To Initiate Radical Photopolymerization with Long-Lived Triplet Charge-Separated State of Electron Donor−Acceptor Thermally Activated Delayed Fluorescence (TADF) Compounds

Abstract

Long-lived charge separated (CS) state was 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 reported for the first time the initiation of photopolymerization with the long-lived ³CS state. After the addition of co-initiator diphenyliodonium hexafluorophosphate (DPI), we monitored the fluorescence and the lifetimes quenching of the TADF initiators. The lifetime quenching study showed that the quenching of the ³CS state is more significant than the ¹CS state. Using nanosecond transient absorption (ns-TA) spectroscopy, we have analyzed in detail the quenching of the ³CS and ³LE states, the Stern-Volmer quenching constants (K_SV) were obtained as K_SV(³CS) = 3.4 × 10⁴ M⁻¹, K_SV(³LE) = 2.9 × 10⁴ M⁻¹, K_SV(¹CS) = 8.6 × 10² M⁻¹, respectively. These results demonstrated that the quenching of the ³CS state is predominant. This concept is different from the conventional photoinitiators (PIs) that using the localized excited state (³LE) to initiate the intermolecular electron transfer with the co-initiators and it represent a paradigm shift of the photoinitiated polymerization mechanism. Finally, the PIs PTZ-DTO was applied to lithography to obtain a series of high-resolution patterns.