Light-harvesting Units Research Articles

Thioxanthone Functionalized NanoTiO2 Composites as Photocatalyst for Degradation of Organic Dyes.

Titanium dioxide (TiO2) photocatalytic technology has the advantages of high catalytic activity, high chemical stability, nontoxicity, and low cost. Therefore, it finds widespread applications in the degradation of organic pollutants in water, antibacterial, environmental purification, and other fields. In this study, we have obtained a photocatalyst by modifying nanoTiO2 with the photosensitizer thioxanthone. The light-harvesting units of thioxanthone and nanoTiO2 can work synergistically to capture light energy. As a heterogeneous photocatalytic material, it can efficiently degrade organic dyes such as Rhodamine B, methyl blue and methyl orange. Specifically, the degradation rate of 0.1 mmol/L Rhodamine B can reach 97% after 35 min of irradiation, and methyl blue and methyl orange can also reach 98 and 56%, respectively.

Noble Metal Plasmon-Molecular Catalyst Hybrids for Renewable Energy Relevant Small Molecule Activation.

Significant endeavors have been dedicated to the advancement of materials for artificial photosynthesis, aimed at efficiently harvesting light and catalyzing reactions such as hydrogen production and CO2 conversion. The application of plasmonic nanomaterials emerges as a promising option for this purpose, owing to their excellent light absorption properties and ability to confine solar energy at the nanoscale. In this regard, coupling plasmonic particles with molecular catalysts offers a pathway to create high-performance hybrid catalysts. In this review, we discuss the plasmonic-molecular complex hybrid catalysts where the plasmonic nanoparticles serve as the light-harvesting unit and promote interfacial charge transfer in tandem with the molecular catalyst which drives chemical transformation. In the initial section, we provide a concise overview of plasmonic nanomaterials and their photophysical properties. We then explore recent breakthroughs, highlighting examples from literature reports involving plasmonic-molecular complex hybrids in various catalytic processes. The utilization of plasmonic materials in conjunction with molecular catalysts represents a relatively unexplored area with substantial potential yet to be realized. This review sets a strong basis and motivation to explore the plasmon-induced hot-electron mediated photelectrochemical small molecule activation reactions. Utilizing in situ spectroscopic investigations and ultrafast transient absorption spectroscopy, it presents a comprehensive template for scalable and sustainable antenna-reactor systems.

Photo- and electrochemical properties of newly synthesized thioxathone-viologen-fluorene ternary compound and its electrochromic device

External stimuli-responsive materials exhibit widespread potential applications in sensors, smart windows, displays, and opto-electronic molecular devices. In this study, we report light-/electrochemically induced chromic behaviors of newly synthesized ternary compound in the device. This compound is composed of a light-harvesting unit of thioxathone (TX), an electron donor of fluorene (FL), and an electron mediator of viologen (VIO), and thus it can act as a photochromic species with an intramolecular electron transfer under irradiation. The precursors of TX and FL substituents give off broad emissions at approximately 437 and 310 − 330 nm, respectively. Both emissions are largely quenched in the TX-VIO-FL ternary compound, which is caused by an intramolecular transduction of energy from the excited singlet FL and TX to the electroactive VIO substituent. Based on cyclic voltammogram of TX-VIO-FL modified electrodes, two reversible waves are observed at around − 0.44 and − 0.89 V (vs Hg/Hg2Cl2), attributed to the electron transport processes of TX-VIO2+-FL ↔ TX-VIO+•-FL and TX-VIO+•-FL ↔ TX-VIO0-FL, respectively. Sandwiched type of photo-/electrochromic devices are constructed, displaying a good transmittance and reversible viologen color change behavior under irradiation or an applied bias of − 2.6 V.

A Molecular Z-Scheme Artificial Photosynthetic System Under the Bias-Free Condition for CO2 Reduction Coupled with Two-electron Water Oxidation: Photocatalytic Production of CO/HCOOH and H2 O2.

