Dependence of the Lifetime upon the Excitation Energy and Intramolecular Energy Transfer Rates: The (5) D(0) Eu(III) Emission Case

In many Eu(III) -based materials, the presence of an intermediate energy level, such as ligand-to-metal charge transfer (LMCT) states or defects, that mediates the energy transfer mechanisms can strongly affect the lifetime of the (5) D(0) state, mainly at near-resonance (large transfer rates). We present results for the dependence of the (5) D(0) lifetime on the excitation wavelength for a wide class of Eu(III) -based compounds: ionic salts, polyoxometalates (POMs), core/shell inorganic nanoparticles (NPs) and nanotubes, coordination polymers, β-diketonate complexes, organic-inorganic hybrids, macro-mesocellular foams, functionalized mesoporous silica, and layered double hydroxides (LDHs). This yet unexplained behavior is successfully modelled by a coupled set of rate equations with seven states, in which the wavelength dependence is simulated by varying the intramolecular energy transfer rates. In addition, the simulations of the rate equations for four- and three-level systems show a strong dependence of the emission lifetime upon the excitation wavelength if near-resonant non-radiative energy transfer processes are present, indicating that the proposed scheme can be generalized to other trivalent lanthanide ions, as observed for Tb(III) /Ce(III) . Finally, the proper use of lifetime definition in the presence of energy transfer is emphasized.     

Energy gap law of electron transfer in nonpolar solvents

We investigate the energy gap law of electron transfer in nonpolar solvents for charge separation and charge recombination reactions. In polar solvents, the reaction coordinate is given in terms of the electrostatic potentials from solvent permanent dipoles at solutes. In nonpolar solvents, the energy fluctuation due to solvent polarization is absent, but the energy of the ion pair state changes significantly with the distance between the ions as a result of the unscreened strong Coulomb potential. The electron transfer occurs when the final state energy coincides with the initial state energy. For charge separation reactions, the initial state is a neutral pair state, and its energy changes little with the distance between the reactants, whereas the final state is an ion pair state and its energy changes significantly with the mutual distance; for charge recombination reactions, vice versa. We show that the energy gap law of electron-transfer rates in nonpolar solvents significantly depends on the type of electron transfer.     

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.     

Dependence of the Lifetime upon the Excitation Energy and Intramolecular Energy Transfer Rates: The 5D0 EuIII Emission Case

In many Eu^III‐based materials, the presence of an intermediate energy level, such as ligand‐to‐metal charge transfer (LMCT) states or defects, that mediates the energy transfer mechanisms can strongly affect the lifetime of the ⁵D₀ state, mainly at near‐resonance (large transfer rates). We present results for the dependence of the ⁵D₀ lifetime on the excitation wavelength for a wide class of Eu^III‐based compounds: ionic salts, polyoxometalates (POMs), core/shell inorganic nanoparticles (NPs) and nanotubes, coordination polymers, β‐diketonate complexes, organic–inorganic hybrids, macro‐mesocellular foams, functionalized mesoporous silica, and layered double hydroxides (LDHs). This yet unexplained behavior is successfully modelled by a coupled set of rate equations with seven states, in which the wavelength dependence is simulated by varying the intramolecular energy transfer rates. In addition, the simulations of the rate equations for four‐ and three‐level systems show a strong dependence of the emission lifetime upon the excitation wavelength if near‐resonant non‐radiative energy transfer processes are present, indicating that the proposed scheme can be generalized to other trivalent lanthanide ions, as observed for Tb^III/Ce^III. Finally, the proper use of lifetime definition in the presence of energy transfer is emphasized.     

Photo-switchable molecular devices based on metal-ionic recognition

A novel calix[4]arene derivative, bearing two spirobenzopyran moieties in the lower rim, can recognize lanthanide ions. Alternating irradiation with ultraviolet and visible light controls the ligand-to-metal charge transfer (LMCT) and energy transfer of the host-Eu³⁺ complexes. Thus, fluorescence of Eu³⁺ can be switched on and off through light. The system may be applied as molecular logic switches.     

