C‑Shaped Diastereomers Containing Cofacial Thiophene-Substituted Quinoxaline Rings: Synthesis, Photophysical Properties, and X‑ray Crystallography
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Abstract
Synthesis
and characterization of two diastereomeric C-shaped molecules
containing cofacial thiophene-substituted quinoxaline rings are described.
A previously known bis-α-diketone was condensed with an excess
of 4-bromo-1,2-diaminobenzene in the presence of zinc acetate to give
a mixture of two C-shaped diastereomers with cofacial bromine-substituted
quinoxaline rings. After chromatographic separation, thiophene rings
were installed by a microwave-assisted Suzuki coupling reaction, resulting
in highly emissive diastereomeric compounds that were studied by UV–vis,
fluorescence, and NMR spectroscopy, as well as X-ray crystallography.
The unique symmetry of each diastereomer was confirmed by NMR spectroscopy.
NMR data indicated that the <i>syn</i> isomer has restricted
rotation about the bond connecting the thiophene and quinoxaline rings,
which was also observed in the solid state. The spectroscopic properties
of the C-shaped diastereomers were compared to a model compound containing
only a single thiophene-substituted quinoxaline ring. Ground state
intramolecular π–π interactions in solution were
detected by NMR and UV–vis spectroscopy. Red-shifted emission
bands, band broadening, and large Stokes shifts were observed, which
collectively suggest excited state π–π interactions
that produce excimer-like emissions, as well as a remarkable positive
emission solvatochromism, indicating charge-transfer character in
the excited state