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Geometry and quadratic optical properties of twisted push–pull biphenyls
Synthetic Metals 124 (2001) 209±211
Geometry and quadratic optical properties of twisted push±pull biphenyls A. Forta,*, A. Boeglina, L. Magera, C. Amyotb, C. Combellasb, A. ThieÂbaultb, V. Rodriguezc a
IPCMS, Groupe d'Optique Non-LineÂaire et d'OptoeÂlectronique, 23 rue du Loess, 67037 Strasbourg Cedex, France b ESPCI, Environnement et Chimie Analytique, 10 rue Vauquelin, 75231 Paris Cedex, France c Laboratoire de Physico-Chimie MoleÂculaire, 351 Cours de la LibeÂration, 33405 Talence Cedex, France
Abstract Push±pull biphenyl compounds appear to be model systems for studying the large sensitivity to the dielectric environment of zwitterionic molecules. We present here a systematic theoretical and experimental study, combining numerical simulations and NMR, IR and NLO measurements on two analogous diaryl molecules dissolved in solvents with varying polarities. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Twisted biphenyls; Geometric structure; Non-linear optical properties
1. Introduction In the past decade, noticeable progress has been achieved in the understanding of correlation between structure and non-linear optical (NLO) properties of charge transfer molecules. These push±pull molecules, formed with donor and acceptor end groups connected via a p-conjugated system, are well suited for quadratic optical effects [1]. Various theoretical models have been developed to calculate the NLO properties of push±pull molecules. A simple two-level formalism, well adapted to real experimental conditions of NLO measurements, has been used as a guideline for the synthesis of molecules possessing very high quadratic hyperpolarizabilities [2,3]. In the case of push±pull polyenes, Marder et al. [4] have described the NLO properties using a geometric parameter BLA (Bond Length Alternation). Other authors have added a vibronic coupling to the basic two-state model to describe the strong IR absorptions [5,6] in these systems and the vibrational contributions to the static response [7,8]. Among the various NLO molecular compounds being studied, the biphenyl derivatives present an interesting feature due to the ability of the phenyl rings to rotate around the central C±C bond. The variation of this dihedral angle of twist induces a modulation of the charge transfer between the two substituents. Computational studies of both NLO properties and absorption spectra of twisted biphenyls demonstrate their tunability both in transition frequency
* Corresponding author. E-mail address: [email protected] (A. Fort).
and NLO amplitudes [9]. Moreover, additional low energy absorption bands have been predicted for strongly twisted chromophores. This extends their potential for applications to the domain of optical limiting for example. However, the respective roles of the environment, the strength of the donor/acceptor substituents, and/or the conjugation length are not yet completely understood. The aim of this work is to analyze, theoretically and experimentally, the correlated role of the twist angle and of the electronic structure on the linear and non-linear optical properties using two analogous biphenyl compounds differing in their zwitterionic character. 2. Results Two similar diaryl compounds were synthesized. For both these zwitterionic molecules, the permanent dipole moment is larger in the ground state than in the ®rst excited state leading to negative quadratic hyperpolarizabilities. Molecule CCP has already been studied [10] whereas molecule CCM was especially designed for this work. In CCM, the nitrogen atom was grafted in meta position in order to lower its symmetry as well as make the neutral resonance form unfavorable (see Fig. 1). One can, therefore, also expect a twist angle greater for CCM than for CCP. Numerical calculations where the twist angle was ®xed arbitrarily, predicted an enhancement of the absolute value of the quadratic hyperpolarizabilty with increasing torsion of the chromophores [9]. Experimentally, however, the molecular environment, i.e. the local electric ®eld, will affect simultaneously the twist angle and the molecular orbitals, and,
0379-6779/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 ( 0 1 ) 0 0 4 5 0 - 7
210
A. Fort et al. / Synthetic Metals 124 (2001) 209±211
Fig. 1. Diaryl zwitterionic molecules studied: CCP (left) with its two resonance forms and CCM (right).
