The Synthesis And Properties Of The Imidazo[1,2-a]pyridine-2-carboxylic Acid Derivatives

Fluorescence sensor has been widely used in the detection of a specific substance in environment quantificationally for its highly sensitivity and selectivity, simplicity of operator and instantaneity.In this paper, six imidazo[1,2-a]pyridine-2-carboxylic acid derivatives had been synthesized with pyridin-2-amine and ethanol-2-oxopropanoate as the raw material. They are N'-?quinolin-2-ylmethylidene?imidazo[1,2-a]pyridine-2-carbohydrazide?L1?, N'-?pyridin-2-yl methylidene?imidazo[1,2-a]pyridine-2-carbohydrazide?L2?, N'-[?2-hydroxyphenyl?methylidene]imidazo[1,2-a]pyridine-2-carbohydrazide?S1?, N'-[?2-hydroxynaphthalen-1-yl?methylidene]imidazo[1,2-a]pyridine-2-carbohydrazide?S2?, N'-[?2-hydroxyquinolin-3-yl?methylidene]imidazo[1,2-a]pyridine-2-carbohydrazide?S3? and N'-[?2,3-dihydroxyphenyl?methylidene]imidazo[1,2-a]pyridine-2-carbohydrazide?S4?. All the compounds were characterized by the1 H NMR, 13 C NMR, FTIR and MS analysis methods. The derivatives could be used as fluorescence sensor for the detection of metal ions according to the analysis of UV-vis spectral and fluorescence spectral.L1 with the quinoline as the receptor could sensitively detect Zn²⁺ by fluorescence method in C₂H₅OH:H₂O?9:1,v:v? HEPES buffer solution. The limit of detection was calculated to be 10-8 M. L1 exhibited a remarkable fluorescence enhancement after bonding with Zn²⁺ and at the same time reduced a change in UV-vis spectral. The absorption band shifted from 325 nm to 375 nm. The 1:1 stiochiometry between L1 and Zn²⁺ was determined by the Job's plot.S1 could act as a Al³⁺ fluorescence sensor in DMSO:H₂O?9:1,v:v? HEPES buffer solution by the effect of deprotonation with the salicylaldehyde as the receptor. The detection limit for Al³⁺ was 10-11 M and other metal ions almost had no influence on the fluorescence of S1-Al³⁺. The stiochiometry between S1 and Al³⁺ was calculated by the DFT. In addition, S1 also had a fluorescence enhancement in the alkaline environment at the p H range of 11 to 13 and the p Ka of S1 was calculated to be 12.15. As a result, S1 could be a good p H sensor and the change in structure of S1 in the alkaline condition was confirmed by the 1H NMR titration experiments.The compound S2 with 2-hydroxynaphthalene as the receptor had a fluorescence response for Zn²⁺ and Al³⁺ in C₂H₅OH:H₂O?9:1,v:v? HEPES buffer solution. The system of S2-Zn²⁺ emission green fluorescence excited by a certain wavelength UV light and the max emission wavelength was 493 nm. By contrast, the system of S2-Al³⁺ emitted blue fluorescence and the max emission wavelength was 480 nm. The two ions could be distinguished by the color of fluorescence and the emission wavelength. What's more, after the addition of Zn²⁺, the solution of the system had a color change from colorless to yellow and the absorption band in UV-vis spectral had a remarkable red shift. The limit of detection for Zn²⁺ and Al³⁺ were 5.6×10-7 M and 3.2×10^-9 M.S3 could also detect Zn²⁺ in C₂H₅OH:H₂O?9:1,v:v? HEPES buffer solution by the fluorescence method and its receptor was 2-hydroxyquinoline. The limit of detection for Zn²⁺was 10^-7 M. However the fluorescence intensity of S3-Zn²⁺ could be interfered by other metal ions like Cu²⁺, Co²⁺ and Ni²⁺.All the optimum structure and molecular orbitals of L1, S1, S2 and S3 before and after binding with ions was calculated by DFT/TDDFT. The detection for L1 towards Zn²⁺ was based on the ICT process and the detection for S1, S2 and S3 towards ions was due to the deprotonation inhibiting the PET process.