Solvent and intermolecular effects on first hyperpolarizabilities of organometallic tungsten-carbonyl complexes, a TDDFT study

The first hyperpolarizability of two tungsten-carbonyl complexes, tungsten pentacarbonyl pyridine and tungsten pentacarbonyl trans-1,2-bis(4-pyridyl)-ethylene, has been studied by the high-level TDDFT method. The consideration of the solvent effect and intermolecular pi-pi weak interaction in the calculations quantitatively improve the final result of both the electronic excitations and the first hyperpolarizabilities. By using the orbital decomposition scheme (J. Phys. Chem. A 2006, 110, 1014-1021), the NLO mechanisms of these two complexes have been ascribed to the dominant contribution from the metal-to-ligand charge transfer, with HOMO --> LUMO character, and the indispensable contribution from the intraligand charge transfer as well. A supplementary formula has been proposed to implement the orbital-pair transition analysis. This study reports the significant influences of solvation and intermolecular interactions on the first hyperpolarizabilities of organometallic NLO chromophores.     

Molecular salts with diquat-based electron acceptors for nonlinear optics

We have used several techniques, including Stark spectroscopy and MO calculations, to investigate the optical and electronic properties of two new dipolar chromophoric cations containing diquat-based electron acceptor groups. Both the Stark data and the calculated parameters show that the strong electron-accepting properties of a diquat unit lead to static first hyperpolarizabilities beta0 which are considerably larger than those of a related stilbazolium chromophore. In addition, one compound has a strongly 2D quadratic NLO response, providing a very rare example of a charged molecule displaying such behavior.     

Diastereoisomeric dinuclear ruthenium complexes of 2,5-di(2-pyridyl)thiazolo[5,4-d]thiazole

The first metal complexes of 2,5-di(2-pyridyl)thiazolo[5,4-d]thiazole (5) are described. X-Ray crystal structures are reported for the free ligand 5, a dinuclear copper complex 6 and the two diastereoisomers, 7meso and 7rac, of the dinuclear bis(2,2'-bipyridine)ruthenium complex. The two diastereoisomers of 7 and the 4,4'-dimethyl-2,2'-bipyridine analogue 8 are readily separated by cation exchange chromatography. 1H NMR and visible absorption spectra and electrochemical data for the four dinuclear ruthenium complexes reveal that these have relatively small HOMO-LUMO energy gaps and exhibit relatively weak metal-metal interactions.     

Quadratic and cubic nonlinear optical properties of salts of diquat-based chromophores with diphenylamino substituents

A series of chromophoric salts has been prepared in which 4-(diphenylamino)phenyl (Dpap) electron donor groups are connected to electron-accepting diquaternized 2,2'-bipyridyl (diquat) units. The main aim is to combine large quadratic and cubic nonlinear optical (NLO) effects in potentially redox-switchable molecules with 2D structures. The chromophores have been characterized as their PF(6)(-) salts by using various techniques including electronic absorption spectroscopy and cyclic voltammetry. The visible absorption spectra are dominated by intense π → π* intramolecular charge-transfer (ICT) bands, and all of the compounds show two reversible or quasireversible diquat-based reductions and partially reversible Dpap oxidations. Single crystal X-ray structures have been obtained for one salt and for the precursor compound (E)-4-(diphenylamino)cinnamaldehyde, both of which adopt centrosymmetric space groups. First hyperpolarizabilities β have been measured by using hyper-Rayleigh scattering (HRS) with a 800 nm laser, and Stark (electroabsorption) spectroscopy of the ICT bands affords estimated static first hyperpolarizabilities β(0). The directly and indirectly derived β values are large and generally increased substantially for the bis-Dpap derivatives when compared with their monosubstituted analogues. Polarized HRS studies show that the NLO responses of the disubstituted species are dominated by "off-diagonal" β(zyy) components. Lengthening the diquaternizing alkyl unit lowers the electron-acceptor strength and therefore increases the ICT energies and decreases the E(1/2) values for diquat reduction. However, compensating increases in the ICT intensity prevent significant decreases in the Stark-based β(0) responses. Cubic NLO properties have been measured by using the Z-scan technique over a wavelength range of 520-1600 nm, revealing relatively high two-photon absorption cross-sections of up to 730 GM at 620 nm for one of the disubstituted chromophores.     

