Synthesis of mono-, di-, tri- and tetracarboxy azaphthalocyanines as potential dark quenchers

Mono-, di-, tri- and tetracarboxy-substituted metal-free azaphthalocyanines (AzaPc) were synthesized from 5,6-bis(diethylamino)pyrazine-2,3-dicarbonitrile and 6-(5,6-dicyano-3-(diethylamino)pyrazin-2-ylamino)hexanoic acid using a statistical condensation approach. AzaPc bearing eight diethylamino peripheral substituents was also isolated from the mixture. Analysis of the distribution of congeners in the statistical mixture using optimized HPLC method (Phenomenex Synergy RP Fusion column, acetonitrile/tetrahydrofuran/water (pH 5.5) 50:20:30) was performed. The analysis showed optimal ratios of starting materials to be 3:1 for AAAB, 1:3 for ABBB and 1:1 for AABB/ABAB types of the congeners. The distribution of the congeners corresponded well with calculated values indicating similar reactivity of both starting materials and no sterical constraint between adjacent isoindole units in the AzaPc ring. All investigated AzaPc showed no fluorescence, extremely low singlet oxygen quantum yields (Φ∆ < 0.005) in monomeric form and strong absorption in a wide range from 300 nm to almost 700 nm. Such properties are highly promising for future investigation of these compounds as dark quenchers of fluorescence in DNA hybridization probes.

► Metal-free azaphthalocyanines with one to four peripheral carboxy substituents.
► Ratio of congeners from statistical condensation optimized by HPLC.
► Compounds exerted no fluorescence and very low singlet oxygen quantum yields.
► Strong absorption in a wide range from 300 nm to almost 700 nm for each compound.
► Properties of azaphthalocyanines are promising for dark quenchers of fluorescence.