Sub-picosecond transient absorption spectroscopy of substituted photochromic spironaphthoxazine compounds

The photochromic reaction dynamics of spiroindolinenaphthoxazine and its 6′CN and 5′CHO substituted compounds is investigated in different solvents by femtosecond transient absorption spectroscopy. In addition to the formation of the merocyanine coloured form (ring-opened trans form, OF), another short-lived intermediate species is produced upon photoexcitation, which is not a precursor to the OF product but which is formed in parallel to it via a competing relaxation process. This species is ascribable to either a relaxed s-cis ring-opened isomer on the ground state potential energy surface or to a metastable minimum of the excited S₁ state potential energy surface of the ring-closed form. The observed kinetics suggest that the production of OF (photocoloraton reaction) is controlled by the efficiency of the competing process rather than by an s-cis - trans isomerisation energy barrier.