Conjugated microporous polymer membranes for light-gated ion transport

Inspired by the light-gated ion channels in cell membranes that play important roles in many biological activities, herein, we developed an artificial light-gated ion channel membrane out of conjugated microporous polymers. Through bottom-up design of the monomer molecular structure and by the electropolymerization method, the membrane pore size and thickness were precisely controlled on the molecular level. The obtained membrane exhibited uniform pore size and highly sensitive light-switchable response. The photoisomerization of the polymer chain resulted in a reversible “on and off” light control over the pore size and subsequently led to light-gated ion transport across the membrane for a series of ions including hydrogen, potassium, sodium, lithium, calcium, magnesium, and aluminum ions.

Then the reaction mixture was heated at 100 °C for 2 h with vigorous stirring. After completion of the reaction, the reaction solution was cooled at room temperature. Then the reaction solution was neutralized with 10% HCl (about 35 mL) solution and after that methanol (35 mL) was added to the reaction mixture and stirred for 15 min. Further, the obtained precipitate was filtered and the filter cake was washed with a copious amount of deionized water. Thereafter, the obtained crude compound was recrystallized in a dichlromethane/methanol mixture to afford the pure compound as a white solid powder 9'H-9, 3':6', 9''-tercarbazole (6) (~2.02 g, 82 % yield). 1  Synthesis of (E)-1, 2-bis (4-(3-(9'H-[9, 3':6', 9''-tercarbazol]-9'-yl)propoxy)phenyl)diazene In 100 mL single necked round bottom flask, a compound (E)-1,2-bis(4-(3-bromopropoxy) phenyl) diazene (2) (0.80 g, 1.75 mmol), 9'H-9,3':6',9''-tercarbazole (6) (1.92 g, 3.86 mmol), and anhydrous potassium carbonate (1.93 g, 14 mmol) and in 25 mL dry dimethylformamide (DMF), the solution was turbid, which was continued the stirred. Then the reaction mixture was heated at 100 °C for 24 h. After completion of the reaction, the reaction solution was cooled at room temperature. After that, the reaction mixture was filtered by suction filtration. The obtained filter cake was washed with a small amount of DMF until the filtrate was colorless and the product had very low solubility in DMF and to remove the impurities and unreactive reactants. After that, the filter cake was dissolved in deionized water (400 mL) and stirred for 1 hour, after that, the solution was filtered out. The obtained crude product was dissolved in dichloromethane (400 mL) and filtered off. The obtained filtrated solution was evaporated under reduced pressure via a rotary evaporator. The obtained crude product was chromatographed on silica gel with n-hexane/DCM (1:1, v/v) as eluent to afford the pure (E)-1, 2-bis (4-(3-(9'H-[9, 3':6', 9''-tercarbazol]-9'-yl) propoxy) phenyl) diazene (7) (~1.29 g, 26 % yield) as a yellow solid powder. 1     The UV-Vis spectra of the monomer in the trans and cis-state are slightly different from that of the azo-CMP membranes. The reason could be that the monomer was dissolved in a mixture of dry dichloromethane (DCM) and acetonitrile (CAN) (4/1, v/v) for the UV-vis spectra analysis, while the membranes were checked in the form of dried solid film (azo-CMP on ITO glass). Although the spectra look different, the change of trend is similar to that of the membranes. The reversible photoisomerization of the monomer could be recycled at least five times as shown in Fig. S9. The supersensitive isomerization of the designed monomer is fundamental for the construction of the light-gated ion channels.
The ratio of trans-to-cis isomer under UV light irradiation was calculated using the equations: where, is the maximum absorbance of the trans-azo-CMP membranes (or trans-azo-CMP monomer) at the peak position, is the absorbance of the membranes (or monomer) at the peak position under UV light irradiation for time t.  Table S1. Optimization of fabrication parameters.
The fabrication parameters including scan rate, number of CV cycles, monomer concentration, solvents ratio, and voltage range were investigated and optimized. When the ratio of the solvents was set at 2:3 or 3:2, the dissolvability of the monomer is poor due to the less component of methylene dichloride. The monomer can be dissolved normally when the solvents ratio of was set at 4:1. When the concentration of the monomer was set at 0.15 mM/L, the monomer showed good dissolvability. Scan range also has great effects on the fabrication. Under a relatively low high voltage (e.g., 0.8V), the electropolymerization cannot occur smoothly. As a result, no continuous membrane was formatted on the working electrode. However, when the high-voltage was set at 1.18 V, the obtained membrane dissolved after the generation. The effects of scan rate and the number of CV cycles on morphology, roughness, and surface hydrophilicity of the membranes were shown in Figs. S11 and S12. It should note that the surface hydrophilicity of the membranes relies heavily on the morphology and the nano-structure of the membrane surface. The rough surface enhanced the hydrophobicity. Because the designed azo-CMP membranes are expected to be used in an aqueous solution for controlled ion transport, a hydrophilic property is required. In order to obtain a hydrophilic membrane surface, the roughness of the membrane requires to further decrease.
As shown in Fig (32). In addition, in the azo-CMP membrane sample, the intensity of the peak at 805 cm −1 is greatly increased, which is ascribed to the increase of the risubstitued carbazole ring (28). It further confirms the polymerization of carbazoles.        The nitrogen-sorption test of the membranes reversed back from cis to trans was also checked by giving the cis-membrane in situ visible light irradiation. The results were shown in Fig. S27. After being reversed to its trans-state, the azo-CMP membrane showed similar pore size distribution compared with that of the initial trans-membrane. Some mesopores (200-100 Å) were generated during the sample drying or evacuation process in the nitrogen-sorption tests. In micropore range, four peaks at 10.5, 12.0, 14.0, and 17.9 Å were obtained, which matched well with that of the initial trans-membrane. It suggests that the pore size can be successfully switched back to the original larger pore.  The current recorded using divalent and trivalent cations is higher than that using monovalent ions.
The reason is that divalent and trivalent cations have higher electric charges. That means the current will be 3 times stronger with one Al 3+ across the membrane than the that obtained when one monovalent ion across the membrane. Therefore, in order to compare the ion permeation rate, the current (Fig. 5e) in the main test was normalized by the number of ion charges.      Tested under concentration-driven process.
Ion permeation rate across the trans-membrane followed the order of K + < Na + < Ca 2+ < Mg 2+ < Al 3+ , which is consistent with their hydrated ionic sizes, suggesting the size sieving effect of the membrane. In addition, the membrane showed light-gated regulation to all the common ions used in the tests. Specifically, after UV light irradiation, the ion permeation rate obtained using K + , Na + ,  compared with that observed using trans-membrane. The ions with a larger hydrated radius exhibited more significant changes in ion permeation rate after the trans to cis photoisomerization.
It indicates that the larger ions suffer greater transfer resistance when crossing the cis-membrane.
The result also confirmed that the ion transport inside the membrane is mainly governed by the size sieving effect. Moreover, the light-gated "on-off-on" transport prosperity is also reversible in the concentration-driven ion permeation process (Fig. S34d), which supports well the results observed in the electrically driven ion permeation tests.  Movie S1 Light-triggered motion of the membrane.