From π–π Stacking to Chain Entanglements: Single Crystals of Oligoether-Substituted Thieno[3,2- <i>b</i> ]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.