  Metal string complexes or extended metal atom chains (EMACs) belong to a
family of molecules that consist of a linear chain of directly bonded metal
atoms embraced helically by four multidentate organic ligands. These four
organic ligands are usually made up of repeating pyridyl units,
single-nitrogen-substituted heterocyclic annulenes, bridged by independent
amido groups. Here, in this paper, we show that these heterocyclic annulenes
are actually nanoscale molecular split-ring resonators (SRRs) that can exhibit
simultaneous negative electric permittivity and magnetic permeability in the
UV-Vis region. Moreover, a monolayer of self-assembled EMACs is a periodic
array of molecular SRRs which can be considered as a negative refractive index
material. In the molecular scale, where the quantum-size effect is significant,
we apply the tight-binding method to obtain the frequency-dependent
permittivity and permeability of these molecular SRRs with their tensorial
properties carefully considered.
