Subscriber access provided by CMU Libraries - http://library.cmich.edu
Article
The effects of extending the #-electron system of pillaring linkers on fluorescence sensing of aromatic compounds in two isoreticular metal-organic frameworks Alireza Azhdari Tehrani, Hosein Ghasempour, Ali Morsali, Gamall Makhloufi, and Christoph Janiak Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.5b01175 • Publication Date (Web): 12 Oct 2015 Downloaded from http://pubs.acs.org on October 15, 2015
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Crystal Growth & Design is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 17
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
Crystal Growth & Design
The effects of extending the π-electron system of pillaring linkers on fluorescence sensing of aromatic compounds
in
two
isoreticular
metal-organic
frameworks Alireza Azhdari Tehrani,† § Hosein Ghasempour,† § Ali Morsali,*† Gamall Makhloufi,‡ and Christoph Janiak‡ † Department of Chemistry, Faculty of Sciences, Tarbiat Modares University, P.O. Box 14115175, Tehran, Iran ‡ Institut für Anorganische Chemie und Strukturchemie, Heinrich-Heine-Universität Düsseldorf, Universitätsstr. 1, D-40225 Düsseldorf, Germany
ABSTRACT. A new porous metal-organic framework (TMU-21) which is isostructural to our recently reported TMU-6, is introduced. The structure of this framework has been determined by X-ray crystallography and further characterized by FT-IR spectroscopy, elemental and thermogravimetric analyses. Its structural features as well as its stability and porosity were studied. These two metal-organic frameworks are interesting candidates for comparative fluorescence study. Thus, their potential abilities for sensing nitrobenzene, benzene and PAHs, namely naphthalene, anthracene and pyrene, were investigated. This study clearly shows an
ACS Paragon Plus Environment
1
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
Page 2 of 17
important contribution of extending the π-electron systems of pillaring linkers in the ability of MOFs to sense aromatic compounds.
Introduction In the last decade, metal-organic frameworks (MOFs) have received considerable interest due to their potential applications in different fields such as catalysis, gas storage and sensing.1-3 This interest is largely based on the ability to tailor the topology, pore size and functionality by judicious selection of the molecular building blocks.4 Taking advantage of this feature, many chemists have tried to design various MOFs with desirable structures and properties. The selective molecular recognition and sensing of small molecules by MOFs have been identified as one of the important and exciting area of research in the recent few years.5-6 Much attention has been focused on sensing and removal of toxic chemicals, such as nitroaromatic compounds (NACs),7-9 small aromatics10-11 and polycyclic aromatic hydrocarbons (PAHs).12-13 Recently, it has been proposed that the aromatic organic ligands in the MOF structure play an important role in interacting with organic analytes via π-interactions, namely π-π stacking and C-H⋯π interactions.12-14 Among a mass of MOFs synthesized to date, isoreticular metal-organic frameworks (IRMOFs) have provided a suitable platform for understanding the structure–function relationship. On account of their structural similarity, one can systematically study the effects of minimal structural variations in organic linker skeleton or secondary building units (SBUs) of MOFs.15 Based on this strategy, herein, we introduce a new metal-organic framework, TMU-21, which is isostructural to a reported TMU-6 that has been reported recently by some of us.16 The study reveals the importance of extending the π-electron systems of pillaring linkers in the ability of MOFs to sense aromatic compounds, Scheme 1a.
