Electronic properties of isoindigo-based conjugated polymers bearing urea-containing and linear alkyl side chains

Brynn P. Charron a, Michael U. Ocheje a, Mariia Selivanova a, Arthur D. Hendsbee b, Yuning Li b and Simon Rondeau-Gagné *a
aDepartment of Chemistry and Biochemistry, University of Windsor, Ontario N9B 3P4, Canada. E-mail: srondeau@uwindsor.ca
bDepartment of Chemical Engineering, University of Waterloo, University Ave West, Ontario N2L 3G1, Canada

Received 12th July 2018 , Accepted 17th September 2018

First published on 17th September 2018


A side-chain engineering study has been performed with isoindigo-based conjugated polymers to modulate their physical and electronic properties through the incorporation of urea functionalities. Easily accessible and versatile, the incorporation of urea moieties in the side chains of conjugated polymers enables the formation of intermolecular hydrogen bonds between the polymer chains, resulting in a significant effect on the solid-state morphology and electronic properties. Incorporation of 0 to 20 mol% of urea-containing side chains was achieved, and the resulting materials were characterized by several techniques. Moreover, a comparison was also performed with similar polymers incorporating 0 to 20 mol% of dodecyl linear side chains. The organic field-effect transistors fabricated from the new materials showed that, at low content of urea moieties, the formation of hydrogen bonds did not allow for a significant improvement of the charge carrier mobility. However, devices made from the polymer with 20 mol% urea moieties showed an enhanced average charge mobility (0.032 cm2 V−1 s−1), in contrast to the polymer with 20 mol% dodecyl side chains (0.0073 cm2 V−1 s−1). The results obtained from the investigation of urea-containing isoindigo-based polymers confirm that the incorporation of urea moieties in polyisoindigos is a promising strategy to control the nanoscale morphology, solubility and polarity of conjugated polymers without reducing their performance in organic field-effect transistors.


1. Introduction

Organic conjugated polymers possess several appealing properties towards the fabrication of electronic devices, such as good charge carrier mobility, easily tunable synthetic design, and good solubility in organic solvents for large-scale solution-based processing techniques.1–4 In order to develop novel devices based on conjugated polymers, several recent works investigated diverse approaches to gain a complete understanding of the relationship between molecular design and properties of the materials, including mechanical,5–8 optical and electronic properties.9–12

Conjugated polymers are modified and designed through side-chain and π-conjugated backbone engineering.13–16 In fact, both design elements have an important effect on the final properties of the materials by modulating and influencing several key factors for electronic and mechanical properties such as backbone planarity, lamellar spacing, π-stacking distances, crystallinity, glass transition temperature, chain alignment, interchain interactions, etc.17 Recently, a new strategy, based on the utilization of non-covalent interactions, has been developed and utilized by several groups to modulate the electronic properties of conjugated polymers.18,19 Through rational chemical design, non-covalent bonding moieties have been introduced in conjugated polymers in order to create intramolecular conformational locks, thus improving backbone planarity allowing for enhanced π-delocalization as well as charge carrier mobility in organic field-effect transistors (OFETs).20–22 Hydrogen bonds have also been investigated to guide the polymer self-assembly and influence the solid-state morphology of electroactive materials.18,23,24 By using this approach, the synthesis of diketopyrrolopyrrole (DPP)-based conjugated polymers with thermally labile side-chains that were removed upon thermal annealing was reported.25,26 The resulting side-chain free polymer thin films were shown to be morphologically stable, solvent-resistant to degradation, and led to good performance in organic electronic devices. However, despite the efficiency of this approach to control the processing of conjugated polymers in thin films, the polymers suffered from reduced solubility after removal of the protecting groups, which limited their compatibility with solution-based deposition techniques.

A few examples of soluble conjugated polymers employing hydrogen-bonding interactions were recently reported.27 Among others, Zhang et al. reported an innovative strategy to incorporate up to 10 mol% urea-containing side-chains onto DPP-based polymers.19 The incorporation of the hydrogen-bonding moieties allowed for a significantly enhanced field-effect mobility of 13.1 cm2 V−1 s−1, which surpasses previously reported charge mobilities for comparable conjugated DPP-based polymers. Another utilization of hydrogen bonding in conjugated polymers has recently been reported by Bao et al. to improve the stretchability of DPP-based semiconducting polymers.28 Incorporation of various amounts of a PDCA conjugation breaking unit allowed for intermolecular hydrogen bonding, which significantly modified the solid-state morphology of the polymer network. The PDCA motif possesses many intriguing properties such as the possibility of coordinating metals and allowing for both intramolecular and intermolecular hydrogen bonding through the non-bonding amides and pyridine. In addition to the formation of dynamic bonding between the polymer chains, the non-conjugated PDCA moieties also reduced the crystallinity and promoted chain alignment upon strain, overall enhancing the molecular stretchability of the synthesized materials.

