Lutetium bisphthalocyanine: the first molecular semiconductor. Conduction properties of thin films of p- and n-doped materials

Thin films of bisphthalocyaninatolutetium derivatives (Pc₂Lu) have been doped using a co-sublimation technique with an electron acceptor (dichlorodicyanoquinodimethane, DDQ) and an electron donor (tetraphenyldithiapyranylidene, DIPSΦ₄). The conduction properties of Pc₂Lu and of p- and n-type doped materials have been determined and correlated with the optical characteristics of the films. Pc₂Lu is the first molecular semiconductor that satisfies the following three criteria: (i) it is a molecular material, (ii) its intrinsic conductivity is in the range 10⁻⁶−10⁻¹Ω⁻¹cm⁻¹, (iii) it can be doped with both electron acceptors and electron donors.     

The Radical Anion,Dianion and Electron Transport Properties of Tetraiodotetraazapentacene

Tetraiodotetraazapentacene I₄TAP , the last missing derivative in the series of halogenated silylated tetraazapentacenes, was synthesized via condensation chemistry from a TIPS-ethynylated diaminobenzothiadiazol in three steps. Single and double reduction furnished its air-stable monoanion and relatively air-stable dianion, both of which were characterized by crystallography. All three species are structurally and spectroscopically compared to non-halogenated TAP and Br₄TAP . I₄TAP is an n-channel material in thin-film transistors with average electron mobilities exceeding 1 cm² (Vs)⁻¹.     

Benzothiophene and benzosulfone fused pyrazino[2,3-g]quinoxaline: Synthesis and semiconducting properties

A novel air-stable n-type benzothiaphene endcapped azaarene (BTPQ) and its sulfonated derivative (BSPQ) were prepared via two pathways and characterized by NMR, UV–vis, fluorescence and cyclic voltammetry spectroscopy. Symmetrically introducing four nitrogen atoms into acenes, the semiconductor properties could be changed from p-type to n-type detected through the space charge limited current (SCLC) method. After sulfonation of BTPQ, BSPQ is with deeper frontier orbital energy levels and enhanced the electron mobility.     

Halogenated 6,13-bis(triisopropylsilylethynyl)-5,7,12,14-tetraazapentacene: applications for ambipolar air-stable organic field-effect transistors

Density functional theory calculations were performed to explore the influence of halogenation on the reorganization energies (λ), adiabatic ionization potentials (IPs), adiabatic electron affinities (EAs), and air stabilities of a series of pentacene (PENT) and tetraceno[2,3-b]thiophene (TbTH) derivatives. According to calculated IP and EA values, all well-known PENT and TbTH derivatives in this paper are air-stable p-channel but not air-stable n-channel organic field-effect transistors (OFETs) due to insufficient EAs, consistent with experimental observations. The calculated results show that attaching two or more halogen atoms onto air-unstable 6,13-bis(triisopropylsilylethynyl)-5,7,12,14-tetraazapentacene (TIPS-N4PENT) is sufficient for promoting ambipolar air-stable properties. The electronic coupling and band structure calculations indicate that halogenated TIPS-N4PENT derivatives have potential applications in high-performance ambipolar air-stable OFETs. They also provide rational guidelines for the design of ambipolar air-stable organic semiconductors (OSCs).     

An n-Type Polythiophene Derivative with Excellent Thermoelectric Performance

Typical n-type conjugated polymers are based on fused-ring electron-accepting building blocks. Herein, we report a non-fused-ring strategy to design n-type conjugated polymers, i.e. introducing electron-withdrawing imide or cyano groups to each thiophene unit of a non-fused-ring polythiophene backbone. The resulting polymer, n-PT1 , shows low LUMO/HOMO energy levels of −3.91 eV/−6.22 eV, high electron mobility of 0.39 cm² V⁻¹ s⁻¹ and high crystallinity in thin film. After n-doping, n-PT1 exhibits excellent thermoelectric performance with an electrical conductivity of 61.2 S cm⁻¹ and a power factor (PF) of 141.7 μW m⁻¹ K⁻². This PF is the highest value reported so far for n-type conjugated polymers and this is the first time for polythiophene derivatives to be used in n-type organic thermoelectrics. The excellent thermoelectric performance of n-PT1 is due to its superior tolerance to doping. This work indicates that polythiophene derivatives without fused rings are low-cost and high-performance n-type conjugated polymers.     

