Revealing the blueprint of photosynthesis

Space-confined mediation of electron transfer for efficient biomolecular solar conversion

Solar-converting nanosystems using self-renewing biomaterial resources carry great potential for developing sustainable technologies to ameliorate climate change and minimize reliance on fossil fuels. By mimicking natural photosynthesis, diverse proof-of-concept biosolar systems have been used to produce green electricity, fuels and chemicals. Efforts so far have focused on optimizing light harvesting, biocatalyst loading and electron transfer (ET), however, the long-term performance of best-performing systems remains a major challenge due to the intensive use of diffusive, toxic mediators. To overcome this limitation, we developed a rationally designed nanosystem based on the entrapment of non-toxic mediator, ferrocene dimethanol (Fc), localized at the abiotic-biotic molecular interface that efficiently promoted ET between electrode surface and two photosynthetic proteins: cytochrome c and photosystem I. We demonstrate that space-confined Fc mediators (1 nM) are as effective in terms of ET kinetics as a 500 000-fold higher concentration of freely-diffusive Fc. The Fc-confined biophotocathodes showed a milestone photocurrent density of 14 μA cm-2 under oxic conditions compared to analogous planar (2D) biophotoelectrodes, with a photoconductive biolayer stable for over 5 months. The space-confined ET mediation reported in this work opens a new avenue for efficiently interfacing biomachineries, providing a benchmark design advancement in the quest for viable biohybrid technologies.

Host-Guest Approach to Promoting Photocatalysis Based on Consecutive Photo-Induced Electron-Transfer Processes via Efficient Förster Resonance Energy Transfer

Supramolecular artificial light-harvesting system with highly efficient host-guest energy transfer pathway provides an ideal platform for optimizing the photochemistry process. The consecutive photo-induced electron transfer (conPET) process overcomes the energy limitation of visible-light photocatalysis, but is often compromised by mismatching between the absorption of ground state dye and its radical, weakening the efficiency of photoredox reaction. By encapsulating a conPET photocatalyst rhodamine 6G into metal-organic cage, the supramolecular approach was undertaken to tackle the intrinsic difficulty of matching the light absorption of photoexcitation between rhodamine 6G and its radical. The highly efficient Förster resonance energy transfer from the photoexcited cage to rhodamine 6G forced by host-guest encapsulation facilitates the conPET process for the single-wavelength light-driven activation of aryl halides by stabilizing and accelerating the production and accumulation of the rhodamine 6G radical intermediate. The tunable and flexible nature of the supramolecular host-guest complex renders the cage-based encapsulation strategy promising for the development of ideal photocatalysts toward the better utilization of solar energy.

Photocatalytic fixation and oxygenation of NAD+/NADP+ and sulfides using solar light: Exploring mechanistic investigations and their impact on synthetic applications

Sulfur-doped Eosin-B (SDE-B) photocatalysts were synthesized for the first time utilizing sublimed sulfur (S8) as a dopant in an in situ thermal copolymerization technique. Sulfur doping not only increased Eosin-B (E-B) absorption range for solar radiation but also improved fixation and oxygenation capabilities. The doped sulfur bridges the S-S bond by substituting for the edge bromine of the E-B bond. The improved photocatalytic activity of SDE-B in the fixation and oxygenation of NAD+/NADP+ and sulfides using solar light is attributed to the photo-induced hole of SDE-B's high fixation and oxygenation capacity, as well as an efficient suppression of electron and hole recombination. The powerful light-harvesting bridge system created using SDE-B as a photocatalyst works extremely well, resulting in high NADH/NADPH regeneration (79.58/76.36%) and good sulfoxide yields (98.9%) under solar light. This study focuses on the creation and implementation of a sulfur-doped photocatalyst for direct fine chemical regeneration and organic transformation.

Transforming Pharmaceutical Synthesis with Sein-E-B Nanocomposite Photocatalyst through 1,4-NAD(P)H Cofactor Regeneration and C-N Bond Activation

The need for sunlight chemical renewal and contemporary organic transformation has fostered the advancement of environmentally friendly photocatalytic techniques. For the first time, we report on the novel crafting of a bright future with selenium-infused Eosin-B (Sein-E-B) nanocomposite photocatalysts in this work. The Sein-E-B nanocomposite materials were created using a hydrothermal process for solar chemical regeneration and organic transformation under visible light. The synthesized samples were subjected to UV-DRS-visible spectroscopy, FT-IR, SEM, EDX, EIS and XRD analysis. The energy band gap of the Sein-E-B nanocomposite photocatalyst was measured using UV-DRS, and the result was around 2.06 eV. to investigate the generated Sein-E-B catalytic activity as a nanocomposite for 1,4-NADH/NADPH re-formation and C-N bond activation. This novel photocatalyst offers a promising alternative for the regeneration of solar chemicals and C-N bond creation between pyrrole and aryl halides.

