Injects an electron

If the semiconductor is sensitized with a dye, the process which injects an electron from the dye into the semiconductor has MMCT character. Consider, for example, the dye mentioned above, Cu(PPh3)2dmp, on Sn02. Irradiation... 

As mentioned earlier, a great deal of literature has dealt with the properties of heterogeneous liquid systems such as microemulsions, micelles, vesicles, and lipid bilayers in photosynthetic processes [114,115,119]. At externally polarizable ITIES, the control on the Galvani potential difference offers an extra variable, which allows tuning reaction paths and rates. For instance, the rather high interfacial reactivity of photoexcited porphyrin species has proved to be able to promote processes such as the one shown in Fig. 3(b). The inhibition of back ET upon addition of hexacyanoferrate in the photoreaction of Fig. 17 is an example of a photosynthetic reaction at polarizable ITIES [87,166]. At Galvani potential differences close to 0 V, a direct redox reaction involving an equimolar ratio of the hexacyanoferrate couple and TCNQ features an uphill ET of approximately 0.10 eV (see Fig. 4). However, the excited state of the porphyrin heterodimer can readily inject an electron into TCNQ and subsequently receive an electron from ferrocyanide. For illumination at 543 nm (2.3 eV), the overall photoprocess corresponds to a 4% conversion efficiency. 

The adsorbed sensitizers in the excited state inject an electron into the conduction band of the semiconductor substrate, provided that the excited state oxidation potential is above that of the conduction band. The excitation of the sensitizer involves transfer of an electron from the metal t2g orbital to the 7r orbital of the ligand, and the photo-excited sensitizer can inject an electron from a singlet or a triplet electronically excited state, or from a vibrationally hot excited state. The electrochemical and photophysical properties of both the ground and the excited states of the dye play an important role in the CT dynamics at the semiconductor interface. 

There is another possible mechanism which really deserves the name supersensitization because the charge injection becomes either only just possible or is largely increased by the interaction between dye molecule and supersensitizer. This is the case if the excited dye molecule reacts at first with the supersensitizer and reaches in this way a state where it can inject an electric charge even in the ground state. The "term scheme for this mechanism is shown in Fig. 20. The assumption here is that the excited state of the molecule as a donor cannot inject an electron into the conduction band or can do this only very ineffectively. It can, however, act as an electron acceptor against the supersensitizer. 

In the case of a semiconductor electrode, the existence of the energy gap makes a qualitatively different location of energy levels quite probable (Figs. 23b, 23c). One of them, either the ground or excited, is just in front of the energy gap, so that the direct electron transition with this level involved appears to be impossible. This gives rise to an irreversible photoelectro-chemical reaction and, as a consequence, to photocurrent iph. The photoexcited particle injects an electron into the semiconductor conduction band... 

The convoluted, high-surface-area interface between the Ti02 and the electrolyte solution is an essential characteristic of DSSCs [1-3,5,17,18]. The photoconversion process begins at this interface when the adsorbed dye, D, absorbs a photon and the resulting excited state, D, injects an electron, 2, into the nanocrystalline Ti02 semiconductor ... 

One other approach to the photoelectrolysis of water that has been adopted involves the photosensitization of semiconductor electrodes such as Ti02,362,364,365 SrTi03365,366 or Sn02 265,365,367 by, for example, [Ru(bipy)3]2+. The photochemically excited state of the chromophore injects an electron into the conduction bond of a semi-conductor this is then passed via an external circuit to a platinum electrode for H2 production. The oxidized form of the quencher then forms 02 apparently in an uncatalyzed reaction. Unfortunately, all such systems... 

The primary intermediates are 4-NC and hydroquinone, and the secondary intermediate is benzenetriol. In both oxygenated and deoxygenated systems, the nitrophenolate ion injects an electron into the conduction band of Ti02,... 

Ohmic contacts. An ohmic contact is defined as one which supplies a particular crystal with an infinite supply of either electrons or positive holes. Under an applied field these charge carriers are drawn into the material setting up a space charge. The subsequent currents are thus termed space-charge limited currents. In general the activation energy required to inject a positive hole from an electrode of work function W into a crystal is Ic — W and that to inject an electron W — Ac. Thus for ohmic contacts the conditions to be satisfied for holes and electrons are respectively (12a) and (12b). Although... 

The excited state of the dye may, provided the energies match, inject an electron into the conduction band of the semiconductor, forming an oxidized species D+ which can then accept an electron from a solution redox couple, which is, in turn, re-reduced at the counter... 

Ru+2 complexes readily react with surface histidine residues to form stable derivatives. Photochemical methods were used to inject an electron into the Ru3+ site followed by monitoring kinetics of ET from Ru2+to the metalloprotein active site. 

Nature of charge carriers. Once injected, an electron and a hole must be brought at small enough distance to allow their recombination. This will depend on the nature and transport properties of the charge carriers. All CPs used in EL are nondegenerate ground-state polymers. The relevant notions of polaron and bipolaron and their relative stability were introduced in Chapter 11, Section IV.C. Some characteristic times pertinent to polaron or bipolaron formation will be discussed first, then the influence of traps. 

When connected to other components of a supramolecular assembly or to a semiconductor electrode through a polypyridine ligand, a metal-polypyridine unit is kinetically especially suited to inject an electron from its MLCT excited state, that is to act as an excited state reductant. Ultrafast rates can be reached. 

More direct ET into the oxidase has been initiated photochemically with an artificial electron donor, tris(2,2 -bipyridyl)ruthenium [17], electrostatically bound to the enzyme, and with Ru-labeled cytochrome c [18]. An objection to these types of experiments is, of course, that they involve unatural electron donors, whose site of interaction with the oxidase may be different from that of cytochrome c itself. This objection does not apply to experiments in which the strongly reducing triplet state of bound Zn-cytochrome c is generated photochemically to inject an electron into the oxidase [19], since this derivative of cytochrome c has the same structure and has been shown to bind to the same site as the native protein [20],... 

Figure 41. Two charge-hopping mechanisms. The donor injects an electron (or hole) into the bridge which consists of discrete redox units, (a) The bridge units are nearly degenerate. Consequently, the injected electron (or hole) moves randomly and reversibly up and down the bridge, finally becoming irreversibly trapped by the acceptor, (b) The bridge units constitute an ordered redox cascade the electron or hole moves essentially irreversibly along the bridge towards the acceptor.

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