Isomeric Yield Research Articles

Ultrafast Excited State Dynamics of the Protonated Schiff Base of All- trans Retinal in Solvents

We present a comparative study of the ultrafast photophysics of all- trans retinal in the protonated Schiff base form in solvents with different polarities and viscosities. Steady-state spectra of retinal in the protonated Schiff base form show large absorption-emission Stokes shifts (6500–8100 cm −1) for both polar and nonpolar solvents. Using a broadband fluorescence up-conversion experiment, the relaxation kinetics of fluorescence is investigated with 120 fs time resolution. The time-zero spectra already exhibit a Stokes-shift of ∼6000 cm −1, indicating depopulation of the Franck-Condon region in ≤100 fs. We attribute it to relaxation along skeletal stretching. A dramatic spectral narrowing is observed on a 150 fs timescale, which we assign to relaxation from the S 2 to the S 1 state. Along with the direct excitation of S 1, this relaxation populates different quasistationary states in S 1, as suggested from the existence of three distinct fluorescence decay times with different decay associated spectra. A 0.5–0.65 ps decay component is observed, which may reflect the direct repopulation of the ground state, in line with the small isomerization yield in solvents. Two longer decay components are observed and are attributed to torsional motion leading to photo-isomerization. The various decay channels show little or no dependence with respect to the viscosity or dielectric constant of the solvents. This suggests that in the protein, the bond selectivity of isomerization is mainly governed by steric effects.

Acid−Metal Balance of a Hydrocracking Catalyst: Ideal versus Nonideal Behavior

n-Alkane hydrocracking has been previously performed in a perfectly mixed flow reactor at temperatures from 473 to 553 K, pressures from 0.5 to 10 MPa, and molar hydrogen-to-hydrocarbon inlet ratios from 50 to 300 on a USY zeolite loaded with 0.5 wt % Pt1. For a given ratio of metal to acid sites, four important causes affecting the ideal character (i.e., the quasi equilibration of the (de)-hydrogenation reactions) were identified. In the investigated range of operating conditions, (1) a decreasing total pressure, (2) an increasing temperature, (3) an increasing molar hydrogen-to-hydrocarbon inlet ratio, and (4) a higher reactant carbon number lead to nonideality. A kinetic model for nonideal hydrocracking has been derived on the basis of a lumped reaction scheme. With literature values for the kinetic parameters, this allows us to rationalize the deviations from ideality in hydrocracking observed as a function of the operating conditions and to describe the high isomerization yield in ideal hydrocracking...

Quantum optimal control of ozone isomerization

We present a feasibility study of ozone isomerization based on a recent ab initio potential energy surface and a model Hamiltonian constructed by holding the bond lengths constant and using the valence angle as the isomerization coordinate. Optimal control theory is used to find an electric field that drives isomerization with a yield of 95% to the symmetric metastable triangular form of ozone. A frequency filter is applied as an additional spectral constraint limiting the field bandwidth. A post-facto analysis is performed showing a degree of inherent robustness of the isomerization yield to field noise.

Photoisomerization of Arylstilbazolium Ligands in the Presence of DNA¶

ABSTRACTThe ability of the DNA duplex to behave as an efficient organized medium for trans‐cis photoisomerization has been explored. The presence of DNA affected isomerization in a variety of ways depending on the aryl moiety properties of the ligand and its DNA‐binding mode. Contrary to intercalating ligands, 9‐[2‐(N‐methylpyridinium‐4‐yl)vinyl)phenanthrene (2) and 9‐[2‐(N‐methylpyridinium‐4‐yl)vinyl]anthracene (3), which gave only cis and trans isomers, the additional products‐cyclobutane photodimers‐were detected for 2‐[2‐(N‐methylpyridinium‐4‐yl)vinyl]naphthalene (1), which binds cooperatively to the minor groove of DNA. Photostationary states (pss) for all ligands were seriously affected by the presence of DNA. A trans isomer‐rich pss and restriction of trans→cis process, observed for ligands 1 and 2, were explained in terms of a different binding affinity of DNA toward particular isomers. On the contrary, 9‐anthryl derivative 3 isomerized against the isomer‐binding preferences, showing cis isomer‐rich pss and enhanced quantum yield of isomerization. The unique behavior of ligand 3‐DNA complex was attributed to different isomerization mechanism that consists in quantum chain isomerization from an excited singlet state possessing a charge transfer character. This is the first example of ligand, which undergoes DNA‐mediated cis‐trans isomerization in the opposite direction than expected from DNA‐binding preferences. The analysis of pss data based on two alternative pathways of photoisomerization showed that investigated ligands follow the model that allows isomerization of both free and DNA‐bound ligands.

