Systematic Tuning of the Luminescence Output of Multicomponent

Article Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Systematic Tuning of the Luminescence Output of Multicomponent Metal−Organic Frameworks Joel Cornelio, Tian-You Zhou, Adil Alkaş, and Shane G. Telfer* MacDiarmid Institute for Advanced Materials and Nanotechnology, Institute of Fundamental Sciences, Massey University, Palmerston North, New Zealand J. Am. Chem. Soc. Downloaded from pubs.acs.org by UNIV OF LOUISIANA AT LAFAYETTE on 11/01/18. For personal use only.

S Supporting Information *

ABSTRACT: Metal−organic frameworks (MOFs) exhibit a broad range of luminescence characteristics due to the vast array of metal ions and organic linkers available as building blocks. Systematic control over the emissive output of MOFs is highly sought after. Methods for tuning emission profiles are emerging based largely on luminescent metal ions and the encapsulation of emissive guests. Herein, we show how the functionalization of the organic linkers of a series of multicomponent MUF-77 (MUF = Massey University Framework) materials can methodically tune their spectral output. This was quantified by chromaticity diagrams. White-light emission was obtained by combining the photophysical characteristics of the three distinct organic fluorophores present in these materials. Our results also show that both (i) energy transfer interactions between the organic components and (ii) noncovalent interactions with guests can also be harnessed to tune the emission. These results establish multicomponent metal−organic frameworks as fluorescent materials with unique spectral characteristics.

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profiles.22−26 The drawback of using lanthanide ions as the source of emitted light, however, is that their spectral profile is narrow, and consequently they have low color rendering indices (CRIs).27 Introducing guest molecules, such as metal complexes, fluorescent dyes, and volatile organic compounds, to MOF pores can tune luminescence profiles and avoid these drawbacks.8,28−30 One of the foremost guiding principles of the design and synthesis of metal−organic frameworks is that of isoreticular chemistry.31 Among other things, this posits that introducing small functional groups to MOF linkers while retaining their length will conserve the lattice structure. The application of this principle has been shown to generate families of materials with rational structure−property relationships. This allows them to be optimized for a variety of applications.32 In this work, we build on the isoreticular principle to show that the emission profiles of multicomponent MOFs can be tuned by systematically introducing functional groups to the linkers. This work takes advantage of the fact that the diamagnetic nature of zinc(II) in general does not influence the intrinsic emission behavior of the linkers.33

INTRODUCTION Metal−organic frameworks (MOFs) often exhibit interesting and useful luminescence properties. These materials typically use one or more of the following strategies to achieve luminescent output: emission from organic linkers, emission from metal clusters, encapsulation of emissive guest molecules, defects or charge transfer processes.1,2 The attractive photophysical characteristics of MOFs have resulted in their applications in chemical and biological sensing, thermometry, second- and third-harmonic generation, and white-light emission.3−8 Obtaining while-light emission from UV input is an active area of luminescence research due to widespread applications in display devices. This process typically uses phosphors, and a variety of materials have been used to generate white light.9−14 Reports of white-light emission from MOFs share some common strategies, for example using MOFs with a controlled combination of red−green−blue or blue−yellow emitters15−19 or making use of photophysical processes among the framework constituents.20,21 From a survey of the field it is evident that control over the emissive output of MOFs is highly sought after. Nevertheless, there are only a few families of MOFs that exhibit emission profiles that can be tuned in a systematic manner. Gai et al. were able to tune the emission characteristics of one series by using MOFs with varying ratios of lanthanide metal ions.16 Other groups have used trivalent lanthanide ions as guests or as components of metal clusters to produce variable emission © XXXX American Chemical Society

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RESULTS AND DISCUSSION MUF-77 (Massey University Framework-77)34,35 is a multicomponent, quaternary MOF comprising three chemically and Received: September 12, 2018

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DOI: 10.1021/jacs.8b09887 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Journal of the American Chemical Society

Figure 1. Schematic of the structure of MUF-77 showing the relative arrangement of the ligands.

