Dennis
Sheberla§
*,
Snehangshu
Patra
,
Yair H.
Wijsboom
,
Sagar
Sharma
,
Yana
Sheynin
,
Abd-Elrazek
Haj-Yahia
,
Adva Hayoun
Barak
,
Ori
Gidron
and
Michael
Bendikov¶
Department of Organic Chemistry, Weizmann Institute of Science, Rehovot, 76100, Israel
First published on 17th October 2014
Polyfurans have never been established as useful conjugated polymers, as previously they were considered to be inherently unstable and poorly conductive. Here, we show the preparation of stable and conducting polyfuran films by electropolymerization of a series of oligofurans of different chain lengths substituted with alkyl groups. The polyfuran films show good conductivity in the order of 1 S cm−1, good environmental and electrochemical stabilities, very smooth morphologies (roughness 1–5 nm), long effective conjugation lengths, well-defined spectroelectrochemistry and electro-optical switching (in the Vis-NIR region), and have optical band-gaps in the range of 2.2–2.3 eV. A low oxidation potential needed for polymerization of oligofurans (compared to furan) is a key factor in achievement of improved properties of polyfurans reported in this work. DFT calculations and experiments show that polyfurans are much more rigid than polythiophenes, and alkyl substitution does not disturb backbone planarity and conjugation. The obtained properties of polyfuran films are similar or superior to the properties of electrochemically prepared poly(oligothiophene)s under similar conditions.
Many researchers have tried to overcome preparation problems by varying the conditions of polymerization.5,11 The main problem associated with electrochemical polymerization is the high oxidation potential required to polymerize furan, which results in irreversible over-oxidation of the resulting polyfurans.5,12 A mixture of trifluoride diethyl etherate and ethyl ether (BFEE/EE) as a solvent was used to reduce this electropolymerization potential.11c,d,13 However, the reported conductivities of the resulting polyfurans were small (up to 10−2 S cm−1), and their absorption spectra (λmax = 420 nm)11d reveal a short conjugation length of about eight furan rings.14 Kanatzidis et al., used terfuran as the starting monomer to lower the potential needed for polymerization to less than 1.0 V (vs. SCE).11b The resulting polyfuran was characterized by a variety of techniques and showed a red shifted absorbance maximum centered at 468 nm. Nevertheless, the polyfuran produced had a low conductivity of 2 × 10−3 S cm−1, which is probably due to defects in the polymer, as were observed by IR spectroscopy. Chemically synthesized polyfuran did not appear to be highly conjugated because of a significant degree of furan ring-opening defects.9,15 It is noteworthy that in contrast to polyfurans, the incorporation of furan blocks in conjugated polymers resulted in high quality materials.16 Overall, despite numerous reports on the preparation of polyfuran (usually lacking full characterization),5 it appears that there are no reports of polyfurans with properties such as high conductivity, stability, high conjugation length and small number of defects, all of which are required for their application as organic electronic materials.
We previously reported the preparation of long unsubstituted α-oligofurans (Fu5-Fu9) and recently alkyl substituted α-oligofurans (Fun-2C6 (n = 4, 6, 8) and Fu16-6C6) that exhibit higher fluorescence, better packing, greater rigidity, and higher solubility than the corresponding oligothiophenes.14,17 We also showed that these oligofurans display good electronic properties, e.g., field effect mobilities and on-off ratios, similar to those of the corresponding oligothiophene analogues.18 Thus, we surmised that polyfurans would not only possess similar properties, but would have advantages over other conductive polymers. The rigidity of their backbone is expected to enable the introduction of large side substituents without breaking planarity and π-conjugation. Moreover, polyfurans can be synthesized from renewable resources19 and may be biodegradable.20 Thus, we believe that the reported poor properties, such as low conductivity,5 blue-shifted absorption,11d and instability,9 of polyfurans were due to defects produced during their preparation rather than intrinsic properties of polyfurans.
