Tuning the Optical and Electronic Properties of 4,8-Disubstituted Benzobisoxazoles via Alkyne Substitution

In an effort to design new electron-deficient building
blocks for
the synthesis of conjugated materials, a series of new trans-benzobisoxazoles bearing halogen or alkynyl substituents at the 4,8-positions
was synthesized. Additionally, the impact of these modifications on
the optical and electronic properties was investigated. Theoretical
calculations predicted that the incorporation of various alkynes can
be used to tune the energy levels and band gaps of these small molecules.
The targeted 4,8-disubstituted benzobisoxazoles were easily prepared
in good yields using a two-step reaction sequence: Lewis acid catalyzed
orthoester cyclization followed by Sonogashira cross-coupling. The
experimentally determined HOMO values for these 4,8-disubstituted
benzobisoxazoles ranged from −4.97 to −6.20 eV and showed
reasonable correlation to the theoretically predicted values, with
a percent deviation that ranged from 2.4–12.8%. However, the
deviation between actual and predicted HOMO values was reduced to
less than 3.5% when the theoretical values were extrapolated to the
long-chain limit and compared to copolymers containing the 4,8-disubstituted
benzobisoxazoles. Collectively, these results indicate that these
4,8-disubstituted trans-benzobisoxazoles can be used
for the synthesis of new conjugated materials with electronic properties
that are variable and predictable.