Conducting polyfurans by electropolymerization of oligofurans

Polyfuran films produced by electropolymerization of a series of oligofurans substituted with alkyl groups show improved properties, such as good conductivity and stability, well-defined spectroelectrochemistry and smooth morphology.


Experimental Materials
Compounds 1-7 and 10-18 were synthesized (see below; for the synthesis of compounds 1, 10 and 11 see elsewhere 1 ). All other compounds were used as is following their purchase from Sigma-Aldrich, with the exception of 3-bromofuran, which was purchased from CAPOT Chemical Company Limited. Heraeus Clevios™ P standard dispersion (PEDOT:PSS) was passed through 1.2μm syringe filter before use. 1 H and 13 C NMR spectra were recorded in solution on Bruker AVANCE III 300 MHz, 400 MHz and Bruker AV-500 MHz spectrometers using tetramethylsilane (TMS) as the external standard. Chemical shifts are expressed in ppm. High resolution mass spectra were measured on a Waters Micromass GCT Premier Mass Spectrometer using field desorption (FD) ionization. Differential scanning calorimetry (DSC) measurements were performed on a TA Q200 DSC instrument. UV-Vis-NIR absorption measurements were made on a JASCO V-570 spectrometer. Et2O, THF and toluene were distilled from sodium/benzophenone under an atmosphere of dry argon prior to use.
The solvent was evaporated in vacuum. The compound was purified by column chromatography (elution with hexane) to give the terfuran 6 as a colorless oil which solidified on cooling at −20C (0.15 g, 51% yield

Synthesis of 5,5'-dibromo-3,3'-dioctyl-2,2'-bifuran (16)
. N-Bromosuccinimide (0.21 g, 1.20 mmol) was added to a solution of 3,3'-dioctyl-2,2'-bifuran (0.20 g, 0.56 mmol) in 30 mL benzene at room temperature under N2. The reaction mixture was kept at room temperature and stirred for 2 h. The mixture was quenched with saturated sodium bicarbonate and saturated sodium thiosulfate pentahydrate (Na2S2O3·5H2O). The product was extracted with hexane, dried (MgSO4), and evaporated. Flash chromatography on a silica column using hexane as eluent gave 16 (0.23 g, 80% yield) as a white crystalline product. 1 H NMR (300MHz, C6D6): The spectroelectrochemical setup consisted of a quartz cuvette as the electrochemical cell in a custom-made holder, an ITO-coated glass electrode used as the working electrode, Ag/AgCl-wire used as a pseudo reference electrode, and Pt-wire used as a counter electrode. Potentials were applied using a Princeton Applied Research 263A potentiostat. Absorption spectra were taken in a quartz cuvette with 10 mm optical path length (100-QX, Hellma) with a JASCO V-570 UV-Vis-NIR spectrophotometer.
FTIR-ATR spectroscopy was performed using a Nicolet 6700 FT-IR (Thermo Fisher Scientific Inc.) equipped with a MIRacle (PIKE Technologies) ATR sampling accessory with a ZnSe crystal using an MCT/A liquid nitrogen cooled detector. The presented spectra were not corrected for ATR intensities.
Scanning electron microscopy (SEM) images were recorded using a Leo Ultra 55 FEG SEM with an operating voltage of 3 keV. AFM topography images were acquired using a P47 AFM (NT-MDT) equipped with a small scanner. Images were recorded in tapping mode in the air at room temperature (20-23°C) using silicon micro cantilevers (OMCL-AC240TS-W2, Olympus). The set point ratio was adjusted to 0.75-0.8 (corresponding to "light" tapping) and the scan rate was set to 1 Hz. Imaging was carried out in different scan directions and at different scales to verify the consistency and robustness of the evaluated structures. The thickness of polymer films was measured by AFM profilometry.

