# Circularly Polarized Luminescence in Rotaxanes and Catenanes

Arthur H. G. David, Juan Cuerva, Victor Blanco

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► To cite this version:

Arthur H. G. David, Juan Cuerva, Victor Blanco. Circularly Polarized Luminescence in Rotaxanes and Catenanes. ChemistryEurope, 2025, pp.e202400107. 10.1002/ceur.202400107. [http://dx.doi.org/10.1002/ceur.202400107](http://dx.doi.org/10.1002/ceur.202400107)

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# Circularly Polarized Luminescence in Rotaxanes and Catenanes

Arthur H. G. David,*[a] Juan M. Cuerva,[b] and Victor Blanco*[b]

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Rotaxanes and catenanes have proven to be key structures in the development of artificial molecular machines with a variety of applications. Among them, the study and modulation of optical properties, mainly light emission, have been widely explored. The growing interest in chiral mechanically interlocked molecules has enabled this research interest to be extended to chiroptical properties, in particular to the study of circularly polarized luminescence. In this review, we discuss the

progress on this topic, from seminal examples to the growing number of examples reported in very recent years. Both rotaxanes and catenanes with a fixed or static CPL response are reviewed, but also designs that exploit the switching mechanisms enabled by these interlocked architectures to modulate and switch their CPL response both in solution and/or in the aggregate or solid state.

# 1. Introduction

In the same way chiral molecules can preferentially absorb left- or right-handed circularly polarized light, giving rise to the circular dichroism process (or electronic circular dichroism (ECD) when UV-vis light is involved), non-racemic chiral fluorophores can exhibit the preferential emission of one hand of circularly polarized light, giving rise to the Circularly Polarized Luminescence (CPL) phenomenon.[1] Therefore, ECD and CPL can be considered as two sides of the same coin, since the former is related to the $S_0 \rightarrow S_n$ absorption electronic transitions, while the latter appears for the $S_n \rightarrow S_0$ emission transitions, usually the $S_1 \rightarrow S_0$ transition in organic molecules. Thus, while ECD can be evaluated by the $g_{abs}$ dissymmetry factor, the CPL response is usually characterized in terms of the emission dissymmetry factor $g_{lum}$, defined as $g_{lum} = 2(I_L - I_R)/(I_L + I_R)$, where $I_L$ and $I_R$ are the intensities of the circularly polarized light of each handedness.[2] However, a relevant difference between ECD and CPL is that the differential emission (Δl) is not a valid parameter for evaluating the CPL of different species as the excitation source determines the emission intensity, while in ECD there is no such dependence.

The study of CPL has grown in parallel with the development of suitable instrumentation from the home-made modified fluorimeters used in the early days to the commercial instruments available nowadays. Among the different possible layouts,[1b] we can consider two main setups. Both rely on the measurement of the light emitted by the chiral species when irradiated with unpolarized light, but they differ in the angle at which the emission is recorded, which can be 90°, like in traditional fluorimeters, or 180°. The first setup is useful for homogeneous isotropic solutions, while the second one minimizes some artifacts, such as photoselection, that can

appear when there is anisotropy in the media, i.e. aggregates, supramolecular systems, liquid crystals or in the solid state (films, pellets). The circularly polarized emission is then converted in both cases to linearly polarized light by a photoelastic modulator (PEM), selected by a linear polarizer and collected by a detector. This way of processing the emitted light is still a limitation of the technique, as any source of linear polarized light introduces a contribution that can alter or even mask the true CPL signal, giving for example signals for two enantiomers that are not mirror images or even show the same sign. This problem is magnified by the weak CPL response in terms of $g_{lum}$ usually displayed by organic molecules, which represents another issue the CPL instrumentation must deal with. This limitation is usually solved with long acquisition times, which is a disadvantage in many applications and explains the continuous efforts to find organic emitters with high $g_{lum}$ values.

In recent years, the study of this chiroptical property has experienced a great rise, gaining much attention by scientists in different fields such as organic and inorganic chemistry, photo-physics, and materials chemistry. As a result, CPL has come to the forefront as an attractive property due to its potential in a variety of applications, such as new smart sensing methodologies[3] and microscopies,[4] or anticounterfeiting materials.[5] All these applications rely on the additional degree of freedom and additional information associated with the circular polarization opening new possibilities. Thus, the analysis of CPL could allow the detection of a chiral emissive analyte in the presence of other achiral fluorophores that would give a fluorescence background, CPL-active probes could be used in bioimaging and labeling of biomolecules if suitable instruments can detect their CPL signal or reinforce the security against counterfeiting by using security inks that include fluorophores with CPL responses, which could be distinguished from other achiral fluorescent materials even with the same emission profile. CPL-OLEDs are usually mentioned as well as another potential application of CPL.[6] For instance, CPL-OLEDs can be envisaged as an interesting alternative to avoid the loss of light intensity associated with antiglare systems. A design used is based on the conversion of incident light into circularly polarized light of one hand. With a suitable filter, this light is suppressed but also is the emission of the OLED, as one of its hands is absorbed. As a possible solution, thermally activated delayed fluorescence (TADF) electroluminescent CPL-OLEDs of the suitable handedness with $g_{lum}$ values as close as possible to 2 would result in a much lower light intensity loss.

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© 2025 The Author(s). ChemistryEurope published by Chemistry Europe and Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

<footnote>[a] Dr. A. H. G. David
Institut des Sciences Chimiques de Rennes (ISCR) – UMR 6226
Université de Rennes, CNRS
263 Av. Général Leclerc, 35700 Rennes, France
E-mail: arthur.david@univ-rennes.fr</footnote>

<footnote>[b] Prof. J. M. Cuerva, Dr. V. Blanco
Departamento de Química Orgánica, Unidad de Excelencia de Química (UEQ)
Universidad de Granada
Facultad de Ciencias, Avda. Fuente Nueva, s/n, 18071, Granada, Spain
E-mail: victorblancos@ugr.es</footnote>
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Thus, the development of CPL-active small molecules[7] or materials[8] based on organic or inorganic structures is a state-of-the-art research topic. In this context, there is an ongoing quest in this field for the development of structures with good CPL activity, as well as for the design of synthetic routes as efficient, direct and versatile as possible, aspects that still represent a challenge in many cases and where innovative approaches are yet sought. Moreover, not only the access to structures with good CPL response is relevant but also the development of systems that would also allow its modulation.[3] Thus, CPL switching is also of great interest for the development of functional devices.

The synthesis and study of [n]rotaxanes[9] and
[n]catenanes[10] (being n the number of components) (Fig-
ure 1a), two prominent members of the family of mechanically
interlocked molecules, have received great interest for the last
30 years.[11] One of the reasons is the possibility that these
interlocked structures offer to control their co-conformation, i.e.
the relative position or orientation of their linear and/or
macrocyclic components, through the application of external
stimuli (Figure 1b–d). This stimuli-induced switching allows, in
many cases, the modulation of their properties or carrying out a
certain task, which was profusely exploited in the development
of a flourishing number of molecular machines based on those
architectures.[12] Despite the huge advance in this topic, the
study of chiroptical properties, especially CPL, in rotaxanes and
catenanes received much less attention.[13] In this sense, the
higher complexity of chiral interlocked molecules could be
blamed as one of the reasons for this slower development.
However, the growing recent attention in chiral rotaxanes and
catenanes,[14] including those with types of chirality that arise

<figure><img src="image_8.png" /><figcaption>**Figure 1.** a) Cartoon representation of the structure of a [2]rotaxane (left) and a [2]catenane (right); b) Stimuli-induced translational movement (shuttling) of the macrocycle and c) pirouetting motion in a [2]rotaxane; d) pirouetting in a [2]catenane.</figcaption></figure>

from their interlocked nature, together with the prominent
interest in the study of structures with CPL and their
applications have led to a very recent but promising takeoff of
the research topic of CPL-active rotaxanes and catenanes.

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Dr. Arthur H. G. David earned his Ph.D. at the University of Granada, Spain, under the supervision of Drs. Victor Blanco and Araceli G. Campaña, investigating CPL emissive rotaxanes and the supramolecular chemistry of saddle-shaped nanographenes. He then traveled to Northwestern University, USA, where he entered Sir Fraser Stoddart's team to study fluorescent catenanes, pumps, and cyclophanes. Afterward, he joined the University of Angers, France, working with Drs. Antoine Goujon and Philippe Blanchard, designing novel molecules for organic electronics. Since November 2024, he has continued his research in supramolecular chemistry as a CNRS researcher at the ISCR in Rennes, France.

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Graduated in Chemistry from University of
Granada (UGR) (1992) in two specialties:
Organic and Technical Chemistry. He got his
PhD from Autonomous University of Madrid in
1997 under the supervision of Prof. Echavar-
ren. In 1998, he joined UGR, where he
currently is Full Professor. In 2015 he was
awarded the "Ignacio Ribas" Medal, from the
Organic Chemistry Group of the Royal Spanish
Chemistry Society. His research career has

been focused on the development of syn-
thetic methodologies, although more recently
he has applied that experience in the develop-
ment of new materials and/or molecular
probes with potential applications in bio-
science and nanoelectronics.

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Dr. Blanco got his PhD from University of A Coruña working on the metal-directed self-assembly of metallacycles, inclusion complexes and catenanes. After a postdoctoral stay at the group of Prof. David Leigh working on rotaxane-based switchable catalysts, he joined University of Granada in 2014. In 2022 he was appointed as Associate Professor at the same University. His main research interests are new synthetic methodologies for rotaxane synthesis, the study and modulation of (chir-)optical properties in rotaxane architectures as well as the supramolecular chemistry of aromatic systems.
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In this review, we discuss the reported [n]rotaxanes and
[n]catenanes that display a CPL response,[15] going from the
seminal works to the many advancements in the topic achieved
in very recent years. We will first review the rotaxanes and
catenanes with static, i.e., non-switchable, CPL response to then
move to discuss those designs in which the CPL signal can be
modulated upon addition of an external stimulus. Finally, we
will consider some examples of macromolecular and
supramolecular systems based on rotaxanes that also display
CPL activity.

## 2. Rotaxanes and Catenanes with Static CPL Response

The first example of a rotaxane- or catenane-based mechan-
ically interlocked molecule with a reported CPL response was a
[4]rotaxane (1a) described in 2014 by Prof. Inouye and co-
workers (Figure 2).[16] The system consisted of two units of a
thread bearing a 1,6-dialkynylpyrene unit threaded through two
γ-D-cyclodextrin (γ-D-CD) macrocycles (Figure 2a). The dialkynyl-
pyrene moiety played a double role as it acted both as
fluorophore and also as recognition site for the γ-D-CD in water,
due to its hydrophobic character. Thus, the design of the

[4]rotaxane relied on the formation of a inclusion complex with 2:2 stoichiometry between the chiral macrocycles and the 1,6-dialkynylpyrene units, which allowed the synthesis of the rotaxane through the threading-and-capping strategy. The pyrenes were encapsulated through the wider outer rims of the γ-D-CD and arranged into a stacked dimer with the two pyrene units twisted. Thus, the emission wavelengths (Figure 2b) and fluorescence lifetimes (τ₁ = 16 ns, τ₂ = 107 ns) matched those corresponding to a pyrene excimer, which also showed a remarkable quantum yield (Φ<sub>F</sub> = 37%). The inclusion of the pyrene dimer within the chiral cavities of the γ-D-CD resulted in a chiral information transfer to the pyrene dimer, inducing a preferential orientation of the pyrene cores within the stacked dimer and enabling chiroptical properties. Therefore, the [4]rotaxane displayed CPL emission between 400 nm and 650 nm (λ<sub>max</sub> CPL = 480 nm), the region of the excimer emission band, with |g<sub>lum</sub>| = 1.5×10<sup>-2</sup> in H<sub>2</sub>O (at pH ca. 9.5, achieved using NH<sub>3</sub>) (Figure 2b). This emission dissymmetry factor value is quite remarkable for a pure organic compound and of the same order of magnitude as the best reported for chiral pyrene excimer.<sup>[17]</sup>

A few years later, this research group applied the same strategy in the preparation of an analogous [4]rotaxane (1b) in which the 1,6-dialkynylpyrene moiety was replaced by a 3,9-dialkynylperylene unit (Figure 2).[18] Likewise 1a, [4]rotaxane 1b showed excimer emission, which is more challenging to obtain with perylene in solution, although with a lower quantum yield (Φ<sub>F</sub> = 15%). A CPL signal was also observed in the 500–700 nm range (λ<sub>max</sub> CPL ≈ 573 nm), with a g<sub>lum</sub> value slightly higher (|g<sub>lum</sub>| = 2.1×10<sup>-2</sup>) (H<sub>2</sub>O containing NH<sub>3</sub>, pH = 9.5) than that exhibited by rotaxane 1a.

