Coordination-Driven Tetragonal Prismatic Cage and the Investigation on Host-Guest Complexation

A coordination-driven host has been reported to encapsulate guests by noncovalent interactions. Herein, we present the design and synthesis of a new type of prism combining porphyrin and terpyridine moieties with a long cavity. The prism host can contain bisite or monosite guests through axial coordination binding of porphyrin and aromatic π interactions of terpyridine. The ligands and prismatic complexes were characterized by electrospray ionization mass spectrometry (ESI-MS), TWIM-MS, NMR spectrometry, and single-crystal X-ray diffraction analysis. The guest encapsulation was investigated through ESI-MS, NMR spectrometry, and transient absorption spectroscopy analysis. The binding constant and stability were determined by UV-Vis spectrometry and gradient tandem MS (gMS²) techniques. Based on the prism, a selectively confined condensation reaction was also performed and detected by NMR spectrometry. This study provides a new type of porphyrin- and terpyridine-based host that could be used for the detection of pyridyl- and amine-contained molecules and confined catalysis.

Supramolecular template-directed synthesis of triazole oligomers

Sandwich complexes formed by two zinc porphyrins and a diamine ligand (DABCO) have been used as a supramolecular template to direct the synthesis of triazole oligomers. Monomer units equipped with two polymerizable functional groups, an alkyne and an azide, were attached to the template via ester bonds between a phenol unit on the monomer and benzoic acid units on the porphyrin. Self-assembly of the zinc porphyrins by addition of DABCO led to a supramolecular complex containing four of the monomer units, two on each porphyrin. CuAAC oligomerisation was carried out in the presence of a chain capping agent to prevent intermolecular reactions between the templated products, which carry reactive chain ends. The templated-directed oligomerisation resulted in selective formation of a duplex, which contains two identical chains of triazole oligomers connecting the porphyrin linkers. The effective molarity for the intramolecular CuAAC reactions on the template is 3–9 mM, and because the triazole backbone has a direction, the product duplex was obtained as a 4 : 1 mixture of the parallel and antiparallel isomers. Hydrolysis of the ester bonds connecting the oligomers to the template gave a single product, the phenol 2-mer, in excellent yield. The introduction of a supramolecular element into the template considerably broadens the scope of the covalent template-directed oligomerisation methodology that we previously developed for the replication of sequence information in synthetic oligomers.

A supramolecular metalloporphyrin assembly was used as a disposable template for controlling the oligomerisation of covalently attached monomer building blocks to give a linear oligomeric product that is not accessible via untemplated reactions.

Two-step template method for synthesis of axis-length-controlled porphyrin-containing hollow structures

π-Conjugated porphyrin-containing hollow structures with defined axis-length were successfully synthesized by a two-step template method, i.e., template-assisted cyclization and oligomerization.

Flexible porphyrin cages and nanorings

We have reviewed recent advances in the synthesis of porphyrin cages and nanorings with flexible links, as well as their applications in host–guest chemistry and catalysis. Template-directed strategies have greatly facilitated the synthesis of porphyrin cages and nanorings with high complexities. Meanwhile, the unique features of flexible porphyrin cages and nanorings having a good balance between rigidity and flexibility make them especially suitable to encapsulate fullerenes, multipyridyl compounds and other guests.

Nanorings with copper(ii) and zinc(ii) centers: forcing copper porphyrins to bind axial ligands in heterometallated oligomers

The affinity of copper(ii) porphyrins for pyridine ligands is extremely weak, but oligo-pyridine templates can be used to direct the synthesis of Cu-containing cyclic porphyrin oligomers when they also have Zn centers. We report the synthesis of two heterometallated nanorings: a six-porphyrin ring prepared from a Zn/Cu/Zn linear trimer and a ten-porphyrin ring prepared from a Zn/Zn/Cu/Zn/Zn pentamer. Both these macrocycles have copper porphyrins at two specific positions across the diameter of the ring and zinc at other sites. The presence of a paramagnetic metal results in broadening of the 1H NMR spectra and reduces the relaxation time constants (T1 and T2). The changes in T1 provide quantitative information on the distance of each proton from the copper atom. The Zn/Zn/Cu/Zn/Zn linear porphyrin pentamer binds strongly to a penta-pyridyl template, despite the weakness of the Cu-N interaction, because of the chelate cooperativity of the neighboring Zn-N coordination. The stabilities of a family of four linear porphyrin pentamer complexes were determined by UV-vis-NIR titration and analyzed using a chemical double-mutant cycle. The results show that the free energy of interaction of a copper center to axial pyridine ligands is -6.2 kJ mol-1 when the entropy cost of bringing together the two molecules has already been paid by pyridine-zinc interactions. The development of template-directed approaches to the synthesis of nanorings with combinations of different metals at specific positions around the ring opens up many possibilities for controlling the photophysical behavior of these supramolecular systems and for probing their conformations by EPR.

