Quantum chemical investigation of excited state aromaticity and antiaromaticity effects in organic molecules
The project is directed towards aromaticity effects in electronically excited states where my research group is internationally leading. We apply quantum chemical calculations using DFT and electron correlated wavefunction methods (primarily CASSCF and CASPT2 but also coupled cluster methods). The calculations are connected to on-going experimental studies in the group. The topic is based on Baird's rule which tells that species with 4n pi-electrons are aromatic and those with 4n+2 pi-electrons are antiaromatic in the lowest pipi* triplet and singlet states. A series of processes and properties can be examined and rationalized in terms of excited state aromaticity or antiaromaticity.
We will now perform quantum chemical computations along four directions; (1) explore fundamental aspects of the excited state aromaticity and antiaromaticity concepts, (2) design molecules for photovoltaics, bioimaging and photocatalysis, (3) explore photoreactivity, particularly directed towards photodegradation of pharmaceuticals and agrochemicals, and (4) develop photoreactions for oligomerizations of volatile alkenes into biofuels.
Direction 1: Here we will explore limitations and complications of the excited state aromaticity and antiaromaticity concepts. Recently, the concepts have been applied by a growing number of research groups internationally, but far from all studies are correct. There is an urgent need to critically examine claims of excited state Baird-aromaticity and antiaromaticity, and to unravel the limitations, complications and pitfalls. More research is especially needed on excited state aromaticity and antiaromaticity in singlet excited states.
Direction 2: During 2020 and 2021 we published two papers on computational design of chromophores for singlet fission photovoltaics. In one paper we utilized a combination of ground state Hückel aromaticity and excited state Baird aromaticity for such design (JACS 2020, 142, 5602), while in the other paper we revealed that Cibalackrot-type chromophores are Hückel-aromatic in their lowest triplet state (Chem. Sci. 2021, 12, 6159). We will continue on similar design, now combined with synthesis of the most promising candidate compounds. We will also continue on design of Baird-aromatic triplet state quenchers for self-healing fluorophores (see PNAS 2020, 117, 24305).
Direction 3: Molecules which are aromatic and stable in the electronic ground state often become antiaromatic when excited. This excited state antiaromaticity can trigger photorearrangements or other photoreactions as we revealed for benzene (JACS 2020, 142, 10942). We will now explore how this impact on the photoreactivity of substituted benzenes and heteroaromatics. The majority of all pharmaceuticals and agrochemicals contain ground state aromatic parts which can photodegrade. We recently reveled this for the amiloride type drugs (Cell Reports Phys. Sci. 2020, 1, 100274), and we will continue along this line with focus on environmental effects.
Direction 4: In the final project direction we will identify photochemical routes for oligomerization of small alkenes, e.g., by design of suitable photoredox catalysts where the initial screening and design is performed through quantum chemical calculations. We believe such photocatalysts can be designed taking advantage of the facile sequential photooxidation that occurs in amiloride (see above).