J. Phys. Chem. B 2005, 109, 10179-10183
10179
Third-Order Nonlinear Optical Figure of Merits for Conjugated TTF-Quinone Molecules I. Fuks-Janczarek,†,§ J. Luc,† B. Sahraoui,† F. Dumur,‡ P. Hudhomme,‡ J. Berdowski,§ and I. V. Kityk*,§ Laboratoire POMA, UFR Science, UMR CNRS 6136, UniVersite´ d’Angers. 2, BouleVard, LaVoisier 49045, Angers, France, Laboratoire Chimie, Inge´ nierie Mole´ culaire et Mate´ riaux d'Angers (CIMMA), CNRS UMR 6200, UniVersite´ d’Angers. 2, BouleVard LaVoisier, 49045 Angers, France, and Institute of Physics, J. Długosz UniVersity of Czestochowa, Pl-42217, Al. Armii Krajowej 13/15, Czestochowa Poland ReceiVed: February 20, 2005; In Final Form: March 24, 2005
We have revealed a substantial enhancement of third-order optical figure of merits by the synthesis of a compact molecule possessing the tetrathiafulvalene (TTF) group with two backside CdO groups. Addition of the saturated methylene chain substantially suppresses the third-order optical figure of merits and even local optical hyperpolarizabilities at λ ) 532 nm. Another TTF-derivative molecule possessing ethylenic and acetylenic chains demonstrates large hyperpolarizabilities; however, generally, the figure of merit factor decreases due to the increasing optical losses as a consequence of enhanced linear absorption. At the same time, both of the chromophores have a large nonlinear optical response. General approaches for search and design of the third-order optical materials with improved properties are given.
Introduction Recently, one can observe great efforts in the creation of organic materials possessing improved third-order optical properties.1-3 Research of new materials, which have great potential in nonlinear optical devices, is one of the current interesting subjects for investigation.4 There is still need for a new transparent material with very low losses and a large nonlinear optical response. The miniaturization of electronic devices has renewed a great interest devoted to molecular electronics.5 Of particular interest at present are degenerate four wave mixing (DFWM) and two-photon absorption.6,7 The mentioned effects are of importance during the design of optical limiters, organic optically recorded gratings, and so forth. One of the most important parameters here is the figure of merit which is equal to a ratio between third-order optical susceptibility to the absorption. For a possibility to compare this parameter for the different compounds, we have renormalized the value by the refractive index and geometry factor and present the parameter in the arbitrary units. It is important to have the maximal value of the parameter for the spectral range covering wavelengths of emission for the typical lasers Nd:YAG and corresponding optical second harmonics. It is well-known that an increasing number of conjugated chemical bond numbers leads to an increase of the corresponding susceptibilities; however, this one is achieved due to the decreasing transparency related to the decrease of the HOMO-first excited state energy gap. As a consequence, different sophisticated molecular engineering approaches were developed to obtain the enhanced figure of merit factors. Another possible solution consists of the addition of long C-C tails substantially localizing the chromophore molecule.9 The main drawback of this solution consists of the necessity of the synthesis of a large molecule which is unstable against the * Corresponding author. E-mail:
[email protected]. † UMR CNRS 6136, Universite ´ d’Angers. 2. ‡ CNRS UMR 6200, Universite ´ d’Angers. 2. § J. Długosz University of Czestochowa.
Figure 1. Chemical structure of the studied compounds TTFdiquinone (1) and TTF-monoquinone (2).
thermal destruction and phototreatment. At the same time, their incorporation into the different host polymer matrixes is a relatively complicated task. Thus, the main strategy in this direction should be devoted to a search of materials with the modified backside groups in the chromophore. Contrary to second-order optics, for the third-order optic susceptibilities, it is not necessary to achieve a non-centrosymmetry in electron charge density distribution; however, the presence of the doubled backside carbon-oxygen bonds situated symmetrically with respect to the tetrathiafulvalene (TTF) plane groups may be crucial because in this case there appears to be large local dipole moments which play a crucial role in the output third-order susceptibility, although for the total molecule, the dipole moments may be compensated, giving a relatively low susceptibility value. Therefore, in the present work, we will try to modify the backside group in the relatively small tetrathiafulvalene-quinone molecule (see Figure 1) to achieve an enhancement of the figure of merits compared to other TTF derivatives such as dithienylethylene, dithienylbutadiene, acetylenic, ethylenic, and bisdithiafulvenyl ones. Within a phenomenological description, the polarization vector of a material medium can be represented in the form of a power series (with respect to successive powers of the electric field amplitude) and different optical phenomena are connected with each term of this expansion. Besides the fundamental importance for the understanding of physical mechanisms in the matter-strong laser field interactions, the research in nonlinear optics has led to numerous important applications. These ones present potential applications motivating extensive
10.1021/jp0508711 CCC: $30.25 © 2005 American Chemical Society Published on Web 04/26/2005
10180 J. Phys. Chem. B, Vol. 109, No. 20, 2005
Figure 2. UV-visible spectra of compound 1 (10-3 M) in different solvents.
