"Manipulation" of Crystal Structure by Methylthiolation Enabling Ultrahigh Mobility in a Pyrene-Based Molecular Semiconductor

Methylthiolation Modified Bay-Annulated Indigo Derivatives for Enhanced Performance in Organic Field-Effect Transistors

Indigo is one of the most well-known natural dyes and has attracted significant research interest due to its low cost and exceptional stability. Notably, bay-annulated indigo (BAI) has been reported as an effective electron acceptor and is widely used in various applications. Herein, a methylthio-substituted BAI derivative, compound 1, is successfully synthesized and the impact of methylthio substitution on its optoelectronic properties is investigated. UV-vis absorption and fluorescence spectra reveal that compound 1 displays a significant redshift compared to the nonmethylthio-substituted compound 2. Cyclic voltammetry measurements and density functional theory calculations indicate that compound 1 has a narrower highest occupied molecular orbital and lowest unoccupied molecular orbital gap, demonstrating the prominent influence of methylthio side chains in modulating molecular electronic properties. Importantly, the organic field-effect transistor device based on compound 1 exhibits a hole mobility 3.5 times higher than that of the nonmethylthio-substituted compound 2. Furthermore, atomic force microscopy characterization reveals the formation of needle-like crystallites in the compound 1 film after annealing, whereas compound 2 forms an amorphous thin film. These results suggest that methylthiolation is an effective strategy for tuning intermolecular interactions in novel BAI derivatives, and compound 1 is a promising hole-transporting material.

Integrating Strong Luminescence and High Mobility in Organic Single Crystals of Covalent Pyrene Dimers

Simultaneously achieving strong luminescence and high mobility in organic semiconductors remains a challenge. Herein, two covalently dimerized pyrene derivatives (1Py-2Py and 1Py-1Py) with distinct chemical linkages and crystal packing arrangements are presented. Remarkably, the radiative transition of pyrene is gradually unforbidden from 1Py-2Py to 1Py-1Py. Moreover, 1Py-2Py showcases 1D long-range π─π stacking, while 1Py-1Py exhibits 2D herringbone packing formed by a vast network of intermolecular C─H∙∙∙π interactions. To the surprise, both high photoluminescence quantum yield (PLQY = 72.17%) and high hole mobility (µ = 32.6 cm2 V-1 s-1) are simultaneously harvested in 1Py-1Py crystal, which are far superior to those in 1Py-2Py crystal (PLQY = 48.66% and µ = 0.05 cm2 V-1 s-1). These findings underscore the potential of covalent pyrene dimer with 1-position linkages as a promising organic semiconductor for the exceptional combination of strong luminescence and high mobility, which is substantially ascribed to the efficiently unforbidden emission and the favorable 2D charge transport pathways.

Hydrogen-Bond-Directed Supramolecular Organic Semiconductor Thin Films Realized via Thermal Precursor Approach

Flexible organic semiconductors are regarded as an important class of functional materials, enabling their fabrication via a solution-processed method. However, supramolecularly designed organic thin-film transistor (OTFT) devices using hydrogen bonds have not been sufficiently developed. This is because of the reduced solubility caused by introducing hydrogen bond sites and the difficulty in controlling molecular orientations and packing structures. Herein, we report a reasonable strategy for the formation of hydrogen-bonded semiconductor thin films of a tetrabenzoporphyrin derivative with amide groups using the thermal precursor approach. The thin films are successfully fabricated by thermal conversion of the corresponding precursor in the film state. Assembly structure with a hydrogen-bond network is revealed by the combination of X-ray diffraction and infrared p-polarized multiple-angle incidence resolution spectrometry. The OTFT devices exhibit high hole mobility of 0.25 cm2 V-1 s-1 among hydrogen-bonded OTFTs reported previously. Notably, we unveiled the high thermal stability of semiconductor films without decreasing device performance in air up to 250 °C. This strategy can become a key methodology to form hydrogen-bond-directed organic electronic devices, namely supramolecular devices.

