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Flipping Motion to Bias the Organized Supramolecular Polymerization of N-Heterotriangulenes Yeray Dorca, Jesús Cerdá, Juan Aragó, Enrique Ortí, and Luis Sanchez Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b01653 • Publication Date (Web): 17 Jul 2019 Downloaded from pubs.acs.org on July 20, 2019

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Chemistry of Materials

Flipping Motion to Bias the Organized Supramolecular Polymerization of N-Heterotriangulenes Yeray Dorca,a,‡ Jesús Cerdá,b,‡ Juan Aragó,*,b Enrique Ortí,*,b and Luis Sánchez*,a Departamento de Química Orgánica, Facultad de Ciencias Químicas, Universidad Complutense de Madrid, 28040 Madrid, Spain; e-mail: [email protected]. b Instituto de Ciencia Molecular, Universidad de Valencia, 46980 Paterna, Spain; e-mail: [email protected]. a

ABSTRACT: Synergistic experimental and theoretical studies have allowed to disentangle the possible pathways for the supramolecular polymerization of a series dicyanovinyl-bridged N-heterotriangulene (NHT) derivatives bearing benzamide units with achiral (1a) and chiral (1b-c) side chains. The synthesis of these bowl-shaped, self-assembling units yields a mixture of monomeric species with C3 and C1 symmetry. Both monomeric species are able to self-assemble into different supramolecular aggregates with sufficient stability to coexist in freshly prepared solutions. The dissimilar ratio of the aggregates initially generated results in different spectroscopic features and, more specifically, in the apparition of non-mirror circular dichroism (CD) spectra for chiral 1b and 1c. The interconversion at room temperature of the aggregates formed by the C3- and C1-symmetry monomeric species is energetically unfavorable due to the steric hindrance between the neighboring dicyanovinyl groups within the aggregate. Heating the aggregates constituted by both monomeric species favors their disassembly and, at the same time, the conversion of the monomeric species with C1 symmetry into that with C3 symmetry by a flipping motion of one dicyanovinyl group. Cooling down back the solutions to room temperature leads to the formation of helical-like columnar aggregates based on C3-symmetry monomers showing specular CD spectra for chiral 1b and 1c. The flipping motion at molecular level described here for the bowl-shaped dicyanovinyl-bridged NHTs 1a-c is, to the best of our knowledge, the only example in which a geometric change in the monomeric species, with no participation of intramolecular noncovalent interactions, is responsible for biasing the pathway complexity that yields two different stable supramolecular architectures from a single self-assembling unit.

INTRODUCTION The development of organic electronics requires the synergy of different scientific areas (physics, engineering, chemistry, …) to achieve high-performance optoelectronic devices. Fieldeffect transistors, light-emitting diodes, and photovoltaic cells are among the most investigated devices constructed by using electroactive organic molecules usually constituted by conjugated building blocks. These optoelectronic devices are generally built up in a sandwich-like architecture in which the active layer, constituted by only one type of organic molecule or by the combination of two or more organic molecules, are placed between two electrodes able to inject or collect electrons and/or holes.1–3 A key issue in the development of efficient devices is the morphology of the active layer, which is strongly conditioned by the chemical nature of the constituent unit(s). The rational molecular design of the electroactive materials is crucial to enhance their mechanical and electronic properties thus biasing the final performance of the device. This performance also drastically depends on the self-assembly pathways that yield an optimal molecular organization upon the processing of the active layer.4 A common procedure for the processing of -conjugated molecules starts with the preparation of solutions in which the active molecules are perfectly solvated, i.e., molecularly dissolved, by the addition of “good” solvents or by increasing the temperature. The spin coating of such solutions or the addition of “bad” solvents are widely utilized protocols to achieve organized active layers in

