Hybrid conjugated polymers with alternating dithienosilole or dithienogermole and tricoordinate boron units

Yohei Adachi a, Yousuke Ooyama a, Yi Ren b, Xiaodong Yin b, Frieder Jäkle *b and Joji Ohshita *a
aDepartment of Applied Chemistry, Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan. E-mail: jo@hiroshima-u.ac.jp; Fax: +81-82-424-5494; Tel: +81-82-424-7743
bDepartment of Chemistry, Rutgers University Newark, 73 Warren Street, Newark, New Jersey 07102, USA. E-mail: fjaekle@rutgers.edu

Received 24th October 2017 , Accepted 16th December 2017

First published on 18th December 2017


Conjugated polymers composed of tricoordinate boron and π-conjugated units possess extended conjugation with relatively low-lying LUMOs arising from pB–π interactions. However, donor–acceptor (D–A) polymers that feature triorganoboranes alternating with highly electron-rich donors remain scarce. We present here a new class of hybrid D–A polymers that combine electron-rich dithienosiloles or dithienogermoles with highly robust tricoordinate borane acceptors. Polymers of modest to high molecular weight are readily prepared by Pd-catalyzed Stille coupling reaction of bis(halothienyl)boranes and distannyldithienosiloles or -germoles. The polymers are obtained as dark red solids that are stable in air and soluble in common organic solvents. Long wavelength UV-vis absorptions at ca. 500–550 nm indicate effective π-conjugation and pronounced D–A interactions along the backbone. The emission maxima occur at wavelengths longer than 600 nm in solution and experience further shifts to lower energy with increasing solvent polarity, indicative of strong intramolecular charge transfer (ICT) character of the excited state. The powerful acceptor character of the borane comonomer units in the polymer structures is also evident from cyclic voltammetry (CV) analyses that reveal relatively low-lying LUMO levels of the polymers, enhancing the D–A interaction. Density functional theory (DFT) calculations on model oligomers further support these experimental observations.


Introduction

There has been growing interest in donor–acceptor (D–A) type π-conjugated polymers in the field of organic materials. Intramolecular electronic interactions between the donor and acceptor units give rise to extended conjugation, resulting in low band gap polymers. Intermolecular through-space interactions in the solid state are also enhanced, which facilitates the hopping carrier mobility between polymer chains. Because of these intriguing optoelectronic properties, applications of conjugated D–A polymers as active materials in organic optoelectronic devices, such as organic photovoltaics (OPVs) and organic field-effect transistors (OFETs), have been widely explored.1–8 For use in electronic devices, controlling the electronic states of the polymers is of critical importance to precisely match those of other materials in the devices. Thus, many types of electron donor and acceptor fragments with different electronic states have been developed. However, they are usually based on common π-conjugated units and strategies that introduce new building blocks are highly anticipated. One approach that has attracted much recent attention involves the development of new conjugated building blocks that feature electron-rich or electron-deficient main group elements, allowing for effective tuning of the electronic structure.9–11

Among different main group elements, boron stands out in that when introduced into π-conjugated systems it dramatically alters the electronic states.12–19 Interactions between the boron vacant p-orbital and π*-orbitals effectively lower the LUMO level of an organic conjugated π-system. In pioneering work, the group of Naka and Chujo demonstrated the formation of divinylbenzene-borane alternating polymers PVPVB (Chart 1) by the hydroboration of diethynylbenzene and mesitylborane as the first example of tricoordinate boron-containing conjugated polymers.12 Following this original finding, several other routes to boron-containing conjugated polymers (e.g., PBThB, P9BF, BPA) have been introduced, resulting in a range of new materials with intriguing electronic and optical properties.20–28 However, a drawback is that many of these polymers show limited stability in ambient atmosphere because of the high reactivity of the tricoordinate boron centers towards oxygen and moisture. Recently, some of us introduced the highly air-stable tricoordinate thienylboranes BDT and FBDT (Chart 1) as robust and versatile building blocks of conjugated polymers.29 In these structures, the central boron atom is protected by the bulky Mes* (2,4,6-tris(tert-butyl)phenyl) or FMes (2,4,6-tris(trifluoromethyl)phenyl) group, making it possible to handle them without special care. In addition to the steric protection, the fluorine-containing FMes group greatly enhances the electron-accepting properties of the tricoordinate boron in the pB–π system.30 Polythiophenes with tricoordinate boron embedded in the main chain were readily prepared via Stille cross coupling (PB2T–PB5T, PFB2T in Chart 1).31 These polymers exhibit intriguing properties, such as red-shifted UV-vis absorptions, derived from the extended conjugation through the p-orbital on boron, and strong emission in solution as well as the film state.31 However, donor–acceptor polymers that combine robust tricoordinate organoborane moieties and highly electron-rich donors in their backbone remain exceedingly rare.32,33


image file: c7py01790a-c1.tif
Chart 1 Examples of electron-deficient tricoordinate organoboron polymers and targeted organoborane-dithienosiloles/dithienogermoles D–A polymers.

