Tuning the absorption spectra and nonlinear optical properties of D‐π‐A azobenzene derivatives by changing the dipole moment and conjugation length: a theoretical study

The optical properties of several azobenzene derivatives were modulated by varying the dipole moments and conjugation lengths of the D‐π‐A systems. The relationship between the structure and absorption spectrum and polarizability was studied in the gas phase, THF and MeOH solutions, respectively, by using the density functional theory. The calculated absorption spectra and second‐order polarizabilities are in good agreement with the available experimental observations. In comparison with the D‐π‐A monomer, the H‐shaped D‐π‐A dimer almost doubles the dipole moments and hence increases the second‐order polarizabilities, without a significant shift in the maximum absorption bands. The addition of another azobenzol group between electron‐donating and ‐accepting groups increases the second‐order polarizabilities by 4–6 times, but leads to an evident red‐shift of about 65–80 nm in spectra. The relative second‐order polarizability of the halogen‐substituted derivatives is in the sequence of ? CF₃ > ? F > ? Cl > ? Br, without obvious substituent effects on the optical transparency. The D‐π‐A chromophores with the strong electron‐donating (amino) and ‐accepting (acetyl) substituent present the larger second‐order polarizabilities, at the cost of about 20 nm red‐shift of the maximum absorption lengths relative to the halogen‐substituted species. It is also demonstrated that both the linear and nonlinear optical properties augment with the increase in solvent polarity, accompanied by a red‐shift in the wavelengths of maximum absorption by about 18 and 23 nm, respectively, in THF and MeOH solutions. The changes in optical properties upon the structural modifications are further rationalized by the electronic structures of various H‐shaped dimers. Copyright © 2010 John Wiley & Sons, Ltd.     

Nanoscale Transition Metal Dichalcogenides: Structures,Properties, and Applications

Transition metal dichalcogenides (TMDs), such as MoS₂, MoSe₂, WS₂, and WSe₂, are layered materials with strong in-plane ionic-covalent bonds and weak out-of-plane van der Waals interactions, enabling formation of various nanostructures, such as nanotubes, nanoribbons, nanoflakes, and fullerene-like nanoparticles. Various remarkable properties have been found recently in these nanostructures, opening up brand new opportunities for their applications in nanoelectronics, optoelectronics, spintronics and structural materials. In this article, we present recent advances in the study of two-dimensional TMDs and their derivatives with special emphasis on structures, morphologies, properties (electronic, magnetic, thermal, mechanical), and applications (transistors, sensors, catalysts, lubricants, and composite materials). In addition, routes for modifying these properties by chemical doping, defect engineering, strain engineering, and electric fields are discussed. Our intent is to present a state-of-the-art view in this fast evolving field, with a balanced theoretical and experimental perspective.     

Modulation of electronic structures of thienylene vinylene oligomers by substituents and solvents: ground and excited states

The effects of substituents on the electronic structures of di(thienylene vinylene) (2TV) in ground and excited states are studied using density functional theory (DFT) and time‐dependent DFT (TD‐DFT), respectively. A representative set of electron donating groups (amino, methoxy and methyl) and withdrawing groups (acetylene, cyano and nitro) are introduced on the vinylene and thienyl moieties to investigate the influence of substituents. Bulk solvent effects are also taken into account by means of the polarizable continuum model (PCM). In contrast to the aromatic structures of 2TV and its derivatives in their ground (S₀) states, the electronic structures of first singlet excited (S₁) states are rather delocalized. The electron‐donating/withdrawing capability, position and number of substituents are important factors in tuning the vertical S₀ → S₁ absorption energies and S₁ → S₀ emission energies of 2TV derivatives. The NO₂‐ and NH₂‐substituents exert significant effects on the geometries of both ground and excited states and hence the absorption and photoluminescence (PL) emission spectra. The solvent polarity introduces modest influence on the excitation energies for most of the 2TV derivatives. But the absorption and PL emission spectra of nitro‐substituted 2TV exhibit noticeable red shifts as the medium polarity increases. Copyright © 2008 John Wiley & Sons, Ltd.     

