Selenium in Diketopyrrolopyrrole-based Polymers: Influence on Electronic Properties and Charge Carrier Mobilities

Diketopyrrolopyrrole (DPP)-based π-conjugated copolymers with thiophene have exceptionally high electron mobilities. This paper investigates electronic properties and charge carrier mobilities of selenophene containing analogues. Two new copolymers, with alternating thiophene DPP (TDPP) and selenophene DPP (SeDPP) units, were synthesized. Two side-chains, hexyl (Hex) and triethylene glycol (TEG) were employed, yielding polymers designated as PTDPPSeDPP-Hex and PTDPPSeDPP-TEG. Selenophene systems have smaller band gaps, with concomitant enhancement of the stability of the reduced state. For both polymers, ambipolar mobilities were observed in organic field-effect transistors (OFET). Grazing incidence X-ray diffraction (GIXD) data indicates preferential edge-on orientation of PTDPPSeDPP-TEG, which leads to superior charge transport properties of the TEG substituted polymer, as compared to its Hex analogue. Time-dependent-density functional theory (TDDFT) calculations corroborate the decrease in the optical band gap with the inclusion of selenophene. Ambipolar charge transport is rationalized by exceptionally wide conduction bands. ΔSCF calculations confirm the larger electron affinity, and therefore the greater stability, of the reduced form of the selenophene-containing DPP polymer in presence of chloroform.     

基于氟硼二吡咯的共轭聚合物研究进展

氟硼二吡咯（BODIPY）类功能染料因具有良好的光子及电子性质已经得到广泛的应用,对其共轭聚合物的研究则在近年受到广泛关注。与BODIPY单体相比,BODIPY的共轭聚合物具有更窄的带隙、吸收光谱的红移及更强的电子传导能力。BODIPY共轭聚合物在有机半导体材料及有机太阳能电池等方面具有广泛的应用前景。总结了BODIPY共轭聚合物的制备方法、结构与性质的关系以及应用等方面,提出了BODIPY共轭聚合物设计合成策略及未来的发展趋势。     

Photoconductive polymers: Structure,mechanisms and properties

Charge carrier transport properties of organic polymers can vary over a wide range. The paper shows that the electron- and hole mobilities of polymers with pendant photoconductive groups (i. e. carbazole) are on the order of 10^-6 cm²/Vs. In these materials the flow of electronic charge is maintained by the overlap of the π-orbitals of the pendant molecular groups. The large variation of this short-range interaction, depending on the local configurations encountered in polymer glasses, leads to a large variation of hopping probabilities and, hence, to wide rate-distributions. These distributions are reflected in the slow algebraic decay characteristics of the observed photocurrents. The typical time exponents α (α < 1) are shown to carry a great deal of physical information, if the dynamical range of the experiments is sufficiently large. The paper also refers to quasi-conjugated polymers (polysilanes) whose dynamic transport parameters are about 10³ times better (faster) as compared to polymers with pendant groups. These new materials open interesting aspects for the development of new polymeric materials with better transport parameters and, hence, shorter 'switching times'.     

