Influence of cluster composition on NLO properties of neutral cubane-like heterothiometallic clusters

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Synthetic Metals 158 (2008) 264–272

Influence of cluster composition on NLO properties of neutral cubane-like heterothiometallic clusters Guodong Tang a,b , Yuan Cao c , Jinfang Zhang a,c , Yu Zhang b , Yinglin Song d,∗∗ , Fengli Bei c , Lude Lu c , Chi Zhang a,c,∗ a

Research Center for Advanced Molecular Materials, School of Chemistry and Chemical Engineering, Scientific Research Academy, Jiangsu University, Zhenjiang 212013, PR China b Jiangsu Key Laboratory for Chemistry of Low-Dimensional Materials, Department of Chemistry, Huaiyin Teachers College, Huaian 223001, PR China c Institute of Molecular Engineering and Advanced Materials, School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, PR China d School of Physical Science and Technology, Suzhou University, Suzhou 215006, PR China Received 8 October 2007; received in revised form 16 January 2008; accepted 23 January 2008 Available online 25 April 2008

Abstract A series of new candidates as nonlinear optical materials, tetra-nuclear heterobimetallic clusters [MOS3 M3  Y(PPh3 )3 ] (M = Mo, M = Ag, Y = Br 1; M = W, M = Ag, Y = I 2; M = Mo, M = Cu, Y = I 3; M = W, M = Cu, Y = I 4), have been synthesized by newly developed ligand-redistribution reaction for third-order nonlinear optical (NLO) studies. Single-crystal X-ray diffraction shows that clusters [MoX(␮3 -S)3 (␮3 -Br)(AgPPh3 )3 ] 1 and [WX(␮3 -S)3 (␮3 -I)(CuPPh3 )3 ] 4 (X = O0.5 S0.5 1, O 4) adopt an isomorphous neutral cubane-like skeleton. Their optical nonlinearities were measured by Z-scan technique with an 8 ns pulsed laser at 532 nm. These clusters were found to exhibit effective nonlinear absorption, self-focusing effects and large optical limiting capabilities. The effective NLO susceptibilities χ(3) and the corresponding second-order hyperpolarizabilities γ of these clusters are also reported. The influence of cluster composition on NLO properties has been discussed accordingly. © 2008 Elsevier B.V. All rights reserved. Keywords: Neutral cubane-like clusters; Cluster composition; Nonlinear optical properties

1. Introduction Much effort for applying transition metal chalcogenides has been made in the past two decades both in developing highperformance catalysts and advanced functional materials, and in demonstrating their potential in logical operation and biological science [1–9]. Synthesis of Mo(W)/S/Cu(Ag) heterobimetallic clusters has attracted considerable ongoing attention because of their rich structural chemistry and potential applications as thirdorder nonlinear optical (NLO) materials [10–12]. It has been

∗

Corresponding author at: Research Center for Advanced Molecular Materials, School of Chemistry and Chemical Engineering, Scientific Research Academy, Jiangsu University, No. 301 Xuefu Road, Zhenjiang 212013, PR China. Tel.: +86 511 88797815; fax: +86 511 88797815. ∗∗ Corresponding author. E-mail addresses: [email protected] (Y. Song), [email protected] (C. Zhang). 0379-6779/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2008.01.015

well-documented that the third-order optical nonlinearity of such heterobimetallic clusters can be attributed to their electronic characters [10a,11a,12f,13]. Among these various clusters, trianionic cubane-like clusters were first found to exhibit very large optical limiting effects, which was ascribed to their highly symmetrical cage skeletons [13]. A recent interesting discovery involving two pyridyl-containing neutral cubic clusters with isomorphous skeleton, they exhibit diametrically opposed NLO refractive properties [14]. However, in contrast with the above tri-anionic and pyridyl-containing neutral counterparts, previous studies on PPh3 -capped neutral cubic tetrathiometalate clusters [MS4 M X(PPh3 )3 ] demonstrated only very weak optical nonlinearity and negligible OL behavior [15]. In order to extend our knowledge on optical nonlinearities of transition metal chalcogenide clusters and to further elucidate correlations between molecular structure (composition) and NLO performance of this skeletal type of clusters, herein, we report the synthesis, characterization and NLO properties of four cubic-cage PPh3 -capped

