基于超快多维振动光谱技术解析分子体系的三维空间构型

超快多维振动光谱技术目前已经被广泛应用到各种凝聚态分子体系中分子的结构以及快速变化动力学过程的测量之中,并有望成为新一代解析分子体系微观结构及超快行为的常规手段。本文从两个主线出发,介绍如何利用超快多维振动光谱技术解析分子体系的三维空间构型。一方面通过测量分子内各个振动模式跃迁偶极矩间的夹角来获得分子体系内不同基团的相对空间取向,并最终确定分子的空间构型。另一方面,通过详细解析分子间振动能量转移的机理,进而将实验中测得的振动能量转移速率转化为分子之间的距离信息。     

Dissipative dynamics of a system passing through a conical intersection: ultrafast pump-probe observables

The dynamics of a system incorporating a conical intersection, in the presence of a dissipative environment, is studied with the purpose of identifying observable ultrafast spectroscopic signatures. A model system consisting of two vibronically coupled electronic states with two nuclear degrees of freedom is constructed. Dissipation is treated by two different methods, Lindblad semigroup formalism and the surrogate Hamiltonian approach. Pump-probe experimental expectation values such as transient emission and transient absorption are calculated and compared to the adiabatic and diabatic population transfer. The ultrafast population transfer reflecting the conical intersection is not mirrored in transient absorption measurements such as the recovery of the bleach. Emission from the excited state can be suppressed on the ultrafast time scale, but the existence of a conical intersection is only one of the possible mechanisms that can provide ultrafast damping of emission.     

Environment-dependent ultrafast photoisomerization dynamics in azo dye

Azoaromatic dyes have been extensively investigated over the past decade due to their potential use in a variety of optical devices that exploit their ultrafast photoisomerization processes. Among the azoaromatic dyes, Disperse Red 19 is a commercially available azobenzene nonlinear optical chromophore with a relatively high ground-state dipole moment. In the present study, we used ultrafast time-resolved spectroscopy to clarify the dynamics of a push-pull substituted azobenzene dye. Solution and film samples exhibited different ultrafast dynamics, indicating that the molecular environment affects the photoisomerization dynamics of the dye.     

Femtosecond studies of tryptophan fluorescence dynamics in proteins: local solvation and electronic quenching

We report our systematic examination of tryptophan fluorescence dynamics in proteins with femtosecond resolution. Distinct patterns of femtosecond-resolved fluorescence transients from the blue to the red side of emission have been characterized to distinguish local ultrafast solvation and electronic quenching. It is shown that tryptophan is an ideal local optical probe for hydration dynamics and protein-water interactions as well as an excellent local molecular reporter for ultrafast electron transfer in proteins, as demonstrated by a series of biological systems, here in melittin, human serum albumin, and human thioredoxin, and at lipid interfaces. These studies clarify the assignments in the literature about the ultrafast solvation or quenching dynamics of tryptophan in proteins. We also report a new observation of solvation dynamics at far red-side emission when the relaxation of the local environment is slower than 1 ps. These results provide a molecular basis for using tryptophan as a local molecular probe for ultrafast protein dynamics in general.     

Sensitivity and lineshape improvement in ultrafast 2D NMR by optimized apodization in the spatially encoded dimension

Ultrafast 2D NMR allows the acquisition of a 2D spectrum in a single scan. However, even when the acquisition of ultrafast spectra is carried out under optimized conditions, the appearance and the sensitivity of 2D spectra are often not satisfactory compared with what one could expect from this promising methodology. This is due to limitations in terms of sensitivity, spectral width and resolution, and also to non-ideal lineshapes characterized by asymmetric sinc wiggles. Here, we identify the origin of these distortions by means of numerical simulations compared with experimental data. We then propose a processing approach to improve lineshapes while increasing the sensitivity of ultrafast experiments. The method consists in multiplying the Fourier transform of ultrafast echoes by an optimized apodization function. The principles of the method are described, and a variety of window functions are tested to determine optimum processing conditions. The approach is finally applied to ultrafast 2D spectra, leading to symmetric lineshapes with a sensitivity increased by a factor of 2.     

Site-selective photoinduced electron transfer from alcoholic solvents to the chromophore facilitated by hydrogen bonding: a new fluorescence quenching mechanism

Solute-solvent intermolecular photoinduced electron transfer (ET) reaction was proposed to account for the drastic fluorescence quenching behaviors of oxazine 750 (OX750) chromophore in protic alcoholic solvents. According to our theoretical calculations for the hydrogen-bonded OX750-(alcohol)(n) complexes using the time-dependent density functional theory (TDDFT) method, we demonstrated that the ET reaction takes place from the alcoholic solvents to the chromophore and the intermolecular ET passing through the site-specific intermolecular hydrogen bonds exhibits an unambiguous site selectivity. In our motivated experiments of femtosecond time-resolved stimulated emission pumping fluorescence depletion spectroscopy (FS TR SEP FD), it could be noted that the ultrafast ET reaction takes place as fast as 200 fs. This ultrafast intermolecular photoinduced ET is much faster than the diffusive solvation process, and even significantly faster than the intramolecular vibrational redistribution (IVR) process of the OX750 chromophore. Therefore, the ultrafast intermolecular ET should be coupled with the hydrogen-bonding dynamics occurring in the sub-picosecond time domain. We theoretically demonstrated for the first time that the selected hydrogen bonds are transiently strengthened in the excited states for facilitating the ultrafast solute-solvent intermolecular ET reaction.     

