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Julia Laudien - One of the best experts on this subject based on the ideXlab platform.
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room temperature ligand and base free heck reactions of aryl diazonium salts at low palladium loading sustainable preparation of substituted stilbene derivatives
ChemInform, 2010Co-Authors: Francoisxavier Felpin, Karinne Miqueu, Jeanmarc Sotiropoulos, Eric Fouquet, Oier Ibarguren, Julia LaudienAbstract:Palladium(II)-catalyzed Heck reactions of aryl diazonium salts with 2-arylacrylates lead to Cis-Stilbenes with good to excellent stereoselectivity.
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room temperature ligand and base free heck reactions of aryl diazonium salts at low palladium loading sustainable preparation of substituted stilbene derivatives
Chemistry: A European Journal, 2010Co-Authors: Francoisxavier Felpin, Karinne Miqueu, Jeanmarc Sotiropoulos, Eric Fouquet, Oier Ibarguren, Julia LaudienAbstract:The Pd(OAc)2-catalyzed Heck reaction of aryl diazonium salts with 2-arylacrylates led to Cis-Stilbenes with good to excellent stereoselectivity. The environmentally friendly protocol developed in this work features low palladium loading in technical-grade methanol at room temperature under base-, additive-, and ligand-free conditions. The same protocol applied to simple Heck coupling of aryl diazonium salts with methyl acrylate allows astonishingly low palladium loading, down to 0.005 mol %. The stereoselectivity experimentally observed for the synthesis of Cis-Stilbenes has been rationalized by DFT calculations. Moreover, the role of methanol in promoting the reaction has been clarified by a computational study.
Tahei Tahara - One of the best experts on this subject based on the ideXlab platform.
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Real-Time Monitoring of Structural Evolution in Cis-Stilbene Photoisomerization Time-Domain Raman Spectroscopy
Springer Series in Chemical Physics, 2009Co-Authors: Satoshi Takeuchi, Tetsuya Taketsugu, Sanford Ruhman, Takao Tsuneda, Mahito Chiba, Tahei TaharaAbstract:We studied the vibrational structure of reactive S1 Cis-Stilbene through wavepacket motions generated impulsively at various delay-times. They showed gradual frequency downshift, demonstrating highly anharmonic nature of the excited-state potential and structural evolution with photoisomerization
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Revised steady-state fluorescence spectrum and nature of the reactive S1 state of Cis-Stilbene in solution
Chemical Physics Letters, 2008Co-Authors: Takumi Nakamura, Satoshi Takeuchi, Noriyuki Suzuki, Tahei TaharaAbstract:Abstract We measured steady-state fluorescence of cis -stilbene in solution, by carefully separating the contribution of coexisting trace amount of trans isomer. The obtained ‘pure’ fluorescence spectrum of cis -stilbene shows a broad structureless band (peaked around 420 nm) extending to the wavelength region as long as 700 nm. Compared to the fluorescence intensity from the coexisting trans isomer, the oscillator strength of the cis fluorescence was evaluated as 0.17, which indicated that the S 1 state of cis -stilbene is an optically allowed state.
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A 40-fs time-resolved absorption study on Cis-Stilbene in solution: observation of wavepacket motion on the reactive excited state
Chemical Physics Letters, 2004Co-Authors: Kunihiko Ishii, Satoshi Takeuchi, Tahei TaharaAbstract:Abstract Time-resolved absorption spectroscopy with ∼40 fs time resolution was applied to the S 1 state of cis -stilbene in solution. An oscillatory signal caused by ∼220 cm −1 wavepacket motion was clearly observed. Its damping rate was much faster than the isomerization rate and insensitive to the change of the solvent. The obtained ultrafast pump–probe data showed that the dephasing of the initial coherent wavepacket motion first takes place rapidly, followed by the ∼1 ps isomerization reaction. The significance of the ∼220 cm −1 mode, intramolecular vibrational energy redistribution (IVR) process, and high anharmonicity of the S 1 potential surface in the initial stage of cis -stilbene photochemistry is discussed.
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A 40-fs Time-Resolved Absorption Study of Cis-Stilbene in Solution: Observation of Coherent Nuclear Wavepacket Motion in Reactive Excited State
Springer series in chemical physics, 2004Co-Authors: Kunihiko Ishii, Satoshi Takeuchi, Tahei TaharaAbstract:Photoisomerization reaction of Cis-Stilbene in solution was studied by time-resolved absorption spectroscopy with 40-fs resolution. Rapidly damping coherence of a ~220 cm-1 vibrational mode was observed in the reactive excited state.