Symmetry-Breaking Charge Transfer in Metal-Organic Frameworks

High quantum-yield charge carrier generation from the initially prepared excitons defines a key step in the light-harvesting and conversion scheme. Photoinduced charge transfer in molecular electron donor-acceptor assemblies is driven by a sizable ΔG0, which compromises the potential of the generated carriers. Reminiscent of the special pair at the reaction center of the natural light-harvesting complex, symmetry-breaking charge transfer (SBCT) within a pair of identical struts of metal-organic framework (MOF) will facilitate the efficient generation of long-lived charge carriers with maximized potentials without incorporating any foreign redox species. We report SBCT in pyrene-based zirconium metal-organic framework (MOF) NU-1000 that leads to efficient generation of radical ions in a polar solvent and bound CT states in a low-polar solvent. The probe unveils the role of the low-lying non-Franck-Condon excitonic states as intermediates in the formation of the SBCT state from the initially prepared Franck-Condon S1 states. Ultrafast and transient spectroscopy─probed over 200 fs-30 μs time scale─evinces a kSBCT = (110 ps)-1 in polar media (εs = 37.5) forming solvated radical ions with recombination rate kCR = (∼45 ns)-1. A slower rate with kSBCT = (203 ps)-1 was recorded in low-polar (εs = 7.0) solvent manifesting a bound [TBAPy•+ TBAPy•-] state with kCR ≈ (17 μs)-1. This discovery, along with other unique photophysical features relevant to light harvesting, should define a MOF-based platform for developing heterogeneous artificial photon energy conversion systems.

Influence of ligand variation on the deactivation process of metal-to-ligand charge transfer excited states in quadruply bonded dimolybdenum complexes

To explore the dynamics of metal-to-ligand charge transfer (MLCT) excited states involving covalently bonded dimetal units, a series of quadruply bonded dimolybdenum (Mo2) complexes, namely, [Mo2]-ph, [Mo2]-naph, and [Mo2]-anth, were synthesized and characterized. Our investigations reveal a non-radiative process associated with the deactivation of the MLCT state into a low-lying dimetal-centered triplet state (3Mo2-δδ*), resulting in the populated MLCT states in these molecular systems exhibiting either extremely weak emission or being non-emissive. The influence of ligand variation on the dynamics of MLCT states was examined using femtosecond transient absorption spectroscopy, with deactivation time constants determined to be 1.9 ps for [Mo2]-ph, 6.5 ps for [Mo2]-naph, and 49 ps for [Mo2]-anth. This electron transfer behaviour follows an inverse energy-gap law, contrary to the general guideline that applies to the decay of the MLCT state back to the electronic ground state. This result offers valuable insights into understanding the photochemical and photophysical properties of covalently bonded dimetal complexes.

Metal-Carbodithioate-Based 3D Semiconducting Metal-Organic Framework: Porous Optoelectronic Material for Energy Conversion

Solar energy conversion requires the working compositions to generate photoinduced charges with high potential and the ability to deliver charges to the catalytic sites and/or external electrode. These two properties are typically at odds with each other and call for new molecular materials with sufficient conjugation to improve charge conductivity but not as much conjugation as to overly compromise the optical band gap. In this work, we developed a semiconducting metal-organic framework (MOF) prepared explicitly through metal-carbodithioate "(-CS₂)_nM" linkage chemistry, entailing augmented metal-linker electronic communication. The stronger ligand field and higher covalent character of metal-carbodithioate linkages─when combined with spirofluorene-derived organic struts and nickel(II) ion-based nodes─provided a stable, semiconducting 3D-porous MOF, Spiro-CS₂Ni. This MOF lacks long-range ordering and is defined by a flexible structure with non-aggregated building units, as suggested by reverse Monte Carlo simulations of the pair distribution function obtained from total scattering experiments. The solvent-removed "closed pore" material recorded a Brunauer-Emmett-Teller area of ∼400 m²/g, where the "open pore" form possesses 90 wt % solvent-accessible porosity. Electrochemical measurements suggest that Spiro-CS₂Ni possesses a band gap of 1.57 eV (σ = 10^-7 S/cm at -1.3 V bias potential), which can be further improved by manipulating the d-electron configuration through an axial coordination (ligand/substrate), the latter of which indicates usefulness as an electrocatalyst and/or a photoelectrocatalyst (upon substrate binding). Transient-absorption spectroscopy reveals a long-lived photo-generated charge-transfer state (τ_CR = 6.5 μs) capable of chemical transformation under a biased voltage. Spiro-CS₂Ni can endure a compelling range of pH (1-12 for weeks) and hours of electrochemical and photoelectrochemical conditions in the presence of water and organic acids. We believe this work provides crucial design principles for low-density, porous, light-energy-conversion materials.

Self-Assembly of Chiro-Optical Materials from Nonchiral Oligothiophene-Porphyrin Derivatives and Random Coil Synthetic Peptides

Biomimetic chiral optoelectronic materials can be utilized in electronic devices, biosensors and artificial enzymes. Herein, this work reports the chiro-optical properties and architectural arrangement of optoelectronic materials generated from self-assembly of initially nonchiral oligothiophene-porphyrin derivatives and random coil synthetic peptides. The photo-physical- and structural properties of the materials were assessed by absorption-, fluorescence- and circular dichroism spectroscopy, as well as dynamic light scattering, scanning electron microscopy and theoretical calculations. The materials display a three-dimensional ordered helical structure and optical activity that are observed due to an induced chirality of the optoelectronic element upon interaction with the peptide. Both these properties are influenced by the chemical composition of the oligothiophene-porphyrin derivative, as well as the peptide sequence. We foresee that our findings will aid in developing self-assembled optoelectronic materials with dynamic architectonical accuracies, as well as offer the possibility to generate the next generation of materials for a variety of bioelectronic applications.

Remembering James Barber (1940-2020)

James Barber, known to colleagues and friends as Jim, passed away in January 2020 after a long battle against cancer. During his long and distinguished career in photosynthesis research, Jim made many outstanding contributions with the pinnacle achieving his dream of determining the first detailed structure of the Mn cluster involved in photosynthetic water oxidation. Here, colleagues and friends remember Jim and reflect upon his scientific career and the impact he had on their lives and the scientific community.

