3.2. Photophysical Characterization

The UV–Vis spectra were recorded from 300 to 800 nm using solutions of POSSPs 1–3 in THF (6.6 µM) and POSSPs 4 and 5 in DMSO (100 µM) in quartz cuvettes (1 cm path length). Similar conditions were used for the parent porphyrins (TPP, ATPP, TAPP and compound 6). The fluorescence spectra for POSSPs and parent porphyrins were obtained in the same solutions described above using an excitation wavelength of 520 nm. The fluorescence spectra were recorded from 600 to 800 nm.

The extinction coefficients were obtained using the Beer’s Law equation from the linear regression of absorption values vs. concentration. For POSSPs 1–3, several concentrations in THF ranging from 0.1 to 10 µM were used. In the case of POSSPs 4 and 5, several concentrations in DMSO ranging from 0.1 to 10 µM were utilized. Similar conditions were used for the parent porphyrins (TPP, ATPP, TAPP and compound 6).

The fluorescence quantum yields for air-saturated solutions (Φ_F) in THF (POSSPs 1–3) or DMSO (POSSPs 4 and 5) were determined using the comparative method. TPP was used as a reference with a fluorescence quantum yield of 0.12 in benzene [42]. The POSSPs concentrations ranged from 0.1 to 10 µM (THF) for POSSPs 1–3 and from 3 to 15 µM (DMSO) for POSSPs 4 and 5. The excitation wavelength was 520 nm and the excitation and emission slit width were 2 nm. The fluorescence quantum yields were measured according to the comparative method described by Equation (1).

The ¹O₂ quantum yields (Φ_Δ) were determined through an indirect method using dimethylanthracence (DMA) as the singlet oxygen probe. Several solutions containing DMF were air saturated and prepared with DMA (50 μM) and the POSSPs or parent porphyrins (TPP, ATPP, TAPP and compound 6) (5 μM). These solutions were covered with aluminum foil to avoid any premature quenching. Quartz cuvettes (1 cm × 1 cm) were filled with 1 mL of the solution, placed in a spectrofluorophotometer (xenon lamp, Shimadzu RF-5301 PC) and irradiated at 515 nm for 600 s. The absorbance decay of DMA was monitored at 380 nm, which was corrected from light scattering by subtracting the spectra of POSSPs. The Φ_Δ was calculated using Equation (2).

where Φ_∆,S is the singlet oxygen quantum yield of the sample and m is the slope of the plotted data relative to the area of the emission peak against the absorption of the reference [42].