Photoreduction quantum yields

TABLE 12 Photoreduction Quantum Yields for Derivatives in Figure 12 in Aerated Ethyl Acetate"... 

The photoreduction quantum yields and the triplet yields of 4-aminobenzophenone in several hydrogen donor solvents, ZH, are listed in Table 4.1, with comparison to a benzophenone which has an n-7T lowest triplet state in all these cases. Although dimethylformamide (DMF) is a far better... 

Free and polymer-bound 2-benzyloxy-thioxanthone exhibit similar flash photolysis behaviour and the same photoreduction quantum yield in the presence of 2-(A, AT-diethylamino) ethanol. This clearly shows that the polymeric nature does not appear to affect photophysical properties of the thioxanthone moiety. The photoinitiated polymerization of MMA in benzene solution, using BOTX and poly(StX-co-St) in combination with 2-(MA -dieffiylamino) ethanol, indicates that the polymer-bound chromophore seems to operate in the same way and with similar efficiency as the free photoinitiator, at least in conditions of dilute chromophore concentration. 

Quantitative photophysical, photochemical and photopolymerisation data are presented on five novel water soluble benzophenone photoinitiators. Phosphorescence quantum yields, triplet lifetimes and transient formation on conventional flash photolysis correlate with the ability of the initiators to photoinduce the polymerisation of 2-hydroxyethylmethacrylate and a commercial monoacrylate resin in aqueous media. The results indicate that the lowest excited triplet state of the initiator is abstracting an electron from the tertiary amine cosynergist probably via a triplet exciplex followed by hydrogen atom abstraction. This is confirmed by a detailed analysis on the effect of oxygen, pH and the ionisation potential of the amine on transient formation and photopolymerisation. Using photocalorimetry a linear correlation is found between the photopolymerisation quantum yields of the initiators and their photoreduction quantum yields in aqueous media. 

One of the photoinitiators, 4-methylsulphoniumbenzo-phenone sodium salt, exhibited a photoreduction quantum yield of 1.5 which is associated with secondary processes due to the reaction of the alkylamino radicals with a ground-state ketone molecule. 

In fact one of the photoinitiators examined here is highly photoactive with a measured photoreduction quantum yield greater than unity. 

The photoreduction quantum yields for the five benzophenone photoinitiators are compared in Table I together with their respective mean triplet lifetimes. 

Table I. Extinction Coefficients (E), Triplet Lifetimes(T) and Photoreduction Quantum Yields (J)r) of the Water Soluble Benzophenones...

It is important to point out at this point that the rate constant k and the quantum yield for a photochemical reaction are not fundamentally related. Since the quantum yield depends upon relative rates, the reactivity may be very high (large kr), but if other processes are competing with larger rates, the quantum yield efficiency of the reaction will be very small. That there is no direct correlation between the quantum yield and the rate is clearly seen from the data in Table 1.2 for the photoreduction of some substituted aromatic ketones in isopropanol ... 

The photoreduction of cyclobutanone, cyclopentanone, and cyclohexanone by tri-n-butyl tin hydride was reported by Turro and McDaniel.<83c> Quantum yields for the formation of the corresponding alcohols were 0.01, 0.31, and 0.82, respectively. Although the results for cyclopentanone and cyclohexanone quenching were not clear-cut (deviations from linearity of the Stem-Volmer plots were noted at quencher concentrations >0.6 M), all three ketone photoreductions were quenched by 1,3-pentadiene, again indicating that triplets are involved in the photoreduction. 

Thus the rates of production of acetone via the photoreduction of 4-tri-fluoromethylacetophenone and 4,4 -dimethylbenzophenone differ by a factor of four, while the quantum yields are essentially identical. 

Coincident with the decrease in the quantum yield of photoreduction with lowest and CT triplet states, there is an increase in the triplet... 

Details of nitrobenzene photochemistry reported by Testa are consistent with the proposal that the lowest triplet excited state is the reactive species. Photoreduction, as measured by disappearance quantum yields of nitrobenzene in 2-propanol is not very efficient = (1.14 0.08) 10 2 iD. On the other hand, the triplet yield of nitro benzene in benzene, as determined by the triplet-counting method of Lamola and Hammond 28) is 0.67 0.10 2). This raises the question of the cause of inefficiency in photoreduction. Whereas Lewis and Kasha 29) report the observation of nitrobenzene phosphorescence, no long-lived emission from carefully purified nitrobenzene could be detected by other authors i4,3o). Unfortunately, the hterature value of Et for nitrobenzene (60 kcal mole i) is thus based on an impurity emission and at best a value between 60 and 66 kcal mole can be envisaged from energy-transfer experiments... 

The efficiency of nitrobenzene photoreduction may be increased remarkably in 2-propanol/hydrochloric acid mixtures. In 50% 2-propanol/water containing 6 moles l i HCl, acetone and a complex mixture of chlorinated reduction products are formed i ). Both HCl and 2-propanol (as hydrogen source) are needed. When sulfuric acid is substituted for HCl, enhanced photoreduction does not occtu . When using mixtures of HCl and LiCl to maintain a constant chloride concentration (6 M) and vary [H+], a constant disappearance quantum yield 366 =0.15 is found within the [H+]-range 0.05—6 moles l i. This strongly suggests that chloride ions play an essential role, probably via electron transfer to 3(n, tt )-nitrobenzene i > [Eq. (1)], but it is also evident from the data presented that the presence of add is probably important in subsequent steps, [Eq. (3)]. 

The photoreduction of eight electron-acceptor- and four electron-donor-substituted nitrobenzenes has been studied and quantum yields for either starting material disappearance or product formation have been reported 7). Photolysis of 4-nitrobenzonitrile and 4-nitrotoluene in air-saturated solutions was completely quenched and thus a triplet multiplicity of the reacting excited state was derived 7). 

From photoreduction (> 280 nm) in diethylamine, low yields of 1-naphthyl-amine and the corresponding azo- and azoxy compounds have been obtained Photolysis (366 nm) in acidified 50% aqueous 2-propanol at varied HCl-concentrations results in remarkable enhancement of photoreduction compared to neutral 2-propanol. The highest disappearance quantum yield measured was 1.28 X 10 2 for 6 M HCl 4-chloro-l-naphthylamine is formed as main product 74.75). 

Photoreduction of Co(NH3)5Br2 + occurs in competition with aquation at 370 mjj., both processes having O 0.1 at 254 mjx only reduction is observed with 4> 0.19. Irradiation of Co(NH3)5I2+ with 370 or 254-mjx lights leads only to reduction. Various quantum yields have been reported as shown in Table IV. The latter complex was studied by Haim and Taube61 who found Ored = 1.97 and proposed ... 

Data obtained from the photoreduction of benzophenone in isopropanol indicates that the quantum yield (for the formation of acetone) is nearly constant using irradiation at several different wavelengths between 366 and 254 mp..16 The quantum yield for phosphorescence emission from the carbonyl n,ir triplet state is independent of exciting wavelength.22 Furthermore, the singlet excitation spectrum for the phosphorescence emission has been found to be very similar (if not identical) to the absorption spectrum.23 There are other examples where irradiation in the region of a given transition has ultimately led to a triplet of a different type.24... 

Table IV summarizes the pertinent characteristics of some of the naphthyl carbonyl compounds. All of these compounds emit from a it,7T triplet very similar to that of naphthalene. Those that have been studied are resistant to photoreduction in isopropyl alcohol and photocycloaddition with 2-methyl-2-butene25 and isobutylene.17 Significant oxetane formation was, however, observed with the aldehydes, albeit with only moderate efficiency (quantum yield approximately one-tenth that of benzaldehyde).25...

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