Sol−Gel Derived Urea Cross-Linked Organically Modified Silicates. 2. Blue-Light Emission

Abstract

The photoluminescence and the local structure of sol−gel derived organic−inorganic hybrids, so-called ureasils, are discussed. Their host matrix is a silica-based network to which different numbers of oxyethylene repeat units 8.5, 15.5, and 40.5 for U(600), U(900), and U(2000), respectively are covalently grafted by means of urea linkages. The small-angle X-ray scattering (SAXS) results suggest a diphasic structure for the morphology of the hybrids induced by local phase separation between siliceous domains and polymeric regions. The estimated interdomain distances, ranging from 27 Å for U(600) to 59−64 Å for U(2000), indicate that the three ureasils are greatly homogeneous on the SAXS scale. The luminescence spectra show a broad light emission (2.0−4.1 eV) with a blue band at ∼2.6 eV and a purplish-blue one at ∼2.8−3.0 eV, clearly distinguished by time-resolved spectroscopy. The energies of these two components are related to the dimension of the backbone inorganic skeleton. The local structure of these amorphous siliceous regions is depicted as a planar structure that combines different proportions of six to eight silica-based chains (blue emission) with three to four organically modified Si−O environments (purplish-blue emission). The calculated coherent diffraction lengths of the siliceous domains for U(600), U(900), and U(2000) 16.6, 16.1, and 20.5 Å, respectively points to an increase of the overall disorder of the inorganic backbone as the quantity of oxyethylene chains increase from 8.5 to 40.5.