Unusual stability of thermally induced colored isomers of photochromic 3-aryl-3-(1-naphthyl)-3H-naphtho[2,1-b]pyrans in solid state

Colorless photochromic 3-aryl-3-(1-naphthyl)-3H-naphtho[2,1-b]pyrans are converted to their colored isomers in molten state. It is found that the 1-naphthyl at the 3-position plays the main role for the unusual stability of solid colored isomers at ambient temperature.