Reversible Cis-Cisoid to Cis-Transoid Helical Structure Transition in

Article pubs.acs.org/Macromolecules

Reversible Cis-Cisoid to Cis-Transoid Helical Structure Transition in Poly(3,5-disubstituted phenylacetylene)s Sheng Wang, Xuanyu Feng, Zhiyuan Zhao, Jie Zhang, and Xinhua Wan* Beijing National Laboratory for Molecular Sciences, Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China S Supporting Information *

ABSTRACT: A series of novel 3,5-disubstituted phenylacetylenes, rM-1, sM-1 to sM-5, bearing an achiral methoxycarbonyl pendant group and various chiral N-alkylamide pendant groups, were synthesized. They were converted to the corresponding polymers, rP-1, sP-1 to sP-5, with high cis-structure contents under the catalysis of [Rh(nbd)Cl]2, aiming to understand how the environmental variation and the structure of pendant group influence the chiroptical properties of polymers. sP-1/rP-1 were soluble in CHCl3 and THF at the molecular level and exhibited much larger optical rotations with opposite signs to those of sM-1/rM-1 and displayed the intense Cotton effects centered at 360 nm in the circular dichroism (CD) spectra, ascribed to the one-handed, contracted cis-cisoid helical polyene backbone. The reversible conformation transition between the contracted cis-cisoid helix and the frustrated, extended cis-transoid helix was achieved by alternately adding trifluoroacetic acid (TFA) and triethylamine into the hydrogen bond donating solvent (i.e., CHCl3), as evidenced by UV−vis absorption and CD spectroscopy, dynamic and static laser light scattering, DSC, and WAXD results. However, the addition of TFA into the sP-1 solution in the hydrogen bond accepting solvent (i.e., THF) caused no discernible halochromism. The competing interaction of THF with TFA was considered to account for the observed difference in acidinduced chromism. The small modification in the chiral alkylamide pendant group was found to remarkably affect the solubility and helical conformation of the polymer. sP-2 was insoluble in all the solvents tested, sP-3 and sP-4 dissolved in polar DMF, while sP-5 dissolved in both polar and apolar solvents. Depending on the nature of solvents and additives, sP-3 and sP-4 took either contracted or frustrated helical conformation, whereas sP-5 took only a stretched helical conformation due to the highly branched alkyl group.

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INTRODUCTION Helix is an important asymmetric structure. The formation and regulation of helical conformation play vital roles in the fundamental biological activities of many biomacromolecules such as proteins and genes.1,2 For example, the primary conformation of DNA in aqueous solution is B-form but converts to A-form as the salt or ethanol content is increased.3,4 Such a stimulus responsive conformation transition is utilized by polymerases and endonucleases to optimize their association with DNA to facilitate DNA replication.5,6 Inspired by Mother Nature, the design and synthesis of helical polymers capable of adapting their conformations to external stimuli have attracted considerable interests7−12 for their wide practical and potential applications in chiral recognition,13−15 asymmetric catalysis,16,17 chirooptical switches,18−22 and molecular devices,23−26 among others.27−29 Substituted poly(phenylacetylene)s (PPAs) are a typical type of dynamic helical polymers, which have been comprehensively studied in the past several decades.8,30−32 Many PPAs bearing a single substituent in the phenyl ring are now available. Yashima and co-workers investigated the helicity induction and memory © XXXX American Chemical Society

effect of PPAs through acid−base or host−guest interactions.33−37 These induced helical PPAs have been used to template the formation of supramolecular helical aggregates of achiral porphyrin and cyanine dyes.38,39 Their pioneering works on the visualization of PPA molecules on highly oriented pyrolytic graphite by AFM have advanced the characterization of helical polymers to the molecular resolution level.40,41 Tang et al. systematically studied the conformation modulation and functionalization of various helical PPAs containing amino acid, sterol, saccharide, and nucleoside appendages through macromolecular design and synthetic route elaboration.42−46 It was found that the inter- and intramolecular hydrogen bonds play a vital role in the stabilization and variation of helical structures, which could be tuned by external stimuli such as solvents, pH, temperature, or achiral additives. Some PPAs bearing lysine dendrons or dendritic oligo(ethylene glycol)s by Zhang et al. were reported to show size-selective colorimetric anion sensing Received: September 27, 2016 Revised: October 23, 2016