We introduce here a new class of hybrid donor–acceptor polymers that incorporate both strongly electron-accepting tricoordinate borane units (BDT, FBDT) and highly electron-rich dithienosilole (DTS) and dithienogermole (DTG) donor units (Chart 1). The electron-rich planar tricyclic π-systems of DTS and DTG give rise to strong emission and enhanced conjugation compared with 2,2′-bithiophene itself. They have been widely used as efficient donor units of conjugated D–A oligomers and polymers for organic electronic device materials, such as OPVs,34–45 OFETs,46–49 dye-sensitized solar cells,50–57 and organic light-emitting diodes.58–64 However, they have not yet been explored as comonomers of boron-containing conjugated polymers. We report here the first examples of polymers containing both tricoordinate boron and electron-rich silole/germole moieties in the backbone. These polymers exhibit pronounced intramolecular charge transfer (ICT) character as indicated by low energy absorptions and strong solvatochromic effects in the photoluminescence spectra. The experimental observations are discussed together with the results of quantum chemical calculations on molecular model compounds.

Results and discussion

Synthesis

The DTS/DTG-containing organoborane polymers were prepared as presented in Scheme 1. BDTI2 was synthesized as reported in the literature29 and the organoborane monomer FBDTBr2 was prepared by the Sn–B exchange reaction of BBr3 with 2 equivalents of (5-bromothien-2-yl)trimethylstannane, followed by the reaction of the resulting intermediate, bis(5-bromothien-2-yl)bromoborane with 2,4,6-tris(trifluoromethyl)phenyllithium. The NMR and mass spectra of the intermediate and monomer FBDTBr2 are available in the ESI (Fig. S1–S7). To obtain polymers with sufficient solubility, we introduced branched 2-ethylhexyl (2ET) groups on the Si and Ge bridging atoms. The Stille cross-coupling reactions of bis(2,6-trimethylstannyl)dithienoheteroles and bis(5-halothienyl)boranes were examined under the conditions similar to those reported for other D–A polymers containing group 14 element bridged bithiophene units.43,65 The reactions proceeded smoothly in refluxing toluene. The resulting solutions containing the polymers were passed through a short silica gel column with chlorobenzene as eluent to remove the Pd catalyst. Polymers pDTSBDT and pDTGBDT were further purified by reprecipitation from toluene/ethanol and then from toluene/acetone, whereas pDTSFBDT and pDTGFBDT were reprecipitated twice from chloroform/ethanol. The modest yields (see Table 1) of polymers pDTSBDT and pDTGBDT are attributed to the formation of low molecular weight oligomers that were mostly removed by reprecipitation. All the polymers are stable in air and well soluble in common organic solvents, such as hexane, toluene, chloroform, and THF.
Table 1 Data for hybrid borane-dithienosilole/dithienogermole copolymers
Polymer Yielda/% M n[thin space (1/6-em)]b Đ T 5d[thin space (1/6-em)]c/°C
a Reprecipitated from toluene/ethanol and then from toluene/acetone (pDTSBDT, pDTGBDT) or twice from chloroform/ethanol (pDTSFBDT, pDTGFBDT). b Determined by GPC analysis (Đ = Mw/Mn). c 5% weight loss temperatures determined by thermogravimetric analysis (TGA) in nitrogen.
pDTSBDT 53 9400 1.9 308
pDTGBDT 52 12[thin space (1/6-em)]400 2.0 308
pDTSFBDT 83 13[thin space (1/6-em)]900 4.2 406
pDTGFBDT 78 15[thin space (1/6-em)]300 3.6 403



image file: c7py01790a-s1.tif
Scheme 1 Synthesis of hybrid borane-dithienosilole/germole copolymers; 2ET = 2-ethylhexyl.

The polymer structures were verified by NMR and MALDI-TOF MS analysis (Fig. S8–S22). 11B NMR measurements at room temperature gave no clear signals, but spectral data acquired at 50 °C revealed broad peaks around 45 ppm (Fig. S12), which is in a similar range as for the corresponding organoboron monomers.29,31 The presence of the FMes substituents on boron in pDTSFBDT and pDTGFBDT was further confirmed by sharp singlets due to the ortho- and para-CF3 groups (−58.4, −65.2 ppm) in the 19F NMR spectra (Fig. S17). For pDTSBDT and pDTSFBDT, the 29Si NMR spectra displayed only one major resonance (−5.4, −5.1 ppm), consistent with incorporation of the silole moieties into the polymers (Fig. S18). Thus, the heteronuclear NMR data clearly confirm the integrity of the building blocks in the polymer structures. In the 1H NMR spectra of the polymers peaks were observed at reasonable chemical shifts and with nearly ideal integral ratios (Fig. S8–S11). For instance, the aromatic protons of the FMes groups appear at a characteristic chemical shift of 8.18 ppm, while those of the Mes* groups resonate at 7.45 ppm. 13C NMR measurements were also performed for all the polymers at 50 °C, providing spectra that are well resolved, even revealing the C–F coupling for the FMes-substituted polymers (Fig. S13–S16). The number of signals and chemical shifts of the major peaks were consistent with the expected structures. Small additional peaks of very low intensity in the aromatic region are tentatively attributed to polymer end groups.

