Synthesis, Structures, and Hydroboration of Oligo- and Poly(3

Article pubs.acs.org/Macromolecules

Synthesis, Structures, and Hydroboration of Oligo- and Poly(3alkynylthiophene)s Frank Pammer,†,‡ Fang Guo,† Roger A. Lalancette,† and Frieder Jak̈ le*,† †

Department of Chemistry, Rutgers UniversityNewark, 73 Warren Street, Newark, New Jersey 07102, United States Institute for Organic Chemistry II and Advanced Materials, University of Ulm, Albert-Einstein-Allee 11, 89081 Ulm, Germany

‡

S Supporting Information *

ABSTRACT: 3-Alkynyl-substituted terthiophenes and polythiophenes were synthesized, and their properties and behavior toward hydroboration with Mes2BH were investigated. The alkynyl-substituted terthiophene was found to crystallize in an intriguing layered structure that mimics an interdigitated polymer wherein the terthiophenes form tightly π-stacked linear pseudopolymer strands. The heptynyl side chains of neighboring stacks interlock ideally, almost completely filling the void inbetween substituents and thus allowing for minimal spacing between neighboring pseudopolymer strands. Poly(3alkynylthiophene)s were accessed through Grignard metathesis polymerization (GRIM) of 2,5-dibromo-3-heptynylthiophene, which yielded polymers of moderate molecular weights (Mn = 6.3; PDI = 1.73) in yields of up to 82%. Further modification by hydroboration with Mes2BH resulted in a material that is partially functionalized with vinylborane groups and as a result exhibits charge transfer bands in the UV−vis spectra. A single-crystal X-ray structure of the corresponding hydroborated terthiophene species shows extensive intermolecular interactions, resulting in π-stacks of π-dimers, despite the steric bulk of the dimesitylborane moieties.

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INTRODUCTION

Chart 1

Polythiophenes (PTs) are among the most widely employed materials in the fabrication of optoelectronic devices such as organic field effect transistors, organic light-emitting diodes, or organic photovoltaic cells.1 Functionalized PTs also play important roles as conjugated polymer sensors. 2 The regioregularity of poly(3-alkylthiophene)s (P3ATs) has been shown to critically affect the materials properties, including the optical band gap and the electrical conductivity after doping. A coplanar arrangement of the thiophene rings in regioregular polythiophenes (rr-P3ATs) generally results in a larger effective conjugation length. Grignard metathesis polymerization (GRIM) in particular has emerged as the synthetic procedure of choice for the preparation of rr-P3ATs with a broad variety of substituents and functional groups in the side chain.3 While regioregularity is critical for a coplanar arrangement of thiophene rings in P3ATs, the less sterically demanding substituents in poly(3-alkynylthiophene)s allow for a perfectly coplanar arrangement even when the monomer units are linked together in a head-to-head fashion. The latter was convincingly demonstrated by Yamamoto and co-workers, who reported on the properties of head-to-head coupled polymers (A, R1 = alkyl) obtained by Stille-type polycondensation (Chart 1).4 Copolymers of thiophene and 3-alkynylthiophene were synthesized by a similar protocol.5 GRIM polymerization of 3-alkynylthiophenes was attempted by Li and co-workers but furnished material (B, R2 = n-octyloxy) of relatively low © 2012 American Chemical Society

molecular weight, high polydispersity (Mn = 3.4 kDa, PDI = 2.14), and unknown regioregularity.6 We recently described the GRIM polymerization of vinylborane-functionalized 2,5-dibromothiophene monomers to furnish low molecular weight polymers C.7 Organoboranefunctionalized conjugated polymers have attracted much recent interest as optoelectronic and sensory materials.8 The presence of the electron-deficient organoborane moieties in C results in strong electronic coupling between the polythiophene main chain and the conjugated side chains. Moreover, the organoborane groups are Lewis acidic and thus lend themselves to the binding of anions,9 which leads to a pronounced response in Received: May 25, 2012 Revised: July 26, 2012 Published: August 10, 2012 6333

dx.doi.org/10.1021/ma3010718 | Macromolecules 2012, 45, 6333−6343

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accomplished with dimesitylborane13 (Mes2BH), which was chosen because steric protection of tricoordinate boron centers with mesityl groups generally furnishes air and moisture stable products.8 Furthermore, Mes2BH has been reported to be highly regioselective in the hydroboration of internal alkynes such as 1-phenylpropyne14 or 1-trimethylsilyl-2-phenylacetylene.13c In the present case, however, the reaction of 3 with Mes2BH proved to be unselective, resulting in isomers 4α/β in a 1:1 ratio. While a possible reason might be the smaller size of the five-membered thiophene ring, electronic effects could also play a role and even precoordination of the borane to the thiophene sulfur is conceivable. Separation by reverse-phase column chromatography with MeCN/Et2O as the eluent gave the isomers 4α and 4β in about 30% yield each.15 The isomers show signals in the 11B NMR spectrum at around 75 ppm (4α: 75 ppm; 4β: 76 ppm), in the typical range for tricoordinate boron compounds.16 The individual isomers were distinguished by 1H NMR spectroscopy. The central thiophene ring and the newly formed vinyl-CH in 4β yield two singlets at 7.52 and 7.51 ppm. The spectrum of 4α also shows a singlet for the central thienyl ring at 6.93 ppm, but the resonance of the vinyl-H is observed as a triplet at 7.00 ppm (3JHH = 7.0 Hz) due to scalar coupling with the allylic methylene group of the pentyl chain. The chemical shift of the γ-methylene group also differs significantly between the two isomers. The signal for 4α appears superimposed with the mesityl- and thienyl-CH3 groups around 2.15 ppm but can be unambiguously assigned by its coupling pattern in the HHCOSY-NMR spectrum (Figure S9 in the Supporting Information). For 4β the corresponding signal is broadened and strongly shifted downfield to 2.83 ppm due to the steric hindrance and ring current effect of the adjacent mesityl groups. In comparison, the propargylic methylene group in 3 yields a sharp triplet at 2.32 ppm (3JHH = 5.6 Hz). Solid-State Structures of Terthiophenes 3 and 4β. Slow evaporation of solutions in hexanes gave single crystals of 3 and 4β suitable for analysis by X-ray diffraction. Terthiophene 3 crystallizes in the triclinic space group P-1 with two symmetry equivalent molecules in the unit cell (Figure 1a). The thiophene rings assume a trans conformation in which the central S2-ring is slightly rotated out of the common plane with torsion angles of 15.3° (S1-ring//S2-ring) and 11.3° (S2-ring// S3-ring) relative to the neighboring rings. This stands in contrast to the perfectly coplanar arrangement in 3,3′-dialkynyl2,2′-bithiophenes reported by Yamamoto and co-workers.4a,c,17 The fact that the angle for the S2//S3 rings is similar to that of the S1//S2 rings suggests that the twisting in 3 is due to packing effects rather than the steric demand of the alkynyl substituent. 3 does not adopt the typically observed “herringbone” packing1d but shows an unusual and interesting supramolecular structure. Based on association into pairs of terthiophenes, an intricate three-dimensional layered structure emerges that mimics a crystalline polymer with interdigitated side chains (Figure 1b,c). The concept of side-chain interdigitation in polythiophenes is of considerable interest because it increases the packing density and affects the size of crystalline domains in the bulk material.19 This in turn has been associated with improved environmental stability20 and increased charge carrier mobility.19b−d Polythiophenes bearing solubilizing alkyl substituents on every ring (e.g., rr-P3HT) cannot interdigitate because the atom density would be too high.19b,c Interdigitation is commonly observed, however, for partially alkylated materials,

the absorption and emission characteristics. While these materials were prepared by polymerization of vinylboranefunctionalized monomers, an alternative approach is to polymerize alkynyl-substituted thiophene monomers first, followed by further elaboration of the alkynyl groups in a postpolymerization modification process. Herein we discuss the synthesis of 3-heptynyl-substituted ter- and polythiophenes, their structural properties, and their functionalization by hydroboration with dimesitylborane.

