Time-resolved Luminescence Measurements Research Articles

Intramolecular charge transfer photoemission of a silicon-based copolymer containing carbazole and divinylbenzene chromophores. Electron transfer across silicon bridges.

A new copolymer consisting of N-isopropylcarbazole/dimethylsilylene bridge/divinylbenzene units was synthesized and characterized. Dual fluorescence was observed in this copolymer in polar solvents. The absence of the second band at the lower transition energy of the two emission maxima in nonpolar solvents and the quantitative correlation of the lower-energy emission band maxima with solvent polarity indicate that the lower-energy emission band arises from an intramolecular charge transfer (ICT) state. A series of model compounds was synthesized to investigate the source of the charge transfer. It was found that the Si-bridged dyad with a single N-isopropylcarbazole and a single divinylbenzene was the minimum structure necessary to observe dual luminescence. The lack of dual luminescence in low-temperature glasses indicates that the ICT requires a conformation change in the copolymer. Analogous behavior in the Si-bridged dyad suggests that the ICT in the copolymer is across the silicon bridge. Results from time-resolved luminescence measurements with picosecond and subnanosecond excitation were used to support the thesis that twisted charge-transfer states are the likely source of the observed dual luminescence.

Formation of Photoluminescent Lead Bromide Nanoparticles on Aluminoborosilicate Glass

A multicomponent aluminoborosilicate photoluminescent glass was synthesized by introducing Pb(II) and NaBr in its composition. The room-temperature photoluminescence is due to the existence of 4 nm nanocrystals, shown using TEM imaging and XRD analysis, which are assigned to PbBr2 nanocrystals. The glasses display a broad emission band with a peak at 2.85 eV by exciting at 3.35 eV, with an anisotropy equal to 0.19 at room temperature. At 77 K, the emission intensity increases 1 order of magnitude and a vibronic structure appears, indicating an electron–phonon coupling with the glass matrix. Time-resolved luminescence measurements of these nanocrystals reveal mixed-order kinetics, with second-order recombination of self-trapped electron centers and a first-order temperature-dependent nonradiative rate constant connected with pathways due to confinement of self-trapped centers.

Ultrafast Charge- and Energy-Transfer Dynamics in Conjugated Polymer: Cadmium Selenide Nanocrystal Blends

Hybrid nanocrystal–polymer systems are promising candidates for photovoltaic applications, but the processes controlling charge generation are poorly understood. Here, we disentangle the energy- and charge-transfer processes occurring in a model system based on blends of cadmium selenide nanocrystals (CdSe-NC) with poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylene vinylene] (MDMO-PPV) using a combination of time-resolved absorption and luminescence measurements. The use of different capping ligands (n-butylamine, oleic acid) as well as thermal annealing allows tuning of the polymer–nanocrystal interaction. We demonstrate that energy transfer from MDMO-PPV to CdSe-NCs is the dominant exciton quenching mechanism in nonannealed blends and occurs on ultrafast time scales (<1 ps). Upon thermal annealing electron transfer becomes competitive with energy transfer, with a transfer rate of 800 fs independent of the choice of the ligand. Interestingly, we find hole transfer to be much less efficient than electron transfer and to extend over several nanoseconds. Our results emphasize the importance of tuning the organic–nanocrystal interaction to achieve efficient charge separation and highlight the unfavorable hole-transfer dynamics in these blends.

Highly enhanced visible luminescence in Zn1−xMgxO nanocrystals

Intensified trap luminescence in Zn1−xMgxO nanocrystals compared with nondoped ZnO nanocrystals is investigated using static and time-resolved luminescence measurements at room temperature. As the Mg concentration x increases from 0 to 0.14, the visible luminescence increases by 10 times and its quantum efficiency reaches 22% while the spectrum blueshifts and broadens. The time-resolved measurements reveal that the spectrum redshifts and narrows with time after the excitation. These temporal changes are ascribed to a large distribution of the trap energy induced by Mg doping, not to the energy distribution in the conduction band caused by crystal size fluctuation.

