Narcissistic Self-Sorting and Enhanced Luminescence via Catenation in Water

Narcissistic self-sorting, namely that components are able to distinguish “self” from “nonself” during self-assembly, was accomplished via catenation by condensing multiple hydrazides and an aldehyde, or a hydrazide and multiple aldehydes in water. The underneath mechanism of this behavior relies on the corresponding homo [2]catenanes are thermodynamically more favored than their hetero counterparts, because the former containing two identical macrocyclic components are able to maximize the intercomponent noncovalent forces. One of these catenanes contains four 4-phenylpyridinium units, which are often considered barely luminescent due to intramolecular rotations and vibrations that lead to nonradiative annihilation of their excited states. These intramolecular motions, however, are restricted upon integrating 4-phenylpyridiniums within the catenane architecture. As a consequence, compared to its non-interlocked counterparts, this catenane exhibits enhanced uorescence, which represents a novel conceptual model for developing luminescent materials.


Introduction
Nature employs self-assembly to obtain many functional molecular architectures, such as DNA double strands, and the protein shells of viruses, by gathering many subcomponents together, without the need of performing compound puri cation or reacting group protection/deprotection. Even though these subcomponents often contain many competitive ligating sites or interacting units such as base pairs, they are still able to undergo selective ligation in a noninterference manner to produce the target products. Chemists have attempted to mimic these biological behaviors of precise syntheses in arti cial systems. Orthogonal reactivity 1 was taken advantage of for this purpose, namely that two or more noninteracting reactions are used in ligation for synthesize complex molecules. Each of these reactions does not interfere with each other. Self-sorting 2 , which could be expressed in the manner of either narcissistic 3 or social 4 , is also accomplished. These events are conceptually more reminiscent of the biological systems. Here, dynamic reactions are used, allowing the systems to perform error checking and search for their thermodynamic minima. 5,8 The target molecules are sophisticatedly designed to represent the most thermodynamically favored products. When a "wrong" subcomponent takes the place of a "right" one in self-assembly, the self-assembled products become less thermodynamically favored, probably due to disturbance of some inter-component noncovalent forces. As a consequence, the "wrong" subcomponents are repelled, enabling the self-assembled systems to distinguish "self" from "nonself", even though both ligands might have the same or competitive ligating groups. While most of the arti cial self-sorting systems reported in the literature rely on metal-ligand coordination systems, 6 using dynamic bonds or noncovalent forces 7 are less extensively studied. In addition, given that Nature chooses water as the life medium, approaches to accomplish self-sorting in aqueous media must be developed, which helps to fully unravel the underneath mechanisms of the biological events.
Recently, a water-compatible dynamic covalent approach based on acylhydrazone condensation 8 was developed by us 9 and other groups. 10 A variety of molecules with complex molecular architectures, including rings, 9c,10e catenanes, 9a,9e cages 9b and knots 10a,10b,10c,10f are successfully obtained. Here, we employed this dynamic approach to self-assemble a series of catenanes in relatively high yields, each of which is composed of two identical macrocycles. Within each framework of these homo [2]catenanes, the intramolecular noncovalent forces between their building blocks are maximized, including hydrophobic effect, π-π interactions, hydrogen bonding, as well as CH-π interactions, as inferred from crystallographic analysis. Such driving forces allow some of these [2]catenanes to be self-assembled in close to quantitative yields. Narcissistic self-sorting behaviors were clearly observed during self-assembly. That is, combining two different formyl precursor and one acylhydrazide, or one formyl precursor and two acylhydrazides in water only yielded the homo [2]catenanes composed of two identical macrocyclic components selectively, without forming their hybridized catenanes containing two different rings.
One of these homo [2]catenanes contains four luminescent 4-phenylpyridinium groups. In the literature, 11a it is reported that the luminescence of 4-phenylpyridinium is relatively weak, because the central C-C single bond allows the occurrence of intramolecular rotations and vibrations that lead to nonradiative annihilation of its excited states. Within the framework of this [2]catenane, these intramolecular motions are e ciently restricted. As a consequence, the luminophore in the catenane exhibits enhanced luminescence compared to that of the noninterlocked counterparts, by nearly six times. We thus envision that the catenation here provides us a novel approach for developing luminescent materials.

