Fluorescence Decay Measurements Research Articles

Determining the Exciton Diffusion Length in a Polyfluorene from Ultrafast Fluorescence Measurements of Polymer/Fullerene Blend Films

Emission quenching is studied in systems composed of [9,9-dioctylfluorene-co-N-(4-butylphenyl)-diphenylamine] (TFB) and various concentrations of three different types of acceptors: [6,6]-phenyl-C61 butyric acid methyl ester (mono-PCBM) and its multiadduct analogues bis-PCBM and tris-PCBM. We find that the degree of emission quenching for a given fullerene concentration decreases as the PCBM adduct number increases, while the microstructure, as observed with transmission electron microscopy, becomes more coarse. These observations are rationalized in terms of possible differences in the miscibility of fullerenes in the polymer and different excitation dissociation rates. We also extract a value for the exciton diffusion length in TFB of 9.0 ± 2 nm from ultrafast fluorescence decay measurements. The results have been confirmed with independent measurements.

Optical spectroscopy of Tm3+ : YAG transparent ceramics

Transparent YAG ceramics doped with different concentrations of Tm3+ have been fabricated by solid state reactive sintering and their spectroscopic properties have been thoroughly investigated. Absorption, emission and fluorescence decay measurements have been carried out in different experimental conditions in order to obtain information about the radiative and non-radiative processes in these materials. The 2 µm stimulated emission performance has been evaluated using for the first time an astigmatically compensated cavity which permitted assessment of the tunability of the laser emission.

Effect of La3+ incorporation on the spectroscopic properties of Yb3+ -doped Y1−x La x Ca4 O(BO3 )3 (x = 0.09 and 0.25) crystals

Through the partial substitution of Y3+ ions with La3+ ions in the structural framework of YCa4O(BO3)3, the Yb3+-doped mixed oxyborate Y1−xLaxCa4O(BO3)3 (x = 0.09 and 0.25) crystals have been grown and characterized. The La3+ content (x = 0.25) in the crystal far exceeds the La3+ doping limit (x = 0.1) previously reported, whereas the crystal remains transparent and free from cracks. Optical absorption, emission and fluorescence decay measurements have been performed at room temperature, and the influence of La3+ doping on the spectroscopic characteristics of the crystals has been presented and discussed in detail. The main spectral and laser parameters pertinent to laser potential have been evaluated and compared with those of other oxyborates. The perspectives for application of the Yb3+:Y1−xLaxCa4O(BO3)3 crystals as laser media have been discussed.

Broadband conversion of ultraviolet to visible and near-infrared emission in Gd3 +/Yb3 + codoped germanate glass

Gd3+/Yb3+ co-doped germanate glasses were fabricated using the melt-quenching method. Broadband visible emission was observed from the gadolinium doped germanate glasses under ultraviolet (UV) excitation, and white-light was obtained by modifying Yb3+ co-dopant concentration. Through this co-doped system, the white-light emission can be further converted into the near-infrared range which is matched well with the energy band gap of commercial silicon solar cell. The possible energy transfer processes involved are proposed and discussed according to the excitation, emission and fluorescence decay measurements. The effect of Yb3+ ion concentration with respect to fixed Gd3+ ions on the visible and near-infrared (NIR) emission has also been studied. The conversion efficiency from visible to near-infrared is as high as 69.3% in the GZN: 0.2Gd2O3–0.6Yb2O3 sample. The results indicate that this material is potential in various applications, such as white LED, enhancing silicon solar cell efficiency, and so on.

Near-infrared luminescence enhancing by co-doping Bi3+ in YVO4:Nd3+

NIR luminescence phosphors Nd3+, Bi3+ co-doped YVO4 were prepared by conventional solid-state reaction and the optical properties are investigated by photoluminescence, photoluminescence excitation spectra and fluorescence decay measurements. Introduction of Bi3+ in YVO4:Nd3+ led to significantly enhancing of the NIR emission of Nd3+ ions, the peak excitation wavelength also red-shifted from 335 to 352nm and match better with the solar spectrum. Energy transfer from VO43-, Bi3+ charge transfer states to the excitation states of Nd3+ accounts for this fluorescence enhancement and the possible energy transfer mechanism was briefly discussed. The peculiar optical properties of YVO4:Bi3+, Nd3+ help us to forecast the feasibility of potential application in enhancing the efficiency of Si-based solar cells.