Here we report the first electrochemically-prepared stable and conducting polyfuran films P1–P7 (Chart 1), which possess all the properties necessary to be considered as conducting polymers suitable for applications in organic electronics. These polymers have several advantages over other conjugated polymers, such as rigidity of the backbone and film smoothness. The use of long oligofurans with solubilizing groups, which have relatively low oxidation potential, as the starting monomers for polymerization was the key factor in obtaining polyfuran with improved properties. The electrochemical and spectroelectrochemical behavior, conductivity, electrochemical stability in air under ambient conditions, as well as the morphology of the prepared polyfuran films were studied and compared with corresponding polythiophene analogues. We also report the development of a synthetic methodology for the preparation of long oligofurans 1–7 substituted with solubilizing groups.
For the synthesis of octafuran 5 (Scheme 2), dibromo derivative 13 was prepared using the procedure described in the literature.22 Interestingly, during the course of the reaction, terfuran derivative 14 was also obtained with a 6% yield. Subsequent reaction of compound 13 and octylmagnesium bromide with Ni(dppp)Cl2 gave 3,3′-dioctyl-2,2′-bifuran (15) with a 74% yield. Stille coupling of dibromo derivative 16 (obtained from the bromination of 15 using NBS) with [2,2′:5′,2′′-terfuran]-5-yltributylstannane (11) yielded dioctyl-substituted octafuran 5.
Oligomer 2 has the highest octyl chain to furan ratio (1:
3), while oligomer 4 has the lowest octyl chain to furan ratio (1
:
7). In order to synthesize polymers having at least a 1
:
1 ratio of alkyl chain to furan ring, bromo groups present in compound 14 were replaced with alkyl chains. Kumada coupling of 14 with methylmagnesium bromide gave compound 6 (Scheme 3) with a moderate yield of 51%. The tetramethyl quaterfuran 7 (Scheme 3) was synthesized by lithiation of the monobromo precursor 18 followed by coupling with CuCl2.
The electrodeposition of polymers P1–P7 has been performed both potentiodynamically and potentiostatically. The best polyfuran films were obtained using a 0.1–1 mM concentration of the oligomer in an ACN solution containing 0.1 M TBACF3SO3 as the electrolyte. In general, for potentiodynamic polymerization, the potential was scanned at 50 mV s−1 from −0.2 V to 0.8 V vs. Ag/AgCl-wire and for potentiostatic polymerization, a potential of 0.75 V vs. Ag/AgCl-wire was applied. Further optimization of polymerization conditions is possible for each of the oligomers (see Table S2, ESI‡). Smooth electrodeposition and the growth of insoluble, highly adhesive continuous polymeric films were observed on different types of working electrodes (i.e., ITO-coated glass, Pt-disk, Au-coated quartz crystal and IDA). It is worth noting that the electrodes were not modified and specially cleaned as they were in the preparation of smooth polythiophenes.24 Representative cyclic voltammograms (CV) for the polymerization of P3 (PFu5-C8) on ITO electrode, showing stepwise increases in the current over the course of polymerization, are displayed in Fig. 1 (for other polymers see Fig. S3, ESI‡).
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Fig. 1 Electrochemical deposition of 3 (0.1 mM) in 0.1 M TBACF3SO3/ACN at 50 mV s−1 on an ITO-coated glass slide. |
Oxidation peak potentials are poorly resolved for oligomers 1, 2, 4 and 5 (Fig. S3‡). Therefore, the onset potentials for oligomer oxidation (Eox), measured from the first CV curve of the polymerization, were used for comparison between different oligomers (Table 1). As expected, both the introduction of an alkyl group into an oligomer and the elongation of the oligomer decrease the oxidation potential. For example, the Eox for P1 (PFu3) is 0.62 V, whereas for P2 (PFu3-C8) and P3 (PFu5-C8), it is 0.51 V and 0.43 V, respectively. Furthermore, polyfurans are obtained at lower oxidation potentials than polythiophenes, as shown by the oxidation onset potential of terfuran, which is 0.33 V lower than that of terthiophene.