Results And Discussion
Two bisdialdehydes 1a 2+ ·2Cl ─ , 1b 2+ ·2Br ─ were prepared via S N 2 reactions. Both of these two aldehydes are water soluble on account of their cationic nature. In 1a 2+ ·2Cl ─ , each of the formyl units and the corresponding pyridinium unit are bridged by a phenyl functional group ( Figure 1). In 1b 2+ ·2Br ─ , the two formyl units are grafted directly onto the electron-withdrawing pyridinium units ( Figure 1). These formyl units in 1b 2+ are more electrophilic than those in 1a 2+ and therefore, the former are fully hydrolyzed in water. A set of bishydrazide linkers, namely 2a, 2b and 2c, were prepared (Schemes S1-3), each of which bears a glycol chain containing two, three and four ─OCH 2 CH 2 ─ units, respectively (Figure 1).
Within the framework of each [2]catenane, the two macrocyclic components undergo circumvolution motion with respect to each other. The rates of this motion vary in different solvents, as inferred from the corresponding 1 H NMR spectra. For example, in the 1 H NMR spectrum of (1a 2+ ·2c) 2 ·4Cl ─ recorded in D 2 O, the corresponding resonances split into two sets of peaks (Figure 2A), an observation indicating that within the catenane framework, the circumvolution occurs in a relatively slow rate on the 1 H NMR timescale. As a consequence, each of the ring components becomes chemically asymmetrical, i.e., the building block in each macrocyclic component that is encircled within another ring is chemically inequivalent relative to the one outside. In contrast, the 1 H NMR spectrum of (1a 2+ ·2c) 2 ·4PF 6 ─ ( Figure 2B) recorded in CD 3 CN at room temperature exhibits only one set of resonances, indicating the circumvolution motion becomes faster on the 1 H NMR timescale. By performing variable-temperature (VT) 1 H NMR spectroscopic experiments, the energy barrier (∆G) of the circumvolution motion of (1a 2+ ·2c) 2 ·4PF 6 ─ in CD 3 CN was calculated to be 58.5 KJ·mol -1 ( Figure S83). Calculating the corresponding (∆G) of (1a 2+ ·2c) 2 ·4Cl ─ in D 2 O by using VT NMR experiments was unsuccessful, because of the relatively high frozen point of water.
Single crystals of [2]catenanes (1a 2+ ·2c) 2 ·4Cl ─ , (1b 2+ ·2b) 2 ·4Cl ─ and (1b 2+ ·2c) 2 ·4Cl ─ (Figures 3, S88-90), were prepared by slow vapor diffusion of THF into their corresponding aqueous solutions, which provided unambiguous evidence to convince their mechanically interlocked architectures. In each of these [2]catenanes, the cavity of each of the two mutually mechanically interlocked rings is almost fully occupied by another ring, leading to a variety of inter-component close contacts. For example, the framework of (1a 2+ ·2c) 2 exhibits a tightly packed sandwich-shaped architecture. Each of the pyridinium building blocks in one ring undergoes π-π interactions with an adjacent phenyl unit in another ring, as inferred from their short interplane distances, i.e., around 3.4 Å (Figure 3). Close contacts also indicate the occurrence of CH•••O hydrogen bonding and CH-π interactions (Figure 3). In the framework of (1b 2+ ·2b) 2 , ring strain is observed in each of the two macrocycles, i.e., some of its aromatic building blocks become bent to some extent. Such ring strain does not occur in either (1a 2+ ·2c) 2 or (1b 2+ ·2c) 2 , an observation consistent with the aforementioned results that the formation of the catenanes containing 2c residues is more favored and higher yielding than the [2]catenane counterparts containing either 2a or 2b.
Such observation con rms our hypothesis that the [2]catenane framework is able to restrict the intramolecular motions of 4-phenylpyridinium. This mechanically interlocked architecture is thus believed to represent a novel conceptual model in attaining highly emissive nano-constructs.
The mechanism of the luminescence enhancement via catenation is essentially not different from the host-guest recognition systems, 12 as well as the aggregation-induced emission 13 (AIE) systems. However, our approach represents one step of advance. First, different from AIE materials that often function in solid state, the luminescence enhanced via catenation occurs in a homogeneous solution, where the catenane molecules are homogeneously dispersed. Second, the supramolecular complexes are essentially a library of mixture, i.e., the ratio of complexes and the dissociated components might differ by modulating the conditions, such as concentrations, temperature, as well as solvent. As a comparison, the luminescent molecules obtained via catenation are essentially pure substances. This feature, at least to some extent, would help to develop luminescent materials with better quantitative controllability and stability.

