Tuning the Electric Field Response of MOFs by Rotatable Dipolar Linkers
- Johannes P. DürholtJohannes P. DürholtComputational Materials Chemistry Group, Fakultät für Chemie und Biochemie, Ruhr-Universität Bochum, Bochum 44801, GermanyMore by Johannes P. Dürholt
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- Babak Farhadi JahromiBabak Farhadi JahromiComputational Materials Chemistry Group, Fakultät für Chemie und Biochemie, Ruhr-Universität Bochum, Bochum 44801, GermanyMore by Babak Farhadi Jahromi
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- Rochus Schmid*Rochus Schmid*E-mail: [email protected]Computational Materials Chemistry Group, Fakultät für Chemie und Biochemie, Ruhr-Universität Bochum, Bochum 44801, GermanyMore by Rochus Schmid
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Abstract
Recently the possibility of using electric fields as a further stimulus to trigger structural changes in metal–organic frameworks (MOFs) has been investigated. In general, rotatable groups or other types of mechanical motion can be driven by electric fields. In this study we demonstrate how the electric response of MOFs can be tuned by adding rotatable dipolar linkers, generating a material that exhibits paraelectric behavior in two dimensions and dielectric behavior in one dimension. The suitability of four different methods to compute the relative permittivity κ by means of molecular dynamics simulations was validated. The dependency of the permittivity on temperature T and dipole strength μ was determined. It was found that the herein investigated systems exhibit a high degree of tunability and substantially larger dielectric constants as expected for MOFs in general. The temperature dependency of κ obeys the Curie–Weiss law. In addition, the influence of dipolar linkers on the electric field induced breathing behavior was investigated. With increasing dipole moment, lower field strengths are required to trigger the contraction. These investigations set the stage for an application of such systems as dielectric sensors, order–disorder ferroelectrics, or any scenario where movable dipolar fragments respond to external electric fields.
Synopsis
Rotatable dipolar groups, used as organic linkers in metal−organic frameworks (MOFs), can substantially alter their electric field response, in terms of both their dielectric and flexible properties.
Introduction
ARTICLE SECTIONS
Metal–organic frameworks (MOFs) are an emerging class of crystalline, hybrid, and porous materials, which are built from oligotopic molecules (linkers) and metal ion nodes. (1−3) Potential applications for MOFs are gas separation and storage, water purification, chemical sensing, catalysis, drug delivery, and imaging. (4−8) In particular, so-called stimuli-responsive MOFs, which undergo physical or chemical “changes of large amplitude in response to external stimulation”, (9) have gained much attention for the design of new functional materials. (10,11) Well-studied external stimuli are temperature, mechanical pressure, guest adsorption or evacuation, and light absorption.
Recently, the possibility to use external electric fields E as a further stimulus to trigger structural changes in MOFs has been reported. By molecular dynamics simulations, the well-known breathing behavior of MIL-53(Cr) could be reproduced, however, only by applying a very high external electric field. (12) One of us proposed a potential mechanism for this transformation, based on field-induced dipole–dipole interactions in a wine-rack-type lattice (see Figure 6). (13) Recently, Kolesnikov and co-workers confirmed this mechanism for a 2D lattice model by statistical mechanics calculations. (14) Furthermore, Knebel et al. demonstrated that an electric field can be used to enhance the molecular sieving capability of a ZIF-8-based MOF-polymer membrane by initiating reversible phase transitions. (15)
On the other hand, in addition to guest adsorption, the regularity and porosity of MOFs make them an ideal platform to realize molecular machines. The void space allows for the motion of either the MOF building blocks, such as rotating linkers, (16) or molecular guests inside the pores. For example, Loeb and co-workers incorporated a rotaxane moiety in the linker of an MOF. (17) Very recently, the unidirectional rotation of a light-driven rotor within an MOF was demonstrated. (18) Again, external electric fields are a facile way to trigger motion of movable components in MOFs. For example, Yazaydin and co-workers investigated two interesting systems in silico, where the transport of guests is modulated by the help of dipolar groups ordering in an external electric field. In the first case, a large dipolar molecule was mounted on the open metal coordination sites in Mg-MOF-74, which acts as a molecular gate, controlling the flow of guest molecules along the hexagonal channels. (19) In the second example, the guest diffusion in an IRMOF variant with a dipolar group at the linker was modulated by applying an external field. (20) Due to the ordering of linkers in the field, a nonisotropic self-diffusion was observed.
To gain a deeper understanding of any kind of field induced motion or structural transformation in a system, its dielectric permittivity κ needs to be known. However, due to the general interest in electrically insulating MOFs as potential low-κ interlayer dielectrics for semiconducting devices, past theoretical investigations were mainly focusing on the electronic polarization of rigid MOFs in the linear response regime. Note that a somewhat related current development is, on the other hand, electrically conducting MOFs, which respond by a current on an electric field. (21) Nevertheless, in the context of MOFs as low-κ materials at first Zagorodniy et al. studied the dielectric response of cubic Zn-based frameworks, employing the semiempirical Clausius–Mossotti model. (22) Later, Warmbier et al. employed density functional perturbation theory to calculate κ for similar rigid cubic frameworks. (23) Inspired by these theoretical studies, the dielectric and optical properties of HKUST-1 (24) and ZIF-8 (25) were investigated experimentally, confirming that MOFs are indeed promising candidates for low-κ dielectrics. (26) Recent theoretical investigations by Ryder and co-workers, again based on static density functional theory (DFT) calculations, established structure–property relations for the dielectric response of MOFs. (27,28) They found only a minor dependency of κ on the chemical composition of the MOF but an almost linear dependency on the porosity.
The dielectric properties of MOFs with incorporated dipolar rotors have, on the other hand, not been investigated up to now. In contrast to the systems studied by Ryder et al., a strong dependency of κ on the chemical composition, and in particular on the dipole strength of the rotor, is expected, since the highly temperature-dependent orientation polarization should dominate over the electronic polarization in these systems. As a consequence, molecular dynamics (MD) simulations at finite temperature are needed for the determination of the static dielectric permittivity κ. In this work we used pillared layer MOFs as reference systems to establish the methodology, and to investigate the influence of increasing dipole strength and temperature on the dielectric constant for a given framework structure. In our reference systems, square lattices of 1,4-benzenedicarboxylate (bdc) linked zinc paddle-wheels are pillared by bispyridine linkers as shown in Figure 1. The parent system (A) with a central phenyl exhibits no dipolar group. In B 1,2-difluorophenyl and in C phtalazine replace the phenyl in the backbone with an increasing dipole strength. This setup allows for an easy manipulation of the dielectric response of the material, by keeping its other properties unaffected. Furthermore, this system is realistic and could even be deposited as a thin film on a device by layer-by-layer synthesis. (29)
Figure 1
Figure 1. Investigated systems with zinc paddle-wheels as inorganic building blocks, which are connected in the pcu topology by phenyl moieties (blue) in the x- and y-direction and pillared in the z-direction by bispyridine units (red). Between the two pyridine units, different organic moieties (A–C) can be mounted to vary the dipole moment of the pillar linker.
For the determination of κ we followed the work by Zhang and Sprik who proposed four different methods to calculate κ from classical MD simulations in the case of liquid water (namely, from polarization fluctuations or as a finite field derivative with either fixed electric field E or fixed displacement charge field D). (30) In contrast to the molecular liquid water, the herein investigated systems are porous crystalline polymers and show a nonisotropic dielectric response, since the dipolar linkers can only rotate around the z-axis. Furthermore, the dipole density is much lower and thus also the possible polarization. To determine the numerically most efficient way of predicting κ for such systems, the four proposed methods were applied to systems B and C, and its performance was validated. In addition, we verified the hypothesis that dipolar groups facilitate the electric field induced breathing (13) which was observed originally for MIL-53.
Methods
ARTICLE SECTIONS
Dielectric Constants from MD Simulations
When a material is exposed to an electric field, it becomes polarized. The induced polarization P is related to the electric field E by the electric susceptibility χ
(1)
The relative permittivity of a material (i.e., its dielectric constant) κ is then given by κ = 1 + χ. In general, this relative permittivity is not a constant but depends on the frequency of the applied field, the temperature T, and other parameters. The induced polarization can be divided into three different contributions: electronic Pel, displacive (ionic) Pion, and orientation polarization Porient. Consequently, the relative permittivity can be written as κ = 1 + χel + χion + χorient. Pel corresponds to the response of the electron density to the electric field. At the high-frequency limit this contribution is crucial, since the nuclei move too slowly to adapt. The ionic polarization is due to the adaptation of the nuclei positions with respect to the electric field. Orientation polarization arises whenever molecules or molecular groups carry a permanent dipole moment μ, which becomes partially aligned to the electric field. For polar liquids this third contribution becomes dominating.
In their investigation on liquid water, Zhang and Sprik proposed four different methods to calculate relative permittivities from classical molecular dynamics simulations, based on two different Hamiltonians. (30) In the constant-E method, the electric enthalpy 
of a system of volume Ω is written as
where HPBC is a classical Hamiltonian with v denoting the set of momenta and positions of the atoms, E is the fixed electric field, and P(v) is the macroscopic polarization for the microscopic state specified by v. Note that the SI unit system is used in this work throughout (conversion formulas to the CGS unit system are given in the SI). Thus, the field-dependent force on atom i is qiE, where qi is the charge assigned to the atom in the FF model.
of a system of volume Ω is written as(2)
In the constant-D method, the electric internal energy 
functional is written as
where D denotes the fixed macroscopic electric displacement field and ϵ0 the vacuum permittivity. (31) From this follows that the field-dependent force on atom i is 
. In contrast to MD simulations using the fixed-E approach, here, the instantaneous polarization P has to be known at every time step, because the force depends explicitly on its value. Since P is a multivalued property and depends on the choice of the unit cell, (32) it has to be assured that all atoms are kept in the same reference frame during the simulation. Furthermore, because MOFs are crystalline periodic materials, it is practically impossible to define a unit cell with a vanishing dipole moment and thus a polarization of zero. Therefore, prerequisite to a simulation with a fixed-D Hamiltonian, a corresponding simulation with E = 0 has to be performed to sample the polarization at zero field for the unit cell of choice, which is then used as reference polarization in eq 3. In general, as discussed already by Stengel et al., the constant-E Hamiltonian corresponds macroscopically to a capacitor in short circuit, since polarization fluctuations have to be compensated by fluctuating charges on the capacitor plates. In contrast, the constant-D Hamiltonian is related to a capacitor in open-circuit conditions with a fixed value of free charge Q on the plates, with the polarization fluctuations leading to a fluctuating field E. (30,31)
functional is written as(3)
. In contrast to MD simulations using the fixed-E approach, here, the instantaneous polarization P has to be known at every time step, because the force depends explicitly on its value. Since P is a multivalued property and depends on the choice of the unit cell, (32) it has to be assured that all atoms are kept in the same reference frame during the simulation. Furthermore, because MOFs are crystalline periodic materials, it is practically impossible to define a unit cell with a vanishing dipole moment and thus a polarization of zero. Therefore, prerequisite to a simulation with a fixed-D Hamiltonian, a corresponding simulation with E = 0 has to be performed to sample the polarization at zero field for the unit cell of choice, which is then used as reference polarization in eq 3. In general, as discussed already by Stengel et al., the constant-E Hamiltonian corresponds macroscopically to a capacitor in short circuit, since polarization fluctuations have to be compensated by fluctuating charges on the capacitor plates. In contrast, the constant-D Hamiltonian is related to a capacitor in open-circuit conditions with a fixed value of free charge Q on the plates, with the polarization fluctuations leading to a fluctuating field E. (30,31)Relative permittivities κ can now be derived from polarization fluctuations at either E = 0 (using the fixed-E Hamiltonian from eq 2) or D = 0 (using the fixed-D Hamiltonian from eq 3). With E = 0, κ can be derived from the fluctuations of the polarization P by
where kB is Boltzmann’s constant, and T is the temperature. Note that due to the anisotropy of the investigated systems we do not average over spatial directions. The brackets ⟨·⟩ denote here the usual ensemble averages. For D = 0, κ is calculated from
(4)
(5)
Furthermore, relative permittivities can also be estimated from simulations for a finite field by extrapolating then to the limit of zero field. For an electric field E, κ can be computed by using the relation
with ⟨Px⟩ the expectation value of the polarization obtained from an MD run using the constant Ex = E Hamiltonian. The finite D derived value for κ follows from the relation
where the expectation value of the polarization is determined from an average over a trajectory generated by the constant Dx = D Hamiltonian. Note that, for both approaches, ⟨Px⟩ is obtained in relation to the average polarization from a trajectory at E = 0, with the same choice for the unit cell.
(6)
(7)
Computational Details
All molecular dynamics simulations were performed using the MOF-FF force-field (33,34) (parameter sets are provided in the SI), employing our in-house developed PYDLPOLY code. (33) The Ewald summation method was used for calculating the electrostatic interactions, and the short-range interactions were truncated smoothly at 12 Å. The unit cells for the materials were chosen as shown in Figure 6 by a rotation of 45° around the crystallographic c-axis of the conventional cell, which results in a unit cell 2 times larger than the conventional one. Simulations in the (N, V, T) ensemble for the calculations of the dielectric constants were performed in a 2 × 2 × 2 supercell of the rotated unit cell at temperatures of 150, 298, and 450 K. For every system and applied field strength the material was first equilibrated for 100 ps using a Berendsen thermostat (35) with a time constant of 200 fs and then further simulated for at least 10 ns using a Nosé–Hoover thermostat (36) with a time constant of 1 ps. For all simulations a time step of 1 fs was used. The polarization was monitored every 10th time step. Statistical errors of the polarization P were calculated by performing a reblocking analysis. (37)
For the investigation of the electric field induced breathing behavior, we always employed a larger 6 × 2 × 2 supercell of the rotated unit cell at 298 K and a pressure of 1 bar. At every field strength the system was first equilibrated for 50 ps in the (N, V, T) ensemble using a Berendsen thermostat (35) with a time constant of 200 fs. Then, it was equilibrated in the (N, σ, T) ensemble for 2 ns by a Berendsen thermostat (time constant of 200 fs) and barostat (time constant of 1 ps) followed by a sampling run of 5 ns using a Nosé–Hoover thermostat (time constant of 1 ps) and barostat (time constant of 1 ps).
Results and Discussion
ARTICLE SECTIONS
Dielectric Constants
The polarization response Px for a finite electric field Ex, applied in the x-direction, is shown in Figure 2 for all three MOF systems (Figure 1) at temperatures T of 150, 298, and 450 K. As expected, system A behaves substantially different from systems B and C. Whereas A shows the typical behavior of a dielectric material, i.e., the polarization P increases as a linear function of the electric field E, systems B and C show a nonlinear response, typical for paraelectric materials. The difference is due to the fact that systems B and C carry rotatable dipolar groups. Thus, the first, relatively steep increase of the polarization P with increasing electric field E corresponds to orientation polarization from an alignment of the dipolar groups. The second, flatter part of the curve corresponds to an increasing ionic polarization, as also observed for system A. With increasing dipole moment of the linker groups, the polarization increases substantially for a given field. In addition, in contrast to A, the polarization for B and C becomes highly temperature-dependent, since the field alignment is hampered by the entropy of the dipolar rotors. It should be noted that, due to the anisotropy of the systems, the same behavior is observed for a field applied in the y-direction, whereas for a field applied in the z-direction, only a dielectric response (as for system A) is observed also for B and C, since the dipolar rotors can only rotate about the z-axis. Note that this rotation is almost barrier-free. During the force-field validation we determined a rotational barrier around the Zn–N bond of only 0.15 kcal mol–1 by DFT calculations of nonperiodic model systems.
Figure 2
Figure 2. Polarization Px as a function of the applied electric field Ex for all investigated systems at temperatures of 150 K (blue), 298 K (orange), and 450 K (red).
Due to the use of a nonpolarizable force-field, only ionic and orientation polarization are taken into account when the relative permittivities κ are predicted in the following. However, the additional electronic contribution can be estimated by the structure–property relations developed recently by Ryder et al. (28) They found an almost linear dependency between porosity and relative permittivity for a variety of MOFs. Using Zeo++, (38) a porosity of about 50% was calculated for all systems. This corresponds to a κ ≈ 1.8 or χ ≈ 0.8, which needs to be added to the here predicted permittivities for a comparison with experimental values. However, since we are mainly interested in relative values, we report in the following on the uncorrected numbers.