The examples just discussed display a static CPL response, i.e. they are chiral non-racemic emissive compounds and, therefore, they display a certain CPL signal, but, as in most organic CPL emitters, it cannot be modulated upon the application of external stimuli. However, the interlocked structure still plays a role because the systems discussed relied on a chiral macrocycle as the source of asymmetry in the system, creating a chiral environment that influenced the originally achiral fluorophore, enabling its CPL. This strategy was also applied by the group of Prof. Wang in two different examples of CPL-active rotaxanes based on pillar[5]arene derivatives as chiral macrocycles published in 2024.[19] Pillar[5]arene derivatives are chiral, but their enantiomers usually exhibit low interconversion or racemization barriers due to the allowed rotation around the single C–C bonds. However, if this rotation is blocked, configurationally stable enantiomers can be obtained. This can be achieved either by functionalizing the pillar[5]arene core with a bulky group or by threading the macrocycle into a mechanically interlocked structure.[20]

In a first work, this group reported the study of hetero[3]rotaxanes with aggregation-induced emission (AIE)[21] that displayed CPL activity in the solid state (2a,b) (Figure 3).[19a] The rotaxanes consisted of a non-symmetric thread based on a C₁₀ diamide with an AIE-active tetraphenylethene (TPE) unit and a bulky benzylamine derivative as stoppers and 24-crown-8 (24C8) and *per*-O-ethyl pillar[5]arene (3) as the macrocycles

<figure><img src="image_13.png" /><figcaption>**Figure 2.** a) Structure of [4]rotaxanes **1a,b**; b) Fluorescence (dashed line, bottom) and CPL (solid line, top) signals of **1a** (H₂O, pH ≈ 9.5).[16] Figure 2b reproduced with permission from M. Inouye, K. Hayashi, Y. Yonenaga, T. Itou, K. Fujimoto, T.-a. Uchida, M. Iwamura, K. Nozaki, A Doubly Alkynylpyrene-Threaded [4]Rotaxane That Exhibits Strong Circularly Polarized Luminescence from the Spatially Restricted Excimer, *Angew. Chem. Int. Ed.* **2014**, *53*, 14392–14396. © Copyright 2014 Wiley-VCH GmbH or related companies. All rights reserved, including rights for text and data mining and training of artificial intelligence technologies or similar technologies.</figcaption></figure>
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<figure><img src="image_15.png" /><figcaption>**Figure 3.** a) Structure of hetero[3]rotaxanes **2a,b** (only the *S*<sub>p</sub> isomers are shown); b) Synthesis of hetero[3]rotaxanes *S*<sub>p</sub>-2*a,b* from [2]rotaxane *S*<sub>p</sub>-4 and their X-ray structures; c) CPL (KBr pellets) in ΔI (left) and *g*<sub>lum</sub> (right) scale of hetero[3]rotaxanes *R*<sub>p</sub>*S*<sub>p</sub>-2*a,b*.<sup>[19a]</sup> Figure 3c has been published in *CCS Chemistry* 2024, Sequence Engineering in Tuning the Circularly Polarized Luminescence of Aggregation-Induced Emission-Active Hetero[3]rotaxanes is available online at https://doi.org/10.31635/ccschem.024.202303738, https://www.chinesechemsoc.org/doi/10.31635/ccschem.024.202303738.</figcaption></figure>

(Figure 3a). The presence of two different macrocycles threaded through the non-symmetric linear component gave rise to two sequence-isomeric hetero[3]rotaxanes depending on whether **3** or **24C8** was the macrocycle closest to each of the stopper units. The rationale behind this design was again the chiral pillar[5]arene enabling a chiral information transfer to the fluorophore, giving rise to a CPL response. The synthetic methodology followed combined the stopper exchange strategy[22] and a metal-free active template approach.[23] Thus, the authors initially reported the synthesis of [2]rotaxane **4** formed by the pillar[5]arene macrocycle and a C₁₀ diester with

pentafluorophenyl stoppers (Figure 3b). This intermediate rotaxane is relevant as it enabled the resolution of the two enantiomers (*R*<sub>p</sub>-4 and *S*<sub>p</sub>-4) derived from each configuration of pillar[5]arene **3**[24] by chiral-stationary phase-HPLC (CSP-HPLC). Moreover, the pentafluorophenyl esters reacted with benzylic amines, enabling the stopper exchange reaction in which the stereochemistry was maintained as the dethreading of the macrocycle was not allowed by the nucleophilic addition-elimination mechanism. Carrying out this amide formation reaction in the presence of the crown ether allowed the introduction of the second macrocycle regioselectively, taking advantage of the ability of this type of macrocycles to catalyze the reaction between amines and electron-poor phenol esters (Figure 3b). As a result, the two separated enantiomers of each hetero[3]rotaxane (*R*<sub>p</sub>*S*<sub>p</sub>-2*a* and *R*<sub>p</sub>*S*<sub>p</sub>-2*b*) were obtained, which enabled the study of their chiroptical properties. Although the rotaxanes showed AIE in acetone/water mixtures with more than 60% water, no significant chiroptical activity was observed under such conditions. However, in the solid state (KBr pellets), both *R*<sub>p</sub>*S*<sub>p</sub>-2*a* and *R*<sub>p</sub>*S*<sub>p</sub>-2*b* rotaxanes displayed a CPL band between 400 nm and 600 nm (λ<sub>max CPL</sub> = 466 nm). Interestingly, whereas for rotaxane *R*<sub>p</sub>*S*<sub>p</sub>-2*a* the |*g*<sub>lum</sub>| value was 4.0×10<sup>-3</sup>, *R*<sub>p</sub>*S*<sub>p</sub>-2*b* displayed an enhanced chiroptical response with |*g*<sub>lum</sub>| = 1.2×10<sup>-2</sup> (*R*<sub>p</sub>-2*b*) and 0.8×10<sup>-2</sup> (*S*<sub>p</sub>-2*b*) (Figure 3c). This result was attributed to the different sequences of the rotaxanes. In **2a**, the crown ether was located between the chiral macrocycle and the TPE unit, thereby inhibiting the chiral influence of the former in the fluorophore. In contrast, in **2b**, the crown ether resided on the opposite side and even restricted the movement of the pillar[5]arene along the thread, enhancing the chiral information transfer to the fluorophore. This hypothesis was supported by the packing in the crystal X-ray structure of both rotaxanes, as for **2b** more intramolecular and intermolecular interactions between the chiral macrocycle and the TPE unit were observed. It should be noted that the CPL signals of *R*<sub>p</sub>*S*<sub>p</sub>-2*b* were not perfect mirror images, suggesting that the signals were partially affected by artifacts, which is not uncommon in solid-state measurements. However, the bands were of opposite signs, ensuring that a true CPL component was present.

More recently, the same group reported a remarkable enantioselective synthesis of planar chiral pillar[5]arenes functionalized with two substituted alkynes (mainly alkynylarene groups) on one of their rings (A1/A2 functionalization according to Ogoshi's nomenclature[25]). Following this methodology based on an asymmetric Sonogashira coupling that uses a chiral piperidine-containing phosphoramidite ligand derived from H<sub>8</sub>BINOL (Figure 4a), a wide variety of dialkynyl pillar[5]arenes were obtained with yields up to 91% and e.e. up to 97%.[19b] In this way, a pillar[5]arene with 2-ethynylanthracene groups (compound *S*<sub>p</sub>-5*b*) was obtained in 72% yield and 95% e.e (Figure 4a). As the almost enantiopure *S*<sub>p</sub>-5*b* obtained incorporated a fluorophore, it exhibited, in the solid state (KBr pellets), a CPL signal between 430 nm and 590 nm (|*g*<sub>lum</sub>| = 1.3×10<sup>-3</sup>). However, its incorporation into [2]rotaxane structures with threads based on C<sub>10</sub> alkyl chains stoppered with esters or pyrene- or anthracene-based benzylic amides (*S*<sub>p</sub>-6*a-c*) en-
<figure><img src="image_16.png" /></figure>

<figure><img src="image_17.png" /><figcaption>**Figure 4.** a) Enantioselective synthesis of pillar[5]arene derivative S<sub>p</sub>-5b; b) Structure of homochiral [2]rotaxanes S<sub>p</sub>-6a-c and synthesis of S<sub>p</sub>-6a,c from S<sub>p</sub>-6a by stopper exchange; c) CPL signal (KBr pellets) in Δl (left) and g<sub>lum</sub> (right) scale of S<sub>p</sub>-5b and [2]rotaxanes S<sub>p</sub>-6a-c.<sup>[19b]</sup> Figure 4c adapted with permission from X.-H. Zhou, X. Zhang, Y.-R. Song, X. Li, L.-T. Bao, W.-T. Xu, X.-Q. Wang, H.-B. Yang, W. Wang, Catalytic Enantioselective Synthesis of Planar Chiral Pillar[5]arenes via Asymmetric Sonogashira Coupling, *Angew. Chem. Int. Ed.*, **2025**, *64*, e202415190. © Copyright 2025 Wiley-VCH GmbH or related companies. All rights reserved, including rights for text and data mining and training of artificial intelligence technologies or similar technologies.</figcaption></figure>

hanced the chiroptical response with |g<sub>lum</sub>| values of 2.7×10<sup>-3</sup>, 2.9×10<sup>-3</sup> and 5.0×10<sup>-3</sup> (Figure 4b).

An interesting difference between these pillar[5]arene rotaxanes and Inouye's [4]rotaxanes lies in the fact that, while in cyclodextrin macrocycles covalent stereogenic centers, i.e. C atoms with four different substituents, are the origin of chirality (Figure 2a inset), pillar[n]arenes show planar chirality since its structure consists of methylene-bridged *p*-dialkoxy- or *p*-dihydroxybenzene units. Induction of CPL in rotaxane architectures by the latter demonstrates that types of chirality other than covalent chirality can induce chiroptical properties in an

achiral fluorophore placed within the chirally perturbing environment created by the chiral motif.

Another approach to obtain CPL-emissive rotaxanes was established by Wang and co-workers and consisted of the incorporation of a rim-differentiated tiara-pillar[5]arene with five pendant pyrene moieties (7a) in [2]rotaxanes with a C<sub>8</sub> or C<sub>10</sub> diester axle with two pentafluorophenyl stoppers (8a,b) (Figure 5).<sup>[26]</sup> The rim-differentiated tiara-pillar[5]arene showed emission between 375 and 625 nm (Φ<sub>F</sub> = 33.4%) corresponding to the pyrene units. Similarly, the rotaxanes exhibited fluorescence responses between 375 nm and 625 nm (λ<sub>max em</sub> = 485 nm) in CH<sub>2</sub>Cl<sub>2</sub> with a Φ<sub>F</sub> of 38.3% and 35.1% for the C<sub>8</sub> and C<sub>10</sub> [2]rotaxanes, respectively. This photoluminescence was assigned to pyrene excimer emissions since all fluorescence lifetimes were over 20 ns. For the isolated pillar[5]arene nonetheless, no CPL activity could be detected because of the smallness of the methoxy group on one side of the pillar[5]arene, which allows the rotation of the phenylene rings, enabling its chiral inversion. However, the mechanical bond created during the rotaxane synthesis hampered this flipping of the phenylene rings and therefore the racemization of the pillar[5]arene. Thus, both [2]rotaxanes displayed mirror-image CPL spectra in a wide variety of solvents with high |g<sub>lum</sub>| values

<figure><img src="image_18.png" /><figcaption>**Figure 5.** a) Structure of pyrene-containing rim-differentiated tiara-pillar[5]arenes 7a,b (only the S<sub>p</sub> isomer is shown) and synthesis of [2]rotaxanes 8a,b (only the S<sub>p</sub> isomer is shown); b) X-ray diffraction structure of [2]rotaxane 8a (only the S<sub>p</sub> isomer is shown). H atoms are omitted for clarity. Color coding: C, gray, light blue; O, red; F, light green.</figcaption></figure>
<figure><img src="image_19.png" /></figure>

reaching 0.015 and 0.013 at 455 nm in CH₂Cl₂ for the C₈ and C₁₀ [2]rotaxanes, respectively. Noteworthy is the fact that these dissymmetry factors are outstanding for small organic molecules. Interestingly, these values are higher than those obtained for a similar rim-differentiated tiara-pillar[5]arene (7b), functionally alized with bulky groups to prevent the flipping of the phenylene rings, which showed |g<sub>lum</sub>| values almost an order of magnitude lower (3.8×10⁻³ and 2.2×10⁻³). These results demonstrate the relevance of synthesizing chiral rotaxanes for CPL applications as their interlocked structure can have a strong influence on the g<sub>lum</sub> values, enhancing them if undesired conformational freedom is restricted as consequence of the mechanical bond.

In contrast to rotaxane architectures, catenanes have
generally been less studied in terms of their CPL properties.
Thus, we can only highlight an example of a [2]catenane (10)
with static CPL activity recently reported by Prof. Wang and co-
workers (Figure 6).[27] Its design consisted of pillar[5]arene 3 and
a macrocycle bearing an aliphatic chain as recognition motif
and a 9,14-diphenyl-9,14-dihydrodibenzo[a,c]phenazine (DPAC)
as fluorophore. This kind of dihydrophenazines are interesting
fluorescent motifs owing to their vibration-induced emission
(VIE) properties.[28] Their structure, including that of the DPAC
core, is curved. However, when they are excited, in their lowest
excited state, their conformation can change, adopting a planar
geometry. This process reduces the energy difference to the
ground state, giving rise to a red-shifted emission. As a result,
VIE-active species can show multiple fluorescence. [2]catenane
10 was obtained as a racemic mixture in 27% yield from
[2]rotaxane rac-4 by reaction of the amine groups of a

functionalized DPAC derivative (9) with the pentafluorophenyl
ester groups of 4 (Figure 6a), demonstrating the versatility of
the synthetic strategy developed by this group, suitable not
only for the synthesis of rotaxanes but also for other
mechanically interlocked molecules (MIMs). The authors man-
aged to separate the enantiomeric forms of the catenane (R<sub>p</sub>-10
and S<sub>p</sub>-10) by CSP-HPLC. Despite the interesting VIE behavior
exhibited by the system in solution, which could have led to
solvent-tunable CPL activity, no CPL signal could be detected.
However, in the solid state (cast films or KBr pellets), both R<sub>p</sub>-10
and S<sub>p</sub>-10 displayed mirror-image CPL signals between 400 nm
and 550 nm (λ<sub>max</sub> CPL ≈ 450 nm), which was attributed to the
DPAC core, demonstrating the chiral information transfer
between the chiral pillar[5]arene ring and the fluorophore in a
rather flexible catenane structure. The wavelengths of the CPL
band correspond to the usual fluorescence emission, i.e. no VIE
process occurred as it was hampered in the solid state. The
emission dissymmetry factor values were in the 10<sup>-3</sup> order of
magnitude, being higher in KBr pellets (|g<sub>lum</sub>| = 4.79–5.08×10<sup>-3</sup>)
than in cast films (|g<sub>lum</sub>| = 2.46–2.68×10<sup>-3</sup>) (Figure 6b).

**3. Rotaxanes and Catenanes with Switchable CPL Response**

The development of rotaxanes and catenanes with static CPL emission, such as those discussed in the previous section, is valuable as it represents an interesting strategy to confer chiral emission to achiral fluorophores and develop new CPL emitters. However, the true potential of these architectures arises from the possibility they offer to modulate different properties, including optical properties,[15,29] through a stimuli-induced switching of the position of the macrocycle, which has been exploited in the development of a variety of molecular machines.[12] Therefore, rotaxane- and catenane-based molecular shuttles have been developed whose CPL emission can be switched in solution or in the aggregate/solid state upon control of the distribution of a macrocycle between different binding sites on the thread or on the second macrocycle in a stimuli-controlled manner.

