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. 2020 Dec 17;5(51):33461–33469. doi: 10.1021/acsomega.0c05514

The Effect of Alkyl Chain Length on Well-Defined Fluoro-Arylated Polythiophenes for Temperature-Dependent Morphological Transitions

Yuto Ochiai 1, Tomoya Higashihara 1,*
PMCID: PMC7774253  PMID: 33403308

Abstract

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Understanding the relationship between the molecular structure and morphological behaviors of well-defined semiconducting polymers is essential for developing novel conjugated building blocks and determining the origin of the functional characteristics of semiconducting polymers. Here, we provide insights into the significant temperature-dependent morphological transitions of novel well-defined polythiophene derivatives with m-alkoxy-substituted fluoro-aryl side units: poly(3-(4-fluoro-3-(hexyloxy)phenyl)thiophene) (PHFPT) and poly(3-(4-fluoro-3-(dodecyloxy)phenyl)thiophene) (PDFPT). We found that these unique morphological transitions depend on the alkyl chain length of the substituted fluoro-aryl side units. In PHFPT with short alkyl chains, the thermal treatment promotes a crowded interdigitated packing structure, resulting in narrow lamellar spacings in its crystalline structure. In contrast, the long alkyl chain of PDFPT acts as a physical spacer and disturbs the crowded interdigitation. In addition, the thermal treatment induces the backbone planarization and an ordered packing morphology in PDFPT. These demonstrations provide a critical milestone for the phase transitions of semiconducting polymers with conjugated side units.

Introduction

The excellent molecular designability of semiconducting polymers (SCPs) enables the fine tuning of optoelectronic properties1 and equipment with specific functions, such as intrinsic stretchability,2 self-hearability,3,4 and biocompatibility.5,6 Owing to these unique advantages, SCPs have attracted attention as next-generation electronic materials.7,8 In terms of optimizing the optoelectronic properties of the SCPs, the introduction of conjugated side units,9−12 a fluorine atom,13−15 or both16−18 into the conjugated system has been used as an effective molecular design strategy. In addition, these bulky conjugated side units and the fluorine atom, which has the highest Pauling electronegativity, can induce alternation of the chain coplanarity and molecular interactions. In addition to the molecular design, the functional characteristics and mesoscopic scale morphology of the SCPs strongly correlate with their primary structures.19−26 Even if the chemical structures of polymers are completely the same, those characteristics can be changed dramatically. For instance, the backbone conformational changes lead to alteration in intermolecular packing and domain ordering.27 The differences in regioregularity28−30 and molecular weight31,32 influence their amorphousness and crystallographic orientation. Therefore, the elucidation of the relationships between the molecular design and morphological behaviors with well-defined SCPs is required for accessing novel conjugated building blocks and a deep understanding of their morphological characteristics.

Herein, we report the significant temperature-dependent morphological transitions of well-defined polythiophene derivatives with m-alkoxy-substituted fluoro-aryl side units. We demonstrate that the length of the alkyl chains on the fluoro-aryl side units is the key factor for determining the molecular conformation, ordering, and temperature-dependent transitions. Specifically, poly(3-(4-fluoro-3-(hexyloxy)phenyl)thiophene) (PHFPT) and poly(3-(4-fluoro-3-(dodecyloxy)phenyl)thiophene) (PDFPT) can be synthesized by a chain-growth catalyst transfer polycondensation via the Kumada coupling reaction,33,34 generally called the Kumada catalyst transfer polycondensation (KCTP). The thiophene backbone is chosen as the main chain structure to obtain the applicability for controlled synthesis methods. In addition, we focused on the fluoro-aryl side unit for optimizing the optoelectronic properties for the above reasons. In terms of the 2D-extended conjugation, several polythiophene derivatives with conjugated side units have been reported.35−46 Additionally, poly(3-(4-fluorophenyl)thiophene) (PFPT), which has a polythiophene backbone and fluoro-aryl side unit, has been reported for electrochemical capacitor applications.47−52 However, these PFPTs were synthesized by electrochemical polymerization and are insoluble in typically used organic solvents. Hence, we introduce the alkyl chains onto the fluoro-aryl unit for the designing of PHFPT and PDFPT, which enable them to be synthesized via solution reactions, affording good solution processability. The temperature-dependent behaviors of PHFPT and PDFPT shed light on the phase transitions of the SCPs with conjugated side chains.

