Zinc azaphthalocyanines with thiophen-2-yl, 5-methylthiophen-2-yl and pyridin-3-yl peripheral substituents: Additive substituent contributions to singlet oxygen production

Abstract

The syntheses of five pyrazine-2,3-dicarbonitriles, substituted with combinations of electron donating substituents, namely 5-methylthiophen-2-yl or thiophen-2-yl and the electron withdrawing substituent pyridin-3-yl, are reported. The compounds were obtained from the condensation of diaminomaleonitrile and vicinal dicarbonyl compounds. Cyclotetramerizations of the monomers with both Zn(OAc) 2 and Zn(quinoline) 2Cl 2, realised substituted zinc azaphthalocyanines (ZnAzaPcs). The UV–vis spectra of these macrocycles revealed that thiophene and methylthiophene peripheral substituents induce substantial, red shifted Q-band absorptions from 636 nm for unsubstituted ZnAzaPc to 679 nm for octa(5-methylthiophen-2-yl)ZnAzaPc. This effect is substituent-specific and additive since the observed red shift for the Q-band of tetra(5-methylthiophen-2-yl)-tetra(pyridin-3-yl)ZnAzaPc is close to the calculated contribution from each substituent. Singlet oxygen quantum yields ( Φ Δ) varied according to the peripheral substituent on ZnAzaPc and was also substituent-specific and additive. Whereas singlet oxygen production increased, the fluorescence quantum yield decreased with increasing number of substituents on ZnAzaPc; however, no quantitative relationship was found for Φ F and substitution patterns for the zinc complexes. Improved solubility was observed for unsymmetrical ZnAzaPcs, substituted with push–pull peripheral substituents.