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neutral clusters containing oxotrithiometalate and coinage-metal atoms. Molecular orbital analysis was used to reach a certain qualitative understanding of underlying physics that gives rise to the observed NLO phenomena. 2. Results and discussion 2.1. Synthetic procedure The synthesis for cubane-like clusters have been reported previously in three routes as two-phase reaction, low-heating temperature solid-state reaction, and self-assembly reaction, by which the cubane-like clusters have been achieved in very low yields or sometime, were unrepeatable [13a,15,17]. We recently developed a new synthetic approach, the ligandredistribution reaction, for the syntheses of heterobimetallic clusters. All titled clusters in this work were obtained in pure form with relatively high yields by introducing this strategy into PPh3 -containing system, while traditionally cubane-like clusters with PPh3 donor ligands were all synthesized through two-phase reaction or low-heating temperature solid-state reaction in a lower yield with notable impurities [16,17]. The first step of the synthetic procedure comprises the adduction of Cu/Ag with PPh3 . There is no need to isolate the intermediate product [Ag(PPh3 )3 NO3 ] or [Cu(PPh3 )3 I] at this stage. When [NH4 ]2 [MOS3 ] (M = Mo, W) was introduced into the reaction system, the ligand-redistribution occurred with part of P–M bonds broken and M(S2 )M bonds forming. It is worth noting that a little bit surplus of PPh3 is necessary in Agcontaining systems, in order to avoid the precipitation of a large amount of Ag2 S or the polymerization. But excessive PPh3 may hamper redistribution reaction and facilitate the formation of byproducts thus decreasing the yields. It should be also pointed out that the addition sequence of reagents has a decisive influence on the final products in dealing with Ag-containing systems. Addition of [NH4 ]2 [MOS3 ] into intermediate mixture will inevitably cause the precipitation of Ag2 S because of the comparatively high concentration of Ag and the inclination of [MOS3 ]2− to produce S2− .

Fig. 1. Molecule structure of [WO(␮3 -S)3 (␮3 -I)(CuPPh3 )3 ]·2H2 O 4, all hydrogen atoms omitted for clarity.

S· · ·S edges. The W atom basically retains the tetrahedral structure of the free [WOS3 ]2− anion with S–W–S(O) angles ranging from 107.09(15)◦ to 111.90(5)◦ . The W–O(1) bond length in 4 ˚ and the three W–S bond lengths are quite similar is 1.741(11) A ˚ which is consistent with the expected (2.252(4)–2.270(4) A), arrangement of one W O double bond and three W–S single bonds. Every of the three Cu atoms in almost equivalent configuration coordinates with two ␮3 -S atoms, one ␮3 -I atom and one PPh3 ligand, forming three distorted tetrahedra. The bond angles of these three Cu atoms are 100.27(13)–123.74(17)◦ for Cu(1), 97.68(12)–123.75(19)◦ for Cu(2), and 100.85(13)–121.08(18)◦ for Cu(3), respectively. The W–Cu lengths, ranging from

2.2. Structure description The cluster structures were determined by single-crystal Xray diffraction with the molecular configurations of clusters 4 and 1 displayed in Figs. 1 and 2, respectively. The crystallographic data and structure refinement of clusters 1 and 4 were listed in Table 1. The molecular structures of clusters 1 and 4 are isomorphous, with structure of cluster [WO(␮3 -S)3 (␮3 -I)(CuPPh3 )3 ] 4 describing in details. Consisting of two interlocked metal tetrahedra [WCu3 ] and nonmetal tetrahedra [IS3 ], cluster 4 can be described as a distorted cube with eight corners occupied by one W, one ␮3 -I, three ␮3 -S and three Cu atoms. Through O(1)–W(1)–I(1) atoms, there is a C3 axis which leads to a crystallographic C3 symmetry for [WO(␮3 -S)3 (␮3 -I)(CuPPh3 )3 ]. Each Cu atom is bound to a peripheral PPh3 ligand. Three [CuPPh3 ] groups are coordinated to [WOS3 ]2− unit across three

Fig. 2. Molecule structure of [MoO0.5 S0.5 (␮3 -S)3 (␮3 -Br)(AgPPh3 )3 ] 1, all hydrogen atoms omitted for clarity.