Highly fluorescent and biocompatible iridium nanoclusters for cellular imaging

Highly fluorescent iridium nanoclusters were synthesized and investigated its application as a potential intracellular marker. The iridium nanoclusters were prepared with an average size of ~2 nm. Further, these nanoclusters were refluxed with aromatic ligands, such as 2,2'-binaphthol (BINOL) in order to obtain fluorescence properties. The photophysical properties of these bluish-green emitting iridium nanoclusters were well characterized by using UV-Visible, fluorescence and lifetime decay measurements. The emission spectrum for these nanoclusters exhibit three characteristic peaks at 449, 480 and 515 nm. The fluorescence quantum yield of BINOL-Ir NCs were estimated to be 0.36 and the molar extinction co-efficients were in the order of 10(6) M(-1)cm(-1). In vitro cytotoxicity studies in HeLa cells reveal that iridium nanoclusters exhibited good biocompatibility with an IC50 value of ~100 μg/ml and also showed excellent co-localization and distribution throughout the cytoplasm region without entering into the nucleus. This research has opened a new window in developing the iridium nanoparticle based intracellular fluorescent markers and has wide scope to act as biomedical nanocarrier to carry many biological molecules and anticancer drugs.

Photochemical trapping heterogeneity as a function of wavelength, in plant photosystem I (PSI–LHCI)

In the present paper the marked changes in photochemical trapping time over the absorption/fluorescence band of isolated PSI–LHCI are studied by means of time resolved fluorescence decay measurements. For emission at 680–690nm the effective trapping time is close to 17–18ps, and represents the effective trapping time from the bulk antenna. At wavelengths above 700nm the effective trapping time increases in a monotonic way, over the entire emission band, to attain values in the range of 70–80ps near 760nm. This is argued to be caused by “uphill” energy transfer from the low energy states to the core antenna and reaction centre. These data, together with the steady state emission spectrum, permit calculation of the overall trapping time for maize PSI–LHCI, which is estimated to be approximately 40ps. The wavelength dependence of the trapping time indicates, that in PSI–LHCI there exists at least one red form which emits at lower energies than the 735nm state. These data indicate that Photosystem I is about 55% diffusion limited.

Self-Assembly in Tailor-Made Polyfluorenes: Synergistic Effect of Porous Spherical Morphology and FRET for Visual Sensing of Bilirubin

Two new fluorene-based homo- (PDP–PF) and copolymers (PDPPF-co-Ph) were synthesized with a bulky 3-pentadecylphenoxy (PDP) group appended hexyl chains at the 9, 9′ position using Suzuki coupling polymerization. Investigation on the morphology of the polymers using microscopic techniques like TEM and AFM indicated formation of self-assembled nanostructures like vesicles by PDP–PF and porous spheres by PDPPF-co-Ph respectively. Dynamic as well as static light scattering studies (DLS, SLS) in THF also indicated the existence of self-assembled nanosized particles in solution with a shape factor (ρ) of 0.76 and 0.96 for PDP–PF and PDPPF-co-Ph, respectively, confirming the existence of vesicles in the case of the former and spherical particles in the case of the latter polymer. The favorable photophysical properties of the polyfluorenes were taken advantage of for the selective sensing of unbound bilirubin (BR) in THF. A high energy transfer efficiency of 86% upon addition of bilirubin with color change from blue (polyfluorene emission) to green (FRET-induced bilirubin emission) was observed with PDPPF-co-Ph. Steady state fluorescence measurements gave a minimum donor–acceptor distance of 36 A0 and time-resolved fluorescence decay measurements showed a reduction in average lifetime of PDPPF-co-Ph (from 450 to 240 ps) upon addition of bilirubin indicating efficient energy transfer. The open porous spherical assembly of PDPPF-co-Ph enabled better adsorption of the analyte, which along with the good spectral overlap resulted in greater efficiency for FRET-induced energy transfer. Sensing of unbound bilirubin was also attempted in THF/water solvent mixture in an effort to simulate the unbound (THF soluble) and bound (water-soluble) bilirubin equilibrium. Enhancement of bilirubin emission coupled with quenching of polyfluorene emission makes this approach adaptable for visual fluorimetric color change (blue to green) based sensor. Structural analogues such as biliverdin and porphyrin showed poor fluorescence quenching efficiency, thus highlighting the selectivity and sensitivity of the FRET-based sensing of bilirubin by the newly designed polyfluorene.

