Photochromic Porous and Nonporous Viologen-Based MetalâOrganic

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Photochromic Porous and Nonporous Viologen-based Metal-organic Frameworks for Visual Detecting Oxygen Shi-Li Li, Min Han, Bin Wu, Jie Wang, and Xian-Ming Zhang Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00189 • Publication Date (Web): 01 Jun 2018 Downloaded from http://pubs.acs.org on June 1, 2018

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Crystal Growth & Design

Photochromic

Porous

and

Nonporous

Viologen-based

Metal-organic Frameworks for Visual Detecting Oxygen Shi-Li Li†, Min Han,† Bin Wu,† Jie Wang† and Xian-Ming Zhang†,‡* †

Key Laboratory of Magnetic Molecules & Magnetic Information Materials (Ministry

of Education), School of Chemistry & Material Science, Shanxi Normal University, Linfen 041004, P. R. China ‡

Institute of Crystalline Materials, Shanxi University, Taiyuan 030006 P. R. China

ABSTRACT: The detection of oxygen molecule is of particular interest due to various applications in chemical, biological and environmental areas, and the typical materials of detecting O2 such as clark-type electodes and optical oxygen sensors suffer from disadvantages such as insensitiveness or usage of expensive instrument. In this paper, we present a nonporous 2D viologen based metal-organic-framework consisting

of

pseudo-left-

and

right-handed

helical

chains

[Cd(CPBPY)(o-BDC)(H2O)]·H2O (CPBPY = N-(3-carboxyphenyl)-4,4′-bipyridinium, o-BDC = o-benzenedicarboxylate) 1, that exhibits a slow photochromism under vaccum, inert atmosphere and enven exposure to oxygen, and a 3D porous 6-connected

pcu

topological

metal-organic-framework

[Cd3(CPBPY)2(BDC)3]·DMF·H2O (BDC = 1,4-benzenedicarboxylate)

2, which

displays rapid photochromism only under vaccum or inert atmosphere. The photochromic product 2’ can quickly and conveniently detect O2 by naked eye recognition of color change. The reason why 2 can fast detect oxygen might be

ACS Paragon Plus Environment

Crystal Growth & Design 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

presence of 1D channels providing pathway for oxygen molecule.

KEYWORDS:

detection, oxygen, porous framework, viologen, naked eye recognition

1. INTRODUCTION Photochromic materials are currently attracting great attention, not only because of their unique photochemical and photophysical properties but also because of their potential technological application in sensors, photography, optical storage, information display, electrochromic display, smart windows, optical switches, rewritable copy papers, solar-to-fuel conversion, photo-mechanics, photomasks, photocatalysis, and so on.1−4 Viologen derivatives is an important class of photochromic system, which can be used as photoactive organic linkers to construct photochromic polymers based on the photo-induced electron transfer from electron-rich species to the viologen acceptors.5−7 Among this system, the zwitterionic viologen derivatives bearing carboxylate, as very promising electron acceptors, have possessed versatile coordination modes and rich flexible structure.8 Such viologen-based materials are of particular importance due to their colourimetric detection of a large variety of small molecules, particularly oxygen.9−11 The determination of molecular oxygen is very important in various fields, such as life science, biology, meteorology, oceanography, medical and environmental fields.12,13 The most common methods for detecting oxygen are the Clark-type electrode and optical approaches while these methods have some drawbacks.14,15 For example, the


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Clark-type sensors are so small that it is practically insensitive to oxygen, and the optical sensors are based on luminescence enhancement or quenching toward O2 molecule, and such detection relies on fluorescence analysis by means of the expensive

fluorescence

spectrophotometer.16−18

However,

the

viologen-based

photochromic materials can show significant color change between dication (V2+) and radical cation (V+•), and they can easily detect oxygen molecules by visual inspection of color change, which is more simple and powerful than luminescence analysis and conventional sensing methods.19 Despite the increasing number of photochromic metal-organic complexes with coordinated pyridinium derivatives, the viologen-based sensor of volatile molecules specially for sensing of O2 is still at the infant stage. Most viologen-based photochromic materials, exposed to oxygen, exhibit slow color change with response time from hours to days, too long for detecting oxygen molecules.20 Thus, it is still highly desirable to design and develop new viologen-based photochromic sensors for fast detecting oxygen molecules. A huge challenge in this domain for chemists is how to synthesize a fast sensor for oxygen. As far as we know, the Lewis acidic sites on viologen-based ligand have strong abilities to interact with electron rich units or guest molecules.21−24 Thus, a promising approach for the rapid and convenient detection of oxygen molecules is to build the porous framework with open Lewis acid sites such as the positively charged N atoms of viologen units.22,25−27 Nevertheless, it is very difficult to construct such functional materials with viologen-based ligands because their cationic sites have great tendency of interactions with electron-rich sites precluding the formation of



