinato-zinc(II) (CuTPPS and ZnTPPS, respectively), which were embedded in aluminosilicate gel hosts. Saturated behavior was found as a function of incident excitation intensity, and then the OL phenomenon was associated with RSA, these effects being attributed to higher excited-state absorption. A dynamic analysis of excited-state processes indicated that ISC and triplet−triplet absorption dominates nonlinear processes. This suggested that RSA was dependent on the ratio of the effective absorption cross section for the excited states to that for the ground state. Copper ions trap excited electrons in CuTPPS-doped gels. This phenomenon of electron trapping decreased polarizability and, consequently, led to weaker OL and quenching of upconversion emission as compared to ZnTPPS gels.1367 Mesostructured Pluronic P105/silica composites (Pluronic P105 is a difunctional block copolymer surfactant terminating in primary hydroxyl groups) were also considered suitable host matrixes for tetraphenylporphyrin (TPP) dye species in OL studies. Such combinations group many of the beneficial properties of sol−gel-derived silica glasses and organic polymer host systems.1368 As compared to the inclusion of porphyrin Chart 129. PMMA−Porphyrin Composite

derivatives in amorphous silica glass, the presence of the mesostructure-directing Pluronic P105 triblock copolymer species permits higher concentrations of dispersed TPP dye molecules into the structure of the resulting composites. This improves optical limiting properties. Furthermore, the optical damage resistance of the TPP dye species incorporated in the Pluronic P105 block copolymer−silica composites remains significantly better than that for organic host matrixes.1368 Radical transfer polymerization was utilized to prepare PMMA-porphyrin composite 617 (Chart 129)1369 with controlled molecular weights and narrow polydispersity. The method showed a potential to resolve the phenomenon of the irregular dispersibility and agglomeration of porphyrins. The third-order nonlinear optical properties were studied by Z-scan technique in the ps time scale. The nonlinear refracting index (η2) and the third-order nonlinear polarizability [χ(3)] were 4.1 × 10−10 and 1.64 × 10−10 esu, respectively. The obtained results implied that the prepared compound featured the large 13153


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Chart 130. Porphyrin−Nanotubes Composites

Two porphyrin-functionalized MWNT nanohybrids (623 and 624, Chart 130)1377 have been prepared through 1,3dipolar cycloaddition reactions, and the respective OPL properties of the materials were compared to those of zinc and free-base meso-tetraphenylporphyrins (ZnTPP and TPP). The nanohybrids showed improved OPL ability. At the highest input fluence used in the experiments (1.65 J cm−2), the output fluences were 0.90, 0.83, 0.75, 0.40, and 0.27 J cm−2 for TPP, ZnTPP, MWNTs, 623, and 624, respectively. This clearly indicated that 623 and 624 exhibited the best OL performance. Graphene, a very recent high flier in materials science, with an atomically thin, 2D structure that consists of sp2-hybridized carbons, is known for exhibiting remarkable electronic and mechanical properties.1378−1380 However, multifunctional hybrid materials that take advantage of both superior properties of sole graphene and a functionalizing material with OL properties have not yet been explored, due to its “young age”. Some authors reported the first covalently bonded and organic soluble graphene hybrid with an amino-substituted meso-tetraphenyl porphyrin (625a, Chart 131).1381 Beside the

Figure 101. OL responses (to 4 ns, 532 nm optical pulses) of MWNTs, 622, TPP, MWCNTs/TPP blend, and 621 in DMF. Reproduced with permission from ref 1376. Copyright 2014 John Wiley and Sons.

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Chart 131. Covalently Linked Graphene−Metal Porphyrin Composites

study of other characteristics, the OL properties of the Porderivatized graphene 625a were better than the benchmark C60. Photoinduced electron- and/or energy-transfer mechanisms played an important role in the determination of the OL performance, opening thus a new pathway in this area of materials chemistry. The NLO properties of covalently linked graphene-metal porphyrins 625b and 625c (Chart 131) were also reported.1382 Effective combination of the different OL mechanisms, such as nonlinear scattering, TPA, and energy transfer in the graphene−porphyrin composites, resulted in an improved OL effect for ns pulses at 532 nm with respect to benchmark C60. For instance, 625b and 625c displayed values for β = 4720 and 3570 cm/GW, respectively. These values were much higher than those found by the authors of the reference materials CuTPP, Zn-TPP, and pristine graphene presenting 132, 166, and 900 cm/GW, respectively. Similarly, the χ(3) values of graphene, 625b, and 625c measured with DFWM were found to be 4.2 × 10−12, 7.1 × 10−12, and 8.5 × 10−12 esu, respectively. In the hybrid systems, the existence of nonlinear scattering, arising from the graphene moiety, could largely increase the damage threshold of the nanocomposites, which were 1.7, 1.3, 0.8, 0.2, and 0.1 J/cm2 for Cu-TPP, Zn-TPP, graphene, 625b, and 625c, respectively. Two graphene oxide (GO)-based nanohybrid materials possessing covalent links to axially coordinated tetraphenylporphyrins (626 and 627, Chart 131) were prepared1383 and evaluated for their NLO properties using ns and ps Z-scan measurements at 532 nm. 626 and 627 were compared to GO, dihydroxotin(IV) tetraphenylporphyrin (SnTPP), and the phosphorus-cored tetraphenylporphyrin (PTPP). GO, SnTPP, 626, PTPP, and 627 exhibited considerable nonlinear absorption. As shown in Figure 102, at low input fluence, they showed a linear response of output fluence, obeying Beer’s law. However, as the incident fluence increased, the output fluence deviated from linearity, consistent with an OL response due to the negative variation of nonlinear transmission. It could be seen that the OL effect of 626 and 627 was more efficacious than that of the individual porphyrins SnTPP and PTPP, and nonfunctionalized GO. The OL performance at 532 nm for with 7 ns pulses of negatively charged gold nanoparticles or GO (acceptors) was

Figure 102. Comparison of the OL performance of GO, SnTPP, 626, PTPP, and 627 in DMF at 532 nm. Reproduced with permission from ref 1383. Copyright 2013 Elsevier.

significantly improved when they were mixed with watersoluble, positively charged porphyrin (donor) giving, respectively, the derivatives 628 (Chart 132) and 629 (Chart 133).1384 On the contrary, when a negatively charged porphyrin was added to the ionic dyads, no OL enhancement was found. This pointed to the existence of additional energy transfer pathways that are responsible for the deactivation of the excited state in the donor−acceptor complexes, as concluded by transient absorption analysis. The authors demonstrated that porphyrin−gold nanoparticle (628) and porphyrin−graphene oxide (629) ionic complexes displayed enhanced NLO properties as compared to the individual porphyrin, Au NPs, and GO, mainly due to nonlinear scattering and faster nonradiative decay. Porphyrinic related materials such as expanded pentaazadentate porphyrin metal complexes 630−633 (Chart 134) have 13155


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Chart 132. Gold Nanoparticle−Porphyrin Hybrid

Chart 134. Pentaazadentate Porphyrin Metal Complexes

Chart 133. Graphene−Porphyrin Hybrid

synthesis of pentaazadentate porphyrin-like complexes having generally good NLO properties. Pentaazadentate Pors exhibit high ISC rates and high yields of triplet excited states formation,1388 at 532 nm with 5 ns laser pulses. The electronic absorption spectra of these materials usually display a strong Q(0−0) band absorption at ∼760 nm, or even more bathochromically shifted, depending on the different numbers of πelectrons. Like other Pors compounds, the nature of central metal ions in pentaazadentate systems has a significant effect on the linear absorption. The variation of the nature of the benzene ring substituents also influences both linear and NLO absorption. At 532 nm, the limiting outputs of the complexes were influenced by the electron donor/acceptor properties of the substituent groups. Moreover, variations in the conjugated bridging group, R, also influence the linear and nonlinear absorptions. Compounds 630b, 631, and 632 (Chart 134) had similar linear transmission at 532 nm but showed different OL performance at the same wavelength, especially for 632. This was probably due to the strong electron-withdrawing ability of the −CN groups. Investigations have also been conducted on pentaazadentate porphyrin-like metal complex 633b (Chart 134).1389 The linear absorption spectrum of this complex revealed a strong Q-band centered at 779 nm and a B-band centered at 429 nm. The Qband of complex 633b was bathochromically shifted by ∼18 nm as compared to that of 630c, while the B-band remained practically at the same position. Consequently, the optical window, between the B-band and Q-band, in which reverse saturable absorption could occur, was wider for complex 633b. The electronic polarizability of 631 in the NLO regime is much higher than that of 630c, due to more extensive conjugation associated with the presence of the benzophenone group. The dynamics of the NLO response showed strong fluence dependence, which indicated the participation of excited states in the nonlinear process. Although 633b exhibited strong RSA for ns laser pulses, it showed a transformation from RSA to saturable absorption at high fluence for ps pulses.1389

been studied for their OL properties.1385−1387 These complexes contain two-dimensional delocalized electron systems and exhibit remarkable third-order NLO properties. The ring structures of these complexes can be readily modified so that a more extended conjugated system with 18−26 π-electrons is obtained. Furthermore, the large core size of the ligand provides a very stable coordination environment for large cations. This means that a larger number of metal ions are available for central coordination with respect to a normal Por while keeping a nearly coplanar configuration. These flexible structural modifications make possible the design and the 13156


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conjugated structure, given the permanent dipole moment and relatively high triplet state quantum yields of these complexes, further investment in the improvement of their photostability and dynamics of excited-state population could improve the OPL ability.

Chart 135. Corrole Derivatives for OPL

7. PHTHALOCYANINES AND RELATED MACROCYCLES 7.1. Advantages of Large π-Electron Macrocycles for OL

Phthalocyanines (Pcs), metallophthalocyanines (PcMs), and their naphthalocyanine (Nc) derivatives have been studied for many years in great detail, mostly for their use as dyes1393,1394 and as electro-, photocatalysts.1393,1395 Recently, the chemistry Table 12. Im[χ(3)] Values for Some Pc-Based Materials compound type SiNc thin film SiNc PMMA film ClInPc FAlPc PbPc NiPc CuTPP

In analogy with porphyrins and phthalocyanines, corroles1390 are now considered an independent class of molecules within Chart 136. Phosphorus Triazatetrabenzcorroles

Im[χ(3)], esu 5.0 8.9 1.5 5.0 2.0 7.0 1.0

× × × × × × ×

10−7 10−9 10−10 10−11 10−11 10−12 10−12

ref 1451 1452 1453 1453 1454 1455 1456

of Pcs and their higher homologues, that is, 1,2- and 3,4naphthalocyanines (1,2- and 2,3-Ncs), has undergone a renaissance, because Pcs and many of their derivatives exhibit interesting properties for applications in biochemistry,1271,1396−1403 materials science,1393−1395,1404−1439 and nonlinear optics.1440−1448 The principal reason for the existence of large optical nonlinearities in Pc-based systems is the presence of an extended network of conjugated π-electrons, the main characteristics of which are a high electrical polarizability and the ability to give rise to several transitions in the ultraviolet− visible (UV−vis) range.1449,1450 As a few examples, listed in Table 12 are comparative Im[χ(3)] values for some materials based on Pcs. From a quick look at Table 12, the main information we can extract is that the conjugation is mostly important on the Im[χ(3)] values. Next, several other conclusions will be drawn to find the molecular structures that are more effective for the definition of optical power limiters. A practical optical limiter must operate over the wide range of incident intensities that might be encountered. The nonlinear response should possess a low threshold and remain large over a wide range of fluences before the nonlinearity saturates. The latter phenomenon normally requires a high concentration of the nonlinear material interacting with the optical beam. This implies that the organic material must be highly soluble in common organic solvents, or be a pure liquid/solid film possessing good optical quality. Pcs used as nonlinear absorbers tend to aggregate at high concentrations. The intermolecular interactions caused by aggregation often result in the decay of the first excited triplet state with high absorption cross-section. The effect of molecular aggregation is particularly relevant when highly concentrated solutions of PcMXs (PcMX = metallophthalocyanine containing an axial ligand) or solid-state modifications are adopted for the determination of their NLO properties.1457−1459 In fact, the spectral properties of PcMX thin films differ from those of diluted solutions for the broadening of the absorption bands

the largest family of tetrapyrrolic compounds. Some of the authors of this Review have synthesized a set of corrole derivatives 634−637 (Chart 135) and investigated their NLO properties. It was generally observed a decrease of transmission with increasing incident fluence for all corroles under investigation.1391 This was indicative of the occurrence of RSA for all corroles 634−637 due to sequential two- or multiphoton absorption. The best performing corroles in terms of OL effect were compounds 635 and 636 because they reached the lowest values of Tmin at about 0.40. A phosphorus triazatetrabenzcorrole 638−640 (Chart 136) has also been prepared.1392 The OL properties have been investigated to examine whether the lower symmetry resulting from the direct pyrrole−pyrrole bond could induce a positive effect in the resulting limiting thresholds. The authors found that such compounds were more suitable for singlet oxygen applications than OL. Although their OPL performance was not as promising as many other materials with similar 13157


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and a slight red shift of the absorption peaks.1460−1462 This is expected as a consequence of aggregation in which intermolecular interactions are of the van der Waals

Chart 138. Low Aggregating Phthalocyanine Complexes

Chart 137. Some Substituted PcM Complexes for OL

type.25,122,125 Molecular aggregation manifests itself with an increase of χ(3) by increasing PcMX concentration, as experimentally determined by several authors.1463,1464 Extensive Pc aggregation needs to be suppressed by modification of the molecular shape to diminish van der Waals interactions between the large π-systems. In addition, the material must possess a high linear transmission and a large nonlinear absorption over a broad spectral bandwidth, as well as a high threshold for damage. Furthermore, the nonlinear absorption must appear with a subnanosecond response time. Meeting all of these criteria is a significant challenge in synthesis. Among the Pc-based nonlinear absorbers that have been studied, (βcumylphenoxy)4PcPb (641, Chart 137),1454,1465,1466 (tertbutyl)4-PcInCl (642, Chart 137),122,125,1455,1460,1461 and axially double substituted SiPc (643, Chart 137)1467 have been used as optical limiters that approach the characteristics necessary for a practical device.

viscous liquid, which displayed self-healing properties after irradiation and an aptitude to fill short path length optical cells by capillary action. Highly nonaggregated series of peripherally substituted phthalocyanines containing different metals (Al, Ga, In, and Zn) (644a−d and 645, Chart 138)1484 were prepared, and their nonlinear optical properties, including OL effect, were investigated in the CW excitation regime, using Z-scan measurements, with ns pulsed laser, carried out at 632.8 and 532 nm. The wavelength at which the Pc complex exhibited the strongest nonlinear effect was determined within the low absorption window region (420−590 nm) comprised between the Q- and B-bands. The authors found the σeff to be wavelength dependent with common double peaks at 505 nm and around 485 nm. The highest value of σeff was determined for 644d (1.1 × 10−16 cm2), having also the largest figure of merit. All compounds revealed high performance in OPL, with low thresholds, probably due to the low aggregation prevented by the presence of steric substituents. Nevertheless, the best performance offered by the indium complex was mainly to heavy metal effect (vide infra). The steric demand of cumbersome substituents definitely enhances the OPL properties of phthalocyanines. Two phthalocyanines enjoying this condition were synthesized (646 and 647, Chart 139).1485 The corresponding NLO properties were evaluated at the wavelengths 600, 640, 680, and 800 nm using picosecond (∼1.5 ps) pulses and femtosecond (∼140 fs) pulses at 800 and 780 nm and utilizing Z-scan experiments in both close- and open-aperture configurations. TPA and saturable absorption were the dominant nonlinear absorption mechanisms observed with ps/fs excitation at different wavelengths of the NIR-vis spectral region. Large TPA cross sections of ∼14 000 GM and η2 values in the range

7.2. Aggregation Issues

The intermolecular interactions caused by aggregation at high concentrations of the Pc can provoke the decay of the upper triplet state. This effect is especially relevant when highly concentrated solutions of Pcs or solid-state configurations are adopted to increase the nonlinear optical effects.1457−1459,1468 The spectral properties of Pc thin films are different from those of diluted solutions, for the broadening of the absorption bands and the possible small red-shifting of the absorption peaks.1461,1469−1476 Such results are expected as an outcome of aggregation in which intermolecular interactions are of the van der Waals type.1477 It was found that the optical susceptibility of Pcs increases with their concentration.1478−1480 To overcome aggregation issues, Snow synthesized a new substituted Pc possessing a low aggregation level,1481−1483 with the purpose of producing thin films for OL with a multifunctional Pc. The Pc opportunely derivatized by Snow for OL purposes was a 13158


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a clear example of the effect of molecular aggregation upon NLO properties.

