ZIF-8 as Nonlinear Optical Material: Influence of Structure and

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ZIF‑8 as Nonlinear Optical Material: Influence of Structure and Synthesis Stijn Van Cleuvenbergen,*,† Ivo Stassen,‡ Elena Gobechiya,‡ Yuexing Zhang,§,# Karen Markey,‡ Dirk E. De Vos,‡ Christine Kirschhock,‡ Benoît Champagne,§ Thierry Verbiest,† and Monique A. van der Veen*,⊥ †

Molecular Imaging and Photonics, Department of Chemistry, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium Department of Bioengineering, Centre for Surface Chemistry and Catalysis, Celestijnenlaan 200F, 3001 Leuven, Belgium § Laboratoire de Chimie Théorique, Unité de Chimie Physique Théorique et Structurale, University of Namur, rue de Bruxelles, 61, B-5000 Namur, Belgium ⊥ Catalysis Engineering, TU Delft, Julianalaan 136, 2628 Delft, The Netherlands ‡

S Supporting Information *

ABSTRACT: Metal−organic framework ZIF-8, from the zeolitic imidazolate framework family, shows a large intrinsic second-order nonlinear optical (NLO) response. In addition, ZIF-8 is a stable, inexpensive material that is transparent in the visible (vis) and nearinfrared (NIR) window. This is crucial for NLO applications. The secondorder NLO activity is due to the noncentrosymmetric octupolar symmetry of the material. We found that fast syntheses lead to a lower second-order NLO response. Consistent with polarized second-harmonic generation (SHG) microscopy measurements, we ascribe this to defects that create local centers of centrosymmetry but do not affect the orientation of the crystal lattice. Syntheses with slow nucleation lead to quasi-perfect crystals with a large average second-order NLO coefficient ⟨deff⟩ of 0.25 pm/V, which is explained and supported by ab initio theoretical calculations.

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INTRODUCTION The development of adequate second-order nonlinear optical (NLO) materials is of key importance for applications such as electro-optical switching, telecommunications, and the development of optical computing.1,2 In biomedical research, NLO materials are becoming popular to image biological tissue with much higher penetration depth than linear optical methods.3 Yet these applications require additional qualities of NLO materials: ultrashort response times, biocompatibility, adaptive NLO behavior, and processability in a variety of films and nanostructures. While the majority of commercial NLO materials are currently inorganic crystals, organic materials offer clear advantages in terms of response times, processability, and tunability.4 However, it is hard to gain control over the macroscopic organization of organic crystals. Since secondorder NLO activity requires a material to be noncentrosymmetric, post-processing techniques such as electric field poling or Langmuir−Blodget deposition are often applied. In this respect, metal−organic frameworks (MOFs) have emerged as a particularly interesting family of NLO materials. They can be conceived as extended metal coordination complexes, consisting of metal ions linked together by multitopic organic linker molecules. Their modular building principle leads to a higher predictability of the structure of the resulting solid compared to typical organic materials.5 This principle has been exploited to © 2016 American Chemical Society

come to rational design of noncentrosymmetric NLO active MOFs,6,7 and even a high contrast nonlinear optical MOF switch has been reported.8 Conversely, NLO techniques such as second-harmonic generation (SHG) microscopy have been employed to study structure and organization in MOFs, due to their remarkable sensitivity to symmetry.9 Such studies often provide information that is complementary10,11 or even hidden12 to traditional characterization techniques such as Xray diffraction (XRD). Here, we report, for the first time, the second-order NLO activity of ZIF-8, the most widely investigated member of the ZIF (zeolitic imidazolate framework) family. This MOF consists of Zn tetrahedra coordinated by 2-methylimidazolate linker units resulting in a zeolite-like sod topology. ZIF-8 displays a strong intrinsic NLO signal, comparable to commercial inorganic crystals such as KDP (potassium dihydrogen phosphate), while the organic nature of the NLO response guarantees ultrafast response times. Other than previously reported NLO-active MOFs, ZIF-8 is remarkably stable, both thermally and chemically, and is transparent in the visible and near-infrared (NIR) window.13 These properties are crucial for practical applications. Moreover, ZIF-8 is biocompatible, highlighting the possibility of sensitive second-order Received: March 16, 2016 Published: April 13, 2016 3203