In the first step, dielectric constants were calculated on the basis of polarization fluctuations obtained from simulations at E = 0 (see eq 4) and D = 0 (see eq 5). Fluctuations were sampled for every system and temperature for 50 ns. The mean of the polarization obtained from the E = 0 simulation was used as reference polarization for the D = 0 case. The results are shown in Table 1. κ is calculated as average over κx and κy, since the system is isotropic in the x- and y-direction. This is demonstrated in Figure 3, where the convergence of the variance s2 = ⟨P2⟩ – ⟨P⟩2 over the time of the MD simulation is shown for system B for E = 0 and D = 0 at 450 K. Both approaches yield the same result in κ, which proves that our treatment of the multivalued polarization (32) by making use of the reference polarization in the case of constant D simulations is indeed correct. In accordance to the observations by Zhang and Sprik, the polarization fluctuations are lower for D = 0, and σ2 converges faster in comparison to E = 0 simulations (see Figure 3). Despite that, it has to be taken into account that, prerequisite to a constant-D simulation, a constant-E simulation needs to be performed to determine the reference polarization. Note that this is not necessary for a molecular dipolar fluid like water but inevitable for a periodic crystalline polymer like an MOF. As a consequence, the constant-D approach is computationally more demanding and also more sensitive to systematic errors due to potentially insufficient sampling of the reference polarization.
Figure 3
Figure 3. Accumulating variance s2 of the total polarization P in x- and y-directions for system B at 450 K for (a) E = 0 and (b) D = 0.
Table 1. Dielectric Constants κ Obtained from Polarization Fluctuationsa
| T = 150 K | T = 298 K | T = 450 K | ||||
|---|---|---|---|---|---|---|
| E = 0 | D = 0 | E = 0 | D = 0 | E = 0 | D = 0 | |
| system B | 2.01(7) | 2.1(1) | 1.52(1) | 1.53(1) | 1.357(5) | 1.362(6) |
| system C | 6.8(3) | 6.8(7) | 3.65(8) | 4.0(2) | 2.65(4) | 2.85(8) |
a
To give an impression for the sampling error, ±2σ is given as statistical error.
Next, we tested the suitability of the proposed approaches to calculate κ as finite field derivative from simulations with an applied E or D field for our case of an MOF with dipolar rotors. Constant-Ex simulations were performed at different field strengths ranging from 0.005 to 0.50 V Å–1, sampling the polarization each time for 10 ns. As a consistency check we selected nine values for Ex (see the first column of Table 2) for the case of system C at 150 K and calculated the polarization Px. The relationship D = ϵ0E + P was employed to obtain the corresponding D values. These values were then taken as the displacement field in constant-Dx simulations using the Hamiltonian of eq 3. The excellent correspondence in the resulting Px as obtained by the two different methods underlines again the suitability of our approach of treating the reference polarization in constant-D simulations. Note that the values for Dx are always larger compared to Px, due to the dielectric effect. After this consistency check, the D derivative estimates of the relative permittivity κ were computed by performing constant-Dx simulations for a field strength ranging from 0.5 × 104 to 20 × 104e Å–2.
Table 2. Simulation Conditions at Constant-Ex or Constant-Dx and the Corresponding Observed ⟨Px⟩ for System C at T = 150 Ka
| Ex (V Å–1) | ⟨Px⟩ (104 e Å–2) | Dx (104 e Å–2) | ⟨Px⟩ (104 e Å–2) |
|---|---|---|---|
| 0.01 | 3(1) | 3.15 | 2.6(6) |
| 0.02 | 3.5(4) | 4.60 | 3.6(4) |
| 0.03 | 3.8(2) | 5.47 | 3.9(1) |
| 0.04 | 4.0(1) | 6.22 | 4.04(9) |
| 0.05 | 4.2(1) | 6.92 | 4.18(8) |
| 0.10 | 4.62(6) | 10.15 | 4.63(5) |
| 0.19 | 5.21(3) | 15.71 | 5.21(3) |
| 0.28 | 5.70(3) | 21.18 | 5.70(2) |
| 0.50 | 6.78(1) | 34.41 | 6.78(1) |
a
To give an impression for the sampling error, ±2σ is given as statistical error.
Figure 4 shows the comparison of κ, obtained as a function of E and D for systems B and C at all investigated temperatures. The relative permittivity has to be obtained by an extrapolation to zero field. All in all, this yields results comparable to those from polarization fluctuations listed in Table 1. As already observed by Zhang and Sprik in the case of liquid water, this extrapolation is somewhat more difficult for the constant-E method due to the steeper slope of the curve at the intercept. However, as it can be seen from the error bars, the sampling error is increasing substantially for decreasing field strengths. This is especially pronounced in the case of the constant-D simulations due to the disadvantageous dependence of the statistical error on the polarization ⟨P⟩ (see eq S6), which makes the extrapolation equally error prone. Furthermore, for the constant-D approach it has to be taken into account that, in the case of periodic polymers like an MOF, the reference polarization has to be determined beforehand, which renders this method numerically less efficient. Overall, from our analysis we recommend computing κ from polarization fluctuations at E = 0 as long as the saturation for finite field is not of relevance.
Figure 4
Figure 4. Static relative permittivity κx obtained from fixed-Ex and fixed-Dx simulations for system B at (a) 150 K, (c) 298 K, and (e) 450 K and system C at (b) 150 K, (d) 298 K, and (f) 450 K. To give an impression for the sampling error, ±2σ is plotted as statistical error.
Further information on the coupling of the dipolar groups can be gained by analyzing the temperature dependence of systems B and C by a Curie–Weiss plot (Figure 5). Here, the inverse of the electric susceptibility χ = κ – 1 is plotted as a function of temperature. According to the Curie–Weiss law for paraelectric materials (or ferroelectrics beyond the Curie temperature) a linear relationship is expected with
where the Curie constant C is equal to the inverse of the slope. As expected, with C = 747 K a substantially larger Curie constant is observed for system C as compared to B with C = 164 K, since C should be proportional to the square of the dipole moment μ. Indeed, the ratio of the Curie constants (
) is roughly equal to the ratio of the squared dipole moments (
). The so-called Weiss constant θ provides information about the ground state of the material at 0 K. A value of θ > 0 indicates a ferroelectric ground state, and for θ < 0 it should be antiferroelectric, whereas for θ = 0 it is paraelectric (no coupling of the dipoles). Our results predict Weiss constants close to zero, indicating only weak interactions between the individual dipoles, which can be rationalized by the porosity of the MOFs and the consequently large spacial separation of the dipoles. For system B, a negative Weiss constant of θB = −10.9 K is found, whereas a positive value is predicted for system C (θC = 24.7 K). However, for both systems an antiferroelectric ground state is to be expected, since this is clearly indicated by model calculations for simple orthorhombic dipole lattices. (39) The slightly positive Weiss constant for system C can be explained by the in general weak interactions of the dipoles and the dependency on statistical errors of the predicted dielectric constants.
(8)
) is roughly equal to the ratio of the squared dipole moments (). The so-called Weiss constant θ provides information about the ground state of the material at 0 K. A value of θ > 0 indicates a ferroelectric ground state, and for θ < 0 it should be antiferroelectric, whereas for θ = 0 it is paraelectric (no coupling of the dipoles). Our results predict Weiss constants close to zero, indicating only weak interactions between the individual dipoles, which can be rationalized by the porosity of the MOFs and the consequently large spacial separation of the dipoles. For system B, a negative Weiss constant of θB = −10.9 K is found, whereas a positive value is predicted for system C (θC = 24.7 K). However, for both systems an antiferroelectric ground state is to be expected, since this is clearly indicated by model calculations for simple orthorhombic dipole lattices. (39) The slightly positive Weiss constant for system C can be explained by the in general weak interactions of the dipoles and the dependency on statistical errors of the predicted dielectric constants.Figure 5
Figure 5. Inverse of the electric susceptibility χ for systems B and C, obtained by the two fluctuation methods as a function of temperature T. The dashed lines show a linear regression curve to the data sets obtained for both systems.
Field Induced Breathing Behavior
As already discussed, Ghoufi and co-workers observed an electric field induced breathing of MIL-53(Cr) in a theoretical simulation at high field strength. (12) One of us proposed a mechanism for this behavior, by invoking a model of a 2D grid of induced dipoles. (13) As shown in the upper panel of Figure 6, the application of an electric field results in a lattice of induced dipoles, oriented along an axis of the unit cell, located mainly at the inorganic building blocks due to the relatively large and alternating charges. The electrostatic interaction between the dipoles depends on the shape of the lattice. By a wine-rack deformation to a rhombic shape the attractive interactions can be increased. Note that the term ΩEP in the electric enthalpy 
does not depend on the volume of the system (eq 2). Thus, the interactions between the induced dipoles are responsible for the transformation, whereas the electric field is just the source for the necessary polarization.
does not depend on the volume of the system (eq 2). Thus, the interactions between the induced dipoles are responsible for the transformation, whereas the electric field is just the source for the necessary polarization.Figure 6
Figure 6. Upper panel: 2D lattice of induced (aligned) dipoles. The gray box in the left figure indicates our choice of the unit cell, which is rotated by 90° in respect to the conventional one (size in respect to the conventional is 2 × 1 × 1). Lower panel: 2D lattice of permanent dipoles. The dipole–dipole interaction is stabilized undergoing the large pore–small pore phase transition.
The here investigated systems with dipolar rotors should in principle give rise to the same wine-rack-type breathing deformation as observed in MIL-53(Cr). However, the critical field strength Ec, which is necessary to trigger the structural transformation, should depend on the strength of the incorporated dipolar organic moieties. Furthermore, since the field strength required to orient the permanent dipoles (lower panel of Figure 6) is lower than that to induce equally strong dipoles, a lower Ec should be expected in the case of a system with rotatable dipolar groups.
To prove this, we performed molecular dynamics simulations in the NσT ensemble, allowing for cell deformations at increasing field strength Ex for systems A–C at 298 K. Figure 7a shows the unit cell volume Ω as a function of the electric field. As expected, in all three cases the structure collapses to a closed pore form at sufficiently high field. It should be noted that the force-field was parametrized with respect to the sterically relaxed building blocks (see Figure S1). Thus, the force-field becomes less accurate for high deformation (see Figure S2) and potentially underestimates the unit cell volumes of the contracted form. However, the critical field strength Ec, at which the contraction sets in, is unaffected by this. The simulations show that dipolar groups favor the transition: whereas system A transforms at ≈0.6 V Å–1, system B already contracts at ≈0.5 V Å–1 and C even at only ≈0.3 V Å–1. In Figure 7b, the polarization Px is plotted as a function of the electric field Ex. For all three systems a jump in Px is observed at the critical field strength due to the volume contraction. Interestingly, systems B and C show almost the same value for the critical polarization needed to trigger the contraction (≈5 × 104e Å–2). A comparison with Figure 2 indicates that these values lie above the region of orientation polarization for both materials, which means that additional buildup of displacive polarization is needed to reach the critical level for the collapse. However, when the amount of ionic polarization is smaller, the dipole of the oriented linkers becomes larger, leading to a lower Ec for system C. Consequently, to reach contraction purely by means of orientation polarization, organic moieties carrying even larger dipole moments would have to be incorporated into the framework. Then, the field-driven breathing would be temperature-dependent, since the required field strength needed to reach the critical polarization would also be temperature-dependent. As expected, system A needs a higher electric field Ec for the contraction, but to our surprise a smaller critical polarization (≈0.2 × 104e Å–2) is necessary. Note that in system A the polarization is mainly induced directly at the inorganic zinc paddle-wheels, which actually serve as hinges during the contraction, potentially lowering the critical polarization.
Figure 7
Figure 7. Simulation of systems A–C at 298 K in the NσT ensemble at different electric field strengths Ex: (a) unit cell volume as a function of the applied electric field Ex; (b) polarization Px as a function of the applied electric field Ex.
Conclusions and Outlook
ARTICLE SECTIONS
In this work, we demonstrated that dipolar rotors used as linkers in MOFs have a strong impact on the electric permittivity κ of the materials and can be used to tune their electric response. The herein investigated systems exhibit substantially larger dielectric constants as expected for MOFs in general and exhibit a strong dependency on the chemical composition of the dipolar linker moiety, i.e., on its dipole moment. In addition, a strong temperature dependence of κ was observed, which obeys the Curie–Weiss law for paraelectric materials. The investigated materials exhibit an anisotropic dielectric response. They are paraelectric in the x- and y-direction and dielectric in the z-direction. This offers new opportunities for the design of specialized dielectric sensors. Future efforts will aim to investigate the effect of adsorbed guest molecules on the dielectric permittivity.
In addition we found that the easiest and most efficient method to calculate κ from molecular dynamics simulations is to compute them from polarization fluctuations at zero field, since this makes the need for a reference polarization obsolete. Reference polarizations are needed to tackle the multivaluedness of the polarization as soon as the computation of the dielectric constant depends somehow on the absolute value of the polarization.
Furthermore, the field induced wine-rack-type lattice contraction of the investigated systems was studied with respect to the influence of the dipolar linkers. As observed before by Ghoufi et al. for MIL-53(Cr), a similar breathing behavior was found to be present for the pillared-layer MOFs. However, the dipolar linkers are able to reduce the critical field strength needed to trigger the contraction, depending on the magnitude of the incorporated dipole moment. With even higher dipole moments and in combination with guest adsorption, even lower critical field values Ec could be possible, allowing for new routes of potential application of MOFs with rotatable dipolar linkers in sensing or gas separation.
Supporting Information
ARTICLE SECTIONS
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscentsci.9b00497.
Detailed information about the MOF-FF energy expression and the parameter sets used within the MD simulations (PDF)
Terms & Conditions
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Author Information
ARTICLE SECTIONS
- Rochus Schmid - Computational Materials Chemistry Group, Fakultät für Chemie und Biochemie, Ruhr-Universität Bochum, Bochum 44801, Germany;
http://orcid.org/0000-0002-1933-3644;
Acknowledgments
ARTICLE SECTIONS
J.P.D. is grateful for the financial support by the Fonds der Chemischen Industrie (FCI). Further financial support from the Deutsche Forschungsgemeinschaft (DFG) within the COORNETs priority program is acknowledged (grant SCHM 1389/9-1).
References
ARTICLE SECTIONS
This article references 39 other publications.
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- 3Farha, O. K.; Hupp, J. T. Rational Design, Synthesis, Purification, and Activation of Metal-Organic Framework Materials. Acc. Chem. Res. 2010, 43, 1166– 1175, DOI: 10.1021/ar1000617[ACS Full Text
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- 5Dias, E. M.; Petit, C. Towards the Use of Metal–Organic Frameworks for Water Reuse: A Review of the Recent Advances in the Field of Organic Pollutants Removal and Degradation and the next Steps in the Field. J. Mater. Chem. A 2015, 3, 22484– 22506, DOI: 10.1039/C5TA05440K[Crossref], [CAS], Google Scholar5https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXhsFGqtr7O&md5=4087ab1abe6a006b03e8f22cbc5a104fTowards the use of metal-organic frameworks for water reuse: a review of the recent advances in the field of organic pollutants removal and degradation and the next steps in the fieldDias, Elton M.; Petit, CamilleJournal of Materials Chemistry A: Materials for Energy and Sustainability (2015), 3 (45), 22484-22506CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)A review. Water reuse is becoming increasingly important as more and more areas in the world are facing water stress issues. Treatment of wastewater to attain the purity required for various usages from culture irrigation to drinking water is therefore key. Several water treatment options are already in place and while we will continue to use them, parallel efforts are required to: (i) address the removal of the most persistent chems. in water and (ii) provide solns. for local communities. Recently, several studies on the use of metal-org. frameworks (MOFs) for the adsorption and photocatalytic degrdn. of orgs. in water have been reported. This enthusiasm originates from the large porosity and chem. tunability of MOFs - beneficial for adsorption - as well as their catalytic nature - beneficial for degrdn. The present review proposes a comprehensive and crit. anal. of the most recent studies on the use of MOFs for orgs. adsorption and photocatalytic degrdn. The potential to use MOFs to catalyze the prodn. of H2 from org. mols., like water contaminants, is also addressed. Overall, the discussion is organized based on the type of org. pollutants targeted and encompasses those released in industrial, domestic and agricultural wastewater streams.
- 6Kreno, L. E.; Leong, K.; Farha, O. K.; Allendorf, M.; Van Duyne, R. P.; Hupp, J. T. Metal–Organic Framework Materials as Chemical Sensors. Chem. Rev. 2012, 112, 1105– 1125, DOI: 10.1021/cr200324t[ACS Full Text], [CAS], Google Scholar6https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXhsVCgtL%252FI&md5=f6ed572725bc067ee8da81880bf1ec90Metal-Organic Framework Materials as Chemical SensorsKreno, Lauren E.; Leong, Kirsty; Farha, Omar K.; Allendorf, Mark; Van Duyne, Richard P.; Hupp, Joseph T.Chemical Reviews (Washington, DC, United States) (2012), 112 (2), 1105-1125CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)A review. The authors present a crit. review of the literature on metal-org. frameworks (MOFs) as chem. sensors. The authors begin by briefly examg. challenges relating to MOF sensor development including the design of MOFs with desirable properties, incorporation of appropriate signal transduction capabilities, and integration of MOFs into devices by employing thin-film growth techniques. Subsequent sections discuss specific examples of MOF sensors, categorized by method of signal transduction. Sensors based on MOF photoluminescence are discussed briefly. The authors have limited the review of luminescence-based sensors to a small no. of recent reports where the porous MOF architecture, or its chem. compn., imparts selective sensing capabilities. Scintillating MOFs that luminesce in the presence of radioactive analytes are also discussed. Other signal transduction schemes that use photons include various kinds of optical interferometry, analyte modulation of localized surface plasmon resonance energies, and solvatochromism. Mech. signal-transduction schemes employed with MOFs include ones based on surface acoustic wave, quartz crystal microbalance, and microcantilever devices. Elec. schemes thus far were limited to ones based on impedance spectroscopy.
