Theoretical Study of the Substituent Effects on the Nonlinear Optical Properties of a Room‐Temperature‐Stable Organic Electride

Excess‐electron compounds can be considered as novel candidates for nonlinear optical (NLO) materials because of their large static first hyperpolarizabilities (β₀). A room‐temperature‐stable, excess‐electron compound, that is, the organic electride Na@(TriPip222), was successfully synthesized by the Dye group (J. Am. Chem. Soc. 2005 , 127, 12416). In this work, the β₀ of this electride was first evaluated to be 1.13×10⁶ au, which revealed its potential as a high‐performance NLO material. In particular, the substituent effects of different substituents on the structure, electride character, and NLO response of this electride were systemically studied for the first time by density functional theory calculations. The results revealed that the β₀ of Na@(TriPip222) could be further increased to 8.30×10⁶ au by introducing a fluoro substituent, whereas its NLO response completely disappeared if one nitryl group was introduced because the nitro‐group substitution deprived the material of its electride identity. Moreover, herein the dependence of the NLO properties on the number of substituents and their relative positions was also detected in multifluoro‐substituted Na@(TriPip222) compounds.     

Theoretical study of a novel organic electride with large nonlinear optical responses

The design of stable organic electrides with high nonlinear optical (NLO) properties is a challenge in organic and materials chemistry. Here we theoretically design of a novel organic molecular electride model, Li⁺(C₂₀H₁₅Li₅)e⁻, by modifying the lithiation and Li-doping based on dodecahedrane (C₂₀H₂₀). Its electride characteristic is verified by the quantum theory of atoms in molecules and electron localization function analyses. For the first time, the strategy of steric protection is applied to improve the stability of the organic electride Li⁺@(C₂₀H₁₅Li₅)e⁻, in which the closed C₂₀ cage serves not only as the ligand with a negative inner electric field to stabilize the Li cation but also as a barrier to prevent the Li cation from escaping. Meanwhile, the released excess electron is firmly captured in the cavity of Li₅. Moreover, Li⁺(C₂₀H₁₅Li₅)e⁻ displays a remarkably large first hyperpolarizability of 1.4 × 10⁴ au with potential application in organic second-order NLO materials.     

Theoretical study on nonlinear optical properties of the Li+[calix[4]pyrrole]Li−dimer,trimer and its polymer with diffuse excess electrons

The static (hyper)polarizabilities of the dimer and trimer with diffuse excess electrons, [Li⁺[calix[4]pyrrole]Li^?]_n, are firstly investigated by the DFT(B3LYP) method in detail. For the dimer and trimer, a Li atom inside each calix[4]pyrrole unit is ionized to form a diffuse excess electron. The results show that the dimer and trimer containing two and three excess electrons, respectively, have very large first hyperpolarizablities as 2.3 × 10⁴ and 4.0 × 10⁴ au, which are 30 and 40 times larger than that of the corresponding [calix[4]pyrrole]_n (n = 2, 3) without Li atom. Also, β values of dimer and trimer are twice and four times as large as that of monomer containing one excess electron. Obviously, not only excess electron but also the number of excess electron plays an important role in increasing the first hyperpolarizability. Moreover, the (hyper)polarizabilities of the [Li⁺[calix[4]pyrrole]Li^?]_n polymer are investigated at ab initio level by using the elongation finite‐field (elongation FF) method. All the oligomers of the [Li⁺[calix[4]pyrrole]Li^?]_n with many excess electrons exhibit very large first hyperpolarizability and large second hyperpolarizability. The present investigation shows that by introducing several and more excess electrons into the nonlinear optical (NLO) materials will be an important strategy for improving their NLO properties, which will be helpful for design of NLO materials. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010     

Three-propeller-blade-shaped electride: remarkable alkali-metal-doped effect on the first hyperpolarizability

