Reversibly Thermochromic Cyclic Dipeptide Nanotubes - Langmuir

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Reversibly Thermochromic Cyclic Dipeptide Nanotubes Min Jeong Seo, Jisoo Song, Chandra Kantha, Mohammed Iqbal Khazi, Umesha Kundapur, Jung-Moo Heo, and Jong-Man Kim Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00743 • Publication Date (Web): 22 Jun 2018 Downloaded from http://pubs.acs.org on June 25, 2018

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Reversibly Thermochromic Cyclic Dipeptide Nanotubes Min Jeong Seo,†§ Jisoo Song,†§ Chandra Kantha,‡ Mohammed Iqbal Khazi,‡ Umesha Kundapur,‡ Jung-Moo Heo,† and Jong-Man Kim*,†,‡ †

Department of Chemical Engineering, Hanyang University, Seoul 04763, Korea.

‡

Institute of Nano Science and Technology, Hanyang University, Seoul 04763, Korea.

KEYWORDS. cyclic dipeptide, polydiacetylene, nanotubes, thermochromism

ABSTRACT. Owing to the capability of forming extensive hydrogen bondings and the facile introduction of chirality, cyclic dipeptides (CDPs) have gained great attention as scaffolds for functional supramolecules. Surprisingly, introduction of a photopolymerizable diacetylene (DA) moiety to the CDP afforded nanotubular structures with enhanced stability and reversible thermochromism. A series of CDP-containing diacetylenes (CDP-DAs) are prepared by coupling 10,12pentacosadiynoic acid with cyclic dipeptides, cyclo(-Gly-Ser) and cis/trans cyclo(-Ser-Ser). Fabrication of CDP-DA self-assemblies in polar chloroform and methanol solvent mixture affords nanotubes comprising single-wall and multi-wall structure. Self-assembly behavior and morphology characteristic are examined by SEM and TEM. Next, XRD analysis confirms wellordered lamellar structures with a perfect agreement with the bilayers formation leading to the tubular structure via lamellar scrolling behavior. Upon UV irradiation, monomeric CDP-DA tubular assemblies result in the blue-colored CDP-polydiacetylene (PDA) nanotubes. Interestingly, CDP-PDA nanotubes exhibit a reversible blue-to-red color change for over ten

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consecutive thermal cycles. The CDP-DA/PDA supramolecular system demonstrates the potential application in developing stimulus-responsive functional materials.

INTRODUCTION The exploitation of molecular self-assembly processes using cyclic dipeptides (CDPs) is a very attractive strategy for the design and creation of ordered and hierarchical functional nanoarchitectures.1-9 CDPs are the simplest class of cyclic peptide, which can be generated by cyclisation of two amino acids, and they characteristically contain 2,5-diketopiperazine skeleton. Extensive studies and mechanistic insights of CDP-driven assemblies have allowed for the harnessing of the self-assembly process for various nano- and bio-technological applications. CDP possesses several meritorious features compared to its linear peptide counterparts. It is often more stable to enzymatic degradation and its conformationally constrained heterocyclic structure affords structural rigidity useful for effective self-assembly. Most importantly, its cisamide functionalities (with two H-bonding acceptors and donors each) allow intermolecular hydrogen bonding between adjacent CDP molecules, which foster the construction of higher order supramolecular architecture. These hydrogen-bonded molecular self-assemblies can be selectively controlled by precisely designing amino acid precursors or CDP substituents. In recent years, the diversity of design strategies explored by combining with smart functional scaffolds has resulted in an extended library of nanoscale morphologies, including molecular gels,10-16 spheres,17 fibers,18 tubes,19 helices20 in nano- and micro-scale. In addition to structural variety, a wide range of stimuli-responsive properties such as thixotropic,10,11,16,21,22 photoresponsive,16 thermo-responsive10-12,21,22 properties have been achieved at various levels. Combining CDPs with other functional moieties such as azobenzene16 and naphthalenediimide20


