Photooxidation and Photoreduction

Surface vs Solution Reactions, Anotliei issue of debate in pliotocatalyzed mineialization of oiganic substrates is whether the initial oxidation occurs on the photocatalyst s surface or in solution. Kinetic data of photooxidations and photoreductions have often been fitted to the simple... 

Photooxidation and photoreduction of an electron donor and an electron acceptor, respectively, as illustrated schematically with a one-electron molecular orbital scheme. 

The photooxidation and photoreduction of water in separate reaction systems are demonstrated in Sections 7.3.1 and 7.3.2, respectively. Since the redox... 

Leaver I.H., Photochemistry of dyed and pigmented polymers Normans. Allen, Photooxidation and Photoreduction of dyes in ploymers, Applied Science Publishers Ltd, England, p. 161 —p.239. (1980)... 

In brief, it is shown that combined photooxidation and photoreduction of organic and inorganic species can considerably enhance the efficiency of photocatalytic reactors. It is also expected that a proper selection of the cation will optimize simultaneous oxidation-reduction processes. 

The operation of an ionic reduction mechanism certainly would not exclude pesticides from also taking part in the well-known free-radical abstraction of hydrogen atoms. Indeed, it seems inevitable that certain reactions such as the light-energized phenylation of the fungicide Phygon (XVI) (53) must take place exclusively via the free-radical route (Equation 4), and sensitized photooxidations and photoreductions in the presence of natural water constituents such as chlorophyll and riboflavin undoubtedly will be shown to be important in pesticide transformations in the environment. 

The mechanism for a whole photocatalysis process is usually quite complicated. In this section, we will describe recent studies of detailed photocatalytic reactions of two important molecules on various Ti02 surfaces oxygen (electron scavenger) and methanol (hole scavenger), which are representative photooxidation and photoreduction reactions oti Ti02. 

Methylene blue, a colorant used in preparations for external use, imdergoes photooxidation (11) and photoreduction in the presence of ethylenediamine tetraacetic acid (EDTA) (12). The photodecomposition of vanillin solutions in ethanol is accompanied by the formation of a yellow color and the development of a slightly bitter taste (13). 

The concentration of functional PSI and PSII reaction centers was estimated from the amounts of P700 and QA, respectively, present in the various samples. The amounts of P700 and QA were determined from the amplitude of the light-m/Mus-dark absorbance change at A.=700 nm (AA70o) and at X =320 nm (AA320), respectively [Melis 1989, Smith et al. 1990]. The functional Chi antenna size of PSI and PSII was measured from the kinetics of P700 photooxidation and QA photoreduction, respectively [Melis and Anderson 1983, Melis 1989]. 

Fig. 3 shows the picosecond kinetics reported by Kaufmann etal.. These workers used Rb. sphaeroides R-26 reaction-center complexes poised either at -1-200 mV to maintain Q in the oxidized state and functional [Fig. 3 (A)], or poised at 400 mV, so that Q is chemically reduced before the flash [Fig. 3 (B)]. Picosecond absorbance changes at 540 run were measured to directly monitor the redox changes of BO. In both cases, BO photoreduction represented by the initial absorbance decrease occurred in < 10 ps. In both cases, the risetime of the 1250-nm absorbance inaease (not shown), which can be assigned exclusively to the photooxidation of P870, also showed a risetime of <10. These results indicate that, independent of the redox state of Q, P870 is photooxidized and loses an electron to BO, by way of the [P BO]-t//v->[P BO ] reaction, in <10ps.

The transient difference spectrum slOps [Fig. 7 (A, a)] is predominantly due to the formation of excited singlet state of the antenna Chi a, with a major bleaching at 685 nm. The transient difference spectrum 200 ps after excitation [Fig 7 (A, b)] is ascribed to contributions from both P680 photooxidation and O photoreduction. The l-ns difference spectrum [Fig. 7 (A,c)], with the maximum bleaching now shifted to 680 nm, is considered to be due only to the remaining photooxidized P680. ... 

The quantum requirements for both P700 photooxidation and P430 photoreduction were measured in PS-I particles using monochromatic light sources for excitation With 671-nm excitation light, the quan-... 

Fig. 9. (A) Light-induced EPR changes due to P700 photooxidation and FeS-A photoreduction at 13 K. (B) EPR spectra of P700 and FeS-A" after the PS-1 particles had been illuminated at 13 K for 20 s (top row) and after the illuminated sample had been maintained at 175 K for 6 m and then recooled to 13 K (bottom row). (C) plot of loss of EPR signals of P700 and FeS-A measured after exposure to various temperatures for various amounts oftime [see table (D)]. Figure source Ke, Sugahara, Shaw, Hansen, Hamilton and Beinert (1974) Kinetics of appearance and disappearance of light-induced EPR signals ofPlOCt and Iron-sulfur protein(s) at low temperatures. Biochim Biophys Acta 368 405,406.

Fig. 5 (D) shows the spectra of light-induced absorbance changes measured 150 and 800 ps after excitation of TSF-I particles poised at 200 mV by 708- or 689-nm flashes. The l50-ps spectrum has a maximum absorbance increase in the 750-nm region the 800-/>.s difference spectrum coincides well with the difference spectrum of [P700 -P700], shown as a dashed curve. The flash-induced AA spectrum in the 450-600 nm region is also similar to that for P700 and is shown in Fig. 5 (E). Based on the assumption that the 150-p difference spectmm consists of changes due to primary-donor photooxidation and primary-acceptor photoreduction, while in the %00-ps difference spectrum only changes due to the photooxidized P700 remain, it should be possible to obtain the difference spectmm of the photore-duced primary acceptor, i.e AA[Aq -Aq], as a temporal difference in absorbance. In Fig. 5 (F), the solid trace derived from the difference between the l50-ps and the WO-ps difference spectra in panels (D) and (E) represents AA[Aq -Aq]. As expected, the plotted A(AA) closely resembles that reported by Fujita et at for the formation ofthe Chl-o anion radical in vitro, and almost coincides with it (the dashed trace)...

Solutions of tetrazolium salts, e.g., 53, have been reported to both become colored and bleached under the influence of both UV and visible light. Several workers have attributed this phenomenon to photoreduction to the corresponding formazan (51) and the formation of a fluorescent colorless compound (152) through photooxidation.240- 243 The reduction of 152 under UV or blue light to the intense green radical structure (153) has also been reported (Scheme 21).244 A one-electron reduction product (154) is proposed as a short-lived intermediate in the photoreduction.245... 

Here pn is 1,2-diaminopropane and bn is 2,3-diaminobutane. Decomposition of the amine cation radicals obtained by photooxidation of the ligands en, bn, and pn have been discussed by Moeller. The products of Co(en)33+ photolysis can be satisfactorily explained by postulating that carbon-carbon bondbreaking is the principal step in decomposition of the cation radical H2NCH2CH2NH2t.58 Presuming a similar mechanism to obtain in photoreduction of Co(pn)33+, there are then two possible reaction pathways leading to different products. 

Photoreduction of cobalt(III) complexes in nonaqueous solvent systems has been little studied because of the limited solubility of cobalt(III) complexes and their tendency to photooxidize the solvent. Irradiation with 365-mjj. light of cis- or trans-Co(en)2C 2 + and Co(en)2Cl(DMSO)2+ in dimethylsulfoxide (DMSO) leads rapidly to production of a green tetrahedral cobalt(II) product apparently with concurrent solvent oxidation.53,71 Irradiation with 365-mjx light of the molecular Co(acac)3 in benzene rapidly gives a red precipitate which may be the cobalt(II) acetylacetonate.53... 

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