which competitive self-assembly pathways are attenuated to avoid dissimilar nanoscale morphologies. These methodologies illustrate the importance of having well-established strategies to regulate the self-assembly pathways of -conjugated materials.5–7 In recent reports, detailed mechanistic insights into the supramolecular aggregation of relatively small -conjugated molecules have been utilized for the optimization of novel selfassembling -conjugated materials.8,9 These studies reveal the correlation between the concept of pathway complexity in supramolecular polymerization and in bulk materials. A direct consequence of the concurrence derived from pathway complexity is the formation of kinetically trapped states that subtract the monomeric species from the initial mixture or solution retarding or impeding the formation of the thermodynamically preferred morphology. The formation of such kinetic states has been described for covalent polymers in which, making use of starting chiral units and by varying the solvent conditions, it is possible to achieve helical polymers of both handedness.10,11 A growing number of supramolecular polymers, in which the monomeric units are joined together by noncovalent forces,12 are found in literature dealing with the formation of kinetic aggregates that corroborate the similitude between covalent and noncovalent macromolecules. The formation of kinetic species is indeed utilized to perform living supramolecular

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polymerization yielding programmable aggregates that evolve to afford supramolecular polymers with a lower degree of polydispersity and enhanced properties.13–15 When studying kinetically controlled supramolecular polymerizations, three different scenarios are found in literature. The first one involves the formation of two different supramolecular structures, the kinetic (off-pathway) and the thermodynamic (on-pathway) aggregates, in which the former evolves upon time to the thermodynamically controlled on-pathway aggregate.16–22 The second one corresponds to those examples in which the kinetic species are dormant monomers that further evolve to the final on-pathway aggregates.20,21,23–25 The third scenario implies the formation of highly stable aggregates that hardly interconvert and are considered as supramolecular polymorphs.26–28 Especially relevant is the second case when the dormant monomers afford off-pathway aggregates that further evolve to generate on-pathway aggregates.20,21 To date, a vast majority of the reported metastable monomers are species stabilized by intramolecular H-bonding interactions giving rise to seven- to nine-membered pseudocycles that, upon applying appropriate conditions of concentration and temperature, open up and generate the more active monomeric species.29,30 It is also possible to find examples of metastable species in the formation of metallosupramolecular polymers, in which the nature of the metastable monomers has not been accurately determined.31 Herein, the synthesis of a series of bowl-shaped, dicyanovinyl-bridged N-heterotriangulenes (NHTs) (compounds 1a-c in Figure 1) is reported and their selfassembling features investigated by means of a combined experimental-theoretical approach. The C3-symmetry and the peripheral decoration of NHTs with achiral and chiral side chains allows achieving chiral helical aggregates.32,33 Different spectroscopic studies realized by UV-Vis and circular dichroism (CD) techniques demonstrate the formation of a initial mixture constituted by both off- and on-pathway supramolecular aggregates that evolve to the more stable supramolecular structure upon a heating/cooling treatment. The final supramolecular assemblies show high stability and dissimilar spectroscopic characteristics compared to the initial aggregates. Theoretical studies reveal that the foreseen C3symmetry geometry of 1a-c can change to a C1-symmetry disposition by the flipping motion of one of the three 2,2dicyanovinyl bridging groups. Both C3- and C1-symmetry monomeric structures can coexist and, thus, generate columnar aggregates of different stability through the initial formation of dimeric units that further grow up to afford helical columnar aggregates (Figure 1b). The synergy between experimental and theoretical techniques demonstrates the formation of metastable monomeric species in which intramolecular noncovalent interactions are nonoperative. The C1-symmetry monomer gives rise to kinetically trapped aggregates that, upon heating the solution to 90 ºC and cooling down to room temperature, evolve to supramolecular assemblies based on the more stable C3-symmetry monomer. The data presented in this manuscript contribute to broaden the knowledge into the organization principles of Nheterotriangulene derivatives, which have been previously reported to undergo pathway complexity,21,34,35 and also have demonstrated to be suitable candidates for the application in organic electronics.36