Alkali metal doping effect on static first hyperpolarizabilities of PMI chains

An ab initio study of the effect on nonlinear optical (NLO) properties of medium-size polymethineimine (PMI) chains caused by doping with an alkali metal atom along the backbone is presented. Both the electronic and (preliminary) vibrational static first hyperpolarizabilities are investigated. Doping leads to the injection of an excess electron into the PMI chain, which is accompanied by major enhancement of its NLO response. Along with the hyperpolarizability, other electronic and structural properties depend strongly upon the position of doping along the chain. The vibrational contribution is larger than the corresponding electronic one for most of the cases studied.     

簇电荷对金属—金属键影响的量子化学研究

在探讨过渡金属原子簇化合物的金属——金属键的本质时,簇电荷的影响已引起人们的注意。簇电荷对金属——金属键的作用比较复杂,其中有价电子的成键效应和金属原子氧化数变化所产生的电荷效应。键长与簇电荷之间很难找到简单的关系。Cotton等曾对此问题做过初步讨论,但尚缺定量或半定量的理论计算依据,本文采用改进的电荷自洽EHMO程序(MAD—SCCO-EHMO)计算一系列Mo,Tc,Ru,Rh,和Re等二核簇的电子结构,根据M(?)lliken重迭集居分析,讨论簇电荷对金属——金属键的影响。     

Two-dimensional, pyrazine-based nonlinear optical chromophores with ruthenium(II) ammine electron donors

Six new nonlinear optical (NLO) chromophores with pyrazinyl-pyridinium electron acceptors have been synthesized by complexing a known pro-ligand with electron donating {Ru(II)(NH(3))(5)}(2+) or trans-{Ru(II)(NH(3))(4)(py)}(2+) (py = pyridine) centers. These cationic complexes have been characterized as their PF(6)(-) salts by using various techniques including electronic absorption spectroscopy and cyclic voltammetry. The visible d → π* metal-to-ligand charge-transfer (MLCT) absorptions gain intensity on increasing the number of Ru(II) centers from one to two, but remain at constant energy. One or two Ru(III/II) redox processes are observed which are reversible, quasi-reversible, or irreversible, while all of the ligand-based reductions are irreversible. Molecular first hyperpolarizabilities β have been determined by using hyper-Rayleigh scattering (HRS) at 1064 nm, and depolarization studies show that the NLO responses of the symmetric species are strongly two-dimensional (2D) in character, with dominant "off-diagonal" β(zyy) components. Stark (electroabsorption) spectroscopic measurements on the MLCT bands also allow the indirect determination of estimated static first hyperpolarizabilities β(0). Both the HRS and the Stark-derived β(0) values increase on moving from mono- to bimetallic complexes, and substantial enhancements in NLO response are achieved when compared with one-dimensional (1D) and 2D monometallic Ru(II) ammine complexes reported previously.     

What Do We Know about the Metal-Metal Bond?

Almost all main group and subgroup metals are able to form metal-metal bonds. The bond order ranges from weak interaction to a quadruple bond, and the degree of aggregation from a dinuclear entity to a three-dimensional network. In spite of numerous physicochemical studies, not all aspects of the metal-metal bond are understood. The ability of metal-metal linked polynuclear complexes to serve as a reservoir for missing or excess electrons enables them to react both with nucleophilic or reducing reagents and with electrophilic or oxidizing reagents. The intermediate position occupied by clusters between simple complexes and the bulk metal is of theoretical and practical significance.     

Proton-induced tuning of metal-metal communication in rack-type dinuclear Ru complexes containing benzimidazolyl moieties

Ru complexes bearing a bis-tridentate benzimidazolyl ligand have been synthesized. The dinuclear ones act as a bibasic acid with pK(a1)=4.36 and pK(a2)=5.90. The protonated form of the dinuclear complex exhibited two one-electron oxidations at +0.91 and +1.02 V versus the ferrocenium/ferrocene (Fc/Fc(+)) couple (the potential difference (ΔE)=0.11 V), but the di-deprotonated form showed two waves at +0.50 and +0.58 V versus Fc/Fc(+) (ΔE=0.08 V). Since the potential difference between two waves reflects the strength of the metal-metal interaction, the deprotonation of the benzimidazole moieties in the complexes weakened the Ru-Ru communication. The degree of electronic coupling between two metal centers, estimated from the intervalence charge transfer (IVCT) band, was greater for the protonated form. DFT calculations for the protonated and deprotonated forms of the dinuclear complex suggest that the Ru(II)-L(H(2)) π* interaction plays a key role in the Ru-Ru interaction.     

