Crystal packing control of a trifluoromethyl-substituted furan/phenylene co-oligomer.

Furan/phenylene co-oligomer single crystals are considered as future materials for organic optoelectronics. Here, the effects of trifluoromethyl substituents on the crystallization, structure and optical properties of furan/phenylene co-oligomer 1,4-bis{5-[4-(trifluoromethyl)phenyl]furan-2-yl}benzene are studied systematically. The solution growth methods and physical vapor transport result in the formation of three polymorphs depending on the growth method and the solvent. Single-crystal X-ray analysis reveals the crystal structures to correspond to H-, J- or mixed aggregates. All obtained crystals exhibit high photoluminescence efficiency and have optical properties which strongly depend on the crystal packing. Variable-temperature X-ray powder diffraction analysis shows the thermal transition of two forms (H- and J-aggregates) into a third one (mixed aggregate). Terminal trifluoromethyl groups induce weak intermolecular interactions which control the crystal packing and optical properties of co-oligomer single crystals.