Photophysics and electrochemistry of a platinum-acetylide disubstituted perylenediimide

The synthesis and photophysical study of a perylene diimide (PDI) functionalised with platinum acetylide units of the type, trans{-C≡C-Pt(PBu3)2-C≡C-Ph} and comparison with a phenylacetylide substituted model compound are reported. The model compound demonstrates typical perylene absorption and photoluminescence spectra characteristic of singlet excited state formation and decay. The Pt-substitution, however, appears to induce spin-orbit coupling into the chromophore and giving rise to a triplet excited state which was confirmed by transient absorption measurements. This excited state is quenched by oxygen, leading to the formation of singlet oxygen in dichloromethane, recorded by time-resolved near-infrared luminescence measurements.

Trap‐Limited Exciton Diffusion in Organic Semiconductors

From time-resolved luminescence measurements it is demonstrated that the background concentration of excitonquenching defects in a range of organic semiconductors is the same as their electron-trap concentration. This observation suggests that the exciton-quenching defects and the electron traps share the same origin. The typical exciton diffusion length of 5–8 nm for organic semiconductors is therefore governed by the distance between the universal electron traps.

Note: Near infrared spectral and transient measurements of PbS quantum dots luminescence

We describe an experimental setup for the characterization of luminescence from nanostructures. The setup is intended for steady-state and time-resolved luminescence measurements in the near-infrared region. The setup allows us to study spectral luminescence properties in the spectral range of 0.8-2.0 μm with high spectral resolution and kinetic luminescence properties between 0.8 and 1.7 μm with a time resolution of 3 ns. The capabilities of the system are illustrated by taking luminescence measurements from PbS quantum dots. We established the size dependencies of the optical properties of the PbS quantum dots over a wide spectral range. Finally, the energy transfer process was studied with a high temporal and spectral resolution.

Luminescence Amplification by Enzymatic Eu2+ Oxidation to Eu3+ for Time-Resolved Peroxidase Activity Measurement

A novel peroxidase activity assay was developed for horseradish peroxidase (HRP) and myeloperoxidase (MPO), in which substrate Eu(2+) was catalytically oxidized to Eu(3+), and the Eu(3+) luminescence was enhanced by the addition of sensitizer 4,4'-bis(1″,1″,1″,2″,2″,3″,3″-hepatafluoro-4″,6″-hexanedione-6″-yl)chlorosulfo-o-terphenyl (BHHCT) for time-resolved measurement of the BHHCT-Eu(3+) complex. Since BHHCT-Eu(3+) has a long lifetime (more than 500 μs), typical of Eu(3+) oxidation state, and the emission wavelength (615 nm) is totally different from those of Eu(2+) complexes, time-resolved luminescence measurement of the Eu(3+) complex enabled suppressed background and high signal/background ratio. The present method was successfully applied to monitor the oxidative stress level, which is closely associated with peroxidase activity level, in rat heart muscle homogenates. Notable parallel temporal change was observed for peroxidase activity and 4-hydroxynonenal (HNE) concentration after lipopolysaccharide (LPS) injection for induction of oxidative stress in rats. Such a relation does not contradict the oxidative stress mechanism that HNE is produced via lipid peroxidation, which is caused by the (•)OH radical generated by peroxidase activity.

Transient Absorption Kinetics Associated with Higher Exciton States in Semiconducting Single-Walled Carbon Nanotubes: Relaxation of Excitons and Phonons

We investigate the transient absorption kinetics due to excitons associated with the first and higher transition states in semiconducting single-walled carbon nanotubes at high excitation densities by means of femtosecond pump–probe spectroscopy. The time evolutions of absorption changes associated with the first, second, and third excitonic transition bands exhibit similar decay behavior; the decay is much longer than the exciton population decay observed by time-resolved luminescence measurements. From spectral shape analysis of the transient absorption spectra, it is found that the absorption change in the short time regime (<∼2 ps) is mainly due to the bimolecular Auger recombination process, reflecting the exciton population decay. The long-lasting decay behavior over ∼1 ns is attributed to the thermalization process of phonons generated during the inter- and intraband relaxation of excitons promoted to higher states by the Auger recombination process and the subsequent thermal diffusion to the surrounding surfactant and solvent.

