A Complex Comprising a Cyanine Dye Rotaxane and a Porphyrin Nanoring as a Model Light-Harvesting System

A nanoring-rotaxane supramolecular assembly with a Cy7 cyanine dye (hexamethylindotricarbocyanine) threaded along the axis of the nanoring was synthesized as a model for the energy transfer between the light-harvesting complex LH1 and the reaction center in purple bacteria photosynthesis. The complex displays efficient energy transfer from the central cyanine dye to the surrounding zinc porphyrin nanoring. We present a theoretical model that reproduces the absorption spectrum of the nanoring and quantifies the excitonic coupling between the nanoring and the central dye, thereby explaining the efficient energy transfer and demonstrating similarity with structurally related natural light-harvesting systems.     

Light Harvesting and White‐Light Generation in a Composite of Carbon Dots and Dye‐Encapsulated BSA‐Protein‐Capped Gold Nanoclusters

Several strategies have been adopted to design an artificial light‐harvesting system in which light energy is captured by peripheral chromophores and it is subsequently transferred to the core via energy transfer. A composite of carbon dots and dye‐encapsulated BSA‐protein‐capped gold nanoclusters (AuNCs) has been developed for efficient light harvesting and white light generation. Carbon dots (C‐dots) act as donor and AuNCs capped with BSA protein act as acceptor. Analysis reveals that energy transfer increases from 63 % to 83 % in presence of coumarin dye (C153), which enhances the cascade energy transfer from carbon dots to AuNCs. Bright white light emission with a quantum yield of 19 % under the 375 nm excitation wavelength is achieved by changing the ratio of components. Interesting findings reveal that the efficient energy transfer in carbon‐dot–metal‐cluster nanocomposites may open up new possibilities in designing artificial light harvesting systems for future applications.     

Non‐Radiative Energy Transfer Mediated by Hybrid Light‐Matter States

We present direct evidence of enhanced non‐radiative energy transfer between two J‐aggregated cyanine dyes strongly coupled to the vacuum field of a cavity. Excitation spectroscopy and femtosecond pump–probe measurements show that the energy transfer is highly efficient when both the donor and acceptor form light‐matter hybrid states with the vacuum field. The rate of energy transfer is increased by a factor of seven under those conditions as compared to the normal situation outside the cavity, with a corresponding effect on the energy transfer efficiency. The delocalized hybrid states connect the donor and acceptor molecules and clearly play the role of a bridge to enhance the rate of energy transfer. This finding has fundamental implications for coherent energy transport and light‐energy harvesting.     

Highly Branched Polymers with Layered Structures that Mimic Light‐Harvesting Processes

Hyperbranched polymers (HBPs) with decorated donor and acceptor chromophores in different domains were constructed to demonstrate the function of light harvesting in a polymeric nanostructure. Taking advantage of our recently developed chain‐growth copper‐catalyzed azide–alkyne cycloaddition polymerization, two structural parameters in the HBPs, for example, the molar ratio of the acceptor Coumarin 343 in the core to the donor Coumarin 2 on the periphery, and the average distance between these two layers, could be independently varied in a one‐pot synthesis. The results demonstrated an efficient energy transfer from the excited Coumarin 2 to the ground‐state Coumarin 343 in the core, with the efficiency of the energy transfer reaching as high as 98 %. The excited Coumarin 343, after receiving energy from donor Coumarin 2 emitted higher fluorescence intensity than when directly excited, indicating an observed light concentration effect from the periphery dye to the central dye in one polymer structure.     

Multichromophoric Organic Molecules Encapsulated in Polymer Nanoparticles for Artificial Light Harvesting

We designed a self‐assembled multichromophoric organic molecular arrangement inside polymer nanoparticles for light‐harvesting antenna materials. The self‐assembled molecular arrangement of quaterthiophene molecules was found to be an efficient light‐absorbing antenna material, followed by energy transfer to Nile red (NR) dye molecules, which was confined in polymer nanoparticles. The efficiency of the antenna effect was found to be 3.2 and the effective molar extinction coefficient of acceptor dye molecules was found to be enhanced, which indicates an efficient light‐harvesting system. Based on this energy‐transfer process, tunable photo emission and white light emission has been generated with 14 % quantum yield. Such self‐assembled oligothiophene–NR systems encapsulated in polymer nanoparticles may open up new possibilities for fabrication of artificial light harvesting system.     

