Redox‐Guest‐Induced Multimode Photoluminescence Switch for Sequential Logic Gates in a Photoactive Coordination Cage

Heterometallic cages: synthesis and applications

High symmetry metallosupramolecular architectures (MSAs) have been exploited for a range of applications including molecular recognition, catalysis and drug delivery. Recently there have been increasing efforts to enhance those applications by generating reduced symmetry MSAs. While there are several emerging methods for generating lower symmetry MSAs, this tutorial review examines the general methods used for synthesizing heterometallic MSAs with a particular focus on heterometallic cages. Additionally, the intrinsic properties of the cages and their potential emerging applications as host-guest systems and reaction catalysts are described.

Secondary Bracing Ligands Drive Heteroleptic Cuboctahedral PdII12 Cage Formation

The structural complexity of self-assembled metal-organic capsules can be increased by incorporating two or more different ligands into a single discrete product. Such complexity can be useful, by enabling larger, less-symmetrical, or more guests to be bound. Here we describe a rational design strategy for the use of subcomponent self-assembly to selectively prepare a heteroleptic cage with a large cavity volume (2631 ų) from simple, commercially available starting materials. Our strategy involves the initial isolation of a tris(iminopyridyl) Pd^II₃ complex 1, which reacts with tris(pyridyl)triazine ligand 2 to form a heteroleptic sandwich-like architecture 3. The tris(iminopyridyl) ligand within 3 serves as a "brace" to control the orientations of the labile coordination sites on the Pd^II centers. Self-assembly of 3 with additional 2 was thus directed to generate a large Pd^II₁₂ heteroleptic cuboctahedron host. This new cuboctahedron was observed to bind multiple polycyclic aromatic hydrocarbon guests simultaneously.

Guest‐Modulated Circularly Polarized Luminescence by Ligand‐to‐Ligand Chirality Transfer in Heteroleptic Pd II Coordination Cages

Multicomponent metallo‐supramolecular assembly allows the rational combination of different building blocks. Discrete multifunctional hosts with an accessible cavity can be prepared in a non‐statistical fashion. We employ our shape‐complementary assembly (SCA) method to achieve for the first time integrative self‐sorting of heteroleptic Pd^II cages showing guest‐tunable circularly polarized luminescence (CPL). An enantiopure helicene‐based ligand (M or P configuration) is coupled with a non‐chiral emissive fluorenone‐based ligand (A or B) to form a series of Pd₂L₂L′₂ assemblies. The modular strategy allows to impart the chiral information of the helicenes to the overall supramolecular system, resulting in CPL from the non‐chiral component. Guest binding results in a 4‐fold increase of CPL intensity. The principle offers potential to generate libraries of multifunctional materials with applications in molecular recognition, enantioselective photo‐redox catalysis and information processing.

A Platform to Evaluate the Effect of Back Charge Transfer on the Electrical Conductivity of TTF Charge Transfer Complexes: TTF3MCl6 (M = In, Sb)

TTF₃MCl₆ (M = In, Sb) series were developed for an ideal platform to investigate the effect of back charge transfer of MCl₆^3- on electrical conductivity depending on the metal ions. They were successfully synthesized by a UV light-induced one-pot reaction where TTF oxidation and formation of MCl₆^3- occurred sequentially. In isostructural TTF₃InCl₆ and TTF₃SbCl₆, the intermolecular interaction between MCl₆^3- and TTF induces back charge transfer, which were confirmed by the crystal structure and spectroscopic analysis. Despite the similar crystal structure in terms of intermolecular distance, TTF₃InCl₆ shows 3-orders of magnitude higher electrical conductivity compared to TTF₃SbCl₆. According to the cyclic voltammograms (CV) and electron spin resonance (ESR) spectra, increased conductivity is because of the higher degree of back charge transfer from MCl₆^3- in TTF₃InCl₆ compared to TTF₃SbCl₆, which is due to the lower electronegativity of In, considering that the only difference between the two compounds is the center metal.

Interconversion between a Pd3L2 trigonal prism and a Pd6L8 cube via anion exchange: binding affinity of monoatomic vs. polyatomic anions

Systematic interconversion between trigonal prisms [Pd₃X₆L₂] (X^- = Cl^-, Br^-, and I^-) and cubic cages [Pd₆L₈]¹²⁺(X^-)₁₂ (X^- = BF₄^- and CF₃SO₃^-) via anion exchange was established. Self-assembly of K₂PdX₄ (X^- = Cl^- and Br^-) with a C₃-symmetric tridentate 1,3,5-tris(2-isonicotinamidephenoxy)benzene (L) produces a trigonal prism, [Pd₃X₆L₂]. Further photoreaction of the [Pd₃X₆L₂] (X^- = Cl^- and Br^-) with CH₂I₂ gives rise to a halide-exchanged species, [Pd₃I₆L₂]. In contrast, anion exchange of [Pd₃X₆L₂] (X^- = Cl^-, Br^-, and I^-) with BF₄^- yields cubic-shaped cages, [Pd₆L₈]¹²⁺(BF₄^-)₁₂, with an inner cavity of 15.9 × 15.9 × 15.9 ų. Successive anion exchange of [Pd₆L₈]¹²⁺(BF₄^-)₁₂ with CF₃SO₃^- gives rises to anion-exchanged [Pd₆L₈]¹²⁺(CF₃SO₃^-)₁₂ and vice versa without cage destruction. Thus, the cage system is specifically sensitive to anions, enabling cage formation to recognize the binding affinity and size of various anions.

