Lifetime- and Color-Tailored Fluorophores in the Micro- to Millisecond Time Regime

Lanthanide-Based Optical Probes of Biological Systems

The unique photophysical properties of lanthanides, such as europium, terbium, and ytterbium, make them versatile molecular probes of biological systems. In particular, their long-lived photoluminescence, narrow bandwidth emissions, and large Stokes shifts enable experiments that are infeasible with organic fluorophores and fluorescent proteins. The ability of these metal ions to undergo luminescence resonance energy transfer, and photon upconversion further expands the capabilities of lanthanide probes. In this review, we describe recent advances in the design of lanthanide luminophores and their application in biological research. We also summarize the latest detection systems that have been developed to fully exploit the optical properties of lanthanide luminophores. We conclude with a discussion of remaining challenges and new frontiers in lanthanide technologies. The unprecedented levels of sensitivity and multiplexing afforded by rare-earth elements illustrate how chemistry can enable new approaches in biology.

Luminescence of Eu3+ ions in hybrid polymer-inorganic composites based on poly(methyl methacrylate) and zirconia nanoparticles

Spherical nanoparticles of ZrO₂ with 2 and 10 mol% EuO_1.5 up to 20 nm size were prepared by the method of hydrothermal synthesis for luminescent functionalization of the polymer-inorganic nanocomposites based on poly(methyl methacrylate). Surface modification of oxide nanoparticles was carried out by 3-(trimethoxysilyl)propyl methacrylate, dimethoxymethylvinyl silane and 2-hydroxyethyl methacrylate to provide uniform distribution and to prevent agglomeration of nanosized filler in the polymer matrix. Polymer-inorganic composites were synthesized by in situ free radical polymerization in bulk. Structuring of ZrO₂ -EuO_1.5 nanoparticles in the poly(methyl methacrylate) was studied by very-small-angle neutron scattering. According to the results, the dependence of photoluminescent properties of ZrO₂ -EuO_1.5 nanoparticles on the content of lanthanide, the symmetry of the crystal field, surface treatment and the polymer matrix were established. A correlation was shown between Stark splitting in luminescence spectra of ZrO₂ -EuO_1.5 nanoparticles and their phase composition. Using MMT-assay it was shown that composites based on poly(methyl methacrylate) and ZrO₂ -EuO_1.5 nanoparticles do not have cytotoxic properties, which makes it possible to use them as prosthesis materials with contrasted and luminescent imaging properties.

Ultrasensitive optical imaging with lanthanide lumiphores

In principle, the millisecond emission lifetimes of lanthanide chelates should enable their ultrasensitive detection in biological systems by time-resolved optical microscopy. In practice, however, lanthanide imaging techniques have provided no better sensitivity than conventional fluorescence microscopy. Here, we identified three fundamental problems that have impeded lanthanide microscopy: low photon flux, inefficient excitation, and optics-derived background luminescence. We overcame these limitations with a new lanthanide imaging modality, transreflected illumination with luminescence resonance energy transfer (trLRET), which increases the time-integrated signal intensities of lanthanide lumiphores by 170-fold and the signal-to-background ratios by 75-fold. We demonstrate that trLRET provides at least an order-of-magnitude increase in detection sensitivity over that of conventional epifluorescence microscopy when used to visualize endogenous protein expression in zebrafish embryos. We also show that trLRET can be used to optically detect molecular interactions in vivo. trLRET promises to unlock the full potential of lanthanide lumiphores for ultrasensitive, autofluorescence-free biological imaging.

Efficient route to pre-organized and linear polyaminopolycarboxylates: Cy-TTHA, Cy-DTPA and mono/di- reactive, tert-butyl protected TTHA/Cy-TTHA

Pre-organized polyaminopolycarboxylate chelators Cy-TTHA and Cy-DTPA were synthesized via modular five-step syntheses from commercially available starting materials in ~ 62% and 47% overall yields, respectively. Furthermore, strategies are reported for the efficient preparation of mono- and di-reactive, tert-butyl-protected TTHA/Cy-TTHA to selectively functionalize central chelators' carboxylic acids.

