Template-Modulated Afterglow of Carbon Dots in Zeolites: Room

May 10, 2019 - Room-temperature phosphorescence (RTP) and thermally activated delayed fluorescence (TADF) materials with long afterglow lifetimes have...
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Template-Modulated Afterglow of Carbon Dots in Zeolites: Room-Temperature Phosphorescence and Thermally Activated Delayed Fluorescence Jiancong Liu,†,§ Hongyue Zhang,† Ning Wang,† Yue Yu,† Yuanzheng Cui,† Jiyang Li,*,† and Jihong Yu*,†,‡ †

State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, China ‡ International Center of Future Science, Jilin University, Qianjin Street 2699, Changchun 130012, China § Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education of the People’s Republic of China, Heilongjiang University, Harbin 150080, China S Supporting Information *

ABSTRACT: Room-temperature phosphorescence (RTP) and thermally activated delayed fluorescence (TADF) materials with long afterglow lifetimes have aroused considerable interest. Here, we successfully achieve the modulation of RTP and TADF properties in a carbon dots (CDs)-inzeolite system based on different organic templates via an in situ solvothermal synthetic method. Benefitting from the efficient stabilizing effect of triplet states of CDs by SBT zeolite matrix, CDs@SBT-1 with a larger singlet−triplet energy gap (ΔEST) of 0.36 eV exhibits predominant RTP with a lifetime of 574 ms, while CDs@SBT-2 with a smaller ΔEST value of 0.18 eV shows TADF with a lifetime of 153 ms. Further investigations reveal that different organic templates result in different CDs structures, thus modulating the ΔEST values of CDs@zeolites. This work demonstrates a facile strategy to modulate the afterglow properties of CDs@zeolite composites, which opens the possibility of designing the novel afterglow materials desired for various advanced applications.

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development of organic afterglow materials with long-lived excited states.7−9 So far, several strategies have been utilized to promote the generation of room-temperature phosphorescence (RTP) materials with ultralong lifetimes, such as incorporating heavy atoms,10 host−guest design,11,12 H-aggregation method,13 and crystallization-induced method.14 For the thermally activated delayed fluorescence (TADF) materials, apart from the suppression of the nonradiative decay process, a small energy gap between the lowest singlet (S1) and triplet (T1) excited state levels (ΔEST) is also a key factor for facilitating the highly efficient reverse intersystem crossing (RISC) from T1 to S1 excited states, thus harvesting triplet excitons for fluorescence.15−19 The strategy to achieve small energy gap is to obtain the spatially separated highest occupied molecular

fterglow materials, which can generate emission for a long time after the removal of excitation light, have attracted extensive attention in the fields of illumination, security, bioimaging, etc.1−3 To date, commercial afterglow materials are mainly attained in inorganic systems with rare earth elements, which suffer from high cost, cytotoxicity, and relatively complicated preparation methods (e.g., high fabrication temperatures).4,5 It is of great importance to develop new kinds of afterglow materials that may solve many of the problems in practical applications. Organic afterglow materials with diversified optoelectronic properties have shown unique advantages for their low cost, high quantum efficiency, and versatile synthetic processes.6 However, as the excited state of organic molecules is usually active and triplet excitons are easily deactivated by nonradiative decay processes, it is challenging to achieve afterglow emission with an ultralong lifetime, usually known as phosphorescence or delayed fluorescence. Effective suppression of the nonradiative decay process is crucial for the © 2019 American Chemical Society

Received: March 19, 2019 Accepted: May 10, 2019 Published: May 10, 2019 58

DOI: 10.1021/acsmaterialslett.9b00073 ACS Materials Lett. 2019, 1, 58−63

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2 with an average diameter of 3.0 and 2.9 nm, respectively. The well-resolved lattice spacing of 0.21 nm in CDs is corresponding to the (100) facet of graphite carbon (Figure 1A−D).22 Figure 1C clearly shows that the CDs are embedded

orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) by introducing a large steric hindrance structure or constructing a donor−acceptor system.20 However, so far, there remains a lack of effective method to modulate the afterglow properties of RTP and TADF with ultralong lifetimes in a specific system. Carbon dots (CDs), known as a new kind of luminescent nanoparticles, have aroused considerable interest in the fields of sensing, optoelectronic devices, and bioimaging because of their merits, such as low toxicity, biocompatibility, and optoelectronic and photocatalytic properties.21−23 The various structures of CDs fabricated from different precursors and synthetic methods provide the possibility to modulate the luminescence properties of CDs-based materials, including fluorescence, phosphorescence, and delayed fluorescence.24−27 More recently, we have synthesized a class of CDs-based TADF and RTP materials with ultralong lifetimes by a “dotsin-zeolites” strategy.28 The nanoconfined space of zeolites plays a key role in stabilizing the triplet excited states by suppressing the nonradiative relaxation of CDs. The considerable flexibility of rational combination of CDs generated from different organic species under the hydrothermal or solvothermal synthetic conditions and zeolite matrices with variable confined nanospaces may facilitate the design of CDs-based afterglow composites with novel photoluminescent properties and applications.29 Here, by utilizing different organic templates as precursors of CDs, we have successfully achieved the modulation of RTP and TADF in a specific “dots-in-zeolite” system via an in situ synthetic method. Two CDs-based composites based on zinc aluminophosphate SBT zeolite matrix are solvothermally synthesized. CDs@SBT-1 prepared with (4-(2-aminoethyl)morpholine as the template emits predominant long-lived RTP with lifetime of 574 ms, while CDs@SBT-2 with 4,7,10-trioxa1,13-tridecanediamine as the template exhibits TADF with lifetime of 153 ms. The introduction of different templates can effectively modulate the structures of CDs confined in the zeolite matrix, further results in the different ΔEST of CDs in the composites, giving rise to RTP or TADF properties. CDs-based composites were in situ synthesized by adopting the “dots-in-zeolites” strategy under solvothermal conditions. The CDs@SBT-1 and CDs@SBT-2 composites were synthesized in the similar reaction system of Al(OiPr)3−H3PO4− Zn(NO3)2·6(H2O)−ethylene glycol−H2O−template at 170 °C for 7 days by using (4-(2-aminoethyl)morpholine and 4,7,10-trioxa-1,13-tridecanediamine as the template, respectively. Powder X-ray diffraction (PXRD) patterns of CDs@ SBT-1 and CDs@SBT-2 reveal the characteristic diffraction peaks of the SBT zeolite (Figure S1). The structure of SBT zeolite is constructed by alternation of (Al/Zn)O4 tetrahedra and PO4 tetrahedra, forming a three-dimensional framework with two 12-ring channels (6.4 × 7.4 and 7.3 × 7.8 Å) along the [001] direction (inset of Figure S1).30 As seen from the scanning electron microscopy (SEM) images, the as-synthesized CDs@SBT-1 and CDs@SBT-2 crystals show a flowerlike morphology, which are formed by the aggregation of hexagonal platelike crystals (Figure S2). Inductively coupled plasma (ICP) analysis gives that the ratios of Al/Zn in CDs@ SBT-1 and CDs@SBT-2 are 1/0.8 and 1/0.7, respectively, which suggests that the zeolite host matrices are basically the same. By the transmission electron microscopy (TEM) analysis, uniform CDs can be detected in CDs@SBT-1 and CDs@SBT-

Figure 1. TEM images, size distributions of CDs in composites, and the excitation-dependent photoluminescence of CDs@SBT-1 (A, B, E) and CDs@SBT-2 (C, D, F) composites (inset, HRTEM image of CDs). The size distributions were obtained by counting about 50 particles.

