Site-specific Characterization of Structure and Dynamics of Complex Materials by EPR Spin Probes

Abstract

With the notable exceptions of magnetic materials and some transition metal catalysts most materials are diamagnetic and thus do not exhibit strong EPR signals. Occasionally, weak signals due to impurities and defects may be of significant interest, as for instance in semiconductors and optical materials, where such irregularities of the structure strongly influence those properties that have to be optimized for typical applications. The majority of materials, however, can be considered as EPR silent. This lack of background signals from the bulk of the sample provides the opportunity to apply EPR spectroscopy as a probe technique. Sites of interest in complex materials can thus be addressed with high specificity and a sensitivity corresponding to bulk concentrations of electron spins between 10 μM and 1 mM. Development of such approaches is strongly aided by similar work on biological systems [1]. Starting from the past decade several groups have focused on direct covalent attachment of stable free nitroxide radicals to interfacial sites in complex materials (spin-labeling) [2-5]. In an alternative concept, the electrostatic and hydrophobic interactions that govern selfassembly of supramolecular systems are utilized to direct spin probes with appropriate functionality to the site of interest [5-7]. Basic considerations about the choice of spin probes and the attachment of labels are discussed in Section “Addressing Specific Sites by Spin Probes and Spin Labels”. Dynamics of such spin probes and labels on time scales between 10 ps and 1 μs can be characterized by the basic, fast and sensitive continuous-wave (CW) EPR experiment and is a rich source of information on both dynamics and structure of the materials themselves [6,8-10]. The most important approaches for analyzing such dynamics are discussed in Section “Detecting Supramolecular Interactions by Changes in Probe Dynamics”.