Introduction

Cucurbit[n]urils (CB[n]s) are a family of macrocyclic host homologues [1,2,3,4,5,6,7,8,9,10,11,12]. CB[n]s are known for their ability to form high affinity inclusion complexes with suitable guests in water. However, further applications of CB[n]s are limited by their inadequate aqueous solubility, and the laborious process of isolating homologues [1, 2, 13,14,15,16]. Acyclic CB[n]s (ACBs), derived from glycoluril oligomers, have been developed to provide new approaches [17,18,19,20,21,22]. Acyclic CB[n]s are composed of glycoluril tetramers linked by methylene bridges, and two aromatic O-xylylene walls bearing substituents. Acyclic CB[n]s are capable of recognizing suitable ammonium salts, dyes and pharmaceuticals, and are widely used for sensing and imaging [23,24,25,26], drug delivery and controlled release [27, 28], and drug sequestration [29, 30]. The investigations well demonstrate that ACBs can create guest binding features that complement those of CB[n]s or other acyclic receptors [31,32,33].

The self-aggregation of organic dyes often occurs in aqueous phase, which is mainly driven by or intermolecular noncovalent interactions like van der Waals force or π–π stacking interactions [33]. This aggregating phenomenon can be observed in UV–vis spectra, manifesting as the shifts in the absorption band compared with the monomeric species. J-aggregation and H-aggregation are two of the most common aggregation patterns. J-aggregation, which was named due to the obvious red-shifted J-bands with high absorbance coefficient, is the head-to-tail stacking of dye molecules in a staircase shape [34,35,36]. H-aggregation is a ladder-like face-to-face stacking of dyes, which exhibits weak blue shift H-bands [37,38,39].

The aggregation behavior of dyes mainly depends on dye structure, temperature and solution composition [40]. In this context, host-guest interactions have been developed as a useful strategy to control the aggregation of various organic dyes [10, 41,42,43,44,45]. Herein we report the use of a highly water-soluble acyclic CB[n] host ACB-01 to form stable complexes with two cyanine dyes pseudoisocyanine (PIC) and pinacyanol (PIN) (Fig. 1). These host-guest complexes could disrupt the aggregation process of dyes. Moreover, dye aggregation could be recovered by adding competing guest methyl viologen (MV) or 1-adamantanamine hydrochloride (AD).

Fig. 1
figure 1

Chemical structures of acyclic CB[n], organic dyes and cationic guests

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Results and discussion

Acyclic CB[n] host ACB-01 was prepared by our previous work [46]. Aromatic tetraesters were synthesized by three-step nucleophilic substitution reactions. The glycoluril tetramer, synthesized from glycoluril and paraformaldehyde, underwent a double electrophilic aromatic substitution reaction with two equivalents of aromatic tetraester to form an octaester, which was hydrolyzed to obtain the final product. ACB-01 contained a C-shape glycoluril tetramer and two terminal substituted aromatic walls with eight carboxylate groups. The glycoluril tetramer provided a hydrophobic cavity and two electron-rich carbonyl portals for the binding of cationic guests. Due to the eight carboxylate groups, ACB-01 had significantly higher solubility of 155 mM than CB [7] (~ 20 mM). The eight carboxylate groups not only increased the aqueous solubility of the host, but also improved the binding towards cationic guests.

In water the cyanine dye PIC showed maximum absorption wavelengths at 485 and 525 nm in monomeric form [47]. At a lower temperature or with higher concentrations of dye and inorganic salts, PIC preferred to form J-aggregates which exhibited a characteristic sharp absorption band at 575 nm [48]. In order to estimate the affinity between ACB-01 and PIC, the J-aggregation has to be inhibited by controlling the condition with a low dye concentration and moderate temperature. In Fig. 2, there were two characteristic absorption bands of the monomeric form of PIC with a concentration of 0.01 mM and at room temperature. No absorption peak appeared at 575 nm, which indicated that no J-aggregation occurred. The absorbance of PIC decreased as host ACB-01 was added, and the absorption band also had a minor red shift by 3 nm. The change of PIC absorption spectra revealed that PIC was complexed by host ACB-01 and stabilized by the strong host-guest interaction. Fitting results from Job plots and the absorbance at 500 nm (Fig. S1) showed that PIC followed a 1:1 binding stoichiometry with host ACB-01. The binding constant (Ka) was determined to be (1.54 ± 0.15 ) × 106 M−1.

