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. 2024 Jul 31;9(32):34719–34724. doi: 10.1021/acsomega.4c03584

Multidimensional Dynamic Control of Supramolecular Phthalocyanine Gear: A Self-Assembly System Responding to Solvent, Temperature, and Hydrostatic Pressure

Tomokazu Kinoshita †, Daisuke Sakamaki ‡,*, Gaku Fukuhara †,*
PMCID: PMC11325503  PMID: 39157123

Abstract

graphic file with name ao4c03584_0004.jpg

Smart supramolecular materials that respond toward various external stimuli hold great promise for various applications in molecular memories, logic gates, and drug delivery systems. In this study, the active control over the self-assembly of phathalocyanine gear was achieved by combining temperature and hydrostatic pressure stimuli with a dynamic solvent. Eventually, we found that the supramolecular gear can behave as a logic gate; “engaged” (+1) or “not” (0) state is switchable by solvent, temperature, and hydrostatic pressure. This paper describes not only new aspects for the rational design of smart stimuli-responsive supramolecular materials but also the significance of multidimensional dynamic control.

1. Introduction

Supramolecular materials that respond to a large variety of external stimuli such as solvent, temperature, photons, pH, and mechanical forces (including pressure, stress, strain, and tension) have attracted considerable attention in the field of multidisciplinary chemistry.1−8 Such smart materials have potential applications in molecular memories, logic gates, and drug delivery systems, which require stimuli-responsive structural/optical/functional changes.9−12 The current mainstream for creating supramolecular materials undoubtedly represents a “bottom-up” approach rather than the “top-down” one with the growing field of supramolecular and supramolecular polymer chemistry.13−17 In general, “nature” assembles a functional monomer in a smart manner to produce complex but highly ordered structures, such as proteins and enzymes.18,19 Namely, it provides a valuable hint that self-assembly (particularly self-dimerization) is the most effective and fundamental bottom-up approach. Hence, incorporating stimuli-responsiveness into the self-dimerization process can lead to the creation of smart supramolecular materials with good responsiveness. This stimulates the exploration of a novel approach toward the discovery of stimuli-responsive supramolecular materials.

In recent years, hydrostatic pressure or solution-state isotropic pressure used as an external stimulus, has become the focus of many researchers20 because its effect on mechanochemical materials21 and mechanobiological living systems22 have not been fully explored yet; hence, we excluded the high-pressure solid chemistry studies conducted using a diamond anvil cell (∼GPa),23 which is beyond the targeted pressure range during hydrostatic pressurization (∼MPa). Historically, hydrostatic pressure effects in solutions have been investigated since the 1960s.24−33 Nevertheless, few studies on the influence of hydrostatic pressure on the self-assembly process have been conducted up to date. Recently, we have reported the solution-state supramolecular polymerization of curved-π sumanene buckybowls and attempted to implement the hydrostatic pressure control of supramolecular polymerization.34 Unfortunately, the effect of hydrostatic pressurization was negligible, as indicated by the small ΔV° value of 2.7 cm3 mol–1 due to dense stacking. Thus, a self-assembly system stimulated by hydrostatic pressure should be developed.

In this study, we focused on the supramolecular self-dimerization of Zn-coordinated phthalocyanine (Pc) containing four pairs of 6,13-dihydro-6,13-diazapentacene (DHDAP) pillars with n-hexyl side chains on the periphery (1Zn),35 as shown in Figure 1. 1Zn forms an H-type dimer 1Zn2 via π–π interactions between two cofacial Pc rings and interdigitated DHDAP pillars in specific solvents at ambient pressure (0.1 MPa), as if a “gear” is perfectly engaged. Figure 1 shows the optimized structures of the model compounds of 1Zn and 1Zn2 without n-hexyl side chains (1Zn’ and 1Zn’2) at the M06-2X/6-31G(d) (for H, C, N) and LANL2DZ (for Zn) level of theory (Cartesian geometries are shown in SI). The stabilization energy upon the dimerization was calculated to be approximately 400 kJ mol–1, which is comparable to that of the covalent bond formation. Two 1Zn’ molecules are closely engaged by filling each other’s voids with DHDAP pillars, and nearly three of the five six-membered rings in each DHDAP unit are inserted into the voids of the counterpart monomer. The close contact of the two Zn atoms (3.315 Å) suggests the existence of a strong attractive interaction between the two 1Zn’ molecules. In addition to the π–π stacking between two Pc planes, some intermonomer short contacts (<3.4 Å) between the DHDAP units were found, indicating the existence of intermonomer π–π interactions among the pillars. In our previous study, we successfully isolated 1Zn and 1Zn2, as well as their Cu-analogues 1Cu and 1Cu2, and thoroughly investigated their structures.35 By using mass spectrometry and spectroscopic measurements, including various 1D and 2D NMR techniques (1H–1H COSY, 13C DEPT, 13C/1H HMBC, 13C/1H HMQC, and 1D DPFGSE-NOE) for 1Zn and 1Zn2 and ESR measurements for 1Cu and 1Cu2, we confirmed that the dimers possess D4 symmetric structures where the DHDAP pillars are interdigitated to the voids of the counterpart monomer. Interestingly, 1Zn underwent only dimerization and did not form higher oligomers.35 The absence of trimers and higher oligomers was explained by the optimized structure of the dimer, in which no sufficient space was left for the interdigitation of the pillars of the third monomer (Figure S16).

