Frequency-domain time-resolved four wave mixing spectroscopy of vibrational coherence transfer with single-color excitation

Triply vibrationally enhanced four-wave mixing spectroscopy is employed to observe vibrational coherence transfer between the asymmetric and symmetric CO-stretching modes of rhodium(I) dicarbonyl acetylacetonate (RDC). Coherence transfer is a nonradiative transition of a coherent superposition of quantum states to a different coherent superposition due to coupling of the vibrational modes through the bath. All three excitation pulses in the experiment are resonant with a single quantum coherence, but coherence transfer results in new coherences with different frequencies. The new output frequency is observed with a monochromator that resolves it from the stronger peak at the original excitation frequency. This technique spectrally resolves pathways that include coherence transfer, discriminates against spectral features created solely by radiative transitions, and temporally resolves modulations created by interference between different coherence transfer pathways. Redfield theory simulates the temporal modulations in the impulsive limit, but it is also clear that coherence transfer violates the secular approximation invoked in most Redfield theories. Instead, it requires non-Markovian and bath memory effects. RDC may provide a simple model for the development of theories that incorporate these effects.     

Photoinduced bimolecular electron transfer investigated by femtosecond time-resolved infrared spectroscopy

Ultrafast infrared transient absorption spectroscopy is used to study the photoinduced bimolecular electron transfer reaction between perylene in the first singlet excited state and 1,4-dicyanobenzene in acetonitrile and dichloromethane. Following vibrational marker modes on both donor and acceptor sides in real time provides direct insight into the structural dynamics during the reaction. A band narrowing on a time scale of a few tens of picoseconds observed on the antisymmetric CN stretching vibration of the dicyanobenzene radical anion indicates that a substantial part of the excess energy is channeled into vibrational modes of the product, despite the fact that the reaction is weakly exergonic. An additional narrowing of the same band on a time scale of several hundreds of picoseconds observed in acetonitrile only is interpreted as a signature of the dissociation of the geminate ion pairs into free ions.     

Unexpected electron transfer in cryptochrome identified by time-resolved EPR spectroscopy

Subtle differences in the local sequence and conformation of amino acids can result in diversity and specificity in electron transfer (ET) in proteins, despite structural conservation of the redox partners. For individual ET steps, distance is not necessarily the decisive parameter; orientation and solvent accessibility of the ET partners, and thus the stabilization of the charge-separated states, contribute substantially.     

Carbon-to-metal hydrogen atom transfer: direct observation using time-resolved infrared spectroscopy

We report the direct spectroscopic observation of hydrogen atom transfer reactions from carbon to metals, in which homolytic cleavage of a C-H bond is accomplished at a single metal center. Laser flash photolysis (355 nm) of a solution of [Cp(CO)2Os]2 leads to homolysis of the Os-Os bond and formation of the osmium-centered radical, Cp(CO)2Os*, as observed by time-resolved infrared (TRIR) spectroscopy. DFT computations on Cp(CO)2Os* support this assignment. Continuous photolysis (lambda > 300 nm) of [Cp(CO)2Os]2 in the presence of excess 1,4-cyclohexadiene produces the osmium hydride Cp(CO)2OsH. The kinetics of this carbon-to-metal hydrogen atom transfer were examined by TRIR spectroscopy. The second-order rate constant for hydrogen atom transfer from 1,4-cyclohexadiene to Cp(CO)2Os* in hexane at 23 degrees C is kH = (2.1 +/- 0.2) x 106 M-1 s-1. The pKa of Cp(CO)2OsH was determined as 32.7 in CH3CN, and use of a thermochemical cycle provided an estimated lower limit of 82 kcal/mol for the Os-H bond dissociation energy, indicating that it is an exceptionally strong M-H bond. Photolysis of [Tp(CO)2Os]2 (Tp = hydridotris(pyrazolyl)borate) results in carbon-to-metal hydrogen atom transfers from even stronger C-H bonds (THF or toluene) and produces Tp(CO)2OsH.     

