Introduction

Mechanochromic luminescent (MCL) materials, referring that the photophysical properties including emission wavelength, emission lifetime, and quantum yield are sensitive to external mechanical stimulation1. As one of the most promising smart materials, MCL materials have attracted great attention and been widely used in construction of sensors and information storage devices2. More and more organic MCL molecules have been developed such as distyrylanthracene3, oligo(p-phenylene vinylene)4,5, boron complexes6, triphenylamine (TPA), tetraphenylethylene derivatives7,8, and others9,10, along with the proposed design strategies including cation–anion interaction11,12, donor–acceptor type compounds13,14, doping15,16, site effect of substituent17,18, etc. These approaches mainly focused on the construction of different mechanoresponsive systems through molecular structure modification. However, even with the same backbone, the subtle change in molecular structure could make great difference on the photophysical properties and mechanoresponsive behavior, highlighting the need to understand the structure-property relationship of specific design methods19,20. In addition, most of the MCL systems reported thus far were generated by transitioning from a crystalline state to an amorphous or a new crystalline form21,22. However, the strict conditions required for crystal cultivation limit the applicability of MCL materials. Therefore, developing amorphous MCL materials is of great significance.

Numerous pioneering works have shown that both molecular packing and conformation are crucial for the mechanoresponsive behavior of materials in the solid state23,24. From this perspective, besides ingenious molecular designs, other strategies that could alter the packing arrangement and/or conformation in the solid state may also be suitable for constructing MCL materials. The host–guest interaction has been proved to be a powerful tool for regulating the packing arrangement and conformation of guests. On one hand, encapsulating chromophores on guest molecules by suitable macrocyclic hosts enables the conversion of monomer, dimer, and supramolecular polymer/assembly, while tuning the packing arrangement usually leads to intriguing emission property25,26,27,28,29,30. On the other hand, host-induced folded or planarized conformation of guests also results in appealing photophysical property31,32,33,34,35. Additionally, the host–guest complex with high binding affinity could maintain the specific binding mode in the solid state. Even in an amorphous state, when part of the twisted guest is located outsides the confined cavity, the host–guest complex may still show responsiveness toward mechanical stimulation. Overall, the host–guest complexation may be an appropriate candidate for designing MCL materials.

Cucurbit[n]uril (CB[n], n = 5–8, 10) is composed of n glycoluril units bridged by 2n methylene and possesses a symmetric structure with a confined cavity, showing high binding affinity towards cationic guests36,37,38. The past few years have witnessed the great achievements of CB-based luminescent materials, including tunable luminescent materials and organic room temperature phosphorescence materials39,40,41. In addition, the supramolecular structure of donor–acceptor (D-A) systems united through intermolecular charge transfer (CT) interaction has attracted tremendous attention and provided a prospective way to induce unique luminescent systems42,43,44. Noncovalent D-A structures usually exhibit spectific charge transfer emission that could be modulated by changing the donor or acceptor groups45,46. Besides, this supramolecular method has been utilized to construct striking MCL materials, in which the mechanical stimulation could modulate the packing of the assembled D-A groups15,16,47. CB[n] could dramatically enhance the intermolecular CT interaction between donors/acceptors to form the heteroternary inclusion complexes48. However, the mechanoresponsive property of the noncovalent charge transfer pair inside a macrocyclic host has not been investigated so far.

In this study, three cationic TPA derivatives named G1–G3 (Fig. 1c) were designed and synthesized based on the mechanoresponsive behavior of TPA molecules7. All of G1–G3 powders show a redshift of emission after grinding. The cationic G1–G3 are able to bind to CB[8] to form 1:2 homoternary inclusion complexes through host-stabilized charge transfer (HSCT) interaction, and the obtained complexes exhibit a redshift of emission upon grinding (Fig. 1a) even though they are amorphous. The planarization of molecular conformation induced by grinding is responsible for the red-shifted emission of homoternary complexes. Additionally, a heteroternary complex formed between CB[8] and a D-A pair also shows MCL characteristic (Fig. 1b).

Fig. 1: The proposed host–guest inclusion strategy for the construction of sequentially red-shifted mechanochromic system.
figure 1

a Schematic of the construction of sequentially red-shifted mechanochromic system. b Schematic of the formation of heteroternary inclusion complex. c Chemical structures of macrocyclic host CB[8] and guests (the counterions Cl- are omitted for clarity).

