star: synthesis, structure and anion binding

J Polym Res (2017) 24:180 https://doi.org/10.1007/s10965-017-1346-9

ORIGINAL PAPER

Thiacalix[3]Triazine-centered regioregular poly(3-hexylthiophene) star: synthesis, structure and anion binding Thu Anh Nguyen 1,2 & Anh Tuan Luu 1 & Tam Huu Nguyen 1 & Nhut Minh Le 1 & Hoan Minh Tran 1 & Le-Thu T. Nguyen 1 & Jun Young Lee 2 & Ha Tran Nguyen 1,3

Received: 7 July 2017 / Accepted: 28 September 2017 # Springer Science+Business Media B.V. 2017

Abstract The first synthesis of a well-defined star polymer consisting of three regioregular poly(3-hexylthiophene) (rrP3HT) arms emanating from an electron acceptor thiacalix[3]wtriazine core via direct arylation polymerization is described. Its structural characteristics as well as optical and thermal properties were evaluated using Fourier transform infrared spectroscopy (FTIR), 1H and 13C nuclear magnetic resonance (NMR), X-ray diffraction (XRD), gel permeation chromatography (GPC), differential scanning calorimetry (DSC) and atomic force microscopy (AFM) methods. The UV-vis and photoluminescence spectroscopy results evidenced the impact of the electron-withdrawing feature of the thiacalix[3]triazine core and the star structure on the optical behavior of the star-shaped material, as compared to the rr-P3HT linear analogue. The anionbinding property of the star-shaped P3HT resulting in fluorescence quenching was also studied.

Keywords Poly(3-hexylthiophene) . Star-shaped conjugated polymer . Thiacalix[3]triazine . Direct arylation

* Ha Tran Nguyen [email protected] 1

Faculty of Materials Technology, Ho Chi Minh City University of Technology, Vietnam National University, 268 Ly Thuong Kiet, District 10, Ho Chi Minh City, Vietnam

2

Department of Chemical Engineering, Sungkyunkwan University, Suwon 16419, Republic of Korea

3

Materials Technology Key Laboratory (Mtlab), Ho Chi Minh City University of Technology, Vietnam National University, Ho Chi Minh City, 268 Ly Thuong Kiet, District 10, Ho Chi Minh City, Vietnam

Introduction Organic semiconductors have recently received consideration due to their advantages of low cost, light weight, processability and high mechanical flexibility. In particular, star-shaped conjugated molecules is a subject of high current interest as active materials for organic electronic devices such as organic field-effect transistors (OFETs), polymeric light emitting diodes (PLEDs), electrochromic displays or organic solar cells (OSCs) [1–7]. Star-shaped conjugated polymers are a class of nonlinear polymers and they exhibited unique properties such as suppressed fluorescence quenching in the solid state and improved light-harvesting [8–14]. Generally, star-shaped conjugated polymers comprise several linear polymers as arms joined together through a central structure as a core. Depending on the bonds between the arms and the core, they can create one of several different shapes. If the arms are rigidrod, a flat inflexible core normally provides an overall twodimensional geometry, whereas a non-planar center results in a three-dimensional architecture. There are two strategies for preparation of star-shaped conjugated polymers, including: the arm-first method and the core-first method. In the armfirst method, linear arm polymers are synthesized first and subsequently end-group functionalized in order to be attached to a reactive core. In contrast, the core-first method is used to prepare a reactive core that can initiate the polymerization of monomers to form arm chains [15–20]. Among various conjugated polymers, regioregular poly(3hexylthiophene) (rr-P3HT) has been widely studied because of the excellent performance in terms of solubility, chemical stability, charge mobility, and commercial availability [21]. Examples of the synthesis of star P3HTs by different synthetic pathways have been reported by several groups. Kim et al. reported the synthesis of multiarmed star P3HTs with microgel core via the crosslinking reaction of a rr-P3HT

