Intermolecular Hydrogen Bonding Controlled Intersystem Crossing Rates of Benzophenone

Solvation plays a critical role in various physicochemical and biological processes. Here, the rate of intersystem crossing (ISC) of benzophenone from its S-1(n pi*) state to its triplet manifold of states is shown to be modified by hydrogen-bonding interactions with protic solvent molecules. We selectively photoexcite benzophenone with its carbonyl group either solvent coordinated or uncoordinated by tuning the excitation wavelength to the band center (lambda = 340 nm) or the long-wavelength edge (lambda = 380 nm) of its pi* <- n absorption band. A combination of ultrafast absorption and Raman spectroscopy shows that the hydrogen-bonding interaction increases the time constant for ISC from <200 fs to 1.7 +/- 0.2 ps for benzophenone in CH3OH. The spectroscopic evidence suggests that the preferred pathway for ISC is from the S-1(n pi) to the T-2(pi pi*) state, with the rate of internal conversion from T-2(pi pi*) to T-1(n pi*) controlled by solvent quenching of excess vibrational energy.