Diacetylene Complexes of Tridentate - ACS Publications - American

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Helical Self-Assembly and Photopolymerization Properties of Achiral Amphiphilic Platinum(II) Diacetylene Complexes of Tridentate 2,6Bis(1-alkylpyrazol-3-yl)pyridines Yongguang Li,†,‡ Keith Man-Chung Wong,‡,§ Hok-Lai Wong,‡ and Vivian Wing-Wah Yam*,†,‡ †

Lehn Institute of Functional Materials, School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, P.R. China ‡ Institute of Molecular Functional Materials (Areas of Excellence Scheme, University Grants Committee, Hong Kong) and Department of Chemistry, The University of Hong Kong, Pokfulam Road, Pokfulam, Hong Kong § Department of Chemistry, South University of Science and Technology of China, 1088 Xueyuan Blvd., Shenzhen 518055, P.R. China S Supporting Information *

ABSTRACT: Amphiphilic platinum(II) diacetylene complexes of the 2,6bis(1-butylpyrazol-3-yl)pyridine pincer ligand were designed and synthesized. Helical fibrous nanostructures were obtained through supramolecular assembly of the achiral platinum(II) diacetylene complexes via intermolecular hydrogen bonding, amphiphilic effects, Pt···Pt interactions, and π−π stacking interactions. In situ post-photopolymerization of the diacetylene unit was shown to occur in the preorganized helical fibers.

KEYWORDS: diacetylene, platinum, helical, self-assembly, photopolymerization

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interactions in both solid and solution states.20−24 The study on the self-assembly properties and the corresponding spectroscopic features of platinum(II) complexes has attracted much attention, and the interesting morphologies of their selfassembled nanostructures have also been systematically investigated in recent years.25−35 On the contrary, the use of achiral square-planar platinum(II) complexes as planar motifs for the unique construction of chiral supramolecular aggregates has been less explored.36−38 In this work, achiral amphiphilic platinum(II) complexes of tridentate 2,6-bis(1-butylpyrazol-3yl)pyridine were designed and explored as versatile building blocks for the exploration of their helical self-assembly properties. On the other hand, polydiacetylenes (PDA) are well-known to exhibit a diverse array of potential applications in chemistry, biology, and materials science.39−41 Amphiphilic diacetylenes are known to undergo self-assembly into a variety of forms which can be modified to incorporate ligands and substrates for detection applications and photopolymerized to generate PDA in situ. So far, self-assemblies of amphiphilic diacetylenes as well as their photopolymerization properties have been systemati-

INTRODUCTION Helical morphology is a rather common and fascinating phenomenon in nature. One of the most famous examples is the double helix formed by DNA. Inspired by nature, there has been a strong interest in the design of molecules that can form helical assemblies at the nanoscale by noncovalent interactions,1,2 for instance, hydrogen bonding, hydrophobic− hydrophobic interactions, π−π stacking, electrostatic interactions, metal−ligand coordination, and van der Waals forces. Generally, construction of helical structures is achieved by the controlled assembly of intrinsically chiral or achiral species in the presence of chiral constituents.3−6 Interestingly, in some specific cases, completely achiral molecules with unique flat and π-conjugation systems or multicomponents could also be capable of self-assembling into helical nanostructures.7−19 Recently, potential applications such as those of chiral recognition, sensing, catalysis, optics, electronics, and chiroptical switches derived from functional supramolecular chiral aggregates have also attracted much attention due to the emergence of new properties that are not commonly found in small chiral molecules.1,5,12,14,18 Due to their square-planar structure, platinum(II) complexes of tridentate π-acceptor ligands are known to exhibit excellent spectroscopic properties and possess a high tendency to form supramolecular aggregates through Pt···Pt and π−π stacking © XXXX American Chemical Society

Received: March 9, 2016 Accepted: June 13, 2016

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DOI: 10.1021/acsami.6b02840 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

ACS Applied Materials & Interfaces

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cally studied;42−49 however, little has been reported on the helical self-assembly from completely achiral diacetylenes.50−53 Herein, we describe the design and synthesis of square-planar platinum(II) diacetylene complexes of tridentate 2,6-bis(1butylpyrazol-3-yl)pyridine ligands (Scheme 1), their supra-