DOI: 10.1021/acs.chemmater.6b01087 Chem. Mater. 2016, 28, 3203−3209

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Chemistry of Materials

Figure 1. Scanning electron microscopy (SEM), second harmonic generation (SHG), and two-photon fluorescence (TPF) images of ZIF-8 crystals from different syntheses (from fast to slow: RT, MW, O). The scale bar is set to 25 μm for all images, except for the SEM image of the RT synthesis, where it is set to 1 μm. For the SHG and TPF images, the relative average intensity level is shown in the lower right corner. We measured smaller crystals, to avoid phase-matching effects in the SHG window.

NLO imaging in biological samples.14 In addition, ZIF-8 is easily processed into high-quality thin films, on glass and silicon substrates, which already has led to a range of optical applications.15 Recently, ZIF-8 was pitched as the first microporous material to be deposited by chemical vapor deposition, making it compatible with microelectronic circuitry as well.16 To explain the second-order NLO activity of ZIF-8, the molecular response and framework symmetry were respectively studied by ab initio calculations of the hyperpolarizabilty β of the unit cell of ZIF-8 and by a combination of SHG microscopy and XRD. We also investigated the influence of the synthesis procedure on the average second-order NLO coefficient, ⟨deff⟩, of ZIF-8 and found that different synthesis protocols lead to distinct differences in NLO activity. To account for this effect, the local crystal symmetry was studied with polarized SHG microscopy.

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Formate-modulated solvothermal microwave synthesis (MW) was carried out using a modified published procedure.18 The molar composition of the 50 mL synthesis solution was ZnCl2:HCOONa:2methylimidazole:methanol = 1:1.5:1.5:250. The solution was transferred to a Milestone PTFE reaction vessel with PEEK shield and subsequently heated in Milestone MicroSYNTH microwave synthesis oven at 8 °C/min to 100 °C for 4 h. Post-treatment of the crystals was performed using identical conditions as the room-temperature procedure. Formate-modulated conventional oven (O) solvothermal synthesis was carried out using a slightly modified published procedure.19 The molar composition of the 20 mL synthesis solution was ZnCl2:HCOONa:2-methylimidazole:methanol = 1:2:2:320. The solution was transferred to a polytetrafluoroethylene (PTFE)-lined steel autoclave. A microscopy slide was added to the autoclave to facilitate nucleation of the crystals. The autoclave was placed in an isothermal oven at 100 °C for 19 h. Post-treatment of the crystals was performed identical to the other procedures. XRD Measurements. Powder X-ray diffraction (XRD) patterns were recorded at room temperature on a STOE STADI MP diffractometer with a focusing Ge(111) monochromator (Cu Kα1 radiation, λ = 1.54056 Å) in Debye−Scherrer geometry with a linear position-sensitive detector (PSD) with a 2θ window of 6°. Data were recorded in the 2θ range of 3°−90.50° with a step width of 0.5°, internal PSD resolution of 0.01°, and a counting time of 200 s per step. All samples of ZIF-8 material obtained from different syntheses correspond to a cubic structure with the I4̅3m or Im3m space group and unit-cell parameters of a = 16.99 Å (RT-synthesis), a = 17.01 Å (MW-synthesis), and a = 17.02 Å (O-synthesis). Single-crystal XRD data of a large (>50 μm) cubic single-crystal from the O-synthesis were collected at 100 K on an Agilent

EXPERIMENTAL SECTION

Synthesis. All chemicals were used as received from commercial suppliers, without further purifications. Room-temperature (RT) ZIF8 synthesis was carried out according to a published procedure.17 The molar composition of the synthesis solution was Zn(NO3)2·6H2O:2methylimidazole:methanol = 1:8:1000. The 200 mL solution was stirred in a sealed glass beaker for 1 h at room temperature to allow nucleation and growth of the crystals. The crystals were recovered by centrifugation, washed with methanol, and dried under air at 100 °C. 3204


