Modulating the luminescence properties of coronene molecules via inclusion complex formation with helical syndiotactic poly(methyl methacrylate)

Abstract

The formation of nanocavities that can confine molecules in limited spaces is crucial for controlling the arrangement, aggregation, reactivity, and function of molecules. It is also one of the foundational elements of molecular systems in life. Herein, we demonstrate that the helical structure of syndiotactic poly(methyl methacrylate) (st-PMMA), a synthetic polymer, functions as a nanocavity that confines coronene, as a guest molecule, in the nanospace, thereby controlling the luminescence properties of coronene. Inclusion complexes are formed between st-PMMA helices and coronene molecules in toluene, as confirmed by UV–vis, DSC, and XRD. When encapsulated within the st-PMMA helical cavity, coronene molecules in a low-dimensional arrangement exhibit aggregation-induced emission and the observed fluorescence is red-shifted. Additionally, when a quencher, C₇₀, is introduced as a second guest molecule into the st-PMMA/coronene inclusion complex gel, C₇₀ is encapsulated within the st-PMMA helical cavity, resulting in the selective quenching of only the aggregation-induced emission of the encapsulated coronene.

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Introduction

The formation of nanocavities in cells and protein assemblies is frequently observed in nature.¹ By confining molecules in the limited spaces of nanocavities, essential life functions, such as the formation and stabilization of protein folding structures by chaperones and DNA replication by polymerases, are achieved. Recently, nanocavities with spatially specific functions have also attracted attention in materials science.^2–5 To date, various host materials with such molecular-scale nanocavities, including carbon nanotubes,² self-assembled organic nanotubes,³ metal–organic frameworks (MOFs),⁴ and many precisely designed supramolecular capsules,⁵ have been reported. These hosts maintain their strong cavities by linking structural units with strong covalent, ionic, or coordination bonds in condensed phases to maximize intermolecular interactions via high-density packing. Nanospace-containing materials that can confine target molecules in limited nanocavities exhibit functions different from simple mixtures, such as molecular arrangement, catalysis, templating, and substance transport (e.g., drug delivery).^2–6 Among these, materials that arrange target molecules in low-dimensional arrays within molecular-scale nanocavities are particularly interesting because of their unique optical properties, electrical properties, and reactivity based on the molecular arrangement.

As seen in the α-helix of proteins, helical structures are the fundamental secondary structures formed by linear polymer chains. The inner cavities of such polymer helices can also be considered as nanocavities with unique functions. For example, amylose, a polysaccharide, forms a 6/1 helix in an aqueous solution, with six glucose monomer units per turn, and encapsulates iodine within its inner cavity.⁷ This is known as the “starch–iodine test,” which is utilized for the simple detection of the amylose helical structure or triiodide using its color change. Furthermore, amylose can form inclusion complexes with low-molecular-weight organic compounds⁸ and synthetic polymers.⁹ Compared to cyclic low-molecular-weight host cavities such as cyclodextrin and calixarene, the notable advantage of helical polymer host cavities is that they exhibit effective inclusion abilities for various guest molecules.

In our previous studies, we reported that syndiotactic poly(methyl methacrylate) (st-PMMA), a stereoregular commodity plastic, can also serve as a helical polymer host cavity.^10,11 st-PMMA forms a thermoreversible physical gel in aromatic solvents such as toluene, in which the st-PMMA chains adopt the shape of a 74/4 helix (i.e., 74 units per 4 turns) with a large inner cavity. The st-PMMA helical cavity size-selectively encapsulates fullerenes, such as C₆₀, C₇₀, C₇₆, C₈₄, C₉₀, and C₉₆, to form supramolecular peapod-like inclusion complexes.¹² Inclusion complexes composed of st-PMMA and fullerenes have been investigated for use as functional materials,¹³ such as polymer alloys,¹⁴ polymer brushes,¹⁵ nonvolatile memory devices,¹⁶ and electrode interlayers in organic solar cells.¹⁷ In addition to fullerenes, polycyclic aromatic hydrocarbons, such as pyrene and phenanthrene, can also be encapsulated in the st-PMMA helical cavity.¹⁸

Herein, we report that coronene, which exhibits fluorescence properties, can also be encapsulated as a novel guest molecule within the st-PMMA helical cavity in toluene. The encapsulation of coronene within the confined nanospace of the inner cavity of the st-PMMA helix significantly alters its absorption and emission properties. Furthermore, by encapsulating C₇₀ as the second guest molecule, which functions as a quencher, the emission properties of coronene can be successfully controlled by forming an inclusion complex.

