Time Resolved CW-EPR Spectroscopy of Powdered Samples: Electron Spin Polarization of a Nitroxyl Radical Adsorbed on NaY Zeolite, Generated by the Quenching of Excited Triplet Ketones

Abstract

Chemically induced dynamic electron polarization (CIDEP) generated in a faujasite zeolite (NaY) by the interaction between a stable free radical (4-oxo-TEMPO) and the triplet state of benzophenone was investigated by time-resolved electron spin resonance spectroscopy (TR-CW-EPR). The TR-CW-EPRs were performed by either pulling a long tube containing powdered zeolite through the EPR cavity during the laser irradiation, or by flowing a liquid transport medium (polydimethylsiloxane) for the zeolite powder, through a flat cell in the EPR cavity. CIDEP was observed for intermolecular triplet quenching (benzophenone triplets with 4-oxo-TEMPO) and intramolecular triplet quenching using a covalently linked TEMPO-benzophenone molecule. The identification of the polarized nitroxide structure was confirmed by employing both 14N and 15N 4-oxo-TEMPO isotopomers. The kinetics of the triplet quenching inside the zeolite were studied by diffuse reflection laser flash photolysis.