Triptycene-Based Ladder Polymers with One-Handed Helical Geometry

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Triptycene-Based Ladder Polymers with One-Handed Helical Geometry Tomoyuki Ikai, Takumu Yoshida, Ken-ichi Shinohara, Tsuyoshi Taniguchi, Yuya Wada, and Timothy M. Swager J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b13865 • Publication Date (Web): 28 Feb 2019 Downloaded from http://pubs.acs.org on February 28, 2019

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Journal of the American Chemical Society

Triptycene-Based Ladder Polymers with One-Handed Helical Geometry Tomoyuki Ikai,†,§,* Takumu Yoshida,† Ken-ichi Shinohara,‡ Tsuyoshi Taniguchi,† Yuya Wada,† and Timothy M. Swager§,* †

Graduate School of Natural Science and Technology, Kanazawa University, Kakuma-machi, Kanazawa 920-1192, Japan School of Materials Science, Japan Advanced Institute of Science and Technology (JAIST), 1-1 Asahi-dai, Nomi 923-1292, Japan § Department of Chemistry, Massachusetts Institute of Technology (MIT), 77 Massachusetts Ave, Cambridge, MA 02139, United States ‡

ABSTRACT: Here we report an efficient synthesis of optically active ladder-type molecules and polymers through intramolecular cyclization of chiral triptycenes containing bis[2-(4-alkoxyphenyl)ethynyl]phenylene units. The electrophile-induced cyclization reactions are directed away from the bridgehead carbon atoms of triptycene by steric factors, thereby producing one-handed twisted ladder units without any detectable byproducts. Moreover, the quantitative and regioselective nature of this intramolecular cyclization allowed us to synthesize optically active ladder polymers with a well-defined one-handed helical geometry, in which homoconjugated dibenzo[a,h]anthracene units are helically arranged along the main chain. This synthetic route enables the construction of a variety of nanoscale helical ladder architectures and provides an entry into new chiroptical materials.

INTRODUCTION Ladder polymers are defined as macromolecules consisting of an uninterrupted sequence of rings with adjacent rings having two or more atoms in common.1 50 years after the first report by Van Deusen,2 ladder polymers remain challenging synthetic targets for organic and polymer chemists.3 To afford structurally well-defined ladder polymers, reactions must be highly selective and quantitative to avoid undesired branching structures and/or imperfect ladder structures, in which the ring sequence in the polymer chain is partially interrupted. To date, several methods for a ladder formation, such as the Scholl reaction,3c,4 Diels–Alder polymerization,5 Friedel–Crafts substitution,6 Schiff base formation,7 Swern reaction,8 double nucleophilic aromatic substitution,3f,9 Tröger’s based formation,3f,10 norbornene-arene annulation,11 and olefin metathesis,3e,12 have been proposed, all of which meet severe synthetic limitations related to selectivity and quantitativity. Our group introduced a facile approach to ladder polymers and polycyclic aromatics using electrophilic aromatic substitution,13 which has now been expanded by others to synthesize pconjugated ladder-type compounds.3d,11b,14 Triptycene is a rigid, bulky, and paddle-wheel-shaped aromatic hydrocarbon with strictly limited conformational freedom because its structure is fixed by a [2.2.2] ring system.15 Purposely designed triptycene molecules and polymers have been synthesized16 and used as functional materials in a wide variety of applications including molecular machines,17 organic electronics,18 two-dimensional assembly,19 host–guest chemistry,20 gas transport/storage materials,21 and sensors.22 Unmodified triptycene has D3h symmetry and thus is an achiral molecule. However, if multiple of the same or different substituents are introduced onto the triptycene’s benzene rings