Rapid access to functionalized nanographenes through a palladium-catalyzed multi-annulation sequence

Nanographenes and polycyclic aromatic hydrocarbons exhibit many intriguing physical properties and have potential applications across a range of scientific fields, including electronics, catalysis, and biomedicine. To accelerate the development of such applications, efficient and reliable methods for accessing functionalized analogs are required. Herein, we report the efficient synthesis of functionalized small nanographenes from readily available iodobiaryl and diarylacetylene derivatives via a one-pot, multi-annulation sequence catalyzed by a single palladium catalyst. This method enables the preparation of small nanographenes bearing various polar functional groups, such as hydroxy, amino, and pyridinic nitrogen atoms, which are otherwise difficult to incorporate. These functional groups provide valuable sites for further derivatization, allowing the modulation of small nanographenes' solubility, optoelectronic properties, and photochromic and vapochromic behaviors. Our new method thus provides a platform for facile access to novel carbon-based materials.

Exploration and development of molecule-based printed electronics materials: an integrated approach using experimental, computational, and data sciences

The challenge in developing molecule-based electronic materials lies in the uncontrollable or unpredictable nature of their crystal structures, which are crucial for determining both electrical properties and thin-film formability. This review summarizes the findings of a research project focused on the systematic development of crystalline organic semiconductors (OSCs) and organic ferroelectrics by integrating experimental, computational, and data sciences. The key outcomes are as follows: 1) Data Science: We developed a method to identify promising materials from crystal structure databases, leading to the discovery of unique molecule-based ferroelectrics. 2) Computational Science: The origin of high layered crystallinity in π-core - alkyl-chain-linked molecules was clarified based on intermolecular interaction calculations. We proposed a stepwise structure optimization method tailored for layered OSCs. 3) Material Development: We developed various alkylated layered OSCs, which exhibit high mobility, heat resistance, and solubility. We discovered several unique phenomena, including frozen liquid crystal phases, significant polar/antipolar control, and phase control through mixing, leveraging the variability of alkyl chain length. We also developed molecule-based ferroelectrics showing peculiar ferroelectricity, including multiple polarization reversal, competing ferroelectric/antiferroelectric order, and spinner-type configurations with π-skeletons. 4) Advanced Structural Analysis: By combining cryo-electron microscopy and X-ray-free electron laser (XFEL), we enabled crystal structure analysis for ultrathin crystals that are usually difficult to analyse. 5) Device Development: Utilizing the self-organized growth of layered OSCs through solution processes, we developed a method to produce exceptionally clean semiconductor - insulator interfaces, achieving field-effect transistors that show sharp (near theoretical limit) and stable switching at low voltages.

Thiophene-fused fulminenes (FuDTs): promising platforms for high-mobility organic semiconductors with a zigzag shape

Zigzag-shaped [6]phenacene isomers with fused thiophene rings, fulmineno[2,1-b:10,9-b']dithiophene (FuDT-α) and fulmineno[1,2-b:9,10-b']dithiophene (FuDT-β), were syntheized and their p-type organic semicondutor properties were studied. Small effective masses of holes were estimated from the crystal structures of both isomers, which was particularly demonstrated by the hole mobility of 10.5 cm2 V-1 s-1 for FuDT-α single crystals.

On the importance of crystal structures for organic thin film transistors

Historically, knowledge of the molecular packing within the crystal structures of organic semiconductors has been instrumental in understanding their solid-state electronic properties. Nowadays, crystal structures are thus becoming increasingly important for enabling engineering properties, understanding polymorphism in bulk and in thin films, exploring dynamics and elucidating phase-transition mechanisms. This review article introduces the most salient and recent results of the field.

Machine Learning-Inspired Molecular Design, Divergent Syntheses, and X-Ray Analyses of Dithienobenzothiazole-Based Semiconductors Controlled by S⋅⋅⋅N and S⋅⋅⋅S Interactions

Inspired by the previous machine-learning study that the number of hydrogen-bonding acceptor (NHBA) is important index for the hole mobility of organic semiconductors, seven dithienobenzothiazole (DBT) derivatives 1 a-g (NHBA=5) were designed and synthesized by one-step functionalization from a common precursor. X-ray single-crystal structural analyses confirmed that the molecular arrangements of 1b (the diethyl and ethylthienyl derivative) and 1c (the di(n-propyl) and n-propylthienyl derivative) in the crystal are classified into brickwork structures with multidirectional intermolecular charge-transfer integrals, as a result of incorporation of multiple hydrogen-bond acceptors. The solution-processed top-gate bottom-contact devices of 1b and 1c had hole mobilities of 0.16 and 0.029 cm2 V-1s-1, respectively.