Results and discussion

Inclusion complex formation between st-PMMA and coronene

The st-PMMA/coronene inclusion complex gel was obtained by adding st-PMMA (M_n = 580 000, rr = 95.2%, 20.0 mg)¹⁹ to a pale-yellow transparent solution of coronene in toluene (1.0 mg mL⁻¹, 2 mL; Fig. 1a(i)), followed by heating to 110 °C to form a homogeneous solution and cooling to 25 °C. Fig. 1a(ii) shows a photograph of the gel after centrifugation, in which the gel has settled. Gelation is driven by the formation of microcrystalline domains of the helical st-PMMA structure, which serve as crosslinking points. Compared to the parent solution, the supernatant had a lighter color, while the gel had a darker color. This implies that similar to the formation of the st-PMMA/C₆₀ inclusion complex,^10–12 coronene molecules are preferentially encapsulated within the st-PMMA helix over toluene molecules, forming the st-PMMA/coronene inclusion complex. The amount of encapsulated coronene in the st-PMMA/coronene inclusion complex, determined via UV–vis analysis of the supernatant, was 1.8 mg, and the encapsulation ratio (amount of encapsulated coronene ÷ the amount of added coronene) of the st-PMMA/coronene inclusion complex gel was 89%. The formation of a crystalline inclusion complex was confirmed by DSC and XRD analyses of the inclusion complex film obtained after solvent removal. For the inclusion complex gel prepared using 1.0 mL of toluene, 10 mg of st-PMMA, and 1.0 mg of coronene, the st-PMMA/coronene inclusion complex film was obtained by first removing the supernatant and then vacuum-drying with stepwise heating up to 160 °C. In the DSC thermogram of the inclusion complex film (Fig. 1b), an endothermic peak was observed at 290.0 °C with a ΔH of 11.8 J g⁻¹ (iv), which was attributed to the melting of the inclusion complex crystals. This melting point (290.0 °C) is higher than that of the st-PMMA/C₆₀ inclusion complex (T_m = 224.9 °C).¹⁰ Additionally, during the second heating of the inclusion complex film, only the glass transition temperature (T_g) of st-PMMA was observed (v), similar to the results of the first heating of the st-PMMA film (iii). The inclusion complex film exhibited an XRD pattern characteristic of the st-PMMA inclusion complex¹⁰ and significantly different from those of the st-PMMA film and coronene (Fig. 1c). These results prove the formation of a crystalline st-PMMA/coronene inclusion complex.

Fig. 1 (a) Photographs of the toluene solution of coronene (1.0 mg mL⁻¹, 2.0 mL) (i) and the st-PMMA/coronene complex gel after the addition of st-PMMA (20.0 mg) to the solution, followed by heating to 110 °C, cooling to 25 °C, and centrifugation (ii). (b) DSC thermograms of the st-PMMA film (iii) and the st-PMMA/coronene inclusion complex film (iv). These films were prepared by evaporating the solvents from the st-PMMA and st-PMMA/coronene gels in toluene, respectively. The measurements were performed from 30 °C to 300 °C. Then, the sample (iv) was cooled to 30 °C and then heated again (v). The heating or cooling rate was 10 °C min⁻¹. (c) XRD patterns of the st-PMMA film (vi), the st-PMMA/coronene complex film (vii), and bulk coronene (viii).

