Solute diffusion in relation to the glass transition temperature of solute-polymer blends

The transport of various dye solute molecules from amorphous donor polymer films to an amorphous acceptor polymer film has been investigated. Dye diffusion was studied by laminating dye-donor and dye-acceptor films under controlled pressure and temperature. The acceptor medium was kept constant, whereas a wide range of polymer structures were used as binders in the donor system. At constant dye concentration, the transport of the dye molecules from the donor sheet was found to be controlled by the glass transition temperature of the dye-polymer mixture in the donor film. This relationship was ubiquitous for all types of chromophore and polymer studied. The data was found to fit well with free volume considerations and the Williams-Landel-Ferry equation by using the glass transition as that of the dye-polymer blend in the donor matrix.     

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.     

Photoinduced intramolecular charge transfer (ICT) reaction in trans-methyl p-(dimethylamino) cinnamate: A combined fluorescence measurement and quantum chemical calculations

The photophysical behaviour of trans-methyl p-(dimethylamino) cinnamate (t-MDMAC) donor–acceptor system has been investigated by steady-state absorption and emission spectroscopy and quantum chemical calculations. The molecule t-MDMAC shows an emission from the locally excited state in non-polar solvents. In addition to weak local emission, a strong solvent dependent red shifted fluorescence in polar aprotic solvents is attributed to highly polar intramolecular charge transfer state. However, the formation of hydrogen-bonded clusters with polar protic solvents has been suggested from a linear correlation between the observed red shifted fluorescence band maxima with hydrogen bonding parameters (). Calculations by ab initio and density functional theory show that the lone pair electron at nitrogen center is out of plane of the benzene ring in the global minimum ground state structure. In the gas phase, a potential energy surface along the twist coordinate at the donor (–NMe₂) and acceptor (–CH = CHCOOMe) sites shows stabilization of S₁ state and destabilization S₂ and S₀ states. A similar potential energy calculation along the twist coordinate in acetonitrile solvent using non-equilibrium polarized continuum model also shows more stabilization of S₁ state relative to other states and supports solvent dependent red shifted emission properties. In all types of calculations it is found that the nitrogen lone pair is delocalized over the benzene ring in the global minimum ground state and is localized on the nitrogen centre at the 90° twisted configuration. The S₁ energy state stabilization along the twist coordinate at the donor site and localized nitrogen lone pair at the perpendicular configuration support well the observed dual fluorescence in terms of proposed twisted intramolecular charge transfer (TICT) model.     

Design and characterization of two-dye and three-dye binary fluorescent probes for mRNA detection

We report the design, synthesis, and characterization of binary oligonucleotide probes for mRNA detection. The probes were designed to avoid common problems found in standard binary probes such as direct excitation of the acceptor fluorophore and overlap between the donor and acceptor emission spectra. Two different probes were constructed that contained an array of either two or three dyes and were characterized using steady-state fluorescence spectroscopy, time-resolved fluorescence spectroscopy, and fluorescence depolarization measurements. The three-dye binary probe (BP-3d) consists of a Fam fluorophore which acts as a donor, collecting light and transferring it as energy to Tamra, which subsequently transfers energy to Cy5 when the two probes are hybridized to mRNA. This design allows the use of 488 nm excitation, which avoids the direct excitation of Cy5 and at the same time provides a good fluorescence resonance energy transfer (FRET) efficiency. The two-dye binary probe system (BP-2d) was constructed with Alexa488 and Cy5 fluorophores. Although the overlap between the fluorescence of Alexa488 and the absorption of Cy5 is relatively low, FRET still occurs due to their close physical proximity when the probes are hybridized to mRNA. This framework also decreases the direct excitation of Cy5 and reduces the fluorescence overlap between the donor and the acceptor. Picosecond time-resolved spectroscopy showed a reduction in the fluorescence lifetime of donor fluorophores after the formation of the hybrid between the probes and target mRNA. Interestingly, BP-2d in the presence of mRNA shows a slow rise in the fluorescence decay of Cy5 due to a relatively slow FRET rate, which together with the reduction in the Alexa488 lifetime provides a way to improve the signal to background ratio using time-resolved fluorescence spectra (TRES). In addition, fluorescence depolarization measurements showed complete depolarization of the acceptor dyes (Cy5) for both BP-3d (due to sequential FRET steps) and BP-2d (due to the relatively low FRET rate) in the presence of the mRNA target.     