Single-Molecule Charge-Transport Modulation Induced by Steric Effects of Side Alkyl Chains

The experimental investigation of side-chain effects on intramolecular charge transport in π-conjugated molecules is essential but remains challenging. Herein, the dependence of intra-molecular conductance on the nature of branching alkyl chains is investigated through a combination of the scanning tunneling microscope break junction (STM-BJ) technique and density functional theory. Three thiophene-flanked diketopyrrolopyrrole (DPP) derivatives with different branching alkyl chains (isopentane, 3-methylheptane, and 9-methylnonadecane) are used with phenylthiomethyl groups as the anchoring groups. The results of single-molecule conductance measurements show that as the alkyl chain becomes longer, the torsional angles between the aromatic rings increase due to steric crowding, and therefore, the molecular conductance of DPP decreases due to reduction in conjugation. Both theoretical simulations and ¹H NMR spectra demonstrate that the planarity of the DPPs is directly reduced after introducing longer branching alkyl chains, which leads to a reduced conductance. This work indicates that the effect of the insulating side chain on the single-molecule conductance cannot be neglected, which should be considered for the design of future organic semiconducting materials.     

Synthesis and mesomorphism of tetrafluoro substituted 4-cyano oligophenyls

ABSTRACT

The synthesis and characterisation of new liquid crystal (LC) compounds based on terphenyl core are described. New family of dielectrically positive compounds presents alternative molecular approach to the conventional LC design. Correlations between molecular structure and mesomorphic properties for compounds being cyano terminated analogues as well as other known from the literature have been drawn. Compounds are characterised by ¹H NMR spectroscopy and mass spectrometry (electron ionisation) analysis. They show monotropic nematic behaviour in broad temperature range, confirmed by a polarising thermomicroscopy, differential scanning calorimetry. Detailed synthetic procedures are attached. Synthesised compounds stand as promising components of medium to highly birefringent liquid crystalline mixtures.     

Synthesis and properties of perfluoroalkylated TIPS-pentacenes

The introduction of perfluoroalkyl (Rf) groups into polycyclic aromatic hydrocarbons (PAHs) is a potential strategy for developing air-stable electron-acceptors that are useful for manufacturing novel n-type organic semiconductors. This paper describes a facile synthetic method for 9,13-bis[(triisopropylsilyl)ethynyl]pentacene (“TIPS-pentacene”) derivatives bearing two or four Rf groups at peripheral regions. The molecular structures of newly synthesized TIPS pentacenes bearing four Rf groups were established by single-crystal X-ray analysis. The Rf-substituted TIPS-pentacenes are stable both in solutions and in the solid state under ambient conditions, probably because of the combination of the TIPS-ethynyl and Rf groups. The Rf groups stabilize the π-orbitals of the TIPS-pentacene framework and improve the electron-accepting properties, as evidenced by electrochemical measurements and photoelectron yield spectroscopy (PYS).     

Novel Thienyl DPP derivatives Functionalized with Terminal Electron-Acceptor Groups: Synthesis,Optical Properties and OFET Performance

