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