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Chemistry of Materials SuperNova diffractometer, equipped with an Atlas CCD detector, using Mo Kα radiation (λ = 0.71073 Å). The images were interpreted and integrated with the CrysAlisPro software from Agilent Technologies. Cubic space group I4̅3m is confirmed. NLO Measurements. All NLO measurements were performed at the 800 nm line of an Insight DS+ system of Spectra Physics (120 fs, 80 MHz pulse rate). The nonlinear optical coefficient, as well as the thermal and photostability, were measured in transmission against a BBO crystal with a widefield SHG microscope, in accordance with earlier publications.10 In this setup, the incident light is focused by a long focal length lens ( f = 75 mm, NA = 0.013) and can, in good approximation, be considered as a collimated beam; the images are formed by a 15× long working distance objective (NA = 0.35). Structural SHG images were obtained with a scanning microscope (Olympus, Model BX61WI-FV1200-M) in transmission mode. A 15× long working distance objective (NA = 0.35) is used in all measurements in combination with a visible-light condenser (NA = 0.9). Further details about calculations and fittings are provided in the Supporting Information (SI). Theoretical Calculations. The time-dependent Hartree−Fock (TDHF) and time-dependent density functional theory (TDDFT) schemes20 were applied for obtaining the dynamic (frequencydependent) first hyperpolarizabilities (β) at an incident wavelength of 800 nm (ℏω = 1.55 eV). The TDDFT calculations employ a longrange corrected (LC) exchange-correlation functional (LC-BLYP),21 characterized by a range-separating parameter (μ) of 0.47. The atomic basis set combines the 6-31+G* basis set for the C, H, and N atoms with the LANL2DZ effective core potential and valence electron basis set for the Zn atoms (abbreviated as LANL2DZ/6-31+G*) for all compounds except C240H324N120Zn2412+, for which the 6-31+G* basis set was replaced by 6-31G*. All calculations were performed with the Gaussian 09 package.22 The noncoordinated N atoms of the imidazolate units were terminated with H atoms, because this most closely resembles the situation in the actual ZIF-8 framework.

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Determination of the Nonlinear Optical Coefficient. The NLO efficiency of differently sized crystals can be compared in an accurate manner by measuring the secondorder NLO coefficient ⟨deff⟩. deff is the experimental standard to quantify the second-order NLO response of materials. This is achieved by measuring a sample’s SHG intensity against a known reference while accounting for the thickness of the ZIF8 crystallites and the reference crystal (see the SI). We selected small crystals of ∼2 μm for this analysis to avoid phasematching effects. The NLO coefficients for the different syntheses are summarized in Table 1. The RT-synthesis Table 1. Second-Order NLO Coefficient (deff) for Different Batches of ZIF-8, Showing a Variation in Efficiency between Different Synthesis Procedures synthesis method

⟨deff⟩a (pm/V)

number of crystals analyzed

room temperature, RT microwave, MW oven, O

0.05 ± 0.02 0.16 ± 0.03 0.25 ± 0.04

28 26 47

⟨deff⟩ is calculated as described in the SI; the standard deviation reflects the variation over different crystals. a

crystals show a modest value of 0.05 pm/V. However, microcrystals from the slower MW-synthesis generate a markedly higher ⟨deff⟩ value (∼0.16 pm/V). In the O-synthesis, for which nucleation is slowed even further, values are enhanced to ∼0.25 pm/V. These values are large, on the order of commercial NLO crystals such as potassium dihydrogen phosphate (KDP). While numerous MOFs have been studied for their secondorder NLO activity as well, comparing their efficiencies is difficult. The majority of publications concerning SHG-active MOFs makes use of the semiquantitative Kurtz powder method, usually only reporting the SHG intensity of a MOF powder against a reference, not taking into account the phase matching conditions, packing, crystallite size, and coherence length.23−25 That being said, it seems that, in terms of pure efficiency, ZIF-8 is outperformed by some of the MOFs reported so far.6 However, ZIF-8 is exceptional, since it combines a high NLO activity with low cost, remarkable stability, and transparency in the optical window. We confirmed the exceptional thermal stability of ZIF-8,13 remaining SHG active until decomposition (above 450 °C under N2; see the SI). In addition, no photodegradation occurred when exposed to intense laser light for prolonged periods of time (see the SI). Structural Characterization. The recorded powder XRD patterns show a very good match with both the I4̅3m and Im3m space groups. A single crystal from the O-synthesis was selected for single-crystal XRD (see the SI) and its diffraction pattern agrees with I4̅3m symmetry. This is consistent with earlier reports in the literature13,26 and explains the NLO activity of ZIF-8 as I4̅3m is noncentrosymmetric (octupolar), while Im3m is centrosymmetric. Octupolar symmetries are advantageous for NLO applications, since they are less sensitive to polarization and exhibit a better transparency/efficiency tradeoff.27 Another method to determine point group symmetry of single crystals was developed by van der Veen et al. and is based on polarized SHG measurements.28 The point group of a ZIF-8 single crystal resulting from the solvothermal O-synthesis was determined with this method (see the SI) and is Td, in agreement with the assignment of the I4̅3m space group.