Long-lived luminogenic probe for detection of RNA in a crude solution of living bacterial cells

A pre-type sensitizer for a lanthanide complex on an oligonucleotide was successfully converted to a perfect final structure in a target DNA/RNA-templated reaction, without any chemical reagent or enzyme, under neutral conditions. The final form of the lanthanide-oligonucleotide provided a long-lived luminescence signal, appropriate for time-gated luminescence analysis and signal amplification. Target DNA/RNA-assisted time-gated luminescence analysis is a powerful tool for elimination of autofluorescence and detection of target RNA in living bacterial cells.

A versatile long-wavelength-absorbing scaffold for Eu-based responsive probes

Coumarin-sensitized, long-wavelength-absorbing luminescent Eu(III)-complexes have been synthesized and characterized. The lanthanide binding site consists of a cyclen-based chelating framework that is attached through a short linker to a 7-hydroxycoumarin, a 7-B(OH)(2)-coumarin, a 7-O-(4-pinacolatoboronbenzyl)-coumarin or a 7-O-(4-methoxybenzyl)-coumarin. The syntheses are straightforward, use readily available building blocks, and proceed through a small number of high-yielding steps. The sensitivity of coumarin photophysics to the 7-substituent enables modulation of the antenna-absorption properties, and thus the lanthanide excitation spectrum. Reactions of the boronate-based functionalities (cages) with H(2)O(2) yielded the corresponding 7-hydroxycoumarin species. The same species was produced with peroxynitrite in a ×10(6)-10(7)-fold faster reaction. Both reactions resulted in the emergence of a strong ≈407 nm excitation band, with concomitant decrease of the 366 nm band of the caged probe. In aqueous solution the methoxybenzyl caged Eu-complex was quenched by ONOO(-). We have shown that preliminary screening of simple coumarin-based antennae through UV/Vis absorption spectroscopy is possible as the changes in absorption profile translate with good fidelity to changes in Eu(III)-excitation profile in the fully elaborated complex. Taken together, our results show that the 7-hydroxycoumarin antenna is a viable scaffold for the construction of turn-on and ratiometric luminescent probes.

Monitoring a coordinated exchange process in a four-component biological interaction system: development of a time-resolved terbium-based one-donor/three-acceptor multicolor FRET system

Hormonal regulation of cellular function involves the binding of small molecules with receptors that then coordinate subsequent interactions with other signal transduction proteins. These dynamic, multicomponent processes are difficult to track in cells and even in reconstituted in vitro systems, and most methods can monitor only two-component interactions, often with limited capacity to follow dynamic changes. Through a judicious choice of three organic acceptor fluorophores paired with a terbium donor fluorophore, we have developed the first example of a one-donor/three-acceptor multicolor time-resolved fluorescence energy transfer (TR-FRET) system, and we have exemplified its use by monitoring a ligand-regulated protein-protein exchange process in a four-component biological system. By careful quantification of the emission from each of the three acceptors at the four channels for terbium donor emission, we demonstrate that any of these donor channels can be used to estimate the magnitude of the three FRET signals in this terbium-donor triple-acceptor system with minimal bleedthrough. Using this three-channel terbium-based, TR-FRET assay system, we show in one experiment that the addition of a fluorescein-labeled estrogen agonist displaces a SNAPFL-labeled antiestrogen from the ligand binding pocket of a terbium-labeled estrogen receptor, at the same time causing a Cy5-labeled coactivator to be recruited to the estrogen receptor. This experiment demonstrates the power of a four-color TR-FRET experiment, and it shows that the overall process of estrogen receptor ligand exchange and coactivator binding is a dynamic but precisely coordinated process.

Synthesis and characterization of coumarin-based europium complexes and luminescence measurements in aqueous media

A series of new ligands suitable for the formation of luminescent lanthanide complexes in water is described. The chelates are designed for analyte labeling and play the role of fluorescent donor in homogeneous time-resolved fluorescence assays using LEDs as a light source for excitation at 370 nm. Ligands are constructed from a coumarin nucleus, for lanthanide sensitization, and different aminomethylenecarboxy moieties are introduced in positions 7 and 5, 6, or 8 of the sensitizer. A reactive spacer arm under biocompatible conditions (maleimide, azide) is introduced at position 3 for ultimate bioconjugation purposes. The synthesis and characterization of the ligands are described, together with the preparation of their corresponding europium complexes. Photophysical properties of the complexes are investigated in water by means of UV-vis and luminescence spectroscopy.