n-Channel semiconductor materials design for organic complementary circuits

Organic semiconductors have unique properties compared to traditional inorganic materials such as amorphous or crystalline silicon. Some important advantages include their adaptability to low-temperature processing on flexible substrates, low cost, amenability to high-speed fabrication, and tunable electronic properties. These features are essential for a variety of next-generation electronic products, including low-power flexible displays, inexpensive radio frequency identification (RFID) tags, and printable sensors, among many other applications. Accordingly, the preparation of new materials based on π-conjugated organic molecules or polymers has been a central scientific and technological research focus over the past decade. Currently, p-channel (hole-transporting) materials are the leading class of organic semiconductors. In contrast, high-performance n-channel (electron-transporting) semiconductors are relatively rare, but they are of great significance for the development of plastic electronic devices such as organic field-effect transistors (OFETs). In this Account, we highlight the advances our team has made toward realizing moderately and highly electron-deficient n-channel oligomers and polymers based on oligothiophene, arylenediimide, and (bis)indenofluorene skeletons. We have synthesized and characterized a "library" of structurally related semiconductors, and we have investigated detailed structure-property relationships through optical, electrochemical, thermal, microstructural (both single-crystal and thin-film), and electrical measurements. Our results reveal highly informative correlations between structural parameters at various length scales and charge transport properties. We first discuss oligothiophenes functionalized with perfluoroalkyl and perfluoroarene substituents, which represent the initial examples of high-performance n-channel semiconductors developed in this project. The OFET characteristics of these compounds are presented with an emphasis on structure-property relationships. We then examine the synthesis and properties of carbonyl-functionalized oligomers, which constitute second-generation n-channel oligothiophenes, in both vacuum- and solution-processed FETs. These materials have high carrier mobilities and good air stability. In parallel, exceptionally electron-deficient cyano-functionalized arylenediimide derivatives are discussed as early examples of thermodynamically air-stable, high-performance n-channel semiconductors; they exhibit record electron mobilities of up to 0.64 cm(2)/V·s. Furthermore, we provide an overview of highly soluble ladder-type macromolecular semiconductors as OFET components, which combine ambient stability with solution processibility. A high electron mobility of 0.16 cm(2)/V·s is obtained under ambient conditions for solution-processed films. Finally, examples of polymeric n-channel semiconductors with electron mobilities as high as 0.85 cm(2)/V·s are discussed; these constitute an important advance toward fully printed polymeric electronic circuitry. Density functional theory (DFT) computations reveal important trends in molecular physicochemical and semiconducting properties, which, when combined with experimental data, shed new light on molecular charge transport characteristics. Our data provide the basis for a fundamental understanding of charge transport in high-performance n-channel organic semiconductors. Moreover, our results provide a road map for developing functional, complementary organic circuitry, which requires combining p- and n-channel transistors.     

Comprehensive approach to intrinsic charge carrier mobility in conjugated organic molecules, macromolecules, and supramolecular architectures

Si-based inorganic electronics have long dominated the semiconductor industry. However, in recent years conjugated polymers have attracted increasing attention because such systems are flexible and offer the potential for low-cost, large-area production via roll-to-roll processing. The state-of-the-art organic conjugated molecular crystals can exhibit charge carrier mobilities (μ) that nearly match or even exceed that of amorphous silicon (1-10 cm(2) V(-1) s(-1)). The mean free path of the charge carriers estimated from these mobilities corresponds to the typical intersite (intermolecular) hopping distances in conjugated organic materials, which strongly suggests that the conduction model for the electronic band structure only applies to μ > 1 cm(2) V(-1) s(-1) for the translational motion of the charge carriers. However, to analyze the transport mechanism in organic electronics, researchers conventionally use a disorder formalism, where μ is usually less than 1 cm(2) V(-1) s(-1) and dominated by impurities, disorders, or defects that disturb the long-range translational motion. In this Account, we discuss the relationship between the alternating-current and direct-current mobilities of charge carriers, using time-resolved microwave conductivity (TRMC) and other techniques including field-effect transistor, time-of-flight, and space-charge limited current. TRMC measures the nanometer-scale mobility of charge carriers under an oscillating microwave electric field with no contact between the semiconductors and the metals. This separation allows us to evaluate the intrinsic charge carrier mobility with minimal trapping effects. We review a wide variety of organic electronics in terms of their charge carrier mobilities, and we describe recent studies of macromolecules, molecular crystals, and supramolecular architecture. For example, a rigid poly(phenylene-co-ethynylene) included in permethylated cyclodextrin shows a high intramolecular hole mobility of 0.5 cm(2) V(-1) s(-1), based on a combination of flash-photolysis TRMC and transient absorption spectroscopy (TAS) measurements. Single-crystal rubrene showed an ambipolarity with anisotropic charge carrier transport along each crystal axis on the nanometer scale. Finally, we describe the charge carrier mobility of a self-assembled nanotube consisting of a large π-plane of hexabenzocoronene (HBC) partially appended with an electron acceptor. The local (intratubular) charge carrier mobility reached 3 cm(2) V(-1) s(-1) for the nanotubes that possessed well-ordered π-stacking, but it dropped to 0.7 cm(2) V(-1) s(-1) in regions that contained greater amounts of the electron acceptor because those molecules reduced the structural integrity of π-stacked HBC arrays. Interestingly, the long-range (intertubular) charge carrier mobility was on the order of 10(-4) cm(2) V(-1) s(-1) and monotonically decreased when the acceptor content was increased. These results suggest the importance of investigating charge carrier mobilities by frequency-dependent charge carrier motion for the development of more efficient organic electronic devices.     

