Achieving Multicolor Long-Lived Luminescence in Dye-Encapsulated

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Achieving Multi-Color Long-Lived Luminescence in Dye-Encapsulated MetalOrganic Frameworks and Its Application to Anti-Counterfeiting Stamps Jianbin Liu, Yixi Zhuang, Le Wang, Tianliang Zhou, Naoto Hirosaki, and Rong-Jun Xie ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13486 • Publication Date (Web): 20 Dec 2017 Downloaded from http://pubs.acs.org on December 21, 2017

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ACS Applied Materials & Interfaces

Achieving Multi-Color Long-Lived Luminescence in Dye-Encapsulated Metal-Organic Frameworks and Its Application to Anti-Counterfeiting Stamps Jianbin Liu, † Yixi Zhuang, *,† Le Wang, * ,‡ Tianliang Zhou, † Naoto Hirosaki, § and Rong-Jun Xie, *,†,§ †

College of Materials, Xiamen University, Xiamen 361005, P. R. China. Email:

[email protected]; [email protected]

‡

College of Optical and Electronic Technology, China Jiliang University, Hangzhou

310018, P. R. China. Email: [email protected]

§

Sialon Group, National Institute for Materials Science 1-1 Namiki, Tsukuba, Ibaraki

305-0044, Japan. Email: [email protected] KEYWORDS:

metal-organic

frameworks,

long-lived

dye-encapsulated, energy transfer, anti-counterfeiting

1

ACS Paragon Plus Environment

luminescence,

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT:

Long-lived luminescent metal-organic frameworks (MOFs) have attracted much attention due to their structural tunability and potential applications in sensing, biological imaging, security systems, and logical gates. Currently, the long-lived luminescence emission of such inorganic-organic hybrids is dominantly confined to short-wavelength regions. The long-wavelength long-lived luminescence emission, however, has been rarely reported for MOFs. In this work, a series of structurally stable long-wavelength long-lived luminescent MOFs have been successfully synthesized by encapsulating different dyes into the green phosphorescent MOFs Cd(m-BDC)(BIM). The multi-color long-wavelength long-lived luminescence emissions (ranging from green to red) in dye-encapsulated MOFs are achieved by the MOFs-to-dye phosphorescence energy transfer. Furthermore, the promising optical properties of these novel long-lived luminescent MOFs allow them to be used as inkpads for advanced anti-counterfeiting stamps. Therefore, in this work it not only offers a facile way to develop new types of multi-color long-lived luminescent materials, but also provides a reference for the development of advanced long-lived luminescent anti-counterfeiting materials.

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1. INTRODUCTION

Counterfeiting of documents, currency, food and branded products is a global problem, causing enormous economic and security implications for the government, businessmen and consumers.1-3 It is estimated that the underground economy brought by counterfeiting each year amounts to billions of dollars. The urgent need for developing next-generation high-tech anti-counterfeiting system has triggered extensive researches on advanced anti-counterfeiting materials in the last decade. Optical functionality, owing to the feature of hard-to-copy and easy-to-authenticate (readout), is considered to be an ideal security element in the anti-counterfeiting systems. So far, a variety of optical-functionality-based anti-counterfeiting materials, including lanthanide-doped luminescent nanomaterials, quantum dots (semiconductor and carbon based), and plasmonic nanomaterials have been successfully developed.4-9 These materials generally possess small/uniform particle size and high fluorescent efficiency, suggesting great promise in the anti-counterfeiting application. However, the anti-counterfeiting optical information in these materials is mainly based on the short-lived fluorescence emission, which suffers serious background fluorescence interference from the common printing media (e.g. office paper or banknotes). Therefore, to discover or develop excellent materials showing long-lived fluorescence (i.e. persistent luminescence and delayed fluorescence) or room-temperature phosphorescence (RTP) is quite important for the development of advanced

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anti-counterfeiting technologies without background fluorescence.

Luminescent materials with long-lived fluorescence and phosphorescence have always been the interest of research due to their important applications in light-emitting diodes, sensing, security systems, and biological imaging.10-13 At present, these materials are mainly inorganic phosphors (e.g., SrAl2O4:Eu2+,Dy3+ or CaAl2O4:Eu2+,Nd3+). These materials, however, contain rare earth elements with limited resources, and also are lack of tunable structure.14-17 Through sensitization of metal ions by organic linkers, organometallic complexes can also exhibit phosphorescence phenomena.18-21 Unfortunately, they have relatively low quantum efficiency, extremely short phosphorescence lifetime (typically several milliseconds) and poor structural stability. It is therefore necessary to find new material systems with desired long-wavelength long-lived luminescence and excellent structural flexibility and stability.

