Supramolecular polymerization behavior of a ditopic self-folding biscavitand

Abstract

Reported herein is the supramolecular polymerization of a mixture of a feet-to-feet connected biscavitand and a homoditopic quinuclidinium guest that is regulated by cooperativity in the host–guest association. Diffusion-ordered NMR spectroscopy (DOSY) was used to evaluate the supramolecular polymerization in toluene, CHCl₃, and tetrahydrofuran (THF). Upon concentrating the solutions of the biscavitand with the quinuclidinium guest in CHCl₃ and THF, the diffusion coefficient (D) values were meaningfully decreased, indicating that the host–guest complexation facilitated supramolecular polymerization. In contrast, the slight change of the D value in toluene suggests that supramolecular polymerization was suppressed, although the binding constant (K) between the cavitand and quinuclidinium guest was reported to be 10⁵ L mol⁻¹ in toluene. The viscosity measurements showed both the critical polymerization concentration (CPC) and entangled concentration (C_e) upon concentrating the CHCl₃ solution of the mixture. Neither the CPC nor C_e was seen in the toluene solution of the mixture. Accordingly, the strong negative cooperativity in the 1:2 host–guest complexation of the biscavitand discouraged the supramolecular polymerization in toluene. These findings are valuable in deepening the understanding of host–guest association-driven supramolecular polymerization behaviors regulated by a combination of cooperativity and K value in solution.

Graphical Abstract

The host–guest association-driven supramolecular polymerization behaviors between a biscavitand and a homoditopic quinuclidinium guest are directed by a combination of cooperativity and binding constants in solution.

cavitand, cooperativity, supramolecular polymers

1. Introduction

Supramolecular polymers are maintained by reversible noncovalent interactions such as hydrogen bonding, π–π stacking interactions, host–guest interactions, donor–acceptor interactions, and other interactions.^1–4 Due to the manner in which they are linked, supramolecular polymers have a stimulus-responsive nature, which has drawn great attention in the development of new functional polymer materials.^5–17 To date, many supramolecular polymers have been investigated by many groups worldwide through judiciously designed supramolecular monomers.^18–31 Supramolecular polymers are formed through self-association of a single species of monomer^32–43 or iterative intermolecular association of 2^44–57 or more^58–61 species of monomers. Among these supramolecular polymers, those consisting of 2 species of monomers yield a two-monomer array within the polymer main chain, which is formed, for example, by intermolecular association of a homoditopic host molecule possessing 2 binding sites and a homoditopic guest molecule possessing 2 guest units.^62–66 When such homoditopic host molecules sequentially bind 2 guest molecules in the 2 binding sites, the first host–guest complexation can promote or suppress the complexation of the other binding site, which is known as a positive or negative cooperative effect, respectively.^67–70 To date, the chain length of supramolecular polymers has been evaluated between host moieties and guest moieties of supramolecular monomers by the binding constant (K).⁷¹ In addition to K values, cooperative effects in host–guest association processes should influence molecular recognition-directed supramolecular polymerization.

Cooperativity in receptor–substrate associations commonly occurs in biomolecules and is a crucial process of biological regulation.^72–76 Recent efforts in this area of investigation have led to the successful development of artificial host molecules that exhibit cooperativity in guest binding. In the 1980s, Rebek et al. demonstrated pioneering work on fully synthetic host molecules that show cooperativity in both heteromeric^77–79 and homomeric⁸⁰ guest binding. A recent line of research has established that one of the driving forces of the cooperativity in guest binding is ascribed to conformational changes upon binding guest molecules.⁸¹ In a homoditopic host molecule, guest binding of one host moiety causes a conformational change in the other binding site. The resulting binding sites are conformationally preorganized to be more or less suitable for second guest binding. Thus, all the binding sites of multitopic host molecules need to be conformationally coupled for the representation of cooperativity in guest binding. Cooperative supramolecular polymerization has been actively debated in the field of supramolecular polymers.^82–86 However, there are limited reports that investigate host–guest association-driven supramolecular polymerization controlled by cooperativity.