Results and Discussion

Synthesis of Polymers

In this study, we prepared well-defined fluoro-arylated polythiophene derivatives with m-substituted alkoxy side chains of different lengths (Scheme 1 and Figure 1a). The detailed monomer and polymer synthesis protocols are available in the Supporting Information (Schemes S1 and S2 and Figures S1–S10). To achieve the controlled polymerization of the thiophene monomer with the electron-withdrawing fluoro-aryl unit, we first investigated the Grignard exchange reaction of 7 and 8. The reaction of the monomers was carried out with an equivalent of iPrMgCl in the presence of LiCl dissolved in THF at 0 °C for 40 min followed by quenching with 5 M HCl aq. The 1H NMR spectra confirmed the quantitative Mg–I exchange at the 5-position of the thiophene ring for the resulting products from 7 and 8. Based on these results, the KCTP of 7 and 8 proceeded with Ni(dppe)Cl2 in THF at 60 °C for 1 h to successfully obtain PHFPT and PDFPT with controlled number-average molecular weights (Mns), narrow molecular weight distributions (Mw/Mns), and high regioregularity (r.r.) of 94 and 93%, respectively (Table 1). These Mn values could be controlled by changing the feed ratio of the monomer and Ni catalyst while maintaining low Mw/Mn values (Figures S11 and S12 and Tables S1 and S2), and the samples shown in Table 1 were used in the latter section. For comparison, we also prepared a well-defined regioregular poly(3-hexylthiophene) (P3HT) (Mn = 12,000, Mw/Mn = 1.07, and r.r. = 98%) by KCTP.

Scheme 1. General Synthetic Procedure of Monomer and Polymer Materials.

Scheme 1

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Figure 1.

Figure 1

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Energy levels and thermal characteristics of polymers. (a) Chemical structures of studied polymers. (b) Temperature-dependent HOMO energy levels of PHFPT and PDFPT. (c) First cooling cycle DSC thermograms of P3HT, PHFPT, and PDFPT. (d) Second heating cycle DSC thermograms of P3HT, PHFPT, and PDFPT.

Table 1. Synthesis of Fluoro-Arylated Polythiophenes.

sample Mna Mw/Mna r.r. (%)b yield (%)
PHFPT 12,100 1.14 94 66
PDFPT 12,100 1.05 93 54
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a

Determined by SEC using a calibration with polystyrene standards in THF at 40 °C.

b

Head-to-tail regioregularity determined by 1H NMR spectra, comparing the signal intensities of thiophene protons at 4-position.

Energy Levels and Thermal Properties

The highest occupied molecular orbital (HOMO) energy levels for the obtained polymers were measured using a photoelectron spectrometer to investigate the effect of introducing electron-withdrawing fluoro-aryl units on the thiophene backbone. As predicted, both PHFPT and PDFPT showed deeper HOMO energy levels than P3HT (−4.7 eV) due to their strong electron-withdrawing side units (Figure 1b and Figure S13). In addition, we found that their backbone coplanarity and energy levels strongly correlate with the thermal annealing temperature and the length of the alkoxy substituents. The HOMO energy level of PDFPT, which has a long alkoxy chain, significantly shifts by thermal annealing. Its HOMO energy level rises to −5.21 from −5.62 eV (as-cast film) following annealing at 100 °C. Furthermore, this increase in the HOMO energy level becomes deeper again after annealing at 150 °C. These results indicate that the PDFPT backbone planarity and chain ordering significantly change with the thermal treatments.53 The morphological ordering of the PDFPT film increased at 100 °C compared to the as-cast film and then decreased again after annealing at 150 °C. This temperature-dependent change in HOMO energy level is only observed in PDFPT. In PHFPT, which has a short alkoxy chain, it exhibited minor changes in the HOMO energy levels at the different annealing temperatures. These unchanged deep HOMO energy levels suggest that PHFPT has a twisted thiophene backbone, which is possibly due to the bulky side units, and this chain conformation is not altered by the thermal treatments.

