飞秒时间分辨的光电子影像技术研究2-氯吡啶超快内转换动力学

利用时间分辨的飞秒光电子影像技术结合时间分辨的质谱技术, 研究了2-氯吡啶分子激发态的超快过程. 实时观察到了2-氯吡啶分子第二激发态(S₂)向第一激发态(S₁)高振动能级的的超快内转换过程,该内转换的时间常数为(162±5)fs. 实验结果表明, 通过S₂/S₀的锥形交叉衰减到基态的衰减通道也是退布居的重要通道, 其时间尺度为(5.5±0.3) ps.     

飞秒时间分辨的光电子影像研究氯化苄分子内转换动力学(英文)

结合时间分辨的飞秒光电子影像(TRPEI)技术和时间分辨的质谱技术,研究了氯化苄(BzCl)分子内转换动力学过程.从光电子影像中获得了光电子动能分布和角度分布.氯化苄分子吸收两个400nm的光子后从基态跃迁到S4态和S2态.获得的母体离子随泵浦-探测时间延迟变化的曲线可以用两个指数函数进行拟合,包括一个时间常数为50fs的快速组分和一个时间常数为910fs的慢速组分.通过分析光电子动能分布随延迟时间的变化,我们认为分子被激发到S4态后在很短的时间内与S2态发生耦合迅速弛豫到S2态,然后再经内转换(IC)弛豫到S1态.最初布居的激发态分子经过内转换弛豫到S1态的时间尺度为50fs.910fs的慢速时间组分反映了分子弛豫到S1态后,经内转换向基态S0的弛豫.光电子角度分布的各向异性参数从零时刻的0.87增加到25fs时的0.94,然后逐渐减小到190fs时刻的0.59的现象,也反映了氯化苄分子从S4态耦合到S2态,然后内转换到S1态的动力学过程.     

飞秒时间分辨的光电子影像对3-甲基吡啶分子超快动力学研究

利用飞秒时间分辨的光电子影像技术结合时间分辨的质谱技术,研究了3-甲基吡啶分子激发态的超快过程. 实时观察到了3-甲基吡啶分子S₂态向S₁态高振动能级的超快内转换过程,该内转换的时间大约为910fs. 二次布居的S₁态主要通过内转换衰减到基态S₀,该内转换的时间尺度为2.77 ps. 光电子能谱分布和光电子角分布显示,S₂态和S₁态在电离的过程中跟3p里德堡态发生偶然共振. 本次实验中还用400 nm两个光子吸收的方法布居了3-甲基吡啶的3s 里德堡态. 研究表明,3s 里德堡态的寿命为62 fs,并主要通过内转换快速衰减到基态.     

氯苯分子第一激发态超快动力学

利用时间分辨飞秒光电子影像技术结合时间分辨质谱技术, 研究了氯苯分子第一激发态的超快过程. 266.7 nm单光子将氯苯分子激发至第一激发态. 母体离子时间变化曲线包括了不同的双指数曲线. 一个是时间常数为(152±3) fs的快速组分, 另一个是时间常数为(749±21) ps的慢速组分. 通过时间分辨的光电子影像得到了时间分辨的光电子动能分布和角度分布. 时间常数为(152±3) fs的快速组分反映了第一激发态内部的能量转移过程, 这个过程归属为氯苯分子第一激发态耗散型振动驰豫过程. 时间常数为(749±21) ps的慢速组分反映了第一激发态的慢速内转换过程. 另外, 实验实时观察到典型的非对称陀螺分子(氯苯)激发态的非绝热准直和转动退相干现象. 并推算出第一次转动恢复时间为205.8 ps (C类型)和359.3 ps (J类型).     

Ultrafast dynamics of UV-excited imidazole

The ultrafast dynamics of UV-excited imidazole in the gas phase is investigated by theoretical nonadiabatic dynamics simulations and experimental time-resolved photoelectron spectroscopy. The results show that different electronic excited-state relaxation mechanisms occur, depending on the pump wavelength. When imidazole is excited at 239.6 nm, deactivation through the NH-dissociation conical intersection is observed on the sub-50 fs timescale. After 200.8 nm excitation, competition between NH-dissociation and NH-puckering conical intersections is observed. The NH-dissociation to NH-puckering branching ratio is predicted to be 21:4, and the total relaxation time is elongated by a factor of eight. A procedure for simulation of photoelectron spectra based on dynamics results is developed and employed to assign different features in the experimental spectra.     