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RESULTS AND DISCUSSION Synthesis of Monomers and Terthiophene Model Compounds. According to Nagarjuna et al.,10 2,5-dibromo-3alkynylthiophenes can be synthesized by selective Sonogashira−Hagihara coupling11 of an alkyne with 3-iodo-2,5dibromothiophene (1). Adopting a procedure described by Prakash and Olah for the direct iodation of electron-deficient arenes,12 we developed a new synthesis of 1 that gives access to the compound on a multigram scale (Scheme 1). The reaction Scheme 1. Synthesis of Monomer 2a

a

Reagents and conditions: (i) 1.1 equiv of NIS, F3B·OEt2 + H2O, RT, 20 h; (ii) 1.2 equiv of 1-heptyne, 5% CuI, 2.5% Pd(PPh3)2Cl2, NEt3, THF, RT, 36 h.

of commercially available 2,5-dibromothiophene with Niodosuccinimide (NIS) in partially hydrolyzed F3B·OEt2 afforded 1 in 58% yield, after recrystallization from EtOH/ MeOH (1:5) at low temperature. 3-(1-Heptynyl)-2,5-dibromothiophene (2) was then obtained by Sonogashira−Hagihara coupling of 1 with 1-heptyne. 1-Heptyne was chosen in order to provide solubilizing side chains in the final oligomeric and polymeric products. Monomer 2 was converted to the terthiophene 3 by Stille coupling with 2-methyl-5-(tributylstannyl)thiophene (Scheme 2). The desired product was obtained in 48% yield and fully characterized by 1H and 13C NMR, UV−vis spectroscopy, and high resolution MALDI-MS. Hydroboration of 3 was Scheme 2. Synthesis and Hydroboration of Terthiophene 3a

a

Reagents and conditions: (i) 5% Pd(PPh3)4, toluene, reflux, 16 h; (ii) Mes2BH, THF, RT, 3 h. 6334


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Figure 1. (a) Molecular structure of a close dimer of 3. Ellipsoids shown at 50% probability level; hydrogen atoms are omitted for clarity. (b) Schematic representation, numbering, and relevant parameters of adjacent close dimers within a pseudopolymer double strand viewed along the zaxis.18 (c) Space-fill representation of two adjacent close dimers within a pseudopolymer double strand viewed along the z-axis. Spheres drawn at 1.0 van der Waals radii.

Figure 2. Polymers pQT-Cn and pBTTT-Cn and a schematic representation of side chain interdigitation in pQT-C12.21

Figure 3. Crystal packing of 3. (a) View along the x-axis (C2−C2″ direction) showing the bricklayer arrangement of four pseudopolymer double strands. Dashed lines indicate π-stacking. (b) View along the y-axis (C2−C2′ direction) showing the ring-slipped π-stacked structure of seven terthiophene dimers. Ellipsoids drawn at 50% probability level; hydrogen atoms are omitted for clarity.

1b). The alkyl chains assume a pseudo-trans conformation relative to the triple bonds, which allows them to almost completely fill the available space between the terthiophenes (Figure 1c; see also Figure S36 in the Supporting Information). Additional coplanar pairs are aligned along the C2−C2″ direction (x-axis) to form pseudopolymer double strands with a

such as poly[5,5′-bis(3-alkyl-2-thienyl)-2,2′-bithiophene)] (pQT-Cn)21,19b and poly(2,5-bis(3-alkylthiophene-2-yl)thieno[3,2-b]thiophene) (pBTTT-Cn, Figure 2).22 The basic motif in the packing structure of 3 consists of pairs of coplanar terthiophenes that face each other in close contact with antiparallel CC axes spaced 4.0 Å apart (D1 in Figure 6335


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Figure 4. Crystal packing of 4β. (a) Side view of two π-stacked unit cells. (b) A layer of terthiophene π-stacks viewed perpendicular to the thiophene ring planes. Ellipsoids are shown at 50% probability level; hydrogen atoms are omitted for clarity.

side-chain repeat height of c = 16.5 Å.23 These double strands are close to planar with a vertical offset of 0.062 Å between the (C2,C13,C2′,C13′) planes (defined as xy-plane) of neighboring dimers. Packing along the y- and z-axes then gives rise to a layered structure of π-stacked double strands spaced ca. 3.4 Å apart (Figure 3). The overall packing distance lies near the lower end of the range usually observed for conjugated polymers (3.4−3.8 Å).24 Within a layer the individual double strands do not show distinctive interactions, as the shortest interstrand contacts (S2···C3* 3.678 Å, S2···H3* 3.203 Å; S2···C4* 3.701 Å, S2···H4* 3.246 Å)18 are longer than the respective van der Waals radii. The formation of the double strands is instead attributed to dispersion interactions between the interlocking heptynyl side chains. The layered superstructure then arises due to extensive π-stacking of the terthiophene strands with closest neighbors in the z-direction. Each terthiophene strand forms a tight π-stack with one neighboring strand in an adjacent layer spaced ca. 3.3 Å apart, based on the distances between coplanar (C2,C7,C13) planes (Figure 3a). Each individual terthiophene interacts with two molecules in one adjacent layer by forming ring-slipped π-stacks (Figure 3b). This gives rise to a bricklayertype arrangement between both the double strands as a whole and the individual close dimers within a double strand. The superstructure found for 3 is insofar unusual, as the arrangement of the terthiophenes and the side chains resembles an almost ideally interdigitated polymer and allows for maximum packing density of the pseudopolymer chains. The latter is evident from the chain−chain distance (d) of 11.1 Å, which is shorter than what is found for polymers with even shorter side chains such as pBTTT-C6 (d = 13.5 Å)19c and poly(3,6-dihexyl[2,2′]bi[thieno[3,2-b]thiophene]) (PDHTTC6) (d = 13 Å)20 or noninterdigitating rr-P3HT (d = 14.4 Å).19c,25 The dominance of π-stacked layers over a “herringbone” structure is mainly observed for substituted acenes26 and oligothiophenes bearing polar functional groups that allow for strong dipolar interactions and hydrogen bonding.27 Rare examples of compounds that adopt layered structures similar to the one described here include β-dialkylated quaterthiophenes introduced by Bäuerle28 and a dialkynylbithiophene reported by Yamamoto.29 Only in the latter case, the almost ideal collinear arrangement of the oligothiophene moieties found for 3 is also observed. An investigation of the hydroboration product 4β by X-ray diffraction yielded a similarly intriguing packing structure (Figure 4). The introduction of the Mes2B groups adds

considerable bulk to the side chain that causes steric interactions with the terthiophene moiety. Close contacts exist between the vinylic hydrogen atom H15 and S1 (d = 2.695(1) Å)30 and between the thienyl proton H8 and the alkyl protons attached to C17 (d = 2.174(1) Å). As a result, the vinylborane fragment (C15−C16−B1) cannot adopt a coplanar arrangement but is rotated by 30° relative to the central S2thiophene ring (Figure 4a). Similarly, the steric pressure causes the S1-ring to be rotated by 24.5° relative to the S2-ring, while the S3-ring is almost coplanar (S2-ring/S3-ring = 8.8°). However, despite these deformations and the considerable bulk of the side chains, extensive π-stacking occurs between terthiophene moieties. 4β crystallizes in the triclinic space group P-1 with two molecules in the centrosymmetric unit cell. The center of inversion is located in-between two terthiophene moieties, which assume a ring-slipped π-stacked structure. The coplanar central thiophene rings of two molecules within the unit cell are spaced ca. 3.55 Å apart, while the distance to the next molecules in an adjacent asymmetric unit is ca. 3.91 Å, resulting in chains of “π-stacked π-dimers”. The terthiophene stacks are packed parallel to one another ca. 4.3 Å apart31 and are offset vertically (normal to the S2-ring plane) by ca. 1.3 Å, relative to neighboring stacks. This pattern results in layers of terthiophenes separated by the heptenylborane side chains (Figure 4b). Metathesis Experiments and Polymer Synthesis. To develop the GRIM polymerization of 2, we first investigated the halogen metal exchange of the monomer with Grignard reagents (Scheme 3). A limiting factor in the polymerization of 3-substituted 2,5-dibromothiophenes is the inherent formation of two isomeric magnesio species in the Grignard metathesis step. For 2,5-dibromo-3-alkylthiophenes typically a selectivity of 75−85% for the sterically less hindered 5-position is observed, regardless of the Grignard reagent employed.3 According to 1H NMR analysis, treatment of 2 with iPrMgCl led to rapid transmetalation in THF and even in the nonpolar solvent C6D6 (Figures S24−S27 in the Supporting Information). Consumption of 75% of the starting material occurred within 5 min, while quantitative conversion was reached in less than 45 min. However, the reaction was found to be less selective than for the corresponding 3-alkylthiophenes. The 5metalated species remains the primary product, but with a slightly lower selectivity of about 55−65%. Experiments with i PrMgCl·LiCl32 and the sterically more demanding tBuMgCl resulted in similar regioselectivity. Given the higher reactivity and faster conversion,3b,d iPrMgCl·LiCl was chosen as the 6336


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Scheme 3. GRIM Polymerization and Transmetalation Experiments

Figure 5. GPC-UV chromatograms of purified 5a (dotted), 5c (solid line), and 8 (dashed). Recorded in THF vs polystyrene standards.

