Introduction

Extensive investigations on [n]cycloparaphenylenes have uncovered their distinct size-dependent photophysical properties due to their unique in-plane radial conjugated structure1,2,3,4,5, and ultimately established [n]CPPs as a remarkably versatile platform that can be tailored for diverse applications, such as chiral recognition6, mechanically interlocked molecules7,8,9,10,11,12,13,14,15,16,17,18, hierarchical self-assembly19,20,21, organic electronic devices22,23,24,25,26,27,28,29,30,31,32, fluorescence imaging33,34, circularly polarized luminescene35,36,37,38,39,40 and beyond2,41,42,43,44,45,46,47,48,49,50. The incorporation of an electron donor (D) or acceptor (A) into the framework of [n]CPPs typically results into a significant separation of the frontier molecular orbitals, which leads to a narrower optical band gap, and in turn, enables effective modulation of the electrochemical and photoelectric properties of [n]CPPs51. The bending of the π-system in CPPs also leads to an elevation of its highest occupied molecular orbital energy (HOMO) compared to that of its linear counterparts, thereby establishing their electron-donating characteristics. Thus, the integration of electron acceptors into the electron donor CPP nanohoop holds significant implications for advancing the development of CPPs in material science4,52,53,54,55 and supramolecular chemistry42,56. In this context, only a few elegant examples on D-A CPPs were reported57,58,59,60,61,62,63 (Fig. 1a). In these reported D-A CPPs, the acceptors were fully embedded into the CPP backbone, which disturbs the electronic structure of the pristine CPPs and consequently leads to a single redshifted emission from the charge transfer (CT) state (Fig. 1a). Therefore, our objective is to obtain a D-A CPP that demonstrates two distinct, tunable emissions from two emissive states: a locally excited (LE) state for blue emission and a CT state for a significantly redshifted emission (as illustrated in Fig. 1). Achieving such a feat faces a significant challenge yet holds great appeal for the development of new applications, for instance, in the fields such as multicolor luminescent materials, particularly in the realm of single-molecule white-light emission.

Fig. 1: Conceptional strategy for designing and constructing D-A CPP nanohoops with dual-emission states.
figure 1

a The D-A CPP nanohoops exhibiting a single-emission in previous work (up) and a tunable dual-emission from the locally excited (LE) and charge transfer (CT) states in this work (bottom). b An effective post-synthesis strategy based on Diels-Alder reaction to synthesize quinone-based D-A [n]CPPs (Nq[10]CPP, Aq[10]CPP and Tq[10]CPP) and the fluorescence emission spectra in CHCl3 in this work. Nq Naphthoquinone, Aq Anthraquinone, Tq Tetracenequinone.

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Multicolor luminescence materials hold significant potential in white light-emitting diodes, flexible displays, information storage and bio-imaging64,65,66,67,68,69,70. The single-molecule organic white-light emission materials, featuring dual-emission peaks spanning 400-700 nm visible light region, exhibit a variety of advantages over multicomponent emitters. These advantages include good light reproducibility, no phase separation and reduced color degradation71,72,73. In principle, dual emission commonly originates from two emissive states: the lowest singlet excited state and one of the excited CT state74,75, the lowest triplet excited state76,77,78, or excimer state79,80, which exhibits a largely redshifted emission. We proposed that by integrating acceptors with adjustable electron-withdrawing capacity into CPP, a single-molecular emitter could be created with inherent and adjustable dual-emission capabilities from LE and CT states when excited at single-wavelength, thus enabling the realization of multicolor including white-light emission (Fig. 1b). We expected that adjusting the LUMO levels of the D-A CPP nanohoop through acceptor engineering would enable us to manipulate the CT state and thus to control the optical properties, which is crucial for creating D-A CPP nanohoops with enhanced optical and electrochemical characteristics.

To realize the aforementioned expectation, 1,4-benzoquinone, 1,4-naphthoquinone and 1,4-anthraquinone have been deliberately chosen as electron acceptors and annulated laterally onto one of benzenes in [10]CPP, rather than spanning across the CPP nanohoop (Fig. 1). This rational design not only retains the structural feature of CPP, but also endows quinone-based [10]CPPs with a CT state, thus leading to the formation of a built-in dual emission from LE and CT states at the single molecular level after single-wavelength excitation. Since quinone acceptors are not compatible with the reductive aromatization that is crucial for the synthesis of CPPs1, a post-modification strategy was adopted to prepare these quinone-based [10]CPP nanohoops. To achieve this goal, thiophene-1,1-dioxide was selected as it is recognized for its versatility and effectiveness as a Diels−Alder diene, capable of engaging in a variety of reactions81,82. Herein, we present the quinone-based acceptor engineering of various D-A CPPs (Nq[10]CPP, Aq[10]CPP and Tq[10]CPP) through post-synthetic strategy based on Diels-Alder reaction of oxTh[10]CPP incorporating thiophene-1,1-dioxide (Figs. 1 and 2). Interestingly, these quinone-based D-A CPP nanohoops demonstrate a distinctive acceptor-dependent dual fluorescence emission in organic solvents when excited at single-wavelength, thus leading to multicolor emission. It is noteworthy that Aq[10]CPP exhibits single-molecular white-light emission in pure CHCl3. Importantly, the ratio of dual-emission intensity in these D-A CPPs can be effectively regulated by multiple factors such as solvent polarity, aggregation states, and redox reaction, thus resulting in a diverse range of fluorescence emissions including white-light emission in single-molecule after single-wavelength excitation. Time-dependent density functional theory (TD-DFT) calculations reveal a significant lowering of the LUMO levels through the precise modifications of acceptors in these D-A nanohoops.

