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Oxygenic photosynthetic organisms evolved mechanisms for thermal dissipation of energy absorbed in excess to prevent formation of reactive oxygen species. The major and fastest component, called non-photochemical quenching, occurs within the photosystem II antenna system by the action of two essential light-harvesting complex (LHC)-like proteins, photosystem II subunit S (PSBS) in plants and light-harvesting complex stress-related (LHCSR) in green algae and diatoms. In the evolutionary intermediate Physcomitrella patens, a moss, both gene products are active. These proteins, which are present in low amounts, are difficult to purify, preventing structural and functional analysis. Here, we report on the overexpression of the LHCSR1 protein from P. patens in the heterologous systems Nicotiana benthamiana and Nicotiana tabacum using transient and stable nuclear transformation. We show that the protein accumulated in both heterologous systems is in its mature form, localizes in the chloroplast thylakoid membranes, and is correctly folded with chlorophyll a and xanthophylls but without chlorophyll b, an essential chromophore for plants and algal LHC proteins. Finally, we show that recombinant LHCSR1 is active in quenching in vivo, implying that the recombinant protein obtained is a good material for future structural and functional studies. [ABSTRACT FROM AUTHOR]

The light-harvesting antenna of higher plant photosystem II ( LHCII) is the major photosynthetic membrane component encoded by an entire family of homologous nuclear genes. On the contrary, the great majority of proteins of photosystems and electron transport components are encoded by the chloroplast genome. In this work, we succeeded in gradually inhibiting the expression of the chloroplast genes that led to the disappearance of the photosystem complexes, mimicking almost total photoinhibition. The treated plants, despite displaying only some early signs of senescence, sustained their metabolism and growth for several weeks. The only major remaining membrane component was LHCII antenna that formed superstructures - stacks of dozens of thylakoids or supergrana. Freeze-fracture electron microscopy revealed specific organization, directly displaying frequently bifurcated membranes with reduced or totally absent photosystem II ( PSII) reaction centre complexes. Our findings show that it is possible to accumulate large amounts of light-harvesting membranes, organized into three-dimensional structures, in the absence of reaction centre complexes. This points to the reciprocal role of LHCII and PSII in self-assembly of the three-dimensional matrix of the photosynthetic membrane, dictating its size and flexible adaptation to the light environment. [ABSTRACT FROM AUTHOR]

Light-harvesting complex ( LHC)-like ( LIL) proteins contain two transmembrane helices of which the first bears a chlorophyll ( Chl)-binding motif. They are widespread in photosynthetic organisms, but almost nothing is known about their expression and physiological functions. We show that two LIL3 paralogues ( LIL3:1 and LIL3:2) in A rabidopsis thaliana are expressed in photosynthetically active tissues and their expression is differentially influenced by light stress. Localization studies demonstrate that both isoforms are associated with subcomplexes of LHC antenna of photosystem II. Transgenic plants with reduced amounts of LIL3:1 exhibited a slightly impaired growth and have reduced Chl and carotenoid contents as compared to wild-type plants. Ectopic overexpression of either paralogue led to a developmentally regulated switch to co-suppression of both LIL3 isoforms, resulting in a circular chlorosis of the leaf rosettes. Chlorotic sectors show severely diminished levels of LIL3 isoforms and other proteins, and thylakoid morphology was changed. Additionally, the levels of enzymes involved in Chl biosynthesis are altered in lil3 mutant plants. Our data support a role of LIL3 paralogues in the regulation of Chl biosynthesis under light stress and under standard growth conditions as well as in a coordinated ligation of newly synthesized and/or rescued Chl molecules to their target apoproteins. [ABSTRACT FROM AUTHOR]

In this article, we are investigating excitation energy transfer (EET) in a basic unit cell of light-harvesting complex II (LH2), named a LH2-type trimer. Calculation of energy transfer efficiency (ETE) in the framework of non-Markovian environment is also implemented. With these achievements, we theoretically predict the environment-assisted quantum critical effect, where ETE exhibits a sudden change at the critical point of quantum phase transition (QPT) for the LH2-type trimer. It is found that highly efficient EET with nearly unit efficiency may occur in the vicinity of the critical point of QPT. [ABSTRACT FROM AUTHOR]

Metal–organic frameworks (MOFs) based on porphyrin or metalloporphyrin components are of particular interest due to their potential applications in molecular sorption, light-harvesting, and heterogeneous catalysis. This perspective is focused on recent advances in the syntheses and functional properties of metal–metalloporphyrin frameworks, which are classified according to coordination moieties on the porphyrin ligands. [ABSTRACT FROM AUTHOR]

The cyanobacterial light-harvesting complex, the phycobilisome, is degraded under nutrient limitation, allowing the cell to adjust light absorbance to its metabolic capacity. This large light-harvesting antenna comprises a core complex of the pigment allophycocyanin, and rod-shaped pigment assemblies emanating from the core. NblA, a low-molecular-weight protein, is essential for degradation of the phycobilisome. NblA mutants exhibit high absorbance of rod pigments under conditions that generally elicit phycobilisome degradation, implicating NblA in degradation of these pigments. However, the vast abundance of rod pigments and the substantial overlap between the absorbance spectra of rod and core pigments has made it difficult to directly associate NblA with proteolysis of the phycobilisome core. Furthermore, lack of allophycocyanin degradation in an NblA mutant may reflect a requirement for rod degradation preceding core degradation, and does not prove direct involvement of NblA in proteolysis of the core pigment. Therefore, in this study, we used a mutant lacking phycocyanin, the rod pigment of Synechococcus elongatus PCC7942, to examine whether NblA is required for allophycocyanin degradation. We demonstrate that NblA is essential for degradation of the core complex of the phycobilisome. Furthermore, fluorescence lifetime imaging microscopy provided in situ evidence for the interaction of NblA with allophycocyanin, and indicated that NblA interacts with allophycocyanin complexes that are associated with the photosynthetic membranes. Based on these data, as well as previous observations indicating interaction of NblA with phycobilisomes attached to the photosynthetic membranes, we suggest a model for sequential phycobilisome disassembly by NblA. [ABSTRACT FROM AUTHOR]

