Chromophore theory

Although it appears possible to explain away the objections to the Ostwald theory, and though it is exceedingly useful, one must admit that the chromophore theory has given us a much deeper insight into the behavior of indicators. Actually a proper understanding of indicator properties is impossible without the latter theory, which will be discussed in the next section. 

The chromophore theory originated with Bernthsen and Friedlander, who proposed, simultaneously and independently, that the colorless form of phenolphthalein present in acid solutions possessed a lactone structure, and that the red salt formed by alkalis contained a chromophore quinone group. Hence the color transformation was accompanied by a change in constitution. Later, Hantzsch and his students extended the theory, and ascertained that unless a structural change occurred, the color remained the same. They stated, however, that in many cases the demonstration of a structural change was too difficult to be decisive. 

The author finds himself at variance with these views. He feels that the chromophore theory fails to explain the transformation satisfactorily, and that it merely considers a phenomenon which occurs simultaneously with the color change. The fact that color and constitution vary at the same time does not necessarily indicate that constitutional rearrangements are the cause of the color change. 

The chief point of departure from the explanation of Ostwald consists in not having to say that the color of the ions differs from that of the pseudo-compounds. It is the ionogenic form which is differently colored. We see here the connection with the chromophore theory which states that the ionogen form and the normal form have different structures. 

Wyatt CN, Buckler KJ. The effect of mitochondrial inhibitors on membrane currents in isolated neonatal rat carotid body type I cells. J Physiol Lond 2000 527 121P-122P. Buerk DG, Chugh DK, Osanai S, Mokashi A, Lahiri S. Dopamine increases in cat carotid body during excitation by carbon monoxide implications for a chromophore theory of chemoreception. J Auton Nerv Syst 1997 67 130-136. 

Ha T-K, Lewerenz M, Marquardt R and Quack M 1990 Overtone Intensities and dipole moment surfaces for the Isolated CH chromophore In CHD3 and CHF3 experiment and ab initio theory J. Chem. Phys. 93 7097-109... 

Most of the qualitative relationships between color and structure of methine dyes based on the resonance theory were established independently during the 1940 s by Brooker and coworkers (16, 72-74) and by Kiprianov (75-78), and specific application to thiazolo dyes appeared later with the studies of Knott (79) and Rout (80-84). In this approach, the absorptions of dyes belonging to amidinium ionic system are conveyed by a group of contributing structures resulting from the different ways of localization of the 2n rr electrons on the 2n l atoms of the chromophoric cationic chain, rather than by a single formula ... 

A great number of monoaza or polyaza. either symmetrica] or unsym-metrical, mono trimethine thiazolocyainines have been synthesized in order to verify or to obtain semiempirical rules, more or less based on the resonance theory, concerning the relation between the color of a thiazolo dye and the number and place of nitrogen atoms in the chromophoric chain. For example. Forster s rule applies to ionic dyes and stipulates that the will increase with the decreasing tendency of chromophoric atoms lying between the two auxochromes to take up the characteristic charges (90). 

The color and constitution of cyanine dyes may be understood through detailed consideration of their component parts, ie, chromophoric systems, terminal groups, and solvent sensitivity of the dyes. Resonance theories have been developed to accommodate significant trends very successfully. For an experienced dye chemist, these are useful in the design of dyes with a specified color, band shape, or solvent sensitivity. More recendy, quantitative values for reversible oxidation—reduction potentials have allowed more complete correlation of these dye properties with organic substituent constants. 

Itishould be noticed that the similarity between the osmophore theory and Witt s chromophore colour theory does not extand much beyond the initial conception and there seems to be no connection between the odour and the colour of a body, it is indeed quite the exception for a body to have both a strong odour and a strong colour. Two prolific sources of colour, viz. the diazo group and a large molecule have no counterpart as regards odour, and it is probably only by chance that quinone and chroman both have pronounced odours and are the sources of colour. 

Experimental work carried out in these laboratories during recent years has been based on the theory that insecticides owe their activity to a toxic nucleus—the toxophore—the properties of which may be modified by auxiliary radicals—the auxotoxes. This nomenclature is suggested by the names of analogous functions in dyestuffs, the chromophore and auxochrome groups. 

The term angular overlap model was first used in 1965 to describe a more general treatment, which may also be called the model (12,13). The relative energies of the orbitals in a partly-filled /-shell are expressed in terms of the parameters ex (X = a, rr, etc.), whose coefficients can be calculated from the geometry of the system. The eK model was developed from perturbation theory, but is equivalent to the E2 model when only a-overlap is considered, and to the Yamatera-McClure model for orthoaxial chromophores with linear ligators. The notation ex is often used, where ... 

On the Theory of Dyes.—In carbon compounds absorption in the visible part of the spectrum, i.e. subjective colour, is conditioned by the presence of a so-called chromophoric group in the molecule. The nitroso-group is strongly chromophoric, the nitro-group much less so, whilst the azo-group is quite considerably chromophoric, but only in aromatic systems. Azomethane is colourless. 

For assemblies of like chromophores in three dimensions in an infinite volume, many theories have provided various expressions of the survival probability Gs(t). Only one of them will be given here, owing to its simplicity and good accuracy. Huber (1981) obtained the following relationship ... 

Kometani etal.(93) used a theory for energy transfer from a donor to acceptors in a plane to determine the location of the retinal chromophore relative to the membrane surface. Another similar study on the location of the active site of chloroplast ATPase relative to the membrane surface has also been carried out.(94)... 

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