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1State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin Uni-versity, 2699 Qianjin Street, Changchun 130012, China. 2International Center ofFuture Science, Jilin University, Changchun 130012, China.*Corresponding author. Email: jihong@jlu.edu.cn (J.Y.); lijiyang@jlu.edu.cn (J.L.)

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Carbon dots in zeolites: A new class of thermallyactivated delayed fluorescence materialswith ultralong lifetimesJiancong Liu,1 Ning Wang,1 Yue Yu,1 Yan Yan,1 Hongyue Zhang,1 Jiyang Li,1* Jihong Yu1,2*

Thermally activated delayed fluorescence (TADF) materials are inspiring intensive research in optoelectronic applica-tions. To date,most of the TADFmaterials are limited tometal-organic complexes andorganicmoleculeswith lifetimesof several microseconds/milliseconds that are sensitive to oxygen. We report a facial and general “dots-in-zeolites”strategy to in situ confine carbon dots (CDs) in zeolitic matrices during hydrothermal/solvothermal crystallization togenerate high-efficient TADF materials with ultralong lifetimes. The resultant CDs@zeolite composites exhibit highquantum yields up to 52.14% and ultralong lifetimes up to 350 ms at ambient temperature and atmosphere. Thisintriguing TADF phenomenon is due to the fact that nanoconfined space of zeolites can efficiently stabilize the tripletstates of CDs, thus enabling the reverse intersystem crossing process for TADF. Meanwhile, zeolite frameworks can alsohinder oxygenquenching topresent TADFbehavior at air atmosphere. This designconcept introduces anewperspectiveto develop materials with unique TADF performance and various novel delayed fluorescence–based applications.

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INTRODUCTIONMaterials with long-lived excited states, such as phosphorescence orthermally activated delayed fluorescence (TADF)materials, have arousedconsiderable interest for their important applications in optoelectronics,photocatalytic reactions, molecular imaging, and security aspects (1–4).In particular, theTADFmaterials have the ability to harvest triplet excitedstates for the highly efficient reverse intersystem crossing (RISC) fromtriplet (T1) to singlet (S1) excited states, thus exhibiting high quantum ef-ficiency and unique optoelectronic properties (5–8). These materials arefinding great use in organic light-emitting diodes (OLED) and sensors(9–11). Todate,most of the reportedTADFmaterials are based onmetal-organic complexes [for example, Cu(I), Ag(I), and Au(I) complexes (12)]and organic molecules [for example, organic donor-acceptor molecules(13, 14) and fullerenes (15)] with the lifetimes limited in the range of sev-eral microseconds to several milliseconds (16–19). Moreover, most ofthem display TADF behaviors in the absence of oxygen because oxygenis a strongquencher of the triplet state emission (20).NewTADFmaterialswith ultralong lifetimes at ambient conditions are highly desired to pro-mote the multifunctional applications of TADF materials. Becausetriplet excitons are easily deactivated by nonradiative decay processesderived from the vibration and rotation of emitters and host molecules,energy transfer to othermolecules, or triplet-triplet annihilationof emitters,to effectively suppress the nonradiative decay processes is crucial to achievethe long-lived excited states for TADF materials.

In recent years, carbondots (CDs) asnewemerging luminescentnano-materials have received extensive attraction because of their low toxicity,biocompatibility, photostability, and optoelectronic and photocatalyticproperties (21–24). Notably, a new kind of room temperature phospho-rescent material has been prepared by incorporating CDs into thematrices of polyvinyl alcohol (25, 26), potash alum (27), and poly-urethane (28). The matrices can restrict the intramolecular vibrationsand rotations of the functional groups on CDs, thus stabilizing thetriplet excited states. Note that the TADF emission with a slow decaylifetime of 676 to 810 ns can be observed in water-soluble CDs from 100

to 250K inwhich the frozen solution acts as a rigidmatrix to suppress thenonradiative relaxation of CDs and thus promotes the TADF behavior(29). Finding a more suitable host matrix with a strong ability to harvesttriplet excited states and generating a new class of CD-based materialshaving unique TADF properties at ambient conditions would be of greatinterest. Ordered nanoporousmaterials with well-confined spaces, espe-cially zeolites with high thermal and chemical stability, are ideal candi-dates to incorporate functional guests (30–35). Generally, the direct insitu incorporation of photoluminescent materials into inorganic porousmaterials requires a strong driving force for the coassembly of guest lu-minescent species and host inorganic frameworks (36–38). Fortunately,the hydrothermal or solvothermal synthetic conditions for zeolites aresuitable for CDs, in which the organic amines and solvents can providethe raw materials for the formation of CDs (39). Therefore, the in situ in-corporationofCDs into zeolite crystals underhydrothermal or solvothermalconditions to produce CDs@zeolite composites is highly possible.