Theoretical studies of the structural,electronic and optical properties of carbazole‐based compounds

Carbazole derivatives have drawn increasing attention recently in organic electronic device applications because of their particular optoelectronic properties. An in‐depth theoretical investigation was elaborated in this paper to reveal the molecular structures, optoelectronic properties, and the structure‐property relationships of different carbazole‐linked functional groups. The geometric and electronic structures in ground and the mobility for the hole and electron are both calculated by density functional theory method. The excited‐state geometries of these compounds were obtained through Single‐excitation Configuration Interaction method, and time‐dependent density functional theory calculation results described the absorption and emission spectra properties, respectively. Some conclusions are as follows: (1) enlarging the π‐conjugated area, the corresponding spectra red shifted markedly; (2) by introducing the electron‐donor such as carbazole, the spectra blue shifted slightly; (3) compared with compound 1, the spectra for these compounds are hardly influenced by introducing an electron‐acceptor or heterocyclic substitution. On all accounts, these compounds are interesting optoelectronic functional materials. On the basis of their structural modifiability, the arylamine derivatives substituted carbazole compounds have great potential in the applications of organic light‐emitting diodes, organic solar cells, and sensors. Copyright © 2011 John Wiley & Sons, Ltd.     

A study of electronic structure and lattice dynamics of CoSb3 skutterudite

The novel filled skutterudite materials have attracted much interest in recent years and experimental studies have revealed that electrical properties (electrical conductivity and Seebeck coefficient) in these materials are dominated by their electronic structure while the effective suppression of thermal conductivity is mainly determined by their lattice dynamics. To clarify the relationship between microstructure and properties in further, we report a systematic study of electronic structures and lattice dynamics of CoSb₃ in this paper using linearized augmented plane waves based on the density functional theory of first principles. By calculating band structure and partial density of states (PDOS), effects of electronic structures of CoSb₃ on electrical properties were investigated. Based on the calculated results of phonon dispersions and phonon density of states of CoSb₃, lattice dynamics of CoSb₃ (heat capacity, Debye temperature, mean free path and lattice thermal conductivity) are discussed in detail. The calculated results are excellently consistent with other work and experimental data.     

Insight into the structural,electronic and elastic properties of AInQ2 (A: K,Rb and Q: S,Se, Te) layered structures from first-principles calculations

First-principles calculations were performed to explore the structural, elastic and electronic properties of the ternary indium chalcogenides AInQ₂ (A: K, Rb and Q: S, Se, Te) in both monoclinic and triclinic phases. This study is carried out by using the first-principles pseudopotential plane-wave (PP–PW) method as implemented in CASTEP code. Both the generalized gradient approximation of Perdew–Burke–Ernzerhof scheme (GGA–PBE) and the Heyd–Scuseria–Ernzerhof (HSE06) hybrid functional were used to treat the exchange-correlation interactions. In order to confirm the previous reports and to understand the effect of symmetry in determining the physical properties of these layered materials we have calculated the structural and the electronic properties at the equilibrium lattice constant for both the systems. The single-crystal elastic constants C_{i j} are calculated using the stress-strain approach. The elastic moduli of the polycrystalline aggregates and their related properties are obtained in the framework of Voigt–Reuss–Hill approximations. Electronic band structure indicates the semiconducting behaviour with a direct band gap at Γ–Γ. The results obtained from the (HSE06) hybrid functional are in excellent agreement with the available experimental data and computed results for the monoclinic and triclinic structures.     

Ab initio investigation of isomeric and conformeric structures of halogen nitrites,XONO (X = Cl,Br, I)

A comparative study of isomers and conformers of halogen nitrites, XONO (X?=?Cl, Br, I), with particular emphasis on the I derivatives, has been carried out using high levels of electronic structure theory. All isomeric and conformeric structures and cis- to trans- conformational barriers were determined for each family. The nitryl halide isomers, XNO₂, were calculated to be the lowest energy compounds. Interesting variations in the structural parameters, harmonic vibrational frequencies and stabilization energies were obtained on halogen substitution, that are particularly pronounced in the iodine family.     

