Donor position driven excited state modulation in benzoylthiophene–carbazole emitters: divergent TTA/RTP pathways for high efficiency blue-cyan OLEDs

Abstract

Positional isomerism offers a powerful strategy for regulating emissive pathways in organic luminophores, enabling access to room-temperature phosphorescence (RTP), aggregation-induced emission (AIE), triplet–triplet annihilation (TTA), dual emission, and through-space charge transfer (TSCT). Despite these advances, a systematic understanding of positional effects in donor–acceptor (D–A) systems capable of supporting both RTP and TTA remains limited. Herein, three D–A positional isomers (4a–4c) are designed by anchoring N-phenylcarbazole at the C3, C4, and C5 positions of thiophene within a benzoylthiophene scaffold, where the benzoyl group is fixed at the C2 position. This regio-modulation induces pronounced variations in molecular conformation, electronic structure, and excited-state dynamics, as revealed by combined experimental studies and density functional theory calculations. All isomers exhibit AIE behaviour, with progressively enhanced emission from the 2,5-isomer (4a) to the 2,3-isomer (4c), driven by increasing restriction of intramolecular rotation. The 2,4-isomer (4b) displays dual locally excited (LE) and charge-transfer (CT) emission in toluene/DCM mixtures, whereas 4c exhibits TSCT emission arising from a large donor–acceptor dihedral angle. All compounds show RTP, while 4b and 4c further demonstrate temperature-induced non-classical dynamic RTP associated with twisted triplet states. Owing to relatively large ΔE_ST values, reverse intersystem crossing is inefficient, suppressing TADF, and the delayed emission originates predominantly from RTP and/or rapid TTA. OLEDs based on these emitters produce deep blue to cyan electroluminescence, with the 4b-based device achieving a maximum EQE of 17%. Overall, this work establishes donor positional control as an effective molecular handle for tuning LE–CT balance and triplet utilization in organic emitters.