Figure 1.

Figure 1

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Structures of the phthalocyanine derivative: (a) monomer (1Zn) and (b) dimer (1Zn2). n-Hexyl groups were omitted for clarity. 3D models were obtained by the DFT-optimization for the model compounds without n-hexyl groups.

The most important characteristics of 1Zn after dimerization is a strong solvent dependency.35 In tetrahydrofuran (THF), both 1Zn and 1Zn2 existed as stable species that were separated from each other, and no interconversion was observed even at 333 K. The results indicated that not only the dissociation of 1Zn2 but also the dimerization of 1Zn requires significant activation energies owing to the steric demand upon the interdigitation of the pillars. 1Zn exhibited an intense absorption band at 675 nm corresponding to the Q-band of Pc in THF, which hypsochromically shifted to 652 nm upon the dimerization to 1Zn2 owing to the H-type configuration of the two Pc units. In dichloromethane, toluene, and o-dichlorobenzene, 1Zn2 was in a metastable state that gradually dissociated to produce monomer 1Zn. In contrast, in ethyl acetate (EA), monomer 1Zn transformed to a metastable state and then dynamically dimerized (1Zn2). These results indicate that the solvation core in 1Zn plays a critical role in the dimerization process, revealing a highly dynamic nature during self-assembly. Such a dynamic dimerization system of 1Zn becomes the most viable candidate for the purpose of the present work. Herein, we report a novel stimuli-responsive supramolecular “gear”, 1Zn, that can be dynamically controlled by varying both the hydrostatic pressure and temperature in the dynamic EA solution. It was found that all cooperative factors, including the multidimensional dynamic control by the solvent, temperature, and hydrostatic pressure, are essential for the self-assembly process. For example, among the representative self-dimerization systems about the oligomeric strands,36 expanded helicenes,37 and merocyanine dyes,38 temperature and/or solvent appear to be effective control factors.

2. Experimental Section

Instruments

Ultraviolet/visible/near-infrared (UV/vis/NIR) absorption spectra were recorded using a JASCO V-770 spectrometer.

Materials

Spectrophotometric grade THF, toluene, dichloromethane, and ethyl acetate were used as received without further purification. 1Zn and 1Zn2 were synthesized according to the literature.35

Hydrostatic Pressure Spectroscopy

UV/vis/NIR absorption spectra were recorded under the hydrostatic pressure using a custom-built high-pressure apparatus.20 Because the utilized method was previously described in detail, we outline it only briefly. A quartz inner cell was filled with a sample solution and then placed into an outer cell with fitted sapphire windows. A tightly closed outer cell was hydrostatically pressurized using water and then placed inside the spectrometer to obtain the hydrostatic pressure spectra presented in this paper.

3. Results and Discussion

Dynamic Self-Assembling of the Gear Induced by Temperature Stimulus

First, before studying the effect of hydrostatic pressurization, we investigated the influence of the temperature stimulus on the dimerization process (particularly in EA) through a metastable state. The temperature-dependent UV/vis/NIR spectra of 1Zn recorded at 313–343 K exhibit distinct shifts to 1Zn2, for which no changes were observed under the same conditions. This indicates the irreversibility of the conversion from 1Zn to 1Zn2 with a significant role of temperature. Thus, the dimerization rate constant kdim can be expressed as follows:

graphic file with name ao4c03584_m001.jpg 1
graphic file with name ao4c03584_m002.jpg 2

