Strongly absorbing chromophores

Most of the reliable and useful data is due to relatively strongly absorbing chromophores (s > 200) which are mainly indicative of conjugated or aromatic systems. Examples listed below encompass most of the commonly encountered effects. 

The first cage lanthanide complexes studied for their photophysics were the simple 2.2.1 cryptands. The lack of a strongly absorbing chromophore, and easy approach of solvent molecules meant that their luminescence properties were disappointing in comparison to many recently studied complexes. The Lehn cryptand (L53) (Scheme 6... 

Assuming that no other strongly absorbing chromophores are present, then the organic C60 adducts tend to be intensely red, whereas the organometallic adducts tend to be intensely green or red. The extinction coefficients (e) have values comparable to those of uncomplexed, with the more intense color arising because of small shifts in the 450-600 nm band. 

Due to the selective nature of the most sensitive detectors, electrochemical and fluorescence, derivatisation is often required to take advantage of these. Derivatisation has also been used to improve sensitivity of methods using UV detection by adding a more strongly absorbing chromophore. Derivatisation can be combined with trace enrichment a detection level of 70ngr for aldicarb (2-methyl-2-(methylthio) propanol 0-[(methyl-amino)carbonyl] oxime), aldicarb sulphoxide and aldicarb sulphone has been quoted for 10-ml sample volumes using this technique. 

DNA strongly absorbs UV radiation, especially mid-range UVB (290 to 320 nm) radiation. Two major DNA lesions are induced following UV exposure, pyrimidine dimers and 6-4 pyrimidine-pyrimidone photoproducts. Because the action spectrum (induction of a biological activity as a function of wavelength) for erythema closely matches the action spectrum for pyrimidine dimer formation, DNA is believed to be the chromophore for sunburn.6 Pyrimidine dimer formation, or more properly, the failure to adequately repair dimers after solar irradiation is also the primary cause of sunlight-induced skin cancer formation.7-8... 

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. 

However, carbenes such as alkylcarbenes that contain no chromophore are generally transparent in the most useful UV region. Those spectroscopically invisible carbenes can be monitored by the ylide method. Here, the carbenes react with a nucleophile Y competitively with all other routes to decay to form a strongly absorbing and long-hved ylide (Scheme 9.5). In the presence of an additional... 

Because methylene-interrupted polyunsaturates do not have strongly absorbing chromo-phores in the UV region, detection by refractive index or far-UV detection (205-214 nm) has been utilized in RP-HPLC separations of free fatty acids and their aliphatic esters. Refractive index detection is less sensitive than UV detection. However, with far-UV detection, solvents absorbing UV below 210 cannot be used. The RP-HPLC separations reported to date have generally involved derivatization designed to incorporate aromatic chromophores allowing detection by fluorescence or UV detection. 

One of the most commonly used class of derivatization agents for diasteromer formation are isothiocyanates and isocyanates. Enantiomers of /3-blockers, amphetamine, epinephrine, methamphetamine, and mexiletine have been resolved after derivatization with these agents. Isothiocyanates produce thiourea derivatives upon reaction with primary and secondary amines. Thiourea derivatives also provide a strong UV absorbance for the detection of enantiomers lacking a strong UV chromophore. Isocyanates produce ureas when reacted with amines. The physical properties of these ureas are similar to thiourea derivatives. Isocyanates will also react with alcohols to yield carbamates. 

The origins of two chirality rules, the Octant Rule for ketones and the Exciton Chirality Rule, have been explained, and it has been shown how the rules can be applied to the determination of molecular absolute configuration. In applications of the Octant Rule, when the absolute configuration of a ketone is known, the rule may be used to determine its conformation. Applications of the Exciton Chirality Rule to determine the absolute configuration of diols follows from the derivatization with strongly UV-visible absorbing chromophores. 

An artificial substrate labeled with a strong UV-absorbing chromophore was synthesized. This substrate and its product, obtained by hydrolysis of the ADP-ribosyl moiety, were separated on a Zorbax-ODS reversed-phase CI8 column (4.6 mm X 250 mm). The initial mobile phase was 70% 0.0435 M sodium acetate (pH 4.0) and 30% acetonitrile. The percentage of acetonitrile... 

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