## 3 Results and Discussion
### Photochemical Properties  
In the UV-Vis spectrum of **2** two absorption bands were noted. The spectrum profile is typical for azaphthalocyanines, with the Soret band in the range of 300-460 nm and the Q band located in the range of 600-750 nm (Figure 5). These bands arise from the \(\pi\)-\(\pi\) * transitions. Slightly increased absorption in the range of 500-600 nm could be the result of aggregates formation. A similar phenomenon has been previously noted for tetrapyrazino-porphyrazines [17]. Compared to Pcs, tetrapyrazinoporphyrazines macrocycles with the same substituents and containing the same central metal cation in the core revealed a higher tendency to form aggregates. In the emission study, macrocycle **2** upon excitation at the Soret as well as the Q band did not emit fluorescence. Figure 4: Fragment of \({}^{1}\)H–\({}^{13}\)C HMBC NMR spectrum of **2** in pyridine-\(d_{5}\). Figure 3: Fragment of \({}^{1}\)H–\({}^{1}\)H COSY NMR spectrum of **2** in pyridine-\(d_{5}\). Tetrapyrazinoporphyrazine derivative **2** revealed low solubility in DMF and DMSO, and it was completely insoluble in the water. Therefore, the quantum yield of singlet oxygen formation was studied only in DMF. An indirect method of singlet oxygen generation was applied using 1,3-diphenylisobenzofuran as a singlet oxygen chemical quencher, whereas an unsubstituted zinc(II) phthalocyanine (ZnPc) was treated as a reference. The mixture of DPBF and tetrapyrazinoporphyrazine derivative **2** was exposed to the monochromatic light at the wavelength adjusted to the maximum of Q band in the absorption spectrum and monitored in time using the UV-Vis spectroscopy. The DPBF decomposition kinetics was calculated and compared with the reference zinc(II) phthalocyanine. The value of the quantum yield of singlet oxygen generation (\(\Phi_{\Delta}\)) approached 0.023. The potential of structurally similar azaphthalocyanine derivative in terms of singlet oxygen generation was previously presented by [PERSON] et al. [28]. They reported the quantum yield of singlet oxygen generation of 0.196 for zinc(II) tetrapyrazinoporphyrazine derivative I (Figure 6), which indicates that the thiophenyl functionalization decreased the \(\Phi_{\Delta}\) value as compared to unsubstituted zinc(II) phthalocyanine. Usually, zinc(II) ion-containing phthalocyanine analogs are considered efficient singlet oxygen generators due to the \"heavy atom effect\" [53]. For example, unsubstituted zinc(II) phthalocyanine used as reference compound reveals significantly higher \(\Phi_{\Delta}\) at the level of 0.56 in DMF and 0.67 in DMSO, whereas unsubstituted magnesium(II) phthalocyanine 0.28 and 0.14, respectively [54]. A similar phenomenon appeared in the case of **2**. An additional issue that could affect a very low singlet oxygen formation by **2** can be combined with terpene moiety. In our latest study, phthalocyanine derivative bearing thymol moieties (II, Figure 6) formed singlet oxygen with the quantum yield of 0.15 in DMF in comparison to unsubstituted zinc(II) phthalocyanine 0.56 [35]. The above-mentioned literature data and the experimental results obtained within current study allow to explain the observed effects. Namely, it seems that the limited singlet oxygen formation ability of **2** can be the result of both magnesium(II) ion presence in the core and bulkymethoxy-thiophenyl periphery, which decrease the solubility of studied macrocycle. Figure 5: The absorption spectrum of studied compound **2** in DMF. In the photostability study, azaphthalocyanine derivative **2** under irradiation with visible light presented very high stability. The calculated quantum yield for photostability (\(\Phi_{\text{P}}\)) of **2** at 9.59 \(\times\) 10\({}^{-6}\) significantly exceeds the border of 10\({}^{-4}\)-10\({}^{-5}\), allowing us to classify this macrocycle as a stable porphyrinoid following [PERSON] and et al. [55].