Redox potentials of excited molecules

We can conclude from these thermodynamic considerations that it is possible to estimate the redox potentials of excited molecules, if we know the equilibrium redox potentials for the molecules in the ground state, as well for reduction as for oxidation, and add or subtract from these redox potentials the excitation energy AE of the lowest singlet or triplet state. For most dye molecules the reduction redox potential is experimentally more easily accessible than the oxidation redox potential. In such cases we have found that an estimation can be made by assuming that the ionisation energy of the dye molecule in crystalline state is similar to the ionisation energy in a polar solvent and gives an approximate value for the absolute redox potential. Such estimations are especially useful for a comparison of molecules with similar structure. 

The redox potentials of an excited molecule can now be derived as follows The stored energy AE is given by the 0-0 transition between the lowest vibrational levels in the ground and excited state, i.e. AE = AE ) ). The excited state of a molecule may be either a singlet or a triplet state. It is possible to estimate the redox potentials of excited molecules by adding or subtracting A () o from the redox potential for the molecule in the ground state. One obtains... 

Other photoelectrochemical application redox potentials of excited molecules (to be explored) electroanalytical probe... 

Many of the dark redox potentials were determined by polarographic methods. One can further easily derive, from Eqs. (10.4) and (10.5), the relative positions of the redox potentials of a molecule in its excited state, i.e. F,redox(M /M ) should occur at higher (or more negative) values compared with F.redox(M/M ) and F.redox(M /M ) at lower (or more positive) energies with respect to F,redox(M/M ), as shown in Fig. 10.3. 

ZnO (suspension) sensitizes the photoreduction of Ag" by xanthene dyes such as uranin and rhodamine B. In this reaction, ZnO plays the role of a medium to facilitate the efficient electron transfer from excited dye molecules to Ag" adsortei on the surface. The electron is transferred into the conduction band of ZnO and from there it reacts with Ag. In homogeneous solution, the transfer of an electron from the excited dye has little driving force as the potential of the Ag /Ag system is —1.8 V (Sect. 2.3). It seems that sufficient binding energy of the silver atom formed is available in the reduction of adsorbed Ag" ions, i.e. the redox potential of the silver couple is more positive under these circumstances. 

The chemical association of the exciplex results from an attraction between the excited-state molecule and the ground-state molecule, brought about by a transfer of electronic charge between the molecules. Thus exciplexes are polar species, whereas excimers are nonpolar. Evidence for the charge-transfer nature of exciplexes in nonpolar solvents is provided by the strong linear correlation between the energy of the photons involved in exciplex emission and the redox potentials of the components. 

Fig. 1. Correlations between redox potentials of the excited and unexcited dye molecule, acting as electron acceptor or electron donor, in absolute (E) and conventional (C/nhe) scale...

In contrast to the electron transfer reactions in question, the efficiency of which is primarily determined by the difference of redox potentials of the excited donor molecule and the acceptor and, hence, by the variation in free energy (21), efficiencies of exothermic energy transfers depend solely on the local concentration of an appropriate quencher. 

The thermodynamics of excited states has been thoroughly discussed by Grabowski50, sl) with particular reference to acid-base equilibria (Forster s cycleS2 ). We will only consider here the problem of evaluating the difference between the redox potentials of M+/M and M+/ M (or M/M and M/M ) couples where M is the ground state molecule and M is an electronically excited state. 

In organic molecules there is generally a great separation between oxidation states, so that an excited organic molecule can usually serve as either an electron donor or an electron acceptor but not both. In transition metal complexes the presence of redox sites on both metal and ligand offers additional possibilities not available to either simple metal ions or organic molecules. The oxidation states are often closely spaced, so that the case is very common in which an excited state can be used as both an electron donor and an electron acceptor. As can be seen from Fig. 7, when the oxidation and reduction potentials of a molecule are close enough,... 

Any endoergonic photochemical reaction converts light energy into chemical energy. If the photoproducts are kinetically stable, they have to be considered fuels as they can be stored, transported and then converted to other chemical species with evolution of energy11 ). As we have seen in Section 5, electronic excitation increases the oxidation and reduction potentials of a molecule. Light absorption can thus drive a redox reaction in the non-spontaneous direction (Fig. 16). 

The excited state of a diamagnetic species with a closed-shell ground state is both a better donor and acceptor than its associated ground state. In addition to the ground-state redox potential, the excited state has the additional redox power of the absorbed photon, i.e. hv. Figure 2.15 illustrates this point, whereby ionization potentials (IPs) and electron affinities (EAs) for a ground- and excited-state molecule are compared. Excitation decreases the IP by AEhomo-lumo/ i.e. 

The redox potentials of the excited-state and gronnd-state molecules can then be related by (Rehmand Weller, 1969) U... 

Such a process is possible at a semiconductor electrode where the conduction band occurs above the redox potential of the excited molecule i.e. in terms of the usual energy scheme that E. > The first experimental results were... 

From Fig. 3, it is clear that the reactivity of molecules in the excited state is directly linked to lifetimes. In general, the redox potentials of an excited state, - s+/s and Es s, can be expressed in terms of the ground state redox potentials, - s+/s and E s/S-J as... 

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