Manifestations of aromaticity in antiaromatic molecules

Aromaticity and antiaromaticity are fundamental concepts in organic chemistry, but there is an ongoing need to understand how these concepts work in non-traditional π-systems. This thesis explores the interplay between these two concepts in unconventional π‑systems, and the techniques used to identify their manifestations, focusing on optical spectroscopy, radical species, and stacked molecules. Optical spectroscopy has been used to assign (anti)aromaticity in species that are either short‑lived or not stable under ambient conditions. In this work, electrochemistry is used to access different π‑electron counts for porphyrin and norcorrole. I show that the spectroscopic features of (anti)aromaticity are most readily identified in the visible and near-infrared spectral regions, while the infrared region is of less value for rigid macrocycles than was suggested by previous reports based on more flexible systems. Mandado’s rules of mixed (anti)aromaticity are the existing theoretical framework for interpreting the (anti)aromaticity of species with an odd number of π‑electrons. In this thesis, I use computational models of charged annulenes to evaluate the efficacy of Mandado’s rules. I report evidence that supports these rules, and examine which computational measures of (anti)aromaticity are most appropriate for evaluating odd π‑electron π-systems. Previous work has suggested that stacked pairs of antiaromatic molecules are stabilised by a phenomenon known as stacked aromaticity. Here, I investigate cofacially stacked dimers of π‑conjugated molecules, and I present a rational modification of the interfragment delocalisation index to quantify the delocalisation between the two molecules. I show that stacked aromaticity is associated with greater delocalisation between the component antiaromatic molecules, and that this delocalisation extends across larger assemblies of antiaromatic molecules. Overall, this work builds on three open areas of research: assignments of (anti)aromaticity based on optical spectroscopy, Mandado’s rules of mixed (anti)aromaticity, and stacked aromaticity. The conclusions herein clarify the use of existing techniques to assign and assess (anti)aromaticity. A key outcome of my work is to improve the ability of future researchers to rationally select measures of (anti)aromaticity for both traditional and non-traditional π-systems, and to enable better control of (anti)aromatic systems for functional materials, such as molecular electronics.