Incorporating Electron-Deficient Bipyridinium Chromorphores to Make

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Incorporating electron-deficient bipyridinium chromorphores to make multiresponsive metal-organic frameworks Ning-Ning Yang, Jia-Jia Fang, Qi Sui, and En-Qing Gao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17381 • Publication Date (Web): 29 Dec 2017 Downloaded from http://pubs.acs.org on December 30, 2017

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ACS Applied Materials & Interfaces

Incorporating electron-deficient bipyridinium chromorphores to make multiresponsive metalorganic frameworks Ning-Ning Yang, Jia-Jia Fang, Qi Sui, En-Qing Gao* †

Shanghai Key Laboratory of Green Chemistry and Chemical Processes, School of Chemistry

and Molecular Engineering, East China Normal University, Shanghai 200062 (P. R. China). KEYWORDS. metal-organic frameworks, electron transfer, charge transfer, logic gates, anion sensors

ABSTRACT. Metal-organic frameworks (MOFs) are versatile platforms to design switchable and sensory materials responsive to external stimulus. Copuling the electron-deficient bipyridinium chromorphore with the pore structures of MOFs is a nice strategy to design multiresponsive MOFs. Here we present a proof-of-concept study. Postsynthetic N,N’cycloalkylation of UiO-67-bpy (bpy = 2,2’-bipyridyl) leads to a novel ionic MOF (UiO-67-DQ) functionalized by the electron-deficient diquat (DQ) chromophore. The combination of porosity, cationic character and electron deficiency imparts UiO-67-DQ with versatile responsive properties. It readily undergoes anion exchange, with selective ionochromism associated with charge-transfer (CT) complexation; it is electrochemically active and shows anion-dependent photochromism associated with radical formation through electron transfer (ET); the iono- and

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photochromism cause efficient luminescence quenching because of energy transfer (EnT) to CT complexes or radicals. The properties of UiO-67-MQ (MQ = N,N'-dimethyl-2,2'-bipyridylium) are also presented for comparison. The CT and ET effects and consequently the EnT efficiency in UiO-67-MQ are weaker than those in UiO-67-DQ because the electron-deficient character is weakened by the severe interannular twist in MQ2+. On the basis of the rich responsive properties, the MOFs are used as sensory and switching materials for facile discrimination of a range of anions, for quantitative detection of I-, and for mimicking of logic operations ranging from simple logic gates to complex integrated logic circuits.

INTRODUCTION Metal-organic frameworks (MOFs) are a newly-emerging class of porous materials built of metal-based nodes and organic linkers.1-3 Thanks to the combination of crystallinity, porosity, diversity and tailorability in structure and properties, MOFs have been vigorously explored as versatile platforms for materials design relevant to many applications.

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In particular, MOFs

have great potentials in the design of sensing and switching materials because they can in principle be functionalized at the metal sites, the organic linker or the pore space with specific motifs that are responsive to various physical or chemical stimuli.8-11 N,N’-diquaternized 4,4'-bipyridiniums (viologens) and the less common 2,2’ analogs are electron-deficient compounds featuring two renowned attributes: the redox activity to form radicals through electron transfer (ET) triggered by a certain stimulus, and the propensity to form electron donor-acceptor (D-A) complexes with electron-rich species through charge transfer (CT).12-13 The properties have allowed for various applications of viologens, ranging from electron relays in photochemical reactions to key building units in supramolecular assemblies


