Charge transfer complexes spectra! features

Figure 47(b) shows the difference spectrum of TCNE at Pt in TBAF/MeCN electrolyte for modulation between + 0.25 V (vs. Ag/Ag+) base potential and - 0.25 V working potential. The spectrum is dominated by the two negative features at 2187 and 2148 cm 1. The latter band was reported as 104 x greater than would be predicted for a simple Beer s Law calculation, using the concentration of substrate present [105]. The former feature was assigned by the authors, on the basis of work by Devlin and co-workers [106], to the C = N stretch enhanced via the formation of a charge-transfer complex between the anion radical and a surface Pt atom. The more intense feature at 2148 cm 1 was attributed to the solution-free C = N fundamental stretch enhanced by the formation of an electron donor/acceptor complex between the anion radical and neutral TCNE. Further studies were carried out on the TCNE system [56] and these demonstrated considerable complexity in the system. 

The dotted lines indicate a weak interaction, that is, possibly not full-fledged bonds. The term charge transfer implies that, although electron transfer or bond formation does not happen in the ground state, an electron does jump from one component to the other in an excited state, giving rise to characteristic features in the optical spectrum. At low temperatures, charge-transfer complexes with even stronger nucleophiles such as phosphines may be isolated ... 

Cationic ferrocene complexes with one, two, and four cationic [B(R)bpy] (bpy = 2,2,-bipyridine) acceptors such as 66 show absorption at Amax = 496-540 nm with the contribution of charge transfer between the ferrocene unit and the B(R)bpy substituent(s) (165). This is confirmed by the EPR spectrum of the monoreduced neutral species, which features a line shape indicating a considerable admixture of the ligand and metal orbitals. Preparation and physical properties of the related polymer, 67, have also been reported (166). 

The other important aspect in dye-sensitized solar cells is water-induced desorption of the sensitizer from the surface. Extensive efforts have been made in our laboratory to overcome this problem by introducing hydrophobic properties in the ligands (48-53). The photocurrent action spectra of these complexes shows broad features covering a large part of visible spectrum and displays a maxima around 550 nm, where the monochromatic IPCE exceeds 80%. The performance of these hydrophobic complexes as charge-transfer photosensitizers in nanocrys-... 

The uv-visible spectrum of the slow band is compared with that of its molybdate, tungstate, and Fe(III) complexes in Figures 4, 5, and 6 respectively. The dominant features of the complex spectra arise from charge transfer bands the absorption maxima are listed in Table IV. 

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).

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