Modulated emission from dark triplet excitons in aza-acene compounds: fluorescence versus phosphorescence

In the field of organic light-emitting diodes (OLEDs), research interests focus on making the optically dark triplet excitons shine in order to increase the electro-optic conversion efficiency of devices. In this work, two kinds of phenazine compounds, i.e. dibenzo[ a,c] phenazine (DBP) and tribenzo[ a,c,i] phenazine (TBP), were synthesized and used as model compounds to regulate the emission efficiency of the dark triplet excitons by chemical modification. Charge-transfer induced ultrafast intersystem crossing (CT-ISC) with a time constant of similar to 1 ps was observed for these two phenazine derivatives upon photoexcitation with a high triplet yield of 77.1% for DBP and 58.7% for TBP. The triplet excited states of DBP can produce ultra-long phosphorescence with lifetime as long as 318 ms at 77 K. The quantum yield for phosphorescence (FP) is determined to be 8.45%. In sharp contrast, the triplet-excited (TBP)-T-3* undergoes an efficient reverse intersystem crossing (RISC) process, resulting in bright delayed fluorescence emission with negligible phosphorescence. A controllable luminescence behavior from the triplet states between fluorescence and phosphorescence in phenazine derivatives is demonstrated. Theoretical calculations reveal that the structure-dependent triplet evolution is due to the charge-transfer induced energy level alignment within these compounds. Our results may have potential applications in the design of OLEDs and high triplet yield pure organic materials.