Holding of planar chirality of pillar[5]arene by kinetic trapping using host-guest interactions with achiral guest solvents

Lighting Up Bispyrene-Functionalized Chiral Molecular Muscles with Switchable Circularly Polarized Excimer Emissions

Aiming at the further extension of the application scope of traditional molecular muscles, a novel bispyrene-functionalized chiral molecular [c2]daisy chain was designed and synthesized. Taking advantage of the unique dimeric interlocked structure of molecular [c2]daisy chain, the resultant chiral molecular muscle emits strong circularly polarized luminescence (CPL) attributed to the pyrene excimer with a high dissymmetry factor (glum) value of 0.010. More importantly, along with the solvent- or anion- induced motions of the chiral molecular muscle, the precise regulation of the pyrene stacking within its skeleton results in the switching towards either "inversed" state with sign inversion and larger glum values or "down" state with maintained handedness and smaller glum values, making it a novel multistate CPL switch. As the first example of chiral molecular muscle-based CPL switch, this proof-of-concept study not only successfully widens the application scopes of molecular muscles, but also provides a promising platform for the construction of novel smart chiral luminescent materials for practical applications.

Diastereoselective Polypseudorotaxane Formation with Planar Chiral Pillar[5]arenes via Co-crystallization Processes

As the number of chiral ring molecules in chiral polyrotaxane increases, the number of possible stereoisomers exponentially increases. Consequently, the selective synthesis of a specific stereoisomer becomes much more challenging. To address this problem, we co-crystallized poly(ethylene glycol) and a diastereomeric ring molecule, pillar[5]arene, in the solid state. The co-crystallization formed polypseudorotaxanes with a high diastereomeric excess (ca. 88 % de), meaning that polypseudorotaxanes containing (S, pS) stereoisomer pillar[5]arene rings were synthesized selectively. By contrast, in solution and evaporation systems, the selectivity remained low (ca. 10 % de). The results suggested that the packing effect by the co-crystallization contributed to the denser assembly of ring molecules on the polymeric chain, resulting in the diastereoselective formation. High diastereoselectivity was also observed even in higher-molecular-weight poly(ethylene glycol)s. These selectivities arose from the cooperative effects of the ring molecules on the polymeric chain, which were supported by calculating the stabilization energy.

Chiral Rotaxane-Branched Dendrimers as Relays in Artificial Light-Harvesting Systems with Boosted Circularly Polarized Luminescence

Starting from AIEgen-functionalized chiral [2]rotaxane building block, we have successfully synthesized a new class of chiral rotaxane-branched dendrimers through controllable divergent strategy for the first time, based on which novel chiral artificial light-harvesting systems (LHSs) were successfully constructed in aqueous phase by sequentially introducing achiral donor and acceptor. More importantly, accompanied by the two-step Förster resonance energy transfer (FRET) process in the resultant artificial LHSs, the sequentially amplified circularly polarized luminescence (CPL) performances were achieved, highlighting that the chiral rotaxane-branched dendrimers could serve as excellent relay for both energy transfer and chirality transmission. Impressively, compared with the sole chiral rotaxane-branched dendrimers, the dissymmetry factors (glum) values of the resultant artificial LHSs were amplified by one order of magnitude up to 0.038, enabling their further applications in information storage and encryption. The proof-of concept study provides not only a feasible approach for the efficient amplification of CPL performances but also a novel platform for the construction of novel chiral luminescent materials.

Pillar[5]arenes decorated with six-membered-ring aromatics at all the substitution positions

Macrocyclic molecules have characteristic properties different from linear ones, such as high symmetry and guest-inclusion ability. To bring drastic changes to these properties, direct introduction of many substituents is a challenging but effective tool. Herein, we attain direct installation of ten six-membered-ring aromatic π-units into both rims of a pillar[5]arene. In contrast to previous pillar[n]arenes with less hindered five-membered-ring units, which showed conformational complexity and crushed crystal structures, the per-phenyl-substituted pillar[5]arene has a cylinder-shaped crystal structure with a dichloromethane inside the cavity and is obtained as a single pair of D 5-symmetric enantiomers. The average dihedral angles between the core and peripheral benzene rings sharply increase from 38° to 66°. These differences indicate the importance of local steric repulsion on both rims for determining the structures and properties of macrocycles.

Chirality Induction and Memory of Pillar[4]arene[1]quinone Derivatives in Visible-Light Range

Pillar[4]arene[1]quinone derivatives (PQXs) were synthesized by the oxidation of pillar[5]arenes, which exhibited notable charge transfer (CT) transitions at approximately 485 nm. Successful chiral resolution of two pairs of enantiomeric conformers was achieved. Despite reduced binding affinity, PQXs demonstrated slower racemization kinetics. Visible-light chiroptical induction with a significant dissymmetry factor was attained by complexing PQXs with a chiral guest. The induced enantiomeric excess could be maintained through competitive binding with an achiral guest, offering a promising strategy for chiral sensing and memory.

