Dynamic Covalent Hydrazone Supramolecular Polymers towardMultiresponsive Self-Assembled Nanowire SystemKyung-su Kim, Hye Jin Cho, Jookyeong Lee, Seonggyun Ha, Sun Gu Song, Seunghun Kim,Wan Soo Yun, Seong Kyu Kim, Joonsuk Huh,* and Changsik Song*

Department of Chemistry, Sungkyunkwan University, 2066 Seobu-ro, Jangan-gu, Suwon-si, Gyeonggi-do 16419, Republic of Korea

*S Supporting Information

ABSTRACT: Stimuli-responsive polymeric systems are of considerable interest due totheir potential applications in environment-adaptive technologies such as smartsurfaces. Traditionally, such systems can be constructed either by dynamic noncovalent(supramolecular) or dynamic covalent chemistry, but the use of both chemistries inone system may offer unique opportunities for structural diversity and variouscontrollability. Herein, we report that hydrazone−pyridinum conjugates, which can bedynamically exchanged by transimination, assemble to form one-dimensionalnanowires due to direct intermolecular interactions (without metal-ion coordination).The self-assembly process can be controlled not only by dynamic covalent chemistrybut also by pH adjustment. The hydrazone−pyridinum conjugates are transformed tomerocyanine-type dyes of distinctive negative solvatochromism via deprotonation,which also affects their self-assembly. Such a dual control of the dynamic molecularassembly will provide unique way to develop diverse smart nanomaterials withmultistimuli-responsiveness.

■ INTRODUCTION

Stimuli-controlled or -responsive self-assembly at the molecularscale has been intensively investigated over the past few decadesfor various applications such as self-healing materials,1,2

biosensing,3−5 drug delivery,6,7 artificial muscles,8−11 andmolecular devices.12 To control the assembly behavior at themolecular scale, the molecules or functional groups capable ofself-assembly should possess stimuli-responsiveness. Manymolecular moieties have been developed for stimuli-responsiveassembled materials: spiropyran,13 azobenzene,14 terpyri-dine,15,16 naphthalene diimide,17 and hydrazone.18 Amongthem, the hydrazone functional group offers an advantage notonly due to its configurational changes via E−Z isomerizationinduced by light,19,20 pH,21 and metal coordination22 but alsodue to the dynamics of its reversible CN bond formation,23,24

which enables structural diversity and dynamicity. Hydrazoneshave been utilized in the construction of stimuli-responsivesupramolecular polymers, in which monomeric units are heldtogether with reversible noncovalent bonds. For example,Hanton et al. reported the controlled self-assembly ofhydrazone−metal complexes depending on the species ofmetal ions.18 Lehn et al. reported a hydrazone-based photo-and thermoresponsive supramolecular metalloassembly whichshowed photoinduced potassium release.25 Samori et al.reported the hydrogen-bond assisted metalloassembly of abis(hydrazone) ligand.26 Precedent research suggests that thehydrazone moieties play a role of either a simple linker or aligand for metal-ion binding. In the function of the simple linker,supramolecular polymerization is enabled by certain functionalgroups capable of (multiple) hydrogen bond or host−guest

interactions which are connected by the dynamic hydrazonemoiety. On the contrary, in the function of the ligand, thebinding of metal ions at the hydrazone moiety may producesupramolecular polymers. The intermolecular interaction thatenables the supramolecular polymerization is based on thereversible hydrazone−metal association.27−30 To the best of ourknowledge, the direct intermolecular interaction as a driving force forsupramolecular polymerization has not yet been reported. Herein,we report novel benzoyl hydrazone−p-pyridinium conjugatesthat show self-assembly by direct intermolecular interactions.Remarkably, the supramolecular polymerization and absorptionproperties can be easily controlled by pH and the dynamiccovalent hydrazone exchange.