Photoisomerization of arylstilbazolium ligands in the presence of DNA.

The ability of the DNA duplex to behave as an efficient organized medium for trans-cis photoisomerization has been explored. The presence of DNA affected isomerization in a variety of ways depending on the aryl moiety properties of the ligand and its DNA-binding mode. Contrary to intercalating ligands, 9-[2-(N-methylpyridinium-4-yl)vinyl]phenanthrene (2) and 9-[2-(N-methylpyridinium-4-yl)vinyl]anthracene (3), which gave only cis and trans isomers, the additional products--cyclobutane photodimers--were detected for 2-[2-(N-methylpyridinium-4-yl)vinyl]naphthalene (1), which binds cooperatively to the minor groove of DNA. Photostationary states (pss) for all ligands were seriously affected by the presence of DNA. A trans isomer-rich pss and restriction of trans --> cis process, observed for ligands 1 and 2, were explained in terms of a different binding affinity of DNA toward particular isomers. On the contrary, 9-anthryl derivative 3 isomerized against the isomer-binding preferences, showing cis isomer-rich pss and enhanced quantum yield of isomerization. The unique behavior of ligand 3-DNA complex was attributed to different isomerization mechanism that consists in quantum chain isomerization from an excited singlet state possessing a charge transfer character. This is the first example of ligand, which undergoes DNA-mediated cis-trans isomerization in the opposite direction than expected from DNA-binding preferences. The analysis of pss data based on two alternative pathways of photoisomerization showed that investigated ligands follow the model that allows isomerization of both free and DNA-bound ligands.

Fission fragment angular momentum in the spontaneous fission of 244Cm

Summary Independent isomeric yield ratios (IYR) of 128Sb, 130Sb, 132Sb, 131Te, 133Te, 132I, 134I, 135Xe and 138Cs have been determined in the spontaneous fission of 244Cm using off-line gamma ray spectrometric technique. From the IYR, fragment angular momenta (J rms) have been deduced using spin dependent statistical model analysis. It was observed that: (i) J rms of fragments having spherical 82n shell and even-Z products are lower than the fragments away from spherical neutron shell and odd-Z products which indicates the effect of nuclear structure. (ii) The yield-weighted average angular momentum shows an inverse correlation with the elemental yield which indicate coupling between collective and intrinsic degrees of freedom. (iii) Comparison of the present data in 244Cm(SF) with similar data for 245Cm(n th, f) shows comparable odd-even effect on fragment angular momentum in both fissioning systems inspite of higher odd-even effect on elemental yield in the former compared to latter. This may be due to the linear correlation of J rms with deformation parameter and thus with deformation energy unlike the exponential dependence of the proton odd-even effect on intrinsic excitation energy.

The molecular basis for the high photosensitivity of rhodopsin.

Based on structural information derived from the F NMR data of labeled rhodopsins, rhodopsin crystal structure, and excited-state properties of model polyenes, we propose a molecular mechanism that accounts specifically for the causes of the well-known enhanced photoreactivity of rhodopsin (increased rates and quantum yield of isomerization). It involves the key features of close proximity of C-187 to H-12 and chromophore bond lengthening upon light absorption. The resultant "sudden punch" to H-12 triggers dual processes of decay of the Franck-Condon-excited rhodopsin, a productive directed photoisomerization and a nonproductive decay returning to the ground state as two separate molecular pathways [based on real-time fluorescence results of Chosrowjan, H., Mataga, N., Shibata, Y., Tachibanaki, S., Kandori, H., Shichida, Y., Okada, T. & Kouyama, T. (1998) J. Am. Chem. Soc. 120, 9706-9707]. The two processes are controlled by the local protein structure: an empty space provided by the intradiscal loop connecting transmembrane helices 4 and 5 and a protein wall composed of amino acid units in transmembrane 3. Suggestions, involving retinal analogs and rhodopsin mutants, to improve the unusually high photosensitivity of rhodopsin are proposed.