Metal-organic frameworks (MOFs) are crystalline materials self-assembled from metal ions or clusters with organic ligands (linkers) to form highly cross-linked structure with chemically adjustable porosities and high internal surface area.22-24 Because they possess both the stable structure of inorganic crystals and the easy tailoring of organic materials, luminescent MOFs (LMOFs) are getting much more attention in recent years and find their potential applications in sensing, laser, bio-imaging, photocatalysis and so on.25-36 Recently, Qian et al. have developed some 4


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fluorescence-color-tunable LMOFs based on a dye-encapsulated strategy for applications in microlaser, white light-emitting diodes and temperature sensing.37-41 In addition, several LMOFs with second-time-scale RTP have also been successively reported, and are thus suggested for applications in temperature/pH sensing.42-47 Unfortunately, these phosphorescent LMOFs mostly show the short-wavelength (ca. 520 nm) green RTP. Although the long-wavelength phosphorescence (650 nm) can be realized by introducing pyridine guest molecules into MOFs at Yan’s group, the crystal structure of MOFs is inevitably destroyed or distorted with the incorporation, and the phosphorescence is also unstable due to the volatilization of guest molecules when the temperature is above 50 °C.42 Additionally, a few red (near 615 nm) phosphorescent MOFs doped or filled with lanthanide Tb3+ ions or organometallic complexes have been reported, but they have more or less problems of low quantum efficiency, short lifetime and poor stability of phosphorescence, which cannot meet the requirements for practical applications.48-50 To expand the application of such types of luminescent materials to other fields such as biological imaging and advanced security systems, long-wavelength long-lived luminescent materials also need to be discovered.

In this work, we successfully synthesized a series of color-tunable and structurally stable LMOFs with second-time-scale long-lived luminescence in the wavelength regions ranging from green, yellow, orange, to red. Multi-color long-lived

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luminescence was achieved by efficient RTP energy transfer from the host (phosphorescent MOFs) to the guest (dyes) in a dye-encapsulated MOFs system. The strategy of RTP energy transfer shows a great potential to obtain enormous novel long-lived luminescent materials by the different combinations of phosphorescent MOFs and dyes in the reported materials library. Also, we attempted to produce an anti-counterfeiting stamp by using the novel dye-encapsulated MOFs with long-wavelength long-lived luminescence. As the stamp has both short-lived fluorescence and long-lived luminescence (i.e. RTP from MOFs and delayed fluorescence from the encapsulated dyes sensitized by the triplet state of the ligands) with different and unduplicated spectral fingerprints, multiplex modes of anti-counterfeiting can be readily achieved.

2.EXPERIMENTAL SECTION

Materials: All chemical reagents and dyes for the syntheses in this work were purchased from commercial sources and used without further purification.

Characterization: Powder X-ray diffraction (PXRD) patterns of all composites were recorded with a Bruker AXS Advance diffractometer using Cu Kα radiation (λ = 1.5418 Å). Measurements were made in a 2θ range of 5°−60° at room temperature with a step of 0.0163° (2θ) and a counting time of 0.1 s/step. The operating power was 40 kV, 40 mA. Thermogravimetric analysis (TGA) experiments were carried out on a 6


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TG209F1 thermal analyzer under air atmosphere from room temperature to 800 °C with a heating rate of 10 °C min-1. The UV-visible diffusive reflectance spectra were measured on a Shimadzu UV-3600-plus spectrophotometer and the fluorescence spectra were recorded by using a Hitachi F-4600 spectrometer. The long-lived luminescence spectra were obtained from an Ocean Optics QE Pro charge coupled device (CCD). The fluorescence quantum yield (QY) at room temperature was measured by using a Teflon-lined integrating sphere (F-M101, Edinburgh, diameter: 150 mm; weight: 2 kg) in FLS980 fluorescence spectrometer. The long-lived luminescence QY was measured by using a Hitachi F-4600 spectrometer. Confocal laser scanning images were taken on a Leica TCS SP5 laser scanning confocal microscope equipped with a Leica TCS SP5 positive confocal microscopy. The photos and supporting movies were respectively recorded by a Canon EOS 5D MarkⅡ equipped with a Canon EF 100 mm f/2.8 Macro and a Canon EOS 700D equipped with a Canon EF 50 mm f/1.8 STM after the products were irradiated by either 254 or 365 nm ultraviolet light.

