Intramolecular photosensitization

Cyclopentenone 56 having a propyl-like substituent undergoes photosensitized intramolecular hydrogen abstraction to give rearrangement product 57 and cycloadduct 58 [30]. 

Kndr, G. (2001) Intramolecular charge transfer excitation of meso-tetrakis (1-pyrenyl) porphyrinatogold(lll) acetate. Photosensitized oxidation of guanine. Inorganic Chemistry Communications,... 

The 977-pyrido[3,4- ]pyrrolizin-9-one 145 has been prepared for its photochemical properties. The preparation involves an intramolecular Friedel-Crafts acylation of the acid chloride formed from 3-(l-pyrrolyl)pyridine-4-carboxy-late (Scheme 40). The product is a photosensitizer, which absorbs visible light its absorption spectra are pH, solvent, and concentration dependent <1994SAA57>. 

The photosensitized electron transfer by 9,10-dicyanoanthracene (DCA) has been shown to initiate the addition of the a-silyl amine 44 to 4,4-dimethylcyclohexenone47 (equation 14). Intramolecular addition of a-silyl amine 45 was also shown to be feasible45,46 (equation 15). The primary step is electron transfer to give the aminium... 

In general, photosensitive protecting groups are removed by photosolvolysis, making use of the different reactivities of the excited and ground states, or by intramolecular, photoactivation reactions. 

In contrast to the photo physical processes just described, photochemical processes produce new chemical species. Such processes can be characterized by the type of chemistry induced by light absorption photodissociation, intramolecular rearrangements, photoisomerization, photodimerization, hydrogen atom abstraction, and photosensitized reactions. 

Fig. 34 Photosensitized singlet oxygen production 1/ r is the general (radiative and non-radiative) rate constant of the transition Si So fcsi is the rate constant of singlet-triplet conversion tt is the lifetime of the triplet, T1, electronic state of PS kj is the second-order rate constant of singlet oxygen quenching of the Ti state of PS tl and nr are the radiative lifetime and rate constant of all intramolecular nonradiative energy relaxation processes of O2 ( Ag)...

It is possible that some of these photochemical cycloadditions take place by a [,2S + 2S] mechanism (which is of course allowed by orbital symmetry) when and if they do, one of the molecules must be in the excited singlet state (St) and the other in the ground state.980 The nonphotosensitized dimerizations of cis- and trans-2-butene are stereospecific,981 making it likely that the [ 2S + 2S] mechanism is operating in these reactions. However, in most cases it is a triplet excited state that reacts with the ground-state molecule in these cases the diradical (or in certain cases, the diionic) mechanism is taking place. In one intramolecular case, the intermediate diradical has been trapped.982 Photosensitized 2ir + 2-rr cycloadditions almost always involve the triplet state and hence a diradical (or diionic) mechanism. 

The direct irradiation of 1,2-diazocines, 40 [R = H, Me (isom. mix., Ph-(frans)] produced disproportionation (102) and coupled (103) products. The ratio of 102/103 increased with increasing amounts of nonketonic photosensitizer, indicating the intermediacy of a relatively long-lived 1,6-diradical in which more time is allowed for intramolecular hydrogen transfer (70JA4922). 

Pandey, G., Karthikeyan, M., and Mumgan, A. (1998) New intramolecular a-arylation strategy of ketones by the reaction of silyl enol ethers to photosensitized electron transfer generated arene radical cations construction of benzannulated and benzospiroannulated compounds. Journal of Organic Chemistry, 63, 2867-2872. 

The involvement of styryl as a diene partner has also been demonstrated in the photosensitized electron transfer [4 + 2] intramolecular cycloaddition of l,l,8,8-tetraphenyl-l-7-octadiene (Scheme 9.29) [43]. This reaction is promoted by 1,4-dicyanobencene (DCB), and a single electron-transfer mechanism has been suggested. 

Abstract. Recent synthetic, spectroscopic and photopysical work related to the problem of light energy conversion is reported. In particular, new isocyano Ru(II) bipyridine photosensitizers are described, and a number of polynuclear complexes are discussed in which intramolecular transport of electronic charge and excitation energy can be studied. 

In this report, we will describe some of our studies aimed at (i) obtaining new inorganic photosensitizers by second-sphere modification of known ones, and (ii) assembling photosensitizer units with other molecular components in discrete, covalently bound supramolecular structures. Studies of type (i), besides their intrinsic interest, have some relevance to the problem of how the properties of a photosensitizer are modified by inclusion in a supramolecular structure. Systems of type (ii) would be useful to study the basic processes of intramolecular electron and energy transfer involved in the performance of molecular photochemical devices. 

Based on their easily tunable photophysical and redox properties, transition metal complexes are versatile components to be used in the construction of photochemical molecular devices. The studies presented in this article show that the combination of the Ru(bpy)22+ photosensitizer and cyanide bridging units allows the synthesis of a variety of polynuclear systems that exhibit interesting photochemical properties. Depending on the nature of the attached metal-containing units, supramolecular systems can be obtained that undergo efficient photoinduced intramolecular energy or electron transfer processes. 

A photosensitized activation of carbon-selenium bonds was also used for performing phenylseleno group transfer reactions. This process involves a photosensitized electron transfer (PET) as the initial step in the reaction sequence. Fragmentation affords a radical and phenylselenolate, which is oxidized to diphenyl diselenide in the presence of oxygen. The cyclized radical is then trapped by diphenyl diselenide to afford the final product. This process is quite general for intramolecular radical reactions.70,266... 

In the solid state, the intramolecular electron transfer to a naked dication may partly take place from the proximal hexacyanoferrate(II) anions. The extent of electron transfer at the ground state becomes large with an increase in the half-redox potential. The low photosensitivity for Bzl-V2+ in the solid state appears to be caused by the fact that the electron transfer already takes place in the ground state and/or the bulky hexaferrate (II) ion is barely brought close to the positive pyridinium nitrogens attached to the benzyl group possessing a relatively bulky molecular size. 

In relation to this, it is interesting to refer to the intramolecularly photosensitized diastereodifferentiating isomerization of 3-, 4-, and 5-benzoyloxycyclooctet-enes 66Z-68Z (Scheme 10) [51]. In particular, the de of photoproduct 66E is a critical function of substrate concentration (Fig. 6), leading to a switching of the diastereoselectivity at 10 mM. This apparently unusual phenomenon is reasonably... 

A PET in intramolecular CPs between pyridinium ions and bromide, chloride or thiocyanate ions for polymerization initiation is described, too [137-139]. As expected, an equilibrium exists among free ions, ion pairs, and CT, which is shifted to the free ions in polar solvents and to the complex in a less polar solvent That complex serves as the photosensitive species for the polymerization (see Scheme 10). The photodecomposition of the CT yields radicals of the former anion and N-alkylpyridinyl radicals. Probably, the photopolymerization is initiated only by X- radicals, whereas latter radicals terminate the chain reaction. By addition of tetrachloromethane, the polymerization rate is increased owing to an electron transfer between the nucleophilic pyridinyl radical and CC14 (indirect PET). As a result, the terminating radicals are scavenged and electrophilic -CQ3 radicals are produced. 

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