Polymorph-Dependent Emission in Cyano Group-Substituted Thiophene/Phenylene Co-Oligomer Crystals

This paper presents the preparation and optical characterization of two crystal polymorphs (light-blue- and green-emitting crystals) of 2,5-bis-(4'-cyanobiphenyl-4-yl)-thiophene (BP1T-CN), a cyano-substituted thiophene/phenylene co-oligomer (TPCO) that has not been previously reported. Light-blue-emitting triclinic and green-emitting monoclinic crystals were fabricated via physical vapor transport (PVT) and solution-growth methods, respectively. Structural analysis using X-ray diffraction (XRD) revealed that the light-blue- and green-emitting polymorphs correspond to triclinic and monoclinic forms, respectively. In the light-blue-emitting crystals, π-π interactions are weakened due to the small overlap of molecular π planes. Meanwhile, in the green-emitting crystals, strong π-π interactions arose because the molecules stack parallel to the a-axis (the shortest axis length), leading to a red shift of 8 nm in the optical spectra due to energy stabilization. Both polymorphs demonstrated high photoluminescence quantum yields (PLQYs) of 58% (light blue) and 60% (green) and exhibited Fabry-Pérot lasing behavior under optical excitation with thresholds of 67 and 75 μJ/cm², respectively. The suitability of both polymorphs as laser media, together with their nearly parallel orientation with respect to the crystal basal plane of the bent-shaped molecular structures induced by Coulomb interactions between cyano groups, opens up further possibilities for applications such as exciton-photon strong coupling, vertical-cavity surface-emitting lasers (VCSELs), and high-performance charge injection and transport in devices, including organic field-effect transistors and organic thin-film solar cells.