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.     

Energy Transfer in Dye‐Coupled Lanthanide‐Doped Nanoparticles: From Design to Application

Surface modification with organic dye molecules is a useful strategy to manipulate the optical properties of lanthanide‐doped nanoparticles (LnNPs). It enables energy transfer between dyes and LnNPs, which provides unprecedented possibilities to gain new optical phenomena from the dye–LnNPs composite systems. This has led to a wide range of emerging applications, such as biosensing, drug delivery, gene targeting, information storage, and photon energy conversion. Herein, the mechanism of energy transfer and the structural‐dependent energy‐transfer properties in dye‐coupled LnNPs are reviewed. The design strategies for achieving effective dye–LnNP functionalization are presented. Recent advances in these composite nanomaterials in biomedicine and energy conversion applications are highlighted.     

Wide‐Range Light‐Harvesting Donor–Acceptor Assemblies through Specific Intergelator Interactions via Self‐Assembly

We have synthesized two new low‐molecular‐mass organogelators based on tri‐p‐phenylene vinylene derivatives, one of which could be designated as the donor whereas the other one is an acceptor. These were prepared specifically to show the intergelator interactions at the molecular level by using donor–acceptor self‐assembly to achieve appropriate control over their macroscopic properties. Intermolecular hydrogen‐bonding, π‐stacking, and van der Waals interactions operate for both the individual components and the mixtures, leading to the formation of gels in the chosen organic solvents. Evidence for intergelator interactions was acquired from various spectroscopic, microscopic, thermal, and mechanical investigations. Due to the photochromic nature of these molecules, interesting photophysical properties, such as solvatochromism and J‐type aggregation, were clearly observed. An efficient energy transfer was exhibited by the mixture of donor–acceptor assemblies. An array of four chromophores was built up by inclusion of two known dyes (anthracene and rhodamine 6G) for the energy‐transfer studies. Interestingly, an energy‐transfer cascade was observed in the assembly of four chromophores in a particular order (anthracene‐donor‐acceptor‐rhodamine 6G), and if one of the components was removed from the assembly the energy transfer process was discontinued. This allowed the build up of a light‐harvesting process with a wide range. Excitation at one end produces an emission at the other end of the assembly.     

Quadruplex-to-duplex transition of G-rich oligonucleotides probed by cationic water-soluble conjugated polyelectrolytes

G-quartet DNA converts to duplex form in the presence of its complementary strand. This conformational change can be detected in real time by a homogeneous assay method based on the signal amplification of conjugated polyelectrolytes and the specific interaction of intercalating dyes with double-stranded DNA (dsDNA). The probe solution contains a cationic, conjugated polymer (CCP), G-quadruplex labeled with a fluorescein at the 5'-terminus (G-quadruplex-Fl), and ethidium bromide (EB). The addition of a complementary target results in the transition from G-quadruplex to duplex (dsDNA-Fl) and EB intercalation within the duplex structure. Excitation of the CCP leads to energy transfer from CCP to dsDNA-Fl (FRET-1) and then energy transfer from dsDNA-Fl to EB (FRET-2). Increasing the number of mismatched bases discourages dsDNA formation, which is detected in the assay.     

New 8-amino-BODIPY derivatives: surpassing laser dyes at blue-edge wavelengths

The development of highly efficient and stable blue‐emitting dyes to overcome some of the most important shortcomings of available chromophores is of great technological importance for modern optical, analytical, electronic, and biological applications. Here, we report the design, synthesis and characterization of new tailor‐made BODIPY dyes with efficient absorption and emission in the blue spectral region. The major challenge is the effective management of the electron‐donor strength of the substitution pattern, in order to modulate the emission of these novel dyes over a wide spectral range (430–500 nm). A direct relationship between the electron‐donor character of the substituent and the extension of the spectral hypsochromic shift is seen through the energy increase of the LUMO state. However, when the electron‐donor character of the substituent is high enough, an intramolecular charge‐transfer process appears to decrease the fluorescence ability of these dyes, especially in polar media. Some of the reported novel BODIPY dyes provide very high fluorescence quantum yields, close to unity, and large Stokes shifts, leading to highly efficient tunable dye lasers in the blue part of the spectrum; this so far remains an unexploited region with BODIPYs. In fact, under demanding transversal pumping conditions, the new dyes lase with unexpectedly high lasing efficiencies of up to 63 %, and also show high photostabilities, outperforming the laser action of other dyes considered as benchmarks in the same spectral region. Considering the easy synthetic protocol and the wide variety of possible substituents, we are confident that this strategy could be successfully extended for the development of efficient blue‐edge emitting materials and devices, impelling biophotonic and optoelectronic applications.     