To further elucidate the microstructure of the polymers, MALDI-TOF mass spectra were acquired. Series with peak spacings that match well with the repeating units consisting of a DTS/DTG (x) and borane (y) moiety were readily identified. As illustrated in Fig. 1, oligomers are detected with up to ca. x, y = 5 units for each building block. Both types of terminal groups are present as suggested by peak series corresponding to x = y, x = y + 1, and x = y − 1 for the number of constituting building blocks. Surprisingly, matching of the peak distributions to calculated peak patterns indicated the absence of additional protons at the chain ends (Fig. S19–S22). This may suggest that cyclic species are formed, at least in case of the relatively low molecular weight species that are detectable by MALDI-TOF MS. For some of the samples relatively much smaller peaks corresponding to oligomers with an additional DTS or DTG moiety are also detected (x = y + 2, Fig. S21–S22). This is most pronounced for pDTGFBDT, but almost completely absent in the spectra of pDTSBDT and pDTGBDT, and indicates the presence of a homo-coupled unit. Consistent is that in the 1H NMR spectra we found a small peak at around 7.1 ppm (Fig. S8–S11), which is similar in chemical shift to DTS/DTG homopolymers.37,43 The ratios of the homo-coupled units to the desired ideal alternating structure, however, is very small, estimated to be approximately 10% based on 1H NMR integral ratios both for pDTSBDT and pDTGBDT and less than 5% for the fluorinated polymers.


image file: c7py01790a-f1.tif
Fig. 1 Section of the MALDI-TOF MS of hybrid borane-dithienosilol/dithienogermole copolymers (positive mode, matrix: trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malononitrile); x = number of DTS/DTG units and y = number of dithienylborane units.

The molecular weights of the polymers were estimated by gel permeation chromatography (GPC) using polystyrene standards (Table 1). Polymers pDTSBDT and pDTGBDT showed monomodal GPC profiles with dispersities (Đ) of 1.9 and 2.0, respectively, whereas bimodal distributions with larger dispersities were found for pDTSFBDT and pDTGFBDT. We also carried out thermogravimetric analysis (TGA) of the polymers to evaluate their thermal stability. The temperatures corresponding to 5% weight loss (T5d) were noted at 308 °C for both pDTSBDT and pDTGBDT in nitrogen. These values are comparable to those of previously reported polymers such as PB4T (Chart 1).31 Notably, the T5d values of pDTSFBDT and pDTGFBDT are over 400 °C and comparable to those of homopolymers of DTS (T5d = 414 °C, Mn = 42[thin space (1/6-em)]000)43 and DTG (T5d = 400 °C, Mn = 33[thin space (1/6-em)]000).38 This difference is likely ascribable to the higher thermal stability of the CF3 in comparison to the tBu groups. We also performed differential scanning calorimetry (DSC) analyses to determine the melting and glass transition points of the polymers in nitrogen. For pDTSBDT and pDTGBDT no transitions could be observed from room temperature to 250 °C, but the scans for pDTSFBDT and pDTGFBDT displayed glass transition points at 107 °C and 127 °C in the second scan (Fig. S23 and S24).

