Article pubs.acs.org/Macromolecules
Helical Poly(5-alkyl-2,3-thiophene)s: Controlled Synthesis and Structure Characterization Hong-Hai Zhang,*,† Chuanxu Ma,† Peter V. Bonnesen,† Jiahua Zhu,† Bobby G. Sumpter,†,‡ Jan-Michael Y. Carrillo,†,‡ Panchao Yin,§ Yangyang Wang,† An-Ping Li,† and Kunlun Hong*,† †
Center for Nanophase Materials Sciences, ‡Computer Science & Mathematics Division, and §Chemical and Engineering Materials Division, Neutron Sciences Directorate, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States S Supporting Information *
ABSTRACT: Whereas poly(3-alkyl-2,5-thiophene)s (P3AT), with many potential applications, have been extensively investigated, their ortho-connected isomers, poly(5-alkyl-2,3-thiophene)s (P5AT), have never been reported because of the difficulty in their syntheses. We herein present the first synthesis of regioregular P5AT via controlled Suzuki cross-coupling polymerization with PEPPSI-IPr as catalyst, affording the polymers with tunable molecular weight, narrow polydispersity (PDI), and well-defined functional end groups at the gram scale. The helical geometry of P5AT was studied by a combination of NMR, small-angle X-ray scattering (SAXS), and scanning tunneling microscopy (STM). Particularly, the single polymer chain of poly(5butyl-2,3-thiophene) (P5BT) on highly oriented pyrolytic graphite (HOPG) substrates with either M or P helical conformation was directly observed by STM. The comparison of UV−vis absorption between poly(5-hexyl-2,3-thiophene) (P5HT) (λ = 345 nm) and poly(3-hexyl-2,5-thiophene) (P3HT) (λ = 450 nm) indicated that the degree of conjugation of the backbone in P5HT is less than in P3HT, which may be a consequence of the helical geometry of the former compared to the more planar geometry of the latter. Moreover, we found that P5HT can emit green fluorescence under UV (λ = 360 nm) irradiation.
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INTRODUCTION Conjugated polyarylenes have found a wide range of applications including optical sensors, electrochromic devices, field effect transistors, and organic solar cells.1−3 Their physical properties are controlled by the manner in which their rings are connected. As a result, ortho-, meta-, and para-linked polyarylenes each show distinctive optoelectronic properties and accordingly constitute different classes of functional polymers.4,5 Although meta- and para-linked polyarylenes have been extensively investigated over the past decades, the study of poly(o-arylene)s is still at an early stage because of the synthesis challenges. It has been reported recently that oligo(o-arylene)s intrinsically possess a compressed helical structure with a helical pitch of 0.3−0.5 nm in the solid state as revealed by single crystal X-ray analysis, including oligo(o-phenylene)s, oligo(o-thiophene)s, oligo(o-thiazole)s, and oligo(o-furan)s.6−10 Moreover, Hartley and co-workers disclosed that this compressed helical structure of oligo(o-arylene)s is stabilized by aromatic stacking interactions,11,12 which is different from traditional helical polymers commonly stabilized by steric effects or hydrogen bonding.13−15 Thus, due to their helical structures, oligo(o-arylene)s are reported to possess certain unique properties with potential applications different from their meta- or para- counterparts.16 © XXXX American Chemical Society
For example, Fukushima and Aida et al. reported that oligo(ophenylene)s possessed a main chain redox active property that responded to electrical inputs by a conformation change and acted as surface modifiers for homeotropic columnar ordering of discotic liquid crystals.8,17 Ito and co-workers reported poly(quinoxaline-2,3-diyl)s can act as a scaffold for chiral polymeric ligands with high enantioselectivities.18,19 In addition, poly(quinoxaline-2,3-diyl) films exhibited selective reflection of righthanded circular polarized light (CPL) in the visible region after annealing.20 Fascinated by the unique structural features, we are interested in poly(5-alkyl-2,3-thiophene)s (P5AT), the ortho-linked isomers of the most studied conjugated polymers, poly(3-alkyl2,5-thiophene)s (P3AT).21−26 However, the lack of an efficient and accessible synthetic methodology has made it difficult to explore the structure and properties of P5AT. The reported ortho-connected aromatic oligomers are usually synthesized by an iterative stepwise approach, which is labor-intensive and typically affords only milligram quantities of the desired oligo(oReceived: June 9, 2016 Revised: June 16, 2016
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DOI: 10.1021/acs.macromol.6b01233 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Scheme 1. Synthetic Route for Monomer 3
arylene)s.6−10 Since the strong repulsion derived from highly angled connections has made the synthesis of poly(o-arylene)s by direct polymerizations particularly challenging, only three such examples have been reported so far.27−31 Ito and co-workers reported the synthesis of poly(2,3-quinoxaline)s via multiple successive insertion reactions of 1,2-diisocyanoarenes.27,28 Recently, the Nozaki and Uchiyama groups independently reported the synthesis of poly(o-arylene)s via polymerization of aryne or [2.2.1]oxabicyclic alkene (aryne equivalent), which make it possible to study the properties of poly(o-phenylene)s.29,30 However, using aryne or [2.2.1]oxabicyclic alkene as starting materials also limited the availability of various poly(oarylene)s via this method. In 2015, Nakano and co-workers reported the synthesis of poly(1,10-phenanthroline-5,6-diyl)s via Yamamoto coupling polymerization, which may be generally applicable to the synthesis of other poly(o-arylene)s.31 However, the control of the polymers regarding molecular weight, chain end groups, and regioregularity, which may greatly influence their properties,32−34 is still elusive. To facilitate further explorations of the structure−property relationship and