N-Benzoyl dithieno[3,2-b:2′,3′-d]pyrrole-based hyperbranched polymers by direct arylation polymerization

Background Although poly(N-acyl dithieno[3,2-b:2′,3′-d]pyrrole)s have attracted great attention as a new class of conducting polymers with highly stabilized energy levels, hyperbranched polymers based on this monomer type have not yet been studied. Thus, this work aims at the synthesis of novel hyperbranched polymers containing N-benzoyl dithieno[3,23,2-b:2′,3′-d]pyrrole acceptor unit and 3-hexylthiophene donor moiety via the direct arylation polymerization method. Their structures, molecular weights and thermal properties were characterized via 1H NMR and FTIR spectroscopies, GPC, TGA, DSC and XRD measurements, and the optical properties were investigated by UV–vis and fluorescence spectroscopies. Results Hyperbranched conjugated polymers containing N-benzoyl dithieno[3,23,2-b:2′,3′-d]pyrrole acceptor unit and 3-hexylthiophene donor moiety, linked with either triphenylamine or triphenylbenzene as branching unit, were obtained via direct arylation polymerization of the N-benzoyl dithieno[3,23,2-b:2′,3′-d]pyrrole, 2,5-dibromo 3-hexylthiophene and tris(4-bromophenyl)amine (or 1,3,5-tris(4-bromophenyl)benzene) monomers. Organic solvent-soluble polymers with number-average molecular weights of around 18,000 g mol−1 were obtained in 80–92% yields. The DSC and XRD results suggested that the branching structure hindered the stacking of polymer chains, leading to crystalline domains with less ordered packing in comparison with the linear analogous polymers. The results revealed that the hyperbranched polymer with triphenylbenzene as the branching unit exhibited a strong red-shift of the maximum absorption wavelength, attributed to a higher polymer stacking order as a result of the planar structure of triphenylbenzene. Conclusion Both hyperbranched polymers with triphenylamine/triphenylbenzene as branching moieties exhibited high structural order in thin films, which can be promising for organic solar cell applications. The UV–vis absorption of the hyperbranched polymer containing triphenylbenzene as branching unit was red-shifted as compared with the triphenylamine-containing polymer, as a result of a higher chain packing degree.


Background
Conjugated polymers have received significant attention in fundamental and applied research owing to their interesting optical and optoelectronic properties. Thus, they have been used in many electronic applications such as light emitting diode (OLED), polymeric solar cells (PSCs), electrochromic devices, organic field-effect transistors (OFETs), chemo-and biosensors [1][2][3][4]. In these extensive applications, the donor-acceptor (D-A) type of conjugated polymers, consisting of both electron donor and electron acceptor substituents along the conjugated backbone with excellent electron mobility, broad absorption spectrum and properly matched energy levels, has generated significant interest in the field of PSCs [5][6][7][8][9][10]. Especially, conjugated polymers composed of various thiophene-based electron donating units have shown promising properties to be suitable as hole-transporting materials in electro-optical devices [11][12][13].
Hyperbranched conjugated polymers with highly branched molecular structure can effectively suppress aggregation and therefore are attractive due to good solubility and processability, low viscosity as well as facile one-pot synthesis and tunable electrical properties. Despite extensive research on the synthesis of hyperbranched conducting polymers in the past [19][20][21], in the last couple of years considerable effort has been put into the development of hyperbranched conjugated structures based on new compositional units. The Cu(I)-catalyzed azide-alkyne click reaction was used to synthesize an ethynyl-capped hyperbranched conjugated polytriazole [22]. Zhou et al. [23] employed Suzuki coupling polymerization to obtain hyperbranched polymers based on alkyl-modified 2,4,6-tris(thiophen-2-yl)-1,3,5-triazine and fluorene units with high molecular weights and enhanced two-photon absorption as compared with their unsubstituted analogues. The Suzuki polymerization was also used to one-pot synthesize a hyperbranched conjugated polymer bearing dimethylamino groups to be used as a PSC cathode interlayer [24]. Sen et al. [25] synthesized hyperbranched conjugated polymers based on 4,4′-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) via Sonogashira cross coupling polymerization reactions. The polymers showed red shifts in absorption and emission maxima upon contact with toluene and benzene vapors. Very recently, hyperbranched thiophene-flanked diketopyrrolopyrrole (TDPP)-based polymers with narrow bandgaps were prepared by direct arylation polymerization method [26]. Knoevenagel condensation and Sonogashira coupling methods were used to synthesize different hyperbranched conjugated polymers, which were tested as chemosensors for detecting nitroaromatic compounds [27][28][29]. The base-catalyzed reactions between α,β-unsaturated ester and aldehyde was employed to synthesize hyperbranched conjugated polymers containing 1,3-butadiene repeating units and carboxylic ester side groups for sensing metal ion Fe 3+ [30].
The structure of monomer 3 was determined via 1 H NMR. The 1 H NMR spectrum of monomer 3 (Fig. 1) shows a doublet peak at 7.73 ppm (peak c), a triplet peak at 7.65 ppm (peak e, Fig. 1) and a triplet peak at 7.55 ppm (peak d) corresponding to the protons of the benzene ring. The doublet peak at 7.1 ppm (peak b) and the singlet peak at 6.85 ppm (peak a) are assigned to the protons of the thiophene rings. The presence of these peaks, along with their integral ratios, indicate that the amidation reaction has taken place successfully to give the N-benzoyl dithieno[3,2-b:2′,3′-d]pyrrole monomer.

