Benzophenone derivatives , excited-state

Photodecomposition of A -l,2,3-triazolines gives aziridines. In cyclohexane the cis derivative (304) gives the cis product (305), whereas photolysis in benzene in the presence of benzophenone as sensitizer gives the same ratio of cis- and trans-aziridines from both triazolines and is accounted for in terms of a triplet excited state (70AHC(ll)i). A -Tetrazo-lines are photolyzed to diaziridines. 

Porter, G., and P. Suppan Primary Photochemical Processes in Aromatic Molecules. Part 12. Excited States of Benzophenone Derivatives. Trans. Faraday Soc. 61, 1664 (1965). 

A different explanation was identified in the quenching properties of these heterocycles. Thiophene and mono methyl derivatives are efficient quenchers of triplet benzophenone. The Stem-Volmer plot showed a linear relationship [104, 105]. On the contrary, 2,5-dimethylthiophene (a compound able to give the cycloaddition reaction) is not a good quencher of benzophene [106]. N-Benzoylpyrrole also does not act as a quencher of the triplet benzophenone [106]. On the contrary, pyrrole and selenophene are quenchers of the excited benzophenone [106]. In this case, the Stern-Volmer plot is not linear. This situation is commonly encountered when the quencher employed quenches two excited states. It seems reasonable that pyrrole acts as quencher of both triplet benzophenone and the exciplex between triplet benzophenone and pyrrole [106]. 

The application of UV absorbers, i.e. compounds absorbing the harmful solar radiation, represents an effective solution of the problem (Rabek, 1990). The absorbed radiation is deactivated by intramolecular radiative and radiationless processes. The ideal UV absorber is expected to absorb all terrestrial UV-A and UV-B radiation but no radiation having wavelengths higher than 400 nm. Different classes of commercialized UV absorbers fulfil requirements on effective plastics protection. A group of UV absorbers acting by excited state intramolecular proton transfer (ESIPT) mechanism (Pospfsil and Nespurek, 1997) includes phenolic derivatives of benzophenone (37), various benzotriazoles, such as 38 or 39, and 1,3,5-triazine 40. Non-phenolic UV absorbers are represented by oxamide 41 and a-cyanoacrylate 42. 

Several molecules which can undergo intramolecular proton transfer in the excited state have been found to be unusually photostable. Thus, for example, 2-hydroxybenzophenone is used as a photostabilizer in polymers while benzophenone itself is photoactive (Kysel, 1969). In crystalline 2-(2 -hydroxyphenyl)benzothiazole and its derivatives [3] a proton is transferred in the excited state from an... 

The malic acid derivative 12 reacted with benzaldehyde to yield the oxetanes 13a,b with a diastereomeric excess of 80% (Scheme 4) [10]. It should also be mentioned that the regioselectivity and the exo/endo selectivity are complete. The favored formation of 13a is explained by the dominant conformation depicted in A, B. The syn approach of benzaldehyde excited in the 3n,7r state with respect to the alkoxy substituent (transition state A) is hindered by electrostatic repulsion between the substituent and the carbonyl group having a reversed polarity in the excited state. The addition of benzophenone to the furan derivative 14 was stereospecific [11]. In this case, however, the attack of the 3n,TT excited ketone occurred in a syn manner with respect to the hydroxy function to yield 15. The conformation indicated in the transition state C was supported by calculations. 

The H-bond strength of the third group—nonlumlnescent molecules—is similar to that of the salicylic derivatives, and no ground state tautomers are observed. Tremendous pK changes are necessary in order to facilitate proton transfer in the excited state. The deactivation path is still a matter of speculation. It is worth noting that a substance of the fluorescent class (phenyl salicylate, Table 2) has been the first ultraviolet stabilizer (9) used on a technical scale, despite its poor absorption intensity (screening effect). This substance is photo-chemlcally converted into 2,2 -dlhydroxy benzophenone (87) in a photo-Fries reaction. This can, however, not explain the total efficiency of this stabilizer (93). 

