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Materials 2020, 13, 2646 3 of 24 for their potential use in photodynamic therapy. Characteristics, such as their singlet oxygen formation, photostability, and aggregation properties were assessed and discussed. Their in vitro anticancer effects have been studied by determining cell viability in BT-474 (human ductal breast carcinoma) and MCF-7 (human breast adenocarcinoma derived from pleural effusion) breast cancer cell lines. The effects of concentration, irradiation and incubation time of the dyes were studied to find the ideal conditions in which the dyes exhibit photodynamic activity against these cancer cell lines. In addition, we also investigated the effects of introducing amino and methylamino groups in the dyes’ structure on the above-mentioned properties. 2. Materials and Methods Starting materials for the preparation of quinoline- and benzoselenazole-derived unsymmetrical squaraine cyanine dyes, 1-iodohexane, 2-methylquinoline (1), squaric acid (3) and 2-methylbenzoselenazole (7), and the analytical grade solvents, were obtained from commercial sources and used as received without further purification, unless otherwise specified. Petroleum ether refers to the fraction 40–60 ◦C. Solvents were dried, as described in the literature [46], and freshly distilled. 3-Hexyl-2-methylbenzoselenazol-3-ium iodide (8) [47], 1-hexyl-2-methylquinolin-1-ium iodide (2) [47] and 3,4-dibutoxycyclobut-3-en-1,2-dione (4) [37] were prepared according to the literature procedures. Analytical thin-layer chromatography (TLC) was conducted on aluminum plates covered with 0.25 mm of silica gel (Merck, Darmstadt, Germany). Recrystallizations were carried out in solvent mixtures mentioned in each case. Melting points (M.p.) were determined in a hot plate binocular microscope apparatus (URA Technic, Porto, Portugal) and were not corrected. After melting the crystals, the possibility of decomposition was monitored by TLC; if there was a possibility of decomposition, it was indicated with the abbreviation dec. Proton and carbon nuclear magnetic resonance (1H and 13C NMR) spectra were obtained at a temperature of 298.15 K on a NMR Avance III 400 spectrometer (Bruker, Bremen, Germany) performing at 9.4 T, observing 1H at 400.13 MHz and 13C at 100.63 MHz, or on a NMR Bruker Avance III 600 spectrometer executing at 14.09 T, observing 1H at 600.13 MHz and 13C at 150.90 MHz. Dye samples were prepared in deuterochloroform or hexadeuterodimethyl sulfoxide (CDCl3 or DMSO-d6). Chemical shifts (δ) were reported in parts per million (ppm) relative to tetramethylsilane or residual solvent signals and the coupling constants (J) are reported in Hz. The proton splittings were described as singlet or broad singlet (s or br s, respectively), doublet (d), triplet or broad triplet (t or br t, respectively), broad quartet (br q), quintet or broad quintet (qt or br qt, respectively), or multiplet (m). Chemical shifts of 13C spectra were presented to the hundredths since rounding to the tenth causes signs of different carbons to appear at the same chemical shift. Distortionless enhancement by polarization transfer 135◦ (DEPT 135) spectra were used to make the assignments of the carbon signals. Visible to near-infrared (Vis-NIR) absorption spectra were recorded in acetonitrile (ACN), acetone (ACT), dichloromethane (DCM), dimethylformamide (DMF), dimethyl sulfoxide (DMSO), 1,4-dioxane (DXN), ethanol (EtOH), methanol (MeOH), tetrahydrofuran (THF), xylene (XLN), Dulbecco’s modified Eagle medium (DMEM), phosphate-buffered saline (PBS) and water on a Lambda 25 spectrophotometer (Perkin Elmer, Whaltam, MA, USA) using a 1 cm quartz cell (Hellma Analytics, Mülheim, Germany); λmax in nanometers (nm). To determine the molar extinction coefficients (ε) of the squaraine dyes in the various solvents, five diluted dye solutions were prepared by taking aliquots of a known concentration stock solution, the absorbance intensities of each dye solution were plotted versus the sample concentration, and a linear fit was applied to determine the slope of the line. For easier comparison, the logarithm of these molar extinction coefficient values was estimated (M−1·cm−1). Infrared (IR) spectra were recorded using potassium bromide pellets on an IRAffinity-1S FTIR instrument (Shimadzu, Duisburg, Germany); νmax in cm−1. The intensity of the bands was described as strong (s), medium (m), or weak (w). High- and low-resolution electrospray ionization time-of-flight mass spectra (HRESI- and LRESI-TOFMS) were assessed with a microTOF (focus) Bruker Daltonics spectrometer (located at CACTI, University of Vigo, Vigo, Spain), and the abbreviation [M]+ refers to the molecular ion peak.PDF Image | Photophysicochemical Light Antiproliferative vs cancer
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