Figure 1 Fluorescence photographs and spectra of the generated
organic difluoroboron complexes. a Fluorescence
photographs of solid powder of representative
difluoroboron complexes under UV
light irradiation (365 nm). b Fluorescence photographs
of representative complexes in DCM (2×10-5 M) under UV
light irradiation (365 nm). c Absorption
(2×10-5 M) and emission (10-7 M)
spectra of 3aa in different solvents.
Next, the absorption and emission spectra of these products in
dichloromethane were investigated sufficiently, the absorption and
emission maxima of these solution vary from 403 nm to 504 nm, and 463 nm
to 598 nm with strong substituent-dependency (Table S1). The Stokes
shift is a fatal parameter for fluorophorre associated closely with
their applicational potential. Fluorophores with large Stokes shifts,
which possess few spectral overlaps between the absorption and the
emission, are desirable for bio-imaging and bio-Sensing due to their
higher sensitivity by eliminating of self-absorption. Herein, the
largest Stokes shifts of these synthesized complexes was evaluated to
reach up to 184 nm (3an ), which provide promising potential
fluorescent probe for bio-sensing. The extraordinary Stokes shifts of3an may be attributed to its high degree of charge separation
caused by the electron donating, and accepting effect between the
electron-deficient difluoroboron motif and the electron-rich naphthalene
group. Since the solid state has a significantly difference luminescence
performance from the solution state influenced by the aggregation state
of the molecules, 3ak and 3ba were selected for the
solid-state fluorescence test, and the emission maxima of these
complexes vary from 580 nm to 600 nm, and the quantum yield of3ba is as high as 61.3% (Table S2). These results elucidate
the dual-phase emission properties of the complexes. Moreover, to
explore the application of the compounds in cell bio-imaging,3ad was selected based on the structure of the complex for
lysosome-targeting experiments (for details, see Supplementary Materials
Figure S1).
Conclusions
We have developed a straightforward and sustainable synthetic
methodology to construct N,O-Bidentate difluoroboron complexes from
quinoxalin-2(1H)-ones and ketones without transition-metal-catalyst.
This approach benefits from easily available starting materials,
excellent step and atom economy, and good functional group
compatibility. In addition, most of these complexes have broad and
intense absorption and emission bands, and display bright and intensive
fluorescence in dual-phase. The results of colocalization experiments of
the selected product also demonstrated the specificity for lysosomes
targeting, indicating potential applications for cell tracking related
to lysosomes function. Ongoing research including further mechanistic
details and applicational exploration are currently underway.
Experimental
Reaction condition A :
Quinoxalin-2(1H)-ones 1 (0.2 mmol, 1.0 equiv), methyl ketone2 (0.4 mmol 2 equiv),
Cu(BF4)2·6H2O (0.5
equiv), HBF4 (2.0 equiv) and DCE (3.0 mL) were added to
a 35.0 mL sealed tube. The reaction mixture was heated to 110oC for 48 hours.
When the reaction was finished, 20 mL saturated ammonium chloride
solution was added and the residue was extracted with DCM (3×5.0 mL).
The pure product was obtained by
flash chromatography on Aluminum oxide using petroleum ether and
Dichloromethane as the eluent (PE/DCM = 5:1 to 1:3).
Reaction condition B : Quinoxalin-2(1H)-ones 1 (0.2
mmol, 1.0 equiv), methyl ketones 2 (0.4 mmol 2 equiv),
HBF4 (2.0 equiv) and DCE (3.0 mL) were added to a 35.0
mL sealed tube. The reaction mixture was heated to 80oC for 48 hours. When the reaction was finished, 20 mL
saturated ammonium chloride solution was added and the residue was
extracted with DCM (3×5.0 mL). The pure product was obtained by flash
chromatography on Aluminum oxide using petroleum ether and
Dichloromethane as the eluent (PE/DCM = 5:1 to 1:3).
Supporting Information
The supporting information for this article is available on the WWW
under https://doi.org/10.1002/cjoc.2023xxxxx.
Acknowledgement (optional)
The author thanks the generous financial support from the Scientifc and
Technological Innovation Project of China Academy of Chinese Medical
Sciences CI2021A05102, the National Natural Science Foundation of China
(21702235, 82141001), the Fundamental Research Funds for the Central
Public Welfare Research Institutes (ZZ13-YQ-098, ZZ14-FL-010,
ZZ15-ND-10).
References