Light-induced Chiral Self-assembly of Achiral Triphenylamines with Controlled Number of Hydrogen Bondings

doi:10.21203/rs.3.rs-1637360/v1

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Light-induced Chiral Self-assembly of Achiral Triphenylamines with Controlled Number of Hydrogen Bondings

https://doi.org/10.21203/rs.3.rs-1637360/v1

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We report the self-assembly of achiral triphenylamine molecules with different numbers of hydrogen bonding sites. The supramolecular chirality was controlled by the symmetry breaking effect of circularly polarized light (CPL) occurring in the nucleation step. As the number of hydrogen bonding sites increased, the stability of supramolecular helices increased, and reversible transformation of the right- and left-handed helices by CPL became feasible only with aid of ultra-sonication. In contrast, triphenylamine molecules without hydrogen bonding sites did not undergo light-induced self-assembly.

Chirality is one of interesting properties found in nature, and its various hierarchical levels can be observed everywhere, from atomic to galaxial scales^1–4. Understanding how chiral matter is generated in nature and accumulated in biological systems such as DNA and the helices of proteins, has been considered a key to finding the origin of life^5–7. Among the many levels of chirality, one of the most interesting and significant subjects is chirality at the supramolecular level. Supramolecular chirality is important to the folding of proteins, the catalytic abilities of enzymes, DNA duplication and so on^8,9. Several recent interesting studies have investigated the formation of chiral supramolecular structures from achiral molecules through symmetry breaking^10–18. Symmetry breaking can be induced in achiral molecules by several types of external stimuli, including the doping of chiral molecules^10,11, shear force^12,13, magnetic fields^14,15 and circularly polarized light (CPL)^16–18. It has been suggested that an initial chiral bias driven by such external chiral stimuli might be amplified during the self-assembly process, resulting in homochirality^16–20. In our previous study, we reported that opposite helical senses could be induced in photo-oxidized triphenylamine stacks by applying circularly polarized light of opposite handedness¹⁶. However, despite locking the helices by the photo-polymerization of diacetylene, the formed supramolecular chirality did not have a thermodynamically stable state at room temperature. The chiral information was lost at temperatures above 263 K and as time went on, making it difficult to pursue further applications of chiral sensing or supramolecular scaffolding for asymmetric reactions and chiral conductors. To form a more stable chiral self-assembled structure for various applications, deep insight into the chirality induction mechanism via self-assembly is indispensable.

One important factor in the formation of helical structures is the hydrogen bonding of amide groups, as seen in the alpha helix structure of protein^21,22. This is also a key factor in the formation of stable self-assembled supramolecular structures through secondary interactions^23–26. In the present study, considering the importance of hydrogen bonding, we synthesized three different achiral triphenylamine (TPA) derivatives shown in Fig. 1, one with doubled hydrogen bonding sites (TPA-2a-SDA) and one without hydrogen bonding sites (TPA-ester) compared to TPA having three hydrogen bonding sites (TPA-SDA), to investigate the effect of hydrogen bonding on the self-assembly of triphenylamine derivatives. CPL induced the formation of one-handed helical self-assembled structures with the TPA derivatives having amide groups (TPA-2a-SDA and TPA-SDA) in aliphatic chlorinated solvents, but not with TPA-ester. We investigated how hydrogen bonding affected their stacking structure, rotational radius, stabilization energy and the thermal stability of their helical self-assembled structures using density-functional-tight-binding (DFTB) calculations and circular dichroism (CD) measurements. Our data suggest that the hydrogen bonding greatly affects the self-assembly behaviour of TPA derivatives by assisting the formation of self-assembled helical fibers and thermally stable structures. Comparison of the three molecules showed that controlling the number of hydrogen bonding sites could be a useful approach in designing achiral molecules for chiral self-assembly with external stimuli.

The synthetic routes of the three achiral compounds containing triphenylamine and diacetylene moieties are shown in Fig. 1. TPA-SDA and TPA-2a-SDA were synthesized using the same EDC, HOBT amide coupling reaction. The TPA-ester was synthesized using Steglich esterification, and the detailed synthetic pathways are shown in Fig. S1. The compounds were characterized by NMR, FT-IR and MALDI-TOF analyses (Fig S2 - S12).

