Author: Felix

Quantum Bio-Inorganic Chemistry

Felix

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

Felix

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

Felix

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

Felix

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.

Talk: excited-state aromaticity

Felix

Today Felix is giving a talk at the Computational PhotoChemistry Online Meeting 2021: Tuning photophysical properties via excited-state aromaticity

The talk discusses the effects of aromaticity on excited-state energies and other properties. Strategies for quantifying and visualising aromaticity are shown as well.

Three recent papers are discussed:

You can download the slides here:

Download

Antiaromatic macrocycles

Felix

How do macrocycles with [4n] electrons behave? Are there signatures of their formal antiaromaticity and how can their properties be tuned for practical applications? A recent study, led by Florian Glöcklhofer (Imperial College, London) endeavours to tackle these questions. A set of macrocycles based on [2.2.2.2]cyclophanetetraenes was synthesised, their redox and optical properties were measured, and a detailed computational analysis was performed.

Clear signatures of the unique properties of these macrocycles was found considering their large Stokes shifts (>1.5 eV) along with the ease of producing doubly charged states. A detailed computational analysis traces these properties back to the aromaticity of the excited and doubly charged states, respectively. In addition, it is illustrated how the properties of the macrocycles can be systematically varied with introduction of functional groups and variation of the aromatic units.

The study just appeared as a preprint on ChemRxiv: Functional Group Introduction and Aromatic Unit Variation in a Set of π-Conjugated Macrocycles: Revealing the Central Role of Local and Global Aromaticity.

Below, electron density difference plots for the charged states of the parent molecule paracyclophanetetraene are shown highlighting the cyclic symmetry of the electron attachment. The 2+/2- and 6+/6- states are aromatic whereas the 4+/4- singlet states are antiaromatic.

  • Dianion
  • Tetraanion
  • Hexaanion
  • Dication
  • Tetracation
  • Hexacation