Designing Spin-Correlated Radical Ion Pairs for Quantum Sensing of Electric Fields: Effect of Electron–Nuclear Hyperfine Coupling

Light-driven formation of radical ion pairs that occurs much faster than their electron spin dynamics results in correlated spins whose coherence properties can be used as a quantum-based electric field sensor. This results from the radical ion pair having charge and spin distributions that track one another. Thus, electric field induced changes in the distance between the two charges are reflected in the spin-spin distance that can be measured directly using out-of-phase electron spin echo envelope modulation (OOP-ESEEM), a pulse-EPR technique. OOP-ESEEM produces oscillations whose frequency depends on the spin-spin exchange (J) and dipolar (D) interactions as well as the electron-nuclear hyperfine couplings of the radicals. To date, analyses of OOP-ESEEM data have focused predominately on determination of J and D, largely because D serves to measure the spin-spin distance. In this study we compare the OOP-ESEEM signals obtained from radical ion pairs that are fully deuterated with those that contain hydrogen. The results show that low modulation amplitudes occur in the OOP-ESEEM data using deuterated radicals, making it difficult to accurately measure radical ion pair distances. Further, the use of hydrogen-containing radicals provides the modulation amplitude enhancement necessary to measure small distances accurately using OOP-ESEEM data despite the somewhat shorter electron spin coherence times of the hydrogen-containing radical ion pairs. This is an important consideration when designing photogenerated radical ion pairs to serve as quantum sensors of electric fields.