Understanding Aromaticity in [5]Helicene-Bridged Cyclophanes: A Comprehensive Study

This study explores the aromaticity of doubly [5]­helicene-bridged
(1,4)­cyclophane and triply [5]­helicene-bridged (1,3,5)­cyclophane via
calculations of the magnetic response and of electronic aromaticity
indices. The primary objective is to assess the π-electron delocalization
to determine whether they sustain global ring currents associated
with π aromaticity. The molecules show local ring currents in
the presence of an external magnetic field. The ring currents flow
diatropically in the stacked six-membered rings and in the helicene
arms. However, these π currents are not interconnected due to
the discontinuity of the π delocalization at the C–C
single bonds connecting the central six-membered rings to the helicene
arms. Electronic indices suggest that the helicene-arm systems have
significantly smaller electron delocalization than benzene. The reduction
in the delocalization does not compromise their ability to exhibit
ring currents in the presence of an external magnetic field. The analysis
provides further evidence that the magnetic criteria yield a different
degree of aromaticity for the helicene arms than obtained in the calculation
of the electronic aromaticity indices. However, both approaches confirm
that the studied molecules are not globally aromatic.