HLCT Driven Twisted Benzo[d]thiazole Based D–A–D Fluorophores for High‐Performance, Two‐Component White LEDs

Design and synthesis of the fluorophores with tailored emission is critical for advanced optoelectronic applications, including white LEDs. Here in this study, we synthesized benzo[d]thiazole-based donor-acceptor-donor (D-A-D) systems to achieve efficient fluorophores with tunable-emission properties. Three luminogens (based on triphenylamine, diphenylamine, and carbazole as donor units and benzo[d]thiazole as acceptor units) were designed, synthesized, and explored as phosphors for white LEDs. Single-crystal X-ray diffraction revealed significant twisting in the molecular geometries. These twisted structures effectively reduced conjugation, which resulted in a greenish-blue emission in nonpolar solvents. The photophysical properties of these compounds were investigated through solvatochromism experiments and Lippert-Mataga plots, and the study reveals that the compounds have a hybridized local and charge transfer (HLCT) character in their excited states. Lifetime studies revealed mono- and bi-exponential decay. Density functional theory (DFT) calculations were performed to optimize the geometries, and a close match between the theoretical and single-crystal structures was observed. Further, the selected luminogens were integrated with a red-emitting Eu(III) phosphor and PMMA matrix to fabricate a white light-emitting diode (LED). The white LED device exhibited a color rendering index (CRI = 74), excellent chromaticity coordinates (x = 0.29, y = 0.30), a correlated color temperature (CCT = 7207 K), and a luminous efficacy of radiation (LER = 320 lm/W). We observed that introducing an additional diphenylamine donor in the D-A-D system (Py-28-2) enhanced the absolute PLQY to 46.33%. These results demonstrate the potential of these donor-acceptor systems for applications in opto-electronics and lighting technologies.