Here, a facile and general strategy has been developed to synthesize anew class of CD-based TADFmaterials with ultralong lifetimes by in situembedding CDs within a series of zeolitic crystalline matrices undersolvothermal/hydrothermal conditions (Fig. 1). The CDs@zeolite com-positeswith high quantumyields (QYs) up to 52.14%andultralongTADFlifetimes up to 350 ms at ambient temperature and atmosphere havebeen successfully prepared. By varying the synthetic conditions of zeo-litic compounds, such as the organic structure-directing agents (SDAs)and the solvents, the CDs@zeolite composites with modulated QYsand lifetimes can be synthesized. A possible mechanism for the forma-tion of these CD-based TADF materials has been proposed, and theirapplication in dual-mode security protection has been demonstrated. Thiswork provides a new “dots-in-zeolites” protocol for the design andpreparation of novel TADF materials and may thus open various de-layed fluorescence–based applications in solar cells, high-resolutionbioimaging, sensing, and security protection.

RESULTS AND DISCUSSIONSynthesis of CDs@zeolite compositesTaking aluminophosphate zeoliteAlPO-5 (AFI) as an example,we havedemonstrated an in situmethod to prepareCDs@zeolite TADFmaterials

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Fig. 1. Proposed in situ formation process of photoluminescent CDs@AlPO-5 composite. The reaction mixture is composed of inorganic precursors and organic species.Under the solvothermal condition, the formation of CDs is accompaniedwith the crystallization of zeolite, and the CDs can be in situ embedded and confined in the zeolitematrixduring the crystal growth, resulting in photoluminescent CDs@zeolite composite.

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Fig. 2. SEM and TEM images and photoluminescence properties of CDs@zeolite composites. (A) Scanning electron microscopy (SEM) image of CDs@AlPO-5 composite(inset, the inorganic framework and the SDA). (B) TEM imageof CDs@AlPO-5 composite showingCDs embedded in the inorganicmatrix (inset, HRTEM imageof a typical CDwith alattice spacing of 0.21 nm). (C) Excitation-emission two-dimensional plot of CDs@AlPO-5 composite. Emission intensity riseswith the color changing fromblue to green and to red.(D) Fluorescence microscopy images of CDs@AlPO-5 with ultraviolet (UV), blue, and green light excitation. (E to L) The SEM, TEM, HRTEM images, excitation-emission two-dimensional plots, and fluorescence microscopy images with UV, blue, and green light excitation of CDs@2D-AlPO and CDs@MgAPO-5, respectively.

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under solvothermal/hydrothermal conditions. The CDs@AlPO-5 com-posite was solvothermally synthesized by using triethylamine (TEA)as the SDA in the reaction system of Al(OiPr)3–H3PO4–TEA–HF–triethylene glycol (TEG) at 180°C for 3 days. The as-synthesizedCDs@AlPO-5 crystals display hexagonal prism–like morphology(Fig. 2A), and their powder x-ray diffraction (PXRD) pattern revealsthe characteristic diffraction peak of theAFI structure (fig. S1A). Liquidchromatography–high-resolution mass spectrometry (LC-HRMS) of thedissolved sample confirms the existence of TEA in the CDs@AlPO-5composite (fig. S2). The transmission electron microscopy (TEM) imageshows that uniform and monodispersed CDs are embedded in the AlPO-5crystals with an average particle diameter of 3.7 nm (Fig. 2B and fig. S3A).Meanwhile, theCDswith a diameter of about 3.8 nmcan also be observedin the synthetic mother liquid of CDs@AlPO-5 (fig. S4, A and B). Thehigh-resolution TEM (HRTEM) image reveals a well-resolved latticespacing of 0.21 nm, which is close to the (100) facet of graphite (27).

