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. 

Getting back to photochemistry, photochemical reactions are kineti-cally controlled conversions ubiquitous in nature where phenomena far from equilibrium are the rule, rather than the exception. They are generally categorized into two groups those from equilibrated excited molecules (with reactive species with lifetimes usually in nanoseconds or... 

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. 

A related view of the physical basis of the solvent coordinate is the well-known reaction field R, which is the field located on the solute dipole due to the dipole induced solvent polarization. For example, the reaction field for the equilibrated excited state dipole Rlq is given by... 

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. 

According to the Franck-Condon principle, the photoexcitation triggers a vertical transition to the excited state, which is followed by a rapid nuclear equilibration. Without donor excitation, the electron transfer process would be highly endothermic. However, after exciting the donor, electron transfer occurs at the crossing of the equilibrated excited state surface and the product state. 

The expression given in Eq. (10) for the work assumes that p = 0, where p is the ionic strength of the medium. AG is the free-energy of the equilibrated excited-state (AG AE00), rD and rA are the molecular radii of the donor and acceptor molecules, e5 is the static dielectric constant or permittivity of the solvent, and z is the charge on each ion. ss is related to the response of the permanent dipoles of the surrounding solvent molecules to an external electrical field. Equation (9), the Bom equation, measures the difference in solvation energy between radical ions in vacuo and solution. 

To a significant extent, the vibrationally equilibrated excited states (VEqES) can be treated as a well-defined thermodynamic system. The molecular geometry, the solvation environment, and so forth can, in principle, be inferred from the emission band shape (e. g., as in Eqs. 17 and 18) or they can be probed by the use of resonance Raman and time-resolved Raman and infrared techniques. Typically, the VEqES is a better oxidant and reductant than the ground state, and this is a very important aspect of the chemistry of charge-transfer excited states. 

Vibrationally Equilibrated Excited States Relaxation Processes... 

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.

In the light of the rapid excited-state relaxation dynamics and the observed <100 fs electron injection component, the fast injection component must occur from non-equilibrated excited states. Distinct singlet and triplet electron-injection pathways have been observed for Ru polypyridyl complexes on SnOa (Iwai et al, 2000) and TiOi (Benko et al, 2002 Kalloinen et al, 2002). In a detailed study of electron injection dynamics in N3/TiOi (Benko et al, 2002 Kalloinen et al, 2002), 55% of electron injection was shown to occur with 50 fs injection time from singlet MLCT... 

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

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