An experimental examination of the competition between polar coupling and local organization in determining vibrational population relaxation

We report on the vibrational population relaxation and rotational diffusion dynamics of perylene and 1-methylperylene in benzene and toluene. For these experiments, the naphthalene-like ring distortion modes of both perylene (1375 cm(-1)) and 1-methylperylene (1370 cm(-1)) were excited selectively using a stimulated emission population scheme, and the efficiency of depopulation of these modes was measured in each solvent. For these systems, there is an IR-active acceptor vibrational mode at the same frequency and the order of the dominant intermolecular vibrational energy transfer process depends on the identity of the solute. For 1-methylperylene, dipole-dipole coupling is operative, and for perylene, quadrupole-dipole coupling mediates the transfer. The importance of solvent organization about the solute can be evaluated by comparing solute T-1 times in the two solvents. In the limit of fast solvent cage exchange, the solute T-1 relaxation times should increase with increasing order of the polar coupling process and decrease with increasing intermolecular alignment. Our experimental data indicate that the T-1 times are similar for all systems studied, implying the importance of persistent intermolecular interactions in determining the efficiency of vibrational population exchange in solution.