Theory and calculation for the electronic coupling in excitation energy transfer

Excitation energy transfer (EET) is a process where the electronically excitation is transferred from a donor to an acceptor. EET is widely seen in both natural and in artificial systems, such as light‐harvesting in photosynthesis, the fluorescence resonance energy transfer technique, and the design of light‐emitting molecular devices. In this work, we outline the theories describing both singlet and triplet EET (SEET and TEET) rates, with a focus on the physical nature and computational methods for the electronic coupling factor, an important parameter in predicting EET rates. The SEET coupling is dominated by the Coulomb coupling, and the remaining short‐range coupling is very similar to the TEET coupling. The magnitude of the Coulomb coupling in SEET can vary much, but the contribution of short‐range coupling has been found to be similar across different excited states in naphthalene. The exchange coupling has been believed to be the major physical contribution to the short‐range coupling, but it has been pointed out that other contribution, such as the orbital overlap effect is similar or even larger in strength. The computational aspects and the subsequent physical implication for both SEET and TEET coupling values are summarized in this work. © 2013 The Authors. International Journal of Quantum Chemistry Published by Wiley Periodicals, Inc.     

Mediation of resonance energy transfer by a third molecule

The influence of a third molecule on the rate of resonance energy transfer is studied using diagrammatic perturbation theory within the framework of molecular quantum electrodynamics. Two distinct mechanisms are identified. One corresponds to direct transfer between donor and acceptor while the other involves relay of energy by the third species. Fermi Golden rule transition rates valid for all separation distances beyond wave function overlap are evaluated for these two processes as well as for the interference term between direct and indirect exchange, thereby extending previous work which was limited to the near-zone only. Short- and long-range limits are also obtained in each case. It is found that in the near-zone the indirect rate contribution exhibits inverse sixth power dependence on relative distances of emitter and absorber relative to the third body, in contrast to its far-zone counterpart, which exhibits inverse square behavior. The interference term, however, displays inverse cubic dependence on all three distance vectors at short-range and inverse behavior in the far-zone. Interestingly, for a collinear arrangement of the three molecules in the near-zone, the interference term is negative, reducing the overall rate of energy transfer. The results obtained are interpreted in terms of microscopic and macroscopic pictures of transfer occurring within a surrounding medium.     

Synthesis and photophysical properties of kinetically stable complexes containing a lanthanide ion and a transition metal antenna group

A series of bimetallic complexes has been prepared in which an octadentate DOTA-monoamide pocket containing a bound lanthanide ion is linked covalently to a Re(I) or Ru(II) bipyridyl unit via an alkyl spacer group. The transition metal chromophores incorporated in this way act as effective sensitisers for lanthanide-centred luminescence. The rate and efficiency of energy transfer are dependent upon the nature of the spacer group, and upon the nature of the lanthanide acceptor. For the RuNd diad, there is a long rise-time associated with the energy-transfer step, such that energy transfer is rate determining in H(2)O, but not in D(2)O. The results also lead us to suggest that energy transfer may precede formation of the (3)M((Ru/Re))L((bpy))CT state and may be a competitive deactivation pathway for the precursor state ((1)M((Ru/Re))L((bpy))CT).     

Role of diffusion in excitation energy transfer and migration

Effect of diffusion on excitation energy transfer and migration in a dye pair sodium fluorescein (donor) and Rhodamine-6G (acceptor) has been studied for different viscosities by both steady state and time domain fluorescence spectroscopic measurements. The donor-donor interaction appears to be weaker as compared to donor-acceptor interaction and thus favors direct Forster-type energy transfer. Interestingly, at low viscosity (water in this case) transfer appears to be controlled by material diffusion/energy migration. Further, acceptor dynamics reveals the fact that direct Forster transfer dominates in viscous media.     

Energy transfer, excited-state deactivation, and exciplex formation in artificial caroteno-phthalocyanine light-harvesting antennas

We present results from transient absorption spectroscopy on a series of artificial light-harvesting dyads made up of a zinc phthalocyanine (Pc) covalently linked to carotenoids with 9, 10, or 11 conjugated carbon-carbon double bonds, referred to as dyads 1, 2, and 3, respectively. We assessed the energy transfer and excited-state deactivation pathways following excitation of the strongly allowed carotenoid S2 state as a function of the conjugation length. The S2 state rapidly relaxes to the S* and S1 states. In all systems we detected a new pathway of energy deactivation within the carotenoid manifold in which the S* state acts as an intermediate state in the S2-->S1 internal conversion pathway on a sub-picosecond time scale. In dyad 3, a novel type of collective carotenoid-Pc electronic state is observed that may correspond to a carotenoid excited state(s)-Pc Q exciplex. The exciplex is only observed upon direct carotenoid excitation and is nonfluorescent. In dyad 1, two carotenoid singlet excited states, S2 and S1, contribute to singlet-singlet energy transfer to Pc, making the process very efficient (>90%) while for dyads 2 and 3 the S1 energy transfer channel is precluded and only S2 is capable of transferring energy to Pc. In the latter two systems, the lifetime of the first singlet excited state of Pc is dramatically shortened compared to the 9 double-bond dyad and model Pc, indicating that the carotenoid acts as a strong quencher of the phthalocyanine excited-state energy.     

