Multidecker Sandwich Complexes VnBenn+1 (n = 1, 2, 3) as Stronger

Mar 4, 2015 - comparison with ferrocene or V1Bz2, the multidecker sandwich complexes VnBzn+1 (n = 2, 3) can be considered as stronger donor group for ...
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Multidecker Sandwich Complexes VnBenn+1 (n = 1, 2, 3) as Stronger Electron Donor Relative to Ferrocene for Designing HighPerformance Organometallic Second-Order NLO Chromophores: Evident Layer Effect on the First Hyperpolarizability and TwoDimensional NLO Character Shu-Jian Wang,*,† Yin-Feng Wang,‡ and Chenxin Cai*,† †

Jiangsu Key Laboratory of New Power Batteries, Jiangsu Collaborative Innovation Center of Biomedical Functional Materials, College of Chemistry and Materials Science, Nanjing Normal University, Nanjing 210097, P. R. China ‡ Jiangxi Province Key Laboratory of Coordination Chemistry, Institute of Applied Chemistry, School of Chemistry and Chemical Engineering, Jinggangshan University, Ji’an, Jiangxi 343009, P. R. China S Supporting Information *

ABSTRACT: During the past 3 decades, significant progress has been made in multidecker sandwich complexes. Replacing metallocene in the metallocene-based second-order nonlinear optical chromophores with multidecker sandwich complexes will create a variety of new second-order NLO organometallic complexes. To explore the possibility of improving NLO properties of organometallic complexes by using multidecker sandwich complexes, we theoretically design five second-order NLO moleculars VnBzn+1-(C2H2)3-NO2 (n = 1, 2, 3) with widely investigated VnBzn+1 (n = 1, 2, 3) by changing the layer number n or position of conjugation π−A system in the D−π−A structure. The NLO properties of these five complexes are investigated in detail. By comparison with the β0 of ferrocenyl derivative Fe(η5-C5H5)2-(C2H2)3-NO2, the larger NLO response (β0, 10215−36917 au) is found in VnBzn+1-(C2H2)3-NO2 (n = 1, 2, 3). The dependence of the first hyperpolarizabilities β0 on the layer number n and the position of conjugation π−A system is discussed in the article. The evident layer effect on the first hyperpolarizabilities β0 is shown. And, the enlargement of the first hyperpolarizability β0 by the layer number n can be rationalized by considering the enhancement of parallel-x-MLCT (from more than one metal center to acceptor, −NO2), perpendicular-x-MLCT (from multiple metal centers to multiple C6H6 ligands) transition and two-dimensional NLO character with the two charge transfer axes. By comparison with ferrocene or V1Bz2, the multidecker sandwich complexes VnBzn+1 (n = 2, 3) can be considered as stronger donor group for designing NLO moleculars. We hope, this groundbreaking work may evoke one’s attention to design new, highly efficient second-order NLO molecular materials with excellent building blocks: multidecker sandwich complexes.

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INTRODUCTION 5

attracted more and more attention because of their intriguing structural, magnetic, transport properties.10,12,28−34 For example, multidecker sandwich complexes VnBzn+1 (Bz = C6H6),9 LnnCOTn+1 (Ln = Ce, Nd, Eu, Ho, and Yb; COT = C8H8),17 metal-carborane multidecker sandwich complexes,14,15 tripledecker complex trans-(CpV)2(μ−η6:η6-C6H6),7 etc. have been

1−3

Since the discovery of ferrocene Fe(η -C5H5)2 in 1951, a great deal of work has been carried out to synthesize new sandwich-like organometallic complexes with different metals and cyclic ligands.4−17 These complexes have found applications in many fields, such as catalysis,18−20 nonlinear optics,21−27 etc. Among them, extended sandwich complexes (triple deckers, tetradeckers, etc.) composed of d-block transition metals or f-block actinide/lanthanide elements and carbocyclic ligands or carborane rings, etc.8,9,12,13,28 have © XXXX American Chemical Society

