Ultrafast Investigation of Excited-State Dynamics in Trans-4-methoxyazobenzene Studied by Femtosecond Transient Absorption Spectroscopy

The ultrafast photoisomerization and excited-state dynamics of trans-4-methoxyazobenzene (trans-4-MAB) in solutions were investigated by femtosecond transient absorption spectroscopy and quantum chemistry calculations. After being excited to the S\begin{document}$_2$\end{document} state, the two-dimensional transient absorptions spectra show that cis-4-MAB is produced and witnessed by the permanent positive absorption in 400\begin{document}$-$\end{document}480 nm. Three decay components are determined to be 0.11, 1.4 and 2.9 ps in ethanol, and 0.16, 1.5 and 7.5 ps in ethylene glycol, respectively. The fast component is assigned to the internal conversion from the S\begin{document}$_2$\end{document} to S\begin{document}$_1$\end{document} state. The other relaxation pathways are correlated with the decay of the S\begin{document}$_1$\end{document} state via internal conversion and isomerization, and the vibrational cooling of the hot S\begin{document}$_0$\end{document} state of the cis-isomer. Comparing of the dynamics in different solvents, it is demonstrated that the photoisomerization pathway undergoes the inversion mechanism rather than the rotation mechanism.     

飞秒时间分辨的光电子谱对苯S2态的超快动力学研究

结合飞秒时间分辨的质谱技术与时间分辨的光电子影像技术对苯S₂激发态的超快动力学进行了研究.苯分子吸收两个400 nm的光子被激发到S₂态,之后再用一个267 nm的光子对其进行探测.获得的母体离子产率随泵浦探测时间延迟的变化曲线包含了两个不同的时间寿命组分.第一个时间寿命组分(90 ± 1) fs被归纳为S₂态到S₁/S₀态的内转换过程;第二个时间寿命组分(5.0 ± 0.2) ps被归纳为S₁态的衰减过程.实验中观察到的第二个寿命组分小于早前的研究结果,这表明了在S₁态的衰减过程中还可能存在其他的过程.从时间分辨的光电子影像提取得到的光电子能谱中发现了一个新的失活过程,该过程被归结为激发态S₁的振动态与“热”三重态T₃之间的系间交叉过程.     

Investigation of Ultrafast Electronic Transfer Process on Organic/Inorganic Heterojunction by Femtosecond Transient Absorption

We detect a relaxation process of excited SQ02 dye in the chlorobenzene solution and an-chor SQ02 on Al₂O₃ and TiO₂ lm, so as to investigate the photophysical properties of pristine SQ02 in the monodisperse system, aggregation state, and the corresponding inter-facial electron transfer process. The experimental data show that the lifetime of SQ02 in the monondisperse system is ~2.0 ns, but that of SQ02 anchored on the Al₂O₃ lm could obviously decrease to ~21 ps. The time of electron transfer from excited SQ02 to TiO₂ lm is estimated to be ~2.6 ps and the yield of electron injection is estimated to be ~89.1%, which matches the incident photon to current e ciency of dye-sensitized solar cell based on SQ02. In addition, some dyes are found to pack on the other dyes anchored on the nanocrystal lm, and their relaxation time could reach ~60 ps. They couldn't participate in the interfacial electron transfer, since they are far away from the TiO₂ interface.     

Ultrafast Excited State Dynamics of Biliverdin Dimethyl Ester Coordinate with Zinc Ions

