Polarity Emergence via Skeletal Topology Rearrangements in Nonbenzenoid Molecular Carbons

Rational design and integration of benzenoid and nonbenzenoid carbon frameworks enable the construction of low-dimensional carbon nanostructures with unconventional topologies and tailored electronic properties. Here, we demonstrate the on-surface synthesis of nanosized pentaheptite (NPH), a nonbenzenoid molecular carbon consisting of equal numbers of penta-heptagonal units, arising from the strategic precursor design and the inherent structural versatility of carbon skeletons. The as-synthesized NPH adopts a C_2h symmetric configuration with largely compensated intramolecular charge polarization, while benzenoid units can be incorporated into the intrinsic penta-heptagonal framework via two possible pathways: an intramolecular Stone-Wales-like skeletal rearrangement and a process involving azulene-naphthalene rearrangements, forming a transformed pentaheptite (T-NPH), as supported by density functional theory simulations. Such a transformation breaks skeletal symmetry, leading to intramolecular charge redistribution and electrostatic asymmetry, as revealed by local probe microscopy measurements and corroborated by density functional theory calculations. Our work highlights the role of skeletal topology rearrangements in governing the local electrostatic properties of nonbenzenoid carbon nanostructures, offering insights into polarity-related electrostatic effects in nonbenzenoid molecular carbons.