Photophysical properties

The photophysical properties of the polymers are summarized in Tables 2 and 3. In addition, representative absorption and fluorescence spectra of the borane-dithienogermole copolymers in solution and in the film state are displayed in Fig. 2 and those of the corresponding borane-dithienosilole copolymers are illustrated in Fig. S25 and S26. The absorption maxima of all polymers (λabsmax in the range of 532 to 562 nm in THF) are red-shifted with respect to those of the borane building blocks BDT (λabsmax = 324 nm in THF) and FBDT (λabsmax = 326 nm in THF)29 and unsubstituted quaterthiophene (λabsmax = 393 nm in CH2Cl2),66 indicating that the conjugation is effectively extended due to pB–π interactions. Importantly, the absorption maxima are also significantly red-shifted from that of thiophene copolymer PB4T (Chart 1, λabsmax = 480 nm in THF),31 which contains the same number of thiophene units between tricoordinate boron centers. This result points to a pronounced effect of the electron-rich DTS/DTG donor units. The spectral data of the polymers are very similar regardless of the heterole bridging element (Si or Ge), but the absorption and emission bands of the FMes-substituted polymers pDTSFBDT and pDTGFBDT are further red-shifted relative to those of pDTSBDT and pDTGBDT containing Mes* substituents on boron. This observation suggests that the D–A interaction between the borane and dithienosilole/germole units strongly depends on the electron-deficient character of boron, which is enhanced for the FMes relative to the Mes*-substituted borane moiety. The absorption bands are not significantly affected by the nature of the solvent, and in the film state they appear at wavelengths similar to those in solution, indicating rather weak intermolecular interactions in the ground state (Table 2). In contrast, the fluorescence spectra show a strong dependence on the solvent polarity (Table 3). In non-polar solvents intense emissions are observed in the range of 592 to 630 nm with quantum yields from 32–41%. As the solvent polarity increases, the emission maxima (λemmax) are shifted to longer wavelengths and the fluorescence quantum yields (Φ) gradually decrease. This tendency is in line with a pronounced ICT character of the photoexcited states.67,68 Again, a dramatic bathochromic shift is observed for the FMes-borane copolymers relative to the Mes*-borane copolymers, resulting in emission maxima up to 670 nm for pDTGFBDT in pyridine as the solvent. We expected the polymers to be also emissive in the film state, because the bulky Mes* and FMes substituents should inhibit intermolecular interactions. However, the emission of the polymers in the film state is weak and the fluorescence quantum yields could not be determined reliably (<2%). Moreover, the emission bands were red-shifted from those in solution, signifying strong intermolecular interactions. As the absorption spectra of the polymers are nearly independent of the state (solution or film), the red-shifted emission for the films is likely due to further stabilization of the excited state by intermolecular interaction, such as excimer or exciplex formation. To further explore this aspect, we prepared 0.5 wt% PMMA films of pDTSBDT and pDTSFBDT. The emission maxima of the PMMA-embedded polymer films are clearly blue-shifted from those of the neat films while the quantum yield increases significantly (pDTSBDTλemmax = 626 nm, Φ = 2%; pDTSFBDTλemmax = 665 nm, Φ = 6%; Fig. S27). This supports the notion that excimer or exciplex formation plays a significant role in the condensed film state.
image file: c7py01790a-f2.tif
Fig. 2 Absorption (solid lines) and fluorescence (dashed lines) spectra of pDTGBDT (left) and pDTGFBDT (right) in solution and as thin film.
Table 2 Absorption data of borane-dithienosilole/germole copolymers
Polymer λ absmax/nm
Hexanea Toluenea THFa Pyridinea CH2Cl2[thin space (1/6-em)]a Filmb
a Carried out in 8.0 mg L−1 solution. b Spin-coated film on quartz glass. c Shoulder peak.
pDTSBDT 525, 495 535, 504 533, 502 537, 510 533, 505 535c, 496
pDTGBDT 527, 497 533, 508 532, 505 537, 510 537c, 507 538c, 500
pDTSFBDT 548, 515c 558, 527 556, 526 525 557, 529 544
pDTGFBDT 550, 517c 558, 527 562c, 523 533 561, 535 540


Table 3 Fluorescence data of borane-dithienosilole/germole copolymers
Polymer λ emmax/nm (Φ/%)
Hexanec Toluenec THFc Pyridinec CH2Cl2[thin space (1/6-em)]c Filmd
a Excited at 500 nm. b Excited at 530 nm. c Carried out in 8.0 mg L−1 solution. d Drop-casted film on quartz glass. e Too weak to enable determination of quantum yield.
pDTSBDT 592 (32) 606 (33) 613 (30) 627 (23) 616 (11) 650 (—e)
pDTGBDT 595 (36) 603 (32) 611 (25) 622 (18) 617 (12) 663 (—e)
pDTSFBDT 627 (41) 616 (30) 654 (19) 665 (6) 622 (14) 697 (—e)
pDTGFBDT 630 (36) 629 (29) 651 (17) 670 (4) 622 (15) 709 (—e)