consequently, the linear and NLO properties of the chromophores. The optical absorption spectra of these two molecules present signi®cant differences. The CCP molecule possesses one strong absorption band corresponding to the charge transfer transition, centered at wavelength 504 nm, followed by vibronic replica. On the contrary, no such vibronic transitions can be observed in the equivalent blue shifted (lCT 449 nm) absorption band of CCM. Moreover, a second, weaker and much broader, band appears around 600 nm. The absence of vibronic structure in CCM is not readily explained but could be attributed to a more variable twist angle y leading to a larger inhomogeneous broadening of the transition. In Fig. 2, we have plotted the results of MOPAC calculations for the rotational potential well versus twist angle, for two molecules CCP0 and CCM0 , equivalent to CCP and CCM respectively, with a simpli®ed structure
(see inset in Fig. 2). These calculations predict a planar geometry for both molecules whereas NMR measurements indicate a torsion angle of about 408 for an analogous compound [10]. However, in any case the stabilization energy is greater for CCP than for CCM. On the other hand, the low energy absorption band is in fact predicted by ZINDO calculations of electronic structures of CCM. We have performed conventional EFISH measurements at wavelength l 2pc=o 1907 nm for both molecules in solution to determine the product mb (2o). The static values were derived using the usual dispersion relation veri®ed in the two-level approximation. In Table 1, we have gathered the results obtained using different solvents of distinct polarities. The local ®eld created around the molecule favors the charge transfer and consequently increases the zwitterionic character of the molecular ground state. One observes an hypsochromic shift for the charge transfer absorption
Fig. 2. Numerical results for N in para (CCP0 ) and N in meta (CCM0 ) positions.
Table 1 EFISH results for CCP (para) and CCM (meta)a Solvents
ET
CCP lCT (nm)
DMSO DMF Acetone CH2Cl2 CHCl3 a
10
0.44 0.40 0.36 0.31 0.26
504 508 512 520 524
CCM mb (2o)
mb (0)
1000 525 600 425 325
670 350 400 285 210
lCT (nm)
mb (2o)
449 435 442 452 462
3500 1000 Not soluble 750 Not soluble
mb (0) 2550 750 550
ET is the normalized parameter scaling the polarity of the solvents, lCT the charge transfer absorption wavelength. The mb products are given in esu.
48
A. Fort et al. / Synthetic Metals 124 (2001) 209±211
211
Fig. 3. IR spectra of CCP in: chloroform (- - -); acetone (Ð); DMSO (± ±); methanol ( ).
band and a corresponding enhancement (in absolute value) for the (negative) quadratic hyperpolarizabilities with increasing polarity of the solvents. This behavior is in agreement with the predictions of the MIX formalism describing the quadratic hyperpolarizabilities of zwitterionic type molecules in the vicinity of the cyanine limit [2,3]. For CCM, the (absolute) higher static quadratic non-linearities compared to the corresponding values for CCP, can be attributed to the more zwitterionic character with perhaps a larger twist angle because of the meta position of the nitrogen atom. Numerical calculations of mb for simpli®ed analogous compounds (see Fig. 2), in agreement with the work of Albert et al. [9], demonstrate the enhancement of mb with the twist angle y. For large y, around 908, the results for CCP and CCM differ considerably. However, for more reasonable values of y, the calculations indicate similar behaviors. Furthemore, these values are comparable to the experimental results obtained on molecules in solution. Both theoretical and experimental results indicate higher non-linearities when the nitrogen atom is in meta position compared to the corresponding para situation. It should be noticed here that a relatively small change in polarity leads to a rather large increase in the quadratic non-linearities. Whether these changes are accompanied by a sizeable modi®cation of the twist angle can not be ascertained at this stage however. In order to investigate the possible link between geometry and electronic properties, we have recorded a series of IR absorption spectra for CCP compound in four different solvents. In Fig. 3, we observe that the para substituted compound exhibits ®ve strong absorption bands between 1100 and 1700 cm 1. To assign these bands we have used numerical simulations based on density functional theory as implemented in the Dmol3 software. While such calculations may not reproduce the energies and the intensities of these bands accurately, they seem to correctly give the normal modes in the right order. In particular, the 1500 cm 1 band corresponds to the C±O stretch, strongly shifted towards low energies because of its partial double
bond nature. Around 1300 cm 1 we ®nd the vibrational mode responsible for the vibronic structure in the optical spectra with a strong weight of the central aryl bond. Overall, the spectra show only small shifts in energy but large variations in intensity as a function of solvent polarity. However, the visible energy shifts in the 1300 cm 1 band seem to point at sizeable changes in inter ring twist angle. 3. Conclusion The numerically predicted enhancement of the quadratic optical response for the more zwitterionic diaryl CCM has been experimentally veri®ed. To discriminate between the respective roles of geometrical and electronic structures, we have used additional experimental information, such as NMR and vibrational spectroscopies. For zwitterionic compounds such as CCP and CCM, we have shown that the dielectric environment has an extremely strong effect on the NLO properties and that it can be characterized, to some extent, through IR spectroscopy.
