Anisotropic conductivity of undoped and [FeCl4]-doped trans-polyacetylene

EHMO/CO (crystal orbital) quantum chemical methods were used to calculate the two-dimensional (2D) energy band structures of highly oriented trans-polyacetylene (PA) undoped and doped with [FeCl₄]⁻. The band gaps (E_g) of undoped PA in directions parallel and perpendicular to the oriented direction were 1.195 and 3.040 eV, respectively, and that of doped PA with [FeCl₄]⁻ were 0.0223 and 0.0504 eV, respectively. Based on the calculational results, the changes in conductive properties from undoped PA to [FeCl₄]⁻ doped PA could be interpreted. The conductivity anisotropy ratio σ/σ decreased when PA was doped by [FeCl₄]⁻, because the PA chain and the dopant showed a strong interchain coupling. The theoretical results for undoped and doped PA were in good agreement with the experiment.     

Soliton dynamics in alternating trans-polyacetylene and in stacked systems

The influence of substitutional disorder, of random forces and of energy dissipation on solitons in alternating trans-polyacetylene using the Su-Schrieffer-Heeger Hamiltonian is discussed. Double ionization in polyene chains was also investigated. Further soliton dynamics within a full Pariser-Parr-Pople Hamiltonian for trans-polyacetylene are presented. The presence of conformational solitons in stacked systems is shown numerically in a polyformamide model system. Possible relations to DNA and carcinogenesis are briefly reviewed.     

Cr、Sn、Co掺杂对层状二维材料MoSi2N4的电子和光学性能的影响(英文)

基于新合成的二维材料MoSi₂N₄(MSN),我们建立了一系列MSN的掺杂模型进行了第一原理计算。首先,我们计算了本征MSN的电子特性,包括其能带结构和态密度。然后我们研究了Cr、Sn和Co掺杂对MSN的电子和光学性质的影响。结果表明,在3种掺杂体系中,Co掺杂体系表现出最低的形成能,这表明Co掺杂体系是最稳定的。通过带隙计算表明,尽管3种掺杂模型都降低了MSN的固有带隙,但却表现出3种不同的电子特性。态密度图也显示,Cr和Co掺杂体系都在导带底(CBM)和价带顶(VBM)附近产生局部尖峰。此外,光学性质的计算中表明,掺杂后体系的光学性质也得到了改善。     

Thin polymer films on semiconductors: From their protection to the realization of molecular electronic devices

When homogeneously grafted as thin films on small band-gap inorganic semiconductors (n-GaAs, n-CdS), organic conjugated polymers such as poly-methylthiophene afford interesting new structures. In their doped conducting state, these polymer films bring a long-term protection of these semiconductors against their photocorrosion in aqueous medium, and the subsequent inclusion of metallic aggregates in these films allows increased catalytic activity for achieving photoelectrochemical reactions. In their undoped semiconducting state, they lead to the realization of “organic-on-inorganic” p-n junctions which show very low leakage current and high allowed current density. As photovoltaic cell, an energy conversion efficiency of 17.5% has been obtained under 100 mW.cm⁻² irradiation with a p-PMeT/n-GaAs cell. The larger number of work already devoted to organic conjugated polymers such as polyacetylene, polypyrrole, polythiophene, has shown that these compounds can be switched between a doped oxidized state, with a nearly metallic conductivity, σ ∼ 10²-10³S.cm⁻¹, and an undoped neutral state which presents semiconducting properties, σ ∼ 10⁻⁷ -10⁻⁹ S. cm⁻¹. Either as free standing film or grafted on an electrode, these polymers exhibit interesting organic electrode properties which have been widely characterized. On the other hand much less is known on their behavior when deposited on inorganic semiconductors. In the following, this area will be discussed on two examples, the protection of narrow band-gap semiconductors against their photo-degradation in aqueous solution, and the new junction properties presented by “organic-on-inorganic” electronic devices.     

