Temperature and orientation dependence of electron spin relaxation in molybdenum(V) porphyrins

Abstract

AbstractElectron spin lattice relaxation rates were measured by saturation recovery for three oxo(5,10,15,20‐tetratolylporphyrinato)molybdenum(V) complexes, OMo(TTP)X, X=Cl−, OH−, OEt−. For O=Mo(TTP)(OEt) in glassy solution between 15 and 130 K the temperature dependence of 1/T1 is consistent with a two‐photon Raman process. For each of the complexes at 100 K 1/T1 is faster when the magnetic field is in the molecular plane than when it is perpendicular to the molecular plane, which is attributed to orientation dependence of intramolecular vibrations. The orientation‐dependent values of 1/T1 are approximately the same for isotopes with I = 0 or I = 5/2, which indicates that molybdenum nuclear hyperfine interaction is not a major factor in the electron spin‐lattice relaxation rate. Phase memory relaxation rates were measured by electron spin echo. Below about 90 K 1/Tm is weakly temperature dependent and becomes more strongly temperature dependent at higher temperature as 1/T1 approaches 1/Tm and as the host lattice softens. For isotopes with I = 5/2 at 100 K 1/Tm is faster at molecular orientations for which the resonant field is more strongly dependent on small changes in molecular orientation.