Efficient Formation of Luminescent Lanthanide(III) Complexes by Solid‐Phase Synthesis and On‐Resin Screening

Time-resolved luminescence measurements of luminescent lanthanide complexes have advantages in biological assays and high-throughput screening, owing to their high sensitivity. In spite of the recent advances in their energy-transfer mechanism and molecular-orbital-based computational molecular design, it is still difficult to estimate the quantum yields of new luminescent lanthanide complexes. Herein, solid-phase libraries of luminescent lanthanide complexes were prepared through amide-condensation and Pd-catalyzed coupling reactions and their luminescent properties were screened with a microplate reader. Good correlation was observed between the time-resolved luminescence intensities of the solid-phase libraries and those of the corresponding complexes that were synthesized by using liquid-phase chemistry. This method enabled the rapid and efficient development of new sensitizers for Sm(III), Eu(III), and Tb(III) luminescence. Thus, solid-phase combinatorial synthesis combined with on-resin screening led to the discovery of a wide variety of luminescent sensitizers.

Photoluminescence properties of red phosphor Gd3PO7:Eu3+ for UV-pumped light-emitting diodes

A red-emitting Gd3PO7:5% Eu3+ phosphor is reported. Through transitions of 5d -> 4f (F-7(1) and F-7(2)) in Eu3+, the phosphor shows a bright red emission under ultraviolet (UV) light excitation. The emission intensity at 615 nm corresponding to D-5(0) -> F-7(2) transition of Eu3+ ion depending on irradiation time under 394 nm excitation increases firstly and then small reduces. After 20 min for UV-light irradiation, it decreases by about 0.5% suggesting that the photostability of the red phosphor Gd3PO7:Eu3+ is excellent. Under 325 nm excitation, the emission intensities grow rapidly as the temperature increases from 75 K to 285 K and then it keeps on growing slowly until 475 K, revealing the excellent thermostability of the red phosphor Gd3PO7:Eu3+. Decay time and time-resolved luminescence measurements have also been studied. All the results reveal that Gd3PO7:Eu3+ is a promising red-emitting phosphor for UV LED applications. (c) 2013 Elsevier B.V. All rights reserved.

Polarized luminescence from Jahn-Teller split triplet states of self-trapped excitons in PbMoO4

The green luminescence from the triplet states of a self-trapped exciton (STE) located on a tetrahedral (MoO${}_{4}$)${}^{2\ensuremath{-}}$ ion in PbMoO${}_{4}$ splits into three bands, I, II, and III, as a result of the symmetry lowering of the (MoO${}_{4}$)${}^{2\ensuremath{-}}$ ion due to a Jahn-Teller effect. Time-resolved luminescence measurements have been performed for polarizations with the electric vector parallel to the crystallographic $c$ axis ($\mathbf{E}\ensuremath{\parallel}\mathbf{c}$) and perpendicular to it (E\ensuremath{\perp}c) in a temperature range between 8 and 225 K. All three bands are confirmed to exhibit significant polarizations. The main band II at 520 nm is polarized with $\mathbf{E}\ensuremath{\parallel}\mathbf{c}$ at low temperatures ($T$ 75 K) but changes its polarization from $\mathbf{E}\ensuremath{\parallel}\mathbf{c}$ to E\ensuremath{\perp}c at $T$ &gt; 125 K. Band III at 600 nm is polarized with E\ensuremath{\perp}c at $T$ = 170 and 225 K. The degree of polarization of these two bands does not depend on the polarization of excitation light. On the other hand, band I at 405 nm is polarized with E\ensuremath{\perp}c ($\mathbf{E}\ensuremath{\parallel}\mathbf{c}$) under the excitation by photons polarized parallel (perpendicular) to the $c$ axis, in the range of $T$ = 150--225 K. The observation of polarized luminescence provides convincing evidence for further lowering of the symmetry of (MoO${}_{4}$)${}^{2\ensuremath{-}}$ ions due to a uniaxial crystal field in addition to the Jahn-Teller distortion. The sublevels of the triplet STE state responsible for the PbMoO${}_{4}$ luminescence are identified on the basis of group-theoretical considerations.

