Water soluble porphyrin-fluorescein triads: Design, DFT calculation and pH-change-triggered fluorescence response

This paper reports the results of the synthesis and identification of water soluble trans-diaxial complexes of Sn(IV)-5,10,15,20-tetra-(4-sulfophenyl)porphyrin with fluorescein, in which the organic ligands are bound to the tin cation of the tetrapyrrolic macrocycle via a carboxyl (triad I) or a hydroxyl (triad II) group. The structures of triads (I, II) were confirmed by the methods of one- and two-dimensional NMR and quantum-chemical modeling. A spectrophotometric method was used to study the spectral, acidic and fluorescence properties of the synthesized compounds. The paper also discusses the possible mechanisms of protolytic and tautomeric equilibria in the processes involving fluorescein ligands in the axial complexes with hydrophilic Sn(IV)porphyrin. It is found that the fractional distribution of the triads with the lactone, quinoid, and zwitterionic forms of fluorescein axial ligands depends on the medium acidity, exhibiting high pH sensitivity in the ranges where this substance and its derivatives in the free state do not exhibit fluorescence. As a result, the ability of the porphyrin-fluorescein triads to determine water acidity can be used to develop an effective method of detecting pH-dependent biological processes and environmental pollution. This method will facilitate the production of new fluorescent sensors for biomedical purposes (drug delivery triggered by pH changes or oxygen distribution in tissues) and engineering applications (wastewater detection).