Direct Evidence Of Energy Transfer Research Articles

Direct evidence of energy transfer from a singlet ligand level to lanthanide ions in their diketonate complexes

We have compared the fluorescence intensity of 2-naphthoyltrifluoroacetonate (NTA) in nanoparticles from Gd(NTA)3phen complexes with the fluorescence intensities of this compound in nanoparticles from similar complexes of Pr, Nd, Sm, Eu, Tb, Dy, Er, Ho, and Tm, which absorb in the NTA fluorescence range. We have proven that there is energy transfer from the S1 level of NTA ligands to Ln(III) ions, which occurs with rates ktr ∼ 1011–1012 s−1. We have also studied the competition between two processes: energy transfer from Ln(NTA)3phen to Ln(III) ions and energy transfer to Nile blue molecules incorporated into nanoparticles from complexes of these ions. It has been shown that, in nanoparticles from complexes of Nd(III), Tb(III), Dy(III), and Tm(III) ions, which are incapable of sensitizing the fluorescence of Nile blue, the values of ktr2 for the energy transfer from NTA to Ln(III) ions, which were obtained from the data on the change in the intensity of the sensitized fluorescence of Nile blue, completely coincide with the values of ktr1 determined from the fluorescence quenching of NTA in these nanoparticles. We have found that, in nanoparticles from Pr, Sm, Eu, Er, and Ho complexes, the efficiency of the sensitized fluorescence of Nile blue is higher than that predicted from the fluorescence quenching of NTA by these ions in their complexes, which indicates that all these ions participate as mediators in the energy transfer from ligands to the dye.

Deep-sea low-light radiometer system

Two single-waveband low-light radiometers were developed to characterize properties of the underwater light field relevant to biological camouflage at mesopelagic ocean depths. Phenomena of interest were vertical changes in downward irradiance of ambient light at wavelengths near 470 nm and 560 nm, and flashes from bioluminescent organisms. Depth profiles were acquired at multiple deep stations in different geographic regions. Results indicate significant irradiance magnitudes at 560 nm, providing direct evidence of energy transfer as described by Raman scattering. Analysis of a night profile yielded multiple examples of bioluminescent flashes. The selection of high-sensitivity, high-speed silicon photomultipliers as detectors enabled measurement of spectrally-resolved irradiance to greater than 400 m depth.

Direct Visualization of Energy-transferred Excited State in Eu Complex

Highly luminescent lanthanide (Ln) complexes have attracted much attention because Ln3+ions show long-lived ff-emissions with narrow band shape. Their unique photo-optical properties are promising for the design of light-emitting materials and sensing devices. Although the ff-emissions are essentially weak because of Laporte forbidden, chelate ligands is effective to strengthen the intramolecular energy transfer from photo-excited organic ligands to Ln3+ions. The direct evidence of energy transfer from ligands to Ln3+and details of excited state, however, are still veiled. Here, we report direct visualization of energy-transferred excited state in Eu complex with a hexadentate ligand (L) consisting of two bipyridine moieties bridged by an ethylendiamine unit, [Eu3+(L)(NO3)2](PF6) (Eu(L))[1] by Maximum Entropy Method (MEM) charge density[2] and electrostatic potential analysis[3] based on SR X-ray diffraction. First, we confirmed that the electron numbers of Eu and ligand L in the excited state are the same as those in the ground state, which is a direct evidence of energy transfer instead of charge transfer. Next, we observed charge re-distribution in the Eu ion and the ligand L. The electrostatic potential distributions calculated using MEM charge density give an experimental evidence for the existence of polarization of ligand L both in the ground and photo-excited states. The orientation polarization in the ground state changed during pumping at 315 nm, and the charge re-distribution are qualitatively consistent with a theoretical prediction. This characteristic luminescence behavior based on the energy relaxation process have not been detected by fundamental crystal structural analysis. We have succeeded in visualization of subtle but important change due to energy transfer in the mononuclear Europium complex with hexadentate ligand at the first time.