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Table 1 Crystallographic data and structure refinement for clusters 1 and 4 Chemical formula

C54 H45 Ag3 BrMoO0.5 P3 S3.5 (1)

C54 H49 Cu3 IO3 P3 S3 W (4)

Formula weight Temperature Radiation Crystal system Space group ˚ a (A) ˚ b (A) ˚ c (A) β (◦ ) ˚ 3) V (A Z d (g m−3 ) μ (mm−1 ) F(0 0 0) Crystal size (mm) θ for data collection (◦ ) Index ranges Reflections collected Unique R (int) Absorption correction Refinement method Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I > 2σ(I)] ˚ −3 ) Largest diff. peak and hole (e A

1409.56 293(2) K ˚ Mo K␣ (λ = 0.71073 A) Monoclinic P21 /c 15.736(3) 11.193(2) 30.357(6) 93.49(3) 5337.0(2) 4 1.750 2.326 2768 0.5 × 0.2 × 0.07 1.34 < θ < 25.98 0 ≤ h ≤ 18, 0 ≤ k ≤ 13, −36 ≤ l ≤ 36 10,823 10,428 0.0549 ψ-Scan Full-matrix least-squares on F2 10,428/0/589 0.903 R1 = 0.0647, wR2 = 0.1614 1.121 and −1.101

1436.44 293(2) K ˚ Mo K␣ (λ = 0.71073 A) Monoclinic P21 /c 11.971(2) 28.978(6) 19.303(4) 101.8(3) 6556(2) 4 1.455 3.383 2815 0.50 × 0.12 × 0.10 2.81 < θ < 25.98 −14 ≤ h ≤ 0, 0 ≤ k ≤ 34, −23 ≤ l ≤ 23 12,570 11,483 0.085 ψ-Scan Full-matrix least-squares on F2 11,483/2/613 0.934 R1 = 0.0686, wR2 = 0.1494 1.418 and −0.0882

˚ are obviously longer than those 2.735(2) to 2.748(2) A, ˚ (2found in [WOS3 Cu3 I(2-pic)3 ] (2.6758(2)–2.7056(2) A) pic = 2-methylpyridine) [14]. The Cu-S distances, varying from ˚ are slightly longer than those in 2.3214(4) to 2.335(5) A, ˚ which is possibly [WOS3 Cu3 I(2-pic)3 ] (2.286(4)–2.320(4) A), attributed to the different steric hindrance of PPh3 and 2-pic. The I atom coordinates with three Cu atoms forming a trigo˚ nal pyramidal, both bond distances of Cu(1)–I(1) (2.881(2) A) ˚ are obviously shorter than that of and Cu(3)–I(1) (2.861(2) A) ˚ such that the cubic skeleton of Cu(2)–I(1) bond (2.981(2) A), cluster 4 is heavily distorted. Although the molecular structures of clusters 1 and 4 are isomorphous, it should be pointed out that the S(1) and O(1) atoms in the cluster 1 are disordered. The O atom occupancy factor is 0.5, while the S atom’s is 0.5.

clusters 2 and 4, a red shift of 11 nm in the spectrum of 2 was also found in comparison with that of 4. This is understandable because the d orbitals of the M (Ag or Cu) atoms have significant contributions to the HOMO (Fig. 4). The relatively low linear absorption in the visible and near IR regions can satisfy the demands from practical third-order NLO applications. The third-order NLO behaviors and OL effects of clusters 1–4 were investigated in 1.0 × 10−4 mol dm−3 CH2 Cl2 solution with 532 nm laser pulses in 8 ns duration. The nonlinear absorption components of four clusters were evaluated by Z-scan method under an open-aperture configuration (Figs. 5(a), 6(a), and Fig. S1(a)) and the experimental NLO absorptive date for

2.3. Linear absorption spectra and nonlinear optical properties The UV–vis absorption spectra of the clusters 1–4 in CH2 Cl2 solution are displayed in Fig. 3. It shows that these clusters exhibit almost negligible linear absorption in the visible region ranging from 500 to 800 nm. As expected, red shifts in the spectra of 1 (364.4 nm) and 3 (379.2 nm) were observed in comparison with those of 2 (328.6 nm) and 4 (317.6 nm), respectively, because of the replacement of the central metal W atom in 2 and 4 with Mo atom in 1 and 3. The first absorption (364.4, 328.6, 379.2 and 317.6 nm) can be assigned as charge-transfer bands of the type (␲)S → (d)M (M = W, Mo) arising from the [MOS3 ] moiety, which is blue-shift compared to those of the free [MOS3 ]2− anion (459 nm for Mo and 375 nm in W). Regarding

Fig. 3. UV–vis spectra of clusters 1–4 in CH2 Cl2 solution.

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Fig. 4. Examples of HOMOs and LUMOs of neutral cubic shaped clusters 1–4, all ligands omitted for clarity.

Fig. 5. Z-Scan measurement of cluster 2 in 1.0 × 10−4 mol dm−3 CH2 Cl2 solution at 532 nm. The filled squares represent the experimental Z-scan data; the solid curve represent theoretical fitting based on Eqs. (1) and (2) to the experimental data. (a) Data collected under the open-aperture configuration and (b) data obtained by dividing the normalized Z-scan data obtained under the closeaperture configuration by the normalized Z-scan data in (a).

