Enhancing Intermolecular Interaction by Cyano Substitution in Copper Phthalocyanine

Abstract

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, CuPc(CN)8,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 CuPc(CN)8 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 CuPc(CN)8. 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 CuPc(CN)8 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 CuPc(CN)8 is ascribed to the cyano groups that participate in antiparallel dipolar coupling, thereby enhancing intermolecular interaction in the CuPc(CN)8 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.