Initiation of radical photopolymerization via the long-lived triplet charge-separated state of donor−acceptor thermally activated delayed fluorescence compounds

Abstract

Long-lived charge-separated (CS) states are observed in a series of organic electron donor–acceptor (D–A) thermally activated delayed fluorescence (TADF) emitters (PTZ-DTO, PSeZ-DTO, and DPTZ-DTO). We report, for the first time, the initiation of radical photopolymerization via the long-lived ³CS state. After adding a co-initiator, diphenyliodonium hexafluorophosphate (DPI), we monitored the fluorescence intensity and lifetime quenching of the TADF initiators. The lifetime quenching study showed that the quenching of the ³CS state is more significant than that of the ¹CS state. Using nanosecond transient absorption (ns-TA) spectroscopy, we analyzed the quenching of the ³CS and the triplet localized excited (³LE) states in detail, and the Stern–Volmer quenching constants (K_SV) were found to be K_SV(³CS) = 3.4 × 10⁴ M⁻¹, K_SV(³LE) = 2.9 × 10⁴ M⁻¹, and K_SV(¹CS) = 8.6 × 10² M⁻¹. These results demonstrated that the quenching of the ³CS state predominates in the intermolecular electron process with DPI as the electron acceptor. This concept is different from the use of conventional photoinitiators (PIs) that use ³LE state to initiate the intermolecular electron transfer with the co-initiator, and thus this work represents a paradigm shift of photoinitiated polymerization. Finally, the PIs PTZ-DTO was applied in lithography to obtain a series of high-resolution patterns.