First-principles prediction of charge mobility in carbon and organic nanomaterials

We summarize our recent progresses in developing first-principles methods for predicting the intrinsic charge mobility in carbon and organic nanomaterials, within the framework of Boltzmann transport theory and relaxation time approximation. The electron-phonon couplings are described by Bardeen and Shockley's deformation potential theory, namely delocalized electrons scattered by longitudinal acoustic phonons as modeled by uniform lattice dilation. We have applied such methodology to calculating the charge carrier mobilities of graphene and graphdiyne, both sheets and nanoribbons, as well as closely packed organic crystals. The intrinsic charge carrier mobilities for graphene sheet and naphthalene are calculated to be 3 × 10(5) and ～60 cm(2) V(-1) s(-1) respectively at room temperature, in reasonable agreement with previous studies. We also present some new theoretical results for the recently discovered organic electronic materials, diacene-fused thienothiophenes, for which the charge carrier mobilities are predicted to be around 100 cm(2) V(-1) s(-1).     

Molecular design and density functional theory investigation of novel low‐band‐gap copolymers between quinoid acceptors and aromatic donors

The structures and electronic properties of furo[3,4‐b]pyridine‐based alternating donor and acceptor conjugated oligomers, in which furan and pyrrole are used as donors, and their periodic polymers were investigated using density functional theory at the B3LYP/6‐31G(d) level. The bond lengths, bond length alternation, bond critical point (BCP) properties, nucleus‐independent chemical shift (NICS) and Wiberg bond index (WBI) were analyzed and correlated with conduction properties. The changes of bond length, BCP properties, NICS and WBI all show that the degree of conjugation increases with main chain extension. The changes of NICS also show that the conjugation is stronger in the central section than in the outer section. Hydrogen bonding interactions and nitrogen atom substitution in the acceptors play very important roles in the geometries, electronic structures and energy gaps. The theoretical results suggest that pyrrole‐based polymers are good candidates for conducting materials, compared with furan‐based polymers. With an increase of nitrogen atom substitution in the acceptors in these polymers, the intermolecular charge transfers along the polymeric axes are enhanced, and the bond length alternations and HOMO–LUMO energy gap for these polymers are decreased. The results suggest that the six polymers studied all have lower energy gaps (in the range 0.81–1.26 eV), which indicate that these proposed polymers are good candidates for n‐doping conductive materials, especially poly(7‐(furan‐2‐yl)furo[3,4‐e][1,2,4]triazine) and poly(7‐(1H‐pyrrol‐2‐yl)furo[3,4‐e][1,2,4]triazine). Copyright © 2011 Society of Chemical Industry     

Probing the nonequilibrium nature of polymer glasses by Fickian diffusion of alkane permeants

Volume relaxation of glassy polymers has been followed by diffusion of simple alkane permeants. The changes in diffusion rates allow direct monitoring of changes in segmental mobilities of the polymers. In general, the diffusion rates decrease linearly with annealing time on double-logarithmic plots and then level off as equilibrium is reached. The reduction in diffusion rates, however, can be recovered by raising the temperature above the glass transition temperature and then cooling to the same annealing temperature. Such behavior indicates that the predominant molecular process responsible for changes in mechanical properties of glassy polymers that accompany volume relaxation is not “structure formation” but is the loss of segmental mobilities arising from a reduction of the total free volume in the polymers.     

Electrochemical gating: A method to tune and monitor the (opto)electronic properties of functional materials

Electrochemical polarization of a crystalline, polymeric or nanoporous system or a single molecule may change the density of charge carriers in a controlled way, and hence the optical and electrical properties. If the system has two contacts, its electronic conductivity can be measured in situ as a function of the charge carrier density that is varied by the electrochemical potential. This is called electrochemical gating. Such investigations can reveal the nature of the charge carriers (mobile or localized) and the mechanism of electronic conduction. Here, we present a brief review of a number of systems including inorganic crystals, polymers, nanoporous quantum-dot solids, and single molecules for which electrochemical gating was used successfully in the study of the electronic properties.     

