Abstract
A new type of fluorogenic and fluorochromic probe based on the reduction of weakly fluorescent 4-azido-6-(4-cyanophenyl)cinnoline to the corresponding fluorescent cinnoline-4-amine was developed. We found that the fluorescence of 6-(4-cyanophenyl)cinnoline-4-amine is strongly affected by the nature of the solvent. The fluorogenic effect for the amine was detected in polar solvents with the strongest fluorescence increase in water. The environment-sensitive fluorogenic properties of cinnoline-4-amine in water were explained as a combination of two types of fluorescence mechanisms: aggregation-induced emission (AIE) and excited state intermolecular proton transfer (ESPT). The suitability of an azide–amine pair as a fluorogenic probe was tested using a HepG2 hepatic cancer cell line with detection by fluorescent microscopy, flow cytometry, and HPLC analysis of cells lysates. The results obtained confirm the possibility of the transformation of the azide to amine in cells and the potential applicability of the discovered fluorogenic and fluorochromic probe for different analytical and biological applications in aqueous medium.
Highlights
The chemistry of azides is a fast-growing research area
The synthesis of azide 5 was achieved by the synthetic protocol reported earlier [35] with a slight modification: the replacement of the chromatographic purification of 4-bromocinnoline 4 with recrystallization improved the yield of compound
This amine has improved fluorescent properties compared to the starting azide: a red shift of the emission was observed in solvents with differing polarities, while an increase in fluorescence intensity was found in polar solvents, with the strongest one in water
Summary
Creation of different materials [12,13] Fluorogenic probes are another important area for the application of organic azides [14,15,16]. This technique allows the conversion of non-fluorescent or weakly fluorescent materials to compounds with higher fluorescent ability. In order to obtain fluorescent material from a non-fluorescent azide probe, the azido group can be modified in two ways: the conversion of azides into a 1,2,3-triazole derivatives by CuAAC [15,18,19,20,21,22,23] or SPAAC [23,24] (Figure 1A) or the reduction of azides to amines [23,24,25,26,27,28,29] (Figure 1B)
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