research for new effective materials for nonlinear optics, that is, materials presenting a large nonlinear response to the external optical field, weak absorption and scattering, and a short response time. In these investigations, an important role is played by different organic compounds, which can possess large optical nonlinearities, are chemically flexible, and can be prepared in different forms.1-9 The third-order nonlinear optical effects have been observed in molecules with highly delocalized π-electrons using a degenerate four wave mixing experiment, in which the chain of conjugated bonds facilitates the polarization of the molecule in the external field. In particular, the organic conjugated molecules can present third-order nonlinearities.10-12 In the present article, as basic chromophore molecule are chosen the new synthesized TTF derivatives which are wellknown as precursors of organic metals presenting remarkable conducting properties.13 Their extended molecular structure promotes the occurrence of delocalized π-electrons created by the conjugated bonds. These properties suggest that the studied compounds are suitable compounds for nonlinear optic studies. In this paper, we report on a systematic study of the thirdorder nonlinear optical properties and second-order nonlinear optical hyperpolarizability of two extended tetrathiafulvalene derivatives (TTF-diquinone triad (1) and TTF-monoquinone dyad (2)) (see Figure 1). A comparison of their properties with other TTF derivatives is given. The TTF-diquinone triad (1) and TTF-monoquinone dyad (2) are of great interest due to their ability to easily donate or accept an electron, thus constituting the basic concept of organic electronic devices. Preparation of the Chromophore The multistep synthesis of the triad TTF-diquinone (1) and the dyad TTF-monoquinone (2) was very recently described.14 Spectroscopic data (13C NMR and infrared spectra) together with the electron charge density distribution obtained by theoretical calculations were in agreement with a strong polarization of the SsCdCsCdO conjugated system.15 However, the X-ray crystallographic analysis of triad (a) revealed that the molecule is nearly planar, and only the carbonyl groups deviated with an angle of 4° with respect to the molecular plane. The UV-visible spectra of 1 and 2 are shown in Figures 2 and 3. A broad absorption band is present in the 500-1100 nm region which clearly indicates a weak charge-transfer interaction resulting from the conjugation involving both donor and acceptor moieties. The absorption maximum of this band demonstrates
Fuks-Janczarek et al.
Figure 3. Visible spectra of compound 2 (10-3 M) in different solvents.
a pronounced negative solvatochromism, which decreases upon going from a low polarity medium to a high polarity medium (CH2Cl2, 782 nm; THF, 739 nm; DMF, 735 nm). For TTFmonoquinone (2), a bathochromic effect of this charge-transfer band was noted (600-1200 nm) according to the following absorption maxima: CH2Cl2, 847 nm; THF, 773 nm; DMF, 758 nm. Third-Order Nonlinear Susceptibility The nonlinear optical properties of the matter are caused by the fact that the electric polarization vector, P, of the sample is a nonlinear function of the electric field, E, of the electromagnetic wave propagating in a medium. In general, the quantum mechanical analysis based on the microscopic model of the matter constituting the medium is necessary to analyze the response of a medium to an external electromagnetic field. A standard approach for considering the phenomena in nonlinear optics is to expand the polarization of the medium P ) P(E) into a power series of E: 〈2〉 〈3〉 〈1〉 〈2〉 Pi ) P〈1〉 i + Pi + Pi + ... ) χij Ej + χijk EjEk +
χ〈3〉 ijklEjEkEl + ... (1) Here, the indices i, j, k, and l indicate the components of the electromagnetic wave polarizations in the laboratory arbitrary setup. The terms beginning from the second describe the nonlinear phenomena of the corresponding order.16,17 The coefficients χ〈n〉 are defined as the electrical (optical) susceptibilities of order n, and their detailed description can be found in classical textbooks on the subject. For us, particular interest presents the third terms corresponding to the third-order susceptibilities. The susceptibility tensor characterizes the nonlinear optical response of the dielectric material on the macroscopic level. The physical quantity that characterizes optical nonlinearities of an individual molecule is its second-order optical hyperpolarizability, γ. In the case of solutions, there exists the following relation between the susceptibility χ〈3〉 of a solution and hyperpolarizability, γ, of an individual molecule:18 4 χ〈3〉 ) χ〈3〉 solvent + F Nγ
(2)
where F ) (n2 + 2)/3 is the so-called Lorentz local field factor coefficient, N ) NAC/M is the number of dissolved molecules per unit volume, M and C are respectively their molar mass
{
Third-Order Nonlinear Optical Figure of Merits
∂A1 R ) - A1 + iHχ〈3〉(A1A/1 + 2A2A/2)A1 ∂z 2 ∂A2 R ) A2 - iHχ〈3〉(2A1A/1 + A2A/2)A2 ∂z 2 ∂A3 ) -ΦA3 + 2iHχ〈3〉A1A2A/4 dz ∂A4 ) ΦA4 - 2iHχ〈3〉A1A2A/3 dz
Figure 4. Schematic diagram of the geometry of the light beams in a four wave mixing experiment.