**3.1. Systems with Dynamic CPL Signal in Solution**

In 2019, our group reported the first example of a rotaxane whose CPL response could be turned “on” or “off” by the switching of the position of the macrocycle induced by the addition of acid or base (Figure 7).[30] The [2]rotaxane (11·PF₆⁻) was formed by a crown ether macrocycle with an embedded fluorescent bispyrene moiety, where the pyrene units were connected through the C-2 position, and a thread incorporating two different binding sites for this macrocycle, a chiral secondary ammonium/amine unit based on D- or L-phenyl-alanine and an achiral triazolium ring. The rationale behind the design was that the bispyrene unit presented two possible chiral atropisomers due to the allowed rotation around the

<figure><img src="image_20.png" /><figcaption>**Figure 6.** a) Chemical structure and synthesis of DPAC-containing [2]catenane 10 as racemic mixture (only the *S*<sub>p</sub> isomer is shown); b) CPL in Δl scale of [2]catenanes *R*<sub>p</sub>/*S*<sub>p</sub>-10 in cast films (left) and KBr pellets (right).<sup>[27]</sup> Reproduced and adapted from ref [27] (https://pubs.rsc.org/en/content/articlelanding/2024/sc/d4sc00650j) with permission from the Royal Society of Chemistry.</figcaption></figure>
<figure><img src="image_21.png" /></figure>

<figure><img src="image_22.png" /><figcaption>**Figure 7.** a) Chemical structure and acid/base-induced operation of rotaxane 11·PF<sub>6</sub><sup>-</sup>; b) UV-vis (black line) and fluorescence (purple line) spectra (CHCl<sub>3</sub>) (top) and CPL in normalized Δl scale (CHCl<sub>3</sub>) (bottom) of [2]rotaxane 11-H<sup>+</sup>·2PF<sub>6</sub><sup>-</sup> (left), [2]rotaxane 11·PF<sub>6</sub><sup>-</sup> obtained upon treatment with K<sub>2</sub>CO<sub>3</sub> (middle) and 11-H<sup>+</sup> restored upon addition of CF<sub>3</sub>CO<sub>2</sub>H (right).<sup>[30]</sup> Adapted with permission from A. H. G. David, R. Casares, J. M. Cuerva, A. G. Campaña, V. Blanco, *J. Am. Chem. Soc.* **2019**, *141*, 18064–18074 (https://pubs.acs.org/doi/10.1021/jacs.9b07143). © Copyright 2019 American Chemical Society. Further permissions related to the material excerpted should be directed to the ACS.</figcaption></figure>

pyrene-pyrene single bond, so that in the absence of a chiral environment, i.e. when the macrocycle was located around the triazolium ring, both conformations had the same energy, so they were equally populated, rendering the moiety achiral and disabling the CPL response. On the contrary, within a chirotopic space, i.e. nearby the chiral secondary ammonium unit, one of the enantiomeric conformations was favored due to the chiral information transfer process from the stereogenic center to the bispyrene, resulting in a chiral structure, thus enabling its CPL response (Figure 7a). To achieve its modulation, a way to control the position of the macrocycle between both binding sites was needed, so its acid/base-promoted switching between secondary ammonium/amine units and triazolium rings was

chosen.[31] Thus, we demonstrated that when the ammonium unit was protonated, the macrocycle was located on this station, and the [2]rotaxane (11-H⁺·2PF₆⁻) showed CPL emission between 390 nm and 460 nm with |g<sub>ium</sub>| = 5×10⁻⁴ in CHCl₃ solution. Mirror-imaged signals were obtained depending on the configuration of the amino acid residue, demonstrating that the chiral information transfer process between the chiral ammonium and the bispyrene was responsible for the CPL signal (Figure 7b bottom left). Addition of K₂CO₃ deprotonated the ammonium unit and promoted the switching of the position of the macrocycle toward the achiral triazolium ring. This station is further away from the stereogenic center, so the chiral information transfer was much weaker, and no CPL signal
<figure><img src="image_23.png" /></figure>

was detected for 11·PF₆⁻ (CPL "off", Figure 7b bottom middle). Addition of CF₃CO₂H switched back the macrocycle toward the chiral station, turning back "on" the CPL signal, with the same profile and g_lum values as the original state (Figure 7b bottom right). It is noteworthy that this CPL switching was performed without altering the fluorescence of the system, as neither a shift in the band (located in the 380–500 nm range with λ_max em = 404 nm) nor a significant change in the quantum yield (Φ_F = 11%) were observed (Figure 7b top). Finally, this CPL switching was carried out in situ for three complete cycles, although with some degradation of the "on" state after each cycle, which was attributed to a possible decomposition of the "off" state. We want to highlight that unlike most rotaxanes discussed in the previous section, the achiral fluorophore was part of the macrocycle, while the chiral moiety was located on the thread. Moreover, a single covalent stereogenic center created the chirotopic space. The induction of a CPL response demonstrated that approaches based on this feature can also be exploited in the development of CPL-active rotaxanes.

A [c2]daisy chain rotaxane was recently employed by the
group of Prof. Wang in the development of a CPL switch
(Figure 8).[32] The design is based on a pillar[5]arene macrocycle
monofunctionalized with a pyrene linker bearing a C8 alkyl
chain (the macrocycle binding site) with a terminal amine. A
double threading followed by a capping reaction with an
isocyanate afforded the dimeric rotaxane with the [c2]daisy
chain structure containing thiourea units (Figure 8a). According

to the characterization by NMR spectroscopy, the pillar[5]arene
macrocycles were located on the alkyl chain, however, around
the methylene groups closer to the thiourea unit as H bond
interactions between the thiourea and the alkoxy groups of the
macrocycle were established. As a result of the daisy chain
topology, the two pyrene units were brought together and
stacked, giving rise to excimer emission. Thus, in addition to the
monomer emission at shorter wavelengths, the fluorescence
spectrum of 12 showed a band between ca. 420 nm and
600 nm (λmax em = 463 nm, ΦF = 3.8%), corresponding to the
excimer (Figure 8b left). The planar chiral nature of the macro-
cycles forced a preferential twisting in the stacked pyrene
dimer, transferring their chiral information to the fluorophores,
thus enabling chiroptical properties. Their study required the
resolution by CSP-HPLC of the three possible stereoisomers
formed, i.e. two chiral with both macrocycles with either Rₚ or
Sₚ configuration (Rₚ-12 and Sₚ-12) and a meso form with one
macrocycle of each configuration. Thus, Rₚ-12 and Sₚ-12 showed
chiroptical properties, in particular, CPL emission between ca.
415 nm and 600 nm in solution (CH₂Cl₂). Interestingly, this CPL
response showed a bisignate character with the more intense
band between 415 and 500 nm (λmax CPL ≈ 463 nm, |g|<sub>lum</sub> =
1×10⁻²) (Figure 8b). Unfortunately, the origin of this dual CPL
response could not be fully unraveled by the authors, although
they pointed to the presence of two different co-conformations
in the first excited state as a possible explanation.

The thiourea moieties were key for the operation of the
system and the modulation of these (chir-)optical properties.
Thioureas are H bond donors that can bind to H bond acceptors
or anions such as carboxylates. In rotaxane architectures, this
binding could influence their co-conformation, i.e. can induce a
change in the position of the macrocycle along different
binding sites on the thread.[33] Thus, addition of trifluoroacetate
salts or DMSO to 12 induced a change in the position of the
macrocycles due to their interaction with the thiourea units
(Figure 9a), as demonstrated by ¹H NMR spectroscopy. The
pillar[5]arenes were still placed around the alkyl chain, but now
they resided closer to the pyrene units. However, although the
influence on the macrocycle position was similar for both
stimuli, the effect on the chiroptical properties was different.
Addition of trifluoroacetate anions resulted in a weaker CPL
response between *ca.* 415 nm and 500 nm (both in terms of Δl
and g<sub>lum</sub> values, the latter decreasing to |g<sub>lum</sub>| = 2×10<sup>-3</sup>) (Fig-
ure 9c). The addition of a Na<sup>+</sup> salt to remove the trifluoroace-
tate anions through the formation of insoluble sodium carbox-
ylate salts restored the macrocycles of the [c2]daisy chain to
their original position and, therefore, the initial chiroptical
properties were recovered (Figure 9d). This switching was
repeated up to five times with little degradation of the CPL
response. In a certain way, this switching mode induced by the
coordination of anions was similar to that described for the
[2]rotaxane 11, as it promoted a change in the intensity of the
CPL signal (in this case a decrease instead a complete turn off)
without a significant change in the fluorescence quantum yield,
which only showed a slight increase up to Φ<sub>F</sub> = 4.0%. In
contrast, the switching induced by the solvent change resulted
in a more unusual outcome. Addition of DMSO to a solution of

<figure><img src="image_24.png" /><figcaption>**Figure 8.** a) Synthesis of [c2]daisy chain **12** (only the *R*<sub>p</sub> isomer is shown) through the threading-and-capping approach; b) Fluorescence spectra (left bottom) and CPL spectra in Δl (left top) and *g*<sub>lum</sub> (right) scale of *R*<sub>p</sub>/*S*<sub>p</sub>-**12** (*CH*<sub>2</sub>*Cl*<sub>2</sub>).<sup>[32]</sup> Figure 8b adapted with permission from X. Li, W.-T. Xu, X.-Q. Xu, Y. Wang, X.-Q. Wang, H.-B. Yang, W. Wang, Lighting Up Bispyrene-Functionalized Chiral Molecular Muscles with Switchable Circularly Polarized Excimer Emissions, *Angew. Chem. Int. Ed.* **2025**, *64*, e202412548 © Copyright 2025 Wiley-VCH GmbH or related companies. All rights reserved, including rights for text and data mining and training of artificial intelligence technologies or similar technologies.</figcaption></figure>
<figure><img src="image_25.png" /></figure>

<figure><img src="image_26.png" /><figcaption>**Figure 9.** a) TD-DFT (PBE0/Def2-SVP) optimized structures of the most stable geometry of the first excited state of *R*<sub>p</sub>-12·DMSO (top), *R*<sub>p</sub>-12 (middle) and *R*<sub>p</sub>-12·CF<sub>3</sub>CO<sub>2</sub><sup>-</sup> (bottom) showing the arrangement of the stacked pyrene units. Atomic coordinates taken from ref. [32]. H atoms, except those of the urea groups, are omitted for clarity. Color coding: C, gray, light blue; O, red; N, blue, S, yellow, F, light green, H, white; b) ΔI (left) and *g*<sub>lum</sub> (right) spectra of *R*<sub>p</sub>*S*<sub>p</sub>-12 in different CH<sub>2</sub>Cl<sub>2</sub>/DMSO ratios showing the inversion of the sign and the increased intensity of the CPL signal at high proportion of DMSO; c) CPL spectra in ΔI (left) and *g*<sub>lum</sub> (right) scale of *R*<sub>p</sub>*S*<sub>p</sub>-12 (CH<sub>2</sub>Cl<sub>2</sub>) in the presence of increasing amounts of CF<sub>3</sub>CO<sub>2</sub><sup>-</sup> showing the decrease of the CPL response; d) Restoration of the CPL signal of the solutions from the experiments in c) upon addition of increasing amounts of Na<sup>+</sup> cations to remove the trifluoroacetate anions. Both the ΔI (left) and *g*<sub>lum</sub> (right) spectra are shown [32]. Figures 9b-d adapted with permission from X. Li, W.-T. Xu, X.-Q. Xu, Y. Wang, X.-Q. Wang, H.-B. Yang, W. Wang, Lighting Up Bispyrene-Functionalized Chiral Molecular Muscles with Switchable Circularly Polarized Excimer Emissions, *Angew. Chem. Int. Ed.* **2025**, *64*, e202412548. © Copyright 2025 Wiley-VCH GmbH or related companies. All rights reserved, including rights for text and data mining and training of artificial intelligence technologies or similar technologies.</figcaption></figure>

daisy chain R<sub>p</sub>/S<sub>p</sub>-12 in CH<sub>2</sub>Cl<sub>2</sub> up to a 1:10 (CH<sub>2</sub>Cl<sub>2</sub>/DMSO v/v) ratio resulted not only in an increase in the fluorescence quantum yield (Φ<sub>F</sub> = 7.2%) and the |g<sub>lum</sub>| value of the CPL signal to 1.5×10<sup>-2</sup>, but, more interestingly, in the inversion of the sign of the main CPL band (Figure 9b). This sign inversion was attributed to the change from a left-handed twist in the pyrene excimer to a right-handed one, which was supported by the DFT-optimized structures of the first excited state of R<sub>p</sub>-12 and its complex with DMSO (Figure 9a). TD-DFT calculations also shed light on the stimuli-induced changes in the |g<sub>lum</sub>| values. In homogeneous systems, the dissymmetry factor depends on the electric (|μ|) and magnetic (|m|) dipole transition moment of the S<sub>1</sub>→S<sub>0</sub> transition and the cosine of the angle between them (cos θ) (g<sub>lum</sub> = (|μ|·|m|·cosθ)/(|μ|<sup>2</sup> + |m|<sup>2</sup>)). Taking this into account, the decrease of θ from 107.2° to 48.1° due to the change in the arrangement of the pyrene units in the excimer upon coordination of the [c2]daisy chain to DMSO can mainly account for the increase in the |g<sub>lum</sub>| value. On the contrary, the more unfavorable values of the parameters in the structure resulting from the binding of the thiourea units

to trifluoroacetate anions explained the decrease in the |g<sub>lum</sub>| observed.

