Postsynthetic Paddle-Wheel Cross-Linking and Functionalization of

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Post-synthetic paddle-wheel crosslinking and functionalization of 1,3-phenylenebis(azanetriyl)tetrabenzoate based MOFs Philipp Müller, Florian Wisser, Volodymyr V. Bon, Ronny Grünker, Irena Senkovska, and Stefan Kaskel Chem. Mater., Just Accepted Manuscript • Publication Date (Web): 05 Mar 2015 Downloaded from http://pubs.acs.org on March 5, 2015

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Chemistry of Materials

Post-synthetic paddle-wheel crosslinking and functionalization of 1,3-phenylenebis(azanetriyl)tetrabenzoate based MOFs. Philipp Müller,a Florian M. Wisser,a Volodymyr Bon,a Ronny Grünker,a,b Irena Senkovska,a Stefan Kaskela,* a

Institute of Inorganic Chemistry, Technische Universität Dresden, Bergstraße 66, D-01062 Dresden, Germany

KEYWORDS Metal-organic framework, stabilization, crosslinking, ethanol adsorption, vapochromism, ethanol detection.

ABSTRACT: A new metal-organic framework Cu2(mpbatb)(H2O)2 (DUT-71, DUT - Dresden University of Technology) (mpbatb - 4,4',4'',4'''-(1,3-phenylenebis(azanetriyl)tetrabenzoate) was prepared by the solvothermal reaction in a mixture of DMF and EtOH. Post-synthetic modification of DUT-71 by using different neutral nitrogen containing bridging molecules in single-crystal to single-crystal transformation fashion results in a series of porous compounds (DUT-72, DUT-73a, DUT73b, DUT-74, DUT-90, DUT-91 and DUT-95) with apparent BET surface area up to 2700 m2g-1. Interestingly, the Cu-Cu distances between paddle-wheels during the transformation are adapted to the bridging ligands, pointing on the flexible character of DUT-71. After solvent removal and resolvation, a drastic color change from green to dark blue and vice versa was observed. This observation was studied in details in situ by monitoring the adsorption of ethanol in low concentration region by UV/Vis technique. The correlation of the ethanol adsorption uptake and the intensity of characteristic absorption peaks in UV/Vis spectra render the new materials as promising candidates for vapochromic sensing.

INTRODUCTION Metal-organic frameworks are modular materials that are assembled from molecular building units in modular fashion and therefore can achieved a high degree of well defined functionality. This makes them very attractive for many applications where porosity1-3 and functionality4,5 is required. Further features of this material class are e.g. (i) crystallinity, which enables controlled design and targeted modification of the framework based on the crystallographic information, (ii) optical functions, due to the presence of transition or rare earth metals.6-8 The optical response can be caused by the coordinating molecules, which do not participate in the network formation and can be removed or introduced without changes in the framework integrity. This solvatochromic/vapochromic property make MOFs interesting for sensing applications.9 The main envisioned application of vapochromic materials is as indicator in sensors for detecting volatile organic compounds in a variety of environments, including industrial, domestic, and medical areas. The origin of solvochromism/vaporchromism in MOFs can be of different nature: charge transfer influenced by

solvent polarity,10 d-d transition effect,11 changes in the coordination sphere of the metal ion due to the solvent removal,12 as well as changes in the coordination sphere induced by ligand exchanges.13 Since the accurate detection of substances at low ppm levels is a huge challenge, the porosity combined with the classical transition metal complexes response can drastically enhance the sensitivity.14-15 Thus, for efficient sensing of vapors, a robust porous MOF system is necessary. Often, highly porous MOFs involve long linker molecules and form open frameworks (void fraction up to 95%), that may collapse during solvent removal. Actually, the most important factors influencing the robustness of the MOFs during activation are not completely understood so far, but it is widely accepted, that significant capillary forces, and hence surface tension, can be created during activation, which in turn can yield fully or partially collapsed frameworks.16 Thus, supercritical drying is especially important for mesoporous MOFs.17 The framework topology,18-20 chemical composition,21, 22 as well as polarity of the framework17 are factors influencing the robustness of the framework during the activation procedure. One useful approach to stabilize the framework against collapse is pillaring of paddle-wheel clusters in MOFs.23, 24