Initial stages of beta-amyloid Aβ1−40 and Aβ1−42 oligomerization observed using fluorescence decay and molecular dynamics analyses of tyrosine

The development of Alzheimer’s disease is associated with the aggregation of the beta-amyloid peptides Aβ1−40 and Aβ1−42. It is believed that the small oligomers formed during the early stages of the aggregation are neurotoxic and involved in the process of neurodegeneration. In this paper we use fluorescence decay measurements of beta-amyloid intrinsic fluorophore tyrosine (Tyr) and molecular dynamics (MD) simulations to study the early stages of oligomer formation for the Aβ1−40 and Aβ1−42 peptides in vitro. We demonstrate that the lifetime distributions of the amyloid fluorescence decay efficiently describe changes in the complex Tyr photophysics during the peptide aggregation and highlight the differences in aggregation performance of the two amyloids. Tyr fluorescence decay is found to be a more sensitive sensor of Aβ1−40 aggregation than Aβ1−42 aggregation. The MD simulation of the peptide aggregation is compared with the experimental data and supports a four-rotamer model of Tyr.

Phonon assisted effective non-resonant energy transfer based 1 μm luminescence from Nd3+–Yb3+ codoped zinc–boro–bismuthate glasses

Phonon assisted non-resonant energy transfer from Nd3+→Yb3+ is reported in a new series of heavy metal oxide based zinc–boro–bismuthate glasses. An intense infrared emission at around 1μm from Yb3+ has been realized upon Nd3+ excitation, with a transfer efficiency reaching up to 87% for fairly low acceptor concentration of 1mol% Yb2O3. Such high energy transfer efficiency despite poor spectral overlap of donor’s emission with acceptor's absorption has been attributed to the excellent matching of host phonon energy with the energy difference among Nd3+ and Yb3+ excitation levels. From the host assisted spectral overlap function, the microscopic energy transfer parameter (CDA) for Nd3+→Yb3+ energy transfer in these glasses have been estimated and is found to be ∼3.6×10−39cm6s−1, which is in good agreement with the CDA value obtained from decay analysis of donor luminescence. The measured fluorescence decay curve showed excellent correlation with the theoretical fit simulated using hopping model suggesting the hopping migration assisted energy transfer. A comparison of the obtained results with other reported host systems has been presented using a performance parameter (ΦET) defining the ability of Nd3+→Yb3+ energy transfer per unit loss occurred due to Nd3+–Nd3+ self-quenching.

Femtosecond Fluorescence Dynamics of Trp-Trp Dipeptide in Water and its Interaction with DNA

We reported here the systematic measurements of fluorescence decay and quenching of tryptophan dipeptide Trp-Trp in water, and its interaction with DNA by using an ultraviolet upconversion fluorescence measurement system with time resolution better than 100 fs together with a time correlated single photon counting apparatus on the 100 ps to 30 ns time scale. We analyzed the effect of acidity and phosphate ion etc upon Trp-Trp fluorescence decay profiles. Fluorescence data reveal multiexponential decay of Trp-Trp dipeptide in water, and Trp-Trp can interact with DNA. It is shown that Trp-Trp is subject to ultrafast quenching with DNA and also senses nearby water dynamics. The time constant for the bulk water “solvent relaxation” remains near 1 ∼ 2 ps, but a new ultrafast decay, fit to an exponential time constant of ∼ 10 ps with associated amplitude positive or negative (>380 nm) depending on the emission wavelength, has been found. Candidate mechanisms (including ET to nearby acceptors and/or collisional quenching etc.) and the analysis of time resolved emission spectra will be discussed.

