Structure and Nonlinear Optical Property of the Cluster Compound V2S6O2Cu6(3-MePy)6①

TANG Guo-Dong ZHANG Jin-Fang SONG Ying-Lin ZHANG Chi ② LU Lu-De

a (Jiangsu Key Laboratory for Chemistry of Low-dimensional Materials, School of Chemistry and Chemical Engineering, Huaiyin Normal University, Huaian 223001, China)

b (Material Chemistry Laboratory, Nanjing University of Science and Technology, Nanjing 210094, China)

c (Research Center for Advanced Molecular Materials, School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjing 212013, China)

d (School of Physical Science and Technology, Suzhou University, Suzhou 215006, China)

1 INTRODUCTION

In the last several decades, nonlinear optical(NLO) materials have attracted considerable interest because of their potential applications in the development of photonic device[1-4]. Some inorganic materials such as LiNbO3and organic molecules are perhaps the famous NLO materials[5-7]. During the last two decades, the chemistry of Mo(W)-Cu(Ag)-S clusters has attracted considerable attention due to their structural diversity and particularity. Those compounds, as NLO materials, combine the advantages of inorganic compounds (the involvement of heavy atoms) and organic compounds (the facility of structural modifications)[8,9]. The tetrathiometalates,[MS4]2-(M = Mo, W), are the simplest synthetic units. Many clusters with good NLO and optical limiting (OL) are discovered[10]. These NLO materials are used not only for applications in protecting optical sensors and human eyes from high-intensity laser hazards, but also for their utilization in optical signal processing and transmission, and optical data acquisition and storage[11-13]. The chemistry of[MoS4]2-and [WS4]2-is by far the most highly developed because some cubane-type MoFe3S4clusters are relevant to the nitrogenase[14]. However,the chemistry of [VS4]3-has not been extensively investigated because it is sensitive to air and moisture[15]. In 1983, Holm first reported the synthesis of a linear trinuclear complex [VS4Fe2Cl2]3-[16]. Subsequently, the planar tetra-, penta- and heptanuclear V-Cu-S heterometallic clusters have been obtained[17]. Zheng and coworkers have obtained two clusters [VS4(CuPPh3)5X2]·nCH2Cl2(X = Cl, Br)using low temperature solid-state reaction[15]; Liu has prepared [VS4Cu4(R2dtc)n(PhS)4–n]3-[18]and Muller has obtained [VS4Cu3(PPh3)4]·nCH2Cl2] in solution[19]. Zhang and Jiang[20,21]have reported the similar structural clusters [V2S6O2(CuPPh3)4(CuMe-CN)2]·2CH2Cl2·2PrOH and [Cu6V2S6O2(C6H8N2)6],but didn’t investigate their properties. The biological functions of these clusters have been widely investigated. However, studies on their NLO property are very limited. In this paper, we wish to report the preparation and crystal structure of a cluster compound V2S6O2Cu6(3-MePy)6, together with its interesting NLO effects and strong self-focusing effect measured by the Z-scan technique.

2 EXPERIMENTAL

2. 1 General

Compound [NH4]3VS4was prepared via a slight modification of the reported procedure[14,18]. Other chemicals were of A. R. grade and used without further purification. IR spectra (KBr pellet) were recorded on a Nexus 870 FT-IR spectrometer. Elemental analyses were performed by a Perkin-Elmer 240C elemental analyzer. X-ray crystal diffraction measurements of the crystals were performed on a Siemens Smart/CCD diffractometer equipped with a graphite-monochromatic Mo-Kα radiation (λ =0.71073 ?) at 153 K. No signi fi cant decay was observed during data collection. The raw data were processed to give structure factors using the SAINT-plus program[22].

2. 2 Synthesis of V2S6O2Cu6(3-MePy)6

A well-ground mixture of [NH4]3VS4(1 mmol,0.24 g), CuI (4 mmol, 0.76 g), and Bu4NBr (2 mmol,0.64 g) was put into a carnelian mortar and skived.The resulting black solid was taken up in 3-methyl pyridine (30 mL) and filtration of the resulting mixture afforded a black-red filtrate. This filtrate was layered with a mixture solvent of i-PrOH/MeCN(V/V = 5:3, i-PrOH = isopropanol, MeCN = methyl cyanide) and allowed to stand at room temperature for a week, over which black-red crystals formed(0.24 g; yield 37%). Anal. Calcd. (%) for C36H42N6O2S6V2Cu6(%): C, 34.1; H, 3.32; N, 6.64.Found (%): C, 34.8; H, 3.27; N, 6.60. IR (cm–1):[ν(V-S)443.1(vs)], [ν(Cu-S)410.2(sh)], 649.6(m),702.2(s), 799.7(s), [ν(V=O)949.5(vs)], 1033.2(w),1106.4(m).