Bio-inspired molecular-engineered systems have been extensively investigated for the half-reactions of H2 O oxidation or CO2 reduction with sacrificial electron donors/acceptors. However, there has yet to be reported a device for dye-sensitized molecular photoanodes coupled with molecular photocathodes in an aqueous solution without the use of sacrificial reagents. Herein, we will report the integration of SnIV - or AlIII -tetrapyridylporphyrin (SnTPyP or AlTPyP) decorated tin oxide particles (SnTPyP/SnO2 or AlTPyP/SnO2 ) photoanode with the dye-sensitized molecular photocathode on nickel oxide particles containing [Ru(diimine)3 ]2+ as the light-harvesting unit and [Ru(diimine)(CO)2 Cl2 ] as the catalyst unit covalently connected and fixed within poly-pyrrole layer (RuCAT-RuC2 -PolyPyr-PRu/NiO). The simultaneous irradiation of the two photoelectrodes with visible light resulted in H2 O2 on the anode and CO, HCOOH, and H2 on the cathode with high Faradaic efficiencies in purely aqueous conditions without any applied bias is the first example of artificial photosynthesis with only two-electron redox reactions.

Metal-Free Highly Stable and Crystalline Covalent Organic Nanosheet for Visible-Light-Driven Selective Solar Fuel Production in Aqueous Medium

Conversion of CO2 into solar fuels via artificial photosynthesis is one of the most promising and sustainable approaches to mitigate global warming and worldwide energy shortage. Covalent organic frameworks (COFs) exhibit well-defined arrangements of building blocks, tunable porosity, and high thermal and chemical stability in harsh conditions. The tunable band gaps of COFs by suitably introducing chromophoric light-harvesting units make them a unique class of metal-free heterogeneous photocatalysts for the successful conversion of CO2 to solar fuel. In this work, we report a simple, efficient, and low-cost 2D COF (TTA-Tz) composed of 1,3,5-tris-(4-aminophenyl)triazine (TTA) and 4,4′-(thiazolo[5,4-d]thiazole-2,5-diyl)dibenzaldehyde (Tz) for photocatalytic CO2 reduction. The 2D-layered COF is exfoliated into ultrathin covalent organic nanosheets (CONs), which shows visible-light-driven photoreduction of CO2 to CO (yield = 2.8 mmol g–1, rate = 82 μmol h–1 g–1, and selectivity >99%) in aqueous medium without an external sacrificial electron donor. Interestingly, for a mixed solvent system, the CO evolution rate (163 μmol g–1 h–1) is found double than the aqueous medium case with 99% selectivity. By introducing both BNAH and TEA as sacrificial electron donors, the significant amount of CH4 (499 μmol g–1) is produced and the rate of CO evolution (310 μmol g–1 h–1) is further enhanced. The mechanistic insight of CO2 reduction is studied by DFT-based theoretical calculation, which is further supported by in situ diffuse reflectance spectroscopy study.

Recent advances on catalysts for photocatalytic selective hydrogenation of nitrobenzene to aniline.

Selective hydrogenation of nitrobenzene (SHN) is an important approach to synthesize aniline, an essential intermediate with extremely high research significance and value in the fields of textiles, pharmaceuticals and dyes. SHN reaction requires high temperature and high hydrogen pressure via the conventional thermal-driven catalytic process. On the contrary, photocatalysis provides an avenue to achieve high nitrobenzene conversion and high selectivity towards aniline at room temperature and low hydrogen pressure, which is in line with the sustainable development strategies. Designing efficient photocatalysts is a crucial step in SHN. Up to now, several photocatalysts have been explored for photocatalytic SHN, such as TiO2, CdS, Cu/graphene and Eosin Y. In this review, we divide the photocatalysts into three categories based on the characteristics of the light harvesting units, including semiconductors, plasmonic metal-based catalysts and dyes. The recent progress of the three categories of photocatalysts is summarized, the challenges and opportunities are pointed out and the future development prospects are described. It aims to give a clear picture to the catalysis community and stimulate more efforts in this research area.

