PICOSECOND TIME RESOLUTION OF VIBRATIONAL-RELAXATION IN MOLECULAR VANDERWAALS COMPLEXES - PERYLENE WITH NAPHTHALENE AND BENZENE

Fluorescence time profiles following excitation of naphthalene, benzene, and some alkane 1:1 complexes with perylene under supersonic jet conditions have been recorded at almost-equal-to 40-ps resolution by using time-correlated single-photon counting techniques. Variations in rate constants over 2-3 orders of magnitude for the naphthalene case show that the vibrational coupling of in-plane modes is substantially weaker than for intermolecular or out-of-plane modes at similar energies. Moreover, vibrational relaxation in perylene/naphthalene is considerably slower than for some other molecular van der Waals complexes having fewer internal modes. For example, at 705-cm-1 internal energy, the naphthalene complex shows > 1 order of magnitude longer vibrational relaxation time (i.e. 4 ns) than is the case for for the corresponding benzene complex (120 ps). At 353 cm-1, the difference is even more pronounced, intramolecular vibrational relaxation (IVR) lifetimes of > 15 ns (naphthalene) and 140 ps (benzene) being observed. On the other hand, the vibrational coupling in the benzene complex is shown to be more extensive than for the pentane complex at 353 cm-1. This behavior would not have been predicted by applying statistical theories based on available potential energy data. It is evident that the role of multipolar interaction in limiting large-amplitude relative motion of the components of such interaromatic complexes needs to be carefully examined with respect to its role in promoting vibrational coupling.