A holographic grating study on a crystal-to-crystal photodimerization and thermal backreaction.

The thermally reversible single-crystal-to-single-crystal [2+2] photodimerization of a styrylpyrylium salt, (E)-2,6-Di-tert-butyl-4-[2-(4-methoxyphenyl)-ethenyl]pyrylium- trifluormethansulfonate, has been investigated by means of a laser-induced holographic grating technique. By irradiation far into the long wavelength absorption tail, the single crystalline character of the sample can be retained both during the photodimerization and the thermal backreaction. Without irradiation the holographic gratings are stable at room temperature, but decay at elevated temperatures. From the temperature dependence of the holographic growth and decay curves an upper limit for the activation energy for the photodimerization is obtained to 15.2 kJ/mol. For the thermal cyclobutane cleavage an activation energy of 99 kJ/mol and an attempt frequency of 1.2×1012 s-1 are found. © 1994 American Institute of Physics. [ABSTRACT FROM AUTHOR]