Ultrafast time-resolved coherent degenerate four-wave-mixing spectroscopy for investigating molecular dynamics in different states

In this paper, ultrafast time-resolved coherent degenerate four-wave-mixing (DFWM) spectroscopy is performed to investigate molecular dynamics in the gaseous phase. Laser pulses lasting for 40 fs are used to create and monitor different vibrational eigenstates of iodine at room temperature (corresponding to a low saturation pressure of about 35 Pa). Using an internal time delay in the DFWM process resonant with the transition between the ground X-state and the excited B-state, the vibrational states of both the electronically excited and the ground states are detected as oscillations in the DFWM transient signal. The dynamics of either the electronically excited or ground state of iodine molecules obtained are consistent with the previous high temperature studies on the femtosecond time-resolved DWFM spectroscopy.     

Bonded exciplex formation from stacked thymine and adenine: semiclassical simulations

The nonradiative decay of a π-stacked adenine and thymine, following excitation of thymine by an ultrafast laser pulse, was studied by semiclassical dynamics simulations. The simulation found that a bonded exciplex is formed after excitation due to strong interaction between the stacked bases. The strong interaction significantly lowers the energy gap between the LUMO and HOMO and consequently facilitates the deactivation of the electronically excited molecule. It also restricts the deformation vibration of the excited thymine molecule, which slows down the coupling between the HOMO and LUMO energy levels and delays the deactivation of the excited adenine molecule to the electronic ground state.     

Femtosecond stimulated Raman spectroscopy

Advances in the field of Femtosecond Stimulated Raman Spectroscopy (FSRS), a new time‐resolved structural technique that provides complete vibrational spectra on the ultrafast timescale, are reviewed. When coupled with a femtosecond optical trigger, the time evolution of a reacting species can be monitored with unprecedented <25 femtosecond temporal and 5 cm^‐1 spectral resolution. New technological and theoretical advances including the development of tunable FSRS and a background‐free FSRS format are discussed. The most recent experimental studies focus on ultrafast reaction dynamics in electronically excited states: isomerization in cyanobacterial phytochrome, ultrafast spin flipping in a solar cell sensitizer, and excited state proton transfer in green fluorescent protein. The use of FSRS to directly map multidimensional reactive potential energy surfaces and to probe the mechanism of reactive internal conversion is prospectively discussed.     

飞秒时间分辨质谱和光电子影像对分子激发态动力学的研究

分子量子态的研究,特别是分子激发态演化过程的研究不仅可以了解分子量子态的基本特性和量子态之间的相互作用,而且可以了解化学反应过程和反应通道间的相互作用.飞秒时间分辨质谱和光电子影像是将飞秒抽运-探测分别与飞行时间质谱和光电子影像相结合的超快谱学方法,为实现分子内部量子态探测,研究分子量子态相互作用及超快动力学过程提供了强有力的工具,可以在飞秒时间尺度下研究单分子反应过程中的光物理或光化学机理.本文详细介绍了飞秒时间分辨质谱和光电子影像的技术原理,并结合本课题组的工作,展示了这两种方法在量子态探测及相互作用研究领域,特别是激发态电子退相、波包演化、能量转移、分子光解动力学以及分子激发态结构动力学研究中的广泛应用.最后,对该技术的发展前景以及进一步的研究工作和方向进行了展望.     

Theoretical study of excited state proton transfer in pyrrole-2-carboxylic acid

Pyrrole 2-carboxylic acid (PCA) shows dual emission (310 nm and 430 nm) in water on photo-excitation, which indicates that more than one species is in the excited state. This paper reports on the quantum chemical analysis of pyrrole 2-carboxylic acid (PCA) in the light of a possible excited state proton transfer. Dipole moment, excited state energy and findings in molecular orbital calculations (HOMO, LUMO) establish that PCA is a likely candidate for transfer of a proton from the pyrrole moiety to the C=O of carboxylic moiety (possible zwitterionic form) in the excited state. Overall, the computed predicted results of intramolecular and intermolecular excited state proton transfer corroborates the experimental results.     