Our group has studied that feet-to-feet connected biscavitands are promising ditopic host molecules that exhibit cooperativity in guest binding.^87–90 The 2 cavitand moieties were found to be conformationally coupled when the 2 cavitand moieties were linked by short alkyl chains. Recently, we found that biscavitand 1 based on Rebek's deep cavitand^91–93 exhibited cooperativity in guest (quinuclidinium 3a) binding depending on solvent polarity (Fig. 1).⁹⁴ The less polar toluene and CHCl₃ facilitated the negative cooperativity of the host–guest complexation, whereas the cooperativity was slightly positive in tetrahydrofuran (THF). In view of this finding, we were keen on studying the supramolecular polymerization behaviors based on biscavitand–quinuclidinium host–guest complexation. The growth of supramolecular polymer chains can be regulated by the cooperativity of the host–guest association. Here, we report the supramolecular polymerization of biscavitand 1 and a ditopic quinuclidinium guest 2, which was influenced by the cooperativity that was directed by the solvent properties.

Fig. 1.

a) Molecular structures of biscavitand 1, monocavitand 4, ditopic quinuclidinium guest 2, and monotopic quinuclidinium guest 3. b) Schematic cartoon of the supramolecular polymer poly-(1•2).

2. Materials and Methods

2.1 General

All solvents were commercial reagent grade and were used without further purification except where noted. Dry N,N-dimethylformamide (DMF) was obtained by distillation over CaH₂. ¹H and ¹³C NMR spectra were recorded on a Bruker Ascend 400 spectrometer for characterization of new chemical entities, and chemical shifts were reported on the delta scale in ppm relative to residual CHCl₃ (δ = 7.26 and 77.0 for ¹H and ¹³C, respectively). Diffusion-ordered NMR spectroscopy (DOSY) experiments were carried out on a JEOL JNM-ECA500 spectrometer, and chemical shifts were reported on the delta scale in ppm relative to residual CHCl₃ (δ = 7.26 for 1H), THF (δ = 3.55 for 1H), and toluene (δ = 6.95 for 1H). Melting point (M.p.) was measured with a Yanagimoto micro melting point apparatus. Preparative separations were performed by silica gel gravity column chromatography (Silica Gel 60 N [spherical, neutral]). Preparative medium-pressure liquid chromatography (MPLC) separations were carried out with a YAMAZEN smart flash EPCLC AL-580S using a Bio-Beads (S-X1) column. Morphological evaluation of the films was performed using atomic force microscopy (AFM) using a Burker Multimode 8HR under ambient conditions with a Bruker cantilever model NCHV. The images were analyzed using NanoScope Analysis 2.0 software. Previously synthesized 1, 3a, 3b, and 4 were used for this work. For their purity and detailed experimental procedures, refer to Ref.⁹⁴ Compounds 5⁹⁵ and 6⁹⁶ were synthesized according to reported methods.

2.1.1 Dimethyl 4,4′-bis-{(3-decyltridecyl)oxy}-[1,1′-biphenyl]-2,2′-dicarboxylate (7)

To a solution of 5 (1.1 g, 3.6 mmol) and K₂CO₃ (2.9 g, 21 mmol) in 18 mL of dry DMF was added 6 (3.5 g, 7.8 mmol). The mixture was stirred at 65 °C for 10 h in an oil bath under an argon atmosphere. The resulting mixture was diluted with dichloromethane, and then the organic layer was washed with water, dried over Na₂SO₄, and concentrated in vacuo. Column chromatography on silica gel (0% to 20% ethyl acetate in n-hexane, eluent) gave the desired product 7 (67%) as a colorless oil: ¹H NMR (400 MHz, CDCl₃): δ 7.48 (doublet [d], 2H, J = 2.6 Hz), 7.09 (d, 2H, J = 8.4 Hz), 7.04 (double doublet [dd], 2H, J = 8.4, 2.6 Hz), 4.03 (t, 4H, J = 7.1 Hz), 3.62 (single [s], 6H), 1.77 (quartet [q], 4H, J = 7.1 Hz), 1.56 (overlapped, 2H), 1.21 to 1.35 (m, 72H), 0.88 (t, 12H, J = 7.1 Hz) ppm; ¹³C NMR (125 MHz, CDCl₃): δ 167.6, 157.9, 135.0, 131.7, 130.6, 118.0, 115.0, 66.6, 51.9, 34.6, 33.7, 33.1, 31.9, 30.1, 29.7, 29.7, 29.4, 26.6, 22.7, 14.1 parts per million (ppm); high-resolution mass spectrometry (HRMS) (electrospray ionization [ESI⁺] calculated (calcd). for C₆₂H₁₀₇O₆  m/z 947.80622 [M + H]⁺, found m/z 947.80725.