Photoisomerization and Fluorescence Properties of Hemiindigo Compounds Having Intramolecular Hydrogen Bonding

Abstract Intramolecularly hydrogen bonded hemiindigo compounds 2–4 were prepared and their photochemical behavior was investigated. Z-2 did not undergo isomerization, while 3 underwent isomerization mutually between the Z-isomer and E-isomer in solution to exhibit a color change between greenish yellow and reddish orange in benzene. Introduction of a formyl group at the 2-position of the pyrrole ring of 3 brought about the increase in the fluorescence quantum yield and fluorescence lifetime by a factor of 10, and the decrease of the quantum yield of isomerization of 4 in benzene. In addition, introduction of the formyl group affects the thermal stability. The intramolecular hydrogen bonding can be used to construct a novel photochromic molecule.

Protonation effect on the excited state behaviour of EE-1-(n-pyridyl)-4-phenylbutadienes (n = 2, 3 and 4).

The acid-base equilibria of three aza-derivatives of EE-1,4-diphenylbutadiene and the excited state properties of their neutral and protonated forms have been studied in aqueous solutions. The prevalent relaxation of the neutral molecules is photoisomerization by a diabatic singlet mechanism accompanied by substantial internal conversion while the quantum yield of the radiative pathway is very small, particularly for the ortho- and para-substituted pyridines. Protonation affects only slightly the isomerization yield of these two compounds while drastically decreasing that of the meta-substituted analogue. Molecular orbital calculations and comparison with the corresponding azastilbenes indicate that the first excited singlet state of the protonated meta compound has a net charge transfer character, which leads to an increase of the torsional barrier in S1. Application of the Forster cycle to the three compounds showed that they become much stronger bases in S1. The 3-pyridyl derivative proved to be peculiar also in this respect since its equilibration with the proton occurs during the S1 lifetime thus allowing the acid-base equilibration in the excited state to be directly monitored by fluorimetric titration.

PhotochemicalE(trans)–Z(cis) Isomerization in 9-Anthraceneacrylic Esters

Several (1–6) 9-anthraceneacrylic esters were synthesized in order to study photochemical E(trans)–Z(cis) isomerization. All of the compounds 1–6 underwent selective E-to-Z isomerization upon direct excitation (> 400 nm) in organic solvents, leading to the formation of a thermodynamically less stable Z isomer over 96%. Triplet sensitized isomerization selectively produces the Z-to-E isomer in over 98%. The higher quantum yield of isomerization observed in the triplet-sensitized Z-to-E isomerization process informs us that a “quantum chain” process is in operation. Fluorescence data generated on all compounds indicate that the E-to-Z isomerization process involves a charge-transfer or polar singlet excited state.

Photoisomerization of 2-(2-pyrrol-2-ylethenyl)benzoxazole and 2-(2-pyrrol-2-ylethenyl)benzothiazole

Hydrogen bonded compounds 2-(2-pyrrol-2-ylethenyl)benzoxazole (4) and 2-(2-pyrrol-2-ylethenyl)benzothiazole (5) were synthesized and their photochemical behavior was studied. The (Z) isomers of 4 and 5 form intramolecular hydrogen bonds as revealed by the 1H NMR studies. The trans isomers are pale yellow in solution, but on photoirradiation, the color changes slightly to a different yellow due to the change of the absorption spectra. The quantum yield of isomerization of 4 and 5 is quite high in the range 0.4–0.6 and is somewhat dependent on the solvent properties. On triplet sensitization 4 and 5 gave T–T absorption spectra with a lifetime of ca. 1 μs in benzene. Since a transient absorption spectrum was not observed on direct irradiation of 4 and 5 with a 308 nm laser pulse, the cis–trans isomerization on direct irradiation should take place in the excited singlet state. On triplet sensitization 4 and 5 underwent cis–trans isomerization. These results indicate that the intramolecular hydrogen bonding is broken in the singlet excited state as well as in the triplet excited state of (Z)-4 and (Z)-5.