CDs@2D-AlPO compositewas synthesized by using 4,7,10-trioxa-1,13-tridecanediamine (TTDDA) as the SDA in the solvothermal syntheticsystem of Al(OiPr)3-H3PO4-TTDDA-TEG. Single-crystal XRD analysisreveals that the CDs@2D-AlPO has a layered [A12P3O12H]

2− structure,which is isostructural with the framework of aluminophosphate UT-5synthesized by using cyclohexylammonium as the SDA (40). Proto-nated TTDDA molecules are located in the layered structure (seeMaterials andMethods formore details on the synthesis and structuredetermination; the crystallographic tables, atomic coordinates,PXRD pattern, and structure of CDs@2D-AlPO are provided in tables S1and S2 and figs. S1B and S5). As shown in Fig. 2E, the CDs@2D-AlPOcrystals display plate-like morphology. TEM and HRTEM imagesshow that the CDs with an average diameter of 3.5 nm are embeddedin the CDs@2D-AlPO crystals (Fig. 2F and fig. S3B).

Liu et al., Sci. Adv. 2017;3 : e1603171 26 May 2017

By using TTDDA as the SDA, CDs@MgAPO-5 was prepared in thehydrothermal reaction system of Al(OiPr)3-MgHPO4·3H2O-H3PO4-TTDDA-H2O at 180°C for 3 days. The obtained material has the AFIstructure, as confirmed by PXRD study (fig. S1C), and displays polyhedralmorphology (Fig. 2I). Well-dispersed CDs with an average particle di-ameter of 3.4 nm can be clearly observed in the zeolitic crystalline matrix(Fig. 2J and fig. S3C).

These CD-based TADFmaterials can be successfully prepared by insitu embedding luminescentCDs into zeolites and relatedopen-frameworkmaterials in the presence of different organic templates and solvents. Thesimultaneous formation of CDs and zeolites under the solvothermal/hydrothermal conditions is crucial for the generation of CDs@zeolitecomposites. In contrast, in the conventional hydrothermal synthesis ofAlPO-5 in the system of Al(OiPr)3-H3PO4-TEA-HF-H2O at 180°C,AlPO-5 was crystallized after 3 hours, but no CDs were observed inthe mother liquid at the same time. Therefore, the resultant productshows no photoluminescent property.

Excitation-dependent fluorescence ofCDs@zeolite compositesThe as-synthesizedCDs@AlPO-5,CDs@2D-AlPO, andCDs@MgAPO-5composites show the characteristic excitation-dependent fluorescencebehavior of CDs (Fig. 2, C, G, and K). The diluted mother liquids ofCDs@zeolite also display similar excitation-dependent fluorescence tothose of CDs@zeolite composites (fig. S4C). The observed emissions ofCDs@zeolite composites can change from blue to green and to red withthe prolonging excitation wavelengths (Fig. 2, D, H, and L). Strong deepblue emissions can be observed under a 370-nm excitation with theCommission Internationale de I’Eclairage (CIE) coordinates of (0.17,0.13), (0.17, 0.14), and (0.17, 0.13) for CDs@AlPO-5, CDs@2D-AlPO,

Fig. 3. TADFproperties of CDs@AlPO-5 composite. (A) The steady-state photoluminescence spectrum (deep blue line) and delayed photoluminescence spectrum (blue line)of CDs@AlPO-5 excited under 370 nm at room temperature. The emission band of the delayed fluorescence centers at 430 nm, which is consistent with that of the promptfluorescence shown in the steady-state spectrum. Inset: The photographs of CDs@AlPO-5 under sunlight, excitedwith a UV lamp (365 nm) and then turned off. a.u., arbitrary units.(B) The time-resolved decay spectra of CDs@AlPO-5 at room temperaturewith the long lifetime of 350ms and short lifetime (inset) of 2.9 ns. (C) Temperature-dependent transientphotoluminescence decay of the CDs@AlPO-5 composite. (D) The steady-state photoluminescence spectrum (deep blue line) and delayed photoluminescence spectrum (oliveline) of CDs@AlPO-5 excited under 370 nm at 77 K. The delayed photoluminescence spectra were collected with the delay time of 1 ms.

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andCDs@MgAPO-5, respectively. TheQYs of these compositematerialscan be modulated by changing the host matrices and the reaction rawmaterials. The QYs are changed from 15.53% for CDs@AlPO-5 to22.77% forCDs@MgAPO-5 andup to 52.14% forCDs@2D-AlPOunderthe 370-nm excitationwavelength. The enhancedQYs ofCDs@2D-AlPOand CDs@MgAPO-5 over that of CDs@AlPO-5may be attributed to theTTDDASDA in the former two composites, which can produce and passi-vate the surface of CDs with enhanced photoluminescence QYs accordingto the literature (41, 42).On the other hand, theQYsof theCDs@2D-AlPOand CDs@MgAPO-5 composite materials are much higher than those ofreported TTDDA-based CDs (12 to 13%). The improvements of QYs aredue to the efficient stabilization of CDs by the zeolite matrices.