ACS Paragon Plus Environment
2
Page 3 of 17
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
Crystal Growth & Design
Experimental Section Synthesis Ligand L1 and TMU-6 were synthesized according to the procedures described by some of us.16 Synthesis of ligand (L2) The L2 ligand, Bis-pyridin-4-ylmethylene-naphtalene-1,5-diamine, was synthesized according to the following procedure. To 4-pyridine carboxaldehyde (1.88 ml, 20.0 mmol) in elthanol (50 ml) was added 1,5-diaminonaphthalene(1.58 g, 10.0 mmol) at room temperature. After the addition of dichloromethane (50 ml) and then two drops of formic acid, the mixture was stirred at room temperature for 24 h and then filtered. The resulting yellow crystalline solid was washed with ethanol (3 × 5 ml) and hexane (3 × 5 ml), and crystallized from CH2Cl2–hexane to give ligand L2. Yield, 76%, M.p. = 242–245 °C. IR (KBr, cm-1): 3034 (w), 1627 (s), 1594(s), 1553 (s), 1500(w), 1406 (s), 1319 (w), 1225(s), 980 (w), 704(s), 650(w). MS (m/z): 336.3 (M+, base peak). Synthesize of [Zn(oba)(4-nbpy)0.5]n.(DMF)1.5 (TMU-21) 0.298 g of Zn(NO3)2.6H2O (1 mmol), 0.181 g of synthesized 4-nbpy (0.5 mmol), 0.258 g of H2oba (1 mmol) were first dissolved in 15 ml of N,N΄-Dimethylformamide (DMF). The mixture was then placed in a Teflon reactor and heated at 120 °C for 3 days. The mixture was then gradually cooled to room temperature during 48 hours. Brown crystals were formed on the walls of reactor with a 38% synthesis yield. FT-IR data (KBr pellet, cm-1): selected bands: 654(m), 763(m), 876(m), 1017(m), 1095(m), 1157(s), 1238(vs), 1404(vs), 1504(m), 1610(vs), 1671(s) and 3416(w). Anal. calcd for ZnC29.5H26.5N3.5O6.5
([Zn(C14H8O5)(C11H8N2)].(C3H7NO)1.5):
C, 59.11; H, 4.46; N: 8.18, Found: C, 58.16; H, 4.28, N: 8.08.
ACS Paragon Plus Environment
3
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
Page 4 of 17
Activation method For activation purpose, synthesized crystals (TMU-6 and TMU-21) were then soaked in a 5 mL of CH3CN solvent for 5 days, with fresh CH3CN added every 24 hours. After 5 days the CH3CN solution was decanted, and activated crystals were dried at 120 °C under vacuum for at least 24 hours. (after activation) FT-IR (KBr pellet, cm-1): 656(w), 779(m), 874(w), 1095(m), 1158(s), 1238(vs),
1403(vs),
1504(m)
and
1604(vs).
Anal.
calcd
for
ZnC25H16N2O5
([Zn(C14H8O5)(C11H8N2)]): C, 61.31; H, 3.29; N: 5.72, Found: C, 60.96; H, 3.24, N: 5.70.
Results and Discussion TMU-21 was synthesized by combining Zn(NO3)2.6H2O, the ditopic H2OBA and L2 ligands using the solvothermal method at 120˚C for 72 h to give suitable X-ray quality crystals. X-ray crystallography revealed that TMU-6 and TMU-21 are isostructural crystallizing in the monoclinic C2/c space group.16 The asymmetric unit of TMU-21 consists of one Zn(II) ion, an OBA ligand and half of the L2 ligand. In both TMU-6 and TMU-21, each Zn(II) in the binuclear secondary building unit (SBU), Scheme 1b, (Zn⋯Zn separation=3.494(1) Å for TMU-6 and 3.509(1) Å for TMU-21) coordinated to four oxygen atoms of carboxylate groups belonging to one chelating and two bridging OBA ligands and one nitrogen atom of L2 ligand. The SBU nodes are connected to other secondary building units through the carboxylate groups of OBA ligands to form two-dimensional (2D) sheets. The L2 ligands act as pillars between 2D sheets to form the 3D framework which can be described as triply interpenetrated pcu alpha-Po primitive cubic network. Due to the V-shaped geometry of the OBA ligand (∠C-O-C =115.4(3)˚ for TMU6 and 117.6(4)˚ for TMU-21), they bridge the SBUs into a 3D network with honeycomb-like hexagonal channels. Both compounds possess large channels, along the [1 0 1ത] direction, with aperture size of 7.5 × 6.4 Å and 7.0 × 5.5 Å (including van der Waals radii of the atoms) for
ACS Paragon Plus Environment
4
Page 5 of 17
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
Crystal Growth & Design
