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

Abstract

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.