A contorted nanographene shelter

Chemically Mediated Artificial Electron Transport Chain

Electron transport chains (ETCs) are ubiquitous in nearly all living systems. Replicating the complexity and control inherent in these multicomponent systems using ensembles of small molecules opens up promising avenues for molecular therapeutics, catalyst design, and the development of innovative energy conversion and storage systems. Here, we present a noncovalent, multistep artificial electron transport chains comprising cyclo[8]pyrrole (1), a meso-aryl hexaphyrin(1.0.1.0.1.0) (naphthorosarin 2), and the small molecules I2 and trifluoroacetic acid (TFA). Specifically, we show that 1) electron transfer occurs from 1 to give I3 - upon the addition of I2, 2) proton-coupled electron transfer (PCET) from 1 to give H 3 2 •2+ and H 3 2 + upon the addition of TFA to a dichloromethane mixture of 1 and 2, and 3) that further, stepwise treatment of 1 and 2 with I2 and TFA promotes electron transport from 1 to give first I3 - and then H 3 2 •2+ and H 3 2 + . The present findings are substantiated through UV-vis-NIR, 1H NMR, electron paramagnetic resonance (EPR) spectroscopic analyses, cyclic voltammetry studies, and DFT calculations. Single-crystal structure analyses were used to characterize compounds in varying redox states.

Stepwise Synthesis of Low-Symmetry Hexacationic Pyridinium Organic Cages

An efficient approach was developed for the synthesis of the well-known BlueCage by pre-bridging two 2,4,6-tris(4-pyridyl)-1,3,5-triazine (TPT) panels with one linker followed by cage formation in a much improved yield and shortened reaction time. Such a stepwise methodology was further applied to synthesize three new pyridinium organic cages, C2, C3, and C4, where the low-symmetry cages C3 and C4 with angled panels demonstrated better recognition properties toward 1,1'-bi-2-naphthol (BINOL) than the high-symmetry analogue C2 featuring parallel platforms.

Helical Cage Rotors Switched on by Brake Molecule with Variable Fluorescence and Circularly Polarized Luminescence

While chiral molecular rotors have unique frames and cavities to possibly generate switchable chiroptical functions, it still remains a formidable challenge to precisely restrict their rotations to activate certain functions such as fluorescence as well as circularly polarized luminescence (CPL), which are strongly related to the local molecular rotations. Herein, we design a pair of enantiopure helical cage rotors, which emit light neither at the molecular state nor in the crystal or aggregation states, although they contain luminophore groups. However, upon mounting with fluoroaromatic borane (TFPB) as a molecular brake, the phenyl rotation of the helical cage can be effectively hindered and fluorescence and CPL activities of the molecular cage are switched on. Crystal structure analysis reveals that the rotation is restricted through synergistic B-O-H-N bonding and a fluoroaromatic-aromatic (ArF-Ar) dipole interaction. Moreover, the helical cages are switched on stepwise with color-variable fluorescence and CPL by the inner brake in the molecular state and the outer brake in the supramolecular assemblies, respectively. This work not only provides the design idea of chiroptical molecular rotors but also unveils how fluorescence and CPL could be generated in cage rotor systems.

Tailored Water-Soluble Covalent Organic Cages for Encapsulation of Pyrene and Information Encryption

Forming pyridine salts to construct covalent organic cages is an effective strategy for constructing covalent cage compounds. Covalent organic cages based on pyridine salt structures are prone to form water-soluble supramolecular compounds. Herein, we designed and synthesized a triangular prism-shaped hexagonal cage with a larger cavity and relatively flexible conformation. The supramolecular cage structure was also applied to the encapsulation of pyrene and information encryption.

Constructing Ultrastable Metallo-Cages via In Situ Deprotonation/Oxidation of Dynamic Supramolecular Assemblies

Coordination-driven self-assembly enables the spontaneous construction of metallo-supramolecules with high precision, facilitated by dynamic and reversible metal-ligand interactions. The dynamic nature of coordination, however, results in structural lability in many metallo-supramolecular assembly systems. Consequently, it remains a formidable challenge to achieve self-assembly reversibility and structural stability simultaneously in metallo-supramolecular systems. To tackle this issue, herein, we incorporate an acid-/base-responsive tridentate ligand into multitopic building blocks to precisely construct a series of metallo-supramolecular cages through coordination-driven self-assembly. These dynamic cagelike assemblies can be transformed to their static states through mild in situ deprotonation/oxidation, leading to ultrastable skeletons that can withstand high temperatures, metal ion chelators, and strong acid/base conditions. This in situ transformation provides a reliable and powerful approach to manipulate the kinetic features and stability of metallo-supramolecules and allows for modulation of encapsulation and release behaviors of metallo-cages when utilizing nanoscale quantum dots (QDs) as guest molecules.

