Clay templating Langmuir–Blodgett films of a nonamphiphilic ruthenium(II) complex

doi:10.1016/j.colsurfa.2005.10.092

Colloids and Surfaces A: Physicochemical and Engineering Aspects

Volumes 284–285, 15 August 2006, Pages 135-139

https://doi.org/10.1016/j.colsurfa.2005.10.092 Get rights and content

Abstract

We fabricated hybrid Langmuir–Blodgett (LB) films of smectite clays and a metal complex (tris(4,7-diphenyl-1,10-phenanthroline)ruthenium(II) perchlorate) which has no long alkyl chain. Characteristics of films fabricated on an aqueous suspension of synthetic saponite were compared with films fabricated on an aqueous suspension of natural sodium montmorillonite by means of nonlinear and linear optical spectroscopy, pressure–area (π–A) isotherm measurement and atomic force microscopy (AFM). AFM studies revealed that the homogeneity of a hybrid LB film fabricated on an aqueous suspension of synthetic saponite is considerably higher than that of a film fabricated on an aqueous suspension of montmorillonite.

Introduction

It is possible for inorganic–organic hybrid films to create new functionalized materials because they possess characteristics such as an easily changeable layer-by-layer structure or precisely controllable film thickness, or a variety of physical properties, which are not present in each of the separate components [1]. Certain physical properties such as second-order nonlinear optics and magnetics of these hybrid films only exist when a noncentrosymmetric molecular alignment and/or an oriented molecular alignment in the film is present. However, the molecules intercalated in the interlamellar spaces of an inorganic layer material and in hybrid films that are fabricated via layer-by-layer assembly tend to cancel any orientational anisotropy in most cases.

One of the most widely used techniques for controlling the alignment and orientation of molecules is the Langmuir–Blodgett (LB) technique. By modifying this technique, attempts were made to realize oriented molecular alignment in inorganic–organic hybrid films [2], [3]. We also fabricated hybrid LB films making use of an aqueous suspension of clay particles as a subphase instead of ultra pure water [4], [5], [6]. When stirred in water, a smectite clay exfoliates into a single sheet consisting of a negatively charged two-dimensional ultra-thin layer (1 nm thick). Therefore, when cationic molecules are spread onto the surface of a clay suspension, the cationic molecules are adsorbed onto the clay sheet through an electrostatic interaction between the clay sheet and the cationic molecules. It should be noted here that the hydrophobicity of a clay sheet which adsorbs cationic molecules should be much higher than that of the free clay sheet. Therefore, once a clay sheet adsorbs cationic molecules of which solubility is low, it should remain at the air–water interface at a high probability. Similar to the case with the typical LB film, the clay-organics hybrid LB film is obtained after compression and deposition of the hybridized clay sheets.

Conventional LB films are mechanically unstable because the component molecules are held together primarily by van der Waals forces [7]. On the other hand, the clay templated hybrid LB films are mechanically stable owing to the electrostatic attractive force between a clay sheet and the cationic guest molecule [4], [5], [6], [8]. In addition, as one of the cohesive forces which determine molecular orientation in the film, the electrostatic attractive interaction can be utilized. Hence, even when the molecules do not possess a long alkyl chain, hybrid LB films with a noncentrosymmetric molecular alignment can be fabricated [4], [5], [6], [8].

Up until now, natural sodium montmorillonite, which is one of the most popular smectite clays, has conventionally been chosen as the template. However, the shape and the size of natural clay particles are quite different between each particle. The maximum particle size of natural sodium montmorillonite is about 1 μm in diameter. Therefore, it is very difficult for clay particles in such a hybrid film to attain close packing. Several gaps and stacks of which diameters are few hundred nano-meters are often seen in the film. This situation might prevent the application of such a hybrid film for optical material. Recently, synthetic smectite clays with a similar particle shape and particle size are easily available. It is considered that the use of a synthetic smectite clay enables the fabrication of a desirable hybrid film especially for an optical application. Thus, in this study, we have attempted to fabricate a hybrid film by employing one of a synthetic smectite clay; synthetic saponite of which averaged particle size is about 30 nm. As for a guest cationic molecule, a nonamphiphilic metal complex [tris(4,7-diphenyl-1,10-phenanthroline)ruthenium(II) perchlorate (Fig. 1)] of which adsorption behavior on a surface of sodium montmorillonite was preliminary studied [4] was chosen. The morphological characteristics of the hybrid films fabricated on suspension of synthetic saponite were compared with those fabricated on suspension of sodium montmorillonite.

Section snippets

Sample preparation

Tris(4,7-diphenyl-1,10-phenanthroline)ruthenium(II) perchlorate was synthesized by following procedures. An ethanol solution of stoichiometric mixture of 4,7-diphenyl-1,10-phenanthroline and Ru(III)Cl₃·nH₂O was refluxed for 4 h under nitrogen atmosphere. The obtained dichloride, tris(4,7-diphenyl-1,10-phenanthroline)ruthenium(II) dichloride, was recrystallized several times from ethanol. The counter ions were exchanged by a treatment in a methanol solution of NaClO₄. This process was repeated

Results and discussion

Pressure–area (π–A) isotherms recorded on an aqueous suspension of synthetic saponite are shown in Fig. 2. After compression, surface pressure was found to be increased to a value over than 25 mN/m, in cases of the concentration of synthetic saponite was higher than 0.01 mg/ml. This fact suggests that the hybrid monolayer was formed at the air–water interface, even though a nonamphiphilic metal complex was used as a guest molecule.

Depending on the concentration of synthetic saponite, the

Concluding remarks

In this study, we demonstrated the fabrication of clay-organics hybrid LB films making use of aqueous suspensions of clays as subphases. This technique enables the realization of an oriented molecular array even when a nonamphiphile is used. It was found that the roughness of the film obtained on the aqueous suspension of synthetic saponite was much smaller than a wavelength of light. Hence, it should be concluded that the use of the synthetic saponite should be one of the most efficient means

Acknowledgements

This work was supported by a Grant-in-Aid for Scientific Research on Priority Areas from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of the Japanese Government.

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Clay templating Langmuir–Blodgett films of a nonamphiphilic ruthenium(II) complex

Abstract

Introduction

Section snippets

Sample preparation

Results and discussion

Concluding remarks

Acknowledgements

Chem. Phys. Lett.

Formation of a clay monolayer at an air–water interface

J. Chem. Soc., Chem. Commun.

Langmuir–Blodgett films of a clay mineral and ruthenium(II) complexes with a noncentrosymmetric structure

J. Am. Chem. Soc.

Second-order nonlinear optical properties of the hybrid film of a clay and a chiral metal complex

Mol. Cryst. Liq. Cryst.

Structural analyses of clay–metal complex hybrid films by observing optical second-harmonic generation

Nonlinear Opt.