Picosecond dynamics of n-hexane solvated trans-stilbene

In this article, we report studies of the time- and frequency-resolved picosecond dynamics of trans-stilbene solvated with one n-hexane molecule in a van der Waals complex. Excitations are made to several overtones of a low-frequency intermolecular vibration and to combination bands of these overtones with the symmetric in-plane ethylene bend mode ν₂₅ in the S₁ state of two distinct conformers, A and C, of these complexes. A Franck-Condon analysis of these spectra allows a characterization of the potential for this intermolecular mode in the ground and excited electronic state of both conformers. From the time-resolved dynamics of the A-isomer, the three different types of IVR behavior in the cluster, as in the bare molecule, are identified and studied. Single exponential fluorescence decays are observed at low excess energies, showing no IVR on the time scale of the lifetime of the molecule. At intermediate energies, between 220 and 260 cm^-1, quantum-beat modulated decay profiles indicate restricted IVR dynamics. At higher excess energies IVR becomes dissipative and biexponential decays are observed. The decrease in the lowest excess energy at which dissipative IVR behavior sets in, from 1200 cm^-1 in the bare t-stilbene molecule to less than 300 cm^-1 in the complex, is attributed to a large increase in the density of vibrational states of the complex due to the six low-frequency intermolecular vibrations of the cluster. A similar study of the C-isomer reveals that the IVR dynamics become even more accelerated. Excitation of the stilbene mode ν₂₅ at 200 cm^-1 leads to dissipative energy flow from the stilbene molecule into the cluster vibrations within tens of picoseconds, becoming faster for higher excess energies. The results present a nice example of the significant impact that low-frequency "solvent-like" cluster vibrations have on the dynamics of vibrational energy redistribution and the coherent transfer of vibrational energy between the solute and the cluster modes.