Fast Energy Transfer Processes Research Articles

Efficient Yellow Emission and Near-Unified Photoluminescence Quantum Yield of Sb3+ in a One-Dimensional Confinement Cadmium Chloride Lattice

Sb3+, with a 5s2 electron configuration, has been widely studied as a dopant for its efficient self-trapped exciton (STE) broadband emission. However, the efficient emission of Sb3+-doped one-dimensional (1D) structures and the energy/charge transfer between the host and dopant are rarely reported. In this paper, a series of Sb3+-doped 1D (TDMP)CdCl4 (TDMP = trans-2,5-dimethylpiperazine) powder crystals were prepared by the solvothermal method. The 1D quantum confinement effect and Jahn–Teller distortion in this organic–inorganic hybrid with a soft lattice generate strong electron–phonon coupling. In addition, we also found a fast energy transfer process between the host ([CdCl6]2–) and the dopant ([SbCl6]3–). The synergy of strong electron–phonon coupling and fast energy transfer enables 8.8% Sb3+:(TDMP)CdCl4 to exhibit efficient broadband yellow emission under UV excitation with a photoluminescence quantum yield (PLQY) of 99.69%. DFT calculations indicate that the emission comes from the triplet self-trapped excitons (STEs) emission of the [SbCl6]3– octahedron. This work provides a deeper understanding of the emission of Sb3+ in doped systems and provides a strategy to guide the design of future efficient luminescent materials.

Enhancing the FRET by tuning the bandgap of acceptor ternary ZnCdS quantum dots.

In this article, we report the band gap tuning of ternary ZnCdS quantum dots (QDs) by varying the concentration of the capping ligand, mercaptoacetic acid (MAA). The functionalization of QDs leads to the control of their size and band gap due to the quantum confinement effect, causing blue shift in the absorption and photoluminescence (PL) spectra with a gradual change in the concentration of the capping ligand from 0.5 to 2.5 M. Ensulizole (2-phenylbenzimidazole-5-sulfonic acid) is an important organic ultraviolet (UV) filter that is frequently used in sunscreen cosmetics. An effective overlapping of the PL spectrum of ensulizole and the absorption spectrum of QDs with 2.5 M MAA is achieved. A formidable decrease in the PL intensity and the PL lifetime of ensulizole promotes an efficient Förster resonance energy transfer (FRET) from sunscreen ensulizole to the QDs. The magnitude of the FRET efficiency (E) is ∼70%. This very high value of E is the signature of the existence of a very fast energy transfer process from ensulizole to the MAA functionalized ZnCdS QDs. The dyad system consisting of ZnCdS QDs and ensulizole sunscreen can serve as a prototype model to develop a better understanding of the photochemistry of ensulizole and consequently the formulation of more efficient sunscreen cosmetics.

Favorable Mixing Thermodynamics in Ternary Polymer Blends for Realizing High Efficiency Plastic Solar Cells

AbstractTernary blends with broad spectral absorption have the potential to increase charge generation in organic solar cells but feature additional complexity due to limited intermixing and electronic mismatch. Here, a model system comprising the polymers poly[5,5‐bis(2‐butyloctyl)‐(2,2‐bithiophene)‐4,4‐dicarboxylate‐alt‐5,5‐2,2‐bithiophene] (PDCBT) and PTB7‐Th and PC70BM as an electron accepting unit is presented. The power conversion efficiency (PCE) of the ternary system clearly surpasses the performance of either of the binary systems. The photophysics is governed by a fast energy transfer process from PDCBT to PTB7‐Th, followed by electron transfer at the PTB7‐Th:fullerene interface. The morphological motif in the ternary blend is characterized by polymer fibers. Based on a combination of photophysical analysis, GIWAXS measurements and calculation of the intermolecular parameter, the latter indicating a very favorable molecular affinity between PDCBT and PTB7‐Th, it is proposed that an efficient charge generation mechanism is possible because PTB7‐Th predominantly orients around PDCBT filaments, allowing energy to be effectively relayed from PDCBT to PTB7‐Th. Fullerene can be replaced by a nonfullerene acceptor without sacrifices in charge generation, achieving a PCE above 11%. These results support the idea that thermodynamic mixing and energetics of the polymer–polymer interface are critical design parameter for realizing highly efficient ternary solar cells with variable electron acceptors.

Spectral and temporal photoluminescence behavior of colloidal PbS quantum dots

We have studied a temporal and spectral behavior of the photoluminescence of the colloidal PbS quantum dots (QDs) with a controlled spacing between QDs. The high-speed streak camera system combined with a monochromator was used to obtain the time evolution of the luminescence spectrum itself.In the close-packed QD sample with a QD edge-to-edge spacing of 4.2nm, the two-step photoluminescence decay with time constants of 30 and 200ns was observed after initial fast change in the luminescence due to the fast energy transfer process. This two-step decay also occurred for the other QD samples with large QD edge-to-edge spacings, i.e., 5.8 and 12.5nm. Here, the observed decay time for these samples showed the same spectral behavior even though the distance between QDs is considerably different. Such characteristic photon energy dependence strongly suggests a decay mechanism that the decay consists of (1) fast non-radiative decay from the excited state to the ground state through surface defect states and (2) slow intrinsic radiative decay. Thus, these results indicate a new aspect of the photoluminescence decay characteristics, which shows the importance of the non-radiative energy transfer path in the PbS QD luminescence.