Electronic structure and band gaps in cationic heterocyclic oligomers. Multidimensional analysis of the interplay of heteroatoms, substituents, molecular length, and charge on redox and transparency characteristics

Oxidative doping of extended pi-conjugated polymers and oligomers produces dramatic changes in optical and electrical properties, arising from polaron and soliton-derived midgap states. Despite the great importance of such changes for materials properties, far less is known about the cationic polaron states than about the neutral, semiconducting or insulating, undoped materials. The systematic, multifactor computational analysis of oligoheterocycles such as oligothiophenes, oligofurans, and oligopyrroles presented here affords qualitative and quantitative understanding of the interplay among skeletal substitution pattern, electronic structure, and the effective band gap reduction on p-doping. A simple linear relation is derived for predicting p-doped oligomer and polymer effective band gaps based on those of the neutral oligomers; this relationship confirms the effectiveness of a "fixed band" approximation and explains the counterintuitive increase of the effective band gap on p-doping of many small band gap oligomers. The present analysis also suggests new candidates for transparent conductive polymers and predicts limiting behavior of ionization potential, electron affinity, and other properties for various polyheterocyclic systems. The results yield insight into materials constraints in electrochromic polymers as well as on p- and n-type conductors and semiconductors.     

Cu掺杂MgF2晶体的电子结构及光学特性

基于密度泛函理论(DFT)的第一性原理平面波超软赝势方法,计算了纯的MgF2晶体和掺杂不同原子分数(2.08%,4.16%,6.24%)Cu的MgF2晶体结构、电学性质以及光学性质.结果表明:Cu的掺入导致MgF2晶体禁带宽度逐渐变窄,并且Cu掺杂使得MgF2晶体折射率和吸收峰增加,特别是在4eV附近区域出现了新吸收峰.同时也给出了引起体系性质变化的物理机制,Cu掺杂MgF2晶体在光电化学方面有着潜在的应用价值.     

Poly(thiophenes) derivatized with linear and macrocyclic polyethers: from cation detection to molecular actuation

The association of linear or macrocyclic polyethers with the electronic properties of the π-conjugated polythiophene backbone leads to functional conducting polymers that exhibit metal cation dependent electronic properties. Based on this concept, various classes of cation sensors have been proposed and investigated for almost two decades. The interactions of metal cations with linear or macrocyclic polyether functional groups lead to modifications of the electronic properties of the π-conjugated backbone through various mechanisms including direct electronic effects on a single conjugated chain, collective electrochemical processes, or conformational changes. Conjugated polymers and oligomers representative of these various processes are discussed with an emphasis on recent examples of derivatized conjugated systems in which the interactions between metal cations and polyether groups serve as driving force to create molecular motion in conjugated systems.     

CdSe的电子结构和光学性质研究

CdSe是Ⅱ-Ⅵ族半导体材料中一种重要的半导体材料,它有闪锌矿和纤锌矿两种不同的结构,带隙较窄,具有优良的电光特性和广泛的应用前景,得到了人们的广泛关注[1-3].     

Metal-containing conjugated oligo- and polythiophenes

Conjugated polymers containing metal groups are an important class of new materials that may find application as sensors, non-linear optical materials and in molecular electronics. Polythiophenes are an extensively studied class of conjugated polymers, that can be easily modified synthetically, and may be prepared using a variety of methods. Metal groups pendant to polythiophene backbones modulate the electronic, optical and redox properties of the conjugated backbone and can introduce novel structural motifs to these materials. Functionalization of oligo- and polythiophenes by Fe, Pd and Ru containing groups is discussed, along with the properties of some of these materials.     

