Chromogenic Tubular Polydiacetylenes from ... - ACS Publications

Jan 3, 2017 - Jong-Man Kim: 0000-0003-0812-2507. Author Contributions. J.-M.H. and Y.K. contributed equally to this work. Notes. The authors declare n...
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Chromogenic Tubular Polydiacetylenes from Topochemical Polymerization of Self-Assembled Macrocyclic Diacetylenes Jung-Moo Heo,† Youngmee Kim,⊥ Seulki Han,‡ Joonyoung F. Joung,# Sang-hwa Lee,§ Sejin Han,‡ Jaegeun Noh,‡,∥ Jaeyong Kim,§,∥ Sungnam Park,# Haiwon Lee,‡,∥ Yoon Mi Choi,% Young-Sik Jung,& and Jong-Man Kim*,†,∥ †

Department of Chemical Engineering, ‡Department of Chemistry, §Department of Physics, and ∥Institute of Nano Science and Technology, Hanyang University, Seoul 04763, Korea ⊥ Department of Chemistry and Nano Science, Ewha Womans University, Seoul 03760, Korea # Department of Chemistry, Korea University, Seoul 02841, Korea % Center for Chemical Analysis and &Center for Medicinal Chemistry, Korea Research Institute of Chemical Technology, Daejeon 34114, Korea S Supporting Information *

ABSTRACT: Tubular materials formed by self-assembly of small organic molecules find great utility in chemical and material science. Conventional tubular structures often lack stability because noncovalent molecular interactions are responsible for their conformational integrities. Herein we report the development of covalently linked chromogenic organic nanotubes which are prepared by using topochemical polymerization of self-assembled macrocyclic diacetylenes (MCDAs). Crystal structures of five MCDAs having different diameters were elucidated, and four of these substances were transformed to tubular polydiacetylenes (PDA) by UV-induced polymerization. Surprisingly, MCDA-1 was found to self-assemble in stacks with a tilt angle of 62.1°, which significantly deviates from the optimal value for polymerization of 45°. This observation suggests that geometric parameters derived using linear diacetylene (DA) models might not be strictly applicable to polymerization of MCDA systems. Blue-phase PDAs obtained by polymerization of MCDA-1 and MCDA-3 have different thermochromic and solvatochromic properties, which enable them to be utilized for colorimetric differentiation of aromatic solvents including isomeric xylenes. The observations made and information obtained in this study should enhance the understanding and design of stimulus-responsive rigid organic nanotubes.

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INTRODUCTION Nature utilizes self-assembling materials and processes to construct systems that function to promote survival. This feature can be readily seen in the structure of cell membranes that are composed of self-assembled lipid bilayers. Nature inspired, self-assembling processes have been actively employed to develop functional biomimetic supramolecular structures.1−5 Especially interesting are cylindrically shaped tubular architectures that have internal pores with defined diameters.6−8 One of the most reliable approaches to obtain tubular supramolecules is by self-assembly of macrocyclic organic molecules.9−13 Accordingly, a variety of functional organic macrocyclic compounds have been designed and subjected to diverse selfassembly conditions to produce functional supramolecular nanotubes. One critical limitation of conventional tubular materials that are prepared by self-assembly of macrocyclic compounds is their lack of stability. This is due to the fact that conformational integrities of most of tubular structures are a consequence of © 2017 American Chemical Society

noncovalent molecular interactions including hydrogen bonding, π−π stacking and hydrophobic interactions. Accordingly, the tubular structures often lose their shapes when they are exposed to an environment where weakening of the noncovalent molecular interactions occurs. One obvious approach to making a tubular structure rigid is to connect the component molecules using covalent bonding. In this regard, macrocyclic compounds containing diacetylene (DA) moieties14−25 are very promising because properly aligned DAs can undergo topochemical polymerization26−28 to produce a conjugated polymer called polydiacetylene (PDA).29−39 As schematically represented in Figure 1, a macrocyclic diacetylene (MCDA) (Figure 1a) can self-assemble to form a tubular structure (Figure 1b). Polymerization of the stacked supramolecular MCDA tube should then lead to formation of tubular PDA, Received: November 17, 2016 Revised: January 3, 2017 Published: January 24, 2017 900

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Figure 1. A polydiacetylene nanotube from a macrocyclic diacetylene (MCDA). (a) Schematic representation of a MCDA. (b) Nanotube from selfassembly of MCDAs. (c) Topochemical polymerization of a MCDA nanotube results in formation of a polydiacetylene (PDA) nanotube. Geometric parameters for topochemical polymerization of DAs are d1 (repeat distance), d2 (contact distance), and α (tilt angle). Optimal values (d1 = 4.9 Å, d2 = 3.5 Å, and α = 45°). (d) Structure of MCDAs investigated for tubular PDA formation.

These geometric parameters have long been accepted as optimal or required for topochemical polymerization of selfassembled linear DAs. However, it is not known if these parameters are a requirement for polymerization of macrocyclic DA systems. This fundamental question and our interest in the design of colorimetric PDAs40−42 stimulated us to initiate the current investigation. In order to gain information about this issue, we prepared the five MCDAs shown in Figure 1d and investigated their crystal structures and polymerization behaviors as well as the morphologies and colorimetric responses of the resultant polymers. In the effort, we found that all five MCDAs afford high quality crystals suitable for single crystal X-ray analyses. With the exception of the largest macrocyclic DA (MCDA-5), the other MCDAs form intriguing tubular structures in the solid state and yield PDAs upon UV-induced topochemical polymerization. Surprisingly, efficient polymerization takes place with the molecularly stacked tubular MCDA-1 even though it has a tilt angle of 62.1° that significantly deviates from the optimal value of 45°. This result suggests that the geometric parameters associated with linear DAs, especially the tilt angle, might not be strictly required for polymerization of MCDAs. In addition, the covalently linked conjugated polymer allowed monitoring of a single tubular PDA chain after extensive dilution of the tubular polymer bundles in organic solvent. We also observed that depending on the structures of the MCDA precursors, the PDA nanotubes have varied thermochromatic and solvatochromatic properties and that their distinct behaviors can be used to differentiate between aromatic solvents including isomeric xylenes. The details of the investigation leading to these observations are described below.