Synthesis of amphiphilic isoindigo co‐polymers for organic field effect transistors: A comparative study

In this work, the amphiphilic isoindigo (am‐iInd) based conjugated polymers namely poly(am‐iInd‐DT) and poly(am‐iInd‐TT) and their regular counterpart poly(reg‐iInd‐DT) and poly(reg‐iInd‐TT) were synthesized to compare their opto‐electronic and charge transport properties. They were used to fabricate organic field effect transistors. Charge transport properties in conjugated polymers depend upon intermolecular interaction which is strongly affected by the nature of side chains. The amphiphilic nature of the conjugated polymers has little impact on the charge transport properties. The charge carrier mobility of amphiphilic conjugated polymers was comparable with the regular polymers except poly(reg‐iInd‐DT) which can be correlated by X‐ray diffraction and thin film morphology. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45461.     

Directional polymerisation and electronic properties of a cyanosubstituted dialkylpolyphenylenevinylene

Summary The synthesis of a monomer allowing for directional polymerisation based on the Knoevenagel condensation reaction is presented. The free running polymerisation gave a molecular weight (M _w = 24650 g mol^-1) polymer product. The polymerisation reaction could be followed by size exclusion chromatography (SEC) and the molecular weight of the polymer product could be controlled by termination of the polymerisation reaction at a suitable time. When terminated before completion a lower molecular weight product was obtained that could be separated into oligomers using preparative SEC. The polymer product and the oligomers were found to have very similar physical properties in terms of the optical band gaps, electronic energy levels and charge carrier mobilities as studied by UV-vis and pulse radiolysis time resolved microwave conductivity (PR-TRMC). Finally the positions of the filled energy levels were determined using ultraviolet photoelectron spectroscopy (UPS).This revised version was published online in September 2004. Due to technical problems, the previous version contained an incomplete PDF.     

Synthesis and photoconductivity study of azo functional polymers

A new type of high photosensitive azo functional polymers was synthesized and characterized by viscometry, infrared (IR) spectroscopy and Raman spectroscopy. Xerographic properties were determined in bilayer devices. Results show that azo polymers have high photosensitivity resulting from the high electronic mobility (AP-1 and AP-3) and a high charge transfer (AP-4). The study of spectral responses indicates that these polymers exhibit good xerographic properties in visible region, and AP-1 also has high photosensitivity in the near-IR region. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 1913–1920, 1999     

Conducting block copolymers. Towards a polymer pn- junction

Summary The synthesis of polyphenylacetylene (1), polypentafluorophenylacetylene (2) and the block copolymer of 1 and 2 polyphenylacetylene-polypentafluorophenylacetylene (3) using a Rhodium catalysed living polymerisation reaction is presented. Photoelectron spectroscopy of thin films of the individual polymers using 50eV photons from a synchrotron allowed for the determination of the position of the the electronic energy levels, ionisation potentials and the vacuum level shift which indicated that the block copolymer organises at the gold substrate surface such that the fluorinated part of the copolymer extends towards the air interface. Pulse radiolysis time resolved microwave conductivity (PR-TRMC) allowed for the determination of the minimum carrier mobilities and the carrier lifetimes. The sum of carrier mobilities, _min, were respectively 5.2·10^–7, 6.3·10^–7 and 3.2·10^–7m² V^–1 s^–1 and the first half life, _1/2, was 2.0, 1.5 and 1.0 s in 1, 2 and 3. The study shows that it is possible to make conducting block copolymers by the rhodium catalysed polymerisation of arylacetylenes with different electronic energies that organises at the surface giving rise to electronic properties that approach analogy to the traditional inorganic semiconductor pn-junctions.     

