Impact of Donor Strength on Thermally Activated Delayed Fluorescence in Heptazine-Based Donor–Acceptor Emitters

Thermally activated delayed fluorescent (TADF) emitters usually feature donor and acceptor units connected in a twisted configuration. This geometry separates the frontier molecular orbitals, resulting in a small energy gap between the lowest singlet and triplet excited states (ΔEST), which promotes reverse intersystem crossing (RISC) and subsequent delayed fluorescence. The electron-donating and electron-accepting strengths of the substituents crucially influence the TADF efficiency. Herein, we report the excited state photophysics of heptazine-based TADF emitters by tuning their intrinsic donor strength and environment polarity. Transient absorption and time-resolved fluorescence spectroscopy reveal that with increasing donor strength, the rates of intersystem crossing (ISC) and RISC decrease, contrary to the trends observed for other D–A TADF systems. This behavior results from a larger energy gap between the charge transfer and high-lying locally excited triplet states of these emitters, which weakens the vibronic coupling between them as the donor strength increases.