A Photoprotective Effect by Cation‐π‐Interaction? Quenching of Singlet Oxygen by an Indole Cation‐π Model System

Abstract

We investigated the effect of the cation‐π interaction on the susceptibility of a tryptophan model system toward interaction with singlet oxygen, that is, type II photooxidation. The model system consists of two indole units linked to a lariat crown ether to measure the total rate of removal of singlet oxygen by the indole units in the presence of sodium cations (i.e. indole units subject to a cation‐π interaction) and in the absence of this interaction. We found that the cation‐π interaction significantly decreases the total rate of removal of singlet oxygen (kT) for the model system, that is, (kT = 2.4 ± 0.2) × 108 m−1 s−1 without sodium cation vs (kT = 6.9 ± 0.9) × 107 m−1 s−1 upon complexation of sodium cation to the crown ether. Furthermore, we found that the indole moieties undergo type I photooxidation processes with triplet excited methylene blue; this effect is also inhibited by the cation‐π interaction. The chemical rate of reaction of the indole groups with singlet oxygen is also slower upon complexation of sodium cation in our model system, although we were unable to obtain an exact ratio due to differences of the chemical reaction rates of the two indole moieties.

Singlet Oxygen Quenching by Resveratrol Derivatives†

“…Similar nonlinear plots were observed in our recent work studying the photooxidation of tryptophan with and without cation‐π interaction when MB was used as the photosensitizer. We hypothesize that this results from electron transfer from the quencher to the excited cationic MB photosensitizer, that is, a type I photooxidation process (55). In our previous work, we had been studying a tryptophan model complex.…”