Cooperation of proton and electron in metal complexes aiming for proto-electronics

Proto-electronics is emerging as an integrated technology that combines protonics and electronics. Moreover, π-conjugated metal complexes are a promising platform for new and more versatile proto-electronics, because the judicious design of component metal ions and organic ligands enables precise control over redox properties while adjusting the Lewis acidic/basic properties. Particularly, the coupling of protons and electrons, along with the dual migration of both in a solid, driven by rational molecular design and component selection, is essential for advancing molecule-based proto-electronics research. This review highlights recent developments in crystalline metal complexes that exhibit proton-concerted electron transfer or proton–electron mixed conduction to assess the future direction of molecule-based proto-electronics. • This review highlights recent developments in metal complexes that exhibit proton-concerted electron transfer or proton–electron mixed conduction aiming for molecule-based proto-electronics. • Ligands and metal substitution effects on proton–electron coupling behavior were systematically investigated in the π-planar metal dithiolene complexes. • Metal–ligand isomerization of π-planar metal complexes has a greater effect on proton–electron coupling behavior compared with metal substitution. • Combination of BEDT-TTF and paddlewheel-type dinuclear platinum complex anion provided a proton–electron mixed conductor with the highest ambipolar conductivity among the structurally-defined materials. • A platinum dithiolene complex with naphthoquinone moieties showed the highest ambipolar conductivity among the neutral metal complexes.