Thermally equilibrated excited

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. ... 

SM = iAM2 a measure of the distortion of the acceptor vibration in the excited state. A is the dimensionless, fractional displacement in normal vibration M between the thermally equilibrated excited and ground states. It is related to AQeq by A AQe ( )l/2, where M is the reduced mass for the vibration. 

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). 

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. 

Initially excess vibrational energy is rapidly lost by radiationless processes, such as collision with solvent molecules, to give the thermally equilibrated excited singlet molecules S t. This has a short lifetime ( 10 8 s) and may then lose its energy by any of the processes 2-5 below. 

Figure 2. Generalized excited state diagram. M represents a ground-state molecule, M and M are the Franck-Condon and thermally equilibrated excited states, respectively, hv is the excitation energy, Eqq the excited state energy, and tq is the inherent excited state lifetime. Relevant ground-and excited state redox couples are shown.

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

Because of the several qualitative diflFerences between the concepts of ligand field and thermally equilibrated excited states, it is useful to develop a distinguishing vocabulary. Conventional ligand field excited states will be called Franck-Condon states since the energies are those of band maxima and the transitions between them are essentially those vertical ones with maximum Franck-Condon overlap. The abbreviation thexi state has been proposed (12) for a thermally equilibrated excited state. The gist of the foregoing is that conventional ligand field theory treats hypothetical electronic excited states which are in reality Franck-Condon states, whereas it is thexi states that are important in photophysical and photochemical processes. New theory or new extensions of present theory are clearly needed to treat the latter type of state. 

Next, there are several indications that the photoreactive species is not in a Franck-Condon state, but rather it is in a thermally equilibrated excited state. One indication is that quantum yields as well as the nature of the photoreaction do not vary appreciably as the irradiating wavelength traverses the width of a ligand field band (although variations may occur on going from one band to another) (13, 14, 15). It appears that a common reactive state is reached, regardless of the degree of vibrational excitation of the initially produced Franck-Condon state. The simplest explanation is that this common state is a thexi state. 

When heavy atoms are introduced into the framework of a molecule, marked changes in the light emission characteristics occur. In particular, for complex molecules which contain metal ions from the second and third transition series, only a single emission from a set of thermally equilibrated excited levels is generally observed (J-7). This behavior is apparently associated with a spin-orbit coupling eflFect, and, indeed, it has been suggested that this effect is so large as to render spin labels inappropriate in second- and third-row metal complexes (8). 

For most practical systems (solid, liquid, and atmospheric pressure gaseous phase) vibrational relaxation occurs in the picosecond time scale. Since most of the interesting chemistry and physics that takes place in electronically excited states occurs on a much longer timescale (see below), thermally equilibrated excited states should he considered as the only relevant intermediates in photochemistry, regardless of the initial amount of vibrational excitation with which they may have been created. 

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