Quasi-Planar π-Extended Cycloparaphenylenes: On-Surface Synthesis, Characterization, and Electronic Properties

Cycloparaphenylenes (CPPs), hoop-shaped conjugated macrocycles composed of para-linked phenyl units, have attracted significant interest due to their curved aromatic frameworks and para-conjugation, which give rise to unique (opto)electronic properties. Full planarization of their π-systems is structurally inaccessible, as it would further increase the inherent ring strain. However, partial planarization can be achieved by π-extension through incorporation of rigid bending units. Here, we report the on-surface synthesis of quasi-planar π-extended [2n]CPPs (n = 8-11) on Au(111), achieved via a bottom-up surface-assisted strategy, and their characterization by scanning tunneling microscopy/spectroscopy (STM/STS) and density functional theory (DFT). These π-extended CPPs are macrocycles with para-connected phenylenes forming the inner backbone and perylenyl groups bridging the phenylene units. The n = 8 CPP adopts a planar geometry on the surface, while the n = 9 to n = 11 homologues remain nonplanar. Nevertheless, substrate-induced flattening enhances π-orbital overlap and leads to a progressive reduction of the HOMO-LUMO gap with increasing ring size. This behavior is specific for planarized CPPs and contrasts with conventional, nonplanar CPPs. Moreover, π-extension results in a reduced gap compared to the unsubstituted planar parent [2n]CPP. dI/dV maps reveal orbital confinement at the edges and fjord regions, attributed to slower wave function decay. In larger CPPs, the tilted perylenyl groups modulate this confinement and affect the STM contrast in fjord regions. The unique structure of these strained, quasi-planar CPPs provides a well-defined platform for studying the interplay of curvature, conjugation, and electronic structure in bent benzenoid macrocycles.