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(such as mechanically interlocked molecules and molecular machines14-15) and to switching/sensing materials related to electro-, photo-, or piezochromism.16-20 Incorporating the bipyridinium moiety into MOFs is a nice approach to responsive MOFs.21 Recently, viologen ligands bearing carboxylate groups have been used to construct coordination polymers showing ET-based responses in color, luminescence, magnetism and other properties.22-27 However, MOFs that allow effective coupling of the porosity of the frameworks and the rich responsive properties of the bipyridinium chromophore are still rare.28-29 It is highly desired to obtain highly porous structures that can gave full play and interplay to the ET and CT functions of the chromophore, but the construction of stable porous MOFs directly from bipyridinium-based ligands is a synthetic challenge because the positive charge of bipyridinium disfavors strong metal coordination.28, 30 An alternative route to bipyridinium MOFs is postsynthetic modification (PSM), which has been a powerful strategy to stable and functionalized MOFs and especially to those otherwise inaccessible. 31 For this purpose, 2,2'-bipyridinium is preferred to the 4,4' analog because 2,2'-bipyridyl-based dicarboxylate ligands can form stable porous MOFs with the N atoms uncoordinated and open to the pores,32-33 which thus allows for postsynthetic Nquaternization. We envision that the 2,2’-bipyridinium-functionalized MOFs can have the following responsive properties (Figure 1). (1) The porosity of the frameworks and the CT propensity of the bipyridinium group can cooperate to allow for selective inclusion and colorimetric sensing of appropriate electron-rich species; (ii) Associated with the ET propensity of the bipyridinium group, the MOFs should be electrochemically active and can be photochromic; (iii) The ET and CT processes can have impacts on other properties, such as luminescence; (iv) The frameworks are cationic with counteranions enclosed in the pores, which can impart anion exchange ability; (v) Anion exchange can provide additional dimensions of


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responsiveness, such as anion dependence in CT, ET and luminescence. The PSM approach to bipyridinium MOFs still remains largely unexplored. The only known example is N,N’dimethylation of UiO-67-bpy (a Zr(IV) MOF derived from 2,2’-bipyridyl-5,5’-dicarboxylic acid), which leads to an electron-deficient cationic framework (hereafter denoted as UiO-67MQ) with N,N'-dimethyl-2,2'-bipyridylium (MQ).34-36 We have recently shown that UiO-67-MQ selectively responds to alkylamines in color and fluorescence because of CT interactions.34 In this article, we report on a new bipyridinium MOF, UiO-67-DQ (DQ = diquat), which was postsynthetically derived from UiO-67-bpy by N,N’-cycloalkylation (Figure 1). Thanks to the more electron-deficient character of DQ than MQ, UiO-67-DQ displays richer and more sensitive responsive behaviors than UiO-67-MQ, illustrating the first MOF showing all of the properties outlined above. The responsive mechanisms are analyzed, and the implications for applications in colorimetric anion discrimination, iodide determination, and logic gate mimicking are also presented.

Figure 1. Synthesis and the potential functions of an ionic MOF bearing the electron-deficient bipyridinium chromophore.


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RESULTS AND DISCUSSION Synthesis and Characterization.UiO-67-DQ was synthesized by a facile PSM reaction under mild conditions: simply stirring UiO-67-bpy and ethylene ditriflate (EDT) in CHCl3 for 5 h at room temperature. The formation of ionic UiO-67-DQ was confirmed by IR spectroscopy (Figure S1). The product shows a new IR band at 3087 cm-1, attributable to the ethylene group, and the strong bands observed at 1262, 1168 and 1030 cm-1 are characteristic ν(SO3) and ν(CF3) absorptions of the triflate (TfO-) counteranion.37 According to 1H NMR with UiO-67-DQ digested in HF (aq.)/d6-DMSO (Figure S2), 65% of the 2,2'-bipyridyl groups are quaternized to DQ. Powder X-ray diffraction (PXRD) profile of UiO-67-DQ is in good agreement with that of UiO-67-bpy (Figure 2a), confirming the retention of the porous framework after PSM. According to nitrogen adsorption (Figure 2b), the Brunner-Emmet-Teller (BET) surface area of UiO-67-DQ is 815 m2 g-1, with a total pore volume of 0.383 cm3 g-1. These values are lower than those for UiO-67-bpy,34 which is owing to the introduction of ethylene groups and TfOcounteranions. The scanning electron microscopy (SEM) images revealed an octahedral shape with an average size of 400 nm (Figure S3). Thermogravimetric analysis (TGA) and PXRD suggest that the framework does not collapse until 300°C (Figure S4).

