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Materials 2020, 13, 2646 10 of 24 bond between the nitrogen atom of N-methylamine substituent and the carbon of the four-membered central ring, occurring from the partial character of the double bond, which should promote the non-equivalence of chemical environments. Our research group recently observed this phenomenon in the NMR spectra of several symmetrical [25,41] and unsymmetrical methylaminosquaraine cyanine dyes [37,48]. Table 1. Relevant proton and carbon nuclear magnetic resonance spectra signals of quinoline- and benzoselenazole-derived unsymmetrical squaraine cyanine dyes 9–12. 13C NMR C=CH 92.85 88.91 92.84 89.13 93.89 89.15 94.40 93.99 89.86 89.30 1H NMR C=CH Dye 9 6.00 (1H, s) 5.77 (1H, s) 10 6.19 (1H, s) 5.75 (1H, s) 11 6.18 (1H, s) 5.95 (1H, s) 12 6.15 (1H, s) 6.07 (1H, s) 5.92 (1H, s) 5.85 (1H, s) 3.1.3. Absorption and Aggregation Behavior The “phototherapeutic window” is the name assigned to the range of wavelength values between 600 and 850 nm to which candidate compounds for photodynamic therapy photosensitizers should absorb with some intensity [37,52,53]. This range of values comprises the wavelengths at which light permeates the tissues more efficiently and, at the same time, sends enough energy to the target tissue to activate the photosensitizing molecules accumulated in it [54,55]. Thus, one of the first approaches to be made in novel candidate molecules for this therapeutic strategy is the study of their absorption capacity as well the determination of their maximum absorption wavelengths, in order to know if, when the light is incurred in the living tissue, these compounds are able to capture energy from the light source, even at some depth. For this purpose, the dyes involved in the present work were dissolved in several organic solvents (acetonitrile, acetone, dichloromethane, dimethylformamide, dimethyl sulfoxide, 1,4-dioxane, ethanol, methanol, tetrahydrofuran and xylene), to study the influence of polarity, as well as their dissolution in protic and aprotic solvents, in this photophysical property. Regardless of the nature of the organic solvent, all squaraine dyes have shown to have maximum absorption wavelengths within the values at which living tissues have high transparency to light (Figure 1 and Table 2). It is also possible to verify that, in general, the synthesized dyes presented maximum absorption wavelength values very close to each other in the various organic solvents, as well as spectral bands with similar behavior, except when dissolved in xylene, the most non-polar solvent used. In the latter, the presence of two visibly distinct bands in squaraine dye 9 is noteworthy, the least intense of which corresponds not only to the typical shoulder of this compounds’ core, as previously reported in the literature, but also to the formation of H aggregates [56,57]. Due to the fact that there is no total inverse linearity between the increase in the maximum absorption wavelength of the dyes and the polarity of the organic solvents studied, and that the compounds do not exhibit marked differences in their absorption spectra, except when displayed in xylene, it appears that the synthesized compounds do not show marked solvatochromism, a behavior previously reported for dyes also derived from cyanines [58]. Whether the solvents are protic or not, it does not seem to play an important role in this photophysical parameter of the synthesized dyes, since in ethanol and methanol, both polar protic solvents, the dyes exhibit low and high maximum absorption wavelengths, respectively.PDF Image | Photophysicochemical Light Antiproliferative vs cancer
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