Applications of Supramolecular Polymers Generated from Pillar[n]arene-Based Molecules

Supramolecular chemistry enables the manipulation of functional components on a molecular scale, facilitating a "bottom-up" approach to govern the sizes and structures of supramolecular materials. Using dynamic non-covalent interactions, supramolecular polymers can create materials with reversible and degradable characteristics and the abilities to self-heal and respond to external stimuli. Pillar[n]arene represents a novel class of macrocyclic hosts, emerging after cyclodextrins, crown ethers, calixarenes, and cucurbiturils. Its significance lies in its distinctive structure, comparing an electron-rich cavity and two finely adjustable rims, which has sparked considerable interest. Furthermore, the straightforward synthesis, uncomplicated functionalization, and remarkable properties of pillar[n]arene based on supramolecular interactions make it an excellent candidate for material construction, particularly in generating interpenetrating supramolecular polymers. Polymers resulting from supramolecular interactions involving pillar[n]arene find potential in various applications, including fluorescence sensors, substance adsorption and separation, catalysis, light-harvesting systems, artificial nanochannels, and drug delivery. In this context, we provide an overview of these recent frontier research fields in the use of pillar[n]arene-based supramolecular polymers, which serves as a source of inspiration for the creation of innovative functional polymer materials derived from pillar[n]arene derivatives.

Alignment and Dynamic Inversion of Planar Chirality in Pillar[n]arenes

Pillar[n]arenes are symmetrical macrocyclic compounds composed of benzene panels with para-methylene linkages. Each panel usually exhibits planar chirality and prefers chirality-aligned states. Because of this feature, pillar[n]arenes are attractive scaffolds for chiroptical materials that are easy to prepare and optically resolve and show intense circular dichroism (CD) signals. In addition, rotation of the panels endows the chirality of pillar[n]arenes with a dynamic nature. The chirality in tubular oligomers and supramolecular assemblies sometimes show time- and procedure-dependent alignment phenomena. Furthermore, the CD signals of some pillar[n]arenes respond to the addition of chiral guests when their dynamic chirality is coupled with host-guest properties. By using diastereomeric pillar[n]arenes with additional chiral structures, the response can also be caused by achiral guests and changes of the environment, providing molecular sensors.

Diastereoselective Rotaxane Synthesis with Pillar[5]arenes via Co-crystallization and Solid-State Mechanochemical Processes

Chiral rotaxanes have attracted much attention in recent decades for their unique chirality based on their interlocked structures. Thus, selective synthesis methods of chiral rotaxanes have been developed. The introduction of substituents with chiral centers to produce diastereomers is a powerful strategy for the construction of chiral rotaxanes. However, in case of a small energy difference between the diastereomers, diastereoselective synthesis is extremely difficult. Herein, we report a new diastereoselective rotaxane synthesis method using solid-phase diastereoselective [3]pseudorotaxane formation and mechanochemical solid-phase end-capping reactions of the [3]pseudorotaxanes. By co-crystallization of stereodynamic planar chiral pillar[5]arene with stereogenic carbons at both rims and axles with suitable end groups and lengths, the [3]pseudorotaxane with a high diastereomeric excess (ca. 92% de) was generated in the solid state because of higher effective molarity with aid by packing effects and significant energy differences between [3]pseudorotaxane diastereomers. In contrast, the de of the pillar[5]arene was low in solution (ca. 10% de) because of a small energy difference between diastereomers. Subsequent end-capping reactions of the polycrystalline [3]pseudorotaxane with high de in solvent-free conditions successfully yielded rotaxanes while maintaining the high de generated by the co-crystallization.

Adaptive Planar Chirality of Pillar[5]arenes Invertible by n-Alkane Lengths

Chirality of host molecules can be induced and/or inverted by the guest molecules. However, the adapting chirality of hosts to the length of n-alkanes remains a great challenge because n-alkanes are neutral, achiral, and linear molecules, resulting in a weak interaction with most compounds. Herein, we report a system with chirality adapted to n-alkane lengths, using a pillar[5]arene-based macrocyclic host, S-Br, which contains five stereogenic carbons and five terminal bromine atoms on each rim. The electron-rich cavity of S-Br could include n-alkanes and the planar-chiral isomers sensitively inverted in response to the lengths of the complexed n-alkanes. The inclusion of a short n-alkane such as n-pentane made S-Br more inclined to be in the pS-form, whereas the inclusion of long n-alkanes such as n-heptane made the pR-form more favorable. The difference in the stability of the isomers was supported by the crystal structures and the theoretical calculations. Furthermore, temperature drives the adaptive chirality of S-Br with n-alkanes. An n-alkane with middle length, n-hexane, showed the dominance of the pR-form of S-Br at a higher temperature, whereas the pS-form was shown at a lower temperature.

