Defects as Color Centers: The Apparent Color of Metal–Organic

Research Article Cite This: ACS Appl. Mater. Interfaces 2017, 9, 37463-37467

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Defects as Color Centers: The Apparent Color of Metal−Organic Frameworks Containing Cu2+-Based Paddle-Wheel Units Kai Müller,† Karin Fink,‡ Ludger Schöttner,† Meike Koenig,† Lars Heinke,*,† and Christof Wöll† †

Institute of Functional Interfaces (IFG) and ‡Institute for Nanotechnology (INT), Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany S Supporting Information *

ABSTRACT: As in the case of other semiconducting materials, optical and electronic properties of metal−organic frameworks (MOFs) depend critically on defect densities and defect types. We demonstrate here that, in addition to the influence of imperfections on MOF chemical properties like guest binding energies and catalytic activity, the optical properties of these crystalline molecular solids also crucially depend on deviations from the perfect crystalline structure. By recording UV−vis absorption spectra for MOF thin films of particularly high quality, we demonstrate that low-defect samples of an important MOF, HKUST-1, are virtually colorless. Electronic structure calculations of the excited states by employing complete active space self-consistent field (CASSCF) calculations show that the d−d excitations in defects result in the typical green color of the MOF material synthesized by conventional methods. KEYWORDS: metal−organic frameworks, defects, color, porous coordination polymers, copper paddle wheel

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INTRODUCTION The architecting of materials and in particular the positioning of molecular subunits, such that the assemblies exhibit properties not already provided by the individual subunits, is among the most interesting challenges in today’s material science. In the past two decades a new class of compounds, metal−organic frameworks, or MOFs, has been introduced, which by following rational guidelines allows to create designer materials from molecular subunits.1 Adding appropriate functionalities to chromophores or organic semiconductors yields multitopic linkers, which are then connected to metal or metal/oxo clusters to yield crystalline MOFs with interesting optical and electronic properties. With the number of experimentally characterized MOFs approaching 100.0002 and the number of simulated MOFs going to the millions,3 this very fast growing class of materials may find numerous applications in electronics (fabrication of MOF-based OFETs),4 optoelectronics (realization of photon up-conversion),5 photovoltaics,6 and thermoelectrics.7 As for other compounds, defects play a crucial role for the properties of electronic and optical devices realized with such materials. To quote from a recent paper by K. Müllen:8 “Unravelling the relationships between defects and material properties is a daunting task, but increased analysis and knowledge could be beneficial in many applications.” In organic electronics, for example, the highest charge carrier mobilities are obtained for molecular crystals with low defect concentrations. Also, the emergence of indirect band gaps, a very favorable property for applications in photovoltaics, can only be observed in the presence of a high degree of crystalline order.6 © 2017 American Chemical Society

In case of MOFs, the importance of structural defects for indiffusion of molecular guests into these porous materials have been demonstrated9 and the often beneficial influence on chemical and catalytic properties have been discussed and exploited.10,11 The role of structural imperfections on optical and electronic properties of MOFs is probably equally important, but so far, the number of detailed investigations of this topic has been rather limited. Here, we provide a detailed analysis on how the optical properties of a prototype MOF, Cu2(BTC)3, or HKUST-1,12 depend on defect densities. The interpretation of the experimental results is supported by precise, ab initio electronic structure calculations. It is demonstrated that the greenish or turquoise color, typical for HKUST-1 powders, results from structural defects in this crystalline framework, which act as color centers.

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RESULTS AND DISCUSSIONS Since the benzene-1,3,5-tricarboxylic acid (BTC) units, the organic linkers from which Cu2(BTC)3 is made, are optically inert, the typical color of these compounds must result from optical absorption within the metal clusters used as connectors, in this case Cu2+ dimers. For the perfect HKUST-1 structure, these metal clusters exhibit a local D2h symmetry with only slight deviations from D4h symmetry. Since the d−d transitions are strongly localized to the copper atoms, the change of D4h to Received: August 12, 2017 Accepted: October 4, 2017 Published: October 4, 2017 37463

DOI: 10.1021/acsami.7b12045 ACS Appl. Mater. Interfaces 2017, 9, 37463−37467

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involving intermediate ultrasonic treatments,20 yields films, referred to as HQ-SURMOFs, which are virtually transparent (Figure 2). A thorough quantitative analysis of the MOF thin

D2h symmetry is not expected to affect the results in a significant way. This assumption is supported by calculations for a model system which reflects this distortion (see section Quantum Chemical Calculations). In this case, only one d−d transition is optically allowed and has a very low intensity (see Symmetry Discussion in Experimental Section). We demonstrate that the color in these systems, as shown in Figure 1, is due to color centers provided by reduction of Cu ions.