Figure 2. (a) PXRD patterns of UiO-67-bpy and UiO-67-DQ. (b) N2 adsorption/desorption isotherms of UiO-67-bpy and UiO-67-DQ.


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Ion Exchange and Ionochromism. The UiO-67-type cationic frameworks have open pores and windows for anion accommodation and diffusion and thus are nice candidate materials for anion exchange. The as-synthesized UiO-67-DQ-TfO and UiO-67-MQ-TfO were stirred in aqueous solutions containing an excess anion (Cl-, Br-, I-, NO3-, SCN-, SO42- or ClO4-, as potassium or sodium salts). The occurrence of ion exchange was confirmed by IR spectra (Figure S5). After the treatment, the characteristic bands of TfO- disappeared, and in the cases of multiatomic anions, the characteristic absorptions (NO3-, 1380 cm-1; SCN-, 2060 cm-1; SO42-, 1135 cm-1; ClO4-, 1100 cm-1) appeared as new strong bands. To our satisfaction, PXRD measurements showed that the UiO-67-type framework retains its structural integrity in UiO-67-DQ-X and UiO-67-MQ-X (X denotes the above-mentioned anions) (Figure 3 and S6).

Figure 3. PXRD patterns of UiO-67-DQ-X. Interestingly, the ion exchange processes can be accompanied by marked color change, depending upon the anion involved. When UiO-67-DQ-TfO was exchanged with Br-, SCN- and I-, the color rapidly turns from white to pale yellow, yellow and orange, respectively. No color change was discerned for the exchange with Cl-, NO3-, SO42- and ClO4- (Figure 4b). The selective


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ionochromism is indicative of electronic interactions between the framework and the (pseudo)halides. Control tests revealed that UiO-67-bpy is completely irresponsive in color to all of the above anions, precluding the possibility that ionochromism arises from the interactions of the anions with the [Zr6O4(OH)4(COO)12] cluster or the unquaternized bpy linker. Definitely, it is the PSM-generated DQ2+ chromophore that interacts with Br-, SCN- or I- to induce color change. The color change suggests that the interactions are more than simple interionic electrostatic forces but include CT from electron-rich anions to electron-deficient DQ2+. That is, a groundstate ion-pair CT complex (or D-A complex) is formed, which causes new UV/visible CT absorption that is not characteristic of either the donor or the acceptor.38 The UV−vis spectra for all UiO-67-DQ-Xs are collected in Figure 4a. For X = Br-, SCN- and I-, CT complexation is evidenced by the obvious absorptions in the visible region. The spectra for X = TfO-, NO3- , SO4 2-

, ClO4- do not evince CT interactions, consistent with the weak electron-donating character of

these oxyanions; the minor difference in the absorption edge could be due to the perturbation by electrostatic interactions. For X = Cl-, a small redshift (compared with the preceding oxyanions) of the whole UV band is discernable, which may indicate very weak CT interactions. Generally, for a given electron acceptor, the donor having a higher electron-donating ability (a smaller oxidation potential) affords a stronger CT interaction with a smaller CT transition energy (∆CT, the gap between the highest occupied molecular orbital (HOMO) of the donor and the lowest unoccupied molecular orbital (LUMO) of the acceptor). Indeed, Milliken dependence

39-40

was

observed for UiO-67-DQ-Xs with X = (pseudo)halides, i.e., a linear correlation between ∆CT (estimated from the CT bands. Table S1) and the oxidation potential of X- (Figure 4a, inset)41. This unequivocally establishes the CT character of the (pseudo)halide-DQ2+ interactions.