CPL on/off control of an assembled system by water soluble macrocyclic chiral sources with planar chirality

Herein, we report the synthesis and planar chiral properties of a pair of water-soluble cationic pillar[5]arenes with stereogenic carbons. Interestingly, although units of the molecules were rotatable, only one planar chiral diastereomer existed in water in both cases. As a new type of chiral source, these molecules transmitted chiral information from the planar chiral cavities to the assembly of a water-soluble extended π-conjugated compound, affording circularly polarized luminescence (CPL). The chirality transfer process and resulting CPL were extremely sensitive to the feed ratio of the chiral pillar[5]arenes owing to the combined action of their planar chirality, bulkiness, and strong binding properties. When a limited amount of chiral source was added, further assembly of the extended π-conjugated compound into helical fibers with CPL was triggered. Unexpectedly, larger amounts of chiral source destroyed the helical fiber assemblies, resulting in elimination of the chirality and CPL properties from the assembled structures.

Readily obtained pillar[5]arenes with pure planar chirality enabled CPL on/off control of an assembled system by varying the feed ratio.

Reversible "On/Off" Chiral Amplification of Pillar[5]arene Assemblies by Dual External Stimuli

We report dual-stimuli, thermo- and photostimuli, responsive chiral assemblies, of planar-chiral pillar[5]arenes with azobenzene groups on their rims. The azobenzene-substituted planar-chiral pillar[5]arenes were synthesized by copper(I)-catalyzed alkyne-azide cycloaddition "click" reaction of azide-substituted planar-chiral pillar[5]arenes containing S or R stereogenic carbon atoms with an alkyne-substituted azobenzene. These decaazides with stereogenic carbons could act as starting points for a large library of planar-chiral pillar[5]arenes. Homeotropic alignment of azobenzenes, caused by the mesogenic property of the azobenzene groups, was induced by annealing a film of the azobenzene-substituted planar-chiral pillar[5]arenes. The alignment resulted in chiral propagation from the planar-chiral pillar[5]arene cores to the azobenzene area and caused significant chiral amplification consequently. These aligned chiral assemblies were collapsed by trans to cis photoisomerization of the azobenzene groups, resulting in chiral amplification off, and reconstructed by cis to trans thermo-isomerization, again turning on the chiral amplification.

Overtemperature-protection intelligent molecular chiroptical photoswitches

Stimuli-responsive intelligent molecular machines/devices are of current research interest due to their potential application in minimized devices. Constructing molecular machines/devices capable of accomplishing complex missions is challenging, demanding coalescence of various functions into one molecule. Here we report the construction of intelligent molecular chiroptical photoswitches based on azobenzene-fused bicyclic pillar[n]arene derivatives, which we defined as molecular universal joints (MUJs). The Z/E photoisomerization of the azobenzene moiety of MUJs induces rolling in/out conformational switching of the azobenzene-bearing side-ring and consequently leads to planar chirality switching of MUJs. Meanwhile, temperature variation was demonstrated to also cause conformational/chiroptical inversion due to the significant entropy change during the ring-flipping. As a result, photo-induced chiroptical switching could be prohibited when the temperature exceeded an upper limit, demonstrating an intelligent molecular photoswitch having over-temperature protection function, which is in stark contrast to the low-temperature-gating effect commonly encountered.

Realizing overtemperature protection with a molecular device is challenging. Here, the authors demonstrate an overtemperature protection function by integrating thermo- and photoresponsive functions into a pillar[6]arene based pseudocatanene.

Pre-regulation of the planar chirality of pillar[5]arenes for preparing discrete chiral nanotubes

Regulating the chirality of macrocyclic host molecules and supramolecular assemblies is crucial because chirality often plays a role in governing the properties of these systems. Herein, we describe pillar[5]arene-based chiral nanotube formation via pre-regulation of the building blocks' chirality, which is different from frequently used post-regulation strategies. The planar chirality of rim-differentiated pillar[5]arenes is initially regulated by chiral awakening and further induction/inversion through stepwise achiral external stimuli. The pre-regulated chiral information is well stored in discrete nanotubes by interacting with a per-alkylamino-substituted pillar[5]arene. Such pre-regulation is more efficient than post-regulating the chirality of nanotubes.