Temperature dependence of ultrafast photoinduced ring-opening and -closure reactions of spironaphthooxazine in crystalline phase

To investigate the dynamics of photoinduced ring-opening and -closure reactions in crystalline phase, we measured transient absorption spectra of spironaphthooxazine microcrystalline powder by femtosecond diffuse reflectance spectroscopy and examined their temperature dependence. The lifetimes of the excited state of the spiro form and the subsequent non-planar open form were about 3 ps and 3 ns at room temperature, respectively. The temperature dependence of their decay constant showed the Arrhenius type behavior and the activation energy of the ring-opening and -closure reactions are estimated to 7 and 12 kJ mol−1, respectively. The yield of the non-planar open form became smaller at lower temperature, and it was zero below about 140 K. The experimental results and kinetic analysis indicate that a short-lived species having a molecular conformation different from the non-planar form should be considered as an intermediate in the ring-opening reaction.