Fig. 2: Synthetic route.
figure 2

Reaction conditions: a Pd(PPh3)4, K2CO3, THF/H2O, 110 °C, overnight; b SnCl2, HCl, THF, r.t., 16 h, 12 % in two steps; c (CH3)2CO, NaHCO3, Oxone, dichloromethane, H2O, -5 °C, 24 h, 33%; d phenyl ether, 200 °C, 2 days, 37%-51%. Th Thiophene. oxTh thiophene-1,1-dioxide.

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Results

Synthesis

Our synthetic strategy involves the synthesis of CPP nanohoop with a thiophene moiety (Th[10]CPP), which can be readily oxidized to yield oxTh[10]CPP. The post-synthesis of oxTh[10]CPP through Diels-Alder reaction with various quinones results in the formation of D-A CPPs (Nq[10]CPP, Aq[10]CPP and Tq[10]CPP). The synthetic procedures for five nanohoops are shown in Fig. 2. Initially, 2,5-dibromothiophene was coupled with C-shaped borate esters 2 by Suzuki-Miyaura cross-coupling in a diluted solution to obtain the macrocyclic precursor 3, which was subsequently treated with the freshly prepared H2SnCl4 in anhydrous tetrahydrofuran at room temperature to afford the macrocycle Th[10]CPP with a yield of 12% over two steps. oxTh[10]CPP was then prepared by oxidizing Th[10]CPP in the presence of prepared dimethyldioxirane, with its structure unequivocally characterized by single crystal X-ray diffraction (Fig. 3a and S1). Finally, Nq[10]CPP, Aq[10]CPP and Tq[10]CPP were synthesized via post-functionalization using Diels-Alder reactions of oxTh[10]CPP with 1,4-benzoquinone, 1,4-naphthoquinone and 1,4-anthraquinone, respectively, in phenyl ether at 200 °C with 37-51% yields for each nanohoop. The structures of these quinone-based [10]CPPs were characterized by 1H NMR, 13C NMR and MALDI-TOF MS (Fig. S47-S66).

Fig. 3: X-ray crystal structures.
figure 3

X-ray crystal structures (left), Herringbone packing model (middle), Long range channel alignment (right) of a Th[10]CPP, b Nq[10]CPP and c Aq[10]CPP; intermolecular noncovalent interactions of d Nq[10]CPP and e) Aq[10]CPP.

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X-ray crystallographic analysis

The structures of Th[10]CPP, Nq[10]CPP and Aq[10]CPP were further confirmed by single-crystal X-ray analysis (Fig. 3, S1–S3 and Table S1-S3). Suitable crystals were obtained by slow diffusion of n-Hexane into dichloromethane solution at room temperature. Crystallographic studies indicate that Th[10]CPP, Nq[10]CPP and Aq[10]CPP feature nearly circular cavities with diameters of about 1.3 nm. All nanohoops adopt a herringbone packing mode, resulting the formation of tubular channels in the solid phase. However, Nq[10]CPP and Aq[10]CPP exhibit distinct packing characteristics as shown in Figs. 3b and 2c. In particular, the quinone units display varying degrees of outward tilt relative to the cavity plane of CPP. For Nq[10]CPP, naphthoquinone moiety is positioned at a dihedral angle of 154° with respect to the cavity plane (Fig. 3b). This configuration promotes π − π stacking interaction between adjacent naphthoquinone moieties, with a contact distance of 3.3 Å. The overlapped area of two neighboring naphthoquinone moieties is estimated to be approximately 61%, indicating a strong π···π interaction (Fig. 3d). In the case of Aq[10]CPP, the anthraquinone unit is tilted at a smaller dihedral angle of 117° relative to the cavity plane, which differs notably from Nq[10]CPP (Fig. 3c). This reduced tilt angle results in the intercalation of anthraquinone moieties within adjacent CPP cavities in the solid phase (Fig. 3e). This arrangement contrasts sharply with Nq[10]CPP, where the π − π stacking between two adjacent naphthoquinone moieties occurs at the outer cavity. Additionally, both Nq[10]CPP and Aq[10]CPP single crystals feature numerous intermolecular hydrogen bonds (C-H···O at distances of 2.523 Å, 2.324 Å and 2.411 Å; C- H···π at distances of 2.599 Å and 2.855 Å) between the quinone and phenylene segments. Furthermore, the unit cells of Th[10]CPP, Nq[10]CPP and Aq[10]CPP belong to the monoclinic P21/c, C2/c and P21 space groups, respectively (Table S1-S3).