Significant alkali-metal-doped effects on the structure and the first hyperpolarizability (β ₀) of effective multi-nitrogen complexant tris[(2-imidazolyl)methyl]amine (TIMA) are investigated. Three imidazoles of TIMA like three blades of propeller connect with methyls by the C–C single bonds. Because of the three C–C single-bond cooperative rotations, the TIMA behaves with great flexibility, and it is a high-performance multi-nitrogen complexant for the alkali metal doping. Thus, the new complexes A_m-TIMA (A_m = Li, Na, and K) with electride characteristic have diffuse excess electron than the reported electride-type system due to the strong interaction between the complexant TIMA and alkali metal. For the first hyperpolarizability, three engaging electrides A_m-TIMA with the diffuse excess electrons exhibit considerably large β ₀ values using the MP2 (full) method and the β ₀ values of new electrides are greatly larger (3,464–29,705 times) than that (338 au) of TIMA. Surprisingly, the K-TIMA sets a new record β ₀ value to be 1.00 × 10⁷ au which far exceeds than that (3,694–76,978 au) of the reported electride-type system Li@calix[4]pyrrole (J Am Chem Soc 127:10977–10981, 2005) and Li_n−H−(CF₂−CH₂)₃−H (n = 1, 2) (J Am Chem Soc 129:2967–2970, 2007) and 31,123 au of the organometallic system (J Am Chem Soc 121:4047–4053, 1999) Ru(trans-4,4′-diethylaminostyryl-2,2′-bipyridine)₃²⁺, as well as 1.23 × 10⁶ au of the large donor-CNT systems (Nano Lett 8:2814–2818, 2008). Clearly, the alkali-metal-doped effect on the first hyperpolarizability is very dramatic for the high-performance multi-nitrogen complexant TIMA. Considering simple possibility from molecule to material, the β ₀ values of optimized Li-TIMA-dimer and Li-TIMA-tetramer are investigated by BHandHLYP method. Interestingly, results show that the order of β ₀ value is Li-TIMA-monomer < Li-TIMA-dimer < Li-TIMA-tetramer. So the new three-propeller-blade-shaped electrides can be considered as candidates for high-performance nonlinear optical materials.     

Intermetallic Electride Catalyst as a Platform for Ammonia Synthesis

Electrides loaded with transition‐metal (TM) nanoparticles have recently attracted attention as emerging materials for catalytic NH₃ synthesis. However, they suffer from disadvantages associated with the growth and aggregation of nanoparticles. TM‐containing intermetallic electrides appear to be promising catalysts with the advantages of both electrides and transition metals in a single phase. LaRuSi is reported here to be an intermetallic electride with superior activity for NH₃ synthesis, and direct evidence is provided supporting its electride‐character‐induced catalytic performance. The discussion is made mainly based on the contrasting synthesis rates over the isostructural compounds LaRuSi, CaRuSi, and LaRu₂Si₂, and the N₂ isotope‐exchange reactions over these compounds. Lattice hydride ions, which can reversibly exchange with anionic electrons, are shown to be indispensable in the promotion of NH_x formation. The mechanism derived from the present findings provides new guidelines for NH₃ synthesis.     

Theoretical studies on structures and nonlinear optical properties of alkali doped electrides B12N12–M (M = Li,Na, K)

The structures and nonlinear optical properties of a novel class of alkali metals doped electrides B₁₂N₁₂–M (M = Li, Na, K) were investigated by ab initio quantum chemistry method. The doping of alkali atoms was found to narrow the energy gap values of B₁₂N₁₂ in the range 3.96–6.70 eV. Furthermore, these alkali metals doped compounds with diffuse excess electron exhibited significantly large first hyperpolarizabilities (β₀) as follows: 5571–9157 au for B₁₂N₁₂–Li, 1537–18,889 au for B₁₂N₁₂–Na, and 2803–11,396 au for B₁₂N₁₂–K. Clearly, doping of the alkali atoms could dramatically increase the β₀ value of B₁₂N₁₂ (β₀ = 0). Furthermore, their transition energies (ΔE) were also calculated. The results showed that these compounds had low ΔE values in the range 1.407–2.363 eV, which was attributed to large β₀ values of alkali metals doped B₁₂N₁₂ nanocage. © 2016 Wiley Periodicals, Inc.     

