Tetraphenylpyrazine Based AIE Luminogens: Unique Excited State Decay and Its Application in Deep-Blue Light-Emitting Diodes

Carbazole-modified tetraphenylpyrazine (TPP) derivatives TPP-Cz, TPP-PhCz, TPP-2Cz, and TPP-2PhCz are prepared with different patterns by inserting a pi-bridge and adopting two-armed forms to produce aggregation-induced emission (AIE)-type deep-blue emitters for organic light-emitting diode (OLED) applications. Systematic spectral analysis and detailed theoretical calculation of these TPP derivatives reveal their special excited state behaviors. Unlike the traditional AIE unit, which features enhanced emission from solution to aggregate with the nonradiative decay channel blocked, a tenfold increase in the radiative decay rate of these TPP-based AIE luminogens (AIEgens) is observed from the solution to film states. The formation of a stable quinone conformation of the TPP center and its peripheral phenyl group in the excited state is the main reason for this behavior. The distribution changes in the total reorganization energy are further integrated, and a feasible molecular design strategy is proposed for the TPP family. Nondoped OLEDs are fabricated, and the AIEgens exhibit good performance with valuable blue emission (x + y < 0.3). The TPP-Cz-based optimized devices exhibit more stable external quantum efficiencies and an ideal roll-off with unchanged Commission Internationale de l'Elcairage (CIE) coordinates of (0.16, 0.11). These findings demonstrate the great potential of TPP with its unique excited state characteristics as a deep-blue AIEgen.