Three novel diketopyrrolopyrrole (DPP) based small molecules have been synthesized and characterized in terms of their chemical-physical, electrochemical and electrical properties. All the molecules consist of a central DPP electron acceptor core symmetrically functionalized with donor bi-thienyl moieties and flanked in the terminal positions by three different auxiliary electron-acceptor groups. This kind of molecular structure, characterized by an alternation of electron acceptor and donor groups, was purposely designed to provide a significant absorption at the longer wavelengths of the visible spectrum: when analysed as thin films, in fact, the dyes absorb well over 800 nm and exhibit a narrow optical bandgap down to 1.28 eV. A detailed DFT analysis provides useful information on the electronic structure of the dyes and on the features of the main optical transitions. Organic field-effect transistors (OFETs) have been fabricated by depositing the DPP dyes as active layers from solution: the different end-functionalization of the dyes had an effect on the charge-transport properties with two of the dyes acting as n-type semiconductors (electron mobility up to 4.4 ⋅ 10⁻² cm²/V ⋅ s) and the third one as a p-type semiconductor (hole mobility up to 2.3 ⋅ 10⁻³ cm²/V ⋅ s). Interestingly, well-balanced ambipolar transistors were achieved by blending the most performant n-type and p-type dyes with hole and electron mobility in the order of 10⁻³ cm²/V ⋅ s     

Molecular design of n-type organic semiconductors for high-performance thin film transistors

This digest aims to provide organic chemists with an overview of recent progress on n-type organic semiconductors for application in organic thin film transistors (OTFTs) with an emphasis on molecular design. Herein, we survey n-type organic semiconductors with field effect mobility of 1 cm²/Vs or higher in OTFTs after a brief introduction to the structure and operation of OTFTs and discussion of two key factors (frontier molecular orbitals and molecular packing) of organic semiconductors. On the basis of this survey, we finally reach conclusions on the current status of n-type organic semiconductors for OTFTs and provide an outlook for molecular design.     

Density functional theory study on electron and hole transport properties of organic pentacene derivatives with electron-withdrawing substituent

Attaching electron-withdrawing substituent to organic conjugated molecules is considered as an effective method to produce n-type and ambipolar transport materials. In this work, we use density functional theory calculations to investigate the electron and hole transport properties of pentacene (PENT) derivatives after substituent and simulate the angular resolution anisotropic mobility for both electron and hole transport. Our results show that adding electron-withdrawing substituents can lower the energy level of lowest unoccupied molecular orbital (LUMO) and increase electron affinity, which are beneficial to the electron injection and ambient stability of the material. Also the LUMO electronic couplings for electron transport in these pentacene derivatives can achieve up to a hundred meV which promises good electron transport mobility, although adding electron-withdrawing groups will introduce the increase of electron transfer reorganization energy. The final results of our angular resolution anisotropic mobility simulations show that the electron mobility of these pentacene derivatives can get to several cm(2) V(-1) s(-1), but it is important to control the orientation of the organic material relative to the device channel to obtain the highest electron mobility. Our investigation provide detailed information to assist in the design of n-type and ambipolar organic electronic materials with high mobility performance.     

Synthesis of novel multi-arm star azobenzene side-chain liquid crystalline copolymers with a hyperbranched core

A series of novel multi-arm star side-chain liquid crystalline (LC) copolymers with hyperbranched core moieties were synthesized by atom transfer radical polymerization (ATRP) using a multi-functional hyperbranched polyether as the initiator and chlorobenzene as the solvent. The multi-functional hyperbranched polyether initiator was prepared from poly(3-ethyl-3-(hydroxymethyl)oxetane) (PEHO) and 2-bromo-2-methylpropionyl bromide. The azobenzene side-chain liquid crystalline arms were designed to have an LC conformation of poly[6-(4-methoxy-4^′-oxy-azobenzene)hexyl methacrylate] with different molecular weights. Their characterization was performed with ¹H NMR, size exclusion chromatograph (SEC), differential scanning calorimetry (DSC) and thermal polarized optical microscopy (POM). The multi-arm star side-chain liquid crystalline copolymers exhibited a smectic and a nematic phase, and the phase transition temperatures from the smectic to the nematic phase and from the nematic to isotropic phase increased with increasing the molecular weight of the multi-arm star side-chain liquid crystalline copolymers from 1.78 × 10⁴ to 9.07 × 10⁴.     

