Bending a photonic wire into a ring

Giant Expanded Porous Metallo-Hexagons

Molecular self-assembly is a widely recognized approach for fabricating biomimetic functional nanostructures. Here, we report the synthesis of two giant hollow coronoid-like supramolecular hexagons, H1 and H2. These hexagons feature large cavities, showcasing unique inner and outer hexagons fixed by specific connectivities for enhanced stability and high metal center density. H1 exhibits properties that can be transformed through the thermodynamic conversion of the metallopolymer formed by L1 and L2. With an edge length of 6.8 nm, H2 is one of the largest hexagons reported to date. 1D and 2D NMR, TEM, ESI-MS, and TWIM-MS experiments provided conclusive evidence for the composition and structure of the assembled hexagons. This work demonstrates the feasibility of constructing giant supramolecular architectures with precise control over their size and shape, opening up new possibilities for the design and synthesis of sophisticated supramolecules and nonbiological materials.

Template-Directed Synthesis of Strained meso-meso-Linked Porphyrin Nanorings

Strained macrocycles display interesting properties, such as conformational rigidity, often resulting in enhanced π-conjugation or enhanced affinity for non-covalent guest binding, yet they can be difficult to synthesize. Here we use computational modeling to design a template to direct the formation of an 18-porphyrin nanoring with direct meso-meso bonds between the porphyrin units. Coupling of a linear 18-porphyrin oligomer in the presence of this template gives the target nanoring, together with an unexpected 36-porphyrin ring by-product. Scanning tunneling microscopy (STM) revealed the elliptical conformations and flexibility of these nanorings on a Au(111) surface.

Impact of Ring-Closing on the Photophysical Properties of One-Dimensional π-Conjugated Molecular Aggregate

The self-assembled state of molecules plays a pivotal role in determining how inherent molecular properties transform and give rise to supramolecular functionalities and has long attracted attention. However, understanding the influence of morphologies spanning the nano- to mesoscopic scales of supramolecular assemblies derived from identical intermolecular interactions has been notoriously challenging due to dynamic structural change and monomer exchange of assemblies in solution. In this study, we demonstrate that curved one-dimensional molecular assemblies (supramolecular polymers) of lengths of around 70-200 nm, originating from the same luminescent molecule, exhibit distinct photoluminescent properties when they form closed circular structures (toroids) versus when they possess chain termini in solution (random coils). By exploiting the difference in kinetic stability between the toroids and random coils, we developed a dialysis protocol to selectively purify the former. It was revealed that these terminus-free closed structures manifest higher energy and more efficient luminescence compared with their mixed state with random coils. Time-resolved fluorescence measurements unveiled that random coils, due to their dynamic structural fluctuation in solution, generate local defects throughout the main chain, leading to luminescence from lower energy levels. In mixtures of the two assemblies, luminescence was exclusively observed from such a lower energy level of random coils, a result attributed to energy transfer between the assemblies. This work emphasizes that for identical supramolecular assemblies, only averaged properties have traditionally been considered, but their structures at the nano- to mesoscopic scale are important especially if they have a certain degree of shape persistency even in solution.

Supramolecular interaction of PCBM with porphyrins in solution: Photophysical insights

This work reports the self-assembly between [6,6]-phenyl C71 butyric acid methyl ester (PCBM) and 2,3,7,8,12,13,17,18-octaethyl-21H,23H porphyrin (1)(and/2,3,7,8,12,13,17,18-octaethyl-21H,23H porphyrin Zn(II) (2) in toluene. Ground state intermolecular interaction is evidenced from absorption spectrophotometric measurements. New absorption bands are observed in the visible region which may be identified due to charge transfer (CT). Several important physicochemical factors are enumerated for PCBM-1 and PCBM-2 systems. Fluorescence investigations elicit complex formation of PCBM with porphyrins (with both 1 and 2) and reveal considerable magnitude of binding constant (K) for PCBM-2 system, i.e., KPCBM-2 = 80,435 dm3⋅mol-1 compared to PCBM-1 system, i.e., KPCM-1 = 12,600 dm3·mol-1 as well as highly ratio of selectivity in binding (KPCBM-2/KPCBM-1 ∼ 6.4). Time resolved fluorescence experiments reveal that photoexcited decay from the excited singlet state of porphyrins (i.e., 1* and 2*) by PCBM is statically controlled compared to dynamic path. Magnitude of solvent reorganization energy indicates possibility of faster charge recombination in case of PCBM-2 system. Both 1H and 13C NMR measurements provide substantial support behind complexation of PCBM with porphyrins (both 1 and 2) in solution. Ab initio calculations in vacuo support the trend in K for PCBM-1 and PCBM-2 systems and establish the proper orientation of PCBM towards 1 (and/ 2) during complexation. Transient absorption measurements establish two different mode of energy transfer pathway from porphyrin to PCBM in toluene.

