Cyano Decoration of π‐Bridge to Boost Photoluminescence and Electroluminescence Quantum Yields of Triazine/Carbazole Based Blue TADF Emitter

Abstract In general, a large donor‐acceptor dihedral angle is required to guarantee sufficient frontier molecular orbitals separation for thermally activated delayed fluorescence (TADF) emitters, which is intrinsically unfavorable for the radiative transition. We present a molecular design method favoring both reverse intersystem crossing (RISC) and radiative transitions even at a moderate D−A angle. A blue TADF emitter TrzBuCz‐CN was designed with triazine/tert‐butylcarbazole as donor/acceptor and cyano (CN) incorporated on the phenylene bridge. In comparison with the methyl decoration in similar way (TrzBuCz−Me), CN decoration reduced the D−A dihedral angle from 70° to 60°, which is intrinsically not favorable for sufficient FMO separation, but unexpectedly reduced the singlet and triplet energy gap (Δ E ST ) and thus facilitated TADF feature by pulling down the lowest singlet state energy. While the reduced distorsion instead improved the HOMO‐LUMO overlap and boosted the fluorescence quantum yield from 41 % to 94 %. The blue organic light‐emitting diode of TrzBuCz‐CN exhibited an external quantum efficiency of 13.7 % with emission peak at 466 nm, greatly superior to 6.0 % of TrzBuCz−Me. The result provides a feasible design strategy to facilitate both RISC and radiation processes by CN decoration of the linking bridge of TADF emitters.