Novel functionalized bis(imino)pyridine cobalt(II) catalysts for ethylene polymerization

This report describes synthesis and ethylene polymerization in the various conditions by two novel 2,6-bis(imino)pyridine (BIMP) catalysts B and C based on cobalt activated by methylaluminoxane (MAO) in a slurry semi-batch reactor. The catalyst activities as well as polymer properties were affected dramatically by electronic effects of the attached substitutions on the para-position of the pyridine ring. Theoretical study exhibited more positive charge on the central metal of the catalyst B resulted in higher activity at the expense of lower thermal stability and lifetime. The polymer obtained using the catalysts exhibited high molecular weight and almost narrow molecular weight distribution (MWD) ranging from 2.35 to 4.10 at the employed polymerization conditions. The highest and lowest molecular weight of the obtained polymers were produced by the catalyst A and C respectively. Hydrogen could slightly increase the catalyst activities with the exception of the catalyst B. The catalyst C bearing electron-donor OMe substitution at the para-position of the pyridine ring, produced PE with narrower PDI relative to the polymer resulted by catalysts A and B.     

Theoretical investigation on the white-light emission from a single-polymer system with simultaneous blue and orange emission

White organic light-emitting devices (WOLEDs) have attracted considerable attention because of their good potential for various lighting applications. Among these devices, WOLEDs based on polymers (WPLEDs) are of particular interest. We report here a theoretical investigation of the white-light emission from a single-polymer system with simultaneous blue (polyfluorene as a blue host) and orange (2,1,3-benzothiadiazole-based derivative as an orange dopant) emission. A variety of theoretical methods are used and evaluated to calculate electronic and optical properties of polyfluorene and 2,1,3-benzothiadiazole-based derivatives. Simulated electronic and optical properties are found to agree well with available experimental measurements. The influence of the “CH”/N heterosubstitution on the electronic and optical properties of the 2,1,3-benzothiadiazole-based derivative is considered. Furthermore, we find that the electronic and optical properties of “CH”/N substitution derivatives can be tuned by symmetrically adding suitable electron-donating groups on N,N-disubstituted amino groups, implying good candidates as orange dopants in WPLEDs with polyfluorene as a blue-light-emitting host. Solvent (dichloromethane) effects on the electronic and optical properties of 2,1,3-benzothiadiazole-based derivatives have been investigated. In addition, low reorganization energy values of holes for designed 2,1,3-benzothiadiazole-based derivatives within the framework of the charge hopping model suggest them to be good hole transfer materials.     

Design of semiconducting indacenodithiophene polymers for high performance transistors and solar cells

The prospect of using low cost, high throughput material deposition processes to fabricate organic circuitry and solar cells continues to drive research towards improving the performance of the semiconducting materials utilized in these devices. Conjugated aromatic polymers have emerged as a leading candidate semiconductor material class, due to their combination of their amenability to processing and reasonable electrical and optical performance. Challenges remain, however, to further improve the charge carrier mobility of the polymers for transistor applications and the power conversion efficiency for solar cells. This optimization requires a clear understanding of the relationship between molecular structure and both electronic properties and thin film morphology. In this Account, we describe an optimization process for a series of semiconducting polymers based on an electron rich indacenodithiophene aromatic backbone skeleton. We demonstrate the effect of bridging atoms, alkyl chain functionalization, and co-repeating units on the morphology, molecular orbital energy levels, charge carrier mobility, and solar cell efficiencies. This conjugated unit is extremely versatile with a coplanar aromatic ring structure, and the electron density can be manipulated by the choice of bridging group between the rings. The functionality of the bridging group also plays an important role in the polymer solubility, and out of plane aliphatic chains present in both the carbon and silicon bridge promote solubility. This particular polymer conformation, however, typically suppresses long range organization and crystallinity, which had been shown to strongly influence charge transport. In many cases, polymers exhibited both high solubility and excellent charge transport properties, even where there was no observable evidence of polymer crystallinity. The optical bandgap of the polymers can be tuned by the combination of the donating power of the bridging unit and the electron withdrawing nature of co-repeat units, alternating along the polymer backbone. Using strong donors and acceptors, we could shift the absorption into the near infrared.     

