7078
J . Phys. Chem. 1991, 95, 7078-7082
stationary states produced by Couette shear flow, Taylor vortex flow, and rigid body rotation. Acknowledgment. We are grateful for financial support from the Natural Sciences and Engineering Research Council of Canada, the Ministere de 1'Education du Quebec, and Esso Imperial Oil. L.T. acknowledges FCAR for a graduate scholarship.
We thank S. BErubE, H. Dumont, S. Gag& J. Gigugre, J. R. Martin, and M. St-Pierre for their help at various stages of this work. Supplementary Material Available: Schematic diagrams for the frequency counter and the valve control of Figure 1 (2 pages). Ordering information is given on any current masthead page.
Molecular ConJugationof Dlmethoxydlphenylpdyenes Studied by Raman Spectroscopy C. T. Lin,* A. M. Mahloudji, Department of Chemistry, Northern Illinois University, DeKalb, Illinois 601 15-2862
B. J. Baer,t and M. F. Nicol Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90024- 1569 (Received: October 4, 1990; In Final Form: January 25, 1991)
Raman spectra of pure crystalline C6H5(CH+!H),,C6HS and CH30C6H4(CH~H)nc6H40CH3were recorded, where the OCH3groups were at either the ortho or the para positions of the aromatic rings, for chain lengths of n = 2.3.4. Variations of the Raman intensities and frequencies of the phenyl ring versus polyenic vibrations were analyzed in terms of molecular r-conjugations. Retentions of r-conjugation in dimethoxydiphenylpolyenes is shown to decrease in the following order: nonsubstituted > para > ortho and also n = 2 > 3 > 4. The effect of molecular geometry on the formation of twisted intramolecular charge-transferconfiguration and enhancement of the &hyperpolarizability of "push-pull" diphenylpolyenes is discussed.
I. Introduction 'Push-pull" diphenylpolyenes display unusually large nonlinear optical effects.I4 In principle, secondsrder (p) nonlinear optical materials must be noncentrosymmetric in order to produce an observable response; that is, they usually have large internal dipole moments like those typical of the conjugated donor-acceptor compounds. The third-order ( 7 ) effects are symmetry-independent, but the materials must be highly polarizable, i.e., a system with highly conjugated backbones. The nonlinearity of such molecules can be enhanced by increasing either the conjugation length (hence, electron delocalization) for 7 or the strength of donor or acceptor groups for 8. In both cases, the limiting factor for push-pull diphenylpolyenes is the extent of molecular uconjugation or, equivalently, molecular planarity resulting from the overlaps of 2p, orbitals. This limiting factor may be affected by (i) the types and positions of electron-donating and/or -accepting substituents, (ii) the length of the polyenic chain, (iii) the crystal lattice in solids, (iv) the solvent polarity in solutions, and (v) the photophysical properties and dynamics of the "resonant" and "nonresonant' photoexcited states. Our initial motivation for the present work was to examine effects i-iii in several diphenylpolyenes. Recently, the photophysical properties of a series of 4,4'-disubstituted 1,Cdiphenyl- 1,3,5-hexatrienes (D,A-DPH) were in~estigated,~ where D and A are the electron-donating and -accepting groups, OCH3, N(CH3)2,and NOz. D,A-DPH have a twisted intramolecular charge-transfer (TICT) configuration in their ground-state potential surface? which are stabilized by the intermolecular charge interactions. Other push-pull polyenes reported to have TICT configurations include p-cyano-p'(methylthio)diphenylacetylene,6 benzodithiapolyene aldehydes, and 4-(dimethylamino)phenylpolyene aldehydes? The molecular is hyperpolarizabilit 8, of (CH3)2NC6H4(CH-=CH)3C6H4N02 (770 & 60) X IO- & cm5/csu,Bwhich is approximately 1700 times that of urea. The large quadratic optical nonlinearities for DCurrent address: School of Chemical Sciences, University of Illinois, Urbana, IL 61801.
