Investigation of protonation effects on the electronic and structural properties of halogenated sulfonated porphyrins

• The presence of halogen atoms reduces the flexibility of the porphyrinic ring.
• The protonation was observed only in extremely acid environment.
• Chlorinated atoms cause a stronger effect on the fluorescence than fluorinated atoms.
• Protonation reduces the fluorescence lifetime of sulfonated porphyrins.
• Protonation causes an enhancement of the absorbance in the therapeutic window.

The present study aimed at evaluating pH effects on the electronic and structural properties of halogenated sulfonated porphyrins. We used UV–Vis absorption, fluorescence emission, and NMR spectroscopies, combined with quantum chemical calculations using Density Functional Theory. We observed that the presence of one or two halogen atoms (chlorine or fluorine) at ortho-position of phenyl ring reduces the flexibility of the porphyrin structures, making their protonation harder. Since the protonation of anionic porphyrins provides a convenient way for their aggregation, the protonation observed only in extremely acid environment is indicative of the low probability for the aggregation of these porphyrins, which is an important characteristic of an efficient photosensitizer. Furthermore, the enhancement in the absorbance (9–13.5 times) in the phototherapeutic window suggests that the protonation of halogenated sulfonated porphyrins could be a possible strategy for a higher photodynamic efficiency.