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Synthesis of Macrocyclic Poly(3-hexylthiophene) and Poly(3heptylselenophene) by Alkyne Homocoupling George R. McKeown,† Yuan Fang,‡,§ Nimrat K. Obhi,† Joseph G. Manion,† Dmitrii F. Perepichka,*,‡ and Dwight S. Seferos*,† †
Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario M5S 3H6, Canada Department of Chemistry and Center for Self-Assembled Chemical Structures, McGill University, 801 Sherbrooke Street West, Montreal, Quebec H3A 0B8, Canada § Division of Molecular Imaging and Photonics, Department of Chemistry, KU Leuven-University of Leuven, Celestijnenlaan 200F, B-3001 Leuven, Belgium ‡
S Supporting Information *
ABSTRACT: Here we report the synthesis of cyclic samples of poly(3-hexylthiophene) (P3HT, degrees of polymerization = 25, 40, and 75) and poly(3-heptylselenophene) (P37S, DP = 30). Cyclization was accomplished using a mild alkyne−alkyne homocoupling procedure. Alkyne-terminated poly(ethylene glycol) was then coupled to residual uncyclized polymers, which were subsequently removed by column chromatography, enabling isolation and characterization of pure cyclic polymers. Cyclization was confirmed by the disappearance of terminal alkyne protons, the decrease in hydrodynamic radius [measured by size exclusion chromatography (SEC)], and the observed identical molecular weight distribution [measured by matrix-assisted laser desorption/ ionization (MALDI) mass spectrometry]. The lower weight macrocyclic polymers have decreased self-assembly as measured by optical absorption and transmission electron microscopy. The highest weight macrocycles were imaged using scanning tunneling microscopy. Cyclic polymers adopted a tightly bent conformation, while their linear analogues assembled as fully extended chains. Our method of cyclization and purification is broadly applicable to conjugated polymers (CPs) and will enable the development of novel optoelectronic materials.
T
One of the most well-studied conjugated polymers is poly(3alkylthiophene) (P3AT), which makes it an ideal candidate to study the effect of cyclic topology in CPs. P3AT can be synthesized using living catalyst-transfer polymerization (CTP), allowing for precise control over polymer chain length and endgroup functionalities.19−22 The selenium analogue poly(3alkylselenophene) (P3AS) is less studied but can similarly be synthesized in a controlled manner using CTP.23−26 P3AT and P3AS are therefore excellent candidates to test the generality of a cyclization procedure. These polymers have numerous applications: in OPVs,27,28 organic field-effect transistors (OFETs),29 and organic light-emitting diodes (OLEDs).30,31 In each of these devices, performance is strongly associated with polymer morphology. Chain loops and cyclic structures are commonly observed defects in conjugated polymers in the solid state. 32 The effects of cyclic topology on the morphological properties of P3AT are thus of direct relevance for their device properties.33
he ability to tailor the morphological and physical properties of a polymer without changing its chemical composition is highly desirable. Physical properties of polymers, such as crystallinity, mechanical strength, and conductivity, are largely governed by topology.1−5 Cyclic polymers are a special class of polymer topology. The chain ends of linear polymers have higher diffusion rates relative to the interior units, which influences polymer properties such as density, intrinsic viscosity, glass transition temperature, refractive index, and host−guest interactions.6,7 Cyclic polymers display different diffusion behavior due to the lack of free end units and thus possess different physical properties. Many examples of cyclic polymers8−11 and cyclic conjugated oligomers 12−16 exist in the literature; however, cyclic conjugated polymers (CPs) are very rare. Recent examples include large annulenes17 and monodisperse porphyrin polymers.18 However, most of these studies involve difficult purification steps or are limited to low molecular weight examples. Unlike traditional polymers, conjugated macrocycles have altered electronic properties. Iyoda et al.12 showed that cyclic oligothiophenes have increased interaction with fullerenes, which is promising for improving the efficiency of organic photovoltaics (OPVs). © XXXX American Chemical Society
Received: August 4, 2016 Accepted: September 6, 2016
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polymerization (DP) of 25, 40, and 75. One sample of α,ωdipentynyl-P37S was synthesized with a DP of 30. The dispersity (Đ) of these samples varied between 1.16 and 1.23 as measured by size exclusion chromatography (SEC). Cyclization was accomplished by alkyne homocoupling, previously used to synthesize macrocycles of poly(ethylene oxide) and poly(styrene) in near-quantitative yields.9 The polymers were cyclized by slowly adding the α,ω-dipentynylsubstituted linear polymer to a stirring solution of copper and palladium catalysts in THF. After 48 h of stirring, alkyneterminated PEG (Mn 5000 Da) was added, and the mixture was stirred for another 24 h to couple with all residual P3HT/P37S alkyne groups before terminating the reaction by precipitation. To isolate the macrocyles, the precipitate was redissolved and passed through a short silica-gel column where the linear PEGfunctionalized polymer remained on the column [based on TLC analysis the retention factor of P3HT in standard-phase column chromatography (silica column, chloroform) decreases from 0.75 to 0.01 after PEGylation], and the pure macrocycles eluted. The yield and other characterization data of the polymers are presented in Table 1.