Synthesis and Functionalization of Porphyrins through Organometallic Methodologies

This review focuses on the postfunctionalization of porphyrins and related compounds through catalytic and stoichiometric organometallic methodologies. The employment of organometallic reactions has become common in porphyrin synthesis. Palladium-catalyzed cross-coupling reactions are now standard techniques for constructing carbon-carbon bonds in porphyrin synthesis. In addition, iridium- or palladium-catalyzed direct C-H functionalization of porphyrins is emerging as an efficient way to install various substituents onto porphyrins. Furthermore, the copper-mediated Huisgen cycloaddition reaction has become a frequent strategy to incorporate porphyrin units into functional molecules. The use of these organometallic techniques, along with the traditional porphyrin synthesis, now allows chemists to construct a wide range of highly elaborated and complex porphyrin architectures.

Structure-Dependent Electronic Interactions in Ethyne-Bridged Porphyrin Arrays Investigated by Single-Molecule Fluorescence Spectroscopy

By using single-molecule fluorescence spectroscopy, we have investigated the electronic interaction of ethyne-bridged porphyrin arrays (ZNE) depending on their structure. The fluorescence dynamics of ZNE show a large amount of one-step photobleaching behaviors, indicating the high degree of π-conjugation. The ratio of one-step photobleaching behavior decreased as the number of porphyrin units increased. This behavior indicates that the linear and shortest Z2E shows a strong electronic coupling between constituent porphyrin moieties. Structural properties and orientation of ZNE were also measured by wide-field excitation fluorescence spectroscopy (ExPFS) and defocused wide-field imaging (DWFI). The ExPFS and DWFI show that the structure of absorbing and emitting units of Z2E and Z3E are linear. On the other hand, star-shaped pentamer with five porphyrins acts as an absorbing unit, but unidirectional trimer moiety acts as an emitting unit in the Z5E molecule. Collectively, these studies provide further information on the electronic interaction depending on their structure and length.

Recent advances in the template-directed synthesis of porphyrin nanorings

This Feature Article reviews recent advances in the template-directed synthesis of porphyrin nanorings, including new templating methods, novel structures, and their applications in host–guest chemistry and artificial light-harvesting.

A Molecular Nanotube with Three-Dimensional π-Conjugation

A π-conjugated twelve-porphyrin tube is synthesized in 32 % yield by a template-directed coupling reaction that joins together six porphyrin dimers, forming twelve new C-C bonds. The nanotube has two bound templates, enclosing an internal volume of approximately 4.5 nm3. Its UV/Vis/NIR absorption and fluorescence spectra resemble those of a previously reported six-porphyrin ring, but are red-shifted by approximately 300 cm-1, reflecting increased conjugation. Ultrafast fluorescence spectroscopy demonstrates extensive excited-state delocalization. Transfer of electronic excitation from an initially formed state polarized in the direction of the nanotube axis (z axis) to an excited state polarized in the xy plane occurs within 200 fs, resulting in a negative fluorescence anisotropy on excitation at 742 nm.

A Molecular Nanotube with Three-Dimensional π-Conjugation

A π-conjugated twelve-porphyrin tube is synthesized in 32 % yield by a template-directed coupling reaction that joins together six porphyrin dimers, forming twelve new C=C bonds. The nanotube has two bound templates, enclosing an internal volume of approximately 4.5 nm³. Its UV/Vis/NIR absorption and fluorescence spectra resemble those of a previously reported six-porphyrin ring, but are red-shifted by approximately 300 cm⁻¹, reflecting increased conjugation. Ultrafast fluorescence spectroscopy demonstrates extensive excited-state delocalization. Transfer of electronic excitation from an initially formed state polarized in the direction of the nanotube axis (z axis) to an excited state polarized in the xy plane occurs within 200 fs, resulting in a negative fluorescence anisotropy on excitation at 742 nm.