The nanosecond-picosecond two-color two-laser flash photolysis method is also useful to study the excited state of radicals, that is, the D, state. We applied nanosecond-picosecond two-color two-laser flash photolysis to detect the absorption and fluorescence spectra of the ketyl radical of benzophenone and its derivatives (BPH and BPDH ) in the excited state in the UV-visible region [111], since BPH and BPDH are well investigated radicals in various fields. Since BPH and BPDH are generated from irreversible ways, such as photoionization, we employed a streak camera to realize the single-shot detection of these intermediates. 

Interest continues in the photochromic behaviour of the spiropyran and related systems both in the patent and the basic research literature. Nanosecond laser kinetic spectroscopy has been used to investigate the intermediates involved in the interconversions between the colourless spiropyrans (35) and (36) and the coloured merocyanines present in solution at room temperature as the -isomers (37) and (38). On excitation (530 nm) of nitro derivative (37) in acetone, two transients were observed, the triplet excited state (lifetime 6ps) and the Z-isomer (lifetime 0.3ms). The same transients were observed on excitation (353 nm) of the spiropyran form (35). Naphthalene or benzophenone sensitisation of the photo-... 

The photoaddition of aldehydes or ketones to furan has been reported over the years. Griesbeck and his co-workers have established that the diaster-eoselectivity of the addition of benzaldehyde to furan is 212 1 for the formation of the exo and endo products (98) and (99). The study was extended to the addition of other carbonyl compounds (100) and the ratio of products from these additions is shown under the appropriate structure (101) and (102). L-Ascorbic acid and some of its derivatives (103) also undergo photochemical addition of aromatic aldehydes and ketones. With benzaldehyde and benzophenone the products obtained are the mixture of (104) and (105) with a preference for the formation of the former. The stereochemistry of the addition of the excited state carbonyl compound to ascorbic acid favours the path where the phenyl and the alkoxy groups are cis on the resultant oxetane. Benzaldehyde adds photochemically to the enamine (106) when the mixture is irradiated in acetonitrile solution. Three products are... 

Chandrasekaran and Whitten362) have recently described the operation of a PEC for H2 based on the reductive quenching of the excited state of a Ru(bpy)3+-derivative and benzophenone. 

Hydrogen abstraction from lipids by triplet states of benzophenone derivatives followed directly by the use of laser flash techniques allows the separation of physical quenching processes from chemical reactions of the excited state. 98 xhe production of 2 by the photochemical decomposition of aromatic endoperoxides has also been studied by ps kinetic procedures. 99 mechanistic study has also been reported on the phototransformation of 3-nitrophenol in aqueous solution. 99 There is a strong wavelength dependence of the low quantum yield for the phototransformation in this system. 

A combination of benzophenone and 1,3-dioxane is a convenient hydrogen abstraction-type photoinitiator system for the free radical photopolymerization of methyl methacrylate, styrene and other monomers. As an advantage, this system does not require an additional hydrogen donor as other conventional initiators. In a similar way, mixtures of thioxanthone derivatives and fluorenes can be used as visible light absorbing oil- and water-soluble photoinitiators for free radical polymerization of methyl methacrylate, ethyl 2-(2-phosphonoethoxymethyl)acrylate and trimethylolpropane triacrylate. Photopolymerization and laser flash photolysis studies reveal that initiation occurs by intra- and intermolecular hydrogen abstraction by the thioxanthone-like triplet excited state. 

The Study of the mechanism of the reactions indicated that for aromatic carbonyl compounds, the reaction occurs through a triplet excited state of the carbonyl compound, whereas for aliphatic carbonyl compounds through both singlet and triplet excited states of the carbonyl compound. The reaction is stereospecific for aliphatic carbonyl compounds and gives syn adduct. For cyclic alkenes, kinetically controlled endo-isomer is the major product. The regioselectivity of this cycloaddition reaction depends on the stability and steric interactions of the intermediate diradical. In the reaction of benzophenone with isobutene, the major product is derived from the stable diradical. 

Hydrogen abstraction by benzophenone or xantone derivatives is a classical problem that continues to attract research interest in this field. The substituent effects on the kinetics of benzophenone photoreduction by added isopropanol have been examined in acetonitrile. In agreement with expectations, both experimental results and DFT theoretical calculations are consistent with the involvement of n- i excited states. 

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