Ultrafast Time‐Resolved Transient Structures of Solids and Liquids by Means of Extended X‐ray Absorption Fine Structure

Detection of ultrafast transient structures and the evolution of ultrafast structural intermediates during the course of reactions has been a long standing goal of chemists and biologists. This article will be restricted to nanosecond, picosecond and shorter time‐resolved extended X‐ray absorption fine structure (EXAFS) studies, its aim being to present the progress and problems encounter in measurements and understanding the structure of transients. The recent advances in source technology has stimulated a wide variety of novel experiments using both synchrotrons and smaller laboratory size systems. With more efficient X‐ray lenses and detectors many of the previously difficult experiments to perform, because of the exposure time required and weak signals, will now be easily performed. The experimental system for the detection of ultrafast time‐resolved EXAFS spectra of molecules in liquids is described and the method for the analysis of EXAFS spectra to yield transient structures is given. We believe that utilizing our table‐top ultrafast X‐ray source and the polycapillary optics in conjunction with dispersive spectrometer and charge coupled devices (CCD) we will be able to determine the structure of many reaction intermediates and excited states of chemical and biological molecules in solid and liquid state.     

Wave function engineering for ultrafast charge separation and slow charge recombination in type II core/shell quantum dots

The size dependence of optical and electronic properties of semiconductor quantum dots (QDs) have been extensively studied in various applications ranging from solar energy conversion to biological imaging. Core/shell QDs allow further tuning of these properties by controlling the spatial distributions of the conduction-band electron and valence-band hole wave functions through the choice of the core/shell materials and their size/thickness. It is possible to engineer type II core/shell QDs, such as CdTe/CdSe, in which the lowest energy conduction-band electron is largely localized in the shell while the lowest energy valence-band hole is localized in the core. This spatial distribution enables ultrafast electron transfer to the surface-adsorbed electron acceptors due to enhanced electron density on the shell materials, while simultaneously retarding the charge recombination process because the shell acts as a tunneling barrier for the core localized hole. Using ultrafast transient absorption spectroscopy, we show that in CdTe/CdSe-anthraquinone (AQ) complexes, after the initial ultrafast (~770 fs) intra-QD electron transfer from the CdTe core to the CdSe shell, the shell-localized electron is transferred to the adsorbed AQ with a half-life of 2.7 ps. The subsequent charge recombination from the reduced acceptor, AQ(-), to the hole in the CdTe core has a half-life of 92 ns. Compared to CdSe-AQ complexes, the type II band alignment in CdTe/CdSe QDs maintains similar ultrafast charge separation while retarding the charge recombination by 100-fold. This unique ultrafast charge separation and slow recombination property, coupled with longer single and multiple exciton lifetimes in type II QDs, suggests that they are ideal light-harvesting materials for solar energy conversion.     

Ultrafast coherent photoexcited dynamics in a trimeric dendrimer probed by X-ray stimulated-Raman signals

The photoinduced ultrafast coherent inter-chromophore energy redistribution in a triarylamine trimer is explored using nonadiabatic excited state molecular dynamics followed by simulations of X-ray Raman signals. The nitrogencentered system ensures strong interchromophore interactions and, thus, the presence of coherences. Nevertheless, the multitude of non-deterministic photoinduced pathways during the ultrafast inter-branch migration of the excitation results in random confinement on some branches and, therefore, spatial exciton scrambling and loss of phase information at long times. We show that the vibronic coherence dynamics evolving into the incoherent scrambling mechanism on ultrafast 50 fs timescale, is accurately probed by the TRUECARS X-ray stimulated Raman signal. In combination with previous results, where the technique has revealed long-lived coherences in a rigid heterodimer, the signal is most valuable for detecting ultrafast molecular coherences or their absence. We demonstrate that X-ray Raman spectroscopy is a useful tool in the chemical design of functional molecular building blocks.

The photoinduced ultrafast coherent inter-chromophore energy redistribution in a triarylamine trimer is explored using nonadiabatic excited state molecular dynamics followed by simulations of X-ray Raman signals.     

Time‐Resolved Extended X‐ray Absorption Fine Structure (EXAFS) Studies by Means of an Energy Dispersive Spectrometer

The structures formed during the photodicosiation of CBr₄ in alcohol solution have been measured by means of a novel ultrafast time‐resolved x‐ray absorption fine structure (EXAFS) experimental system, suitable for the direct determination of ultrafast transient structures of molecules in the liquid and solid phase. The EXAFS spectra of the starting and final products of the reaction were measured and analyzed.