References [1] J. Zyss, I. Ledoux, J.-F. Nicoud, Molecular Nonlinear Optics, Academic Press, New York, 1994. [2] M. Barzoukas, C. Runser, A. Fort, M. Blanchard-Desce, Chem. Phys. Lett. 257 (1996) 531. [3] M. Barzoukas, C. Runser, J. Nonlin. Opt. Phys. Mater. (1996) 757. [4] S.R. Marder, C.B. Gorman, F. Meyers, J.W. Perry, G. Bourhill, J.-L. BreÂdas, B.L. Pierce, Science 265 (1994) 632. [5] A. Painelli, Chem. Phys. Lett. 285 (1998) 352. [6] A. Painelli, F. Terenziani, Chem. Phys. Lett. 312 (1999) 211. [7] M. Del Zoppo, C. Castiglioni, P. Zuliani, G. Zerbi, Handbook of Conducting Polymers, Marcel Dekker, New York, 1998, p. 765. [8] D.M. Bischop, Adv. Chem. Phys. 104 (1998) 1. [9] I.D. Albert, T.J. Marks, M.A. Ratner, J. Am. Chem. Soc. 120 (1998) 1174. [10] C. Runser, A. Fort, M. Barzoukas, C. Combellas, C. Suba, A. ThieÂbault, R. Graff, J.P. Kintzinger, Chem. Phys. 193 (1994) 309.
Geometry and quadratic optical properties of twisted push±pull biphenyls A. Forta,*, A. Boeglina, L. Magera, C. Amyotb, C. Combellasb, A. ThieÂbaultb, V. Rodriguezc a
IPCMS, Groupe d'Optique Non-LineÂaire et d'OptoeÂlectronique, 23 rue du Loess, 67037 Strasbourg Cedex, France b ESPCI, Environnement et Chimie Analytique, 10 rue Vauquelin, 75231 Paris Cedex, France c Laboratoire de Physico-Chimie MoleÂculaire, 351 Cours de la LibeÂration, 33405 Talence Cedex, France
Abstract Push±pull biphenyl compounds appear to be model systems for studying the large sensitivity to the dielectric environment of zwitterionic molecules. We present here a systematic theoretical and experimental study, combining numerical simulations and NMR, IR and NLO measurements on two analogous diaryl molecules dissolved in solvents with varying polarities. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Twisted biphenyls; Geometric structure; Non-linear optical properties
1. Introduction In the past decade, noticeable progress has been achieved in the understanding of correlation between structure and non-linear optical (NLO) properties of charge transfer molecules. These push±pull molecules, formed with donor and acceptor end groups connected via a p-conjugated system, are well suited for quadratic optical effects [1]. Various theoretical models have been developed to calculate the NLO properties of push±pull molecules. A simple two-level formalism, well adapted to real experimental conditions of NLO measurements, has been used as a guideline for the synthesis of molecules possessing very high quadratic hyperpolarizabilities [2,3]. In the case of push±pull polyenes, Marder et al. [4] have described the NLO properties using a geometric parameter BLA (Bond Length Alternation). Other authors have added a vibronic coupling to the basic two-state model to describe the strong IR absorptions [5,6] in these systems and the vibrational contributions to the static response [7,8]. Among the various NLO molecular compounds being studied, the biphenyl derivatives present an interesting feature due to the ability of the phenyl rings to rotate around the central C±C bond. The variation of this dihedral angle of twist induces a modulation of the charge transfer between the two substituents. Computational studies of both NLO properties and absorption spectra of twisted biphenyls demonstrate their tunability both in transition frequency
* Corresponding author. E-mail address: [email protected] (A. Fort).