In this study we report a CTP polymerization, end-capping, and cyclization method for synthesis of cyclic conjugated polymers. This general new approach for preparing cyclic conjugated polymers uses alkyne homocoupling, followed by the selective end-capping of uncyclized byproduct with poly(ethylene glycol) (PEG) to facilitate purification. The separation of linear and cyclized materials allows us to fully test the morphological differences in the cyclic samples. We also explore the effect of cyclization on the physical properties of these polymers at the macro, nano, and molecular scale. Macrocyclic poly(3-hexylthiophene) (P3HT) has been previously reported by Coulembier et al.;34 however, in that previous example linear and cyclic polymers were not separated, and individual cyclic polymers were not directly imaged. Here we report a general procedure that overcomes these previous limitations and study three different molecular weight samples of cyclic P3HT, including imaging individual cyclic P3HT chains, as well as the first example of cyclic poly(3heptylselenophene) (P37S). The preparation of conjugated macrocycles involves CTP of thiophene or selenophene monomers, followed by end-capping with reactive alkyne groups. Coupling these alkyne-terminated linear polymers in dilute solution forms conjugated macrocycles. Following cyclization, PEG with a reactive alkyne terminus was added and subsequently coupled to any residual uncyclized macromolecules to increase their polarity. This allowed facile separation of cyclized and uncyclized polymers by standard-phase column chromatography (Scheme 1). Regioregular P3HT and P37S were synthesized by CTP, followed by end-capping with trimethylsilyl (TMS)-protected pentynyl groups to create the linear precursors to the cyclic products. Pentynyl groups were chosen for their stability and easy installation during CTP. Three samples of α,ω-dipentynylP3HT35 were synthesized with number-average degrees of
Table 1. Molecular Weights of Linear and Cyclic Polymer Samples as Measured by SEC and MALDI sample namea
yield (%)b
SEC Mp (kg/mol)c
SEC Mn (kg/mol)d
SEC Đ
MALDI Mn (kg/mol)e
l-P3HT25 c-P3HT25 l-P3HT40 c-P3HT40 l-P3HT75 c-P3HT75 l-P37S30 c-P37S30
63 58 41 52 47 41 45 54
7.6 6.0 14.5 11.5 25.6 20.0 12.6 9.0
7.3 7.5 12.9 12.0 21.3 22.5 10.6 8.8
1.16 1.67 1.17 1.37 1.20 1.44 1.23 1.44
3.67 3.63 7.38 7.15 12.56 12.53 6.69 6.87
a
Sample prefixes use l for linear and c for cyclic. bYield of polymerization for linear polymers, yield of cyclization for cyclic polymers. cPeak molecular weight. dNumber-average molecular weight. eCalculated using m/z peaks of the MALDI spectra.
