Ultrafast dynamics of photoinduced ring-opening and the subsequent ring-closure reactions of spirooxazines in crystalline state

We investigated photoisomerization dynamics of spironaphthooxazine (SNO) and its chloro-substituted derivative (Cl–SNO) in crystalline phase by femtosecond diffuse reflectance absorption spectroscopy. Although the photoinduced ring-opening reaction occurred in 3 ps and <1 ps for SNO and Cl–SNO, respectively, photoisomerization to trans-planar photomerocyanine did not take place. The generated non-planar open form of SNO lived for 2 ns, while that of Cl–SNO returned to the spiro form in 1 ps. The crystalline phase results are compared to the analogous experiments in amorphous solids with a view of restricted conformational changes of molecules embedded in a crystal lattice.