Through bond energy transfer (TBET)-based fluorescent chemosensors

Excitation energy transfer is one of the crucial issues in photophysical and photochemical process of any muti-chromophoric molecular systems, such as energy harvester and fluorescent chemosensor. Through bond energy transfer (TBET)-based fluorescent chemosensors are composed of three main parts: energy donor, energy acceptor, and rigid linker. Comparing with the often used Förster resonance energy transfer (FRET) mechanism, TBET does not require spectral overlap, thus it may enable more possible combination of energy donors and acceptors to be employed and afford higher sensitivity toward targets through ratiometric fluorescence. In this review, we highlight the recent progress in the design and biological applications of the organic TBET-based fluorescent chemosensors during 2014–2019, which will provide profound guidance for designing powerful chemosensors as well as exploring further biological applications.     

Luminescence Resonance Energy Transfer in a Multiple‐Component,Self‐Assembled Supramolecular Hydrogel

Cascade energy transfer from a sensitizer to Tb^III then to fluorescent dyes was studied for the first time in a supramolecular hydrogel. Efficient energy transfer from Tb^III to the dyes was observed, as established by time‐delayed emission and excitation spectral analysis, lifetime data, and microscopic studies.     

Luminescence Resonance Energy Transfer in a Multiple‐Component,Self‐Assembled Supramolecular Hydrogel

Cascade energy transfer from a sensitizer to Tb^III then to fluorescent dyes was studied for the first time in a supramolecular hydrogel. Efficient energy transfer from Tb^III to the dyes was observed, as established by time‐delayed emission and excitation spectral analysis, lifetime data, and microscopic studies.     

Förster resonance energy transfer in poly(methyl methacrylates) copolymers bearing donor–acceptor 1,3‐thiazole dyes

The Förster resonance energy transfer (FRET) properties in poly(methyl methacrylate) copolymers containing 2‐(pyridine‐2‐yl) thiazole dyes were studied upon systematic variation of the donor‐to‐acceptor ratio. To this end, 2‐(pyridine‐2‐yl) thiazole dyes specially designed for the usage as energy donor and acceptor molecules were incorporated within one polymer chain. Poly(methyl methacrylate) copolymers containing these donor and acceptor dyes were synthesized using the RAFT polymerization method. Copolymers with a molar mass (M_n) of nearly 10,000 g/mol were achieved with dispersity index values (?) under 1.3. The presented copolymers act as a model system for the FRET investigation. Förster resonance energy transfer properties of the copolymers are characterized by steady state as well as time resolved fluorescence spectroscopy. The results indicate that the energy transfer rates and the transfer efficiencies are tunable by variation of the donor‐acceptor‐ratio. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 4765–4773     

Intramolecular triplet energy transfer in multi-chromophoric dyes and its influence on the photostability

The photostability of organic dyes plays a very important role in practically all aspects of the development and applications of these dyes. In recent years, intramolecular transfer of triplet excitation energy between isolated chromophores on the same molecule has been a subject of intensive studies. Many multi-chromophoric organic dyes with covalent linkage between the chromophores, one of which can act as a triplet acceptor of the excess energy, have been synthesized and studied. The significant increases in photostability of such assembled dyes have been reported, suggesting that some chromophores can act as internal photostabilizers. These modified dyes have enhanced photostability and hence potential applications in a wide range of areas such as laser dyes, electroluminescent (EL) devices and solar cells.     

Phosphorescence Energy Transfer: Ambient Afterglow Fluorescence from Water‐Processable and Purely Organic Dyes via Delayed Sensitization

Ambient afterglow luminescence from metal‐free organic chromophores would provide a promising alternative to the well‐explored inorganic phosphors. However, the realization of air‐stable and solution‐processable organic afterglow systems with long‐lived triplet or singlet states remains a formidable challenge. In the present study, a delayed sensitization of the singlet state of organic dyes via phosphorescence energy transfer from organic phosphors is proposed as an alternative strategy to realize “afterglow fluorescence”. This concept is demonstrated with a long‐lived phosphor as the energy donor and commercially available fluorescent dyes as the energy acceptor. Triplet‐to‐singlet Förster‐resonance energy‐transfer (TS‐FRET) between donor and acceptor chromophores, which are co‐organized in an amorphous polymer matrix, results in tuneable yellow and red afterglow from the fluorescent acceptors. Moreover, these afterglow fluorescent hybrids are highly solution‐processable and show excellent air‐stability with good quantum yields.     

Efficient Light‐Harvesting Systems with Tunable Emission through Controlled Precipitation in Confined Nanospace

Light harvesting is a key step in photosynthesis but creation of synthetic light‐harvesting systems (LHSs) with high efficiencies has been challenging. When donor and acceptor dyes with aggregation‐induced emission were trapped within the interior of cross‐linked reverse vesicles, LHSs were obtained readily through spontaneous hydrophobically driven aggregation of the dyes in water. Aggregation in the confined nanospace was critical to the energy transfer and the light‐harvesting efficiency. The efficiency of the excitation energy transfer (EET) reached 95 % at a donor/acceptor ratio of 100:1 and the energy transfer was clearly visible even at a donor/acceptor ratio of 10 000:1. Multicolor emission was achieved simply by tuning the donor/acceptor feed ratio in the preparation and the quantum yield of white light emission from the system was 0.38, the highest reported for organic materials in water to date.     