One of the important characteristics of helical polymers is their chirality that arises from controlling the helical sense (right-handed helix (P helix) and left-handed helix (M helix)). Using chiral compounds such as 1-phenylethanol (1) or 1-phenylethylamine either instead of or along with toluene, an optically active inclusion complex gel with a controlled helical sense of st-PMMA helices was obtained.^10,20 In the case of the optically active st-PMMA/C₆₀ inclusion complex gel with C₆₀ as the guest molecule, repeated extraction operations with toluene completely removed the added chiral compounds. Even after removing these chiral compounds, the helical sense of st-PMMA remained biased in one direction. Consequently, CD was observed in the vibrational region in the CD (VCD) spectrum for PMMA absorption and in the electronic transition region in the CD (ECD) spectrum for C₆₀ absorption. Moreover, an optically active st-PMMA/C₆₀ inclusion complex gel can be obtained even when the st-PMMA gel is initially prepared in a mixture of chiral compounds and toluene, followed by the removal of the chiral compounds through toluene washing and subsequent addition of C₆₀. In this work, an optically active st-PMMA/coronene inclusion complex gel was fabricated using the latter method, in which an optically active st-PMMA gel was prepared in advance (Fig. 2a). st-PMMA was gelled in a mixed solvent of toluene and 1 (9/1, v/v), and the resulting gel was thoroughly washed with toluene to completely remove the chiral compound 1. Subsequently, a coronene solution was added to the st-PMMA gel. Fig. 2b shows CD spectra of the st-PMMA/coronene inclusion complex gels (encapsulation ratio = 77–78%). When (R)-1 and (S)-1 were used to induce helicity in the st-PMMA gel, induced CD (mirror images of each other) was observed in the absorption region of coronene. In contrast, no CD was observed when the racemic mixture, (RS)-1, was used. The formation of a preferred-handed helical structure of st-PMMA in the presence of 1 confirmed that the helical structure functions as a chiral nanocavity, allowing coronene molecules to be placed in a chiral environment. The observed induced CD of coronene is presumed to originate from the stack of coronene molecules slightly shifting in either the clockwise or counterclockwise direction.²¹ The crystalline nature of the inclusion complex, confirmed by DSC and XRD, and the experimentally observed CD imply that the encapsulated coronene is arranged in a low-dimensional array within the st-PMMA helix.

Fig. 2 (a) Schematic illustration of the induction of preferred-handed helicity in st-PMMA in the presence of chiral compound 1. The induced helicity was retained after complete removal of (R)-1 and subsequent encapsulation of coronene molecules. (b) CD (top) and absorption (bottom) spectra of the st-PMMA/coronene inclusion complex gels in toluene obtained after the addition of coronene to the isolated st-PMMA gels prepared using racemic 1 (black lines), (R)-1 (red lines) and (S)-1 (blue lines).

The results mentioned above confirm that in toluene, coronene molecules are preferentially encapsulated within the st-PMMA helix over toluene molecules and the formation of the st-PMMA/coronene inclusion complex occurs spontaneously. Next, the effects of the preparation conditions, such as the concentrations of the guest (coronene) and host (st-PMMA) molecules, on the amount of encapsulated coronene were investigated. The change in the amount of encapsulated coronene was investigated using 2.0 mL of toluene and 1.0 mg of the polymer (polymer concentration = 0.50 mg mL⁻¹) at different coronene concentrations. Fig. 3a shows the plots of the encapsulated coronene amount and the encapsulation ratio as a function of the coronene concentration. At low coronene concentrations (<0.025 mg mL⁻¹), the UV–vis spectra of the parent solution and supernatant after gel formation were the same, indicating that no preferential encapsulation of coronene was observed (indicated by the arrow in Fig. 3a). The amount of encapsulated coronene increased almost linearly with the coronene concentration in the range of 0.10–0.30 mg mL⁻¹, and it saturated at ∼0.28 mg at a coronene concentration of ≤0.40 mg mL⁻¹. After reaching this saturation amount, the encapsulation ratio also decreased. At the saturation amount of encapsulated coronene, the coronene content in the inclusion complex was 22 wt% (0.28 mg of coronene encapsulated in 1.0 mg of st-PMMA). If coronene was inserted into the 18.5/1 helix of st-PMMA perpendicular to the helical axis at a distance equivalent to the intermolecular distance in a single crystal (0.34 nm),²² the maximum coronene content would be 29.8 wt% (Fig. S1). Therefore, 29.8 wt% was considered as a 100% filling ratio, and the filling ratio of the inclusion complex obtained at a coronene concentration of 0.20 mg mL⁻¹ (addition amount = 0.40 mg) was 74%.