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expanded the potential of CDPs as promising candidates for developing a new supramolecular architecture with structural diversity and functional versatility. Polydiacetylenes (PDAs) are intriguing conjugated polymers that have meritorious features of self-assembly and extended π-conjugation.23–32 PDAs are intrinsically self-organized supramolecular architectures which can be prepared by polymerization of self-assembled diacetylene (DA) monomers. Owing to the existence of extended delocalized π- electron networks, PDAs typically absorb visible light. Interestingly, a colorimetric transition (typically blue-to-red) can be triggered by various stimuli such as heat, solvent, and molecular recognition and this has been the basis for the design of PDA-based colorimetric sensors.33-47 Following the principles of supramolecular chemistry, DAs are incorporated into various molecular selfassemblies driven by specific non-covalent interactions such as π-π stacking, ionic bonds or aromatic interactions, hydrophobic interactions and extended hydrogen bond networks. When coupled with the judicious choice of functional groups, structural cohesion and stability of the PDA assemblies along with various synergistic effects such as chromic or fluorescence shift can be achieved. Although tremendous progress has been made with CDPs and PDAs for generating supramolecular

architectures

that

display

specific

assembly-related

properties

(both

morphological and electronic), the scope of CDP-PDA hybrids has not yet been explored. For instance, there are no previous examples in which CDP and PDA are either combined in a single molecular framework or as a hybrid mixture/capsulation to form supramolecular assemblies. In this context, we envision to extend the scope of CDP-PDA hybrids by constructing a system which combines both CDP and PDA in a single molecular framework to explore the additive structural and functional properties. It should be noted that one elegant design of PDA linked to


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macrocyclic tetra-β-peptide has been reported, which utilizes hydrogen-bond assisted columnar stacking of macrocyclic peptide structure to generate tubular morphology.

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The nanotubes

formed by this concept are non-responsive in nature. In addition, the self-assembly process forming into nanotubes and tubular characteristic significantly differ from our present concept of CDP-PDA nanotubes. As part of our ongoing efforts for the development of PDA-based smart materials,49-56 we now report the fabrication of the first CDP/PDA self-assembled nanotubes. A series of CDP-DA monomers were prepared by coupling CDPs; cyclo(-Gly-Ser) and cis/trans cyclo(-Ser-Ser) with 10,12-pentacosadiynoic acid (PCDA). The results of this study show that self-assembled CDPDAs affords tubular structures and can undergo UV-induced polymerization leading to the bluecolored CDP-PDA nanotubes without any structural change. In addition, CDP-PDA nanotubes exhibit reversible thermochromism which can be attributed to intermolecular hydrogen bonding of CDP motifs in the CDP-PDA supramolecular system. EXPERIMENTAL SECTION Materials. 10,12-Pentacosadiynoic acid (PCDA) was purchased from GFS Chemicals. Cyclo(Gly-Ser) and Cyclo(-Ser-Ser) were purchased from Bachem(Switzerland). D-serine and N-BocL-serine were purchased from TCI (Korea). Propylphosphonic anhydride (50% in ethyl acetate), ammonia solution (2M in MeOH), oxalyl chloride and 4M HCl (in dioxane) was purchased from Sigma-Aldrich (Korea). All the other chemicals used were analytical grade reagent. Instruments. Optical/fluorescence microscopic images were collected with Olympus BX 51W/DP70, TEM investigations were carried out using a JEOL TEM-2100F microscope. SEM of nanotube morphology was conducted with a JEOL (JSM-6330F) FE-SEM. The UV absorption spectra were recorded on a single beam Agilent 8453 UV−vis spectrometer (Agilent


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Technologies, Waldbronn, Germany). The absorption spectra were recorded on a USB2000 miniature fiber-optic spectrometer (Ocean Optics). IR spectra were recorded on a Thermo Nicolet NEXUS 470 FTIR using an ATR accessory (Thermo Fisher Scientific, Inc.). The 1H and 13

C NMR spectra were recorded on a Varian Unitylnova (300 and 600MHz) spectrometer at 298