Figure 2. Representative PXRD pattern (Cuα radiation) of the MUF77 nanocrystals with an SEM image (inset, scale bar: 1 μm). The PXRD patterns of large MUF-77 crystals synthesized under solvothermal conditions and simulated from the SCXRD structure are shown for comparison.

Scheme 1. Preparation of Luminescent MUF-77 Nanocrystals (1−16)

Scheme 1): (a) As a starting point, we used an unfunctionalized set of linkers to build up the parent MUF-77 frameworks, [Zn4O(hxtt)4/3(bpdc)1/2(bdc)1/2], where x is an alkyl group on the truxene linker (Figure 1). (b) A second set of frameworks was made by introducing a guanidine functional group to the bpdc linker (gua). This linker is a fluorophore that emits light in the yellow region (λem = 570 nm). (c) A third set of frameworks were constructed using the strong blue emitter bdc-NH2 (λem = 427 nm) in place of bdc. (d) Finally, we combined both gua and bdc-NH2 linkers in the same framework. In contrast to the traditional solvothermal synthesis of MUF-77, which produces crystals of around 500 μm in size, we sought to develop a synthetic protocol to make nanocrystals of these materials which would be better suited to the luminescence studies. This was achieved by conducting the synthesis at room temperature and using zinc acetate in place of zinc nitrate (Scheme 1). For each combination of three organic linkers, their ratio had to be carefully optimized to avoid the formation of other products. The successful realization of phase-pure MUF-77 analogues 1−16 was demonstrated by PXRD (Figure 2 and Figure S31− S35) and 1H NMR spectroscopy of acid-digested samples. The PXRD patterns can be indexed to the lattice planes predicted for MUF-77 (Figure S31), which shows that the arrangement of the linkers is the same as that deduced by single-crystal Xray diffraction for the parent MUF-77 (Figure 1). Furthermore, single crystals of selected MUF-77 derivatives (1, 5, and 13) were grown, and their unit cell parameters were found to be virtually identical to those of the parent material (Table S4). Since the ligand functional groups are located in large pores, rotational disorder prevents their coordinates from being pinpointed by XRD methods. NMR spectroscopy, on the other hand, is an excellent tool for showing that they are introduced to the framework without any degradation or side reactions. In addition, 1H NMR spectroscopy confirms that the ratio of the ligands exactly matches the [Zn4O(hxtt)4/3(bpdc/gua)1/2(bdc-

geometrically distinct ligands connected by Zn4O clusters. Its overall formula is [Zn4O(hxtt)4/3(bpdc)1/2(bdc)1/2] (bdc = 1,4-benzenedicarboxylate, bpdc = 4,4′-biphenyldicarboxylate, hxtt = alkyl-functionalized truxene-2,7,12 tricarboxylate). Importantly, derivatives of MUF-77 are isoreticular with the parent: all three linkers can be replaced by analogues that bear functional groups, while the lattice maintains an ith-d topology. The materials reported herein were generated in accord with this isoreticular principle: when functional groups are introduced to the linkers, the overall lattice structure is unperturbed. We employed the following synthetic strategy to develop MUF-77 analogues with tunable luminescence (Figure 1, B


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Figure 3. Emission spectra of MUF-77 nanocrystals 1−16, suspended in DMF, with an excitation wavelength of 390 nm.