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macroinitiator with a divinyl compound [22]. However, the arm numbers of these star P3HTs are not well-defined. Senkovskyy et al. [23] have reported the core-first method based on the externally initiated Kumada catalyst-transfer polycondensation (KCTP) to synthesize a six-arm star P3HT having a hexaphenylbenzene core with a dispersity of 1.98. Also using the core-fisrt KCTP method, Luscombe and coworker succeeded [24] in synthesizing a well-defined threearm star P3HT with a low dispersity (Đ = 1.15) using a trifunctional Ni complex-based core-initiator. On the other hand, star polymers containing copolymers of P3HT as arm segments have also been reported [25, 26]. Despite significant efforts being made to enable the well-controlled synthesis of star-shaped P3HTs via the core-first approach, the finding of a suitable core-initiator and a preparation process for the Nicomplex-based core-initiator is challenging. Moreover, the preparation of star-shaped P3HTs with electron acceptor cores remains limited by undeveloped synthetic method. Heteracalixarenes have gained considerable attention in supramolecular chemistry in recent years owing to their selfassembling ability [27, 28]. In particular, thiacalix[3]triazine is a subclass within the calixarene family, which has been proven to be suitable as macrocyclic scaffolds depending on anion binding moieties [29–31]. Thiacalix[3]triazine is constructed from 1,3,5-triazines, enforced as electron-deficient host for halide ion binding through anion-π interactions [32–34]. Thiacalix[3]triazine can be prepared by condensation of a Scheme 1 Synthesis of the rrP3HT linear precursor and starshaped s-P3HT-T3A

dichloro-1,3,5-triazine with sulfide ion. The synthesis of thiacalix[3]triazines with peripheral phenol or tert-butyl substituents from the reaction of corresponding 2,4-dichloro1,3,5-triazine with NaSH or alternatively Na2S has been reported [35, 36]. Thiacalix[3]triazine has been shown to interact with non-protic and less-acidic protic anions via the anion association mechanism, and with more-acidic protic anions following the protonation mechanism [35]. In this contribution, we synthesized via the arm-first method a novel star-shaped P3HT comprising electron acceptorthiacalix[3]triazine (T3A) core and three branched motifs of P3HT. The star architecture was obtained via direct-arylation coupling reactions between a T3A derivative and α-bromopoly(3-hexylthiophene) (Scheme 1). The synthesis and preliminary results on the characterization of the optical and thermal properties of the star polymer are presented, together with a comparison with those of the corresponding rr-P3HT linear precursor.

Experiment Materials 3-Hexylthiophene (>98%) was purchased from TCI (Tokyo, Japan). N-bromosuccinimide was purchased from Acros Organics. Palladium(II) acetate (Pd(OAc) 2 , ≥99.9%),

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tricyclohexylphosphine tetrafluoroborate (Pcy3.HBF4, 97%), pivalic acid (PivOH, 99%), 4′-bromoacetophenone (98%), iodobenzene I,I-diacetate (98%), iodine (≥99.8%), [1,3bis(diphenylphosphino)propane]dichloronickel(II) (Ni(dppp)Cl2), tetra-n-ethylammonium hydrogen carbonate (≥95%) and potassium disulfate (K2S2O7, 99%) were purchased from Sigma-Aldrich (St. Louis, MO, USA) and used as received. Cyanuric chloride (99.8%), phenol (99.8%), NaSH (99%), potassium acetate (KOAc), sodium carbonate (99%) and magnesium sulfate (98%) were purchased from Acros (Bridgewater, NJ, USA) and used as received. Chloroform (CHCl3) (99.5%), toluene (99.5%) and tetrahydrofuran (THF) (99%) were purchased from Fisher/Acros (Bridgewater, NJ, USA) and dried using molecular sieves under N 2. Dichloromethane (CH 2Cl2) (99.8%), n-heptane (99%), methanol (99.8%), ethyl acetate (99%) and diethyl ether (99%) were purchased from Fisher/Acros (Bridgewater, NJ, USA) and used as received. Measurements 1

H NMR and 13C NMR spectra were recorded in deuterated chloroform (CDCl3) with tetramethylsilane as an internal reference, on a Bruker Avance 500 MHz. Fourier transform infrared (FTIR) spectra, collected as the average of 64 scans with a resolution of 4 cm−1, were recorded from KBr disks on the FTIR Bruker Tensor 27. SEC measurements were performed on a Polymer PL-GPC 50 gel permeation chromatography (GPC) system equipped with an RI detector, with THF as the eluent at a flow rate of 1.0 mL min−1. Molecular weights and molecular weight distributions were calculated with reference to polystyrene standards. UV–visible absorption spectra of polymers in solution and polymer thin films were recorded on a Shimadzu UV-2450 spectrometer over the wavelength range 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 instrument under nitrogen flow (heating rate 10 °C min−1). Wide-angle powder X-ray diffraction (XRD) patterns were recorded at room temperature on a Bruker AXS D8 Avance diffractometer using Cu Kα radiation (K = 0.15406 nm), at a scanning rate of 0.05o s−1. The data were analyzed using DIFRAC plus Evaluation Package (EVA) software. The d-spacings were calculated from peak positions using Cu Kα radiation and Bragg’s law. Atomic force microscopy (AFM) images were obtained using a Bruker Dimension 3100 atomic force microscope. Synthesis of 2-bromo-3-hexylthiophene (1) To a solution of 3-hexylthiophene (5 g, 29.7 mmol) in anhydrous THF (50 mL) in a 200 mL flask, a solution of Nbromosuccinimide (5.29 g, 29.7 mmol) was added slowly at