2. 3 X-ray structure determination

A well-developed single crystal of cluster V2S6O2Cu6(3-MePy)6with suitable dimensions was selected and mounted on a glass fiber with epoxy resin. The diffraction data of V2S6O2Cu6(3-MePy)6were collected on a Siemens Smart/CCD diffractometer within the range of 3.08≤θ≤25.01oby using the ω-scan technique and Mo Kα radiation (λ =0.71073 ?) at 153 K. A total of 10679 reflections were collected, and 3904 independent reflections were used in the subsequent structure solution and refinement. The structure of cluster V2S6O2Cu6(3-MePy)6was solved by direct methods and refined by full-matrix least-squares on F2using the SHELXTLPC package of crystallographic software[23]. The positions of V and Cu atoms were solved by direct methods. C, N and O atoms were confirmed by difference Fourier synthesis, and the hydrogen atoms were placed in their calculated positions. Crystallographic data for the cluster compound V2S6O2Cu6(3-MePy)6are summarized as follows: P1, Z = 1, a =9.7278(4), b = 10.4180(5), c = 11.9230(6) ?, α =108.240(3), β = 98.120(2), γ = 99.500(2)o, R = 0.052 and wR = 0.104.

2. 4 Nonlinear optical measurement

The single-crystal of the title cluster was solved in a dimethylformamide (DMF) solvent. A DMF solution of cluster V2S6O2Cu6(3-MePy)6was placed in a 2 mm quartz cell for NLO measurement. The property of the compound was measured with a linearly polarized laser light (λ = 532 nm; pulse width = 8 ns) generated from a Q-switched and frequency-doubled Nd-YAG laser. The spatial profiles of the optical pulses were nearly Gaussian after passing through a spatial filter. The laser beam was focused with a 25 cm focal-length focusing mirror. The radius of the beam waist was measured to be 50 μm. The input and output energies of the laser pulse were measured simultaneously by two energy detectors (Laser Precision Rjp-735) which were linked to a computer by an IEEE interface[24].

3 RESULTS AND DISCUSSION

3. 1 Synthesis of the cluster

[NH4]3VS4has been proven to be a useful starting material for V-M-S (M = Cu, Fe) clusters containing organic ligands[25]. Many people have done the work using the liquid synthesis method to prepare the compound[18,19,21]. In the present work, (NH4)3VS4,CuI, and Bu4NBr were mixed together and skived,and then extracted with 3-methyl pyridine. After filtration, the filtrate was layered with a mixture solvent of i-PrOH/MeCN (V/V = 5:3) and black-red crystals were obtained after a week. In this work, the cluster was synthesized by solid grinding with the yield up to 37%, and the operation is simple.Meanwhile, there is shorter time and smaller amount of solvent than that in the liquid synthesis method[21].

3. 2 Crystal structure of V2S6O2Cu6(3-MePy)6

The molecular configuration of the cluster is shown in Fig. 1. Selected bond lengths and bond angles are listed in Table 1.

Table 1. Selected Bond Lengths (?) and Bond Angles (o) for the V2S6O2Cu6(3-MePy)6 Cluster

Fig. 1. Molecular structure of V2S6O2Cu6(3-MePy)6. All hydrogen atoms are omitted for clarity

The V2S6Cu6unit forms a cage structure; the core of the cluster structure consists of six Cu and two V atoms which form a cage structure through μ4-S of each edge. The vanadium atom is found at the center of a [VOS3]3-tetrahedral moiety, with three sulfur atoms for extensive coordination with three copper atoms. Each copper atom is bonded to three μ4-S atoms and one N atom from the 3-methyl pyridine group to form a tetrahedral S3Cu(3-MePy) unit.

The cluster consists of two centrosymmetric units VS3O[Cu(3-MePy)]3, which are bonded with the Cu-Cu’ distances which are ranging from 2.6607(10)to 2.7316(11) ? and slightly shorter than that reported[21](from 2.693(1) to 2.772(10) ?), indicating the existence of weak metal-metal interactions.In this cluster, one S atom of the [VS4]3-unit is replaced by an O atom and the [VS4]3-turns into[VS3O]3-which has a tetrahedral moiety with S-V-X (X = S, O) angles varying from 109.13(13)oto 109.89(16)o, similar to our reports[21]that the S-V-S angle is 109.97(5)o and the S-V-O angle 108.97(5)o. The bond lengths of V-μ4-S change from 2.2386(17) to 2.2423(18) ?, somewhat longer than those of the free [VS4]3-anion as expected(2.17 ? in the ammonium salt). The V-O bond length in the cluster is 1.604(4) ?, which is the same as that of V=O double bond. The whole molecule retains the Cisymmetry. The IR band at 949.5 cm-1of this cluster is consistent with the reported typical V=O bond frequency. In each S3CuN unit, the copper atom is bonded to three μ4-S atoms and a N atom from the 3-methyl pyridine group to form a distorted tetrahedron, with S-Cu-X (X = S, N)angles ranging from 102.35(14)o to 123.88(15)o.This phenomenon may arise from the steric effect of the bulky 3-methyl pyridine ligands.