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Results

Mechanochromic property of guests

The synthesis and characterization of new compounds are presented in the supporting information (Supplementary Figs. 1–19). Compound G1 exhibits a higher ΦF value (8.6%) and shorter emission wavelength (560 nm) in dichloromethane, and a significantly reduced ΦF value (0.3%) with longer emission wavelength (611 nm) in acetonitrile (Supplementary Fig. 20). These observations suggest its twisted intramolecular charge transfer (TICT) nature49. The luminescent behaviors and MCL properties of G1–G3 in the solid state were investigated next. As shown in Fig. 2a, G1 is highly emissive in the solid state, exhibiting yellow fluorescence at 566 nm with an ΦF of 32%, and the maximum emission wavelength is red-shifted to 578 nm (ΦF = 22%) after grinding. Powder X-ray diffraction (PXRD) results disclose that the sharp and intense diffraction peaks of G1 powder are significantly weakened by grinding (Supplementary Fig. 21b), suggesting that grinding could partially destroy the ordered packing arrangement. Fuming with water leads to the return of its fluorescence to the initial state, accompanied by the PXRD pattern of the fumed sample being identical to the initial samples, revealing the recovery of its crystalline structure (Supplementary Fig. 21). G2 powder shows similar spectral and PXRD pattern changes upon grinding and fuming: the emission wavelength is slightly red-shifted from 547 nm to 556 nm by grinding and restores to the initial state via fuming with water; weakened diffraction peaks after grinding restore as that of the initial sample upon fuming (Fig. 2b and Supplementary Fig. 22). G3 emits yellow fluorescence with the maximum emission wavelength at 560 nm (ΦF = 28%). After grinding, the emission wavelength is red-shifted to 596 nm (Fig. 2c and Supplementary Fig. 23a) with a decreased ΦF value of 19%, accompanied by the emission color changing to orange. Subsequent treating the ground sample with water makes the blueshift of emission wavelength to 571 nm (Supplementary Fig. 23a). Compared to the initial powder of G3, the PXRD results of the fumed sample show that the diffraction peaks at 9.96°, 14.14°, and 15.61° disappear, accompanied by the observation of new intense signals at 8.93°, 11.90°, 18.43°, and 29.92° (Supplementary Fig. 23b), implying that a new crystalline state could form when the amorphous ground sample is treated with water.

Fig. 2: The MCL behaviors of guests and homoternary complexes.
figure 2

Photoluminescence spectra for the as-prepared and ground samples of G1 and CB[8]·G12 (a), G2 and CB[8]·G22 (b), G3 and CB[8]·G32 (c) (λex = 420 nm) (insert: photographs for the as-prepared and ground samples of G and CB[8]·G2 under 365 nm UV light excitation). d Powder X-ray diffraction (PXRD) patterns for the as-prepared and ground samples of CB[8]·G2.

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As presented above, all of G1–G3 exhibit MCL characteristic: the red-shifted emission is observed upon grinding, along with a decrease in fluorescence quantum yield (Table 1). These mechanoresponsive behaviors in the solid state are highly related to phase transition.

Table 1 Photoluminescence data of G1–G3 and their host–guest complexes in different states at room temperature
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Host–guest complexation between CB[8] and guests

Firstly, 1H NMR titration experiments were conducted to verify the binding mode (Fig. 3a). Upon gradually increasing the equiv. of CB[8], the signals of H7-11 on the 4-phenylpyridine group of G1 undergo upfield shifts (∆δ = −1.30 ppm, −1.16 ppm, −0.99 ppm, −1.07 and −0.30 ppm, respectively), while the signal of H1 on aldehyde undergoes downfield shift (∆δ = 0.17 ppm), indicating that the 4-phenylpyridine group is located inside the cavity, while the aldehyde group is near the outside of the cavity. Besides, both the proton signals of bound and free G1 are observed when the equiv. of CB[8] is 0.25, revealing that the recognition process displays slow kinetics of exchange on the 1H NMR time scale. Electrospray ionization mass spectroscopy (ESI-MS) test confirms the formation of the CB[8]·G12 complex in aqueous solution (Fig. 3b): the ion at m/z = 1029.355 corresponds to the 1:2 complex ([CB[8] + 2G1]2+ = 1029.362). Taking into account the strong electrostatic repulsion of pyridinium units and the lack of observation of extra split signals of CB[8] in 1H NMR spectra, it can be concluded that the 4-phenylpyridine groups of two G1 should arrange in an anti-parallel manner in the cavity of CB[8]50. As shown in Fig. 3c, the absorbance of G1 at 251 and 353 nm decrease gradually upon adding CB[8] into the aqueous solution, and the absorbance at 415 nm exhibits an apparent bathochromic shift by 23 nm upon continuous addition of CB[8] until 0.5 equiv. This spectral change is similar to the related host–guest systems involving the formation of CT dimer in the cavity of CB[8] driven by the HSCT interaction51,52. Furthermore, the negligible fluorescence of G1 in diluted aqueous solution with the maximum emission wavelength at 601 nm (ΦF ˂ 0.1%) is greatly enhanced upon addition of CB[8] (Fig. 3d), with an ΦF of 0.71% for CB[8]·G12. This enhancement can be attributed to the restricted intramolecular rotation of the chromophore (AIE effect) by the confinement effect from the non-polar cavity of CB[8] and the weakened TICT interaction caused by complexation49. Also, G1 shows an obvious increased fluorescence lifetime upon binding with CB[8]. The results of fluorescence lifetime are shown in supplementary information (Supplementary Figs. 24, 25), and the photophysical data are summarized in Table 1. Subsequently, isothermal titration calorimetry (ITC) experiment was carried out to verify the binding affinity of CB[8] towards G1. The data is well fitted by using a sequential binding model, showing a host–guest stoichiometry of 1:2 (Fig. 3e). According to the thermodynamic data obtained from ITC, the host–guest complexation is mainly governed by enthalpy (Supplementary Table 1). The resulted binding constant of CB[8]·G12 is as high as 7.98 × 1012 M−2, indicating the high stability of this homoternary complex.

