Hyperconjugative Aromaticity-Based Circularly Polarized Luminescence Enhancement in Polyaurated Heterocycles

Hyperconjugative aromaticity (HA) frequently appears
in metalla-aromatics,
but its effect on photophysical properties remains unexplored to date.
Herein, we reveal two different HA scenarios in nearly isostructural
triaurated indolium and benzofuranylium compounds. The biased HAs
show a discernible effect on the spatial arrangement of metal atoms
and thus tailor metal parentage in frontier orbitals and the HOMO–LUMO
energy gap. Theoretical calculations and structural analyses demonstrate
that HA not only influences the degree of electron delocalization
over the trimetalated aromatic rings but also affects π-coordination
of Au(I) and intercluster aurophilic interaction. Consequently, the
triaurated benzofuranylium complex shows better photoluminescence
performance (quantum yield up to 49.7%) over the indolium analogue.
Furthermore, four pairs of axially chiral bibenzofuran-centered trinuclear
and hexanuclear gold clusters were purposefully synthesized to correlate
their HA-involved structures with the chiroptical response. The triaurated
benzofuranylium complexes exhibit strong circular dichroism (CD) response
in solution but CPL silence even in solid film. In contrast, the hexa-aurated
homologues display strong CD and intense CPL signals in both aggregated
state and solid film (luminescence anisotropy factor glum up to 10–3). Their amplified chiroptical
response is finally ascribed to the dominant intermolecular exciton
couplings of large assemblies formed through the HA-tailored aggregation
of hexanuclear compounds.