Products 5a−c dissolve in hot chloroform or THF without substantial insoluble residue. However, at ambient temperature the higher molecular weight material (5c) showed only limited solubility in THF, chloroform, and aromatic solvents, while being completely insoluble in methanol, hexanes, and dichloromethane. The moderate solubility is consistent with the relatively high molecular weight and in part a result of polymer aggregation due to π-stacking interactions (vide inf ra). In comparison, the corresponding perfectly regioregular polymers A were found to be completely insoluble even at 140 °C in 1,2dichlorobenzene unless functionalized with an n-decyl group.4b Preliminary kinetic studies indicate that the polymerization may initially proceed via a fast chain-growth mechanism considering that polymer of Mn = ca. 1500−2000 Da and a low PDI of 1.1−1.2 is formed within just a few minutes even at room temperature. However, further monomer consumption proved to be sluggish, which possibly reflects the formation of complexes between the Ni catalyst and the alkynyl groups33 that impede the polymerization. The complexes should break up more readily at elevated temperature, which explains the higher yields when kept under reflux. However, the molecular weight increased only very gradually even at high temperature with significant broadening of the molecular weight distribution (see Figure S28). This is indicative of the involvement of stepgrowth condensation processes including chain−chain coupling (see also discussion of MALDI-MS data).3g The loss of control could also be related to the lower stability of the nickel catalyst in the absence of more sterically demanding alkyl groups as discussed by Luscombe and co-workers.34 The high yields of 60% and 80% for 5b and 5c indicate that, unless the two initially formed magnesio species interconvert, both regioisomers must have been incorporated into the polymer. Indeed, in comparison to poly(3-alkylthiophene)s

metallating agent for the polymerization experiments described in the following. Polymerization of 2 did not occur readily under typical GRIM conditions (Table 1). Although a rapid color change indicated successful initiation of the polymerization, stirring at ambient temperature followed by the usual work-up (precipitation into MeOH, Soxhlet extraction with hexanes and further extraction with CHCl3) afforded only material of low molecular weight (Mn = 3.8 kDa, PDI = 1.36, DP = 21.6, 5a) in poor yield (10−15%, up to 75% of monomeric 6/7 recovered). Similarly low molecular weights have been reported in the syntheses of B6 and related alkenylthiophenes.6,33 Stefan and co-workers have attributed this behavior to competing interactions of the vinyl groups with the Ni(0) species generated during the catalytic cycle,7,33 and similar complexation processes may be expected for the alkynyl monomer used in this study. However, raising the reaction temperature to reflux overnight allowed the isolation of a polymer of considerably higher molecular weight (Mn = 6.3 kDa, PDI = 1.62, DP = 35.7, 5b) in 57% yield. Heating the reagents to 110 °C in a THF/toluene (1:1) mixture in a sealed flask gave a polymer with similar properties (Mn = 6.1 kDa, PDI = 1.73, DP = 34.6, 5c, Figure 5) but in further improved yield of more than 80%.

Table 1. Reaction Conditions and Results for the Isolated Polymers 5a−c and the Hydroborated Polymer 8 polymer

a

1 2 3

5a 5b 5c

4

8

solvent THF THF THF/ toluene THF

λmax/nm (eV) solutionb

λedge/nm (eV) solutionb

T/°C

rct time/h

Mn/ kDaa

PDIa

yield/%

RT 68 110

16 16 16

3.8 6.3 6.1

1.36 1.62 1.73

10−15 57 82

514 (2.41) 517 (2.40) 517 (2.40)

587 (2.11) nd 609 (2.04)

68

48

7.6

1.85

65d

323 (3.84), 478 (2.60)

580 (2.14)

λmax/nm (eV) thin filmc nd nd 556 (2.23), 600 (2.07) 323 (3.84), 466 (2.66)

λedge/nm (eV) thin filmc nd nd 652 (1.90) 598 (2.07)

GPC-UV vs polystyrene standards. bSolvent THF. cSpin-cast from THF solution. dYield based on 30% hydroboration. 6337


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dissociates from the chain end after a final reductive elimination that ends the polymerization process. Yet another reason might be that coupling occurs with residual dibromothiophene monomer 2, but this is unlikely given that less than 3% of unreacted monomer were present based on the quenching experiments of the initially generated Grignard species. We next explored the possibility of introducing borane functionalities into 5 by hydroboration. Heating a solution of 5c in THF to reflux in the presence of an excess of Mes2BH resulted in partial hydroboration of the triple bonds and yielded the polymer 8 as a red solid with significantly altered physical properties (Scheme 4, Table 1). Polymer 8 can be isolated and purified by precipitation into methanol and, in contrast to the precursor 5c, is very well soluble in common organic solvents (THF, CHCl3, benzene) even at room temperature. Analysis of 8 by GPC-UV revealed only a slight increase in the number-average molecular weight to Mn = 7.6 kDa (PDI = 1.85). This is to be expected, since the elution time is correlated to the hydrodynamic volume of the polymers, which is dominated by the rodlike character of the polymer backbone for both 5c and 8.38 The 1H NMR spectrum of 8 clearly shows peaks due to the newly introduced Mes2B groups in the aryl and alkyl regions (see Figure S22). The Mes2B content can be estimated by comparison of the integrated signals of the terminal butyl fragment (9 H) and the remaining alkyl signals between 1.8 and 3.0 ppm that represent the mesityl-CH3 groups (18 H) and allylic and propagylic methylene groups (2 H). The observed integral ratio of 7.6:9 is considerably lower than the theoretical value of 20:9 expected for quantitative functionalization and corresponds to a degree of hydroboration of ca. 30%. This finding is corroborated by the results from thermogravimetric analysis (TGA) (see Figure S34 and associated comments in the Supporting Information). Upon heating, 8 rapidly loses ca. 35% of its mass between 190 and 280 °C, attributed to loss of Mes2BH through retro-hydroboration.39 In the 11B NMR spectrum 8 yields a broadened signal at 63 ppm. Addition of an excess of tetrabutylammonium fluoride (TBAF) results in a shift of the resonance to 3.7 ppm (8) that is accompanied by sharpening of the signal. The peaks lie in the typical range for tricoordinate boron and tetrahedral borate systems, respectively, which is consistent with binding of fluoride anions to the boron centers.16 Electronic Structure. Polymers 5a−c show almost identical UV−vis spectra with longest wavelength absorption maxima around λmax = 515 nm (Figure 8a, Table 1). However, the absorption onset is significantly lower in energy for 5c (609 nm, 2.04 eV) in comparison to 5a (587 nm, 2.11 eV). This is attributed to the considerably higher average molecular weight of polymer 5c. Indeed, GPC-PDA analysis of 5c in THF provides clear evidence for this molecular weight dependence of the optical gap (Figure S29). The energy gaps in solution are similar to those reported for A (ca. 600 nm, 2.07 eV)4 and are significantly lowered compared to regioregular poly(3alkylthiophene)s (rr-P3HT: λmax = 445−456, onset ca. 560 nm/2.22 eV).40 Unlike in rr-P3HT, the head-to-head coupling in A still allows for a coplanar conformation of the thiophene rings and does not lower the effective conjugation length. The tendency of conjugated polymers to aggregate in the solid state via π−π−stacking is much more pronounced for regioregular than for random polymers. The π−π−stacking typically results in a distinct bathochromic shift in the

differentiation between the two isomers is less likely to occur during the polymerization, due to the lower steric demand of the alkynyl as compared to alkyl chains.4 Our kinetic studies (Figure S28) reveal that in a THF/toluene mixture at 110 °C initially the 5-magnesiated isomer is preferentially converted, but over time the sterically more hindered 2-isomer also reacts. Typically, the 1H NMR resonances of the thienyl protons are used to probe the regioregularity of the resulting polythiophenes. However, the respective signals in the spectra of 5b and 5c recorded in CDCl3 appear broadened and superimposed with the solvent (Figure S17). In THF-d8 two broad signals are visible in the aromatic region at 7.39 and 7.20 ppm (Figure S19),35 which indicates a lack of regioregularity. The 13C NMR spectra of 5a and 5c (CDCl3, Figure 6) closely resemble each

Figure 6. 13C NMR spectra of 5c (top), 5a (center), and terthiophene 3 recorded in CDCl3.

