Lifetime of singlet

Becker et al. (1985 a) also investigated the photolytic dediazoniation of arenediazonium salts sensitized by zinc tetraphenylporphyrin (ZnTPP). Due to the very different lifetimes of singlet and triplet ZnTPP (1.8 ns and 1.4 ms, respectively) it is possible to differentiate between singlet and triplet electron transfer reactions using different diazonium salt concentrations. 

The use of emission (fluorescence and phosphorescence) as welt as absorption spectroscopy. From these spectra the presence of as well as the energy and lifetime of singlet and triplet excited states can often be calculated. 

Generation of 78 by thermolysis or photolysis of a diazoalkane or diazirine precursor, however, affords the singlet carbene, whose 1,2-H shift to ethene is opposed by a barrier of only 0.678 to 1.298 kcal/mol. Consequently, even in cryogenic matrices, singlet 78 rearranges more rapidly than it intersystem crosses to the triplet, which has therefore not been detected by UV or ESR in either an Ar matrix at 8 K or a Xe matrix at 15 K." The lifetime of singlet 78 at ambient temperature has been estimated at <0.5 ns.89,98b (Note the enormous spectator substituent effect of Cl the lifetime of MeCCl is 740 ns,60 at least 1500 times longer than that of MeCH.)... 

The lifetime of formylcarbene was determined by transient absorption and grating spectroscopies.39 Photolysis of formyldiazomethane produces formylcarbene which can isomerize, kr, to ketene or react with pyridine (Scheme 4). Using the pyridine ylide method, the lifetime of singlet formylcarbene was estimated to be 150-730 ps in CH2CI2. This is in reasonable agreement with the lifetime of 900 ps determined by transient grating spectroscopy. 

In solution photochemistry in the presence of acids, the primary process is also the same except that both NND and the aminyl radical are protonated the recombination of the aminium radical and NO to give 295A is too slow to compete with bond scissions174 (Scheme 11). The failure of oxygen to quench nitrosamine photoreactions in either solution (see below) or gas phases under various conditions must also mean a very short lifetime of singlet excited nitrosamines, in agreement with the fast dissociation159,160. 

Moan J, Berg K (1991) The photodegradation of porphyrins in cells can be used to estimate the lifetime of singlet oxygen. Photochem Photobiol 53 549-553. 

J. G. Parker and W. D. Stanbro, Optical determination of the collisional lifetime of singlet molecular oxygen in acetone and deuterated acetone, J. Am. Chem. Soc. 104, 2067-2069 (1982). 

The change of p with solvents may be due to either a change of the lifetime of singlet oxygen (1 /kB) or a change of the reactivity of the substrate toward singlet oxygen (kg), or both. Systematic studies of solvent effects are, therefore, very much needed. 

In the case of insertion toward fluorine (41), there is an even greater amount of positive-positive charge repulsion between the ortho and ipso carbons than in the transition state and this effect is responsible, in part, for a higher activation barrier for insertion toward F to form 41 than away from fluorine to form 40. Therefore, the origin of the pronounced influence of ortho,ortho-difluoro substitution on the lifetime of singlet arylnitrene and the increased activation energy of its cyclization is the result of combination of the steric effect and the extraordinary electronegativity of fluorine atom. 

The deprotonation time is therefore often so long that protolytic equilibrium cannot become established during the lifetime of singlet excited states (say... 

An electronically excited molecule may, under some conditions, absorb another quantum and be raised to a higher excited state. Usually the population of excited species is so low that the probability of this occurrence is very slight. However, in recent years the technique of flash photolysis has been developed, which allows us to investigate the absorption properties of excited states. An extremely high intensity laser, which has approximately one million times the power of a conventional spectroscopic lamp, is turned on for a tiny fraction of a second, and a large population of excited species is produced. Immediately after this photolysis flash is turned off, a low-power spectroscopic flash may be turned on and the absorption spectrum of the already-excited system determined. By varying the delay between photolysis and spectroscopic flashes, much can be learned about the absorption and lifetime of singlet and triplet excited states. 

Since the lifetime of singlets is very short, it has a quasi-stationary solution... 

Triplet diphenylcarbene has a Tl/2 of 2 ps in the presence of 1.0 M isoprene (Eisenthal et al., 1980). The lifetime of singlet phenylchlorocar-bene ranges from 5 to 500 ns in the presence of 1.0 M alkenes with various substituents (Turroetal., 1980). In 1.0M methanol singlet fluorenylidene and singlet diphenylcarbene have half-lives of 0.77 ns and 0.02 ns respectively (Zupancic and Schuster, 1980 Eisenthal et al., 1980). Triplet carbenes react very rapidly with 02. 

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