Fig. 6. Z-Scan measurement of cluster 3 in 1.0 × 10−4 mol dm−3 CH2 Cl2 solution at 532 nm. The filled squares represent the experimental Z-scan data; the solid curve represent theoretical fitting based on Eqs. (1) and (2) to the experimental data. (a) Data collected under the open-aperture configuration; (b) data obtained by dividing the normalized Z-scan data obtained under the close-aperture configuration by the normalized Z-scan data in (a).

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clusters 1–4, obtained under the condition used in this study, can be adequately described by Eqs. (1) and (2) [18,19] which can be used to describe a third-order NLO absorptive process:  +∞ 1 2 √ T (Z) = ln[1 + q(Z)e−τ ] dτ (1) q(Z) π −∞ +∞  +∞ 

α2

q(z)= 0

0

−αo L Io 2 2 1−e e[−2(γ/ω0 ) −(t/to ) ] r dr dt 2 αo 1+(z/zo )

(2)

where α0 and α2 are 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 cluster 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, r is the radial coordinates, t is the time, and t0 is the pulse width. The nonlinear refractive properties of clusters were assessed by dividing the normalized Z-scan data obtained under the close-aperture configuration by the normalized Z-scan data obtained under the open-aperture configuration (Figs. 5(b), 6(b), Figs. S1(b) and S2). The valley/peak patterns of the corrected transmittance curves show the characteristic self-focusing behaviors of the propagating light in the cluster samples. An effective third-order nonlinear refractive index n2 of the clusters can be derived from the difference between the normalized transmittance values at the valley and peak positions ( Tv–p ) using Eq. (3) [20]. n2 =

λα0

Tv–p 0.812πI(1 − e−α0 L )

(3)

where I is the incident pulsed light intensity. In CH2 Cl2 solutions, clusters 1–3 show strong NLO absorptive properties as depicted in Figs. 5(a), 6(a) and S1(a) while 4 displays only very weak NLO absorption. Figs. 5(a), 6(a) and S1(a) all show a good fit between the experimental data and theoretical curves. The NLO absorptive coefficients, α2 of clusters, were calculated to be 2.13 × 10−10 1, 2.12 × 10−10 2 and 1.44 × 10−10 m W−1 3, respectively. With the measured values of the difference of normalized transmittance values at valley and peak positions Tv–p , the NLO refractive indices n2 of clusters 1–4 were calculated to be 7.29 × 10−11 1, 8.73 × 10−11 2, 5.13 × 10−11 3 and 2.46 × 10−11 esu 4, respectively. The positive values of the third-order nonlinear refraction of clusters 1–4 indicate self-focusing refractive behaviors of those four clusters. In accordance with the obtained α2 and n2 values, the modulus of the effective third-order susceptibility χ(3) can be calculated by Eq. (4) [20] ⎛   2   ⎞     cn2 2 8 2 c2 9 × 10 ε n  0 0     |χ(3) | = ⎝ (4) α 2  +  0 n2  ⎠    80π  2ν where ν is frequency of the laser light, n0 is the linear refractive index of the sample; ε0 and c are the permittivity and the speed

