Charge-transfer absorption band definition

In many reactions of MA, e.g., photochemical, radical, Diels-Alder, and polymerizations, the intermediacy of a charge-transfer (CT) complex is invoked. By this is meant rapid and reversible interaction of two molecules in solution, the presence of which could be detected by the appearance of a broad intense band in the visible or ultraviolet absorption spectrum at wavelengths longer than those due to the individual components. In other words, this definition really defines complexes showing charge-transfer absorption.This method-limited definition could be misleading. For example, we also know that in some cases complexes may be inferred e.g., in case of MA acetone, but no distinct UV or visible absorption may be observed.Mulliken has used the term electron donor-acceptor (EDA) complexes. ... 

The charge transfer argument appears to be more logical. An article on calculated energy levels of Ru-doped Ti02 supports this approach " . CrNiSbTi-DR pigment shows a shift of the absorption band intermediate between that of a NiSbTi-DR and a CrSbTi-DR, and does not appear to be a composite of two separate absorption bands. However, more definitive evidence is needed. 

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