CO2 Adsorption in Azobenzene Functionalized Stimuli Responsive

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CO Adsorption in Azobenzene Functionalized Stimuli Responsive Metal–Organic Frameworks Runhong Huang, Matthew R Hill, Ravichandar Babarao, and Nikhil V. Medhekar J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b03541 • Publication Date (Web): 08 Jul 2016 Downloaded from http://pubs.acs.org on July 11, 2016

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The Journal of Physical Chemistry

CO2 Adsorption in Azobenzene Functionalized Stimuli Responsive Metal–Organic Frameworks

Runhong Huang†, Matthew R. Hill‡♯, Ravichandar Babarao‡¶* and Nikhil. V. Medhekar†*

†

Department of Materials Science and Engineering, Monash University, Clayton, Victoria, Australia.

‡

Commonwealth Scientific and Industrial Research Organisation (CSIRO) Manufacturing, Clayton, Victoria, Australia. ♯

Department of Chemical Engineering, Monash University, Clayton, Victoria, Australia. ¶

School of Science, RMIT University, Melbourne, Victoria 3001, Australia

*Contact Information: Dr. Nikhil V. Medhekar Department of Materials Engineering, Monash University. Room 109, Building 69, 22 Alliance Lane, Clayton, VIC-3800, Australia. Tel: +61 3 9905 1421, Email: [email protected] Dr. Ravichandar Babarao Commonwealth Scientific and Industrial Research Organisation (CSIRO), Private Bag 10, Clayton South, VIC-3169, Australia. Tel: +61 3 9545 2943, Email: [email protected]

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ABSTRACT: Recent reports of externally triggered, controlled adsorption of carbon dioxide (CO2) have raised the prospects of using stimuli responsive metal–organic frameworks (MOFs) for energy efficient gas storage and release. Motivated by these reports, here we investigate CO2 adsorption mechanisms in photoresponsive PCN-123 and azo-IRMOF-10 frameworks. Using a combination of grand canonical Monte Carlo and first principles quantum mechanical simulations, we find that the CO2 adsorption in both frameworks is substantially reduced upon light induced isomerization of azobenzene, in agreement with the experimental measurements. We show that the observed behavior originates from inherently weaker interactions of CO2 molecules with the frameworks when azobenzene groups are in cis state, rather than due to any steric effects that dramatically alter the adsorption configurations. Our studies suggest that even small changes in local environment triggered by external stimuli can provide a control over the stimuli responsive gas adsorption and release in MOFs.


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1. INTRODUCTION A rapid increase in the atmospheric CO2 in recent years has led to several acute environmental issues, ranging from global warming to sea level rise and ocean acidification.1-3 Carbon capture and storage technologies represent a promising solution towards mitigating the anthropogenic CO2 emission.3-4 Conventionally, these processes can consume up to 25%–40% of the production capacity of the power plant, thereby rendering the conventional technologies energy-inefficient and costly.3 Metal–organic frameworks (MOFs), have attracted a growing interest as a promising class of materials for low-energy CO2 capture and release.2-3, 5 MOFs are microporous crystalline materials characterized with an ultra high surface area, a large free volume and a low density.4 Furthermore, several recent studies have demonstrated that external stimuli such as pressure, heat, and in particular, light, can enable the changes in conformations and chemistry of a host of chemical compounds that can be introduced into the crystal structure of MOFs.6-9 These demonstrations, coupled with high gas adsorption capacities of MOFs, have prompted investigations into externally triggered and energy-efficient gas capture and release. Among several compounds that respond to external stimuli, azobenzene (C12H10N2) and diarylethene (DArE) are the most extensively explored for use in stimuli-responsive MOFs.6, 8 When exposed to light or heat, these compounds undergo significant changes in their conformations, which can be exploited to control the adsorption capacity of the microporous structures hosting these molecules.10-14 A few early works introduced these stimuli-responsive molecules as guest molecules in MOFs as well as porous aromatic frameworks, and reported a photoresponsive and photochromic behavior that led to significant changes in gas adsorption patterns.15-19 However, this approach of using non-covalently bonded stimuli-responsive molecules suffers from the difficulties associated with controlling their distribution within the pores.20 On the other hand, the stimuli-responsive molecules when integrated into MOF crystal structure as functional ligands or as groups attached to the ligands provide a robust control over the stimuli-responsive behavior than the non-covalently bonded molecules.20-21 Recently, a few noteworthy studies have reported the synthesis of MOFs with 3 ACS Paragon Plus Environment


