(a) Reversible formation upon light irradiation of a covalent thioether bond between the sulfur atom from the C426 side chain of LOV2 and the C4a atom of the isoalloxazine ring of the FMN. (b) Schematic of the time domains explored in this study. The ground state of LOV2 is probed in the dark (grey). A photostationary equilibrium between the ground state and the photoadduct is built up under steady blue-light illumination for 60 s (blue). Relaxation of the photoadduct is monitored over 1620 s (cyan to yellow gradient). The existence of a crystalline phase transition (light grey) is probed during the relaxation process.