Absorption spectra, Rose Bengal

Every polymer, no matter what the rose bengal concentration, shows a maximum absorption for the rose bengal moiety at 571-2 nn when the spectrum is taken in a non-polar solvent such as methylene chloride. RB benzyl ester, on the other hand, shows a maximum absorption at 564 nm in MeOH. This phenomenon is explained by a relatively large influence of hydrogen bonding solvents on the absorption maxima. The absorption maximum of RB benzyl ester (essentially a monomeric rose bengal polymer unit) in different solvents was measured. The positions of the absorption maxima, as a function of solvent, are shown in Thble IV. 

Fig. 3. An absorption spectrum of the polypyrrole film containing Rose Bengal (a) and that of 1 x 10 mol dm Rose Bengal aqueous solution (b).

Rose bengal is colored at the pH of plasma with an absorption peak at 545 m/A (S30). However, the absorption spectrum is affected by absorption of dye on albumin so that, in diluted plasma, the absorption peak is at 560 rufi (S2). 

Direct measurement of the absorption spectra of heterogeneous photosensitizers is impossible because of light dispersion from the polymer surface. Schaap, Thayer, Blossey and Neckers reported the use of diffuse reflectance techniques to approximate absorption spectra (14), but this technique does not produce "true" absorption spectra. The sample for reflectance is prepared in a MgO tablet. The (p)-RB is not, therefore, dispersed in a solvent and the spectra obtained are the spectra of ( -RB without ideal dispersion of the rose bengal moieties in solution. It is impossible to observe the true spectrum in the absence of perturbing effects using reflectance techniques. 

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