A

DOI: 10.1021/acs.macromol.6b02116 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Synthetic Route of Monomers and Polymers

ability and low critical solution temperature.47,48 Most of the monosubstituted PPAs take stretched cis-transoid helical conformations in both bulk and solution states due to the strong steric repulsion of neighboring side groups, and only a few of them are contracted cis-cisoid helices.8,30−32 Percec and co-workers reported the transition from the cis-cisoid conformation in Φh,k and Φhio phases to the cis-transoid one in Φh phase in some dendronized cis-PPAs.49−52 Tabata et al. obtained both cis-transoid and cis-cisoid PPAs in solid by varying the polymerization conditions, but no differences were found in UV−vis absorption and 1H NMR spectra once these two kinds of PPAs dissolved in chloroform, probably due to the instability of cis-cisoid conformation in solution.53,54 Moreover, Riguera et al. wonderfully got cis-cisoid and cis-transoid PPAs in different solvents by changing the polar or donor character of the solvents to switch the conformation of the amide and (OC)C−C(−O) bonds.55−57 Based on the interactions of chiral PPAs with different mono- and divalent cations, various chiral nanostructures, such as chiral nanospheres, nanotubes, gels, and toroids, were conveniently obtained.58,59 Despite these marvelous achievements in monosubstituted poly(phenylacetylene)s, the research on the PPAs with two or more substituents in a phenyl ring is rare although such polymers could have more structural variety and function capacity.60−63 The through-space chirality transfer by means of dynamic rotaxane mobility was used to tune the helicity of cis-transoid 3,5-disubstituted poly(phenylacetylene)s.62,64−66 Optically active polymers with distinct main chain structures were prepared through the polymerization of an optically active phenylacetylene derivative bearing a meta-azide residue with a rhodium catalyst followed by the click reaction of the pendant azides or by the click polymerization of the monomer.61 The helix-sense-selective polymerizations of achiral phenylacetylenes having two hydroxyl or N-alkylamide groups at meta positions were demonstrated by using a chiral rhodium catalyst. The obtained polymers showed intense circular dichroism signals in the absorption region of the main chain, suggesting the formation of helical conformations with an excess of screw sense. The chiral helix structures of the formed polymers were cis-cisoid conformations with good stability in toluene at room temperature for a long time because of intramolecular hydrogen bonds but unstable in CHCl3.63,67−71 In the present work, we studied the synthesis, helical structures, and conformation regulation of poly(3,5-disubstituted

phenylacetylenes), aiming to understand how the bulkiness of pendant group and the environmental variations will influence their chiroptical properties. A series of novel 3,5disubstituted phenylacetylenes, rM-1, sM-1 to sM-5, bearing an achiral methoxycarbonyl pendant group and various chiral N-alkylamide pendant groups, were designed and synthesized. The [Rh(nbd)Cl]2-catalyzed polymerization converted these monomers to the corresponding polymers, rP-1, sP-1 to sP-5, with high cis-structure contents. Depending on the structure of pendant groups as well as the nature of solvents and additives, contracted cis-cisoid and frustrated cis-transoid helical conformations were observed, and the reversible transition between them was achieved.

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RESULTS AND DISCUSSION Synthesis and Polymerization. Six monomers, rM-1, sM-1 to sM-5, were synthesized in the present work (Scheme 1). Many studies on the conformation regulation of substituted poly(phenylacetylene)s are involved in the intermolecular hydrogen bonding, which usually lead to the aggregation of polymer chains.42−46 Long alkyl groups in rM-1 and sM-1 were chosen to guarantee the molecular dissolution of their polymers, rP-1 and sP-1, in apolar solvents like THF and chloroform. The structures of chiral alkylamide pendant groups in sM-2 to sM-5 were varied to investigate their effects on the helical conformations and chiroptical properties of the polymers. The monomer synthesis began with dimethyl 5-bromoisophthalate. It was first converted to dimethyl 5-[(trimethylsilyl)ethynyl]isophthalate through Pd-catalyzed Sonogashira coupling reaction and then to 3-methoxycarbonyl-5-carboxyphenylacetylene through partially hydrolysis in the mixture of THF, methanol, and water containing 1.1 equiv of NaOH. The amidation of the resultant acid with the selected chiral amine, in the presence of 1-ethyl-3-(3-(dimethylamino)propyl)carbodiimide hydrochloride (EDC) and 4-(dimethylamino)pyridine (DMAP), yielded the target monomers. Each reaction proceeded smoothly. All the key intermediates and monomers were identified by 1 H NMR and 13C NMR spectroscopy as well as high-resolution mass spectrometry (Figures S1−S21). The polymerizations were carried out in THF at 25 °C by using [Rh(nbd)Cl]2 as the catalyst with a constant monomer/ catalyst molar ratio of 40/1. The bidentate−Rh complex was chosen as the catalyst because it had been proved to produce the stereoregular poly(phenylacetylene)s with high cis-contents. B

