Synthesis,luminescence And AIE Mechanism Of A Totally Planar AIE Molecule

In 2001,B.Z.Tang discovered the Aggregation-induced Emission(AIE)effect for the first time.Compounds with AIE effects show no fluorescence or very weak fluorescence in solution but strong fluorescence in solid or aggregated states.Their potential applications in chemical sensing and stimulus-response materials have attracted wide attention.AIE molecules have a twisted non-planar structure with multiple substituents.In solution,the excitation energy is lost through the rotation of the substituent.In the aggregated state,the excitation energy can be converted into light energy due to the blockage of the rotation of substituent,and the molecule with non-planar structure will not quench the fluorescence due to the tight pi-pi stacking.Thus,strong fluorescence can be emitted in the aggregated state.This is the widely accepted AIE mechanism named restriction of intramolecular motions(RIM).Therefore,it is generally believed that AIE molecules must have a distorted and non-planar structure.However,this work found that even completely planar organic molecules can exhibit the AIE activities.Since many organic luminogens have a planar structure,this discovery will effectively expand the scope of AIE molecules,enrich the content of AIE mechanism and promote the development of AIE research.In this work,1,2,4,5-tetrahydroxybenzene and tetrafluoroterephthalonitrile were subjected to a nucleophilic substitution reaction to obtain A-D-A triplet compound at a yield of 88%which is formed by five six-membered rings connected in parallel,containing two tetrafluoroterephthalonitriles and one 1,2,4,5-tetrahydroxybenzene.After optimizing the reaction conditions,the crude product does not require column chromatography and can be purified by simple recrystallization,and can be prepared on a large scale.The single crystal structure indicates that this triplet compound has a completely planar structure.However,the triplet is almost no fluorescence in water-soluble and highly polar solvents such as DMF,THF,and acetone.After adding water,turbidity was generated and the fluorescence was greatly enhanced,indicating that it is a typical AIE molecule.A series of photophysical performance tests and further investigation of the crystal structure revealed that this A-D-A triplet molecule is easily solvated by polar solvent molecules due to dipole interaction,so that the excitation energy is transferred to the solvent and lost.Thus,there are almost no fluorescence in polar solvents.In the solid state,the accumulation of triplets will exclude solvent molecules,and due to the repulsion of the lone electron pair on the four oxygen atoms on the two rings of the triplet molecule with the oxygen atom lone electron pair on other molecules,the triplet molecule cannot perform the tight pi-pi stacking but has a very large sliding pi-pi stacking with the large stagger of molecular planes.Thus,it can emit strong fluorescence after aggregation.In addition,it was found that the fluorescence intensity of the A-D-A triplet compound in 1,2-dichloroethane increases with increasing temperature,which is contrary to the general rule that the fluorescence intensity decreases with increasing temperature.It mainly due to the decreasing polarity of dichloroethane with the increasing temperature and reducing the solvation of triplets to causing the enhancement of fluorescence.In the range of 0-80~°C,the fluorescence intensity shows a very good linear relationship with temperature changes,which allows the potential application of the triplet as fluorescence thermometer.Meanwhile,the triplet contains a tetraoxobenzene-rich electronic unit and can interact with nitroaromatic hydrocarbon electron-deficient molecules,especially its fluorescence is more easily quenched by dinitrophenol and trinitrophenol and its detection limit for the analysis of 2,4,6-trinitrophenol can be as low as 0.168 nM.