other. Four broadened signals corresponding to the thiophene rings appear at 136.5, 133.4, 128.7, and 119.0 ppm, while the signals of the alkynyl-carbon atoms are found at 97.7 ppm (CC−CH2) and 76.1 ppm (CC−CH2). By comparison with 3 the peak at 128.7 ppm is assigned to the thienyl-CH group, while the other signals correspond to the quaternary carbon atoms. The close similarity between the 13C NMR spectra indicates that the reaction conditionsambient temperature vs 110 °Cdo not affect the regioregularity of the polymers. The lower resolution of the spectrum of 5c is merely owing to the higher molecular weight and the ensuing reduced solubility at room temperature. Further analysis of 5c by mass spectrometry revealed the presence of two types of polymeric species (Figure 7). According to the high-resolution MALDI-MS data, the two series of signals correspond to H/Br- and Br/Br-terminated polymers, while H/H-terminated species are not present. Matching isotope patterns were observed for species of up to m/z = 2370 Da (H/Br, DP = 13) and 2540 Da (Br/Br, DP = 13).36 There are several possible explanations for the presence of Br/Br-terminated polymers in 5c. As discussed above, the high temperatures required in the formation of 5c could result in chain−chain coupling processes.3g,37 Another possibility is that the Ni(0) catalyst does not undergo oxidative addition but 6338


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Figure 7. (a) High-resolution MALDI-FT-MS spectrum of 5c. (b) Close-ups of peaks with H/Br- and Br/Br-end groups at n = 11. Top: experimental MALDI-MS spectra; bottom: corresponding theoretical isotope patterns.

longest wavelength transition is most likely centered on the polythiophene main chain.42 Upon hydroboration, the steric bulk of the vinylborane groups prevents a coplanar conformation of the polymer backbone, resulting in the observed blue shift of the absorption. A second absorption band also emerges at 322 nm. This band is attributed to charge transfer from the thiophene main chain into the empty pz-orbital on boron.42 Consistent with this assignment is that this band disappears upon addition of tetrabutylammonium fluoride (TBAF), which binds to the tricoordinate boron center and thus makes the charge transfer pathway unavailable (Figure S31). These findings further corroborate the incorporation of the borane functionalities into the polymer. We further investigated the electronic structure of polymers 5 (lower molecular weight batch) and 8 by cyclic voltammetry in CH2Cl2/[(n-Bu)4N]PF6 solution. We found that the onset of oxidation for both 5 (Eox,onset = 0.23 V vs Fc/Fc+) and 8 (Eox,onset = 0.32 V vs Fc/Fc+) is significantly shifted to higher potential relative to rr-P3HT (Eox,onset = 0.03 V vs Fc/Fc+, Figure S32). This effect, which is attributed to the electronwithdrawing nature of the sp-hybridized CC carbon atoms, is desirable as it should improve the oxidative stability relative to P3HT and other alkylthiophene polymers. Experiments on thin films of a higher molecular weight batch of 5 drop-cast from 1,1,2,2-tetrachloroethane solution further confirmed our results and generally gave better reversibility (Eox,onset = 0.46 V vs Fc/ Fc+, CH2Cl2/[(n-Bu)4N]PF6), although the peak currents decreased in successive redox cycles (Figure S33). Prior studies on Mes2B-functionalized polythiophenes showed that borane

Scheme 4. Hydroboration of 5c

absorption spectra and may give rise to a vibronic fine structure.1,24 Indeed, thin films of A have been reported to exhibit a pronounced shift (ca. 80 nm) as well as a vibronic fine structure.4a,b In comparison, thin films of 5c exhibit a more modest bathochromic shift of the absorption maximum of ca. 40 nm from 517 to 556 nm (Figure 8a). Also, a shoulder band centered at ca. 600 nm emerges, but a vibronic fine structure is not observed. On the basis of the NMR and UV−vis data, it can therefore be surmised that 5c does not possess a regioregular structure as well-defined as that of A. Hydroboration results in a shift of the longest wavelength absorption maximum from 517 to 478 nm (Figure 8b). This hypsochromic shift is somewhat unexpected given that incorporation of a tricoordinate boron center into a conjugated system typically leads to extension of conjugation and thus results in a bathochromic shift.41 In our case, however, the 6339


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gives access to significantly higher molecular weights than the synthesis of C from hydroborated derivatives of 2.

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EXPERIMENTAL SECTION

Materials and Methods. All reactions and manipulations of sensitive compounds were carried out under an atmosphere of prepurified nitrogen using either Schlenk techniques or an inertatmosphere glovebox (Innovative Technologies). Ether solvents were distilled from Na/benzophenone prior to use. Hydrocarbon solvents were purified using a solvent purification system (Innovative Technologies; alumina/copper columns for hydrocarbon solvents). Mes2BH13 and rr-P3HT3 were synthesized according to literature procedures. Other reagents were commercially available (Aldrich, Acros, Strem) and were either used as obtained or purified by standard procedures.43 iPrMgCl·LiCl was generated in situ from iPrMgCl and LiCl. iPrMgCl was prepared from 2-chloropropane and Mg in Et2O, insolubles and metal flakes were filtered off, and the solvent was removed in vacuo to leave a homogeneous gray slurry of iPrMgCl· xEt2O, the Et2O content of which was determined by 1H NMR. All 499.9 MHz 1H, 125.7 MHz 13C, and 160.4 MHz 11B NMR spectra were recorded at ambient temperature on a Varian INOVA spectrometer equipped with a boron-free 5 mm dual broadband gradient probe (Nalorac, Varian Inc., Martinez, CA). Solution 1H and 13 C NMR spectra were referenced internally to solvent signals.44 Unless stated otherwise, 11B NMR spectra were acquired with boronfree quartz NMR tubes and referenced externally to BF3·Et2O (δ = 0 ppm). Individual signals are referred to as singlet (s), doublet (d), triplet (t), quintet (quint), multiplet (m), centrosymmetrical multiplet (mc), and broadened (br). GPC-RI/UV analyses were performed with THF (1 mL/min) as eluent on a Waters Empower GPC system equipped with a 717plus autosampler, a 1525 binary HPLC pump, a 2487 dual λ absorbance or a photodiode array (PDA) detector, a 2414 refractive index detector, and styragel columns (Polymer Laboratories; two columns 5 μm/Mixed-C). The columns were kept in a column heater at 35 °C and were calibrated with polystyrene standards (Polymer Laboratories). High-resolution mass spectrometry measurements were performed on a Bruker 7-T FT-MS using either APCI (atmospheric pressure chemical ionization) or MALDI (matrixassisted laser desorption ionization) for sample ionization. Anthracene, 9,10-diphenylanthracene, benzo[a]pyrene, or α-cyano-3-hydroxycinnamic acid was used as matrix in combination with silver(I) trifluoromethanesulfonate (AgOTf) as oxidant. EI-MS low-resolution mass spectrometry data of volatile compounds were acquired using a Hewlett-Packard GC/MS system consisting of a HP 5973 mass selective detector and a HP 6890 Series GC system. UV−vis absorption data were acquired on a Varian Cary 500 UV−vis/NIR spectrophotometer. Elemental analyses were obtained from Intertek/ Quantitative Technologies Inc., Whitehouse, NJ. Cyclic voltammetry studies in solution were carried out on a CV-50W analyzer from BAS. The three-electrode system consisted of an Au disk as working electrode, a Pt wire as secondary electrode, and a Ag wire as the pseudoreference electrode. The voltammograms were recorded in CH2Cl2 containing [(n-Bu)4N]PF6 (0.1 M) as the supporting electrolyte. The scans were referenced after the addition of a small amount of ferrocene (Fc) or decamethylferrocene (Fc*) as internal standard. The potentials are reported relative to the Fc/Fc+ couple (Fc*/Fc*+ = −550 mV vs Fc/Fc+ in CH2Cl2/[(n-Bu)4N]PF6). Thin film voltammetric experiments were carried out on a computercontrolled Autolab PGSTAT30 potentiostat with a three-electrode system consisting of a platinum working electrode, a platinum wire counter electrode, and an Ag/AgCl reference electrode. The working electrode was coated by dipping into a saturated polymer solution in 1,1,2,2-tetrachloroethane (5c), followed by drying in air. The scans were referenced to Ag/AgCl (Ag/AgCl vs Fc/Fc+ = 0.65 V (CH2Cl2).45 X-ray diffraction intensities of 3 and 4β were collected on a Bruker SMART APEX2 CCD diffractometer. Crystallographic data and details of the X-ray diffraction experiments and crystal structure refinement for 3 and 4β are given in Tables S1−S3. SADABS46 absorption

Figure 8. (a) UV−vis spectra of 5c in THF solution (solid line) and as thin film (dashed line). (b) UV−vis spectra of 8 in THF solution (solid line) and as thin film (dashed line). Spectra normalized to absorption maxima in the visible range.

substitution results in significantly lower energy LUMO levels.41c Hence, electrochemical reduction of 8 was attempted using THF/[(n-Bu)4N]PF6 as the supporting electrolyte, but the scans only showed overlapping irreversible processes. The fact that polymer 8 is only partially hydroborated may be responsible for the ill-defined features in the reduction scans, which do not allow for an unambiguous electrochemical determination of the LUMO levels.

