Synthesis And Application Of Squaraine-Based Optical Chemosensors

Squaraines are important functional fluorescent dyes and have extensive applications in solar cells,fluorescent probes,photodynamic therapy,and nonlinear optics.Squaraines are very suitable for the design of optical chemosensors due to the excellent and charming optical properties,such as large molar absorption coefficients,strong fluorescence and attractive emission in near-infrared region.In this paper,the development of synthesis and application of squaraine-based optical chemosensors were reviewed.Base on the literatures and reports,several new squaraine-based optical chemosensors were designed and developed to detect analytes such as CO₂ gas,Al³⁺ and so on.And the plausible response mechanisms of these new synthetic optical chemosensors were proposed by combining theoretical calculations and experimental tests.The main results were as follow:Firstly,an unsymmetrical squaraine dye incorporating vinylamine moiety,SQ-NH₂,was synthetized.This simple and novel squaraine-based system was demonstrated to fluorescently and colorimetrically sense CO₂ gas with high sensitivity in DMSO in the presence of fluoride ions?added as the tetrabutylammonium salt,TBAF?.From results,large bathochromic shifts were observed both in the absorption spectra and fluorescence spectra,and the color change of solution could be fluorescently and colorimetrically observed clearly by the naked eye when the deprotonated SQ-NH₂ with fluoride ions responsed to carbon dioxide gas.Combining 1H NMR titration tests and density functional theory?DFT?calculations,a plausible sensing mechanism that the deprotonated SQ-NH₂ with fluoride ions would rapidly restore with the aid of water molecules on condition that acidic carbon dioxide gas was bubbled in DMSO was proposed and further revealed.Moreover,theoretical studies agreed with experimental data well.Secondly,a cationic squaraine-based chemosensor SQM had been synthesized,and its sensing behavior towards nucleophiles followed by CO₂ gas was described in detail by UV-Vis,ESI-MS and 1H NMR spectral analysis.The results indicated that there were two different paths for the bleaching of SQM depending on whether water existed or not when the nucleophilic addition reaction between SQM and superbase such as TBD or DBU occurred.More interestingly,the bleached SQM in MeCN-H₂O?V : V = 90 : 10?could be colorimetrically restored when bubbling CO₂ gas,which enabled it to act as a highly sensitive “naked-eye” CO₂ gas detector in aqueous medium,whereas that in MeCN,treated in the same way,still remained unchanged,unexpectedly.Combining theoretical analyses and experimental tests,a plausible sensing mechanism was proposed to illustrate the bleaching process of SQM and the response of the bleached SQM to CO₂ gas in aqueous medium.Finally,a novel multi-response squaraine-based fluorescent chemosensor SQP was designed and synthesized,which exhibited a high sensitivity for detecting Al³⁺,Zn²⁺ and Cd²⁺ in ethanol-water?9 : 1?buffer solution?10 mM HEPES,pH = 7.0?and selectively responded to them in an order of Al³⁺ to Zn²⁺ to Cd²⁺ due to the differences of stability and binding modes when forming their respective metal-ligand complexes.Furthermore,in solution,SQP can restore A?1-42-Al complex involving the potential relationships with the conformational aspects of Alzheimer's disease?AD?to A?1-42 efficiently by binding Al³⁺ in A?1-42-Al complex,suggesting SQP may act as a protective agent by preventing or reverting A? aggregation in the extracellular spaces,which can provide a better biological understanding for developing successful therapeutics for the treatment of AD.The binding properties of sensor SQP with metal ions were investigated by UV-Vis,fluorescence,1H NMR and mass spectra.The results of various experiments and density functional theory?DFT?calculations showed the chemosensor SQP followed a turn on mechanism with the inhibition of C=N isomerization?cis-trans?and the activation of chelation enhanced fluorescence?CHEF?.