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0 °C under nitrogen. The mixture was stirred at 0 ∘C for 1 h. After that, 50 mL of distilled water was added to the reaction mixture, and the mixture was extracted with diethyl ether. The organic layer was washed with a solution of Na2S2O3 (10%), and then the mixture was washed with a solution of KOH (10%) and dried over anhydrous MgSO4. The mixture was distilled to give a colorless oil (6.7 g, 92% yield). 1 H NMR (300 MHz, CDCl3), δ (ppm): 7.19 (d, 1H), 6.82 (d, 1H), 2.59 (t, 2H), 1.59 (quint, 2H), 1.33 (m, 6H), 0.91 (t, 3H). 13C NMR (75.5 MHz, CDCl3), δ (ppm): 141.0, 128.2, 125.1, 108.8, 31.6, 29.7, 29.4, 28.0, 22.6, 14.1. Synthesis of 2-bromo-3-hexyl-5-iodothiophene (2) Iodine (1.42 g, 11.18 mmol) and iodobenzene I,I-diacetate (1.965 g, 6.1 mmol) were added to a solution of 2-bromo-3hexylthiophene (1) (2.5 g, 11.1 mmol) in dichloromethane (25 mL) at 0 °C. The mixture was stirred at room temperature for 4 h. Then, aqueous Na2S2O3 (10%) was added, and the mixture was extracted with diethyl ether and dried over anhydrous MgSO4. The solvent was evaporated to obtain a crude product. The residue was purified by silica column chromatography (eluent n-heptane) to give pure 2-bromo-3 hexyl-5iodothiophene (2) as a pale yellow oil (3 g, 86% yield). 1 H NMR (300 MHz, CDCl3), δ (ppm): 6.97 (s, 1H), 2.52 (t, 2H), 1.56 (quint, 2H), 1.32 (m, 6H), 0.89 (t, 3H). 13C NMR (75.5 MHz, CDCl3), δ (ppm): 144.3, 137.0, 111.7, 71.0, 31.5, 29.6, 29.2, 28.8, 22.5, 14.1. Synthesis of regioregular head-to-tail poly(3-hexylthiophene) (rr-P3HT) with H/Br end-groups (3) A dry 500 mL three-neck flask was flushed with nitrogen and charged with 2-bromo-3-hexyl-5-iodothiophene (2) (15 g, 40 mmol). After three azeotropic distillations by toluene, anhydrous THF (220 mL) was added via a syringe, and the mixture was stirred at 0 °C for 1 h. i-PrMgCl (2 mol L−1 solution in THF, 19.14 mL, 38.28 mmol) was added via a syringe and the mixture was continuously stirred at 0 °C for 1 h. The reaction mixture was kept at 0 °C. The mixture was transferred to a flask containing a suspension of Ni(dppp)Cl2 (760 mg, 1.4 mmol) in THF (25 mL). The polymerization was carried out for 24 h at 0 °C, followed by addition of a 5 mol L−1 HCl solution. After termination, the reaction was stirred for 15 min and extracted with CHCl3. The polymer was precipitated in cold methanol and washed several times with nhexane. The polymer was characterized by 1H NMR and GPC. The yield was 70%. FTIR (cm−1): 721, 819, 1376, 1454, 1510, 2853, 2922, 2953. 1H NMR (300 MHz, CDCl3), δ (ppm): 6.96 (s, 1H), 2.90 (t, 2H), 1.79 (sex, 2H), 1.52 (q, 6H), 0.94 (t, 3H). GPC:

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Mn = 4500 gmol−1. Ð = Mw/Mn = 1.18. m/z: 1409, 1574, 1740, 1906, 2072, 2238, 2404, 2570, 2736, 2902. Synthesis of 2,4-dicloro-6-phenoxy-1,3,5-triazine (5) Cyanuric chloride (7) (1.840 g, 10 mmol) was dissolved in acetone (100 mL) and cooled to 0 °C. In a separate flask, phenol (0.94 g, 10 mmol) was reacted with NaOH (0.400 g, 10 mmol) in water (100 mL) to form a clear aqueous solution. Then, the aqueous solution was added dropwise to the cyanuric chloride solution. After stirring at 0 °C for 8 h, the mixture was poured into water (100 mL) to form a white precipitate. The white precipitate was filtered and washed with water and ethanol. The product was purified by recrystallization with n-hexane to give a white solid. Yield: 80%. 1 H NMR (300 MHz, CDCl3) δ (ppm): 7.43–7.36 (m, 4H), 7.28 (dd, J = 7.8, 1.4 Hz, 2H), 7.17–7.11 (m, 4H). Analysis calculated for C9H5Cl2N3O: C, 44.66; H, 2.08; Cl, 29.29; N, 17.36; O, 6.61. Found: C, 44.06; H, 2.18; Cl, 29.21; N, 17.35; O, 6.69.