Twisting of conjugated oligomers and polymers: case study of oligo- and polythiophene

Interring twisting (change in the dihedral angle between conjugated rings) of polythiophene was studied theoretically using periodic boundary conditions (PBC) at the B3LYP/6-31G(d) level. We find that the band gap of polymers is strongly dependent on the interring twist angle; yet twisting requires very little energy. A twist of 30 degrees increases the band gap by 0.75 eV in polythiophene, while requiring only 0.41 kcal mol(-1) per monomer unit. Such a small energetic value is of the order of crystal packing or van der Waals forces. These results are compared with calculations performed on model oligomers. Sexithiophene, its radical cations, and its dication are optimized at 0-180 degrees end-to-end twist angles (which correspond to 0-36 degrees interring dihedral angles) using the B3LYP/6-31G(d) method. The theoretical results suggest that the HOMO-LUMO gap, ionization potential, and charge distribution of oligomers are strongly dependent on twisting, whereas, similar to the case of polythiophene, twisting of neutral oligothiophenes costs very little energy. In the case of the radical cation, the lowest energy transition is shifted to a longer wavelength region on twisting, while the second-lowest energy transition is shifted to a shorter wavelength region. This implies that twisted, doped conducting polymers (modeled here by an oligomer radical cation), in contrast to planar, doped polymers, should be transparent within a certain optical window (in the far-visible region, at approximately 1.5 eV). This observation is explained on the basis of changes in the shape and overlap of the frontier molecular orbitals.     

Structure and properties of non-classical polymers II. Band structure and spin densities

Theoretical investigation of the band structure of three types of nonclassical polymers, namely alternant (one- and two-dimensional), nonalternant and heteroatomic, are carried out. Although polyradicals, these polymers have a considerable delocalization energy which may determine their relative stability.The spin-density distribution of the alternant type of non-classical polymers corresponds to a ferrimagnetic ground state at 0 K.The non-classical polymers represent a new class of organic systems as their band structure and magnetic properties essentially differ from those of common polymers.     

Transition-metal-templated synthesis of rotaxanes and catenanes: From small molecules to polymers

This article provides an overview of the fundamental principles of the synthesis of metallocatenanes and metallorotaxanes. It also describes the synthesis and properties of electronic conducting polymers—polypyrrole and polythiophene—built around metallocatenanes and metallorotaxanes. The particular properties of this new class of polymers, including the possibility of transmetallation reactions being performed with them and the observation of electronic coupling between the metal centers and the conducting matrix, are discussed. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 3470–3477, 2003     

Modulating the Electronic and Optical Properties of Tetragonal ZnSe Monolayers by Chalcogen Dopants

The recently proposed three‐atom‐thick single‐layer ZnSe sheet demonstrates a strong quantum confinement effect by exhibiting a large enhancement of band gap relative to the zinc blende (ZB) bulk phase. In this work, we aim at investigating the electronic and optical properties of this ultrathin tetragonal ZnSe single‐layer sheet with various chalcogen dopant atoms, based on density functional theory (DFT). We find that these single‐layer sheets with dopant atoms are still direct‐band semiconductors with tunable band gaps, which can lead to strong light absorption and potential applications in solar energy harvesting. Theoretical optical absorbance results show that the S‐doped ZnSe monolayer exhibits a higher absorption performance compared to other doped and undoped ZnSe monolayers. These findings pave a way for the modulation of novel ultrathin tetragonal ZnSe monolayers for a wealth of potential optoelectronic applications.     

共轭聚合物的结构缺陷及对光电性能的影响

共轭聚合物是由大量重复基元通过化学键连接的一维体系,具有独特的光、电、电化学等性质,已经引起学术界的广泛关注.由于共轭聚合物结构(链段、构象、聚集态)的复杂性,即使在非常精细的合成条件下,少量结构缺陷的形成也是难免的.共轭聚合物,特别在其固态状态下激发能量能够有效传递,使得少量缺陷的影响被放大,对其光电性质产生巨大影响.因此对共轭聚合物结构缺陷的研究,包括缺陷成因与控制、缺陷密度的分析、缺陷的分子结构与电子结构特征等,对于高品质材料的研发具有重要的意义.本文对国内外研究进展进行了比较详尽的介绍.     