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azobenzene and diarylethene attached to the organic ligands with controllable switching of their conformations.21-28 Park et. al. synthesized a porous organic PCN-123 structure using a photoswitchable azobenzene functionalized ligand 2-(phenyldiazenyl)terephthalate and reported a considerable drop in CO2 capacity as azobenzene isomerizes from trans to cis under UV exposure.22 Azobenzene was also incorporated within the ligand backbone of Zn-based MOF (Zn(AzDC)(4,4’BPE)0.5).29 This as-synthesized structure demonstrated the release of CO2 triggered by the irradiation of light.29 In addition to gas adsorption, azobenzene functionalized porous structures were also investigated for externally triggered capture and release of guest molecules such as methylene blue and butanediol.2425

While these experiments suggest the promise of using stimuli-responsive groups within the crystal structure of MOFs for externally triggered CO2 capture and release, the underlying molecularlevel mechanisms are yet unknown.22 In particular, it is not clear whether the observed drop in CO2 adsorption capacities originates from the steric effects that lead to the changes in pore morphology and the blocking of stronger metal-oxide binding sites, or due to the changes in the local interactions arising from the charge accumulation on metal oxides.22 Identifying the underlying mechanisms of the observed behavior is crucial for designing the externally triggered CO2 capture and release functionality in MOFs, and yet, such studies are seldom reported in the literature due to the inherent difficulties in investigating molecular-level mechanisms via experiments. In this regard, molecular simulations are increasingly being pursued to complement the experimental studies of gas adsorption in MOFs in a variety of ways, including in predicting gas adsorption isotherms, investigating adsorption sites and enthalpies, as well as in computational characterization of strucutres.30-33 However, to the best of our knowledge, so far no simulation study has investigated the mechanisms of CO2 adsorption in MOFs functionalized with stimuli responsive groups. Here we systematically investigate the adsorption of CO2 in two representative MOFs with organic ligands functionalized with stimuli responsive azobenzene, namely, PCN-123 and azo-IRMOF4 ACS Paragon Plus Environment

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10, using a combination of grand canonical Monte Carlo simulations and first principles density functional theory calculations. We consider PCN-123 framework not only because the reversible alteration to the CO2 adsorption under external stimuli was first demonstrated in this structure, but also due to its potential for use drug delivery.22 To test whether the mechanisms for reversible CO2 adsorption observed in the case of PCN-123 MOF are transferrable to other similar structures, we also investigated IRMOF-10 framework with azobenzene functional group attached to its ligands, which has a similar topology but a larger pore size than PCN-123.25 In both these structures, we examine the changes in CO2 adsorption capacities as a result of conformational changes in azobenzene functional groups. We find that our predicted CO2 adsorption capacities are in excellent agreement with the available experimental data. Our analysis of the binding energies and radial distribution functions reveals that the observed stimuli responsive CO2 adsorption in these frameworks can be attributed to inherently stronger binding of CO2 molecules in the trans configuration of the azobenzene groups than in the cis configuration.

2. MODELS AND METHODS 2.1 Crystal Structures of PCN-123 and azo-IRMOF-10 Frameworks. PCN-123 is an azobenzene functionalized MOF composed of ZnO4 metal oxide nodes and 2(phenyldiazenyl)terephthalate ligands. Since X-ray diffraction measurements are typically unable to distinguish between the randomly oriented azobenzene functional groups and free solvent within the pores, the crystal structure of this framework cannot be directly obtained from the experiments.22 Hence we built the crystal structure by functionalizing azobenzene groups on the ligands of the topologically similar IRMOF-1 framework, followed by the structural optimization using first principles density functional theory (DFT) methods. This approach of constructing the crystal structure is consistent with several previous works investigating the gas adsorption in porous materials using computational techniques.31, 34 Figure 1 shows the lowest energy optimized structure of PCN-123 with azobenzene 5 ACS Paragon Plus Environment


