Does Marcus-Hush theory really work? Optical studies of intervalence transfer in acetylene-bridged biferrocene monocation at infinite dilution and at finite ionic strengths

Robert L. Blackbourn, Joseph T. Hupp*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

76 Scopus citations

Abstract

Intervalence charge-transfer data for acetylene-bridged biferrocene monocation (Bf+) have been collected in five solvents in the presence and absence of excess electrolyte and in the limit of infinite chromophore dilution. The study was motivated by earlier work which demonstrated that the intervalence absorption maximum for Bf+ in methylene chloride could vary substantially with both chromophore concentration and added electrolyte concentration. In the present study similar (but smaller) variations are found in other solvents. The variations are ascribable to both ion pairing and higher order ionic association. For the available solvents the logarithms of the Bf+X- pairing constant (X- = PF6 -, BF4 -, or ClO4 -) is found to vary inversely with the solvent's static dielectric constant, Ds. When the ion-pairing effects are eliminated by dilution, a genuine test of the Marcus-Hush prediction of the solvent dependence of the intervalence charge-transfer energy (Eop MMCT) is possible. For highly polar solvents (Ds ≥ 20) a good fit of Eop MMCT to 1/Dop - 1/Ds is found (where Dop is the optical dielectric constant and 1/Dop is the main variant in the two-term dielectric parameter). For methylene chloride, however, Eop MMCT falls well below the best-fit line for five other solvents. The deviation is tentatively attributed to dielectric saturation and is expected, therefore, to be manifest in other media possessing limited dielectric strength. In light of these findings, a speculative interpretation of pressure-induced solvent freezing effects is offered. It is suggested that, in frozen CD3CN, Eop MMCT is simultaneously subjected to ion-pairing perturbations and to dielectric saturation effects and that the effects act in an offsetting, largely compensatory fashion. The speculation is supported by semiquantitative extrapolations from known liquid solvent behavior and by an analysis of intervalence bandwidths.

Original languageEnglish (US)
Pages (from-to)1788-1793
Number of pages6
JournalJournal of physical chemistry
Volume94
Issue number5
DOIs
StatePublished - Jan 1 1990

ASJC Scopus subject areas

  • Engineering(all)
  • Physical and Theoretical Chemistry

Fingerprint Dive into the research topics of 'Does Marcus-Hush theory really work? Optical studies of intervalence transfer in acetylene-bridged biferrocene monocation at infinite dilution and at finite ionic strengths'. Together they form a unique fingerprint.

Cite this