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pores.28,29 In order to obtain such porous functional materials, we have recently modified the rigidity of ligands and the number of Lewis acid sites. In this context, utilizing 1,4-benzenedicarboxylic acid (H2BDC),

o-benzenedicarboxylic acid

(o-H2BDC) as the rigid ancillary ligands and N-(3-carboxyphenyl)-4,4′-bipyridinium chloride (HCPBPY·Cl) with only single Lewis acid site as photochromic functional group, nonporous 2D framework [Cd(CPBPY)(o-BDC) (H2O)]·H2O 1 and porous 3D-framework [Cd3(CPBPY)2(BDC)3]·DMF·H2O 2 are synthesized. Compound 1 shows pseudo-homochiral layers constructed by pseudo-left or right-handed helical chains, which is the condensing packing structure. Compared to condensing packing structure of 1, compound 2 possesses 1D channel in the uninodal 6-connected pcu primitive cubic frameworks. Interestingly, 1 is photochromic in air and anaerobic conditions and its photoproduct can slowly reverse to original color in air, while 2 shows photochromic transformation only under anaerobic condition and its photoproduct can immediately go back, which can be used as a promising candidate for the rapid and convenient detection of O2 molecule. Furthermore, these results will promote the development of fast sensors for oxygen and their applications in chemical and biological systems.

2. EXPERIMENTAL SECTION 2.1. Synthesis and Characterization. The ligand HCPBPY•Cl was synthesized according to previously reported literature.30 Synthesis of 1: HCPBPY·Cl (0.137 g, 0.5 mmol), Cd(NO3)2·4H2O (0.155 g, 0.5


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mmol), o-H2BDC (0.033 g, 0.2 mmol), DMF (4 mL), EtOH (2 mL) and H2O (1 mL) were stirred and sealed in a 15 mL Teflon reactor, and then then heated at 100 ºC for 7 days. After cooling to room temperature, Yellow block crystals of complex 1 were obtain in 34% yield based on HCPBPY·Cl. Anal. Calcd (%) for 1 C25H20CdN2O8: C, 50.95; H, 3.40; N, 4.76. Found: C, 51.01; H, 3.36; N, 4.81. IR (KBr, cm−1): ν 3453(s), 2907(w), 2340(w), 1625(s), 1569(w), 1391(s), 1131(m), 988(w), 886(w), 829(w), 743(w), 615(w), 518(w).

Synthesis of 2:

This compound was synthesized with a method similar to that of

compound 1, except that the initial ancillary ligand o-H2BDC was instead of H2BDC (0.086 g, 0.5 mmol). Yellow block crystals of 2 were isolated with a yield of 60% based on HCPBPY·Cl . Anal. Calcd (%) for 2 C61H45Cd3N5O18: C, 49.73; H, 3.08; N, 4.75. Found: C, 49.71; H, 3.13; N, 4.77. IR (KBr, cm−1): ν 3439(s), 2922(w), 2366(w), 1615(s), 1375(s), 1125(m), 829(w), 748(w), 641(w), 513(w). 2.2. Single Crystal X-ray Diffraction. A Agilent Technologies Gemini EOS diffractometer at 293(2) K with Cu−Kα radiation (λ = 1.5418 Å) is used for data collection at 298 K. All non-hydrogen atoms of compound 1 and 2 were refined with anisotropic thermal parameters. All hydrogen atoms on carbon and oxygen atoms were generated geometircally. The detailed crystallographic data and structural refinement parameters for complex 1 and 2 are summarized in Table 1. Selective bond distances and bond angles of compound 1 and 2 are listed in Table S1. Table 1. Crystallographic Data and Structural Refinement for 1 and 2



Compound

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1

2

Empirical

C25H20CdN2O8

C58H36Cd3N4O16

Fw

588.83

1382.11

Crystal system

monoclinic

monoclinic

Space group

P2/c

P21/c

a (Å)

11.14375(15)

