Chiral Inductions in Excited State Reactions: Photodimerization of Alkyl 2‐Naphthoates as a Model

Enantioselectivity in organic transformations continues to be a topic major interest in organic photochemistry. In the last decade, synergistic combination of photocatalysis and organocatalysis has emerged as a powerful strategy to gain enantioselectivity in photochemical reactions, and remarkable achievements have been obtained. In this strategy, the asymmetric induction is provided in ground state. In contrast, in the conventional enantioselective photochemistry, the chiral induction is controlled in electronic excited state, and to achieve high stereoselectivity is still a formidable challenge. Because the reactions of excited states often yield strained products with unique structures in single step that are difficult to form by thermal reactions, the development of new strategies attempted to achieve enantioselectivity in excited state reactions is still highly desired. Since the short excited state lifetime and low activation energy for reaction in excited state leave little room for manipulating the chiral induction, in order to gain enantioselectivity the substrate molecule has to already reside in a chiral environment during the excitation step. Chiral auxiliaries and chiral supramolecular hosts can provide such environments. In this presentation, we summarize the studies employing chiral auxiliary and chiral microreactor approaches to achieve high asymmetric inductions in excited state reactions performed in our laboratory. We chose the photodimerization of alkyl 2‐naphthoates as a reaction model to give deeper insights into the basic factors controlling chiral induction in excited state.     

Theoretical analysis of the triplet excited state of the [Pt2(H2P2O5)4]4- ion and comparison with time-resolved X-ray and spectroscopic results

A full understanding of the nature of excited states of transition metal complexes is important for understanding their chemical reactivity and role as intermediates in photochemically induced reactions. The ground and excited states of the [Pt(2)(pop)(4)](4-) ion are investigated using density functional theory (DFT). Calculations with different functionals employing quasi-relativistic Pauli and ZORA formalisms all predict a Pt-Pt bond shortening and a slight Pt-P lengthening upon excitation to the lowest triplet state, the latter in apparent contradiction to experimental EXAFS results. The PW86LYP functional with the ZORA relativistic treatment is found to produce good agreement with time-resolved crystallographic and spectroscopic results. A topological bond path between the Pt atoms is found in both the ground and the excited states, though the electron localization function (ELF) indicates weak Pt-Pt covalent bonding for the excited state only. The spin density is mainly localized on the Pt atoms, giving insight into the ability of the triplet excited state to abstract hydrogen and halogen atoms from organic substrates.     

2-(2'-羟基-5'-甲基苯基)苯并三氮唑质子转移的光谱和反应机理

用AM1和INDO/CI方法研究了2-(2'-羟基-5'-甲基苯基)苯并三氮唑的激发态质子转移反应, 求得基态和激发态反应的位能面、势垒和过渡态; 研究了异构体的稳定性、氢键强度及光谱的指认, 计算结果均与实验结果符合。讨论了对光诱导质子转移的机理和应用前景。     

NH_2基态和激发态的SAC－CI和量子拓扑方法研究

ＮＨ_２基态和激发态的ＳＡＣ－ＣＩ和量子拓扑方法研究郑世钧，蔡新华，宋天乐，孟令鹏，中迁博，波田雅彦（河北师范学院化学系，石家庄，０５００９１）（京都大学工程学院京都，日本）关键词ＳＡＣ－ＣＩ方法，激发态，电子密度，拓扑分析用量子拓扑学方法研究激发态...     

HL-LH2中色素分子间的单重激发态能量传递

采用飞秒时间分辨吸收光谱手段观测了在500和800 nm激发下高光培养的紫色光合细菌Rhodopseu-domonas(Rps). palustris外周捕光天线LH2(HL-LH2)中不同共轭链长类胡萝卜素(Carotenoid, 简称Car)和细菌叶绿素a(Bacteriachlorophyll a, 简称BChl a)的特征吸收光谱. 光谱动力学分析结果表明, HL-LH2中不同Car分子间可能存在复杂的单重激发态能量平衡过程, Car分子同时向BChl a分子发生多途径的单重激发态能量传递, B800主要接受来自Car的S2和S1态能量; B850则主要接受来自长共轭链Car(共轭双键数目n=13)的S1态和B800的激发态能量, 整个能量传递过程在3~5 ps内完成.     

