Role of Excited-State Structural Reorganization in Achieving Enhanced TADF in Molecular Aggregates for Efficient Cyan-Green and Yellow OLEDs

Abstract

Achieving high-efficiency thermally activated delayed fluorescence (TADF) in the solid state remains a major challenge for next-generation optoelectronics. While molecular design strategies focus on tuning the singlet-triplet energy gap (ΔEST) and spin-orbit coupling (SOC), the role of excited-state structural reorganization in the molecular aggregates remains largely overlooked. Here, Density Functional Theory (DFT) and Quantum Mechanics/Molecular Mechanics (QM/MM) calculations along with experimental evidence, are employed to investigate the interplay of ΔEST, SOC, and structural reorganization in both monomeric and aggregated forms of novel phenoxazine- and carbazole-based luminogens. For the first time, it is revealed that low-frequency vibrational modes (<50 cm−1), which induce large molecular distortions between the S1 & T1 states in solution, are markedly suppressed upon aggregation, leading to enhanced TADF efficiency. This enables highly efficient cyan-green and yellow Organic Light Emitting Diodes (OLEDs) with outstanding EQEmax (24.4% and 22.8%), low turn-on voltages (3–3.5 V), and high luminance (>11 000 cd m−2). Beyond optoelectronics, the carbazole-based emitters exhibit Mechanochromic luminescence (MCL)-TADF with >50 nm shifts. They also show strong lipid-droplet targeting (Pearson's r∼0.95) for bioimaging, along with efficient two-photon upconversion. The findings render the crucial understanding for the rational design of solid-state TADF systems, enabling efficient optoelectronic applications.