ORIENTATION DEPENDENCE OF ELECTRON-SPIN PHASE MEMORY RELAXATION-TIMES IN COPPER(II) AND VANADYL COMPLEXES IN FROZEN SOLUTION

The orientation dependence of the electron spin phase memory relaxation time (T(m)) was measured for vanadyl 5-(4-carboxyphenyl)-10,15,20-tritolylporphyrin at 22, 50, and 100 K, copper(II) bis(diethyldithiocarbainate) at 50 and 100 K, and copper(II) 5,10,15,20-tetratolylporphyrin at 50, 70, and 100 K in frozen solution. T(m) was determined by fitting a single exponential to two-pulse electron spin echo data. The values of T(m) were strongly dependent on the orientation of the molecule in the magnetic field. Longer values were obtained when the magnetic field was along a principal axis or along a non-canonical turning point in the spectrum. Shorter values of T(m) were observed at intermediate orientations. The orientation dependence of T(m) is attributed to molecular motion. The EPR spectra for the three systems examined are approximately axial, so the relevant motion is motion of the molecular z axis with respect to the external magnetic field. Longer values of T(m) (slower relaxation) occur for orientations at which the resonant condition is less sensitive to a change in orientation of the molecular z axis. Shorter values of T(m) (faster relaxation) occur at orientations for which the resonant condition is more sensitive to a change in orientation of the molecular z axis.