Figure 2. UV−vis absorption spectra of the high-quality (blue) and conventional low-quality (red) HKUST-1 MOF films. The dotted line is the spectrum of the quartz substrate. The inset shows the spectra in the visible range. The absorbance is shown in absorbance units (a.u.). The absorption spectra are normalized to a thickness of 100 nm. For comparison, the absorption spectrum of copper(II) acetate in acetic acid solution is shown in Figure S2.

film optical properties was carried out by recording UV−vis absorption spectra for the different samples. Note that obtaining such high-quality UV−vis data in a reproducible fashion for MOF powders is rather difficult due to diffraction effects caused by the powder particles. Inspection of the absorption data shown in Figure 2 clearly reveals that the absorption in the visible range, responsible for the color, is substantially reduced for the high-quality thin films grown with the ultrasonication method. The absorbance of the HQ SURMOF at 700 nm corresponds to a molar extinction coefficient of the copper dimers of approximately 1000 L mol−1 cm−1. The absorbance of the low-quality SURMOF is significantly larger over the entire visible range. To aid the assignment of the features in the optical absorption spectra, high-quality ab initio electronic structure calculation were carried out for perfect (modeled with full D4h symmetry) and defective (PW units containing a Cu+ ion) PW units, see Figure 3. As expected, the results of the calculations reveal that optical absorption in the visible range is only possible by d−d excitations at the Cu2+ pairs in the PW units.21 Inspection of Figure 3 shows, however, that the excitation probabilities of these electronic transitions are very small. Please note that, in Figure 3, the intensity of the absorption for the perfect D4h case has been enlarged by a factor of 100. To study the importance of structural imperfections, the optical activity of a typical defect in HKUST-1, which was already described in previous work,22 has been calculated. Although numerous further defects, like additional (linker) molecules coordinated to the metal nodes or copper monomers may occur, we focus on defects which were found to be prevailing in HKUST-1.22 These defects are obtained by reducing one Cu2+ ion to Cu+, resulting in the loss of one of the four carboxylate ligands in the PW units (see models in the Quantum Chemical Calculations section). Here, transitions with tremendously higher intensity are present. For such a distorted geometry, the symmetry is reduced, and additional transitions are optically allowed, resulting in a huge increase of the optical absorption by more than 2 orders of magnitude. So far, the geometry was fixed when the linker molecule was removed. In this case, the

Figure 1. (a) Synthesis of high-quality MOF thin films by using a dipping robot equipped with an ultrasonic bath. The functionalized substrate surface is alternatively immersed in the metal node solution, pure ethanol, organic-linker solution and pure ethanol. Ultrasonication is applied in the ethanol bath. (b) Photographs of solvothermally synthesized HKUST-1 powder (left), HKUST-1 SURMOF (center), and high-quality (HQ) HKUST-1 SURMOF (right). While HKUST-1 powder and low-quality SURMOFs have the typical strong turquoise color, HQ-SURMOFs are almost transparent and only slightly turquoise. (c) Sketch of the defect-free HKUST-1 structure. C is plotted in gray, O red, and Cu orange. Hydrogen is not shown.

HKUST-1 is a MOF consisting of Cu2+ dimers connected to the carboxylate groups of BTC to yield the structure shown in Figure 1c. The paddle wheel (PW) units are a characteristic structure element of this and many other Cu2+- and Zn2+-based MOFs.13 After synthesis, HKUST-1 powders exhibit a greenish color, which is converted to blue when the material is activated by heating in an inert atmosphere.12,14 Because of the defect density in MOF powder materials, the standard form of these compounds obtained by solvothermal synthesis is difficult to control (e.g., surface defects may play an important role);9 the studies reported here were carried out for highly oriented, thin films of HKUST-1 grown on transparent substrates using a layer-by-layer (lbl) method (Figure 1). The resulting films exhibit a high degree of crystalline order, as evidenced by the Xray diffraction (XRD) patterns recorded in out-of-plane geometries (see Figure S1). Raman spectra obtained for these films exhibit the characteristic splitting of the symmetric carboxylate stretch vibration, a direct consequence of the dinuclear PW.15 This splitting is absent when the D2h-symmetry of the PW units is distorted.16 Whereas HKUST-1 MOF thin films grown with the lbl method, also referred to as SURMOFs,17−19 show the greenishturquoise color known from the powders, high-quality films fabricated using a recently introduced variant of the lbl-method 37464

DOI: 10.1021/acsami.7b12045 ACS Appl. Mater. Interfaces 2017, 9, 37463−37467

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Moreover, pristine transparent HQ-SURMOF can be destroyed for instance by exposure to elevated temperatures23 or by exposure to water vapor resulting in defect SURMOFs with higher absorption (Figure S3).