2.1.2 4,4’-Bis-{(3-decyltridecyl)oxy}-[1,1’-biphenyl]-2,2’-dimethanol (8)

Product 7 (2.3 g, 2.4 mmol) was dissolved in dry THF (22 mL), and added dropwise to a solution of LiAlH₄ (280 mg, 7.4 mmol) in 42 mL of dry THF at room temperature. The mixture was stirred for 15 h at room temperature under an argon atmosphere. Ethyl acetate (2 mL) was added to the reaction vessel to quench the residual LiAlH_4, and the resulting mixture was extracted with dichloromethane. The organic layer was washed with water, dried over Na₂SO₄, and concentrated in vacuo. Column chromatography on silica gel (0% to 10% ethyl acetate in n-hexane, eluent) afforded the desired product 8 (89%) as a colorless oil: ¹H NMR (400 MHz, CDCl₃): δ 7.05 (d, 2H, J = 8.4 Hz), 7.03 (d, 2H, J = 2.3 Hz), 6.86 (dd, 2H, J = 8.4, 2.3 Hz), 4.38 (d, 2H, J = 11.8 Hz), 4.32 (d, 2H, J = 11.8 Hz), 4.02 (t, 4H, J = 7.2 Hz), 1.77 (q, 4H, J = 7.2 Hz), 1.56 (m, 2H), 1.21 to 1.36 (m, 72H), 0.88 (t, 12H, J = 7.1 Hz) ppm; ¹³C NMR (125 MHz, CDCl₃): δ 158.8, 140.3, 131.6, 131.2, 114.8, 113.8, 66.5, 63.3, 34.6, 33.7, 33.2, 31.9, 30.1, 29.7, 29.7, 29.4, 26.6, 22.7, 14.1 ppm; HRMS (ESI⁺) calcd. for C₆₀H₁₀₆O₄Na m/z 913.79888 [M + Na]⁺, found m/z 913.79868.

2.1.3 2,2’-Bis(bromomethyl)-4,4’-bis-{(3-decyltridecyl)oxy}-1,1’-biphenyl (9)

To a solution of 8 (1.4 g, 1.6 mmol) and PPh₃ (0.99 g, 3.8 mmol) in 99 mL of dichloromethane was added CBr₄ (2.8 g, 8.4 mmol) at room temperature. The mixture was stirred for 10 h under an argon atmosphere at room temperature. The resulting mixture was directly passed through silica gel (0% to 20% ethyl acetate in n-hexane, eluent) to give rise to desired product 9 (74%) as a colorless oil: ¹H NMR (400 MHz, CDCl₃): δ 7.15 (d, 2H, J = 8.3 Hz), 7.04 (d, 2H, J = 2.4 Hz), 6.86 (dd, 2H, J = 8.3, 2.4 Hz), 4.30 (d, 2H, J = 10.0 Hz), 4.17 (d, 2H, J = 10.0 Hz), 4.02 (t, 4H, J = 6.9 Hz), 1.77 (q, 4H, J = 6.9 Hz), 1.56 (overlapped, 2H), 1.20 to 1.35 (m, 72H), 0.88 (t, 12H, J = 7.0 Hz) ppm; ¹³C NMR (125 MHz, CDCl₃): δ 158.9, 137.4, 131.7, 131.4, 116.0, 114.7, 66.5, 34.6, 33.7, 33.1, 32.4, 31.9, 30.1, 29.7, 29.7, 29.4, 26.6, 22.7, 14.1 ppm; HRMS (ESI⁺) calcd. for C₆₀H₁₀₄O₂Br₂  m/z 1014.63976 [M]⁺, found m/z 1014.63995.

