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 []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)

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Version: 2.4

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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

Elucidating the Electronic Structure of TADF emitters

Thermally activated delayed fluoresence (TADF) is an exciting modern research area aimed at producing new OLED emitters. From a theoretical perspective TADF is particularly fascinating because it requires a detailed understanding of the different terms that contribute to the singlet and triplet excitation energies of the molecules studied. In a recent study led by Yihan Shao from the University of Oklahoma, we investigated a recently developed TADF emitter and showed how a combination of different wavefunction analysis tools provides deep insight into its excited-state properties. The paper just appeared in J. Phys. Chem. Lett.: Elucidating the Electronic Structure of a Delayed Fluorescence Emitter via Orbital Interactions, Excitation Energy Components, Charge-Transfer Numbers, and Vibrational Reorganization Energies.