Photophysical Properties

The optical properties of five [10]CPP derivatives in this work were examined by UV-Vis absorption spectroscopy, fluorescence emission spectroscopy and time-resolved fluorescence decay (Fig. 4, S5 and S8). The key data were compiled in Table 1. To establish a connection between the electronic structure and photophysical properties, TD-DFT calculations were performed at the MN15/6-31 G*/SMD level. The UV-Vis absorption spectra of the acceptor-based CPPs oxTh[10]CPP, Nq[10]CPP, Aq[10]CPP and Tq[10]CPP in chloroform displayed comparable absorption maxima (λmax) around 329-343 nm, accompanying by a red-shifted broad band ranging from 430 nm to 620 nm (Fig. 4a). These absorption maxima were very close to that of [10]CPP (338 nm)3. According to the DFT calculations, the main absorption peak can be attributed to the HOMO-1 → LUMO transition for oxTh[10]CPP at 343 nm, HOMO-1 → LUMO + 1 and HOMO → LUMO + 2 transitions for Nq[10]CPP at 338 nm, HOMO-1 → LUMO + 1 and HOMO → LUMO + 3 transitions for Aq[10]CPP at 339 nm, and HOMO → LUMO + 3 and HOMO-1 → LUMO + 2 transitions for Tq[10]CPP at 329 nm (Fig. 5). The red-shifted broad bands in these spectra can be ascribed to the characteristic absorption peaks of the ICT states mainly associated with the transitions of HOMO-2 → LUMO and HOMO → LUMO, indicating the formation of D-A CPPs (Table S11 and S15). Additionally, as shown in Fig. 5, the HOMOs of oxTh[10]CPP, Nq[10]CPP, Aq[10]CPP and Tq[10]CPP were predominantly located on the donor phenylene backbone, while the LUMOs were mainly situated on thiophene-1,1-dioxide and quinone acceptors, respectively. Notably, despite the HOMO energies of these D-A CPPs being around 5.830 eV, close to that of [10]CPP, the LUMO energies of these D-A CPPs are obviously lower: -1.880 eV for oxTh[10]CPP, -1.967 eV for Tq[10]CPP, -2.074 eV for Aq[10]CPP, and -2.310 eV for Nq[10]CPP. These values are significantly lower than that of [10]CPP (-1.072 eV), leading to a narrower HOMO-LUMO band gap in these D-A nanohoops (Table S18). However, the UV-Vis absorption spectrum of Th[10]CPP only showed a main absorption peak at 341 nm, attributed to the transitions of HOMO-2 → LUMO and HOMO → LUMO + 2 (Table S11). Furthermore, based on the onset of the longest wavelength absorption (λonset), the optical gaps of Th[10]CPP, oxTh[10]CPP, Nq[10]CPP, Aq[10]CPP and Tq[10]CPP were estimated to be 2.87, 2.34, 2.02, 2.24 and 2.31 eV, respectively (Table 1). This trend of the optical gap variation aligned with the theoretical calculations (Table S18).

Fig. 4: Photophysical Properties.
figure 4

The a UV-Vis spectra, b Fluorescence spectra of Th[10]CPP, oxTh[10]CPP, Nq[10]CPP, Aq[10]CPP and Tq[10]CPP (1.0 × 10−5 M) in CHCl3, Pictures of Th[10]CPP, oxTh[10]CPP, Nq[10]CPP, Aq[10]CPP and Tq[10]CPP in CHCl3 c at ambient light and d at λ = 365 nm.

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Table 1 The photophysical and electrochemical data of Th[10]CPP, oxTh[10]CPP, Nq[10]CPP, Aq[10]CPP and Tq[10]CPP
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Fig. 5: Theoretical calculations.
figure 5

Energy diagrams and pictorial representations of the frontier molecular orbitals for a oxTh[10]CPP, b Nq[10]CPP, c Aq[10]CPP and d Tq[10]CPP calculated by TD MN15/6-31 G*/SMD. The values of f represent the oscillator strengths.

