Abstract
Single crystals of furan/phenylene co-oligomers are among the most promising highly-emissive materials for applications in various optoelectronic devices. In this work, we synthesized and studied furan/phenylene co-oligomers with the same conjugated core 1,4-bis(5-phenylfuran-2-yl)benzene and methyl substituents at p- and m-positions of the terminal phenyls. The effect of substituents on the crystal packing, charge transport and luminescence of the single crystals was studied. Compared to the unsubstituted compound, the methyl-substituted co-oligomers demonstrated improved thermostability and enhanced photoluminescence, which we assign to J-aggregation resulting from the strong inclination of the molecules against the main crystal facet. The charge mobility in single crystal organic field-effect transistors decreased upon the inclination of the molecules. We conclude that the molecular tilt angle, intermolecular distances and interactions in crystals of heteroaryl-containing linear conjugated oligomers can be controlled by the introduction of end methyl groups in the appropriate positions.
Highlights
Single crystals of small conjugated molecules, due to their highly ordered and low-defect structure, provide a unique opportunity for the investigation of the fundamental structure– property relationship via systematic design and the study of libraries of related compounds with peculiar molecular packing
We conclude that the molecular tilt angle, intermolecular distances and interactions in crystals of heteroaryl-containing linear conjugated oligomers can be controlled by the introduction of end methyl groups in the appropriate positions
Note that assuming the direction of transition dipole moments is parallel to the long molecular axes, the tilt angles of 2Me- and 4Me-BPFB correspond to J-aggregation,[35] which is beneficial for high luminescence.[1]
Summary
Single crystals of small conjugated molecules, due to their highly ordered and low-defect structure, provide a unique opportunity for the investigation of the fundamental structure– property relationship via systematic design and the study of libraries of related compounds with peculiar molecular packing. By varying a number of aromatic rings in thiophene/phenylene co-oligomers, single crystals with high mobility[6] and high luminescence efficiency were found.[7,8] The introduction of terminal substituents in thiophene/phenylenes was demonstrated to be a powerful tool for solubility and morphology control.[9,10] Distyrylbenzenes (DSB) were shown to give highly emissive single crystals and besides terminal substitution could be modified using vinylene units resulting in a tremendous library of DSB derivatives having different intra- or intermolecular interactions and demonstrating specific optical and charge transport properties.[1] Aggregation motifs in DSB single crystals were intensively explored by substitution of the backbone by methyl,[11,12] tertbutyl,[13] methoxy,[14] cyano,[14,15] fluorine,[13,14,16] trifluoromethyl[15,17] and other[1] groups. We compare vapor-grown single crystals of co-oligomers substituted at pand m-positions of the terminal phenyls and demonstrate that the tilt angle and intermolecular distances could be effectively tuned without distortion of the molecular electronic structure
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