in the interrupted spaces of zeolite matrix because CDs are much larger than the micropores of SBT zeolite. The interrupted framework of SBT zeolite can act as a suitable matrix to accommodate CDs with narrow size distribution and effectively prevent the aggregation and fluorescence quenching of CDs. In addition, CDs with similar sizes have been found in the mother solutions (Figure S3). This suggests that the solvothermal synthetic condition for SBT zeolite is suitable for the formation of CDs. In the synthesis process, CDs could be in situ formed in the mother liquid by using the solvents and templates as raw materials.31,32 With the crystallization time increasing, the CDs with the graphitic core are gradually formed. Meanwhile, some CDs are embedded into the solid zeolite matrix during the crystal growth process. As shown in Figure 1E and F, both CDs@SBT-1 and CDs@ SBT-2 exhibit excitation-dependent fluorescence behavior, which is common for CDs.33 When excited at 370 nm, CDs@SBT-1 displays the strongest emission at around 453 nm, while CDs@SBT-2 shows the strongest emission centered at around 425 nm, under 350 nm excitation, and strong emission at around 440 nm, under 370 nm excitation. The quantum yields (QYs) of the CDs@SBT-1 and CDs@SBT-2 excited at 370 nm are 30.72% and 29.45%, respectively. 59

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Figure 2. (A, E) Photographs of CDs@SBT-1 and CDs@SBT-2 composites under sunlight, excited with a 365 nm UV lamp, and turned off. (B, F) Steady-state photoluminescence spectra (deep blue line) and delayed photoluminescence spectra (blue line) of CDs@SBT-1 and CDs@SBT-2, excited under 370 nm at room temperature. (C, G) Time-resolved decay spectra of CDs@SBT-1 composite at 525 nm and CDs@SBT-2 composite at 440 nm at room temperature and air atmosphere. The black dots are the experimental decay and the red dots are the corresponding fitting curves. (D, H) Temperature-dependent transient photoluminescence decay of CDs@SBT-1 composite at 525 nm and CDs@SBT-2 composite at 440 nm in air atmosphere.

corresponds to the emission from T1 state. However, when the temperature increases to 300 K, the afterglow emission changes to 440 nm, corresponding to the delayed fluorescence from S1 state (Figure S4B). A similar TADF phenomenon has also been found in CDs-based composites.36 Consequently, the predominant afterglow emissions of the CDs@SBT-1 and CDs@SBT-2 composites can be attributed to RTP and TADF, respectively. In addition, CDs@SBT-1 exhibits excitationindependent phosphorescence centered at around 525 nm; while CDs@SBT-2 exhibits excitation-dependent delayed fluorescence, the emissions are red shifted from 435 to 470 nm with the increasing excitation wavelength from 330 to 400 nm (Figure S7). Notice that the photoluminescent property of UCSB-10 (SBT topology) with 1,13-diamino-4,7,10-trioxatridecane as the template has been reported before, but its afterglow property was not studied, and the luminescent mechanism was not unambiguously known at that time.37 On the other hand, in addition to the different organic templates, other synthetic factors, such as the solvents, can also influence the properties of the resulting CDs-based materials. For example, when we change the ratio of ethylene glycol: H2O (i.e., 9:0, 5:4, and 3:6) in the synthesis, the resulting CDs@ SBT composites show similar RTP or TADF properties, but the afterglow lifetimes of the composites decrease with the increase of H2O (Figure S8). The ΔEST values have been studied to elucidate different afterglow properties. By comparing the steady-state and delayed photoluminescence spectra at 77 K, we calculated the ΔEST of CDs@SBT-1 to be 0.36 eV (S1, 2.74 eV; T1, 2.38 eV) and the ΔEST of CDs@SBT-2 is 0.18 eV (S1, 2.82 eV; T1, 2.64 eV) (Figure S9). Different ΔEST values of CDs@SBT-1 and CDs@SBT-2 are responsible for their different photophysical process of RTP and TADF. As the host matrices of CDs@SBT-1 and CDs@SBT-2 are essentially same, the different ΔEST values should predominantly come from the confined CDs.