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groups in trans as well as in corresponding cis configurations. As in experiments, it is evident that the optimized structure is characterized by the presence of a concave as well as convex pores.22 It should be noted that a periodic unit cell of the PCN-123 crystal considered here contains 12 azobenzene groups, each of which can point either in or out of concave pores, thereby giving rise to several metastable configurations of the PCN-123 crystal. In order to ensure that our investigation of stimuli responsive CO2 adsorption in PCN-123 is representative of the experimental observations, we also considered several low energy metastable structures of PCN-123 crystal. These structures were obtained by a systematic identification of all distinct configurations, followed by a structural optimization using DFT methods. Figure 2 shows two metastable configurations with energies next to the lowest energy configuration. Azobenzene functionalized IRMOF-10 has a similar topology with the PCN-123 framework, the only difference being that the metal oxide nodes in azo-IRMOF-10 are connected by a longer 2azobenzene 4,4’-biphenyldicarboxylic ligand rather than a 2-(phenyldiazenyl)terephthalic ligand.25 Similar to the structure generation process of PCN-123, we obtained the crystal structure of azobenzene functionalized IRMOF-10 by first functionalizing azobenzene groups on IRMOF-10 framework, and then optimizing the crystal structure using DFT methods. Figure 3 shows the lowest energy optimized crystal structure of IRMOF-10 with azobenzene groups in trans and cis configurations. Finally, for both PCN-123 and azo-IRMOF-10 frameworks, we quantified their structural features such as porosity, accessible surface area, and maximum and pore-limiting diameters for both trans and cis states of the azobenzene functional groups. The numerical values of these structural parameters are presented in Table 1. The values for the accessible surface area, porosity, the maximum and pore limiting diameters for each structure considered in this study are obtained using the program Poreblazer_v.3.0.2.33


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2.2 Density Functional Theory (DFT) Calculations. Our first principles calculations were performed using density functional theory within generalized gradient approximations as employed in Vienna Ab Initio simulation package (VASP).35 For both structural optimization and total energy calculations, the projector augmented wave method was used to describe core and valence electrons,36 while electron exchange and correlation were described using Perdew-Burke-Ernzerhof formulation.37 All calculations were performed using a primitive cell with periodic boundary conditions. The ionic positions and cell vectors were relaxed until ionic forces were less than 0.01 ev/Å. The Brillouin zone was sampled using a 1×1×1 and 3×3×3 Γ-centered kpoints mesh for structural optimization and accurate total energy calculations, respectively. We used a plane wave kinetic energy cutoff of 500 eV and 800 eV for structural optimization and total energy calculations, respectively. To calculate the partial charges on every atom of the framework, which are essential for simulations of CO2 adsorption isotherms, we implemented the density-derived electrostatic and chemical charge (DDEC) method using a fully periodic primitive cell.38 Figures S1–S4 in the Supporting Information (SI) present the atomic partial charges for all frameworks considered in this work. It is well known that standard DFT methods based on generalized gradient approximation do not fully account for the long range dispersion interactions between the framework and the weakly bound gaseous adsorbates.39 To accurately estimate static binding energies for CO2 molecules with PCN-123 and azo-IRMOF-10 frameworks, we implemented dispersion corrections using DFT-D2 method.39 Static binding energies ∆E of CO2 molecules at 0 K were then calculated as ∆‫ܧ = ܧ‬୘ − ‫ܧ‬୊ − ‫ܧ‬୥ , where ET, EF, and Eg denotes for the total energy of the framework and adsorbate complex, the framework, and an isolated gas molecule, respectively. With this definition, a negative value of the adsorption energy indicates a thermodynamically favorable adsorption of CO2 in the framework.