K in CDCl3 and DMSO-d6 High-resolution mass spectra (HRMS) were recorded on a SYNAPT G2 (water, U.K.) using a time-of-flight (TOF) analyzer and MALDI-TOF using AXIMA (SHIMADZU). XRD spectra were recorded with a HR-XRD, D8 discover (Bruker). Raman spectra were recorded on a LabRAM HR Evolution Raman Spectrometer (Horiba Scientific, 785nm laser source). Specimen preparation. Fabrication of dandelion-like microspheres: For examining the specimen of dandelion-like microsphere structure, 10 mg of CDP-DA-1 was dissolved in 1 mL of chloroform. This solution was heated and stirred to clarity (~55 oC) and then dropped (4 µL) on the glass slide. The specimen was dried at room temperature. Fabrication of nanotube structures: For examining the specimen of nanotube structure, a quantity of 5 mg of CDP-DA-1/CDP-DA-2 was dispersed in 1 mL of 50:50 (v:v) mixture of chloroform and methanol. This solution was heated and stirred to clarity (~55 oC) and then cooled to -7 oC. After the solution was stabilized at -7 oC for 3 hours, educted monomers were dropped (4 µL) on the glass slide. The specimen was dried at -7 oC slowly and then UV irradiated (254 nm, 1 mWcm-2) for 10 s for polymerization.


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Figure 1. Structures of CDP-DAs: mono-substituted CDP-DA-1 and di-substituted CDP-DA-2 (cis) and CDP-DA-3 (trans).

Figure 2. Schematic for formation of CDP-PDA-1 (a) Structure of CDP-DA-1 (b) Selfassembled CDP-DA-1 via hydrogen bond (c) Formation of CDP-PDA-1, polymerized CDPDA-1, upon UV irradiation. RESULTS AND DISCUSSION


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Molecular Design and Synthesis. The initial goal of this study was to prepare chiral CDPDA-1 system and to study the additive structural and functional influence on morphology and self-assembly behavior. As shown in Figure 1, for CDP-DA-1, cyclo(-Gly-Ser); (3S)-3(hydroxymethyl)piperazine-2,5-dione, is introduced to 10,12-pentacosadiynoic acid (PCDA) via ester linkage. The synthetic protocol for CDP-DA-1 begins with acylation of PCDA and then coupled with (3S)-3-(hydroxymethyl)piperazine-2,5-dione (see the supporting information for the scheme and experimental details). The CDP-DA-1 supramolecular architectures originating from hydrogen bonding network is schematically presented in Figure 2. The two amide functionalities in the CDP-DA-1 monomer enable self-assembly with strong intermolecular hydrogen bonding. A chiral center in the head group provides helical packing of the amphiphiles to form into tubular structures. Further, these self-assemblies are expected to undergo topochemical polymerization to form a rigid structure. Next, we extended our strategy to prepare the CDP-DA analogs; CDP-DA-2 and CDP-DA-3, by varying the number of substituents and stereochemistry (Figure 1) to get more mechanistic insights on structure, morphology, and self-assembly behaviors. The synthetic strategy similar to CDP-DA-1 was employed for CDP-DA-2 by bis-coupling of cis-cyclo(-Ser-Ser); (3S, 6S)-3,6bis(hydroxymethyl)piperazine-2,5-dione with PCDA (see the supporting information for the scheme and experimental details). In case of CDP-DA-3, trans-cyclo(-Ser-Ser); (3S, 6R)-3,6bis(hydroxymethyl)piperazine-2,5-dione, was first synthesized starting from D-serine by employing a multistep synthetic route and finally bis-coupled with PCDA (see the supporting information for the scheme and experimental details). The structures of synthesized products were confirmed by using IR, 1H, 13C NMR and high-resolution mass spectra (HRMS) /MALDITOF spectral methods (see Figures S1–S18).


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Fabrication of self-assembly structure and morphology analysis.

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A

detail

and

comparative study of CDP-DA/PDAs on structure, self-assembly behavior and morphology were carried out. To investigate the self-assembled characteristics of CDP-DAs, the aggregation morphologies were examined first by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and then by X-ray diffraction (XRD) measurements. Mono-substituted CDP-DA-1.

