Substitution, Environment, and Excitation Wavelength Effects on the

Subscriber access provided by Caltech Library Services

Article

Substitution, Environment and Excitation Wavelength Effects on the Optical Nonlinearities of Some Novel cis-, trans- #-Conjugated Azobenzenes Nikos Liaros, Tomas Marangoni, Fabrizio Cattaruzza, Davide Bonifazi, and Stelios Couris J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp508714b • Publication Date (Web): 30 Sep 2014 Downloaded from http://pubs.acs.org on October 6, 2014

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Title: Substitution, Environment and Excitation Wavelength Effects on The Optical Nonlinearities of Some Novel cis-, trans- π-Conjugated Azobenzenes N. Liaros1,2, T. Marangoni3, F. Cattaruzza3,4, D. Bonifazi3,4 , S. Couris*1,2 1

Department of Physics, University of Patras, 26504 Patras, Greece

2

Institute of Chemical Engineering Sciences (ICE-HT), Foundation for Research and Technology-Hellas (FORTH), P.O. Box 1414, 26504 Patras, Greece.

3

Department of Chemical and Pharmaceutical Sciences, INSTM UdR Trieste, University of Trieste, Piazzale Europa 1, 34127 Trieste, Italy 4

Department of Chemistry and Namur Research College (NARC), University of Namur (UNamur) Rue de Bruxelles 61, 5000 Namur, Belgium.

*

Corresponding Author:

Prof. Stelios Couris Department of Physics University of Patras 26504, Patras, Greece Tel: (+30) 2610 996086 Email: [email protected] , [email protected]

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract The transient nonlinear optical response of a series of highly π-conjugated 4,4’diethynylazobenzene derivatives, symmetrically substituted with different electron rich aromatic moieties (i.e., a N,N’-dihexylaniline, 4-ethynyl-(N,N’,dihexylaniline) and a Zn-(II)porphyrin fragment respectively) is investigated by means of the Z-scan technique. The systematic study of their nonlinear optical properties, using 4 ns laser excitation, revealed that all molecules possess very large third-order nonlinearity excited at 532 nm, whereas only the aniline derivatives showed NLO response under 1064 nm excitation. In-depth investigation, both in solutions and in thin films established that the NLO response of these azobenzene (AB) derivatives is totally attributed to their high refractive nonlinearity, presenting negligible nonlinear absorption. These findings strongly suggest that these molecules can be exploited for the development of new materials suitable for photonic/optoelectronic devices, since their strong nonlinear refraction combined with the absence of any nonlinear absorption ensure low losses, very low heating of the organic material and eventually longer operative-lifetimes.

Keywords: Z-scan, nonlinear optical response, third-order susceptibility, azobenzene-based molecular systems ACS Paragon Plus Environment

Page 2 of 30

Page 3 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.

Introduction Since the discovery of the second harmonic generation (SHG) in 1961,1 where two

photons interacting with a “nonlinear medium” can generate a photon at doubled frequency, intensive scientific investigations on nonlinear optical materials have been performed.2-9 Indeed, among other things, it has been shown that materials possessing high nonlinear optical (NLO) response can allow an effective manipulation of the fundamental properties of light, giving rise to a plethora of new phenomena ranging from frequency doubling or tripling (SHG, THG) and self-focusing or self-defocusing to phase conjugation, optical limiting, solitons generation, etc.10-18 As a result, in the past few years, materials possessing high NLO response have emerged as promising candidates for the realization and development of photonic integrated devices, which can be exploited in optoelectronics and nanophotonics applications.19-21 In particular, a strong interest in organic NLO materials has emerged due to their superior speed and responsive magnitude.22-27 More recently, great efforts have been dedicated to the investigation of switchable molecular materials, the NLO response of which can be reversibly modulated by means of an external stimuli such as redox, pH or light.28-32 In the latter family of compounds, azobenzene (AB) and its derivatives represent one of the most important class of molecular switches that, through a light or heat induced stimuli, can selectively and reversibly isomerize form the trans (E) to the cis (Z) configuration and vice-versa.33-38 At the molecular level, the isomerization reaction dramatically modifies the spatial redistribution of the electron density and affecting its polarizability, thus providing an efficient tool for controlling and inducing large variations of the molecular optical nonlinearities.39 In a very recent work,40 our groups reported on the NLO response of three different azobenzene derivatives 1, 2 and 3 (Fig. 1a) bearing electron-donating moieties, as studied by means of the Z-Scan technique41 employing picosecond laser excitation. All the derivatives exhibited very large optical nonlinearities under both visible and infrared excitation, both in solutions and in the form of thin films. Moreover, a confirmation of the modulation effect of the photoswitching process on the NLO response of AB-derivative 1 has been demonstrated. In this context, an enhancement of the third-order susceptibility was observed when passing from the trans to the cis photostationary state (PSS). However, to efficiently exploit these azobenzenebased materials at the level of application, they must satisfy additional physical requirements.42 Specifically, the majority of the applications (i.e. optical computing or alloptical signal processing) require NLO materials possessing large nonlinear refraction and minimal, if not at all, nonlinear absorption since the latter is associated with high losses and ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

