Intra- and inter-monomeric transfers in the light harvesting LHCII complex: the Redfield-Förster picture

We further develop the model of energy transfer in the LHCII trimer based on a quantitative fit of the linear spectra (including absorption (OD), linear dichroism (LD), circular dichroism (CD), and fluorescence (FL)) and transient absorption (TA) kinetics upon 650 nm and 662 nm excitation. The spectral shapes and relaxation/migration rates have been calculated using the combined Redfield-F?rster approach capable of correctly describing fast relaxation within strongly coupled chlorophyll (Chl) a and b clusters and slow migration between them. Within each monomeric subunit of the trimeric complex there is fast (sub-ps) conversion from Chl's b to Chl's a at the stromal side accompanied by slow (>10 ps) equilibration between the stromal- and lumenal-side Chl a clusters in combination with slow (>13 ps) population of Chl's a from the 'bottleneck' Chl a604 site. The connection between monomeric subunits is determined by exciton coupling between the stromal-side Chl's b from the two adjacent subunits (Chl b601'-608-609 cluster) making a simultaneous fast (sub-ps) population of the Chl's a possible from both subunits. Final equilibration occurs via slow (>20 ps) migration between the Chl a clusters located on different monomeric subunits. This migration includes up-hill transfers from the red-most Chl a610-611-612 clusters located at the peripheral side in each subunit to the Chl a602-603 dimers located at the inner side of the trimeric LHCII complex.     

Electronic and vibrational relaxation of porphycene in solution

The relaxation of electronically excited porphycene in acetonitrile solution has been studied by transient absorption spectroscopy supported by global analysis techniques. Three processes following the femtosecond pulse excitation to the S 2 state have been identified: the intramolecular vibrational redistribution on the time scale of tens of femtoseconds, the internal conversion S 2 right arrow-wavy S 1 (750 fs) and thermal equilibration of the molecule by energy exchange with the solvent (16 ps). The recorded transient absorption kinetics exhibit oscillations which have been assigned to the evolution of wavepackets in both S 1 and S 0 states.     

Solvent-induced modulation of collective photophysical processes in fluorescent silica nanoparticles

In this paper we show how it is possible to control the nature and the efficiency of collective photophysical processes in a network composed of two different fluorescent units organized on the surface of silica nanoparticles. Such a structure is obtained by covering nanoparticles with a layer of dansyl moieties (Dns) and by partially protonating them in solution. The two fluorophores Dns and Dns.H(+) have very different photophysical properties and can be selectively excited and detected. The interaction between the two units Dns and Dns.H(+) has been first investigated in a reference compound obtained by derivatizing 1,6-hexanediamine with two dansyl units. The photophysical characterization of this compound (absorption spectra, fluorescence spectra, quantum yield, and lifetime) showed that the two moieties can be involved both in energy and electron-transfer processes. Dansylated nanoparticles were prepared by modifying preformed silica nanoparticles with dansylated (3-aminopropyl)trimethoxysilane. Photophysical studies indicated that protonation has a dramatic effect on the fluorescence of the nanoparticles, leading to the quenching of both the protonated units and the surrounding nonprotonated ones. This amplified response to protonation, due to charge-transfer interactions, is solvent-dependent and is less efficient in pure chloroform with respect to acetonitrile/chloroform (5/1 v/v) mixtures. The reduced efficiency of the electron-transfer processes responsible for the quenching makes energy transfer competitive to such an extent that in pure chloroform excitation energy migration takes place from Dns.H(+) to Dns with great efficiency.     

Theory of laser-simulated surface processes

Theoretical techniques for describing laser-stimulated surface processes in a vacuum and at a gas-surface interface are presented. For adspecies-surface systems, the laser excitation of vibrational degrees of freedom is considered, and quantum-mechanical and classical models and also an “almost first-principles” treatment of the competition between multiphoton absorption and multiphonon relaxation are discussed. The laser excitation of electronic degrees of freedom is considered with respect to surface states of semiconductors and metals, for the predissociation of diatomic adspecies on metal substrates, for ionization, and for resonance fluorescence of a gaseous atom near a metal. In connection with gas-surface interactions, the influence of laser radiation on diffraction patterns and energy transfer in atom-surface scattering is explored. Collisional ionization and ion neutralization in the presence of laser radiation are discussed. The roles of partial pressure and surface coverage in laser-stimulated surface processes are analyzed. Finally, some ideas on surface waves and annealing are presented.     

