Quantum chemical investigation of excited state aromaticity and antiaromaticity effects in organic molecules
Abstract
The project is directed to aromaticity 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 link to 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 are antiaromatic in the lowest pipi* triplet and singlet states. Numerous excited state processes and properties can be examined and rationalized by this rule. We will now, (1) explore fundamental aspects of excited state aromaticity and antiaromaticity, (2) design new chromophores for singlet fission photovoltaics, (3) explore photodegradation of aromatic pharmaceuticals and agrochemicals, (4) develop photocatalysts for oligomerizations of volatile alkenes into biofuels, and (5) design new high-spin Baird-aromatic molecules.
Direction 1: We will explore limitations and complications of the excited state aromaticity and antiaromaticity concepts. The concepts are applied by a growing number of researchers worldwide, but not all studies are correct. Thus, there is a need to scrutinize claims of excited state Baird-aromaticity/antiaromaticity, and to unravel limitations, complications and pitfalls. Research is especially needed on the antiaromaticity of singlet pipi* excited states of regular heteroaromatic molecules.
Direction 2: We earlier showed in a theoretical study how chromophores for singlet fission (SF) photovoltaics can be designed based on excited state Baird-aromaticity (JACS 2020, 142, 5602), but together with synthetic and spectroscopic groups we have identified that substituents alone will not turn monobenzopentalenes into SF chromophores (ChemPhysChem 2024, 25, e202300737). A dilemma is the low absorptivity of their first singlet excited state. We will now address this issue computationally before going to experiments.
Direction 3: Molecules which are aromatic in their ground state often become antiaromatic when excited, which triggers various photoreactions as we revealed for benzene (JACS 2020, 142, 10942). In ongoing experimental/computational projects we explore the photoreactivity of substituted benzenes and heteroaromatics. Most pharmaceuticals and agrochemicals contain ground state aromatic moieties which can photodegrade. We will continue explorations along this line.
Direction 4: Our fourth direction addresses the photochemical formation of sustainable aviation fuels from volatile alkenes formed by cyanobacteria directly from CO2, water and sunlight (Green Chem 2022, 24, 9602). Two subdirections will now be explored; (i) design of new photoacid catalysts for photocatalyzed alkene oligomerizations, and (ii) design of new triplet sensitizer chromophores with small singlet-triplet energy gaps suitable for solar-light initiated isoprene dimerization. The latter study builds on an experimental/computational collaboration in which we involved (ACIE 2021, 60, 21817).
Direction 5: This direction builds on a recent paper where we reported that macrocyclic molecules with low-lying states of very high total spin (nonet states) can be based on the 2,3-dimethylenecyclobut-1-ene scaffold (Chem. Eur J 2024, 30, e202303549). We identified species with singlet-nonet energy gaps of merely 0.4 eV. We will now design similar macrocycles, but with high-spin ground states.
