Quantum yields, of photoproducts

The PFR also takes place with aryl alkyl carbonates [125]. Methoxy-substi-tuted dervatives can undergo substitution of MeO by the acyl moiety, as has been reported for esters and amides. This is shown in Scheme 47 for 2-methoxyphenyl ethyl carbonate (168) [126]. On the other hand, the overall quantum yield of photoproducts is 10-fold lower in the para-than in the ortho- or (meta- me-thoxyphenyl. Chlorophenyl ethyl carbonates do not rearrange, but undergo C—Cl homolysis. The efficency of photodechlorination follows the order para < meta < ortho [127],... 

When quenching of an excited state is detected by lower quantum yields of photoproducts that arise from that state rather than by lower quantum yields of emission, Equation 13.22 becomes... 

Quantum yields of photoproduction of ana-quinones of acetoxy-substituted anthraquinones with amino substituents in the anthraquinone ring as well as with an acetyl group proved to be the lowest (Table 7.2) As in the case of photochromic alkylanthraquinones, the reverse photoreaction, from ana-quinone to para-quinone for derivatives of acetoxyanthraquinone, proved to be impossible.21 The transition from ana-quinone to para-quinone occurred during freezing out of a sample owing to thermal bleaching. 

Aliphatic sulfides can be efficient co-initiators for the photoinduced polymerization induced by benzophenone [185, 186]. An exceptionally strong effect was observed for 2,4,6-trimethyl-1,3,5-trithiane (TMT). A model reaction for free-radical formation during photoreduction of an initiator triplet state by a sulfide is the photoreduction of benzophenone by dimethyl sulfide [171, 187-189]. In this process it was established that electron transfer from the sulfur atom to the triplet state of the benzophenone is a primary photochemical step. In this step, radical ions are formed. The overall quantum yields of photoproducts (ketyl radicals and radical anions) are low (Ed) 0.26) in aqueous solution, in the range 0.16-0.20 in mixed water-acetonitrile solution and less then 0.01 in pure acetonitrile. These results suggest that, in organic solvents, back electron transfer within the radical-ion pair to regenerate the reactants is the dominant process. 

Anthracenes linked by a much longer chain also give internal photodimers, and this is demonstrated by the formation (with reasonably high quantum yield) of photoproducts from bis(9-anthryl)polyoxa-alkanes (187) with linking chain lengths of up to 19 atoms. Kinetic parameters for this reaction have also been reported. An elaboration of this system to the cyclophane (188), which has one... 

The question of turn-over numbers of the photocatalyst in particulate systems was raised. Professor Bard commented that these were usually very good, even for difficult processes such as photoinitiated polymerization, which normally cause extensive electrode fouling. It is possible that microparticulate semiconducting photoelectrodes are self-cleansed of surface poisons by strongly oxidizing intermediates, such as hydroxyl radials, produced in the course of the photoelectrochemical process. However, the efficiency (i.e. the quantum yield of photoproduct) of such devices was still very low. Until this was improved, such devices could not be practical and there would be little incentive to improve specificity. 

Fig. 2 Effect of the laser intensity on the quantum yield of formation of oxidized bases (j)ox) and pyrimidine dimeric photoproducts (0dim)...

Quantum Yields (p) for Product Formation. The quantum yields for formation of the corresponding photoproducts b (arylamine), c (ortho photo-Fries), and d (para photo-Fries) upon photolysis of la-4a are given in Table II. In each case, for photolysis at 254 nm the sum (S>total) of the quantum yield for photoproducts (fy,... 

Energy transfer to photoreactive acceptors has also been widely utilized for excitation quantum yield determination (chemical titration), mainly in the decomposition of dioxetanes ° . The quantum yields are calculated from the photoproduct yield obtained at infinite energy acceptor concentrations ( = 1.0) by extrapolation of the double-reciprocal relationship between the photochemically active energy acceptor concentration and the photoproduct yield ( Lp ). H the quantum yield of the photochemical reaction (excitation quantum yield (< > ) can be calculated (equation 8) . ... 

Schmidt, S., R. N. Schindler, and T. Benter, Photodissociation Dynamics of CIO and CIOOCI Branching Ratios, Kinetic Energy and Quantum Yield of Primary Photoproducts, Presented at the XXIII Informal Conference on Photochemistry, May 10-15,... 

Flash photolysis with nanosecond laser excitation, monitoring absorption [112-114] and optoacoustic signals [82,90,115] of the transients, showed that in the primary reaction of P, two photoproducts are formed, presumably simultaneously in parallel pathways (see below and [136] for a caveat), with a total quantum yield of d>r 700 > 0.5 38% of I o decaying with a... 

Pr is converted to Pfr in (at least) two parallel pathways each comprising several consecutive steps. The primary photoreaction afTords in a quantum yield of >0.5 two products, I700 and I o, with microsecond lifetimes. These photoproducts form a photoequilibrium with Pr within the... 