Chart 139. Sterically Crowded PcZns

7.3. Effect of the Combination Central Atom-Axial Ligand

Inclusion of axial ligand X to a tri- or tetra-valent central atom M represents an additional tool for the fine modulation of NLO properties in the resulting PcMXs.1461 This is due to several effects associated with the presence of the entity MX, such as the introduction of a dipole moment perpendicularly oriented with respect to the Pc ring plane, which alters the electronic structure of the macrocycle,1488−1490 and the introduction of new steric effects modifying the packing properties of PcMXs.1460 The first study of the metal effect on RSA mechanism was provided by Gires, in 1966,1491 later followed by many other groups. Collected data122,125,127,1457,1460,1488,1492−1494 have shown that the presence of an axial ligand brings about a considerable improvement of the third-order Pc optical properties. This is especially Chart 140. PcMCls for OL

−16

true for the χ(3) values of PcMXs with MX = VO, InCl as compared to the χ ( 3 ) of non axially substituted PcMs.122,125,1460,1461 In these particular cases, the reason for such large variations is the central moieties such as VO and InCl, which possess the favorable combination of high dipole moments and paramagnetism as well as the presence of a central heavy atom.125,1460,1461,1492 These systems resulted in improved performance of PcMX-based optical absorbers due to the enhancement of the ISC crossing process through the socalled heavy atom and paramagnetic effects.731,1495 Other authors detected1496 opposite signs of nonlinear refraction in methanol solutions of chloroaluminum phthalocyanine (648, Chart 140), using the Z-scan technique with two output modes of a frequency-doubled, Q-switched, modelocked Nd:YAG laser (operation frequency: 10 Hz). With the single pulse switched out of a train of ps pulses, it was possible to observe positive nonlinear refraction of 648, which was attributed to the population of excited singlet states giving transitions. With the utilization of ns pulse trains, a sign change of nonlinear refraction relative to that of single ps-pulse excitation was observed. This was attributed to the heatinduced temperature rise and triplet state relaxation in 648. Thermally induced optical nonlinearity of 649 (Chart 140)1497 in ethanol was studied in the regime of low optical power using a CW He−Ne laser as source of excitation (Z-scan technique). The optical response was determined by measuring the intensity-dependent refractive index (η2), which was found to be 8.9 × 10−8 cm2/W, a reasonably large value. At low power, the energy output from the 649-based optical limiter varied linearly with the input, thus obeying Beer’s law. At higher power, the self-modulation effect was used to demonstrate aperture-limited OL with a threshold of 3 mW. Still, nowadays

−1

of (1−7) × 10 cm W were obtained for these molecules from ps Z-scan data. The largest σ2/β values were obtained at 800 nm for both of these molecules with 647 presenting a σ2 value about 4 times higher than that of 646. Pcs display fast optical nonlinearities with low response times (quasi-instantaneous response)1426 and possess small dielectric constants.125 Such general features render feasible the use of Pcs as active species in the development of OL devices for eye and sensor protection from hostile energetic light pulses.1486 In the following, the dependence of the OL properties on the structural features of Pcs is described with the aim of showing where possible which molecular features are associated with improved OL properties. The introduction and the variation of peripheral substituents in phthalocyanines can modify the spatial arrangement of the molecule and the extent of molecular interactions between neighboring molecules. Both the level of aggregation and the supramolecular structure of the aggregates are strongly dependent on the nature of peripheral substituents, the relevant NLO properties being heavily influenced by the level of aggregation. A study reported the effect of the variation of the peripheral substituents in a metal-free phthalocyanine with formula RxPcH2 with R = t-Bu and x = 4, in one case, and R = OCH2CF3 and x = 8, in the other case, on the NLO properties of their solutions.1487 The peripheral group OCH2CF3 was a good aggregation suppressor. By increasing the concentration of the phthalocyanines, the nonlinear absorption coefficient of (t-Bu)4PcH2 started to decrease when 0.5% in weight ratio, whereas the analogue substituted with (OCH2CF3) showed a nearly constant value of the nonlinear absorption coefficient within the same range of concentration. This study constituted 2

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Chart 141. Axially Substituted PcInXs

for such variations is due to the ability of the central moieties, such as VO, TiO, or InCl, to introduce dipole moments perpendicularly oriented to the Pc ring with values in the order of 10−15 D,1499 which alter the electronic structure of the macrocycle in both ground and excited states, and provoke new steric effects that modify the aggregation of PcMXs.1460,1505 One of the first systematic studies on the effect of the combination central atom-axial ligand in PcMXs on the resulting OL effect was reported by Shirk and our group in 1998 for the series of compounds (t-Bu)4PcInX1460 with X = Cl, Br, I, p-trifluoromethylphenyl (p-TMP), m-TMP, pfluorophenyl (p-FP), phenyl, and pentafluorophenyl (PFP) (649, Chart 140 and 650a−f, Chart 141).1461,1469,1470 Among various Pcs, the soluble and axially substituted RxPcInX displayed the largest nonlinear optical absorption coefficients. A comparison between 649 and 650b showed that these two species have similar linear optical spectra. However, in transient absorption studies, it was found that the ISC process rate of 650b was higher than that for 649, whereas the quantum yields for the formation of the absorbing triplet excited state were approximately the same for both 650b and 649.1460,1461,1469,1470 This could be ascribed to the stronger heavy-atom effect of the heavier iodine axial ligand, which accelerates the formation of triplet excited states in the nonlinear optical regime. On the other hand, the species (t-Bu)4PcInXs having alkyl groups as axial ligands resulted to be photochemically unstable when

Pc 649 is considered a state-of-the-art compound for OPL, following the work of Perry et al. on the remarkable OL properties of (t-Bu)4PcInCl.122 The OL properties of a large variety of Pcs differing in the nature of the central metal M and axial ligand X have been reported.1498 In this study, the comparison of the linear and NLO parameters of 40 different PcMs was presented. It was found the merit factor σ24/σ13 varied in the approximate range 1−30 when the analyzed species had different values of concentration in solution. The largest values of κ were found for the complexes with M = In, Zn, Ga, H2, Pd, and Pb, whereas nickel and cobalt phthalocyanine complexes displayed the lowest values as determined with the Z-scan technique in the open aperture configuration.158 From the analysis of the OL trends in terms of Pcs molecular parameters, it became clear that no single molecular parameter could be used in an exclusive manner to describe quantitatively the efficacy of an optical limiter based on photoactive Pcs.1498 This was proved, for example, by the absence of a clear dependence of the merit coefficient κ on the atomic mass of the central atom M or the number of Pc rings per molecule. On the other hand, κ displayed a more regular dependence on the absorption coefficient of the ground state (α0) of Pc at the wavelength of detection of NLO effects. In fact, it was a linear relationship between log κ and log α0 was found. Such an experimental relationship can be used for establishing new design rules in terms of molecular engineering, and for the further development of molecules for OL.1498 An important outcome from this compared evaluation is the idea of combining Pcs with different NLO properties in multilayered configurations. Such configurations are made of layers of Pcs having different values of saturation fluences, which can reduce about 80% the overall transmitted intensity with respect to a homogeneous single layered structure. The reason for such an improvement is related to the specificity of the response of every layer to laser sources with agile features.1036,1498 From the comparison of OL responses of so many different Pcs under the same conditions of irradiation and characterization, it is generally concluded that structural factors influence indeed the OL properties of Pcs but not in a straightforward manner. In fact, the variation of the central atom can introduce significant changes in the NLO behavior as far as OL is concerned.1499−1504 This is especially true in the case of PcMXs with MX = VO, TiO, or InCl.122,1284,1461,1469,1470 The reason

Figure 103. Nonlinear absorption coefficients as a function of wavelength for Pcs 649 (■), 650c (▲), and 650e (○). Reproduced with permission from ref 1460. Copyright 2000 American Chemical Society. 13160


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exposed to both intense light and daylight. These facts led to the conclusion that the In−C bond is more stable against irradiation when the C atom is sp2 hybridized. The complexes 650c and 650e displayed higher solubility than (t-Bu)4PcInCl (649), and exhibited higher nonlinear absorption coefficients, lower limiting thresholds, and lower optical transmissions at high fluences when compared to 649.122,1460 For example, at 532 nm, solutions of 650c and 649 with the same linear transmittance (=80%) showed a substantially different behavior in terms of limiting threshold being the latter parameter twice lower for 650c with respect to 649 (Figure 103). The photophysical properties of 649, 650c, and 650e have been studied in the NLO regime with nanosecond light pulses.1460,1461,1469,1470 In this analysis, it was found that excitation at around 500 nm produces initially excited singlet states, which are converted into excited triplet states within a time lapse of about 300 ps.1460 Such a fast conversion provoked by the process of ISC indicates that the excited triplet states are the main responsible for the absorption in the nonlinear optical regime. In addition to that, the lifetime of the absorbing triplet excited state was relatively longer than the duration of the light pulses in the range 1−10 ns. Consequently, these materials resulted in effective optical limiters with a photodynamic behavior that is fluence dependent. In the approximate wavelengths range 400− 600 nm, 649, 650c, and 650e behave as reverse saturable absorbers. The differences could be associated with the higher electronic polarizability of the axial aryl ligand with respect to chlorine atom.1469,1470 Other facts were mostly explained in terms of reduced aggregation in the highly concentrated solutions used for the determination of OL properties, thus reducing the deleterious effects associated with molecular aggregation. This mostly induces a faster decay of the excited absorbing states.1460 The diminution of molecular aggregation in phthalocyaninatoindium(III) complexes with axial aryl groups could be verified by showing the poorer dependence of the molar extinction coefficient at the different wavelengths

Chart 142. Axially Substituted PcMXs (M = Mn, In) for OPL

and triplet state lifetimes were not significantly affected by the nature of the axial ligand, remarkable differences in the variation of nonlinear transmittance were observed for the different 651−652 complexes due to significantly different ISC rates (Figure 104). Heavier axial ligands in phthalocyanines 651c and 651b produced the largest variations of nonlinear transmission due to heavy-atom effect and displayed also the best limiting threshold [quite better than the state of the art 2,(3)-tetra-tertbutyl-phthalocyaninato indium chloride (649)]. Metals with lower capacity to induce fluorescence decay, such as Mn, have also been used as central moiety in phthalocyanines. Pcs 653a,b (Chart 142)1508 were prepared, and the third-order NLO properties of 653a and 653b in toluene solution were studied with Z-scan technique, giving Im[χ(3) ] = 1.44 × 10−11 and 2.27 × 10−11 esu, respectively. Compounds with the formula PcTiXn (654a−e, Chart 142) in

Figure 104. Nonlinear transmittance at 532 nm, T532, for PcInXs excited at 532 nm with ns pulses. Reproduced with permission from ref 1506. Copyright 2008 American Chemical Society.

Figure 105. Nonlinear transmission of compounds 654b−e. Reproduced with permission from ref 1470 Copyright 2001 John Wiley and Sons.

of these molecules with respect to (t-Bu)4PcInCl (649) upon variation of the complex concentration in the linear optical regime. Other axially substituted indium phthalocyanines, 651a−c and 652a and 652b (Chart 141), have been investigated, and their OL properties with nanosecond light pulses were evaluated.1506,1507 All complexes behaved as reverse saturable absorbers in the range of 400−625 nm due to a triplet−triplet excited-state transition. While ESA cross sections

which the central Ti atom possesses a +4 oxidation state, offer the possibility of binding two different ligands, for example, Cl or O atoms belonging to (thio)catechol, or O and S as single ligands, due to the bivalent nature of Ti(IV) coordinated inside the cavity of the Pc macrocycle.1469,1470,1509−1511 Our approach was the study of the NLO properties in PcTiXs as a function of the substituent in the axial aromatic ligand starting from PcTiO and axially substituted systems like (catecholato)-2,(3)-(tetra13161


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tert-butylphthalocyaninato)titanium(IV) and analogues (654a− e). The molecules of the type 654 with an electron-withdrawing group as substituent of the axial ligand showed the better OL response (Figure 105).1469,1470 The large functionalized catechol ligands introduce steric hindrance and reduce the tendency to form aggregates. Electron-withdrawing groups like CN, CHO, CH2CN, and Br in the axial catechol originate a dipole moment perpendicular to the macrocycle. The four complexes named above were considered as model complexes for the investigation of their OL performance. It was found the higher was the dipole moment, the better was the OL effect. In fact, the best performance in terms of OL was achieved with 654d, followed by the 654e, and 654b. The OL performance of these materials at 532 nm varies markedly with the different electron-withdrawing character of the substituents at the aromatic axial ligand. In ranging from compound 654c to compound 654e (Figure 105), the electron-withdrawing character of the axial ligand increases, and a corresponding lowering of the limit transmission is observed. Such correlation is not trivial because the changes of limit transmission in

Chart 144. Peripherally Substituted Sulfonic Phthalocyanines

Chart 143. Some Axially Substituted Pcs for OPL

enhanced in comparison to the chloro analogue 656b, showing that the usual tendency of Pcs to form aggregates can be effectively suppressed by axial substitution. The NLO behavior for 656b and 656c at 532 nm in toluene indicated that they behave as reverse saturable absorbers. The presence of p-TMP as axial ligand in 656c is associated with the enhancement of the OL response with respect to 656b.