Fig. 3 (a) Dependence of the encapsulated coronene content (top) and encapsulation ratio (bottom) of the inclusion complex of st-PMMA (1.0 mg) in toluene (2.0 mL) on the coronene concentration. (b) Changes in the encapsulation ratio of the inclusion complex as a function of the st-PMMA concentration at fixed coronene concentrations of 0.50 (○), 0.20 (◆), and 0.025 mg mL⁻¹ (■). (c) Changes in the encapsulation ratio of the inclusion complex with time as the coronene concentration decreases from 0.20 (◊) to 0.025 mg mL⁻¹ by adding toluene (21 mL) to the inclusion complex (st-PMMA = 3.0 mg, coronene = 0.60 mg, and toluene = 3.0 mL).

Next, the effect of the st-PMMA (host) concentration on the amount of encapsulated coronene was examined (Fig. 3b). At a coronene concentration of 0.025 mg mL⁻¹, as previously mentioned, no encapsulation was observed at a polymer concentration of 0.025 mg mL⁻¹. Even when the polymer concentration was increased 20-fold to 5.0 mg mL⁻¹ at a coronene concentration of 0.025 mg mL⁻¹, no coronene encapsulation was observed (plot indicated by ■, Fig. 3b). When the coronene concentration was increased to 0.20 mg mL⁻¹, the encapsulation ratio increased as the polymer concentration increased, and the amount of encapsulated coronene saturated at ∼0.11 mg when the polymer concentration was ≤2.0 mg mL⁻¹. The encapsulation ratio of coronene at saturation was ∼55%, indicating that ∼45% of the added coronene remained unencapsulated in toluene (plot indicated by ◆, Fig. 3b). When the coronene concentration was further increased to 0.50 mg mL⁻¹, the amount of encapsulated coronene saturated at a polymer concentration of ≤2.0 mg mL⁻¹, the encapsulation ratio increased to ∼80%, and ∼20% of the added coronene remained unencapsulated (plot indicated by ○, Fig. 3b). These results clearly demonstrate that the amount of encapsulated coronene reached saturation with increasing polymer concentration and the coronene concentration significantly affected the encapsulation amount at saturation.

At low coronene concentrations (≤0.025 mg mL⁻¹), the spontaneous encapsulation of coronene was not observed even at high polymer concentrations. Therefore, an experiment was performed to release encapsulated coronene by changing the coronene concentration. An inclusion complex gel with a coronene encapsulation ratio of 52% was prepared at a coronene concentration of 0.20 mg mL⁻¹. After removing the supernatant, toluene was added to reduce the coronene concentration to 0.025 mg mL⁻¹. Fig. 3c shows the time-dependent change in the encapsulation ratio of the inclusion complex, demonstrating that the coronene encapsulated in the polymer chains was completely released into the toluene within ∼5 h. This result indicates that the polymer gel, which is a soft material, is an open system and hence facilitates the transport of substances to and from the helical inner cavity, which functions as the host site.

