Host-Guest Chemistry in Binary and Ternary Complexes Utilizing π-Conjugated Carbon Nanorings

DFT insights into assembling [8]MCPP with [14]pyridine nanobelts for amino acid sensing

The progress in designing nanoscale electronic sensors for detecting amino acids (AAs) has attracted considerable interest due to their ability to enable label-free and real-time detection. In this study, the [14]pyridine@[8]MCPP system formed by assembling [8]cycloparaphenylene ([8]MCPP) with [14]pyridine methylene-bridged nanobelts was investigated using density functional theory (DFT) calculations as a potential sensor for five amino acids: glycine (Gly), alanine (Ala), threonine (Thr), leucine (Leu), and aspartic acid (Asp). The sensing capabilities of the assembled structure were evaluated through various analyses, including frontier molecular orbital (FMO), density of states (DOS), quantum theory of atoms in molecules (QTAIM), non-covalent interactions (NCI), and electron density difference (EDD). The energy gap of the [14]pyridine@[8]MCPP assembly was influenced by the presence of amino acids, with the most significant change (-8.75 %) observed in the [14]pyridine@[8]MCPP/Asp complex. Furthermore, QTAIM and NCI analyses indicated that the interactions between AAs and the [14]pyridine@[8]MCPP assembly are primarily governed by van der Waals (vdW) forces. The short recovery times (3.47 × 10-10 to 1.27 × 10-6 s) and favorable sensor responses (0.09-0.17) of the [14]pyridine@[8]MCPP/AA complexes at 298 K suggest that this assembly could serve as an effective material for detecting amino acids. These findings underscore the potential of assembled nanostructures as valuable candidates for amino acid sensing applications.

Ring-in-Ring Assembly Facilitates the Synthesis of a [12]Cycloparaphenylene ABC-Type [3]Catenane

Cycloparaphenylenes (CPPs) represent a significant challenge for the synthesis of mechanically interlocked architectures, because they lack heteroatoms, which precludes traditional active and passive template methods. To circumvent this problem and explore the fundamental and functional properties of CPP rotaxanes and catenanes, researchers have resorted to unusual non-covalent and even to labor-intensive covalent template approaches. Herein, we report a ring-in-ring non-covalent template strategy that makes use of the surprisingly strong non-covalent inclusion of crown ethers into suitably sized CPPs. By threading a secondary ammonium salt through the crown ether and closing the third ring via CuAAC click reaction, we obtained a rare ABC-type hetero-[3]catenane comprising [12]CPP, 24-crown-8 and a dibenzylammonium macrocycle. X-ray crystallography shed light on the ring-in-ring pre-organization and the [3]catenane topology was confirmed by NMR and MS-MS studies. Molecular simulations provided insights into the intriguing ring-vs.-ring-vs.-ring dynamics of the [3]catenane, which are highly dependent on the protonation state of the dibenzylammonium site. This ring-in-ring assembly strategy opens new avenues for the synthesis of complex CPP architectures and their use in functional supramolecular systems.

Optical Axial Chirality Enhancement and Transfer within Aromatic Micelles upon (Co-)encapsulation

Axial chirality is the key physiochemical element, yet its chiroptical utilities have been largely limited to covalent synthesis and infinitely assembled systems so far. Here we report a new application of axially chiral binaphthyls for efficient, optical chirality enhancement and transfer upon noncovalent encapsulation by achiral aromatic micelles in water. The CD activities of dialkoxy binaphthyls are significantly enhanced (up to 7-fold) upon encapsulation by an anthracene-based aromatic micelle. Large emission enhancement (∼4-fold) and efficient guest-to-guest, optical chirality transfer are achieved through coencapsulation of the binaphthyls with achiral cycloparaphenylenes, in a guest-within-guest fashion, by the micelle. The observed unusual properties are derived from the tight inclusion of the chiral guests into the macrocyclic guests, efficiently generated only in the aromatic cavity. Moderate CPL can be observed from the coencapsulated macrocycles within the ternary composites. Furthermore, more than ∼4-fold enhanced guest-to-guest chiroptical transfer is demonstrated with a functionalized cycloparaphenylene through the present coencapsulation strategy.

