Lifetime-Engineered Phosphorescent Carbon Dots-in-Zeolite Composites for Naked-Eye Visible Multiplexing

Open AccessCCS ChemistryRESEARCH ARTICLE1 Dec 2021Lifetime-Engineered Phosphorescent Carbon Dots-in-Zeolite Composites for Naked-Eye Visible Multiplexing Xiaowei Yu, Kaikai Liu, Hongyue Zhang, Bolun Wang, Guoju Yang, Jiyang Li and Jihong Yu Xiaowei Yu State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012 , Kaikai Liu *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Henan Key Laboratory of Diamond Optoelectronic Materials and Devices, Key Laboratory of Material Physics, Ministry of Education, School of Physics and Microelectronics, Zhengzhou University, Zhengzhou 450001 , Hongyue Zhang State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012 International Center of Future Science, Jilin University, Changchun 130012 , Bolun Wang State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012 , Guoju Yang State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012 , Jiyang Li *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012 and Jihong Yu *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012 International Center of Future Science, Jilin University, Changchun 130012 https://doi.org/10.31635/ccschem.020.202000639 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Lifetime-coded optical multiplexing has attracted wide attention due to avoiding spectral overlap and background interference. At present, most of the materials used for lifetime-coded multiplexing involve rare-earth metal ions with their lifetime domains in the microsecond range, thus greatly limiting their application scope. Herein, nine kinds of green, room-temperature phosphorescent (RTP) carbon dots-in-zeolite ([email protected]) composites with engineered lifetimes from 0.38 to 2.1 s are thermally prepared under solvent-free conditions by systematically adjusting the reaction conditions and host–guest interactions. The regulation mechanism of various reaction factors of RTP lifetime has been elucidated. Varying crystallization temperature and time or introducing different amounts of C=O, C=N, or N-H bonds through CDs precursors would tune intersystem crossing rate (KISC) and nonradiative decay rate (Knr) of [email protected] composites. In addition, changing zeolite matrices would generate different host-guest interactions, then result in variable Knr, thus giving tunable RTP lifetimes. In view of the second-level lifetime tunability of as-made [email protected] composites, naked-eye visible multiplexing by lifetime coding has been demonstrated, realizing portrait encryption. This work opens a new opportunity for the application of CDs-based materials in optical multiplexing. Download figure Download PowerPoint Introduction Optical multiplexing can realize simultaneous transmission of multiple signals on one channel by taking advantage of the tunable properties of light, including wavelength, frequency, polarization, lifetime, and angular momentum, which has been widely applied in the fields of biology and biomedicine, optical data storage, and information anti-counterfeiting.1–5 With the conventional approaches for optical multiplexing, luminescent color coding may result in spectral overlap, while luminescent intensity coding is greatly limited by background interference. As a temporal coding dimension, luminescent lifetime coding can increase multiplexing capability because it is independent of luminescent color and intensity, thus effectively avoiding the interference of spectral overlap and background signals.6–8 A key problem for lifetime-coded multiplexing is realizing tunable emission lifetimes over a wide range. At present, the most common materials used in lifetime-coded multiplexing are lanthanide-doped nanoparticles with excellent optical properties and easily tunable emission lifetimes.2,9 Such examples include blue fluorescent Tm-doped upconversion nanocrystals with tunable emission lifetimes from 25.6 to 662.4 μs achieved by varying sensitizer-emitter distances, and core or multishell structured upconversion nanoparticles with variable emission lifetimes (e.g., from 632 to 836 μs, 278 to 390 μs, 437 to 770 μs, 78 to 2157 μs, and 44.5 to 7.21 ms) achieved by changing the thickness of energy relay layer.7,8,10,11 In addition, CdTe and CdS quantum dots (QDs) with two kinds of emission lifetimes (100 ns and 1 μs) and Cu(I) complexes with tunable lifetimes from 12.9 to 22.3 μs have also been reported to obtain lifetime-coded multiplexing.12,13 However, the rare-earth metal or heavy metal