Efficient Light‐Harvesting Antennae Resulting from the Dense Organization of Dyes into DNA Junctions through d‐Threoninol

Herein we report the construction of efficient light‐harvesting antennae by hybridization of DNA oligonucleotides containing high densities of fluorophores into DNA junctions through d ‐threoninol. Six pyrene donors could be incorporated into each arm without self‐quenching. A perylene acceptor was located at the center of the junction. Antenna effects of a duplex and three‐ to eight‐way junctions were systematically compared. Six‐ and eight‐way junctions had the highest antenna effects, and their effective absorption coefficients were 8.5 times higher than that of perylene. Interestingly, even‐numbered junctions had higher efficiencies than odd‐numbered junctions. Nondenaturing gel analyses and fluorescence lifetime measurements demonstrated that the strong odd–even effects were derived from differences in the stability of junctions. The results presented will guide the design of efficient artificial photosynthetic systems.     

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.     

Light‐Harvesting Nanotubes Formed by Supramolecular Assembly of Aromatic Oligophosphates

A 2,7‐disubstituted phosphodiester‐linked phenanthrene trimer forms tubular structures in aqueous media. Chromophores are arranged in H‐aggregates. Incorporation of small quantities of pyrene results in the development of light‐harvesting nanotubes in which phenanthrenes act as antenna chromophores and pyrenes as energy acceptors. Energy collection is most efficient after excitation at the phenanthrene H‐band. Fluorescence quantum yields up to 23 % are reached in pyrene doped, supramolecular nanotubes.     

DNA‐Based Oligochromophores as Light‐Harvesting Systems

The chromophores ethynyl pyrene as blue, ethynyl perylene as green and ethynyl Nile red as red emitter were conjugated to the 5‐position of 2′‐deoxyuridine via an acetylene bridge. Using phosphoramidite chemistry on solid phase labelled DNA duplexes were prepared that bear single chromophore modifications, and binary and ternary combinations of these chromophore modifications. The steady‐state and time‐resolved fluorescence spectra of all three chromophores were studied in these modified DNA duplexes. An energy‐transfer cascade occurs from ethynyl pyrene over ethynyl perylene to ethynyl Nile red and subsequently an electron‐transfer cascade in the opposite direction (from ethynyl Nile red to ethynyl perylene or ethynyl pyrene, but not from ethynyl perylene to ethynyl pyrene). The electron‐transfer processes finally provide charge separation. The efficiencies by these energy and electron‐transfer processes can be tuned by the distances between the chromophores and the sequences. Most importantly, excitation at any wavelength between 350 and 700 nm finally leads to charge separated states which make these DNA samples promising candidates for light‐harvesting systems.     

Structure‐Dependent Photoinduced Electron Transfer in Fullerodendrimers with Light‐Harvesting Oligophenylenevinylene Terminals

Oligophenylenevinylene (OPV)‐terminated phenylenevinylene dendrons G1 – G4 with one, two, four, and eight “side‐arms”, respectively, were prepared and attached to C₆₀ by a 1,3‐dipolar cycloaddition of azomethine ylides generated in situ from dendritic aldehydes and N‐methylglycine. The relative electronic absorption of the OPV moiety increases progressively along the fullerodendrimer family C₆₀G1 – C₆₀G4 , reaching a 99:1 ratio for C₆₀G4 (antenna effect). UV/Vis and near‐IR luminescence and transient absorption spectroscopy was used to elucidate photoinduced energy and electron transfer in C₆₀G1 – C₆₀G4 as a function of OPV moiety size and solvent polarity (toluene, dichloromethane, benzonitrile), taking into account the fact that the free‐energy change for electron transfer is the same along the series owing to the invariability of the donor–acceptor couple. Regardless of solvent, all the fullerodendrimers exhibit ultrafast OPV→C₆₀ singlet energy transfer. In CH₂Cl₂, the OPV→C₆₀ electron transfer from the lowest fullerene singlet level (¹C₆₀*) is slightly exergonic (ΔG_CS≈0.07 eV), but is observed, to an increasing extent, only in the largest systems C₆₀G2 – C₆₀G4 with lower activation barriers for electron transfer. This effect has been related to a decrease of the reorganization energy upon enlargement of the molecular architecture. Structural factors are also at the origin of an unprecedented OPV→C₆₀ electron transfer observed for C₆₀G3 and C₆₀G4 in apolar toluene, whereas in benzonitrile, electron transfer occurs in all cases. Monitoring of the lowest fullerene triplet state by sensitized singlet oxygen luminescence and transient absorption spectroscopy shows that this level is populated through intersystem crossing and is not involved in photoinduced electron transfer.     

