Effects of linking chain length and magnetic field on the kinetics of photoprocesses in covalently bonded porphyrin—viologen dyads

Abstract

The formation and decay kinetics of chain linked triplet radical pairs derived from photo-induced electron transfer reactions in a series of 21 zinc porphyrin-flexible spacer-viologen (ZnP-Sp n -Vi2+) dyads containing 2–138 atoms (n) in the spacer, have been examined by nanosecond laser flash photolysis techniques in an external magnetic field. In non-viscous polar solvents (acetone and CHCl3 plus CH3OH = 1:1 v/v), the effect of the spacer length on the rate constant of forward electron transfer can be described by the equation: k et = k 0 et(n + 6)−1.5, with k 0 et = 3 × 1010 s−1 and 1.2 × 1010 s−1 for electron transfer from the singlet and triplet states of ZnP, respectively. In zero magnetic field, the value of the triplet radical pair recombination rate constant, k r(0) = 0.7 × 106-8 × 106 s−1, is significantly smaller than k et. The dependence of k r(0) on n has an extremum with the maximum near n = 20. In a strong magnetic field (B = 0.21 T), significant retardation of triplet radical pair recombination is observed. In strong magnetic fields the effect of the chain length on triplet radical pair recombination rates is rather small and k r(B) may vary in the range 0.3 × 106-1 × 107 s−1. The phenomena observed are discussed in terms of the interplay of molecular and spin dynamics in the limits of slow and fast encounters, taking into account the exchange-interaction.