poly(MCDA) (Figure 1c). Topochemical polymerization of the self-assembled MCDA not only makes the polymer nanotube rigid but also leads to a polymer that contains a conjugated ene−yne backbone. By employing this strategy, several PDA nanotubes have been prepared by using topochemical polymerization of MCDAs.14−25 Shimizu and co-workers reported single-crystalto-single-crystal polymerization of a self-assembled MCDA that contains aromatic amide moieties.14 Recently, Lauher and coworkers described a polyether MCDA that stacks into a columnar structure and reported that formation of PDA nanotubes was achieved by thermal annealing.15 The Morin group described preparation of rigid organic nanotubes with a phenylene−butadinylene macrocycle.16 Although Morin did not provide crystal structures for the monomer, he suggested that the polymerized material is best described as having a tubular architecture based on the monomer structure and TEM images. Nagasawa et al. prepared a series of macrocyclic diacetylenedicarboxamides and investigated their gelation and polymerization properties.17 Baughman described a strategy for fabrication of hydrocarbon nanotubes by sequential selfassembly polymerization−thermal annealing of MCDAs.18,19 Finally, fully conjugated annulene-type macrocycles with DA moieties have also been employed as good sources of polymer tubes.20−25 However, PDA-like tubular formation using these macrocycles has been very rare. In spite of the significance of tailor-made molecular nanotubes and the potential flexibility in designing these structures, only limited information is available about the precise molecular orientation of the MCDAs in the self-assembled columnar state that are required for PDA nanotube formation. In general, polymerization is known to proceed effectively when DA units are preorganized with a repeat distance (d1) of 4.9 Å (C1−C1′), contact distance (d2) of 3.5 Å (C1−C4′), and a tilt angle (α) of 45° (Figure 1). 901

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RESULTS AND DISCUSSION Molecular Design and Synthesis. Among many possible MCDA candidates, we have selected those whose structures are shown in Figure 1d because of the following reasons. First, each MCDA consists of two aromatic benzoate moieties linked with two DA-containing carbon chains, both of which enable π−π and hydrophobic interactions, respectively. These molecular interactions are expected to aid self-assembly of the MCDAs to form tubular structures. Second, the MCDA unit in these substances can be readily constructed with facile control of ring size using commercially available methyl 3,5-dihydroxybenzoate and different diacetylene diols. Third, the presence of carbon chains of different lengths between the diacetylene and the ether linker in the MCDA skeleton will enable an investigation of a possible odd−even number of carbon effect on PDA formation. Finally, the methyl ester group in the MCDAs can be potentially used to introduce additional functional groups before and after polymerization. The strategies employed for the synthesis of macrocyclic diacetylenes MCDA-1−5 are shown in Scheme 1. A sequential monoprotection−coupling−deprotection−macrocycliczation procedure was found to produce the desired MCDAs (see Supporting Information for details). Diacetylene diols 1−5, employed in these routes, were monoprotected with tertbutyldimethylsilyl chloride (TBDMS) to give the corresponding monoprotected diacetylenic alcohols 6−10. The alcohols were then converted to the corresponding mesylates 11−15, which were treated with methyl 3,5-dihydroxybenzoate to form the respective bis-diacetylenes 16−20. The TBDMS protecting groups in 16−20 were removed using of tetrabutylammonium fluoride (TBAF) to give bis-diacetylenic diols 21−25, which

were then converted to intermediate bis-bromides 26−30. Reactions of 26−30 with 3,5-dihydroxymethylbenzoate generated the target macrocyclic diacetylene monomers MCDA1−5. All intermediates in these sequences and the MCDAs were fully characterized by using spectroscopic methods. Importantly, analysis of MCDA-1−5 by using single-crystal X-ray crystallography unambiguously confirmed their structures. Structural Analysis. High quality single crystals suitable for X-ray crystallographic analysis were obtained by slow evaporation (at 30 °C) of solutions of MCDAs in tetrahydrofuran (for MCDA-1 and MCDA-3) and a 1:1 mixture (v/v) of tetrahydrofuran and n-hexane (for MCDA-2, MCDA-4, and MCDA-5). This procedure yielded micrometersized, transparent, and colorless crystals of all MCDAs (see Figure 4, inset). The cell constants and other crystallographic parameters for the MCDAs are presented in Tables S1 and S2. The geometric parameters for MCDAs in the solid state are summarized in Table 1. In Figure 2 are shown crystal structures of the three macrocyclic DAs MCDA-1, MCDA-2, and MCDA-3. Top views of a single molecule of each macrocyclic diacetylene unit in the crystal are displayed in Figures 2a−c (see Figures S1−S3 for detailed ORTEP drawings). In these units, the methyl Table 1. Geometric Parameters for Macrocyclic Diacetylenes

d1 (Å) d2 (Å) α (deg) 902

MCDA-1

MCDA-2

MCDA-3

MCDA-4

MCDA-5

4.694 4.438 62.09

5.325 3.946 47.74

4.777 4.108 56.04

5.031 (5.031) 4.017 (3.575) 52.01 (45.19)

9.869 7.544 42.65

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Figure 2. Crystal structures of macrocyclic diacetylenes MCDA-1−3. (a−c) Ellipsoid diagrams showing the crystal structures of macrocyclic DAs MCDA-1 (a), MCDA-2 (b), and MCDA-3 (c). Diacetylene moieties are colored green. (d−f) View along the column axis of crystal structures derived from MCDA-1 (d), MCDA-2 (e), and MCDA-3 (f). Hydrogen atoms and solvent molecules are omitted for clarity. (g−i) Side views of four consecutive macrocyclic DAs derived from MCDA-1 (g), MCDA-2 (h), and MCDA-3 (i). Macrocyclic DAs adopt a chairlike conformation in the crystal lattice. DAs are colored green to show how these moieties are in close proximity.