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CONCLUSION 3-Alkynyl-substituted ter- and polythiophenes have been synthesized, and their properties and behavior toward hydroboration with Mes2BH have been investigated. The terthiophene 3 exhibits an intriguing three-dimensional superstructure in the solid state that mimics an ideally interdigitated polythiophene. The alkynyl triple bonds contribute significantly to this self-assembly, as they reduce the steric demand near the pseudopolymer backbone and thus allow for effective interlocking of the side chains. Hydroboration of 3 yields the isomeric vinylborane-functionalized terthiophenes 4α/β. The solid-state structure of 4β confirms that the orientation of the vinylborane group is suitable for strong electronic interactions with the polythiophene backbone. Despite the bulk of the boryl side group, extensive π-stacking results in an interesting extended structure. Starting from the dibromothiophene precursor 2 GRIM polymerization allowed us to isolate poly(3-alkynylthiophene)s (5) in high yield. A lack of regioregularity is attributed both to unselective transmetalation of 2 and to the low steric demand of the triple bond that renders head-to-head coupling less unfavorable. Reacting 5 with Mes2BH yielded a partially vinylborane-functionalized material (8). This postpolymerization modification approach is preparatively more facile and 6340


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3 as a pale yellow solid. Yield: 247 mg (667 μmol, 48%). Rf = 0.28 (SiO2, hexanes). 1H NMR (500 MHz, C6D6): δ = 7.35 (d, 3JHH = 3.6 Hz, 1 H), 7.15 (s, 1 H), 6.78 (d, 3JHH = 3.5 Hz, 1 H), 6.46 (d, 3JHH = 2.7 Hz, 1 H), 6.32 (d, 3JHH = 2.5 Hz, 1 H), 2.32 (t, 3JHH = 7.1 Hz, 2 H, CC−CH2), 2.12 (s, 3 H, Th−CH3), 2.01 (s, 3 H, Th-CH3), 1.52 (quint, 3JHH = 7.3 Hz, 2 H, CC−CH2CH2), 1.34 (mc, 2 H, CH2CH2CH3), 1.22 (sext, 3JHH = 7.3 Hz, 2 H, CH2CH3), 0.84 (t, 3JHH = 7.3 Hz, 3 H, CH3) ppm. 1H NMR (500 MHz, CDCl3): δ = 7.24 (d, 3 JHH = 3.6 Hz, 1 H), 6.95 (s, 1 H, Th−H), 6.93 (d, 3JHH = 3.5 Hz, 1 H), 6.69 (dd, 3JHH/4JHH = 3.5 Hz/0.9 Hz, 1 H), 6.66 (dd, 3JHH/4JHH = 3.5 Hz/0.9 Hz, 1 H), 2.50 (s, 3 H, Th−CH3), 2.49 (t, superimposed, 2 H, CC−CH2), 2.47 (s, 3 H, Th−CH3), 1.67 (quint, 3JHH = 7.3 Hz, 2 H, CC−CH2CH2), 1.48 (mc, 2 H, CH2CH2CH3), 1.39 (sext, 3JHH = 7.3 Hz, 2 H, CH2CH3), 0.94 (t, 3JHH = 7.3 Hz, 3 H, CH3) ppm. 13C NMR (125 MHz, C6D6): δ = 134.0, 139.7, 137.9, 134.7, 134.6, 134.4, 127.3 (Th−CH), 126.4 (Th−CH), 125.8 (Th−CH), 125.6 (Th− CH), 124.3 (Th−CH), 118.9, 95.7 (CC−CH2), 77.2 (CC− CH2), 31.5, 28.6, 22.6, 20.2 (CC−CH2), 15.1 (Th−CH3), 15.0 (Th−CH3), 14.2 (CH3) ppm. APCI-MS for [C21H22S3]+: calcd [m/z] = 370.0884 Da, found [m/z] = 370.0888 Da. UV−vis (THF): λmax = 380 nm, ε380 = 4700 M−1 cm−1. Metathesis Experiments with 2,5-Dibromo-3-(1-heptynyl)thiophene, 2. In a glovebox under inert gas atmosphere 2 (33.6 mg, 100 μmol) and iPrMgCl·0.86Et2O (16.7 mg, 100 μmol) were weighed in, dissolved in 0.6 mL of C6D6, and transferred into an NMR tube equipped with a PTFE screw cap. The NMR tube was then quickly taken out of the glovebox, and the reaction progress was monitored by 1 H NMR. After complete conversion of 2 the reaction mixture was quenched with degassed H2O and dried by filtration over Na2SO4. The NMR spectra of 6 and 7 were then recorded. 6: 1H NMR (500 MHz, C6D6): δ = 6.89 (d, 4JHH = 1.4 Hz, 1 H, H-4), 6.79 (d, 4JHH = 1.4, Hz, 1 H, H-2) ppm. 7: 1H NMR (500 MHz, C6D6): δ = 6.68 (d, 3JHH = 5.5 Hz, 1 H, H-4), 6.42 (d, 3JHH = 6.0 Hz, 1 H, H-5) ppm; superimposed/ unassigned signals: δ = 2.18 (t, 2 H, 3JHH = 7.0 Hz, CC−CH2), 2.14 (t, 2 H, 3JHH = 6.8 Hz, CC−CH2), 1.36−1.46 (m, 4 H, CH2), 1.24− 1.34 (m, 4 H, CH2), 1.14−1.22 (m, 4 H, CH2), 0.83 (t, 6 H, 3JHH = 7.3 Hz, CH3) ppm. EI-MS (C11H13Br, m/z (%)): 258 (71, [M]+). GRIM Polymerization of 2. In a typical procedure a solution of i PrMgCl·0.86Et2O (167.0 mg, 1.0 mmol) and LiCl (42.4 mg, 1.0 mmol) in dry THF with an overall volume of 9 mL was stirred for 30 min at ambient temperature. The mixture was then cooled to −15 °C, the monomer 2 (336 mg, 1.0 mmol) was added, and the mixture was stirred for another 1 h, while warming to ambient temperature. Subsequently, Ni(dppp)Cl2 (10 μmol, 5.42 mg), suspended in 1 mL of THF, was added, and the mixture was left to react for 16 h, either by stirring at ambient temperature (5a) or heating to 68 °C in a sealed flask (oil bath temperature, 5b). For purification the reaction solution was poured into MeOH (100 mL) that was acidified with a few drops of concentrated hydrochloric acid. A purple precipitate formed that was collected in a Soxhlet thimble and then washed on a Soxhlet extractor with methanol and hexanes. Soxhlet extraction with CHCl3 followed by evaporation of the solvent gave the product as a purple solid. The polymers proved to be moderately soluble in THF, CHCl3, toluene, and benzene at room temperature and insoluble in CH2Cl2. For 5c the Grignard metathesis was carried out in 4 mL of THF, followed by addition of the catalyst suspended in 1 mL of THF. The reaction mixture was then diluted by addition of 5 mL of dry toluene and heated to 110 °C (oil bath temperature) for 16 h in a sealed flask. 5a: Yield: 10−15%, up to 75% of 6/7 recovered. GPC-UV: Mn = 3.8 kDa, Mw = 5.3 kDa, PDI = 1.38. 5b: Yield: 57%, GPC-UV: Mn = 6.3 kDa, Mw = 10.2 kDa, PDI = 1.62. 5c: Yield 82%, GPC-UV: Mn = 6.1 kDa, Mw = 10.6 kDa, PDI = 1.73. 1H NMR (500 MHz, CDCl3): δ = 7.31 (br, 0.5 H, Th−H), 7.06 (br, 0.5 H, Th−H), 2.51 (br, 2 H, C C−CH2), 1.68 (br, 2 H, CH2), 1.43 (br, 4 H, CH2), 0.94 (br, 3 H, CH3) ppm. 1H NMR (500 MHz, THF-d8): δ = 7.39 (br, 0.5 H, Th− H), 7.19 (br, 0.5 H, Th−H), 2.57 (br, 2 H, CC−CH2), 1.49 (br, 6 H, CH2), 0.95 (br, 3 H, CH3) ppm. 13C NMR (125 MHz, CDCl3): δ = 136.5 (Th−C), 133.4 (Th−C), 128.7 (Th−CH), 119.0 (Th−C), 97.7 (CC−CH2), 31.4 (CH2), 28.2 (CH2), 22.4 (CH2), 20.1 (CH2), 14.1 (CH3) ppm; CC−CH2 could not be observed for 5c due to