TADF property of CDs@zeolite compositesUnderUVexcitation at 370 nm, theCDs@AlPO-5 emits deep blue lightwith emission centered at about 430 nm (deep blue line in Fig. 3A). Thepersistent luminescence can be seen by naked eyes at room temperaturewhen the UV light is turned off. The emission band of the delayed flu-orescence centers at 430 nm (blue line in Fig. 3A), which is consistent withthatof theprompt fluorescence showninthesteady-state spectrum.The time-resolved decay spectra reveal the presence of both short- and long-livedspecies with lifetimes of 2.9 ns and 350 ms at room temperature, respec-tively (Fig. 3B). This demonstrates the coexistence of prompt fluorescenceand delayed fluorescence for CDs@AlPO-5 composite. The temperature-dependent transient photoluminescence decay conducted from100 to 350K(as shown in Fig. 3C) shows that the ratio of the delayed component in-creases with the rising of temperature from100 to 350K, demonstratingthat the delayed fluorescence phenomenon is influenced by the presenceof thermal activation energy, which is characteristic for TADF-activematerials (5, 19). The energy gap between the S1 andT1 excited state levels(DEST) has been further studied. The delayed spectrummeasured at 77 Kshows an emissionband at 466nm,which canbe assigned as the emissionof a triplet excited state (olive line in Fig. 3D). Compared with the steady-state photoluminescence spectrum at 77K (deep blue line in Fig. 3D)withthe emission band at 430 nm (the emission of a singlet excited state), anDEST value of 0.22 eV (S1, 2.88 eV; T1, 2.66 eV) can be obtained. Thesmall energy gap (DEST) may make the RISC process from the T1 toS1 state possible when induced by room temperature thermal activation,thus facilitating the generation of TADF phenomenon.

As with CDs@AlPO-5, CDs@2D-AlPO and CDs@MgAPO-5 alsoexhibit the TADF behavior with long lifetimes of 197 and 216 ms,and their calculated DEST are 0.23 and 0.22 eV, respectively (figs. S6and S7). Photophysical characteristics of these CDs@zeolite compositesunder ambient conditions excited at 370 nmare summarized inTable 1.Note that there is a negligible amount of adsorbed organic solvents/water in theCDs@zeolites that has an influence on theTADFproperties(fig. S8 and table S3).

Liu et al., Sci. Adv. 2017;3 : e1603171 26 May 2017

To understand the origin of TADF behavior, we studied the struc-ture of CDs@AlPO-5 composite by x-ray photoelectron spectroscopy(XPS),UV-visible (UV-vis) absorption spectroscopy, and Fourier trans-form infrared (FTIR) spectroscopy (fig. S9). On the basis of the XPSanalysis, the C–C/C==C bonds (284.6 eV), C–O/C–N bonds (286.3 eV),C==O/C==N bonds (287.6 eV), and pyridine-like (400.2 eV) and pyrrolic-likeN (402.2 eV) atoms canbe identified (28).UV-vis absorption spectrumof CDs@AlPO-5 indicates the presence of p→p* transition of C==C bonds(263 nm) and n→p* transition of C==O/C==N bonds (350 nm) (25). TheFTIR spectrum also shows the presence of C–O/C–N and C==O/C==Nbonds (26). The C==O or C=N groups on CDsmay facilitate the intrinsictriplet generation through intersystem crossing (43–46). As withCDs@AlPO-5, similar functional groups of CDs can also be found inCDs@2D-AlPO and CDs@MgAPO-5 composites (figs. S10 and S11).

