Effects of spacer structure and external magnetic fields on the recombination kinetics of radical ion pair states of zinc porphyrin–viologen dyads

Abstract

Magnetic field effects on the recombination kinetics of the triplet radical ion pair state (RIPS) of the Zn porphyrin viologen dyad (P–Sp–Vi²⁺) with Zn porphyrin (P) and viologen (Vi²⁺) linked by a flexible [(CH₂)_n, n= 3, 6, 10] or semi-rigid (with the 1,4-phenylene and 4,4′-biphenylene moieties) spacer (Sp) have been studied by the nanosecond laser flash photolysis technique in acetonitrile. As the Sp lengthens the rate constant (k_r) of the RIPS recombination in zero magnetic field (B= 0) increases from 0.80 × 10⁶ to 5.9 × 10⁶ s^–1, while the k_r value in a strong magnetic field (B= 0.24 T) is almost invariant at (0.3–0.7)× 10⁶ s^–1. The experimental results are discussed in terms of a simple kinetic scheme including only two subensembles of the RIPS conformers (‘closed’ and ‘open’). The rate-determining processes of the RIPS recombination are concluded to be the singlet–triplet conversion stimulated by the hyperfine coupling modulated by the exchange interaction in zero magnetic field and spin–orbit coupling induced intersystem electron transfer in a strong magnetic field. The considerations of the spin dynamics based on the relative yields and the formation kinetics of the triplet RIPS are involved.