  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.