Chloroplasts utilize photons from solar radiation to synthesize energy-rich molecules of ATPs and NADPHs, which are further used in active cellular processes. Multiprotein complexes (MPCs), including photosystems (PSII and PSI), and the cellular architecture responsible for generation of the proton motive force and the subsequent photophosphorylation, mediate the task of ATP and NADPH synthesis. Both photosystems and other multiprotein assemblies are embedded in thylakoid membranes. Advances in techniques used to study structural biology, biophysics, and comparative genomics and proteomics have enabled us to gain insights of structure, function, and localization of each individual component of the photosynthetic apparatus. An efficient coordination among MPCs is essential for normal functioning of photosynthesis, but there are various stressors that might directly or indirectly interact with photosynthetic components and processes. Cadmium is one of the toxic heavy metals that interact with photosynthetic components and damage photosystems and other MPCs in thylakoids. In plants, iron deficiency shows similar symptoms as those caused by Cd. Our article provides a general overview of chloroplast structure and a critical account of Cd-induced changes in photosystems and other MPCs in thylakoids, and suggests the possible mechanisms involved in mediating these changes. The connection between Cd-induced Fe deficiency and the elevated Cd toxicity under the Fe-deficient condition was also discussed. [ABSTRACT FROM AUTHOR]

A list of excellent articles published in the sister journals of the "European Journal of Organic Chemistry" is presented on topics including photochemical bleaching of an elaborate artificial light-harvesting antenna, sulfur-doped carbon aerogel as a metal-free oxygen reduction catalyst, and synthesis and characterization of frustrated spin ladders.

To investigate the feasibility of using near-infrared absorption to predict stability of light-harvesting complex 2 from the purple bacteria, we compared the structural stability of light-harvesting complex 2 with different absorption properties. The spectral changes showed that light-harvesting complex 2 fromRhodobacter azotoformans(absorption maxima at 799 and 847 nm) was more susceptible to both sodium-dodecylsulfate and acid than light-harvesting complex 2 fromRhodopseudomonas palustris(absorption maxima at 803 and 857 nm). It is concluded that a relatively redshifted near-infrared absorption tends to indicate a more stable structure of light-harvesting complex 2. [ABSTRACT FROM PUBLISHER]

Photosynthesis is one of the ancient biological processes, playing crucial role converting solar energy to cellular usable currency. Environmental factors and external perturbations has forced nature to choose systems with the highest efficiency and performance. Recent theoretical and experimental studies have proved the presence of quantum properties in biological systems. Energy transfer systems like Fenna-Matthews-Olson (FMO) complex shows quantum entanglement between sites of Bacteriophylla molecules in protein environment and presence of decoherence. Complex biological systems implement more truthful mechanisms beside chemical-quantum correlations to assure system's efficiency. In this study we investigate thermal quantum correlations in FMO protein of the photosynthetic apparatus of green sulfur bacteria by quantum discord measure. The results confirmed existence of remarkable quantum correlations of of BChla pigments in room temperature. This results approve involvement of quantum correlation mechanisms for information storage and retention in living organisms that could be useful for further evolutionary studies. Inspired idea of this study is potentially interesting to practice by the same procedure in genetic data transfer mechanisms. [ABSTRACT FROM AUTHOR]

Styrene-maleic acid copolymer was used to effect a non-detergent partial solubilization of thylakoids from spinach. A high density membrane fraction, which was not solubilized by the copolymer, was isolated and was highly enriched in the Photosystem (PS) I-light-harvesting chlorophyll (LHC) II supercomplex and depleted of PS II, the cytochrome b6/f complex, and ATP synthase. The LHC II associated with the supercomplex appeared to be energetically coupled to PS I based on 77 K fluorescence, P700 photooxidation, and PS I electron transport light saturation experiments. The chlorophyll (Chl) a/b ratio of the PS I-LHC II membranes was 3.2 ± 0.9, indicating that on average, three LHC II trimers may associate with each PS I. The implication of these findings within the context of higher plant PS I antenna organization is discussed. [ABSTRACT FROM AUTHOR]

Light-induced infrared difference spectroscopy (IR-DS) has been used, especially in the last decade, to investigate early photophysics, energy transfer and photoprotection mechanisms in isolated and membrane-bound light harvesting complexes (LHCs). The technique has the definite advantage to give information on how the pigments and the other constituents of the biological system (proteins, membranes, etc.) evolve during a given photoreaction. Different static and time-resolved approaches have been used. Compared to the application of IR-DS to photosynthetic Reaction Centers (RCs), however, IR-DS applied to LHCs is still in an almost pioneering age: very often sophisticated techniques (step-scan FTIR, ultrafast IR) or data analysis strategies (global analysis, target analysis, multivariate curve resolution) are needed. In addition, band assignment is usually more complicated than in RCs. The results obtained on the studied systems (chromatophores and RC-LHC supercomplexes from purple bacteria; Peridinin-Chlorophyll-a-Proteins from dinoflagellates; isolated LHCII from plants; thylakoids; Orange Carotenoid Protein from cyanobacteria) are summarized. A description of the different IR-DS techniques used is also provided, and the most stimulating perspectives are also described. Especially if used synergically with other biophysical techniques, light-induced IR-DS represents an important tool in the investigation of photophysical/photochemical reactions in LHCs and LHC-containing systems. [ABSTRACT FROM AUTHOR]

The [PbI 6 ] inorganic framework in the CH 3 NH 3 PbI 3 could be affected by the oxidization effect of I − ions in atmosphere. The [PbI 6 ] octahedral torsion angle increased ∼4.94° after the newly prepared CH 3 NH 3 PbI 3 powders were stored in dry atmosphere for 30 days at 20 °C according to the results of XRD Rietveld refinement and X-ray Absorption Fine Structure. Analyses based on density of states indicated that UV–vis adsorption edge at ∼850 cm –1 corresponded to electron transition from I-5p orbitals to empty Pb-6p orbitals. The increased [PbI 6 ] octahedral torsion angle made the band gap energy of CH 3 NH 3 PbI 3 increased according to First-principle calculation and UV–vis absorption spectra. As a result, the band gap energy of CH 3 NH 3 PbI 3 were strongly related to the structure of [PbI 6 ] inorganic framework because the electrons diffused along the [PbI 6 ] octahedral chains in CH 3 NH 3 PbI 3 perovskite. [ABSTRACT FROM AUTHOR]

In photosynthetic organisms, light energy is converted into chemical energy through the light absorption and excitation energy transfer (EET) processes. These processes start in light-harvesting complexes, which contain special photosynthetic pigments. The exploration of unique mechanisms in light-harvesting complexes is directly related to studies, such as artificial photosynthesis or biosignatures in astrobiology. We examined, throughab initiocalculations, the light absorption and EET processes using cluster models of light-harvesting complexes in purple bacteria (LH2). We evaluated absorption spectra and energy transfer rates using the LH2 monomer and dimer models to reproduce experimental results. After the calibration tests, a LH2 aggregation model, composed of 7 or 19 LH2s aligned in triangle lattice, was examined. We found that the light absorption is red shifted and the energy transfer becomes faster as the system size increases. We also found that EET is accelerated by exchanging the central pigments to lower energy excited pigments. As an astrobiological application, we calculated light absorptions efficiencies of the LH2 in different photoenvironments. [ABSTRACT FROM AUTHOR]