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2.3 Grand Canonical Monte Carlo (GCMC) Simulations. To study CO2 adsorption in stimuli responsive PCN-123 and azo-IRMOF-10 frameworks, we performed grand canonical Monte Carlo simulations using RASPA code.40 CO2 molecules were modeled as threesite rigid molecules. CO2-CO2 and CO2-framework interactions were modeled using a combination of pairwise Lennard-Jones and Coulombic potentials. A Dreiding force field was employed to model the framework.41 In all cases, the cross parameters of the Lennard-Jones potential were calculated by the Lorentz-Berthelot combining rules. All Lennard-Jones potential parameters were rescaled to fit experimental isotherm of pure N2 at 77 K (see Fig. S5, SI).22 This approach of rescaling the force field parameters is consistent with earlier works on investigating CO2 adsorption in metal–organic frameworks.31, 42-43 As mentioned earlier, the density-derived electrostatic and chemical charge (DDEC) method was employed to calculate the partial atomic charges for both frameworks from the electronic charge densities obtained from highly accurate DFT calculations.38 Tables S1–S2 in the SI present the values of the relevant parameters for the Lennard-Jones and Dreiding force fields employed in this work. All our GCMC simulations were run for 500,000 equilibrium cycles, followed by 500,000 cycles for statistical data collection. Every cycle included N steps, where N is the number of the CO2 molecules in the system; we used a minimum 20 steps where the number of CO2 molecules was less than 20. Every GCMC step consisted of five types of trial moves for gas molecules, namely, insertion, rotation, translation, deletion and random re-insertion. Finally, the statistical uncertainties in our GCMC calculations were smaller than the symbol sizes used in the figures.

3. RESULTS and DISCUSSION 3.1 Structural Features. Figures 1 and 3 show the DFT-optimized crystal structure of lowest energy configurations of PCN-123 and azo-IRMOF-10 in both trans and cis configurations, while Table 1 presents the values of the relevant structural parameters for these frameworks. Due to the longer 2-azobenzene 4,4’8 ACS Paragon Plus Environment

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biphenyldicarboxylic ligand in azo-IRMOF-10 than the 2-(phenyldiazenyl)terephthalic ligand in the topologically similar PCN-123 framework, azo-IRMOF-10 framework demonstrates larger pore size as well as accessible pore volume and surface area than the PCN-123 framework. It can also be readily observed that both PCN-123 and azo-IRMOF-10 frameworks are characterized by the alternating concave and convex pores. Due to the topological constraints imposed by these pores, the azobenzene groups in trans configurations can only point into the concave pores, resulting in the blocking of concave pores, leaving the convex pores remain largely unaffected. On the other hand, upon isomerization to cis configuration, the azobenzene groups remain within the convex pores in both PCN123 and azo-IRMOF-10 frameworks. This behavior is evident in the reduction of maximum pore diameters in both these frameworks upon isomerization of azobenzene groups as shown in Table 1. It is also interesting to note that both the free volume and the accessible surface area are significantly affected by this isomerization in the PCN-123, whereas they are largely unchanged in the azo-IRMOF10 framework. The structural parameters obtained by simulations and presented in Table 1 could not be directly benchmarked with experiments due unavailability of the experimental structural data.22 However, we have estimated the BET surface area from experimentally measured N2 isotherm at 77 K to be 1656 m2/g, in good agreement with the accessible surface area obtained from simulations.22

3.2 CO2 Adsorption in PCN-123 Framework. Next we investigate how the isomerization of azobenzene functional groups influences CO2 adsorption in PCN-123 framework. Using GCMC simulations, we first calculated the CO2 adsorption isotherms for the DFT optimized structure with azobenzene groups in both trans and cis configurations as shown in Figure 4. These predictions were compared with the experimentally measured isotherms for PCN-123 synthesized in dark conditions so that nearly all azobenzene groups were initially in trans configuration, followed by an exposure to UV light at 365 nm to isomerize them to cis configuration.22 As shown in Fig. 4, the predicted adsorption isotherms are in a good agreement with experiments, confirming the 9 ACS Paragon Plus Environment