It is well known that the electrolyte, solvent, oligomer concentration, and method of electropolymerization drastically affect the quality of the resulting polymeric films.1d,23,25 We found that TBACF3SO3/ACN electrolyte (see Fig. S4‡ for others) and concentrations of monomers lower than usually used3b,11b produced better polyfurans. Electrolyte anions and solvent could affect the stability of intermediate σ-dimers, while lower concentrations could reduce π-dimerization of cation radicals that are produced during electropolymerization.1d
CVs of polyfurans in monomer-free TBACF3SO3/ACN electrolyte showing broad oxidation and reduction peaks (Fig. 2 and S5‡), are similar to those of other conducting polymers.1d,3b It is difficult to determine accurate oxidation potentials for polyfurans from the obtained CVs. Nevertheless, oxidation potentials could be estimated from in situ conductivity measurements (see below).
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Fig. 2 CV of P3 film on an ITO coated glass electrode in monomer-free 0.1 M TBACF3SO3/ACN electrolyte at different scanning rates. |
The measured λmax values of neutral polyfurans are in the range of 456–466 nm (Table 2). This range is red-shifted compared with most of previously reported values of <420 nm.10c,11a,c,d,12b This suggests that much longer effective conjugation lengths are obtained in the current work. We note that Kanatzidis et al. reported11b an absorbtion maximum of 468 nm for PFu3. However, their spectrum lacks vibronic shoulders, which are observed in all our spectra. We estimate the effective conjugation length in prepared polyfurans to be more than 25 furan units, as can be deduced from extrapolation of the λmax values of α-oligofurans (see Fig. S25‡).
Polymer | λ max, nm | Optical band gapa, eV | E g (calc)b, eV | HOCO (calc)b, eV | LUCO (calc)b, eV |
---|---|---|---|---|---|
a The optical band gaps were obtained from the extrapolation of the linear part of the (αhν)2vs. hν plot to (αhν)2 = 0, where α is the absorption coefficient. b Calculated using the PBC/B3LYP/6-31G(d) level. c For comparison, PT3 was prepared and measured in this work at same conditions as polyfurans. | |||||
PFu3 (P1) | 466 | 2.31 | 2.41 | −4.36 | −1.95 |
PFu3-C8 (P2) | 462 | 2.31 | 2.35 | −4.20 | −1.85 |
PFu5-C8 (P3) | 463 | 2.32 | 2.37 | −4.23 | −1.88 |
PFu7-C8 (P4) | 458 | 2.29 | 2.38 | −4.29 | −1.90 |
PFu8-2C8 (P5) | 460 | 2.31 | 2.36 | −4.23 | −1.88 |
PFu3-3C1 (P6) | 464 | 2.22 | 2.25 | −4.02 | −1.78 |
PFu4-4C1 (P7) | 456 | 2.22 | 2.26 | −4.03 | −1.77 |
PT3c | 475 | 2.02 | 2.05 | −4.61 | −2.56 |
The optical band gaps of polyfurans estimated from the absorption spectra (Table 2) for unsubstituted P1 and partially alkyl substituted P2–P5 are ca. 2.3 eV. By contrast, for polyfurans P6 and P7 substituted with one methyl group per furan ring, the band gaps are decreased to ca. 2.2 eV. This observation is consistent with DFT calculations, which showed that alkyl substitution reduces band gaps (Table 2 and S3‡). Moreover, calculations (Table 2) showed that this reduction in band gap occurs because of the increase in the highest occupied crystal orbital (HOCO) energy (i.e. the energy of the valence band), which is partially offset by an increase in the lowest occupied crystal orbital (LUCO) energy (i.e. the energy of the conduction band). Consequently, the oxidation potential of alkyl-substituted polyfurans decreased. The increase of the HOCO energy is consistent with the electron donating property of alkyl groups.
