Molecular switches and real-time ion sensing in pyridinium circuits via a single-molecule STM-break junction

The functional electronic and spectro-electrochemical properties of two structural pyridinium isomers, Py_Down-BF₄ and Py_Up-BF₄, were studied at the single-molecule level using the STM-BJ technique. These isomers differ in the position of the redox-active pyridinium core. The aim was to identify the role of core's position in promoting reversible switching between electromers (redox isomers) in solution and at the gold-pyridinium-gold junction circuit. We measured the single-molecule conductance of each pyridinium isomer in various electrolyte environments using tetrabutylammonium salts (TBABF₄, TBAPF₆, TBABr, and TBACl). The choice of electrolytes played a crucial role in the histograms’ shapes—junction distribution, width, and peak position—which act as unique conductance fingerprints for each isomer. During STM-BJ measurements, a dynamic evolution in the conductance histograms was determined, particularly with the electrolytes TBAPF₆ and TBABF₄. This behavior was attributed to the real-time detection of interactions between the positively charged pyridinium core and the electrolyte anions within the gold-pyridinium-gold junction. The dynamic evolution in single-molecule conductance was rationalized by the Gibbs free energies (ΔG) for the anion-cation pairs obtained from density functional theory (DFT) calculations. Furthermore, the theoretical trend predicted by DFT combined with the Keldysh nonequilibrium Green's function (NEGF) formalism (DFT-NEGF) was consistent with the experimental results.