Thermosolvatochromism of betaine-30 in CH3CN

The solvatochromic dye betaine-30 is thermochromic as well, due to the temperature dependence of solvent polarity, which strongly influences the wavelength of the visible absorption band. We report an analysis of the temperature-dependent absorption spectrum of betaine-30 in CH3CN, incorporating the internal-mode displacements determined from the resonance Raman profiles at room temperature. The temperature-dependent solvent reorganization energy lambda (solv) associated with the visible transition of betaine-30 influences the width and position of the absorption spectrum and is relevant to theories for the rate of return electron transfer. We have previously determined lambda (solv) for betaine-30 in acetonitrile and deuterated acetonitrile from analysis of the room-temperature absorption and resonance Raman profiles using time-dependent spectroscopy theory. In this work, we present a revised set of normal-mode displacements, including the contribution from a torsional mode of betaine-30 at 133 cm(-1), obtained from an analysis of the room-temperature Raman profiles in CH3CN and CD3CN. These displacements are then kept fixed, and the temperature-dependent absorption spectrum of betaine-30 in acetonitrile is modeled to obtain the solvent reorganization energy, 0-0 energy, and transition moment as a function of temperature. The solvent reorganization energy lambda (solv) is found to decrease with increasing temperature, consistent with decreasing solvent polarity but opposite to the prediction of dielectric continuum theory. In contrast to our previous analysis, the nonlinear solvent response is included in the model, and the amplitude of the solvent response is found to be smaller in the excited than the ground electronic state, due to the decrease in solute dipole moment in the excited electronic state.