Morphology and Photophysical Properties of One-Dimensional Arrayed Porphyrin Aggregates Assisted by Cyclodextrin Inclusion Complexation

In this study, we investigated zinc-tetraphenylporphyrin/γ-cyclodextrin complex formation based on absorption, on fluorescence and circular dichroism spectroscopy, and on fluorescence decay measurement. When the cyclodextrin concentration increased, a new absorption and fluorescence peak appeared and then circular dichloism spectra suggested a chairal aggregate formation. These results indicated that the addition of γ-CD enhanced J-aggregate formation. The morphology of the obtained aggregates was directly observed by SEM and TEM, which showed one-dimensional rod-like structures of 30 nm in diameter and ca. 200 nm in length.

ChemInform Abstract: Spectroscopic Assessment of Dy3+:LiLa(MoO4)2 Crystal as an Active Medium for All‐Solid‐State Direct Yellow‐Emitting Lasers.

AbstractThe title crystal, grown by the Czochralski method, is characterized by absorption, excitation, and emission spectroscopy together with fluorescence decay measurements at room temperature.

Investigation of intra‐cellular metabolism by time‐resolved autofluorescence

AbstractPurpose Changes in metabolism are first signs of pathological processes. The extra‐cellular metabolism can be estimated by parameters of microcirculation. The fluorescence of endogenous fluorophores characterizes the intracellular metabolism. Besides of NADH and FAD which act as electron transporter in the respiratory chain, several other fluorophores like lipofuscin and advanced glycation end products, or collagen and elastin form the autofluorescence of the fundus.Methods The influence of single fluorophores can be determined combining measurements of fluorescence decay after pulse excitation with spectral measurements. Based on fluorescence and lifetime measurements on isolated fluorophores and separated anatomical structures of porcine eyes, a laser scanner ophthalmoscope was modified for Fluorescence Lifetime Imaging Ophthalmoscopy (FLIO). The eye‐ground of human eyes is excited at 448 nm (70 ps FWHM, 80 MHz) and the fluorescence is detected in 2 spectral channels (490‐560 nm and 560‐700 nm) in TCSPC in 1024 time channels.Results A certain relation was found between exponents of triple‐exponential fit and anatomical structures. The lifetime tau 1 (about 70 ps) corresponds to the fluorescence in retinal pigment epithelium. The lifetime tau 2 (about 400 ps) originates from layers in the neuronal retina (receptors, bi‐polares, ganglion cells). The lifetime tau 3 (about 3ns) is determined by connective tissue in the nerve fiber layer and to a certain degree by the fluorescence of the crystalline lens, too.Conclusion The changes of tau 2 in the short wavelength spectral range which were found in the neuronal retina for diabetic patients having no signs of diabetic retinopathy,are interpretable as a loss of free NADH and a shift to protein‐bound NADH. Commercial interest

Spectroscopic assessment of Dy3+:LiLa(MoO4)2 crystal as an active medium for all-solid-state direct yellow-emitting lasers

The spectroscopic properties of the Dy3+:LiLa(MoO4)2 crystal prepared by the Czochralski method, have been investigated so as to assess its potentialities for direct yellow laser operation upon excitation with blue–violet or near UV laser diodes. Optical absorption, excitation, emission and fluorescence decay measurements have been performed at room temperature. An intense yellow emission band with favorably high yellow-to-blue luminescence intensity ratio has been observed in the visible region, and relevant parameters for laser applications have been determined in accordance with the Judd–Ofelt theory and Füchtbauer–Ladenburg formula. Based on analysis of Dy3+ concentration dependence of fluorescence lifetime, the energy transfer among Dy3+ ions by the dipole–dipole interaction has been found to be responsible for the fluorescence self-quenching of the 4F9/2 upper laser level at higher dopant concentration. The prospect of the Dy3+:LiLa(MoO4)2 crystal for the realization of efficient yellow laser systems has been discussed.

Fluorescence Turn-On Sensing of Anions Based on Disassembly Process of Urea-Functionalized Poly(phenylenebutadiynylene) Aggregates

A rationally designed poly(phenylenebutadiynylene) bearing urea functionalities (poly-1) was demonstrated to be a novel fluorescence turn-on probe for various anions. Poly-1 itself exhibits an extremely weak emission that was undetectable by the naked eye, whereas the fluorescence emission of poly-1 was significantly enhanced by the addition of anions. Based on the NMR, dynamic light scattering, and fluorescence decay measurements, the observed fluorescence turn-on response was determined to be realized by the fluorescence recovery based on the disassembly of the poly-1 aggregates, which was originally triggered by the anion recognition event at the urea units of poly-1.