Recently attracting a lot of attention in the literature as a π-conjugated building block for the design of high mobility conjugated polymers, isoindigo has been shown to be particularly interesting for the preparation of intrinsically stretchable conjugated polymers.29,30 In fact, as reported by Bao et al. in 2014, poly(isoindigo) can be stretched up to two times more than DPP-based polymers before suffering from a major decrease in charge mobility.31 Furthermore, as demonstrated by multiple examples through the recent literature, isoindigo is easily tunable via rational synthesis and represents a promising building block for organic electronics.32–34

Herein, we report the synthesis and characterization of novel isoindigo-based conjugated polymers incorporating hydrogen bonding moieties within the solubilizing side chains. Specifically, urea functional groups were selected as hydrogen bonding moieties given their strong hydrogen bonding and directionality. Moreover, the urea groups showed great promise for the enhancement of electronic properties in π-conjugated polymers and can be easily installed in a limited number of synthetic steps.19 In order to gain complete insight on the influence of urea moieties on the optical and electronic properties, multiple characterization techniques were utilized, including UV-Vis spectroscopy, gel-permeation chromatography (GPC), FT-IR spectroscopy and X-ray diffraction. Moreover, OFETs were fabricated to investigate the thin film formation and electronic properties of the urea-containing isoindigo-based polymers. Finally, since the introduction of linear side chains is known to drastically influence the solid-state morphology of conjugated polymers, a careful comparison was performed with different control polymers, incorporating various ratios of linear side-chains (dodecyl) (Fig. 1).


image file: c8tc03438a-f1.tif
Fig. 1 Structures of isoindigo-based conjugated polymers P1 to P7.

2. Experimental section

2.1 Materials

Commercial reactants were used without further purification unless stated otherwise. All the solvents used in these reactions were distilled prior to use. Tris(dibenzylideneacetone)dipalladium(0)-chloroform adduct (Pd2(dba)3·CHCl3) was purchased from Sigma Aldrich and recrystallized following a reported procedure.35 (E)-1,2-Bis(5-(trimethylstannyl)thiophen-2-yl)ethene (TVT), (E)-6,6′-dibromo-1,1′-didodecyl-[3,3′-biindolinylidene]-2,2′-dione, and (E)-6,6′-dibromo-1,1′-didodecyl-[3,3′-biindolinylidene]-2,2′-dione were synthesized according to literature procedures.34,36,37

2.2 Measurements and characterization

Nuclear magnetic resonance (NMR) spectra were recorded on a Bruker 300 MHz spectrometer. The spectra for all polymers were obtained in deuterated 1,1,2,2-tetrachloroethane (TCE-d2) at 120 °C. Chemical shifts are given in parts per million (ppm). Number average molecular weight (Mn), weight average molecular weight (Mw), and polydispersity index (PDI) were evaluated by high temperature size exclusion chromatography (SEC) using 1,2,4-trichlorobenzene and performed on a PL-GPC 120 instrument (Agilent Technologies) equipped with a single TSKgel GPC column (GMHHR-H; 300 mm × 7.8 mm) also calibrated by monodisperse polystyrene standards. Thermogravimetric analysis (TGA) was performed on a Mettler Toledo AG-TGA/SDTA851e. FTIR spectroscopy was performed on a Bruker ALPHA FTIR Spectrometer using a Platinum ATR sampling module. UV visible spectroscopy was performed on a Varian UV/visible Cary 50 spectrophotometer. The surface structure of polymer film was obtained using a Multimode atomic force microscope (AFM, Digital Instruments) operated in the tapping mode at room temperature. Images were collected using Nanoscope 6 software and processed using WSxM 5.0 Develop 8.0 software. All measurements were conducted using a Keithley 4200 semiconductor parameter analyzer (Keithley Instruments Inc.) under dry Ar (glove box). A BASi Epsilon Potentiostat was used to obtain the cyclic voltammetry measurements. Cyclic voltammograms were taken in a 0.1 M solution of TBAPF6 in anhydrous acetonitrile at room temperature. Pt was used as working electrode and counter electrode. Ag|AgCl was used as non-aqueous reference. The scan rate of measurements was 100 mV s−1. X-ray diffraction was performed on a Proto AXRD Benchtop Powder Diffractometer with a Cu source (λ = 1.5406 Å).

