Substituent effects in the excited state (anti)aromaticity of aromatic carbocycles
Title: Substituent effects in the excited state (anti)aromaticity of aromatic carbocycles
SNIC Project: SNIC 2022/22-525
Project Type: SNIC Small Compute
Principal Investigator: Stefano Crespi <>
Affiliation: Uppsala universitet
Duration: 2022-05-12 – 2023-06-01
Classification: 10405


Aromaticity is a fundamental property of unsaturated compounds. The presence of a precise number of delocalized electrons on the conjugated pi scaffold (4n+2 electrons, following the Hueckel rule) allows for an enhanced stabilization of the cyclic structure. On the other hand, the rules regulating the stability at the ground state have a mirror counterpart at the excited state, in a way that an aromatic molecule, when excited, will produce an anti-aromatic compound. These structures are characterized by instability and a higher reactivity to alleviate the antiaromaticity. The same is valid for antiaromatic compounds at the ground state (4n electrons) forming aromatic structures at the excited state, which will be stabilized. The role and nature of the substituent effects in aromatic cycles (e.g. benzene) are crucial to predicting and understanding their chemical reactivity. Hence, with this project, I would like to study how substituents of different nature (electron-withdrawing or donating or electron neutral, following the Hammett substituent scale) affect the properties of a series of increasingly larger aromatic and antiaromatic structures (starting from cyclobutadiene with 4 carbon atoms and arriving at cyclooctatetraene with 8), both in the ground and first triplet excited state. I am expecting the antiaromatic compounds to be more severely affected by the presence and nature of the substituents. I will study these effects by applying DFT optimizations on structures that were previously pre-screened conformationally using semi-empirical sampling (i.e. using the crest approach developed by Grimme). Structural, energetic and magnetic aromaticity indices will be used to probe the difference in properties and stabilization that the substituent will impart on the aromatic and antiaromatic counterpart of the cycles.