Modulation of Delayed Fluorescence Guided by Conformational Effect-Mediated Thermally Enhanced Phosphorescence in Phenothiazines–Quinoline–Cl Conjugates
Suvendu S. Dey, Arun K. Pal, Manoj Upadhyay, Ayan Datta, Debdas Ray
Abstract
Triplet energy harvesting via thermally activated delayed fluorescence (TADF) from pure organic systems has attracted great attention in organic light-emitting diodes, sensing, and photocatalysis. However, the realization of thermally enhanced phosphorescence (TEP)-guided efficient TADF with a high rate of reverse intersystem crossing ( k RISC ) still needs to be discovered. Herein, we report two phenothiazine–quinoline conjugates ( P2QC, P2QMC ) comprising two phenothiazine donors covalently attached to the chlorine-substituted quinolinyl acceptor. Spectroscopic analysis in conjunction with quantum chemistry calculations reveals that TEP in P2QC originated due to slow internal conversion from higher-lying triplet to lowest triplet (T2′ → T1′) of the quasi-axial (QA) conformer and TADF ( k RISC = 1.44 × 10 8 s –1 ) originated from the quasi-equatorial (QE) conformer caused by a low singlet–triplet gap (Δ E S1–T1 = 0.11 eV) and triplet energy transfer from QA to QE owing to the degenerate ground state of the conformers. In contrast, TADF ( k RISC = 0.74 × 10 8 s –1 ) and dual phosphorescence under ambient conditions are observed in P2QMC . This study provides a sustainable guideline for developing efficient TADF emitters via conformation effects and energy transfer mechanisms.