The Synthesis And Applications Of The Imidazo[2,1-b]thiazole-6-carboxylic Acid Derivatives

In this paper,the heterocyclic compound,imidazo[2,1-b]thiazole-6-carboxylic acid,was designed and synthesized based on thiazol-2-amine and ethyl 3-bromo-2-oxopropanoate.Six derivatives of imidazo[2,1-b]thiazole-6-carboxylic acid were designed and synthesized by introducing different recognition groups,which were?E?-N'-?2-hydroxybenzylidene?imidazo[2,1-b]thiazole-6-carbohydrazide?X1?,?E?-N'-??2-hydroxynaphthalen-1-yl?methylene?imida-zo[2,1-b]thiazole-6-carbohydrazide?X2?,?E?-N'-?2,3-dihydroxybenzylidene?imidazo[2,1-b]t-hiazole-6-carbohydrazide?X3?,?E?-N'-?3-ethoxy-2-hydroxybenzylidene?imidazo[2,1-b]thiaz-ole-6-carbohydrazide?X4?,?E?-N'-?4-?diethylamino?-2-hydroxybenzylidene?imidazo[2,1-b]t-hiazole-6-carbohydrazide?X5?and?E?-N'-??8-hydroxy-2,3,6,7-tetrahydro-1H,5H-pyrido[3,2,1-ij]quinolin-9-yl?methylene?imidazo[2,1-b]thiazole-6-carbohydrazide?X6?,respectively.The structures of the above six derivatives were analyzed and characterized by means of ¹H NMR,¹³C NMR,FTIR and MS.All data results were basically consistent with the compound structure.Moreover,the optical properties of the above six derivatives were investigated by fluorescence emission spectrum and ultraviolet absorption spectrum.It was found that the above six compounds could be used as fluorescent probes to identify specific metal ions under different test systems,displaying good optical performance.X1 based on salicylaldehyde could be used as a sensor to identify Al³⁺through a significant fluorescence enhancement and to detect Cu²⁺through a highly efficient quenching behavior without interference from other metal ions in MeOH/H₂O?v/v=9/1,pH=5?TRIS buffer solution.The detection limits of X1 for Al³⁺and Cu²⁺were calculated to be 3.1×10^-10 M and9.8×10^-11 M,respectively.X2 based on 2-hydroxy-1-naphthaldehyde could be used as a sensor to detect Al³⁺in MeOH/H₂O?v/v=9/1,pH=7.4?TRIS buffer solution and to detect Zn²⁺in Et OH/H₂O?v/v=9/1,pH=7.4?TRIS buffer solution with detection limits of 1.45×10^-7 M and 1.29×10^-8 M,respectively.The fluorescence intensity of X2 was reversible even after six cycles with the sequentially alternative addition of metal ions(Al³⁺and Zn²⁺)and PPi,showing an efficient“off-on-off”fluorescence behavior.X3 based on 2,3-dihydroxy-benzaldehyde showed weak fluorescence intensity in DMF/H₂O?v/v=1/1,pH=7.4?TRIS buffer solution.X3 could be used as“turn-on”fluorescent probe for selective detection of Al³⁺.And the resultant complex X3[Al³⁺]could be used as a new multifunctional sensor to identify F^-by further enhancement of fluorescence intensity and detect PPi by fluorescence quenching.X4 based on 3-ethoxy-2-hydroxybenzaldehyde could be used to sequentially detect Zn²⁺and PPi in Et/H₂O?v/v=9/1,pH=7.4?TRIS buffer solution with lower detection limit of 10^-9 M.X4 could be used as sensor for Zn²⁺even in the presence of other competitive metal ions,while it was less effective in the presence of Co²⁺and Cu²⁺.X5 based on4-?diethylamino?-2-hydroxybenzaldehyde could be used as a sensor to detect In³⁺in DMF/H₂O?v/v=9/1,pH=7.4?TRIS buffer solution and detect Zn²⁺in EtOH/H₂O?v/v=9/1,pH=7.4?TRIS buffer solution through fluorescence enhancement with detection limits of1.02×10^-9 M and 5.5×10^-9 M,showing good selectivity and sensitivity.X5 still maintained a good response to In³⁺after two cycles with a negligible fluorescence loss.X6,which was based on 8-hydroxy-2,3,6,7-tetrahydro-1H,5H-pyrido[3,2,1-ij]quinoline-9-carbaldehyde,could sequen-tially identify Zn²⁺and PPi in acetonitrile/H₂O?v/v=9/1,pH=7.4?TRIS buffer solution through the presence or absence of fluorescent signals and could detect In³⁺in DMF/H₂O?v/v=9/1,pH=7.4?TRIS buffer solution with obvious fluorescence enhancement,displaying different fluorescence properties.All the stoichiometry between the fluorescent probes and metal ions was 1:1,which was proved by fluorescence titration curve and mass spectrometry.In order to verify the binding model and sensing mechanism,all the optimal structure and molecular orbitals of fluorescent probe in the absence and presence of metal ions were obtained by the Gaussian 09.