# Sunlight Bleaching of Subporphyrazine Dye Films
## 3 Results and Discussion
### Photobleaching  
#### 3.4.1 On Bare Glass Substrate  
The intensity of the strong Q-band, which basically accounts for the pink color of subphthalocyanine films, decreases after exposure to light, i.e., the films become bleached. The degree of bleaching in different conditions was quantified by measuring the ratio of the optical density at \(\lambda^{\text{Q}}_{\text{max}}\) in the pristine and in the irradiated films of SubPzS\({}_{3}\)H\({}_{0}\) (D\({}^{\text{Q}}_{\text{pristine}}\)/D\({}^{\text{Q}}_{\text{irradiated}}\))--Table 1. The corresponding deconvolutions of the Q-band are shown in Figure S5. When the films were stored in the dark, they retained their color over the entire time of observation, regardless of the atmosphere. The deconvolution results showed that the composition of the Q-band, including positions and relative intensities of subbands, did not change in the irradiated samples under any conditions, even when the Q-band intensity was reduced by a factor of two or more. The CT-band at 393 nm experienced a clear blue shift of 12 nm in the SubPzS\({}_{3}\)H\({}_{0}\) films irradiated in air (Figure S6). This suggests an increase in the energy of the corresponding intramolecular transition, likely to be due to the interaction of the macrocycle with (oxidized?) decomposition products. In some experiments, the samples were placed into a carefully purged cuvette filled with the so-called zero-gas, i.e., synthetic air prepared by mixing ultrapure oxygen and nitrogen in atmospheric proportions. We thereby minimized the influence of moisture, carbon oxides, and any kind of uncontrolled air components during irradiation; all other experimental conditions remained the same (the cuvette window attenuates the sunlight by about 10%). As seen from Table 2, the photobleaching of films under direct irradiation in synthetic air weakened, but did not cease completely. This result highlights the importance of controlling the laboratory atmosphere during measurements on thin-film samples [28,29]. Otherwise, the trends in the photo-assisted degradation of dye molecules may be misjudged. As seen from Table 2, the photobleaching of SubPzS\({}_{3}\)H\({}_{0}\) films on glass was deeper in argon than it was in air. This is somewhat unexpected [8], and one has to assume that \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & **Pristine** & **Direct** & **Filtered** & **Direct** & **Filtered** & **Direct** \\  & **Sunlight** & **Sunlight** & **Sunlight** & **Sunlight** & **Sunlight in** \\  & **in Air** & **in Air** & **in Air** & **in Ar** & **in Ar** & **Synth. Air** \\ \hline \(\lambda^{\text{Q}}_{\text{max}}\), nm & 553 & 553 & 553 & 553 & 554 & 555 \\ D\({}^{\text{Q}}_{\text{pristine}}\)/D\({}^{\text{Q}}_{\text{irradiated}}\) & - & 1.6 & 1.2 & 2.2 & 1.2 & 1.4 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Maximum of Q-band (\(\lambda^{\text{Q}}_{\text{max}}\)) and ratio of the optical density at \(\lambda^{\text{Q}}_{\text{max}}\) in pristine and irradiated SubPzS\({}_{3}\)H\({}_{0}\) films on glass (D\({}^{\text{Q}}_{\text{pristine}}\)/D\({}^{\text{Q}}_{\text{irradiated}}\)). some of the atmospheric air components prevented the photoexcited state of SubPzS\({}_{3}\)H\({}_{0}\) from further destruction. To prove this assumption, we conducted similar degradation tests with thin films of the two nearest analogs of SubPzS\({}_{3}\)H\({}_{0}\), shown in Figure 1. The idea was to see if the chemical structure of the molecule, here, the presence of 1,2,5-thiadiazole units, is responsible for the resistance to photodestruction [6]. The absorption spectra of the films are shown in Figure 5a; the results are summarized in Table 3. As in solutions, there was a hypsochromic shift of the Q-band in the pristine films of SubPzS\({}_{3}\)H\({}_{0}\) and SubPzS\({}_{2}\)H\({}_{4}\) relative to the conventional SubPcS\({}_{0}\)H\({}_{12}\). The reasons for these band shifts, as well as the redox behavior of these compounds, were considered in our previous studies [14; 16; 18].