Two dimensional inorganic electride-promoted electron transfer efficiency in transfer hydrogenation of alkynes and alkenes

A simple and highly efficient transfer hydrogenation of alkynes and alkenes by using a two-dimensional electride, dicalcium nitride ([Ca₂N]⁺·e^–), as an electron transfer agent is disclosed. Excellent yields in the transformation are attributed to the remarkable electron transfer efficiency in the electride-mediated reactions. It is clarified that an effective discharge of electrons from the [Ca₂N]⁺·e^– electride in alcoholic solvents is achieved by the decomposition of the electride via alcoholysis and the generation of ammonia and Ca(OⁱPr)₂. We found that the choice of solvent was crucial for enhancing the electron transfer efficiency, and a maximum efficiency of 80% was achieved by using a DMF mixed isopropanol co-solvent system. This is the highest value reported to date among single electron transfer agents in the reduction of C–C multiple bonds. The observed reactivity and efficiency establish that electrides with a high density of anionic electrons can readily participate in the reduction of organic functional groups.     

Investigation on the electronic structures and nonlinear optical properties of alkali metal atom doped all‐cis 1,2,3,4,5,6‐hexafluorocyclohexane

On the basis of stable all‐cis 1,2,3,4,5,6‐hexafluorocyclohexane, a series of alkali metal atom doped M F₆C₆H₆ (M = Li, Na, and K) compounds were theoretically constructed and studied by using ab initio quantum chemistry method. The calculated results show that the HOMO–LUMO gap of the M F₆C₆H₆ conspicuously narrowed from 10.41 eV of pure F₆C₆H₆ to about 2.00 eV of M F₆C₆H₆. The electride characteristics of M F₆C₆H₆ are verified by their electronic structures, HOMOs, and small VIE values. As expected, these electrides possess considerable static first hyperpolarizabilities (β₀). Among the studied electrides, the largest β₀ of the Li F₆C₆H₆ is 7.00 × 10⁵ au, which is about 3030 times larger than pure F₆C₆H₆. TD‐M06‐2X calculations show that these larger β₀ values are attributed to lower transition energies for the crucial excited states of M F₆C₆H₆ systems. Further, the vibrational contributions to the static first hyperpolarizabilities of these molecules are also estimated. Moreover, Li atom doped dimer and trimer of F₆C₆H₆ also present unusual electride's features and exhibit dramatically large β₀. Thus, the F₆C₆H₆ interacting with the alkali metal atoms may be a potential promising NLO nanomaterial.     

Alkali metal atom-aromatic ring: a novel interaction mode realizes large first hyperpolarizabilities of M@AR (M=Li, Na, and K, AR=pyrrole, indole, thiophene, and benzene)

Several new electride compounds M@pyrrole (M = Li, Na, and K), Li@AR (AR = indole, thiophene, and benzene), Li@tryptophan and Li@serotonin were designed and investigated, which exhibit considerably large first hyperpolarizabilities (β(0)) (6705, 1116, 11399, 5781, 4808, 1536, 8106, and 9389 au, respectively) by comparison with their corresponding sole molecules pyrrole (β(0) = 30 au), indole (104 au), thiophene (6 au), benzene (0 au), tryptophan (159 au) and serotonin (151 au), respectively. The computational results revealed that the interaction of the alkali metal atom with π-conjugated aromatic ring (AR) is one effectively new approach to produce diffuse excess electron to get a large β(0) value, which is advantageous for the design of the novel high-performance NLO materials with π-conjugated AR: alkali metal atoms doped nanomaterials and biomolecules.     