Multiparametric Homogeneous Method for Identification of Ligand Binding to G Protein-Coupled Receptors: Receptor–Ligand Binding and β-Arrestin Assay

Two homogeneous assay systems have been combined to provide a new cell-based functional assay. The assay can be used to identify ligand binding to β(2)-adrenergic receptors, but also the downstream response can be determined in the same assay. Both the quenching resonance energy transfer (QRET) and the DiscoveRx PathHunter assay formats allow the use of intact cells. The homogeneous QRET technique is a single-label approach based on nonspecific quenching of the time-resolved luminescence, enabling agonist and antagonist receptor binding measurements. The commercial PathHunter assay is in turn based on enzyme fragment complementation, which can be detected on the basis of chemiluminescence signal. In the PathHunter technology the enzyme complementation is recorded immediately downstream of agonist-induced receptor activation. The new multiparametric detection technology combines these two assay methods enabling the identification of agonist, and antagonist binding to the receptor, and the agonist-induced response. Using the QRET and the PathHunter methods a panel of β(2)-adrenergic receptor ligands (epinephrine, terbutaline, metaproterenol, salmeterol, propranolol, alprenolol, bisoprolol, ICI 118,551, and bucindolol) was tested to prove the assay performance. The signal-to-background ratio for tested ligands ranged from 5 to 11 and from 6 to 18 with QRET and PathHunter, respectively. Combined homogeneous assay technique can provide an informative method for screening purposes and an efficient way to monitor receptor-ligand interaction, thus separating agonist from antagonist.

Optical and Time-Resolved Electron Paramagnetic Resonance Studies of the Excited States of a UV-B Absorber (4-Methylbenzylidene)camphor

The excited states of UV-B absorber (4-methylbenzylidene)camphor (MBC) have been studied through measurements of UV absorption, phosphorescence, triplet-triplet (T-T) absorption, and steady-state and time-resolved electron paramagnetic resonance spectra in ethanol. The energy level and lifetime of the lowest excited triplet (T(1)) state of MBC were determined. The energy level of the T(1) state of MBC is much lower than that of photolabile 4-tert-butyl-4'-methoxydibenzoylmethane. The weak phosphorescence and strong time-resolved EPR signals, and T-T absorption band of MBC were observed. These facts suggest that the significant proportion of the lowest excited singlet (S(1)) molecules undergoes intersystem crossing to the T(1) state and the deactivation process from the T(1) state is predominantly radiationless. The quantum yields of singlet oxygen production by MBC determined by time-resolved near-IR luminescence measurements are 0.05 ± 0.01 and 0.06 ± 0.01 in ethanol and in acetonitrile, respectively. The photostability of MBC arises from the (3)ππ* character in the T(1) state. The zero-field splitting parameters in the T(1) state are D = 0.0901 cm(-1) and E = -0.0498 cm(-1). The sublevel preferentially populated by intersystem crossing is T(y) (y close to in-plane short axis and to the C═O direction).

Does photodissociation of molecular oxygen from myoglobin and hemoglobin yield singlet oxygen?