Light Harvesting and Amplification of Emission of Donor Perylene–Acceptor Perylene Aggregates in Aqueous Medium

With the aid of rational design, we have synthesized a pair of water‐soluble donor‐ and acceptor‐type twisted perylene bisimide units, which together form aggregates upon lowering the pH of the medium, providing bright yellow fluorescence. The light‐harvesting efficiency of the co‐assembled system can be tuned by controlling the ratio of donor to acceptor and 98.1 % efficiency has been achieved.     

The determination of the Förster distance (R0) for phenanthrene and anthracene derivatives in poly(methyl methacrylate) films

Experiments that employ direct resonance energy transfer (DET) to obtain information about distances or domain sizes in polymer systems require independent information about the magnitude of the characteristic (F?rster) energy transfer distance R(0). Values of R(0) are relatively straightforward to obtain by the traditional spectral overlap method (R(0)(SO)) for dyes in fluid solution, but are much more difficult to obtain for dyes in rigid polymer films. Here one can obtain a value for R(0) as a fitting parameter (R(0)(FF)) for donor fluorescence decay experiments for samples containing a random distribution of donor and acceptor dyes in the polymer film. In previous experiments from our group, we needed values of R(0) for various phenanthrene (Phe, donor) and anthracene (An, acceptor) derivatives. In this paper, we describe experiments which determine R(0) values by both methods for a series of Phe-An donor-acceptor pairs in poly(methyl methacrylate) and polystyrene films. Both the location of substituents on the donor and acceptor as well as the choice of the medium had an effect on the measured R(0), which varied between 2.0 and 2.6 nm. We also ascertained that there is some unknown factor, also prevalent in the work of others, which results in the F?rster radius being larger when determined by the F?rster fit method than by the method of spectral overlap.     

Porphyrin‐Ryleneimide Hybrids: Low‐Bandgap Acceptors in Energy‐Transfer Cassettes

Energy‐transfer cassettes consisting of naphthaleneimide‐fused metalloporphyrin acceptors (M=Zn and Pd) and BODIPY donors have been designed and synthesized. These systems have rigid pseudo‐tetrahedral structures with a donor‐acceptor separation of ca. 17.5 Å. Spectroscopic investigations, including femtosecond transient absorption measurements, showed efficient excitation energy transfer (EET) occurring according to the Förster mechanism. Strong fluorescence of the donor units and significant spectral overlap of the donor and acceptor subunits are prerequisites for the efficient EET in these systems.     

FRET-based modified graphene quantum dots for direct trypsin quantification in urine

A versatile nanoprobe was developed for trypsin quantification with fluorescence resonance energy transfer (FRET). Here, fluorescence graphene quantum dot is utilized as a donor while a well-designed coumarin derivative, CMR2, as an acceptor. Moreover, bovine serum albumin (BSA), as a protein model, is not only served as a linker for the FRET pair, but also a fluorescence enhancer of the quantum dots and CMR2. In the presence of trypsin, the FRET system would be destroyed when the BSA is digested by trypsin. Thus, the emission peak of the donor is regenerated and the ratio of emission peak of donor/emission peak of acceptor increased. By the ratiometric measurement of these two emission peaks, trypsin content could be determined. The detection limit of trypsin was found to be 0.7 μg/mL, which is 0.008-fold of the average trypsin level in acute pancreatitis patient's urine suggesting a high potential for fast and low cost clinical screening.     

Optimization of the molecular orbital energies of conjugated polymers for optical amplification of fluorescent sensors

Cationic water-soluble poly(fluorene-co-phenylene)s with electron withdrawing or donating substituents on the conjugated backbone were designed and synthesized. Fluorescence resonance energy transfer (FRET) experiments between these conjugated polymers and dye-labeled single-stranded DNA (ssDNA-C*) reveal the importance of matching donor and acceptor orbital energy levels to improve the sensitization of C* emission. Quenching of polymer fluorescence with ssDNA-C* and differences in C* emission suggest involvement of photoinduced charge transfer (PCT) as an energy wasting mechanism. The HOMO and LUMO energy levels of the conjugated polymers and C serve as a preliminary basis to understand the competition between FRET and PCT. Dilution of C in polymer/ssDNA-C complexes by addition of ssDNA yields insight into C*...C self-quenching. Under optimized conditions, where there is no probe self-quenching and minimum PCT, efficient signal amplification is demonstrated despite poor spectral overlap between polymer and C.     

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.     