The structure and the large nonlinear optical properties of Li@calix[4]pyrrole

A new compound with electride characteristics, Li@calix[4]pyrrole, is designed in theory. The Li atom in Li@calix[4]pyrrole is ionized to form a cation and an excess electron anion. Its structure with C(4v) symmetry resembles a cup-like shape. It may be a stable organic electride at room temperature. The first hyperpolarizability of the cup-like electride molecule is first investigated by the DFT (B3LYP) method. The result shows that this electride molecule has a considerably large first hyperpolarizability with beta(0) = 7326 au (63.3 x 10(-30) esu), while the beta(0) value of the related calix[4]pyrrole system is only 390 au. Obviously, the Li atom doped in calix[4]pyrrole brings a dramatic change to the electronic structure, so that the first hyperpolarizability of Li@calix[4]pyrrole is almost 20 times larger than that of calix[4]pyrrole. We find that the excess electron from the Li atom plays an important role in the large first hyperpolarizability of Li@calix[4]pyrrole. The present investigation reveals a new idea and different means for designing and synthesizing high-performance NLO materials.     

NLO‐X (X = I–III): New Gaussian basis sets for prediction of linear and nonlinear electric properties

New adjusted Gaussian basis sets are proposed for first and second rows elements (H, B, C, N, O, F, Si, P, S, and Cl) with the purpose of calculating linear and mainly nonlinear optical (L–NLO) properties for molecules. These basis sets are new generation of Thakkar‐DZ basis sets, which were recontracted and augmented with diffuse and polarization extrabasis functions. Atomic energy and polarizability were used as reference data for fitting the basis sets, which were further applied for prediction of L–NLO properties of diatomic, H₂, N₂, F₂, Cl₂, BH, BF, BCl, HF, HCl, CO, CS, SiO, PN, and polyatomic, CH₄, SiH₄, H₂O, H₂S, NH₃, PH₃, OCS, NNO, and HCN molecules. The results are satisfactory for all electric properties tested; dipole moment (µ), polarizability (α), and first hyperpolarizability (β), with an affordable computational cost. Three new basis sets are presented and called as NLO‐I (ADZP), NLO‐II (DZP), and NLO‐III (VDZP). The NLO‐III is the best choice to predict L–NLO properties of large molecular systems, because it presents a balance between computational cost and accuracy. The average errors for β at B3LYP/NLO‐III level were of 8% for diatomic molecules and 14% for polyatomic molecules that are within the experimental uncertainty. © 2014 Wiley Periodicals, Inc.     

Efficient,Reversible Redox‐Switching of Molecular First Hyperpolarizabilities in Ruthenium(II) Complexes Possessing Large Quadratic Optical Nonlinearities

Low‐energy metal‐to‐ligand charge‐transfer (MLCT) excitations are associated with the very large molecular nonlinear optical (NLO) coefficients, β, of the complexes [Ru^II(NH₃)₅(N‐R‐4,4′‐bipyridinium)]³⁺ (R=methyl, phenyl, or 4‐acetylphenyl). Chemical oxidation to the Ru^III forms causes bleaching of the MLCT absorptions and marked attenuation of the NLO responses. Both effects are completely reversed upon re‐reduction, and the extent of the β switching is about 10‐ to 20‐fold.     

Lithium salt electride with an excess electron pair--a class of nonlinear optical molecules for extraordinary first hyperpolarizability

A new lithium salt electride with an excess electron pair is designed, for the first time, by means of doping two sodium atoms into the lithium salt of pyridazine. For this series of electride molecules, the structures with all real frequencies and the static first hyperpolarizability (beta 0) are obtained at the second-order M?ller-Plesset theory (MP2). Pyridazine H 4C 4N 2 becomes the lithium salt of pyridazine Li-H 3C 4N 2 as one H atom is substituted by Li. The lithium salt effect on hyperpolarizability is observed as the beta 0 value is increased by about 170 times from 5 to 859 au. For the electride effect, an electride H 4C 4N 2...Na 2 formed by doping two Na atoms into pyridazine, the beta 0 value is increased by about 3000 times from 5 to 1.5 x 10 (4) au. Furthermore, combining these two effects, that is, lithium salt effect and electride effect, more significant increase in beta 0 is expected. A new lithium salt electride Li-H 3C 4N 2...Na 2 is thus designed by doping two Na atoms into Li-H 3C 4N 2. It is found that the new lithium salt electride, Li-H 3C 4N 2...Na 2, has a very large beta 0 value (1.412 x 10 (6) au). The beta 0 value is 2.8 x 10 (5) times larger than that of H 4C 4N 2, 1644 times larger than that of Li-H 3C 4N 2, and still 93 times larger than that of the electride H 4C 4N 2...Na 2. This extraordinary beta 0 value is a new record and comes from its small transition energy and large difference in the dipole moments between the ground state and the excited state. The frequency-dependent beta is also obtained, and it shows almost the same trends as H 4C 4N 2 < Li-H 3C 4N 2 < H 4C 4N 2...Na 2 < Li-H 3C 4N 2...Na 2. This work proposes a new idea to design potential candidate molecules with high-performance NLO properties.     

