Excitation energy dependence of luminescent sol-gel organically modified silicates

The light emission features of a new class of intensely luminescent organic/inorganic materials are reported. These sol-gel-derived hybrids contain short highly solvating oxyethylene units (OCH(2)CH(2)) grafted onto a silica network by means of urea cross-links. Three xerogels series designated as ureasils U(600), U(900) and U(2000), have been synthesized incorporating different units of oligopolyoxyethylene chains, approximately 8.5, 15.5 and 40.5, respectively. The emission spectra of these ureasils are characterized by a large broad band between 1.82 and 3.44 eV, with a blue (approximate to 2.6 eV) and a purplish-blue (approximate to 2.8-3.0 eV) component. These bands are associated with planar oxygen-based rich-silicon domains with different numbers of Si-O structural units. The emission peak position, E(P), of the three ureasils shift to higher energies with the increasing of the excitation energy, E(X). For excitation energies between 3.10 and 3.76 eV a linear relation with a slope approximately equal to 0.5 has been found between E(P) and E(X). (C) 1999 Elsevier Science S.A. All rights reserved.