Biswaranjan Sahu obtained his master’s degree at the University of Hyderabad. He is currently a PhD Scholar working under the guidance of Dr. Sivakumar Vaidyanathan in the Department of Chemistry at IITH. He is also currently conducting research at NIMS as part of a visiting program at the NIMS-IIT Hyderabad Joint Research Center. His research interests are centered on both organic chiral and achiral luminescent materials for optoelectronic applications.

Efficient exciton utilization and low hole-injection barriers remain key challenges in organic emissive materials. Hybridized local and charge-transfer (HLCT)-based D–A–D emitters can overcome the 25% spin limit, but the role of donor strength in excited-state character and efficiency remains unclear. Herein, we present four benzo[4,5]thieno[3,2-d]thiazole-based deep-blue fluorophores (OXM 03, 04, 07, 08) by keeping triphenylamine as the primary donor while systematically varying the strength of the secondary donor. The selected benzo[4,5]thieno[3,2-d]thiazole acceptor core has shallower HOMO levels (−5.93 eV) than those of conventional thiophene or benzo[b]thiophene, thereby reducing hole-injection barriers. Natural transition orbitals (NTOs) analysis reveals a clear progression from locally excited (LE) to HLCT to CT character from OXM 03 to OXM 08. The temperature-dependent photoluminescence (TD-PL) studies revealed that OXM 03 and OXM 08 exhibit dual emission peaks that merge at temperatures above 60 ℃ and 30 ℃, respectively, due to thermal broadening, while OXM 04 and OXM 07 maintain their broad, featureless emission regardless of temperature. The quantum yield result shows that the balanced LE and CT character, along with high rigidity, yields the best emission efficiency. OXM 03 exhibits the highest PL quantum yield (PLQY) of 63.3% in toluene and was therefore selected for fabrication on a commercial 370 nm UV LED. The fabricated device demonstrated that high solution-state PLQY translates into outstanding device performance, with a color rendering index (CRI) of 83 and a luminous efficacy of radiation (LER) of 308 lm W−1.