C6H4(CHdH),&H4-A (D,A-DDP) in solution appear to be related to the observed photophysical properties.' These relationships imply that D,A-DDP develops a stable TICT conformation; that is, the molecular ranjugation is r e d u d in solution. Several reports's9 have been published that correlate the effect of side-chain substitution on the geometrical twisting and molecular r-conjugation of push-pull polyenes. Torruelles et a1.9 reported that x()) and the magnitude of the absorption peaks reduce for a series of D,A-DDP going from n = 2 to 3. They attributed this observation to a partially broken wonjugation in the side-chain-disubstituted diphenylpolyenes. Also, for stilbene-type molecules, theoretical calculations show a "sudden polarization" or an energy minimization in orthogonally twisted conformation.'*'* This phenomenon is observed in push-pull polyenes, producing a TICT state.5 Raman intensities and molecular r-conjugation have been related for substituted styrenes, biphenyls, and ~tilbenes,'~ linear (1) Nonlinear Optical Properties of Organic and Polymeric Materials; William, D. J., Eds.; ACS Symposium Series 233; American Chemical Society: Washington, DC, 1983. ( 2 ) Nonlinear Opfical Properties of Organic Molecules and Crystals; Chemla, D. S., Zya, J., Eds.;Academic Prerr: Orlando, FL, 1987. (3) Nonlinear @tical and Elecrroacrive Polymers; Prasad, P. N.. Ulrich, D. R., Eds.; Plenum: New York, 1988. ( 4 ) Nonlinear Optical Effects in Organic Polymers; Merrier, J., Kajzar, F., Prasad, P., Ulrich, D., Ma.; NATO AS1 Series 162, Series E, Applied Sciences; Kluwer Academic Publishers: Boston, MA, 1989. (5) Lin, C. T.; Guan, H. W.; McCoy, R. K.; Spangler, C. W. J . Phys. Chem. 1989, 93, 39. (6) Khundkar, L. R.; Stiegman, A. E.; Perry, J. W. J. Phys. Chem. 1990, 94, 1224. (7) Slama-Schwok, A.; Elanchard-Desce,M.; Lehn, J. M. J . Phys. Chem. 1990, 94, 3894. ( 8 ) Spangler, C. W.; McCoy, R. K.; Dembek, A. A.; Sapochak, L. S.; Gates, E. D. J . Chem. Soc., Perkin Trans. I 1989, 151. (9) Torruelles, W. E.; Zanoni, R.; Marques, M. E.;Stegeman, 0. I.; Mbhlmann, G.R.; Erolhuiaen, E. W. P.; Horsthuis, N . H. 0 . Chem. Phys. Lett. 1990, 175, 261. (IO) Retting, W.; Majenz, W. Chem. Phys. Lett. 1989, 154, 335. ( 1 1) Lippert, E.; Retting, W.; Bonacic-Koutecky, V.; Heisel, F.; Micke, J. A. Adv. Chem. Phys. 1987, I . (12) Hochstrasser, R. M. Pure Appl. Chem. 1980. 52, 2683.
0022-365419112095-7078$02.50/0 Q 1991 American Chemical Society
The Journal of Physical Chemistry, Vol. 95, No. 18, 1991 7079
Raman Spectroscopy of Dimethoxydiphenylpolyenes
TABLE k W a v m ( c d ) of okcned h n u n Bands for DPH, DPO, and C H @ C f i ( C H - C H ) , C f i M , n=2
n-3
P9’
0,o‘
1629 (vs) 1581 (w) 1608 (vi)
1625 (vs)
1523 (w)
1490 (m)
1591 (vs)
I318 (w) 1282 (m) 1254 (vw) 1 I80 (s)
1146 (s) 1 I10 (vw)
1288 (s) 1237 (w) 1179 (w) 1 I59 (m) I I50 (s) 1094 (m) 1048 (vw) 1029 (vw)
DPH 1593 (vs) 1569 (m) 1607 is). 1587 (vs) 1490 (w) 1444 (w) 1338 (vs) 1327 (vw) 1305 (vw) 1257 (s) 1238 (vw) 1211 (vw) 1181 (m)
1143 (s)
999 (m) 943 (vw)
948 (w)
P P’
0,o‘
1592 (vs) 1570 (w) . .
1582 (vs) 1567 (w) 1604 (m.)
1587 (vs) 1516 (vw)
I490 (w) 1468 (vs) 1322 (vw) 1307 (w) 1303 (wi) 1256 (5) 1244 (s)
1261 (w) 1244 (m)
DPO 1574 (vs)
1545 (w)
1581 (vs) 1498 (vw) 1455 (vs) 1326 (vw) 1309(vw) 1300 (vw)
1580 (s) 1516 (w)
1551 (s) 1483 (vw)
1238 (m)
1228 (m) 1211(vw) 1179 (w) 1157 (s) 1143 (s)
1180 (vw) 1186 (vw) 1157 (m) 1147 (s) 1148 (s) 1148 (s) 1 1 1 1 (vw) I103 (m) I055 (vw) 1029 (vw) 1005 (w) 958 (vw)
polyenes,14 and diphenylp~lyenes.’~Schmid and Topsom” established that the fourth root of the Raman intensity is linearly related to the length of conjugation in a series of planar conjugated hydrocarbons, provided that the intensity is corrected for the resonance Raman effect. Deviations from this relationship were used as a measure of deviation of ranfiguration from pure planar conjugated hydrocarbons. For diphenylpolyenes C6Hs(CH= CH),C6H5 with n varying from 1 to 4, the influence of the *conjugation on the Raman intensities of the ring vibrations is particularly large. The extent of molecular *-conjugation for unsubstituted diphenylpolyenes increases as the polyenic chain length increases.I5 In this article, we describe the effects of substitution on the molecular *-conjugation of solid diphenylpolyenes observed by using Raman spectroscopy. Methoxy substituents were placed at either the ortho or para positions of the aromatic rings of diphenylpolyenes for chain lengths of n = 2, 3,4. The molecular ranjugation deduced will be compared with those available data obtained from theoretical calculations and X-ray crystallography. 11. Experimental Section The diphenylpolyenes were purchased from Aldrich Chemical Co. The corresponding 4,4’- or 2,2’disubstituted compounds of CH30C&(CH-CH),C&40CH3 were synthesized and purified by modifications of known methods? The samples in powder form were assembled in capillary tubes, and the laser was focused on the sample to -50 pm. The incident laser power was less than 50 mW since the Raman intensities of samples investigated were unusually high. Most Raman spectra were obtained by using the 514.5-nm line of a Spectra Physics 165 Ar+ laser. The 632.8-nm line of a low-power (