Investigation of the weak binding of a tetrahistidine‐tagged peptide to quantum dots by using capillary electrophoresis with fluorescence detection

Capillary electrophoresis with fluorescence detection was utilized to probe the self‐assembly between cyanine group dye labeled tetrahistidine containing peptide and CdSe/ZnS quantum dots, inside the capillary. Quantum dots and cyanine group dye labeled tetrahistidine containing peptide were injected into the capillary one after the other and allowed to self‐assemble. Their self‐assembly resulted into a measurable Förster resonance energy transfer signal between quantum dots and cyanine group dye labeled tetrahistidine containing peptide. The Förster resonance energy transfer signal increased upon increasing the cyanine group dye labeled tetrahistidine containing peptide/quantum dot molar ratio and reached a plateau at the 32/1 molar ratio. Additionally, the Förster resonance energy transfer signal was also affected by the increment of the interval time of injection and the sampling time. Online ligand exchange experiments were used to assess, the potential of a monovalent ligand of imidazole and a hexavalent ligand peptide, to displace surface bound cyanine group dye labeled peptide ligands from the quantum dots surface. Under optimal conditions, a linear relationship between the integrated peak areas and hexavalent ligand peptide was obtained at a hexavalent ligand concentration range of 0−0.5 mM. Therefore, the present assay has the potential to be applied in the online ligands detection.     

Integrating DNA Photonic Wires into Light‐Harvesting Supramolecular Polymers

An approach combining DNA nanoscaffolds with supramolecular polymers for the efficient and directional propagation of light‐harvesting cascades has been developed. A series of photonic wires with different arrangements of fluorophores in DNA‐organized nanostructures were linked to light‐harvesting supramolecular phenanthrene polymers (SPs) in a self‐assembled fashion. Among them, a light‐harvesting complex (LHC) composed of SPs and a photonic wire of phenanthrene, Cy3, Cy5, and Cy5.5 chromophores reveals a remarkable energy transfer efficiency of 59 %. Stepwise transfer of the excitation energy collected by the light‐harvesting SPs via the intermediate Cy3 and Cy5 chromophores to the final Cy5.5 acceptor proceeds through a Förster resonance energy transfer mechanism. In addition, the light‐harvesting properties are documented by antenna effects ranging from 1.4 up to 23 for different LHCs.     

Light‐Harvesting Three‐Chromophore Systems Based on Biphenyl‐Bridged Periodic Mesoporous Organosilica

Three‐chromophore systems with light‐harvesting behavior were prepared, which are based on periodic mesoporous organosilica (PMO) with crystal‐like ordered structure. The organic bridges of biphenyl‐PMO in the pore walls act as donors and two types of dye are incorporated in the one‐dimensional channels. Consecutive two‐step‐Förster resonance energy transfer is observed from the biphenyl moieties to mediators (diethyl‐aminocoumarin or aminoacridone), followed by energy transfer from mediators to acceptors (dibenzothiacarbocyanine, indodicarbocyanine, sulforhodamine G). High energy‐transfer efficiencies ranging from 70 to 80 % are obtained for two‐step‐FRET, indicating that the mesochannel structure with one‐dimensional ordering provides spatial arrangement of chromophore pairs for an efficient direct energy transfer. The emission wavelength can be tuned by a choice of acceptor dye: 477 nm (diethylaminocoumarin), 519 nm (aminoacridone), 567 nm (sulforhodamine G), 630 nm (dibenzothiacarbocyanine), and 692 nm (indodicarbocyanine).     

