Pathways for the photochemical hydrogen abstraction by n,π*-excited states

As a result of recent experimental and theoretical investigations, new aspects of the photochemical hydrogen abstraction by n,pi*-excited states have emerged. The absolute reactivity of singlet-excited n,pi* states exceeds that of the corresponding tripler states, but the singlet reaction turns out to be chemically much less efficient than the tripler reaction. Hence, radiationless deactivation of the singlet states represents the dominant pathway for interaction with hydrogen donors. The fraction of singlet encounters undergoing deactivation appears to increase with decreasing reaction exothermicity. Hence, when relatively inert solvents like methanol or chloroform act as hydrogen donors towards singlet-excited azoalkanes, the reaction becomes entirely inefficient. This constitutes an example of a novel fluorescence quenching mechanism, which is referred to as an "aborted" hydrogen abstraction. The inefficiency of the singlet reaction can be rationalized, making use of the most recent theoretical findings, in terms of the occurrence of a conical intersection, i.e., a real surface crossing, rather than the previously implicated avoided crossing. The conical intersection provides an efficient channel for nonadiabatic return to the ground-state reactants and, thus, serves to account for the observed inefficiency of product formation from the singlet state. In general, the conical intersection appears to follow the transition state for hydrogen abstraction, but the consequences of a situation, where the conical intersection precedes the transition state are also discussed. The latter becomes reasonable for endothermic hydrogen abstractions, for which one would then expect a unit efficiency of singlet deactivation. The absolute reactivities in hydrogen abstractions, the involvement of CT interactions and tunneling, and the behavior of different n,pi* chromophores (azoalkanes versus ketones) are also discussed in view of the most recent experimental data. General rules for photochemical hydrogen abstractions by n,pi*-excited states are expressed.