抄録
In contrast to photochromism and thermochromism, piezochromism and tribochromism have been known only in limited compounds. We now demonstrate a novel compound which shows color change in the solid-state on grinding or applying pressure. A novel piezochromic compound is 11-(3'-oxodihydro[1]benzothiophen-2'-ylidene)cyclopenta[1,2-b:4,3-b']di[1]benzothiophene (1) which bears a partial structure of thioindigo. This compound is dark red in color, while undergoes a color change from red to black when a mechanical stress is imposed on powders in a mortar with a pestle. The black form shows the reflectance band in a range of 600-1000 nm. The reverse process, from black to red form, is realized upon heating or dissolving the black solids of 1. The X-ray crystal structure has been carried out to clarify the mechanism of the color change. The molecule is entirely planar and stacks in a columnar structure by head-to-head overlapping. There is an inverted disorder between the carbonyl moiety and the sulfur atom; the molecules of the one form are stacked in one column while the inverted molecules are likewise stacked in the neighboring column. Based on the crystal structure of 1, the mechanism of reversible chromotropism is discussed.