ions involved in these materials are high-toxicity, high-cost, or nonrenewable resources. Moreover, the lifetime domains of these materials are mainly limited to microseconds. The instantaneous images are difficult to capture by a common camera, thereby necessitating expensive time-resolved imaging scanning system consisting of spectrometers, time-correlated single-photon counting spectrometers, and other complicated devices, which seriously limits the application of lifetime-coded multiplexing. Therefore, developing low-cost and low-toxicity materials with long and tunable emission lifetimes that can be captured by the naked eye is highly desired in lifetime-coded multiplexing. In recent years, carbon dots (CDs)-based room-temperature phosphorescent (RTP) materials have attracted extensive attention because of the low-toxicity, low-cost, and excellent optical properties.14–19 More importantly, CDs-based RTP materials can easily achieve long emission lifetime on the second timescale by promoting efficient intersystem crossing (ISC) process and stabilizing triplet excited states,16,18–23 thus becoming the ideal materials for naked-eye visible optical multiplexing. However, compared with lanthanide-doped nanoparticles, realizing the controllable regulation of RTP lifetime of CDs is still a challenge,24 and especially, the regulation in a wide range, such as from the second to millisecond timescale, has not been reported. Recently, a "carbon dots-in-zeolite" ([email protected]) strategy proposed by our group was proven a feasible strategy to prepare CDs-based RTP composites.25–28 The confinement effect of zeolite matrices to CDs can stabilize triplet excited states of CDs to achieve extralong RTP lifetimes up to 2.1 s. Significantly, the diversity of CDs precursors and the adjustability of zeolite matrices provide a possibility for tuning the RTP lifetime in a wide range. However, the systematic regulation of RTP lifetime of [email protected] composites has not been studied, and the regulation mechanism of various reaction factors of RTP lifetime is still unclear. Herein, nine kinds of [email protected] composites with engineered RTP lifetimes from 0.38 to 2.1 s are thermally synthesized under solvent-free conditions by adopting different CDs precursors, zeolite matrices, as well as reaction temperature and time. The influences of various factors on the structure and chemical groups of CDs (e.g., C=O, C=N, and N–H bonds), as well as the interactions between CDs and zeolite framework have been elucidated. Based on this, composites with tunable KISC and Knr have been achieved, thus leading to variable RTP lifetimes. As a proof of concept, naked-eye visible optical multiplexing based on the tunable emission lifetimes of [email protected] composites has been demonstrated, exhibiting visible portrait encryption capability. Experimental Methods Chemicals and reagents All the following reagents were used without any purification. Ethylenediamine (EDA; 99 wt %) was bought from Shanghai Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Triethanolamine (TEOA; 99 wt %) and phosphoric acid (H3PO4;85 wt %) were supplied by Beijing Chemical Works (Beijing, China). Diethylenetriamine (DETA; 99 wt %) and α-lipoic acid (99 wt %) were purchased from Shanghai Aladdin Reagent Co., Ltd. (Shanghai, China). Dipropylamine (DPA; analytical reagent) was bought from Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China). Tetraethylammonium bromide (TEABr; 99 wt %) was supplied by Tianjin Guangfu Fine Chemical Research Institute (Tianjin, China). Pseudo boehmite (AlOOH) was bought from Guizhou More Advanced Materials Co., Ltd. (Guizhou, China). NH4H2PO4 (99 wt %) was purchased from Tianjin Guangfu Technology Development Co., Ltd. (Tianjin, China). Tetramethylammonium hydroxide pentahydrate (TMA·5H2O; 99 wt %) was purchased from Beijing Innochem Technology Co., Ltd. (Beijing, China). Fumed silica (99.8 wt %) was bought from Shanghai Aladdin Reagent Co., Ltd. (Shanghai, China). We synthesized DPA·phosphoric acid (DPA·H3PO4). Synthesis of [email protected] composites In the synthesis, α-lipoic acid and three kinds of amine analogs with different structures, including EDA with two –NH2, DETA with two –NH2 and one –NH–, and TEOA with three –OH and one –(C)3–N, were selected as CDs precursors, and AlPO-5 (AFI zeotype) with one-dimension 12-ring channels and SAPO-20 (SOD zeotype) with six rings and sod cages were selected as zeolite matrices (Figure 1a).29,30 All the CDs precursors and zeolite precursors were mixed and