The singlet HOMO/LUMO transition (S2, 1La) is shown to be strongly aromatic whereas the triplet HOMO/LUMO transition (T1, 3La) is antiaromatic. Does this mean states reached by the same kind of orbital transition behave differently depending on their spin-multiplicity?
The aromatic S2 lies above the antiaromatic S1 even though S2 is the HOMO/LUMO transition. Does this mean that singlet antiaromaticity is actually a stabilising effect?
We have discussed the excited states of naphthalene from an entirely different viewpoint in a recent J. Chem. Theory Comput. article. It would be fascinating to combine the two viewpoints.
A recent study, lead by Florian Glöcklhofer from Imperial College London, explores the effect of methoxy and thiomethyl subtitutions on a formally antiaromatic macrocycle. The corresponding paper “[126.96.36.199]Paracyclophanetetraenes (PCTs): cyclic structural analogues of poly(p‑phenylene vinylene)s (PPVs)” is available via Open Research Europe, 1, 111, 2012.
The above figure compares the orbitals and aromaticity descriptors for different charge and spin states. Importantly, the symmetry is broken in the T1 state, inhibiting Baird aromaticity. By comparison, the symmetry is retained for the neutral singlet, dianion, and dication states all of which exhibit aromaticity.
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:
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]).
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 [188.8.131.52]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.
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.
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.