Communication pubs.acs.org/IC
A Polar Titanium−Organic Chain with a Very Large SecondHarmonic-Generation Response Bongsu Kim, Seung-Jin Oh, Hongil Jo, and Kang Min Ok* Department of Chemistry, Chung-Ang University, 84 Heukseok-ro, Dongjak-gu, Seoul 06974, Korea S Supporting Information *
large SHG efficiency. Topotactic transformations to TiO2 microrods and water-molecule-driven reversible centricity changes are also presented. Pure colorless needle crystals of CAUMOF-18 were synthesized through a solvothermal reaction by using titanium(IV) isopropoxide (Ti[OCH(CH3)2]4), 2-pyridinecarboxylic acid [2-PC; NC5H4(CO2H)], 2,6-pyridinedicarboxylic acid [2,6-PDC; NC5H3(CO2H)2], dimethylammonium iodide [N(CH3)2H2I], and anhydrous N,N′-dimethylformamide at 130 °C for 1 day. Single-crystal structure analysis using synchrotron radiation reveals that CAUMOF-18 has a unidimensional structure (see Figure 1). The unique Ti4+ cation in an asymmetric unit is in the distorted TiO5N2 pentagonal-bipyramidal moiety with Ti−O and Ti−N bond lengths of 1.696(8)−2.128(11) and 2.251(8)−2.295(11) Å, respectively. The Ti4+ cation is coordinated by 2-PC and 2,6-PDC ligands along the equatorial
ABSTRACT: A noncentrosymmetric (NCS) titanium− organic compound, [H2N(CH3)2]TiO{[NC5H3(CO2)2][NC5H4(CO2)]} (CAUMOF-18), has been synthesized by a solvothermal reaction. The aligned unidimensional polar chain structure of CAUMOF-18 consisting of corner-shared distorted TiO5N2 pentagonal bipyramids is attributed to strong hydrogen-bonding and π−π interactions. CAUMOF-18 reveals a very strong secondharmonic-generation efficiency of 400 times that of αSiO2 and is phase-matchable (type I). Water-moleculedriven reversible centricity conversion and topotactic transformation to TiO2 microrods for CAUMOF-18 are also presented.
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evising smart functional noncentrosymmetric (NCS) materials is of huge current interest attributable to a wide variety of applications in laser systems, optical communications, photolithography, energy harvesting, detectors, and memories.1−3 The relevancies mainly originate from symmetrydependent fundamental properties such as piezoelectricity, second-harmonic generation (SHG), ferroelectricity, and pyroelectricity.4−6 Until now, many synthetic chemists have combined NCS chromophores as building units to increase the possibility of NCS materials. A few well-established asymmetric units in NCS oxide materials include anions with aligned πconjugated groups (NO3−, CO32−, BO33−, etc.),7−17 d10 cations under a greatly polarizable environment,18−22 and second-order Jahn−Teller distortive cations,23−30 i.e., d0 metal cations with distortive octahedral moieties (Ti4+, Nb5+, W6+, etc.) and cations possessing stereoactive lone pairs (Pb2+, Bi3+, Se4+, etc.). To discover further superior-performing NCS materials, several NCS metal−organic hybrid materials that can be constructed in a controlled manner with various metal cations and organic linkers have been suggested.31−33 Most of the NCS metal−organic hybrid materials have been developed by a rational synthesis on the basis of different network topologies such as helical chains, polymeric grids, and diamondoid frameworks.32,34−39 Although several interesting NCS hybrid materials have been reported, to our surprise, NCS titanium−organic materials with extended structures are extremely rare perhaps attributed to the instability and high reactivity of the titanium precursors. A couple of recently reported NCS titanium−organic frameworks have shown photoactive properties and photocatalyzed polymerization of methyl methacrylate.40,41 Herein, we report a novel NCS polar titanium−hybrid chain, [H 2 N(CH 3 ) 2 ]TiO{[NC5H3(CO2)2][NC5H4(CO2)} (CAUMOF-18), with a very © XXXX American Chemical Society
Figure 1. Ball-and-stick and polyhedral representations of CAUMOF18 in the (a) ac and (b) ab planes (blue, Ti; gray, C; red, O; cyan, N). H atoms have been omitted for clarity. Note that the strong hydrogen bonds (red dotted lines) and π−π interactions from each chain lead CAUMOF-18 to crystallize in a polar structure. Received: October 19, 2016
A
DOI: 10.1021/acs.inorgchem.6b02536 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry position, i.e., in the approximate ab plane. Each corner of the TiO5N2 pentagonal bipyramid is then linked through O(1), and infinite zigzag chains are produced along the [001] direction. As shown in Figure 1, the “short” and “long” Ti−O bonds alternate through the chain along the [001] direction. Interestingly, all of the chains in the structure reveal the same alternation. A more detailed structural examination suggests that strong hydrogenbonding interactions occur between the dimethylammonium cations and the carboxylate groups of the 2-PC and 2,6-PDC ligands [see the red dashed lines in Figure 1a; N(5)···O(3) 2.788(14) Å; N(5)···O(4) 2.76(2) Å; N(5)···O(7) 2.966(18) Å]. In addition, π−π interactions between the pyridine rings and carboxylate groups are observed from each chain. Both of the hydrogen bonds and π−π interactions have distorted TiO5N2 pentagonal bipyramids that are aligned along the [001] direction and generate a macroscopic polar structure. The backbone of CAUMOF-18 can be described as an anionic {TiO2/2[NC5H3(CO2)2][NC5H4(CO2)]}− chain, and charge balance is retained by the incorporated [H2N(CH3)2]+ cation. The NCS polar structure of CAUMOF-18 led us to monitor the nonlinear-optical (NLO) properties. Powder SHG measurements using 1064 nm radiation on polycrystalline CAUMOF-18 revealed a very strong SHG response of 400 times that of α-SiO2, which compares well to those of BaTiO3 (400 × α-SiO2) and LiNbO3 (600 × α-SiO2).42 Additional measurements on the graded polycrystalline sample indicated that CAUMOF-18 is type I phase-matchable (see Figure 2). On the basis of the SHG
C−H and CC bonds for the pyridine ring are observed at ca. 3026−3087 and 1605−1711 cm−1, respectively. The multiple peaks found at ca. 1302−1424 cm−1 may be assigned to the COO stretching vibrations. The vibration for the N−H bonds in the ammonium cation is observed around 2793 cm−1. The UV−vis diffuse-reflectance and IR spectra for CAUMOF-18 can be found in the Supporting Information. The thermogravimetric analysis (TGA) diagram and powder X-ray diffraction (PXRD) patterns measured at different temperatures revealed that CAUMOF-18 is thermally stable up to 290 °C (see Figure 3). Above the temperature, the framework
Figure 3. TGA diagram for CAUMOF-18. The PXRD patterns at different temperatures and SEM images indicate that CAUMOF-18 transforms to TiO2 upon heating, while the rod-shaped morphology is maintained.