Promotion of Förster Resonance Energy Transfer in a Saponite Clay Containing Luminescent Polyhedral Oligomeric Silsesquioxane and Rhodamine Dye

A new hybrid photostable saponite clay with embedded donor–acceptor dyes was prepared and characterized in this work. The saponite is intercalated with a luminescent polyhedral oligomeric silsesquioxane, which transfers the photoexcitation energy directly to an acceptor dye (rhodamine B). The obtained composite material was characterized by means of XRD, TEM microscopy, and UV/Vis and photoluminescence spectroscopy. A physicochemical study showed that the system behaved as an efficient Förster resonance energy transfer pair, owing to the very good spectral overlap of donor emission (λ_em=510–540 nm) and acceptor absorption in the λ=530–570 nm range. The hybrid material represents the first example of a photonic antenna based on a synthetic saponite clay and can be considered a step forward in the search for new, efficient, and stable materials suitable for light‐harvesting applications.     

羧脒盐桥介导的单重态能量传递直接观察

设计构建了以羧脒盐桥联接的萘和蒽超分子组装体系,NA-(脒基-羧基)-An以及相应的模型体系.稳态和时间分辨荧光光谱研究表明,置于羧脒盐桥两端的萘和蒽基团之间发生了从萘到蒽的单重态能量传递,NA-(脒-羧)-An超分子体系中单重态能量传递效率和速率常数分别大于0.99和9.9×10⁹s^-1.推测羧脒盐桥介导了体系中的单重态能量传递过程,单重态能量传递‘通过键’以电子交换机制进行.     

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.     

Energy transfer--a tool for probing micellar media

The non-ionic polyoxyethylene chain-containing surfactant Triton X-100 (TX-100) forms well-defined micelles and reverse micelles in aqueous and hydrocarbon media, respectively. Nonradiative energy transfer between two charged fluorescent dyes, fluorescein (FL) and acridine orange (AO) has been used to probe the micelles and reverse micelles of TX-100. In the energy transfer system employed, FL acts as the donor and AO as the acceptor. This is borne out by the fluorescence spectral data. Time-resolved studies further corroborate the steady-state results. As the fluorescence emission spectra of the two dyes show a considerable amount of overlap, they are resolved into individual donor and acceptor components using the principal component analysis (PCA) method. This study also focuses on the more important role played by hydrophobic forces (compared with electrostatic interactions) in promoting energy transfer between charged species in micellar media.     

Design and synthesis of core and peripherally functionalized with 1,8-naphthalimide units fluorescent PAMAM dendron as light harvesting antenna

This paper reports on the design, synthesis and spectral characteristics of a novel PAMAM dendron (7), core and peripherally functionalized with 1,8-naphthalimide fluorophores. The novel compound 7 was configured as light harvesting antenna where the system surface is functionalized with “donor” dyes (blue emitting 1,8-naphthalimides) that are capable of absorbing light and efficiently transferring the energy to a single “acceptor” dye (yellow-green emitting 1,8-naphthalimide) in the focal point of the dendron. The overlap between the emission of the donor and the absorbance of the acceptor was more than 95%. As a result of the energy transfer, the blue emission intensity of the periphery in the donor–acceptor system was decreased with 93%, while the yellow-green core fluorescence enhancement (λ_ex = 360 nm) of the system was more than 26 times with respect to the fluorescence intensity of the comparative yellow-green emitting 1,8-naphthalimide. This indicates efficient energy transfer between the donor and acceptor dye fragments and that the novel compound 7 would be able to act as a highly efficient light harvesting antenna.     

Development of a FRET assay for monitoring of HIV gp41 core disruption

The fusogenic core assembly of human immunodeficiency virus type 1 (HIV-1) fusion protein gp41 is a critical transformation for viral entry. Molecules that are able to intercept this process are of great therapeutic value as HIV-1 fusion inhibitors. In the search for such molecules, assay systems that can be adapted to high-throughput screens are valuable. Given that gp41 fusogenic transformation is characterized by the hexameric association of heptads located at the N and C terminal regions of the protein ectodomain, the corresponding heptad peptides (CHR and NHR), known to form the six-helix bundle core of gp41 fusion active form, are potentially useful in developing a fluorescence resonance energy transfer (FRET) system for identification of HIV fusion inhibitors. We demonstrate that by strategically placing two FRET probes on these two peptides, we are able to monitor the intermolecular co-association by fluorescence quenching between the fluorescence donor and acceptor. The utility of the system is that it should be adaptable to high-throughput screening (HTS) toward peptide or small-molecule HIV fusion inhibitors targeting the gp41 core. Herein, we report the design, synthesis, and development of a N- and C- terminal peptide FRET pair for screening of gp41 six-helix bundle disruption.     

