Quantum Interference in Radical and Neutral Single-Molecule Junctions

Single organic molecules facilitate bottom-up functionalization and atomically precise engineering of their properties that are not accessible in other materials. Despite over a decade of development, the value for S in single organic molecules at room temperature is usually below ±20 μV/. This is because the frontier orbitals of most molecules are far from the Fermi energy (EF) of the electrodes.

Quantum interference (QI) in single-molecule junctions creates an anti-resonance in the transmission function near EF which usually translates in a decrease of the conductance (G) and in an increase of the Seebeck coefficient (S) (1). In particular, this phenomenon appears in molecules with meta connections (2). Here we present an experimental study of the G and S in single molecules by comparing para- and meta-connected fluorene derivatives (2) and Blatter radicals (3) using a scanning tunneling microscope (STM) at room temperature and ambient conditions. Our results reveal the importance of the intrinsic spin state of radicals for a simultaneous enhancement of G and S and the influence of QI in these transport properties.