11.5706(3)

b (Å)

10.79790(14)

19.5715(5)

c (Å)

18.8341(3)

14.0456(4)

α (°)

90

90

β (°)

97.4407(13)

94.576(2)

γ (°)

90

90

V (Å3)

2247.20(5)

3170.53(14)

Z

4

2

ρcalc,(g cm-3)

1.740

1.448

µ, (mm-1)

8.284

7.025

F(000)

1184

1368

Reflections

12557/4508

16421/7422

Crystal size(mm)

0.28×0.18×0.10

0.21×0.15×0.07

Tmax/Tmin

0.4913/0.2051

0.9294/0.8078

Data/parameters

4508/329

7422/367


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S

1.048

1.169

R1 a

0.0283

0.0399

wR2b

0.0812

0.1322

∆ρmax/∆ρmin(eÅ-3)

0.982/-0.542

1.416/-0.553

a

R1 = ∑Fo-Fc/∑Fo. bwR2 = [∑[w(Fo2-Fc2)2]/∑[w(Fo2)2]]1/2.

2.3. Electron Paramagnetic Resonance (EPR). EPR spectra of 1 and 2 were recorded with a Bruker A300-10/12 electron paramagnetic resonance spectrometer at room temperature. 2.4. Gas Adsorption Experiments: Gas uptake experiments were measured using a Micromeritics ASAP 2020 surface area and pore size analyzer at 77 K. The as-synthesized samples of compound 2 were immersed in dry acetone at least 10 times within three days, and then evacuated at 373 K for one day. The N2 and O2 sorption isotherms were measured at liquid nitrogen temperature.

3. RESULTS AND DISCUSSION 3.1. Crystal Structures of compound 1. Crystal structural analysis reveals that compound 1 crystallizes in the monolclinic crystal system, with a space group of P2/c. The Cd2+ ion is coordinated by six O atoms with one N atom from one water molecule, one CPBPY ligand, and two o-BDC2– ligands forming a seriously distorted pentagonal bipyramid, with Cd-O bond distances ranging from 2.2962(18) – 2.447(2) Å and Cd-N bond distance of 2.378(2) Å (Figure 1a). The O-Cd-O bond angles range from 53.63(7) to 140.29(8)°, and O-Cd-N bond angles range from 78.00(8) to 138.66(9)°. Cd2+ ions



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are bridged by the o-BDC2– ligand to fabricate a pseudo-left- and right-helical chain [Cd(o-BDC)]n (Figure 1b). The neighboring chains are further linked by CPBPY ligands to form a 2D layer (Figure 1c). Along the a-axis, the 2D layers stack together through the face to face π⋅⋅⋅π stacking interactions and C-H⋅⋅⋅O hydrogen bonds, in which the π⋅⋅⋅π stacking interactions exist between two neighboring benzene rings from two CPBPY ligands to form the 3D supramolecular array, and the shortest C-H⋅⋅⋅O distance between two neighboring layers is 2.18 Å shorter than the sum of their van der waals radii (2.60 Å), implying electrostatic interactions indeed exist between the layers (Figure 1d). The structural analysis by PLATON program indicates that there are no channels, meaning that the O2 molecules cannot diffuse inside the framework of 1.

. (a)

(b)


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(c)

(d)

Figure 1 (a) view of compound 1 showing the surrounding environment of the Cd(II) atoms, (b) the pseudo-left- and right-handed helical chains bridged by the o-BDC ligands, (c) top view of the 2D layer constructed by only pseudo-helical chains, (d) the packing structure of 1 showing the two different layers arranged alternately. Crystal Structures of compound 2. Compound 2 crystallizes in a monoclinic crystal system, with a space group of P21/c, and the asymmetric unit contains one and a half Cd2+ ions, one CPBPY ligand, one and a half BDC ligands (Figure 2a). Cd(1) ion adopts a seven coordination geometry, with Cd-O bond distances within the range of 2.278(2) – 2.519(3) Å and Cd-N bond distance of 2.463(3) Å, coordinated by six carboxylic oxygen atoms and one nitrogen atom. Among the six O atoms, four O atoms are from three different BDC linkers and two O atoms are from one bidentate CYBPY ligand. And one nitrogen atom is from another CPBPY ligand (Figure 2a). While Cd(2) exhibits an octahedral coordination geometry surrounded by six



