Transient Time-resolved Fluorescence Research Articles

In situ construction of rich oxygen vacancy Bi/Bi3TaO7 heterojunction photocatalysts

Oxygen vacancy-rich Bi/Bi3TaO7 binary nanomaterials were obtained by a simple in situ reduction. The as-prepared composite exhibited superior photocatalytic activity for the degradation of RhB compared to the single material under the visible light range. The degradation rate of the optimal sample reached 92% within 120 min, 12.25 times that of the pure Bi3TaO7. XRD and HRTEM characterization showed that the Bi metal obtained through in situ reduction was loaded on the Bi3TaO7 nanostructure. In addition, XPS and EPR indicated a significant increase in the oxygen vacancy content of the composites. The time-resolved transient fluorescence spectroscopy and photocurrent results proved that the separation rate of photoexcited carriers and the corresponding ability of photoelectrons are greatly increased. It is due to the synergistic effect of the unique photo-reaction excitation pathway induced by bismuth metal plasma resonance and oxygen vacancies, which not only extends the photo-reaction to the infrared region but also increases the carrier residence time, and this is the fundamental reason for the excellent photocatalytic performance of the composite material. The h+ is the main reactive radical of the reaction, and •O2− also plays a role. Based on the above characterization study, we propose a possible photoreaction mechanism for the synergistic oxygen vacancy and Bi metal of Bi/Bi3TaO7.

Singlet exciton fission and its magnetic field effect in ordered and random systems

Utilizing singlet exciton fission strategy is generally considered as a feasible way to improve the quantum efficiency of organic photovoltaic devices. However, the orientations of organic molecules in amorphous thin films are random. Whether or not efficient fission process can occur between randomly packed molecules is an important problem of this method. In this experiment, the magnetic field effect of steady-state photoluminescence and time-resolved transient fluorescence decay of rubrene-doped organic films were measured at room temperature. Then, the magnetic field effect of singlet fission rate between doped rubrene molecules was determined. Based on the Merrifield’s phenomenological theory, the difference between the B-field effects of fission rates in ordered and random systems were analyzed in detail. Comparing the experimental results and theoretical curves showed that singlet fission process mainly took place on those randomly arranged rubrene molecules in doped films. This experimentally confirmed the occurrence of efficient singlet fission in amorphous organic solid. Additionally, the fission rate was expected to be related with the charge mobility of material.

Application of Visible-to-UV Photon Upconversion to Photoredox Catalysis: The Activation of Aryl Bromides.

The activation of aryl-Br bonds was achieved by sequential combination of a triplet-triplet annihilation process of the organic dyes, butane-2,3-dione and 2,5-diphenyloxazole, with a single-electron-transfer activation of aryl bromides. The photophysical and chemical steps were studied by time-resolved transient fluorescence and absorption spectroscopy with a pulsed laser, quenching experiments, and DFT calculations.

Ultrafast energy transfer pathways in R-phycoerythrin from Polysiphonia urceolata

Energy transfer (ET) processes between chromophores in R-phycoerythrin (R-PE) from Polysiphonia urceolata were studied by use of ultrafast spectroscopic methods. Several primary ET pathways were elaborated. A fluorescence decay component with a time constant of several hundred picoseconds observed by streak camera is tentatively assigned to the reversible formation of exciton traps between α84 and β84 pigment pairs. In order to investigate much faster ET processes in R-PE, a noncollinear optical parametric amplifier based femtosecond time-resolved transient fluorescence spectrometer was employed. The results reveal that the ET between α84 and β84 pigment pair has a time constant of 1-2ps; the energy migration between α84 and β84 pairs within the R-PE trimer has a time constant of 30-40ps. We also demonstrated an ET process from phycourobilin to phycoerythrobilin with a time constant as fast as 2.5-3.0ps, which was directly observed in fluorescence kinetics by selective excitation of the phycourobilin molecules acting as the energy donor.

Ultrafast Energy Transfer in Artificial Antenna Molecule Measured by Transient Fluorescence Spectroscopy

We have reported previously the ultrafast energy transfer process with a time constant of 0.8 ps from a monomeric to a dimeric subunit within a perylenetetracarboxylic diimide trimer, which was derived indirectly from a model fitting into the transient absorption experimental data. Here we present a direct ultrafast fluorescence quenching measurement by employing fs time-resolved transient fluorescence spectroscopy based on noncollinear optical parametric amplification technique. The rapid decay of the monomer's emission due to energy transfer was observed directly with a time constant of about 0.82 ps, in good agreement with the previous result.

