J . Phys. Chem. 1991, 95, 7858-7862
7858
Mesomorphic Molecular Materials. Dielectric and Nonlinear Optlcal Properties of Donor-Acceptor Azomethine Nematogens F. Tournilhac,* J. Simon, Chimie et Electrochimie des Matiriaux Molbculaires, ESPCI-CNRS, 10 Rue Vauquelin, 75231 Paris Cedex 05, France
M. Barzoukas, D. Josse, CNET, 196 Rue H . Ravera, 92220 Bagneux, France
and Z. Belarbi Laboratoire d'Electronique des Solides, Universiti de Lyon I , 43 Boulevard du 1 I Novembre 191 8, 69622 Villeurbanne Cedex, France (Received: February 26, 1991)
N-(4-Methoxybenzylidene)-4-cyanoaniline(MBCA) and N-(4-~yanobenzylidene)-e"thoxyaniline(CBMA) are highly polar molecular units forming nematic mesophases in the temperature ranges 106-121 and 115-125 OC, respectively. The dielectric properties of single domains of these two nematogens were determined. The Kirkwood factors found indicate a rather weak local antiferroelectric coupling between the molecular units. The molecular hyperpolarizability coefficients of MBCA and CBMA were measured in solution by the electric field induced second harmonic (EFISH) technique. The second harmonic generation (SHG) properties of the nematic phases of MBCA and CBMA have been characterized as a function of an external electric field. The macroscopic SHG properties have been compared to those calculated from the molecular characteristics.
Introduction Integrated optoelectronics necessitate, in many cases, the use of active materials under the form of thin films. Molecular materials seem particularly appropriate for such a purpose.'s2 Thin films may be easily formed in various ways: spin coating, sublimation under vacuum, liquid crystals in cells, etc. Liquid crystalline molecular materials are indeed currently employed in visualization devices. Second harmonic generation (SHG) in nonlinear optics (NLO) necessitates a noncentrosymmetric medium. Within a single domain of a liquid crystal, the amount of molecular units that are noncentrosymmetrically oriented is proportional to the applied electric field. A maximum of 5% of the molecular units can be noncentrosymmetrically oriented with reasonable values of the field ( IO5V/cm) and of the ground-state dipole moment ( 5 D). A significant amount of the molecular nonlinearity may therefore be recovered at a macroscopic scale. Liquid crystalline phases are also highly birefringent; an electrically adjustable birefringence could be used for obtaining the phase matching conditions in SHG. Liquid crystal NLO devices therefore seem promising in integrated optics. For such a purpose, poled polymers may be alternatively used.*' Only a few publications3-' relate the use of liquid crystalline phases for SHG. Early publications were primarily concerned with the determination of the symmetry of the medium and the study of the mechanism (dipolar or quadrupolar) associated with SHG. A second series of publications clearly demonstrated the quadrupolar origin of SHG in the absence of field;&1°it was found, ( I ) Simon. J.; Bassoul, P.; Norvez, S . New J . Chem. 1989, 13, 13. (2) For a recent review see: Chemla, D. S.; Zyss, J . Nonlineur Optical Properties of Organic Molecules and Crystals; Academic Press: Orlando, FL. 1987. (3) Freund, 1.; Rentzepis, P. M . Phys. Rev. Lett. 1967, 18. 393. (4) Durand, G.;Lee, C . H . C. R. Acad. Sci. 1967, 264, 1397. Durand. G.;Lee, C. H . Mol. Cryst. Liq. Cryst. 1968, 5, 171. ( 5 ) Goldberg, L. S.; Schnur, J . M . Appl. Phys. Lett. 1969. 14, 306. (6) Arakelyan, S . M.;Grigoryan, G.L.; Nersisyan, S. Ts.; Nshanyan, M. A.; Chilingaryan, Yu. S. J E T P Lett. 1978, 28, 186 and 296. Arakelyan, S. M.;Grigoryan, G. L.; Nersisyan, S . Ts.; Chilingaryan, Yu. S . Sou. Pfiys. J E T P 1981, 53, 977. (7) For a review see: Khoo, 1. C. Progress in Optics XXVI; Wolf, E., Ed.; Elsevier Science: Amsterdam, 1988. (8) Saha, S. K.; Wong, G.K. Appl. Phys. Lett. 1979, 34,423. Saha. S. K.;Wong. G . K. Opt. Commun. 1979, 30, 119.