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As for sP-1, only the protons of long alkyl chains appear in pure CDCl3, whereas the other protons are badly resolved, making it impossible to analyze its structure (Figure 1b). Given the good solubility of sP-1 in CHCl3 (this will be addressed again in the following section), the poor resolution of its 1H NMR spectrum is probably due to its contracted helix stabilized by strong intramolecular hydrogen bonding. To destroy the hydrogen bonds, 10 vol % trifluoroacetic acid (TFA) was added into the CDCl3 solution. As a result, a much better resolved 1H NMR spectrum was observed (Figure 1c). It is evident that the ethynyl proton signal of sM-1 disappears after polymerization. Instead, a new olefinic proton peak at δ = 5.62 ppm appears from the conjugated main chain. The cis-structure content of sP-1 was estimated as 93% according to the method developed by Percec and co-workers.49−52 Consistent with other [Rh(nbd)Cl]2-catalyzed polyacetylenes, all the resultant polymers have high cis-contents as demonstrated in Table 1 and Figures S24−S27. Figure 2 displays the FTIR spectra of sM-1 and sP-1 measured in CHCl3. sM-1 exhibits absorption bands at 3437 and

After about 30 min, the reaction mixture became very viscous. The polymerizations were allowed to continue for 12 h. The obtained polymers except sP-5 were precipitated in acetone, which is a good solvent of the monomer but a nonsolvent of the polymer. Methanol is used for the precipitation of sP-5. All the monomers were converted to the corresponding polymers, rP-1, sP-1 to sP-5, with high yields and molar masses (Table 1). The resultant polymers are thermally stable Table 1. Polymerization Results and Properties of Polymersa monomer rM-1 sM-1 sM-2 sM-3 sM-4 sM-5

convb (%) 95 82 80 85 94 84c

Mn × 10−4 d

13.7 7.9d −e 23.5f 22.1f 27.2d

PDI d

1.97 2.60d −e 1.61f 1.64f 1.46d

cisg (%)

Tdh (°C)

92 93 −e 96 99 93

396 390 358 367 376 372

a

Carried out in THF at room temperature under nitrogen for 12 h; [M] = 0.06 M, [cat.] = 1.5 mM. bAcetone-insoluble part. cMethanolinsoluble part. dEstimated by GPC in THF against a polystyrene calibration. eInsoluble. fEstimated by GPC in DMF against a polystyrene calibration. gDetermined by 1H NMR analysis. h5% weight loss temperature under nitrogen at a heating rate of 10 °C/min.

and have the temperature of 5% weight loss under a nitrogen atmosphere above 350 °C (Table 1 and Figure S22). The polymers rP-1 and sP-1 are readily dissolved in apolar solvents like CHCl3 and THF, while sP-3 to sP-5 are soluble in polar solvents like DMF. Although sP-2 is very similar in structure to the others, it is insoluble in all the solvents tested. Structural Characterization. The structures of all the polymers except for sP-2 were characterized by 1H NMR and FTIR spectroscopy. Figure 1 shows the 1H NMR spectra of

Figure 2. Solution-state FTIR spectra of monomer sM-1 in CHCl3 (a) and polymer sP-1 in CHCl3 without (b) or with the addition of 1.0 vol % TFA (c). c = 5−10 mg/mL.

3305 cm−1 associated with the N−H and H−CC stretching vibrations, respectively. After polymerization, the H−CC vibration disappears completely, suggesting the full consumption of acetylene triple bond of sM-1. The N−H stretching vibrations shift to 3258 cm−1 in sP-1. Moreover, the amide I absorption (CO stretching) peak of sP-1 in CHCl3 is observed at 1632 cm−1, about 29 cm−1 lower than that of sM-1. The amide II absorption (N−H bending) peak of sP-1 is observed at 1545 cm−1, 30 cm−1 higher than that of sM-1. The strong intramolecular hydrogen bonds within sP-1 are indicated. Optical Rotation. Table 2 summarizes the optical rotations of polymers and monomers measured in CHCl3, THF, and DMF. All the monomers exhibit good solubility in these three solvents and display specific optical rotations [α]20D ranging from −39.8° to +40.7°. Except for sP-2 and sP-3, the polymers present rather larger optical rotations than their corresponding monomers in the same solvents, indicating the formation of chiral secondary structures. For example, sM-1 shows an [α]20D value of +23.1° in CHCl3, whereas sP-1 shows an [α]20D value of −691.2°, implying that the polyene backbone has been induced to twist in a dominant direction by the chiral pendants. Moreover, rP-1 exhibits a positive optical rotation, ascribed to the opposite screw sense of polymer main chain. As for sP-2, it is insoluble in all the common solvents tested, so it is difficult to investigate its optical activity in solution. The low [α]20D value of sP-3 in DMF is probably due to the existence of helical