Synthesis of MOFs PM 1: PM 1 was synthesized following a literature procedure with some modifications. A mixture of Cd(NO3)2·4H2O (160 mg, 0.5 mmol), m-BDC (83 mg, 0.5 mmol), BIM (26 mg, 0.2 mmol), CH3COONa (287 mg, 3.5 mmol) and H2O (10 mL) were sealed into a 50 mL Teflon reactor, heated under autogenous pressure at 150 °C for 36 h. After slow cooling of the reaction mixture to room

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temperature, colorless crystals were collected by filtration and washed with distilled water and ethanol in turn, then dried at 60 °C for 12 h. Yield: 29.5% (based on Cd).

Synthesis of Dye-Encapsulated MOFs Composites PM 2-6: Dye-encapsulated MOFs composites were synthesized in a similar procedure as PM 1 crystal, except for the different dye solutions in distilled water or ethanol with different concentration were additionally injected into the precursor solution of PM 1. Namely, 200 µL ethanol solutions of 4-Methylumbelliferone (BMU; 2 g · L-1), 200 µL aqueous solutions of Fluorescent Green B (FGB; 180 mg·L-1), 200 µL aqueous solutions of Rhodamine 123 (Rh 123; 200 mg·L-1), 200 µL aqueous solutions of Rhodamine 6G (Rh 6G; 100 mg·L-1), and 1500 µL aqueous solutions of Rhodamine B (Rh B; 100 mg·L-1) were injected into the precursor solution of PM 1 to yield PM 2, PM 3, PM 4, PM 5, PM 6, respectively. The products were washed thoroughly with distilled water and ethanol for several times to remove residual dyes on the surface of MOFs, and dried at 60 °C for 12 h. Yield: 26.5% for PM 2, 23 % for PM 3, 26.1% for PM 4, 25.7% for PM 5, 25.8% for PM 6 (based on Cd).

Preparation of Inkpads for the Anti-Counterfeiting stamp: A mixture of 500 mg PVP (Polyvinylpyrrolidone, Mw=58000), 2 mL Baifb skin care glycerin and 2 mL ethanol were sealed in a 10 mL glass vial, and preserved after the ultrasonic treatment for 15 min. Subsequently, colored inkpad was prepared by mixing 150 mg ground PM 6 and 0.5 mL preservation solutions described above under the ultrasonic treatment 8


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for 15 min.

3.RESULTS AND DISCUSSION

3.1. Synthesis and Structure

Micrometer-sized crystals of Cd-based MOFs, Cd(m-BDC)(BIM) (hereafter referred to phosphorescent MOFs PM 1), were synthesized via a solvothermal reaction following a literature procedure with some modifications.47 A mixture of Cd(NO3)2 · 4H2O, 1,3-benzenedicarboxylic acid (m-BDC), benzimidazole (BIM), CH3COONa and H2O were sealed into a 50 mL Teflon reactor and heated under autogenous pressure at 150 °C for 36 h. After slow cooling of the reaction mixture to room temperature (RT), colorless crystals were collected by filtration and washed with distilled water and ethanol in turn, then dried at 60 °C for 12 h. Inspired by the one-pot synthesis strategy of guest molecules encapsulated in ZIF-8 crystals, such as caffeine,51 fluorescein,52 porphyrin,53 doxorubicin,54 and coronene,43 the thermally and chemically stable PM 1 emitting green RTP was selected as the host to encapsulate organic fluorescent dyes. Dye-encapsulated PM 1 composites were synthesized in a similar procedure as PM 1 crystals (see Experimental Section). The resulting crystals of dye-encapsulated PM 1 composites (hereafter referred to PM 2, PM 3, PM 4, PM 5, PM 6) were washed with distilled water and ethanol in turn to remove the excess dyes adsorbed on the MOFs surface. 9



