Enhancement of electron spin relaxation rates of metalloporphyrins due to interaction with a faster relaxing metal bound to an appended bipyridyl

Abstract

Complexes of 5,10,15-tri- p-tolyl 20-4-(4-methyl) bipyridyl) porphyrin were prepared with Zn(II), Cu(II) or vanadyl coordinated to the porphyrin and bis(hexafluoroacetylacetonato)copper(II), Cu(hfac) 2, or bis(ethylxanthato)nickel(II), Ni(ex) 2, bound to the bipyridyl. Continuous wave EPR, electron spin echo and saturation recovery measurements were performed to determine the effect of metal-metal interaction on the EPR lineshapes between 6 K and room temperature and on the electron spin relaxation rates between about 10 and 120 K. Metal-metal exchange was larger for the Cu(II) porphyrins than for the vanadyl porphyrins. For the Cu(hfac) 2 adducts the spin-spin interaction caused the relaxation rate for the slower relaxing Cu(II) or vanadyl in the porphyrin to become approximately equal to that for the faster relaxing Cu(hfac) 2 moiety. The zero-field splitting for the Ni(II) complex was too large to permit detection of Ni(II) EPR signals at X-band, but the effect of the Ni(II) on the slower relaxing Cu(II) or vanadyl provided information on the relaxation rates for the Ni(II). Although the electron spin relaxation rates for the Ni(II) were orders of magnitude faster than for the Cu(II) in the Cu(hfac) 2 moiety, this did not result in proportionately larger effects of the Ni(II) on the relaxation rates for the slower relaxing centers because of the large separation between the Ni(II) and Cu(II) or vanadyl transition energies.