0022-365419 1 /2095-7858$02.50/0
however, that dipolar SHG could take place when an electrical field or a flexoelectric stress was applied. The molecular hyperpolarizability may be semiquantitatively predicted from the molecular structure.'," The mesogens used in the previous studies were not suitable for efficient SHG and were not designed for such a purpose. Highly polarizable ~mectogens'~-" have been synthesized. However, smectic liquid crystalline phases are difficult to organize in single domains. In this paper the NLO properties of two highly polarizable nematogens are studied; experiments on related compounds have been published.36 The molecular hyperpolarizability coefficients fl are determined by the electric field induced second harmonic (EFISH) t e c h n i q ~ e 'in~ ~solution. '~ Single domains of the nematic phases are obtained by surface treatment of glass slides with poly(viny1 alcohol). The characterization of the SHG properties as a function of an applied electric field on the liquid crystalline phases is described. Synthesis and Properties of the Mesogens
N-(4-Methoxybenzylidene)-4-cyanoaniline (MBCA, 1) and N-(Ccyanobenzylidene)-4-methoxyaniline (CBMA, 2) have been chosen for our studies (Figure 1). The synthesis of 1 and 2 was previously described.j6 The products were purified by recrystallization from ethanol and sublimation under vacuum to obtain good dielectric properties of the materials (see ref 23). The nematic phases of these compounds were observed by optical microscopy in the temperature ranges 106-121 O C for MBCA (1) and 115-125 O C for CBMA (2) as previously reported." On cooling, the nematic phase are met(9) Gu. S.T.; Saha, S. K.; Wong. G. K. Mol. Cryst. Li9. Cryst. 1981,69, 287. (IO) Shtykov, N . M.; Blinov, L. M.; Dorozhkin, A. M.; Barnik, M. I. J E T P Letf. 1982.35, 171. Barnik, M.I.; Blinov, L. M.; Dorozhkin, A. M.: Shtykov, N . M. Sou. Phys. J E T P 1982. 54, 935. ( I I ) See: Nicoud, J . F.; Twieg, R. J . Reference 2, p 23. (12) Tournilhac, F.; Nicoud, J . F.; Simon, J.; Weber, P.; Guillon, D.; Skoulios. A. Liq. Cryst. 1987, 2, 55. (13) Fouquey, C.; Lehn. J . M.; Malthite. J . J . Chem. Soc., Chem. Commum 1987, i424. (14) Levine, B.F.; Bethea, C. G . Appl. Phys. Lett. 1974, 24, 445. Levine, B. F.;Bethea, C. G . J . Chem. Phys. 1975, 63. 2666. (15) Barzoukas, M.; Josse, D.; FrCmaux, P.; Zyss. J.; Nicoud, J. F.; Morley, J . 0.J . Opt. SOC.Am. 1987, 84, 977. (16) Janini. G . M.;Katrib, A. H . J . Chem. Educ. 1983, 60, 1087. ( I 7) Malthite, J.; Billard, J.; Canceill. J.; Gabard, J.; Jacques, J . J . Phys., Colloq. 1976. C3. I.
0 1991 American Chemical Societv
Mesomorphic Molecular Materials
The Journal of Physical Chemistry, Vol. 95, No. 20, 1991 7859
TABLE I: Determination of the Dielectric Properties of MBCA and CBMAa
MBCA (1) 1 MBCA
Figure 1.
