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Materials 2020, 13, 2646 18 of 24 symmetrical benzoselenazole derivatives reported in the same study, the quinoline heterocycle had an impact on this photophysical character of unsymmetrical squaraine dyes, since the compounds herein presented exhibited, on average, a red shift of about 20–30 nm. In aqueous media, the dyes showed broader bands resulting from the aggregation in these media (Figure 2). Nevertheless, their capacity for absorbing energy remained quite high. This finding has been observed previously for other symmetrical and unsymmetrical squaraine cyanine dyes derived from other heterocycles [41,48]. The occurrence of the J-type aggregates formation, as we saw for the aminosquaraine dyes 11 and 12 in specific aqueous media, until then, was not observed by us in other squaraine dyes. We found that the constitution of aqueous media, such as the salts present in them, can have significant effects on their aggregation behavior. Regarding the dyes’ light-stability, an approach that has been explored in our prior studies [25,41,48], the unsubstituted dye in the zwitterionic dye 9 was the one that stood out the most, as it is very stable to light (Figure 3). Aminosquaraine dyes had a worse photostable performance, since there was a more pronounced decrease in the absorbance of dyes 11 and 12 over the irradiation periods. In our last study, in which benzothiazole- and iodoquinoline-derived unsymmetrical squaraine dyes loaded with the same substituents on the squaric ring were involved [48], although the zwitterionic dye revealed less firmness than the one presented here, the aminosquaraine dyes revealed an absorbance evolution similar to that of zwitterionic. The relevance of the choice of heterocycles in this property has already been evidenced by us [76], and in this study, compared to benzothiazole-derived dyes [48], we found that the introduction of the benzoselenazole heterocycle moiety significantly improved dyes’ light stability of the zwitterionic one. Establishing a comparison between benzothiazole and benzoselenazole-derived aminosquaraine dyes, after 20 min of light exposure, benzothiazole-based dyes suffer a less marked decrease in absorbance over the irradiation time [48]. In our experience, and within this dyes’ scaffold, the dyes under study only lose to the indolenine-based squaraine dyes, since these molecules have very high stability for long periods of irradiation [25,38,41]. However, it should be noted that these comparisons may not be so linear, as light sources with different properties were used for the other studies. The comparison of the dyes’ stability in organic and aqueous solvents led us to conclude that, although the four-membered central ring unsubstituted squaraine dye 9 has a worse performance in aqueous media, and the aminosquaraine dyes 11 and 12 reduce their sensitivity to light, the zwitterionic dye 9 was the dye with the best photostability in this study. Hypothetically explanatory reasons for the differences observed in the light-stability of the synthesized dyes in organic and aqueous media are the significant reduction in dyes’ molar absorptivity coefficient in aqueous media, so these dyes will not have as much ability to absorb light energy in these solvents, as well as the tendency for the aggregates’ formation that may have increased the resistance of aminosquaraine dyes to the presence of light. The singlet oxygen formation ability of the quinoline- and benzoselenazole-derived squaraine dyes was weak (0.03 ≤ Φ∆ ≤ 0.06). These values are within those previously observed for unsymmetrical analogs [37,48]; however, it was expected that compounds with the selenium atom exhibit increased capacity for the formation of this reactive oxygen species. This speculation is explained by the fact that symmetrical dyes derived from benzoselenazole have exhibited high singlet oxygen quantum yields (0.05 ≤ Φ∆ ≤ 0.31) in a previous study [77], which were determined using the same technique. Despite being reported by many as a very relevant property, studies have shown that photosensitizer candidates with low singlet oxygen production capacity can reveal very significant antitumor photodynamic activity [37,48,78]. The dyes that showed greater in vitro photodynamic activity on the selected breast cancer cell lines were the aminosquaraine dyes 11 and 12 (Figures 4 and 5). Contrary to what has been reported with HepG2 and Caco-2 cell lines [37,48], in which the introduction of amines in the four-membered central ring induced excessive cytotoxicity, and the introduction of zwitterionic dyes increased photodynamic activity, in our study the unsymmetrical squaraines showed the innocuousness of the unsubstituted squaraine dye and the selective photocytotoxicity of the aminosquaraine dyes for BT-474 and MCF-7PDF Image | Photophysicochemical Light Antiproliferative vs cancer
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