and NLO amplitudes [9]. Moreover, additional low energy absorption bands have been predicted for strongly twisted chromophores. This extends their potential for applications to the domain of optical limiting for example. However, the respective roles of the environment, the strength of the donor/acceptor substituents, and/or the conjugation length are not yet completely understood. The aim of this work is to analyze, theoretically and experimentally, the correlated role of the twist angle and of the electronic structure on the linear and non-linear optical properties using two analogous biphenyl compounds differing in their zwitterionic character. 2. Results Two similar diaryl compounds were synthesized. For both these zwitterionic molecules, the permanent dipole moment is larger in the ground state than in the ®rst excited state leading to negative quadratic hyperpolarizabilities. Molecule CCP has already been studied [10] whereas molecule CCM was especially designed for this work. In CCM, the nitrogen atom was grafted in meta position in order to lower its symmetry as well as make the neutral resonance form unfavorable (see Fig. 1). One can, therefore, also expect a twist angle greater for CCM than for CCP. Numerical calculations where the twist angle was ®xed arbitrarily, predicted an enhancement of the absolute value of the quadratic hyperpolarizabilty with increasing torsion of the chromophores [9]. Experimentally, however, the molecular environment, i.e. the local electric ®eld, will affect simultaneously the twist angle and the molecular orbitals, and,
0379-6779/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 ( 0 1 ) 0 0 4 5 0 - 7
210
A. Fort et al. / Synthetic Metals 124 (2001) 209±211
Fig. 1. Diaryl zwitterionic molecules studied: CCP (left) with its two resonance forms and CCM (right).
consequently, the linear and NLO properties of the chromophores. The optical absorption spectra of these two molecules present signi®cant differences. The CCP molecule possesses one strong absorption band corresponding to the charge transfer transition, centered at wavelength 504 nm, followed by vibronic replica. On the contrary, no such vibronic transitions can be observed in the equivalent blue shifted (lCT 449 nm) absorption band of CCM. Moreover, a second, weaker and much broader, band appears around 600 nm. The absence of vibronic structure in CCM is not readily explained but could be attributed to a more variable twist angle y leading to a larger inhomogeneous broadening of the transition. In Fig. 2, we have plotted the results of MOPAC calculations for the rotational potential well versus twist angle, for two molecules CCP0 and CCM0 , equivalent to CCP and CCM respectively, with a simpli®ed structure
(see inset in Fig. 2). These calculations predict a planar geometry for both molecules whereas NMR measurements indicate a torsion angle of about 408 for an analogous compound [10]. However, in any case the stabilization energy is greater for CCP than for CCM. On the other hand, the low energy absorption band is in fact predicted by ZINDO calculations of electronic structures of CCM. We have performed conventional EFISH measurements at wavelength l 2pc=o 1907 nm for both molecules in solution to determine the product mb (2o). The static values were derived using the usual dispersion relation veri®ed in the two-level approximation. In Table 1, we have gathered the results obtained using different solvents of distinct polarities. The local ®eld created around the molecule favors the charge transfer and consequently increases the zwitterionic character of the molecular ground state. One observes an hypsochromic shift for the charge transfer absorption
Fig. 2. Numerical results for N in para (CCP0 ) and N in meta (CCM0 ) positions.
Table 1 EFISH results for CCP (para) and CCM (meta)a Solvents
ET
CCP lCT (nm)
DMSO DMF Acetone CH2Cl2 CHCl3 a
10
0.44 0.40 0.36 0.31 0.26
504 508 512 520 524
CCM mb (2o)
mb (0)
1000 525 600 425 325
670 350 400 285 210
lCT (nm)
mb (2o)
449 435 442 452 462
3500 1000 Not soluble 750 Not soluble
mb (0) 2550 750 550
ET is the normalized parameter scaling the polarity of the solvents, lCT the charge transfer absorption wavelength. The mb products are given in esu.