Molecular "backbone surgery" of electron-deficient heteroarenes based on dithienopyrrolobenzothiadiazole: conformation-dependent crystal structures and charge transport properties

Electron-deficient heteroarenes based on dithienopyrrolobenzothiadiazole (BTP) have been highly attractive due to their fascinating packing structures, broad absorption profiles, and promising applications in non-fullerene organic solar cells. However, the control of their crystal structures for superior charge transport still faces big challenges. Herein, a conformation engineering strategy is proposed to rationally manipulate the single crystal structure of BTP-series heteroarenes. The parent molecule BTPO-c has a 3D network crystal structure, which originates from its banana-shaped conformation. Subtracting one thiophene moiety from the central backbone leads to a looser brickwork crystal structure of the derivative BTPO-z because of its interrupted angular-shaped conformation. Further subtracting two thiophene moieties results in the derivative BTPO-l with a compact 2D-brickwork crystal structure due to its quasi-linear conformation with a unique dimer packing structure and short π-π stacking distance (3.30 Å). Further investigation of charge-transport properties via single-crystal organic transistors demonstrates that the compact 2D-brickwork crystal structure of BTPO-l leads to an excellent electron mobility of 3.5 cm2 V-1 s-1, much higher than that of BTPO-c with a 3D network (1.9 cm2 V-1 s-1) and BTPO-z with a looser brickwork structure (0.6 cm2 V-1 s-1). Notably, this study presents, for the first time, an elegant demonstration of the tunable single crystal structures of electron-deficient heteroarenes for efficient organic electronics.

Statistical analysis of interatomic transfer integrals for exploring high-mobility organic semiconductors

Charge transport in organic semiconductors occurs via overlapping molecular orbitals quantified by transfer integrals. However, no statistical study of transfer integrals for a wide variety of molecules has been reported. Here we present a statistical analysis of transfer integrals for more than 27,000 organic compounds in the Cambridge Structural Database. Interatomic transfer integrals were used to identify substructures with high transfer integrals. As a result, thione and amine groups as in thiourea were found to exhibit high transfer integrals. Such compounds are considered as potential non-aromatic, water-soluble organic semiconductors.

The Photosalient Effect and Thermochromic Luminescence Based on o-Carborane-Assisted π-Stacking in the Crystalline State

Herein, we report the unique multiple-stimuli responsiveness of anthracene-tethered o-carborane derivatives. We designed and synthesized anthracene derivatives with different substitution positions and numbers of the o-carborane units. Two compounds had characteristic crystal structures involving the columnar π-stacking structures of the anthracene units. From the analysis of crystalline-state structure-property relationships, it was revealed that the crystals exhibited the photosalient effect accompanied by photochemical [4+4] cycloaddition reactions and temperature-dependent photophysical dual-emission properties including excimer emission of anthracene. Those properties were considered as non-radiative and radiative deactivation pathways through the excimer formation in the excited state and the formation of excimer species was facilitated by the π-stacking structure of anthracene units. Moreover, we found unusual temperature dependency on the occurrence of the photosalient effect. According to the data from variable temperature X-ray crystallography, a strong correlation between lattice shrinkage and strain accumulation is suggested.

High Sensitivity and Ultra-Broad-Range NH3 Sensor Arrays by Precise Control of Step Defects on The Surface of Cl2-Ndi Single Crystals

Vapor sensors with both high sensitivity and broad detection range are technically challenging yet highly desirable for widespread chemical sensing applications in diverse environments. Generally, an increased surface-to-volume ratio can effectively enhance the sensitivity to low concentrations, but often with the trade-off of a constrained sensing range. Here, an approach is demonstrated for NH3 sensor arrays with an unprecedentedly broad sensing range by introducing controllable steps on the surface of an n-type single crystal. Step edges, serving as adsorption sites with electron-deficient properties, are well-defined, discrete, and electronically active. NH3 molecules selectively adsorb at the step edges and nearly eliminate known trap-like character, which is demonstrated by surface potential imaging. Consequently, the strategy can significantly boost the sensitivity of two-terminal NH3 resistance sensors on thin crystals with a few steps while simultaneously enhancing the tolerance on thick crystals with dense steps. Incorporation of these crystals into parallel sensor arrays results in ppb-to-% level detection range and a convenient linear relation between sheet conductance and semi-log NH3 concentration, allowing for the precise localization of vapor leakage. In general, the results suggest new opportunities for defect engineering of organic semiconductor crystal surfaces for purposeful vapor or chemical sensing.