Mechanistic Insights into Cobalt-Based Water Oxidation Catalysis by DFT-Based Molecular Dynamics Simulations

We report the mechanistic details of the water oxidation process by the complex, [Co^II(bpbH₂)Cl₂], where bpbH₂ = N, N'-bis(2'-pyridinecarboxamide)-1,2-benzene. An experimental study reported the complex as the efficient catalyst for the water oxidation process. We performed density functional theory calculations at the M06-L level and first-principles molecular dynamics simulations to study the catalytic nature of the complex. We investigated the energetics of the total catalytic cycle, which combines the oxygen-oxygen bond formation, proton-coupled electron transfer (PCET) steps, and release of oxygen molecule. The formed peroxide and superoxide intermediates in the catalytic cycle were characterized with the help of the Mulliken spin density parameters. Mulliken spin densities of the metal-oxo bond reveal that the triplet state of Co^V═O has a double-bond nature, but the quintet state of the complex has a radical nature (Co^IV-O^•-). In an alternative way, the deprotonation of the amide groups of the ligand is also considered. The deprotonation and formation of higher oxidation metal-oxo intermediates are also possible. In addition to this, we have considered the effect of phosphate buffer on water nucleophilic addition. The oxygen-oxygen bond formation is favorable by the catalyst with the deprotonated form of the ligand, with the addition of water as the nucleophile. In the oxidation process, the C═O bonds of the ligand transfer the electron density to nitrogen atoms, stabilizing the higher order oxo, peroxide, and superoxide bonds. The oxygen-oxygen bond formation is the rate-determining step in the overall water oxidation process. This bond was further investigated using first-principles molecular dynamics at the PBE-D2 level. The dynamics of proton, hydroxide ion, and the nature of the ligand structure on the oxygen-oxygen bond were examined. We find that the oxygen molecule is released from the superoxide complex with the addition of water molecules.

Biocatalytic conversion of sunlight and carbon dioxide to solar fuels and chemicals

This review discusses the progress in the assembly of photosynthetic biohybrid systems using enzymes and microbes as the biocatalysts which are capable of utilising light to reduce carbon dioxide to solar fuels. We begin by outlining natural photosynthesis, an inspired biomachinery to develop artificial photosystems, and the rationale and motivation to advance and introduce biological substrates to create more novel, and efficient, photosystems. The case studies of various approaches to the development of CO2-reducing microbial semi-artificial photosystems are also summarised, showcasing a variety of methods for hybrid microbial photosystems and their potential. Finally, approaches to investigate the relatively ambiguous electron transfer mechanisms in such photosystems are discussed through the presentation of spectroscopic techniques, eventually leading to what this will mean for the future of microbial hybrid photosystems.

Computational Approach for Probing Redox Potential for Iron-Sulfur Clusters in Photosystem I

Photosystem I is a light-driven electron transfer device. Available X-ray crystal structure from Thermosynechococcus elongatus showed that electron transfer pathways consist of two nearly symmetric branches of cofactors converging at the first iron-sulfur cluster FX, which is followed by two terminal iron-sulfur clusters FA and FB. Experiments have shown that FX has lower oxidation potential than FA and FB, which facilitates the electron transfer reaction. Here, we use density functional theory and Multi-Conformer Continuum Electrostatics to explain the differences in the midpoint Em potentials of the FX, FA and FB clusters. Our calculations show that FX has the lowest oxidation potential compared to FA and FB due to strong pairwise electrostatic interactions with surrounding residues. These interactions are shown to be dominated by the bridging sulfurs and cysteine ligands, which may be attributed to the shorter average bond distances between the oxidized Fe ion and ligating sulfurs for FX compared to FA and FB. Moreover, the electrostatic repulsion between the 4Fe-4S clusters and the positive potential of the backbone atoms is lowest for FX compared to both FA and FB. These results agree with the experimental measurements from the redox titrations of low-temperature EPR signals and of room temperature recombination kinetics.

Novel fluorescent self-assembling material with gel properties: ion recognition and energy transfer

A novel fabrication strategy for preparing fluorescent nanomaterials has been proposed based on supramolecular self-assembly complexes and energy transfer. Here a dual acylhydrazone-functionalized molecule (DAF) was designed and synthesized by...

Photoinduced Charge Transfer with a Small Driving Force Facilitated by Exciplex-like Complex Formation in Metal-Organic Frameworks

Photoinduced charge transfer (PCT) is a key step in the light-harvesting (LH) process producing the redox equivalents for energy conversion. However, like traditional macromolecular donor-acceptor assemblies, most MOF-derived LH systems are designed with a large ΔG⁰ to drive PCT. To emulate the functionality of the reaction center of the natural LH complex that drives PCT within a pair of identical chromophores producing charge carriers with maximum potentials, we prepared two electronically diverse carboxy-terminated zinc porphyrins, BFBP(Zn)-COOH and TFP(Zn)-COOH, and installed them into the hexagonal pores of NU-1000 via solvent-assisted ligand incorporation (SALI), resulting in BFBP(Zn)@NU-1000 and TFP(Zn)@NU-1000 compositions. Varying the number of trifluoromethyl groups at the porphyrin core, we tuned the ground-state redox potentials of the porphyrins within ca. 0.1 V relative to that of NU-1000, defining a small ΔG₀ for PCT. For BFBP(Zn)@NU-1000, the relative ground- and excited-state redox potentials of the components facilitate an energy transfer (EnT) from NU-1000* to BFBP(Zn), forming BFBP(Zn)_S1* which entails a long-lived charge-separated complex formed through an exciplex-like [BFBP(Zn)_S1*-TBAPy] intermediate. Various time-resolved spectroscopic data suggest that EnT from NU-1000* may not involve a fast Förster-like resonance energy transfer (FRET) but rather through a slow [NU-1000*-BFBP(Zn)] intermediate formation. In contrast, TFP(Zn)@NU-1000 displays an efficient EnT from NU-1000* to [TFP(Zn)-TBAPy], a complex that formed at the ground state through electronic interaction, and thereon showed the excited-state feature of [TFP(Zn)-TBAPy]*. The results will help to develop synthetic LHC systems that can produce long-lived photogenerated charge carriers with high potentials, i.e., high open-circuit voltage in photoelectrochemical setups.

