Efficient Intermolecular Electron Transfer-Mediated Radical Photopolymerization via Controlled Accessibility of Triplet Excited States in Donor–Acceptor Sensitizers

In this work, we initially explored the mechanistic origin of radical photopolymerization mediated by intermolecular electron transfer between compact donor–acceptor (D–A)-structured sensitizers (Sens) and co-initiators. Using transient absorption spectroscopy, electrochemical methods, and time-dependent density functional theory (TD-DFT) calculations, we demonstrated that the key factor is ensuring that Sens has a long-lived triplet excited state, which can be achieved by tuning molecular conformation, solvent polarity, and D–A strength. The results revealed that thermally activated delayed fluorescence (TADF) materials with through-space charge transfer (TSCT) characteristics exhibit significantly prolonged charge transfer triplet excited state (3CT) lifetimes compared with their through-bond charge transfer (TBCT) counterparts. Stern–Volmer quenching experiments indicate that both the locally triplet excited state (3LE) and the 3CT state of the sensitizer can be used for efficient electron transfer with the co-initiator, but the 3CT state is more favorable. Notably, the intersystem crossing (ISC) efficiency of the Sens significantly influences the subsequent photopolymerization efficiency of monomers. Based on these findings, a highly efficient photoinitiation system with polymerization efficiency comparable to that of the commercial 2-Isopropylthioxanthone (ITX) system was developed. Furthermore, the photoresist derived from this system was explored for photolithography applications, producing a series of well-defined patterns.