€11 CL
2 CBMA
Two mesogens used in the present study: MBCA (1); CBMA
(2).
C
CBMA (2)
17.8
15.8
6.0 9.6
6.0
8.8
dielectric constant in the isotropic phase (T= 130 "C). and el, the values measured at 80 OC have been indicated (metastable state).
0
and c, its value at infinite frequency (e, is taken equal to 1.05n2, as indicated in ref 22). The molecular dipole moments tend locally to couple in a ferroelectric or antiferroelectric manner. The average effective dipole moment pen interacting with the local field is affected by this phenomenon. The Kirkwood factor g
(3)
1:
E
N
N
111
It
0
Figure 2. Representation of the charge-transfer states in MBCA (1).
astable down to 65 OC over long periods of time. The compounds 1 and 2 present a charge transfer from the electron donor (MeO) to the electron acceptor (CN) subunits. The dipole moment of MBCA (1) (5.04D) is slightly larger than that for CBMA (2) (4.23D).I6 This has been rationalized by postulatingI6 a stabilization of the charge-transfer state by the electronegative nitrogen atom in 1 (Figure 2). Similar differences between 1 and 2 were found in the optical absorption spectra: 1 Lx = 317 nm (t = 2.6 X 104 L mol-' cm-I), 2 A,, = 353 nm (1.6 X IO4). Solvatochromism experiments'* have been camed out to determine the difference in dipole moment between the ground state and the excited state. The spectral shifts were too small to lead to significant values of A p (MBCA, A p = -2.3 D;CBMA, Ap = -0.5 D). Moreover, a change of the twisting angle between the two phenyl rings19i20as a function of the solvent probably makes solvatochromism inappropriate for the determination of Ap. Dielectric Properties The proportion T of the mesogens which are noncentrosymmetrically oriented in an electric field is given, in a first approximation, by the Langevin law T = p,El/nkT (1) where p is the dipole moment of the mesogen (in esu-cm), E, is the locaf field in 300 V cm-I, and kT is in erg. For usual electric fields (103-IVV/cm) and ground-state dipole moments (3-5 D), between 0.01 and 5% of the mesogens are noncentrosymmetrically oriented at room temperature. The coefficient n depends on the order parameter of the mesophase and is between one and three2! El is the local field, which is related to the external electric field E and to the field generated by the oriented neighboring molecular units. In the approximation of a spherical cavity in a medium constituted of polar units, El is given by the Onsager expression21v22 €(em + 2) El = E t, + 2c where c is the dielectric constant of the medium at zero frequency (18) Varma, C. A. G . 0.;Groenen, E. J. J. R e d . Trau. Chim. Pays-Bas 1972, 91, 296. (19) Minkin, W . I.; Zhdanov, Y.A.; Medyantzeva. E. A.; Ostroumov, Yu. A. Tetrahedron 1967. 23. 3651. (20) Ashraf El-Bayoumi, M.; El-Aasser, M.; Abdel-Halim, F. J. Am. Chem. Soc. 1971. 93, 586. (21) Singer, K. D.; Kuzyk, M. G.; Sohn. J. E. J. Opt. Soc. Am. B 1987, 4. 968. (22) Bijttcher, C. J. F. Theory of Electric Polarization; Elsevier: Amsterdam, 1973.
