Here
we describe the synthesis and spectroscopic and structural
characterization of various borazine-doped polyphenylenes displaying
high doping dosages (16–18%). Capitalizing on the condensation
reaction approach, the desired products were formed using a mixture
of
p
-phenylendiamine and aniline with BCl
3
, followed by the addition of an aryl lithium derivative. The use
of mesityl lithium (MesLi) yields strained multiborazine derivatives,
which proved to be unstable in the presence of moisture. However,
when xylyl lithium (XylLi) was used, chemically stable multiborazines
were obtained, with oligomers showing molecular weight up to 10
4
, corresponding to 16–18 monomer units. While the dimer,
trimer, and tetramer could be isolated as pure products and their
structure characterized by mass and NMR analysis, higher oligomers
could only be isolated as mixtures of
B
-hydroxy-substituted
derivatives and characterized by gel permeation chromatography. The
structures of the dimer and trimer derivatives were confirmed by X-ray
analysis, which nicely showed the presence of the two and three borazine
rings spaced by one and two 1,4-aryl bridges, respectively. Notably,
the trimer forms a porous crystalline clathrate. The peripheral xylyl
and phenyl moieties of each molecule intramolecularly embrace each
other through C–H and π–π stacking interactions.
Steady-state UV–vis absorption characterization suggested that
the molecules are UV absorbers, with the extinction coefficient linearly
scaling with the degree of oligomerization. On the other hand, low-emission
quantum yields were obtained for all derivatives (<7%), suggesting
that high BN-doping dosages dramatically affect the emission properties
of the doped polyphenylenes.