Multidecker Sandwich Cluster VnBenn+1 (n = 1, 2, 3, 4) as a

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Multidecker Sandwich Cluster VnBenn+1 (n = 1, 2, 3, 4) as a Polarizable Bridge for Designing 1D Second-Order NLO Chromophore: Metal−π Sandwich Multilayer Structure as a Particular Charge-Transfer Axis for Constructing Multidimensional NLO Molecules 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 ABSTRACT: Multidecker sandwich π-coordination complexes with multiple metal atoms and cyclic π-ligands can be considered as building blocks for designing NLO molecules. In a previous work, we found that multidecker sandwich complexes of the form VnBzn+1 (n = 1, 2, 3) can be considered as stronger electron donors than metallocene Fe(η5-C5H5)2 for the design of second-order NLO molecules, and a layernumber dependence of the first hyperpolarizability of VnBzn+1−(C2H2)3−NO2 (n = 1, 2, 3) and the two-dimensional NLO character was observed from our calculation results. In the present article, by analogy with traditional donor−acceptor substituted conjugated polyenes, we took the multidecker sandwich complexes VnBzn+1 (n = 1, 2, 3, 4) as the polarizable bridge instead of the traditional π-electron conjugated bridge and designed several novel push−pull systems of the form donor−(VnBzn+1)−acceptor (n = 1, 2, 3, 4) by introducing electrondonating and electron-withdrawing groups at the terminal benzene ring of VnBzn+1. The NLO properties of these new species were investigated in detail. It was found that the first hyperpolarizability β0 of the donor−(VnBzn+1)−acceptor complexes increases with the number of layers n for n = 1−3. However, when the number of layers was increased to 4, there was an apparent decrease in β0, which is different from the behavior of traditional push−pull π-systems. To understand the optical transition and NLO response, time-dependent density functional theory (TD-DFT) calculations were carried out for these 1D chromophores. By analyzing the excited state that is crucial to the NLO response, we found that metal (d orbital) to π-ligand (antibonding π*orbital) charge-transfer (MLCT) transitions indeed occur in the metal−π sandwich multilayer structure and that small VnBzn+1 clusters (n = 2 or 3) could play a role in the polarizable bridge or CT axis. The findings from this quantum-chemical study should provide hints to scientists designing new organometallic multidimensional NLO molecules with metal−π sandwich multilayer structures as new charge-transfer axes.

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INTRODUCTION Materials that can interact strongly with the electric field of incident laser radiation and generate nonlinear optical (NLO) responses are currently of great scientific and technological interest.1−6 During the past three decades, a variety of materials have been investigated for their nonlinear optical properties, including inorganic materials, organic molecules and polymers, and organometallic compounds. Among them, organometallics are potentially interesting candidates for NLO materials, and interest in these materials has grown tremendously in the past two decades.5,6 Organometallic groups can be considered as excellent donors or acceptors because they are among the strongest oxidants or reductants known, and sometimes, they can be considered as polarizable bridges in D−π−A dipolar structures. During the past two decades, significant progress has been made in multidecker sandwich π-coordination complexes. Extended sandwich complexes (triple-deckers, tetradeckers, © XXXX American Chemical Society

etc.) composed of d-block transition metals or f-block actinide/ lanthanide elements and carbocyclic ligands or carborane rings have been synthesized and extensively studied.7−23 Multidecker sandwich π-coordination complexes with multiple metal atoms and cyclic π-ligands could be considered as building blocks for designing NLO molecules. In a previous work,24 we found that widely investigated multidecker sandwich complexes of the form VnBzn+1 (n = 1, 2, 3)13,14,25−27 can be considered as stronger electron donors than metallocene Fe(η5-C5H5)2 for the design of NLO chromophores. From our calculation results, we found that, when the number of layers, n, of a multidecker sandwich complex VnBzn+1 increases from 1 to 2 or 3, large offdiagonal β tensor components of the organometallic secondorder NLO molecules VnBzn+1−(C2H2)3−NO2 (n = 1, 2, 3) Received: May 15, 2015 Revised: June 18, 2015

