Predistortion amplified in the excited state

•A new principle explains photochemical observations such as chiral induction.•It accounts for the accordancy principle and the choice between different channels.•The improved regioselectivity and quantum yield in rhodopsin is rationalized.•It suits the partly anti-Woodward-Hoffmann photochemistry of cyclobutenes.•Dependences on the initial state are clarified.

If a molecule is distorted by steric factors or by its environment along a photochemically active coordinate, vertical excitation puts it to a down-slope in the upper state, so that it is accelerated into the same direction. If the photoreaction is barrierless or nearly so, this acceleration can be decisive for selecting one of several possible paths. This principle is used to explain the accordancy principle, for which examples in photochemical electrocyclic reactions are abundant. It also rationalizes the selection between several allowed pericyclic reactions and/or cis–trans isomerizations including the regioselectivity in rhodopsin. Because the Woodward–Hoffmann rules are activated only with delay, the origin of the stereospecificity is reconsidered. In a generalization of the model, the predistortion is generated in a preceding electronic state Sn or on transition from there to the lowest excited singlet state S1; another variant uses selective vibrational preexcitation.

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