of the light in vacuum. For 1.00 × 10−4 mol dm−3 CH2 Cl2 solution of clusters 1–4, the χ(3) parameters were calculated to be 7.19 × 10−12 for 1, 7.25 × 10−12 for 2, 6.21 × 10−13 for 3, 5.24 × 10−12 esu for 4, respectively. From χ(3) = γNF4 , the corresponding modulus of the second-order hyperpolarizabilities γ of clusters 1–4 were 3.67 × 10−29 for 1, 3.71 × 10−29 for 2, 3.17 × 10−30 for 3, 2.30 × 10−29 esu for 4, where N is the number density (concentration) of clusters in the sample solutions, F4 is the local field correction factor (F = (n20 + 2)/3). All NLO results in this work and some literature date for the NLO properties of the related clusters are listed in Table 2. The γ values in Table 2 indicate that clusters 1–4 have effective third-order NLO performance. The hyperpolarizability γ values of clusters 1–4, are larger than those of the other skeletal types of clusters, such as nest-shaped clusters [MoOS3 Cu3 (4-pic)6 ]·Br (3.0 × 10−31 esu), [WOS3 Cu3 (4-pic)6 ]·Br (1.0 × 10−31 esu) [9], linear-shaped clusters [MoAu2 S4 (PPh2 Py)2 ] (8.7 × 10−31 esu), [WAu2 S4 (PPh2 Py)2 ] (1.4 × 10−30 esu) [21], tri-anionic cubane-like clusters [13] [Et4 N]3 [MoOS3 (␮3 I)(AgI)3 ] (9.1 × 10−31 esu), [Et4 N]3 [WOS3 (␮3 -I)(AgI)3 ] (7.8 × 10−31 esu) [22]. In comparison with neutral cubic [MoS4 Ag3 Cl(PPh3 )3 ], clusters [MoS4 Cu3 Cl(PPh3 )3 ], [WS4 Ag3 Cl(PPh3 )3 ], and [WS4 Ag3 Br(PPh3 )3 ] [15] whose NLO performances were not detected, clusters 1–4 have the same structural type as that of the aforementioned clusters, except that a terminal S atom in the above clusters is replaced with a terminal O atom in 1–4. Clusters 1–4 possess the same molecule configuration but have a difference either in their central metal atoms or in their skeletal coinage-metal atoms. An obvious improvement in optical capability has been demonstrated by substituting Cu atoms with Ag atoms in 1–4. Similar advancement is also found when a Mo atom is replaced by a W atom in 1–4. These can be attributed to the heavy metal atom effect, arising from the fact that W and Ag atoms can facilitate intersystem crossing more effectively via spin-orbit coupling than Mo and Cu atoms. As for the interrelation between structure and NLO performance, the fact that the four neutral cubane-like shaped clusters have better NLO properties than the reported nest-shaped, linear-shaped and anion cubane-like shaped clusters can be attributed to the presence of large delocalized electron cloud in the cluster skeleton and peripheral ligands of the title cluster compounds. In order to explore and understand the relationships between molecular composition and NLO properties of neutral heterothiometallic clusters, the frontier molecules orbitals of those clusters 1–4 (Fig. 4) were calculated. According to the frontier molecules orbital’s theory, orbitals of HOMO–LUMO and related orbitals are important to the electronic properties of the clusters. HOMO is usually as the donors and the LUMO is as the acceptors. In the title clusters, the HOMOs were composed of the orbitals from M (M = Cu, Ag), ␮3 -S and X (X = Br, I) atoms, while the LUMOs were composed of those orbitals from central metal M (M = W, Mo), and Ot atoms. These frontier orbitals were primarily composed of d orbitals of M, M atoms and p orbitals of O, S, X atoms. Ag atom is polarized easier than Cu atom, and O atom has larger electronegativity than S atom. The larger electrons are delocalized, the

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Table 2 Third-order non-linear optical properties of clusters 1–4 and the related clusters Compounds [MoXS3 (␮3 -Br)(AgPPh3 )3 ] [WOS3 (␮3 -I)(AgPPh3 )3 ] [MoOS3 (␮3 -I)(CuPPh3 )3 ] [WOS3 (␮3 -I)(CuPPh3 )3 ] [MoS4 Cu3 Cl(PPh3 )3 ] [MoS4 Ag3 Cl(PPh3 )3 ] [WS4 Ag3 Cl(PPh3 )3 ] [WS4 Ag3 Br(PPh3 )3 ] [MoOS3 Cu3 (4-pic)6 ]·Br [WOS3 Cu3 (4-pic)6 ]·Br [MoAu2 S4 (PPh2 Py)2 ] [WAu2 S4 (PPh2 Py)2 ] [Et4 N]3 [MoOS3 (␮3 -I)(AgI)3 ] [Et4 N]3 [WOS3 (␮3 -I)(AgI)3 ] a b

E (eV)

α2 (m W−1 )

n2 (esu)

χ(3) (esu)

γ (esu)

N (mol dm−3 )

Ref.

1.77 1.48 2.15 1.84 2.29 2.15 2.53 2.56 – – – – – –

2.1 × 10−10

7.3 × 10−11

7.2 × 10−12

3.6 × 10−29

1.0 × 10−4a

This work This work This work This work [15] [15] [15] [15] [12j] [12j] [21] [21] [8] [8]

2.1 × 10−10 1.4 × 10−10 – nd nd nd nd 1.6 × 10−10 2.8 × 10−10 6.0 × 10−12 4.3 × 10−11 1.0 × 10−10 1.2 × 10−10

8.7 × 10−11 5.1 × 10−11 2.5 × 10−11 nd nd nd nd 3.2 × 10−8 3.3 × 10−8 3.6 × 10−10 2.60 × 10−9 – –