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Subsequently, the fluorescence emission spectroscopies in solution were also conducted (Figs. 4b and 4d). Interestingly, in chloroform, the emission spectrum of Aq[10]CPP exhibits two obvious peaks with comparable intensities at 460 and 628 nm, resulting in the emission of a near white light with a CIE coordinate (0.33, 0.25). Similarly, Tq[10]CPP displayed a strong emission peak at 465 nm, along with a redshifted peak at 598 nm with lower intensity, leading to sky blue emission. Unexpectedly, Nq[10]CPP exhibited a main emission peak at 465 nm, no obvious redshifted emission was detected. This phenomenon can be attributed to the fact that 1,4-naphthoquinone moiety in Nq[10]CPP is more exposed to solvent than other quninone units in Aq[10]CPP and Tq[10]CPP, which renders the redshifted emission of Nq[10]CPP more sensitive to the polarity of solvents. Pleasingly, the redshifted emission of Nq[10]CPP was observed in a nonpolar solvent (vide infra). It is noteworthy that the quinone-based [10]CPPs exhibit distinct characteristics of a built-in dual emission at the single molecular level after single wavelength excitation, which is sharply different from the other reported D-A [n]CPPs57,58,59,60,61,62,63. It is worth noting that the emission wavelengths of Nq[10]CPP (465 nm), Aq[10]CPP (460 nm) and Tq[10]CPP (465 nm) at blue wavelength are all close to that of [10]CPP (466 nm)3. Accordingly, the emission at blue wavelength can be attributed to the LE state, similar to [10]CPP. Furthermore, theoretic calculations of excited-states revealed that the quinone-based [10]CPPs possess dual emission from LE and CT states. For instance, the blue emission peak of Aq[10]CPP at 460 nm is attributed to transitions from LUMO + 2 → HOMO and LUMO + 1 → HOMO-1, corresponding to the LE state of oligophenylene backbone, while the redshifted emission peak at 628 nm is assigned to the transition of LUMO → HOMO, which is attributed to the CT state between oligophenylene backbone and anthraquinone (Table S17). The same argument holds for Tq[10]CPP and Nq[10]CPP. In contrast, oxTh[10]CPP exhibits a single emission peak at 464 nm in chloroform without a redshifted emission peak even in nonpolar solvents, indicating the absence of CT state (Figure S6). Additionally, Th[10]CPP also displays one peak at 493 nm. Theoretic calculations revealed that the emission at 464 nm of oxTh[10]CPP primarily arises from the LUMO + 1 → HOMO transition, while the emission at 493 nm of Th[10]CPP is mainly due to the LUMO → HOMO-1 transition. These transitions are attributed to the LE state but no transition for CT states was revealed for Th[10]CPP and oxTh[10]CPP (Table S13). In addition, the fluorescence transient decay spectra of Th[10]CPP and oxTh[10]CPP only showed a prompt decay with a lifetime of 1.3 ns and 2.6 ns, respectively (Table 1 and Figure S5). While the quinone-based [10]CPPs exhibited a double-exponential decay feature, composing of a short-lived species with a lifetime of about 0.5 ns and a long-lived species with a lifetime of about 4.0 ns (Table 1 and Figure S8). The fluorescence quantum yields of Th[10]CPP, oxTh[10]CPP, Nq[10]CPP, Aq[10]CPP and Tq[10]CPP were measured and listed in Table 1. Furthermore, phosphorescence emissions of Nq[10]CPP, Aq[10]CPP and Tq[10]CPP were observed at about 650 nm for Aq[10]CPP and Tq[10]CPP at 77 K in 2- methyltetrahydrofuran (Figure S10-S12), with lifetimes of 15.8 and 30.5 ms, respectively. In contrast, no phosphorescence was detected for Nq[10]CPP under the same conditions.

Given Nq[10]CPP, Aq[10]CPP and Tq[10]CPP exhibit the D-A characteristics of CT state, solvatofluorochromic behavior were expected. Subsequently, fluorescence spectra of Nq[10]CPP, Aq[10]CPP and Tq[10]CPP were further measured in various solvents including tetrachloromethane (CCl4), tetrahydrofuran (THF) and dimethyl sulfoxide (DMSO) (Figure S9 and Table S6). Aq[10]CPP shows obvious dual-emissions at 453 nm and 580 nm in nonpolar solvent CCl4 with an intensity ratio of the redshift emission to the blue one to be 1.6 (Table S8). In THF, the blue emission wavelength of Aq[10]CPP did not exhibit obvious changes, while the emission peak at longer wavelength displays a redshift to 600 nm, and the intensity ratio of the dual-emission decreases to 0.4 (Table S8). Variations in the dual-emission intensities caused by the polarity of solvents eventually led to the multicolor emissions, including pale purplish-pink, white and blue (Figure S9e and S9f). Similarly, Nq[10]CPP and Tq[10]CPP also display two emission peaks in CCl4 (Figure S9b and S9h). In THF, Tq[10]CPP show a main peak at 465 nm accompanying with a shoulder peak at 600 nm. However, the emission peak of Nq[10]CPP at longer wavelength is quenched. Additionally, the emissions of these CPPs at longer wavelengths were all markedly quenched in DMSO. These observations further support the fact that the emission peaks at blue wavelength are attributed to the LE state, whereas the peaks at redshifted wavelength are associated with the CT state, which is notably influenced by the solvent polarity. Furthermore, we also calculated the dipole moments of excited states using the Lippert Mataga formalism, thereby verifying the presence of the LE and CT states (Table S19-S24 and Figure S42-S46).