RESULTS AND DISCUSSION

We studied ZIF-8 crystals synthesized according to three distinct published protocols. A fast synthesis protocol at room temperature by reaction of Zn(NO3)2 in methanol (RTsynthesis) yields small crystallites in the nanometer range, with a rather narrow size distribution centered at ∼90 nm. A second slower, sodium formate-modulated solvothermal microwave synthesis (MW-synthesis) starting from ZnCl2 generates crystals in the micrometer range of varying size, generally ∼1−25 μm.18 The third protocol is a modification of the MWsynthesis using a conventional oven instead of a microwave oven (O-synthesis).19 This slows the nucleation of ZIF-8; hence, a longer crystallization time was maintained with respect to the MW-synthesis (19 h instead of 4 h). The resulting crystals are generally 1−20 μm in size. Typical SEM images are shown in Figure 1. The formation of the framework structure is confirmed for all respective samples by powder XRD measurements (see the SI). ZIF-8 is NLO active, displaying strong SHG and two-photon fluorescence (TPF), as can be seen in Figure 1. The O crystals show higher SHG activity than their MW counterparts, while the two-photon fluorescence intensity is similar. Signal levels are much lower for the smaller RT crystals. Since we measured crystals of similar size for the O-synthesis and MW-synthesis (2.0 ± 0.8 and 2.3 ± 0.7 μm, respectively), their relative SHG activity can be evaluated directly from the average relative intensities displayed in the images. However, we cannot directly compare with the RT crystals, since crystal size and aggregation for these smaller (67 ± 10 nm) crystallites must be taken into account. 3205


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Figure 2. Structure of the C16H24N8Zn2+, C28H41N14Zn23+, C48H68N24Zn44+, and C240H324N120Zn2412+ ZIF-8 structures used in ab initio calculations.

Figure 3. Structural SHG images for the MW (left) and O (right) synthesis (scale bars = 50 μm). For each pixel, the color represents the position of the maximal polarization response agreeing with the framework orientation. The brightness correlates with the measured SHG intensity.

Theoretical Calculation of the SHG Response. To further confirm that the SHG activity arises from the Td structure of ZIF-8, its SHG-response is modeled theoretically. The SHG activity can be understood on the macroscopic level, in terms of the effective measured nonlinear coefficient deff or the theoretically assessed nonlinear susceptibility χ(2) of the bulk medium and in terms of the hyperpolarizability (β) on the molecular level. To model the macroscopic nonlinear susceptibility, following a recent investigation,29 the cluster approach was adopted to characterize the SHG response of ZIF-8. It consists of evaluating the first hyperpolarizability β of increasingly large clusters to assess the χ(2) ijk response. It presents advantages over the crystalline orbitals30 and electrostatic interaction methods,31 which suffer from the large unit-cell size or from difficulties inherent to the separability of the crystals into non-overlapping entities. Calculations were performed on the C 16 H 24 N 8 Zn 2+ , C28H41N14Zn23+, C48H68N24Zn44+, and C240H324N120Zn2412+ structures (Figure 2) at the TDDFT/LC-BLYP (TDHF) level of approximation within the T convention. The values calculated at the TDHF level are always given between brackets. In particular, the hyperpolarizability β of C48H68N24Zn44+ amounts to 39 a.u. (49 a.u.). In order to assess the intrinsic unit contribution to the first hyperpolarizability, the cluster β values have been divided either by the number of zinc atoms (NZn) or by the number of 2methylimidazole (mIM) ligands (NL). From the smallest C16H24N8Zn2+ cluster to C48H68N24Zn44+, β/NZn and β/NL decrease from 28 (41) a.u. to 10 (12) a.u. and 7 (10) a.u. to 3 (4) a.u. at the TDDFT/LC-BLYP (TDHF) level of approximation. For the C240H324N120Zn2412+ cluster, the TDDFT/LC-BLYP (TDHF) β calculated with the

