Developing Through‐Space Charge Transfer Polymers as a General Approach to Realize Full‐Color and White Emission with Thermally Activated Delayed Fluorescence

Through‐space charge transfer polymers (TSCT polymers) that contain a non‐conjugated polystyrene backbone and spatially separated donor and acceptor units for solution‐processed OLEDs with full‐color and white emission is reported. By tuning the charge transfer strength between donor and acceptors with different electron‐accepting ability, emission color spanning from deep blue to red can be achieved. By incorporating two kinds of donor/acceptor pairs in one polymer to create duplex through‐space charge‐transfer channels, blue and yellow emission can be simultaneously obtained to realize white electroluminescence from a single polymer. The TSCT polymers exhibit thermally activated delayed fluorescence effect with delayed‐component lifetimes in range of 0.36–1.98 μs, and unexpected aggregation‐induced emission (emission intensity enhancement of up to 117 from solution to aggregation state).     

Amphiphilic Fluorescence Resonance Energy-Transfer Dyes: Synthesis,Fluorescence, and Aggregation Behavior in Water

Amphiphilic pyrene/perylene bis-chromophore dyes were synthesized from unsymmetrically substituted perylene bisimide dyes, which were obtained through three synthetic methods. The optical and aggregation behaviors of these functional dyes were studied by means of UV/Vis absorption and fluorescence spectroscopy, dynamic light scattering, and TEM. These dyes are highly fluorescent and cover the whole visible-light region. A donor/acceptor dye displays intramolecular fluorescence resonance energy transfer (FRET), with a high efficiency of up to 96.4 % from pyrene to perylene bisimide chromophores, which leads to a high fluorescence color sensitivity to environmental polarity. Under a λ=365 nm UV lamp, the light-emitting colors of the donor/acceptor dye change from green to yellow with increasing solvent polarity, which demonstrates application potential as a new class of FERT probes. The donor/acceptor dye in water was assembled into hollow vesicles with a narrow size distribution. The bilayer structure of the vesicular wall was directly observed by means of TEM. These vesicular aggregates in water are fluorescent at λ=650–850 nm within the near-infrared region.     

Fluorescent Peptide Beacons for the Selective Ratiometric Detection of Heparin

Heparin is extensively used as an anticoagulant drug during surgery. Two fluorophore‐functionalized cationic oligopeptides HS 1 and HS 2 were developed to monitor heparin ratiometrically in aqueous media. Upon binding to heparin, HS 1 and HS 2 undergo a conformational change from an open form to a folded form, which leads to a distinct change in the fluorescence properties. HS 1 switches from pyrene monomer emission to an excimer emission. For HS 2 , a fluorescence resonance energy transfer (FRET) process is enabled between a naphthalene donor and a dansyl acceptor. This method is highly selective for heparin relative to other similar biological analytes such as hyaluronic acid or chondroitin sulfate. HS 1 and HS 2 could also detect heparin ratiometrically in diluted bovine serum. The strong ratiometric emission color change can also be observed by the naked eye. Addition of the polycationic protein protamine releases both HS 1 and HS 2 from their heparin complex, which simultaneously restores pyrene monomer emission for the first case and decreases the FRET process for the latter case, respectively. Dynamic light scattering (DLS) and AFM studies confirm aggregate formation of heparin with HS 1 and HS 2 .     

Energy transfer and fluorescence quenching in complexes of polymethine dyes with human serum albumin

Electronic excitation energy transfer (EET) between molecules of polymethine dyes bound to human serum albumin (HSA) has been established and studied by absorption and fluorescence spectroscopy as well as by fluorescence decay measurements. In this system, excitation of the donor dye molecule leads to fluorescence of the acceptor dye molecule, both bound to HSA, with donor fluorescence quenching by the acceptor. The short distance between the donor and the acceptor (25-28 A) revealed from the Forster model of EET as well as some spectroscopic data show that both molecules are probably located in the same binding domain of HSA. The role of HSA is to bring donor and acceptor molecules together to a distance adequate to achieve EET as well as to increase the donor and acceptor fluorescence quantum yields. Efficient quenching of the intrinsic HSA fluorescence by some polymethine dyes (oxonols) is observed. The experimental results fit well a model for the formation of a weakly fluorescent dye-HSA complex; the quencher in this complex should be located in the immediate vicinity of the HSA fluorophore group (Trp(214)).     