Crystal-Structure Control of Molecular Semiconductors by Methylthiolation: Toward Ultrahigh Mobility

ConspectusThe crystal structure of organic semiconductors has been regarded as one of the crucial factors for realizing high-performance electronic devices, such as organic field-effect transistors. However, although the control of crystal structures of organic semiconductors has been examined in the last two decades of intensive efforts of the development of organic semiconductors, active measures to control crystal structures enabling high carrier mobility are still limited. In 2016, our research group noticed that regioselective methylthiolation could provide a selective crystal structure change from an ordinary herringbone structure to a pitched π-stacking structure, similar to the crystal structure of rubrene, in the benzo[1,2-b:4,5-b']dithiophene (BDT) system. Following this serendipitous finding, our group systematically investigated the relationship between the molecular and crystal structures of a range of methylthiolated aromatic and heteroaromatic compounds.This Account provides a comprehensive overview of our research efforts and advancements in the development of methylthiolated small-molecule-based organic semiconductors (molecular semiconductors). We first describe the outline of the past development of molecular semiconductors, focusing on the types of crystal structures of high-performance molecular semiconductors. Then, we describe our findings on the drastic crystal structure change in the BDT system upon methylthiolation, detailing the causes of the change in terms of the intermolecular contacts and intermolecular interaction energies. This is followed by the confirmation of the generality of the crystal-structure change by methylthiolation of a series of acene and heteroacenes, where the herringbone structure in the parent system is unexceptionally transformed into the pitched π-stacking structure, a promising crystal structure for high-mobility molecular semiconductors well exemplified by the prototypical molecular semiconductor, rubrene. In fact, the methylthiolated anthradithiophene afforded comparable high mobility to rubrene in single-crystal field-effect transistors. Then, we demonstrate that the sandwich herringbone structures of peri-condensed polycyclic aromatic hydrocarbons, including pyrene, perylene, and peropyrene, change into brickwork crystal structures upon methylthiolation and that, among these compounds, very promising molecular semiconductors, methylthiolated pyrene and peropyrene, showing ultrahigh mobility of 30 cm2 V s-1, are realized.Through the studies, by gaining insights into the underlying mechanisms driving the crystal structure changes, we lay a strong foundation for tackling challenges related to controlling the crystal structures and developing high-performance molecular semiconductors. This will be a distinct approach from the past activities in the development of molecular semiconductors that mainly focused on molecules themselves, including their synthesis, properties, and characterization. We thus anticipate that our findings and the present Account will open the door to a new era of the development of molecular semiconductors.

Eclipsed and Twisted Excimers of Pyrene and 2-Azapyrene: How Nitrogen Substitution Impacts Excimer Emission

Due to their unique photophysical and electronic properties, pyrene and its analogues have been the subject of extensive research in recent decades. The propensity of pyrene and its derivatives to form excimers has found wide application in various fields. Nitrogen-substituted pyrene derivatives display similar photophysical properties, but for them, excimer emission has not been reported to date. Here, we use time-dependent density functional theory (TD-DFT) calculations to investigate the low-lying exciton states of dimers of pyrene and 2-azapyrene. The excimer equilibrium structures are determined and the contribution of charge transfer (CT) excitations and intermolecular interactions to the exciton states is disclosed using a diabatization procedure. The study reveals that the dimers formed by the two molecules have quite similar exciton-state patterns, in which the relevant CT contributions govern the formation of excimer states, along with the La/Lb state inversion. In contrast with pyrene, the dipole-dipole interactions in 2-azapyrene stabilize the dark eclipsed excimer structure and increase the barrier for conversion into a bright twisted excimer. It is suggested that these differences in the nitrogen-substituted derivative might influence the excimer emission properties.

Crystal-structure simulation of molecular semiconductors: brickwork-related crystal structures of methylthiolated peri-condensed polycyclic aromatic hydrocarbons

Despite the critical importance to carrier transport properties, studies on the control and prediction of crystal structures of molecular semiconductors have not been well-matured. To tackle this issue, we have developed "in silico crystallization" (ISC) protocols for simulating the brickwork (BW) crystal structures of methylchalcogenolated polycyclic aromatic hydrocarbons (PAHs). In this study, by carefully analyzing a BW-related polymorph of experimental crystal structures, an inclined brickwork (iBW) structure, we further extend the ISC protocol to simulate various BW-related crystal structures including iBW structures. Rational conditional branching in the simulation not only makes it possible to simulate eight polymorph candidates of methylchalcogenolated PAHs but also helps understand the relationship between the polymorphs. Furthermore, the relative favorability of each polymorphic candidate, i.e., the likelihood of the appearance among the polymorph candidates, can also be evaluated.