In situ conductivity
In situ conductivity measurements were performed on a Bio-Logic SAS VSP potentiostat equipped with two independent channels using an IDA (interdigitated array) microelectrode. The IDA electrode is consisting of two arrays of 50 gold digits (strips), each 10 μm wide and 5 mm long with an interdigit distance of 10 μm (Figure S1), where only the microarrays are accessible to the electrolyte solution. The first channel of the potentiostat was used in the standard three electrode mode using Pt-wire as the counter electrode, Ag/AgCl-wire as the reference electrode, and one of the arrays of the IDA electrode as the working electrode (W1 in Figure S2). A second channel was used to supply a small constant probe potential of 10 mV between two set of digits (working electrodes W1 and W2, Figure S2). Both channels were synchronized and voltammetric (CV) and probe current data were acquired simultaneously. Polymer films were grown potentiodynamicaly or potentiostatically. During electropolymerization, the increasing conductance of the growing film was monitored. It was noticed, that conductance saturates reaching some value. Therefore, the polymerization was stopped when the increase in conductance was small (less than 10%). Once grown, the films were washed with ACN and then placed in monomer-free electrolyte solutions. We found that using such setup low conductance values (<10 -2 S) could not be measured reliably because of the small negative contribution from faradic current. Therefore the on-off ratio cannot be extracted from the experimental data.

Electrolyte Effect
Polyfurans prepared in 0.1 M TBABF4/ACN and 0.1 M TBAClO4/ACN electrolyte showing almost no optical activity with blue-shifted maximum absorption peak, meaning that non-conjugated polyfuran is obtained ( Figure   S4).

CV of polymers
The CV of P1 clearly shows a two-step oxidation process (Figure 2a). For P2, P3 and P7, however, the same twostep oxidation, which is similar to that observed previously in polythiophenes, 8 is less resolved (Figure S5, S6).

Conductivity measurements
A value of 10-80 S/cm was reported by Tourillon and Garnier, 9 who first claimed to have prepared conjugated polyfuran via anodic electrochemical polymerization of furan. However, such high value seems to be unreliable, as Ohsawa et al., 10 were unable to reproduce such a highly conductive polyfuran.
Nalwa 11 also reported conductivity as high as 20 S/cm. However, this value was measured on ill-defined samples as no characterization (other than EPR) was undertaken. As well, such high conductivity contradicts with his conclusion that that the polyfuran has only a low degree of conjugation.
In any case, the high voltage required for anodic coupling of furan should result in irreversible oxidation of the polymer backbone. Hence, in neither of these earlier reports were the conductivities measured on well-defined samples of polyfuran.

Morphology study
AFM data were processed using NT-MDT Nova 12 and Gwyddion 13 software.    PFu 5 -C 8

S30
Estimation of polyfuran effective π-conjugation length from UV-Vis spectroscopy The effective conjugation length in P1 (max=466 nm) is approximated to be about 33 furan units as deducted from second order polynomial extrapolation of absorption maxima of unsubstituted polyfurans as function of the reciprocal of the chain length. (Fig. S25a). Similarly, effective conjugation lengths in alkyl substituted polyfurans P2-P7 (average max=460 nm) is about 27 furan units based on extrapolation of max of dihexyl substituted oligofurans (Fig. S25b). We note that such analysis could serve only an approximation to effective π-conjugation length. EQCM measurements were carried out in a Teflon cell consisting of an Au coated quartz crystal purchased from CH Instruments as the working electrode, a Pt-wire counter electrode, and a Ag/AgCl pseudo reference electrode.
A quartz crystal of 13.7 mm diameter was AT-cut and sandwiched between 5.11 mm diameter vapor-deposited gold disks. The resonating frequency of the crystal in air was 8 MHz, and the surface area of the Au electrode was 0.205 cm 2 . The shear modulus (µ) of the crystal was 2.947×10 11 g cm −1 s −2 and the density (ρ) was 2.648 g cm −3 .
Before the electrochemical experiments, the working electrode was run in double distilled water to obtain a stable