Photoinduced processes molecular rearrangements

Electron, energy and proton transfer or molecular rearrangements are the most important events that occur in interfacial supramolecular assemblies. In this chapter, the general theories of electron transfer, both within ISAs and across the film/electrode interface, are described. Moreover, photoinduced electron, energy and proton transfer processes are discussed. As this book focuses on supramolecular species, the treatment is restricted to intramolecular or interfacial processes without the requirement for prior diffusion of reactants. 

In terms of photophysics, electron transfer reactions create an additional non-radiative pathway, so reducing the observed emission lifetimes and quantum yields in A-L-B dyads in comparison with a model compound. However, there are other processes, such as molecular rearrangements, proton transfer and heavy-atom effects, which may decrease the radiative ability of a compound. One of the most important experimental methods for studying photoinduced processes is emission spectroscopy. Emission is relatively easy to detect and emission intensities and lifetimes are sensitive to competing processes. Studying parameters such as emission quantum yields and lifetimes for a given supramolecular species and associated... 

The strategy devised in order to obtain the photoinduced shuttling movement of R between the two stations Ai + and A2 + is based on a four stroke synchronized sequence of electron transfer and molecular rearrangement processes, as illustrated in Figure 27(b).Light excitation of the photoactive unit P + (process 1) is followed by the transfer of an electron from this unit to Ai + (process 2) which competes with the intrinsic decay of the P + excited state (process 3). After the reduction of Ai +, with the consequent deactivation of this station, the ring moves (process 4) by 1.3 nm to encircle A2 +, a step that is in competition with the back electron transfer from Ai+ (still encircled by R) to the oxidized unit P + (process 5). Eventually, a back electron transfer from the free reduced station Ai + to the oxidized unit P + (process 6) restores the electron-acceptor power to this radical cationic station. As... 

Several reviews have been published within the year which are of general relevance to the photoreactions of aromatic compounds. The subjects of these reviews include photochemistry in ionic liquids and in isotropic and anisotropic media, organic synthesis utilizing photoinduced electron-transfer reactions," heteroatom-directed photoarylation processes, photochromism, and photochemical molecular devices. Reviews more directly pertinent to the sections in the present chapter include those of the photoisomerization of five-membered heteroaromatic azoles, the photocycloaddition of benzene derivatives to alkenes, Diels-Alder additions of anthracenes, advances in the synthesis of polycyclic aromatic compounds, diarylethene-based photochromic switches, the photo-Fries rearrangement, and the application of Diels-Alder trapping of photogenerated o-xylenols to the synthesis of novel compounds. " A number of chapters in the two recently published handbooks of photochemistry and photobiology and in the revised edition of the text on photochromism are also pertinent to the current subject matter. 

The three methylenecyclobutanes, 51,52, and 53, do not interconvert under direct irradiation or SET-photosensitized conditions. However, 3,3-diaryl-4,4-dideuterio-l-methylenecyclobutane (d -Sd) does undergo electron-transfer photoinduced degenerate methylene-cy-clobutane rearrangement when either DCN, DCA, or 2,6,9,10-tetracy-anoanthracene is used as sensitizer. These processes involve the allylically stabilized 1,4-cation radical intermediates dj-SS and d -58/-+28 tjjg 1 4-cation radical 58 is efficiently captured by molecular oxygen, giving rise to 73 as shown in Scheme 15. 

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