2.3 FET device fabrication and characterization

A bottom-gate bottom-contact (BGBC) configuration was used for all of the OTFT devices. A heavily n+-doped SiO2/Si wafer with a 300 nm thick SiO2 dielectric layer was patterned with a gold source and drain pairs having a channel width (W) of 1000 μm and length (L) of 30 μm, using conventional photolithography and thermal deposition techniques. The wafers were cleaned via oxygen plasma treatment with C18H37SH for 2 minutes, followed by sonication in acetone and isopropanol. Subsequently, the wafers were treated with acid (aq.) and dodecyltrichlorosilane was deposited by soaking the cleaned wafers in a solution of 20 mL toluene and 7 drops of DDTS for 20 minutes. The prepared wafers were then rinsed with toluene, dried with nitrogen and further dried on a hotplate at 120 °C for 30 minutes before spin-coating of the polymer films. The organic semiconductor thin films were spin-cast on the DDTS-treated substrates and controlled the thickness at ∼40 nm from prepared polymer solutions in chlorobenzene (3 mg mL−1). Before bringing the samples into the glove box, all samples were heated at 30 °C for 20 minutes on a hot plate to help remove the residual solvent. The thermal annealing process was carried out inside a Ar-filled glove box. All measurements were conducted using a Keithley 4200-SCS semiconductor parameter analyzer (Keithley Instruments Inc., Cleveland, OH, USA) in an Ar-filled glove box at room temperature.

3. Results and discussion

The synthesis of polymers P1 to P7 is depicted in Scheme 1. To gain access to urea-containing isoindigo-based polymers, a procedure developed by Zhang et al. was utilized.19 Starting from the isoindigo core, alkylation was performed in basic conditions with 1,6-dibromohexane to afford compound 1 in 87% yield. The resulting materials were then reacted with sodium azide to afford compound 2 in with a yield of 58%, followed by a Staudinger reaction to reduce the azide moieties into amines. Since the amine-containing compound had a low solubility in common organic solvents, it was directly reacted with hexylisocyanate to afford compound 3 in 21% yield over 2 steps. Monomer 3 was then copolymerized with (E)-6,6′-dibromo-1,1′-bis(2-decyltetradecyl)-[3,3′-biindolinylidene]-2,2′-dione under Stille polymerization conditions to afford polymers P2 to P4. To gain access to polymers P5 to P7, incorporating 5 to 20 mol% of linear dodecyl chain, a similar strategy was utilized with monomer (E)-6,6′-dibromo-1,1′-didodecyl-[3,3′-biindolinylidene]-2,2′-dione.34 The resulting conjugated polymers, incorporating different ratios of urea-containing side-chains (P2 to P4) or linear dodecyl side-chains (P5 to P7) were purified by Soxhlet extraction and their physical properties are summarized in Table 1.
image file: c8tc03438a-s1.tif
Scheme 1 Synthetic pathway to urea-containing monomer 3.
Table 1 Molecular weight, polydispersity, optical properties, and energy levels of isoindigo-based polymers P1 to P7
M n (kDa) M w (kDa) PDIb λ max solnc (nm) λ max filmc (nm) E optg [thin space (1/6-em)] (eV) HOMOe (eV) LUMOf (eV)
a Number-average molecular weight and weight-average molecular weight estimated by high temperature gel permeation chromatography in 1,2,4-trichlorobenzene at 140 °C using polystyrene as standard. b Weight dispersity defined as Mw/Mn. c Absorption maxima determined in solution (2.5 × 10−4 g mL−1 in CHCl3) and spin-coated thin film. d Calculated by the following equation: gap = 1240/λonset of polymer film. e Calculated from cyclic voltammetry (Potentials vs. Ag/AgCl) using 0.1 M TBAPF6 in CH3CN as electrolyte. f Estimated from calculated Eoptg and HOMO.
P1 13.0 59.5 4.6 700 698 1.65 −5.40 −3.75
P2 12.2 75.2 6.1 698 695 1.64 −5.35 −3.71
P3 8.7 33.8 3.9 699 697 1.64 −5.33 −3.69
P4 9.6 17.9 1.9 692 694 1.62 −5.38 −3.76
P5 9.9 64.0 6.4 700 700 1.66 −5.38 −3.72
P6 16.8 102.7 6.1 700 700 1.66 −5.54 −3.88
P7 7.0 21.1 3.0 696 699 1.67 −5.29 −3.62


