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).

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.

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.

Baird aromaticity

Can you sort these molecules according to increasing triplet excitation energies?

Some basic considerations might suggest that energies go down as the size of the molecule increases. But this is incorrect. The decisive feature of these molecules is their ground-state antiaromaticity along with their potential for excited-state Baird aromaticity. Triplet excitation energies increase sharply going from 1 (0.1 eV) via 2 (1.9 eV) to 3 (2.6 eV). This can be understood in the sense that antiaromaticity is blurred as the molecule becomes larger.

More strikingly, when going from 3 to 4 or 5, the energy drops again dramatically down to 1.0 eV. This effect is explained following Ayub et al. by the simple fact that these molecule possess resonance structures with simultaneous quartets and sextets.

In a recent paper, Exploitation of Baird Aromaticity and Clar’s Rule for Tuning the Triplet Energies of Polycyclic Aromatic Hydrocarbons, we investigate these phenomena in detail using a recently developed method for the visualisation of chemical shielding tensors (VIST) along with an analysis of natural transition orbitals. In addition, a model for rationalising the dia- and paramagnetic shielding effects observed in (anti)aromatic systems is presented.

See also TCA, 2020, 139, 113 on a discussion of related bipenylene derivatives.

Highly sensitive Al measurements

A new study led by J. Lachner from the Helmholtz-Zentrum Dresden describes a method for detecting 26Al via Ion-Laser Interaction Mass Spectrometry using a particle accelerator.

Quantum chemical calculations highlight the different energetics of 26MgO and 26AlO, which are separated with high specificity despite being isobars.

The article just appeared in the International Journal of Mass Spectrometry: Highly sensitive 26Al measurements by Ion-Laser-InterAction Mass Spectrometry