We first began studying self-assembly behavior and

morphology of the CDP-DA-1 nanostructures. In order to fabricate CDP-DA-1 nanostructures, the self-assembly behavior of CDP-DA-1 monomer was investigated using different solvent systems such as chloroform, dichloromethane, and various solvent mixtures like chloroform with methanol/ hexane/ DMSO, dichloromethane/methanol, and THF/methanol/DMSO (Figure S19). However, chloroform and chloroform/methanol solvent system appeared as an ideal system for fabricating nanostructures. As a result, various supramolecular self-assemblies including plate-like, dandelion-like, and tube-like microspheres were afforded by using a simple drop casting technique.


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Figure 3. SEM images of the different morphologies of CDP-DA-1 (a) Dandelion-like microsphere structures generated in chloroform solution. (b) Magnified image of the white marked region in panel-a. (c,d) Magnified image of the red- and blue-marked region in panel-b, respectively. Microspheres composed of plate-like nanosheets (panel-c) and hollow nanotubes (panel-d) are observed (e) Magnified image of panel-d. (f) Nanosheets scrolling into nanotubes at different stages. Individual nanosheets are scrolled up to form a single-wall nanotube or multiwall nanotube.


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Figure 4. SEM(a,b), TEM(c,d) images of CDP-PDA-1 nanotubes generated in mixed chloroform and methanol solution (1:1 v:v).

In weak polar chloroform, the CDP-DA-1 self-assembles into microspheres when casted directly on a glass slide and dried at room temperature. Examining of the SEM and acquired image shown in Figure 3a, revealed Dandelion-like microsphere structures. Under a closer inspection of the SEM image in Figure 3b, Dandelion-like microspheres were observed to contain two distinct coexisting morphologies (regions separately marked with red and blue color). As seen in magnified images of marked regions displayed in Figures 3c and d, microsphere assemblies with flat plate-like ends (sheet-like) and the curved tubular structure, are clearly identifiable. For a deeper morphological insight, magnified images of tubular structures shown in Figure 3d were collected. The image displayed in Figure 3e confirms the formation of tubular structures of a nanometric diameter which are packed together as a microsphere. As can be seen in Figure 3f,


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the nanotubes appeared to be formed from single-wall and multi-wall structure. In order to maximize the yield of tubular formation of CDP-DA-1, experimental conditions such as solvent system, polarity, concentration, and temperature were optimized. The addition of a polar solvent such as methanol into the sample chloroform solution stabilizes the assembled morphologies of CDP-DA-1 and favorable morphologic transformation was observed. The flat plate-like ends of microsphere turned into more curved and the tube-like self-assemblies of nanoscale sizes are predominantly observed in the mixed solvent of chloroform and methanol (1:1 v:v) (Figures 4a and b). The tubular characteristic of CDP-PDA-1 is further confirmed by TEM images (Figure 4c and d). The XRD pattern of CDP-PDA-1 provides a further understanding for the evolution of hollow nanotubes from plate-like lamellar substructures. The lamellar structures with interlamellar layer distance of 6.79 nm (using Bragg’s law) calculated at 2θ = 2.6o, 5.35o, 8o, 10.7o, 13.4o, 16.15o, which is in perfect agreement with the bilayers formed into a tubular structure (see Figure S20). A schematic representation of CDP-DA-1 bilayers formed into a tubular structure is shown in Figure 5. The formation of these tubular structures can be regarded as the polarity driven scrolling-up conversion of their corresponding lamellar structures. There are two stages that have been assigned to CDP-DA-1 lamellar structures formed into nanotubes with consistently high quality. CDP-DA-1 is first self-assembled into bilayer plates of perfect lamellar structures stacked with a head-to-head interface (head, CDP moiety; tail, DA chain), which are tightly packed with extensive hydrogen bonding of CDP moieties and properly aligned DAs. The hydrogen bonding network acts as a stitching process that forces CDP-DA-1 monomer units into lamellar morphology. Next, as the polarity of the surrounding solvent changes, i.e. ratio of methanol to chloroform, the bilayer plates are gradually scrolled-up; starting with