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Not only the stabilization, but also the functionalization of the MOF can be performed by exposing parent MOF crystals to a concentrated solution of (functional) linker of choice in a carefully selected solvent.25, 26 Already in 2004 Kitagawa and co-workers followed by Chen and coworkers performed a series of experiments involved linking 2D paddlewheel layers with bipyridyl pillars to obtain 3D structures.27, 28 Pillared-paddle-wheel MOFs have been often utilized later as model systems by many authors to control pore dimensions and surface functionality.29, 30 One impressive illustration of the framework strengthening was reported by Klein et al. DUT-34, a very fragile framework with pto topology, which could not be desolvated under retaining of porosity even by supercritical CO2 drying was stabilized by incorporation of neutral nitrogen containing cross linker. After supercritical drying the material (DUT-23) with permanent porosity and remarkable surface area of 4850 m2/g was obtained.31 The group of Suh performed insertion of a linear nitrogen containing ligand post-synthetically into the SNU-30, a MOF based on the tetracarboxylic linker. In excess of DEF, the framework is able to release the pillar in a single crystal to single crystal reaction.32 Recently, Hupp and coworkers shown that incorporated into the pillared-layer structure nitrogen containing linker could be even replaced by longer one, with retention of the parent topology.33 In this contribution we demonstrate, that the post synthetic cross-linking of paddle-wheels in a singlecrystal to single-crystal fashion leads to significant stability increase in three dimensional 1,3phenylenebis(azanetriyl)tetrabenzoate (mpbatb) based MOFs, resulting in a series of vapochromic sensing materials. Using the new MOF Cu2(mpbatb) (further named as DUT-71) as starting material, several stabilizing N-donor ligands with various lengths were introduced into the framework. By tuning of the composition, crystal structures, solvent removal conditions, and porosity of all modified compounds were studied. All derivative materials show different robustness during the solvent removal as well as different gas and vapor adsorption properties.

EXPERIMENTAL SECTION Tetraethyl-4,4',4'',4'''-(1,3-phenylenebis(azanetriyl)) tetra benzoate A two necked round bottom flask was charged with palladium acetate (83 mg, 0.317 mmol, 4 mol%), 2,2'– bis(diphenylphosphino)–1,1'–binaphthyl (BINAP, 461 mg, 0.74 mmol, 8 mol%), cesium carbonate (18.08 g, 55.48 mmol, 6 eq) and filled with argon. Ethyl 4-bromobenzoate (10.55 g, 46.236 mmol, 5 eq, 7.55 ml) dissolved in dioxane (25 ml) was added and the mixture was heated up to 100°C. After 20 min 1,3-phenylendiamine (1 g, 9.25 mmol) dissolved in dioxane (30 ml) was added. The reaction was stopped after 7 days (TLC control) and the cooled mixture was filtered over a Celite® frit. The filter cake was washed with THF. The solvent was evaporated and a mixture of diethyl ether and pentane was added. The solvents were evaporated again and the solid crude product was washed with a diethyl ether/pentane mixture and finally with small amounts of ethanol (Yield: 1 4.785 g, 74%). H-NMR (CDCl3, 500 MHz): δ (in ppm): 1.38 (t,