Luminescence and upconversion from thulium(iii) species in solution

Thulium salts and complexes are shown to be emissive from three states in the excited state manifold of Tm(3+). Formation of the (1)D(2) state can result in luminescence, or in energy transfer to the lower energy (1)G(4) and (3)H(4) emissive states. Where chromophores are present in the ligand structure, emission is restricted to thulium centred emissive states that are lower in energy than the chromophore centred donor state. We have also observed direct multi-photon excitation of the thulium excited state manifold. Furthermore, additional transitions are observed in the multi-photon excitation spectra that are consistent with upconversion as a consequence of sequential single photon absorption and relaxation processes within the thulium excited state manifold.     

Changes in the structure of chlorophyll-protein complexes and excitation energy transfer during photoinhibitory treatment of isolated photosystem I submembrane particles

The activity of light-induced oxygen consumption, absorption spectra, low temperature (77 K) chlorophyll fluorescence emission and excitation spectra were studied in suspensions of photosystem (PS) I submembrane particles illuminated by 2000 microE m(-2) s(-1) strong white light (WL) at 4 degrees C. A significant stimulation of oxygen uptake was observed during the first 1-4 h of photoinhibitory treatment, which rapidly decreased during further light exposure. Chlorophyll (Chl) content gradually declined during the exposure of isolated PSI particles to strong light. In addition to the Chl photobleaching, pronounced changes were found in Chl absorption and fluorescence spectra. The position of the major peak in the red part of the absorption spectrum shifted from 680 nm towards shorter wavelengths in the course of strong light exposure. A 6-nm blue shift of that peak was observed after 5-h illumination. Even more pronounced changes were found in the characteristics of Chl fluorescence. The magnitude of the dominating long-wavelength emission band at 736 nm located in untreated particles was five times reduced after 2-h exposure, whereas the loss in absolute Chl contents did not exceed 10% of its initial value. The major peak in low-temperature Chl fluorescence emission spectra shifted from 736 to 721 nm after 6-h WL treatment. Individual Chl-protein complexes differed in the response of their absorption spectra to strong WL. Unlike light-harvesting complexes (LHC), LHCI-680 and LHC-730, which did not exhibit changes in the major peak position, its maximum was shifted from 678 to 671 nm in CPIa complex after PSI submembrane particles were irradiated with strong light for 6 h. The results demonstrated that excitation energy transfer represents the stage of photosynthetic utilization of absorbed quanta which is most sensitive to strong light in isolated PSI particles.     

Bidirectional "ping-pong" energy transfer and 3000-fold lifetime enhancement in a Re(I) charge transfer complex

The synthesis and photophysics of a new Re(I)-carbonyl diimine complex, Re(PNI-phen)(CO)(3)Cl, where the PNI-phen is N-(1,10-phenanthroline)-4-(1-piperidinyl)naphthalene-1,8-dicarboximide is reported. The metal-to-ligand charge transfer (MLCT) emission lifetime was increased approximately 3000-fold at room temperature with respect to that of the model complex [Re(phen)(CO)(3)Cl] as a result of thermal equilibrium between the emissive (3)MLCT state and a long-lived triplet ligand-centered ((3)LC) state on the PNI chromophore. This represents the longest excited state lifetime (τ = 651 μs) that has ever been observed for a Re(I)-based CT photoluminescence at room temperature. The energy transfer processes and the associated rate constants leading to the establishment of the excited state equilibrium were elucidated by a powerful combination of three techniques (transient visible and infrared (IR) absorption and photoluminescence), each applied from ultrafast to the micro/milliseconds time scale. The MLCT excited state was monitored by transient IR using CO vibrations through time intervals where the corresponding signals obtained in conventional visible transient absorption were completely obscured by overlap with strong transients originating from the pendant PNI chromophore. Following initial excitation of the (1)LC state on the PNI chromophore, energy is transferred to form the MLCT state with a time constant of 45 ps, a value confirmed in all three measurement domains within experimental error. Although transient spectroscopy confirms the production of the (3)MLCT state on ultrafast time scales, Fo?rster resonance energy transfer calculations using the spectral properties of the two chromophores support initial singlet transfer from (1)PNI* to produce the (1)MLCT state by the agreement with the experimentally observed energy transfer time constant and efficiency. Intersystem crossing from the (1)MLCT to the (3)MLCT excited state is believed to be extremely fast and was not resolved with the current experiments. Finally, triplet energy was transferred from the (3)MLCT to the PNI-centered (3)LC state in less than 15 ns, ultimately achieving equilibrium between the two excited states. Subsequent relaxation to the ground state occurred via emission resulting from thermal population of the (3)MLCT state with a resultant lifetime of 651 μs. The title chromophore represents an interesting example of "ping-pong" energy transfer wherein photon excitation first migrates away from the initially prepared (1)PNI* excited state and then ultimately returns to this moiety as a long-lived excited triplet which disposes of its energy by equilibrating with the photoluminescent Re(I) MLCT excited state.     