To check the formation of supramolecular structures, self-assembly of the TPA compounds prepared in this study was attempted in aliphatic chlorinated solvents by cooling the solutions from room temperature to -10 ^oC without light. Interestingly, only the TPA-2a-SDA underwent self-assembly and produced fibrous supramolecular products (Fig. 2). The TPA-SDA and TPA-ester did not undergo self-assembly in the same condition. However, TPA-SDA underwent light-induced self-assembly when exposed to ambient light as reported previously¹⁶. The TPA-ester did not show any self-assembly behaviour even with light, and with increase of the concentration three times (8.0 mg∙mL^− 1 in CH₂Cl₂) (Fig. S13 and S14). The self-assembly of TPA-2a-SDA was further tested in aliphatic chlorinated solvents with light (Fig. S15). The color change and UV-vis spectrum of the solution (Fig. S16a) indicate the formation of triphenylammonium radicals, which is also supported by ¹H nuclear magnetic resonance spectrum (NMR) of TPA-2a-SDA in dichloromethane (CD₂Cl₂) showing the suppression of aromatic protons after being irradiated with light from a fluorescent lamp for 10 min (Fig. S17). These observations are consistent with the literature^27,28 and our previous study for TPA-SDA¹⁶.

Unlike the behaviour of the TPA-SDA and TPA-2a-SDA, TPA-ester which does not have a hydrogen bonding site did not show suppression of the aromatic proton peaks (Fig. S18) despite the light irradiation. However, the light induced the same blue shift of UV-vis spectrum and color change of the TPA-ester solution like the TPA-SDA and TPA-2a-SDA solutions due to the formation of triphenylammonium radicals (Fig. S16b). It seems that the molecular interaction without hydrogen bondings is not strong enough for the stacking of TPA-ester.

The self-assembled aggregates of TPA-2a-SDA in various aliphatic chlorinated solvents was observed with scanning electron microscopy (FE-SEM) after spin coating of the solutions. Figure 2a and 2c showed the self-assembled fibrous structures of TPA-2a-SDA formed in dichloromethane. The self-assembled supramolecular fibers were observed with and without light irradiation, indicating that TPA-2a-SDA had stronger intermolecular interactions than TPA-SDA, which required light irradiation to self-assemble¹⁶.

The self-assembled supramolecular fibers formed in the TPA-2a-SDA solution without light and with irradiation of non-polarized light did not exhibit CD activity, revealing that it contained a racemic mixture of both left- and right-handed helical aggregates of TPA-2a-SDA. However, the solutions irradiated by CPL showed CD activity which followed the direction of light, with maximum points at 325 nm and 254 nm (Fig. 3a).

These observations indicate that the direction of the CPL can induce the handedness of the helix formed by self-assembly of TPA-2a-SDA. Interestingly, the handedness of the helix was not affected by irradiation of CPL with opposite handedness. The CD spectrum did not change by exposing the self-assembled supramolecules of the TPA-2a-SDA to CPL with opposite handedness (Fig. 3b, 3c). This observation is strikingly different compared to the result for TPA-SDA in our previous work, in which the supramolecular chirality could be reversibly switched by irradiation with the opposite directional CPL¹⁶.

In order to reverse the supramolecular chirality, we applied ultra-sonication to the solution followed by light irradiation in the direction opposite to that of the CPL, and observed that the CD activity changed in the opposite direction, according to CPL (Fig. 3d). Considering that ultrasound has been known to break the hydrogen bond networks and other non-covalent interactions^29–31, these results indicate that the introduction of additional hydrogen bonding sites reinforces the intermolecular interactions, making them more stable after symmetry breaking by the CPL in their nucleation step. In addition, it was observed that the supramolecular chirality of TPA-SDA decreased completely after a month at 263 K, but the supramolecular chirality of TPA-2a-SDA remained intact after a month (Fig. S19). TPA-2a-SDA is more resistant to external stimuli than TPA-SDA when their one-handed helix is formed by CPL driven symmetry breaking.

To evaluate the hydrogen bonding effect, temperature-dependent circular dichroism measurements was carried out with the self-assembled supramolecules of TPA-SDA and TPA-2a-SDA, at a heating rate of 2 K∙min^-1 with intervals of 5 K. For TPA-2a-SDA, the CD activity maintained up to 288 K, with a sharp decrease at 293 K, and finally little CD activity was observed at room temperature (298 K) (Fig. 4a). But for TPA-SDA, the CD activity decreased at 263 K, and almost entirely collapsed at 278 K. This thermodynamic experiment could not be carried out with TPA-ester, because it did not show any CD signal in aliphatic chlorinated solvents (Fig. S20).