As one of the biological endogenous pigments, biliverdin (BV) and its dimethyl ester (BVE) have extremely weak fluorescence in solution with quantum yield less than 0.01%. However, the situation reverses with the addition of zinc ions. The strength for fluorescence of BVE-Znbegin{document}$ ^{2+} $end{document} complex is greatly enhanced and fluorescence quantum yield can increase to begin{document}$ sim $end{document}5%. Herein, we studied ultrafast excited state dynamics of BVE-Znbegin{document}$ ^{2+} $end{document} complex in ethanol, begin{document}$ n $end{document}-propanol, and DMSO solutions in order to reveal the mechanism of fluorescence quantum yield enhancement. The results show that BVE can form a stable coordination complex with zinc with 1:1 stoichiometry in solution. BVE is structurally and energetically more stable in the complex. Using picosecond time-resolve fluorescence and femtosecond transient absorption spectroscopy, we show that smaller non-radiative rate constant of BVE-Znbegin{document}$ ^{2+} $end{document} complex in DMSO is the key to increasing its fluorescence quantum yield and the excited state decay mechanism is also revealed. These results provide valuable information about the fluorescence property change after BVE binding to metal ions and may provide a guidance for the study of phytochromes or other fluorescence proteins in which BV/BVE acts as chromophores.     

Ultrafast Excited State Relaxation Dynamics of New Fuchsine- a Triphenylmethane Derivative Dye

An all-inclusive investigation of the ultrafast excited state relaxation dynamics of a triphenylmethane derivative molecule, New Fuchsine (NF), using a combined approach of density functional theory (DFT), femtosecond transient absorption spectroscopy (fs-TAS), and photoluminescence spectroscopy is presented in this work. The DFT calculations confirmed the formation of twisted molecular structure in the excited state of NF in ethanol solution with bond rotation of ≈86°. TAS measurements of NF solution exhibited ultrafast ground state-recovery pathway via a conical intersection confirming an ultrafast structural reorientation. On the contrary, TAS measurements of NF thin-film exhibited a longer excited-state lifetime suggesting a hindered molecular twisted state formed as an intermediate step. Photophysical kinetic models are proposed to globally fit the fs-TAS data establishing the twisted intramolecular charge transfer (TICT) state mediated ground state recovery for NF in solution and thin film, respectively. Temperature-dependent photoluminescence study of NF film provided a clear insight into the effect of rotational motion of phenyl rings in NF molecules over the TICT mediated emission.     

Hot-Electron Photodynamics in Silver-Containing BEA-Type Nanozeolite Studied by Femtosecond Transient Absorption Spectroscopy

Silver cations were introduced in nanosized BEA-type zeolite containing organic template by ion-exchange followed by chemical reduction towards preparation of photoactive materials (Ag⁰-BEA). The stabilization of highly dispersed Ag⁰ nanoparticles with a size of 1–2 nm in the BEA zeolite was revealed. The transient optical response of the Ag-BEA samples upon photoexcitation at 400 nm was studied by femtosecond absorption. The photodynamic of the hot electrons was found to depend on the sample preparation. The lifetime of the hot electrons in the Ag−BEA samples containing small Ag nanoparticles (1–2 nm) is significantly shortened in comparison to bear Ag nanoparticles with a size of 10 nm. While for the larger Ag nanoparticles, the energy absorbed in the conduction band is decaying by electron-phonon coupling into the metal lattice, the high surface-to-volume ratio of the small Ag nanoparticles favors the dissipation of the energy of the hot electrons from the metal nanoparticles (Ag⁰) towards the zeolitic micro-environment. This finding is encouraging for further applications of Ag-containing zeolites in photocatalysis and plasmonic chemistry.     

Excited‐State Dynamics of the Metal String Complex Co3(dpa)4(NCS)2 from Femtosecond Transient Absorption Spectra

Transient absorption spectroscopy is used to study the excited‐state dynamics of Co₃(dpa)₄(NCS)₂, where dpa is the ligand di(2‐pyridyl)amido. The ππ*, charge‐transfer, and d–d transition states are excited upon irradiation at wavelengths of 330, 400 and 600 nm, respectively. Similar transient spectra are observed under the experimental temporal resolution and the transient species show weak absorption. We thus propose that a low‐lying metal‐centered d–d state is accessed immediately after excitation. Analyses of the experimental kinetic traces reveal rapid conversion from the ligand‐centered ππ* and the charge‐transfer states to this metal‐centered d‐d state within 100 fs. The excited molecule then crosses to a second d–d state within the ligand‐field manifold, with a time coefficient of 0.6–1.4 ps. Because the ground‐state bleaching band recovers with a time coefficient of 10–23 ps, we propose that an excited molecule crosses from the low‐lying d–d state either directly within the same spin system or with spin crossing via the state ²B to the ground state ²A₂ (symmetry group C₄). In this trimetal string complex, relaxation to the ground electronic surface after excitation is thus rapid.     