一类五核平面开口型过渡金属原子簇电子吸收光谱和二阶非线性光学性质的TDDFT研究

运用TDDFT B3LYP/LanL2DZ方法, 研究了一类具有非中心对称的五核平面开口构型过渡金属原子簇化合物[MoS₄Cu₄(py)₆X₂] (X＝Br, I)的电子吸收光谱和静态二阶非线性极化率, 估算了晶体的宏观二阶非线性光学系数. 电子吸收光谱的计算结果与实验测量结果比较符合; 碘系簇合物的静态二阶非线性极化率大于溴系. 详细讨论了该类金属簇合物电子吸收光谱的归属及其相关联的电子跃迁方式; 在微观水平上阐述了其二阶非线性光学性质的起源. 研究结果表明外围无机卤素配体4p/5p轨道到簇芯[MoS₄]杂化轨道的电子转移对静态二阶非线性极化率的贡献大于有机配体的贡献; 而过渡金属簇芯内的电子转移也有较大的贡献. 这对于理解过渡金属原子簇化合物内的电子转移对光学激发的作用以及用来设计新的无机-有机杂化二阶非线性光学材料有较大的帮助.     

Theoretical study on the considerable second-order nonlinear optical properties of naphthylimido-substituted hexamolybdates

The static first hyperpolarizabilities and origin of nonlinear optical (NLO) properties of [(2-methylnaphthyl)imido]hexamolybdates derivatives have been investigated by density functional theory (DFT). The [(2-methylnaphthyl)imido]hexamolybdate has considerable large first hyperpolarizability, 6.780 x 10(-30) esu, and it is larger than that of [(2,6-dimethylphenyl)arylimido]hexamolybdate due to the double aromatic rings in the naphthylimido ligand. The naphthylimido ligand acts as an electron-donor and the polyanion acts as an electron-acceptor. The substituent position on the naphthylimido is a key factor to determine the first hyperpolarizability of (naphthylimido)hexamolybdate derivatives. The derivative, which the iodine atom locates on the para nitrogen on the naphthylimido ligand, has the largest betao(o) value among the iodine-substituted derivatives. It suggests that the iodine atom is quasi linear with nitrogen and Mo, which is bonded to thenitrogen atom, could generate a large static electronic field and give the large contribution to NLO response.The introducing of electron-donors significantly enhances the first hyperpolarizabilities of (naphthylimido)hexamolybdates comparing with the electron-acceptors as the electron-donating ability is significantly enhanced when the electron-donor is attached to the naphthylimido segment. The present investigation provides important insight into NLO properties of (arylimido)molybdate derivatives.     

Switchable Nonlinear Optical Properties of η(5)-Monocyclopentadienylmetal Complexes: A DFT Approach

Density functional theory (DFT) calculations have been carried out to investigate the switching of the second-order nonlinear optical (NLO) properties of η(5)-monocyclopentadienyliron(II) and ruthenium(II) model complexes presenting 5-(3-(thiophen-2-yl)benzo[c]thiophen-1-yl)thiophene-2-carbonitrile as a ligand. The switching properties were induced by redox means. Both oxidation and reduction stimulus have been considered, and calculations have been performed both for the complexes and for the free benzo[c]thiophene derivative ligand in order to elucidate the role played by the organometallic fragment on the second-order NLO properties of these complexes. B3LYP, CAM-B3LYP, and M06 functionals were used for our calculations. The results show some important structural changes upon oxidation/reduction that are accompanied by significant differences on the corresponding second-order NLO properties. TD-DFT calculations show that these differences on the second-order NLO response upon oxidation/reduction are due to a change in the charge transfer pattern, in which the organometallic iron and ruthenium moieties play an important role. The calculated static hyperpolarizabilities were found to be strongly functional dependent. CAM-B3LYP, however, seems to predict more reliable structural and optical data as well as hyperpolarizabilities when compared to experimental data. The use of this functional predicts that the studied complexes can be viewed as acting as redox second-order NLO switches, in particular using oxidation stimulus. The β(tot) value of one-electron oxidized species is at least ～8.3 times (for Ru complex) and ～5.5 times (for Fe complex) as large as that of its nonoxidized counterparts.     