To understand the correlation between the thermal transition behavior and chemical structures of the studied polymers, the differential scanning calorimetry (DSC) measurement was performed. The melting and crystalline temperatures of the fluoro-arylated polymers largely decreased compared to P3HT (Figure 1c,d). Note that the melting temperatures (Tm) of PHFPT and PDFPT are observed to be below 150 °C. The temperature-dependent energy level changes of PDFPT can be explained based on this result. The thermal treatment over the Tm destroys the planarized backbone conformation and ordered morphology, which was formed by annealing at 100 °C. The relative crystalline information can be extracted from the heat of fusion of each polymer. The heat of fusion of PHFPT clearly shows a lower value than those of PDFPT and P3HT. This result also supports the twisted backbone conformation of PHFPT, disturbing the well-ordered morphology.

Optical Properties

Indeed, we observed that the backbone planarity of the fluoro-arylated polymers can be readily alternated by thermal treatment, even in the film state based on investigation of the optical properties. These temperature-dependent conformational changes strongly correlate with the length of the alkoxy substituents. To understand their intra- and interpolymer chain conformations, ultraviolet–visible (UV–vis) spectroscopy was performed. All the polymers were well dissolved in chloroform, and their solution state spectra showed similar monomodal absorbance peaks associated with the coil-like chain conformation (Figure 2a). The peaks for both fluoro-arylated polymers clearly redshifted compared with those for P3HT, indicating the extended conjugation of the isolated polymer chains. On the other hand, the spin-coated films of the fluoro-arylated polymers exhibit almost the same absorbance spectra as their solution state (Figure 2c,d). These results indicate that the twisted coil-like backbone conformation is maintained in the film state. This is similar to the chain conformation in solution and solid state of the regio-irregular P3HTs, which have twisted backbones due to the large steric hindrance of each alkyl side chain.54 In general, the film spectra for the regioregular P3HT deposited from good solvents show a significant redshift and defined vibronic peaks due to the backbone planarization with an extended conjugation system and well-ordered intermolecular π-stacked aggregate formation (Figure 2b). Thus, the bulky m-alkoxy-substituted fluoro-aryl units may disturb the backbone coplanarity and intermolecular aggregations. From the spectra of PHFPT films, negligible changes were observed at both thermal annealing temperatures of 100 and 150 °C. In contrast, the spectra for PDFPT films show drastic improvements in the backbone planarity and interchain aggregation via thermal annealing even in the spin-coated films (Figure 2d and Table 2). Surprisingly, by measuring the film spectra of PDFPT in the range of room temperature to 115 °C, we observed that this coil-to-rod conformational transformation starts at a low temperature of approximately 60 °C (Figure S15).

Figure 2.

Figure 2

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Optical properties of polymers. (a) Normalized absorption spectra of P3HT, PHFPT, and PDFPT solutions in CHCl3. (b–d) Normalized absorption spectra of P3HT, PHFPT, and PDFPT thin films annealed at various temperatures. (e) Schematic illustration of the twisted backbone of PHFPT. (f) Schematic illustration of the planarized backbone of PDFPT.

Table 2. Absorbance Characteristics of Polymers.

sample condition λmax (nm) λonset (nm) Eg (eV)a
P3HT solution 452 539 2.30
as-cast film 555 645 1.92
100 °C annealed film 555 645 1.92
PHFPT solution 468 557 2.23
as-cast film 461 534 2.32
100 °C annealed film 458 533 2.33
PDFPT solution 467 555 2.23
as-cast film 475 540 2.30
100 °C annealed film 544 638 1.93
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a

Calculated from onset wavelengths in UV–vis spectra, where Eg = 1240/λonset.