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monodentate oxygen atoms from two CPBPY ligands in the axial positions and four different BDC linkers in the equatorial plane. Subsequently, two Cd(1) atoms and one Cd(2) are bridged by six carboxylate groups coming from two CPBPY ligands and four BDC ligands to constitute a centrosymmetric linear trinuclear unit, namely [Cd3(COO)6] second building unit (SBU) (Figure 2b). Each linear SBU has been linked by four mixed-BDC2--CPBPY ligands and two BDC2– ligands to six neighboring linear SBUs, forming a uninodal 6-connected pcu primitive cubic framework with topology (412.63) (Figure 2c and 2d). To be noted, along the c-axis, in the host framework there are 1D channels occupied by a few of disordered solvent molecules, confirmed by elemental analysis and TG data, which is accounting for as much as 24.4% of the total volume calculated by PLATON software. Such void space has been beneficial for viologen radicals contacting oxygen molecules, which should be the reason why the photoproducts 2’ can recover the primary color in few seconds.

(a)

(b)


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(c)

(d)

Figure 2 (a) view of compound 1 showing the surrounding environment of the Cd(II) atoms, (b) The linear trimeric Cd3(COO)6 cluster SBU, (c) view of the Cd3(COO)6 SBU as 6-connected nodes linked by BDC2- and CPBPY ligands, (d) simplified representation of a pcu network in 2. 3.2. Adsorption Properties. In order to verify the permanent porosity in compound 2, the nitrogen and oxygen sorption behaviors have been further investigated. Before the adsorption studies, the collected crystals of compound 2 were immersed in dry acetone at least 10 times within 3 d. Then, the sample was activated by drying at 373 K under a dynamic vacuum for 12 h. The N2 adsorption measurements at 77 K exhibit typical type-I gas uptake isotherms (Figure S4), which is typical for microporous materials. A very sharp uptake under low relative pressure (P/P0 < 0.01) and the following plateau at around 57.7 cm3 g−1 reveal the permanent porosity of compound 2. On the basis of the N2 sorption data, the Brunaurer–Emmett–Teller (BET) and Langmuir surface areas are 191.3 and 247.1 m2 g−1, respectively. Using the t-plot method model, the microporous volume is calculated to be 0.086 cm3 g−1, which



accounts for about 96.6% of the total pore volume. As shown in Figure S5, the pore size distribution curve using the N2 uptake data at 77 K exhibits three narrow micropore width distributions located at 8.0, 10.9 and 12.7 Å. The sorption behaviors of compound 2 for O2 have been further investigated. It is noteworthy that the amounts of O2 uptake for 2 is 163.2 cm3 g−1 at 77 K under 1 bar and also displays type-I sorption profiles (Figure S4). The value of O2 uptake is higher than that of N2 under the same conditions, which could be mainly attributed to the fact that the Lewis acidic sites on viologen-based ligand interact with O2 molecules more strongly than with N2. Moreover, the O2 molecules can diffuse inside the micropores and induce the oxidative quenching of viologen radicals, which is the reason why the pale-blue photoproducts 2’ can go immediately back to yellow with exposure to pure O2 or air atmosphere. 3.3. Photochromic Properties. Viologen-based photo-responsive materials can usually reverse to the primary color in the presence of oxygen. To be noted, the color reversing time of these photo-responsive compounds generally vary considerably.31−34 It is interesting that these two compounds exhibit different photo-responsive properties in air. With exposure ultraviolet light in air, compound 1 shows a rapid visible photochromic transformation, while compound 2 does not undergo the photochromic process. With the duration of UV irradiation, the block crystals of 1 turn from yellow into gray (1’), and after continuous irradiation for 75 min no further color change was observed (Figure 3a). With exposure to pure O2, gray crystals (1’) can go slowly (within 5 min) from gray back (1’) to yellow (1), while exposing to the