Aggregations in THF/H2O and the Film of PMMA

Encouraged by the aforementioned results, we were motivated to explore the potential of D-A nanohoops as a single-molecular emitter for full-color emission, including white-light emission upon aggregation. The luminescence properties were further investigated in THF with different water fractions (fw) (Fig. 6). As shown in Fig. 6b, Aq[10]CPP exhibits emission peaks at 457 nm and 600 nm in pure THF. As the water content increased from 0% to 50%, the fluorescence intensity at 457 nm gradually decreased, while that at 600 nm remained relatively constant. When the water content exceeds 53%, the intensity of emission peak at 600 nm steadily increases as the water fraction rises. The maximum emission was observed at fw = 80 %, eventually accompanying with disappearance of the emission peak at 464 nm. Moreover, the fluorescent color of solution transitions from blue to orange, covering a region near white light with a CIE coordinate of (0.33, 0.26) at fw = 55%. Furthermore, the Tyndall effect, which refers to the scattering of light by particles suspended in a colloid or fine suspension83, was observed in our experiments. Specifically, the Tyndall phenomena were detected as the water fraction increases from 50% to 90%, indicating that aggregation occurred in the mixed solvents (Figure S17). Moreover, the dynamic light scattering (DLS) investigations suggested that nanoparticles were formed with sizes ranging approximately from 111 nm to 1793 nm, dependent on the water fractions (Figure S18). As shown in Figs. 6a and 6c, the fluorescence spectra of Nq[10]CPP and Tq[10]CPP follow a similar trend. As the water content increased, the fluorescence intensity of Nq[10]CPP decreased at 465 nm, while a new emission peak appeared at 667 nm with enhanced fluorescence intensity. For Tq[10]CPP, it shows a major emission peak at 465 nm accompanying with a shoulder peak at 600 nm in pure THF. At fw = 0%-70%, the fluorescence intensity at 465 nm decreased markedly, whereas the intensity at 600 nm showed little variation. When the water fraction is over 70%, the emission intensity at 600 nm is obviously enhanced. These spectral changes in the mixture of THF/H2O lead to the significant changes in the emission color from blue to red for Nq[10]CPP and from blue to orange for Tq[10]CPP, respectively. The Tyndall phenomena were also observed when the water fractions were beyond 70% and 60% for Nq[10]CPP and Tq[10]CPP, respectively. The formations of nanoparticles were further demonstrated by DLS experiments (Figure S15-S16 and S19-S20). More importantly, Tq[10]CPP show the dual-emissive feature and emit white light with the CIE coordinates of (0.33, 0.28) at fw = 72 %. As for oxTh[10]CPP, it exhibits blue emission at 464 nm in pure THF (Fig. 6d). When the water fraction exceeds 55 %, a new emission peak emerges at 575 nm, with its intensity steadily increasing while the emission intensity at 464 nm decreases. The appearance of a new peak at 575 nm is also associated with the Tyndall phenomenon and the formation of nanoparticles as indicated by DLS (Figure S13-S14). Simultaneously, the fluorescent color of solution changes significantly from blue to yellow, covering a region near white light with a CIE coordinate of (0.32, 0.30) at fw = 60%. The intensity ratios of the redshift emission to the blue one in all these CPPs can be precisely adjusted through aggregation, ranging from 0 (fw = 0 %) to a maximum of 70 (fw = 90 %) (Fig. 6, middle). For quinone-based CPPs, the spectral changes observed in THF/H₂O indicate that aggregation substantially facilitates the emission of CT states. As for oxTh[10]CPP, the emission at red-shifted wavelength after aggregation is speculated to the formation of the intermolecular charge transfer.

Fig. 6: Aggregations in THF/H2O.
figure 6

Fluorescence emission spectra with different water fraction in THF solvent (left), Curve of fluorescent intensity ratio of the redshifted emission to the blue one vs. water fraction. Insert: photos of changes in fluorescence color at λ = 365 nm (middle), and CIE 1931 chromaticity diagram in THF/H2O mixtures (right) of (a) Nq[10]CPP (1.0 × 10−5 M, λex = 338 nm), b Aq[10]CPP (5.0 × 10−6 M, λex = 331 nm), c Tq[10]CPP (5.0 × 10-6 M, λex = 332 nm), d oxTh[10]CPP (1.0 × 10−5 M, λex = 343 nm).

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In order to study the morphology of aggregated [10]CPPs, scanning electron microscopy (SEM) was employed (Fig. 7 and S21-S24). SEM images demonstrated that the morphologies of the nanoparticles varied as the water fraction was adjusted. For Aq[10]CPP, fibrous aggregates initially form at 50% water fraction (Figure S19a). At 90% water fraction, the morphology is characterized by smooth, angular regular polyhedra (Fig. 7d). As the water fraction increases, the morphologies of Nq[10]CPP, Tq[10]CPP and oxTh[10]CPP transition from sphere to square and finally to polyhedron. (Figure S21, S22 and S24).

Fig. 7: Scanning electron microscopy.
figure 7

The SEM images of a Nq[10]CPP in THF/water (20:80, v/v, fw = 80 %); b Nq[10]CPP in THF/water (5:95, v/v, fw = 95 %); c Aq[10]CPP in THF/water (45:55, v/v, fw = 55 %); d Aq[10]CPP in THF/water (5:95, v/v, fw = 90 %); e Tq[10]CPP in THF/water (28:72, v/v, fw = 72 %); f) Tq[10]CPP in THF/water (10:90, v/v, fw = 90 %). g oxTh[10]CPP in THF/water (40:60, v/v, fw = 60 %); h) oxTh[10]CPP in THF/water (10:90, v/v, fw = 90 %).