Thermodynamic Parameters Governing the Self-Assembly of Head–Head–Head Lanthanide Bimetallic Helicates

The heterobitopic ligands L ABX (X=1, 2, 3, 4 or 5), differing only by a Cl or NEt(2) substituent, have been designed to complex with a pair of lanthanide ions to form triple-stranded bimetallic helicates of overall composition [Ln2(L ABX)3]6+. The percentage of HHH (head-head-head) isomer, in which each of the three ligand strands coordinates to the same lanthanide ion with the same coordination unit, is deciding the ability of the ligands to selectively form heterobimetallic complexes containing one luminescent and one magnetic or two different luminescent ions. It deviates significantly from the statistical value of 25 % and ranges from 6-20 % for L AB2 complexes to 93-96 % for L AB4 complexes. The equilibrium between HHT (head-head-tail) and HHH isomers has been investigated in detail for homobimetallic helicates (Ln=Y, La, Ce, Pr, Nd, Sm, Eu, Lu) by means of variable temperature NMR and thermodynamic parameters have been determined. The equilibrium is characterized by small values of DeltaH and DeltaS, which vary in opposite direction along the lanthanide series for complexes with the same ligand in a way that keeps the value of DeltaG almost constant. The results are interpreted in terms of differences in interstrand stacking, ion-dipole interactions and metal-metal repulsion.

Europium tetracycline as a luminescent probe for nucleoside phosphates and its application to the determination of kinase activity

The determination of enzyme activities and the screening of enzyme regulators is a major task in clinical chemistry and drug development. A broad variety of enzymatic reactions is associated with the consumption of adenosine triphosphate (ATP), including, in particular, phosphorylation reactions catalyzed by kinases, formation of adenosine cyclic monophosphate (cAMP) by adenylate cyclases, and ATP decomposition by ATPase. We have studied the effect of a series of adenosine (ATP, ADP, AMP, cAMP) and guanosine (GTP, GDP) phosphoric esters, and of pyrophosphate (PP) on the fluorescence emission of the europium tetracycline (EuTC) complex. We found that these compounds have strongly different quenching effects on the luminescence emission of EuTC. The triphosphates ATP and GTP behave as strong quenchers in reducing the fluorescence intensity of EuTC to 25 % of its initial value by formation of a ternary 1:1:1 complex. All other phosphate esters showed a weak quenching effect only. The applicability of this fluorescent probe to the determination of the activity of phosphorylation enzymes is demonstrated by means of creatine kinase as a model for non-membrane-bound kinases. In contrast to other methods, this approach does not require the use of radioactively labeled ATP substrates, additional enzymes, or of rather complex immunoassays.

Lanthanide Complexes of a Picolinate Ligand Derived from 1,4,7-Triazacyclononane with Potential Application in Magnetic Resonance Imaging and Time-Resolved Luminescence Imaging

The new potentially octadentate ligand, 1-(carboxymethyl)-4,7-bis[(6-carboxypyridin-2-yl)methyl]-1,4,7-triazacyclononane (H(3)bpatcn), in which two picolinate arms and one acetate arm are connected to the 1,4,7-triazacyclonane core, has been prepared. Potentiometric studies show an increased stability of the Gd(III) complex of H(3)bpatcn (logK(GdL)=15.8(2)) with respect to the Gd(III) complex of the analogous ligand 1,4,7-triazacyclononane-N,N',N''-triacetic acid (H(3)nota) (logK(GdL)=13.7), associated with an increased selectivity of H(3)bpatcn for gadolinium over calcium. The H(3)bpatcn ligand sensitises the terbium ion very efficiently, leading to a long-lived and highly luminescent terbium complex (quantum yield=43 %), in spite of the presence of a coordinated water molecule. (1)H proton NMR studies indicate that the metal ion is rigidly encapsulated by the three arms of the octadentate ligand H(3)bpatcn and that the macrocycle framework remains bound (through the five nitrogen and the three oxygen atoms) even at high temperature. A new theoretical method for interpreting the water proton relaxivity is presented. It is based on recent progresses in the description of the electronic spin relaxation and on an auxiliary probe solute. It replaces the Solomon, Bloembergen and Morgan (SBM) framework, which is questionable at low field, while avoiding resorting to simulations and/or sophisticated theories with additional unknown zero-field splitting (ZFS) parameters. The inclusion of two picolinate groups on a triazacyclononane framework affords the mono-aquo gadolinium complex [Gd(bpatcn)(H(2)O)] with favourable electron-relaxation properties (tau(eff)(S0)=125 ps). The optimisation of the electronic relaxation by ligand design is especially important to achieve high relaxivity in the new generation macromolecular complexes with long rotational correlation times.