氮杂苯并菲类n-型有机半导体材料研究进展

苯并菲盘状液晶是一类新型的有机电子学材料.该类材料多数以空穴传输功能为主,能传输电子的n-型材料较少.氮杂苯并菲是与苯并菲衍生物非常相似的一种杂环化合物,材料结构中引入了氮原子,吸电子能力得到增强,是潜在的n-型有机半导体材料,具有重要的应用价值.本文系统回顾了氮杂苯并菲类盘状液晶材料的研究进展,分类讨论了材料的分子结构,其中包括二、四、六氮杂苯并菲,以及它们的合成方法和物理化学性能,论述了材料在光电子领域的最新使用进展,并在此基础上,对该类液晶材料作为n-型有机半导体在光电子器件领域的应用前景进行了展望.     

4,5,9,10‐Pyrene Diimides: A Family of Aromatic Diimides Exhibiting High Electron Mobility and Two‐Photon Excited Emission

The design and synthesis of high‐performance n‐type organic semiconductors are important for the development of future organic optoelectronics. Facile synthetic routes to reach the K‐region of pyrene and produce 4,5,9,10‐pyrene diimide (PyDI) derivatives are reported. The PyDI derivatives exhibited efficient electron transport properties, with the highest electron mobility of up to 3.08 cm² V⁻¹ s⁻¹. The tert‐butyl‐substituted compounds (t‐PyDI) also showed good one‐ and two‐photon excited fluorescence properties. The PyDI derivatives are a new family of aromatic diimides that may exhibit both high electron mobility and good light‐emitting properties, thus making them excellent candidates for future optoelectronics.     

Diindenopyrazines: Electron-Deficient Arenes

The syntheses, properties and application of the air-stable electron acceptors, diindenopyrazines 4 a – g are reported demonstrating the introduction of functional aryl groups in the 6- and 12-positions. The targets are accessible on the hundred milligram to gram scale. The structure of the aryl groups in 4 a – g modulates their solubility, redox potentials and optical properties. The introduction of electron-poor aryl groups to the electron-poor diindenopyrazine backbone reduces the electron affinity to −4 eV, making the compounds attractive as n-semiconductors. A simple organic field-effect transistor of 4 e –without optimization– shows electron transport with a mobility of up to 0.037 cm² V⁻¹ s⁻¹.     

Semiconducting Polymers Based on Simple Electron-Deficient Cyanated trans-1,3-Butadienes for Organic Field-Effect Transistors

Developing high-performance but low-cost n-type polymers remains a significant challenge in the commercialization of organic field-effect transistors (OFETs). To achieve this objective, it is essential to design the key electron-deficient units with simple structures and facile preparation processes, which can facilitate the production of low-cost n-type polymers. Herein, by sequentially introducing fluorine and cyano functionalities onto trans-1,3-butadiene, we developed a series of structurally simple but highly electron-deficient building blocks, namely 1,4-dicyano-butadiene ( CNDE ), 3-fluoro-1,4-dicyano-butadiene ( CNFDE ), and 2,3-difluoro-1,4-dicyano-butadiene ( CNDFDE ), featuring a highly coplanar backbone and deep-positioned lowest unoccupied molecular orbital (LUMO) energy levels (−3.03–4.33 eV), which render them highly attractive for developing n-type semiconducting polymers. Notably, all these electron-deficient units can be easily accessed by a two-step high-yield synthetic procedure from low-cost raw materials, thus rendering them highly promising candidates for commercial applications. Upon polymerization with diketopyrrolopyrrole ( DPP ), three copolymers were developed that demonstrated unipolar n-type transport characteristics in OFETs with the highest electron mobility of >1 cm² V⁻¹ s⁻¹. Hence, CNDE , CNFDE , and CNDFDE represent a class of novel, simple, and efficient electron-deficient units for constructing low-cost n-type polymers, thereby providing valuable insight for OFET applications.     

