Stark Spectroscopy of Donor/Acceptor Substituted Polyenes Gerold U. Bublitz, † Rafael Ortiz, ‡ Seth R. Marder, ‡,§ and Steven G. Boxer* ,† Contribution from the Department of Chemistry, Stanford UniVersity, Stanford, California 94305-5080, The Beckman Institute, California Institute of Technology, Pasadena, California 91125, and the Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109 ReceiVed NoVember 25, 1996 X Abstract: The electronic structure of donor/acceptor polyenes (merocyanines) is studied by means of electroabsorption (Stark) spectroscopy. Molecules belonging to this class exhibit an extreme variability of their linear and nonlinear optical properties. Depending on the nature of the donor and acceptor end groups and the polarity of their solvent environment they can adopt ground state molecular structures between limits with mostly localized double bonds (polyene-like) or fully delocalized double bonds (cyanine-like). By comparing the results obtained for the change in dipole moment, Δµ, and change in polarizability, ΔR, upon optical excitation of 12 donor/acceptor polyenes with theoretical predictions, each molecule’s location in between these limits can be assigned. The results demonstrate that this position can be understood by considering only the relative electron donating and accepting strengths of the end groups. The solvent dependence of the electronic structure is also studied by measuring Stark spectra for two compounds in different frozen solvents. The results are consistent with a change of the molecule’s ground state toward more dipolar structures in polar solvents. Taken together, the donor, acceptor, and solvent dependences of the electronic structure of donor/acceptor polyenes can be described by a simple model based on the gradual change of the molecule's ground state between polyene-like and cyanine-like structures. Donor/acceptor substituted polyenes or merocyanine dyes are of widespread interest. Their intense absorption bands in the visible region of the spectrum and the remarkable sensitivity of these bands to solvent polarity have been studied for a great number of dyes. 1-3 Recently the nonlinear optical coefficients of these molecules have been linked to the influence of the donor and acceptor group on the connecting polyene bridge, providing deeper insight into structure-function relationships in nonlinear optical materials. 4-7 In addition to the prospect of practical applications, these molecules also provide an ideal case for testing and furthering our theoretical understanding of the electronic structure of conjugated systems. The effects of the electron donating and accepting end groups on the electronic structure of the bridging polyene can be modeled by the simple picture shown below. If both the donor and acceptor are weak, the molecular structure will be effectively like an unperturbed polyene (A). With increasing donor and/or acceptor strength the ground state structure of the bridge becomes both more delocalized and dipolar, that is it changes toward a cyanine-like fully delocalized structure (C). Further increase of the donor and/or acceptor strength leads to changes toward the limit of a charge-separated structure (E) where the double bonds are again localized, but their position has shifted relative to form A. There should exist a full range of intermediate cases between these limits, for example B would have a structure with contributions from A and C, while D would be an intermediate between structures C and E. Two closely related concepts to quantitate these effects are the average bond length alternation (BLA) and the average bond order alternation (BOA) between adjacent bonds in the bridge. Limit A corresponds to a BLA of about 0.11 Å (BOA )-0.6), the value found for a polyene like octatetraene, BLA ) 0 Å (BOA ) 0) at the cyanine limit C, and the BLA is negative from C toward the limit E (the BOA is positive now). 8,9 Another way to view the effects of the donor and acceptor groups, which ties in with the experimental results presented in this paper, is that they create a substantial internal electric field applied along the long molecular axis. The further effect of solvent polarity on the molecular structure can be viewed as the effect of a matrix electric field. 10 Depending on the polarity of the solvent, the ground state structure of the solvated molecule is more (for polar solvents) or less (for nonpolar solvents) dipolar in character. This effect has been demonstrated by both IR and NMR measurements of merocyanines in different solvents, 11 each demonstrating a change in the molecular structure with † Department of Chemistry, Stanford University. ‡ The Beckman Institute, California Institute of Technology. § The Jet Propulsion Laboratory, California Institute of Technology. X Abstract published in AdVance ACS Abstracts, March 15, 1997. (1) Brooker, L. G. S.; Keyes, G. H.; Williams, W. W. J. Am. Chem. Soc. 1942, 64, 199-210. (2) Brooker, L. G. S.; Keyes, G. H.; Sprague, R. H.; VanDyke, R. H.; VanLarge, E.; VanZandt, G.; White, F. L. J. Am. Chem. Soc. 1951, 73, 5326-5332. (3) Brooker, L. G. S.; Keyes, G. H.; Sprague, R. H.; VanDyke, R. H.; VanLarge, E.; VanZandt, G.; White, F. L.; Cressman, H. W. J.; Dent, S. G. J. J. Am. Chem. Soc. 1951, 73, 5332-5350. (4) Marder, S. R.; Perry, J. W.; Bourhill, G.; Gorman, C. B.; Tiemann, B. G.; Mansour, K. Science 1993, 261, 186-189. (5) Gorman, C. B.; Marder, S. R. Proc. Natl. Acad. Sci. U.S.A. 1993, 90, 11297-11301. (6) Marder, S. R.; Cheng, L.-T.; Tiemann, B. G.; Friedli, A. C.; Blanchard-Desce, M.; Perry, J. W.; Skindhøj, J. Science 1994, 263, 511- 514. (7) Marder, S. R.; Gorman, C. B.; Meyers, F.; Perry, J. W.; Bourhill, G.; Bre ´das, J.-L.; Pierce, B. M. Science 1994, 265, 632-635. (8) Marder, S. R.; Perry, J. W.; Tiemann, B. G.; Gorman, C. B.; Gilmour, S.; Biddle, S. L.; Bourhill, G. J. Am. Chem. Soc. 1993, 115, 2524-2526. (9) Meyers, F.; Marder, S. R.; Pierce, B. M.; Bre ´das, J.-L. J. Am. Chem. Soc. 1994, 116, 10703-10714. (10) Boxer, S. G. In The Photosynthetic Reaction Center; Deisenhofer, J., Norris, J. R., Eds.; Academic Press: San Diego, 1993; pp 179-220. (11) Reichardt, C. SolVents and SolVent Effects in Organic Chemistry, 2nd ed.; VCH: Weinheim, 1988, and references therein. 3365 J. Am. Chem. Soc. 1997, 119, 3365-3376 S0002-7863(96)04081-4 CCC: $14.00 © 1997 American Chemical Society