Excitation dynamics in the LHCII complex of higher plants: modeling based on the 2.72 Angstrom crystal structure

We have modeled steady-state spectra and energy-transfer dynamics in the peripheral plant light-harvesting complex LHCII using new structural data. The dynamics of the chlorophyll (Chl) b-->Chl a transfer and decay of selectively excited "bottleneck" Chl a and b states have been studied by femtosecond pump-probe spectroscopy. We propose an exciton model of the LHCII trimer (with specific site energies) which allows a simultaneous quantitative fit of the absorption, linear-dichroism, steady-state fluorescence spectra, and transient absorption kinetics upon excitation at different wavelengths. In the modeling we use the experimental exciton-phonon spectral density and modified Redfield theory. We have found that fast b-->a transfer is determined by a good connection of the Chls b to strongly coupled Chl a clusters, i.e., a610-a611-a612 trimer and a602-a603 and a613-a614 dimers. Long-lived components of the energy-transfer kinetics are determined by a quick population of red-shifted Chl b605 and blue-shifted Chl a604 followed by a very slow (3 ps for b605 and 12 ps for a604) flow of energy from these monomeric bottleneck sites to the Chl a clusters. The dynamics within the Chl a region is determined by fast (with time constants down to sub-100 fs) exciton relaxation within the a610-a611-a612 trimer, slower 200-300 fs relaxation within the a602-a603 and a613-a614 dimers, even slower 300-800 fs migration between these clusters, and very slow transfer from a604 to the quasi-equilibrated a sites. The final equilibrium is characterized by predominant population of the a610-a611-a612 cluster (mostly the a610 site). The location of this cluster on the outer side of the LHCII trimer probably provides a good connection with the other subunits of PSII.     

Chloroplast biogenesis 87: Evidence of resonance excitation energy transfer between tetrapyrrole intermediates of the chlorophyll biosynthetic pathway and chlorophyll a

The thorough understanding of photosynthetic membrane assembly requires a deeper knowledge of the coordination of chlorophyll (Chl) and thylakoid apoprotein biosynthesis. As a working model for future investigations, we have proposed three Chl-thylakoid apoprotein biosynthesis models, namely, a single-branched Chl biosynthetic pathway (SBP) single-location model, an SBP multilocation model and a multibranched Chl biosynthetic pathway (MBP) sublocation model. Rejection or validation of these models can be probed by determination of resonance excitation energy transfer between various tetrapyrrole intermediates of the Chl biosynthetic pathway and various thylakoid Chl-protein complexes. In this study we describe the detection of resonance energy transfer between protoporphyrin IX (Proto), Mg-Proto and its monomethyl ester (Mp(e)) and divinyl and monovinyl protochlorophyllide a (Pchlide a) and several Chl-protein complexes. Induction of various amounts of tetrapyrrole accumulation in green photoperiodically grown cucumber cotyledons and barley leaves was achieved by dark incubation of excised tissues with delta-aminolevulinic acid (ALA) and various concentrations of 2,2'-dipyridyl for various periods of time. Controls were incubated in distilled water. After plastid isolation, treated and control plastids were diluted in buffered glycerol to the same Chl concentration. Excitation spectra were then recorded at 77 K at emission maxima of about 686, 694 and 738 nm. Resonance excitation energy transfer from Proto, Mp(e) and Pchlide a to Chl-protein complexes emitting at 686, 694 and 738 nm was observed by calculation of treated minus control difference excitation spectra. The occurrence of resonance excitation energy transfer between anabolic tetrapyrroles and Chl-protein complexes appeared as well-defined excitation bands with excitation maxima corresponding to those of Proto, Mp(e) and Pchlide a. Furthermore, it appeared that resonance excitation energy transfer from multiple short-wavelength, medium-wavelength and long-wavelength Proto, Mp(e) and Chlide a sites to various Chl-protein complexes took place. Because resonance excitation transfer from donors to acceptors cannot take place at distances larger than 100 A, it is proposed that the observed resonance excitation energy transfers are not compatible with the SBP single-location Chl biosynthesis thylakoid membrane biogenesis model. The latter assumes that a single-branched Chl biosynthetic pathway located in the center of a 450 x 130 A photosynthetic unit generates all of the Chl needed for the assembly of all Chl-protein complexes.     