Photoluminescence properties of the st-PMMA/coronene inclusion complex gel

Because coronene is a fluorescent molecule, the absorption and emission properties of coronene encapsulated in the nanocavity of the st-PMMA helix are of interest. Therefore, the absorption and fluorescence spectra of the st-PMMA/coronene inclusion complex gel were measured. Fig. 4a shows the UV–vis spectra of the toluene solution of coronene with a concentration of 0.50 mg mL⁻¹ (i) and the inclusion complex gel (encapsulation ratio = 83%) (ii) obtained by adding PMMA to the solution. To obtain the UV–vis spectrum of the inclusion complex gel, it was vigorously stirred and measured as a dispersion. Because st-PMMA does not have absorption bands at wavelengths above 300 nm, the absorption observed in the UV–vis spectrum of the inclusion complex gel (ii) originated from coronene molecules encapsulated in the st-PMMA helix (83%) and coronene molecules in the toluene solution (17%). The UV–vis spectrum shows that the formation of the inclusion complex reduced the characteristic absorption of coronene in the low-wavelength region (e.g., at 326 nm and 341 nm) while increasing the absorption in the long-wavelength region (e.g., at ∼357 nm).

Fig. 4 (a) Absorption and PL (λ_ex = 340 nm) spectra and (b) photographs under 365 nm UV light of the toluene solution of coronene (i, iii, and v) and the st-PMMA/coronene complex gel (encapsulation ratio = 83%) (ii, iv, and vi) obtained at PMMA and coronene concentrations of 5.0 and 0.50 mg mL⁻¹, respectively.

Fig. 4a(iii) and 4a(iv) show the fluorescence spectra of the toluene solution of coronene and the st-PMMA/coronene complex gel measured at an excitation wavelength of 340 nm. In these spectra, changes due to the formation of the inclusion complex were observed, with a decrease in the emission intensity at shorter wavelengths (433.5 nm and 450.5 nm) and an increase in the emission intensity at longer wavelengths (478 nm and 495 nm) in the spectrum of the inclusion complex gel, exhibiting a broad excimer-like emission.²³ When both samples were visually inspected under 365 nm light irradiation, the toluene solution of coronene appeared cobalt blue and the st-PMMA/coronene inclusion complex gel (encapsulation ratio = 83%) appeared sky blue (Fig. 4b). The excitation spectrum of the inclusion complex gel (encapsulation ratio = 83%) was measured by monitoring the emission at 433.5 nm (Fig. S2b), which is the emission maximum of coronene without st-PMMA. The spectrum closely matched the absorption spectrum of the toluene solution of coronene, implying that the fluorescence at 433.5 nm could be attributed to unencapsulated coronene molecules. However, the excitation spectrum observed at an emission wavelength of 495 nm did not match the absorption spectrum of the toluene solution (Fig. S2b). According to these experimental results, the spectral changes observed for the st-PMMA/coronene inclusion complex gel may be due to aggregation caused by the arrangement of coronene molecules within the inner cavity of the st-PMMA helix.

The coronene inclusion complex exhibits a larger Stokes shift compared with coronene in toluene; the Stokes shift increases from 6240 cm⁻¹ to 7090 cm⁻¹.²⁴ This observation is consistent with a previous study by Seko et al., which suggested that a microcrystalline arrangement may facilitate excimer formation.²⁵ Such excimer formation can explain both the red-shifted emission and the enlarged Stokes shift. The enlarged Stokes shift in the inclusion complex is attributed to stabilization of the excited state by excimers formed within the microcrystalline arrangement of coronene in the helical nanocavity of st-PMMA.

Notably, compared to the toluene solution, the absorbance at 340 nm in the UV–vis spectrum of the inclusion complex gel decreased to ∼28%, while the emission intensity in the fluorescence spectrum of the inclusion complex at an excitation wavelength of 340 nm increased, implying an improvement in the quantum yield. Using an absolute PL quantum yield spectrometer, the quantum yield at an excitation wavelength of 330 nm was measured. The quantum yield of the toluene solution of coronene was 1.8% whereas that of the inclusion complex gel (encapsulation ratio = 83%) with the same coronene concentration was 12.6%, an increase of approximately sevenfold. Additionally, in the fluorescence spectra at an excitation wavelength of 360 nm, where the absorption intensity was weak for the toluene solution of coronene, a more significant difference was observed. Almost no emission was detected in the spectrum of the coronene toluene solution, whereas fluorescence with peaks at 478 and 495 nm was observed in the spectrum of the inclusion complex gel, with the intensity ratio reaching ∼20-fold (Fig. S3). These results confirm that spectral changes require the encapsulation of coronene molecules in the st-PMMA helix, supporting the occurrence of aggregation-induced emission caused by the arrangement of coronene molecules in the helical inner cavity.

