4386
J . Phys. Chem. 1990, 94, 4386-4387
A Photophysical and Structural Study on Dye-Type Organic Molecules with Potentially Useful Nonllnear Opticat Properties Malcolm R. McLean, Mamoun Bader, Larry R. Dalton,* Department of Chemistry, University of Southern California, Los Angeles, California 90089-0482
Robert L. S. Devine, and William H. Steier Department of Electrical Engineering, University of Southern California, Los Angeles, California 90089-0483 (Received: January I I , 1990; In Final Form: March 13, 1990)
We have initiated a photophysical and structural study of organic dye-type compounds containing various donor/acceptor groups and heteroatoms for use in nonlinear optical applications. Trends in the values of the second-order hyperpolarizability coefficient, @, have been determined by the solvatochromatic method. Possible applications as organic crystals for second harmonic generation were explored with an X-ray diffraction structure analysis on one member of the series; 1,2,4-trichloro-8-nitrophenoxazin-3-one (1) crystallized in the space group P 2 , / n (monoclinic), with four molecules in the unit cell.
Introduction Recent activity in the study of the second-order nonlinear optical (NLO) behavior of organic molecules has concentrated on either polyene- or azo-type. dyes. Very large values of the second-order hyperpolarizability, 0, have been obtained, with the corresponding expectation of high second-order NLO coefficients [x(*)]for either electrooptic or second harmonic generation (SHG) applications. The successful exploitation of these high 0 values has, however, encountered several difficulties. In the case of thin-film electrooptic applications using doped polymers, problems such as relaxation of the molecular orientation are well-kn0wn.l For SHG applications, utilization of bulk organic crystals is the most common approach. Here, however, there are two fundamental problems which must be overcome before a useful crystal can be obtained. The first is to obtain a noncentrosymmetric crystalline s t r u c t u r e a necessary condition for second-order NLO properties in crystalline solids.2 Unfortunately, approximately 70% of second-order N L O molecules are expected to crystallize in a centrosymmetric ~ t r u c t u r e . ~The second restraint is that the crystal must be phase-matchable if useful intensities of the second harmonic are to be ~ b t a i n e d . ~ As part of a continuing effort to develop and evaluate materials with useful NLO properties, we are currently studying a series of dye-like aromatic compounds. The rationale for investigating such systems arose from two considerations. The first concerned the stability of the dyes, there being some evidence that the polyene-based chromophores are susceptible to photodecomposition. Yet another example of instability is the high-0 merocyanine chromophore, which decomposes on prolonged exposure to the atmosphere.s Precedent for enhanced durability of the heterocyclic molecules 1-3 (Figure 1) came from our own experience with ladder polymers such as poly(quinoxa1ine) (PQL), poly( phenothiazine) (PTL), and the benzimidazobenzophenanthroline ladder polymers BBL and BBB, which exhibit exceptional thermal and photostability? The second consideration concerned the attachment of the N L O active chromophores to polymer backbones. In order to obtain a useful material, the resulting functionalized polymer must have a high glass transition ( I ) Singer, K. D.; Kuzyk, M. G.; Holland, W. R.; Sohn, J. E.; Lalama, S. R. B.; Katz, H. E.; Schilling, M.L.Appl. Phys. Left. 1988.53.
J.; Comizzoli, 1800.
(2) Williams, D. J. Angew. Chem., Int. Ed. Engl. 1984, 23, 690. (3) Mighell, A . D.; Hines, V. L.; Rodgers, J. R. Acta. Crystallogr. 1983, A39, 731. (4) Ledoux, 1.; Josse, D.; Vidakovic, P.; Zyss, J. Opt. Eng. 1986, 25, 2. (5) Daniel, M. F.;Smith, G. W. Mol. Cryst. Liq. Cryst. 1984, 102, L193. (6)(a) See, for example, Coter, F.; Belaish, Y.; Davidov, D.; Dalton, L. R.; Ehrenfreund, E.; McLean, M. R.; Nalwa, H. S.Synrh. Met. 1989, 29, E471. (b) Dalton, L. R.; Thomson, J.; Nalwa, H. S.Polymer 1987, 28, 534. (c) Yu, L. P.;Dalton, L. R. J . Am. Chem. Soc. 1989, 111, 8699.
