Benzophenone, laser flash photolysis

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... 

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

Laser flash photolysis of benzophenone in pure SC CO2 at 33°C and 110 bar resulted in generation of triplet benzophenone within the laser excitation pulse (Figure... 

The TTA spectrum of benzophenone observed by laser flash photolysis indicates that strong TTA was completely quenched by the addition of MeTND. Energy transfer is indeed 100% efficient. 

The analysis of similar processes with benzophenone (1) and benzil (7) requires a higher time resolution of the experimental setup. Using ns-laser flash photolysis, we observed the formation of radical ion intermediates, depending on solvent polarity, added salts and competing H-abstraction [36]. Summarizing all these experiments, one can draw the following conclusions (cf. Figs. 3—5, see also Ref. [33]) ... 

Various compounds were shown to sensitize the photochemical decomposition of pyridinium salts. Photolysis of pyridinium salts in the presence of sensitizers such as anthracene, perylene and phenothiazine proceeds by an electron transfer from the excited state sensitizer to the pyridinium salt. Thus, a sensitizer radical cation and pyridinyl radical are formed as shown for the case of anthracene in Scheme 15. The latter rapidly decomposes to give pyridine and an ethoxy radical. Evidence for the proposed mechanism was obtained by observation of the absorption spectra of relevant radical cations upon laser flash photolysis of methylene chloride solutions containing sensitizers and pyridinium salt [64]. Moreover, estimates of the free energy change by the Rehm-Weller equation [65] give highly favorable values for anthracene, perylene, phenothiazine and thioxanthone sensitized systems, whilst benzophenone and acetophenone seemed not to be suitable sensitizers (Table 5). The failure of the polymerization experiments sensitized by benzophenone and acetophenone in the absence of a hydrogen donor is consistent with the proposed electron transfer mechanism. 

In a similar way, anthracene triplet (4>,gj3=0.71, z =6A,700Mr cmr ) and the naphthalene triplet (4>jg = 0.75, e j = 24,500 M" cm" ) in cyclohexane solution have been introduced as transient chemical actinometers for the third-harmonic (355 run) and fourth-harmonic (266 nm) output of Nd YAG lasers, respectively (44). In summary, transient chemical actinometers are ideal for accurately measuring the energy of single laser pulses, provided the quantum yields and extinction coefficients of the transients are well known (45 7). Thus, the well-established benzophenone actinometer (42-44) has been used as a reliable reference to calibrate the azobenzene actinometer (see section "Laser Intensity Measurements with the Azobenzene Actinometer" Doherty S, Hubig SM, unpublished results) and the Aberchrome 540 actinometer (48,49) for intensity measurements with pulsed Nd YAG and/or XeCl excimer lasers. However, such actinometer can only be used when a complete set of laser flash photolysis equipment including a kinetic spectrometer is available. 

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... 

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]. 

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. 

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... 

Enolization of triplet 5,8-dimethyl-1-tetralone by hydrogen transfer effects" ", spin-lattice relaxation of pentacen-6,13-quinone triplets in a benzoic acid host crystal involves proton tunnelling", and photoreduction of benzophenone and thioxanthen-9-one by amines" are other researches of photochemical significance. Interactions of formate ions with triplet states of benzophenone-4-carboxylate and -4-sulphonate, 1,4-naphthoquinone, and anthraquinone-2-sulphonate have been examined systematically by laser flash photolysis". ... 

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 data show that the dynamics of this reaction involve hydrogen bonding effects . Fs-ps laser photolysis has been deployed to follow the details of the photoreduction of excited benzophenone by N,N-dimethylaniline in acetonitrile solution . Laser flash photolysis also has been used to examine the chromophore assisted peroxy-bond breakage in the case of a benzophenone perester . ... 

---