Next, the dependence of spectral changes on the encapsulation ratio was investigated. By fixing the coronene concentration at 0.50 mg mL⁻¹ and varying the polymer concentration, continuous changes in the absorption and emission spectra of the st-PMMA/coronene inclusion complex gels were observed (Fig. S4 and S5). In the fluorescence spectra (excitation wavelength = 340 nm), with an increase in the encapsulation ratio, the fluorescence intensity in the low-wavelength region decreased because of unencapsulated coronene whereas the fluorescence intensity in the long-wavelength region increased (Fig. S5a). The gels prepared with 5.0 mg mL⁻¹ st-PMMA and 0.025 mg mL⁻¹ coronene exhibited absorption and fluorescence emission spectra similar to those of the toluene solution of coronene without PMMA (Fig. S6).

In the fluorescence spectra excited at 360 nm, the PL was primarily observed from the inclusion complex, while emission from unencapsulated coronene was weak (Fig. S5b). Therefore, the fluorescence intensities at 502.5 nm in these spectra were normalized by the amount of encapsulated coronene and plotted against the filling ratio (normalized I_{PL at 502.5 nm}) (Fig. S5c). As the polymer concentration increased from 0.50 to 5.0 mg mL⁻¹, the encapsulation ratio of coronene increased from 27% to 83%, whereas the filling ratio decreased from 71% to 26%, indicating a reduction in the proportion of st-PMMA helices involved in encapsulation (Table S1). Interestingly, despite the filling ratio decreasing from 71% to 26%, the normalized I_{PL at 502.5 nm} values remained nearly constant (Fig. S5c). This result suggests that even at low filling ratios, the encapsulated coronene molecules are not sparsely distributed within the st-PMMA helix, but rather are positioned in close proximity to one another.

We also measured the absorption and fluorescence emission spectra of gels with coronene concentrations ranging from 0.025 to 0.50 mg mL⁻¹ at a fixed PMMA concentration of 5.0 mg mL⁻¹ (Fig. S7). As the coronene concentration increased, the absorption at longer wavelengths, i.e., ∼360 nm, increased and fluorescence emission at longer wavelengths increased due to aggregate formation. These results confirm that, as noted earlier, at low coronene concentrations, coronene is not encapsulated in the st-PMMA helix cavities and behaves like a toluene solution. In contrast, at higher concentrations, it is encapsulated within the st-PMMA helix cavities.

Next, we examined the changes in the absorption and fluorescence spectra when C₇₀ was introduced as the second guest molecule in the st-PMMA/coronene inclusion complex. C₇₀ was also encapsulated in the inner cavity of st-PMMA, forming an inclusion complex. The fluorescence of C₇₀ is relatively weak,²⁶ and C₇₀ acts as a fluorescence quencher of coronene when both coexist.

A toluene solution of C₇₀ was added to a preprepared st-PMMA/coronene inclusion complex gel (coronene encapsulation ratio = 82%), and the time-dependent changes in the encapsulation ratios of each guest molecule were studied (Fig. 5a). At the final st-PMMA, coronene, and C₇₀ concentrations of 2.0 mg mL⁻¹, 0.50 mg mL⁻¹, and 0.14 mg mL⁻¹ (corresponding to 10 mol% relative to coronene; adjusted after C₇₀ addition), respectively, C₇₀ encapsulation occurred rapidly, reaching an encapsulation ratio of 90% within 7 min. In contrast, the encapsulation ratio of coronene did not considerably change after C₇₀ addition, confirming that both guest molecules were encapsulated within the st-PMMA helix. Notably, while C₇₀ is preferentially encapsulated in the st-PMMA helix when C₆₀ and C₇₀ are used as guest molecules,¹² herein, coronene and C₇₀ were quantitatively encapsulated.