temperature, T,. Experience in our own laboratories has shown that pendant functionalization of polymer backbones often leads to low T, polymers.’ However, a dramatic increase in T, was observed on attaching a two-ring heterocycle similar in structure to 1, namely 4-chloro-7-nitrobenzofuran,onto a 25/75% styrenelp-aminostyrene copolymer backbone; the increase in T, was on the order of 160 “C, from 130 (parent) to 290 OC (quantitative substitution on p-aminostyrene).* We expect the same order of increase for the series under investigation; this is a current area of research. In addition, the present series contains the same terminal group as merocyanine (carbonyl), which should enhance the 0 values. We will present values of pLpo for these molecules (where p is the ground-state dipole moment and Po the zero-frequency value of p) and discuss the effects of different substituents. In addition, an X-ray diffraction crystallographic study of compound 1 is presented as definitive proof of structure and as part of our attempts to generate noncentrosymmetric crystals of the dyes for x@) applications. Experimental Results Compounds 1, 2a-c, and 3b were synthesized in-house, according to literature methcds?JO via condensation reaction of the appropriate monomers in refluxing acetic acid or ethanol. Purification was achieved by sublimation, recrystallization, or column chromatography. Compounds 3a and 4 are commercially available dyes (methylene violet and phenol blue, Aldrich Chemical Co.) and were recrystallized from toluene before use. Compounds were characterized by comparison of their ‘H N M R (JgOL FX 90 Q FTNMR), UV-visible (Perkin-Elmer Lambda 4C) and FTIR (Perkin-Elmer 1760) spectra with literature values?JO In addition, 1 was submitted for elemental analysis: [Anal. Calcd for CI2H3N2O4Cl3:C, 41.71; H, 0.88; N, 8.11. Found: C, 42.05; H, 0.97; N, 7.891, 4-Dimethylamino-4’-nitrostilbene (DANS) and 4-methoxy-4’-nitrostilbene(MONS) were used as references for the ppo measurements.” Initial measurement of the pp0 value of 1 appeared promising, with a value comparable to 2-methyl-4-nitroaniline (MNA), but, more importantly, with minimal thermal or photochemical deg(7) Yu, L. P.; Chen, M.; Dalton, L. R.submitted for pubtication in Chem. Mater. (8)(a) Polis, D. W.; Bader, M.; Dalton, L. R. The Effects of Matrix Attachment on the Third Order Nonlinear Optical Properties of Dyes. In Materials Research Society Proceedings, Electrical, Optical and Magnetic Properties of Organic Solid State Materials; Chiang, L. Y., Cowan, D., Chaikin. P., Eds.; Materials Research Society: Pittsburgh, PA, 1990. (b) Unpublished results. (9)Mital, R. L.;Jain, S. K. J . Chem. SOC.C 1971, 1875. (IO) Jain, S. K.; Mital, R. L. Z . Naturforsch. 1977, 326, 821. ( I I ) We thank Prof. C. W. Spangler, Northern Illinois University, for providing the reference molecules.
0022-3654/90/2094-4386$02.50/0 0 1990 American Chemical Society
The Journal of Physical Chemistry, Vol. 94, No. 11, 1990 4381
Letters
Et0
NOz
CI
NM%+
-
e
N
0
[3] a X = S b
141
X=O
Figure 1. Schematic of the donor-acceptor compounds, series 1-4. TABLE I: pp0 and Lx Values Reflecting the Effects of Differing Bridging and Donor Atoms compd
28 2b 2c 38 3b
4
X
donor Et0
Et0
Et0 NMe, NMe, NMe,
S S
S S 0 none
Cl’s 0 I 3 0 0 0
A,,
pb0, nm
483 496 519 533 508 567
10-30
D esu 85 105 50 250 75 200
radation (i.e., 1 has a melting/sublimation point of 249 OC and exhibited no photochemical degradation during our measurements). This prompted us to perform an X-ray diffraction crystal structure on 1, to confirm the assigned structure and to investigate whether 1 crystallizes in a noncentrosymmetric form, essential for bulk x ( ~ ) The . orange-red crystal (0.60 X 1.04 X 0.80 mm) used in the X-ray diffraction structural analysis was grown by vapor diffusion from a saturated solution of 1 in benzene utilizing hexane as a cosolvent. Data were collected on a Nicolet/Syntex diffractometer utilizing Mo K a radiation. A data set consisting of 1740 reflections was collected, of which 754 satisfied the I > 3 4 4 criteria. These reflections were used in the refinement, which was solved with the S H E L X ~program.” ~ Data: space group PZ,/n; a = 9.829 (3) A, b = 8.300 (2) A, c = 15.542 (4) A; V = 1253.7 (7) AS;2 = 4. Final agreement factors are R(F) = 0.0939 and R(wF) = 0.1091. The unit cell consisted of four molecules of 1 and contained a crystallographic center of symmetry. Compound 1 therefore crystallizes in a centrosymmetric space group, in this case P2,/n, and is not expected to exhibit a bulk x(*). Table I shows values of ppo obtained for two different series of molecules, 2a-c and 3a, 3b,and 4. Note that, although, strictly speaking, phenol blue (4) is out of place in the present series, we have included it as a reference for the effect of bridging atoms. The values were obtained by using solvatochromism, a technique of limited accuracy. The aim here is to show trends in ppo for various substituents, rather than obtain accurate absolute values. We have, however, confirmed the usefulness of the solvatochromic method using a series of polyene based molecules for which values have been measured by using the standard electric field induced second harmonic generation (EFISH) technique.” The largest uncertainty in the application of the solvatochromic method is in the value of the interaction radius of the solvent cage.I3 Hence, we have compared polyene-based molecules with the same number of carbon-carbon double bonds linking the rings, but with different end groups. In this manner, we have reproduced the trends in /3 to within *30%. The interaction radii for the set of (12) Sheldrick, G. M. SHELX programs 1976,University of Cambridge, Cambridge, U.K. ( I 3) Liptay, W . Agnew. Chem., Int. Ed. Engl. 1969. 8, 177. (14) Devine, R. L. S.;Steier, W.H.; McLean, M. R.; Bader, M.; Dalton, L. R. Manuscript in preparation.
