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Solid-State Conformation, Molecular Packing, and Electrical and Optical Properties of Processable β-Methylated Sexithiophenes
Abstract
Newly synthesized sexithiophenes, di- and tetramethylated at the β-positions, are shown to be soluble and processable compounds, giving single crystals suitable for X-ray structure determination. The molecular packing was characterized in terms of crystal cohesion and short CH··· S and CH···π intermolecular distances. In particular, the dimethylated sexithiophene displayed very compressed molecular packing and a thin film field effect transistor, fabricated with this material, was characterized by high charge mobility [2 × 10-2 cm2/(V s)]. The tetramethylated compound crystallizes at room temperature in two different systems and with different conformations. The conformational polymorphs, which are easily interconverted at room temperature, are characterized by different wavelengths of light emission and excitation decay rates.
... Even more interesting is the behaviour of oligothiophene-S,S-dioxides with flexible backbone, that, in marked contrast with the non functionalised molecules, show the typical AIEgens behaviour, with low PLQY in solution but very high PLQY (up to 70%) in the solid state [17,38,41,[44][45][46]. In addition, high tunable optical gain has been demonstrated [47], thus making oligothiophene-S,S-dioxides excellent candidates as active materials for light emitting devices, such as LEDs and lasers [20,22,[48][49][50][51]. ...
The development of organic molecules showing high photoluminescence quantum yield (PLQY) in solid state is a fundamental step for the implementation of efficient light emitting devices. In this work the origin of the high PLQY of two trimers and two pentamers having one central thiophene-S,S-dioxide unit and two and four lateral thiophene or phenyl groups, respectively, is investigated by temperature dependent photoluminescence and time resolved photoluminescence measurements. The experimental results demonstrate that the molecules with lateral phenyl rings show higher PLQY due to a weaker coupling with intramolecular vibrations-related to variations in the radiative and non-radiative decay rates-and indicate different molecular rigidity as the main factors affecting the PLQY of this class of molecules.
... 61 It should also be noted that packing and contact effects occurring in molecular crystals or amorphous structures are known to inuence the encountered solid-state conformation and exibility for geometrical relaxation. 26,62,63 Potentially, generative ML models trained on condensed phase data could therefore help producing more realistic conformer ensembles. ...
The molecular reorganization energy λ strongly influences the charge carrier mobility of organic semiconductors and is therefore an important target for molecular design. Machine learning (ML) models generally have the potential to strongly accelerate this design process (e.g. in virtual screening studies) by providing fast and accurate estimates of molecular properties. While such models are well established for simple properties (e.g. the atomization energy), λ poses a significant challenge in this context. In this paper, we address the questions of how ML models for λ can be improved and what their benefit is in high-throughput virtual screening (HTVS) studies. We find that, while improved predictive accuracy can be obtained relative to a semiempirical baseline model, the improvement in molecular discovery is somewhat marginal. In particular, the ML enhanced screenings are more effective in identifying promising candidates but lead to a less diverse sample. We further use substructure analysis to derive a general design rule for organic molecules with low λ from the HTVS results.
... 56 It should also be noted that packing and contact effects occurring in molecular crystals or amorphous structures are known to influence the encountered solid-state conformation and flexibility for geometrical relaxation. 26,57,58 Potentially, generative ML models trained on condensed phase data could therefore help producing more realistic conformer ensembles. ...
The molecular reorganization energy $\lambda$ strongly influences the charge carrier mobility of organic semiconductors and is therefore an important target for molecular design. Machine learning (ML) models generally have the potential to strongly accelerate this design process (e.g. in virtual screening studies) by providing fast and accurate estimates of molecular properties. While such models are well established for simple properties (e.g. the atomization energy), $\lambda$ poses a significant challenge in this context. In this paper, we address the questions of how ML models for $\lambda$ can be improved and what their benefit is in high-throughput virtual screening (HTVS) studies. We find that, while improved predictive accuracy can be obtained relative to a semiempirical baseline model, the improvement in molecular discovery is somewhat marginal. In particular, the ML enhanced screenings are more effective in identifying promising candidates but lead to a less diverse sample. We further use substructure analysis to derive a general design rule for organic molecules with low $\lambda$ from the HTVS results.
... [28] More recently, we have reported the polymorphs of a thiomethyl substituted quaterthiophene [29] as well as of a tetramethyl substituted sexithiophene. [30] Conformational polymorphism between amorphous and crystalline phase is at the origin of the fabrication of a bicolored pixel structure LED based on a single luminescent oligothiophene. [31] Polymorphic needle-shaped crystals and crystalline nanofibers with very different functional properties and reproducibly grown on different substrates have also recently been reported. ...
The implementation of nano/microelectronic devices requires efficient strategies for the realization of supramolecular structures with desired function and supported on appropriate substrates. This article illustrates a strategy based on the synthesis of thiophene oligomers having the same “sulfur‐overrich” quaterthiophene inner core (non bonding interactional algorithm) and different terminal groups. Nano/microfibers are formed on surfaces having a morphology independent of the nature of the deposition substrate and displaying a wide tuning of properties that make the fibers optoelectronically suitable for application in devices. Thiophene‐based hexamers and octamers having the same identical tetrameric ‘sulfur‐overrich’ inner core acting as an interactional algorithm, spontaneously self‐assemble into surface‐independent, crystalline, nanostructured microfibers. The microfibers, rod‐like or helical depending on the packing forces induced by the groups linked to the inner core, are fluorescent, conductive and chiral at the nanoscale even in the lack of chiral groups in the molecular skeleton.
... Generally, different polymorphs of crystals might exhibit different properties, such as different habits, mechanical properties, 37,38 and electrical and optical properties. 39 Solid state photochemical reactions are also affected by crystal structure. Bertmer et al. 40 reported the lower kinetics of the photoreaction of the β-polymorph than that of α-cinnamic acid, which can be ascribed to the higher amount of molecular reorientation of functional groups upon photoreaction and the larger distance between the reacting double bonds. ...
Although diverse molecule structures have been designed to realize photoresponsive motion, the effects of polymorphism on photoresponsiveness are rarely reported. Here we utilize trans-4,4’-azopyridine (trans-4AP) as model compound to investigate the effect of crystal structures on photomechanical motion. A new polymorph of the trans-4AP crystal, Form 2, was discovered and compared with known Form 1 from the perspective of molecular arrangement and intermolecular interactions. It was found that different molecular arrangements resulted in distinct performance of photoisomerization in two solids. Under the UV light, Form 1 crystal bent away from the UV light, while Form 2 crystals exhibited diverse photomechanical motions including quasi-bidirectional bending and twisting motion, depending on different sizes and shapes of the used crystals. Such deformation originated from the diverse strain distribution in the irradiated surfaces in Form 1 and Form 2, which was verified by powder X-ray diffraction, solid-state UV/Vis absorption spectra and atomic force microscopy (AFM). Various contribution and energy of intermolecular interactions make two polymorphs of trans-4AP crystals deform in different modes. Relatively weak intermolecular interaction resulted in larger deformation in Form 2, which was further proved by the Hirshfeld Surface and energy framework analysis.
... π-π stacking has in fact been observed in many oligothiophenes and their derivatives. [42][43][44][45][46][47][48] Experimental measurements indicate that π-π interaction plays an important role in the control of the solid-state and self-assembled structures of thiophene oligomers and polymers. [49,50] To this end, we report the synthesis and characterization of a coil-rod-coil π-conjugated LC compound (quaterthiophene (4T)/ PEO4) consisting of oligothiophene and ethylene oxide moieties. ...
In this work, a joint experimental and computational study on the synthesis, self‐assembly, and ionic conduction characteristics of a new conjugated liquid crystal quaterthiophene/poly(ethylene oxide) (PEO4) consisting of terminal tetraethyleneglycol monomethyl ether groups on both ends of a quaterthiophene core is performed. In agreement with molecular dynamic simulations, temperature‐dependent grazing‐incidence wide angle X‐ray scattering and X‐ray diffraction indicate that the molecule spontaneously forms a smectic phase at ambient temperature as characterized both in bulk and thin film configurations. Significantly, this smectic phase is maintained upon blending with bis(trifluoro‐methanesulfonyl)imide as ion source at a concentration ratio up to r = [Li⁺]/[EO] = 0.05. Nanosegregation between oligothiophene and PEO moieties and π–π stacking of thiophene rings lead to the formation of efficient 2D pathways for ion transport, resulting in thin‐film in‐plane ionic conductivity as high as 5.2 × 10⁻⁴ S cm⁻¹ at 70 °C and r = 0.05 as measured by electrochemical impedance spectroscopy. Upon heating the samples above a transition temperature around 95 °C, an isotropic phase forms associated with a pronounced drop in ionic conductivity. Upon cooling, partial and local reordering of the conducting smectic domains leads to an ionic conductivity decrease compared to the as‐cast state.