where pLeis the ground-state dipole moment in solution, permits one to estimate the nature and the magnitude of the local electric coupling (g > 1, ferroelectric coupling; g < 1, antiferroelectric couplingz2). In the framework of the Onsager approximation for an isotropic medium (eq 2),pen is related to the macroscopic dielectric constant c via the Kirkwood-Frohlich equation22 (4) where N is the number of molecules per unit of volume. The Kirkwood factor is an indication of the local electrical couplings in which only the nearest dipolar neighbors are conIn the isotropic phase, the Kirkwood factor is g = 0.63 for CBMA and g = 0.50 for MBCA. Both values indicate an antiferroelectric coupling, the largest molecular dipole moment giving rise to the lower g factor. The dielectric properties of the liquid crystalline phases of MBCA (1) and CBMA (2) have been determined on single domains of the liquid crystals oriented in a magnetic field following conventional technique^.^'^^^ In this way, the parallel (q) and perpendicular (el) components of the dielectric constant could be measured (Table I). The transverse permittivities tL of CBMA and MBCA have approximately the same values over the whole domain of stability of the nematic phase. This indicates that the mesogens have nearly equal transverse dipole moments. The longitudinal permittivity ell of MBCA is larger than that for CBMA as expected from the values of the molecular dipole moments. Similar studies were carried out on N-(4-cyanobenzylidene)-4-octyloxyaniline (CBOOA),2S which presents a smectic A and a nematic liquid crystalline phase. A strong decrease in the longitudinal permittivity is observed at the N SA transition, indicating a higher antiferroelectric correlation in the smectic phase. Nematic mesophases therefore demonstrate a rather weak antiferroelectric coupling of the cyano groups. This is probably also the case for the nematogens 1 and 2 as evidenced by the value of the g factor.
-
Nonlinear Optical Properties (a) Molecular Hyperpolarizabilities. Second-order molecular hyperpolarizabilities of MBCA and CBMA were measured in solution (acetone) at 1.06 pm by using the EFlSH method14J5 (Table 11). The experimental conditions were identical with those previously described.I5 (23) Belarbi, 2.;Guillaud, G.;Tournilhac, F.; Aourag. H.;Khelifa, B. Jpn. J . Appl. Phys. 1991, 30, 711. (24) See, for example: Maier, W.; Meier. G. %. Naturforsch. 1961, 16A, 1 Zoo.
(25) Belarbi, Z.; Guillaud, G.; Maitrot, nilhac, F. Reo. Phys. Appl. 1988, 23, 143.
M.;Huck, J.; Simon, J.; Tour-
7860 The Journal of Physical Chemistry, Vol. 95, No. 20, 1991
Tournilhac et al.
TABLE Ik Molecular Nonlinear Optical Properties of tbe Mesogem Used in the Resent Study Determined by EFISHC Technique" Pa, D b,nm flex,, esu A, nm Pc, esu 317
16 f 8
1060
9*5
4.23
353
23 f 10
1060
11
(5.46)b
292
20
1060
13
( 5S 5 ) b
342
2w
1890
15
4.8
247
5
1890
6.2
320
35
1060
MBCA (1)
i s
CMBA (2)
3 (ref 26)
C N-&'
4
(ref 29)
-
4.5
5 (ref 26)
20
6 (ref 28) @pS, ground-state dipole moment; &,
A;
maximum absorption wavelength; fl, second-order hyperpolarizability coefficient measured at the wavelength Calculated according to the group additivity model.27 Calculated from pfl reported
Pc,frequency-independent hyperpolarizability coefficient.
in ref 29.
The molecular hyperpolarizabilitymay be considered28as the sum of a group additive contribution (@add) and a charge-transfer term ( P C T ) : (5) @ = @add + PCT The charge-transfer term is predominant'*2for the type of moiecular units used in the present study; PCTis related to the characteristics of the charge-transfer absorption band by the equa tion28
where Wo is the energy corresponding to the charge-transfer absorption band at the wavelength Xo (Wo = hc/Xo), w is the pulsation of the incident radiation, f is the oscillator strength of the charge-transfer band,30and A p is the difference of the dipole moment between the excited state (pe)and the ground state (pJ. The term PCTdepends on the wavelength of the incident radiation. A zero-frequency term gTmay be defined which characterizes the intrinsic properties of the molecular unit @CT = P& k X 0 ) (7)
where X is the wavelength of the incident radiation and
glass
polarizer
nematic
analyzer
'
Figure 3. Schematic representation of the experimental setup used to measure the SHG properties of nematic liquid crystalline phases. .