Electrochemical properties

To further evaluate the electronic structures of the polymers, cyclic voltammetry measurements were performed on the polymer films in acetonitrile containing tetrabutylammonium perchlorate as the supporting electrolyte (Table 4). The cathodic cyclic voltammograms (CVs) of pDTGBDT and pDTGFBDT are shown in Fig. 3 and those of pDTSBDT and pDTSFBDT occur at very similar potentials (Fig. S28(a)). They reveal pseudo-reversible reductive couples. As borane-quarterthiophene copolymer PB4T (Chart 1) solutions exhibited reversible cathodic profiles,31 the modest electrochemical reversibility for the present polymers is likely due to the different experimental conditions (solution vs. film) and possibly also associated with cathodic instability of the DTS/DTG units. The LUMO levels as estimated from the onsets of reduction (Ered) are significantly lower for pDTSFBDT and pDTGFBDT in comparison to those of pDTSBDT and pDTGBDT, reflecting the strong electron-withdrawing ability of the FMes group (Table 4). We also investigated the anodic properties of the polymers. The polymer films gave irreversible anodic peaks and, in contrast to their cathodic behaviors, no clear dependence of the anodic peak potentials on the polymer structures was seen (Fig. 4 and S28(b)). This is ascribed to the fact that the polymer HOMOs are mainly localized on the DTS and DTG units, as predicted by the theoretical studies (vide infra).35 Similarity of the electronic states for DTS and DTG has been demonstrated. On the basis of the CV onset potentials (Eg(CV)), the HOMO–LUMO gaps decrease in the order of pDTSBDT > pDTGBDTpDTSFBDT > pDTGFBDT, which agrees well with the trend for the optical bandgaps (Eg(Opt)). We further deduce that the narrowed Eg of pDTSFBDT and pDTGFBDT compared to those of pDTSBDT and pDTGBDT is primarily due to the lower LUMO levels.
Table 4 Electrochemical data of borane-dithienosilole/germole copolymers
Polymer E red/V (Fc/Fc+) LUMOb/eV E ox[thin space (1/6-em)]c/V (Fc/Fc+) HOMOd/eV E g(CV)e/eV E g(Opt)f/eV
a Onset of reductive wave in CV using 0.1 M tetrabutylammonium perchlorate in MeCN as supporting electrolyte and a scan rate of 50 mV s−1. b Determined as −(4.8 + Ered). c Onset of oxidative wave in CV. d Determined as −(4.8 − Eox). e Determined as EoxEred. f Obtained from the onset of absorption in THF. g [thin space (1/6-em)]Ref. 31.
pDTSBDT −1.99a −2.81 0.66 −5.46 2.65 2.16
pDTGBDT −1.98a −2.82 0.63 −5.43 2.61 2.15
pDTSFBDT −1.75a −3.05 0.67 −5.47 2.42 2.07
pDTGFBDT −1.74a −3.06 0.64 −5.44 2.38 2.02
PB4T −1.89 −2.90 2.29



image file: c7py01790a-f3.tif
Fig. 3 Cyclic voltammetry data (reductive waves) of borane-dithienogermole copolymer films in acetonitrile with 0.1 M tetrabutylammonium perchlorate as supporting electrolyte at a scan rate of 50 mV s−1. The spectra are normalized at the top of the first reductive wave.

image file: c7py01790a-f4.tif
Fig. 4 Cyclic voltammetry data (oxidative waves) of borane-dithienogermole copolymer films in acetonitrile with 0.1 M tetrabutylammonium perchlorate as supporting electrolyte at a scan rate of 50 mV s−1. The spectra are normalized at the top of the first oxidative wave.

Theoretical studies

To gain further insights into the electronic structures of the present polymers, density functional theory (DFT) calculations were carried on the model compounds DTGBDT2 and 2TBDT2 at the B3LYP/6-31G(d,p) level of theory using the Gaussian 09 program. The tBu and 2-ethylhexyl groups were replaced with methyl groups in the models to simplify the calculations. The frontier orbitals of DTGBDT2 and 2TBDT2 derived from these calculations are shown in Fig. 5. The two model compounds exhibit very similar HOMO and LUMO orbital distributions regardless of whether a Si/Ge-fused or unfused bithiophene bridge is present. The HOMOs and LUMOs are mainly distributed on the central thiophene rings and the adjacent boron atoms, but the HOMOs are more localized on the quaterthiophene cores. The LUMOs have significant contributions from the boron p-orbitals, indicating effective pB–π* conjugation. However, σ*–π* conjugation is not evident for the DTG unit. It is known that such a conjugation tends to be suppressed in highly conjugated π-systems.43,69 The HOMO energy level of DTGBDT2 is significantly higher than that of 2TBDT2. This is in part due to the better planarity of DTGBDT2, as shown in Fig. 5, but the electron donating effect of the germanium atom in the DTG unit may also contribute to raising the HOMO. It may also be speculated that the bithiophene moiety in the DTG unit is fixed in a syn geometry, inducing a through-space anti-bonding interaction between the 3,3′-positions in DTG to raise the HOMO level of DTGBDT2. While the higher planarity of DTGBDT2 might be expected to also lower the LUMO level due to more effective conjugation, the calculated LUMO energy of DTGBDT2 is comparable to that of 2TBDT2. A possible reason is that the electron-donating character of DTG raises not only the HOMO but also the LUMO level.
image file: c7py01790a-f5.tif
Fig. 5 Frontier orbital depictions of model compounds (B3LYP/6-31G(d,p)).

AFM measurements

We investigated the morphological properties of thin films of the present polymers by atomic force microscopy (AFM). The films were prepared by spin-coating toluene solutions of pDTGBDT and pDTGFBDT on glass substrates. The thus obtained images are shown in Fig. S29. In these images, any ordered structures such as lamellar orientations and segment formation cannot be observed, indicating no specific chain–chain interactions of the polymers are involved. The surface of the pDTGFBDT film was very smooth and flat, and no aggregates were observed, but the pDTGBDT film displayed aggregates. This may indicate stronger intermolecular interactions for pDTGBDT, which could be advantageous when applying the polymer as semi-conducting material.

