Linear and non-linear optical properties of hetero-cyclic aromatic polymers

We investigated linear and non-linear optical properties of non-fused and fully fused hetero-cyclic thiophene, selenophene and pyrrole oligomers. We found that these oligomers show relatively better environmental stability in their fused form than non-fused geometry. Linear extrapolation of calculated energy gap between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) in the polymer limit exhibits extremely good agreement with experimentally reported band gap for thiophene, selenophene and pyrrole oligomers in their non-fused form. The lowest singlet dipole allowed excited state was calculated using time-dependent density functional theory and extrapolated optical gap estimated were in good agreement with experimental observation. Static linear and first hyperpolarisabilities were also studied as a function of chain length for donor–acceptor substituted polymers. The (hyper)polarisabilities are increasing as a function of chain length in both forms although the non-fused pyrrole oligomers show slow variation. Our study on hyperpolarisability also shows that donor–acceptor substituted non-fused selenophene oligomers are most promising candidates to contribute significantly in non-linear photonics and all fully fused hetero-cyclic oligomers show strong NLO responses.     

A density functional theory investigation of the stability,aromaticity, and photophysical behavior for the highly conjugated macrocycles containing 4 pyrroles

Porphyrin ( Pr ), porphycene ( Pc ), and [22]porphyrin(2.2.2.2) ( P[22] ) have been theoretically investigated. We design 2 highly conjugated macrocycles containing 4 pyrroles with different linkage bridges, which are named for 4 pyrrole ( Pf ) and methylene‐dipyrrolidine ( Pm ), as the theoretical model so as to investigate the stability, aromaticity, and photophysical behavior of these porphyrin derivatives, and the influence of getting or losing electron to the neutral molecule. The geometric structures of the molecules are optimized by density functional theory method. The absorptions are calculated by the time‐dependent density functional theory method. Based on the optimized structures, the nucleus‐independent chemical shifts (NICS) are calculated. The molecule with negative NICS value possesses larger highest occupied molecular orbital (HOMO)‐lowest occupied molecular orbital (LUMO) gap than that with positive NICS value, the molecule with bigger positive NICS value possesses smaller HOMO‐LUMO gap, and the molecule with bigger negative NICS value (in absolute value) possesses bigger HOMO‐LUMO gap. The current density indicates that the π‐electron delocalization is more effective in Pr and Pc than in Pf , Pm , and P[22] and corresponds to the stability of molecules. The absorptions of the molecules are all in the UV‐visible and infrared regions. The major transitions for most of the molecules are all from HOMO to LUMO. Compared with Pf ^2? , Pr ^2? , Pc ^2? , and P[22] ^2? , Pm ^2? shows distinctive photophysical properties, which is due to the reduced HOMO‐LUMO gap, structural distortion, and strong antiaromaticity.     

NLO characteristics of D-π-A coumarin-thiophene bridged azo dyes by Z-scan and DFT methods

ABSTRACT

The structural, electronic, intramolecular charge transfer (ICT) and nonlinear optical (NLO) properties of the donor-π-acceptor (D-π-A) azo linked dyes bearing coumarin thiophene bridge with different acceptors were inspected by Z-scan and DFT methods. The dye 3a exhibits bathochromic absorption maxima (649 and 650?nm) in the near IR region in DMF and DMSO. The dye 3a holds low HOMO–LUMO gap elucidated by CV and DFT indicating strong ICT character. The thermal stability is high for 3a and it shows enhanced NLO property by Z-scan and DFT methods as predicted in both global and range-separated hybrid functionals. The molecular geometry was optimised using B3LYP/6-311?+?g(d,p). The ICT characteristics are correlated with NLO properties obtained by Z-scan and DFT techniques.     

