Electronic and structural characterization of a piezochromic indigoid:: 11-(3'-oxodihydrobenzothiophen-2'-ylidene)cyclopenta[1,2-b:4,3-b']dibenzothiophene

The title compound, a novel thioindigoid, undergoes a color change in the solid state from red to black due to mechanical shearing ("piezochromism"), and the color is also recovered either by heat or solvent treatment. The mechanism of the piezochromic effect has been studied in the present investigation on the basis of the crystal structure and thermal analysis. The absorption band in the initial red state is found to be similar to the one in solution, indicating that the molecular nature is well preserved even in the solid state. The color change (red --> black) is then induced by mechanical shearing, accompanied by an additional, new band around 750 nm. The new band covers the whole visible region together with the band in the red phase to make the color black. The new band has been interpreted as arising from excitonic interactions between transition dipoles based on the re-organized molecular arrangement caused by mechanical shearing.