Figure 1. 1H NMR spectra of monomer sM-1 (a, CDCl3, 400 MHz, 25 °C) and polymer sP-1 in CDCl3 without (b) or with (c) 10 vol % TFA (500 MHz, 50 °C).

sM-1 and sP-1 in CDCl3. The resonance absorptions of sM-1 at δ = 5.95, 3.95, and 3.17 ppm are ascribed to the protons of NH, OCH3, and ≡CH groups, respectively (Figure 1a). C

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Macromolecules Table 2. Chiroptical Properties of Monomers and Polymers [α]20Da (deg)

a

[α]20Db (deg)

monomer

CHCl3

THF

DMF

polymer

CHCl3

THF

DMF

sM-1 rM-1 sM-2 sM-3 sM-4 sM-5

+23.1 −22.3 +22.5 +20.1 +21.3 +21.7

+29.2 −27.7 +26.2 +23.4 +26.8 +25.0

+40.7 −39.8 +38.5 +28.3 +25.1 +11.0

sP-1 rP-1 sP-2 sP-3 sP-4 sP-5

−691.2 +734.1 × × × +1466.4

−622.8 +564.5 × × × +2810.2

× × × +95.7 +695.0 +2311.5

c = 0.2 g/dL. bc = 0.02 g/dL. ×: insoluble.

reversal along polymer main chain as well as the presence of helices with opposite screw senses. UV−Vis Absorption and Circular Dichroism (CD). Figure 3 shows the UV−vis absorption and CD spectra of

Here, the entropic cost may be compensated by the strong intramolecular hydrogen bonds formed between the amide groups in the pendants of the polymer. Namely, the contracted helix could be tuned by controlling the formation of hydrogen bonds. To shed light on this point, UV−vis absorption and CD spectra of sP-1 in CHCl3 containing various content of TFA were recorded. As indicated previously, sP-1 in pure CDCl3 displays a long tailed absorption band and exhibits a conservative Cotton effects at 283 nm and a nonconservative Cotton effects at 360 nm. When TFA was added into the CHCl3 solution, remarkable variations were observed in both UV−vis absorption and CD spectra. With an increase of TFA contents in CHCl3, a new absorption gradually appears at 436 nm, and the CD intensities at 283 and 360 nm gradually decrease as shown in Figure 4. Accompanied by the changes in the UV−vis absorption

Figure 3. UV−vis absorption and CD spectra of sP-1/rP-1 and sM-1/ rM-1 in THF. The spectra were measured in a 10 mm quartz cell at ambient temperature with a concentration of 5.0 × 10−5 mol/L.

sP-1/rP-1 and sM-1/rM-1 in CHCl3. sM-1 and rM-1 exhibit weak absorptions at 300 nm, probably associated with the substituted phenyl rings,42−46 and no absorption beyond 325 nm was observed in the UV−vis absorption spectra. However, intense absorption tails with shoulders at around 283 and 360 nm are observed for sP-1 and rP-1. The former absorption is again ascribed to the substituted phenyl rings,67−70 which blue-shifts due to the steric crowding around polymer main chains, while the latter to the conjugated polyene backbone. The cutoff absorption wavelengths of sP-1 or rP-1 are 430 nm, so short that their CHCl3 solutions are colorless. Moreover, sM-1 and rM-1 are CD-inactive at the wavelength longer than 270 nm. Their polymers, sP-1 and rP-1, however, present the strong conservative Cotton effects at 283 nm and the nonconservative Cotton effects at 360 nm, respectively, with perfect mirror images, indicating again the formation of tight, one-handed helical structures, most probably cis-cisoid helices for stereoregular polyacetylenes, which will be further characterized later. Surprisingly, though sP-1 just possesses one amide group, its contracted helical structure in CHCl3 is much more stable than the cis-cisoid PPAs with two achiral amides at 3- and 5-positions in Aoki’s work.71 No obvious change in the CD and UV−vis absorption spectra of sP-1 in CHCl3 was observed even after a week (Figure S28). sP-1 and rP-1 in THF show the UV−vis absorption and CD spectra similar to those in CHCl3 (Figure S29), suggesting again the existence of a contracted helical structure. Reversible Acid-Induced Chromism. The formation of contracted helical conformation is usually entropically unfavorable.45

Figure 4. UV−vis absorption and CD spectra of sP-1 in CHCl3/TFA mixtures with various compositions.

