Diphenylcarbene intersystem crossing

The kinetics of the reaction of triplet diphenylcarbene with pyridine has been studied.108 Laser flash photolysis of diphenyldiazomethane in the presence of pyridine results in the formation of pyridinium ylide 200. This reactive species shows an absorption band at 500 nm. Based on the low activation energy of the reaction, it was concluded that the data were inconsistent with a simple preequilibrium model. Instead, it was suggested that the intersystem crossing step occurs at a point other than at the minimum of singlet diphenylcarbene.108... 

Sitzmann, E. V. Langan, J. Eisenthal, K. B. Intermolecular effects on intersystem crossing studied on the picosecond timescale the solvent polarity effect on the rate of singlet-to-triplet intersystem crossing of diphenylcarbene, J. Am. Chem. Soc. 1984, 106, 1868-1869. 

Interconversion of the triplet components can be frozen out when the triplet is produced photochemically at very low temperatures in a matrix or host crystal. In these conditions, photolysis of diphenyldiazomethane produces the Ty(6i) component of diphenylcarbene selectively [8]. This finding is consistent with Fig. 9.1 if the carbene is produced in its closed shell singlet state and then crosses to the more stable triplet. It is also consistent with direct reactive intersystem crossing RISC from the photoexcited reactant molecule [9, footnote on p. 285]. 

Sitzmann, E.V., Langan, J., and Eisenthal, K.B., Intermolecular Effects on Intersystem Crossing Studied on the Picosecond Timescale The Solvent Polarity Effect on the Rate of Singlet-to-Triplet Intersystem Crossing of Diphenylcarbene,/. Am. Chem. Soc., 106, 1868,1984. 

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