... a-Oligofurans with engineered 3-hexylfuran head-to-head defects remained essentially planar ( Figure 3). [11a] For comparison, in relatedt hiophene systems, head-to-head sites have dihedrala ngles of around2 6 8. [12] Another important consequence resultingf rom the increased rigidity of furans, along with the smaller size of the oxygen atom, is that the spacingb etween furan oligomers in the solid state is smaller than in thiophene oligomers. [11d] Optically,t he onseto fa bsorption for oligofurans is blueshifted compared to thiophenes. ...
Thiophene is one of the most ubiquitous moieties in organic conjugated materials, however, furan, its oxygen congener, and furan derivatives, have received comparatively less attention. This is primarily due to the intrinsic instability of furan and its tendency to decompose in the presence of oxygen and light. Incorporating furan into conjugated systems can confer many benefits, including increases in conjugation, improved solubility, and better transport properties. In this Concept Article, advances in furan containing conjugated materials are presented. The impact of furan on the properties of conjugated materials is discussed, recent advances in synthetic methods are overviewed, and strategies for improving the stability of conjugated furans are detailed.
... 16 The structural flexibility of oligothiophenes has been shown to result in conformational polymorphism 17,18 and variations in electronic properties in the solid state. 19,20 Similar polymorphic effects can also be expected to play a role at molecule−metal interfaces, with potentially significant consequences for charge transport in oligothiophene-based devices. ...
Charge transport in electronic applications involving molecular semiconductor materials strongly depends on the electronic properties of molecular-scale layers interfacing with external electrodes. In particular, local variations in molecular environments can have a significant impact on the interfacial electronic properties. In this study, we use scanning tunneling microscopy and spectroscopy to investigate the self-assembly regimes and resulting electronic structures of alkyl-substituted quaterthiophenes adsorbed on the Au(111) surface. We find that at dilute molecular concentrations, dimerized cis conformers were formed, while at higher concentrations corresponding to small fractions of a submonolayer, the molecular conformation converted to trans, with the molecules self-assembled into ordered islands. At approximately half-monolayer concentrations, the structure of the self-assembled islands transformed again showing a different type of the trans conformation and qualitatively different registry with the Au(111) lattice structure. Molecular distributions are observed to vary significantly due to variations in local molecular environments, as well as due to variations in the Au(111) surface reactivity. While the observed conformational diversity suggests the existence of local variations in the molecular electronic structure, significant electronic differences are found even with molecules of identical apparent adsorption configurations. Our results show that a significant degree of electronic disorder may be expected even in a relatively simple system composed of conformationally flexible molecules adsorbed on a metal surface, even in structurally well-defined self-assembled molecular layers.
... 5 A potential complication that may impact oligothiophene-based applications is that the relative flexibility of oligothiophene backbones has been found to lead to conformational polymorphism, 6,7 resulting in variations of the electronic properties in the solid state. 8,9 Similarly, the effects of conformational polymorphism specific to interfaces may be expected to affect the chargetransport properties in oligothiophene-based devices. This expectation is supported by several scanning tunneling microscopy (STM) studies, which demonstrated that substituted oligothiophenes can show conformational polymorphism, 10−12 as well as diverse self-assembly regimes caused by the presence of substituent groups. ...
Alkyl-substituted quaterthiophenes on Au(111) form dimers linked by their alkyl substituents and, instead of adopting the trans-conformation found in bulk oligothiophene crystals, assume cis-conformations. Surprisingly, the impact of the conformation is not decisive in determining the LUMO energy. Scanning Tunneling Microscopy and Spectroscopy of the adsorption geometries and electronic structures of alkyl-substituted quaterthiophenes show that the orbital energies vary substantially due to the local variations in the Au(111) surface reactivity. These results demonstrate that interfacial oligothiophene conformations and electronic structures may differ substantially from those expected based on the band structures of bulk oligothiophene crystals.
... Continuous investigations on tailoring of the molecular structure are giving rise to considerable structural diversity. At that, major efforts are directed towards improving chemical stability and processability, facilitation of self-ordering, and adjustment of optical and electronic properties [7][8][9][10][11]. ...
We study the morphology and optical properties of vacuum deposited films of the α-ω-bis-(dicyanovinylen)-sexithiophene, comprising four butyl side chains DCV6T-Bu(1,2,5,6) (DCV6T-Bu). An absorption band showing vibronic substructure indicates ordered molecular arrangement in the solid state. The room temperature (RT) self-organization is confirmed by X-ray diffraction (XRD). For films grown on heated substrates, XRD analysis and atomic force microscopy display increased crystallinity with larger domain size. In correlation to the XRD data, with increasing substrate temperature the absorption of the heated films becomes more structured and continuously shifts to longer wavelengths. Further, the hole mobility in DCV6T-Bu/C60 planar heterojunction (PHJ) devices, utilizing DCV6T-Bu films grown at RT and elevated substrate temperature is investigated using the charge extraction by linearly increasing voltage method. The derived values of the activation energy are consistent with the corresponding DCV6T-Bu film morphology. However, the charge carrier mobility does not increase with improving molecular order, as is evident by the obtained mobility values of 1.0×10−6cm2/Vs for the RT and 3.1×10−7cm2/Vs for the heated device, respectively. Finally, DCV6T-Bu/C60 PHJ solar cells consisting of absorber layers deposited on heated and unheated substrates are compared.
As an effective method to modulate the physicochemical properties of materials, crystal phase engineering, especially hetero‐phase, plays an important role in developing high‐performance photocatalysts. However, it is still a huge challenge but significant to construct porous hetero‐phase nanostructures with adjustable band structures. As a kind of unique porous crystalline materials, metal‐organic frameworks (MOFs) might be the appropriate candidate, but the MOF‐based hetero‐phase is rarely reported. Herein, we developed a secondary building unit (SBU) regulating strategy to prepare two crystal phases of Ti‐MOFs constructed by titanium and 1,4‐dicarboxybenzene, i.e., COK and MIL‐125. Besides, COK/MIL‐125 hetero‐phase was further constructed. In the photocatalytic hydrogen evolution reaction, COK/MIL‐125 possessed the highest H2 yield compared to COK and MIL‐125, ascribing to the Z‐scheme homojunction at hetero‐phase interface. Furthermore, by decorating with amino groups (i.e., NH2‐COK/NH2‐MIL‐125), the light absorbing capacity was broadened to visible‐light region, and the visible‐light‐driven H2 yield was greatly improved. Briefly, the MOF‐based hetero‐phase possesses periodic channel structures and molecularly adjustable band structures, which is scarce in traditional organic or inorganic materials. As a proof of concept, our work not only highlights the development of MOF‐based hetero‐phase nanostructures, but also paves a novel avenue for designing high‐performance photocatalysts.
As an effective method to modulate the physicochemical properties of materials, crystal phase engineering, especially hetero‐phase, plays an important role in developing high‐performance photocatalysts. However, it is still a huge challenge but significant to construct porous hetero‐phase nanostructures with adjustable band structures. As a kind of unique porous crystalline materials, metal–organic frameworks (MOFs) might be the appropriate candidate, but the MOF‐based hetero‐phase is rarely reported. Herein, we developed a secondary building unit (SBU) regulating strategy to prepare two crystal phases of Ti‐MOFs constructed by titanium and 1,4‐dicarboxybenzene, i.e., COK and MIL‐125. Besides, COK/MIL‐125 hetero‐phase was further constructed. In the photocatalytic hydrogen evolution reaction, COK/MIL‐125 possessed the highest H2 yield compared to COK and MIL‐125, ascribing to the Z‐Scheme homojunction at hetero‐phase interface. Furthermore, by decorating with amino groups (i.e., NH2‐COK/NH2‐MIL‐125), the light absorbing capacity was broadened to visible‐light region, and the visible‐light‐driven H2 yield was greatly improved. Briefly, the MOF‐based hetero‐phase possesses periodic channel structures and molecularly adjustable band structures, which is scarce in traditional organic or inorganic materials. As a proof of concept, our work not only highlights the development of MOF‐based hetero‐phase nanostructures, but also paves a novel avenue for designing high‐performance photocatalysts.