Direct arylation polycondensation
The chemical structures of hyperbranched conjugated polymers PBDP3HTTPA and PBDP3HTTPB and corresponding monomers are shown in Schemes 1 and 2, respectively. The monomers N-benzoyl dithieno[3,2b:2′,3′-d]pyrrole (3) and 2,5-dibromo-3-hexylthiophene (4) underwent direct arylation polycondensation with tris(4-bromophenyl)amine (1) (or 1,3,5-tris(4-bromophenyl)benzene (2)) to build hyperbranched conjugated polymer structures. The polycondensation was carried out in the DMAc solvent at 100 °C with Pd(OAc) 2 and PCy 3 .HBF 4 as the catalyst and ligand, respectively. The PBDP3HTTPA hyperbranched polymer was synthesized by polymerization of a mixture of monomers (1), (3) and (4), the solution of which became dark orange after 2 h, and gradually turned into black accompanying the appearance of a solvent-insoluble black solid. After 24 h, the hyperbranched polymer was obtained by purification via extraction, filtration via a Celite layer to remove the Pd catalyst, subsequent washing and precipitation in cold n-heptane. On the other hand, the polymerization mixture of monomers (2), (3) and (4) showed a red color in 3 h after initiation, which then gradually changed into dark red. The obtained PBDP3HTTPB was purified in a similar way to PBDP3HTTPA. The yield of both reactions were in the range of 80-90% after 24 h. It should be noted that the solvent-insoluble part (about 5%) and soluble oligomer fraction were removed via filtration through Celite layer and via washing with acetone, respectively. The number average molecular weights (M n s) Scheme 1 Direct arylation polycondensation of N-benzoyl dithieno[3,2-b:2′,3′-d]pyrrole, 3-hexylthiophene and tris(4-bromophenyl)amine monomers, resulting in PBDP3HTTPA as determined by GPC relative to polystyrene standards of PBDP3HTTPA and PBDP3HTTPB were 18,000 and 16,700 g mol −1 , with polydispersities of 2.1 and 2.3, respectively (Fig. 2, Table 1). These hyperbranched conjugated polymers were soluble well in common organic solvents such as chloroform, THF, toluene, DMAc and insoluble in methanol, diethyl ether and n-heptane.

