Magnesium tetrapyrazinoporphyrazines: tuning of the pKa of red-fluorescent pH indicators

Abstract

Magnesium(II) tetrapyrazinoporphyrazines (TPyzPzs) are excellent red fluorophores (λ_F ∼ 663 nm, Φ_F ∼ 0.53 in THF). In this work, a series of magnesium(II) complexes of unsymmetrical TPyzPzs bearing one or two phenol substituents was prepared. Suitable substitutions on the phenolic moiety tuned its pK_a in the range of 5.5 to 13. Deprotonation of the phenolic group at higher pH induced a strong donor (phenolate) in the macrocycle that led to pH-dependent quenching of the red fluorescence of these indicators. pH sensing was proved in water solutions after the incorporation of TPyzPzs into two delivery systems – microemulsions and liposomes. The latter also serves as a simple model of biomembranes. Finally, a wavelength-ratiometric probe was constructed by the incorporation of a TPyzPz indicator and lipophilic pH-nonsensitive BODIPY dye into liposomes. Synthetic precursors for TPyzPzs, substituted pyrazine-2,3-dicarbonitriles, also represent donor–acceptor systems and the pH-dependent changes in absorption spectra may be easily visible to the naked eye.