Fluorescence halogenated solvents

Some photoionizations occur only in halogenomethane solvents whereas in other solvents (e.g. ethanol, benzene) only fluorescence occurs in the halogenated solvents ionization is monophotonic. This is the case with di- and tetraphenyl-p-phenylenediamine in chloroform and carbon tetrachloride (Fitzgerald et al., 1971), and with TMPD in chloroform, bromoform, methylene bromide and chloride (Meyer, 1970), and dimethylaniline in bromobenzene (Gradowski and Latowski, 1974 et al., 1972). Electron transfer may occur from the excited aromatic singlet to ground state solvent [(54) and... 

Equation (55) may also be an oversimplification in some cases. That is, there is fluorescence evidence that an excited aromatic compound may form an excited complex (an exciplex) with a halogenated solvent prior to dissociative electron transfer. Irradiation of triethylamine in chloro-, bromo-, and iodobenzene resulted in the quenching of triethylamine fluorescence, and in the appearance of a new fluorescence at higher wavelength (Tosa et al., 1969). Irradiations of triethylamine and a halogenobenzene in methanol solution led to fluorescence quenching again but also to the formation of halide ion, biphenyl, and other free radical products. The several reactions are summarized in eqns (57)-(61), in which D represents... 

Another method of preparation is as follows 1 33 parts of fluorescein are dissolved in 5 parts of ether and treated with 25 parts of selenium chloride in the same solvent. A yellowish-red precipitate separates, and after long stirring at the ordinary temperature the ether is distilled off. The residue is stirred with water, the mixture filtered and the residue now dissolved in sodium hydroxide. After further filtration the filtrate is treated with hydrochloric acid, which precipitates seleno-fluorescei n. Further purification is effected by solution in alkali and reprecipitation. A reddish-brown powder is obtained, soluble with fluorescence in alcohol, but insoluble in water. In concentrated sulphuric acid it dissolves to give an orange solution. Its alkali salts are very soluble in wrater, giving red solutions. This process may also be applied to phthalins, which are obtained by the reduction of phthaleins and their halogen derivatives. If the selenium chloride is replaced by the oxychloride similar products are obtained.2 In place of the phthalins specified in the patents quoted, their O-acetyl compounds or O-acetyl compounds of the phthaleins may be used in indifferent solvents. The products are different from those obtained by the action of selenium on fluoresceins in aqueous alkali solutions.3... 

The mobile phase plays an important part in the fluorescence of a molecule. Unless chosen with care, the mobile phase can quench the fluorescence of the molecule of interest. Most of the non-halogen-containing solvents used in HPLC can be used with fluorescence detection. However, dissolved oxygen or other impurities in the eluent can cause quenching. The solvent polarity and the pH of the mobile phase can also affect the fluorescent process if they influence the charge status of the chromophore. For example, aniline fluoresces at pH 7 and at pH 12, but at pH 2, where it is cationic, it does not fluoresce. Table 3.5 shows wavelength selections for some common LC-fluorescence applications.30... 

Here, k( and k0 are the rate constants of the fluorescent and nonactivated radiationless relaxation steps, respectively. Using Eq. (8) Saltiel and D Agostino [9] determined an activation energy of approximately 13kJmol and an zlf-factor of the order of 10 2-1013s for stilbene in nonpolar solvents. Similar results have been reported for halogen- and cyano-substituted stilbenes (Table 12a). As examples, plots of log d>f versus 1/T are shown in Figure 12 for stilbene, 4-chloro-, and 4-bromostilbene. 

The presence of heavy atoms (e.g., halogens) in solvents generally tends to decrease fluorescence intensity. The same is true with increasing temperature, where a rC increase in temperature decreases fluorescence intensity by 2% [40],... 

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