spectra, the Cotton effects at around 280 and 360 nm completely vanish, while a broad and weak CD peak centered at 450 nm with the opposite sign emerges in the visible spectral region. Bathochromic shifts and increases in intensity have long been known to be associated with increased conjugation. This conjugation is larger for poly(phenylacetylene) with cis-transoid helical structure than for that with cis-cisoid helical structure. Therefore, the observed increased conjugation in the mixed solvent of CHCl3/TFA can be logically ascribed to a more loosely wound cis-transoid helix. Such a helix would allow more motion and give rise to the better resolved proton NMR spectrum. The strong interactions between sP-1 and TFA molecules are proved by the intensive saturation transfer difference (STD) effects (Figure S40).55 When the acid is gradually neutralized by adding a certain amount of triethylamine (TEA)46,72 into the solution, the UV−vis absorption and CD spectra of sP-1 recover completely (Figure 5), revealing a reversible helical conformation transition. D

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Figure 5. (a) UV−vis absorption and CD spectra of sP-1 in CHCl3 with the addition of TFA and TEA. (b) Dependence of absorption and CD intensities of main chain on switching times for sP-1 in CHCl3 with alternate addition of TFA and TEA. (c) Visible change of solution colors with the addition of TFA and TEA in CHCl3. (d) Pictures of solid sP-1 and sP-1TFA, which were precipitated in methanol from CHCl3 and CHCl3/TFA solution, respectively.

Figure 6. UV−vis absorption and CD spectra of sP-1 in CHCl3/HCOOH (a) and CHCl3/AcOH (b) mixtures with various compositions.

Light Scattering. The explanation to the origin of the conformation variation of PPAs is usually complexed by the multiple intermolecular hydrogen bonds, which may induce polymer chains to aggregate.42−46 To exclude this possibility, dynamic light scattering (DLS) and static light scattering (SLS) were employed to investigate the overall dimension of sP-1. Figure 7 presents the size distributions of sP-1 in CHCl3 and the CHCl3/TFA mixture (400/1, v/v), respectively. No matter whether TFA is added or not, sP-1 exhibits a monodistribution at a concentration of 0.5 mg/mL. Moreover, the excessive scattered intensity shows almost no angular dependence. These results imply that sP-1 stays as single polymer chains in these two solvents probably due to the presence of long chiral alkyl chains. The average effective hydrodynamic radius (Rh,app) values are 25 nm in CHCl3 and 7 nm in the mixed solvent. The large decrease in Rh,app means that the addition of TFA lead to the collapse of polymer chains due to the failure of intramolecular hydrogen bonding. Figure 8 shows the angular dependence of It/(Is − I0) of sP-1 in CHCl3. The root-mean-square radius of gyration Rg was determined to be 86 nm. The polymer conformation can be evaluated from the prefactor ρ (= Rg/Rh).

The changes in the absorption spectra of the polymers induced by the alternate addition of TFA and TEA are so obvious that we can see by the naked eye. The chloroform solution of sP-1 is colorless and changes to yellow when TFA is added (Figure 5c). It should be noted that the time required to complete helix− helix transition is different although the above-mentioned acidinduced chromism is reversible. The contracted helix converts to a stretched helix immediately with the addition of 0.05 vol % TFA. However, the reverse transition takes 24 h after adding 0.07 vol % TEA. The increased transition time may be due to the slow formation of intramolecular hydrogen bonds. Such a halochromism greatly relies on the acidity of additives. As shown in Figure 6, the addition of 0.05 vol % HCOOH caused no obvious change in UV−vis absorption and CD spectra. The transition from the contracted helix to the stretched helix was observed only when 0.40 vol % HCOOH was added. As for AcOH, such a transition did not happen even though 4.0 vol % of AcOH was introduced into the CHCl3 solution. Compared to TFA, HCOOH presents weaker acidity and AcOH is the weakest. It may be conclusive that the stronger the acidity is the less amount of acid is required to realize the helix−helix transition. E

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Figure 7. DLS traces of sP-1 in chloroform solution (0.5 mg/mL) with and without addition of TFA.

Figure 9. Reversible transition from single-handed, contracted cis-cisoid to frustrated, stretched cis-transoid helical conformation.

assumed that the precipitation in nonsolvent may quench the structure of sP-1 in the mixed solvent. sP-1 as-obtained from polymerization is platelike red solids, while sP-1TFA is yellow powders (Figure 5d). As depicted in Figure 10, the

Figure 8. SLS result of sP-1 in chloroform solution (0.5 mg/mL).