Molecular structures reveal the origin of spectral variation in cryptophyte light harvesting antenna proteins.

In addition to their membrane-bound chlorophyll a/c light-harvesting antenna, the cryptophyte algae have evolved a unique phycobiliprotein antenna system located in the thylakoid lumen. The basic unit of this antenna consists of two copies of an αβ protomer where the α and β subunits scaffold different combinations of a limited number of linear tetrapyrrole chromophores. While the β subunit is highly conserved, encoded by a single plastid gene, the nuclear-encoded α subunits have evolved diversified multigene families. It is still unclear how this sequence diversity results in the spectral diversity of the mature proteins. By careful examination of three newly determined crystal structures in comparison with three previously obtained, we show how the α subunit amino acid sequences control chromophore conformations and hence spectral properties even when the chromophores are identical. Previously we have shown that α subunits control the quaternary structure of the mature αβ.αβ complex (either open or closed), however, each species appeared to only harbor a single quaternary form. Here we show that species of the Hemiselmis genus contain expressed α subunit genes that encode both distinct quaternary structures. Finally, we have discovered a common single-copy gene (expressed into protein) consisting of tandem copies of a small α subunit that could potentially scaffold pairs of light harvesting units. Together, our results show how the diversity of the multigene α subunit family produces a range of mature cryptophyte antenna proteins with differing spectral properties, and the potential for minor forms that could contribute to acclimation to varying light regimes.

Heterogenization of Salen Metal Molecular Catalysts in Covalent Organic Frameworks for Photocatalytic Hydrogen Evolution.

Integrating a molecular catalyst with a light harvester into a photocatalyst is an effective strategy for solar light conversion. However, it is challenging to establish a crystallized framework with well-organized connections that favour charge separation and transfer. Herein, we report the heterogenization of a Salen metal complex molecular catalyst into a rigid covalent organic framework (COF) through covalent linkage with the light-harvesting unit of pyrene for photocatalytic hydrogen evolution. The chemically conjugated bonds between the two units contribute to fast photogenerated electron transfer and thereby promote the proton reduction reaction. The Salen cobalt-based COF showed the best hydrogen evolution activity (1378 μmol g-1 h-1 ), which is superior to the previously reported nonnoble metal based COF photocatalysts. This work provides a strategy to construct atom-efficient photocatalysts by the heterogenization of molecular catalysts into covalent organic frameworks.

Monitoring Electron Flow in Nickel Single-Atom Catalysts during Nitrogen Photofixation.

An efficient catalytic system for nitrogen (N2) photofixation generally consists of light-harvesting units, active sites, and an electron-transfer bridge. In order to track photogenerated electron flow between different functional units, it is highly desired to develop in situ characterization techniques with element-specific capability, surface sensitivity, and detection of unoccupied states. In this work, we developed in situ synchrotron radiation soft X-ray absorption spectroscopy (in situ sXAS) to probe the variation of electronic structure for a reaction system during N2 photoreduction. Nickel single-atom and ceria nanoparticle comodified reduced graphene oxide (CeO2/Ni-G) was designed as a model catalyst. In situ sXAS directly reveals the dynamic interfacial charge transfer of photogenerated electrons under illumination and the consequent charge accumulation at the catalytic active sites for N2 activation. This work provides a powerful tool to monitor the electronic structure evolution of active sites under reaction conditions for photocatalysis and beyond.

Performance evaluation of DSSCs using naturally extracted dyes from petals of Lantana repens and Solidago canadensis flowers as light-harvesting units

Performance evaluation of DSSCs using naturally extracted dyes from petals of Lantana repens and Solidago canadensis flowers as light-harvesting units

Enhancing the temporal stability of DSSCs with novel vinylpyrimidine anchoring and accepting group