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accuracy of the various force field parameters used in this study. It is evident that isomerization of azobenzene leads to a significant reduction in CO2 adsorption capacity of PCN-123 framework. For example, the experimentally measured capacity at 1 bar pressure drops from 23.0 cm3/g to 16.9 cm3/g upon isomerization to cis, which corresponds to a 26% reduction in CO2 adsorption capacity. The simulations of CO2 isotherms also predict a similar drop from 23.5 cm3/g and 18.0 cm3/g at 1 bar. It is worth noting that the simulated isotherms for both trans and cis configurations of azobenzene are linear, whereas the experimental isotherm for cis configuration shows a marginally nonlinear behavior. This could possibly be attributed to an incomplete isomerization of azobenzene groups upon UV exposure in experimental structures, while the azobenzene groups in simulated structures were exclusively either in trans or cis configurations. Although the predicted adsorption isotherms presented in Figure 4 correspond to the lowest energy configuration of the azobenzene functionalized PCN-123, we also calculated the CO2 isotherms for two additional next-to-lowest energy isomers of PCN-123 depicted in Figure 2. As shown in Figure S6 of the SI, it is evident that the two additional low energy isomers also demonstrate a substantial reduction in CO2 capacity in the range of 25%–33% as azobenzene groups isomerize from trans to cis configuration, in good agreement with the predictions for the lowest energy structure as well as with experiments.22 This observation suggests that the DFT-optimized structures considered in our study and subsequent predictions for adsorption isotherms are representative of the experimental observations for PCN-123 framework. It should be noted that while the CO2 adsorption capacity for azobenzene functionalized PCN-123 frameworks is generally lower than other high capacity metal-organic frameworks (up to 180 cm3/g at 298k, 1 bar),44-45 it compares favorably with the adsorption capacities of photo-responsive frameworks (up to 31.36 cm3/g a 1 bar).18 In order to identify the preferential adsorption sites of CO2 molecules in PCN-123, we calculated radial distribution functions between the center of mass of CO2 molecules and two specific framework atoms, namely, the oxygen atom of the carboxylate group and the nitrogen atom of the azobenzene group. As shown in Fig. 5, in both trans and cis configurations of the azobenzene, CO2 molecules 10 ACS Paragon Plus Environment

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occupy a similar binding site approximately 4 Å away from the oxygen atom of the carboxylate group. This observation is in distinct contrast with earlier investigations into CO2 adsorption in PCN-123 frameworks,22 which suggested shielding of the metal oxide binding sites in cis configurations of azobenzene as the origin of the reduction in CO2 adsorption capacity. A close observation of Figure 5 reveals that the distance between CO2 molecules and nitrogen atom of the azobenzene groups varies considerably between trans and cis configurations—CO2 molecules are found 4 Å away from the nitrogen atom in trans configuration and but are located much further at 6 Å in cis configuration. This suggests that extra interactions provided by azobenzene in trans configuration can provide explanation for the higher CO2 uptake shown in Figure 4. To further investigate the nature of the interactions between CO2 molecules and PCN-123 framework, we also calculated CO2 adsorption isotherms in trans and cis configurations of azobenzene groups in the absence of any electrostatic interactions, thereby including only long range dispersion or van der Waals interactions. As seen in Figure S7 in the SI, CO2 adsorption reduces in the absence of electrostatic interactions for both trans and cis states of the azobenzene groups. However, since the CO2 adsorption drop reduces from 27% to 13.9% (from 17.3cm3/g to 14.9cm3/g at 1 bar) when the electrostatic interactions were switched off, it is clear that both electrostatic and long range dispersion interactions play a role in the adsorption of CO2 in azobenzene functionalized PCN-123 frameworks. While several factors such as accessible surface area, pore morphology, free volume, adsorption sites and adsorption enthalpy can all influence uptake of CO2 in a porous structure, it has been established that the adsorption enthalpy largely controls the capacity of a porous structure to adsorb CO2 at low operating pressures up to 1 bar.46 To accurately estimate the strength of binding interactions between CO2 molecules and the framework, we further investigated the binding sites for CO2 molecules and determined their static binding energies at 0 K using dispersion corrected DFT methods. Figure 6 shows an isolated CO2 molecule located in its strongest binding site in PCN-123 with azobenzene groups for trans and cis configurations. In trans configuration, the CO2 molecule is adsorbed close to 11 ACS Paragon Plus Environment