correction was applied, and the structures were solved using direct methods and completed by subsequent difference Fourier syntheses and refined by full matrix least-squares procedures on reflection intensities (F2). All non-hydrogen atoms were refined with anisotropic displacement coefficients. The H atoms were placed at calculated positions and were refined as riding atoms. All software and sources scattering factors are contained in the SHELXTL program package.47 Crystallographic data for the structures of 3 and 4β have been deposited with the Cambridge Crystallographic Data Center as supplementary publications CCDC-883709 and CCDC-883710, respectively. Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: (+44) 1223-336-033; e-mail: [email protected]). Synthesis of 2,5-Dibromo-3-iodothiophene, 1. A mixture of 2,5-dibromothiophene (2.42 g, 10.0 mmol) and 2.47 g of Niodosuccinimide (11.0 mmol) was prepared in a reaction vessel under the exclusion of light and oxygen and cooled in an ice bath. Precooled BF3·H2O·Et2O (prepared from 5 mL (40.5 mmol) of BF3·Et2O and 737 μL (40.9 mmol) of H2O) was added dropwise, and the reaction mixture was then let warm to ambient temperature and stirred for 24 h. Subsequently, the reaction mixture was poured into 100 mL of ice water, and 150 mL of diethyl ether was added. The organic phase was separated, washed with water, sodium thiosulfate solution, saturated sodium bicarbonate solution, and brine, and dried over sodium sulfate. After evaporation of the solvent the crude product was obtained as a brown oil 2.96 g (purity ≈90%, GCMS), which was purified further by crystallization from EtOH/MeOH (ca. 1:5, v:v) at −30 °C. Yield: 2.13 g (5.8 mmol, 58%). 1H NMR (500 MHz, CDCl3): δ = 6.93 (s, 1 H, Th−H) ppm. 13C NMR (125 MHz, CDCl3): δ = 137.2 (Th−CH), 116.4 (C−Br), 113.5 (C−Br), 85.3 (C−I) ppm. EIMS (C4HBr2I, m/z (%)): 367.7 (100, [M]+). Synthesis of 2,5-Dibromo-3-(1-heptynyl)thiophene, 2. In a Schlenk flask, copper(I) iodide (952 mg, 5.0 mmol), Pd(PPh3)2Cl2 (1.76 g, 2.5 mmol), and 1 (19.06 g, 51.8 mmol) were suspended in a mixture of dry THF (150 mL) and dry triethylamine (150 mL). The reaction mixture was cooled to 0 °C in an ice bath, and 1-heptyne (1.2 equiv, 5.98 g, 62.2 mmol) was then added. After the addition the ice bath was removed and the greenish-black solution was stirred at ambient temperature for 35 h. Subsequently the solvent was removed in vacuo, and the residue was taken up in 500 mL of Et2O. The organic phase was washed with dilute hydrochloric acid, water, saturated sodium bicarbonate solution, and brine, dried over sodium sulfate, and filtered through a Celite plug. Evaporation of the solvent yielded the crude product as a black oil that was purified by vacuum distillation. Yield: 10.33 g (30.7 mmol, 59% of a colorless oil); bp 117−119 °C (0.2 mbar). 1H NMR (500 MHz, CDCl3): δ = 6.88 (s, 1 H, CH), 2.41 (t, 3JHH = 7.1 Hz, 2 H, CC−CH2), 1.61 (quint, 3JHH = 7.3 Hz, 2 H, CH2), 1.44 (mc, 2 H, CH2), 1.36 (sext, 3JHH = 7.2 Hz, 2 H, CH2), 0.92 (t, 3JHH = 7.3 Hz, 3 H, CH3) ppm. 1H NMR (500 MHz, C6D6): δ = 6.60 (s, 1 H, Th−H), 2.13 (t, 3JHH = 7.0 Hz, 2 H, CC−CH2), 1.40 (quint, 3JHH = 7.2 Hz, 2 H, CH2), 1.28 (mc, 2 H, CH2), 1.20 (sext, 3JHH = 7.3 Hz, 2 H, CH2), 0.83 (t, 3JHH = 7.3 Hz, 3 H, CH3) ppm. 13C NMR (125 MHz, CDCl3): δ = 132.4 (4-CH), 125.9, 114.7, 110.6, 95.5 (CC), 73.5 (CC), 31.1, 28.3, 22.3, 19.6, 14.1 ppm. 13C NMR (125 MHz, C6D6): δ = 132.7 (Th−CH), 126.5, 115.1, 110.0, 95.6 (CC), 74.2 (CC), 31.2, 28.5, 22.5, 19.7, 14.2 ppm. EI-MS (C 11H12Br 2, m/z (%)): 335.9 (100, [M] +). MALDI-MS for [C11H12Br2S+Ag]+: calcd [m/z] = 442.8057 Da; found [m/z] = 442.8090 Da. Elemental analysis (C11H12Br2S): calcd C 39.31 H 3.60, found C 39.17 H 3.33. Synthesis of 3. 2-Methyl-5-(tributylstannyl)thiophene (1.35 g, 3.5 mmol), 2 (468 mg, 1.4 mmol), and Pd(PPh3)4 (80.3 mg, 70 μmol) were suspended in 15 mL of dry toluene under a nitrogen atmosphere, the flask was sealed, and the reaction mixture was heated to 115 °C (oil bath temperature) overnight. The solution was then poured into dilute hydrochloric acid, and another 25 mL of toluene was added. The organic phase was separated, washed with saturated sodium bicarbonate solution and brine, and dried over sodium sulfate. Evaporation of the solvent gave the crude product as a brown oil, which was purified by column chromatography (SiO2, hexanes) to give 6341


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superimposition with the solvent peak, but was found at 76.1 ppm for 5a. UV−vis: λmax (THF) = 517 nm, ε517 = 8830 L mol−1 cm−1; λmax (thin film) = 556 nm. Elemental analysis (C11H12S·0.1CHCl3): calcd C 70.83 H 6.48 S 17.04, found C 70.76 H 6.47 S 15.00. Hydroboration of 5c. To polythiophene 5c (75.0 mg, 425 μmol) and Mes2BH (1.5 equiv, 159.6 mg, 628 μmol) were added 5 mL of dry THF, and the mixture was heated to reflux for 36 h in a sealed flask. The reaction mixture was poured into 40 mL of methanol to give an orange-red precipitate that was collected by filtration. The latter was taken up in chloroform and precipitated into methanol again, collected by filtration, and then freeze-dried from benzene. Yield: 69 mg (65%, based on 30% hydroboration). GPC-UV: Mn = 7.6 kDa, Mw = 14.0 kDa, PDI = 1.85. UV−vis: λmax(THF) = 245, 323, 478 nm, ε245 = 18 390 L mol−1 cm−1, ε323 = 9460 L mol−1 cm−1, ε478 = 7202 L mol−1 cm−1; λmax(thin film) = 471 nm. 1H NMR (500 MHz, C6D6): δ = 7.3− 7.5 (vinyl-/thienyl-H), 6.6−6.95 (br, Mes-CH), 2.65−3.00 (br, allylCH2), 2.40 (br, s, Mes-o-CH3), 2.18 (br, s, Mes-p-CH3), 1.50−1.80 (br, CH2), 1.00−1.50 (br, CH2), 0.80−1.00 (br, CH3) ppm. 11B NMR (160.4 MHz, C6D6): δ = 61 ppm (3.7 ppm after addition of 2 equiv of TBAF). Elemental analysis (C11H12S·0.5Mes2BH·0.5CHCl3): calcd C 68.19 H 6.70 S 8.88, found C 67.92 H 7.05 S 8.39.