Excited‐State Intramolecular Proton Transfer in a Blue Fluorescence Chromophore Induces Dual Emission

Compared with green fluorescence protein (GFP) chromophores, the recently synthesized blue fluorescence protein (BFP) chromophore variant presents intriguing photochemical properties, for example, dual fluorescence emission, enhanced fluorescence quantum yield, and ultra‐slow excited‐state intramolecular proton transfer (ESIPT; J. Phys. Chem. Lett., 2014 , 5, 92); however, its photochemical mechanism is still elusive. Herein we have employed the CASSCF and CASPT2 methods to study the mechanistic photochemistry of a truncated BFP chromophore variant in the S₀ and S₁ states. Based on the optimized minima, conical intersections, and minimum‐energy paths (ESIPT, photoisomerization, and deactivation), we have found that the system has two competitive S₁ relaxation pathways from the Franck–Condon point of the BFP chromophore variant. One is the ESIPT path to generate an S₁ tautomer that exhibits a large Stokes shift in experiments. The generated S₁ tautomer can further evolve toward the nearby S₁/S₀ conical intersection and then jumps down to the S₀ state. The other is the photoisomerization path along the rotation of the central double bond. Along this path, the S₁ system runs into an S₁/S₀ conical intersection region and eventually hops to the S₀ state. The two energetically allowed S₁ excited‐state deactivation pathways are responsible for the in‐part loss of fluorescence quantum yield. The considerable S₁ ESIPT barrier and the sizable barriers that separate the S₁ tautomers from the S₁/S₀ conical intersections make these two tautomers establish a kinetic equilibrium in the S₁ state, which thus results in dual fluorescence emission.     

Intersystem Crossing of 2-Methlypyrazine Studied by Femtosecond Photoelectron Imaging

2-methylpyrazine was excited to the high vibrational dynamics of the S₁ state with 260 nm femtosecond laser light, and the evolution of the excited state was probed with 400 nm light. Because it was unstable, the S₁ state decayed via intersystem crossing to the triplet state T₁, and it may have decayed to the ground state S₀ via internal conversion. S₁-to-T₁ intersystem crossing was observed by combining time-resolved mass spectrometry and time-resolved photoelectron spectroscopy. The crossover time scale was 23 ps. Rydberg states were identified, and the photoelectron spectral and angular distributions indicated accidental resonances of the S₁ and T₁ states with the 3s and 3p Rydberg states, respectively, during ionization.     

Theoretical study of the photochemical reaction mechanism of bicyclo[4.1.0]hept‐2‐ene upon direct photolysis

Ab initio multiconfigurational CASSCF/MP2 method with the 6‐31G* basis set has been employed in studying the photochemistry of bicyclo[4.1.0]hept‐2‐ene upon direct photolysis. Our calculations involve the ground state (S₀) and excited states (S₁, T₁, and T₂). The ground‐state reaction pathways corresponding to the formation of the six products derived from bicyclo[4.1.0]hept‐2‐ene via two important diradical intermediates (D1 and D2) were mapped. It was found that there are various crossing points (conical intersections and singlet–triplet crossings) in the regions near D1 and D2. These crossing points imply that direct photolysis can lead to two possible radiationless relaxation routes: (1) S₁ → S₀, (2) S₁ → T₂ → T₁ → S₀. Computation indicates that the second route is not a competitive path with the first route during direct photolysis. The first route is initiated by barrierless cyclopropane bond cleavage to form two singlet excited diradical intermediates, followed by efficient decay to the ground‐state surface via three S₁/S₀ conical intersections in the regions near the diradical intermediates. All six ground‐state products can be formed via the three conical intersections almost without barrier after the decays. The barriers separating the diradical minima on S₁ from the S₁/S₀ conical intersections were found to be very small with respect to the vertical excitation energy, which can explain why the product distribution is independent of excitation wavelength. Triplet surfaces are not involved in the first route, which agrees with the fact that the product contribution was unchanged by the addition of naphthalene. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Photoisomerization of Arylazopyrazole Photoswitches: Stereospecific Excited‐State Relaxation

Electronic structure calculations and nonadiabatic dynamics simulations (more than 2000 trajectories) are used to explore the Z–E photoisomerization mechanism and excited‐state decay dynamics of two arylazopyrazole photoswitches. Two chiral S₁/S₀ conical intersections with associated enantiomeric S₁ relaxation paths that are barrierless and efficient (timescale of ca. 50 fs) were found. For the parent arylazopyrazole (Z8) both paths contribute evenly to the S₁ excited‐state decay, whereas for the dimethyl derivative (Z11) each of the two chiral cis minima decays almost exclusively through one specific enantiomeric S₁ relaxation path. To our knowledge, the Z11 arylazopyrazole is thus the first example for nearly stereospecific unidirectional excited‐state relaxation.     