PcTiXs solutions are mainly caused by the electronic effects associated with the dipole moment of the axial ligand. In these cases, the steric effects associated with the different catechol based axial ligands do not play a relevant role in determining the OL performance due to the similar sizes and the similar packing properties of the different PcTiXs here considered. Such results represent the first demonstration that substituents with electron-withdrawing character at the axial ligand improve the OL performance of Pcs in an analogous fashion of electronwithdrawing substituents attached to the conjugated Pc ring. The comparison of the OL effect produced by the paramagnetic species nitrido(2,3-octa-n-pentylphthalocyaninato)-tungsten(V) (655, Chart 143) and nitrido(tetra-tertbutylphthalocyaninato)-rhenium(V) (656a, Chart 143) shows that the tungsten complex with paramagnetic properties has a better NLO performance.1512 This is ascribed to an acceleration of the ISC process, which allows a faster successive absorption of the second photon. The presence of electronically polarizable axial ligands generally improves the NLO properties of the resulting molecule in terms of OL effectiveness.1513 The soluble species 656b−c were also synthesized for OL studies.1514 These two materials have similar linear absorption spectra. However, the solubility of 656c in organic solvents was

7.4. Effect of Peripheral Substitution in Phthalocyanines

The OL effect produced by tetrasulfonated derivatives 657a−c (Chart 144)1515 was investigated with ns pulses at the wavelength of 532 nm. Pc 657a exhibited the best OL performance, which resulted from strong RSA and nonlinear refraction as a consequence of the presence of highly

Figure 106. Variation of the normalized transmission of Pcs 658a−c at 532 nm (cuvette thickness: 2 mm). Sample was excited with 9 ns pulses. Linear transmittance was 0.75 for all three solutions. Reproduced with permission from ref 1516. Copyright 2010 Elsevier. 13162


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Chart 145. Tetrasubstituted Phthalocyanines for OPL

Figure 107. Plots of normalized transmission against energy density for compounds 661a−e. Reproduced with permission from ref 1518. Copyright 2003 The Royal Society of Chemistry.

polarizable 5d-electrons in the valence orbitals of the central atom. The nonlinear transmission (NLT) of ns pulses at 532 nm was reported for the sulfonamide substituted Pcs 658a−c.1516 All three phthalocyanines behaved as reverse saturable absorbers with increasing efficiency of OL in the order 658a < 658b < 658c (Figure 106).1516 From the experimental data, the best performing material, 658c, presented σexc = 9.6 × 10−17 cm2 and the figure of merit σexc/σ0 = 8. Tetrasubstituted Pcs 659 and 660 (Chart 145), bearing aryl and alkyl substituents,1517 respectively, with different central metals and axial chloride ligands, have been studied for their OL properties using nanosecond pulses. It was shown that all of the compounds exhibit good OL behavior, and Pcs with heavier central metals displayed better performance due to the faster ISC caused by enhanced spin−orbit coupling. The OL properties of a series of metalated and unmetalated 1,4-octaalkyl or octaalkoxy-substituted phthalocyanines have been also investigated.1518−1520 The substitution pattern of these materials inferred a beneficial effect concerning the aggregation aspect, due to a partial ring distortion caused by the 1,4 -substitution pattern. All compounds have shown RSA at the wavelength of 532 nm. For instance, from the OPL evaluation of phthalocyanines 661a−e (Chart 146),1518 a representative OL plot is presented in Figure 107. A subset of the hexyl-substituted compounds (661a, 661b, 661c, 661d, and 661e) was case studied. The OL of the Pd phthalocyanine 661d, which exhibits the largest β value of 9.6 × 10−8 cm W−1, was the best performing compound. In the case of phthalocyanines 662a−e (Chart 146),1519,1520 bearing octadecyloxy substituents, their NLO measurements revealed that the Pd compound (662a) was the best performer, probably due to the heavy atom effect. Besides Pcs with 1,4-substitution patterns, Pcs bearing substituents at 2,3- positions have also shown important OPL properties. For instance, the OPL performance of compounds

663−673b (Chart 147),1521−1523 bearing differentiated groups, was investigated by using ns pulses at 532 nm. The experimental results have shown that the OL capabilities in the two kinds of substituted Pcs were not very much dependent on the lengths of the side chains, and presented similar behavior in terms of OPL. In addition,1522,1523 compound 669 with Pb as central metal (Chart 147) demonstrated a very low limiting threshold of 27 mJ/cm2 with linear transmittance of 65%. The same authors studied also the NLO transmission in the irradiation condition approaching the saturation of the first excited triplet state population at different linear transmissions of 65%, 76%, and 87%. For an optical path of 5 mm, the transmitted fluences were clamped, respectively, at 50, 100, and 150 mJ/cm2 when the incident fluence reached 2.2 J/cm2 (Figure 108). Other octa-substituted metallophthalocyanines 670a,b (Chart 147)1524 were studied for nonlinear absorption and refraction properties, in the picosecond regime using Z-scan technique at 1064 nm. While TPA seemed to be the dominant nonlinear mechanism for 670a, the process causing the optical nonlinearity of 670b was ESA. As consequence of that, the ESA cross section was estimated to be 1.4 × 10−18 cm2. Attempts to establish the influence of peripheral and nonperipheral substitution pattern on the OL properties were carried out using carbohydrate tetra-substituted Pcs 671 and 672 (Chart 147) using ns laser pulses at 532 nm.1525 When compared to nonsubstituted PcZn, both compounds presented a much lower OL threshold. Optically active phthalocyanines 673a,b (Chart 147)1526,1527 were also tested for OPL at 532 nm with 10 ns pulses utilizing the Z-scan technique, when 673a and b were either dispersed in liquid solution or embedded in epoxy resin thin plates. All samples possessed σexc values larger than σ0, which is indicative of RSA capability. The authors found that incorporation of Cu in the phthalocyanine cavity (673b) had a

Chart 146. 1,4-Substituted Phthalocyanines

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Chart 147. Peripherally Substituted Phthalocyanines

Chart 148. Phthalocyanine Derivatives for OPL

Figure 108. OL results (for compound 669) for the optical path length of 5 mm with different concentrations corresponding to different linear transmissions of 65%, 76%, and 87%, respectively. Reproduced with permission from ref 1522. Copyright 2001 Elsevier.

positive effect on the OL performance, reaching a low limit threshold of 0.3 J/cm2, at the quite high linear transmittance of 76%. They also found that the thin films of the Pcs had slightly better performance than the chloroform solutions. Several other examples appeared in the literature regarding the optical limiting measurements of different types of Pcs, which, almost exclusively, undergo RSA upon irradiation with pulsed lasers, as a result of multiphotonic absorption.1528−1533 Most of the times, the substitution pattern demonstrated its importance, mostly inducing specific electronic effects1530,1531 or acting as aggregation preventer.1532,1533 Frequently, the effect of metalation produces materials with enhanced OPL performance, mostly due to the “so-called” heavy atom effect.1529,1531

The induction of electronic effects and their influence on the NLO properties is another important aspect related to the presence of peripheral substituents on phthalocyanines. The comparison of the OL behavior at 532 nm with 10 ns laser pulses for the series of compounds 674−677 (Chart 148) proved that the species possessing electron-withdrawing groups as peripheral substituents displayed a more effective OL.1534 In fact, compounds 675 and 676 displayed the lowest values of optical transmittance at high levels of irradiation.1534 Other 13164


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Chart 149. Double and Poly-Decked Lanthanide Phthalocyanines

materials were tested in the same perspective, to establish a direct correlation between these electronic factors.1535−1539 These experimental facts proved that the alteration of the electronic charge distribution in the Pc ring provoked by ring substituents with strong electronic properties affected markedly the absorption properties of the complex in the excited state.1540 It is expected that ring substituents with electronwithdrawing properties produce larger variations of the transition dipole moment in correspondence of the electronic transition responsible for the OL effect.1541 This would result in a net increase of the excited-state absorption cross-section and an increase of the merit factor κ because the ground-state absorption cross section is not as affected as the excited-state cross-section by the presence of these substituents. 7.5. Dimeric and Oligomeric Phthalocyanines

Figure 109. Output−input characteristics of 678b (○) and 678a (●) in chloroform solutions of 74% linear transmittance at 532 nm. Reproduced with permission from ref 1551. Copyright 2001 Springer Science and Business Media.

The NLO properties of dimeric Pcs constituted by sandwiched bis(phthalocyaninato)lanthanides have been studied and analyzed by several groups.1542−1549 It is generally found that the spectral region of effective RSA is red-shifted with respect to the corresponding monomeric Pc.1542 Moreover, modifications of the energy levels scheme involved in the NLO processes are usually taken into account due to the existence of cofacial interactions between the two Pc rings.1550 The main structural limitation associated with the use of lanthanide Pcs is the impossibility of varying the electronic properties of the sandwiched coordinating atom through, for example, axial substitution, due to the fixed valence of the lanthanides and their large size that impede π−π stacked interaction.

The OL properties of sandwich-type lanthanides diphthalocyanine were investigated at 532 and 1064 nm.1551,1552 In one of these studies, it was demonstrated that 678a (Chart 149) exhibited better OL properties than 678b at 532 nm,1551 with limiting thresholds of 1.2 and 1.8 J/cm2 for 678a and 678b, respectively (Figure 109). Compounds 679a,b (Chart 149)1553 were investigated by measurements of fluence-dependent transmittance using 10 ns pulses produced by a Nd:YAG laser at the second harmonic of 532 nm. The results demonstrated that the 679b exhibited better OPL performance than its congener Gd. This was attributed to the smaller ionic radius of Gd, shortening the ringto-ring separation, thus enforcing the intramolecular π−π 13165


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means of the harmonic oscillator model of aromaticity index.1566 It was found that the two condensed Pc rings show almost independent behavior in terms of π-electrons delocalization, and no mesomeric structures of the dimers including both rings could be drawn. This was in agreement with the experimental spectral features of the binuclear complexes when compared to the spectrum of the mononuclear species. The absence of any electronic delocalization implied that this dimeric complex was equivalent to a species with a double concentration of Pc ring and central moieties per single molecular unit with no lateral interaction. The OL properties were consistent with the fact that the binuclear Pcs behave as reverse saturable absorbers, having less effectiveness with respect to tBu4PcInCl partially due to the higher linear absorption of the ground state.1565,1566 This was apparently due to a greater propensity for near planar dimeric structures to aggregate in solution. The greater degree of aggregation may well have also led to a shortening of the excited-state lifetime relative to the mononuclear tBu4PcInCl. It was concluded that enlargement of a π-electron system by itself was not sufficient to enhance the OL properties of a Pc. This was because the variations in solubility and excited-state dynamics resulted in a less efficient mechanism of excited-state formation when passing from mononuclear to binuclear fused rings. Among trimeric Pcs, a linear phthalocyaninatoindium(III) acetylacetonate trimer1507 and triazine centered trimeric Zn and CuPc complexes1567 are the only examples whose OL behavior has been investigated behaving in any case as reverse saturable absorbers. The OL effect at 532 nm generated by the peripherally bridged dimeric species (t-Bu)3PcMX(t-Bu)3PcM with X = alkynyl (−CC−), and M = Co, Zn (687a−c, Chart 150), has been also studied.1568,1569 The species 687a displayed the highest merit factor among the various peripherally alkynyl bridged Pc dimers σexc/σ0 = 11. Moreover, the comparison between the OL performance of 687a and its starting monomer demonstrated that the dimerization leads to molecular systems with a larger merit factor. Plots of normalized transmission against incident pulse energy density for the butadiynyl-bridged bisphthalocyanines 687a,b are presented in Figure 111, where again the solid lines are the theoretical curve fits. It is very interesting that in this case the cobalt dimer 687b exhibits a much larger nonlinear absorption than the zinc dimer 687a. The trimers formed by μ-oxo bridges are another type of phthalocyanine derivatives that can be used for optical limiting purposes. Compounds 688, 689, and 690 (Chart 151) were prepared, and their nonlinear optical properties and OL effects were investigated at 532 nm.1570 To do so, the relationships between the incident light intensity (Iin) and the transmitted intensity (Iout) were investigated at 532 nm and at the excitation wavelength corresponding to the Q-band of the oligomeric Pc (720, 670, and 660 nm for the dimer, trimer, and tetramer, respectively). The ratio of β(Q‑band) to β(532) increased with the number of phthalocyanine units (0.74, 1.3, and 2.2 for dimer, trimer, and tetramer). This tendency was consistent with the transient absorbance ratio of the excited state Q-band to 532 nm, which increased due to oligomerization. Therefore, the OL effects mainly originated from the T−T absorption bands. A conjugated phthalocyanine pentamer 691 (Chart 152)1571 was studied for OPL performance using 10 ns pulses at 532 nm. By adding methyl iodide to chloroform, this solvent mixture was revealed to be crucial for the enhancement of the OPL properties based on triplet state absorption. Such an enhance-

interaction. Furthermore, the nonlinear absorption properties of double-decker phthalocyanine rare earth complex 679c

Figure 110. NLO absorptive property of 680 in toluene solution under an open-aperture configuration. Reproduced with permission from ref 1555. Copyright 2013 The Royal Society of Chemistry.

(Chart 152)1554 were analyzed by performing Z-scan measurements with femtosecond pulses at 800 nm, which were radially polarized. The observed TPA process originating from strong intramolecular π−π interaction yielded a large TPA cross section of 2.54 × 10−45 cm4/s photon. A sextuple-decker complex (680, Chart 149)1555 with six phthalocyanine ligands bridged by two samarium(III) and three cadmium(II) ions, representing the largest stacked tetrapyrrole metal molecule in the sandwich series in terms of the tetrapyrrole-deck number, has been synthesized. The investigation of the NLO properties revealed the excellent OL properties of this compound that displayed a limiting threshold of 0.37 J cm−2 for 680. As shown in Figure 110,1555 compound 680 in toluene under an open-aperture configuration displays reverse saturation absorption, attributed to the decrease of the linear transmittance along with the increase of the concentration, which in turn resulted in an enlargement of the range of light intensity values at which nonlinear absorption occurred. A less studied group of dimeric Pcs for nonlinear optics is constituted by cofacially stacked Pcs with atoms other than lanthanides. To our knowledge, the only reported example of this kind is represented by Pc2Ti whose third-order NLO properties were determined at 800 nm.1556 Among dimeric Pcs, the NLO properties of μ-oxo bridged Pc dimers with the general structure Pc(X)M−L−M(X)Pc (681, 682, and 683, Chart 150) have been studied at 532 nm.1557−1561 The larger OL effect generated by the dimers of type 681 with respect to the other type of dimers 682 and 683 indicates the sharing of the common axial ligand reduces the excited absorption cross-section of the lower triplet excited state, which is responsible for the oserved NLO absorption.1470,1560 Successively the dimers with a direct M−M bond (684, Chart 154) displayed OL properties with improved features with respect to the single Pc ring coordinated by a single metal atom.1498,1562−1564 Similarly, the Pc dimer with central Ti atoms bridged by tetrahydroxy-p-benzenoquinone (685, Chart 150) also displays an OL effect with improved characteristics if compared to the parent monomer. Such findings are explained in terms of the higher number of active Pc rings per singular molecular unit.1470 Fused phthalocyanine dimers were also studied for OL purposes.1565,1566 The theoretical evaluation of the extent of electronic delocalization and aromaticity in axially substituted binuclear Pc (686a,b, Chart 150) has been accomplished by 13166


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Chart 150. Dimeric Phthalocyanines

aggregation issues, the incorporation of other organic or inorganic moieties has been a field with an important progress. Suppression of aggregation of Pcs has been tackled by using dendritic phthalocyanines.1572 For instance, a low aggregating type of phthalocyanines was prepared (692a−c, Chart 153),1573 and nonlinear absorptions of the compounds in chloroform solution were investigated

ment was due to the increase of the ISC efficiency through the external heavy atom effect. 7.6. Phthalocyanine Hybrids

The conjugation of phthalocyanines with other materials, which might function as property enhancers, is a topic of wide interest in the more recent history of Pcs for OL and NLO applications. Whether by combination of electronic effects or by suppressing 13167


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Chart 152. Phthalocyanine Pentamer for OPL

Figure 111. Plots of normalized transmission against incident pulse energy density for butadiynyl-bridged bisphthalocyanines 687a and 687b. The solid lines are theoretical curve fits. Reproduced with permission from ref 1568. Copyright 2003 The Royal Society of Chemistry.

Chart 153. Polysilsesquioxane Phthalocyanine Derivatives for OPL

with Z-scan technique (pulse duration, 4 ns; wavelength of irradiation, 532 nm). While PcCu showed very high nonlinear absorption, PcMs 692a and 692c did not show considerable nonlinear absorption, following the order 692c < 692a < 692b (Figure 112). Because of this reason, only the Cu complex could be considered for OPL. One of the first studies on the OL effect generated by Pcs embedded in a sol−gel host (solid-state configuration)1574 reported the results obtained with Sn, Ge, and Cu phthalocyanines with a PcSnX2 (X was not defined by the authors) showing the best OL performance.1575 An important achievement was the realization of solid-state limiters having properties similar to those shown by optical limiters based on Chart 151. Phthalocyanine Oligomers

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of 1.8 J/cm2 for 693a and 693b, 1.9 J/cm2 for 693c, and 2 J/ cm2 for the undoped copolymer were found. Germanium and tin phthalocyanines containing peripheral and axial different substituents (694a−c and 695a−c, Chart 154)1593 were synthesized, and thin multilayer films of poly(methyl methacrylate) (PMMA) and poly(vinyl chloride) Chart 155. Gel Incorporated Phthalocyanine Derivative

Figure 112. Typical open-aperture Z-scan spectra with normalized transmittance plotted as a function of sample position z for 692a−c. Reproduced with permission from ref 1573. Copyright 2008 The Royal Society of Chemistry.

phthalocyanine solutions. Later, the OL effect generated by Pcs embedded in polymers, such as poly(methyl methacrylate), was also studied in large detail by several groups.1576−1588 The spectral properties of PcMX thin films are different from those of diluted solutions, for the broadening of the absorption bands and the possible shifting of the absorption peaks. Such results are expected as an outcome of aggregation in which intermolecular interactions are of the van der Waals type. Later, femtosecond degenerate pump−probe spectroscopy was carried out on thin films of a series of axially substituted phthalocyanines,1589 bearing gallium and indium as central metals, demonstrating that the long-lived state of the dynamics was the triplet state and the behavior of the degenerate states. Pump−probe was determined by exciton−exciton annihilation, nonradiative recombination, and ISC processes, concluding that indium-contained samples had a larger ISC rate than the others, due to the heavy metal effect. This type of measurements was also performed for thermally evaporated polycrystalline unsubstituted vanadyl and lead phthalocyanine films.1590 The influence of local heating on the transient spectra was observed. Exciton−exciton annihilation with a time-dependent rate was observed in both films. This was explained by one-dimensional diffusion-limited annihilation. The OL effect on ns pulses at 532 nm produced by rare-earth metallo-phthalocyanines (693a−d, Chart 154), which doped a copolymeric matrix of poly(methyl methacrylate) and methyl2-cyanoacrylate, has been studied.1591,1592 The response of the material was attributed to RSA due to ESA. Damage thresholds

Figure 113. Comparison of the optical limiting effects of A1 sample with 1.80 mm thickness (■), A2 sample with 2.24 mm thickness (▲), A3 sample with 2.06 mm thickness (◆), and A3 sample with 1.39 mm thickness (●). Reproduced with permission from ref 1595. Copyright 2000 Elsevier.

