Constitutional isomers of a C-3-symmetric molecule showing different piezochromic behaviours: on-off switching and colour tuning

The piezochromic behaviours of C-3-symmetric molecules with p-bromophenyl side groups connected to a phenyl ring core through cyano-vinylene bridges were studied. Upon grinding, a-BPAN-Br having cyano groups at the a-position to the phenyl ring core exhibited substantial quenching of a bluish green emission (on-off switching) and its constitutional isomer, b-BPAN-Br, showed an emission colour change from bluish green to deep blue (colour tuning). When the molecules were exposed to an organic vapour, the initial emission of each molecule was restored. The powder X-ray diffraction and DSC studies revealed that the as-synthesized and vapour-annealed samples had the same crystalline structures, while the ground samples had amorphous structures. Their structural analogues, a-BPAN-H and b-BPAN-H which had no bromo groups, did not show any piezochromic or vapochromic behaviour. Poly(b-BPAN) consisting of covalently linked b-BPAN units were prepared by Ullmann reaction of bBPAN- Br. The polymer showed similar luminescence to that of crystalline b-BPAN-Br, but its initial emission was not changed by grinding.