Photoreactive Carbon Dioxide Capture by a Zirconium-Nanographene Metal-Organic Framework

The mechanism of photochemical CO₂ reduction to formate by PCN-136, a Zr-based metal-organic framework (MOF) that incorporates light-harvesting nanographene ligands, has been investigated using steady-state and time-resolved spectroscopy and density functional theory (DFT) calculations. The catalysis was found to proceed via a "photoreactive capture" mechanism, where Zr-based nodes serve to capture CO₂ in the form of Zr-bicarbonates, while the nanographene ligands have a dual role of absorbing light and storing one-electron equivalents for catalysis. We also find that the process occurs via a "two-for-one" route, where a single photon initiates a cascade of electron/hydrogen atom transfers from the sacrificial donor to the CO₂-bound MOF. The mechanistic findings obtained here illustrate several advantages of MOF-based architectures in molecular photocatalyst engineering and provide insights on ways to achieve high formate selectivity.

The Green Box: Selenoviologen-Based Tetracationic Cyclophane for Electrochromism, Host-Guest Interactions, and Visible-Light Photocatalysis

The novel selenoviologen-based tetracationic cyclophanes (green boxes 3 and 5) with rigid electron-deficient cavities are synthesized via S_N2 reactions in two steps. The green boxes exhibit good redox properties, narrow energy gaps, and strong absorption in the visible range (370-470 nm), especially for the green box 5 containing two selenoviologen (SeV²⁺) units. Meanwhile, the femtosecond transient absorption (fs-TA) reveals that the green boxes have a stabilized dicationic biradical, high efficiency of intramolecular charge transfer (ICT), and long-lived charge separation state due to the formation of cyclophane structure. Based on the excellent photophysical and redox properties, the green boxes are applied to electrochromic devices (ECDs) and visible-light-driven hydrogen production with a high H₂ generation rate (34 μmol/h), turnover number (203), and apparent quantum yield (5.33 × 10^-2). In addition, the host-guest recognitions are demonstrated between the green boxes and electron-rich guests (e.g., G1:1-naphthol and G2:platinum(II)-tethered naphthalene) in MeCN through C-H···π and π···π interactions. As a one-component system, the host-guest complexes of green box⊃G2 are successfully applied to visible-light photocatalytic hydrogen production due to the intramolecular electron transfer (IET) between platinum(II) of G2 and SeV²⁺ of the green box, which provides a simplified system for solar energy conversion.

Conformationally Confined Emissive Cationic Macrocycle with Photocontrolled Organelle-Specific Translocation

The optimization of molecular conformation and aggregation modes is of great significance in creation of new luminescent materials for biochemical research and medical diagnostics. Herein, a highly emissive macrocycle (1) is reported, which is constructed by the cyclization reaction of triphenylamine with benzyl bromide and exhibits very distinctive photophysical performance both in aqueous solution and the solid state. Structural analysis reveals that the 1 can form self-interpenetrated complex and emit bright yellow fluorescence in the crystal lattice. The distorted yet symmetrical structure can endow 1 with unique two-photon absorption property upon excitation by near-infrared light. Also, 1 can be utilized as an efficient photosensitizer to produce singlet oxygen (1 O2 ) both in inanimate milieu and under cellular environment. More intriguingly, due to the strong association of 1 with negatively charged biomacromolecules, organelle-specific migration is achieved from lysosome to nucleus during the 1 O2 -induced cell apoptosis process. To be envisaged, this conformationally confined cationic macrocycle with photocontrolled lysosome-to-nucleus translocation may provide a feasible approach for in situ identifying different biospecies and monitoring physiological events at subcellular level.

Recent trends in organic cage synthesis: push towards water-soluble organic cages

Research on organic cages has blossomed over the past few years into a mature field of study which can contribute to solving some of the challenging problems. In this review we aim to showcase the recent trends in synthesis of organic cages including a brief discussion on their use in catalysis, gas sorption, host-guest chemistry and energy transfer. Among the organic cages, water-soluble analogues are a special class of compounds which have gained renewed attention in recent times. Due to their advantage of being compatible with water, such cages have the potential of showing biomimetic activities and can find use in drug delivery and also as hosts for catalysis in aqueous medium. Hence, the synthetic strategies for the formation of water-soluble organic cages shall be discussed along with their potential applications.