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.

A Novel N-Type Molecular Dopant With a Closed-Shell Electronic Structure Applicable to the Vacuum-Deposition Process.

Rational design, synthesis, and characterization of a new efficient versatile n-type dopant with a closed-shell electronic structure are described. By employing the tetraphenyl-dipyranylidene (DP0) framework with two 7π-electron systems modified with N,N-dimethylamino groups as the strong electron-donating substituent, 2,2',6,6'-tetrakis[4-(dimethylamino)phenyl]-4,4'-dipyranylidene (DP7), a closed-shell molecule with an extremely high-lying energy level of the highest occupied molecular orbital, close to 4.0 eV below the vacuum level, is successfully developed. Thanks to its thermal stability, DP7 is applicable to vacuum deposition, which allows utilization of DP7 in bulk doping for the development of n-type organic thermoelectric materials and contact doping for reducing contact resistance in n-type organic field-effect transistors. As vacuum-deposition processable n-type dopants are very limited, DP7 stands out as a useful n-type dopant, particularly for the latter purpose.

Enantiopure 2-(2-ethylhexyl)dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophenes: synthesis, single-crystal structure and a surprising lack of influence of stereoisomerism on thin-film structure and electronic properties.

Starting from a chiral resolution of 2-ethylhexanoic acid followed by conversions of functional groups without interfering with the enantiopurity, we have successfully introduced an enantiopure 2-ethylhexyl group on to dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DNTT) via a Negishi-coupling reaction to synthesize 2-(R)-(2-ethylhexyl)- and 2-(S)-(2-ethylhexyl)-DNTT (R- and S-EH-DNTT, respectively). Then, the crystallinities, thin-film structures, and the organic field-effect transistors (OFETs) based on R-, S- and racemic EH-DNTT (rac-EH-DNTT) were studied to elucidate the effect of stereoisomerism in the 2-ethylhexyl group. The crystal structures of the R- and S-EH-DNTTs are classified as herringbone packing and contain two crystallographically independent molecules connected by edge-to-face CH-π intermolecular interactions, and the molecules' directly opposite directions avoid steric repulsion between the 2-ethylhexyl groups. Thin films of the EH-DNTTs fabricated using both the spin-coating and vacuum-deposition methods were revealed to have similar but slightly different packing structures to that in the single crystal. Intriguingly, the packing structures in the thin-film state depend on the deposition method, and not on the stereoisomers of EH-DNTT. Consistent with the packing structures in the thin-film state, the performance of OFETs based on the thin films of the R-, S- and rac-EH-DNTTs were affected by the deposition method, and not by the stereoisomerism. This means that the stereoisomerism in the alkyl side chain has a marginal effect on the packing structure and electronic properties in the thin-film state. This is endorsed by the theoretical calculations using the functional-group symmetry-adapted perturbation theory (F-SAPT), which indicated that the intermolecular interactions between the DNTT cores are dominant in the total intermolecular interaction energies, and implies that the crystallization process in the thin-film deposition could be governed by intermolecular interactions between the DNTT cores. We conclude that in 2-ethylhexyl-substituted organic semiconductors with a large and highly aggregative π-conjugated core, like EH-DNTT, the enantiopurity in the 2-ethylhexyl group does not significantly affect the thin-film structure and thus the performance of thin-film OFETs.

Quinoid-Aromatic Resonance for Very Small Optical Energy Gaps in Small-Molecule Organic Semiconductors: A Naphthodithiophenedione-oligothiophene Triad System.

Organic semiconductors with very small optical energy gaps have attracted a lot of attention for near-infrared-active optoelectronic applications. Herein, we present a series of donor-acceptor-donor (D-A-D) organic semiconductors consisting of a highly electron-deficient naphtho[1,2-b:5,6-b']dithiophene-2,7-dione quinoidal acceptor and oligothiophene donors that show very small optical energy gaps of down to 0.72 eV in the solid state. Investigation of the physicochemical properties of the D-A-D molecules as well as theoretical calculations of their electronic structures revealed an efficient intramolecular interaction between the quinoidal acceptor and the aromatic oligothiophene donors in the D-A-D molecules; this significantly enhances the backbone resonance and thus reduces the bond length alternation along the π-conjugated backbones. Despite the very small optical energy gaps, the D-A-D molecules have low-lying frontier orbital energy levels that give rise to air-stable ambipolar carrier transport properties with hole and electron mobilities of up to 0.026 and 0.043 cm2 V-1 s-1 , respectively, in field-effect transistors.

