A New Class of Donor‐π‐Acceptor Phosphonium Salt Exhibiting Intriguing Thermally Activated Delayed Fluorescence

Abstract Thermally activated delayed fluorescence (TADF) behaviors in metal halide perovskite and perovskite derivatives are rarely reported due to the absence of suitable TADF molecules that can serve as the A‐site cations. Herein, a series of novel donor‐ π ‐acceptor type of molecules are developed by adopting substituted phosphine and phosphonium as the electron donor and acceptor linked by the polarized π ‐spacer. The prepared ((diphenylphosphaneyl)phenyl)diphenylphosphonium‐X (BzDPP‐X, X = counterions: Cl − , Br − , I − , NO 3 − , CH 3 COO − etc.) salts show bright emission in the visible range with TADF characteristics due to the strong intramolecular charge transfer (ICT). Benefiting from the spatially separated highest occupied molecular orbital and lowest unoccupied molecular orbital, small singlet‐triplet energy splitting (Δ E ST ) is obtained, facilitating the reverse intersystem crossing process. It turns out that the emission of BzDPP‐X is dependent on the counter anions, and the counterions help to stabilize the ICT state. Intriguingly, the emission can be tuned by integrating metal‐halide units into the above phosphonium salt. Remarkably, the zinc‐based halide exhibits a higher photoluminescence quantum yield (36.2%) than the pure benzyl(2‐(diphenylphosphaneyl)phenyl)diphenylphosphonium bromide (26.9%). This work exemplifies an easy design concept and a facile synthetic method toward ionic luminescent materials.