Resonance Raman Evidence for Multistate Contributions to the Lowest Optical Transitions of Azulenic−Thiobarbituric Acid Donor−Acceptor Chromophores

Abstract

Resonance Raman spectra have been obtained for two electron donor−acceptor substituted “push−pull” conjugated molecules possessing guaiazulene donor groups and thiobarbituric acid acceptor groups. One of these was the subject of a recent detailed hyper-Rayleigh scattering excitation profile throughout the two-photon resonant region [Hsu, C.-C. ; Liu, S.; Wang, C. C.; Wang, C. H. Dispersion of the first hyperpolarizability of a strongly charge-transfer chromophore investigated by tunable wavelength hyper-Rayleigh scattering. J. Chem. Phys. 2001, 114, 7103−7108] and an effort to calculate this profile with a Kramers−Kronig technique based on the two-state model for the first hyperpolarizabilities of strongly charge-transfer molecules [Kelley, A. M. Frequency-dependent first hyperpolarizabilities from linear absorption spectra. J. Opt. Soc. Am. B 2002, 19, 1890−1900]. The resonance Raman spectra of both azulene donor molecules show strong dispersion in the relative intensities of Raman lines of similar frequency as the excitation is tuned across the absorption band. This suggests that the broad visible absorption band has significant contributions from more than one electronic transition, although the presence of more than one molecular species differing in charge distribution and/or conformation cannot be ruled out. These effects are not observed in a chromophore having julolidine instead of guaiazulene as the donor group.