Prof. He, Prof. Wang, Prof. Yang and co-workers also reported an impressive example of CPL switching in a mechanically planar chiral[14c] [2]catenane (13) (Figure 10).[34,35] In this sense, orthogonal "on-off" and intensity switching of the CPL activity were achieved. The catenane structure consisted of two interlocked macrocycles bearing a diethylene glycol, a dibenzyl amine unit and a dibenzyl ether within the ring and an appended pyrene moiety as the fluorophore. The incorporation of the amine and the ether on each side of the macrocycle resulted in C<sub>1h</sub> symmetry, which gave rise to the mechanically planar chirality of the catenane (Figure 10a). The [2]catenane adopted a co-conformation ("closed" form, Figure 10a,b middle) that placed the pyrene units close in space, allowing the excimer formation. This spatial arrangement was demonstrated by NOESY NMR experiments and by fluorescence spectroscopy, which showed, in addition to the typical monomeric pyrene fluorescence band, another emission band between ca. 410 and 650 nm (λ<sub>max em</sub> = 480 nm) in CH<sub>2</sub>Cl<sub>2</sub>, typical of the pyrene
<figure><img src="image_27.png" /></figure>

<figure><img src="image_28.png" /><figcaption>**Figure 10.** a) Chemical structure of [2]catenane 13 (only the R<sub>mp</sub> isomer is shown) and switching mechanism between its different co-conformations induced by coordination/removal of Na<sup>+</sup> cations (to 13·Na<sup>+</sup>, open co-conformation) or acid/based addition (13·2H<sup>+</sup>, protonated co-conformation); b) DFT (LC-ωHPBE/6-311G(d,p)) optimized structures (ground state) of the open (left), closed (middle) and protonated (right) co-conformations of R<sub>mp</sub>-13. Atomic coordinates taken from ref. [36]. H atoms, except those of the ammonium groups, are omitted for clarity. Color coding: C, gray, light blue; O, red; N, blue, H, white; c) Switching of the fluorescence (bottom) and CPL (Δl scale, top) bands of R<sub>mp</sub>/S<sub>mp</sub>-13 upon addition of Na<sup>+</sup> ions;<sup>[34]</sup> d) CPL spectra (Δl scale, top) and fluorescence (bottom) of R<sub>mp</sub>/S<sub>mp</sub>-13 in CH<sub>3</sub>Cl<sub>2</sub> showing the main band corresponding to the excimer emission;<sup>[34]</sup> e) CPL spectrum (Δl scale) of S<sub>mp</sub>-13 showing its bisignated character due to the emission from the monomer and excimer states;<sup>[37]</sup> f) Switching of the fluorescence (bottom) and CPL signal (Δl scale, top) of R<sub>mp</sub>/S<sub>mp</sub>-13 upon addition of CF<sub>3</sub>CO<sub>2</sub>H.<sup>[34]</sup> Figures 10c,d,f adapted with permission from Y. Wang, J. Gong, X. Wang, W.-J. Li, X.-Q. Wang, X. He, W. Wang, H.-B. Yang, Multistate Circularly Polarized Luminescence Switching through Stimuli-Induced Co-Conformation Regulations of Pyrene-Functionalized Topologically Chiral [2]Catenane, *Angew. Chem. Int. Ed.* **2022**, *61*, e202210542. © Copyright 2022 Wiley-VCH GmbH or related companies. All rights reserved, including rights for text and data mining and training of artificial intelligence technologies or similar technologies. Figure 10e adapted with permission from X. Wang, P. Wu, Y. Wang, T. Cui, M. Jia, X. He, W. Wang, H. Pan, Z. Sun, H.-B. Yang, J. Chen, Unraveling the Origin of Multichannel Circularly Polarized Luminescence in a Pyrene-Functionalized Topologically Chiral [2]Catenane, *Angew. Chem. Int. Ed.* **2024**, *63*, e202407929. © Copyright 2024 Wiley-VCH GmbH or related companies. All rights reserved, including rights for text and data mining and training of artificial intelligence technologies or similar technologies.</figcaption></figure>

excimer emission (Φ<sub>F</sub> = 5.0%). The ratio between excimer and monomer emission was 1.33, 3-fold higher than the 0.44 ratio observed for the non-catenated macrocycle at the same concentration, demonstrating the major intramolecular nature of the excimer. DFT theoretical calculations by the authors and later by Dr. Bella and co-workers also supported this structure (Figure 10b).<sup>[36]</sup> The optimized structure in the ground state by the latter showed that the pyrene units stacked in a twisted manner with an interplanar distance of 3.2 Å (further reduced to 3.0 Å in the S<sub>1</sub> excited state), and a displacement of 0.75 Å. S<sub>mp</sub>-13 and R<sub>mp</sub>-13 [2]catenanes showed in this "closed" form mirror-imaged CPL emission within the spectral region of the excimer emission (λ<sub>max CPL</sub> = 480 nm) with a |g<sub>lum</sub>| = 1.2×10<sup>-2</sup> (Figure 10d top). However, a deeper follow-up photophysical

study in collaboration with the group of Prof. Chen revealed a more complex behavior.<sup>[37]</sup> Thus, a closer inspection of the CPL signal by time-resolved CPL (TRCPL) and transient-absorption (TA) spectroscopy showed again a bisignate character, demonstrating a dual chiral emission. Thus, intense CPL emission at λ<sub>max CPL</sub> = 480 nm (τ<sub>1</sub> = 8.5 ns; τ<sub>2</sub> = 27 ns) was observed along with a weak CPL band of opposite sign centered at λ<sub>max CPL</sub> ≈ 400 nm (τ = 8.0 ns) (Figure 10d bottom). The analysis of the data allowed the attribution of the former to the excimer emission and the latter to the monomer-like emission from the S<sub>1</sub> excited state. In view of these results, the authors concluded that the [2]catenane exhibited two types of chirality: Euclidean chirality as result of the twisted stacked arrangement of the pyrene units in the excimer responsible for the main CPL band,
<figure><img src="image_29.png" /></figure>

and mechanically planar chirality, which is transferred to the fluorophore inducing another CPL response, much weaker, however, than the excimer one. At this point, it should be noted that the types of chirality that arise from the mechanical bond in mechanically interlocked structures, which have been almost unexplored in CPL emitters, could result in CPL emission, although only a weak response was observed in this example. Whether higher g<sub>lum</sub> values or useful signals can be achieved is an unanswered question that requires further investigation.

The presence of the oligo(ethylene glycol) units in catenane 13 enabled a co-conformational switching induced by their binding to Na⁺ cations, which were in fact used as a synthetic template.[38] As a result of this complexation, the systems adopted an “open” co-conformation with the pyrene moieties far away from each other, as supported by NMR spectroscopy and more recent DFT calculations (Figure 10a,b left). The latter showed that the distance between the pyrene moieties is 21 Å, which meant they were too far away from each other to form intramolecular excimers. Therefore, the photoluminescence spectrum showed only the band corresponding to the monomer-like pyrene emission between 360 nm and 450 nm with a tail up to 550 nm (Φ<sub>F</sub>=5.5%), with the disappearance of the excimer emission. As a result, in the S<sub>mp</sub>/R<sub>mp</sub>-13·Na⁺ complex, the CPL response was almost switched “off” as the main CPL band in the “closed” form, corresponding to the excimer emission, was no longer present (Figure 10c). TRCPL showed that, in fact, 13·Na⁺ was not completely CPL-silent as a weak band at λ<sub>max CPL</sub>=400 nm (τ=8.0 ns) could still be observed.[37] This band was attributed to the monomer pyrene emission that arises from the transfer of the mechanically planar chirality of the catenane to the fluorophore, as discussed above. However, the drastic change in the CPL intensity at λ=480 nm justifies the structure being considered an “on-off” CPL switch. Addition of [2.2.2]cryptand to remove the Na⁺ cations restored the optical and chiroptical properties as the “open” co-conformation was recovered. Five consecutive “on-off” cycles could be achieved without significant degradation of the response.

Finally, the presence of amine units enabled a second mechanism to modulate the CPL response. Thus, the protonation of the amine by addition of trifluoroacetic acid promoted significant changes in the emissive properties of the system. The fluorescence spectrum maintained the components originated by the emission of both the excimer and the monomeric pyrene, however, with a decrease in the excimer/monomer emission ratio from the original 1.33 to 0.25, which suggested a change in the spatial arrangement of the fluorophores (Figure 10e). DFT calculations were in accordance with these results, as they showed that the pyrene stacking in the “closed” form was disturbed due to the electrostatic repulsion between the ammonium salts. As a result, the interplanar distance of the pyrene units increased to 3.75 Å in the ground state (Figure 10b right) and to 3.9 Å in the S<sub>1</sub> excited state. On the other hand, a ca. three-fold increase in the fluorescence quantum yield (Φ<sub>F</sub> = 17.1%) was observed, which was attributed to the limitation of the deactivation of the fluorescence by a photo-induced electron transfer (PET) process involving the amine lone electron pair. Regarding the CPL response, the protonation of

the amines induced a red shift of the main CPL signal (450-650 nm, λ<sub>max CPL</sub> ≈ 510 nm) as well as an improvement of the dissymmetry factor, which nearly doubled (|g<sub>lum</sub>| = 2.2×10<sup>-2</sup>). Again, the wavelength of the band pointed to the chiral excimer as the origin of the CPL. In this sense, the CPL signal maintained its bisignate character, even more pronounced than in the “closed” form, with a second band of opposite sign located between ca. 400 nm and 450 nm (λ<sub>max CPL</sub> = 430 nm) with a reported |g<sub>lum</sub>| ≈ 1×10<sup>-3</sup> (Figure 10e). All these changes in the (chir-)optical properties were fully reversible by the addition of DBU, which allowed the modulation of the intensity of the CPL response for several cycles. TD-DFT calculations succeeded in reproducing the experimental behavior of the excimer band as they predicted both the enhancement and redshift of the signal upon protonation and its disappearance upon Na<sup>+</sup> coordination.[36]

Based on the same mechanically planar chiral [2]catenane scaffold, Prof. Li, Prof. Wang, Prof. Cheng, Prof. Yang and co-workers developed a multiple-resonance thermally activated delayed fluorescence (MR-TADF) emitter with switchable circularly polarized emission (compound 14, Figure 11).[39] This structure was similar to the [2]catenane 13 but bore a CzBN-type MR-TADF emitter (Figure 11a inset) instead of the pyrene unit. An alkynyl moiety was also intercalated between the amine and the luminescent core to prevent the undesired PET that took place in the previous design. The [2]catenane showed interesting optical properties. It exhibited an emission band between ca. 460 nm and 600 nm (λ<sub>max em</sub> = 496 nm, Φ<sub>PL</sub> = 100% in N<sub>2</sub>-purged toluene, 55% in air-purged toluene) corresponding to the CzBN unit (Figure 11b left). This band is quite narrow compared with typical organic fluorophores, although the full-width at half-maximum (FHMW) ranges 37 nm (toluene)-40 nm (CH<sub>2</sub>Cl<sub>2</sub>) instead of the FHMW=22 nm observed for the monomeric macrocycle. This result was attributed to the formation of an intramolecular excimer forced by the catenane co-conformation. Two different lifetimes were observed in toluene (τ<sub>1</sub>=9.3 ns, fractional quantum yield η<sub>F</sub>=81%; τ<sub>2</sub> = 38.4 μs, η<sub>DF</sub>=19%), the longer one corresponding to the delayed fluorescence. Due to its chiral nature, R<sub>mp</sub>/S<sub>mp</sub>-14 also displayed CPL emission in the same range as the fluorescence band both in solution (Figure 11b right) and in doped films of 1,3-dihydro-1,1-dimethyl-3-(3-(4,6-diphenyl-1,3,5-triazin-2-yl) phenyl)indeno-[2,1-b]carbazole (DMIC-TRZ) (Figure 11c, right). The dissymmetry factors in solution (toluene) were 3.6×10<sup>-3</sup> for S<sub>mp</sub>-14 and -3.8×10<sup>-3</sup> for R<sub>mp</sub>-14, in good agreement with the TD-DFT value (|g<sub>lum</sub>| = 2.67×10<sup>-3</sup>) calculated for the “closed” co-conformation of the [2]catenane scaffold. As with [2]catenane 13, the co-conformation could be switched in CH<sub>2</sub>Cl<sub>2</sub> solution by the coordination of Na<sup>+</sup> cations to the oligo(ethylene glycol) units, reaching an “open” co-conformation where the emissive units did not longer interact. This co-conformation switching influenced the (chir-)optical properties. On the one hand, the emission band suffered a slight blue-shift and its FHMW decreased to 28 nm as the CzBN units did not longer stack. In contrast, the CPL response was switched off (Figure 11b). Addition of [2.2.2]cryptand restored the original CPL activity,
<figure><img src="image_30.png" /></figure>

<figure><img src="image_31.png" /><figcaption>**Figure 11.** a) Co-conformational switching of catenane 14 through Na⁺ coordination/removal; b) Modulation of the fluorescence spectra (left) and CPL spectra in Δl (right) in CH₂Cl₂ by addition of Na⁺ cations; c) Electroluminescence (left) and circularly polarized electroluminescence (CPEL) spectra of a CPL-OLED device incorporating as one of its components an emitting layer of 14 in DMIC-TRZ.[39] Figures 11b,c adapted with permission from Y. Wang, W.-L. Zhao, Z. Gao, C. Qu, X. Li, Y. Jiang, L. Hu, X.-Q. Wang, M. Li, W. Wang, C.-F. Chen, H.-B. Yang, Switchable Topologically Chiral [2]Catenane as Multiple Resonance Thermally Activated Delayed Fluorescence Emitter for Efficient Circularly Polarized Electroluminescence *Angew. Chem. Int. Ed.* **2025**, *64*, e202417458. © Copyright 2025 Wiley-VCH GmbH or related companies. All rights reserved, including rights for text and data mining and training of artificial intelligence technologies or similar technologies.</figcaption></figure>

confirming the operation of the system as an "on-off" CPL switch. In this sense, several "on-off" cycles were performed.

[2]catenanes $R_{mp}/S_{mp}-14$ also showed good CPL in the solid state. Thus, in the DMIC-TRZ films (3 wt% of 14) the $g_{lum}$ values improved to $1.1 \times 10^{-2}$ and $-1.6 \times 10^{-2}$, although at the cost of the signals no longer being perfect mirror images, which indicate that some artifacts were distorting the CPL response. Nevertheless, its MR-TADF behavior and its chiral emission made the [2]catenane a potential candidate for the development of CPL-OLED devices, which was also investigated. Thus, the authors prepared a series of devices incorporating an emitting layer of $R_{mp}/S_{mp}-14$ in DMIC-TRZ as one of the multiple

components of the device. The devices with 3 wt% $R_{mp}$- or $S_{mp}-14$ showed electroluminescence (EL) centered at 500 nm with an FHMW of 36 nm–37 nm (for $R_{mp}/S_{mp}-14$ and *rac*-14, respectively), an external quantum efficiency (EQE) of 15.6%-17.6% and circularly polarized electroluminescence (CPEL) with a dissymmetry factor in the order of $10^{-3}$ ($|g_{EL}| \approx 2 \times 10^{-3}$) (Figure 11c).

## 3.2. Systems with Dynamic CPL Signal in the Aggregate or Solid State

The same switching mechanism based on the binding of anions to thiourea units in pillar[5]arene-based rotaxanes previously discussed for the operation of the [c2]daisy chain 12 has been exploited in a series of interlocked systems with a CPL response in the solid state that can be switched.

