Modulation of Delayed Fluorescence Guided by Conformational Effect-Mediated Thermally Enhanced Phosphorescence in Phenothiazines-Quinoline-Cl Conjugates.

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.