H NMR (300 MHz, acetone-d6) δ (ppm): 7.46–7.35 (m, 2H), 7.34–7.23 (d, 1H), 7.22–7.12 (m, 2H). 13C NMR (75 MHz, acetone-d6) δ (ppm): 181, 171, 152, 130, 127, 122. Analysis calculated for C9H5Cl2N3O: C, 50.77; H, 2.37; Cl, 8.33; N, 19.73; O, 7.51; S, 11.29. Found: C, 49.86; H, 2.48; Cl, 8.21; N, 19.35; O, 7.59, S, 12.51.2,4-dichloro-6-phenoxy-1,3,5-triazine (5) (2 g, 8.26 mmol) was dissolved in dry THF and the solution was purged with nitrogen for 10 min. NaSH (0.86 g, 15.30 mmol) was added to the solution and the reaction was carried out at 60 °C for 72 h. After completion of the reaction, the solution was dissolved in a mixture of dichloromethane and distilled water. The organic fraction was then washed with water, dried with K2CO3, filtered and solvent evaporated to dryness. The crude product was purified over a silica column with n-heptane/ethyl acetate (v/v: 3/1) as eluent to obtain a light yellow powder as the pure product. Yield: 18%. 1 H NMR (300 MHz, acetone-d6) δ (ppm): 7.46–7.35 (m, 2H), 7.34–7.23 (d, 1H), 7.22–7.12 (m, 2H). 13C NMR (75 MHz, acetone-d6) δ (ppm): 181, 171, 152, 130, 127, 122. Analysis calculated for C9H5Cl2N3O: C, 50.77; H, 2.37; Cl, 8.33; N, 19.73; O, 7.51; S, 11.29. Found: C, 49.86; H, 2.48; Cl, 8.21; N, 19.35; O, 7.59, S, 12.51. 1

Synthesis of 4,6,10,12,16,18,19,20,21-nonaaza-5,11,17triphenoxy-2,8,14-trithiacalix [3]arene (6) 2,4-dichloro-6-phenoxy-1,3,5-triazine (5) (2 g, 8.26 mmol) was dissolved in dry THF and the solution was purged with nitrogen for 10 min. NaSH (0.86 g, 15.30 mmol) was added to the solution and the reaction was carried out at 60 °C for 72 h. After completion of the reaction, the solution was dissolved in a mixture of dichloromethane and distilled water. The organic fraction was then washed with water, dried with K2CO3, filtered and solvent evaporated to dryness. The crude product was purif ied over a silica column with n-heptane/ethyl acetate (v/v: 3/1) as eluent to obtain a light yellow powder as the pure product. Yield: 18%.

Synthesis of star-shaped poly(3-hexylthiophene) -4,6,10,12,16,18,19,20,21-Nonaaza-5,11,17-triphenoxy2,8,14-trithiacalix[3]arene (s-P3HT-T3A) (7) 100 mg (0.022 mmol) of P3HT (3) was dissolved in 6 mL of DMAc. Then 4,6,10,12,16,18,19,20,21-nonaaza-5,11,17triphenoxy-2,8,14-trithiacalix [3]arene (6) (4.5 mg, 7.4 × 10−3 mmol) in DMAc (4 mL) was dropped slowly at 100 °C for 2 h. To the solution, 7.6 mg (0.055 mmol) of K2CO3 was added. Then, Pd(OAc)2 (0.25 mg, 1.1 × 10−3 mmol), PCy3.HBF4 (0.81 mg, 2.2 × 10−3 mmol) and PivOH (0.033 mmol, 3.37 mg) were added to the solution.

