NMR studies of the structure of the photoinduced forms of photochromic spironaphthoxazines

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Abstract
Irradiation at 355 nm with a pulsed laser of the colourless 1,3-dihydro-1,3,3-trimethylspiro[2H-indole-2,3′-[3H ]-naphth[2,1-b][1,4]oxazine] results in formation of photomerocyanines (coloured forms of photochromic compounds); this and its chloro derivative were studied by NMR spectroscopy. This has allowed us to confirm the structure of the stereoisomers. The colourless and coloured forms exist in thermal equilibrium. Integration of certain photomerocyanine signals allowed us to calculate the thermal kinetics of bleaching k, the half-life τ and the activation enthalpy ωH‡ at different low temperatures and from these temperature dependence studies of the thermal decay rate, the thermal energy barrier for the decay of the coloured metastable state to the colourless form was determined.

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