Quinones photoreduction

A multicomponent positive-imaging process using ammonia release has been described by Ricoh.211 The components of the system are (1) a cobalt(III) hexaammine complex, (2) a quinone photoreductant, (3) a chelating agent such as dimethylglyoxime, (4) a leuco dye (triarylmethane type), (5) a photooxidant (biimidazole) and (6) an organic acid (toluenesulfonic acid). 

Quinaldine zinc complexes, 952 Quinolines metal complexes, 952 zinc complexes, 952 Quinones photoreduction zinc complexes, 994... 

Brennan, J.F., and Beutel, J. (1969) Quinone photochemistry. II. The mechanism of photoreduction of 9,10-phenanthrenequinone and 2-tert-butyl-9,10-anthraquinone in ethanol. J. Phys. Chem. 73, 3245-3249. 

In a study of the benzophenone sensitized photoreduction of camphor-quinone (16), Monroe and Weiner108-110 found that the quantum yield of photoreduction decreased from 0.386 to 0.087 as [16] increased from 0.003 to 0.20M. 

The photoreduction of carboxylic acids and their derivatives has not been widely reported, although cyclic imides do behave like ketones in such reactions (4.39). Quinones are readily photore-duced, and this is of particular importance for anthraquinone dyes or pigments used on cellulose materials such as cotton. Some quinone dyes cause what is known as phototendering—the fabric progressively weakens and can be torn easily after exposure of the dyed... 

Figure 23-17 The zigzag scheme (Z scheme) for a two-quantum per electron photoreduction system of chloroplasts. Abbreviations are P680 and P700, reaction center chlorophylls Ph, pheophytin acceptor of electrons from PSII QA, Qg, quinones bound to reaction center proteins PQ, plastoquinone (mobile pool) Cyt, cytochromes PC, plastocyanin A0 and Aj, early electron acceptors for PSI, possibly chlorophyll and quinone, respectively Fx, Fe2S2 center bound to reaction center proteins FA, FB, Fe4S4 centers Fd, soluble ferredoxin and DCMU, dichlorophenyldimethylurea. Note that the positions of P682, P700, Ph, Qa/ Qb/ Ay and A, on the E° scale are uncertain. The E° values for P682 and P700 should be for the (chlorophyll / chlorophyll cation radical) pair in the reaction center environment. These may be lower than are shown.

Figure 4. Light intensity dependence of the photoreduction current of anthra-quinone in acetonitrile. An anodic wave grows in at about 0.0 V (vs. SSCE) with...

Fig. 14. Schematic representation of light-driven (2e + 2H+) symport across a membrane via the quinone carrier molecule vitamin Kj and its hydroquinone form proflavine (PF)-sen-sitized photoreduction of methyl-viologen MV2+ in the RED phase, yields the reducing species MV+, with simultaneous oxidative decomposition of EDTA used as electron donor the OX phase contains ferricyanide as electron acceptor [6.49].

The behaviour of compound F in HPC (Figure 8) parallels the observations made when it is adsorbed on paper where bleaching of the intense red colour was seen [5]. Some photoreduction of the quinone part may therefore be involved during the bleaching. 

The kinetics of photoreactivity measured at 390 nm for compounds A, D, E, and A - D and Q - H mixtures are reported in figure 10. The main features are the negative values associated with the photoreduction of quinones and the large reactivity in HPC films of the methoxyhydroquinone A. 

Photochromic compounds functioning by an oxidation-reduction mechanism (electron transfer), especially a photoreduction mechanism, are known in inorganic materials such as silver halides, which are utilized in eyewear lenses. Although the number of organic photochromic compounds operating via electron transfer is fewer than those by isomerization, heterolytic (or homolytic) cleavage, and pericyclic reactions, several classes of compounds have been reported, such as thiazines,1 viologens,2 and polycyclic quinones.3... 

On the other hand, the overwhelming success of the radical-pair mechanism is well supported by many experimental investigations (2,18,49,106,119). In a systematic study of the photoreduction of quinones by phenols, Adeleke and Wan (2) have confirmed the effect of the sign of the Ag on the polarization of the semiquinone and the phenoxy radicals as dictated by eq. 10, and Elliot and Wan (49) have obtained relative individual enhancement factors of the esr lines of the durosemiquinone radical formed in the photoreduction of the parent quinone by isopropanol which agree very well with the values predicted by eq. 16. 

The photochemist is rather familiar with the photoexcited triplet states and the associated intersystem crossing processes. It is well documented that the photoexcited triplet state plays an important role in organic photochemistry. It is thus conceivable that the electron spin polarization of the photoexcited triplet can be further transferred to a radical pair formed by the reactions of the triplet with a suitable substrate. Such a photoexcited triplet mechanism was first proposed by Wong and Wan in 1972 (135) to account for the "initial polarization" observed in the naphthosemiquinone radical formed in the photoreduction of the parent quinone in isopropanol. It was further considered that the triplet mechanism might also lead to CIDNP if such initially polarized radicals react rapidly to give products with nuclear spin polarization induced via the Overhauser mechanism. 

Irradiation from the side with a high-intensity uv source may also introduce some problems. The volume of the solution within the receiver coil may be heated by the accompanying light, and the polarized products may diffuse faster away from the receiver coil zone. This effect can be illustrated in Fig. 7, which shows the polarization of the recombined product p-benzo-quinone in the photoreduction of benzoquinone obtained under two different experimental conditions. In the upper curve the polarization observed was greatly reduced and showed only a "diminished absorption" when the sample was allowed to heat up by light... 

The photoreduction of quinones by phenols has been employed by us as a model photochemical system to probe the polarization phenomena. In all the cases we were observing both the neutral semiquinone and the phenoxy radicals, and we were confident that the primary reaction is the triplet abstraction of a hydrogen atom from phenol. It is also generally recognized that in a basic solution the semiquinone radical readily undergoes the following reaction ... 

If the original semiquinone radical QH is polarized, the semi-quinone radical anion derived from eq. 52 can be expected to retain much of the initial polarization. Thus in the CIDEP studies of the photoreduct ion of quinones in triethylamine solution, the primary photochemical process was thought to involve the possible exciplexes (42) ... 

The application of the CIDEP technique to the kinetic isotope effect in the photoreduction of quinones was made by Adeleke and Wan (2). Photolyses of naphthaquinone in isopropanol and in isopropanol-dg were carried out under identical experimental conditions. Analysis of the polarization data indicates that the ratio kjj/kj = 2.6. However, in a separate series of experiments in which the naphthaquinone was photoreduced by substituted phenol and phenol-O,, the kinetic isotope effect drops to a value of kjj/kj = 1.6. 

Polarization Transfers and Reaction Mechanisms. Polarization transfers include the previously mentioned electron-nuclear Over-hauser effect and the nuclear-nuclear Overhauser effect. In this section we will discuss only electron-electron polarization transfer via a secondary chemical reaction involving a primary polarized radical. Again we shall use the photoreduction of quinone (t-butyl-p-benzoquinone) as an example. In solvent containing isopropanol, reaction of triplet quinone by phenols leads to two structural isomers, radicals I and II ... 

It has been a most satisfying experience when we could combine the CIDEP and CIDNP techniques to look at the photoreductions of certain quinones. In these experiments CIDEP provides information on the primary reactions involving the excited triplet and the secondary reactions involving the primary polarized radicals. However, esr experiments only "see" those radicals which escape the cage the CIDNP experiments fill in the missing information about the initial radical pair reacting in the primary cage. 

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