The development of biologically templated artificial light harvesting antennae and energy transfer devices is a highly active research area with exceptional challenges. Natural energy harvesting complexes have exquisite spectrally- and spatially-tuned systems with high redundancy to maximize their ability to gather, channel, and distribute electromagnetic radiation. Attempting to mimic these highly efficient systems requires at the very least (sub)nanoscale precision in the positioning of light sensitive molecules, the latter of which must also possess carefully selected photophysical properties; in essence, these two fundamental properties must be exploited in a synergistic manner. First, the scaffold must be highly organized, ideally with multiple symmetrical components that are spatially arranged with nanoscale accuracy. Second, the structure must be amenable to chemical modification in order to be (bio)functionalized with the desired light sensitive moieties which have expanded greatly to now include organic dyes, metal chelates, fluorescent proteins, dye-doped and noble metal nanoparticles, photoactive polymers, along with semiconductor quantum dots amongst others. Several families of biological scaffolding molecules offer strong potential to meet these stringent requirements. Recent advances in bionanotechnology have provided the ability to assemble diverse naturally derived scaffolds along with manipulating their properties and this is allowing us to understand the capabilities and limitations of such artificial light-harvesting antennae and devices. The range of scaffold or template materials that have been used varies from highly symmetrical virus capsids to self-assembled biomaterials including nucleic acids and small peptides as well as a range of hybrid inorganic–biological systems. This review surveys the burgeoning field of artificial light-harvesting and energy transfer complexes that utilize biological scaffolds from the perspective of what each has to offer for optimized energy transfer. We highlight each biological scaffold with prominent examples from the literature and discuss some of the benefits and liabilities of each approach. Cumulatively, the available data suggest that DNA is the most versatile biological material currently available, though it has challenges including precise dye placement and subsequent dye performance. We conclude by providing a perspective on how this field will progress in both the short and long term, with a focus on the transition to applications and devices. [ABSTRACT FROM AUTHOR]

Light-harvesting 2 (LH2) complexes from a genetically modified strain of the purple photosynthetic bacterium Rhodobacter ( Rba. ) sphaeroides were studied using static and ultrafast optical methods and resonance Raman spectroscopy. Carotenoid synthesis in the Rba. sphaeroides strain was engineered to redirect carotenoid production away from spheroidene into the spirilloxanthin synthesis pathway. The strain assembles LH2 antennas with substantial amounts of spirilloxanthin (total double-bond conjugation length N = 13) if grown anaerobically and of keto-bearing long-chain analogs [2-ketoanhydrorhodovibrin ( N = 13), 2-ketospirilloxanthin ( N = 14) and 2,2′-diketospirilloxanthin ( N = 15)] if grown semi-aerobically (with ratios that depend on growth conditions). We present the photophysical, electronic, and vibrational properties of these carotenoids, both isolated in organic media and assembled within LH2 complexes. Measurements of excited-state energy transfer to the array of excitonically coupled bacteriochlorophyll a molecules (B850) show that the mean lifetime of the first singlet excited state (S 1 ) of the long-chain ( N ≥ 13) carotenoids does not change appreciably between organic media and the protein environment. In each case, the S 1 state appears to lie lower in energy than that of B850. The energy-transfer yield is ~ 0.4 in LH2 (from the strain grown aerobically or semi-aerobically), which is less than half that achieved for LH2 that contains short-chain ( N ≤ 11) analogues. Collectively, the results suggest that the S 1 excited state of the long-chain ( N ≥ 13) carotenoids participates little if at all in carotenoid-to-BChl a energy transfer, which occurs predominantly via the carotenoid S 2 excited state in these antennas. [ABSTRACT FROM AUTHOR]

Photosynthesis converts solar energy to chemical energy by means of two large pigment-protein complexes: photosystem I (PSI) and photosystem II (PSII). In higher plants, the PSI core is surrounded by a large light-harvesting complex I (LHCI) that captures sunlight and transfers the excitation energy to the core with extremely high efficiency. We report the structure of PSI-LHCI, a 600-kilodalton membrane protein supercomplex, from Pisum sativum (pea) at a resolution of 2.8 angstroms. The structure reveals the detailed arrangement of pigments and other cofactors--especially within LHCI--as well as numerous specific interactions between the PSI core and LHCI. These results provide a firm structural basis for our understanding on the energy transfer and photoprotection mechanisms within the PSI-LHCI supercomplex. [ABSTRACT FROM AUTHOR]

Singlet oxygen (1O2) is of great interest because of its potential applications in photodynamic therapy, photooxidation of toxic molecules, and photochemical synthesis. Herein, we report novel metallophthalocyanine (MPc) based conjugated microporous polymers (MPc-CMPs) as photosensitizers for the generation of 1O2. The rigid microporous structure efficiently improves the exposure of the majority of the MPc units to oxygen. The MPc-CMPs also exhibit an enhanced light-harvesting capability in the far-red region through their extended π-conjugation systems. Their microporous structure and excellent absorption capability for long-wavelength photons result in the MPc-CMPs showing high efficiency for 1O2 generation upon irradiation with 700 nm light, as evident by using 1,3-diphenylisobenzofuran as an 1O2 trap. These results indicate that MPc-CMPs can be considered as promising photosensitizers for the generation of 1O2. [ABSTRACT FROM AUTHOR]

A wedge structured rotating disk photocatalytic (PC) reactor was used for post-treatment of actual textile wastewater collected after activated sludge treatment. Compared with a conventional planar disk, the wedge structured disk had larger surface area and exhibited better light utilization efficiency. Treatment conditions such as pH and rotating speed were optimized. The COD content in effluent was reduced from 380 mgO 2 L −1 to 109 and 56 mgO 2 L −1 respectively after 3 h and 5 h treatment at pH 6.0 and rotating speed of 20 rpm. The effect of anions (NO 3 − , SO 4 2− and Cl − ) in actual textile wastewater on treatment was investigated. Benefit from high light utilization efficiency, the wedge rotating disk reactor could compensate COD removal inhibition caused by anions. A 1.5 V voltage was also applied to perform photoelectrocatalysis process to further improve the performance, and no additional electrolyte was needed. COD in effluent was reduced to as low as 75 mgO 2 L −1 after 3 h treatment. In addition, this reactor was used to treat other water samples including the actual textile wastewaters collected before activated sludge treatment (initial COD of 880 mgO 2 L −1 ) and after activated carbon adsorption post-treatment (initial COD of 108 mgO 2 L −1 ). Experimental results suggested that wedge structured rotating disk photocatalysis process was stable, efficient and worth further scale-up investigation. [ABSTRACT FROM AUTHOR]