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CONCLUSION We conclude that defects in MOFs clearly are not only relevant for their chemical properties such as binding energies of guest species and catalytic activities but also strongly influence optical and electronic properties. In HKUST-1, optical excitations at the metal nodes are very weak by optical selection rules. Electronic structure calculations reveal that only when the symmetry is reduced, intense optical excitations are present, causing this material to display its typical green color. We are presently optimizing growth conditions for SURMOFs to also be able to produce HQ-SURMOFs for other type of MOFs in order to unravel the influence of such defects on charge carrier mobility and conductivity24 in these molecular frameworks.

Figure 3. d−d excitations obtained at CASSCF-level for different model systems. The line spectra were broadened with a width of 0.1 eV. Excitations of the defect-free HKUST-1 (Cu2+ C4v and Cu2+/Cu2+ D4h, black and gray) are very small and are magnified by a factor 100. Defect PW units (Cu2+/Cu+ D4h), Cu2+/Cu+ symmetric (structure fixed, just one linker molecule removed), and Cu2+/Cu+ distorted, blue, red, and green, respectively) have absorption bands with significantly increased intensities. Note that in the distorted geometry a peak with low intensity (similar to the intensity of the defect-free HKUST-1 peaks) is also present at 1.5 eV.

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EXPERIMENTAL SECTION

Sample Synthesis. The HKUST-1 MOF films were prepared using a lbl approach employing liquid-phase epitaxy (LPE) on appropriately functionalized solid substrates. Prior to the SURMOF deposition, the quartz glass substrates were functionalized by an oxygen plasma treatment for 20 min, resulting in a high density of functional OH groups on the surface. The lbl MOF growth process consists of alternately immersing the substrate in ethanolic 1 mM copper(II) acetate (Alfa Aesar 99.9%) solution for 15 min and in the ethanolic 0.2 mM BTC solution for 30 min (BTC = benzene-1,3,5tricarboxylic acid; also referred to as trimesic acid, Alfa Aesar 98%). In between the immersion in the solutions of the MOF components, the sample was thoroughly rinsed with ethanol. While the regular SURMOFs was simply immersed in pure ethanol for 2 min, during the fabrication of the high-quality (HQ) SURMOFs, the sample was ultrasonicated (40 kHz ultrasonic bath from Weber Ultrasonic GmbH) in ethanol for 2 min. Regular and high-quality HKUST-1 SURMOFs were prepared, respectively, in 50 and 86 synthesis cycles. All SURMOFs described here were synthesized at room temperature using a dipping robot.20 The thickness of the MOF thin films was determined by means of ellipsometry; see Figure S4. The optical data shown in Figure 2 were obtained for a regular and HQ-SURMOF with 104 and 82 nm, respectively. X-ray diffraction of the samples verify the crystalline HKUST-1 MOF structures of the samples; see Figure S1. The UV−vis transmission spectra were recorded by means of a Cary5000 spectrometer from Agilent. X-ray photoelectron spectroscopy (XPS) experiments were carried out in an ultrahigh-vacuum (UHV) apparatus under a base pressure of 10−10 mbar in order to characterize the behavior of copper ions in the HKUST-1 structure. The binding energies were calibrated on the O 1s energy level of carboxylates at 532.1 eV.25 Quantum Chemical Calculations. Quantum chemical calculations have been performed for the PW model complexes Cu2tetrabenzoate and Cu2-tribenzoate (see Figure 5). Similar model clusters have been introduced in ref 21. For the system with four of the linker molecules, D4h symmetry was enforced for the geometric structure. An analogous system with one linker molecule less was investigated. We compared for both systems the low-lying electronic states of two Cu2+ centers as well as those for a Cu2+/Cu+ system. The latter can be expected for the system with three linker molecules if removal of a neutral linker is assumed. All structure optimizations have been performed with the program package Turbomole26 using density functional theory (DFT) with the B3LYP functional. The CASSCF calculations27 were performed with the Bochum program package. In all CASSCF calculations the active space is restricted to the 3d orbitals of the Cu centers. These excitations are responsible for transitions in the low energy range,

ab initio calculations provide a symmetric ground state as linear combination of Cu2+/Cu+ and Cu+/Cu2+. However, core hole spectroscopy indicates a localized ground state. The latter was enforced in the calculations by changing the Cu−O distances at one center and relaxing the rest of the structure. Though only one specific type of defect has been considered, the calculations lead to the following conclusions: Even in highly ordered HKUST-1, a very weak d−d excitation is present. As soon as there are defects involving Cu+ in the material, additional transitions with tremendously higher intensity are allowed and dominate the spectra. (See the Quantum Chemical Calculations and Symmetry Discussion in the Experimental Section for a detailed discussion of the symmetry.) Obviously, also for the HQ-SURMOFs, the experimental data show a residual absorption in the regime of 1.5−2.5 eV, which can be attributed either to the allowed d−d transition or to a remaining very small concentration of defects. We attribute them mainly to a small residual density of Cu+ ions. This hypothesis is collaborated by X-ray photoelectron spectroscopy (XPS) data; see Figure 4.

