Synthesis And Fluorescence Sensing Properties Of Novel Functional Fluorene Compounds With Aggregation-induced Emission (AIE) Effect

Organic fluorescent materials have attracted great attention from scientists in the fields of chemistry,medicine and biotechnology,including fluorene,tetrastyrene,pyrene,perylene,biphenyl,triazole and pyrazine,etc.In recent years,the aggregation-induced emission（AIE）effect discovered by Tang Benzhong et al.is quite different from the traditional aggregation-caused quenching（ACQ）effect,since relevant materials could emit strong fluorescence in the state of aggregation,It has gained important applications in cell imaging,bioluminescence sensing,trace recognition,smart response materials,and polarized light materials have caused an international research interest.The more important organic fluorescent material,fluorene,is formed by two benzene rings connected by a five-membered ring,which has a stronger rigid plane than biphenyl.The overlap ofπelectron orbitals between the rings has the density of its electron cloud highly averaged.Therefore,fluorene has good fluorescence properties.Fluorene can react at the active 2 and 7 positions to produce a series of fluorene derivatives.Based on the unique conjugated structure and electron-rich properties of fluorene derivatives,it is of great significance to study their performance as fluorescent sensors.In this paper,five fluorene derivatives and two benzoic acid pinacol ester intermediates were synthesized by using mesitylene,fluorene,iodine and bisboronic acid pinacol ester as main raw materials.All the compounds were characterized by infrared spectroscopy,¹H NMR,¹³C NMR and elemental analysis.Some of the compounds were characterized by UV-vis spectroscopy,X-ray single crystal diffraction,X-ray powder diffraction（PXRD）,high resolution mass spectrometry,differential scanning calorimetry（DSC）.The1,3,5-tris（9,9-dimethylfluorene-2-yl）-2,4,6-trimethylbenzene（TFTB）and 1,3,5-tri（9,9-dimethylfluorene-2-yl）benzene（TFB）was optimized by semi-empirical PM3 method and its aggregation-induced emission mechanism was studied by density functional theory（DFT）calculation.The fluorescence sensing properties of nitroaromatics,metal cations and small biomolecules（amino acids）were studied.The specific work of this paper is as follows:1.Using mesitylene as the starting material,novel functional fluorene compounds was synthesized by iodination,boric acid esterification and Suzuki coupling reaction:1,3,5-tris（9,9-dimethylfluorene-2-yl）-2,4,6-trimethylbenzene and 1,3,5-tris（9,9-dimethyl-fluorene-2-yl）benzene.Concurrently,six intermediates were synthesized,among which1,3,5-tris（4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl）-2,4,6-trimethylbenzene and 1,3-bis-（4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl）-5-iodo-2,4,6-trimethylbenzene have not been reported in the literature.2.The relationship between the aggregation-induced emission characteristics of TFTB and TFB and its molecular structure was studied and analyzed.3.TFTB and TFB were used as fluorescence sensors for nitroaromatics,metal cations and twenty natural amino acids.The experimental results suggested that TFTB is suitable for2,4,6-trinitrophenol（TNP）and tryptophan（Trp）and has higher sensitivity.Its sensitivity（Ksv）to TNP is 37550.7 M^-1,the detection limit is 6.15μM;the sensitivity（Ksv）to Trp is 13138.2M^-1,and the detection limit is 1.64μM.TFB has high sensitivity to the detection of Fe³⁺,its sensitivity（Ksv）is 127572.6 M^-1,and the detection limit is 6.35μM.4.The fluorescence quenching mechanism between TFTB with TNP and Trp was analyzed by density functional theory（DFT）.