Impact of structure and connectivity of thiophene-based bicycles stacking with fluoroarenes in conjugated materials

Abstract

Noncovalent interactions between aromatic rings can dictate the crystal structures of organic optoelectronic materials. While cofacial interactions of fluorinated and non-fluorinated arenes are useful supramolecular synthons, the impacts of the structural details on these interactions remain unpredictable, particularly with the types of heterocycles common in materials derived from π-conjugated molecules. In this work, a combination of optical spectroscopy and X-ray crystallography demonstrates that both the degree of fluorination of a benzyl ester side chain and the regiochemical connectivity of a benzothiophene (BT) to an arylene-ethynylene backbone impact whether intramolecular cofacial stacking occurs or not. Surprisingly, the thienothiophene (TT) analog, which has an isoelectronic p-system with BT while a stronger electron donor when a part of donor-acceptor π-systems, did not show any evidence of cofacial stacking with fluoroarene side chains, regardless of the extent of fluorination. Quantum-chemical modelling rationalizes the dependence on BT regiochemistry, suggesting that while dispersion interactions comprise the largest individual component of attractive forces in stacking interactions, the strength of electrostatic correlates best with the likelihood of intramolecular stacking interactions occurring in the solids. Finally, many of these molecules show polymorphic behaviour, with examples of blue-shifting and red-shifting mechanofluorochromism. Overall, this work enhances our ability to harness local structural details in deploying non-covalent interactions for designing solid state structures of optoelectronic materials.