Functional Coupling of Biohybrid Photosynthetic Antennae and Reaction Center Complexes: Quantitative Comparison with Native Antennae

Light-harvesting (LH) complexes in photosynthetic organisms absorb photons within limited wavelength ranges over a broad solar spectrum. Extension of the LH wavelength has been realized by attaching artificial fluorophores to LH complexes (biohybrid LH complexes) for complementing the limited-wavelength regions. However, how efficiently such fluorophores in biohybrid LH complexes function to drive the photocatalytic reaction center (RC) has not been quantitatively evaluated, specifically in comparison with native LH antenna complexes. In this study, we prepared various biohybrid LH1-RC complexes (from Rhodopseudomonas palustris), to quantitatively evaluate the LH activity of the attached external chromophores through a photocurrent generation reaction by LH1-RC on an electrode. For a direct comparison of the LH activity among the LH chromophores that were examined, we introduced the k1 term, which represents the extent of the functional coupling of LH and the photochemical reactions in the RC. We determined that the hydrophobic fluorophore ATTO647N attached to LH1 possesses the highest LH activity among the examined hydrophilic fluorophores such as Alexa647, and its activity is comparable to that of native LH1(-RC). The LH activity of LH2 (from Rhodoblastus acidophilus strain 10050) and its biohybrid LH2s were examined for the comprehensive assessment of their LH activity.

Nucleic-Acid-Templated Synthesis of Smart Polymer Vectors for Gene Delivery

Nucleic acids are information-rich and readily available biomolecules, which can be used to template the polymerization of synthetic macromolecules. Here, we highlight the control over the size, composition, and sequence one can nowadays obtain by using this methodology. We also highlight how templated processes exploiting dynamic covalent polymerization can, in return, result in therapeutic nucleic acids fabricating their own dynamic delivery vector - a biomimicking concept that can provide original solutions for gene therapies.

Engineering Soluble Diketopyrrolopyrrole Chromophore Stacks from a Series of Pd(II)-Based Ravels

A strategy to engineer the stacking of diketopyrrolopyrrole (DPP) dyes based on non-statistical metallosupramolecular self-assembly is introduced. For this, the DPP backbone is equipped with nitrogen-based donors that allow for different discrete assemblies to be formed upon the addition of Pd(II), distinguished by the number of π-stacked chromophores. A Pd3 L6 three-ring, a heteroleptic Pd2 L2 L'2 ravel composed of two crossing DPPs (flanked by two carbazoles), and two unprecedented self-penetrated motifs (a Pd2 L3 triple and a Pd2 L4 quadruple stack), were obtained and systematically investigated. With increasing counts of stacked chromophores, UV/Vis absorptions red-shift and emission intensities decrease, except for compound Pd2 L2 L'2 , which stands out with an exceptional photoluminescence quantum yield of 51 %. This is extraordinary for open-shell metal containing assemblies and explainable by an intra-assembly FRET process. The modular design and synthesis of soluble multi-chromophore building blocks offers the potential for the preparation of nanodevices and materials with applications in sensing, photo-redox catalysis and optics.

Site-Selective Ligand Bridging among Multiple Internal Coordination Sites of a Metallomacrocycle and Its Conformational Regulation

Metallomacrocycles with internal coordination sites have a high potential to precisely control the positions of the guest ligands and the overall shape of the assemblies by utilizing the directionality and reversibility of the coordination bonds. However, when such coordinative hosts possess multiple coordination sites, it was difficult to control to which coordination sites the incoming guest ligands bind, because such systems often result in a random, uncontrolled mixture. The metallomacrocycle that we now report, a hexanuclear palladium complex of hexapap possessing six internal coordination sites, can take two different conformations depending on the guests. One is an Alternate conformation, in which six coordination sites of pap alternatively point to Up-Down-Up-Down-Up-Down. The other is a Twisted conformation, in which the coordination sites direct Up-Middle-Down-Up-Middle-Down. Interestingly, linear ditopic α,ω-diamines are captured in three distinct cross-linking modes, and regulations between the two macrocyclic conformations have been realized by the lengths of the diamines. Furthermore, the heteroleptic site-selective bridging of two kinds of diamines has been achieved. It has been demonstrated that a slight difference in the diamine lengths leads to a significant change in the structure and selection of the produced host-guest macrocyclic complexes.

Covalent Template-Directed Synthesis of a Spoked 18-Porphyrin Nanoring

Rings of porphyrins mimic natural light-harvesting chlorophyll arrays and offer insights into electronic delocalization, providing a motivation for creating larger nanorings with closely spaced porphyrin units. Here, we demonstrate the first synthesis of a macrocycle consisting entirely of 5,15-linked porphyrins. This porphyrin octadecamer was constructed using a covalent six-armed template, made by cobalt-catalyzed cyclotrimerization of an H-shaped tolan with porphyrin trimer ends. The porphyrins around the circumference of the nanoring were linked together by intramolecular oxidative meso-meso coupling and partial β-β fusion, to give a nanoring consisting of six edge-fused zinc(II) porphyrin dimer units and six un-fused nickel(II) porphyrins. STM imaging on a gold surface confirms the size and shape of the spoked 18-porphyrin nanoring (calculated diameter: 4.7 nm).

Strained Porphyrin Tape-Cycloparaphenylene Hybrid Nanorings

V-Shaped porphyrin dimers, with masked p-phenylene bridges, undergo efficient oxidative coupling to form meso-meso linked cyclic porphyrin oligomers. Reductive aromatization unmasks the p-phenylenes, increasing the strain. Oxidation then fuses the porphyrin dimers, providing a nanoring with curved walls. The strain in this macrocycle bends the p-phenylene and fused porphyrin dimer units (radii of curvature of 11.4 and 19.0 Å, respectively), but it does not significantly alter the electronic structure of the fused porphyrins.