Triplet carbenes fluorescence

More direct evidence for the intervention of excited states of triplet carbenes in reactions in solution is obtained by spectroscopic studies. Thus, picosecond lasers make it possible to study the quenching of carbene fluorescence by various substrates in solution at room temperature. Diphenylcarbene is generated upon laser photolysis of 30 and a second UV laser pulse is time delayed by 8 ns and is used to excite the carbene, thereby producing the excited triplet DPC (Scheme 9.32). The fluorescence of DPC is then monitored with a streak camera. The fluorescence... 

Valence isomerization of diazoalkanes into diazirines and vice versa is possible by photolysis, although it is always accompanied by dediazoniation. The photolysis of diazirines was investigated intensively in recent years (e.g., by O Gara and Dailey, 1992, and by Modarelli and Platz, 1993 see also the book of Michl and Bonacic-Koutecky, 1990). An electronically excited state is obtained that can, in principle, decay by at least four competitive pathways (Scheme 5-16) a) fluorescence, b) intersystem crossing with the production of triplet carbene, c) formation of an excited diradical followed by internal conversion and production of singlet carbene, and d)... 

Carbenes are prominent in synthetic and polymer chemistry as well as in interstellar and atmospheric chemistry [2-5]. The great interest stems largely from the chemical differences between singlet and triplet carbenes, particularly in their reactivity and rich spectroscopy and photochemistry. The latter were recentiy discussed and summarized in a review by Kable et al. [6]. Laser-induced fluorescence (LIE) studies in particular have yielded a wealth of structural and spectroscopic data as well as useful information on the nature of the potential energy surface of the excited A state, such as barriers to dissociation and... 

Fluorescence quenching studies can establish the rate constant at which a certain substrate interacts with the excited carbene, but they cannot provide any independent mechanistic information. Absorption studies are somewhat more informative in that the primary product of reaction can sometimes be detected directly. In the reaction of di-p-tolylcarbene with CCI4, the radical, (MeC6H4)2CCl, obviously formed as a result of abstraction of Cl atom from the substrate, is detected. Its formation can be monitored to give a rate constant of 1.1 x 10 M s for the excited state, which should be compared with a rate of 2 x 10 M s for ground-state triplet DPC. ... 

Conventional EPR techniques have been successfully used to measure the D and E values of matrix-isolated carbenes in the ground triplet state because the steady-state concentration of triplet species is sufficiently high in the system. The technique cannot be used, however, for excited species having triplet hfetimes of the order of 10-100 ns, since their steady-state concentration is too low. The D parameters are estimated from the external magnetic field effect on the T—T fluorescence decay in a hydrocarbon matrix at low temperamre. The method is based on the effect of the Zeeman mixing on the radiative and nonradiative decay rates of the T -Tq transition in the presence of a weak field. The D values are estimated by fitting the decay curve with that calculated for different D values. The D T ) values estimated for nonplanar DPC (ci symmetry) is 0.20... 

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