7.3 × 10−12 6.2 × 10−13 5.2 × 10−12 nd nd nd nd 5.4 × 10−12 5.5 × 10−12 – – 5.8 × 10−10 7.3 × 10−10

3.7 × 10−29 3.2 × 10−30 2.3 × 10−29 nd nd nd nd 3.0 × 10−31 1.0 × 10−31 1.0 × 10−31 1.4 × 10−30 9.0 × 10−31 8.0 × 10−31

1.0 × 10−4a 1.0 × 10−4a 1.0 × 10−4a – – – – 3.9 × 10−4b 8.9 × 10−4b 2.7 × 10−4a 2.7 × 10−4a 3.2 × 10−4b 4.7 × 10−4b

In CH2 Cl2 solution. In CH3 CN solution; X = O0.5 S0.5 .

bigger the NLO coefficient is. Thus, clusters [MOt (␮3 -S)3 (␮3 Y)(M PPh3 )3 ] should have better NLO properties than clusters [MSt (␮3 -S)3 (␮3 -Y)(M PPh3 )3 ] due to the replacement of Ot atom with St atom in [MS4 M3  Y(PPh3 )3 ]. In principal, the NLO properties were affected by the energy gap between HOMO and LUMO. The smaller the energy gap, the better the NLO performance. The theoretical calculations (in Table 2) showed that clusters [MSt (␮3 -S)3 (␮3 -Y)(M PPh3 )3 ] tend to have larger energy gap than clusters [MOt (␮3 -S)3 (␮3 -Y)(M PPh3 )3 ]. So the latter type of clusters should have better NLO properties than the former clusters. This is consistent with the experimental NLO results in this study (Table 2). The optical limiting effects of clusters 1, 3 and 4 have also been investigated. Both linear and nonlinear transmission data were measured at a concentration of 1.0 × 10−4 mol dm−3 CH2 Cl2 solution. Fig. 7 shows that at very low incident fluence, the transmitted fluence is proportional to the incident intensities as Beer’s law describes. The light energy transmitted starts to deviate from Beer’s law when the incident fluence reaches certain values with respect to each cluster, and the solution became increasingly less transparent as the incident energy

rose. Experiment with CH2 Cl2 solvent alone afforded no observable OL effect. This indicates that solvent contributions are negligible. The presence of strong nonlinear optical absorption and effective refraction effects in clusters may enhance the overall OL performance. The values of the limiting threshold, which is defined as the incident fluence at which the actual transmittance falls to 50% of the corresponding linear transmittance, were measured as 1.41 J cm−1 1, 1.91 J cm−1 3 1.30 J cm−1 4 with the transmittance of 84, 75 and 85%, respectively. As a comparison, the OL thresholds of some clusters and the benchmark optical limiting material C60 are listed in Table 3. From the perspective of the OL capability, clusters 1, 3 and 4 are obviously better than the reported clusters [NBu4 ]2 [MoOS3 Cu3 (NCS)3 ] (7.0 J cm−1 ) [22], [MoOS3 Cu3 (PPh3 )3 ][S2 P(OBu)2 ] (5.0 J cm−1 ) [23], [MoS4 Cu3 Cl(PPh3 )3 ], [MoS4 Ag3 Cl(PPh3 )3 ], [WS4 Ag3 Cl(PPh3 )3 ], [WS4 Ag3 Br(PPh3 )3 ] [15], and their OL performances locate in the same level with those of some tri-anionic cubane-like clusters [13], twin-nest cluster [NEt4 ]4 [Mo2 O2 S6 Cu6 Br2 I4 ] (2.0 J cm−1 ) [24], and C60 [25]. This fact implies that clusters 1, 3 and 4 with neutral cubane-like skeleton and oxotrithiometalate building blocks can act as promising candidates for optical limiting applications. 3. Experimental All reactions and manipulations were carried out by using standard Schlenk techniques under an inert atmosphere of argon. The starting compounds [NH4 ]2 [MOS3 ] (M = Mo, W) were prepared as described in the literature [26]. The solvents were carefully dehydrated prior to use. Other chemicals were generally of A.R. grade and used as commercially available. 3.1. Chemical and physical measurements

1.0 × 10−4

Fig. 7. Optical limiting effects of clusters 1, 3 and 4 in CH2 Cl2 solution: cluster 1 (), cluster 3 ( ), cluster 4 ( ).

mol dm−3

Elemental analysis for carbon and hydrogen were performed on a PerkinElmer 240C elemental analyzer. Infrared spectra were recorded with a Nexus 870 FT-IR Fourier transform spec-

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Table 3 The limiting threshold of clusters 1–4 and the related clusters measured at 532 nm with ns laser pulses Compounds

Structure

Solvent

T (%)

Fth (J cm−2 )

Ref.

