Resonance Raman study of solvent dynamics on the spectral broadening and intramolecular charge transfer of a hemicyanine dye in aqueous solution

The spectroscopic properties of 4-[2-(4-dimethylaminophenyl)ethenyl]-1-methyl-pyridinium iodide (HR) in different solvents reveal the important effects of solvent dynamics-on the-spectral broadening and the intramolecular charge transfer of HR. In this-article, Raman excitation profiles for 18 vibrational modes of HR are reported in aqueous solution at:wavelengths-that span the S-0-->S-1 charge transfer transition. The absorption spectra, fluorescence spectra and resonance Raman profiles of HR are modeled using:time-dependent wave packet theory and the:Brownian oscillator solvent dephasing model. The solvent reorganization energy in the-absorption process is much greater than that due to internal vibrational modes, and the,solvent reorganization energy for the emission process is considerably smaller than that for the.absorption process;,The, fluorescence : spectrum is mainly broadened by the inhomogeneous Gaussian distribution of the electronic energy, perhaps due to internal rotations in the molecule.; The results.suggest similar polarity:of the emission. state and the ground state, and strong coupling between the torsional motion and solvent relaxation; The different dependence of the torsional potential solvent polarity in the S-0 and S-1 state is the: cause of different absorption and fluorescence spectral width. In D2O,the.absorption cross section. of HR is slightly lower,.and the,absorption and fluorescence spectra are slightly narrower, than in H2O. The smaller absorption spectral:linewidth and generally increased Raman cross sections in D2O are accounted forby smaller amplitude of solvent dephasing, perhaps due to the larger inertial moment and stronger hydrogen bonding in D2O,compared to H2O. The.magnitude and direction of the solvent isotope effect on Raman intensity varies with normal mode, suggesting that the solvent-induced dephasing is mode dependent Vibrational modes which are strongly:coupled to the electronic transition are most sensitive to the solvent isotope. (C) 1998 American Institute of Physics.