Synthesis and electronic properties of pentacene derivatives as promising n-type semiconductor candidates

Two pentacene derivatives 1 and 2 were synthesized from the Diels-Alder reactions of furan derivatives with 1,4-benzoquinone. They were characterized by the methods of ¹H — nuclear magnetic resonance spectroscopy (¹H-NMR), matrix-assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF MS), ultraviolet and visible spectrophotometry (UV-VIS), photoluminescence (PL) spectrometry and cyclic voltammetry (CV). The energy gaps of 1 and 2, taken directly from spectroscopic measurements, are broad as 2.72 and 2.46 eV, leading to blue and greenish blue photoluminescence, respectively. The LUMO and HOMO energy levels are −2.77 and −5.49 eV for 1, and −2.91 and −5.37 eV for 2, respectively. The low energy levels make both 1 and 2 good air-stabilities and promising n-type semiconductor candidates for use in organic electronics.     

Cholesteric star-shaped liquid crystals induced by a maltose core: synthesis and characteristics

Three esters of maltose (A1–A3) with 4–(4–(alkoxybenzoyloxy)phenoxy) –6–oxohexanoic acid (b1–b3) side-arms have been synthesised. All the maltose derivatives were laevo-rotationary, unlike their parent cores. The side arm b1 did not display liquid crystalline (LC) properties, and b2 and b3 displayed thermotropic nematic LC properties. The star-shaped compound (SSC) A1 with b1 side-arms did not display a mesomorphic phase. Unlike the nematic schlieren texture provided by side-arms b2 and b3, the star-shaped liquid crystals (SSLC) A2 and A3 displayed Grandjean and oily texture in the cholesteric phase. The results suggest that the LC properties of the side-arms have an important influence on the formation of LC properties in an SSC, and that the maltose core is important in determining the mesomorphic phase type. In other words, the SSC displayed LC properties only when the side-arms were also LC, and the maltose core induced a cholesteric phase in the SSLC with nematic side-arms. The mesomorphic regions for A2 and A3 were 39.1 and 53.7°C during the heating cycle and 63.8 and 107.0°C during the cooling cycle, respectively. The longer terminal chain rendered the mesomorphic region broader.     

An Amorphous n-Type Conjugated Polymer with an Ultra-Rigid Planar Backbone

High charge carrier mobility polymer semiconductors are always semi-crystalline. Amorphous conjugated polymers represent another kind of polymer semiconductors with different charge transporting mechanism. Here we report the first near-amorphous n-type conjugated polymer with decent electron mobility, which features a remarkably rigid, straight and planar polymer backbone. The molecular design strategy is to copolymerize two fused-ring building blocks which are both electron-accepting, centrosymmetrical and planar. The polymer is the alternating copolymer of double B←N bridged bipyridine (BNBP) unit and benzobisthiazole (BBTz) unit. It shows a decent electron mobility of 0.34 cm² V⁻¹ s⁻¹ in organic field-effect transistors. The excellent electron transporting property of the polymer is possibly due to the ultrahigh backbone stiffness, small π-π stacking distance, and high molecular weight.     

Synthesis and properties of side-chain liquid crystalline polymers grafted with chiral dimers containing cholesteryl groups

ABSTRACT

Two series of novel side-chain liquid crystal (LC) polysiloxanes grafted with chiral liquid crystalline dimers containing cholesteryl mesogens were synthesised. The chemical structure and LC properties of comonomers and polymers were characterised by FTIR, ¹H-NMR, DSC, TGA, POM and XRD. M₁ and M₂ were chiral nematic (N*) dimers, and M₃ was an achiral LC monomer displaying nematic mesophase in a narrow mesomorphic temperature range, while the copolymers exhibited N* mesophase whose mesomorphic temperature ranges were much wider than those of the comonomers. Moreover, the glass transition temperatures and isotropization temperatures of the polymers all decreased with decreasing the dimer components. Reflection spectra showed that P_a series tend to attain wide-band selective reflection at long wavelengths, while P_b series were more potential at short wavelengths with narrow bandwidths. Decreasing the dimer components led the wavelength of the selective reflection to blue shift, which was an abnormal phenomenon in chiral mixture system.