Afterglow emissions of CDs@SBT-1 and CDs@SBT-2 composites can be observed by naked eyes after the removal of ultraviolet (UV) excitation light. CDs@SBT-1 emits sky blue light under UV lamp excitation and green afterglow when excitation is turned off (Figure 2A). Under the excitation of 370 nm, the fluorescence emission centers at 453 nm (deep blue line in Figure 2B), while the afterglow emission centers at 525 nm, along with a weak emission at around 453 nm (blue line in Figure 2B). The time-resolved decay spectrum indicates that the long-lived emission at 525 nm has a lifetime of 574 ms at room temperature and air atmosphere (Figure 2C). As shown in Figure 2D, the afterglow intensities in the decay decrease as the temperatures increase from 100 to 300 K, which is characteristic for RTP materials as the rate constant of nonradiative deactivation becomes larger with the increase of temperature.34 The delayed photoluminescence spectra show the phosphorescence emission centers at 525 nm from 100 to 300 K, but the intensities decrease with the increase of temperatures (Figure S4A). Besides, the weak emission at 453 nm is found to possess TADF behavior with the lifetime of 117 ms (Figures 2B, S5, and S6), suggesting the existence of a small amount of RISC process. CDs@SBT-2 shows different afterglow properties; it displays blue fluorescence under UV excitation, and a blue afterglow when the excitation is turned off (Figure 2E). The afterglow emission centered at 440 nm is consistent with that of the prompt fluorescence of CDs@SBT-2 with a lifetime of 153 ms at room temperature and air atmosphere (Figure 2F and G). As shown by the temperature-dependent transient photoluminescence decay of CDs@SBT-2, the ratio of the delayed component rises with the increase of temperature from 100 to 300 K. This indicates that thermal activation energy can activate the afterglow emission, which is the character of TADF materials (Figure 2H).19,35 In addition, the delayed photoluminescence spectra at various temperatures show that the afterglow emissions of CDs@SBT-2 center at about 470 nm, when the temperatures change from 100 to 225 K, which 60

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peak at 1588 cm−1 (G band), corresponding to the in-plane stretching vibration E2g mode of graphite and the sp2-bonded carbon atoms. The peak at 1358 cm−1 (D band) originates from the presence of disordered carbon, which often refers to as the carbon atoms located at the edge of CDs (Figure S11).39,40 The intensity of D band in CDs@SBT-2 is stronger than that of CDs@SBT-1, indicating that more disordered carbon in CDs of CDs@SBT-2, which may lead to larger steric hindrance structure. On the other hand, XPS spectra indicate the presence of C−C/CC bonds (284.6 eV), C−O/C−N bonds (286.1 eV), CO/CN bonds (288.1 eV); pyridinic N (399.2 eV), amino N (401.0 eV), pyrrolic N (401.9 eV); CO/PO bonds (530.5 eV), C−O−C bonds (532.0 eV),41−43 P−O/PO bonds (132.6 eV), and P−N bonds (133.7 eV) in the structure of CDs (Figure 3C−J).24,25 Obviously, the contents of specific bonds in CDs isolated from the mother liquids of CDs@SBT-1 and CDs@SBT-2 are quite different, which reveals the different structural characters of CDs generated from different organic precursors. Considering that long-lived emission cannot be observed for isolated CDs from the purified mother liquids of the composites at room temperature, effective suppression of the nonradiative decay process and stabilization of triplet excited states by the zeolite host matrix are crucial to obtain the ultralong afterglow emission. The solid-state 13C MAS NMR spectra of CDs@SBT-1 and CDs@SBT-2 composites suggest that the structures of organic templates remain intact in the final composites (Figure S12). Then H-bonding interactions should be formed between the surface functional groups of CDs and organic templates/interrupt structure of zeolite host, thus suppressing the nonradiative relaxation. Meanwhile, the triplet states of CDs can be efficiently stabilized by the nanoconfined space of zeolites through locking the emissive species and inhibiting intramolecular vibrations and rotations. Therefore, the effective confinement effect of zeolite matrix, coupled with extensive H-bonding interactions, plays an essential role in harvesting triplet excited states to achieve the long afterglow lifetimes. As a result, CDs@SBT-1 with larger ΔEST value can emit predominant long-lived RTP, while CDs@SBT-2 with the smaller ΔEST value allows the RISC process from the T1 state to S1 state upon the room temperature thermal activation, giving the TADF phenomenon (Figure 4A). Note that the modulation of ΔEST has been studied in RTP system, and several methods, such as the introduction of electron-withdrawing atoms, varying the cations and anions in the host inorganic phase, tuning subtle structural changes, and intramolecular electronic coupling can successfully modulate the singlet/triplet excited states.44−47 However, the mechanistic understanding of the RTP and TADF of CDs@zeolite composites on the molecular level remains challenging. RTP and TADF materials with ultralong lifetimes may find applications in optoelectronics, bioimaging,3,48,49 and security aspects. Here, we illustrate a smart security protection pattern composed of modes of fluorescence, lifetime-encoded afterglow emission, and color-encoded afterglow emission. As shown in Figure 4B, the security pattern (logo of the International Center of Future Science at Jilin University) is coded with two parts: the CDs@SBT-1 as the “I, F, S” parts and CDs@SBT-2 as the “C” and swan parts. When UV excitation is on, the pattern exhibits blue emission, which can act as the fluorescence security protection mode. When the UV excitation is off, it can be clearly observed with naked eye that