Fig. 2
figure 2

UV–vis absorbance of PIC (0.01 mM) with ACB-01 (0–4.0 eq.) added at 25 °C. Inset: Variation of the UV–vis absorbance of PIC (0.01 mM) with ACB-01 (0–4.0 eq.) added at 505 nm

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1H NMR titration was used to illustrate the binding mode between PIC and ACB-01. PIC had a large symmetrical π-conjugated structure with only one positive charge, resulting in its poor aqueous solubility. We mixed 10% (v/v) of DMSO-d6 with D2O to solubilize the dye to a higher concentration of 1.0 mM, and reduced the influence of solvent effect as low as possible (Fig. S2). As ACB-01 was added, the chemical resonances of PIC shifted upfield (Fig. 3a). The resonance of hydrogen near positive charge like Ha showed a relatively minor shift (~ 0.45 ppm), while those of terminal hydrogen like Hf underwent a larger upfield shift (~ 0.66 ppm). This result showed that the hydrophobic cavity of ACB-01 was capable to encapsulate the quinolinic ring of PIC, and the more electropositive part was closer to the edge of the cavity where carboxylate oxygen could provide a stronger electrostatic interaction. Job’s plot based on the variation of Hf chemical shift also confirmed that PIC and ACB-01 formed a 1:1 complex (Fig. 3b).

Fig. 3
figure 3

a 1H NMR spectrum (400 MHz) of PIC (1.0 mM) and ACB-01 (0-1.5 mM) in D2O (10% DMSO-d6) at 25 °C (Aromatic moiety). b Job’s plots of PIC and ACB-01 based on chemical shift of PIC proton Hf ([ACB-01] + [PIC] = 1.0 mM). The binding ratio is 1:1

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The well-matched structure between ACB-01 and PIC and strong host-guest interaction made it possible for ACB-01 to depress PIC’s J-aggregation in aqueous phase. At low temperature such as 5 °C with 1.0 M NaCl added, the spectrum of 0.05 mM PIC exhibited a clear sharp absorption peak at 574 nm (Fig. 4a). As the solution was heated up to 70 °C, the peak disappeared completely. The aggregation of PIC molecules is a negative enthalpy change process. According to the van ‘t Hoff equation, the increase in environmental temperature led to a decrease in the equilibrium constant K, resulting in the deaggregation of PIC. After the solution was cooled down back to 5 °C, 1.0 equivalent of ACB-01 was added and we observed the disappearance of J-band. This difference showed that the host-guest interaction between PIC and ACB-01 was stronger than the π–π stacking interaction of PIC itself. As a result, ACB-01 could disturb the J-aggregation of PIC by forming stable PIC@ACB-01 complexes.

Fig. 4
figure 4

UV–vis spectra of a PIC (0.05 mM) and b PIN (0.05 mM) in the presence or absence of ACB-01 (1.0 eq.) at different temperatures. All solutions were prepared in (a) 1.0 M NaCl or (b) 0.05 M NaCl with 1% MeOH

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The disturbance from ACB-01 could also be observed in fluorescence spectra (Fig. 5). PIC showed an emission band around 580 nm at 5 °C with the presence of 1.0 M NaCl on excitation at 400 nm. The J-aggregation band disappeared as the temperature rose up to 70 °C. The addition of excess ACB-01 could also quench the fluorescence of J-aggregates, preventing PIC molecules from π–π stacking.

Fig. 5
figure 5

Fluorescence spectra of PIC (0.05 mM) in the presence or absence of ACB-01 at different temperatures. Excitation wavelength: 400 nm. All solutions were prepared in 0.2 M NaCl