of the material collapsed because of thermal decomposition of the organic ligands. PXRD patterns obtained at higher temperatures indicated that TiO2 anatase occurred at 600 °C. The anatase started a phase transition at 700 °C and changed to rutile above the temperature. Interestingly, the rod-shaped morphology of the crystallites has been topotactically maintained during thermal decomposition, and TiO2 microrods were successfully prepared (see Figure 3). It should be noticed that pure bulk TiO2 anatase starts to transform to rutile at ca. 600 °C at atmospheric pressure.45,46 The higher anatase-to-rutile transition temperature observed from the thermally decomposed TiO2 from CAUMOF-18 may be attributed to the intact rodshaped morphology during calcination. We found that CAUMOF-18 reveals a very interesting reversible centricity conversion that is driven by water molecules. If exposed to water vapor at room temperature, NCS polar CAUMOF-18 transformed to a centrosymmetric (CS) structure, which was confirmed by both PXRD and SHG measurements on the transformed phase (see Figure 4). Although the transformed phase still exhibited high crystallinity, many cracks existing inside the crystals inhibited single-crystal X-ray diffraction analysis. The PXRD data on the material transformed by water may be indexed on an orthorhombic cell with a ∼ 12.78 Å, b ∼ 20.83 Å, and c ∼ 3.73 Å. The observed centricity conversion from NCS to CS might be influenced by the effectiveness of hydrogen-bonding interactions. As soon as water molecules were introduced into CAUMOF-18, strong interactions between generated dimethylamine and hydronium ions occur, which should impede effective hydrogen-bonding interactions between dimethylammonium cations and carboxylate ligands and result in a CS structure. However, as shown in Figure 4, occluded water molecules could be removed upon heating. If the temperature is increased to 220 °C, the CS material goes back to the original NCS CAUMOF-18.
Figure 2. Phase-matching curve (type I) for CAUMOF-18. The curve is to a guide to the eye and not a fit to the data. A net moment arising from the alignment of the chains composed of corner-shared distorted TiO5N2 pentagonal bipyramids along the [001] direction is responsible for the very strong SHG response for CAUMOF-18.
efficiency as well as the phase-matching behavior, the bulk NLO susceptibility, ⟨deff⟩exp, for the class A SHG material CAUMOF18 is estimated to be approximately 23 pm/V. The very strong SHG efficiency of CAUMOF-18 should be attributable to the large net moment arising from the alignment of the chains composed of corner-shared distorted TiO5N2 pentagonal bipyramids along the [001] direction (see Figure 2). Absorption data (K/S) calculated from the Kubelka−Munk function43,44 using the UV−vis diffuse-reflectance spectrum indicated that CAUMOF-18 has a band gap of ca. 3.7 eV, which may be attributable to the degree of Ti 3d orbital associated in the conduction band. The IR spectrum of CAUMOF-18 revealed Ti−O vibrations at ca. 453 and 670 cm−1. The bands due to the B
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Figure 4. (a) PXRD patterns and (b) schematic representation revealing a water-molecule-driven reversible centricity conversion.
The NCS structure of the regenerated CAUMOF-18 was confirmed by measuring the strong SHG signal. In summary, a new unidimensional NCS polar titanium− organic material, CAUMOF-18, was successfully synthesized through a solvothermal reaction. CAUMOF-18 reveals a very large SHG efficiency of 400 times that of α-SiO2 and is type I phase-matchable. The strong SHG signal originated from the aligned polar geometry formed by both hydrogen bonds and π−π interactions. CAUMOF-18 also exhibited a topotactic transformation and a water-molecule-driven reversible centricity conversion.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02536. Experimental details and data (PDF) Crystallographic data (CIF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Kang Min Ok: 0000-0002-7195-9089 Notes
The authors declare no competing financial interest. Crystals of NCS CAUMOF-18 have been deposited to Noncentrosymmetric Materials Bank (http://ncsmb.knrrc.or. kr).
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ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea, funded by the Korean Government (Grants 2014M3A9B8023478 and 2016R1A2A2A05005298).
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