Excited state aromaticity and antiaromaticity: Fundamental Aspects

Research on excited state aromaticity and antiaromaticity (ESA and ESAA) has grown significantly in recent years.

For the lowest pipi* excited states, Baird’s rule tells that the electron counts for aromaticity and antiaromaticity are reversed to those in the closed-shell electronic ground state, i.e., [4n+2]annulenes are antiaromatic and [4n]annulenes are aromatic.

However, effects of ESA or ESAA can be more complex than given by Baird’s rule, and in the proposed project we will explore not only the scopes but also the complications, limitations and pitfalls of the concepts.

Thus, the proposed research focuses on the purely fundamental aspects because more knowledge within this realm is needed if the ESA & ESAA concepts are to be utilized properly in more applied areas of photochemistry and related fields.

Our studies will involve organic photoreactivity investigations, optical spectroscopy, qualitative theory, and quantitative quantum chemical computations.

We will explore a series of different types of molecules, ranging from small heteroaromatics and (homo-)aromatic carbocations to three-dimensional cage compounds (e.g., closo-boranes) and stacked dimers of 4npi-electron macrocycles.

Both their fundamental photophysical properties and the photochemical reactivities as given by ESA gain or ESAA relief will be studied.

The knowledge gained through the project will be highly valuable for the growth of the area and for its use within applied chemistry.