Synthesis of alkyl and N-alkyl-substituted polyanilines: A study on their spectral properties and thermal stability

Polyaniline (PANI), N-methyl- and N-ethyl-PANI, 2- and 3-ethyl-PANI as well as 2,6-dimethyl- and 3,5-dimethyl-PANI have been synthesized using ammonium persulfate as catalyst. These polymers have been studied in the undoped and doped state by FT-IR and electronic spectroscopy in the range from the UV-VIS to the NIR portion of the spectrum. As doping agent camphorsulfonic acid was used. The chemical structures of the N-methyl- and N-ethyl-PANI have been found to contain quinoneimine units along the polymer chain. In the doped state the N-alkyl-PANIs show the polaronic band transition in the NIR part of the spectrum, at longer wavelength than doped unsubstituted PANI. A similar but less pronounced phenomenon is observed in the case of ethyl-substituted phenyl rings of PANI. Dimethyl-substitution at the phenyl ring of PANI hinders the formation of high molecular weight polymer. Only oligomers are formed.The thermal stability of unsubstituted PANI in the undoped state is very high and N-alkyl substitution and phenyl ring substitution lowers the thermal stability of PANI. Doped samples show a significantly worse thermal resistance in comparison to the corresponding undoped samples, this is due essentially to the volatility and the decomposition of the protonic acid used as dopant. Good general agreement has been found between the predicted thermal stability on the basis of increment groups calculations and the experimental results.The thermal decomposition of unsubstituted and undoped PANI has been followed by FT-IR spectroscopy.     

Combination of MOMM and VEH methods to calculate electronic properties of polymers

A Molecular-Orbital-Based Molecular Mechanics method (MOMM) has been employed to calculate the structures of cyclopentadiene, pyrrole, furan, thiophene, dibenzocyclopentadiene (fluorene), dibenzopyrrole, dibenzofuran, and dibenzothiophene. The dimer structures of the above compounds have also been calculated using the same method to derive the unit cells of polycyclopentadiene, polypyrrole, polyfuran, polythiophene, polydibenzocyclopentadiene, polydibenzopyrrole, polydibenzofuran, and polydibenzothiophene. The band structures, densities of states, ionization potentials, band gaps, reduction potentials, and oxidation potentials of these polymers then have been calculated by using the Valence Effective Hamiltonian method (VEH). The structural effects on electronic properties are discussed.     

Understanding the synergistic effects,optical and electronic properties of ternary Fe/C/S‐doped TiO2 anatase within the DFT + U approach

Although TiO₂ is an efficient photocatalyst, its large band gap limits its photocatalytic activity only to the ultraviolet region. An experimentally synthesized ternary Fe/C/S‐doped TiO₂ anatase showed improved visible light photocatalytic activity. However, a theoretical study of the underlying mechanism of the enhanced photocatalytic activity and the interaction of ternary Fe/C/S‐doped TiO₂ has not yet been investigated. In this study, the defect formation energy, electronic structure and optical property of TiO₂ doped with Fe, C, and S are investigated in detail using the density functional theory + U method. The calculated band gap (3.21 eV) of TiO₂ anatase agree well with the experimental band gap (3.20 eV). The defect formation energy shows that the co‐ and ternary‐doped systems are thermodynamically favorable under oxygen‐rich condition. Compared to the undoped TiO₂, the absorption edge of the mono‐, co‐, and ternary‐doped TiO₂ is significantly enhanced in the visible light region. We have shown that ternary doping with C, S, and Fe induces a clean band structure without any impurity states. Moreover, the ternary Fe/C/S‐doped TiO₂ exhibit an enhanced photocatalytic activity, a smaller band gap and negative formation energy compared to the mono‐ and co‐doped systems. Moreover, the band edges of Fe/C/S‐doped TiO₂ align well with the redox potentials of water, which shows that the ternary Fe/C/S‐doped TiO₂ is promising photocatalysts to split water into hydrogen and oxygen. These findings rationalize the available experimental results and can assist the design of TiO₂‐based photocatalyst materials.     

