Bathochromic shifts chromophores

The replacement of a benzene nucleus by a thiophene nucleus in compounds containing other chromophores causes more or less pronounced bathochromic shifts. These effects have been noticed in the thiophene analogs of malachite green (23), in cumulenes such as... 

Aminoquinoxaline exists predominantly as such rather than in the tautomeric imino form. This is indicated by a comparison of the basic strength of the 2-amino compound (pKo 3.90) and those of its fixed methylated tautomers, 2-dimethylaminoquinoxaline pKa 3.72) and l-methylquinoxalin-2-oneimine (pfCn 8.70). The ultraviolet spectrum of the neutral molecule of 2-dimethylaminoquinoxaline shows the expected bathochromic shifts compared to that of 2-aminoquinoxaline these spectra differ from the ultraviolet spectrum of the neutral molecule of l-methylquinoxalin-2-oneimine (Fig. 1). The mono-cations (68) and (69) derived from 2-aminoquinoxaline and l-methylquinoxalin-2-oneimine have a similar chromophoric system and show almost identical ultraviolet absorption (Fig. 2). 

The reduced symmetry of the chromophore, which still contains 187t-electrons and is therefore an aromatic system, influences the electronic spectrum which shows a bathochromic shift and a higher molar extinction coefficient of the long-wavelength absorption bands compared to the porphyrin, so that the photophysical properties of the chlorins resulting from this structural alteration render them naturally suitable as pigments for photosynthesis and also make them of interest in medical applications, e.g. photodynamic tumor therapy (PDT).2... 

In general, auxochromic substitution of chromophores causes bathochromic shifts and increases in intensity for 7t —> k transitions, and hypsochromic or blue shifts (to shorter wavelengths) for n K transitions. The shifts are explainable in terms of mesomeric resonance) effects caused by interaction of lone pair electrons associated with such auxochromes as —OH, —Cl, —NH2 with the k system of the chromophore. This leads to... 

The spectrum of ketoprofen is shown in Figure 4.9. In this case the simple benzoid chromophore has been extended by four double bonds and thus the symmetry of the benzene ring has been altered. In addition, the strong absorbance band present in benzene at 204 nm has undergone a bathochromic shift giving a X max for ketoprofen at 262 nm having an /I (1%, 1 cm) value of 647. 

Figure 4.10 shows the UV absorption spectra of a solution of procaine in 0.1 M HCl and O.IM NaOH. In procaine, the benzene chromophore has been extended by addition of a C = O group and under acidic conditions, as in Figure 4.10, the molecule has an absorption at 279 nm with an A (1%, 1 cm) value of 100. In addition to the extended chromophore, the molecule also contains an auxochrome in the form of an amino group, which under basic conditions has a lone pair of electrons that can interact with the chromophore producing a bathochromic shift. Under acidic conditions the amine group is protonated and does not function as an auxochrome but when the proton is removed from this group under basic conditions a bathochromic shift is produced and an absorption with A, max at 270 nm with an A (1%, 1 cm) value of 1000 appears. 

We conclude our short discussion of relationships between chemical structure and light absorbance by considering some cases in which an acid or base function forms part of a chromophore. Important examples of compounds exhibiting such chro-mophores are phenols and anilines. As is evident from the spectra shown in Fig. 15.5, deprotonation of a phenolic group results in a substantial bathochromic shift... 

The UV and CD curves of optically active (S)-9-hydroxy-6-methyl-3,4-dihydro-2//-pyrido[2,1 -6] [1,3]thiazinium-4-carboxylate were measured in 1 N hydrochloric acid and in 1 N sodium hydroxide solutions (73ACS1059). The CD spectrum in acid solution exhibits positive bands at 212 and 233 nm and negative bands at 257 and 345 nm, and UV maxima were found at 211, 239, and 342 nm. The two latter bands are associated with the aromatic chromophore. In alkaline solution a bathochromic shift occurred, and the... 

For both cis- and Irans-dianthrylethylenes, the degree of deconjugation, which parallels the degree of deviation from coplanarity of the ethylene and anthracene ir-systems, is borne out in the shape of the electronic absorption spectra. Thus, the long-wavelength absorption of cis-dianthrylethylene 38a is characterized by the fine structure pattern which is typical of the anthracene chromophore, while the bathochromically shifted spectrum of the transisomer 39a is virtually structureless (see Figure 7). Substitution of the ethylene double bond is absorption spectroscopically noticeable for the cis-isomers 38b-f by distortion of the anthracene absorption. The absorption... 

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