A Brief Overview of Some Physical Studies on the Relaxation Dynamics and Förster Resonance Energy Transfer of Semiconductor Quantum Dots

This article highlights some physical studies on the relaxation dynamics and Förster resonance energy transfer (FRET) of semiconductor quantum dots (QDs) and the way these phenomena change with size, shape, and composition of the QDs. The understanding of the excited‐state dynamics of photoexcited QDs is essential for technological applications such as efficient solar energy conversion, light‐emitting diodes, and photovoltaic cells. Here, our emphasis is directed at describing the influence of size, shape, and composition of the QDs on their different relaxation processes, that is, radiative relaxation rate, nonradiative relaxation rate, and number of trap states. A stochastic model of carrier relaxation dynamics in semiconductor QDs was proposed to correlate with the experimental results. Many recent studies reveal that the energy transfer between the QDs and a dye is a FRET process, as established from 1/d⁶ distance dependence. QD‐based energy‐transfer processes have been used in applications such as luminescence tagging, imaging, sensors, and light harvesting. Thus, the understanding of the interaction between the excited state of the QD and the dye molecule and quantitative estimation of the number of dye molecules attached to the surface of the QD by using a kinetic model is important. Here, we highlight the influence of size, shape, and composition of QDs on the kinetics of energy transfer. Interesting findings reveal that QD‐based energy‐transfer processes offer exciting opportunities for future applications. Finally, a tentative outlook on future developments in this research field is given.     

The role of exciton hopping and direct energy transfer in the efficient quenching of conjugated polyelectrolytes

The dynamics of fluorescence quenching of a conjugated polyelectrolyte by a cyanine dye are investigated by femtosecond fluorescence up-conversion and polarization resolved transient absorption. The data are analyzed with a model based on the random walk of the exciton within the polymer chain and a long-range direct energy transfer between polymer and dye. We find that rapid intrachain energy migration toward complex sites with the dye leads to the highly efficient energy transfer, whereas the contribution from direct, long-range energy transfer is negligible. We determine the actual density of complexes with the dye along the polymer chain. A clear deviation from calculations based on a constant complex association constant is found and explained by a reduced effective polymer concentration due to aggregation. Altogether, the quenching efficiency is found to be limited by (i) the energetic disorder within the polymer chain and (ii) the formation of loose polymer aggregates.     

DNA‐Organized Light‐Harvesting Antennae: Energy Transfer in Polyaromatic Stacks Proceeds through Interposed Nucleobase Pairs

DNA‐based light‐harvesting antennae with varying arrangements of light‐absorbing phenanthrene donor units and a pyrene acceptor dye were synthesized and tested for their light‐harvesting properties. Excitation of phenanthrene is followed by rapid transfer of the excitation energy to the pyrene chromophore. A block of six light‐absorbing phenanthrenes was separated from the site of the acceptor in a stepwise manner by an increasing number of intervening AT base pairs. Energy transfer occurs through interposed AT base pairs and is still detected when the phenanthrene antenna is separated by 5 AT base pairs.     

Efficient Excitation‐Energy Transfer in Ion‐Based Organic Nanoparticles with Versatile Tunability of the Fluorescence Colours

Organic nanoparticles consisting of 3,3′‐diethylthiacyanine (TC) and ethidium (ETD) dyes are synthesized by ion‐association between the cationic dye mixture (10 % ETD doping) and the tetrakis(4‐fluorophenyl)borate (TFPB) anion, in the presence of a neutral stabilizing polymer, in aqueous solution. Doping with ETD makes the particle size smaller than without doping. Size tuning can also be conducted by varying the molar ratio (ρ) of the loaded anion to the cationic dyes. The fluorescence spectrum of TC shows good overlap with the absorption of ETD in the 450–600 nm wavelength region, so efficient excitation‐energy transfer from TC (donor) to ETD (acceptor) is observed, yielding organic nanoparticles whose fluorescence colours are tunable. Upon ETD doping, the emission colour changes significantly from greenish‐blue to reddish or whitish. This change is mainly dependent on ρ. For the doped nanoparticle sample with ρ=1, the intensity of fluorescence ascribed to ETD is ～150‐fold higher than that from pure ETD nanoparticles (efficient antenna effect). Non‐radiative Förster resonance‐energy transfer (FRET) is the dominant mechanism for the ETD fluorescence enhancement. The organic nanoparticles of a binary dye system fabricated by the ion‐association method act as efficient light‐harvesting antennae, which are capable of transferring light energy to the dopant acceptors in very close proximity to the donors, and can have multi‐wavelength emission colours with high fluorescence quantum yields.     