To obtain exact kinetic data, the maximum CD intensity of each of the compounds was measured at intervals of 0.5 K as the temperature increased by 2 K∙min^-1. Figure 4b and 4d show the normalized maximum CD intensity of each compound with a non-sigmoidal curve. Like some biological self-assembled molecules including amino acids, peptides and proteins, triphenylamine-based TPA-2a-SDA and TPA-SDA also follow the cooperative nucleation-elongation self-assembly mechanism^6,32.

\({\varphi }_{n}\) =\({\varphi }_{SAT}\left(1-exp\left[(-{h}_{e}\right.\right.)\times (T-{T}_{e})/(R\times {T}_{e}^{2}\left)\right])\) (Eq. 1)

The thermodynamic parameters of both TPA-2a-SDA and TPA-SDA self-assembly processes were obtained using the Meijer-Schenning-van-der-Schoot mathematical model (Eq. 1)^33,34. As shown in Table S1, TPA-2a-SDA has higher T_e and h_e values than TPA-SDA despite the lower concentration, indicating the formation of a more stable self-assembled structure. Since the center cores of the two compounds are the same, it is the differences in the number of hydrogen bonds that contribute to the stabilization of the chiral self-assembled structures, prepared from an achiral monomer by symmetry breaking of the CPL. Furthermore, the solutions did not show CD activity above the elongation temperature, even when the temperature was lowered again to 263 K (Fig. S21). Circularly polarized light is necessary to induce symmetry breaking when assembly occurs.

The self-assembled structures of the three TPAs were determined using computational based density functional tight binding (DFTB) calculations. To calculate the interaction energy of the self-assembled structures, optimization of the geometries of both the monomer and pentamer structures was conducted. For the initial pentamer structures, the monomers were stacked using a geometrical factor, such as the N-N distance and a rotation angle. Because self-assembled structures have huge numbers of atoms, the DFTB calculation was performed using the DFTB + program^35,36. For all DFTB calculations, the 3ob-3-1 parameter was used and the DFTB3-D3H5 method was applied to describe hydrogen bonds better^37,38.

Figure 5 depicts the stacking of the three compounds with an optimized pentamer, showing that the compounds self-assemble into helical stacks which have different rotational angles due to the hydrogen bonding of amides. As the number of amide groups increased, the rotational angle of the helical self-assembly structure increased, as a result of the amide hydrogen bonding geometry. Table S2 represents the stabilization energy, N-N distance and rotational angle of the self-assembled structures of the three compounds, obtained by DFTB calculations. The stabilization energy value per single stack, calculated from the gas state to the stacked state, became higher as the number of hydrogen bonding sites increased, with the largest value of -310.0 kJ∙mol^-1 for TPA-2a-SDA. This tendency is consistent with the enthalpy value in Table S1.

For the TPA-ester without hydrogen bonds, the rotational angle was 6.24°, which was relatively small compared to 13.04° and 16.12° for TPA-SDA and TPA-2a-SDA. This indicates that the hydrogen bonds contributed geometrically to the formation of the helix in the self-assembly structures, which is believed to make symmetry breaking by CPL easier at the nucleation step.

Also, the X-ray diffraction (XRD) pattern of the TPA-2a-SDA solid obtained from the light-irradiated self-assembled sample showed a single broad peak at 2θ = 22° corresponding to a spacing (d) of 0.40 nm, similar to the DFTB calculation value, which reflects the intermolecular distance between the stacked triphenylamine core (Fig. S22).

Based on these thermodynamic studies and the DFTB calculation results, the energy diagrams for TPA-2a-SDA and TPA-SDA are presented in Fig. 6. Both of the TPA molecules were self-assembled by cooperative self-assembly, followed by a nucleation step and an elongation step. Symmetry breaking occurs relative to the direction of the irradiated CPL, and a one-handed chiral helix is formed through the elongation step^39–41.