Synthesis and Characterization of Copolymers of Methyl Methacrylate and 11-(4-Ethoxyazobenzene-4′-oxy)undecyl Methacrylate

Copolymers of methyl methacrylate and 11-(4-ethoxyazobenzene-4′-oxy)undecyl methacrylate (azo-MA) were synthesized and characterized with respect to their photochemical and free volume properties. The isomerization yield at 360 nm was measured by UV spectroscopy. The cis yield after illumination was not significantly suppressed in the solid state when compared to the polymer solution. The free volume characteristics were measured by positron annihilation. The ortho-positronium (o-Ps) lifetime, which is associated with the intersegmental distance in the polymer, increased with increasing azo-MA content up to about 4 mol-% azo-MA. The results are consistent with density and DSC measurements, which show that both the density and glass transition temperature of the copolymers decrease with increasing azo-MA content.

Photochemical E (trans)-->Z (cis) isomerization in substituted 1-naphthylacrylates.

Substituted naphthylacrylates, 1-3, not showing rotamerism have been synthesized with a view to study photochemical E (trans)-->Z (cis) isomerization. Photostationary state composition of the isomers upon direct excitation, triplet sensitized isomerization, quantum yield of isomerization, and steady state and time-resolved fluorescence behavior have been studied for these naphthylacrylates. The direct excitations of the compounds yield high Z (approximately 80%) isomer composition, whereas the triplet sensitization results in less Z (approximately 20%) isomer composition. This indicates that the singlet pathway is very efficient in converting the E isomer to the Z isomer. The naphthylacrylates 1 and 2 exhibit structured fluorescence at room temperature in hexane and upon changing the solvent to CH3CN; the structure of the fluorescence is lost, indicating that the singlet excited-state develops a polar character in a polar environment. The polar nature of the singlet excited state becomes more clear in the case of 3 from its fluorescence solvatochromism. The naphthylacrylates did not exhibit excitation wavelength-dependent fluorescence at room temperature suggesting that the ground state conformers (rotamers) are not involved. Fluorescence lifetimes measured for these compounds displayed biexponential behavior, which is explained using a two-state model.

Free radical 4‐nitrophenylation of thieno[2,3‐b]pyridine. Part 2: Isolation and structural studies of three samples of mono (4‐nitrophenyl) products; relative yields of the five possible isomers

AbstractFrom the crude mixtures of isomeric 4‐nitrophenylthieno[2,3‐b]pyridines (3) previously reported [1] were isolated three analytically pure samples, viz. the 2‐isomer (yellow needles, mp 258°, 3a), the 6‐isomer (red prisms, mp 182°, 3e), and a ternary mixture of the 2‐, 3‐, and 4‐isomers (orange needles, mp 213°, 3a:3b:3c = 1.3:1.0:0.5). The 258° compound was identified as either 3a or 3b by its 1H nmr spectrum and definitively as the former by its x‐ray crystallographic analysis. The isomeric identities of the 182° and 213° samples were established from their 1H nmr spectra only. No 5‐isomer (3d) was identified. Semi‐quantitatively, relative isomeric yields fit the pattern 2‐ (64%)>>6‐ (14%)≥3‐ (12%)>4‐ (6%)≥5‐(≤4%). Crystallographic data for 3a are presented.

Fission fragment angular momentum in ODD-Z fissioning systems

Independent isomeric yield ratios of 128Sb, 130Sb, 132, 131Te, 133Te, 132I, 134I, 136I, 135Xe and 138Cs have been determined in the fast neutron induced fission of 237Np and 241Am using radiochemical and gamma spectrometric technique. From the independent isomeric yield ratios, fragment angular momenta (Jrms) have been deduced using spin-dependent statistical model analysis. Comparison of these data with the literature data for even-Z fissioning systems shows the following important features: (i) Angular momenta for fragments with spherical 82n shell and even-Z products are lower compared to the fragments with out the 82n shell and odd-Z products indicating the effect of nuclear structure. (ii) Angular momentum of even-Z products in all the fissioning systems are comparable where as for odd-Z products it is slightly higher in the odd-Z fissioning systems than in the adjacent even-Z fissioning systems. This indicates the role of single particle on fragment angular momentum in odd-Z fissioning systems.