2.1.4 Compound 10

To a solution of 9 (1.2 g, 1.2 mmol) and NaI (0.32 g, 2.1 mmol) in 20 mL of dichloromethane was added quinuclidine (0.26 g, 2.3 mmol) at room temperature. The mixture was stirred for 10 h under an argon atmosphere at room temperature. The resulting mixture was concentrated in vacuo. MPLC (CHCl₃, eluent) yielded the desired product 10 (32%) as a yellow solid: M.p.: 112 to 117 °C; ¹H NMR (400 MHz, CDCl₃): δ 7.54 (d, 2H, J = 2.6 Hz), 7.38 (d, 2H, J = 8.5 Hz), 7.16 (dd, 2H, J = 8.3, 2.4 Hz), 5.28 (d, 2H, J = 11.9 Hz), 4.15 (m, 4H), 3.98 (d, 2H, J = 11.9 Hz), 3.56 (m, 6H), 3.19 (m, 6H), 2.15 (m 2H), 1.92 (m, 12H), 1.80 (m, 4H), 1.61 (m, 2H), 1.20 to 1.40 (m, 72H), 0.90 (t, 12H, J = 7.2 Hz) ppm; ¹³C NMR (125 MHz, CDCl₃): δ 159.1, 133.8, 133.5, 125.7, 121.1, 118.1, 67.5, 65.6, 55.2, 34.5, 33.7, 33.6, 33.2, 32.0, 30.2, 29.8, 29.8, 29.7, 29.4, 26.5, 24.1, 22.7, 19.5, 14.2 ppm; HRMS (ESI⁺) calcd. for C₇₄H₁₃₀O₂N₂  m/z 539.50607 [M–2I]²⁺, found m/z 539.50574. HRMS (ESI⁻) calcd. for I m/z 126.90502 [I]⁻, found m/z 126.90488.

2.1.5 Compound 2

Compound 10 (480 mg, 36 mmol) was dissolved in dichloromethane (20 mL), and AgSbF₆ (270 mg, 79 mmol) was added at room temperature. The reaction mixture was stirred for 2 h under an argon atmosphere at room temperature. The resulting mixture was passed through Celite (dichloromethane, eluent) and then concentrated in vacuo to afford the desired product 2 (92%) as a yellow solid: M.p.: 99 to 103 °C; ¹H NMR (400 MHz, CDCl₃): δ 7.32 (d, 2H, J = 8.7 Hz), 7.15 (d, 2H, J = 8.7 Hz), 7.04 (s, 2H), 4.33 (d, 2H, J = 11.5 Hz), 4.07 (m, 4H), 3.83 (d, 2H, J = 11.5 Hz), 3.12 (m, 6H), 3.00 (m, 6H), 2.10 (m 2H), 1.89 (m, 12H), 1.78 (m, 4H), 1.57 (m, 2H), 1.19 to 1.28 (m, 72H), 0.88 (t, 12H, J = 7.2 Hz) ppm; ¹³C NMR (125 MHz, CDCl₃): δ 159.3, 134.1, 133.2, 125.2, 120.3, 117.9, 67.1, 66.0, 55.2, 34.6, 33.6, 33.6, 33.2, 31.9, 30.1, 30.1, 29.7, 29.7, 29.7, 29.5, 26.5, 23.8, 22.7, 19.2, 14.1 ppm; HRMS (ESI⁺) calcd. for C₇₄H₁₃₀O₂N₂  m/z 539.50607 [M–2SbF₆]²⁺, found m/z 539.50663. HRMS (ESI⁻) calcd. for SbF₆  m/z 234.89479 [SbF₆]⁻, found m/z 234.89452.

3. Results and discussion

The synthesis of 2 was accomplished as follows (Scheme 1). The 2-fold Williamson etherification of dialcohol 5⁹⁵ with branched alkyl iodide 6⁹⁶ provided diether 7. The reduction of the methyl esters gave dialcohol 8 in good yield. Subsequent bromination of the alcohol groups produced dibromide 9, which was followed by N-alkylation of quinuclidine in the presence of sodium iodide to give rise to guest molecule 2 after counteranion exchange.

Scheme 1.

Synthesis of ditopic quinuclidinium guest 2. DMF = N,N-dimethylformamide; LAH = lithium aluminum hydride; THF = tetrahydrofuran.

This molecular association between 1 and 2 was studied by DOSY. The hydrodynamic radius of molecular species in solution is known to be inversely proportional to their diffusion coefficient (D), which is described by the Stokes–Einstein equation.^97–99 Thus, the formation of supramolecular polymers can be estimated by monitoring the D values of molecular species in solution. 1 and 2¹⁰⁰ were individually dissolved in CHCl₃, THF, and toluene at various concentrations. The resulting solutions were separately placed in NMR test tubes (3 mmϕ), which were subjected to DOSY experiments. The concentration-independent nature of the D values of 1 and 2 was clearly illustrated in all 3 solvents (Fig. 2a to c and Supplementary Figs. S7 to S42), indicating that 1 and 2 were freely moving in the solutions without any specific intermolecular interactions. On the other hand, a significant decrease in the D values was observed in the CHCl₃ solutions of a 1:1 mixture of 1 and 2 upon concentrating the solution (Figs. 2a and Supplementary Fig. S18), indicating the formation of poly-(1•2) in the solution in the high concentration region. The decrease in the D values in CHCl₃ was observed to be steeper than those in THF and toluene (Fig. 2b,c, Supplementary Figs. S30, and S42). The difference in the D values reflects the difference in their degree of polymerization (DP). Judging from the rigid molecular skeleton of poly-(1•2), a cylindrical model^101–106 should be appropriate to estimate the DP based on the diffusion coefficient values (Fig. 2d). In the cylindrical model, the correlation between D and L is described as follows:

Fig. 2.

Plot of diffusion coefficients of 1 (filled circle), 2 (filled triangle), and a 1:1 mixture of 1 and 2 (square) in a) CHCl₃, b) THF, and c) toluene at 25 °C, where C denotes concentrations of 1 for the solution of 1 (circle), 2 for the solution of 2 (triangle), or ([1] + [2]/2) for the solution of the 1:1 mixture of 1 and 2 (square). d) Cartoon representation of poly-(1•2) fitted by a cylindrical model. The diameter (d) of 9.8 Å is consistent with that of the cavitand rim, which was estimated based on the crystal structure of 3b₂•1 that we previously reported in Ref.⁹⁴ e) Plot of the degree of polymerization (DP) of poly-(1•2) against concentrations of a 1:1 mixture of 1 and 2 when assuming that poly-(1•2) is cylindrical in shape. The square, triangle, and circle denote DP in CHCl₃, THF, and toluene, respectively.

$$\begin{array}{c}D={\displaystyle \frac{k_{\mathrm{B}}T}{3\pi \eta L}\{\ln p+\nu \}}\end{array}$$

(1)

where k_B, T, η, and L denote the Boltzmann constant, temperature, solvent viscosity, and length of the cylinder, respectively. p is the axial ratio of the cylinder (p = L/d). ν is the end-effect correction term, which is expressed as follows:

$$\begin{array}{c}\nu =0.312+0.565p^{-1}-0.100p^{-2}\end{array}$$

(2)

Premised on Equations (1) and (2), the DP of poly-(1•2) was estimated in CHCl₃, THF, and toluene. As seen in the DPs plotted in Fig. 2e, efficient growth of the supramolecular polymer chains was observed in CHCl₃ (DP ≈ 44 at 70 mmol L^–1) and THF (DP ≈ 17 at 70 mmol L^–1) solutions of a 1:1 mixture of 1 and 2, whereas short oligomeric species (DP ≈ 5 at 70 mmol L^–1) dominated in the toluene solution of a 1:1 mixture of 1 and 2 (Supplementary Table S1). The K values between 4 and 3a were reported to be 500 L mol⁻¹ in CHCl₃, 205 L mol⁻¹ in THF, and 103,000 L mol⁻¹ in toluene (Table 1).⁹⁴ Assuming that there is no cooperativity in the polymerization process, the DP was estimated to be 7 in CHCl₃, 4 in THF, and 85 in toluene based on the K values at a concentration of 70 mmol L^–1 by an isodesmic model⁷¹ (Table 1), which was not consistent with the experimentally obtained DP values from DOSY experiments at a concentration of 70 mmol L^–1 (Fig. 2e). This observation led to an inference that the DP of the supramolecular polymer is determined not only by the binding constant but also by cooperativity in the guest binding. In CHCl₃, the slightly negative cooperativity of the host–guest complexation for 1 did not interfere with the supramolecular polymerization at high concentrations, resulting in large supramolecular polymers. Slightly positive cooperativity in the quinuclidinium binding of 1 promoted the growth of supramolecular polymer chains in THF. In the case of supramolecular polymerization in CHCl₃ and THF, the K values determined the DP of poly-(1•2) in the solutions rather than the cooperative effect. In contrast, the strong negative cooperativity in toluene totally prevented the second guest binding of 1 even in the concentrated solution, leading to the huge difference in the observed DP of 5 and the estimated DP of 85 at a concentration of 70 mmol L^–1. The first quinuclidinium binding of one cavitand moiety of 1 significantly inhibited the second quinuclidinium binding of another cavitand moiety in toluene, resulting in no polymeric species in the toluene solution.¹⁰⁷

Table 1.