A

DOI: 10.1021/acs.jpcc.5b04656 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C emerge. Why? A reasonable explanation is that, assuming that multidecker sandwich complexes VnBzn+1 can be considered as bridges for charge transfer, the organometallic compounds VnBzn+1−(C2H2)3−NO2 (n = 1, 2, 3) with two charge-transfer (CT) axes can show large off-diagonal β tensor components similarly to the previously reported two-dimensional (2D) chromophores.28−47 Could the multidecker sandwich complexes VnBzn+1 thenbe considered as polarizable bridges (or charge-transfer bridges) instead of traditional π-electron bridges for the design of one-dimensional (1D) second-order NLO molecules? This is a question that requires further investigation. In addition, at present, there is growing interest in developing molecule-based materials with multidimensional NLO responses. Two-dimensional (2D) second-order NLO chromophores28−47 with large off-diagonal β tensor components have been found to have several possible advantages. For example, 2D chromophores can display better phase-matching than 1D chromophores and offer large macroscopic NLO responses. In addition, they can enhance the second-order NLO responses without undesirable visible transparency losses. Therefore, searching among various structures in organometallic complexes for a new CT axis that is different from the traditional πconjugation bridge is helpful for designing novel and highperformance multidimensional NLO chromophores. To investigate the possibility of taking multidecker sandwich complexes [such as the lanthanide triple-decker sandwich complex (η8-COT″)Nd(μ−η8:η8-COT″)Nd(η8-COT″), where COT″ represents C 8 H 6 −(SiMe 3 ) 2 -1,4; 7,16 multidecker EunCOT″n+1 complexes;48 homoleptic and heteroleptic phthalocyaninato or porphyrinic lanthanide triple-decker sandwich complexes;19,20,49,50 or the vanadium-based triple-decker sandwich complex trans-(CpV)2(μ−η6:η6-C6H6)]12,51 as polarizable bridges for the design of NLO molecules, we designed several novel one-dimensional molecules of the form 3NH2− (VnBzn+1)−3CN (n = 1, 2, 3, 4) with the donor−(multidecker sandwich complex)−acceptor structure (see Figure 1) by introducing electron-donating and electron-withdrawing groups at the terminal benzene of VnBzn+1 (n = 1, 2, 3, 4). In the present work, our investigation aimed at (1) predicting the structures of these one-dimensional second-order NLO organometallic complexes, (2) determining the dependence of the first hyperpolarizabilities on the number of layers n and the main optical transitions that contribute dominantly to the first hyperpolarizabilities β0, and (3) discussing the feasibility of designing new NLO molecules with metal−π sandwich multilayer structures as new charge-transfer axes. The results of this work should provide hints to scientists developing new organometallic NLO materials.

Figure 1. Optimized geometrical structures of the four 1D secondorder NLO organometallic complexes 3NH2−(VnBzn+1)−3CN (n = 1, 2, 3, 4).

VnBzn+1 increases linearly with size n and that the V atoms are coupled ferromagnetically.52 Therefore, the spin multiplicities of the ground states of the four 1D second-order NLO complexes 3NH2−(VnBzn+1)−3CN (n = 1, 2, 3, 4) are 2 for n = 1, 3 for n = 2, 4 for n = 3, and 5 for n = 4. For the calculations of hyperpolarizabilities, the Coulombattenuated hybrid exchange-correlation functional (CAMB3LYP), which was recently developed for long-range interactions and charge-transfer systems, is an excellent density functional theory (DFT) method.53−55 For small and mediumsized systems without metal atoms, the Møller−Plesset perturbation (MP2) method is more reliable than DFT methods,56−60 but it is very costly for medium-sized systems with multiple metal atoms and large systems. It has been reported that, for some π-conjugated systems61 or small model systems,62 the value of β0 calculated at the CAM-B3LYP level is close to that obtained at the MP2 level, and the CAM-B3LYP method has been confirmed to be a proper method for calculating first hyperpolarizabilities compared with other DFT methods.63 In addition, in a previous work,24 we found the CAM-B3LYP method to be suitable for calculating the β0 values of the similar system VnBzn+1−(C2H2)3−NO2 (n = 1, 2, 3), Hence, in the present work, the CAM-B3LYP method was employed. However, spin contamination is large for systems containing spin electrons, so the ROCAM-B3LYP method was used to reveal changes in the NLO properties with the number of layers. To obtain well-converged values in this study, slightly small basis sets64,65 (the 6-31G* basis set for the C, H, and N atoms, and the lanl2dz basis set for the transition-metal atoms) were chosen. The NLO properties were evaluated by a finitefield approach.66−68 The magnitude of the applied electric field was set as 0.0010 au, which was checked for the similar system VnBzn+1−(C2H2)3−NO2 (n = 1, 2, 3).