Donor-acceptor conjugates derived from cobalt porphyrin and fullerene via metal-ligand axial coordination: Formation and excited state charge separation

Two types of cobalt porphyrins, viz., meso-tetrakis(tolylporphyrinato)cobalt(II), (TTP)Co (1), and meso-tetrakis(triphenylamino porphyrinato)cobalt(II), [(TPA)4P]Co, (2) were self-assembled via metal-ligand axial coordination of phenyl imidazole functionalized fulleropyrrolidine, ImC[Formula: see text] to form a new series of donor–acceptor constructs. A 1:2 complex formation with ImC[Formula: see text] was established in the case of (TTP)Co while for [(TPA)4P]Co only a 1:1 complex was possible to positively identify. The binding constants [Formula: see text] and [Formula: see text] for step-wise addition of ImC[Formula: see text] to (TTP)Co were found to be 1.07 × 105 and 3.20 × 104 M[Formula: see text], respectively. For [(TPA)4P]Co:ImC[Formula: see text], the measured [Formula: see text] values was found to be 6.48 × 104 M[Formula: see text], slightly smaller than that observed for (TTP)Co. Although both cobalt porphyrins were non-fluorescent, they were able to quench the fluorescence of ImC[Formula: see text] indicating occurrence of excited state events in the supramolecular donor-acceptor complexes. Electrochemistry coupled with spectroelectrochemistry, revealed the formation of cobalt(III) porphyrin cation instead of a cobalt(II) porphyrin radical cation, as the main product, during oxidation of phenyl imidazole coordinated cobalt porphyrin. With the help of computational and electrochemical results, an energy level diagram was constructed to witness excited state photo-events. Competitive energy and electron transfer from excited CoP to coordinated ImC[Formula: see text], and electron transfer from Im1C[Formula: see text]* to cobalt(II) porphyrin resulting into the formation of PCo[Formula: see text]:ImC[Formula: see text] charge separated state was possible to envision from the energy diagram. Finally, using femtosecond transient absorption spectroscopy and data analysis by Glotaran, it was possible to establish sequential occurrence of energy transfer and charge separation processes. The lifetime of the final charge separated state was [Formula: see text] 2 ns. A slightly better charge stabilization was observed in the case of [(TPA)4P]Co:ImC[Formula: see text] due to the presence of electron rich, peripheral triphenylamine substituents on the cobalt porphyrin.

Mechanism and Dynamics of Formation of Bisoxo Intermediates and O-O Bond in the Catalytic Water Oxidation Process

This work elucidates the reactivity of water molecules toward the tridentate nitrogen-containing iron complex in the water oxidation process. Here, we consider the Fe^V-bisoxo complex {[Fe^V(Me₃tacn)(OH₂)(═O)₂]⁺} to be responsible for the oxygen-oxygen bond formation. This O-O bond formation happens through the addition of water as a nucleophile. The transition state was determined by the synchronous transit-guided quasi-Newton method using reactants and products and verified by intrinsic reaction coordinates (IRCs). From the IRC calculations, we observe that the Fe^V═O moiety is attacked by water and assisted by the H-bonded interaction with the oxygen atom of the bisoxo complex. The hydrogen atom is transferred to the oxygen atom of the bisoxo complex through the transition state, and subsequently, the hydroxide is transferred to another oxygen of the bisoxo complex, resulting in the formation of the oxygen-oxygen bond. This work also explains the effect of explicit water molecules on the oxygen-oxygen bond formation. Our results also show how the formation of superoxide plays an essential role in O₂ evolution. We used the potential energy scan method to compute the transition state in the oxygen evolution step. In the present work, we study the effect of chlorine on the formation of the oxygen-oxygen bond formation. In this study, the changes in the oxidation state, spin density, and spin multiplicity of the complexes are investigated for each successive step. Apart from these static theoretical calculations, we also studied the oxygen-oxygen bond formation through first-principles molecular dynamics with the aid of the well-tempered metadynamics sampling technique. From the observation of the free energy surfaces from metadynamics simulations, it is evident that the hydroxide transfer has a lesser free energetic reaction as compared to the proton transfer. This complete mechanistic study may give an idea to design a suitable water oxidation catalyst.

Eosin-Y and sulfur-codoped g-C3N4 composite for photocatalytic applications: the regeneration of NADH/NADPH and the oxidation of sulfide to sulfoxide

The successful development of eosin-Y and sulfur-codoped g-C3N4 composite as a highly efficient photocatalyst for the regeneration of NADH/NADPH (64.38%/81.14%) and the light-driven oxidation of sulfide to sulfoxide with an yield of 99.6%.

