Synthesis of the ligand
Ligand 5-((2,4-dimethylphenyl)diazenyl)isophthalic acid) was synthesized as per our earlier report.33 Initially, 2,4-dimethylaniline solution (2.4 g, 20 mmol in 80 mL of DCM) was mixed with the solution of oxone (2.5 g, 40 mmol in 20 and 180 mL of DCM and water), and stirred in an open condition. The formation of nitroso-compound can be observed after 12 h when the solution turns eventually green. Next, the DCM layer was separated and washed with 1M HCl, twice. The product (2,4-dimethyl-1-nitrosobenzene) was obtained by drying the DCM solution in anhydruous MgSO4, and finally DCM was removed by rota evaporation. Afterward, the product (0.81 g, 6 mmol) with dimethyl-5-aminoisophthalate (0.84 g, 4 mmol) was mixed in acetic acid (60 mL) and stirred further for 48 h at 90 °C. The mixture was cooled down and mixed with 300 mL of water to obtain orange precipitate which is then filtered, washed with water, and dissolved in chloroform. After removal of chloroform by drying in MgSO4, the precipitate (dimethyl 5-((2,4-dimethylphenyl)diazenylisophthalate) was purified by column chromatography. Next, 0.5 g of it was dissolved in 30 mL solution of THF, MeOH and 20% KOH (1:1:1), and stirred at 50 °C overnight. After the removal of organic phase, the aqueous phase was acidified with 6 M HCl to obtain yellow precipitate as ligand. Yield (2 %, 0.42 g). 1H NMR (400 MHz, CDCl3): δ 2.36 (s, 3H); 2.67 (s, 3H); 7.19 (d, 1H); 7.30 (s, 1H); 7.61 (d, 1H); 8.54 (d, 2H); 8.58 (t, 1H). FTIR (cm− 1): 2970, 2679, 2559, 1698, 1610, 1441, 1411, 1382, 1278, 1223, 1101, 1027, 917, 818, 763, 691, 667.
Synthesis of the PMOP
The synthesis of PMOP was carried out by following a procedure in our earlier report.34 A Cu2(OAc)4∙2H2O solution (120 mg, 0.3 mmol in 20 mL of DMA) was added in a DMA solution (10 mL) of 5-((2,4-dimethylphenyl)diazenyl)isophthalic acid (90 mg, 0.3 mmol) confined in vial and kept in dark for two days. Afterward, methanol (MeOH, 30 mL) was added to the solution to obtain a green precipitate, PMOP which is further washed with MeOH (three times) and kept in chloroform prior to use. FTIR (cm− 1): 3423, 2927, 1633, 1442, 1409, 1366, 1231, 1105, 1034, 924, 816, 778, 724, 664.
Synthesis of the ionic liquid (IL)
The IL, [PEG-linked bis(imidazoliumbutyl) di(trifluoromethanesulfonyl)amide, IL(NTf2)] was synthesized in three steps. First, a biphasic system was formed by dissolving PEG-400 (1 M, 10 g) in a mixture of THF (25 mL) and 2.5 M aqueous solution of NaOH (2.5 g). Next, TOCl (1.1 M, 10.48 g) was added to this mixture while incubating in ice bath. Then, 1 M HCl (1.06 mL) was added to the reaction system at room temperature for 4 h. The product was extracted using DCM and water, and the solvent was removed using vacuum evaporation to obtain PEG-400-ditosylate which is later dissolved in a mixture of THF (25 mL) and 3 M aqueous solution of NaOH (3 g). Afterward, 2.2 M (7.5 g) of imidazole was added in the mixture and stirred at 80 °C for 4 h. In order to remove impurity from the resulting material, DCM and water were used, and then filtered. The product (PEG-functionalized bis-imidazole) was dried using vacuum evaporator to remove THF and DCM to obtain a viscous liquid. Second, in a round bottom flask, butyl bromide (16.5 mL) and PEG-linked bis-imidazole (8.2 mL) were added in dry toluene (20 mL), and kept on reflux under N2 atmosphere at 80 °C for 24 h in a dark condition. This biphasic liquid medium was separated and the bottom fraction was washed with MeOH which is removed using vacuum evaporator to obtain a viscous product [PEG-linked bis(imidazolium butyl) di(trifluoromethanesulfonyl)amide, IL(Br)]. Third, to exchange the anion (Br−), IL-Br (8.2 mL) and LiNTf2 were dissolved in MeOH:H2O (4:1) solution and stirred at room temperature for 4 h. The obtained material was washed with water and EtOAc twice, and separated using separating funnel while EtOAc was removed by vacuum evaporation to obtain pale yellow liquid [PEG-linked bis(imidazoliumbutyl) di(trifluoromethanesulfonyl)amide, IL(NTf2)].Yield (5 %, 6.1 mL). 1H NMR (400 MHz, DMSO-d6): δ 0.91 (t, 3H); 1.28 (m, 2H); 1.79 (m, 2H); 3.51 (s, 8H); 3.79 (t, 2H); 4.20 (t, 2H); 4.35 (t, 2H); 7.76 (d, 1H); 7.79 (d, 1H); 9.13 (s, 1H). 13C NMR (100 MHz, DMSO-d6): δ 13.61, 19.23, 31.72, 49.07, 60.19, 70.21, 122.89, 123.29, 136.79 and 170.78. FTIR (cm− 1): 3153, 2951, 1567, 1462, 1353, 1194, 1133, 1051, 941, 792, 732.
