(Multi)ferrocenyl Five-Membered Heterocycles: Excellent Connecting Units for Electron Transfer Studies

The electro- and spectroelectrochemical behavior of diverse (multi)ferrocenyl five-membered heterocyclic compounds, including furan, thiophene, pyrroles, phospholes, etc. is discussed, giving a close insight into the electron transfer processes of these organometallic compounds in their mixed-valence state, whereby electronic and structural modification of the heterocyclic connecting units and/or of the redox-active ferrocenyl termini directly influences the electron transfer properties. In addition, the structural features of these compounds can be correlated with their electrochemical behavior, allowing calculation of the effective electron transfer distance within a specific series of molecules. Tendencies in molecular wire molecules based on bi-, ter-, quarter-, quinque-, and sexithiophene connecting building blocks and the appropriate pyrrole derivatives are discussed as well. The consequences of introducing an additional redox-active transition- metal building block, such as a titanocene or a zirconocene moiety, respectively, into the heterocyclic ring, on the electrochemical behavior of the resulting five-membered heterocycles are also highlighted.