Note that the CDs obtained in diluted mother liquid only show theshort-lived fluorescence lifetime (5.1 ns) at room temperature (fig. S12).This suggests that the inorganic frameworks of zeolites play an essentialrole in stabilizing the long-lived triplet states. In the mother liquid, theexcitations of isolated CDs aremainly deactivated through nonradiativeroutes at room temperature due to the active intramolecular vibrationsand rotations. In contrast, the nanoconfined space in the rigidmatrix ofzeolites can effectively prevent the nonradiative relaxation by lockingthe emissive species and inhibiting their intramolecular motions. Notethat the size of the CDs ismuch larger than themicropores in the zeolitestructures; the CDs must be confined in the interrupted nanospaces.The interrupted framework of zeolites contains extensive danglingOH groups, which may form complex H-bonding interactions withthe surface functional groups of CDs, such as C==N and C==O groups,thus suppressing the nonradiative relaxation of CDs. Meanwhile, theorganic SDAs are crucial for the TADF property of CDs@zeolites thatcan also form H bonds with the functional groups of CDs. When theCDs@AlPO-5 composite was heated at 200°C under vacuum for5 hours to sublime the organic SDAs, both the TADF lifetime and theQY are decreased. No TADF phenomenon was observed when thecomposite was vacuum-calcined at 500°C (figs. S13 and S14 and tableS4). Therefore, zeolite is not only a desirable host for accommodatingCDs to effectively prevent the aggregation ofCDs but is also an excellentconfinement matrix to harvest triplet excited states.

As for the TADF materials, the energy level is essential for effi-cient transition and RISC process (T1 to S1) stimulated via thermal ac-tivities (5, 47). The abovementioned study shows that the as-preparedCDs@zeolite composites have small DEST values (0.22 to 0.23 eV),which make it possible for an efficient TADF emission facilitated bythe RISC process. The possible mechanism for the formation of theseCD-basedTADFmaterials has been proposed in Fig. 4A. Significantly, theCDs@AlPO-5 exhibits ultralong delayed fluorescent lifetime (350 ms),and the delayed fluorescence can be clearly seen by naked eyes. As faras we know, the reported lifetimes of delayed fluorescence are mainly ofseveral microseconds/milliseconds for the TADF materials of metal-organic complexes and organic molecules. The enhanced lifetimes ofCDs@zeolite composites may be caused by the highly efficient stabiliza-tion of the zeolite framework and organic SDAs to effectively prevent thenonradiative relaxation and result in ultralong TADF behavior. Moreover,the CDs@zeolite TADF composite materials can work in air atmosphere,which is different frommost traditional organic TADFmaterials in whichthe long-lived decay component would be greatly quenched at the airatmosphere because oxygen is a strongquencher of the triplet state emis-sion. The zeolitematrix is a good oxygen barrier that can effectively hinderthe collisions between the functional groups and oxygen molecules.

Table 1. Photophysical characteristics of CDs@zeolite composites un-der ambient conditions excited at 370 nm. PL, photoluminescence.

Compounds

PL (nm) CIE QYs (%) tTADF (ms) DEST (eV)

CDs@AlPO-5

430 (0.17, 0.13) 15.53 350 0.22

CDs@2D-AlPO

440 (0.17, 0.14) 52.14 197 0.23

CDs@MgAPO-5

425 (0.17, 0.13) 22.77 216 0.22

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Moreover, the as-synthesized CDs@zeolite composites can keep theirTADFproperty formore than half a year at air condition, showing theirhigh stability (fig. S15 and table S5).

Dual-responsive security protection applicationThe advantages of CDs@zeolite TADF materials may open new per-spectives for various delayed fluorescence–based applications. Here,we simply demonstrate a promising application of CDs@zeolite com-posites as a smart material for use in dual-mode security protection, inwhich traditional photoluminescence acts as one mode of security pro-tection, whereas the visible TADF acts as the other mode with thepersistent luminescence after the excitation for the lifetime-encodingfeature. As shown in Fig. 4B, the security pattern of a rose is coded withtwo parts: theCDs@2D-AlPOas the flower part and the benzil dyemol-ecule as the part of leaves and stems, which exhibit blue and green colorsunder a 365-nm UV excitation, respectively. When switching off theexcitation, only the CDs@2D-AlPO–patterned flower part with ultra-long TADF can be clearly observed with naked eyes. Besides the

Liu et al., Sci. Adv. 2017;3 : e1603171 26 May 2017

application as a smart material in dual-mode security protection, it isbelieved that the CDs@zeolite nanocomposites with long-lived TADFphenomenon can serve as ideal contrast agents for future high-sensitivitybioimaging for efficient avoidance of short-lived background fluores-cence. TADF materials with long-lived excited states are also expectedto be promising for solar cells because their long-lived excitons areenabled to extend diffusion distances for the efficient generation of freecharges, which is a prerequisite for high-performance solar cells (9, 48).

