Nathalie Proos Vedin: Shining Light on the Molecular Jewellery: Excited State (Anti)Aromaticity Assessment of Small Heterocycles

Abstract

This thesis explores the lowest excited states of heteroaromatics. Focus is put on how to properly examine and define their degree of aromaticity or antiaromaticity, as well as on the factors influencing this excited state (anti)aromaticity (ES(A)A) and other photochemical phenomena of importance.

Firstly, the inorganic version of cyclooctatetraene (COT), borazocine (B4N4H8), and intermediate isosteres are studied. The determination of aromatic character in the T1 state is found to be ambiguous, with different conclusions drawn depending on the descriptor employed. The importance of applying several aromaticity indices to ensure proper characterisation is reaffirmed, and it is suggested that more such challenging cases should be identified.

Focus is then shifted to the character of the lowest excited states, which in heterocycles is not limited to π,π*. Because of the in-plane lone-pair nσ orbital(s) provided by the heteroatom(s), the photochemically most relevant states (S1 and T1) are often of n,π* character. This is also the case for equivalent compounds like carbenes. The aromatic character of such odd-π-electron states, which do not conform to the well-known Baird's rule for excited (π,π*) states, is explored. The analysis requires separation of the π-electron spins (α and β), in accordance with Mandado's rule. When the two contributions are added together a residual may emerge, and the result is found to depend on factors such as electronegativity and symmetry. Examples of all possible outcomes are identified, with some ground state aromatic compounds even becoming more aromatic in the excited state than in S0.

The findings are also related to the order of the lowest excited states and whether this is possible to predict. Factors affecting the energy of the nσ orbital, such as electronegativity, bond angle and intraorbital repulsion, influence this. Possibilities for tuning the order are presented.

Finally, the photochemical process of triplet state photoionisation is investigated. No general relation between the degree of ES(A)A and the photoionisation potential is observed, and thus, the factors that affect these parameters are explored more closely. Aspects related to how strongly the electrons are held and to symmetry are found to influence both.

The work presented in this thesis enhances our understanding of the degree of aromaticity and antiaromaticity in excited heterocycles, and clarifies the approach with which to assess it. Insight into how this parameter relates to the character of the lowest excited state and other significant properties is also provided. The generality of the findings are discussed, paving the way towards predictive schemes for future applications within fields such as catalysis, medicine and beyond.