Synthesis of 1,4-dihydropyridine derivatives as novel fluorescent sensors

Abstract

This research involves the synthetic preparation of a series of 1,4-dihydropyridine (DHP) by varying substituent of nitrogen atom. The DHP triester (1) can be easily prepared from the cyclotrimerization of β-amino acrylate by treatment of TiCl4 at room temperature. Hydrolysis of 1 was carried out using KOH to convert triester to tricarboxylic acid (2), which readily soluble in aqueous media. The photophysical properties of these DHP derivatives were studied in pH 8.0 phosphate buffer solution. The absorption spectra displayed absorption maximum in the range of 280 to 300 and 347 to 358 nm. All of these compounds exhibited an emission peak in a range of 432-445 nm with fluorescence quantum efficiencies (f) of 0.07-0.23. Since methoxyphenyl N-substituted DHP showed the highest blue emission appearance also observable by naked-eye, it was therefore selected for further investigation in fluorescence sensing applications. This derivative was found to selectively interact with Hg2+ and the fluorescent signal decrease was assumed to be the result of an oxidation of the DHP into a pyridinium ring specifically induced by Hg2+ that brought about its remarkable selectivity over other metal ions. The decrease of fluorescence signal was proportional to Hg2+ concentration with high quenching efficiency (Ksv = 78,300 M-1) providing a detection limit of 0.2 µM. According to the results, of the use of tricarboxylic acid 2 Fe2+ and Fe3+ detection with protein BSA, as the sensitivity of BSA to Fe2+ and Fe3+ was increased two-fold-higher in the presence of 2. This tricarboxylic acid 2 was also tested with various types of surfactant. Its fluorescent signal was enhanced selectively by SDBS, and the CMC of SDBS could be determined by this method as 1.4 mM (literature CMC values is 1.2 mM). Additionally, the different fluorescence behavior of compound 2 in THF by nitroaromatic explosive compounds, such as trinitrotoluene, 4-nitrotoluene and 2,4-nitrotoluene might be useful for the explosive detection.