Fig. 3: Host–guest complexation of CB[8] with G1.
figure 3

a 1H NMR titration spectra of G1 (2.0 × 10−3 M, 600 MHz, D2O, 298 K) with (a) 0 equiv., (b) 0.25 equiv., (c) 0.5 equiv. of CB[8]. b ESI-HRMS spectrum of CB[8]·G12. UV-Visible absorption (c) and photoluminescence spectra (d) of G1 (2.0 × 10−5 M in H2O) with different amount of CB[8] (0~0.5 equiv.) at 298 K (λex = 420 nm). e ITC data for G1 with CB[8] in H2O ([G1] (syringe) = 2.0 × 10−3 M, [CB[8]] (cell) = 8.0 × 10−5 M, 298 K).

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Afterwards, we used the same methods to investigate the binding mode between CB[8] and G2. NMR titration indicates that, similar to G1, the binding of G2 with CB[8] exhibits slow kinetics of exchange on the 1H NMR time scale, and two included G2 molecules should be arranged in an anti-parallel orientation in the CB[8] cavity (Supplementary Fig. 26). The ESI-MS result (Supplementary Fig. 27) also discloses the ion at m/z = 1057.355 which corresponds to the 1:2 inclusion complex ([CB[8] + 2G2]2+ = 1057.361). Binding of G2 with CB[8] leads to a redshift of its absorbance from 388 nm to 427 nm, and 65-fold/6-fold enhancement of its fluorescence lifetime/quantum yield (Supplementary Fig. 28). The two cation sites endow G3 with better water solubility and may result in peculiar binding mode towards CB[8]. 1H NMR titration spectra demonstrate slow-exchange kinetics for the complexation between CB[8] and G3 (Supplementary Fig. 29). Considering the electrostatic repulsion and steric hindrance of the TPA group, we infer that only one of the 4-phenylpyridine groups for each G3 is located inside the cavity, and two included G3 molecules are also arranged in a head-to-tail manner, thus resulting in the observation of two sets of proton signals of 4-phenylpyridine moiety. As shown in Supplementary Fig. 30, the absorbance of G3 at 437 nm in aqueous solution shows an apparent redshift by 31 nm, and the emission is greatly enhanced upon binding with CB[8], along with the enhancement of fluorescence lifetime and quantum yield (Table 1). The above phenomena give a clear picture on the host–guest binding of G1–G3 with CB[8]: all three guests bind with CB[8] to form stable 1:2 host–guest complexes, and the CB[8]-mediated homoternary complexes in aqueous solution exhibit longer fluorescence lifetime and higher fluorescence quantum yields than that of free guests (Table 1).

Mechanochromic behavior of CB[8]·G2

Based on the above results of host–guest binding, we investigated the MCL properties of host–guest complexes. All of the host–guest complex powders were prepared by lyophilization since lyophilization is an effective way to obtain the solid sample of host–guest complex. As shown in Fig. 2a, the CB[8]·G12 powder exhibits orange fluorescence under 365 nm UV light excitation, with the maximum emission wavelength at 586 nm (ΦF = 20%). It should be noted that the CB[8]·G12 powder has a longer emission wavelength and lower ΦF compared to free G1. Strikingly, the emission wavelength of CB[8]·G12 is further red-shifted to 599 nm with a reduced ΦF of 10% after grinding. Both solid samples of G1 and CB[8]·G12 show a sequentially red-shifted emission upon grinding (566, 578, 586 and 599 nm represent the emission wavelength of G1 and CB[8]·G12 powder before and after grinding, respectively), along with sequentially decreased fluorescence quantum yield (32%, 22%, 20% and 10% represent the quantum yield of G1 and CB[8]·G12 powder before and after grinding, respectively). Fuming with water leads to the recovery of the emission of the ground CB[8]·G12 powder to its initial state (Supplementary Fig. 31a). PXRD results show that all the CB[8]·G12 powders, with different treatments, are in an amorphous state, indicating that the MCL process of CB[8]·G12 is not related to the phase transition (Fig. 2d and Supplementary Fig 31b). The CB[8]·G22 powder also emits bright yellow fluorescence (573 nm) under UV light excitation with an ΦF of 10%, and its emission wavelength is red-shifted to 584 nm after grinding (Fig. 2b). Both the initial and ground powder of CB[8]·G22 do not show observable diffraction peaks (Fig. 2d). While after the ground powder is treated with water, the emission wavelength is blue-shifted to 545 nm. The observation of many sharp signals in PXRD patterns indicates that the fumed sample is in crystalline state or partially crystalline state (Supplementary Fig. 32), hence the blue-shifted emission of the fumed sample could be ascribed to the more twisted conformation of crystalline sample than the amorphous one53,54. Particularly, the CB[8]·G32 powder shows a redshift of emission by 51 nm compared to the free G3 powder, and its maximum emission wavelength is further red-shifted to 626 nm after grinding (Fig. 2c). Subsequent treating the ground sample with water leads to the recovery of initial fluorescence (Supplementary Fig. 33).

