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.
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