in an asymmetric manner, the derivatives are inherently chiral. Fundamental studies of the synthesis and characterization of optically active triptycenes began in the 1960s,23 and over the last few years, their practical applications as chiral materials have attracted increasing attention.24 Recently, we reported that optically active triptycenes can be used in chiral separation25 and as circularly polarized luminescence materials.26 Helix construction at molecular and supramolecular levels is a rational approach to materials that exhibit high performance and advanced functions.27 Examples from nature give evidence to this statement and single-handed helices in biological polymers, such as DNA and proteins, play vital roles in molecular recognition, catalytic activity, and genetic function.28 Thus, the fabrication of three-dimensional molecular architectures possessing both ladder and helix geometries may hold great potential to develop an unexplored domain of polymer materials. Although several groups have made ingenious synthetic attempts,14a,29 helical ladder polymers with a well-defined cyclic repeating unit have not yet been developed except for the noteworthy examples of polyhelicenes and their analogues.30 Furthermore, among polyhelicene-based helical ladder polymers, perfect control of helix handedness has only been achieved using metal–ligand coordination to realize a onehanded twisting ladder structure.30a To develop untapped opportunities in chirality, specially shaped polymers, and nanosized architectures, we have developed single-handed helical ladder polymers utilizing the rigid chiral framework of triptycenes and highly efficient ladder formation using electrophilic aromatic substitution. The triptycene unit is a key structural element and the inherently three-dimensional geometry favors our desired specific secondary structure. The expansive molecular designs

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developed around the triptycene scaffold are also a great advantage for the extension of this method to families of chiral materials. RESULTS AND DISCUSSION The synthesis of the key materials begins with (R,R)-2,6diiodotriptycene ((R,R)-1; see SI section 4 for assignment of absolute configuration), which is reacted with a pinacol boronate ester containing two p-alkoxyphenylethynyl pendants (2) via Suzuki–Miyaura coupling to give (R,R)-3 in 56% yield (Figure 1A). When (R,R)-3 was treated with trifluoroacetic acid (TFA) according to our previously reported procedure,13a an intramolecular multicyclization through electrophilic aromatic substitution proceeded in a regioselective manner and gave (R,R)-4 without producing any detectable regioisomers and/or byproducts (75% isolated yield). The complete regioselectivity was confirmed by the fact that the 1H NMR spectrum of the assynthesized product, which was measured after a simple extraction of the reaction system followed by removal of volatile substances, did not show any signals except for peaks from (R,R)-4 in the aromatic and bridgehead proton regions (Figure 1B; see Figure S1 for peak assignments). In situ monitoring by NMR (dichloromethane-d2, RT) also confirmed that (R,R)-3 was converted into a single product within 30 min of TFA addition (Figure S2). The cyclization reaction exclusively occurred at the 3- and 7-positions of the triptycene unit rather than at the

Figure 1. (A) Syntheses of optically active (R,R)-4. (B) 1H NMR spectra (500 MHz, CDCl3, rt) of (R,R)-3 (a), as-synthesized (R,R)4 (b), and isolated (R,R)-4 (c). (C) X-ray crystal structure of (R,R)4. Thermal ellipsoids are drawn at the 50% probability level.

