Photoinduced electron transfer kinetics

Micelles and microemulsions have been explored as membrane mimetic systems since they possess charged microscopic interfaces which act as barriers to the charge recombination process (Fendler et al., 1980 Hurst et al., 1983). Namely, the influence of the location of the sensitizer on photoinduced electron transfer kinetics and on charge separation between photolytic products in reversed micelles has been studied (Pileni etal., 1985). 

Li F, Unwin PR (2015) Scanning electrochemical microscopy (SECM) of photoinduced electron transfer kinetics at Uquid/tiquid interfaces. J Phys Chem C 119 4031 043... 

Sub-picosecond photoinduced absorption studies were employed to demonstrate the speed of the photoinduced electron transfer. Upon addition of C(M to P30T, the P1A spectrum, decay kinetics, and intensity dependence all change dramatically 36J. Already at 1 ps after photoexcitation by a 100 fs pump pulse at... 

C. Shen and N.M. Kostic. Kinetics of photoinduced electron-transfer reactions within sol-gel silica glass doped with zinc cytochrome c. Study of electrostatic effects in confined liquids. J. Am. Chem. Soc. 119, 1304-1312 (1997). 

On the other hand, the cation radical of the kinetic ESE+ is readily converted to the thermodynamic cation radical by a 1,3-prototropic shift in the course of the photoinduced electron transfer with chloranil.41 As such, the efficient isomerization of the kinetic ESE cation radical as an intermediate in equation (19) accounts for the observed lack of regioselectivity in equation (18).37... 

In classical kinetic theory the activity of a catalyst is explained by the reduction in the energy barrier of the intermediate, formed on the surface of the catalyst. The rate constant of the formation of that complex is written as k = k0 cxp(-AG/RT). Photocatalysts can also be used in order to selectively promote one of many possible parallel reactions. One example of photocatalysis is the photochemical synthesis in which a semiconductor surface mediates the photoinduced electron transfer. The surface of the semiconductor is restored to the initial state, provided it resists decomposition. Nanoparticles have been successfully used as photocatalysts, and the selectivity of these reactions can be further influenced by the applied electrical potential. Absorption chemistry and the current flow play an important role as well. The kinetics of photocatalysis are dominated by the Langmuir-Hinshelwood adsorption curve [4], where the surface coverage PHY = KC/( 1 + PC) (K is the adsorption coefficient and C the initial reactant concentration). Diffusion and mass transfer to and from the photocatalyst are important and are influenced by the substrate surface preparation. 

Apply the Marcus theory to photoinduced electron transfer and show how experimental evidence gives a relationship between the kinetics of the process and the thermodynamic driving force. 

Until now, the isotopic effect was discnssed only in relation to the reactants. In electron-transfer reactions, the solvent plays an eqnally important role. As mentioned, different solvate forms are possible for reactants, transition states, and products. Therefore, it seems important to find a reaction where the kinetic effect resulting from the introduction of an isotope would be present for solvents, but absent for reactants. For a published work concerning this problem, refer Yusupov and Hairutdinov (1987). In this work, the authors studied photoinduced electron transfer from magnesium ethioporphyrin to chloroform followed by a dark recombination of ion-radicals in frozen alcohol solutions. It was determined that the deuteration of chloroform does not affect the rate of transfer, whereas deuteration of the solvent reduces it. The authors correlate these results with the participation of solvent vibrational modes in the manner of energy diffraction during electron transfer. 

The rate of Sc -promoted photoinduced electron transfer from Ceo to CI4Q determined from the decay rate of the absorbance due to Ceo at 740 nm (inset of Fig. 11) obeys pseudo-first-order kinetics and the pseudo-first-order rate constant increases linearly with increasing the p-chloranil concentration [CI4Q] [135]. From the slope of the linear correlation, the second-order rate constant of electron transfer ( et) in Scheme 15 was obtained. The A et value increases linearly with increasing the Sc + concentration. This indicates that CUQ produced in the photoinduced electron transfer forms a 1 1 complex with Sc + (Scheme 15) [78]. When CI4Q is replaced by p-benzoquinone (Q), the value for electron transfer from Ceo to Q increases with an increase in [Sc " ] to exhibit a first-order dependence on [Sc ] at low concentrations, changing to a second-order dependence at high concentrations, as shown in Fig. 13 (open circles) [135]. Such a mixture of first-order and second-order dependence on [Sc ] was also observed in electron transfer from CoTPP (TPP = tetraphenylporphyrin dianion) to Q... 

Photoinduced electron transfer from eosin and ethyl eosin to Fe(CN)g in AOT/heptane-RMs was studied and the Hfe time of the redox products in reverse micellar system was found to increase by about 300-fold compared to conventional photosystem [335]. The authors have presented a kinetic model for overall photochemical process. Kang et al. [336] reported photoinduced electron transfer from (alkoxyphenyl) triphenylporphyrines to water pool in RMs. Sarkar et al. [337] demonstrated the intramolecular excited state proton transfer and dual luminescence behavior of 3-hydroxyflavone in RMs. In combination with chemiluminescence, RMs were employed to determine gold in aqueous solutions of industrial samples containing silver alloy [338, 339]. Xie et al. [340] studied the a-naphthyl acetic acid sensitized room temperature phosphorescence of biacetyl in AOT-RMs. The intensity of phosphorescence was observed to be about 13 times higher than that seen in aqueous SDS micelles. 

