Kinetic Effect on Stability of the Triplet Diphenylcarbene

Carbene is a very important reactive intermediate to explain organic reaction mechanism that cannot be considered by generally; on the other hand, the triplet carbene has been considered to the unite of organic magnetic materials owing to its special electronic structure that has a parallel electronic configuration described asσ1ρ1. However the triplet carbene is too reactive to trap, for instance, the lifetime of triplet diphenylcarbene is only 0.002ms. For this reason the stabilization of carbene would be necessary for organ chemistry and advance organic materials. Two basic strategies, i.e., thermodynamic and kinetic stabilization, are possible for the stabilization of reactive species. Studies examining the relationship between structure and reactivity have shown that thermodynamic stabilization (electronic conjugation effect) usually play an important role in stabilizing the singlet state and that the singlet state undergoing the stabilization becomes less reactive due to the contribution of the conjugation to such an extent that the species can be isolated under ambient condition. On the other hand, kinetic stabilization (steric protection) is more effective in stabilizing the triplet counterpart since the introduction of sterically bulky groups around the carbenic center results in an increase in the carbene bond angle and protection the carbenic center attacked by minor impurities in the surrounding medium, i.e., a trace of oxygen. To explorer the stable effect of substituents on the triplet diphenylcarbene, in the preceding paper, we had generated a series of triplet diphenylcarbenes, in which have various substituents at the para- or ortho-position by photolysis of the corresponding diphenyldiazomethane at room temperature, and studied their reactivities with oxygen by laser flash technique. And revealed that the substituents whether para- or ortho-position can cause the stable effect on the triplet diphenylcarbenes, and the steric protective effect in ortho-position is more effective. In order to further investigate the steric protective effect (kinetic stabilization) of substituents on the triplet diphenylcarbene, a series of triplet diphenylcarbenes having a varied size of substituents at orthoand para-positions have been generated by the irradiation of diazo precursors, and observed them directly by UV/vis spectroscopy. And measured the lifetime of the triplet diphenylcarbenes by means of laser flash photolysis in degassed benzene at room temperature. It has been shown that the triplet diphenylcarbenes have farther stabitities with the bigger substituents at ortho- and para-positions, because the steric effect, that the bigger substituents have made carbene have a bigger central bond angle, protect the attack on the carbenic center by minor impurities.