Reversible mechanochromism in dipyridylamine-substituted unsymmetrical benzothiadiazoles

We report the design and synthesis of push-pull benzothiadiazoles (BTDs) of type D-1-pi-A-pi-D-2 and D-1-pi-A-D-2. These BTDs show strong charge transfer interaction. BTD 3 shows reversible mechanochromism with color contrast between yellow (crystalline state) and orange (amorphous state). Photophysical and computational studies reveal that the planar orientation of the pyridyl and BTD unit in 2 results in no change in solid state emission whereas non-planar orientation of the dipyridylamine and BTD unit in 3 results in efficient mechanochromism.