Effects of the environment on the photochromic behaviour of a novel indeno-fused naphthopyran

The photochromism of a new chromene-type compound, an indeno-fused naphthopyran, was studied under steady irradiation in solvents of different polarity and/or proticity, in microheterogencous systems (micelles and gel) and in a nematic liquid crystal. The solutions change from colourless to coloured upon UV irradiation, due to cleavage of the carbon-oxygen pyran bond. The photoreaction is thermally reversible. Spectra, molar absorption coefficients of the colourless and coloured forms, quantum yield of photocolouration and kinetic parameters of the thermal bleaching (rate constant and activation energy) were determined. Compared with other chromenes, the spectra of both the coloured and colourless forms are red-shifted and the coloured form exhibits a marked positive solvatochromism. The photocolourability is good, even at ambient temperature, and is better in a polar and/or protic medium where the entropy loss due to solvent reorganisation around the transition state decreases the rate of the bleaching process. Thus, the best media for colouration are ethanol and the liquid crystals. In the gel (Aerosol-OT [sodium bis(2-ethylhexyl)sulfosuccinate] isooctane-animal gelatin-water) and microemulsion (AOT-isooctane-water) the non-polar ground state molecules (in both the open and closed forms) occupy the hydrophobic sites and therefore the behaviour is similar to that observed in the pure hydrocarbon.