Thermally Activated Delayed Fluorescence Sensitizer with Through‐Space Charge Transfer Manipulated by Atom Distribution for Solution‐Processed Deep‐Blue Multi‐Resonance OLEDs

Abstract Organic light‐emitting diodes (OLEDs) employing multi‐resonance (MR) emitters have attracted much attention due to their high luminescent efficiency and color purity, however, the development of deep‐blue multi‐resonance OLEDs is hindered by the lack of suitable sensitizers. Here a novel design strategy is reported for thermally activated delayed fluorescence sensitizer with high reverse intersystem crossing (k_RISC) and radiative transition (k_R) rate by regulating atom distribution of a spatially‐aligned carbazole donor/diphenylpyrimidine acceptor structure to manipulate through‐space charge transfer (CT) process. It is found that diphenylpyrimidine acceptor with 1,3‐position nitrogen atoms is un‐conjugated to bridging phenyl moiety by the conjugation node of pyrimidine, while the acceptor with 1,5‐position nitrogen atoms is conjugated to bridging phenyl through conjugation site, opening through‐bond CT pathway via bridging phenyl besides through‐space CT from spatial donor‐acceptor interactions. As a result, the sensitizer with 1,5‐position nitrogen atoms exhibits enhanced oscillator strength by 3.5 folds and accelerated k_R of 3.57 × 10⁷ s⁻¹, while keeping high k_RISC of 4.03 × 10⁶ s⁻¹ and high singlet (S₁) state (2.88 eV). Solution‐processed OLEDs using the sensitizer and deep‐blue multi‐resonance emitter exhibit efficient narrowband electroluminescence with CIE coordinates of (0.14, 0.13) and EQE_max of 15.6%, representing the state‐of‐the‐art device performance for deep‐blue multi‐resonance OLEDs by solution process.