Ferrocene-based nanoelectronics:: Regioselective syntheses and electrochemical characterization of α-monothiol and α,ω-dithiol, phenylethynyl-conjugated, 2,5-diethynylpyridyl- and pyridinium-linked diferrocene frameworks having an end-to-end distance of ∼4 nm

Regiospecific synthetic methods have been developed for the assembly of unsymmetric conjugated molecular frameworks containing 2,5-diethynylpyridyl- and 2,5-diethynylpyridinium-linked diferrocene structures and possessing either mono- or dithioacetate end-groups that are suitable for chemisorption onto Au(III) substrates after conversion to the corresponding thiol derivatives. Electronic spectra and solution electrochemistry of these and model compounds establish the electron-withdrawing character of a 2,5-dimethoxyphenylethynyl substituent on ferrocene that serves to shift the Fe(II)/Fe(III) redox couple to higher potentials. Further, while the unsymmetric nature of the 2,5-diethynylpyridyl bridge in 3 does not differentially perturb the redox couples of the two ferrocenes (Delta E-1/2 < 10 mV), upon methylation, the corresponding pyridinium moiety of 4 now produces a large separation in the two redox potentials (Delta E-1/2 = 190 mV). For the two regioisomeric monothioacetate compounds bearing a terminal 2,5-diethynylpyridyl-linked diferrocene unit, 5 and 6 (and their respective pyridinium counterparts, 7 and 8), redox potentials of the two ferrocenes ate found to be either widely separated or similar in value depending upon the added influence of the 2,5-dimethoxyphenylethynyl group (e.g., Delta E-1/2 = 310 mV in 7 vs similar to 50 mV in 8).