Polymer structure
The polymer structures were characterized by transmission FT-IR and 1 H NMR spectroscopies. The FT-IR spectra of PBDP3HTTPA and PBDP3HTTPB displayed several bands between 2850 and 3060 cm −1 asigned to CH stretching modes of n-hexyl groups and ring C-H stretching vibrations. The bands at 1585 and 1492 cm −1 are ascribed to the aromatic C=C stretching and aromatic C-H deformation vibrations, respectively, while the bands at 1323 and 1274 cm −1 are assigned to the C-N stretching of triphenylamine units. The appearance of a strong absorption band at 1700 cm −1 indicates the existence of C=O group of the N-benzoyl dithieno[3,2b:2′,3′-d]pyrrole moiety in the polymer structure. The bands at 696 and 628 cm −1 are ascribed to the thiophene C-S-C bending and S-C stretching vibrations, respectively.
In the 1 H NMR spectrum of hyperbranched conjugated polymer PBDP3HTTPA (Fig. 3a) pyrrole repeating units. These results indicate that direct arylation coupling polymerization successfully took place to form the expected polymers. Additionally, we noted clearly the disappearance of the signal at 7.35 ppm in the spectrum of PBDP3HTTPA, which was originally aromatic protons closest to bromide in tris(4-bromophenyl) amine (compound 1). Similarly, the signal at 7.51 ppm disappears in the spectrum of PBDP3HTTPB, which was originally aromatic protons closest to bromide in 1,3,5-tris(4-bromophenyl)benzene (compound 2). These suggest that all three bromide groups of compound 1 and 2 were consumed, suggesting the formation of hyperbranched structures.
To reach more insights into the polymer structures, the unit ratio of 3-hexylthiophene (3HT) versus N-benzoyl dithieno[3,2-b:2′,3′-d]pyrrole (BD) was calculated based on the integration values of the thiophene-CH 2 proton signal at 2.6 ppm (peak f, Fig. 2a) and the benzoyl ortho Fig. 3a). Taking into account that the molar ratio between the total number of 3HT and BD units versus the number of TPA units is 1.5, a compositional molar ratio (r) between BD, 3HT and TPA units of 1:1.18:1.45 was determined. In the case of PBDP3HTTPB, r was calculated based on the integration ratio between the thiophene-CH 2 proton signal at 2.6 ppm (peak f, Fig. 3b) and the overlapping shift range of aromatic proton signals around 7.75 ppm of BD (peak q corresponding to 1 proton, Fig. 2b) and triphenylbenzene (peak l, m, n corresponding to 3 protons, Fig. 3b) moieties, taking into acount the molar ratio between the total number of 3HT and BD units versus the number of TPB units being 1.5. PBDP3HTTPB had a compositional molar ratio (r)  Table 1. However, we could not determine the degree of branching by the use of 1 H NMR integration, since the chemical shifts of branching, terminal, and linear units could not be differentiated.
In addition to the NMR results, which indirectly confirm the formation of hyperbranched structures, controlled experiments were also performed. Accordingly, one reactive site of the monomer 3-hexylthiophene (monomer 4) was blocked with a carbaldehyde (-CHO) group to give in 3-hexylthiophene-2-carbaldehyde. Direct arylation reaction between 3-hexylthiophene-2-carbaldehyde and tris(4-bromophenyl)amine (compound 1) was then conducted. Attributed to the non-participation of the carbaldehyde group in the direct arylation reaction, no hyperbranched structure was obtained, as indicated by the low molecular weight (below 1000 g mol −1 ) of the product determined by GPC and mass spectroscopic analysis. The 1 H and 13 C NMR results also indicated a corresponding star-structure formed from 3-hexylthiophene-2-carbaldehyde and tris(4-bromophenyl)amine. These results suggest that a hyperbranched structure could only be generated with the participation of both reactive sites of the monomer.
It should also be noted that in other controlled experiments, the direct arylation reaction between tris(4-bromophenyl)amine and N-benzoyl dithieno[3,2b:2′,3′-d]pyrrole provided a polymer product with a poor solubility, suggesting that a hyperbranched structure was formed. On the other hand, the direct arylation reaction between tris(4-bromophenyl)amine and 3-hexylthiophene resulted in a polymer product with M n of around 15,000 g mol −1 and Đ of 2.1.

Thermal properties
The thermal properties of hyperbranched PBDP3H-TTPA and PBDP3HTTPB were investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). TGA under nitrogen flow was used to evaluate the thermal stability of the purified hyperbranched conjugated polymers in the range from room temperature to 800 °C. PBDP3HTTPA exhibited good thermal stability with decomposition temperature (5% weight loss) around 250 °C (see Fig. 4). The TGA diagram of PBDP3HTTPB showed a mass loss of 5 wt% at 300 °C as the threshold for thermal decomposition, and a loss of about 70 wt% at 500 °C.   The second heating run DSC diagram in the range from 0 to 250 °C of the conjugated hyperbranched polymers are shown in Fig. 5. No transition in this temperature range was detected for PBDP3HTTPB, while a relatively broad endotherm ascribed to a melting peak at 110 °C was observed for PBDP3HTTPA. It is well-known that linear poly(3-hexylthiophene) and poly(dithieno[3,2b:2′,3′-d]pyrrole) chains are generally stiff with very strong intermolecular π-π stacking interactions, resulting in high melting temperatures normally above 200 °C [40][41][42]. Thus, the branching structure hindered the stacking of polymer chains, leading to crystalline domains with less ordered packing and so a low melting temperature range in comparison with the linear analogous polymers. On the other hand, because of the planar structure of TPB units, PBDP3HTTPB exhibits a higher oder of chain stacking than PBDP3HTTPA with propeller-like TPA moieties.

