Excimer photodimerization reactions

A group of papers dealing with the intramolecular photodimerization reaction of a,a>-bis(9-anthryl)alkanes (185 n = 2—10) fails to produce agreement about the detailed mechanism of the reaction. Measurements of quantum yields for fluorescence, photoreaction, and intramolecular deactivation as a function of temperature are said to provide no support for a biradical intermediate, but rather to support a concerted mechanism. In reply, the proposers of the biradical mechanism reinterpret these data and find them consistent with their mechanism. A third research group reports results in fluid solution at room tempera-ture " their concern is more with the question of excimer involvement in the mechanism, and they report that in many of the systems unidentified photoproducts are formed via excimers that do not lead to the normal 9,9 -linked photodimer. The internal photodimer from (185 n = 3) has been studied in a matrix at 10 and photodissociation is shown to lead to two different modifications of (185 n = 3) with different reactivities. A geometrical constraint on the intramolecular photoreactions of 9,9 -linked bisanthracenes is demonstrated by the failure of the di-substituted ethylenes (186) to give internal dimers. 

The stereospecificity of these reactions is surprising in light of the large energies absorbpd by these molecules. Indeed, the major photochemical product of these photolyses was the alternate olefin isomer (1-butene was also observed). These results indicate that free rotation about the photo-excited double bond does not occur in those molecules that dimerize. This suggests the participation of ground state complexes or excimers in the photodimerization. This view is supported by the observations that dilution of cw-2-butene with neopentane (1 1) decreased the yield of dimers and a 1 4 dilution almost completely suppressed dimerization. 

Anthracene undergoes a photochemical 9,10,9, 10 -cycloaddition which goes through the excimer as intermediate. Many aromatic molecules follow similar cycloaddition paths. The close approach of the molecules in the excimer is essential for bond formation, and steric hindrance can prevent the reaction unsubstituted anthracene dimerizes so fast that no excimer fluorescence can be detected, 9,10-dimethylanthracene shows both excimer fluorescence and photodimerization, but 9,10-diphenylanthracene shows neither excimer emission nor photodimerization (Figure 4.52). 

Quina, F H., and Wliitten, D. G. Photochemical Reactions in Organized Monolayer Assemblies. 4. Photodimerization, Photoisomerization, and Excimer Formation with Surfactant Olefins and Dienes in Monolayer Assemblies, Crystals, and Micelles. J. Am. Chem. Soc. 99, 877 (1977). 

Photodimerization often involves an excimer that can be treated as a su-permolecule. (Cf. Section 6.2.3.) Then, the state correlation diagram for the singlet process (Figure 7.27a) ordinarily calls for a two-step return from S, to So along the concerted reaction path. First, an excimer intermediate E is formed. Second, a thermally activated step takes the system to the diagonally distorted pericyclic funnel P" (cf. Section 4.4.1), and the return to So that follows is essentially immediate. The reaction will be stereospecific and concerted in the sense that the new bonds form in concert. However, it will not be concerted in the other sense of the word, in that it involves an intermediate E. ... 

The intensity dependence of UV laser flash excitation of diphenylaraine in methanol shows the occurrence of two quantum photoionization involving the triplet state.Also the binding energies of the triplet excimers observed in poly(N-vinylcarbazole) films between 15 and 55 K have been estimated.Triplet states, biradicals, radical ions and heavy atom effects can all be involved in the photodimerization of aceanthrylene. It is established that the triplet state reaction mechanisms give rise to four stereoisomeric dimers. The basicity of the Ti state of phenazine is found to have a pKa of 1.9, which is appreciably different from an earlier value, by flash photolysis. The discordant results obtained in earlier work are satisfactorily explained. 

It has been observed that piperylene catalyses the photodimerization of 9-phenylanthracene.177 The process appears to be an example of a general phenomenon, for the reaction can be catalysed by other dienes, and improvement of dimer yields has been noted with other anthracenes, including those which undergo dimerization in the absence of dienes. From kinetic data, a scheme is proposed which involves the excimer as the reactive intermediate. 

The rates or efficiencies of bimolecular photoreactions such as excimer formation, photodimerization, and photoaddition can be strongly affected by using micellar solvents when the reactants associate with the micelles When low concentrations of reactants are solubilized in solutions of high concentrations of micelles, the reactants will be separated by association to different micelles and the reaction will be inhibited. Under opposite conditions high local concentrations will cause an increase of quantum yields compared to homogeneous solutions of equal analytical concentration. Thus in micellar solution the efficiency of bimolecular... 

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