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 carbenes, 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 offresonance 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 ~SJfor 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.