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Figure 1. (a) Chemical structure of the bowl-shaped, dicyanovinyl-bridged NHTs 1a-c. (b) Schematic energy landscape for the formation of the off-pathway and on-pathway aggregates of 1a-c. RESULTS AND DISCUSSION Synthesis and self-assembly in solution. The synthesis of the target NHTs 1a-c was accomplished by reacting the previously reported carbonyl-bridged tryarylamines 2a-c37 with the Lehnert’s reagent (malononitrile, pyridine, TiCl4) (Scheme S1 in the Supporting Information).38,39 The ketone functional group experiences a nucleophilic addition followed by an elimination reaction by the action of the carbanion generated in situ from the acid-base reaction of malononitrile and pyridine. The chemical structure of the 2,2-dicyanovinyl NHTs was confirmed by spectroscopic techniques and mass spectrometry (see the Supporting Information). The FTIR spectra of compounds 1 show a net band at 2215 cm–1 that confirms the functionalization of the carbonyl groups of NHTs 2 with the cyano groups (Figure S1a). In addition, the N−H and Amide I stretching bands and the Amide II bending band, appearing at 3300, 1640, and 1535 cm–1, respectively, indicate the intermolecular H-bonding interaction between the amide groups (Figure S1a).40,41 To further confirm the H-bonding interaction between the amide groups, the FTIR spectra of compounds 1a-c was registered in toluene (Tol) as solvent at a total concentration (cT) of 500 M. The formation of H-bonding arrays between the amide functional groups in toluene solution is unambiguously confirmed by the band corresponding to the N−H stretching that appears at 3287 cm–1 and remains unaltered after applying a heating/cooling cycle. In contrast, in the FTIR spectra recorded for 1a-c in a “good” solvent like CHCl3 at cT = 500 M, the N−H stretching band appears at 3456 cm–1, a wavenumber value characteristic of non-bonded amides (Figure S1b).40,41


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Furthermore, concentration-dependent 1H NMR experiments performed in CDCl3 (Figure 2 and S2) show the downfield shift of the triplet at  ~ 6.3 ppm, ascribable to the amide protons, and the shielding effect experienced by all the aromatic protons. These shifts indicate the noncovalent interaction of the NHT scaffolds by the synergy of a triple array of H-bonding interactions between the peripheral amide groups and the πstacking of the aromatic core units, in good agreement with some referable C3-symmetry self-assembling units.42–44

Figure 2. Partial 1H NMR spectra of 1b showing the aromatic and amide protons, and the protons of the methylene group linked to the amide group (298 K, CDCl3, 300 MHz). To further investigate the supramolecular polymerization of the reported NHTs, we firstly performed variable-temperature (VT) UV-Vis experiments by using diluted solutions with mixtures of methylcyclohexane (MCH) and toluene (Tol) as solvent.45 At room temperature, the UV-Vis spectra of compounds 1a-c in a mixture MCH/Tol 1/1 show a broad band centered at 543 nm with a shoulder at 590 nm (Figure 3). Heating up the solution to 90 ºC results in the depletion of these two optical features that convert into a broad band peaking at 535 nm. Plotting the variation of the molar fraction of aggregate () against temperature results in a sigmoidal curve (inset Figure 3 and Figure S3) that, in contrast to previous reports on the self-assembly of related NHTs,34,35,37 could be indicative of an isodesmic supramolecular polymerization mechanism.12 The application of the equilibrium (EQ) model, which considers the equilibrium between monomeric, oligomeric, and macromolecular species,46 to the cooling curves obtained at different concentrations allowed to extract a complete set of thermodynamic parameters (nucleation enthalpy (Hn), elongation enthalpy (He), elongation entropy (S), nucleation (Kn) and elongation (Ke) binding constants, and cooperativity factor ()) (Table 1). Especially relevant is the  value derived from these measurements that was estimated to be 0.006 and 0.004 for compounds 1a and 1b, respectively. These  values are indicative of a slightly cooperative mechanism.12

Figure 3. VT-UV-Vis spectra of 1a (MCH/Tol, 1/1; 6 M; heating rate 1 K min–1). The arrows indicate the changes in the absorption bands upon increasing temperature. The inset shows the molar fraction of aggregate (α) upon heating and cooling at 1 K min–1 for this diluted solution of 1a. The red and blue lines depict the fitting to the EQ model. Table 1. Thermodynamic parameters estimated for compounds 1a and 1b (MCH/Tol, 1/1). 1a