Enhanced Light‐Harvesting Capacity by Micellar Assembly of Free Accessory Chromophores and LH1‐like Antennas

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.     

Light‐Harvesting Systems Based on Organic Nanocrystals To Mimic Chlorosomes

We report the first highly efficient artificial light‐harvesting systems based on nanocrystals of difluoroboron chromophores to mimic the chlorosomes, one of the most efficient light‐harvesting systems found in green photosynthetic bacteria. Uniform nanocrystals with controlled donor/acceptor ratios were prepared by simple coassembly of the donors and acceptors in water. The light‐harvesting system funneled the excitation energy collected by a thousand donor chromophores to a single acceptor. The well‐defined spatial organization of individual chromophores in the nanocrystals enabled an energy transfer efficiency of 95 %, even at a donor/acceptor ratio as high as 1000:1, and a significant fluorescence of the acceptor was observed up to donor/acceptor ratios of 200 000:1.     

A Supramolecular Artificial Light‐Harvesting System with Two‐Step Sequential Energy Transfer for Photochemical Catalysis

An artificial light‐harvesting system with sequential energy‐transfer process was fabricated based on a supramolecular strategy. Self‐assembled from the host–guest complex formed by water‐soluble pillar[5]arene (WP5), a bola‐type tetraphenylethylene‐functionalized dialkyl ammonium derivative (TPEDA), and two fluorescent dyes, Eosin Y (ESY) and Nile Red (NiR), the supramolecular vesicles achieve efficient energy transfer from the AIE guest TPEDA to ESY. ESY can function as a relay to further transfer the energy to the second acceptor NiR and realize a two‐step sequential energy‐transfer process with good efficiency. By tuning the donor/acceptor ratio, bright white light emission can be successfully achieved with a CIE coordinate of (0.33, 0.33). To better mimic natural photosynthesis and make full use of the harvested energy, the WP5?TPEDA‐ESY‐NiR system can be utilized as a nanoreactor: photocatalyzed dehalogenation of α‐bromoacetophenone was realized with 96 % yield in aqueous medium.     

Kinetic study of free‐radical polymerization photoinitiated by cyanine‐borate salts. II.

A series of cyanine butyltriphenylborate salts were prepared and tested as initiators of free‐radical polymerization photoinitiated via a photoinduced electron‐transfer process. For the majority of the tested series, the highest rate of photoinitiated free‐radical polymerization was observed when sec‐butyl radicals were formed. Essentially, there was no influence of the quantum yield of the free‐radical formation on the rate of the free‐radical polymerization initiated by the cyanine‐borate salts. The experimental data revealed that the relationship between the rate of polymerization and the free energy change for the electron transfer displayed typical Marcus region kinetic behavior. The photoreduction of the cyanine butyltriphenylborate salts produced colorless products. The efficiency of the bleached‐dye formation had no effect on the overall efficiency of photoinitiated polymerization. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 2365–2374, 2000     

Energy and hole transfer between dyes attached to titania in cosensitized dye-sensitized solar cells

Cosensitization of broadly absorbing ruthenium metal complex dyes with highly absorptive near-infrared (NIR) organic dyes is a clear pathway to increase near-infrared light harvesting in liquid-based dye-sensitized solar cells (DSCs). In cosensitized DSCs, dyes are intimately mixed, and intermolecular charge and energy transfer processes play an important role in device performance. Here, we demonstrate that an organic NIR dye incapable of hole regeneration is able to produce photocurrent via intermolecular energy transfer with an average excitation transfer efficiency of over 25% when cosensitized with a metal complex sensitizing dye (SD). We also show that intermolecular hole transfer from the SD to NIR dye is a competitive process with dye regeneration, reducing the internal quantum efficiency and the electron lifetime of the DSC. This work demonstrates the general feasibility of using energy transfer to boost light harvesting from 700 to 800 nm and also highlights a key challenge for developing highly efficient cosensitized dye-sensitized solar cells.     