48
A. Fort et al. / Synthetic Metals 124 (2001) 209±211
211
Fig. 3. IR spectra of CCP in: chloroform (- - -); acetone (Ð); DMSO (± ±); methanol ( ).
band and a corresponding enhancement (in absolute value) for the (negative) quadratic hyperpolarizabilities with increasing polarity of the solvents. This behavior is in agreement with the predictions of the MIX formalism describing the quadratic hyperpolarizabilities of zwitterionic type molecules in the vicinity of the cyanine limit [2,3]. For CCM, the (absolute) higher static quadratic non-linearities compared to the corresponding values for CCP, can be attributed to the more zwitterionic character with perhaps a larger twist angle because of the meta position of the nitrogen atom. Numerical calculations of mb for simpli®ed analogous compounds (see Fig. 2), in agreement with the work of Albert et al. [9], demonstrate the enhancement of mb with the twist angle y. For large y, around 908, the results for CCP and CCM differ considerably. However, for more reasonable values of y, the calculations indicate similar behaviors. Furthemore, these values are comparable to the experimental results obtained on molecules in solution. Both theoretical and experimental results indicate higher non-linearities when the nitrogen atom is in meta position compared to the corresponding para situation. It should be noticed here that a relatively small change in polarity leads to a rather large increase in the quadratic non-linearities. Whether these changes are accompanied by a sizeable modi®cation of the twist angle can not be ascertained at this stage however. In order to investigate the possible link between geometry and electronic properties, we have recorded a series of IR absorption spectra for CCP compound in four different solvents. In Fig. 3, we observe that the para substituted compound exhibits ®ve strong absorption bands between 1100 and 1700 cm 1. To assign these bands we have used numerical simulations based on density functional theory as implemented in the Dmol3 software. While such calculations may not reproduce the energies and the intensities of these bands accurately, they seem to correctly give the normal modes in the right order. In particular, the 1500 cm 1 band corresponds to the C±O stretch, strongly shifted towards low energies because of its partial double
bond nature. Around 1300 cm 1 we ®nd the vibrational mode responsible for the vibronic structure in the optical spectra with a strong weight of the central aryl bond. Overall, the spectra show only small shifts in energy but large variations in intensity as a function of solvent polarity. However, the visible energy shifts in the 1300 cm 1 band seem to point at sizeable changes in inter ring twist angle. 3. Conclusion The numerically predicted enhancement of the quadratic optical response for the more zwitterionic diaryl CCM has been experimentally veri®ed. To discriminate between the respective roles of geometrical and electronic structures, we have used additional experimental information, such as NMR and vibrational spectroscopies. For zwitterionic compounds such as CCP and CCM, we have shown that the dielectric environment has an extremely strong effect on the NLO properties and that it can be characterized, to some extent, through IR spectroscopy.
References [1] J. Zyss, I. Ledoux, J.-F. Nicoud, Molecular Nonlinear Optics, Academic Press, New York, 1994. [2] M. Barzoukas, C. Runser, A. Fort, M. Blanchard-Desce, Chem. Phys. Lett. 257 (1996) 531. [3] M. Barzoukas, C. Runser, J. Nonlin. Opt. Phys. Mater. (1996) 757. [4] S.R. Marder, C.B. Gorman, F. Meyers, J.W. Perry, G. Bourhill, J.-L. BreÂdas, B.L. Pierce, Science 265 (1994) 632. [5] A. Painelli, Chem. Phys. Lett. 285 (1998) 352. [6] A. Painelli, F. Terenziani, Chem. Phys. Lett. 312 (1999) 211. [7] M. Del Zoppo, C. Castiglioni, P. Zuliani, G. Zerbi, Handbook of Conducting Polymers, Marcel Dekker, New York, 1998, p. 765. [8] D.M. Bischop, Adv. Chem. Phys. 104 (1998) 1. [9] I.D. Albert, T.J. Marks, M.A. Ratner, J. Am. Chem. Soc. 120 (1998) 1174. [10] C. Runser, A. Fort, M. Barzoukas, C. Combellas, C. Suba, A. ThieÂbault, R. Graff, J.P. Kintzinger, Chem. Phys. 193 (1994) 309.