Photochromic reactions of crystalline spiropyrans and spirooxazines induced by intense femtosecond laser excitationDedicated to Professor Frank Wilkinson on the occasion of his retirement.

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Abstract
Photocoloration of microcrystalline powder of spirooxazine and spiropyran compounds induced by femtosecond laser excitation was investigated using time-resolved and steady state diffuse reflectance spectroscopy. The intermediate products were observed in a ns time region, and assigned to non-planar open forms. On the other hand, photocoloration can occur even in the crystalline phase by intense femtosecond pulse excitation, and the yield of colored forms increased nonlinearly with laser fluence. The mechanism of photochromic reaction induced by high-density excitation is discussed on the basis of excitation fluence and shot number dependences of photocoloration, thermal bleaching of colored forms, and transient absorption spectra. It is considered that free volume around molecules to change the molecular conformation is generated due to the local deformed crystal lattice following the densely formed excited states and intermediate products.

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