Coherent spin-control of S = 1 vanadium and molybdenum complexes

The burgeoning field of quantum sensing hinges on the creation and control of quantum bits. To date, the most well-studied quantum sensors are optically active, paramagnetic defects residing in crystalline hosts. We previously developed analogous optically addressable molecules featuring a ground-state spin-triplet centered on a Cr⁴⁺ ion with an optical-spin interface. In this work, we evaluate isovalent V³⁺ and Mo⁴⁺ congeners, which offer unique advantages, such as an intrinsic nuclear spin for V³⁺ or larger spin-orbit coupling for Mo⁴⁺, as optically addressable spin systems. We assess the ground-state spin structure and dynamics for each complex, illustrating that all of these spin-triplet species can be coherently controlled. However, unlike the Cr⁴⁺ derivatives, these pseudo-tetrahedral V³⁺ and Mo⁴⁺ complexes exhibit no measurable emission. Coupling absorption spectroscopy with computational predictions, we investigate why these complexes exhibit no detectable photoluminescence. These cumulative results suggest that design of future V³⁺ complexes should target pseudo-tetrahedral symmetries using bidentate or tridentate ligand scaffolds, ideally with deuterated or fluorinated ligand environments. We also suggest that spin-triplet Mo⁴⁺, and by extension W⁴⁺, complexes may not be suitable candidate optically addressable qubit systems due to their low energy spin-singlet states. By understanding the failures and successes of these systems, we outline additional design features for optically addressable V- or Mo-based molecules to expand the library of tailor-made quantum sensors.