苯乙炔分子电子激发态超快动力学研究

采用飞秒时间分辨质谱技术结合飞秒时间分辨光电子影像技术研究了苯乙炔分子电子激发态超快非绝热弛豫动力学.用235 nm光作为泵浦光,将苯乙炔分子激发到第二激发态S2,用400 nm光探测激发态的演化过程.时间分辨的母体离子的变化曲线用指数和高斯函数卷积得到不同的两个组分,一个是超快衰减组分,时间常数为116 fs,一个是慢速组分,时间常数为106 ps.通过分析时间分辨的光电子影像得到光电子动能分布,结合时间分辨光电子能谱数据发现,时间常数为116 fs的快速组分反映了S2态向S1态的内转换过程.实验还表明S1态通过内转换被布局后向T1态的系间窜跃过程为重要的衰减通道.本工作为苯乙炔分子S2态非绝热弛豫动力学提供了较清晰的物理图像.     

Spectral and luminescent properties of chlorine-substituted derivatives of aniline

The effect of the position and number of chlorine atoms on spectral, geometrical, and photophysical characteristics of individual molecules of chlorine-substituted derivatives of aniline is studied by electronic spectroscopy and quantum-chemical calculations. It is shown that the long-wavelength absorption band of these compounds is formed by several electronic transitions of different nature and intensity. It is ascertained that electronically excited π σ* states involve C-Cl bonds, which weaken upon excitation. According to quantumchemical calculations, the quenching of fluorescence of chlorine-substituted derivatives of aniline is determined by the fact that intersystem crossing is more effective than radiative decay of the S ₁ state.     

CCl2(A1B1和a3B1)被酮类分子猝灭速率常数的测定

对CCI4／Ar混合气体脉冲直流高压放电产生CCI2自由基,经过约110μs后,再用541．52nm激光将电子基态CCI2激励到激发态A^1B1(0,4,0)振动态K＝0级以上,通过检测激发态CCI2（A）时间分辨荧光信号,测得室温下CCI2（A^1B1和a^3B1)被酮类分子猝灭的实验结果,用所提出的三能级模型分析处理实验数据,获得态分辨速率常数KA和Kα值。     

Hot-electron-driven enhancement of spin-lattice coupling in Gd and Tb 4f ferromagnets observed by femtosecond x-ray magnetic circular dichroism

Femtosecond x-ray magnetic circular dichroism was used to study the time-dependent magnetic moment of 4f electrons in the ferromagnets Gd and Tb, which are known for their different spin-lattice coupling. We observe a two-step demagnetization with an ultrafast demagnetization time of 750 fs identical for both systems and slower times which differ sizeably with 40 ps for Gd and 8 ps for Tb. We conclude that spin-lattice coupling in the electronically excited state is enhanced up to 50 times compared to equilibrium.     

Excited-state hydrogen bonding dynamics of methyl isocyanide in methanol solvent: A DFT/TDDFT study

The time-dependent density functional theory (TDDFT) method was performed to investigate the hydrogenbonding dynamics of methyl cyanide (MeNC) as hydrogen bond acceptor in hydrogen donating methanol (MeOH) solvent. The ground-state geometry optimizations and electronic transition energies and corresponding oscillation strengths of the low-lying electronically excited states for the isolated MeNC and MeOH monomers, the hydrogen-bonded MeNC-MeOH dimer and MeNC-2MeOH trimer are calculated by the DFT and TDDFT methods, respectively. An intermolecular hydrogen bond N≡C…H-O is formed between MeNC and methanol molecule. According to Zhao’s rule on the excited-state hydrogen bonding dynamics, we find the intermolecular hydrogen bonds N≡C…H-O are strengthened in electronically excited states of the hydrogen-bonded MeNC-MeOH dimer and MeNC-2MeOH trimer, with the excitation energy of a related excited state being lowered and electronic spectral redshifts being induced. Furthermore, the hydrogen bond strengthening in the electronically excited state plays an important role on the photophysics and photochemistry of MeNC in solutions     

Ultrafast dynamics of a charge-transfer dimer as a model for the photoinduced phase transition of charge-transfer compounds

By applying ultrafast pump-probe spectroscopy with 15 fs temporal resolution to (TMTTF(+))(2) dimers we provide a full picture of the structural relaxation following photoexcitation of their CT transition. Both population and coherent phonon dynamics allow tracking wave packet motion onto the multidimensional excited state potential energy surface, as obtained by density functional theory calculations. We show that the vibrations that are strongly coupled to the charge-transfer transition of the dimer correspond to those driving the photoinduced phase transition occurring in charge-transfer crystalline solids.     