CONCLUSIONSWe have successfully demonstrated a facile and general dots-in-zeolitesstrategy to prepare CD-based TADF materials with ultralong lifetimesby embedding CDs in nanoconfined spaces of zeolitic crystallinematrices. This approach is effective for the in situ incorporation ofCDs derived from different organic species into various zeolitic hostmaterials under hydrothermal/solvothermal conditions. The uniqueTADF property of these materials with ultralong-lived excited statesat ambient conditions is benefited from the fact that zeolitic hostmatricescan effectively stabilize the triplet state by suppressing the nonradiativeprocesses and hindering the oxygen quenching. The as-preparedCDs@zeolite composites display high QYs and ultralong lifetimes ofup to 52.14% and 350ms, respectively, which affirms their potential ap-plications in security protection and future high-resolution bioimagingapplication. This method is generally applicable for other luminescentnanomaterials. The considerable flexibility of rational combination ofphotoluminescentnanodots andawide rangeof suitablehostmatriceswithwell-confined nanospaces and electrical transport properties (for example,perovskite andTiO2)may facilitate the design of a variety of highly efficientTADF materials for advanced optoelectronic device applications (for ex-ample, solar cells and LED). This discovery of a new class of CD-basedTADFmaterials with ultralong lifetimes and high efficiency is developedas a proof of concept, which we believe will promote both fundamentalunderstanding and future practical applications of TADF materials.

MATERIALS AND METHODSMaterialsAll reagents were used without purification. Aluminum isopropoxide(≥98% purity) and TTDDA (97% purity) were obtained from Sigma-Aldrich. H3PO4 [85 weight % (wt %) in water] and hydrogen fluoride(HF; 40 wt % in water) were supplied by the Beijing Chemical ReagentFactory. TEG andMgHPO4·3H2Owere obtained fromAladdin Chem-ical Reagent Co. High-purity water was obtained from Milli-Q purifi-cation system. TEA (99% purity) was obtained from Tianjin Fuyu FineChemical Co. Ltd.

Synthesis of CDs@zeolitesCDs@AlPO-5was synthesized usingTEAas the SDAunder solvothermalconditions. In a typical synthesis of CDs@AlPO-5, aluminum isopro-poxide was dispersed in a mixture of TEG and H3PO4. Then, TEA andHFwere dispersed into themixturewith stirring.Ahomogeneous gelwasformed with an overall molar composition of 1.0 Al2O3:1.3 P2O5:1.6TEA:1.3 HF:17 TEG. After crystallizing at 180°C for 3 days, the crystal-line product was separated, thoroughly washed with distilled water, anddried at room temperature overnight.

CDs@MgAPO-5 and CDs@2D-AlPO were synthesized by usingTTDDA as the SDA. In a typical synthesis of CDs@MgAPO-5, alumi-num isopropoxide was dispersed in a mixture of water and H3PO4.

Fig. 4. ProposedTADFmechanismofCDs@zeolite composites anddual-responsivesecurityprotectionapplication. (A) Left: CDs dispersed in themother liquid showonlythe prompt fluorescence (PF) with short-lived lifetime. The triplet state would bequenched by nonradiative processes such as the active intramolecular vibrations(vib) and rotations. Right: The terminal OHgroups of the interrupted zeolite framework,along with the occluded organic SDAs may form extensive H bonds with the surfacefunctional groups of CDs to suppress the nonradiative relaxation of CDs derived fromintramolecular vibrations and rotations and thus stabilize the triplet excited states. Thesmall DEST value of the CDsmakes it possible for an efficient TADF emission facilitatedby the RISC process. P, phosphorescence. (B) The security pattern of a rose is codedwith the CDs@2D-AlPO as the flower part and the benzil dye molecule as the leaf andstem parts. Under a 365-nm UV excitation, the flower patterned with CDs@2D-AlPO isblue, and the leaves and stem patterned with benzil dye molecules are green. Whenswitching off the excitation, only the flower represents ultralong delayed fluorescencethat can be clearly observed by naked eyes, showing the time-resolved security features.

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Then, MgHPO4·3H2O and TTDDA were dispersed into the mixturewith stirring. A homogeneous gel was formed with an overall molarcomposition of 1.0 Al2O3:0.1MgO:2.0 P2O5:2.1 TTDDA:139 H2O. Aftercrystallizing at 180°C for 3 days, the crystalline product was separated bywashing with distilled water and drying at room temperature overnight.As for the synthesis of CDs@2D-AlPO, aluminum isopropoxidewas dis-persed in amixture ofTEGandH3PO4.Then,TTDDAwasdispersed intothemixture with stirring. A homogeneous gel was formed with an overallmolar composition of 1.0 Al2O3:1.9 P2O5:2.0 TTDDA:18 TEG:3.7 H2O.After crystallizing at 180°C for 3 days, the plate-like crystalline productwas obtained by washing with distilled water and drying at room tem-perature overnight.