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COMPUTIONAL DETAILS In a previous work,24 we found that the long-range-corrected pure exchange-correlation functional LC-BLYP performs well in describing the large orbital-overlap effects or covalent interactions in the sandwich π-coordination complex ferrocene, which is analogous to the sandwich systems VnBzn+1 (n = 1, 2, 3, 4) investigated in this work. Therefore, the optimized geometrical structures of all molecules 3NH2−(VnBzn+1)−3CN (n = 1, 2, 3, 4) were obtained at the LC-BLYP level. The 631G* basis set was employed for the C, H, and N atoms, and the lanl2dz basis set including a corrected effective core potential (ECP) to take into account relativistic effects was employed for the transition-metal V atoms. Theoretical and experimental studies showed that the magnetic moment of B


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The Journal of Physical Chemistry C

Table 1. Spin Multiplicities, Dipole Moments (μ), Polarizabilities (α0), First Hyperpolarizabilities (β0), and Main Components of the Hyperpolarizability β for the Four 1D Second-Order NLO Organometallic Complexes 3NH2−(VnBzn+1)−3CN (n = 1, 2, 3, 4) and Four Traditional 1D Organic NLO Chromophores NH2−(C2H2)n−CN (n = 1, 2, 3, 4) 3NH2−V1Bz2−3CN 3NH2−V2Bz3−3CN 3NH2−V3Bz4−3CN V3Bz4−1CN V3Bz4−2CN V3Bz4−3CN 3NH2−V4Bz5−3CN NH2−(C2H2)1−CN NH2−(C2H2)2−CN NH2−(C2H2)3−CN NH2−(C2H2)4−CN

spin multiplicity

μ

α0

β0

βx

βxxx

2 3 4 4 4 4 5

1.284 6.545 6.829 2.293 2.994 5.392 7.003 2.546 3.158 3.660 4.085

200 312 460 413 442 421 639 43 77 123 179

8 2395 4088 677 5170 6638 3170 114 578 1827 4373

−8 2395 4084 659 5170 6638 3155

466 4656 7536 1391 9191 11627 6068

The dipole moment μ, polarizability α0, and static first hyperpolarizability β0 are defined as

comparison with the donor−acceptor substituted polyenes, it can be seen that the multidecker metal−π sandwich bridge VnBzn+1 is similar to the π-conjugated bridge (C2H2)n, and the linear polarizabilities of the four organometallic complexes 3NH2−(VnBzn+1)−3CN increase with increasing length of the multidecker sandwich complexes VnBzn+1. In other words, the multidecker sandwich complexes become polarizable upon introduction of donor and acceptor groups at the two ends. Dependence of the First Hyperpolarizabilities β0 on the Number of Layers n and Intracluster Metal-toLigand Charge Transfer (MLCT) in the Multidecker Sandwich Complexes VnBzn+1. In a previous work,24 we found that the organometallic compounds VnBzn+1−(C2H2)3− NO2 (n = 1, 2, 3) constructed with a D−π−A structure are twodimensional (2D) NLO chromophores, as confirmed by the observation of large off-diagonal β tensor components. Could the multidecker sandwich complexes VnBzn+1 be considered as polarizable bridges (or charge-transfer bridges) instead of traditional π-electron bridges for the design of one-dimensional second-order NLO molecules? This is a question that requires further investigation. As shown in Table 1, the first hyperpolarizabilities β0 of the four one-dimensional NLO chromophores 3NH2−(VnBzn+1)−3CN (n = 1, 2, 3, 4) increased in the order 3NH2−(V1Bz2)−3CN (8 au) < 3NH2−(V2Bz3)−3CN (2395 au) < 3NH2−(V4Bz5)−3CN (3170 au) < 3NH2− (V3Bz4)−3CN (4088 au). The dependence of the first hyperpolarizability β0 on the number of layers n in VnBzn+1 is shown in Figure 2. From Figure 2, it can be seen that the first

μ = (μx + μy + μz )1/2 α0 = (αxx + αyy + αzz)/3 β0 = (βx 2 + βy 2 + βz 2)1/2

(1)

where βi =

3 (β + βijj + βikk ) 5 iii

i, j, k = x, y, z

All calculations were carried out using the Gaussian 09 program package.69 The molecular structures and orbitals were plotted with the GaussView program.70

