Nonlinear Optical Rod Indium-Imidazoledicarboxylate Framework as

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Nonlinear Optical Rod Indium-Imidazoledicarboxylate Framework as Room-Temperature Gas Sensor for Butanol Isomers Xiao-Ying Bai,† Wen-Juan Ji,‡ Shu-Ni Li,† Yu-Cheng Jiang,† Man-Cheng Hu,† and Quan-Guo Zhai*,† †

Key Laboratory of Macromolecular Science of Shaanxi Province, School of Chemistry & Chemical Engineering, Shaanxi Normal University, Xi’an, Shaanxi 710062, PR China ‡ School of Chemistry & Material Science, Shanxi Normal University, Linfen, Shanxi 041004, PR China S Supporting Information *

ABSTRACT: The mimic of classic [Co(en)3]-like (en = ethylenediamine) structure generates a 3D chiral indium-imidazoledicarboxylate framework (SNNU-50) exhibiting significant SHG effects. 1D rod building units in SNNU-50 lead to a band gap value comparable to that of In2O3. Different from the high temperature metal oxide semiconducting sensor, MOF sensor prepared by the SNNU-50 sample demonstrates remarkable selectivity and low detection limit to four butanol isomers at room temperature. Notably, SNNU-50 represents the first MOF sensor showing the ability for structural isomer identification, which may open up a new promising application for porous metal−organic frameworks.

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Herein, we report such a novel rod InOF, {[In4(IMDC)3(H2O)6](NO3)3} (SNNU-50). As expected, 1D In-IMDC chains effectively improve the conductivity of SNNU-50, which exhibits a band gap of 3.41 eV (this value is 3.55−3.75 eV for In2O31). Compared to the weak response to common volatile organic compounds (VOC), SNNU-50 exhibits a remarkable sensitivity to butyl alcohol vapor. SNNU-50 also exhibits high selectivity and low detection limit to butanol isomers (n-BA, i-BA, s-BA, and t-BA) at room temperature. To the best of our knowledge, SNNU-50 is the first MOF sensor which can effectively identify the structural isomers. Moreover, an interesting mimic of classic [Co(en)3]like (en = ethylenediamine) coordination fashion make SNNU50 crystallizing in a chiral space group and exhibiting significant nonlinear optical property. Reaction of In(NO3)3·xH2O and 4,5-imidazoledicarboxylic acid (IMDC) in NMF/1,4-dioxane/DMI mixtures at 100 °C for 5 days gave rise to the polyhedral crystals of SNNU-50. It is worth noting that the addition of angular dicarboxylic acid such as 5-hydroxyisophthalic acid, isophthalic acid, and 2,5furandicarboxylic acid as additive can effectively help for the formation of large single crystal samples. X-ray single-crystal structure analysis shows that SNNU-50 exhibits a 3D cationic porous framework generated by In-IMDC rod SBUs and crystallizes in the chiral space group I4132 with the absolute structure parameter of 0.05(14) (Table S1). The asymmetric unit contains two crystallographically independent In3+ ions,

etal oxides such as In2O3 have showed inspiring effects in the gas sensor field because of their good semiconducting property.1 However, such materials are always limited by their low surface areas and high operating temperatures since chemical sensing behavior is prominently a surface-dependent phenomenon.2 It is essential to develop gas sensing materials that simultaneously possess large surface area, good porosity, and uniformity. Metal−organic frameworks (MOFs), constructed by metal-containing nodes and organic bridges, are promising candidates as gas sensors due to their large surface areas, adjustable pore sizes, as well as acceptable thermal stability.3−17 However up to now, very limited MOFs have been developed for semiconducting sensors because of their typically high electrical resistivity.18−23 Rod MOFs24 are metal−organic frameworks containing infinite secondary building units (SBUs) such as metal−oxygen chains. The formation of rod MOFs is supposed to effectively improve the conductivity of metal−organic materials because 1D metal−oxygen chains may retain the good semiconducting property of metal oxides to the greatest extent. Among various metal centers, indium should be an ideal selection to produce rod MOFs since the In-oxygen chain is a distinct SBU for the development of indium-organic frameworks (InOFs).25 On the other hand, 4,5-imidazoledicarboxylic acid (IMDC), a wellknown versatile ligand, has linked indium ions to form several zeolite-like InOFs.26,27 Thus, the combination of In3+ and IMDC is speculated to be a promising strategy for the construction of rod indium-organic frameworks, which may keep the semiconducting property of In2O3 and further act as MOF sensors. © XXXX American Chemical Society

Received: November 29, 2016 Revised: December 13, 2016 Published: December 16, 2016 A