A comparison of crystalline- and graft polymer-based chemosensors

Herein, we report a highly sensitive luminescent thin film chemosensor constructed out of a small-molecule donor/acceptor system. Two types of films were compared: one using a small-molecule crystalline donor/acceptor pair and the other using a donor-graft polymer/small-molecule acceptor pair. The acceptor selected for this proof of concept responds to acid, causing its absorption and emission bands to red-shift, which increases spectral overlap with the donor. This increase in overlap greatly enhances energy transfer from the acceptor to the donor. Signal amplification was ascertained by measuring the ratio of acceptor fluorescence when the donor was excited versus direct excitation of the acceptor. Both types of films exhibited large amplification. For the polymeric system, the mechanism of energy migration was investigated by the use of steady-state fluorescence spectroscopy. The mechanism was determined to be dominated by an exciton-hopping process.     

Fluorescently Labeled Substrates for Monitoring α1,3‐Fucosyltransferase IX Activity

Fucosylation is often the final process in glycan biosynthesis. The resulting glycans are involved in a variety of biological processes, such as cell adhesion, inflammation, or tumor metastasis. Fucosyltransferases catalyze the transfer of fucose residues from the activated donor molecule GDP‐β‐L ‐fucose to various acceptor molecules. However, detailed information about the reaction processes is still lacking for most fucosyltransferases. In this work we have monitored α1,3‐fucosyltransferase activity. For both donor and acceptor substrates, the introduction of a fluorescent ATTO dye was the last step in the synthesis. The subsequent conversion of these substrates into fluorescently labeled products by α1,3‐fucosyltransferases was examined by high‐performance thin‐layer chromatography coupled with mass spectrometry as well as dual‐color fluorescence cross‐correlation spectroscopy, which revealed that both fluorescently labeled donor GDP‐β‐L ‐fucose‐ATTO 550 and acceptor N‐acetyllactosamine‐ATTO 647N were accepted by recombinant human fucosyltransferase IX and Helicobacter pylori α1,3‐fucosyltransferase, respectively. Analysis by fluorescence cross‐correlation spectroscopy allowed a quick and versatile estimation of the progress of the enzymatic reaction and therefore this method can be used as an alternative method for investigating fucosyltransferase reactions.     

Intramolecular electronic energy transfer via exchange interaction in bichromophoric molecules

A theory is presented for intramolecular electronic energy transfer in bichromophoric molecules. Expressions are given for the donor moiety fluorescence (phosphorescence) decay and for its fluorescence (phosphorescence) quantum yield in terms of the average distance between the donor and acceptor moieties and the donor—acceptor bridge flexibility. Comparison with available experimental data supports the predictions of the analysis.     

Quantum dot-based multiplexed fluorescence resonance energy transfer

We demonstrate the use of luminescent quantum dots (QDs) conjugated to dye-labeled protein acceptors for nonradiative energy transfer in a multiplexed format. Two configurations were explored: (1) a single color QD interacting with multiple distinct acceptors and (2) multiple donor populations interacting with one type of acceptor. In both cases, we showed that simultaneous energy transfer between donors and proximal acceptors can be measured. However, data analysis was simpler for the configuration where multiple QD donors are used in conjunction with one acceptor. Steady-state fluorescence results were corroborated by time-resolved measurements where selective shortening of QD lifetime was measured only for populations that were selectively engaged in nonradiative energy transfer.     