Effects of delocalised π-electrons around the linear acenes ring (n = 1 to 7): an electronic properties through DFT and quantum chemical descriptors

Π-electrons in chemical structure are the unique part of the fundamental particles that modify many interesting properties among the organic semiconductor molecules. By comparing the ground state energy, electronic properties and chemical indices within RHF/6-311G, B3LYP/6-311(G), B3LYP/6-311G(d,p), MP2/6-311G* and Cam-B3LYP/aug-cc-pvdz basis set at level of the theory, we identify that the resonance and the inductive effect of the delocalisation of electrons around the acene molecules could be responsible for acenes electronic and chemical properties. The total energies, energy gaps, HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) energy gaps, electron affinity and ionisation potential are close to the experimental and theoretical results. Among the chemical indices, electrophilicity (ω), electronegativity (χ) and chemical hardness (η) observed to decrease as the acenes ring increasing, whereas the softness (S) and chemical potential (μ) increase with increasing the number of carbons around the acene molecules. The study is extended to electronics and chemical properties of the acene.     

外电场下GaN的特性研究

以6-311G(3df,3pd)为基组,采用B3PW91方法优化得到GaN基态分子的几何结构,并探究了电场对GaN分子基态能量、电荷布居数、键长、偶极矩、振动频率、红外光谱强度、HOMO、LUMO能级影响.研究表明:无电场时,谐振频率值为576.2218 cm~(-1),与实验值484.9 cm~(-1)很接近.有电场时,键长、偶极矩、能隙Eg、电荷布居数、红外谱强度、HOMO和LUMO能级随电场的增大而减小;谐振频率和分子总能量随电场的增加而增加.谐振频率和红外谱强度对电场有着明显的依赖关系,这对材料的光学特性研究有提供理论参考.     

Electronic properties of a silicon carbide nanotube under uniaxial tensile strain: a density function theory study

The electronic properties of an armchair (4,4) single-walled silicon carbide nanotube (SWSiCNT) with the length and diameter of 22.4 and 6.93 Å, respectively under different tensile strains are investigated by density functional theory (DFT) calculation. The change of highest occupied molecular orbital and lowest unoccupied molecular orbital (HOMO–LUMO) gap of the nanotube has been observed during the elongation process. Our results show that the gap will significantly decrease linearly with the increase of axial strain. Two different slopes are found before and after an 11% strain in the profiles of the HOMO–LUMO gap. The radial buckling has been performed to investigate the radial geometry of nanotube. The partial density of states (PDOS) of two neighboring Si and C atoms of the nanotube are further studied to demonstrate the strain effect on the electronic structure of SiC nanotube. The PDOS results exhibit that the occupied states of Si atom and the unoccupied states of C atom are red-shifted and blue-shifted under stretching, respectively. Mulliken charge analysis reveals that Si and C atoms will become less ionic under the larger strain. The electron differences of silicon carbide nanotube (SiCNT) on tensile loading are also studied.     

Tuning HOMO–LUMO levels: trends leading to the design of 9‐fluorenone scaffolds with predictable electronic and optoelectronic properties

Highest occupied molecular orbital–lowest unoccupied molecular orbital (HOMO–LUMO) tuning is an important consideration in the development of organic‐based semiconducting materials. A study of the specific effects and overall trends for the HOMO–LUMO tuning of a diverse series of 9‐fluorenones by means of extended conjugation and substituent effects is described. Trends were explored in a range of compounds, beginning with structures having highly electron‐withdrawing substituents and progressing to structures having highly electron‐donating substituents. Compounds with an incremental increase in conjugation were also examined. Electrochemical and optical measurements were used to calculate the HOMO–LUMO levels and HOMO–LUMO bandgap (HLG) for each structure. Results from both methods were compared and correlated with the differences in molecular structure. Increasing the electron‐donating character of the substituents was observed to decrease the HLG and increase the energy levels of the HOMO and the LUMO, whereas an increase in the electron‐withdrawing character produced the opposite results. Increasing conjugation decreased the HLG, increased the HOMO energy level, but decreased the LUMO energy level. Spectroscopic evidence of substituent influence on the carbonyl suggests that substituents directly impact the HLG by influencing the availability of nonbonding electrons within the carbonyl, which impacts the probability of an nπ* transition. The data presented not only elaborate on the HOMO–LUMO tuning of 9‐fluorenone systems but also enable the consideration of 9‐fluorenones as analogous models for HOMO–LUMO tuning in other more complex polyaromatic systems such as bifluorenylidenes. These trends may provide insight into developing materials with specifically tuned HLGs and HOMO–LUMO levels for a variety of applications. Copyright © 2011 John Wiley & Sons, Ltd.     