■ RESULTS AND DISCUSSIONBis-p-pyridinium benzoyl hydrazone BH1 (Figure 1) can beeasily synthesized by a nucleophilic substitution reaction with ahigh yield from commercially available p-xylene dibromide and(E)-N′-(pyridine-4-ylmethylene)benzohydrazide, which wasprepared by the hydrazone formation reaction31 with benzohy-drazide and 4-pyridinecarboxaldehyde (Supporting Informa-tion). Interestingly, we found that the hydrazone p-pyridiniumconjugate BH1 showed pH-responsive color change and helicalwire-type self-assembly (see below). In the presence of a base(e.g., diisopropylethylamine), initially transparent BH1 indimethyl sulfoxide (DMSO) changed to orange-red and becamecolorless again with the addition of an acid (e.g., trifluoroacetic

Received: September 5, 2018Published: October 11, 2018

Article

pubs.acs.org/MacromoleculesCite This: Macromolecules 2018, 51, 8278−8285

© 2018 American Chemical Society 8278 DOI: 10.1021/acs.macromol.8b01909Macromolecules 2018, 51, 8278−8285

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acid) (Figure 1a). We attributed this color change to theformation of a merocyanine-type dye32−34 with deprotonationof cationic BH1. Merocyanine dyes can be described as aresonance hybrid between zwitterionic and quinoid forms andgenerally show large solvatochromic shifts. In addition, the wire-type supramolecular polymerization of BH1 was observed uponthe slow evaporation of the solvent (Figure S1 and Figure 1d),which strongly suggests the presence of intermolecularinteraction between the hydrazones.To determine in detail the molecular origins of the absorption

change and nanowire formation of the hydrazone, we designedand synthesized the derivatives (BH2, BH(Me), and BHm) ofBH1 and their corresponding monohydrazone model com-pounds (H1, H2, H(Me), and Hm, respectively) (Figure 1b).The bulky tert-butyl was attached to the hydrazone group tospecifically investigate the steric effect on the nanowireformation (BH2 and H2). Because the amide hydrogen mayplay a role in intermolecular assembly, a methyl group wassubstituted in BH(Me) and H(Me). Finally, m-pyridiniumhydrazone conjugate was prepared, instead of the p-pyridiniumhydrazone conjugate, to examine the effect of the configurationof π-electrons (BHm andHm) on the electronic absorption andself-assembly.As revealed by the UV−vis absorption spectra of the

monohydrazones (Figure 1c), it was confirmed that thepresence of amide N−H and the conjugation in the para-direction of the pyridinium moiety are responsible for theformation of the merocyanine-type dyes from the hydrzones.The similar absorption peak at around 482 nm of H1 and H2(20 μM in DMSO) disappeared in solutions of the methyl-substitutedH(Me) andmeta-substitutedHm. These results maybe easily understood by examining the resonance canonicalstructure of BH1 or H1 when deprotonated (Figure 1a); thecharge at the amide moiety can be transferred to the electron-deficient pyridinium moiety through a proper π-electronconjugation (i.e., para-direction). Furthermore, the absorption

peak at 482 nm of H1 was more evident with addition of a base(diisopropylethylamine, Figure S2). Thus, we designated theabsorption peak at 482 nm in DMSO as the charge-transfer(CT) band, which cannot be achieved by meta-conjugation andis limited by the methyl substitution. Interestingly, the CT bandof the hydrazones showed large negative solvatochromism(hypsochromic shifts) with solvent polarity (Figure S3), whichappears to be similar to that of Brooker’s merocyanine.35,36 Weassume the dipole moment of the ground state is greater thanthat of the excited state, resulting in the greater stabilization ofthe ground state in highly polar solvents. It should be noted thatthe hypsochromic shift was more pronounced in protic solventsthan in aprotic solvents since protic solvents form hydrogenbonds with the zwitterionic resonance structure of H1,promoting more the charge separation in zwitterionic groundstate of H1 than the case of aprotic solvents.The self-assembled nanowire formation of the bis-hydrazones

(BHs) was also affected by their molecular structures (Figure1d). Interestingly, the ability to form the CT band seemed to bestrongly related to the wire formation. The methyl-substitutedBH(Me) and meta-substituted BHm, which were absent in theCT band, produced irregular aggregates rather than self-assembled wires. Importantly, however, the self-assembledwires were not observed with the completely deprotonatedmerocyanine form of BH1; the presence of amide N−H(protonated) appears to be required in the self-assembly (FigureS4). A different type of self-assembly from bulky BH2 wasobserved, presumably due to the steric hindrance of the tert-butyl group. To examine the degree to which the charge densityof H1 affects dimerization, we calculated the molecularelectrostatic potential (MEP) map using Spartan’16 packagesoftware (DFT calculation at B3LYP/6-31+G*) (Figure S5). Asexpected, the most negative MEP value was observed in theregion of carbonyl oxygen and the most positive MEP value wasobserved in the region of nitrogen of the pyridinium ring,generating its molecular dipole moment from the pyridinium

Figure 1. (a) Formation of a pH-responsive, merocyanine-type dye from the hydrazone (H), which has a resonance structure of zwitterionic andneutral forms. (b)Molecular structures of synthesized hydrazone−pyridinium conjugates. (c) UV−vis absorption spectra ofH1,H2,Hm, andH(Me)in DMSO. (d) SEM images of bis-hydrazones, BHs (10−4 M in DMSO), drop-casted on glass slides, and dipole moments of correspondingmonohydrazones, Hs.

