Imidazole-Based Excited-State Intramolecular Proton-Transfer (ESIPT) Materials: Observation of Thermally Activated Delayed Fluorescence (TDF)

We report the first observation of thermally activated delayed fluorescence (TDF) from an excited-state intramolecular proton-transfer (ESIPT) molecule, a hydroxyl-substituted tetraphenyl imidazole derivative (HPI−Ac), in degassed solutions as well as in low-temperature organic matrixes. In the absence of oxygen, the blue emission of an identical spectral feature was observed in the nanosecond (∼4.4 ns) and microsecond (∼25 μs) time domains, and the fluorescence intensity increased with temperature. From the temperature dependence of the time-resolved spectra of HPI−Ac, the energy gap between the first-excited singlet state and the lowest triplet state was determined to be 7.6 ± 0.3 kJ/mol (630 ± 25 cm^(-1)), and the limiting rate constant of intrinsic reverse intersystem crossing was estimated to be 1.3 (±0.5) × 10^7 s^(-1).