Picosecond time-resolved spectroscopy of the photocolouration reaction of photochromic naphthoxazine-spiro-indolines

The photochemical formation of the open merocyanine forms of several naphthoxazine-spiro-indolines in different solvents have been studied using both picosecond transient absorption (PTA) and picosecond time-resolved resonance Raman (PTR³) methods. The PTA studies have established the presence of several metastable species in the photochemical formation of the coloured merocyanine form of these photochromic compounds. The primary photochemical step occurs on the sub-ps timescale and is followed by the formation of a cisoid intermediate over the next 6-30 ps. This cisoid species then isomerises to the equilibrated distribution of transoid isomers of the merocyanine form with a lifetime that is dependent upon both solvent viscosity and polarity as well as the nature of the substituents on the naphthalene part of the molecule. However, the rate of this cis → trans isomerisation is unaffected on changing the N-alkyl group on the indoline part of the molecule from a methyl to an isobutyl group. The PTR³ studies have demonstrated that, in butan-1-ol, there are at least three different transient species with characteristic vibrational spectra which evolve with different lifetimes to give the final isomeric distribution over the first few ns of the reaction. In cyclohexane it is likely that the formation of a single species is being probed, which is fully developed after the first 200 ps of the reaction. This single species has a spectrum which is the same as the equilibrated steady-state resonance Raman merocyanine spectrum. It is likely that the evolution in the PTR³ spectra obtained here, in butan-1-ol, results from an equilibration of initially formed transoid merocyanine isomers to give a more stable distribution in this polar hydrogen-bonded solvent. Such an equilibration appears to be unnecessary in a non-polar solvent such as cyclohexane and it is suggested that this is because the transoid isomer, initially formed, is already in its most stable form.