Interestingly, the peak of neutral polyfuran films shows red-shifted shoulders (Fig. 3 and S8‡), probably because of the vibronic coupling.27 The presence of vibronic coupling may indicate that the polymer backbone is highly rigid, planar and effectively π-conjugated. We have shown that oligofurans exhibit high rigidity,14,28 as evident from calculated twisting potentials which are higher than for oligothiophenes. Similar computational results have been recently shown for polyfurans.29 Consequently, the π-conjugation and planarity of polyfurans should not be distorted by various substitution of the polyfuran backbone as strongly as it is in the polythiophene family. Indeed, DFT geometry optimization (PBC/B3LYP/6-31G(d)) of polyfurans P1–P7 shows that the polyfuran backbone remains planar upon alkyl substitution (Fig. 4a and S10‡). In contrast, in polythiophene PT4-4C1, which is a thiophene analogue of P7, the polymer backbone deviates strongly from planarity (Fig. 4b), distorting the π-conjugation. As a consequence, the calculated band gap for PT4-4C1 increases to 2.9 eV from 2.0 eV for planar unsubstituted polythiophene.29,30 Whereas measured optical and calculated band gaps for polyfurans P1–P7 are not increased upon alkyl substitution.
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Fig. 4 Calculated (PBC/B3LYP/6-31G(d)) geometry of (a) P7 and (b) polythiophene analog of P7, PT4-4C1. A side view and a view along the polymer backbone. |
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Fig. 5 Experimental FTIR-ATR spectra: (a) of P1 and (b) of P3 films electrodeposited on platinum foil. (c) DFT B3LYP/6-31G(d) calculated IR spectrum of Fu20 (frequencies were scaled by 0.96 (ref. 31)). |
Fig. 6 shows simultaneous measurements of CV and conductance of P1–P4, P6 and P7 films. All obtained polyfurans show maximum conductances comparable to those of PT3 and PEDOT:PSS films (Table 3, Fig. S15‡). Thus electrical conductivities of doped polyfurans are in the order of 1 S cm−1. This conductivity is much higher than most published values, which range from 10−2 to 10−5 S cm−1.5
Polymer | Conductance (max), mS | E hc , V |
---|---|---|
a Half-maximal conductance potential (vs. Ag/AgCl-wire) for the forward scan. b Limited by the resistance of the IDA electrode contacts (Fig. S15). c Drop-cast film. | ||
PFu3 (P1) | 43 | 0.69 |
PFu3-C8 (P2) | 32 | 0.58 |
PFu5-C8 (P3) | 38 | 0.62 |
PFu7-C8 (P4) | 37 | 0.55 |
PFu3-3C1 (P6) | 46 | 0.30 |
PFu4-4C1 (P7) | 32 | 0.44 |
PT3 | 42 | 0.87 |
PEDOT | >72b | −0.21 |
PEDOT:PSSc | 50 | — |
Each polymer undergoes transition to conductive state at different potentials (Fig. 6 and S15‡). We used the potential at half-maximal conductance (for the forward scan) (Ehc) for comparison between polymers. As evident from Table 3, Ehc depends on the degree of alkyl substitution, i.e., Ehc decreases with increasing degree of alkyl substitution (Table 3). For example, the value of Ehc for fully methylated polyfuran P6 is 0.39 V is lower than that of the unsubstituted P1. Since, the transition to conductive state is associated with the oxidation of a conjugated polymer, we can conclude that the oxidation potential of polyfurans is also decreased by alkyl substitution. This is in the agreement with DFT calculations, which show increase in energy of the HOCO as the degree of alkyl substitution increases (Table 2 and S3‡). Furthermore, for polyfurans, the Ehc potentials are lower than that of PT3 but higher than that of PEDOT, which is consistent with the higher oxidation potential of PT3 (ref. 38) and the lower oxidation potential of PEDOT.39
We observe that the conductivity of fully methylated polyfurans P6 and P7 reaches maximum at 0.5 V and 0.6 V, respectively, and decreases at higher potentials. This observation may suggest that the charge carriers are polarons. Upon further electrochemical oxidation, their conductivity decreases (reversibly) as bipolarons are formed. However, this behavior is not observed in polyfurans P1–P4 due to degradation of the polyfuran films at potentials above 0.8 V, where their conductivity decreases irreversibly.