Matlab编程的Hückel分子轨道法计算研究掺杂纳米碳管的化学反应性

使用Matlab自编简单Hückel分子轨道法（SHMO）计算程序,分析空位、Stone-Wales缺陷位、N和B原子掺杂的CNT（5,5）碳纳米管,计算π电子密度和前线分子轨道（HOMO和LUMO）为研究掺杂相对碳纳米管的化学反应性提供依据.具有不同电特性的掺杂相打破了碳纳米管的π电子、HOMO和LUMO的均衡分布.掺杂相和/或邻近的碳原子为HOMO或LUMO贡献了较其它原子更大的轨道系数,在不同的化学反应中表现出良好的亲核性或亲电性.此外,HOMO-LUMO能量差很好地反映了掺杂纳米碳管的导电性.计算结果与已报道的实验和理论结果吻合良好.     

Structure and properties of some chiralanes and chirolanes

The molecular structures, spectra and properties of six chiralanes and chirolanes (approximately spheroidal, saturated, cage hydrocarbons) have been determined by density functional theory (DFT) quantum chemistry calculations. The main features determined are: molecular geometry, partial atomic charges, standard enthalpy of formation, IR, nuclear magnetic resonance (NMR) and circular dichroism (CD) spectra. On the basis of the calculated standard enthalpies of formation and highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) gaps, we suggest that chiralanes/chirolanes are potential synthetic targets. We have calculated the anomalously large downfield ¹³C-NMR shifts for endohedral carbons in the spectra of [5.5] and [5.7]chiralanes.     

DFT study of dihydrogen interactions with lithium containing organic complexes C4H4-mLim and C5H5-mLim （m = 1, 2）

The interactions of dihydrogen with lithium containing organic complexes C₄H_4-mLi_m and C₅H_5-mLi_m (m = 1, 2) were studied by means of density functional theory (DFT) calculation. For all the complexes considered, each bonded lithium atom can adsorb up to five H₂ molecules with the mean binding energy of 0.59 eV/H₂ molecule. The interactions can be attributed to the charge transfer from the H₂ bonding orbitals to the Li 2s orbitals. The kinetic stability of these hydrogen-covered organolithium molecules is discussed in terms of the energy gap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). The results indicate that these organiclithium structures can perhaps be used as building units for potential hydrogen storage materials.     

Electronic properties of phenanthrimidazoles as hole transport materials in organic light emitting devices and in photoelectron transfer to ZnO nanoparticles

Phenanthrimidazoles as hole transport materials have been synthesized, characterized, and applied as nondoping emitters in organic light emitting devices. The synthesized molecules possess high fluorescent quantum yield and thermal properties and display film forming abilities. The highest occupied molecular orbital (HOMO) energies of these materials are shallower than the reported tris(8‐hydroxyquinoline)aluminum (Alq₃), which enables the hole transport ability of these phenanthrimidazoles. Taking advantage of the thermal stability and hole transporting ability, these compounds can be used as a functional layer between NPB [4,4‐bis(N‐(1‐naphthyl)‐N‐phenylamino)biphenyl] and Alq₃ layers and show that these phenanthrimidazoles can be alternatively used as novel hole transport materials and to improve the device performances. Geometrical, optical, electrical, and electroluminescent properties of these molecules have been probed. Further, natural bond orbital, nonlinear optical materials (NLO), molecular electrostatic potential, and HOMO–lowest unoccupied molecular orbital (LMO) energy analysis have been made by density functional theory (DFT) method to support the experimental results. Hyperpolarizability analysis reveals that the synthesized phenanthrimidazoles possess NLO behavior. The chemical potential, hardness, and electrophilicity index of phenanthrimidazoles have also been computed by DFT method. Photoinduced electron transfer explains the enhancement of fluorescence by nanoparticulate ZnO, and the apparent binding constant has been obtained. Adsorption of the fluorophore on ZnO nanoparticle lowers the HOMO and LUMO energy levels of the fluorophore. The strong adsorption of the phenanthrimidazoles on the surface of ZnO nanocrystals is likely due to the chemical affinity of the nitrogen atom of the organic molecule to Zn(II) on the surface of nanocrystal. Copyright © 2013 John Wiley & Sons, Ltd.     

