Hydrogen-Bond-Dynamics-Based Switching of Conductivity and Magnetism: A Phase Transition Caused by Deuterium and Electron Transfer in a Hydrogen-Bonded Purely Organic Conductor Crystal

A hydrogen bond (H-bond) is one of the most fundamental and important noncovalent interactions in chemistry, biology, physics, and all other molecular sciences. Especially, the dynamics of a proton or a hydrogen atom in the H-bond has attracted increasing attention, because it plays a crucial role in (bio)chemical reactions and some physical properties, such as dielectricity and proton conductivity. Here we report unprecedented H-bond-dynamics-based switching of electrical conductivity and magnetism in a H-bonded purely organic conductor crystal, kappa-D-3(Cat-EDT-TTF)(2) (abbreviated as kappa-D). This novel crystal kappa-D, a deuterated analogue of kappa-H-3(Cat-EDT-TTF)(2) (abbreviated as kappa-H), is composed only of a H-bonded molecular unit, in which two crystallographically equivalent catechol-fused ethylenedithiotetrathiafulvalene (Cat-EDT-TTF) skeletons with a +0.5 charge are linked by a symmetric anionic [O center dot center dot center dot D center dot center dot center dot O](-1)-type strong H-bond. Although the deuterated and parent hydrogen systems, kappa-D and kappa-H, are isostructural paramagnetic semiconductors with a dimer-Mott-type electronic structure at room temperature (space group: C2/c), only kappa-D undergoes a phase transition at 185 K, to change to a nonmagnetic insulator with a charge-ordered electronic structure (space group: P (1) over bar). The X-ray crystal structure analysis demonstrates that this dramatic switching of the electronic structure and physical properties originates from deuterium transfer or displacement within the H-bond accompanied by electron transfer between the Cat-EDT-TTF pi-systems, proving that the H-bonded deuterium dynamics and the conducting TTF pi-electron are cooperatively coupled. Furthermore, the reason why this unique phase transition occurs only in kappa-D is qualitatively discussed in terms of the H/D isotope effect on the H-bond geometry and potential energy curve.