Tuning Molecular Packing, Charge Transport, and Luminescence in Thiophene–Phenylene Co-Oligomer Crystals via Terminal Substituents

Organic semiconductors combining efficient charge transport and bright luminescence are promising materials for organic optoelectronic devices. However, our limited understanding of the interplay among the molecular structure, molecular packing, charge transport, and luminescence of organic semiconductors hampers the progress in the field. In this work, this interplay was theoretically and experimentally studied on a series of thiophene–phenylene co-oligomers, namely 5,5′-diphenyl-2,2′-bithiophene (PTTP) with seven different terminal substituents (bare, fluorine, methyl, trifluoromethyl, trimethylsilyl, tert-butyl, and methoxy), in crystals and field-effect devices. We revealed how the terminal substitution tunes the molecular packing, charge transport, and luminescence properties of the crystals. We found that the differences in the resolved crystal structures are mainly related to the inclination angle of PTTP molecules in the molecular layers, which is perfectly correlated with the van der Waals volume of the terminal groups and impacts the intermolecular interactions and molecular orbital overlap. We studied how charge transport depends on the inclination angle and found that PTTP with trimethylsilyl terminal groups has an optimal inclination supporting bipolar charge transport, as demonstrated in organic light-emitting transistors. Moreover, this optimal inclination provides relatively weak dipole–dipole interactions corresponding to J-aggregation and results in the most resilience of crystal luminescence to molecular self-dopants efficiently capturing the excitation energy in the crystals with less inclined molecules forming H-aggregates. Finally, organic phototransistors based on the studied PTTP oligomers were demonstrated. We anticipate that our strategy and the revealed structure–property relationships would greatly benefit the rational design of efficient organic optoelectronic materials.