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1- and 5-positions. A density functional theory calculation indicated that the carbon atoms at the 3- and 7-positions are not more nucleophilic than those at the 1- and 5-positions (Figure S3), suggesting that the observed perfect regioselectivity is the result of steric repulsion between the bridgehead proton and phenoxy pendant.31 The twisted ladder structure of (R,R)-4 was unambiguously determined by X-ray crystallographic analysis (Figure 1C).32 Circular dichroism (CD) and absorption spectra of (R,R)-3 and (R,R)-4 in chloroform are presented in Figure 2A. A very weak CD signal (|De| ≤ 35) was observed for (R,R)-3 in the absorption region of the dibenzo[a,h]anthracene unit (250–400 nm). In sharp contrast, (R,R)-4 exhibited an intense split-type CD band with a positive first Cotton effect in the corresponding region, which can be attributed to chiral exciton coupling between the dibenzo[a,h]anthracene units. This indicates that the two dibenzo[a,h]anthracene units within (R,R)-4 are arranged in a clockwise twisting manner, which is in good agreement with X-ray crystallographic analysis. It is noteworthy that the CD intensity was greatly enhanced after ladder formation and that the maximum |De| value of (R,R)-4 reached 346 at 303 nm, which was approximately ten times higher than that of (R,R)-3 (|Demax| = 35 at 305 nm). Similar chiroptical enhancement was observed in their specific optical rotations ([a]25D: +67 for (R,R)-3 and +763 for (R,R)-4). We also synthesized the opposite enantiomer, (S,S)-4, in the same way as (R,R)-4 (Scheme S1C) and confirmed that their CD spectral patterns were perfect mirror images of each other (Figure 2A). Furthermore, quantitative conversion into the larger ladder molecule (R,R,R,R)-8 containing two chiral triptycene units was achieved through regioselective ladder formation of the corresponding precursor (R,R,R,R)-7 (Scheme 1 and Figure S4). Again, (R,R,R,R)-8 showed a CD signal and specific optical rotation that were much larger than those of (R,R,R,R)-7 (Figure 2B). With these findings in mind, we have applied quantitative and regioselective cyclization of chiral triptycenes to the synthesis of helical ladder polymers with a well-defined molecular structure. To this end, poly-1R consisting of alternating chiral triptycene and bis[2-(4-alkoxyphenyl)ethynyl]phenylene units was synthesized via Suzuki–Miyaura coupling copolymerization of (R,R)-1 and diboronic acid bis(pinacol) ester 5 (Figure 3A). The number-average molecular mass, Mn(SEC), of the obtained poly-1R was estimated to be 1.05 × 104 g mol−1 by size-exclusion chromatography (SEC), which corresponds to a degree of polymerization (DP) of ca. 13 (Figure S5). Poly-1R was then treated with TFA to transform its backbone conformation from a random-coil into a rigid ladder structure with one-handed helical geometry to afford the helical ladder polymer, poly-2R. After reaction for 12 h at room temperature, the consumption of ethynyl groups in poly-1R was confirmed by IR spectral measurements, in which the C≡C stretching band at 2210 cm−1 observed for poly-1R were not detectable (Figure S6). 1H NMR spectra of poly-1R and its ladder product poly-2R are shown in Figure 3C and D, respectively (see Figure S7 for peak assignments). The proton resonances of poly-2R are broader than those of poly-1R, probably reflecting the highly rigid backbone conformation.13a,14c,30c,d As was the case with the abovementioned triptycene molecules, the NMR signal of the poly1R’s bridgehead protons observed at 5.54 ppm shift downfield to 5.74 ppm (peak c) after cyclization. A series of low-intensity signals at 5.43 ppm and 5.42 and 5.63 ppm in Figure 3C and D,


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Figure 2. CD and absorption spectra of 3 and 4 (A), 7 and 8 (B), and poly-1 and poly-2 (C) in chloroform at 25 °C. The spectra of molecules/polymers with (R,R)- and (S,S)-configurations are indicated by red and blue lines, respectively. The spectra indicated by dashed and solid lines were obtained before (3, 7, and poly-1) and after (4, 8, and poly-2) ladder formation, respectively. [3, 4, 7, 8, or triptycene unit in polymers] = 1.0 × 10−4 M.

respectively, was considered to originate from terminal triptycene units with an iodo group. This interpretation is supported by the fact that 1H NMR signals of analogous ladder molecules with iodo substituents ((R,R,R,R)-6 and (R,R,R,R)6’) were observed at almost the same positions as these small peaks (Figure S8). If half of the end groups for poly-1R and poly-2R are iodo groups, their NMR integrations provide approximate DP of 12 in both cases (Figure S8E and G), which are in accord with the SEC estimate of poly-1R. In agreement with a previously reported results for ladder polymer analysis,13a the SEC curve was shifted toward lower molecular mass regions after ladder formation (Figure S5). The Mn(SEC) value of poly-2R was estimated to be 0.80 × 104 g mol−1, a value three-fourths that of poly-1R. This decrease of Mn(SEC) is probably because the radius of gyration of poly-1R was lowered through cyclization to form poly-2R, as a result of its restricted structural freedom. The shifted SEC curve also confirms that undesired cross-linking reactions involving intermolecular electrophilic aromatic additions are not problematic. The characteristic resonance signals observed around 9.01 and 8.85

ppm (peak f and g, respectively) in Figure 3D suggested the formation of dibenzo[a,h]anthracene ring systems. In addition, the (peak c)/(peak f and g) area ratio was 1:2 (Figure S8A),

Scheme 1. Syntheses of Optically Active (R,R,R,R)-6’ and (R,R,R,R)-8

Figure 3. (A) Synthesis of helical ladder poly-2R. (B) Structure of poly-2S. (C, D) 1H NMR spectra (500 MHz, CDCl3, 55 °C) of poly1R (C) and poly-2R (D). Asterisks denote resonance peaks derived from iodo-appended terminal triptycene units.



