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New approaches towards optical materials: Highly efficient molecular emitters dispersed in sol-gel mesoporous hosts.

A.S.S. de Camargo

Sol-gel silicate glasses and ordered mesoporous materials hosting luminescent molecular species such as organic dyes and lanthanide or transition metal complexes, have been at the focus of much attention for photonic devices and bionalytical applications. The ability to exploit the synergy between the intrinsic characteristics of sol–gel hosts (highly controlled purity, versatile shaping and patterning, excellent optical quality, easy control of the refractive index, encapsulation of large amounts of isolated emitting centers protected by the host) and the luminescence features of the molecular emitters (high quantum yields, narrow (or large) bandwidths, long- (or short-) lived emissions, ligand-dependent luminescence sensitization and large Stokes shifts), allows the design and fabrication of materials with tunable properties. Promising applications include light emitting devices, active waveguides in the visible and near-IR spectral regions, active coatings, bio-medical actuators and sensors and biological markers, opening up exciting directions in materials science and related technologies, with significant implications in the integration, miniaturization, and multifunctionalization of devices. In all these applications, and particularly for the biological ones, it is very important to assure high luminescence efficiency while avoiding contamination of the surrounding environments by leakage of the active species. In this sense, the sol-gel silicate hosts with high surface areas and easy functionalization offer vast possibilities to disperse the guest molecules so as to decrease, or completely prevent, the formation of non-emissive molecular aggregates (as common for dye molecules), and to protect them from exposure to well known luminescence quenching agents such as OH- (for most lanthanide complexes) and singlet oxygen (e.g. for iridium complexes). Recently, our research group at the University of São Paulo has dedicated efforts to the development of modern host-guest hybrid materials, and to their structural and photophysical studies. Examples include ordered mesoporous silica (MCM-41), organo-modified silica xerogels, and sodium-aluminophosphate glasses, incorporated with Rhodamine 6G dye, Ir(III)- and Eu(III)-complexes. In all cases, new approaches are sought to achieve host-guest interactions leading to high dispersion of the guest molecules (as probed by various spectroscopic techniques), and avoidance of leakage. In this way, we have succeeded in obtaining optical materials with remarkable improvements of photophysical properties in comparison to those of the molecular species in solutions. In this presentation, I will give an overview of the field and its recent advances, and present our most relevant contributions.


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