Borazino-Doped Polyphenylenes

The divergent synthesis
of two series of borazino-doped polyphenylenes,
in which one or more aryl units are replaced by borazine rings, is
reported for the first time, taking advantage of the decarbonylative
[4 + 2] Diels–Alder cycloaddition reaction between ethynyl
and tetraphenylcyclopentadienone derivatives. Because of the possibility
of functionalizing the borazine core with different groups on the
aryl substituents at the N and B atoms of the borazino core, we have
prepared borazino-doped polyphenylenes featuring different doping
dosages and orientations. To achieve this, two molecular modules were
prepared: a core and a branching unit. Depending on the chemical natures
of the central aromatic module and the reactive group, each covalent
combination of the modules yields one exclusive doping pattern. By
means of this approach, three- and hexa-branched hybrid polyphenylenes
featuring controlled orientations and dosages of the doping B3N3 rings have been prepared. Detailed photophysical
investigations showed that as the doping dosage is increased, the
strong luminescent signal is progressively reduced. This suggests
that the presence of the B3N3 rings engages
additional deactivation pathways, possibly involving excited states
with an increasing charge-separated character that are restricted
in the full-carbon analogues. Notably, a strong effect of the orientational
doping on the fluorescence quantum yield was observed for those hybrid
polyphenylene structures featuring low doping dosages. Finally, we
showed that Cu-catalyzed 1,3-dipolar cycloaddition is also chemically
compatible with the BN core, further endorsing the inorganic benzene
as a versatile aromatic scaffold for engineering of molecular materials
with tailored and exploitable optoelectronic properties.