Figure 4. PL spectra of (R,R)-3 and (R,R)-4 (A), (R,R,R,R)-6 and (R,R,R,R)-6’ (B), and poly-1R and poly-2R (C) in chloroform at 25 °C. Photographs of the corresponding solutions under 365-nm irradiation are also shown. [3, 4, 6, 6’, or triptycene units in polymers] = 1.0 × 10−5 M.

which indicates that the ideal equimolar ratio of dibenzo[a,h]anthracene to triptycene units was present in poly2R. The photoluminescence (PL) behavior of poly-1R also changed apparently upon structural conversion into poly-2R (Figure 4), which showed a 38 nm red-shift in the PL maxima due to a higher degree of electronic delocalization. This PL spectral change was in good agreement with that observed for the model compounds 3 and 4. The small shoulder peak around 400 nm in the PL spectrum of poly-2R was most likely derived from the iodo-substituted terminal triptycene units, which is also consistent with comparisons with the PL spectrum of (R,R,R,R)-6’ (Figure 4B). Meanwhile, unlike the cases of (R,R)4 and (R,R,R,R)-6’, a broad PL band in the wavelength region of 470–600 nm was notably observed in the poly-2R’s spectrum, which leads to a characteristic bluish white emission. In sharp contrast to the IR, NMR, and PL spectra, which showed clear differences after the ladder formation process, poly-1R and its reaction product displayed similar MALDITOF mass spectra (Figure S9). A series of peaks were observed

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at a regular interval of 784.4 amu in both cases, which corresponds to the calculated molecular mass of each repeating unit. These spectrometric observations revealed that the intramolecular multicyclization of poly-1R proceeded almost perfectly without side reactions and that the helical ladder poly2R containing chiral triptycene units was successfully produced. The limited volatilization of compounds under MALDI conditions precludes this method from determining Mn. To obtain a better understanding of the helical ladder structure of poly-2R, an all-atom molecular dynamics (MD) simulation of a 12-mer model of poly-2R, which includes two helical turns, was conducted (Figure 5A and Movies S1–S3). The initial structure was constructed based on the X-ray structural analysis of (R,R)-4. After equilibration at 298 K (see the Supporting Information for details), the simulation was carried out for 2000 ps in the microcanonical ensemble (NVE) as the production run. The molecular models in the final (2000 ps) state are presented in Figure 5B and C. The corresponding molecular models in the initial (0 ps) and middle (1000 ps) stages are also shown in Figure S10. The time-dependent changes of the torsion angle between the two dibenzo[a,h]anthracene planes (qi; see Figure 5D), which are linked through the bridgehead carbons of the ith triptycene unit from the terminal one, are plotted in Figure 5E. The average torsion angles (𝜃̅i) in the 12-mer model and their standard deviations (SD) are summarized in Table S1. The 𝜃̅i values fell within the range from +108.1° to +123.3°, which is considered to be reasonable given the corresponding torsion angle in the crystal structure of (R,R)-4 (+117.3°). This suggests that all pairs of the neighboring dibenzo[a,h]anthracene planes were perfectly arranged in a clockwise twisting manner during the whole calculation period, as observed in the crystal structure of (R,R)-4, thus producing a right-handed helical ladder geometry. In other words, the backbone conformation of poly2R cannot depart from the right-handed helix structure because of the consecutive clockwise twisting ladder structure even though the polymer has a certain degree of mobility, as depicted in Movie S3. The generation of such a helical ladder structure was also supported by atomic force microscopy (AFM) measurements. Figure 6 shows a representative AFM image of poly-2R on a mica substrate, in which perforated film structures with a height of ca. 3 nm were observed. It should be noted that this film thickness is in agreement with the helix diameter of a

Figure 5. (A) Structure of the poly-2R model (12-mer) used for the computational study. (B) Side view and (C) top view of the molecular model of the helical ladder poly-2R in chloroform at 2000 ps in an all-atom MD simulation after equilibration at 298 K represented by spacefilling (backbone) and line (side chains) models. The carbon atoms in the dibenzo[a,h]anthracene units and the bridgehead carbons are highlighted in blue and the chloroform solvent molecules are omitted to simplify the view. (D) Definition of the torsion angle between the two dibenzo[a,h]anthracene planes (θi), which are linked through bridgehead carbons of the ith triptycene unit from the terminal one. (E) Plots of θi as a function of calculation time.


