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 D₁ -A-D₂ 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 ) (λ_em 680 nm). The dyes exhibited good quantum yields, large Stoke shifts (upto 293 nm) and were utilized in nitroaromatics and Cu²⁺ sensing.