Further studies on the same system by Sutin and coworkers158 implied some variation on the mechanisms proposed by Lehn.157 Sutin found that whilst the quantum yield of [Ru(bipy)3]3+ formation with [Rh(bipy)3]3+ as quencher was 0.15, the quantum yield of [Rh(bipy)3]2+ was 0.3. This was interpreted in terms of reactions (43) to (46), where the TEOA- radical reduces the Rh111 species rather than the Rh11 species. Furthermore, the fact that the quantum yield of the final Rh1 photoproduct is 0.13 suggests that it is formed via disproportionation of [Rh(bipy)3]2+... 

A kinetic scheme was proposed [122] with the fluorescent exciplex as precursor of the photoproducts (ortho as well as meta adducts). Quantum yields of adduct formation, exciplex emission, and benzene fluorescence were measured as a function of alkene concentration. The kinetic data fit the proposed reaction scheme. The authors have also attempted to prove the intermediacy of the exciplex in the photoaddition by adding a quencher to the system benzene + 2,2-dimethyl-... 

It should be noted here that thymine photodimerization may occur by a non-concerted mechanism, involving free radical intermediates. Indeed, photoproducts other than cis-syn dimer, such as the next most abundant thymine dimer, so-called 6 4 adduct, were observed in irradiated DNA. However, the quantum yield of cis-syn photodimer formation (r/j 0.02) is more than an order of magnitude higher than that of the 6 4 adduct ( 0.0013) which in turn is an order of magnitude higher than the quantum yields for other thymine isomers [68]. This specificity can lead to the conclusion that the thymine photodimerization occurs predominantly via concerted 2 + 2 cycloaddition mechanism. A time-resolved study of thymine dimer formation demonstrated that thymine cyclobutane dimers are formed on a timescale of less than 200 nsec, while the 6 4 adduct is formed on a timescale of few milliseconds [69]. The delay in the formation of the latter was attributed to the mechanism of its formation through a reactive intermediate. 

Quantum yields of product formation, Ov, can range from <10 6 to >106, depending on the reaction system a quantum yield >1 suggests the secondary reaction courses. A secondary step allows another reactant molecule to be consumed by the primary photoproduct and then Op could reach 2. The quantum yield >2 suggests a chain reaction mechanism. A good example of this is the photochemical synthesis of HC1, for which Op = 106, which means that absorption of one photon by a Cl2 molecule results in the production of a million molecules of HC1. 

The UV absorption spectra, in neutral aqueous medium, of pyrimidone-2 and its dimer reduction product, 6,6 -6u-(3,4-dihydropyrimidone-2), are exhibited in Fig. 2 Particularly interesting was the finding that irradiation of the dimer at 254 nm under these conditions led to the stepwise disappearance of its characteristic absorption spectrum, with the simultaneous appearance o the spectrum of the parent monomer. Additional evidence for the identity of the photoproduct with the parent pyrimidone-2 was furnished by chromatography and polarographic behaviour. The photochemical conversion reaction was shown to be quantitative (under these conditions pyrimidone-2 itself is quite radiation resistant), and to proceed with a quantum yield of 0.1, both in the presence and absence of oxygen 2). This value was unchanged when irradiation was conducted in 2H20 the absence of an isotope effect is clearly of relevance to the mechanism of the photodissociation reaction. 

Irradiation at 254 nm of an aqueous neutral solution of the reduction product of wave I led to stepwise appearance of a new band in the UV with 305 nm, with a quantum yield of 0.08 (Fig. 5), with on overall yield of 80%. Chromatography and polarographic analysis demonstrated that the photoproduct was the parent l-methyl-l,2-dihydro-2-iminopyrimidine... 

The carbene thus reacts with O2 to form an orffio-benzoquinone O-oxide, and with an aliphatic alcohol as H-donor to form a phenoxyl radical (plus an aliphatic radical not shown in Scheme 1). The ground state triplet electronic configuration of this carbene accounts for its reaction behavior, in particular for the fact that it reacts very slowly with the solvent, H2O. In agreement with the intrinsically faster intersystem crossing of 2-bromophenol compared to 2-chlorophenol, the quantum yield of the carbene pathway was higher for the former = 0.04) than for the latter compound (< = 0.003). In contrast, the quantum yields of photo contraction were comparable (< = 0.04). The transient absorption data were confirmed by photoproduct analysis, showing the formation of phenol from 4-bromophenol in the presence of H-donors [16]. 

A number of other, but minor primary photoproducts was also found, among them the products expected from a radical (photo-Claisen) rearrangement and from photohydrolysis of the ortho chlorine 2- and 4-chlorophenol were detected too, but their formation remained unexplained. The photodegradation quantum yield of dichlorprop did not depend on pH and was 50 times smaller than that of the anionic form of the related monohalo-genated compound mecoprop (see above) [77]. This is another example of the marked influence of the pattern of ring halogen substitution on the course and on the efficiency of photodegradation. 

Dicamba is a dichlorinated 2-methoxybenzoic acid derivative. It was found to be photodegraded in aqueous solution with a quantum yield of 0 = 0.0215 at A, = 275nm [99]. The photoproducts were the dihydroxylated derivative and an unexpected dicyclic compound, only formed in the presence of O2 (Scheme 10). 

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