unwanted heating of the material contributing to material deterioration, both effects being undesirable for photonic application.43 Herein, we present a natural evolution of this work, aiming to extend our previous investigations on the electron-rich AB derivatives 1-3, investigating their transient NLO response under nanosecond timescale excitation. As it will be shown, these investigations shed further light on the fact that the previously reported AB library is characterized by a very high nonlinear refractive response under irradiation with ns laser pulses, while exhibiting remarkably low nonlinear absorption, which can be practically considered as negligible, and thus of great potential for materials application.

2. Experimental Methods The transient nonlinear optical response of AB derivatives 1-3 has been investigated by means of the Z-scan technique,41 employing the visible (532 nm) and infrared (1064 nm) outputs of a 4 ns Q-switched Nd:YAG laser operating at a repetition rate 1-10 Hz. For the measurements, several different concentration solutions of the AB derivatives 1-3 in CH2Cl2 were prepared. Representative UV-Vis-NIR optical absorption spectra of these solutions (0.14 mM) are presented in Figure 1b. The samples were contained in 1 mm path length quartz cells and the laser beam was focused into the cells by means of a 20 cm focal length quartz plano-convex lens. The spot diameter of the laser beam at the focus (i.e., full width at 1/e2 of the maximum of irradiance) was measured using a CCD camera and it has been determined to be (36±6) and (62±6) µm at 532 and 1064 nm, respectively, while the Rayleigh length z0 was calculated to be 1.9 (532 nm) and 2.8 mm (1064 nm), that is much longer than the thickness of the quartz cells. More details about the Z-scan experimental setup and the procedure for the analysis of the experimental Z-scan data can be found elsewhere.40,41,44 Briefly, from the “open-aperture” (OA) Z-scan transmission measurements, the nonlinear absorption coefficient β of a sample can be determined. From the division of the “closed-aperture” (CA) Z-scan by the corresponding OA scan, the so-called “divided” Z-scan can be obtained, from which under low nonlinear absorption conditions and the nonlinear absorption coefficient β being previously determined from the OA Z-scan measurement, the nonlinear refractive parameter γ′ of the sample can thus be obtained. Having determined the nonlinear optical parameters, γ΄ and β, the real and imaginary parts of the third-order nonlinear susceptibility χ(3) can be easily calculated from the following relations: ACS Paragon Plus Environment

Page 4 of 30

Page 5 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Im χ ( 3) ( esu ) =

10−7 c 2 n02 β ( cmW −1 ) 96π 2ω

(1a)

Re χ (

10−6 c n02 γ ' ( cm2 W −1 ) 2 480 π

(1b)

3)

( esu ) =

where ω (in s-1) is the frequency of the laser excitation light. Since the determined third-order susceptibility χ(3) of a sample depends on the concentration, in order to compare the nonlinear optical responses of the AB derivatives 1-3, the corresponding second hyperpolarizability γ values have been calculated; these values are independent of the concentration, being molecular constant, revealing the third-order optical nonlinearity per molecule:

γ=

χ (3) N L4

(2)

where N is the number of molecules per unit volume, L is the Lorentz–Lorenz local field correction factor defined as L=(n02+2)/3, and n0 being the refractive index of the solvent.

