Quantum Sensing of Electric Fields Using Spin-Correlated Radical Ion Pairs.

Quantum sensing affords the possibility of using quantum entanglement to probe electromagnetic fields with exquisite sensitivity. In this work, we show that a photogenerated spin-correlated radical ion pair (SCRP) can be used to sense an electric field change created at one radical ion of the pair using molecular recognition. The SCRP is generated within a covalent donor-chromophore-acceptor system PXX-PMI-NDI, , where PXX = -xanthenoxanthene, PMI = 1,6-bis(--butylphenoxy)perylene-3,4-dicarboximide, and NDI = naphthalene-1,8:4,5-bis(dicarboximide). The electron-rich PXX donor in acts as a guest molecule that can be encapsulated selectively by a tetracationic cyclophane ExBox host to give a supramolecular complex ⊂ . Selective photoexcitation of the PMI chromophore results in ultrafast generation of the PXX-PMI-NDI SCRP. When PXX is encapsulated by ExBox, the cyclophane generates an electric field that repels the positive charge on PXX within PXX-PMI-NDI, reducing the SCRP distance, i.e., the distance between the centers-of-charge on the donor and acceptor. Pulse-EPR measurements are used to measure the coherent oscillations created primarily by the electron-electron dipolar coupling in the SCRP, which yields the distance between the two charges (spins) of PXX-PMI-NDI. The experimental results show that the distance between PXX and NDI decreases when ExBox encapsulates PXX, which demonstrates that the SCRP can function as a quantum sensor to detect electric field changes in the vicinity of the radical ions.