Urea hydrogen peroxide determination in whole blood using europium tetracycline probe

We introduce the use of a lanthanide complex, tetracycline-europium, for the clinical diagnosis of urea hydrogen peroxide in human whole blood. The values obtained agree with the urea concentration variation verified in 49 patients, including 12 predialysis, 12 peritoneal, and 15 dialysis subjects, and 10 controls. This method is noninvasive and can help in the identification of renal and cardiac diseases.

Engineering emissive europium and terbium complexes for molecular imaging and sensing

Emissive f-block coordination complexes constitute an important class of optical probes, with applications ranging from sensing of bioactive species, high throughput assays and screening protocols in vitro, to time-resolved imaging studies in cellulo or in vivo. The key chemistry issues to be addressed in complex design and characterisation are defined, with an emphasis on the use of emissive europium and terbium complexes and their conjugates in molecular imaging. Both luminescent 'tags' useful in energy transfer studies and 'responsive' systems for sensing are discussed.

Fluorescence quenching of the europium tetracycline hydrogen peroxide complex by copper (II) and other metal ions

The europium-tetracycline complex [Eu(Tc)] is known to show only weak fluorescence with an emission maximum at 615 nm. On addition of hydrogen peroxide (HP), the strongly fluorescent [Eu(Tc)(HP)] complex is formed, which displays a 15-fold stronger luminescence intensity. This study describes the decrease in luminescence intensity of the [Eu(Tc)(HP)] complex in aqueous solution in the presence of Cu2+, Fe3+, Ag+, Al3+, Zn2+, Co2+, Ni2+, Mn2+, Ca2+, and Mg2+. Static and dynamic quenching can be induced by Cu2+, and these processes were quantified by means of their quenching constants. Stern-Volmer plots were also derived from lifetime imaging measurements accomplished by the rapid lifetime determination (RLD) technique based on microwell plate assays, and also by the time-correlated single photon counting (TCSPC) technique. According to those data, a time-resolved fluorescent method for copper determination can be proposed that is based on dynamic quenching of the [Eu(Tc)(HP)] complex by Cu2+ ions. The response to copper concentrations is linear up to 1.6 micromol L(-1), providing a detection limit of 0.2 micromol L(-1).

Progress in Lanthanides as Luminescent Probes

Lanthanides have recently found applications in different fields of biomolecular and medical research. Luminescent lanthanide chelates have created interest mainly due to their unique luminescent properties, such as their long Stokes' shift and exceptional decay times allowing efficient temporal discrimination of background interferences in the assays, such as immunoassays. Recently, new organometallic complexes have been developed giving opportunities to novel applications, in heterogeneous and homogeneous immunoassays, DNA hybridization assays, high-throughput screening as well as in imaging. In addition, encapsulating the chelates into suitable matrix in beads enables the use of new members of lanthanides extending the emission wavelength to micrometer range and decays from a few microseconds to milliseconds. As the luminescence is derived from complicated intra-chelate energy transfer, it also gives novel opportunities to exploit these levels in different types of energy transfer based applications. This review gives a short overview of recent development of lanthanide chelate-labels and discusses in more details of energy levels and their exploitation in new assay formats.

Supramolecular Recognition of Heteropairs of Lanthanide Ions: A Step toward Self-Assembled Bifunctional Probes

