Synthesis and Characterization of Ru(II) Tris(1,10-phenanthroline)-Electron Acceptor Dyads Incorporating the 4-Benzoyl-N-methylpyridinium Cation or N-Benzyl-N‘-methyl Viologen. Improving the Dynamic Range, Sensitivity, and Response Time of Sol−Gel-Based Optical Oxygen Sensors

Abstract

The title compounds were synthesized by Sonogashira coupling reactions of appropriate Ru(II) complexes with the electron acceptors. Characterization was conducted in solution and in frozen matrixes. Finally, the title compounds were evaluated as dopants of sol−gel materials. It was found that the intramolecular quenching efficiency of 4-benzoyl-N-methylpyridinium cation in solution depends on the solvent: photoluminescence is quenched completely in CH3CN, but not in methanol or ethanol. On the other hand, intramolecular emission quenching by 4-benzyl-N-methyl viologen is complete in all solvents. The difference between the two quenchers is traced electrochemically to the solvation of the 4-benzoyl-N-methylpyridiniums by alcohol. In frozen matrixes or adsorbed on the surfaces of silica aerogel, both Ru(II) complex/electron acceptor dyads of this study are photoluminescent, and the absence of quenching has been traced to the environmental rigidity. When doped aerogels are cooled at 77 K, the emission intensity increases by ∼4×, and the spectra shift to the blue, analogous to what is observed with Ru(II) complexes in solutions undergoing fluid-to-rigid transition. However, in contrast to frozen solutions, the luminescent moieties in the bulk of aerogels kept at low temperatures are still accessible to gas-phase quenchers diffusing through the mesopores, leading to more sensitive platforms for sensors than other room-temperature configurations. Thus, the photoluminescence of our Ru(II) complex dyads adsorbed on aerogel is quenchable by O2 both at room temperature and at 77 K. Furthermore, it was also found that O2 modulates the photoluminescence of aerogels doped with 4-benzoyl-N-methylpyridinium -based dyads over a wider dynamic range compared with aerogels doped with either our viologen-based dyads or with Ru(II) tris(1,10-phenanthroline) itself.