(PVC) containing monomolecular phthalocyanines were obtained by spin-coating on glass substrates. The OL properties of the phthalocyanines in solution and in polymer films were measured by the open aperture Z-scan technique, using 6 ns pulses from a Q-switched Nd:YAG laser at 532 nm (pulse repetition rate: 10 Hz). The OL properties of these Pcs dispersed in multilayer films of PMMA and PVC were generally

Chart 154. Phthalocyanine Derivatives Used for Thin Films

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Chart 156. Phthalocyanine Derivatives for Gel Glass Fabrication

ties were investigated to analyze the effect of triplet−triplet annihilation of the phthalocyanine both in a solid matrix and in a tetrahydrofuran solution at different concentrations. Because of the rigid substituents present on the phthalocyanine, a lower degree of aggregation resulted in much lower probability of the triplet−triplet annihilation process to occur. However, in solution, the first-order exponential decay rate constants were larger than those in the solid matrix, and the percentages of triplet−triplet annihilation in THF solutions were much higher (7.74−50.9%) than those in solid matrixes (1.05−6.51%).

better when compared to the same properties in solution or in the single polymer film. Several other examples of phthalocyanine-polymer thin films have been reported, and effects of saturable absorption and RSA were found and analyzed. 1594,1595 For instance, phthalocyanine 696 (Chart 155),1595 incorporated in gel, was studied for OL. Figure 113 shows the NLO transmission data for samples having different film thickness: sample A1 with thickness 1.80 mm; sample A2 with thickness 2.24 mm; and sample A3 with thicknesses 2.06 and 1.39 mm. The linear transmission values at 532 nm were 16%, 49%, 63%, and 73% for these four samples. Upon increase of the incident optical power, the transmittance of the samples reduced gradually (Figure 113). The expected change of transmittance occurred between 0.2 and 1.7 J/cm2 of incident fluence. Silica gel glasses are also suitable matrixes for embedding phthalocyanines, to produce NLO films of generally good quality.1596−1599 For example, the comparison of the OL effect at 532 nm for 8 ns pulses, which was obtained with several palladium phthalocyanines (697a−c, Chart 156) encapsulated in silica gel glass,1596 revealed that, although all PcPds had better OL effect as composites rather than in solution, 697c showed the best OL performance due to its larger peripheral substituent. Using similar conditions, the OL behaviors of the composites 698a−d and 699 (Chart 156) were also tested for OPL,1597 with 698c displaying the best OL performance, due to the heavy atom effect. Compound 700 (Chart 156),1598 added as a dopant in an ormosil gel glass, demonstrated improved selfdiffraction under CW Ar+ laser irradiation. The largest diffraction pattern was obtained at the wavelength of 457.9, when compared to measurements carried out at 488.0 and 514.5 nm. Similarly, the OL effect was more pronounced at the same optimized wavelength. A novel solid matrix doped with 701 (Chart 156) was also prepared,1599 and photophysical proper-

Chart 157. Metallophthalocyanine−Fullerene Adduct

In the attempt of enhancing the OL properties of materials, the formation of charge-transfer complexes including phthalocyanines has been also considered.25,183,1600,1601 A great enhancement of the NLO properties has been predicted and successively verified when several components in solutions or solid films were mixed to form intermolecular charge-transfer complexes.183 One example is given by C60, which is an electron-deficient three-dimensional cage. In connection with an electron-rich molecule, such as a phthalocyanine, intramolecular or intermolecular charge-transfer can be easily obtained. As a result,1600 a study has shown that in a highly concentrated solution, a Diels−Alder complex 13170


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of PcCu-C60 (702, Chart 157) possesses optical limiting behavior similar to that of its precursor copper phthalocyanine (PcCu), while at lower concentrations, PcCu shows optical limiting similar to that of C60 when input fluence is 200 mJ/ cm2. At higher input fluence (>200 mJ/cm2), the OL effect is similar to that of substituted PcCu. This was an interesting result, because it showed that the enhanced OL effect of 702 derives from the synergy between PcCu and C60, located in two distinct parts of the same molecule. This demonstrated that NLO properties such as OL can be robustly enhanced by the formation of an intramolecular charge-transfer complex.1600 Further linking of C60 derivatives to phthalocyanines was effectuated by some authors.1602,1603 Investigation on a copper phthalocyanine-fullerene dyad using ns pulses at 532 nm resulted in an enhancement on the OL performance of the nanoparticle sample as compared to that of the solution sample. The formation of ordered aggregates with a well-defined “faceto-face” packing fashion was then proposed to be responsible for the enhancement of the OL performance of the nanoparticle sample.1602 On the other hand, the covalent linkage between C60 and tBu4PcTi indicated poorer ESA of the dyad than those of the corresponding individual units producing a correspondingly poorer OL performance.1603

Figure 114. Typical open aperture Z-scan curves of 703 with the concentration of 0.5 mg/mL. Solid lines are the numerical fittings. Reproduced with permission from ref 1608. Copyright 2009 American Chemical Society.

found to be due to RSA, while that of the nanocomposite was due to both RSA and nonlinear scattering. The two mechanisms could be conflicting for OL for the suppression of the whole nonlinear response of PcH2-MWNTs. Figure 114 shows the open aperture Z-scan curves of 703 for different input fluences with the normalized transmission plotted as a function of sample position with respect to the beam focus. All Z-scans exhibited a reduction in the transmission about the beam focus. The minimal normalized transmittance reached 83% for a 0.5 mg/mL solution at the on-focus intensity of 0.17 GW/cm2 (1.02 J/cm2), which further decreased to 68% as the intensity increased to 0.61 GW/cm2 (3.66 J/cm2).1608 The NLO behavior of 704 (Chart 158) in DMSO and DMF solutions was also reported. An improved nonlinear absorption was observed for 704,1610 when compared to the nonlinked phthalocyanine congener, when recorded in DMSO (βeff was 300 vs 221.6 cm/GW, respectively). However, when DMF was used as solvent, the βeff values were lower for 704, when compared to the phthalocyanine alone (120 vs 190 cm/GW, respectively). The authors attributed this occurrence to the fact that DMF could undergo resonance stabilization, thus causing phthalocyanine triplet absorption quenching. Regarding the OL effect of this complex, this was dominated by a strong ESA from a two-photon pumped state. For this system, it was found that the limiting threshold was affected also by the solvent used. So, while 704 presented a threshold of 0.21 J/cm2 in DMSO, its value was only 0.57 J/cm2 in DMF.1610 The latest addition to the carbon allotrope family, graphene, was naturally a platform for producing well-performing optical limiters. For instance, graphene oxide(GO)−zinc phthalocyanine hybrid material 705 (Chart 159) and reduced graphene oxide−zinc phthalocyanine hybrid material r705 have been prepared via covalent functionalization.1611 The NLO properties of the hybrid 705 and r705 were investigated using Z-scan 532 nm with 4 ns laser pulses. The results showed that the efficient functionalization of GO caused hybrid 705 to possess much larger NLO properties and OL performance than those of individual GO, ZnPc. These improved properties were ascribed to a combination of different NLO absorption mechanisms for hybrid 705, including TPA, saturable absorption and ESA from reduced GO moiety, RSA arising from ZnPc moiety, and the contribution of efficient photoinduced electron transfer or energy transfer process between ZnPc and reduced GO.

Chart 158. Carbon Nanotubes−Phthalocyanines Composites

The combination of phthalocyanines and fullerenes may also be achieved via dye immobilization in a suitable host matrix, consequently offering a solid-state optical limiter composite material.1604,1605 One way seems to be the immobilization of fullerene and phthalocyanine dyes in transparent and ordered mesoporous molecular sieve silica films whereby the incorporation of the dye molecules was achieved via sol−gel processes or postsynthesis impregnation.1604,1605 Carbon nanotubes in combination with Pcs were also tested as OL materials. Several reports appeared on the synthesis and NLO properties of covalently functionalized single-walled nanotubes with metallophthalocyanines.1606−1610 For instance, nanocomposite 703 (Chart 158), constituted by unsymmetrically substituted metal-free phthalocyanines covalently functionalized multiwalled carbon nanotubes, was synthesized for OL studies.1608 This material exhibited strong scattering at higher intensities, which clearly comes from the MWNT complement. The nonlinear response of tBu3NH2PcH2 was 13171


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In Figure 115, the normalized transmittance was plotted as functions of the input energy densities, OL behavior of r705, 705, GO, and ZnPc. The OL threshold values for the different samples were also determined. It could be clearly seen that at the same level of linear transmittance of 80%, the hybrid r705 exhibited a lower threshold value (0.98 J/cm2) in comparison with the GO (2.12 J/cm2), ZnPc (1.97 J/cm2), and 705 hybrid (1.74 J/cm2), indicative of much better OL performance.1611 A soluble graphene oxide (GO) covalently functionalized with zinc phthalocyanine (PcZn), 706 (Chart 160), was synthesized by an amidation reaction and evaluated for NLO.1612,1613 Hybrid 706 displayed much larger nonlinear optical extinction coefficients and broadband OL performance than GO at both 532 and 1064 nm, due to the accumulation effect caused by the covalent link between GO and PcZn. While 706 showed a βeff value of ∼51.2 and ∼31.0 cm/GW at 532 and 1064 nm, GO only displayed βeff = ∼30.2 and ∼6.2 cm/GW at the same wavelengths. On the other hand, PcZn had βeff of about ∼31.1 cm/GW at 532 nm. In the same fashion, χ(3) values were higher for 706 at both wavelengths (∼17.6 and ∼21.4 cm/GW at 532 and 1064 nm, respectively). The NLO and OL properties of grapheme-based materials (pristine graphene, graphene oxide, reduced graphene oxide) covalently functionalized by a zinc phthalocyanine derivative (707 and r707, Chart 160)1614 were studied in nanosecond and femtosecond regimes. While in the ns regime the hybrid r707 produced a better OL than the other hybrids, 707 was revealed to be the best OL performer in the fs regime. Results show that NLO properties of covalently linked materials were improved due probably to the strong interaction between graphene and phthalocyanine. The larger nonlinear attenuation coefficient of r707 (βeff = 230 cm/GW), when compared to the other hybrid 707 (βeff = 84 cm/GW) and its higher third-order nonlinear coefficient (χ(3) = 25.22 × 10−12 vs 9.17 × 10−12 esu of 707), make the hybrid r707 a better candidate as nonlinear photoactive material in an optical limiter.1614 The effect of the presence of quantum dots (QDs) on the OPL ability of phthalocyanines was also evaluated. For instance, a variety of metallophthalocyanine complexes (708−710, Chart 161),1477,1615−1617 containing different metals and several substituent types, were tested. Regarding the substituents, the OL parameters of Pcs were higher for tert-butylphenoxy (708a and 709a) when compared to benzyloxyphenoxy (708b and 709b) and phenoxy substituents (708c and 709c) in DMSO, while nonperipheral substitution decreased the OL parameters. Third-order susceptibility (Im[χ(3)]) values of Pcs in the absence and presence of CdTe QDs were in the 10−12−10−10 esu range, slightly improving in the presence of QDs. However, no definitive conclusion could be withdrawn about the OL performance and nature of Pc−QD combinations. Very recently, enhanced nonlinear optical properties (in DMSO) were observed for composite 711 (Chart 162),1618 constituted by a PcInCl covalently linked to CdSe/ZnS or CdSe QD. The NLO parameters were obtained from Z-scan measurements. Contributions on the corresponding OL effect were derived from TPA of the PcInCl moiety and from freecarrier absorption by QDs. This synergy enhanced the effect of OL in the dyad 711. The effective nonlinear absorption coefficient for InPc-CdSe/ZnS was recorded (700.0 cm/GW), presenting free-carrier absorption cross sections for composites 711-CdSe/ZnS and 711-CdSe composites, respectively, of 1.52 × 10−19 and 6.00 × 10−20 cm2. A much lower limiting threshold

Chart 159. Graphene Oxide−Zinc Phthalocyanine Hybrid Material

Figure 115. Optical limiting of r705, 705, ZnPc, and GO excited at 532 nm with 4 ns pulses. Reproduced with permission from ref 1611. Copyright 2014 Elsevier. 13172


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Chart 160. Reduced Graphene Oxide−Zinc Phthalocyanines

Chart 161. Metallophthalocyanine Complexes for Coupling with QD

Chart 162. PcInCl−QD Hybrid

Chart 163. Tetrapyrazino-tetraazaporphyrins

of 92 mJ/cm2 was observed for 711-CdSe/ZnS nanocomposite with respect to the non-nanostructured system. PcZn solutions in the presence of gold nanoparticles (AuNP) showed very good nonlinear absorption properties in the ns regime at 532 nm at high fluences.1619 The enhanced nonlinear response was due to the self-healing of gold nanoparticles, which did not suffer the fast photoinduced fragmentation usually observed during irradiation with intense laser pulses. The self-healing mechanism can be of general interest when intense laser pulses interact with plasmonic nanostructures in the presence of organic chromophores.

important electronic differences from their parent Pcs and Pors analogues. This renders such a class of conjugated macrocycles very interesting materials for NLO studies.1461,1534,1620−1625 The nonlinear transmission properties of some of this type of macrocycles (712a−d, Chart 163) were determined.1624,1625 These specific examples lack peripheral substituents, a feature that can lead to heavy aggregation in solution. It is here stated again that the occurrence of aggregation can influence the

7.7. Other Types of Phthalocyanine Related Compounds

Tetrapyrazino-tetraazaporphyrins are a special subspecies of Pcs. These materials have molecular structural features with 13173


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Chart 164. Pyrazinoporphyrazinato−Copper Complexes

Figure 116. Nonlinear transmission measurements in open aperture configuration of 0.1 mM solutions of 712a−d in DMSO at 532 nm. Reproduced with permission from ref 1624. Copyright 2004 American Chemical Society.