We further study the photoluminescence of unconfined CDs. The CDs were isolated from the mother liquids of CDs@ SBT-1 and CDs@SBT-2 by dialyzing. The purified mother liquids also display excitation-dependent fluorescence in an air atmosphere (Figure S10). However, no long-lived emission is observed for the CDs in the purified mother liquids at room temperature. On the basis of the steady-state and delayed photoluminescence spectra at 77 K, the calculated ΔEST values of CDs in the purified mother liquids are 0.39 eV for CDs@ SBT-1 (S1, 2.85 eV; T1, 2.46 eV) and 0.21 eV for CDs@SBT-2 (S1, 2.88 eV; T1, 2.67 eV) (Figure 3A, B). The structures of CDs were studied by Raman and X-ray photoelectron spectroscopy (XPS) spectra. To obtain the typical bands of carbon species of CDs with Raman spectra, the core−shell Ag@CDs nanoparticles were fabricated according to the literature to quench the fluorescence of CDs in the mother liquids.38 Graphene fragment structures can be found with the

Figure 3. Steady-state (deep blue line) and delayed (olive line) photoluminescence spectra of the purified mother liquids of CDs@ SBT-1 (A) and CDs@SBT-2 (B) composites excited under 370 nm at 77 K. The high-resolution XPS spectra of C 1s, N 1s, O 1s, and P 2p for CDs in the purified mother liquids of CDs@SBT-1 (C, E, G, I) and CDs@SBT-2 (D, F, H, J) composites. 61

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luminescence, and Raman spectra of CDs in the purified mother liquids of CDs@SBT-1 and CDs@SBT-2 (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Jiancong Liu: 0000-0002-1207-6833 Yue Yu: 0000-0002-8189-8291 Jiyang Li: 0000-0002-5176-8939 Jihong Yu: 0000-0003-1615-5034 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Natural Science Foundation of China (Grants 21621001, 21835002, 21671075, 21801069) and the 111 Project (B17020) for supporting this work.

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Figure 4. (A) Proposed RTP/TADF mechanism of CDs@SBT-1 and CDs@SBT-2. (B) Application of CDs@SBT-1 and CDs@SBT2 in security protection. ICFS logo was adapted with permission from the International Center of Future Science at Jilin University.