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RESULTS AND DISCUSSION Geometrical Structures of One-Dimensional SecondOrder NLO Chromophores with a Donor−(VnBzn+1)− Acceptor Dipolar Structure. The optimized geometries of the four organometallic complexes 3NH2−(VnBzn+1)−3CN (n = 1, 2, 3, 4) with a donor−(VnBzn+1)−acceptor dipolar structure are displayed in Figure 1. As shown in Figure 1, these complexes are all one-dimensional linear structures. Because of the metal−π interactions, a slight deformation of the benzene molecules in the multidecker sandwich π-coordination complexes VnBzn+1 is observed. Dipole Moment and Linear Polarizibility. The dipole moments μ, polarizibilities α0, and first hyperpolarizabilities β0 of the four organometallic complexes 3NH2−(VnBzn+1)−3CN (n = 1, 2, 3, 4) were calculated at the ROCAM-B3LYP level, and the results are listed in Table 1. From Table 1, the dipole moments μ are in the order 3NH2−(V1Bz2)−3CN (1.284 au) < 3NH2−(V2Bz3)−3CN (6.545 au) < 3NH2−(V3Bz4)−3CN (6.829 au) < 3NH2−(V4Bz5)−3CN (7.003 au). That is, as expected, the dipole moment increased with increasing number of layers n. For the linear polarizabilities α0, the values increased in the order 3NH2−(V1Bz2)−3CN (200 au) < 3NH2− (V2Bz3)−3CN (312 au) < 3NH2−(V3Bz4)−3CN (460 au) < 3NH2−(V4Bz5)−3CN (639 au). For the purpose of comparison, the corresponding properties of donor−acceptor substituted polyenes NH2−(C2H2)n−CN (n = 1, 2, 3, 4) with a πelectron system as the polarizable bridge were also calculated at the same level of theory, and the first- or second-order polarizibility values are listed in Table 1. Through this

Figure 2. Dependence of the static first hyperpolarizability β0 on the number of layers n. C


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Table 2. Transition Energies (ΔE), Oscillator Strengths (f 0) of Selected Excited States with Large f 0 Value, Main Transitions of the Crucial Excited States of the Four 1D Second-Order NLO Chromophores 3NH2−V1Bz2−3CN 3NH2−V2Bz3−3CN

3NH2−V3Bz4−3CN

3NH2−V4Bz5−3CN

Na

ΔE (eV)

λ (nm)

f0

main transitionc

28b 5b 9 10 8b 9 10 11 11 13 16 17b

4.555 1.965 2.240 2.241 1.754 1.899 1.975 1.993 1.769 1.903 2.026 2.118

272 631 553 553 707 653 628 622 701 652 612 585

0.2169 0.0271 0.0171 0.0229 0.0597 0.0419 0.0326 0.0452 0.0528 0.0466 0.0927 0.1047

Hα → L + 1α (0.39), H − 1α → Lα (0.39) Hβ → L + 1β (0.37), H − 1β → L + 2β (0.34)

H − 1β → Lβ (0.66)

H − 1α → L + 8α (0.20)

a

N is the ordinal number of exited states corresponding to the strong absorption peak in the UV−vis spectrum. bExcited transition that is crucial to the NLO response among the selected states. cHα/β represents highest occupied molecular orbital of α/β orbitals.

hyperpolarizability β0 of the donor−(VnBzn+1)−acceptor complexes increases with the number of layers n for n = 1, 2, 3. However, when the number of layers is increased to 4, there is an apparent decrease in β0. It is known that, for traditional organic second-order NLO chromophores, such as NH2− (C2H2)n−CN, the first hyperpolarizability β0 increases with the length of the π-conjugation bridge (see Table 1). Clearly, there is a difference between multidecker metal−π sandwich VnBzn+1 bridges and π-electron (C2H2)n bridges, and short VnBzn+1 bridges with n = 2 or 3 layers are more suitable for designing new NLO molecules. To determine the influence of the number of substituent groups on the first hyperpolarizability, the NLO properties of the three complexes (V3Bz4)−nCN (n = 1, 2, 3), constructed by introducing only the electronwithdrawing group −CN at the terminal benzene, were calculated at the same level of theory. The β0 values were found to be 677 au for (V1Bz2)−1CN, 5170 au for (V2Bz3)− 2CN, and 6638 au for (V3Bz4)−3CN. It is easy to see that, with increasing number of acceptor groups, the first hyperpolarizability of (V3Bz4)−nCN (n = 1, 2, 3) increases. In addition, it is worth noting that the β0 values of the metal complexes (V3Bz4)−2CN and (V3Bz4)−3CN are larger than that of the complex 3NH2−(V3Bz4)−3CN, which means that the method of introducing only several acceptor groups in the multidecker sandwich complex VnBzn+1 is a better approach for obtaining larger NLO responses. To understand the electronic transitions and NLO responses in the metal complexes 3NH2−(VnBzn+1)−3CN, a two-level model (two-state simplification) is the most appropriate approach, which relates the static molecular first hyperpolarizability β0 to the crucial excited state. The two-level expression is given by the equation

excited states are listed in Table 2, and the UV−vis absorption spectra and main transitions of the crucial excited states of these metal complexes are displayed in Figures 3 and 4,