Biosynthesis of asymmetric carotenoids such as α-carotene and lutein in plants and green algae involves the two enzymes lycopene β-cyclase ( LCYB) and lycopene ε-cyclase ( LCYE). The two cyclases are closely related and probably resulted from an ancient gene duplication. While in most plants investigated so far the two cyclases are encoded by separate genes, prasinophyte algae of the order Mamiellales contain a single gene encoding a fusion protein comprised of LCYB, LCYE and a C-terminal light-harvesting complex ( LHC) domain. Here we show that the lycopene cyclase fusion protein from Ostreococcus lucimarinus catalyzed the simultaneous formation of α-carotene and β-carotene when heterologously expressed in Escherichia coli. The stoichiometry of the two products in E. coli could be altered by gradual truncation of the C-terminus, suggesting that the LHC domain may be involved in modulating the relative activities of the two cyclase domains in the algae. Partial deletions of the linker region between the cyclase domains or replacement of one or both cyclase domains with the corresponding cyclases from the green alga Chlamydomonas reinhardtii resulted in pronounced shifts of the α-carotene-to-β-carotene ratio, indicating that both the relative activities of the cyclase domains and the overall structure of the fusion protein have a strong impact on the product stoichiometry. The possibility to tune the product ratio of the lycopene cyclase fusion protein from Mamiellales renders it useful for the biotechnological production of the asymmetric carotenoids α-carotene or lutein in bacteria or fungi. [ABSTRACT FROM AUTHOR]

Microalgae represent promising organisms for the sustainable production of commodities, chemicals or fuels. Future use of such systems, however, requires increased productivity of microalgal mass cultures in order to reach an economic viability for microalgae-based production schemes. The efficiency of sunlight-to-biomass conversion that can be observed in bulk cultures is generally far lower (35–80%) than the theoretical maximum, because energy losses occur at multiple steps during the light-driven conversion of carbon dioxide to organic carbon. The light-harvesting system is a major source of energy losses and thus a prime target for strain engineering. Truncation of the light-harvesting antenna in the algal model organism Chlamydomonas reinhardtii was shown to be an effective way of increasing culture productivity at least under saturating light conditions. Furthermore engineering of the Calvin–Benson cycle or the creation of photorespiratory bypasses in A. thaliana proved to be successful in terms of achieving higher biomass productivities. An efficient generation of novel microalgal strains with improved sunlight conversion efficiencies by targeted engineering in the future will require an expanded molecular toolkit. In the meantime random mutagenesis coupled to high-throughput screening for desired phenotypes can be used to provide engineered microalgae. [ABSTRACT FROM AUTHOR]

Plants collect light for photosynthesis using light-harvesting complexes (LHCs)-an array of chlorophyll proteins that are able to reversibly switch from harvesting to energy-dissipation mode to prevent damage of the photosynthetic apparatus. LHC antennae as well as other members of the LHC superfamily evolved from cyanobacterial ancestors called high light-inducible proteins (Hlips). Here, we characterized a purified Hlip family member HliD isolated from the cyanobacterium Synechocystis sp. PCC 6803. We found that the HliD binds chlorophyll-a (Chl-a) and β-carotene and exhibits an energy-dissipative conformation. Using femtosecond spectroscopy, we demonstrated that the energy dissipation is achieved via direct energy transfer from a Chl-a Qy state to the β-carotene S1 state. We did not detect any cation of β-carotene that would accompany Chl-a quenching. These results provide proof of principle that this quenching mechanism operates in the LHC superfamily and also shed light on the photoprotective role of Hlips and the evolution of LHC antennae. [ABSTRACT FROM AUTHOR]

The effect of spermine on proton transport across large unilamellar liposomes containing incorporated complexes of the PSII antenna has been studied with the application of a pH-sensitive dye entrapped inside the vesicles. Both monomeric LHCbs and trimeric LHCII increased the permeability of proteoliposomes to protons when in a partly aggregated state within the lipid membrane. We have previously shown that a spermine-induced conformational change in LHCII results in its aggregation and ultimately in the enhancement of excitation energy as heat ( qE ). In this paper, spermine-induced aggregation of LHCII was found to facilitate proton transport across the proteoliposomes, indicating that a second protective mechanism (other than qE ) might exist and might be regulated in vivo by polyamines when photosynthesis is saturated in excess light. [ABSTRACT FROM AUTHOR]

Green plant photosystem II (PSII) and light-harvesting complex II (LHCII) in the stacked grana regions of thylakoid membranes can self-organize into various PSII-LHCII supercomplexes with crystalline or fluid-like supramolecular structures to adjust themselves with external stimuli such as high/low light and temperatures, rendering tunable solar light absorption spectrum and photosynthesis efficiencies. However, the mechanisms controlling the PSII-LHCII supercomplex organizations remain elusive. In this work, we constructed a coarse-grained (CG) model of the thylakoid membrane including lipid molecules and a PSII-LHCII supercomplex considering association/dissociation of moderately bound-LHCIIs. The CG interaction between CG beads were constructed based on electron microscope (EM) experimental results, and we were able to simulate the PSII-LHCII supramolecular organization of a 500 × 500 nm2 thylakoid membrane, which is compatible with experiments. Our CGMD simulations can successfully reproduce order structures of PSII-LHCII supercomplexes under various protein packing fractions, free-LHCII:PSII ratios, and temperatures, thereby providing insights into mechanisms leading to PSII-LHCII supercomplex organizations in photosynthetic membranes. [ABSTRACT FROM AUTHOR]

Light harvesting and excitation energy transfer in photosynthesis are relatively well understood at cryogenic temperatures up to ~100 K, where crystal structures of several photosynthetic complexes including the major antenna complex of green plants (LHC II) are available at nearly atomic resolution. The situation is much more complex at higher or even physiological temperatures, because the spectroscopic properties of antenna complexes typically undergo drastic changes above ~100 K. We have addressed this problem using a combination of quasielastic neutron scattering (QENS) and optical spectroscopy on native LHC II and mutant samples lacking the Chl 2/Chl a 612 pigment molecule. Absorption difference spectra of the Chl 2/Chl a 612 mutant of LHC II reveal pronounced changes of spectral position and their widths above temperatures as low as ~80 K. The complementary QENS data indicate an onset of conformational protein motions at about the same temperature. This finding suggests that excited state positions in LHC II are affected by protein dynamics on the picosecond time scale. In more detail, this means that at cryogenic temperatures the antenna complex is trapped in certain protein conformations. At higher temperature, however, a variety of conformational substates with different spectral position may be thermally accessible. At the same time, an analysis of the widths of the absorption difference spectra of Chl 2/Chl a 612 reveals three different reorganization energies or Huang-Rhys factors in different temperature ranges, respectively. These findings imply that (dynamic) pigment-protein interactions fine-tune electronic energy levels and electron-phonon coupling of LHC II for efficient excitation energy transfer at physiological temperatures. [ABSTRACT FROM AUTHOR]