- 7Lee, J.; Farha, O. K.; Roberts, J.; Scheidt, K. A.; Nguyen, S. T.; Hupp, J. T. Metal–Organic Framework Materials as Catalysts. Chem. Soc. Rev. 2009, 38, 1450– 1459, DOI: 10.1039/b807080f[Crossref], [PubMed], [CAS], Google Scholar7https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1MXkvVamu7k%253D&md5=d9cef4eb52ffb1c73fe1195e11bace26Metal-organic framework materials as catalystsLee, Jeong Yong; Farha, Omar K.; Roberts, John; Scheidt, Karl A.; Nguyen, Son Binh T.; Hupp, Joseph T.Chemical Society Reviews (2009), 38 (5), 1450-1459CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)A review; a crit. review of the emerging field of MOF-based catalysis is presented. Discussed are examples of: (a) opportunistic catalysis with metal nodes, (b) designed catalysis with framework nodes, (c) catalysis by homogeneous catalysts incorporated as framework struts, (d) catalysis by MOF-encapsulated mol. species, (e) catalysis by metal-free org. struts or cavity modifiers, and (f) catalysis by MOF-encapsulated clusters.
- 8Horcajada, P. Porous Metal–Organic-Framework Nanoscale Carriers as a Potential Platform for Drug Delivery and Imaging. Nat. Mater. 2010, 9, 172– 178, DOI: 10.1038/nmat2608[Crossref], [PubMed], [CAS], Google Scholar8https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3cXhtVOnt70%253D&md5=74f3e60d481f39e0000fd93db0e40a64Porous metal-organic-framework nanoscale carriers as a potential platform for drug delivery and imagingHorcajada, Patricia; Chalati, Tamim; Serre, Christian; Gillet, Brigitte; Sebrie, Catherine; Baati, Tarek; Eubank, Jarrod F.; Heurtaux, Daniela; Clayette, Pascal; Kreuz, Christine; Chang, Jong-San; Hwang, Young Kyu; Marsaud, Veronique; Bories, Phuong-Nhi; Cynober, Luc; Gil, Sophie; Ferey, Gerard; Couvreur, Patrick; Gref, RuxandraNature Materials (2010), 9 (2), 172-178CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)In the domain of health, one important challenge is the efficient delivery of drugs in the body using non-toxic nanocarriers. Most of the existing carrier materials show poor drug loading (usually less than 5 wt% of the transported drug vs. the carrier material) and/or rapid release of the proportion of the drug that is simply adsorbed (or anchored) at the external surface of the nanocarrier. In this context, porous hybrid solids, with the ability to tune their structures and porosities for better drug interactions and high loadings, are well suited to serve as nanocarriers for delivery and imaging applications. Here we show that specific non-toxic porous iron(III)-based metal-org. frameworks with engineered cores and surfaces, as well as imaging properties, function as superior nanocarriers for efficient controlled delivery of challenging antitumoral and retroviral drugs (i.e., busulfan, azidothymidine triphosphate, doxorubicin or cidofovir) against cancer and AIDS. In addn. to their high loadings, they also potentially assoc. therapeutics and diagnostics, thus opening the way for theranostics, or personalized patient treatments.
- 9Coudert, F.-X. Responsive Metal–Organic Frameworks and Framework Materials: Under Pressure, Taking the Heat, in the Spotlight, with Friends. Chem. Mater. 2015, 27, 1905– 1916, DOI: 10.1021/acs.chemmater.5b00046[ACS Full Text], [CAS], Google Scholar9https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXisF2hurw%253D&md5=4c0a4fb50b0c9eefcfd9e1dbc3fca990Responsive Metal-Organic Frameworks and Framework Materials: Under Pressure, Taking the Heat, in the Spotlight, with FriendsCoudert, Francois-XavierChemistry of Materials (2015), 27 (6), 1905-1916CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)A review. Recent years have seen a large increase of the research effort focused on framework materials, including the nowadays-ubiquitous metal-org. frameworks but also dense coordination polymers, covalent org. frameworks, and mol. frameworks. With the quickly increasing no. of structures synthesized and characterized, one pattern emerging is the common occurrence of flexibility. More specifically, an important no. of framework materials are stimuli-responsive: their structure can undergo changes of large amplitude in response to phys. or chem. stimulation. They can display transformations induced by temp., mech. pressure, guest adsorption or evacuation, light absorption, etc. and are sometimes referred to as smart materials, soft crystals, or dynamic materials. This Perspective highlights recent progress in this field, showcasing some of the most novel and unusual responses to stimuli, as well as advances in the fundamental understanding of flexible framework materials.
- 10Férey, G.; Serre, C. Large Breathing Effects in Three-Dimensional Porous Hybrid Matter: Facts, Analyses, Rules and Consequences. Chem. Soc. Rev. 2009, 38, 1380– 1399, DOI: 10.1039/b804302g[Crossref], [PubMed], [CAS], Google Scholar10https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1MXkvVamu70%253D&md5=a620fc681f135bdce4044f16b18c60b1Large breathing effects in three-dimensional porous hybrid matter: facts, analyses, rules and consequencesFerey, Gerard; Serre, ChristianChemical Society Reviews (2009), 38 (5), 1380-1399CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)A review. This crit. review focuses on a strange behavior of crystd. solid matter: its reversible swelling with large magnitude. This will be of interest for experts in porous solids but also for solid state chemists and physicists. Some examples, classified according to the dimensionality of the inorg. subnetwork, present the general requirements and the structural rules which govern the existence of this phenomenon. Its consequences concern specific applications related to sensors, energy savings, sustainable development and health (100 refs.).
- 11Schneemann, A.; Bon, V.; Schwedler, I.; Senkovska, I.; Kaskel, S.; Fischer, R. A. Flexible Metal–Organic Frameworks. Chem. Soc. Rev. 2014, 43, 6062, DOI: 10.1039/C4CS00101J[Crossref], [PubMed], [CAS], Google Scholar11https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXht1Slu7nI&md5=b824d2bff6379c9662b50e86739b42f7Flexible metal-organic frameworksSchneemann, A.; Bon, V.; Schwedler, I.; Senkovska, I.; Kaskel, S.; Fischer, R. A.Chemical Society Reviews (2014), 43 (16), 6062-6096CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)A review. Advances in flexible and functional metal-org. frameworks (MOFs), also called soft porous crystals, are reviewed by covering the literature of the five years period 2009-2013 with ref. to the early pertinent work since the late 1990s. Flexible MOFs combine the cryst. order of the underlying coordination network with cooperative structural transformability. These materials can respond to phys. and chem. stimuli of various kinds in a tunable fashion by mol. design, which does not exist for other known solid-state materials. Among the fascinating properties are so-called breathing and swelling phenomena as a function of host-guest interactions. Phase transitions are triggered by guest adsorption/desorption, photochem., thermal, and mech. stimuli. Other important flexible properties of MOFs, such as linker rotation and sub-net sliding, which are not necessarily accompanied by crystallog. phase transitions, are briefly mentioned as well. Emphasis is given on reviewing the recent progress in application of in situ characterization techniques and the results of theor. approaches to characterize and understand the breathing mechanisms and phase transitions. The flexible MOF systems, which are discussed, are categorized by the type of metal-nodes involved and how their coordination chem. with the linker mols. controls the framework dynamics. Aspects of tailoring the flexible and responsive properties by the mixed component solid-soln. concept are included, and as well examples of possible applications of flexible metal-org. frameworks for sepn., catalysis, sensing, and biomedicine.
- 12Ghoufi, A.; Benhamed, K.; Boukli-Hacene, L.; Maurin, G. Electrically Induced Breathing of the MIL-53(Cr) Metal–Organic Framework. ACS Cent. Sci. 2017, 3, 394– 398, DOI: 10.1021/acscentsci.6b00392[ACS Full Text], [CAS], Google Scholar12https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXls1Wmsb4%253D&md5=220c934e3db4391ff037ec15beddc416Electrically Induced Breathing of the MIL-53(Cr) Metal-Organic FrameworkGhoufi, Aziz; Benhamed, Karima; Boukli-Hacene, Leila; Maurin, GuillaumeACS Central Science (2017), 3 (5), 394-398CODEN: ACSCII; ISSN:2374-7951. (American Chemical Society)The breathing behavior of the MIL-53(Cr) metal-org. framework (MOF) was explored previously upon guest-adsorption and thermal and mech. stimuli. Here, advanced mol. simulations based on the use of an accurate force field to describe the flexibility of this porous framework demonstrate that the application of an elec. field induces the structural switching of this MOF leading to a 1st-order transition and a vol. change of >40%. This motivated the authors to elec. tune the pore size of MIL-53(Cr) with the idea to propose a new concept to selectively capture CO2 over CH4 via a mol. sieving that paves the way toward the optimization of current sepn.-based processes.
- 13Schmid, R. An Electric Field Induced Breath for Metal–Organic Frameworks. ACS Cent. Sci. 2017, 3, 369– 371, DOI: 10.1021/acscentsci.7b00162[ACS Full Text], [CAS], Google Scholar13https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXmtlaqsrw%253D&md5=944583213e7ba2ef79ee22887927d95aAn Electric Field Induced Breath for Metal-Organic FrameworksSchmid, RochusACS Central Science (2017), 3 (5), 369-371CODEN: ACSCII; ISSN:2374-7951. (American Chemical Society)There is no expanded citation for this reference.
- 14Kolesnikov, A. L.; Budkov, Y. A.; Möllmer, J.; Kiselev, M. G.; Gläser, R. Metal-Organic Framework Breathing in Electric Field: A Theoretical Study. J. Phys. Chem. C 2019, 123, 10333– 10338, DOI: 10.1021/acs.jpcc.8b11630[ACS Full Text], [CAS], Google Scholar14https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXmtV2jt7Y%253D&md5=9e9e7ea63bfa8ee86339473668f6f800Metal-Organic Framework Breathing in the Electric Field: A Theoretical StudyKolesnikov, Andrei L.; Budkov, Yury A.; Moellmer, Jens; Kiselev, Michael G.; Glaeser, RogerJournal of Physical Chemistry C (2019), 123 (16), 10333-10338CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)In this manuscript, the authors study the elec. induced breathing of a metal-org. framework (MOF) within a 2-dimensional lattice model. The Helmholtz free energy of the MOF in the elec. field consists of 2 parts: the electrostatic energy of the dielec. body in the external elec. field and the elastic energy of the framework. The 1st contribution is calcd. from the 1st principles of statistical mechanics with an account of MOF symmetry. By minimizing the obtained free energy and solving the resulting system of equations, the authors obtain the local elec. field and the parameter of the unit cell (angle α). The paper also studies the cross-sectional area of the unit cell and the polarization as functions of the external elec. field. The authors obtain the hysteresis in the region of the structural transition of the framework. Results are in qual. agreement with the literature data of the mol. dynamics simulation of MIL-53(Cr).
- 15Knebel, A.; Geppert, B.; Volgmann, K.; Kolokolov, D. I.; Stepanov, A. G.; Twiefel, J.; Heitjans, P.; Volkmer, D.; Caro, J. Defibrillation of Soft Porous Metal-Organic Frameworks with Electric Fields. Science 2017, 358, 347– 351, DOI: 10.1126/science.aal2456[Crossref], [PubMed], [CAS], Google Scholar15https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXhs1Kns7%252FE&md5=65943f86316595f45313607e789d3d52Defibrillation of soft porous metal-organic frameworks with electric fieldsKnebel, A.; Geppert, B.; Volgmann, K.; Kolokolov, D. I.; Stepanov, A. G.; Twiefel, J.; Heitjans, P.; Volkmer, D.; Caro, J.Science (Washington, DC, United States) (2017), 358 (6361), 347-351CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)Gas transport through metal-org. framework membranes (MOFs) was switched in situ by applying an external elec. field (E-field). The switching of gas permeation upon E-field polarization could be explained by the structural transformation of the zeolitic imidazolate framework ZIF-8 into polymorphs with more rigid lattices. Permeation measurements under a direct-current E-field poling of 500 V per mm showed reversibly controlled switching of the ZIF-8 into polar polymorphs, which was confirmed by x-ray diffraction and ab initio calcns. The stiffening of the lattice causes a redn. in gas transport through the membrane and sharpens the mol. sieving capability. Dielec. spectroscopy, polarization, and deuterium NMR studies revealed low-frequency resonances of ZIF-8 that we attribute to lattice flexibility and linker movement. Upon E-field polarization, we obsd. a defibrillation of the different lattice motions.
- 16Gonzalez-Nelson, A.; Coudert, F.-X.; van der Veen, M. A. Rotational Dynamics of Linkers in Metal–Organic Frameworks. Nanomaterials 2019, 9, 330, DOI: 10.3390/nano9030330[Crossref], [PubMed], [CAS], Google Scholar16https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXhtFWrs7bO&md5=ff79c23c592c32d12d1589028651b842Rotational dynamics of linkers in metal-organic frameworksGonzalez-Nelson, Adrian; Coudert, Francois-Xavier; Van Der Veen, Monique A.Nanomaterials (2019), 9 (3), 330CODEN: NANOKO; ISSN:2079-4991. (MDPI AG)A review. Among the numerous fascinating properties of metal-org. frameworks (MOFs), their rotational dynamics is perhaps one of the most intriguing, with clear consequences for adsorption and sepn. of mols., as well as for optical and mech. properties. A closer look at the rotational mobility in MOF linkers reveals that it is not only a considerably widespread phenomenon, but also a fairly diverse one. Still, the impact of these dynamics is often understated. In this review, we address the various mechanisms of linker rotation reported in the growing collection of literature, followed by a highlight of the methods currently used in their study, and we conclude with the impacts that such dynamics have on existing and future applications.
- 17Vukotic, V. N.; Harris, K. J.; Zhu, K.; Schurko, R. W.; Loeb, S. J. Metal–Organic Frameworks with Dynamic Interlocked Components. Nat. Chem. 2012, 4, 456– 460, DOI: 10.1038/nchem.1354[Crossref], [PubMed], [CAS], Google Scholar17https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XmvFeqs7s%253D&md5=11793f062005687c50583f59d9f35372Metal-organic frameworks with dynamic interlocked componentsVukotic, V. Nicholas; Harris, Kristopher J.; Zhu, Kelong; Schurko, Robert W.; Loeb, Stephen J.Nature Chemistry (2012), 4 (6), 456-460CODEN: NCAHBB; ISSN:1755-4330. (Nature Publishing Group)The dynamics of mech. interlocked mols. such as rotaxanes and catenanes were studied in soln. as examples of rudimentary mol. switches and machines, but in this medium, the mols. are randomly dispersed and their motion incoherent. As a strategy for achieving a higher level of mol. organization, the authors have constructed a metal-org. framework material [Cu2(L)(H2O)2]·3H2O using a [2]rotaxane (H4L) (composed of a 3,5-dicarboxybenzyl-3,5-dicarboxyaniline axle with [24]crown-6-ether macrocyclic wheel) as the org. linker and binuclear carboxylate bridged Cu(II) units as the nodes. Activation of the as-synthesized material creates a void space inside the rigid framework that allows the soft crown ether macrocyclic ring of the [2]rotaxane to rotate rapidly, unimpeded by neighboring mol. components. Variable-temp. 13C and 2H solid-state NMR expts. were used to characterize the nature and rate of the dynamic processes occurring inside this unique material. These results provide a blueprint for the future creation of solid-state mol. switches and mol. machines based on mech. interlocked mols.
- 18Danowski, W.; van Leeuwen, T.; Abdolahzadeh, S.; Roke, D.; Browne, W. R.; Wezenberg, S. J.; Feringa, B. L. Unidirectional Rotary Motion in a Metal–Organic Framework. Nat. Nanotechnol. 2019, 14, 488– 494, DOI: 10.1038/s41565-019-0401-6[Crossref], [PubMed], [CAS], Google Scholar18https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXms1Ogtrk%253D&md5=861d9bbbd1eafb32a0004a698d9d741aUnidirectional rotary motion in a metal-organic frameworkDanowski, Wojciech; van Leeuwen, Thomas; Abdolahzadeh, Shaghayegh; Roke, Diederik; Browne, Wesley R.; Wezenberg, Sander J.; Feringa, Ben L.Nature Nanotechnology (2019), 14 (5), 488-494CODEN: NNAABX; ISSN:1748-3387. (Nature Research)Overcrowded alkene-based light-driven mol. motors are able to perform large-amplitude repetitive unidirectional rotations. Their behavior is well understood in soln. However, Brownian motion precludes the precise positioning at the nanoscale needed to harness cooperative action. Here, authors demonstrate mol. motors organized in cryst. metal-org. frameworks (MOFs). The motor unit becomes a part of the org. linker (or strut), and its spatial arrangement is elucidated through powder and single-crystal x-ray analyses and polarized optical and Raman microscopies. They confirm that the light-driven unidirectional rotation of the motor units is retained in the MOF framework and that the motors can operate in the solid state with similar rotary speed (rate of thermal helix inversion) to that in soln. These 'moto-MOFs' could in the future be used to control dynamic function in cryst. materials.
