Visible Spectra Color in Compounds

Some famihar compounds may serve to imderscore these relationships between the absorption spectrum and the observed color. The structural formulas of these examples are shown. Notice that each of these substances has a highly extended eonjugated system of electrons. Such extensive conjugation shifts their electronic spectra to such long wavelengths that they absorb visible hght and appear colored. 

RELATIONSHIP BETWEEN THE COLOR OF LIGHT ABSORBED BY A COMPOUND AND THE OBSERVED COLOR OF THE COMPOUND 

Color of Light Absorbed Wavelength of Light Absorbed (nm) Observed Color 

Cyanidin chloride (an anthocyanin, another class of plant pigments) max = 545 nm 

Copyright 2013 Cengage Learning. AH Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. 

---

Electrochromic materials are electroactive compounds whose visible spectra depend on the oxidation state. Possible applications are smart windows, displays, mirrors, and so on. Among the most important performance aspects in electrochromic materials, the reversibility and lifetime of the material to repeated cycles, the time of response (usually in order of seconds), the colors of the oxidized/reduced forms and the change in absorbance upon redox switching (contrast) are of interest. 

The noreugenin chromophore makes an important contribution to the UV spectrum of these compounds, giving peaks at 225, 257, 295 and 318 nm. In some of the compounds the conjugation is extended so that appreciable absorb-tion occurs in the visible range, giving a yellow color to the compounds. This is particularly seen in the A -methyl quaternary compounds 31 and 32. The chromophore is also extended in schumannificine and its analogs (25-28). Details of the UV spectra for each compound are given in Table VI. 

Some colored compounds have also been linked to peptides or proteins for the detection and measurement within visible spectra ranges. Colorimetric methods are relatively nonspecific but they can be made specific by linking them to enzymatic or immunologic reactions (see Immunoassay section). In these cases, enzymes produce colored products from colorless substrates, rather than radionuclides, are used to label particular antigens and antibodies (Winder and Gent 1971). 

Diazoxine, a red product which accompanies the formation of 8-methoxy-quinoline from 8-hydroxyquinoline and diazomethane, was first encountered by Caronna and Sansone in 1939. Later, it was suggested that the properties of this product were consistent with structure 189 (R = Me, R = R = H). ° Subsequently, this product has been prepared by treatment of the iodide 190 with potassium carbonate, and the betaine structure has been confirmed by spectroscopic and chemical studies. Compound 189 (R = Me, R = R = H) is isolated as hydrated violet-red needles. The UV and visible spectra are strongly dependent on the nature of the solvent the colors of solutions vary from yellow to blue. Bromination gives the 5,7-dibromo derivative (189 R = Me, R = R = Br), which is also obtained from... 

TLC/scanning densitometry is a useful tool for the identification of the target compounds on a TLC plate, because the combined methods can separate and then directly measure ultraviolet-visible absorption spectra of the compounds without the laborious and time-consuming procedures described above. In this paper, we deal with the identification of synthetic and natural colors in foods using TLC/scanning densitometry. 

The identification of unknown pesticide zones is initially based on the comparison of the migration of sample zones relative to standards developed on the same layer and colors obtained with selective chromogenic and fluorogenic detection reagents. Many densitometers can record in situ UV and visible absorption and fluorescence excitation spectra to confirm compound identification by the comparison of unknown spectra with stored standard spectra obtained under identical conditions or spectra of standards measured on the same plate. Additional confirmation methods include off-line and on-line combination of TLC with infrared, Raman,... 

Dithizone is also used for the determination of metallic compounds such as copper (II) (Fig. 28). Because of the slight aqueous solubility of this compound, UV-visible spectra have been acquired in methanol/water (50/50) solution (Fig. 29). In strong acid medium (pH = 2.0), the maximum of absorption in the visible region is located at X = 588 nm (e = 13,700 Lmol-1 cm-1). A protonation may occur on the sulphur atom. The coloration of the solution is dark blue. In basic medium (pH = 12.0), the colour turns to orange X = 472 nm, e = 8100 Lmol-1 cm-1). Two isosbestic points, located at X = 425 nm and X = 517 nm, confirms this phenomenon. 

The crystals that are obtained from the cluster formation reactions are intensely colored. In fact, the intensity of the color increases when going from sulfur- to selenium- to tellurium-bridged compounds (see below), as might be expected for an increase in the covalent or (semi-) metallic binding properties. Small copper sulfide and selenide clusters form light red, orange, or purple crystals, but with increasing cluster size the color varies from dark red to reddish-black to (finally) black with a metallic sheen. The optical spectra of some copper selenide cluster compounds have been studied by means of solid-state UV-visible spectroscopy. 

The infrared spectra of adducts such as (5) formed between alkyl picrates and alkoxides show changes relative to the parent compounds that are consistent with a fully covalent structure. A series of strong bands between 1225 and 1040cm" is typical of ketals [118, 119]. Many sigma adducts are intensely colored, and attempts have been made to correlate UV-visible spectra with structural features [120]. However, these spectra do not generally provide such precise information as do NMR studies. Nevertheless, the absorption intensity usually obeys the Beer-Lambert law, so that absorption is directly proportional to concentration, and this is very useful in the determination of concentrations in kinetic and equilibrium studies. 

The 5/ to 6d bands are orbitally allowed and therefore more intense than those of the / to / transitions. They are also usually broader and often observed in the ultraviolet region. The metal to ligand charge-transfer bands are also fully allowed transitions that are broad and occur commonly in the ultraviolet region. When these bands trail into the visible region, they produce the intense colors associated with many of the actinide compounds. Metal-ligand frequencies are also observed in the infrared and Raman spectra of actinide compounds. 

---