Conclusions

We have prepared a new class of conjugated hybrid polymers, in which electron-deficient tricoordinate borane moieties are for the first time combined with electron-rich dithienosilole and dithienogermole donor units. The resulting conjugated hybrid polymers are stable under ambient conditions and well soluble in common organic solvents, including hexane, toluene, and THF. Effective D–A interactions are evidenced by strong bathochromic shifts of the absorption and emission maxima and solvatochromic effects in the fluorescence spectra, indicating a pronounced ICT character at the excited state. Emission maxima of up to 700 nm demonstrate the effectiveness of our approach of combining electron-deficient boranes with relatively low-lying LUMO levels and electron-rich dithienosilole/dithienogermole building blocks with high-lying HOMO levels. Our results further suggest potential utility of these new hybrid D–A polymers in fluorescence imaging and optoelectronic applications, such as photovoltaic or non-linear optical materials.

Experimental

General

NMR spectra were recorded on Varian System 600, 500 and 400MR spectrometers. BF3·Et2O and fluorobenzene were used as the external and internal standards for 11B and 19F NMR measurements, respectively. MALDI-TOF mass spectra were obtained on a Bruker Ultraflextreme instrument in positive mode, using trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malononitrile as the matrix. Molecular weights of the polymers were determined by gel permeation chromatography (GPC) using THF as eluent and serially connected Shodex KF2001 and KF2002 columns, relative to polystyrene standards. TGA was carried out on an SII TG/DTA-6200 analyzer under gentle nitrogen flow (100 mL min−1) at a heating rate of 10 °C min−1. DSC data were acquired with an Exstar DSC6200 thermal analyzer (Seiko Instruments). The sample was packed in an aluminum pan and heated at a rate of 10 °C min−1 under nitrogen from 25 to 250 °C (pDTSBDT and pDTGBDT) or 300 °C (pDTSFBDT and pDTGFBDT), taking the decomposition temperature of the polymers by TGA into account, then cooled to 25 °C at the same rate. This operation was repeated twice and the thermal transitions were derived from the second cycle. UV-vis absorption spectra were measured with a Shimadzu UV-3600 plus spectrometer. Photoluminescence (PL) spectra were measured with a HORIBA FluoroMax-4 spectrophotometer. The PL quantum yields were determined by using a HORIBA FluoroMax-4 spectrophotometer attached to an integration sphere. CVs were measured with an AMETEK VersaSTAT 4 potentiostat/galvanostat in a solution of 0.1 M tetrabutylammonium perchlorate (TBAP) in acetonitrile using a three-electrode system with a Pt plate counter electrode, a Pt wire working electrode, and an Ag/Ag+ reference electrode. The polymer (1 mg) and TBAP (10 mg) were dissolved in 1 mL of chlorobenzene, and the solution was drop-casted on the working electrode and dried under vacuum for 2 h at 50 °C. DFT calculations were performed using the Gaussian 09 program at the B3LYP/6-31G(d,p) level of theory. AFM images were obtained by Agilent Technologies PicoPlus 5500 in AC mode using a soft cantilever with a small force constant (Nanosensors, PPP-FM, force constant = 1.9 N m−1). All reactions were carried out under dry argon. The reaction solvents were purchased from Kanto Chemical Co., Ltd and were distilled from calcium hydride and stored over activated molecular sieves under argon until use. Starting materials, DTSSn,70DTGSn,35 and BDTI229 were prepared according to the literature.

Synthesis of FBDTBr2

In a glovebox, to a solution of BBr3 (0.31 mL, 3.3 mmol) in 5 mL toluene was added a solution of (5-bromothien-2-yl)trimethylstannane (2.1 g, 6.4 mmol) in 5 mL toluene at room temperature. The mixture was stirred overnight at room temperature. The volatile components were then removed under high vacuum. The intermediate was used in the next step without further purification. 1H NMR (499.9 MHz, δ in CDCl3, 25 °C): 7.90 (d, J = 5.0 Hz, 4H), 7.33 (d, J = 5.0 Hz, 4H). 11B NMR (160.3 MHz, δ in CDCl3, 25 °C) 47.8 ppm. To 1,3,5-tris(trifluoromethyl)benzene (1.48 g, 5.3 mmol) in 100 mL of dry ether, n-BuLi (3.3 mL, 1.6 M in hexane, 5.3 mmol) was added dropwise at −78 °C. The mixture was stirred for another 0.5 h at this temperature, then warmed to room temperature and stirred for 4 h. The solvent was removed under high vacuum to obtain 1-lithio-2,4,6-tris(trifluoromethyl)benzene as a light yellow solid. The solid was suspended in 5 mL toluene and transferred to a solution of the bromoborane intermediate in toluene at room temperature. The reaction mixture was stirred for 24 hours at room temperature. Water was added and the aqueous layer was extracted with DCM (3 × 10 mL). All solvents were removed via rotary evaporation. The crude product was purified by flash column chromatography using hexanes as eluent. The product is obtained as a light yellow solid (42% yield). 1H NMR (599.7 MHz, δ in CDCl3, 25 °C): 8.15 (s, 2H), 7.25 (d, J = 4 Hz, 2H, Th), 7.12 (d, J = 4 Hz, 2H, Th). 13C NMR (150.8 MHz, δ in CDCl3, 25 °C): 143.7 (br, C–B), 143.3, 143.3 (br, C–B), 134.8 (q, JC–F = 32 Hz, o-[C with combining low line]CF3), 132.8, 132.4 (q, JC–F = 35 Hz, p-[C with combining low line]CF3), 127.2, 126.4 (br, Th), 123.6 (q, JC–F = 276 Hz, o-CF3), 122.9 (q, JC–F = 273 Hz, p-CF3). 11B NMR (192.4 MHz, δ in CDCl3, 25 °C): 49.7 (w1/2 = 1000 Hz). 19F NMR (470.4 MHz, δ in CDCl3, 25 °C): −56.2 (s), −63.2 (s). HRMS: m/z = 615.8573 ([M], calcd for C17H5BBr2F9S2 615.8204). Elemental analysis calcd (%) for C17H6BBr2F9S2: C 33.15, H 0.98; found: C 33.62, H 0.99.