The degraded optical activities (i.e., difference in transmittance of undoped and doped state) obtained after 120 min of cycling (for a total of 360 cycles) were found to be 39%, 45%, and 51% for P1, P3 and PT3 of the starting activity, respectively. Thus, polyfurans P1 and P3 showed only slightly lower stabilities compared with PT3, but are much better than that reported for a PFu/PEDOT device.10c
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Fig. 9 Stability of the conductive state of polymer films: (a) P1 at 0.7 V, (b) P3 at 0.7 V, and (c) PT3 at 0.9 V on an IDA electrode in monomer-free 0.1 M TBACF3SO3/ACN electrolyte. |
As shown in Fig. 9, polyfurans P1 and P3 show much higher stabilities toward continuous potential application than PT3, whose lower stability is likely due to the high potential (0.9 V) needed for oxidation (doping), which in turn, is expected to break the conjugation of the backbone via side reactions.42 Thus, the lower potential required for doping the polyfurans is advantageous because it minimizes the occurrence of backbone degradation under ambient conditions.
As can be seen in Fig. 10 showing SEM and AFM images of representative example of P3 on HOPG electrode (for the additional images of polyfurans on different electrodes see ESI‡), the prepared polyfuran films show excellent surface smoothness and complete coverage of all working electrodes used. In contrast, polythiophene exhibits separate islands of film and areas of bare electrode surface (see Fig. S17‡ for similarly prepared PT3).47 Additionally, polyfuran films have high adhesion to the electrode surface and are not easily detached. For example, they cannot be pulled off by sticking adhesive tape to the electrode, as in case of poly(3-methylthiophene) films.48 Additionally, films were found to be completely insoluble in organic solvents, such as THF and toluene.
The thickness of prepared polyfuran films can be easily controlled and tailored for the specific application. For instance, the film thickness of P3 linearly depends on the number of CV cycles used in the polymerization (Fig. 11). The thickness of 10 nm on HOPG electrode was obtained after 1 cycle (Fig. 10), 45 nm after 5 cycles (Fig. S19‡) and 90 nm after 10 cycles (Fig. S20‡). The films show full coverage of electrodes with very smooth and homogeneous morphology. Although the rrms value for the 10 nm thick film was only 1 nm, it increased with film growth, as evidenced by the rrms values of 3.5 nm and 5 nm for 45 and 90 nm thick films, respectively (Fig. S20‡). For polyfuran films with thickness above 50 nm, formation of globular structures on the top of smooth film is observed on both HOPG and ITO electrodes (see Fig. S20–S22‡). Polyfuran film roughness on an ITO electrode (Fig. S21–S22‡) is higher than on HOPG and is due to inherent higher roughness of the electrode (i.e., rrms = 4.6 nm for bare ITO, Fig. S23‡).
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Fig. 11 Dependence of (a) thickness and (b) roughness on number of CV cycles used for polymerization of P3 on HOPG electrode. |
The measured high conductivity on the order of 1 S cm−1 for doped polyfuran films may be attributed to better quality of the prepared polyfurans when compared with previous reports.5 This increased quality is manifested in the longer effective conjugation lengths (estimated to be above 25 furan units based on oligomer data, Fig. S25‡) revealed by absorption spectra of the reduced (dedoped) polyfuran films, which show considerable red-shifted (456–466 nm) absorption maxima compared to most reported polyfurans.11a,c,d,12b Additionally, IR spectroscopy showed almost no structural defects, including ring opening and β-coupling in the backbone. Moreover, absorption spectra of the neutral polyfurans show vibronic coupling, indicative of well-ordered, planar, and rigid polymers. Although similar vibronic coupling is also observed in highly conductive samples of PT49 and PEDOT,27b they were not previously observed in electrochemically prepared polyfurans. Furthermore, the measured conductivity of polyfurans is comparable to the conductivity of electrochemically prepared poly(oligothiophene)s.7a Thus, we are convinced that the previously reported low conductivities5 are due to preparation problems, and are not an intrinsic property of polyfurans.