dynamics of the excited states with the activation of different optical processes, as discussed before. From the nonlinear transmission (NLT) profiles presented in Figure 116, it can be seen that the NLT data for the ZnII complex are very similar to those of the MgII complex. This indicates that the presence of a heavier metal center (ZnII), which would favor ISC and a higher triplet population, does not significantly influence the OL results. The NLO data obtained for 712a−c could be interpreted in terms of a sequential TPA mechanism. Nonlinear transmission measurements revealed that aggregation of the molecules certainly induces a variation of their NLO response. To explain these findings, it was proposed that a cooperation mechanism was active for the aggregates because of the high values of excited-state absorption cross section.1624 For the MgII complex, it was found that the OL performance of the Mg(II) complex of tetrakis-2,3-[5,6-di(2-pyridyl)pyrazino]porphyrazine ([Py8TPyz-PzMg(H2O)] in DMSO solution was very good up to fluences of 2200 mJ cm−2.1625 Moreover, the molecule showed very good photostability against possible light-induced decomposition. This was probably related to its larger stability against oxidation. Pump and probe measurements have shown that a multiphoton absorption gives the result of RSA involved triplet excited states only at low fluences, similar to the case of phthalocyanines, while another molecular species, the reduced monoanion [Py8TPyzPzMg(H2O)]•−, dominated the NLO scenario at high fluences. At the molecular level, the nonlinear absorption process was understood in terms of two sequential processes: (a) a four-level RSA mechanism starting from the neutral species, which is active at low intensities, and (b) TPA from an excited state of the monoanion that is generated from the excited triplet state. The latter species has to be considered as a product of photoreduction, which becomes photoactive at high intensities.1625 The third-order NLO properties of another type of porphyrazine-based macrocycles, tetraoctanoyl-/tetraoctanylsubstituted tetra-2,3-[5,6-(9,10-phenantro)pyrazino]-porphyrazinato-copper complex 713 (Chart 164), as well as of tetra-tertbutylpyrazinoporphyrazinato-copper 714 (Chart 164), were also reported.1633,1634 Their excited triplet-state (σexcT) and singlet-state (σexcS) absorption cross sections were determined in solutions at 532 nm with 8 ns laser pulses. The values of σexT for these porphyrazines were found to be an order of magnitude lower than those of PcCu analogues, and their

RSA behavior was attributed mainly to the excited singlet-state absorption. It was concluded that octaazasubstitution in the annulated conjugated rings results in shortening the fluorescence lifetime and decreasing the ISC rate. The comparison of closed-aperture and open-aperture Z-scan results has also shown the stronger nonlinear refraction effects of these compounds when compared to PcCu derivatives. Other classes of compounds related to Pcs are tetraazaporphyrins, or porphyrazines (Pz) complexes,1626 synthetic analogous of porphyrins, in which the four meso-C atoms are substituted by nitrogen. The structural modification caused by azasubstitution introduces differences between the electronic structures of porphyrazines and Pors as observed when comparing their absorption spectra.1627−1629 According to the theory, not only the number of the observed absorption bands but also their origin differs when passing from Pors to tetraazaporphyrins. On the other hand, the absorption bands of Pcs only experience a red shift with respect to tetraazaporphyrins due to the extension of the conjugated π-system upon tetrabenzo-annulation, with the origin of the absorptions, which remains the same as in tetraazaporphyrins. The ground-state absorption spectra of porphyrazines and Pcs, as well as of Pors, have been calculated, and the UV−vis absorption data are tabulated for many compounds.1627−1629 Different from porphyrins and phthalocyanines, the NLO properties of which received much attention for the time being, the NLO data obtained on monomeric tetraazaporphyrins species are much less. In fact, despite the great structural diversity achievable with modified porphyrazines, relatively few works on their third-order NLO behavior were published, including the studies on third-order NLO susceptibility χ(3).1630−1635 For instance, some authors observed a high enhancement of simultaneous TPA in a series of substituted tetraazaporphyrins (715a−c, Chart 165) with multiple electron-accepting groups.1636 Absorption cross sections σ2 were 1.6 × 10−47 13174


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latter compounds have shown also a high photostability, which is an important issue to take into account for the practical realization of optical limiters. Another desirable feature for optical limiters is their low ground-state absorption at the wavelength of irradiation. In case of indium-octaarylporphyrazines, their ground-state absorption in the region 400−550 nm caused by low-intensity CT-transitions increases when passing from electron-withdrawing to electron-donating substituents in peripheral aryls.1637 Also, the OL effect originated from the benzo-annulated derivatives of m-trifluoromethylphenyl (m-CF3Ph)-substituted (chloro)indium-porphyrazines (717−721, Chart 166)1638 was investigated.1639 The values of the effective exited-state absorption cross section σexeff, which includes contributions from both singlet and triplet excited-state absorption cross sections, were measured at 532 nm with 5 ns pulses at the frequency 20 Hz and increased with the benzo-annulation in the following order: 717 (σexeff = 2.39 × 10−17 cm2) < 718 (σexeff = 3.00 × 10−17 cm2) < 719 (σexeff = 3.80 × 10−17 cm2) < 720 (σexeff = 4.55 × 10−17 cm2) < 721 (σexeff = 5.60 × 10−17 cm2). The best OL performance at 532 nm, comparable to that of tBu4PcIn(Cl), was observed for 721.1639 Hemiporphyrazines (Hps) are conjugated macrocycles with C2v symmetry based on two cofacial pyridine rings and two cofacial isoindoline units linked through four aza bridges.1640 One of the most intriguing aspects that emerges from the analysis of the physicochemical properties of Hps is their strong structural resemblance to Pcs, but, contrary to these molecules, they show a nearly complete absence of absorptions in the visible and NIR spectrum. In fact, the presence in Hps of an extended aromatic ring with 18 conjugated π-electrons and C2v symmetry determines transitions only in the near UV.1628,1641−1645 Some incursions on the study of secondorder NLO properties were made by some authors.1632,1646 This feature possibly combined with strong nonlinear absorption motivated research on these materials because an OPL device should possess high optical transmission in the rest state and very low nonlinear transmission in the activated state within the same spectral range. The combination of optical and coordination properties of Hps has recently stimulated our interest toward Hps as photoactive materials for nonlinear optical (NLO) applications.26,1647 We found, for the first time, that an hemiporphyrazinato indium chloride complex (HpInCl) shows reverse saturable absorption (RSA),1647 in the ns time regime. In analogy with the behavior of other conjugated macrocyclic complexes, such an NLO phenomenon was rationalized with a multiphoton absorption process, which, in the lower intensity regime, was due to a stronger absorption of an excited triplet state. Later,26 the NLO properties of several hemiporhyrazines (722a−e and 723, Chart 167) have been determined using nanosecond laser pulses at 532 nm. The nonlinear transmittance (T532) for the toluene solutions of the studied compounds has been recorded as a function of the incident fluence (Fin) within the broad range 0 < Fin < 6 J cm−2 (Figure 118). For a direct comparison of the different nonlinear optical behaviors of Hps 722a−e and 723, it has been plotted in the same figure the data for those Hps solutions having the same value of linear transmittance (T0(532), 0.96 and 0.83 in parts a and b of Figure 118, respectively). All species under investigation behaved as reverse saturable absorbers at 532 nm for nanosecond laser pulses in the

Chart 165. Porphyrazine Complexes for OPL

Figure 117. Plot of the nonlinear transmission of (Ar)8TAPInXs. Reproduced with permission from ref 1637. Copyright 2002 John Wiley and Sons.

cm4 s/photon for 715c at 770 nm, while compounds 715a and 715b displayed absorption cross-section values of 70 and 380 cm4 s/photon, at 783 and 802 nm, respectively. This large value was ascribed to both resonance enhancement by a strong Q(0− 0) linear transition and the presence in the same spectral region of a strong two-photon allowed transition. The OL performance of tetraazaporphyrins in solutions at 532 nm with 3−7 ns laser pulses (ferequency of impinging: 10 Hz) was described for the first time for the series of octaaryl substituted axially coordinated indium-porphyrazines (716a−f, Chart 165).1637 The nature of the axial ligand was found to be not as important as the nature of peripheral groups for the OL effect presented by these porphyrazines (Figure 117). The increase in their OL effect was observed when converting the substituents from peripheral aryls to electron-withdrawing groups, with the octa(trifluoromethylphenyl)-substituted Inporphyrazines having the lowest nonlinear transmittance. The 13175


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Chart 166. Benzo-Annulated Porphyrazine Derivatives

Table 13. Values of Ground-State Absorption Cross Section at 532 nm [σ0(532)] and Limiting Threshold at 532 nm [Flim(532)] for the Toluene Solutions of Compounds 722a−e and 723

Chart 167. Hemiporphyrazine Complexes for OPL

compd

[σ0(532)] ×10−20 cm2

[Flim(532)] (J cm−2)

722a 722b 722c 722d 722e 723

11.8 118.5 1070.9 101.3 6.1 64.6

>5.4 0.45 0.60 1.85 1.65 >5.6

cm−2, that is, (tBu2PhO)2HpH2 (722b) and (HexO)2HpPt (722c) (Figure 118). The indium chloride complex 722d shows a value of F lim (532) = 1.85 J cm −2 , which is intermediate,1647 whereas for HpGeCl2 (722a) and 723, Flim(532) could be only estimated because in the adopted experimental conditions the highest permitted value of Fin(532) did not allow one to reach the lower transmission levels because of cuvette damage and damaging of the focusing optics.26 The nonlinear transmission curves have been fitted with two different models for the dynamics of the excited states. It was found that simultaneous TPA from an excited state must be considered to model the high fluence region for all compounds. In the sole case of (BuO)2HpInCl (722d), the triplet manifold is involved, whereas the singlet manifold has to be considered in all other cases. Confirmation of this behavior has been obtained from pump and probe measurements, which show that only in the case of the InCl complex is the formation of a long-lived excited state involved. Very large values of absorption cross sections for a TPA process from an excited state were found for the metal-free, platinum, and lead complexes (tBu2PhO)2HpH2 (722b), (HexO)2HpPt (722c), and (BuO)2HpPb (722e)). Therefore, the nonlinear absorption behavior was found to be different from other similar systems like phthalocyanines and analogue molecules. Subphthalocyanines (SubPcs)1648,1649 are conjugated complexes possessing special NLO properties due to the simultaneous presence of an aromatic electronic structure,1650 nonplanar skeleton,1651 and a multipolar distribution of electrical charge.1652,1653 Despite the many studies on the

Figure 118. (a) Nonlinear transmission at 532 nm (T532) of HpGeCl2 (722a), (BuO)2HpInCl (722d); (b) (tBu2PhO)2HpH2 (722b), (HexO)2HpPt (722c), (BuO)2HpPb (722e), and compound 723 upon increasing incident fluence (Fin). Linear transmittance at 532 nm (T0(532)) is 0.96 for complexes 722a and 722d, and 0.83 for compounds 722b, 722c, 722e, and 723. Reproduced with permission from ref 26. Copyright 2008 American Chemical Society.

investigated fluence range, because their transmission decreases at larger fluences. In Table 13 are listed the values of limiting threshold Flim(532), for compounds 722a−e and 723. The lowest values of Flim(532) are found around 0.5 J cm−2 and are associated with the NLO behavior of those hemiporphyrazinic complexes, which reach transmission saturation for Fin(532) < 5 J 13176


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shown at 532 nm and TPA at 1.064 mm, with χ(3) values of 6.7 × 10−12 and 6.1 × 10−12 esu, respectively.1656 Similarly, SubPc 725 (Chart 168) has been synthesized,1657 and its third-order NLO properties in ethanol solution have been studied by Zscan technique using 8 ns laser pulses at 532 nm. The complex exhibited a strong NLO absorption effect (α2 = 3.7 × 10−10 mW−1), self-focusing refractive performance (η2 = 2.1 × 10−13 cm2 W−1), and effective third-order NLO susceptibility χ(3) = 1.12 × 10−11 esu.

second- and third-order NLO properties of SubPcs, the only evidence of their OL properties for ns pulses at 532 nm or any other wavelength was reported by the group of Hanack.1654 The effect of RSA could be produced at 532 nm when SuPcs afford excited electronic states with particularly high absorption cross sections at this wavelength, say larger than 1 × 10−16 cm2, because SubPcs generally display quite high linear absorption at 532 nm.1652 On the other hand, the presence of a nonheavy central atom such as boron, and the bright fluorescence in the visible, are features that do not tend to favor effective ISC in excited SubPcs.1655 Consequently, the excited triplet state of SubPcs would be populated with moderate yields, and excited triplet state absorption would thus be comparatively weak. In

7.8. Naphthalocyanines

As analogues of phthalocyanines, the naphthalocyanines (Ncs) have a more extended conjugated π-system due to the additional tetra-benzoannulation. In case of 2,3-Ncs, such a structural modification with respect to Pcs is reflected in the strong bathochromic shift of the Qband in the UV−vis spectra as well as the enlargement of the highly transparent window between the Q- and B-bands so desirable for optical limiters operating in the visible range. To the contrary, UV−vis spectra of 1,2-Ncs do not differ very much from those of Pcs: the position of the Q-band is almost unaffected by 1,2-annulation, indicating no strong conjugation of the annulated benzo-rings with the Pc macrocycle.1658,1659 As we have pointed out before, the extended π-conjugation is considered to be necessary for the attainment of higher thirdorder nonlinearities. Thus, it is expected that the real and imaginary values of χ(3) measured at different wavelengths for many peripherally and axially substituted Ncs containing different central atoms or groups should be, in general, higher than those of analogues Pcs. Here, we will consider the OL effects based on the RSA of Ncs. For the reasons given above, only 2,3-Ncs received attention in connection with their OL properties. To our knowledge, no OL studies on 1,2-naphthalocyanines were reported until now. The approaches for the fine-tuning of OL properties of Ncs are quite similar to those used for the modification of OL properties in Pcs, for exmple, peripheral and/or axial ligand substitution, variation of the central atom, etc. However, one should take into account that the negative effects connected with their tendency to aggregate and/or to decompose photochemically are stronger than in the case of Pcs. From the synthetic styandpoint, these complexes also seem more complicated to achieve, because the preparation is somewhat duller and time-consuming. This is the main reason

Chart 168. Subphthalocyanines for OPL

any case, it was found that both effects of saturable absorption and RSA could be obtained with a solution of subphthalocyanine at 532 nm depending on the intensity of 9 ns laser pulses; saturable absorption occurred at lower intensity levels, whereas the reverse effect prevailed at higher levels. Contrary to expectations, SubPcs did behave as reverse saturable absorbers at 532 nm, despite the high linear absorption at this wavelength. Data have been fitted with a five-level model, which considers three consecutive electronic transitions with absorption crosssection values of 1.4 × 10−16, 1.0 × 10−16, and 40 × 10−16 cm2, respectively.1655 Alkoxy-substituted SubPc 724 (Chart 168) was synthesized, and its NLO properties were evaluated.1656 Compound 724, embedded in thin films, was investigated with the Z-scan technique with 38 ps pulses at 532 nm and 1064 mm. RSA was Chart 169. Ncs Studied for OPL

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Chart 170. Fluorinated Ncs for OPL

why OL studies on these fine materials are scarce, even if the disadvantages seem much less than the advantages when operators use these complexes especially when Ncs inclusion in solid matrixes is considered.1660,1661 The OL properties of Ncs were first reported in 1991,1662 and later presented more in detail.1663 The authors concluded that, due to the specificity of the Nc structure, improved OL could be achieved. Later, Shirk et al. studied several Pcs and Ncs, with silicon and lead as central atoms, and these Ncs displayed the highest nonlinear absorption coefficient at 532 nm1664 in a very reproducible manner.1665 A broadband OL study with nanosecond laser pulses on soluble NcSnX2 (the axial ligands X and/or peripheral substituents were not specified) was carried out in THF, demonstrating the RSA behavior of NcSnX2 in the region 470− 650 nm, which fits with the day vision response.1666 A five-level model of the RSA mechanism was considered to fit the experimental nonlinear transmission curves. In these calculations, both excited singlet−singlet and triplet−triplet transitions were found to contribute in the OL response. It was demonstrated that a 0.26 mm thick 5.4 × 10−3 M solution of NcSnX2 in THF with photonic transmission ∼60% in the linear regime, used as an optical limiter built in a magnifying sight, increased the damage threshold for a charge coupled devices array placed at the focal plane of the objective, by a factor ≥60 under 550 nm laser single shots.1666 The influence of the central atom on the OPL of Ncs 726− 728 (Chart 169) in solutions at 532 nm was found to be not as pronounced as for Pcs. Consequently, among the Ncs studied, only the indium compounds 727b−e possessed enhanced OL. The differences between the OL performances of Si, Sn, and Pb derivatives were not substantial.22,1667 These conclusions were drawn by comparing the values of the ratios of effective excitedstate to ground-state absorption cross sections (σexeff/σg, where σexeff is obtained as an average of the triplet and singlet excitedstates absorption cross sections), and the saturation fluences Fsat of the examined systems. It was considered that the larger ring of Nc leads to a diminished effect of heavy central metal due to the decrease in orbital mixing with increasing size of the macrocycle, and that indium is more strongly coupled to the ring orbitals as compared to the group(IV-a) elements probably by virtue of axial dipole moments oriented perpendicularly with respect to macrocycle plane.

