Low and High Molecular Mass Anthraquinone Derivatives Containing Substituents of Varying Electron Donating Properties: Electrochemical and Spectroelectrochemical Properties

Abstract

Two series of donor–acceptor compounds were investigated, consisting of the same anthraquinone acceptor substituted in either position 1 or in position 2 with donors of varying electron donating properties, namely, phenoxazine (Anth-1-Phenox and Anth-2-Phenox), carbazole (Anth-1-Carb and Anth-2-Carb), or diphenylamine (Anth-1-NPh2 and Anth-2-NPh2). In the negative potential range (vs Fc⁺/Fc) all studied compounds exhibited two reversible redox couples corresponding to two 1e reductions of the anthraquinone unit. These reduction processes showed very little dependence on the donor chemical nature and the positional isomerism, yielding E_1/2(0/–1) in the range from −1.33 V to −1.43 V and E_1/2(−1/–2) in the range from −1.75 V to −1.83 V vs Fc⁺/Fc. To the contrary, their redox potentials of the oxidation processes were strongly dependent on the type of donor, decreasing from 0.82 V for Anth-1-Carb to 0.36 V for Anth-1-Phenox. The corresponding potentials measured for 2-substituted anthraquinones were systematically higher by 90 to 210 mV as compared to their 1-substituted counterparts. Anth-1-Phenox and Anth-2-Phenox showed interesting ambipolar properties, undergoing two consecutive reversible 1e reductions at negative potentials and one reversible 1e oxidation at positive ones. The four remaining compounds did not oxidize reversibly. Quantum chemical (DFT) calculations of the HOMO and LUMO energies as well as the ionization potentials (IPs) and electron affinities (EAs) were in legitimate agreement with the experimental data, in each case reflecting the same trend. Equally good agreement was also found between the experimental UV–vis–NIR spectra and the theoretically calculated transitions. None of the synthesized compounds could be electropolymerized. However, fine-quality thin films of p(Anth-2-NPh2) could be synthesized and deposited on a suitable substrate starting from the oxidative chemical vapor deposition (oCVD) of (Anth-2-NPh2). The obtained polymer, p(Anth-2-NPh2), was of an ambipolar nature and showed a relatively narrow band gap (E_g = 1.52 eV). Combined UV–vis–NIR and Raman spectroelectrochemical investigations revealed that the electrochemical oxidation of this polymer thin film can be considered as a two-step process in which the semiquinone radical cation type of structure is formed in the first step, being then transformed into the diiminium dication form in the second one.