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

Control and prediction of crystal structures of molecular semiconductors are considered challenging, yet they are crucial for rational design of superior molecular semiconductors. It is here reported that through methylthiolation, one can rationally control the crystal structure of pyrene derivatives as molecular semiconductors; 1,6-bis(methylthio)pyrene keeps a similar sandwich herringbone structure to that of parent pyrene, whereas 1,3,6,8-tetrakis(methylthio)pyrene (MT-pyrene) takes a new type of brickwork structure. Such changes in these crystal structures are explained by the alteration of intermolecular interactions that are efficiently controlled by methylthiolation. Single crystals of MT-pyrene are evaluated as the active semiconducting material in single-crystal field-effect transistors (SC-FETs), which show extremely high mobility (32 cm2 V-1 s-1 on average) operating at the drain and gate voltages of -5 V. Moreover, the band-like transport and very low trap density are experimentally confirmed for the MT-pyrene SC-FETs, testifying that the MT-pyrene is among the best molecular semiconductors for the SC-FET devices.

Carbonyl-Terminated Quinoidal Oligothiophenes as p-Type Organic Semiconductors.

A series of quinoidal oligothiophenes terminated with carbonyl groups (nTDs, n = 2-4) are studied as p-type organic semiconductors for the active materials in organic field-effect transistors (OFETs) both by the theoretical and experimental approaches. The theoretical calculations clearly show their high-lying highest occupied molecular orbital (HOMO) energy levels (EHOMOs), small reorganization energies for hole transport (λholes), and large contribution of sulfur atoms to HOMOs, all of which are desirable for p-type organic semiconductors. Thus, we synthesized nTDs from the corresponding aromatic oligothiophene precursors and then evaluated their physicochemical properties and structural properties. These experimental evaluations of nTDs nicely proved the theoretical predictions, and the largest 4TDs in the series (4,4'''-dihexyl- and 3',4,4″,4'''-tetrahexyl-5H,5'''H-[2,2':5',2″:5″,2'''-quaterthiophene]-5,5'''-dione) can afford solution-processed OFETs showing unipolar p-type behaviors and hole mobility as high as 0.026 cm2 V-1 s-1.

Relating Selectivity and Separation Performance of Lamellar Two-Dimensional Molybdenum Disulfide (MoS2) Membranes to Nanosheet Stacking Behavior.

Increased demand for highly selective and energy-efficient separations processes has stimulated substantial interest in emerging two-dimensional (2D) nanomaterials as a potential platform for next-generation membranes. However, persistently poor separation performance continues to hinder the viability of many novel 2D-nanosheet membranes in desalination applications. In this study, we examine the role of the lamellar structure of 2D membranes on their performance. Using self-fabricated molybdenum disulfide (MoS2) membranes as a platform, we show that the separation layer of 2D nanosheet frameworks not only fails to demonstrate water-salt selectivity but also exhibits low rejection toward dye molecules. Moreover, the MoS2 membranes possess a molecular weight cutoff comparable to its underlying porous support, implying negligible selectivity of the MoS2 layer. By tuning the nanochannel size through intercalation with amphiphilic molecules and analyzing mass transport in the lamellar structure using Monte Carlo simulations, we reveal that small imperfections in the stacking of MoS2 nanosheets result in the formation of catastrophic microporous defects. These defects lead to a precipitous reduction in the selectivity of the lamellar structure by negating the interlayer sieving mechanism that prevents the passage of large penetrants. Notably, the imperfect stacking of nanosheets in the MoS2 membrane was further verified using 2D X-ray diffraction measurements. We conclude that developing a well-controlled fabrication process, in which the lamellar structure can be carefully tuned, is critical to achieving defect-free and highly selective 2D desalination membranes.

Crystal Structures of Dimethoxyanthracens: A Clue to a Rational Design of Packing Structures of π-Conjugated Molecules.

We carried out a systematic investigation of packing structures of a series of dimethoxyanthracenes, i. e., 1,4- (1), 1,8- (2), 1,5- (3), 2,6- (4), and 2,7-derivatives (5). The packing structures of the dimethoxyanthracenes are classified into two types, a rubrene-like pitched π-stack (1-3) and a typical herringbone packing (4 and 5), which evidently show that the position of methoxy groups is crucial to determine the packing structure of dimethoxyanthracenes. Effects of the substitution position on intermolecular interactions are analyzed by the noncovalent intermolecular interaction (NCI) method, Hirshfeld surface analysis, and symmetry-adapted perturbation theory (SAPT) method, thus clarifying active roles of the methoxy groups in the formation of rubrene-like pitched π-stack. The present results shed light on a molecular design strategy to realize the rubrene-like pitched π-stack in the solid state, which had been regarded as a packing structure limited for rubrene and its closely related derivatives.