Francoisxavier Felpin - One of the best experts on this subject based on the ideXlab platform.
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room temperature ligand and base free heck reactions of aryl diazonium salts at low palladium loading sustainable preparation of substituted stilbene derivatives
ChemInform, 2010Co-Authors: Francoisxavier Felpin, Karinne Miqueu, Jeanmarc Sotiropoulos, Eric Fouquet, Oier Ibarguren, Julia LaudienAbstract:Palladium(II)-catalyzed Heck reactions of aryl diazonium salts with 2-arylacrylates lead to Cis-Stilbenes with good to excellent stereoselectivity.
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room temperature ligand and base free heck reactions of aryl diazonium salts at low palladium loading sustainable preparation of substituted stilbene derivatives
Chemistry: A European Journal, 2010Co-Authors: Francoisxavier Felpin, Karinne Miqueu, Jeanmarc Sotiropoulos, Eric Fouquet, Oier Ibarguren, Julia LaudienAbstract:The Pd(OAc)2-catalyzed Heck reaction of aryl diazonium salts with 2-arylacrylates led to Cis-Stilbenes with good to excellent stereoselectivity. The environmentally friendly protocol developed in this work features low palladium loading in technical-grade methanol at room temperature under base-, additive-, and ligand-free conditions. The same protocol applied to simple Heck coupling of aryl diazonium salts with methyl acrylate allows astonishingly low palladium loading, down to 0.005 mol %. The stereoselectivity experimentally observed for the synthesis of Cis-Stilbenes has been rationalized by DFT calculations. Moreover, the role of methanol in promoting the reaction has been clarified by a computational study.
Anne B. Myers - One of the best experts on this subject based on the ideXlab platform.
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Solvent Polarity Effects on Cis-Stilbene Photochemistry from the First Femtoseconds to the Final Products
Reaction Dynamics in Clusters and Condensed Phases, 1994Co-Authors: Anne B. Myers, Jon Marc Rodier, David Lee PhillipsAbstract:The ultrafast torsional isomerization and ring closure reactions of photoexcited Cis-Stilbene have been examined in hexane, cyclohexane, methanol, and acetonitrile solvents through a combination of ground state resonance Raman intensities, two-color uv picosecond anti-Stokes resonance Raman scattering, and product quantum yield measurements. The quantum yields for ring closure to ground state dihydrophenanthrene are factors of two to three higher in the nonpolar solvents than in the polar ones, while the yield of trans-stilbene is slightly higher in the polar solvents. Vibrationally hot ground-state trans-stilbene is formed within the 10 ps time resolution of the time-resolved Raman experiments in both solvents, but the subsequent vibrational cooling is about a factor of two faster in methanol than in cyclohexane. The resonance Raman spectra of the Cis-Stilbene ground state exhibit slightly greater relative intensities in modes involving primarily out-of-plane and/or hydrogen wagging or rocking motions in the polar solvents than in the hydrocarbons. These observations, together with the femtosecond transient absorption and fluorescence data from other groups, indicate that increasing solvent polarity causes vertically excited Cis-Stilbene to distort more rapidly along the torsional isomerization coordinate, shortening the excited state lifetime and decreasing the quantum yield for the competing ring closure reaction.
