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Materials 2020, 13, 2646 12 of 24 The combined analysis of the absorption spectra of the dyes in an organic solvent in which the dyes remain in the form of monomers (e.g., dimethyl sulfoxide) with the spectra obtained in aqueous solutions allows the assessment of the occurrence of the aggregates’ formation in these solvents (Figure 2). Figure 2. Normalized red and near-infrared absorption spectra of quinoline- and benzoselenazole- derived unsymmetrical squaraine cyanine dyes 9, 11 and 12 acquired in dimethyl sulfoxide (DMSO), Dulbecco’s modified Eagle medium (DMEM), water and phosphate-buffered saline (PBS) at room temperature. Absorption is presented as arbitrary units (a.u.). The three dyes showed behavior in aqueous media utterly different from each other. For the zwitterionic squaraine dye 9, regardless of the aqueous medium studied, there is the formation of blue-shifted aggregates, called H-aggregates. It is also observed that the tendency to form aggregates is higher than the monomeric form, since the absorbance found for the deviated band is higher than the band corresponding to the free form of the squaraine dye. Of the three aqueous solvents, the absorption spectrum in water should be highlighted, in which dye 9 is more prone to the formation of these dimers, since the band deviated to red has reduced absorbance compared to the culture medium and buffer solutions. Dye 10 was not evaluated for its aggregation character, since this dye hydrolyzes in its precursor dye 9. Zwitterionic and aminosquaraine dyes 9 and 11, respectively, in addition to H aggregates, the band of which is quite intense, show a tendency to form J aggregates, since a red-shifted lower intensity band at about 750 nm is presented in water. In the culture medium, the latter dye showed a profile similar to the zwitterionic dye. Interestingly, the constitution of aqueous media had a major impact on the aggregation character of aminosquaraine dyes 11 and 12. These dyes presented very wide bands in the three aqueous media but, comparing the spectra obtained in water with those of phosphate-buffered saline and culture medium, differences are observed in the intensity with which the aggregates are formed. While in the culture medium, the tendency to form H aggregates remains, and in water a residual amount of J aggregates can be seen, in phosphate-buffered saline and culture medium, these dyes are more prone to dimerization in J aggregates, since more intense red-shifted bands are displayed at approximately 800 nm. Regardless of the aggregate character, all dyes showed proper absorption in the electromagnetic regions where the tissues are transparent to light, as well as good absorption coefficients (Figure 2 and Table 2).PDF Image | Photophysicochemical Light Antiproliferative vs cancer
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