Electron spin dynamics as a probe of molecular dynamics: Temperature-dependent magnetic field effects on charge recombination within a covalent radical ion pair

The electron spin-spin exchange interaction, 2J, in radical pairs (RPs) is exquisitely sensitive to the details of molecular structure and can thus serve as an important probe of structural dynamics in RPs of potential interest to photonic and electronic devices. Photoinitiated ultrafast two-step charge separation produces ¹(MeOAn^+•-6ANl-Nl ^-•), where MeOAn = p-methoxyaniline, 6ANl = 4-(N-piperidinyl)- naphthalene-1,8-dicarboximide, and Nl = naphthalene-1,8:4,5-bis(dicarboximide). Radical pair intersystem crossing subsequently produces ³(MeOAn ^+•-6ANl-Nl^-•), and the total RP population decays with ∼10 ns lifetime at 140 K, which increases to nearly 30 ns at 300 K in toluene. The activation energy observed for this process is negative and can be explained by a mechanism involving a conformational preequilibrium of the RP followed by charge recombination. Over the same temperature range, the magnetic field effect (MFE) on yield of the triplet recombination product, MeOAn-6ANl-^3*Nl, yields the magnitude of 2J, which directly monitors the superexchange electronic coupling for charge recombination. A single resonance in the MFE plot is observed at 300 K, which splits into two resonances at temperatures below 230 K, suggesting that there are two distinct groups of RP conformations at low temperature. The magnitude of 2J for the lower field resonance (10 mT) at 140 K is 5 times smaller than that of the high field resonance. At 300 K the equilibrium is shifted almost entirely to the set of conformers with the stronger electronic coupling. The motion that couples these two groups of conformations is the motion that most effectively gates the donor-acceptor electronic coupling.