It is well established in the literature that ρ is larger than 2 for a stiff chain.73,74 In CHCl3 solution, the ρ value of sP-1 was estimated as 3.44. A quite rigid chain conformation was therefore suggested. SLS measurement was also carried out in the mixed solvent. Rg could not be fitted from Zimm plots, possibly ascribed to the weaker scattered intensity and the smaller size. Besides, the main-chain absorption intensities at 360 nm present a linear relationship with the concentrations of sP-1 in CHCl3 (Figure S30), further excluding the formation of aggregates. Therefore, the halochromism in sP-1 is ascribed to the change in polyene conjugated lengths with the formation and destruction of intramolecular hydrogen bonds modulated by acids. Mechanism Scenario. Based on NMR, FTIR, UV−vis, CD, and light scattering results described above, an acid-induced conformation transition is proposed as shown in Figure 9. sP-1 takes a contracted cis-cisoid conformation in pure CHCl3, stabilized by the intramolecular hydrogen bonds. With the induction of the chiral N-alkylamide pendant, the polyene backbones twist in a single-handed way and display strong Cotton effects. The addition of TFA into the CHCl3 solution destroys the intramolecular hydrogen bonds and makes the cis-cisoid conformation unstable. The transition to cis-transoid helical structure is therefore triggered. The much smaller Rh,app value and weaker molar ellipticity in CHCl3/TFA mixture than those in CHCl3 suggest such a structure may have some helical reversals along the main chain and cause the collapse of polymer main chains. The neutralization of TFA with TEA recovers the intramolecular hydrogen bonds and affects the reappearance of contracted cis-cisoid structure. To prove this speculation, sP-1 in CHCl3/TFA mixture was precipitated in methanol and designated as sP-1TFA. It was

Figure 10. Raman spectra of polymers sP-1 and sP-1TFA.

Raman spectrum of sP-1 exhibits two characterized vibration peaks of cis CC bonds at 1625 and 1598 cm−1, assigned to the contracted cis-cisoid main chain and the phenyl groups in a restricted state, respectively. As for sP-1TFA, the cis CC bond vibration which peaked at 1585 cm−1 overlaps with the phenyl ring, indicating the stretched polyene backbone. Furthermore, the selected precipitant does not affect the structure and solid morphology of sP-1. No matter sP-1 was precipitated in acetone or methanol, their Raman spectra (Figure S44) or DSC curves (Figure S46) are almost the same, convincing this viewpoint. Differential scanning curve (DSC) is an efficient method to distinguish cis-cisoid and cis-transoid conformations.49−54 As shown in Figure 11a, sP-1 shows an endothermic peak at 227 °C and an exothermic peak at 240 °C corresponding to the thermal isomerizations from cis-cisoid to cis-transoid and cis-transoid to trans-transoid, respectively, according to the work published by Percec and co-workers.49−52 Probably due to the rigidity of main chain, no phase transition was observed. On the other hand, sP-1TFA undergoes the glass transition at 136 °C. Its DSC trace also presents a strong exothermic peak at 210 °C corresponding to the isomerization from cis-transoid to trans-transoid. The transition temperature from cis-transoid to trans-transoid in sP-1TFA is lower than that in sP-1, possibly due to the lack of intramolecular hydrogen bonding. F

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Figure 11. DSC traces (a) and WAXD patterns (b) of sP-1 and sP-1TFA which were precipitated in methanol.

Figure 12. Relationship between the TFA volume to induce chromism and the THF content in the mixed solvent.

To rationalize this interesting phenomenon, we studied the acid-induced conformation changes of sP-1 in the solvents by NMR spectroscopy. As previously stated, sP-1 exhibits very broad proton resonance peaks in CHCl3. When TFA is added, sharper resonance peaks with fine structures emerge. In a sharp contrast, little change is observed in the NMR spectrum of this polymer in the THF system. That is to say, no signal of the polyene backbone is present in the downfield spectral region even after the addition of TFA (Figure S23). Solution-state FTIR spectroscopy is a good method to monitor the change of intramolecular hydrogen bonds upon environmental variation. As shown in Figure 13, with the addition of 1.0 vol % TFA in CHCl3 solution of sP-1, the N−H

Besides, WAXD patterns of sP-1 and sP-1TFA were measured at room temperature to determine the crystal and geometrical structures. As shown in Figure 11b, sP-1 exhibits a sharp and intensive reflection peak at d = 19.2 Å and two wide and weak peaks at 10.1 and 6.2 Å, respectively. They could be assigned to the (100), (200), and (300) reflections, assuming the formation of columnar tetragonal crystal.53,54 Moreover, it shows a reflection peak at 3.7 Å belong to the π-stacking of the neighbor phenyl rings. It implies that the contracted single-handed helical conformation favors the ordered packing of polymer backbone and the distance between the neighbor phenyl rings is short enough to interact with each other. In other words, the collapsed polymer chain as sP-1TFA would be difficult to pack orderly. This is what has been observed in the WAXD patterns of sP-1TFA, where the reflection at about 3.7 Å disappears and that at 19.2 Å becomes weak and confused. Solvent Effect. One may ask, is the holochromism only observable in chloroform and how will the solvent affect the chromic process? To answer these questions, the effect of TFA on the CD spectrum of sP-1 in THF was first investigated. As indicated previously, the UV−vis absorption and CD spectra of sP-1 in THF are similar to those in CHCl3. However, the addition of TFA into the THF solution caused no change (Figure S31). In the CHCl3/THF mixture (v/v, 90/10), only did the transition from the contracted helix to the stretched one take place when 1.5 vol % TFA was added (Figure S32). Figure 12 shows the relationship between the required amount of TFA to destroy intramolecular hydrogen bonds and the composition of mixed solvent. It seems that the required amount of TFA increases with the added amount of THF, indicating the strong interaction between THF and TFA.