Molecular modeling for Cu(II)‐aminopolycarboxylate complexes: Structures,conformational energies,and ligand binding affinities

A ligand field molecular mechanics (LFMM) force field (FF) has been developed for d⁹ copper(II) complexes of aminopolycarboxylate ligands. Training data were derived from density functional theory (DFT) geometry optimizations of 14 complexes comprising potentially hexadentate N₂O₄, tetrasubstituted ethylenediamine (ed), and propylenediamine cores with various combinations of acetate and propionate side arms. The FF was validated against 13 experimental structures from X‐ray crystallography including hexadentate N₂O₄ donors where the nitrogens donors are forced to be cis and bis‐tridentate ONO ligands which generate complexes with trans nitrogen donors. Stochastic conformational searches for [Cu{ed(acetate)_n (propionate)_4‐n}]^2?, n = 0–4, were carried out and the lowest conformers for each system reoptimized with DFT. In each case, both DFT and LFMM predict the same lowest‐energy conformer and the structures and energies of the higher‐energy conformers are also in satisfactory agreement. The relative interaction energies for n = 0, 2, and 4 computed by molecular mechanics correlate with the experimental log β binding affinities. Adding in the predicted log β values for n = 1 and 3 suggest for this set of complexes a monotonic decrease in log β as the number of propionate arms increases. © 2013 Wiley Periodicals, Inc.     

The Unique Electronic Structure of Mg2Si: Shaping the Conduction Bands of Semiconductors with Multicenter Bonding

The electronic structures of the antifluorite‐type compound Mg₂Si is described in which a sublattice of short cation–cation contacts creates a very low conduction band minimum. Since Mg₂Si shows n‐type conductivity without intentional carrier doping, the present result indicates that the cage defined by the cations plays critical roles in carrier transport similar to those of inorganic electrides, such as 12 CaO⋅7 Al₂O₃:e⁻ and Ca₂N. A distinct difference in the location of conduction band minimum between Mg₂Si and the isostructural phase Na₂S is explained in terms of factors such as the differing interaction strengths of the Si/S 3s orbitals with the cation levels, with the more core‐like character of the S 3s leading to a relatively low conduction band energy at the Γ point. Based on these results and previous research on electrides, approaches can be devised to control the energy levels of cation sublattices in semiconductors.     

Cooperation of Three Chromophores Generates the Water‐Resistant Nitrate Nonlinear Optical Material Bi3TeO6OH(NO3)2

Nitrates have long been ignored for practical uses as nonlinear optical (NLO) materials because they are usually very easy to dissolve in water; despite this, the π‐conjugated [NO₃]⁻ is among the most desirable NLO‐active structural units. The cooperation of three structural chromophores, namely, Bi₃O₆OH short chains with 6s² lone pair electrons, distorted TeO₆ octahedra with d¹⁰ electrons, and π‐conjugated [NO₃]⁻ triangles, generates a new nitrate NLO material, Bi₃TeO₆OH(NO₃)₂, which exhibits an enhanced phase‐matchable NLO response of three times that of KH₂PO₄ (3×KH₂PO₄), exceeding those of most nitrate NLO materials. Remarkably, the new material did not show obvious weight loss and degeneration of NLO response after being dipped in de‐ionized water for 24 h, indicating that it is highly resistant to water. Theoretical calculations reveal that foreign water molecules cannot stably stay in the crystal lattice of Bi₃TeO₆OH(NO₃)₂. These findings highlight the introduction of diverse chromophores into the nitrate systems as an effective approach for developing practical nitrate NLO materials that are of high water‐resistance and good optical performance.     

