PICOSECOND LASER STUDIES ON THE EFFECT OF STRUCTURE AND ENVIRONMENT ON INTERSYSTEM CROSSING IN AROMATIC CARBENES

A large solvent polarity effect on the rate of singlet to triplet intersystem crossing (ksT) has been observed in the car-benes, diphenylcarbene (DPC) and dicycloheptadienylidene (DCHD)- it is found that both ksT and the energy splitting (A&T) separtaing the singlet and triplet states decrease as the solvent polarity increases for the aromatic carbenes. This “inverse ” gap effect, i.e. the time for intersystem crossing decreases with increasing energy gap, is esplained by an off-resonance intersystem crossing from the singJet to a sparse triplet vibronic manifold characteristic of a smalJ energy gap. The trend in AEsT, which is proposed to be responsible for the variation in ~SJ- for DPC, DCHD and structurally related aromatic carbenes, is suggested to arise from the variation in the bond a&e of the central methylene carbon atom. 1