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1214. (f) Loewe, R. S.; Ewbank, P. C.; Liu, J.; Zhai, L.; McCullough, R. D. Macromolecules 2001, 34, 4324−4333. (g) Bilbrey, J.; Sontag, S. K.; Huddleston, N. E.; Allen, W. D.; Locklin, J. ACS Macro Lett. 2012, 1, 995−1000. (4) (a) Yamamoto, T.; Sato, T.; Iijima, T.; Abe, M.; Fukumoto, H.; Koizumi, T.-A.; Usui, M.; Nakamura, Y.; Yagi, T.; Tajima, H.; Okada, T.; Sasaki, S.; Kishida, H.; Nakamura, A.; Fukuda, T.; Emoto, A.; Ushijima, H.; Kurosaki, C.; Hirota, H. Bull. Chem. Soc. Jpn. 2009, 82, 896−909. (b) Sato, T.; Kishida, H.; Nakamura, A.; Fukuda, T.; Yamamoto, T. Synth. Met. 2007, 157, 318−322. (c) Sato, T.; Cai, Z.; Shiono, T.; Yamamoto, T. Polymer 2006, 47, 37−41. (d) Sato, T.; Kokubo, H.; Fukumoto, H.; Yamamoto, T. Bull. Chem. Soc. Jpn. 2005, 78, 1368−1370. (5) Tan, Z.; Zhou, E.; Yang, Y.; He, Y.; Yang, C.; Li, Y. Eur. Polym. J. 2007, 43, 855−861. (6) Zhou, E.; Hou, J.; Yang, C.; Li, Y. J. Polym. Sci., Part A: Polym. Chem. 2006, 2206−2214. (7) Pammer, F.; Jäkle, F. Chem. Sci. 2012, 3, 2598−2606. (8) (a) Lorbach, A.; Hübner, A.; Wagner, M. Dalton Trans. 2012, 41, 6048−6063. (b) Jäkle, F. Chem. Rev. 2010, 110, 3985−4022. (c) Noriyoshi, N.; Chujo, Y. Polym. J. 2008, 40, 77−89. (d) Jäkle, F. Coord. Chem. Rev. 2006, 250, 1107−1121. (e) Entwistle, C. D.; Marder, T. B. Chem. Mater. 2004, 16, 4574−4585. (f) Entwistle, C. D.; Marder, T. B. Angew. Chem., Int. Ed. 2002, 41, 2927−2931. (g) Chen, P.; Jäk le, F. J. Am. Chem. Soc. 2011, 133, 20142−20145. (h) Lukoyanova, O.; Lepeltier, M.; Laferrière, M.; Perepichka, D. F. Macromolecules 2011, 44, 4729−4734. (i) Popere, B. C.; Della Pelle, A. M.; Thayumanavan, S. Macromolecules 2011, 44, 4767−4776. (j) Caruso, A.; Tovar, J. D. J. Org. Chem. 2011, 76, 2227. (u) Thivierge, C.; Loudet, A.; Burgess, K. Macromolecules 2011, 44, 4012−4015. (k) Gao, L.; Senevirathna, W.; Sauvé, G. Org. Lett. 2011, 13, 5354− 5357. (l) Fabre, B.; Hao, E. H.; LeJeune, Z. M.; Amuhaya, E. K.; Barriere, F.; Garno, J. C.; Vicente, M. G. H. ACS Appl. Mater. Interfaces 2010, 2, 691−702. (m) Peterson, J. J.; Davis, A. R.; Werre, M.; Coughlin, E. B.; Carter, K. R. ACS Appl. Mater. Interfaces 2011, 3, 1796−1799. (n) Hoven, C. V.; Elbing, H. P.; Garner, M.; Winkelhaus, L.; Wang, D.; Bazan, G. C. Nat. Mater. 2010, 9, 249−252. (o) Li, H.; Jäkle, F. Macromol. Rapid Commun. 2010, 31, 915−920. (p) Tokoro, Y.; Nagai, A.; Chujo, Y. Macromolecules 2010, 43, 6229−6233. (q) Liu, W.; Pink, M.; Lee, D. J. Am. Chem. Soc. 2009, 131, 8703−8707. (r) Nagai, A.; Murakami, T.; Nagata, Y.; Kokado, K.; Chujo, Y. Macromolecules 2009, 42, 7217−7220. (s) Li, H.; Jäkle, F. Angew. Chem., Int. Ed. 2009, 48, 2313−2316. (t) Lorbach, A.; Bolte, M.; Li, H.; Lerner, H.-W.; Holthausen, M. C.; Jäkle, F.; Wagner, M. Angew. Chem., Int. Ed. 2009, 48, 4584−4588. (9) (a) Wade, C. R.; Broomsgrove, A. E. J.; Aldridge, S.; Gabbaï, F. P. Chem. Rev. 2010, 110, 3958−3984. (b) Zachary, M. H.; Wang, S. Acc. Chem. Res. 2009, 42, 1584−1596. (c) Yamaguchi, S.; Wakamiya, A. Pure Appl. Chem. 2006, 78, 1413−1424. (10) Nagarjuna, G.; Yurt, S.; Jadhav, K. G.; Venkataraman, D. Macromolecules 2010, 43, 8045−8050. (11) (a) Heravi, M. M.; Sadjadi, S. Tetrahedron 2009, 65, 7761− 7775. (b) Sonogashira, K. J. Organomet. Chem. 2002, 653, 46−49. (12) Prakash, G. K. S.; Mathew, T.; Hoole, D.; Esteves, P. M.; Golam Rasul, Q. W.; Olah, G. A. J. Am. Chem. Soc. 2004, 126, 15770−15776. (13) Mes2BH: (a) Yuan, Z.; Entwistle, C. D.; Collings, J. C.; AlbesaJove, D.; Batsanov, A. S.; Howard, J. A. K.; Taylor, N. J.; Kaiser, H. M.; Kaufmann, D. E.; Poon, S.-Y.; Wong, W.-Y.; Jardin, C.; Fathallah, S.; Boucekkine, A.; Halet, J.-F.; Marder, T. B. Chem.Eur. J. 2006, 12, 2758−2771. (b) Marder, T. B.; Smith, P. S.; Howard, J. A. K.; Fox, M. A.; Mason, S. A. J. Organomet. Chem. 2003, 680, 165−172. (c) Pelter, A.; Smith, K.; Parry, D. E.; Jones, K. D. Aust. J. Chem. 1992, 45, 57−70. (d) Hooz, J.; Akiyama, S.; Cedar, F. J.; Bennett, M. J.; Tuggle, R. M. J. Am. Chem. Soc. 1974, 96, 274−276. (14) Pelter et al. reported a 2:98 selectivity for the hydroboration of 1-phenylpropyne, (Z)-(1-phenyl-2-dimesitylboryl)propene being the major product. Pelter, A.; Smith, K.; Buss, D.; Norbury, A. Tetrahedron Lett. 1991, 32, 6239−6242.

ASSOCIATED CONTENT

S Supporting Information *

Additional experimental data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based upon work supported by the National Science Foundation under Grants CHE-0809642 and CHE1112195 and the Rutgers University Research Council. The Xray diffractometer used in these studies was acquired with funds from the National Science Foundation (CRIF-0443538). F.P. thanks the Alexander von Humboldt foundation for a FeodorLynen postdoctoral fellowship, and F.J. thanks the Alexander von Humboldt Foundation for a Friedrich Wilhelm Bessel research award. We also thank Roman Brukh for assistance with mass spectrometry data collection.

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REFERENCES

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MacKinnon, C. D.; Oakley, R. T.; Reed, R. W. Chem. Mater. 1997, 9, 981−990. (28) 3,3‴-Dialkyl-2,2′:5′,2″:5″,2‴-quaterthiophenes, alkyl = ndodecyl, n-hexyl, n-propyl. Azumi, R.; Götz, G.; Debaerdemaeker, T.; Bäuerle, P. Chem.Eur. J. 2000, 6, 735−744. (29) 4,4′-Bis(1-decynyl)-2,2′-bithiophene. See ref 4a. (30) Numbering analogous to 3 (Figure 2a,b). For complete numbering see Figure S37 in the Supporting Information. (31) Distance between coplanar planes in adjacent unit cells. The planes are defined by the atoms S1, S3, and S2′ within any one unit cell. S1 and S3 correspond to one molecule; S2′ is generated from S2 by inversion at the center of symmetry. (32) (a) Ren, H.; Krasovskiy, A.; Knochel, P. Chem. Commun. 2005, 543−545. (b) Krasovskiy, A.; Knochel, P. Angew. Chem., Int. Ed. 2004, 43, 3333−3336. (33) (a) Bhatt, M. P.; Huynh, M. K.; Sista, P.; Nguyen, H. Q.; Stefan, M. C. J. Polym. Sci., Part A 2012, 50, 3086−3094. (b) Hundt, N.; Palaniappan, K.; Sista, P.; Murphy, J. W.; Hao, J.; Nguyen, H.; Stein, E.; Biewer, M. C.; Gnade, B. E.; Stefan, M. C. Polym. Chem. 2010, 1, 1624−1632. (34) Boyd, S. D.; Jen, A. K.-Y.; Luscombe, C. K. Macromolecules 2009, 42, 9387−9389. (35) Yamamoto observed a single resonance for 5,5′-poly(3,3′heptynyl-2,2′-bithiophene), A, at 7.03 ppm in CDCl2CDCl2. (36) These masses are significantly lower than those estimated by GPC. However, by linear mode MALDI-TOF MS species of up to m/ z = 5500 Da were observed at low resolution. (37) Br/Br-terminated polymer chains could also be generated by chain−chain coupling during the work-up (see refs 3b and Li, Z.; Ono, R. J.; Wu, Z.-Q.; Bielawski, C. W. Chem. Commun. 2011, 47, 197−199 ). However, this should result in only small amounts of higher molecular weight material that can be observed as a tailing in the GPC chromatogram. (38) Since the hydroboration adds steric bulk only to the heptynyl side chains, the significantly increased molecular weight of 8 (from 176.28 to 426.46 Da per hydroborated repeat unit) is not accurately reflected in the GPC data. (39) (a) Benedikt, G.; Kö ster, R.; Larbig, W.; Reinert, K.; Rotermund, G. Angew. Chem. 1963, 75, 1079−1090. (b) Mikhailov, B. M.; Bubnov, Y. N. Organoboron Compounds in Organic Synthesis; Harwood Academic Publishers GmbH: Chur, Switzerland, 1984. (40) (a) Adachi, T.; Brazard, J.; Ono, R. J.; Hanson, B.; Traub, M. C.; Wu, Z. Q.; Li, Z.; Bolinger, J. C.; Ganesan, V.; Bielawski, C. W.; Vanden Bout, D. A.; Barbara, P. F. J. Phys. Chem. Lett. 2011, 2, 1400− 1404. (b) Chen, T.-A.; Wu, R.; Rieke, R. D. J. Am. Chem. Soc. 1995, 117, 235−244. (41) (a) Chen, P.; Lalancette, R. A.; Jäkle, F. J. Am. Chem. Soc. 2011, 133, 8802−8805. (b) Li, H.; Jäkle, F. Polym. Chem. 2011, 1, 897−905. (c) Li, H.; Sundararaman, A.; Venkatasubbaiah, K.; Jäkle, F. Macromolecules 2011, 44, 95−103. (d) Li, H.; Sundararaman, A.; Venkatasubbaiah, K.; Jäkle, F. J. Am. Chem. Soc. 2007, 129, 5792− 5793. (42) Similar absorption bands were observed for terthiophenes 4α/β and related vinylborane-functionalized oligomers; the assignment as a CT band was corroborated by TD-DFT calculations. See ref 7. (43) Armarego, W. L. F.; Perrin, D. D. Purification of Laboratory Chemicals, 4th ed.; Butterworth-Heinemann: Oxford, 1997. (44) Fulmer, G. R.; Miller, A. J. M.; Sherden, N. H.; Gottlieb, H. E.; Nudelman, A.; Stoltz, B. M.; Bercaw, J. E.; Goldberg, K. I. Organometallics 2010, 29, 2176−2179. (45) Connelly, N. G.; Geiger, W. E. Chem. Rev. 1996, 96, 877−910. (46) Sheldrick, G. M. SADABS, University of Göttingen, Germany, 2008. (47) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112−122.