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bending to curling and finally formed into nanotubes of either single-wall structure or further scrolled-up to form multi-wall structure. Upon exposure to 254 nm UV lights, the white color CDP-DA nanotubes turn into blue color as polymerization occurs. Raman spectroscopic analysis shows that characteristic bands associated with conjugated ene-yne in the blue-phase PDA appear at 1450 cm-1 (C=C) and 2084 cm-1 (C≡C) while yne-yne monomer band at 2265 cm-1 (C≡C) disappeared (see Figure S21). Polymerized nanotubes connected by the covalently-linked backbone, found to be more stable than monomer nanotubes under solvent exposure. Polymerized CDP-PDA-1 nanotubes well retain their tubular morphology before and after exposure to chloroform, whereas unpolymerized nanotubes lost their structure (see Figure S22)

Figure 5. Schematic representation of the self-assembly process of CDP-DA-1 nanotube. CDPDAs are first self-assembled into (a) bilayered plates, followed by (b,c) gradual scrolling-up process to ultimately form either (d) single-wall or (e,f) multi-wall nanotubes.

Di-substituted CDP-DA-2(cis) and CDP-DA-3(trans). After

confirming

the

nanotube

structure from mono-substituted cyclic dipeptide (CDP-DA-1), we were curious if di-substituted cyclic dipeptides would afford the similar self-assembling pattern. As a result, as shown in


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Figure 1, we expanded our study and prepared two di-substituted CDPs in cis and trans forms. Based on the result driven from CDP-DA-1, we supposed CDP-DA-2 would afford the tubular structure whereas CDP-DA-3 would not. Accordingly, CDP-DA-2 afforded nanotubular structures in chloroform and methanol solution (see Figure S23). These monomeric tubes undergo facile polymerization upon UV irradiation, as shown in Figure 6a. TEM images also confirmed the formation of the tubular structure of CDP-PDA-2 (Figure 6b). Analysis of X-ray diffraction pattern of CDP-PDA-2 revealed the well-ordered lamellar packing of the nanotube (see Figure S24). By using Bragg’s law, the interlamellar distance calculated at 2θ = 2.95o, 4.45o, 6.05o, 7.45o, 8.95o, 10.6o, 12o, 13.4o was found to be 6.00 nm, which is assignable to the bilayers formation. In a similar manner to CDP-DA-1 nanotube formation, the CDP-DA-2 also follows lamellar scrolling behavior where two CDP-DA-2 molecules stacked with a head-tohead interface (see Figure S25). UV-irradiation induced polymerization of the self-assembled CDP-DA-2 monomer to blue-phase polymeric CDP-PDA-2 is confirmed by Raman spectroscopic method. The Raman spectrum of blue-phase shows two characteristic bands at 1455 cm-1 and 2090 cm-1 corresponding to conjugated C=C and C≡C bonds respectively (see Figure S26).


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Figure 6. SEM(a) and TEM(b) images of CDP-PDA-2 nanotubes generated in mixed chloroform and methanol solution (1:1 v:v).

Figure 7. SEM images of CDP-DA-3 (trans) structures self-assembled in CHCl3:MeOH solution (1:1 v:v) (a) 1 k, (b) 5 k, (c) 10 k, (d) 30 k magnification.

While CDP-DA-2 was observed to show very similar self-assembling pattern through hydrogen bonding, CDP-DA-3 (trans), as we expected, did not afford any tubular structures. CDP-DA-3 self-assembled into onion-like microspheres instead of nanotubes under the same condition of chloroform and methanol mixture (Figure 7). This is can be attributed transconformation of CDP-DA-3, where the cyclo(-Ser-Ser) residue with trans geometry forces CDPDA-3 to adopt a non-planar conformation. As a result, the two symmetric amide groups are oriented in the different direction, which prevents the necessary intermolecular closeness of the CDP moieties and appears to satirically impracticable for the for hydrogen-bond interactions. This clearly suggests orientation-dependent hydrogen bonding function, derived from the geometric conformation of cyclo(-Ser-Ser) residue to construct tubular assemblies.