In the first example, Prof. He, Prof. Wang and co-workers reported a [3]rotaxane (compound 15) with a 9,10-distyrylanthracene unit as the fluorophore in which the acetate-induced switching of the position of the macrocycle could modulate the intensity of its CPL signal in the solid state (Figure 12).[40] The design was based on the bisfunctionalization of the 9,10-distyrylanthracene core with $C_6$ alkyl chains as binding sites for the pillar[5]arene 3. Thiourea units were introduced at both ends as a result of the stoppering reaction. Again, the authors demonstrated by NMR studies and DFT calculations that the coordination of carboxylates, in this case acetate anions, to these thioureas triggered a change in the preferential position on the alkyl chain of the ring from the vicinity of the thioureas towards the central fluorophore (Figure 12a). This switching, which could be reversed by the removal of the acetates through precipitation with Na⁺, altered the distance between the chiral macrocycle and the fluorophore, which could result in a strengthening or weakening of the chiral information transfer process, thus modulating the chiroptical properties. However, the inspection of the electronic circular dichroism (ECD) spectra in solution of $R_p-15$ and $S_p-15$, separated by CSP-HPLC and displaying both macrocycles in the same configuration, did not show any evidence of induction of chiroptical response on the fluorophore in the presence of acetate anions. However, taking advantage of the AIE-character of the 9,10-distyrylanthracene unit, the behavior of the [3]rotaxane in the solid state was also investigated. The increase in its quantum yield ($\Phi_F$ = 36.6% in solid state versus $\Phi_F$ = 1.0% in CH₂Cl₂ solution) was accompanied by a more effective chiral information transfer from the chiral macrocycle to the fluorophore that enabled chiroptical signals from the latter in both absorption and emission. In particular, mirror-image CPL signals were observed in film for $R_p/S_p-15$ between 450 nm and 700 nm ($\lambda_{max}$ CPL = 535 nm) with a $|g_{lum}| = 2.14 \times 10^{-3}$, corresponding to the emission band of the 9,10-distyrylanthracene moiety (Figure 12c). Addition of nBu₄NOAc increased the $|g_{lum}|$ value up to $1.36 \times 10^{-2}$. This enhancement of the CPL was attributed to the macrocycle being closer to the fluorophore, which was now more influenced by the chirotopic space generated by the chiral pillar[5]arene (Figure 12d). The system was restored to its
<figure><img src="image_32.png" /></figure>

<figure><img src="image_33.png" /><figcaption>**Figure 12.** a) Chemical structure of [3]catenane 15 (only the *R*<sub>p</sub> isomer is shown) and acetate-induced switching of the position of the macrocycle; b) X-ray crystal structure of meso-15, showing the H bonds (black dashed lines) between thiourea H atoms and O atoms of pillar[5]arene derivative. H atoms except those of the thiourea groups, are omitted for clarity. Color coding: C, gray, light blue; O, red; N, blue, S, yellow, F, light green, H, white; c) CPL spectra (film) in Δl (left) and g<sub>lum</sub> (right) scale of *R*<sub>p</sub>*S*<sub>p</sub>-15; d) CPL spectra (film) in Δl (left) and g<sub>lum</sub> (right) scale of *R*<sub>p</sub>*S*<sub>p</sub>-15 after addition of OAc<sup>-</sup> ions; e) CPL spectra (film) in Δl (left) and g<sub>lum</sub> (right) scale of *R*<sub>p</sub>*S*<sub>p</sub>-15 after addition of acetate anions followed by their subsequent removal with Na<sup>+</sup> cations.<sup>[40]</sup> Figures 12c-e adapted with permission from W.-J. Li, Q. Gu, X.-Q. Wang, D.-Y. Zhang, Y.-T. Wang, X. He, W. Wang, H.-B. Yang, AIE-Active Chiral [3]Rotaxanes with Switchable Circularly Polarized Luminescence, *Angew. Chem. Int. Ed.* **2021**, *60*, 9507–9515. © Copyright 2021 Wiley-VCH GmbH or related companies. All rights reserved, including rights for text and data mining and training of artificial intelligence technologies or similar technologies.</figcaption></figure>

original state by adding a Na⁺ salt, demonstrating the reversible switching of the intensity of the CPL response (Figure 12e). Five complete CPL switching cycles were performed without significant performance loss.

A similar strategy was applied by Prof. Wang and co-workers in the switching of the CPL emission of [2]rotaxanes based on a DPAC unit (16a,b) (Figure 13).[41] The interlocked molecule incorporated structural elements that were analogous to those in [3]rotaxane 15, such as the pillar[5]arene macrocycle 3 as the source of chirality and the thiourea-containing alkyl chain as the recognition motif, with the difference that, in this work, both C₅ and C₆ alkyl chains were tested to evaluate the influence of length. A DPAC derivative was the fluorophore, although it also played the role of stopper (Figure 13a). The [2]rotaxanes 16a,b were obtained in 66% and 86% yield using the thiourea formation reaction as the capping step. This thiourea unit enabled the anion-induced mechanism discussed above for the switching of the macrocycle, as demonstrated by NMR spectroscopy. The enantiomeric structures were resolved by CSP-HPLC and characterized by single-crystal X-ray diffraction (Figure 13b). As in previous examples, CPL activity was not detected in solution, but appeared in the solid state, in which the chiral information transfer is favored. Thus, rotaxanes 16a,b showed CPL bands between 400 nm and 600 nm (λ<sub>max CPL</sub> = 450 nm), corresponding to the emission of the DPAC unit, with g<sub>lum</sub> values of 4.10×10<sup>-3</sup> and -4.02×10<sup>-3</sup> for *R*<sub>p</sub>-16a and *S*<sub>p</sub>-16a, respectively (Figure 13c), and 3.12×10<sup>-3</sup> and -3.32×10<sup>-3</sup> for *R*<sub>p</sub>-

16b and *S*<sub>p</sub>-16b, respectively. For each rotaxane, the enantiomers show reasonably similar |g<sub>lum</sub>| values, considering that the measurements were carried out in the solid state (KBr pellets), which is prone to introduce artifacts or distort the intrinsic CPL signal of the fluorophores.[42] In this sense, the protocol followed by the authors included rotating and flipping the sample to check (and average, if needed) the signal in an attempt to minimize those potential problems. The slightly higher |g<sub>lum</sub>| values for *R*<sub>p</sub>*S*<sub>p</sub>-16a compared to *R*<sub>p</sub>*S*<sub>p</sub>-16b were attributed to the shorter length of the alkyl chain in the former, which results in the fluorophore being more influenced by the chiroptic space generated by the macrocycle. Addition of a trifluoroacetate salt improved the g<sub>lum</sub> values to 7.87×10<sup>-3</sup> and -7.68×10<sup>-3</sup> for *R*<sub>p</sub>-16a and *S*<sub>p</sub>-16a (Figure 13d), respectively, and 7.19×10<sup>-3</sup> and -7.95×10<sup>-3</sup> for *R*<sub>p</sub>-16b and *S*<sub>p</sub>-16b, respectively, as a result of the chiral macrocycle being closer to the fluorophore, as discussed, enhancing the chiral information transfer. Addition of a Na<sup>+</sup> salt restored the initial response, confirming the operation of the system as a CPL intensity switch (Figure 13e).

It should be highlighted that translating the switching mechanisms and modulation of the CPL activity to the solid state would be a key milestone, as it would open a promising research avenue for the future incorporation of such systems into functional devices. In this sense, the examples just discussed represent a remarkable step forward in this direction, as they achieve the CPL signals in the solid state required for these devices. However, an important limitation that still should
<figure><img src="image_34.png" /></figure>

<figure><img src="image_35.png" /><figcaption>**Figure 13.** a) Structure of [2]catenanes **16a,b** (only the $R_p$ isomer is shown) and translocation of the macrocycle along the thread induced by the coordination of a trifluoroacetate anion to the thiourea; b) X-ray crystal structure of $R_p$-**16a**, showing the H bond (black dashed lines) between the thiourea and the OEt groups of pillar[5]arene 3. H atoms except those of the thiourea are omitted for clarity. Color coding: C, gray, light blue; O, red; N, blue, S, yellow, F, light green, H, white; c) CPL spectra (KBr pellets) in ΔI (left) and $g_{\text{lum}}$ (right) scale of $R_p/S_p$-**16a**; d) CPL spectra (KBr pellets) in ΔI (left) and $g_{\text{lum}}$ (right) scale of $R_p/S_p$-**16a** upon addition of $CF_3CO_2^-$; e) CPL spectra (KBr pellets) in ΔI (left) and $g_{\text{lum}}$ (right) scale of $R_p/S_p$-**16a** after addition and subsequent removal of trifluoroacetate anions.[41] Figures 13c-e adapted with permission from W.-T. Xu, X. Li, P. Wu, W.-J. Li, Y. Wang, X.-Q. Xu, X.-Q. Wang, J. Chen, H.-B. Yang, W. Wang, Dual Stimuli-Responsive [2]Rotaxanes with Tunable Vibration-Induced Emission and Switchable Circularly Polarized Luminescence, *Angew. Chem. Int. Ed.* **2024**, *63*, e202319502. © Copyright 2024 Wiley-VCH GmbH or related companies. All rights reserved, including rights for text and data mining and training of artificial intelligence technologies or similar technologies.</figcaption></figure>

be overcome is the fact that, according to the authors.[41] the switching was carried out in solution before preparing the KBr pellets. In this context, an interesting alternative was explored.[41] [2]rotaxanes $R_p/S_p$-**16a** and $R_p/S_p$-**16b** were added as dopants into a polymer, in particular, a poly(ε-caprolactone) (PCL) stretchable film, maintaining their CPL activity ($g_{\text{lum}}$ = -5.31×10⁻³ for $S_p$-**16a**, and -4.59×10⁻³ for $S_p$-**16b**. The stretching of the film promoted a slight shift of the CPL band attributable to the tunable VIE behavior of the DPAC unit, along with an enhancement of the emission dissymmetry factor to -3.46×10⁻² for $S_p$-**16a**, and -2.99×10⁻² for $S_p$-**16b** (Figure 14).

Another relevant improvement within this context is the incorporation of a light-triggered switching mechanism for the CPL activity in the solid state into a [2]rotaxane recently achieved by Prof. Wang and co-workers (Figure 15).[43] Thus, they developed two [2]rotaxanes based on pillar[5]arene 3 with two different stoppers, an AIE-active TPE unit and a spiropyran (SP) derivative, while a $C_5$ or $C_{10}$ alkyl chain functionalized with a urea moiety was employed as recognition motif. Due to their photochromic character, non-fluorescent spiropyrans can be photoisomerized to luminescent merocyanine (MC) groups by irradiation with UV light (Figure 15a). Interestingly, the authors

<figure><img src="image_36.png" /><figcaption>**Figure 14.** ΔI (left) and $g_{\text{lum}}$ (right) CPL spectra of PCL films doped with [2]rotaxanes $S_p$-**16a,b** unstretched (0%) or with stretching (400%).[41] Adapted with permission from W.-T. Xu, X. Li, P. Wu, W.-J. Li, Y. Wang, X.-Q. Xu, X.-Q. Wang, J. Chen, H.-B. Yang, W. Wang, Dual Stimuli-Responsive [2]Rotaxanes with Tunable Vibration-Induced Emission and Switchable Circularly Polarized Luminescence, *Angew. Chem. Int. Ed.* **2024**, *63*, e202319502. © Copyright 2024 Wiley-VCH GmbH or related companies. All rights reserved, including rights for text and data mining and training of artificial intelligence technologies or similar technologies.</figcaption></figure>

demonstrated that this process can be performed in the solid state (KBr pellets), opening the door to the modification of the (chir-)optical properties of the [2]rotaxanes depending on the SP or MC form of the stopper. In their spiropyran form, rotaxane **17a** showed in the solid state (powder) a main emission band
<figure><img src="image_37.png" /></figure>

<figure><img src="image_38.png" /><figcaption>**Figure 15.** a) Photoisomerization between the spiropyran form (17a-18a) and the merocyanine form (17b-18b) of [2]rotaxanes 17,18 (only the $R_p$ isomers are shown); inset: structure of the SP and MC groups; b) Super-imposed emission spectra of the TPE unit in 17a (powder) and absorption spectra of the MC moiety in 17b (powder) (left) and emission spectra of 17a (powder) before (black line) and after (red line) irradiation with UV light (365 nm) (right); c) CPL spectra (in PMMA) in ΔI (left) and $g_{lum}$ (right) scale of rotaxanes $R_p/S_p$-17a-1,2; d) CPL spectra (KBr pellets) in ΔI (left) and $g_{lum}$ (right) scale of rotaxanes $R_p/S_p$-17b-1,2.[43] Adapted with permission from W.-T. Xu, X. Li, X.-Q. Xu, X.-Q. Wang, H.-B. Yang, W. Wang, Multistate Circularly Polarized Luminescence Switches Based On Dual Stimuli-Responsive Chiral [2]Rotaxanes, *Sci. China Chem.* **2024**, 67, 4202–4211 (https://link.springer.com/article/10.1007/s11426-024-2197-1). © Springer Nature.</figcaption></figure>

between 400 and 600 nm ($\lambda_{max,em} \approx 480$ nm) corresponding to the TPE unit. After irradiating at 365 nm to isomerize the SP unit to the MC form, the fluorescence of the resulting [2]rotaxane 17b was dominated by a band at longer wavelengths ($\lambda_{em} > 600$ nm), while the original TPE emission was

quenched (Figure 15b right). The absence of TPE emission was attributed to a Förster resonance energy transfer (FRET) process that took place between the TPE and MC units due to the overlap of the emission spectrum of the former and the absorption spectrum of the latter (Figure 15b left). The presence of the planar chiral macrocycle enabled chirality in the fluorescence, i.e. CPL emission, due to the chiral information transfer mechanism to either stopper. As a result, after resolution by CSP-HPLC, rotaxanes $R_p/S_p$-17a and $R_p/S_p$-18a, in their SP form, displayed a CPL signal in the solid state (measured in PMMA films) between ca. 400 and 550 nm ($\lambda_{max, CPL} = 450$ nm), corresponding to the TPE emission, with reported $g_{lum}$ values of $1.46 \times 10^{-2}/1.44 \times 10^{-2}$ for $R_p$-17a, $-1.59 \times 10^{-2}/-1.54 \times 10^{-2}$ for $S_p$-17a (Figure 15c) and $1.59 \times 10^{-2}/1.44 \times 10^{-2}$ for $R_p$-18a, $-1.56 \times 10^{-2}/-1.49 \times 10^{-2}$ for $S_p$-18a. It should be noted that two $g_{lum}$ values are given for each configuration of the macrocycle because the SP unit is also chiral, giving rise to two different diastereomers for each macrocycle configuration depending on the configuration of the SP unit. However, the results showed that this configuration of the spiropyran unit had little effect on the $g_{lum}$ value, demonstrating that the chiral information transfer from the macrocycle was the main source of chiral influence on the fluorophore. In this state, the length of the alkyl chain also had little effect on the CPL response, as in both cases the macro-cycle is located around the alkyl chain region close to the thiourea unit, so the effective distance between the chiral source and the fluorophore is similar. After isomerization of the SP core to the MC unit, the CPL band underwent a pronounced redshift in accordance with that of the fluorescence. Therefore, CPL emission (measured in KBr pellets) was now detected above 625 nm ($\lambda_{max, CPL} = 680$ nm) with $g_{lum}$ values of $3.01 \times 10^{-3}/2.94 \times 10^{-3}$ for $R_p$-17b, $-3.06 \times 10^{-3}/-2.86 \times 10^{-3}$ for $S_p$-17b (Figure 15d) and $1.30 \times 10^{-3}/1.28 \times 10^{-3}$ for $R_p$-18b, $-1.30 \times 10^{-3}/-1.26 \times 10^{-3}$ for $S_p$-18b.[44] In this state, the lower dissymmetry factors compared to the rotaxanes with the SP unit could be explained by the distance from the pillar[5]arene ring to the MC moiety being longer than that to the emissive TPE unit in the SP form, weakening the chiral information transfer. The lower $g_{lum}$ values for $R_p/S_p$-18b compared to $R_p/S_p$-17b supported this explanation. Therefore, the authors demonstrated a light-promoted switching mechanism to influence the CPL response of the [2]rotaxanes. Although the influence of the CPL signal was not a consequence of the change in the position of the macrocycle, the rotaxane architecture was still required as the source of the chirotopic space to enable the chiral emission of the fluorophore.

