How energy-offset affects the Seebeck coefficient and conductance of molecular junctions with pyridyl isomers

This study investigates how an energy offset modulates the thermoelectric conversion and electronic properties in molecular junctions based on phenylpyridyl isomers (PYPHE derivatives). By systematically varying the positions of nitrogen (N) atoms in the PYPHE groups, three structurally similar isomers were designed, and their charge transport behaviors in both electrical and temperature fields were examined. Experimental results reveal that the positional isomerization of N atoms can enhance thermoelectric properties by more than twofold while simultaneously improving the rectification ratio. Ultraviolet photoelectron spectroscopy (UPS) measurements and density functional theory (DFT) calculations demonstrate that the highest occupied molecular orbital (HOMO) of the molecules can be tuned closer to the Fermi level of the electrode, thereby reducing the energy offset. This optimization leads to improved thermoelectric conversion efficiency and rectification performance. This work elucidates precise nanoscale control over thermoelectric and conductance properties through molecular isomerization, offering new strategies for designing high-performance dual-functional molecular devices.