Excited-state intramolecular proton transfer (ESIPT) in 2-(2′-hydroxyphenyl)-oxazole and -thiazole

The azoles 2-(2′-hydroxyphenyl)oxazole (HPO) and 2-(2′-hydroxyphenyl)-4-methylthiazole (HPT) have been synthesised and studied in order to compare their photophysical characteristics. Their absorption and emission properties are reported in non-polar, alcoholic and aqueous media. Ground and excited state pK data were determined by spectroscopy and a model is proposed to explain the behaviour of HPT and HPO as a function of the pH. Excitation spectra and quantum chemical calculations suggest an equilibrium of ground state conformers. The calculations also predict a small energy barrier for rotation in the first excited singlet state for the proton transferred tautomers. The resulting twisted structure of the tautomer form possesses a biradicaloid nature, and is near-degenerate in energy with the first excited triplet state.     

Quasiclassical Trajectory Study of Collisional Energy Transfer between Highly Excited C6F6 and N2 ,O2 and Ground State C6F6

Quasiclassical trajectory calculation (QCT) is used frequently for studying collisional energy transfer between highly vibrationally excited molecules and bath gases. In this paper, the QCT of the energy transfer between highly vibrationally excited C6F6 and N2 ,O2 and ground state C6F6 were performed. The results indicate that highly vibrationally excited C6F6 transferred vibrational energy to vibrational distribution of N2, O2 and ground state C6F6, so they are V-V energy transfer. Especially it is mainly V-V resonance energy transfer between excited C6F6 andground state C6F6, excited C6F6 transfers more vibrational energy to ground state C6F6 than to N2 and O2. The values of QCT, - (ΔEvib) of excited C6F6 are smaller than those of experiments.     

PROTOLYTIC EQUILIBRIA IN THE LOWEST EXCITED SINGLET STATES OF 2-HYDROXYQUINOLINE AND 4-HYDROXYQUINOLINE

Abstract— Fluorescence spectra of 2- and 4-hydroxyquinoline in a variety of solvents indicate that the lactam-lactim tautomerism favoring the lactam structure in the ground state also favor the lactam structure in the lowest excited singlet state. Fluorimetric titrations show that the cations of both isomers become slightly more acidic in the lowest excited singlet state and that the anion of the 2 isomer becomes slightly more basic in the excited state. The excited anion of the 4 isomer is either identical in basicity to the ground state anion or does not equilibrate with the quinolone during the lifetime of the excited state. The Forster cycle is found to give a pK _α * values in generally poor agreement with those obtained from fluorimetric titrations. The reasons for this are discussed.     

Electronic quenching of OH A 2Sigma+ radicals in single collision events with H2 and D2: a comprehensive quantum state distribution of the OH X 2Pi products

A pump-probe laser-induced fluorescence technique has been used to examine the nascent OH X (2)Pi product state distribution arising from non-reactive quenching of electronically excited OH A (2)Sigma(+) by molecular hydrogen and deuterium under single-collision conditions. The OH X (2)Pi products were detected in v'=0, 1 and 2; the distribution peaks in v'=0 and decreases monotonically with increasing vibrational excitation. In all vibrational levels probed, the OH X (2)Pi products are found to be highly rotationally excited, the distribution peaking at N'=15 when H(2) was used as the collision partner and N'=17 for D(2). A marked propensity for production of Pi(A') Lambda-doublet levels was observed, while both OH X (2)Pi spin-orbit manifolds were equally populated. These observations are interpreted as dynamical signatures of the nonadiabatic passage of the OH + H(2)/D(2) system through the seams of conical intersection that couple the excited state (2 (2)A') and ground state (1 (2)A') surfaces.     

Visible light induced photocleavage of DNA by a mixed-metal supramolecular complex: [[(bpy)(2)Ru(dpp)](2)RhCl2]5+

The mixed-metal supramolecular complex, [[(bpy)(2)Ru(dpp)](2)RhCl(2)](PF(6))(5) (bpy = 2,2'-bipyridine and dpp = 2,3-bis(2-pyridyl)pyrazine) coupling two ruthenium light absorbers (LAs) to a central rhodium, has been shown to photocleave DNA. This system possesses a lowest lying metal to metal charge transfer (MMCT) excited state in contrast to the metal to ligand charge transfer states (MLCT) of the bpm and Ir analogues. The systems with an MLCT excited state do not photocleavage DNA. [[(bpy)(2)Ru(dpp)](2)RhCl(2)](PF(6))(5) is the first supramolecular system shown to cleave DNA. It functions through an excited state previously unexplored for this reactivity, a Ru --> Rh MMCT excited state. This system functions when irradiated with low energy visible light with or without molecular oxygen.     