Stepwise Increase of NdIII-Based Phosphorescence by AIE-Active Sensitizer: Broadening the AIPE Family from Transition Metals to Discrete Near-Infrared Lanthanide Complexes**

We designed two near-infrared (NIR) lanthanide complexes [( L )₂-Nd(NO₃)₃] ( L =TPE₂-BPY for 1 , TPE-BPY for 2 ) by employing aggregation-induced emission (AIE)-active tetraphenylethylene (TPE) derivatives as sensitizers, which possessed matched energy to Nd^III, prevented competitive deactivation under aggregation, even shifted the excitation window toward 600 nm by twisted intramolecular charge transfer. Furthermore, benefiting from the 4 f electron shielding effect and antenna effect, the enhanced excitation energies of the AIE-active sensitizers by structural rigidification transferred into the inert Nd^III excited state through ³LMCT, affording the first aggregation-induced phosphorescence enhancement (AIPE)-active discrete NIR-emitting lanthanide complexes. As 1 equipped with more AIE-active TPE than 2 , L →Nd energy transfer efficiency in the former was higher than that in the latter under the same conditions. Consequently, the crystal of 1 exhibited one of the longest lifetimes (9.69 μs) among Nd^III-based complexes containing C−H bonds.     

Energy transfer and migration in highly forbidden transitions of lanthanide ion doped crystals

Förster–Dexter theory for resonant energy transfer is extended to higher order and applied to explain the rates of energy transfer and migration processes in highly forbidden transitions for some solid-state lanthanide (Ln) ion systems for which experimental results are available. The second-order two-body energy transfer mechanism involves two inter-ion correlated dipole electrostatic interactions, i.e. dipole dipole–dipole dipole (dd–dd) energy transfer, also termed Axe–Axe energy transfer in view of the similarity of the theoretical formalism with that for two-photon transitions. Each of the dipolar transitions consists of a transition from the 4fⁿ configuration to an opposite-parity configuration, taken to be 4fⁿ⁻¹5d. dd–dd energy transfer is a short-range (R⁻¹²) interaction so that it is most important in systems with short donor Ln–acceptor Ln separations. The energy transfer formalism is extended to include spin-forbidden transitions at one or two sites, the so-called Axe–Judd–Pooler (Axe–JP) and JP–JP energy transfer. In some cases the dd–dd mechanism is the dominant energy transfer process, as exemplified herein for energy migration in the ⁵D₀ state of Sm²⁺ in SrF₂, and also in the ⁵D₀ state of Eu³⁺ in Cs₂NaEuCl₆.     

Intramolecular energy transfer in actinide complexes of 6-methyl-2-(2-pyridyl)-benzimidazole (biz): comparison between Cm and Tb systems

Coordination of the 6-methyl-2-(2-pyridyl)-benzimidazole ligand with actinide and lanthanide species can produce enhanced emission due to increased efficiency of intramolecular energy transfer to metal centers. A comparison between the curium and terbium systems indicates that the position of the ligand's triplet state is critical for the enhanced emission. The energy gap between the ligand's triplet state and the acceptor level in curium is about 1000 cm⁻¹, as compared to a ~600 cm⁻¹ gap in the terbium system. Due to the larger gap, the back transfer with curium is reduced and the radiative yield is significantly higher. The quantum yield for this “sensitized” emission increases to 6.2%, compared to the 0.26% value attained for the metal centered excitation prior to ligand addition. In the terbium case, the smaller donor/acceptor gap enhances back transfer and the energy transfer is less efficient than with the curium system.     

