Triplet Reactions

Photochemical studies on the ring degradation of 3-hydroxy-l,2-benzisoxazole also yielded benzoxazolone, and (40), (41) and (42) (Scheme 14) were believed to be potential intermediates. Low temperature IR measurements indicated the presence of (42) during the photochemical reaction (73JA919, 71DIS(B)4483, 71JOC1088). Sensitization studies indicate that the rearrangement is predominantly a triplet reaction, and the keto tautomer is believed to be the key orientation for the photolysis. 

A simple example serves to illnstrate the similarities between a reaction mechanism with a conventional intermediate and a reaction mechanism with a conical intersection. Consider Scheme 9.2 for the photochemical di-tt-methane rearrangement. Chemical intnition snggests two possible key intermediate structures, II and III. Computations conhrm that, for the singlet photochemical di-Jt-methane rearrangement, structure III is a conical intersection that divides the excited-state branch of the reaction coordinate from the ground state branch. In contrast, structure II is a conventional biradical intermediate for the triplet reaction. 

This key paper was followed by a flurry of activity in this area, spanning several years." " "" A variety of workers reported attempts to deconvolute the temperature dependence of carbene singlet/triplet equilibria and relative reactivities from the influence of solid matrices. Invariably, in low-temperature solids, H-abstraction reactions were found to predominate over other processes. Somewhat similar results were obtained in studies of the temperature and phase dependency of the selectivity of C-H insertion reactions in alkanes. While, for example, primary versus tertiary C-H abstraction became increasingly selective as the temperature was lowered in solution, the reactions became dramatically less selective in the solid phase as temperatures were lowered further. Similar work of Tomioka and co-workers explored variations of OH (singlet reaction) versus C-H (triplet reaction) carbene insertions with alcohols as a function of temperature and medium. Numerous attempts were made in these reports to explain the results based on increases in triplet carbene population... 

This reaction was run in the presence of the triplet quencher 1,3-pentadiene to prevent epimerization due to a-cleavage (a triplet reaction). 

The preceding discussion applied to aromatic ketone triplet reactions. With aliphatic ketones the situation is quite dilferent. As stated previously, aliphatic ketones undergo type II cleavage from both the excited singlet and the triplet state. By studying the reaction with and without added quencher, one can determine the characteristics of the reaction for each state, that is, the singlet reaction can be studied in the presence of a strong triplet quencher while the triplet reaction characteristics can be obtained by the difference between the reaction without quencher and that when quencher is added. For example, for the reaction... 

Using this equation, Wagner and Spoerke assumed that ft, = ftA + K + kK = 1 jr, that is, the triplet lifetime is determined solely by the triplet reaction. In other words, deactivation of the triplet ketone by internal conversion back to the ground state was assumed to be unimportant. The expression then becomes... 

The decarbonylation of dibenzyl ketone has been shown to result from the carbonyl triplet state by its ability to be quenched by 1,3-cyclohexadiene or l,3-pentadiene.<66) Using 1,3-cyclohexadiene as quencher, photodimers of the cyclohexadiene were obtained. Since these are formed only by triplet sensitization,<66) the quenching of ketone triplet states, rather than their excited singlets, was assured. Further evidence for a triplet reaction follows from the fact that decarbonylation could be sensitized by acetone under conditions where the sensitizer absorbed 93% of the light. 

Although essentially any acceptor triplet reaction can potentially be used for determining 18C of the donor if its quantum yield is accurately known, some precautions must be observed in order to assure success. First, the lowest excited singlet level of the acceptor should be above that of the donor to eliminate the possibility of singlet energy transfer. Second, the acceptor should be chosen such that the donor molecules absorb all of the incident light. 

It is possible to obtain a Stern-Volmer plot specifically for the triplet reaction from the equation ... 

Thus a plot of (j)T/Q(t)T against [Q] for low values of [Q] should be a linear plot with intercept 1 and slope for the triplet reaction, from which the rate constant, kR = l/kQ3T, may be determined. 

The triplet reaction of 2-nitrodibenzo[fc,primary amines (n-propylamine and benzylamine) was studied110 in polar and apolar solvents. In polar solvents, the irradiation results in the formation of two isomeric compounds, (alky-lamino)hydroxynitrodiphenyl ether andiV-(alkylamino)-2-nitrophenoxazine (equation 54). In apolar solvents, only the nitrophenoxazine is obtained. In polar solvents, the exciplex formed between the 2-n i trodi benzol h,e [ 1,4]dioxin triplet state and amines dissociates to the solvated radical ions, from which the diphenyl ether arises. 1-Nitrodibenzo[fr,e][l,4]dioxin is stable even on prolonged irradiation. 

Usually only the lowest-lying states So and Ti are considered. However, triplet reactions have never been interpreted eis involving the excited singlet state Si, as well this should behave as a diradical and may show a chemistry much like that of Ti, but should be not detected in ESR-studies. 

Quantitative data on rates of reaction have been obtained for some of the triplet reactions. Assuming triplet quenching to be approximately diffusion controlled, the rate constants for the reactions between excited species and nucleophile are 10 -10 1 mole s . The data show that in comparing and interpreting quantum yields—even in the case of related systems—one should proceed to determine separately rate constants as well as intersystem crossing efficiencies and lifetimes of the reacting excited species. 

It is not easy to explain why the triplet reactions that are energetically much less favored than those of the singlets become dominant at low temperature. Based on Ea and log A measured for triplet carbene abstraction (see Section 5.3), one can estimate the rate constant at 77 K to be <10 M s, suggesting that triplet carbene reactions in matrices at 77 K should not occur. Obviously, reactions of carbenes within matrices are controlled by factors that are not operating in solution phase, as one might expect from dramatic changes in reaction conditions. 

This reaction can be viewed as an internal hydrogen abstraction which takes place through a highly strained four-membered transition state instead of the usual six-mem bered one. The formation of the enol is wavelength dependent and is retarded by triplet quenchers.58-80 In water, excitation into the second excited singlet of biacetyl formed the enol with a quantum yield of 0.10, while excitation into the first excited singlet formed the enol with a quantum yield of 0.01, possibly via a higher excited state formed by a triplet-triplet reaction. Therefore, the authors conclude that the second excited triplet is the state that isomerizes to the enol. 

Hence, provided Ia is increased sufficiently to allow conditions corresponding to the limit to be attained, the value of dm is independent of all triplet reaction rates. It attains a maximum value of 0.02 for a compound having 4>n = 4>t = 0.5. 

At concentrations of dye > 10 5 M in water and ethanol, triplet reaction of the triplet with ground state dye is a primary reaction, and this results in electron transfer forming the semireduced and semioxidized radicals [287]. At lower dye concentrations, triplets may be intercepted by reducing agents... 

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