- 19Tam, B.; Yazaydin, O. Design of Electric Field Controlled Molecular Gates Mounted on Metal–Organic Frameworks. J. Mater. Chem. A 2017, 5, 8690– 8696, DOI: 10.1039/C7TA00101K[Crossref], [CAS], Google Scholar19https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXmtFGnt7g%253D&md5=29147fdd502b12161350e9f89743280eDesign of electric field controlled molecular gates mounted on metal-organic frameworksTam, Benjamin; Yazaydin, OzgurJournal of Materials Chemistry A: Materials for Energy and Sustainability (2017), 5 (18), 8690-8696CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)We propose and computationally demonstrate the concept of elec. field controlled mol. gates mounted on the open-metal coordination sites in metal-org. framework (MOF) materials. The MOF-mol. gate complex functions by opening and closing under the effect of an elec. field. Our design involves Mg-MOF-74, a MOF with hexagonal channels with open-metal coordination sites at each corner, and a multifunctional gate mol. with permanent dipole which anchors itself on the host MOF material and responds to changes in the direction of an elec. field by rotating around its backbone which acts as an axle. By carrying out d. functional theory (DFT) calcns. and mol. dynamics (MD) simulations we show that the MOF-mol. gate complex can be switched between two stable configurations, open and closed, by turning on and off an external elec. field. We further show that the mol. gate can be controlled to block or allow the diffusion of methane mols. through the channels of the MOF like a nanoscale butterfly valve. Elec. field controlled mol. gates mounted on MOFs can pave the way for new mol. machines and nanodevices which can store, deliver or select mols. on demand and with at. precision.
- 20Namsani, S.; Yazaydin, O. Electric Field Induced Rotation of Halogenated Organic Linkers in Isoreticular Metal-Organic Frameworks for Nanofluidic Applications. Mol. Syst. Des. Eng. 2018, 3, 951– 958, DOI: 10.1039/C8ME00030A[Crossref], [CAS], Google Scholar20https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXitVaqur3M&md5=729e2a2ec60e4974afc98201d72d67f7Electric field induced rotation of halogenated organic linkers in isoreticular metal-organic frameworks for nanofluidic applicationsNamsani, Sadanandam; Yazaydin, A. OzgurMolecular Systems Design & Engineering (2018), 3 (6), 951-958CODEN: MSDEBG; ISSN:2058-9689. (Royal Society of Chemistry)We present a systematic computational study which provides a plausible route to control the rotation of org. linkers in isoreticular metal-org. frameworks (IRMOF) by using an external elec. field in order to manipulate the diffusion of mols. in nanopores. We achieve this by halogenating the org. linkers of IRMOF-1 and IRMOF-7 to create permanent dipole moments on the linkers, hence making them responsive to changes in the strength and direction of an elec. field. More importantly we show that by varying the ligand size and the halogen type, no. and substitution positions, the strength of the elec. field required to control the rotation of linkers can be reduced significantly. Cl substitution is most effective in making the org. linkers elec. field responsive since a greater dipole moment is created compared to those obtained by F or Br substitution. Cl substitution of a larger org. linker, i.e. 1,4-naphthalenedicarboxylate (IRMOF-7) rather than 1,4-benzenedicarboxylate (IRMOF-1), results in a greater dipole moment and reduces the elec. field strength required for the rotation of the ligand. Furthermore, double Cl substitution and the optimization of the Cl substitution positions enable controlled rotation of the IRMOF-7 linkers with an elec. field strength as low as 0.5 V nm-1. Finally, using the elec. field induced rotation of org. linkers we show that it is possible to enhance the diffusion of methane mols. in a chosen direction while limiting their mobility in other directions. Our study hints at the potential of using MOFs for flow control in nanofluidic systems.
- 21Sun, L.; Campbell, M. G.; Dincă, M. Electrically Conductive Porous Metal–Organic Frameworks. Angew. Chem., Int. Ed. 2016, 55, 3566– 3579, DOI: 10.1002/anie.201506219[Crossref], [CAS], Google Scholar21https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XmslGisw%253D%253D&md5=185401e793b619c797286fe8df7c641eElectrically Conductive Porous Metal-Organic FrameworksSun, Lei; Campbell, Michael G.; Dinca, MirceaAngewandte Chemie, International Edition (2016), 55 (11), 3566-3579CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)A review. Owing to their outstanding structural, chem., and functional diversity, metal-org. frameworks (MOFs) have attracted considerable attention over the last two decades in a variety of energy-related applications. Notably missing among these, until recently, were applications that required good charge transport coexisting with porosity and high surface area. Although most MOFs are elec. insulators, several materials in this class have recently demonstrated excellent elec. cond. and high charge mobility. Herein, the authors review the synthetic and electronic design strategies that have been employed thus far for producing frameworks with permanent porosity and long-range charge transport properties. In addn., key expts. that have been employed to demonstrate elec. transport, as well as selected applications for this subclass of MOFs, are discussed.
- 22Zagorodniy, K.; Seifert, G.; Hermann, H. Metal-Organic Frameworks as Promising Candidates for Future Ultralow-k Dielectrics. Appl. Phys. Lett. 2010, 97, 251905, DOI: 10.1063/1.3529461[Crossref], [CAS], Google Scholar22https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3cXhsF2rt7nE&md5=3b75fad9d5a4e0723082c4dd11fbb093Metal-organic frameworks as promising candidates for future ultralow-k dielectricsZagorodniy, K.; Seifert, G.; Hermann, H.Applied Physics Letters (2010), 97 (25), 251905/1-251905/2CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)A class of Zn4O(CO2)6-based metal-org. frameworks (MOFs) is theor. analyzed with respect to suitability as an interlayer dielec. material for applications in semiconducting devices. The static dielec. const. is calcd. using the Clausius-Mossotti approach. For 3 out of ∼30 of the considered MOFs excellent combinations of ultralow dielec. const., elastic bulk modulus, and gap energy are found favoring these materials as outstanding candidates for future ultralow-k dielec. materials. (c) 2010 American Institute of Physics.
- 23Warmbier, R.; Quandt, A.; Seifert, G. Dielectric Properties of Selected Metal–Organic Frameworks. J. Phys. Chem. C 2014, 118, 11799– 11805, DOI: 10.1021/jp5029646[ACS Full Text], [CAS], Google Scholar23https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXnvV2jsb4%253D&md5=01301ccd5a3872a2f73fe74e2e721ff7Dielectric Properties of Selected Metal-Organic FrameworksWarmbier, Robert; Quandt, Alexander; Seifert, GotthardJournal of Physical Chemistry C (2014), 118 (22), 11799-11805CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)The electronic structure of a class of [Zn4O(CO2)6] based metal-org. frameworks (MOFs) is theor. analyzed by d. functional perturbation theory. The calcd. static dielec. consts. vary at 1.33-1.54, characterizing the structures as ultralow-k dielec. materials and confirming earlier ests. qual. The authors also present the results of 1st-principle calcns. of the real and imaginary parts of the dielec. function and give the frequency-dependent dielec. const. up to the near-UV, which is important for high frequency semiconductor and optical applications of MOFs. The dielec. and electronic properties are governed by the linker mols., so that the band gap and the dielec. const. can be engineered.
- 24Redel, E.; Wang, Z.; Walheim, S.; Liu, J.; Gliemann, H.; Wöll, C. On the Dielectric and Optical Properties of Surface-Anchored Metal-Organic Frameworks: A Study on Epitaxially Grown Thin Films. Appl. Phys. Lett. 2013, 103, 091903, DOI: 10.1063/1.4819836[Crossref], [CAS], Google Scholar24https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXhtlKrt7rF&md5=f7fa09852ee8a75548f833c5f14fff44On the dielectric and optical properties of surface-anchored metal-organic frameworks: A study on epitaxially grown thin filmsRedel, Engelbert; Wang, Zhengbang; Walheim, Stefan; Liu, Jinxuan; Gliemann, Hartmut; Woell, ChristofApplied Physics Letters (2013), 103 (9), 091903/1-091903/5CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)We det. the optical consts. of two highly porous, cryst. metal-org. frameworks (MOFs). Since it is problematic to det. the optical consts. for the std. powder modification of these porous solids, we instead use surface-anchored metal-org. frameworks (SURMOFs). These MOF thin films are grown using liq. phase epitaxy (LPE) on modified silicon substrates. The produced SURMOF thin films exhibit good optical properties; these porous coatings are smooth as well as crack-free, they do not scatter visible light, and they have a homogeneous interference color over the entire sample. Therefore, spectroscopic ellipsometry (SE) can be used in a straightforward fashion to det. the corresponding SURMOF optical properties. After careful removal of the solvent mols. used in the fabrication process as well as the residual water adsorbed in the voids of this highly porous solid, we det. an optical const. of n = 1.39 at a wavelength of 750 nm for HKUST-1 (stands for Hong Kong University of Science and Technol.-1; and was first discovered there) or Cu3(BTC)2. After exposing these SURMOF thin films to moisture/EtOH atm., the refractive index (n) increases to n = 1.55-1.6. This dependence of the optical properties on water/EtOH adsorption demonstrates the potential of such SURMOF materials for optical sensing. (c) 2013 American Institute of Physics.
- 25Van Cleuvenbergen, S.; Stassen, I.; Gobechiya, E.; Zhang, Y.; Markey, K.; De Vos, D. E.; Kirschhock, C.; Champagne, B.; Verbiest, T.; van der Veen, M. A. ZIF-8 as Nonlinear Optical Material: Influence of Structure and Synthesis. Chem. Mater. 2016, 28, 3203– 3209, DOI: 10.1021/acs.chemmater.6b01087[ACS Full Text], [CAS], Google Scholar25https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XlvV2isrY%253D&md5=8bd97fb5bfa0c308f1614ac8be6f7320ZIF-8 as Nonlinear Optical Material: Influence of Structure and SynthesisVan Cleuvenbergen, Stijn; Stassen, Ivo; Gobechiya, Elena; Zhang, Yuexing; Markey, Karen; De Vos, Dirk E.; Kirschhock, Christine; Champagne, Benoit; Verbiest, Thierry; van der Veen, Monique A.Chemistry of Materials (2016), 28 (9), 3203-3209CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)Metalorg. framework ZIF-8, from the zeolitic imidazolate framework family, shows a large intrinsic 2nd-order nonlinear optical (NLO) response. ZIF-8 is a stable, inexpensive material that is transparent in the visible (visible) and near-IR (NIR) window. This is crucial for NLO applications. The 2nd-order NLO activity is due to the noncentrosym. octupolar symmetry of the material. Fast syntheses lead to a lower 2nd-order NLO response. Consistent with polarized 2nd-harmonic generation (SHG) microscopy measurements, this is ascribed 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 av. 2nd-order NLO coeff. 〈deff〉 of 0.25 pm/V, which is explained and supported by ab initio theor. calcns.
- 26Eslava, S.; Zhang, L.; Esconjauregui, S.; Yang, J.; Vanstreels, K.; Baklanov, M. R.; Saiz, E. Metal-Organic Framework ZIF-8 Films As Low-κ Dielectrics in Microelectronics. Chem. Mater. 2013, 25, 27– 33, DOI: 10.1021/cm302610z[ACS Full Text], [CAS], Google Scholar26https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XhslOlsr7E&md5=e60e478be39f045ee8b13761f2bdfebbMetal-Organic Framework ZIF-8 Films As Low-κ Dielectrics in MicroelectronicsEslava, Salvador; Zhang, Liping; Esconjauregui, Santiago; Yang, Junwei; Vanstreels, Kris; Baklanov, Mikhail R.; Saiz, EduardoChemistry of Materials (2013), 25 (1), 27-33CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)ZIF-8 films were deposited on silicon wafers and characterized to assess their potential as future insulators (low-κ dielecs.) in microelectronics. SEM and gas adsorption monitored by spectroscopic ellipsometry confirmed the good coalescence of the crystals, the absence of intergranular voids, and the hydrophobicity of the pores. Mech. properties were assessed by nanoindentation and tape tests, confirming sufficient rigidity for chip manufg. processes (elastic modulus >3 GPa) and the good adhesion to the support. The dielec. const. was measured by impedance anal. at different frequencies and temps., indicating that κ was only 2.33 (±0.05) at 100 kHz, a result of low polarizability and d. in the films. Intensity voltage curves showed that the leakage current was only 10-8 A cm2 at 1 MV cm-1, and the breakdown voltage was >2 MV cm-1. In conclusion, metal-org. framework ZIF-8 films were exptl. found to be promising candidates as low-κ dielecs. in microelectronic chip devices. This opens a new direction for research into the application of metal-org. frameworks.
- 27Ryder, M. R.; Zeng, Z.; Titov, K.; Sun, Y.; Mahdi, E. M.; Flyagina, I.; Bennett, T. D.; Civalleri, B.; Kelley, C. S.; Frogley, M. D.; Cinque, G.; Tan, J.-C. Dielectric Properties of Zeolitic Imidazolate Frameworks in the Broad-Band Infrared Regime. J. Phys. Chem. Lett. 2018, 9, 2678– 2684, DOI: 10.1021/acs.jpclett.8b00799[ACS Full Text], [CAS], Google Scholar27https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXovVOisbs%253D&md5=fb72a8cfeb08163818b11ea0984249efDielectric Properties of Zeolitic Imidazolate Frameworks in the Broad-Band Infrared RegimeRyder, Matthew R.; Zeng, Zhixin; Titov, Kirill; Sun, Yueting; Mahdi, E. M.; Flyagina, Irina; Bennett, Thomas D.; Civalleri, Bartolomeo; Kelley, Chris S.; Frogley, Mark D.; Cinque, Gianfelice; Tan, Jin-ChongJournal of Physical Chemistry Letters (2018), 9 (10), 2678-2684CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)The field of metal-org. framework (MOF) materials is rapidly advancing toward practical applications; consequently, it is urgent to achieve a better understanding and precise control of their phys. properties. Yet, research on the dielec. properties of MOFs is at its infancy, where studies are confined to the static dielec. behavior or lower-frequency response (kHz-MHz) only. Herein, the authors present the pioneering use of synchrotron-based IR reflectivity expts. combined with d. functional theory (DFT) calcns. to accurately det. the dynamic dielec. properties of zeolitic imidazolate frameworks (ZIFs, a topical family of MOFs). The authors show, for the 1st time, the frequency-dependent dielec. response of representative ZIF compds., bridging the near-, mid-, and far-IR (terahertz, THz) broad-band frequencies. The authors establish the structure-property relations as a function of framework porosity and structural change. Comprehensive results will pave the way for novel ZIF-based terahertz applications, such as IR optical sensors and high-speed wireless communications.
- 28Ryder, M. R.; Donà, L.; Vitillo, J. G.; Civalleri, B. Understanding and Controlling the Dielectric Response of Metal–Organic Frameworks. ChemPlusChem 2018, 83, 308– 316, DOI: 10.1002/cplu.201700558[Crossref], [PubMed], [CAS], Google Scholar28https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXlt1Ontro%253D&md5=3c01a0a7182d6892712e608730e1f650Understanding and Controlling the Dielectric Response of Metal-Organic FrameworksRyder, Matthew R.; Dona, Lorenzo; Vitillo, Jenny G.; Civalleri, BartolomeoChemPlusChem (2018), 83 (4), 308-316CODEN: CHEMM5; ISSN:2192-6506. (Wiley-VCH Verlag GmbH & Co. KGaA)Metal-org. framework (MOF) materials have recently been shown to have promising electronic and dielec. properties. This study involves investigating a diverse range of MOFs to rationalise how the different building blocks that form the structure can affect the electronic properties and dielec. response. The results confirm that the band gap is primarily due to the electronic levels of the org. linkers and that tuning the band gap can be easily achieved either by linker functionalisation or by increasing the aromaticity. The relevance of simple structure-property relationships for different families of isoreticular MOFs through the use of Hammett sigma consts. is also highlighted. It is also shown that the polarisability of the framework can be tuned comparably to the band gap. However, the expected low static dielec. const. is less influenced by the compn. of the MOF and can be modified by acting on the crystal structure. Indeed, it is shown that it can be directly linked to the framework porosity. The anal., based on quantum mech. calcns., includes the contribution from the metals involved, the org. linkers and the symmetry and topol.of the framework and makes suggestions for future work on low-κ dielec. MOFs.