Synthesis of the photoresponsive porous ionic liquids (PPILs)
A parent PPIL was prepared by dispersing PMOP (20 mg) in the solution of IL(NTf2) (1 mL) and MeOH (1 mL), and ultra-sonicated for 20 min. Then, the resulting colloidal solution was dried at 70 °C for 1 h to obtain type III PPIL-2%. Next, to obtain the PPIL with PMOP contents 0.2, 0.1 and 0.05%, the parent PPIL-2% was diluted with IL(NTf2) and MeOH accordingly, and ultra-sonicated for 20 min. These solutions were further kept in over at 70 °C for 2 h to remove MeOH so as to obtain lucid type II PPIL-0.2, 0.1 and 0.05%. Note: MeOH was used to lower the viscosity of IL(NTf2) so that PMOP could be dispersed properly under sonication.
Gas adsorption measurements
Gas uptake analyses were measured on ASAP 2020 analyzer for low pressure adsorption (0 − 1 bar). CO2 adsorption isotherms at 273 K were detected in an ice-water bath and after degas, the liquid samples in the quartz tube were irradiated with UV (365 nm) and Visible light (450 nm) using a xenon lamp (CEL-HXUV300) outfitted with a filter. For high pressure adsorption under UV and Visible light irradiations, BELSORP-HP instrument was employed within the pressure range of 0 − 10 bar at 25 °C. Prior to analyses; the liquid samples were evacuated at 100 °C for at least 2 h.
Simulation studies
Molecular dynamics simulations were conducted using the Forcite module in Accelrys Materials Studio 7.0 software.38 The initial configuration was constructed with one photoresponsive metal-organic polyhedron (PMOP) cluster and one cation and two anions of the ionic liquid packed in a cubic box. The simulations were carried out by implementing Universal force field (UFF)2 along with charge equilibration method (Qeq) addressing point charges of the system. The calculation details are as follows: NPT ensembles with fixed pressure (100 kPa) and temperature (25 °C) were selected to conduct the equilibrium run for both systems (PMOPs with cis and trans structures), where the time step and total equilibrium time-run were set as 1.0 fs and 50 ns, respectively. Two simulated cubic boxes (28.565 and 28.585 Å) were obtained for trans- and cis- cages, respectively. The final configurations of the equilibrium run were retaken as the initial models for the production run under NVT conditions, where the time step and total equilibrium time-run were set as 1.0 fs and 25 ns, respectively. Radial distribution functions analysis was performed for the final confirmation of the production run. For all the dynamic calculations, a cut off distance of 15.5 Å were applied to both van der Waals and electrostatic potential. For each computed frame within the production run, movies were generated to interpret the movements of the studied models. Topological analysis tool Zeo + + 3 was used to compute the pore size distribution map to distinguish the cavity space of the optimized configurations. A probe size of 0.5 Å and 100000 Monte Carlo cycles was implemented.
Data availability
All relevant data supporting the findings of this study are available from the corresponding authors on request.