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.