Unraveling Photocatalytic Mechanism and Selectivity in PET-RAFT Polymerization

The photoredox catalysts pheophorbide a (PheoA) and zinc tetraphenylporphine (ZnTPP) under illumination display strong selectivity toward reversible addition-fragmentation chain transfer (RAFT) agents containing thiocarbonylthio groups, namely dithiobenzoates, xanthates, and trithiocarbonates. The underlying mechanism for the process-whether via energy or electron transfer from the photoexcited catalyst to RAFT agent-has remained unclear, as has the reason for the remarkable selectivity. Quantum chemistry and molecular dynamics calculations are utilized to provide strong evidence that none of the common energy-transfer mechanisms (Forster resonance energy transfer; Dexter electron exchange; or internal conversion followed by vibrational energy transfer) are likely to facilitate polymerization, let alone explain the observed selectivities. In contrast, extensive quantum chemical characterizations of the excited-state orbitals associated with the catalyst-RAFT agent complexes uncover a clear selectivity pattern associated with charge-transfer states that is highly consistent with experimental findings. The results shed light on the intrinsic catalytic role of the photocatalysts and provide a strong indication that a reversible electron/charge-transfer mechanism underpins the remarkable photocatalytic selectivity.