Chemistry - A European Journal
10.1002/chem.202003248
COMMUNICATION
of Scheme 1 suggested that the diamine 5 could form DNK
adducts in situ with boronic acids and act as a fluorogenic
chemosensor. The strong fluorescence of DNK 3, along with the
observation that phenyl ring substitution does not exert a
significant influence on fluorescence in solution, indicate that this
trace quantitation strategy could be both highly sensitive and
robust to a wide variety of boronic acid intermediates.
In summary, we have described the synthesis, structure,
photophysical properties, and applications of a new luminescent
polyaromatic boronic scaffold (diazaboryl-naphthyl-ketones, or
DNKs). These compounds differ from BDAN
4
and
benzodiazaboroles in that they are capable of extremely bright
fluorescence, yet small structural modification unlocks
dramatically and qualitatively different photophysical properties
like aggregation-induced emission, positive solvatochromism,
and solid state fluorescence. Photophysical data in solution and
solid state revealed blue-shifted, narrowed, and intense
emissions for DNKs 1-3 (up to 89% quantum yield) much
superior to 4 and diamine 5. DFT calculations of molecular
orbitals revealed that unlike PhBDAN (4), the LUMO of 1-3 is
localized onto the DNK unit, turning it into an independent
chromophore, which supports the experimental observation that
substitution on the B-phenyl group does not alter luminescence.
The DNKs’ advantageous properties were demonstrated with a
preliminary analytical method for the low-ppm quantitation of
trace boronic acid contaminants in pharmaceutical intermediates.
To eliminate background noise from the intrinsic fluorescence of
diamine 5, HPLC analysis was chosen for peak separation and
fluorescence detection. To this end, DNK 3 demonstrated good
stability to a variety of HPLC conditions, including acidic eluent.
Next, samples were prepared and reaction conditions were
designed to model quantitation of trace boronic acid starting
material in a pharmaceutical intermediate arising from a typical
Suzuki-Miyaura reaction. Thus, a series of standards were
prepared, each as a solid “cake” of trace phenylboronic acid in a
matrix of biphenyl with a range of 1–20 ppm concentrations. To
ensure quantitative conversion to DNK 3 in reasonable time, a
large excess of diamine 5 in a concentrated solution were found
to be essential. Acetonitrile was chosen as the solvent with
gentle heating (Figure 6A).
Acknowledgements
A
COCH3
COCH3
This work was funded by the Natural Sciences and Engineering
Research Council (NSERC) of Canada (Discovery Grant to
D.G.H.) and Alberta Innovates (Scholarship to HH). We thank Dr.
Michael J. Ferguson and Dr. Yuqiao Zhou (X-ray
Crystallography Laboratory, University of Alberta) for X-ray
crystallographic analysis of compounds 1 and 2, and, for advice
and support with photophysical measurements: Prof. Rik
Tykwinski, Prof. E. Rivard, Dr. Sarah Parke, and Wayne Moffatt.
PhB(OH)2
+
(1-20 ppm)
HN
NH
CH3CN (0.1 mL)
40 °C, 3 h
B
H2N
NH2
in biphenyl
Ph
5
(1.0 mg)
3
fluorescence peak integration by HPLC
B.
Keywords: AIEgen • boronic acids • diazaborines •
luminescence • trace analysis
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2
accurate (R ~ 0.99) and linear down to the lowest 1 ppm
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3
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2
5
other reagents, diamine 5 provides much higher quantum
yields and was demonstrated on a crude solid sample.
5
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