TMU-6 and TMU-21, respectively, Figure 1(a) and 1(d). The calculated void space per unit cell for guest-free TMU-6 and TMU-21 frameworks are 34.2% (1852.4 Å3) and 32.9% (1864.1 Å3), respectively.17 Similar to TMU-6, TMU-21 is also non-porous to N2 at 77 K, but is porous to CO2 at 195K [118.0 cm3/g at STP, 5.27 mmol/g); Brunauer−Emmett−Teller (BET) surface area of 490 m2/g], Figure 1(b) and Figure 1(e). Powder X-ray diffraction reveals slight but significant difference between the simulated patterns and the as-synthesized, solvent exchanged and activated materials (Fig. S5). This points to TMU-6 and TMU-21 as being flexible or breathing frameworks and helps to explain why both are non-porous to N2 but adsorb CO2 and aromatic guests. Guests which give rise to stronger host-guest interactions because of a quadrupole moments like CO2 and aromatic molecules are capable of inflating such flexible frameworks again. Also, N2 sorption is measured at 77 K. In small pores N2 cannot penetrate at this cryogenic temperature. CO2 and aromatic guest adsorption is taking place at room temperature at which much more thermal and vibrational flexibility of the framework for the inclusion of even large guest molecules is possible. A survey in Cambridge Structural Database (CSD)18 reveals that both L1 (ref code: PEXXEW)19 and L2 (ref code: XAPTEO)20 crystallize in triclinic space group Pī. Hirshfeld surface analysis was carried out for the study of intermolecular interactions and their relative contribution to the Hirshfeld surface area.21 The contributions of different intermolecular interactions to the Hirshfeld surface areas are illustrated as a histogram in Figure S2. Noteworthy, when the benzene core has been replaced by naphthalene core, the contribution percentages of C-H⋯π and π⋯π interactions increases, while that of H⋯H interactions decreases. As depicted in Figures 1(c), 1(f) and Figure S1, the L1 and L2 pillar ligands are arranged inside the honeycomb channel walls of the TMU-6 and TMU-21, respectively. In these frameworks, the central benzene (for
ACS Paragon Plus Environment
5
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
Page 6 of 17
L1) and naphthalene (for L2) core of pillar ligands is mainly involved in pyC-H···π interactions as hydrogen acceptors with pyridinic hydrogen atoms on different pillar ligand. These intermolecular interactions strengthen the pore walls and therefore avoid channel collapse upon guest removal. Investigation of the intermolecular contacts and crystal packing of these two MOFs via Hirshfeld surface analysis also reveals that the contribution of C-H⋯π and π⋯π interactions increases on going from TMU-6 to TMU-21, Figure S2 and Table S1. Thermogravimetric analysis (TGA) indicates that TMU-21 has a slightly better thermal stability compared to TMU-6. In both cases, the TGA data shows an initial weight loss (~1%, after heating from room temperature to 100˚C) which is attributed to the loosely bound water molecule. The other weight loss occurred between 100 and 380˚C (~14.5%) corresponding to the removal of ~1.5 DMF guest molecules. The third weight loss was observed starting at 380°C and 400°C for TMU-6 and TMU-21, respectively, corresponding to the decomposition of the frameworks, Figure S3. These data are also confirmed by elemental analysis data. To remove the guest DMF molecules from the frameworks, the as-synthesized TMU-6 and TMU-21 were immersed in acetonitrile for 5 days, filtered and vacuum-dried at 120˚C for 24 h. The activation of these two frameworks was confirmed by FT-IR spectroscopy, elemental and powder X-ray diffraction (PXRD) analysis, Figure S4 and Figure S5. The water stability of these two frameworks was also examined by soaking activated samples in water for 24 h. The PXRD patterns of water-treated samples confirmed that these two MOFs are stable in water, Figure S5. Since the composition of these two MOFs is the same, except for the pillar ligand, these frameworks may be good candidates for comparative fluorescence study. TMU-6 has two absorption bands at 373 and 474 nm, while TMU-21 shows two absorption bands located at 353 and 466 nm. These MOFs exhibit strong fluorescence at 434 nm upon excitation at 373 and 353
ACS Paragon Plus Environment
6
Page 7 of 17
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
Crystal Growth & Design
nm for TMU-6 and TMU-21, respectively. The observed enhancement in the fluorescence intensity and the hypsochromic shifts relative to the free L1 and L2 (for both L1 and L2, λmax =528 nm) may be due to the charge transfer transitions between the ligands and central metal atoms, which is previously described in the case of [Zn2(OBA)2(bpeb)].2DMF.H2O framework by Vittal and his co-workers.22 The fluorescence properties of TMU-6 and TMU-21 were preliminary evaluated by immersing the activated MOFs in three aromatic solvents, ie, benzene, toluene, nitrobenzene, Figures 2(a), S6 and S7. Notably, these two MOFs exhibit almost complete fluorescence quenching when immersed into nitrobenzene, with the quenching efficiency of more than 98% for both MOFs. Also, the quenching efficiency follows the order of toluene
