Synthesis and photochromic properties of a dithiazolylindole

Abstract A novel photochromic 2,3-dithiazolylindole has been synthesized and its photochromic properties was elucidated. This compound showed reversible photochromism both in solution and in a crystal with no spontaneous bleaching under dark conditions. The photocyclization quantum yield of the dithiazolylindole was as high as 83% in hexane, suggesting the photoreactive conformation was stabilized through intramolecular interactions. X-ray crystal structure analysis revealed that the dithiazolylindole adopted the photoreactive conformation with quasi-C2 symmetry around the hexatriene moiety in the crystalline state. This conformation was assisted by presumed intramolecular CH/N hydrogen bonding between the central N-methylindole and the side-chain thiazolyl units in combination with steric interactions between methyl groups at the reactive center carbons. The suggestion of intramolecular hydrogen bonding in solution was also supported by a 1H NMR variable temperature study.