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Next, we explored solid-state emission through film fabrication (Fig. 8 and Table 1). We prepared the films by dispersing 5.0 wt.% weight fractions of each nanohoop into the poly(methylmethacrylate) (PMMA) matrix. The emission spectra of the acceptor-based [10]CPPs in films exhibited significant differences compared to those observed in solution. Particularly, the quinone-based [10]CPP display dual-emission peaks at 440 and 662 nm for Nq[10]CPP, 446 and 600 nm for Aq[10]CPP and 462 and 568 nm for Tq[10]CPP. Moreover, compared to those in solution, the emissions at the longer wavelength were notably enhanced in the PMMA films, with an intensity ratio of the redshift emission to the blue one to be 0.8, 8.2 and 2.1 for Nq[10]CPP, Aq[10]CPP and Tq[10]CPP, respectively (Table S8). Upon excitation at 365 nm, the films of Nq[10]CPP, Aq[10]CPP and Tq[10]CPP emit red, orange and yellow emission, respectively. On the other hand, oxTh[10]CPP exhibits a red-shifted emission at 570 nm, producing a yellow color. In contrast, Th[10]CPP demonstrates a green emission at 487 nm, similar to its behavior in solution. These findings clearly indicate that manipulating acceptors in these D-A [10]CPPs readily enables the achievement of multicolor, including white-light emission both in isolated and aggregated states.

Fig. 8: Emission spectra and colors in PMMA films.
figure 8

a The Fluorescence spectra of 5.0 wt.% Th[10]CPP, oxTh[10]CPP, Nq[10]CPP, Aq[10]CPP and Tq[10]CPP / PMMA films, b Pictures of Th[10]CPP, oxTh[10]CPP, Nq[10]CPP, Aq[10]CPP and Tq[10]CPP in PMMA films at λ = 365 nm.

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Electrochemical Properties and Redox

The electrochemical properties were investigated by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) (Figure S25 and Table S9). Th[10]CPP exhibits irreversible oxidation processes at 0.77 V and two reduction processes with quasi-reversible peaks at -1.65 and -2.41 V. oxTh[10]CPP exhibits an oxidation peak at 1.05 V and two irreversible reduction processes at -1.55 and -2.43 V. As for quinone-based [10]CPPs, irreversible oxidation peaks were observed for Nq[10]CPP at 0.99 V, Aq[10]CPP at 0.78 V and Tq[10]CPP at 0.64 V (Figure S25), indicating an obvious acceptor-dependent oxidation potentials. On the other hand, two reduction peaks were detected at about -1.50 V and -2.00 V, attributable to the introduction of quinone. Given the facile reducibility of quinone, its reduction can serve as a stimulus for effectively modulating the properties of the quinone-based [10]CPPs. Subsequently, Aq[10]CPP was selected and chemically reduced using NaBH4 in a mixed solution of THF and MeOH to produce red-Aq[10]CPP containing anthracenediol (characterized by 1H NMR, Figure S57). The modulation of optical properties through chemical reduction was carried out by the UV-Vis absorption spectroscopy and fluorescence emission spectroscopy in CHCl3 (Fig. 9). The absorption spectrum of red-Aq[10]CPP exhibits a prominent peak at 334 nm, while the absorption band at the range from 440 to 620 nm corresponding to the ICT state is absent compared to Aq[10]CPP. Simultaneously, the solution undergoes a color transition from orange to light yellow. Meanwhile, the red-Aq[10]CPP exhibits a maximum emission peak at 460 nm, very close to the LE emission in Aq[10]CPP, while the emission of CT state at 629 nm becomes imperceptible, resulting in a significant change in fluorescence color from white to blue.

Fig. 9: Photophysical properties of Aq[10]CPP upon reduction.
figure 9

a The UV-Vis spectra and Fluorescence emission spectra of Aq[10]CPP and red-Aq[10]CPP in CHCl3. b Pictures of Aq[10]CPP and red-Aq[10]CPP in CHCl3 under b ambient light and c UV light at λ = 365 nm. red: reduction.

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Discussion

In summary, a series of quinone-based [10]CPPs (Nq[10]CPP, Aq[10]CPP and Tq[10]CPP) have been successfully synthesized by an effective post-synthetic strategy using Diels-Alder reaction of oxTh[10]CPP. This versatile synthetic strategy, facilitated by the high reactivity of thiophene-1,1-dioxides, offers significant advantages in the efficient synthesis of various functionalized CPP nanohoops. Single-crystal analysis reveals that different acceptors allow for various intermolecular interactions between CPP nanohoops, leading to diverse stacking modes. Experimental results and theoretical calculations indicate that quinone-based [10]CPP nanohoops exhibit distinct D-A characteristics with a narrowed HOMO-LUMO band gap. Moreover, their photophysical properties can be regulated by the acceptor quinone. All quinone-based CPPs exhibit inherent dual-emission in purely organic solvents, attributed to the LE and CT states. It is noteworthy that Aq[10]CPP shows single-molecular white-light emission in CHCl3. Furthermore, the intensity ratio of the dual-emission peaks can be adjusted by solvent polarity, aggregation and chemically reduction, emitting a wide range of fluorescent colors, including white light. We anticipate this study can provide valuable insights into the efficient synthesis of nanohoops with a broad range of properties and applications.