Effect of Concentration on the Photophysics of Dyes in Light‐Scattering Materials.

Photoactive materials based on dye molecules incorporated into thin films or bulk solids are useful for applications as photosensitization, photocatalysis, solar cell sensitization and fluorescent labeling, among others. In most cases, high concentrations of dyes are desirable to maximize light absorption. Under these circumstances, the proximity of dye molecules leads to the formation of aggregates and statistical traps, which dissipate the excitation energy and lower the population of excited states. The search for enhancement of light collection, avoiding energy wasting requires accounting the photophysical parameters quantitatively, including the determination of quantum yields, complicated by the presence of light scattering when particulate materials are considered. In this work we summarize recent advances on the photophysics of dyes in light‐scattering materials, with particular focus on the effect of dye concentration. We show how experimental reflectance, fluorescence and laser‐induced optoacoustic spectroscopy data can be used together with theoretical models for the quantitative evaluation of inner filter effects, fluorescence and triplet formation quantum yields and energy transfer efficiencies.     

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.     

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.     

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.     

New Red‐Emitting Tetrazine‐Phenoxazine Fluorogenic Labels for Live‐Cell Intracellular Bioorthogonal Labeling Schemes

The synthesis of a set of tetrazine‐bearing fluorogenic dyes suitable for intracellular labeling of proteins in live cells is presented. The red excitability and emission properties ensure minimal autofluorescence, while through‐bond energy‐transfer‐based fluorogenicity reduces nonspecific background fluorescence of unreacted dyes. The tetrazine motif efficiently quenches fluorescence of the phenoxazine core, which can be selectively turned on chemically upon bioorthogonal inverse‐electron‐demand Diels–Alder reaction with proteins modified genetically with strained trans‐cyclooctenes.     

Azo‐Based Fluorogenic Probes for Biosensing and Bioimaging: Recent Advances and Upcoming Challenges

T he use of nonfluorescent azo dyes as dark quenchers in activatable optical bioprobes based on the Förster resonance energy transfer (FRET) mechanism and designed to target a wide range of enzymes has been established for over two decades. The key value of the azo moiety (−N=N−) to act as an efficient “ON–OFF” switch of fluorescence once introduced within the core structure of conventional organic‐based fluorophores (mainly fluorescent aniline derivatives) has recently been exploited in the development of alternative reaction‐based small‐molecule probes based on the “profluorescence” concept. These unprecedented “azobenzene‐caged” fluorophores are valuable tools for the detection of a wide range of reactive (bio)analytes. This review highlights the most recent and relevant advances made in the design and biosensing/bioimaging applications of azo‐based fluorogenic probes. Emphasis is also placed on relevant achievements in the synthesis of bioconjugatable/biocompatible azo dyes used as starting building blocks in the rational and rapid construction of these fluorescent chemodosimeters. Finally, a brief glimpse of possible future biomedical applications (theranostics) of these “smart” azobenzene‐based molecular systems is presented.     

Excitation Energy Transfer in Dendritic Host–Guest Donor–Acceptor Systems

We report on a study of a physically formed host–guest system, which was designed to be investigated by fluorescence energy transfer. All donor and acceptor molecules used were cyanine dyes. Investigation was performed at the ensemble level as well as at the single‐molecule level. The ensemble measurements revealed a distribution of binding sites as well for the donor as for the acceptor. Accordingly, we found a distribution of the energy transfer efficiency. At the single‐molecule level, these distributions are still present. We could discriminate entities that show very efficient energy transfer, some that do not show any energy transfer and systems whose energy transfer efficiency is only about 50 %. The latter allowed the time‐resolved detection of energy transfer of single entities through the acceptor decay. Finally, we discuss the observation that the energy transfer efficiency fluctuates as a function of time.     

Antenna behavior of donor–acceptor dye‐loaded novel supramolecular framework hosts

New guest dye cations within the channel cavities of supramolecular hosts are studied. The guest organic dyes are intercalated in the supramolecular hosts by a coprecipitation reaction to give new dye‐sensitizer coordination polymers. The absorption spectra for the dye molecules within the supramolecular hosts show intense bands in the region from 500 to 700 nm due to the presence of the dyes within the parallel channels in the monomeric forms. The properties of the resulting colored polymers were investigated by IR, UV–vis, fluorescence spectra and X‐ray powder diffraction, indicating the excitation energy transfer from neutral red or pyronine as donors to methylene blue or thionine as acceptors within a supramolecular system filled with a mixture of both dyes. The wide‐ranging tenability of these highly organized materials offers fascinating new possibilities for exploring excitation energy transfer phenomena, and challenges for developing new photonic devices. Copyright © 2005 John Wiley & Sons, Ltd.