Preparation And Photoluminescent Properties Of Afterglow Materials Based On Microporous Crystals

Afterglow materials is a kind of materials which show long-persistent luminescence when the excitation source is stopped.Their unique luminescent properties endow them with a broad range of applications in photoelectric devices,illumination,anti-counterfeiting,precision detection and biological imaging fields.In recent years,organic-based room temperature phosphorescence?RTP?and thermally activated delayed florescence?TADF?materials have aroused enormous attention because of their high quantum efficiency,easy to synthesize and cheap precursors.However,their excited triplet excitons are easy to lose by means of the vibration and rotation of flexible carbon bonds,triplet-triplet quenching and intermolecular energy transfer in emitters.Therefore,the effective suppression of non-radiative transitions and stabilization of triplet excited states are the key to achieve the long-persistent luminescence.Carbon dots?CDs?,a new kind of luminescent nano-materials,show a wide range of applications in biological imaging,anti-counterfeiting and photoelectric devices aspects,because of their high quantum efficiency,resistance to photobleaching and good biocompatibility.Generally,at room temperature,CDs give strong fluorescent emission,but do not show phosphorescent emission.Recent studies find that CDs have strong RTP emission when confined into PVA,KAl?SO₄?₂,mesoporous materials,zeolites and metal-organic frameworks?MOFs?matrices by forming hydrogen bonds to confine the vibration and rotation of CDs and stabilize the triplet state,thus achieving long lifetime RTP emission.The advantages of this strategy lie in that the synthetic method is simple,the materials are cheap,the host and guest can be designed and synthesized as needed.Therefore,a variety of materials with long-persistent luminescence are expected to be synthesized by using this strategy.Metal-organic frameworks?MOFs?materials are a class of inorganic-organic hybrid coordinated networks formed by metal ions or metal clusters connected via organic ligands.In particular,MOFs can form covalence bonds between metal center and ligand to restrain the vibration and rotation of the organic luminescent ligand to achieve effective RTP emission.Comparing with the initial organic ligand,the MOFs can give rise to long-lived RTP emission by forming covalence bonds and crystallization effect.Also,MOFs present tunable phosphorescent properties?wavelength and decay lifetime?based on the alternation of metal ions and stacking fashion of the organic ligands.Therefore,this strategy provides an effective way to realize long lifetime and high performance RTP materials.In this thesis,according to the above two strategies,zeolites and MOFs are chosen as the research systems,and two kinds of long-persistent microporous crystals are designed and synthesized.The specific results are as follows:1.Utilizing the‘dots in zeolite'strategy,we develop CDs-based TADF materials with tunable emissions by confining two kinds of CDs into zeolites via one-step hydrothermal synthesis.At room temperature and atmosphere,the as-made CDs@zeolite materials exhibit tunable TADF emission colors from blue to green,and variable lifetimes from 200 ms to 450 ms.Their quantum yields vary from 20%to42%by changing the component ratio of different CDs in zeolites.Moreover,we find the existence of energy transfer between these two kinds of CDs in zeolites,which is the key point to realize color tunable TADF emission in the composites.Meanwhile,the as-synthesized CDs@zeolite composite exhibits temperature-responsive photoluminescence property.2.By using the strategy of stabilizing the organic luminescent ligand through building MOFs,a two-dimensional?2D?layered MOF with multi-emissive room temperature phosphorescence has been synthesized.The 2D MOF is constructed by benzenetricarboxylic acid?H₃BTC?as organic ligand and Zn/Na as metal centers,which can be stable for more than one year after the removal of mother liquid.Such2D MOF exhibits unique multi-emissive room temperature phosphorescence?RTP?behavior centered at 455 nm,482 nm and 525 nm with lifetimes of 92 ms,97 ms and351 ms,respectively.Also,it shows temperature-responsive photoluminescence performance.