From π-π Stacking to Chain Entanglements: Single Crystals of Oligoether-Substituted Thieno[3,2‑b]thiophenes

Knowledge of the molecular arrangement in the solid state is essential for designing high-performance conjugated polymers. The crystal structures and thermal behavior of their monomers provide important insight into their chain conformation and solid-state structure. Here, single crystals of aromatic thieno-[3,2-b]-thiophene units bearing mono- to tetraethylene glycol chains, building blocks of some high-performance organic mixed ionic-electronic conductors, are isolated using a robust crystallization protocol and analyzed using single-crystal X-ray diffraction and thermal analysis. Increasing oligoethylene glycol chain length shifts packing from π-π stacking to chain entanglement, accompanied by a melting temperature decrease from 149 to 41 °C. Molecular dynamics simulations using force fields parametrized with density functional theory show greater crystal stability for shorter chains, consistent with stronger π-π interactions relative to chain entanglement. Single crystals of a more extended conjugated system, thiophene-flanked thieno-[3,2-b]-thiophene with triethylene glycol, show mixed packing motifs and significant disruption of expected S···O interactions, revealing the importance of both side chain and π-π interactions. This work can be anticipated to aid the workup of monomers for synthesis, clarify packing motifs that govern structure-property relationships in conjugated polymers, and enable force-field implementation to guide organic semiconductor design and deepen understanding of their microstructure.