Controlling vibrational energy flow in liquid alkylbenzenes

Ultrafast infrared (IR) Raman spectroscopy was used to study vibrational energy in φ-S alkylbenzenes, where φ = C₆H₅ and substituents S were CH₃- (toluene), (CH₃)₂CH- (isopropylbenzene, IPB), or (CH₃)₃C- (t-butylbenzene, TBB). Using methods described previously,1 the normal modes were classified as phenyl (φ), substituent (S), or global (G). IR pulses were tuned to find conditions that maximized the localization of initial CH-stretch excitations on φ or S. Anti-Stokes Raman spectroscopy measured transient energy content of Raman-active S, φ, and G modes, to determine the rates of phenyl to substituent (Φ → S) or substituent to phenyl (S → Φ) transfer during the first few picoseconds, when energy transfer was mainly intramolecular. Since phenyl CH-stretches were 90-130 cm^-1 uphill in energy from substituent CH-stretches, of interest were S → Φ processes where molecular structure and local couplings were more important than energy differences. The Φ → S process efficiencies were small and about equal with all three substituents. The S → Φ transfer efficiencies could be increased by increasing substituent size. This was opposite to what would be predicted on the basis of the larger density of states of larger substituents, and it provides a path toward controlling forward-to-backward vibrational energy transfer ratios. The S → Φ transfer efficiency is understood as resulting from an increase in the local anharmonic couplings. A heavier substituent, when vibrating, transfers energy more effectively to the phenyl group.