1b

–40.2 ± 0.5

–48.9 ± 0.5

S [J K–1 mol–1]

–24.0 ± 1

–53 ± 1

Hn [kJ

–12.6 ± 0.6

–13.4 ± 0.2

He [kJ



mol–1] mol–1]

6.2 ×

[a]

10–3

4.3 × 10–3

Ke [L mol–1] [a]

6.3 × 105

6.4 × 105

mol–1] [a]

103

2.8 × 103

Kn [L

3.8 ×

[a] The equilibrium constants for elongation and dimerization, Ke and Kn, and the cooperativity factor,  = Kn/Ke, are calculated at 298 K. Intriguingly, we noticed that the UV-Vis spectra of freshly prepared solutions of compounds 1a-c are not identical to those registered upon applying a heating/cooling (h/c) cycle (Figure 4a and S4). The depletion of the shoulder at 595 nm, which is more pronounced for the chiral congeners 1b and 1c, after h/c treatment and the crossing points between the different spectra are spectroscopic signatures of the formation of different aggregated species. In addition, dissimilar UV-Vis spectra are observed regardless the concentration or the solvent utilized (Figures S4). Considering the differences between these UVVis spectra, we also registered the heating/cooling curves for MCH/Tol 1/1 solutions of NHTs 1a and 1b at a cT = 6 M and applying two different scan rates of 1 and 0.1 K min–1. In the former conditions, the heating curves show a clear hysteresis regarding the cooling curves with a difference in the temperature of elongation (Te) of around 20 ºC (inset in Figure 3 and Figure S5a). In the latter conditions, the slower rate applied results in a negligible hysteresis thus confirming the influence of scan rate on the kinetics of the process (Figure S5b).27,29 Furthermore, the presence of hysteresis between the heating and cooling curves in relation to the supramolecular



polymerization mechanism is associated to the presence of kinetically trapped species. Most commonly, these kinetic species correspond to the formation of inactivated monomers, usually stabilized by intramolecular H-bonding interactions giving rise to pseudocycles, which retard the generation of the thermodynamically controlled supramolecular polymer.20– 25,29,30 Obviously, the chemical structure of the reported NHTs 1a-c impedes the formation of such intramolecularly H-bonded pseudocycles. However, as discussed below on the basis of theoretical calculations, they exhibit a pathway complexity in their supramolecular polymerization due to its bowl-shaped structure. To further confirm the formation of different aggregate architectures from the supramolecular polymerization of compounds 1, we investigated the chiroptical properties of the chiral derivatives 1b and 1c. To optimize the intensity of the dichroic response, we utilized a MCH/Tol 3/7 mixture as solvent and a cT = 20 M to register the CD spectra. In these experimental conditions, the UV-Vis spectrum of the freshly prepared solutions (Figure 4a and S4b) shows, once again, a broad band (peaking at ca. 540 nm) together with a shoulder (590 nm), which suggest the formation of two different

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aggregated species in a different ratio. After applying the h/c cycle, only one broad absorption band (at ca. 535 nm) is observed indicative of the formation of the thermodynamically controlled assembly. These spectroscopic signatures are also found at other concentrations (Figure S4). In addition, the heating and cooling curves for solutions of achiral 1a and chiral 1b in MCH/Tol 3/7 at cT = 20 M were also registered by applying a rate of 1 K min–1. In these experimental conditions, a hysteresis between the heating and cooling curves indicative of the operation of a kinetically controlled supramolecular polymerization was also observed (Figure S6). The CD spectra of the freshly prepared solutions of 1b and 1c display no mirror image of the dichroic features despite the fact that these chiral NHTs are enantiomers (Figure 4b). This erratic dichroic behavior could be initially justified by the macroscopic alignment of the aggregates formed from these NHTs, which would give rise to an intense linear dichroism (LD) effect that would contaminate the CD response.47–49 However, the LD spectra recorded for the freshly prepared solutions of 1b and 1c show no LD response thus ruling out the macroscopic alignment effect (Figure S7).