A longstanding challenge in the field of optoelectronic materials, the effects of solid-state arrangement and morphology are still a prominent factor associated with small-molecule and polymer-based device performance. Here, mixed...
To access the applications of a conformationally flexible molecule, thorough understanding of even the weak contributors to a particular conformation is necessary. We have synthesized and X-ray characterized a series of six diaryl-substituted heteroarenes in order to analyze the role of unconventional intramolecular CAr-H···N/O vs conventional H-bonds on the prefered planar geometry of such molecules. The distance well below the sum of van der Waals radii in their solid state structures indicated the existence of intramolecular CAr-H···N/O interactions. Consequently, the theoretical study was carried out to analyze the energies associated to each of these bonds using the quantum theory of "atoms-in-molecules" (QTAIM) and also the NCI plot. The results indicate that the unconventional C-H···O H-bonds involving aromatic H-atoms and the O-atoms of o-methoxy or o-hydroxy substituents in 2, 3a and 3b despite being weaker (dissociation energies around 3-5 kcal/mol) are relevant and contributing to their planar geometry along with various other factors. However, the electronic repulsion between the lone pair of the nitrogen/oxygen atoms and the neighboring π cloud in these molecules likely contributes more to their preferred planar conformation.
Organic electronics are particularly susceptible to minor impurities within a materials sample because any amount of impurity can alter device performance. Here we outline and explore a specific example of how the sudden decrease in our organic solar cell (OSC) performance was attributed to purchased material with a minor impurity not acknowledged by an external supplier. Had there been no standard procedure in place to evaluate a new batch of materials, the issue could have gone unchecked and inadvertently affected other results in the future. In this specific example, we confirmed the presence of impurities in an alpha-sexithiophene sample purchased from a supplier that drastically affected baseline device data. Our goal is to bring awareness to the issue and emphasise the importance of verifying the purity of any materials purchased for organic electronics by establishing a baseline which would flag any major fabrication changes or the presence of impurities. We hope these analogous issues and methodologies are reported in pursuit of advancing the pace of research and development.
The polymorph selection in a continuous crystallization process combined with wet milling is investigated. To this end, a dimensionless population balance equation model accounting for secondary nucleation, crystal growth and breakage is formulated and solved numerically. We show that a surprisingly small number of dimensionless parameter groups (combinations of kinetic parameters and operating conditions) is decisive to control the polymorphic outcome. Specifically, we show how the operating region where the stable polymorph is obtained can be enlarged by tuning the milling intensity, feed concentration, and residence time. We further rationalize the dependence of the mean size of the particles obtained, the fraction of solute recovered and the productivity of such a process on the dimensionless variables. We showcase this for the model system L-glutamic acid crystallized from water and show that our analysis is in agreement with previously reported experimental studies. Summarizing, the analysis approach introduced here can be used to identify operating spaces for single stage continuous crystallization processes where the right polymorph is reliably obtained and where size, solute recovery and productivity are guaranteed to desired levels.
Three β,β’‐dihexylsexithiophenes were synthesized, one without end group modification (T6.1), one with dialdehyde (T6.2) and one with diester (T6.3) end‐capping. Their properties were investigated by cyclic voltammetry and Ultraviolet–visible spectroscopy, and it was found that while they were nonconducting in undoped form, the conductivity reached 0.001 S cm−1 (T6.1), 0.017 S cm−1 (T6.2), and 0.003 S cm−1 (T6.3) when doped with iodine vapor. The solubilities of T6.2 and T6.3 in chloroform were excellent, whereas the solubility of T6.1 was low and in line with earlier literature values. Moreover, the studies revealed that T6.2 is also highly soluble in tetrahydrofuran. For the dialdehyde end‐capped β,β’‐dihexylsexithiophene T6.2 association constants for self‐association in chloroform and with silica were determined to be 1.6 and 2.3 (mol/kg)−1, respectively. In contrast, the two other oligothiophenes did not show such associations. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46830.
Mediating the backbone coplanarity and solubility of oligothiophenes, especially the head-to-head (HH) disubstituted bithiophene, to achieve an optically and electronically advantageous building block for organic semiconductor materials is a vital yet challenging task. On the other hand, exploring polymer solar cells (PSCs) processed from non-halogenated solvent is necessary toward their large-scale applications. In this contribution, we develop a HH-type bithiophene analogue (TIPS-T2) by strategically applying the triisopropylsilylethynyl (TIPS) scaffold as the side chain. TIPS can serve to narrow optical band gaps, to lower HOMO level, to reduce intrachain steric hindrance, and to guarantee sufficient solubility of the involving polymers. Upon alternating with difluorobenzotriazole (FTAZ) or benzodithiophene-4,8-dione (BDD) acceptor units, two polymers named PT4Si-FTAZ and PT4Si-BDD are synthesized. Encouragingly, non-fullerene PSCs incorporating PT4Si-FTAZ yield a PCE of 6.79% when processed from an environment-friendly solvent of trimethylbenzene due to its promoted backbone planarity as demonstrated by density functional theory (DFT), the involving higher hole mobility, and superior film morphology. The results indicate that TIPS-T2 is a promising building block for constructing photovoltaic polymers and our findings offer an avenue to the ingenious use of TIPS as functional side chains.
The design and synthesis of coordination polymers with a self-penetrating architecture has attracted much interest not only due to their interesting structures but also due to their potential applications. 5,5′-Bis(pyridin-4-yl)-2,2′-bithiophene (bpbp), as a conjugated bithiophene ligand, can exhibit trans and cis conformations and this can lead to the construction of a self-penetrating architecture. In addition, the semi-rigid ancillary ligand 4,4′-oxybis(benzoic acid) (H2oba) can adopt different coordination modes, resulting in coordination polymers with high-dimensional skeletons. A new CdII coordination polymer based on mixed ligands, namely poly[diaquapentakis[μ-5,5′-bis(pyridin-4-yl)-2,2′-bithiophene-κ2N:N′]bis(nitrato-κ2O,O′)tetrakis(μ3-4,4′-oxydibenzoato)-κ10O:O,O′:O′′,O′′′;κ6O:O′:O′′-pentacadmium(II)], [Cd5(C14H14O5)4(NO3)2(C18H12N2S2)5(H2O)2]n, (I), has been synthesized under solvothermal conditions and characterized by single-crystal X-ray diffraction, IR spectroscopy and elemental analysis. Single-crystal X-ray diffraction indicates that there are three crystallographically independent CdII cations, three bpbp ligands, two deprotonated oba2− ligands, one nitrate ligand and one coordinated water molecule in the asymmetric unit. One CdII centre is seven-coordinated, exhibiting a distorted {CdN2O5} pentagonal bipyramidal geometry, while the other two Cd centres are both six-coordinated, showing slightly distorted {CdN2O4} octahedral geometries. The most interesting feature is the co-existence of trans and cis conformations in a single net, allowing structural interpenetration via self-threading and yet the expected self-penetrating structure was obtained. Topological analysis shows that the whole three-dimensional framework can be classified as a 3-nodal (4,6,6)-c net with Schläfli symbol {613.82}2{66}, which is a new topology. Furthermore, the luminescence properties of (I) were examined in the solid state at room temperature.
This thesis explored how, by blending of materials with different electrical characteristics, it is possible to fabricate transistors with new or improved performances. First, organic field-effect transistors based on a single oligothiophene, DH4T, were fabricated and optimized until the measured mobility was superior to that observed in vacuum deposited films. This was achieved through careful tuning of the interfaces using self-assembled monolayers and by strong control of the solvent- evaporation rate. P-type polymers were blended with an n-type polymer. Each resulting solution was used for the fabrication of ambipolar field-effect transistors. These devices were characterized and it was found that for each pair of p- and n-type polymers, a transistor with balanced mobilities and high Ion/Ioff could be fabricated. Finally field-effect transistors based on a blend of P3HT and a photoswitchable diarylethene (DAE-Me) were fabricated. The current was measured during and between irradiations and it was demonstrated that a non-volatile multilevel memory could be fabricated.
Methods of synthesis of oligothiophenes using thiophene lithium, copper, magnesium, zinc, boron, silicon and tin derivatives and also cyclization reactions have been reviewed.
