# Multi-Orbital Charge Transfer into Nonplanar Cycloarenes Revealed with CO-Functionalized STM Tips

## TL;DR

Scientists used a special scanning technique to observe how electrons transfer into complex molecules on metal surfaces, revealing new insights into molecular behavior.

## Contribution

A novel STM-based method is introduced to directly observe multi-orbital charge transfer in nonplanar cycloarenes on metal surfaces.

## Key findings

- STM simulations reveal charge transfer into multiple molecular orbitals of kekulene and isokekulene on copper surfaces.
- Photoemission confirms high selectivity for isokekulene formation on Cu(110).
- The method works for strongly interacting surfaces and low-yield compounds.

## Abstract

On-surface synthesis
enables the tunable preparation of diverse
molecular systems with tailored properties. Recently, the highly selective
synthesis of kekulene (>99%) on Cu(111) and isokekulene (92%) on
Cu(110)
from the same molecular precursor was demonstrated. Scanning tunneling
microscopy (STM) with CO-functionalized tips can identify individual
molecules based on their geometric structure at low coverage on Cu(110)
but also reveals complex features arising from electronic contributions
near the Fermi energy. Here, we investigate the origin of these features
by simulating STM images based on a weighted sum of multiple molecular
orbitals, for which we employ weights based on the calculated molecular-orbital
projected density of states. This analysis provides direct experimental
evidence for charge transfer from the surface into multiple formerly
unoccupied molecular orbitals for single molecules of kekulene as
well as isokekulene in its two nonplanar adsorption configurations.
In comparison, the area-integrating photoemission orbital tomography
technique confirms the charge transfer as well as the high selectivity
for the formation of a full monolayer of predominantly isokekulene
on Cu(110). Our STM-based approach is applicable to a wide range of
adsorbed molecular systems and specifically also suited for strongly
interacting surfaces, nonplanar molecules, and compounds accessible
only in extremely low yields.