Syntheses And Photoluminescent Properties Of Novel Carbon Dots@Inorganic Microporous Materials

Afterglow materials,which can glow for an appreciable time after the removal of the excitation source,have aroused considerable interests in the field of illumination,security,bioimaging,etc.Recently,afterglow materials based on organic materials,such as phosphorescence or delayed fluorescence materials,have attracted much attention because of their low cost,environmental friendly,and high quantum efficiency.However,the excited state of organic molecules is usually active,where triplet excitons are easily deactivated by nonradiative decay processes derived from the vibration and rotation of emitters and host molecules,energy transfer to other molecules,or triplet-triplet annihilation of emitters.The lifetimes of phosphorescence or delayed fluorescence for the organic molecules are usually in the range of several microseconds to several milliseconds.Effective suppression of the nonradiative decay processes is crucial to achieve the long-lived excited states for long afterglow materials.In recent years,carbon dots(CDs)as new emerging luminescent nanomaterials have received extensive attraction because of their low toxicity,biocompatibility,photostability,and optoelectronic and photocatalytic properties.Notably,a new kind of room temperature phosphorescent material has been prepared by incorporating CDs into the solid matrices.Finding a suitable host matrix with a strong ability to harvest triplet excited states and generating a new class of CD-based materials with unique afterglow properties at ambient conditions would be of great interest.Inorganic microporous crystalline materials,especially zeolites with well-confined spaces,high thermal and chemical stability,are ideal candidates to incorporate functional guests.It is expected that the ordered nanoporous matrices can restrict the intramolecular vibrations and rotations of the functional groups on CDs,thus stabilizing the triplet excited states.Meanwhile,the hydrothermal or solvothermal synthetic conditions for zeolites are suitable for CDs,in which the organic amines and solvents can provide the raw materials for the formation of CDs.Therefore,the in situ incorporation of CDs into zeolite crystals under hydrothermal or solvothermal conditions to produce CDs@zeolite composites is highly possible.In this thesis,we report a “dots-in-zeolites” strategy to in situ confine CDs in zeolitic matrices during hydrothermal/solvothermal crystallization.The nanoconfined space of zeolites can efficiently stabilize the triplet states of CDs,thus enabling the reverse intersystem crossing process and generating high-efficient thermally activated delayed fluorescence(TADF)materials with ultralong lifetimes.Meanwhile,two long-afterglow CDs@zeolites composites with different room temperature phosphorescence(RTP)and TADF properties are synthesized by using different organic structure directing agents during the solvothermal crystallization process.The resultant composites are with the same inorganic matrix of SBT zeolite topology but different structures of carbon dots,which significantly influences the energy gap between the S1 and T1 excited state levels(?EST)of luminescent carbon dots and results in different RTP and TADF phenomenon.In addition,we present a novel germanate compound with in situ embedded CDs prepared in the solvothermal system.The as-synthesized composite exhibits excitation-dependent photoluminescence and the temperature-responsive photoluminescent behavior.The main research results of this thesis are as follows:1.We report a facile and general “dots-in-zeolites” strategy to in situ confine CDs in zeolitic matrices during hydrothermal/solvothermal crystallization to generate high-efficient TADF materials(CDs@Al PO-5,CDs@Mg APO-5,and CDs@2D-Al PO composites)with ultralong lifetimes.The resultant CDs@zeolite composites exhibit high quantum yields up to 52.14% and ultralong lifetimes up to 350 ms at ambient temperature and atmosphere.This intriguing TADF phenomenon is due to the fact that nanoconfined space of zeolites can efficiently stabilize the triplet states of CDs,thus enabling the reverse intersystem crossing process for TADF.Meanwhile,zeolite frameworks can also hinder oxygen quenching to present TADF behavior at air atmosphere.2.Two long-afterglow CDs@zeolites composites with different RTP and TADF properties at ambient temperature and atmosphere have been prepared by using 4-(2-Aminoethyl)morpholine and 4,7,10-trioxa-1,13-tridecanediamine as the organic structure directing agents with the same inorganic matrix of SBT zeolite topology(denoted as CDs@SBT-1,CDs@SBT-2,respectively).The introduction of different structure directing agents results in different structures of carbon dots,which significantly differ in the ?EST of luminescent carbon dots with 0.36 e V and 0.18 e V,respectively.With the efficient stabilization of the triplet states of carbon dots by nanoconfined space of zeolites,CDs@SBT-1 with the larger ?EST exhibits predominant RTP with the lifetime of 574 ms,while CDs@SBT-2 with the smaller ?EST exhibits TADF with the lifetime of 153 ms by the efficient reverse intersystem crossing process.3.A new germanate |H2(C4N3H13)|3[Ge7O14.5F2][Ge7O14F3]·2.5H2O(denoted as JLG-16)has been synthesized by using diethylenetriamine as the structure-directing agent under solvothermal conditions.Single-crystal structural analysis reveals that JLG-16 crystallizes in the monoclinic space group C2/c with a = 38.2008(15)?,b = 8.8262(4)?,c = 31.1789(13)?,and ? = 108.5470(10)°.Its structure is built up from 4-and 5-coordinated Ge7 clusters.The alternating connection of 4-and 5-coordinated Ge7 clusters gives rise to a double-layered structure with 16-and 10-ring channels with a low framework density(11.2 Ge/1000 ?3).CDs formed in the mother liquid are in situ embedded in the JLG-16 crystals during the solvothermal crystallization process.The resulting CDs@JLG-16 composite thus exhibits excitation-dependent photoluminescent and temperature-responsive photoluminescent performances,which makes it possible to be used in optical temperature sensing.This “dots-in-zeolites” strategy introduces a new perspective to develop materials with unique long afterglow performance and photoluminescent properties.The considerable flexibility of rational combination of photoluminescent nanodots with different raw materials and suitable host matrices with well-confined nanospaces may facilitate the design of a variety of efficient photoluminescent and long afterglow composites for advanced optoelectronic device and bioimaing applications.