Oxidation Pathways Involving a Sulfide-Endcapped Donor–Acceptor–Donor π‑Conjugated Molecule and Antimony(V) Chloride

The oxidation pathways and products of a discrete, sulfide-endcapped donor–acceptor–donor (D/A/D) molecule, namely, propylenedioxythiophene–benzothiadiazole–propylenedioxythiophene, are investigated. The electrochemical and chemical oxidations proceed by two distinct routes. Specifically, electrochemical oxidation undergoes a sequential two-step, one-electron (1e–) oxidation route with a 117 mV difference between consecutive half-wave potentials. In contrast, chemical oxidation by antimony­(V) chloride (SbCl5) causes the generation of four different oxidized species: (a) the 1e– oxidation state, (b) a decomposition product, (c) the 2e– oxidation state, and (d) a chloride adduct of the 2e– oxidation state. The decomposition product is generated by the reaction of the 1e– oxidation state with residual water, resulting in nucleophilic aromatic substitution at the sulfide group terminal positions. This reaction leads to the formation of a 2e– oxidized, oxygen atom (ketone) terminated decomposed molecule. The chloride adduct is determined to be produced by electrophilic chloronium ion (2e–) oxidation by the SbCl4+ complex, which is a product of SbCl5 ligand disproportionation. The formation of the 2e– oxidized chlorine adduct shows to be linearly dependent on the molarity of SbCl5 in dichloromethane, giving new insight into the concentration dependent reactivity of SbCl5 as a 2e– oxidant. The electronic, optical, and magnetic properties and geometric structures of the 1e– and 2e– oxidized hexachloroantimonate salts are fully characterized by a combination of electrochemistry, X-ray crystallography, UV–vis–NIR, electron paramagnetic resonance, NMR spectroscopies, and density functional theory calculations. The aim of this study is to provide a thorough understanding of the redox pathways of a D/A/D π-conjugated organic molecule for potential application in organic electrochromic devices.