Three unsymmetrical ditopic hexadentate ligands coded for the recognition of trivalent lanthanide ions have been synthesized, L(AB), L(AC), and L(BC), where A represents a benzimidazole-pyridine-benzimidazole coordination unit, B a benzimidazole-pyridine-carboxamide one, and C a benzimidazole-pyridine-carboxylic acid moiety. Under stoichiometric 2:3 (Ln:L) conditions, these ligands self-assemble with lanthanide ions to yield triple-stranded bimetallic helicates having a sizable stability in acetonitrile: log beta(23) values for Eu are equal to 23.9 +/- 0.5 (L(AB)), 23.3 +/- 0.7 (deprotonated L(AC)), and 29.8 +/- 0.5 (deprotonated L(BC)). The crystal structure of the EuEu helicate with L(AB) shows 9-coordinate metal ions and an HHH (H stands for head) configuration of the helically wrapped ligand strands. In the presence of equimolar quantities of Ln and Ln' ions, L(AB) displays a remarkable predisposition to form HHH-heterobimetallic edifices, as proved both in the solid state by the crystal structures of the LaEu, LaTb, PrEr, and PrLu helicates and in solution by NMR spectroscopy. In all cases, the benzimidazole-pyridine-carboxamide units of the three ligands are bound to the smaller lanthanide ion, a fact further ascertained by high-resolution luminescence data on LaEu and by (1)H NMR. Analysis of the lanthanide-induced (1)H NMR shifts and of the spin-lattice relaxation times of the [LnLu(L(AB))(3)](6+) series (Ln = Ce, Pr, Nd, Sm, Eu) demonstrates the isostructural nature of the complexes in solution and that the crystal structure of LaTb is a good model for the solution structure. The selectivity of L(AB) for heteropairs of Ln(III) ions increases with increasing difference in ionic radius, resulting in 70% of the heterobimetallic species for deltar(i) = 0.1 A and up to 90% for LaLu (deltar(i) = 0.18 A), and corresponding to delta(deltaG) in the range 3-10 kJ.mol(-)(1). The origins of this stabilization are discussed in terms of the donor properties of the coordinating units and of the preferential formation of HHH isomers.

Lengthening of fluorescence lifetimes in self-organized metal-organic assemblies

Resonance energy transfer from tris(2,2'-bipyridyl)ruthenium (II) ([Ru(bpy)3](2+)) to nile blue A is demonstrated in aqueous solution in the presence of sodium dodecyl sulfate (SDS). At SDS concentrations below the critical micelle concentration, aggregates that permit energy transfer between these dyes at optically dilute (10 microM) concentrations with nearly 100% efficiency are formed. The disparity between the lifetimes of the donor and acceptor results in the lengthening of the photoluminescence lifetime of the sensitized emission observed from nile blue A. Time-resolved luminescence measurements confirm that the long-lived components of the emission originate from sensitized acceptor emission.

Real-time background suppression during frequency domain lifetime measurements

We describe real time background suppression of autofluorescence from biological samples during frequency domain or phase modulation measurements of intensity decays. For these measurements the samples were excited with a train of light pulses with widths below 1 ps. The detector was gated off for a short time period of 10 to 40 ns during and shortly after the excitation pulse. The reference signal needed for the frequency domain measurement was provided by a long-lifetime reference fluorophore which continues to emit following the off-gating pulse. Both the sample and the reference were measured under identical optical and electronic conditions avoiding the need for correction of the photomultiplier tube signal for the gating sequence. We demonstrate frequency domain background suppression using a mixture of short- and long-lifetime probes and for a long-lifetime probe in human plasma with significant autofluorescence.

Principles and biophysical applications of lanthanide-based probes

Using luminescent lanthanides, instead of conventional fluorophores, as donor molecules in resonance energy transfer measurements offers many technical advantages and opens up a wide range of new applications. Advantages include farther measurable distances ( approximately 100 A) with greater accuracy, insensitivity to incomplete labeling, and the ability to use generic relatively large labels, when necessary. Applications highlighted include the study of ion channels in living cells, protein-protein interaction in cells, DNA-protein complexes, and high-throughput screening assays to measure peptide dimerization associated with DNA transcription factors and ligand-receptor interactions.

Highly luminescent, visible-emitting lanthanide macrocyclic chelates stable in water and derived from the cyclen framework

Two new tetraazamacrocyclic ligands are designed with the aim of sensitizing the luminescence of Tb(III) and Eu(III) ions in water: L5 [1,4,7,10-tetrakis[N-(phenacyl)carbamoylmethyl]-1,4,7,10-tetraazacyclododecane] and L6 [1,4,7,10-tetrakis[N-(4-phenylphenacyl)carbamoylmethyl]-1,4,7,10-tetraazacyclododecane]. These ligands react with lanthanide trifluoromethanesulfonates to yield stable 1:1 complexes in water (log K = 12.89 +/- 0.15 for EuL5). X-ray diffraction on [Tb(L5)(H(2)O)](CF(3)SO(3))(3) (P1 macro, a = 13.308(3) A, b = 14.338(3) A, c = 16.130(3) A, alpha = 101.37(3) degrees, beta = 96.16(3) degrees, gamma = 98.60(3) degrees ) shows the Tb(III) ion lying on a C(4) axis and being 9-coordinate, with one water molecule bound in its inner coordination sphere. The absolute quantum yields are determined in aerated water for the complexes formed with ions used in fluoroimmunoassays (Ln = Sm, Eu, Tb, and Dy). Large values are found for [Tb(H(2)O)(L5)](3+) and [Eu(H(2)O)(L6)](3+), in line with the molecular design of the receptors: 23.1% and 24.7%, respectively. The intense luminescence of these ions results from efficient intersystem crossing and L --> Ln energy transfer processes, as well as from a suitable shielding of the emitting ions from radiationless deactivation.

