The effects of protonation of meso-tetraarylporphyrins bearing N,N-diethylamino groups and the resulting hyperporphyrins on fluorescence and ROS production.
Abstract
Hyperporphyrins formed by protonation of free-base porphyrins with electron-donating groups show strong absorption in the far-red region of the EM spectrum, which could be interesting for applications such as diagnostics and photodynamic therapy (PDT). Red wavelengths penetrate deepest into tissue and are used in PDT to excite photosensitizers, generating singlet oxygen and other reactive oxygen species (ROS) that destroy tumor cells. However, hyperporphyrins and their potential ROS production remain underexplored. Therefore, new porphyrins with varying numbers and arrangements of 4-(N,N-dimethylamino)phenyl meso-substituents, as well as previously known symmetric porphyrin (B4), were synthesized to investigate their behavior under conditions of varying acidity. The formation of hyperporphyrins upon titration with methanesulfonic acid (MSA) in dimethyl sulfoxide (DMSO) or a mixture of DMSO and dichloromethane was confirmed by 1H NMR and UV/vis spectroscopy. Singlet oxygen (1O2)/ROS production was evaluated by monitoring the photodegradation of 1,3-diphenylisobenzofuran during 10 minutes of red-light irradiation (675 nm) of the porphyrin without, and with varying amounts of added MSA; the results showed that the stronger the hyperporphyrin effect, the greater the suppression of production of 1O2 and other ROS. With a very similar trend, protonation of the porphyrins resulted first in a strong decrease in fluorescence, and then in a recovery of fluorescence in accordance with the reduction of the hyperporphyrin effect due to further protonation. The final emission spectra of fully protonated porphyrins showed different shifts compared to the nonprotonated porphyrin, depending on the structure. Two porphyrins with the weakest hyperporphyrin effect (trans-A2B2 and cis-A2B2) showed the highest 1O2/ROS production-upon complete protonation.
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