To confirm the successful encapsulation of dyes into MOFs, typical PM 1 and PM 6 were chosen to be characterized by PXRD, TGA, and laser scanning confocal microscopy (LSCM). PXRD patterns of these dye-encapsulated MOFs are nearly identical to that of PM 1 (Figure S1), indicating that the one-pot synthesis process does not destroy the crystal structure. TGA results show that no substantial weight loss originates from the sublimation of Rhodamine B (Rh B) in PM 6 at about 290 °C (the temperature for Rh B starting to sublime), which implies that dye molecules are tightly restricted by the MOFs matrix (Figure S2). A comparison of Rh B aqueous solution and PM 6 dispersed in aqueous solution before and after centrifugation was conducted to verify the integration of dyes and MOFs. The simultaneous sedimentation of the dye Rh B and MOFs after centrifugation indicates that Rh B is tightly bonded to MOFs (Figure 1a). LSCM displays a uniform dye distribution in a single crystal, suggesting that the dye Rh B molecules are dispersed throughout MOFs and not adsorbed onto the outside surface (Figure 1b).

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Figure 1. (a) Photographs of Rh B aqueous solution and PM 6 dispersed in aqueous solution before and after centrifugation. The colorless suspension solution of PM 6 after centrifugation (up right) confirms the successful conjunction of MOFs and Rh B. (b) Confocal image of PM 6 at different depths recorded on a Leica TCS SP5 laser scanning confocal microscope (λex = 561 nm).

3.2. Optical Properties and Tuning of Emission Colors

A schematic diagram for the illustration of photo-physical processes in dye-encapsulated MOFs is shown in Scheme 1. The incident radiation is absorbed (purple arrow A in Scheme 1) by typically conjugated organic compounds called 11



linkers or ligands. The fluorescence emission in the MOFs (blue F) is subsequently observed due to the electron radiative transition from the excited singlet state S1 to the ground singlet state S0 of ligands. Besides, intersystem crossing (ISC) between the S1 and T1 states of the ligand occurs. Afterwards the phosphorescence emission in the MOFs (green P) occurs owing to the radiative transition from the triplet state T1 to S0 of ligands, and this emission usually has a lifetime in the range of several microseconds to seconds. For the host-guest system of dye-encapsulated MOFs, after the absorption of incident radiation by the ligands in the host matrix MOFs, possible Dexter energy transfer (blue and green ET) from MOFs to guest dyes may take place in two routes:1) S1→S0 (MOFs) to S’0→S’n (dye) and 2) T1→S0 (MOFs) to S’0→S’1 (dye). Therefore, successful encapsulation of dyes into MOFs may result in the quenching of the original fluorescence (blue F) and phosphorescence (green P) emissions from the ligand, and the appearance of additional fluorescence (red F) and delayed fluorescence (red DF) emissions from the dye. On the basis of this strategy, luminescent colors and relative intensities can be flexibly tuned by controlling the species and concentrations of dyes.

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Scheme 1. Schematic diagram for the photo-physical processes in dye-encapsulated MOFs. (A = Absorption; F = Fluorescence; P = Phosphorescence; DF = Delayed Fluorescence; ISC = Intersystem Crossing; ET = Energy Transfer)

As shown in Figure 2a, the synthesized PM 1-6 powders show a variety of colors under natural light, which suggests different absorptions in the visible region associated with the dyes incorporation. Specifically, the UV-visible diffuse reflection spectra of PM 1-6 (Figure S3) evidence that the PM 1-6 powders have absorption bands in the visible region (> 400 nm). The changes in these spectra also indicate the formation of host-guest systems between MOFs and dyes.

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Figure 2. (a) Photographs of PM 1-6 with a black plastic board as background under natural light (left), UV excitation on (middle) and UV excitation off (right). Exposure time was 1/2000 s for PM 1-6 (left), 1/10 s for PM 1, 1/100 s for PM 2, 1/50 s for PM 3, 1/60 s for PM 4 and PM 5, 1/125 s for PM 6 (middle), 1/4 s for PM 1-6 (right), respectively. Other parameters were manually kept constant: ISO value-3200, aperture value-3.5, and white balance-sunlight. (b) Fluorescence emission spectra of different dye species (pink solid line), fluorescence (white solid line), and long-lived luminescence emission spectra (colored solid line) and (c) corresponding CIE chromaticity coordinates of PM 1-6 with different dye species excited at 365 nm under ambient conditions.