The Z-scan experiments at 640 nm were carried out for 2.4 mM solutions of 727d, and the results were compared to those for 2.4 mM solution of 727b.1461 Both compounds show RSA behavior, however exhibiting relatively small nonlinear absorption coefficients at this concentration due to the formation of aggregates with short excited-state relaxation times. Additionally, 727d appeared to be photochemically unstable upon extensive irradiation. It was also shown that the substitution of axial halide by bulky group, for example, ptrifluoromethylphenyl (p-TMP) (727e), allowed one to increase the aggregation threshold for tetra-tert-butyl naphthalocyaninato indium(III) species in solutions.1284,1461,1470,1668 OL measurements were carried out for the first time also on axially substituted gallium tetra-tert-butyl naphthalocyanines 727f, 727g, and μ-oxo-dimer 728 (Chart 169) in chloroform solutions.1470,1461,1668,1669 The compounds exhibited RSA behavior at 532 nm excitation, and their transient absorption spectra recorded under 350 nm laser excitation display positive ESA in the 500−700 nm spectral range. The substitution of axial chlorine by p-TMP group (727g) resulted in an increased value of merit coefficient κ for the studied gallium naphthalocyanines almost by a factor of 2 due to the decreased linear absorption at 532 nm. The dimeric species 728 exhibited the lowest value of κ (4.1 ± 0.1) but, as well, the lowest saturation density Fsat among this series of NcGaX. In

Figure 119. OL performance of hexadecafluorinated gallium naphthalocyanine and its μ-oxo-dimer in comparison to several other OL-active compounds. Reproduced with permission from ref 1670. Copyright 2003 John Wiley and Sons. 13178


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fluorine substituents, which results in the augmentation of the difference in dipole moments between the excited states involved in second photon absorption. Following the expeditious synthesis carried out on the preparation of effective OL materials at 532 nm irradiation,1670 an improved material (731a−c, Chart 170) was afterward synthesized.1671,1672 The Ncs previously reported have not shown a satisfactory performance at both 532 and 1064 nm wavelength (Figure 120) simultaneously,1666 but in this case, the new complexes 731a, 731b, and 731c exhibited OPL simultaneously at both wavelengths, when irradiated with nanosecond pulse lasers. The materials possessed high transparency over a wide range in the UV−vis range, showing interesting OL thresholds (Table 14).1672 Through the appropriate peripheral substitution, the Ncs can give an increased transmitting window, shifted more to the NIR, which is desirable for the fabrication of optical limiters effective in the broad visible-light range including the NIR region. A strong bathochromic shift of the Ncs Q-band can be observed upon alkoxy-substitution in the peripheral positions closest to the Ncs core (α- or 1,6-positions) due to mixing the oxygen pz-orbitals with π-HOMO of the macrocycle and distortion of the Nc-core geometry from the planar, causing the destabilization of π-HOMO. Thus, α-substituted alkoxy naphthalocyanines 732a−c (Chart 171) have the maximum transmittance at 635 nm1662,1663 in contrast to 560 nm for 726a,22,1667 and the absorption maximum of 732a at 910 nm leaving transparent the 520−780 nm window in which RSA is observed.1673,1674 The σexeff/σg values found for 732b and 732c in toluene solutions at 630 nm with 5 ns laser pulses were 22 and 17,1662,1663 respectively. These values are comparable to those exhibited by analogues Pcs at 532 nm.22,1667 Other substitution patterns, which provoke absorption peak shift in the NIR and infer high solubility to NcInX as materials for OL, were also realized (compounds 733a−c, Chart 171).1668 Among these octa-substituted Ncs, the mixed substituent derivative 733b, and its axially substituted analogue 733c, have shown high solubility and no aggregation up to the concentration of 2 × 10−3 M in chloroform. The OL measurements at 580 nm on the 733c with 4 × 10−2 M concentration revealed its RSA behavior with OL energy threshold of 10 nJ.1461 At incident energies higher than 104 nJ, the photochemical transformation of the compound into another product was observed. Such a transformation resulted in a drastic decrease of the sample transmittance with increasing incident light intensity, and the formation of this

comparison with analogous gallium-phthalocyanines, all reported NcGaX1498,1669 show the lower κ and Fsat values. Hexadecafluorinated gallium-2,3-naphthalocyanine 729 (Chart 170) and a μ-oxo-dimer 730 (Chart 170) were also prepared and studied.1670 Although the solubility of the monomeric species appeared to be too low for OL characterization in a wide range of concentrations, the OL properties of the dimeric 730 were studied in THF and compared to those of 727f, 728, and C60 at 532 nm irradiation (Figure 119). The peripherally fluorinated species exhibited enhanced OL proper-

Figure 120. Optical limiting properties of reported Ncs at 1064 nm in CS2 (T0 = 75%). Reproduced with permission from ref 1671. Copyright 2005 John Wiley and Sons.

ties as compared to nonfluorinated gallium t-Bu4Ncs and C60, as follows from their ratios of effective excited to ground state absorption cross sections κ = σexeff/σg and OL thresholds. This improvement in OL performance was associated with the involvement of the strong electron-withdrawing effect of the Table 14. OL Thresholds for Reported Ncs in THF (at 532 nm) and CS2 (at 1064 nm) with Transmission (T0) of 75% optical limiting threshold (J cm−2) material

532 nm

1064 nm

731a 731b 731c

1.55 4.64 2.65

2.96 3.65 2.66

Chart 171. Some Peripherally Alkyl- and Alkoxy-Substituted Ncs for OPL

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Chart 172. Some Peripherally Aryl-, Aryloxy- and Halogen-Substituted Ncs for OPL Studies

Table 15. OPL Parameters for the Studied Ncs1675 ns compd 734a 734b 734c

σ0a

−17

(×10

0.33 1.71 1.63

2

cm )

ps

Fth (J/cm )

Tnonlinc

σeff/σ0d

Fth (J/cm )

Tnonline

σeff/σ0d

0.27 0.11 0.08

0.26 0.19 0.16

>3.24 >4.15 >4.41

0.100 0.075 0.030

0.41 0.39 0.33

>2.14 >2.27 >2.67

b

2

b

2

Ground-state absorption cross section at 532 nm. bAt T/Tlin = 70%. cAt incident fluence of 1.5 J/cm2. dRatio of effective excited-state absorption cross section to that of ground-state absorption. eAt incident fluence of 0.17 J/cm2. a

successively carried out, with irradiation at 532 nm.1676 Halogenated Ncs displayed the best OPL performances. It was also recognized that the concentration increase of the solutions produced the general reduction on the OPL efficacy, due to intermolecular aggregation and consequent excited-state quenching. The presence of heavy atoms like bromine in the periphery of Ncs for OL purposes was also considered by some researchers (736a,b, Chart 172).1677 It could be seen that the influence of Br substituent on ESA process was significant, and therefore the effectiveness of the OL effect of 736a was better than that of 736b. On the basis of this finding, some groups analyzed the nonlinear transmission of tetrabrominated naphthalocyaninato indium-axial halogen (737, Chart 172), in which some solubilizing substituents were also added to decrease the normal aggregation that these complexes usually show.1678 Complexes 734a and 737 were then evaluated with the Z-scan technique in both open and closed aperture configurations at 532 nm for ns pulsed radiation. Once again, it was confirmed that the bromination of the Nc ring considerably improves the OL properties of such a system when high intensity radiations are produced by ns laser pulses at 532 nm. Indeed, the onset of NLO regime occurred at relatively low values of fluence of the incident radiation, and in correspondence of NLO behavior 734a and 737 displayed unusually fast changes of slope for their optical transmission. Such a behavior has been rationalized and modeled through the modification of a simpler model taking into account additional excited-state transitions that take place at higher light intensities. These additional transitions were associated with the possible occurrence of a charge transfer process between excited molecules. Partial bromination of the naphthalocyaninato chloro-indium complex (737) in the peripheral positions of the macrocycle has led to the general improvement of the nonlinear transmission properties of such a system in comparison to the nonhalogenated chloro-indium naphthalo-

photochemical product was found to be only partially reversible. To acquire the differences in the OPL capability upon variation of the axial ligand on Ncs, some authors reported a study about the effect of axial halogen ligand on the OPL of some indium naphthalocyanines (734a−c, Chart 172).1675 OPL of nanosecond (ns) and picosecond (ps) laser pulses at 532 nm using these complexes has been demonstrated. All complexes displayed strong Q(0,0) absorption and measurable emission in the NIR region and exhibit strong ESA in the range of 470−700 nm upon ns laser excitation.1675 The different axial ligands showed negligible effect on the linear absorption, emission, and transient difference absorption spectra. However, the excited-state lifetime, triplet excited-state quantum yield, and efficiency to generate singlet oxygen are affected significantly by the heavier axial ligand. Brominated (734b) and iodinated (734c) complexes have shown higher triplet excited-state quantum yield, while chlorinated complex had longer excited-state lifetime and more efficient singlet oxygen generation. The iodinated complex displayed therefore the best OL due to the higher ratio of excited-state absorption cross section to ground-state absorption cross section (σeff/σ0).1675 From Table 15, all three complexes displayed a noteworthy transmission loss with increased incident fluence.1675 For ns laser pulses, the limiting threshold decreased from 0.27 J/cm2 for 734a to 0.08 J/cm2 to 734c. The transmittance has fallen to 26% at incident fluence of 1.5 J/cm2 for 734a, and to 16% for 734c. For ps laser pulses, the limiting threshold and limiting throughput also followed the same trend, with 734c showing the lowest limiting threshold and limiting throughput. The enhanced OL performance of 734c for ns laser pulses has to be related to the higher triplet excited-state quantum yield, while for ps laser pulses, the better OPL performance is likely due to higher ESA cross section.1675 A comparison of the nonlinear transmission of a series of vanadyl, chloroindium, and lead Ncs (735a−c, Chart 172) was 13180


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Chart 173. Some Peripherally tert-Butyl-, n-Butoxy-, Aryloxy-Substituted Ncs and Nc-Based Sandwich-Dimers for OPL Studies

cyanine (734a) when high fluence radiations were produced by nanosecond laser pulses at 532 nm.1678 Going even further on the halogenation effect, our group studied the photophysical parameters and nonlinear optical properties of two synthesized silicon naphthalocyanines, tetrabrominated 738a and octabrominated 738b (Chart 172).1679 In particular, the molar extinction coefficients of the ground and excited states, the quantum yield of the excitedstate formation, the lifetime of the triplet excited state, and the transient absorption spectra of these compounds were determined. The increased number of peripheral bromides from 738a to 738b resulted in the shortening of the triplet excited-state lifetime and the increase of the quantum yield of the triplet excited-state formation, as expected on the basis of the heavy-atom effect. The enhanced nonlinear transmission behavior of the octabrominated complex could be ascribed to the increased ratio of the triplet excited-state absorption to ground-state absorption cross sections, although the slightly increased triplet excited-state quantum yield of the octabrominated complex could also play a role. In addition, the variation of the peripheral bromide substituents influenced both the sign and the magnitude of the nonlinear refraction of these two complexes for nanosecond laser pulses at 532 nm.1679 In contrast to these promising findings, the OL study on (tBu)4NcAlCl (739a) and (t-Bu)4NcZn (739b) (Chart 173) in alcohol solutions using 5 ns pulses at 532 nm has shown their degradation under air and light exposure.1486 Additionally, the lower chemical stability of some Ncs as compared to Pcs can result also in their decomposition when incorporating them into sol−gel matrixes to manufacture the solid optical limiters, as observed in the cases of α-octabutoxy naphthalocyanines 740a,b (Chart 173).1680 Other requirements for the realization of solid-state guest−host optical limiters are the high linear transmittance and high damage threshold values of the matrixes themselves. Good results in that respect for the guest−host systems were obtained when using the optical grade epoxy thermoset EPO-TEK 310 doped with commercially available 2,3-NcSi[OSi(C6H13)3]2, the OL performance of which was compared to that of other host systems.1575,1681 Ncs 741a,b (Chart 173) were also synthesized, and their OL properties were investigated.1682 The compounds exhibited good OPL performance, with OL thresholds of 0.26 and 0.15 J cm−2 for 741a and 741b, respectively. On the other hand, the third-order nonlinear susceptibility χ(3) of an europium bisnaphthalocyanine 742 (Chart 173) was also measured in dimethylformamide solution using DFWM at 532 nm under

nanosecond pulse excitation.1683 Effective nonlinear absorption coefficient, βeff = 39 cm GW−1, and imaginary part of nonlinear susceptibility, Im(χ(3)) = 1.6 × 10−13 esu, were obtained using open aperture Z-scan technique at the same wavelength. OL properties of the sample were also investigated, but the results were not outstanding, probably due to relatively low solubility and higher aggregation of the complex in solution. Third-order optical nonlinearities of several polymer films doped with Nc derivatives have been measured under resonant conditions by femtosecond DFWM.1452,1684 The metal substitution and the peripheral groups influenced both the extent and the response of the third-order optical nonlinearities. Temporal profiles of the DFWM signal were measured with a time resolution of 0.3 ps and were found to consist of at least two components: the coherent instantaneous nonlinear response and the slow response. The slow response for the films decayed much faster than that for the solutions due to effects of aggregation or intermolecular interactions. The Chart 174. Lead Naphthalocyanines for OPL

electronic component of the effective third-order nonlinear optical susceptibilities, χe(3), was evaluated for the sole polymer film and a film of poly(methyl methacrylate) doped with 20 wt % octabutoxy-substituted zinc naphthalocyanine (740c, Chart 173). The latter system showed the largest χe(3) value of 8.9 × 10−9 esu.1452 On the other hand, the estimated χe(3) value for the 740c neat film was 2.1 × 10−7 esu, which was in the same order of magnitude of the resonant χ(3) value of 5.0 × 10−7 esu for a film of a silicon naphthalocyanine derivative measured by DFWM at 800 nm.1451 The χ(3) value for axial substituted silicone naphthalocyanine was 10.7 × 10−11 esu at 1500 nm, and it decreased to 1.8 × 10−11 esu at 1050 nm, measured by the THG method. Our group also prepared the tetrabromi13181


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compounds represent some of the most effective OPL molecular systems known to date.1685 Upon comparison of the OPL performances of 743 in liquid toluene and in polymeric solid Plexiglas, the onset of NLO behavior occurred at much lower fluences for the toluene sample as compared to the Plexiglas sample (4 × 10−3 vs 7 × 10−2 J cm−2). Once the NLO regime has been reached, the decrease in nonlinear transmission was much steeper when the complex was dispersed in the polymeric matrix (Figure 122).1685 This difference was attributed to the intermolecular aggregation in the polymeric sample, generating additional relaxation channels for excited 743, as well as the polymeric environment, which inevitably influenced the dynamics of the formation/relaxation of excited states of the embedded molecules. The effect of π-conjugation extension with respect to the reference structure of Pc on the determination of NLO properties was studied for the series of structures: transdihydroxy-tetra(t-butyl) porphyrazino-germanium (744, Chart 175), trans-dihydroxy-tetra(t-butyl) phthalocyaninato-germanium (745, Chart 175), and an axially substituted naphthalocyanino-germanium derivative with siloxyl groups (746, Chart 175).1686 As expected, a progressive red-shift of the linear absorption bands was observed together with the increase of the NLO absorption coefficients in going from porphyrazine to naphthalocyanine according to the increase of the degree of benzoannulation. This comparative study clearly demonstrated that larger aromatic systems in Pcs and Ncs provide additional π-electron delocalization and increase of polarizability in both linear and nonlinear regimes.