Molecular matter-wave amplifier

We describe a matter-wave amplifier for vibrational ground-state molecules which uses a Feshbach resonance to first form quasibound molecules starting from an atomic Bose-Einstein condensate. The quasibound molecules are then driven into their stable vibrational ground state via a two-photon Raman transition inside an optical cavity. The transition from the quasibound state to the electronically excited state is driven by a classical field. Amplification of ground state molecules is then achieved by using a strongly damped cavity mode for the transition from the electronically excited molecules to the molecular ground state.     

Photoinduced electron transfer of Rhodamine 6G/N,N-diethylaniline revealed by multiplex transient grating and transient absorption spectroscopies

The dynamics and spectroscopic characteristics of the ultrafast photoinduced electron transfer (ET) of Rhodamine 6G (Rh6G⁺) in N,N-diethylaniline (DEA) were studied using femtosecond time-resolved multiplex transient grating and transient absorption spectroscopies. The ultrafast photoinduced forward ET from DEA to the Rh6G⁺* cation radical excited state has a time constant of τ _FET = 219–318 fs. The much slower backward ET from the neutral radical Rh6G^· to DEA⁺ with a time constant of τ _BET = 22.76–42.31 ps occurs in the inverted region. Intramolecular vibrational relaxation of the excited state takes place in τ _IVR = 2.18–6.91 ps.     

Spectral and luminescent properties of ammonia cyclopalladated complexes

The synthesis of ammonia cyclometalated palladium(II) complexes [Pd(NH₃)₂C^N]ClO₄ (C^N is the deprotonated form of 2-phenylpyridine, 2-(para-tolyl)pyridine, 7,8-benzo(h)quinoline, 2,6-diphenylpyridine, and 4-phenylpyrimidine) is developed. The IR and electronic absorption and emission spectra of these complexes are studied. It is found that the ammine and analogous ethylenediamine cyclometalated Pd(II) complexes have similar spectral and luminescent properties and the same nature of the electronically excited ³(π-π*)-type state responsible for the long-lived luminescence, the π and π* orbitals being localized on the corresponding cyclometalating ligand. The efficient temperature quenching of the luminescence of Pd(II) complexes at room temperature is assigned to the thermally activated population of metal-centered electronically excited states with subsequent nonradiative deactivation.     

Coulombic energy transfer and triple ionization in clusters

Using neon and its dimer as a specific example, it is shown that excited Auger decay channels that are electronically stable in the isolated monomer can relax in a cluster by electron emission. The decay mechanism, leading to the formation of a tricationic cluster, is based on an efficient energy-transfer process from the excited, dicationic monomer to a neighbor. The decay is ultrafast and expected to be relevant to numerous physical phenomena involving core holes in clusters and other forms of spatially extended atomic and molecular matter.     

黄芩素激发态分子内质子转移耦合电荷转移反应的实验和理论研究

通过稳态光谱实验和量子化学计算相结合,研究了黄芩素激发态质子转移耦合电荷转移的反应. 实验和计算中S₁态吸收峰的缺失表明S₁态是暗态. S₁暗态导致在实验中观察不到黄芩素在乙醇溶液中的荧光峰,且固体的荧光峰很弱. 黄芩素分子的前线分子轨道和电荷差异密度表明S₁态是电荷转移态,然而S₂态是局域激发态. 计算的黄芩素分子的势能曲线在激发态只有一个稳定点,这表明了黄芩素激发态分子内质子转移的过程是一个无     

Time‐dependent density functional theory study on the excited‐state hydrogen bonding strengthening of photoexcited 4‐amino‐1,8‐naphthalimide in hydrogen‐donating solvents

The time‐dependent density functional theory (TDDFT) method was performed to investigate the excited‐state hydrogen bonding dynamics of 4‐amino‐1,8‐naphthalimide (4ANI) as hydrogen bond acceptor in hydrogen donating methanol (MeOH) solvent. The ground‐state geometry optimizations, electronic transition energies and corresponding oscillation strengths of the low‐lying electronically excited states for the isolated 4ANi and hydrogen‐bonded 4ANi‐(MeOH)_1,4 complexes were calculated by the DFT and TDDFT methods, respectively. We demonstrated that the intermolecular hydrogen bond C═O···H–O and N–H···O–H in the hydrogen‐bonded 4ANi‐(MeOH)_1,4 is strengthened in the electronically excited state, because the electronic excitation energies of the hydrogen‐bonded complex are correspondingly decreased compared with that of the isolated 4ANi. The calculated results are consistent with the mechanism of the hydrogen bond strengthening in the electronically excited state, while contrast with mechanism of hydrogen bond cleavage. Furthermore, we believe that the transient hydrogen bond strengthening behavior in electronically excited state of fluorescent dye in hydrogen‐donating solvents exists in many other systems in solution. Copyright © 2013 John Wiley & Sons, Ltd.     