R)1/2] stoichiometry (Figures S9−S24). Scanning electron microscopy (SEM) showed that the particles were fairly monodisperse with sizes ranging from 100 to 200 nm (Figure 2 and Figure S37). Overall, this synthetic procedure offers multiple advantages including: (a) the formation of MUF-77 nanocrystals which are nearly uniform in size, (b) the potential to incorporate ligands into MUF-77 that are sensitive to the solvothermal method, (c) facile suspension of the crystalline MOFs in solvents to allow their emission properties to be measured in a straightforward way, and (d) ease of use in applications such as coatings on surfaces like LEDs or in membranes. On measuring the fluorescence of a DMF suspension of the MUF-77 nanocrystals with an excitation wavelength of 390 nm we made the following observations: The various MUF-77 frameworks showed unique emission spectra. This observation immediately demonstrates how the multicomponent nature of MUF-77 can be effectively employed to tune the framework luminescence by introducing simple functional groups to the framework linkers. The emission spectra of MOFs 1, 2, 3, and 4 are dominated by the output from their truxene-based tritopic linkers with peaks centered between 430 and 445 nm (Figure 3a and Figure S6). Upon illumination with UV light the frameworks appear blue in color (Figure S38).

No contributions to the emission spectra are observed from the bpdc and bdc linkers since these components are essentially non-emissive. Because the Zn4O cluster is also nonemissive and coordination of the linkers to zinc(II) results in low spin−orbit coupling,33 the emission profile of the frameworks is largely derived from the individual components. The results from MOFs 5, 6, 7, and 8 show that the emission profiles of the frameworks can be systematically tuned by introducing functional groups to the linkers. The gua ligand features a guanidine-based fluorophore and gives rise to yellow emission (λem = 570 nm, Figures 3b and S7). The incorporation of gua produces a yellow emission band for 5 (575 nm). 6 and 7 also showed slightly blue-shifted emission profiles centered at 565 nm, with a minor peak at 468 nm (Figure 3b). A further blue-shift moving to 560 nm was observed for 8. Here, it is evident that increasing the length of the alkyl functional groups on the tritopic linker results in a shift to lower wavelengths for the yellow emission. Very little contribution from the characteristic blue peaks of the truxene component was noted for these frameworks. We tentatively ascribe this to excited-state energy transfer to the gua ligand, which subsequently enhances emission from this component. The highly emissive nature of the bdc-NH2 linker (Figure S7) endows 9, 10, 11, and 12 with an intense blue emission (Figures 3c and S38). The relevant peaks in the emission spectra fall in the range 430−440 nm. Moreover, the emission C


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Figure 5. Emission spectra of bdc-NH2 and hmtt showing a spectral overlap (gray) with the absorption spectrum of the gua ligand.

Scheme 2. Preparation of MUF-77 Nanocrystals 17−22 by Systematically Including the Yellow Fluorescent gua with Nonfluorescent bpdc in Different Ratios

Figure 4. (a) CIE diagram for the emission spectra of 5−8 and 9−12. (b) CIE diagram for 13−16 showing high tunability.

profiles are influenced by the tritopic linker, which also fluoresces in the same region, making the emission maxima somewhat sensitive to its identity. MOFs 13, 14, 15, and 16 incorporate both the blue (bdcNH2) and yellow (gua) emissive ligands (Scheme 1) together with the four tritopic truxene linkers. A complex interplay between all three components produces frameworks with highly tunable emission profiles (Figures 3d). By increasing the length of the alkyl chain on the tritopic linkers, we obtained distinct spectra for each framework. The blue band showed a steady blue shift from 468 nm for 13 to 445 nm for 16. This was accompanied by an increased intensity relative to the yellow emission band. Additionally, the emission peak of gua at 570 nm seen for 13 moves to lower wavelengths, with 16 having a broad peak centered at 556 nm. This shift is similar to that seen for frameworks 5, 6, 7, and 8. It is important to note

Figure 6. Emission spectra of MUF-77 nanocrystals with various ratios of gua and bpdc linkers (λex = 390 nm). The formula for these frameworks is [Zn4O(hmtt)4/3(gua)x(bpdc)y(bdc)1/2], and the x:y ratio is shown in the figure legend.