polymerization, the repeat distance is somewhat larger than the ideal value. The benzoate ring centroids in MCDA-2 are separated by 5.33 Å. As is the case with MCDA-1, effective aromatic and hydrophobic molecular interactions stabilize the tubular structure of self-assembled MCDA-2. The crystal of MCDA-3 has the following packing parameters that relate to those required for butadiyne topochemical polymerization: repeat distance 4.78 Å, contact distance 4.11 Å, and tilt angle 56.0° (Figure 2i). The tilt angle of 56.0° also significantly deviates from the optimal angle for topochemical polymerization of DAs. The benzoate ring centroids in MCDA-3 are separated by 4.78 Å. Single crystal X-ray diffraction analysis of MCDA-4 revealed the existence of two conformationally distinct polymorphs (Forms I and II) (Figure 3a) (see Figure S4 for detailed ORTEP drawing). The two forms display different packing arrangements and geometric parameters. Solvent molecules are also present in the cavity of MCDA-4, and the corresponding electron density in the disordered solvent regions was removed using the PLATON SQUEEZE. The cavity sizes in MCDA-4 of 14.99 Å × 11.78 and 17.20 Å × 10.68 Å were estimated by measuring the distances between the centers of double bonds on either side and the distances between two phenyl hydrogen atoms pointing to the center of the cycle. The packing parameters for Form I of MCDA-4 are repeat distance 5.03 Å, contact distance, 4.02 Å, and tilt angle 52.1° (Figure 3b). The benzoate ring centroids are separated by 5.03 Å. Interestingly, Form II of MCDA-4 has same repeat distance of 5.03 Å but a different contact distance (3.58 Å) and tilt angle (45.2°). Although Forms I and II have different geometric parameters,

benzoate moieties project outward from the macrocycle core. The cavity sizes, estimated by measuring the distance between the centers of double bonds on either side and the distances between the two phenyl hydrogen atoms pointing to the center of the macrocyclic ring, were found to be 9.69 Å × 8.71 Å for MCDA-1, 11.96 Å × 9.88 Å for MCDA-2, and 11.98 Å × 9.49 Å for MCDA-3. Packing diagrams showing views along the column axis are presented in Figures 2d−f. Viewing these diagrams shows that three MCDAs form nanoporous channels with solvent molecules filling the interior cavity of the tube. Electron density associated with the disordered solvent regions was removed using the PLATON SQUEEZE. Arrays of aromatic−aromatic and hydrophobic interactions stabilize the MCDA molecules in the crystalline state. Inspection of the side view of four consecutive MCDA-1 molecules shows that they adopt a chairlike conformation in the solid state (Figure 2g). The distance between the centroids of aromatic pairs is 4.69 Å, a value that is identical to the repeat distance (d1) between C1 and C1′ carbons. A contact distance (d2) of 4.44 Å (C1−C4′) and a tilt angle (α) of 62.1° were observed for MCDA-1. The monomer repeat distance of 4.69 Å is within the range (ca. 4.9 Å) of that required for facile topochemical polymerization. The contact distance of 4.44 Å, however, is considerably larger than the optimal value of 3.5 Å for polymerization. Moreover, the tilt angle 62.1° significantly deviates from the optimal value (45°) for polymerization. The geometric parameters for MCDA-2 are 5.33 Å for the repeat distance, 3.95 Å for the contact distance, and 47.7° for the tilt angle (Figure 2h). Although the contact distance and the tilt angle are those considered optimal values for 903

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Figure 3. Crystal structure of macrocyclic diacetylenes MCDA-4 and MCDA-5. (a) Ellipsoid diagrams showing the two different (Forms I and II) crystal structures of MCDA-4. Diacetylene moieties are colored green. (b) Side views of four consecutive MCDA-4 molecules in Forms I and II. Both Forms I and II adopt a chairlike conformation in the crystal lattice with different interchain distances and tilt angles. Hydrogen atoms and solvent molecules are omitted for clarity. Diacetylenes are colored green to show how these moieties are in close proximity. (c) Space-filling models of five consecutive macrocyclic DAs in Forms I and II of MCDA-4. (d) View along the column axis. (e) Ellipsoid diagram showing the crystal structure of MCDA-5. Diacetylene moieties are colored green. (f) Top-down view of MCDA-5 molecules in the solid state. Hydrogen atoms and solvent molecules are omitted for clarity. Diacetylene moieties are colored green. (g) Side views of four consecutive MCDA-5 molecules. The distance (d1) between two diacetylene moieties is too large for polymerization.

they share chairlike conformations. The space-filling models for Forms I and II show the DA moieties in each are in a proximity that is favorable for polymerization (Figure 3c). The tubular arrays in Forms I and II are clearly visible by inspecting perspective views along the column axis (Figure 3d). The largest macrocyclic DA investigated in this study, MCDA-5, displays unique molecular orientations in the solid state. An ellipsoid diagram for MCDA-5 in the crystal is shown in Figure 3e (see Figure S5 for detailed ORTEP drawing). Inspection of the top-down view of molecules indicates that two DA moieties in each MCDA-5 adopt zigzag orientations with neighboring DA groups with a plane of symmetry (Figure 3f). X-ray diffraction analysis of MCDA-5 reveals that the macrocyclic DA does form a very narrow channel structure with a cavity size of a 7.36 Å × 16.86 Å estimated by measuring the distance between one side double bond and the opposite side phenyl hydrogen atom and the distance between two alkyl hydrogen atoms pointing to the center of the cycle. Molecular

