Photoinduced electron transfer thermodynamics

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... 

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. 

Electronic excitation of molecules lead to a drastic change of their reactivities. One effect of the excitation is the powerfiil change of the redox properties, a phenomenon which may lead to photoinduced electron transfer (PET) [1-4] The electron-donating as well as the electron-accepting behavior of the excited species are approximately enhanced by excitation energy. This can be explained by means of a simple orbital scheme. By excitation of either the electron donor (D) or the acceptor (A) of a given pair of molecules, the former thermodynamically unfavorable electron transfer process becomes exergonic (A et) (Scheme 1). 

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... 

The fundamental theories behind electron transfer were discussed above in Section 2.1. Indeed, some of the most important empirical proofs for these theories have originated from photoinduced electron transfer in supramolecular donor-acceptor complexes. The difference between thermally and photochemi-cally induced electron transfer lies in both the orbitals participating in the reaction and in the additional thermodynamic driving force provided by the excited state. It is therefore important to consider the redox properties of excited-state species. 

For a photoinduced electron transfer and charge separation to be efficient in a supramolecular device, some structural and energetic prerequisites must be fulfilled. First, the electron transfer must be thermodynamically feasible, i.e. it must be exergonic. The free energy of a photoinduced electron transfer, AGpet, may be calculated according to the following equation ... 

A study by Sessler and coworkers of P-Pzn-Q triads with structures closely related to that of tetrad 53 (see below) has shown remarkably rapid photoinduced electron transfer from the distal free base porphyrin to the quinone. Singlet energy transfer from the free base to the proximal zinc porphyrin on such a time scale is deemed unlikely for thermodynamic reasons, since the first excited singlet state of the zinc... 

Lastly, electron transfer in D—[H]—A assemblies is not a perquisite of the excited states of metal complexes. Organic ensembles 38 and 39 (R = SiMe2 Bu), containing a dimethylaniline-anthracene redox pair, have been synthesized recently [124]. Preliminary time-resolved and steady-state fluorescence experiments indicate the occurrence of photoinduced electron transfer. In work related to Watson Crick base-paired systems, the excited state of the fluorescent pyrene derivative 40 is efficiently quenched (94-99 %) by 2 -deoxyguanosine (dG), 2 -deoxycytidine (dC), or 2 -deoxythymidine (dT) in aqueous solution [125]. A PCET mechanism is thought to be responsible for this process, as the thermodynamics of electron transfer are unfavorable unless coupled to a rapid proton-transfer step. The quenched lifetime of 40 in the presence of dC and dT in H2O is significantly extended by a factor of 1.5-2.0 in D2O this isotope effect is similar to that observed in the kinetics studies of 1 [70]. The invoked PCET reaction mechanism also accounts for the inability of dC and dT to quench the fluorescence of 40 in the aprotic organic solvent DMSO. 

Figure 2. Thermodynamic cycles for evaluating the free energy changes AG°et associated with the intramolecular photoinduced electron transfer processes in system 13 in its oxidized (a) and reduced (b) form. The quantity, the spectroscopic energy, is obtained from the emission spectrum the E values, electrode potentials associated with the given redox change, can be determined through voltammetry experiments. The Coulombic term (e /er) has been considered negligible under the present circumstances.

Direct photolysis of 50-53 in 02-saturated acetonitrile solution also leads to the corresponding carbazoles with a quantum yield of ca 0.64 in all cases (equation 13)161. Apparently, substituents have only a little effect on the chemical yield of carbazole produced by steady-state irradiation in aerated acetonitrile. However, an attempt to carry out such a photocyclization reaction by using photoinduced electron-transfer sensitization has failed, presumably due to fast back electron transfer that quenches the net reaction. It is also interesting to note that chemical oxidation and electrochemical oxidation of 50-53 does not result in carbazoles. Instead, benzidine products are formed. These results are consistent with the AMI calculations, which suggest that the cyclization reaction is both kinetically and thermodynamically more favorable from the triplet state than from the cation radical or dication. 

If ET rate coefficients at metal electrodes tend to a limit rather than decreasing with increasingly negative -AG , there is little opportunity for decreasing the rate of a thermodynamically favourable back ET reaction following a forward PET reaction. The implication for photoinduced electron transfer reactions at metal electrodes is that they will be ineffective as a means of net charge separation, as indeed they are. Metals are... 

Transfer of calcium cations (Ca2 + ) across membranes and against a thermodynamic gradient is important to biological processes, such as muscle contraction, release of neurotransmitters or biological signal transduction and immune response. The active transport can be artificially driven (switched) by photoinduced electron transfer processes (Section 6.4.4) between a photoactivatable molecule and a hydroquinone Ca2 + chelator (405) (Scheme 6.194).1210 In this example, oxidation of hydroquinone generates a quinone to release Ca2+ to the aqueous phase inside the bilayer of a liposome, followed by reduction of the quinone back to hydroquinone to complete the redox loop, which results in cyclic transport of Ca2 +. The electron donor/acceptor moiety is a carotenoid porphyrin naphthoquinone molecular triad (see Special Topic 6.26). 

The thermodynamic driving force for these photoinduced electron-transfer reactions were determined by eq 3 in which Ei/2(Cu(II)/Cu(I)) = 0.343 V vs, NHE is the reversible ground-state reduction potential of the copper site as ... 

Eugster, N., D.J. Fermin, and H.H. Girault (2002). Photoinduced electron transfer at hquid/hquid interfaces. Part VI. On the thermodynamic driving force dependence of the phenomenological electron transfer rate constant. J. Phys. Chem. B 106, 3428-3433. 

Molecule l20-2i employs a general design. In this case, the fluorophore (anthracene) and the receptor (amine) have to communicate across the short cr-bond spacer. Photoinduced electron transfer (PET) can perform this communication if the thermodynamics are favorable. PET usually outruns fluorescence under such conditions. Experimentally, this means that the fluorescence decay rates are slower when compared with the rates of PET. However, PET thermodynamics can be altered to become unfavorable... 

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