Structural Dynamics of Myosin Investigated by Transient Time Resolved Fluorescence

We are using transient time resolved fluorescence (TR)2F to map conformational transitions in the catalytic domain of Dictyostelium myosin II. The photophysical state of fluorescent probes, site specifically attached to proteins, shadows the underlying protein structure, dynamics and local electrochemical environment. Changes in these underlying constraints, affect the excited state lifetime of the probe. Because the lifetime decays exponentially, different populations of photophysical states in the sample ensemble can be isolated by analyzing the time resolved decay. In a typical experiment, a probe (or pair of probes in the case of a FRET experiment) are innocuously attached to specific sites on myosin and actin. We then measure underlying protein dynamics by time resolved anisotropy and accessibility which reflect the local restricted motion of the probe, and by FRET which reflects the distance (or when κ2 does not = 2/3, distance and the relative dipole orientation) between two probe pairs. When these measurements are performed during a biochemical transient initiated by stopped flow, we gain incite into the millisecond scale “structural kinetics” associated with the transition. These studies are made possible by the recent development of a high throughput time resolved fluorescence spectrometer which can measure a complete, high resolution, time resolved fluorescence decay, in 0.1 ms. The high throughput character of this instrument allows us to measure changes in the time resolved fluorescence decay and thus structural dynamics of a probe during a stopped flow initiated biochemical transient. The added dimensionality afforded by time resolved fluorescence isolates discrete populations of structural states in the ensemble. It also increases the measurement precision. We are using this approach to measure millisecond scale structural transitions that occur in myosin during nucleotide binding, hydrolysis and product dissociation and during actin binding and detachment.

Noncollinear optical parametric amplifier based femtosecond time-resolved transient fluorescence spectra: characterization and correction

Spectral distortion frequently occurs in a recently developed femtosecond time-resolved fluorescence spectroscopy based on noncollinear optical parametric amplification, which would limit its applications if it is not treated appropriately. We report the mechanism for broadening and distortion of the amplified spectrum, and it was found that the amplified fluorescence spectral width and the corresponding pulse duration were limited by the uncertainty principle. We demonstrated that the nonuniform gain curve of nonlinear optical crystal in the wide spectral region is the main cause leading to the distorted spectrum. Theoretical analysis shows that by carefully adjusting the experimental parameters, such as propagation and noncollinear angles, the spectral fidelity can be achieved in a broad region. Moreover, we also proposed a method for retrieving the genuine spectrum from the distorted amplified spectrum by experimentally measuring the gain curve encoded in the spectrum of the parametric superfluorescence, which is collected during propagation at noncollinear angles the same as those for amplified fluorescence.

Picosecond Time-resolved Infrared Imaging by a Nonscanning Two-color Infrared Super-resolution Microscope

Abstract A nonscanning two-color infrared super-resolution microscope combining a laser fluorescence microscope and picosecond time-resolved transient fluorescence detected IR (TFD-IR) spectroscopy has been developed. Picosecond time-resolved infrared super-resolution images were obtained with 2 ps time resolutions and 1.4 μm spatial resolutions, without XY scanning of the sample or the excitation light of infrared and visible. A demonstration measurement on Arabidopsis thaliana roots labeled by Rhodamine 6G is presented.

Far-field infrared super-resolution microscopy using picosecond time-resolved transient fluorescence detected IR spectroscopy

A new far-field infrared super-resolution microscopy combining laser fluorescence microscope and picosecond time-resolved transient fluorescence detected IR (TFD-IR) spectroscopy is proposed. TFD-IR spectroscopy is a kind of IR–visible/UV double resonance spectroscopy, and detects IR transitions by the transient fluorescence due to electronic transition originating from vibrationally excited level populated by IR light. IR images of rhodamine-6G solution and of fluorescent beads were clearly observed by monitoring the transient fluorescence. Super-resolution twice higher than the diffraction limit for IR light was achieved. The IR spectrum due to the transient fluorescence was also measured from spatial domains smaller than the diffraction limit.

Vibrational energy relaxation of the 7-azaindole dimer in CCl 4 solution studied by picosecond time-resolved transient fluorescence detected IR spectroscopy

The picosecond time-resolved IR spectra of the 7-azaindole dimer in a carbon tetrachloride solution was measured by using picosecond time-resolved IR/UV double resonance spectroscopy. This spectroscopy selectively detects the IR transition by transient fluorescence due to an electronic transition from a vibrationally excited level. The time-evolution of the IR spectrum is a single exponential with a 19 ps lifetime, which does not correspond to fast nonstatistical decay due to the intramolecular vibrational redistribution found in a gas-phase cluster. From a comparison with the time-resolved IR spectrum of a jet-cooled dimer, this decay is assigned to vibrational cooling from the dimer to the solvent.