Syntheses of octasubstituted zinc azaphthalocyanines with thiophene or thiophene combined with sulfanyl,amino or imido substituents: Influence of the substituents on photochemical and photophysical properties

Several octasubstituted zinc azaphthalocyanines (ZnAzaPcs) of the tetrapyrazinoporphyrazine type have been synthesized as potential sensitizers for photodynamic therapy (PDT). Octasubstituted complexes, with thiophen-2-yl, thiophen-3-yl or benzo[b]thiophen-3-yl peripheral groups, were synthesized and characterized. Octa(thiophen-2-yl) ZnAzaPc is a better singlet oxygen producer and has a red shifted UV absorption Q-band compared to both thiophen-3-yl and benzo[b]thiophen-3-yl substituted ZnAzaPcs. Thus, the thiophen-2-yl substituent is better suited for our purpose. Unsymmetrically substituted ZnAzaPcs were synthesized by cyclotetramerisations of pyrazine-2,3-dicarbonitriles attached to one thiophen-2-yl group and one alkylsulfanyl, thiomorpholinyl or imide group. Constitutional isomers were detected by NMR spectroscopy for some of these complexes. Compared to unsubstituted ZnAzaPc, red shifted Q-bands were observed for all these complexes, due to the presence of thiophen-2-yl groups. The least promising complexes are ZnAzaPcs with thiomorpholine or imide peripheral substituents, i.e. where the peripheral substituents are attached to the macrocycle through nitrogen atoms. Low singlet oxygen quantum yields (Φ_Δ) and also low fluorescence quantum yields (Φ_F) were observed for these ZnAzaPcs. In the case of combined thiophen-2-yl and alkylsulfanyl substituents, the values of Φ_Δ were the highest and reached values of approximately 0.69.