Energy decomposition analysis of metal-metal bonding in [M2X8]2- (X=Cl, br) complexes of 5f (U, Np, Pu), 5d (W, Re, Os), and 4d (Mo, Tc, Ru) elements

The electronic structures of a series of [M2X8]2- (X=Cl, Br) complexes involving 5f (U, Np, Pu), 5d (W, Re, Os), and 4d (Mo, Tc, Ru) elements have been calculated using density functional theory, and an energy decomposition approach has been used to carry out a detailed analysis of the metal-metal interactions. The energy decomposition analysis involves contributions from orbital interactions (mixing of occupied and unoccupied orbitals), electrostatic effects (Coulombic attraction and repulsion), and Pauli repulsion (associated with four-electron two-orbital interactions). As previously observed for Mo, W, and U M2X6 species, the general results suggest that the overall metal-metal interaction is considerably weaker or unfavorable in the actinide systems relative to the d-block analogues, as a consequence of a significantly more destabilizing contribution from the combined Pauli and electrostatic (prerelaxation) effects. Although the orbital-mixing (postrelaxation) contribution to the total bonding energy is predicted to be larger in the actinide complexes, this is not sufficiently strong to compensate for the comparatively greater destabilization originating from the Pauli-plus-electrostatic effects. A generally weak electrostatic contribution accounts for the large prerelaxation destabilization in the f-block systems, and ultimately for the weak or unfavorable nature of metal-metal bonding between the actinide elements. There is a greater variation in the energy decomposition results across the [M2Cl8]2- series for the actinide than for the d-block elements, both in the general behavior and in some particular properties.     

Origin of bulklike structure and bond length disorder of Pt37 and Pt6Ru31 clusters on carbon: comparison of theory and experiment

We describe a theoretical analysis of the structures of self-organizing nanoparticles formed by Pt and Ru-Pt on carbon support. The calculations provide insights into the nature of these metal particle systems-ones of current interest for use as the electrocatalytic materials of direct oxidation fuel cells-and clarify complex behaviors noted in earlier experimental studies. With clusters deposited via metallo-organic Pt or PtRu(5) complexes, previous experiments [Nashner et al. J. Am. Chem. Soc. 1997, 119, 7760; Nashner et al. J. Am. Chem. Soc. 1998, 120, 8093; Frenkel et al. J. Phys. Chem. B 2001, 105, 12689] showed that the Pt and Pt-Ru based clusters are formed with fcc(111)-stacked cuboctahedral geometry and essentially bulklike metal-metal bond lengths, even for the smallest (few atom) nanoparticles for which the average coordination number is much smaller than that in the bulk, and that Pt in bimetallic [PtRu(5)] clusters segregates to the ambient surface of the supported nanoparticles. We explain these observations and characterize the cluster structures and bond length distributions using density functional theory calculations with graphite as a model for the support. The present study reveals the origin of the observed metal-metal bond length disorder, distinctively different for each system, and demonstrates the profound consequences that result from the cluster/carbon-support interactions and their key role in the structure and electronic properties of supported metallic nanoparticles.     

Theoretical Studies on the First Hyperpolarizabilities of One-dimensional Donor-bridge-acceptor Chromophores

The ground-state dipole moments and second-order nonlinear optical （NLO） properties of a series of one-dimensional （1D） chromophores with donor-bridge-acceptor （D-B-A） structures have been investigated by using the second-order MФller-Plesset （MP2） and density functional theory （DFT） methods with the basis set of 6-31＋G（d）. According to the calculated results, the relationship between the molecular static first hyperpolarizability （βμ） and the directions of electron transition has been summarized. In terms of the sign of βμ, these 1D organic chromophores were classified into two categories： type Ⅰ with negative βμ and type Ⅱ bearing positive βμ. The analyses show that the remarkable difference of the first hyperpolarizabilities between Ⅰ and Ⅱ chromophores is associated mainly with the electrostatic interaction between terminal groups and the transport electrons in excited states. Moreover, different from the popular viewpoint, the obtained results also show that most of this series of 1D D-B-A molecules are more charge-separated in the ground states than in the excited states. As a whole, this theoretical investigation, to some extent, can be considered as a useful reference in designing the NLO chromophores with large first hyperpolarizabilities.     

Extended Fe₄ butterfly complexes: theoretical analysis of magnetic properties and magnetostructural maps

The inclusion of additional metal atoms in Fe? butterfly complexes drastically modifies their magnetic properties. Exchange interactions of a Fe?Y? complex have been calculated using theoretical methods based on density functional theory. The calculated values are in good agreement with experimental data showing that the change in the nature of bridging ligands induces a dramatic decrease of the antiferromagnetic wing-body interaction while the body-body interaction between the two central iron atoms is ferromagnetic. Finally, we propose a new tool to facilitate the understanding of the magnetic properties in polynuclear iron complexes. Magnetostructural maps allow us to correlate the calculated exchange coupling constants with metal-metal distances for the dinuclear or polynuclear iron complexes that we have studied.     