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Figure 4. Color change (b) and UV-vis spectra (a and c) of UiO-67-DQ after anion exchange and Xe-light irradiation. Inset of (a): Milliken plot for the MOFs with (pseudo)halides: correlations of the CT transition energy (∆CT) to the oxidation potential (E° (X-/X) vs. NHE) in aqueous solution. The ESR signals after Xe-light irradiation are given beside the corresponding photographs in (b). The anion-exchange material can be easily regenerated and recycled. Anion exchange can be reversibly accomplished with any pair of the anions and can be monitored by IR spectra, by UV spectra or simply by the naked eye in cases of ionochromism. The framework remains intact after repeated exchange, as confirmed by PXRD after ten consecutive exchange cycles (Figure S7). The exchange rate is dependent upon the anions involved. As an example, the exchange between TfO- and I- is examined in details. White UiO-67-DQ-TfO turns immediately orange in contact with aqueous I-, while orange UiO-67-DQ-I fades very slowly when immersed in solutions containing excess OTf-. Actually, among the anions studied, I- is the easiest to be exchanged in but the most difficult to be exchanged out, reflecting the strong CT interactions contribute much to the affinity of the cationic framework for I-.


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To gain more insight into the structure-property relationship, anion exchange was also performed with UiO-67-MQ. Here ionochromism only occurs to SCN- (pale yellow) and I(yellow). The other UiO-67-MQ-Xs we tested are all nearly white (Figure S8). In the UV-vis spectra, Br- leads to a small red-shift in UV absorption, while SCN- and I- cause obvious visiblelight absorption (Figure S9). Comparing the MQ and DQ series reveals that the MQ MOF shows lighter color and larger ∆CT (higher LUMO energy) than the DQ counterpart with the same (pseudo)halide, consistent with the more negative reduction potential (weak electron-acceptor strength) of MQ2+. The two series show a single Milliken linear correlation when plotting∆CT against ∆E° (Figure S10. ∆E° = E°(X-/X) - E°(Q2+/Q•+) with Q = MQ and DQ). The different CT ability of MQ2+ and DQ2+ can be well justified by considering the steric effects of the different N-alkyl groups. The steric hindrance between the two N-methyl groups in MQ2+ dictates a severe non-coplanar twist between the two pyridinium rings, the twist angles ranging from 50° to nearly perpendicular according to crystallographic and theoretical calculations for MQ2+ derivatives.42-43 By contrast, the conformational restraints imposed by the annulating N,N’-ethylene group in DQ2+ leads to much smaller twist angles (15-24°).44-45 The approximate coplanar structure in DQ2+ enjoys electron delocalization and hence the electronaccepting LUMO is lowered to the advantage of CT. Photochromism. The postsynthetic quaternization not only imparts the MOFs with ionexchange and CT coloration properties, but also leads to photoresponsive properties. The UiO67-DQ-X MOFs exhibit obvious and anion-dependent color change upon irradiation with a 300 W xenon lamp. For example, the white MOFs with X = TfO-, Cl- and NO3- turn yellow, brown and brownish yellow, respectively. The CT colorated MOFs with X = Br-, SCN- and I- also change color after irradiation (Figure 4b). All materials show significant changes in the UV-vis


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spectra after irradiation, with medium to strong absorption in the visible region (Figure 4c). Because the precursor MOF (UiO-67-bpy) is completely inert to xenon light, the photochromic properties of UiO-67-DQ-X must be ascribed to the PSM-generated DQ2+ chromophore, which is prone to form the DQ•+ radical through photoinduced electron transfer (PET). The formation of radicals was unequivocally confirmed by electron spin resonance (ESR) spectroscopy. All these MOFs are ESR silent before photoirradiation, but obvious signals appear after irradiation (Figure 4b). Potential electron donors are the carboxylate groups and the counteranions. The fact that the photogenerated color and absorption spectra are anion dependent could imply the involvement of anions, but the possibility of intraligand PET from the carboxylate group to DQ2+ cannot be ruled out. The effective electron donors are open to further investigation. For comparison, the UiO-67-MQ-X MQ series show much weaker response to photoirradiation. Prolonged xenon-light irradiation can lead to deepened color for X = SCN- and I- and faint color for most others (Figure S8 and S11). The color change for X = TfO- and Cl- is indiscernible. The weak photo-response of the MQ series relative to the DQ counterparts can be explained as follows. Generally, the PET processes involve photoexcitation either at the bipyridinium units (MQ2+ and DQ2+) or at the anion-bipyridinium CT complexes, followed by ET and radical formation.12,

46

Both the excited states and the radical states feature electron

occupation of the LUMOs of the bipyridinium units. As mentioned above, the MQ unit shows a severely twisted structure owing to the steric hindrance between the two methyl groups. Compared with the nearly coplanar structure of DQ, the twist of MQ leads to a higher LUMO and hence destabilizes the excited and radical states to the disadvantage of PET.


