Thermally modulated triplet–triplet annihilation: Deciphering the temperature dependence of upconversion dynamics

Abstract

The quantum efficiency ceiling of triplet–triplet annihilation upconversion is intrinsically governed by the spin statistical factor (f), yet the physical origin of its anomalously elevated values (f > 0.6) remains a subject of intense debate. By conducting systematic variable-temperature kinetic studies, integrating time-resolved transient absorption spectroscopy with steady-state fluorescence spectral analysis, we elucidated the temperature-dependent reaction dynamics of the benchmark PtOEP/DPA system to investigate the interaction between f and high-energy excited-state (T2) decay pathways. Notably, the positive temperature dependence of f (rising from 0.575 at 190 K to 0.808 at 280 K) and a small activation energy (Ea = 1.92 kJ/mol) demonstrated the dominance of a direct 3(AA)* → S1 reverse intersystem crossing (RISC) mechanism while excluding the hypothesized 3(AA)* → T2 → S1 cascade. These results not only resolve longstanding debates concerning f-enhancement but also provide a rationale for designing annihilators that exploit low-barrier RISC pathways to circumvent spin-statistical constraints.