Direct Observation of a Plasmon-Induced Hot Electron Flow in a Multimetallic Nanostructure

Plasmon hot carriers are interesting for photoredox chemical synthesis but their direct utilization is limited by their ultrafast thermalization. Therefore, they are often transferred to suitable accepting materials that expedite their lifetime. Solid-state photocatalysts are technologically more suitable than their molecular counterparts, but their photophysical processes are harder to follow due to the absence of clear optical fingerprints. Herein, the journey of hot electrons in a solid-state multimetallic photocatalyst is revealed by a combination of ultrafast visible and infrared spectroscopy. Dynamics showed that electrons formed upon silver plasmonic excitation reach the gold catalytic site within 700 fs and the electron flow could also be reversed. Gold is the preferred site until saturation of its 5d band occurs. Silver-plasmon hot electrons increased the rate of nitrophenol reduction 16-fold, confirming the preponderant role of hot electrons in the overall catalytic activity and the importance to follow hot carriers' journeys in solid-state photosystems.

Gene Loss and Evolution of the Plastome

Chloroplasts are unique organelles within the plant cells and are responsible for sustaining life forms on the earth due to their ability to conduct photosynthesis. Multiple functional genes within the chloroplast are responsible for a variety of metabolic processes that occur in the chloroplast. Considering its fundamental role in sustaining life on the earth, it is important to identify the level of diversity present in the chloroplast genome, what genes and genomic content have been lost, what genes have been transferred to the nuclear genome, duplication events, and the overall origin and evolution of the chloroplast genome. Our analysis of 2511 chloroplast genomes indicated that the genome size and number of coding DNA sequences (CDS) in the chloroplasts genome of algae are higher relative to other lineages. Approximately 10.31% of the examined species have lost the inverted repeats (IR) in the chloroplast genome that span across all the lineages. Genome-wide analyses revealed the loss of the Rbcl gene in parasitic and heterotrophic plants occurred approximately 56 Ma ago. PsaM, Psb30, ChlB, ChlL, ChlN, and Rpl21 were found to be characteristic signature genes of the chloroplast genome of algae, bryophytes, pteridophytes, and gymnosperms; however, none of these genes were found in the angiosperm or magnoliid lineage which appeared to have lost them approximately 203-156 Ma ago. A variety of chloroplast-encoded genes were lost across different species lineages throughout the evolutionary process. The Rpl20 gene, however, was found to be the most stable and intact gene in the chloroplast genome and was not lost in any of the analyzed species, suggesting that it is a signature gene of the plastome. Our evolutionary analysis indicated that chloroplast genomes evolved from multiple common ancestors ~1293 Ma ago and have undergone vivid recombination events across different taxonomic lineages.

Light-Driven Proton Transfer for Cyclic and Temporal Switching of Enzymatic Nanoreactors

Temporal activation of biological processes by visible light and subsequent return to an inactive state in the absence of light is an essential characteristic of photoreceptor cells. Inspired by these phenomena, light-responsive materials are very attractive due to the high spatiotemporal control of light irradiation, with light being able to precisely orchestrate processes repeatedly over many cycles. Herein, it is reported that light-driven proton transfer triggered by a merocyanine-based photoacid can be used to modulate the permeability of pH-responsive polymersomes through cyclic, temporally controlled protonation and deprotonation of the polymersome membrane. The membranes can undergo repeated light-driven swelling-contraction cycles without losing functional effectiveness. When applied to enzyme loaded-nanoreactors, this membrane responsiveness is used for the reversible control of enzymatic reactions. This combination of the merocyanine-based photoacid and pH-switchable nanoreactors results in rapidly responding and versatile supramolecular systems successfully used to switch enzymatic reactions ON and OFF on demand.

Decelerating Charge Recombination Using Fluorinated Porphyrins in N,N-Bis(3,4,5-trimethoxyphenyl)aniline-Aluminum(III) Porphyrin-Fullerene Reaction Center Models

In supramolecular reaction center models, the lifetime of the charge-separated state depends on many factors. However, little attention has been paid to the redox potential of the species that lie between the donor and acceptor in the final charge separated state. Here, we report on a series of self-assembled aluminum porphyrin-based triads that provide a unique opportunity to study the influence of the porphyrin redox potential independently of other factors. The triads, BTMPA-Im→AlPorF_n-Ph-C₆₀ (n = 0, 3, 5), were constructed by linking the fullerene (C₆₀) and bis(3,4,5-trimethoxyphenyl)aniline (BTMPA) to the aluminum(III) porphyrin. The porphyrin (AlPor, AlPorF₃, or AlPorF₅) redox potentials are tuned by the substitution of phenyl (Ph), 3,4,5-trifluorophenyl (PhF₃), or 2,3,4,5,6-pentafluorophenyl (PhF₅) groups in its meso positions. The C₆₀ and BTMPA units are bound axially to opposite faces of the porphyrin plane via covalent and coordination bonds, respectively. Excitation of all of the triads results in sequential electron transfer that generates the identical final charge separated state, BTMPA^•+-Im→AlPorF_n-Ph-C₆₀^•-, which lies energetically 1.50 eV above the ground state. Despite the fact that the radical pair is identical in all of the triads, remarkably, the lifetime of the BTMPA^•+-Im→AlPorF_n-Ph-C₆₀^•- radical pair was found to be very different in each of them, that is, 1240, 740, and 56 ns for BTMPA-Im→AlPorF₅-Ph-C₆₀, BTMPA-Im→AlPorF₃-Ph-C₆₀, and BTMPA-Im→AlPor-Ph-C₆₀, respectively. These results clearly suggest that the charge recombination is an activated process that depends on the midpoint potential of the central aluminum(III) porphyrin (AlPorF_n).