Tryptophan Fluorescence Yields and Lifetimes as a Probe of Conformational Changes in Human Glucokinase

Five variants of glucokinase (ATP-D-hexose-6-phosphotransferase, EC 2.7.1.1) including wild type and single Trp mutants with the Trp residue at positions 65, 99, 167 and 257 were prepared. The fluorescence of Trp in all locations studied showed intensity changes when glucose bound, indicating that conformational change occurs globally over the entire protein. While the fluorescence quantum yield changes upon glucose binding, the enzyme’s absorption spectra, emission spectra and fluorescence lifetimes change very little. These results are consistent with the existence of a dark complex for excited state Trp. Addition of glycerol, L-glucose, sucrose, or trehalose increases the binding affinity of glucose to the enzyme and increases fluorescence intensity. The effect of these osmolytes is thought to shift the protein conformation to a condensed, high affinity form. Based upon these results, we consider the nature of quenching of the Trp excited state. Amide groups are known to quench indole fluorescence and amides of the polypeptide chain make interact with excited state Trp in the relatively unstructured, glucose-free enzyme. Also, removal of water around the aromatic ring by addition of glucose substrate or osmolyte may reduce the quenching.     

超短脉冲激光场中间二氯苯的激发态动力学

利用飞秒时间分辨的飞行时间质谱技术研究了间二氯苯的激发态动力学.间二氯苯分子吸收一个200 nm或者267 nm的光子被抽运到激发态,随后再吸收多个800 nm的光子被电离.实验获得了电离产生的离子质谱信号及其随抽运探测激光延迟时间的变化曲线.在200 nm时,分子被抽运到激发态(π,π*),可观察到三个相互竞争的解离通道的寿命:内转换到排斥态(n,σ*)或者(π,σ*)并发生快速解离,其寿命约(0.15±0.01)ps;内转换到基态的高振动态,能量在基态"热"振动态间弛豫的寿命约为(4.94±0.08)ps;系间窜越到相邻的三重态从而发生预解离过程,其寿命约为(110.09±4.33)ps.在267 nm时,分子被抽运到第一激发态的低振动态,可观察到一个长寿命(约(1.06±0.05)ns)的系间窜越过程.除此之外,在碎片离子信号中还观察到了激发态与基态的高振动态之间的内转换过程.     

采用非绝热动力学模拟探究溶剂效应对桨-轮形状的给体-受体复合物的激发态弛豫动力学的影响

充分理解给体-受体复合物的激发态动力学在实验和理论上都具有非常重要的意义. 本文首次结合电子结构计算和非绝热动力学模拟,探索了最近合成的一个桨-轮形状的给体-受体复合物的光致动力学过程,该复合物由三个氟硼二吡咯基团和一个六氧杂并三苯构成. 根据计算结果,得出结论,桨-轮形状的给体-受体复合物共轭物在激发时将被提升到氟硼二吡咯片段的局部激发状态,然后发生超快局部激发电子态到电荷转移电子态的激子转移. 与先前实验工作中提出的光致电子转移机理不同,这种激子转移过程伴随着从氟硼二吡咯基团到六氧杂并三苯基团上的空穴. 此外,还发现溶剂效应在该体系的光致动力学中起着重要作用. 具体而言,强极性的乙腈溶剂加速了空穴转移动力学,这可以归因于溶剂对电荷转移状态的显著影响,即局部激发和电荷转移激子之间的能隙大大减小,而与此同时非绝热耦合增加,这两个因素都可以促进空穴转移过程. 目前的工作不仅为桨-轮形状的给体-受体复合物的潜在光诱导机制提供了有价值的见解,而且有助于未来设计具有更好光电性能的新型给体-受体复合物.