[Extract] Introduction: Thiophene chemistry is a flourishing area of organic semiconductors as they are useful for field effect transistors, solar cells and light-emitting diodes.1 This interest stems from the unique optical and electronic properties of thiophene oligomers and polymers coupled with the ease of synthesis and their unique reaction selectivity. Therefore, the preparation of thiophenes is pivotal in this burgeoning area of science. Although there have been several reviews that have focused on the preparation of poly(alkylthiophene)s, this is not the case for oligomeric and monomeric materials. Halo-substituted alkylthiophenes are very useful compounds that react under various conditions to give a plethora of derivatives, which can then be used in the field of materials chemistry.
This review summarizes recent advances (from 1986-2013) in the synthesis and reactivity of these systems to provide a stepping-stone for the design and preparation of more complex thiophene-based molecular architectures for the next generation of materials in photonics and electronics. We will focus first on the preparation of halo-substituted alkylthiophenes by electrophilic aromatic substitution and then their reactivity using various metal-catalyzed cross-coupling methods.
This chapter illustrates the relationships between structural and optical properties in oligothiophenes. In particular, it highlights the proper functionalization of the thienyl units with oxygen atoms; it is possible to increase the photoluminescence efficiency up to values comparable and even better than those of the best organic materials commonly used as active compounds in light emitting devices. Such a characteristic, together with the high chemical stability and the wide color tunability, makes oligothiophene-5,5'-dioxides excellent candidates not only for applications to electronic devices, but also to optoelectronic devices, such as multi-color LEDs and lasers. Recent achievement of high-performances thiophene-based devices is discussed.
Semiconductors based on organic molecular components have been the focus of intense investigation for the past half century.1 During most of that time, these materials, primarily consisting of carbon, hydrogen, and oxygen were considered to be merely a scientific curiosity. The solid state structure of these materials is based on individual molecular components bound together by weak interactions, principally van der Waals and dipole-dipole forces, imparting within them the properties of both semiconductors and insulators. Initial studies deepened the fundamental understanding of the physics of these solids, in particular electrical, electronic, and optical properties. Furthermore, organic semiconductors attracted industrial interest when it was recognized that many of them are photoconductive under visible light. This discovery led to their use in xerography and as light valves in liquid crystal displays (LCDs). There were even hopes that very low-cost thin-film solar cells and superconductors could be fabricated using such substances. Unfortunately, the potential of active electronic devices such as solar cells, light emitters, and thin-film transistors (TFTs) remained unfulfilled for decades because organic materials have often proved to be environmentally unstable. Furthermore, making reliable electrical contacts to organic thin-films is difficult, and when exposed to air, water, or ultraviolet light, their electronic properties can degrade rapidly. Finally, initial low carrier mobilities characteristic of organic materials obviated their use in high-frequency applications.
This chapter focuses on the development of organic semiconductors, with particular emphasis on the design strategy of novel semiconductors with a high mobility and stability. The organic semiconductor is a key component of an organic field-effect transistor (OFET). In terms of molecular weight, organic semiconductors can be subdivided into small molecules and polymers while, on the basis of the main charge carriers transporting in OFET channels, organic semiconductors can be further divided into p-type, n-type, and bipolar semiconducting materials. The details of some polymer semiconductors are described in the chapter. As most organic semiconductors possess symmetrical molecular structures, the general synthetic techniques employed in this field will not involve complicated chiral syntheses. The organic semiconductors obtained from multistep syntheses always require to be further purified, using a variety of techniques include recrystallization, column chromatography, physical vapor deposition (PVD) and soxhlet extraction.Controlled Vocabulary TermsOFETs; organic semiconductors; semiconductor doped polymers
We present the fabrication of a pixels structure by a well-defined pattern replication of a micrometer template driven by a surface free-energy lithographic technique, realized by molecular aggregation in dewetting conditions and by confining the liquid solution with geometric boundaries. The organization in the solid-state of the selected thiophene-based molecular materials allows to realize a bicoloured, green and red-emitting pixels structure, by exploiting the molecular structural arrangement, induced during a dewetting process, and the great conformational flexibility of DTT7Me.
High-performance n-type organic field-effect transistors (OFETs) based on 2-dimensional (2D) crystalline layered films of the novel dicyanodistyrylbenzene (DCS) derivative (2Z,2′Z)-3,3′-(1,4-phenylene)bis(2-(3,5-bis(trifluoromethyl)phenyl)acrylonitrile) (CN-TFPA) were fabricated using a simple solution process. The OFETs showed electron mobilities of up to 0.55 cm2 V-1 s-1, which was attributed to the appropriate electron affinity and dense molecular packing in the well-ordered 2D terrace structure. Because of the easy exfoliation capabilities of the CN-TFPA 2D crystalline layers, 2-10 CN-TFPA molecular monolayers could be successfully transferred onto the substrates, enabling the fabrication of ultrathin OFET devices with an active layer thickness of ∼30 nm.Keywords: single-crystalline organic field-effect transistor; n-type semiconductor; two-dimensional single-crystal; solution-process; mechanical cleavage
A study was conducted to turn the crystal polymorphs of alkyl thienoacene via solution self-assembly toward air-stable and high-performance organic field-effect transistors (OFET). The study demonstrated controllable solution self-assembly of two different crystal phases of dihexyl-substituted DBTDT (C6-DBTDT), which was synthesized in a new, facile, and efficient method. The simple solution tailoring of different polymorphs with air operation stability provided information about the investigation of phase-dependent optoelectronic properties of thienoacenes, paving the way to construction of high-performance micro- and nanoelectronic devices based on a definite crystal polymorph. Two different crystal phases of C6-DBTDT could be simply self-assembled by drop casting of different concentration of solutions in chlorobenzene or toluene.
We developed fluorescent turn-on probes containing a fluorescent nucleoside, 5-(benzofuran-2-yl)deoxyuridine (dUBF) or 5-(3-methylbenzofuran-2-yl)deoxyuridine (dUMBF), for the detection of single-stranded DNA or RNA by utilizing DNA triplex formation. Fluorescence measurements revealed that the probe containing dUMBF achieved superior fluorescence enhancement than that containing dUBF. NMR and fluorescence analyses indicated that the fluorescence intensity increased upon triplex formation partly as a consequence of a conformational change at the bond between the 3-methylbenzofuran and uracil rings. In addition, it is suggested that the microenvironment around the 3-methylbenzofuran ring contributed to the fluorescence enhancement. Further, we developed a method for detecting RNA by rolling circular amplification in combination with triplex-induced fluorescence enhancement of the oligonucleotide probe containing dUMBF.
A study reviews the applications of organic semiconductors in two of the most active fields of organic optoelectronics: organic thin-film transistors (OTFTs) and organic solar cells (OSCs). Polymer semiconductors having a common structural component, the imide or amide functional group, connected to π-conjugated cores are specifically discussed in the study. Three critical OTFT performance parameters are charge carrier mobility, current modulation ratio, and threshold voltage. The performance parameters can be derived from the OTFT output and transfer characteristics.
We investigated the arrangement of methyl-terminated terthiophenes inside a nanotube by using density functional theory (DFT) including dispersion corrections. After DFT calculations were conducted, a variety of arrangements of the inner terthiophene chains was found, depending on host-tube diameters and the number of chains. Because of the various inner thiophene arrangements, the terthiophene chains interact differently. The interactions in a smaller nanotube are stronger than those within a larger nanotube, indicating the importance of nanotube confinements to the interchain couplings. The interchain interactions split the orbitals of the multimeric terthiophene chains, which are built from single-chain frontier orbitals, broadening their energy levels. Therefore, nanotube confinements are key factors in determining the energy levels of the frontier orbitals of contained multimeric terthiophenes. As a result, their electronic transitions are affected by the encapsulation in a restricted nanotube space. According to time-dependent DFT calculations, a specific electronic transition occurs from a HOMO-built orbital to a LUMO-built orbital. The broadening of the orbital energies by the aggregation of terthiophene chains in a nanotube leads to a widened range of excitation energies (Ex) in their electronic transitions relative to the single-chain. With respect to the strongest transition of multimeric terthiophenes, the excitation energy is enhanced by confinement to a nanotube. The Ex enhancement within a smaller nanotube is more significant than that within a larger nanotube because of the stronger interchain interactions in a smaller nanotube. Therefore, it is proposed from the DFT calculations that nanotube confinements can modulate electronic and absorption properties of multimeric terthiophene chains by changing the interchain interactions.