Modification on C217 by auxiliary acceptor toward efficient sensitiser for dye-sensitised solar cells: a theoretical study

In this work, to develop efficient organic dye sensitisers, a series of novel donor–acceptor–π–acceptor metal-free dyes were designed based on the C217 dye by means of modifying different auxiliary acceptors, and their photovoltaic performances were theoretically investigated with systematic density functional theory calculations coupled with the incoherent charge-hopping model. Results showed that the designed dyes possess lower highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels as well as narrower HOMO–LUMO gaps compared to C217, which indicate their higher light-harvesting efficiency. In addition, using the (TiO₂)₃₈ cluster and bidentate bridging model, we predicted that the photoelectric conversion efficiency (PCE) for the C217 dye is as high as 9.92% under air mass (AM) 1.5 illumination (100 mW·cm^?2), which is in good agreement with its experimental value (9.60%–9.90%). More interestingly, the cell sensitised by the dye 7 designed in this work exhibits a middle-sized open-circuit voltage of 0.737 V, large short-circuit photocurrent density of 21.16 mA?cm^?2 and a fill factor of 0.801, corresponding to a quite high PCE of 12.49%, denoting the dye 7 is a more promising sensitiser candidate than the C217, and is worth further experimental study.     

Ab initio study for structure, electric properties and light emission of linear-trans-quinacridone

The structure, electric properties and emission rate of linear-trans-quinacridone are investigated within the density functional theory (DFT) calculations. We find the structure of the molecule to be planar with an energy gap of 3.06 eV. The emission lifetime from the lowest unoccupied molecular orbital (LUMO) to the highest occupied molecular orbital (HOMO) of this material is found to be 24 ns, which is in good agreement with experimental results.     

GeTe和GeSe 分子在外电场下的特性研究

对Ge原子采用6-311++G^**基函数,Te和Se原子采用SDB-cc-pVTZ基函数,利用密度泛函理论的局域自旋密度近似方法优化得到了GeTe和GeSe分子的稳定构型,并计算了外电场作用下GeTe和GeSe基态分子的平衡核间距、总能量、最高已占据分子轨道能量E_H、最低未占分子轨道能量E_L、能隙、谐振频率和红外谱强度. 在上述计算的基础上利用单激发组态相互作用-局域自旋密度近似方法研究了GeTe和GeSe分子在外电场下的激发特性. 结果表明:随着正向电场强度的增大,分子核间距逐渐增大,分子总能量逐渐降低,谐振频率逐渐减小,红外谱强度则逐渐增大. 在0-2.0569×10¹⁰ V·m^-1的电场范围内,GeTe分子的E_H 均高于GeSe分子的E_H;随着正向电场的增大,GeTe与GeSe的E_H差逐渐变大,GeTe的E_L低于GeSe的E_L,它们的E_L均随正向电场的增大而增大. 无外场时,GeTe分子的能隙比GeSe分子的能隙要小;在外电场反向增大的过程中, GeTe和GeSe的分子能隙始终减小. 外电场的大小和方向对GeTe和GeSe分子的激发能、振子强度及跃迁的波长均有较大影响. 关键词： GeTe GeSe 外电场 激发态     

Phenylazoimidazole as a possible optical molecular switch: An ab initio study

By applying nonequilibrium Green's function formalism combined first-principles density functional theory, we investigate the electronic transport properties of the phenylazoimidazole optical molecular switch. The molecule that comprises the switch can convert between the cis and the trans forms upon photoexcitation. The influence of HOMO–LUMO gaps and the spatial distributions of molecular orbitals on the electronic transport through the molecular device are discussed in detail. Theoretical results show that the current through the trans form is significantly larger than through the cis form, which suggests this system has attractive potential application in future molecular switch technology.     