Isomeric yield ratios of 134I and 136I in the proton-, 12C- and 19F-induced fission of 235U, 237Np and 238U

Short-lived nuclides of iodine produced in the proton-, 12C- and 19F-induced fission of 235U, 237Np and 238U were separated with a rapid chemical separation system SISAK. Relative yields of the nuclides were determined by γ -ray spectrometry to obtain the isomeric yield ratios of 134mI/134gI and 136mI/136gI. Angular momenta of the fission fragments, 134I and 136I, were derived from the measured isomeric yield ratios by the spin-dependent statistical model. In the light- and heavy-ion fission, variation of the observed angular momenta can be explained by the excitation energies of the fissioning nuclei and the spherical shell structure with N = 82.

Some important aspects of fragment angular momentum in medium energy fission of 238U

Independent isomeric yield ratios of 131Te, 133T and 134I hava been determined at five different energies in the range of 25–44 MeV alpha particle induced fission of 238U using radiochemical and gamma spectrometric techniques. From the independent isomeric yield ratios, fragment angular momenta ( J rms) have been deduced using a statistical model analysis. The J rms were also calculated theoretically based on thermal equilibration of various collective modes after considering the occurrence of multichance fission. These data and the literature data for various fragments in the mass region 126–136 in 238U(α,f), 238U(p,f) and 238U(γ,f) show the following important features: (i) Both the entrance channel excitation energy and input angular momentum affect the fragment angular momentum in the exit channel. (ii) There are two groups of fission products from the point of view of change of fragment angular momentum with increase in excitation energy and input angular momentum. (iii) Fragment angular momentum depends on nuclear structure effect such as shell closure proximity and odd-even effect. (iv) The fragment angular momentum calculated theoretically based on statistical equilibration of various collective modes are in good agreement with the experimental values indicating the validity of such an assumption.

Photochemical Isomerization of trans-Dichlorotetrakis(dimethylphenylphosphine)osmium(II)

Abstract Photochemical isomerization of trans-dichlorotetrakis(dimethylphenylphosphine)osmium(II) to its cis-form was confirmed by NMR and IR spectroscopy. The quantum yield of isomerization was 0.9 at 410 nm which was much larger than that (0.03 at 410 nm) of trans-dichlorobis(diphenylphosphinoethane)osmium(II).

Isomeric yield ratios of fission products in proton-induced fission of232Th

Isomeric yield ratios of 11 fission products were measured in the system of 13 MeV proton-induced fission of232Th by an on-line ion-guide isotope separator. It was found that the closed shell structures of primary fragments and their complementary fragments affect the isomeric yield ratios. Isomeric yield ratios of121Cd (11/2−, 3/2+) and135Xe (11/2−, 3/2+) were measured precisely in the proton energy range of 13 to 26 MeV to investigate their energy dependence. It was found that the isomeric yield ratios increased slightly with proton energy. The results were discussed in connection with the deformation of fission fragments and fission modes.

Studies of some isomeric yield ratios produced with bremsstrahlung

The experimental isomeric ratios for 52m,gMn, 86m,gY, 87m,gY, 89m,gZr, 110m,gIn, 111m,gIn, 112m,gIn, 152m1,gPm, 152m2,m1Eu, 162m,gHo, 164m,gHo and 178m,gLu measured by the activation technique from different targets in ( γ, xnp) reactions ( x≤3) at the bremsstrahlung end-point energy of 43 MeV are presented. The predictions of calculations performed by means of compound nucleus particle evaporation and final γ-deexcitation were critically discussed. The importance of inclusion in the calculations of nonequilibrium particle emission and an adequate γ-decay mode of isomeric nuclei was considered for some of the reactions investigated.