In view of the unique 1:2 binding mode and different photophysical properties of G and CB[8]·G2, it is reasonable to suggest that the decreased ΦF and longer emission wavelengths of G upon complexation in solid state could be owing to the destruction of aggregation of free G and the occurrence of HSCT interaction. Moreover, due to the twisted nature of G and the confinement effect of CB[8], a conformational change may also occur during the host–guest complexation and further affect the luminescent behavior. Notably, all of the PXRD patterns of CB[8]·G2 before and after grinding do not show any clear and intense diffraction peaks (Fig. 2d), indicating that grinding does not alter the amorphous state of initial sample. Based on the twisted geometry and the location of TPA moiety (outside the cavity) in the complex, we speculate that the conformation variation may also play a pivotal role in the MCL process of host–guest complex.

Analysis of single crystals

Single crystals suitable for X-ray structural analysis of G1 and CB[8]·G12 were successfully obtained, allowing us to directly explore the effect of conformation/packing on their photophysical properties and MCL mechanism. Compound G1 crystallized in a triclinic structure with a space group of P-1. The planarity of the twisted molecule can be quantitatively characterized by the dihedral angles of adjacent (hetero)aryl groups55. As illustrated in Fig. 4a, there are two slightly different conformations, G1-a and G1-b, in the G1 crystal with the sum of three dihedral angles of 187.43° (G1-a) and 184.13° (G1-b), demonstrating the highly twisted conformation of G1. Intermolecular interactions including C−H···O (2.441 Å), C−H···π (3.268 Å), and C−H···Cl (3 types, 2.801–2.948 Å) are observed (Fig. 4b). Notably, no π–π interaction is observed in the G1 crystal due to the twisted geometry of G1, which blocks the overlap of the π-plane. Since the loose packing of the crystal induced by the steric hindrance and twisted conformation can be readily amorphized under mechanical stimulation56, the observation of MCL behavior of G1 is reasonable. Upon amorphization by grinding, the high twisting stress and weak interactions are destroyed, generating a relatively planarized conformation, which is accompanied by a decreased quantum yield and a redder emission (Table 1).

Fig. 4: Crystal structure of G1 and CB[8]·G12.
figure 4

a Two conformations of G1 (G1-a and G1-b). b Intermolecular C−H···O, C−H···Cl and C−H···π interactions in G1 crystal. c The packing mode of G1 (view along a-axis). d Molecular arrangement of CB[8]·G12 in crystal (view along b-axis). e Side view and top view of CB[8]·G12-a. f The π–π interactions of G1 in the cavity of CB[8] (the centroid-centroid distances between the neighboring benzene and pyridine rings are presented). g Conformations of G1 in the CB[8]·G12 complex (G1-c, G1-d, G1-e, and G1-f). Hydrogen atoms and counterions are omitted in a and c–g for clarity.

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In agreement with the conclusions based on the 1H NMR titration experiments, the crystal structure of CB[8]·G12 directly shows that the 4-phenylpyridine units of two G1 are located inside the cavity of CB[8] in a head-to-tail manner (Fig. 4d, e). There are four different conformations of G1 (named G1-c, d, e, f, respectively) in the CB[8]·G12 crystal, and two G1 with similar conformations are paired to form two types of homoternary host–guest complexes (Fig. 4d, f). Intriguingly, the sum of the three dihedral angles for G1-c, d, e, f are smaller than those of free G1, indicating that G1 in the inclusion complex is less twisted (Supplementary Table 2). Especially for G1-c and G1-d, the 4-phenylpyridine groups are almost coplanar, with dihedral angles of 5.09° and 7.77°, respectively (Fig. 4g). The data of the dihedral angles in G1-involved crystals are collected in Supplementary Table 2 for clear comparison. Crucially, since two 4-phenylpyridine groups of two G1 molecules are located inside the confined cavity, π–π interactions are observed in the CB[8]·G12 crystal. The centroid-centroid distances between the neighboring benzene and pyridine groups range from 3.712 Å to 3.979 Å (Fig. 4f). According to the results presented above, the host–guest complexation not only arranges the conformation of G1 in a more planar state but also enhances the π–π interaction due to the unique binding mode, which is responsible for the red-shifted emission of CB[8]·G12 compared to free G1 in solid state. In addition, the strong ion–dipole interactions (Supplementary Fig. 34) and hydrophobic effect are synergistically to stabilize the ternary host–guest complex. And each CB[8]·G12 interacts with adjacent CB[8] through multiple C−H···O interactions (Supplementary Fig. 35).