DOI: 10.1021/acs.cgd.6b01744 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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As depicted in Figure 1a, each In(1) center links adjacent three different In(2)-IMDC helical chains through the sevenmembered rings and thus generates the 3-D framework of SNNU-50. Intersecting quadrilateral channels with dimensions of about 9.0 Å × 9.0 Å exist along a-, b-, or c-axis directions (Figure 1c). Moreover, triangular channels with dimensions of about 13.5 Å × 13.5 Å × 13.5 Å are also observed along the [1 1 1] direction (Figure S2). The coordinated water molecules around each In(2) atoms distribute on the inner wall of these channels, which are further occupied by charge-balanced nitrate anions. From the topological point of view, In(1) and IMDC can be regarded as three-connected nodes, and In(2) can be simplified as linker. The overall structure of SNNU-50 can be simplified as a (3, 3)-connected chiral net with the point symbol of {62.10}3{63} (Figure S3). Alternatively, according to the rod-like secondary building unit concept, the 3D net of SNNU-50 exhibits a rather simple topological framework based on 1D rod-shaped In(2)-IMDC building blocks. Each 1D rodshaped building block links the adjacent 3-connected In(1) nodes to generate the 3D rod-packing structure as given in Figure 1d. The phase purity of the bulk sample was confirmed by matching the powder X-ray diffraction (PXRD) patterns of SNNU-50 and the PXRD patterns simulated from single crystal analysis (Figure S4). The solid-state PL spectrum study at room temperature shows that SNNU-50 emits a blue light centered at 390 nm upon photoexcitation at 235 nm. The fluorescence spectrum may be assigned as an intraligand n → π* transition due to the similar emissions of pure IMDC ligand (Figure S5). Moreover, SNNU-50 was found to be nonporous with respect to N2, but exhibits a CO2 uptake capacity of 18.3

one IMDC ligand, and one coordinated H2O molecule (Figure S1). In(1) is symmetrically coordinated with six carboxylate O(1) atoms to form an octahedral geometry. As depicted in Scheme 1, three IMDC ligands around each In(1) generate Scheme 1. Mimic of [Co(en)3] Structure for In(1) Center in SNNU-50

three seven-membered rings. Such distribution is an interesting mimic of classic [Co(en)3]-like (en = ethylenediamine) structure, which should be the origin of the chirality for SNNU-50. To the best of our knowledge, only one similar example is observed in the Cd-IMDC framework, which also crystallizes in the chiral space group.28 In(2) is octahedrally coordinated with two imidazole N(1), two carboxylate O(2), and two O(3) atoms of coordinated water. Each independent IMDC ligand connects two In(2) ions in a bis(bidentate) mode though N,O-heterocoordination, forming two rigid fivemembered chelate rings coplanar with the imidazole ring. Such linkage between In(2) atoms and IMDC ligands leads to a 1-D helical chain with the screw pitch of about 25 Å (Figure S1). Adjacent In(2) atoms are separated to 6.5 Å.

Figure 1. Polyhedral diagrams (a and c) and simplified presentations (b and d) showing the linkage between In(1) center and In(2)-IMDC rod SBUs (a and b), and the 3D microporous framework (c and d) of SNNU-50. B

DOI: 10.1021/acs.cgd.6b01744 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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cm3/g at 273 K and 1 atm (Figure S8). The isosteric heat of adsorption (Qst) of CO2 was determined by fitting the adsorption data collected at 273 and 298 K to the virial model. SNNU-50 exhibits Qst for CO2 of 36.2 kJ/mol (Figure S9) at zero coverage, which is comparable to a lot of famous MOF materials29−31 and indicative of a strong interaction of CO2 with the framework of SNNU-50. The second-order nonlinear optical effects were investigated to confirm the physical properties deriving from the assignment of SNNU-50 to a chiral space group introduced by [Co(en)3]like In(1) center. Approximate estimations were carried out using a powdered sample on a pulsed Q-switched Nd:YAG laser at a wavelength of 1064 nm, which showed that SNNU-50 has a 0.3−0.8 times SHG effect32−34 compared to that of KDP (Figures 2 and S10). Although such SHG effect is not striking,