stacking unfavorable for topochemical polymerization in the crystal of MCDA-5 is associated with the long repeat distance of 9.87 Å (Figure 3g). In fact, UV irradiation or thermal treatment to the MCDA-5 crystals does not promote polymerization. The 46-membered ring structure of MCDA5, in the absence of additional favorable bonding interactions, is presumably too large to form effective self-assembled supramolecules needed for topochemical polymerization. Topochemical Polymerization. Upon exposure to 254 nm UV light, the colorless and transparent MCDA crystals become blue (for MCDA-1), orange (for MCDA-2), bluish purple (for MCDA-3), and wine-red (for MCDA-4) (Figures 4a−d insets). No color change occurs when crystals of MCDA5 are irradiated (data not shown). Irradiation promoted generation of color for MCDA-1−4 is indicative of PDA formation. This proposal is supported by the results of Raman spectroscopic analysis. The appearance of characteristic conjugated ene−yne bands at 1465 cm−1 (CC) and 2113 904

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Figure 4. Topochemical polymerization. (a−d) Raman spectra of single crystals of MCDA-1 (a), MCDA-2 (b), MCDA-3 (c), and MCDA-4 (d) before (black line) and after (green line) 254 nm UV light irradiation (2 mW/cm2) for 60 min and (25 mW/cm2) for 10 min. Inset photos show color changes of the macrocyclic DA crystals upon UV irradiation. (e) Visible absorption spectra of polymerized MCDAs. (f) Proportion of polymer formed as a function of time for irradiation of the MCDAs. Polymer proportion was calculated from the intensities of monomer and polymer peaks in the Raman spectra. It should be noted that a hand-held laboratory UV lamp (2 mW/cm2) was used for the first 60 min irradiation period and a high intensity UV source (25 mW/cm2) was employed for an additional 10 min irradiation period. This approach was employed because the monomer crystals MCDA-1 and MCDA-3 are more sensitive to UV light than are those of MCDA-2 and MCDA-4. Thus, polymerization occurs rapidly for MCDA-1 and MCDA-3 crystals when irradiated with a hand-held laboratory UV lamp while a high intensity UV illuminator is required to achieve a substantial amount of polymerization of MCDA-2 and MCDA-4.

cm−1 (CC) and near complete disappearance of the monomer band at 2258 cm−1 for MCDA-1 is strong evidence for formation of the corresponding PDAs (Figure 4a). Formation of a PDA was found to be less efficient for MCDA-2 under identical UV irradiation conditions, as reflected

in the observation that the monomer Raman band at 2261 cm−1 is seen even after prolonged UV irradiation (2 mW/cm2, 60 min and 25 mW/cm2, 10 min). The Raman spectrum of the orange-colored PDA formed from MCDA-2 contains conjugated ene−yne backbone bands at 1490 cm−1 (CC) and 905

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Macromolecules 2068 cm−1 (CC), respectively (Figure 4b). UV irradiation of a crystal of MCDA-3 leads to the efficient formation of the corresponding PDA that has Raman ene−yne backbone bands at 1484 cm−1 (CC) and 2130 cm−1 (CC) (Figure 4c). Finally, UV irradiation to the polymorph crystal of MCDA-4 results in generation of a PDA with Raman bands at 1470 and 2075 cm−1 and a retained band at 2261 cm−1 corresponding to unreacted monomer (Figure 4d). As expected, the polymers derived from MCDA-1−4 absorb visible light with wavelength maxima that decrease in the following order: poly(MCDA-1), poly(MCDA-3), poly(MCDA-4), and poly(MCDA-2) (Figure 4e). Interestingly, the rate of polymer formation was found to parallel this absorption trend. Thus, MCDA-1 is most rapidly transformed to the corresponding polymer while the rate of polymer production for MCDA-2 is the lowest (Figure 4f and Figure S6). It should be noted that similar size crystals of MCDAs were used for the kinetic experiment since different thickness can lead to the different responses to UV light. We performed GPC analyses to gain information about the molecular weight of the polymer. Most of the polymerized samples were found to be insoluble in organic solvents. Fortunately, the least polymerizable MCDA-2 resulted in formation of soluble materials that were suitable for GPC analysis after removal of insoluble aggregates. The molecular weight (Mw) of the polymerized and soluble MCDA-2 was measured to be 32 000 (Figure S7). Since aggregated particles are removed, the molecular weights observed do not represent the actual molecular weights of the polymer. However, the GPC data suggest that the UV irradiation to the crystalline macrocycles affords a substantial degree of polymerization. In our previous report,43 we observed the molecular weights of six soluble PDAs that display a blue-color are Mws of 50 000− 200 000. Because polymerized MCDA-1 and MCDA-3 show a blue color, we believe that polymeric PDAs are indeed generated upon UV irradiation. A combination of the GPC data and our experience with soluble PDAs strongly suggests that polymeric PDAs are produced from the UV-irradiated macrocyclic crystals. Information obtained from the Raman and visible absorption spectroscopic studies suggests that an odd−even carbon number effect exits in the monomer/polymer systems. Specifically, the observations show that macrocyclic monomers such as MCDA-1 and MCDA-3, which have odd numbers of carbon in chains between DA group and ether oxygen, undergo faster polymerization and generate longer wavelength absorbing PDAs than do MCDA-2 and MCDA-4, which have even numbers of carbon in chains. In addition, monomer Raman bands near 2260 cm−1 for MCDA-1 and MCDA-3 almost completely disappear upon 70 min UV irradiation while residual monomer bands are seen for MCDA-2 and MCDA4 even after prolonged (>2 h) UV illumination. It should be noted that the intensities of Raman bands for the polymerized ene−yne backbone are much larger than those for the monomer bands owing to the highly polarizable delocalized nature of the polymer. Thus, in many cases, Raman bands for the monomers are not observable even though small amounts of these substances are present in the resulting polymer matrix. The presence of significant amounts of unreactive monomers following UV irradiation of MCDA-2 and MCDA-4 indicates that DA packing in these substances is not ideal for polymerization in contrast to those of MCDA-1 and MCDA-3.

