Synthesis and Optical Power Limiting Properties of Heteroleptic Mo 3 S 7 Clusters

Substitution of the halide ligands in (Bu 4 N) 2 [Mo 3 S 7 X 6 ] (X = Cl, Br) by diimine ligands, such as 4,4’-dimethyl-2,2’-bipyridine (dmbpy), 2,2’-bipyridine (bpy) and 1,10-phenanthroline (phen), affords the neutral heteroleptic clusters Mo 3 S 7 Cl 4 (dmbpy) ( 1 ), Mo 3 S 7 Br 4 (dmbpy) ( 2 ), Mo 3 S 7 Br 4 (bpy) ( 3 ), and Mo 3 S 7 Br 4 (phen) ( 4 ). Further substitution of the halide ligands in Mo 3 S 7 Br 4 (diimine) clusters by dmit (1,3-dithiole-2-thione-4,5-dithiolate) allows the preparation of the mixed diimine-dithiolene neutral cluster complexes and Mo acid). The optical limiting properties of complexes 1 – 7 have been assessed by the open-aperture Z-scan technique at 570 nm, employing a nanosecond optical parametric oscillator. In order to investigate the effect of increasing the π -system, complexes 1 – 4 , with general formula Mo 3 S 7 X 4 (diimine), (X = Cl, Br), were compared to clusters 5 – 7 , containing the dmit ligand. The influence of the metal content on the optical power limiting properties was also investigated by comparing the trinuclear series of complexes prepared herein with the bis(dithiolene) dinuclear cluster (Et 4 N) 2 [Mo 2 O 2 S 2 (BPyDTS 2 ) 2 ], which has been recently prepared by our group. All trinuclear clusters 1 – 7 are efficient optical limiters ( σ eff > σ 0 ) with threshold limiting fluence F 15% decreasing on proceeding from dinuclear to trinuclear clusters and, generally, on extending the π -system.