Ultrafast Decay Dynamics of N-Ethylpyrrole Excited to the S1 Electronic State: A Femtosecond Time-Resolved Photoelectron Imaging Study

N-ethylpyrrole is one of ethyl-substituted derivatives of pyrrole and its excited-state decay dynamics has never been explored. In this work, we investigate ultrafast decay dynamics of N-ethylpyrrole excited to the S₁ electronic state using a femtosecond time-resolved photoelectron imaging method. Two pump wavelengths of 241.9 and 237.7 nm are employed. At 241.9 nm, three time constants, 5.0±0.7 ps, 66.4±15.6 ps and 1.3±0.1 ns, are derived. For 237.7 nm, two time constants of 2.1±0.1 ps and 13.1±1.2 ps are derived. We assign all these time constants to be associated with different vibrational states in the S₁ state. The possible decay mechanisms of different S₁ vibrational states are briefly discussed.     

Ultrafast Dynamics of Defect-Assisted Carrier Capture in MoS2 Nanodots Investigated by Transient Absorption Spectroscopy

MoS₂ nanodots are emerging as promising semiconductor materials for optoelectronic devices. However, most of the recent attention is focused on the fabrication of MoS₂ nanodots, and the survey for exciton dynamics of MoS₂ nanodots remains less explored. Herein, we use femtosecond transient absorption spectroscopy to investigate the carrier dynamics of MoS₂ nanodots. Our results show that defect-assisted carrier recombination processes are well consistent with the observed dynamics. The photo-excited carriers are captured by defects with at least two different capture rates via Auger scattering. Four processes are deemed to take part in the carrier relaxation. After photoexcitation, carrier cooling occurs instantly within ~0.5 ps. Then most of carriers are fast captured by the defects, and the corresponding time constant increases from ~4.9 ps to ~9.2 ps with increasing pump fluence, which may be interpreted by saturation of the defect states. Next a small quantity of carriers is captured by the other kinds of defects with a relatively slow carrier capture time within ~65 ps. Finally, the remaining small fraction of carriers relaxes via direct interband electron-hole recombination within ~1 ns. Our results may lead to deep insight into the fundamentals of carrier dynamics in MoS₂ nanodots, paving the way for their further applications.     

结合光电化学和瞬态吸收光谱技术研究光电化学分解水载流子动力学

半导体光电化学制氢是一种重要的、有前景的太阳能应用技术. 其产氢效率取决于光生载流子的产生、分离和传输效率. 深入理解光生载流子的动力学过程对于设计高效的太阳能产氢器件有重要的指导意义. 光电化学和瞬态吸收光谱技术是研究光催化反应微观动力学和机理的强有力手段. 本文介绍作者应用这些技术在半导体光电化学制氢方面所取得的部分最新研究结果, 并对存在的问题和今后研究重点提出了一些看法.     

Molecular Vibronic Structures of HDITC in Solutions Studied by Femtosecond Wavelength-Resolved Pump-Probe Spectroscopy

Wavelength-resolved pump-probe measurements using 11 fs duration pulses were performed to study the vibronic structures of HDITC, a cyanine dye, in ethylene glycol. Ten vibrational modes were observed in the forms of quantum beats. The frequencies of these ten vibrational modes ranged from 135 cm^?1 to 1300 cm^?1. The relative potential displacements along these ten vibrational coordinates were estimated by comparison with theoretical calculations.     

Ultrafast Dynamics of Water Molecules Excited to Electronic F States: A Time-Resolved Photoelectron Spectroscopy Study

The ultrafast dynamics of water molecules excited to the two F states is studied by combining two-photon excitation and time-resolved photoelectron imaging techniques. The lifetimes of the F₁A₁ and F₁B₁ states of H₂O (D₂O) were derived to be 1.0±0.3 (1.9±0.4) and 10±3 (30±10) ps, respectively. We propose that the F₁A₁ state mainly decays through the D state, due to the nonadiabatic coupling between them, while the F₁B₁ state decays through the F₁A₁ state via Coriolis interaction.     