Fig. 1 1H NMR spectrum of 4,6,10,12,16,18,19,20,21-nonaaza-5,11,17-triphenoxy-2,8,14-trithiacalix [3]arene (6, Scheme 1)

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The mixture was bubbled with N2 for 30 min, followed by freeze-pump-thaw cycling. The reaction was carried out at 100 °C for 24 h. After completion of the reaction, the mixture was extracted with CHCl3. The organic layer obtained was passed through Celite to remove the Pd catalyst and any trace of insoluble polymer fraction, and subsequently precipitated in cold methanol to obtain the polymer as green solid. The product was finally dried under reduced pressure at 50 °C for 24 h. A yield of 90% was obtained. FTIR (cm −1): 721, 819, 1376, 1454, 1510, 2853, 2922, 2953. 1H NMR (300 MHz, CDCl3), δ (ppm): 7.61 (s, 2H), 7.49 (s, 2H), 6.96 (s, 1H), 2.90 (t, 2H), 1.79 (sex, 2H), 1.52 (q, 6H), 0.94 (t, 3H). 13C NMR (75.5 MHz, CDCl3), δ (ppm): 143.5, 141.0, 135.5, 129.5, 127.0, 126.0, 119.0, 32.0, 30.5, 29.0, 22.5, 14.0. GPC: Mn = 7800 g mol−1. Ð = Mw /Mn = 1.45.

Fig. 2 1H NMR (a) and 13C NMR (b) spectra of s-P3HT-T3A

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Results and discussion Synthesis and characterization The synthetic route for the rr-P3HT linear precursor (structure 3) and the star-shaped P3HT (s-P3HTs-T3A) containing 4,6,10,12,16,18,19,20,21-nonaaza-5,11,17-triphenoxy2,8,14-trithiacalix [3]arene (T3A) as the core (structure 8) is shown in Scheme 1. First, the rr-P3HT was synthesized via a controlled ‘quasi-living’ Grignard metathesis (GRIM) polymerization of 2-bromo-5-iodo-3-hexyl thiophene monomers in the presence of Ni(dppp)Cl2 to form α-bromo-poly(3hexylthiophene) (Br-P3HT-H), according to a previously reported procedure [37]. The obtained P3HT has a number average molecular weight (M nexp ) of 4500 g mol −1 as

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Fig. 3 GPC traces of s-P3HT-T3A and the rr-P3HT linear precursor

determined by GPC, which is in good correlation with the theoretical value (Mnth = 4700 g mol−1). The control over the GRIM polymerization was further confirmed by a symmetrical and narrow molecular weight distribution characterized by a low dispersity index (Ð =1.18). A high regioregularity content of 99% was determined by 1H NMR, while the presence of the expected end-groups (H/Br) was fully evidenced by MALDI-TOF analysis. 4,6,10,12,16,18,19,20,21-nonaaza-5,11,17-triphenoxy2,8,14-trithiacalix [3]arene was synthesized from cyanuric chloride with a yield of about 20%. As shown in Fig.1, the 1 H NMR spectrum of the synthesized compound reveals characteristic peaks of the corresponding structure. Then, s-P3HT-T3A (7) was synthesized with a yield of 93% via direct arylation coupling reaction between rr-P3HT (3) and T3A (6). To obtain a high reaction conversion, we established a rr-P3HT to T3A molar ratio of 1 to 0.3. The formation of s-P3HT-T3A was controlled by slow addition of a diluted solution of T3A at 100 °C in the presence of Pd(OAC)2 and PCy3.HBF4 as catalyst system in anhydrous toluene. The star-shaped structure of s-P3HT-T3A was characterized via 1H NMR and 13C NMR spectra. 1H and 13C NMR peak assignment and integration were obvious. As shown in Fig. 2a, all the characteristic peaks of P3HT are clearly observed. In the aromatic region, besides the signals of internal thienyl rings (6.96 ppm), the signals of the T3A core were found at 7.61 and 7.49 ppm. From a comparison of the 1H NMR spectra of s-P3HT-T3A and T3A (6), the peak at Table 1 Macromolecular characterization of rr-P3Ht and sP3HT-T3A with a comparison of the shrinking factor g’ as a function of number of arms