(2)

Figure 3. UV−vis absorption spectra and crucial excited states of the four organometallic complexes 3NH2−(VnBzn+1)−3CN (n = 1, 2, 3, 4).

where ΔE, f 0, and Δμ are the transition energy of the crucial excited state, the oscillator strength, and the difference between the ground- and excited-state dipole moments, respectively. From the two-level model, one can deduce that the low-energy excited states with large oscillator strengths f 0 will contribute significantly to the β0 values of the NLO molecules, and these states are the so-called crucial excited states. Therefore, we performed time-dependent TD-CAM-B3LYP computations. The ΔE and f 0 values and main components of the crucial

respectively. As shown in Figure 3, there is an obvious difference between the single-layer and multilayer 1D NLO chromophores in the electronic absorption spectra. For 3NH2− (V1Bz2)−3CN with n = 1 layer, there is only one strong absorption peak (corresponding to state 28) with its largest f 0 value at 272 nm in the UV region. Therefore, the crucial excited state is state 28. For 3NH2−(VnBzn+1)−3CN with n = 2 layers, there are three low-energy and one high-energy strong

β0 ∝

Δμf0 ΔE

3

D


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molecules with metal−π sandwich multilayer structures as new charge-transfer axes.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel.: 86-25-85891780. Fax: 86-25-85891767. *E-mail: [email protected]. Tel.: 86-25-85891780. Fax: 86-2585891767. Notes

The authors declare no competing financial interest.

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Figure 4. Main transitions of the crucial excited state responsible for the second-order NLO response of organometallic complexes 3NH2− (VnBzn+1)−3CN (n = 1, 2, 3, 4).

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 acknowledge the computational resource at State Key Laboratory of Theoretical and Computational Chemistry, Jilin University.

absorption peaks in the UV−vis spectrum. Among these states, state 5 has the largest f 0 value and a low transition energy. Thus, the crucial excited state should be state 5. As shown in Figure 4, the main components are all charge-transfer transitions from metal (d orbital) to π-ligands (antibonding π*-orbital), which suggests that metal-to-ligand charge-transfer (MLCT) transitions indeed occur in triple-decker V2Bz3. This also indicates that V2Bz3 can be considered as a CT axis for constructing multidimensional NLO chromophores. For 3NH2−(VnBzn+1)−3CN with n = 3 layers, there are four lowenergy strong absorption peaks in UV−vis spectra. The crucial excited state should be state 8, in which the main transition is also an MLCT transition. Therefore, tetradecker sandwich cluster V3Bz4 can also be considered as a CT axis. As for the last 1D chromophore with n = 4 layers, the situation is different. As shown in Figure 3, state 17 should be the crucial excited state, but the main transition is not an MLCT transition (see Figure 4). Therefore, it can be concluded that multidecker sandwich clusters VnBzn+1 with n ≥ 4 might be unsuitable as CT axes for the design of multidimensional NLO molecules. In a word, the MLCT transition can occur in metal−π sandwich multilayer structures, and short multidecker sandwich clusters VnBzn+1 with n = 2 or 3 can play the role of a polarizable bridge or CT axis. The findings from this quantumchemical study should provide hints to scientists designing new organometallic multidimensional second-order NLO molecules with metal−π sandwich multilayer structures as new chargetransfer axes.

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REFERENCES

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CONCLUSIONS In the present work, by using linear multidecker sandwich πcoordination complexes VnBzn+1 (n = 1, 2, 3, 4) as polarizable bridges, four novel 1D organometallic second-order NLO molecules 3NH2−(VnBzn+1)−3CN (n = 1, 2, 3, 4) were constructed in theory. The dependence of the first hyperpolarizabilities β0 of these novel dipolar molecules on the number of layers n was investigated. For n = 1, 2, and 3, the β0 value increases with the number of layers. However, for n = 4, an apparent decrease of β0 occurs, which is different from the behavior of traditional donor−acceptor substituted π-systems. To determine the electronic transitions and NLO responses, TD-DFT calculations were carried out. The main components of metal-to-π-ligand charge-transfer (MLCT) transitions in the crucial exited state indicate that the multidecker sandwich complexes VnBzn+1 (n = 2, 3) can play a role of CT axis indeed. The findings from this quantum-chemical study should provide hints to scientists designing new multidimensional NLO E


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Multidecker Sandwich Cluster VnBenn+1 (n = 1, 2, 3, 4) as a

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