Magnetic field effects on electron transfer reactions involving sextet-spin (S = 5/2) intermediates generated on photoexcitation of a Cr(III)-porphyrin complex

Abstract

The excited trip-sextet (6T1) state of chloro-(3-methylimidazol)-(meso-tetraphenylporphyrinato) chromium(III) (CrIIIP) is quenched by 1,1′-dibenzyl-4,4′-bipyridinium (BV2+) in acetonitrile through electron transfer to give 5(CrIIIP·+) and 2BV·+. The intermediate is a geminate ion pair in the sextet (Sx) state 6[5(CrIIIP·+) 2BV·+], which decays through either the escape from a solvent cage to give the free ions or the spin conversion to the quartet (Qa) state followed by back electron transfer. The free ion yield (ΦFI) increased with increasing magnetic field from 0 to 4 T and then slightly decreased from 4 T to 10 T. These magnetic field effects are explained as follows. Under low fields where the Zeeman splitting of the spin sublevels is lower than or comparable with the electron spin dipole—dipole interaction within 5(CrIIIP·+), this interaction effectively induces the Sx → Qa conversion of [5(CrIIIP·+)2BV+] to result in low ΦFI values. Under high fields where the Zeeman splitting is larger than the dipole—dipole interaction, the Sx → Qa conversion is decreased with increasing field to cause higher ΦFI values. The slight decrease in ΦFI above 4 T may be due to the Δg mechanism.