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ring to the benzoyl group. Although our hydrazones did notform self-assembly at their merocyanine form, the dipole−dipoleinteraction may be responsible for the self-assembled wireformation. Thus, dipole moment values of monohydrazoneswere estimated by density functional theory (DFT) calculations(B3LYP/6-31+G*). We found that the dipole moments ofhydrazonemoieties (H1 andH2 forBH1 andBH2) that formedself-assembled wires were higher than those of that did not formself-assembled wires (H(Me) and Hm for BH(Me) and BHm,respectively). However, the least dipole moment value of 11.5 Dfor H(Me) seems sufficient in the case for dimerization ofmerocyanine dyes by dipole−dipole interaction.37 Furthermore,we cannot conclude that the tendency to self-assemble appearsto increase as the dipole moments of the hydrazone moietyincrease, since the dipole moment ofHm (14.1 D) is fairly closeto that of H1 (14.4 D). We suspect that the dipole−dipoleinteraction cannot fully explain the self-assembly behavior ofBHs. In addition, the doubly charged bis-hydrazones stillformed wires in highly polar solvents such as DMSO (stronghydrogen-bond acceptor), which suggests that the dipole−dipole interaction and the hydrogen bond through amide N−Hmay be weakened. The above results motivated a detailedinvestigation of the way in which the hydrazones were dimerizedor aggregated to form the self-assembled wires (Figures 2 and 4).

To understand the intermolecular interaction for thesupramolecular polymerization of the hydrazone conjugatesBHs, the concentration-dependent UV−vis absorption spectraof the model compound H1 was investigated (Figure 2a). Theconcentration of H1 in DMSO was varied from 0.004 to 0.7mM, and the resulting spectra were plotted as the molarabsorptivity (L/(cm mol)) of H1. We found that (1) the molarabsorptivity of the CT band (∼482 nm) decreased and (2) themolar absorptivity of the π−π* band (∼336 nm) increased, asthe concentration of H1 increased (similar trend was observedin UV−vis absorption spectra of BH1 as shown in Figure S6);however, these molar absorptivities were expected to remainunchanged, regardless of the concentration if the solutioncomposition does not change. Interestingly, the variations of themolar absorptivities were not dramatic in protic solvent such aswater (Figure S7). Following the method suggested byCrampton and Robotham,38 we were able to obtain the aciddissociation constant of H1 (pKa = 11.5) in DMSO bufferedwith n-butylamine and n-butylamine hydrochloride, furnishingthe molar absorptivity of merocyanine Z at 482 nm (ϵZ = 4.98 ×104 L/(cm mol)) and consequently the mole fraction of Z (αZ)at each total concentration (c0) (Figure S8). The observed αZseemed to be close to the case of pKa ∼ 5 at very lowconcentration, while it shifted to the less acidic case of pKa (>8)

Figure 2. (a) Concentration-dependent UV−vis absorption spectra of H1 in DMSO solutions from 4 μM to 0.7 mM. (b) Mole fractions of thezwitterionic merocyanine form (Z) in the concentration range 6−600 μM (red) with lines of ideal (calculated) Z fractions with various pKa values(lines from left to right: pKa = 8, 7, 6, 5, and 4). (c) Normalized UV−vis absorption spectra of BH1 in DMSO solution (6 μM) and film state. (d)Estimated transition energy (S0 to S1) difference from monomer to dimer calculated from the arrangements of transition dipole vectors. (e) Plot oftransition energy (S0 to S1) difference (cm

−1) frommonomer to dimer in DMSO according to the slip angle of the transition dipoles. (f)Mole fractionsof the aggregates (A) (αagg) against KcT determined by the nonlinear regression analysis of UV−vis spectra in (a) with the isodesmic model at 336 nm(blue). Calculated αagg against KcT with different σ (= K/K2) values were also presented in lines. (g) Plausible chemical equations showing theequilibrium of hydrazone-pyridinium conjugate H1 (H) and its aggregates (A) with the deprotonated merocyanine form (Z).