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RESULTS AND DISCUSSION

Synthesis, Characterization, X-ray Crystal Structures, and Photophysical Properties. The alkynylplatinum(II) diacetylene complexes (1 and 3−6) were prepared by the reaction of chloroplatinum(II) precursors with the respective alkynes according to the previously reported method.54 Complex 2 was prepared by deprotection of the trimethylsilyl group in 1 by using K2CO3 in methanol and tetrahydrofuran. All of the complexes were characterized by 1H NMR spectroscopy, fast atom bombardment (FAB) mass spectrometry, and elemental analyses. The crystal packing of the complex cation of 1 is shown in Figure 1. The crystallographic and structural refinement data are given in Table S1. The platinum(II) atom has a distorted square-planar geometry, and the N−Pt−N bond angles show deviations from the theoretical values of 90° (78.1° and 79.0°) and 180° (157.1°), respectively, due to the steric demand of the 2,6-bis(1-butylpyrazol-3-yl)pyridine ligand (Table S2). The crystal packing diagram shows that the cationic molecules are stacked in a partial head-to-head configuration with alternating Pt···Pt distances of 3.666 and 4.873 Å. Due to the presence of a bulky trimethylsilyl group, the platinum planes in two complex cations are laterally shifted with the interplanar distance of 3.472 Å, indicating the presence of a weak intermolecular π−π stacking and Pt···Pt interactions in the crystal lattice.

Scheme 1. Structures of Complexes 1−6

molecular assembly properties, and the study of their in situ photopolymerization properties in the preorganized helical nanofibers in mixed solvents through hydrogen bonding, amphiphilic effects, π−π stacking interactions, and Pt···Pt interactions. These may provide insight into their potential applications in materials and polymer science.

Figure 1. (a) Perspective drawing of the complex cation of 1. Hydrogen atoms and counter anions are omitted for clarity. Thermal ellipsoids are shown at the 30% probability level. (b) Dimeric structure. (c) Crystal packing diagram of the complex cations showing a partial head-to-head configuration. B


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Figure 2. (a) Electronic absorption spectra of complexes 1−6 in MeCN. (b) Normalized emission spectra of complexes 1 and 2 in a degassed MeCN solution at room temperature.

Figure 3. Photographs (a) before and (b) after UV irradiation of complex 6 (4.8 × 10−5 mol dm−3) in THF-H2O (1/9 v/v) at room temperature. (c) Time-dependent electronic absorption spectra upon UV irradiation of complex 6 (4.8 × 10−5 mol dm−3) in THF-H2O (1/9 v/v) at room temperature. (d) Emission spectra (1) after UV irradiation at room temperature and (2) before and (3) after UV irradiation of complex 6 (4.8 × 10−5 mol dm−3) in THF-H2O (1/9 v/v) at 77 K. (e) CD spectra before and after UV irradiation of complex 6 (4.8 × 10−5 mol dm−3) in THF-H2O (1/9 v/v) at room temperature.

Strong absorption bands at 270−332 nm and moderately intense absorption bands at 357−410 nm have been observed for the complexes in MeCN at 298 K (Figure 2a). According to the previously reported platinum(II) system,54 the relatively higher energy absorption bands with extinction coefficients (ε) of the order of 104 dm3 mol−1 cm−1 are assigned to [π → π*] intraligand (IL) transitions of the alkynyl ligands and the 2,6bis(1-alkylpyrazol-3-yl)pyridine pincer ligands, whereas the much lower energy, moderately intense absorption bands are tentatively assigned as an admixture of [dπ(Pt) → π*(2,6bis(1-alkylpyrazol-3-yl)pyridine)] metal-to-ligand charge transfer (MLCT) and [π(CC) → π*(2,6-bis(1-alkylpyrazol-3yl)pyridine)] ligand-to-ligand charge transfer (LLCT) transitions.54,55 These assignments have been further substantiated by the observation of two rather distinct bands in complexes 3−6 in the low-energy electronic absorption region. The slightly higher energy absorption band at ∼360 nm appears to be relatively less sensitive to the ancillary substituents on the phenylethynyl ligand, which originates predominantly from the spin-allowed MLCT transition. However, the relatively lower energy absorptions in the region of ∼390−450 nm are more