ground, followed by a solvent-free thermal crystallization process. The schematic diagram of the preparation process is shown in Figure 1b, and detailed information can be seen in the Supporting Information Section "Experimental Details." Figure 1 | (a) Molecular formula diagrams of CDs and zeolite precursors. (b) Schematic diagrams of preparation process of CDs and [email protected] composites. (c) Pseudo-color-mapped phosphorescence images of nine kinds of [email protected] composites with various RTP lifetimes. (d) Modified Jablonski transition diagrams of CDs (left) and [email protected] composites (right). Download figure Download PowerPoint Characterizations Scanning electron microscopy (SEM) images and energy-dispersive spectroscopy (EDS) dates of points analysis were obtained from JSM-7800F (JEOL Ltd., Tokyo, Japan). Transmission electron microscopy (TEM) and high-resolution TEM (HR-TEM) images were recorded on a Tecnai G2 S-Twin F20 (FEI Company, Oregon, USA). Inductively coupled plasma (ICP) emission spectrometry was carried out on an Agilent 725 (NYSE: A, California, USA). Powder X-ray diffraction (XRD) patterns were obtained from the Rigaku Ultima IV diffractometer (Rigaku Corporation, Tokyo, Japan) with Cu Kα radiation (λ = 1.5418 Å). X-ray photoelectron spectroscopy (XPS) was measured by an ThermoESCALAB 250 spectrometer (Thermo Scientific, New York, USA). UV–vis adsorption spectroscopy was measured by a UV/vis/near-infrared (NIR) spectrometer Lambda 950 (PerkinElmer, Massachusetts, USA). Fourier transform infrared (FTIR) spectra were obtained from Bruker VERTEX 80/80v FTIR spectrometers (Bruker Company, Karlsruhe, Germany) with KBr as background. Fluorescence and phosphorescence emission spectra at room temperature and low temperature (77 K) were determined by HORIBA Scientific FluoroMax-4 spectrofluorometer (HORIBA Jobin Yvon Company, Paris, France). RTP decay curves were obtained from the HORIBA Scientific FluoroMax-4 and FluoroHub-B (HORIBA Jobin Yvon Company, Paris, France) fluorescence lifetime system equipped with a photomultiplier tube detector, and the curves were fitted by DAS6 software (HORIBA Jobin Yvon Company, Paris, France). Temperature-dependent phosphorescence decay curves were measured by an Edinburgh FLS920 fluorescence spectrophotometer (Edinburgh Instruments, Edinburgh, England), and the curves were fitted by Origin software (OriginLab Company, Massachusetts, USA). The total quantum yields (QYs) were obtained from the Edinburgh FLS920 with an integrating sphere. The excitation-emission fluorescence and phosphorescence contour plots were measured by HORIBA Scientific FluoroMax-4 spectrofluorometer and Hitachi F-7000 spectrophotometer (Hitachi Company, Tokyo, Japan), respectively. Microscope images were obtained on a Caikon XTL-5500D (Caikon, Shanghai, China). The pictures applied in optical multiplexing were taken by a mobile phone under ZF-5 UV lamp (Shanghai Jiapeng Technology Co., Ltd, Shanghai, China). Results and Discussion By tuning CDs precursors, zeolites matrices, as well as crystallization temperature and time during synthesis, nine kinds of [email protected] composites with variable RTP lifetimes can be achieved ( Supporting Information Table S1). These [email protected] composites are named as C–A/S–X (C represents CDs, A represents AlPO-5 zeolite, S represents SAPO-20 zeolite, and X represents RTP lifetime). To vividly display the difference of RTP lifetime of [email protected] composites, the phosphorescence images of nine kinds of composites are mapped with different pseudo colors, so that the different RTP lifetimes can be represented by various colors (Figure 1c). Note that the CDs obtained by the same CDs precursors but not confined in a zeolite matrix can only emit fluorescence without phosphorescence (illustrated in the left of Figure 1b), indicating that the zeolite matrix plays a key role in the generation of phosphorescence. The modified Jablonski diagrams of the transition and decay process of CDs without and with zeolite matrix are shown in Figure 1d. In CDs without zeolite matrix, following light absorption, the electrons in the single state would transfer to the triplet state by the ISC process and generate triplet excitons, but the triplet excitons would be depopulated through nonradiative processes including intramolecular vibration-rotation and transitions to triplet oxygen (3O2) due to the high nonradiative recombination rate.22 Thus, only blue fluorescence can be observed. While for [email protected] composites, the nonradiative pathways