Fig. 5 (a) Changes in the encapsulation ratios of coronene (○) and C₇₀ (▲) with time after adding C₇₀ to the st-PMMA/coronene gel with an initial coronene encapsulation ratio of 83% (final concentrations: st-PMMA = 2.0 mg mL⁻¹, coronene = 0.50 mg mL⁻¹, and C₇₀ = 0.14 mg mL⁻¹). (b) Changes in the absorption and PL (excitation wavelength = 340 nm) spectra of the st-PMMA/coronene gel before (i and iv) and after adding 0.014 mg (ii and v) and 0.14 mg (iii and vi) of C₇₀, followed by stirring for 30 min (st-PMMA = 2.0 mg mL⁻¹ and coronene = 0.50 mg mL⁻¹). (c) Stern–Volmer plots showing the quenching of the PL of coronene after the addition of C₇₀ to the st-PMMA/coronene inclusion complex gel. Emission intensities at 433.5 nm (×) and 495 nm (●) were used for the analysis. (d) Schematic illustration of the formation of an inclusion complex, in which coronene and C₇₀ are encapsulated within the st-PMMA helix after the addition of C₇₀ to the st-PMMA/coronene inclusion complex.

When C₇₀ was added to the st-PMMA/coronene inclusion complex gel at a ratio of only 1.0 mol% relative to coronene (1.2 mol% relative to encapsulated coronene), the excimer-like emission in the long-wavelength region was significantly quenched in the fluorescence spectrum of the inclusion complex gel at an excitation wavelength of 340 nm after sufficient time had elapsed (30 min; Fig. 5b, (iv) and (v)). Similar results were obtained from fluorescence measurements at an excitation wavelength of 360 nm (Fig. S8). While significant changes were observed in the fluorescence spectra of the inclusion complex gel, the UV–vis absorption spectra showed almost no change before and after C₇₀ addition (Fig. 5b, (i) and (ii)), indicating that coronene remained encapsulated within the st-PMMA helix cavity even after C₇₀ introduction. These results support the conclusion inferred from the encapsulation ratio calculated using the absorbance of the supernatant. When the amount of C₇₀ added increased to 10 mol% relative to coronene (12 mol% relative to encapsulated coronene), only the emission of unencapsulated coronene was observed in the fluorescence spectrum (vi) (note: because of the increased amount of C₇₀ added, the broad characteristic absorption of C₇₀ (e.g., 387 nm) was observed in the UV–vis spectrum (iii)). Such a significant quenching effect due to C₇₀ addition was not observed in the toluene solution of coronene (Fig. S9).

Furthermore, the Stern–Volmer analysis of the quenching behavior of the inclusion complex gel after C₇₀ addition showed a Stern–Volmer constant (K_SV) of 2.3 × 10³ M⁻¹ at 433.5 nm (the emission from unencapsulated coronene), similar to that of the toluene solution. In contrast, the K_SV at 495 nm (the emission from encapsulated coronene) was 4.2 × 10⁴ M⁻¹, approximately 18 times higher than that at 433.5 nm (Fig. 5c). These results imply that in tightly packed coronene aggregates confined within the helical nanocavities, excited states are delocalized across adjacent molecules because of π–π orbital overlap. When a small amount of the quencher (C₇₀) is coencapsulated in the same cavity, these delocalized excited states are rapidly quenched via Förster resonance energy transfer (FRET), much more efficiently than in unencapsulated coronene (Fig. 5d). This indicates that quenching occurs selectively for coronene located within the helical cavities, where C₇₀ is coconfined near the aggregates, enabling efficient energy transfer. In contrast, coronene molecules outside the cavities remain mostly unaffected because of limited interactions with the quencher. Therefore, the helical nanospace not only promotes coronene aggregation but also enables site-selective quenching by C₇₀.^27,28

