Leicestershire, U K LEZZ 3TU Central Laser Facility, Rutherford Appleton Laboratory, Didcot, Oxfordshire, U K O X 1 2 OQX 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-1-01, 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-1-01, 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.Naphthoxazine-spiro-indoline (NOSI) compounds that comprise two heterocyclic moieties connected by a tetrahedral spiro linkage represent an important class of photochromic compounds which have greatly improved resistance to prolonged UV irradiation compared with, for example, the structurally related spiro-pyrans. 'L' Both classes of compounds undergo a photoisomerisation reaction following irradiation in the UV region of the spectrum, which causes a breaking of the carbon-oxygen bond at the spiro linkage. Subsequent rotation about the carbon-carbon and carbon-nitrogen bonds, which remain joining together the two parts of the molecule, forms a transoid, more planar-type, structure which has a structure resembling that of a merocyanine dye, and absorbs in the visible region of the spectrum.2 Quinoid, zwitterion and even biradical structures have been proposed for the open merocyanine forms. There are several near-planar isomers arising from rotatio...

No abstract

No abstract

No abstract

No abstract