-
where Wo is in electronvolts, A p is in Debyes, f 4.6 X cmxA71/2,cmx is the extinction coefficient for the wavelength A, in L mol-' cm-I, and AyIl2 is the half-peak width in cm-I. In the previous equations, the hyperpolarizabilitycoefficients should be written @, in the case z is the ground-state dipole moment axis. It is the vectorial part of the third-order tensor:
kt! = + I / 3 ( k t ! x x + 2P0,zx + E y y + 282y) (1 0) The gT values of MBCA 1 and CBMA 2 are approximately e 2 2
(9)
or numerically
(26) Combellas, C.; Gautier, H.;Simon, J.; Thitbault. A.; Tournilhac, F.; Barzoukas, M.;Josse, D.; Ledoux, 1.; Amatore, C.; Verpeaux, J. N. J. Chem. Soc., Chem. Commun. 1988, 203. (27) Minkin, V. 1.; Osipov, 0. A.; Zhdanov, Y . A. Dipole Moments in Organic Chemisrry; Plenum Press: New York, 1970. (28) Oudar, J. L.; Chemla, D. S . J . Chem. Phys. 1977.66. 2664. (29) Dulcic, A.; Flytzanis, C.; Tang, C. L.; Pepin, D.; Fetizon, M.; Hoppillard, Y. J. Chem. Phys. 1981, 74, 1559. (30) Bacquct. G.; Bassoul. P.; Combellas, C.; Simon, J.; Thitbault, A.; Tournilhac, F. Ado. Mater. 1990, 2, 31 1.
equal (Table 11). They are 2 times higher than the one corresponding to the benzene derivative 5. Comparison with the result obtained by solvatochromism = -3.7 X esu for MBCA; Rz2= -5 X esu for CBMA) confirms the nonapplicability of this method in this case. The methoxycyanostilbene derivative 4 has a hyperpolarizability coefficient 50% higher than those of 1 and 2. This is probably related to the nonplanar conformation of the azomethine deriva t i v e ~ . ' ~The . ~ weak electron-withdrawingand electron-donating abilities of nitrile and methoxy groups yield significantly lower Po values than the one determined for nitroaniline 6. However, the mesogens I and 2 possess NLO properties I order of magnitude higher than those previously used in similar ~tudies.~-'O (b) Second Harmonic Generation in Nematic Phases. A cell has been designed to measure the SHG properties of nematic
(ez2
The Journal of Physical Chemistry, Vol. 95, No. 20, 1991 7861
Mesomorphic Molecular Materials
/
-40
-20
0
20
40 8
Figure 4. Interference fringes obtained with CBMA (2) at 122 O C (nematic mesophase domain). The light intensity at the pulsation 2 0 (6) as a function of the incidence angle (e) is represented. (Experimental setup is shown in Figure 3.)