The light-harvesting complex (LHC) is an essential component in light energy capture and transduction to facilitate downstream photosynthetic reactions in plant and algal chloroplasts. The unicellular dinoflagellate alga Symbiodinium is an endosymbiont of cnidarian animals, including corals and sea anemones, and provides carbohydrates generated through photosynthesis to host animals. Although Symbiodinium possesses a unique LHC gene family, called chlorophyll a-chlorophyll c2-peridinin protein complex (acpPC), its genome-level diversity and evolutionary trajectories have not been investigated. Here, we describe a phylogenetic analysis revealing that many of the LHCs are encoded by highly duplicated genes with multi-subunit polyprotein structures in the nuclear genome of Symbiodinium minutum. This analysis provides an extended list of the LHC gene family in a single organism, including 80 loci encoding polyproteins composed of 145 LHC subunits recovered in the phylogenetic tree. In S. minutum, 5 phylogenetic groups of the Lhcf-type gene family, which is exclusively conserved in algae harboring secondary plastids of red algal origin, were identified. Moreover, 5 groups of the Lhcr-type gene family, of which members are known to be associated with PSI in red algal plastids and secondary plastids of red algal origin, were identified. Notably, members classified within a phylogenetic group of the Lhcf-type (group F1) are highly duplicated, which may explain the presence of an unusually large number of LHC genes in this species. Some gene units were homologous to other units within single loci of the polyprotein genes, whereas intergenic homologies between separate loci were conspicuous in other cases, implying that gene unit ‘shuffling’ by gene conversion and/or genome rearrangement might have been a driving force for diversification. These results suggest that vigorous intra- and intergenic gene duplication events have resulted in the genomic framework of photosynthesis in coral symbiont dinoflagellate algae. [ABSTRACT FROM AUTHOR]

Nickel oxide based p-type dye-sensitized solar cells (DSCs) are limited in their efficiencies by poor fill factors (FFs). This work explores the origins of this limitation. Transient absorption spectroscopy identifies fast recombination between the injected hole and the dye anion under applied load as one of the predominant reasons for the poor FF of NiO-based DSCs. A reduced hole injection efficiency, ηINJ, under applied load is found to play an equally important role. Both, the dye regeneration yield, ΦREG, and ηINJ decrease by approximately 40%-50% when moving from short- to open-circuit conditions. Spectroelectrochemical measurements reveal that the electrochromic properties of NiO are a further limiting factor for the device performance leading to variable light-harvesting efficiencies, ηLH, under applied load. The peak light-harvesting efficiency decreases from 63% at short circuit to 57% at 600 mV reducing the FF of NiO DSCs by 5%. This effect is expected to be more pronounced for future devices with higher operating voltages. Incident, photon-to-electron conversion efficiency front-back analysis at applied bias is utilized to characterize the interfacial charge recombination. It is found that the recombination between the injected hole and the redox mediator has a surprisingly small effect on the FF. [ABSTRACT FROM AUTHOR]

Fucoxanthin–chlorophyll protein (FCP) is the key molecular complex performing the light-harvesting function in diatoms, which, being a major group of algae, are responsible for up to one quarter of the total primary production on Earth. These photosynthetic organisms contain an unusually large amount of the carotenoid fucoxanthin, which absorbs the light in the blue–green spectral region and transfers the captured excitation energy to the FCP-bound chlorophylls. Due to the large number of fucoxanthins, the excitation energy transfer cascades in these complexes are particularly tangled. In this work we present the two-color two-dimensional electronic spectroscopy experiments on FCP. Analysis of the data using the modified decay associated spectra permits a detailed mapping of the excitation frequency dependent energy transfer flow with a femtosecond time resolution. [ABSTRACT FROM AUTHOR]

The photo-excited triplet states of carotenoids, sensitised by triplet–triplet energy transfer from the chlorophyll triplet states, have been investigated in the isolated Photosystem II (PSII) core complex and PSII–LHCII (Light Harvesting Complex II) supercomplex by Optically Detected Magnetic Resonance techniques, using both fluorescence (FDMR) and absorption (ADMR) detection. The absence of Photosystem I allows us to reach the full assignment of the carotenoid triplet states populated in PSII under steady state illumination at low temperature. Five carotenoid triplet ( 3 Car) populations were identified in PSII–LHCII, and four in the PSII core complex. Thus, four 3 Car populations are attributed to β -carotene molecules bound to the core complex. All of them show associated fluorescence emission maxima which are relatively red-shifted with respect to the bulk emission of both the PSII–LHCII and the isolated core complexes. In particular the two populations characterised by Zero Field Splitting parameters |D| = 0.0370–0.0373 cm − 1 /|E| = 0.00373–0.00375 cm − 1 and |D| = 0.0381–0.0385 cm − 1 /|E| = 0.00393–0.00389 cm − 1 , are coupled by singlet energy transfer with chlorophylls which have a red-shifted emission peaking at 705 nm. This observation supports previous suggestions that pointed towards the presence of long-wavelength chlorophyll spectral forms in the PSII core complex. The fifth 3 Car component is observed only in the PSII–LHCII supercomplex and is then assigned to the peripheral light harvesting system. [ABSTRACT FROM AUTHOR]

Nine genes (LHCBM1-9) encode the major light-harvesting system of Chlamydomonas reinhardtii. Transcriptomic and proteomic analyses have shown that those genes are all expressed albeit in different amounts and some of them only in certain conditions. However, little is known about the properties and specific functions of the individual gene products because they have never been isolated. Here we have purified several complexes from native membranes and/or we have reconstituted them in vitro with pigments extracted from C. reinhardtii. It is shown that LHCBM1 and -M2/7 represent more than half of the LHCBM population in the membrane. LHCBM2/7 forms homotrimers while LHCBM1 seems to be present in heterotrimers. Trimers containing only type I LHCBM (M3/4/6/8/9) were also observed. Despite their different roles, all complexes have very similar properties in terms of pigment content, organization, stability, absorption, fluorescence and excited-state lifetimes. Thus the involvement of LHCBM1 in non-photochemical quenching is suggested to be due to specific interactions with other components of the membrane and not to the inherent quenching properties of the complex. Similarly, the overexpression of LHCBM9 during sulfur deprivation can be explained by its low sulfur content as compared with the other LHCBMs. Considering the highly conserved biochemical and spectroscopic properties, the major difference between the complexes may be in their capacity to interact with other components of the thylakoid membrane. [ABSTRACT FROM AUTHOR]