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Figure 8. Photographs of an UV LED before and after coating with 15, which acts as a phosphor to generate white light.

coordinates were close to each other (Figure 4a). All of them lie in the yellow area of the CIE diagram with (x, y) CIE coordinates ranging from (0.45−0.42 to 0.49−0.48). The CCT values for these yellow emission profiles range from 3300 K for 5 to 3828 K for 8, which translates to warm colors (Table S2). The interplay of photophysical processes of all three components and the trend in relative contributions of blue and yellow peaks from 13, 14, 15, and 16 resulted in whitelight emission with tunable CCT values (Figure 4b and Table S2). 15 shows white-light emission with CIE coordinates (0.3218, 0.3590) and CCT of 5935 K close to that of pure white light (0.33, 0.33). This white-light emission is plainly evident to the naked eye (Figure S38). The distinctive contribution of the blue band in 16 translates to a cool (CCT = 16 033 K), bluish-white emission, while 13 (CIE 0.4129, 0.4535) and 14 (CIE 0.3569, 0.4156) have warm, yellow-white emissions (CCT 3780 and 4837 K, respectively). We performed preliminary studies on the photophysical processes which underlie white-light outputs from 13, 14, 15, and 16. One prerequisite for energy transfer is the spectral overlap of donor emission with the absorption profile of the acceptor.20,41 The absorption spectrum of the gua ligand (the acceptor) is compared with the emission spectra of the donors, i.e., hmtt and bdc-NH2 in Figure 5. The clear spectral overlap indicates that energy transfer is possible in these multicomponent materials. This energy transfer behavior proved to be advantageous in that it gave us another method to tune the spectral characteristics of these materials. By methodically diluting the proportion of the yellow-emissive gua ligand in the framework by partially replacing it with the nonluminescent bpdc linker, we prepared nanocrystals of 17, 18, 19, 20, 21, and 22 (Scheme 2 and Table S3). PXRD patterns showed that the MUF-77 structure was retained (Figure S36), and 1H

Figure 7. Changes in the emission spectra on adding (a) nitrobenzene and (b) benzaldehyde to a suspension of nanocrystals of 15, [Zn4O(hmtt)4/3(gua)1/2(bdc-NH2)1/2], in dichloromethane (λex = 390 nm).

that this range of tunability with these systems is achievable only when all three fluorophores work in tandem, which draws on the multicomponent nature of MUF-77. We translated the emission spectra into chromaticity coordinates and plotted them onto CIE (Commission Internationale de l’É clairage) diagrams. This is useful not only to visualize the perceived color and tunability of the emissions but also to show that certain MOFs exhibit highly desirable chromaticity coordinates in the blue, yellow, and white regions. Also, since some emissions had chromaticity coordinates close to the Planckian locus, calculating correlated color temperature (CCT) was possible. Spectral profiles with CCT values above 5000 K, referred to as “cool colors”, have applications in indoor lighting. On the other hand, materials with “warm” CCT values below 5000 K can be used for outdoor lighting.36 MOFs 1, 2, 3, 4, 9, 10, 11, and 12, showed CIE coordinates which were all in the blue region (Figure 4a and Figure S1). Among these materials, saturated deep-blue emission37 with CIE coordinates of y < 0.10 was observed for 1, 9, 11, and 12 (Table S2). Such blue emitters may have potential applications in displays and solid-state lighting.38,39 Yellow emissive materials are useful as they can be used for making physiologically friendly lighting sources.40 Due to the similarities in the emission behavior of 5, 6, 7, and 8, the CIE E


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realize this experimentally, we selected 15, which has a whitelight output. We exchanged the solvent occluded in nanocrystals of 15 with acetone by repeated centrifugation and resuspension. On coating an UV LED (λem = 393 nm) with cyanoacrylate glue and then by repeatedly dipping it into the acetone suspension, we obtained a coating of 15 on the LED. On illumination, the LED glowed white, indicating that MUF77 nanocrystals are effective and robust solid-state phosphors (Figure 8).