Figure 4. (a) UV-Vis spectra of freshly prepared (black line), heated to 90 ºC (red line), and upon cooling down to 20 ºC (green line) solutions of 1c. CD spectra of freshly prepared (b) and upon applying a heating/cooling cycle (c) solutions of 1b and 1c. Experimental conditions: MCH/Tol, 3/7, cT = 20 M. Heating these freshly prepared solutions to 90 ºC results in CD spectra with no dichroic response, as corresponds to the formation of the monomeric species molecularly dissolved. Interestingly, upon cooling down the solutions of 1b and 1c to 20 ºC, the erratic initial CD signal shown in Figure 4b completely changes and a rich dichroic pattern, with maxima at 350 and 585 nm and zero-crossing points at 510 and 545 nm, is obtained (Figure 4c). In addition, and most important, the CD spectra of 1b and 1c display mirror image as corresponds to the formation of helical structures of opposite handedness for both chiral NHTs. The data extracted from the CD experiments suggest that the freshly prepared solutions of the NHTs contain different aggregated species in a different ratio that upon heating the sample disassemble into the monomeric species. Cooling down these samples to 20 ºC favors the supramolecular polymerization into the thermodynamically controlled helical aggregate. The combination of dynamic light scattering (DLS) and atomic force microscopy (AFM) shed some light into the dissimilar morphological features of the possible supramolecular aggregates generated upon self-assembly of the NHTs 1a-c. The hydrodynamic radius (RH) estimated for the aggregates present in the freshly prepared solutions is of around

500 nm with a narrow distribution (Figure 5a and S8). In contrast, the assemblies formed upon applying a heating/cooling cycle exhibit a broader distribution of sizes centered at larger RH values around 2 m. The AFM images display a sharp difference between the morphology of the initial supramolecular aggregates formed in the freshly prepared solutions and that obtained upon performing the h/c cycle (Figure 5b and 5c, respectively). For the freshly prepared solutions, a large number of circular micelles of variable size is observed together with scarce rod-like supramolecular structures (Figure 5b). The application of the h/c cycle modifies the morphology of the aggregates giving rise to the formation of rod-like supramolecular assemblies with a length of around 1 m and a height of 10 nm (Figure 5c and S9). In this case, together with the rod-like structures, it is also possible to observe micelles. This would justify the broad distribution of RH obtained in the DLS measurements (Figure 5c and S9).


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Chemistry of Materials planar geometry owing to the steric hindrance between the dicyanovinyl groups and the adjacent hydrogen atoms of the NHT core. To minimize this interaction, the conjugated core folds down adopting a bowl shape and the three dicyanovinyl groups point up in the opposite direction (Figure 6a and S10). The fully optimized structure exhibits a C3 molecular symmetry.

Figure 5. (a) Normalized distribution of RH for compound 1c (Tol, 500 µM, 298 K). AFM images of the aggregates formed by 1c in the freshly prepared solution (a) and after applying a heating/cooling cycle (b) (Tol, 500 µM, 298 K; silicon wafer as surface). To justify the experimental data, we have considered the dynamic behavior exhibited by the dicyanomethylenated quinacridone compound, that undergoes a flipping motion of the dicyanovinyl groups in solution.50 This motion allows the coexistence of different conformations for such strained, curved, butterfly-shaped aromatic pentacycle. However and unlike to that observed here for NHTs 1a-c, concentrationdependent 1H NMR studies for the dicyanomethylenated quinacridone display no variation in the chemical shifts of all the aromatic protons, which implies the suppression of the selfaggregation ability of this compound as a consequence of its conformational flexibility. As discussed below on the basis of theoretical calculations, in the case of the NHTs studied here, the expected C3-symmetry structure, with the three dicyanovinyl groups pointing to the same side of the bowlshaped molecules, turns into a C1-symmetry structure through the flipping motion of one of the dicyanovinyl groups. Both C3and C1-symmetry monomers generate different aggregated species, one of them less stable and able to evolve upon applying the heating/cooling cycle. Theoretical calculations. To explore in more detailed the aforementioned hypothesis about the self-assembly of compounds 1a-c in different supramolecular entities, we performed quantum-chemical calculations at the density functional theory (DFT) B3LYP-D3/6-31G** and semiempirical GFN2-xTB levels. A simplified model of the dicyanovinyl-bridged NHTs 1a-c, in which the side alkyl chains were replaced by methyl groups, was used in all the calculations (see the Theoretical Calculations section in the SI for full computational details). The geometric structure of the monomer was first optimized at the B3LYP-D3/6-31G** level. Unlike other carbonylbridged NHTs,21,34,35,37 the monomeric species shows a non-