Meijer et al. reported a kinetic study of supramolecular polymerization, in which the formed supramolecular fibers can be dynamically exchanged with other monomers⁴². In light of this study, the chirality of TPA-SDA formed by CPL can be racemized by thermal energy, losing their chirality information due to their low thermal activation energy. But in the case of TPA-2a-SDA, their relatively high thermal activation energy hinders the racemization by thermal energy.

In summary, we synthesized three achiral triphenylamine derivatives (TPA-SDA, TPA-2a-SDA and TPA-ester) with different number of hydrogen bonding sites. As the number of hydrogen bonds increased, the stabilization energy and rotational angle became larger and the chirality formed by CPL remained more stable, as confirmed by DFTB calculations and CD measurements. These results provide insights on the molecular design for chiral self-assembly of achiral molecules using external stimuli.

General information

The ¹H and ¹³C nuclear magnetic resonance (NMR) spectra of the synthesized materials were recorded on a Bruker Science Avance (400 MHz) and Agilent DD-2 (600 MHz) NMR spectrometer with the residual signals as an internal reference. Field-emission scanning microscopy (FE-SEM) was performed on a FEI Company Inspect F. Sample was prepared by dropping the solution on the silicon wafer and evaporated the solvent by spin coating with 1500 rpm. The sample was coated with Pt prior to imaging. Circular dichroism (CD) spectra were obtained using Jasco-815 Spectropolarimeter (150-L type) equipped with temperature controller. Measurement was conducted with aliphatic chlorinated solution in 0.5 mm path length and 0.2 mm path length using a quartz cell at 263 K, with scanning speed of 200 nm∙min^-1 and D.I.T of 0.125 sec. For variable temperature measurement, scanning speed was fixed at 2 K∙min^-1 to prevent fluctuation of data and interval was 0.5 K. Field-emission transmittance electronic microscopy (FE-TEM) was performed on FEI company Tecnai F20 FE-TEM. Samples were prepared on a 200-mesh carbon film by dropping dichloromethane solution of TPA-2a-SDA (1.0 mg∙mL^-1) and through evaporation of the solvent. Before imaging, the samples were stained by exposing ruthenium tetroxide vapor. X-ray diffraction (XRD) pattern were obtained with Rigaku D/MAX-2500 X-ray diffractometer with scan speed of 4^o∙min^-1 and sampling width of 0.01^o. Cu K_α (λ = 0.154 nm) was used as light source. Samples for wide-angle XRD were prepared by drop casting of dichloromethane solution of TPA-2a-SDA (1.0 mg∙mL^-1) on a glass substrate. To measure absorbance, ultraviolet–visible (UV–vis) spectroscopy was performed on a Shimadzu UV-2600 spectrometer equipped with a temperature controller system. The analysis was performed under a 0.013 mg∙mL^-1 dichloromethane solution at 1 cm x 1 cm path length cell. Matrix-assisted laser absorption ionization time-of-flight (MALDI-TOF) mass spectroscopy study was performed on a Bruker autoflex maX using reflector mode for operation. Ionization was performed with a 355 nm pulsed Nd YAG laser, and samples were prepared with dithranol as a matrix and sodium trifluoroacetate as an ion source. The Fourier-transform infrared (FT-IR) spectra was performed in attenuated total reflection (ATR) mode using the Thermo Fisher Scientific Instrument Nicolet IS50 FTIR spectrometer.

Materials.

Triphenylamine (98%), acetyl chloride (98%), aluminum chloride (98%), triethylamine (≥ 99.5%), 1-hydroxybenzotriazole hydrate (≥ 97.0%), ethylenediamine (≥ 99%), 1,4-dioxane (≥ 99.0%), dichloromethane anhydrous (99.8%), 4-(dimethylamino)pyridine (≥ 99%), ethanol (99.8%), sodium borohydride (99%) were purchased from Sigma-Aldrich. The reagents, bromine (> 98.0%), 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (> 98.0%), methanol (> 99.8%), tris(4-formylphenyl)amine (> 97.0%) were purchased from Tokyo Chemical Industry, and hydrochloric acid (35%), sulfuric acid (98.0%), sodium chloride (99.5%) were purchased from Daejung chemicals. N,N`-Dicyclohexylcarbodiimide (99%) was purchased from Acros Organics. Trideca-4,6-diynoic acid (SDA) was synthesized according to the previously reported procedure1