"Disrupt and induce" intermolecular interactions to rationally design organic semiconductor crystals: from herringbone to rubrene-like pitched π-stacking.

The packing structures of organic semiconductors in the solid state play critical roles in determining the performances of their optoelectronic devices, such as organic field-effect transistors (OFETs). It is a formidable challenge to rationally design molecular packing in the solid state owing to the difficulty of controlling intermolecular interactions. Here we report a unique materials design strategy based on the ß-methylthionation of acenedithiophenes to generally and selectively control the packing structures of materials to create organic semiconductors rivalling rubrene, a benchmark high-mobility material with a characteristic pitched π-stacking structure in the solid state. Furthermore, the effect of the ß-methylthionation on the packing structure was analyzed by Hirshfeld surface analysis together with theoretical calculations based on symmetry-adapted perturbation theory (SAPT). The results clearly demonstrated that the ß-methylthionation of acenedithiophenes can universally alter the intermolecular interactions by disrupting the favorable edge-to-face manner in the parent acenedithiophenes and simultaneously inducing face-to-face and end-to-face interactions in the ß-methylthionated acenedithiophenes. This "disrupt and induce" strategy to manipulate intermolecular interactions can open a door to rational packing design based on the molecular structure.

Synthesis of Soluble Dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DNTT) Derivatives: One-Step Functionalization of 2-Bromo-DNTT.

For developing dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DNTT) derivatives as solution processable organic semiconductors, we synthesized 2-brominated DNTT (Br-DNTT) as a common precursor to 2-substituted DNTT derivatives. The synthesis of Br-DNTT features chemoselective metalation and cross-coupling reactions that enable us to keep the 2-bromo group intact from the starting material, 2-bromo-6-methoxynaphthalene, to Br-DNTT. We demonstrated one-step functionalization of Br-DNTT by various palladium- and copper-catalyzed cross-coupling reactions to introduce a variety of substituents, including ethynyl, aryl, heteroaryl, alkyl, alkoxy, and alkylthio groups, in yields of 73 to 98%. The resulting 12 examples of 2-substituted DNTT derivatives, which have bulky or flexible solubilizing groups, have improved solubilities of up to 200 times the solubility of unsubstituted DNTT. Some of the soluble 2-substituted DNTT derivatives were applied to the solution-processed fabrication of organic field-effect transistor (OFET) devices. Most of the OFET devices exhibited average hole mobilities in the order of 10-1 to 10-2 cm2 V-1 s-1. Among the DNTT derivatives, the one substituted with 4-(2-(2-methoxyethoxy)ethoxy)butyl group has the highest solubility of 8.45 g L-1 and also exhibited the highest average hole mobility of 0.28 cm2 V-1 s-1 in the OFET devices.

Precise nanofiltration in a fouling-resistant self-assembled membrane with water-continuous transport pathways.

Self-assembled materials are attractive for next-generation membranes. However, the need to align self-assembled nanostructures (e.g. cylinders, lamellae) and the narrow stability windows for ordered bicontinuous systems present serious challenges. We propose and demonstrate a novel approach that circumvents these challenges by exploiting size-selective transport in the water-continuous medium of a nanostructured polymer templated from a self-assembled lyotropic H1 mesophase. Optimization of the mesophase composition enables high-fidelity retention of the H1 structure on photoinduced cross-linking. The resulting material is a mechanically robust nanostructured polymer possessing internally and externally cross-linked nanofibrils surrounded by a continuous aqueous medium. Fabricated membranes show size selectivity at the 1- to 2-nm length scale and water permeabilities of ~10 liters m-2 hour-1 bar-1 µm. Moreover, the membranes display excellent antimicrobial properties due to the quaternary ammonium groups on the nanofibril surfaces. These results represent a breakthrough for the potential use of polymerized lyotropic mesophase membranes in practical water purification applications.

Single crystal texture by directed molecular self-assembly along dual axes.