Light‐Harvesting Hybrid Assemblies

Light‐harvesting hybrids have gained much importance as they are considered as potential mimics for photosynthetic systems. In this Concept article we introduce the design concepts involved in the building up of light‐harvesting hybrids; these resemble the well‐studied organic‐based assemblies for energy transfer. We have structured this article into three parts based on the strategies adopted in the synthesis of hybrid assemblies, as covalent, semicovalent, and noncovalent procedures. Furthermore, the properties and structural features of the hybrids and analogous organic assemblies are compared. We also emphasize the challenges involved in the processability of these hybrid materials for device applications and present our views and results to address this issue through the design of soft‐hybrids by a solution‐state, noncovalent, self‐assembly process.     

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.     

Supramolecular Light‐Harvesting Antennas of Metal‐Coordinated Bis(8‐Hydroxyquinoline)‐Substituted Porphyrin Networks

Artificial antenna complexes of metal‐coordinated bis(8‐hydroxyquinoline)‐substituted porphyrin networks that mimic antenna chromophores in plants were organized on titanium dioxide electrodes in photoelectrochemical cells. The generated photocurrents can be optimized according to the two ways of porphyrin self‐assembly due to the “antenna effect”: changing the number of assembled porphyrin monolayers and the number of generations of the metal‐coordinated porphyrin networks.     

Light‐Harvesting Nanoparticle Probes for FRET‐Based Detection of Oligonucleotides with Single‐Molecule Sensitivity

Controlling the emission of bright luminescent nanoparticles by a single molecular recognition event remains a challenge in the design of ultrasensitive probes for biomolecules. Herein, we developed 20‐nm light‐harvesting nanoantenna particles, built of a tailor‐made hydrophobic charged polymer poly(ethyl methacrylate‐co‐methacrylic acid), encapsulating circa 1000 strongly coupled and highly emissive rhodamine dyes with their bulky counterion. Being 87‐fold brighter than quantum dots QDots 605 in single‐particle microscopy (with 550‐nm excitation), these DNA‐functionalized nanoparticles exhibit over 50 % total FRET efficiency to a single hybridized FRET acceptor, a highly photostable dye (ATTO665), leading to circa 250‐fold signal amplification. The obtained FRET nanoprobes enable single‐molecule detection of short DNA and RNA sequences, encoding a cancer marker (survivin), and imaging single hybridization events by an epi‐fluorescence microscope with ultralow excitation irradiance close to that of ambient sunlight.     

Highly Efficient Singlet–Singlet Energy Transfer in Light‐Harvesting [60,70]Fullerene–4‐Amino‐1,8‐naphthalimide Dyads

New C₆₀ and C₇₀ fullerene dyads formed with 4‐amino‐1,8‐naphthalimide chromophores have been prepared by the Bingel cyclopropanation reaction. The resulting monoadducts were investigated with respect to their fluorescence properties (quantum yields and lifetimes) to unravel the role of the charge‐transfer naphthalimide chromophore as a light‐absorbing antenna and excited‐singlet‐state sensitizer of fullerenes. The underlying intramolecular singlet–singlet energy transfer (EnT) process was fully characterized and found to proceed quantitatively (Φ_EnT≈1) for all dyads. Thus, these conjugates are of considerable interest for applications in which fullerene excited states have to be created and photonic energy loss should be minimized. In polar solvents (tetrahydrofuran and benzonitrile), fluorescence quenching of the fullerene by electron transfer from the ground‐state aminonaphthalimide was postulated as an additional path.     