Copper-bipyridine grid frameworks incorporating redox-active tetrathiafulvalene: structures and supercapacitance

Redox active tetrathiafulvalene (TTF) and its derivatives when used as electrode additives have exhibited improved energy efficiency and sustainability in batteries. However, the structure-property relationship has not been investigated in detail until very recently. In this work, three redox-active TTF compounds were synthesized, and formulated as [Cu(HL)₂(bpa)₂]_n (1), [Cu(bpe)₂(H₂O)₂]_n·2n(HL)·nMeOH·nH₂O (2), and [Cu(bpp)₂(H₂O)₂]_n·2n(HL) (3) (L = dimethylthio-tetrathiafulvalene-bicarboxylate) for this work. The effects of conjugated state and spacer length of the linkers on structural assembly and band gap as well as the interactions of TTF-TTF/TTF-bpy are discussed. Compound 1 is a bpa and HL co-coordinated 1D Cu(ii) polymer. Compounds 2 and 3 are 2D Cu(ii)-bipyridine (4,4) MOFs incorporating HL (1^-) as free anion columns. The photocurrent density of 2 is larger than those of 1 and 3 due to a strong charge transfer from TTF to bpe in compound 2. The supercapacitance performances of these compounds were evaluated by cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) techniques. The results revealed that the 2D MOF structures of 2 and 3 are beneficial for good specific capacitance values (C_sp). This work revealed the structure-property relationships of TTF derivatives for use as electrode active materials in energy transfer and storage.

Tetrathiafulvalene-based double metal lead iodides: structures and electrical properties

Introducing electronically active organic components into lower dimensional metal halide compounds is an effective strategy to improve the electronic properties of hybrid metal halide materials. We have previously used this strategy to explore hybrid halides with tetrathiafulvalenes (TTFs) and a series of lead iodides and bismuth halides were isolated. The electronic properties were improved notably using this modification. In this work, we expand the study of TTF based main-group metal halides to double metal halides with mixed lead and copper transition metals. Two hybrid TTF-lead-cuprous iodides, formulated as [TTF]5[Pb2Cu2I10]·H2O (1) and [TTF]2[PbCu2I6] (2), and two monometal analogues of [TTF]2[Cu4I6]·H2O (3) and [TTF]2[Ag4I6] (4) were crystallographically characterized. The anion of 1 is a 0D cluster, while that of the others is a 1D chain structure. The anion structures of 1-4 are novel and are reported for the first time. The TTF moieties are stacked to form a 2D framework in 1 and 1D columns in 2-4. We found that the semiconductor properties of the hybrids are related to electron donation from an anion to a cation. The electronic state of the TTF cations is another significant factor that affects the electronic properties of the materials. More notably, this work proved that the conductivity and photoconductivity of the mixed metal iodides are superior to those of the monometal iodides.

Efficient Photoinduced Energy and Electron Transfers in a Tetraphenylethene-Based Octacationic Cage Through Host-Guest Complexation

Artificial photofunctional systems with energy and electron transfer functions, inspired from photosynthesis in nature, have been developed for many promising applications including solar cell, biolabeling, photoelectric materials, and photodriven catalysis. Supramolecular hosts including macrocycles and cages have been explored for simulating photosynthesis based on a host-guest strategy. Herein, we report a host-guest approach by using a tetraphenylethene-based octacationic cage and fluorescent dyes to construct artificial photofunctional systems with energy and electron transfer functions. The cage traps various dyes within its hydrophobic cavity to form 1:1 host-guest complexes via CH-π, π-π, and/or electrostatic interactions in solution. The efficient energy transfer and ultrafast photoinduced electron transfer between the cage and dyes are competitive processes with each other in artificial photofunctional systems. Spectroscopic techniques that confirm energy transfer from the fluorescent cage to dyes (e.g., NiR, R700, and R800) are efficient, which induce the red shift of fluorescence. On the other hand, ultrafast photoinduced electron transfer from dyes (e.g., ICG, AG, and AV) to the fluorescent cage can induce fluorescence quenching. This study provides an insight into the construction of artificial photofunctional systems with energy and electron transfer functions via a host-guest approach in solution.

One-/Two-Photon Excited Cell Membrane Imaging and Tracking by a Photoactive Nanocage

Cell membrane imaging by predesigned molecular and supramolecular photoluminescence probes is of great importance in understanding the nano-biointeractions for potential applications in cellular tracking, drug delivery, cancer diagnosis, and treatment. Herein, we report an effective strategy for cell membrane imaging in both living cell and tissue levels on the basis of a multifunctional nanocage (MOC-16) integrating one-/two-photon active phosphorescence, high charges, balanced hydrophobicity/lipophilicity, and proton sensitivity attributes. The intrinsic optical characters, including strong one-/two-photon excitation and pH-dependent red emission, make MOC-16 powerful optical probes for membrane imaging in living cell and tissue levels under both visible and near-infrared irradiations. Meanwhile, the highly positive charges of +28 endow MOC-16 with adequate water solubility and deprotonation ability while still maintaining its hydrophobicity, thus enabling balanced hydrophobic-lipophilic interactions at the nano-biointerface to facilitate a pH-dependent membrane absorption within the biological pH range of 5.3-7.4. However, the low-charged RuL₃ metalloligand or polyethylene glycol (PEG)-modified MOC-16^PEG with less hydrophobicity cannot offer enough nano-biointeractions for cell membrane tracking. These findings advance the fundamental understanding of nano-biointerface interactions of MOCs with cell membranes and provide further guidance in their biological applications.