Recognition properties of donor- and acceptor-modified biphenyl-DNA

The recognition properties of DNA duplexes containing single or triple incorporations of eight different donor-modified (OMe, NH(2)) and acceptor-modified (NO(2)) biphenyl residues as base replacements in opposite positions were probed by UV-melting and by CD and fluorescence spectroscopy. We found a remarkable dependence of duplex stability on the natures of the substituents (donor vs. acceptor). The stabilities of duplexes with one biphenyl pair increase in the order donor/donor < acceptor/donor < acceptor/acceptor substitution. The most stable biphenyl pairs stabilize duplexes by up to 6 degrees C in T(m). In duplexes with three consecutive biphenyl pairs the stability increases in the inverse order (acceptor/acceptor < donor/acceptor < donor/donor) with increases in T(m), relative to an unmodified duplex, of up to 10 degrees C. A thermodynamic analysis, combined with theoretical calculations of the physical properties of the biphenyl substituents, suggests that in duplexes with single biphenyl pairs the affinity is dominated by electrostatic forces between the biphenyl/nearest neighbor natural base pairs, whereas in the triple-modified duplexes the increase in thermal stability is predominantly determined by hydrophobic interactions of the biphenyl residues with each other. Oligonucleotides containing amino biphenyl residues are fluorescent. Their fluorescence is largely quenched when they are paired with themselves or with nitrobiphenyl-containing duplex partners.     

Preparation and Characterization of a π-Conjugated Donor–Acceptor System Containing the Strongly Electron-Accepting Tetraphenylborolyl Unit

A novel thiophene-bridged donor–acceptor system was synthesized with a carbazole as donor and a borole as acceptor unit. The borole group was successfully installed via the tin–boron exchange reaction of 1,1-dimethyl-2,3,4,5-tetraphenylstannole with 9-(5-(dibromoboryl)thiophen-2-yl)carbazole. The effect of the borole on the optoelectronic properties of the donor–acceptor system was explored by spectroscopic (UV/Vis and fluorescence spectroscopy), electrochemical (cyclic voltammetry) and theoretical (TD-DFT) methods as well as by modifying its structure. The corresponding donor–acceptor compound bearing the widely employed dimesitylboryl acceptor group was also synthesized for comparison.     

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.     

Charge‐Transfer Interactions in Tris‐Donor–Tris‐Acceptor Hexaarylbenzene Redox Chromophores

Symmetric‐ and asymmetric hexaarylbenzenes (HABs), each substituted with three electron‐donor triarylamine redox centers and three electron‐acceptor triarylborane redox centers, were synthesized by cobalt‐catalyzed cyclotrimerization, thereby forming compounds with six‐ and four donor–acceptor interactions, respectively. The electrochemical‐ and photophysical properties of these systems were investigated by cyclovoltammetry (CV), as well as by absorption‐ and fluorescence spectroscopy, and compared to a HAB that only contained one neighboring donor–acceptor pair. CV measurements of the asymmetric HAB show three oxidation peaks and three reduction peaks, whose peak‐separation is greatly influenced by the conducting salt, owing to ion‐pairing and shielding effects. Consequently, the peak‐separations cannot be interpreted in terms of the electronic couplings in the generated mixed‐valence species. Transient‐absorption spectra, fluorescence‐solvatochromism, and absorption spectra show that charge‐transfer states from the amine‐ to the boron centers are generated after optical excitation. The electronic donor–acceptor interactions are weak because the charge transfer has to occur predominantly through space. Moreover, the excitation energy of the localized excited charge‐transfer states can be redistributed between the aryl substituents of these multidimensional chromophores within the fluorescence lifetime (about 60 ns). This result was confirmed by steady‐state fluorescence‐anisotropy measurements, which further indicated symmetry‐breaking in the superficially symmetric HAB. Adding fluoride ions causes the boron centers to lose their accepting ability owing to complexation. Consequently, the charge‐transfer character in the donor–acceptor chromophores vanishes, as observed in both the absorption‐ and fluorescence spectra. However, the ability of the boron center as a fluoride sensor is strongly influenced by the moisture content of the solvent, possibly owing to the formation of hydrogen‐bonding interactions between water molecules and the fluoride anions.     

