Metivier (PPSM) studies photochromic and fluorescent molecules at the single-molecule level - photoswitching kinetics, energy transfer and orientation-resolved imaging - underpinning super-resolution (RESOLFT/STORM-type) probes and molecular sensors. In the broader landscape of NV-centre ensemble quantum sensing (DEER, nano-NMR, T1 relaxometry) operating near pT/sqrt(Hz) sensitivity, this work is paralleled by molecular photoswitches enabling optical super-resolution.