molecules in the present study should all be similar, and we therefore expect trends in pLpo to be well reproduced. Inference of real pp0 values from comparison with the reference set of polyene molecules is, however, more problematic. In the present study we have used the value of ppo for DANS as given by Spangler et al. and estimated the ratio of the interaction radii to be approximately 1.4. Given these uncertainties, we expect the accuracy of the pp0 values to be no better than *50%. We note, however, that initial studies of second harmonic generation in polymer films doped with these molecules generally confirm the magnitudes obtained from the solvatochromism. A more detailed study of the doped polymer layers is currently in pr0gre~s.I~ The effect of replacing hydrogen by a more electron withdrawing atom (chlorine) can be observed in Table I. As expected, when all three hydrogens are substituted the resulting pb0 value is at its lowest within the series.15 This was of interest because altering substituents can introduce further asymmetry thereby altering the crystalline habit, possibly resulting in a noncentrosymmetric space group. The effect of halogen substituents on the value of 0 has been examined in a theoretical study by Cheng et aI.,l5 who also found that their effect on disubstituted benzenes was to generally lower 0.The results obtained for the heterocyclics are not so straightforward, where we can see that the addition of a halogen to the 2-position can actually enhance 0. Analysis of the spectroscopic components obtained from the solvatochromism shows this to be due to an increase in the transition dipole moment. Substitution of three halogens, however, leads to a reduction in the transition dipole moment. Interestingly, compound 2b gave the highest pp0 value. The substitution of the stronger electron donating group, dimethylamino, for ethoxy resulted in an increase of pLpo from 105 X to 250 X D esu, illustrating the general trend of increasing values with stronger electron donating groups (Table I). The proportional increase is similar to that observed for polyene-type molecules, hence again confirming the utility of the solvatochromatic method. A second trend is noted where the main difference in structure is the heteroatom, either sulfur or oxygen. The respective values for pPo are 250 X D esu for the sulfur compound (3a) and 75 X D esu for the oxygen compound (3b); therefore the choice of bridging heteroatom is very important in maximizing the value, with the larger and more easily polarizable sulfur atom providing the larger number. Interestingly, the analogous open compound (4), i.e., no bridging atom, has a ppo value between the oxygen- and sulfur-bridged compounds. Conclusions The three-ring systems examined exhibited high pao values, comparable to or better than MNA, but with improved thermal and photochemical stability. The increase in thermal stability is due in part to the high melting points of the three-ring series 1-3 which range from 190 OC (3a) to 249 OC (1). Compound 1 crystallizes in a centrosymmetric space group and is therefore unsuitable as a crystalline bulk x(*)material. Studies including attachment of similar three-ring compounds as pendant groups on a polymer backbone, utilization of stronger electron donor and/or acceptor groups, and the effect of expanding the backbone to a fused five-ring system are underway and will be described elsewhere. Acknowledgment. This research was supported by the Air Force Office of Scientific Research through contract no. F49620-87-C-0100 and F49620-88-C-007 1 and grant no. 87-0338 (R.L.S.D. and W.H.S.) and by National Science Foundation grant DMR-88-15508.We thank Jin-an Feng, whose work contributed to this article (X-ray diffraction crystal structure). (IS) Cheng, L. T.;Tam, W.;Meredith, E. J.; Rikken, G. L. J. A,; Meijer, E. W.S.P.I.E. Proc. 1989,1147, 61.