Conclusion
In summary, a series of homo [2]catenanes were self-assembled in water via hydrazone condensation. Some of their yields are remarkably high, on account of a variety of intramolecular noncovalent interactions, including hydrophobic effect, π-π interactions, CH-π interactions, as well as hydrogen bonding. When two competitive bisaldehydes namely 1a 2+ and 1b 2+ were combined with a bishydrazide, or two competitive bishydrazides namely 2c and 2d with sharply different structures were combined with a bisaldehyde, narcissistic self-sorting occurred, yielding the corresponding homo [2]catenanes selectively. Such narcissistic self-sorting results from the inherent tendency of a macrocycle to template another version of "itself", in order to maximize the inter-component noncovalent forces that might be disturbed in the putative hetero [2]catenanes. In contrast, combining 1a 2+ with two structurally analogous hydrazides namely 2b and 2c in aqueous media, yielded both homo-and hetero- [2]catenanes. Such selfsorting accomplished in arti cial systems in aqueous solution improves our fundamental understanding on how nature takes advantage of noncovalent interactions to achieve precise syntheses of molecular entities with complex three-dimensional structures when the subcomponents contain same or competitive ligating sites. That is, the target molecules are able to maximize the inter-component noncovalent driving forces.
Within the catenane framework, the intramolecular motions of the 4-phenylpyridinium units, such as rotation around the central C-C bond, are e ciently restricted. Such behaviour helps to suppress nonradiative annihilation of the luminophore, helping to enhance the uorescence of the [2]catenanes. Different from the luminescence enhancement based on supramolecular manner which allows host-guest association/dissociation, the uorescent molecules obtained via catenantion are pure substances. We envision that the usage of such strategy could be expanded to a broader eld of developing luminescent materials, which requires better repeatability and quantitative controllability.  Figure 1 Structural formulas of two dicationic bisaldehydes 1a2+, 1b2+ and three bishydrazides 2a, 2b and 2c. Upon combining one bisaldehyde and one bishydrazide in water, a set of [2]catenanes including (1a2+·2a)2, (1a2+·2b)2, (1a2+·2c)2, (1b2+·2a)2, (1b2+·2b)2 and (1b2+·2c)2 are self-assembled. Charges are balanced by Cl─ or Br─ counteranions, which are omitted here for the sake of clarity.

Figure 2
Partial 1H NMR spectra (500 MHz, 298 K) of A) (1a2+·2c)2·4Cl─ and B) (1a2+·2c)2·4PF6─, which were recorded in D2O and CD3CN respectively. The assignment of each resonance was made by the corresponding two-dimensional NMR spectra shown in the SI. Counterions are omitted in the gure for the sake of clarity.
Counteranions and solvent molecules are omitted for clarity. Some of the close contacts were labeled with dash lines, indicating the occurrence of intramolecular interactions.

Figure 5
Partial high resolution ESI-MS spectrum of a 2:1:1 mixture of 1a2+·2Cl─, 2c and 2d in water. The signals labeled in the spectrum correspond to molecular cations of the homo [2]catenanes namely (1a2+·2c)2 and (1a2+·2d)2 that contain four charges. Counterions are omitted in the gure for the sake of clarity.

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