This switching of the CPL response by light-induced isomerization of one of the stoppers was combined with the anion-induced switching already exploited in previous examples, taking advantage of the urea group present in the structure and giving rise to a four-state CPL-active [2]rotaxane (Figure 16).[43] Thus, addition of trifluoroacetate anions to $R_p/S_p$-17a and $R_p/S_p$-18a resulted in the macrocycle moving away from the urea unit and the TPE stopper and decreasing the $g_{lum}$ values ($6.57 \times 10^{-3}/6.22 \times 10^{-3}$ for $R_p$-17a·CF$_3$CO$_2^-$, $-6.66 \times 10^{-2}/-6.48 \times 10^{-2}$ for $S_p$-17a·CF$_3$CO$_2^-$ and $2.84 \times 10^{-3}/2.40 \times 10^{-3}$ for $R_p$-
<figure><img src="image_39.png" /></figure>

<figure><img src="image_40.png" /><figcaption>**Figure 16.** Switching of the position of the macrocycle in [2]rotaxanes **17a,b** and **18a,b** (only the *R*<sub>p</sub> isomers are shown) induced by the addition/removal of the trifluoroacetate anions.</figcaption></figure>

**18a·CF<sub>3</sub>CO<sub>2</sub><sup>-</sup>**, -2.81×10<sup>-3</sup>/-2.72×10<sup>-3</sup> for S<sub>p</sub>-**18a·CF<sub>3</sub>CO<sub>2</sub><sup>-</sup>**). The same switching of the macrocycle location was achieved by the coordination of the trifluoroacetate anions to the urea in rotaxanes *R*<sub>p</sub>/S<sub>p</sub>-**17b** and *R*<sub>p</sub>/S<sub>p</sub>-**18b**. However, in this case, the chiral macrocycle got closer to the fluorophore unit, enhancing the chiral information transfer, which was reflected in an increase of the *g*<sub>lum</sub> to *ca*. 6.00×10<sup>-3</sup> for all the rotaxanes in comparison to the starting *R*<sub>p</sub>/S<sub>p</sub>-**17b** and *R*<sub>p</sub>/S<sub>p</sub>-**18b**. Addition of *Na*<sup>+</sup> reverted the CPL response to its original state.

# 4. Rotaxane-Based Macromolecular and Supramolecular Materials with CPL Activity

## 4.1. CPL-Emissive Macromolecular Rotaxanes

Polyrotaxanes[45] are the most prevalent macromolecular materials based on MIMs. Although the first examples of polyrotaxanes were based on cyclodextrin macrocycles,[46] the investigation of their CPL properties only emerged in 2022, when Prof. Liu, Dr. Zhu and co-workers built a two-dimensional polyrotaxane forming large monolayers.[47] In order to synthesize this macromolecular material, the authors pre-formed a pseudorotaxane between β-D-cyclodextrin (β-D-CD) and 1,6-diaminopyrrole in water and reacted this inclusion complex with 1,3,5-triformylphloroglucinol in water to create the two-dimensional polyrotaxane **19** (Figure 17a) via imine bond formation stabilized by tautomerism in a dynamic covalent synthetic strategy. To maximize the number of pyrene units threaded within β-D-cyclodextrin rings in the final two-dimensional polyrotaxane, one, two, four, eight or ten equivalents of β-D-CD were employed during the synthesis, yielding 8%, 43%, 73%, 75% and 78% of threaded pyrene units, respectively. The two-dimensional polyrotaxanes displayed alternated Cotton effects between 260 and 460 nm on their ECD spectra and negative blue CPL emission between *ca.* 430 nm and 550 nm (λ<sub>max</sub> CPL = 450 nm), indicating a chiral information transfer from the β-D-

<figure><img src="image_41.png" /><figcaption>**Figure 17.** a) Structure of two-dimensional chiral polyrotaxane **19**. It should be noted that the number of β-D-cyclodextrin per unit of two-dimensional chiral polyrotaxane is lower than six; b) ECD and c) CPL spectra of the polyrotaxane **19** with 73% (green line) and 43% (purple line) of threaded pyrene units, and the covalent organic framework obtained in the absence of β-D-CD (magenta line); d) Plot of the absorption and emission dissymmetry factor, *g*<sub>abs</sub> and *g*<sub>lum</sub>, of polyrotaxane **19** versus the percentage of threaded pyrene units.[47] Figures 17b-d adapted with permission from L. Hu, X. Zhu, C. Yang, M. Liu, Two-Dimensional Chiral Polyrotaxane Monolayer with Emergent and Steerable Circularly Polarized Luminescence Angew. Chem. Int. Ed. **2022**, *61*, e202114759. © Copyright 2022 Wiley-VCH GmbH or related companies. All rights reserved, including rights for text and data mining and training of artificial intelligence technologies or similar technologies</figcaption></figure>

cyclodextrin to the pyrene units, while the covalent organic framework obtained in the absence of CD showed red-shifted absorption and emission spectra on account of strong π-π interactions between the monolayers and exhibited no chiroptical activity (Figure 17b,c). The difference in the percentage of threaded pyrene units in the two-dimensional polyrotaxanes had a critical impact on their chiroptical performances. When the material included 8% threaded pyrene units, a moderate *g*<sub>lum</sub> of *ca.* 1×10<sup>-3</sup> was attained. The emission dissymmetry factor kept increasing until 75% of pyrene units were threaded in the polyrotaxane, reaching a value of *ca.* 8×10<sup>-3</sup> although dropped to *ca.* 3×10<sup>-3</sup> if the number of threaded pyrene units was further increased to 78% (Figure 17d). The reason behind
<figure><img src="image_42.png" /></figure>

this *g*<sub>lum</sub> decrease was attributed to the change of nanostructure adopted by the polyrotaxane, changing from a covalent organic framework-like with 75% of pyrene units threaded to a branched polymer-like nanostructure when the number of threaded pyrenes increased to 78% (see structures in figure 17d).

In addition, Prof. Terao, Prof. Masai and colleagues have reported the synthesis of polymers containing chiral [3]rotaxanes with a pyrene recognition site and two α-D-cyclodextrin rings.[48] These polymeric materials exhibited thermoresponsive emission properties and, according to their structure, they could exhibit CPL emission. Thus, such polymers bearing chiral [3]rotaxanes could potentially be employed to create thermoresponsive CPL materials. Unfortunately, their CPL properties were not investigated.

Rotaxane dendrimers[49] are another type of MIM-based macromolecules that have recently emerged as excellent CPL

emitters. Indeed, Dr. Wang, Prof. Wang, Prof. Yang and co-workers have synthesized chiral rotaxane-branched dendrimers **21-Gn** (n = 1, 2, 3) (Figure 18a), which are type III-A rotaxane dendrimers,[50] incorporating planar chiral pillar[5]arene macrocycles with pendant 9,10-distyrylanthracene units, which act as AIE fluorophores.[51] The authors first synthesized, following a threading-and-capping approach, the [2]rotaxane monomer **20** composed of a pillar[5]arene functionalized with two 9,10-distyrylanthracene units in an A1/A2 pattern as the macrocycle and a thread with a recognition site constituted by an aliphatic chain and a urea unit (Figure 18a). The axle showed two different stoppers, one with two TIPS-protected alkynes and another with a Pt monoacetylide complex. The enantiomeric resolution of this [2]rotaxane monomer could be achieved by CSP-HPLC, affording enantiopure planar chiral [2]rotaxanes on a gram scale. Reaction of the monosubstituted Pt acetylide stopper in the enantiopure [2]rotaxane monomer with 1,3,5-

<figure><img src="image_43.png" /><figcaption>**Figure 18.** a) Structure of monomer rotaxane 20 (only the *R*<sub>p</sub> isomer is shown) and synthesis of the chiral rotaxane-branched dendrimers **21-G1**, **21-G2** and **21-G3**, showed in cartoon representations. Inset: structures of the donor TPEC and the acceptor TTMN molecules; b) CPL spectra of aggregated **21-G3** (blue line), **TPEC-21-G3** (green line), **21-G3-TTMN** (orange line) and **TPEC-21-G3-TTMN** (red line) in THF/H<sub>2</sub>O (5:95); c) Diagram showing the differences in *g*<sub>lum</sub> values (aggregates formed in THF/H<sub>2</sub>O (5:95)) depending on the generation of the rotaxane-branched dendrimers **21** and the addition of TPEC and TTMN.[51] Figures 18b,c adapted with permission from X.-Q. Xu, W.-J. Li, D.-Y. Zhang, Y. Zhu, W.-T. Xu, Y. Wang, X.-Q. Wang, W. Wang, H.-B. Yang, Chiral Rotaxane-Branched Dendrimers as Relays in Artificial Light-Harvesting Systems with Boosted Circularly Polarized Luminescence; *Angew. Chem. Int. Ed.* **2025**, *64*, e202419434. © Copyright 2025 Wiley-VCH GmbH or related companies. All rights reserved, including rights for text and data mining and training of artificial intelligence technologies or similar technologies.</figcaption></figure>
<figure><img src="image_44.png" /></figure>

tris(4-ethynylphenyl)benzene in the presence of Cul afforded the first generation of the enantiopure rotaxane-branched dendrimer $R_p/S_p$-21-G1 in excellent yield (90%). Removal of the TIPS groups of the stoppers of enantiopure $R_p/S_p$-21-G1 employing tetrabutylammonium fluoride followed by transmetallation reaction with the monosubstituted Pt acetylide stopper of the enantiopure [2]rotaxane monomer yielded the second generation of the rotaxane dendrimer $R_p/S_p$-21-G2 (80% yield). The same strategy was utilized for growing the third generation of the enantiopure rotaxane dendrimer $R_p/S_p$-21-G3, again in excellent yield (82%) (Figure 18a). On account of the presence of the 9,10-distyrylanthracene units attached to the pillar[5]arene macrocycles, all generations of the rotaxane dendrimers exhibited AIE, with weak luminescence when dissolved in THF ($\Phi_F$ < 1%) and a huge increase of their $\Phi_F$ reaching 59.2%, 60.9%, and 50.5% for, respectively, 21-G1, 21-G2, and 21-G3 ($\lambda_{max em}$=550 nm for all three generations) when forming aggregates in THF/H$_2$O (5:95). The [2]rotaxane monomer also displayed AIE, but unlike the rotaxane dendrimers, its $\Phi_F$ only attained 20.7% ($\lambda_{max em}$=550 nm, $\lambda_{ex}$=415 nm). The CPL spectra of the enantiopure planar chiral branched-rotaxane dendrimers $R_p/S_p$-21-Gn were recorded on the aggregates in THF/H$_2$O (5:95) and gave mirror-image responses between ca. 480 and 675 nm (Figure 18b for 21-G3). Remarkably, their emission dissymmetry factor increased with the generation growth. Thus, $R_p/S_p$-21-G1 and the [2]rotaxane monomer 20 showed the same $|g_{lum}|$ values of $4.0 \times 10^{-3}$, while $R_p/S_p$-21-G2 attained $|g_{lum}|$ values of $5.5 \times 10^{-3}$, and $R_p/S_p$-21-G3 presented $|g_{lum}|$ values of $8.0 \times 10^{-3}$ (Figure 18c), demonstrating the influence of the structure on the obtained $|g_{lum}|$ value.