Lateral diffusion coefficients in membranes measured by resonance energy transfer and a new algorithm for diffusion in two dimensions

We describe measurements of lateral diffusion in membranes using resonance energy transfer. The donor was a rhenium (Re) metal-ligand complex lipid, which displays a donor decay time near 3 micros. The long donor lifetime resulted in an ability to measure lateral diffusion coefficient below 10(-8) cm(2)/s. The donor decay data were analyzed using a new numerical algorithm for calculation of resonance energy transfer for donors and acceptors randomly distributed in two dimensions. An analytical solution to the diffusion equation in two dimensions is not known, so the equation was solved by the relaxation method in Laplace space. This algorithm allows the donor decay in the absence of energy transfer to be multiexponential. The simulations show that mutual lateral diffusion coefficients of the donor and acceptor on the order of 10(-8) cm(2)/s are readily recovered from the frequency-domain data with donor decay times on the microsecond timescale. Importantly, the lateral diffusion coefficients and acceptor concentrations can be recovered independently despite correlation between these parameters. This algorithm was tested and verified using the donor decays of a long lifetime rhenium lipid donor and a Texas red-lipid acceptor. Lateral diffusion coefficients ranged from 4.4 x 10(-9) cm(2)/s in 1,2-dimyristoyl-sn-glycero-3-[phospho-rac-(1-glycerol)] (DMPG) at 10 degrees C to 1.7 x 10(-7) cm(2)/s in 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) at 35 degrees C. These results demonstrated the possibility of direct measurements of lateral diffusion coefficients using microsecond decay time luminophores.

RNA conformation and folding studied with fluorescence resonance energy transfer

Fluorescence resonance energy transfer (FRET) results from nonradiative coupling of two fluorophores and reports on distances in the range 10-100 A. It is therefore a suitable probe to determine distances in RNA molecules and define their global structure, to follow kinetics of RNA conformational changes during folding in real time, to monitor ion binding, or to analyze conformational equilibria and assess the thermodynamic stability of tertiary structure conformers. Along with the basic principles of steady-state and time-resolved fluorescence resonance energy transfer measurements, approaches to investigate RNA conformational transitions and folding are described and illustrated with selected examples. The versatility of FRET-based techniques has recently been demonstrated by implementations of FRET in high-throughput screening of potential drugs as well as studies of energy transfer that monitor RNA conformational changes on the single-molecule level.

On the possibility of long-wavelength long-lifetime high-quantum-yield luminophores

We describe an approach to creating a new class of luminophores which display both long wavelength emissions exceeding 600 nm and long lifetimes. These luminophores are based on resonance energy transfer (RET) from a long lifetime donor to a short lifetime but long wavelength acceptor. We demonstrated the possibility of obtaining these desirable spectral properties using donors and acceptors noncovalently bound to DNA. The donor was a ruthenium (Ru) metal-ligand complex in which one of the diimine ligands intercalated into double-helix DNA. The acceptors were either nile blue, TOTO-3, or TO-PRO-3. Upon binding of the acceptor to donor-labeled DNA, we found that the acceptor quantum yield was remarkably enhanced so that the wavelength-integrated intensities of the donor and acceptor bound to DNA were many-fold greater than the intensity of the donor and acceptor alone when separately bound to DNA. The origin of this effect is efficient energy transfer from the donor. Under these conditions the effective overall quantum yield approaches that of the acceptor. Importantly, the increased quantum yield can be obtained while maintaining usefully long apparent acceptor lifetimes of 30 to 80 ns. The effect of an increased quantum yield from a low quantum yield donor may find use in assays to detect macromolecular binding interactions. These results suggest the synthesis of covalently linked donor-acceptor pairs with the desirable spectral properties of long wavelength emission, high quantum yield, and moderately long lifetimes for gated detection.