PATHWAYS FOR THE PHOTOCHEMICAL HYDROGEN ABSTRACTION BY N,PI-ASTERISK-EXCITED STATES

As a result of recent experimental and theoretical investigations, new aspects of the photochemical hydrogen abstraction by n,pi-excited st ates have emerged. The absolute reactivity of singlet-excited n,pi st ates exceeds that of the corresponding tripler states, but the singlet reaction turns out to be chemically much less efficient than the trip ler 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 relativel y inert solvents like methanol or chloroform act as hydrogen donors to wards singlet-excited azoalkanes, the reaction becomes entirely ineffi cient. This constitutes an example of a novel fluorescence quenching m echanism, 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 occurrenc e of a conical intersection, i.e., a real surface crossing, rather tha n the previously implicated avoided crossing. The conical intersection provides an efficient channel for nonadiabatic return to the ground-s tate reactants and, thus, serves to account for the observed inefficie ncy of product formation from the singlet state. In general, the conic al intersection appears to follow the transition state for hydrogen ab straction, but the consequences of a situation, where the conical inte rsection precedes the transition state are also discussed. The latter becomes reasonable for endothermic hydrogen abstractions, for which on e would then expect a unit efficiency of singlet deactivation. The abs olute reactivities in hydrogen abstractions, the involvement of CT int eractions and tunneling, and the behavior of different n,pi chromopho res (azoalkanes versus ketones) are also discussed in view of the most recent experimental data. General rules for photochemical hydrogen ab stractions by n,pi-excited states are expressed.