Electronic Properties of Vinylene-Linked Heterocyclic Conducting Polymers: Predictive Design and Rational Guidance from DFT Calculations

The band structure and electronic properties in a series of vinylene-linked heterocyclic conducting polymers are investigated using density functional theory (DFT). In order to accurately calculate electronic band gaps, we utilize hybrid functionals with fully periodic boundary conditions to understand the effect of chemical functionalization on the electronic structure of these materials. The use of predictive first-principles calculations coupled with simple chemical arguments highlights the critical role that aromaticity plays in obtaining a low band gap polymer. Contrary to some approaches which erroneously attempt to lower the band gap by increasing the aromaticity of the polymer backbone, we show that being aromatic (or quinoidal) in itself does not ensure a low band gap. Rather, an iterative approach which destabilizes the ground state of the parent polymer toward the aromatic ? quinoidal level crossing on the potential energy surface is a more effective way of lowering the band gap in these conjugated systems. Our results highlight the use of predictive calculations guided by rational chemical intuition for designing low band gap polymers in photovoltaic materials.     

Spectroscopic and theoretical study of the molecular and electronic structures of a terthiophene-based quinodimethane.

The UV/Vis, infrared absorption, and Raman scattering spectra of 3',4'-dibutyl-5,5"-bis(dicyanomethylene)-5,5"-dihydro-2,2':5',2"-terthiophene have been analyzed with the aid of density functional theory calculations. The compound exhibits a quinoid structure in its ground electronic state and presents an intramolecular charge transfer from the terthiophene moiety to the C(CN)2 groups. The molecular system therefore consists of an electron-deficient terthiophene backbone end-capped with electron-rich C(CN)2 groups. The molecule is characterized by a strong absorption in the red, due to the HOMO-->LUMO pi-pi* electronic transition of the terthiophene backbone that shifts hypsochromically on passing from the solid state to solution and with the polarity of the solvent. The analysis of the vibrational spectra confirms the structural conclusions and supports the existence of an intramolecular charge transfer. Vibrational spectra in several solvents and as a function of temperature have also been studied. Significant frequency upshifts of the vibrations involved in the pi-electron-conjugated pathway have been noticed upon solution in polar solvents and with the lowering of the temperature. Finally, we propose a quinoid molecule as a reliable structural and electronic model for dication species in doped oligothiophenes or for bipolaron charged defects in doped polythiophene.     

Synthesis and properties of perfluorinated conjugated polymers based on polyethylenedioxythiophene, polypyrrole, and polyfluorene. Toward surfaces with special wettabilities

The electrochemical deposition of organic materials is a convenient and straightforward method that affords rough films in mild conditions. The presence of fluorinated chains covalently attached on the polymer backbone allows the control of the second criterion which plays a role on the wetting properties of the surface, that is, the chemical composition. By modification of the nature of the polymer, films with different surface energies were obtained. Thus, original semifluorinated polypyrrole (PPy- RF n ), polyfluorene (PFl- RF n ), and polyethylenedioxythiophene (PEDOT- RF n ) have been chemically and electrochemically synthesized and characterized. On one hand, the chemical polymerization affords highly fluorinated soluble polymers. Soluble PFl- RF n exhibits blue fluorescence in solution while soluble PEDOT- RF n presents optical properties similar to those of PEDOT. Consequently, they represent interesting candidates for optical devices (OLEDs for PFl- RF n , electrochromic materials for PEDOT- RF n ). On the other hand, surface properties have been investigated on the electroformed polymers by goniometry and microscopy. Fluorinated surfaces of electrodeposited polypyrrole, like polythiophene, give birth to high hydrophobic and oleophobic surfaces, while the use of polyethylenedioxythiophene as the polymer increases sufficiently the surface energy to get combined superhydrophobicity and superoleophilicity. The influence of the chemical composition is discussed through the comparison of the wetting properties of polyethylenedioxythiophene and semifluorinated polythiophene and polyethylenedioxythiophene.