Summary of K between 4 and 3a and DP of poly-(1•2). The DPs of poly-(1•2) were estimated by K values assuming that the supramolecular polymerization is in an isodesmic process.

		DP at the concentrations of 1a
solvent	Kb (L mol−1)	10 mM	25 mM	40 mM	55 mM	70 mM
CHCl3	500 ± 8	3	4	5	6	7
THF	205 ± 6	2	3	3	4	4
toluene	103,000 ± 1,000	33	51	65	76	85

^aThe concentrations denote the concentration of 1 ([1] = [2]). ^bThe K values are reported in Ref.⁹⁴

Specific viscosities (η_sp) of the solutions of a 1:1 mixture of 1 and 2 were measured using a rolling-ball viscometer at various concentrations (Fig. 3), giving rise to detailed insight into the supramolecular polymerization of poly-(1•2) in the solutions. The CHCl₃ solution became viscous upon concentrating the solution (Fig. 3a). In the log–log plot, 2 transition concentrations, namely, the critical polymerization concentration (CPC) (6.7 mmol L^–1) and entangled concentration (C_e) (45 mmol L^–1), were clearly illustrated.¹⁰⁸ Below the CPC, monomeric and short oligomeric species exist without overlap in the solution, resulting in a slope value of 0.99 in the log–log plot. Above the CPC, polymer chains come into contact, leading to a slope change from 0.99 to 1.48 in the log–log plot. Concentrating the solution over the CPC yields the second slope change in the log–log plot from 1.48 to 3.24, where the transition concentration corresponds to C_e. Above the C_e, a slope exponent of 3.24 is consistent with that predicted by the mixed reptation-breakage regime of a living polymer chain, where the lifetime of the host–guest interaction is longer than the reptation time.¹⁰⁹ Sizable polymeric chains become entangled in the solution. The CPC was observed in the log–log plot of the THF solution of a 1:1 mixture of 1 and 2, whereas C_e was not observed in the log–log plot (Fig. 3b). This observation reflects that the length of the chain was too short for entanglement of the chain to occur within the measurement concentration range in THF. No transition in the slope of the log–log plot was displayed in the solution of a 1:1 mixture of 1 and 2 in toluene in the measurement window (Fig. 3c). However, the rolling-ball viscometer did not allow measurement of viscosity at concentrations of more than 20 mmol L^–1 due to the solubility issue¹¹⁰; thus, we measured viscosity at higher temperature as follows.

Fig. 3.

Plots of specific viscosity (η_sp) of a 1:1 mixture of 1 and 2 in a,d) CHCl₃, b,e) THF, and c,f) toluene at (blue) 25 °C and (magenta) 40 °C. C = ([1] + [2]/2). The slope values, CPC, and C_e are shown in each graph.

CPC and C_e emerged at higher concentrations in the log–log plot of η_sp measured at 40 °C compared with those measured at 25 °C (Fig. 3d,f), which reflects this well-known feature of host–guest complexations. As a general rule, host–guest complexations are inhibited by heating the solution due to the entropic penalty. Thus, heating the solutions to 40 °C suppressed the growth of the supramolecular polymer chains. This resulted in the solutions of the mixture of 1 and 2 becoming less viscous at every concentration compared with those at 25 °C, leading to the high concentration shift of CPC and C_e. The single slope value of 1.08 was seen in the log–log plot in toluene from the dilute to concentrated concentration region. This observation indicates that monomeric or short oligomeric species were dominant in the solution, resulting in no apparent polymer chain entanglement.

