Excimer intermediates

As early as 1964, Corey had suggested the intermediacy of an oriented pi complex in the cycloaddition of enones (43) and soon thereafter Hammond and coworkers, on the basis of arene fluorescence quenching by dienes, suggested the possible involvement of a polar excited state complex with substantial charge transfer character (44). Since then the possibility of cycloadditions occurring through the intervention of exciplex or excimer intermediates per se or as precursors for radical ions pairs, eq. 12,... 

Excimers may undergo processes other than emission. Nonradiative decay apparently occurs more easily from an excimer than from an excited monomer. Photodimerization may also occur.34 Anthracene, for example, gives 11 through an excimer intermediate. It may be that transient photodimerization is, in fact, responsible for all nonradiative decay of excimers.35... 

There is one report of a concentration dependence of the 313 nm photo-stationary state of azobenzene and 4-methoxyazobenzene in cyclohexane— not in benzene or CCI2F-CCIF2—in the literature.A bimolecular excimer intermediate was postulated. Further work is needed to elucidate whether the absorption coefficients or the quantum yields are concentration-dependent, for instance by ground or excited-state association (cf. Equation 1.3). 

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. ... 

There may well be systems in which the excimer minimum occurs in the S2 rather than the S, surface (Figure 7.27b). The approach to the pericyclic funnel P on S, may then be barrierless, and an excimer intermediate will... 

The intramolecular photocyclodimerization of anthracenes and aryl-substituted alkenes having silyl groups via excimer intermediates has been reported by several groups. In these photoreactions, the silyl groups either activate alkenyl or aryl groups or act as a spacer. In some cases, two alkenyl groups are temporarily connected by a silyl... 

Fig. 5 Diffusional intermolecular singlet fission, (a) Energy diagram of singlet fission via a [T-T] intermediate. In TIPS-pentacene (13) formation of two triplet excitons is exothermic whilst for TIPS-tetracene (14) this process is endothermic, reflected in the respective triplet yields for the two molecules (b). (c) Schematic depicting the collision of TIPS-pentacene in solution to form the excimer intermediate. Adapted from ref, 28,...

Better resolution of an excimer intermediate in fission was achieved in a more recent study of TIPS-tetracene solutions (Fig. 7). In this work, the diffusion-limited dynamics, endothermic energetics and unusually sharp triplet exciton absorption features enabled identification of a spectroscopically distinct intermediate state in transient absorption and photoluminescence. In contrast to TIPS-pentacene, TIPS-tetracene represents a typical tetracene system where singlet fission is endothermic by 200 meV. This endothermicity is well known to have a drastic effect on the rate of fission in the solid state where triplet formation occurs three orders of magnitude slower in films of tetracene than in pentacene. Recent work suggests that fission in tetracene may not require thermal activation. Notably, the decay of singlet excitons and the rise of triplet exciton absorption have been shown to occur independent of temperature. A low-lying, dark intermediate state in tetracene was invoked to explain these observations, however it had been difficult to isolate such a state experimentally. ... 

Transient absorption measurements of the TIPS-tetracene solutions revealed the presence of three distinct states in concentrated solutions. An intermediate was observed that displayed a lifetime identical to the emissive excimer from PL. Formation of the intermediate quenched the singlet exciton emission with a 70 ps lifetime and led to subsequent triplet exciton formation in a 120% yield. The free triplet exciton population rose over the first ten nanoseconds, consistent with thermal dissociation of the excimer intermediate over the endothermic energy barrier. 

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