Received: December 11, 2014 Revised: February 6, 2015

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The Journal of Physical Chemistry C synthesized in the gas phase or liquid phase and extensively studied. Nonlinear optical materials play an important role in telecommunication, optical information processing, extending the spectral coverage of laser source, etc. During the past three decades, a great deal of work has been carried out to study the nonlinear optical properties of many different types (e.g., inorganic materials, organic moleculars, polymers, organometallic compounds, etc.) of NLO materials.27,35 Among them, organometallics are potentially interesting candidates for NLO materials. Compared with organic moleculars, they can offer a larger variety of NLO molecular structures in relation to the metal nd configuration, oxidation state, spin state, etc. One of the most widely investigated classes of NLO metal complexes is metallocene derivatives.22−26,36 And, the relative large first hyperpolarizability of these compounds have been found. How to further improve the NLO properties of metallocene derivatives? This is a question worthy of careful consideration. Because of the strong ability of donating electron, which is related to low-energy electrons from the metal center, the metallocene unit, such as ferrocene, generally is considered as the donor group in the D−π−A structure. Clearly, the multidecker sandwich π-coordination complexes mentioned above include more metals and cycle ligands with π electrons. Can we take the multidecker sandwich complexes as better donor group instead of metallocene for improving the NLO properties of metallocene derivatives or designing new highperformance second-order NLO organometallic complexes? Could the new possible ways of further improving the NLO properties be found along with introduction of the group of multidecker sandwich complexes? In addition, it has been reported that two-dimensional (2D) chromophores with large off-diagonal β tensor components and two charge transfer axes can display the better phasematched than the 1D chromophore, enhance the second-order NLO responses without undesirable visible transparency losses, and offer large macroscopic NLO responses.37−46 Could the 2D chromophores be designed with multidecker sandwich complexes as another one charge transfer axis? To explore the possibility of improving NLO properties or designing new NLO organometallic complexes by using multidecker sandwich complexes, we design five second-order NLO moleculars VnBzn+1-(C2H2)3-NO2 (n = 1, 2, 3) with widely investigated VnBzn+1 (n = 1, 2, 3)47−49by changing the layer number n or position of conjugation π−A system in the D−π−A structure, as shown in Figure 1. In the present work, our investigation aims at (1) predicting the structure of five second-order NLO organometallic complexes, (2) exhibiting the new structure-hyperpolarizability relationships and the dependence of the first hyperpolarizabilities on the layer number n and the position of conjugation π−A system, (3) understanding the nature of improvement of the first hyperpolarizability by the layer number and position of introduced conjugation π−A system. We hope, this groundbreaking work may evoke one’s attention to design new, highly efficient second-order NLO molecular materials with excellent building blocks: multidecker sandwich complexes VnBzn+1 (Bz = C6H6) and LnnCOTn+1 (Ln = Ce, Nd, Eu, Ho, and Yb; COT = C8H8), or metal-carborane multidecker sandwich complexes, etc.

Figure 1. Optimized geometrical structures of the five second-order NLO organometallic complexes VnBzn+1(C2H2)3NO2 (n = 1, 2, 3) designed with widely investigated multidecker sandwich complexes VnBzn+1.

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COMPUTIONAL DETAILS The optimized geometric structures of five second-order NLO organometallic complexes V1Bz2-(C2H2)3-NO2, V2Bz3-mid(C2H2)3-NO2, V2Bz3-end-(C2H2)3-NO2, V3Bz4-mid-(C2H2)3NO2, V3Bz4-end-(C2H2)3-NO2 with all real frequencies are obtained by using the LC-BLYP/gen method. The -mid means that the conjugation π−A system, −(C2H2)3NO2, positions at the middle benzene molecular, and -end means that the conjugation π−A system positions at the terminal benzene molecular, as shown in Figure 1. The 6-31G* basis set is employed for the C, H, N, and O atoms, and the lanl2dz basis set including a corrected effective core potential (ECP) to take into account relativistic effects is employed for the transition metal V atoms. On the basis of our calculation, it is found that the LC-BLYP method perform well in describing the large orbital overlap effects or covalent interactions in the analogous sandwich complex, ferrocene. As shown in Table S1 in the Supporting Information, by comparison with the BDE (bond dissociation energies) values obtained with the other methods, such as B3LYP (D0 = 124.8 kcal/mol) and CAM-B3LYP (D0 = 130.0 kcal/mol), the BDE value of Fe(η5-C5H5)2 is D0 = 150.5 kcal/mol with ZPE correction at the LC-BLYP level, which agrees very well with experimental BDE value50 of ferrocene (158 ± 2 kcal/mol). So, the LC-BLYP method is suitable to optimize the geometry structure. Theoretical and experimental studies showed that the magnetic moment of VnBzn+1 increases linearly with size n, and V atoms are coupled ferromagnetically.10 So, the spin multiplicity of the ground state of five VnBzn+1-(C2H2)3-NO2 (n = 1, 2, 3) second-order NLO organometallic complexes is 2 for n = 1, 3 for n = 2, 4 for n = 3, respectively. For the calculations of hyperpolarizabilities, by comparison with the more accurate but computationally expensive CC techniques (such as, CCSD(T) method), the MP2 level with the relative merit of computationally inexpensive methods is reliable in hyperpolarizabilities calculations51−53 for a mediumsize system without metal atoms, but it is very costly for these multidecker sandwich π-coordination systems with more than one transition metal. The CAM-B3LYP method has been proved to be proper in calculating the first hyperpolarizability of B

