MAGNETIC-FIELD EFFECT ON THE RECOMBINATION OF THE TRIPLET RADICAL ION-PAIR STATE OF THE SEMIRIGID ZN-PORPHYRIN-VIOLOGEN DYAD

Laser flash photolysis technique was used to record the recombination kinetics of the triplet radical pair state of the new semi-rigid Zn-porphyrin-viologen (P-Ph-Vi2+) dyad in which Zn-meso-4-methoxyphenyl-tris (4-tolyl) porphine (P) and methylbenzylviologen (Vi2+) are linked by the 1,4′-phenylene spacer (Ph). In polar solvents the 3[P+•-Ph-Vi+•] decay kinetics fits fairly into the first-order law and the corresponding rate constants (k2) equal (1.8-3.3) x 106 s-1 and do not correlate to a solvent polarity. Application of an external magnetic field (B≤0.24 T) decreases the k2 value to (0.3-0.7) x 106 s-1. The B 1 2 values (defined as the B values at which the magnetic field effect reaches half of its saturated value) are 15-30 mT. The largest magnetic field effect on the k2 value is observed in glycerol, k2(B=0)/k2(B=0.24 T)= 11. In ethanol solution the quenching rate constant of 3[P+•-Ph-Vi+•] by 3O2 equals (8±0.8) × 109 M-1 s-1 and is close to 1 3 of the diffusion limit. The mechanism of the triplet-singlet (T-S) evolution of RP and the magnetic field effect as well as high efficiency of the RP quenching by molecular oxygen (3O2) are discussed with respect to the different behaviour of the RP in several conformers. In opened conformer of RP (with low value of exchange interaction, J, between radical centers) the hyperfine coupling (hfc) and the relaxation mechanism of T-S evolution of the RP are active; in closed conformer of RP where 2J is large enough the spin-orbit coupling dominates. The stimulation of the T-S transitions in closed RP by the exchange interaction of the radicals with 3O2 is very efficient. © 1990.