Conclusions

This study demonstrates that the inner cavity of the helical structure formed by st-PMMA, a synthetic polymer, functions as a nanocavity that confines guest molecules in a nanospace, allowing the tuning of the emission properties of the guest molecules. The st-PMMA helix and coronene form an inclusion complex in toluene, and the coronene molecules encapsulated in the inner cavity of the helix exhibit unique long-wavelength fluorescence emission because of their proximity in the nanospace. It is hypothesized that the guest molecules are arranged in a low-dimensional array within the helix, and this spatially specific arrangement control of guest molecules is likely one of the characteristic functions of the helical polymer host. Unlike static low-molecular hosts, such as cyclodextrins and calixarenes, which are described by the so-called “lock and key” relation, the second characteristic function of the helical polymer host is its flexibility in response to the presence of multiple guest molecules. When C₇₀ is added to the st-PMMA/coronene inclusion complex gel, it is also encapsulated in the st-PMMA helix. The unique fluorescence emission of encapsulated coronene is selectively quenched by encapsulated C₇₀, demonstrating that the fluorescence emission of coronene can be controlled by forming the inclusion complex. In our previous study, we reported that when C₆₀ and C₇₀ are used as guest molecules, each forms an inclusion complex with st-PMMA when used alone. However, when a mixture of C₆₀ and C₇₀ is used, an inclusion complex is preferentially formed with C₇₀. Owing to its flexibility, the st-PMMA helix forms inclusion complexes with various guests, and when guest molecules are used in combination, they exhibit extremely interesting behaviors, for example, the simultaneous encapsulation of both guest molecules or the encapsulation of only one. The polymer helical inclusion complex system, which responds to multiple guest molecules and modulates its physical properties depending on the selected guests, can be regarded as a highly versatile functional polymer material with broad potential applicability.^29,30 The third characteristic function of the helical polymer host is that because it is a polymer, it can construct a system as a gel. Because the gel is an open system, it facilitates the transport of guest molecules. We also demonstrated that by changing the concentration of guest molecules, their encapsulation ratio can be controlled and the fluorescence emission of coronene can be easily modulated. The fluorescence from encapsulated coronene can be selectively quenched after C₇₀ addition owing to the ease of material transport in the gel. The fourth characteristic function of the helical polymer host is that the chirality can be controlled by controlling the helical sense, thereby constructing a chiral nanocavity. Using a toluene/1-phenylethanol mixed solvent to control the helical sense of the st-PMMA helix, an optically active st-PMMA/coronene inclusion complex gel is obtained, in which CD is observed in the absorption region of coronene. Such optically active materials with emission properties have strong potential in the development of polarized emission materials.³⁰

As mentioned above, the helical polymer host, st-PMMA, possesses structure-specific functions that are distinct from static low-molecular hosts. In the prepared st-PMMA/coronene inclusion complex gel, we can successfully control the fluorescence properties of the guest coronene using the unique functions of the polymer helix. This system is a gel that facilitates material transport, can respond to multiple triggers (i.e., multiple guest molecules), and possesses a unique nanospace formed by its inner cavity, positioning it as a smart gel similar to the advanced functions of biological soft tissues. By combining with various guest molecules, it can be developed into electronic and optical materials via molecular arrangements in the nanocavity, optically active materials by controlling the helical sense, high-sensitivity sensors via inclusion complex formation, and molecular flasks (nanoreactors).

Author contributions

K. N., R. Y., I. T., and A. K. conducted the experiments of inclusion complex formation. T. A. performed the molecular modeling of the st-PMMA/coronene inclusion complex. T. M. analyzed the photoluminescence spectral data. T. K. conceived and started the project. T. K. administered the project. K. N., T. M., and T. K. wrote the paper with input from all authors. All authors discussed the results and contributed to the paper.

Conflicts of interest

There are no conflicts to declare.

Data availability

Supplementary information: details of methods, experimental procedures, and additional data. See DOI: https://doi.org/10.1039/d5py00796h.

Acknowledgements

This work was supported by JSPS KAKENHI (No. 22K05233 and 25K08753).

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