phases oriented with an electric field (Figure 3). The cell is constituted of two glass slides separated by 130 pm thick poly(tetrafluoroethylene) spacers. Two 100 pm thick silver electrodes were used to apply electrical fields. The temperature was regulated within *0.5 OC with an electrically heated steel block and a thermistance. The temperature was measured near the sample with a platinum resistance. In this cell, the electrical component of the exciting radiation (E") is in all cases parallel to the direction of the nematic mesophase. The nematic mesophase is planarly oriented by a surface treatment of the glass slides with poly(viny1 alcohol) and rubbing and also by application of a 1000-Hzelectric field of 550 V/cm. Under these conditions, a single domain was obtained as checked by optical microscopy. The noncentrosymmetric orientation of the sample was achieved with high-field electrical pulses (9 kV/cm, 2 c(s) synchronized with the laser pulses. The 1000-Hz field was removed during the laser pulses. Maker fringes3' were recorded by rotating the sample (angle B in Figure 3) while keeping the direction of the nematic phase, the orientating electrical field and the optical electrical field all parallel. A nematic phase belongs to the symmetry group Dmh;under the influence of an electric field the symmetry becomes Cmv,and the tensor coefficients different from zero1are P,,,, Pzxx,P,,,, P,, and @yzy (the axes taken are shown in Figure 3). In the experimental setup used Pz# E; = Py= 0. Typical Maker fringes obtained with 2 are shown in Figure 4. The macroscopic susceptibility r is related to the maximum light intensity at zero angle IL(see Figure 4) and to the coherence length 1, of the medium. The absolute values of I' were obtained by recording the Maker fringes of a quartz single crystal dp, = 1.2 X esu, 1: = 20.6 pm in the same conditions). For B = 0, the transmission coefficients are approximately the same for the sample and for the quartz reference. r in electrostatic units is given by the ratio
(e),
(e)
(11) = (IL/re, I / * / ( ~ C E O / @ ) where Eo is the orientating static electric field applied (in 300 V/cm), dpl is in electrostatic units, and the Q indices indicate the quartz values. The coherence length can be calculated from the refractive indices of the medium at 2w and w3I (nh and nw, respectively):
(12) where X is the fundamental excitating wavelength (1.06 pm). I, can also be calculated from the optical path length difference between two successive minima Bi and Oi+l of the Maker fringes (Figure 5 )
r
1
(31) Jerphagnon, J.; Kurtz, S.K. J . Appl. Phys. 1970, 41, 1667.
\
Figure 5. Optical pathway and parameters used for calculating the coherence length I,.
TABLE III: Nonlinear Optical Properties of the Mesogens 1 and 2 in Isotropic or Nematic Phases' rap,
rcalc?
n~
nzo
IC, pm
1.57'
1.65'
3.3
9
1.49 1.57'
1.55 1.67'
4.3 2.7
3
8
6
12
esu
esu
MBCA (1)
nematic phase (107 "C)
9
CMBA (2)
isotropic phase (128 "C) nematic phase (1 12 O C )
"The n,, value is indicated for the nematic liquid crystalline phases instead of the mean value. where L is the thickness of the sample. The refractive indices n2" and n0 are not known. For a YAG laser source (1.06 pm), the wavelength corresponding to the radiation at 2w is in the visible range (530 nm). Refractive indices of MBCA and CBMA have been calculated from refractivity datal6 in benzene solutions (sodium D line). For MBCA and CBMA the isotropic refractive index is found to be 1.57. This value is in agreement with the value calculated by the group additivity (nD = 1S 5 ) . In further calculations, this last value will be taken. In both cases, the dispersion of the index between the wavelength at which the determination has been carried out (589 nm) and the one corresponding to 2w (530 nm) has been neglected. Because of the accuracy of the NLO measurements, only approximate values of the indices (f2%) are necessary in the calculations; the dispersion is therefore expected to contribute negligibly. Equation 12 permits us to calculate f l when n2wand 1, are known. In the nematic range, the ordinary indices nw were calculated from the relation
(q+ 2n",/3 Anw = "7 - n;
nw =
rexp/4
I, = X/4(nZw- n")
\
(14) (15)
-
The value of An for closely related compounds was previously reported34 (An 0.2). The values nw and n2wused for the calculation of l, with eqs 12 and 13 were fitted with the whole set of successive minima of the Maker fringes (Table 111). Determinations in the isotropic phase of MBCA could not be carried out because the low viscosity of the medium did not enable us to fill the cell. The coherence lengths found are of the order of a few micrometers, in agreement with those measured8.l0 for related compounds. The susceptibility rcalc can also be calculated from the microscopic hyperpolarizability y with the equation (32) Vogel, A. I . J . Chem. SOC.1938, 1323. (33) Vogel, A. 1.; Cresswell. W.T.; Jeffery, G. H.; Leicester, J. J . Chem. Sor. 1952, 514. (34) Huang, C. C.; Pindhak, R.S.;Ho, J. T.J . fhys. Lett. 1974, 35, L185.