Single crystals of CB[8]·G22 and CB[8]·G32 were also obtained by slowly evaporating their aqueous solution at room temperature. Similar to G1, the 4-phenylpyridine groups of G2 adopt an anti-parallel orientation inside the cavity of CB[8], and two types of inclusion complexes are formed by pairs of two different conformations of G2 (Fig. 5a, c). The benzene rings of two G2 inside the cavity are perfectly parallel with interplanar distances of 3.419 (G2-a) and 3.245 Å (G2-b), respectively (Fig. 5d, e). The distances between the centroid of pyridine and the plane of benzene on adjacent G2 molecules in two inclusion complexes are 3.352 (G2-a) and 3.530 Å (G2-b), respectively (Fig. 5d, e). These close distances reveal strong π–π interactions of the G2 dimer inside the cavity. The crystal structure of CB[8]·G32 shows that one of the 4-phenylpyridine groups on each G3 is encapsulated by CB[8], and the two different conformations are paired to form two types of ternary complex, respectively (Fig. 5b, f). Also, the close stacking of G3 molecules inside the cavity implies strong π–π interactions (Fig. 5g, h). The main crystallographic parameters are summarized in Supplementary Tables 3–5. Although, besides the CB[8]·G32 complex, there are two cavity-free CB[8] molecules that interact with neighboring G3 through ion–dipole and C − H···O interactions in each unit cell, this crystal structure still sheds light on the analysis of the host–guest binding mode and the conformation of G3 in the inclusion complex.

Fig. 5: Crystal structure of CB[8]·G22 and CB[8]·G32.
figure 5

a Overview of the binding mode of CB[8]·G22. b Overview of the binding mode of CB[8]·G32. c Unit cell of CB[8]·G22 (view along a-axis). d Conformation of G2-a and π–π interactions of G2-a inside the cavity. e Conformation of G2-b and π–π interactions of G2-b inside the cavity (the distances between benzene ring of two G2 in the cavity, between the centroid of pyridine and the plane of benzene on adjacent G2 in the cavity are presented). f Partial molecular packing of CB[8]·G32. g Conformation of G3-a and π–π interactions of G3-a inside the cavity. h Conformation of G3-b and π–π interactions of G3-b inside the cavity (the distances between benzene ring of two G3 in the cavity, between the centroid of pyridine and the plane of benzene on adjacent G3 in the cavity are presented). Hydrogen atoms and counterions are omitted for clarity.

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The X-ray crystallographic analysis mentioned above indicates that although the 1:2 anti-parallel binding makes the guests adopt a more planar state, the TPA moieties still exhibit twisted conformation in the inclusion complex. Although the powders of host–guest complexes in their amorphous states do not show regular packing as crystals, each inclusion complex maintains its specific binding mode at molecule level to form the macroscopic amorphous powder. The introduction of the host–guest inclusion strategy in producing this type of amorphous organic MCL system may be an enlightening principle in the design of future MCL materials, as it could bypass the complicated and demanding conditions for the preparation of crystalline MCL samples.

Considering that the mechanochromic process of the ternary complex does not involve phase transition, and part of the twisted TPA moiety located outside the cavity is still sensitive to the mechanical stimulation, we believe the planar conformation induced by grinding is the most reasonable mechanism. As changes in molecular conformation often lead to changes in absorption peaks57, solid state absorption spectra of G1 and CB[8]·G12 in different states were performed to validate our speculation. As shown in Supplementary Fig. 36, both G1 and CB[8]·G12 exhibit obvious red-shifted absorbances after grinding, confirming that the alternated molecular conformation is the key origin of the MCL behavior. The host–guest complexation enables the encapsulated guests to achieve extra red-shifted luminescence upon grinding by introducing HSCT interaction.

Formation of heteroternary complex CB[8]·Py·G1 and its mechanochromic behavior

Since the CT interaction between donor and acceptor molecules can be enhanced by forming hetero-guest pairs in the cavity of CB[8]48, we continued to explore whether the host–guest inclusion strategy is applicable to hetero-guest pairs for the construction of MCL system. 1-Pyrenemethylamine hydrochloride (Py-Cl, chlorine is omitted for clarity), retaining electron-rich nature of the pyrene unit and showing adequate solubility in water, was chosen as an ideal electron-donor molecule to construct a heteroternary complex. The binding of Py with CB[8] was investigated using 1H NMR titration spectroscopy (Supplementary Fig. 37). Upon addition of CB[8] to the solution of Py, all proton signals of the pyrene moiety show upfield shifts, demonstrating the encapsulation of the pyrene moiety inside the cavity. While the signal of methylene protons exhibits a slight downfield shift, from 4.88 to 5.08 ppm, suggesting that the NH3+ group is outside but near the carbonyl portal of CB[8]. Additionally, the absorbance and emission in diluted aqueous solution decrease upon addition of CB[8] (Supplementary Fig. 38). These results are consistent with a previous report that the cavity of CB[8] can accommodate one pyrene group58.

