Intra- and intermolecular charge transfer: twin themes and simultaneous competing transitions involving ferrocenes.

Electronic absorption spectra of ferrocene, ferrocenecarboxylaldehyde, butylferrocene, and 1,1'-diacetylferrocene in pure organic polar and non-polar solvents, pure halocarbon solvents and in several hexane-halocarbon solvent mixtures have been recorded. The investigated ferrocenes have shown several intra-molecular electronic transitions of the types pi-pi*, n-pi*, and d-d*. On using protonic solvents (HA) each of the ferocenes (Fc) acquires a proton exported from the solvent to form a complex with the formula [FcH](+)[A](-). However, on using halocarbon solvents each of the ferrocenes has shown an intermolecular charge-transfer-to-solvent (CTTS) which is characterized by the appearance of new absorption spectral band(s) for each ferrocene-halocarbon solvent interaction. Formation constants (K(CT)) and molar absorption coefficients (epsilon(CT)) of these interactions have been determined and discussed. The study has indicated that the observed different transitions are dependent upon the number and nature of the substituents involved in the ferrocenes.