In Figure 5 are displayed schematic illustrations of spatial arrangements of two DAs for topochemical polymerization.

Figure 5. Schematic representation of two DAs for topochemical polymerization in the solid state. The parallelogram indicated using black lines represents two DAs that have been considered as having optimal geometric arrangements for topochemical polymerization. The optimal values are based on those gained in studies of linear DAs. Other colored parallelograms represent two DA positions in the single crystal of MCDAs (MCDA-1: blue; MCDA-2: orange; MCDA-3: purple; MCDA-4 (Form I): yellow; MCDA-4 (Form II): green). The colored dots represent carbon atoms.

The colored dots and lines in the illustrations represent carbon atoms and distances between two carbon atoms, respectively. The geometric arrangement that has long been accepted to be optimal for topochemical polymerization is shown in black lines and dots. The geometric arrangements of carbon positions for the MCDAs, obtained from X-ray crystallographic analysis, are also displayed in different colors. In this way, it is possible to create a schematic comparison of the geometric parameters of the MCDAs and those considered to be optimal values for polymerization. Surprisingly interesting information can be deduced from inspection of the geometric arrangements shown in Figure 5. For example, it can be seen that the position of second DA for MCDA-1 (blue color) deviates significantly from the optimal position as a consequence of a large tilt angle (62.1°), which differs from the angle of 45° that is desired for topochemical polymerization. Moreover, a crystal of MCDA-3 also has a large tilt angle of 56.0°, which deviates by more than 10° from the optimal value. As far as we are aware, only one report exists in which two DAs having a tilt angle larger than 60° undergo topochemical polymerization.44 Specifically, successful polymerization of a diiodobutadiyne-oxalamide cocrystal, which has a tilt angle of 65°, was reported by Goroff and co-workers.44 These workers observed that a colorless cocrystal of these substances becomes blue when first subjected to a high external pressure (2.8 GPa) followed by irradiation. It is possible that the high pressure treatment brings about molecular rearrangements in the crystal lattice prior to polymerization. Because the geometric parameters possessed by polymerizable self-assembled MCDAs reported to date have been within the desired ranges, the efficiencies with which MCDA-1 and MCDA-3 polymerize are both unexpected and surprising. Although this phenomenon is difficult to understand, relevant information comes from the work of Goroff on diiodobutadiyne cocrystals44 and Lauher on a terminal butadiyne.45 The results of these efforts suggest that the repeat distance is more important than the contact distance 906

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Macromolecules in governing DA topochemical polymerization. Importantly, the repeat distances for MCDA-1 (4.7 Å), MCDA-2 (5.3 Å), MCDA-3 (4.8 Å), and MCDA-4 (5.0 Å for Forms I and II) are close to the optimal value of 4.9 Å. Thus, observations made in our studies with the MCDAs support the previous findings and suggestions that the monomer repeat distance contributes more significantly to the topochemical polymerization of macrocyclic DAs than do the contact distance and tilt angle. Furthermore, the significant deviation of the tilt angle for self-assembled MCDA-1 from what is assumed to be optimal implies that perhaps the geometric parameters arising from studies of linear DAs might not be applicable to MCDA systems. In fact, the irradiation of macrocyclic monomers MCDA-1 and MCDA-3, which have larger that optimal tilt angles, results in generation of longer wavelength absorbing PDAs than those coming from MCDA-2 and MCDA-4. Because experimental difficulty exists in directly determining the crystal structures of polymerized MCDAs, quantum chemical calculations were used to estimate their optimized structures. To determine the general characteristics of the optimized structures, MCDA polymers having 10 MCDA repeating units with four CH2 end groups were used for the calculations. The initial structures of polymerized MCDAs were estimated using the crystal structures of the self-assembled MCDAs. In addition, the ONIOM (QM/QM) schemes with high and low layers implemented in Gaussian 09 package were used to obtain the final optimized structures of polymerized MCDAs.46,47 Density functional theory method (B3LYP) with a 6-31g basis set was used to treat the ene−yne backbone as the high layer in the ONIOM schemes whereas a semiempirical method (PM6) was used to treat the remaining low layer. The detailed, two-layered ONIOM schemes are presented in Figures S7−S11. In Figure 6 are shown the optimized structures of topochemically polymerized MCDA-1 and MCDA-2, which exhibit well-defined tubular structures. Interestingly, the polymerized MCDA-1 tube has a relatively linear conformation, which results in efficient overlap of the porbitals in the ene−yne backbones (Figures 6a,c). This shape is likely responsible for the observation that the polymer absorbs longer wavelength visible light. In contrast, polymerized MCDA-2 has a bent tubular conformation (Figures 6b,d), which brings about less efficient overlap of the p-orbitals in the ene−yne backbone. This structural difference is directly reflected in the fact that polymerized MCDA-2 absorbs the shortest wavelength light (Figure 4e). In addition, the quantum chemical calculations indicate that polymerized MCDA-3 and MCDA-4 have relatively straight tubular structures (Figure S13). As described in the Introduction, conventional tubular structures prepared by self-assembly of macrocyclic organic molecules are prone to lose their conformations when exposed to environments where noncovalent molecular interactions are disrupted. In contrast, the polymerized MCDAs are stable in various organic solvents. For instance, sonication of polymerized MCDA-1 crystals in chloroform leads to formation of nanowires (Figure 7a). Since MCDA-1 is highly soluble in chloroform, the monomer crystals do not form nanowires in this solvent. Thus, it is likely that each nanowire of polymerized MCDA-1 (Figure 7b) consists of bundles of individual tubular poly(MCDA) (Figure 7c). The Cryo-TEM image displayed in Figure 7d further demonstrates the stability of the PDA tubular nanobundles. In addition, a single polymer tube is observed after extensive dilution of the polymer aggregate in benzene

Figure 6. Optimized tubular structures. (a−d) Optimized structures of polymerized MCDA-1 (a, c) and MCDA-2 (b, d) models. Side (a, b) and tubular (c, d) views obtained by using a density functional theory (DFT) method shows the polymerized MCDA-1 and MCDA-2 adopt different molecular shapes. The tubular structure of the polymer obtained from MCDA-1 possesses a relatively straight channel while the MCDA-2 derived tube adopts a bent conjugated ene−yne backbone conformation (ene−yne backbones are colored green).