The structure of P1 to P7 was confirmed by 1H NMR at 120 °C in deuterated 1,1,2,2-tetrachloroethylene-d2 (TCE), and shown important similarities with other isoindigo-based conjugated polymer.30,38 Since the feed ratios (amount of urea-containing block added to the polymerization reaction) do not necessarily correlate precisely with the experimental ratios, 1H NMR experiments at 120 °C in deuterated 1,1,2,2-tetrachloroethylene-d2 (TCE) was also used to determine the relative incorporation of the urea-containing isoindigo block (Fig. S1, ESI). This was performed by comparing a peak related to the urea groups (7.8 ppm) with a peak attributed to the protons in alpha position of the lactam ring (3.9 ppm). The corresponding ratios were calculated by assuming 10 repeating units. Physical characterization of the new polymers containing hydrogen bonding and linear side chains was performed by various techniques, and the results are summarized in Table 1. The new urea-containing isoindigo polymers were shown to have moderate molecular weights, as measured by high-temperature size-exclusion chromatography (SEC). The molecular weights (Mn) remained in the same range of value, despite increasing the ratio of urea side-chains (from 13.0 kDa for P1 to 9.6 kDa for P4). A similar trend was also observed for the dodecyl-containing polymers. The polymers were also shown to be strongly aggregating in solution, as demonstrated by the broad dispersity index measured. The moderate molecular weights and broad PDI can be directly attributed to the low solubility of the polymers upon increasing the amount of either urea-containing or linear side chains. Based on thermogravimetric analysis, the thermal decomposition temperatures (measured at 5% weight loss) of all polymers were higher than 350 °C. Finally, UV-visible spectroscopy and cyclic voltammetry were used to gain information concerning the HOMO/LUMO levels and bandgap (Fig. 2a–d).


image file: c8tc03438a-f2.tif
Fig. 2 UV-Vis spectrum of (a) P1 to P4 and (b) P5 to P7 in solution (CHCl3), and oxidation peaks obtained from cyclic voltammetry of (c) P1–P4, and (d) P5–P7 in 0.1 M TBAPF6 in MeCN as electrolyte (scan rate of 100 mV s−1).

In order to gain insight into the effect of incorporating the urea motif into the side chains of isoindigo-based polymers, FTIR spectroscopy was first utilized to confirm the presence of urea moieties, and the results are presented on Fig. 3a. The broad band, ranging from 3450 and 3325 cm−1, was identified as the N–H stretch of the urea groups.19 Due to the relatively small amount of urea moieties incorporated into the polymer, the N–H stretching signal was found to be relatively weak as compared to the other bands associated with the isoindigo backbone. As expected, the intensity of the band increased with the amount of urea moieties. Importantly, this band was not observed for P1 and P5 to P7 (Fig. 3b). Despite its broadness, the N–H stretching band in P2 to P4 showed the presence of a weak shoulder around 3400 cm−1. Previously observed in other hydrogen bonds-containing polymers, this signal was assigned to the N–H stretch of non-hydrogen bonded amide or ‘free’ amide groups, thus supporting the hypothesis that an important fraction of urea moieties are involved in hydrogen bonds in the solid-state.39–41


image file: c8tc03438a-f3.tif
Fig. 3 (a) FT-IR spectra of P1 to P7; (b) FTIR spectra of P1 to P7 zoomed on NH stretching region; (c) UV-Vis spectroscopy of P1 to P4 thin films and (d) UV-Vis spectroscopy of P5 to P7 thin films.

