J. Am. Chem. Soc. 135, 12857–12860 (2013)

Corannulene (C20H10) is a large, bowl-shaped polycyclic aromatic molecule that can be considered as a fullerene fragment. It has attracted attention for a variety of reasons, including its five-fold symmetry, and its packing and coordination behaviours, as well as its potential to exhibit superconductivity with high transition temperatures. Superconductivity is not common in organic species, but has been observed in a number of charge-transfer complexes and doped carbon materials — and has been predicted for alkali-doped corannulene. Now, a team led by Kim Baldridge and Karl-Heinz Ernst from the University of Zurich have prepared films comprising corannulene and caesium, proposing that they have a Cs4C20H10 composition with a dimeric structure in which four caesium atoms are sandwiched between two corannulenes. Li4C20H10 compounds with a similar sandwich structure have previously been observed in solution, but it was thought that analogues with ions larger than lithium would not form.

Credit: © 2013 ACS

Baldridge and Ernst co-deposited corannulene and caesium at 100 K on a copper surface in an ultra-high-vacuum chamber. Analysis by ultraviolet photoelectron spectroscopy revealed layer-by-layer growth, in which a metallic caesium film formed that was then progressively covered by corannulene molecules. At low corannulene coverage, charge transfer from the caesium to the corannulene was observed. This is in agreement with corannulene's good electron-accepting properties — it can accept four electrons into its doubly degenerate lowest unoccupied molecular orbital. At higher coverage, an undoped corannulene multilayer formed on the caesium film. On heating in the 200–300 K temperature range, however, the caesium ions increasingly diffused into the corannulene multilayer. Above 300 K both species desorbed.

Analysis of the film by X-ray photoelectron spectroscopy obtained after annealing supported a composition with a caesium/corannulene ratio of 4:1. Theoretical calculations showed that the most stable structure is a dimer adopting a stacked conformation, with four caesium cations in a square geometry located between two distorted tetraanionic corannulenes, and two additional caesium atoms both above and below the sandwich structure.