Excited-State Proton Transfer and Decay in Hydrogen-Bonded Oxazole System:MS-CASPT2//CASSCF Study

Herein we have employed high-level multi-reference CASSCF and MS-CASPT2 electronic structure methods to systematically study the photochemical mechanism of intramolecularly hydrogen-bonded 2-(2'-hydroxyphenyl)-4-methyloxazole. At the CASSCF level, we have optimized minima, conical intersections, minimum-energy reaction paths relevant to the excited-state intramolecular proton transfer (ESIPT), rotation, photoisomerization, and the excited-state deactivation pathways. The energies of all structures and paths are refined by the MS-CASPT2 method. On the basis of the present results, we found that the ESIPT process in a conformer with the OH…N hydrogen bond is essentially barrierless process; whereas, the ESIPT process is inhibited in the other conformer with the OH…O hydrogen bond. The central single-bond rotation of the S₁ enol species is energetically unfavorable due to a large barrier. In addition, the excited-state deactivation of the S₁ keto species, as a result of the ultrafast ESIPT, is very efficient because of the existence of two easily-approached keto S₁/S₀ conical intersections. In stark contrast to the S₁ keto species, the decay of the S₁ enol species is almostly blocked. The present theoretical study contributes valuable knowledge to the understanding of photochemistry of similar intramolecularly hydrogen-bonded molecular and biological systems.     

The non-adiabatic nanoreactor: towards the automated discovery of photochemistry

The ab initio nanoreactor has previously been introduced to automate reaction discovery for ground state chemistry. In this work, we present the nonadiabatic nanoreactor, an analogous framework for excited state reaction discovery. We automate the study of nonadiabatic decay mechanisms of molecules by probing the intersection seam between adiabatic electronic states with hyper-real metadynamics, sampling the branching plane for relevant conical intersections, and performing seam-constrained path searches. We illustrate the effectiveness of the nonadiabatic nanoreactor by applying it to benzene, a molecule with rich photochemistry and a wide array of photochemical products. Our study confirms the existence of several types of S₀/S₁ and S₁/S₂ conical intersections which mediate access to a variety of ground state stationary points. We elucidate the connections between conical intersection energy/topography and the resulting photoproduct distribution, which changes smoothly along seam space segments. The exploration is performed with minimal user input, and the protocol requires no previous knowledge of the photochemical behavior of a target molecule. We demonstrate that the nonadiabatic nanoreactor is a valuable tool for the automated exploration of photochemical reactions and their mechanisms.

The nonadiabatic nanoreactor is a tool for automated photochemical reaction discovery that extensively explores intersection seams and links conical intersections to photoproduct distributions.     

Excited-State Double Proton Transfer of 1, 8-Dihydroxy-2-Naphthaldehyde: a MS-CASPT2//CASSCF Study

Excited-state double proton transfer (ESDPT) is a controversial issue which has long been plagued with theoretical and experimental communities. Herein, we took 1, 8-dihydroxy-2-naphthaldehyde (DHNA) as a prototype and used combined complete active space self-consistent field (CASSCF) and multi-state complete active-space second-order perturbation (MS-CASPT2) methods to investigate ESDPT and excited-state deactivation pathways of DHNA. Three different tautomer minima of S1-ENOL, S1-KETO-1, and S1-KETO-2 and two crucial conical intersections of S1S0-KETO-1 and S1S0-KETO-2 in and between the S₀ and S₁ states were obtained. S1-KETO-1 and S1-KETO-2 should take responsibility for experimentally observing dual-emission bands. In addition, two-dimensional potential energy surfaces (2D-PESs) and linear interpolated internal coordinate paths connecting relevant structures were calculated at the MS-CASPT2//CASSCF level and confirmed a stepwise ESDPT mechanism. Specifically, the first proton transfer from S1-ENOL to S1-KETO-1 is barrierless, whereas the second one from S1-KETO-1 to S1-KETO-2 demands a barrier of ca. 6.0 kcal/mol. The linear interpolated internal coordinate path connecting S1-KETO-1 (S1-KETO-2) and S1S0-KETO-1 (S1S0-KETO-2) is uphill with a barrier of ca. 12.0 kcal/mol, which will trap DHNA in the S₁ state while therefore enabling dual-emission bands. On the other hand, the S₁/S₀ conical intersections would also prompt the S₁ system to decay to the S₀ state, which could be to certain extent suppressed by locking the rotation of the C5$-$C8$-$C9$-$O10 dihedral angle. These mechanistic insights are not only helpful for understanding ESDPT but also useful for designing novel molecular materials with excellent photoluminescent performances.     

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.     

Nonradiative deexcitation dynamics of 9H-adenine: an OM2 surface hopping study

The nonradiative relaxation of 9H-adenine was studied at the semiempirical OM2/MR-CI level using the surface-hopping approach. Geometry optimizations of energy minima and conical intersections as well as single-point calculations of excitation energies at critical points were performed to characterize the relevant potential energy surfaces of 9H-adenine and to assess the accuracy of the OM2 results. Surface-hopping calculations were performed to describe the nonradiative dynamics of 9H-adenine after vertical excitation into the optically active state. They showed that the deexcitation process is mainly governed by a two-step relaxation consisting of an ultrashort component and a longer component. These findings compare well with experimental results from time-resolved photoelectron spectroscopy.     