Polymorphic crystalline microfibers from an achiral octithiophene with one S-hexyl substituent per ring are separately and reproducibly grown with the same characteristics on various solid surfaces, including the interdigitated electrodes/SiO2 surface of a bottomcontact field-effect transistor. The arrangement of the same molecule in two diverse supramolecular structures leads to markedly different electronic, optical, and charge mobility properties. The microfibers—straight and yellow emitting or helical and red emitting—exhibit p-type charge transport characteristics, with the yellow ones showing a charge mobility and on/off current ratio of one and three orders of magnitude, respectively, greater than the red ones. Both forms show circular dichroism signals with significant differences from one form to the other. DFT calculations show that the octithiophene exists in two different quasi-equienergetic conformations aggregating with diverse orientations of the substituents. This result suggests that the observed polymorphism is conformational in nature. The self-assembly, driven by sulfur–sulfur non-bonding interactions, amplifies the small property differences between conformers, leading to quite different bulk properties.
Oligothiophenes provide a highly controlled and adaptable platform to explore various synthetic, morphologic, and electronic relationships in organic semiconductor systems. These short chain systems serve as models for establishing valuable structure-property relationships to their polymer analogs. In contrast to their polymer counterparts, oligothiophenes afford high-purity and well-defined materials which can be easily modified with a variety of functional groups. Recent work by a number of research groups has revealed functionalized oligothiophenes to be the up-and-coming generation of advanced materials for organic electronic devices. In this review, we discuss the synthesis and characterization of linear oligothiophenes with a focus on applications in organic field effect transistors and organic photovoltaics. We will highlight key structural parameters, such as crystal packing, intermolecular interactions, polymorphism, and energy levels, which in turn define the device performance.
Well-defined monodisperse conjugated oligomers, which own planar backbones and are free from the disturbance of substituents attracted broad interest. Herein, we report a series of symmetrical, isomerically pure oligofurans, namely, the 16-mer 16F-6C6 together with the related nF-2C6 (n = 4, 6, 8). Through computational studies and detailed spectroscopic and X-ray characterization, for the first time we show that the planarity of the furan backbone is almost unaffected by the head-to-head defect which is known to cause serious twists in the oligo- or polythiophene analogues. We present that the properties of these rigid oligo(alkylfuran)s are strongly influenced by the conjugation length. As the longest monodisperse α-oligofuran synthesized to date, nanoscale 16F-6C6 exhibits properties ap-proaching its corresponding polymers. It was observed to be highly fluorescent and stable in neutral and oxidized states. The experimental and computational studies of the redox states of these oligo(alkylfuran)s reveal that 16F-6C6 has singlet biradical (polaron-pair) character in the doubly oxidized ground state: the open-shell singlet ( =0.989) is 3.8 kcal/mol more stable than closed-shell dication.
A series of three thiophene-naphthalene-based asymmetric oligomers-5-decyl-2,2':5',2'':5'',2'''-quaterthiophene (DtT), 5-decyl-5''-(naphthalen-2-yl)-2,2':5',2''-terthiophene (D3TN), and 5-(4-decylphenyl)-5'-(naphthalen-2-yl)-2,2'-bithiophene (DP2TN)-was synthesized by Suzuki cross-coupling reactions. The long alkyl side chains improved both the solubility of the oligomers in solvents and their tendency to self-assemble. UV/Vis absorption measurements suggested that DtT, D3TN, and DP2TN form H-type aggregates with a face-to-face packing structure. In addition, the three oligomers were found to adopt vertically aligned crystalline structures in films deposited on substrates, as revealed by grazing-incidence wide-angle X-ray scattering. These oligomers were used as the active layers of p-type organic field-effect transistors, and the resulting devices showed field-effect mobilities of 3.3×10(-3) cm(2) V(-1) s(-1) for DtT, 1.6×10(-2) cm(2) V(-1) s(-1) for D3TN, and 3.7×10(-2) cm(2) V(-1) s(-1) for DP2TN. The differences in transistor performances were attributed to the degree of π overlap and the morphological differences determined by the molecular structures.
Organic molecules/polymers with a π-conjugated (hetero)aromatic backbone are capable of transporting charge and interact efficiently with light. Therefore, these systems can act as semiconductors in opto-electronic devices similar to inorganic materials. However, organic chemistry offers tools for tailoring materials' functional properties via modifications of the molecular/monomeric units, opening new possibilities for inexpensive device manufacturing. This article reviews the fundamental aspects behind the structural design/realization of p- (hole transporting) and n-channel (electron-transporting) semiconductors for organic field-effect transistors (OFETs). An introduction to OFET principles and history, as well as of the state-of-the-art organic semiconductor structure and performance of OFETs is provided.
The transport properties of three fluorenone derivatives used as active layers in organic field effect transistor (OFET) have been studied. A full structural characterization, evidencing the packing features due to the fluorenone moiety, has been carried out. To rationalize the charge carrier mobility values, it is shown that in addition to structural considerations the redox behavior of molecules has to be taken into account. Structure–property relationships are identified, which might drive the future development of oligomers suitable for OFET applications.
We describe the synthesis, characterization and field effect transistor (FET) properties of a series of furan-based conjugated oligomers such as unsubstituted, hexyl- and styryl-capped linear oligofurans and oligofuran-substituted anthracene derivatives. All studied oligofurans show high fluorescence and good thermal stability. Top contact organic FETs (OFETs) fabricated with oligofurans as the active layer show hole mobilities (0.01 to 0.07 cm2 V−1 s−1) and on/off ratios (104 to 106) on a par with the corresponding oligothiophene analogues, while the threshold voltages displayed by oligofuran-based OFETs are significantly reduced due to higher HOMO energies as compared to those of oligothiophenes. Electroluminescence observed in oligofuran-based OFETs in a bottom-contact geometry is limited by electron injection. Overall, we find that furan building blocks are excellent candidates for replacing thiophene in optoelectronic materials.
3′-methyl-(5,5′′-bis[3-ethyl-3-(6-phenyl-hexyloxymethyl)-oxetane])-2,2′:5′,2′′-terthiophene (5T(Me)Ox) is a solution processable small molecule semiconductor displaying smectic-C and nematic liquid crystal phases. The pendant oxetane group can be polymerized in situ in the presence of a suitable photoacid at concentrations ≥1% by weight. Spin-coated films of pure 5T(Me)Ox and 5T(Me)Ox doped with the soluble photoacid were characterized by absorption and photoluminescent spectroscopy. Thick pristine films showed absorption and emission from a crystalline phase. Thin monolayer (
2,2'-Thenil crystallizes in P21/c with a = 7.2501(12) Å, b = 4.7846(8) Å, c = 13.9867(23) Å, ß = 96.897(3)?, V = 481.67(14) Å3, and Z = 2. The molecule resides on an inversion center and is planar. 3,3'-Thenil also crystallizes in P21/c with a = 3.9904(8) Å, b = 21.310(4) Å, c = 11.618(2) Å, ß = 101.83(3)?, V = 966.9(3) Å3, and Z = 4. Refinement of 3,3'-thenil data indicated that 10.3(2)% of both thienyl rings are flip-disordered in this nonplanar molecule. A brief discussion of disorder in molecules containing terminal, unsubstituted 2- and 3-thienyl rings is presented.
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A new class of oligothiophene–anthracene co-oligomers, 9,10-bis(2-dodecyloligothienylvinyl)anthraceneTnA (n = 1, 2 and 3), were synthesized and characterized. The photophysical and electrochemical properties of these oligomers demonstrated an increased conjugation length with increasing number of thiophene units. These oligomers also exhibited relatively larger ionization potential values compared with pentacene and sexithiophene and thus are expected to show improved stability. The strong aggregation tendency and excimer formation for T1A and obvious aggregation effect for T2A in solution were observed from the concentration and temperature-dependent fluorescence spectra. In contrast, no obvious aggregation behavior could be observed for T3A in solution. Thermal behavior and self-assembly of these co-oligomers in solid state were studied by a combination of TGA, DSC, powder X-ray diffraction and polarizing optical microscope. Liquid crystalline phases were observed in T2A and T3A at elevated temperature while only a crystalline phase was found for T1A. Single-crystal structures of T1A and T2A revealed a lamellar packing mode with ordered intermolecular anthracene-to-anthracene and oligothiophene-to-oligothiophene π–π stacking and these qualify them as potential semiconductors in electronic devices.