which is an important n-type semiconductor metal oxide and has high sensitivity to carbon monoxide, methanol, ethanol, acetone, and so forth. Thus, we speculate that SNNU-50 may also be a good semiconducting sensor. In our opinion, 1-D chain generated by rigid five-membered chelate rings coplanar with the imidazole ring and In(2) centers may be a key contribution to the good semiconducting behavior of SNNU50. The as-synthesized powder sample of SNNU-50 was coated on the Ag−Pd interdigitated electrodes for the gas sensing measurements.35 Gas sensing measurements were first performed by exposing the obtained sensor to 100 ppm diverse VOC gases (dichloromethane, trichloromethane, carbon tetrachloride, acetone, methanol, ethanol and n-propanol, and n-butanol) at room temperature. Figure S12 displays that SNNU-50 sensor has the highest response to n-butanol. The sensitivity of SNNU-50 sensor versus different n-butanol concentrations at 25 °C operating temperature has been further investigated. With an increase of n-butanol concentration from 20 to 120 ppm, the sensitivity promptly increases (Figure 3a). Notably, the sensitivity values and the concentration of n-butanol are of linear relation with the R2 value of about 0.97 (Figure 3b). This result indicates that SNNU-50 can be used for the quantitative detection of n-butanol vapor. Figure 3c shows the response−recovery characteristics of SNNU-50 sensor to 100 ppm n-butanol. There was almost no apparent signal attenuation after five cycles at room temperature, which further demonstrates the practice application of SNNU-50 as n-butyl alcohol gas sensor. Owing to its good sensitivity to n-butyl alcohol, SNNU-50 has drawn our attention for use as a gas sensor in detecting four kinds of butanol isomers (n-butyl alcohol = n-BA, iso-butyl alcohol = i-BA, sec-butyl alcohol = s-BA, tert-Butyl alcohol = tBA). To compare and analyze the selectivity, gas sensing measurements were performed by exposing the obtained sensor to butyl alcohol isomers with given 100 ppm concentrations. As demonstrated in Figure 3d, SNNU-50 sensor has the highest response to n-BA at room temperature. In contrast, the response to t-BA is lowest. The response to i-BA is nearly equal to the s-BA. These results clearly reveal that SNNU-50 sensor can easily identify four butyl alcohol isomers at room temperature. To the best of our knowledge, SNNU-50 is the first MOF sensor showing the ability for structural isomer identification, which may open up a new promising application for porous MOF materials. In general, the porous structure is beneficial to gas sensing performance by increasing the surface reactive sites and facilitating the diffusion of target gases. In our opinion, the coordinated water molecules on the inner wall of channels in SNNU-50 should be a key factor for its unique sensor behavior. The formation of hydrogen bonds between hydroxyl groups of butanol and coordinated water molecule may affect the conductivity of SNNU-50. The stronger hydrogen bonds may decrease the electron density around the In(2) center, and thus increase the electrical resistivity of the MOF material. With four butyl alcohol isomers taken into account, the hydroxyl group is in a different position and the steric hindrance of alkyl groups are different, so the ability to form a hydrogen bond with coordinated water molecules is different. For n-BA, the smallest steric hindrance of an alkyl group on the hydroxyl group leads to the strongest hydrogen bond and thus the highest response. In contrast, the response of SNNU-50 to t-BA is lowest. Our supposed sensor mechanism is further proven by the fact that

Figure 2. (a) Oscilloscope traces of the SHG signals of SNNU-50 at 1064 nm with particles size of 0.08−0.063 mm. (b) Plots of SHG intensity vs particle size. KDP samples serve as the references at 1064 nm.

the origin of chirality of SNNU-50 from the [Co(en)3]-type structure may provide a promising way for the development of chiral or noncentrosymmetric MOF materials by the centrosymmetric ligands. As stated above, rod SBUs may effectively improve the conductivity of rod MOF materials. In order to explore the conductivity of SNNU-50, the measurements of diffuse reflectivity for powder crystal sample were used to obtain its band gap. The diffuse reflection spectrum of SNNU-50 reveals the presence of a band gap of 3.41 eV (Figure S11). It should be pointed out that the band value is 3.55−3.75 eV for In2O3, C

DOI: 10.1021/acs.cgd.6b01744 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Figure 3. Semiconducting sensor performance of SNNU-50 to butanol isomers at room temperature: (a,b) sensitivity to n-butanol with different concentrations and their linear relation; (c) response−recovery characteristics of sensor to 100 ppm n-butanol with five cycles; (d) different sensitivities to four butyl alcohol isomers with same concentration.

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the SNNU-50 sample that lost water cannot be identified as a butanol isomer. In summary, a chiral rod indium−organic framework was successfully synthesized, which exhibits strong blue photoluminescence, significant SHG effect, as well as semiconductive property. More importantly, SNNU-50 demonstrated remarkable identification ability for butanol isomers at room temperature, which may open up a new promising application for porous metal−organic frameworks. Further work on rod MOF gas sensing material with more specific binding sites for guest molecules is ongoing in our lab.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Quan-Guo Zhai: 0000-0003-1117-4017 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by the National Natural Science Foundation of China (21671126, 21271123, and 21601115), and the Natural Science Foundation of Shaanxi Province (2014KJXX-50).

ASSOCIATED CONTENT

S Supporting Information *

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.6b01744. Details about synthesis and general characterizations, crystal data, additional graphics, PXRD and TG data, N2 and CO2 adsorption and desorption isotherms, solidstate FT-IR, PL, NLO, and UV/vis spectra (PDF)

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CCDC 1519834 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033. D

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DOI: 10.1021/acs.cgd.6b01744 Cryst. Growth Des. XXXX, XXX, XXX−XXX