(Figure 7e). High-resolution scanning tunneling (STM) image in Figure 7e clearly demonstrated the formation of a sing poly(MCDA-1) nanotube with a 3.35 nm width on HOPG surface (Figure 7f). Polymers of MCDA-2 (Figure 7g), MCDA3 (Figure 7h), and MCDA-4 (Figure 7i) also generate nanotubular bundles. In addition, single polymer tube are observed for polymerized MCDA-3 and MCDA-4 (Figure S14) but not for polymerized MCDA-2. Colorimetric Responses. One fascinating feature of PDAs is the brilliant color change they undergo (typically blue-to-red) in response to physical and chemical/biochemical stimuli.48−59 The stimulus-responsive color transition of PDAs has served as the basis for the design of efficient colorimetric sensing systems.48−59 In order to utilize this colorimetric property for sensing, it is important that PDA initially have a blue color. In this regard, formation of blue or bluish-purple colored PDAs from MCDA-1 and MCDA-3 is interesting. Accordingly, colorimetric responses to heat and organic solvent of these polymer crystals were investigated. Thermochromism60−64 and solvatochromism65−68 are two frequently explored colorimetric properties of PDA. The thermochromic response of polymerized MCDA-1 was investigated first. A crystal of the polymer derived from MCDA-1 was found to undergo reversible thermochromism. Thus, the original dark blue color of the crystal becomes purple red at 130 °C and the original blue color is recovered upon cooling of the heated polymer (Figure 8a, inset photos). The color changes correspond to changes taking place in visible absorption spectrum during the heating−cooling cycle (Figure 8a). However, irreversible thermochromism occurs when the polymer crystal is heated to 150 °C, which is the melting temperature of MCDA-1 (Figure S15). In this case, the formed purple-red colored polymer retains its color even after cooling 907

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Figure 7. Microscope images of polymerized MCDAs. (a) TEM image of polymerized MCDA-1 nanowires obtained after sonication in chloroform. (b) TEM image of a polymerized MCDA-1 nanowire. (c) Schematic representation of a bundle of nanotubes that are composed of an individual PDA nanotube. (d) Cryo-TEM image of a polymerized MCDA-1 nanowire in chloroform. (e) STM image of a single poly(MCDA-1) chain on HOPG surface obtained after sonication of a poly(MCDA-1) nanowire suspension in benzene (scale bar: 20 nm). (f) Width profile of polymerized MCDA-1. (g−i) TEM images of polymerized MCDA-2 (g), MCDA-3 (h), and MCDA-4 (i) obtained after sonication in chloroform.

partially recovered upon cooling to 30 °C (Figure S16). Monitoring polymerized MCDA-3 during the thermal cycle shows that the Raman bands associated with the ene−yne backbone broaden at 120 °C and undergo a slight band shift (Figure 8d). Interestingly, the monomeric acetylene band at 2254 cm−1 appears at 120 °C while it is unobservable initially at 30 °C. This observation suggests that unreactive MCDA-3 molecules in the polymer crystal are released from the crystal lattice upon heating. Raman bands in the C−C stretching region give useful information about the conformations of alkyl chains in the PDA backbone.43,69 The spectra of blue-phase MCDA derived PDAs contain several weak bands at 1037, 1046, 1058, 1070, and 1089 cm−1 (Figure S17a), which are associated with alkyl chains containing trans conformations. Heating polymerized MCDA-1 to 130 °C does not induce significant spectral changes in the region containing bands that are characteristic of alkyl chains. However, upon heating this polymer to 150 °C, these bands disappear being replaced by one major band at

to 30 °C (Figure S15, inset photos). Unlike polymerized MCDA-1, which displays reversible thermochromism between 30 and 130 °C, polymerized MCDA-3 undergoes an irreversible colorimetric transition in response to thermal stimulation. Specifically, the bluish purple-color of poly(MCDA-3) at 30 °C becomes purple red when the polymer is heated to 120 °C, and the initial color is not recovered upon cooling the sample to 30 °C (Figure 8b). This reversible thermochromism of polymerized MCDA-1 up to 130 °C was also evaluated by using Raman spectroscopy (Figure 8c). The bands of polymerized MCDA-1 at 1465 cm−1 (CC) and 2113 cm−1 (CC) were found to shift to 1467 and 2104 cm−1 at 130 °C. These changes are promoted by thermally induced conformational distortion of the PDA backbone. The Raman bands for the ene−yne groups in the red phase PDAs return to their original positions upon cooling to 30 °C. At temperatures above the monomer melting point (150 °C), the Raman bands at 1465 cm−1 (CC) and 2113 cm−1 (CC) shift to 1497 and 2105 cm−1 and are only 908

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Figure 8. Colorimetric response of nanotube. (a, b) Visible absorption spectra showing the thermochromic properties of polymerized MCDA-1 and MCDA-3 upon heating and cooling. Photos of polymer crystals at the designated temperature are also displayed. (c, d) Raman spectra of single crystals of polymerized MCDA-1 and MCDA-3 upon heating and cooling. (e, f) Visible absorption spectra of polymerized MCDA-1 (e) and MCDA-3 (f) upon exposure to aromatic organic solvents. It should be noted that the absorption spectra for the polymer exposed to o-xylene (e) and p-xylene (f) were recorded using powders owing to the insolubility of the polymer in these solvents.