The formation of heteroternary complex was then checked by 1H NMR titration, UV/vis, and fluorescence spectroscopies in aqueous solution. As shown in Supplementary Fig. 39a–c, the proton signals of G1 slightly shift when G1 is mixed with Py in equal molar ratio. Intriguingly, new proton signals that are different from CB[8]·Py and CB[8]·G12 are observed in the 1H NMR spectrum of the 1:1:1 mixture of CB[8], Py, and G1, confirming the formation of the CB[8]·Py·G1 complex. Although the recognition process of CB[8] towards the hetero-guest pair displays fast exchange kinetics on the 1H NMR time scale (Supplementary Fig. 40), based on the fact that the binding between CB[8] and G1 displays slow-exchange kinetics and no proton signals of CB[8]·G12 are found in the titration NMR spectra, we highly believe that the heteroternary inclusion complex is exclusively formed in aqueous solution. ESI-MS also provides strong evidence for the formation of the heteroternary complex. In the ESI-MS of an equimolar mixture of G1, Py, and CB[8], the ion at m/z = 962.835 corresponding to the 1:1:1 complex ([CB[8] + Py + G1]2+ = 962.835) is observed (Supplementary Fig. 41). Upon gradually adding CB[8] to the aqueous solution of 1:1 mixture of Py and G1, the absorbances at 241, 275 and 340 nm decrease, along with a redshift of absorbance from 414 nm to 437 nm (Supplementary Fig. 42). Due to the strong blue emission of Py in diluted solution (2.0 × 10−5 M), the weak emission signal of G1 is hardly visible in the 1:1 mixture of Py and G1 under the same experimental condition (Fig. 6d). At high concentration (1.0 × 10−3 M), the mixture of Py and G1 shows a weak emission band at 514 nm with a broad full width at half maxima and a long lifetime (τ = 37.97 ns) (Supplementary Figs. 43, 44), which could be ascribed to the formation of the exciplex Py·G159. A broad emission band at 560 nm emerges and is enhanced by gradually adding CB[8] (from 0 to 1.0 equiv.) to the 1:1 mixture of Py and G1 even in diluted aqueous solution (Fig. 6d). The excitation spectrum of CB[8]·Py·G1, collected by monitoring the emission at 560 nm, shows a maximum at about 490 nm, which closely matches with the corresponding charge transfer absorption band of CB[8]·Py·G1 (Supplementary Fig. 44). Therefore, the newly generated yellow emission at 560 nm could originate from the HSCT interaction. We believe that the heteroternary complexation narrows the distance and corrects the orientation between Py and G1, resulting in the stronger charge transfer interaction between Py and G144.

Fig. 6: Crystal structure and MCL behavior of the heteroternary complex CB[8]·Py·G1.
figure 6

a Unit cell of CB[8]·Py·G1 crystal (two types of heteroternary complex are named (CB[8]·Py·G1)-a and (CB[8]·Py·G1)-b, respectively). b Binding mode of (CB[8]·Py·G1)-a, conformation of G1-g and the distance between the pyrene plane of Py and centroid of pyridinium on G1 in (CB[8]·Py·G1)-a. c Binding mode of (CB[8]·Py·G1)-b, conformation of G1-h and the distance between the pyrene plane of Py and centroid of pyridinium on G1 in (CB[8]·Py·G1)-b (hydrogen atoms and counterions are omitted for clarity). d Fluorescence emission spectra of Py + G1 (1:1, 2.0 × 10−5 M in H2O, 298 K) with different equiv. of CB[8] (λex = 340 nm). e Photoluminescence spectra of CB[8]·Py·G1 powder under different treatments (λex = 420 nm).