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Figure 5. Fluorescence spectra of UiO-67-DQ-X before (a) and after (b) irradiation. Luminescent Response. The ionic MOFs also show changes in luminescence in response to ion exchange and photoirradiation. UiO-67-DQ-TfO fluoresces in solid state at λmax = 405 nm. After exchange with other anions, the emission shows no significant shift in wavelength but the intensity is quenched to different degree depending on the anion used. The intensity across the series varies as TfO- > NO3-~ClO4- > SO42- > Cl- > Br- > SCN- > I- (Figure 5a). There is obvious overlap between the emission band and the absorption spectra of anion-exchanged MOFs, and the anion dependence of the emission intensity is in good agreement with the variation of the absorbance above 400 nm (Figure 4a) and with the ionochromic phenomena for Br-, SCN- and I-. Therefore, the fluorescence quenching occurs through the energy transfer (EnT) mechanism.47


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The strong quenching effects of (pseudo)halides can be related to efficient EnT to CT complexes. The fluorescence spectra after photochromism were also measured (Figure 5b). While the weakly-fluorescing MOFs with X = Br-, SCN- and I- show no significant change, the fluorescence of the other materials is significantly quenched after photochromism. The phenomena are not surprising and could be due to EnT to photogenerated radicals, considering that the emission band is covered by the radical absorption (compare Figure 4c and 5b). The EnT mechanism is confirmed by comparison of the emission spectra of different MOFs and their ligands (Figure S12a). The main emission bands of UiO-67-DQ, UiO-67-MQ and UiO-67-bpy are very similar in wavelength to one another and to those of the ligands (2,2’-bipyridyl-5,5’dicarboxylic acid and its N-alkylation derivatives). The results indicate that N-alkylation and the Zr ion have little influence on the emission. In addition, the emission spectra of the DQ and MQ salts without carboxylic substituents are significantly different in wavelength (Figure S12b), suggesting that the N-alkyl groups have strong effects on the fluorescence arising from the bipyridinium chromophores. Therefore, it is most likely that the similar emission of the MOFs arises from the carboxylate group rather than the bipyridyl/bipyridinium groups or the whole ligands. Thus, the luminescence quenching after photochromism is not because of the nonemissive nature of the DQ•+ or MQ•+ radical, and the most likely mechanism is energy transfer from the emissive chromophore (carboxylate) to the radical. The luminescence properties of the MQ series are also investigated. The anion dependence of emission intensity is similar to that for the DQ series, with X = I- and SCN- leading to the strongest quenching (Figure S13a). The quenching effects of (pseudo)halides on the MQ MOFs are less efficient than on the DQ counterparts, in accordance with the weaker CT ability of


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MQ2+. The quenching after photoirradiation is also weaker in the MQ series (Figure S13b), consistent with the weaker PET effects. Electrochemical Properties. Cyclic voltammetry was performed with MOF-modified GCEs (Figure 6 and S14). The UiO-67-DQ-X series (X= TfO-, Cl-, Br-, SCN-, I-) show two sets of quasi-reversible redox peaks with formal potentials in the narrow ranges E1 = -0.36 ̴ -0.33 and E2 = -0.72 ̴ -0.70 V vs Ag/AgCl. The two-step ET behavior is characteristic of bipyridinium compounds (DQ2+ ↔ DQ•+ ↔ DQ)