Emission of Au nanoparticles with and without rhodamine 6G dye

We have observed Stokes and anti-Stokes emission of Au nanoparticles suspended in methanol and rhodamine 6G dye solution. Photoluminescence of Au nanoparticles is a three-step process involving single-photon or three-photon excitation of electron-hole pairs, relaxation of excited electrons and holes, and emission from electron-hole recombination, possibly enhanced by surface plasmons. In the presence of dye, the excitation of anti-Stokes emission of gold involves two-photon absorption in rhodamine 6G molecules followed by the energy transfer to Au nanoparticles with simultaneous absorption of one pumping photon by Au. This mechanism significantly enhances anti-Stokes emission of gold nanoparticles in the presence of dye.     

Time-resolved spectroscopy of energy transfer and trapping upon selective excitation in membranes of Heliobacillus mobilis at low temperature

Transient absorption difference spectra in the Qy absorption band of bacteriochlorophyll (BChl) g and in the 670 nm absorption band of the primary acceptor A0 in membranes of Heliobacillus mobilis (Hc. mobilis) were measured at 20 K upon selective excitation at 668, 793, 810, and 815 nm with a 5 nm spectral bandwidth. When excited at 793 nm, the spectral equilibration of excitations from shorter to longer wavelength-absorbing pigments occurred within 3 ps and mostly localized at the band centered around 808 nm. When excited at 668 nm, the excitation energy transfer from the 670 nm absorbing pigment to the Qy band of BChl g took less than 0.5 ps, and the energy redistribution occurred and localized at 808 nm as in the case of the 793 nm excitation. All of the excitations were localized at the long wavelength pigment pool centered around 810 or 813 nm when excited at 810 or 815 nm. A slower energy transfer process with a time constant of 15 ps was also observed within the pool of long wavelength-absorbing pigments upon selective excitation at different wavelengths as has been observed by Lin et al. (Biophys. J. 1994, 67, 2479) when excited at 590 nm. Energy transfer from long wavelength antenna molecules to the primary electron donor P798 followed by the formation of P+ took place with a time constant of 55-70 ps for all excitations. Direct excitation of the primary electron acceptor A0, which absorbed at 670 nm, showed the same kinetic behavior as in the case when different forms of antenna pigments were excited in the Qy region. This observation generally supports the trapping-limited case of energy transfer in which the excitations have high escape probability from the reaction center (RC) until the charge separation takes place. Possible mechanisms to account for the apparent "uphill" energy transfer from the long wavelength antenna pigments to P798 are discussed.     

Photophysical study of new versatile multichromophoric diads and triads with BODIPY and polyphenylene groups

The photophysical properties of multichromophoric dyes with borondipyrromethene (BODIPY) and poly- p-phenylene (di- p-phenylene and tri- p-phenylene) groups in the same molecule are studied in detail. The excitation of the polyphenylene moiety in the UV region leads to a strong visible fluorescent emission of the BODIPY chromophore, via intramolecular excitation energy transfer between both groups. Consequently, these multichromophoric dyes are characterized by a large "virtual" Stokes shift, with a high fluorescence capacity and an efficient laser emission. On the other hand, the photophysical properties of a related dichromophoric dye with a hydroxy end group at the di- p-phenylene moiety show an important decrease in the fluorescent emission due to a photoinduced electron transfer process in basic media. Therefore, its photophysical properties are sensitive to the environmental acidity/basicity and could be applied as a proton sensor.     