As demonstrated in this review, photoinduced electron transfer reactions are accelerated by appropriate third components acting as catalysts when the products of electron transfer form complexes with the catalysts. Such catalysis on electron transfer processes is particularly important to control the redox reactions in which the photoinduced electron transfer processes are involved as the rate-determining steps followed by facile follow-up steps involving cleavage and formation of chemical bonds. Once the thermodynamic properties of the complexation of adds and metal ions are obtained, we can predict the kinetic formulation on the catalytic activity. We have recently found that various metal ions, in particular rare-earth metal ions, act as very effident catalysts in electron transfer reactions of carbonyl compounds [216]. When one thinks about only two-electron reduction of a substrate (A), the reduction and protonation give 9 spedes at different oxidation and protonation states, as shown in Scheme 29. Each species can... 

Quantitative investigations of the photoinduced electron transfer from excited Ru(II) (bpy)3 to MV2 + were made in Ref. [54], in which the effect of temperature has been studied by steady state and pulse photolysis techniques. The parameters ve and ae were found in Ref. [54] by fitting the experimental data on kinetics of the excited Ru(II) (bpy)3 decay with the kinetic equation of the Eq. (8) type. It was found that ae did not depend on temperature and was equal to 4.2 + 0.2 A. The frequency factor vc decreased about four orders of magnitude with decreasing the temperature down to 77 K, but the Arrhenius plot for W was not linear, as is shown in Fig. 9. 

A kinetic scheme can be written, Scheme 1, in which the rates of back electron transfer from the contact, solvent-separated, and free ions must each be individually considered. Clearly, the greater the tendency of a given solvent to separate the ions, the less significant will be the electrostatic interaction between them, and the greater tendency to suppress back electron transfer for a period sufficiently long to allow radical ion chemistry to ensue. A major concern of this article will be in defining the efficacy of supramolecular assemblies which create inhomogeneous arrays to controll the secondary chemistry of radical ions formed via photoinduced electron transfer. 

Photoexcitation of a deaerated PhCN solution of Acr+-Mes by a nanosecond laser light flash at 430 nm results in the formation of Acr -Mes+ with a quantum yield close to unity (98 %) via photoinduced electron transfer from the mesitylene moiety to the singlet excited state of the acridinium ion moiety ( Acr -Mes) [54]. The decay of Acr -Mes+ obeyed second- rather than first-order kinetics at ambient temperature as observed in the case of Fc+-ZnP-H2P-C60 , when the bimo-lecular back electron transfer predominates owing to the slow intramolecular back electron transfer (see above) [50]. In contrast, the decay of Acr -Mes+ obeys first-order kinetics in PhCN at high temperatures (e.g. 373 K). This indicates that the rate of the intramolecular back electron transfer of Acr -Mes4 becomes much faster than the rate of the intermolecular back electron transfer at higher tempera-... 

Electrochemical studies on SAMs have proven invaluable in elucidating the impact of various molecular parameters such as bridge structure, molecular orientation or the distance between the electroactive species and electrode surface. As described above in Section 5.2.1, the kinetics of heterogeneous electron transfer have been studied as a function of bond length for many systems. Similarly, the impact of bridge structure and inter-site distances have been studied for various supramolecu-lar donor-acceptor systems undergoing photoinduced electron transfer in solution. In both types of study, electron transfer is observed to increase as the distance between the donor and acceptor decreases. As discussed earlier in Chapter 2, the functional relationship between the donor-acceptor distance and the electron transfer rate depends on the mechanism of electron transfer, which in turn depends on the electronic nature of the bridge. 

The EPR technique is a valuable tool for gaining a better understanding of the mechanism and kinetics of photoinduced electron-transfer reactions in colloidal Ti02 systems. There are two main features of the EPR technique which are indispensable in these studies. 

Primary steps of photoinduced electron transfer have been studied in plant reaction centers (PS-I and PS-II), by flash absorption and EPR. In PS-I two questions wereinvestigated i) the properties of the primary radical pair P-700+, A0 (kinetics of decay nature of A0, presumably a specialized chlorophyll a decay by recombination to populate the P-700 triplet state) and ii) the nature of the secondary acceptor A,. Extraction-reconstitution experiments indicate that A, is very probably a molecule of vitamin K,. 

Detailed kinetics of some of these and other processes have been studied, e.g. oxidation by [Os(bipy)3]3+ of J , SCN",167 OH" 122 reaction of [Os(LL)3]3+ with OH- to give 02 (LL = bipy, phen, 5-Mephen, 4,4-Me2bipy),168 oxidation of ascorbic acid by [Os(LL)3]3+ (LL = bipy, 4,4 -Me2bipy) 169 and the photoinduced electron transfer reactions of acidopentaamminecobalt(III) complexes with [Os(bipy)3]2+.170 The outer-sphere oxidation of ascorbic acid by [Os(bipy)3]2+ incorporated in a coating on a graphite electrode has recently been reported.170... 

Kinetic studies have demonstrated that photo-induced electron transfer between the zinc and the gold porphyrin occurs at a rate of (1.7 ps)-1 in Cu(I)-complexed [2]-rotaxane 102, which is much higher than in the case of the free rotaxane 107 (36 ps)-1.73 The higher photoinduced electron transfer rate in the Cu(I) complex 102 than in the demetallated system 107 was explained also in terms of a superexchange mechanism. 

Recent studies of photoinduced electron transfer reactions have demonstrated that the reactivity of a radical ion pair is different from that of free radical ions. Farid showed that in the photoreaction of 1,1-diphenylethene the reactivity of the radical ion pair can be distinguished from that of the free radical cation by kinetic measurements [22]. Mizuno and Otsuji showed that the reactivity feature of a radical ion pair in solvent cage and that of solvent separated radical... 

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