7.46 ppm (Fig. 1, peak c) related to the protons of benzene ring was shifted to 7.61 ppm for s-P3HT-T3A (Fig. 2a, peak 9). This was confirmed by the disappearance of the signal corresponding to the ortho-benzene protons (Fig. 1, peak a) at 7.28 ppm in the 1H NMR spectrum of s-P3HT-T3A. It should be note that, the crude product may contain the unreacted linear P3HT chains and two-arm species, besides the three-arm star-shaped polymer. However, two-arm P3HT chains of high molecular weight (about ~10,000 gmol-1) tend to aggregate and precipitate as an insoluble form (below 10 wt% of the crude product), which was removed via filtration through Celite and re-dissolution and filtration processes. The elimination of the two-arm chains and the unreacted T3A (6) was evidenced by the complete absence of the signal at 7.28 ppm (Fig. 1, peak a) in the spectrum of s-P3HT-TPA. Taking into account the known Mn of 4500 g mol−1 of P3HT, an estimation of the integration ratio between the repeating units of P3HT (peak 1 at 0.94 ppm or peak 7 at 6.96 ppm, Fig. 2a) and the T3A protons (peak 9 at 7.61 ppm, Fig. 2a) resulted in a P3HT chain-to-core molar ratio of 1.09. This suggests that the product contained about 9% of linear P3HT chain contaminant. In addition, all 13C NMR characteristic signals of P3HT and T3A core, indicated by the peaks at 181, 171, 152, 130, 127 and 122 ppm (Fig. 2b), confirm the successful coupling reaction. The number-average molecular weight (Mn) of s-P3HTT3A was 7800 g mol−1 and the dispersity index (Ð) was 1.45 as determined by GPC relative to polystyrene standards. A shift of the GPC signal of the product toward a higher molecular weight value than that of the rr-P3HT precursor, as shown in Fig. 3, suggests successful direct arylation coupling reaction providing the star-shaped structure. Moreover, s-P3HT-T3A was well soluble in common organic solvents such as chloroform, THF, toluene, dichloromethane and was insoluble in methanol and n-heptane. For insight into the structures of the polymers, their intrinsic viscosities [η] were collected form the SEC data as shown in Table 1. The [η] values of s-P3HT-T3A was lower than that of the linear P3HT, suggesting the existence of a branching architecture. The shrinking factor for the intrinsic viscosity of branched polymers g’ can be denoted by: g’ ¼ ½ηBr =½ηLin

ð1Þ

Here, we denote the intrinsic viscosities of branched and linear polymers by [η]Br and [η]Lin. The equation has been extended

Type of polymers

Mna (gmol−1)

Đ = Mn/Mw

[η] (dg L−1)

g’

No. of arms

Linear P3HT s-P3HT-T3A

4500 7800

1.18 1.45

0.127 0.089

1 0.70

1 4

a

Mn was determined by SEC

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Fig. 4 UV − visible spectra of sP3HT-T3A in different solvents and in solid film. The spectrum of rr-P3HT film is shown for comparison

for coiled polymers in a theta solvent by Roovers [38]. Consistently, CHCl3 is the theta solvent for rigid polymers such as P3HT. i gθ η ðempiricalÞ≈½ð3 f –2Þ=f 2 o:58 ð2Þ

Using Eq. (3), the f value was estimated to be 4.0 for s-P3HTT3A, which clearly differs from that of the rr-P3HT linear precursor. Although some deviation is expected for rigid polymers such as P3HT, these results suggest that the star-shaped structure was obtained.

Douglas et al. [39] have developed an empirical relationship between g’ and f for coiled polymers in good solvents, where f is the number of arms as exhibited in Eq. (3)

Optical property of s-P3HT-T3A

g* h ðempiricalÞ≈gθ h ½1−0:267–0:015ð f –1Þ=ð1−0:276Þ i ¼ ½ð3f –2Þ= f 2 o:58 ð1−0:02f Þ Fig. 5 Fluorescence spectra of sP3HT-T3A and rr-P3HT in solid films excited at 515 and 520 nm, respectively

ð3Þ

Figure 4 shows the UV-visible spectra of s-P3HT-T3A, respectively, measured in different solvents and in solid state films. s-P3HT-T3A solutions showed absorption maxima corresponding to the π-π* transition of P3HT chains at around 450 nm in non-polar and poorly polar solvents such as

