Charge-transfer absorption band contact

The addition of small amounts of inert salts such as tetrabutylammo-nium perchlorate (TBAP) or hexafluorophosphate (TBAH) to solutions of charge-transfer salts induces large changes in the intensity of the charge-transfer absorption bands. The magnitude of the salt effect is most pronounced in nonpolar solvents (THF, CH2C12). The monotonic decrease in the CT absorbance with increasing amounts of added TBAP is characteristic of the facile competition for the contact ion pair (42), namely,... 

Finally, care must be exercised in the photochemical activation of contact ion pairs to irradiate only the charge-transfer absorption bands, and not those of the (colored) products. For example, the irradiation of either Cp2Co+ Co(CO)4 or Q+ Co(CO)4 in the presence of PBu3 at wavelengths beyond 510 nm leads only to the dimeric Co2(CO)6(PBu3)2, despite the fact that the CT photochemistry of the same solution at wavelengths below 550 nm leads smoothly to only the disproportionation products. In fact, control experiments demonstrate that the carbonylcobalt dimer arises from a secondary process by the adventitious excitation of the disproportionation salt, namely,... 

The existence of the charge-transfer absorption bands is characteristic of important electron donor-acceptor contributions to the contact ion pair that is the direct precursor in the formation of metal-metal dimers by the mutual annihilation of carbonylcobalt(I) cations and carbonylcobal-tate(—I) anions (79). The diverse results cannot be explained by any single process in which the metal-metal bond for the dimer is formed by the... 

Similar photoinduced dimerizations and ligand substitutions in the presence of additives such as triphenylphosphine are observed with ion-pairs salts of Mn(CO)s and V(CO)6" with cobaltocenium or other cationic acceptors such as Ph2Cr", pyr-idinium, quinolinium, etc [118]. Most importantly, all photochemical transformations of the various carbonyl metallate salts are initiated by actinic light that solely excites the charge-transfer absorption bands of the contact ion pairs whereas local excitation of the separate ions is deliberately excluded. 

Thermal or photochemical activation of the [D, A] pair leads to the contact-ion pair D+, A-, the fate of which is critical to the overall efficiency of donor/acceptor reactivity as described by the electron-transfer paradigm in Scheme 1 (equation 8). In photochemical reactions, the contact ion pair D+, A- is generated either via direct excitation of the ground-state [D, A] complex (i.e., CT path via irradiation of the charge-transfer (CT) absorption band in Scheme 13) or by diffusional collision of either the locally excited acceptor with the donor (A path) or the locally excited donor with the acceptor (D path). 

Hashimoto, S. and Akimoto, H. (1987). Absorption spectra of contact-charge-transfer bands and photochemical reactions of simple alkenes in the cryogenic oxygen matrix. J. Phys. Chem. 91, 1347-1354... 

Independent support for interflavin o-contacts comes from recent chemical studies by Favaudon and Lhoste (13,14). The french authors describe, as already anticipated, a nearby diffusion-controlled dimer formation in aprotic polar medium as the first step in the interflavin contact between oxidized and reduced states, which would finally yield two flavin radicals. This dimer was shown to be not identical in any respect with the well known quinhydrone which can only be obtained in aqueous systems at high flavin concentrations. The long wave band in the absorption spectrum of the new dimer appears to be of charge transfer type, but with a highly reduced half width and better resolved shape than the flavoquinhydrone spectrum. 

---