Electronic bandgap calculations are presented for 2400 experimentally known materials from the Materials Project database and the bandgaps, obtained with different types of functionals within density functional theory and (partial) self-consistent GW approximation, are compared for 20 randomly chosen compounds forming an unconventional set of ternary and quaternary materials. It is shown that the computationally cheap GLLB-SC potential gives results in good agreement (around 15%) with the more advanced and demanding eigenvalue-self-consistent GW. This allows for a high-throughput screening of materials for different applications where the bandgaps are used as descriptors for the efficiency of a photoelectrochemical device. Here, new light harvesting materials are proposed to be used in a one-photon photoelectrochemical device for water splitting by combining the estimation of the bandgaps with the stability analysis using Pourbaix diagrams and with the evaluation of the position of the band edges. Using this methodology, 25 candidate materials are obtained and 5 of them appear to have a realistic possibility of being used as photocatalyst in a one-photon water splitting device. [ABSTRACT FROM AUTHOR]

A table of contents for the issue is presented.

Summary In their natural environment plants and algae are exposed to rapidly changing light conditions and light intensities. Illumination with high light intensities has the potential to overexcite the photosynthetic pigments and the electron transport chain and thus induce the production of toxic reactive oxygen species (ROS). To prevent damage by the action of ROS, plants and algae have developed a multitude of photoprotection mechanisms. One of the most important protection mechanisms is the dissipation of excessive excitation energy as heat in the light-harvesting complexes of the photosystems. This process requires a structural change of the photosynthetic antenna complexes that are normally optimized with regard to efficient light-harvesting. Enhanced heat dissipation in the antenna systems is accompanied by a strong quenching of the chlorophyll a fluorescence and has thus been termed non-photochemical quenching of chlorophyll a fluorescence, NPQ. The general importance of NPQ for the photoprotection of plants and algae is documented by its wide distribution in the plant kingdom. In the present review we will summarize the present day knowledge about NPQ in higher plants and different algal groups with a special focus on the molecular mechanisms that lead to the structural rearrangements of the antenna complexes and enhanced heat dissipation. We will present the newest models for NPQ in higher plants and diatoms and will compare the features of NPQ in different algae with those of NPQ in higher plants. In addition, we will briefly address evolutionary aspects of NPQ, i.e. how the requirements of NPQ have changed during the transition of plants from the aquatic habitat to the land environment. We will conclude with a presentation of open questions regarding the mechanistic basis of NPQ and suggestions for future experiments that may serve to obtain this missing information. [ABSTRACT FROM AUTHOR]

Leaves from three different Arabidopsis lines with different expression levels of PsbS protein showed different levels of non-photochemical quenching. The PsbS deficient plant npq4 showed remarkable reduction of electron transport rate, while the other two lines with a moderate amount (wild type) or an overexpression of PsbS ( L17 ) presented unchanged electron transport rates under the same range of high light intensities. Biochemical investigation revealed that the plant with the highest PsbS content ( L17 ) sustained the highest level of stable PSII–LHCII supercomplex structure, and displayed the smallest fluorescence quenching in the thylakoid membranes, the most efficient linear electron transport and the smallest cyclic electron transport. Based on these observations, it is proposed that the remodeling of PSII–LHCII supercomplexes affected by PsbS plays important roles in regulating the energy balance in thylakoid membrane and in ensuring the sophisticated coordination between energy excitation and dissipation. [ABSTRACT FROM AUTHOR]

Photosynthetic eukaryotes exhibit very different light-harvesting proteins, but all contain membrane-intrinsic light-harvesting complexes (Lhcs), either as additional or sole antennae. Lhcs non-covalently bind chlorophyll a and in most cases another Chl, as well as very different carotenoids, depending on the taxon. The proteins fall into two major groups: The well-defined Lhca/b group of proteins binds typically Chl b and lutein, and the group is present in the ‘green lineage’. The other group consists of Lhcr/Lhcf, Lhcz and Lhcx/LhcSR proteins. The former are found in the so-called Chromalveolates, where they mostly bind Chl c and carotenoids very efficient in excitation energy transfer, and in their red algae ancestors. Lhcx/LhcSR are present in most Chromalveolates and in some members of the green lineage as well. Lhcs function in light harvesting, but also in photoprotection, and they influence the organisation of the thylakoid membrane. The different functions of the Lhc subfamilies are discussed in the light of their evolution. [ABSTRACT FROM AUTHOR]

The pigment composition of Phaeocystis antarctica was monitored under various conditions of light, temperature, salinity, and iron. 19′- Hexanoyloxyfucoxanthin ( Hex-fuco) always constituted the major light-harvesting pigment, with remarkably stable ratios of Hex-fuco-to-chl a under the various environmental conditions. Increased pigment-to-chl a ratios at low irradiance confirmed the light-harvesting function of Fucoxanthin ( Fuco), 19′- Hexanoyloxy-4-ketofucoxanthin ( Hex-kfuco), 19′-butanoyloxyfucoxanthin ( But-fuco), and chl c2 and c3. Increased pigment-to-chl a ratios at high irradiance, low iron concentrations, and to a lesser extent at high salinity confirmed the photoprotective function of diadinoxanthin, diatoxanthin, and ß,ß-carotene. Pigment ratios were not always according to expectations. The consistent increase in But-fuco/chl at high temperature, high salinity, and low iron suggests a role in photoprotection rather than in light harvesting. Low Hex-kfuco/chl ratios at high salinity were consistent with a role as light harvester, but the high ratios at high temperature were not, leaving the function of Hex-kfuco enigmatic. Dedicated experiments were performed to test whether or not the light-harvesting pigment Fuco could be converted into its structural relative Hex-fuco, and vice versa, in response to exposure to light shifts. Rapid conversions could not be confirmed, but long-term conversions cannot be excluded. New pigment ratios are proposed for chemotaxonomic applications. The ratios will improve pigment-based diagnosis of algal species in waters dominated by P. antarctica. [ABSTRACT FROM AUTHOR]

