Evolution of the ionisation energy with the stepwise growth of chiral clusters of [4]helicene

• Published in: Nature communications Vol. 15; no. 1; pp. 4928 - 8
• Main Authors: Domingos, Sérgio R., Tikhonov, Denis S., Steber, Amanda L., Eschenbach, Patrick, Gruet, Sebastien, Hrodmarsson, Helgi R., Martin, Kévin, Garcia, Gustavo A., Nahon, Laurent, Neugebauer, Johannes, Avarvari, Narcis, Schnell, Melanie
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 10.06.2024; Nature Publishing Group; Nature Portfolio
• Subjects: 639/638/11; 639/638/440/94; 639/766/36/1121; 639/766/94; Broken symmetry; Chemical Sciences; Chirality; Clustering; Clusters; Enantiomers; Evolution; Humanities and Social Sciences; Interstellar chemistry; Interstellar matter; Ionization; Isomers; multidisciplinary; Photoelectron spectroscopy; Photoelectrons; Physics; Polycyclic aromatic hydrocarbons; Quantum chemistry; Science; Science (multidisciplinary); Spectroscopy; Spectrum analysis; Structural models; Topology; Vacuum
• ISSN: 2041-1723; 2041-1723
• DOI: 10.1038/s41467-024-48778-0

Polycyclic aromatic hydrocarbons (PAHs) are widely established as ubiquitous in the interstellar medium (ISM), but considering their prevalence in harsh vacuum environments, the role of ionisation in the formation of PAH clusters is poorly understood, particularly if a chirality-dependent aggregation route is considered. Here we report on photoelectron spectroscopy experiments on [4]helicene clusters performed with a vacuum ultraviolet synchrotron beamline. Aggregates (up to the heptamer) of [4]helicene, the smallest PAH with helical chirality, were produced and investigated with a combined experimental and theoretical approach using several state-of-the-art quantum-chemical methodologies. The ionisation onsets are extracted for each cluster size from the mass-selected photoelectron spectra and compared with calculations of vertical ionisation energies. We explore the complex aggregation topologies emerging from the multitude of isomers formed through clustering of P and M , the two enantiomers of [4]helicene. The very satisfactory benchmarking between experimental ionisation onsets vs . predicted ionisation energies allows the identification of theoretically predicted potential aggregation motifs and corresponding energetic ordering of chiral clusters. Our structural models suggest that a homochiral aggregation route is energetically favoured over heterochiral arrangements with increasing cluster size, hinting at potential symmetry breaking in PAH cluster formation at the scale of small grains. Polycyclic aromatic hydrocarbons such as [4]helicenes are important interstellar molecules. Here, the authors show their evolution of chiral clusters upon aggregation through coincidence spectroscopy and quantum chemical calculations.