A Helical Tubular Dyad of [9]Cycloparaphenylene: Synthesis, Chiroptical Properties and Post-Functionalization

The bottom-up synthesis of discrete tubular molecules that mimic the structural features of carbon nanotubes has been a long-standing pursuit for synthetic chemists. As the shortest segments of armchair-type carbon nanotubes, cycloparaphenylenes are regarded as ideal macrocyclic building blocks for achieving this goal. Here we report the synthesis of a helical tubular molecule featuring three diyne linkers between two site-specifically functionalized [9]cycloparaphenylenes. Its C3-symmetrical, radially conjugated structure and solid-state packing have been elucidated by spectroscopic and crystallographic characterizations. Notably, the resolved enantiomers display a circularly polarized luminescence brightness value of 1.47×103 M-1 cm-1, which is among the highest values for chiral organic molecules. Furthermore, the diyne-linked molecule could be directly converted into a thiophene-linked helical molecule, demonstrating the post-functionalization approach for the construction of chiral tubular molecules from cycloparaphenylenes.

Naphthalene Diimide-Embedded Donor-Acceptor Carbon Nanohoops: Photophysical, Photoconductive, and Charge Transport Properties

Designing the architecture of donor-acceptor (D-A) pairs is an effective strategy to tailor the electronic structure of conjugated macrocycles for optoelectronic devices. Herein, we present the synthesis of three D-A nanohoops NDI-[n]CPP (n = 7, 8, 9) containing a naphthalene diimide (NDI) unit as an acceptor and [n]cycloparaphenylenes ([n]CPPs) moieties as donors. The D-A characteristics of NDI-[7-9]CPPs were substantiated through absorption and fluorescence spectroscopic studies, electrochemical investigations, and computational analysis. The device investigations demonstrated that the D-A nanohoops NDI-[7-9]CPPs can serve as the photoconductive layer and demonstrate a significant generation of photocurrent with a fast response upon exposure to light. The magnitude of photocurrent shows high dependence on the size of their rings, with an increasing tendency as the ring size decreases. The generation of photocurrent in free acceptor-based CPP has rarely been reported in the previous literature. Significantly, the C60 complexes of NDI-[7-9]CPPs exhibited a marked enhancement in photocurrent under identical conditions; in particular, the photocurrent of C60⊂NDI-[7]CPP is ca. 3.5 times greater than that of NDI-[7]CPP alone. Furthermore, the potential applications of NDI-[7-9]CPPs in electron- and hole-transport devices have also been explored, revealing the clear evidence of their bipolar behavior as an active charge transport layer.

Acceptor engineering of quinone-based cycloparaphenylenes via post-synthesis for achieving white-light emission in single-molecule

Developing donor-acceptor [n]cycloparaphenylenes (D-A [n]CPPs) with multiple emissions from different emissive states remains challenging yet crucial for achieving white-light emission in single-molecule. Here, we report our explorations into acceptor engineering of quinone-based D-A [10]CPPs (Nq/Aq/Tq[10]CPPs) via a post-lateral annulation using Diels-Alder reactions of oxTh[10]CPP. X-ray analysis reveals that Nq[10]CPP displays a side by side packing via naphthoquione stacking while Aq[10]CPP adopts an intercalated conformation through anthraquinone interaction. Fluorescence investigations reveal that the quinone-based [10]CPPs display distinctive acceptor-dependent dual-emission from both the locally excited state and charge transfer state after single-wavelength excitation in organic solvents, consequently leading to multicolor emissions, in particular, white-light emission in CHCl3 for Aq[10]CPP. In THF/water mixture, quinone-based [10]CPPs and oxTh[10]CPP display a wide range of fluorescence emissions including white-light emission as increasing the fraction of water, accompanying by the formation of nanoparticles as demonstrated by Tyndall effect and SEM. Interestingly, the fluorescence of Aq[10]CPP can be switched from white to blue in CHCl3 upon redox. Our investigations demonstrate that acceptor engineering not only endows quinone-based [10]CPPs with two distint emissive states for achieving white-light emission but also highlights an effective post-synthetic strategy for functionalizing CPP nanohoops with desirable properties.

Synthesis and Photophysical Properties of Small Helical Carbon Nanohoops Combined with Computational Studies on Racemization Pathway

We herein report the facile synthesis of two helical carbon nanorings with small ring sizes, cyclo[6]paraphenylene-1,5-naphthylene ([6]CPPNap1,5), and cyclo[6]paraphenylene-1,5-anthrylene ([6]CPPAn1,5). The structures were determined by NMR and HR-MS. X-ray single-crystal data of [6]CPPNap1,5 was also achieved. The strain energy and racemization processes were investigated by DFT calculations. The reduced ring sizes result in increased ring strain and elevated energy barriers. The photophysical properties were studied by UV-Vis absorption, fluorescence emission, and time-resolved fluorescence decay.