Figure 13. Solution-state FTIR spectra of sP-1 in THF without or with the addition of TFA. c = 5−10 mg/mL. G

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Macromolecules stretching at 3258 cm−1 disappears and amide I absorption shifts to the lower position, attributed to the hydrogen-bond interaction between TFA and amide group. The solution-state IR spectra clearly evince that TFA destroys the intramolecular hydrogen bonds in sP-1, which leads to a stretched helix. sP-1 also presents strong intramolecular hydrogen bonds in THF like in CHCl3. However, after adding 2.0 vol % TFA in THF solution, no obvious change occurs to the IR vibrations of amide group, evincing that no interaction between TFA and amide exists in THF. Like THF, DMF, diethyl ether (Et2O), EtOAc, and acetone are common hydrogen bond acceptors (HBAs), and the HBA ability β follows the order β(DMF) > β(THF) > β(Et2O) > β(EtOAc) ∼ β(acetone).75,76 The required amount of TFA to cause conformation transition is proportional to the HBA ability. The addition of DMF or Et2O into CHCl3 retarded the action of TFA. As shown in Figure 14, about 0.20 vol % of

Chemical Structure Effect. The small modification in chiral alkylamide pendant group was found to remarkably affect the solubility and helical conformation of the polymer. Upon the precipitation from acetone, sP-2, sP-3, and sP-4 were red platelike solids like sP-1, while sP-5 was yellow powders like sP-1TFA. sP-2 is not able to dissolve in all the solvents we tested, while sP-3 to sP-5 are readily dissolved in polar DMF. The intensive long wavelength absorptions with the cutoff at about 550 nm suggest that these polymers have stretched polyene backbones in polar solvent (Figure 15). In the absorption regions of main chain, the three polymers sP-3, sP-4, and sP-5 exhibit Cotton effects attributed to the stretched polyene backbone. However, the strength of Cotton effects distinguishes remarkably. The molar ellipticity of sP-5 at 460 nm reaches 3.0 × 104 deg cm2 dmol−1, and that of sP-4 is only 0.67 × 104 deg cm2 dmol−1 at 440 nm, whereas the CD signal of sP-3 is very weak in the same absorption region. The stretched polymer backbones for sP-3, sP-4, and sP-5 may be rationalized by the strong interactions between the pendant groups and DMF molecules, which cause the polymers to accommondate solvent molecules into the helical structures and disfavor the asymmetric induction chiral pendant groups to polymer backbone. The strong STD effects of sP-3 and sP-4 in DMF-d7 solution support such a speculation (Figures S41 and S42). On the other hand, the increased steric repulsion of β-methyl groups favors the formation of one-handed helical conformation. sP-3 and sP-4 cannot dissolve well in THF or CHCl3 but have a good solubility in the mixture of DMF/THF (v/v, 10/90). In this mixed solvent, sP-3 presents a contracted helical conformation and displays UV−vis absorption and CD spectra similar to those of sP-1 in CHCl3 or THF (Figure 15b and Figure S37). In the identical solvent, however, both stretched and contracted helices seem to coexist along the main chain of sP-4 (Figure 15b and Figure S38). In the polyene backbone absorption region, a strong negative Cotton effect at 360 nm and a weak positive Cotton effect at 438 nm appear. As for sP-5, it exhibits very good solubility in both THF and CHCl3 (Figure S39). The intense absorbance of the main chain at 460 nm accompanied by the strong positive Cotton effects indicates the existence of dominant stretched helix with an excess screw sense. The competition of steric hindrance and hydrogen bonding interaction of pendant groups are considered to account for the difference of sP-3, sP-4, and sP-5 in chiroptical property. All these three polymers are soluble in DMF. Because of the strong interactions with DMF molecules, the polymers can accommodate solvent into the helical structures and favor extended helical conformations. The increased isolation between neighboring pendant groups may weaken the hydrogen bonding

Figure 14. UV−vis absorption and CD spectra of sP-1 in CHCl3 added 0.20 vol % DMF and various volumes of TFA.

