Multifunctional asymmetric D-A-D′ compounds: Mechanochromic luminescence, thermally activated delayed fluorescence and aggregation enhanced emission

Herein, we report synthesis and properties of new multifunctional materials, containing quinoxaline as an acceptor core and differently substituted carbazole moieties as the donors within unsymmetrical D-A-D' type structures. The compounds exhibit thermally activated delayed fluorescence, luminescence variation in response to external stresses (from green to orange) and emission enhancement in the aggregated state. The effect of substitutions on the different properties is discussed. The investigation suggests that the strength of the donor determines the optical properties of unsymmetrical bipolar emitters. Density functional theory calculations revealed that in the ground and the excited state, electrons of the highest occupied molecular orbitals (HOMOs) are mainly localized on the stronger donor. Mechanochromism studies demonstrated that the bulky tert-butyl groups attached to the carbazole moiety give rise to a hypsochromic effect. It is also associated with the strong reversible colour contrast, in the range of 524-583 nm, in response to external stresses. Using the synthesized compound as an emitter in organic light emitting diode, maximum external quantum efficiency of 10.5% and luminance of 48800 cd.cm(-2) were observed.