Conformational equilibria and photoinduced tautomerization in 2-(2′-pyridyl)pyrrole

Depending on the polarity and protic abilities of the solvent, 2-(2′-pyridyl)pyrrole can exist in either syn or anti rotameric forms. In nonpolar solvents, intramolecular excited state single proton transfer is observed, manifested by the appearance of low-energy tautomeric emission. The solvent-assisted excited state double proton transfer reaction is also detected. DFT calculations confirm low barriers for both single and double proton transfer processes in the lowest excited singlet state and show different character of the tautomerization in both cases: in the intramolecular reaction, mutual approach of two nitrogen atoms plays an important role.     

Excited States of Xanthene Analogues: Photofragmentation and Calculations by CC2 and Time‐Dependent Density Functional Theory

Action spectroscopy has emerged as an analytical tool to probe excited states in the gas phase. Although comparison of gas‐phase absorption properties with quantum‐chemical calculations is, in principle, straightforward, popular methods often fail to describe many molecules of interest—such as xanthene analogues. We, therefore, face their nano‐ and picosecond laser‐induced photofragmentation with excited‐state computations by using the CC2 method and time‐dependent density functional theory (TDDFT). Whereas the extracted absorption maxima agree with CC2 predictions, the TDDFT excitation energies are blueshifted. Lowering the amount of Hartree–Fock exchange in the DFT functional can reduce this shift but at the cost of changing the nature of the excited state. Additional bandwidth observed in the photofragmentation spectra is rationalized in terms of multiphoton processes. Observed fragmentation from higher‐lying excited states conforms to intense excited‐to‐excited state transitions calculated with CC2. The CC2 method is thus suitable for the comparison with photofragmentation in xanthene analogues.     

Reactive cysteine is protonated in the triplet excited state of the LOV2 domain in Adiantum phytochrome3

Phototropin is a plant blue-light sensor protein that possesses a flavin mononucleotide (FMN) as the chromophore in LOV domains. Its photoreaction is an adduct formation between FMN and a nearby cysteine that takes place in the triplet excited state of FMN. In this communication, we revealed that the reactive cysteine is protonated in the triplet excited state of the LOV2 domain of Adiantum phytochrome3 by means of low-temperature FTIR spectroscopy. Its hydrogen-bonding interaction is strengthened in the triplet excited state, presumably with the FMN chromophore. Such strong interaction drives adduct formation on a microsecond time scale.     

On the nature of the lowest triplet excited state of the [Rh 2(1,3-diisocyanopropane)(4)]2+ ion

The nature of the ground state and the lowest triplet excited state of the [Rh(2)(1,3-diisocyanopropane)(4)](2+) ion have been investigated by the density functional theory. Two locally stable geometrical conformations are found on the potential energy surfaces of both the ground and excited states, corresponding to the eclipsed and twisted conformations, the eclipsed conformation being more stable and having the shorter Rh-Rh bond length. While the Rh-Rh distances of the two conformations differ by approximately 0.4 A, they shorten to the same value upon excitation ( approximately 3.1 A). The excited state originates from the d(z)()()2 (metal antibonding) to p(z)() (ligand-metal bonding) electronic transition. The Mayer Rh-Rh bond order increases from approximately 0.2 to more than 0.8 upon excitation, while the Rh-C(N) bond order shows a slight decrease. A topological bond path between the Rh atoms is found in both the ground and excited states, while the electron localization function (ELF) indicates weak Rh-Rh covalent bonding for the excited state only.     

Ultrafast energy delocalization and electron transfer dynamics in 2-aminopurine-containing trinucleotides

The fate of electronically excited states in DNA base stacks is of tremendous importance for subsequent photochemical damage reactions in the genome. In this study we present a femtosecond broadband pump-probe study on the adenine isomer 2-aminopurine (Ap) incorporated into trinucleotides. After selective excitation of Ap we can monitor energy delocalization between neighboring Ap moieties as well as excited state electron transfer, depending on the sequence of the trinucleotide. Our results establish the time scale for intrastand excimer formation and reveal the lifetime of the excimer state.     

Ultraviolet absorption and vibrational spectra of 2-fluoro-5-bromopyridine

The ultraviolet absorption spectrum in the range 340-185 nm in the vapour and solution phase has been measured for 2-fluoro-5-bromopyridine. Three fairly intense band systems identified as the pi* <-- pi transitions II, III and IV have been observed. A detailed vibronic analysis of the vapor and solution spectra is presented. The first system of bands is resolved into about sixty-two distinct vibronic bands in the vapour-phase spectrum. The 0,0 band is located at 35944 cm(-1). Two well-developed progressions, in which the excited state frequencies nu'25 (283 cm(-1)) and nu'19 (550 cm(-1)) are excited by several quanta, have been observed. The corresponding excited state vibrational and anharmonicity constants are found to be omega'i = 292 cm(-1), x'ii = 4.5 cm(-1) (i = 25) and omega'i = 563.8 cm(-1), x'ii = 6.9 cm(-1) (i = 19). The other two band systems show no vibronic structure, the band maxima being located at 48346 and 52701 cm(-1), respectively. The oscillator strength of the band systems in different solutions and the excited state dipole moments associated with the first two transitions have been determined by the solvent-shift method. The infrared spectrum in the region 4000-130 cm(-1) and the laser Raman spectrum of the molecule in the liquid state have been measured and a complete vibrational assignment of the observed frequencies is given. A correlation of the ground and excited state fundamental frequencies observed in the UV absorption spectrum with the Raman or infrared frequencies is presented.     

