Self-assembly synthesis, crystal structure and nonlinear optical properties of cluster compound containing PPh₂Py ligand

ZHOU Jian-liang(周建良), WEI Chuan-jun(位传军), TONG Hai-xia(童海霞),

CHUN Xiao-gai(春晓改), CHEN Qi-yuan(陈启元)

School of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China

Received 10 July 2007; accepted 4 September 2007

Abstract: Self-assembly cluster compound [WS₄Cu₃(PPh₂Py)₃Br]₂?CH₃OH (1) was synthesized by the reaction of (NH₄)₂WS₄, CuBr and diphenyl-2-pyridyl-phosphine (PPh₂Py) in CH₃OH solution under a purified nitrogen atmosphere using standard Schlenk techniques. Its structure was determined by X-ray crystallography. It crystallizes in the triclinic crystal system P-1 space group with a=1.178 6 (1) nm, b=1.302 6 (1) nm, c=1.991 7 (2) nm, α=74.671 (7)?, β=86.188 (8)?, γ=64.141 (6)?, V=2.649 5 (5) nm³, Z=1. The W center is slightly distorted from tetrahedral coordination geometry, and the structure is built up from three [Cu(PPh₂Py)]⁺ units bridged by  multifunctional ligand to form a tetranuclear symmetrical cube-like molecule. Measurement of the nonlinear optical (NLO) properties using the Z-scan technique with an 8 ns pulsed laser at 532 nm shows that the compound possesses NLO absorption and effective self-focusing effect at a₂=6.7×10^-11 m/W and n₂=5.64×10^-18 m²/W in a 1.5×10^-4 mol/L DMF solution.

Key words: self-assembly; cluster; Schlenk techniques; Z-scan; self-focusing

1 Introduction

Transition metal-sulfur cluster chemistry develops rapidly, because clusters have interesting electronic, optical, structural and catalytic properties and are of biological interest and industrial significance in advanced materials[1-3]. Great interest in these clusters has recently been aroused by the search for better materials with third-order non-linear optical(NLO) properties because of their potential applications not only in the protection of optical sensors and the human eye from high-intensity laser beams, but also in the development of optical signal detection techniques such as those utilized in optical computers and broad-band communication[4-5]. For example, [NBu₄][MoOS₃Cu₃- BrCl₂], (M=Mo, W; M′=Cu, Ag; X=Cl, Br, I), [NBu₄]₃[MoOS₃Cu₃BrI₃], [WS₄Cu₄- (SCN)₂(Py)₆], [Mo₂Ag₄S₈(PPh₃)₄] and [NEt₄]₄[Mo₂O₂- S₆Cu₆Br₂I₄] exhibit good NLO properties[6-9].

Although the coordination chemistry of Ph₂PPy is well developed, the corresponding chemistry with heterothiometallic clusters has received less attention [10-11]. In this work, we report the self-assembly synthesis, X-ray crystal structure and NLO properties of cluster compound containing the PyPPh₂ ligand.

2 Experimental

(NH₄)₂WS₄ was prepared according to Ref.[12]. Other chemicals were of A. R. grade and used without further purification. IR spectra were recorded on a Fourier Nicolet FT-170SX spectrophotometer with pressed KBr pellets. Carbon, nitrogen and hydrogen analyses were performed on a PE 240C Elemental Analyser.

2.1 Preparation of compound 1

The synthesis of Compound 1 was performed under a purified nitrogen atmosphere using standard Schlenk techniques. A solution of (NH₄)₂WS₄ (0.174 g, 0.5 mmol) in 40 mL CH₃OH was added dropwise to a mixture of CuBr (0.216 g, 1.5 mmol) and PPh₂Py (0.234 g, 1.5 mmol). The solution immediately turned deep-red and was stirred for 10 h at room temperature. Single crystal (yield 70%) was obtained after several days by laying the filtrate with i-PrOH. The analytical calculation result for [WS₄Cu₃(PPh₂Py)₃Br]₂? CH₃OH (%) is C 44.58, H 3.17, N 3.06. The testing result is C 44.55, H 3.19, N 3.03. IR(cm^-1) of this compound is 448.57(s), 422.72(s), 1 480.22(s), 2 960.64(m).

2.2 X-ray crystal structure determination

X-ray measurements and data collection were performed on a Siemens Smart CCD diffractometer with graphite monochromated Mo K_a (λ=0.071 073 nm) radiation at 20 ℃. Intensity data for the crystal were obtained in the range of 4.04?-51.10? using an ω-scan technique. The structure was solved by direct methods and refined by full-matrix least-square methods on F² using SHELXTL software. All H atoms were geometrically fixed and allowed to ride on their attached atoms. Crystallographic data for the structure analysis have been deposited with the Cambridge Crystallographic Data Center, CCDC No.270021. Copies of these information may be obtained free of charge from: The Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK. Fax +44-1223-336033 or E-mail:deposit@ ccdc.cam.ac.uk or www: http://www.ccdc.cam.ac.ck

2.3 Optical measurement

A well-ground sample was dissolved in 1.5×10^-4 mol/L DMF solution and placed in a 5 mm-thick quartz cuvette for NLO measurements. Its NLO properties were measured with an 8 ns pulsed laser at 532 nm generated from a Q-switched frequency-doubled Nd-YAG laser. The spatial profiles of the optical pulses were nearly Gaussian after passing through a filter. The pulsed laser was focused onto the sample cell with a 30 cm focal length mirror. Incident and transmitted pulsed energies were measured simultaneously by two energy detectors (RJP-735 energy probes, laser precision). The NLO properties of the sample were determined by performing Z-scan measurements[13-14].  The sample was mounted on a translation stage that was controlled by the computer to move along the Z-axis with respect to the focal point. An aperture of 0.5 mm in radius was placed in front of the transmission detector. The transmittance was recorded as a function of the sample position on the Z-axis (closed aperture Z-scan). For measuring the NLO absorption, the transmittance of Z-dependent sample was taken without the aperture (open aperture Z-scan).