Figure 121. Nonlinear transmittance at 532 nm, T532, for Nc 743 in toluene (C, 2.8 × 10−4 M; cuvette thickness, 2 mm). The sample was excited with 9 ns pulses. For comparison, the variations of T532 are also shown for 2.3 × 10−4 M Br4(tBu2C6H3O)4NcInCl (737)1675 and 7.8 × 10−4 M (tBu2C6H3O)8NcPb (735a).1676 Reproduced with permission from ref 1685. Copyright 2010 John Wiley and Sons.

nated lead complex 2,(3)-tetrabromo-3,(2)-tetra[(3,5-ditertbutyl)phenyloxy] naphthalocyaninato lead (743, Chart 174),1685 studied for OPL purposes in a Plexiglas matrix with formulation specifically designed and prepared for this purpose by Degussa [naphthalocyanine content: 5.0 × 10−4 M (0.1% by weight)]. A reversible NLO absorption was observed for a fluence above 0.4 J cm−2, through fast excited-state dynamics. As far as the limiting of nanosecond laser pulses is concerned, lead naphthalocyanines have the advantage of more rapid formation of the strongly absorbing excited state T2 (Figure 3) when compared to analogous complexes of lighter central metals (normally in the order of picosecond).1536 Figure 121 shows the nonlinear variation of the transmittance of a solution of complex 743 in toluene with the incident fluence (532 nm, 9 ns laser pulses). The transmittance profiles reveal the occurrence of RSA with a limiting threshold of 0.1 J cm−2 for 743.1685 The NLO transmission profile of tetrabrominated lead complex 743 was similar to that of the analogous indium chloride complex 737 (see Chart 172) under the same experimental conditions of incident light intensity and linear optical transmittance at the wavelength of irradiation. Nc

8. CONCLUDING REMARKS AND PERSPECTIVES A compilation of NLO and OPL parameters is presented in Table 16. The full control of the luminous radiation represents one of the most important issues and challenges for modern and future photonic technologies destined to energy conversion, communication, computing, as well as medicine when these technologies recognize the ultrafast delivery and processing of data, images, and energy via photon transmission as their major strength. This control of ultrafast switching in a time scale non superior to 10−4 s is particularly desired when radiations are emitted by sources with erratic or even hostile character, and then need to be opportunely tamed in a way that is still exploitable by the light-sensing element. The most important light-sensing elements to consider are the human eye (our reference photosensor) and photovoltaic devices for energy conversion purposes. Attenuation, frequency shift, refraction, shuttering, and polarization are the main processes that alter, respectively, the intensity, the wavelength, the speed of the transmitted light, the duration, and the plane of oscillation of the electromagnetic vectors traveling with the optical radiation. The corresponding optical devices that traditionally control these four different parameters of the optical radiation are optical filters, nonlinear optical materials, shutters, monochromators, and polarizers, that is, elements that can be easily found in a more or less correct position on the optical benches that kingly occupy most of the space of our optics laboratories. These quite common objects present the evident limit of being nondynamic. This is equivalent to saying that they in their operative state do not change the properties of attenuating, frequency shifting, blocking, and polarizing the radiation upon variation of the features of radiation itself. For this reason, the

Figure 122. Variations of normalized transmittance at 532 nm for 743 in toluene (▲) and Plexiglas (○). Reproduced with permission from ref 1685. Copyright 2010 John Wiley and Sons.

737 showed a better OL effect with respect to the Pb complex 735a (see Chart 172), which was ascribed to the heavy peripheral atom effect imparted by the bromines present in the structure. Because complex 743 represents an optimized system for OPL with NLO behavior comparable to that of the Pb analogous (Figure 121), it is here recognized that these two 13182


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Chart 175. Porphyrazine, Phthalocyanine, and Naphthalocyanine for OPL Comparison

this urgent information to the brain with the latter that, in turn, will transmit another nervous impulse to shut down the palpebra. Such an example is fascinating and represents an efficacious display of how smart optical device should operate taking into account the eventual existence of a quick feedback mechanism of activation. It has been recognized that smart optical materials should combine together the three basic functions of (i) sensing, (ii) activating a feedback mechanism, and (iii) actuating the required change of optical properties. These materials do exist, and it is shown that their number is very large if the specific optical function of the palpebra has to be reproduced artificially. Moreover, plenty of substances embrace practically all of the different classes of materials in condensed phases, in pure or dispersed states. Deliberately, we have not taken into account the thorough analysis of smart optical devices because this would have led the reader in the field of optical engineering rather than in the more crucial field of materials science, which is at the basis of the initial definition and the successive design and realization of smart optical systems. In optical terms, the function of the palpebra, that is, a natural shutter, is reported as the optical limiting (OL) or optical power limiting effect. The OL effect is at the basis of smart optical filtering and consists of the full transmission of light intensities that are sustainable by the light-sensing element, and in the partial or denied transmission of continuously increasing intensities that could become a harm for the light-sensing element. The OL materials will have to be chosen according to the intensity threshold above which the light-sensing element can no longer interact with the external radiation with its unmodified original characteristics. Historically, the earliest realizations of the OL effect in laboratories appeared only after the advent of high intensity lasers in the late 1960s with high intensity lasers (either continuous or pulsed) being the sole, at least so far, light sources capable of generating such a nonlinear optical effect. We have attempted a classification of material for OL purposes considering the broad classes of the inorganic materials, hybrid systems containing both organic and inorganic moieties, and organic materials. In the framework of inorganic materials, certainly nanostructured systems like nanocrystals, nanocomposites, nanoparticles, quantum dots, metallic/semiconducting clusters, or nanowires display the most appealing optical features by virtue of the modulability of their absorption and diffraction properties through the variation of their size or their aspect ratio. Moreover, nanosized systems can reach rapidly the thermal equilibrium following the exchange of energy with the luminous radiation because the zone of interaction between matter and optical wave practically comprises the whole structure of these strongly confined systems with sizes in the

scientific community has turned their attention toward the development of dynamic optical systems that somehow alter their optical properties with the change of the parameters of an external radiation generally considered as an uncontrollable event and presenting random features. Because of that, the four main phenomena of attenuation, frequency shift, shuttering, and polarization need to be conveniently controlled by an operator through the realization of smart optical systems that modify opportunely their optical properties upon a change (either controlled or unwanted) of the features of the radiation. In doing so, we have defined the meaning of smart optical system, and we have stressed that these systems will have to warrant the maintenance or even the establishment of a safe, comfortable, and active state for a given light-sensing element that will interact with the radiation already modified by the smart optical system. The latter term has a 2-fold significance: we can indicate either a device or a photoactive material, which displays smart optical properties under opportune conditions of irradiation. In this Review, among the various smart optical systems we have focused our attention toward all classes of materials (either molecular, polymeric, or crystalline), which are capable of altering their optical properties according to the given definition of smart. These materials usually present photochromic, electrochromic, or nonlinear optical properties with the characteristic of being reversible properties, but in this Review, it has been made clear that being photochromic, electrochromic, or nonlinear constitutes only the necessary condition for the materials aspiring to be a smart optical systems, yet it is does represent not a sufficient condition. This distinction derives from the fact that some photoactive materials need to be activated by a feedback mechanism to change their optical properties in a convenient way for the light-sensing element. This implies that the simple and direct alteration of the parameters of a radiation induced by lowintensity light in photochromic materials, or by a slowly variable electric field in electrochromic materials, or by an intense light in nonlinear optical materials is not useful and smart in itself unless the change of the radiation feature(s) induced by the optical material is a convenient (or safe) modification for the light-sensing element. Actually, the smart optical material should behave as an optical switch that spontaneously transforms the uncontrolled radiation into a new optical wave with appropriate features for the light-sensing element without the need of being activated by another light-sensing element with such an ancillary function. The object that describes well a smart optical function is represented by the natural palpebra. Such an optical device shutters the too intense radiation for the eye once this light-sensing element has detected on its own the dangerous radiation. The feedback will be the transmission of 13183


section 2.1 2.1 2.2 2.2 2.3 2.3 2.4 2.5 2.5 2.5 2.5 2.5 2.5 2.6 2.6 2.6 2.6 3.1 3.1 3.1 3.1 3.2 4.1 4.1 4.1 4.1 4.2 4.4 4.4 4.5 4.5 4.5 4.5 4.5 5.1 5.1 5.1 5.2 5.2 5.2 5.2 5.3 5.3

compd

TeO2−ZnO glass Au NPs ZnO−Ag NPs Cu2Se crystals CdSe QDs Zn1−xCdxS QDs CdO nanosphere 2-D cluster [WS4Cu4I2(py)6] {[La(Me2SO)8][(μ-WSe4)3Ag3]}n [(μ6-WSe4)Cu6I4(py)4]n selenide cluster 3 stabilized Au NPs Ag NPs Fe2O3 NPs Pd nanowire {[Co(fcz)2Cl2]2MeOH}n (2-D) [Pb(bbbm)2(NO3)2]n (2D polymer) 21b 35a 86c 87 151 200 245a 316c 422 G2-Au-PVB film 432 437 452 487 494 C60 C70 520 MWNTs MWNTs 521 533 graphene oxide GO/Au-NP n.r. n.r. 0.000123 n.r. −177 000 n.r. n.r. 0.0029 n.r. 0.0068 0.000081 −0.000053 0.000022 −0.000001 0.00006 −0.000005 n.r. 0.0000042 0.0000047 n.r. n.r. n.r. 0.0000032 0.000162 15.9 12.93 n.r. n.r. n.r. n.r. n.r. n.r. −0.095 −0.068 n.r. n.r. n.r. n.r. n.r. n.r. n.r. n.r. n.r.

η (10−8 cm2/W) 32.3 n.r. 20.7 n.r. 0.0255 n.r 2 n.r. n.r. 22 6.5 6.8 4.6 n.r. 7.25 0.1 n.r. n.r. 58 n.r. n.r. n.r. 0.215 0.11 1 480 000 −53 5000 n.r. n.r. n.r. 9 600 000 0.597 n.r. 0.4 3.19 n.r. n.r. n.r. n.r. n.r. n.r. n.r. 53 0.11

β (10−8 cm/W) n.r. n.r. n.r. n.r. 0.65 n.r. n.r. 1.6 n.r. n.r. n.r. n.r. 0.24 −0.04 1900 n.r. n.r. 0.005 0.23 0.13 n.r. 1.8 0.0052 13 630 1.7 n.r. n.r. 25 n.r. n.r. n.r. 0.0146 2.0 0.014 0.06 n.r. n.r. n.r. n.r. n.r. 5.1 n.r.

χ(3) (10−6 cm2/V2)

Table 16. NLO and OPL Parametric Values for Different Types of Compoundsa n.r. n.r. n.r. n.r. n.r. n.r. n.r. n.r. 1.27 n.r. 1.8 19 n.r. n.r. n.r. 0.001 n.r. n.r. n.r. 0.091 9.3 n.r. 0.0004 n.r. n.r. n.r. n.r. n.r. n.r. n.r. 0.0073 n.r. n.r. n.r. n.r. n.r. n.r. n.r. n.r. n.r. n.r. n.r. n.r.

σg (10−18 cm2)

n.r. n.r. n.r. n.r. n.r. n.r. n.r. 2.3 n.r. n.r. 5095 n.r. n.r. n.r. n.r. n.r. n.r. n.r. 251 n.r. n.r. n.r. n.r. n.r. n.r. n.r. n.r. n.r. n.r. n.r. n.r.

n.r. n.r. n.r. n.r. n.r. n.r. n.r. n.r. 20 n.r. n.r.

σexc/σgs 0.25 0.08 0.08 5.47 2.4 0.72 3.5 0.4 0.08 0.7 0.28 1 n.r. 0.009 1.8 n.r. 0.9 0.16 n.r. 0.075 2.0 n.r. 7 0.025 0.06 n.r. 0.03 1.03 n.r. n.r. n.r. 0.18 2.45 3.3 n.r. n.r. 0.05 1 13 0.088 0.15 n.r. 0.32

Flim (J/cm2) 532 532 532 532 532 532 532 532 532 532 532 532 532 1064 532 1060 532 532 532 532 532 532 532 532 633 514.5 532 532 532 632.8 800 532 532 532 532 532 532 532 1064 532 532 532 532

λ (nm) techn.

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Z-scan Z-scan

Z-scan Z-scan Z-scan

Z-scan Z-scan DFWM DFWM

DFWM Z-scan Z-scan

Z-scan Z-scan Z-scan Z-scan Z-scan Z-scan Z-scan Z-scan Z-scan Z-scan Z-scan Z-scan Z-scan Z-scan Z-scan Z-scan Z-scan Z-scan Z-scan Z-scan Z-scan DFWM Z-scan Z-scan Z-scan Z-scan Z-scan

ref 286 315 331 352 373 379 393 419 448 482 483 497 501 531 538 570 584 629 630 638 652 712 719 799 826 845 879 947 952 958 964 992 1012 1016 1060 1060 1123 1125 1125 1144 1185 1207 1212

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534 539a carbon nanodots CoTPP 576a 587 589a 595 608 613f 618 625c 630b InClPc 641 Ni Pc 642 650c 644d 649 651c 658c 661d 669 673b EuPc2 678a 687c 702 703 704 705 711−CdSe/ZnS 721 722b 724 725 Si Nc thin film 726 730 731a 731a 734c

compd

Table 16. continued 5.3 5.3 5.3 6 6 6 6 6 6 6 6 6 6 7.1 7.1 7.1 7.1 7.1 7.2 7.3 7.3 7.4 7.4 7.4 7.4 7.5 7.5 7.5 7.6 7.6 7.6 7.6 7.6 7.7 7.7 7.7 7.7 7.8 7.8 7.8 7.8 7.8 7.8

section n.r. n.r. n.r. n.r. n.r. n.r. n.r. n.r. −0.222 −0.000001 n.r. n.r. n.r. n.r. n.r. n.r. n.r. n.r. n.r. 8.9 n.r. n.r. n.r. 2100 n.r. n.r. n.r. n.r. n.r. n.r. n.r. n.r. n.r. n.r. n.r. n.r. 0.000021 n.r. n.r. n.r. n.r. n.r. n.r.

η (10−8 cm2/W) n.r. n.r. 210 n.r. n.r. 280 n.r. 2.7 1.9 0.375 10.5 472 n.r. n.r. n.r. −1.8 n.r. n.r. n.r. n.r. n.r. n.r. 9.6 n.r. n.r. n.r. n.r. 0.56 n.r. 3.9 30 150 70 n.r. n.r. 4.8 0.037 n.r. n.r. n.r. n.r. n.r. n.r.

β (10−8 cm/W) n.r. n.r. 0.000001 n.r. n.r. n.r. n.r. 1.3 0.0415 5.8 n.r. 1.1 n.r. 18 250 0.95 n.r. n.r. 380 n.r. n.r. n.r. 5.0 n.r. n.r. 0.017 n.r. 0.28 0.016 2.1 150 n.r. 34 n.r. n.r. 94 1.6 70 000 n.r. n.r. n.r. n.r. n.r.