are "blocked" to some extent by the zeolite matrix, so that the triplet excitons of CDs can return to ground state by emission of a photon and green phosphorescence production. Based on the above generation process of RTP in [email protected] composites and the previous studies about RTP lifetime, we speculate that the variable RTP lifetimes of such materials may be associated with the ISC and nonradiative recombination process.31–33 Generally, small energy gap (ΔEST) values between the lowest triplet (T1) state and the lowest singlet (S1) state and more C=O/C=N bonds that can provide n orbits and generate an effective spin–orbit coupling (SOC) between S1 and Tn can greatly promote the ISC process.31–33 Furthermore, efficient immobilization of CDs, such as embedding CDs into host matrix, or constructing a more compact carbon core that can behave as matrix to self-immobilize triplet excitons would restrain the nonradiative recombination process.23,34 This suggests the tunability of RTP lifetime may in essence derive from the tunable ΔEST values, chemical groups, and structure of CDs, as well as the interactions between CDs and zeolite framework. To verify this, various experiments and characterizations are adopted to analyze the composition, structure, and luminescence of the nine kinds of [email protected] composites. In view of the difference of reaction conditions and host–guest interactions in synthesis, these composites can be classified into three types: (1) C–A–1.97, C–A–1.82, C–A–1.56, C–A–1.36, and C–A–1.22 composites have the same CDs precursors and zeolite matrices, but different crystallization times and temperatures; (2) C–A–2.10 and C–A–1.68 composites have the same crystallization time and temperature, as well as the zeolite matrices, but different CDs precursors; and (3) C–S–0.62 and C–S–0.38 composites have the same CDs precursors, crystallization time and temperature, but different Si/Al ratios of zeolite framework as confirmed by EDS of points analysis and ICP results ( Supporting Information Figures S1 and S2 and Table S2). Composition and structure of nine kinds of [email protected] composites Due to the interference of zeolite framework and organic templates occluded by the zeolite matrix, it is difficult to characterize the chemical groups of CDs within the composites. Thus, we prepared the CDs solutions with the same amine/α-lipoic acid/H2O ratio, crystallization temperature, and time as the corresponding composites to explore the possible chemical groups generated by CDs precursors. These CDs solutions are named as C–A/S–X–CDs ( Supporting Information Table S3). There is no obvious difference in the FTIR spectra of CDs solutions prepared under different reaction temperatures and times ( Supporting Information Figure S3a and Figure 2a). However, obvious differences can be found in the FTIR spectra of CDs solutions prepared from different kinds of CDs precursors ( Supporting Information Figure S3b and Figure 2a). The FTIR peaks at 1650–1662 cm−1 derive from the stretching vibration of C=O/C=N bonds, and the peaks at 1540–1580 cm−1 are attributed to the deformation vibration of N–H bonds.16,35 Obviously, the FTIR peak associated with C=O/C=N bonds in C–A–1.68–CDs (TEOA as precursor) is weaker than those in C–A–2.10–CDs (EDA as precursor) and C–A–1.97–CDs (DETA as precursor), and the FTIR peaks associated with N–H bonds in C–A–2.10–CDs and C–A–1.68–CDs are stronger than those in C–A–1.97–CDs. The XPS are measured to further characterize the chemical groups in C–A–2.10–CDs, C–A–1.97–CDs, and C–A–1.68–CDs. The C1s spectra of three kinds of CDs demonstrate the peaks associated with C=O bonds in C–A–2.10–CDs and C–A–1.97–CDs are obviously stronger than those in C–A–1.68–CDs ( Supporting Information Figure S4a and Figure 2b), which indicates there are more C=O bonds in C–A–2.10–CDs and C–A–1.97–CDs.36,37 The N 1s spectra display the peaks associated with N–H bonds (amino N and pyrrolic N) in which C–A–2.10–CDs and C–A–1.68–CDs are stronger than those in C–A–1.97–CDs ( Supporting Information Figure S4b and Figure 2b), which indicates there are more N–H bonds in C–A–2.10–CDs and C–A–1.68–CDs.22,37 These results are consistent with those of FTIR analyses. In addition to amino N, the N atoms in C–A–1.97–CDs mainly exist in the form of pyridinic N (C=N–C), indicating there are more C=N bonds in C–A–1.97–CDs than those in C–A–2.10–CDs and C–A–1.68–CDs.37 Moreover, the contents of N and O atoms in C–A–1.68–CDs are less than and more than those in C–A–2.10–CDs and C–A–1.97–CDs, respectively; this