Helic[6]arene-Based Chiral Pseudo[1]rotaxanes and [1]Rotaxanes

Recent advances in novel chiral macrocyclic arenes

Chiral macrocyclic arenes possess confined three-dimensional asymmetric cavities, electron-rich structures, chiral luminescence properties and excellent enantioselective recognition properties and have become a frontier and hotspot of macrocyclic chemistry and supramolecular chemistry. In recent years, there has been growing interest in the development of novel chiral macrocyclic arenes, which have found applications in various research areas. In this review, the construction, properties and functional applications of novel chiral macrocyclic arenes in enantioselective recognition, chiral sorting and construction of chiral luminescent materials according to their chiral types, including central, axial, planar, and inherent chiralities, are summarized. It is expected that this review will be helpful for research on supramolecular chemistry and for promoting the development of synthetic chemistry, materials chemistry and biochemistry.

Self-Inclusion Complexation of Electron-Accepting Guest into Electron-Donating Cyclic Host by Photoexcitation

Self-inclusion complexes consisting of host-guest conjugates are one of the unique supramolecular structures because they form in-state and out-state depending on the external stimuli. Despite many reports of the stimuli-responsive self-inclusion complex formation, study of the structural relaxation from out-state to in-state by photoexcitation has been unexplored. Herein, we report that an electron-donating host and an electron-accepting guest conjugate exhibits the structural relaxation from out-state to in-state by photoexcitation. Formation of the in-state in the excited state resulted in exciplex emission along with the locally excited emission from the out-state. Moreover, this structural relaxation by photoexcitation was suppressed not only by temperature, but also by the presence of guest molecules, resulting in changes in the ratio of the dual emission intensities.

Ruthenium(II)-Catalyzed Oxidative Annulation of Imidazo[1,5-a]quinolin-2-iums Salts and Internal Alkynes via C-H Bond Activation

Ruthenium(II)-catalyzed synthesis of π-conjugated fused imidazo[1,5-a]quinolin-2-ium derivatives have been achieved via C-H activation of quinoline-functionalized NHC (NHC=N-heterocyclic carbene) and oxidative coupling with internal alkynes. The reaction occurred with high efficiency, broad substrate scope, tolerates a wide range of functional groups and utilized into a gram-scale. Synthetic applications of the coupled product have been exemplified in the late-stage derivatization of various highly functionalized scaffolds. Moreover, most of the annulated products exhibit intense fluorescence and have potential applications in optoelectronic devices. Mechanistic studies have provided insights into the spectroscopic characterization of key five-membered ruthenacycle intermediate and Ru(0) sandwich species. Based on several control experiments, deuterium-kinetic isotope effect, and thermodynamic activation parameters the mechanistic finding demonstrated that fused imidazo-[1,5-a]quinolin-2-ium C(2)-H bond cleavage is the rate-determining step and ruling out the possibility of reductive elimination for controlling the rate of reaction.

Synthesis of 'Impossible' Rotaxanes

'Impossible' rotaxanes, which are constituted by interlocked components without obvious binding motifs, have attracted the interest of the mechanically interlocked molecules (MIMs) community. Within the synthetic efforts reported in the last decades towards the preparation of MIMs, some innovative protocols for accessing 'impossible' rotaxanes have been developed. This short review highlights different selected synthetic examples of 'impossible' rotaxanes, as well as suggests some future directions of this research area.

Highly Luminescent Chiral Carbon Nanohoops via Symmetry Breaking with a Triptycene Unit: Bright Circularly Polarized Luminescence and Size-Dependent Properties

Chiral carbon nanohoops with both high fluorescence quantum yield and large luminescence dissymmetry factor are essential to the development of circularly polarized luminescence (CPL) materials. Herein, the rational design and synthesis of a series of highly fluorescent chiral carbon nanohoops TP-[8-13]CPPs via symmetry breaking with a chiral triptycene motif is reported. Theoretical calculations revealed that breaking the symmetry of nanohoops causes a unique size-dependent localization in the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular obtitals (LUMOs) as the increasing of sizes, which is sharply different from those of [n]cycloparaphenylenes. Photophysical investigations demonstrated that TP-[n]CPPs display size-dependent emissions with high fluorescence quantum yields up to 92.9% for TP-[13]CPP, which is the highest value among the reported chiral conjugated carbon nanohoops. The high fluorescence quantum yields are presumably attributed to both the unique acyclic, and radial conjugations and high radiative transition rates, which are further supported by theoretical investigations. Chiroptical studies revealed that chiral TP-[n]CPPs exhibit bright CPL with CPL brightness up to 100.5 M-1 cm-1 for TP-[11]CPP due to the high fluorescence quantum yield. Importantly, the investigations revealed the intrigued size-dependent properties of TP-[n]CPPs with regards to (chir)optical properties, which follow a nice linear relationship versus 1/n. Such a nice linear relationship is not observed in other reported conjugated nanohoops including CPPs.