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Cis-Stilbene photochemistry: solvent dependence of the initial dynamics and quantum yields
Journal of the American Chemical Society, 1993Co-Authors: Jon Marc Rodier, Anne B. MyersAbstract:Photochemical quantum yields for the formation of Trans-stilbene and 4a,4b-dihydrophenanthrene from Cis-Stilbene have been measured in cyclohexane, hexane, acetonitrile, and methanol solvents. Degassed Cis-Stilbene samples at 27 o C were irradiated at 280 nm, and the formation of photoproducts was monitored by absorption spectroscopy. The quantum yield for formation of dihydrophenanthrene is two to three times greater in the nonpolar solvents than in the polar ones (15-19% vs 5-8%), while a slight solvent dependence of the yield of trans-stilbene is observed (35-39%in the polar solvents vs 32-35% in the hydrocarbons)
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Cis-Stilbene photochemistry: direct observation of product formation and relaxation through two-color UV pump-probe Raman spectroscopy
Chemical Physics, 1993Co-Authors: David Lee Phillips, Jon Marc Rodier, Anne B. MyersAbstract:Abstract The formation of hot trans-stilbene by photoisomerization of Cis-Stilbene, and its subsequent vibrational cooling, have been monitored by picosecond two-color UV pump-probe anti-Stokes Raman spectroscopy. Pairs of pump (295 nm) and probe (278 nm) pulses at a 500 Hz repetition rate interrogate flowing samples of Cis-Stilbene in cyclohexane or methanol, and the anti-Stokes Raman scattering due mainly to the trans-stilbene product is detected. Hot ground-state trans-stilbene appears promptly within our 10 ps experimental time resolution, and the anti-Stokes intensity decays with a time constant of 39 ± 10 ps in cyclohexane and 17 ± 5 ps in methanol. The decay of the anti-Stokes intensity is accompanied by a shift of the C=C stretching vibrations by about 20 cm−1 to higher frequencies. The question of whether or not the initial vibrational distribution is statistical is addressed, but the present data do not allow firm conclusions to be drawn.
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Ultraslow and Ultrafast Time-Resolved Resonance Raman Spectra of Cis-Stilbene Photoproducts
Springer Proceedings in Physics, 1992Co-Authors: David Lee Phillips, Jon Marc Rodier, Anne B. MyersAbstract:The photochemistry of stilbene has been widely studied as a paradigm for cis-trans isomerization in general and solvation effects on rapid large-amplitude motions in particular. Early time-resolved studies focused mainly on the trans isomer; the excited state dynamics of Cis-Stilbene, with its much shorter lifetime (≤2 ps), have been probed by analyzing ground-state resonance Raman intensities to extract the excited-state dynamic information contained therein [1–3], but have only recently been observed directly on the femtosecond time scale [4–9]. While the time-resolved absorption and fluorescence techniques employed thus far can generate high quality kinetic data, they provide relatively little detailed information about structural relaxation and energy disposal in the photoproducts. To address these questions we are applying time resolved resonance Raman spectroscopy, already used extensively to probe dynamics in the S1 excited state of trans-stilbene [10–12], to the faster photochemistry of the cis isomer.
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Resonance Raman spectra of 4a,4b-dihydrophenanthrene, the photocyclization product of Cis-Stilbene
Chemical Physics Letters, 1991Co-Authors: Jon Marc Rodier, Xiaopei Ci, Anne B. MyersAbstract:Abstract Resonance Raman spectra 4a,4b-dihydrophenanthrene have been obtained by irradiating degassed samples of Cis-Stilbene in cyclohexane and in methanol at 266 nm and subsequently exciting the Raman spectrum within the absorption band of the dihydrophenanthrene product at 436 and 477 nm. We believe that these are the first published vibrational spectra of this species. The Raman spectra at both wavelengths are dominated by a strong band at 1474 cm −1 which is presumably the mostly in-phase stretch of the double bonds in the conjugated polyene chain, shifted down in frequency by about 100 cm −1 from the corresponding all-trans linear polyene. Other weaker vibrations are observed at 769, 1035, 1080, 1147, 1201, 1298, 1534, and 1555 cm −1 , and some assignments are suggested based on comparison with QCFF/PI calculations. The nonresonantly enhanced Raman lines of the other Cis-Stilbene photoproduct, trans-stilbene, relative to dihydrophenanthrene are much stronger in methanol than in cyclohexane under the same irradiation conditions. This observation, together with the results of simple laser photolysis experiments in which the 436 nm absorbance is used to monitor the rates of dihydrophenanthrene formation and destruction, indicate a large solvent effect on the quantum yield for photocyclization of Cis-Stilbene and little or no solvent effect on the yield for photochemical ring opening of dihydrophenanthrene. The ground state resonance Raman intensities of Cis-Stilbene excited at 266 nm indicate slightly faster distortion of the vertically excited state along torsional coordinates in methanol relative to cyclohexane solution.
Robin M. Hochstrasser - One of the best experts on this subject based on the ideXlab platform.