Energy Migration in Dendritic Oligothiophene-Perylene Bisimides

A series of novel oligothiophene-perylene bisimide hybrid (DOTPBI) dendrimers up to the second generation (G0, G1, and G2) were investigated. Optical measurements such as nonlinear optical and time-resolved spectroscopy, including two-photon absorption, fluorescence upconversion, and excited state transient absorption were carried out. Results of these measurements revealed the ability of these molecules to undergo intramolecular fluorescence resonance energy transfer (FRET) from the dendritic oligothiophenes (DOT) to the perylene bismide (PBI) moiety. The delocalization length and the photoinduced electron transfer (PET) rate were investigated as a function of dendrimer generation. A fast energy transfer process from the DOT dendron to the PBI core was observed. For the case of the G2 dendrimer, with relatively large thiophene dendrons attached to the bay area of the perylene bisimide, the PBI core is highly twisted and its ability to self-assemble into π-π stacked aggregates is destroyed. As a result, among the three generations studied, G1, which has the best two-photon cross section and the most efficient energy transfer, is the best light harvesting material.

Excitation energy migration in covalently linked perylene bisimide macrocycles

A series of acetylene-linked perylene bisimide (PBI) macrocycles 3a–d with various ring sizes from trimer to hexamer have been synthesised by a palladium-catalysed homocoupling reaction of perylene bisimide 1. Photophysical properties of these PBI macrocycles have been examined by steady-state absorption, fluorescence, fluorescence lifetime, fluorescence anisotropy decay, and transient absorption measurements. Both pump-power dependence on the femtosecond transient absorption and the transient absorption anisotropy decay profiles are directly related with the excitation energy migration processes within PBI macrocycles where the exciton-exciton annihilation time and the polarization anisotropy decay time are well described in terms of the Forster-type incoherent energy hopping model. Consequently, the excitation energy hopping times of macrocycles become slower and then saturated as the ring size increases. Nevertheless, the intrinsically large transition dipole moment of PBI leads to fast energy transfer processes as compared to other artificial light-harvesting complexes such as those constructed from porphyrin building blocks.

KeV fullerene interaction with hydrocarbon targets: Projectile penetration, damage creation and removal

The physics of energetic fullerene projectile penetration, damage creation and sputtering in organic solids is investigated via molecular dynamics simulations. Two models are used, the first one based on a full atomistic description of the target and the second one, using a coarse-grain prescription that was recently developed and tested for a benzene molecular crystal [E. Smiley, Z. Postawa, I.A. Wojciechowski, N. Winograd, B. J. Garrison, Appl. Surf. Sci. 252 (2006) 6436]. The results explore the mechanism of energy transfer from the C 60 projectile to the organic target atoms/molecules through the comparison with significantly different projectiles (Argon) and samples (Ag crystal). The effects of the projectile energy on the penetration and fast energy transfer processes (200 fs) are also delineated. The second part of the results investigates the ‘long term’ consequences (20–50 ps) of fullerene impacts in hydrocarbon sample surfaces. In an icosane (C 20H 42) solid, a 5 keV C 60 projectile induces a crater of ∼10 nm diameter surrounded by a ∼4 nm wide rim and ejects ∼70 intact molecules. More than 75% of the fragments generated by the fullerene in the surface are also sputtered away by the end of the event. The perspective considers the capabilities of fullerene projectiles for depth profile analysis of molecular samples by particle-induced desorption mass spectrometry.

Radiative relaxation in 2p-excited argon clusters

The emission of ultraviolet fluorescence radiation from variable size argon clusters is investigated with high spectral resolution in the Ar 2p-excitation regime. The fluorescence excitation spectra reveal strong fluorescence intensity in the Ar 2p-continuum, but no evidence for the occurrence of discrete low-lying core-exciton states in the near-edge regime. This finding is different from the absorption and photoionization cross section of argon clusters and the solid. The dispersed fluorescence shows a broad molecular band centered near 280 nm. The present work indicates that double and triple ionization via the LMM-Auger decay are required to initiate the fluorescence processes in the Ar 2p-continuum. The present results are consistent with the formation of singly charged, excited moieties within the clusters, which are assigned as sources of the radiative emission in the 280 nm regime. A fast energy transfer process (interatomic Coulombic decay (ICD)), which has been proposed by recent theoretical work, is assigned to be primarily the origin of these singly charged, excited cations besides intra-cluster electron impact ionization by the Auger electrons. Our findings give first possible experimental evidence for ICD in the core level regime.