Distorted cage structures of Si36 cluster

Using full-potential linear-muffin-tin-orbital molecular-dynamics (FP-LMTO-MD) method, we have performed calculations on the fullerene cage structures and the binding energies of Si₃₆ cluster. It is found that their atomic arrangement tends towards tetrahedral geometry. The distorted structures are very stable. In addition, we have also investigated a stacked structure by tricapped trigonal prisms (TTP). The stacked structure is slightly more stable than the distorted fullerene cages. Their electronic states suggest that they present different electric properties.     

On the properties of new benzothiazole derivatives for organic light emitting diodes (OLEDs): A comprehensive theoretical study

The present investigation explored theoretically the geometry structures, the electronic and the optical properties of new benzothiazole derivatives with incorporated triphenylamine/diphenylnaphthylamine or (4-vinylphenyl)acrylonitrile as highly efficient emitting molecular materials for organic light emitting diodes (OLEDs). This study is accomplished in order to provide an in-depth understanding of the structure-properties correlation and their effects on optoelectronic devices.In this contribution, we apply quantum-chemical methods (semiempirical Austin Model 1 (AM1), ab-initio Hartree-Fock (HF) and density functional theory (DFT)) to elucidate the photophysical properties of these molecules. First of all, the geometry structures of these compounds in the ground and the excited states were fully optimized, showing a non-planar configuration structures. Whereas, the geometrical structure of benzothiazole with cyano-PV unit is more planar. Structural parameters and vibrational properties of these compounds are then derived. Moreover, absorption and luminescence properties, lying in a bluish white or red light emission, were elucidated by ZINDO/S methods. The molecular orbital (highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO)), the ionization potentials (IPs) and the electron affinities (EAs) of compounds under study were also investigated. It was found that the detailed results obtained from theoretical simulations are consistent with the available experimental data. This kind of theoretical approach has been proved to be reliable for the structure and spectroscopic properties, and predicts the favorable qualities of these benzothiazole derivatives, which make them as the materials of choice for high performance applications.     

Spectroscopy properties of the amide group in valpromide and some derivatives with antiepileptic activity

Infrared spectra at 300 and 77 K and Raman spectra at 300 K of the valpromide (Vpd), N‐substituted derivatives, N‐ethylvalpromide (Etvpd), N‐isopropylvalpromide (Ipvpd) and the N,N‐disubstituted derivative, N,N‐dimethylvalpromide (Dmvpd) with antiepileptic activity, have been measured and analyzed with results derived from computational chemistry calculation. In agreement with theoretical predictions, experimental data indicate that while in Etvpd, Dmvpd and Ipvpd there are four different conformational co‐existing components (Etvpd: TTCG⁺, TCCG⁻, TTTC, G⁺G⁺C G⁺; Dmvpd: TTCC, G⁻TTA⁺, G⁺A⁻TC, G⁺A⁻C A⁺; Ipvpd: TTCT, TCCT, TCCC, G⁻ TTT) in the Vpd there are only three distinct stable conformations of C₁ symmetry group: TTC, TCT, G⁺G⁺T. Based on the accuracy of the B3LYP calculation, with the 6‐31 + G** basis set estimated by comparison between the predicted values of the vibrational modes and the available experimental data, we performed a structural and vibrational study of the amide group in the Vpd and their derivatives. We found that small nonplanarity deviations of C(O)N backbone induce significant changes on the structural and spectroscopic properties. These are not compatible with the decreasing of the resonance effect as it is produced when the twisting around the C(O) N increases. From the Natural Bond Orbital (NBO) analysis the existence of stabilizing electrostatic interactions of type C H···O/N and C H···H N/C, which induce significant structural changes and a complex electronic redistribution of charge on the π‐system in those structures becomes evident. We view this as a consequence of the filled electron density change Lewis‐type NBOs type lp_{O1, 2}, lp_N1, σ_{(C H)N acyl} and empty non‐Lewis NBOs type σ*_{(C H)N acyl}, σ*_{N H}. Copyright © 2009 John Wiley & Sons, Ltd.     