Introduction
Inorganic materials have been widely used in optics and electronics for many years. With the development of optoelectronics and photonics, the discovery of compounds able to meet the demands of technology has become even more important. In this context, the search for materials with improved third-order nonlinear optical (NLO) properties, which include the phenomena of nonlinear refraction and nonlinear absorption, is of utmost importance. Optimizing such materials will permit further developments in a number of photonic technologies. One such application where the existing materials are still lacking the appropriate performance is optical limiting (power limiting). 1 Optical limiting is a physical phenomenon that involves clamping the optical power transmitted through a material when the incident light intensity increases, which corresponds to the material transmittance decreasing at higher intensities. Power limiting devices based on a variety of materials such as, for example, organic dyes, polymers, inorganic semiconductors, and carbon-based systems like carbon nanotubes or fullerenes have been suggested. A large number of coordination and organometallic molecular complexes that may be useful as NLO materials in photonics have also been reported. 2 Less attention has been devoted to inorganic clusters. This is mainly due to two reasons: on the one hand, their (usually) deep color makes them unsuitable for NLO applications necessitating transparency in the visible region, and on the other hand, their frequent instability in the presence of high intensity radiation prohibits applications involving lasers. 3 However, the past two decades have witnessed significant progress in the field since a series of cubane-like heterobimetallic molybdenum and tungsten cluster sulfides were found to be superior optical limiters to fullerene, C60. [4][5][6] The presence of bridging sulfide ligands enhances the photochemical stability of metal clusters by reinforcing the metal-metal bonds.
Despite the drawbacks mentioned above, metal clusters present several advantages over other inorganic/organic compounds traditionally used in nonlinear optics. Firstly, their constituent heavy atoms introduce more energy sublevels, and consequently more allowed transitions, as compared to organic molecules, and secondly, their NLO properties can be easily tuned by changing the constituent elements, oxidation state, structural type and/or outer ligands. Zhang et al. have systematically explored the structure-property correlation within a series of Mo(W)/S(Se)/Ag clusters. Five different skeletal cluster cores have been shown to display significant third-order NLO properties.
In addition, two different heavy atom effects on the third-order NLO properties of this system have been identified. Substitution of S by Se causes a stronger NLO performance of the system, while there is a sign alternation from the self-defocusing performance upon replacing Mo by W. 7 Further systematic investigations are needed to establish additional correlations that will ultimately allow us to identify the optimal NLO material.
Insert Table 2 As we have discussed previously, closed-aperture Z-scan is usually used to derive the nonlinear refractive index n2 by examining self-focusing or self-defocusing phenomena, while open-aperture Z-scan can afford the nonlinear absorption properties by monitoring the total transmission through a sample. 32 Both were employed in the present studies. The closed-aperture scans show characteristic self-defocusing traces. The negative refractive nonlinearity may possess contributions from a variety of different mechanisms, including thermal effects. Therefore, no further comment is made here.
Transmission vs. fluence plots were generated, where the fluence is the energy of the laser pulse per unit area. Examples of each class of complex are shown in Figure 3.
For comparative purposes, a threshold limiting fluence F15% (defined as the incident fluence needed to reduce the transmittance through a sample by 15%) has been used to assess the relative optical limiting merit of these clusters. The results are tabulated in Table 2. 18.1) proved more efficient than the clusters, but further comment is not warranted as the suspension of C60 in our study is likely to be functioning as an optical limiter largely by scattering, rather than attenuation by excited-state absorption.
Insert Table 3 In all structures, the non-hydrogen atoms were refined anisotropically. Despite some exceptions, the hydrogen atoms bonded to carbon were included at their idealized positions and refined as riders with isotropic displacement parameters assigned as 1.2 times the Ueq value of the corresponding bonding partner. The structure of (Bu4N) [1·Cl] was refined according to the monoclinic space group P21/n. Dichloromethane was found cocrystallized with the cluster complex. Compound (Bu4N)[2·Br] was refined according to the triclinic space group P-1. Dichloromethane and toluene were found cocrystallized with the cluster complex. In the dichloromethane molecule centered at C(40), one chlorine atom was found disordered between two positions and therefore the hydrogen atoms were not included. The sum of the occupation factors of the chlorine atoms was constrained to 1 and their Uij parameters were equated. Structure (Bu4N)[3·Br] was refined according to the monoclinic space group P21. Dichloromethane and toluene were also found cocrystallized. In the tetrabutylammonium anion centered at N(200), one terminal carbon was found to be disordered over two positions. Consequently, their occupation factors were expressed in terms of a "free variable" so that their sum was constrained to 1 and their Uij parameters were equated using the EADP constraint. Owing to disorder, we considered as justified the omission of the hydrogen atoms in this molecule. Finally, the cluster complex (Bu4N)[4·Br] was refined according to the triclinic space group P-1. In the tetrabutylammonium cation, three terminal carbons (C104, C108 and C112) were refined with a partial occupancy (0.5, 0.5 and 0.75, respectively).
Additionally, the bond distances between three pairs of terminal atoms (C103-C104, C107-C108 and C111-C112) were constrained to a fixed value. The structural figures were drawn by using ORTEP3 v2.02. 43

Conclusions.
In conclusion, novel heteroleptic trinuclear molybdenum cluster complexes containing either mixed diimine-halides or diimine-dithiolene ligands have been prepared and characterized. Their optical power limiting properties have also been investigated and compared to those exhibited by a bis(dithiolene) dinuclear molybdenum cluster prepared formerly by our group, namely (Et4N)2[Mo2O2S2(BPyDTS2)2]. 13 The optical limiting merit increases on increasing metal content, and in most cases it increases when the π-system is extended by coordination of dmit, an electron-rich ligand. All trinuclear clusters presented herein are optical limiters with σeff values larger than those of the corresponding ground-state cross-sections σ0.
Supporting Information. CCDC