Photochromism of spiropyrans. Part 1.—Mechanism of photocolouration

The sensitized and unsensitized yields of phosphorescence and photoisomerisation of 6-nitro-1′,3′,3′-trimethyl-2′,3′-dihydro-2H-benzo[b]pyran-2-spiro-2′-indole in poly(methyl methacrylate) have been measured at various temperatures. The intersystem crossing yield which equals the unsensitized phosphorescence yield divided by the sensitized phosphorescence yield observed under conditions where energy transfer to form the triplet state of the spiro compound is 100 % efficient varies very little with temperature, being 0.30 at 210 K and 0.50 at 75 K. The yield of the unsensitized triplet reaction is given by the product of the intersystem crossing yield and the yield of photosensitized colouration reaction observed when triplet energy transfer produces the triplet state of the spiro compound with unit efficiency and at all temperatures this value was found to be equal to the measured unsensitized colouration yield demonstrating that the unsensitized reaction occurs exclusively via reaction of a triplet state. Studies of the ability of triplet energy donors of variable energy to sensitized photoisomerisation and/or phosphorescence of 6-nitro-1′,3′,3′-trimethyl-2′,3′-dihydro-2H-benzo[b]pyran-2-spiro-2′-indole in poly(methyl methacrylate) and polystyrene show that a higher triplet state and not the phosphorescent state is the reactive state in the colouration reaction.