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Finally, in order to further clarify the origin of tubular morphology, we examined SEM for pristine CDP aggregates cyclo(-Gly-Ser) and cis/trans cyclo(-Ser-Ser). Inspection of SEM images shows flake-like and rod-like morphology in chloroform/methanol system (see Figure S27). These combined results unambiguously support the concept of the lamellar scrolling-up behavior of CDP-DA-1/2 into nanotube formation. Reversible thermochromic Property.

Very interestingly, the polymerized nanotubes

derived from CDP-DA-1 displayed a reversible thermochromism. Accordingly, a reversible blue-to-red color transition occurred during the thermal cycle at 25° ↔ 90° C correspondingly. The color transition between blue-red-blue occurred within 10 s and is sufficiently clear for naked-eye detection of designated temperature variation. As seen in Figure 8a, this reversible color transition occurs without a noticeable change in absorption intensity. Photographs of polymerized CDP-DA-1 at designated temperature are displayed as an inset. The reversible blueto-red color transition process was repeated in cycles to evaluate the colorimetric reversibility and stability of the CDP-PDA-1. A plot of absorbance at 650 nm as a function of thermal cycling for the blue-to-red color transition over ten consecutive cycles demonstrates thermoresponsive colorimetric reversibility with no obvious decrease in original intensity, as seen in Figure 8b. The phenomenon of reversible thermochromism for polymerized CDP-PDA-1 is further evidenced by using Raman spectroscopy method. As shown in Figure 8c, the bands appearing at 1453 and 2082 cm-1 for ene-yne respectively in blue phase shows noticeable shift upon heating to 90 °C and appears at 1500 and 2109 cm-1 for respective ene-yne bands in the red phase PDA. When the temperature is gradually cooled to 25 °C, the shifted bands return to the original ene-yne bands with the same intensity.


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Previously, we have reported that strong headgroup interactions enable a PDA to display a reversible thermochromism.57 The reversible thermochromism observed with the CDP-PDA-1 nanotube is presumably due to the extensive hydrogen bonding among CDP moieties in the polymer matrix. In order to analyze the nature of hydrogen bonding among the CDP head groups, 1H NMR (CDCl3) spectra were examined upon increasing concentration, as seen in Figure 8d. We observed

Figure 8. Reversible thermochromic property of CDP-PDA-1; (a) Absorbance spectra of CDPPDA-1 showing thermochromic properties upon heating and cooling. The inset photographs of CDP-PDA-1 shows color transition at a designated temperature. (b) Plots of absorbance intensity at 650 nm as a function of thermal cycles(25 ↔ 90 oC). (c) Raman spectra of CDPPDA-1 upon heating and cooling. Hydrogen-bonding effect; (d) 1H NMR (CDCl3) peak shift of CDP-DA-1 solution dependent on concentration. (e,f) FT-IR spectra of CDP-DA-1 solution in chloroform (black) and powder (red).


