Photochromic Heteroaromatic Thiofulgides and Dimethoxybutanoic Acid Lactones.

Thiofulgides 1, (X = S) cyclise on irradiation with UV light to form thermally stable photochromes 2, (X = S) that absorb at much longer wavelengths than photochromes 2, (X = O), formed from corresponding fulgides 1, (X = O), reverse reactions occur on exposure to white light; photochromic thiofulgides are prepared by reaction of fulgides with sodium hydrosulfide in hot toluene and photochromic lactones 9 and 12 are prepared by reaction of fulgides with sodium hydrosulfide in cold methanol followed by cyclisation with a carbodiimide.