UV-Vis absorption spectroscopic experiments were then performed in solution as well as thin films, to assess the optical properties of the novel isoindigo-based conjugated polymers. All polymers, in both solution and thin films, exhibited two distinct absorption bands, as shown in Fig. 3c and d. For P1 to P7, a band centered at 450 nm was observed, and can be attributed to the π–π* transition. This transition was not impacted by the side-chain engineering with urea moieties or simple dodecyl chain. Furthermore, for all polymers, a broad band centered at 700 nm was attributed to the intramolecular donor–acceptor charge transfer, and showed two vibrational peaks (0–0 and 0–1).42 For P1 to P4, it was noted that, in thin film, the intensity of the 0–0 peak gradually decreased upon the addition of the hydrogen-bonding side chains. Previously observed for other π-conjugated materials incorporating hydrogen bonding moieties, this finding indicates that an increased incorporation of urea-containing side chains can significantly affect the aggregation of conjugated polymers.18 This trend was also observed in solution. In contrast, no such trend was observed for P5 to P7, indicating that the degree of aggregation was not directly impacted by the incorporation of the linear alkyl side chain.

To further investigate the influence of the intermolecular hydrogen bonds on the as-spun films of the polymers after annealing at 200 °C, X-ray diffractometry (XRD) was used. This technique allowed for an investigation into the interlamellar distances, which can be assigned to diffraction peaks near 2θ = 4° (distances of approximately 20 Å). As observed on Fig. 4a, for urea-containing isoindigo-based polymers P2 to P4, the interlamellar spacing slightly increased as compared to P1; whereas the increased amount of alkyl side chain (P5–P7) did not significantly impact the interlamellar spacing. Although it was hypothesized that the interlamellar spacing would decrease upon increased hydrogen bonding moiety incorporation, this trend was not exhibited for the urea isoindigo-based polymers. Additionally, the full width at half maximum (FWHM), a parameter directly correlated to the coherence length in solid-state, was calculated for the interlamellar diffraction peak of P1 to P7, as shown in Fig. 4b.43 Interestingly, the FWHM for the interlamellar diffraction peak initially increased upon incorporating 5 mol% (P2) of urea-containing alkyl chain as compared to 0 mol% (P1). However, the FWHM then decreased upon adding a greater amount of urea moieties. In comparison, the FWHM for P5 to P7, incorporating dodecyl alkyl chains, exhibited a similar trend. However, it was noted that the FWHM significantly decreased when comparing P3 (10 mol% urea moieties) and P4 (20 mol% urea moieties), to P6 (10 mol% dodecyl alkyl chains) and P7 (20 mol% dodecyl alkyl chains). These findings suggest that the hydrogen bonds formed between urea moieties can help to increase the order in the π-conjugated system, a phenomenon also observed for dodecyl side chains, but to a lesser extent.


image file: c8tc03438a-f4.tif
Fig. 4 (a) Powder X-ray diffraction of P1 to P7 in thin films, zoomed on small angle (2θ = 3° to 2θ = 7°) and (b) full width at half maximum (FWHM), calculated for P1, P2 to P4 (urea-containing alkyl chains) and P5 to P7 (dodecyl-containing alkyl chains). FWHM is reported in angstroms (Å).

To further investigate the nanoscale morphology of P2 to P4, atomic force microscopy (AFM) was performed on thin films, before and after thermal annealing at 200 °C. A comparison was also performed with reference polymers P5 to P7, and the results are summarized in Fig. 5 and Fig. S2 (ESI). All polymers, with or without urea-containing side-chains, were shown to form relatively smooth thin films before annealing, without the formation of a specific ordered/structured morphology. This observation indicates that no phase separation occurs in the solid-state, despite the formation of intermolecular hydrogen bonds in P2 to P4. Interestingly, after annealing at 200 °C, the RMS roughness of the films of P2 to P7 decreased, which can be attributed to an enhancement in the crystallinity. As observed for the non-annealed samples, no phase separation at the nanoscale nor fibre-based structures were observed for the conjugated polymers with or without urea moieties.


image file: c8tc03438a-f5.tif
Fig. 5 Atomic force microscopy (AFM) height images of P2 to P7 after thermal annealing at 200 °C. Scale bars are 500 nm.