Chromium(III) Complexes for Photochemical Nitric Oxide Generation from Coordinated Nitrite: Synthesis and Photochemistry of Macrocyclic Complexes with Pendant Chromophores, trans-[Cr(L)(ONO)2]BF4

Several new dinitritochromium(III) complexes of the type trans-[Cr(L)(ONO)(2)]BF(4), where L is a derivative of the macrocyclic ligand cyclam having pendant aromatic chromophores attached (L = 5,7-dimethyl-6-(substituted)-1,4,8,11-tetraazacyclotetradecane), have been prepared and characterized. Photoexcitation of aqueous solutions containing these complexes at wavelengths corresponding to the pendant chromophore absorption bands led to the generation of NO as detected by an electrochemical sensor. Photophysical data show that the expected fluorescence of the pendant chromophores is largely quenched when the macrocyclic ligand is coordinated to these Cr(III) centers, and this is interpreted in terms of fast energy transfer processes from the ligand-centered pipi states to the Cr(III)-centered ligand field states leading to subsequent cleavage of the Cr(III)-coordinated nitrito ligand. Thus, the chromophores tethered to the coordinated cyclam serve as light-gathering antennae for the intramolecular sensitization of the NO-generating photoreactions at the metal center.

Energy transfer within perylene-terrylene dendrimers evidenced by polychromatic transient absorption measurements.

The time dependent spectral properties of a first and a second generation dendrimer with peryleneimide chromophores at the rim and a terrylenediimide chromophore in the core were investigated by time resolved polychromatic transient absorption measurements. The obtained results of the dendritic structures were compared with those of three model compounds. In the perylene-terrylene dendrimers a very fast energy transfer process was observed. Besides energy transfer singlet-singlet annihilation could be observed. It could be concluded that competition between the rates of energy transfer and of singlet-singlet annihilation is dependent on the dendrimer generation.

Energy transfer from π-conjugated ligand to rare-earth ions in (diethienyl-bipyridazine) eu3+ complex

We have carried out energy transfer studies of 5,5′-(2-dithienyl)-2,2′-bipyridazine Eu+3 (DTBPZ-Eu) complexes. Tuning the excitation to the oligomer absorption band, we have obtained characteristic Eu+3 photoluminescence. Long lived excitations that are photogenerated in the pure oligomer are absent in the complex, indicating a fast energy transfer process.

The role of covalency and bridging ligands in two‐ion cooperative optical transitions in lanthanide ion‐coupled systems

AbstractThe influence of covalency and bridging ligands on cooperative optical effects in dimers with closely coupled lanthanide ions in insulating crystals and binuclear complexes is theoretically treated. It is shown that there is a specific interaction between Ln3+ ions (which may be called covalent coupling) in tight ion pairs originating from virtual electron hoppings between lanthanide ions via common bridging ligands. This interaction can be regarded as a generalization of the superexchange interaction mechanism. A general microscopic model and the corresponding formalism for the covalent coupling mechanism are developed. A general expression to estimate the intensity of electric dipole two‐ion cooperative optical transitions is derived. Both the nature of bridging ligands and the local geometry of the Ln3+ (A)–ligandLn3+ (B) dimer are found to affect strongly the intensities of cooperative transitions. Our model predicts rapid increase in the contribution of this mechanism to the intensities of two‐ion transitions when going from small “ionic” ligands (F− and O2−) to large “covalent” ligands (Br−, I−). Possible manifestations of the covalent coupling mechanism in fast energy transfer processes and unusually strong up‐conversion fluorescence observed in CsCdBr3 : Ln3+ systems and as well as in electronic spectra of the U2Cl10 binuclear compound are discussed.

Site-selective spectroscopy and level ordering in C-phycocyanine

We present a combined fluorescence and hole-burning study of the biliprotein C-phycocyanin. Sharp zero-phonon holes compare with a broad structureless fluorescence. This finding is rationalized in terms of the special level structure in this pigment, the fast energy-transfer processes and a lack of correlation of the energies of the emissive states.

Picosecond time-resolved fluorescence spectra of photosystem I and II in Chlorella pyrenoidosa

Picosecond time-resolved fluorescence spectra emitted from intact cells of the green alga Chlorella pyrenoidosa have been measured by means of a new detection technique using a microchannel-plate photomultiplier. A fluorescence band (F700) was observed at 690–730 nm in the initial time region (0–180 ps), in addition to the well-known spectrum (F685) of photosystem II (PS II)-chlorophyll a (Chl a) with a peak at 685 nm. F700 decays rapidly with lifetime of 104 ps, while F685 decays much more slowly in bi-exponential form with lifetimes of 0.64 and 1.7 ns. Appearance of F700 is independent of closure of the reaction center II (RC II). F700 is thus assigned to the fluorescence from PS I-Chl a, whose decay is governed by a fast energy transfer process from the antenna Chl aof PS I to P700 of RC I.

Fast electrical discharge tube for time dependent spectoscopy

An improved transverse discharge tube for time resolved spectroscopy is described. Current densities of 1000 A/cm2 with risetime of less than 5×10−9 sec and total pulse length of 3×10−8 sec were achieved in pressures up to 500 Torr of noble gas mixtures. This improved performance, compared with the Tanaka longitudinal tube, is essential for observation of fast energy transfer processes originating at highly excited atomic and molecular levels.