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12 H), 4.35 (q, 8 H), 6.88-6.90 (m, 3 H), 7.07 (d, 8 H), 7.25 (t, 13 1H), 7.91 (d, 8 H). C-NMR (CDCl3, 125 MHz):δ (in ppm): 14.35 (CH3), 60.79 (CH2), 122.41 (CH), 122.59 (CH), 123.48 (CH), 124.91 (Cq), 130.90 (CH), 130.97 (CH), 147.52 (Cq), 150.45 (Cq), 166.00 (Cq). 4,4',4'',4'''-(1,3-phenylenebis(azanetriyl))tetrabenzoic acid (H4mpbatb) In a round bottom flask tetraethyl-4,4',4'',4'''-(1,3phenylenebis(azanetriyl))tetrabenzoate (2.7 g, 3.96 mmol) was dissolved in THF (55 ml), to this solution potassium hydroxide (4.44 g, 79 mmol, 20 eq) in water (27 ml) was added and heated up to 70°C for 20 h. After cooling down to room temperature the organic phase was evaporated and the aqueous phase was acidified with 6 M HCl. The precipitate was filtered, washed with water, small amounts of ethanol and diethyl ether. The product was dried over night in vacu1 um (2.24 g, 96%). H-NMR (DMSO-d6, 500 MHz): δ (in ppm): 6.87 (t, 1 H), 6.90 (dd, 2 H), 7.11 (d, 8 H), 7.39 (t, 1 H), 7.85 (d, 13 8 H), 12.73 (br, 4 H). C-NMR (DMSO-d6, 125 MHz): δ (in ppm): 122.27 (CH), 122.53 (CH), 123.05 (CH), 125.00 (Cq), 131.01 (CH), 131.56 (CH), 147.04 (Cq), 149.95 (Cq), 166.74 (Cq). Synthesis of DUT-71 [Cu2(mpbatb)]. For a typical synthesis of DUT-71, H4mpbatb (100 mg, 0.17 mmol) and copper(II)chloride dihydrate (86 mg, 0.5 mmol) were dissolved in a mixture of DMF and ethanol (1:1, 20 ml) using ultrasonic bath in a Pyrex tube. The solution was heated in an oven at 80°C for 2 days. After cooling down to room temperature the solvent was replaced with a fresh mixture of DMF and ethanol (1:1). Yield: 61 mg (43.4 % based on H4mpbatb)). Elemental analysis calc. (%) for Cu2(C34O8N2H20)(C2H6O)2(H2O)0.4(C3ONH7)0.2 [Cu2(mpbatb)(EtOH)2(H2O)0.4(DMF)0.2]: C 54.9, H 3.9, N 3.6, found C 54.2, H 3.2, N 3.5. Synthesis of DUT-72 [Cu4(mpbatb)2(dabco)3.5]. To the washed DUT-71 crystals 1,4-diazabicyclo[2.2.2]octane (dabco, 114 mg, 1.02 mmol) dissolved in a mixture of DMF and ethanol (1:1, 6 ml), was added and heated to 80°C for 7 days. After cooling down to room temperature the solvent was replaced with a fresh mixture of DMF and ethanol (1:1). Yield: based on DUT-71. Elemental analysis calc. (%) for Cu4(C68O16N4H40)(C6N2H12)1.45(C2H6O)1.8(H2O)2.9(C3ONH7)1.3 [Cu4(mpbatb)2(dabco)1.45(EtOH)1.8(H2O)2.9(DMF)1.3]: C 56.2, H 4.5, N 6.3, found C 56.0, H 4.2, N 6.2. Synthesis of DUT-73a [Cu4(mpbatb)2(1,3-bib)0.5]. To the washed crystals of DUT-71, 1,3-bis(1H-imidazol-1-yl)benzene (1,3-bib, 71 mg, 0.34 mmol) dissolved in a mixture of DMF and ethanol (1:1, 6 ml), was added and heated to 80°C for 7 days. After cooling down to room temperature the solvent was replaced with a fresh mixture of DMF and ethanol (1:1). Yield: based on DUT-71. Elemental analysis calc. (%) for Cu4(C68O16N4H40)(C12N4H10)0.5(C2H6O)1.45(H2O)1.3(C3ONH7)1.95 [Cu4(mpbatb)2(1,3-bib)0.5(EtOH)1.45(H2O)1.3(DMF)1.95]: C 56.4, H 4, N 6.3, found C 56.7, H 3.9, N 6.3. Synthesis of DUT-73b [Cu4(mpbatb)2(1,4-bib)0.5]. To the washed DUT-71 crystals, 1,4-bis(1H-imidazol-1-yl)benzene (1,4-bib, 71 mg, 0.34 mmol) dissolved in a mixture of DMF and ethanol (1:1, 6 ml), was added and heated to 80°C for 7 days. After cooling down to room temperature the solvent was replaced with a fresh mixture of DMF and ethanol (1:1). Yield: based on DUT-71. Elemental analysis calc. (%) for Cu4(C68O16N4H40)(C12N4H10)0.5(C2H6O)2(C12H10N4)0.3(C3ONH7) 2.25 [Cu4(mpbatb)2(1,4-bib)0.5(EtOH)2(BIB)0.3(DMF)2.25]: C 57.4, H 4.1, N 7.2, found C 57.6, H 4.4, N 7.1.