- 29Bétard, A.; Fischer, R. A. Metal–Organic Framework Thin Films: From Fundamentals to Applications. Chem. Rev. 2012, 112, 1055– 1083, DOI: 10.1021/cr200167v[ACS Full Text], [CAS], Google Scholar29https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXht1Slt7bF&md5=e166d873cc42ed503be528d261270effMetal-Organic Framework Thin Films: From Fundamentals to ApplicationsBetard, Angelique; Fischer, Roland A.Chemical Reviews (Washington, DC, United States) (2012), 112 (2), 1055-1083CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)A review. The field of metal-org. frameworks (MOFs), which are often also called porous coordination polymers (PCPs), has been growing tremendously over the last 15 years. This fascinating class of cryst. hybrid materials, which are formed by assocn. of metal centers or clusters and org. linker(s), offers a unique chem. versatility combined with a designable framework and an unprecedentedly large and permanent inner porosity. In this review, MOF thin films will be examd. with respect to three main themes. Fabrication methods are important and will be described in the first section. Characterization, which ranges from compositional and structural identification to lateral homogeneity and porosity, is an important issue that will be discussed together with various strategies aimed at improving the quality and applicability of films. Finally, potential applications of MOF films in several fields will be reviewed.
- 30Zhang, C.; Sprik, M. Computing the Dielectric Constant of Liquid Water at Constant Dielectric Displacement. Phys. Rev. B: Condens. Matter Mater. Phys. 2016, 93, 144201, DOI: 10.1103/PhysRevB.93.144201[Crossref], [CAS], Google Scholar30https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XhsFGlsb%252FO&md5=b40de5cd71861e2310dd5c39e1fb6cd5Computing the dielectric constant of liquid water at constant dielectric displacementZhang, Chao; Sprik, MichielPhysical Review B (2016), 93 (14), 144201/1-144201/13CODEN: PRBHB7; ISSN:2469-9950. (American Physical Society)The static dielec. const. of liq. water is computed using classical force field based mol. dynamics simulation at fixed elec. displacement D. The method to constrain the elec. displacement is the finite- temp. classical variant of the const. D method developed by Stengel. Spaldin, and Vanderbilt [Nat. Phys. 5, 304 (2009)]. There is also a modification of this scheme imposing fixed values of the macroscopic field E. The method is applied to the popular SPC/E model of liq. water. We compare four different ests. of the dielec. const., two obtained from fluctuations of the polarization at D = 0 and E = 0 and two from the variation of polarization with finite D and E. It is found that all four ests. agree when properly converged. The computational effort to achieve convergence varies, however, with const. D calcns. being substantially more efficient. We attribute this difference to the much shorter relaxation time of longitudinal polarization compared to transverse polarization accelerating const. D calcns.
- 31Stengel, M.; Spaldin, N. A.; Vanderbilt, D. Electric Displacement as the Fundamental Variable in Electronic-Structure Calculations. Nat. Phys. 2009, 5, 304– 308, DOI: 10.1038/nphys1185[Crossref], [CAS], Google Scholar31https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1MXktVKjurY%253D&md5=bbd45d002ef3292bc6b4f44211ccb5b3Electric displacement as the fundamental variable in electronic-structure calculationsStengel, Massimiliano; Spaldin, Nicola A.; Vanderbilt, DavidNature Physics (2009), 5 (4), 304-308CODEN: NPAHAX; ISSN:1745-2473. (Nature Publishing Group)Finite-field calcns. in periodic insulators are tech. and conceptually challenging, owing to fundamental problems in defining polarization in extended solids. Although significant progress has been made recently with the establishment of techniques to fix the elec. field E or the macroscopic polarization P in first-principles calcns., both methods lack the ease of use and conceptual clarity of std. zero-field calcns. Here we develop a new formalism, in which the elec. displacement D, rather than E or P, is the fundamental elec. variable. Fixing D has the intuitive interpretation of imposing open-circuit elec. boundary conditions, which is particularly useful in studying ferroelec. systems. Furthermore, the analogy to open-circuit capacitors suggests an appealing reformulation in terms of free charges and potentials, which dramatically simplifies the treatment of stresses and strains. Using PbTiO3 as an example, we show that our technique enables full control over the elec. variables within the d. functional formalism.
- 32Spaldin, N. A. A Beginner’s Guide to the Modern Theory of Polarization. J. Solid State Chem. 2012, 195, 2– 10, DOI: 10.1016/j.jssc.2012.05.010[Crossref], [CAS], Google Scholar32https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XhsVSqt77J&md5=40ead8f8398a94a18104e422d00a7224A beginner's guide to the modern theory of polarizationSpaldin, Nicola A.Journal of Solid State Chemistry (2012), 195 (), 2-10CODEN: JSSCBI; ISSN:0022-4596. (Elsevier B.V.)The so-called modern theory of polarization, which rigorously defines the spontaneous polarization of a periodic solid and provides a route for its computation in electronic structure codes through the Berry phase, is introduced in a simple qual. discussion.
- 33Bureekaew, S.; Amirjalayer, S.; Tafipolsky, M.; Spickermann, C.; Roy, T. K.; Schmid, R. MOF-FF – A Flexible First-Principles Derived Force Field for Metal-Organic Frameworks. Phys. Status Solidi B 2013, 250, 1128– 1141, DOI: 10.1002/pssb.201248460[Crossref], [CAS], Google Scholar33https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXjt1ajurs%253D&md5=a034c9370905858f7dfd6f3b2d2d66daMOF-FF - A flexible first-principles derived force field for metal-organic frameworksBureekaew, Sareeya; Amirjalayer, Saeed; Tafipolsky, Maxim; Spickermann, Christian; Roy, Tapta Kanchan; Schmid, RochusPhysica Status Solidi B: Basic Solid State Physics (2013), 250 (6), 1128-1141CODEN: PSSBBD; ISSN:0370-1972. (Wiley-VCH Verlag GmbH & Co. KGaA)A review. In this contribution the development, definition and selected applications of a new force field (FF) for metal-org. frameworks MOF-FF is presented. MOF-FF is fully flexible and is parameterized in a systematic and consistent fashion from first principles ref. data. It can be used for a variety of different MOF-families and in particular - due to the reparametrization of a variety of org. linkers - also to explore isoreticular series of systems. The history of the development, leading to the final definition of MOF-FF is reviewed along with the application of the previous incarnations of the FF. In addn., the parametrization approach is explained in a tutorial fashion. The currently parametrized set of inorg. building blocks is constantly extended. Formate models of currently covered inorg. building blocks.
- 34Dürholt, J. P.; Fraux, G.; Coudert, F.-X.; Schmid, R. Ab Initio Derived Force Fields for Zeolitic Imidazolate Frameworks: MOF-FF for ZIFs. J. Chem. Theory Comput. 2019, 15, 2420– 2432, DOI: 10.1021/acs.jctc.8b01041[ACS Full Text], [CAS], Google Scholar34https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXkslait70%253D&md5=d327855e6f4cbf2ab21f5d2310e25f58Ab Initio Derived Force Fields for Zeolitic Imidazolate Frameworks: MOF-FF for ZIFsDuerholt, Johannes P.; Fraux, Guillaume; Coudert, Francois-Xavier; Schmid, RochusJournal of Chemical Theory and Computation (2019), 15 (4), 2420-2432CODEN: JCTCCE; ISSN:1549-9618. (American Chemical Society)In this paper, we parametrized in a consistent way a new force field for a range of different zeolitic imidazolate framework systems (ZIF-8, ZIF-8(H), ZIF-8(Br), and ZIF-8(Cl)), extending the MOF-FF parametrization methodol. in two aspects. First, we implemented the possibility to use periodic ref. data in order to prevent the difficulty of generating representative finite clusters. Second, a new optimizer based on the covariance matrix adaptation evolutionary strategy (CMA-ES) was employed during the parametrization process. We confirmed that CMA-ES, as a state-of-the-art black box optimizer for problems on continuous variables, is more efficient and versatile for force field optimization than the previous genetic algorithm. The obtained force field was then validated with respect to some static and dynamic properties. Much effort was spent to ensure that the FF is able to describe the crucial linker swing effect in a large no. of ZIF-8 derivs. For this reason, we compared our force field to ab initio mol. dynamic simulations and found an accuracy comparable to those obtained by different exchange-correlation functionals.
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- 38Willems, T. F.; Rycroft, C. H.; Kazi, M.; Meza, J. C.; Haranczyk, M. Algorithms and Tools for High-Throughput Geometry-Based Analysis of Crystalline Porous Materials. Microporous Mesoporous Mater. 2012, 149, 134– 141, DOI: 10.1016/j.micromeso.2011.08.020[Crossref], [CAS], Google Scholar38https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXht12jurjN&md5=28e97cac049214d18c5a964ab405c4d9Algorithms and tools for high-throughput geometry-based analysis of crystalline porous materialsWillems, Thomas F.; Rycroft, Chris H.; Kazi, Michaeel; Meza, Juan C.; Haranczyk, MaciejMicroporous and Mesoporous Materials (2012), 149 (1), 134-141CODEN: MIMMFJ; ISSN:1387-1811. (Elsevier Inc.)Cryst. porous materials have a variety of uses, such as for catalysis and sepn. Identifying suitable materials for a given application can, in principle, be done by screening material databases. Such a screening requires automated high-throughput anal. tools that calc. structural properties for all materials contained in a database so they can be compared with search queries, grouped or classified. One important aspect of the structural anal. of materials such as zeolites and metal org. frameworks is the investigation of the geometrical parameters describing pores. Here, we present algorithms and tools to efficiently calc. some of these important parameters. Our tools are based on the Voronoi decompn., which for a given arrangement of atoms in a periodic domain provides a graph representation of the void space. The resulting Voronoi network is analyzed to obtain the diam. of the largest included sphere and the largest free sphere, which are two geometrical parameters that are frequently used to describe pore geometry. Accessibility of nodes in the network is also detd. for a given guest mol. and the resulting information is later used to retrieve dimensionality of channel systems as well as in Monte Carlo sampling of accessible surfaces and vols. The presented algorithms are implemented in a software tool, Zeo++, which includes a modified version of the Voro++ library. We present example applications of our algorithms and tools using zeolite frameworks currently listed in the Atlas of Zeolite Frameworks.
- 39Massidda, V.; Mirasso, C. R. Least-Energy Configuration in Orthorhombic Lattices of Dipoles. J. Magn. Magn. Mater. 1992, 116, 277– 281, DOI: 10.1016/0304-8853(92)90173-L[Crossref], [CAS], Google Scholar39https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK38XmsVChu78%253D&md5=11120882cb8d70b00cc702ecaecd113eLeast-energy configuration in orthorhombic lattices of dipolesMassidda, Vittorio; Mirasso, Claudio R.Journal of Magnetism and Magnetic Materials (1992), 116 (1-2), 277-81CODEN: JMMMDC; ISSN:0304-8853.The authors consider simple, body-centered and face-centered lattices of freely reorientable point dipoles. The configurations of least dipole-dipole energy are detd. for arbitrary values of the lattice consts. a, b, c. The results can be summarized as follows (the authors took a ≤ b ≤ c). For simple lattices the most stable configuration is always of the antiferro type, with the dipoles in the a direction. For body-centered lattices the most stable configuration is of the ferro type in the a or in the c direction, or antiferro in the a direction, depending on the a, b, c values. Analogously, for face-centered lattices it is of the ferro type in the a or b direction, or it takes 1 of 2 different antiferro arrangements in the b direction.
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Abstract
Figure 1
Figure 1. Investigated systems with zinc paddle-wheels as inorganic building blocks, which are connected in the pcu topology by phenyl moieties (blue) in the x- and y-direction and pillared in the z-direction by bispyridine units (red). Between the two pyridine units, different organic moieties (A–C) can be mounted to vary the dipole moment of the pillar linker.
Figure 2
Figure 2. Polarization Px as a function of the applied electric field Ex for all investigated systems at temperatures of 150 K (blue), 298 K (orange), and 450 K (red).
Figure 3
Figure 3. Accumulating variance s2 of the total polarization P in x- and y-directions for system B at 450 K for (a) E = 0 and (b) D = 0.
Figure 4
Figure 4. Static relative permittivity κx obtained from fixed-Ex and fixed-Dx simulations for system B at (a) 150 K, (c) 298 K, and (e) 450 K and system C at (b) 150 K, (d) 298 K, and (f) 450 K. To give an impression for the sampling error, ±2σ is plotted as statistical error.
Figure 5
Figure 5. Inverse of the electric susceptibility χ for systems B and C, obtained by the two fluctuation methods as a function of temperature T. The dashed lines show a linear regression curve to the data sets obtained for both systems.
Figure 6
Figure 6. Upper panel: 2D lattice of induced (aligned) dipoles. The gray box in the left figure indicates our choice of the unit cell, which is rotated by 90° in respect to the conventional one (size in respect to the conventional is 2 × 1 × 1). Lower panel: 2D lattice of permanent dipoles. The dipole–dipole interaction is stabilized undergoing the large pore–small pore phase transition.
Figure 7
Figure 7. Simulation of systems A–C at 298 K in the NσT ensemble at different electric field strengths Ex: (a) unit cell volume as a function of the applied electric field Ex; (b) polarization Px as a function of the applied electric field Ex.
References
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- 3Farha, O. K.; Hupp, J. T. Rational Design, Synthesis, Purification, and Activation of Metal-Organic Framework Materials. Acc. Chem. Res. 2010, 43, 1166– 1175, DOI: 10.1021/ar1000617[ACS Full Text], [CAS], Google Scholar3https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3cXosVyjtbY%253D&md5=be96ac34a74a551550ba08c66dcbd4b6Rational Design, Synthesis, Purification, and Activation of Metal-Organic Framework MaterialsFarha, Omar K.; Hupp, Joseph T.Accounts of Chemical Research (2010), 43 (8), 1166-1175CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)A review of recent advances in the synthetic design, purifn., and activation of metal-org. framework materials. The rational design of a series of org. struts to limit framework catenation and thereby produce large pores is described. In addn., the rapid sepn. of desired metal-org. frameworks from cryst. and amorphous contaminants cogenerated during synthesis based on their different densities is demonstrated. Finally, the mild and efficient activation of initially solvent-filled pores with supercrit. carbon dioxide, yielding usable channels and high internal surface areas is discussed. Now constituting thousands of distinct examples, metal-org. frameworks are an intriguing class of hybrid materials that exist as infinite cryst. lattices with inorg. vertices and mol.-scale org. connectors. Useful properties such as large internal surface areas, ultralow densities, and the availability of uniformly structured cavities and portals of mol. dimensions characterize functional metal-org. frameworks. Researchers have effectively exploited these unusual properties in applications such as hydrogen and methane storage, chem. sepns., and selective chem. catalysis. One of the most attractive features of metal-org. frameworks is the simplicity of their synthesis. Typically they are obtained via one-pot solvothermal prepns. However, metal-org. framework materials, esp. more complex ones, can be difficult to obtain in pure form and with the optimal degree of catenation, the interpenetration or interweaving of identical independent networks. Once these two issues are satisfied, the removal of the guest mols. (solvent from synthesis) without damaging the structural integrity of the material is often an addnl. challenge. We expect that the advances in the synthesis, sepn., and activation of metal-org. frameworks could lead to metal-org. frameworks with new structures and functions, better and faster sepn. and purifn. of these materials, and processing methods that avoid pore blockage and pore collapse.
- 4Furukawa, H.; Cordova, K. E.; O’Keeffe, M.; Yaghi, O. M. The Chemistry and Applications of Metal-Organic Frameworks. Science 2013, 341, 1230444, DOI: 10.1126/science.1230444[Crossref], [PubMed], [CAS], Google Scholar4https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A280%3ADC%252BC3sbjs1yrsw%253D%253D&md5=7eb2a10c62623ff8a27708ffe9561d45The chemistry and applications of metal-organic frameworksFurukawa Hiroyasu; Cordova Kyle E; O'Keeffe Michael; Yaghi Omar MScience (New York, N.Y.) (2013), 341 (6149), 1230444 ISSN:.Crystalline metal-organic frameworks (MOFs) are formed by reticular synthesis, which creates strong bonds between inorganic and organic units. Careful selection of MOF constituents can yield crystals of ultrahigh porosity and high thermal and chemical stability. These characteristics allow the interior of MOFs to be chemically altered for use in gas separation, gas storage, and catalysis, among other applications. The precision commonly exercised in their chemical modification and the ability to expand their metrics without changing the underlying topology have not been achieved with other solids. MOFs whose chemical composition and shape of building units can be multiply varied within a particular structure already exist and may lead to materials that offer a synergistic combination of properties.