is due to fewer N atoms and greater O atoms in the precursor TEOA. The UV–vis absorbance spectra of nine kinds of [email protected] composites display two major peaks at approximately 252 and 357 nm ( Supporting Information Figure S5), which are attributed to the π–π* transition and n–π* transition, respectively.38,39 High crystallization temperature and long crystallization time produce stronger π–π* and n–π* absorbance peaks, indicating the generation of more C=C bonds and C=O/C=N bonds (Figure 2c and Supporting Information Figure S5a). The obvious red shift of the π–π* absorbance peak of the C–A–1.68 composite compared with those of the C–A–2.10 and C–A–1.97 composites may indicate a larger conjugated structure existing in the C–A–1.68 composite ( Supporting Information Figure S5b).40,41 Moreover, the n–π* absorbance peak of the C–A–1.68 composite is obviously weaker than those of the C–A–2.10 and C–A–1.97 composites, which may suggest fewer C=O/C=N bonds existing in the C–A–1.68 composite ( Supporting Information Figure S5b). These results are consistent with those of XPS and FTIR spectra. Note that the Si/Al ratio of zeolite matrix has no obvious influence on the absorbance spectra, as shown in Supporting Information Figure S5c. Hence, we infer the kinds of CDs precursors as well as crystallization temperature and time affect the amounts of O and N atoms, C=O bonds, C=N bonds, and N–H bonds of CDs within composites. Figure 2 | The comparisons of (a) FTIR, (b) XPS, and (c) UV–vis absorbance results of various samples with different reaction temperatures and times or CDs precursors. Here, more pentacle stars and sunflowers represent higher contents of corresponding chemical groups. Note that only the comparisons between the same chemical groups under the same regulation factors are effective. Download figure Download PowerPoint The XRD patterns confirm that seven kinds of composites possess AlPO-5 zeolite matrix (named as [email protected] composites, Figure 3a) and two others possess SAPO-20 zeolite matrix (named as [email protected] composites, Figure 3b). A small number of impurity peaks (asterisks in Figures 3a and 3b) are due to the coexistence of other zeolite crystals. The C–A–2.10 and C–S–0.62 composites are taken as the examples for detailed characterizations of CDs in AlPO-5 and SAPO-20 zeolite matrices, respectively. The XRD patterns of isolated CDs from the mother solutions of C–A–2.10 and C–S–0.62 composites display a characteristic peak of graphited CDs ( Supporting Information Figure S6).42 The morphology and size of [email protected] composites are characterized by SEM. The crystals of [email protected] composites show flower-like shape, and the size is approximately 1–2 μm (Figure 3c and Supporting Information Figures S7a–S7f). The [email protected] composites show spherical crystal shapes approximately 20–30 μm (Figure 3d and Supporting Information Figure S7g). The TEM images of C–A–2.10 and C–S–0.62 composites show that spherical CDs are uniformly embedded into the zeolite crystals (Figures 3e and 3f), and the average size of CDs is about 2.85 (inset of Figure 3e) and 2.55 nm (inset of Figure 3f), respectively. Notice that there are obviously fewer CDs in the TEM image of the C–S–0.62 composite than in the TEM image of C–A–2.10 composite; this might be due to the uncrystallized floccules on the surface of SAPO-20 zeolite crystals that influence the observation of CDs embedded in the zeolite crystals. The obvious lattice fringes with 0.21 nm spacing can be observed in their HR-TEM images (insets of Figures 3e and 3f). The HR-TEM images and XRD studies of CDs indicate the CDs in composites are highly crystalline. The TEM images of isolated CDs from mother solutions of C–A–2.10 and C–S–0.62 composites show that the CDs are spherical, and the average size is about 3.4 and 3.46 nm, respectively ( Supporting Information Figures S8a–S8d), larger than the CDs confined in zeolite crystals. This can be attributed to the space confinement effect of zeolite matrix limiting the growth of CDs within zeolite crystals. Figure 3 | (a) XRD patterns of [email protected] composites and simulated AlPO-5. (b) XRD patterns of [email protected] composites and simulated SAPO-20. SEM images of (c) C–A–2.10 composite and (d) C–S–0.62 composite. TEM images of (e) C–A–2.10 composite and (f) C–S–0.62 composite; the insets are corresponding size distribution diagrams and HR-TEM images. Microscope images of C–A–2.10 composite: (g) sunlight, (h) with 365 nm UV light, and (i) after 365 nm UV