χ(3) (10−6 cm2/V2) n.r. n.r. n.r. 50 85 28 20 n.r. n.r. n.r. n.r. 0.83 9.98 n.r. n.r. 104 1.8 n.r. 110 1.6 3.1 12 n.r. n.r. 6.9 n.r. n.r. n.r. n.r. n.r. n.r. n.r. n.r. 6.4 1.19 n.r. n.r. n.r. n.r. n.r. n.r. n.r. 16.3

σg (10−18 cm2) n.r. n.r. n.r. 3.6 7.08 4.6 6 n.r. n.r. n.r. n.r. 45.78 5 n.r. n.r. n.r. n.r. n.r. n.r. 30 43 8 5.9 n.r. 5 n.r. n.r. 1.8 n.r. n.r. n.r. n.r. n.r. 8.76 14.4 n.r. n.r. n.r. n.r. n.r. n.r. n.r. 4.4

σexc/σgs 1.55 0.93 0.074 n.r. n.r. 0.24 0.01 n.r. 0.28 0.5 0.070 0.1 0.044 n.r. 0.07 n.r. 0.15 0.080 0.7 0.47 0.09 0.13 2.1 0.027 0.3 n.r. 1.2 n.r. n.r. n.r. 0.21 0.98 0.092 0.13 0.45 n.r. n.r. n.r. 2.15 0.45 1.55 2.96 0.08

Flim (J/cm2) 532 532 800 532 532 532 532 532 532 532 532 532 532 1907 532 665 570 560 485 532 532 532 532 532 532 1064 532 532 830 532 532 532 532 532 532 532 532 800 532 532 532 1064 532

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THG Z-scan Z-scan Z-scan Z-scan Z-scan n.r. Z-scan Z-scan Z-scan Z-scan Z-scan DWFM

Z-scan Z-scan Z-scan

Z-scan Z-scan Z-scan Z-scan

Z-scan

ref 1230 1237 1245 24 1315 1323 1324 1337 1341 1362 1373 1382 1388 1453 1454,1467 1455 1460 1460 1484 122 1506 1516 1518 1522 1526 1552 1551 1568 1600 1608 1610 1611 1618 1639 26 1656 1657 1451 1669 1670 1672 1672 1675

techn. Z-scan

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738b 743

ref

Review

order of the radiation wavelength. The chemical nature of the species forming the inorganic nanostructure is also relevant especially in terms of chemical-physical stability of the nanostructure under the quite drastic conditions of realization of the OL effect. Among nanosized inorganic systems, the chlorides of metals bearing an intermediate oxidation state, thiocyanates of alkaline, or transition metals and metallic Se or Cu dispersed in opportune media ranging from amines to halogeno-alkenes displayed OL effects at different radiation wavelengths and to a different extent. Other interesting examples of inorganic OL materials because of their high stability and integrability in more ample glass-based architectures are those represented by the dispersions of semiconducting oxides (e.g., TiO2, ZnO, NiO, or PbO) in nanoparticulated form within silica matrixes. Other nanocrystalline systems like the mixed oxides ErMn1−xCrxO3 and TeO2− Nb2O5−ZnO or the mixed selenide/sulfide of zinc are quite important examples because they combine strong OL properties with high chemical stability. Literature shows that the chemical constituent of the solid matrixes that disperse inorganic nanoparticles with OL properties is not limited to silica because the niobates of alkaline metals and bismuth oxide are also two feasible choices for the entrapment of OL active inorganic nanomaterials. It has been observed that in some cases there is a beneficial synergy between host-matrix and guest photoactive material that leads to an ameliorated performance with respect to poorly interacting host−guest combinations in terms of OL effect (cases of zinc oxide or lead fluoride/lead oxide dispersed in boron glasses). This is the case of those binary systems that upon intimate contact modify quite thoroughly their electronic states in correspondence with the interface between host and guest. In the ambit of inorganic systems that are not nanostructured and have macroscopic features, the semiconductors are particularly important OL materials. This derives mainly from the typical band structure of semiconductors, the bandgap of which usually ranges in the UV−vis−NIR interval. The optical absorption in semiconducting materials is characterized by the very strong coupling between the optical wave and the characteristic frequency of the charge carriers. This occurs because of the large extension of the frontier orbitals defined by the charge carriers, which occupy the whole structure of the macroscopic semiconductor. Such a peculiarity in semiconductors has been widely recognized as fundamental for the realization of strong OL effects. The presence of very large molecular orbitals hosting a high number of mobile electronic carriers (as many as, at least, the number of atoms forming a macroscopic system) as a structural requirement for the design of efficient OL materials has successfully motivated the investigations on fullerene-based systems like nanotubes (single- or multiwalled), nano-onions, graphene, and the like. In the realization of OL materials based on C60 and on its more or less distorted versions, it can be seen a kind of evolutional link bridging inorganic systems (like semiconducting crystals that historically constitute the first recognized examples of OL materials) with molecular organic systems. From the mole of results, the variety of the examples, and the rapid development of their OL performances, molecular organic materials having an extended network of conjugated electrons can be considered as the protagonists of the present research on OL materials (and probably also of the future one). Among conjugated organic molecules, the highly symmetric systems based on the skeletons of porphyrins and phthalocyanines are unsurpassed as OL materials in the visible

a η = nonlinear refractive index; β = nonlinear absorption coefficient; Im χ(3) = imaginary part of third-order susceptibility; σg = ground-state absorption cross section; κ = σexc/σg = excited-state cross section/ground-state absorption cross section ratio; Flim = limiting threshold; λ = wavelength of measurement; n.r. = not reported.

Z-scan

techn. λ (nm)

532 532 n.r. 0.1 2.3 2.65 1.9 10.2 n.r. n.r. n.r. n.r. 0.000033 n.r. 7.8 7.8

Flim (J/cm2) σexc/σgs σg (10−18 cm2) χ(3) (10−6 cm2/V2) β (10−8 cm/W) η (10−8 cm2/W) section compd

Table 16. continued

1679 1685

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photon absorption coefficient (or the imaginary part of fifthorder nonlinear susceptibility) for a fifth-order nonlinear optical process of simultaneous multiphoton absorption. If one considers also the mechanisms of nonlinear optical refraction and scattering, we are destined to read tables of nonlinear index coefficients, that is, an intensity-dependent coefficient correlated to the real part of the third-order nonlinear susceptibility, or first hyperpolarizability (alias the second-order nonlinear susceptibility). Susceptibilities of the various orders as well as absorption coefficients (or cross sections) of the different electronic states involved in nonlinear optical processes can be evaluated with several techniques and methods (photothermal techniques, Kerr effect, Hyper-Rayleigh scattering, two-photon fluorescence, three-wave mixing, DFWM, chirped-pulse pump− probe, or transmission measurements), but it will never be stressed enough that the evaluation of the OL action of a material has to be conducted only with the direct measurements of the optical transmission at the different levels of irradiation. In particular, the OL effect is evaluated properly with the Z-scan method, nonlinear absorption spectroscopy, or transient absorption spectroscopy with the pump and probe technique. This is because the evaluation of high-orders susceptibilities or high-orders absorption cross sections via methods that do not consider the phenomena of optical absorption at the wavelength(s) of analysis can lead to severe discrepancies between the expected behavior of the material and the actual one in an optical limiter due to the necessarily different beam characteristics and optical configurations that the realization of distinct nonlinear optical experiments requires. Also, within the framework of the experimental method or technique, the operator must pay attention to the physical features of the OL active material, that is, if this is encapsulated in a sample that constitutes a thick or thin absorber with respect to the geometry of beam focalization, and to the features of the optical systems, the f number. This renders generally very difficult the comparison of the OL performances by classes of photoactive materials that are acting according to different mechanisms. To circumvent in part this problem, the definition of figures of merit (FOM) has been considered taking into account the criterion that a FOM should be a dimensionless parameter, which is not dependent on the mechanism of OL action. A meaningful and very practical choice for applicative purposes has been the ratio of the optical transmission at low intensity level (Tlow) to the optical transmission at high level of intensity (Thigh) considering the different characteristics of the testing radiation for every OL experiment. In fact, a comparative work on OL effect has to specify the wavelength, the level of incident intensity of the radiation. Moreover, if the radiation source is a pulsed laser, beam shape (i.e., if CW, Gaussian, or squared hyperbolic secant), the pulse shapes (i.e., if top hat or Gaussian), frequency of irradiation, and duration (which ranges usually between 0.1 ps and 1 ms) have to be declared. Concerning the temporal features of the testing radiation, the choice of an OL active material under a predefined regime of irradiation should be considered very carefully because the corresponding mechanism of the OL effect should have a kinetics compatible with that imposed by pulse duration (and eventually by the frequency of pulse impinging). Another crucial issue in comparative studies of OL by materials of different nature is related to the relative importance of refractive/thermal effects with respect to absorptive effects in nonlinear optical regimes at the high intensity levels typical of nonlinear optics. Because

and NIR ranges for a variety of important reasons: high chemical stability, high electronic polarizability (both linear and nonlinear), strong one-photon and multiphoton absorption, well-defined electronic structure, versatility of their synthetic chemistry, possibility of varying the nature of the coordinating center, possibility of being chemically anchored on matrixes, modulation of the orientation of dipole moment changes during ground- and excited-state transitions for the presence of axial groups, possibility of retaining single molecule properties also in solid (polymeric) matrixes, possibility of varying in a controlled fashion the linear and nonlinear absorption properties, possibility of altering the distribution of the conjugated electrons by adding peripheral or axial substituents with strong electronic effects [either electron-donating (EDGs) or electronwithdrawing groups (EWGs)], and strong absorption properties in the solid state when deposited in the configuration of thin films. It does not surprise if the most common OL materials actually utilized in commercialized goggles or optical components for eye and photosensor protection against pulsed agile lasers are derivatives of these photoactive conjugated macrocycles. We have been able to collect the structures of more than 200 porphyrin- and phthalocyanines-based complexes in the present work to witness the plethora of working examples among these conjugated molecules for OL finalities. Interesting OL behavior has been shown also by push−pull organic molecules, which are characterized by strong permanent dipole moments in both linear and nonlinear optical regimes. This is the consequence of the simultaneous presence of EDGs and EWGs in push−pull systems, which impart high electronic polarizability at various different orders with occurrence of strong nonlinear absorption and refraction due to multiple photon effects. A variety of structures and symmetries, for example, linear, star-like, cross-like, multibranched, or dendrimer, can be of the push−pull type (or donor−π−acceptor) provided that the EWGs and the EDGs are symmetrically distributed on opposite sides of the molecular core. Most common examples of molecular cores with conjugated electrons for OL purposes are arylenes, fluorenes, indenoquinoxalines, carbazoles, (diphenylamino)stilbenes, oligothiophenes, oligofuranes, anthraquinones, indotricarbocyanines, or indigo dyes, just to cite a few classes. To such a great diversity of materials for OL applications, it corresponds an analogous variety of mechanisms to realization of the OL effect. These OL active mechanisms span from reverse saturable absorption, simultaneous two (or even three)photon absorption with the possible involvement of excitedstate absorption, to nonlinear refraction, nonlinear scattering, or thermal refraction, and in the special case of crystalline/ polycrystalline vanadium oxide(s), there is the possibility of producing OL also through a solid-state transition of the type semiconductor-to-metal, which brings about the increase of the optical reflectance and the accompanying diminution of transmittance. Therefore, the optical constants that change during the OL action generated by a given material vary with the nature of the material itself. It is not hard to find in the literature tables of parameters that are not correlatable straightforwardly when different classes of materials are compared. Constants of relevance for the evaluation of the OL effect can be the ground- and excited-state absorption cross sections in the case of sequential multiphoton absorption mechanism, two-photon absorption coefficient (or the imaginary part of the third-order nonlinear susceptibility) in the case of simultaneous multiphoton absorption, or three13187


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several OL generating mechanisms can take place either simultaneously or in a consecutive manner, the effectiveness of the OL action can be drastically altered if the configuration of the optics of radiation collection is open or closed in the actual OL device (also called optical limiter). If the OL action is mostly attained via absorptive mechanisms, one should expect that the photoactive material itself produces a very high value of the ratio Tlow/Thigh. On the other hand, a material acting as optical limiter prevailingly via refractive/scattering mechanisms will display a high value of the parameter Tlow/Thigh provided that the optics of collection has a closed geometry (characteristic obviously not dependent on the nature of the photoactive material but related to device configuration). The continuous search of materials for OL applications along so many different directions of materials science is a clear indication that a single material or even a single class of materials cannot satisfy all requirements imposed by the problem of optical power limiting. Because of the inexistence of a unique ideal material for OL applications, it makes more sense to combine in the same device OL active materials acting at complementary wavelengths or operating with different mechanisms (absorptive and refractive), and having complementary ranges of attenuation of the incident radiation intensity. At the theoretical level, with the ever-increasing computational power of machines, it is expected that the design of new molecular systems for OL applications will be progressively easier if modeling will consider the calculation of linear and nonlinear optical properties at the different wavelengths directly from the designed structure. Alternatively, computational chemistry/physics can take advantage of the continuously augmenting amount of tabulated nonlinear optical data, and conceive then programs that predict the nonlinear optical properties of relevance for OL by comparing the analogous parameters obtained experimentally for materials having a structure similar to those under design and theoretical investigation.

of conjugated macrocycles in the particular sector of optical limiting. In this context, Dini collaborated with several research groups from Europe, the U.S., and Singapore. Before joining the University of Rome in 2010 as research group leader, Dini got involved in the study of semiconductors with nanostructured features for a variety of electronic applications including not only nonlinear optics but also solar energy conversion. For the realization of this last part of research, Dini was hosted by Professor Johannes Gerhard Vos at the School of Chemical Sciences of Dublin City University in Ireland’s capital. Dini has authored and coauthored more than 100 papers. Mário J. F. Calvete received his Industrial Chemistry diploma from University of Coimbra in 2000 and his Ph.D. in Natural Sciences− Organic Chemistry in 2004, from Eberhard Karls University of Tübingen, Germany, supervised by Prof. Michael Hanack, with a thesis on the field of the synthesis of phthalocyanine derivatives for NLOOptical Limiting. After a two-year stay at Tübingen as a postdoctoral fellow, working in related fields of Optical Limiting, he returned to Portugal for a postdoctoral stay at University of Aveiro, working on the synthesis of porphyrin−phthalocyanine dyads. In 2010 he was appointed as Auxiliary Researcher at University of Coimbra. He is also Invited Auxiliary Professor, and his current research interests are in tetrapyrrolic macrocycle design and other heterocyclic ligands and their uses in homogeneous/heterogeneous catalysis, theranostics, and light-driven applications. He has published more than 60 peerreviewed papers in international journals, 1 book, and 4 book chapters. Michael Hanack received his Ph.D. in Chemistry from the University of Tübingen, Germany in 1957, and his Habilitation in Organic Chemistry soon after, in 1961. From 1962 to 1968 he was PrivatDozent, and in 1970 he was appointed as a Full Professor and Head of the Department of Organic Chemistry at the University of Saarland in Saarbrücken, Germany. In 1975 Michael Hanack was appointed Full Professor of Organic Chemistry at the University of Tübingen, Germany, until 2000, the occasion when he retired from the Chair of Organic Chemistry. Since then, he maintains fruitful activity as Emeritus Professor at the same university. He began his research work on organofluorine chemistry and stereochemistry. He very soon switched to physical organic chemistry, which remained his prime interest for the first half of his research period. From 1978 he gradually changed his topics to the synthesis of phthalocyanines and their derivatives, and the study of their properties, at first electrical, than conductive, and finally nonlinear optical properties. During his appointment at the University of Tübingen, he supervised over 230 Ph.D. students and almost as many Post-Docs. He was the Editor of many publications, wrote several books, and published more than 700 peer-review papers in international journals.

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies Danilo Dini obtained his Ph.D. in Materials Science in 1998 at the University of Rome LA SAPIENZA presenting a thesis on the electrochemical synthesis and chemical-physical characterization of conducting polymers with electrochromic properties. His Ph.D. mentor was Professor Franco Decker. In the postdoctoral time Dini dealt initially with the analysis of the electrochemiluminescent properties of conjugated systems in liquid dispersions and in the immobilized state at Fritz-Haber-Institut of the Max Planck Gesellschaft in Berlin-Dahlemdorf. Successively, the almost serendipitous encounter with Professor Michael Hanack in 2001 led Dini to consider the beautifully blue-colored phthalocyanines as a class of conjugated materials for an application quite more diverse than electrochemistry: nonlinear optics. In the successive five years spent in Tuebingen at the Institut für Organische Chemie of its university, Dini developed some synthetic skills and, more importantly, became progressively more confident with pulsed lasers for the characterization

ACKNOWLEDGMENTS For the realization of some parts of this Review, D.D. gratefully acknowledges financial support from Ateneo LA SAPIENZA (protocol no. C26A142SCB). M.J.F.C. thanks FCT-Portugal (Portuguese Foundation for Science and Technology) and FEDER − European Regional Development Fund through the COMPETE Programme (Operational Programme for Competitiveness) for funding (SFRH/BPD/99698/2014). D.D. is particularly indebted to Kathy at the office of Professor Josef Michl during the time of his Chemical Reviews editorship, for having provided hardly accessible bibliographic material in the field of optical limiting. 13188


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