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The Journal of Physical Chemistry C π-conjugated systems compared with the other methods.54−56 To learn the suitability of the CAM-B3LYP level for calculation of the first hyperpolarizability of the investigated systems VnBzn+1-(C2H2)3-NO2, we compared the CAM-B3LYP value of second-order NLO organometallic complex FeCP2-(C2H2)3NO2 with those calculated by using the B3LYP, BHandHLYP, LC-BLYP, and more reliable MP2 methods with the same basis sets. As shown in Table 1, the CAM-B3LYP result is close to

μ = (μx 2 + μy 2 + μz 2 )1/2 α0 = (αxx + αyy + αzz)/3

β0 = (βx 2 + βy 2 + βz 2)1/2

Here

Table 1. Polarizability α0 (au) and First Hyperpolarizability β0 (au) of Second-Order NLO Metallocene Derivative FeCP2(C2H2)3NO2 Calculated by Using Different Methods FeCP2(C2H2)3NO2

β0

α0

MP2 LC-BLYP CAM-B3LYP B3LYP BH and HLYP

4760 3046 4375 6576 4167

222 217 232 250 228

(2)

βi =

3 (β + βijj + βikk ); 5 iii

i, j, k = x, y, z

It should be noted that the tensor components βiii of first hyperpolarizability is calculated with the y axis parallel to the molecular central axis of multidecker sandwich complexes VnBzn+1 (n = 1, 2, 3) and the key word nosymm is added in the route section of Gaussian input file. All calculations are carried out by using the Gaussian09 program package.68 The molecular structure and orbitals were plotted with the GaussView program.69

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RESULTS AND DISCUSSION A. Equilibrium Geometries. The optimized geometries of five organometallic complexes VnBzn+1-(C2H2)3-NO2 (n = 1, 2, 3) are displayed in Figure 1. As shown in Figure 1, the conjugation π−A system, -(C2H2)3-NO2, with a slight incline is approximately perpendicular to the molecular axis (y-axis) of multidecker sandwich complexes VnBzn+1 (n = 1, 2, 3). In addition, due to metal-π interaction, the slight deformation of benzene moleculars in VnBzn+1 is observed. B. The dipole moment and polarizibility. The electric properties of the five metal complexes calculated at the ROCAM-B3LYP level are listed in Table 2. From Table 2, the dipole moment μ is 3.090 au (V2Bz3-mid-(C2H2)3NO2) < 3.272 au (V1Bz2(C2H2)3NO2) < 4.563 au (V2Bz3-end-(C2H2)3-NO2) < 4.573 au (V3Bz4-mid-(C2H2)3NO2) < 5.256 au (V3Bz4-end(C2H2)3NO2). The layer effect is not shown. For the polarizability, the α0 is 285 au (V1Bz2(C2H2)3NO2) < 399 au (V2Bz3-mid-(C2H2)3NO2) < 466 au (V2Bz3-end-(C2H2)3NO2) < 647 au (V3Bz4-mid-(C2H2)3NO2) < 684 au (V3Bz4-end(C2H2)3NO2). As shown in Figure 2, it can be seen that the layer number enhances the polarizability α0 of multidecker sandwich complex derivatives VnBzn+1(C2H2)3NO2 (n = 1, 2, 3), and the evident layer effect is shown. Comparing the polarizability α0 of V2Bz3-mid-(C2H2)3NO2 and V2Bz3-end(C2H2)3NO2, it can be found that the position of introduced conjugation π−A system, −(C2H2)3NO2, can also effect the polarizability α0, and the α0 (466 au) value of V2Bz3-end(C2H2)3NO2 with conjugation π−A system positioned at the terminal benzene is larger than that (399 au) of V2Bz3-mid(C2H2)3NO2 with conjugation π−A system positioned at the

