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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]