Polymer synthesis

A mixture of 225 mg (0.303 mmol) of DTSSn, 204 mg (0.303 mmol) of BDTI2, 13.9 mg (5 mol%) of Pd2(dba)3, 18.3 mg (20 mol%) of P(o-tol)3, and 10 mL of toluene was heated to reflux for 4 days. The mixture was cooled to room temperature and subjected to short silica gel chromatography with chlorobenzene as the eluent. The eluate was evaporated and the residue was reprecipitated from 3 mL of toluene into 200 mL of EtOH and then from 3 mL of toluene into 200 mL of acetone to give 133 mg (52% yield) of pDTSBDT as a red powder: 1H NMR (400 MHz, δ in CDCl3): 7.58 (br s, 2H, thiophene), 7.45 (s, 2H, Mes*), 7.32 (s, 2H, DTS), 7.26–7.21 (m, 2H, thiophene), 1.51–1.42 (2H, CH in 2ET = 2-ethylhexyl), 1.40 (s, 9H, CH3 in Mes*), 1.35–0.92 (m, 20H, CH2 in 2ET), 1.23 (s, 18H, CH3 in Mes*), 0.88–0.72 (m, 12H, CH3 in 2ET). 11B NMR (160.4 MHz, δ in CDCl3, 50 °C): 46.5 (w1/2 = 4500 Hz). 13C NMR (125.7 MHz, δ in CDCl3, 50 °C): 151.8 (sp2), 148.5 (sp2), 147.7 (sp2), 146.7 (sp2), 144.7 (DTS), 142.3 (sp2), 138.4 (DTS), 134.7 (sp2), 127.8 (sp2), 126.2 (sp2), 124.7 (sp2), 122.7 (sp2), 38.7 (tBu), 35.9 (2ET), 35.6 (2ET), 35.0 (tBu), 34.7 (tBu), 31.4 (tBu), 29.0 (2ET), 28.9 (2ET), 23.1 (2ET), 17.7 (2ET), 14.2 (2ET), 10.8 (2ET). 29Si NMR (99.3 MHz, δ in CDCl3, 50 °C): −5.4. TGA: T5d 308 °C (in nitrogen). Melting point was not observed up to 250 °C by DSC in nitrogen.

Polymer pDTGBDT was prepared from 238 mg (0.301 mmol) of DTGSn and 203 mg (0.302 mmol) of BDTI2 as a red powder (137 mg, 53% yield) in a manner similar to that above. The polymer was purified by reprecipitating from 3 mL of toluene into 200 mL of EtOH and then from 3 mL of toluene into 200 mL of acetone: 1H NMR (400 MHz, δ in CDCl3): 7.58 (br s, 2H, thiophene), 7.45 (s, 2H, Mes*), 7.32 (s, 2H, DTG), 7.26–7.21 (m, 2H, thiophene), 1.60–1.48 (m, 2H, CH in 2ET), 1.40 (s, 9H, CH3 in Mes*), 1.38–1.12 (m, 20H, CH2 in 2ET), 1.24 (s, 18H, CH3 in Mes*), 0.88–0.72 (m, 12H, CH3 in 2ET). 11B NMR (160.4 MHz, δ in CDCl3, 50 °C): 45.8 (w1/2 = 4700 Hz). 13C NMR (125.7 MHz, δ in CDCl3, 50 °C): 151.8 (sp2), 148.5 (sp2), 147.5 (sp2), 146.7 (sp2), 146.1 (DTG), 142.3 (sp2), 138.2 (DTG), 134.7 (sp2), 127.9 (sp2), 126.2 (sp2), 124.6 (sp2), 122.7 (sp2), 38.7 (tBu), 37.0 (2ET), 35.5 (2ET), 35.0 (tBu), 34.7 (tBu), 31.4 (tBu), 28.9 (2ET), 28.8 (2ET), 23.1 (2ET), 20.8 (2ET), 14.2 (2ET), 10.9 (2ET). TGA: T5d 308 °C (in nitrogen). Melting point was not observed up to 250 °C by DSC in nitrogen.