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at higher concentrations (Figure 2b). We suspect that suchdramatic decrease of the “apparent” Ka is due to an event athigher concentrations, presumably a certain type of aggregation.Thus, we examined the aggregation model39,40 to explain theconcentration behavior of the UV−vis absorption spectra ofH1.The band assignment should have been performed prior to

the analysis, since the CT band (∼482 nm) was assigned to theabsorption of zwitterionic merocyanine Z while the π−π* band(∼336 nm) was initially assigned to the absorption of hydrazoneH. The position of the absorption peak of the aggregates Aseems rather obscure, but we assume that the aggregates A canalso contribute to the π−π* band (∼336 nm) according to thefollowing reasons. First, we observed absorption of BH1 in filmstate, which shows similar location of absorption with π−π*band derived from the protonated hydrazone (H) (Figure 2c).We can presume safely that all hydrazone moieties of BH1 existas aggregates (similar to A) in the film state. Second, wemeasured the temperature-dependent UV−vis spectra of H1 inDMSO (10 and 60 μM), which suggests the existence ofadditional chemical equilibrium at higher concentrations(Figure S9). At the lower concentration of 10 μM withincreasing temperature, the decreasing amount of the absorptionat 336 nm was comparable to the increasing amount at 482 nm,suggesting one species converting to the other. However, in thecase of the higher concentration of 60 μM, we did not observethe absorbance increase at 482 nm as much as the decrease at

336 nm, suggesting the possible existence of another speciesresponsible for the absorption at 336 nm. Lastly, as shown inFigure 2d, we derived a model for a centrosymmetric dimerstructure of H1, where the distance was 3.599 Å and the slipangle was 48.5° (from the X-ray crystallographic structure ofH1; see Figure 3b). With this model and transition dipole vector(21.06 D, TD-DFT calculation at B3LYP/6-31+G*) we couldcalculate the difference in the transition energy (cm−1) betweenH and dimerD (Figure 2e). We obtained a fairly small difference(13 cm−1 in DMSO with dielectric constant of 46.7) accordingto the dipole−dipole approximation,41 which supports theassumption that the bands ofH, D, and Amay appear in similarlocations in the UV−vis spectra (around the π−π* band).After assigning the absorptions of both aggregates A and

hydrazone H to the π−π* band, we were able to perform thenonlinear regression analysis of the concentration-dependentUV−vis absorption spectra of H1 using the isodesmicmodel.42,43 First, the concentration sum (cT) of [H] + [A]was determined easily by subtracting the obtained value of [Z]from the total concentration. The baseline shifting of the bandsof H and A (shown in Figure 2a) was corrected prior to theanalysis. Then, the isodesmic model was applied to analyze theπ−π* band (∼336 nm) (Figure S10) with eq 1.

Figure 3. (a) ORTEP diagram of H1 (thermal ellipsoids are set at 50% probability, and the iodide anion and methanol are omitted for clarity). (b)Interaction energy of the assembled dimer observed in the solid state ofH1 and the distances of interactions (CH−π: 3.451 Å; π−π: 3.561 and 4.565Å). (c) Hirshfeld surface ofH1 with the shape index function showing π−π and CH−π interactions. (d) Hirshfeld surface ofH1 with the curvednessfunction demonstrating good fitness between molecules. (e) Contributions of the specific contacts from the Hirshfeld surface analysis of H1. (f)Schematic representation of supramolecular polymerization of BH1 via hydrazone’s self-assembly.

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ϵ =+ − +

ϵ − ϵ + ϵ++ +

+

Kc Kc

Kc

2 1 4 1

2( )( )H A

H A H A

H AH A A2

(1)

where K denotes the aggregation equilibrium constant, cH+A andϵH+A denote [H] + [A] and the molar absorptivity at 336 nm,respectively, and ϵH and ϵA refer to the molar absorptivity of thehydrazone H and aggregates A, respectively.According to the result of the nonlinear regression analysis of

data from Figure 2a at 336 nm with the isodesmic model, thefraction of aggregatesA in Figure 2a was obtained. The obtainedfraction αagg was plotted against KcT (K: equilibrium constant),with the theoretical lines from the nucleation−elongation modelwith various σ values (σ = K2/K) from 0.001 to 1000 (Figure2b).41,44 If the elongation equilibrium constant (K) and thenucleation equilibrium constant (K2) are the same (K2 = K, or σ= 1), it is noncooperative or isodesmic to which ourUV−vis dataappear to fit very well. The fitted results of the mole fractions ofeach species (A,H, and Z) were plotted as a function of the totalconcentration of H1 (c0) (Figure S11). The merocyanine orzwitterionic Z decreases rapidly as the concentration increases.The hydrazone H increases first but then also decreases due tothe formation of its aggregatesA. In the concentrated solution ofH1 (above ∼200 μM), the aggregates A appeared to assume themost abundant portion (>90%). We were also able to obtain thevalue of the aggregation equilibrium constant K = 8.06 × 104