Creating well-defined single-crystal textures in materials requires the biaxial alignment of all grains into desired orientations, which is challenging to achieve in soft materials. Here we report the formation of single crystals with rigorously controlled texture over macroscopic areas (>1 cm2) in a soft mesophase of a columnar discotic liquid crystal. We use two modes of directed self-assembly, physical confinement and magnetic fields, to achieve control of the orientations of the columnar axes and the hexagonal lattice along orthogonal directions. Field control of the lattice orientation emerges in a low-temperature phase of tilted discogens that breaks the field degeneracy around the columnar axis present in non-tilted states. Conversely, column orientation is controlled by physical confinement and the resulting imposition of homeotropic anchoring at bounding surfaces. These results extend our understanding of molecular organization in tilted systems and may enable the development of a range of new materials for distinct applications.

Controlling orientational order in block copolymers using low-intensity magnetic fields.

The interaction of fields with condensed matter during phase transitions produces a rich variety of physical phenomena. Self-assembly of liquid crystalline block copolymers (LC BCPs) in the presence of a magnetic field, for example, can result in highly oriented microstructures due to the LC BCP's anisotropic magnetic susceptibility. We show that such oriented mesophases can be produced using low-intensity fields (<0.5 T) that are accessible using permanent magnets, in contrast to the high fields (>4 T) and superconducting magnets required to date. Low-intensity field alignment is enabled by the addition of labile mesogens that coassemble with the system's nematic and smectic A mesophases. The alignment saturation field strength and alignment kinetics have pronounced dependences on the free mesogen concentration. Highly aligned states with orientation distribution coefficients close to unity were obtained at fields as small as 0.2 T. This remarkable field response originates in an enhancement of alignment kinetics due to a reduction in viscosity, and increased magnetostatic energy due to increases in grain size, in the presence of labile mesogens. These developments provide routes for controlling structural order in BCPs, including the possibility of producing nontrivial textures and patterns of alignment by locally screening fields using magnetic nanoparticles.

Polymer Nanosheets from Supramolecular Assemblies of Conjugated Linoleic Acid-High Surface Area Adsorbents from Renewable Materials.

We present a strategy for robustly cross-linking self-assembled lamellar mesophases made from plant-derived materials to generate polymer nanosheets decorated with a high density of functional groups. We formulate a supramoleclar complex by hydrogen-bonding conjugated linoleic acid moieties to a structure-directing tribasic aromatic core. The resulting constructs self-assemble into a thermotropic lamellar mesophase. Photo-cross-linking the mesophase with the aid of an acrylate cross-linker yields a polymeric material with high-fidelity retention of the lamellar mesophase structure. Transmission electron microscopy images demonstrate the preservation of the large area, highly ordered layered nanostructures in the polymer. Subsequent extraction of the tribasic core and neutralization of the carboxyl groups by NaOH result in exfoliation of polymer nanosheets with a uniform thickness of ∼3 nm. The nanosheets have a large specific area of ∼800 m2/g, are decorated by negatively charged carboxylate groups at a density of 4 nm-2, and exhibit the ability to readily adsorb positively charged colloidal particles. The strategy as presented combines supramolecular self-assembly with the use of renewable or sustainably derived materials in a scalable manner. The resulting nanosheets have potential for use as adsorbents and, with further development, rheology modifiers.

Highly Selective Vertically Aligned Nanopores in Sustainably Derived Polymer Membranes by Molecular Templating.

We describe a combination of molecular templating and directed self-assembly to realize highly selective vertically aligned nanopores in polymer membranes using sustainably derived materials. The approach exploits a structure-directing molecule to template the assembly of plant-derived fatty acids into highly ordered columnar mesophases. Directed self-assembly using physical confinement and magnetic fields provides vertical alignment of the columnar nanostructures in large area (several cm2) thin films. Chemically cross-linking the mesophase with added conventional vinyl comonomers and removing the molecular template results in a mechanically robust polymer film with vertically aligned 1.2-1.5 nm diameter nanopores with a large specific surface area of ∼670 m2/g. The nanoporous polymer films display exceptional size and charge selectivity as demonstrated by adsorption experiments using model penetrant molecules. These materials have significant potential to function as high-performance nanofiltration membranes and as nanoporous thin films for high-density lithographic pattern transfer. The scalability of the fabrication process suggests that practical applications can be reasonably anticipated.

Dithienyl Acenedithiophenediones as New π-Extended Quinoidal Cores: Synthesis and Properties.