Synthesis, properties and photovoltaic performance in dye-sensitized solar cells of three meso-diphenylbacteriochlorins bearing a dual-function electron-donor

Bacteriochlorins are crucial to photosynthesis in bacteria. Studies of air-stable, meso-substituted bacteriochlorins are rare. We herein report the synthesis, properties, and photovoltaic performance of three new air-stable, meso-substituted bacteriochlorins bearing a dioctylfluorenylethyne (denoted as LS-17), a dioctylaminophenylethynylanthrylethyne (LS-43), and a diarylaminoanthrylethyne (LS-45) as the electron-donating groups. Among these LS-bacteriochlorins, LS-17 displays sharp UV-visible absorption bands whereas LS-43 and LS-45 give rise to broadened and red-shifted absorptions. Electrochemical and DFT results suggest that the first oxidation and reduction reactions of these bacteriochlorins are consistent with the formation of the cation and anion radicals, respectively. For dye-sensitized solar cell applications, photovoltaic performance of the LS-45 cell achieves an overall efficiency of 6.04% under one-sun irradiation.

Thiazolothiazole-Based Luminescent Metal-Organic Frameworks with Ligand-to-Ligand Energy Transfer and Hg2+-Sensing Capabilities

Photoinduced electron and energy transfer through preorganized chromophore, donor, and acceptor arrays are key to light-harvesting capabilities of photosynthetic plants and bacteria. Mimicking the design principles of natural photosystems, we constructed a new luminescent pillared paddle wheel metal-organic framework (MOF), Zn₂(NDC)₂(DPTTZ), featuring naphthalene dicarboxylate (NDC) struts that served as antenna chromophores and energy donors and N,N'-di(4-pyridyl)thiazolo-[5,4-d]thiazole (DPTTZ) pillars as complementary energy acceptors and light emitters. Highly ordered arrangement and good overlap between the emission and absorption spectra of these two complementary energy donor and acceptor units enabled ligand-to-ligand Förster resonance energy transfer, allowing the MOF to display exclusively DPTTZ-centric blue emission (410 nm) regardless of the excitation of either chromophore at different wavelengths. In the presence of Hg²⁺, a toxic heavy metal ion, the photoluminescence (PL) of Zn₂(NDC)₂(DPTTZ) MOF underwent significant red-shift to 450 nm followed by quenching, whereas other transition metal ions (Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, and Cd²⁺) caused only fluorescence quenching but no shift. The free DPTTZ ligand also displayed similar, albeit less efficient, fluorescence changes, suggesting that the heavy atom effect and coordination of Hg²⁺ and other transition metal ions with the DPTTZ ligands were responsible for the fluorescence changes in the MOF. When exposed to a mixture of different metal ions, including Hg²⁺, the MOF still displayed the Hg²⁺-specific fluorescence signal, demonstrating that it could detect Hg²⁺ in the presence of other metal ions. The powder X-ray diffraction studies verified that the framework remained intact after being exposed to Hg²⁺ and other transition metal ions, and its original PL spectrum was restored upon washing. These studies demonstrated the light-harvesting and Hg²⁺ sensing capabilities of a new bichromophoric luminescent MOF featuring a seldom-used photoactive ligand, which will likely spark an explosion of TTZ-based MOFs for various optoelectronic applications in near future.

Antenna-like Ring Structures via Self-Assembly of Octaphosphonate Tetraphenyl Porphyrin with Nucleobases

Supramolecular self-assembly of an octaphosphonate tetraphenyl porphyrin with three different nucleobases (adenine, cytosine, and thymine) was studied. Porphyrin 1 with 8 and 10 equiv of cytosine produces light-harvesting ring-like structures, that is, architectures similar to those observed in natural light-harvesting antenna. However, porphyrin assembled with adenine or thymine resulted in prisms and microrods, respectively. UV-vis absorption, fluorescence, and dynamic light scattering were used to determine the mode of aggregation in solution. Scanning electron microscopy and X-ray diffraction spectroscopy used to visualize the self-assembled nanostructures and their behavior in the solid state, respectively. Thus, we believe that this study may demonstrate a deeper understanding on how one needs to manipulate donor/acceptor subunits in supramolecular assemblies to construct artificial antenna architectures.

Effect of different shading materials on grain yield and quality of rice

Light is a basic environmental factor required for plant growth and development; however, these are not only affected by light quantity, but also by light quality - light and radiation of different wavelengths and different compositions. In four different rice varieties (Oryza sativa L.), two kinds of shading materials, white cotton yarn (Shading (W)) and black nylon net (Shading (B)) were used to simulate cloudy days. Yield decreased under Shading (W) (15.3-17.7%) and Shading (B) (16.6-20.0%) compared to under sunny day (CK), and different effects on rice quality, which is mainly affected by changes in light quality, were observed. The change in light quality (Blue, Green, Red and R/FR proportions) represented under Shading (W) was significantly different from that under CK and Shading (B) conditions. Red light composition under Shading (W) was closer to that of the cloudy day condition. The proportion of blue light under Shading (W) was significantly lower than that under CK conditions; under Shading (B), it was higher than that under all conditions. The differences in light quality may affect photosynthesis in leaves and final starch synthesis, resulting in increased chalky grain rate, chalkiness, and poor rice quality. White cotton yarn as the shading material for further research used to simulate the influence of the light environment on rice growth under cloudy conditions will be better than black net.

Surface anchored self-assembled reaction centre mimics as photoanodes consisting of a secondary electron donor, aluminium(iii) porphyrin and TiO2 semiconductor

Vertically assembled photoanodes, consisting of aluminum(iii) porphyrin, an electron donor, and semiconductor TiO₂, have been fabricated and their photophysical properties investigated.