Regulating Charge-Transfer in Conjugated Microporous Polymers for Photocatalytic Hydrogen Evolution

Bandgap engineering in donor–acceptor conjugated microporous polymers (CMPs) is a potential way to increase the solar-energy harvesting towards photochemical water splitting. Here, the design and synthesis of a series of donor–acceptor CMPs [tetraphenylethylene (TPE) and 9-fluorenone (F) as the donor and the acceptor, respectively], F_0.1CMP , F_0.5CMP , and F_2.0CMP , are reported. These CMPs exhibited tunable bandgaps and photocatalytic hydrogen evolution from water. The donor–acceptor CMPs exhibited also intramolecular charge-transfer (ICT) absorption in the visible region (λ_max=480 nm) and their bandgap was finely tuned from 2.8 to 2.1 eV by increasing the 9-fluorenone content. Interestingly, they also showed emissions in the 540–580 nm range assisted by the energy transfer from the other TPE segments (not involved in charge-transfer interactions), as evidenced from fluorescence lifetime decay analysis. By increasing the 9-fluorenone content the emission color of the polymer was also tuned from green to red. Photocatalytic activities of the donor–acceptor CMPs ( F_0.1CMP , F_0.5CMP , and F_2.0CMP ) are greatly enhanced compared to the 9-fluorenone free polymer ( F_0.0CMP ), which is essentially due to improved visible-light absorption and low bandgap of donor–acceptor CMPs. Among all the polymers F_0.5CMP with an optimum bandgap (2.3 eV) showed the highest H₂ evolution under visible-light irradiation. Moreover, all polymers showed excellent dispersibility in organic solvents and easy coated on the solid substrates.     

Fluorescence resonance energy transfer between unnatural amino acids in a structurally modified dihydrofolate reductase

The cleavage of a substrate protein by HIV-1 protease has been monitored in real time by the use of a dihydrofolate reductase fusion protein in which a fluorescence donor and a fluorescence acceptor were introduced into sites flanking the HIV-1 protease cleavage site. The amino acids 7-azatryptophan and dabcyl-1,2-diaminopropionic acid were introduced into specific sites of the DHFR fusion protein in an in vitro protein biosynthesizing system using two misacylated suppressor tRNAs, each of which recognized a specific, unique codon introduced into the mRNA. Excitation of the fluorescence acceptor in the initially expressed protein afforded no light production, consistent with quenching by fluorescence resonance energy transfer. Treatment of the elaborated protein with HIV-1 protease cleaved the protein between the fluorescence donor and acceptor, affording a time-dependent increase in fluorescence that was equal in magnitude to that produced by admixture of a stoichiometric amount of free 7-azatryptophan to the solution containing the intact protein.     

A Novel Fluorescent Probe for Lead Ions

A new fluorescent probe for lead ions, p-nitrophenyl 3H-phenoxazin-3-one-7-yl phosphoric acid (NPPA), has been synthesized by linking resorufLn (serving as a fluorophore and electron acceptor) to p-nitrophenol (serving as a fluorescence quencher and electron donor) through phosphodiester bonds. When NPPA was irradiated with light, intramolecular fluorescence self-quenching took place due to the PET (photoinduced electron transfer) from the donor to the acceptor. However, upon addition of Pb^Ⅱ, the phosphate ester bonds in the probe were cleaved and the fluorophore was released, accompanying the retrievement of fluorescence.     

Interlayer triplet-triplet energy transfer in nanosized molecular layers

The results of the study of interlayer triplet-triplet energy transfer from anthracene molecules to Nile Red molecules in Langmuir-Blodgett films are presented. The observed sensitized delayed fluorescence of the energy acceptor is shown to be due to annihilation of migrating triplet excitons. It has been found that the decay kinetics of delayed fluorescence of the donor and the acceptor has a complex form and is described by a combination of the power and exponential functions. The dependence of the energy transfer efficiency on the distance between the donor and acceptor layers was studied.     