13, 44

and suggests that the bipyridinium group in the

framework is electrochemically addressable. The MQ series also show the two-step redox character, but the peaks shift towards more negative potentials, confirming the weaker electronacceptor strength of MQ2+ than DQ2+. In addition, the peak currents of the second reduction and the two oxidation processes for the MQ series are much weaker, suggesting lower stability of the reductive products of MQ2+. The LUMO and HOMO energies have been calculated from the onset potential of the first reduction peak and the energy gap obtained from UV-vis spectra (Table S1).48-49 The LUMOs are located at the bipyridinium moieties, and in cases of CT complexation, the HOMOs should be at (pseudo)halides. The schematic energy diagrams drawn in Figure 6 clearly show that the HOMO energy increases in the order Cl- < Br- < SCN- < I- and that the LUMO energy decreases from MQ2+ to DQ2+, consistent with the CT and ET trends observed in the ionochromic and photochromic studies.


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Figure 6. (a) Cyclic voltammograms of UiO-67-DQ-X with the corresponding salts as supporting electrolytes (0.1 M, KCl, KBr, KI, KSCN and NaOTf). Scan rate ν = 100 mV s-1.(b) HOMO-LUMO energy levels for UiO-67-MQ-X and UiO-67-DQ-X. Detection of I- In Solution Phases. Convenient and efficient detection of iodine has been urged for its importance to life, for the health and environmental problems caused by its discharge during applications, and for radionuclide monitoring in nuclear power industry.50 Encouraged by the anion-dependent chromogenic and luminescence response of the cationic MOFs, we investigated the MOFs as chemical sensors for I-.


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Figure 7. (a) Relative fluorescence intensity (at 423 nm) of UiO-67-DQ-TfO/EtOH dispersions in response to various anions (4 mM). Variation of fluorescence spectra (b) and I0/I (c, 423 nm) upon incremental addition of KI. Dispersing as-synthesized UiO-67-DQ in water or ethanol gives a stable milk-white suspension fluorescing at λ

max

= 423 nm. Notably, the solid isolated by centrifuging the

suspension retains the UiO-67-type framework (Figure S15), and the supernatant does not fluoresce, indicating that the MOF (rather than any dissolved species) in the suspension is responsible for fluorescence. Addition of I-, SCN- or Br- causes obvious and anion-dependent color change and fluorescence quenching (Figure 7a and S16). By contrast, NO3-, ClO4-, SO42and Cl- do not lead to significant changes in color and fluorescence intensity. The phenomena are similar to those observed in solid states, except that the latter group of anions causes different degree of fluorescence quenching in the solid states (vide supra). We ascribe the difference to solvent effects: the weak host-anion interactions that influence the solid-state fluorescence are overcome by solvation of the cationic host and the anions, but solvation is not sufficient to overcome the relatively strong CT interactions between the host and I-, SCN- or Br-. Similar solvent effects were also observed for UiO-67-MQ (Figure S17). The solvent effects are beneficial to the selective fluorescent detection of I-, SCN- or Br-. For comparative analysis on I--detecting performance, fluorescence titration was performed with three systems: UiO-67-DQ in water and in ethanol, and UiO-67-MQ in water (Figure 7 and


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S18). For all systems, incremental addition of KI leads to gradual quenching of fluorescence. The I0/I versus [I-] plots are linear at low concentrations and have been fitted to the Stern– Volmer equation I0/I = KSV[I-] + 1, where KSV is the quenching constant measuring the sensitivity.47 The limit of detection (LOD) was estimated according to the 3σ convention (σ is the standard deviation of the blank measurements). The most sensitive system is UiO-67-DQ (EtOH), with LOD < 1 ppm (Table 1). The quenching efficiency and the KSV constant increase and the LOD decreases form UiO-67-MQ (aq) to UiO-67-DQ (aq) and to UiO-67-DQ (EtOH). The differences between UiO-67-MQ (aq) and UiO-67-DQ (aq) are attributable to the higher electron-accepting strength of DQ2+ than MQ2+ (vide supra). The better performance of UiO-67DQ-TfO in ethanol than in water can be justified in terms of solvent effects. In water, the anionic donor (I-) and the cationic acceptor (DQ2+) are solvated to the disadvantage of CT complexation. In ethanol, which is less polar, the solvation effect is weaker for the benefit of CT complexation. Therefore, the color and fluorescence response to I- is more sensitive in EtOH than in water. Table 1. Selected data for I- detection.

