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Molecular doping is an extremely useful approach for the optoelectronic properties tuning of organic materials, particularly crystalline luminescent semiconductors. Usually, dopant materials with good luminescence properties are used to improve the optoelectronic properties of host materials. However, the host-dopant compatibility and their selection criteria remain unclear. It is generally considered that the host and dopant should have a "similar" structure, which does not allow one to perform a thorough systematic design and choice of materials for obtaining low-defect doped organic semiconductors. Herein, we report a systematic experimental study coupled with existing literature data on host-dopant compatibility in a series of symmetric linear conjugated small molecules. Molecular planarity, short molecular axis and side polar substituents were demonstrated to be critical for successful host–guest molecular doping. Heterocyclic unit type and arrangement, molecular long axis, conjugation chain length and terminal substituents were demonstrated to not affect the host-dopant compatibility. An elaborated molecular doping guide is highly demanded for the systematic smart design of novel doped high-performance luminescent organic semiconductor materials. [ABSTRACT FROM AUTHOR]

Linearly conjugated oligomers attract ever‐growing attention as promising systems for organic optoelectronics because of their inherent lucky combination of high charge mobility and bright luminescence. Among them, furan‐phenylene co‐oligomers (FPCOs) are distinguished by outstanding solubility, very bright luminescence, and good hole‐transport properties; however, furan‐containing organic semiconductors generally lack electron transport, which makes it impossible to utilize them in efficient light‐emitting electronic devices, specifically, ambipolar light‐emitting transistors. In this work, 1,4‐bis(5‐phenylfuran‐2‐yl)benzene (FP5) derivatives are synthesized with the fully/partially fluorinated central and edge phenyl rings. It is shown that the selective fluorination of FPCOs lowers the energies of frontier molecular orbitals, maintaining the bandgap, solubility, and bright luminescence, dramatically improves the photostability, tunes the π‐π stacked packing, and allows the first realization of electron transport in FPCOs. It is found that selectively fluorinated 2,2′‐(2,3,5,6‐tetrafluoro‐1,4‐phenylene)bis[5‐(3,5‐difluorophenyl)furan] demonstrates well‐balanced ambipolar charge transport and efficient electroluminescence in an organic light‐emitting transistor (OLET) with external quantum and luminous efficiencies as high as 0.63% and 5 cdA−1, respectively, which are among the best reported for OLETs. The findings show that "smart" fluorination is a powerful tool to fine‐tune the stability and performance of linearly conjugated small molecules for organic optoelectronics. [ABSTRACT FROM AUTHOR]

Aggregation-induced emission (AIE) materials find their applications in organic optoelectronics, bio-imaging and sensors. In this work, we introduced alkyl substituents in AIE-active bis(4-((9H-fluoren-9-ylidene)methyl)phenyl)thiophene and elucidated their effect on the molecular and crystal structures, crystallization and optical properties of materials. Ethyl-containing (C2-BFMPT) and octyl-containing (C8-BFMPT) 2,5-bis(4-((2,7-dialkyl-9H-fluoren-9-ylidene)methyl)phenyl)thiophenes were synthesized in 4 steps. The introduction of alkyl groups weakened intermolecular interactions, decreased the crystal quality, melting point, and density, and increased the solubility of materials. Octyl-containing derivative was demonstrated to generate two crystal forms obtained by the native (form I) and additive-assisted (form II) crystallizations. The latter appeared to have a better crystal quality. C–H⋯π interactions and an extensive positional disorder were revealed for both derivatives. In C8-BFMPT crystals (form II) only a half of molecular backbones were well localized due to multiple intermolecular C–H⋯π interactions, whereas another half demonstrated a high positional disorder. The AIE effect with a negligible photoluminescence (PL) quantum yield (QY) in solution and a PL QY of 5% for C2-BFMPT and 2% for C8-BFMPT (form II) crystals was demonstrated. The cooling of the C8-BFMPT form II resulted in 10-fold increase of PL QY. The introduction of alkyl-substituents and additive-assisted crystallization are highlighted as powerful tools for the control of crystal packing, morphology, polymorphism and the optical performance of AIE-materials. [ABSTRACT FROM AUTHOR]

The design of light-emitting crystalline organic semiconductors for optoelectronic applications requires a thorough understanding of the singlet exciton transport process. In this study, we show that the singlet exciton diffusion length in a promising semiconductor crystal based on furan/phenylene co-oligomers is 24 nm. To achieve this, we employed the photoluminescence quenching technique using a specially synthesized quencher, which is a long furan/phenylene co-oligomer that was facilely implanted into the host crystal lattice. Extensive Monte-Carlo simulations, exciton–exciton annihilation experiments and numerical modelling fully supported our findings. We further demonstrated the high potential of the furan/phenylene co-oligomer crystals for light-emitting applications by fabricating solution-processed organic light emitting transistors. [ABSTRACT FROM AUTHOR]

Heteroaryl-containing co-oligomers are considered among the most promising materials for organic optoelectronics. In this work we synthesized a series of linear furan/phenylene co-oligomers (FPs) with different molecular length and studied their structure, luminescence and charge transport properties. The X-ray diffraction experiments revealed the higher distortion of molecular planarity for the longer FPs which is assigned to the crystal environment effect. All FPs demonstrated stability and high photoluminescence quantum yield (PL QY) both in solution and vapor-grown single crystals. The absolute PL QY as high as 70% was demonstrated for FP6 single crystal. The top-contact top-gate organic field-effect transistors (OFETs) demonstrated virtually the same mobility ∼0.35 cm 2 /Vs for a FPs with different conjugation chain. However the threshold voltage for the studied OFETs was demonstrated to be significantly reduced for a co-oligomer FP7 with longer conjugation chain. Thus we conclude that the increase of conjugation chain for co-oligomers does not benefit an increase of charge carrier mobility but improve the crystal/dielectric interface. [ABSTRACT FROM AUTHOR]