Studies on piezochromic photochromic (E )-5-dicyanomethylene-3-[1-(2,5-dimethyl-3-furyl)ethylidene]-4-diphenylmethylenetetrahydrofuran-2-one and related photochromic compounds

Yellow crystals of the title compound 2b gave a dark red powder on mechanical grinding. The yellow and red solids turned deep blue on irradiation (366 nm) and the red and blue solids turned yellow on exposure to white light. The yellow crystals gave a deep red solution in toluene, which turned blue on irradiation with ultraviolet light. The blue solution reverted to red on exposure to white light. The chemistry of these colour changes is discussed. Related photochromic compounds 2a, 2c, 2d, 4a, 4b, 7a and 7, with a lesser degree of molecular overcrowding, are not piezochromic. The related non photochromic compound 1, with the greatest degree of overcrowding, is tribochromic.