a

System

QE/% a

KSV/M-1

LOD/ppm

UiO-67-MQ (aq)

48

338

17

UiO-67-DQ (aq)

87

1739

1.7

UiO-67-DQ (EtOH)

94

4292

0.91

Quenching efficiency (QE = (1-I/I0)×100%) at [I-] = 4 mM.

Test Paper for Anion Discrimination. Chemically specific colorimetric test papers are the simplest and most convenient devices for quick and on-site qualitative chemical discrimination. Thanks to the rich ionochromic and photochromic properties, UiO-67-DQ can be used to prepare test papers for naked-eye discrimination of different anions. The white test papers were obtained


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simply by dip-coating MOF suspensions onto filter papers. Dropping the solutions of I-, SCNand Br- on the test papers or dipping the papers in the solutions leads to distinct, steady colors within a few seconds (Figure 8). Reversible switch between different colors (including white) can be achieved simply by dipping the paper in an appropriate solution. Furthermore, putting the ion-exchanged papers under a xenon lamp leads to new colors characteristic of the ions exchanged.

Figure 8. Photographs of the ion-indicator test papers upon exposure to various anions before and after under xenon lamp irradiation. Chemical Logic Gates. Implementation of Boolean logic operations using sensory molecular systems that produce measurable physical outputs in response to external chemical inputs are becoming an inspiring subject in recent years,51-52 but the study using MOFs is still rare.53-54 The multi-responsive switching properties of the present MOFs can be used to implement the functions of different logic gates. The output signal can be either light absorption (OA) or fluorescence (OF), with low and high absorbance/intensity as 0 and 1 states, respectively. Within this convention, a chromogenic anion such as I- operating on UiO-67-DQ mimics a YES gate for OA at CT wavelengths or a NOT gate for OF. With both I- and Br- as inputs, three different logic


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gates can be fabricated. The output signals for different input combination, the circuit symbols and the truth tables are shown in Figure 9. Either I- or Br- or both cause strong absorption at 400 nm (OA1 = 1) and significant fluorescence quenching (OF = 0), so an OR and a NOR logic gates are achieved. When the absorption above 500 nm is recorded as output (OA2), the TRANSFER logic is satisfied, which is equivalent to the YES logic for input I-, with input Br- has no effect. Including SCN- as the third input would lead to three-input OR and NOR logic gates for OA1 and OF, respectively.

Figure 9. The OR, NOR and YES logic gates based on UiO-67-DQ. (a) Logic circuits. (b) UVVis spectra measured with test papers for different input combinations. The fluorescence spectra are given in the supporting information (Figure S19). (c) Truth table. (d) Pictures of the test papers and column plots for different outputs. Similar logic gates can be fabricated using UiO-67-MQ. More sophisticated logic operations can be accomplished by introducing alkylamines as inputs. As shown recently by us,34 UiO-67MQ absorbs alkylamines through CT complexation. The photophysical changes concomitant with amine absorption are similar to what we described above for anion exchange with I-, but the


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Figure 10. The INHIBIT and IMPLICATION logic gates based on UiO-67-MQ with BA (BA = butylamine) and H+ as inputs. (a) Logic circuits. (b) UV-Vis spectra measured with test papers for different input combinations. The fluorescence spectra are given in the supporting information (Figure S20). (c) Truth table. (d) Pictures of the test papers and column plots for different outputs. changes can be reverted by H+, which protonates the amines and destroys CT. Here we are focused on designing logic gates utilizing the responsive behaviors. Just like a chromogenic anion, a single amine input produces a YES or NOT logic depending upon the output recorded. The combination of two amines or an amine plus a chromogenic anion leads to OR or NOR. Because H+ alone does not cause effects but disables amines, the results of an amine and an acid (such as HCl, HOTf) acting on UiO-67-MQ correspond to an INHIBIT gate for OA and an IMPLICATION gate for OF (Figure10). With a chromogenic anion as additional input, the acid/amine/anion 3-input system mimics two integrated logic circuits, in which an INHIBIT gate is followed by an OR or NOR gate for OA and OF, respectively (Figure S21). If two or more


