Chlorine extinction coefficient

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... 

The basis of this method is that when normal seawater is chlorinated at the usual levels of 1 to 10mg/l of chloride, the bromine in seawater (8.1 x 10 4 M, 65 mg/1 at salinity = 35%o) is rapidly and quantitatively oxidised to Br() and HBrO. If 50 mg/1 of bromide is added to distilled or fresh waters containing HCIO plus C1CT, then HBrO plus BrO" are both formed. The HBrO plus BrO" will in turn rapidly brominate fluorecein (9-[o-carboxyphenyl]-6-hydroxy-3-isoxanthenone) to give the pink tetrabromo derivative eosin yellow (2,4,5,7-tetrabromo-9-[o-carboxyphenyl]-6-hydroxy-3-isoxanthenone), provided the molar ratio of bromide to fluorescein is 4 1. The resultant increase in eosin can be measured visually or spectrophotometrically, and the decrease in fluo-roscein measured fluorometrically. If the molar ratio of bromide to fluoroscein is < 4 1, then the mono-, di-, and tri-bromo derivatives are formed repro-ducibly. These derivatives have extinction coefficients close to eosin and are accounted for in the standardisation. 

Chlorine dioxide readily degrades in aqueous solutions imder ultraviolet light. It has a broad UV absorption band with a maximum near 360 nm and a molar extinction coefficient of about 1,150 (M X cniy (Aieta and Berg 1986). It is postulated that the reaction in solution proceeds as in the gas phase, to give CIO and O. The initial photodissociation reaction is followed by rapid dark and light reactions to produce the products, chlorate (CIO3 ), hypochlorite (OCl ), and chloride (Cl ) (Zika et al. 

Figure 5 (a)-(c) shows the absorption spectra of some halogenated ketones in the near ultraviolet. In the case of acetone itself the absorption may be attributed to an n - tt transition associated with the carbonyl group. The substitution of halogen atoms leads to an increase in the molecular extinction coefficient and a shift of the absorption maximum toward the red. The ketones containing both chlorine and fluorine atoms show absorption curves with some fine structure but it is not possible to find any correlation between the structure and the shape of the absorption curve. 

The beautiful colors associated with porphyrin and chlorophyll systems are manifest in their characteristic electronic absorption spectra. The most intense band in the spectra, around 400 nm, is known as the Soret band it is intrinsic to the large macrocyclic conjugated pathway and has molar extinction coefficients usually between 150 000-400 000. This extinction value is lower in chlorins than in porphyrins, and the band is absent in porphyrinogens (6) and ring-opened bile pigments. 

The naphthalimide ring system has the drawback of low effectiveness, which is mainly attributable to the low molar extinction coefficient. The industrial synthesis of alkoxynaphthalimides begins with acenaphthene. Chlorination and subsequent oxidation with dichromate give the corresponding naphthalic acids [116], which are converted to the anhydrides on drying. Mild reaction with methyla-mine, followed by reaction with sodium methoxide or sodium ethoxide, gives, e.g., 64 or 65. 

Haloperoxidase activity is usually determined spectrophotometri-cally by the bromination or chlorination of 2-chloro-5,5-dimethyl-1,3-cyclohexanedione (monochlorodimedone MCD) using hydrogen peroxide as the oxidant of the halide (28) (Figure 2). This assay is convenient because of the large change in extinction coefficients between the enolate and the product. However, its use is constrained by the need for conditions that stabilize the enolate. In the early work on V-BrPO, the... 

Direct calculation of extinction coefficients (and hence oscillator strengths) requires a knowledge of both intensity of absorption and concentration of absorbing species. For the transition described above, concentrations of OH were calculated from the known fractional dissociation of water at elevated temperatures. In a few other instances, it may be possible to estimate the concentration of the intermediate from chemical considerations. Thus, Lipscomb et alP were able to calculate the extinction coefficient at A = 2577 A of the radical CIO from flash photolysis studies of chlorine dioidde. It was shown that the disappearance of CIO obeyed a second order rate law, so that... 

A method for the estimation of phosgene in commercial chlorine is based on a knowledge of the apparent extinction coefficient of the carbonyl stretching band in a high pressure cell designed to contain liquid chlorine [1664]. 

A number of investigators have examined di-hydroporphyrins (chlorins) since these compounds have Q-bands in the 650-700 nm region with extinction coefficients some 15-20 times those of porphyrins. Morgan et al. [49,50] have examined a Sn-etiopurpurin (Figure 14), which appears to have favorable pharmacokinetic properties (i.e., is taken up and retained by experimental tumors in rodents for at least 24h but is not retained for long periods of time in the skin rendering the animal photosensitive). This material is undergoing further preclinical development for PDT and may represent a useful alternative to Photofrin. 

Hydroxyphenyl benzotriazoles have the structure (XV) where X is H or Cl (chlorine shifts the absorption to longer wavelengths), Ri is H or branched alkyl, and R2 is CH3 to CgHi7 linear and branched alkyl (Ri and R2 increase the affinity to polymers). Some technically important materials in this class are 2-(2 -hydroxy-5 -methyl-phenyl)-benzotriazole, 2-(2 -hydroxy-3 -5 -(Ji.fert-butyl-phenyl)-benzotriazole, and 2-(2 -hydroxy-3, 5 -di-tert-butyl-phenyl)-5-chlorobenzotriazole. In comparison with 2-hydroxybenzophenones, the 2-(2 -hydroxyphenyl) benzotriazoles have higher molar extinction coefficients and steeper absorption edges towards 400 nm. 

Lenzi (133) reports that the extinction coefficient exhibits only a slight variation with the acidity of the medium and remains unchanged in the presence of chloride ion, chlorate ion, and chlorine. Kieffer and Gordon (120) found that the extinction coefficient was independent of temperature (25-50 C), acid concentration (0.2-4M), chloride ion concentration (up to 0.3M), and ionic strength (2-4M). 

Chlorin 17 deserves a special mention. It was expected that by modifying the chromophore from a porphyrin to a chlorin an increase in the extinction coefficients, especially for the Q bands, should be observed as is typical for chlorins. Actually a drastic (almost threefold) decrease of the Soret band occurs in the chlorin in comparison to the analogous porphyrins. However, no Qy band increase was observed. This effect has been ascribed in the past as an increase of the Q to Soret band relative ratios of transition moments. After self-assembly, broad bands in the visible region with about the same intensity are obtained from both the chlorin and the porphyrins. As chlorins are less readily accessible (in the case of 17 a protective group strategy had to be employed which considerably increased the number of required synthetic steps) this allows us to state that for applications where large absorption coefficients are needed, the use of simple chlorins is not warranted and the cheaper porphyrins can be used instead. 

Hydrogenation of a carbon-carbon double bond in one or two pyrrole rings to generate chlorins and, respectively, bacteriochlorins — this modification results in a shift of the red absorption to around 690 to 700 nm and around 780 nm, as weU as in a marked increase in the molar extinction coefficient... 

Substitution of the Si-Cl bonds in the chlorinated polymers by MeOH leads to a red shift, relative to the other polysilanes, in the absorption maximum (X, = 348 nm, esi-si = 1802 cm- M- Figure Id) along with a slight decrease in the extinction coefficient relative to the methylphenyl congener. The poly(methoxyphenylsilane) 4 fluoresces at room temperature (Xem= 394 nm). 

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