Influence of Torsional Vibrations on the Photoluminescence Characteristics of Dye Solutions

The influence of torsional vibrations of fluorophore molecule on polarization spectra and absorption/emission vibronic band profiles of isotropic dye solutions has been considered. Basing on the concept of luminescence center (LC) and assuming that (1) electronic transitions in the LC may be assisted by torsion-vibrational excitations, and (2) reorientations of the LC can be described in terms of Stokes–Einstein rotational diffusion, the formula for time-dependent emission anisotropy, as a function of excitation, , and observation, frequencies has been obtained. It comprises depolarization by combined reorientations of the fluorophore molecule, i.e., its torsional vibrations with respect to the LC, and rotational diffusion of the LC. This approach is a generalization of the appropriate results obtained earlier by Ehrenberg and Rigler and, independently, by Chuang and Isenthal. The considered model has specific property that the torsional vibrations appear both as depolarizing factor for and as shaping factor for absorption/emission bands, resulting in the variation of the emission anisotropy across appropriate band profiles. This is demonstrated graphically using numerical results obtained for a simplified, one-dimensional torsional oscillator. It is also shown that observed absorption and emission spectra of coumarin solutions can be reproduced using this model with appropriate potentials for restoring forces.