Hot-Exciplex: A New Regime of Triplet Harvesting Beyond Conventional TADF Advancing Hyperfluorescence

TADF- or exciplex-sensitized hyperfluorescence (HF)-organic light-emitting diodes (OLEDs) can enhance color purity and accelerate singlet radiative decay from a terminal emitter, offering new insights into future material development, photophysics, and device performance. While the development of wide-energy-gap exciplexes for deep-blue OLEDs remains challenging, this work presents a unique approach to generating a blue exciplex and using it as a sensitizer in HF-OLEDs. Considering the benefits of efficient triplet confinement in sensitizer-based OLEDs, we developed a new "hot-exciplex" system that leverages higher-lying triplet states, confirmed by ultrafast picosecond and nanosecond transient absorption studies. To achieve this system, a simple bispyrimidine motif, BisPMD, and a carbazole derivative, CCP, are used as the acceptor and donor, respectively, owing to their similar high triplet energy levels. This design enables access to the hot excitonic states, reducing the singlet-triplet energy gap (ΔE_ST) and enhancing the upper-level reverse intersystem crossing (RISC) rate (k_hRISC). The "hot-exciplex" system proves to be an effective sensitizer for blue TADF and narrowband multi-resonance (MR) emitters, enabling efficient Förster energy transfer (FRET) and improving rolloff and color purity at high efficiency. This advanced exciplex-driven HF-OLED is expected to enhance device performance and open further opportunities.