Figure 6. Top and side views of the B3LYP-D3/6-31G**optimized C3- (a) and C1-symmetry (b) monomers. Short noncovalent Ar−H···N≡C interactions are denoted with dashed red lines. (c) Energy profile calculated at the B3LYP-D3/631G** level for the interconversion between the C3- and C1symmetry monomers through the transition state (TS) associated to the flipping motion of one dicyanovinyl group. The deviation from planarity of the NHT core is characterized by the distance between the central nitrogen atom and the plane defined by the carbon atoms to which the benzamide groups are attached, which has a value of 0.76 Å, and by the internal dihedral angle ϕ (Figure 6a), which has a value of 9.9º. The peripheral benzamide units are twisted by 34.7º with respect to the NHT core (β angle in Figure 6a) to avoid the interactions with the hydrogen atoms of the core. This twisting angle is similar to that previously reported for benzamide-substituted, carbonyl-bridged NHTs (β = 34.0º).37 The amide groups are additionally rotated with respect to the benzene plane by 23.3° (γ angle in Figure 6a). The rupture of planarity of the NHT core makes possible a second minimum-energy structure with C1symmetry, in which two of the dicyanovinyl groups point upwards and the third one undergoes a flipping motion pointing downwards in the same direction of the conjugated core (Figure 6b). The lower symmetry of this monomer implies a NHT core more distorted from planarity, although the values obtained for the dihedral angles β and γ (around 35 and 23º, respectively) are similar to those computed for the C3-symmetry monomer. Two additional C1-symmetry monomers differing in the relative orientation of the peripheral benzamides are possible (Figure