apparent in the hydroxyl- and amino-substituted phenylethynyl complexes, indicating the predominant involvement of an LLCT character. Complexes 1 and 2 were found to exhibit luminescence in a degassed MeCN solution at 298 K (Figure 2b). The large Stokes shift and the long emission lifetimes (2.2 and 2.4 μs for 1 and 2, respectively) in the microsecond range suggest that they are coming from triplet excited states. Therefore, the emissions are assigned to a mixture of the [dπ(Pt) → π*(2,6-bis(1-alkylpyrazol-3-yl)pyridine)] 3MLCT excited state and the 3[π(CC) → π*(2,6-bis(1-alkylpyrazol3-yl)pyridine)] LLCT excited state origins, similar to that of previously reported alkynylplatinum(II) complexes.54,55 In contrast, complexes 3−6 with ester/amide groups as well as much longer alkyl chains on the alkynyl ligands were nonemissive in degassed MeCN solution. Because previous work has demonstrated that phenylethynylplatinum(II) 2,6bis(1-alkylpyrazol-3-yl)pyridine complexes are emissive in degassed MeCN solutions at 298 K,55 it is likely that the floppiness of the long alkyl chains decorated on complexes 3−6 would lead to efficient nonradiative decay, resulting in dissipation of the emissive excited-state energy. It is reasonable C


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Figure 4. SEM images of complex 6 (a) before and (b) after UV irradiation, and TEM images of complex 6 (c) before and (d) after UV irradiation in THF-H2O (1/9 v/v, 4.8 × 10−5 mol dm−3).

that an energetically accessible or lower lying nonemissive 3 LLCT excited state, due to a relatively higher lying pπ(C CR) orbital brought about by the ester/amide substituent, would quench the 3MLCT excited state. Photopolymerization Property. To force the molecules into close proximity and to align and preorganize them for possible photopolymerization studies, a poor solvent such as water was added to the solutions of the complexes in THF. Complex 6 was found to form a stable and clear solution in THF-H2O (1/9 v/v) and can be kept for more than half a year without any precipitate formation. On the contrary, the THFH2O (1/9 v/v) solutions of complexes 1−5 became opaque, and precipitate started to appear in a day. Therefore, complex 6 was chosen for the subsequent studies. The electronic absorption spectrum of complex 6 in the THF-H2O (1/9 v/ v) mixed solution at 298 K shows strong absorption bands at 264, 296, 318, and 334 nm and a moderately intense absorption band at 390 nm. Given the fact that the free ligand (L4) also shows strong absorptions at 258 nm (Figure S1c), the highenergy intense absorption bands of complex 6 are assigned as the [π → π*] IL transitions of alkynyl ligands and 2,6-bis(1butylpyrazol-3-yl)pyridine pincer ligands, whereas the lowenergy, moderately intense absorption band originated from an admixture of [dπ(Pt) → π*(2,6-bis(1-butylpyrazol-3-yl)pyridine)] MLCT and [π(CC) → π*(2,6-bis(1-butylpyrazol-3-yl)pyridine)] LLCT excited states. At dilute concentrations, the absorption band with the lowest energy appears at ∼390 nm, while an absorption tail beyond 450 nm grows with increasing concentration (Figure S1a). Violation of Beer’s law at 450 nm (Figure S1b) indicates that the ground state complex would aggregate in concentrated solutions. The electronic absorption at λ > 450 nm probably originates from a metal−

metal-to-ligand charge-transfer (MMLCT) transition as a result of intermolecular π−π stacking and Pt···Pt interactions in a relatively concentrated THF-H2O mixed solvent. On prolonged irradiation at λ = 254 nm, the color of complex 6 in the THF-H2O (1/9 v/v) mixed solvent changes from colorless to red (Figures 3a and b). Two new absorption bands near 488 (yellow phase) and 532 (red phase) nm, which are attributed to two different backbone conformational states,56 appear in the THF-H2O mixed solvent at room temperature (Figure 3c), signifying that an efficient photopolymerization of complex 6 has occurred to form the ene-yne conjugated PDA.57 The newly appeared absorption bands are assigned as the π → π* electronic transition. The intensity of the absorption is found to reach its maximum within 3 h with a slight red shift and a color change from colorless to red (Figures 3a and b). The as-prepared red PDA solution has also been subjected to temperature-dependent electronic absorption studies. The intensity of the absorption bands near 488 and 532 nm decreased, and the bands are slightly blue-shifted in energy when the temperature increases from 25 to 75 °C. In addition, a vibronic shoulder appears at about 565 nm with an increase in absorbance at 390 nm, which is assigned as the MLCT absorption band (Figure S2), suggesting that the platinum(II) complex moieties are not close to each other at elevated temperatures, giving rise to a decrease in the extent of aggregation. During the cooling process, the intensity of the MLCT absorption band decreased, while the absorption bands corresponding to the yellow phase (488 nm) and red phase (532 nm) together with the MMLCT absorption band increased.58 This is further supported by the negligible change in the temperature-dependent electronic absorption spectra for the corresponding diacetylene-containing alkyne L4 (Figure D


