Modular Redox Switching of Dinuclear Organometallic Molecular Junctions

Molecular switch is one of the essential functional units of molecular electronics. Here, we report development of new molecular switches based on the electron-rich diruthenium complexes with the (2,5-di-R-substituted 1,4-diethynylbenzene)diyl linkers. The dinuclear molecular switches, {µ-p-C≡C-(2,5-R2-C6H2)-C≡C}{Ru(dppe)2(C≡C-C6H4-p-SMe)}2 1R (R= OMe, H, CF3), with various substituents (R) on the bridging phenylene rings showed two successive reversible 1e-oxidation waves, indicating stability of 1e-oxidized mixed-valence species. The solid-state structure of [1H]+ showed the charge-localized Robin-Day class II nature, while that of [1OMe]+ revealed the fully charge-delocalized class III nature. These characters were also evident from the spectroscopic data in solutions. Single-molecule conductance measurements by the scanning tunneling microscope break junction method revealed a significant dependence of the conductance on R, i.e. 1OMe turned out to be >100-times more conductive than 1H and 1CF3, whereas the substituent effect of the monocationic complexes was within a fold-change of 2. As a result, the ON/OFF ratios (the ratios of the conductance of the cationic species [1R]+ to that of the neutral species 1R) were critically dependent on R (as large as 191 when R = CF3) and even reversed (0.4 when R = OMe). Furthermore, the neutral and monocationic complexes 1H and [1H]+ fabricated into the nanogap devices showed in situ ON/OFF switching behavior. The present study demonstrates not only the rare examples of the mixed-valence complexes which were subjected to the break junction measurements but also the first examples of molecular switch, the ON/OFF ratio of which was controlled by tuning the organic linker parts.