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Figure 6. (A) AFM image (1.80 µm × 1.80 µm) of poly-2R on mica in air at 25 ± 1 °C. (B) Height profiles measured along the dashed lines labeled a–d in (A).

computer-generated molecular model of poly-2R (Figure 5 and S10). Thus, the film structures observed in Figure 6A would likely be derived from a monomolecular film consisting of the helical ladder poly-2R. Further investigations on the direct observation of the helix structure of poly-2R by high-resolution AFM and scanning tunneling microscopy is now in progress in our laboratories. We next studied the chiroptical properties of the helical ladder polymer (see Figure 2C). As inferred from the above discussion of the chiral ladder molecules, the ladder-type poly-2R showed a more intense CD signal than that of poly-1R. Poly-2S was also synthesized from (S,S)-1 through polymerization and ladder formation in the same way as poly-2R (Figure 3B and Scheme S1D), and poly-2R and poly-2S showed perfect mirror-image CD spectra as a result of their enantiomeric relationship (Figure 2C). The CD pattern and intensity of poly-2R remained unchanged over the temperature range of −10 to 55 °C, owing to its rigid helical ladder structure (Figure S11). Poly-1R and poly-2R also showed a clear difference in vibrational CD spectra (Figure S12), particularly in the region of aromatic skeletal vibration (1400–1500 cm−1). To investigate how the chiroptical properties of poly-2R were influenced by the molecular mass, three samples with different Mn(SEC) values were prepared by SEC fractionation and then their CD signal intensities were compared. As shown in Figure S13, the maximum CD values reached a plateau when Mn(SEC) exceeded 0.79 × 104 g mol−1, where the influence of the terminal end-group weakening CD intensity was almost negligible (Figure S14). We compared the CD signal intensities of the fraction of poly-2R with Mn(SEC) of 1.58 × 104 g mol−1 and the corresponding ladder molecules (R,R)-4 and (R,R,R,R)-8. Kuhn’s dissymmetry factor (gabs = ΔAbs/Abs), which can be interpreted as the CD intensity per molar absorption coefficient, was used for this chiroptical comparison because the numbers of dibenzo[a,h]anthracene units contained in each single molecule or repeating unit are different, which results in large variation of molar absorption coefficients. The gabs spectra of (R,R)-4, (R,R,R,R)-8, and poly-2R (Figure S15) revealed that the maximum |gabs| value increased with ladder length in both positive and negative directions (Table S2). In particular, the long-wavelength region shows a positive first Cotton effect with an average gabs value of poly-2R (4.1 × 10−3) that was approximately twice the value determined for the monomeric model compound (R,R)-4 (1.8 × 10−3). Although the absorption regions and patterns are different from each other and a simple

