Synthesis of symmetrical and unsymmetrical zinc phthalocyanines from two different precursors 4,5-bis(tert-butylsulfanyl)phthalonitrile (A) and N-(3-azidopropyl)-2,3-dicyanoquinoxaline-6-carboxamide (B) is described. Congeners of AAAA, AAAB, ABAB and AABB type were isolated by a chromatographic technique, however, the congener of BBBB type had to be prepared in a separate reaction. The adjacent and opposite isomers were also separated and fully characterized. Isolated phthalocyanines contained different number of azide groups, a substrate for highly efficient Cu(I) -catalyzed azide-alkyne 1,3-dipolar cycloaddition ("click chemistry"). All phthalocyanines absorbed strongly (ε over 150 000 M-1.cm-1) over 700 nm. Their singlet oxygen quantum yields were determined in DMF and ranged from 0.63 to 0.79, fluorescence quantum yields in DMF were considerably lower in the range 0.03–0.06. All these properties make them suitable building blocks for a simple modification and a synthesis of phthalocyanines with better tuned properties for photodynamic therapy.
In this work we present a rational design of the active part of third generation photosensitizers for photodynamic therapy based on phthalocyanine and an azaphthalocyanine core. The preferred zinc complexes of the AAAB type that contain bulky tert-butylsulfanyl substituents (A) and one carboxy group (B) have been synthesized by statistical condensation and fully characterized. The tetramerization was performed using magnesium(ii) butoxide followed by demetalation and insertion of ZnII. Compound 1 synthesized from 4,5-bis(tert-butylsulfanyl)phthalonitrile (A) and 2,3-dicyanoquinoxaline-6-carboxylic acid (B) exerted very promising photophysical properties (Q-band absorption at 726 nm, ϵ = 140000 M–1 cm–1), which allowed strong absorption of light at long wavelengths where the penetration of the light through human tissues is deeper. The very high singlet oxygen quantum yield of 1 (ΦΔ = 0.80) assures efficient photosensitization. As a result of bulky peripheral substituents, compound 1 shows good solubility in organic solvents with a low degree of aggregation, which makes it potentially viable for non-complicated modification. One carboxy group in the final structure of 1 allows simple binding to possible carriers. This compound is suitable for binding to targeting moieties to form the highly active part of a third-generation photosensitizer.