Vibrational coherence transfer characterized with Fourier-transform 2D IR spectroscopy

Two-dimensional infrared (2D IR) spectroscopy of the symmetric and asymmetric C[Triple Bond]O stretching vibrations of Rh(CO)(2)acac in hexane has been used to investigate vibrational coherence transfer, dephasing, and population relaxation in a multilevel vibrational system. The transfer of coherence between close-lying vibrational frequencies results in extra relaxation-induced peaks in the 2D IR spectrum, whose amplitude depends on the coherence transfer rate. Coherence transfer arises from the mutual interaction of the bright CO stretches with dark states, which in this case reflects the mutual d-pi(*) back bonding of the Rh center to both the terminal carbonyls and the acetylacenonate ligand. For 2D IR relaxation experiments with variable waiting times, coherent dynamics lead to the modulation of peak amplitudes, while incoherent population relaxation and exchange results in the growth of the relaxation-induced peaks. We have modeled the data by propagating the density matrix with the Redfield equation, incorporating all vibrational relaxation processes during all three experimental time periods and including excitation reorientation effects arising from relaxation. Coherence and population transfer time scales from the symmetric to the asymmetric stretch were found to be 350 fs and 3 ps, respectively. We also discuss a diagrammatic approach to incorporating all vibrational relaxation processes into the nonlinear response function, and show how coherence transfer influences the analysis of structural variables from 2D IR spectroscopy.     

Bidirectional electron transfer in photosystem I: direct evidence from high-frequency time-resolved EPR spectroscopy

Efficient charge separation occurring within membrane-bound reaction center proteins is the most important step of photosynthetic solar energy conversion. All reaction centers are classified into two types, I and II. X-ray crystal structures reveal that both types bind two symmetric membrane-spanning branches of potential electron-transfer cofactors. Determination of the functional roles of these pairs of branches is of fundamental importance. While it is established that in type II reaction centers only one branch functions in electron transfer, we present the first direct spectroscopic evidence that both cofactor branches are active in the type I reaction center, photosystem I.     

Probing side-chain dynamics in the proteasome by relaxation violated coherence transfer NMR spectroscopy

A pair of experiments is presented for measuring intra-methyl 1H-1H dipolar cross-correlated spin relaxation rates in highly deuterated, methyl protonated proteins with significantly improved sensitivity relative to previously developed experiments that measure dynamics via 1H spin relaxation. In applications to proteins with correlation times in the macromolecular limit, these cross-correlation rates are related directly to order parameters, characterizing the amplitude of motion of methyl-containing side-chains. The experimental approach is validated by comparing extracted order parameters with those obtained via 2H and 13C spin relaxation methods for both protein L (7.5 kDa) and malate synthase G (82 kDa), with excellent correlations obtained. The methodology is applied to study Ile, Leu, and Val side-chain dynamics in a 360 kDa "half-proteasome" complex. In particular, order parameters obtained from the WT complex and from a second complex where the proteasome gating residues are deleted establish that the relative levels of dynamics in each of the two molecules are very similar. It thus becomes clear that there is no communication between gating residues and other regions of the molecule involving pico- to nanosecond time-scale dynamics of these methyl-containing side-chains.     

Theory of ultrafast time-resolved absorption spectroscopy

In this paper, the density matrix formalism has been applied to treat ultrafast time-resolved absorption spectroscopy. We have shown that in the femto-second (fs) pump-probe experiments, the observed time-resolved absorption spectra consist of the contributions from the population (i.e., incoherent contribution) and the coherence (i.e., the phase of the system). The adiabatic approximation has been used to derive the expressions for ultrafast time-resolved spectra. We have also shown that the dynamics of the coherence will result in quantum beat. Numerical calculations have been performed to demonstrate the theoretical results.     

Accurate time-resolved laser spectroscopy on lithium atoms

The lifetime of the lithium 2p ² P states has been measured with high accuracy using the delayed coincidence technique with a continuous mode-locked dye laser as the source of the excitation light. The value 27.22 (0.20) ns was obtained. In addition, the hyperfine structure of the⁷Li 2p ² P _3/2 state, which can normally scarcely be resolved, has been studied by recording the slow quantum beats at zero field and the very fast beats in the Paschen-Back regime. New accurate values for the²³Na 3p ² P _3/2 state hyperfine structure constants are also presented.     

Methyl-radical association studied by time-resolved mass spectroscopy

Time-resolved mass spectroscopy has been utilized to study the kinetics of methylradical association. The rate coefficient for the association process was determined by following the time dependence of ethane formation after flash photolysis of CH₃I in the presence of N₂. The net rate coefficient for the process 2CH₃ (+M) → C₂H₆(+M) has a high-pressure limit of (4.0 ± 0.3) × 10^?11 cm³ molecule^?1 sec^?1 at 313°K. This rate coefficient has found to be insensitive to the third-body number densities for pressures ranging from 13 Torr down to below 0.5 Torr, indicating that the lifetime for dissociation of the intermediate C₂H₆* species is greater than 2 × 10^?7 sec at 313°K for a gas pressure of 0.5 Torr.     