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Synthesis of DUT-74 [Cu4(mpbatb)2(bpta)0.5]. To the washed crystals of DUT-71, 3,6-di(4-pyridyl)-1,2,4,5-tetrazine (bpta, 80 mg, 0.34 mmol) dissolved in a mixture of DMF and ethanol (1:1, 12 ml), was added and heated to 80°C for 7 days. After cooling down to room temperature the solvent was replaced with a fresh mixture of DMF and ethanol (1:1). Yield: based on DUT-71. Elemental analysis calc. (%) for Cu4(C68O16N4H40)(C12N6H8)0.5(C2H6O)2.5(H2O)(C3ONH7) [Cu4(mpbatb)2(bpta)0.5(EtOH)2.5(H2O)(DMF)]: C 56.3, H 3.9, N 6.4, found C 56.5, H 3.6, N 6.2. Synthesis of DUT-90 [Cu4(mpbatb)2(1,3-bib)0.5 (dabco)2.5]. To the washed crystals of a DUT-73a, dabco (57.4 mg, 0.51 mmol) dissolved in a mixture of DMF and ethanol (1:1, 6 ml), was added and heated to 80°C for 7 days. After cooling down to room temperature the solvent was replaced with a fresh mixture of DMF and ethanol (1:1). Yield: based on DUT-71. Elemental analysis calc. (%) for Cu4(C68O16N4H40)(C12N4H10)0.5(C6N2H12)1.85(C2H6O)(H2O)2.9 [Cu4(mpbatb)2(1,3-bib)0.5(dabco)1.85(EtOH)(H2O)2.9]: C 57.0, H 4.3, N 7.4, found C 57.1, H 4.4, N 7.2. Synthesis of DUT-91 [Cu4(mpbatb)2(1,4-bib)0.5 (dabco)2.5]. To the washed crystals of a DUT-73b, dabco (57.4 mg, 0.51 mmol) dissolved in a mixture of DMF and ethanol (1:1, 6 ml), was added and heated to 80°C for 7 days. After cooling down to room temperature the solvent was replaced with a fresh mixture of DMF and ethanol (1:1). Yield: based on DUT-71. Elemental analysis calc. (%) for Cu4(C68O16N4H40)(C12N4H10)0.5(C6N2H12)1.55(C2H6O)0.3(H2O)3 [Cu4(mpbatb)2(1,4-bib)0.5(dabco)1.55(EtOH)0.3(H2O)3]: C 56.9, H 4.0, N 7.2, found C 56.6, H 4.4, N 7.1. Synthesis of DUT-95 [Cu4(mpbatb)2(bpta)0.5 (dabco)2.5]. To the washed crystals of a DUT-74, dabco (57.4 mg, 0.51 mmol) dissolved in a mixture of DMF and ethanol (1:1, 6 ml), was added and heated to 80°C for 7 days. After cooling down to room temperature the solvent was replaced with a fresh mixture of DMF and ethanol (1:1). Yield: based on DUT-71. Elemental analysis calc. (%) for Cu4(C68O16N4H40)(C12N6H8)0.5(C6N2H12)1.7(C2H6O)3(H2O) [Cu4(mpbatb)2(bpta)0.5(dabco)1.7(EtOH)3(H2O)]: C 57.3, H 4.5, N 7.7, found C 57.2, H 4.4, N 7.1. UV/Vis experiments. For a typical experiment ca. 9 mg supercritically dried MOF material (for more details see ESI) was mixed with 150 mg KBr in a glovebox. The mixture was transferred into a sample holder equipped with a dome, to prevent the contact with the ambient atmosphere. The sample holder was connected with the equipment for dynamic vapour dosing. Nitrogen was used as carrier gas. The total -1 flow was 100 ml min (Tab. S1 and Fig. S1).

RESULTS AND DISCUSSION Tetratopic linkers are very promising building blocks in MOF chemistry giving a plethora of highly porous and functional materials.34, 35 A new tetratopic linker 4,4',4'',4'''-(1,3-phenylenebis(azanetriyl)tetrabenzoic acid (H4mpbatb) was designed and synthesized in a two step synthesis: reaction of 1,3-phenylendiamine and ethyl-4bromobenzoate in presence of a palladium catalyst was followed by a ester cleavage.Solvothermal reaction of CuCl2·2H2O and H4mpbatb (Fig. 1) leads to the formation of green rod-like crystals of Cu2(mpbatb)(H2O)2. The single crystal X-ray diffraction reveals the tetragonal space group P4/mnc. The crystal structure contains two symmetry independent Cu-paddle wheel units. Their

centers of gravity are located in the 4e (site symmetry 4.. for Cu1—Cu2) and 4c (site symmetry 2/m.. for Cu3—Cu3) Wyckoff position, correspondingly. The interconnection of these square-planar inorganic building blocks by tetratopic ligands in its tetrahedral conformation results in a trinodal 4,4,4-connected 3D framework with nou topology (Fig. 2 and Fig. 3).36

Figure 1: Linker molecules used for the synthesis and postsynthetic modification of DUT-71.