Surface doping of rubrene single crystals by molecular electron donors and acceptors

The surface molecular doping of organic semiconductors can play an important role in the development of organic electronic or optoelectronic devices. Single-crystal rubrene remains a leading molecular candidate for applications in electronics due to its high hole mobility. In parallel, intensive research into the fabrication of flexible organic electronics requires the careful design of functional interfaces to enable optimal device characteristics. To this end, the present work seeks to understand the effect of surface molecular doping on the electronic band structure of rubrene single crystals. Our angle-resolved photoemission measurements reveal that the Fermi level moves in the band gap of rubrene depending on the direction of surface electron-transfer reactions with the molecular dopants, yet the valence band dispersion remains essentially unperturbed. This indicates that surface electron-transfer doping of a molecular single crystal can effectively modify the near-surface charge density, while retaining good charge-carrier mobility.

An Unsymmetrical 5,15-Disubstituted Tetrabenzoporphyrin: Effect of Molecular Symmetry on the Packing Structure and Charge Transporting Property

Molecular design strategy to control the crystal structure of two-dimensional (2D) π-extended organic semiconductors has not been intensively explored. We synthesized an unsymmetric tetrabenzoporphyrin derivative (TIPS-Ph-BP) to demonstrate the effect of molecular symmetry on crystal packing. TIPS-Ph-BP formed an antiparallel slipped π-stacking and 2D herringbone-like structure. An unsymmetric structure would make 2D π-stacking more stable than a one-dimensional (1D) columnar structure to counteract steric and electronic imbalance in the crystal. As a result, TIPS-Ph-BP achieved the high hole mobility of 0.71 cm2 V-1 s-1.

Precise Control of the Molecular Arrangement of Organic Semiconductors for High Charge Carrier Mobility

Organic semiconductors are well-known to exhibit high charge carrier mobility based on their spread of the π-orbital. In particular, the π-orbital overlap between neighboring molecules significantly affects their charge carrier mobility. This study elucidated the direct effect of subtle differences in the π-orbital overlap on charge carrier mobility, by precisely controlling only molecular arrangements without any chemical modifications. We synthesized disulfonic acid composed of a [1]benzothieno[3,2-b][1]benzothiophene (BTBT) moiety, and prepared organic salts with four butylamine isomers. Regardless of the type of butylamine combined, electronic states of the constituent BTBT derivative were identical, and all BTBT arrangements were edge-to-face herringbone-type. However, depending on the difference of steric hindrance, center-to-center distances and dihedral angles between neighboring BTBT moieties slightly varied. Despite a similar arrangement, the photoconductivity of four organic salts differed by a factor of approximately two. Additionally, theoretical charge carrier mobilities from their crystal structures exhibited a strong correlation with their photoconductivity.

Modulating the Alkylation Position on Terminal Thiophene Ring of Naphtho[2,3-b:6,7-b'] Bithieno[2,3-d] Thiophene (NBTT) for High-Performance Organic Optoelectronic Devices

Several thiophene terminated thienoacenes with high mobilities in organic thin-film transistors (OTFTs) have been reported; however, the structure-property relationship of thiophene terminated thienoacenes was unclear, especially the impact of α or β position substitution of terminal thiophene ring on molecular packing and physicochemical properties. Here, we report the synthesis and characterization of a six-ring-fused naphtho[2,3-b:6,7-b'] bithieno[2,3-d] thiophene (NBTT) and its derivatives 2,8-dioctyl-naphtho[2,3-b:6,7-b'] bithieno [2,3-d] thiophene (2,8-C8NBTT) and 3,9-dioctyl-naphtho[2,3-b:6,7-b'] bithieno [2,3-d] thiophene (3,9-C8NBTT). It is found that the alkylation on terminal thiophene ring can effectively tune the molecular stacking from a cofacial herringbone stacking mode (NBTT) to layer-by-layer packing (2,8-C8NBTT and 3,9-C8NBTT). Impressively, a hopping to "band-like" charge transport mechanism evolution of vacuum deposited films is realized by modulating the alkylation position on the terminal thiophene rings. As a result, the OTFTs based on 2,8-C8NBTT characterized by a "band-like" transport presents the highest mobility of 3.58 cm² V^-1 s^-1 together with a remarkably high current on/off ratio around 10⁹. Furthermore, organic phototransistors (OPTs) based on 2,8-C8NBTT thin film also exhibits higher photosensitivity (P) of 2.0 × 10⁸, photoresponsivity (R) of 3.3 × 10³ A W^-1, and detectivity (D*) of 1.3 × 10¹⁶ Jones than those based on NBTT and 3,9-C8NBTT.