LANL2DZ/6-31G* basis set amounts to 218 a.u. (315 a.u.), which corresponds to β/NZn = 9 (13) or β/NL = 4 (5). A depolarization ratio of 1.50 confirms the octupolar structure of the NLOphore. These calculations demonstrate the nonnegligible β and χ(2) response of ZIF-8. By treating the ZIF-8 framework structure as being built up from its repetitive secondary building blocks (SBUs), welldefined relationships between the hyperpolarizabilty β of the SBUs and the macroscopic NLO coefficient of the perfect framework structure can be found (see the SI).32 The calculated average NLO coefficient (⟨deff⟩) for ZIF-8 is 0.29 (0.42) pm/V. While these values should be treated as an estimate, they agree very well with the experimental values that we have found for ZIF-8 crystals resulting from the O-synthesis. Structural SHG Imaging. To account for the variation in deff between crystals of different syntheses, we used SHG microscopy to study their local structure.33 The micrometersized crystals from the O-synthesis and the MW-synthesis are ideally suited for this type of optical microscopy. We can map variation in local symmetry (point group, lattice orientation), because these will give rise to a different dependency of the SHG on the polarization of the incident light.34 Single crystals will show a uniform polarization dependency, in contrast to irregularities such as polycrystalline domains, grain boundaries, amorphous regions, crystal defects, twinned regions, and so on. We mapped the variation in SHG intensity while rotating the linear polarization of the incident laser light over 360°, while only the SHG light polarized linearly along a single orientation is detected (see the SI for theoretical treatment).34 In Figure 3, the result is shown for images of both syntheses (optical, multiphoton, and SHG images can be found in the SI). We call these structural SHG images. If the angle (color) is continuous over a crystal, then the symmetry and orientation of the 3206


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intrinsically linked to the occurrence of crystal defects. This would explain the distinct difference in second-order NLO efficiency between different synthesis procedures: fast nucleation kinetics, as in the RT-synthesis, lead to more defective crystals and, hence, a lower second-order coefficient. The slowest synthesisnamely, the O synthesisleads to the most perfect crystals with optimal second-order NLO efficiency.

crystallographic axes also are continuous. First, we observe that this is generally the case for both the O-synthesis and the MWsynthesis. Second, since SHG is generated over the entire set of crystals, there are no obvious amorphous regions (these would not show SHG), at least on the scale resolvable by this optical technique (∼1 μm). There are differences in absolute SHG intensity within individual crystals: linear optical effects such as reflection/refraction at different crystal planes, different optical path length (phase matching/scattering) as well as local amorphization (beneath the optical resolution) can all contribute to this end. From these observations, we deduce that the variance in deff between the O-synthesis and the MWsynthesis is not explained by polycrystallinity or large-scale amorphization, and the orientation of the crystallographic axes is maintained over individual crystals. An explanation may be found in the crystal structure of ZIF8. The SBU of ZIF-8 is a (truncated) octahedron, where each Zn atom is coordinated by four 2-methylimidazolate linkers. The orientation of these linkers ultimately defines the point and space group symmetry of the crystal (see Figure 4). In the case

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CONCLUSION ZIF-8 is a hybrid organic−inorganic second-order nonlinear optical (NLO) material that is transparent, stable, and built up from economical constitutents (Zn and 2-methylimidazolate). A ⟨deff⟩ value on the order of commercial inorganic NLO crystals is reached (namely, 0.25 pm/V). The NLO response can be understood, in terms of the noncentrosymmetric crystal structure (I4̅3m, octupolar) and the hyperpolarizabilty of the unit cell. Moreover, the NLO response is dependent on local crystal ordering. We attribute the lower efficiency found for fast synthesis procedures to crystal defects with a centrosymmetric organization of the organic linkers, while syntheses with slow nucleation lead to quasi-perfect crystals with a high NLO activity. Its stability, ease of processing, and biocompatibility show the potential of ZIF-8 in emerging applications such as optical computing and frequency doubling crystals for deep tissue imaging.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b01087. Determination of the second-order NLO coefficient, thermal and photostability; point group determination by SHG microscopy; theoretical calculation of deff; structural SHG imaging (PDF) XRD measurements (CIF)