their potential applications, it is important to get a clear insight into the secondary structure of P5AT. Although 2,2′,3,3′-linked oligo(o-thiophene)s have been reported to adopt a helical structure based on single crystal X-ray analysis, one could not presume that 2,3-linked P5AT would necessarily adopt the same helical geometry due to the difference in ring connections and chain length.6,35 Compared with oligomers, direct analysis of helical geometry of poly(o-arylene)s is more difficult. Although the secondary structure of poly(2,3quinoxaline)s and poly(1,10-phenanthroline-5,6-diyl)s have been experimentally studied by NMR, small-angle X-ray scattering (SAXS), and circular dichroism (CD),31,36,37 it is still challenging to provide exact helical structure information such as helical pitch and helical sense with these methods. The microscopic observation of polymers by high-resolution atomic force microscopy (AFM), scanning tunneling microscopy (STM), and related techniques is one of the most straightforward methods to resolve this problem, and the observations of helical structures of single polymer chains including polyisocyanates38,39 and polyacetylenes40−42 and other helical polymers43−45 with helical pitch above 2 nm have been reported. However, direct observation of helical conformation of single polymer chains of poly(o-arylene)s has never been reported, since the smaller helical pitch of poly(o-arylene)s of around 0.5 nm is approaching the limits of microscopic techniques.46 Controlled Suzuki cross-coupling polymerization has been demonstrated as one of the most powerful tools for producing a variety of poly(p-arylene)s and poly(m-arylene)s, including P3AT, in a controlled manner.47−51 On the basis of our experience, we envisioned that such method is plausible for the synthesis of poly(o-arylene)s, such as P5AT.52−55 Although Ito and co-workers reported that their attempts to synthesize poly(o-
quinoline) via Suzuki cross-coupling polymerization only yielded oligomers (mainly with six units),56 we believe that judiciously selecting a catalyst with the optimal balance of properties may be the key to accomplishing successive cross-coupling between the sterically hindered ortho-substituted substrates.57−59 In this paper, we report the successful synthesis of regioregular P5AT with tunable molecular weight, narrow polydispersity (PDI), and well-defined chain end-functional groups in gram quantities via controlled palladium-catalyzed Suzuki crosscoupling polymerization with commercially available PEPPSIIPr as the catalyst. Then, we studied the secondary structure of P5AT by a combination of methods that included NMR, computational simulation, SAXS, and STM. Particularly, we directly observed the helical geometry of poly(5-butyl-2,3thiophene) (P5BT) on highly oriented pyrolytic graphite (HOPG) substrates by STM. We also explored the optical properties of poly(5-hexyl-2,3-thiphene)s (P5HT) by comparison with well-studied P3HT.
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RESULTS AND DISCUSSION Controlled Synthesis of P5AT. Our study began with the synthesis of the thiophene monomer 2-bromo-5-alkylthiophen3-ylboronic acid pinacol ester (monomer 3) through three steps including alkylation, bromination, and boronation from commercially available thiophene in gram scale quantities. In the borylation step, an in situ quenching strategy was employed to prevent the “halogen dance”, to afford the desired monomers.60 The structure of the obtained monomer was confirmed by NMR (see Figures S1 and S2 for details). We then carefully tested polymerization with different catalysts (see Table S1) and identified the commercially available PEPPSI-IPr as the best overall catalyst.61−64 Accordingly, the polymerization was performed with potassium phosphate as base, a mixture of THF and water as solvent, and PEPPSI-IPr as the catalyst, affording P5HT with a molecular weight of 6300 g/mol and a PDI of 1.34 (see Table S2). For practical purposes, the polymerization of 3a was performed at gram scale and afforded P5HT with molecular a weight of 6400 g/mol and PDI of 1.35 (Table 2, entry 1). The analogous monomer with an n-butyl group at the 5-position (3b) was also tested for polymerization with PEPPSI-IPr as catalyst, affording P5BT with molecular weight of 5500 g/mol and PDIs of 1.42. The P5ATs obtained here are soluble in common organic solvents such as chloroform, dichloromethane, tetrahydrofuran, and toluene. We then investigated the polymerization process in more detail with PEPPSI-IPr as the catalyst. As shown in Table 2, when PEPPSI-IPr was employed as catalyst, higher molecular weights of P5HT were obtained with smaller amounts of catalyst. In general, the molecular weight of P5HT could be modulated by the feed ratio of monomer 3a and the catalyst PEPPSI-IPr (see Figure S3). This result indicated that PEPPSI-IPr-catalyzed B
DOI: 10.1021/acs.macromol.6b01233 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules
monomer conversion and the molecular weight of the generated P5HT and found a nearly linear correlation between them with slightly increased PDIs for P5HT of increased conversions (see Figure S4), a characteristic of chain-growth polymerization. The end groups of P5HT were analyzed by matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry. We prepared P5HT with PEPPSI-IPr as catalyst and then in situ capped with phenylboronic acid after polymerization. Two series of peaks were observed, and the m/z values of the major one (>95%) can be accurately expressed as 166.3n (repeat unit) + 77.1 (Ph) + 77.1 (Ph), where n is the number of repeat units. The expected peak for the 20-mer, for example, is at m/z 166.3 × 20 (repeat unit) + 77.1 (Ph) + 77.1 (Ph) = 3480.2, and a peak was observed at m/z 3480.3. This quantitative relationship reveals that the major series of peaks corresponds to P5HT with a phenyl group at each end (designated as Ph/Ph). The minor series (
