Tuning iron-amine electronic coupling by different aromatic bridges based on ferrocene-ethynyl-triarylamine systems

Abstract

The syntheses of a series of ferrocene (Fc)–ethynyl–triarylamine (TAA) inorganic–organic mixed-valence systems with different aromatic bridge linkers (Aryl = 1,4-phenyl [1], 1,4-naphthyl [2], 9,10-anthryl [3]) are reported in this study. These systems were accessible with good yields through the classical Sonogashira cross-coupling reactions of brominated intermediates, 1a, 2a, and 3a, with ethynylferrocene, respectively. Target compounds 1–3 were fully characterized by nuclear magnetic resonance, elemental analysis, and single-crystal X-ray diffraction analysis. Redox and electronic coupling properties were investigated by anodic voltammetry, UV–visible–near-infrared (NIR) spectroelectrochemistry, and density functional theory (DFT) calculations. Experimental results revealed that the first anodic steps of compounds 1–3 occur in ethynylferrocene units and the second redox process comes from triphenylamine moieties. This result is in accordance with the spin density distribution results. The Fe–N electronic coupling of singly oxidized species 1+–3+ observed by characteristic NIR bands proved the predicted electronic excitation of TAA to Fc from the time-dependent DFT calculations. The degree of electron coupling in 1+–3+ decrease with the increase in aromatic conjugation (phenyl, naphthyl, and anthryl).