Two different polymorphic forms of the titled compound, which has the molecular conformation of complete S-syn-anti-syn in oligothiophene backbone, have been grown on glass substrate by vacuum deposition. The two phases are the single-crystal phase (Type I) and the new high-temperature phase (Type II) observed only when the thin film process in the physical vapor transport (vacuum evaporation) is carried out. The ratio of the two phases can be controlled with the substrate temperature and deposition rate. The spacing of Type II by X-ray diffraction measurement is shorter than that of Type I, indicating that the long axis of the molecule in Type II is more inclined against the substrate than those in Type I. Infrared and Raman spectra indicated that Type II is attributed to the conformational polymorphism: conversion from S-syn-anti-syn to S-all-anti. Therefore, the polymorphs originate from the different molecular packing involving the conformational change of the molecule. This unique property is attributed to the extra bulky terminal groups of the compounds. The origin of the transformation from Type I to Type II is that the vapor phase conversion caused by reduction of the activation energy of rotational isomerization barrier. However, in spite of the extra bulky terminal groups, the mentioned polymorphism is not observed in the titled compound analogue, which has S-all-anti conformation. The origin is discussed with the difference of rotational isomerization barrier from syn to anti conformation.
Thienyl substituted 2H-1,4,2-diazaphosphole complexes 3a,b were prepared via highly selective ring-expansion reactions of 2H-azaphosphirene complex 1 and nitriles with our new synthetic protocol using triflic acid and NEt3. The single-crystal X-ray structures of 3a,b show that the 3,5-substituents adopt a coplanar arrangement with the diazaphosphole ring resulting in extended π-conjugation, thus giving rise to absorptions at long wavelengths in their UV/Vis spectra. On the basis of Time-Dependent DFT (TD-DFT) calculations the longest-wavelength absorption could be assigned to a metal–ligand charge transfer (MLCT) process and another low-energy band was interpreted as a superposition of π–π* and n–π* transitions. Protonation of the ring nitrogen yields a pronounced bathochromic shift of all bands along with an increase in their intensity. These effects can be explained by the different extent to which the orbital energies are affected by protonation.
New head-to-head type polythiophenes with acetylenic –CCR side groups, HH-P3(CCR)Th (R=n-C10H21, n-C6H13, n-C4H9), were prepared by palladium-catalyzed polycondensation of the corresponding dibromo-monomers by using Me3SnSnMe3 as the polycondensing agent. The single crystal structure of the monomer revealed high coplanarity of the bithiophene unit, and the derived polymer showed a UV–vis absorption peak at approximately 520nm. The λmax position was red-shifted from those of regioregular poly(3-alkylthiophene)s (385 and 430nm for HH- and HT-type polymers, respectively). These data indicate that the newly synthesized polythiophene with the –CCR group has a highly coplanar structure with a large effective π-conjugation system.
A series of thiophene-based homologues with a silicon core surrounded by mono-, bi-, terthiophene, and their derivatives with alkylsilyl linkages has been prepared using hydrosilylation and Stille coupling methods.
The α-conjugated and structurally defined dodecithiophene 1 well crystallizes from solution into single crystals which were investigated by X-ray structure analysis. Molecular conformations and packing of the longest oligothiophene ever structurally characterized were determined. Surprisingly, rather rare syn-conformations at the terminal thiophene rings and partial twisting of alkyl side chains for compact packing were found, whereby the molecules form a layer-like arrangement with partial overlap of their heteroaromatic systems. In comparison, the molecular properties of the oligomer in a 2D self-assembled monolayer adsorbed at the liquid–solid interface were investigated by scanning tunnelling microscopy (STM). In comparison to the 3D bulk material, on graphite, a different molecular arrangement was found. A lamella-type ordering of molecularly resolved oligomers which exhibit an all-anti conformation could be imaged over wide areas. Larger lattice constants were determined which originate from molecule–substrate interactions.
Trimethylsilyl end-capped bi- and terthiophene carbaldehydes were prepared by reaction of bi- and terthienyl lithium with DMF. Condensation of 5-trimethylsilyl-2,2′-bithiophene-5′-carbaldehyde with dinitrile of malonic acid gave silylbithienyl methylidenedinitrile in good yield, while reaction with hydroxylamine was accompanied by desilylation. The reaction of hydroxylamine with silylbithienyldinitrile leads to the formation of 2-[5-(5′-trimethylsilyl-2,2′-bithienyl)methylidene]malonic acid bisamidoxime. The cytotoxic effect of bi- and terthiophene derivatives was investigated in vitro on two monolayer tumor cell lines: MG-22A (mouse hepatoma) and HT-1080 (human fibrosarcoma). The molecular structure of 5-trimethylsilyl-2,2′-bithiophene-5′-carbaldehyde was studied by X-ray diffraction.
A chiral oligothiophene, possessing in-chain chirality, was
prepared and its racemate was characterized by single crystal X-ray
crystallography; the in-chain chiral 1,1′-binaphthyl moiety
interrupts the π-conjugation and affects the solid state properties of
the oligothiophene.
Organic field-effect transistors (OFETs) were first described in 1987. Their characteristics have undergone spectacular improvements during the last two or three years. At the same time, several models have been developed to rationalize their operating mode. In this review, we examine the performance of OFETs as revealed by recently published data, mainly in terms of field-effect mobility and on–off current ratio. We compare the various compounds that have been used as the active component, and describe the most prominent fabrication techniques. Finally, we analyze the charge transport mechanisms in organic solids, and the resulting models of OFETs.
Oligothiophenes in solution and in the solid state have different conformational properties, as can be concluded from the spectroscopic and theoretical data and X-ray structure of three di- and tetramethyl alpha-conjugated quaterthiophenes (see Figure) reported here.
The optical properties of polysubstituted α-conjugated quaterthiophene crystals display marked differences depending on substitution pattern and molecular conformation. The combination of optical spectroscopy and x-ray diffraction elucidates the correlation among molecular functionalization, crystalline structure, and electronic states. The data are quantitatively interpreted by a semiempirical technique that provides the excited state energies starting from the measured molecular geometry. © 1998 American Institute of Physics.
We report an optical study of polymorphic single crystals of a polysubstituted α-conjugated quaterthiophene. The crystals exhibit two crystalline symmetries (monoclinic and triclinic) with different energy gaps and lifetimes of the elementary excitations. Their optical properties are correlated to the molecular structure, showing that the functionalization process determines an intrinsic tuning of the crystal properties. © 1998 American Institute of Physics.
Single crystals of unsubstituted sexithiophene (6T) were produced by a sublimation technique. X-ray structure shows that the molecule has a quasi-planar all-trans configuration, the torsional angles between adjacent rings being lower than their respective standard deviation. The unit cell belongs to space group P2(1)/n and presents the herringbone packing common to a great deal of planar molecules. Owing to the well-defined orientation of the molecules in single crystals, the polarization of vibration modes and coupling due to crystalfield effects could be unambiguously determined from polarized light IR spectroscopy. Raman spectra of the single crystals are also presented and compared to that of polycrystalline evaporated films.
The thiophene oligomer alpha-hexathienylene (alpha-6T) has been successfully used as the active semiconducting material in thin-film transistors. Field-induced conductivity in thin-film transistors with alpha-6T active layers occurs only near the interfacial plane, whereas the residual conductivity caused by unintentional doping scales with the thickness of the layer. The two-dimensional nature of the field-induced conductivity is due not to any anisotropy in transport with respect to any molecular axis but to interface effects. Optimized methods of device fabrication have resulted in high field-effect mobilities and on/off current ratios of > 10(6). The current densities and switching speeds are good enough to allow consideration of these devices in practical large-area electronic circuits.
Polymorphism of oligothiophenes, especially quaterthiophene (4T)—see previous communication—has also been studied by another group. They report the growth of large single crystals of 4T from the vapor phase and their characterization by single-crystal X-ray diffraction spectroscopy. The Figure shows the shift between two nearest neighbor molecules in the vapor-grown polymorph of 4T necessary for close packing.
Knowledge of the crystal structures of oligothiophenes is necessary if progress is to be made towards understanding the structure property relationships of polythiophenes, since single crystals of these oligothiophene derivatives (one of which is shown in the Figure) have been designed, synthesized, structurally characterized.
Conformational polymorphism of an oligothiophene, the title compound (T4S), is reported for the first time. Efficient synthesis of T4S produced, in addition to the triclinic, deep orange crystals reported previously, a second (yellow monoclinic) form of T4S. whose structure as revealed by single-crystal X-ray diffraction is presented here. The synthetic pathway is outlined and the molecular structure and crystal packing are dis cussed. The significantly different pi-delocalization in the two conformations indicates the need for very careful molecular characterization.