that of the MP2 levels, whereas BHandHLYP and LC-BLYP level underestimate the β0 values, and the B3LYP level overestimates the β0 values. Thus, the CAM-B3LYP method was considered proper to predict the first hyperpolarizabilities β0 of these multidecker sandwich complex derivatives. In the calculations of electric properties, the spin contamination is in an acceptable range for the layer number n = 1, 2, but a little big for n = 3. Therefore, the ROCAM-B3LYP method is used to calculate the first hyperpolarizabilities. In general, in order to obtain reasonable calculated values of NLO properties, choosing a suitable basis set for further calculation that is sufficiently converged and computationally affordable for an investigated system is primarily important.57−64 In order to obtain well-converged values in this study, the slightly small basis set (the 6-31G* basis set for the C, H, N, and O atoms, the lanl2dz basis set for the transition metals) is choosed for hyperpolarizability calculations. The dipole moment μ, polarizabilities α0, and static first hyperpolarizabilities β 0 are evaluated by a finite-field approach65−67 at the ROCAM-B3LYP/gen level. In order to find proper AEF (Applied Electric Field), the first hyperpolarizabilities β0 of organometallic complex V2Bz3-mid(C2H2)3-NO2 are calculated at the ROCAM-B3LYP/gen level in a series of fields (see Table S2 in the Supporting Information). It can be seen that there is a plateau for β0 in an AEF range from 0.0005 to 0.0015 au Hereby, the 0.0010 au AEF is suitable in the calculation of NLO properties. The dipole moment, μ, polarizability, α0, static first hyperpolarizability, β0, are defined as follows:

Table 2. Spin Multiplicity, Dipole Moment μ, Polarizability α0, First Hyperpolarizability β0, the Main Tensor Components of Hyperpolarizability β of the Five NLO Organometallic Complexes Designed with Multidecker Sandwich Complexes VnBzn+1(C2H2)3NO2 (n = 1, 2, 3) and Metallocene Derivative FeCP2(C2H2)3NO2 V1Bz2(C2H2)3NO2 V2Bz3-end-(C2H2)3NO2 V3Bz4-end-(C2H2)3NO2 V2Bz3-mid-(C2H2)3NO2 V3Bz4-mid-(C2H2)3NO2 FeCP2(C2H2)3NO2

spin multiplicity

μ

α0

β0

βxxx

βxxy

βyyy

2 3 4 3 4 1

3.272 4.563 5.256 3.090 4.573 2.824

285 466 684 399 647 232

10215 22642 36917 10857 31156 4375

−15990 −34364 −45214 −17001 −49852 −7001

−3466 10316 21335 −3187 12069 −1221

−13 −494 4222 531 −2172 −66

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Figure 2. Dependence of the (a) polarizability α0 and (b) static first hyperpolarizability β0 on the layer number n and the position of conjugation π− A system.

where ΔE, f 0 and Δμ are the transition energy of the crucial excited state, oscillator strength, and the difference between the ground and excited-state dipole moments. From the two-level model, moleculars with unidirectional intense low-energy charge transfer transitions will have large tensor component βiii and large β. However, there exists three low-energy CT states in this type of metal complexes VnBzn+1(C2H2)3NO2 (n = 1, 2, 3), such as, intraligand charge transfer transition (ILCT), metal-to-ligand charge transfer transition (MLCT) perpendicular and parallel to the molecular axis (y-axis, see Figure 1). The molecular orbitals relevant to the ILCT and MLCT (perpendicular and parallel to the y-axis) transitions of V1Bz2(C2H2)3NO2 and V2Bz3-end-(C2H2)3NO2 are shown in Figue 3. For the metallocene-based second-order NLO chromophore, it has been reported by Green et al.25 that due to two lowenergy transitions contributing significantly to the observed optical nonlinearity, the two-level model is prohibited for understanding the trends in β. On the basis of experimental and theoretical results, they concluded that M → A CT transition I (from nonbonding, nearly degenerate dy2/dx2−y2/dxy orbitals (M) of the metal center of metallocene, to the orbital (A) localized on the acceptor, such as the group, −NO2) and π → A CT transition II (from the orbital (π) formed from a combination of the highest occupied cyclopentadienyl C5H5 orbital and the highest occupied π-bridge orbital, to orbital (A) localized on the acceptor) contributes significantly to the observed optical nonlinearity (see ref 25). M → A (from one metal center to acceptor) transition is equal to MLCT transition, and π → A (almost from C5H5 ligand to acceptor) transition is equal to ILCT transition. It can be inferred that analogy to metallocene-based second-order NLO chromophore, MLCT (from one metal center to acceptor, parallel to the x-axis) and ILCT (almost from C6H6 ligand to acceptor, parallel to the x-axis) transitions also contributes significantly to the first hyperpolarizability of V1Bz2(C2H2)3NO2. When the layer number increases from 1 to 2 or 3, the enhancement in β0 can be rationalized by considering the enhancement of parallelx-MLCT (from more than one metal center to acceptor, −NO2), perpendicular-x-MLCT (from multiple metal centers to multiple C6H6 ligands) transition (see Figure 3) and twodimensional NLO character with the two charge transfer axes. As shown in Table 2, the obvious increase of diagonal βxxx, βyyy, and off-diagonal βxxy values with layer number n or changing position of conjugation π−A system can confirm that parallel-xMLCT and perpendicular-x-MLCT transitions are strengthened and two-dimensional NLO character37−46 appears. It is first found that the perpendicular-x-MLCT transition in the multidecker sandwich complexes VnBenn+1 (n = 1, 2, 3) can