Hence, we hypothesize that the applied thermal energy may be consumed for the entanglement and interdigitation of the side chains in PHFPT due to the bulky side units and twisted backbone (Figure 2e). In addition, the long alkyl chain of PDFPT acts as a physical spacer and disturbs the crowded interdigitation of the side chain. Instead, the applied thermal energy is consumed for the thiophene backbone planarization in PDFPT (Figure 2f). Similar temperature-dependent morphological changes have been observed in a few previous reports focusing on the poly[3-(4-alkylphenyl)thiophene]s.55−57 However, the detailed mechanism for the conformational transformation and the molecular structural effects remains unexplained. Therefore, we scrutinized the nanolevel molecular ordering and packing structures, as discussed in a later section.

Temperature-Dependent Morphological Characteristics

Grazing incidence X-ray diffraction (GIXD) measurements were carried out to confirm our hypothesis and provide insights into the contrasting morphological behaviors between the PFHPT and PDFPT thin films. All the calculated crystallographic parameters are summarized in Table 3.

Table 3. GIXD Profiles of Polymers.

sample film condition lamellar spacing (nm) π–π stacking (nm)
PHFPT as-cast 1.62 NA
100 °C annealed 2.16, 1.61, 1.27 0.42
150 °C annealed 1.61, 1.34 0.38
PDFPT as-cast 2.23 NA
100 °C annealed 2.95 0.42
150 °C annealed 2.87 NA
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In the case of PHFPT, the as-cast film shows an isotropic diffraction, and it is difficult to find a (010) diffraction peak due to the π–π stacking (Figure 3a). A lamellar distance of 1.62 nm was calculated from the (100) diffraction peak in the out-of-plane (qz) direction. This value is smaller than the lamellar distance of P3HT (1.65 nm) (Figure S15 and Table S3); even PHFPT has bulky side units. These results indicated that a less ordered packing structure is due to their twisted backbone and interdigitation of the side chains. Additionally, the m-substituted alkyl chain might be twisted away from the backbone to pack between the lamellar distances along with the neighboring thiophene main chains.40 This behavior is consistent with a previous report for the head-to-head, tail-to-tail regiostructure-based P3HT, which has a twisted backbone and shows a narrow lamellar distance of 1.25 nm due to its side-chain interdigitation.54 Interestingly, the 100 °C-annealed PHFPT film shows small additional (100) peaks on either side of the main peak in the qz direction (Figure 3b,d). Additionally, a (010) diffraction peak can be observed in the in-plane (qxy) direction, giving a π–π stacking distance of 0.42 nm (Figure 3e). This suggests that the PHFPT chains are packed in an edge-on orientation, and their ordered structure is improved by the thermal treatment. The lamellar distance is calculated to be 1.61 nm from the main (100) peak, which is almost the same as that for the as-cast film. The other additional peaks give the lamellar distances of 2.16 and 1.27 nm. This result demonstrates that several phase transitions compete between avoiding and accelerating the crowded interdigitation for annealing at 100 °C. Notably, the newly presented wider lamellar distance disappeared in the 150 °C-annealed PHFPT film, where the thermal treatment temperature is over its Tm (Figure 3c). The lamellar distances were determined to be 1.61 and 1.34 nm from the main (100) peak and the side peak at the higher qz, respectively. Accordingly, the interdigitation of the side chains was accelerated by the thermal treatments in the PHFPT films, as we hypothesized. In terms of PDFPT, the as-cast film also shows an isotropic diffraction, and it is difficult to find the (010) diffraction peak due to π–π stacking similar to that of PHFPT. On the other hand, the calculated lamellar distance is 2.23 nm, which corresponds to a wider lamellar distance than P3HT and PHFPT (Figure 3f). Note that the lamellar distance in the PDFPT film annealed at 100 °C was determined to be 2.95 nm without any side peaks, which is wider than that in the as-cast film (Figure 3g,i). Furthermore, the edge-on orientation of the PDFPT chains and increasing ordered packing structures are confirmed due to the observation of the highly ordered diffraction peaks in the qz direction and the (010) diffraction peak in the qxy direction (Figure 3j). In the annealed film at 150 °C, a relatively wide lamellar distance is maintained, but the highly ordered structures disappear (Figure 3h). This is probably because of the melting impact of the PDFPT crystalline domains as the Tm value of PDFPT is found below 150 °C from the DSC measurement (Figure 1d). The long alkyl chain on PDFPT physically disturbs the crowded interdigitation of each side chain, forming a wide lamellar distance. Simultaneously, these physical spacers at the lamellar spacing enable enhancement of the backbone planarity and ordered polymer chain aggregations. This morphological behavior is in agreement with the optoelectrical and thermal properties, as already discussed above.