comparison is difficult, these CD studies provided further convincing evidence that poly-2R backbone has the proposed helical ladder structure without a detectable levels of structural defects. In 2004, Pu and co-workers reported the first attempt at the synthesis of a helical ladder polymer by the same cyclization reaction used in the present study.14a In this case, an optically active 1,1’-binaphthyl framework was used as a chiral source and two possible reaction sites existed in the corresponding naphthyl rings. Thus, perfect regioselectivity was required to construct a defect-free helical ladder polymer, as pointed out by the authors. Because there was no experimental evidence for the integrity of regioselectivity in their report, we verified it here using an analogous 1,1’-binaphthyl-based model molecule ((R)Bi-b; see Figure S16A). Cyclization of (R)-Bi-b under the same conditions as described by Pu et al.14a led to the formation of at least two regioisomers, as confirmed by NMR measurements (Figure S16B and C). This indicates that the regioselectivity of the corresponding macromolecular cyclization is not perfect and the resulting ladder polymer contains irregular structures. The present poly-2R therefore stands as the second successful example of a one-handed helical ladder polymer with a regular repeating unit, with the first example being a helicene-based coordination polymer.30a In the present helical ladder polymers, the p-conjugated planes of the repeating unit are basically oriented parallel to the helix axis. Thus, a p-electronic surface defines a helical cavity with the potential to form p-p interactions with guest substances, including molecules, polymers, and carbon nanotubes. Such a p-electron environment is critically different from the previously reported polyhelicene-type ladders, in which p-conjugated planes are arranged perpendicular to the helix axis and p-p interactions are mostly self-contained within each molecule. Preliminary attempts for material application revealed that poly-2R exhibited an apparent resolution ability when applied to a chiral stationary phase for high-performance liquid chromatography (Figure S17A), whereas poly-1R did not (Figure S17B). CONCLUSIONS We have described a highly efficient ladder formation using a chiral triptycene framework, in which an intramolecular multicyclization reactions occur in a quantitative and regioselective manner to provide single-handed twisted ladder molecules 4 and 8. In addition, this ladder formation could be applied to polymers containing chiral triptycene units in the main chain without losing the integrity of reaction selectivity. In the macromolecular ladder structure of the resulting poly-2R and poly-2S, dibenzo[a,h]anthracene units were contiguously arranged along the main chain through triptycene-based two bridgehead carbons, and the backbones were twisted in one direction to construct a helical ladder shape. We believe that these ladder polymers, which can fall into a new category of helical polymers, represent a promising class of advanced materials for use as nanochannels for molecular/ion transport, organic electronics, specific reaction fields, and functional hosts through further modification of the backbone and pendant units. Work towards these goals is now underway in our laboratory.

ASSOCIATED CONTENT Supporting Information



The Supporting Information is available free of charge on the ACS Publications website. Experimental procedures, characterizations of monomers, polymers, and model compounds, and additional spectroscopic, chromatographic, and computational data (PDF) Crystallographic data for rac-4 (CIF) Movie S1 (AVI) Movie S2 (AVI) Movie S3 (AVI)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the Japan Society for the Promotion of Science (JSPS) KAKENHI through a Grant-in-Aid for Scientific Research (C) (Grant no. 17K05875). Work at MIT was supported in part by the Air Force Office of Scientific Research (Grant no. 17RT0904). Computation time was provided by the supercomputer system, Research Center for Advanced Computing Infrastructure, Japan Advanced Institute of Science and Technology (JAIST). We are appreciative of Dr. Akio Ohta for DFT calculations, Mr. Taro Mori for preparing chiral columns, and Ms. Mari Ikurumi for her assistance with HRMS measurements.

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Figure 1. (A) Syntheses of optically active (R,R)-4. (B) 1H NMR spectra (500 MHz, CDCl3, rt) of (R,R)-3 (a), as-synthesized (R,R)-4 (b), and isolated (R,R)-4 (c). (C) X-ray crystal structure of (R,R)-4. Thermal ellipsoids are drawn at the 50% probability level. 164x226mm (300 x 300 DPI)


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Figure 4. PL spectra of (R,R)-3 and (R,R)-4 (A), (R,R,R,R)-6 and (R,R,R,R)-6’ (B), and poly-1R and poly-2R (C) in chloroform at 25 °C. Photographs of the corresponding solutions under 365-nm irradiation are also shown. [3, 4, 6, 6’, or triptycene units in polymers] = 1.0 × 10−5 M. 165x119mm (300 x 300 DPI)



Figure 5. (A) Structure of the poly-2R model (12-mer) used for the computational study. (B) Side view and (C) top view of the molecular model of the helical ladder poly-2R in chloroform at 2000 ps in an all-atom MD simulation after equilibration at 298 K represented by space-filling (backbone) and line (side chains) models. The carbon atoms in the dibenzo[a,h]anthracene units and the bridgehead carbons are highlighted in blue and the chloroform solvent molecules are omitted to simplify the view. (D) Definition of the torsion angle between the two dibenzo[a,h]anthracene planes (θi), which are linked through bridgehead carbons of the ith triptycene unit from the terminal one. (E) Plots of θi as a function of calculation time.


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Figure 6. (A) AFM image (1.80 μm × 1.80 μm) of poly-2R on mica in air at 25 ± 1 °C. (B) Height profiles measured along the dashed lines labeled a–d in (A).


Triptycene-Based Ladder Polymers with One-Handed Helical Geometry

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