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.

Talk: excited-state aromaticity

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:

Antiaromatic macrocycles

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.

Release of TheoDORE 2.4

Version 2.4 of the TheoDORE wavefunction analysis package is available. Download the current version below.

New features of TheoDORE 2.4:

TheoDORE – Download

Download the newest release of the TheoDORE wavefunction analysis program – TheoDORE 2.4 (22 April 2021)

Size: 12 MB
Version: 2.4

Full release notes:

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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