Figure 3.

Figure 3

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GIXD profiles of polymers. (a–c) 2D GIXD images of PHFPT thin films annealed at various temperatures. (d, e) 1D GIXD profiles of PHFPT along with the out-of-plane direction (d) and in-plane direction (e). (f–h) 2D GIXD images of PDFPT thin films annealed at various temperatures. (i, j) 1D GIXD profiles of PDFPT along with the out-of-plane direction (i) and in-plane direction (j).

Conclusions

In summary, we have successfully developed conjugated building blocks, which can be synthesized by a controlled polymerization, with tunable optoelectrical characteristics by introducing the m-alkoxy-substituted fluoro-aryl moieties onto a polythiophene backbone. Our designed polymers show deep-lying HOMO energy levels (−5.62to −5.21 eV) owing to the presence of electron-withdrawing side units and a twisted backbone conformation. Furthermore, we find that the twisted backbones are readily planarized by a thermal treatment even in the film state. These temperature-dependent conformational transitions strongly correlate with the length of the alkyl substituents on the fluoro-aryl side units. The intermolecular interdigitation and narrow lamellar spacing are promoted by thermal annealing due to their twisted backbone in the PHFPT film, which has a short alkyl chain. On the other hand, the long alkyl chains in PDFPT behave as physical spacers and disturb the crowded interdigitation. Consequently, the applied thermal energy is consumed for the thiophene backbone planarization in PDFPT. Hence, our demonstrated insights into the phase-transition behaviors present a significant milestone in the design of semiconducting polymers with conjugated side-chain units.

Experimental Section

Materials

All reagents were purchased from Tokyo Chemical Industry Co., Ltd., Wako Chemicals Co., Ltd., and Sigma-Aldrich. THF (99.5%, stabilizer-free, Wako Chemicals Co., Ltd.) was refluxed over sodium benzophenone under nitrogen for 2 h and then distilled just before use. Other commercial reactants were used without further purification.

General Procedure for the Synthesis of Polymers

A 30 mL two-neck flask with lithium chloride (5 equiv) was heated using a heat gun under reduced pressure. After the flask was cooled down to room temperature under a N2 gas atmosphere, 3-(3-alkoxy-4-fluorophenyl)-2-bromo-5-iodothiophene (7 or 8) (0.2 g, 1 equiv) was added. After purging with N2 gas, anhydrous THF (15 mL) was added, and the solution was cooled down to 0 °C. The iPrMgCl solution (1.3 M in THF, 1.05 equiv) was then slowly added dropwise via a syringe. After complete addition, the reaction was stirred for 30 min at room temperature. To another 10 mL two-neck flask, bis(triphenylphosphino)Ni(II) dichloride and 1,2-bis(diphenylphosphino)ethane were added. After purging with N2 gas, the mixture was dissolved in anhydrous THF (5 mL). Afterward, the prepared Ni(dppe)Cl2 solution was quickly injected into the prepared Grignard thiophene monomer precursor solution via a syringe to start the Kumada catalyst transfer polycondensation. Polymerization was carried out for 1 h at 60 °C followed by quenching with a 5 M HCl solution. The crude solution was poured into a mixture solution of methanol (200 mL) and water (100 mL), and the precipitate was filtered and loaded to an extraction thimble and left in a Soxhlet extractor. The crude polymer was successively extracted with methanol and acetone, and then the objected polymer was collected by chloroform. The obtained chloroform solution was concentrated and dried under high vacuum overnight to give poly(3-(4-fluoro-3-(hexyloxy)phenyl)thiophene) (PHFPT) and poly(3-(4-fluoro-3-(dodecyloxy)phenyl)thiophene) (PDFPT).

