Correlation of structural and functional properties in optical materials using modern magnetic resonance techniques

Abstract

The development of new optical materials based on inorganic hosts doped with luminescent rare-earth ions or coordination compounds is an active area of current materials research. To facilitate the design of materials with optimized emission properties, detailed structural information at the atomic level is essential, regarding the local bonding environment (siting) of the luminophores, their second-nearest coordination spheres, and their extent of local clustering or molecular aggregation, as well as the distribution over separate micro- or nanophases in composite materials. Owing to their element-selectivity, inherently quantitative character, and their sensitivity to local interactions, magnetic resonance techniques such as NMR and EPR are ideally suited for addressing such structural questions. The goal of this project is the development of a comprehensive magnetic resonance strategy for obtaining a fundamental understanding of the spectroscopic and photophysical properties of such material systems on a structural basis. Because the luminophore species themselves are not accessible to standard solid state NMR investigations owing to their inherent paramagnetic character, three complementary approaches will be used: (i) NMR studies of diamagnetic mimics (45Sc, 89Y, 139La, 171Yb, and 175Lu) incorporated into the systems of interest, (ii) analysis of the paramagnetic interactions of framework nuclei in the vicinity of the luminophore species, and (iii) measurement of electron-nuclear dipole-dipole couplings via pulsed EPR methods (electron spin echo envelope modulation (ESEEM) spectroscopy) on the paramagnetic ions themselves. Systems to be studied include aluminoborate, aluminophosphate, oxyfluoride, and fluorophosphate laser host glasses, ceramics and crystals doped with luminescent rare-earth or transition metal ions, with the goal of obtaining quantitative information about site occupancies, local environments and distance correlations, as well as micro- and nanophase populations. We will further apply this methodology to inorganic-organic hybrid systems based on the incorporation of luminescent rare earth metal complexes into a variety of porous inorganic frameworks (zeolites, mesoporous silicas, clays), in order to quantify intermolecular guest-host orientations and interactions and molecular aggregation processes in these materials. This information will serve as an important structural basis for optimizing compositional, synthesis and processing parameters towards improved optical materials. (AU)