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.