Electronic and conformational properties of 2,3-benzodiazepine derivatives

Summary The molecular geometries and electronic structures of 2,3-benzodiazepine derivatives have been studied by means of the MNDO-PM3 method. A number of electronic properties have been computed and examined in order to find indication of the role of the electronic characteristics of the different molecules and their pharmacological properties. Theoretical data indicate that both electronic and structural properties appear responsible for the varying degree of anticonvulsant activity exhibited by compounds 1–4.     

New spectral assignment of n‐propanol in the C―H stretching region

The C―H stretching vibration serves as an important probe for characterizing molecular structures and properties of hydrocarbons. In this work, we present a detailed study on gas‐phase Raman spectrum of n‐propanol in the C―H stretching region using stimulated photoacoustic Raman spectroscopy. A complete assignment was carried out with the aid of quantum chemistry calculations and depolarization ratio measurement as well as isotope substitutions, i.e. CH₃CD₂CD₂OH, CD₃CH₂CD₂OH and CD₃CD₂CH₂OH. It is shown that the spectra of three C―H groups of n‐propanol overlap each other because of Fermi resonance coupling and different molecular conformations, leading to complex features that were not determined previously. In addition, the comparisons between the spectra of three isotopologues reveal that the C―H vibrations at different sites of carbon chain exhibit different sensitivity to conformational change of n‐propanol. The CH₃ stretching vibration at terminated γ‐carbon is not sensitive whereas the CH₂ stretching vibrations at both α‐carbon and β‐carbon atoms are sensitive. Furthermore, Raman spectra of liquid propanol recorded by conventional spontaneous Raman technique are reassigned on the basis of gas‐phase analysis. Copyright © 2016 John Wiley & Sons, Ltd.     

New ring-like models and ab initio DFT study of the medium-range structures,energy and electronic properties of GeSe2 glass

Ab initio DFT calculations were performed on Ge_nSe_m nanoclusters (n?=?2, 3, 5, 6, 12; m?=?6–9, 14, 16, 30) that represent the local structure of GeSe₂ glass and on some ‘defect’ Ge_nSe_m clusters that are thought to be related to the inhomogeneity of the structure at the nanoscale. The optimal geometries, total energies and their derivatives as well as the electronic properties of Ge_nSe_m nanoclusters were calculated using traditional DFT method. In addition, the TD-DFT method has been applied to calculate the electronic band gaps of the clusters. The calculated physico-chemical properties of Ge_nSe_m nanoclusters and their couplings with the local-and medium-range order structure formations in GeSe₂ glass are analysed and discussed.     

Ab initio study of the electronic structures and conduction properties of some novel low band-gap donor-acceptor polymers

The results of the electronic structures and conduction properties of four novel donor-acceptor polymers based on polysilole, obtained on the basis of ab initio Hartree-Fock crystal orbital method using their optimized geometries, are reported. The repeat unit of these polymers consists of bicyclopentadisilole unit bridged by an electron-accepting group Y(Y=CCH₂ in PSICH, CO in PSICF, CCF₂ and CC(CN)₂ in PSICN). All the polymers on the basis of their geometries and π-bond order values are found to have benzenoid-type electronic structures. Comparison of the important electronic properties such as ionization potential, electron affinity and band-gap of these polymers indicates PSICN to be the best candidate for intrinsic conductivity and reductive (n-) doping while PSICH is predicted to be the best candidate for oxidative (p-) doping. All these polymers are estimated to have band-gap values ranging between 1 and 2 eV. The low band-gap values of these polymers are rationalised on the basis of the patterns of their frontier orbitals.     

FP-LMTO study of structural,electronic and optical properties of wurtzite (CdS)n/(CdSe)n superlattices

In this paper, we have conducted a first-principles study of the structural, electronic and optical properties of (CdS)_n/(CdSe)_n superlattices (where n is numbers of monolayers) in the wurtzite phase (B4), using the Full-Potential Linear Muffin-Tin Orbital (FP-LMTO) method within the Local Density Approximation (LDA) technique, in order to describe the exchange correlation energy. The calculated electronic properties indicate that all (CdS)_n/(CdSe)_n superlattices configurations, possess a semiconductor behavior with same energy gaps. We have seen more carefully and accurately that the different superlattices configurations have no effect on the electronic properties; in particular, we did not observe any dependence between the band gap behavior and the used layers.     