CdS量子点与曙红Y间的荧光共振能量转移研究

在水溶液中,量子点与有机荧光染料之间可能发生荧光共振能量转移(FRET)。本文以发射波长470nm的Cd S量子点为供体,曙红Y为受体,建立了Cd S量子点-曙红Y的FRET体系,研究了该体系的FRET参数。该体系受体供体数目比为8,猝灭效率为45.6%,增强效率为20.1%;供体-受体间的距离为4.4nm;临界能量转移距离为2.4nm。     

A novel design method of ratiometric fluorescent probes based on fluorescence resonance energy transfer switching by spectral overlap integral

A ratiometric measurement, namely, simultaneous recording of the fluorescence intensities at two wavelengths and calculation of their ratio, allows greater precision than measurements at a single wavelength, and is suitable for cellular imaging studies. Here we describe a novel method of designing probes for ratiometric measurement of hydrolytic enzyme activity based on switching of fluorescence resonance energy transfer (FRET). This method employs fluorescent probes with a 3'-O,6'-O-protected fluorescein acceptor linked to a coumarin donor through a linker moiety. As there is no spectral overlap integral between the coumarin emission and fluorescein absorption, the fluorescein moiety cannot accept the excitation energy of the donor moiety and the donor fluorescence can be observed. After cleavage of the protective groups by hydrolytic enzymes, the fluorescein moiety shows a strong absorption in the coumarin emission region, and then acceptor fluorescence due to FRET is observed. Based on this mechanism, we have developed novel ratiometric fluorescent probes (1-3) for protein tyrosine phosphatase (PTP) activity. They exhibit a large shift in their emission wavelength after reaction with PTPs. The fluorescence quenching problem that usually occurs with FRET probes is overcome by using the coumarin-cyclohexane-fluorescein FRET cassette moiety, in which close contact of the two dyes is hindered. After study of their chemical and kinetic properties, we have concluded that compounds 1 and 2 bearing a rigid cyclohexane linker are practically useful for the ratiometric measurement of PTPs activity. The design concept described in this paper, using FRET switching by spectral overlap integral and a rigid link that prevents close contact of the two dyes, should also be applicable to other hydrolytic enzymes by introducing other appropriate enzyme-cleavable groups into the fluorescein acceptor.     

Isomers of Dithienocyclopentapyrene-Based Non-Fullerene Electron Acceptors: Configuration Effect on Photoelectronic Properties

One pair of isomers, centrosymmetric anti- Py - 1 and axisymmetric syn- Py - 2 , was designed and synthesized with an acceptor–donor–acceptor (A–D–A) structure by choosing dithienocyclopentapyrene with four 4-hexylphenyl side chains as the D unit, and 2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene) malononitrile as the A unit. In-depth structure–property relationship studies revealed that the isomers have similar UV/Vis absorption, fluorescence emission, and energy levels but significant differences in molecular shape, polarity, and charge mobility. Solution-processed bulk-heterojunction (BHJ) small-molecule organic solar cells with Py - 1 as the electron-acceptor material and PTB7-Th as the electron-donor material exhibit a power conversion efficiency (PCE) of 6.07 %, or 60 % higher than that of Py - 2 (3.7 %), which could be mainly attributed to the higher and more balanced hole/electron mobilities and better phase separation of the Py - 1 -based active layer.     

Crossover from superexchange to hopping as the mechanism for photoinduced charge transfer in DNA hairpin conjugates

The mechanism and dynamics of photoinduced charge separation and charge recombination have been investigated in synthetic DNA hairpins possessing donor and acceptor stilbenes separated by one to seven A:T base pairs. The application of femtosecond broadband pump-probe spectroscopy, nanosecond transient absorption spectroscopy, and picosecond fluorescence decay measurements permits detailed analysis of the formation and decay of the stilbene acceptor singlet state and of the charge-separated intermediates. When the donor and acceptor are separated by a single A:T base pair, charge separation occurs via a single-step superexchange mechanism. However, when the donor and acceptor are separated by two or more A:T base pairs, charge separation occurs via a multistep process consisting of hole injection, hole transport, and hole trapping. In such cases, hole arrival at the electron donor is slower than hole injection into the bridging A-tract. Rate constants for charge separation (hole arrival) and charge recombination are dependent upon the donor-acceptor distance; however, the rate constant for hole injection is independent of the donor-acceptor distance. The observation of crossover from a superexchange to a hopping mechanism provides a "missing link" in the analysis of DNA electron transfer and requires reevaluation of the existing literature for photoinduced electron transfer in DNA.