Key-to-lock halogen bond-based tetragonal pyramidal association of iodonium cations with the lacune rims of beta-octamolybdate

The structure-directing "key-to-lock" interaction of double σ-(IIII)-hole donating iodonium cations with the O-flanked pseudo-lacune rims of [β-Mo8O26]4- gives halogen-bonded iodonium-beta-octamolybate supramolecular associates. In the occurrence of their tetragonal pyramidal motifs, deep and broad σ-(IIII)-holes of a cation recognize the molybdate backbone, which provides an electronic pool localized around the two lacunae. The halogen-bonded I⋯O linkages in the structures were thoroughly studied computationally and classified as two-center, three-center bifurcated, and unconventional "orthogonal" I⋯O halogen bonds. In the latter, the O-atom approaches orthogonally the C-IIII-C plane of an iodonium cation and this geometry diverge from the IUPAC criteria for the identification of the halogen bond.

Synthesis, Characterization, and Resistive Memory Behaviors of Highly Strained Cyclometalated Platinum(II) Nanohoops

Strained carbon nanohoops exhibit attractive photophysical properties due to their unique π-conjugated structure. However, incorporation of such nanohoops into the pincer ligand of metal complexes has rarely been explored. Herein, a new family of highly strained cyclometalated platinum(II) nanohoops has been synthesized and characterized. Strain-promoted C-H bond activation has been observed during the metal coordination process, and Hückel-Möbius topology and random-columnar packing in the solid state are found. Transient absorption spectroscopy revealed the size-dependent excited state properties of the nanohoops. Moreover, the nanohoops have been successfully employed as active materials in the fabrication of solution-processable resistive memory devices, including the use of the smallest platinum(II) nanohoop for the fabrication of a binary memory, with low switching threshold voltages of ca. 1.5 V, high ON/OFF current ratios, and good stability. These results demonstrate that strain incorporation into the structure can be an effective strategy to fundamentally fine-tune the reactivity, optoelectronic, and resistive memory properties.

A Conjugated Phenylene Nanocage with a Guest-Adaptive Deformable Cavity

The highly strained, phenylene-derived organic cages are typically regarded as very rigid entities, yet their deformation potential and supramolecular properties remain underexplored. Herein, we report a pliable conjugated phenylene nanocage by synergistically merging rigid and flexible building blocks. The anisotropic cage molecule contains branched phenylene chains capped by a calix[6]arene moiety, the delicate conformational changes of which endow the cage with a remarkably deformable cavity. When complexing with fullerene guests, the cage showcases excellent guest-adaptivity, with its cavity volume capable of swelling by as much as 85 %.

Modular Synthesis of Improbable Rotaxanes with All-Benzene Scaffolds

"Improbable" rotaxanes consisting of interlocked conjugated components represent non-trivial synthetic targets, not to mention those with all-benzene scaffolds. Herein, a modular synthetic strategy has been established using an isolable azo-linked pre-rotaxane as the core module, in which the azo group functions as a tracelessly removable template to direct mechanical bond formations. Through versatile connections of the pre-rotaxane and other customizable modules, [2]- and [3]rotaxanes derived from all-benzene scaffolds have been accomplished, demonstrating the utility and potential of the synthetic design for all-benzene interlocked supramolecules.

Monitoring Hierarchical Assembly of Ring-in-Ring and Russian Doll Complexes Based on Carbon Nanoring by Förster Resonance Energy Transfer

We presented the construction of the ring-in-ring and Russian doll complexes on the basis of triptycene-derived carbon nanoring (TP-[12]CPP), which not only acts as a host for pillar[5]arene (P5A) but also serves as an energy donor for building Förster resonance energy transfer (FRET) systems. We also demonstrated that their hierarchical assembly processes could be efficiently monitored in real time using FRET. NMR, UV-vis and fluorescence, and mass spectroscopy analyses confirmed the successful encapsulation of the guests P5A/P5A-An by TP-[12]CPP, facilitated by C-H···π and ···π interactions, resulting in the formation of a distinct ring-in-ring complex with a binding constant of Ka = 2.23 × 104 M-1. The encapsulated P5A/P5A-An can further reverse its role to be a host for binding energy acceptors to form Russian doll complexes, as evidenced by the occurrence of FRET and mass spectroscopy analyses. The apparent binding constant of the Russian doll complexes was up to 3.6 × 104 M-1, thereby suggesting an enhanced synergistic effect. Importantly, the Russian doll complexes exhibited both intriguing one-step and sequential FRET dependent on the subcomponent P5A/P5A-An during hierarchical assembly, reminiscent of the structure and energy transfer of the light-harvesting system presented in purple bacteria.