Carboxyl-functionalized flavin as efficient heavy-atom-free triplet photosensitizers: A Theoretical Investigation

Abstract

The electronic structure 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 with extensive density functional theory (DFT) and time-dependent DFT (TD-DFT) based calculations. We showed that non-radiative decay from the lowest singlet (S1) and triplet excited states (T1) of these McFLs are dominant over their radiative counterparts. The rate constants of internal conversion (IC) from S1 of McFLs are one order of magnitude larger than the competing fluorescent emission (FE) and this can be attributed to the intrinsic low frequency normal modes of isoalloxazine backbone, and their coupling with those of the functional groups. The conjugation of carboxyl and -Br moieties with isoalloxazine backbone and heavy-atom-effect of -Br contribute to transition of n→π* nature satisfying El-Sayed’ rules, and the rate constants for corresponding intersystem crossing (ISC) of 7McFL, 8McFL, DcFL and DBFL all increase, and are about one order of magnitude faster than those of the competing IC processes, turning the evolution of Tn dominant the photocycle of these McFLs. As the McFLs at Tn will be populated to T1 via fast IC, 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 the vibronic coupling, etc. The high triplet excited state population at steady state and the triplet quantum yield of 7McFL comparable to DBFL suggest its superiority as efficient heavy-atom-free triplet sensitizer for application in photocatalysis. We expect the findings would help to design of novel flavin-based triplet sensitizers for photoredox catalytic organic transformations.