Abstract
A set of tetracyanobutadiene (TCBD)- and dicyanoquinodimethane (DCNQ)-bridged ferrocenyl dimers 5–8 were designed and synthesized by the [2 + 2] cycloaddition–retroelectrocyclization reaction of diferrocenyl complexes 3 and 4 with tetracyanoethylene (TCNE) and 7,7,8,8 tetracyanoquinodimethane (TCNQ), respectively. The effect of constitutional isomers (para vs meta) and different acceptors on their donor–acceptor interactions and photophysical and redox properties as well as electronic structures was evaluated using a variety of experimental and theoretical methods. The single-crystal X-ray structures of TCBD- and DCNQ-bridged ferrocenyl dimers 6 and 7 are reported. The DCNQ-bridged ferrocenyl dimers 7 and 8 have lower HOMO–LUMO gap values with red-shifted absorption bands in comparison to those of TCBD-bridged ferrocenyl dimers 5 and 6. Mössbauer spectra of 3–8 are suggestive of very similar isomer shifts and quadrupole splittings in all diferrocene complexes despite their different proximities to the electron-withdrawing fragment. Spectroelectrochemical data on 5–8 are suggestive of the presence of ferrocene-centered HOMOs in these compounds as well as a lack of electronic coupling between ferrocene groups.