Squarely Mesoporous and Functional Nanocomposites by Self‐Directed Assembly of Organosilane

Mesoporous silica nanocomposites attracted increasing attention in recent years due to their wide applications in many fields, such as optoelectronic devices, catalysis, and drug delivery. These porous materials are normally synthesized by co-assembling bridged silsesquioxanes and surfactants, followed by the removal of surfactant molecules. Self-directed assembly of bridged silsesquioxanes which exploits non-covalent interactions generally produces mesoscopically ordered lamellar solid materials. To the best of our knowledge, mesoporous materials by self-directed assembly of bridged silsesquioxanes have not been reported to date. Recently, porphyrin derivatives are attracting increasing attentions not only because of their versatile optical and electrochemical properties as well as high chemical and thermal stabilities, but their interesting assembly to form desirable structures, mainly due to the large and flat conjugated tetrapyrrole macrocycle. Here we explores the unique assembly behavior of porphyrin-bridged silsesquioxane and first reports the synthesis of mesoporous materials by a directed assembly of organosilane, without using any surfactant. Particularly, the resultant mesopores are uniform and show a square structure, which provide them with potential applications as separation media that combine both shape selectivity and enantioselectivity. The building molecule, porphyrin-bridged silsesquioxane (PBS, Fig. 1), was readily synthesized by reacting 4,4’,4”,4’”(21H,23H-porphine-5,10,15,20-tetrayl)tetrakis(benzoic acid) with c-isocyanatopropyltriethoxysilane in tetrahydrofuran. It is well-known that disk-shaped macrocycle structure can make PBS molecules to stack together easily. As a result, the engineered building blocks organize into closely stacked and highly ordered superstructures after evaporation of the solvent from PBS tetrahydrofuran/H3O + solutions. As shown in the optical micrograph at Figure 2a, the solvent evaporation (air dry in minutes) leads to the formation of flake-like and transparent films. X-ray diffraction analysis (Fig. 2b) demonstrates the ordered lamellar structure in the films with a d-spacing of 2.07 nm which can be related to the length of bridging organic units of the building blocks. A side view of transmission electron microscopy (TEM) (shown in Fig. 2c) further confirms the ordered lamellar structure with an interlamellar distance of 2.1 nm. This value agrees well with that obtained from XRD analysis. Mesoporous structure of hybrid thin films is demonstrated in Figure 3. Due to the unique chemical structure of PBS molecules, they assemble in a square arrangement during the evaporation of solvents, producing mesopores among PBS stacks inside the materials (schematically shown in Fig. 3a). Atomic force microscopy further confirms this square and porous organization (Fig. 3b). In addition, the N2 adsorption isotherms indicates that the average diameter of these mesopores calculated through the BJH model is ∼ 1.7 nm (Fig. 3c), which agrees with the results from the atomic force microscope image in Figure 3b. It should be noted that the film sample had been repeatedly evacuated and flushed with N2 for several times before the nitrogen adsorption/desorption characterization. So the observed square pores are indeed derived from the unique assembly of PBS building molecules, C O M M U N IC A IO N