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air, the discoloration is slower for about 40 min. This reverse process of the crystal color can be repeated at least 10 times. When 1’ was heated in air, it could turn back to 1 much more quickly. Interestingly, under anaerobic conditions 1’ is very stable and keeps its pale-blue color for at least a year. The reason why the fading process of 1’ only occurs under aerobic conditions should be due to oxidative bleaching from MV+• to MV2+ by O2, as observed in other viologen-based polymers.19,25 This striking phenomenon of 1 reminds us why compound 2 does not display the photochromic transformation under the air atmosphere, perhaps because under aerobic conditions the photo-induced radicals of 2 is oxidized so quickly that the photochromic process cannot be observed by naked eyes. As we have predicted, under a high purity nitrogen or vacuum with exposure to constant irradiation for 5 min the color change is observed from yellow (2) to pale-blue (2’) (Figure 3b). The photoproducts 2’ are also very stable, but with exposure to pure O2 atmosphere the pale-blue product 2’ goes immediately back to yellow within 5 seconds. It is noteworthy that the discoloration is closely related to the concentration of O2. When the oxygen concentration is 0.01%, 0.1%, 21% and 100%, the response time for color change is 150, 40, 15 and 5 seconds, respectively. The color change does not occur under the oxygen concentration of less than 0.01%, thus the limiting concentration of O2 for 2 is 0.01%. In view of the above-mentioned facts, compound 2 is more suitable for detecting O2 than that of 1.



(a)

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(b)

Figure 3 Color change of compound 1 (a) and 2 (b) in the photochromic process under inert atmosphere. As shown in Figure 4, the UV-vis absorption spectroscopy of 1 contains two new absorption peaks centered at around 385 nm and 565 nm when irradiated by UV light, and with continuous irradiation the intensities of two new absorption bands have kept growing until their intensities reach their saturation points after illumination for 10 min. Based on the relative literature, new adsorption peaks can be ascribe to the reduction of CPBPY ligand and the formation of the CPBPY·- radicals caused by the photoinduced electron transfer. 35−37 As previously discussed, the UV-vis data of 1 are changed before and after irradiation, but PXRD and IR data remains unchanged (Figure S6 and S7). These experimental data illustrate that the photochromic process is not because of photoinduced isomerization or photolysis but because of an electron-transfer process. The kinetics of the photochemical reactions for 1 is studied using the reported equation38: ln(

A0 ି‫ۯ‬ಮ ‫ۯି ܜۯ‬ಮ

) = kt where A0, At, and A ∞ are the


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absorptivity at different irradiation time, and k is the kinetics constant, and As shown in Figure S8, the linear fit of the data indicates that the solid state light reversion of 1 at 565 nm exhibits first-order reaction kinetics with a rate constant as 8.64 × 10−4 s-1. Its rate is lower than the prveiously reported results of viologen hybrids,35, 39 which might be due to that photochromic 1’ tends to return to its original color in air. With prolonged irradiation the photochromic materials might lose their photochromic reactivity, and thus photo-fatigue resistance is an important factor for the performance evaluation of photochromic materials. The photochromic reversible processes of compound 1 can be repeated for at least 10 continuous cycles, demonstrating that compound 1 has a high photo-fatigue resistance to promote its application in various optoelectronic devices. Compound 2 can exhibit the photochromic process only in inert atmosphere after continuous UV-irradiation. To confirm the photoinduced radicals, the EPR data are measured in detail. Before irradiation, compound 1 is essentially EPR silent. Upon UV-irradiation, the color of the crystals can gradually deepen with the appearance of an EPR signal at g = 2.0058 (Figure 5a), further indicating that the photochromism of 1 presumably arises from photo-induced free viologen radical. When the colored crystals are exposed to pure O2 or air, they could return slowly to their initial color, accompanied by the EPR signal to quiet down. This reversible photochromic phenomenon can be also found in the similar viologen-based coordination compounds.25, 40 In presence of air the color of crystal 2 remains unchanged after UV-irradiation with EPR silent, while under a N2 atomosphere or vacuum the color of 2 can change with the appearance of an EPR



signal at g = 2.0043 (Figure 5b), which is generally considered to be associated with the generation of the bipyridinium radical in an inert atmosphere. When upon exposure to pure O2 or air, the photoproduct 2’ can return immediately to their initial color with the EPR signal disappearing, which should be related to the fact that the O2 molecules can diffuse inside the micropores and strongly interact with the Lewis acidic sites on CPBPY·- radical, inducing the oxidation process of the bipyridinium radicals. Thus photochromic product 2’ can rapidly detect O2 by the naked eye, and might be as a promising oxygen sensor applied in various fields such as chemical industries and fuel combustion.

Figure 4 (a) the time-dependent UV-vis absorption spectroscopy of 1 upon irradiation, (b) the photoswitching of the color change in 10 cycles.