We have studied the effect of alkyl substitution on the properties of polyfurans. DFT calculations predict reductions of the band gaps, as well as oxidation potentials of polyfurans, with increasing alkyl substitution ratio (see Table S3‡). Indeed, compared to P1, we experimentally observe a reduction of around 0.08 eV (Table 2) in optical band gaps in fully methylated polyfurans P6 and P7. Although we cannot reliably measure the oxidation potentials from cyclic voltammetry, we do qualitatively observe that the onset of the broad oxidation peak is shifted to lower potentials for P6 and P7. Another estimation of oxidation potentials can be obtained from the potential at half-maximum conductance Ehc, which is significantly decreased upon alkyl substitution (e.g., Ehc = 0.69 V vs. Ag/AgCl-wire for PFu3 (P1) and Ehc = 0.30 V for PFu3-3C1 (P6)). Furthermore, Ehc for polyfurans is lower than that of PT3 (Ehc = 0.87 V). Therefore, when compared to polythiophenes, a lower oxidation potential of polyfurans serves as an advantage. The lower doping potential minimizes the occurrence of the destructive side reactions (e.g., with water and oxygen). Moreover, oligofurans show lower oxidation potential compared to oligothiophenes, as evidenced by the onset potentials for oligomer oxidation (0.62 V for terfuran and 0.95 V for terthiophene).
As shown by DFT calculations29 and absorption spectra (Fig. 4 and S11‡), polyfurans have the further advantage of being more rigid than polythiophenes. High rigidity of polyfurans is also consistent with the experimental observation that regioirregular methylated polyfurans P6 and P7 show lower band gaps and oxidation potentials. This is advantageous for future development of furan-based polymers, where regioregular arrangement of different functional groups (otherwise needed in the case of polythiophenes to keep the efficient π-conjugation, e.g., rrP3HT)50 is no longer mandatory.
We have carefully studied the stability of electrochemical, optical, and conductive properties of polyfurans in open air under ambient conditions. The stability of the redox activity and conductivity over time showed that polyfurans are actually more stable than polythiophene PT3 under similar conditions. This stability is attributed to lower potential needed for doping of polyfurans. The optical switching stability of polyfurans is found to be comparable with that of PT3, and it is much better than the previously reported stability of the optical switching of a PFu/PEDOT device at 415 nm. Previous studies showing optical switching degradation after only 90 s then is likely attributable to the polyfuran's poor quality, as suggested by the low absorption maximum (λmax = 420).10c Therefore, we believe that the earlier reported instabilities of polyfurans are also due to problems in their preparation.
We have identified two phases of the polyfuran film growth. Initially (up to ca. 50 nm thickness), we observe a smooth thin film covering the whole electrode surface is formed. As the film continues to grow, globular bumps and aggregates are observed on top of the smooth film surface. As a result, the roughness of the surface increases. For instance, in P3 on HOPG, the rrms value was ca. 5 nm for the underlying film and ca. 17 nm for the full scanned area (Fig. S20‡). The formation of the structures on top of homogenous film is consistent with electrochemical polymerization where a new polymer forms at surface points with a higher conductivity.
It is worth noting that prepared polyfuran films have lower roughness than most other electrochemically-prepared films under similar conditions,24,25c,44–46i.e., in organic media electrolyte, in ambient environment, without electrode surface modifications and extensive electrode cleaning. This is certainly an advantage for technological applications of polyfurans.
Polymerization of oligofurans using considerable lower potential than parent furan allowed us to obtain stable polyfurans with very good properties. This is in contrast to the previous belief that polyfurans are intrinsically unstable and possess low conductivity. We believe that this work will establish polyfurans as a fundamental series of conductive polymers with important properties, opening up future applications for polyfurans as conductive polymers.
Footnotes |
† This work is dedicated to the memory of Professor Michael Bendikov. |
‡ Electronic supplementary information (ESI) available: Full experimental and computational details, synthesis of 1–18, Fig. S1–S26 and Tables S1–S4. See DOI: 10.1039/c4sc02664k |
§ Current address: Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, US. E-mail: E-mail: denn@mit.edu |
¶ Deceased July 2, 2013. |
This journal is © The Royal Society of Chemistry 2015 |