Photoinduced energy and electron-transfer processes in porphyrin-perylene bisimide symmetric triads

The photophysics of two symmetric triads, (ZnP)2PBI and (H2P)2PBI, made of two zinc or free-base porphyrins covalently attached to a central perylene bisimide unit has been investigated in dichloromethane and in toluene. The solvent has been shown to affect not only quantitatively but also qualitatively the photophysical behavior. A variety of intercomponent processes (singlet energy transfer, triplet energy transfer, photoinduced charge separation, and recombination) have been time-resolved using a combination of emission spectroscopy and femtosecond and nanosecond time-resolved absorption techniques yielding a very detailed picture of the photophysics of these systems. The singlet excited state of the lowest energy chromophore (perylene bisimide in the case of (ZnP)2PBI, porphyrin in the case of (H2P)2PBI) is always quantitatively populated, besides by direct light absorption, by ultrafast singlet energy transfer (few picosecond time constant) from the higher energy chromophore. In dichloromethane, the lowest excited singlet state is efficiently quenched by electron transfer leading to a charge-separated state where the porphyrin is oxidized and the perylene bisimide is reduced. The systems then go back to the ground state by charge recombination. The four charge separation and recombination processes observed for (ZnP)2PBI and (H2P)2PBI in dichloromethane take place in the sub-nanosecond time scale. They obey standard free-energy correlations with charge separation lying in the normal regime and charge recombination in the Marcus inverted region. In less polar solvents, such as toluene, the energy of the charge-separated states is substantially lifted leading to sharp changes in photophysical mechanism. With (ZnP)2PBI, the electron-transfer quenching is still fast, but charge recombination takes place now in the nanosecond time scale and to triplet state products rather than to the ground state. Triplet-triplet energy transfer from the porphyrin to the perylene bisimide is also involved in the subsequent deactivation of the triplet manifold to the ground state. With (H2P)2PBI, on the other hand, the driving force for charge separation is too small for electron-transfer quenching, and the deactivation of the porphyrin excited singlet takes place via intersystem crossing to the triplet followed by triplet energy transfer to the perylene bisimide and final decay to the ground state.     

Thermal behavior of annealed organic glasses

The influence of annealing processes on the thermal behavior of organic glasses in the glass-transition interval has been investigated and analyzed quantitatively. In detailed annealing studies of atactic polystyrene and Aroclor 5460, the absorption of thermal energy superposed on the increase in the specific heat at the glass transition, observed with suitably chosen heating rates, was followed by the differential thermal method. It is concluded that the absorption of thermal energy observed under these conditions parallels the extent of molecular relaxation that has taken place during the annealing period. It is not necessary to postulate a first-order process to account for the energy absorption. Moreover, such a postulate leads to severe conceptual difficulties regarding the development of crystallinity in crystallizable materials. The areas and the shapes of the endotherms are considered in terms of the original physical properties of the quenched glasses and the anticipated equilibrium properties. Relationships between the extent of energy absorption and time-dependent processes such as volume relaxation are discussed.     

An investigation into the excitation migration in polyfluorene solutions via temperature dependent fluorescence anisotropy

Fluorescence anisotropy of dilute polyfluorene polymer solutions has been used to show that two processes, exciton migration and conformational relaxation (twisting of part of the chain), occur within polyfluorene polymers of 20 or more repeat units. The former process is dominant and temperature independent for high excitation energy but, as the chain length is decreased, exciton migration is eliminated and the conformational relaxation becomes the only mechanism by which excited state energy relaxation to the emission site can occur. When the polymers are excited in the absorption band tails, again no migration is observed but conformational relaxation is still present.     

Lifetime Shortening and Fast Energy‐Tansfer Processes upon Dimerization of a A‐π‐D‐π‐A Molecule

Time‐resolved fluorescence and transient absorption experiments uncover a distinct change in the relaxation dynamics of the homo‐dimer formed by two 2,5‐bis[1‐(4‐N‐methylpyridinium)ethen‐2‐yl)]‐N‐methylpyrrole ditriflate ( M ) units linked by a short alkyl chain when compared to that of the monomer M . Fluorescence decay traces reveal characteristic decay times of 1.1 ns and 210 ps for M and the dimer, respectively. Transient absorption spectra in the spectral range of 425–1050 nm display similar spectral features for both systems, but strongly differ in the characteristic relaxation times gathered from a global fit of the experimental data. To rationalize the data we propose that after excitation of the dimer the energy localizes on one M branch and then decays to a dark state, peculiar only of the dimer. This dark state relaxes to the ground state within 210 ps through non‐radiative relaxation. The nature of the dark state is discussed in relation to different possible photophysical processes such as excimer formation and charge transfer between the two M units. Anisotropy decay traces of the probe‐beam differential transmittance of M and the dimer fall on complete different time scales as well. The anisotropy decay for M is satisfactorily ascribed to rotational diffusion in DMSO, whereas for the dimer it occurs on a faster time scale and is likely caused by energy‐transfer processes between the two monomer M units.     

