Glycosylated Porphyrins: Characterization of Association in Aqueous Solutions by Absorption and Fluorescence Spectroscopies and Determination of Singlet Oxygen Yield in Organic Media

Abstract

The behavior of a series of porphyrins substituted at meso positions by para-phenyl-β-d-glucose groups is examined in aqueous solutions by absorbance and fluorescence spectroscopies. This series includes tetra-, tri- and diglycosylated compounds, the last being substituted at adjacent or opposite meso positions. The other subsituents are alkyl chains and, in one case, a fluorophenyl group. Although all these compounds dimerize in aqueous solutions, the dimerization constants span over four orders of magnitiude (Kd= 1.3 × 105–1.9 × 109 M−1). The dimer stability is strongly dependent on the nature and the arrangement of the substituents. The diglycosylated derivative substituted at adjacent meso positions yields the most stable dimer that is likely to present an offset stacked geometry. The dimer structures expected from the symmetry of the various compounds correlated well with the optical spectra that display exciton splitting in the Soret band. The fluorescence of the dimers is quenched in all derivatives with the noticeable exception of the adjacent diglycosylated derivative. All the compounds studied photosensitize the formation of singlet oxygen with yields ranging between 0.54 and 0.81. A fairly linear correlation between the yield and the number of aryl groups at meso positions is observed.