Photochromism of nitrospironaphthoxazines and spiroanthroxazine

The photocolouration of spiroanthroxazine (3) and two nitrosubstituted spironaphthoxazines (1 and 2) and the subsequent thermal relaxation back to the ring-closed (spiro: Sp) form were studied by time-resolved techniques in solvents of low and high polarity. The photochemical ring opening of 1 or 2 occurs via singlet states, whereas the photomerocyanine of 3 is populated from a triplet state. The relaxation times at 25 °C range from 3 s for 1–3 in methylcyclohexane (MCH) to 200 s for 2 in ethanol. They are due to differences in the activation energy (77–90 kJ mol−1) and the pre-exponential factor (0.3–6)×1013 s−1. The phosphorescence properties of 1–3 in MCH, methyltetrahydrofuran (MTHF) or ethanol at −196 °C are described. Deactivation of the excited merocyanine of 1 or 2, upon excitation at 530 nm, into the Sp form occurs mainly in the excited singlet state, as indicated by bleaching at lower temperatures. For 3, an additional bleaching due to the population of the triplet state takes place. The merocyanines of 1–3 exhibit fluorescence in glassy media at −196 °C, competing with the radiationless deactivation pathways of the excited singlet state. The effects of structure and medium properties on the photoprocesses and the mechanisms of photochromism and thermal decolouration are discussed.