Photoluminescence of Mid-Infrared HgTe Colloidal Quantum Dots

The photoluminescence quantum yield of HgTe colloidal quantum dots is measured from 1800 to 6500 cm–1. There is a steep drop to low energy reminiscent of the generic gap law. However, direct evidence of energy transfer to the C–H stretch and overtone vibrations is apparent when temperature tunes the PL wavelength of a given sample through the vibrational resonances. Calculations based on the radiative rate and resonant energy transfer to the ligand vibrations appear to account for much of the quantum yield drop. Power-dependent photoluminescence lifetime measurements on 3.7 nm particles show fast, ∼50 ps, biexciton lifetime similar to other colloidal quantum dot systems of similar sizes.

Factors on the separation of photogenerated charges and the charge dynamics in oxide/ZnFe2O4composites

Four oxide/ZnFe2O4 composites (WO3/ZnFe2O4, α-Bi2O3/ZnFe2O4, TiO2/ZnFe2O4 and SnO2/ZnFe2O4) were synthesized with 2 : 1, 4 : 1, 6 : 1 and 8 : 1 molar ratios of (W, Bi, Ti and Sn)/Zn. From the energy band structure, all composites can satisfy the energy level matching between their components (oxides and ZnFe2O4). For WO3/ZnFe2O4 and α-Bi2O3/ZnFe2O4, almost all of the molar ratios have stronger photovoltaic responses than each component before compounding, but the other two composites present the opposite result. Due to the different charge dynamics, the photovoltaic characteristics of four composites with molar ratio 4 : 1 did not satisfy the “inverted region effect”. Time-resolved fluorescence spectroscopy provided direct evidence of energy transfer between components. Then we discussed the factors affecting the separation of photogenerated charges and the charge dynamics from surface area, band gap, conduction band edge, energy level matching and overlapping of wave functions. The research results also provide a way to select composite systems as photoanodes for dye-sensitized solar cells.

Emission characteristics of a barrier discharge in an Ar-H2O mixture

We have experimentally studied the UV radiation of a low-temperature barrier discharge plasma in an Ar-H2O mixture. The spectral interval 300–400 nm has been examined in detail. Addition of argon with a pressure of 24 kPa to a barrier discharge in water vapor at a pressure of ∼0.1 kPa leads to a ninefold increase in the UV radiation power of excited hydroxyl molecules. An increase in the duration of the UV radiation pulse of the mixture in the longitudinal discharge decay has been achieved for the first time, which may be direct evidence of energy transfer from metastable argon atoms to water molecules. An estimate of the upper boundary of the dissociative excitation rate constant of hydroxyl molecules OH*(A2Σ+) upon interaction of metastable argon atoms with water molecules has been obtained.

Direct evidence of energy transfer from Er3+ to Sm3+ in Er3+/Sm3+ co-doped system

Sm3+ and Er3+ co-doped NaYF4 microcrystals were synthesized through a facile hydrothermal method. The samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), upconversion (UC) emission spectra, and dynamic analysis. In the co-doped samples, the bridging effect of Er3+ ions played an important role in populating excited Sm3+. The energy transfer (ET) from Er3+ to Sm3+ was confirmed conclusively under 383nm excitation. Experiments on concentration variation and dynamic analysis revealed the ETs from Er3+ to Sm3+ in detail. The possible routes for populating excited Sm3+ ions were proposed based on spectral and dynamic analysis.

Broadband Near-Infrared Quantum Cutting in Eu2 + and Yb3 + Ions Co-doped CaAl2O4 Phosphor

A and coactivated phosphor that can cut one photon in UV and visible regions into two near-infrared (NIR) region photons could be used as a downconversion luminescent convertor in front of crystalline silicon solar cells to reduce the thermalization loss. The luminescence spectra in both visible and NIR regions and the luminescence decay curves of were recorded to provide a direct evidence of energy transfer from to ions. The highest energy transfer efficiency was estimated to be 55.05%. The results indicate that is a potential ion couple which may be used to improve the solar cell efficiency.

