Chromicity conformation

Experimental results corroborate that shifts of 1.2 eV are always present if any of the variables acting on the electrochemical process are changed the solvent, the salt, or the temperature of work. We cannot attribute the observed shift to solvatochromic, counter-ion-chromic, or thermochromic effects taking place inside the film during oxidation-reduction processes. So, as predicted, these shifts are a consequence of the way the chains store or relax energy through conformational changes stimulated by electrochemical oxidation or reduction, respectively. 

Hexahelicene is oxidized by chromic acid under very mild conditions (stirring for 24 h at room temperature in an acetic acid-water solution) giving hexahelicene-5,6-quinone(109)in 70%yield 138). The increased oxidation rate of 1,3,14,16-tetramethyl-hexahelicene has to be the ascribed to the electronic effect of the multiple methyl substitution and not to the disturbance of the helical conformation as was concluded from the polarographic oxidation potentials (see Sect. 5) as well as from charge-transfer spectra138). 

A mixture of molybdenum hexacarbonyl and 4-chlorophenol is effective in performing al-kyne metathesis of dipropynylated dialkylbenzenes. Alkyne metathesis of these precursors leads to the clean formation of dialkyl poly( paraphenyleneethynylene)s (PPEs) in high yield and with high molecular weights. This facile yet effective access to the PPEs has allowed study of their spectroscopic, structural, and thermal properties. While PPEs have been made before, the dialkyl-PPEs turned out to have particularly interesting optical and liquid-crystalline properties that can be explained in terms of the conformation of the main chains. The PPEs have also been utilized to construct light-emitting diodes and other semiconductor devices. This chapter discusses the interplay of structure, chromicity, and electronic properties of the dialkyl-PPEs. 

The ring A conformation of a series of 2,3-disubstituted 24-noroleananes has been investigated.The rates of oxidation with chromic acid of a series of 1-, 2-, and 3-hydroxy- (both a and f3) oleananes, -24-noroleananes, and -lupanes have been measured. 

The preferred conformation of the alkyl /3-D-ribopyranosides (83) is the Cl form, in which only the hydroxyl group at C-3 is axial. The cataljdic oxidation of methyl or benzyl 8-D-ribopyranoside in neutral solution gives the expected alkyl /3-D-er2/i/iro-pentopyranosid-3-ulose (84). " - The methyl glycoside (84 R = CH3) is identical with the chromic acid oxidation product of methyl jS-D-xylopyranoside. The benzyl glycoside (84 R = CH2C6H6) does not form a crystalline hydrate, is less stable than (82), and epimerizes very easily in alkaline solution to the pentopyranosid-... 

Thermograms of precipitated chromic oxide gels show endothermic effects indicating that part of the water is loosely bound and part rig-dly bound. Maximum surface area is found at a temperature at which complete expulsion of water seems to take place. These endothermic effects are followed by an exothermic peak due to crystallization of Cr20s. D.T.A. of gels heated up to 600° in nitrogen atmosphere fails to show any exothermic peak. X-ray diffraction and surface area results are in conformity with those of D.T.A. 

Even if the torsion of the chain of polythiophene could account for the chromic phenomena, we must still find good theoretical reasons why such a torsion should be possible. An ab initio theoretical study of the barriers to torsional flexibility in conjugated polymers in the gas phase [27], demonstrated that they are of the order of 0.7 ey clearly allowing conformational flexibility. This rationalizes the solubility of the polymers, but also warns that the polymer solid, where chain-chain interactions must be taken into account, is a rather different system. 

The coupling of the electronic structure of the conjugated polythiophene to the conformation can account for chromic phenomena as well as for aspects of electronic transport of the materials in their undoped or doped states. The order-disorder transition of the polymer chains, between a planar and a non-planar state, is governed by molecular mechanisms related to the nature and regularity of side-chain substituents. 

Van der Veen G, Prins W. 1974. Photoregulation of polymer conformation by photo chromic moieties I. Anionic ligand to an anionic polymer. Photochem Photobiol 19 191 196. 

It was observed that the ions Cd chromic transition of the polymerized liposomes in the presence of E. coli. The ions Cu, Zn, Ni, and Fe + inhibited the transition. It is hypothesized that the later group of metal ions bind strongly to the carboxylic acid head groups, making the ene-yne backbone of the polymerized hposomes more rigid for conformational changes. 

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