Bioinspired construction of light-harvesting antenna via hierarchically co-assembling approach

Porphyrin-Based Metal-Organic Framework Materials: Design, Construction, and Application in the Field of Photocatalysis

Metal-organic frameworks (MOFs) are a novel category of porous crystalline materials with an exceptionally high surface area and adjustable pore structure. They possess a designable composition and can be easily functionalized with different units. Porphyrins with conjugated tetrapyrrole macrocyclic structures can absorb light from ultraviolet to visible light regions, and their structures and properties can be facilely regulated by altering their peripheral groups or central metal ions. Porphyrin-based MOFs constructed from porphyrin ligands and metal nodes combine the unique features of porphyrins and MOFs as well as overcoming their respective limitations. This paper reviewed the design and construction, light absorption and charge transfer pathways, and strategy for improving the photocatalytic performance of porphyrin-based MOFs, and highlighted the recent progress in the field of CO2 reduction, hydrogen evolution, organic synthesis, organic pollutant removal, and nitrogen fixation. The intrinsic relationships between the structure and the property of porphyrin-based MOFs received special attention, especially the relationships between the arrangements of porphyrin ligands and metal nods and the charge transfer mechanism. We attempted to provide more valuable information for the design and construction of advanced photocatalysts in the future. Finally, the challenges and future perspectives of the porphyrin-based MOFs are also discussed.

Porphyrin-Based Covalent Organic Frameworks: Design, Synthesis, Photoelectric Conversion Mechanism, and Applications

Photosynthesis occurs in high plants, and certain organisms show brilliant technology in converting solar light to chemical energy and producing carbohydrates from carbon dioxide (CO₂). Mimicking the mechanism of natural photosynthesis is receiving wide-ranging attention for the development of novel materials capable of photo-to-electric, photo-to-chemical, and photocatalytic transformations. Porphyrin, possessing a similar highly conjugated core ring structure to chlorophyll and flexible physical and chemical properties, has become one of the most investigated photosensitizers. Chemical modification and self-assembly of molecules as well as constructing porphyrin-based metal (covalent) organic frameworks are often used to improve its solar light utilization and electron transfer rate. Especially porphyrin-based covalent organic frameworks (COFs) in which porphyrin molecules are connected by covalent bonds combine the structural advantages of organic frameworks with light-capturing properties of porphyrins and exhibit great potential in light-responsive materials. Porphyrin-based COFs are expected to have high solar light utilization, fast charge separation/transfer performance, excellent structural stability, and novel steric selectivity by special molecular design. In this paper, we reviewed the research progress of porphyrin-based COFs in the design, synthesis, properties, and applications. We focused on the intrinsic relationship between the structure and properties, especially the photoelectric conversion properties and charge transfer mechanism of porphyrin-based COFs, and tried to provide more valuable information for the design of advanced photosensitizers. The applications of porphyrin-based COFs in photocatalysis and phototherapy were emphasized based on their special structure design and light-to-electric (or light-to-heat) conversion control.

Design and preparation of highly active TiO2 photocatalysts by modulating their band structure

Titanium dioxide photocatalysts with high reduction potential and visible light response hold great promise in photochemical conversion. Here, we used a simple glycine doping method to synthesize novel N-TiO2@C photocatalysts with upward shifted conduction bands and narrowed band gaps as well as inhibited recombination of photoinduced electron-hole pairs. The N-TiO2@C photocatalysts exhibited higher visible light response and remarkably enhanced photocatalytic activity in the production of nicotinamide adenine dinucleotide (NADH) by photomediated reduction of NAD+ without any electron mediator. The yield of NADH was up to 70.3 % far greater than that of the undoped TiO2 (11.3 %), and it stabilized at ca. 60 % after 10 cycles. The viability of coupling NADH regeneration with enzymatic reaction (alcohol dehydrogenase) was established in aldehyde reduction where formaldehyde was specifically reduced to methanol. These findings shed new light on the modulation of the band structure of semiconductors and develop an electron mediator free strategy for NADH-dependent artificial photosynthesis through coupled photocatalytic and enzymatic approaches.