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(a)

(b)

Figure 5 Electron Paramagnetic Resonance (EPR) spectra of 1 (a) and 2 (b) in original, colored and decolored state, respectively. The electron transfer to viologens in the solid state generally can be related to several factors (i.e., the ionization potentials of the inclusion guest molecules, the short contact between the pyridinium N atom and the donor, short contact between the hydrogen atom at the α-carbon atom and donor, weak hydrogen bonds short C-H···π contact between pyridinium C-H and the donor, π-π stacking interaction between the aromatic rings in the neighboring bipyridinium ligands or the dihedral angle between the pyridine rings).20, 41−43 Among these factors, the distance and orientation between the donor and acceptor units and the degree of coplanarity of the bipyridinium moieties play a key role in the photochromic properties.34, 44,45 From the structural data of 1, there are no π-π stacking interactions exists between the bipyridinium and adjacent benzenecarboxylate units. The nearest distance between carboxylate oxygen atoms (O6) and viologen nitrogen atoms (N2) is 4.072 Å, which is quite far for the through-space electron transfer between viologen acceptor and carboxylate group donor units. The dihedral angle between the normal of the pyridinium plane and the donor-N+ plane is 30.934°, which seems not to be powerful enough to stabilize the viologen monocation radicals.20,

46

However, we notice that the shortest C-H···π

distance (2.680 Å) between carboxylate group and a pyridinium C-H group, and a short contact between the oxygen atom O6 and the hydrogen atom at the α-carbon atom of pyridinium part [d(O6···H9) = 2.178 Å] (Figure S9), acting as reasonable ACS Paragon Plus Environment


electron-transfer pathway for the photochromic process. While for compound 2, there are no edge-to-face C-H···π interactions between carboxylate donor and pyridinium C-H, however, the shortest distance between the hydrogen atom at the α-carbon atom of pyridinium group and the carboxylate oxygen atom [d(C8-H8···O8)] is 2.257 Å (Figure S10), which may be a possible electron transfer pathway from a donor unit to viologen acceptor. And the dihedral angle between the normal of the pyridinium plane and the donor-N+ plane is only 10.800°, which is much smaller than that of 1 and close to 0°. Such geometry is believed to be suitable for the photo-induced reduction and the stability to the viogen radicals. Compare with 1, one notable structural feature of 2 is the host framework with one-dimensional channel, which may be favorable for viologen radicals contacting oxygen molecules, confirmed by the oxygen adsorption studies of compound 2.5, 47 Thus, the above results indicate that the void space is the reason why the compound 2 can just exhibit the photochromic properties in inert atomosphere.

4. CONCLUSIONS In conclusion, utilizing the rigid ligands such as H2BDC, o-H2BDC and HCPBPY·Cl, two new viologen-based compounds have been synthesized via solvothermal synthesis, in which complex 1 and 2 show nonporous 2D-framework and porous framework, respectively. Interestingly, 1 is photochromic in air or under anaerobic conditions and its photoproduct can slowly reverse to its original color in air, while 2 shows photochromic transformation only under anaerobic conditions and its photoproduct can change immediately back, which can be easily examined by the ACS Paragon Plus Environment

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naked eye and UV-vis spectra, thus it provide a rapid and convenient method for detection of oxygen. Furthermore, these results will open a new vista in searching for fast sensors for oxygen and be favorable for the understanding of the underlying mechanism for the detection of O2. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Selected bond distances, PXRD patterns, IR, TGA curves, UV/Vis spectra and additional figures. (PDF) Accession Codes CCDC 1821841-1821842 contain the supplementary crystallographic data for this paper.

These

data

can

be

obtained

free

of

charge

via

www.ccdc.cam.ac.uk/data_request/cif, by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], [email protected].

ACKNOWLEDGMENTS



This work was financially supported by NSFC (Grants 21701105, 20925101), 10 000 Talents Plan, Shanxi Province Science Foundation for Youths (201701D221034) and Shanjin Scholars.

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For Table of Contents Use Only Photochromic

Porous

and

Nonporous

Viologen-based

Metal-organic Frameworks for Visual Detecting Oxygen Shi-Li Li†, Min Han,† Bin Wu,† Jie Wang† and Xian-Ming Zhang†,‡*

Porous and Nonporous Viologen-based compounds have been synthesized, in which only porous crystals can quickly and conveniently detect O2 by naked eye recognition of color change.


Photochromic Porous and Nonporous Viologen-Based MetalâOrganic

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