Thexi states

Those organometallic thexi states which have been detected have involved compounds where the quantum yield for photodissociation is very low. Time-resolved uv-visible absorption and emission studies have been made on W(CO)5L and W(CO)4L species (L = acetylpyridine, L = o-phenanthroline) (54), but, as in the case of intermediates, these studies provided lifetimes but no structural information. 

Here, C(s) is a solid reduced form of A, and the double line denotes a liquid junction of negligible potential. Also, A is a thermally equilibrated excited (or thexi) state and, as such, is essentially a different chemical species from A. We suppose, therefore, that it is possible to find an electrode M that is reversible to the reduction of A to C(s), but completely polarized with respect to the reduction of A to C(s). Operation of this cell under steady state conditions should then give the desired reversible work available from the photoproduction of A. ... 

Some future directions in inorganic photochemistry have been outlined by Adamson (56). A pessimistic picture of the practical uses of solar energy conversion systems is painted, but a rosy view of the academic future of the subject is held. It is anticipated that there will be further examination of thermally equilibrated excited (thexi) states—their lifetimes, and spectroscopic and structural properties—and an extension of present efforts to organometallics and metalloproteins is also envisaged (56). The interpretation of spectroscopic data from excited states will continue to be controversial and require future experimentation (57). 

Thext States and DOSENCO States 7.3.2.2.1 Thexi states... 

In solution, the excess vibrational energy following an FC transition is lost very quickly — there are indications that only a few picoseconds are needed for the complex to come to thermal equilibrium with the medium with respect to vibrational excitation.19 We speak of the thermally equilibrated excited state, or, as an abbreviation, of the thexi state. Photochemical and photophysical processes very often involve thexi states. 

A final point is that the energy of a thexi state will be less than that of the preceding FC state. The former has been difficult to estimate, but it can be seen from Figures 4 and 6 that it should... 

There is a possibility that an FC state will react before complete thermal equilibration. In the case of diatomic molecules, the process is usually known as predissociation — a dissociative state crosses the excited state potential surface. The situation is more complicated in the case of a coordination compound, but one can imagine an FC state relaxing along some nuclear coordinate leading to bond breaking. A state capable of such a process has been called a DOSENCO state, an acronym for Decay On SElected Nuclear Coordinates .21 The same authors use the term DERCOS (DEcay via Random Coordinate Selection) for a thexi state. 

The central process in photochemistry is that of chemical reaction, ordinarily from a thexi state. The quantum yield, , is defined by equation (4), where n denotes the moles of reaction (corrected for any ground state or thermal reaction) and is the einsteins, that is, moles of light quanta absorbed. 

If A is a thexi state, its reactions should obey conventional chemical kinetics, and we can examine several simple, important cases. Suppose firstly that A is produced by a flash or laser pulse technique in a time short compared to the time scale of the other processes. The produced A will disappear with a rate constant k which is the sum of the rate constants for all applicable processes. In the absence of quencher, we write k° = knr + kT + kcr the time for [A ] to decrease by a factor of e, r°, is just jk°. With quencher present, we have k = knr + kT + kCT + fcq[Q] and i = 1 jk. The ratio of lifetimes in the absence and presence of quencher is given by equation (10). A plot of t°/t versus [Q] should thus be linear, with slope kqr° this product is often designated as Kgy and called the Stem—Volmer constant. 

Very little is known about the excited-state properties of cA-Ru(dcb)2(NCS)2 anchored to semiconductor surfaces. However, the photophysical properties of cis-Ru(bpy)2(ina)2 ", where ina is isonicotinic acid, 4-COOH-pyridine, on nanocrystalline Ti02 and Zr02 surfaces were recently reported. An intriguing observation was that the activation energy for internal conversion from the thexi state to the ligand field states increased dramatically compared with fluid solution [129]. Similar behavior might be expected for cw-Ru(dcb)2(NCS)2. 

Among the chromium(III) octahedra discussed here, the emission band is uniformly narrow, showing that the vibrational extension of the thexi state is projected down on the part of the A2 potential surface running almost strictly parallel with the potentialsurface (in spite of both being U-levels in and hence containing... 

In a mobile solvent, the amount of solvent movement depends upon the relative rates of solvent motion and depopulation of the excited state. For the long-lived E and T, states, there is ample time for the solvation shell to adjust to the equilibrium geometry. The situation in the shorter-lived T2 and Ti states is more problematical. Prompt intersystem crossing is probably too fast for appreciable solvent motion, even in a low viscosity medium. Whether, the same is true in the thexi T2 is uncertain. No reliable estimates of rii are available and the lifetime of the thexi state is unknown. 

The lifetime of the thexi state of Ru(bpy)32+ is 1 ps in water at room temperature.31 The radiative rate constant (kr 105 s-1) is typically about two orders of magnitude smaller than the nonradiative rate constant (km 107s-1) and hence the excited-state lifetime is controlled by the latter.31 Ru(II) and Os(II) polypyridyl excited states have been shown to follow Jortner s energy gap law, wherein the nonradiative rate constant increases exponentially with decreased energy separation... 

MLCT Excited States on Ti02 Experimental studies of MLCT states on Ti02 (and other semiconductors) are few mainly because of rapid interfacial charge separation that shortens their lifetimes considerably. Some aspects of MLCT excited states anchored to nanocrystalline Ti02 thin films are now becoming available through studies where the semiconductor acceptor states lie above (toward the vacuum level) the excited-state reduction potential of the sensitizer such that excited-state electron transfer from the thexi state is unfavorable. 

FIGURE 12.17 An idea of how ultrafast excited-state injection could be used to reduce electron acceptors that would be energetically unfavorable for the thexi state. An example of this was realized with Ru and Os sensitizers with 4,4 -(COOH)2-2,2 -biquinoline ligands. These ligands were reduced prior to Ti02, a necessary condition for realization of this behavior. 

The Thermally Equilibrated Excited (Thexi) State Chemistry of Some Co(III) Ammines... 

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