Ultrafast Decay Dynamics of 2-Hydroxypyridine Excited to S1 Electronic State

The S\begin{document}$_1$\end{document} state decay dynamics of 2-hydroxypyridine following UV excitation at a wavelength range of 276.9\begin{document}$-$\end{document}250.0 nm is investigated using femtosecond time-resolved photoelectron imaging technique. Based on pump wavelength dependence of the decay dynamics, a refined decay picture is proposed. At pump wavelength of 276.9 nm, the S\begin{document}$_1$\end{document} state is depopulated through intersystem crossing to lower triplet state(s). At 264.0 nm, both intersystem crossing to lower triplet state(s) and internal conversion to the ground state are in operation. At 250.0 nm, internal conversion to the ground state becomes dominated.     

飞秒泵浦-探测质谱方法研究苯内转换动力学

以飞秒400及266 nm激光脉冲结合泵浦-探测飞行时间质谱方法研究了苯S2态内转换动力学. 400 nm双光子过程将苯分子激发到S2电子态,布居在S2电子态的分子由于能级耦合无辐射弛豫到S1电子态. 通过测定C6H6+讯号强度随泵浦-探测延迟时间的改变,获得苯S2及S1电子态的衰减寿命分别为(48±1)fs及(6.5±0.2)ps, S2态及S1电子态的消激发机理被认为是相应势能面间的锥形交叉引起的内转换.     

Characterization of Locally Excited and Charge-Transfer States of the Anticancer Drug Lapatinib by Ultrafast Spectroscopy and Computational Studies

Lapatinib (LAP) is an anticancer drug, which is metabolized to the N- and O-dealkylated products (N-LAP and O-LAP, respectively). In view of the photosensitizing potential of related drugs, a complete experimental and theoretical study has been performed on LAP, N-LAP and O-LAP, both in solution and upon complexation with human serum albumin (HSA). In organic solvents, coplanar locally excited (LE) emissive states are generated; they rapidly evolve towards twisted intramolecular charge-transfer (ICT) states. By contrast, within HSA only LE states are detected. Accordingly, femtosecond transient absorption reveals a very fast switching (ca. 2 ps) from LE (λ_max=550 nm) to ICT states (λ_max=480 nm) in solution, whereas within HSA the LE species become stabilized and live much longer (up to the ns scale). Interestingly, molecular dynamics simulation studies confirm that the coplanar orientation is preferred for LAP (or to a lesser extent N-LAP) within HSA, explaining the experimental results.     

金纳米颗粒在不同包裹介质中的超快等离子体动力学

Herein, we prepared four samples, namely gold/poly(sodium-p-styrenesulfonate) (Au/PSS), gold/silicon dioxide (Au/SiO₂), gold/titanium dioxide (Au/TiO₂), and gold/cuprous oxide (Au/Cu₂O) core/shell nanocomposites, to investigate how the surrounding medium affects the ultrafast plasmon dynamics of Au nanoparticles (NPs). We recorded femtosecond transient absorption spectra of Au NPs in Au/PSS, Au/SiO₂, Au/TiO₂, and Au/Cu₂O core/shell nanocomposites at various time delays. We found that the spectral features in the femtosecond transient absorption spectra of Au NPs in Au/TiO₂ and Au/Cu₂O core/shell nanocomposites were dramatically different from those of Au NPs in Au/PSS and Au/SiO₂ core/shell nanocomposites. A comprehensive analysis of the ultrafast plasmon dynamics of Au NPs in the core/shell nanocomposites revealed that following excitation of the resonance plasmon band of Au NPs, the exited electrons could be efficiently transferred into the conduction bands of TiO₂ and Cu₂O in Au/TiO₂ and Au/Cu₂O core/shell nanocomposites.     

