Intramolecular Hydrogen Bonding in a Triangular Dithiazolyl-Azaindole for Efficient Photoreactivity in Polar and Nonpolar Solvents

A triangular dithiazolyl-azaindole derivative has been synthesized as a highly sensitive photochromic molecule. 2,4-Dimethyl-5-phenylthiazol derivative la, and a reference compound, 5-methyl-2-phenylthiophene derivative 2a, both showed photochromism with quantum yields of 90 and 45 %, respectively, in hexane, and 90 and 35%, respectively, in methanol. In the single-crystal state, la revealed photochromic coloration. Its conformation was assigned to a photoreactive state with C-2-symmetry around the hexatriene reaction center. Temperature-dependent NMR studies and quantum chemical calculations indicated the contribution of intramolecular hydrogen-bonding between the central azaindole unit and the side thiazole units, which stabilizes the photoreactive C-2-symmetric conformation and elevates the photoreactivity in both non-polar and polar solvents.