Such framework configuration leads to formation of two types of cages (Fig. 2d): the larger one with 20.6 Å x 9.4 Å in dimensions and the smaller cage with 25.5 Å x 3.7 Å in diameter. All distances are measured from atom centre to atom centre without considering of Van der Waals radii of the atoms. In accordance with PLATON report, the structure of DUT-71 contains the solvent accessible void of 14629.9 Å3 (76.2% of the unit cell volume).37 Moreover, the Poreblazer 3.0 software suggests a theoretical accessible surface area of 4712 m2g-1, a total pore volume of 1.56 cm3g-1 and limiting pore diameter of 6 Å.38 Such outstanding calculated textural properties motivated us to prove the porosity experimentally. Unfortunately, solvent exchange to ethanol or dichloromethane with further applying of dynamic vacuum at room temperature results in amorphous material, not accessible for nitrogen at -196°C. Also the supercritical carbon dioxide activation procedure does not result in a porous material, pointing on the high fragility of the framework. Since the paddle-wheel units in the structure were aligned along the c axis, we decided, inspired by our previous work,31 to stabilize the structure of DUT-71 by insertion of neutral diamine cross-linkers of appropriate length between the neighboring paddle-wheels. Detailed analysis of the crystal structure of DUT-71 results in two different spacing distances between Cu2…Cu2 and Cu1…Cu1 atoms from neighboring paddle-wheels along [001] direction, which could be used for hypothetical cross-linker integration: 13.84(1) Å (“large gap”) and 8.00(1) Å (“small gap”) (Fig. 2, S2, S3, S21). Insertion of pillars into one or both gaps should strengthen the framework in the c direction. One of the suitable molecules for incorporation into a short gap could be 1,4diazabicyclo[2.2.2]octane (dabco), having 2.58 Å between nitrogen atoms.


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Figure 2: View on the structure of DUT-71, a) along a axis, b) along b axis, c) along c axis and d) distance between copper paddle-wheels in direction of c. Light blue (Cu), dark blue (N), red (O), grey (C), red and yellow spheres indicate the pore volume.

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co into the network, resulting in the new compound with the framework composition Cu4(mpbatb)2(dabco)3.5 (DUT-72) (Fig. S4, S5 and S22). The small gaps in DUT-72 are occupied by dabco molecules, connecting two neighboring paddle-wheels along c direction. All other paddle-wheels are coordinated by the dabco in monodentate manner. Interestingly, the framework adapts oneself to the cross-linker and as result, the corresponding Cu–Cu distance decreases from 8.00(1) Å in DUT-71 to 7.02(1) Å in DUT-72. Such changes lead to an increase of the “large gap” from 13.84(1) Å to 14.65(1) Å. For insertion of the cross-linker into a large gap of 13.84(1) Å in DUT-71, following neutral diamine ligands were chosen: bent 1,3-bib (1,3-bi(1H-imidazol-1yl)benzene) (N-N distance 9.1 Å), linear 1,4-bib (1,4-bi(1Himidazol-1-yl)benzene) (N-N distance 9.8 Å) and bpta (3,6-bi(pyridin-4-yl)1,2,4,5-tetrazine) (N-N distance 10.3 Å) (Fig. 1). The crystals of DUT-71 were soaked in the solution of corresponding cross-linker at elevated temperature. The successful incorporation of the 1,3-bib cross-linker into the “large gape” of DUT-71 could be proven by single crystal X-ray diffraction. The Cu2—Cu2 distance decreases from 13.84(1) Å in DUT-71 to 13.38(1) Å in Cu4(mpbatb)2(1,3-bib)0.5(H2O)3 (further named as DUT73a; Fig. S6, S7, and S23). Using the same protocol, 1,4-bib and bpta were introduced into the DUT-71 framework giving Cu4(mpbatb)2(1,4-bib)0.5(H2O)2(DMF) (DUT-73b) and Cu4(mpbatb)2(bpta)0.5(H2O)3 (DUT-74), respectively (Fig. S8, S9, and S24 for DUT-73b and Fig. S10, S11 and S25 for DUT-74). Both compounds are isoreticular to DUT-73a, showing different Cu1—Cu1 and Cu2—Cu2 distances after embedding of the long cross-linkers. The increase of the N…N distance in the linker leads to an increase of c and decrease of a lattice parameters (Tab.1). Interestingly, the incorporation of the 1,4-bib into the DUT-71 framework is accompanied by slight distortion of the 4e paddle-wheels from the 4-fold axis. This leads to a reduction of the space group symmetry from P4/mnc (DUT-71) to Pccn (DUT-73b). Since the simplest groupsubgroup relation between these two space groups P4/mnc → Cccm → Pccn involves the face centering, the unit cell volume of DUT-73b is doubled in comparison with the pristine compound. The single crystal-to-single crystal transformation to DUT-72, DUT-73a and DUT-74 proceed under preservation of the space group of the parent DUT-71 compound. Unfortunately, all these post-synthetic modifications of DUT-71 by only cross-linking of one gap type do not lead to the sufficient robustness. Obviously, the framework is too soft and the strengthening of only “large” or “small” spacing is not sufficient to achieve the porous solvent free framework. Therefore, crystals of DUT-73a, DUT-73b, and DUT-74 were subjected to further post synthetic modification step with introducing of additional dabco linkers for interconnecting the “small gaps” (Fig. 3).

