Solvent-Controlled Product Branching in Riboflavin Uncovered by Time-Resolved Infrared and Transient Absorption Spectroscopy.

Riboflavin (vitamin B2) is an important visible chromophore in flavoproteins and a popular photosensitizer in organic photocatalysis. Despite its importance, a complete understanding of riboflavin's full photocycle remains undetermined, specifically the factors that govern the relative yields of its singlet and triplet excited states─both playing important roles in electron transfer chemistry. A combination of transient absorption spectroscopy and time-resolved infrared spectroscopy is used to address this problem, tracking the photoexcited states of riboflavin over 9 decades of time under the same experimental conditions. These studies reveal that the triplet quantum yield increases by a factor of 2.7 by merely changing the solvent from DMSO to H2O. Electronic structure calculations show that this enhancement is not likely to originate from a change in the associated singlet and triplet state potential surfaces. Instead, we discern solvent-dependent branching, with more efficient internal conversion in DMSO, likely mediated by its stronger hydrogen bonding acceptor character than water. This strong modulation shows that the triplet branching, and thus the reservoir of long-lived states required for electron transfer reactions, can be carefully tuned by the immediate environment of the flavin chromophore.