钠蒙脱石水化膨胀和层间结构的分子动力学模拟

运用Cerius^2软件中能量最小化和分子动力（molecular dynamics,MD）模块研究了钠蒙脱石的水化膨胀性能和层间结构特征。计算结果表明：当蒙脱石粘土中的水含量从0增加剑300mg／g时,平衡时蒙脱石的层间距d001值也随之增大,并且和实验数据吻合良好。通过对含有一、二、三层水分子钠蒙脱行的MD模拟可以看出,Na^＋在所有水化物中和四面体电荷位置成内层配位作用,而和八面体电荷位置成外层配位作用的Na^＋只住一层水化物中位置比较固定,在二层和三层水化物中逐渐向层中其它方向扩散。对所有3种水合物自扩散系数的计算结果说明：层间阳离子和水分子的活动性比水溶液中的低。Na-O,O-O和H-O的径向分布函数分析表明层间水分子的结构和水中水分子的结构不同。     

π-Conjugated cyanostilbene derivatives: a unique self-assembly motif for molecular nanostructures with enhanced emission and transport

π-Conjugated organic molecules represent an attractive platform for the design and fabrication of a wide range of nano- and microstructures for use in organic optoelectronics. The desirable optical and electrical properties of π-conjugated molecules for these applications depend on their primary molecular structure and their intermolecular interactions such as molecular packing or ordering in the condensed states. Because of the difficulty in satisfying these rigorous structural requirements for photoluminescence and charge transport, the development of novel high-performance π-conjugated systems for nano-optoelectronics has remained a challenge. This Account describes our recent discovery of a novel class of self-assembling π-conjugated organic molecules with a built-in molecular elastic twist. These molecules consist of a cyano-substituted stilbenic π-conjugated backbone and various terminal functional groups, and they offer excellent optical, electrical, and self-assembly properties for use in various nano-optoelectronic devices. The characteristic "twist elasticity" behavior of these molecules occurs in response to molecular interactions. These large torsional or conformational changes in the cyanostilbene backbone play an important role in achieving favorable intermolecular interactions that lead to both high photoluminescence and good charge carrier mobility in self-assembled nanostructures. Conventional π-conjugated molecules in the solid state typically show concentration (aggregation) fluorescence quenching. Initially, we describe the unique photoluminescence properties, aggregation-induced enhanced emission (AIEE), of these new cyanostilbene derivatives that elegantly circumvent these problems. These elastic twist π-conjugated backbones serve as versatile scaffolds for the preparation of well-defined patterned nanosized architectures through facile self-assembly processes. We discuss in particular detail the preparation of 1D nanowire structures through programmed self-assembly. This Account describes the importance of utilizing AIEE effects to explore optical device applications, such as organic semiconducting lasers (OSLs), optical memory, and sensors. We demonstrate the rich electronic properties, including the electrical conductivity, field-effect carrier mobility, and electroluminescence of highly crystalline 1D nanowire and coaxial donor-acceptor nanocable structures composed of elastic twist π-conjugated molecules. The electronic properties were measured using various techniques, including current-voltage (I-V), conducting-probe atomic force microscopy (CP-AFM), and space-charge-limited-current (SCLC) measurements. We prepared and characterized several electronic device structures, including organic field-effect transistors (OFETs) and organic light-emitting field-effect transistors (OLETs).     

The Effects of Methoxylated Isoindigo on the Optical and Charge Transport Properties of the Corresponding Polymers

Although the effects of electron-deficient group substitution on isoindigo on the corresponding conjugated polymers are extensively studied, the modification of isoindigo core with electron-rich groups has not been investigated. It is envisioned that the introduction of the methoxy group on isoindigo will not only tune the highest occupied molecular orbital (HOMO) energy level of the corresponding polymers but also introduce O···S “conformation lock” to increase the coplanarity of the polymers, which should facilitate hole transport. Herein, the syntheses of two methoxylated isoindigos and the investigations on the charge transport behaviors of their copolymers with bisthiophene ( 2T ) and bisthiazole ( 2Tz ) are reported. It is found that the substitution positions have a drastic influence on the UV–vis absorption and electrochemical properties for both monomers and polymers. Theoretical calculations and single crystal structure analysis confirm the existence of O···S “conformation lock”, however, both methoxy substitutions also change the aggregation behaviors of the corresponding polymers to a mixed face-on/edge-on orientation which has an adverse effect for charge transport. Among the four polymers, the polymer of 5,5'-methoxylated isoindigo and 2T exhibit the best hole mobility of 1.9 × 10⁻¹ cm² V⁻¹ s⁻¹.