Quantum sensing using fluorescent atomic-scale defects, such as the nitrogen–vacancy (NV) center in diamond, provides novel characterization of molecular systems and fresh insight into chemical functionality. Manipulated by light and microwaves, NV centers enable highly localized detection of nuclear and electron spins and their dynamics in molecular targets to probe structure–function relationships as well as spin and charge transport. In this talk, I highlight diamond surface engineering strategies to mitigate decoherence and charge-state instability of near-surface NV centers for chemical sensing. I further show how single NV centers can be used for nuclear magnetic resonance detection of surface-tethered molecules and to probe transient paramagnetic intermediates of photochemical reactions, such as spin-correlated radical pairs.