Structure effects on the photobehaviour of 2,2-diphenyl(2H)chromenes

The photochromic behaviour of a series of 2,2-diphenyl(2H)chromenes, bearing different substituents (methyl, thienyl, B(OH)2 and nitro groups), has been investigated. The photochromism of these molecules is described in terms of absorption spectra, photocolouration and photobleaching quantum yields, kinetics of thermal bleaching and competition between photochemical and photophysical relaxation. The quantum yield of UV photocolouration is generally fairly high (0.7–1.0), but decreases for compounds with thienyl and nitro substituents. The ring-open coloured form thermally rearranges to the closed uncoloured form, whereas visible photobleaching is substantially negligible (≤10−3). The thermal bleaching process is markedly affected by the substituents and, in the case of the nitro-derivatives, is characterised by a drastic decrease of activation entropy and enthalpy.