Femtosecond time-resolved transient absorption spectroscopy of xanthophylls

Xanthophylls are a major class of photosynthetic pigments that participate in an adaptation mechanism by which higher plants protect themselves from high light stress. In the present work, an ultrafast time-resolved spectroscopic investigation of all the major xanthophyll pigments from spinach has been performed. The molecules are zeaxanthin, lutein, violaxanthin, and neoxanthin. beta-Carotene was also studied. The experimental data reveal the inherent spectral properties and ultrafast dynamics including the S(1) state lifetimes of each of the pigments. In conjunction with quantum mechanical computations the results address the molecular features of xanthophylls that control the formation and decay of the S* state in solution. The findings provide compelling evidence that S* is an excited state with a conformational geometry twisted relative to the ground state. The data indicate that S* is formed via a branched pathway from higher excited singlet states and that its yield depends critically on the presence of beta-ionylidene rings in the polyene system of pi-electron conjugated double bonds. The data are expected to be beneficial to researchers employing ultrafast time-resolved spectroscopic methods to investigate the mechanisms of both energy transfer and nonphotochemical quenching in higher plant preparations.     

Accurate time-resolved laser spectroscopy on silver atoms

The lifetimes of the silver 5p ² P states have been measured with high accuracy by time-resolved laser spectroscopy using the delayed coincidence technique. Also, the hyperfine structure of¹⁰⁷Ag and¹⁰⁹Ag in these two states has been studied by observing quantum beats.     

Amine-functionalized lanthanide-doped zirconia nanoparticles: optical spectroscopy, time-resolved fluorescence resonance energy transfer biodetection, and targeted imaging

Ultrasmall inorganic oxide nanoparticles doped with trivalent lanthanide ions (Ln(3+)), a new and huge family of luminescent bioprobes, remain nearly untouched. Currently it is a challenge to synthesize biocompatible ultrasmall oxide bioprobes. Herein, we report a new inorganic oxide bioprobe based on sub-5 nm amine-functionalized tetragonal ZrO(2)-Ln(3+) nanoparticles synthesized via a facile solvothermal method and ligand exchange. By utilizing the long-lived luminescence of Ln(3+), we demonstrate its application as a sensitive time-resolved fluorescence resonance energy transfer (FRET) bioprobe to detect avidin with a record-low detection limit of 3.0 nM. The oxide nanoparticles also exhibit specific recognition of cancer cells overexpressed with urokinase plasminogen activator receptor (uPAR, an important marker of tumor biology and metastasis) and thus may have great potentials in targeted bioimaging.     

Intramolecular electron and energy transfer in an axial ZnP-pyridylfullerene complex as studied by X- and W-band time-resolved EPR spectroscopy

Light-driven electron transfer (ET) and energy transfer (EnT) in a self-assembled via axial coordination Zn-porphyrin-pyridylfullerene (ZnP-PyrF) complex were studied by time-resolved electron paramagnetic resonance (TREPR) spectroscopy at 9.5 GHz (X-band) and 95 GHz (W-band). The studies over a wide temperature range were carried out in media of different polarity, including isotropic toluene and tetrahydrofuran (THF), and anisotropic nematic liquid crystals (LCs), E-7 and ZLI-4389. At low temperatures (frozen matrices), photoexcitation of the ZnP donor results mainly in singlet-singlet EnT to the pyridine-appended fullerene acceptor. In fluid phases ET is the dominant process. Specifically, in isotropic solvents the generated radical pairs (RPs) are long-lived, with lifetimes exceeding that observed for covalently linked donor-acceptor systems. It is concluded that in liquid phases of both polar and nonpolar solvents the separation of the tightly bound complex into the more loosely bound structure slows down the back ET (BET) process. Photoexcitation of the donor in fluid phases of LCs does not result in the creation of the long-lived RPs, since the ordered LC matrix hinders the separation of the complex constituents. As a result, fast intramolecular BET takes place in the tightly bound complex. Contrarily to the behavior of covalently linked donor-acceptor systems in different LCs, the polarity of the LC matrix affects the ET process. Moreover, in contrast to covalently linked D-s-A systems, utilization of LCs for the coordinatively linked D-s-A complexes does not reduce the ET rates significantly.     