lower frequency (1047 cm−1). The results suggest that the trans C−C conformation of the alkyl chain is altered during the color transition and becomes at least partially gauche. Although not as clear as in the case of MCDA-1, the Raman bands for polymerized MCDA-3 in the alkyl chain region also display gauche conformational characteristics at high temperature (Figure S17b). The thermochromic behaviors of polymerized MCDA-1 and MCDA-3 are worthy of further comment. Polymerized MCDA-1 was found to display reversible blue-to-red thermochromism between 30 and 130 °C. The reversible thermochromism is presumably a result of the fact that thermal energy absorbed by the polymer results in the increase in chain mobility and a consequent distortion of the ene−yne backbone.

Upon cooling to 30 °C, the original conformation of the backbone is restored with recovery of the blue color. However, when the polymer is heated to the monomer melting temperature of 150 °C (see Figure S18 polymer DSC curves), the original blue color of the polymer is not recoverable. In this situation, unpolymerized MCDA-1 molecules are released causing voids to be generated in the polymer structures. This change enables the polymer chains to undergo severe twisting to form conformation(s) that cannot reconvert to the original upon cooling. Polymerized MCDA-3 displays an irreversible colorimetric response to thermal stimulation. The difference between the thermochromic behavior of polymers derived from MCDA-1 and MCDA-3 is presumably a result of a ring-size effect. The 909

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from commercially available methyl 3,5-dihydroxybenzoate and diacetylene diols. Second, the facile generation of high quality crystals suitable for the single crystal analysis is another meritorious feature of the macrocyclic systems employed in this investigation. Third, except for MCDA-5, the largest macrocyclic diacetylene investigated, all MCDAs undergo UVinduced topochemical polymerization. Fourthly, the effective topochemical polymerization of self-assembled MCDAs with unfavorable geometrical parameters, especially with the significant deviation of tilt angle (>60°) from optimal 45°, suggests that the stacking parameters derived from linear DA models might not be strictly applicable for the MCDAs. Lastly, topochemical polymerization of the macrocyclic diacetylene monomers was found to generate blue-phase polydiacetylenes that have different thermochromic and solvatochromic properties. As a consequence of these properties, the blue-phase tubular polymers can be utilized for colorimetric differentiation of aromatic solvents including isomeric xylenes. Accordingly, the potential application of the tubular polymer as a colorimetric sensor was demonstrated. The combined results suggest that the macrocyclic scaffold derived from a dihydroxybenzoate and a diacetylene diol along with introduction of proper modification of the ester moieties will be a potentially useful framework for constructing functional tubular conjugated polymer structures. Thus, we believe the study described above opens a new avenue for the design of covalently linked functional organic nanotubes.

relatively smaller ring size of MCDA-1 imparts a greater rigidity to the self-assembled structure and a lower mobility to the alkyl chain. The densely packed nature and restricted mobility of the alkyl chain gives the polymer derived from MCDA-1 the ability to recover its original conformation upon cooling. In contrast, polymerized MCDA-3 has a larger ring size, and as a result, its molecular packing has a lesser effect on restricting chain mobility than in polymerized MCDA-1. Consequently, release of unreacted MCDA-3 molecules from the polymer crystal even at temperatures below the melting temperature causes unrecoverable disruption of the chain conformation. The observation of thermochromism for the poly(MCDA-1) and poly(MCDA-3) suggests that flexible alkyl spacers that allow some movements upon heating are necessary. In addition, unreacted monomers and/or oligomers present in the polymer matrix provide some space for the backbone twisting when the polymer absorbs thermal energy. Morin’s observation of no thermochromism with a rigid macrocyclic diacetylene-derived polymer further supports these requirements.25 The solvatochromic behaviors of polymer tubes derived from MCDA-1 and MCDA-3 were also investigated. A polymerized MCDA-1 crystal does not undergo a color change in the presence of acetonitrile, ethanol, ethyl acetate, acetone, and hexane. In contrast, the crystal displays a blue-to-pink color change in toluene and a blue-to-yellow color change in THF, benzene, dichloromethane, and chloroform (Figure S19). A polymerized MCDA-3 crystal undergoes colorimetric responses to a wide range of common organic solvents, including blue-topink in m-xylene, acetonitrile, ethyl acetate, and acetone and blue-to-yellow in o-xylene, toluene, THF, benzene, dichloromethane, and chloroform (Figure S20). Interestingly, polymerized MCDA-1 can be employed to differentiate colorimetrically between benzene, toluene, and o-xylene (Figure 8e, inset). Specifically, the crystal has a blue color in o-xylene, pink in toluene, and yellow in benzene. These phenomena are directly reflected in the corresponding absorption spectral changes (Figure 8e). Moreover, interesting observations are made when poly(MCDA-3) crystals are exposed to isomeric xylenes. An immediate blue-to-yellow color transition occurs when the polymer crystals are treated with o-xylene while the polymer becomes pink in m-xylene, and no color change takes place in p-xylene. The absorption spectral changes displayed in Figure 8f parallel these observations. The solvatochromic phenomenon experienced by PDAs occurs when free monomers in the polymer matrix are released to solvent and consequent twisting takes place in the PDA chains. This change lessens the degree of overlap of the arrayed p-orbitals, which results in the color transition. Thus, monomer solubility directly affects the solvatochromic properties of PDAs. The solvatochromic properties of macrocyclic DA derived tubular PDAs are also governed by the same general principle that applies to other types of PDAs. Accordingly, the blue PDAs obtained from MCDA-1 and MCDA-3 display colorimetric responses that depend on the solubility of the corresponding monomeric DAs in solvents (Table S3).