Structure and electronic properties of a heterojunction between a single wall carbon nanotube and a TiC cluster

Atomic structures and electronic properties of heterojunctions of Ti-TiC and TiC-single wall carbon nanotube, Ti₄₈-Ti₁₉C₂₆ and Ti₁₉C₂₄-C₃₀, are studied by the first principles calculation based on the density functional theory. At the junctions, these substrates are smoothly connected with each other and keep their original structures and electronic properties. The structures of the junctions obtained in the present work give a realistic model to ab initio study for electronic transport properties through the junction of a carbon nanotube and an electrode.     

Electronic structure of α-Al2O3 slabs: A local environment study

In this work we performed an ab initio/Density Functional Theory (DFT) study of structural and electronic properties of the (0 0 1) α-Al₂O₃ surface. For this study we used two methods with different basis set: the Full-Potential Augmented Plane Wave plus local orbital (FP-APW+lo) and a linear combination of numerical localized atomic orbital basis sets, employing the WIEN2k code and the SIESTA code, respectively. In order to calculate the structural and electronic properties of the reconstructed surface, we calculated the final equilibrium atomic position with the SIESTA code and then the electric-field gradient (EFG) at Al sites was calculated with the FP-APW+lo code using the optimized positions. Using this procedure we found equilibrium structures with comparative lower energy than those obtained using only the FP-APW+lo method. The EFG tensor and the local structure for Al were studied as a function of the depth from the surface for the relaxed structures. We found that distances down to 6 Å from the surface are sufficient to converge the EFG and the Al–O distances to bulk values. The predicted bulk EFG at the Al site is in good agreement with available experimental values. These results can be used for local probes purposes, e.g., in the case of doping, with important sensitivity for probes located close to the top of the surface, in particular for distances smaller than 6 Å.     

Tuning the electronic properties of the fullerene C<Subscript>20</Subscript> cage via silicon impurities

The fullerene C₂₀ represents one of the most active classes of nanostructures, and they have been widely used as active materials for important applications. In this study, we investigate and discuss the tuning of the electronic properties of the fullerene C₂₀ cage via various consternations and locations of silicon atoms. All calculations are based on the density functional theory (DFT) at the B3LYP/3-21G level through the Gaussian 09W program package. The optimized structures, density of state (DOS) analysis, total energies, dipole moments, HOMO energies, Fermi level energies, LUMO energies, energy gaps, and the work functions were performed and discussed. Our results show that the electronic properties of C₂₀ cage do not only depend on the silicon impurity concentrations, but also depend on the geometrical pattern of silicon impurities in the C₂₀ cage. The tuning of the electronic properties leads to significant changes in the charge transport and the absorption spectra for C₂₀ cage via engineering the energy gap. So, we suggest that substitutional impurities are the best viable option for enhancement of desired electronic properties of C₂₀ cage for using these structures in nanoelectronics and solar cell applications.     

Electronic structure and magnetic properties of Laves phase LuFe2

The electronic structure and magnetic properties of the Laves phase of LuFe₂ with C14, C15, and C36 structures has been investigated using the full-potential linearized augmented plane wave method. In order to study the stability of magnetic phases, nonmagnetic and spin-polarized calculations for ferromagnetic ordering were performed. It is found that the ferromagnetic hexagonal C14 phase is the ground-state structure and the C15 phase is an intermediate state between the C14 and C36 structures. There is an increase in the average magnetic moment on the Fe sites in the order of C15 →C14 →C36 structures, whereas the Lu-moment is not significantly different. We also find that there exist both localized and itinerant d electrons, resulting in antiferromagnetic ordering in the three structures. Their density-of-states, equilibrium volumes, and elastic properties are discussed, which is important for the understanding of the physical properties of LuFe₂ and may inspire future experimental research.