Solid structure
The photophysical properties of conjugated polymers are affected by their nanostructures of solid state films. Therefore, the molecular ordering of the PBDP3HTTPA and PBDP3HTTPB hyperbranched polymers in the solid state was investigated by powder X-ray diffraction (XRD) measurements (Fig. 6). The XRD patterns of PBDP3H-TTPA and PBDP3HTTPB exhibit two distinctive diffractions at 2θ = 5.5° and 27.0°, corresponding respectively to an interchain d-spacing of 16.1 Å between neighboring polymer chains separated by n-hexyl side chains [40,43] and a π-π stacking distance of 3.3 Å. This π-π stacking distance is slightly smaller than that observed for classical poly(3-hexylthiophene) [44,45] and is close to that observed for dithieno[3,2-b:2′,3′-d]pyrrole-based oligomers and polymers [42,46]. Because of the difference in the planar geometry of TPB and TPA units, PBDP3HTTPB exhibits a slightly higher ordered packing, indicated by the somewhat higher intensities of diffraction peaks. In addition, the XRD pattern of PBDP3HTTPA shows a broad amorphous halo centered ca. 21° associated with scattering from a disordered packing of n-hexyl side chains [47,48] whereas this amorphous diffraction is less prominent for PBDP3HTTPB. Figure 7a, b present the UV-vis spectra of PBDP3H-TTPA and PBDP3HTTPB, respectively, measured in In the case of PBDP3HTTPB, an absorption maximum at around 410-420 nm was observed in CHCl 3 , THF as well as toluene. However, an addition absorption peak at 750 nm was found for PBDP3HTTPB in THF and toluene, indicating the co-existence of a small fraction of polymer molecules in a more aggregated form. In solid film, besides an absorption maximum at 410 nm, PBDP3HTTPB exhibited an absorption peak at 700 nm, broadening to 850 nm. This reveals that PBDP3HTTPB has a high aggregation degree than PBDP3HTTPA in the solid state, which is in agreement with the DSC and XRD results.

Optical properties
The photoluminescent (PL) spectra of the hyperbranched conjugated polymers in solutions excited at the absorption maxima are shown in Fig. 8a, b. In CHCl 3 , PBD3HTTPA displayed an emission peak at 545 nm upon excitation at 310 nm, whereas in toluene solution, PBD3HTTPA exhibited triplet peaks at 460, 500 and 560 nm upon excitation at 360 nm. In THF solution, PBD3HTTPA exhibited double peaks at 450 and 500 nm upon excitation at 400 nm. In the case of PBDP3HTTPB upon excitation at 410 nm, the polymer showed peaks at 380 nm and 475 nm in CHCl 3 , whereas it displayed triplet peaks at 380, 475 and 520 nm in both THF and toluene.
The fluorescence quantum yields (φ F ) of the polymers in dilute CHCl 3 were measured in comparison to 9,10-diphenylanthracence as a standard (in cyclohexane  φ F = 0.9), and the results are summarized in Table 2. The quantum yields increased from 0.57 for PBDP3HTTPA to 0.62 for PBDP3HTTPB. It is likely that the stronger π-π stacking effect in the structure of PBDP3HTTPB led to a higher quantum yield as a result of a lower nonradiative decay rate [49,50].

Solvent-induced aggregation
The aggregation state induced by intermolecular interactions based on π-stacking affects strongly the optical properties of conjugated polymers [51]. In solution, the H-aggregates (with parallel aligned transition dipoles) and J-aggregates (with head-to-tail aligned transition dipoles) show distinct changes in the absorption band, i.e. bathochromic (red) shifts or hypsochromic (blue) shifts, respectively, compared to the monomeric species [52,53]. Molecular aggregation can possibly be induced by addition of a non-solvent to a polymer solution. Figure 9 shows the absorption spectra of the PBDP3HTTPA and PBDP3HTTPB hyperbranched polymers, measured in CHCl 3 /methanol mixtures. The π-π* absorption band of PBDP3HTTPA was located at 310 nm in pure CHCl 3 , indicating the coil conformation of polymer chains. The addition of methanol from 10 to 90% to polymer solution induced slight bathochromic shifts, indicating conformational changes toward the formation of aggregates. A similar effect was observed for PBDP3HTTPB when adding small amounts of methanol from 10 to 40%. Moreover, at higher methanol contents, strong red shifts were observed, indicating significant chain aggregation. Correspondingly, the absorption maximum of P3HTBTTPA shifted to 500, 520 and 550 nm at methanol contents of 60, 80 and 90%, respectively. These results also confirm that PBDP3HTTPB exhibits a higher tendency to form aggregate than PBDP3HTTPA.

