Effect of Thermally Reversible Conformers on Delayed Fluorescence and Temperature Sensing
Nirmalya Acharya, Manoj Upadhyay, Raktim Deka, Debdas Ray
Abstract
Achieving thermally enhanced delayed emission (TEDE) through thermally reversible conformational changes in organic conjugate acid–base pairs poses significant challenges in the design process. Here, we present a novel approach utilizing thermally reversible conformational isomerization to attain TEDE from three distinct twisted donor–acceptor conjugates (PTzQF, PTzQCl, and PTzQBr). These conjugates feature a phenothiazine covalently linked to the 8-position of the quinoline segment via a C–N single bond, with 4-fluorophenyl, 4-chlorophenyl, and 4-bromophenyl substituents at the 2,4-positions and a naphthyl unit at the 8-position. Through detailed spectroscopic studies and quantum chemistry calculations, we unveil that the doped conjugates within the host [1,3-bis( N -carbazolyl)benzene] (mCP) exhibit thermally reversible luminescence, involving both host locally excited (LE) and guest charge transfer (CT) emissions. Upon heating, a luminescence quenching effect occurs on the violet LE emission, while the orange delayed fluorescence (DF) (CT) experiences enhancement at elevated temperatures ranging from 10 to 350 K. Conversely, cooling triggers a reversal back to LE emission. The thermally induced conformational change within the guests leads to the modulation of the DF due to a slight energy difference between the conformers (7.64 kcal/mol). Meanwhile, the control of nonradiative transitions of the host during heating (350 K) and cooling (10 K) influences the LE band. Employing the ratiometric luminescence response (RLR) of the blended films, we devise RLR-thermometers showcasing absolute and relative sensitivities between 0.06973 and 0.52839 and 0.017 and 0.025%, with temperature resolutions ranging from 0.0012 to 0.063 K. Through this study, we aim to provide insights into temperature sensing via host–guest emission modulation.