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

Abstract

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 S0→S1 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 on solvent polarity in the S0and S1 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 for by 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.