7862 The Journal of Physical Chemistry, Vol. 95, No. 20, 1991
where N is the number of molecules per unit of volume. In the isotropic phases, the local field factors f may be calculated from the dielectric constants and the refractive indices of the medium. For f'the Onsager approximation is taken:
p
= e(€-
+ 2)/(€, + 2€)
(17)
For the frequency-dependent factorsf" andf2" the orientation polarization can be neglected and the local field factor is given by the Lorenz-Lorentz relation:'I
f" = (n:
+ 2)/3
(18)
The microscopic hyperpolarizability coefficients y can be related to the effective ground-state dipole moment and the second-order hyperpolarizability of the molecule in solution
-
where perfis in esucm, 0 and y are in electrostatic units, kT is in erg; n = 5 isotropic phase, and n 2 nematic phase. In this equation, n depends on the order parameter of the mesophase; a value n 2 is typical for conventional nematic liquid crystals, whereas n = 5 is taken for isotropic phases.'' The electronic third-order hyperpolarizability yeis negligible for usual pp values. The effective dipole moment hefris related to the molecular dipole moment in solution p and the Kirkwood factor g of the condensed phase (eq 3). In the nematic phases the Onsager and Lorenz-Lorentz approximations are not applicable; therefore, the same local fields and the g factor as in the isotropic liquids will be taken to calculate rcalc. Equations 16 and 19 allow us to estimate the microscopic from microscopic parameters pfl and B (Table susceptibility rcalc 111). The calculated and experimental values are in fairly good agreement. The correction due to the light diffusion has not been taken into account in the determination of rexp and should lead to higher values. For the case in which ~ c f and f ~ the local field are constant, a 2.5-fold increase of r is expected from eq 19 when going from the isotropic to the nematic phase. A value of 2 is experimentally obtained. This value may be taken as the "nematic effect" on
-
Tournilhac et al. SHG, independently of the models used to estimate the local fields, the dielectric constants, the order parameters, etc. Conclusion In this paper, it is demonstrated that polarizable nematogens can be used to generate second harmonic. The amount of mesogens that are noncentrosymmetrically oriented in an electric field is not incompatible with efficient SHG. However, a even higher proportion of the head-to-head arrangement of mesogens can be favored by using the amphiphilic properties of suitably designed molecules (the polyphiles see ref 35). Such studies are in progress.
Experimental Section p-Anisidine, p-anisaldehyde, 4-cyanophenol, and 4-aminobenzonitrile were purchased from Aldrich Chemical Co. CBMA and MBCA were recrystallized from absolute ethanol (SDS); further purification of MBCA was achieved by sublimation. The purity of the compounds was checked by NMR spectroscopy (Varian EM 390). Solvatochromic experiments were carried out by using dichloromethane, carbon tetrachloride, tetrahydrofuran, acetonitrile, and trichloroethylene (SDS, Quality Purex); absolute molar extinction coefficients and bandwidths were determined in acetonitrile solutions. All spectra were recorded by using a Kontron-Uvikon 860 UV-visible spectrometer. Dielectric measurements were performed as indicated in ref 25. Nonlinear optical measurements were carried out by using a Q-switch YAG-Nd laser (pulse, 12 ns, 4-5 mJ, 10 Hz). The second harmonic generated a t 2w was compared to a solid reference (NPP powder). The diameter of the incident beam was 4 mm, and it was focused with a 1.5-diopter lens. Acknowledgment. We thank I. Ledoux and P. Fremaux for helpful discussions. Registry NO. 1, 13036-19-6; 2, 20256-89-7. (35) Tournilhac, F.; Bosio, L.; Nicoud, J. F.; Simon, J. Chcm. Phys. Lett. 1988. 145. 452. (36) Cheng, L. T.;Tam, W.; Meredith, G. R.; Rikken, G. L. J. A.; Meijer, E. W.Proc. SPIE-In?.SOC.Eng. 1989, 1147,61.
