Transfer Absorption Spectra

The concept of optical CT in metal complexes has been pioneered by Jorgensen [19]. CT transitions are classified according to the redox sites 

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Fig. 1 Charge-transfer absorption spectra of enol silyl ethers complexes with re-acceptors. (a) Spectral changes accompanying the incremental additions of cyclohexanone enol silyl ether [2] to chloranil in dichloromethane. Inset Benesi-Hildebrand plot, (b) Charge-transfer absorption spectra of chloranil complexes showing the red shift in the absorption maxima with decreasing IP of the enol silyl ethers, (c) Comparative charge-transfer spectra of EDA complexes of a-tetralone enol silyl ether [6] showing the red shift in the absorption maxima with increasing EAs of the acceptors tetracyanoben-zene (TCNB), 2,6-dichlorobenzoquinone (DCBQ), chloranil (CA), and tetracyanoqui-nodimethane (TCNQ). Reproduced with permission from Ref. 37.

To present the equations relating half-wave reduction potentials and charge transfer absorption spectra to electron affinities. 

Fig. 12. Band analysis of the (rans-[IrCl4F2]2 charge transfer absorption spectrum (10 K in KC1 discs) by deconvolution into Gaussians (points, experimental, solid line superposition of components)...

Fig. 5. Charge-transfer absorption spectrum of 3.5 mMCo(CO)3(PBu3)2+ Co(CO)4 (A) Solvent effect in THF, CH2C12, MeCN, and Et20. (B) Salt effect of added TBAP (top to bottom) none, 1, 2, 3, 4, 5, 10, and 20 equivalents, relative to 10 mM Co(CO)3(PBu3)2+ Co(CO)4 in THF (27).

Charge-transfer absorption is important because it produces very large absorbances, providing for a much more sensitive analytical method. One important example of a charge-transfer complex is that of o-phenanthroline with Fe +, the UV/Vis spectrum for which is shown in Figure 10.17. Charge-transfer absorption in which the electron moves from the ligand to the metal also is possible. 

The occurrence of energy transfer requires electronic interactions and therefore its rate decreases with increasing distance. Depending on the interaction mechanism, the distance dependence may follow a 1/r (resonance (Forster) mechanism) or e (exchange (Dexter) mechanisms) [ 1 ]. In both cases, energy transfer is favored by overlap between the emission spectrum of the donor and the absorption spectrum of the acceptor. 

UV-vis spectra obtained for W0x-Zr02 samples, (NH4)6H2Wi204o, and bulk WO3 standards are shown in Figure 6. The absorption spectrum for (NH4)6H2Wi204o exhibits two bands with maxima at 4.9 and 4.0 eV. The band at 4.9 eV has been assigned to electron transfer from 0 to W in 0=W species, based on previous studies for MoOx samples [20]. The band at 4.0 eV probably reflects similar processes in W-O-W species. Individual bands are not apparent in bulk WO3 because of its polymeric nature and wide range of W-O-W distances. 

The rhodamine B-bound complex of Ir1 (387) shows only minor alterations in the absorption spectrum of bound rhodamine B as opposed to free dye however, its fluorescence is strongly quenched.626 Fluorescence is intense when the rhodamine dye is attached to an Ir111 center. The authors conclude that the excited-state quenching mechanism is via electron transfer. 

Porter and Wilkinson(56) measured the rates of quenching for a variety of triplet donors with triplet acceptors at room temperature in fluid solution by flash photolysis. The appearance of the triplet-triplet absorption spectrum of the acceptor and the simultaneous disappearance of the donor triplet-triplet absorption spectrum provided unequivocal evidence for the triplet-triplet energy transfer process. Table 6.5 provides some of the quenching rate constants reported in this classic paper. 

As examples. Table 8 records some observations on d—d and charge transfer absorption bands in metal/protein systems. The examination of the spectrum of cobalt carbonic anhydrase (d—d) and of iron conalbumin (charge-transfer) permitted a prediction of the ligands from the protein to the metal. The predictions have now been substantiated by other methods. 

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