[MoXS3 (␮3 -Br)(AgPPh3 )3 ] [MoOS3 (␮3 -I)(CuPPh3 )3 ] [WOS3 (␮3 -I)(CuPPh3 )3 ] [MoS4 Cu3 Cl(PPh3 )3 ] [MoS4 Ag3 Cl(PPh3 )3 ] [WS4 Ag3 Cl(PPh3 )3 ] [WS4 Ag3 Br(PPh3 )3 ] [NBu4 ]2 [MoOS3 Cu3 (NCS)3 ] [NBu4 ]2 [WS4 Cu3 Br4 ] {MoOS3 Cu3 (PPh3 )3 [S2 P(OBu)2 ]} C60

Cubane-like Cubane-like Cubane-like Cubane-like Cubane-like Cubane-like Cubane-like Nest-shaped Cubane-like Cubane-like –

CH2 Cl2 CH2 Cl2 CH2 Cl2 CH2 Cl2 CH2 Cl2 CH2 Cl2 CH2 Cl2 MeCN MeCN CH2 Cl2 Toluene

84 75 85 – – – – 71 70 90 62

1.4 1.9 1.3 nd nd nd nd 7 1.6 5 1.6

This work This work This work [15] [15] [15] [15] [22] [27] [23] [25]

T: linear transmission; Fth : limiting threshold; X = O0.5 S0.5 .

trometer (KBr pellets). Electronic spectra were measured on a Shimadzu UV-3100 spectrophotometer. 3.2. MO calculations Geometry optimization is one of the most important steps in the theoretical calculations, this procedure proceeds in two steps: firstly, the geometry was constructed by MM+ molecular dynamics in HyperChem 6.0 package [28] and then optimized by the ADF (Amsterdam Density Functions) package 2005 [29]. Results from GGA (BP) computations are presented here. For the full systems, basis sets used for the transition metal atoms and S, P atoms were from set TZP and for C, N and H were from set DZ. Preliminary results show that coordination of PPh3 or PH3 to metal atoms in the clusters introduces little difference to the molecular orbitals of interest. PH3 was, therefore, used instead of PPh3 for all the members of the clusters 1–4 in the calculations [15]. 3.3. Synthesis of [MoX(μ3 -S)3 (μ3 -Br)(AgPPh3 )3 ] (X = O0.5 S0.5 ) (1) AgNO3 (0.6 mmol, 0.102 g) was added dropwise to a solution of PPh3 (1.5 mmol, 0.393 g) in 40 mL CH2 Cl2 with thorough stirring followed by an addition of [Bu4 N]Br (0.2 mmol, 0.064 g). The resulting well-dispersed suspension was rapidly injected into a solution of [NH4 ]2 [MoOS3 ] (0.2 mmol, 0.049 g) in 2 mL DMF with stirring for only 1 min. Immediate filtration of the resulting solution afforded a red filtrate, which was subsequently laid on the surface with i-PrOH. Red platelet single crystals were obtained after several days. Yield: 0.17 g (61% based on Mo). Anal. calcd. for C54 H45 Ag3 BrMoO0.5 P3 S3.5 (%): C 46.1; H 3.2; found: C, 45.9; H, 3.0. IR (cm−1 ): 921(vs.) [ν(Mo–Ot )], 439(vs.) [ν(Mo–␮3 -S)]. UV–vis (CH2 Cl2 ) λmax (nm) (103 ε/dm3 cm−1 mol−1 ): 364.4 (5.1). 3.4. Synthesis of [WO(μ3 -S)3 (μ3 -I)(AgPPh3 )3 ] (2) Cluster 2 was prepared in the same way as cluster 1 except for using [NH4 ]2 [WOS3 ] and [Bu4 N]I instead of [NH4 ]2 [MoOS3 ] and [Bu4 N]Br, respectively. Yellow platelet crystals were

