A Thermochemical and <scp>TDDFT</scp> Exploration of the Electropolymerization of Thiophene

The electropolymerization of thiophenes can yield high-quality polymer films with tunable properties and a wide array of potential applications. Nevertheless, despite decades of research, the growth mechanism, which involves a complex interplay of interfacial processes including heterogeneous electrochemical reactions, homogeneous chemical reactions, and oligomer solubility, is not well understood. Here, we use experimentally validated density functional theory calculations to survey the geometry, electron distribution, redox, and electronic absorption characteristics of a dataset of thiophene species involved in electropolymerization. The landscape of possible 3-alkylthiophene isomers up to terthiophene is explored, considering three possible redox states with predicted redox potentials. Free energy calculations support radical-radical coupling as the more favorable mechanism, at least in the early stages of polymerization. Time-dependent density functional theory reveals energies and oscillator strengths that can be used for experimental elucidation of the mechanism, for example, via time-resolved spectroscopy, and explains broad absorption features in spectroelectrochemical data. Energetic implications for the formation of branched and regioregular polymers are drawn with new insight into how regioregularity might be achieved via electropolymerization.