Chromophores aromatic ring

Appreciable interaction between chromophores does not occur unless they are linked directly to each other, or forced into close proximity as a result of molecular stereochemical configuration. Interposition of a single methylene group, or meta orientation about an aromatic ring, is sufficient to insulate chromophores almost completely from each other. Certain combinations of functional groups afford chromophoric systems which give rise to characteristic absorption bands. 

Compounds 167-171 outlined in Fig. 43 form another series of diboronic acids that form complexes with mono- and disaccharides. In these cases the asymmetrical immobilization of chromophoric functional groups, e.g., aromatic rings in 167-170 or Fe -complexation with the related boronate 171, can be analyzed by circular dichroism measurements [256-262]. 

Scheme 1. Principle of cyanine dye synthesis leading to trimethine (n = l), pentamethine (n = 2) and heptamethine (n = 3) chromophores. Structures comprising indolic subunits are usually named indocarbocyanine, indodicarbocyanine and indotricarbocyanine, respectively. Formic acid, malonic aldehyde, glutaconic aldehyde are used in their protected dianUide or orthoester form. They can be applied as substituted derivatives to introduce residues into the polymethine unit. The indolic substructure might bear further residues or annelated aromatic rings...

Practically all the commercially important reactive dyes based on activated double bonds employ the vinyl sulfone reactive group (2.35), usually as the sulfatoethylsul-fone derivative (2.34), which converts into (2.35) in the dyebath under the influence of alkali at pH 11 to 12.5. They are usually applied onto the fibre by exhaustion from the dyebath at around 40-60 °C or by cold padding at 30 °C. The vinylsulfone group is most commonly attached to the dye chromophore via an aromatic ring, e.g. Cl Reactive Black 5 (2.2), but in a few cases a bridging aliphatic group is employed. ... 

The chromophore is not only extended by conjugation the presence of an electron-donating substituent on an aromatic ring also has the effect of increasing the wavelength of absorption, as we can see for the examples in Box 2.1. 

Notice how substituents with Ione pairs on the atom attached to the aromatic ring (e.g. OH, NH ) extend the chromophore by resonance of the lone pair electrons into the aromatic ring and, in general, lead to an increase in both A ax and , and have a similar effect to that of conjugation (e.g. CO2H). 

Postcolumn photochemical reactions are another approach to the detection problem. High-intensity UV light, generally provided by a Hg or Zn lamp, photolyzes the HPLC effluent, which passes through a Teflon (47) or quartz tube. The photolysis reaction determines the nature of the subsequent detection. If the compound has a UV chromophore, such as an aromatic ring, and an ionizable heteroatom, such as chlorine, then the products of the reaction can be detected conductometrically. Busch et al. (48) have examined more than 40 environmental pollutants for applicability to detection with photolysis and conductance detection. Haeberer and Scott (49) found the photoconductivity approach superior to precolumn derivatization for the determination of nitrosoamines in water and waste water. The primary limitation of this detection approach results from the inability to use mobile phases that contain ionic modifiers, that is, buffers and... 

The chemical deactivation of photoexcited anthracenes by dimerization usually proceeds by 4re + 4re cycloaddition [8]. However, exceptions to this rule have become known in recent years [8], and a multitude of steps, including the formation of metastable intermediates such as excimers, may actually be involved in a seemingly simple photochemical reaction such as the dimerization of 9-methylanthracene [9, 10]. Moreover, substitution of the anthracene chromophore may affect and alter its excited state properties in a profound manner for a variety of reasons. For example, in 9-tert-butylanthracene the aromatic ring system is geometrically distorted [11,12] and, consequently, photoexcitation results in the formation of the terf-butyl-substituted Dewar anthracene [13-15], The analogous photochemical isomerization of decamethylanthracene [16] probably is attributable to similar deviations from molecular planarity. 

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