General Measurements and Characterizations

1H NMR spectra were recorded with a JOEL JNM-ECX400 spectrometer at 25 °C. Deuterated chloroform was used as a solvent with trimethylsilane as a standard. Number- and weight-average molecular weights (Mn and Mw) were measured by SEC on a JASCO GULLIVER 1500 equipped with a pump, an absorbance detector (UV, λ = 254 nm), and two polystyrene gel columns, based on a conventional calibration curve using polystyrene standards. THF (40 °C) was used as a carrier solvent at a flow rate of 1.0 mL/min. The highest occupied molecular orbital (HOMO) energy level was determined with a PESA: Riken AC-2 photoelectron spectrometer. Thermal analysis was performed on a Seiko EXSTAR 6000 TG/DTA 6300 thermal analyzer at a heating rate of 10 °C/min for thermogravimetry (TG) and a TA Instruments Q-100 connected to a cooling system at a heating rate of 10 °C/min for differential scanning calorimetry (DSC). UV–vis absorption spectra of the polymer solution and thin films were recorded using a Hitachi U-4100 spectrophotometer. The polymer thin film samples for UV–vis measurements were spin-coated at 1000 rpm for 60 s onto glass substrates from chloroform solutions (20 mg mL–1). Tapping mode AFM observation was performed with an Agilent AFM 5500 using microfabricated cantilevers with a force constant of 34 N/m. The grazing incidence X-ray diffraction (GIXD) measurements were conducted at the beamline BL46XU of SPring-8, Japan. The polymer thin film samples for GIXD measurements were spin-coated at 1000 rpm for 60 s onto Si wafers from chloroform solutions (20 mg mL–1). The sample was irradiated at a fixed incident angle αi on the order of 0.12° through a Huber diffractometer with an X-ray energy of 12.398 keV (λ = 0.10002 nm), and the GIXD patterns were recorded with a 2D image detector (Pilatus 300 K) with the sample-to-detector distances of 174.1 mm.

Acknowledgments

Y.O. was supported by Japan Society for the Promotion of Science (JSPS) Research Fellowship for Young Scientists (proposal no. 18J21080) and Innovative Flex Course for Frontier Organic Material Systems (iFront) at Yamagata University. GIXD measurement was carried out at the BL46XU of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (proposal no. 2018A1794). We thank Dr. Keisuke Tajima (RIKEN, Japan) for the kind assistance in measuring PESA AC-2 and for productive discussions. We thank Dr. Tomoyuki Koganezawa (JASRI, Japan) for the kind assistance in measuring GIXD.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.0c05514.

  • Experimental details of monomer and polymer syntheses, 1H and 13C NMR spectra of monomer compounds, 1H NMR spectra of polymers, controlled polymerization results, photoelectron spectra, TGA thermograms, UV–vis absorption spectra, 2D GIWAXS profiles, and AFM images (PDF)

Author Contributions

Y.O. and T.H. designed the project and experiments. Y.O. synthesized and characterized all of the monomer compounds and semiconducting polymers. Y.O. organized the data and wrote the manuscript. All authors reviewed and commented on the manuscript. T.H. directed the project.

The authors declare no competing financial interest.

Supplementary Material

ao0c05514_si_001.pdf (1.2MB, pdf)

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