Synthesis of a helical π-conjugated polymer with a dynamic hydrogen-bonded network in the helical cavity and its circularly polarized luminescence properties

Tomoyuki Ikai *a, Seiya Awata a and Ken-ichi Shinohara b
aGraduate School of Natural Science and Technology, Kanazawa University, Kakuma-machi, Kanazawa 920-1192, Japan. E-mail: ikai@se.kanazawa-u.ac.jp
bSchool of Materials Science, Japan Advanced Institute of Science and Technology (JAIST), 1-1 Asahi-dai, Nomi 923-1292, Japan

Received 6th November 2017 , Accepted 5th December 2017

First published on 6th December 2017


An optically active π-conjugated polymer (poly-1) containing glucose-linked biphenyl units in the main chain was synthesized via polymerization using an amide-appended 4,6-diiodothieno[3,4-b]thiophene as a cross-coupling partner. Based on the characteristic chiroptical properties of poly-1, combined with the result of all-atom molecular dynamics simulation, a cooperative intramolecular hydrogen-bonded network among amide pendants was considered to form in the helical cavity and to play a crucial role in the stabilization of the helically folded state. The helical poly-1 efficiently emitted circularly polarized light upon photoirradiation in solution and the film state.


Artificial oligomers and polymers capable of adopting a specific compact conformation owing to cooperative intramolecular interactions, such as a solvophobic interaction, metal coordination and hydrogen bonding, are known as foldamers.1,2 Since the seminal examples reported by Moore and co-workers,3–6 foldamer-related studies have attracted considerable attention in the fields of supramolecular chemistry and polymer chemistry because of the unique features of foldamers, such as the formation of an inclusion complex and the functional switching properties associated with an environment-dependent structural alteration.7 Thus far, a wide variety of foldamers consisting of a number of different structural motifs have been synthesized.7–14

To create novel functional materials containing unique structural motifs in naturally occurring products, we recently synthesized optically active π-conjugated polymers bearing a glucose-linked biphenyl (GLB) unit in the main chain, whose molecular design was inspired by natural saccharides, ellagitannins.15–17 A GLB-based polymer containing thieno[3,4-b]thiophene (TT) units (poly-A in Chart 1) is one of the examples.18 We demonstrated that poly-A behaved as a foldamer and could fold into a preferred-handed helical conformation in a certain exterior environment. When the backbone was folded into a helical structure, the TT-bound pendants were found to be located inside the helical cavity. In addition, the optically active poly-A capable of undergoing a coil-to-helix transition was applied to a chiral stationary phase. However, because the helical folding of the reported GLB-based polymers was mainly attributed to just a solvophobic interaction, helix formation could only be achieved under a specific condition involving a large amount of poor solvents, such as acetonitrile. This situation would limit the application potential of the helical GLB-based polymers because of solubility issues. To expand the helix-forming conditions and exploit the full potential of chiral functions inherent to the GLB-based polymers, an additional driving force for promoting their helix formation is considered necessary.


image file: c7py01867c-c1.tif
Chart 1

In this study, to achieve the stabilization of a preferred-handed helical conformation by enhancing an intramolecular interaction, we synthesized a GLB-based polymer (poly-1 in Scheme 1) containing an amide group at the 2-position of the TT ring, in which an intramolecular hydrogen-bonded network among the amide pendants is expected to form in the helical cavity. The significant effect of the amide pendants on the helix-forming ability of the resulting poly-1 was investigated by a combination of the (chir)optical property measurements under various conditions and all-atom molecular dynamics (MD) simulation.


image file: c7py01867c-s1.tif
Scheme 1 Synthesis of the TT-based monomer 3 (A) and poly-1 (B).