Figure 4. Reorientation of imidazolate linker molecules in ZIF-8 introduces a center of inversion, making the material SHG inactive.

of Td symmetry, they are oriented without an inversion center. However, if these linkers take a different orientation, it can lead to the appearance of a local inversion center and alter the symmetry to centrosymmetric Oh. This could happen, e.g., by creating an inversion center within the truncated octahedron, or when Td octahedra are oriented such that an inversion center occurs between them. We cannot distinguish between these two situations, but either will locally quench the SHG activity. The long-range orientation of the crystal lattice axes would not be affected, as the position of the Zn centers remains identical. This would explain the difference in deff between the Osynthesis and MW-synthesis: more local inversion centers of Oh symmetry occur for the MW-synthesis (and the RT-synthesis), with respect to the O-synthesis, while the orientation of the octahedrons is preserved almost over the entire set of crystals. It has been shown that certain sorbates induce a gate-opening mechanism in ZIF-8 by forcing a reorientation of the linker units.35−38 However, this gate opening does not involve full rotation of the linker units and the space group symmetry is not affected. Note that if full rotation would be thermodynamically possible, the orientation of the linker units would randomize, resulting in an averaged centrosymmetric (Oh) structure. Also note that ZIF-8 remains SHG-active upon heating until decomposition occurs at ∼450 °C (under nitrogen), showing that the energy barrier for linker rotation is indeed high. This is further supported by the high rotation barrier of the linker in C28H41N14Zn23+ (22 eV), as calculated with the B3LYP exchange-correlation functional and the LANL2DZ/6-31+G* basis set. However, we assume that local randomization (i) can occur during the crystallization process of ZIF-8 and (ii) is

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S. Van Cleuvenbergen). *E-mail: [email protected] (M. A. van der Veen). Present Address #

Guangxi Normal University, Department of Chemistry and Pharmaceutical Sciences, Key Laboratory for the Chemistry and Molecular Engineering of Medicinal Resources, 15 Yucai Road, Guilin 541004, PRC. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge J. Jacobs and L. Van Meervelt of the Biochemistry Department of KU Leuven for their help with the single-crystal XRD measurements. T.V., C.K., and M.v.d.V. acknowledge the Scientific Research Fund−Flanders FWO for a research grant (Research Project No. G.0927.13), while S.V. acknowledges the FWO for a postdoctoral fellowship. M.v.d.V., 3207


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Chemistry of Materials T.V., and D.D.V. are grateful for the financial support from the Hercules Foundation. I.S. thanks FWO for a Ph.D. fellowship. K.M. and D.D.V. are grateful to Belspo for funding in the IAP project Functional Supramolecular Systems. Y.Z. acknowledges financial support of the Fonds Spéciaux de Recherche of the Académie Universitaire Louvain (AUL), cofunded by the Marie Curie Actions of the European Commission (No. ADi/DB/ 986.2011) for his postdoctoral grant. C.E.A.K. acknowledges the Flemish government for long-term structural funding (Methusalem). This work has also been supported by funds from the Belgian Government (IUAP No. P7/5 “Functional Supramolecular Systems”) and the Francqui Foundation. The calculations were performed on the computers of the Consortium des Équipements de Calcul Intensif, including those of the Technological Platform of High-Performance Computing, for which we gratefully acknowledge the financial support of the FNRS-FRFC (Convention Nos. 2.4.617.07.F and 2.5020.11) and of the University of Namur.

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ABBREVIATIONS NLO nonlinear optics; SHG second harmonic generation; TPF two-photon fluorescence; MOF metal−organic framework; ZIF zeolitic imidazolate framework; XRD X-ray diffraction

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REFERENCES

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ZIF-8 as Nonlinear Optical Material: Influence of Structure and

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