Ruthenium(II)–Polyimine–Coumarin Light‐Harvesting Molecular Arrays: Design Rationale and Application for Triplet–Triplet‐Annihilation‐Based Upconversion

Ru^II–bis‐pyridine complexes typically absorb below 450 nm in the UV spectrum and their molar extinction coefficients are only moderate (ε<16 000 M ^?1 cm^?1). Thus, Ru^II–polyimine complexes that show intense visible‐light absorptions are of great interest. However, no effective light‐harvesting ruthenium(II)/organic chromophore arrays have been reported. Herein, we report the first visible‐light‐harvesting Ru^II–coumarin arrays, which absorb at 475 nm (ε up to 63 300 M ^?1 cm^?1, 4‐fold higher than typical Ru^II–polyimine complexes). The donor excited state in these arrays is efficiently converted into an acceptor excited state (i.e., efficient energy‐transfer) without losses in the phosphorescence quantum yield of the acceptor. Based on steady‐state and time‐resolved spectroscopy and DFT calculations, we proposed a general rule for the design of Ru^II–polypyridine–chromophore light‐harvesting arrays, which states that the ¹IL energy level of the ligand must be close to the respective energy level of the metal‐to‐ligand charge‐transfer (M LCT) states. Lower energy levels of ¹IL/³IL than the corresponding ¹M LCT/³M LCT states frustrate the cascade energy‐transfer process and, as a result, the harvested light energy cannot be efficiently transferred to the acceptor. We have also demonstrated that the light‐harvesting effect can be used to improve the upconversion quantum yield to 15.2 % (with 9,10‐diphenylanthracene as a triplet‐acceptor/annihilator), compared to the parent complex without the coumarin subunit, which showed an upconversion quantum yield of only 0.95 %.     

Long‐Distance Electronic Energy Transfer in Light‐Harvesting Supramolecular Polymers

The efficient collection of solar energy relies on the design and construction of well‐organized light‐harvesting systems. Herein we report that supramolecular phenanthrene polymers doped with pyrene are effective collectors of light energy. The linear polymers are formed through the assembly of short amphiphilic oligomers in water. Absorption of light by phenanthrene residues is followed by electronic energy transfer along the polymer over long distances (>100 nm) to the accepting pyrene molecules. The high efficiency of the energy transfer, which is documented by large fluorescence quantum yields, suggests a quantum coherent process.     

Interfacing Click Chemistry with Automated Oligonucleotide Synthesis for the Preparation of Fluorescent DNA Probes Containing Internal Xanthene and Cyanine Dyes

Double‐labeled oligonucleotide probes containing fluorophores interacting by energy‐transfer mechanisms are essential for modern bioanalysis, molecular diagnostics, and in vivo imaging techniques. Although bright xanthene and cyanine dyes are gaining increased prominence within these fields, little attention has thus far been paid to probes containing these dyes internally attached, a fact which is mainly due to the quite challenging synthesis of such oligonucleotide probes. Herein, by using 2′‐O‐propargyl uridine phosphoramidite and a series of xanthenes and cyanine azide derivatives, we have for the first time performed solid‐phase copper(I)‐catalyzed azide–alkyne cycloaddition (CuAAC) click labeling during the automated phosphoramidite oligonucleotide synthesis followed by postsynthetic click reactions in solution. We demonstrate that our novel strategy is rapid and efficient for the preparation of novel oligonucleotide probes containing internally positioned xanthene and cyanine dye pairs and thus represents a significant step forward for the preparation of advanced fluorescent oligonucleotide probes. Furthermore, we demonstrate that the novel xanthene and cyanine labeled probes display unusual and very promising photophysical properties resulting from energy‐transfer interactions between the fluorophores controlled by nucleic acid assembly. Potential benefits of using these novel fluorescent probes within, for example, molecular diagnostics and fluorescence microscopy include: Considerable Stokes shifts (40–110 nm), quenched fluorescence of single‐stranded probes accompanied by up to 7.7‐fold light‐up effect of emission upon target DNA/RNA binding, remarkable sensitivity to single‐nucleotide mismatches, generally high fluorescence brightness values (FB up to 26), and hence low limit of target detection values (LOD down to <5 nM ).