Synthesis of TPA-2a-SDA and TPA-ester

Synthesis of tris(4-acetylphenyl)amine : In a 250 mL two-neck round bottom flask, triphenylamine (0.5 g, 2.04 mmol) was dissolved in anhydrous dichloromethane (10 mL) under nitrogen atmosphere. Slowly aluminium chloride (AlCl₃) (0.84 g, 6.32 mmol) dissolved in dichloromethane (5 mL) was added under stirring, and the whole mixture was cooled to 273 K. Then acetyl chloride (0.5 g, 6.32 mmol) was added dropwise at 273 K. After the addition was complete, and the reaction mixture was stirred and allowed to reach room temperature overnight. The reaction mixture was poured into ice water (125 mL), the organic layer was extracted with dichloromethane (3 x 50 mL), dried on MgSO₄, collected and evaporated (630 mg, 83%). ¹H NMR (CDCl₃, 400 MHz, ppm): 7.92 (d, 6H, J = 8.6 Hz, Ar), 7.17 (d, 6H, J = 8.6 Hz,Ar), 2.81 (s, 9H, COCH₃).

Synthesis of tris(4-carboxyphenyl)amine : Tris(4-acetylphenyl)amine (450 mg, 1.21 mmol) was dissolved in 1,4-dioxane (12.5 mL) in a three-neck round bottom flask. Meantime, in a beaker, bromine (0.62 mL, 12.12 mmol) was added dropwise to an aqueous (7.5 mL) solution of NaOH (1.59 g, 40 mmol) cooled by ice bath. After the addition of the bromine solution to round bottom flask, the solution was stirred for 20 min at room temperature. The solution was transferred in a dropping funnel and dropped into the solution of tris(4-acetylphenyl)amine. The mixture was stirred and heated at 313 K overnight. Then, the mixture was cooled at 273 K using an ice bath and saturated under stirring with hydroxylamine HCl to deoxidize the excess of formed sodium bromite (NaBrO₂). The solution was acidified by HCl (37%) till pH = 1.5. The pale yellow precipitate was filtered and dried under vacuum. The crude product was recrystallized from acetic acid to afford pure one. (432 mg, 94%) ¹H NMR (DMSO-d₆, 400 MHz, ppm): 7.90 (d, 6H, J = 8.6 Hz, Ar), 7.16 (d, 6H, J = 8.6 Hz, Ar), 12.71 (broad singlet, 3H, COOH).

Synthesis of Tris(4-carbomethoxyphenyl)amine : 50 mL three round bottom flask fitted with a magnetic stirrer and a heating oil bath was charged with tris(4-carboxyphenyl)amine (0.15 g, 0.40 mmol) 5 mL of methanol, and 0.3 mL of H₂SO₄. The mixture was heated up to 343 K and refluxed for 6h, and finally cooled to room temperature. A green solid Tris(p-carbomethoxyphenyl)amine was obtained after ethyl acetate/brine extraction and evaporation of organic layer. (161 mg, 96%) ¹H NMR (DMSO-d₆, 400MHz, ppm): 7.92 (d, 6H, J = 8.4 Hz, Ar), 7.16 (d, 6H, J = 8.4 Hz, Ar), 3.83 (s, 9H, COOCH₃).

Synthesis of tris(4-((2-aminoethyl)carbamoyl)phenyl)amine (TPA-EDA) : Flesh ethylene diamine of 20 mL (excess, 0.225 mol) was placed in a 50 mL three flask, and then Tris(4-carbomethoxyphenyl)amine (0.15 g, 0.36 mmol) dissolved in 50 mL methanol in advance was slowly added into the solution through a constant pressure funnel. The mixture was stirred under nitrogen atmosphere at 273 K for 30 min, and then reflux at 343 K for 18h. Excess ethylene diamine was removed by rotary evaporation. The crude product was dissolved in methanol/toluene (1/10 by volume) mixed solvents and re-evaporated to remove the residual ethylene diamine three times. Small amount of methanol was poured into the residual solution and the solution was poured into 1 L of ethylacetate to precipitate the product. Yellowish solid was obtained. (152 mg, 84%) ¹H NMR (DMSO-d₆, 400 MHz, ppm): 8.40 (s, 3H, NHCO), 7.82 (d, 6H, J = 8.6 Hz, Ar), 7.09 (d, 6H, J = 8.6 Hz, Ar), 3.27 (d, 6H, J = 5.9 Hz, CONHCH₂), 2.71 (d, 6H, J = 6.4 Hz, CH₂NH₂).

