Synthesis and Optical Properties of Organic-Inorganic Hybrid Mesoporous Materials with Naphthalene Bridging Groups

Ordered naphthalene-bridged hybrid periodic mesoporous organosilicas (PMOs) were synthesized by co-condensation of 2,7-bis(3-triethoxysilylpropylaminocarbonyloxy) naphthalene (NIS) and tetraethoxy orthosilane (TEOS) using cationic trimeric surfactant [C10H21N+(CH3)(2)(CH2)(2)N+(CH3)(C10H21)-(CH2)(2)N+(CH3)(2)C10H21]center dot 3Br(-) as a structure-directing agent. The resulting samples were characterized by powder X-ray diffraction, high resolution transmission electron microscopy, nitrogen adsorption-desorption, and differential scanning calorimetry/thermogravimetric analysis. Ordered mesoporous hybrid materials with a crystal-like pore wall formed when the molar ratio of NIS to the sum of NIS and TEOS was 40%. When this value is below or above 40%, ordered mesoporous hybrid materials with amorphous phase in the pore walls, and nonporous hybrid materials are obtained, respectively. As the number of naphthyl groups in the pore walls increases, the thermal stability of the hybrid materials is enhanced through the strong p-p interactions between organic groups. Because of the fluorescent naphthyl groups in the silica framework, the PMOs exhibit optical behavior consistent with excimer formation. Absorption spectra of the PMOs show blue shifts compared with that of the precursor (NIS), suggesting the formation of aggregates in the pore walls of the hybrid materials. As the molar ratio of NIS to the sum of NIS and TEOS increases, the fluorescence quantum yield of the PMOs decreases through fluorescence quenching caused by aggregation of naphthyl groups.