3. 3 Nonlinear optical properties

Some single crystals of cluster V2S6O2Cu6(3-MePy)6were selected and solved in the DMF solvent. The NLO properties of cluster V2S6O2Cu6(3-MePy)6were investigated with 532 nm laser pulses of 8 ns duration in a 1.58 × 10-4mol·dm-3DMF solution. The nonlinear absorption component of the cluster was evaluated by Z-scan method under an open-aperture configuration (Fig. 2 (left)) and the NLO absorptive experimental data for the cluster V2S6O2Cu6(3-MePy)6, obtained under the condition used in this study, can be adequately described by equations (1) and (2)[22,26], which was used to describe a third-order NLO absorptive process:

Here, α0and α2are respectively linear and effective third-order NLO absorptive coefficients, light transmittance T is a function of the sample’s Z-position (with respect to focal point Z = 0), Z is the distance of the sample from the focal point, L is the sample thickness, I0is the peak irradiation intensity at focus, Z0= πω02/λ, where ω0is the spot radius of the laser pulse at focus and λ is the laser wavelength,r is the radial coordinates, t is the time, and t0is the pulse width.

Fig. 2. Z-scan measurement of the cluster V 2S6O2Cu6(3-MePy)6 in a 1.58×10–4 mol·dm–3 DMF solution at 532 nm with I0= 8.2 × 1012 W·m–2. The small diamonds represent the Z-scan experimental data; the solid curve represents theoretical fitting based on equations (1) and (2) to the experimental data. Data (left) are collected under the open-aperture configuration; data (right) are obtained by dividing the normalized Z-scan data obtained under the closed-aperture configuration by the normalized Z-scan data in (left)

The nonlinear refractive properties of the cluster V2S6O2Cu6(3-MePy)6were assessed by dividing the normalized Z-scan data obtained under the closedaperture configuration by the normalized Z-scan data obtained under the opened-aperture configuration(Fig. 2 (right)). The valley/peak patterns of the corrected transmittance curves show the characteristic self-focusing behavior of the propagating light in the sample. An effective third-order nonlinear refractive index, n2, of the cluster V2S6O2Cu6(3-MePy)6can be derived from the difference between the normalized transmittance values at the valley and peak positions(Δ Tv-p) using equation (3)[27].where I is the incident pulsed light intensity.

Based on our previous time-resolved nonlinear transmission studies[28]and the NLO experimental results reported here, we can conclude that the physical origin of the observed reverse saturable absorption (RSA) in this cluster can be attributed to excited-state absorptive nonlinear. It is obvious that theoretical curves (solid curve) based on equations(1) and (2) qualitatively reproduce well the general pattern of the observed experimental data (squares).This fact suggests that the experimentally obtained NLO effects are effectively third-order in nature. Fig.2 (left) gives the NLO absorptive properties of the cluster V2S6O2Cu6(3-MePy)6in a DMF solution under an open-aperture configuration. It shows large nonlinear optical absorption. The NLO absorptive coefficient (α2) of the cluster was calculated as 1.7 ×10–10m·W–1. The cluster also shows a strong selffocusing effect. Fig. 2 (right) gives the NLO refractive behavior of the cluster in a DMF solution.According to equation (3), the NLO refractive index n2was calculated to be 1.27×10-18m2·W–1. The selffocusing effect due to the excitation state’s refraction volume is bigger than the ground state’s.

4 CONCLUSION

The cage-shaped cluster has been prepared by the skiving method. The structure of the cluster compound was characterized by X-ray diffraction; the NLO properties were studied by Z-scan technique with 8 ns pulsed laser at 532 nm. The cluster compound V2S6O2Cu6(3-MePy)6exhibits strong NLO absorptive ability (α2= 1.7×10–10m·W–1) and effective self-focusing performance (n2= 1.27×10-18m2·W–1) in a 1.58×10-4mol·dm-3DMF solution.

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