To evaluate the semiconducting performance of the new isoindigo-based conjugated polymers incorporating urea-containing alkyl chains, organic field-effect transistors (OFETs) with a bottom-gate bottom-contact (BGBC) structure were fabricated with P1 to P4. Devices fabricated from dodecyl-containing polymers P5 to P7 were also investigated to get an accurate comparison, and to gain insight into the influence of the urea moieties on the polymers’ semiconducting properties. Results are summarized in Table 2 and the detailed fabrication procedure can be found in the ESI. It is important to mention that, despite bottom-gate top-contact (BGTC) architecture is known to minimize the contact resistance and can allow for higher mobilities, a BGBC architecture was used due to its ease of fabrication (pre-patterned substrate) and compatibility with large-scale device fabrication (minimal exposure of the device components to solvents and chemicals).44 Transistor devices fabricated from P1 to P7 showed the typical output and transfer characteristics for thin film transistors (Fig. S3 to S6, ESI). Since hydrogen bonding has a direct effect on the thin film morphology, and our previous annealing data indicated that this changes upon heating of the films, OFETs were first evaluated without thermal annealing. As expected, all devices showed low charge mobilities and on/off current ratios, independent of the type of side chains incorporated on the π-conjugated backbone. This can be attributed to a suboptimal morphology and lower crystallinity, which does not favor charge transport through the thin film.45 To promote crystallinity and enhance charge mobilities, OFETs were also evaluated after annealing at 200 °C for P1 to P7. Interestingly, the addition of urea-containing side-chains did not significantly influence charge transport, as demonstrated by the results obtained from OFETs fabricated with P2 (5 mol%), P3 (10 mol%) and P4 (20 mol%), which showed an average charge mobility of 0.022, 0.022 and 0.032 cm2 V−1 s−1, respectively. In contrast to the urea-containing polymers, OFETs fabricated from P5 to P7, incorporating from 5 to 20 mol% of linear dodecyl side chains, showed a clear decrease in charge mobility upon incorporating a greater ratio of dodecyl chains. In fact, the average charge mobility of P7 (20 mol%, 0.0073 cm2 V−1 s−1) was shown to be one order of magnitude lower than that of P5 (5 mol%, 0.041 cm2 V−1 s−1) and P4 (20 mol% urea, 0.032 cm2 V−1 s−1). In contrast to the trend observed for previous reported urea-containing DPP-based polymers, our findings suggest that the intermolecular hydrogen bonding enabled in isoindigo-based π-conjugated polymers by the urea moieties does not significantly enhance charge transport.19 However, in contrast to dodecyl-containing conjugated polymers, the charge mobility was maintained upon incorporating additional urea-containing side chains. This result can be potentially attributed to the intermolecular hydrogen bonds between the polymer chains, which can allow for the conservation of a desirable morphology in the solid-state.

Table 2 Average and maximum hole mobility (μaveh, μmaxh), threshold voltages (Vth), Ion/Ioff, and ratios for OFETs fabricated from P1 to P7 before and after thermal annealing. The device performances were averaged from 5 devices
Polymer Annealing temperature [°C] μ aveh/μmaxh [cm2 V−1 s−1] I ON/IaveOFF V aveth [V]
P1 (0 mol%) 30 6.7 × 10−5 ± 3.8 × 10−5/9.9 × 10−5 103 −95.1
100 0.0077 ± 0.0016/0.010 105 −51.4
200 0.016 ± 0.015/0.037 106 −54.9
P2 (5 mol%) 30 0.0033 ± 0.0009/0.0044 104 −44.0
100 0.030 ± 0.015/0.0053 106 −35.5
200 0.022 ± 0.013/0.042 106 −54.3
P3 (10 mol%) 30 2.9 × 10−4 ± 3.1 × 10−5/3.4 × 10−4 103 −14.6
100 0.0062 ± 0.0011/0.0072 105 −47.9
200 0.022 ± 0.006/0.028 106 −49.9
P4 (20 mol%) 30 8.0 × 10−5 ± 2.9 × 10−5/1.1 × 10−4 103 −47.6
100 0.0043 ± 0.0029/0.0092 105 −54.4
200 0.032 ± 0.012/0.051 105 −54.6
P5 (5 mol%) 30 2.1 × 10−4 ± 0.00030/7.6 × 10−4 103 −33.7
100 0.011 ± 0.003/0.015 105 −57.9
200 0.041 ± 0.012/0.057 106 −58.7
P6 (10 mol%) 30 2.43 × 10−5 ± 2.27 × 10−5/5.3 × 10−5 103 −10.8
100 0.0017 ± 0.0006/0.0024 104 −50.5
200 0.010 ± 0.003/0.014 105 −60.8
P7 (20 mol%) 30 4.7 × 10−6 ± 5.5 × 10−6/8.6 × 10−6 103 −116.7
100 0.00025 ± 3.9 × 10−5/0.00031 103 −50.1
200 0.0073 ± 0.0058/0.016 105 −54.9


