Thermally Reversible Photoisomerizations

This article consists of a programmatic part and a pragmatic one. The programmatic aspects are the establishment of photochemistry, in the broadest definition of its scope, as a developing field directed toward the elucidation of photobiological phenomena, and an appeal for the wide development of the hitherto neglected dimension of electronically excited states. The pragmatic part is the detailed description of the photochemistry of linearly conjugated cyclohexadienones as an example of a thermally reversible photoisomerization system. The logical use of methods for obtaining information about the formal kinetics of light‐induced reactions and for the identification of short‐lived transients by low‐temperature spectroscopy is of value to both parts.