Spectroscopic studies,Density Functional Theory calculations,and non-linear optical properties of binuclear Fe(III), Co(II), Ni(II), Cu(II), and Zn(II) complexes of OONN Schiff base ligand

The binuclear complexes of Fe(III), Co(II), Ni(II), Cu(II), and Zn(II) with 2-[3-(benzylideneamino)-2-(benzylidenehydrazono)-4-oxothiazolidin-5-yl] acetic acid ligand (HL) were prepared and their stoichiometry was determined by elemental analysis. The stereochemistry of the studied binuclear metal complexes was confirmed by analyzing their infrared spectra, ¹H NMR, and magnetic moment. Thermal decomposition studies of the binuclear complexes have been performed to demonstrate the status of water molecules present in these binuclear complexes and their general decomposition pattern. The equilibrium geometry of the ligand and its studied complexes were calculated using density function theory (DFT) calculations at the B3LYP/GENECP level of the theory. The results show that the ligand and its complexes are nonplanar structures as indicated from the values of the dihedral angles. Extent of distortion from regular geometry has been performed and discussed in terms of the values of the angles between the central metals and the coordinated sites. The E_HOMO and E_LUMO energies of the studied ligand and its complexes are used to calculate the global properties. The nonlinear optical parameters (NLO), anisotropy of the polarizibility (Δα), and the mean first-order hyperpolarizability (<β>) were calculated. The (<β>) values were compared with Urea as a reference molecule and the results of (<β>) values showed that the ligand and the studied complexes have good NLO behaviors.     

Side‐chain conformations in the isomorphous polyfluorinated {4,4′‐bis[(2,2‐difluoroethoxy)methyl]‐2,2′‐bipyridine‐κ2N,N′}dichloridopalladium and ‐platinum complexes

The polyfluorinated title compounds, [M Cl₂(C₁₆H₁₆F₄N₂O₂)] or [4,4′‐(HCF₂CH₂OCH₂)₂‐2,2′‐bpy]M Cl₂ [M = Pd, ( 1 ), and M = Pt, ( 2 )], have –C(H_α)₂OC(H_β)₂CF₂H side chains with H‐atom donors at the α and β sites. The structures of ( 1 ) and ( 2 ) are isomorphous, with the nearly planar (bpy)M Cl₂ molecules stacked in columns. Within one column, π‐dimer pairs alternate between a π‐dimer pair reinforced with C—H…Cl hydrogen bonds (α,α) and a π‐dimer pair reinforced with C—H_β…F(—C) interactions (abbreviated as C—H_β…F—C,C—H_β…F—C). The compounds [4,4′‐(CF₃CH₂OCH₂)₂‐2,2′‐bpy]M Cl₂ [M = Pd, ( 3 ), and M = Pt, ( 4 )] have been reported to be isomorphous [Lu et al. (2012). J. Fluorine Chem. 137 , 54–56], yet with disorder in the fluorous regions. The molecules of ( 3 ) [or ( 4 )] also form similar stacks, but with alternating π‐dimer pairs between the (α,β; α,β) and (β,β) forms. Through (C—)H…Cl hydrogen‐bond interactions, one molecule of ( 1 ) [or ( 2 )] is expanded into an aggregate of two inversion‐related π‐dimer pairs, one pair in the (α,α) form and the other pair in the (C—H_β…F—C,C—H_β…F—C) form, with the plane normals making an interplanar angle of 58.24 (3)°. Due to the demands of maintaining a high coordination number around the metal‐bound Cl atoms in molecule ( 1 ) [or ( 2 )], the ponytails of molecule ( 1 ) [or ( 2 )] bend outward; in contrast, the ponytails of molecule ( 3 ) [or ( 4 )] bend inward.