obtained. Yield: 0.18 g (58% based on W). Anal. calcd. for C54 H45 Ag3 IOP3 S3 W (%): C, 42.3; H, 3.0; found: C, 42.1; H, 3.2. IR (cm−1 ): 935(vs.) [ν(W–Ot )], 429(vs.) [ν(W–␮3 -S)]. UV–vis (CH2 Cl2 ) λmax (nm) (103 ε/dm3 cm−1 mol−1 ): 328.6 (6.2). 3.5. Synthesis of [MoO(μ3 -S)3 (μ3 -I)(CuPPh3 )3 ]·2H2 O (3) CuI (3 mmol, 0.572 g) and PPh3 (3 mmol, 0.787 g) was added to CH2 Cl2 (50 mL) and stirred for ca. 1.5 h at room temperature. Then (NH4 )2 [MoOS3 ] (1 mmol, 0.261 g) was added. The reacting system immediately turned to black-red and was stirred for additional 10 min. Black-red crystals were obtained after 2 weeks by layering the filtrate with i-PrOH. Yield: 0.19 g (70% based on Mo). Anal. calcd. for C54 H49 IO3 P3 S3 Cu3 Mo (%): C, 48.1; H, 3.7; found: C, 47.9; H, 3.6; IR (KBr pellets, cm−1 ): 914(vs.) (ν(Mo–Ot )), 445(vs.) (ν(Mo–␮3 -S)). UV–vis (CH2 Cl2 ) λmax (nm) (103 ε/dm−3 cm−1 mol−1 ): 379.2 (2.3). 3.6. Synthesis of [WO(μ3 -S)3 (μ3 -I)(CuPPh3 )3 ]·2H2 O (4) The same procedure as for the preparation of cluster 3 was employed to synthesize cluster 4 except that (NH4 )2 [WOS3 ] (1 mmol, 0.332 g) was used instead of (NH4 )2 [MoOS3 ] (1 mmol, 0.261 g). Orange crystals were obtained after 2 weeks by layering the filtrate with i-PrOH. Yield: 0.20 g (70% based on W). Anal. calcd. for C54 H49 IO3 P3 S3 Cu3 Mo (%): C, 45.1; H, 3.4; found: C, 44.9; H, 3.2; IR (KBr pellets, cm−1 ): 914(vs.) (ν(W–Ot )), 445(vs.) (ν(W–␮3 -S)). UV–vis (CH2 Cl2 ) λmax (nm) (103 ε/dm−3 cm−1 mol−1 ): 317.6 (2.5). 3.7. Crystal structure determinations A well-developed single crystal of cluster 1 with suitable dimensions was selected and mounted on a glass fiber, while 4 sealed in a quartz capillary with mother liquor to avoid efflorescence. The diffraction data of 1 and 4 were collected on an Enraf-Nonius CAD-4 diffractometer with unit cell parameters determined from automatic centering of 25 reflections by least-squares methods. Intensities were collected for Lorentz polarization effects and absorption correction was applied using

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the ψ-scan technique. The structures of clusters 1 and 4 were solved by direct method and refined by full matrix least-squares refinement on the basis of F2 using the SHELXL-97 package of crystallographic [30]. All non-hydrogen atoms were refined anisotropically. The hydrogen atoms were allowed to ride on their parent atoms with fixed isotropic thermal parameters. 3.8. Nonlinear optical measurements All optical measurements were conducted at room temperature with clusters 1–4 dissolved in CH2 Cl2 and placed in a 2 mm quartz cell. The samples were irradiated with linearly polarized 8 ns laser pulses at 532 nm generated from a Q-switched frequency-doubled Nd:YAG laser. The spatial profiles of the optical pulses were nearly Gaussian. The pulsed laser beam was focused onto the sample cell by use of a focusing mirror (focal length 30 cm). The spot radius of the laser beam was measured to be 55 ␮m (half-width at 1/e2 maximum). The energy of incident pulses varied by a Newport Com. attenuator was measured with a laser detector (Rjp-735 energy probe), which was linked to a computer by an IEEE interface and simultaneously the transmitted energy gauged with another detector. The interval between the laser pulses was chosen to be 1 s to avoid the influence of thermal and long-term effects. The thirdorder NLO absorptive and refractive behaviors of clusters 1–4 were determined by taking advantage of Z-scan technique [18]. The samples were mounted on a computer controlled translation stage, which moved along the axis of the incident laser beam (Zdirection) with respect to the focal point. A 0.2-mm diameter aperture was installed in front of the transmission detector for determination of the sign and magnitude of the nonlinear refraction (close-aperture Z-scan) whereas for nonlinear absorption measurements no aperture was applied (open-aperture Z-scan). Transmittance was recorded as a function of the sample position on the Z-axis. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (No. 50472048), the Program for New Century Excellent Talents in University (NCET-05-0499) and Foundation of UJS and NJUST. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.synthmet.2008.01.015. References [1] [2] [3] [4]

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