Downconversion for Solar Cells in Sr<sub>3</sub>Gd(PO<sub>4</sub>)<sub>3</sub>:Tb, Yb Phosphors

An efficient near-infrared (NIR) quantum cutting (QC) Tb3+ and Yb3+ co-doped phosphor Sr3Gd(PO4)3 has been synthesized by conventional high temperature solid technique. Upon excitation of Tb3+ with a visible photon at 485 nm, two NIR photons could be emitted by Yb3+ through cooperative energy transfer (CTE) from Tb3+ to two Yb3+ ions. Excitation and emission spectra as well as fluorescence decay measurements have been carried out to examine the occurrence of cooperative energy transfer (CET ) from Tb3+ to Yb3+ ions. The result indicates Tb3+ and Yb3+ co-doped Sr3Gd(PO4)3 is potentially used as down-converter layer in silicon-based solar cell.

CH/π-Interaction-Guided Self-Assembly in π-Conjugated Oligomers

We report CH/π hydrogen-bond-driven self-assembly in π-conjugated skeletons based on oligophenylenevinylenes (OPVs) and trace the origin of interactions at the molecular level by using single-crystal structures. OPVs were designed with appropriate pendants in the aromatic core and varied by hydrocarbon or fluorocarbon tails along the molecular axis. The roles of aromatic π-stack, van der Waals forces, fluorophobic effect and CH/π interactions were investigated on the theromotropic liquid crystallinity of OPV molecules. Single-crystal structures of hydrocarbon OPVs provided direct evidence for the existence of CH/π interactions between the π-ring (H-bond acceptor) and alkyl C-H (H-bond donor). The four important crystallographic parameters, d(c-x)=3.79 Å, θ=21.49°, φ=150.25° and d(Hp-x)=0.73 Å, matched in accordance with typical CH/π interactions. The CH/π interactions facilitate the close-packing of mesogens in x-y planes, which were further protruded along the c axis producing a lamellar structure. In the absence of CH/π interactions, van der Waals interactions drove the assembly towards a Schlieren nematic texture. Fluorocarbon OPVs exhibited smectic liquid-crystalline textures that further underwent Smectic A (SmA) to Smectic C (SmC) phase transitions with shrinkage up to 11%. The orientation and translational ordering of mesogens in the liquid-crystalline (LC) phases induced H- and J-type molecular arrangements in fluorocarbon and hydrocarbon OPVs, respectively. Upon photoexcitation, the H- and J-type molecular arrangements were found to emit a blue or yellowish/green colour. Time-resolved fluorescence decay measurements confirmed longer lifetimes for H-type smectic OPVs relative to that of loosely packed one-dimensional nematic hydrocarbon-tailed OPVs.

Probing Local Environments by Time-Resolved Stimulated Emission Spectroscopy

Time-resolved stimulated emission spectroscopy was employed to probe the local environment of DASPMI (4-(4-(dimethylamino)styryl)-N-methyl-pyridinium iodide) in binary solvents of different viscosity and in a sol-gel matrix. DASPMI is one of the molecules of choice to probe local environments, and the dependence of its fluorescence emission decay on viscosity has been previously used for this purpose in biological samples, solid matrices as well as in solution. The results presented in this paper show that time-resolved stimulated emission of DASPMI is a suitable means to probe the viscosity of local environments. Having the advantage of a higher time resolution, stimulated emission can provide information that is complementary to that obtained from fluorescence decay measurements, making it feasible to probe systems with lower viscosity.

Spectroscopic properties of Tm3+/Ho3+ co-doped NaLa(WO4)2 single crystal

Tm3+/Ho3+ co-doped NaLa(WO4)2 single crystal was successfully grown by the Czochralski method. The crystal was characterized by room temperature absorption spectra, fluorescence spectra around 2μm, up-conversion fluorescence and luminescence decay measurements. Spectroscopic properties related to the laser operation around 2μm of Ho3+ ions have been evaluated. The energy level scheme and energy transfer processes of Tm3+ and Ho3+ were analyzed. The obtained spectroscopic results show the crystal is a potentially host for Ho3+ 2μm infrared laser.