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 [220.127.116.11]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.
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,