3. Results and discussions Structurally, the backbone of 1-3 is formed by a photoswitchable core consisting of a 4,4’diethynylated azobenzene (Fig. 1a), to which different π-conjugated moieties are linked (N,N’-dihexylaniline, 4-ethynyl-(N,N’,dihexylaniline) and a Zn-(II)-porphyrin fragment for 1, 2 and 3, respectively. The choice to adopt highly conjugated substituent was principally aimed to increase the electron delocalization along the molecular scaffolds, which is known to be responsible for the enhancement of the molecular polarizability and therefore the NLO response.45,46 As previously described,47 the UV-Vis-NIR optical absorption spectra of 1-3 (Fig. 1b) in their thermally equilibrated state, show that these derivatives mainly exist as trans AB isomers which subsequently, upon irradiation at the maximum of their intense absorption (λmax trans: 450 nm for 1 and 2, and 360 nm for molecule 3), are able to undergo a trans→cis photoisomerization leading to a cis-enriched PSS state. The first assessment of the NLO response of 1-3 was performed in solution using CH2Cl2 as solvent, which restrains effective isomerization of the AB core to take place,40,47 therefore leading to the evaluation of the NLO response for the trans-enriched PSS species. In Figures 2a-c, the schematic structures (upper part) and some representative “divided” Z-scan recordings of different concentrations CH2Cl2 solutions of 1-3 are presented. Z-scan measurements were performed separately under visible (532 nm) and infrared (1064 nm) laser ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

excitation. Due to the lower NLO response exhibited by all three AB derivatives under infrared excitation, about two orders of magnitude higher concentration solutions and laser energies were employed (see e.g. Fig. 2). As shown, under 532 nm laser excitation, all 1-3 molecules exhibited important nonlinear refraction of negative sign, as indicated by the peakvalley configuration of all the “divided” Z-scans, suggesting a clear self-defocusing behavior (i.e. negative Reχ(3) or n2). Simultaneously, all studied molecules were found to exhibit negligible nonlinear absorption for both visible and infrared excitation wavelengths, for the range of laser energies employed. This finding, being of great importance for several applications, denotes that under visible and/or infrared nanosecond laser excitation conditions the NLO response of molecules 1-3 is entirely dominated by the nonlinear refraction. From the “divided” Z-scans shown in Figures 2a-c, and taking into account the laser energy employed in each case and the concentration of the corresponding sample, it becomes evident that the NLO response of molecules 1-3 under 532 nm laser excitation was found to be significantly much larger than that observed for 1064 nm excitation. In fact, for infrared excitation, almost two orders of magnitude higher laser energy and concentration were used in order to obtain similar magnitude ∆Τp-v values from the “divided” Z-scan recordings with those employed in the case of visible excitation. Interestingly, molecule 3, in which the AB core is linked with the Zn-(II)-porphyrin moiety, was found to exhibit negligible NLO response under 1064 nm excitation. In Figures 2d-f the variation of the ∆Tp-v parameter as a function of laser energy, for different concentration solutions, is presented. As can be seen from the graphs, the values of the ∆Tp-v parameter for all solutions were found to increase with the concentration and scale linearly with the laser energy, as indicated by the straight lines, corresponding to the linear best fits of the experimental data, also shown in these graphs. From the slopes of these straight lines, the nonlinear refractive index parameter γ′ was deduced for each sample.48 Similar graphs have been prepared in the case of infrared excitation. Finally, the real part of the third-order susceptibility, Reχ(3), was calculated from Equation 1b and the magnitude of the third-order nonlinear susceptibility χ(3) was obtained. Then, from the concentration dependence of the third-order susceptibility χ(3), the value of the second hyperpolarizability γ has been determined. In Figures 3a and 3b, the variation of the third-order susceptibility of the CH2Cl2 solutions of molecules 1-3 versus concentration is depicted for 532 and 1064 nm laser excitation respectively. As shown, in all cases, a good linear correlation was found to hold ACS Paragon Plus Environment