Synthesis of tris(4-((2-(trideca-4,6-diynamido)ethyl)carbamoyl)phenyl)amine (TPA-2a-SDA) : TPA-EDA (0.1 g, 0.20 mmol), SDA (0.20 g, 1.00 mmol) and 1-Hydroxybenzotriazole (HOBT) (0.16 g, 1.20 mmol) were dissolved in 20 mL of DMF. The reaction mixture was stirred at 273 K for 30min. 1-Ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) (0.23 g, 1.20 mmol) and Et₃N (0.16 mL, 1.20 mmol) were added into the solution. The solution was warmed up to RT. It was stirred for 24h at 343 K. The reaction mixture was poured into brine and extracted with dichloromethane. The organic layer was washed with water (2 times) and the precipitate which formed on the boundary was filtered off. The organic layer was evaporated and precipitated with hexane. The precipitate was purified by column chromatography two times (dichloromethane: methanol = 19:1 and subsequently dichloromethane: methanol = 9:1). (57 mg, 25%) Matrix-assisted laser absorption ionization time-of-flight (MALDI-TOF) mass spectroscopy, m/z 1090.74 [M + Na]⁺ (Calcd: 1067.62) ¹H NMR (DMSO-d₆, 400 MHz, ppm): 8.47 (t, 3H, J = 5.2 Hz, CONH), 8.07 (t, 3H, J = 5.2 Hz, NHCO), 7.81 (d, 6H, J = 8.5 Hz, Ar), 7.10 (d, 6H, J = 8.6 Hz, Ar), 3.29 (t, 6H, J = 5.8 Hz, NHCH₂), 3.21 (t, 6H, J = 5.3 Hz, CH₂NH), 2.29 (m, 12H, COCH₂, C ≡ CCH₂) 1.42 (m, 6H, CH₂CH₂), 1.29 (m, 6H, CH₂CH₂), 1.24 (m, 12H, CH₂CH₂), 0.85 (t, 9H, CH₂CH₃) ¹³C NMR (DMSO-d₆, 150 MHz, ppm): 170.74, 166.04, 135.91, 129.87, 129.34, 123.74, 78.74, 77.57, 38.79, 34.26, 31.09, 28.31, 28.11, 22.40, 18.71, 15.30, 14.33.

Synthesis of tris(4-(hydroxymethyl)phenyl)amine : In a 250 mL two-neck round bottom flask, tris(4-formylphenyl)amine (100 mg, 0.30 mmol) and sodium borohydride (50 mg, 1.32 mmol) was dissolved in dichloromethane (40 mL) and ethanol (15 mL). After stirring for 2 h at room temperature, the solution was extracted with dichloromethane and brine. The organic layer was collected and evaporated, which subsequently was precipitated in hexane and filtered. (99 mg, 99%) ¹H NMR (DMSO-d₆, 400 MHz, ppm): 7.21 (d, 6H, J = 8.6 Hz, Ar), 6.91 (d, 6H, J = 8.5 Hz, Ar), 5.09 (t, 3H, J = 5.7 Hz, CH₂OH), 4.44 (d, 6H, J = 5.7 Hz, CH₂OH).