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azobenzene, with the distance from the nitrogen atom of azobenzene and the nearest carboxylate oxygen being 3 Å and 4.1 Å, respectively. On the other hand, in its strongest binding site in cis configuration of azobenzene, the CO2 molecule is located away from azobenzene and closer to the carboxylate oxygen. In fact, this binding site is very close to site I, the conventional primary CO2 adsorption site close to metal oxide and along the corner diagonal of the convex pores.47 The extra attractive interaction provided when azobenzene groups are in trans configuration therefore leads to drifting of the CO2 molecule away from the site I position. These extra interactions are also confirmed by the static binding energies at 0 K obtained using DFT calculations. We found the CO2 binding energies to be –31.3 kJ/mol and –27.5 kJ/mol with the azobenzene groups in trans and cis configurations, respectively. These distinct binding energies for CO2 molecules for trans and cis configurations of azobenzene within the framework are particularly noteworthy since CO2 molecules show no appreciable difference in binding with trans and cis configurations for azobenzene molecules when not attached as functional groups in the framework (see Figure S8 in SI). While the distances for the adsorbed CO2 molecules as predicted by the DFT calculations at 0 K are generally smaller than those obtained by radial distribution functions at 298 K as shown in Figure 5, both GCMC simulations and DFT calculations nevertheless provide a consistent description of adsorption configurations for CO2 molecules in azobenzene functionalized PCN-123 framework. These findings again confirm that the reduction in CO2 adsorption capacity in PCN-123 framework upon isomerization of azobenzene functional groups is driven by the stronger electrostatic and long range interactions of CO2 molecules with PCN-123 framework when azobenzene groups are in trans configuration. Finally, to confirm whether the shielding of the metal cluster by the azobenzene groups in cis configuration plays any role in the reducing the CO2 uptake in PCN-123 framework,22 we also investigated the accessible surfaces for CO2 adsorption. Figure 7 shows a cross section of the accessible surface, specifically sliced along face diagonals to include the main CO2 adsorption site I along the corner to diagonal of the convex pore. It is evident that in cis configuration of the azobenzene groups, 12 ACS Paragon Plus Environment

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type I sites are still available for gas adsorption instead of being shielded by the functional groups— there is no appreciable difference between the accessibility of site I for trans and cis configurations of azobenzene. This observation further illustrates that the stronger interactions between CO2 molecules and PCN-123 framework with azobenzene functional groups in trans configuration give rise to its higher in CO2 uptake compared with the cis configuration.

3.3 CO2 Adsorption in Azo-IRMOF-10 Framework. Next we discuss the influence of isomerization of azobenzene functional groups on the CO2 adsorption capacity

of

azo-IRMOF-10

framework,

which

contains

a

longer

2-azobenzene

4,4’-

biphenyldicarboxylic ligand rather than a shorter 2-(phenyldiazenyl)terephthalic ligand in PCN-123 framework. By using GCMC simulation methods, we calculated CO2 adsorption isotherms for DFT optimized structures of IRMOF-10 with azobenzene in trans and cis configurations shown in Figure 3. From CO2 adsorption isotherms at 298 K presented in Figure 8, it is evident that the isomerization of azobenzene groups to cis configuration leads to a much smaller reduction in CO2 uptake in azo-IRMOF10 framework than in the PCN-123 framework. For example, the CO2 uptake for trans and cis configuration is 16.0 cm3/g and 14.6 cm3/g at 1 bar pressure, representing a drop of 8.3%. Figure 9 shows the radial distributions functions calculated between the adsorbed CO2 molecules and a few specific framework atoms obtained using GCMC calculations. We find that CO2 molecules are adsorbed closer to the oxygen atom of the carboxylate group than the nitrogen atom of azobenzene in its trans as well as cis configurations, revealing the metal oxide site as the primary CO2 binding site in azo-IRMOF10 framework. Moreover, the adsorbed CO2 molecules are much closer to the azobenzene in trans (~4.7 Å) than in cis configuration (~6.3 Å), which indicate stronger interactions in trans configuration leading to a higher CO2 uptake. While these observations are qualitatively similar to those observed in azobenzene functionalized PCN-123 framework, we find that due to the longer 2-azobenzene 4,4’-