Conclusions
We have demonstrated the successful synthesis of novel hyperbranched conjugated polymers containing N-benzoyl dithieno[3,2-b:2′,3′-d]pyrrole and 3-hexylthiophene monomer units, linked with the triphenylamine or triphenylbenzene moiety (PBDP3HTTPA and PBDP3HTTPB, respectively), via direct arylation polycondensation in 80-90% yields. The molecular weights of the obtained polymers were 18,000 g mol −1 for PBDP3HTTPA and 16,700 g mol −1 for PBDP3HTTPB. Both polymers exhibited high structural order in thin films, which can be promising for organic solar cell applications. The UV-vis absorption of PBDP3HTTPB containing triphenylbenzene as branching unit was red-shifted as compared with PBD-P3HTTPA, as a result of a higher chain packing degree. Generally, the results proved that the optical properties of these hyperbranched conjugated polymers could be controlled via alteration of the branching unit, which is useful for potential application as optoelectronic materials.
Characterization 1 H NMR spectra were recorded in deuterated chloroform (CDCl 3 ) with TMS as an internal reference, on a Bruker Avance 300 MHz. Fourier transform infrared (FT-IR) spectra, collected as the average of 64 scans with a resolution of 4 cm −1 , were recorded from KBr disk on the FT-IR Bruker Tensor 27. Size exclusion chromatography (SEC) measurements were performed on a Polymer PL-GPC 50 gel permeation chromatograph system equipped with an RI detector, with tetrahydrofuran as the eluent at a flow rate of 1.0 mL min −1 . Molecular weight and molecular weight distribution were calculated with reference to polystyrene standards. UV-vis absorption spectra of polymers in solution and polymer thin films were recorded on a Shimadzu UV-2450 spectrometer over a wavelength range of 300-700 nm. Fluorescence spectra were measured on a HORIBA IHR 325 spectrometer. Differential scanning calorimetry (DSC) measurements were carried out with a DSC 204 F1-NETZSCH instruments under nitrogen flow (heating rate 10 °C min −1 ). Thermogravimetric analysis (TGA) measurements were performed under nitrogen flow using a STA 409 PC Instruments with a heating rate of 10 °C min −1 from ambient temperature to 800 °C. Wide-angle powder X-ray diffraction (XRD) patterns were recorded at room temperature on a Bruker AXS D8 Advance diffractometer using Cu-Kα radiation (k = 0.15406 nm), at a scanning rate of 0.05 degrees per second. The data were analyzed using DIFRAC plus Evaluation Package (EVA) software. The d-spacing was calculated from peak positions using Cu-Kα radiation and Bragg's law.

Synthesis of tris(4-bromophenyl)amine (1)
N-bromosuccinimide (2.17 g, 12.2 mmol) and triphenylamine (1 g, 4.08 mmol) were added to anhydrous THF (10 mL) at 0 °C under nitrogen. The mixture was stirred at 50 °C for 1.5 h. After completion of the reaction, 10 mL of distilled water was added to the reaction mixture, which was extracted with dichloromethane.  3 were added to the monomer solution. The vial was sealed with a rubber cap and then removed from the glove box. The vial was heated in a 100 °C oil bath for 24 h. After being cooled to room temperature, the reaction mixture was diluted with 30 mL of chloroform. The obtained organic layer was passed through Celite to remove the Pd catalyst and the insoluble polymer fraction, subsequently washed with 10% solution of Na 2 S 2 O 3 and distilled water, dried over Na 2 CO 3 , concentrated and finally poured into a large amount of cold n-heptane to precipitate the polymer. The resulting polymer was isolated by filtration, washed with acetone to remove oligomers, and finally dried under reduced pressure at 50 3 were added to the monomer solution. The vial was sealed with a rubber cap and then removed from the glove box. The vial was heated in a 100 °C oil bath for 24 h. After being cooled to room temperature, the reaction mixture was diluted with 30 mL of chloroform. The obtained organic layer was passed through Celite to remove the Pd catalyst and the insoluble polymer fraction, subsequently washed with 10% solution of Na 2 S 2 O 3 and distilled water, dried over Na 2 CO 3 , concentrated and finally poured into a large amount of cold n-heptane to precipitate the polymer. The resulting polymer was isolated by filtration, washed with acetone to remove oligomers, and finally dried under