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fibers does not have anobvious effect on the alteration of the macroscopic morphologies. Similar to that in the THF-H2O (1/9 v/v) mixed solvent, complex 6 also shows helical aggregation behavior and polymerization properties in MeCN, i-PrOH, DMF, MeOH, and DMSO-H2O (1/9 v/v) mixed solvents (Figures S2, S8, and S9). Although the chirality of the nanofibers prepared from complex 6 was produced by chance, the supramolecular chirality is sensitive to temperature and shows chiral memory after the supramolecular chirality was formed. The intensities of the CD peaks decrease and finally disappear as the temperature was increased from 25 to 75 °C. It was also found that the complex does not possess photopolymerization properties at 75 °C because no new absorption bands or color changes appeared after irradiation at λ = 254 nm for more than 1 h. However, during the cooling process, CD signals reappeared gradually and were recovered in the same handedness after the complex was cooled back to 25 °C (Figure S10), indicating formation of helical nanostructures. It is likely that the heating and cooling processes allow the molecules to undergo different degrees of supramolecular assembly, which would lead to changes in the CD spectra. However, as the changes may not follow the same path every time, the spectral changes may not be completely recovered. The recovered aggregates show similar polymerization properties upon UV irradiation at 254 nm with new absorption bands appearing at 488 and 532 nm and color changes from colorless to red. The probable reason is that the noncovalent interactions involving the intermolecular hydrogen bonding, Pt···Pt interactions, π−π stacking, and hydrophobic−hydrophobic interactions are weakened and destroyed as the temperature increases, leading to deaggregation of the self-assembled nanofibers and silencing of the CD signal. The system lacks preorganized aggregates at distances that match the repeat distances required for the topochemical 1,4-polymerization of diacetylene. The helical structure undergoes reassembly gradually via reformation of the noncovalent interactions when it is allowed to cool back to 25 °C. The retention of the handedness of the supramolecular chirality, though with incomplete intensity recovery, may result from the presence of residual seeds which provide the template for the subsequent formation of the helical nanostructures (Figure S6a).59,62,63 To provide a systematic investigation of the formation of the supramolecular chirality, all other complexes 1−5 were also subjected to the study of their polymerization properties and aggregation behaviors. However, none of them could be polymerized or could form well-organized nanostructures with the observable chirality under CD spectroscopy (Figures S11 and S12). The probable reason for the lack of polymerization properties in complexes 1−3 may be attributed to the absence of hydrogen bonding as well as the presence of the relatively bulky trimethylsilyl group in complex 1,64 resulting in much larger intermolecular spacing and misalignment of the complexes and preventing polymerization of the diacetylene motifs, which require appropriate intermolecular spacing as well as preorganization for the topochemical reaction to occur. Although complexes 4 and 5 contain an amide group attached to the diacetylene moiety, the noncovalent interaction is probably not strong enough for the formation of well-organized intermolecular structures. It is noteworthy that the fine distinction between the structures of 5 and 6 is that an amide group is present in 6 whereas an ester group is present in 5. This leads to quite different polymerization properties and

