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

Magnesium(ii) tetrapyrazinoporphyrazines (TPyzPzs) are excellent red fluorophores (λF ∼ 663 nm, ΦF ∼ 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 pKa 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.