light. Download figure Download PowerPoint Luminescence of nine kinds of [email protected] composites The optical properties of the nine kinds of [email protected] composites are fully investigated. Microcosmic fluorescence and phosphorescence photographs of C–A–2.10 composite are taken by a microscope, and bright blue fluorescence can be observed under 365 nm UV light, while obvious green phosphorescence can be displayed when the 365 nm UV light is turned off (Figures 3g–3i). The fluorescence and afterglow spectra (Ex = 360 nm) of nine kinds of [email protected] composites are shown in Figures 4a and 4b, and the fluorescence and phosphorescence peaks are centered at approximately 430–446 and 516–520 nm, respectively. Notice that another peak near 440 nm can be observed in afterglow spectra, which is assigned to thermally activated delayed fluorescence (TADF) of CDs.28 The ΔEST values of these samples are given in Figure 4c by calculating the energy gap between fluorescence and phosphorescence peaks in low-temperature (77 K) spectra (Ex = 400 nm) ( Supporting Information Figure S9). The ΔEST values are in the range of 0.32–0.55 eV, which favors populating triplet excitons to generate RTP.20,43 This result proves that tuning the ΔEST values by changing reaction conditions or host–guest interactions is available. The photoluminescence QYs (PLQYs) (Ex = 400 nm) of [email protected] composites vary from 43.14% to 7.87% (Figure 4d). Although the RTP lifetimes of C–S–0.62 and C–S–0.38 composites are shorter than those of C–A–1.56 and C–A–1.22 composites, the stronger fluorescence intensity of C–S–0.62 and C–S–0.38 composites ( Supporting Information Video S1) results in their higher PLQYs. The fluorescence spectra of nine kinds of [email protected] composites are fitted by Gauss, and their phosphorescence QYs can be obtained by calculating the proportions of phosphorescence peak area ( Supporting Information Figure S10).44 By taking C–A–2.10 composite as an example, both the excitation-emission fluorescence and phosphorescence contour plots display excitation-independent behaviors (Figures 4e and 4f), suggesting the as-made [email protected] composites have a single emissive center. Lifetime decay curves (Ex = 360 nm) of C–A–2.10 composite under various temperatures (from 78 to 353 K) are illustrated in Figure 4g. The average lifetime decreases from 2.45 to 1.04 s with temperature increasing from 78 to 353 K ( Supporting Information Figure S11 and Table S4), which is attributable to the gradual activation of molecules by the increasing temperature and the strengthening of nonradiative transitions process, resulting in the reduction of long-lifetime triplet-state ingredients.25 Figure 4 | (a) Fluorescence and (b) afterglow emission spectra of nine kinds of [email protected] composites. (c) ΔEST values and (d) PLQYs of nine kinds of [email protected] composites. Excitation-emission (e) fluorescence and (f) phosphorescence contour plots of C–A–2.10 composite. Note that the Ex and Em represent excitation wavelength and emission wavelength, respectively. (g) Lifetime decay curves of C–A–2.10 composite under different temperatures (78–353 K). Variation of fluorescence intensity, phosphorescence intensity, and RTP lifetime of C–A–2.10 composite after (h) illumination by 365 nm UV light for different times and (i) heating at different temperatures for 30 min. Note: power of UV light is 12 W and radiation distance is 18 cm. Download figure Download PowerPoint The photo- and thermal-stability of C–A–2.10 and C–S–0.62 composites are further explored. The two composites almost maintain RTP lifetime after continuously irradiating for 8 h by 365 nm UV lamp or heated at 180 °C for 30 min (Figures 4h and 4i and Supporting Information Figure S12). The fluorescence and phosphorescence intensity of the two composites are also nearly invariable when the heating temperature is below 150 °C or the irradiating time is no more than 8 h, and they display a slight decline only when the heating temperature is below 180 °C. The above results indicate the [email protected] composites have relatively high stability, which is a key factor for lifetime-coded optical multiplexing. Regulation mechanism of reaction factors to RTP lifetime The Knr of the nine kinds of [email protected] composites is calculated according to eqs 1 and 2.22 Here, the Knr, Kp, τp, and QY represent nonradiative decay rate, phosphorescence decay rate, RTP lifetime, and phosphorescence QYs, respectively. The calculated results illustrate that change of reactio