Page 6 of 30

Page 7 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


between the two quantities. From the slopes of the straight lines shown in Figure 3 (corresponding to the linear best fits of the experimental data points), the second hyperpolarizability γ values have been calculated. The results are summarized in Tables 1 and 2, for 532 and 1064 nm laser excitation respectively. In particular, the third-order susceptibility χ(3),

the

nonlinear refractive

index parameter γ′ and the second

hyperpolarizability γ values of molecules 1-3 are presented. All AB molecules were found to possess very large nonlinear refraction, among the largest reported for similar molecular systems,49-53 and more importantly, they all exhibited negligible nonlinear absorption. The combination of important nonlinear refraction with negligible nonlinear absorption is a particularly advantageous situation for the exploitation of these molecules in integrated devices, making them very promising candidates for different photonic and opto-electronic applications. Molecules 1-3 were found to also exhibit strong, negative sign nonlinear refraction at both excitation wavelengths, corresponding to a strong self-defocusing behavior. Whereas molecule 1 possesses the largest response, 2 and 3 exhibited similar size response for visible excitation. Interestingly, extension of the measurements in the infrared region, have revealed a different picture. In particular, molecule 2 was found to exhibit the largest NLO response, while 3 did not exhibit any measurable response, even for the highest laser intensities employed (up to 0.6 GW/cm2). Concerning the larger NLO response of 1-3 determined under visible laser excitation, compared to that for infrared excitation, this situation can be understood by considering the one-photon resonant enhancement which can be operative only for visible excitation conditions. In fact, as it is shown in the absorption spectra of Figure 1b, molecules 1 and 2 exhibit two strong absorption peaks at about 450 nm and 350 nm, associated to the typical π-

π* transitions of azobenzene based molecules, corresponding to transitions from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). In the case of visible laser excitation, i.e. at 532 nm, the laser wavelength falls at the long wavelength absorption tail of the 450 nm absorption band (Fig. 1b), thus providing significant oscillator strength, facilitating one-photon absorption, explaining the significant enhancement of the NLO response. In the case of molecule 3 however, the 450 nm π-π* absorption band is shifted at shorter wavelengths (i.e. higher energy), at about 360 nm, making improbable one-photon resonance enhancement. Another, relatively intense absorption band appearing at



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

the optical absorption spectrum of 3 at about 550 nm, associated with the Q-band of the zincporphyrin moiety, seems not to facilitate any enhancement of the NLO response of 3. From the values of the second hyperpolarizabilities given in Tables 1 and 2, the relative enhancement of the NLO response of 1 and 2 in terms of the ratio γ532/γ1064 can be calculated. So, for AB 1 an enhancement factor of 18×103 was obtained, while a factor of 103 was found for 2. Molecule 3 displaying a sizeable response for visible excitation, similar to that of 2, was found to exhibit negligible response for infrared excitation. Taking into account the very similar structures for molecules 1 and 3, it becomes evident that the presence of the two Zn– porphyrin termini is switching off the NLO response of molecule 3 when excited in the infrared light, thus providing a chemical way to control the strength of the NLO response. As shown in Figure 1b, the solutions of 1-3 were presenting insignificant absorption in the spectral region 600-1200 nm, suggesting the absence of any one photon resonant enhancement. Moreover, the possibility of two-photon absorption is rather improbable because of the low laser intensities used. Therefore, the difference between the observed nonlinearities of 1 and 2, under 1064 nm excitation, should reflect the effects of the nature of the substituent attached to the AB core and the conjugation length. In that respect, the presence of two ethynyl spacers in 2, increasing the conjugation length (compared to the one ethynyl spacer of 1) results in larger NLO response under infrared excitation. In fact, the present findings suggest that longer conjugation length result to larger NLO response, when all other factors influencing the NLO response remain unchanged (i.e., excluding resonance enhancement). This is in excellent agreement with the results reported in previous works on AB based molecules,23 in which increasing the conjugation length resulted in larger second hyperpolarizability values. In the current case, molecule 2 was found to exhibit almost one order of magnitude larger second hyperpolarizability compared to 1 (see Table 2). Furthermore, comparing the response of molecules 1 and 2, under visible excitation, the second hyperpolarizability of AB 1 was found to be much higher than that of 2 (see e.g. Table 1). Taking into account the opposite behavior found for infrared excitation, in the absence of any enhancement, it is reasonable to conclude that the larger NLO response of 1 in the visible spectral range can be explained by evoking the larger absorption of 1 compared to 2 at the laser excitation wavelength, leading to more efficient resonant enhancement therefore masking the effect of the longer conjugation length.54