We have synthesized two isomeric pairs of benzo- and naphthodithiophenediones with two flanking thiophenes and characterized them by single-crystal X-ray analysis, cyclic voltammetry, steady-state optical electronic absorption and emission spectroscopies, transient absorption spectroscopy, and vibrational spectroscopies with in situ spectroelectrochemistry techniques, and then compared them with the thieno[3,2-b]thiophene-2,5-dione counterpart that we previously reported. The results show that the central acenedithiophenedione cores have quinoidal conjugation with closed-shell character. The π-extension of the quinoidal core raises (lowers) the HOMO (LUMO) energy levels of the triads, resulting in the drastic reduction of their energy gaps from approximately 2.0 eV to 1.1 eV. Owing to the electron-withdrawing nature of the carbonyl terminal group at the quinoidal core, the triads have low-lying LUMO energy levels ranging from -3.9 eV to -4.3 eV, and can be regarded as strong electron-acceptor building units. Interestingly, the pairs of structural isomers have similar electronic structures in both the neutral and charged states despite the different shapes (linear and angular) and/or symmetry (C2h and C2v ) of the acenedithiophenedione cores.

Very Small Bandgap π-Conjugated Polymers with Extended Thienoquinoids.

The introduction of quinoidal character to π-conjugated polymers is one of the effective approaches to reducing the bandgap. Here we synthesized new π-conjugated polymers (PBTD4T and PBDTD4T) incorporating thienoquinoids 2,2'-bithiophene-5,5'-dione (BTD) and benzo[1,2-b:4,5-b']dithiophene-2,6-dione (BDTD) as strong electron-deficient (acceptor) units. PBTD4T showed a deep LUMO energy level of -3.77 eV and a small bandgap of 1.28 eV, which are similar to those of the analog using thieno[3,2-b]thiophene-2,5-dione (TTD) (PTTD4T). PBDTD4T had a much deeper LUMO energy level of -4.04 eV and a significantly smaller bandgap of 0.88 eV compared to those of the other two polymers. Interestingly, PBDTD4T showed high transparency in the visible region. The very small bandgap of PBDTD4T can be rationalized by the enhanced contribution of the resonance backbone structure in which the p-benzoquinodimethane skeleton in the BDTD unit plays a crucial role. PBTD4T and PBDTD4T exhibited ambipolar charge transport with more balanced mobilities between the hole and the electron than PTTD4T. We believe that the very small bandgap, i.e., the high near-infrared activity, as well as the well-balanced ambipolar property of the π-conjugated polymers based on these units would be of particular interest in the fabrication of next-generation organic devices.

Horizontal and Vertical Orientation of Polythiophenes by Electrochemical Polymerization in Magnetically Aligned Smectic Liquid Crystal.

Macroscopically highly oriented polythiophene derivative films are synthesized by electrochemical polymerization in a smectic liquid crystal reaction medium. Control of orientation direction of conjugated main chains is performed by use of magnetic field which is applied parallel, obliquely, and perpendicularly to the substrate electrodes to align the liquid crystal reaction medium.

Synthesis of Isothianaphthene (ITN) and 3,4-Ethylenedioxy-Thiophene (EDOT)-Based Low-Bandgap Liquid Crystalline Conjugated Polymers.

Copolymers, consisting of isothianaphthene and phenylene derivatives with liquid crystal groups, were synthesized via Migita-Kosugi-Stille polycondensation reaction. IR absorption, UV-vis optical absorption, and PL spectroscopy measurements were carried out. Thermotropic liquid crystallinity of the polymers with bandgap of ~2.5 eV was confirmed.

Dynamically controllable emission of polymer nanofibers: electrofluorescence chromism and polarized emission of polycarbazole derivatives.

Electrochemical polymerization of a series of N-alkyl-2,7-di(2-thienyl)carbazoles in acetonitrile was performed to obtain conjugated polymers with fluorescence. Scanning electron and atomic force microscopies revealed that the surface morphology of the polymer films significantly depends on the alkyl chain lengths of the polymers. Particularly, a homopolymer bearing hexyl groups and copolymers with an average alkyl chain length of six carbon atoms show nanofiber morphology. The polymer nanofibers were stacked on a substrate electrode. The fluorescence of the polymer nanofiber film was tunable with application of voltage, with good repeatability. The X-ray diffraction pattern of the fibers showed the structural order. The polymer nanofibers thus prepared showed an electrochemically driven change in polarized photoluminescence.