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, κ-D3(Cat-EDT-TTF)2 (abbreviated as κ-D). This novel crystal κ-D, a deuterated analogue of κ-H3(Cat-EDT-TTF)2 (abbreviated as κ-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···D···O]−1-type strong H-bond. Although the deuterated and parent hydrogen systems, κ-D and κ-H, are isostructural paramagnetic semiconductors with a dimer-Mott-type electronic structure at room temperature (space group: C2/c), only κ-D undergoes a phase transition at 185 K, to change to a nonmagnetic insulator with a charge-ordered electronic structure (space group: P1̅). 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 π-systems, proving that the H-bonded deuterium dynamics and the conducting TTF π-electron are cooperatively coupled. Furthermore, the reason why this unique phase transition occurs only in κ-D is qualitatively discussed in terms of the H/D isotope effect on the H-bond geometry and potential energy curve.

氢键（hydrogen bond, H-bond）是化学、生物学、物理学及所有分子科学领域中最基础且关键的非共价相互作用之一。其中，氢键内质子或氢原子的动力学行为日益受到学界关注，因其在（生物）化学反应以及介电性、质子传导率等物理性质调控中发挥着至关重要的作用。本文报道了首例基于氢键动力学的电导率与磁性开关现象，该现象发现于纯有机导体晶体κ-D3(Cat-EDT-TTF)2（简称为κ-D）。该新型晶体κ-D是κ-H3(Cat-EDT-TTF)2（简称为κ-H）的氘代类似物，仅由氢键连接的分子单元构成：两个电荷为+0.5、晶体学等价的儿茶酚稠合乙撑二硫四硫富瓦烯（catechol-fused ethylenedithiotetrathiafulvalene, Cat-EDT-TTF）骨架，通过对称的阴离子型[O···D···O]⁻¹型强氢键相互连接。尽管氘代体系κ-D与母体氢体系κ-H在室温下均为同结构顺磁半导体，且具有二聚体莫特型电子结构（空间群：C2/c），但仅κ-D在185 K时发生相变，转变为具有电荷有序电子结构的非磁性绝缘体（空间群：P-1）。X射线晶体结构分析证实，电子结构与物理性质的这种显著转变，源于氢键内氘原子的转移或位移，同时伴随Cat-EDT-TTF π体系间的电子转移，证明了氢键结合的氘原子动力学与导电四硫富瓦烯（tetrathiafulvalene, TTF）π电子之间存在协同耦合效应。此外，本文还从氢键几何结构与势能曲线的H/D同位素效应视角，定性讨论了该独特相变仅发生于κ-D的内在原因。