Oligomerization of photochemically oxidized phenothiazine by nanostructured -Fe₂O₃

Phenothiazines (PTZs) and their derivatives are a group of compounds that have diverse applications in medicine, the dye industry and advanced materials. Due to the electron-hole donating characteristic, these compounds have also been applied in batteries and solar energy devices. PTZs can be converted into stable radical cations by chemical and photochemical reactions. The PTZ radical cations are reactive for polymerization. The photochemical formation of PTZ radical cations is favored in aggregates by the Type I mechanism in which one molecule in the triplet excited state abstracts one electron from a neighbor molecule. In the present study, 10H-phenothiazine (PHT) was dissolved in chloroform and converted to the radical cation species (PHT center dot +) by UV light irradiation. The radical cation was characterized by UV-visible absorption and electron paramagnetic resonance techniques. Addition of alpha-Fe2O3 nanorods and nanowires to PHT & BULL;+ chloroform solution promoted its color change from orange to blue-green. Absorption measurement revealed a significant decrease of the UV and visible bands assigned to radical cation species and the appearance of a broad band at the spectral region of 650 nm. The blue-green solution was dried, leading to the formation of crystals that were characterized using the X-ray diffraction technique. Considering possible PHT oligo(poly)merization, the crystals were analyzed using matrix-assisted laser desorption ionization time-of-flight mass spectrometry that corroborated the formation of oligomers containing three, four and five PHT residues and their sulfoxide derivatives. The PHT oligomers retained photosensitivity and exhibited an electron paramagnetic resonance signal indicating possible applications as hole donors in electronic devices. Oligomerization was also obtained by aggregation of PHT & BULL;+ in sodium dodecylsulfate micelles indicating that proximity and orientation of the free radical molecules is the mechanism for the oligomerization. Therefore, a new route for PHT oligomerization catalyzed by alpha-Fe2O3 nanowires is proposed. The method is readily available, inexpensive and environmentally safe. (c) 2022 Society of Industrial Chemistry.