middle benzene. The same result is found in another two metal complexes with n = 3. C. The Evident Layer Effect on the First Hyperpolarizabilities and Two-Dimensional NLO Character. What are we most interesting with is the static first hyperpolarizabilities β0 of the five moleculars. As shown in Table 2, the β0 of the five moleculars constituted by the D−π− A structure is 10215 au for V1Bz2(C2H2)3NO2, 10857 au for V2Bz3-mid-(C2H2)3NO2, 22642 au for V2Bz3-end-(C2H2)3NO2, 31156 au for V3Bz4-mid-(C2H2)3NO2, 36917 au for V3Bz4-end(C2H2)3NO2, respectively. The dependences of the first hyperpolarizability β0 on the layer number n and position of conjugation π−A system are exhibited in Figure 2. The nonlinear optical responses of these five complexes are large, and the evident layer effect on the first hyperpolarizability is shown (see Figure 2). The layer number n of the donor group VnBzn+1 increases from 1 to 3, the β0 of NLO chromophore increases about 3 times. Comparing the β0 of V2Bz3-mid(C2H2)3NO2 and V2Bz3-end-(C2H2)3NO2, it can be seen that like polarizability α0, the position of introduced conjugation π− A system, −(C2H2)3NO2, can also obviously effect the first hyperpolarizability β0. And the β0 (22642 au) of V2Bz3-end(C2H2)3NO2 is about 2 times as large as that (10857 au) of V2Bz3-mid-(C2H2)3NO2. However, the unobvious change of the first hyperpolarizabiliy β0 is found in another two metal complexes with n = 3. By comparison with the β0 of ferrocenyl derivative Fe(η5-C5H5)2(C2H2)3NO2 (see Table 2), it can be deduced that the ability of donating electron of VnBzn+1 (n = 1, 2, 3) is stronger than ferrocene, and the multidecker sandwich complexes VnBzn+1 (n = 2, 3) is stronger donor relative to sandwich complexes ferrocene or V1Bz2 for designing secondorder NLO organometallic complexes. If one wants to further improve the NLO response of the metallocene-based secondorder nonlinear optical chromophore, replacing the metallocene group with multidecker sandwich complexes, such as VnBzn+1 (n = 1, 2, 3), LnnCOTn+1 (Ln = Ce, Nd, Eu, Ho, and Yb; COT = C8H8), etc., may be a good approach. What is the nature of enhancement of the first hyperpolarizability by the layer number? This is a question worthy of consideration. The SOS method and the related two-state simplification is the most useful approachs for chemists to understand structure-hyperpolarizability relationships. When a single charge transfer state or one tensor component βiii dominates the second-order NLO response β of molecular, the trends in β can be inferred by the two-level expression as given by the eq 1

β0 ∝

Δμf0 ΔE3

(1) D

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designing second-order NLO organometallic molecular will be proposed by our research group in the future.

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ASSOCIATED CONTENT

S Supporting Information *

Complete refs 33, 44, and 68, reaction energy for the FeCp2 → Fe (3d64s2) + 2Cp dissociation channel obtained with different theory methods, and AEF (applied electric field) dependence of the first hyperpolarizibility β0 of V2Bz3-mid-(C2H2)3-NO2. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S.-J.W.). Telephone: (86)-2585891780, Fax: (86)-25-85891767. *E-mail: [email protected] (C.C.). Telephone: (86)-2585891780, Fax: (86)-25-85891767. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Nos. 21362015, 221175067, 21273117, 21375063, 21335004, and 21405083), Natural Science Foundation of the Jiangsu Higher Education Institutions (13KJB150024, and 14KJB150012), Program of Outstanding Innovation Research Team of Universities in Jiangsu Province, and Priority Academic Program Development of Jiangsu Higher Education Institutions. The authors espress thanks for the use of the computational resource at State Key Laboratory of Theoretical and Computational Chemistry, Jilin University.