Visible‐Light‐Induced Organic Photochemical Reactions through Energy‐Transfer Pathways

Visible‐light photocatalysis is a rapidly developing and powerful strategy to initiate organic transformations, as it closely adheres to the tenants of green and sustainable chemistry. Generally, most visible‐light‐induced photochemical reactions occur through single‐electron transfer (SET) pathways. Recently, visible‐light‐induced energy‐transfer (EnT) reactions have received considerable attentions from the synthetic community as this strategy provides a distinct reaction pathway, and remarkable achievements have been made in this field. In this Review, we highlight the most recent advances in visible‐light‐induced EnT reactions.     

Light‐Controlled Ion Transport through Biomimetic DNA‐Based Channels

Light‐controlled nanochannels are fabricated through self‐assembling azobenzene‐incorporated DNA (Azo‐DNA) strands to regulate ion transport. By switching between collapsed and relaxed states using visible and ultraviolet light alternately, the Azo‐DNA channels can be opened and closed because the conformation of Azo‐DNA changes, that is, Azo‐DNA is used as switchable controlling unit. In addition to sharing short response time and reversibility with other photoresponsive apparatuses, the Azo‐DNA‐based nanochannel system has advantages in good biocompatibility and versatile design, which could potentially be applied in light‐controlled drug release, optical information storage, and logic networks.     

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.     

A Mechanically Interlocked [3]Rotaxane as a Light‐Harvesting Antenna: Synthesis,Characterization, and Intramolecular Energy Transfer

Mimicking photosynthesis : The concept of light‐harvesting by using a mechanically interlocked [3]rotaxane is developed through synthesis and characterization. Our results provide a new candidate for light‐harvesting systems and also open up the possibility of creating intelligent or controllable energy‐collecting machines (see figure).

     

Photochemical Bleaching of an Elaborate Artificial Light‐Harvesting Antenna

The target artificial light‐harvesting antenna, comprising 21 discrete chromophores arranged in a logical order, undergoes photochemical bleaching when dispersed in a thin plastic film. The lowest‐energy component, which has an absorption maximum at 660 nm, bleaches through first‐order kinetics at a relatively fast rate. The other components bleach more slowly, in part, because their excited‐state lifetimes are rendered relatively short by virtue of fast intramolecular electronic energy transfer to the terminal acceptor. Two of the dyes, these being close to the terminal acceptor but interconnected through a reversible energy‐transfer step, bleach by way of an autocatalytic step. Loss of the terminal acceptor, thereby switching off the energy‐transfer route, escalates the rate of bleaching of these ancillary dyes. The opposite terminal, formed by a series of eight pyrene‐based chromophores, does not bleach to any significant degree. Confirmation of the various bleaching steps is obtained by examination of an antenna lacking the terminal acceptor, where the autocatalytic route does not exist and bleaching is very slow.     

Light‐Harvesting Ytterbium(III)–Porphyrinate–BODIPY Conjugates: Synthesis,Excitation‐Energy Transfer,and Two‐Photon‐Induced Near‐Infrared‐Emission Studies

Based on a donor–acceptor framework, several conjugates have been designed and prepared in which an electron‐donor moiety, ytterbium(III) porphyrinate (YbPor), was linked through an ethynyl bridge to an electron‐acceptor moiety, boron dipyrromethene (BODIPY). Photoluminescence studies demonstrated efficient energy transfer from the BODIPY moiety to the YbPor counterpart. When conjugated with the YbPor moiety, the BODIPY moiety served as an antenna to harvest the lower‐energy visible light, subsequently transferring its energy to the YbPor counterpart, and, consequently, sensitizing the Yb^III emission in the near‐infrared (NIR) region with a quantum efficiency of up to 0.73 % and a lifetime of around 40 μs. Moreover, these conjugates exhibited large two‐photon‐absorption cross‐sections that ranged from 1048–2226 GM and strong two‐photon‐induced NIR emission.     

Hierarchical Self‐Assembly of a Biomimetic Light‐Harvesting Antenna Based on DNA G‐Quadruplexes

A new modular approach to an artificial light‐harvesting antenna system is presented. The approach involves the hierarchical self‐assembly of porphyrin acceptor molecules to G‐quadruplexes tethered to coumarin donor moieties.