Since the 9,10-distyrylanthracene moiety can play the role of donor or acceptor, the authors have selected two AIE luminogens as donor and acceptor, 4-(1,2,2-triphenylethenyl) benzoic acid (TPEC) and 2-((5-(4-(Diphenylamino)phenyl)thio)phen-2-yl)methylene)-malononitrile (TTMN) (Figure 18a inset), respectively, to investigate the potential energy transfer between these molecules and the rotaxane dendrimers and create artificial light-harvesting systems. While TPEC can bind through its carboxylate group to the urea moieties on the rotaxane dendrimers, TTMN can be encapsulated in hydrophobic pockets of the aggregated rotaxane dendrimers. Thus, the dyad systems **TPEC-21-Gn** with a 9:1 TPEC/dendrimer ratio exhibited in THF/H$_2$O (5:95) yellow emission ($\lambda_{max em}$=550 nm) corresponding to the 9,10-distyrylanthracene units when exciting the TPEC units ($\lambda_{ex}$=360 nm) on account of a FRET mechanism from the latter to the former. The associated energy transfer efficiencies ($\Phi_{ET}$) were calculated to range between 17.9% and 22.7% and their $\Phi_F$ reached 35.4% to 39.4% depending on the rotaxane dendrimer generation. Similarly, 21-Gn-TTMN (rotaxane dendrimer/TTMN 6:1 ratio) displayed in THF/H$_2$O (5:95) the typical orange-red emission ($\lambda_{max em}$=620 nm) of TTMN when exciting the 9,10-distyrylanthracene moieties ($\lambda_{ex}$=415 nm). This FRET mechanism was more efficient with $\Phi_{ET}$ attaining values between 63.6% and 76.3%, nonetheless their $\Phi_F$ ranged between 17.3% and 20.2% depending on the generation of rotaxane dendrimers. To go further and create artificial light-harvesting systems with a

cascade FRET process, triad **TPEC-21-Gn-TTMN** systems with a 54:6:1 TPEC/dendrimer/TTMN ratio were investigated. Thus, when the TPEC was excited by irradiating at 360 nm, an energy transfer took place to the rotaxane dendrimers (21-Gn), followed by a second energy transfer to TTMN, resulting in the orange-red emission centered at 620 nm. These cascade energy transfers were slightly more efficient than those observed for the 21-Gn-TTMN systems, reaching $\Phi_{ET}$ up to 77.9%. The $\Phi_F$ of these artificial light-harvesting systems decreased lightly and ranged between 12.6% and 16.2%. CPL spectra of all these artificial light-harvesting systems were recorded in THF/H$_2$O (5:95) and showed a similar trend. **TPEC-R$_p$/S$_p$-21-Gn** exhibited mirror-image CPL spectra with a yellow emission between 450 nm and 750 nm ($\lambda_{max CPL}$=550 nm) originated from the 9,10-distyrylanthracene units, while $R_p/S_p$-21-Gn-TTMN and **TPEC-R$_p$/S$_p$-21-Gn-TTMN** displayed mirror-image CPL spectra with an orange-red emission between ca. 550 nm and 725 nm ($\lambda_{max CPL}$=620 nm) belonging to the TTMN (Figure 18b). For the **TPEC-R$_p$/S$_p$-21-Gn** systems, the $|g_{lum}|$ were measured to be $2.5 \times 10^{-2}$, $2.3 \times 10^{-2}$, and $2.6 \times 10^{-2}$ when exciting the TPEC units ($\lambda_{ex}$=360 nm) and $0.8 \times 10^{-2}$, $0.9 \times 10^{-2}$, and $1.1 \times 10^{-2}$ when exciting directly the 9,10-distyrylanthracene units ($\lambda_{ex}$=415 nm), for, respectively, **TPEC-R$_p$/S$_p$-21-G1**, **TPEC-R$_p$/S$_p$-21-G2**, and **TPEC-R$_p$/S$_p$-21-G3**. Furthermore, for the $R_p/S_p$-21-Gn-TTMN system, the $|g_{lum}|$ attained $1.7 \times 10^{-2}$, $1.5 \times 10^{-2}$, and $2.0 \times 10^{-2}$ when exciting the 9,10-distyrylanthracene units ($\lambda_{ex}$=415 nm), for, respectively, $R_p/S_p$-21-G1-TTMN, $R_p/S_p$-21-G2-TTMN, and $R_p/S_p$-21-G3-TTMN and the CPL signal became almost silent when exciting the TTMN unit ($\lambda_{ex}$=520 nm). Regarding the triad systems **TPEC-R$_p$/S$_p$-21-Gn-TTMN**, the $|g_{lum}|$ values reached their pinnacle for all three generations of chiral branched-rotaxane dendrimers: $3.1 \times 10^{-2}$, $3.5 \times 10^{-2}$, and $3.8 \times 10^{-2}$ when exciting the TPEC units ($\lambda_{ex}$=360 nm), for, respectively, **TPEC-R$_p$/S$_p$-21-G1-TTMN**, **TPEC-R$_p$/S$_p$-21-G2-TTMN**, and **TPEC-R$_p$/S$_p$-21-G3-TTMN**.

Using these artificial light-harvesting systems, the authors have been able to exploit their luminescence differences to create multi-color polymethyl methacrylate films displaying QR Codes that can be scanned under excitation at 365 nm but are unreadable under excitation at 450 nm. In addition, by capitalizing on their CPL emission, a display with eight digits was constructed using these artificial light-harvesting systems. This display can show a different pattern under excitation at 365 nm when observing its luminescence at 620 nm and its CPL emission with a right circular polarizer at 550 nm. Thus, these artificial light-harvesting systems based on chiral rotaxane-branched dendrimers are promising materials for encryption and information storage.

## 4.2. CPL-Emissive Rotaxane-Based Supramolecular Assemblies

While supramolecular materials composed by inclusion complexes and pseudo-rotaxanes exhibiting CPL activity are legion,[52] examples of CPL-emissive supramolecular assemblies based on MIMs are extremely rare and, to the best of our knowledge, consist exclusively of a single rotaxane architecture. In 2022, Dr. Liu, Prof. Tian, Prof. Zhu and co-workers, described
<figure><img src="image_45.png" /></figure>

the synthesis and the self-assembly of [1]rotaxane 22 (Fig-
ure 19a) constructed from mono-6-deoxy-6-(amino)-β-D-cyclo-
dextrin and dansyl chloride.[53] [1]Rotaxane 22 displayed an
emissive band between 430 nm and 650 nm with a single
maximum centered at 516 nm in DMSO (Φ<sub>F</sub>=64.0%) and MeOH
(Φ<sub>F</sub>=36.7%), or at 530 nm in H<sub>2</sub>O (Φ<sub>F</sub>=17.3%). The CPL
spectrum (Figure 19b) of [1]rotaxane 22 showed a negative
response in all three solvents with distinct g<sub>lum</sub> values of
-6.4×10<sup>-4</sup>, -3.1×10<sup>-4</sup> and -2.6×10<sup>-4</sup> for, respectively, H<sub>2</sub>O,
MeOH and DMSO, while a reference dansyl compound
displayed no CPL activity, demonstrating the chiral information
transfer from the β-D-cyclodextrin to the fluorescent dansyl unit
in the [1]rotaxane. The differences in the optical and chiroptical
properties of [1]rotaxane 22 depending on the solvent were
attributed to the formation of different conformers according to
the solvent utilized. 22 was further engaged into self-assembly
experiments. In H<sub>2</sub>O, the [1]rotaxane self-assembled into two
types of nanostructures: (i) a channel-type nanostructure, which
was the thermodynamic self-assembled material, and (ii) a
cage-type nanostructure, which was the kinetic self-assembled
material. The self-assembly of [1]rotaxane 22 resulted in a

<figure><img src="image_46.png" /><figcaption>**Figure 19.** a) Structure of chiral [1]rotaxane 22; b) CPL spectra of [1]rotaxane 22 in H₂O (blue line), MeOH (red line) and DMSO (green line), and a reference dansyl compound in H₂O. CPL spectra in solid state of the c) channel and the d) cage-type nanomaterials based on [1]rotaxane 22 depending on the temperature. Switching of the fluorescence (red line) and the CPL emission (blue line) of the e) channel and the f) cage-type nanomaterials based on [1]rotaxane 22 following heating and cooling cycles in the solid state.[53] Figures 19b-f adapted with permission from X. Song, X. Zhu, S. Qiu, W. Tian, M. Liu, Self-Assembly of Adaptive Chiral [1]Rotaxane for Thermo-Rulable Circularly Polarized Luminescence *Angew. Chem. Int. Ed.* **2022**, *61*, e202208574. © Copyright 2022 Wiley-VCH GmbH or related companies. All rights reserved, including rights for text and data mining and training of artificial intelligence technologies or similar technologies.</figcaption></figure>

hypochromic shift of its emission (λ<sub>maxem</sub>=459 and 493 nm, in
the case of the channel and the cage-type materials, respec-
tively) accompanied by an extremely important increase in
fluorescence quantum yields, which reached values of 92% and
80% for the thermodynamic and the kinetic self-assembled
materials, respectively. Both supramolecular materials exhibited,
this time, positive CPL signals in the solid state with g<sub>lum</sub> values
of 2.4×10<sup>-2</sup> and 0.88×10<sup>-2</sup> for the channel and the cage-type
nanostructure, respectively. These differences are attributed to
the fact that a stronger nanoconfinement effect is present in
the case of the thermodynamically self-assembled product. In
addition, for the channel-type nanomaterial, increasing the
temperature from 25°C to 90°C enhanced its emission
dissymmetry factor (g<sub>lum</sub>=4.0×10<sup>-2</sup>) and resulted in a bath-
ochromic shift (λ<sub>max em</sub>=484 nm) of its fluorescence (Figure 19c)
and a decrease of its quantum yield (Φ<sub>F</sub>=80%). Remarkably,
the switching of the CPL properties of this thermorespective
nanomaterial was reversible for at least five heating-cooling
cycles without loss of performance (Figure 19e). The reason
behind this increase in the emission dissymmetry factor is the
reduction of the nanospaces of the channel, as suggested by
variable-temperature powder X-ray diffraction. In contrast,
heating the cage-type nanomaterial, obtained from [1]rotaxane
22, caused a decrease in the g<sub>lum</sub> values, reaching 0.26×10<sup>-2</sup>,
accompanied again by a bathochromic shift of its fluorescence
(Figure 19d) and a lower quantum efficiency (Φ<sub>F</sub>=66%). In the
case of the kinetic self-assembled material, the fluorescence
and CPL properties were not recovered after cooling down the
material to 25°C. After each heating-cooling cycle, the CPL
performances declined, whereas the fluorescence properties
remained stable after the first heating cycle (Figure 19f).

**5. Summary and Outlook**

After discussing the current state of the field, we can draw the
conclusion that chiral rotaxanes and catenanes are promising
scaffolds for the development of CPL organic emitters.
Compared to other organic structures, these interlocked
architectures offer different advantages that make them very
attractive for this purpose. On the one hand, as discussed in
section 2, they allow the induction of CPL on an achiral
fluorophore by its inclusion in an interlocked structure with a
chiral component. As we showed, this chiral component can be
the macrocycle, either with covalent stereogenic centers, such
as cyclodextrins, or with other types of chirality, such as planar
chirality in the case of pillar[5]arenes. However, it was also
demonstrated that chiral elements on the thread could also
influence a CPL response in the macrocycle. This strategy
represents an interesting approach for obtaining CPL emission
from different achiral fluorophores, which may be difficult to
embed into other chiral scaffolds but can be easily incorporated
into rotaxanes or catenanes. As discussed, this induction was
achieved mainly in the solid state with the use of AIE
fluorophores, although it is also possible in solution with
excimers if a preferential orientation is forced or with
fluorophores in which a chiral conformation can be favored. On
<figure><img src="image_47.png" /></figure>

the other hand, the g<sub>lum</sub> values of a CPL-active unit can be influenced when embedded into an interlocked structure, opening the door to the enhancement of the chiroptical properties. Thus, the restriction of molecular motions due to the interlocked structure can block racemization pathways, enabling a CPL response or favoring a certain geometry with enhanced signals. Of course, this is not an effect that can be achieved in any system. If no structural parameters of the fluorophore or their arrangements are modified, then the g<sub>lum</sub> values are not expected to be altered, so a rational design is needed. Finally, the possibility of precisely controlling the conformation in those interlocked structures by external stimuli has proven to be a powerful approach to modulate CPL emission, mainly its intensity (either an "on-off" switching or a change in the g<sub>lum</sub> value) but also the wavelength or the sign of the signal, both in solution and in the solid state, thus opening the way to their incorporation into functional devices such as CPL-OLEDs.

Considering the potential of this kind of structures, it is not
difficult to understand the huge development experienced by
this topic in very recent years. Nevertheless, there are still
plenty of challenges ahead that need to be addressed if
rotaxanes and catenanes are set to play a key role in the
development of CPL materials and devices. Thus, more diverse
structures, based on other chiral subunits are yet to be
explored. For example, cyclodextrins and pillar[5]arenes are by
far the most exploited macrocycles, but many other chiral
macrocycles could be evaluated.[54] Another key point that
remains almost completely underexplored is the study of the
CPL response of MIMs that display the specific types of chirality
that arise from the mechanical bond, such as mechanical planar
chirality in rotaxanes or those derived from co-conformational
stereogenic centers. This is an almost unexplored vast area that
needs to be explored, where different designs have to be
synthesized and studied to get insight into how good the CPL
responses that can be obtained are, what the structural
requirements are, what the suitable positions where the
fluorophores can be placed are or which kinds of chirality, if
any, are more efficient. Therefore, many questions remain open
within this topic. Additionally, investigation of the CPL response
in other interlocked architectures, such as higher order links,[55]
is yet to be tackled, although promising structures have already
been reported. For instance, Leigh and co-workers built a chiral
luminescent Star of David [2]catenane, yet its CPL activity was
not examined.[56] However, perhaps we could point as the
bigger challenge, associated with one of the biggest prospects
that is the possibility of CPL modulation, the further implemen-
tation of switching mechanisms that would enable this
modulation directly in the solid state, maybe by the use of light
as the external stimuli or in an electric-induced fashion.
Therefore, we believe that although excellent research has been
carried out so far, we are still at the dawn of the field and a vast
number of options are yet to be explored, which will, for sure,
lead to outstanding advances in this field.

**Acknowledgements**

This work was supported by grant PID2023-146433NB-100
funded by MICIU/AEI/10.13039/501100011033 and by ERDF/EU.
A.H.G.D acknowledges the CNRS and the University of Rennes
for their support.

**Conflict of Interests**

The authors declare no conflict of interest.

**Data Availability Statement**

Data sharing is not applicable to this article as no new data
were created or analyzed in this study.

Keywords: circularly polarized luminescence · rotaxanes · catenanes · chirality

[1] a) J. P. Riehl, F. S. Richardson, *Chem. Rev.* **1986**, *86*, 1–16; b) W. R. Kitzmann, J. Freudenthal, A.-P. M. Reponen, Z. A. VanOrman, S. Feldmann, *Adv. Mater.* **2023**, *35*, 2302279.

[2] a) G. Longhi, E. Castiglioni, J. Koshoubu, G. Mazzeo, S. Abbate, *Chirality* **2016**, *28*, 696–707; b) L. Arrico, L. Di Bari, F. Zinna, *Chem. Eur. J.* **2021**, *27*, 2920–2934.

[3] a) J.-L. Ma, Q. Peng, C.-H. Zhao, *Chem. Eur. J.* **2019**, *25*, 15441–15454; b) Y. Gao, C. Ren, X. Lin, T. He, *Front. Chem.* **2020**, *8*, 458; c) L. Zhang, H.-X. Wang, S. Li, M. Liu, *Chem. Soc. Rev.* **2020**, *49*, 9095–9120; d) K. Takaishi, C. Maeda, T. Ema, *Chirality* **2023**, *35*, 92–103.

[4] a) G. Albano, A. Taddeucci, G. Pescitelli, L. Di Bari, *Chem. Eur. J.* **2023**, *29*, e202301982; b) D. F. De Rosa, P. Stachelek, D. J. Black, R. Pal, *Nat. Commun.* **2023**, *14*, 1537.

