A New Series of D₁-A-D₂-type ESIPT-TICT-AIE Active Orange-to-red Emissive Unsymmetrical Azines: Their All-throughout Mechanochemical Synthesis and Exploration of Photophysical Properties

Azines are interesting compounds exhibiting aggregation-induced-emission (AIE) and twisted-intramolecular-charge-transfer (TICT) properties that have exciting prospects in chemosensing and bioimaging applications. They commonly have symmetrical structures and there is no report on red-emissive unsymmetrical azines. Herein, we report a novel class of orange-to-red emissive hydroxybenzothiazole(HBT)-based unsymmetrical azines (BTDPA) with triple photophysical characteristics of ESIPT-TICT-AIE. The dyes were synthesized by an all-throughout mechanochemical route in a sustainable way. They displayed D-1-A-D-2 character and fluoresce strongly both in organic solvents due to ESIPT and in the solid state by AIE via TICT. The incorporation of different electron-withdrawing groups (EWGs) and electron-donating groups (EDGs) at either HBT or diphenyl-methylene moiety resulted in tunable fluorescence properties. The red-emissive character was achieved by keeping EDG both at HBT (-OMe) and the diphenyl-methylene moiety (-NMe2) (lambda(em) 680 nm). The dyes exhibited good quantum yields, large Stoke shifts (upto 293 nm) and were utilized in nitroaromatics and Cu2+ sensing.