Recent Research Progress of Organic Small-Molecule Semiconductors with High Electron Mobilities

Organic electronics has made great progress in the past decades, which is inseparable from the innovative development of organic electronic devices and the diversity of organic semiconductor materials. It is worth mentioning that both of these great advances are inextricably linked to the development of organic high-performance semiconductor materials, especially the representative n-type organic small-molecule semiconductor materials with high electron mobilities. The n-type organic small molecules have the advantages of simple synthesis process, strong intermolecular stacking, tunable molecular structure, and easy to functionalize structures. Furthermore, the n-type semiconductor is a remarkable and important component for constructing complementary logic circuits and p-n heterojunction structures. Therefore, n-type organic semiconductors play an extremely important role in the field of organic electronic materials and are the basis for the industrialization of organic electronic functional devices. This review focuses on the modification strategies of organic small molecules with high electron mobility at molecular level, and discusses in detail the applications of n-type small-molecule semiconductor materials with high mobility in organic field-effect transistors, organic light-emitting transistors, organic photodetectors, and gas sensors.

Benzothiadiazole versus Thiazolobenzotriazole: A Structural Study of Electron Acceptors in Solution-Processable Organic Semiconductors

Despite the rapid progress of organic electronics, developing high-performance n-type organic semiconductors is still challenging. Donor-acceptor (D-A) type conjugated structures have been an effective molecular design strategy to achieve chemically-stable semiconductors and the appropriate choice of the acceptor units determines the electronic properties and device performances. We have now synthesized two types of A1 -D-A2 -D-A1 type conjugated molecules, namely, NDI-BTT-NDI and NDI-TBZT-NDI, with different central acceptor units. In order to investigate the effects of the central acceptor units on the charge-transporting properties, organic field-effect transistors (OFETs) were fabricated. NDI-TBZT-NDI had shallower HOMO and deeper LUMO levels than NDI-BTT-NDI. Hence, the facilitated charge injection resulted in ambipolar transistor performances with the optimized hole and electron mobilities of 0.00134 and 0.151 cm2  V-1  s-1 , respectively. In contrast, NDI-BTT-NDI displayed only an n-channel OFET performance with the electron mobility of 0.0288 cm2  V-1  s-1 . In addition, the device based on NDI-TBZT-NDI showed a superior air stability to that based on NDI-BTT-NDI. The difference in these OFET performances was reasonably explained by the contact resistance and film morphology. Overall, this study demonstrated that the TBZ acceptor is a promising building block to create n-type organic semiconductors.

Mixed Ionic and Electronic Conduction in Small-Molecule Semiconductors

Small-molecule organic semiconductors have displayed remarkable electronic properties with a multitude of π-conjugated structures developed and fine-tuned over recent years to afford highly efficient hole- and electron-transporting materials. Already making a significant impact on organic electronic applications including organic field-effect transistors and solar cells, this class of materials is also now naturally being considered for the emerging field of organic bioelectronics. In efforts aimed at identifying and developing (semi)conducting materials for bioelectronic applications, particular attention has been placed on materials displaying mixed ionic and electronic conduction to interface efficiently with the inherently ionic biological world. Such mixed conductors are conveniently evaluated using an organic electrochemical transistor, which further presents itself as an ideal bioelectronic device for transducing biological signals into electrical signals. Here, we review recent literature relevant for the design of small-molecule mixed ionic and electronic conductors. We assess important classes of p- and n-type small-molecule semiconductors, consider structural modifications relevant for mixed conduction and for specific interactions with ionic species, and discuss the outlook of small-molecule semiconductors in the context of organic bioelectronics.