As previously observed for other π-conjugated systems incorporating non-covalent bonding moieties, temperature has an important effect on the stability and formation of the supramolecular interaction.19,24 Therefore, OFET characteristics were also evaluated at 100 °C for P1 to P7. Interestingly, despite an important enhancement in charge mobility in contrast to the devices tested at room temperature, polymers P1 to P7 all demonstrated a lower charge mobility after annealing at 100 °C in comparison to annealing at 200 °C. Therefore, this observation indicates that intermolecular hydrogen bonds, even if more stable at lower temperature, are not important enough to overcome the influence of crystallinity and promote an optimal charge transport. However, as observed for after annealing at 200 °C, the incorporation of greater amounts of urea-containing side-chains did not significantly influence the charge mobility in contrast to the incorporation of dodecyl side chains, which drastically reduced the charge transport in OFETs upon increasing the content of linear side chains (from 0.011 cm2 V−1 s−1 for P5 to 0.00025 cm2 V−1 s−1 for P7). Once again, this observation indicates that hydrogen bonding can help to maintain the charge transport properties, even when the solid-state packing of π-conjugated polymers is affected by the addition of various side chains.

4. Conclusion

In conclusion, a novel series of isoindigo-based conjugated polymers were prepared by incorporating various ratios of urea-containing side chains (P2 to P4) and dodecyl side chains (P5 to P7). The physical properties of these new polymers were characterized by various techniques, including UV-Vis spectroscopy, FT IR spectroscopy and X-ray diffraction. The urea moieties were shown to enable the formation of hydrogen-bonding in thin films and to affect the aggregation properties of the polymers in the solid-state in comparison to the polymers incorporating their saturated side chains counterparts. Moreover, the incorporation of these functional groups also significantly impacted the polymer morphology and stacking order, which was also observed with linear side chain-containing polymers to a lesser extent. The resulting polymers were used to fabricate OFET devices. Interestingly, the urea-containing polymers showed a maximum charge mobility of 0.032 cm2 V−1 s−1 (P4, 20 mol% urea), which is fairly similar as that observed for the linear side chain-containing polymers. However, upon increased incorporation of the urea-containing side chains the charge mobility remained stable whereas the charge mobility suffered drastically upon increased incorporation of dodecyl chains. This phenomenon was attributed to the intermolecular hydrogen bonds between urea moieties, which helped to maintain an effective morphology for charge transport despite the effect of the linear side chains. Based on the results obtained, we believe that the incorporation of hydrogen bonding moieties in π-conjugated polymers is a promising strategy to tune the solubility, morphology, and mechanical and electronic properties of π-conjugated systems. Previously shown to be beneficial to mechanical properties, and to induce healing properties, the incorporation of high contents of hydrogen bonding moieties in conjugated polymers without sacrificing the charge mobility could also become an important design strategy to design new polymers for stretchable and flexible electronics.27,28

Funding sources

This work was supported by NSERC through Discovery Grants (RGPIN: 2017-06611 and RGPIN-2016-04366). S. R.-G. also acknowledges the Faculty of Science and the Department of Chemistry and Biochemistry at the University of Windsor for financial support.

Author contributions

All authors contributed to the manuscript. All authors have given approval to the final version of the manuscript.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

The authors also thank Ms Dandan Miao and Prof. Jean-François Morin (Université Laval) for GPC measurements, and Tricia B. Carmichael and her group (University of Windsor) for AFM experiments. B. P. C. thanks NSERC for financial support through an Undergraduate Student Research Award (USRA). M. U. O. thanks the Ministry of Advanced Education and Skills Development of Ontario for an Ontario Graduate Scholarship. ADH thanks NSERC for financial support from a postdoctoral fellowship.

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Footnote

Electronic supplementary information (ESI) available: Detailed experimental procedures and a complete characterization of materials. See DOI: 10.1039/c8tc03438a

This journal is © The Royal Society of Chemistry 2018