# Sunlight Bleaching of Subporphyrazine Dye Films
## 3 Results and Discussion
### Photobleaching
#### 3.4.1 On Bare Glass Substrate
The replacement of one thiaadiazole unit by a benzene ring reduces the stability of the SubPz molecule in air, while increasing it in argon (the films on bare glass). SubPzS\({}_{3}\)H\({}_{4}\) is equally stable in both atmospheres and takes an intermediate position--Table 3. The films of \"perhydrogenated\" subphthalocyanine SubPcS\({}_{0}\)H\({}_{12}\) are less resistant to irradiation in air than in inert atmosphere, which agrees with the trend noted earlier in Refs. [7; 8; 30]. Figure 5: Absorption spectra of thin films of compounds in Figure 1 before (left) and after exposure to the direct sunlight in air (middle) and argon (right). The molecular structures of pigments are shown in Figure 1. (**a**) On glass; (**b**) On glass/ITO. Consequently, thiadiazole units, or rather, their intermediates with gas molecules, can stabilize the photoexcited state of the macroheterocycle. Although photobleaching of films composed by SubPcS\({}_{0}\)H\({}_{12}\) molecules is known ([7; 8; 21] and references therein), both the pathways and the end products of the reactions underlying this process appear to be not quite understood. It is clear that the end-products must be transparent in the visible range, and the most suitable candidate is phtallimide [7]. We examined this assumption in the following experiment. Solutions of SubPcS\({}_{0}\)H\({}_{12}\) and phthalimide in DSMO (10\({}^{-3}\) M) were prepared and placed in the quartz cuvettes. The cuvette with SubPcS\({}_{0}\)H\({}_{12}\) was subjected to a xenon arc lamp irradiation with continuous argon purging until the solution became visually colorless. The UV/Vis spectra of phthalimide and thus bleached SubPcS\({}_{0}\)H\({}_{12}\) solution were measured (Figure S7). In the latter case, a band at 291 nm was registered, which coincides with the pure phthalimide. Then, the bleached solution was evaporated, and the residue was collected and pressed into a KBr pellet. The FTIR spectra were measured and compared with the spectra of similar pellets containing neat SubPcS\({}_{0}\)H\({}_{12}\) and phthalimide powders (Figure S7). In the spectrum of the residue, the characteristic stretching vibrations of NH- and B-OH groups at 3211 cm\({}^{-1}\), vC = O at 1740 cm\({}^{-1}\) and vB-O at 1384 cm\({}^{-1}\), were observed (Figure S7). It indicates, again, the presence of phthalimide, and, possibly, boric acid as reaction products. However, the destruction of porphyzines SubPzS\({}_{3}\)H\({}_{0}\) and SubPzS\({}_{2}\)H\({}_{4}\) may proceed in a different way, and yield different end products, such as (4H-pyrrolo[3,4-c][1,2,5]thiadiazole-4,6(5H)-dione)--Figure S7. However, the identification of the reaction scheme, as well as redox chemistry, is beyond the scope of this work. UV filtering equally inhibits the photobleaching of SubPzS\({}_{3}\)H\({}_{0}\) films in air and argon--Table 2. Filtered sunlight lacks the wavelengths responsible for the excitation of high-energy electronic transitions from HOMO-1 to LUMO in SubPc/SubPz (Figure S4). A 20% decrease in the Q-band intensity, observed in the spectra of films exposed to filtered irradiation, can be due to soft annealing of films rather than chemical destruction of molecules. As seen from Figure 2, the filtering itself does not prevent warming-up of a molecular film by incident light; the attenuation of the steady-state temperature is only 4-6 \({}^{\circ}\)C. When the subphthalocyanines were in a dissolved state where annealing effects were excluded, the photobleaching of solutions exposed to a filtered light was almost negligible, whereas without a filter, the Q-band intensity decreased noticeably (Figure S8). Hence, only the photons absorbed in the range of the Soret-band are responsible for the destructive reactions. Turning to the comparison of differently substituted macroheterocycles from Figure 1, the fine structure of the Q-band in SubPzS\({}_{2}\)H\({}_{4}\) and SubPcS\({}_{0}\)H\({}_{12}\) films was weakly modified by irradiation in any atmosphere, similarly to SubPzS\({}_{3}\)H\({}_{0}\) (Figure S9).