Three compounds based on aniline, thiophene and vinylpyrimidine as anchoring group have been prepared and studied as light harvesting units for dye-sensitized solar cells (DSSCs). They comprise thiophenyl π-spacer systems with one or two thiophenes (AT-Pyri, ATT-Pyri) and aniline as donor which is functionalized, in one case, with an hydroxyl group (HO-AT-Pyri). The bithiophene based dye (ATT-Pyri) shows higher extinction coefficients and smaller transition energy, as well as broader absorption spectrum. Thus, despite lower dye uptake, devices based on one thiophene and with the hydroxyl group (HO-AT-Pyri) have slightly higher power conversion efficiencies (η) as compared with the analogous dye without this group (AT-Pyri). However, the best performing device is based on the dye with bithiophene bridge (ATT-Pyri) when a co-adsorbent is added to avoid the aggregation of the dye. The photovoltaic parameters of cells based on these vinylpyrimidine dyes are enhanced over 1000 h, especially for ATT-Pyri devices, which reach a η = 6,4%.

Cover Feature: Amphiphilic Fullerene‐BODIPY Photosensitizers for Targeted Photodynamic Therapy (ChemMedChem 6/2022)

The Cover Feature illustrates the amphiphilic glucose-BODIPY-fullerene nanotheranostic carriers as targeting cancer cells via photodynamic therapy (PDT, i.e. the true source of the hero's power). These developed heavy-atom-free phototheranostic agents were fabricated with a universal spin converter, fullerene, a light-harvesting unit, BODIPY, and an active targeting site, glucose. When irradiated with a red-light source to disrupt cancer cells, they produced efficient singlet oxygen and proved to be a promising treatment strategy as a fullerene-mediated PDT approach. Cover design by Dr. Günaydın, Dr. Gedik, and Dr. Okutan. More information can be found in the Research Article by Ezel Öztürk Gündüz, Elif Okutan et al.

Photoprotective energy quenching in the red alga Porphyridium purpureum occurs at the core antenna of the photosystem II but not at its reaction center

Photosynthetic organisms have evolved light-harvesting antennae over time. In cyanobacteria, external phycobilisomes (PBSs) are the dominant antennae, whereas in green algae and higher plants, PBSs have been replaced by proteins of the Lhc family that are integrated in the membrane. Red algae represent an evolutionary intermediate between these two systems, as they employ both PBSs and membrane LHCR proteins as light-harvesting units. Understanding how red algae cope with light is not only interesting for biotechnological applications, but is also of evolutionary interest. For example, energy-dependent quenching (qE) is an essential photoprotective mechanism widely used by species from cyanobacteria to higher plants to avoid light damage; however, the quenching mechanism in red algae remains largely unexplored. Here, we used both pulse amplitude-modulated (PAM) and time-resolved chlorophyll fluorescence to characterize qE kinetics in the red alga Porphyridium purpureum. PAM traces confirmed that qE in P. purpureum is activated by a decrease in the thylakoid lumen pH, whereas time-resolved fluorescence results further revealed the quenching site and ultrafast quenching kinetics. We found that quenching exclusively takes place in the photosystem II (PSII) complexes and preferentially occurs at PSII’s core antenna rather than at its reaction center, with an overall quenching rate of 17.6 ± 3.0 ns−1. In conclusion, we propose that qE in red algae is not a reaction center type of quenching, and that there might be a membrane-bound protein that resembles PsbS of higher plants or LHCSR of green algae that senses low luminal pH and triggers qE in red algae.

A nano-enzymatic photoelectrochemical L-cysteine biosensor based on Bi2MoO6 modified honeycomb TiO2 nanotube arrays composite

