Synthesis and characterization of ran-copoly (p-phenylene sulfide sulfone/ketone)s

Random copoly(p-phenylene sulfide sulfone/ketone)s (PPSS/K) are prepared in high yield by the polycondensation of sodium hydrosulfide (NaSH) with bis(4-chlorophenyl) sulfone (BCPS) and 4,4′-dichlorobenzophenone (DCBP). The polymerization is conducted between 200–220°C, depending on the composition of the copolymer, and in the presence of water without any detrimental effects to the molecular weight. The copolymers with sulfone/ketone mole ratios (S : K) > 25 : 75 are amorphous, while the copolymers with S : K ratios ≤ 25 : 75 are crystalline. These materials form tough, creaseable films and exhibit a linear increase in the glass transition temperature with increasing sulfone content. Sulfuric acid solutions of the copolymers are dark orange to red and display an increasing λ_max in the uv-visible spectra as the S:K ratio of the copolymers decreases. © 1994 John Wiley & Sons, Inc.     

Isomeric Schiff bases related by dual imino‐group reversals

Two isomeric pairs of Schiff bases, N,N′‐bis­(2‐methoxy­benzyl­idene)‐p‐phenyl­enediamine, C₂₂H₂₀N₂O₂, (I), and 2,2′‐dimeth­oxy‐N,N‐(p‐phenyl­enedimethyl­ene)dianiline, C₂₂H₂₀N₂O₂, (II), and (E,E)‐1,4‐bis­(3‐iodo­phen­yl)‐2,3‐diaza­buta‐1,3‐diene (alternative name: 3‐iodo­benzaldehyde azine), C₁₄H₁₀I₂N₂, (III), and N,N′‐bis­(3‐iodo­phen­yl)ethylenedi­imine, C₁₄H₁₀I₂N₂ [JAYFEV; Cho, Moore & Wilson (2005). Acta Cryst. E 61 , o3773–o3774], differ pairwise only in the orientation of their imino linkages and in all four individual cases occupy inversion centers in the crystal, yet all four compounds are found to assume unique packing arrangements. Compounds (I) and (II) differ substantially in mol­ecular conformation, possessing angles between their ring planes of 12.10 (15) and 46.29 (9)°, respectively. Compound (III) and JAYFEV are similar to each other in conformation, with angles between their imino linkages and benzene rings of 11.57 (15) and 7.4 (3)°, respectively. The crystal structures are distinguished from each other by different packing motifs involving the functional groups. Inter­molecular contacts between meth­oxy groups define an R₂²(6) motif in (I) but a C(3) motif in (II). Inter­molecular contacts are of the I⋯I type in (III), but they are of the N⋯I type in JAYFEV.     

Fringe skeletonizing using an improved derivative sign binary method

A modified derivative sign binary method is proposed to extract fringe skeletons from interferometric fringe patterns. A fringe direction map ranging from 0° to 360° is obtained with an unambiguous relationship between the grayscale and fringe tangent direction. Using this approach, the derivative sign binary map is detected without a fringe direction jump. The dark (light) fringe intensity minimum (maximum) can be extracted automatically to define fringe skeletons. In addition, two different anisotropic one-dimensional filters are described that can be used for further improving the fringe quality. Examples are provided for a holographic interferogram and a Moiré fringe pattern to illustrate applications and benefits of the approach.     

Dipolar excitations at the LIII x-ray absorption edges of the heavy rare-earth metals

We report measured dipolar asymmetry ratios at the LIII edges of the heavy rare-earth metals. The results are compared with a first-principles calculation and excellent agreement is found. A simple model of the scattering is developed, enabling us to reinterpret the resonant x-ray scattering in these materials and to identify the peaks in the asymmetry ratios with features in the spin and orbital moment densities.     

Coherent control of the isomerization of retinal in bacteriorhodopsin in the high intensity regime

Coherent control protocols provide a direct experimental determination of the relative importance of quantum interference or phase relationships of coupled states along a selected pathway. These effects are most readily observed in the high intensity regime where the field amplitude is sufficient to overcome decoherence effects. The coherent response of retinal photoisomerization in bacteriorhodopsin to the phase of the photoexcitation pulses was examined at fluences of 10(15) - 2.5 ×?10(16) photons per square centimeter, comparable to or higher than the saturation excitation level of the S(0) - S(1) retinal electronic transition. At moderate excitation levels of ～6 ×?10(15) photons/cm(2) (<100 GW/cm(2)), chirping the excitation pulses increases the all-trans to 13-cis isomerization yield by up to 16% relative to transform limited pulses. The reported results extend previous weak-field studies [Prokhorenko et al., Science 313, 1257 (2006)] and further illustrate that quantum coherence effects persist along the reaction coordinate in strong fields even for systems as complex as biological molecules. However, for higher excitation levels of ～200 GW/cm(2), there is a dramatic change in photophysics that leads to multiphoton generated photoproducts unrelated to the target isomerization reaction channel and drastically changes the observed isomerization kinetics that appears, in particular, as a red shift of the transient spectra. These results explain the apparent contradictions of the work by Florean et al. [Proc. Natl. Acad. Sci. U.S.A. 106, 10896 (2009)] in the high intensity regime. We are able to show that the difference in observations and interpretation is due to artifacts associated with additional multiphoton-induced photoproducts. At the proper monitoring wavelengths, coherent control in the high intensity regime is clearly observable. The present work highlights the importance of conducting coherent control experiments in the low intensity regime to access information on quantum interference effects along specific reaction coordinates.     

Femtosecond dynamics of the ring closing process of diarylethene: a case study of electrocyclic reactions in photochromic single crystals

The cyclization reaction of the photochromic diarylethene derivative 1,2-bis(2,4-dimethyl-5-phenyl-3-thienyl)perfluorocyclopentene was studied in its single crystal phase with femtosecond transient absorption spectroscopy. The transient absorption measurements were performed with a robust acquisition scheme that explicitly exploits the photoreversibility of the molecular system and monitors the reversibility conditions. The crystalline system demonstrated 3 × 10(4) repeatable cycles before significant degradation was observed. Immediately following photoexcitation, the excited state absorption associated with the open-ring conformation undergoes a large spectral shift with a time constant of approximately 200 fs. Following this evolution on the excited state potential energy surface, the ring closure occurs with a time constant of 5.3 ps, which is significantly slower than previously reported measurements for similar derivatives in the solution phase. Time resolved electron diffraction studies were used to further demonstrate the assignment of the transient absorption dynamics to the ring closing reaction. The mechanistic details of the ring closing are discussed in the context of prior computational work along with a vibrational mode analysis using density functional theory to give some insight into the primary motions involved in the ring closing reaction.     

Excitonic effects in two-dimensional vibrational spectra of liquid formamide

The linear and two-dimensional infrared (2DIR) responses of the amide I vibrational mode in liquid formamide are investigated experimentally and theoretically using molecular dynamics simulations. The recent method based on the numerical integration of the Schr?dinger equation is employed to calculate the 2DIR spectra. Special attention is devoted to the interplay of the structural dynamics and the excitonic nature of the amide I modes in determining the optical response of the studied system. In particular, combining experimental data, simulated spectra and analysis of the simulated atomic trajectory in terms of a transition dipole coupling model, we provide a convincing explanation of the peculiar features of the 2DIR spectra, which show a substantial increase of the antidiagonal bandwidth with increasing frequency. We point out that, at variance with liquid water, the 2DIR spectral profile of formamide is determined more by the excitonic nature of the vibrational states than by the fast structural dynamics responsible for the frequency fluctuations.