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, 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/T-1 is consistent with a two-photon Raman process. For each of the complexes at 100 K 1/T-1 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/T-1 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/T-m is weakly temperature dependent and becomes more strongly temperature dependent at higher temperature as 1/T-1 approaches 1/T-m and as the host lattice softens, For isotopes with I=5/2 at 100 K 1/T-m is faster at molecular orientations for which the resonant field is more strongly dependent on small changes in molecular orientation.