Reversible Mutarotation in a Macrocyclic Ytterbium Complex

M40 is a four‐fold symmetry macrocyclic ligand endowed with axial and central chiral elements, all of R configuration. It promptly binds late lanthanides (Yb III and Lu III ) yielding a negative helicity, as witnessed by NIR‐electronic circular dichroism. In the course of a few hours, a new conformation of the complex takes over, which has opposite helicity and allows for a dynamic free‐bound equilibrium. Upon slow solvent evaporation, the original conformation is retrieved and the whole dynamic process can be started again, as in a sandclock, allowing one to envisage applications as a time‐marker chiral switch.