The use of discrete organic compounds as active materials in
transistors is described, beginning with α-sexithiophene
(α-6T) and progressing to other thiophene oligomers and nonthiophene
semiconductors. Device operation, molecular design, synthesis, film
morphology and transport of holes and electrons are covered.
The alkyl-substituted oligothiophenes with varying degree of polymerization have been characterized both in their neutral and in their doped forms. Large blade-shaped crystals of these compounds are readily accessible through recrystallization. Thin films of the oligothiophenes are also obtained from casting or vacuum evaporation. The oligothiophene compounds can be doped (oxidized) with an acceptor such as iodine and TCNQF4(7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane). The doping makes the yellow transparent oligothiophene materials dark green or purple and highly conducting. The oligothiophenes can also be doped with an acceptor in solution.
Novel soluble sexithiophenes (6Ts) β-functionalized with electron-donating methylsulphanyl groups are reported. The resulting processable sexithiophenes, two examples of which are shown in the Figure, have optical gaps that can be finely tuned around the value for unsubstituted α-6T. The observed liquid-crystal behavior described here indicates a high degree of molecular organization in the solid in at least one case.
Communication: alpha-Octithiophene, the highest non-substituted member of the alpha-oligothiophene family known to date, has been obtained as razor-blade thin, bright red single crystals-shown between crossed polarizers on the cover of this issue. The synthesis, purification, crystal growth, and X-ray analysis of the crystals are reported. The ratio of the a and c cell parameters is found to be exceptionally high (a/c is nearly 10), making alpha-oligothiophene the record holder for structural anisotropy among organic crystals.
The structure of a sexithiophene derivative end-substituted by two bulky triisopropylsilyl groups has been determined in an attempt to better understand the relationship between structure and the electronic and optical properties of polythiophenes. The synthesis and structural characterization of derivative are described in detail and the unusual crystal system that involves staggered parallel arrays of molecules and a strong deviation from coplanarity of the pi-conjugated backbone is discussed.
The application of heterostructure concepts to organic field effects transistors (FETs) is briefly reviewed. In particular, the physics of operation of α-sexithiophene- and C60-based FETs—which can act as p- or n-channel transistors depending on the sign and magnitude of the gate bias—and of isotype heterostructure FETs is outlined. In the latter, both active materials, e.g., an aromatic diamine and α-sexithiophene, transport the same (majority) carrier type.
Yields of functionalized sexithiophenes through the Stille reaction are greatly ameliorated using in situ generated Pd(AsPh3)4 as the catalyst. The regiospecific synthesis of a new head-to-head/tail-to-tail hexa(methylsulfanyl)sexithiophene is reported. All sexithiophenes were distilled under high vacuum to reach the degree of purity required for use in electronic applications.
Electrical properties of thin films of sexithiophene derivatives are directly dependent on the structural order of oligothiophene molecules; the order can be controlled by conditions of film deposition, and chemically by grafting of self-assembling groups. Current densities, switching time and dynamic range of these film transistors make them potentially attractive for use in large scale electronic circuits.
The cross-coupling of organotin reagents with a variety of organic electrophiles, catalyzed by palladium, provides a novel method for generating a carbon-carbon bond. Because this mild, versatile reaction is tolerant of a wide variety of functional groups on either coupling partner, is stereospecific and regioselective, and gives high yields of product, it is ideal for use in the synthesis of elaborate organic molecules. When the coupling reaction is carried out in the presence of carbon monoxide, instead of a direct coupling, carbon monoxide insertion takes place, stitching the two coupling partners together and generating a ketone.
A conjugated, fused-thiophene oligomer, bis(dithienothiophene) (BDT), has been synthesized and deposited by vacuum sublimation as the active layer in organic thin film transistors (TFTs). The TFTs show exceptionally high ON/OFF ratios up to 108 between accumulation and depletion with sharp turn-on characteristics comparable to that of amorphous silicon TFTs (subthreshold slope S=0.6 V/decade). Field-effect mobilities are 0.02–0.05 cm2/V s. The good performance is explained by the relatively high π-π∗ gap of the short-chain BDT molecule and the favorable coplanar π-π stacking in BDT, differing from the herringbone stacking in the oligothiophenes. © 1997 American Institute of Physics.
High-quality single crystals of α-quaterthiophene (α-4T) and α-hexathiophene (α-6T) were investigated to compare intrinsic bulk and surface electrical properties. The bulk properties of these organic p-type semiconductors are derived from extended current–voltage characteristics and the surface properties from single crystal field-effect transistor measurements. Most significantly, charge carrier mobilities as high as 0.5 cm2/V s are observed in α-6T both in the bulk and at the surface. The high quality and purity of the crystals are evident from the low trap densities(<1015 cm−3) and the even lower dopant concentrations (2×1013 for α-4T and 7×1010 cm−3 for α-6T). These intrinsically high performance figures, together with the ease of processing, make these oligothiophenes attractive materials for “plastic electronic” devices. © 1998 American Institute of Physics.
Two stereoregular dialkyl-substituted sexithiophenes have been synthesized and their X-ray structures revealed. Both molecules have neighbouring thiophene rings placed antiparallel and the molecules are almost planar. The torsional angles between the rings vary from 5 to 11°. The alkyl side chains are in the planar zigzag form and almost confined to the plane of the thiophene backbone.
For the fabrication of organic transistors sexithiophene (6T) is one of the most promising materials. Since high purity improves the transistor performance of 6T, the FET described here was built on an ultrapure sexithiophene single crystal, shown in the figure as a gray block. The characteristics of this transistor-better than those of FETs based on evaporated polycrystalline sexithiophene-are presented.
Sexithiophene (6T) is a promising material for organic-based electronic devices and its photoluminescence spectra remain a topic of interest. 6T exhibits two different crystal forms, for which photoluminescence spectra and stimulated emission were measured at various temperatures. The authors observe a strong dependence of the spectra on crystal structure. Precise determination of the electronic transitions of both structures shows that most of the steady-state photoluminescence arises from crystal defects.
α-hexathienyl (α–6T) is a highly promising material for application in thin film transistor devices. Recently, record high mobilities, together with record high current on/off ratios, have been reported.1 Thus far, structural information on this exciting material is sketchy. The crystal structures of several such hexamers have been investigated, but only with powder samples, since the crystal growth has proven exceedingly difficult.2-5 Powder Rietveld refinements on these materials are severely hampered by the large number of overlapping reflections, preferred orientation, ambiguities in symmetry, etc. Here, we present a crystal structure of the high-temperature polymorph of α–6T (α–6T/HT), as determined from a single-crystal structure analysis. In this polymorph, the hexamer crystallizes in the smallest unit cell so far reported for this material, but the molecule is flat. Extended Hückel theory (EHT) band structure calculations show that α–6T/HT is an indirect gap semiconductor, with the conduction band minimum at Y and the valence band maximum at Γ. The conduction and valence bands both show a remarkable degree of dispersion along X and Y for a molecular crystal. The electronic band structure of this material is strikingly similar to that of the two-dimensional organic superconductors based on bis(ethylenedithio)tetrathiafulvalene (ET), such as κ−(ET)2 Cu(NCS)2.
The synthesis, thin-film morphology, and hole mobility in thin-film transistors (TFTs) of compounds based on the novel anthradithiophene (ADT) ring system are reported. The parent compound and its 2,8-dihexyl, didodecyl, and dioctadecyl derivatives (DHADT, DDADT, and DOADT, respectively), synthesized via alkylated thiophene dicarboxaldehyde acetals, were investigated. They all form highly ordered polycrystalline vacuum-evaporated films with mobilities as high as 0.15 cm2/(V s), as high as has ever been observed for a polycrystalline organic material. DOADT has a mobility of 0.06 cm2/(V s) even though 70% of its molecular volume is occupied by hydrocarbon chains. DHADT was cast from solution under atmospheric conditions onto a TFT giving a mobility of 0.01−0.02 cm2/(V s). Thus, the alkylated ADTs combine a pentacene-like intrinsic mobility with greater solubility and oxidative stability.
Starting from known molecular geometries and atomic radii, a fast and accurate method for calculating the volume of a molecule is proposed. The same procedure used to calculate this molecular volume is used to perform a volume analysis on various kinds of molecular systems and structured media. This analysis allows the location of empty and filled spaces and the precise evaluation of their volumes. The method is applied here to three different problems: (i) the calculation of cavity volume in cage compounds, and the study of their compatibility with complexed ions; (ii) the prediction of stoichiometry and stability of inclusion compounds in crystalline matrices; and (iii) the analysis of steric factors influencing the solid-state reactivity of organic compounds. Other applications in related fields are briefly examined.