Indeed, soaking of DUT-71 crystals in the solution of dabco in DMF/ethanol leads to the incorporation of dab-


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Figure 3: Topological representation of single crystal-to-single crystal transformations in DUT-71 series.

Table 1: Unit cell constants and “short” and “long” gaps for DUT-71 and derivates. For full crystallographic data see SI. Compound

a in Å

c in Å

DUT-71# DUT-72# DUT-73a# DUT-73b* DUT-74# DUT-90# DUT-91# DUT-95#

26.600 26.540 26.700 37.441 26.450 26.810 26.600 26.610

27.120 27.010 26.720 25.250 27.880 25.860 26.380 26.810

#

Small gap Cu1-Cu1 in Å 8.00 7.02 8.01 8.16 8.02 7.05 7.03 7.02

Large gap Cu2-Cu2 in Å 13.84 14.65 13.38 13.85 14.58 13.44 13.99 14.45

*

Space group: P4/mnc; Pccn

This results in a series of novel materials with ideal compositions Cu4(mpbatb)2(1,3-bib)0.5(dabco)2.5 (DUT90), Cu4(mpbatb)2(1,4-bib)0.5(dabco)2.5 (DUT-91) and Cu4(mpbatb)2(bpta)0.5(dabco)2.5 (DUT-95) (Fig. S12 - S17). According to the single crystal X-ray diffraction, all compounds crystallize in the space group of parent material with slightly different unit cell parameters (Tab. 1, Tab. S5-S8). Noteworthy the orthorhombic DUT-73b phase

converts again to the tetragonal phase (DUT-91) after dabco insertion. The elemental and DTA/TG analyses performed on the supercritically activated materials show a chemical composition of Cu4(mpbatb)2(1,3-bib)0.5(dabco)1.85, Cu4(mpbatb)2(1,4-bib)0.5(dabco)1.55 and Cu4(mpbatb)2(bpta)0.5(dabco)1.7 for DUT-90, DUT-91 and DUT-95, respectively. Obviously, not all paddle-wheels are occupied by dabco molecules after second postsynthetic modification step (Fig. S26 - S28). The durability of crosslinker insertion was proven for DUT-95. In contrast to the results of Suh and co-workers on SNU-30, where the post synthetic bpta linker embedding is reversible, the DUT-95 shows no leaching of bpta or dabco after soaking of the sample for 5 days in DEF. The supernatant solution was monitored by 1H-NMR that shows no signals of bpta and dabco molecules.32 After solvent exchange to acetone and drying using supercritical CO2, XRPD patterns of DUT-91 and DUT-95 do not change, demonstrating that frameworks stability (Fig. S14-S17). Interestingly, the PXRD of activated DUT90 contains only hk0 reflections indicating the leak of long range order in the [001] direction. Obviously, in contrast to DUT-91 and DUT-95, containing linear crosslinker, the bent 1,3-bib ligand could not provide the necessary degree of robustness to the framework in the