Poly-1 bearing amide pendants on the TT units as hydrogen-bonding sites was synthesized via the copolymerization of GLB-1 and the TT-based monomer (3) with a palladium catalyst, similar to a previously reported method (Scheme 1).18 The number-average molecular weight (Mn) and polydispersity index (PDI) of the obtained poly-1, determined by size-exclusion chromatography (SEC), were 5.0 × 103 and 1.8, respectively. Circular dichroism (CD) and absorption spectra of poly-1 recorded in various solvents at 25 °C are presented in Fig. 1A. A clear solvent dependence was observed for both the optical and chiroptical properties of poly-1. Similar to the previously reported findings for poly-A bearing TT-bound ester units,18,19 the acetonitrile solution of poly-1 showed an intense CD signal in the main-chain chromophore region (350–500 nm). Considering the close similarity of the CD spectral patterns, poly-1 most likely has a left-handed helical structure similar to that of poly-A, and its pendant groups on the TT units are likely located inside the helical cavity. In acetonitrile, a solvophobic interaction between aromatic rings is considered to play a key role in the helical folding. We also observed that poly-1 exhibited a strong CD band even in chloroform, and its pattern was similar to that in acetonitrile. This finding is completely different from the result observed for poly-A, which showed a weak CD signal and did not form a specific higher-ordered structure in a good solvent, chloroform (ESI Fig. S1).18 There are many examples of optically active π-conjugated polymers exhibiting a drastic CD enhancement derived from chiral aggregation.7,20–22 However, the intense CD absorption of poly-1 observed in chloroform was considered to be entirely unrelated to such aggregate formations because the spectral change was hardly observed in the concentration range of 0.01–1.0 mM (ESI Fig. S2). The dynamic light scattering analysis of poly-1 also supports this reasoning (ESI Fig. S3). These results indicate that poly-1 can fold into a preferred-handed helical conformation in chloroform as well as in acetonitrile. We also confirmed that the helical backbone of poly-1 formed in chloroform was mostly maintained even at 55 °C (ESI Fig. S4), judging from the invariance of the CD intensity. Because the only difference between the chemical structures of poly-1 and poly-A is the pendant groups on the TT units (amide or ester), the helical folding of poly-1 in chloroform is expected to originate from the intramolecular hydrogen-bonding interactions among the pendant amide groups. The contribution of the hydrogen bonding to the helix formation in chloroform was also confirmed by infrared measurements; the stretching vibrational band of the carbonyl group of poly-1 in chloroform was shifted to lower energy by approximately 10 cm−1 relative to that of the corresponding TT-based monomer 3 (ESI Fig. S5B). The clear spectral difference between poly-1 and 3 was also observed in the region of the NH stretching bands (ESI Fig. S5A). In view of the orientation of the pendant groups on the TT units in the helically folded state, the intramolecular hydrogen-bonded network among the side-chain amide groups can be considered to form in the helical cavity, as illustrated in Fig. 1B.


image file: c7py01867c-f1.tif
Fig. 1 (A) CD and absorption spectra of poly-1 in various solvents at 25 °C. [Glucose unit] = 1.0 × 10−4 M. (B) Schematic illustration of a folded structure of poly-1 with a hydrogen-bonded network in the helical cavity.

To better understand the stabilization of the helical conformation through the intramolecular hydrogen bonding, an all-atom MD simulation of poly-1 in chloroform at 298 K was conducted using a corresponding 20-mer model (Fig. 2 and ESI Movies S1 and S2). A molecular model possessing a left-handed helical backbone with the torsion angle across the ethyne axes of θ = −134° was used as the initial polymer structure based on the findings of a previous report.18 After an equilibration process, as described in the ESI, the simulation in the NVE ensemble (constant number of atoms, volume and energy) was conducted for 1000 ps as the production run. The molecular models in the final (1000 ps) state are presented in Fig. 2B–E, where the hydrogen bonds between amide pendants are indicated by blue dashed lines.23 The combinations of the NH hydrogen and the carbonyl oxygen forming a hydrogen bond are also highlighted in ESI Fig. S6. The corresponding molecular models in the initial (0 ps) and middle (500 ps) stages are also shown in ESI Fig. S7 and S8, respectively. This simulation demonstrates that appreciable numbers of amide pendants in poly-1 contribute to the formation of the hydrogen-bonded network in the helical cavity and that the multiple N–H⋯O[double bond, length as m-dash]C interactions work not only between the adjacent repeating units but also between the nth and (n + 4 or 5)th repeating units present in adjacent helical turns (ESI Fig. S6). It can also be observed that the hydrogen bonding did not always occur between the exact same pairs of amide pendants; however, some of the hydrogen-bonding pairs newly appeared or disappeared in each moment (ESI Movie S3). These hydrogen bonding dynamics were also confirmed by the time-dependent changes in the interatomic distances between the NH hydrogen and the carbonyl oxygen in the initially formed hydrogen-bonding pairs at 0 ps, as shown in ESI Fig. S9. The number of hydrogen bonds formed in the poly-1 model are plotted as a function of time in ESI Fig. S10. Although there was an increase or decrease in the number of hydrogen bonds, the polymer chain was observed to form more than eight hydrogen bonds on average at a certain moment. This cooperative intramolecular hydrogen-bonded network in the helical cavity most likely allows poly-1 to maintain the helical conformation in chloroform. To obtain further insight into the hydrogen-bonding cooperativity, the molecular-weight dependence of the helix-forming ability in chloroform was investigated using poly-1 with different molecular weights, which were prepared by SEC fractionation (Fig. 3). We observed that if the molecular weight was greater than 5.0 × 103 g mol−1, the (chir)optical properties of poly-1 were almost independent of the molecular weight. This finding means that the intramolecular interaction among approximately 6 repeating units (corresponding to less than two helical turns) is adequate to sufficiently stabilize the helically folded state.


image file: c7py01867c-f2.tif
Fig. 2 (A) Structure of the 20-mer model of poly-1. (B) Top-view and (C) side-view of the molecular model of the helically folded poly-1 in chloroform at 1000 ps in the all-atom MD simulation represented by stick models. The poly-1 backbone is highlighted in purple. (D, E) Simplified molecular models displaying only the TT units in (B) and (C), respectively. The hydrogen bonds are indicated by blue dashed lines. The chloroform solvent molecules are represented by line models in (B) and (C). All the scale bars represent 1.0 nm.

image file: c7py01867c-f3.tif
Fig. 3 SEC traces of as-synthesized poly-1 (Mn: 5.0 × 103 g mol−1, PDI: 1.8) (A) and its fractionated components with different molecular weights (B) (eluent, chloroform; polystyrene standards). (C) Molecular-weight dependence of the CD and absorption spectra of poly-1 in chloroform at 25 °C.