A New Robust and Highly Sensitive FRET Donor–Acceptor Pair: Synthesis,Characterization, and Application in a Thrombin Assay

The synthesis of a new, robust fluorescence‐resonance‐energy‐transfer (FRET) system is described. Its donor chromophore is derived from an N‐allyl‐substituted quinolinone attached to 4‐bromophenylalanine via Heck cross‐coupling. The resulting Fmoc‐protected derivative 11 was used as building block in solid‐phase peptide synthesis (SPPS). As FRET acceptor, a sulfonylated ruthenium(II)–bathophenanthroline complex with a peripheral COOH function was prepared for covalent attachment to target molecules. The UV/VIS absorption and emission spectra of peptides bearing only the donor (D) or acceptor (A) dye showed a good overlap of the emission band of the donor with the absorption band of the acceptor. The fluorescence spectra of a peptide bearing both dyes revealed an additional emission after excitation of the donor, which is due to indirect excitation of the acceptor via FRET. The long fluorescence lifetime of the Ru^II complex (0.53 μs) makes it well‐suited for time‐resolved measurements. As a first application of this new FRET system, the peptide 18 , with the recognition sequence for the protease thrombin, flanked by the two dyes, was synthesized and successfully cleaved by the enzyme. The change in the ratio of the fluorescence intensities could be determined.     

Electronic excitation energy transfer between molecules of carbocyanine dyes in complexes with DNA

Electronic excitation energy transfer has been carried out between molecules of carbocyanine dyes bound noncovalently to DNA. 3,3′,9-Triethyl-5,5′-dimethyloxacarbocyanine iodide was used as an energy donor and 3,3′-diethylthiacarbocyanine iodide as an acceptor dye. In this process, the band belonging to the donor is observed in the fluorescence excitation spectrum of the acceptor. Donor fluorescence quenching by the acceptor in the presence of DNA was studied. The results of the experiments are discussed in terms of the Dye-DNA stoichiometric complex formation and with respect to concentrating the dyes in the microphase (pseudophase) of the biopolymer.     

A novel FRET approach for in situ investigation of cellulase–cellulose interaction

A novel real-time in situ detection method for the investigation of cellulase–cellulose interactions based on fluorescence resonance energy transfer (FRET) has been developed. FRET has been widely used in biological and biophysical fields for studies related to proteins, nucleic acids, and small biological molecules. Here, we report the efficient labeling of carboxymethyl cellulose (CMC) with donor dye 5-(aminomethyl)fluorescein and its use as a donor in a FRET assay together with an Alexa Fluor 594 (AF594, acceptor)–cellulase conjugate as acceptor. This methodology was successfully employed to investigate the temperature dependency of cellulase binding to cellulose at a molecular level by monitoring the fluorescence emission change of donor (or acceptor) in a homogeneous liquid environment. It also provides a sound base for ongoing cellulase–cellulose study using cellulosic fiber.     

Three-dimensional orientational colocalization of individual donor--acceptor pairs

We report on the determination of the three-dimensional orientation of the donor and acceptor transition dipoles in individual fluorescence resonance energy transfer (FRET) pairs by means of scanning optical microscopy with annular illumination. Knowledge of the mutual orientation of the donor and acceptor dipole is mandatory for reliable distance determination based on FRET efficiency measurements. In our model system perylenediimide as the donor and terryelenediimide as the acceptor are coupled via a stiff p-terphenyl linker. The absorption dipoles of the donor and acceptor are selectively addressed by the 488 nm and 647 line of an Ar/Kr mixed gas laser, respectively. A clear deviation from collinearity is observed with a distribution of misalignment angles peaked around 22 degrees.