Largely Negative Energy Difference between the Singlet and Triplet Excited States and Ultrafast Delayed Fluorescence

Abstract

Abstract Molecules exhibiting inversion of the lowest singlet and triplet excited states, referred to as inverted singlet–triplet (IST) molecules, can show a short delayed lifetime below 1 μs and are therefore promising candidates for next-generation emitters in organic light-emitting diodes. However, the structural factors that govern this inversion remain insufficiently understood, limiting the rational molecular design of compounds with tunable photophysical properties. Here, we analyzed the relationships between excited-state energy levels and frontier molecular-orbital energy levels in heptazine derivatives. The results indicate that degenerate HOMOs and LUMOs are key factors for reducing ΔEST through enhanced mixing of multiple excitation configurations, whereas phenyl groups slightly increase ΔEST by enhancing the exchange interaction, K. Wave function-based quantum chemical calculations using EOM-CCSD estimated large negative ΔEST values of −0.244 eV and −0.238 eV for the heptazine derivatives, HzDA3 and HzTFE3, respectively. Temperature-dependent time-resolved photoluminescence, together with the onsets analysis of photoluminescence and phosphorescence spectra, confirmed their negative ΔEST values. HzDA3 and HzTFE3 exhibited fast reverse intersystem crossing rate constants exceeding 107 s–1, resulting in ultrafast delayed lifetimes of 76 and 341 ns, respectively. These results clarify the key factors governing inverted singlet–triplet (IST) gap in heptazine derivatives and provide a basis for the development of IST molecules.