## 3 Results and Discussion
### Photodegradation Evaluation  
Since photosensitizers are ideally tools that produce ROS only in the presence of radiation, the stability evaluation of these compounds when exposed to this condition is of particular interest. Photodegradation of the potential photosensitizing candidate is considered a disadvantage since the destruction of the light-sensitive molecule in other products may complicate the interpretation of the experimental results as well as lead to decreased efficiency of ROS production [68; 69]. Nevertheless, most commercial photosensitizing drugs used in tumor photodynamic treatment, such as porphyrins and chlorines, have poor photostability [70]. It is known that squaraine dyes are a class of compounds with fair photostability and that this photochemical property is mainly related to the nature of heterocyclic units [71].  
For the photodegradation monitoring, solutions of each unsymmetrical squaraine dye in DMSO were irradiated by a light-emitting diode (LED) system centered at \(630.8\pm 0.8\) nm for periods of 1 min from 0 to 20 min, 2 min from 20 to 40 min and 5 min from 40 min to 60 min (Figure 3). All dyes showed moderate photostability, except for the precursor \(O\)-methylated dye **11**, which degradation over the irradiation time was more pronounced. The remaining dyes showed very similar light stability over the studied time period. Unsubstituted dye **10** and aminosquaraines **12** and **13**, after 14 min of irradiation, longer irradiation time performed in in vitro cytotoxicity assays, showed reduced degradation. Thus, we can consider that the in vitro photodynamic effects produced up to this irradiation period are of the dye in question, without significant interference of degradative compounds.  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Solvent**} & \multicolumn{2}{c}{**Dye 10**} & \multicolumn{2}{c}{**Dye 11**} & \multicolumn{2}{c}{**Dye 12**} & \multicolumn{2}{c}{**Dye 13**} \\ \cline{2-9}  & \(\mathbf{\lambda_{\text{max}}}\) & **log \(\varepsilon\)** & \(\mathbf{\lambda_{\text{max}}}\) & **log \(\varepsilon\)** & \(\mathbf{\lambda_{\text{max}}}\) & **log \(\varepsilon\)** & \(\mathbf{\lambda_{\text{max}}}\) & **log \(\varepsilon\)** \\ \hline
**ACN** & 714 & 5.25 & 665 & 5.28 & 686 & 5.36 & 699 & 5.25 \\ \hline
**ACT** & 720 & 5.12 & 668 & 5.24 & 690 & 5.36 & 703 & 5.21 \\ \hline
**DCM** & 723 & 5.16 & 672 & 5.33 & 694 & 5.38 & 707 & 5.30 \\ \hline
**DMF** & 725 & 5.14 & 675 & 5.23 & 700 & 5.31 & 711 & 5.20 \\ \hline
**DXN** & 729 & 5.15 & 680 & 5.11 & 706 & 5.27 & 716 & 5.15 \\ \hline
**EtOH** & 701 & 5.22 & 668 & 5.27 & 693 & 5.38 & 702 & 5.28 \\ \hline
**MeOH** & 695 & 4.45 & 665 & 4.60 & 687 & 5.36 & 698 & 5.25 \\ \hline
**THF** & 733 & 5.06 & 671 & 5.26 & 702 & 5.26 & 714 & 5.17 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Vis/NIR absorption data of the squaraine cyanine dyes **10–13** in acetonitrile (ACN), acetone (ACT), dichloromethane (DCM), dimethylformamide (DMF), 1,4-dioxane (DXN), ethanol (EtOH), methanol (MeOH) and tetrahydrofuran (THF) (\(\lambda_{\text{max}}\) in nm and log \(\varepsilon\) in M\({}^{-1}\)-cm\({}^{-1}\).  
Figure 3: Photodegradation evaluated of squaraine cyanine dyes **10–13**. DMSO squaraine dye solutions were irradiated with a LED system centered at \(630.8\pm 0.8\) nm with the power of \(4.3\pm 0.5\) mW and their absorbance read over 60 min (Abs\({}_{\text{t}}\) (dye): absorbance of squaraine dye when irradiated at time t; Abs\({}_{0}\) (dye): absorbance of squaraine dye without irradiation).