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biphenyldicarboxylic ligand, the attached azobenzene functional groups exert a much smaller influence on the CO2 adsorption in azo-IRMOF-10 framework. Next we performed dispersion corrected DFT calculations to identify the CO2 adsorption sites and to estimate the strength of the interactions between the adsorbed molecules and azo-IRMOF-10 framework. In contrast to PCN-123, CO2 molecules in azo-IRMOF-10 with both trans and cis configurations of azobenzene functional groups preferentially adsorb in site III (see Fig. 10), conventionally described as the site near metal oxide and located in the plane separating convex and concave pores.47 Since site III is not the most preferred CO2 adsorption site in pure, unfunctionalized cubic IRMOF-1 framework,47 this observation points to the stabilizing effect of azobenzene groups. Moreover, the adsorption site shown in Figure 10 is closer to the oxygen atom of the carboxylate group than azobenzene, in qualitative agreement with the radial distribution functions obtained shown in Figure 9. Using dispersion corrected DFT calculations, we obtained the static binding energy of CO2 molecules at 0 K to be –29.5 kJ/mol and –26.3 kJ/mol for azobenzene in trans and cis configurations, respectively. The weaker interactions of CO2 molecules with the framework when azobenzene groups are in cis configuration are therefore responsible for a reduction in the CO2 uptake upon isomerization. Finally, for completeness, we also investigated the accessible surfaces for CO2 molecules in azobenzene functionalized IRMOF-10 framework. Figure 11 shows the cross-sectional view of the accessible surface, along the plane separating convex and concave pores that includes adsorption site III. Similar to PCN-123, the most preferred type III adsorption sites remain accessible for CO2 molecules in both trans and cis configurations of azobenzene groups in azo-IRMOF-10 framework. A comparison of the changes in CO2 adsorption upon isomerization of azobenzene functional groups in PCN-123 and azo-IRMOF-10 frameworks highlights the general mechanism responsible for the reduction in CO2 uptake in cis configurations of azobenzene. While the lowest energy CO2 adsorption sites are distinctly different in these two frameworks, the carboxylate oxygen attached to the metal oxide node remains accessible for CO2 adsorption in both trans and cis configurations of 14 ACS Paragon Plus Environment

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azobenzene. This observation provides little support for the shielding of the binding sites close to metal oxides as the main cause for the reduction in CO2 uptake upon isomerization of the azobenzene as suggested in the literature.22 In contrast, the reduced CO2 uptake can be ascribed to the relatively weaker interactions of the adsorbed CO2 molecules with the framework when the azobenzene groups are isomerized to cis configuration. Since the strength of these interactions crucially depends on the distance of adsorbed CO2 molecules from the azobenzene, the isomerization has a stronger influence on CO2 adsorption in PCN-123 due to its shorter ligand than in azo-IRMOF-10 framework.

5. CONCLUSIONS To summarize, employing a combination of grand canonical Monte Carlo and first-principles simulations, we investigated CO2 adsorption in photoresponsive frameworks PCN-123 and IRMOF-10 functionalized with azobenzene. Our calculations predicted a 23% and 8% reduction of CO2 uptake upon isomerization of azobenzene in PCN-123 and azo-IRMOF-10 frameworks, respectively, in good agreement with the available experimental data. Radial distribution functions and accurate DFT calculations show that the observed drop in CO2 uptake in both frameworks can be directly attributed to the stronger interactions provided when azobenzene groups are in their trans configurations compared to cis configurations. Furthermore, CO2 molecules preferentially adsorb near metal oxide carboxylate groups in type I and type III binding sites in PCN-123 and azo-IRMOF-10 frameworks, respectively, and these binding sites remain accessible for both trans and cis configurations of the azobenzene functional groups. These findings suggest that an ability to block CO2 binding sites need not be an essential criteria for the functional linkers in order to achieve low energy stimuli-responsive CO2 adsorption and release in metal–organic frameworks, and furthermore, a sufficient control over CO2 adsorption can be obtained by using linkers that demonstrate differential interactions with bound CO2 molecules in their topological states driven by the external stimuli.



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SUPPORTING INFORMATION Details of computational methods, rescaled force field parameters, simulated N2 isotherm, interaction energies between azobenzene and CO2 molecules, comparison of CO2 isotherms of force field parameters and electrostatic interactions.

ACKNOWLEDGEMENTS Authors gratefully acknowledge computational support from Monash Sun Grid, MASSIVE, CSIRO Burnet cluster, Pawsey Supercomputing Facility and the National Computing Infrastructure funded by the Government of Australia. R.B. acknowledges Australian Research Council for the DECRA fellowship and support from the Science and Industry Endowment Fund by the Government of Australia. R.H. acknowledges top up scholarship from CSIRO OCE Science Leader.


