In Situ Synthesis,Formation And Luminescence Mechanism Of CDs/Phthalamide Room Temperature Phosphorescent Materials

Room temperature phosphorescence(RTP)materials,a class of materials with long afterglow,have gained extensive attention owing to their wide applications in bioimaging,data encryption,optoelectronic devices,and other fields.At present,RTP materials mainly include inorganic compounds with rare earth elements,transition metal complexes,and organic compounds.These materials usually have high luminous efficiency and long lifetimes.However,the current organic RTP materials present high cost,non-renewable resources,strong toxicity,complex preparation process,low yield,and poor stability,which significantly limit their potential applications.Therefore,it is of great significance to find an environment-friendly RTP material with excellent RTP properties.In the past decade,photoluminescent carbon dots(CDs)have triggered tremendous attention and become a good candidate for novel RTP materials because of their small size,low toxicity,high photostability,adjustable luminescence,and mild synthesis conditions.Generally,CDs do not exhibit phosphorescence owing to their unstable triplet excited states.Great effort has been devoted to stabilizing triplet excited states by embedding CDs into some matrix.However,most of these methods need to synthesize CDs or matrix first and then compound CDs with the matrix to achieve RTP materials by chemical or physical means,which makes their preparation complex.In addition,compared with inorganic crystals,organic crystals are more likely to produce hydrogen bonds with the surface functional groups in CDs,which is more conducive to stabilize the luminescence center and produce phosphorescence.In view of the above shortcomings,this work puts forward a novel strategy,that is,CDs@organic crystal composites are in-situ synthesised by one step method.The formation and phosphorescence mechanism of CDs@organic crystal composites are investigated.The main content of this thesis is concluded as follows:1.Preparation of RTP materials and their applications in white light-emitting diodes(LED)and information securityWith phthalic acid as carbon source,formamide as nitrogen source,and ethylene glycol as solvent,CDs@Phthalamide were prepared by one-step solvothermal method.Then the RTP materials of CDs embedded in organic crystal were obtained by cooling and adding poor solvent.Through orthogonal experiment and further reaction optimization,it was found that the RTP CDs obtained at 220℃ for 4 h exhibit the strongest phosphorescence emission.Under these conditions,as-prepared RTP materials not only exhibit an excitation-independent phosphorescence emission peaking at 497 nm but also an ultralong RTP lifetime of 410 ms(more than 5 s as observed by the naked eye).The RTP CDs have fluorescence-phosphorescence double emission ability and the phosphorescent quantum yield was 8.25%.2.Formation mechanism,phosphorescence mechanism,and applications of CDs@PhthalamideFirst,the formation mechanism of CDs was inferred by combining optical properties and structural characterization.CDs are mainly formed by carbonization of phthalic acid and formamide under high temperature and high pressure.Phthalamide crystal is formed by the dehydration of phthalic acid to form phthalic anhydride,which then reacts with ammonia from the decomposition of formamide.This conclusion was confirmed by calculation and experiment.Combining recrystallization experiments with optical and structural characterization,it is considered that CDs are the source of luminescence and C=O is the luminescence center of RTP CDs.CDs can be used as the nucleation sites of phthalamide crystal,and eventually the structure of CDs embedded in the phthalamide is formed.Phthalamide crystal,as a rigid structure,can inhibit the vibration of luminescence center and non-radiative transition in CDs.N element can reduce the band gap between triplet excited states and singlet excited states,which promotes intersystem crossing.The RTP material was used as a phosphor for white LED with wide emission peak and good thermal stability.In addition,because of RTP nature,it can also be used for information safety.