Pentacyanoiron(II) as an electron donor group for nonlinear optics: medium-responsive properties and comparisons with related pentaammineruthenium(II) complexes

In this article, we describe a series of complex salts in which electron-rich {Fe(II)(CN)(5)}(3)(-) centers are coordinated to pyridyl ligands with electron-accepting N-methyl/aryl-pyridinium substituents. These compounds have been characterized by using various techniques including electronic absorption spectroscopy and cyclic voltammetry. Molecular quadratic nonlinear optical (NLO) responses have been determined by using hyper-Rayleigh scattering (HRS) at 1064 nm, and also via Stark (electroabsorption) spectroscopic studies on the intense, visible d --> pi* metal-to-ligand charge-transfer (MLCT) bands. The relatively large static first hyperpolarizabilities, beta(0), increase markedly on moving from aqueous to methanol solutions, accompanied by large red-shifts in the MLCT transitions. Acidification of aqueous solutions allows reversible switching of the linear and NLO properties, as shown via both HRS and Stark experiments. Time-dependent density functional theory and finite field calculations using a polarizable continuum model yield relatively good agreement with the experimental results and confirm the large decrease in beta(0) on protonation. The Stark-derived beta(0) values are generally larger for related {Ru(II)(NH(3))(5)}(2+) complexes than for their {Fe(II)(CN)(5)}(3)(-) analogues, consistent with the HRS data in water. However, the HRS data in methanol show that the stronger solvatochromism of the Fe(II) complexes causes their NLO responses to surpass those of their Ru(II) counterparts upon changing the solvent medium.     

三核欠完整立方烷金属原子簇二阶非线性光学性质的理论研究

研究了具有欠完整立方烷构型的过渡金属原子簇分子的二阶非线性光学性质。利用TDDFT方法计算了选取的簇分子及相应模拟构造分子的静态和动态的一阶非线性光学超级化率(ijk);并计算了不同金属、桥原子和配体以及簇芯对该类化合物一阶超级化率的影响。选取其中的一个簇分子为基本模型,分析了该分子的电子结构和分子轨道,在微观水平上阐述了其非线性光学性质的可能起源。认为由过渡金属和硫原子组成的簇芯和与桥原子相连的配体对该类簇合物的二阶非线性光学性质的起决定性的作用。     

Large changes of static electric properties induced by hydrogen bonding: an ab initio study of linear HCN oligomers

We report the partitioning of the interaction-induced static electronic dipole (hyper)polarizabilities for linear hydrogen cyanide complexes into contributions arising from various interaction energy terms. We analyzed the nonadditivities of the studied properties and used these data to predict the electric properties of an infinite chain. The interaction-induced static electric dipole properties and their nonadditivities were analyzed using an approach based on numerical differentiation of the interaction energy components estimated in an external electric field. These were obtained using the hybrid variational-perturbational interaction energy decomposition scheme, augmented with coupled-cluster calculations, with singles, doubles, and noniterative triples. Our results indicate that the interaction-induced dipole moments and polarizabilities are primarily electrostatic in nature; however, the composition of the interaction hyperpolarizabilities is much more complex. The overlap effects substantially quench the contributions due to electrostatic interactions, and therefore, the major components are due to the induction and exchange-induction terms, as well as the intramolecular electron-correlation corrections. A particularly intriguing observation is that the interaction first hyperpolarizability in the studied systems not only is much larger than the corresponding sum of monomer properties, but also has the opposite sign. We show that this effect can be viewed as a direct consequence of hydrogen-bonding interactions that lead to a decrease of the hyperpolarizability of the proton acceptor and an increase of the hyperpolarizability of the proton donor. In the case of the first hyperpolarizability, we also observed the largest nonadditivity of interaction properties (nearly 17%) which further enhances the effects of pairwise interactions.     

A DFT study on dinuclear metallocenes RMMR [R= (BCO)5, (BNN)5; M = Be, Mg, Ca, Zn, Cd

A series of dinuclear metallocenes with formula RMMR [R= (BCO)5, (BNN)5; M = Be, Mg, Ca, Zn, Cd] are investigated via density functional theory. All these compounds contain two M[eta5-(BCO)5] fragments or two M[eta5-(BNN)5] fragments with a direct metal-metal single sigma bond. Detailed NBO analyses show that the metal-ligand interactions are predominantly ionic in nature and each metal atom is in its +1 oxidation state. NBO analyses indicate that the interaction between the B-B bonds and metal-metal antibond has played a role in the stabilities of these compounds.