Ultraviolet/visible radiation chromophores

Chromophore (Section 13 21) The structural unit of a mole cule principally responsible for absorption of radiation of a particular frequency a term usually applied to ultraviolet visible spectroscopy... 

Absorption of ultraviolet and visible radiation in organic molecules is restricted to certain functional groups (chromophores) that contain valence electrons of low excitation energy (Figure 4). The spectrum of a molecule containing these chromophores is complex. This is because the superposition of rotational and vibrational transitions on the electronic transitions gives a combination of overlapping lines. This appears as a continuous absorption band. 

Unsubstituted polysaccharides do not appreciably absorb ultraviolet and visible radiation, but they can be made to do so intensely by combining them with chromophores and chromogens (e.g., a-naphthol, dihydroxynaph-thalein, anthrone, carbazole, phenol-sulfuric acid, 2-thiobarbituric acid, tolu-idine blue, diphenylamine, Congo red, aniline blue, and methyl orange), usually in acidic or basic media. Coloration is normally preceded by depoly-... 

Chromophores are any parts of molecules which contain 7t-electrons (Table 10.8) and absorb ultraviolet and/or visible radiation. Applications of UV/VIS... 

The concept of a chromophore is analogous to that of a group vibration, discussed in Section 6.2.1. Just as the wavenumber of a group vibration is treated as transferable from one molecule to another so is the wavenumber, or wavelength, at which an electronic transition occurs in a particular group. Such a group is called a chromophore since it results in a characteristic colour of the compound due to absorption of visible or, broadening the use of the word colour , ultraviolet radiation. 

The significance of vibrational optical activity becomes apparent when it is compared with conventional electronic optical activity in the form of optical rotatory dispersion (ORD) and circular dichroism (CD) of visible and near-ultraviolet radiation. These conventional techniques have proved most valuable in stereochemistry, but since the electronic transition frequencies of most structural units in a molecule occur in inaccessible regions of the far-ultraviolet, they are restricted to probing chromophores and their immediate intramolecular environments. On the other hand, a vibrational spectrum contains bands from most parts of a molecule, so the measurement of vibrational optical activity should provide much more information. 

The theory underlying electronic spectroscopy with lasers is essentially the theory of visible or ultraviolet photon interactions. The distinctive features that arise with the deployment of laser light in electronic spectroscopy are principally those that relate to or exploit the qualities of the electric field produced by the laser beam. Laser electronic spectroscopy is primarily based on coupling (usually of dipolar character) between the electron clouds of individual ions, atoms, chromophores or molecules of the sample with the electric field of the impinging laser radiation. The high level of monochromaticity affords the means to obtain high-resolution data. 

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