M−1, which seems fairly high and comparable to the

dimerization constant of the tetrasulfonated magnesiumphthalocyanine, which is known to undergo cation-inducedself-assembly.45

To further investigate the intermolecular interaction of thehydrazone−pyridinium conjugates, Hirshfeld surface analysiswas utilized in the crystal network ofH146 obtained in a mixtureof acetonitrile and methanol. H1 crystallized in the P21/n spacegroup, in which the pyridinium cation was charge-balanced withI− anion (Figure 3a; other atoms are omitted for clarity).Interestingly, we found that theH1molecules were directionallypacked normal to the aromatic rings (Figure S12), and theneighboring molecules were positioned antiparallel to eachother or centrosymmetrically, facing the para-hydrogen of thebenzoyl group toward the center of the pyridinium ring of theneighboring molecule, with a distance of around 3.5 Å (Figure3b). This packing mode may explain why sterically bulky-substituted BH2 showed a different type of self-assembly(Figure 1d); the tert-butyl group may prevent the efficientpacking of hydrazone molecules.Figures 3c and 3d show the Hirshfeld surfaces mapped over

the shape index and curvedness47 of H1, respectively. In thegenerated Hirshfeld surface ofH1with the shape index function,we observed “bow-tie” patterns which indicate the presence ofπ−π stacking,48 and CH−π interactions between benzene andpyridinium ring (Figure 3c). The curvedness map showed flatareas in the contacted surface, which indicates the possibility of awell-matched dimer structure (Figure 3d). The decomposed

Figure 4. (a) Transimination (dynamic covalent chemistry) of hydrazone 1 to BH1 catalyzed by trifluoroacetic acid (TFA) and the formation of pH-responsive, merocyanine-type dyeBZ1 fromBH1 by triethylamine (TEA). (b) SEM images of transimination reactionmixtures at the initial (left) andafter 24 h (middle) and TEA treatment (right), forming zwitterionic merocyanine BZ1. (c) Selected time-dependent nuclear magnetic resonance(NMR)measurements of the transimination (DMSO-d6) from 0 to 21 h. (d) Percent conversion and yield of hydrazone 1 andBH1, respectively, uponproceeding of the transimination reaction.

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fingerprint plots indicate that the types of interactions in thepacking of H1 in the crystal were van der Waals forces (H−Hcontacts, 49.6%), CH−π interactions (C−H contacts, 15.2%),and π−π stacking interactions (C−C contacts, 7.4%) (Figure3e). For the pair of H1 molecules, we were able to calculate theinteraction energy using CrystalExplorer3.1 software (B3LYP/6-31G(d,p)). The total interaction energy appeared to be−40.7kcal/mol, which can be further divided in terms of electrostatic(−7.3 kcal/mol), polarization (−3.1 kcal/mol), dispersion(−48.8 kcal/mol), and exchange repulsion (18.5 kcal/mol). Inshort, the dimerization of H1 appears to be large due to thestabilization by van der Waals, π−π, and CH−π interactions,rather than by electrostatic dipole−dipole interaction only. Inaddition, since self-assembled wires of BH1 could be formedeven in water that can strongly interrupt the intermolecularhydrogen bonding (Figure S13), it can be assumed that directintermolecular interactions (π−π, CH−π, and van der Waalsinteractions identified by Hirshfeld surface analysis) have agreater influence than the hydrogen bonding in self-assembly.The morphology of the self-assembled wire of BH1 was

investigated by scanning emission microscopy (SEM) andatomic force microscopy (AFM) (Figure S14). The SEM andAFM images of BH1 prepared by drop-casting on glasssubstrates or silicon wafers displayed self-assembled helicalwires that supposedly consisted of individual one-dimensional(1-D) polymers. It appeared that as shown in Figure 3f, theinitially formed individual 1-D polymers were combinedtogether progressively, eventually resulting in thick, twistedwires with lengths of up to 10 μm, widths ranging from 5 to 45nm, and pitches of over 65 nm (Figure S14). Furthermore, itappeared that the two lateral directions of growth were notisotropic; one direction seemed slightly more favorable than theother, resulting in rectangular-type twisted wires (Figure 3f).Although the helicity of the assembled wires seems to becontrolled as left-handed (M) in SEM images (Figures 1d and4b, Figure S4), we identified the right-handed (P) helices also.Currently, the control of the helicity of self-assembled nanowiresis under investigation.One of the most important properties of hydrazone-based

