Photochromism of spirobenzopyranindolines and spironaphthopyranindolines

Abstract

The photocolouration of five spiro[2H-1-benzopyran-2,2′-indoline] compounds (1–5) and two spironaphthopyranindolines (6, 7) to give the ring-opened merocyanine(s) and the subsequent thermal relaxation back to the closed form was studied. A trans-merocyanine but no cis-photoisomer were detected for 1–7 using λ_exc = 308 nm and a singlet pathway accounts for photocolouration. In contrast to nitrospiropyrans the yield of colouration of spiropyrans is essentially independent of solvent polarity. The relaxation times at 25°C range from 0.01 s for 6 in methylcyclohexane to 17 s for 5 in ethanol, which is due to differences in both activation energies (65–80 kJ mol⁻¹) and the pre-exponential factor (10¹²–10¹⁴ s⁻¹). The photomerocyanine yield is substantially increased upon naphthalene- or acetone-sensitized excitation, where otherwise the same spectral changes, relaxation time and activation parameters were found. Upon excitation of the trans-merocyanine (λ_exc = 530 nm), a cis-merocyanine with lifetime in the sub ms range could be detected at lower temperatures. Deactivation of the excited trans-merocyanine occurs mainly in the excited singlet state. For 4 and 5, due to an internal heavy-atom effect of bromine substituents, and for 1, a triplet state as a precursor of the cis- and trans-merocyanines could be observed. The effects of structure and medium properties on the photoprocesses are described and the mechanisms of photochromism and thermal decolouration are discussed.