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The cube-shaped yellow crystals of CB[8]·Py·G1 were successfully obtained by slow evaporation of the aqueous solution (1:1:1 mixture of CB[8], Py, and G1) at room temperature over a period of one month. It is worth mentioning that the CB[8]·Py·G1 crystal provides another X-ray crystallography evidence for heteroternary complex, following the report by Kim’s group on the first CB-based hetero-guest pair crystal in 200148. As shown in Fig. 6a, two types of CB[8]·Py·G1 (designated as a and b, respectively) with different binding sites are arranged alternately in each unit cell. In (CB[8]·Py·G1)-a, the pyrene group and methylated pyridinium ring of G1 are located inside the cavity, and two guests are arranged in a head-to-tail manner (Fig. 6b). The pyrene plane is almost perpendicular to the equatorial plane of CB[8] with a dihedral angle of 88.93°. G1-g shows smaller dihedral angles of its adjacent (hetero)aryl groups than G1-a/b, suggesting that the heteroternary binding style also results in a less twisted geometry of G1 (Supplementary Table 2). The methylated pyridinium ring of G1 and pyrene moiety of Py are almost parallel, showing a dihedral angle of 1.14° and a mean separation of 3.287 Å (Fig. 6b). These values are typical for the charge transfer pair inside CB[8] in Kim’s work (additionally, the arrangement of pyridinium and π-moiety of pyrene in the cavity also fits the category of cation–π interaction60). In (CB[8]·Py·G1)-b, the dihedral angle of pyrene plane and the equatorial plane of CB[8] is 51.73°, and two guests are arranged in a head-to-head manner (Fig. 6c). Similarly, G1-h also shows a less twisted conformation than free G1, and the dihedral angle between methylated pyridinium ring of G1 and pyrene moiety is 4.71°. Besides the HSCT interaction, the stronger ion–dipole interactions between guests and C=O of CB[8] are also responsible for the stability of CB[8]·Py·G1 complex (Supplementary Fig. 45). Additionally, a large amount of intermolecular C−H···O interactions between adjacent inclusion complexes are observed (Supplementary Fig. 46).

With the observation of a bright yellow emission induced by HSCT interaction in aqueous solution, we decided to explore whether the HSCT interaction could affect the MCL behavior of the D-A system. Before beginning our investigation, the luminescent property of a 1:1 mixture of Py and G1 (the solid sample was prepared by lyophilization of a homogeneous solution of 1:1 mixture of Py and G1) was examined. The powder shows slightly red-shifted absorption and emission compared to the individual Py and G1 (Supplementary Fig. 47), suggesting the existence of a D-A structure61. However, grinding the sample leads to a negligible redshift of emission by 4 nm (Supplementary Fig. 48), suggesting that the mechanical stimulation could not be able to alter the arrangement of this D-A structure. In contrast, the CB[8]·Py·G1 powder emits yellow fluorescence at 569 nm with an ΦF of 18% (Fig. 6e). After grinding, the emission is red-shifted to 583 nm, and treating the ground powder with water restores the emission wavelength to its initial state. Only few weak diffraction signals are observed in the PXRD pattern of CB[8]·Py·G1 powder, and grinding transforms the morphology of the sample into an amorphous state (Supplementary Fig. 49). The weak diffraction signals reappear after the ground powder is treated with water. Grinding also leads to a red-shifted absorption of the CB[8]·Py·G1 powder (Supplementary Fig. 50). As Py and 4-phenylpyridine of G1 are located inside the confined cavity, the mechanical stimulation is more likely to cause a conformation planarization of the unencapsulated TPA groups and subsequently induce stronger CT interaction, resulting in the red-shifted emission of the heteroternary complex. Theoretical calculations were then conducted to gain a deeper insight into the MCL behavior of the heteroternary complex. As shown in Supplementary Fig. 51, two optimized structures of heteroternary complex, (CB[8]·Py·G1)-α and (CB[8]·Py·G1)-β, were built based on the single crystal of CB[8]·Py·G1. Among them, G1 in (CB[8]·Py·G1)-β is more planar than that in (CB[8]·Py·G1)-α. Importantly, (CB[8]·Py·G1)-β with a planar G1 exhibits a red-shifted emission compared to (CB[8]·Py·G1)-α with a more twisted G1, suggesting that the planar conformation of G1 may be responsible for the red-shifted emission under mechanical stimuli. This calculation result is in line with the experimental observation of the red-shifted emission of the heteroternary complex CB[8]·Py·G1 upon grinding62. The independent gradient model (IGM) analysis63 for (CB[8]·Py·G1)-α/β clearly reveals the existence of multiple noncovalent interactions (the green region) between the host and guests (Supplementary Fig. 52), which is in agreement with the X-ray crystallographic analysis. This calculation result could also explain the red-shifted emission of the homoternary complexes upon grinding, since the TPA groups of G1–G3 in the homoternary complexes are also located outside the cavity, as shown in the crystal structures. Besides, the steric effect of CB[8] may lead to a loose packing, increasing the likelihood of mechanical stimulation inducing a change in molecular conformation. Moreover, the destruction of a significant amount of intermolecular interactions upon grinding, particularly the intermolecular hydrogen bonds between adjacent inclusion complexes, could synergistically result in a larger spectral shift compared to the D-A structure without the host64.