NMR spectroscopy on their acid-digested solutions showed that the ratios of gua to bpdc were 1:0.25, 1:1, 1:3, 1:5.6, 1:9, and 1:19, respectively (Figures S25−S30). Our hypothesis was that the emission spectra of 17−22 would show a steady increase in the truxene-based blue emission with diminishing levels of the acceptor gua component in the MOFs. This arises from some of the truxene-based blue emission being unquenched by energy transfer to gua. This was borne out by the emission spectra of their DMF suspensions at 390 nm (Figure 6). A reduction in the energy transfer from the truxene linker when lower levels of gua are present leads to a drop in intensity of the yellow peak around 575 nm and a corresponding increase in the blue emission peak. Consequently, the CIE coordinates derived from these emission spectra (Figures S4−S5 and Table S3) vary linearly and traverse across the CIE diagram from the yellow region for 17 (0.4206, 0.4681) to the blue region for 22 (0.2120, 0.1750). The CIE coordinates for 20 were (0.3519, 0.3780) which are indicative of white light. These coordinates move to the blue region (0.2900, 0.2998) for 21. This demonstrates that adjusting linker ratios in these multicomponent frameworks is an effective method for tuning the spectral output and achieving white-light emission. Guanidine groups are well-known for their ability to engage in hydrogen bonding with various functional groups.42−44 Since the luminescent gua ligand contains a guanidinyl moiety, we considered whether it would be sensitive to the inclusion of guest molecules capable of H-bonding. The chiral center of gua may also be responsive to the stereochemistry of the substrate molecule. As a proof of principle, we took nanocrystals of 5 ([Zn4O(hmtt)4/3(gua)1/2(bdc)1/2]), which were originally in DMF, and redispersed them in dichloromethane by repeated centrifugation and resuspension. This mitigated any competition for H-bonding sites on the frameworks between occluded DMF and the added guest molecules. On adding small quantities of the H-bond acceptor nitrobenzene as a guest molecule, we found that the emission was steadily quenched (Figure 7a). The quenching was more pronounced for the yellow peak (570 nm) compared to the smaller blue peak (468 nm). This caused the CIE coordinates to shift from (0.4354, 0.4848) at 0 mM nitrobenzene to (0.3855, 0.4330) at 160 mM nitrobenzene (Figure S2). The addition of other H-bonding guests such as benzaldehyde also impacted the emission of 5. In addition to reducing the intensity of the yellow band, the addition of benzaldehyde produced another feature, viz., a steady increase in the blue component of the emission. This originates from the tritopic hmtt ligand (Figure 7b) and implies that energy transfer from the truxene to the gua is attenuated by the benzaldehyde guest. Consequently, higher tunability is achievable in this case. The CIE coordinates move from (0.3714, 0.4292) in 16 mM benzaldehyde to near white light (0.3109, 0.3692) in 32 mM benzaldehyde. The coordinates move further to the blue region of the CIE diagram at higher concentrations of benzaldehyde (Figure S3). As a control experiment, the emission was unaffected on adding the πelectron-deficient non-H-bonding guest benzonitrile (Figure S8). These features make MUF-77 systems unique in that their emission profiles can be finely tuned by addition of H-bonding guests which readily diffuse into the framework. One of the key advantages of nanocrystalline MUF-77 is its suitability for coating substrates such as LED devices. To

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CONCLUSION These results establish multicomponent metal−organic frameworks as fluorescent materials with unique spectral characteristics. The emissive properties of multicomponent MOFs can be tuned using three different approaches: linker modification, interligand energy transfer, and guest binding. These draw on the multicomponent nature of MUF-77, which generates an array of three photoactive ligands in precise positions with well-defined orientations.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.8b09887. CIE diagrams, CIE coordinates, ligand and MOF synthesis and characterization data, emission spectra of ligands, representative SEM images, and photos of MOFs under UV irradiation (PDF)

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AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Shane G. Telfer: 0000-0003-1596-6652 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to Seok June (Subo) Lee and David Perl for valuable discussions and to the staff of the Manawatu Microscopy and Imaging Centre (MMIC) for the SEM images. The RSNZ Marsden Fund supported this study (contract 14-MAU-024).

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REFERENCES

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Systematic Tuning of the Luminescence Output of Multicomponent

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