Spectral and kinetic properties of a red–blue pH-sensitive photochromic spirooxazine

By means of UV/Vis absorption spectroscopy, NMR measurements and kinetic modelling, two spironaphthoxazine compounds bearing 5-OAlk substituent have been investigated in neutral and acidified ethanol media. The kinetics of ring opening/ring closure and the equilibrium constant, the quantum yield of photocoloration, the molar absorption coefficients of the neutral and protonated open forms and pKa values have been determined. A three-species photoreversible transformation between the closed form, the open form and the protonated open form has been observed in slightly acidified solutions.