- 5Dias, E. M.; Petit, C. Towards the Use of Metal–Organic Frameworks for Water Reuse: A Review of the Recent Advances in the Field of Organic Pollutants Removal and Degradation and the next Steps in the Field. J. Mater. Chem. A 2015, 3, 22484– 22506, DOI: 10.1039/C5TA05440K[Crossref], [CAS], Google Scholar5https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXhsFGqtr7O&md5=4087ab1abe6a006b03e8f22cbc5a104fTowards the use of metal-organic frameworks for water reuse: a review of the recent advances in the field of organic pollutants removal and degradation and the next steps in the fieldDias, Elton M.; Petit, CamilleJournal of Materials Chemistry A: Materials for Energy and Sustainability (2015), 3 (45), 22484-22506CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)A review. Water reuse is becoming increasingly important as more and more areas in the world are facing water stress issues. Treatment of wastewater to attain the purity required for various usages from culture irrigation to drinking water is therefore key. Several water treatment options are already in place and while we will continue to use them, parallel efforts are required to: (i) address the removal of the most persistent chems. in water and (ii) provide solns. for local communities. Recently, several studies on the use of metal-org. frameworks (MOFs) for the adsorption and photocatalytic degrdn. of orgs. in water have been reported. This enthusiasm originates from the large porosity and chem. tunability of MOFs - beneficial for adsorption - as well as their catalytic nature - beneficial for degrdn. The present review proposes a comprehensive and crit. anal. of the most recent studies on the use of MOFs for orgs. adsorption and photocatalytic degrdn. The potential to use MOFs to catalyze the prodn. of H2 from org. mols., like water contaminants, is also addressed. Overall, the discussion is organized based on the type of org. pollutants targeted and encompasses those released in industrial, domestic and agricultural wastewater streams.
- 6Kreno, L. E.; Leong, K.; Farha, O. K.; Allendorf, M.; Van Duyne, R. P.; Hupp, J. T. Metal–Organic Framework Materials as Chemical Sensors. Chem. Rev. 2012, 112, 1105– 1125, DOI: 10.1021/cr200324t[ACS Full Text], [CAS], Google Scholar6https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXhsVCgtL%252FI&md5=f6ed572725bc067ee8da81880bf1ec90Metal-Organic Framework Materials as Chemical SensorsKreno, Lauren E.; Leong, Kirsty; Farha, Omar K.; Allendorf, Mark; Van Duyne, Richard P.; Hupp, Joseph T.Chemical Reviews (Washington, DC, United States) (2012), 112 (2), 1105-1125CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)A review. The authors present a crit. review of the literature on metal-org. frameworks (MOFs) as chem. sensors. The authors begin by briefly examg. challenges relating to MOF sensor development including the design of MOFs with desirable properties, incorporation of appropriate signal transduction capabilities, and integration of MOFs into devices by employing thin-film growth techniques. Subsequent sections discuss specific examples of MOF sensors, categorized by method of signal transduction. Sensors based on MOF photoluminescence are discussed briefly. The authors have limited the review of luminescence-based sensors to a small no. of recent reports where the porous MOF architecture, or its chem. compn., imparts selective sensing capabilities. Scintillating MOFs that luminesce in the presence of radioactive analytes are also discussed. Other signal transduction schemes that use photons include various kinds of optical interferometry, analyte modulation of localized surface plasmon resonance energies, and solvatochromism. Mech. signal-transduction schemes employed with MOFs include ones based on surface acoustic wave, quartz crystal microbalance, and microcantilever devices. Elec. schemes thus far were limited to ones based on impedance spectroscopy.
- 7Lee, J.; Farha, O. K.; Roberts, J.; Scheidt, K. A.; Nguyen, S. T.; Hupp, J. T. Metal–Organic Framework Materials as Catalysts. Chem. Soc. Rev. 2009, 38, 1450– 1459, DOI: 10.1039/b807080f[Crossref], [PubMed], [CAS], Google Scholar7https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1MXkvVamu7k%253D&md5=d9cef4eb52ffb1c73fe1195e11bace26Metal-organic framework materials as catalystsLee, Jeong Yong; Farha, Omar K.; Roberts, John; Scheidt, Karl A.; Nguyen, Son Binh T.; Hupp, Joseph T.Chemical Society Reviews (2009), 38 (5), 1450-1459CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)A review; a crit. review of the emerging field of MOF-based catalysis is presented. Discussed are examples of: (a) opportunistic catalysis with metal nodes, (b) designed catalysis with framework nodes, (c) catalysis by homogeneous catalysts incorporated as framework struts, (d) catalysis by MOF-encapsulated mol. species, (e) catalysis by metal-free org. struts or cavity modifiers, and (f) catalysis by MOF-encapsulated clusters.
- 8Horcajada, P. Porous Metal–Organic-Framework Nanoscale Carriers as a Potential Platform for Drug Delivery and Imaging. Nat. Mater. 2010, 9, 172– 178, DOI: 10.1038/nmat2608[Crossref], [PubMed], [CAS], Google Scholar8https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3cXhtVOnt70%253D&md5=74f3e60d481f39e0000fd93db0e40a64Porous metal-organic-framework nanoscale carriers as a potential platform for drug delivery and imagingHorcajada, Patricia; Chalati, Tamim; Serre, Christian; Gillet, Brigitte; Sebrie, Catherine; Baati, Tarek; Eubank, Jarrod F.; Heurtaux, Daniela; Clayette, Pascal; Kreuz, Christine; Chang, Jong-San; Hwang, Young Kyu; Marsaud, Veronique; Bories, Phuong-Nhi; Cynober, Luc; Gil, Sophie; Ferey, Gerard; Couvreur, Patrick; Gref, RuxandraNature Materials (2010), 9 (2), 172-178CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)In the domain of health, one important challenge is the efficient delivery of drugs in the body using non-toxic nanocarriers. Most of the existing carrier materials show poor drug loading (usually less than 5 wt% of the transported drug vs. the carrier material) and/or rapid release of the proportion of the drug that is simply adsorbed (or anchored) at the external surface of the nanocarrier. In this context, porous hybrid solids, with the ability to tune their structures and porosities for better drug interactions and high loadings, are well suited to serve as nanocarriers for delivery and imaging applications. Here we show that specific non-toxic porous iron(III)-based metal-org. frameworks with engineered cores and surfaces, as well as imaging properties, function as superior nanocarriers for efficient controlled delivery of challenging antitumoral and retroviral drugs (i.e., busulfan, azidothymidine triphosphate, doxorubicin or cidofovir) against cancer and AIDS. In addn. to their high loadings, they also potentially assoc. therapeutics and diagnostics, thus opening the way for theranostics, or personalized patient treatments.
- 9Coudert, F.-X. Responsive Metal–Organic Frameworks and Framework Materials: Under Pressure, Taking the Heat, in the Spotlight, with Friends. Chem. Mater. 2015, 27, 1905– 1916, DOI: 10.1021/acs.chemmater.5b00046[ACS Full Text], [CAS], Google Scholar9https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXisF2hurw%253D&md5=4c0a4fb50b0c9eefcfd9e1dbc3fca990Responsive Metal-Organic Frameworks and Framework Materials: Under Pressure, Taking the Heat, in the Spotlight, with FriendsCoudert, Francois-XavierChemistry of Materials (2015), 27 (6), 1905-1916CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)A review. Recent years have seen a large increase of the research effort focused on framework materials, including the nowadays-ubiquitous metal-org. frameworks but also dense coordination polymers, covalent org. frameworks, and mol. frameworks. With the quickly increasing no. of structures synthesized and characterized, one pattern emerging is the common occurrence of flexibility. More specifically, an important no. of framework materials are stimuli-responsive: their structure can undergo changes of large amplitude in response to phys. or chem. stimulation. They can display transformations induced by temp., mech. pressure, guest adsorption or evacuation, light absorption, etc. and are sometimes referred to as smart materials, soft crystals, or dynamic materials. This Perspective highlights recent progress in this field, showcasing some of the most novel and unusual responses to stimuli, as well as advances in the fundamental understanding of flexible framework materials.
- 10Férey, G.; Serre, C. Large Breathing Effects in Three-Dimensional Porous Hybrid Matter: Facts, Analyses, Rules and Consequences. Chem. Soc. Rev. 2009, 38, 1380– 1399, DOI: 10.1039/b804302g[Crossref], [PubMed], [CAS], Google Scholar10https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1MXkvVamu70%253D&md5=a620fc681f135bdce4044f16b18c60b1Large breathing effects in three-dimensional porous hybrid matter: facts, analyses, rules and consequencesFerey, Gerard; Serre, ChristianChemical Society Reviews (2009), 38 (5), 1380-1399CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)A review. This crit. review focuses on a strange behavior of crystd. solid matter: its reversible swelling with large magnitude. This will be of interest for experts in porous solids but also for solid state chemists and physicists. Some examples, classified according to the dimensionality of the inorg. subnetwork, present the general requirements and the structural rules which govern the existence of this phenomenon. Its consequences concern specific applications related to sensors, energy savings, sustainable development and health (100 refs.).
- 11Schneemann, A.; Bon, V.; Schwedler, I.; Senkovska, I.; Kaskel, S.; Fischer, R. A. Flexible Metal–Organic Frameworks. Chem. Soc. Rev. 2014, 43, 6062, DOI: 10.1039/C4CS00101J[Crossref], [PubMed], [CAS], Google Scholar11https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXht1Slu7nI&md5=b824d2bff6379c9662b50e86739b42f7Flexible metal-organic frameworksSchneemann, A.; Bon, V.; Schwedler, I.; Senkovska, I.; Kaskel, S.; Fischer, R. A.Chemical Society Reviews (2014), 43 (16), 6062-6096CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)A review. Advances in flexible and functional metal-org. frameworks (MOFs), also called soft porous crystals, are reviewed by covering the literature of the five years period 2009-2013 with ref. to the early pertinent work since the late 1990s. Flexible MOFs combine the cryst. order of the underlying coordination network with cooperative structural transformability. These materials can respond to phys. and chem. stimuli of various kinds in a tunable fashion by mol. design, which does not exist for other known solid-state materials. Among the fascinating properties are so-called breathing and swelling phenomena as a function of host-guest interactions. Phase transitions are triggered by guest adsorption/desorption, photochem., thermal, and mech. stimuli. Other important flexible properties of MOFs, such as linker rotation and sub-net sliding, which are not necessarily accompanied by crystallog. phase transitions, are briefly mentioned as well. Emphasis is given on reviewing the recent progress in application of in situ characterization techniques and the results of theor. approaches to characterize and understand the breathing mechanisms and phase transitions. The flexible MOF systems, which are discussed, are categorized by the type of metal-nodes involved and how their coordination chem. with the linker mols. controls the framework dynamics. Aspects of tailoring the flexible and responsive properties by the mixed component solid-soln. concept are included, and as well examples of possible applications of flexible metal-org. frameworks for sepn., catalysis, sensing, and biomedicine.
- 12Ghoufi, A.; Benhamed, K.; Boukli-Hacene, L.; Maurin, G. Electrically Induced Breathing of the MIL-53(Cr) Metal–Organic Framework. ACS Cent. Sci. 2017, 3, 394– 398, DOI: 10.1021/acscentsci.6b00392[ACS Full Text], [CAS], Google Scholar12https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXls1Wmsb4%253D&md5=220c934e3db4391ff037ec15beddc416Electrically Induced Breathing of the MIL-53(Cr) Metal-Organic FrameworkGhoufi, Aziz; Benhamed, Karima; Boukli-Hacene, Leila; Maurin, GuillaumeACS Central Science (2017), 3 (5), 394-398CODEN: ACSCII; ISSN:2374-7951. (American Chemical Society)The breathing behavior of the MIL-53(Cr) metal-org. framework (MOF) was explored previously upon guest-adsorption and thermal and mech. stimuli. Here, advanced mol. simulations based on the use of an accurate force field to describe the flexibility of this porous framework demonstrate that the application of an elec. field induces the structural switching of this MOF leading to a 1st-order transition and a vol. change of >40%. This motivated the authors to elec. tune the pore size of MIL-53(Cr) with the idea to propose a new concept to selectively capture CO2 over CH4 via a mol. sieving that paves the way toward the optimization of current sepn.-based processes.
- 13Schmid, R. An Electric Field Induced Breath for Metal–Organic Frameworks. ACS Cent. Sci. 2017, 3, 369– 371, DOI: 10.1021/acscentsci.7b00162[ACS Full Text], [CAS], Google Scholar13https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXmtlaqsrw%253D&md5=944583213e7ba2ef79ee22887927d95aAn Electric Field Induced Breath for Metal-Organic FrameworksSchmid, RochusACS Central Science (2017), 3 (5), 369-371CODEN: ACSCII; ISSN:2374-7951. (American Chemical Society)There is no expanded citation for this reference.
- 14Kolesnikov, A. L.; Budkov, Y. A.; Möllmer, J.; Kiselev, M. G.; Gläser, R. Metal-Organic Framework Breathing in Electric Field: A Theoretical Study. J. Phys. Chem. C 2019, 123, 10333– 10338, DOI: 10.1021/acs.jpcc.8b11630[ACS Full Text], [CAS], Google Scholar14https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXmtV2jt7Y%253D&md5=9e9e7ea63bfa8ee86339473668f6f800Metal-Organic Framework Breathing in the Electric Field: A Theoretical StudyKolesnikov, Andrei L.; Budkov, Yury A.; Moellmer, Jens; Kiselev, Michael G.; Glaeser, RogerJournal of Physical Chemistry C (2019), 123 (16), 10333-10338CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)In this manuscript, the authors study the elec. induced breathing of a metal-org. framework (MOF) within a 2-dimensional lattice model. The Helmholtz free energy of the MOF in the elec. field consists of 2 parts: the electrostatic energy of the dielec. body in the external elec. field and the elastic energy of the framework. The 1st contribution is calcd. from the 1st principles of statistical mechanics with an account of MOF symmetry. By minimizing the obtained free energy and solving the resulting system of equations, the authors obtain the local elec. field and the parameter of the unit cell (angle α). The paper also studies the cross-sectional area of the unit cell and the polarization as functions of the external elec. field. The authors obtain the hysteresis in the region of the structural transition of the framework. Results are in qual. agreement with the literature data of the mol. dynamics simulation of MIL-53(Cr).
- 15Knebel, A.; Geppert, B.; Volgmann, K.; Kolokolov, D. I.; Stepanov, A. G.; Twiefel, J.; Heitjans, P.; Volkmer, D.; Caro, J. Defibrillation of Soft Porous Metal-Organic Frameworks with Electric Fields. Science 2017, 358, 347– 351, DOI: 10.1126/science.aal2456[Crossref], [PubMed], [CAS], Google Scholar15https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXhs1Kns7%252FE&md5=65943f86316595f45313607e789d3d52Defibrillation of soft porous metal-organic frameworks with electric fieldsKnebel, A.; Geppert, B.; Volgmann, K.; Kolokolov, D. I.; Stepanov, A. G.; Twiefel, J.; Heitjans, P.; Volkmer, D.; Caro, J.Science (Washington, DC, United States) (2017), 358 (6361), 347-351CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)Gas transport through metal-org. framework membranes (MOFs) was switched in situ by applying an external elec. field (E-field). The switching of gas permeation upon E-field polarization could be explained by the structural transformation of the zeolitic imidazolate framework ZIF-8 into polymorphs with more rigid lattices. Permeation measurements under a direct-current E-field poling of 500 V per mm showed reversibly controlled switching of the ZIF-8 into polar polymorphs, which was confirmed by x-ray diffraction and ab initio calcns. The stiffening of the lattice causes a redn. in gas transport through the membrane and sharpens the mol. sieving capability. Dielec. spectroscopy, polarization, and deuterium NMR studies revealed low-frequency resonances of ZIF-8 that we attribute to lattice flexibility and linker movement. Upon E-field polarization, we obsd. a defibrillation of the different lattice motions.
- 16Gonzalez-Nelson, A.; Coudert, F.-X.; van der Veen, M. A. Rotational Dynamics of Linkers in Metal–Organic Frameworks. Nanomaterials 2019, 9, 330, DOI: 10.3390/nano9030330[Crossref], [PubMed], [CAS], Google Scholar16https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXhtFWrs7bO&md5=ff79c23c592c32d12d1589028651b842Rotational dynamics of linkers in metal-organic frameworksGonzalez-Nelson, Adrian; Coudert, Francois-Xavier; Van Der Veen, Monique A.Nanomaterials (2019), 9 (3), 330CODEN: NANOKO; ISSN:2079-4991. (MDPI AG)A review. Among the numerous fascinating properties of metal-org. frameworks (MOFs), their rotational dynamics is perhaps one of the most intriguing, with clear consequences for adsorption and sepn. of mols., as well as for optical and mech. properties. A closer look at the rotational mobility in MOF linkers reveals that it is not only a considerably widespread phenomenon, but also a fairly diverse one. Still, the impact of these dynamics is often understated. In this review, we address the various mechanisms of linker rotation reported in the growing collection of literature, followed by a highlight of the methods currently used in their study, and we conclude with the impacts that such dynamics have on existing and future applications.