A total of 204 pairs of different crystal structures for the same organic molecule (polymorphs), determined at room conditions, were retrieved from the Cambridge Structural Database. Crystallographic, chemical, and pharmaceutical aspects of the phenomenon were considered. Correlations between differences in density, calculated packing energy, and lattice-vibrational entropy, and other crystal properties, are presented. Indices to quantify conformational polymorphism and differences in coordination sphere in the crystal are proposed. Differences in lattice-vibrational entropy between polymorphs are seldom, if ever, large enough to equal or to exceed differences in packing energy (enthalpy) at room temperature. Although few experimental estimates of energy differences between polymorphs are available, the overall results and some detailed comparisons with calculated lattice energies confirm the good performance of the parameters of the crystal potential. A tentative polymorph for aspirin is proposed by a structure generation procedure. The occurrence of polymorphism in organic crystals is very frequent, if the proper temperature range is explored, but at room conditions, the appearance of several polymorphic forms is not as pervasive as it is sometimes said to be.
In order to analyze the correlation between charge transport and structural properties in conjugated oligomers, sexithiophene, 6T, was substituted by hexyl groups, both on the terminal alpha positions (alpha,omegaDH6T) and as pendant groups in the beta position (beta,beta'DH6T). Structural characterizations by X-ray diffraction show that vacuum-evaporated thin films of 6T and alpha,omegaDH6T consist of layered structures in a monoclinic arrangement, with all-trans planar molecules standing on the substrate. When compared to 6T, alpha,omegaDH6T is mainly characterized by a very large increase of molecular organization at the mesoscopic level, evidenced by a much longer range ordering. Electrical characterizations indicate that the conductivity of alpha,omegaDH6T is largely anisotropic, with a ratio of 120 in favor of the conductivity parallel to the substrate plane, i.e. along the stacking axis. The charge carrier mobility, determined on field-effect transistors fabricated from these conjugated oligomers, also shows an increase by a factor of 25 when passing from 6T to alpha,omegaDH6T, reaching a value of 5 x 10(-2) cm2 V-1 s-1. In contrast, beta,beta'DH6T presents very low conductivity and mobility, the latter being below detection limit. These results are attributed to the self-assembly properties brought by alkyl groups in the alpha,omega position.
Several new p-type semiconducting materials with lower electron-donating ability than the parent sexithiophene were synthesized and their thermal, morphological, and FET properties were investigated. The incorporation of thiazole rings into oligothiophenes was designed to lower the highest occupied molecular orbital (HOMO) level of the molecules and hence make them less susceptible to p-doping. FET devices based on a dihexylated six-ring compound with thiazoles as the central rings indeed showed enhanced stability to p-doping over those of typical sexithiophenes. Relatively high on/off current ratios (greater than 104 for gate voltages of −100 and 0 V) were routinely obtained from devices operating in air, eliminating the need for strict exclusion of oxygen. The threshold voltage of devices made from this compound showed no signs of shifting toward more positive gate voltages after more than one month's exposure to the air. The thiazole-containing oligomers generally had lower field-effect mobilities than the corresponding oligothiophenes, possibly due to larger charge injection barrier and less favorable morphologies of the evaporated films. The electrical characteristics of the previously uninvestigated α,ω-dihexyl quinquethiophene (DHα5T) are also reported in this paper. Single-crystal morphologies were observed in evaporated films of DHα5T. Films evaporated at an optimal deposition temperature (TD = 155 °C) gave mobilities as high as 0.1 cm2/Vs. As in the cases of pentacene and DHα4T, we believe that the greatly enhanced mobility is probably correlated with the single-crystal morphology. DHα5T molecules also have an orientation different from the essentially perpendicular one of other oligothiophenes; they were found to be tilted at 30° to the substrate.
End-substitution of quaterthiophene with hexyl groups leads to a highly soluble conjugated oligomer, α,ω-dihexylquaterthiophene (DH4T), which has been characterized for its thermal, structural, and electrical properties. Differential scanning calorimetry indicates the existence of a 3-dimension (3D)-to-mesophase transition, occurring at 84 °C, below the melting temperature of the material (179 °C). X-ray diffraction patterns performed on crude and thermally treated films of DH4T confirm the existence of a smectic phase with two spacings that increase with temperature. This result is interpreted by a model involving alkyl chain movements, which result in spacing shrinkage, whereas the thienylene sequence remains faced at typical van der Waals distances. DH4T thus forms a 2-dimensional (2D) semiconductor with a liquid crystal-like structural organization. DH4T can be deposited as active semiconducting layer in thin-film transistors, either by vacuum evaporation or by spin coating on an octylsilane-pretreated surface. A high-field-effect carrier mobility has been obtained for both deposition techniques, μ = 3 × 10-2 and 1.2 × 10-2 cm2 V-1 s-1, respectively, together with an interesting Ion/Ioff ratio of 105. These data are discussed together with literature results on unsubstituted quaterthiophene (4T) and the parent sexithiophenes (DH6T and 6T). On one hand, results show that the semiconducting quaterthiophene core in DH4T is large enough to allow high charge transport properties, comparable to those of a sexithiophene core, whereas the core is also short enough to allow its α,ω-dialkylated derivative to be highly soluble and solution processable, contrary to the case of DH6T. Results also suggest that the larger band gap of the shorter conjugated quaterthiophenes is responsible for their lower dopant concentration, and hence of their higher dynamic ratio.
Our investigation of thiophene oligomers as organic thin film transistor (TFT) semiconductors is extended to the hexamer with dodecyl and octadecyl end-substituents and with side chains containing ethereal oxygens. Two thiophene tetramers are studied as well. All of the new compounds are prepared via polar, monosubstituted half-oligomers that are purified, further elaborated, and dimerized. Properties are reported in comparison with the previously reported dihexyl compounds. All of the compounds form ordered films with orientation perpendicular to the substrate. For the hexamers, the longer chains decrease the TFT mobility of evaporated films, while the oxygens have very little electronic effect, even though the oxygen does cause an approximate doubling of the solubility. A tetramer with an ether side chain has a mobility below 0.01 cm2/Vs. Films were also cast from dilute solution and showed mobilities at or above 0.01 cm2/Vs in several cases. This casting process may be useful in devising all-liquid-phase fabrication protocols for organic-based electronic circuits.
The thin film structure and hole mobility of 5,5'''-dihexylquaterthiophene are reported. Films sublimed onto Si/SiO2 or carbon grids held at 30-100 degrees C are single-crystal-like over tens of micrometers as shown by electron diffraction. The orientation has the long ards of the quaterthiophene core nearly perpendicular to the substrate. The mobility ranges from 0.05 to 0.23 cm(2)/Vs, depending on the deposition temperature, much higher than would be expected based on earlier thiophene oligomer data.
The influence of deposition temperature on structure, orientation, and morphology of vacuum-evaporated sexithiophene films has been studied by using X-ray diffraction, UV-visible spectroscopy and scanning electron microscopy. We correlate this with the mobility of these films as measured in field-effect transistors. The X-ray study based on meridional OOl reflections shows evidence for various crystalline phases depending on the substrate temperature during vapor deposition. A high degree of orientation can be achieved even in several-micrometer-thick films deposited above 190 degrees C. It is shown that the field-effect mobility is substantially enhanced for deposition temperatures close to the melting point (290 degrees C), which is associated with a suitable orientation (002 being the single contact plane) and eventually a favorable crystalline structure and coalescent lamellae morphology.
A crystal of an organic compound is the ultimate supermolecule, and its assembly, governed by chemical and geometrical factors, from individual molecules is the perfect example of solid-state molecular recognition. Implicit in the supramolecular description of a crystal structure is the fact that molecules in a crystal are held together by noncovalent interactions. The need for rational approaches towards solid-state structures of fundamental and practical importance has led to the emergence of crystal engineering, which seeks to understand intermolecular interactions and recognition phenomena in the context of crystal packing. The aim of crystal engineering is to establish reliable connections between molecular and supramolecular structure on the basis of intermolecular interactions. Ideally one would like to identify substructural units in a target supermolecule that can be assembled from logically chosen precursor molecules. Indeed, crystal engineering is a new organic synthesis, and the aim of this
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