(1) Xu, S.; Chen, R.; Zheng, C.; Huang, W. Excited State Modulation for Organic Afterglow: Materials and Applications. Adv. Mater. 2016, 28, 9920−9940. (2) Pan, Z.; Lu, Y.-Y.; Liu, F. Sunlight-Activated Long-Persistent Luminescence in the Near-Infrared from Cr3+-doped Zinc Gallogermanates. Nat. Mater. 2012, 11, 58−63. (3) Zhen, X.; Tao, Y.; An, Z.; Chen, P.; Xu, C.; Chen, R.; Huang, W.; Pu, K. Ultralong Phosphorescence of Water-Soluble Organic Nanoparticles for In Vivo Afterglow Imaging. Adv. Mater. 2017, 29, 1606665. (4) Li, Y.; Gecevicius, M.; Qiu, J. Long Persistent Phosphors-from Fundamentals to Applications. Chem. Soc. Rev. 2016, 45, 2090−2136. (5) Wang, J.; Wang, S.; Su, Q. Synthesis, Photoluminescence and Thermostimulated-Luminescence Properties of Novel Red LongLasting Phosphorescent Materials β-Zn3(PO4)2:Mn2+,M3+ (M = Al and Ga). J. Mater. Chem. 2004, 14, 2569−2574. (6) Kabe, R.; Adachi, C. Organic Long Persistent Luminescence. Nature 2017, 550, 384−387. (7) Ma, X.; Xu, C.; Wang, J.; Tian, H. Amorphous Pure Organic Polymers for Heavy-Atom-Free Efficient Room-Temperature Phosphorescence Emission. Angew. Chem., Int. Ed. 2018, 57, 10854− 10858. (8) Bian, L.; Shi, H.; Wang, X.; Ling, K.; Ma, H.; Li, M.; Cheng, Z.; Ma, C.; Cai, S.; Wu, Q.; Gan, N.; Xu, X.; An, Z.; Huang, W. Simultaneously Enhancing Efficiency and Lifetime of Ultralong Organic Phosphorescence Materials by Molecular Self-Assembly. J. Am. Chem. Soc. 2018, 140, 10734−10739. (9) Cai, S.; Shi, H.; Li, J.; Gu, L.; Ni, Y.; Cheng, Z.; Wang, S.; Xiong, W. W.; Li, L.; An, Z.; Huang, W. Visible-Light-Excited Ultralong Organic Phosphorescence by Manipulating Intermolecular Interactions. Adv. Mater. 2017, 29, 1701244. (10) Bolton, O.; Lee, K.; Kim, H.-J.; Lin, K. Y.; Kim, J. Activating Efficient Phosphorescence from Purely Organic Materials by Crystal Design. Nat. Chem. 2011, 3, 205−210. (11) Oster, G.; Geacintov, N.; Ullah Khan, A. Luminescence in Plastics. Nature 1962, 196, 1089−1090. (12) Jiang, K.; Zhang, L.; Lu, J.; Xu, C.; Cai, C.; Lin, H. Triple-Mode Emission of Carbon Dots: Applications for Advanced Anti-Counterfeiting. Angew. Chem., Int. Ed. 2016, 55, 7231−7235. (13) An, Z.; Zheng, C.; Tao, Y.; Chen, R.; Shi, H.; Chen, T.; Wang, Z.; Li, H.; Deng, R.; Liu, X.; Huang, W. Stabilizing Triplet Excited States for Ultralong Organic Phosphorescence. Nat. Mater. 2015, 14, 685−690.

“I, F, S” parts coded with CDs@SBT-1 show green RTP, while the “C” and swan parts coded with CDs@SBT-2 show blue TADF. In summary, two novel afterglow CDs@zeolite composites with distinct RTP and TADF properties have been in situ solvothermally synthesized by using the same SBT zeolite matrix but different organic templates. The nanoconfined space of zeolites coupled with H-bonds among confined CDs, organic templates, and matrix can efficiently suppress the nonradiative processes of CDs, thus stabilizing the long-lived triplet states for the afterglow emission. The introduction of different templates results in different CDs structures, thus leading to different ΔEST of CDs confined in CDs@zeolites. Unique RTP and TADF phenomena can be modulated based on different ΔEST and their lifetimes reach 574 ms and 153 ms, respectively. This work suggests that tuning the ΔEST values of photoluminescent CDs by organic templates is a powerful methodology to prepare and modulate novel afterglow materials. Considering the great variety of organic templates and a wide range of zeolitic matrices, it is anticipated that many more CDs-based materials with desired photoluminescent properties and innovative applications, such as bioimaging, sensing, and light emitting, will be discovered in the future.

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ASSOCIATED CONTENT

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmaterialslett.9b00073. Methods, powder XRD patterns, SEM images, steadystate photoluminescence spectra, delayed photoluminescence spectra, time-resolved decay spectrum, solid-state 13 C MAS NMR spectra of CDs@SBT-1 and CDs@SBT2 composites, TEM images, excitation-dependent photo62

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DOI: 10.1021/acsmaterialslett.9b00073 ACS Materials Lett. 2019, 1, 58−63