Polymer pDTSFBDT was prepared from 235 mg (0.318 mmol) of DTSSn and 195 mg (0.317 mmol) of FBDTBr2 as a purple powder (216 mg, 78% yield) in a manner similar to that above. The polymer was purified by reprecipitating twice from 3 mL of chloroform into 200 mL of EtOH: 1H NMR (400 MHz, δ in CDCl3): 8.18 (s, 2H, FMes), 7.54–7.40 (m, 2H, thiophene), 7.35 (br s, 2H, DTS), 7.33–7.28 (m, 2H, thiophene), 1.50–1.38 (m, 2H, CH in 2ET), 1.38–0.92 (m, 20H, CH2 in 2ET), 0.88–0.74 (12H, CH3 in 2ET). 11B NMR (160.4 MHz, δ in CDCl3, 50 °C): 44.7 (w1/2 = 4000 Hz). 13C NMR (125.7 MHz, δ in CDCl3, 50 °C): 150.1 (sp2), 149.3 (sp2), 145.4 (DTS), 145.0 (sp2), 143.6 (sp2), 140.6 (sp2), 138.1 (DTS), 134.9 (q, JC–F = 32 Hz, o-[C with combining low line]CF3), 131.9 (q, JC–F = 34 Hz, p-[C with combining low line]CF3), 128.7 (sp2), 126.1 (sp2), 125.4 (sp2), 123.7 (q, JC–F = 276 Hz, o-CF3), 123.0 (q, JC–F = 273 Hz, p-CF3), 36.1, 35.9, 29.1, 29.0, 23.0, 18.0, 14.0, 10.8. 19F NMR (470.4 MHz, δ in CDCl3): −58.4 (m), −65.2 (s). 29Si NMR (99.3 MHz, δ in CDCl3, 50 °C): −5.1. TGA: T5d 406 °C (in nitrogen). DSC: Tg 107 °C (in nitrogen).

Polymer pDTGFBDT was prepared from 250 mg (0.317 mmol) of DTGSn and 195 mg (0.317 mmol) of FBDTBr2 as a purple powder (241 mg, 83% yield) in a manner similar to that above. The polymer was purified by reprecipitating twice from 3 mL of chloroform into 200 mL of EtOH: 1H NMR (400 MHz, δ in CDCl3): 8.18 (s, 2H, FMes), 7.54–7.40 (m, 2H, thiophene), 7.35 (br s, 2H, DTG), 7.34–7.28 (m, 2H, thiophene), 1.58–1.45 (m, 2H, CH in 2ET), 1.38–1.10 (m, 20H, CH2 in 2ET), 0.91–0.77 (12H, CH3 in 2ET). 11B NMR (160.4 MHz, δ in CDCl3, 50 °C): 46.0 (w1/2 = 3900 Hz). 13C NMR (125.7 MHz, δ in CDCl3, 50 °C): 150.1 (sp2), 146.9 (sp2), 146.8 (DTG), 145.1 (sp2), 143.6 (sp2), 140.5 (sp2), 137.9 (DTG), 134.9 (q, JC–F = 32 Hz, o-[C with combining low line]CF3), 131.9 (q, JC–F = 34 Hz, p-[C with combining low line]CF3), 128.7 (sp2), 126.0 (sp2), 125.3 (sp2), 123.7 (q, JC–F = 276 Hz, o-CF3), 123.0 (q, JC–F = 273 Hz, p-CF3), 37.2, 35.7, 29.1, 28.9, 23.1, 21.2, 14.0, 10.9. 19F NMR (470.4 MHz, δ in CDCl3): −58.4 (m), −65.2 (s). TGA: T5d 403 °C (in nitrogen). DSC: Tg 127 °C (in nitrogen).

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by JSPS KAKENHI Grant Number JP24102005, JP16K14066 and JP17H06890. F. J., Y. R. and X. Y. thank the National Science Foundation for support (Grant CHE-1362460 and CHE-1664975). We thank Abdullah Alahmadi and Dr Roman Brukh for assistance with acquisition of MALDI-TOF mass spectral data. We also thank Dr Ichiro Imae for assistance with AFM measurements.

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Footnote

Electronic supplementary information (ESI) available: Characterization details including NMR and mass spectra, DSC curves, absorption spectra, fluorescence spectra, and cyclic voltammograms. See DOI: 10.1039/c7py01790a

This journal is © The Royal Society of Chemistry 2018