Delayed fluorescence

Patrick’s first paper as first author just appeared in PCCP: The role of excited-state character, structural relaxation, and symmetry breaking in enabling delayed fluorescence activity in push-pull chromophores. Well done Patrick!

We were interested in understanding the difference in thermally activated delayed fluorescence (TADF) between two closely related donor-acceptor-donor systems using either an anthraquinone and benzodithiophenedione acceptor units, respectively. The first one was known to be an effective TADF emitter [JACS 2014, 136, 18070] whereas the second one had significantly lower quantum yield for TADF [PCCP 2019, 21, 10580].

Rather than just presenting energies, it was the purpose of this paper to shed detailed insight into the wavefunctions involved. Notable differences in the wavefunctions and charge-transfer character were found between the two molecules. Even more striking differences existed between different computational methods.

After evaluating electronic structure methods, we presented geometry optimisations in solution, highlighting the importance of symmetry breaking for producing an emissive lowest singlet state. The role of different solvation models was discussed as well.

Highly efficient photodynamics simulations

Molecules interacting with light undergo fascinating photodynamical processes inducing chemical reactions, transferring energy, or converting electronic energy into heat. These processes can be elucidated computationally via photodynamics simulations. However, these can be computationally highly demanding making the simulation of many interesting processes unfeasible.

A possible route to overcome this problem and to allow for efficient dynamics simulations is by combining vibronic coupling models (describing the energies) with the surface hopping method (describing the dynamics). We have introduced this idea two years ago [PCCP, 2019, 21, 57]. A new paper in Accounts of Chemical Research summarises developments since then: Surface Hopping Dynamics on Vibronic Coupling Models Acc. Chem. Res. 2021, 52, 3760.

Conjugated macrocycles

A recent study led by F. Glöcklhofer from Imperial College, London, investigates the properties of substituted conjugated macrocycles. The first (pre-review) version just appeared on Open Research Europe:

[2.2.2.2]Paracyclophanetetraenes (PCTs): cyclic structural analogues of poly(p‑phenylene vinylene)s (PPVs) by M. Pletzer, F. Plasser, M. Rimmele, M. Heeney, and F. Glöcklhofer.

Computations reveal the local and global aromaticity of the macrocycles in various electronic states by using the VIST method along with a discussion of the nodal structures of the orbitals.

Dominant natural difference orbitals (NDOs) (blue/red for electron detachment; green/orange for attachment) for different electronic states of O-PCT (C2 rotamer) and VIST plots for the S0 and T1 states at the S1 geometry. Copyright 2021 Pletzer M. et al. (CC-BY 4.0).

Quantum Bio-Inorganic Chemistry

Next Wednesday, 29 September 2021, Felix will host a discussion session for the quantum bio-inorganic chemistry society together with Ilaria Ciofini and Sebastian Mai.

Topic: Classification and Analysis of Excited-state Wavefunctions in Transition Metal Complexes

You can find more information here and a preview of the introduction slides here.

This discussion will cover formal aspects – what do we even mean by excited-state character and is it physically observable – along with more practical aspects of how to characterise states effectively.

Let me know if you have an questions you would like us to discuss.

Oxygen Harvesting from Carbon Dioxide

A recent study, led by Benjamin Buckley and Felipe Iza from Loughborough University, presents an innovative use of carbon dioxide. Using a plasma, carbon dioxide is turned into a source of atomic oxygen, which is used as a waste-free oxidant for the oxidation of alkenes to epoxides. The study, a collaborative work between engineering, synthesis and computation, just appeared in Chemical Science: Oxygen Harvesting from Carbon Dioxide: Simultaneous Epoxidation and CO Formation.

Visualisation of Aromaticity and Antiaromaticity

Dylan has finished his MChem project entitled “Visualisation of Aromaticity and Antiaromaticity via the Computation of the Chemical Shielding on Multi-Dimensional Grids.” You can find his report here. The purpose of his project was to develop a convenient method for computing shielding tensors on a grid around a molecule. The developed code is available via github.

Below, an analysis of biphenylene is shown in the singlet (a) and triplet (b) state. For the singlet this representation highlights the aromaticity (red) of the benzene rings whereas the central 4-membered ring is found to be antiaromatic (blue). In the triplet (b), the whole molecule is found to be aromatic (red) according to Baird’s rule.

An analysis of norcorrole using either its doubly protonated form (a) or a nickel complex (b) highlights the antiaromaticity at the centre of this molecule whereas an aromatic pathway is found at the perimeter (see also [P. B. Karadakov, Org. Lett. 2020, 22, 8676]).

For more examples of how this type of analysis is used in the literature, see e.g., [Angew. Chemie – Int. Ed. 2020, 59, 19275] and [ChemPhysChem 2021, 22, 741].

Excited-state symmetry breaking

A recent study led by Zoltan Szakacs and Eric Vauthey from the University of Geneva explores excited-state symmetry breaking in donor-acceptor-donor systems. The associated paper just appeared in PCCP: Excited-state symmetry breaking in 9,10-dicyanoanthracene-based quadrupolar molecules: the effect of donor-acceptor branch length

The main idea behind this work is to use symmetry-selection rules and the associated forbidden transitions to probe how inversion symmetry is broken during the photodynamics. See [JPCL 2021, 12, 4067] for an initial discussion of the idea.