Improving light harvesting behavior and recognition sites of sensor is an effective way to achieve highly sensitive and selective detection of nano-enzymatic photoelectrochemical (PEC) L-cysteine (Cyst). In this work, the Bi2MoO6-TiO2 composite (BMO-HTNTs) with honeycomb TiO2 nanotube arrays (HTNTs) as light harvesting unit and Bi2MoO6 nanoparticles (BMO) as recognition unit was prepared. The typical honeycomb-nanoparticle heterostructure between HTNTs and BMO improves the separation efficiency of the electron-hole pairs. Moreover, BMO nanoparticles fully dispersed on HTNTs with large specific surface area provide more sites for Cyst recognition. The composite exhibited the enhanced light absorption and higher photocurrent density at 0.2 V (vs. Ag/AgCl), which was 4 times than that of HTNTs under simulated light. Ingeniously, Cyst can not only be specifically recognized through the Bi-S bonds but also act as electron donor to scavenge the photogenerated holes of BMO-HTNTs, resulting in the increase of PEC responses. The PEC sensor showed superior analytical performance for Cyst detection with two linear regression equations in the range of 500 nM to 500 μM and a detection limit of 150 nM. The results of the enzyme-free sensor on practical urine and serum samples demonstrated its promising application in biological monitoring.

Amphiphilic Fullerene-BODIPY Photosensitizers for Targeted Photodynamic Therapy.

Nanotheranostic tailor-made carriers are potent platforms for the treatment of cancer that propound a number of advantages over conventional agents for photodynamic therapy (PDT). Herein, four new heavy atom free amphiphilic glucose-BODIPY-fullerene dyads (14-17) endowed with carbohydrate units in the styryl units, which can also form nanomicelles (14-17NM) with Tween 80 for PDT are reported. Glucose-BODIPY-fullerene systems (14-17) and related nanomicelles (14-17NM) have been prepared to emcee efficient singlet oxygen generation upon light irradiation. In vitro anti-tumor effects of the compounds 14-17 and 14-17NM in the presence of light and in darkness have been investigated with K562 human chronic myelogenous leukemia suspension cells. Anti-tumor toxicity upon light irradiation was due to the formation of singlet oxygen and reactive oxygen species (ROS). This study may provide an accomplished example of efficient PDT applications based on nanovehicles fabricated with universal spin converter, fullerene, light harvesting unit, BODIPY dyes conjugated with targeting units to fight against cancer.

Bias-Free Photoelectrochemical Water Splitting Cells Constructed by Calixarene Dyes and Molecular Ru Catalysts via Pyridyl Anchoring Groups

Two metal-free calix[4]arene dyes with pyridyl anchoring groups (C4BTP and C4STP) and a comparative single-chain dye (M-BTP) for comparison have been synthesized and applied in dye-sensitized photoelectrochemical cells in corporation with a pyridyl anchor-based molecular water oxidation catalyst (RuPy). All the devices based on the three dyes can conduct overall water splitting reaction under visible light with low or zero bias. It is confirmed that the cone structure and multiple light-harvesting units of calixarene dyes bring superior durability and photoelectroconversion property compared with chain dyes M-BTP, and the introduction of alkalinous pyridyl anchoring groups improves the adsorption firmness on TiO2 in aqueous solution through the Ti–N bond. The optimized TiO2|C4BTP+RuPy photoanode gives the highest photocurrent density of over 800 μA cm–2, while the TiO2|C4STP+RuPy photoanode exhibits the best Faradaic efficiency of 64.9% for O2 evolution. To meet the needs of practical applications, the TiO2|C4BTP+RuPy photoanode is utilized in a two-electrode system with 0 V bias, giving a steady photocurrent density of 42 μA cm–2 in 3600 s and a Faradaic efficiency of 45.9% for O2 evolution. This study offers an inspiration for developing efficient and durable organic photosensitizers in photocatalytic water-splitting devices.

Light-Induced Organic Transformations by Covalent Organic Frameworks as Reticular Platforms for Selective Photosynthesis

