Aromatic poly(pyridinium salt)s. Part 3. Photoreduction in amide solvents

UV–Vis irradiation of bis(pyridinium salt) model compounds and poly(pyridinium salt)s in amide solvents result in dramatic changes in their UV–Vis spectra. The absorption maximum near 342 nm, characteristic of pyridinium rings, disappears, while an absorption maximum near 500 nm appears and then disappears. During this process, the solutions change from yellow to dark red to light yellow. The inherent viscosities of the poly(pyridinium salt)s also decrease dramatically under UV–Vis irradiation. Both the color changes and the viscosity decreases are unaffected by the nature of the pyridinium salt counter anion. However, they are dramatically affected by the presence of pendent substituents. Electron-withdrawing groups increase the rate at which these changes take place, while electron-donating groups retard them. It is speculated that the photodecomposition proceeds through the initial formation of radical cations. It is also speculated that the amide groups may serve as reducing agents providing electrons in the electron transfer process. Evidence for this comes from the fact that a poly(pyridinium salt) containing pendent amide groups underwent photoreduction in DMSO in marked contrast to other poly(pyridinium salt)s that are photostable in this solvent.