and concentration, NA is Avogadro’s number, and n is the refractive index of the medium. In our experiments, χ〈3〉 solvent was negligibly small. 〈3〉 In our case, χ〈3〉 solvent which is very weak compared to χsolution 18 at the concentration used could be neglected, and the reliability of our experiment and the second-order hyperpolarizability, γ, characterizing molecular optical nonlinearities could be estimated through the following equation:
γ ) χ〈3〉/F4N
(3)
(4)
The wave amplitudes and thus the value of R depend on polarizations of the beams, and one should write Rijkl instead of R to indicate that the polarization states of interacting waves 〈4〉, 〈1〉, 〈2〉, and 〈3〉 are i, j, k, and l, respectively. To simplify the notation, we will omit the corresponding polarization indices in the following formulas and we present only the general idea of the theoretical model used. More details of the model used can be found in refs 21-24. To evaluate the third-order optical parameters described by the third-order susceptibility, we use the experimental data of the DFWM efficiency and the theoretical formula for DFWM output coefficient calculated in the frame of a model based on the nonlinear propagation equations.21,22 The laser waves propagating in the medium are taken as plane waves E〈i〉(r,t) ) A〈i〉(r) exp[i(kir - ωit)] + c.c. (i ) 1, 2, 3, 4). Using the slowly varying amplitude approximation and assuming that all three incident waves are linearly polarized along the x axis, one can obtain the following equations describing the wave propagation in the medium (in cgs units):
{ } p2 +
DFWM Setup
R ) I〈4〉(z ) 0)/I〈3〉(z ) 0)
(5)
〈3〉 ) χ〈3〉 , H ) 12π2/nλ, Φ ) R/2 - 2iHχ〈3〉where Ai ) A〈i〉 x , χ xxxx / / (A1A1 + A2A2), n is the linear refractive index of the material, λ is the wavelength of the laser light, and R is the linear absorption coefficient. The intensity of the beam is now Ii ) (nc/2π)AiA/i . If the medium shows only linear absorption, the third-order susceptibility is a real value and the above system can be solved to give the DFWM efficiency (reflectivity):
where N is the number density of the molecules in solution.