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Vibrational Coherence in Photoisomerization Reaction of Cis-Stilbene in Solution
Ultrafast Processes in Spectroscopy, 1996Co-Authors: Dipak K. Palit, Arpad Z. Szarka, N. Pugliano, Robin M. HochstrasserAbstract:An important goal of the study of chemical reaction dynamics in solution is to characterize reactive coordinates and understand how the solvent influences the reaction pathways and energy flow. The Photoisomerization of Cis-Stilbene, in which a significant structual change occurs without severing any chemical bond, is an ideal one for this purpose. The isomerization of Cis-Stilbene in the excited singlet state involves motion on a surface that is close to being barrierless along the reaction coordinate1. The time dependence of the disappearance of the electronically excited state has been shown to be sensitive to solvent and temperature2. Vibrational dephasing times longer than 200 fs, as can be estimated from the full Raman line width of 50 cm-1 for the lowest frequency (165 cm-1) mode of the ground state3, dictates the possibility of probing vibrational wave-packet dynamics on the reactive surface of photoisomerization reaction of Cis-Stilbene using femtosecond (fs) laser pulses. Such an experiment is made possible by the existence of a strong excited state absorption band of Cis-Stilbene in the 650 nm region which is spectroscopically isolated from any ground state absorption as well as the gain spectra4.
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Femtosecond laser studies of the cis‐stilbene photoisomerization reactions
Journal of Chemical Physics, 1993Co-Authors: Roseanne J. Sension, Arpad Z. Szarka, S. T. Repinec, Robin M. HochstrasserAbstract:Femtosecond laser studies have been performed on the photoisomerization reactions of cis‐stilbene to obtain the most detailed understanding to date of a polyatomic isomerization reaction in a condensed phase environment. These experiments demonstrate that vibrationally hot product molecules are formed within a few hundred femtoseconds of the escape of the molecule from the cis* region of the potential energy surface. Although the cis to trans reaction may proceed via a twisted intermediate structure, this intermediate is not intercepted on the ∼150 fs time scale. The frictional effects on the cis to trans reaction coordinate are found to be important and account for the anisotropy of the trans product molecules. Specific experiments presented in detail are the absorption spectrum of electronically excited cis molecules (cis*); the anisotropy decays for cis* showing motion along the reaction coordinate; the detection of the trans‐stilbene product using transient fluorescence and transient absorption, confi...
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Vibrational energy redistribution and relaxation in the photoisomerization of cis‐stilbene
Journal of Chemical Physics, 1992Co-Authors: Roseanne J. Sension, Arpad Z. Szarka, Robin M. HochstrasserAbstract:Trans‐stilbene photoproduces appearing from the photoisomerization of cis‐stilbene within 150 fs are observed to be vibrationally hot, but with a nonthermal population distribution. The product electronic spectrum depends on the incident photon energy. Clearly, intramolecular vibrational energy redistribution is not ultrafast.
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Femtosecond laser studies of the Cis-Stilbene photoisomerization reactions. The Cis-Stilbene to dihydrophenanthrene reaction
The Journal of Physical Chemistry, 1991Co-Authors: S. T. Repinec, Arpad Z. Szarka, Roseanne J. Sension, Robin M. HochstrasserAbstract:After optical excitation of Cis-Stilbene, ground state dihydrophenantrene is formed in 1.7 ps (hexadecane) via kinetically required intermediate(s) present during the first 300±200 fs.
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The viscosity dependence and reaction coordinate for isomerization of cis‐stilbene
Journal of Chemical Physics, 1990Co-Authors: S.a. Abrash, S. T. Repinec, Robin M. HochstrasserAbstract:Femtosecond spectroscopic methods are used to study the dynamics following optical excitation of cis‐stilbene molecules in hydrocarbon solvents. Transient absorption spectra of cis‐stilbene over the range 320–1100 nm are reported. The anisotropies of these transients permit the assignment of the various excited electronic states in this region to A type in C2 symmetry. The excited state absorptions disappear at rates that are weakly dependent on solvent friction and comparisons with simple statistical mechanical theories and various potentials indicate that there is likely to be a barrier crossing process responsible for the observed decay times being in the range 0.7–1.4 ps. For observation times longer than ca. 100 fs an exponential decay of the cis population is observed and the transient spectrum does not appear to change in shape. A Kramers model fits the frictional dependence of the decay rates. Direct observation of what appears to be trans isomer ground states at 335 nm is reported following excit...