Achieving Highly Efficient Luminescence and Multi‐Photon Absorption Character in Thermally Activated Delayed Fluorescence Emitters Based on a Tetrabenzo[a,c]phenazine Core

The integration of multiphoton absorption phenomenon (MPA) with thermally activated delayed fluorescence (TADF) materials opens new avenues for bio-optoelectronic applications. However, the development of TADF emitters that simultaneously exhibit triplet harvesting and MPA is hindered by the challenge of balancing molecular planarity, reduced orbital overlap, and large oscillator strength. In this study, we demonstrate a quadrupolar molecular design based on the tetrabenzo[a,c]phenazine (TBPZ) to accomplish efficient MPA and TADF performance. Two new TADF emitters, TBPZ2TPA and TBPZ2NP, were synthesized, featuring an electron-accepting TBPZ core and electron-donating triarylamine (TAA) units to achieve an optimal balance between TADF and MPA characteristics. Particularly, TBPZ2TPA blended film exhibits orange-red emission with a maximum at 589 nm, a high photoluminescence quantum yield (ФPL) of 71%, a sufficiently small singlet-triplet energy gap (ΔEST) of 0.16 eV and reversed intersystem crossing rate (kRISC) of 1.3 × 104 s-1. Furthermore, it demonstrated a remarkable two-photon absorption (2PA) cross-section (σ2) of 892 GM at 780 nm and an excellent 2PA brightness of 785 GM, along with evident three-photon absorption (3PA) at 1200 nm. These results provide a compelling strategy for developing high-performance TADF emitters with exceptional MPA properties, enabling dual functionality for both OLEDs and high-resolution bioimaging.