Self-assembly of amino acids toward functional biomaterials

Biomolecules, such as proteins and peptides, can be self-assembled. They are widely distributed, easy to obtain, and biocompatible. However, the self-assembly of proteins and peptides has disadvantages, such as difficulty in obtaining high quantities of materials, high cost, polydispersity, and purification limitations. The difficulties in using proteins and peptides as functional materials make it more complicate to arrange assembled nanostructures at both microscopic and macroscopic scales. Amino acids, as the smallest constituent of proteins and the smallest constituent in the bottom-up approach, are the smallest building blocks that can be self-assembled. The self-assembly of single amino acids has the advantages of low synthesis cost, simple modeling, excellent biocompatibility and biodegradability in vivo. In addition, amino acids can be assembled with other components to meet multiple scientific needs. However, using these simple building blocks to design attractive materials remains a challenge due to the simplicity of the amino acids. Most of the review articles about self-assembly focus on large molecules, such as peptides and proteins. The preparation of complicated materials by self-assembly of amino acids has not yet been evaluated. Therefore, it is of great significance to systematically summarize the literature of amino acid self-assembly. This article reviews the recent advances in amino acid self-assembly regarding amino acid self-assembly, functional amino acid self-assembly, amino acid coordination self-assembly, and amino acid regulatory functional molecule self-assembly.

Mechanistic process understanding of the self-assembling behaviour of asymmetric bolaamphiphilic short-peptides and their templating for silica and titania nanomaterials

Investigation of the self-assembly of peptides is critically important to clarify certain biophysical phenomena, fulfill some biological functions, and construct functional materials. However, it is still a challenge to precisely predict the self-assembled structures of peptides because of their complicated driving forces and various assembling pathways. In this work, to elucidate the effects of noncovalent interactions including hydrogen bonding, molecular geometry, and hydrophobic and electrostatic interactions on the peptide self-assembly, a series of asymmetric bolaamphiphilic short peptides consisting of Ac-EI₃K-NH₂ (EI₃K), Ac-EI₄K-NH₂ (EI₄K), Ac-KI₃E-NH₂ (KI₃E) and Ac-KI₄E-NH₂ (KI₄E) were designed and their self-assembling behaviors at different solution pH values were investigated systematically. The peptides self-assembled into twisted nanofibers under most conditions except for EI₄K in a strongly alkaline solution and KI₄E under a strongly acidic condition, in which they self-assembled into nanotubes via helical monolayer nanosheet intermediates. In particular, KI₄E nanotubes are formed under acidic conditions, and its diameters are ∼500 nm much greater than most of the self-assembled structures from bolaamphiphilic peptides. Moreover, reversible morphological transition between the nanotubes and twisted nanofibers was observed with the change in solution pH. Such tunable self-assembled structures and switchable surface properties of the asymmetric bolaamphiphilic short-peptides allow them to be used as templates to construct advanced materials. Silica and titania nanomaterials faithful to the peptide templates in morphology were prepared at ambient temperature. This work clearly elucidates the effects of noncovalent interactions on the peptide self-assembly and also provides new insights into the design and preparation of complicated inorganic materials from tunable organic templates.

Peptide-Tetrapyrrole Supramolecular Self-Assemblies: State of the Art

The covalent and noncovalent association of self-assembling peptides and tetrapyrroles was explored as a way to generate systems that mimic Nature's functional supramolecular structures. Different types of peptides spontaneously assemble with porphyrins, phthalocyanines, or corroles to give long-range ordered architectures, whose structure is determined by the features of both components. The regular morphology and ordered molecular arrangement of these systems enhance the photochemical properties of embedded chromophores, allowing applications as photo-catalysts, antennas for dye-sensitized solar cells, biosensors, and agents for light-triggered therapies. Chemical modifications of peptide and tetrapyrrole structures and control over the assembly process can steer the organization and influence the properties of the resulting system. Here we provide a review of the field, focusing on the assemblies obtained from different classes of self-assembling peptides with tetrapyrroles, their morphologies and their applications as innovative functional materials.