Malononitrile based ternary AIE-ML materials: Experimental proof for emission switch from non-TADF to TADF

Abstract Three malononitrile based D-A-D compounds with TPA, PhCz and DMAC donor (DPMM-2TPA, DPMM-2PhCz and DPMM-2DMAC) at the para-position of phenyl group on the DPMM unit are synthesized and their photophysical properties are systematically investigated. All compounds display positive solvatochromic behaviour, good thermal and electrochemical stability. Moreover, pronounced aggregation-induced emission and mechanochromic luminescence are observed in their solid states. DPMM-2TPA and DPMM-2DMAC realized red fluorescence, extremely small ΔEST of 0.06 and 0.01 eV and significant thermally activated delayed fluorescence character, due to the operative RISC process via spin orbit coupling from 3LE to 1CT and hyperfine coupling between 3CT and 1CT, respectively. Nevertheless, TADF cannot be detected for DPMM-2PhCz because of its large 1CT and 3CT gap (0.34 eV) and inaccessible endothermic energy transfer. These results suggest that the energy level of 1CT, 3LE and 3CT is adjustable by carefully designing the molecular configuration and thus enable the emission switch from non-TADF to TADF. Multilayer OLEDs based on DPMM-2TPA and DPMM-2DMAC exhibit a maximum EQE of 16.5% and 12.7%, respectively, which is 3–4 times higher than DPMM-2PhCz based non-TADF OLEDs (EQE = 4.4%).