Minor light-harvesting complexes (Lhcs) CP24, CP26 and CP29 occupy a position in photosystem II (PSII) of plants between the major light-harvesting complexes LHCII and the PSII core subunits. Lack of minor Lhcs in vivo causes impairment of PSII organization, and negatively affects electron transport rates and photoprotection capacity. Here we used picosecond-fluorescence spectroscopy to study excitation-energy transfer (EET) in thylakoid membranes isolated from Arabidopsis thaliana wild-type plants and knockout lines depleted of either two (koCP26/24 and koCP29/24) or all minor Lhcs (NoM). In the absence of all minor Lhcs, the functional connection of LHCII to the PSII cores appears to be seriously impaired whereas the “disconnected” LHCII is substantially quenched. For both double knock-out mutants, excitation trapping in PSII is faster than in NoM thylakoids but slower than in WT thylakoids. In NoM thylakoids, the loss of all minor Lhcs is accompanied by an over-accumulation of LHCII, suggesting a compensating response to the reduced trapping efficiency in limiting light, which leads to a photosynthetic phenotype resembling that of low-light-acclimated plants. Finally, fluorescence kinetics and biochemical results show that the missing minor complexes are not replaced by other Lhcs, implying that they are unique among the antenna subunits and crucial for the functioning and macro-organization of PSII. [ABSTRACT FROM AUTHOR]

The processes of recovering colored-carotenoid (Car) biosynthesis in Car-less cells of the purple sulfur bacterium Ectothiorhodospira haloalkaliphila grown with diphenylamine (DPA-cells) have been studied. It has been found that (1) the rate of recovering colored-Car biosynthesis in the lag-phase is far ahead of the growth rate of the cells themselves; (2) several Cars (ζ-carotene, neurosporene etc.) act as intermediates in Car biosynthesis; (3) because filling the “empty” Car pockets in the LH1-RC complexes is faster than in LH2, available spirilloxanthin is preferentially incorporated into the nascent LH1-RC core particles; (4) as a consequence of the resulting lack of spirilloxanthin availability, the biosynthetic intermediates (anhydrorhodovibrin, rhodopin and lycopene) fill the empty nascent LH2 Car pockets. In the present report, we further discuss the process of colored Car incorporation into LH complexes during the recovery of Car biosynthesis in the DPA-cells of Ect . haloalkaliphila . [ABSTRACT FROM AUTHOR]

In this paper, we reported for the first time that Rhodobacter azotoformans was capable of synthesizing a spectral variant of peripheral light-harvesting complex (LH3), besides a high light form (LH2), in response to low light intensity. Carotenoid components in these complexes were analyzed by absorption spectra, high-pressure liquid chromatography and mass spectroscopy analysis. Only spheroidene carotenoid was detected in LH2, while LH3 possessed three kinds of carotenoids, spheroidene, spirilloxanthin, and anhydrorhodovibrin. The spirilloxanthin and anhydrorhodovibrin predominated in LH3 and were rarely found in Rhodobacter species. Carotenoid-to-bacteriochlorophyll energy transfer efficiency in LH3 increased by 4% compared to that in LH2. Raman spectroscopic properties of carotenoids in both complexes supported the view that carotenoids altered their planar configuration to a distorted form by interaction with protein matrix in response to low light conditions. In conclusion, the low light adaptation mechanism of Rba. azotoformans involved regulating the synthesis of LH3 and additional carotenoids as well as the configuration change of incorporated carotenoids. [ABSTRACT FROM AUTHOR]

Quantum dots (QDs) can absorb ultraviolet and long-wavelength light energy much more efficiently than natural light-harvesting proteins and transfer the excitation energy to photosynthetic reaction centers (RCs). Inclusion into liposomes of RC membrane pigment-protein complexes combined with QDs as antennae opens new opportunities for using such hybrid systems as a basis for artificial energy-transforming devices that potentially can operate with greater efficiency and stability than devices based only on biological components. RCs from Rhodobacter sphaeroides and QDs with fluorescence maximum at 530 nm (CdSe/ZnS with hydrophilic covering) were embedded in lecithin liposomes by extrusion of a solution of multilayer lipid vesicles through a polycarbonate membrane or by dialysis of lipids and proteins dispersed with excess detergent. The dimensions of the resulting hybrid systems were evaluated using dynamic light scattering and by transmission cryoelectron microscopy. The efficiency of RC and QD interaction within the liposomes was estimated using fluorescence excitation spectra of the photoactive bacteriochlorophyll of the RCs and by measuring the fluorescence decay kinetics of the QDs. The functional activity of the RCs in hybrid complexes was fully maintained, and their stability was even increased. [ABSTRACT FROM AUTHOR]

A light-harvesting complex of photosystem II (LHCII), isolated from spinach, was immobilized onto a gold electrode modified with self-assembled monolayers (SAMs) of alkanethiols, NH-(CH)-SH, n = 2, 6, 8, 11; HOOC-(CH)-SH; and CH-(CH)-SH; and a bare electrode. The extent of LHCII complex adsorption according to surface treatment decreased in the order amino groups > carboxylic acid groups > methyl groups and increased with the methylene chain length in NH-(CH)-SH. Interestingly, the photocurrent density depended on the terminal group and the methylene chain length in NH-(CH)-SH and decreased in the order amino groups > methyl groups > carboxylic acid groups. An efficient photocurrent response of the LHCII complex on SAMs of NH-(CH)-SH, n = 8 was observed upon illumination at 680 nm. These results indicated that the LHCII complexes were well organized on the cationic surfaces of the gold electrodes modified with amino alkanethiols. The quantum yield depended on the methylene chain length ( n), where the maximum photocurrent response was observed at n = 8, which corresponded to a distance of 1.7 nm between the terminal amino group in NH-(CH)-SH and the gold surface. [ABSTRACT FROM AUTHOR]

Appropriate experimental platforms are required to clarify the structure-function relationships of membrane protein assemblies. In photosynthetic bacteria, light-harvesting complex 2 and light-harvesting/reaction center core complex play key roles in capturing and transferring light energy and facilitating subsequent charge separation. These photosynthetic apparatuses form a supramolecular assembly in the photosynthetic membrane. However, the mechanism through which this assembly influences the efficiency of energy conversion remains to be clarified. We review our recent studies that were conducted to evaluate the structure-function relationship of the supramolecular assembly of photosynthetic antenna complexes in various lipid bilayer systems, as well as the construction of novel systems of planar lipid membranes for use as experimental platforms. [ABSTRACT FROM AUTHOR]

