Quantum chemical investigation of excited state aromaticity and antiaromaticity effects in organic molecules
Abstract
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 (CASSCF, CASPT2, DLPNO-UCCSD(T) and EOM-CC). 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.
In the next period we will perform quantum chemical computations along four directions; (1) explore fundamental aspects of excited state aromaticity and antiaromaticity, (2) design molecules for photovoltaics and photocatalysis, (3) explore photoreactivity directed towards photodegradation of pharmaceuticals and agrochemicals, and (4) develop photoreactions for oligomerizations of volatile alkenes into biofuels.
Direction 1: Here we 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, 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: We earlier 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 designs, now combined with synthesis of the most promising candidate compounds. We focus on new chromophores that to various extents are aromatic in their lowest excited states while nonaromatic in their ground states.
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, and we will continue along this line with focus on environmental effects.
Direction 4: In the fourth 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 have promising experimental results and want to determine computationally to what extent the observed photoreactivity is triggered by excited state antiaromaticity. This information will provide guidance for further optimization of the photocatalysts.