Advances in circularly polarized luminescence materials based on chiral macrocycles

Development of circularly polarized luminescence (CPL) materials utilizing supramolecular strategies has recently attracted increasing interest in supramolecular chemistry and materials science. Chiral macrocycles, especially chiral macrocyclic hosts, have stable structures, adjustable internal cavities to encapsulate different guests, and host-guest complexation to induce special photophysical properties. Consequently, various CPL materials based on chiral macrocycles have been developed during the last decade. To gain a better understanding of this rapidly developing research area, it is necessary and also important to summarize the advances in CPL materials based on chiral macrocycles. In this review, CPL materials from different chiral macrocycles, especially classical and newly reported chiral macrocyclic hosts and their derivatives, will be comprehensively summarized. It is believed that this review will be of guiding significance and also very helpful for the development of macrocyclic chemistry and CPL materials.

Tunable Fluorescence and Morphology of Aggregates Built from a Mechanically Bonded Amphiphilic Bistable [2]Rotaxane

Advanced Organic Chemical Materials Co-constructed Mechanically bonded amphiphiles (MBAs), also known as mechanically interlocked molecules (MIMs), have emerged as an important kind of functional building block for the construction of artificial molecular machines and soft materials. Herein, a novel MBA, i. e., bistable [2]rotaxane H2 was designed and synthesized. In the solution state, H2 demonstrated pH and metal ion-responsive emissions due to the presence of a distance-dependent photoinduced electron transfer (PET) process and the fluorescence resonance energy transfer (FRET) process, respectively. Importantly, the amphiphilic feature of H2 has endowed it with unique self-assembly capability, and nanospheres were obtained in a mixed H2 O/CH3 CN solvent. Moreover, the morphology of H2 aggregates can be tuned from nanospheres to vesicles due to the pH-controlled shuttling motion-induced alternation of H2 amphiphilicity. Interestingly, larger spheres with novel pearl-chain-like structures from H2 were observed after adding stoichiometric Zn2+ . In particular, H2 shows pH-responsive emissions in its aggregation state, allowing the visualization of the shuttling movement by just naked eyes. It is assumed that the well-designed [2]rotaxane, and particularly the proposed concept of MBA shown here, will further enrich the families of MIMs, offering prospects for synthesizing more MIMs with novel assembly capabilities and bottom-up building dynamic smart materials with unprecedented functions.

Design and Synthesis of New Type of Macrocyclic Architectures Used for Optoelectronic Materials and Supramolecular Chemistry

Supramolecular chemistry has received much attention for decades. Macrocyclic architectures as representative receptors play a vital role in supramolecular chemistry and are applied in many fields such as supramolecular assembly and host-guest recognition. However, the classical macrocycles generally lack functional groups in the scaffolds, which limit their further applications, especially in optoelectronic materials. Therefore, developing a new design principle is not only essential to better understand macrocyclic chemistry and the supramolecular behaviors, but also further expand their applications in many research fields. In recent years, the doping compounds with main-group heteroatoms (B, N, S, O, P) into the carbon-based π-conjugated macrocycles offered a new strategy to build macrocyclic architectures with unique optoelectronic properties. In particular, the energy gaps and redox behavior can be effectively tuned by incorporating heteroatoms into the macrocyclic scaffolds. In this Minireview, we briefly summarize the design and synthesis of new macrocycles, and further discuss the related applications in optoelectronic materials and supramolecular chemistry.

Chiral Nanocluster Complexes Formed by Host-Guest Interaction between Enantiomeric 2,6-Helic[6]arenes and Silver Cluster Ag20: Emission Enhancement and Chirality Transfer

A pair of chiral nanocluster complexes were formed by the host−guest interaction between the enantiomeric 2,6-helic[6]arenes and nanocluster Ag₂₀. The formation and stability of the nanocluster complexes were experimentally and theoretically confirmed. Meanwhile, the chiral nanocluster complexes exhibited enhanced luminescence and induced CD signals at room temperature in the solid state, revealing the stable complexation and chirality transfer from the chiral macrocycles to the nanocluster Ag₂₀.