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that the amide peaks in 1H NMR which originally appeared at 6.022 and 5.859 ppm in the 1 mM of CDP-DA-1 solution were merged and shifted to 6.567 ppm in 20 mM. By gradually increasing the concentration of the solution composed of CDP-DA-1, two peaks of the corresponding amide group were gradually shifted downfield. This result indicates that as the concentration of the solution increases, the CDP-DA-1 monomers become more efficient in selfassembly, thus forming more robust hydrogen bonding. Likewise, the effect of hydrogen bonding was analyzed by the change in absorption spectra of FT-IR. In this case, we compared the spectra of a chloroform solution of CDP-DA-1 and the powder itself. The robust hydrogenbonded N-H stretching band of the amide group was shifted from 3340 cm-1 in the solution state to 3194 and 3053 cm-1 in the powder state, indicating enhancement of its strength (Figure 8e). The region of 1600 to 1700 cm-1 also demonstrates the effect of hydrogen bonding on terminal carbonyl in ester group and amide group (Figure 8f). In case of ester group which is less susceptible to hydrogen bonding, the shift of the carbonyl peak is small, as shown in 1745 cm-1 in the solution and 1749 cm-1 in the powder state. In contrast, the carbonyl of the amide group is largely affected by hydrogen bonding, as illustrated by the shift from 1693 cm-1 in the solution to 1685 cm-1 in the powder state. Further, CDP-PDA-2 also featured reversible thermochromism upon heating and cooling, showing the similar visible absorbance spectra with a maximum absorption peak at 637 nm (see Figure S28a). After confirming reversible thermochromism using absorption spectroscopy and Raman spectroscopy techniques (see Figures S28b and c), analysis of hydrogen bonding was similarly performed by inspecting 1H NMR and FT-IR spectra. The amide peak of 1H NMR (CDCl3) is shifted downfield as hydrogen bonding effect is enhanced upon increasing concentration (see Figure S28d). We were also able to confirm the shift of ester and amide peaks


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in the FT-IR spectrum as CDP-DA-2 exists in solution and as powder each (see Figures S28e and f). CONCLUSIONS We have fabricated chromogenic CDP-PDA nanotubes with higher structural cohesion and stability, and reversible thermochromic property. A series of cyclic dipeptide-containing diacetylenic amphiphiles (CDP-DAs) were synthesized and their supramolecular structures were studied in different solvent systems. The CDP-DAs forms self-assembled architectures through strong intermolecular hydrogen bonding interactions between adjacent CDP moieties. Under the optimized condition, we were able to obtain the self-assembled nanotubes from CDP-DA-1 (mono-substituted) and CDP-DA–2 (cis-, di-substituted) with high yield and consistency. We also showed that CDP-DA-3 (trans-, di-substituted) did not afford any tubular forms due to the unfavorable structural geometry. The structural analysis of nanotubes was conducted by using SEM, TEM, and powder XRD measurements. The XRD pattern analysis revealed the wellordered lamellar structure formed into single/multi-wall nanotubes via lamellar scrolling process, which consists of bilayers with a corresponding interlamellar distance of 6 to 7 nm. The CDPDA-1 and CDP-DA-2 nanotubes undergo facile UV polymerization without any structural damage. Formation of polymeric CDP-PDA nanotubes was confirmed by Raman spectral analysis. The reversible thermochromism for the nanotubes derived from CDP-DA-1 and CDPDA-2 was spectroscopically demonstrated using UV absorption and Raman spectral analysis. The intermolecular hydrogen bonding responsible for this reversible thermochromic behavior was confirmed by 1H NMR and FT-IR studies. The CDP-DA/PDA molecular system brings to attention a new design capable of generating stimuli-responsive chromogenic nanotubes. The results of the current study convincingly


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demonstrate the significance of the strategy to combine both CDP and PDA in a supramolecular network to exploit additive structural and functional features. We hope that this study will serve as a stimulus for future research on CDP-DA/PDA in developing responsive nanomaterials for a diverse range of applications. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Detailed information on synthesis scheme and experimental procedures; spectroscopic data (NMR, HRMS and MALDI-TOF); X-ray diffraction spectra of CDP-DA-1 and CDP-DA-2; Raman spectra of CDP-DA-1 and CDP-DA-2; SEM images of CDP-DA-1 vs CDP-PDA1upon chloroform exposure, CDP-DA-3; schematic of the CDP-DA-2 nanotube formation; reversible thermochromism of CDP-DA-2 (UV absorption spectra, Raman spectra, NMR and FTIR spectra). AUTHOR INFORMATION Corresponding Author *Email: [email protected] Author Contributions §Min Jeong Seo and Jisoo Song contributed equally to this work. Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This investigation was supported financially by the National Research Foundation of Korea (NRF) grant funded by a Korea government (MSIP) (NRF-2017R1A2A1A05000752).


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Table of Contents Graphic


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Reversibly Thermochromic Cyclic Dipeptide Nanotubes - Langmuir

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