Laser flash photolysis studies benzophenone

Laser Flash Photolysis Studies of Benzophenone in Supercritical C02... 

Photoreduction of benzophenone by primary and secondary amines leads to the formation of benzpinacol and imines [145]. Quantum yields greater than unity for reduction of benzophenone indicated that the a-aminoalkyl radical could further reduce the ground state of benzophenone. Bhattacharyya and Das confirmed this in a laser-flash photolysis study of the benzophenone-triethylamine system, which showed that ketyl radical anion formation occurs by a fast and a slow process wherein the slow process corresponds to the reaction of a-aminoalkyl radical in the ground state of benzophenone [148]. Direct evidence for similar secondary reduction of benzil [149] and naphthalimides [150] by the a-aminoalkyl radical have also been reported. The secondary dark reaction of a-aminoalkyl radicals in photo-induced electron-transfer reactions with a variety of quinones, dyes, and metal complexes has been studied by Whitten and coworkers [151]. 

Direct evidence for the formation of radical ion intermediates in the benzophenone-amine system was also obtained by Peters et al. [153-156]. Picosecond laser-flash photolysis studies have indicated the formation of ketyl radical anions concomitant with the decay of the benzophenone triplet. For 1.0 M dime-thylaniline and diethylaniline the rate of electron transfer to the benzophenone triplet was 3.6 x 10 and 4.2 x 10 m s , respectively. On the basis of their studies Peters et al. proposed a mechanism in which a solvent-separated ion pair... 

Polymerization of butane-1,4-diol dimethacrylate, sensitized by benzophenone in the presence of three different sulfides, has been described by Andrzejewska et al. [190]. The measurements show that in the absence and in the presence of propyl sulfide and 2,2 -thiobisethanol no polymer was formed. This can be explained by the effective back electron transfer process that occurs in the radical-ion pair in organic solvents. Effective polymerization was observed only in the presence of TMT. Laser flash photolysis studies performed for the benzophenone-TMT pair allow one to construct a scheme (Scheme 23) explaining characteristic features of the mechanism of polymerization initiated by the system. The results prompted the authors to study other symmetrically substituted 1,3,5-trithianes as electron donors for benzophenone-sensitized free-radical polymerization (Figure 38 Table 12) [191]. 

The mechanism of the quenching of triplet benzophenenone by both electron and hydrogen donors involves change transfer effects". Diffuse reflectance laser flash photolysis studies of the reactions of triplet benzophenone with hydrogen atom donors on surfaces have also been reported". A dual phosphorescence of benzophenone at 77K in HjO/EtOH in glasses indicates that in this environment a... 

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. 

Cao, H., Akimoto, Y, Fujiwara, Y., Xanimoto, Y., Zhang, L.P., and Xung, C.H., A laser flash photolysis study of the intramolecular energy transfer reaction from benzophenone to norbornadiene covalently bonded by a rigid steroid bridge. Bull Chem. Soc. Jpn., 68, 3411-3415, 1995. 

The most important common features in the photodecarboxylation of ketoprofen, suprofen, tiaprofenic acid, tohnetin, and benoxaprofen in neutral aqueous solutions are the following (1) All of them produce carbanion intermediates, which upon protonation give rise to the decarboxylated photoproducts (Scheme 4), and (2) the photodecarboxylation quantum yields are much higher from the carboxylate forms than from the non-dissociated acids. However, there are also some differences. Thus, while photodecarboxylation of benoxaprofen is produced from its singlet excited state, in the case of the benzophenone derivatives (ketoprofen, suprofen, tiaprofenic acid, tohnetin), the process seems to take place from the triplet excited states.Clear evidence for the participation of these states has been provided by laser flash photolysis studies. Thus, picosecond laser excitation of ketoprofen in aqueous... 

Several researchers have investigated the photoenolization of various o-methyl acetophenone and o-methyl benzophenone derivatives. The mechanism of photoenolization of o-methyl benzophenone, 1, has been studied with laser flash photolysis and can be described as follows (Scheme 2) Irradiating 1 forms its first... 