S11), but they are less relevant in terms of supramolecular aggregation. In terms of stability, the C3-symmetry monomer is calculated to be more stable than the C1-symmetry monomer by 2.51 kcal/mol. The interconversion between the two monomers via the flipping motion of the dicyanovinyl group is thermally accessible with a maximum energy barrier of 5.4 kcal/mol (Figure 6c). Therefore, both monomeric structures are energetically competitive and can coexist in solution. To gain more insight into the supramolecular organization of the studied NHTs, we built up different supramolecular dimers based on the previously optimized C3- and C1-symmetry monomers. After geometry optimization at the B3LYP-D3/631G** level, the different structures computed converged to the three supramolecular dimers (D1, D2, and D3) shown in Figure 7. Dimers D1 and D2 correspond to stacked concave–convex arrangements of the C3- and C1-symmetry monomers, respectively, in which the monomers are fitted in an almost perfectly eclipsed disposition (Figure 7a and 7b, respectively). In contrast, dimer D3 implies a concave–concave arrangement of the C3-symmetry monomers (Figure 7c). This disposition resembles to that described for other referable self-assembling units responsible for generating anti-cooperative supramolecular polymerizations.51 In terms of stability, dimer D1 is computed to be the most stable, and dimers D2 and D3 are found to be 8.3 and 10.4 kcal/mol higher in energy, respectively. It is to be mention that heterodimers formed by one C3- and one C1-symmetry monomer were also considered but they were calculated significantly higher in energy (see the Supporting Information for a detailed discussion). Regarding the intermolecular interactions, dimers D1 and D2 show favorable π‒π interactions between the NHT cores with short intermolecular contacts (Figure S12 and S13). For instance, the distance between the central nitrogen atoms is predicted to be 3.57 Å for both D1 and D2. The peripheral benzamide rings are also oriented to facilitate the π-stacking with interplanar distances between the benzene rings of 3.37 (D1) and 3.32 Å (D2). In these dimers, the monomer units are slightly rotated along the stacking axis (i.e., the axis through the central N atoms) with small twisting angles of 0.5 and 2.3º for D1 and D2, respectively. It is worth noting that larger rotation angles between the bowl-shaped monomers, as those predicted for planar carbonyl-bridged NHTs (21.7º),37 are here hampered by the significant steric hindrance between the dicyanovinyl bridging groups of one molecule and the benzamide benzenes of the neighboring molecule (Figure 7a and 7b). Despite this small intermolecular rotation, dimers D1 and D2 exhibit strong intermolecular H-bonding interactions between the amide groups due to the inverted orientation of these groups. Although the eclipsed disposition of the monomers in the dimer precludes the formation of linear N−H···O=C bonds, the H-bonding interactions between the two monomers take place at short distances of 1.94 and 1.85 Å for D1 and D2, respectively (Figure S12 and S13). In addition to these strong interactions, weak intermolecular H-bonding-type interactions between the nitrogen atoms of the dicyanovinyl groups of one molecule and the hydrogen atoms of the pendant benzene rings in the neighboring molecule are also generated (Figure 7a and b). In fact, the main difference between D1 and D2 in terms of intermolecular interactions is the number of these weak Hbonding-type interactions. Whereas D1 shows three intermolecular Ar−H···N≡C interactions at 2.28 Å, D2 displays only two at ca. 2.33 Å. This partially explains the higher stability of D1 compared to D2 by 8.3 kcal/mol. It is important

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to remark that, despite this relatively large energy difference, both dimers may be formed as a nucleus for further supramolecular growth due to the coexistence and easy interconversion between C3- and C1-symmetry monomers (Figure 6).

Figure 7. Side views of the minimum-energy structures calculated at the B3LYP-D3/6-31G** level for dimers D1 (a), D2 (b), and D3 (c). Blue and red dashed lines emphasize the intermolecular N−H···O=C and Ar−H···N≡C interactions, respectively. On the other hand, dimer D3, which shows a concave– concave arrangement of the C3-symmetry monomers (Figure 7c), presents an intermolecular distance between the central N atoms of 3.98 Å and a rotation angle along the C3-axis of 34.8º that favors the formation of strong intermolecular H-bonds between the amide groups (1.91 Å, Figure S14). Dimer D3 also exhibits an optimal π-stacking of the peripheral benzamide units with interplane distances of only 3.20 Å. The concave-concave disposition of the monomers in dimer D3 makes the π‒π interactions between the dicyanovinyl NHT moieties less efficient and makes dimer D3 10.4 kcal/mol less stable than dimer D1. In contrast to D1 and D2, the close structure of dimer D3 does not allow the incorporation of additional monomers and impedes the formation of longer supramolecular structures. Dimer D3 therefore could correspond to a metastable species. In a further step, and using the previously-optimized dimers D1 and D2 as building blocks, supramolecular decamers were constructed and fully relaxed with the modern and efficient semiempirical GFN2-xTB method.52 The resulting DM1 and DM2 decamers are, therefore, based on the self-assembly of the C3- and C1-symmetry monomers, respectively. As sketched in Figure 8 (see also Figure S16 and S17), the fully optimized decamers give rise to columnar helical-type aggregates with small rotation angles between adjacent monomers. Monomers in the stack show favorable π‒π interactions between both the NHT cores, with intermolecular contacts between the central nitrogen atoms around 3.6 Å, and the peripheral benzamide groups. They form an additional triple zig-zag array of short intermolecular H-bonds in the 1.80−1.95 Å range between the amide groups in adjacent monomers. The main difference between DM1 and DM2 is the rotation angle along the growing axis, which is larger for DM2 compared to DM1 (Figure 8). In