To measure the third-order susceptibilities, we use the degenerate four wave mixing (DFWM) method.18-21 A schematic diagram for realization of the DFWM is presented in Figure 4. The nonlinear medium is irradiated by three waves of frequency ω: two of them (beams 1 and 2) are strong counterpropagating beams traveling in opposite directions (pump beams), and the third one (probe beam) is a much weaker beam which makes a small angle (5° in our experiments) with respect to the direction of pump beam propagation.. The nonlinear polarization of the medium contains thus a component which is the source of the fourth wave (signal wave) of the same frequency, ω, and propagating in a direction backward to the probe beam. In the DFWM experiment, the intensities of the light beams satisfied the following relations: I〈1〉 (z ) 0) ) I〈2〉 (z ) l) and I〈3〉, I〈4〉, I〈1〉. The main parameter of the DFWM process is its efficiency defined as
}
J. Phys. Chem. B, Vol. 109, No. 20, 2005 10181
R)
I〈4〉(0) 〈3〉
)
I (0)
[ [
R2 4
p2 g 0 R2 p(cot(pl)) + 2 R2 2 p + 4 p2 < 0 R2 q(cth(ql)) + 2
]
(6)
]
where
p2 )
(
)
48π3 〈3〉 2 〈1〉 2 R2 χ I (0) exp(-Rl) - , q ) ip. 2 4 n cλ
A general discussion of this model and its validity limits, taking into account linear and nonlinear absorption, was recently presented.23-25 The values of the third-order susceptibility, χ〈3〉, can be obtained by adjustment of the theoretical curve given by eqs 5 and 6 to the experimentally measured values of the efficiency, R. Third-Order Optical Figure of Merits Organic chromophore molecules 1 and 2 were solvated in chloroform or tetrahydrofuran (THF) to do the DFWM measurements (see Figure 1). The thickness of the cell containing the sample was l ) 10-3 m. Their UV-visible spectra are presented in Figures 2 and 3. We studied the influence of different solvents on the UV absorption of the sample. The excitation was provided by 30 ps laser pulses at λ ) 532 nm generated by an amplified mode-locked Quantel Nd:YAG laser operating with a 1 Hz repetition rate. The first step of the experimental procedure was to measure an intensity of the signal wave I〈4〉 for several solution concentrations to determine the concentration for which the output I〈4〉 is maximal.21-23 In the second step, we measured the DFWM efficiency versus the intensity of the pump beam. The analysis of transmission for the range of the pump intensities up to 1.2 GW/cm2 has shown that the studied molecules exhibited only linear absorption (see Figure 5). Taking into account the values of the absorption coefficient at a given concentration, C, and the value R for the four waves mixing, the values of χ〈3〉 could be evaluated from eq 5 as the only free parameter in this equation. The efficiency, R, was measured as a function of the intensity of wave 〈1〉 (in
10182 J. Phys. Chem. B, Vol. 109, No. 20, 2005
Fuks-Janczarek et al. TABLE 1: Experimental Results from the Theoretical Expression of Degenerated Four Wave Mixing Given by eq 5 Being Fitted to the Experimental Data Shown in Figure 6, in Order to Deduce the Nonlinear Optical Susceptibilities for Both Studied Compounds
Figure 5. Transmission measurements for the studied compounds 1 and 2 at 532 nm.
Figure 6. DFWM efficiency of the studied compounds 1 (2) and 2 (9) at a wavelength of 532 nm.
all experiments, I〈3〉 ) 0.01I〈1〉) for linear polarizations of the interacting waves 〈1〉, 〈2〉, and 〈3〉. Figure 6 presents typical experimental behavior for the studied compounds which clearly show a strong third-order optical response. From Table 1, one can see that chromophore 1 possess the largest figure of merit compared to the other TTF-derivative chromophores such as dithienylethylene, dithienylbutadiene, acetylenic, ethylenic, bisdithiafulvenyl, and so forth. Thus, enhancement of the π-conjugated tails leading to enhancement of the microscopic hyperpolarizabilities is not so high to compensate the increasing optical losses due to narrowing of the HOMO-first excited state energy gap. At the same time, addition of the saturated chains such as for sample 2 substantially enhances the losses caused by the substantial screening of the local dipole moments. Conclusion We have presented studies of third-order nonlinear optical figure of merits for two new TTF-derivative small moleculess TTF-diquinone (1) and TTF-monoquinone (2)swhich belong to tetrathiafulvalene derivatives. Molecular orbital calculations
of these compounds suggested a strong polarization of the conjugated system certainly resulting from electronic delocalization over the extended framework. In consequence, a strong nonlinear optical response of the molecule to the external electric (light) field can be expected. Chromophore 1 possesses the largest figure of merit compared to the other TTF derivatives such as dithienylethylene, dithienylbutadiene, acetylenic, ethylenic, bisdithiafulvenyl, and so forth. Thus, for polyenic derivatives, enhancement of the π-conjugated tails leading to enhancement of the microscopic hyperpolarizabilities is not so high to compensate the increasing optical losses during narrowing of the HOMO-first excited state energy gap. At the same time, addition of the saturated methylene chainlike fragments for sample 2 substantially enhances the optical losses caused by the substantially larger total state dipole moments. The second-order hyperpolarizabilities of the studied molecules are about 103 times greater than γ of CS2, which is the reference material for the DFWM measurements. The increase of the conjugation length is in favor of an increase of its hyperpolarizability. In conclusion, we have shown that the small molecules under consideration present small absorption and their merit factor is relatively large compared to polyenic derivatives of TTF. References and Notes (1) Kost, A.; Tutt, L.; Klein, M. B.; Dougherty, T. K.; Elias, W. E. Opt. Lett. 1993, 18, 334.
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