Optical charge transfer

Much of chemistry occurs in the condensed phase solution phase ET reactions have been a major focus for theory and experiment for the last 50 years. Experiments, and quantitative theories, have probed how reaction-free energy, solvent polarity, donor-acceptor distance, bridging stmctures, solvent relaxation, and vibronic coupling influence ET kinetics. Important connections have also been drawn between optical charge transfer transitions and thennal ET. 

Fig. 20 The proposed mechanism of ESPT incorporating a solvent-polarity-induced barrier in protic solvents following optical charge transfer and solvent relaxation. See full name of each compound in text (reprint from ref. [141], Copyright 2008 Wiley-VCH)...

A theoretical formalism is available for understanding optical charge transfer processes in a variety of chemical systems (mixed-valence ions, donor-acceptor complexes, metal-ligand charge transfer chromophores, etc) where the extent of charge transfer is large and where electronic coupling between the electron donor and acceptor sites is relatively small. 

Optical charge transfer (CT) is commonly observed in un-symmetrical molecules or molecular complexes in which there are sites of distinctly different ionization energies and electron affinities. The origin and properties of optical charge transfer transitions provide the basis for this account. A convenient place to begin chemically is with mixed-valence compounds and two examples are shown below (1-3). In the first (eq 1), the sites of different oxidation states are held in close... 

Eqs 3-5 provide a relatively simple theoretical basis for understanding optical charge transfer and the relationship between optical and thermal electron transfer. One implication... 

With the limitations above noted, let us return to the application of eqs 3-5 to IT transitions and to optical charge transfer (CT) in general. These equations, in fact, prove to be remarkably successful in providing a basis for understanding optical CT processes in a number of chemical systems. It was suggested above that the vibrational term, x> has both intramolecular and medium contributions. From dielectric loss and related measurements, the collective vibrations of the medium occur at low frequencies for most solvents, the energy spacings between levels are small, and equations based on the classical... 

Eq 7 calculates the energy difference arising from the medium between the thermally equilibrated mixed-valence ground state and a vibrationally nonequilibrium, mixed-valence excited state. The value of Ae depends on the nonequilibrium state 1) For optical charge transfer, Ae = e, the unit electron charge. 2) For thermal electron transfer between chemically symmetrical sites, Ae = e/4. 3) For a chemically unsymmetrical electron transfer... 

General Discussion—Optical Charge-Transfer Transitions... 

Mechanistically the colour is formed by an optical charge-transfer between metal centres in the solid-state lattice, e.g. in tungsten trioxide this involves partial reduction of the pale yellow W" to the blue state. This reduction requires partial insertion of a balancing cation, as shown schematically in Figure 1.32, where M is usually lithium or hydrogen. 

Fig. 3. Relationship between the electron transfer pathways for deactivation of the excited [2, 2, 3] complex and the optical charge transfer transitions.

Complexes I 20) and II (68) are unstable with respect to internal electron transfer, and the rates of the thermal reactions have been measured complex III (2e) is stable with respect to electron transfer. Complex IV is symmetrical and presumably subject to rapid internal transfer (103). Optical charge transfer has not been detected in any of these systems. In the case of the two cobalt(III) complexes, comparison with the data of Table III suggests that the bands should be... 

It is normally unnecessary for the electrochemist to be concerned with the mobility of carriers in most of the semiconductors whose properties have been studied, since the very low conductivity of "small polaron samples would normally preclude their measurement. However, a proviso must be entered here in the case of binary and, more especially, ternary samples. It may well be the case that the majority carriers in a particular material are indeed itinerant (i.e. have mobilities in excess of ca. 1 cm2 V 1s 1), but there is no guarantee that this will be true of the minority carriers generated by optical absorption. Thus, the oxide MnTi03 shows a marked optical charge transfer absorption from Mn(II) to Ti(IV), the latter being the CB. The resultant holes reside on localised sites in the Mn levels, presumably as local Mn(III) centres, and are comparatively immobile. The result is that there is... 

Fig. 20. Electronic spectrum of a rhenium(I) carbonyl complex (23) featuring an optical charge transfer transition at Xj =lW nm involving a coordinated quinone acceptor ligand (181).

The expressions derived from the traditional two-state model are useful in rationalizing a variety of electron transfer processes. Both thermal and optical charge transfer can be treated and, although not discussed here, electrochemical processes... 

Characterized by optical charge transfer band S-to-bead... 

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