Scheme 1. Synthesis of c-P3HT and c-P37S Using PEG to Separate Linear Polymers
Cyclic polymers have a lower SEC-determined molecular weight than their linear counterparts, while their mass distributions are identical when measured by MALDI (Figure 1a,b). This is consistent with cyclization, where the hydrodynamic radius decreases while molecular weight remains unchanged. In general cyclization causes the SEC-determined peak molecular weight of all three P3HT samples to decrease by ∼20% upon cyclization (Figure 1a and S1). Interestingly the sample of P37S decreases more dramatically upon cyclization, by around 30%, which is likely a result of stronger heteroatom interactions in this polymer.36 In all cases SEC indicates the presence of higher molecular weight cyclic species, which we attribute to the n = 2 macrocycles. 1H NMR confirms end group coupling as the terminal alkyne protons disappear, while adjacent protons shift upfield from 2.29 to 2.36 ppm in CHCl3 (Figure 1c,d). The disappearance of the terminal alkyne protons is accompanied by a loss of coupling between the former terminal alkyne protons and the adjacent methylene protons (4J = 2.6 Hz). The solution optical absorption spectra of cyclic and linear P3HT are identical when dissolved at 80 °C in a marginal solvent (anisole, 1,2-dimethoxybenzene) for all high molecular weight samples. In the case of the low molecular weight 1076
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Figure 2. (a) Large-scale STM image of l-P3HT75. (b) High-resolution STM image of l-P3HT75. The inset shows a magnified area with unit cell and molecular model overlay. (c,d) STM image of l-P3HT40 and lP37S30, respectively. (e) Large-scale STM image of c-P3HT75. (f,g) Zoomed-in images of a single macrocycles where pentynyl linker groups appear as dark areas located at different location of the rings. Figure 1. (a) SEC profiles and (b) MALDI spectra of l-P3HT75 and cP3HT75. c-P3HT75 has a reduced hydrodynamic radius with identical molecular weight. (c) 1H NMR of l-P3HT25 in CDCl3 (d) 1H NMR of c-P3HT25 in CDCl3.
pyrolytic graphite (HOPG) was selected as the substrate to minimize interfacial interaction effects. The polythiophenes were applied onto a freshly cleaved HOPG surface in CHCl3 (5 μg/mL), followed by annealing at 80 °C for 10 min prior to imaging. Long-chain l-P3HT75 self-assembles into large domains of crystallized polymer strands (Figure 2a). The degree of order is far greater than that observed for commercial regioregular P3HT.32,38 The conjugated backbone of l-P3HT75 is shown as bright lamellae, with very few deformations associated with bending and folding of the chain. The high-resolution STM image (Figure 2b) depicts the internal structure of the polymer skeleton which is composed of regularly spaced rod-like features assigned to a pair of thiophene rings that correspond to a 1D unit cell of P3HT. The alkyl side chains are located in the darker regions. The dimension of the 2D unit cell is measured as a = 0.80 ± 0.06 nm, b = 1.34 ± 0.06 nm, and γ = 87 ± 2°. This is in good agreement with the internal periodicity of the polymer (0.79 nm) and the interchain separation assuming interdigitation of the alkyl chain of the neighboring lamellae (1.35 nm)39 (Figure 2b). Such highly ordered packing is consistent with the TEM results that show formation of extended microscopic lamellae by l-P3HT75. The polymer chain length distribution measured from the STM images (36 nm ± 9 nm) is in agreement with the calculated length of the numberaverage DP = 75 (29.6 nm, based on 0.79 nm per bithiophene).40 Shorter-chain l-P3HT40 (Figure 2c) and l-P37S30 (Figure 2d) also formed ordered lamellae with conjugated backbones aligned parallel to each other and interdigitated alkyl chains. Compared to l-P3HT75, these two polymers form shorter lamellae (∼10−15 nm) with no discernible structure, implying their higher mobility. The self-assembled structure of c-P3HT75 is less ordered, and more bending is observed. c-P3HT75 folds itself to promote intramolecular alkyl chain interdigitation, which is supported by the unit cell measurements: a = 0.75 ± 0.06 nm, b = 1.24 ± 0.06 nm, and γ = 89 ± 2°. Individual macrocycles can be discerned in the STM images (Figure 2f,g). It was noticed that the conjugated backbone is capable of making sharp bends. The bis(pentynyl) linker appear as ∼1.0 nm darker area of the