Methods

All starting chemicals were obtained from commercial sources and used without further purification, unless indicated otherwise. All reactions were performed with dry solvents under Argon in dried glassware with standard vacuum-line techniques. Anhydrous THF and 1,4-dioxane were obtained from the Solvent Purification System. NMR spectra were obtained with JEOL Delta (400 MHz and 600 MHz) spectrometer at 298 K, using dichloromethane-d (CD2Cl2) and chloroform-d (CDCl3) as solvent. Mass spectra of MALDI-TOF were acquired on Bruker Daltonics Autoflex III. UV-Vis spectra were recorded on Shimadzu UV-2450. Fluorescence and phosphorescence spectra were measured on FS5 and Edinburg FLS-980 fluorescence spectrometer. The absolute fluorescence quantum yield was measured by using an absolute PL quantum yield spectrometer (Quantaurus-QY C11347-11). Scanning electron microscopy (SEM) was performed on a Hitachi SU-8000 with an accelerating voltage of 5 kV. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) experiments were carried out at room temperature with an electrochemical workstation from Chenhua Instruments Co. (Shanghai, China). Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were measured with a glass carbon electrode for THF solution of a sample (1 mM) in the presence of a supporting electrolyte (Bu4NPF6, 0.1 M) under nitrogen atmosphere and the scan rates at 100 mV/s. The potential was externally calibrated against the Fc/Fc+ couple versus a Ag electrode.

Synthesis of Th[10]CPP

To a dried 250 mL flask containing a magnetic stirring bar were added 1 (50 mg, 0.21 mmol, 1.0 eq), 2 (237 mg, 0.21 mmol, 1.0 eq), Pd(PPh3)4 (48 mg, 0.04 mmol, 0.2 eq) and K2CO3 (4.1 g, 30 mmol, 144.2 eq). The flask was evacuated and filled with argon three times. Dry THF (45 mL) and the degassed distilled water (15 mL) was added via syringe. Then, the mixture was deoxygenated and freeze-pump-thawed (3×) and then was allowed to react at 110 °C overnight. After the reaction mixture was cooled, the mixture was extracted with ethyl acetate, washed with water and saturated brine. The combined organic phase was dried over sodium sulfate and consequently concentrated under reduced pressure. The crude product was initially purified by column chromatography on silica gel (petroleum ether/ethyl acetate = 4/1, v/v) to afford a yellow solid as the crude macrocyclic precursor that was used in the next step without further purification.

A H2SnCl4/THF solution was freshly prepared by dissolving anhydrous SnCl2 (217 mg, 1.15 mmol) in 15 mL anhydrous THF under Ar atmosphere and then adding concentrated HCl (aq) (3 mL) to the solution. The resulting solution was deoxygenated and stirred for 30 min. Crude product (91 mg) was dissolved in anhydrous THF (38 mL) under the Ar atmosphere and the freshly prepared H2SnCl4/THF solution was added to this solution. The reaction mixture was stirred at room temperature for 24 h before being quenched with NaOH/H2O solution. The aqueous layer was extracted with dichloromethane and the organic layers were combined and dried with anhydrous sodium sulfate. After purifying by column chromatography on silica gel (petroleum ether/CH2Cl2 = 2/1, v/v) to get Th[10]CPP as a yellow solid (Yield: 19 mg, 12 % in two steps). M.p. > 300 °C. 1H NMR (600 MHz, CDCl3, 298 K) δ: 7.59-7.55 (m, 28H), 7.53 (d, J = 8.6 Hz, 4H), 7.40 (d, J = 8.6 Hz, 4H), 7.17 (s, 2H). 13C NMR (150 MHz, CDCl3, 298 K) δ: 148.0, 139.2, 138.6, 138.5, 138.3, 138.3, 133.7, 127.6, 127.6, 127.5, 127.4, 122.5. HR-MS (MALDI-TOF) calculated for C58H38S M: 767.2694, found 767.2773.

Synthesis of oxTh[10]CPP

(Method A) Th[10]CPP (50 mg, 0.064 mmol, 1.0 eq) was dissolved in acetone-H2O-CH2Cl2 (1:1:1, 18 mL) in a two-necked 50 mL glass round bottom flask and added NaHCO3 (862 mg, 10.26 mmol, 158.4 eq). The solution was stirred at -5 °C for 15 min. Then, the oxone (881.2 mg, 1.44 mmol, 22.1 eq) was added in 5 times and stirred at -5 °C for 14 h. The reaction was quenched with NaHSO3, and extracted with dichloromethane. The organic layer was washed with brine and dried over anhydrous sodium sulfate. After purifying by column chromatography on silica gel (petroleum ether/CH2Cl2 = 1/2, v/v) to get oxTh[10]CPP as an orange solid (Yield: 16.9 mg, 33 %).

(Method B) To a dried 5 mL Schlenk containing a magnetic stirring bar was added Th[10]CPP (10 mg, 0.013 mmol, 1.0 eq) and the flask was evacuated and filled with argon three times. Subsequently, CH2Cl2 (0.5 mL), HCO2H (50 µL) and H2O2 (100 µL, aq. 30%) were added via syringe at -5 °C and stirred for 1 h. Then, the mixture was allowed to react at 50 °C overnight. The reaction was extracted with dichloromethane. The organic layer was washed with brine and dried over anhydrous sodium sulfate. After purifying by column chromatography on silica gel (petroleum ether/CH2Cl2 = 1/2, v/v) to get oxTh[10]CPP as an orange solid (Yield: 2.5 mg, 24 %). M.p. > 300 °C. 1H NMR (600 MHz, CDCl3, 298 K) δ 7.63 (d, J = 8.8 Hz, 4H), 7.58 (d, J = 6.5 Hz, 28H), 7.53 (d, J = 8.8 Hz, 4H), 6.87 (s, 2H). 13C NMR (150 MHz, CDCl3, 298 K) δ: 143.2, 141.4, 139.3, 138.7, 138.4, 138.3, 138.2, 138.1, 137.8, 128.9, 127.9, 127.7, 127.7, 127.6, 125.9, 119.6. HR-MS (MALDI-TOF) calculated for C58H38O2S [M + H]+: 799.2593, found 799.2669.