In contrast to the results in acetonitrile and chloroform, poly-1 showed less intense CD signals in dimethylformamide (DMF) and dimethyl sulfoxide (DMSO), accompanied by the disappearance of an absorption peak at approximately 480 nm (Fig. 1A). Judging from the relationship between the main-chain conformations and the CD/absorption patterns of poly-A,18 poly-1 is expected to have a random-coil conformation in DMF and DMSO. This is most likely because effective intramolecular interactions to stabilize the helical conformation, such as hydrogen bonding and solvophobic interactions, did not work in these solvents due to their high polarity and moderate affinity for the π-conjugated backbone.

Photographs of poly-1 in solution under irradiation at 365 nm are presented in the inset of Fig. 4. Poly-1 showed an apparent green emission in solution, which was attributed to its π-conjugated backbone. The fluorescence quantum yields of poly-1 in chloroform, acetonitrile and DMF were determined to be 6%, 3% and 4%, respectively. Considering the chiroptical and photoluminescence (PL) behavior of poly-1, we next investigated its circularly polarized luminescence (CPL) performance. The PL, CPL and dissymmetry factor (glum) spectra of poly-1 in solution are presented in Fig. 4. Here, glum = 2(ILIR)/(IL + IR), where IL and IR are the PL intensities of the left- and right-handed circularly polarized light, respectively. The solvent dependence of the CPL intensity is consistent with the result obtained in the CD spectral analysis; poly-1 exhibited better CPL performances in chloroform and acetonitrile than in DMF. The glum maximum value reached 1.6 × 10−2 in chloroform. To the best of our knowledge, the resulting glum value is the highest value among those of previously reported one-handed helical polymers in molecularly dispersed solutions and non-oriented films.24–34 As anticipated, the CPL properties of poly-A in chloroform were inferior to those of poly-1, reflecting their conformational difference (ESI Fig. S1B). A poly-1 film was also prepared by spin coating its chloroform solution onto a quartz plate. Prior to recording the film-state spectra, we confirmed that the influence of the optical anisotropy on the film was almost completely negligible (ESI Fig. S11). The film-state CPL spectrum of poly-1 is presented in Fig. 4. The emission maximum wavelength in the film state was red-shifted by approximately 50 nm compared with that in chloroform. Thus, the poly-1 film exhibited a yellow emission, most likely because the conjugation length of the main chain was more expanded in the film state through intermolecular π–π stacking interactions. We confirmed that poly-1 exhibited a high glum value of more than 1.0 × 10−2 in the film state as well as in solution.


image file: c7py01867c-f4.tif
Fig. 4 PL (bottom), CPL (middle) and glum (top) spectra of poly-1 in solution ([glucose unit] = 1.0 × 10−5 M) (solid line) and the film state (dashed line) at room temperature. λex = 300 nm. Insets: photographs of the corresponding poly-1 solutions and film under irradiation at 365 nm.

In summary, we have demonstrated that the amide group on the TT ring significantly promoted the helical folding of the GLB-based polymer. Experimental and computational investigations revealed that when poly-1 folded into a left-handed helical structure in chloroform, multiple N–H⋯O[double bond, length as m-dash]C hydrogen bonds were formed in its helical cavity. This dynamic hydrogen-bonded network is considered to be of key importance for stabilizing a helical conformation. We also observed that the helically folded poly-1 exhibited an excellent CPL with a green and yellow color in solution and the film state, respectively, and that the glum value reached greater than 1.0 × 10−2 in both states. Considering the outstanding features of the investigated polymer, such that the pendant groups on the TT units are arranged in the helical cavity, we believe that new GLB-based polymers capable of being applied for chiral sensing and asymmetric catalysis can be developed through an appropriate design of the TT units. Related work is currently in progress and will be reported in due course.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Japan Society for the Promotion of Science (JSPS) KAKENHI Grants-in-Aid for Scientific Research (C), Grant No. 17K05875 and the Ogasawara Foundation for the Promotion of Science & Engineering. Computation time was provided by the supercomputer system, Research Center for Advanced Computing Infrastructure, JAIST.

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Footnote

Electronic supplementary information (ESI) available: Detailed experimental procedures, characterization of monomers/polymers and additional spectroscopic and computational data. See DOI: 10.1039/c7py01867c

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