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 π–π*, n–π*, 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 (KCT) and molar absorption coefficients (ɛCT) 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.

Graphical abstractElectronic absorption spectra of ferrocenes in mixed solvents (hexane-chlorinated solvent) have performed amazing behaviour. On increasing the concentration of the chlorinated solvent and decreasing the hexane concentration the band characterizing to MLCT (n–π* intra-transition within the ferrocenyl moieties) in hexane is gradually diminishing meanwhile the other new band which is characterized to the CTTS (charge-transfer-to-solvent) is growing with increasing of its intensity. The mobile electrons of the iron(II) atom of the ferrocenes are reoriented from being transferred to the cyclopentadienyl rings to be transferred toward the chlorinated solvent. Undoubtedly, CTTS wins the competition with MLCT.Figure optionsDownload full-size imageDownload as PowerPoint slide