The free m-BDC ligand emits the blue fluorescence centered at 394 nm under UV 14


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excitation (Figure S4), and immediately turns into green phosphorescence at 510 nm after the removal of UV excitation (Figure S5). After the ligands coordinated with metal Cd clusters to form PM 1, the fluorescence peak shifts to 401 nm upon excitation at 365 nm (Figure S6). Interestingly, after the removal of UV excitation, PM 1 emits bright green phosphorescence centered at 520 nm (Figure S7), which should be attributed to the radiative transition from the triplet state T1 to S0 of m-BDC ligands.55 The phosphorescence duration could be perceived for approximately 10 s by the naked eyes (see the movie S1). The above results are similar to those reported in the literature.47

With the integration of dyes into MOFs, the fluorescence color in PM 2-6 varies from blue to green, yellow, orange, and red under UV irradiation (Figure 2a-middle). As shown in the fluorescence spectra (Figure 2b-white solid line and pink solid line), the spectra of PM 2-6 show double-band characteristics: one peaked at approximately 400 nm from the original MOFs and another at longer wavelengths from the dyes, respectively. The emission maximum for the long-wavelength fluorescence reveals obvious a guest-dependent feature covering a wide range from 410 nm for PM 2 to 630 nm for PM 6. Accordingly, the fluorescence of PM 2-6 shifts from blue to red in the Commission Internationale deL’ Eclairage (CIE) chromaticity diagram (Figure 2c), which proves the superiority of dye-encapsulated MOFs as luminophores with different emissions by modulating dye species. Charmingly, after the removal of UV

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excitation, PM 3-6 exhibit MOFs-dye dual long-lived luminescence emissions that last for a few seconds as traced by the naked eyes, and show yellowish green, yellow, orange, and red delayed fluorescence from the encapsulated dyes (Figure 2a and 2b, also see the movie S1). These changes of photoluminescence spectra are also indicative of the successful encapsulation of dyes into PM 1. As PM 1 itself shows a green phosphorescence emission while the dye only emits short-lived fluorescence, the long-lived delayed fluorescence of dye may be achieved by the efficient MOFs-to-dye phosphorescence energy transfer in dye-encapsulated MOFs. Compared with PM 1 itself, PM 2-6 possess higher photoluminescence quantum yield (Table S1). The fluorescence and long-lived luminescence quantum yields for PM 6 are estimated as 18.99% and 12.35%, which are higher than those of PM 1 (with quantum yields of 2.38% for fluorescence and 5.1% for phosphorescence). The increase in quantum yields can be ascribed to efficient light harvesting of dyes. It is unprecedented that the excellent long-wavelength long-lived luminescence emission can be realized by simply encapsulating dyes into MOFs, thus providing a new and facile strategy for the synthesis of long-wavelength long-lived luminescent MOFs materials.

In order to get deeper insights into the energy transfer mechanism in PM 2-6, PM 6 is selected as a typical case for analysis. As shown in Figure S8, the absorption of the dye Rh B and the fluorescence of PM 1 show an obvious spectral overlap,

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therefore the energy transfer mentioned above may occur from PM 1 to dyes when the two luminescent species are close to each other. Moreover, although Rh B has much less absorption at 365 nm, the intense emission of Rh B in PM 6 can be observed, which is due to the efficient sensitization of the ligand m-BDC and the isolation effect from the MOFs matrix (Figure S9). Additionally, the fluorescence intensity and color of PM 6 can be finely tuned by just changing the dye concentration. Upon increasing the dye concentration from 0.006 to 0.15 mg, the emission intensity of MOFs increases and reaches its maximum at 0.07 mg, and the emission spectra redshift monotonically (Figure S10).

Similarly, as shown in Figure 3a, due to the significant spectral overlap between the absorption of Rh B in PM 6 and the phosphorescence of PM 1, the phosphorescence energy transfer would occur from MOFs to Rh B effectively, leading to long-lived red delayed fluorescence assigned to Rh B (Figure S11). The delayed fluorescence color and intensity of PM 6 can be systematically tuned by altering the dye concentration under 365 nm excitation (Figure 3b and 3c). Upon increasing the dye concentration from 0.006 to 0.15 mg, the phosphorescence emission intensity of MOFs gradually quenches; while the emission intensity of Rh B rises firstly and then decreases accompanied by the redshift of the emission wavelength due to the partial dye aggregation caused by the excessive concentration. Phosphorescence decay curves upon excitation at 365 nm show that the lifetime monitored at 520 nm sharply

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decreases from 765 ms for PM 1 to 293 ms for PM 6 (Figure 3d). The changes in the emission lifetime also confirm the presence of the MOFs-to-dye phosphorescence energy transfer in PM 6. The decay time of MOFs itself is mainly depended on the phosphorescence energy transfer from the MOFs to Rh B, which is shortened in MOFs but elongated in Rh B.