TFA is required to realize the conformational transition when adding 0.20 vol % DMF in CHCl3 solution of sP-1. The addition of EtOAc or acetone exerted less influence on role of TFA (Figures S34 and S35). Furthermore, the damaged hydrogen bonds also can recover when DMF or THF is added. As shown in Figures 12a and 14, the stretched helix converts to the contracted helix partially or completely when additional THF or DMF is added, further evincing the interaction between TFA and these HBA solvents. Toluene is neither a hydrogen bond donating nor a hydrogen bond accepting solvent. Similar conformation manipulation to that in THF is expected. As shown in Figure S36, contracted cis-cisoid helix changes to an extended cis-transoid helix with the addition of TFA, suggesting that toluene does not influence the interaction between TFA and amide groups.

Figure 15. UV−vis absorption and CD spectra of sP-3 to sP-5 in DMF (left), THF, and DMF/THF (v/v, 10/90) mixture (right). H

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Macromolecules and disfavor a single-handed helical structure. On this occasion, the steric repulsion of pendant groups has the effect to raise the rotation barriers and stabilize the helical structure. sP-3 has the smallest chiral alkyl groups and the lowest steric hindrance. As a result, it shows very weak optical activity in DMF, probably due to the existence of helical reversals along polymer main chain as well as the mixing of helices with opposite screw senses. The β-methyl group of sP-4 increases the steric repulsion of pendant groups and stabilizes the helical conformation of this polymer. sP-5 has two β-methyl groups and the largest steric repulsion among the three polymers. Therefore, it has the highest optical activity in DMF. However, in THF containing 10 vol % DMF, the hydrogen bonding is less interrupted than in pure DMF. The polymer with weakest steric hindrance, i.e. sP-3, takes singlehanded, contracted helical structure, while sP-4 with medium steric hindrance takes either contracted or extended helical structures. For sP-5, only extended helical structure can form because the strong steric hindrance heavily interrupts hydrogen bonds. Raman spectra (Figure S45), DSC curves (Figure S47), and WAXD patterns (Figures S48−51) were also measured to confirm the structures of sP-2 to sP-5 in solid state.

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AUTHOR INFORMATION

Corresponding Author

*(X.W.) Tel 86-10-62754187, Fax 86-10-62751708, e-mail [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support of the National Natural Science Foundation of China (No. 21274003) and the Research Fund for Doctoral Program of Higher Education of MOE (No. 20110001110084) is greatly appreciated.

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REFERENCES

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CONCLUSIONS We have synthesized a series of optically active helical poly(3,5disubstituted phenylacetylene)s, rP-1, sP-1 to sP-5, bearing an achiral methoxycarbonyl substituent and various chiral N-alkylamide substituents. The effects of the environmental variation and the structure of pendant group on the formation and stabilization of helical structures of such polymers were systematically investigated through a combinatory analyses of 1H NMR, FTIR, Raman, UV−vis absorption, CD, DLS and SLS, DSC, and WAXD. Contracted cis-cisoid helical conformations were formed by sP-1/rP-1 in CHCl3 and THF due to the strong intramolecular hydrogen bonds. The interruption of intramolecular hydrogen bonds by adding TFA into hydrogen bond donating solvent (e.g., CHCl3) induced the immediate transition to frustrated cis-transoid helical conformations, which could recover slowly by the neutralization of TEA. In hydrogen bond accepting solvent (e.g., THF), however, such a halochromism did not happen. The competing interaction of THF with TFA was considered to account for the observed difference in acid-induced chromism. The small modification in chiral alkylamide pendant remarkably influenced the solubility and helical conformation of the polymer. sP-3 and sP-4 were soluble in polar DMF, while sP-5 dissolved in both polar and apolar solvents. However, sP-2 was insoluble in all the solvents tested. Depending on the nature of solvent and additive, sP-3 and sP-4 took either contracted or frustrated helical conformation, whereas sP-5 took only a stretched helical conformation because of the large steric hindrance of highly branched alkyl groups. The interactions of neighboring structure units and the electronic energy levels of polyene backbone between contracted cis-cisoid helix and elongated cis-transoid helix would be different. Taking the advantage of such a reversible helical transition, switchable chiral amplification and novel stimulus-responsive chiral electroptical materials could be anticipated. The related work is under way.

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Materials and instruments, synthetic and polymerization procedures, 1H NMR, 13C NMR, and FTMS of monomers and the related intermediates, 1H NMR of PPAs, CD spectra, UV−vis spectra, Raman spectra, DSC and WAXD patterns, and solution-state FTIR spectra of sP-5 (PDF)

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DOI: 10.1021/acs.macromol.6b02116 Macromolecules XXXX, XXX, XXX−XXX

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