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terms of energetics, decamer DM1 based on the C3-symmetry monomer is found to be more stable than DM2 by 13.2 kcal/mol. Theoretical calculations therefore indicate that the flipping motion of the dicyanovinyl groups, which gives rise to the coexistence of different monomeric species, biases the selfassembly of the molecular units resulting in different helicallike columnar aggregates. Unlike the monomers, an efficient conversion of the less stable supramolecular architecture (DM2) to the most stable one (DM1) would be only achieved after the disassembly of the supramolecular aggregate because a collective flipping motion of dicyanovinyl groups in the aggregate would be highly unlikely. In this regard, helical columnar supramolecular structures similar to the DM1 and DM2 decamers would be responsible for the spectroscopic signatures of the freshly-prepared solutions whereas only the thermodynamically-controlled supramolecular polymer (DM1type structure) would be the species present after the heating/cooling cycle.

Heating the diluted solutions of 1a-c causes the disassembly of all the supramolecular species and the conversion of the initially formed C1-symmetry monomers into the more stable C3symmetry monomers that, upon cooling, give rise to helical-like columnar aggregates showing specular CD spectra for the chiral 1b and 1c. An efficient conversion between the proposed helical-like columnar assemblies at room temperature would be energetically unfavorable due to the steric hindrance between the neighboring dicyanovinyl groups within the aggregate. The data presented herein contribute to broaden the knowledge concerning the organization principles that govern the supramolecular polymerization of N-heterotriangulenes, a scaffold that has been previously reported to experience pathway complexity by the formation of intramolecular Hbonding interactions, and has also been demonstrated to be suitable for the application in organic electronics. To the best of our knowledge, the flipping motion at molecular level described here for the bowl-shaped dicyanovinyl-bridged NHTs is the only example in which a geometric change in the monomeric species biases the pathway complexity of a self-assembling process to yield different stable supramolecular architectures.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Full experimental details, additional FTIR, 1H NMR, UV-Vis, and CD measurements, AFM images, and theoretical calculations, including Figures S1-S18, are provided.

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

ORCID Luis Sánchez: 0000-0001-7867-8522 Enrique Ortí: 0000-0001-9544-8286 Juan Aragó: 0000-0002-0415-9946

Author Contributions

Figure 8. Lateral and top views of the right-handed, helical-like columnar decamers DM1 (a) and DM2 (b) based on the selfassembly of the C3- and C1-symmetry dicyanovinyl-bridged NHT monomers computed at the GFN2-xTB level. CONCLUSIONS The synergy between experimental techniques and quantumchemical calculations has allowed to disentangle the possible pathways for the supramolecular polymerization of a series of dicyanovinyl-bridged N-heterotriangulene (NHT) derivatives (1a-c) bearing benzamide units. In these NHT compounds, two monomeric species with C3 and C1 symmetry, interconvertible via a flipping motion of one dicyanovinyl group, are able to self-assemble into different supramolecular aggregates with sufficient stability to coexist in freshly prepared solutions. The different supramolecular aggregates are initially generated in different ratios for all the three NHTs 1a-c, which provokes the apparition of non-mirror CD spectra for chiral 1b and 1c.

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡ These authors contributed equally.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT Financial support by the MINECO of Spain (CTQ2017-82706-P, CTQ2015-71154-P, and Unidad de Excelencia María de Maeztu MDM-2015-0538), the Generalitat Valenciana (PROMETEO/2016/135 and SEJI/2018/035), the Comunidad de Madrid (NanoBIOCARGO, P2018/NMT-4389), and European Feder funds (CTQ2015-71154-P) is acknowledged. Y.D. and J.C. are respectively grateful to Comunidad de Madrid and MINECO for their predoctoral fellowships. J.A. is also thankful to the MICIU for a “Ramon-y-Cajal” fellowship (RyC-2017-23500).

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GRAPHICAL ABSTRACT

10 ACS Paragon Plus Environment

Flipping Motion To Bias the Organized ... - ACS Publications

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