The application of molecular modelling techniques in the prediction of the photochromic behaviour of spiroindolinonaphthoxazines

Two isomeric photochromic compounds, spiroindolinonaphth[2,1-b] [1,4] oxazine 1 and spiroindolinonaphth[1,2-b] [1,4] oxazine 2, were synthesised. During the synthesis of spirooxazine 2, a coloured, non-photochromic spirooxazine 6 was isolated as an associated product and characterised as a mixture of geometric isomers. A range of readily available molecular modelling techniques were applied to provide an account of the behaviour of the photochromic materials. Heats of formation calculated using AM1, following MM2 geometry optimisation, for ring-closed and ring-opened spirooxazines, provide a method for assessing whether a spirooxazine might be expected to show photochromic behaviour. ZINDO calculations based on the molecular geometry optimised in this way provide a reasonable account of the UV-vis absorption profile of the ring-closed forms 1 and 2. An approach that uses MM2 calculations to predict the most stable of the isomer(s) of the transient ring-opened photomerocyanines, used in combination with PPP-MO calculations using optimised parameterisation, may be used to provide a prediction of the λmax values, although the method may not produce an unequivocal result when the energies of the geometrical isomers are similar.