Triptycene based organometallic complexes: a new class of acceptor synthons for supramolecular ensembles

New triptycene based organometallic complexes have been synthesized and subsequently used for design of the triptycene motif containing nanoscalar trigonal prisms. Flexible 3,3′-bipyridine is employed as a donor unit.

Modular self-assembled multiporphyrin cages with tunable shape

Three new molecular cages-a trigonal prism (2) and two cubic-shaped boxes of increasing size (3, 4)-featuring as many as 10 porphyrins were obtained in quantitative yields by self-assembly of a single bis-zincporphyrin metallacycle (1) with appropriate polytopic N-linkers: the trigonal planar tpt or the cruciform porphyrins 4'-TPyP and 4'-TPhPyP.

Template-directed synthesis of a covalent organic capsule based on a 3 nm-sized metallocapsule

A 4 nm-sized covalent organic capsule having 24 pyridyl groups was synthesized in extremely high yield (43% in three steps) using an octahedron-shaped metallocapsule as a template molecule. The entity was fully characterized by NMR and MALDI-TOF MS measurements. N-Methylation of the 24 pyridyl groups of the organic capsule produced a 5 nm-sized polycationic capsule, which is larger than the neutral precursory capsule because of the electrostatic repulsion between the positive charges on the pyridinium groups of the capsule.

Acidification and assembly of porphyrin at an interface: counterion matching, selectivity, and supramolecular chirality

The interfacial diprotonation and assemblies of a free-base achiral porphyrin, 5,10,15,20-tetrakis(3,5-dimethoxyphenyl)-21H,23H-porphine, on various acidic subphases were investigated. It has been shown that the compound could be diprotonated in situ on an acidic subphase and can form assemblies. The interfacially organized supramolecular assemblies were transferred onto a solid substrate, and the assemblies showed supramolecular chirality. Interestingly, the supramolecular chirality of the assemblies of the diprotonated species showed a counterion-dependent behavior. For the assemblies fabricated from the aqueous HCl subphases, a strong Cotton effect (CE) could be observed, although the porphyrin itself is achiral. When an aqueous HBr solution was used as the subphase, the assemblies showed a weak CE, whereas no CE could be detected for the assemblies formulated from the HNO3 or HI subphase. Interestingly, when a mixture of HBr and NaCl, or HNO3 and NaCl, was employed as the subphase, the formed assemblies displayed chiral features similar to those fabricated on the HCl subphase, suggesting that the Cl(-) could be preferentially visualized in terms of supramolecular chirality, although the system itself is composed of achiral species. On the basis of the experimental facts and a theoretical calculation, an explanation with regard to the different sizes of the counterions and the distinct binding affinities of the counteranions to the diprotonated porphyrin species has been proposed. Our findings provide new insights into the assembly of the diprotonated porphyrins as well as the interfacially occurring symmetry breaking.

Toward plasmonic solar cells: protection of silver nanoparticles via atomic layer deposition of TiO2

Plasmonic silver nanoparticles have unique properties that lend themselves to unusual optical applications, potentially including use as absorption amplifiers in dye-sensitized solar cells (DSSCs). However, these particles are easily damaged under oxidizing conditions. Atomic layer deposition of TiO2 onto transparent-conductive-oxide-supported silver particles was examined as a means of protecting particles while simultaneously incorporating them into DSSC-functional photoelectrodes. The resulting assemblies were exposed to corrosive I-/I3- solutions, and the degree of silver etching was determined via scanning electron microscopy and ultraviolet-visible spectroscopy. To form a pinhole-free (i.e., fully protective) crystalline TiO2 layer, 7.7 nm (300 cycles) must be deposited. If, however, a 0.2 nm (2 cycles) Al2O3 adhesion layer is included, only 5.8 nm (211 cycles) of TiO2 are necessary for the formation of a pinhole-free coating.