- 17Vukotic, V. N.; Harris, K. J.; Zhu, K.; Schurko, R. W.; Loeb, S. J. Metal–Organic Frameworks with Dynamic Interlocked Components. Nat. Chem. 2012, 4, 456– 460, DOI: 10.1038/nchem.1354[Crossref], [PubMed], [CAS], Google Scholar17https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XmvFeqs7s%253D&md5=11793f062005687c50583f59d9f35372Metal-organic frameworks with dynamic interlocked componentsVukotic, V. Nicholas; Harris, Kristopher J.; Zhu, Kelong; Schurko, Robert W.; Loeb, Stephen J.Nature Chemistry (2012), 4 (6), 456-460CODEN: NCAHBB; ISSN:1755-4330. (Nature Publishing Group)The dynamics of mech. interlocked mols. such as rotaxanes and catenanes were studied in soln. as examples of rudimentary mol. switches and machines, but in this medium, the mols. are randomly dispersed and their motion incoherent. As a strategy for achieving a higher level of mol. organization, the authors have constructed a metal-org. framework material [Cu2(L)(H2O)2]·3H2O using a [2]rotaxane (H4L) (composed of a 3,5-dicarboxybenzyl-3,5-dicarboxyaniline axle with [24]crown-6-ether macrocyclic wheel) as the org. linker and binuclear carboxylate bridged Cu(II) units as the nodes. Activation of the as-synthesized material creates a void space inside the rigid framework that allows the soft crown ether macrocyclic ring of the [2]rotaxane to rotate rapidly, unimpeded by neighboring mol. components. Variable-temp. 13C and 2H solid-state NMR expts. were used to characterize the nature and rate of the dynamic processes occurring inside this unique material. These results provide a blueprint for the future creation of solid-state mol. switches and mol. machines based on mech. interlocked mols.
- 18Danowski, W.; van Leeuwen, T.; Abdolahzadeh, S.; Roke, D.; Browne, W. R.; Wezenberg, S. J.; Feringa, B. L. Unidirectional Rotary Motion in a Metal–Organic Framework. Nat. Nanotechnol. 2019, 14, 488– 494, DOI: 10.1038/s41565-019-0401-6[Crossref], [PubMed], [CAS], Google Scholar18https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXms1Ogtrk%253D&md5=861d9bbbd1eafb32a0004a698d9d741aUnidirectional rotary motion in a metal-organic frameworkDanowski, Wojciech; van Leeuwen, Thomas; Abdolahzadeh, Shaghayegh; Roke, Diederik; Browne, Wesley R.; Wezenberg, Sander J.; Feringa, Ben L.Nature Nanotechnology (2019), 14 (5), 488-494CODEN: NNAABX; ISSN:1748-3387. (Nature Research)Overcrowded alkene-based light-driven mol. motors are able to perform large-amplitude repetitive unidirectional rotations. Their behavior is well understood in soln. However, Brownian motion precludes the precise positioning at the nanoscale needed to harness cooperative action. Here, authors demonstrate mol. motors organized in cryst. metal-org. frameworks (MOFs). The motor unit becomes a part of the org. linker (or strut), and its spatial arrangement is elucidated through powder and single-crystal x-ray analyses and polarized optical and Raman microscopies. They confirm that the light-driven unidirectional rotation of the motor units is retained in the MOF framework and that the motors can operate in the solid state with similar rotary speed (rate of thermal helix inversion) to that in soln. These 'moto-MOFs' could in the future be used to control dynamic function in cryst. materials.
- 19Tam, B.; Yazaydin, O. Design of Electric Field Controlled Molecular Gates Mounted on Metal–Organic Frameworks. J. Mater. Chem. A 2017, 5, 8690– 8696, DOI: 10.1039/C7TA00101K[Crossref], [CAS], Google Scholar19https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXmtFGnt7g%253D&md5=29147fdd502b12161350e9f89743280eDesign of electric field controlled molecular gates mounted on metal-organic frameworksTam, Benjamin; Yazaydin, OzgurJournal of Materials Chemistry A: Materials for Energy and Sustainability (2017), 5 (18), 8690-8696CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)We propose and computationally demonstrate the concept of elec. field controlled mol. gates mounted on the open-metal coordination sites in metal-org. framework (MOF) materials. The MOF-mol. gate complex functions by opening and closing under the effect of an elec. field. Our design involves Mg-MOF-74, a MOF with hexagonal channels with open-metal coordination sites at each corner, and a multifunctional gate mol. with permanent dipole which anchors itself on the host MOF material and responds to changes in the direction of an elec. field by rotating around its backbone which acts as an axle. By carrying out d. functional theory (DFT) calcns. and mol. dynamics (MD) simulations we show that the MOF-mol. gate complex can be switched between two stable configurations, open and closed, by turning on and off an external elec. field. We further show that the mol. gate can be controlled to block or allow the diffusion of methane mols. through the channels of the MOF like a nanoscale butterfly valve. Elec. field controlled mol. gates mounted on MOFs can pave the way for new mol. machines and nanodevices which can store, deliver or select mols. on demand and with at. precision.
- 20Namsani, S.; Yazaydin, O. Electric Field Induced Rotation of Halogenated Organic Linkers in Isoreticular Metal-Organic Frameworks for Nanofluidic Applications. Mol. Syst. Des. Eng. 2018, 3, 951– 958, DOI: 10.1039/C8ME00030A[Crossref], [CAS], Google Scholar20https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXitVaqur3M&md5=729e2a2ec60e4974afc98201d72d67f7Electric field induced rotation of halogenated organic linkers in isoreticular metal-organic frameworks for nanofluidic applicationsNamsani, Sadanandam; Yazaydin, A. OzgurMolecular Systems Design & Engineering (2018), 3 (6), 951-958CODEN: MSDEBG; ISSN:2058-9689. (Royal Society of Chemistry)We present a systematic computational study which provides a plausible route to control the rotation of org. linkers in isoreticular metal-org. frameworks (IRMOF) by using an external elec. field in order to manipulate the diffusion of mols. in nanopores. We achieve this by halogenating the org. linkers of IRMOF-1 and IRMOF-7 to create permanent dipole moments on the linkers, hence making them responsive to changes in the strength and direction of an elec. field. More importantly we show that by varying the ligand size and the halogen type, no. and substitution positions, the strength of the elec. field required to control the rotation of linkers can be reduced significantly. Cl substitution is most effective in making the org. linkers elec. field responsive since a greater dipole moment is created compared to those obtained by F or Br substitution. Cl substitution of a larger org. linker, i.e. 1,4-naphthalenedicarboxylate (IRMOF-7) rather than 1,4-benzenedicarboxylate (IRMOF-1), results in a greater dipole moment and reduces the elec. field strength required for the rotation of the ligand. Furthermore, double Cl substitution and the optimization of the Cl substitution positions enable controlled rotation of the IRMOF-7 linkers with an elec. field strength as low as 0.5 V nm-1. Finally, using the elec. field induced rotation of org. linkers we show that it is possible to enhance the diffusion of methane mols. in a chosen direction while limiting their mobility in other directions. Our study hints at the potential of using MOFs for flow control in nanofluidic systems.
- 21Sun, L.; Campbell, M. G.; Dincă, M. Electrically Conductive Porous Metal–Organic Frameworks. Angew. Chem., Int. Ed. 2016, 55, 3566– 3579, DOI: 10.1002/anie.201506219[Crossref], [CAS], Google Scholar21https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XmslGisw%253D%253D&md5=185401e793b619c797286fe8df7c641eElectrically Conductive Porous Metal-Organic FrameworksSun, Lei; Campbell, Michael G.; Dinca, MirceaAngewandte Chemie, International Edition (2016), 55 (11), 3566-3579CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)A review. Owing to their outstanding structural, chem., and functional diversity, metal-org. frameworks (MOFs) have attracted considerable attention over the last two decades in a variety of energy-related applications. Notably missing among these, until recently, were applications that required good charge transport coexisting with porosity and high surface area. Although most MOFs are elec. insulators, several materials in this class have recently demonstrated excellent elec. cond. and high charge mobility. Herein, the authors review the synthetic and electronic design strategies that have been employed thus far for producing frameworks with permanent porosity and long-range charge transport properties. In addn., key expts. that have been employed to demonstrate elec. transport, as well as selected applications for this subclass of MOFs, are discussed.
- 22Zagorodniy, K.; Seifert, G.; Hermann, H. Metal-Organic Frameworks as Promising Candidates for Future Ultralow-k Dielectrics. Appl. Phys. Lett. 2010, 97, 251905, DOI: 10.1063/1.3529461[Crossref], [CAS], Google Scholar22https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3cXhsF2rt7nE&md5=3b75fad9d5a4e0723082c4dd11fbb093Metal-organic frameworks as promising candidates for future ultralow-k dielectricsZagorodniy, K.; Seifert, G.; Hermann, H.Applied Physics Letters (2010), 97 (25), 251905/1-251905/2CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)A class of Zn4O(CO2)6-based metal-org. frameworks (MOFs) is theor. analyzed with respect to suitability as an interlayer dielec. material for applications in semiconducting devices. The static dielec. const. is calcd. using the Clausius-Mossotti approach. For 3 out of ∼30 of the considered MOFs excellent combinations of ultralow dielec. const., elastic bulk modulus, and gap energy are found favoring these materials as outstanding candidates for future ultralow-k dielec. materials. (c) 2010 American Institute of Physics.
- 23Warmbier, R.; Quandt, A.; Seifert, G. Dielectric Properties of Selected Metal–Organic Frameworks. J. Phys. Chem. C 2014, 118, 11799– 11805, DOI: 10.1021/jp5029646[ACS Full Text], [CAS], Google Scholar23https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXnvV2jsb4%253D&md5=01301ccd5a3872a2f73fe74e2e721ff7Dielectric Properties of Selected Metal-Organic FrameworksWarmbier, Robert; Quandt, Alexander; Seifert, GotthardJournal of Physical Chemistry C (2014), 118 (22), 11799-11805CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)The electronic structure of a class of [Zn4O(CO2)6] based metal-org. frameworks (MOFs) is theor. analyzed by d. functional perturbation theory. The calcd. static dielec. consts. vary at 1.33-1.54, characterizing the structures as ultralow-k dielec. materials and confirming earlier ests. qual. The authors also present the results of 1st-principle calcns. of the real and imaginary parts of the dielec. function and give the frequency-dependent dielec. const. up to the near-UV, which is important for high frequency semiconductor and optical applications of MOFs. The dielec. and electronic properties are governed by the linker mols., so that the band gap and the dielec. const. can be engineered.
- 24Redel, E.; Wang, Z.; Walheim, S.; Liu, J.; Gliemann, H.; Wöll, C. On the Dielectric and Optical Properties of Surface-Anchored Metal-Organic Frameworks: A Study on Epitaxially Grown Thin Films. Appl. Phys. Lett. 2013, 103, 091903, DOI: 10.1063/1.4819836[Crossref], [CAS], Google Scholar24https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXhtlKrt7rF&md5=f7fa09852ee8a75548f833c5f14fff44On the dielectric and optical properties of surface-anchored metal-organic frameworks: A study on epitaxially grown thin filmsRedel, Engelbert; Wang, Zhengbang; Walheim, Stefan; Liu, Jinxuan; Gliemann, Hartmut; Woell, ChristofApplied Physics Letters (2013), 103 (9), 091903/1-091903/5CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)We det. the optical consts. of two highly porous, cryst. metal-org. frameworks (MOFs). Since it is problematic to det. the optical consts. for the std. powder modification of these porous solids, we instead use surface-anchored metal-org. frameworks (SURMOFs). These MOF thin films are grown using liq. phase epitaxy (LPE) on modified silicon substrates. The produced SURMOF thin films exhibit good optical properties; these porous coatings are smooth as well as crack-free, they do not scatter visible light, and they have a homogeneous interference color over the entire sample. Therefore, spectroscopic ellipsometry (SE) can be used in a straightforward fashion to det. the corresponding SURMOF optical properties. After careful removal of the solvent mols. used in the fabrication process as well as the residual water adsorbed in the voids of this highly porous solid, we det. an optical const. of n = 1.39 at a wavelength of 750 nm for HKUST-1 (stands for Hong Kong University of Science and Technol.-1; and was first discovered there) or Cu3(BTC)2. After exposing these SURMOF thin films to moisture/EtOH atm., the refractive index (n) increases to n = 1.55-1.6. This dependence of the optical properties on water/EtOH adsorption demonstrates the potential of such SURMOF materials for optical sensing. (c) 2013 American Institute of Physics.
- 25Van Cleuvenbergen, S.; Stassen, I.; Gobechiya, E.; Zhang, Y.; Markey, K.; De Vos, D. E.; Kirschhock, C.; Champagne, B.; Verbiest, T.; van der Veen, M. A. ZIF-8 as Nonlinear Optical Material: Influence of Structure and Synthesis. Chem. Mater. 2016, 28, 3203– 3209, DOI: 10.1021/acs.chemmater.6b01087[ACS Full Text], [CAS], Google Scholar25https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XlvV2isrY%253D&md5=8bd97fb5bfa0c308f1614ac8be6f7320ZIF-8 as Nonlinear Optical Material: Influence of Structure and SynthesisVan Cleuvenbergen, Stijn; Stassen, Ivo; Gobechiya, Elena; Zhang, Yuexing; Markey, Karen; De Vos, Dirk E.; Kirschhock, Christine; Champagne, Benoit; Verbiest, Thierry; van der Veen, Monique A.Chemistry of Materials (2016), 28 (9), 3203-3209CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)Metalorg. framework ZIF-8, from the zeolitic imidazolate framework family, shows a large intrinsic 2nd-order nonlinear optical (NLO) response. ZIF-8 is a stable, inexpensive material that is transparent in the visible (visible) and near-IR (NIR) window. This is crucial for NLO applications. The 2nd-order NLO activity is due to the noncentrosym. octupolar symmetry of the material. Fast syntheses lead to a lower 2nd-order NLO response. Consistent with polarized 2nd-harmonic generation (SHG) microscopy measurements, this is ascribed 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 av. 2nd-order NLO coeff. 〈deff〉 of 0.25 pm/V, which is explained and supported by ab initio theor. calcns.
- 26Eslava, S.; Zhang, L.; Esconjauregui, S.; Yang, J.; Vanstreels, K.; Baklanov, M. R.; Saiz, E. Metal-Organic Framework ZIF-8 Films As Low-κ Dielectrics in Microelectronics. Chem. Mater. 2013, 25, 27– 33, DOI: 10.1021/cm302610z[ACS Full Text], [CAS], Google Scholar26https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XhslOlsr7E&md5=e60e478be39f045ee8b13761f2bdfebbMetal-Organic Framework ZIF-8 Films As Low-κ Dielectrics in MicroelectronicsEslava, Salvador; Zhang, Liping; Esconjauregui, Santiago; Yang, Junwei; Vanstreels, Kris; Baklanov, Mikhail R.; Saiz, EduardoChemistry of Materials (2013), 25 (1), 27-33CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)ZIF-8 films were deposited on silicon wafers and characterized to assess their potential as future insulators (low-κ dielecs.) in microelectronics. SEM and gas adsorption monitored by spectroscopic ellipsometry confirmed the good coalescence of the crystals, the absence of intergranular voids, and the hydrophobicity of the pores. Mech. properties were assessed by nanoindentation and tape tests, confirming sufficient rigidity for chip manufg. processes (elastic modulus >3 GPa) and the good adhesion to the support. The dielec. const. was measured by impedance anal. at different frequencies and temps., indicating that κ was only 2.33 (±0.05) at 100 kHz, a result of low polarizability and d. in the films. Intensity voltage curves showed that the leakage current was only 10-8 A cm2 at 1 MV cm-1, and the breakdown voltage was >2 MV cm-1. In conclusion, metal-org. framework ZIF-8 films were exptl. found to be promising candidates as low-κ dielecs. in microelectronic chip devices. This opens a new direction for research into the application of metal-org. frameworks.