Structural and photophysical properties of coordination networks combining [Ru(bipy)(CN)4]2- anions and lanthanide(III) cations: rates of photoinduced Ru-to-lanthanide energy transfer and sensitized near-infrared luminescence

Co-crystallization of K2[Ru(bipy)(CN)4] with lanthanide(III) salts (Ln = Pr, Nd, Gd, Er, Yb) from aqueous solution affords coordination oligomers and networks in which the [Ru(bipy)(CN)4]2- unit is connected to the lanthanide cation via Ru-CN-Ln bridges. The complexes fall into two structural types: [{Ru(bipy)(CN)4}2{Ln(H2O)m}{K(H2O)n}] x xH2O (Ln = Pr, Er, Yb; m = 7, 6, 6, respectively), in which two [Ru(bipy)(CN)4]2- units are connected to a single lanthanide ion by single cyanide bridges to give discrete trinuclear fragments, and [{Ru(bipy)(CN)4}3{Ln(H2O)4}2] x xH2O (Ln = Nd, Gd), which contain two-dimensional sheets of interconnected, cyanide-bridged Ru2Ln2 squares. In the Ru-Gd system, the [Ru(bipy)(CN)4]2- unit shows the characteristic intense (3)metal-to-ligand charge transfer luminescence at 580 nm with tau = 550 ns; with the other lanthanides, the intensity and lifetime of this luminescence are diminished because of a Ru --> Ln photoinduced energy transfer to low-lying emissive states of the lanthanide ions, resulting in sensitized near-infrared luminescence in every case. From the degree of quenching of the Ru-based emission, Ru --> Ln energy-transfer rates can be estimated, which are in the order Yb (k(EnT) approximately 3 x 10(6) sec(-1), the slowest energy transfer) < Er < Pr < Nd (k(EnT) approximately 2 x 10(8) sec(-1), the fastest energy transfer). This order may be rationalized on the basis of the availability of excited f-f levels on the lanthanide ions at energies that overlap with the Ru-based emission spectrum. In every case, the lifetime of the lanthanide-based luminescence is short (tens/hundreds of nanoseconds, instead of the more usual microseconds), even when the water ligands on the lanthanide ions are replaced by D2O to eliminate the quenching effects of OH oscillators; we tentatively ascribe this quenching effect to the cyanide ligands.     

Direct observation of rotational energy transfer in ammonia by time-resolved infrared—microwave double resonance

An N₂O laser is used to pump the ground vibrational state (8,7) inversion doublet of ¹⁴NH₃ while simultaneously monitoring other ground state doublets. Time-resolved rotational energy transfer signals are observed in accordance with known selection values. Absolute rates of rotational energy transfer processes are estimated.     

Ligand-passivated Eu:Y2O3 nanocrystals as a phosphor for white light emitting diodes

Eu(III)-doped Y(2)O(3) nanocrystals are prepared by microwave synthetic methods as spherical 6.4 ± 1.5 nm nanocrystals with a cubic crystal structure. The surface of the nanocrystal is passivated by acetylacetonate (acac) and HDA on the Y exposed facet of the nanocrystal. The presence of acac on the nanocrystal surface gives rise to a strong S(0) → S(1) (π → π*, acac) and acac → Ln(3+) ligand to metal charge transfer (LMCT) transitions at 270 and 370 nm, respectively, in the Eu:Y(2)O(3) nanocrystal. Excitation into the S(0) → S(1) (π → π*) or acac → Ln(3+) LMCT transition leads to the production of white light emission arising from efficient intramolecular energy transfer to the Y(2)O(3) oxygen vacancies and the Eu(III) Judd-Ofelt f-f transitions. The acac passivant is thermally stable below 400 °C, and its presence is evidenced by UV-vis absorption, FT-IR, and NMR measurements. The presence of the low-lying acac levels allows UV LED pumping of the solid phosphor, leading to high quantum efficiency (～19%) when pumped at 370 nm, high-quality white light color rendering (CIE coordinates 0.33 and 0.35), a high scotopic-to-photopic ratio (S/P = 2.21), and thermal stability. In a LED lighting package luminosities of 100 lm W(-1) were obtained, which are competitive with current commercial lighting technology. The use of the passivant to funnel energy to the lanthanide emitter via a molecular antenna effect represents a new paradigm for designing phosphors for LED-pumped white light.     

Statistical approaches to the transient populations and to the macroscopic energy transfer rate for the upconversion phenomenon in monodoped amorphous solid

In this article, statistical approaches to the first and the second excited state transient populations and to the temporal macroscopic energy transfer rate for the upconversion process in amorphous solid generic systems monodoped with trivalent lanthanide ions are reached. The plots of the expressions show general tendencies reported in the literature. The derivation and the analysis of the formalism allowed us to fulfill our main objective, that is, to make a theoretical study about the microscopic and statistical mechanisms present in the phenomenon and their relation with the classic kinetic analysis. The study shows that the inclusion of the minimum possible radius between two optical centers in a solid affects the initial slopes of the decay curves of the luminescence from the intermediate state. We also corroborate that the usual treatment of experimental data using direct equations for the dynamics of the populations in laser pulsed excitation experiments falls in the mistake of not considering the temporality of the macroscopic energy transfer rate. Finally, physical explanations are formulated about this temporal behavior and about the main factors that generate the characteristic simple exponential decay loss of the luminescence from the intermediate state.     