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anions or amines were applied as input to the acid/amine/anion system, further integrated circuits with four or more inputs can be mimicked. Two 5-input logic circuits are illustrated in Figure 11.

Figure 11. UiO-67-MQ-based 5-input logic operations successively integrating OR, INHIBIT and OR/NOR gates (BA = butylamine, DEA = dietnyleamine). (a) Logic circuits. (b) UV-Vis spectra measured with test papers for different input combinations. The fluorescence spectra are given in the supporting information (Figure S22). (c) Truth table and pictures of the test papers and column plots for different outputs.

CONCLUSIONS In summary, multistimuli-responsive ionic MOFs featuring electron-deficient bipyridinium functionalities have been synthesized by postsynthetic N-alkylating modification and the


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multipurpose use in anion sensors and chemical logic gates have been described. The combination of porosity, the cationic nature, and the electron-accepting propensity affords UiO67-DQ with versatile responsive properties. It can undergo reversible anion exchange, with sensitive ionochromism for Br-, SCN- and I- because of CT interactions; it is electrochemically active and shows anion-dependent photochromism associated with ET; the CT-based ionochromic and ET-based photochromic phenomena have efficient quenching effects on the photoluminescence of the framework owing to EnT to the CT complexes or the radicals. The responses of UiO-67-MQ are weaker because the severely twisted conformation of MQ2+ elevates the LUMO and thereby weakens the electron-deficient character. Based on the rich responsive properties, we demonstrated that the MOFs can be used as anion sensors for facile colorimetric discrimination of different anions and for quantitative detection of I-. The multistimuli-responsive properties also make it possible to mimic various logic operations with a single material, including YES, NOT, OR, NOR, INHIBIT, IMPLICATION and even more complex integrated logic circuits. This work demonstrates that integrating bipyridinium chromophores into porous MOFs can allow full play to the electron-deficient character and provides a blueprint for the design of sensory and switching materials. EXPERIMENT SECTION Synthesis of UiO-67-DQ. EDT (3.5 mL) and CHCl3 (12 ml) was added to a 50 mL vial containing 500 mg UiO-67-bpy (~0.23 mmol) .The mixture was stirred for 5.5 h at room temperature. The solid was isolated by filtration, washed with ethanol for several times, and dried at 80°C for 2 h under vacuum to obtain a white solid powder.


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Ion Exchange. UiO-67-DQ/UiO-67-MQ (50 mg) was added to a salt solution (100 mL, 0.1 M KCl, KBr, KI, KNO3, KSCN, Na2SO4, or NaClO4). The mixture was stirred for 1 h at room temperature, and the solid was isolated by filtration and dried at room temperature. Fluorescence Sensor Experiments. Stable milk-white dispersions were prepared by ultrasonicating a MOF (6.0 mg) in ethanol or water (8.0 mL) for 30 min. After recording the fluorescence using 2.5 mL of the dispersion, 2.5 µL of iodide solution with a given concentration was added to the cuvette and stirred for 5 min, and then fluorescence was measured. The titration procedure was repeated until the total iodide concentration reached 4.0 mM. According to a dynamic study, the emission intensity does not show appreciable change after 5 min. ASSOCIATED CONTENT The supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/. Supplementary graphic material (PDF) AUTHOR INFORMATION Corresponding Author [email protected]

ACKNOWLEDGMENT This work is supported by the National Natural Science Foundation of China (Grant No. 21471057 and 21773070).


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Incorporating Electron-Deficient Bipyridinium Chromorphores to Make

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