Exciplex formation photochemistry

All of the photochemical cycloaddition reactions of the stilbenes are presumed to occur via excited state ir-ir type complexes (excimers, exciplexes, or excited charge-transfer complexes). Both the ground state and excited state complexes of t-1 are more stable than expected on the basis of redox potentials and singlet energy. Exciplex formation helps overcome the entropic problems associated with a bimolecular cycloaddition process and predetermines the adduct stereochemistry. Formation of an excited state complex is a necessary, but not a sufficient condition for cycloaddition. In fact, increased exciplex stability can result in decreased quantum yields for cycloaddition, due to an increased barrier for covalent bond formation (Fig. 2). The cycloaddition reactions of t-1 proceed with complete retention of stilbene and alkene photochemistry, indicative of either a concerted or short-lived singlet biradical mechanism. The observation of acyclic adduct formation in the reactions of It with nonconjugated dienes supports the biradical mechanism. 

The photochemistry of tethered alkenes is more predictable than the nontethered situation. There is evidence of Ti-stacking for closely held moieties which presumably improves the orbital interactions between the alkenes. Exciplex formation is likely involved when the reacting groups are within 5 angstroms. Cornil et al. have shown that k systems can couple within this distance [11]. This exciplex could lead to a concerted cycloaddition from the excited state which would be consistent with the observed products. Although stepwise addition (see Sch. 3) cannot be ruled out even in these tethered singlet reactions. Ring closure must be very rapid if diradicals are involved, since no radical-trapped species have been found. 

Although the phenomenon is more common in organic photochemistry, a coordination entity can also act in the process of excimer or exciplex formation as an excited molecule AB or quencher (Q).The second-sphere donor-acceptor interaction with an acceptor quencher causes oxidative quenching of AB, whereas interaction with a donor quencher yields reductive quenching. 

As mentioned earlier in the discussion of exciplex formation, electron transfer between an excited state species and a ground state molecule (Equation 2.8 and Equation 2.9) is frequently observed in the photochemistry of systems containing an electron donor-acceptor combination. As a result, a pair of radical ions is formed that react with oxygen but with different rates. The reaction of ground state oxygen with radical anions occurs rapidly and yields superoxide anion (Equation 2.16). The superoxide then adds to the radical cation forming D02 (Equation 2.17). When D is an olefin, D02 is a dioxetan that is liable to cleave to yield ketones as products. 

Enhancement of PCB photodegradation also is observed in the presence of t r i f luoroace t i c acid (40), presumably via the protonated intermediates (ArClH ). Dienes, such as 1,3-cyclohexadiene, accelerate haloaromatic photoreactions (41.) and modify chloronaphthalene photochemistry by enhancing reductive dechlorination and suppressing dimerization (42). The mechanism is believed to involve exciplex formation with the diene or protonation by the olefin (43). 

Photochemistry, photophysics Solvent-solute interactions, dynamics of micelles, polymer structure and dynamics, characterization of excited states, excimer and exciplex formation Capability to provide information on rapid phenomena... 

The photochemistry of this class of complexes has also been studied extensively. For example, the high-energy XLCT emission of 4a in benzene has been found to be quenched by pyridine [47], The bimolecular quenching rate constant was estimated to be 5.9 0.5 X 10 dm- mol" s". The mechanism for the quenching has been ascribed to the exciplex formation between the excited complex and the quencher. 

In this context, it needs to be remembered that gas-phase spectroscopy constitutes a reductionist approach to the study of basic chemistry. Extrapolation to bulk conditions requires consideration of hydrogen bonding and stacking interactions. These interactions modify the photochemistry. For example, it stacking can lead to exciplex formation, which opens up additional deexcitation channels [107]. 

Morley, K. and Pincock, J. A., The photochemistry of 2-(l-naphthyl)ethyl benzoates cycloaddition and intramolecular exciplex formation, /. Org. Chem., 66, 2995-3003, 2001. 

The SET between amine and acceptor may be enhanced by photoexcitation and may lead to the formation of exciplexes2 or molecular complex with charge transfer character3. The photochemistry between aromatic acceptors and amines via the exciplexes has been discussed earlier (Scheme l)4. 

The present article reviews the photochemical deactivation modes and properties of electronically excited metallotetrapyrroles. Of the wide variety of complexes possessing a tetrapyrrole ligand and their highly structured systems, the subject of this survey is mainly synthetic complexes of porphyrins, chlorins, corrins, phthalocyanines, and naphthalocyanines. All known types of photochemical reactions of excited metallotetrapyrroles are classified. As criteria for the classification, both the nature of the primary photochemical step and the net overall chemical change, are taken. Each of the classes is exemplified by several recent results, and discussed. The data on exciplex and excimer formation processes involving excited metallotetrapyrroles are included. Various branches of practical utilization of the photochemical and photophysical properties of tetrapyrrole complexes are shown. Motives for further development and perspectives in photochemistry of metallotetrapyrroles are evaluated. 

Photocycloaddition and photoaddition can be utilized for new carbon-carbon and carbon-heteroatom bond formation under mild conditions from synthetic viewpoints. In last three decades, a large number of these photoreactions between electron-donating and electron-accepting molecules have been appeared and discussed in the literature, reviews, and books [1-10]. In these photoreactions, a variety of reactive intermediates such as excimers, exciplexes, triplexes, radical ion pairs, and free-radical ions have been postulated and some of them have been detected as transient species to understand the reaction mechanism. Most of reactive species in solution have been already characterized by laser flash photolysis techniques, but still the prediction for the photochemical process is hard to visualize. In preparative organic photochemistry, the dilemma that the transient species including emission are hardly observed in the reaction system giving high chemical yields remains in most cases [11,12]. 

The importance of bioexcimers (bioexciplexes) in the photochemistry of biological compounds has been also emphasized. Computation of potential energy curves modeling the complex pheophytin-quinone shows the relevance that stabilization caused by the formation of rr-stacked excited dimers, that is, excimers (exciplexes) and the corresponding presence of conical intersections, have to provide... 

The photochemistry of p-diketones is dramatically altered if they are converted to their boron difluoride complexes (58). The reduction potential of the complex is lowered from that of the diketone or its enol so that their excited states can act as electron transfer sensitisers of alkene photochemistry or will form exciplexes with benzene derivatives, leading to the formation of products which are apparently produced by ortho addition to the arene... 

We ve seen that bimolecular processes involving excited states can take many forms. Collision can facilitate relaxation to the ground state (quenching) or formation of an excited state complex (exciplex or excimer). Alternatively, bimolecular association can occur prior to excitation, leading to an absorption complex. In this and the next two sections we consider a new outcome for the interactions of an excited state, D, with another molecule, A (Eq. 16.8). Now the result is energy transfer from one molecule to another, producing electronically excited A (A ). Different mechanisms are possible, and these energy transfer processes are very important in photochemistry and other fields. 

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