Optical gating in electronically bistable spin systems for quantum science

Quantum sensing utilizes the exquisite sensitivity of quantum states to external field (magnetic, electric, thermal and electromagnetic) to detect and measure physical quantities of interest with unprecedented sensitivity and accuracy. Here we propose an indirect strategy for quantum sensing in molecular systems based on the response of structurally bistable ligands to external stimuli, which in turn modulate the quantum states of central transition metal ions. We have recently reported the optical gating of spin-charge states in cobalt semiquinones from an ls-Co(III)catSQ•- (doublet S = ½ qubit state) to an hs-Co(II)(SQ•-)2 (hs-Co(II) S = 3/2 state) via photochromic ligands. Here, we demonstrate optical gating by CW X-band electron paramagnetic resonance (EPR) spectroscopy between the ls-Co(III)SQ•- state with an isotropic g ~ 2.00 to the hs-Co(II)(SQ)2 state with g ~ 2.48 and 5.0. Pulsed EPR experiments via inversion recovery and Hahn echo sequences reveal slow spin dynamics (T1 and Tm) of the doublet Co(III)catSQ•- state that change subtly as a function of irradiation. Contributions from cross-relaxation, rotational averaging of hyperfine and nuclear diffusion are proposed. By leveraging bistable systems that respond to the local environment, an indirect strategy for quantum sensing is proposed for quantum information protocols. This article is part of the discussion meeting issue 'Excitonic frontiers'.