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Host-guest complexation was a reversible process. The introduction of a competing guest could displace PIC from PIC@ACB-01 complexes and re-establish J-aggregation. Previous researches [47] had reported that methyl viologen (MV) and 1-adamantanamine hydrochloride (AD) showed high binding affinity with acyclic CB[n]s. We first confirmed the binding stoichiometry between ACB-01 and two guests (Fig. S3-4). The host could form 1:1 supramolecular complex with the two guests. We also estimated the binding constants of MV@ACB-01 and AD@ACB-01 by UV–vis competing titration (Fig. S5-6), which was determined to be (1.15 ± 0.31) × 108 M−1 and (2.23 ± 0.05) × 106 M−1, respectively (Table 1). The Ka value for MV@ACB-01 was about two orders of magnitude higher than that for PIC@ACB-01, while the Ka value for AD@ACB-01 was slightly higher than that of the dye. Therefore, these two competing guests were capable to displace the dye from the cavity of ACB-01. As expected, the J-band around 574 nm was regenerated after MV was added gradually into the solution of PIC@ACB-01 complexes at 5 °C (Fig. 6a). The absorbance of PIC J-aggregates continued to increase until the concentration of MV reached one equivalent. AD had similar affinity with ACB-01 compared with PIC, which made it difficult to displace ACB-01. The J-band fully regenerated until 1.4 equivalents of AD was added (Fig. 6b). Therefore, we were certain that J-aggregation of PIC could be precisely controlled by the host ACB-01 as an inhibitor and competing guests MV or AD as regenerants.

Fig. 6
figure 6

UV–vis spectra of PIC (0.05 mM) with ACB-01 (1.0 eq.) by adding a MV or b AD at 5 °C. Inset: absorbance at 573 nm of the dyes against [MV] or [AD]. All solutions were prepared in aqueous solution of NaCl (1.0 M)

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Table 1 Binding constants (Ka) for ACB-01 with cationic guests
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PIN was pooly soluble in water, and 1% volume ratio of methanol was added to solubilize PIN. UV–vis absorption measurement was first carried out to reveal the PIN@ACB-01 complexation properties. In the concentration of 0.01 mM PIN showed absorption maxima at 551 and 600 nm (Fig. S7). These two absorption bands underwent obvious red shift (16 nm and 11 nm respectively) as the concentration of ACB-01 increased, and the absorbance decreased slightly at first and then increased. The binding stoichiometry was fitted to be 1:1 and the corresponding Ka value for PIN@ACB-01 complex was determined to be (6.09 ± 0.82) × 105 M−1. This value was lower than that of PIC. 1 H NMR spectroscopy was unable to be used to study the binding mode due to the poor solubility of PIC. We propose that the large conjugative structure of PIN molecule spread out the only one positive charge, which relatively weakened the electrostatic interaction between host and guest.

With 0.05 mM NaCl added, the solution of 0.05 mM PIN showed a new absorption band at 473 nm (Fig. 4b) at 5 °C, which should be ascribed to H-aggregation as it was blue-shifted from original [49]. Another new red-shifted peak at 642 nm was ascribed to J-aggregation [50]. The two absorption bands disappeared after the solution was heated up to 70 °C, which again suggested the decrease in equilibrium constant on H- or J- aggregation caused by enthalpy effect, similarly as PIC. Addition of one equivalent of ACB-01 caused significant changes on the UV–vis spectrum. Both new absorption bands were diminished and the other two underwent a red shift like low dye concentration. The two phenomena both indicated the formation of PIN@ACB-01 complexes and the inhibition of aggregation.

MV and AD were used to remove ACB-01 from the complexes as competing guests. PIN was relatively weakly associated with ACB-01 compared to PIC. The binding constant of the former was three and two orders of magnitude lower than that of MV and AD, respectively. As a result, one equivalent of competing guest was adequate to displace the dye from the complexes, observed in the reappearance of H-band (Fig. S8-9). The concentration of the competing guests required for complete regeneration of aggregation absorption band showed the difference of stability between two dyes.

All the optical phenomena could be summarized as three relevant equilibria:

$${\rm{dye}}\rightleftharpoons{\rm{aggregates}}$$
(1)
$${\rm{dye }} + {\rm{ACB}} - 01\rightleftharpoons{\rm{dye}}@{\rm{ACB}} - 01$$
(2)
$${\rm{ACB}} - 01 + {\rm{ competing}}\,{\rm{guest}}\rightleftharpoons{\rm{competing}}\,{\rm{guest}}@{\rm{ACB}} - 01$$
(3)

Conclusions

In conclusion, we use a highly water-soluble acyclic CB[n] with eight carboxylate tails to bind cyanine dyes PIC and PIN. The formation of dye@ACB-01 complexes could inhibit dyes’ aggregation in aqueous solutions (J-aggregation for PIC and H-aggregation for PIN), which could be reversed by stronger competing guests MV and AD as regenerants. All these processes were under direct influence of binding ability of ACB-01, which could help to realize the precise control of the equilibrium between dye monomers and aggregates. These results showed us the potential application of acyclic cucurbiturils in aqueous dye stabilization.