Ultrafast Twisting Dynamics of Thioflavin‐T: Spectroscopy of the Twisted Intramolecular Charge‐Transfer State

Understanding the excited‐state properties of thioflavin‐T (ThT) has been of immense importance, because of its efficient amyloid‐sensing ability related to neurodegenerative disorders. The excited‐state dynamics of ThT is studied by using sub‐pico‐ and nanosecond time‐resolved transient absorption techniques as well as density functional theory (DFT)/time‐dependent DFT calculations. Barrierless twisting around the central C?C bond between two aromatic moieties is the dominant process that contributes to the ultrafast dynamics of the S₁ state. The spectroscopic properties of the intramolecular charge‐transfer state are characterized for the first time. The energetics of the S₀ and S₁ states has also been correlated with the experimentally observed spectroscopic parameters and structural dynamics. A longer‐lived transient state populated with a very low yield has been characterized as the triplet state.     

Effect of Structural Defects and Impurities on the Excited State Dynamics of 2D BA2PbI4 Perovskite

In this work, we show that the quality of the precursor and the thin film preparation strongly affect the optoelectronic properties of the 2D perovskite BA₂PbI₄. 2D perovskites with alkylammonium organic cations such as butylammonium (BA) are relatively soft structures that exhibit large dynamic disorder and phase variations. Here we show, by a variety of spectroscopy techniques (steady state absorption, photoluminescence and ultrafast transient absorption), that at temperatures below the phase transition (253 K) the material exhibits excitonic features from the room temperature phase (due to incomplete structural transition) and a broadband emission at 560–600 nm (due to self-trapped excitons) with varied relative intensities depending on the precursors and processing conditions. This suggests that the processing conditions have a large influence on the crystallization and introduction of extrinsic defect impurities directly affecting the optoelectronic properties. Making absolute statements about the properties of BA₂PbI₄ requires improved control over the materials thin film deposition and a better understanding of the role of the lattice vibrational dynamics and extrinsic defects on the exciton dynamics.     

4-硝基咪唑A-带激发态结构动力学

通过共振拉曼光谱实验和量子化学计算的方法研究了4-硝基咪唑(4NI)A-带激发态衰变动力学. 对4NI的振动光谱、紫外电子吸收光谱、荧光光谱和共振拉曼光谱进行了指认. 在全活化空间自洽场法(CASSCF)/6-31G(d)计算水平下获得了单重激发态S₁(n_Oπ^*)和S₂(ππ^*)和势能面交叉点S₁(n_Oπ^*)/S₂(ππ^*)的优化几何结构和能量, 分析了A-带共振拉曼光谱的强度模式特征, 获得了短时结构动力学, 并结合全活化空间自洽场法(CASSCF)理论计算结果确定了4NI 在S₂(ππ^*)态衰变通道主要是S_{2, FC}→S_{2, min}(ππ^*)→S₀辐射弛豫.     

Ultrafast Nonadiabatic Photoisomerization Dynamics Mechanism for the UV Photoprotection of Stilbenoids in Grape Skin

Natural UV photoprotection plays a vital role in physiological protection. It has been reported that UVC radiation can make resveratrol (RSV) and piceatannol (PIC) accumulate in grape skin. In this work, we demonstrated that RSV and PIC could significantly absorb UVA and UVB, and confirmed their satisfactory photostability. Furthermore, we clarified the UV photoprotection mechanism of typical stilbenoids of RSV and PIC for the first time by using combined femtosecond transient absorption (FTA) spectroscopy and time‐dependent density functional theory (TD‐DFT) calculations. RSV and PIC can be photoexcited to the excited state after UVA and UVB absorption. Subsequently, the photoisomerized RSV and PIC quickly relax to the ground state via nonadiabatic transition from the S₁ state at a conical intersection (CI) position between potential energy surfaces (PESs) of S₁ and S₀ states. This ultrafast trans‐cis photoisomerization will take place within a few tens of picoseconds. As a result, the UV energy absorbed by RSV and PIC could be dissipated by an ultrafast nonadiabatic photoisomerization process.