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.     

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.     

Substituent effects on dynamics at conical intersections: alpha,beta-enones

Femtosecond time-resolved photoelectron spectroscopy and high-level theoretical calculations were used to study the effects of methyl substitution on the electronic dynamics of the alpha,beta-enones acrolein (2-propenal), crotonaldehyde (2-butenal), methylvinylketone (3-buten-2-one), and methacrolein (2-methyl-2-propenal) following excitation to the S2(pipi*) state at 209 and 200 nm. We determine that following excitation the molecules move rapidly away from the Franck-Condon region, reaching a conical intersection promoting relaxation to the S1(npi*) state. Once on the S1 surface, the trajectories access another conical intersection, leading them to the ground state. Only small variations between molecules are seen in their S2 decay times. However, the position of methyl group substitution greatly affects the relaxation rate from the S1 surface and the branching ratios to the products. Ab initio calculations used to compare the geometries, energies, and topographies of the S1/S0 conical intersections of the molecules are not able to satisfactorily explain the variations in relaxation behavior. We propose that the S1 lifetime differences are caused by specific dynamical factors that affect the efficiency of passage through the S1/S0 conical intersection.     

Photodeactivation paths in norbornadiene

The first high level ab initio quantum‐chemical calculations of potential energy surfaces (PESs) for low‐lying singlet excited states of norbornadiene in the gas phase are presented. The optimization of the stationary points (minima and conical intersections) and the recalculation of the energies were performed using the multireference configuration interaction with singles (MR‐CIS) and the multiconfigurational second‐order perturbation (CASPT2) methods, respectively. It was shown that the crossing between valence V₂ and Rydberg R₁ states close to the Franck–Condon (FC) point permits an easy population switch between these states. Also, a new deactivation path in which the doubly excited state with (π₃)² configuration (DE) has a prominent role in photodeactivation from the R₁ state due to the R₁/DE and the DE/V₁ conical intersections very close to the R₁ and DE minima, respectively, was proposed. Subsequent deactivation from the V₁ to the ground state goes through an Olivucci–Robb‐type conical intersection that adopts a rhombic distorted geometry. The deactivation path has negligible barriers, thereby making ultrafast radiationless decay to the ground state possible. © 2013 Wiley Periodicals, Inc.     

The effect of molecular distortions on spin–orbit coupling in simple hydrocarbons

Recent work [D.N.S. Parker et al., Chem. Phys. Lett. 469 (2009) 43–49] has found intersystem crossing (ISC) on an ultrafast timescale in electronically excited benzene, a surprise as hydrocarbons generally have small spin–orbit coupling. In this paper, the effect of molecular distortions on spin–orbit coupling (SOC) is calculated for cyclobutadiene and benzene. At equilibrium the SOC in both molecules is negligible, and therefore terms arising from molecular distortions must play a significant role in any fast ISC. We show that out-of-plane C–H bends, which leads to the hybridisation of σ and π orbitals, are responsible for the most significant effect. The S₁/S₀ conical intersection is an important feature for understanding the photochemistry of these molecules. We examine the SOC along the vector from the Franck–Condon point to the lowest energy point on the crossing seam and discuss the potential importance of the SOC to the ultrafast dynamics.     

Computational Photochemistry of the Azobenzene Scaffold of Sudan I and Orange II Dyes: Excited‐State Proton Transfer and Deactivation via Conical Intersections

We employed the complete active space self‐consistent field (CASSCF) and its multistate second‐order perturbation (MS‐CASPT2) methods to explore the photochemical mechanism of 2‐hydroxyazobenzene, the molecular scaffold of Sudan I and Orange II dyes. It was found that the excited‐state intramolecular proton transfer (ESIPT) along the bright diabatic ¹ππ* state is barrierless and ultrafast. Along this diabatic ¹ππ* relaxation path, the system can jump to the dark ¹nπ* state via the ¹ππ*/¹nπ* crossing point. However, ESIPT in this dark state is largely inhibited owing to a sizeable barrier. We also found two deactivation channels that decay ¹ππ* keto and ¹nπ* enol species to the ground state via two energetically accessible S₁/S₀ conical intersections. Finally, we encountered an interesting phenomenon in the excited‐state hydrogen‐bonding strength: it is reinforced in the ¹ππ* state, whereas it is reduced in the ¹nπ* state. The present work sets the stage for understanding the photophysics and photochemistry of Sudan I–IV, Orange II, Ponceau 2R, Ponceau 4R, and azo violet.