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toluene, chloroform and THF, and at 490 nm in a more polar solvent such as ethyl acetate. As the solubility of P3HT chains decreases in increased polar solvents, the red-shift behavior of s-P3HT-T3A in ethyl acetate is caused by a conformational change from coil to rod. In addition, the presence of a shoulder peak at around 600–610 nm, which also appears for thin films of both s-P3HT-T3A and rr-P3HT, could be attributed to the formation of a small fraction of a highly aggregated form arising from high molecular weight polymer chains. The film of s-P3HT-T3A showed a main absorption with maximum at 515 nm, which is hyperchromic compared to the absorption maximum at 523 nm of the rr-P3HT linear precursor. A comparison of the relative integrated absorption area of the shoulder peak at 610 nm with that of the main peak suggested similar highly aggregated contents for the s-P3HT-T3A and rr-P3HT thin films. It seems that for very high molecular weight P3HT chains, the star-shaped structure did not effectively hinder chain aggregation. Nevertheless, it should be emphasized that when comparing the major conformations corresponding to the main absorptions (at 515 nm for sP3HT-TPA and 523 nm for rr-P3HT), the more hyperchromic blue shift in the absorption wavelength of s-P3HT-TPA is a strong indication of a lower aggregation degree of most polymer chains as a result of the star-shaped structure. This is further confirmed by the XRD result (vide infra) revealing a larger π−π stacking distance between P3HT arm chains in the solid state of s-P3HT-TBA. This observation indicates a low aggregation degree in the thin film state of s-P3HT-T3A as a result of the star-shaped structure, as compared with the rr-P3HT linear precursor. The photoluminescent spectra of s-P3HT-T3A and rrP3HT in thin films excited at their absorption maxima, i.e. 515 and 520 nm, respectively, are shown in Fig. 5. In solid state films, s-P3HT-T3A as well as the rr-P3HT linear precursor displayed an emission peak at around 727 nm. However, the s-P3HT-T3A exhibited an additional peak at around 480 nm attributed to the T3A core. The fluorescence quantum yields (ΦF) of the polymers in dilute CHCl3 were measured in comparison to 9,10-diphenylanthracence as a standard (in cyclohexane, ΦF = 0.9). The fluorescence quantum yield of polymers were determined following the equation: Φ F ðPolymersÞ ¼ Φ F ðRef Þ :η2 =η2 ðRef Þ :I=A:AðRef Þ =I ðRef Þ

Thermal property of s-P3HT-T3A The thermal property of the s-P3HT-T3A in the solid state was studied by DSC. The DSC second-heating traces in the range from 0 to 250 °C of s-P3HT-T3A and rr-P3HT are shown in Fig. 6. It is well known that linear P3HT chains are generally stiff chain molecules with very strong intermolecular interactions, resulting in a high melting temperature (Tm) normally above 200 °C [40]. The rr-P3HT linear precursor has a Tm of around 239 °C with a melting enthalpy (ΔHm) of 17.14 J g−1. On the other hand, the DSC heating trace of s-P3HT-T3A shows a relatively broad melting endotherm at around 207 °C with a ΔHm of 10.2 J g−1, suggesting less aggregation than the rr-P3HT linear precursor. Nitrogen-containing aromatic rings, such as triazine, are known to exhibit significantly stronger stacking tendencies than aromatic hydrocarbon rings [41]. Therefore, despite the star-shaped structure, the πstacking between the electron-deficient rings of the T3A core allows for certain stacking order of P3HT arm chains. Solid structure of s-P3HT-T3A The optoelectronic property of thiophene-based conjugated polymers is strongly related to their structural order in the solid state. Thus, it is of critical importance to assess the morphology of conjugated polymers for successful application of these materials in organic optoelectronics. The molecular order of s-P3HT-T3A and rr-P3HT in the solid state was investigated by powder XRD measurements (Fig. 7). Powder samples were prepared similarly to those for DSC analysis, i.e. by precipitation from solution. The XRD pattern of the rr-P3HT linear precursor revealed characteristic reflection peaks observed for classical P3HT materials [42, 43]. These include the (100) reflection peak at 2θ = 5.5o attributed to an interlayer spacing of 16.1 Å between linear conjugated segments

ð4Þ

where ΦF (Ref) is the quantum yield of the reference compound, η is the refractive index of the solvent, I is the integrated fluorescence intensity and A is the absorbance at the excitation wavelength. Based on above fluorescence quantum yield equation, the quantum yield was estimated to be about 0.44 for s-P3HT-T3A, which is similar to that of the rr-P3HT linear precursor (ΦF (rrP3HT) = 0.42).

Fig. 6 DSC second-heating traces (exo up) of rr-P3HT and s-P3HT-T3A

J Polym Res (2017) 24:180 Fig. 7 XRD patterns of rr-P3HT (a) and s-P3HT-T3A (b), and a schematic representation of sP3HT-T3A structure

