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Enhancing Intermolecular Interaction by Cyano Substitution in Copper Phthalocyanine
On-surface
molecular self-assembly is one of the key paradigms
for understanding intermolecular interactions and molecule–substrate
interactions at the atomic scale. Phthalocyanines are planar π-conjugated
systems capable of self-assembly and can act as versatile, robust,
and tunable templates for surface functionalization. One of the ways
to tailor the properties of phthalocyanines is by pendant group substitution.
How such a scheme brings about changes in the properties of the phthalocyanines
at the nanoscale has not been greatly explored. Here we present an
atomic-scale picture of the self-assembly of copper phthalocyanine,
CuPc, and compare it with its cyano analogue, CuPcÂ(CN)<sub>8,</sub>on Au(111) using scanning tunneling microscopy (STM) and scanning
tunneling spectroscopy (STS) in ultrahigh vacuum (UHV) at 77 K. STM
imaging reveals a tetramer unit cell to be the hallmark of each assembly.
The periodicity of herringbone reconstruction of Au(111) is unchanged
upon CuPcÂ(CN)<sub>8</sub> adsorption, whereas for CuPc adsorption
this periodicity changes. STS measurements show an increment in the
highest occupied–lowest unoccupied molecular orbital (HOMO–LUMO)
gap from CuPc to CuPcÂ(CN)<sub>8</sub>. Extensive ab initio calculations
within density functional theory (DFT) match well with the experimental
observations. STM imaging shows adsorption-induced organizational
chirality for both assemblies. For CuPcÂ(CN)<sub>8</sub> at LUMO energy,
the individual molecule exhibits an orbital-energy-dependent chirality
on top of the existing organizational chirality. It remains achiral
at HOMO energy and within the HOMO–LUMO gap. No such peculiarity
is seen in the CuPc assembly. This energy-selective chiral picture
of CuPcÂ(CN)<sub>8</sub> is ascribed to the cyano groups that participate
in antiparallel dipolar coupling, thereby enhancing intermolecular
interaction in the CuPcÂ(CN)<sub>8</sub> assembly. Thus, our atomically
resolved topographic and spectroscopic studies, supplemented by DFT
calculations, demonstrate that pendant group substitution is an effective
strategy for tweaking intermolecular interactions and for surface
functionalization