Quantum chemical investigation of excited state aromaticity and antiaromaticity effects in organic molecules
Abstract
The project is directed to aromaticity and antiatomaticity in electronically excited states of cyclic pi-conjugated molecules 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), and these computations 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 rationalized by this rule. The proposed project builds on the previous with similar five directions. That is, we will (1) explore the fundamentals of excited state aromaticity and antiaromaticity (ESA & ESAA), (2) design new chromophores for singlet fission photovoltaics (SF PV), (3) design new high-spin Baird-aromatic molecules, (4) explore photodegradation of aromatic pharmaceuticals, agrochemicals and organic photovoltaics materials, and (5) develop organic photoacid catalysts.
Direction 1: The fundamentals of the ESA & ESAA concepts need to be further explored, and their limitations, complications and pitfalls of ESA & ESAA determined as they are applied by a growing number of researchers, sometimes incorrectly. Research in the next year is especially needed on the antiaromaticity of singlet pipi* of heteroaromatic molecules, but also to explain why different aromaticity descriptors can lead to very different results for one and the same molecule.
Direction 2: We earlier concluded in a theoretical study that 4npi-electron chromophores for SF can be designed based on excited state Baird-aromaticity (JACS 2020, 142, 5602). However, during the last year we have revealed factors that make such species less promising, and describe this in a manuscript which is about to be submitted (see project 7, activity report). We have identified alternative approaches that utilize the ESA & ESAA concepts and that will be explored in the next.
Direction 3: Expanding from a paper by us in 2024 (Chem. Eur J 2024, 30, e202303549) we now design macrocyclic molecules with high-spin ground state where computations guide synthesis and further experiments. The macrocycles are composed of triplet state Baird-aromatic moieties coupled in ferromagnetic manner so as to yield organic polyradicals with very high-spin ground states.
Direction 4: 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 various derivatives of benzenes and heteroaromatics triggered by ESAA relief. Many pharmaceuticals, agrochemicals and organic photovoltaics materials contain ground state aromatic moieties which can photodegrade. Improved predictability of the photodegradations can have relevance for environmental chemistry.
Direction 5: Expanding on direction 4, we will use computations to design photoacids that operate based on ESAA relief. We predict that these can be used in experimental areas such as green chemistry to produce larger functionalized molecules from gaseous or volatile compounds produced by cyanobacteria (see Green Chem 2022, 24, 9602).
