Search results for "Isomerization"

Photoresponsive Ferroelectric Liquid-Crystalline Polymers

2007

The photoresponse of ferroelectric smectic side-chain liquid-crystalline (LC) polymers containing a photoisomerizable azobenzene derivative as a covalently linked photochromic side group is investigated. By static measurements in different photostationary states, the effect of trans-cis isomerization on the material's phase-transition temperatures and its ferroelectric properties (spontaneous electric polarization Ps and director tilt angle 0) are analyzed. It turns out that the Curie temperature (transition S C * to S A ) can be reversibly shifted by up to 17 °C. The molecular mechanism of this "photoferroelectric effect" is studied in detail using time-resolved measurements of the dye's o…

Some photochemical transformations of polymers

1969

Abstract Several light-induced reactions of polymers are described. The light-sensitive groups are part of the macromolecules. Photoisomerization of polyvinylbutyral produces a polymer with vinylbutyrate-groups. At exposure to light, furan and benzofuran are added to the carbonyl-groups of polyvinylbenzophenone. Polymers with azido-groups are prepared, which eliminate nitrogen at irradiation. Light-sensitive films can be cast from polymers with o-quinonediazide-groups.

Conformational properties of N-acetyl-N-methyl-alpha,beta-dehydroalanine N'-methylamide.

2006

The conformational properties of Ac-Delta(Me)Ala-NHMe (N-acetyl-N-methyl-alpha,beta-dehydroalanine N'-methylamide), as the simplest model of N-methyl-alpha,beta-dehydroamino acids, was examined with theoretical methods and in comparison with Ac-DeltaAla-NHMe and Ac-DeltaAla-NMe(2). The N-terminal amide of the Delta(Me)Ala residue easily adopts the configuration cis and the torsion angles phi, psi are highly flexible. The Delta(Me)Ala residue is a conformational flexibilizer as compared to the parent DeltaAla, which is a conformational stiffener. This seems to be the reason why Delta(Me)Ala is found in small natural cyclic peptides, where it ensures the conformational flexibility necessary f…

Heterocyclic rearrangements in constrained media. A zeolite-directed photorearrangement of 1,2,4-oxadiazoles

2005

[reaction: see text] The first intrazeolite-photoinduced rearrangement of a five-membered heterocycle is reported. A completely different behavior compared to solution irradiations has been observed. The zeolite's role in directing the photoreaction of 3-phenyl-1,2,4-oxadiazoles toward the formation of the corresponding 1,3,4-oxadiazoles in a ring contraction-ringexpansion route is discussed.

N-Methyldehydroamino acids promote a configuration cis of N-methylamide bond

2008

Abstract Dehydroamino acids with a methylated N-terminal tertiary amide bond occur in natural small cyclic peptide toxins. To investigate their conformational preferences a systematic theoretical analysis was performed on N ′-methylamides of N -acetyl- N -methyldehydroamino acids (Ac-Δ(Me)Xaa-NHMe, where Xaa = ( Z )-Abu, ( E )-Abu, Val, ( Z )-Phe, and ( E )-Phe) considering the configuration trans and cis of the tertiary amide bond. The ϕ , ψ potential energy surfaces were calculated at the B3LYP/6-31+G ∗∗ //HF/3-21G level with inclusion of the solvent (water) effect (SCRF method). The conformers localised were fully optimised at the B3LYP/6-31+G ∗∗ in vacuo. The accessible areas of the pot…

Living Light-Induced Crystallization-Driven Self-Assembly for Rapid Preparation of Semiconducting Nanofibers.

2018

Well-defined nanostructures composed of conjugated polymers have attracted significant attention due to their intriguing electronic and optical properties. However, precise control of the size and uniformity of these semiconducting nanostructures is still rare and challenging, despite recent advances in strategies to obtain self-assembled nanostructures with narrow dispersions. Herein, we demonstrate the preparation of fluorescent conjugated block copolymers by one-shot polymerization and rapid formation of nanofibers in a few minutes via light-induced crystallization-driven self-assembly, driven by facile cis-to- trans photoisomerization of its poly( p-phenylenevinylene) blocks. Furthermor…

Synthesis of all-syn Functionalized Triphenylene Ketals

2011

The stereoselective synthesis of triphenylene ketals offers access to unique scaffolds. For a good performance in supramolecular applications an all-syn orientation of the functional groups is essential. The oxidative trimerization of catechol ketals by molybdenum pentachloride or mixtures with titanium tetrachloride leads to a template-directed formation. Several heterocyclic moieties are suitable for this transformation. A template-directed isomerization of anti,anti,syn isomers to the desired C 3 -symmetric derivative was demonstrated in two cases.

2021

Ruthenium(II) polypyridine complexes are among the most popular sensitizers in photocatalysis, but they face some severe limitations concerning accessible excited-state energies and photostability that could hamper future applications. In this study, the borylation of heteroleptic ruthenium(II) cyanide complexes with alpha-diimine ancillary ligands is identified as a useful concept to elevate the energies of photoactive metal-to-ligand charge-transfer (MLCT) states and to obtain unusually photorobust compounds suitable for thermodynamically challenging energy transfer catalysis as well as oxidative and reductive photoredox catalysis. B(C6F5)(3) groups attached to the CN- ligands stabilize t…

Modified photobehavior of carboxylic acid derivatives induced by protonation

1987

Abstract A series of carboxylic acid derivatives 1–6 containing a second interacting function have been converted into the corresponding O -protonated species or acylium ions upon treatment with 96% sulfuric acid or oleum, respectively, at room temperature. The resulting stable cations have been spectroscopically characterized and submitted to irradiation. Cis-trans photoisomerization was observed in the ionic speciesderived from trans -crotonic, fumaric and maleic acids, and maleic anhydride. By contrast protonated crotonolactone 5a+ was found to be photostable. Finally, protonated α-bromobutyrolactone 6a+ was clean and efficiently dehydrohalogenated to 5a+ upon irradiation. Since the norm…

Photoswitchable coordination compounds

2001

Photoswitchable compounds represent an attractive class of materials in coordination chemistry. Recent progress dealing with transition metal compounds involving photo-induced changes of the magnetic and/or optical properties to long-lived metastable states are covered in the present review article. The basic photophysical phenomena together with representative examples such as nitroprusside derivatives, relevant spin crossover complexes, stilbenoid complexes and finally Prussian blue analogues are discussed. Some possible applications regarding energy and information storage are suggested at the end.