Production of Excited Molecules by Electron Transfer

As studied by Bard and co-workers, the chemiluminescence of Ru(bipy)3 can be observed by electron transfer at metal electrodes when alternatively polarizing the Pt electrode between potentials corresponding to the redox potentials of Ru /Ru and Ru VRu [60]. The emission was interpreted as an electron transfer according to the reaction  

In this case, a Ru(bipy)3 in its triplet state was formed by the annihilation reaction (Eq. 10.27). 

In principle, an excited molecule can also be produced by an electron transfer from the conduction band of a semiconductor to the oxidized species of an organic molecule (e.g. Ru(bipy)3 ). Instead of the annihilation reaction given by Eq. (10.27) we have then for the Ru complex 

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 

Bard and co-workers have pointed out that it is frequently difficult to attribute the electrogenerated luminescence unambiguously to the process discussed above [62]. In several cases, instead of reaction (Eq. 10.29), the reduced species is also formed at a semiconductor electrode leading to the annihilation process (Eq. 10.27). This difficulty is caused by the fact that the reduction potential of a molecule in the dark ( Frejjox(M/ M )) is frequently rather close to the oxidation potential of the excited molecule ( predox(MVM)) (see e.g. Fig. 10.3). Luttmer and Bard found one system, rubrene, for which these two potentials are well separated. These authors observed a luminescence due to electron transfer from a ZnO electrode to the oxidized species of rubrene [62j. Another interesting example is the formation of an excited molecule by transfer of hot electrons, as already discussed in Section 7.8. 

Such a process is possible at a semiconductor electrode where the conduction band occurs above the redox potential of the excited molecule that is in terms of the usual energy scheme that j ( Ru /Ru ). The first experimental results were published by Gleria etal, who observed luminescence of Ru(bipy)j upon cathodic polarization of an n-SiC electrode in an electrolyte containing Rufbipyij [14]. This result was interpreted on the basis ofthe processes given in Eqs. (10.29) and (10.28). 

In the previous sections, we have discussed electron transfer reactions between optically excited molecules and semiconductor electrodes. The question arises whether the opposite effect, the production of an excited state by electron transfer, is also possible. In principle it can be realized if we select a semiconductor, the conduction band (c.b.) of which is located above the energy level of the excited species [ f(M /M )]. The reaction process is then given by 

This condition can be met, for instance, by using n-SiC and an aqueous solution of Ru(II)(bipy)3. In this case, luminescence was observed during cathodic polarization. This result proved the formation of an excited Ru(II)(bipy)3 by electron transfer.  

Bard and co-workers have analyzed the whole problem in more detail and emphasized that it is frequently difficult to attribute the electrogenerated luminescence unambiguously to the process discussed above. In several cases, instead of reaction (74), electron transfer occurred to the neutral molecule according to 

Here a radical anion (M ) is formed which reacts with the cation (M ) in the solution ( radical ion annihilation ). This process leads also to the production of excited molecules which are detected by luminescence measurements. The process can also be observed with metal electrodes.  

In the latter process, rubrene is formed in its excited singlet state (R ) which was detected by fluorescence measurements. 

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Reactions of Complex Ions. For reactions of systems containing H2 or HD the failure to observe an E 1/2 dependence of reaction cross-section was probably the result of the failure to include all products of ion-molecule reaction in the calculation of the experimental cross-sections. For reactions of complex molecule ions where electron impact ionization probably produces a distribution of vibrationally excited states, kinetic energy transfer can readily open channels which yield products obscured by primary ionization processes. In such cases an E n dependence of cross-section may be determined frequently n = 1 has been found. 

The vast majority of works that study the impurity ionization of excited molecules are confined to highly exergonic electron transfer specified by inequality (3.261). Under this condition the reverse electron transfer regenerating the excited state can be forgotten. All photogenerated ions recombine uniquely into the ground state of the neutral products. An important exception to this rule was demonstrated in the pioneering work of Rehm and Weller [53]. This... 

Willig and co-workers used near-infrared spectroscopy to measure excited-state interfacial electron transfer rates after pulsed light excitation of cis-Ru(dcb)2(NCS)2-Ti02 in vacuum from 20 to 295 K [208]. They reported that excited-state electron injection occurred in less than 25 fs, prior to the redistribution of the excited-state vibrational energy, and that the classical Gerischer model for electron injection was inappropriate for this process. They concluded that the injection reaction is controlled by the electronic tunneling barrier and by the escape of the initially prepared wave packet describing the hot electron from the reaction distance of the oxidized dye molecule. It appears that some sensitizer decomposition occurred in these studies as the transient spectrum was reported to be similar to that of the thermal oxidation product of m-Ru(dcb)2(NCS)2. 

In some cases the excited states of molecules can also be achieved by means of heterogeneous electron transfer. Typically, electron transfer to or from an electrode results in formation of an excited state of the electrode [18-20]. However, the oxidized form of some luminescent species may be reduced by electrons transferred from the conduction band of an n-type semiconductor, showing evidence for the production of triplet states [21-23]. 

The authors proposed a mechanism based on a cage-mediated guest-to-host electron transfer (Fig. 9.30) in which the cage acted as a photosensitizing molecular flask. Excitement of the coordination cage, followed by electron transfer from alkyne to an electron-deficient cage and the reaction of a molecule of water (solvent) with the obtained phenyl alkyne radical cation, results in benzylic radicals and subsequently the anti-Markovnikov product. 

A special feature of the kinetics of reactions initiated by electron pulses is that in the early stages the reaction proceeds mainly in relatively small confined regions of the solution, known as spurs , within which the concentrations of electrons, radicals and excited molecules are larger by orders of magnitude than those that obtain in the bulk solution. That this is the state of affairs may be expected from the physics of absorption of electrons by liquids sketched above. Much experimental evidence on product yields shows good agreement with a model based on the assumption of equilibrated macroscopic values of rate constants within the spurs, coupled with diffusion-controlled transfer of product molecules to the bulk solution. A brief account of the theory follows. 

According to the electron-transfer mechanism of spectral sensitization (92,93), the transfer of an electron from the excited sensitizer molecule to the silver haHde and the injection of photoelectrons into the conduction band ate the primary processes. Thus, the lowest vacant level of the sensitizer dye is situated higher than the bottom of the conduction band. The regeneration of the sensitizer is possible by reactions of the positive hole to form radical dications (94). If the highest filled level of the dye is situated below the top of the valence band, desensitization occurs because of hole production. 

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