where [M]0 and [M] represent the monomer concentrations at the initial time and arbitrary time (t), respectively. Upon applying temperature in the range of 313–343 K to an EA solution of 1Zn (29 μM), the time-course absorbance monitored at the Q-band (675 nm) decreased (Figure S1 in SI). Combining Figure S1 and eq 2 enabled fitting the data presented in Figure S3, which resulted in straight lines (correlation coefficients: r = 0.999). The slopes in Figure S3 represent the kdim values obtained at four temperatures (Table S1). Moreover, a similar temperature dependence was observed for the time-course 1Zn concentration changes (Figure 2a). To quantitatively evaluate the enthalpic and entropic contributions to the self-assembly process, the kdim data obtained at the four temperatures were subjected to the Eyring analysis procedure (Figure 2b). For this purpose, the natural logarithm of kdim was plotted as a function of the reciprocal temperature. According to Figure 2b, each set of data points falls on a single straight line (r = 0.999), indicating that the self-dimerization mechanism did not change in the tested temperature range. The activation enthalpy ΔH‡ of 91.5 kJ mol–1 and entropy ΔS‡ of 47.9 J K–1 mol–1 were estimated from the slope and intercept of the straight line, and the activation Gibbs free energy ΔG‡ of 77.2 kJ mol–1 (298 K) was calculated using the two kinetic parameters. The positive ΔG‡ value indicates that dimerization does not proceed spontaneously at room temperature; however, elevating the temperature promotes the dimerization process by exceeding the activation energy in the transition state (TS), which is a critical factor for engaging each “gear” (Figure 2c). In fact, the ΔG‡ value observed here necessitates a temperature condition at least over 313 K for the spontaneous dimerization, as shown in Figure 2a. Among the main ΔG‡ components, the positive ΔS‡ value drives the dimerization process. This phenomenon can be reasonably explained considering that the broad-range solvent EA core around monomer 1Zn is highly likely to be blown off along with the gear formation in the TS (desolvation). The desolvation process leads to the endothermic reaction (positive ΔH‡) in the TS although various gear pieces fully engage with each other. This result indicates that solvation (for 1Zn) or desolvation (for 1Zn2) plays a decisive role in the self-assembly process, confirming its dynamic nature.

Figure 2.

Figure 2

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(a) Time-dependent concentration changes of 1Zn in EA observed at 0.1 MPa. Temperature applied: 313, 323, 333, and 343 K (from blue to red). (b) Eyring plot of the dimerization process of 1Zn (29 μM) in EA conducted at 0.1 MPa (r = 0.999). (c) Energy diagram of the dimerization in EA conducted at 0.1 MPa.

Hydrostatic Pressure Stimulus Controls the Gear Formation

Next, we investigated the gear responses to the hydrostatic pressure stimulus. In this experiment, UV/vis/NIR absorption spectra were recorded at high pressures up to 320 MPa and room temperature (298 K) using a previously reported optical system (see Experimental Section). Measurements were performed from the NIR (1300 nm) to UV (300 nm) region in THF, toluene, dichloromethane, and EA. Gradual pressure-induced bathochromic shifts and hyperchromic effects were observed for both 1Zn and 1Zn2 (Figures S4 and S7 in SI, respectively). The former behavior originates from the pressure-induced solvent polarizability (density) changes that cause the stabilization of the π* orbital,24,25 and the latter phenomenon occurs because of the increasing effective concentration during solution compression. Considering that the hydrostatic pressure effect on the bathochromic shift is generally observed at approximately 1 cm–1 MPa–1 for common π-conjugated organic molecules (such as anthracene, pyrene, and perylene),20,39−41 the resulting shifts of 0 ∼ −0.39 cm–1 MPa–1 for 1Zn and −0.11 ∼ −0.25 cm–1 MPa–1 for 1Zn2 were relatively small (Figures S6 and S9 in SI, respectively). This remarkable pressure effect was likely responsible for the characteristic Q-band obtained for the Pc chromophore, which revealed that the hydrostatic pressure responses in the Q-band were considerably different from those obtained for the regular π–π* transition. In addition, the normalized UV/vis/NIR spectra of 1Zn2 and 1Zn in all solutions obtained at the hydrostatic pressure are almost superimposable (Figures S5 and S8 in SI, respectively), indicating that hydrostatic pressurization did not cause dissociation/association, particularly in specific solvation EA. These hydrostatic pressurization experiments conducted at 298 K ultimately enable dynamic control by combining the temperature and hydrostatic pressure stimuli (multidimensional control).