[5] a) D.-Y. Liu, L.-Y. Xiong, X.-Y. Dong, Z. Han, H.-L. Liu, S.-Q. Zang, *Angew. Chem. Int. Ed.* **2024**, *63*, e202410416; b) N. Yang, J. Zhao, W. Liu, Y. Li, Y. Yang, *Adv. Opt. Mater.* **2024**, *13*, 2402128; c) L. E. MacKenzie, R. Pal, *Nat. Rev. Chem.* **2021**, *5*, 109–124; d) I. López-Sicilia, A. M. Ortuño, P. Reine, D. Otero, M. T. Martín-Romero, L. Camacho, L. Álvarez de Cienfuegos, A. Orte, J. J. Giner-Casares, D. Miguel, J. M. Cuerva, *J. Mat. Chem. C* **2023**, *11*, 2591–2599.

[6] F. Furlan, J. M. Moreno-Naranjo, N. Gasparini, S. Feldmann, J. Wade, M. J. Fuchter, *Nat. Photonics* **2024**, *18*, 658–668.

[7] T. Zhang, Y. Zhang, Z. He, T. Yang, X. Hu, T. Zhu, Y. Zhang, Y. Tang, J. Jiao, *Chem. Asian J.* **2024**, *19*, e202400049.

[8] Z.-L. Gong, X. Zhu, Z. Zhou, S.-W. Zhang, D. Yang, B. Zhao, Y.-P. Zhang, J. Deng, Y. Cheng, Y.-X. Zheng, S.-Q. Zang, H. Kuang, P. Duan, M. Yuan, C.-F. Chen, Y. S. Zhao, Y.-W. Zhong, B. Z. Tang, M. Liu, *Sci. China Chem.* **2021**, *64*, 2060–2104.

[9] M. Xue, Y. Yang, X. Chi, X. Yan, F. Huang, *Chem. Rev.* **2015**, *115*, 7398–7501.

[10] a) G. Gil-Ramírez, D. A. Leigh, A. J. Stephens, *Angew. Chem. Int. Ed.* **2015**, *54*, 6110–6150; b) Q. Chen, K. Zhu, *Chem. Soc. Rev.* **2024**, *53*, 5677–5703.

[11] a) J.-P. Sauvage, *Angew. Chem. Int. Ed.* **2017**, *56*, 11080–11093; b) J. F. Stoddart, *Angew. Chem. Int. Ed.* **2017**, *56*, 11094–11125.

[12] a) S. Erbas-Cakmak, D. A. Leigh, C. T. McTernan, A. L. Nussbaumer, *Chem. Rev.* **2015**, *115*, 10081–10206; b) I. Aprahamian, *ACS Cent. Sci.* **2020**, *6*, 347–358; c) S. Zhang, Y. An, X.-M. Chen, Q. Li, *Smart Mol.* **2023**, *1*, e20230015.

[13] A. H. G. David, J. F. Stoddart, *Isr. J. Chem.* **2021**, *61*, 608–621.

[14] a) N. H. Evans, *Chem. Eur. J.* **2018**, *24*, 3101–3112; b) E. M. G. Jamieson, F. Modicom, S. M. Goldup, *Chem. Soc. Rev.* **2018**, *47*, 5266–5311; c) S. M. Goldup, *Acc. Chem. Res.* **2024**, *57*, 1696–1708.

[15] a) R. Gu, C.-X. Zhao, D.-H. Qu, *Mol. Syst. Des. Eng.* **2023**, *8*, 832–841; b) S. Zhang, Y. An, X.-M. Chen, I. Aprahamian, Q. Li, *Responsive Mater.* **2023**, *1*, e20230023.
<figure><img src="image_48.png" /></figure>

[16] M. Inouye, K. Hayashi, Y. Yonenaga, T. Itou, K. Fujimoto, T.-a. Uchida, M. Iwamura, K. Nozaki, *Angew. Chem. Int. Ed.* **2014**, *53*, 14392–14396.

[17] Y. Ohishi, M. Inouye, *Tetrahedron Lett.* **2019**, *60*, 151232.

[18] K. Hayashi, Y. Miyaka, Y. Ohishi, T.-a. Uchida, M. Iwamura, K. Nozaki, M. Inouye, *Chem. Eur. J.* **2018**, *24*, 14613–14616.

[19] a) Z. Peng, P.-P. Jia, X.-Q. Wang, X.-L. Zhao, H.-B. Yang, W. Wang, *CCS Chem.* **2024**, *6*, 2489–2501; b) X.-H. Zhou, X. Zhang, Y.-R. Song, X. Li, L.-T. Bao, W.-T. Xu, X.-Q. Wang, H.-B. Yang, W. Wang, *Angew. Chem. Int. Ed.* **2025**, *64*, e202415190.

[20] S. Fa, T. Kakuta, T.-a. Yamagishi, T. Ogoshi, *Chem. Lett.* **2019**, *48*, 1278–1287.

[21] J. Mei, N. L. C. Leung, R. T. K. Kwok, J. W. Y. Lam, B. Z. Tang, *Chem. Rev.* **2015**, *115*, 11718–11940.

[22] a) M. Rémy, I. Nierengarten, B. Park, M. Holler, U. Hahn, J.-F. Nierengarten, *Chem. Eur. J.* **2021**, *27*, 8492–8499; b) N. Becharguia, I. Nierengarten, J.-M. Strub, S. Cianférani, M. Rémy, E. Wasielewski, R. Abidi, J.-F. Nierengarten, *Chem. Eur. J.* **2024**, *30*, e202304131.

[23] S. D. P. Fielden, D. A. Leigh, C. T. McTernan, B. Pérez-Saavedra, I. J. Vitoria-Yrezabal, *J. Am. Chem. Soc.* **2018**, *140*, 6049–6052.

[24] Strictly, each of the aromatic rings in the pillar[5]arene can adopt $R_p$ or $S_p$ configuration. However, in this kind of systems, pillar[5]arenes generally appear as $(R_{pr}, R_{pr}, R_{pr}, R_p)$ or $(S_{pr}, S_{pr}, S_{pr}, S_p)$ stereoisomers, i.e. with all of the rings in the same configuration, so the enantiomers of 3 can be shortened as $R_{pr}-3$ and $S_p-3$.

[25] T. Ogoshi, K. Kitajima, S. Fujinami, T.-a. Yamagishi, *Chem. Commun.* **2011**, *47*, 10106–10108.

[26] J.-L. Song, C. Chen, X. Li, Y. Jiang, Z. Peng, X.-Q. Wang, H.-B. Yang, W. Wang, *Nat. Commun.* **2024**, *15*, 10531.

[27] W.-T. Xu, Z. Peng, P. Wu, Y. Jiang, W.-J. Li, X.-Q. Wang, J. Chen, H.-B. Yang, W. Wang, *Chem. Sci.* **2024**, *15*, 7178–7186.

[28] a) Z. Zhang, G. Sun, W. Chen, J. Su, H. Tian, *Chem. Sci.* **2020**, *11*, 7525–7537; b) Z. Zhang, X. Jin, X. Sun, J. Su, D.-H. Qu, *Coord. Chem. Rev.* **2022**, *472*, 214768.

[29] D. A. Leigh, M. Á. F. Morales, E. M. Pérez, J. K. Y. Wong, C. G. Saiz, A. M. Z. Slawin, A. J. Carmichael, D. M. Haddleton, A. M. Brouwer, W. J. Buma, G. W. H. Wurpel, S. León, F. Zerbetto, *Angew. Chem. Int. Ed.* **2005**, *44*, 3062–3067.

[30] A. H. G. David, R. Casares, J. M. Cuerva, A. G. Campaña, V. Blanco, *J. Am. Chem. Soc.* **2019**, *141*, 18064–18074.

[31] F. Coutrot, *ChemistryOpen* **2015**, *4*, 556–576.

[32] X. Li, W.-T. Xu, X.-Q. Xu, Y. Wang, X.-Q. Wang, H.-B. Yang, W. Wang, *Angew. Chem. Int. Ed.* **2025**, *64*, e202412548.

[33] Y.-L. Huang, W.-C. Hung, C.-C. Lai, Y.-H. Liu, S.-M. Peng, S.-H. Chiu, *Angew. Chem. Int. Ed.* **2007**, *46*, 6629–6633.

[34] Y. Wang, J. Gong, X. Wang, W.-J. Li, X.-Q. Wang, X. He, W. Wang, H.-B. Yang, *Angew. Chem. Int. Ed.* **2022**, *61*, e202210542.

[35] The chirality present in this kind of catenanes used to be described as topological chirality (see, for example, ref 14b). However, in more recent works, such as ref. 14c, the term mechanically planar chirality has been recommended. Therefore, the latter is used in this review.

[36] G. Bella, M. Milone, G. Bruno, A. Santoro, *J. Mat. Chem. C* **2024**, *12*, 3005–3012.

[37] X. Wang, P. Wu, Y. Wang, T. Cui, M. Jia, X. He, W. Wang, H. Pan, Z. Sun, H.-B. Yang, J. Chen, *Angew. Chem. Int. Ed.* **2024**, *63*, e202407929.

[38] a) A. Inthasot, S.-T. Tung, S.-H. Chiu, *Acc. Chem. Res.* **2018**, *51*, 1324–1337; b) T.-Y. Tai, Y.-H. Liu, C.-C. Lai, S.-M. Peng, S.-H. Chiu, *Org. Lett.* **2019**, *21*, 5708–5712.

[39] Y. Wang, W.-L. Zhao, Z. Gao, C. Qu, X. Li, Y. Jiang, L. Hu, X.-Q. Wang, M. Li, W. Wang, C.-F. Chen, H.-B. Yang, *Angew. Chem. Int. Ed.* **2025**, *64*, e202417458.

[40] W.-J. Li, Q. Gu, X.-Q. Wang, D.-Y. Zhang, Y.-T. Wang, X. He, W. Wang, H.-B. Yang, *Angew. Chem. Int. Ed.* **2021**, *60*, 9507–9515.

[41] W.-T. Xu, X. Li, P. Wu, W.-J. Li, Y. Wang, X.-Q. Xu, X.-Q. Wang, J. Chen, H.-B. Yang, W. Wang, *Angew. Chem. Int. Ed.* **2024**, *63*, e202319502.

[42] G. Albano, G. Pescitelli, L. Di Bari, *Chem. Rev.* **2020**, *120*, 10145–10243.

[43] W.-T. Xu, X. Li, X.-Q. Xu, X.-Q. Wang, H.-B. Yang, W. Wang, *Sci. China Chem.* **2024**, *67*, 4202–4211.

[44] Although the MC is not chiral and, therefore, diastereomers were no longer possible, two $g_{lum}$ values were given for each macrocycle configuration depending on the initial diastereomer used for the SP to MC isomerization.

[45] a) L. Chen, X. Sheng, G. Li, F. Huang, *Chem. Soc. Rev.* **2022**, *51*, 7046–7065; b) J. S. W. Seale, Y. Feng, L. Feng, R. D. Astumian, J. F. Stoddart, *Chem. Soc. Rev.* **2022**, *51*, 8450–8475.

[46] a) A. Harada, J. Li, M. Kamachi, *Nature* **1992**, *356*, 325–327; b) G. Wenz, B. Keller, *Angew. Chem. Int. Ed. Engl.* **1992**, *31*, 197–199.

[47] L. Hu, X. Zhu, C. Yang, M. Liu, *Angew. Chem. Int. Ed.* **2022**, *61*, e202114759.

[48] T. Nakagawa, S. Ishino, D. Inamori, H. Masai, J. Terao, *ACS Macro Lett.* **2023**, *12*, 751–758.

[49] X.-Q. Wang, W.-J. Li, W. Wang, H.-B. Yang, *Acc. Chem. Res.* **2021**, *54*, 4091–4106.

[50] K. C.-F. Leung, K.-N. Lau, *Polym. Chem.* **2010**, *1*, 988–1000.

[51] X.-Q. Xu, W.-J. Li, D.-Y. Zhang, Y. Zhu, W.-T. Xu, Y. Wang, X.-Q. Wang, W. Wang, H.-B. Yang, *Angew. Chem. Int. Ed.* **2025**, *64*, e202419434.

[52] a) Y. Zhang, D. Yang, J. Han, J. Zhou, Q. Jin, M. Liu, P. Duan, *Langmuir* **2018**, *34*, 5821–5830; b) C. Yang, D. Yang, X. Zhu, Y. Meng, M. Liu, *Langmuir* **2020**, *36*, 12366–12374; c) Y. Guo, Y. Han, X.-S. Du, C.-F. Chen, *ACS Appl. Polym. Mater.* **2022**, *4*, 3473–3481; d) M. Kazem-Rostami, A. Orte, A. M. Ortuño, A. H. G. David, I. Roy, D. Miguel, A. Garci, C. M. Cruz, C. L. Stern, J. M. Cuerva, J. F. Stoddart, *J. Am. Chem. Soc.* **2022**, *144*, 9380–9389; e) C. Tu, W. Wu, W. Liang, D. Zhang, W. Xu, S. Wan, W. Lu, C. Yang, *Angew. Chem. Int. Ed.* **2022**, *61*, e202203541; f) K. Velmurugan, A. Murtaza, A. Saeed, J. Li, K. Wang, M. Zuo, Q. Liu, X.-Y. Hu, *CCS Chem.* **2022**, *4*, 3426–3439.

[53] X. Song, X. Zhu, S. Qiu, W. Tian, M. Liu, *Angew. Chem. Int. Ed.* **2022**, *61*, e202208574.

[54] Z. Sun, H. Tang, L. Wang, D. Cao, *Chem. Eur. J.* **2025**, *31*, e202404217.

[55] a) R. S. Forgan, J.-P. Sauvage, J. F. Stoddart, *Chem. Rev.* **2011**, *111*, 5434–5464; b) Z. Ashbridge, S. D. P. Fielden, D. A. Leigh, L. Pirvu, F. Schaufelberger, L. Zhang, *Chem. Soc. Rev.* **2022**, *51*, 7779–7809.

[56] D. P. August, J. Jaramillo-Garcia, D. A. Leigh, A. Valero, I. J. Vitorica-Yrezabal, *J. Am. Chem. Soc.* **2021**, *143*, 1154–1161.

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<figure><img src="image_49.png" /></figure>

**Static CPL Response**

<figure><img src="image_50.png" /></figure>

**CPL Modulation**

<figure><img src="image_51.png" /></figure>

**Supramolecular and 2D Materials**

<figure><img src="image_52.png" /></figure>

Dr. A. H. G. David*, Prof. J. M. Cuerva, Dr. V. Blanco*

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**Circularly Polarized Luminescence in Rotaxanes and Catenanes**

This review highlights the chiral rotaxanes and catenanes possessing static or stimuli-responsive circularly polarized luminescence (CPL) activity both in solution and in the solid state.

It also discusses macromolecular and supramolecular CPL-emissive materials derived from these mechanically interlocked molecules.