Аннотация:Single crystals of thiophene-phenylene co-oligomers (TPCO) are promising optoelectronic materials for light-emitting devices as they combine efficient charge carrier transport and high luminescence. These electronic properties are controlled by the chemical structure of the molecules and their crystal packing; however, the structure-property relationships in TPCO single crystals have not yet been properly understood.
Here, we study the impact of molecular end-groups (H, F, CH3, CF3, Si-(CH3)3, C-(CH3)3, O-CH3) on the structural and electronic properties of vapor-grown TPCO single crystals with the same phenylene-thiophene-thiophene-phenylene (PTTP) conjugated core using optical/atomic force microscopy, x-ray diffraction, DFT and other calculations, and field-effect transistor (FET) measurements.
Our experimental data show that the variation of the end-group types results in a significant impact on the crystal habit and on the molecular orientation vs their basal plane, thus not affecting much the herringbone packing itself. Almost all the molecules tend to form layered plate-like crystals, which is confirmed by morphology simulations. TPCOs with H and F, CH3 and CF3, Si-(CH3)3 and C-(CH3)3 have similar crystal habit, respectively. Calculated energies of frontier molecular orbitals of the TPCOs show their small variation with the end-group type; however, the static dipole moment of half TPCO changes its direction, probably affecting charge injection. FETs based on non-fluorine TPCO single-crystals exhibited p-type semiconductor behavior with charge carrier mobility in the range 0.03–0.1 cm2/Vs, while other TPCO single crystals (with F and CF3 end-groups) worked as n-type FETs. We discuss the FET performance in relation with structural features of the TPCO single crystals.
This work was supported by RFBR (project № 17-02-00841).