Thus, we investigated the hydrostatic pressure effects on the dimerization process in the dynamic EA solvent at an appropriate temperature. According to Figure 3a, spectral measurements were conducted every 10 min at 333 K under 0.1 MPa. As a result, the absorbance in the monomeric band (675 nm) continuously decreased, and a dimeric band (649 nm) appeared; the dimerization process spontaneously occurred under these conditions (vide supra). Meanwhile, when a similar experiment was performed at a hydrostatic pressure of 100 MPa, the monomer band at 0 min remained nearly intact during pressurization (Figure 3b, colored lines). This means that the pressurized monomer does not dimerize. After applying a pressure of 100 MPa for 60 min, the pressurized system was depressurized to the original atmospheric pressure, and the spectrum obtained in 20 min contained the dimer band (Figure 3b, black dotted line). This indicates that the supramolecular gear is either “engaged” (+1 state) or “not engaged” (0 state) depending on the hydrostatic pressure (Figure 3c).

Figure 3.

Figure 3

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Time-dependent UV/vis/NIR absorption spectra of 1Zn (32 μM) obtained in EA at a temperature of 333 K and pressures of (a) 0.1 MPa (in a 2 mm cell) and (b) 100 MPa (in a high-pressure cell). The colored lines represent the spectrum after standing at 0, 10, 20, 30, 40, 50, and 60 min (from black to light blue). The black dotted line in (b) represents the depressurized spectrum recorded after applying a pressure of 100 MPa for 60 min and then 0.1 MPa for 20 min. (c) Dimerization mechanism of 1Zn. (d) Pressure-dependent concentration changes of 1Zn observed in EA at a temperature of 333 K and various pressures of 20, 40, 60, 80, and 100 MPa (from black to purple). (e) Pressure dependence of kdim obtained for EA at 333 K (r = 0.972).

To finally elucidate the governing mechanism for the stimuli-responsive supramolecular gear, we investigated the high-temperature dimerization kinetics (kdim) under the hydrostatic pressure in the dynamically solvated EA. The time-course absorbance curves recorded in 20 MPa increments from 20 to 100 MPa (Figure S10) produced the pressure-dependent kdim constant (the fitting results are presented in Figure S15, and the obtained data are summarized in Table S2). In addition, the time-course 1Zn concentration changes (Figure 3d) clearly depend on the applied pressure.

The correlation between the activation volume (ΔV‡) and kinetic constant can be expressed as follows:

graphic file with name ao4c03584_m003.jpg 3

According to eq 3, the natural logarithm of kdim plotted against P (Figure 3e) results in a straight line (r = 0.972), indicating that in this pressure range, the hydrostatic pressure effect does not change the dimerization TS. The ΔV‡ value estimated from the slope is equal to +62.0 cm3 mol–1, which is larger than those reported previously for pressure-responsive chemosensor systems (<10 cm3 mol–1) owing to the desolvation/solvation processes ΔV20,42 and above-mentioned sumanene-based supramolecular polymerization. Hence, such a large change in ΔV‡ can be easily explained by not the regular solvation process but the highly dense or “expanded” (positive ΔV) gear engaging during dimerization (Figure 3c). This gear expansion scenario in the TS was further supported by the above-mentioned positive ΔS‡ value; the dynamic solvation core around the monomer is excluded (positive ΔS‡) to open up an effective space that enables the perfect engagement of each gear (positive ΔV‡). Incidentally, the limitation of this multidimensional dynamic control was based on the boiling point of EA (77 °C) and the upper limitation of our high-pressure apparatus (∼400 MPa).

4. Conclusions

In conclusion, a smart dynamic stimuli-responsive supramolecular gear has been discovered for the first time. The gear worked the temperature- and volume-correlated ΔG‡ as 77.2 kJ mol–1 and ΔV‡ as +62.0 cm3 mol–1, both of which were mutually compensated in dynamic EA solvent. Such dynamic responses were achieved by combining the temperature and hydrostatic pressure stimuli in the dynamic solvation shell, which can be considered multidimensional dynamic control. Our findings not only provide new guidelines for exploring smart materials that respond to various stimuli but also proposes a dynamic control concept using applicable external stimuli. For further expanding the multidimensional dynamic control concept, systems that are controlled by temperature, solvent, pressure, and other stimuli are currently in progress.

Acknowledgments

This work was supported by Grant-in-Aid (No. 20H02726 and 20H05866 to D.S. and No. 23H04020 to G.F.) from the Japan Society for the Promotion of Science (JSPS), the Asahi Glass Foundation (to G.F.), Ajinomoto Co., Inc. (to G.F.), and Ogasawara Toshiaki Memorial Foundation (to D.S). T.K. acknowledges JSPS Fellowships for Young Scientists (No. 23KJ0906).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.4c03584.

  • Calculation method, temperature-dependent kinetics, hydrostatic pressure spectroscopy, pressure-dependent kinetics, and computational simulations (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao4c03584_si_001.pdf (5.7MB, pdf)

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Supplementary Materials

ao4c03584_si_001.pdf (5.7MB, pdf)

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