Synthesis of Nq[10]CPP

To a dried 5 mL Schlenk containing a magnetic stirring bar were added oxTh[10]CPP (19 mg, 0.02 mmol, 1.0 eq), 1,4-benzoquinone (26 mg, 0.2 mmol, 10.0 eq) and the flask was evacuated and filled with argon three times. Subsequently, phenyl ether (400 µL) was added via syringe. Then, the mixture was allowed to react at 200 °C for 2 days. After cooling to r.t, the reaction mixture was poured into hexane and the precipitate was collected. Dissolve the precipitate in dichloromethane. After purifying by column chromatography on silica gel (petroleum ether/CH2Cl2 = 1/4, v/v) to get Nq[10]CPP as a red solid (Yield: 6.2 mg, 37 %). M.p. > 300 °C. 1H NMR (600 MHz, CD2Cl2, 298 K) δ: 7.59–7.58 (m, 28H), 7.55 (d, J = 8.6 Hz, 4H), 7.29 (d, J = 8.6 Hz, 4H), 7.05 (s, 2H), 6.97 (s, 2H). 13C NMR (150 MHz, CD2Cl2, 298 K) δ: 185.6, 142.1, 141.3, 139.1, 139.1, 138.8, 138.7, 138.6, 138.5, 138.5, 130.5, 129.6, 127.8, 127.8, 127.7, 127.7, 127.4. HR-MS (MALDI-TOF) calculated for C64H40O2 M: 840.3028, found 840.3027.

Synthesis of Aq[10]CPP

To a dried 5 mL Schlenk containing a magnetic stirring bar were added oxTh[10]CPP (17 mg, 0.02 mmol, 1.0 eq), 1,4-Naphthoquinone (32 mg, 0.2 mmol, 10.0 eq) and the flask was evacuated and filled with argon three times. Subsequently, phenyl ether (400 µL) was added via syringe. Then, the mixture was allowed to react at 200 °C for 2 days. After cooling to r.t, the reaction mixture was poured into hexane, and the precipitate was collected. Dissolve the precipitate in dichloromethane. After purifying by column chromatography on silica gel (petroleum ether/CH2Cl2 = 1/4, v/v) to get Aq[10]CPP as an orange solid (Yield: 9.0 mg, 51 %). M.p. > 300 °C. 1H NMR (600 MHz, CD2Cl2, 298 K) δ: 8.23-8.22 (m, 2H), 7.83-7.82 (m, 2H), 7.60–7.56 (m, 32H), 7.33 (d, J = 8.6 Hz, 4H), 7.11 (s, 2H). 13C NMR (150 MHz, CD2Cl2, 298 K) δ 184.0, 142.9, 142.1, 138.9, 138.8, 138.8, 138.7, 138.6, 138.6, 138.5, 134.5, 134.3, 132.6, 129.5, 127.8, 127.8, 127.8, 127.7, 127.5, 127.3. HR-MS (MALDI-TOF) calculated for C68H42O2 M: 890.3185, found 890.3173.

Synthesis of Tq[10]CPP

To a dried 5 mL Schlenk containing a magnetic stirring bar were added oxTh[10]CPP (11 mg, 0.01 mmol, 1.0 eq), 1,4-Anthraquinone (29 mg, 0.1 mmol, 10.0 eq) and the flask was evacuated and filled with argon three times. Subsequently, phenyl ether (300 µL) was added via syringe. Then, the mixture was allowed to react at 200 °C for 2 days. After cooling to r.t, the reaction mixture was poured into hexane and the precipitate was collected. Dissolve the precipitate in dichloromethane. After purifying by column chromatography on silica gel (petroleum ether/CH2Cl2 = 1/4, v/v) to get Tq[10]CPP as an orange solid (Yield: 4.0 mg, 43 %). M.p. > 300 °C. 1H NMR (600 MHz, CD2Cl2, 298 K) δ: 8.77 (s, 2H), 8.15-8.14 (m, 2H), 7.74-7.72 (m, 2H), 7.60-7.56 (m, 32H), 7.36 (d, J = 8.7 Hz, 4H), 7.14 (s, 2H). 13C NMR (150 MHz, CD2Cl2, 298 K) δ: 184.0, 143.0, 142.2, 138.9, 138.8, 138.6, 135.6, 133.8, 131.0, 130.4, 129.8, 129.4, 129.4, 127.8, 127.7, 127.5. HR-MS (MALDI-TOF) calculated for C72H44O2 M: 940.3341, found 940.3332.