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cis

Figure 1. DFT-optimized atomic structure of PCN-123 framework with azobenzene groups in trans and cis configurations. Hydrogen atoms and the azobenzene groups pointing out of pores are not shown for clarity. Color code for atoms: Zn, slate; C, gray; O, red; N, blue.

Figure 2. Low-energy isomers of PCN-123 framework with azobenzene in trans configuration. (a) Atomic structure and the schematic representation of the lowest energy crystal structure. (b,c) Schematic representations of the DFT-optimized structures of metastable isomers with next lowest energies. In all cases, the cubic box with metal oxide clusters at its corners denotes concave pores, while red and blue sticks denote the ligands with an azobenzene group pointing outward from and inward into the concave pore, respectively. The energy of each isomer per unit cell is relative to the energy of the lowest energy isomer shown in (a).


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trans

cis

Figure 3. DFT-optimized atomic structure of azo-IRMOF-10 with azobenzene groups in trans and cis configurations. Hydrogen atoms and the azobenzene groups pointing out of pores are not shown for clarity. Color code for atoms: Zn, slate; C, gray; O, red; N, blue.

Table 1. Framework density ρ, porosity Ø, pore limiting and maximum pore diameter, and accessible surface area for trans and cis configurations of the azobenzene functional groups in PCN-123 and azoIRMOF-10 frameworks. ρ (g/cm3) trans PCN-123

cis

0.82

trans azo-IRMOF-10

cis

0.43

Ø

pore limiting diameter (Å)

maximum pore diameter (Å)

accessible surface area (m2/g)

0.26

3.9

14.2

1482.0

0.20

5.3

10.0

1628.5

0.49

10.1

19.1

4086.2

0.50

10.3

16.1

4092.6



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Figure 4. CO2 adsorption isotherm at 298 K for PCN-123 framework with azobenzene groups in trans and cis configuration. Closed symbols represent experimentally measured values,25 and the solid lines indicate the predicted values.

Figure 5. Radial distribution functions g(r) computed between the center of mass of CO2 molecules and the various PCN-123 framework atoms at 298 K and 1 bar pressure, with azobenzene functional groups in trans and cis configurations.


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Figure 6. DFT-D2 optimized low energy binding sites of an isolated CO2 molecule in PCN-123 framework with azobenzene groups in trans and cis configurations. Atomic clusters are cleaved from the periodic crystal for clarity and hydrogen atoms are shown implicitly. Color code for atoms: Zn, slate; C, gray; O, red; N, blue. All distances are in Å. ∆E denotes the binding energy of the CO2 molecule.

trans

cis

Figure 7. Schematic representation of the PCN-123 unit cell and the accessible surface for CO2 adsorption with azobenzene groups in trans and cis configurations. The yellow sphere shows a convex pore. In the cross sectional view of the accessible surface area along a plane that includes adsorption site I, blue and red colors denote free and occupied regions, respectively.



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trans cis

Figure 8. CO2 adsorption isotherms at 298 K for azo-IRMOF-10 with azobenzene groups in trans and cis configurations as obtained using GCMC calculations. Closed symbols represent predicted values and the solid lines are guide to eye.

Figure 9. Radial distribution functions g(r) computed between the center of mass of CO2 molecules and various azo-IRMOF-10 framework atoms at 298 K and 1 bar pressure, with azobenzene functional groups in trans and cis configurations.


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Figure 10. DFT-D2 optimized low energy binding site of an isolated CO2 molecule in azo-IRMOF-10 with azobenzene groups in trans and cis configurations. Atomic clusters are cleaved from the periodic crystal for clarity and hydrogen atoms are shown implicitly. Color code for atoms: Zn, slate; C, gray; O, red; N, blue. All distances are in Å. ∆E denotes the binding energy of the CO2 molecule.

trans

cis

Figure 11. Schematic representation of the azo-IRMOF-10 unit cell and the accessible surface for CO2 adsorption with azobenzene groups in trans and cis configurations. The yellow sphere shows a convex pore. In the cross sectional view of the accessible surface area along a plane that includes adsorption site III, blue and red color denotes free and occupied regions, respectively.



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Heat

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CO2 Adsorption in Azobenzene Functionalized Stimuli Responsive

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