Photoassisted synthesis of value-added organic products has developed greatly in the last decades in response to the pressing need for a transition toward sustainable processes and renewable energy. One of the formidable challenges of the light-induced chemical steps is provided by the control of the catalytic efficiency and selectivity under photocatalytic conditions. An attractive perspective is foreseen by triggering the photoreaction events in confined spaces, wherein light harvesting and photocatalytic units are framed into functional architectures. Division of tasks among specialized compartments responds to a bioinspired strategy with the final aim to orchestrate the rate of concurrent and sequential events, to maximize performance while directing the reaction selectivity. Covalent organic frameworks (COFs) are a class of emerging materials that can meet these requirements, with the potential to bridge the existing gap between molecular and heterogeneous photocatalysis. Here, a rich pool of molecular building blocks and chemical linkages is available to afford crystalline porous solids with tailored photophysical properties emerging from the interconnected COF structure walls, while catalytic cofactors can be provided by engineering of the pore surface. In this Perspective, we highlight recent developments where COFs have been successfully employed as photocatalysts for selective organic transformations. The relationship between the COF reticular structure and its photocatalytic behavior is discussed, in terms of the light-conversion pathways and photoredox events, including electron and/or energy transfer mechanisms. The possible role of confinement effects, intrinsic in long-range order porous COF materials, remains largely unexplored in photocatalytic applications. New progress is expected to arise from close interdisciplinary cooperation involving synthetic chemistry and materials science communities.

Precise Construction of Stable Bimetallic Metal–Organic Frameworks with Single-Site Ti(IV) Incorporation in Nodes for Efficient Photocatalytic Oxygen Evolution

Precise Construction of Stable Bimetallic Metal–Organic Frameworks with Single-Site Ti(IV) Incorporation in Nodes for Efficient Photocatalytic Oxygen Evolution

Photocatalytic activity of hybrid P450 Enzymes

The Cytochromes P450 are heme-thiolate enzymes that carry out selective oxyfunctionalization of a wide range of substrates using molecular dioxygen and two reducing equivalents provided by redox partner enzymes. Alternative approaches to activate P450 enzymes have received recent attention in an attempt to circumvent the use of redox partners and cofactors. In particular, the use of light-harvesting units has proven useful in triggering P450 activity upon visible light-excitation. Our laboratory has taken advantage of the inorganic Ru(II)-diimine photosensitizers and their unique excited properties to harness P450 synthetic potential. Their covalent attachment to non-native single cysteine residues has enabled rapid electron injection into the heme domain of several P450 enzymes leading to high photocatalytic activity and coupling efficiency. Recently, we also developed a bimolecular system to activate P450 with peroxygenase activity. The crystal structure of the most efficient hybrid enzyme reveals that the photosensitizer is ideally positioned to deliver electrons to the heme active site utilizing the natural electron transfer pathway. A combination of rational and directed evolution approaches are currently employed to optimize the biocatalyst photocatalytic activity and expand the scope of the light-driven P450 activity. In addition, the tunability and derivatization of the inorganic complexes have promoted the formation of dimeric structures and heterogeneous biocatalysts and provided insights into the rate-limiting steps in the light-driven hybrid P450 enzyme.

Natural dyes extracted from Inthanin bok leaves as light-harvesting units for dye-sensitized solar cells

The dye-sensitized solar cells (DSSC) deliver a low-cost and dependable alternative for various photovoltaic devices. The extraction process of natural pigments is simple and inexpensive compared with synthetic dyes. Natural plant pigments were extracted from flowers, leaves, roots, and fruits. Thus, this research focuses on the potential of natural dye using cold extraction with methanol. A UV–visible spectrometer was used for analyzing the Inthanin bok (Lagerstroemia macrocarpa) pigments absorption wavelength for the DSSC application. Carotenoids have the highest content, which is 10.666 ± 0.324 μg/ml; followed by chlorophyll-a with 2.708 ± 0.251 μg/ml and lastly chlorophyll-b with 2.500 ± 0.102 μg/ml. Elemental of the TiO2 nanoparticles and natural dyes was confirmed by energy-dispersive X-ray spectroscopy (EDX). The scanning electron microscope (SEM) and the laser scanning microscope used for analyzed morphological characteristics of TiO2 nanoparticles and natural dyes. Moreover, the highest efficiency of the pigments extracted from Inthanin bok leaves is 1.138% ± 0.018, which the condition of 1 layer of TiO2 nanoparticles and the temperature; 300 °C.