3 Results and discussion

3.1 Structural descriptions

Table 1 lists the crystallographic data and structure refinements for Compound 1. Selected bond lengths and angles are given in Table 2. The structure of Compound 1 is shown in Fig.1.

Table 1 Crystal data and structure refinement for C₁₀₃H₈₈Br₂- Cu₆N₆OP₆S₈W₂

Table 2 Selected bond lengths and angles for C₁₀₃H₈₈Br₂Cu₆N₆OP₆S₈W₂

Fig.1 ORTEP drawing of WS₄Cu₃(PyPPh₂)₃Br with ellipsoids drawn at 30% probability level

The core of [WS₃Cu₃Br] is a slightly distorted cube, in which the four metal atoms and the four non-metal atoms are alternatively distributed. One S is terminal and the other three S and Br are μ₃-bridging atoms. The length of the W=S bond [0.213 49(18) nm] is typical for W=S double bond. The other three W—S bonds are single bonds, and their average bond length of 0.225 07 nm is longer than that of W=S double bond. The central unit of the crystal can be described as a slightly distorted cube, in which four corners are occupied by one W and three Cu atoms, and the other four corners are occupied by one Br atom and three m₃-S atoms. All the Cu atoms are coordinated by one P, two m₃-S, and one m₃-Br atoms. The Cu-S distances are similar, but Cu-P distances are longer than those in compound [MoS₄Cu₃(PPh₃)₃Cl][15]. This can be contributed to the difference of PPh₃ and PPh₂Py.

3.2 Nonlinear optical properties

The NLO properties of Compound 1 were determined by using Z-scan techniques. A Z-scan measurement is shown in Fig.2 and Fig.3. The optical propagation equation for the pulsed light intensity is given by Eqn.(1)[16]:

                                 (1)

where  α is the non-linear absorption coefficient of the sample, which can be expressed as the function of the incident pulsed light intensity I from Eqn.(2)[16]:

Fig.2 Open-aperture Z-scan results of Compound 1 in 1.5×10^-4 mol/L DMF solution

Fig.3 Z-scan results of Compound 1 in 1.5×10^-4 mol/L DMF solution observed under closed-aperture configuration

                      (2)

where  α₀ is the linear absorption coefficient of the sample; σ₀ is the absorption cross-section of the ground-state molecular solution; N is the concentration of the sample solution; I_s=(hγ/σ₀τ_e0) is the saturation intensity, with τ_e0 being the lifetime of the excited-state, K_α=σ_e/σ₀ is the ratio of the excited-state absorption cross-section to the ground-state cross-section.

The nonlinear absorption components were evaluated by the Z-scan method under an open aperture configuration and the NLO absorptive experimental data obtained under the conditions used in this study can be well described by Eqns.(3) and (4)[17], which are used to describe a third-order NLO absorptive process:

      (3)

                          (4)

where  light transmittance T is a function of the Z-position (against the focal point Z=0) of the sample; Z is the distance of the sample from the focal point; L is the sample thickness; I₀ is the peak irradiation intensity at focus. Z₀=πω₀²/λ, where ω₀ is the spot radius of the laser pulse at focus and λ is the laser wavelength; γ is the radial coordinate; t is the time; and t₀ is the pulse width.

The nonlinear refractive properties were assessed by dividing the normalized Z-scan data obtained under the closed aperture configuration by the normalized Z-scan data obtained under the open aperture configuration. The valley/peak patterns of the corrected transmittance curves show characteristic self-focusing behaviors of the propagating light in the sample. An effective third-order nonlinear refractive index n₂ of this compound can be derived from the difference between normalized transmittance values at valley and peak position (ΔT_v-p ) by use of Eqn.(5)[18]:

              (5)

where  I is the peak irradiation intensity at focus and λ is the wavelength of the laser. The filled boxes in Fig.2 and Fig.3 are the optical limiting experimental data measured with linear transmittance. The effective nonlinear absorptive indexes α₂ and n₂ of this compound are 6.7×10^-11 m/W and 5.64×10^-18 m²/W. The n₂ value of the title compound is comparable with that of the heterobimetallic polymeric compound {[n-Bu₄N]- [W₂Ag₃S₈]}_n (3.67×10^-18 m²/w)[19]. Therefore, this cluster compound exhibit considerable NLO absorption properties. The valley/peak pattern of the normalized transmittance curve, obtained under a closed aperture configuration, shows characteristic self-focusing behavior of propagating light in the sample. This property shows that this cluster compound can be promising material for applications such as protection of optical sensors.

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Foundation item: Project(2006FJ4235) supported by the Post-Doctoral Foundation of Hunan Province, China

Corresponding author: ZHOU Jian-liang; Tel: +86-13755009868; Fax: +86-731-8879616; E-mail: zhoujl@mail.csu.edu.cn

(Edited by LI Xiang-qun)