Figure 4. X-ray photoelectron spectra of a pristine HQ-SURMOF (black) and of the sample after exposure to water vapor for 15 min. In the pristine sample, the Cu+ content is estimated to be smaller than 3%. The Cu+ ratio increases to about 9% after the exposure to water. 37465

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Figure 5. Model systems for HKUST-1 and possible defects. D4h symmetric paddle wheel unit (left), one linker molecule removed, and unchanged structure (center). The three Cu−O bonds of one Cu center are significantly shortened (right) by fixing the O atoms at (0 1.81 1.12) and (±1.810 0 1.12). Cu silver, C blue, H white, and O red. while ligand-to-metal charge transfer states as well as ligand excitations are located at higher energies.21 In all calculations, a def2-TZVPP basis set28 was applied. Though dynamical correlation is not included at CASSCF level, typical errors compared to higher level methods seem to be in the order of only 0.2 eV.29−31 In our previous work on the optical properties of HKUST-1, we observed a Cu d−d transition absorption peak in the range of 2 eV by TD-DFT level. This peak had only a very low intensity in CASSCF calculations. Here, we extend our CASSCF calculations on systems with one missing linker molecule. The remaining charge can be either distributed symmetrically over both Cu centers or localized on one of the Cu centers. These two possibilities make a significant difference for the electronic states. In the first case, the ground state of the PW unit is a linear combination of the unpaired electron located at either the first or the second Cu atom.

φ1/2 =

In the crystal structure, the angles between the linker molecules deviate slightly from 90°. In calculations on a model system considering this distortion but only possessing one linker molecule, (Model 2 in ref.21) the two components of the 1Eu state were split by 5 meV only and one of the d−d transitions forbidden in D4h got an intensity which is a factor of 100 smaller than the intensity of the 1Ag → 1Eu transition. The excitation energies of both models are identical (1.6 eV) and agree well to calculations on Cu acetate monohydrate.32 For the Defect PW Units. If a Cu2+/Cu+ system in D4h symmetry is considered (blue line), then one peak with high intensity shows up at very low energies. This is the allowed excitation between the wave functions φ1 and φ2. The second allowed transition is again the local excitation from dxz, dyz to dx2−y2 at 1.8 eV with a very weak intensity. Taking away one of the linker molecules reduces the symmetry and additional transitions occur (red line). In the case that the symmetry is further reduced (green line) the number of peaks is further increased. Taking into account that various types of defects appear in most HKUST-1 MOF materials, broad and strong absorption in the visible range has to be expected.

1 (|Cu12 +Cu+2 | ± |Cu1+Cu 22 +|) 2

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These states are rather close in energy, and a transition between them is allowed and rather intense. In addition, there is the possibility to excite one of the Cu centers at the same time. A second possibility is that one Cu center is reduced and that the orbitals as well as the local structure can relax to this state.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b12045. X-ray diffraction, ellipsometry and UV−vis spectroscopy data (PDF)

φ = |Cu12 +Cu 2+| Possible excited states are either local d−d excitations at one center or metal-to-metal charge-transfer (MMCT) states. Energetically, the latter are expected to be higher in energy. The latter situation is simulated by a manual distortion of the symmetry. The positions of the O atoms coordinating one of the Cu centers are fixed at positions which correspond to a shorter Cu−O distance (1.8 Å) in case Cu remains in its original position. Then the positions of all other atoms are relaxed (right-hand side of Figure 5). Here, the hole in the Cu 3d shell should be localized at the Cu with the short distances. The aim of this calculation was not to obtain exact peak positions but to identify how the asymmetric coordination influences the spectra in form of additional peaks. Symmetry Discussion. Pristine, Defect-Free PW Unit. In Figure 3, the d−d excitations are systematically investigated. In the first calculation (gray line in Figure 3), only local d−d transitions at one Cu2+ center have been considered for a D4h symmetric structure. The isolated Cu2+ center has a local C4v symmetry, the five d orbitals correspond to the irreproducible representations a1, b1, b2, and e. In the ground state, the dx2−y2 orbital is singly occupied (b1). Therefore, only excitations from the e-orbitals dxz and dyz are allowed. Thus, only one excitation is optically allowed in this highly symmetric system. However, the intensity of such local d−d excitations remains small. In the D4h-symmetric Cu2+/Cu2+ system, the antiferromagnetic coupled singlet ground state belongs to A1g symmetry. The only allowed d−d transition leads to an electronic state of 1Eu-symmetry and is again an excitation from the dxz and dyz orbitals (black line in Figure 3).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Christof Wöll: 0000-0003-1078-3304 Notes

The authors declare no competing financial interest.

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REFERENCES

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