Biohybrid light-harvesting antennas are an emerging platform technology with versatile tailorability for solar-energy conversion. These systems combine the proven peptide scaffold unit utilized for light harvesting by purple photosynthetic bacteria with attached synthetic chromophores to extend solar coverage beyond that of the natural systems. Herein, synthetic unattached chromophores are employed that partition into the organized milieu ( e.g. detergent micelles) that house the LH1-like biohybrid architectures. The synthetic chromophores include a hydrophobic boron-dipyrrin dye (A1) and an amphiphilic bacteriochlorin (A2), which transfer energy with reasonable efficiency to the bacteriochlorophyll acceptor array (B875) of the LH1-like cyclic oligomers. The energy-transfer efficiencies are markedly increased upon covalent attachment of a bacteriochlorin (B1 or B2) to the peptide scaffold, where the latter likely acts as an energy-transfer relay site for the (potentially diffusing) free chromophores. The efficiencies are consistent with a Förster (through-space) mechanism for energy transfer. The overall energy-transfer efficiency from the free chromophores via the relay to the target site can approach those obtained previously by relay-assisted energy transfer from chromophores attached at distant sites on the peptides. Thus, the use of free accessory chromophores affords a simple design to enhance the overall light-harvesting capacity of biohybrid LH1-like architectures. [ABSTRACT FROM AUTHOR]

Perovskite solar cells with submicrometre-thick CH3NH3PbI3 or CH3NH3PbI3-xClx active layers show a power conversion efficiency as high as 15%. However, compared to the best-performing device, the average efficiency was as low as 12%, with a large standard deviation (s.d.). Here, we report perovskite solar cells with an average efficiency exceeding 16% and best efficiency of 17%. This was enabled by the growth of CH3NH3PbI3 cuboids with a controlled size via a two-step spin-coating procedure. Spin-coating of a solution of CH3NH3I with different concentrations follows the spin-coating of PbI2, and the cuboid size of CH3NH3PbI3 is found to strongly depend on the concentration of CH3NH3I. Light-harvesting efficiency and charge-carrier extraction are significantly affected by the cuboid size. Under simulated one-sun illumination, average efficiencies of 16.4% (s.d. ± 0.35), 16.3% (s.d. ± 0.44) and 13.5% (s.d. ± 0.34) are obtained from solutions of CH3NH3I with concentrations of 0.038 M, 0.050 M and 0.063 M, respectively. By controlling the size of the cuboids of CH3NH3PbI3 during their growth, we achieved the best efficiency of 17.01% with a photocurrent density of 21.64 mA cm-2, open-circuit photovoltage of 1.056 V and fill factor of 0.741. [ABSTRACT FROM AUTHOR]

The major light-harvesting complex of photosystem II (LHCII) is an important regulatory protein in photosynthetic membranes. In vivo LHCII forms stable trimers and is found either associated to photosystem II or in LHCII-only containing domains. It was suggested that in native thylakoid membrane LHCII changes its conformation and macroorganization upon switching from light-harvesting to photoprotective state. Herein we have analyzed LHCII Langmuir monolayers at different subphase salt composition and in two different states – partly deprotonated (LHCII), at low basic pH 7.8, and highly protonated (p-LHCII), at pH 5.2, mimicking the functional light-harvesting and light-protective states of the protein, respectively. We have found strong difference in the supramolecular organization of the protein in these two functional states, the protonated monolayer exhibiting higher order of organization and significantly higher stability compared to the partly deprotonated one. Both LHCII and p-LHCII monolayers are composed of trimers self-assembling in aggregates with different packing density – loosely packed compiling homogeneous well-ordered monolayer areas and tightly packed organized in heterogeneous disordered phase. These two types of macroorganization are found in different proportions in protonated and partly deprotonated LHCII monolayers, the p-LHCII monolayer being much more heterogeneous than LHCII one. [ABSTRACT FROM AUTHOR]

The trimeric light-harvesting complexes II (LHCII) of plants and green algae are pigment-protein complexes involved in light harvesting and photoprotection. Different conformational states have been proposed to be responsible for their different functions. At present, detergent-solubilized LHCII is used as a model for the 'light-harvesting conformation', whereas the 'quenched conformation' is mimicked by LHCII aggregates. However, none of these conditions seem to perfectly reproduce the properties of LHCII in vivo. In addition, several monomeric LHC complexes are not fully stable in detergent. There is thus a need to find conditions that allow analyzing LHCs in vitro in stable and, hopefully, more native-like conformations. Here, we report a study of LHCII, the major antenna complex of plants, in complex with amphipols. We have trapped trimeric LHCII and monomeric Lhcb1 with either polyanionic or non-ionic amphipols and studied the effect of these polymers on the properties of the complexes. We show that, as compared to detergent solutions, amphipols have a stabilizing effect on LHCII. We also show that the average fluorescence lifetime of LHCII trapped in an anionic amphipol is ~30 % shorter than in α-dodecylmaltoside, due to the presence of a conformation with 230-ps lifetime that is not present in detergent solutions. [ABSTRACT FROM AUTHOR]

Side-chains at the imidic position of naphthalimide rendered a firm control over (i) the degrees of π–π overlap and (ii) distances between the perylenimide units in a crystalline naphthalimide–perylenimide dyad as determined using single crystal XRD and Hirshfeld surface analyses. Steady-state and time-resolved electronic spectroscopy in addition to DFT calculations revealed a decline in intermolecular excitonic interaction due to interfering alkyl chains. [ABSTRACT FROM AUTHOR]

Artificial photosynthetic systems that contain light-harvesting coordination complexes may one day replace conventional non-renewable sources of energy with renewable solar energy sources. Light-Harvesting Complexes (LHC) are important components of natural photosynthetic systems and are also sought after in artificial systems as well. Polynuclear photoactive complexes are therefore very attractive, and those based on stereogenic [Ru(2,2'-bipyridine)3]2+ are photophysically appealing, but difficult to obtain in a stereochemically pure form. On the other hand, polynuclear complexes based on the achiral [Ru(2,2':6',2"-terpyridine)2]2+ motif are easy to synthesise, however, these complexes are devoid of attractive excited-state properties. Hence strategies to increase the r.t. excited-state lifetime of these complexes would be of practical importance in vectorial electron and/or electron transfer in various optoelectronic applications. This tutorial review will report on the sophisticated synthetic strategies currently in use to enhance the photophysical properties of mononuclear Ru(II) complexes of tridentate ligands at room temperature. [ABSTRACT FROM AUTHOR]

We analyze a one-dimensional ring structure composed of many two-level systems, in the limit where only one excitation is present. The two-level systems are coupled to a common environment, where the excitation can be lost, which induces super- and subradiant behavior, an example of cooperative quantum coherent effect. We consider time-independent random fluctuations of the excitation energies. This static disorder, also called inhomogeneous broadening in literature, induces Anderson localization and is able to quench superradiance. We identify two different regimes: (i) weak opening, in which superradiance is quenched at the same critical disorder at which the states of the closed system localize; (ii) strong opening, with a critical disorder strength proportional to both the system size and the degree of opening, displaying robustness of cooperativity to disorder. Relevance to photosynthetic complexes is discussed. [ABSTRACT FROM AUTHOR]