Figure 3. (a) The phosphorescence emission spectrum (green solid line) of PM 1 and the UV-visible absorption spectrum of Rh B in aqueous solution (red solid line). (b) The long-lived luminescence emission spectra and (c) the CIE chromaticity coordinates for PM 6 with different dye concentrations excited at 365 nm under ambient conditions. (d) Time-resolved emission decay curves for PM 1 and PM 6 monitored at 520 nm and 630 nm under ambient conditions. 18


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3.3. Application to the Anti-Counterfeiting Stamp

Encouraged by the excellent long-lived luminescent performance and ultra high photostability of PM 2-6 (Figure S12), an anti-counterfeiting inkpad was fabricated by mixing PM 6, polyvinylpyrrolidone (PVP), ethanol and glycerol with appropriate mass ratio (see Experimental Section). Figure 4a presents the pattern (the logo of College of Materials, Xiamen University) sealed by using the home-made inkpad, and the high-quality pink pattern can be easily identified after the rapid evaporation of alcohol under natural light. In addition, when excited at 365 nm, the stamp shows the red fluorescence emission, on a printing paper exhibiting the blue fluorescence background. As expected, it immediately turns into the dark orange long-lived luminescence emission after the removal of UV excitation, which can be perceived within several seconds by the naked eyes (Figure 4a; the movie S2). The fluorescence spectrum also displays a strong MOFs-dye dual fluorescence emission with a peak value intensity ratio of 7.11 (Ired/Iblue) and the calculated chromaticity coordinates are (0.57, 0.32) (Figure 4b). Similarly, in the long-lived luminescence spectrum, the strong MOFs-dye dual long-lived luminescence emission (Ired/Igreen = 9.48) is also observed, with the CIE values being (0.58, 0.36) (Figure 4c). By the way, in comparison with PM 6 itself, the emission of the inkpad is subjected to a slight blueshift, which is possibly due to the lower energy transfer efficiency caused by the partial structure destroy during the grinding for the inkpad preparation. The

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time-resolved emission decay curves upon excitation at 365 nm show that the lifetime is measured to be 293 and 363 ms when monitored at 520 and 630 nm, respectively (Figure 4d). PXRD patterns also confirm the stability of PM 6 crystallinity in the stamp forming solution for 5 months (Figure S13). From these results, we believe that the luminescent properties and crystalline integrity of PM 6 are well maintained in the inkpad.

Typically, the highly designable attributes make luminescent materials a popular class of advanced anti-counterfeiting agents, which give prescribed luminescent features upon exposure to a specific illumination wavelength. Compared to a single fluorescence-based anti-counterfeiting, additional security information, such as rare spectra and lifetime features, can be provided by long-lived luminescence, thus making long-lived luminescent anti-counterfeiting materials more difficult to be replaced

by

others.

In

addition,

upconversion

or

downconversion

based

anti-counterfeiting inks can usually be excited by a single wavelength, and they subsequently emit at only a single wavelength too. This anti-counterfeiting measure appears to be comparatively less effective in protecting documents. On the other hand, the dye-encapsulated MOFs PM 6 anti-counterfeiting inkpad has at least two excitations and color emissions (viz. MOFs itself and dyes, respectively). In particular, there is fluorescence but no phosphorescence emission for PM 6 anti-counterfeiting inkpad under 520 nm excitation (i.e. the excitation only for the dye). Herein such

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distinctive and unduplicated spectral fingerprint makes the stamp a highly reliable anti-counterfeiting measure that is very tough to break through. Predictably, upgraded levels of anti-counterfeiting measures can be achieved if a combination of different-color long-lived luminescent MOFs is used or a single MOFs material is encapsulated with a variety of dyes. Furthermore, because of their excellent luminescence properties, the security information can be easily detected by a simple time-resolved charge-coupled device (CCD), which is of great significance for practical applications. Clearly, such a dye-encapsulated MOFs platform enables us to readily fabricate high performance advanced anti-counterfeiting materials by modulating the amounts and components of encapsulated dyes. We believe that this simple but facial synthetic strategy provides a new path for the development of advanced anti-counterfeiting materials, which would be a new challenge for counterfeiting technology.

