Correlation of Kondo effect and molecular conformation of the acceptor molecule in the TTF-TCNE charge transfer complex

TL;DR

This study explores how the molecular conformation of TCNE influences the presence of Kondo resonance in a TTF-TCNE charge transfer complex on gold, revealing a correlation between structure and electronic properties.

Contribution

It demonstrates a direct link between molecular conformation of TCNE and the occurrence of Kondo resonance in a charge transfer complex, using advanced microscopy techniques.

Findings

Kondo resonance observed in one TCNE conformation but not the other.

Two distinct conformations of TCNE exhibit different electronic properties.

Both conformations are singly charged despite differing Kondo behavior.

Abstract

A Kondo resonance has been observed on purely organic molecules in several combinations of charge transfer complexes on a metal surface. It has been regarded as a fingerprint of the transfer of one electron from the donor to the extended $\pi$ orbital of the acceptor's LUMO. Here, we investigate the stoichiometric checkerboard structure of tetrathiafulvalene (TTF) and tetracyanoethylene (TCNE) on a Au(111) surface using scanning tunneling and atomic force microscopy at 4.8 K. We find a bistable state of the TCNE molecules with distinct structural and electronic properties. The two states represent different conformations of the TCNE within the structure. One of them exhibits a Kondo resonance, whereas the other one does not, despite of both TCNE types being singly charged.