S3). However, the electronic absorption spectrum does not completely recover back to its original state upon cooling to 25 °C, and the newly appeared vibronic shoulder at about 565 nm had no obvious changes (Figure S2). The lack of complete recovery indicates that the extent of π-conjugation along the polymer backbone probably changed upon thermal annealing. Complex 6 was emissive at 532 nm in the THF-H2O (1/9 v/v) mixed solvent at 77 K (Figure 3d). After prolonged irradiation at λ = 254 nm, the PDA exhibited well-resolved vibronicstructured emission bands at 524 and 561 nm (Figure 3d) at room temperature and 77 K, respectively. On the basis of the emission results at room temperature and at 77 K as well as the related studies on the emission of red PDA,43 the new emission band of the complex after UV irradiation is believed to originate from red PDA with the typical vibronic-like split band pattern of the emission separated by about 1258 cm−1, resulting from the 1Bu and 2Ag states.48 On the contrary, an emission origin of an admixture of the [dπ(Pt) → π*(2,6-bis(1-alkylpyrazol-3yl)pyridine)] 3MLCT and [π(CC) → π*(2,6-bis(1-alkylpyrazol-3-yl)pyridine)] 3LLCT excited states was assigned at 77 K before UV irradiation. After polymerization, emission bands typical of red PDA and the [dπ(Pt) → π*(2,6-bis(1alkylpyrazol-3-yl)pyridine)] 3MLCT/[π(CC) → π*(2,6bis(1-alkylpyrazol-3-yl)pyridine)] 3LLCT excited states become apparent. Helical Self-Assembly Studies. It is interesting to note that the aggregates of complex 6 in the mixed THF-H2O (1/9 v/v) solution are shown to be circular dichroism (CD) active with band maxima at approximately 258, 319, 333, and 450 nm (Figures 3e and S4), suggesting that the aggregates show supramolecular chirality probably due to the symmetry breaking process. The supramolecular chirality obtained from the self-assembly of achiral complexes is expected to be random following a statistical distribution.59−61 Interestingly, the supramolecular aggregates show a higher preference for one handedness over another under our experimental conditions according to the CD results (Figure S5) and the morphologies from scanning electron microscopy (SEM) images (Figures S6a and b); for example, there is a higher number of incidences where certain handed forms are detected more than the other handed forms using the conditions we used for the sample preparation. The reasons for this are unknown, and further studies will be made in the future. The in situ polymerized PDA solution was also investigated by CD spectral studies. The CD spectra show new Cotton effects at 488 and 532 nm after polymerization (Figure 3e), suggesting that the chiral information was transferred to the polymerized PDA moieties.59,61 To further investigate their supramolecular assembly properties, SEM and transmission electron microscopy (TEM) were performed on complex 6 before and after irradiation to study its morphological changes. In line with the CD results, complex 6 without chiral constituents was shown to self-assemble into nanoscale helical fibers, as shown in Figures 4 and S6. Helical nanofibers were formed in both the monomer and polymer assemblies. Helical structures of widths in the range of 12−60 nm were observed. The helical nanofibers were singular (Figure S6b), twisted with each other (Figure S6c), or entangled with others (Figures S6a and d). There were no detectable changes in the diameters and morphologies of the nanofibers before and after irradiation according to the SEM, TEM, and dynamic light scattering (DLS) results (Figures 4 and S7), further indicating that the in situ topochemical reaction happening inside the E


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Figure 5. (a and b) Two possible schematic models of molecular packing. (c) Schematic drawing of a helically stacked array of cations of complex 6 driven by intermolecular double hydrogen bonding, hydrophobic−hydrophobic interactions, Pt···Pt interactions, and π−π stacking interactions.

the diacetylene moieties for polymerization.65 The 1H NMR spectrum reveals that complex 6 undergoes the aggregation process in the mixed solvents. Well-resolved proton signals with chemical shifts and splitting patterns consistent with the chemical formulation are observed in DMSO−d6. The chemical shifts move upfield, and the signals corresponding to the pyridine and pyrazole moieties become broad, suggestive of the presence of molecular interactions and aggregate formation in the self-assembly process in DMSO−d6-D2O (1/9 v/v) mixed solvent (Figure S14). Cross peaks between the pyrazole and pyridine proton signals with NOE interactions were observed, suggesting the possible adoption of a head-to-head packing conformation in mixed solutions. This is further supported by fact that the electronic absorption band at ∼450 nm suggests the presence of intermolecular π−π stacking and Pt···Pt interactions in the system (Figure S1). When the temperature was increased, the absorption tail at ∼500 nm gradually disappeared, indicating that the aggregates may undergo a deaggregation process (Figure S15).28 Thus, a head-to-head