Investigation of Macrocyclic Polymers as Artificial Light Harvesters: Subpicosecond Energy Transfer in Poly(9,9-dimethyl-2-vinylfluorene)

The spectroscopy and dynamics of a novel molecular architecture that mimics natural light harvesting have been characterized. The deployment of 9,9-dimethyl-2-fluorenyl (DMF) chromophores in atactic macrocyclic poly(9,9-dimethyl-2-vinylfluorene) is similar to that in the light harvesting antenna LH2 of the purple photosynthetic bacteria. A variety of spectroscopic probes are used to study the dynamics in these novel polymer systems. The number of chromophores is tuned from 12-142 identical chromophore units. Steady-state absorption and emission measurements, time-resolved fluorescence, and ultrafast transient absorption anisotropy techniques provide evidence for distinct differences in the photophysics of matching molecular weight linear and cyclic polymers and of the occurrence of energy transfer in these polymers. There is direct evidence of energy transfer in these macrocycles manifested in the depolarization decay components, which are characterized by two exponentials and are substantially faster than observed for reorientation of the free DMF chromophore. The time constants for the macrocycles are 700-900 fs and 7-8 ps and are size dependent; the biexponential decay arises from conformational and stereochemical disorder and can be well described by a master equation simulation assuming Förster incoherent hopping on model polymer structures. The results suggest energy hopping between adjacent chromophores on a 1 ps time scale. The pathway for energy migration is shown to be primarily between nearest neighbors along the cyclic backbone, but there is a considerable spread in the site-to-site hopping rates. Small cycles adopt a pseudoplanar ring type arrangement of the chromophore transition dipoles as observed in bacterial light harvesting antenna, and it is found that the linear polymers also show similar short-range planarity of transition dipoles. Overall, it is found that such small macrocyclic polymers possess excellent characteristics for light harvesting among identical chromophores and behave as a circular photonic wire.

Photo- and electroluminescence of SrS:Cu and SrS:Ag,Cu,Ga thin films

Photoluminescence (PL) and electroluminescence (EL) of blue and green SrS:Cu thin films prepared by atomic layer epitaxy and molecular beam epitaxy, and blue SrS:Ag,Cu,Ga thin films prepared by sputtering were studied in a temperature range of 80–320 K. Two bands are present both in the PL and EL spectra of SrS:Cu films. The low energy band (L band) at about 520 nm can be observed throughout the temperature range studied. The high energy band (H band) at about 460 nm is seen only at higher temperatures and it disappears at 80 K. Decay studies at 80 K reveal at least two types of centers having lifetimes of 100 and 17 μs with a 10% variation. Several luminescence processes are proposed for further discussions. The L band may be attributed to the emission from an isolated Cu+ ion replacing the host Sr2+ ion but at an off-center site, and it undergoes a redshift caused by aggregated Cu centers. The H band is tentatively described as due to the Cu+ ion at different site symmetry. Blue luminescence of SrS:Ag,Cu,Ga most probably originates from Ag+ pairs. There is no direct evidence of energy transfer from Cu+ to Ag+ but Cu does contribute to the EL.

Functional interactions between subunits of aspartate aminotransferase: Formation of monoliganded dimers during titration of the apoenzyme by pyridoxal 5′-phosphate

The interaction between apoaspartate aminotransferase and pyridoxal 5′-phosphate at either pH 8.3 (active form of holoenzyme) or pH 5.0 (inactive form) corresponds to a strong quenching of tryptophan fluorescence. The hybrid molecule containing one pyridoxal 5′-phosphate bound per dimer has been prepared both by electrofocusing and by ion exchange chromatography. At both pH values, the fluorescence of the hybrid is 80 to 85% of the arithmetic mean between the fluorescence of the symmetrical holoenzyme and apoenzyme. This is direct evidence of energy transfer from tryptophan residues of the subunit of apoenzyme to the coenzyme of the other subunit. Fluorescence intensity was used to determine the quantity of hybrid holoapoenzyme formed during titration of the apoenzyme by pyridoxal 5′-phosphate. At pH 8.3 a non-linear decrease in the fluorescence is observed, corresponding to 60% of hybrid for the point of half reactivation; this value corresponds to the percentage obtained by electrofocusing (Schlegel & Christen, 1974). At pH 5.0, the decrease in fluorescence is linear during pyridoxal binding; this indicates that at this pH the hybrid is never obtained at detectable concentrations. These results indicate strong interactions between subunits of aspartate aminotransferase corresponding to a weakly negative co-operativity at alkaline pH and a positive cooperativity at acidic pH for the binding of the coenzyme.