Photochromism of naphthoxazine-spiro-indolines by direct excitation and following sensitisation by triplet-energy donors

Abstract

Quantum efficiencies of merocyanine formation, (Φ_A), have been measured for several naphthoxy-spiro-indolines, (NOSIs) in a variety of solvents as well as in several polyurethane matrices having different glass-transition temperatures. In toluene solution Φ_A values range from 0.74 to 0.33 with those compounds substituted with electron-donating substituents in the 6′-position on the oxazine ring giving the highest yields. Quantum yields for merocyanine formation are also sensitive to change of solvent, being the highest in toluene and the lowest in methanol. Interestingly, in highly viscous polyurethane matrices quantum yields are typically in the range 0.1 to 0.2. Surprisingly, replacing the methyl group on the nitrogen in the indoline part of the molecule with an isobutyl group has no effect on the quantum yield in either toluene or in a polyurethane matrix. Triplet-energy donation has been used to establish that a triplet pathway leading to merocyanine formation is available and that the efficiency of merocyanine formation via triplet-state sensitization is close to unity for each of the three derivatives studied in toluene as solvent. By using various triplet-energy donors and measuring the values of the resulting quenching rate constants the lowest triplet energy of 1,3,3-trimethylspiro(indoline-2,3′[3H]naphth[2,1-b]-6′-indolino[1,4]oxazine) has been determined as 200 ± 5 kJ mol^–1 which is consistent with sensitization via the triplet state associated with the oxazine part of the molecule. For donors with triplet-state energies > 210 kJ mol^–1 quenching constants were shown to reach a plateau value of 4.5 ± 0.3 × 10⁹ dm³ mol^–1 s^–1.