This work discusses the recombination mechanisms and the chemical nature of the emitting centers subjacent to the white-light emission of sol−gel derived amine-functionalized hybrids lacking metal activator ions, such as those based on 3-aminopropyl)triethoxysilane (APTES), 3-glycidyloxypropyltrimethoxysilane (GPTES), and on urea and urethane precursors. The white-light photoluminescence (PL) results from a convolution of the emission originated in the NH (NH2) groups of the urea or urethane bridges (APTES- and GPTES-based hybrids) with electron−hole recombinations occurring in the siloxane nanoclusters. These two components reveal a radiative recombination mechanism typical of donor−acceptor pairs, mediated by some localized centers. Photoinduced proton-transfer between defects such as NH3 + and NH- (GPTES- and APTES-based hybrids) or NH2 + and N- (di-ureasils and di-urethanesils) is proposed as the mechanism responsible for the NH-related component. Electron paramagnetic resonance data suggest that the specific PL mechanism subjacent to the component associated with the siliceous nanodomains involves oxygen-related defects.

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 unitss8.5, 15.5, and 40.5 for U(600), U(900), and U(2000), respectivelysare 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)s16.6, 16.1, and 20.5 Å, respectivelyspoints to an increase of the overall disorder of the inorganic backbone as the quantity of oxyethylene chains increase from 8.5 to 40.5.

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