Optical spectroscopy charge-transfer transitions

In this paper we will describe and discuss the metal-to-metal charge-transfer transitions as observed in optical spectroscopy. Their spectroscopic properties are of large importance with regard to photoredox processes [1-4], However, these transitions are also responsible for the color of many inorganic compounds and minerals [5, 6], for different types of processes in semiconductors [7], and for the presence or absence of certain luminescence processes [8]. 

Most charge-transfer transitions show less vibronic resolution than the examples in Figure 2. Resonance Raman spectroscopy has often been used in these cases to analyze the structural changes between the initial and final states of the transition, an approach especially relevant to metal centers in enzymes and to bioinorganic model compounds. The full ensemble of optical spectroscopic techniques has been applied to the study of the lowest-energy metal-to-ligand charge-transfer (MLCT) bands in Ru(bipyridine)3 and related complexes. Other well-studied cases of MLCT transitions with resolved vibronic structure include a number of W(CO)sL complexes. "... 

In closing this paragraph we mention the emission spectroscopy of the U6 + ion. This ion shows emission spectra with clear vibrational structure. It is mentioned here because the optical transition is of the charge-transfer type, and the U6+ ion may be described in the present context as a 5f° ion. The vibrational structure in the emission spectra depends strongly on the ligands of... 

Electro-absorption (EA) spectroscopy, where optical absorption is observed under the application of an electric field to the sample, is another method that can distinguish between localised and inter-band excitations. The electric field produces a Stark shift of allowed optical absorptions and renders forbidden transitions allowed by mixing the wavefunctions of the excited states. Excitons show a quadratic Stark (Kerr) effect with a spectral profile that is the first derivative of the absorption spectrum for localised (Frenkel) excitons and the second derivative for charge transfer excitons, i.e. 

It usually turns out that there are several such vibrations. They will help electron transfer from A to B. The reason is obvious e.g., the empirical formula for Vjip says that a vibration that makes the AB distance smaller will increase the transfer probability. This could be visible in what is known as resonance Raman spectroscopy close to a charge transfer optical transition. In such spectroscopy, we have the opportunity to observe particular vibronic transitions. The intensity of the vibrational transitions (usually from u = 0 to u = 1) of those normal modes that facilitate electron transfer will be highest. 

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