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desolvated state. After resolvation with a fresh mixture of DMF and ethanol (1:1) all reflexes in the PXRD pattern reappear, indicating recovery of the long range order after resolvation.

fluences the absorption behavior. So, the absorption maximum for Cu(OAc)2·H2O, a molecular complex based on paddle-wheel motif is located at 705 nm and shifts to 672 nm in Cu(OAc)2 after removal of the coordinated water molecules (Fig. S29).40 De Vos et al. showed that the maximum corresponding to the d-d transition in HKUST-1, a paddle-wheel containing MOF, is shifting to higher wavelengths by coordination of guest molecules to the cluster.11

Figure 4: Nitrogen physisorption isotherms of DUT-95 (triangle), DUT-91 (square) and DUT-90 (circle) at 77K.

Figure 5: Color change of activated (left) to solvated DUT95 (right).

Nitrogen physisorption experiments reveal type-I isotherms for DUT-90, DUT-91 and DUT-95 (Fig. 4). The highest specific BET area was determined for DUT-95 (2701 m2g-1), followed by DUT-91 (2652 m2g-1) and DUT-90 (2278 m2g-1) (Fig. S18 - S20). The pore volumes estimated from nitrogen adsorption isotherms at p/p0 = 0.9 are: 1.07 cm3g-1 for DUT-95, 1.04 cm3g-1 DUT-91 and 0.9 cm3g-1 for DUT-90 (Tab. 2).

Ethanol physisorption measurements at 25°C were carried out for DUT-90, DUT-91 and DUT-95. For all samples a type-I isotherm was recorded with saturation uptake of 102 cm3·g-1 for DUT-90, 104 cm3·g-1 for DUT-91 and 123 cm3·g-1 for DUT-95 (Fig. S35 - S37).

Table 2: Specific BET surface area and pore volume of investigated MOFs after post-synthetic modification with crosslinkers. The surface area was calculated using consistency criteria (for more details see ESI). Compound DUT-90 DUT-91 DUT-95

Surface area in 2 -1 mg 2278 2652 2701

Pore volume in 3 -1 cm g 0.90 1.04 1.07

Interestingly, activation/resolvation of the DUT-90, DUT-91 and DUT-95 materials was accompanied by a color change from green (for as made or solvent exchanged materials) to blue (after activation) and vice versa (Fig. 5). Therefore the compounds can be potentially used as vapochromic sensors. Since the sensing of alcohols vapor, especially of ethanol, is of wide interest, the behavior of the materials in presence of ethanol was investigated. The interaction between the frameworks of DUT-90, DUT-91 and DUT-95 and ethanol was studied by solid state UV/Vis spectroscopy. As shown in figure 6, broad maxima is observed in the spectra of activated materials between 500 and 600 nm indicative for the d-d transition of paddle-wheel copper ions.11, 39, 40 (UV/Vis spectra of all used ligands are presented in Fig. S30- S31 of the ESI.) It is known, that coordination environment of the Cu atoms of a paddle-wheel in-

Figure 6: Top: UV/Vis spectra of dried DUT-90 (dotted line), DUT-91 (dashed line), and DUT-95 (solid line).

Since all DUT samples investigated in this study undergo a significant color change during resolvation, the changes in UV/Vis spectra were expected after exposure to ethanol. In order to study kinetic of the ethanol vapor adsorption on DUT-90 (Fig. 7a, S32), DUT-91 (Fig. S33, S34), and DUT-95 (Fig. 7b, 7c) the UV/Vis spectra were measured in situ during the adsorption experiments. For this purpose a certain concentration of ethanol (from 100 ppm up to 700 ppm) was introduced to the measuring cell containing defined amount of activated sample. All investigated materials show similar changes in the UV/Vis spectra (Figure 7 and S32 - S34) during the ethanol adsorption. For DUT-90 (Fig. 7a), very low ethanol concentrations (100 ppm (p/p0 = 0.00128) and 250 ppm (p/p0 = 0.00321) cause small changes in the intensity of the absorbance peak at 566 nm. If the concentration


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increase up to 400 ppm (p/p0 = 0.00541), an abrupt decrease in absorbance at 566 nm takes place. Further stepwise concentration increases up to 700 ppm (p/p0 = 0.00901) does not influence the UV/Vis spectra, indicating that the highest ethanol loading is reached. It is also confirmed by ethanol physisorption experiments (Table S2 - S4).

