Photochemie substituierter Cycloheptatriene. IX Zum photoreaktiven Zustand der sigmatropen Wasserstoffverschiebung

Photochemistry of Substituted Cycloheptatrienes. IX. About the Photoreactive State in the Sigmatropic Hydrogen Shift We have studied the sigmatropic hydrogen shift in mono- and diarylcycloheptatrienes. From quenching and senzitation experiments is assumed that the sigmatropic H-shift occurs in the singlet state. The population of the photoreactive state is in concurrence with the population of the fluorescent singlet state. We postulate a common pericyclic minimum for hydrogen shift of 7,3 and 1,4-diarylcycloheptatrienes, respectively. In this minimum the excited Cycloheptatrienes have a more covalent than an ionic character. This was concluded from measurements of reaction quantum yields in dependence on substituents and solvents.