Multi-stimuli response of a novel half-cut cruciform and its application as a security ink†
(E)-2′-[2-(Benzo[d]thiazol-2-yl)vinyl]-N,N-dimethyl-(1,1′-biphenyl)-4-amine 2, a novel V-shaped molecule (i.e. a half-cut cruciform), and its line-shaped isomer, (E)-4′-[2-(benzo[d]thiazol-2-yl)vinyl]-N,N-dimethyl-[1,1′-biphenyl]-4-amine 4, were successfully prepared via a two-step route of Suzuki coupling and Witting reaction. The much more profound ICT effect on 2 than on 4 means that the former has largely reduced quantum yields in highly polar solvents, in which AIE effect is enhanced for 2 but not for 4. Morphology-dependent fluorochromism in highly contrasted colours induced by mechanical force or surrounding pH stimuli can reversibly occur on 2 but not on 4. X-ray crystallography revealed loose molecular packing caused by the twisted conformation of 2, which is assumed to account for the intense solid-state emission and mechano-chromic activity. An organic solution of 2 and CF3COOH mixture shows potential for use as a security ink without the need for a covering reagent.