morphologies (Figures 3, 4, and S11), suggesting that the double amide groups also play an important role in the formation of well-organized helical nanostructures. The morphology of the corresponding diacetylene-containing alkyne was also studied by TEM to understand the mechanism of the formation of helical nanostructures of complex 6. The alkyne derivative formed only ribbon-like nanostructures (Figure S13), suggesting that the Pt···Pt and π−π interactions associated with the aggregation behavior of the square-planar platinum(II) moieties are crucial in the formation of the helical nanostructures in addition to the double hydrogen bonding that was discussed (vide supra). Square-planar platinum(II) complexes of tridentate N-donor pincer ligands usually prefer a head-to-tail packing model.20−22 If the head-to-tail form is adopted in this system (Figure 5a), the rigid topo-polymerization of diacetylenes would not occur in this situation because the spacing of the adjacent diacetylenes would largely deviate from the ideal spacing (4.9 Å) for repeat distances and 3.4 Å for the neighboring C(1)−C(4) atoms of F



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packing model is assumed and proposed in the scheme in Figure 5b. In the formation of such a supramolecular helical array of monomers, intermolecular hydrogen bonding together with Pt···Pt and π−π stacking interactions act cooperatively and generate a different helical sense by chance through symmetry breaking. A small helical sense in the initial stage may yield a considerably larger excess of helical sense, resulting in a macroscopic supramolecular chirality.50,61 It is likely that in a water/organic solvent (9/1 v/v) mixed media, the complex molecules come close to each other as a result of the aggregation of the poorly solvated hydrophobic moieties, forming a rather hydrophobic microenvironment and thus facilitating the formation of double hydrogen bonds between the amide groups on the alkyl chains with the positively charged square-planar platinum(II) moieties adopting a headto-head conformation that is directed toward the hydrophilic environment and stabilized by Pt···Pt and π−π stacking interactions. This has been supported by the 1H−1H NOESY spectra, where cross-peaks between the pyrazole proton (Hb) and the pyridine signals (Hd) with NOE interactions are evident (Figure S14), suggesting the formation of a slipped head-to-head J-aggregate-like stacking configuration of lower energy that leads to the formation of helical nanostructures (Figure 5c) with high stability properties that remain in solution in the mixed solvent system for at least half a year. The in situ polymerization of diacetylenes occurs in the preorganized nanofibers. X-ray diffraction measurements were also used to evaluate the nanostructures of the supramolecular assembly (Figure S16). According to Bragg’s equation, the dspacing of the fibers is estimated to be 5.8 nm, whose length is shorter than twice the length of a single molecule (2l = 6.4 nm), suggesting the formation of a bilayer structure in an interdigitated fashion of the alkyl chains, consistent with the proposed stacking model.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

V.W.-W.Y. acknowledges support from The University of Hong Kong under the URC Strategic Research Theme on New Materials and the Sun Yat-Sen University. This work was supported by the University Grants Committee Areas of Excellence Scheme (Grant AoE/P-03/08) and the General Research Fund (GRF) (Grant HKU17304715) from the Research Grants Council of Hong Kong Special Administrative Region, China. Y.L. acknowledges support from the National Natural Science Foundation of China (Grant 21503284). Dr. A.Y.-Y. Tam and Dr. S.Y.-L. Leung are gratefully acknowledged for helpful discussions. Dr. Z.W. Wei and Mr. K. Wu in the Lehn Institute of Functional Materials are gratefully acknowledged for technical assistance in the crystal structure determination.

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CONCLUSION In conclusion, platinum(II)-containing diacetylene complexes with tridentate N-donor pincer ligands were designed and synthesized. The use of mixed solvents was demonstrated to force the complex molecules into close proximity, while the double hydrogen bonding serves to align and preorganize the complex molecules to adopt a head-to-head packing to facilitate the topological photopolymerization of the diacetylenes to give PDA assemblies. Helical fibrous nanostructures were obtained through supramolecular assembly of the achiral platinum(II) diacetylene complex via intermolecular hydrogen bonding, amphiphilic effects, and Pt···Pt and π−π stacking interactions. In situ post-photopolymerization of the diacetylene unit was shown to give organometallic platinum(II) polymers in the preorganized helical fibers. These results may provide important insights into the potential applications of the complexes in materials and polymer chemistry.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b02840. Experimental details, electronic absorption spectra, CD spectra, DLS results, XRD data, 1H−1H NOESY spectra, SEM images, TEM images (PDF) X-ray crystallography (CCDC 1432216) (CIF) G


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