Blog - Page 6 of 12 - Comp-Photo-Chem

Baird aromaticity

FelixApril 20, 2021

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.

Visualisation of chemical shielding tensors (VIST)

FelixApril 8, 2021

Our paper Visualisation of chemical shielding tensors (VIST) to elucidate aromaticity and antiaromaticity just appeared in the European Journal of Organic Chemistry. You can find more information here.

The figure below shows the shielding tensors overlaid over the ACID isosurface for phenanthrene (computed using GIMIC according to JPCA 2016, 120, 5658).

Fluorescence Activity of Fingerprinting Reagents

FelixApril 1, 2021

A new paper on the photochemistry of fingerprinting reagents, led by L. Hunnisett and P. Goddard at Loughborough, just appeared in J. Chem. Phys.: Mechanistic Insight into the Fluorescence Activity of Forensic Fingerprinting Reagents. The paper investigates the reason for the different fluorescent activities observed in chemically related fingerprinting reagents.

Highly sensitive Al measurements

FelixMarch 30, 2021

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

Talk at Faraday Symposium

FelixMarch 29, 2021

Today at 13:00, Felix will give a talk at the Faraday Joint Interest Group Conference.

New Analysis Tools for Excited-State Quantum Chemistry: Turning Numbers into Chemical Insight

You can download the slides here.

Elucidating the Electronic Structure of TADF emitters

FelixMarch 16, 2021

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.

3D visualisation of chemical shielding tensors

FelixJanuary 20, 2021

Aromaticity is a ubiquitous yet elusive concept in chemistry and chemists have spent a great deal of effort on developing methods to quantify and visualise aromaticity. One particularly popular method is the nucleus independent shift (NICS), which can be seen as a virtual NMR experiment carried out within a conjugated ring to evaluate the enhanced chemical shielding induced by aromatic ring-currents. Strikingly NICS also allows to quantify antiaromaticity, as this induces a net deshielding effect within the ring. NICS provides a powerful quantitative aromaticity criterion but the main challenge for its graphical representation is that the chemical shielding is a 3×3 tensor, which is difficult to visualise with the existing methods.

Therefore, we have developed a new method for the visualisation of chemical shielding tensors (VIST), which provides a local representation of the shielding tensor along with the molecular structure. The method, thus, allows to probe local aromaticity along with the underlying anisotropy of the shielding. The method is described in the preprint “3D Visualisation of chemical shielding tensors to elucidate aromaticity and antiaromaticity” available on ChemRxiv.

Within the preprent we exemplify the main concepts in the benzene and phenanthrene molecules and continue by studying

the interplay of ground state antiaromaticity and Baird triplet state aromaticity in the potential singlet fission chromophore cyclobuta[l]phenanthrene,
local aromaticity in the neutral formally antiaromatic ground state along with global aromaticity in the doubly reduced state of paaracyclophanetetraene,
global aromaticity in the dication of [8]cycloparaphenylene
through-space aromaticity in a stacked norcorrole dimer.

The underlying code is scheduled to be released within the next version of the TheoDORE wavefunction analysis package.