Atomic force microscopy (AFM) provided the morphological insight into a 1:1 mixture of 1 and 2 in the solid state, which is supportive of the solvent-dependent nature of supramolecular polymerization. Stock solutions of the 1:1 mixtures of 1 and 2 in CHCl₃, THF, and toluene were spin-coated onto freshly cleaved mica. The resulting mica substrates were dried in vacuo for 10 h and subjected to AFM measurements. Widespread sheet-like morphologies were clearly observed in the AFM image of a 1:1 mixture of 1 and 2 prepared from its CHCl₃ solution (Fig. 4a,d). The large supramolecular polymers grown in the CHCl₃ solution uniformly covered the mica surface upon evaporating the solvent, resulting in sheet-like morphologies. The height of the sheets was measured to be approximately 3 to 4 nm, which is consistent with the contour length of poly-(1•2) (Fig. 4g). The sheet-like morphologies with heights of approximately 3 to 4 nm were also observed in the AFM image of a 1:1 mixture of 1 and 2 prepared from its THF solution (Fig. 4b,e). The well-grown polymeric sheets were most likely derived from the concentration change when the specimen was prepared. Upon evaporating THF on the mica substrate, the THF solution of the 1:1 mixture of 1 and 2 was concentrated. The growth of poly-(1•2) was enhanced in the condensed solution, resulting in sheet-like morphologies consistent with the morphologies seen in the AFM image shown in Fig. 4a.¹¹¹ On the other hand, randomly aggregated particles were observed in the AFM image of a 1:1 mixture of 1 and 2 prepared from its toluene solution (Fig. 4c). Judging from the size distribution of the particles, approximately 1 to 3 molecules of 1 and 2 were aggregated on the mica (Fig. 4f). Although the binding constant between the cavitand and quinuclidinium in toluene was the highest out of the 3 solutions, the negative cooperativity prevented the growth of the polymer chains even in the condensed solution, resulting in particle-like morphologies. The results of the AFM study are in good agreement with the solution-phase analyses described above.

Fig. 4.

AFM images (3 μm × 3 μm) of cast films of a 1:1 mixture of 1 and 2 prepared from their a) CHCl₃, b) THF, and c) toluene solutions. Height profiles along the d) green and e) blue lines shown in a) and b), respectively. f) Histogram of the height of particles seen in c). The particles were randomly selected within the image shown in c), and the selected particles are indicated in Supplementary Fig. S44. g) Schematic cartoon of poly-(1•2) showing a contour length of approximately 3 nm. 1 and 2 are drawn in stick and space-filling models, respectively.

4. Conclusion

In summary, we reported the supramolecular polymerization behaviors of a mixture of biscavitand 1 and ditopic quinuclidinium guest 2. The formation of poly-(1•2) was carefully studied by a combination of DOSY, viscosity, and AFM measurements, which revealed that the assembly behavior between 1 and 2 was environmentally responsive, with the extent of the chain length of poly-(1•2) being modulated through changes in solvent. The present work underscores that the cooperativity in guest binding significantly influences the iterative guest binding; slightly positive cooperativity and slightly negative cooperativity did not prevent the growth of the supramolecular polymer chains in the condensed solutions, where the DP of poly-(1•2) was determined by the K value between the host and guest moieties. In contrast, strong negative cooperativity resulted in monomeric or short oligomeric species even though the K value between the host and guest moieties was very large. These results clearly show that the solvent properties determine the growth of the supramolecular polymer chain in solution. Although cooperative supramolecular polymerization has been extensively researched in the field of supramolecular polymers, there are limited reports that investigate host–guest association-driven supramolecular polymerization controlled by cooperativity. The present work is thus valuable for deepening the understanding of host–guest association-driven supramolecular polymerization behaviors regulated by a combination of cooperativity and binding constants in solution and is expected to set the stage for the design of supramolecular polymers that can control the chain length in response to the preparation solvent.

Acknowledgments

The authors are grateful to Ms. Tomoko Amimoto and Mr. Hitoshi Fujitaka of the Natural Science Center for Basic Research Development (N-BARD), Hiroshima University for facilitating the HRMS and DOSY measurements, respectively.

Supplementary data

Supplementary material is available at Bulletin of the Chemical Society of Japan online.

Funding

This work was supported by JSPS KAKENHI, Grants-in-Aid for Transformative Research Areas, “Condensed Conjugation” Grant Number JP21H05491 and “Materials Science of Meso-Hierarchy” Grant Number JP23H04873, Grant-in-Aid for Scientific Research (A) Grant Number JP21H04685, and Grant-in-Aid for Young Scientists Grant Number JP22K14727. We also acknowledge the support by the KEIRIN JKA, Grant Number 2023 M-419. Funding from the Toshiaki Ogasawara Memorial Foundation, the Tobe Maki Scholarship Foundation, the Urakami Scholarship Foundation, the Proterial Materials Science Foundation, ENEOS Tonengeneral Research/Development Encouragement & Scholarship Foundation, and Takahashi Industrial and Econimic Research Foundation is gratefully acknowledged.

Data availability

The data underlying this article are available in the article and in its online supplementary material.

Author notes

H. Fujimoto and T. Hirao contributed equally to this work.

Conflict of interest statement. None declared.