Planar π-extended cycloparaphenylenes featuring an all-armchair edge topology

The [ n ]cycloparaphenylenes ([ n ]CPPs)— n para -linked phenylenes that form a closed-loop—have attracted substantial attention due to their unique cyclic structure and highly effective para -conjugation leading to a myriad of fascinating electronic and optoelectronic properties. However, their strained topology prevents the π -extension of CPPs to convert them either into armchair nanobelts or planarized CPP macrocycles. Here we successfully tackle this long-standing challenge and present the bottom-up synthesis and characterization of atomically precise in-plane π -extended [12]CPP on Au(111) by low-temperature scanning probe microscopy and spectroscopy combined with density functional theory. The planar π -extended CPP is a nanographene with an all-armchair edge topology. The exclusive para -conjugation at the periphery yields delocalized electronic states and the planarization maximizes the overlap of p orbitals, which both reduce the bandgap compared to conventional CPPs. Calculations predict ring currents and global aromaticity in the doubly charged system. The intriguing planar ring topology and unique electronic properties make planar π -extended CPPs promising quantum materials. The strained topology of [ n ]paracyclophenylenes ([ n ]CPPs) typically prevents their π sysytem from being extended, but now the formation of a planar π -extended CPP has been achieved through a bottom-up on-surface synthesis approach. The planar π -extended [12]CPP produced by this method is a nanographene featuring an all-armchair edge, which leads to delocalized electronic states around the entire ring.