Kinetics of reversible photoisomerization: determination of the primary quantum yields for the E-Z photoisomerization of silylenephenylenevinylene derivatives.

The kinetics scheme for directly excited, photoreversible reactions is solved exactly under the assumptions of no irreversible side reactions and constant excitation intensity for the duration of the reaction. The advantages of the methodology over the extrapolation-to-zero-time and the back-reaction correction methods are (i) that the quantum yields of both the forward and reverse photoreactions can be obtained starting from either pure reactant or pure product and (ii) the conversion percentage is not limited to a narrow domain in the neighborhood of small conversions. Examples of E-Z photoisomerizations are given to illustrate the fitting procedures required. The results from these examples are compared to the photoisomerization method of extrapolating the empirical quantum yields to zero time and the back-reaction correction. The exact equations are used to justify the extrapolation-to-zero-time method and to establish criteria on extrapolation ranges for the conversion percentage of starting material.