Page 8 of 30

Page 9 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


In an effort to elucidate and quantify the contribution of each isomer on the NLO response of these AB-based molecules, trans-enriched and cis-enriched solutions have been prepared and their NLO response has been investigated in details. For this, molecule 1 was chosen as it was found exhibiting the largest NLO response among the three molecules studied. So, for that purpose, trans-enriched (herein named simply trans-PSS) solutions of 1 have been prepared in CH2Cl2. The use of this solvent was based on the fact that it is known to restrain the effective isomerization of the AB core, therefore leading to trans-enriched solutions. For the case of the cis-isomer of 1, some cis-enriched solutions (herein named simply cis-PSS) were prepared in cyclohexane (CHX), since this solvent is known to allow the trans-cis isomerization to take place upon suitable irradiation of the sample, i.e. by photoinduced isomerization. For the irradiation of the samples the output from a 150 Watts Xe arc lamp was used, after having passed through an appropriate optical filter centered at 460 nm and having bandwidth of 10 (±2) nm. In Figure 4a, the UV-Vis-NIR optical absorption spectra of a 0.22 mM solution of 1 in CHX are shown before (i.e. trans-PSS solution) and after irradiation (i.e. cis-PSS solution). In Figure 4b, the temporal evolution of the trans- and cis- populations are shown, where the decrease of the trans isomer population is indicated by the significant lowering of the 450 nm band and the simultaneous increase of the cis isomer population is indicated by the gradually increment of the 310 nm absorption shoulder. Within about 10 minutes of irradiation, the population of the cis isomer was observed to attain a cisenriched PSS state. Under nanosecond laser excitation, the NLO response of a material is characterized as transient response rather than an instantaneous one, being determined essentially by the photophysical parameters of the system, the linear absorption being the most important. In that view, samples exhibiting the same absorbance at the wavelength where excitation is occurring (Fig. 4), they should exhibit a priori the same response. In that case, any difference in the NLO response might be attributed e.g. to the different NLO response of the isomers. However, the NLO response of both the trans-PSS and cis-PSS solutions were found similar, indicating that the two isomers possess similar NLO response under 532nm laser excitation, at least within the experimental error. It is interesting to note that similar solutions revealed different NLO response using 35 ps laser excitation, the cis-PSS solutions exhibiting larger NLO response compared to that of the corresponding trans-PSS ones.40 The difference of the NLO response between ns and ps laser excitation is arising from the different physical mechanisms occurring. In particular,



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

under ps laser excitation, electronic and vibrational effects and/or molecular redistribution are the main mechanisms contributing to the NLO response (instantaneous NLO response), as it has been suggested elsewhere.48,55 However, under ns laser excitation population redistribution among the excited states dominates (transient NLO response).48 In the latter case, significantly larger values of the NLO parameters are usually obtained. Thin films’ transient NLO response. In order to investigate the influence of the environment of the AB-based molecules on their NLO response, thin films of molecule 1 have been prepared by spin coating. In general, in films, the interactions between the molecules are expected to be stronger (often leading to increase aggregation) than in solutions, therefore affecting significantly their NLO response. The films were developed on glass substrates and were homogeneous having a thickness of 3-3.5 µm. Z-scan measurements were performed on these films and have revealed strong NLO response under visible excitation but insignificant response in the infrared. More in detail, the films were found to exhibit negative nonlinear refraction and saturable absorption (i.e., β

Substitution, Environment, and Excitation Wavelength Effects on the

Recommend Documents