外电场下CdSe的基态性质和光谱特性研究

采用密度泛函(DFT)B3PW91方法在Lanl2dz基组下优化得到CdSe分子的基态稳定构型, 并研究了外电场对CdSe基态分子的总能量、HOMO能级、LUMO能级、能隙、电偶极矩μ、电荷布居、红外光谱的影响. 在相同的基组下用TD-DFT 方法计算了外电场下CdSe分子的前9个激发态的激发能、激发波长和振子强度. 结果表明: 无电场时CdSe分子的激发波长与实验结果符合较好, 相应的激发能也很接近. 随着电场增加, CdSe基态分子键长、偶极矩、红外谱强度先减小后增大; HOMO能级、LUMO能级、能隙随电场增加而减小; 总能量、谐振频率则是先增大后减小. 此外, 外电场对CdSe分子的激发能, 激发波长和振子强度均有较大影响.     

外电场作用下FO分子的特性研究

采用密度泛函的B3P86方法,以6-311+G(3df)为基函数优化得到不同外电场下FO基态分子的稳定几何结构、键长、总能量、HOMO能级、LUMO能级、费米能级、能隙、红外光谱和谐振频率.结果表明,分子结构与外电场有着强烈的依赖关系,且对电场方向的依赖呈现出不对称性;随着正向电场的增大,HOMO能级、LUMO能级和费米能级是减小的,能隙是先增大后减小;红外光谱和谐振频率是增大的,而频率间隔是不断减小的.     

Charge transfer interaction and nonlinear optical properties of barium chloride and thiourea complexes: a vibrational spectroscopic study

The nonlinear optical (NLO) semiorganic crystals barium thiourea chloride (BTC), bis(thiourea)barium chloride (BTBC) and barium(tetrakisthiourea) chloride (BTTC) were grown by the slow evaporation technique. FT‐Raman and IR spectra of the crystallized NLO materials were recorded and analyzed. The equilibrium geometry, bonding features and harmonic vibrational wavenumbers were investigated with the help of B3LYP density functional theory (DFT) method. From the optimized geometry, the decrease in C N bond length indicates the charge delocalization over the region of the molecules. Lengthening of CS bond and the deviation of CS···Cl angles clearly show the coplanarity of the amide planes of the complexes. The lowering of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy gap appears to be the cause for its enhanced charge transfer interactions. Copyright © 2011 John Wiley & Sons, Ltd.     

Electronic structure and molecular orbital study of hole-transport material triphenylamine derivatives

Recently, triphenylamine (TPA), 4,4′-bis(phenyl-m-tolylamino)biphenyl (TPD), 4,4′-bis(1-naphthylphenylamino)biphenyl (NPB) and their derivatives are widely used in the organic light-emitting diode (OLED) devices as a hole-transporting material (HTM) layer. We have optimized twenty different structures of HTM materials by using density functional theory (DFT), B3LYP/6-31G method. All these different structures contain mono-amine and diamine TPA derivatives. The energies of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) along with molecular orbitals for these HTMs are also determined. We have found that the central amine nitrogen atom and the phenyl ring, which is next to the central amine nitrogen atom, show significant contribution to the HOMO and LUMO, respectively. The sum of the calculated bond angles (α+β+γ) of the central amine nitrogen atom has been applied to describe the bonding and the energy difference for HOMO and LUMO in these TPA derivatives. Electronic structure calculations have been performed for these TPA derivatives. Again, the LCAO-MO patterns of HOMO and LUMO levels of these derivatives are used to investigate their electron density. A series of electron-transporting steps are predicted for these compounds employing these calculated results.