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Figure 4. (a) Photographs of the anti-counterfeiting stamp under natural light (left), UV excitation on (middle) and UV excitation off (right). Exposure time was 1/400 s for the left, 1/100 s for the middle, and 1/4 s for the right, respectively. Other parameters were manually kept constant: ISO value-3200, aperture value-2.8, and white balance-sunlight. (b) Fluorescence and (c) long-lived luminescence emission spectra. (d) Time-resolved emission decay curves monitored at 520 nm and 630 nm under ambient conditions.

4.CONCLUSION

In summary, we successfully synthesized novel dye-encapsulated MOFs that exhibit the long-wavelength (red) second-time-scale long-lived luminescence owing to the efficient MOFs-to-dye phosphorescence energy transfer. The dye-encapsulated MOFs were applied as anti-counterfeiting stamps with multiple spectral features that are superior to others based on a single fluorescence. Additionally, a large number of organic fluorescent dyes with high quantum efficiency, such as cyanine, coumarin, and boron dipyrromethene, can also be encapsulated into various phosphorescent MOFs to systematically tune the long-lived luminescent emission color and quality. Furthermore, the facile approach for preparing dye-encapsulated MOFs enables to design and develop novel long-lived luminescent materials as well as advanced anti-counterfeiting system or potential biological imaging.

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ASSOCIATED CONTENT

Supporting Information

The Supporting Information is available free of charge on the ACS Publications website. Figure S1 shows the XRD patterns of PM 1 (black line), PM 2(blue line), PM 3 (green line), PM 4 (yellow line), PM 5 (orange line), and PM 6 (red line). Figure S2 shows the TGA data of Rh B (blue), PM 1 (black), and PM 6 (red). Figure S3 shows the UV−visible diffuse reflection spectra of PM 1-6 in the solid state at room temperature. Figure S4 shows the fluorescence normalized excitation (pink, monitored at 394 nm) and emission (blue, excited at 353 nm) spectra of m-BDC in the solid state at room temperature. Figure S5 shows the phosphorescence emission spectrum of m-BDC excited at 365 nm in the solid state at room temperature. Figure S6 shows the fluorescence emission spectrum of PM 1 excited at 365 nm in the solid state at room temperature. Figure S7 shows the phosphorescence emission spectrum of PM 1 excited at 365 nm in the solid state at room temperature. Figure S8 shows the fluorescence emission spectrum (blue solid line) of PM 1 and the UV-visible absorption spectrum of Rh B in aqueous solution (red solid line) at room temperature. Figure S9 shows the emission spectra of Rh B in PM 6 (red solid line), in aqueous solution (pink solid line) and the Rh B powder (black solid line) excited at 365 nm in the solid state at room temperature. Figure S10 shows the fluorescence emission spectra (up) and the 1931 CIE chromaticity coordinates (below) for PM 6 with different dye concentrations 23



excited at 365 nm in the solid state at room temperature. Figure S11 shows the excitation spectra of dyes in MOFs (monitored at 410 nm for PM 2, 547 nm for PM 3, 567 nm for PM 4, 590 nm for PM 5, 630 nm for PM 6) and phosphorescence emission spectrum of PM 1 in the solid state at room temperature. Figure S12 shows the fluorescence emission spectra of (a) PM 6 (b) PM 5 (c) PM 4 and (d) PM 3 monitored every half an hour. Figure S13 shows the XRD patterns of PM 6 (black line) and PM 6 in the inkpad for 5 months (red line). Table S1 shows the fluorescence quantum yield of PM 1-6 with different dye species. Movie S1 shows the fluorescence and long-lived luminescence of PM 1-6 samples with different dye species. Movie S2 shows the fluorescence and long-lived luminescence of the anti-counterfeiting stamp.

AUTHOR INFORMATION

Corresponding Authors

E-mail: [email protected]

E-mail: [email protected]

Email: [email protected]

Notes

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The authors declare no competing financial interest. ACKNOWLEDGMENTS

This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 51572232, 51561135015, 51502254, 61575182, 61405183), and National Key Research and Development Program of China (2017YFB0404301).

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TOC figure:

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Achieving Multicolor Long-Lived Luminescence in Dye-Encapsulated

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