We present recent spectroscopic results on a group of compounds consisting of ferrocene (Fc) donors and 2,2'-bipyridine-based (B-bpy) acceptors with tetracoordinated boron bridges (see illustration). Such compounds are interesting candidates for materials where doping and/or charge-transfer optical excitation can lead to a variety of interesting electronic interactions, such as (photo-)conductivity, spin-spin interactions and strong correlation phenomena.
For the mono-ferrocene compounds, we analyze the solvent dependence of the visible-range Fc→B-bpy MLCT band (metal-to-ligand charge transfer) using a modified solvatochromism model that takes into account the cationic charge of the solute. This analysis supports the assignment of an MLCT excited state involving almost complete displacement of a unit electronic charge from the Fc donor to the B-bpy acceptor. We have studied the excited-state dynamics of the MLCT state using two-color pump-probe transient absorption spectroscopy, including the effect of solvent polarity on the initial sub-picosecond dynamics and sub-nanosecond back-electron transfer rate.
For the Fc-polymer and di-ferrocene monomer, in addition to the study of the MLCT excited state, we present results of the changes during chemical oxidation of the Fc centers in solution and neat-polymer drop-cast thin films (using iodine, NOPF₆). During oxidation of the polymer, a near-infrared absorption band arises (with λ_max=1780 nm, Δν=4300 cm^-1, ε_max=65 M^-1cm^-1) which we assign to a Fc→Fc⁺ intervalence charge transfer transition between neighboring Fc/Fc⁺ groups along the polymer chain. Based on Marcus-Hush theory, this yields an estimate of the effective interaction parameter of α~0.03 between adjacent Fc/Fc+ groups. Hence, in their oxidized state, these polymers possess both a ground-state and MLCT excited-state mechanism for in-chain conductivity.

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