Conjugated chromophoric groups

Table 4.4. Characteristics of Simple Conjugated Chromophoric Groups...

Both evidence of the browning caused by the high-intensity absorption of conjugated chromophoric groups and upward shifts in the ultraviolet-visible spectral baselines indicate that higher hydroquinone concentrations produce more polymers. Free radical coupling is probably involved in the polymerization since the existence of free radicals is correlated to hydroquinone concentration. The ultraviolet-visible absorption profiles of these polymers formed in the systems that are not sterilized (Kung and McBride, 1988) appear to be similar to those of phenol-derived polymers in the systems free... 

The conjugated chromophore (color-causing) system can be extended by electron-donor groups such as —NH2 and —OH and by electron-acceptor groups such as —NO2 and —COOH, often used at opposite ends of the molecule. An example is the aromatic compound alizarin [72-48-0], also known since antiquity as the ted dye madder. 

Hydroxyl groups — Since conjugated hydroxyl groups do not have any influence on the chromophore of the molecule, they do not have any effect on the UV-Vis spectrum. Therefore, p-carotene, p-cryptoxanthin, and zeaxanthin all... 

It gives an absorption plateau between 282 mp and 295 mp and an absorption peak at 315 mp. It has been amply demonstrated that absorption above 300 mp is due to the presence of a chromophoric group conjugated with an oxygen substituted benzene ring (51), (52), (69), (75). To ascertain the nature of the chromophoric group (carbonyl or ethylenic... 

Thus the weak n — n band in a saturated carbonyl compound is shifted from below 300 nm to above 300 nm with an increase in s. Conjugation of additional chromophoric groups moves mx progressively... 

Photochemical cis-trans isomerization in a conjugated polyene system is thought to be the crucial primary process in vision. The visual pigment (rhodopsin) is derived from 11 -crs-retinal by reaction of the aldehyde group with an amino substituent in a protein (opsin). There is considerable distortion in the geometry of this chromophoric group anyway, because of the spatial requirements of the protein... 

Heat catalyzes free radical formation in cellulose. Aldehydes form from C2 and C3 hydroxyls. Aldehydes oxidize to carboxyls, and with dehydration, carbon monoxide (CO) and carbon dioxide (C02) form as well as conjugated carbonyl-ethylenic chromophoric groups that selectively absorb blue light and impart yellowness (35). During the induction stage of cellulose oxidation, yellowness may increase steadily with selective carbonyl and ethylene group formation. By artificially aging... 

The first law of photochemistry [the Grotthus-Drapper principle (30)] states that for a photochemical reaction to occur, some component of the system must first absorb light. The second law of photochemistry [the Stark-Einstein principle (3J)j states that a molecule can only absorb one quantum of radiation. The absorbed energy causes the dissociation of bonds in the molecules of the wood constituents. This homolytic process produces free radicals as the primary photochemical products. This event, with or without the participation of oxygen and water, can lead to depolymerization and to formation of chromophoric groups such as carbonyls, carboxyls, qui-nones, peroxides, hydroperoxides, and conjugated double bonds. 

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