Superprotonic conduction of intrinsically zwitterionic microporous polymers based on easy-to-make squaraine, croconaine and rhodizaine dyes

Porous organic polymers (POPs) have been prepared via a novel metal free polycondensation between a tritopic indole-based monomer and squaric, croconic and rhodizonic acids. Each of the three POPs exhibited high BET surface areas (331–667 m2 g−1) and zwitterionic structures. Impedance measurements revealed that the intrinsic POPs were relatively weak proton conductors, with a positive correlation between the density of oxo-groups and the proton conduction. Doping the materials with LiCl vastly improved the proton conductivity up to a value of 0.54 S cm−1 at 90 °C and 90% relative humidity.

The flask was fitted with a Dean-Stark condenser containing molecular sieves (3 Å) and then attached to a condenser. The reaction was heated to 130 °C and left to react over 3 days.
After this time, a dark, insoluble solid had formed. The precipitate was filtered and washed with H 2 O, acetone and THF. The resulting polymer was purified via Soxhlet extraction with toluene (24 h) and chloroform (24 h) and dried in vacuo to afford 350 mg (96%) of a metallic red powder.

RH 1
Quinoline Butanol/toluene, 72 h, 3 Å molecular sieves 1,3,5-Tris(2,3,3-trimethyl-3H-indol-5-yl)benzene (300 mg, 0.545 mmol), rhodizonic acid (169 mg, 0.2 mmol), and quinolone (16 mg, 0.11 mmol) were added to a flask, dissolved in a butanol/toluene mixture (20 mL; v/v 1:1), and deoxygenated via bubbling with N 2 for 10 min. The flask was fitted with a Dean-Stark condenser with molecular sieves (3A) and then attached to a condenser. The reaction was heated to 130 °C and left to react over 3 days.     Procedures for the surface area analyses for PSQ, PCR and PRH. The porosity analyses were performed using a Quantachrome Autosorb iQ gas sorption analyzer. Each polymeric samples was outgassed at 0.03 torr with a 2 °C/min ramp to 100 °C and held at 100 °C for 12 hours. Pore analysis was first performed using CO 2 at 273.15 K. Afterwards, each sample was reactivated at 0.03 torr with a 2 °C/min ramp to 100 °C and held at 100 °C for 12 hours. Pore analysis was then performed using N 2 at 77.35 K (P/P 0 range of 1×10 -5 to 0.995).

Impedance Analysis
AC impedance data were obtained using a pellet (3 mm in diameter) pressed at 5,000 kg for a couple of minutes. The thickness of the pellet was ranging from 0.5 to 1.0 mm. The pellet was placed in a sample holder composed of electrode and the sample holder was put in a chamber controlling temperature and humidity. Data points were obtained after the condition to be stable over 30 min. Measurements were carried out using a Solartron SI 1260 Impedance/Gain-Phase Analyzer and 1296 Dielectric Interface and applied AC voltage amplitude of 100 mV and frequency range of 10 MHz -1 Hz.

Synthesis of LiCl@PSQ
PSQ (15 mg) was ground for 30 min and then washed with deionised water (5 mL × 3) and acetone (5 mL × 3). After being dried in air, PSQ was immersed into saturated lithium chloride in THF (2 mL) for 2 days at room temperature. Then, the sample was filtered, washed with THF (5 mL × 3) and acetone (5 mL × 3), dried in air to afford the LiCl@PSQ sample.

Synthesis of LiCl@PCR
PCR (15 mg) was immersed into saturated lithium chloride in THF (2 mL) for 2 days at room temperature. Then, the sample was filtered, washed with THF (5 mL × 3) and acetone (5 mL × 3), dried in air to afford the LiCl@PCR sample.

Synthesis of LiCl@PRH
PRH (15 mg) was ground for 30 min and then washed with deionised water (5 mL × 3) and acetone (5 mL × 3). After being dried in air, PRH was immersed into saturated lithium chloride in THF (2 mL) for 2 days at room temperature. Then, the sample was filtered, S22 washed with THF (5 mL × 3) and acetone (5 mL × 3), dried in air to afford the LiCl@PRH sample.

Calculation for proton conductivity and activation energy.
The Nyquist plots (Z'' vs. Z') of proton-conducting MOF often show a single semicircle at high frequency, representing proton resistivity contributions of bulk sample. The proton conductivity was deduced from the semicircle by fitting an equivalent circuit which consists of Rs, R1 and W1 in the frequency range from 10 MHz to 1 Hz. Rs corresponds to wire and electrode resistance, R1 is proton resistance and W1 attributes to the resistivity of grain boundary. Sometimes W1 is not necessary, because the impedance plot of the capacitive tail may not appear in the measured range due to the high magnitude of the resistivity. The water-assisted conductivities of synthesized materials were measured under different relative humidity and temperature conditions and were further fitted with different fitting circuits using the ZView software. Proton conductivity (σ, S cm -1 ) was calculated from the impedance spectra with the equation of σ = l/RS, where l is the thickness (mm) and S is the cross-sectional area (mm 2 ) of the pellet, while R (Ω) can be calculated from the impedance plots. The activation energy values were calculated using the Arrhenius equation σT = σ 0 exp(-Ea/kT) by the slope of the plots of ln(σT) versus 1000/T.        Theoretically, the proton conductivity has a good correlation with the activation energy, i.e., the proton conductivities follow the order PSQ < PCR < PRH, the activation energy follow the order PSQ > PCR > PRH (J. Am. Chem. Soc. 2011, 133, 2034-2036. However, the measured activation energies of PSQ, PCR and PRH at 90 o C were 0.43, 0.42 and 1.08 eV, respectively. As for the higher activation energy of PRH, one possibility is onset of a glass or phase transition that causes the sites preferred by protons in the polymer to become disordered. (J. Am. Ceram. Soc. 2002, 85, 2637 S28

pH-Measurement of PSQ, PCR and PRH
10 mg of sample polymer was immersed into deionized water (0.2 mL). After sonication, the pH was measured using universal indicator paper (see Figure S37). Two key observations were drawn from this experiment: 1) The pH values all point to weak acidity (no lower than 4), indicating low concentrations of protons, and consistent with the low proton conductivities of all three as-made samples at room temperature.
2) The pH values of the PRH and PCR are slightly lower than that of PSQ, which could be caused by the greater number of electron-withdrawing C=O groups in the croconic (3 C=O groups) and rhodizonic (4 C=O groups) units, relative to the squaric (2 C=O groups) unit.