Controlling the Electronic States and Physical Properties of MMX-Type Diplatinum-Iodide Chain Complexes via Binary Countercations

MMX-type quasi-one-dimensional iodide-bridged dinuclear
Pt complexes (MMX chains) with binary countercations show a new alternating
charge-polarization + charge-density-wave (ACP+CDW) electronic state
and reversible switching of the electronic states and physical properties
upon dehydration and rehydration process. By comparing several MMX
chains with various binary countercations with previous chains, we
found that the short backbone of the aliphatic diammonium ion was
indispensable for realizing the ACP+CDW state because it induces a
2-fold periodicity along the chain axis via twisting of the ligands.
Moreover, the reversibility of the changes in the structure and electrical
conductivity upon dehydration and rehydration depend on the length
of aliphatic diammonium ion. Short diammonium ions support a robust
framework, which undergoes reversible structural changes. On the other
hand, long and bent aliphatic diammonium ions weaken the framework,
which causes partial degradation of the crystal and a decrease in
the electrical conductivity when the structure changes. However, the
decrease in the activation energy of the electrical conductivity after
the dehydration process is independent of the robustness of the complex,
indicating that the orbital overlap in MMX chains with binary countercations
increases upon dehydration. Controllable electronic states and physical
properties provide a platform for designing the multifunctional materials
based on MMX chains.