Thermally Activated Delayed Fluorescence and Room-Temperature Phosphorescence in Asymmetric Phenoxazine-Quinoline (D2-A)Conjugates and Dual Electroluminescence

Triplet energy harvesting via either thermally activated delayed fluorescence (TADF) or room-temperature phosphorescence (RTP) from pure organic systems has attracted great attention in the field of organic light-emitting diodes, sensing, and bioimaging. However, the realization of dual electroluminescencevia TADF and RTP in single molecules remains elusive. Herein, we report two phenoxazine-quinoline conjugates (DPQandDPQM) in which two phenoxazinedonors are covalently attached to the 6,8-positions of 2,4-diphenylquinoline and/or 7-methyl-2,4-diphenylquinoline acceptors. Experimental and quantum chemistrycalculations combining reference compounds (o-PQP,p-PQP,Phox, andQPP)reveal that both conjugates show TADF (with different rate constants of reverseintersystem crossing,krISC= 0.43-1.30x106s-1) via reverse intersystem crossingfrom the charge transfer triplet (3CT) to singlet (1CT) states mediated by vibroniccoupling among1CT, local triplet (3LE), and3CT states due to close energy gaps.Further, RTP with quantum yields (phi P) of ca. 21-24% features was also observed dueto the radiative decay of3LE states. Phosphorescence measurements of DPQMat low temperatures (T= 77, 10 K) ensure a distinctzero-field splitting ofT1(CT) into substates. Both compounds showed dual electroluminescence with external quantum efficiencies of ca. 11-12% due to the efficient triplet harvesting from both TADF and RTP channels.