Chromophore groups

Numerous inorganic salts containing electrons engaged in d orbitals are responsible for transitions of weak absorption located in the -visible region. These transitions are generally responsible for their colours. That is why the solutions of metallic salts of titanium [Ti(H20)g] + or of copper [Cu(H20)g] + are blue, while potassium permanganate yields violet solutions, and so on. 

The functional groups of organic compounds (ketones, amines, nitrogen derivatives, etc.), responsible for absorption in UV/Vis are called chromophores (Table 9.1). A species formed from a carbon skeleton transparent in the near UV on which are attached one or several chromophores constitutes a chromogene. 

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Some of the simple chromophoric groups, together with the absorption maxima of simple compounds containing these groups, are collected in... 

Chain transfer is an important consideration in solution polymerizations. Chain transfer to solvent may reduce the rate of polymerization as well as the molecular weight of the polymer. Other chain-transfer reactions may iatroduce dye sites, branching, chromophoric groups, and stmctural defects which reduce thermal stabiUty. Many of the solvents used for acrylonitrile polymerization are very active in chain transfer. DMAC and DME have chain-transfer constants of 4.95-5.1 x lO " and 2.7-2.8 x lO " respectively, very high when compared to a value of only 0.05 x lO " for acrylonitrile itself DMSO (0.1-0.8 X lO " ) and aqueous zinc chloride (0.006 x lO " ), in contrast, have relatively low transfer constants hence, the relative desirabiUty of these two solvents over the former. DME, however, is used by several acryhc fiber producers as a solvent for solution polymerization. 

The monoazo and disazo pigments contain one or more chromophoric groups usually referred to as the azo group. However, it... 

Dyestuff organic chemistry is concerned with designing molecules that can selectively absorb visible electromagnetic radiation and have affinity for the specified fiber, and balancing these requirements to achieve optimum performance. To be colored the dyestuff molecule must contain unsaturated chromophore groups, such as a2o, nitro, nitroso, carbonyl, etc. In addition, the molecule can contain auxochromes, groups that supplement the chromophore. Typical auxochromes are amino, substituted amino, hydroxyl, sulfonic, and carboxyl groups. 

Ultraviolet absorption spectra of tetrahy dro-1,3-oxazines do not show any maximum. Only after the introduction of a chromophoric group do bands appear. Thus 5-nitro derivatives show a strong maximum near 270 m/A, which is typical for a nitro group, and another one near 200 m(x which is probably also produced by the nitro group.In the instance of 5-nitro-5-hydroxymethyl derivatives, the absorption is much weakened this was explained by Urbanski in terms of a hydrogen bond between the hydroxyl and the nitro group. Other chromophores, such as C=0, C=NH, C—C, also cause the appearance of absorption maxima in the range 210-265 m/A and near 360... 

Furthermore, photochemically induced homolytical bond cleavage can also be applied when the prepolymer itself does not contain suitable chromophoric groups [113-115]. Upon thermolysis of ACPA in the presence of styrene, a carboxyl-terminated polystyrene is formed. This styrene-based prepolymer was reacted with lead tetraacetate and irradiated with UV light yielding free radicals capable of initiating the polymerization of a second monomer (Scheme 33) [113]. 

Since the electrostatic potential sharply decreases with increasing distance from the polyelectrolyte cylinder, the degree of reactivity modification by functional groups fixed to the polyion is strongly dependent on the distance from the cylinder surface. Considerable electrostatic potential effects on the photoinduced forward and thermal back electron transfer reactions, which will be discussed in the following chapters, can be attributed to the functional chromophore groups directly attached to the polyelectrolyte back-bone through covalent bonds. 

Fig. 42. A series of boronic acids 159-166 with chromophoric groups has been evaluated with respect to its capacity to selectively bind d-glucose or other saccharides...

Many biologically important sugars are derivatives having a chromophoric group that absorbs within the range of commercial instrumentation. Not only is the c.d. spectrum of such molecules easier to measure, but the interpretation of the spectrum is simplified, because only the chromophore is involved. Many laboratories have concentrated on the c.d. of such monomers and their polymers, and the results will be discussed. 

The [Ru(bpy)3] + core has more recently been used to construct first-generation dendrimers containing coumarin-450 chromophoric groups. 

Self-assembly of functionalized carboxylate-core dendrons around Er +, Tb +, or Eu + ions leads to the formation of dendrimers [19]. Experiments carried out in toluene solution showed that UV excitation of the chromophoric groups contained in the branches caused the sensitized emission of the lanthanide ion, presumably by an energy transfer Forster mechanism. The much lower sensitization effect found for Eu + compared with Tb + was ascribed to a weaker spectral overlap, but it could be related to the fact that Eu + can quench the donor excited state by electron transfer [20]. 

In dichloromethane solutions, excitation of the chromophoric groups of the dendrons causes singlet-singlet energy transfer processes that lead to the excitation of the porphyrin core. It was found that the dendrimer 17, which has a spherical morphology, exhibits a much higher energy transfer quantum yield (0.8) than the partially substituted species 13-16 (quantum yield <0.32). Fluo-... 

UV/VIS spectrophotometry can be used to determine many physico-chemical characteristics of compounds and thus can provide information as to the identity of a particular compound. Although UV/VIS spectra do not enable absolute identification of an unknown, they are frequently used to confirm the identity of a substance through comparison of the measured spectrum with a reference spectrum. However, UV spectrophotometry is not highly specific, and can obviously only be applied to polymer additives which are absorbers of UV radiation, i.e. contain chromophoric groups. Both UV and IR monitor functional entities rather than the entire molecular structure. A functional group s proximity to other electropositive or electronegative structures in a molecule affects the absorbance spectrum, allowing one to infer some details of molecular structure. 

While pure erucamide does not exhibit absorption in the 250-280 nm range all commercial products showed UV absorption maxima at 230, 257, 267 and 278 nm (with variable intensities), indicative of the presence of oxidation products (up to 8 wt%) as impurities with chromophoric groups. Colourless impurities do not have an interference effect. 1JV/VIS was also used to evaluate discoloration of 50wt/wt% mixtures of commercial erucamide and inorganic antiblock agents [59]. Apart from the interference by impurities from solvents chemical methods suffer from lengthy procedures. 

The stoichiometry of the enzyme-inactivator complex has historically been most commonly determined using radiolabeled versions of the inactivator. Alternative methods include incorporation of a fluorescent or chromophoric group into the inactivator, or the use of quantitative LC/MS methods. 

The chromophoric groups can bear one or more dissociable protons or can be nonionic. In the former, the ion exchange between the proton and appropriate metal cations causes the color change, while in the latter the coordination of the metal ion to the chromophoric donor or acceptor of the dye molecule induces a change of the charge transfer band of the dye. 

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