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Fig. 8 AFM images of s-P3HTT3A (5 μm × 5 μm)

separated by n-hexyl side chains, and the (020) peak at 2θ = 26.3o corresponding to an interplanar distance of 3.4 Å between P3HT chains within the main chain layers. Such a short stacking spacing of 3.4 Å indicates a tilted chain packing structure with strong π−π stacking interactions [43, 44]. The XRD pattern of the star-shaped s-P3HT-TBA also exhibits (100) and (020) P3HT crystalline diffraction peaks corresponding to spacings of 16.1 Å (5.5o) and 3.8 Å (23.2o), respectively. In comparison with the rr-P3HT linear precursor, the larger d(020) value, of 3.8 Å, reveals a larger π−π stacking distance between the P3HT arm chains due to lower intermolecular interactions. In the XRD pattern of s-P3HT-T3A, there is a peak at 2θ = 37.7o attributed to a spacing of 2.38 Å between thiacalix[3]triazine core macrocycles stacked on top of one another [36, 45]. The stacking of T3A planar core units, originating from CH-π and π-π ring-ring interactions between the triazine rings as well as between the phenoxy substituents Fig. 9 Emission spectra of sP3HT-T3A in THF (CM = 0.1 M) in the presence of tetra-nethylammonium hydrogen carbonate

[36, 45], facilitates packing of P3HT arms. This was indicated by intense sharp diffraction signals associated with P3HT crystallites. The (100) diffraction peaks at 2θ = 5.5o corresponding to an end-to-end packing of the hexyl side chains of 16.1 Å, together with the (020) peak at 23.3o (3.8 Å) measuring the π-π stacking spacing between successive polythiophene backbones, suggest a packing motif of P3HT arms similar to the classical P3HT polymorph with negligible hexyl side-chain interdigitation. We note the presence of a sharp diffraction peak located at 2θ = 19.4o (peak (020)* in Fig. 9c). This may be indicative of another P3HT crystal structure with a slightly larger lattice spacing of 4.6 Å, as samples with multiple polymorphic phases have been observed in polyalkylthiophenes [43, 44, 46]. The coexistence of two polymorphs might also explain the presence of the broad and asymmetric melting transition in the DSC curve of sP3HT-T3A (Fig. 6). Furthermore, the (020) diffraction is

J Polym Res (2017) 24:180

superimposed on a broad amorphous halo centered ca. 21° associated with scattering from a disordered packing of side chains [46, 47], whereas this amorphous diffraction is not obvious for the rr-P3HT linear precursor. AFM characterization The micro-and nanoscopic morphology of a thin deposit of sP3HT-T3A was examined by AFM in tapping mode contact. The film has been prepared by drop-casting onto a wafer substrate from a good solvent (THF). Fig. 8 shows the AFM height and phase images of s-P3HT-T3A thin film surface after solvent annealing with THF vapour at 70 °C, followed by annealing at 150 °C. Linear rr-P3HTs are well-known to exhibit nanofibril morphology with the lateral length corresponding to intermolecular π-π stacking interaction [22]. Thus, the absence of fibril structure in the AFM images of sP3HT-T3A strongly suggests the formation of the star-shaped structure [22]. Moreover, the film of s-P3HT-T3A shows relatively large size domains with a high roughness. These crystalline domains are a result of strong self-organization of rr-P3HT arm chains of s-P3HT-T3A, which could be beneficial to ordered structure formation and charge transport in thin films [48, 49].

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arylation polymerization. It was found that both the star structure and the core play a critical role in the aggregation of P3HT chains and hence optical properties, as evidenced by the UV − vis, DSC, XRD and AFM results. Moreover, the star-shaped P3HT with T3A core exhibited anion association property, and therefore could be useful as a chemo-sensor to detect certain chemicals in the environment due to fluorescence quenching properties caused by effective energy transfer from P3HT moieties to T3A core-anion complexes. Acknowledgements This research was fully supported by Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number “104.02-2016.56”.

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2.

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4.

The anion association property of s-P3HT-T3A 5.

In order to explore the optical property of s-P3HT-T3A related to fluorescence switching caused by an anion association with the core, a solution of s-P3HT-T3A was prepared in THF (CM = 0.1 M) in the presence of tetra-n-ethylammonium hydrogen carbonate. A 10−3 M solution of tetra-n-ethylammonium hydrogen carbonate was prepared with the host stock solution to remain a constant host concentration throughout the anion association experiment. In the solution of s-P3HT-T3A, an addition of 10−4 mmol of tetra-n-ethylammonium hydrogen carbonate in the solution of polymer resulted in a decrease in fluorescence intensity, dropping by 11% of the initial value. This phenomenon is referred to fluorescence quenching, caused by effective energy transfer from P3HT moieties to the anion complex formed by the T3A core and [HCO3−]. Simultaneously, an absorbance peak around 310 nm corresponding to the formed anion complex emerged. In addition, the fluorescence intensity was measured as a function of [HCO3−] concentration. As shown in Fig. 9, the fluorescence intensity was reduced with increasing [HCO3−] concentration and reached a limit of 60% of the initial intensity.

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