REACTIVITY AND EFFICIENCY OF SINGLET-EXCITED AND TRIPLET-EXCITED STATES IN INTERMOLECULAR HYDROGEN ABSTRACTION REACTIONS

The rate constants for quenching of the singlet- and triplet-excited s tate of acetone and a cyclic azoalkane by the hydrogen donors tributyl tin hydride, 1,4-cyclohexadiene, and 2-propanol have been determined b y time-resolved spectroscopy. It is concluded, in variance with previo us studies, that singlet-excited states are significantly more reactiv e than triplet-excited states and that the reactivity difference betwe en the two states of different spin multiplicity increases (i) with de creasing reactivity of the hydrogen donor and (ii) with increasing sin glet-triplet energy gap of the excited state. This result is corrobora ted by semiempirical calculations. The relative efficiency for photore duction by tributyltin hydride, which was determined by monitoring the formation of tributyltin radicals upon flash photolysis, was found to be four times lower for singlet-excited acetone than for the triplet state. The discrepancy between higher reactivity but lower efficiency in the intermolecular interaction of n,pi-excited singlet states with hydrogen donors is attributed to efficient radiationless deactivation , which has been predicted by correlation diagrams as a viable pathway for singlet-excited states.