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 S₁(nπ*) 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 (λ = 340 nm) or the long-wavelength edge (λ = 380 nm) of its π* ← 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 S₁(nπ*) to the T₂(ππ*) state, with the rate of internal conversion from T₂(ππ*) to T₁(nπ*) controlled by solvent quenching of excess vibrational energy.