molecular systems is that the covalent hydrazone bond is alsodynamic. Dynamic covalent chemistry (DCvC) has beenintensively studied in applications such as adaptive surfaces,polymer actuators, drug delivery, and drug discovery sys-tems.49−53 To investigate the morphology control of supra-molecular wires from bis-hydrazones, a hydrazone transimina-tion experiment was performed between hydrazone 117 andcompeting bis-aldehyde 2 (Figure 4a). Bis-aldehyde 2 (0.5equiv) was added to the DMSO solution of hydrazone 1 (15mM) in the presence of catalytic trifluoroacetic acid (TFA), andthe mixture was stirred at 100 °C. 1H nuclear magneticresonance (NMR) spectroscopy confirmed that the compoundBH1 was produced as the exchange reaction proceeded, whichwas also verified by SEM. At the initial stage of the reaction, noself-assembled wire was present, but after a few hours, self-assembled helical wires were observed that were identical tothose directly formed from BH1 (Figure 4b). We also furtherinvestigated the kinetics of DCvC (transimination) for BH1 bytime-dependent NMR spectra (Figure 4c and Figure S15). Byassigning the amide proton of BH1 (12.69 ppm) and hydrazone1 (11.87 ppm), we were able to calculate the molar ratio ofhydrazone 1 to BH1, which showed that the equilibrium oftransimination from hydrazone 1 to BH1 was established after21 h at the 80% yield for hydrazone 1 (Figure 4d). The above

results clearly demonstrated that the hydrazone’s reversible ordynamic covalent chemistry controls the morphology of self-assembled nanowires from supramolecular polymerization; theinitially nondirectional aggregates of 1 transformed into 1-Dself-assembled nanowires of BH1 through the DCvC (transimi-nation) of the hydrazones.

■ CONCLUSIONWe found that “dynamic covalent” hydrazone−pyridiniumconjugates (BHs) also underwent “dynamic noncovalent”(supramolecular) polymerization in the formation of stimuli-responsive nanowires. The hydrazone−pyridinium conjugatesshowed pH-dependent color changes, resulting from theirconversion from the protonated hydrazone to the deprotonatedmerocyanine-type zwitterion. The concentration-dependentUV−vis absorption spectra of the model hydrazone (H1)were analyzed by nonlinear regression analysis using theisodesmic model of aggregation to show that the dimerization(or further aggregation) was prevalent at higher concentration(aggregate fraction >90% above ∼200 μM in DMSO). Theintermolecular interactions between the hydrazone−pyridiniumconjugates were analyzed with the X-ray crystal structure ofH1,indicating that the van der Waals, π−π, and CH−π interactionsmight work together with the dipole−dipole interaction. Thecovalently dynamic character of the hydrazones was utilized indemonstrating control of the morphology in self-assemblednanowires; dynamic transimination enabled the conversion ofirregular aggregates to twisted wires. We believe that suchcontrol of dynamic molecular self-assembly will provide asignificant contribution to the development of novel multi-stimuli-responsive materials for applications such as drugdelivery system, actuators, and biosensors.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.macro-mol.8b01909.

Experimental methods, detailed synthetic procedures,supporting figures, 1H and 13C NMR spectra (PDF)CheckCIF/PLATON report (PDF)X-ray crystal structure of methylpyridinium hydrazone(CIF)

■ AUTHOR INFORMATIONCorresponding Authors*(J.H.) E-mail joonsukhuh@skku.edu.*(C.S.) E-mail songcs@skku.edu.ORCIDChangsik Song: 0000-0003-4754-1843NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was supported by the Nano Material DevelopmentProgram (2012M3A7B4049644) and the Small Grant Explor-a t o r y R e s e a r c h ( S G E R ) P r o g r a m ( N R F -2015R1D1A1A02062095) through the National ResearchFoundation of Korea (NRF) funded by the Ministry ofEducation, Science and Technology (MEST), Republic ofKorea. J.H. also acknowledges support by Basic Science

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Research Program through the NRF funded by the MEST(NRF-2015R1A6A3A04059773 and 2017R1A4A1015770).

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