samples, P3HT25, the linear and cyclic topologies differ (Figure S4). The absorption peak of c-P3HT25 is blue-shifted to 435 nm relative to 444 nm for l-P3HT25 (all samples measured in 1,2-dimethoxybenzene at 80 °C). This is consistent with the expectation that the increased ring strain of lower molecular weight cyclic polymers will have a greater effect on the optoelectronic properties,37 and also that at longer polymer lengths this strain is alleviated. The solid-state optical absorption spectra reveal a difference in self-assembly between linear and cyclic polymers of all sizes except for the highest sample, DP 75 (Figure S4). The lowerweight circular samples all have blue-shifted absorption bands, relative to their linear counterparts, in addition to variations in the intensities of their vibronic shoulders (at 600 nm for P3HT and 650 nm for P37S). The former indicates less conjugation along cyclic polymer backbones, while the latter is associated with different degrees of interchain π−π interactions. Decreased aggregation of the cyclic polymers is also evident in transmission electron miscroscopy (TEM) images of structures grown in anisole or 1,2-dimethoxybenzene at room temperature (Figure S5). While l-P3HT25 assembles into ordered nanoribbons, c-P3HT25 appears to form less-ordered amorphous aggregates. Both l-P3HT40 and c-P3HT40 form nanowires, but nanowires of c-P3HT40 are shorter, indicating less efficient self-assembly of the cyclic polymer. Similarly, both l-P37S30 and c-P37S30 form nanoribbons; however, l-P37S30 nanoribbons possess nanowire cores, indicating a higher degree of self-assembly. On the other hand, both linear and cyclic P3HT75 samples form long nanowires that are indistinguishable. These results support our assertion that cyclization of low molecular weight conjugated polymers decreases their propensity to self-assemble at the nanoscale and that this trend disappears as polymer molecular weight increases. The molecular structure of the linear and cyclic polythiophenes was further examined by high-resolution scanning tunnelling microscopy (STM) (Figure 2). Highly oriented 1077
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macrocycle ring which is in agreement with its calculated length (1.15 nm). The width of c-P3HT75 is approximately half of that of l-P3HT75 (15 nm ± 5 nm over 95 measurements). No selfassembly was observed of l-P3HT25 as well as c-P3HT25, cP3HT40, and c-P37S30. This could be explained by the expected higher mobility of these structures precluding their imaging by STM at room temperature. Differences between the cyclic and linear samples with identical degrees of polymerization are quite striking. In summary, three samples of α,ω-dipentynyl-P3HT with DP 25, 40, and 75 and one sample of α,ω-dipentynyl-P37S with DP 30 were synthesized and cyclized via alkyne homocoupling. Alkyne-terminated PEG was used to functionalize residual uncyclized polymers, which were then removed on a silica column. SEC, 1H NMR, and MALDI provide evidence that the polymers were successfully cyclized: cyclic polymers have a lower peak molecular weight by SEC, lose their terminal alkyne protons by 1H NMR, and have identical molecular weights to their linear precursors by MALDI. STM showed that selfassembled monolayers of c-P3HT are sharply bent, while lP3HT forms monolayers with fully extended chains. These observations are consistent with the expected decreased order in cyclized polymers relative to linear polymers. This is the first use of an alkyne homocoupling procedure to prepare conjugated polymer macrocycles. It should be an effective general strategy for cyclizing the numerous polymers prepared by CTP. This approach not only serves as a simple synthetic route for cyclization of linear polymers but also facilitates their separation from uncyclized polymers, enabling further investigation of polymer conformation on physical properties of conjugated polymers.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.6b00603. Supporting Information includes an experimental section and supplementary figures (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. Author Contributions
All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by the NSERC of Canada, The Canadian Foundation for Innovation, the Ontario Research Fund, and MESRST of Québec. GRM is grateful to the University of Toronto Department of Chemistry for a Special Opportunity Travel Award.
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