Time-resolved luminescence measurements in the near-infrared region indicate that photodissociation of molecular oxygen from myoglobin and hemoglobin does not produce detectable quantities of singlet oxygen. A simple and highly sensitive method of luminescence quantification is developed and used to determine the upper limit for the quantum yield of singlet oxygen production. The proposed method was preliminarily evaluated using model data sets and confirmed with experimental data for aqueous solutions of 5,10,15,20-tetrakis(4-N-methylpyridyl) porphyrin. A general procedure for error estimation is suggested. The method is shown to provide a determination of the integral luminescence intensity in a wide range of values even for kinetics with extremely low signal-to-noise ratio. The present experimental data do not deny the possibility of singlet oxygen generation during the photodissociation of molecular oxygen from myoglobin and hemoglobin. However, the photodissociation is not efficient to yield singlet oxygen escaped from the proteins into the surrounding medium. The upper limits for the quantum yields of singlet oxygen production in the surrounding medium after the photodissociation for oxyhemoglobin and oxymyoglobin do not exceed 3.4×10−3 and 2.3×10−3, respectively. On the average, no more than one molecule of singlet oxygen from every hundred photodissociated oxygen molecules can succeed in escaping from the protein matrix.

Rapid energy transfer in non-porous metal–organic frameworks with caged Ru(bpy)32+ chromophores: oxygen trapping and luminescence quenching

Two non-porous metal–organic frameworks (MOFs) with caged Ru(bpy)32+ chromophores, [Ru(bpy)3][Zn2(C2O4)3] (1) and [Ru(bpy)3][NaAl(C2O4)3] (2), were synthesized and characterized. Their emission properties were studied by both steady-state and time-resolved luminescence measurements. Air-free microcrystals of 1 and 2 exhibit long-lived triplet metal-to-ligand charge transfer (3MLCT) excited states with lifetimes of 760 and 1305 ns, respectively. Lifetimes are significantly shortened (to 92 ns for 1 and 144 ns for 2) by trapping of trace amounts of oxygen in the non-porous MOFs, presumably due to amplified luminescence quenching of Ru(bpy)32+*. Following MLCT excitation, Ru(bpy)32+*/Ru(bpy)32+ energy transfer migration in 1 and 2 results in efficient quenching of Ru(bpy)32+* by Os(bpy)32+ added as an energy transfer trap at doping levels of 0.2–1.0%. A kinetic analysis indicates that the three-dimensional chromophore connectivity in 1 and 2 provides a network for rapid excited state energy transfer migration among Ru(bpy)32+ units, ultimately, finding an Os(bpy)32+ trap site. These crystalline frameworks with caged chromophores have proven to be ideal systems for studying light harvesting processes in artificial supramolecular systems.

Enhanced ultraviolet emission and improved spatial distribution uniformity of ZnO nanorod array light-emitting diodes via Ag nanoparticles decoration

Localized surface plasmon (LSP) enhanced ultraviolet (UV) light-emitting diodes (LEDs) were fabricated by embedding a ZnO nanorod array/p-GaN film heterostructure into a Ag-nanoparticles/PMMA composite. By optimizing the concentration of Ag nanoparticles in PMMA, two distinct changes in electroluminescence (EL) spectra were observed: (1) the UV EL component from ZnO excitons was selectively enhanced more than 13-fold and the entire spectral lineshape was changed and (2) the spatial uniformity of the output photon intensity was improved and the linewidth of an angular distribution curve was increased by ∼2 times. These observations can be attributed to near-field optical coupling between Ag LSPs and ZnO excitons. Time-resolved luminescence measurements and a model calculation reveal that the optical coupling results in the increase of the spontaneous emission rate and internal quantum efficiency of Ag-nanoparticles-decorated ZnO nanorod arrays. Moreover, the LSP-exciton interaction allows the device's EL to be coupled out of the nanorod waveguide and to be isotropically scattered into every direction, thus broadening the angular distribution of the EL intensity.