The percentages of CDs incorporated in the CDs@zeolite compositeswere evaluated according to the CHN analysis by comparing theCDs@zeolites with the pure zeolite analogs synthesized with the sameSDAs. The percentages of CDs calculated on the basis of C content inCDs@AlPO-5 andCDs@2D-AlPOwere 0.55 and 0.64wt%, respectively.However, thepercentageofCDs inCDs@MgAPO-5wasdifficult toprovidebecause no pure MgAPO-5 could be synthesized by using the same SDA.

Apparatus and measurementsPXRD data were collected on a Rigaku Ultima IV diffractometer withCu Ka radiation (l = 1.5418 Å). The TEM and HRTEM images weretaken on an FEI Tecnai G2 S-Twin F20 TEM and FEI Titan G2 60-300scanning TEM. The infrared absorption spectra were recorded on aBruker FTIR IFS-66V/S spectrometer in the range of 400 to 4000 cm−1

with KBr pellets. A baseline correction was applied after measurement.UV-vis adsorption spectra were recorded on a Shimadzu UV-2550spectrophotometer. The XPS measurements were performed using aThermoESCALAB250 spectrometerwithmonochromatizedAlKa ex-citation. LC-HRMSwas performed on a high-performance liquid chro-matography (Agilent 1290) andHRMS (microTOF-Q II, BrukerDaltonics)].Thermogravimetric (TG) analysis was performed on aTAQ500 analyzerin air from room temperature to 800°C at 5°C/min. CHN analysis wasconducted on a PerkinElmer 2400 elemental analyzer.

Photoluminescence measurementsExcitation-emission two-dimensional plots, steady-state spectra, anddelayed photoluminescence spectra were obtained by using a HORIBAScientific FluoroMax-4 spectrofluorometer. The fluorescence lifetimespectra were performed with a HORIBA Scientific FluoroMax-4 byusing FluoroHub-B fluorescence lifetime system equippedwith a photo-multiplier tube detector. A nanoLED and amicrosecond flash xenon arclamp were used as the excitation sources. Ludox was used to calculateinstrument response factor (IRF). The data were analyzed using DAS6software using a multiexponential model for lifetime. The average life-times (tav) can be calculated with tav = ∑Aiti

2/∑Aiti, where Ai is the pre-exponential for lifetime ti (Ai and ti of the CDs@zeolites and dilutedmother liquid are shown in table S6). The fluorescent images were takenusing an Olympus BX51 at UV, blue, and green light excitations throughband-pass filters of 450-, 550-, and 580-nm wavelengths. Temperature-dependent transient photoluminescence decay was measured using anEdinburgh FLS920 fluorescence spectrophotometer equipped with amicrosecond flashlamp. The fluorescence QY was measured on an Ed-inburgh FLS920 with an integrating sphere. To identify the accuracy ofthe measured QY, a tetraphenylethene thin film was used as reference.The measured QY (50.2%) was close to its standard QY of 49.2% (49).The CDs@zeolite composites were measured in solid state, and all pho-toluminescencemeasurements were conducted under the air condition.

Liu et al., Sci. Adv. 2017;3 : e1603171 26 May 2017

Single-crystal XRDSuitable single crystal of CDs@2D-AlPOwith dimensions of 0.23mm×0.21 mm × 0.20 mm was selected for single-crystal XRD analysis. Theintensity data were collected on a Bruker SMARTAPEXII CCDdiffrac-tometer using graphite-monochromatedMoKa radiation (l =0.71073Å)at a temperature of 23° ± 2°C. Cell refinement and data reduction wereaccomplished with the SAINT processing program. The structure wassolved in the P21/c space group by direct methods and refined by full-matrix least-squares technique with the SHELXTL crystallographicsoftware package. The heaviest atoms of Al, P, and O could be unam-biguously located, and the C and N atoms were subsequently located inthe difference Fourier maps.