Excited State Intramolecular Proton Transfer of 1-Hydroxyanthraquinone

The excited state intra-molecular proton transfer dynamics of 1-hydroxyanthraquinone in solution are investigated by femtosecond transient absorption spectroscopy and quantum chemistry calculations. Two characteristic bands of excited state absorption and stimu-lated emission are observed in transient absorption spectra with the excitation by the pump wavelength of 400 nm. From the delayed stimulated emission signal, the time scale of the intra-molecular proton transfer is determined to be about 32 fs. The quantum chemistry calculations show that the molecular orbits and the order of the S₂ and S₁ states are rever-sal and a conical intersection is demonstrated to exist along the proton transfer coordinate. After proton transfer, the second excited state of tautomer populated via the conical intersection undergoes the internal conversion with ～200 fs and the following intermolecular energy relaxation with ～16 ps. The longer component 300 ps can be explained in terms of the relaxation from excited-state tautomer to its ground state. From our observations, two proton transfer pathways via a conical intersection are proposed and the dominated one preserves the molecular orbits.     

An MRD CI investigation of the photochemical isomerization of cyclohexadiene to hexatriene

The photoisomerization of cyclohexadiene to cis-hexatriene has been investigated using the multi-reference configuration interaction (MRD CI) method involving all active valence electrons. It has been determined that the initial absorption process in cyclohexadiene is nonvertical, and that the excitation energy and oscillator strength obtained are in good agreement with gas phase absorption measurements. The excited state which is initially populated in the absorption process can undergo rapid nonradiative relaxation to a lower-lying excited state with a barrierless potential leading to formation of the ground state hexatriene photoproduct. The large absorption linewidth which is experimentally observed is correlated with rapid nonradiative relaxation in the excited state which is both photophysical and photochemical in nature. The effect of orbital symmetry upon the reaction, as well as the dipole moments of the excited states are also discussed.     

Steps to demarcate the effects of chromophore aggregation and planarization in poly(phenyleneethynylene)s. 2. The photophysics of 1,4-diethynyl-2-fluorobenzene in solution and in crystals

Crystals of 1,4-bis(2-hydroxy-2-methyl-3-butynyl)-2-fluorobenzene 4 have a rich packing structure with four distinct molecules in the unit cell. A complex hydrogen bonding network results in the formation of cofacial trimers, cofacial dimers, and monomers within the same unit cell. Given a remarkable opportunity to investigate the effect of aggregation on the photophysics of 1,4-diethynylbenzenes, we analyzed the absorption, diffuse reflectance, and emission spectra of compound 4 in solutions and in crystals. Diffuse reflectance and fluorescence excitation revealed a red-shifted absorption that is absent in dilute solution but becomes observable at high concentrations and low temperatures. The fluorescence emission in the solid state is dual with components assigned to monomers and aggregates. The excitation and emission assigned to the monomer are nearly identical in crystals and dilute solutions. The absorption and emission bands assigned to aggregates are broad and red-shifted by 60--80 nm. As expected for a sample with absorbers and emitters with different energies and incomplete equilibration, efficient monomer-to-aggregate energy transfer was observed by a proper selection of excitation wavelengths. The fluorescence quantum yield of 4 in solution is relatively low (Phi(F) = 0.15) and the singlet lifetime short (tau(F) = 3.8 ns). A lower limit for the triplet yield of Phi(T) = 0.64 was determined indirectly in solution by (1)O(2) sensitization, and a relatively strong and long-lived phosphorescence was observed in low-temperature glasses and in crystals at 77 K.     

Quantum yields of luminescent lanthanide chelates and far-red dyes measured by resonance energy transfer.

Luminescent lanthanide chelates have unusual spectroscopic characteristics that make them valuable alternative probes to conventional organic fluorophores. However, fundamental parameters such as their quantum yield, and radiative and nonradiative decay rates have been difficult or impossible to measure. We have developed a simple and robust method based on resonance energy transfer to accurately measure these parameters. In addition, the excitation/emission process in lanthanide chelates involves several steps, and we are able to quantify each step. These include excitation of an organic antenna, transfer of energy from the antenna to lanthanide, and then lanthanide emission. Overall, the parameters show that lanthanide chelates can be efficient long-lived emitters, making them sensitive detection reagents and excellent donors in resonance energy transfer. The method is also shown to be applicable to photophysical characterization of red-emitting dyes, which are difficult to characterize by conventional means.