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.