A Study of Boratriazaroles: An Underdeveloped Class of Heterocycles

Boratriazaroles
were discovered in the late 1960s, and since then,
a variety of substituted boratriazarole derivatives have been prepared.
However, no study has compared the properties of these BN heterocycles
with their carbon-based analogues. In this work, we have prepared
a series of boratriazarole derivatives and have investigated how structural
variations in the five-member heterocycle affect photophysical and
electronic properties. Boratriazaroles exhibit absorption and emission
spectra comparable to those of their azacycle analogues but have a
markedly lower quantum yield. The quantum yield can be increased with
the incorporation of a 2-pyridyl substitution on the boratriazaroles,
and the structural and optoelectronic properties are further influenced
by the nature of the B-aryl substituent. Introducing an electron-deficient p-cyano group on the B-phenyl substituent creates a twisted
intramolecular charge transfer state that causes a large Stokes shift
and positive solvatochromism. Our work should serve to guide future
synthetic efforts toward the application of boratriazaroles in materials
science.