Two-Photon Photodynamic Therapy by Water-Soluble Self-Assembled Conjugated Porphyrins

Studies on two-photon absorption (2PA) photodynamic therapy (PDT) by using three water-soluble porphyrin self-assemblies consisting of ethynylene-linked conjugated bis (imidazolylporphyrin) are reviewed. 2PA cross-section values in water were obtained by an open aperture Z-scan measurement, and values were extremely large compared with those of monomeric porphyrins such as hematoporphyrin. These compounds were found to generate singlet oxygen efficiently upon one- as well as two-photon absorption as demonstrated by the time-resolved luminescence measurement at the characteristic band of singlet oxygen at 1270 nm and by using its scavenger. Photocytotoxicities for HeLa cancer cells were examined and found to be as high as those of hematoporphyrin, demonstrating that these compounds are potential candidates for 2PA-photodynamic therapy agents.

Unusual x-ray excited luminescence spectra of NiO suggest self-trapping of thed-dcharge-transfer exciton

Luminescence spectra of NiO have been investigated under vacuum ultraviolet (VUV) and soft x-ray (XUV) excitation (DESY, Hamburg). Photoluminescence (PL) spectra show broad emission violet and green bands centered at about 3.2 and 2.6 eV, respectively. The PL excitation (PLE) spectral evolution and lifetime measurements reveal that the two mechanisms with short and long decay times, attributed to the $d({e}_{g})$-$d({e}_{g})$ and $p(\ensuremath{\pi})$-$d$ charge transfer (CT) transitions in the range 4--6 eV, respectively, are responsible for the observed emissions. The XUV excitation makes it possible to avoid the predominant role of the surface effects in luminescence and reveals a bulk violet luminescence with a puzzling well-isolated doublet of very narrow lines. These lines with close energies near 3.3 eV are attributed to recombination transitions in the self-trapped $d$-$d$ CT excitons formed by the coupled Jahn-Teller Ni${}^{+}$ and Ni${}^{3+}$ centers. The conclusion is supported by a comparative analysis of the luminescence spectra for NiO and solid solution Ni${}_{x}$Zn${}_{1\ensuremath{-}x}$O and by a comprehensive cluster model assignment of different $p$-$d$ and $d$-$d$ CT transitions and their relaxation channels. Our paper shows that the time-resolved luminescence measurements provide an instructive tool for the elucidation of the $p$-$d$ and $d$-$d$ CT excitations and their relaxation in $3d$ oxides.

Novel Red-Emitting Ba2Tb(BO3)2Cl:Eu Phosphor with Efficient Energy Transfer for Potential Application in White Light-Emitting Diodes

A novel red-emitting Ba(2)Tb(BO(3))(2)Cl:Eu phosphor possessing a broad excitation band in the near-ultraviolet (n-UV) region was synthesized by the solid-state reaction. Versatile Ba(2)Tb(BO(3))(2)Cl compound has a rigid open framework, which can offer two types of sites for various valence's cations to occupy, and the coexistence of Eu(2+)/Eu(3+) and the red-emitting luminescence from Eu(3+) with the aid of efficient energy transfer of Eu(2+)-Eu(3+)(Tb(3+)) and Tb(3+)-Eu(3+) have been investigated. Ba(2)Tb(BO(3))(2)Cl emits green emission with the main peak around 543 nm, which originates from (5)D(4) → (7)F(5) transition of Tb(3+). Ba(2)Tb(BO(3))(2)Cl:Eu shows bright red emission from Eu(3+) with peaks around 594, 612, and 624 nm under n-UV excitation (350-420 nm). The existence of Eu(2+) can be testified by the broad-band excitation spectrum, UV-vis reflectance spectrum, X-ray photoelectron spectrum, and Eu L(3)-edge X-ray absorption spectrum. Decay time and time-resolved luminescence measurements indicated that the interesting luminescence behavior should be ascribed to efficient energy transfer of Eu(2+)-Eu(3+)(Tb(3+)) and Tb(3+)-Eu(3+) in Ba(2)Tb(BO(3))(2)Cl:Eu phosphors.