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EXPERIMENTAL SECTION

Materials. Deca-4,6-diyne-1,10-diol (1) and dodeca-5,7-diyne-1,12diol (2) were used as received from GFS Chemical (Powell, OH). Tetradeca-6,8-diyne-1,14-diol (3), hexadeca-7,9-diyne-1,16-diol (4), and octadeca-8,10-diyne-1,18-diol (5) were prepared using literature procedures.70,71 3,5-Dihydroxybenzoate, methanesulfonyl chloride, and tetrabromomethane were purchased from Sigma-Aldrich (Korea). tert-Butyldimethylsilyl chloride and tetrabutylammonium fluoride solution (1 M in THF) were purchased from Tokyo Chemical Industry (Korea). Instrumentation. Optical/fluorescence microscopic images were collected with an Olympus BX 51W/DP70. Raman spectra were recorded on a LabRAM HR Evolution Raman spectrometer (Horiba Scientific, 785 nm laser source). TEM investigations were carried out using a JEOL TEM-2100F microscope. Cryo-TEM investigations were carried out using a Tecnai F20 G2. STM images were colleccted with a NanoScopeE (Veeco, Santa Barbara, CA) and a commercially available Pt/Ir tip. The UV absorption spectra were recorded on a single beam Agilent 8453 UV−vis spectrometer (Agilent Technologies, Waldbronn, Germany). The absorption spectra were recorded on an USB2000 miniature fiber-optic spectrometer (Ocean Optics). A homemade Peltier device was used to investigate the thermochromism of the polydiacetylene at high temperatures. IR spectra were recorded on a Thermo Nicolet NEXUS 470 FTIR uisng an ATR accessory (Thermo Fisher Scientific, Inc.). The 1H and 13C NMR spectra were recorded on a Varian Unitylnova (300 MHz) spectrometer at 298 K in CDCl3. DSC spectra were obtained on a DSC2010 (TA Instruments. Inc.). High-resolution mass spectra (HRMS) were recoreded on a SYNAPT G2 (water, U.K.) using a time-of-flight (TOF) analyzer. Mass spectra (MS) were recorded on an AXIMA (Shimadzu). XRD spectra were recorded with a HR-XRD, D8 discover (Bruker). Synthesis of Macrocyclic Diacetylene (MCDA). The macrocyclic diacetylenes investigated in this study were prepared from commercially available methyl 3,5-dihydroxybenzoate and diacetylene diols. A sequential monoprotection−coupling−deprotection−macrocyclization procedure afforded desired MCDAs (see Supporting Information for details).

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CONCLUSIONS We have developed new types of covalently linked chromogenic organic nanotubes, which are prepared by using topochemical polymerization of self-assembled macrocyclic diacetylenes. Final comments about the diacetylene containing macrocycles MCDA-1−5 employed in this effort are in order. First, the macrocycles described above are readily prepared 910

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X-ray Crystallography. High quality single crystals suitable for Xray crystallographic analysis were obtained by slow evaporation (at 30 °C) of solutions of MCDAs in tetrahydrofuran (for MCDA-1 and MCDA-3) and a 1:1 mixture (v/v) of tetrahydrofuran and n-hexane (for MCDA-2, MCDA-4, and MCDA-5). This procedure yielded micrometer-sized, transparent, and colorless crystals of all MCDAs. The X-ray diffraction data for all five compounds were collected on a Bruker APEX-II diffractometer equipped with a monochromator in the Mo Kα (λ = 0.710 73 Å) incident beam. Each crystal was mounted on a glass fiber. The CCD data were integrated and scaled using the Bruker-SAINT software package, and the structures were solved and refined using SHELXTL V6.12.72 All hydrogen atoms were placed in the calculated positions. The crystallographic data for compounds MCDA-1−MCDA-5 are listed in Table S1. Structural information was deposited at the Cambridge Crystallographic Data Center (CCDC reference numbers are 1497281−1497285 for MCDA-1−MCDA-5, respectively). Quantum Chemical Calculations. The optimized structures of polymerized MCDAs were obtained by using quantum chemical calculations with density functional theory and semiempirical methods implemented in commercially available Gaussian 09 package.46 First, the optimized repeating units of polymerized MCDAs, which were estimated from the crystal structures of self-assembled MCDAs, were obtained by using the periodic boundary condition method (PBEPBE/ 3-21g). And then, the initial structures of polymerized MCDAs were constructed by adding 10 optimized repeating units with four CH2 end groups. Finally, the ONIOM (QM/QM) schemes with high and low layers were used to calculate the optimized structure of polymerized MCDAs.47 The ene−yne backbones in polymerized MCDAs were quantum mechanically calculated as a high layer by the density functional theory method (b3lyp) with a 6-31g basis set while the rest part was calculated as a low layer by the semiempirical method (PM6). The detailed ONIOM schemes of polymerized MCDAs are presented in Figures S8−S12.

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REFERENCES

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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/acs.macromol.6b02493. Detailed information on experimental procedures; computational, crystallographic, and spectroscopic data, including Figures S1−S49 and Tables S1−S3 (PDF) X-ray crystal structure of MCDA-1 (CIF) X-ray crystal structure of MCDA-2 (CIF) X-ray crystal structure of MCDA-3 (CIF) X-ray crystal structure of MCDA-4 (CIF) X-ray crystal structure of MCDA-5 (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (J.-M.K.). ORCID

Jong-Man Kim: 0000-0003-0812-2507 Author Contributions

J.-M.H. and Y.K. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported financially by Samsung Research Funding Center of Samsung Electronics under Project SRFCMA1501-06. 911

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DOI: 10.1021/acs.macromol.6b02493 Macromolecules 2017, 50, 900−913

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DOI: 10.1021/acs.macromol.6b02493 Macromolecules 2017, 50, 900−913