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Figure 3. Representations of charge-transfer transitions responsible for the second-order NLO properties of organometallic complexes V1Bz2(C2H2)3NO2 and V2Bz3-end-(C2H2)3NO2.

REFERENCES

(1) Kealy, T. J.; Pauson, P. L. A New Type of Organo-Iron Compound. Nature 1951, 168, 1039−1040. (2) Fischer, E. O.; Pfab, W. Z. Cyclopentadien-Metallkomplexe, ein neuer typ Metallorganischer Verbindungen. Z. Naturforsch. B 1952, 7, 377−379. (3) Wilkinson, G.; Rosenblum, M.; Whiting, M. C.; Woodward, R. B. The Structure of Iron Bis-Cyclopentadienyl. J. Am. Chem. Soc. 1952, 74, 2125−2126. (4) Long, N. J. Metallocenes: An Introduction to Sandwich Complexes; Blackwell Science: Oxford: U.K., 1998. (5) Salzer, A.; Werner, H. A New Route to Triple-Decker Sandwich Compounds. Angew. Chem., Int. Ed. 1972, 11, 930−932. (6) Schildcrout, S. M. High-Pressure Mass Spectra and Gaseous Ion Chemistry of Ferrocene. J. Am. Chem. Soc. 1973, 95, 3846−3849. (7) Duff, A. W.; Jonas, K.; Goddard, R.; Kraus, H. J.; Krueger, C. The First Triple-Decker Sandwich with a Bridging Benzene Ring. J. Am. Chem. Soc. 1983, 105, 5479−5480. (8) Kurikawa, T.; Takeda, H.; Hirano, M.; Judai, K.; Arita, T.; Nagao, S.; Nakajima, A.; Kaya, K. Electronic Properties of Organometallic Metal−Benzene Complexes [Mn(benzene)m (M = Sc−Cu)]. Organometallics 1999, 18, 1430−1438. (9) Hoshino, K.; Kurikawa, T.; Takeda, H.; Nakajima, A.; Kaya, K. Structures and Ionization Energies of Sandwich Clusters (Vn(benzene)m). J. Phys. Chem. 1995, 99, 3053−3055. (10) Miyajima, K.; Nakajima, A.; Yabushita, S.; Knickelbein, M. B.; Kaya, K. Ferromagnetism in One-Dimensional Vanadium−Benzene Sandwich Clusters. J. Am. Chem. Soc. 2004, 126, 13202−13203. (11) Miyajima, K.; Yabushita, S.; Knickelbein, M. B.; Nakajima, A. Stern−Gerlach Experiments of One-Dimensional Metal−Benzene Sandwich Clusters: Mn(C6H6)m (M = Al, Sc, Ti, and V). J. Am. Chem. Soc. 2007, 129, 8473−8480.

influence the hyperpolarizability β0 value. Therefore, modification for adjusting and controlling this MLCT transition may be a new possible way to further improve the NLO properties of this type of NLO moleculars. Now, this work is being done in our research group.

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CONCLUSIONS In the present work, by means of density functional theory, five novel organometallic second-order NLO moleculars with multidecker sandwich π-coordination complexes VnBzn+1 (n = 1, 2, 3) as the donor group in the D−π−A structure are designed and their NLO properties are investigated in detail. The evident layer effect on the first hyperpolarizibility is first found, and the essence of enhancement of the first hyperpolarizability by the layer number n is revealed. When the layer number increases from 1 to 2 or 3, the parallel-x-MLCT (from more than one metal center to acceptor, −NO2) and perpendicular-x-MLCT (from multiple metal centers to multiple C6H6 ligands) transition are enhanced, and two-dimensional NLO character appears. The obvious increase of diagonal βxxx, βyyy, and off-diagonal βxxy with changing the layer number n or position of conjugation π−A system can confirm this. In addition, it is first found that the perpendicular-x-MLCT transition in the multidecker sandwich complexes VnBenn+1 (n = 1, 2, 3) can influence the β0 value. A new possible way for E

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