Picosecond and nanosecond transient spectroscopic studies to elucidate the nature of various intermediates formed in the photoinduced electron-transfer reaction of amines have been most extensive for the ketone-amine systems. These studies have been described in detail in a recent review by Yoon et al. [10]. Some aspects of these studies are briefly described here. In the earlier studies of Cohen and coworkers, the photochemical reactions of benzophenone with aliphatic amines were probed by fluorescence quenching, determination of product quantum yields, and nanosecond laser-flash photolysis [143-147]. They proposed that the reactions of amines with... 

Photoreactions between triplet benzophenone 104 and l-A,A-dialkylaminonaphthalene (105) are studied by means of the laser flash photolysis technique in CH3CN (at X = 355 nm) in the presence and in the absence of MeOH or H2O. The produced triplet exciplexes 3 (105- 104) show electron transfer phenomena which occur due to an increase in the electron affinity of 104 in 3(105 104) by proton donor molecules, such as methanol or water172. 

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. 

The use of supercritical fluids as reaction media for organometallic species is also investigated. Reactions include photochemical replacement of carbon monoxide with N2 and H2 in metal carbonyls, where the reaction medium is supercritical xenon. Also, photochemical activation of C-H bonds by organometallic complexes in supercritical carbon dioxide is investigated. More recent studies on photochemical reactions also include laser flash photolysis of metal carbonyls in supercritical carbon dioxide and ethane and laser flash photolysis of hydrogen abstraction reaction of triplet benzophenone in supercritical ethane and... 

Mataga et al. (18) studied energy transfer from the excited triplet of benzophenone to naphthalene by laser flash photolysis at 77K and showed that the non-exponential decay curves of the benzophenone triplet obey the Inokuti-Hirayama equation (eq (ID). Inokuti and Hirayama (17) themselves compared the data on triplet-triplet transfer between certain aromatic molecules obtained by Terenin and Ermolaev with eq (11) and reported that a good fit was found with an appropriate choice of the parameters C and 7. 

Other triplet state characterization studies involve phenazine phosphorescence in alkane solvents in the glass transition range, establishment of the cis-keto form of the triplet 2-(2 -hydroxyphenyl)benzothiazole in nonpolar glass , 1,5-diaminoanthraquinone and l-acetylaminoanthraquinone have been studied by laser flash photolysis in solution. The formation of the triplet state of oxazine and the consequent delayed fluorescence in acetonitrile has been sensitized by benzophenone. The intersystem crossing efficiency of molecules with small S,-T,... 

The role of the isomers of 2,4,6-trimethyl-l,3,5-trithiane as co-initiators in benzophenone-induced photopolymerizations was pursued by studying the primary photochemical reactions by nanosecond laser-flash photolysis <2001JPH133> it was found that the polymerization efficiency of the co-initiator did not follow the efficiency of photo induced formation of the initiating radicals however, detailed mechanisms of the various stages of polymerization were proposed <1998JPH21, 1999MM2173, 2001JPH133>. 

CF,)jC,H4, 0-, m-, and p-FC,H4Me, o- and m-FC,HiCF PhF, - CeFe, and C,F6H has been studied, and trends in the radiative and non-radiative decay rates of the excited molecules are discussed. - c/s-But-2-ene is effective in increasing the fluorescent and triplet yields of pentafluorobenzene. The decay rate of triplet benzophenone, produced by laser flash photolysis, is ca. ten times faster in hexafluorobenzene than in benzene, and reversible addition of the triplet species to the aromatic ring may play an important role. The phosphorescence spectrum of decafluoro-benzophenone has been studied, and the triplet state of this ketone (see also p. 380) in inert solvents e.g. perfluoromethylcyclohexane) has a lifetime of 20 ps. U.v. irradiation of decafluorobenzophenone in isopropyl alcohol... 

A laser study of the prototropic equilibrium of triplet benzophenone (BP) has been reported [111]. Acid-base properties in the triplet state of aromatic ketones in HjO-CHjCN (4 1) mixtures have been studied by means of nanosecond laser flash photolysis. The acidity constants p /T) in the triplet state are determined by means of the T T, absorbance titration curve, the Ware plot, and the Rayner-Wyatt plot, whose values agree well among them, showing that the acid-base equilibrium in the... 

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