Synthesis of tris(4-((trideca-4,6-diynoyloxy)methyl)phenyl)amine (TPA-ester) : In a two-neck round bottom flask, tris(4-(hydroxymethyl)phenyl)amine (75 mg, 0.23 mmol) was dissolved in dichloromethane. To the solution, 4-(dimethylamino)pyridine (4-DMAP) (136 mg, 1.12 mmol), SDA (230 mg, 1.12 mmol) and DCC (230 mg, 1.12 mmol) were added at 0^oC, and the reaction mixture was stirred for 30 min. The reaction mixture was warmed to room temperature and stirred for 16 h. The solution was washed by brine and evaporated. The washed solution was precipitated in hexane to remove urea. After the solution was purified by column chromatography three times, 1) hexane, 2) hexane: ethyl acetate = 1:1 and 3) hexane: ethyl acetate = 1:3 eluents. (46 mg, 23%). Matrix-assisted laser absorption ionization time-of-flight (MALDI-TOF) mass spectroscopy, m/z 900.64 [M + H]⁺ (Calcd: 899.52) ¹H NMR (Acetone-d₆, 400MHz, ppm): 7.34 (d, 6H, J = 8.7 Hz, Ar), 7.05 (d, 6H, J = 8.7 Hz, Ar), 5.12 (s, 6H, CH₂OOC), 2.61 (m, 12H, CH₂C ≡ CC ≡ CCH₂), 2.23 (t, 6H, J = 6.4 Hz, COCH₂), 1.48 (m, 6H, CH₂CH₂), 1.30 (m, 18H, CH₂CH₂CH₂), 0.88 (t, 9H, J = 6.9 Hz, CH₂CH₃). ¹³C NMR (Acetone-d₆, 100 Hz, ppm): 170.99, 147.41, 131.07, 129.45, 123.91, 77.86, 75.51, 65.79, 65.55, 65.15, 37.70, 31.09, 29.57, 28.08, 22.29, 18.45, 14.64, 13.40.

Formation of chiral supramolecular helical nanofibers by light.

To irradiate light during the self-assembly, Lumatec SUV-DC-P which had a wavelength range of 280–500 nm and a maximum intensity of 370 nm was used as a light source. To irradiate visible light, a filter with 400 nm of cut-off wavelength was placed in front of the light source. Circularly polarized light was formed by placing a linear polarizer and a λ/4 wave-plate in front of the light source. Samples were placed at 5 cm from the lamp and external light was blocked to precisely investigate the effect of CPL. For dissolution of TPA-2a-SDA, ultra-sonication was applied to the solution of TPA-2a-SDA (1.0 mg∙mL^-1) at room temperature, and the solution was cooled to -10 ^oC while exposing to CPL.

Mathematical models to characterize supramolecular polymerization process.

To investigate the self-assembly behavior of TPA-SDA and TPA-2a-SDA, variable temperature CD measurement was conducted. Non-sigmoidal curves were obtained by plotting the degree of aggregation (\({\alpha }\)_agg) against temperature for two monomers, which indicate cooperative supramolecular polymerization process. By fitting the graph with Meijer-Schenning-van der Schoot model, the detailed thermodynamic parameters for self-assembly processes were obtained. We used their elongation regime for fitting of the graph by Eq. 1:

\({\varphi }_{n}\) =\({\varphi }_{SAT}\left(1-exp\left[(-{h}_{e}\right.\right.)\times (T-{T}_{e})/(R\times {T}_{e}^{2}\left)\right])\) (Eq. 1)

In this equation, \({h}_{e}\) means the molecular enthalpy release due to the noncovalent interactions during elongation. T is absolute temperature, \({T}_{e}\) is the elongation temperature and R is the gas constant. \({\varphi }_{SAT}\) is introduced as a parameter to ensure that \({\varphi }_{n}/{\varphi }_{SAT}\) does not exceed unity.

DFT calculation.

The self-assembled structures of the three TPA derivatives were determined using computational based density functional tight binding (DFTB) calculations. To calculate the interaction energy of the self-assembled structures, optimization of the geometries of both the monomer and pentamer structures was conducted^35,36. For the initial pentamer structures, the monomers were stacked using a geometrical factor, such as the N-N distance and a rotation angle. Because self-assembled structures have huge numbers of atoms, the DFTB calculation was performed using the DFTB + program. For all DFTB calculations, the 3ob-3-1 parameter was used and the DFTB3-D3H5 method was applied to describe hydrogen bonds better^37,38.

Acknowledgements

This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (NRF- 2021R1A4A1052070) and by Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government (MOTIE) (20191510301070).

Competing interests

The authors declare no competing interests.

Author contributions

S. Y. Kim conceived and supervised the project. D. H. Kim carried out the monomer synthesis and finding self-assembly condition, NMR, FE-SEM, UV-Vis, CD data analysis and writing the original draft. T. H. Kim carried out FE-TEM, ATR-IR and MALDI-TOF data analysis. Y. J. Kim and W. Y. Kim performed DFTB simulations. M. E. Seo, H. S. Lee and S. Y. Kim reviewed and edited the manuscript.

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Light-induced Chiral Self-assembly of Achiral Triphenylamines with Controlled Number of Hydrogen Bondings

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