Ultrafast time-resolved spectroscopy of xanthophylls at low temperature

Many of the spectroscopic features and photophysical properties of xanthophylls and their role in energy transfer to chlorophyll can be accounted for on the basis of a three-state model. The characteristically strong visible absorption of xanthophylls is associated with a transition from the ground state S0 (1(1)Ag-) to the S2 (1(1)Bu+) excited state. The lowest lying singlet state denoted S1 (2(1)Ag-), is a state into which absorption from the ground state is symmetry forbidden. Ultrafast optical spectroscopic studies and quantum computations have suggested the presence of additional excited singlet states in the vicinity of S1 (2(1)Ag-) and S2 (1(1)Bu+). One of these is denoted S* and has been suggested in previous work to be associated with a twisted molecular conformation of the molecule in the S1 (2(1)Ag-) state. In this work, we present the results of a spectroscopic investigation of three major xanthophylls from higher plants: violaxanthin, lutein, and zeaxanthin. These molecules have systematically increasing extents of pi-electron conjugation from nine to eleven conjugated carbon-carbon double bonds. All-trans isomers of the molecules were purified by high-performance liquid chromatography (HPLC) and studied by steady-state and ultrafast time-resolved optical spectroscopy at 77 K. Analysis of the data using global fitting techniques has revealed the inherent spectral properties and ultrafast dynamics of the excited singlet states of each of the molecules. Five different global fitting models were tested, and it was found that the data are best explained using a kinetic model whereby photoexcitation results in the promotion of the molecule into the S2 (1(1)Bu+) state that subsequently undergoes decay to a vibrationally hot S1 (1(1)Ag-) state and with the exception of violaxanthin also to the S* state. The vibrationally hot S1 (1(1)Ag-) state then cools to a vibrationally relaxed S1 (2(1)Ag-) state in less than a picosecond. It was also found that a portion of the S* population is converted into S1 (2(1)Ag-) during deactivation, but this process and the relative yield of S* was found to depend on temperature, consistent with it being associated with a twisted conformation of the xanthophyll. The results of the global fitting suggest that subpopulations of twisted conformers of xanthophylls already exist in the ground state prior to photoexcitation.     

时间分辨发射光谱的应用研究进展

时间分辨荧光技术是研究超分子组装微环境变化的主要手段之一。荧光体与蛋白质、内膜及胶束等相互作用引起最大荧光强度和量子效率急剧变化反映了激发态反应情况,激发态过程通常用时间分辨发射光谱(TRES)测定。国内在这一领域的研究虽有成果,但与国外相比仍有一定差距。本文对时间分辨荧光光谱的应用研究进展进行了综述,阐述了其历史、原理、检测方法及近几年国内外的应用研究进展情况。     

Breaking the nanosecond barrier in FTIR time-resolved spectroscopy

The state of the art in broad spectral bandwidth infrared time-resolved spectroscopy (IRTRS) is reviewed, with particular regard to time resolution in the nanosecond and sub-nanosecond regime. While step-scan Fourier transform infrared (S²FTIR) has been successful in pushing the time resolution of IRTRS to sub-microsecond limits, and is, in principle, applicable for monitoring time-dependent phenomena on any time scale, a practical limit for S²FTIR is currently about 1 ns, due to the limitations of parts of the instrument other than the interferometer itself. For the particular case of IRTRS of transient photo-excited states illustrated here and other photo-excitation studies, it is proposed that the most effective way to breach the nanosecond barrier and to push the time resolution limit of IRTRS to 10 ps, or even lower, while still maintaining the spectral bandwidth and resolution and the multiplex and throughput advantages of interferometry, is to turn to constant velocity, continuous-scan (CS) FTIR, in the pump–probe asynchronous sampling mode. In the method described, the pump is provided by the picosecond UV pulse of an electron storage ring-powered free electron laser and the infrared probe is the picosecond `white light' synchrotron pulse from the same storage ring. The design specifications of this system are 10 ps time resolution with 3 cm⁻¹ spectral resolution.     

飞秒时间分辨荧光非共线光参量放大光谱技术

时间分辨荧光光谱技术是研究激发态弛豫、能量传递以及电荷转移等光化学过程的重要且直接的工具．飞秒时间分辨荧光非共线光参量放大光谱技术是一种新发展的具有高时间分辨率、宽探测带宽、高增益的时间分辨光谱技术．本文对该技术的基本原理与工作特性、系统配置、荧光收集和会聚、数据采集模式、时问分辨光谱数据处理等进行了系统阐述．最后简单介绍了飞秒时间分辨荧光非共线光参量放大光谱技术在物理、生物和化学领域的3个典型应用,并以此展现该技术的高时间分辨率、宽光谱探测及高增益优势．