SUPPLEMENTARY MATERIALSSupplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/3/5/e1603171/DC1fig. S1. PXRD patterns.fig. S2. LC-HRMS of dissolved CDs@AlPO-5 composite.fig. S3. The particle diameter distributions of CDs in CDs@zeolite composites.fig. S4. TEM analysis and photoluminescence property of CDs in the diluted mother liquid ofCDs@AlPO-5.fig. S5. Structure of 2D-AlPO synthesized by using TTDDA as SDA.fig. S6. TADF properties of CDs@2D-AlPO composite.fig. S7. TADF properties of CDs@MgAPO-5 composite.fig. S8. TADF properties of CDs@zeolite composites after vacuum drying for 12 hours at roomtemperature.fig. S9. The compositional analysis of the organic species in CDs@AlPO-5 composite.fig. S10. The compositional analysis of the organic species in CDs@2D-AlPO composite.fig. S11. The compositional analysis of the organic species in CDs@MgAPO-5 composite.fig. S12. The time-resolved decay spectrum of the CDs in the diluted mother liquid ofCDs@AlPO-5 at room temperature.fig. S13. PXRD and TG characterizations of CDs@AlPO-5 composites vacuum-calcined atdifferent temperatures.fig. S14. TADF properties of CDs@AlPO-5 composites upon vacuum calcination at differenttemperatures.fig. S15. TADF properties of CDs@zeolite composites kept under ambient conditions for morethan half a year.table S1. Crystal data and structure refinement for CDs@2D-AlPO.table S2. Atomic coordinates (× 104) and equivalent isotropic displacement parameters (Å2 ×103) for CDs@2D-AlPO.table S3. The photoluminescence emission bands and lifetimes of CDs@zeolite compositesafter vacuum drying at room temperature.table S4. The photoluminescence emission bands and lifetimes of CDs@AlPO-5 compositesafter vacuum calcination at different temperatures.table S5. The photoluminescence emission bands and lifetimes of CDs@zeolite compositesbefore and after half a year under ambient conditions.table S6. The multiexponential lifetime (ti) and preexponential for lifetime (Ai) of theCDs@zeolite composites and the diluted mother liquid.data file S1. CIF file for CDs@2D-AlPO.data file S2. CheckCIF file for CDs@2D-AlPO.

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Acknowledgments: We thank Y. Wang (Jilin University), Q. Zhang (Zhejiang University),Y. Tao (Nanjing Tech University), and R. Chen (Nanjing University of Posts andTelecommunications) for helpful discussions. Funding: This work was supported by theNational Key Research and Development Program of China (grant no. 2016YFB0701100), theNational Natural Science Foundation of China (grant nos. 21320102001, 21621001,21671075, and 91122029), and the “111” Project of the Ministry of Education of China

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(B17020). Author contributions: J.Y. supervised and coordinated all aspects of theproject. J. Li also coordinated this work. J. Liu carried out synthesis and characterization,interpreted the data, and wrote the paper. N.W. performed the TEM and HRTEMmeasurements. Y. Yu conducted the dual-mode security protection experiments. Y. Yansolved crystal structures and performed data fitting. H.Z. did part of the experimentsand related analyses. J.Y. and J. Li revised the manuscript. All authors discussed andcommented on the manuscript. Competing interests: The authors declare that they haveno competing interests. Data and materials availability: CCDC 1517735 contains thesupplementary crystallographic data for this paper. These data can be obtained freeof charge from the Cambridge Crystallographic Data Centre (www.ccdc.cam.ac.uk/data_request/cif). All data needed to evaluate the conclusions in the paper are present in

Liu et al., Sci. Adv. 2017;3 : e1603171 26 May 2017

the paper and/or the Supplementary Materials. Additional data related to this paper maybe requested from the authors.

Submitted 14 December 2016Accepted 22 March 2017Published 26 May 201710.1126/sciadv.1603171

Citation: J. Liu, N. Wang, Y. Yu, Y. Yan, H. Zhang, J. Li, J. Yu, Carbon dots in zeolites: A new classof thermally activated delayed fluorescence materials with ultralong lifetimes. Sci. Adv. 3,e1603171 (2017).

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with ultralong lifetimesCarbon dots in zeolites: A new class of thermally activated delayed fluorescence materials

Jiancong Liu, Ning Wang, Yue Yu, Yan Yan, Hongyue Zhang, Jiyang Li and Jihong Yu

DOI: 10.1126/sciadv.1603171 (5), e1603171.3Sci Adv 

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MATERIALSSUPPLEMENTARY http://advances.sciencemag.org/content/suppl/2017/05/22/3.5.e1603171.DC1

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