Phosphorescent resonant energy transfer between iridium complexes

The mechanism for triplet energy transfer from the green-emitting fac-tris[2-(4'-tert-butylphenyl)pyridinato]iridium (Ir(tBu-ppy)3) complex to the red-emitting bis[2-(2'-benzothienyl)pyridinato-N,C3')(acetylacetonato)iridium (Ir(btp)2(acac)) phosphor has been investigated using steady-state and time-resolved photoluminescence spectroscopy. [2,2';5,'2' ']Terthiophene (3T) was also used as triplet energy acceptor to differentiate between the two common mechanisms for energy transfer, i.e., the direct exchange of electrons (Dexter transfer) or the coupling of transition dipoles (F?rster transfer). Unlike Ir(btp)2(acac), 3T can only be active in Dexter energy transfer because it has a negligible ground state absorption to the 3(pi-pi*) state. The experiments demonstrate that in semidilute solution, the 3MLCT state of Ir(tBu-ppy)3 can transfer its triplet energy to the lower-lying 3(pi-pi*) states of both Ir(btp)2(acac) and 3T. For both acceptors, this transfer occurs via a diffusion-controlled reaction with a common rate constant (ken = 3.8 x 10(9) L mol-1 s-1). In a solid-state polymer matrix, the two acceptors, however, show entirely different behavior. The 3MLCT phosphorescence of Ir(tBu-ppy)3 is strongly quenched by Ir(btp)2(acac) but not by 3T. This reveals that under conditions where molecular diffusion is inhibited, triplet energy transfer only occurs via the F?rster mechanism, provided that the transition dipole moments involved on energy donor and acceptor are not negligible. With the use of the F?rster radius for triplet energy transfer from Ir(tBu-ppy)3 to Ir(btp)2(acac) of R0 = 3.02 nm, the experimentally observed quenching is found to agree quantitatively with a model for F?rster energy transfer that assumes a random distribution of acceptors in a rigid matrix.     

Energy and photoinduced electron transfer in a wheel-shaped artificial photosynthetic antenna-reaction center complex

Functional mimics of a photosynthetic antenna-reaction center complex comprising five bis(phenylethynyl)anthracene antenna moieties and a porphyrin-fullerene dyad organized by a central hexaphenylbenzene core have been prepared and studied spectroscopically. The molecules successfully integrate singlet-singlet energy transfer and photoinduced electron transfer. Energy transfer from the five antennas to the porphyrin occurs on the picosecond time scale with a quantum yield of 1.0. Comparisons with model compounds and theory suggest that the F?rster mechanism plays a major role in the extremely rapid energy transfer, which occurs at rates comparable to those seen in some photosynthetic antenna systems. A through-bond, electron exchange mechanism also contributes. The porphyrin first excited singlet state donates an electron to the attached fullerene to yield a P(*+)-C(60)(*-) charge-separated state, which has a lifetime of several nanoseconds. The quantum yield of charge separation based on light absorbed by the antenna chromophores is 80% for the free base molecule and 96% for the zinc analogue.     

Characterization of the triplet state of tris(8-hydroxyquinoline)aluminium(III) in benzene solution

The lowest triplet state of tris(8-hydroxyquinoline)aluminium(III) (Alq3) has been prepared by pulse radiolysis/energy transfer from appropriate donors in benzene solutions and has an absorption maximum around 510 nm with a lifetime of about 50 mus. It is quenched by molecular oxygen, leading to singlet oxygen formation. From flash photolysis and singlet oxygen formation measurements, a quantum yield of triplet formation of 0.24 was determined for direct photolysis of the complex. A value of 2.10 +/- 0.10 eV was determined for the energy of the lowest triplet state by energy transfer studies and was confirmed by phosphorescence measurements on Alq3, either in the heavy atom solvent ethyl iodide or photosensitized by benzophenone in benzene. Dexter (exchange) energy transfer was observed from triplet Alq3 to platinum(II) octaethylporphyrin.