Properties of the organic semiconductors can be finely tuned via changes in their molecular structure. However, the relationship between the molecular structure, molecular packing, and (opto)electronic properties of the organic semiconductors to guide their smart design remains elusive. In this study, we address computationally and experimentally the impact of subtle modification of a thiophene–phenylene co-oligomer CF₃-PTTP-CF₃ on the molecular properties, crystal structure, charge transport, and optoelectronic properties. This modification consists in the substitution of two C–H atom pairs by N atoms in the thiophene units and hence converting them to thiazole units. A dramatic effect of the N-substitution on the crystal structure—the crossover from the herringbone packing motif to π-stacking—is attributed to significant changes in the molecular electrostatic potential. The changes in the molecular and crystal structure resulting from the N-substitution clearly reveal themselves in the Raman spectra. The increase of the calculated electron mobility in the corresponding crystals as a result of the N-substitution is rationalized in terms of the changes in the molecular and crystal structure. The charge transport, electroluminescence, and photoelectric properties are compared in thin-film organic field-effect transistors based on CF₃-PTTP-CF₃ and its N-substituted counterpart. An intriguing similarity between the effects of N-substitution in the thiophene rings and fluorination of the thiophene–phenylene oligomer is revealed, which is probably associated with a more general effect of electronegative substitution. The obtained results are anticipated to facilitate the rational design of organic semiconductors.

Emissive materials that radiate polarized light are in high demand for a broad variety of light-emitting electronic devices. In organic light-emitting devices—light-emitting diodes and transistors—highly polarized emission from the device surface, the surface emission, can be achieved by the in-plane co-linear orientation of molecular transition dipole moments (TDMs) in the device active layer. Furthermore, such a TDM arrangement strongly alleviates the waveguiding effect, enhancing the performance of the device. In this work, we report the first practical implementation of these ideas in an organic light-emitting transistor. Highly emissive semiconductor single crystals were grown from a thiophene–phenylene co-oligomer 1,4-bis{5-[4-(trimethylsilyl)phenyl]thiophen-2-yl}benzene (TMS-PTPTP-TMS) with trimethylsilyl (TMS) terminal substituents, which promote the in-plane TDM orientation. Organic light-emitting transistors based on TMS-PTPTP-TMS showed ambipolar charge transport with efficient electroluminescence, and the single-crystal devices demonstrated linearly polarized electroluminescence with the polarization degree of 0.78 ± 0.06. The observed polarization properties of both electro- and photoluminescence are in full agreement with the solved crystal structure and the corresponding calculations. Our findings indicate that smart control of in-plane ordered TDMs via molecular packing is a promising approach to designing materials for highly efficient light-emitting electronic devices.

Two-dimensional (2D) organic semiconductor macroscopic single crystals have recently demonstrated their excellent charge-transport properties competing with their 3D counterparts. However, the combination of efficient charge transport and prominent luminescent properties in an organic 2D material is still a challenge. Here, we report on 2D emissive semiconductor single crystals based on four and five-ring thiophene–phenylene co-oligomers with didecyl terminal substituents—DD-PTTP and DD-PTPTP, where P and T stand for phenylene and thiophene, respectively—and investigate how the insertion of a P-ring in the PTTP conjugated core impacts the molecular and various solid-state properties of the material. DD-PTTP and DD-PTPTP were synthesized and characterized, and then macroscopic 2D single crystals of both oligomers in the form of monolayer and few-layer films were grown from solution and studied by optical interference, photoluminescence (PL), and atomic-force microscopies. Polarization-sensitive PL microscopy revealed the orientation of the conjugated cores in 2D crystals relative to their facets and indicated that the molecules are H-aggregated. 2D organic field-effect transistors (OFETs) based on both oligomers showed good stability, and their performance nicely followed the Shockley model with a charge-carrier mobility of up to 0.2 cm² V⁻¹ s⁻¹, which is among the best values for various materials based on linearly conjugated oligomers. The highest charge-carrier mobility was demonstrated for the monolayer devices indicating high molecular order in the monolayer single crystals. 2D organic light-emitting transistors (OLETs) with DD-PTTP and DD-PTPTP active layers were demonstrated, and the DD-PTPTP-based 2D OFETs and OLETs showed higher performance. The latter was assigned to the stronger intermolecular interactions in DD-PTPTP stemming from its longer conjugation length despite the almost equal optical gaps of DD-PTTP and DD-PTPTP. The results obtained show that macroscopic 2D single crystals of thiophene–phenylene co-oligomers are a promising platform for 2D organic optoelectronics.

Merging the functionality of an organic field-effect transistor (OFET) with either a light emission or a photoelectric effect can increase the efficiency of displays or photosensing devices. In this work, we show that an organic semiconductor enables a multifunctional OFET combining electroluminescence (EL) and a photoelectric effect. Specifically, our computational and experimental investigations of a six-ring thiophene-phenylene co-oligomer (TPCO) revealed that this material is promising for OFETs, light-emitting, and photoelectric devices because of the large oscillator strength of the lowest-energy singlet transition, efficient luminescence, pronounced delocalization of the excited state, and balanced charge transport. The fabricated OFETs showed a photoelectric response for wavelengths shorter than 530 nm and simultaneously EL in the transistor channel, with a maximum at ~570 nm. The devices demonstrated an EL external quantum efficiency (EQE) of ~1.4% and a photoelectric responsivity of ~0.7 A W^–1, which are among the best values reported for state-of-the-art organic light-emitting transistors and phototransistors, respectively. We anticipate that our results will stimulate the design of efficient materials for multifunctional organic optoelectronic devices and expand the potential applications of organic (opto)electronics.