Charge-separation in panchromatic, vertically positioned bis(donor styryl)BODIPY-aluminum(iii) porphyrin-fullerene supramolecular triads

Three, broad band capturing, vertically aligned supramolecular triads, R2-BDP-AlPorF3←Im-C60 [R = H, styryl (C2H2-Ph), C2H2-TPA (TPA = triphenylamine); ← = coordinate bond], have been constructed using BODIPY derivative (BDP, BDP-Ph2 or BDP-TPA2), 5,10,15,20-tetrakis(3,4,5-trifluorophenyl)aluminum(iii) porphyrin (AlPorF3) and fullerene (C60) entities. The C60 and BDP units are bound to the Al center on the opposite faces of the porphyrin: the BDP derivative through a covalent axial bond using a benzoate spacer and the C60 through a coordination bond via an appended imidazole. Owing to the bis-styryl functionality on BDP, the constructed dyads and triads exhibited panchromatic light capture. Due to the diverse absorption and redox properties of the selected entities, it was possible to demonstrate excitation wavelength dependent photochemical events. In the case of the BDP-AlPorF3 dyad, selective excitation of BDP resulted in singlet-singlet energy transfer to AlPorF3 (kEnT = 1.0 × 1010 s-1). On the other hand, excitation of the AlPorF3 entity in the BDP-AlPorF3←Im-C60 triad revealed charge separation leading to the BDP-(AlPorF3)˙+-(C60)˙- charge separated state (kCS = 2.43 × 109 s-1). In the case of the Ph2-BDP-AlPorF3 dyad, energy transfer from 1AlPorF3* to 1(Ph2-BDP)* was witnessed (kEnT = 1.0 × 1010 s-1); however, upon assembling the supramolecular triad, (Ph2-BDP)-AlPorF3←Im-C60, electron transfer from 1AlPorF3* to C60 (kCS = 3.35 × 109 s-1), followed by hole shift (kHS = 1.00 × 109 s-1) to Ph2-BDP, was witnessed. Finally, in the case of the TPA2-BDP-AlPorF3←Im-C60 triad, only electron transfer leading to the (TPA2-BDP)˙+-AlPorF3←Im-(C60)˙- charge separated state, and no energy transfer, was observed. The facile oxidation of Ph2-BDP and TPA2-BDP compared to AlPorF3 in the latter two triads facilitated charge separation through either an electron migration or hole transfer mechanism depending on the initial excitation. The charge-separated states in these triads persisted for about 20 ns.

Oxidation of P700 Ensures Robust Photosynthesis

In the light, photosynthetic cells can potentially suffer from oxidative damage derived from reactive oxygen species. Nevertheless, a variety of oxygenic photoautotrophs, including cyanobacteria, algae, and plants, manage their photosynthetic systems successfully. In the present article, we review previous research on how these photoautotrophs safely utilize light energy for photosynthesis without photo-oxidative damage to photosystem I (PSI). The reaction center chlorophyll of PSI, P700, is kept in an oxidized state in response to excess light, under high light and low CO2 conditions, to tune the light utilization and dissipate the excess photo-excitation energy in PSI. Oxidation of P700 is co-operatively regulated by a number of molecular mechanisms on both the electron donor and acceptor sides of PSI. The strategies to keep P700 oxidized are diverse among a variety of photoautotrophs, which are evolutionarily optimized for their ecological niche.

Excited-State Electronic Properties in Zr-Based Metal-Organic Frameworks as a Function of a Topological Network

Molecular assemblies in metal-organic frameworks (MOFs) are reminiscent of natural light-harvesting (LH) systems and considered as emerging materials for energy conversion. Such applications require understanding the correlation between their excited-state properties and underlying topological net. Two chemically identical but topologically different tetraphenylpyrene (1,3,6,8-tetrakis( p-benzoicacid)pyrene; H₄TBAPy)-based Zr^IV MOFs, NU-901 ( scu) and NU-1000 ( csq), are chosen to computationally and spectroscopically interrogate the impact of topological difference on their excited-state electronic structures. Time-dependent density functional theory-computed transition density matrices for selected model compounds reveal that the optically relevant S₁, S₂, and S _n states are delocalized over more than four TBAPy linkers with a maximum exciton size of ∼1.7 nm (i.e., two neighboring TBAPy linkers). Computational data further suggests the evolution of polar excitons (hole and electron residing in two different linkers); their oscillator strengths vary with the extent of interchromophoric interaction depending on their topological network. Femtosecond transient absorption (fs-TA) spectroscopic data of NU-901 highlight instantaneous spectral evolution of an intense S₁ → S _n transition at 750 nm, which diminishes with the emergence of a broad (580-1100 nm) induced absorption originating from a fast excimer formation. Although these ultrafast spectroscopic data reveal the first direct spectral observation of fast excimer formation (τ = 2 ps) in MOFs, the fs-TA features seen in NU-901 are clearly absent in NU-1000 and the free H₄TBAPy linker. Furthermore, transient and steady-state fluorescence data collected as a function of solvent dielectrics reveal that the emissive states in both MOF samples are electronically nonpolar; however, low-lying polar excited states may get involved in the excited-state decay processes in polar solvents. The present work shows that the topological arrangement of the linkers critically controls the excited-state electronic structures.

