On-off switchable thermally activated delayed fluorescence controlled by multiple channels: Understanding the mechanism behind distinctive polymorph-dependent optical properties

[Display omitted] •Emitter shows on–off switchable crystal-dependent TADF, AIE and multicolor MCL.•On-off TADF behavior can be controlled via a variety of simple external stimuli.•The underlying mechanisms of the tunable luminescent features are explored deeply. To develop multifunctional emitters, a pair of structural isomers, TRZ-c-BPXZ and TRZ-a-BPXZ, with 7H-benzo[c]phenoxazine (c-BPXZ) and 12H-benzo[a]phenoxazine (a-BPXZ) as donors, and triphenyltriazine as acceptor, are designed and synthesized. Impressively, TRZ-c-BPXZ not only exhibits on–off switchable crystal-dependent thermally activated delayed fluorescence (TADF) characteristic, but the on–off TADF emission in different aggregation states can be reversibly manipulated via a lot of channels, such as grinding, heating, fuming, recrystallizing, vacuum evaporation and sublimation. Through the detailed theoretical simulation, as well as thoroughly analysis of packing arrangements and molecular conformations of single crystals, it reveals that not only is the planar donor crucial to designing TADF molecules, but also the quasi-equatorial configuration of molecules is significant. And the appropriate π···π stacking in dimers also lengthens the delayed lifetime. Overall, these results shed light on the underlying mechanism of the tunable photophysical properties and on–off switchable TADF behavior. To the best of our knowledge, TRZ-c-BPXZ is the first example of achieving molecular on–off controllable TADF characteristic by a variety of convenient methods.