- 27Ryder, M. R.; Zeng, Z.; Titov, K.; Sun, Y.; Mahdi, E. M.; Flyagina, I.; Bennett, T. D.; Civalleri, B.; Kelley, C. S.; Frogley, M. D.; Cinque, G.; Tan, J.-C. Dielectric Properties of Zeolitic Imidazolate Frameworks in the Broad-Band Infrared Regime. J. Phys. Chem. Lett. 2018, 9, 2678– 2684, DOI: 10.1021/acs.jpclett.8b00799[ACS Full Text], [CAS], Google Scholar27https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXovVOisbs%253D&md5=fb72a8cfeb08163818b11ea0984249efDielectric Properties of Zeolitic Imidazolate Frameworks in the Broad-Band Infrared RegimeRyder, Matthew R.; Zeng, Zhixin; Titov, Kirill; Sun, Yueting; Mahdi, E. M.; Flyagina, Irina; Bennett, Thomas D.; Civalleri, Bartolomeo; Kelley, Chris S.; Frogley, Mark D.; Cinque, Gianfelice; Tan, Jin-ChongJournal of Physical Chemistry Letters (2018), 9 (10), 2678-2684CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)The field of metal-org. framework (MOF) materials is rapidly advancing toward practical applications; consequently, it is urgent to achieve a better understanding and precise control of their phys. properties. Yet, research on the dielec. properties of MOFs is at its infancy, where studies are confined to the static dielec. behavior or lower-frequency response (kHz-MHz) only. Herein, the authors present the pioneering use of synchrotron-based IR reflectivity expts. combined with d. functional theory (DFT) calcns. to accurately det. the dynamic dielec. properties of zeolitic imidazolate frameworks (ZIFs, a topical family of MOFs). The authors show, for the 1st time, the frequency-dependent dielec. response of representative ZIF compds., bridging the near-, mid-, and far-IR (terahertz, THz) broad-band frequencies. The authors establish the structure-property relations as a function of framework porosity and structural change. Comprehensive results will pave the way for novel ZIF-based terahertz applications, such as IR optical sensors and high-speed wireless communications.
- 28Ryder, M. R.; Donà, L.; Vitillo, J. G.; Civalleri, B. Understanding and Controlling the Dielectric Response of Metal–Organic Frameworks. ChemPlusChem 2018, 83, 308– 316, DOI: 10.1002/cplu.201700558[Crossref], [PubMed], [CAS], Google Scholar28https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXlt1Ontro%253D&md5=3c01a0a7182d6892712e608730e1f650Understanding and Controlling the Dielectric Response of Metal-Organic FrameworksRyder, Matthew R.; Dona, Lorenzo; Vitillo, Jenny G.; Civalleri, BartolomeoChemPlusChem (2018), 83 (4), 308-316CODEN: CHEMM5; ISSN:2192-6506. (Wiley-VCH Verlag GmbH & Co. KGaA)Metal-org. framework (MOF) materials have recently been shown to have promising electronic and dielec. properties. This study involves investigating a diverse range of MOFs to rationalise how the different building blocks that form the structure can affect the electronic properties and dielec. response. The results confirm that the band gap is primarily due to the electronic levels of the org. linkers and that tuning the band gap can be easily achieved either by linker functionalisation or by increasing the aromaticity. The relevance of simple structure-property relationships for different families of isoreticular MOFs through the use of Hammett sigma consts. is also highlighted. It is also shown that the polarisability of the framework can be tuned comparably to the band gap. However, the expected low static dielec. const. is less influenced by the compn. of the MOF and can be modified by acting on the crystal structure. Indeed, it is shown that it can be directly linked to the framework porosity. The anal., based on quantum mech. calcns., includes the contribution from the metals involved, the org. linkers and the symmetry and topol.of the framework and makes suggestions for future work on low-κ dielec. MOFs.
- 29Bétard, A.; Fischer, R. A. Metal–Organic Framework Thin Films: From Fundamentals to Applications. Chem. Rev. 2012, 112, 1055– 1083, DOI: 10.1021/cr200167v[ACS Full Text], [CAS], Google Scholar29https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXht1Slt7bF&md5=e166d873cc42ed503be528d261270effMetal-Organic Framework Thin Films: From Fundamentals to ApplicationsBetard, Angelique; Fischer, Roland A.Chemical Reviews (Washington, DC, United States) (2012), 112 (2), 1055-1083CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)A review. The field of metal-org. frameworks (MOFs), which are often also called porous coordination polymers (PCPs), has been growing tremendously over the last 15 years. This fascinating class of cryst. hybrid materials, which are formed by assocn. of metal centers or clusters and org. linker(s), offers a unique chem. versatility combined with a designable framework and an unprecedentedly large and permanent inner porosity. In this review, MOF thin films will be examd. with respect to three main themes. Fabrication methods are important and will be described in the first section. Characterization, which ranges from compositional and structural identification to lateral homogeneity and porosity, is an important issue that will be discussed together with various strategies aimed at improving the quality and applicability of films. Finally, potential applications of MOF films in several fields will be reviewed.
- 30Zhang, C.; Sprik, M. Computing the Dielectric Constant of Liquid Water at Constant Dielectric Displacement. Phys. Rev. B: Condens. Matter Mater. Phys. 2016, 93, 144201, DOI: 10.1103/PhysRevB.93.144201[Crossref], [CAS], Google Scholar30https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XhsFGlsb%252FO&md5=b40de5cd71861e2310dd5c39e1fb6cd5Computing the dielectric constant of liquid water at constant dielectric displacementZhang, Chao; Sprik, MichielPhysical Review B (2016), 93 (14), 144201/1-144201/13CODEN: PRBHB7; ISSN:2469-9950. (American Physical Society)The static dielec. const. of liq. water is computed using classical force field based mol. dynamics simulation at fixed elec. displacement D. The method to constrain the elec. displacement is the finite- temp. classical variant of the const. D method developed by Stengel. Spaldin, and Vanderbilt [Nat. Phys. 5, 304 (2009)]. There is also a modification of this scheme imposing fixed values of the macroscopic field E. The method is applied to the popular SPC/E model of liq. water. We compare four different ests. of the dielec. const., two obtained from fluctuations of the polarization at D = 0 and E = 0 and two from the variation of polarization with finite D and E. It is found that all four ests. agree when properly converged. The computational effort to achieve convergence varies, however, with const. D calcns. being substantially more efficient. We attribute this difference to the much shorter relaxation time of longitudinal polarization compared to transverse polarization accelerating const. D calcns.
- 31Stengel, M.; Spaldin, N. A.; Vanderbilt, D. Electric Displacement as the Fundamental Variable in Electronic-Structure Calculations. Nat. Phys. 2009, 5, 304– 308, DOI: 10.1038/nphys1185[Crossref], [CAS], Google Scholar31https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1MXktVKjurY%253D&md5=bbd45d002ef3292bc6b4f44211ccb5b3Electric displacement as the fundamental variable in electronic-structure calculationsStengel, Massimiliano; Spaldin, Nicola A.; Vanderbilt, DavidNature Physics (2009), 5 (4), 304-308CODEN: NPAHAX; ISSN:1745-2473. (Nature Publishing Group)Finite-field calcns. in periodic insulators are tech. and conceptually challenging, owing to fundamental problems in defining polarization in extended solids. Although significant progress has been made recently with the establishment of techniques to fix the elec. field E or the macroscopic polarization P in first-principles calcns., both methods lack the ease of use and conceptual clarity of std. zero-field calcns. Here we develop a new formalism, in which the elec. displacement D, rather than E or P, is the fundamental elec. variable. Fixing D has the intuitive interpretation of imposing open-circuit elec. boundary conditions, which is particularly useful in studying ferroelec. systems. Furthermore, the analogy to open-circuit capacitors suggests an appealing reformulation in terms of free charges and potentials, which dramatically simplifies the treatment of stresses and strains. Using PbTiO3 as an example, we show that our technique enables full control over the elec. variables within the d. functional formalism.
- 32Spaldin, N. A. A Beginner’s Guide to the Modern Theory of Polarization. J. Solid State Chem. 2012, 195, 2– 10, DOI: 10.1016/j.jssc.2012.05.010[Crossref], [CAS], Google Scholar32https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XhsVSqt77J&md5=40ead8f8398a94a18104e422d00a7224A beginner's guide to the modern theory of polarizationSpaldin, Nicola A.Journal of Solid State Chemistry (2012), 195 (), 2-10CODEN: JSSCBI; ISSN:0022-4596. (Elsevier B.V.)The so-called modern theory of polarization, which rigorously defines the spontaneous polarization of a periodic solid and provides a route for its computation in electronic structure codes through the Berry phase, is introduced in a simple qual. discussion.
- 33Bureekaew, S.; Amirjalayer, S.; Tafipolsky, M.; Spickermann, C.; Roy, T. K.; Schmid, R. MOF-FF – A Flexible First-Principles Derived Force Field for Metal-Organic Frameworks. Phys. Status Solidi B 2013, 250, 1128– 1141, DOI: 10.1002/pssb.201248460[Crossref], [CAS], Google Scholar33https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXjt1ajurs%253D&md5=a034c9370905858f7dfd6f3b2d2d66daMOF-FF - A flexible first-principles derived force field for metal-organic frameworksBureekaew, Sareeya; Amirjalayer, Saeed; Tafipolsky, Maxim; Spickermann, Christian; Roy, Tapta Kanchan; Schmid, RochusPhysica Status Solidi B: Basic Solid State Physics (2013), 250 (6), 1128-1141CODEN: PSSBBD; ISSN:0370-1972. (Wiley-VCH Verlag GmbH & Co. KGaA)A review. In this contribution the development, definition and selected applications of a new force field (FF) for metal-org. frameworks MOF-FF is presented. MOF-FF is fully flexible and is parameterized in a systematic and consistent fashion from first principles ref. data. It can be used for a variety of different MOF-families and in particular - due to the reparametrization of a variety of org. linkers - also to explore isoreticular series of systems. The history of the development, leading to the final definition of MOF-FF is reviewed along with the application of the previous incarnations of the FF. In addn., the parametrization approach is explained in a tutorial fashion. The currently parametrized set of inorg. building blocks is constantly extended. Formate models of currently covered inorg. building blocks.
- 34Dürholt, J. P.; Fraux, G.; Coudert, F.-X.; Schmid, R. Ab Initio Derived Force Fields for Zeolitic Imidazolate Frameworks: MOF-FF for ZIFs. J. Chem. Theory Comput. 2019, 15, 2420– 2432, DOI: 10.1021/acs.jctc.8b01041[ACS Full Text], [CAS], Google Scholar34https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXkslait70%253D&md5=d327855e6f4cbf2ab21f5d2310e25f58Ab Initio Derived Force Fields for Zeolitic Imidazolate Frameworks: MOF-FF for ZIFsDuerholt, Johannes P.; Fraux, Guillaume; Coudert, Francois-Xavier; Schmid, RochusJournal of Chemical Theory and Computation (2019), 15 (4), 2420-2432CODEN: JCTCCE; ISSN:1549-9618. (American Chemical Society)In this paper, we parametrized in a consistent way a new force field for a range of different zeolitic imidazolate framework systems (ZIF-8, ZIF-8(H), ZIF-8(Br), and ZIF-8(Cl)), extending the MOF-FF parametrization methodol. in two aspects. First, we implemented the possibility to use periodic ref. data in order to prevent the difficulty of generating representative finite clusters. Second, a new optimizer based on the covariance matrix adaptation evolutionary strategy (CMA-ES) was employed during the parametrization process. We confirmed that CMA-ES, as a state-of-the-art black box optimizer for problems on continuous variables, is more efficient and versatile for force field optimization than the previous genetic algorithm. The obtained force field was then validated with respect to some static and dynamic properties. Much effort was spent to ensure that the FF is able to describe the crucial linker swing effect in a large no. of ZIF-8 derivs. For this reason, we compared our force field to ab initio mol. dynamic simulations and found an accuracy comparable to those obtained by different exchange-correlation functionals.
- 35Berendsen, H. J. C.; Postma, J. P. M.; van Gunsteren, W. F.; DiNola, A.; Haak, J. R. Molecular Dynamics with Coupling to an External Bath. J. Chem. Phys. 1984, 81, 3684, DOI: 10.1063/1.448118[Crossref], [CAS], Google Scholar35https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaL2cXmtlGksbY%253D&md5=5510dc00297d63b91ee3a7a4ae5aacb1Molecular dynamics with coupling to an external bathBerendsen, H. J. C.; Postma, J. P. M.; Van Gunsteren, W. F.; DiNola, A.; Haak, J. R.Journal of Chemical Physics (1984), 81 (8), 3684-90CODEN: JCPSA6; ISSN:0021-9606.In mol. dynamics (MD) simulations, the need often arises to maintain such parameters as temp. or pressure rather than energy and vol., or to impose gradients for studying transport properties in nonequil. MD. A method is described to realize coupling to an external bath with const. temp. or pressure with adjustable time consts. for the coupling. The method is easily extendable to other variables and to gradients, and can be applied also to polyat. mols. involving internal constraints. The influence of coupling time consts. on dynamical variables is evaluated. A leap-frog algorithm is presented for the general case involving constraints with coupling to both a const. temp. and a const. pressure bath.
- 36Hoover, W. G. Canonical Dynamics: Equilibrium Phase-Space Distributions. Phys. Rev. A: At., Mol., Opt. Phys. 1985, 31, 1695, DOI: 10.1103/PhysRevA.31.1695[Crossref], [PubMed], [CAS], Google Scholar36https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A280%3ADC%252BC2sjotlWltA%253D%253D&md5=99a2477835b37592226a5d18a760685cCanonical dynamics: Equilibrium phase-space distributionsHooverPhysical review. A, General physics (1985), 31 (3), 1695-1697 ISSN:0556-2791.There is no expanded citation for this reference.
- 37Flyvbjerg, H.; Petersen, H. G. Error Estimates on Averages of Correlated Data. J. Chem. Phys. 1989, 91, 461– 466, DOI: 10.1063/1.457480[Crossref], [CAS], Google Scholar37https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaL1MXkslOis7s%253D&md5=a424bd42502aa2989d102aa4c87b8ecaError estimates on averages of correlated dataFlyvbjerg, H.; Petersen, H. G.Journal of Chemical Physics (1989), 91 (1), 461-6CODEN: JCPSA6; ISSN:0021-9606.A description is given on how the true statistical error on an av. of correlated data can be obtained with ease and efficiency by a renormalization group method. The method is illustrated with numerical and anal. examples, having finite as well as infinite range correlations.
- 38Willems, T. F.; Rycroft, C. H.; Kazi, M.; Meza, J. C.; Haranczyk, M. Algorithms and Tools for High-Throughput Geometry-Based Analysis of Crystalline Porous Materials. Microporous Mesoporous Mater. 2012, 149, 134– 141, DOI: 10.1016/j.micromeso.2011.08.020[Crossref], [CAS], Google Scholar38https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXht12jurjN&md5=28e97cac049214d18c5a964ab405c4d9Algorithms and tools for high-throughput geometry-based analysis of crystalline porous materialsWillems, Thomas F.; Rycroft, Chris H.; Kazi, Michaeel; Meza, Juan C.; Haranczyk, MaciejMicroporous and Mesoporous Materials (2012), 149 (1), 134-141CODEN: MIMMFJ; ISSN:1387-1811. (Elsevier Inc.)Cryst. porous materials have a variety of uses, such as for catalysis and sepn. Identifying suitable materials for a given application can, in principle, be done by screening material databases. Such a screening requires automated high-throughput anal. tools that calc. structural properties for all materials contained in a database so they can be compared with search queries, grouped or classified. One important aspect of the structural anal. of materials such as zeolites and metal org. frameworks is the investigation of the geometrical parameters describing pores. Here, we present algorithms and tools to efficiently calc. some of these important parameters. Our tools are based on the Voronoi decompn., which for a given arrangement of atoms in a periodic domain provides a graph representation of the void space. The resulting Voronoi network is analyzed to obtain the diam. of the largest included sphere and the largest free sphere, which are two geometrical parameters that are frequently used to describe pore geometry. Accessibility of nodes in the network is also detd. for a given guest mol. and the resulting information is later used to retrieve dimensionality of channel systems as well as in Monte Carlo sampling of accessible surfaces and vols. The presented algorithms are implemented in a software tool, Zeo++, which includes a modified version of the Voro++ library. We present example applications of our algorithms and tools using zeolite frameworks currently listed in the Atlas of Zeolite Frameworks.
- 39Massidda, V.; Mirasso, C. R. Least-Energy Configuration in Orthorhombic Lattices of Dipoles. J. Magn. Magn. Mater. 1992, 116, 277– 281, DOI: 10.1016/0304-8853(92)90173-L[Crossref], [CAS], Google Scholar39https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK38XmsVChu78%253D&md5=11120882cb8d70b00cc702ecaecd113eLeast-energy configuration in orthorhombic lattices of dipolesMassidda, Vittorio; Mirasso, Claudio R.Journal of Magnetism and Magnetic Materials (1992), 116 (1-2), 277-81CODEN: JMMMDC; ISSN:0304-8853.The authors consider simple, body-centered and face-centered lattices of freely reorientable point dipoles. The configurations of least dipole-dipole energy are detd. for arbitrary values of the lattice consts. a, b, c. The results can be summarized as follows (the authors took a ≤ b ≤ c). For simple lattices the most stable configuration is always of the antiferro type, with the dipoles in the a direction. For body-centered lattices the most stable configuration is of the ferro type in the a or in the c direction, or antiferro in the a direction, depending on the a, b, c values. Analogously, for face-centered lattices it is of the ferro type in the a or b direction, or it takes 1 of 2 different antiferro arrangements in the b direction.
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ARTICLE SECTIONSThe Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscentsci.9b00497.
Detailed information about the MOF-FF energy expression and the parameter sets used within the MD simulations (PDF)
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