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 1a , 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, 1a 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.