(15) The optical and electronic properties of 4α/β are discussed in detail in ref 7. (16) Noeth, H.; Wrackmeyer, B. Nuclear Magnetic Resonance Spectroscopy of Boron Compounds. In NMR - Basic Principles and Progress; Diehl, P., Fluck, E., Kosfeld, R., Eds.; Springer-Verlag: Berlin, 1978; Vol. 14, p 461. (17) The torsion angles are also larger than those reported for unsubstituted terthiophene (6°−9°) or head-to-tail 3,4′,4″-trimethylterthiophene (7°−8°). (a) Van Bolhuis, F.; Wynberg, H.; Havinga, E. E.; Meijer, E. W.; Staring, E. G. J. Synth. Met. 1989, 30, 381−389. (b) Barbarella, G.; Zambianchi, M.; Bongini, A.; Antolini, L. Adv. Mater. 1994, 6, 561−564. (18) Primes indicate atoms within a close dimer or double strand that are generated through symmetry operations. Asterisks indicate corresponding atoms in adjacent double strands. Relevant parameters are abbreviated as follows: c = side chain repeating height along the C2−C2″ line; d = interchain distance between C2−C2″ lines; D1/D2 = distances between parallel side chains. The x-and y-axes span the (C2,C13,C2′,C13′) plane, the x-axis being aligned with the C2,C2″ line. The z-axis is orthogonal to the (C2,C13,C2′,C13′) plane. (19) (a) Zhao, L.-H.; Png, R.-Q.; Zhuo, J.-M.; Wong, L.-Y.; Tang, J.C.; Su, Y.-S.; Chua, L.-L. Macromolecules 2011, 44, 9692−9702. (b) Kline, J. R.; DeLongchamp, D. M.; Fischer, D. A.; Lin, E. K.; Richter, L. J.; Chabinyc, M. L.; Toney, M. F.; Heeney, M.; McCulloch, I. Macromolecules 2007, 40, 7960−7965. (c) Northrup, J. E. Phys. Rev. B 2007, 76, 245202 (1−6). (d) Heeney, M.; Bailey, C.; Genevicius, K.; Shkunov, M.; Sparrowe, D.; Tierney, S.; McCulloch, I. J. Am. Chem. Soc. 2005, 127, 1078−1079. (20) Choi, D.; Jeong, B.-S.; Ahn, B.; Chung, D. S.; Lim, K.; Kim, S. H.; Park, S. U.; Ree, M.; Ko, J.; Park, C. E. ACS Appl. Mater. Interfaces 2012, 4, 702−706. (21) (a) Ong, B. S.; Wu, Y. L.; Liu, P.; Gardner, S. J. Am. Chem. Soc. 2004, 126, 3378−3379. (b) Salleo, A.; Chen, T. W.; Völkel, A. R.; Wu, Y.; Liu, P.; Ong, B. S.; Street, R. A. Phys. Rev. B 2004, 70, 115311(1− 10). (22) (a) Heeney, M.; Bailey, C.; Genevicius, K.; Shkunov, M.; Sparrowe, D.; Tierney, S.; McCulloch, I. J. Am. Chem. Soc. 2005, 127, 1078−1079. (b) McCulloch, I.; Heeney, M.; Bailey, C.; Genevicius, K.; MacDonald, I.; Shkunov, M.; Sparrowe, D.; Tierney, S.; Wagner, R.; Zhang, W.; Chabinyc, M. L.; Kline, R. J.; McGehee, M. D.; Toney, M. F. Nat. Mater. 2006, 5, 328−333. (23) The terthiophenes (i.e., the C2−C13 axis) stand at an angle of 2.52° relative to the overall orientation of the double strand defined by the C2−C2″−C2″″ line. (24) Yamamoto, T.; Komarudin, D.; Arai, M.; Lee, B.-L.; Suganuma, H.; Asakawa, N.; Inoue, Y.; Kubota, K.; Sasaki, S.; Fukuda, T.; Matsuda, H. J. Am. Chem. Soc. 1998, 120, 2047−2058. (25) To the best of our knowledge, reference data for the more suitable analogue pQT-C6 are not available. (26) (a) Hinoue, T.; Shigenoi, Y.; Sugino, M.; Mizobe, Y.; Hisaki, I.; Miyata, M.; Tohnai, N. Chem.Eur. J. 2012, 18 (2012), 4634−4643. (b) Payne, M. M.; Parkin, S. R.; Anthony, J. E.; Kuo, C. C.; Jackson, T. N. J. Am. Chem. Soc. 2005, 127, 4986−4987. (c) Moon, H.; Zeis, R.; Borkent, E. J.; Besnard, C.; Lovinger, A. J.; Siegrist, T.; Kloc, C.; Bao, Z. N. J. Am. Chem. Soc. 2004, 126, 15322−15323. (d) Anthony, J. E.; Eaton, D. L.; Parkin, S. R. Org. Lett. 2002, 4, 15−18. (27) (a) Fitzner, R.; Elschner, C.; Weil, M.; Uhrich, C.; Körner, C.; Riede, M.; Leo, K.; Pfeiffer, M.; Reinold, E.; Mena-Osteritz, E.; Bäuerle, P. Adv. Mater. 2012, 24, 675−680. (b) Fitzner, R.; Reinold, E.; Mishra, A.; Mena-Osteritz, E.; Ziehlke, H.; Körner, C.; Leo, K.; Riede, M.; Weil, M.; Tsaryova, O.; Weiß, A.; Uhrich, C.; Pfeiffer, M.; Bäuerle, P. Adv. Funct. Mater. 2011, 21, 897−910. (c) Janzen, D. E.; Burand, M. W.; Ewbank, P. C.; Pappenfus, T. M.; Higuchi, H.; da Silva Filho, D. A.; Young, V. G.; Brédas, J.-L.; Mann, K. R. J. Am. Chem. Soc. 2004, 126, 15295−15308. (d) Pappenfus, T. M.; Burand, M. W.; Janzen, D. E.; Mann, K. R. Org. Lett. 2003, 5, 1535−1538. (e) Bader, M. M.; Custelcean, R.; Ward, M. D. Chem. Mater. 2003, 15, 616−618. (f) Yassar, A.; Demanze, F.; Jaafari, A.; El Idrissi, M.; Coupry, C. Adv. Funct. Mater. 2002, 12, 699−708. (g) Barclay, T. M.; Cordes, A. W.; 6343


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