Electronic Structure and Photophysical Properties of 2-(N,N-diethylanilin-4-yl)-4,6-bis(3,5-dimethylpyrazol-1-yl)-1,3,5-triazine

Electronic structure and photophysical properties of 2-(N,N-diethylanilin-4-yl)-4,6-bis(3,5-dimethylpyrazol-1-yl)-1,3,5-triazine are studied theoretically with quantum chemical methods as well as 2D site and 3D cube representations. The theoretical results reveal that the first excited state is an intramolecular charge transfer excited state. The change in dipole moment for the first excited state of the excitation is fitted, and the calculated result the change in dipole moment ￠1=6.40 D is consistent with the experimental result ￠1=6.90 D. The polarizability is also fitted. The large changes in dipole moment and the polarizability of the excitation show that S1 is of large nonlinear optical (NLO) effect. The NLO will promote efficient two-photon-absorption cross sections. The excited state properties of dpbt with different external electronic fields are also discussed theoretically.     

First observation in the gas phase of the ultrafast electronic relaxation pathways of the S(2) states of heme and hemin

The time evolution of electronically excited heme (iron II protoporphyrin IX, [Fe(II) PP]) and its associated salt hemin (iron III protoporphyrin IX chloride, [Fe(III) PP-Cl]), has been investigated for the first time in the gas phase by femtosecond pump-probe spectroscopy. The porphyrins were excited at 400 nm in the S(2) state (Soret band) and their relaxation dynamics was probed by multiphoton ionization at 800 nm. This time evolution was compared with that of the excited state of zinc protoporphyrin IX [Zn PP] whose S(2) excited state likely decays to the long lived S(1) state through a conical intersection, in less than 100 fs. Instead, for [Fe(II) PP] and [Fe(III) PP-Cl], the key relaxation step from S(2) is interpreted as an ultrafast charge transfer from the porphyrin excited orbital π* to a vacant d orbital on the iron atom (ligand to metal charge transfer, LMCT). This intermediate LMCT state then relaxes to the ground state within 250 fs. Through this work a new, serendipitous, preparation step was found for Fe(II) porphyrins, in the gas phase.     

萘甲腈和2,3-二甲基丁烯-2-光环加成反应:三氟醋酸的效应

在低转光解情况下,1-萘甲腈(1-NpCN)和2,3-二甲基丁烯-2(DMB)在环已烷或苯中的光环加成反应主要得到环丁烷3,但2-NpCN主要得到氨杂环丁烯6和少量的环丁烷5,三氟醋酸(TFA)量增加抑制1-NpCN和DMB的光环加成反应,表明TFA与基态及单激发态1-NpCN生成非光反应的复合物,它也淬灭^*(1-NpCN-DMB)激基复合物.在低浓度范围(0.1M以下)TFA加速2-NpCN和DMB光环加成反应,但在高浓度范围(大于0.1M)时又抑制反应.这些结果被解析如下:^*(2-NpCN-DMB)激基复合物和TFA反应增加5和6的得率,反之,是由于生成非光反应的基态复合物及淬灭单激发态2-NpCN.在非极性溶剂中二种单激发态萘甲腈生成TFA激基复合物的萤光与母体^*NpCN萤光区域几乎相同.     

Product branching between reactive and nonreactive pathways in the collisional quenching of OH A 2Sigma+ radicals by H2

The collisional quenching of OH radicals in their excited A 2Sigma+ electronic state by molecular hydrogen is examined to determine the partitioning between reactive and nonreactive pathways. This is achieved using a pump-probe laser technique to compare the population prepared in the excited OH A 2Sigma+ state with that produced in the OH X 2Pi ground state from nonreactive quenching. Only a small fraction of the products, less than 15%, arise from nonreactive quenching; reactive quenching is the dominant product channel. The branching between the product channels provides a new dynamical signature of the conical intersection region(s) that couple the excited state potential for OH A 2Sigma++H2 with OH X 2Pi+H2 and H2O+H products.