Discussion

We have proposed a host–guest inclusion-directed strategy for the rational design of sequentially red-shifted mechanochromic materials. A series of TPA derivatives (G1–G3) show redshifts of emission upon grinding owing to planarization of TPA unit under mechanical stimulation. These three guests bind with CB[8] host in a 1:2 stoichiometry to form stable homoternary complex CB[8]·G2, which is evidenced by 1H NMR, ESI-MS, ITC, UV/vis, fluorescence spectroscopies, and X-ray crystallography. The host–guest complexes display bathochromic-shifted emission compared to free guests in solid state, due to the host–guest inclusion inducing enhanced intermolecular charge transfer interaction and less twisted conformation of guests. Furthermore, grinding leads to a further redshift of emission of homoternary complexes, resulting from the planarization of unencapsulated moieties by mechanical stimulation. The sequentially red-shifted mechanochromic system has been successfully achieved through the host–guest inclusion strategy without complicated molecular design and synthesis, and the construction of an amorphous mechanoresponsive complex (CB[8]·G2) provides a more convenient procedure for the preparation of MCL systems. Additionally, the host–guest encapsulation of a donor–acceptor pair in CB[8] endows the heteroternary complex with better responsiveness toward mechanical stimulation, with a large redshift of emission. The host–guest inclusion strategy in this work expands the spectral shift compared to free mechanoresponsive guest upon mechanical stimulation by introducing the HSCT interaction, which we believe could be an inspirational and relatively universal method for the construction of novel MCL materials.

Methods

Materials and characterization methods

ESI-MS spectra were recorded on a 9.4 T high-resolution FT-MS mass spectrometer (Solarix 9.4T) from Bruker in the positive polarity. Fluorescence spectra were measured on a PerkinElmer LS-55 machine at 298 K, and a Suprasil Quartz (QS) cuvette with 1 cm pathlength was used for the fluorescence spectra measurements in aqueous solution. Milli-Q water (18.2 MΩ∙cm) was used for preparation of all nondeuterated aqueous solutions. The UV/Vis experiments were performed on a SHIMADZU UV-36002 instrument with 1 cm pathlength cells at 298 K. All 1H NMR and 13C NMR spectra were collected on Agilent 600 MHz DD2 at 298 K. The absolute fluorescence quantum yields were determined by using a Hamamatsu C9920-02G Instruments Integrating Sphere Module (SC-30) on the FS5 spectrofluorometer. Fluorescence spectra analysis software Fluoracle and Quantum Yield Wizard were used to calculate quantum yields. Fluorescence quantum yield measurements were done for 1 mL aqueous solutions with an optical density in the range of 0.06–0.1. ITC experiments were carried out on a Nano ITC (TA Instruments) at 298 K (the first data point was removed from the data set prior to curve fitting; the intervals between injections were set as 200 s). The results of PXRD were collected on the Bruker D8 Advance powder-X diffractometer. The copper target provided Kα rays (λ = 1.5418 Å). All chemical reagents were purchased commercially and used directly without further purification. The synthesis and characterization are summarized in the supplementary information. The time-resolved fluorescence decay curves were obtained on a FS5 time-correlated single-photon-counting (TCSPC) instrument.

Crystal growth

15.0 mg G1 was dissolved in 5.0 mL H2O. After two weeks at room temperature, yellow flake crystals were precipitated out from the concentrated solution. The single crystals of CB[8]·G2 and CB[8]·Py·G1 were provided by standing the corresponding aqueous solution at room temperature, and the concentration of guests in these samples is 2.5 × 10−3 M.

Solid samples preparation

(1) CB[8]·G2: G1 (20 mg, 0.050 mmol) was dissolved in 15 mL deionized water. After added CB[8] (33.22 mg, 0.025 mmol) to the solution, the mixture was sonicated for about 5 min until CB[8] was dissolved, and then the solid sample of CB[8]·G12 was obtained by lyophilization. Solid samples of CB[8]·G22 and CB[8]·G32 were obtained by the same method. (2) CB[8]·Py·G1: Py (13.39 mg, 0.050 mmol), G1 (20 mg, 0.050 mmol) was dissolved in 25 mL deionized water. After added CB[8] (66.44 mg, 0.050 mmol) to the solution, the mixture was sonicated for about 8 min until CB[8] was dissolved, and then the solid sample of CB[8]·Py·G1 was obtained by lyophilization. (3) 1:1 mixture of Py and G1: Py (13.39 mg, 0.050 mmol) and G1 (20 mg, 0.050 mmol) were added in 25 mL deionized water, and the mixture was sonicated for about 5 min until Py and G1 was dissolved. The solid sample was obtained by lyophilization.

Theoretical calculations

The specific process for conformational search was performed by using ORCA5.0.3 package65. The ground state geometry optimization of (CB[8]·Py·G1)-α/β was conducted using the B3LYP-D3/def2SVP level66. The first excited state geometry optimization of (CB[8]·Py·G1)-α/β was conducted by the wB97X-D3/def2SVP level62,67. The Visual Molecular Dynamics (VMD) software was employed for visualization68.