Carboxyl-functionalized flavin as efficient heavy-atom-free triplet photosensitizers: a theoretical investigation

The electronic structures and photophysical properties of a set of designed flavin derivatives (McFLs), namely, methyl flavin-7-carboxylate (7McFL), methyl flavin-8-carboxylate (8McFL), flavin-7,8-dicarboxylic anhydride (DcFL), flavin (FL) and 7,8-dibromoflavin (DBFL), were investigated using extensive density functional theory (DFT) and time-dependent DFT (TD-DFT)-based calculations to understand the role of carboxyl groups in the photophysical processes. We showed that non-radiative decay from the lowest singlet (S₁) and triplet excited states (T₁) of these McFLs is dominant over their radiative counterparts. The rate constants of internal conversion (IC) from S₁ of McFLs are one order of magnitude larger than those of competing fluorescent emission (FE), which can be attributed to the intrinsic low-frequency normal modes of the isoalloxazine backbone and their coupling with those of the functional groups. The conjugation of carboxyl and Br moieties with the isoalloxazine backbone, together with the heavy-atom effect of Br, contributes to the n → π* transition, in accordance with El-Sayed' rules. All the rate constants for the corresponding intersystem crossing (ISC) processes of 7McFL, 8McFL, DcFL and DBFL increase from 8.50 × 10⁸ s^-1 for FL to the order of 10⁹ s^-1 and are about one order of magnitude higher than those of the competing IC processes, making T_n evolution dominant in the photocycle of these McFLs. The McFLs at T_n are populated to T₁via fast IC, and the slow phosphorescent emission from T₁ can be attributed to the limited spin-orbit coupling and adiabatic excitation energy of the electronic transition involved, while the faster non-radiative decay from T₁ may be attributed to vibronic coupling. The triplet quantum yields of 7McFL, 8McFL, DcFL and DBFL also change from 73.16% for FL to ∼90%. Furthermore, the concentration of the triplet excited state in a 3.30 × 10^-2 mol L^-1 CH₃CN solution at steady state under continuous radiation is on the order of 10^-2 mol L^-1. The high triplet excited state population at steady state and the triplet quantum yield of 7McFL, comparable to that of DBFL, suggest its superiority as an efficient heavy-atom-free triplet sensitizer for photocatalytic applications. These findings highlight the ability of carboxyl groups to enhance spin-orbit coupling, promoting ISC and triplet quantum yields of these photosensitizers. We expect that these findings will help in the design of novel flavin-based triplet sensitizers for photoredox catalytic organic transformations.