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

Electron spin lattice relaxation rates were measured by saturation recovery for three oxo(5,10,15,20-tetratolylporphyrinato)molybdenum(V) complexes, OMo(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.