In the region between 250 ppm (p/p0 = 0.00321) and 550 ppm (p/p0 = 0.00707) a large absorbance decrease at 566 nm in the UV/Vis spectra was observed, corresponding to the stepwise increase in the ethanol uptake in the physisorption measurement. For DUT-90 the largest gain in the ethanol uptake of 17 cm3g-1, from p/p0 = 0.0035 (268 ppm) to p/p0 = 0.00533 (394 ppm) take place corresponding to the largest change in the UV/Vis spectra (Fig. 8 top). At 400 ppm (p/p0 = 0.0054, Fig. 8 top) and 550 ppm (Fig. 8 bottom) the saturation is reached for DUT-90 and DUT-95, respectively, and a plateau is observed in both experiments. Similar behavior could be also observed for DUT-91 (Fig. S38). Thus, the DUT-90, DUT-91 and DUT-95 can be considered as promising materials for sensing and for estimation of ethanol vapor concentrations in ppm region, where the detection is based on color change.

Figure 8: Correlation between ethanol physisorption measurement (red graph) and 1/absorbance at 566 nm from UV/Vis experiments (black graph) in DUT-90 (top) and DUT-95 (bottom) at 298 K.

CONCLUSION Figure 7: Ethanol loading monitored by UV/Vis: DUT-90 (a), kinetic of adsorption on DUT-95 at 400 ppm (b), and DUT-95 (c).

Since the UV/Vis spectra undergo a continuous change during the ethanol physisorption, the intensity of the absorbance at 566 nm of DUT-90, DUT-91 and DUT-95 was correlated to the relative ethanol vapor pressure (inverse of absorbance was used) (Fig. 8 and S38). The UV/Vis experiment and the ethanol physisorption follow the same trend: at low concentration, up to 250 ppm (p/p0 = 0.00321), only small changes were observed, in the UV/Vis spectra of DUT-90, DUT-91 and DUT-95, as well as low uptake in the physisorption isotherms.

A series of new copper paddle-wheel containing MOFs based on tetratopic carboxylate linker molecule could be obtained using solvothermal synthesis and post-synthetic functionalization. The flexible starting compound DUT-71 could be permanently functionalized by coordinating of different nitrogen containing neutral ligands. Such interconnection increases the rigidity of the framework drastically and generates a permanent porosity accessible for gas molecules. The pillared MOFs show color change from green to blue during drying process and back from blue to green by exposure to ethanol vapor. The color change was followed in situ by UV/Vis spectroscopy during the ethanol physisorption experiments. The intensity of the absorbance peak at 566 nm could be correlated


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with ethanol concentration rendering these materials as promising candidates for sensing applications at ethanol concentrations from 50 up to 600 ppm.

ASSOCIATED CONTENT Supporting Information. Details of supercritical drying, linker synthesis, PXRDs, BET plots, TGA, additional UV/Vis spectra, ethanol physisorption isotherms, crystallographic tables. This material is available free of charge via the Internet at http://pubs.acs.org. CCDC 1033167 – 1033171 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge form The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

AUTHOR INFORMATION Corresponding Author * [email protected] b

Present Addres:

Department of Molecular Functional Materials, Technische Universität Dresden, Mommsenstraße 4, D-01062 Dresden, Germany

Funding Sources Financial support from the Deutsche Forschungsgemeinschaft (DFG SPP1362) as well as from HZB is gratefully acknowledged.

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Supporting Information Available: Synthetic procedures, PXRD patterns, physisorption data, TG analysis data, UV/Vis data, and crystallographic data. This information is available free of charge via the Internet at http://pubs.acs.org/ CCDC-1033164 – 1033171 contain the supplementary crystallographic data for DUT-71, DUT-72, DUT-73a, DUT-73b, DUT74, DUT-90, DUT-91 and DUT-95. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

ACKNOWLEDGMENT The authors thank Dr. C. Meyer (ILR Dresden) for providing software for automatic MFCs controlling. Dedicated to Professor Manfred Scheer on the occasion of th his 60 birthday.

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Postsynthetic Paddle-Wheel Cross-Linking and Functionalization of

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