Dual-Function Electrocatalytic and Macroporous Hollow-Fiber Cathode for Converting Waste Streams to Valuable Resources Using Microbial Electrochemical Systems

Dual-function electrocatalytic and macroporous hollow-fiber cathodes are recently proposed as promising advanced material for maximizing the conversion of waste streams such as wastewater and waste CO₂ to valuable resources (e.g., clean freshwater, energy, value-added chemicals) in microbial electrochemical systems. The first part of this progress report reviews recent developments in this type of cathode architecture for the simultaneous recovery of clean freshwater and energy from wastewater. Critical insights are provided on suitable materials for fabricating these cathodes, as well as addressing some challenges in the fabrication process with proposed strategies to overcome them. The second and complementary part of the progress report highlights how the unique features of this cathode architecture can solve one of the intrinsic bottlenecks (gas-liquid mass transfer limitation) in the application of microbial electrochemical systems for CO₂ reduction to value-added products. Strategies to further improve the availability of CO₂ to microbial catalysts on the cathode are proposed. The importance of understanding microbe-cathode interactions, as well as electron transfer mechanisms at the cathode-cell and cell-cell interface to better design dual-function macroporous hollow-fiber cathodes, is critically discussed with insights on how the choice of material is important in facilitating direct electron transfer versus mediated electron transfer.

Efficient Energy Transfer (EnT) in Pyrene- and Porphyrin-Based Mixed-Ligand Metal-Organic Frameworks

Designing and synthesizing the ordered light-harvesting systems, possessing complementary absorption and energy-transfer process between the chromophores, are essential steps to accomplish successful mimicking of the natural photosynthetic systems. Metal-organic frameworks (MOFs) can be considered as an ideal system to achieve this due to their highly ordered structure, superior synthetic versatility, and tailorable functionality. Herein, we have synthesized the new light-harvesting mixed-ligand MOFs (MLMs, MLM-1-3) via solvothermal reactions between a Zr₆ cluster and a mixture of appropriate ratio of 1,3,6,8-tetrakis(p-benzoic acid)pyrene and [5,10,15,20-tetrakis(4-carboxy-phenyl)porphyrinato]-Zn(II) ligands. The identical symmetry and connectivity of the two ligands of the MLMs was the key parameter of successful synthesis as a single MOF form, and the ample overlap between the emission spectrum of pyrene and the absorption spectrum of porphyrin provided the ideal platform to design an efficient-energy transfer (EnT) process within the MLMs. We obtained the nanoscale maps of the fluorescence intensities and lifetimes of microsize MLM grains for unambiguous visualization of EnT phenomena occurring between two ligands in MLMs. Moreover, due to complementary absorption and energy transfer between the two ligands in the MLMs, our MLMs performed as superior photoinduced singlet-oxygen generators, verifying the enhanced light-harvesting properties of the pyrene- and porphyrin-based MLMs.

Efficiency of Dinucleosides as the Backbone to Pre-Organize Multi-Porphyrins and Enhance Their Stability as Sandwich Type Complexes with DABCO

Flexible linkers such as uridine or 2′-deoxyuridine pre-organize bis-porphyrins in a face-to-face conformation, thus forming stable sandwich complexes with a bidentate base such as 1,4-diazabicyclo[2.2.2]octane (DABCO). Increased stability can be even greater when a dinucleotide linker is used. Such pre-organization increases the association constant by one to two orders of magnitude when compared to the association constant of DABCO with a reference porphyrin. Comparison with rigid tweezers shows a better efficiency of nucleosidic dimers. Thus, the choice of rigid spacers is not the only way to pre-organize bis-porphyrins, and well-chosen nucleosidic linkers offer an interesting option for the synthesis of such devices.

Cascade energy transfer and tunable emission from nanosheet hybrids: locating acceptor molecules through chiral doping

A donor–acceptor–donor triad with excellent intramolecular energy transfer (99%) exhibits cascade energy transfer in the presence of second acceptors leading to tunable emission colours. Chiral acceptor doping enables chiral induction and to locate acceptors in the donor scaffold.

A switchable self-assembling and disassembling chiral system based on a porphyrin-substituted phenylalanine-phenylalanine motif

Artificial light-harvesting systems have until now not been able to self-assemble into structures with a large photon capture cross-section that upon a stimulus reversibly can switch into an inactive state. Here we describe a simple and robust FLFL-dipeptide construct to which a meso-tetraphenylporphyrin has been appended and which self-assembles to fibrils, platelets or nanospheres depending on the solvent composition. The fibrils, functioning as quenched antennas, give intense excitonic couplets in the electronic circular dichroism spectra which are mirror imaged if the unnatural FDFD-analogue is used. By slightly increasing the solvent polarity, these light-harvesting fibres disassemble to spherical structures with silent electronic circular dichroism spectra but which fluoresce. Upon further dilution with the nonpolar solvent, the intense Cotton effects are recovered, thus proving a reversible switching. A single crystal X-ray structure shows a head-to-head arrangement of porphyrins that explains both their excitonic coupling and quenched fluorescence.

Color-Tunable Resonant Photoluminescence and Cavity-Mediated Multistep Energy Transfer Cascade

Color-tunable resonant photoluminescence (PL) was attained from polystyrene microspheres doped with a single polymorphic fluorescent dye, boron-dipyrrin (BODIPY) 1. The color of the resonant PL depends on the assembling morphology of 1 in the microspheres, which can be selectively controlled from green to red by the initial concentration of 1 in the preparation process of the microspheres. Studies on intersphere PL propagation with multicoupled microspheres, prepared by micromanipulation technique, revealed that multistep photon transfer takes place through the microspheres, accompanying energy transfer cascade with stepwise PL color change. The intersphere energy transfer cascade is direction selective, where energy donor-to-acceptor down conversion direction is only allowed. Such cavity-mediated long-distance and multistep energy transfer will be advantageous for polymer photonics device application.