Abstract
The photoexcited state of the corrin-ligand of vitamin B(12) is an old puzzle. We show here that the metal-free corrin-ligand emits dual fluorescence in its singlet excited state. As a specific consequence of the asymmetry of the natural corrin-ligand, its strongly emitting singlet excited state exists as a pair of isomers that interconvert rapidly in an unprecedented H/D-Isotope sensitive way in competition with their fluorescent decay.