Synthesis and Characterization of Water-Soluble Phenylene−Vinylene-Based Singlet Oxygen Sensitizers for Two-Photon Excitation

The synthesis and characterization of water-soluble singlet oxygen sensitizers with a phenylene−vinylene motif is presented. The principal motivation for this study was to better understand specific
features of a water-soluble molecule that influence the photosensitized production of singlet oxygen
upon nonlinear, two-photon excitation of that molecule. To achieve water solubility, sensitizers
were synthesized with ionic as well as nonionic substituents. In the ionic approach, salts of
N-methylated pyridine, benzothiazole, and 1-methyl-piperazine moieties were used, as were aryl-substituted sulfonic acid moieties. In the nonionic approach, aryl-substituted triethylene glycol
moieties were used. Selected photophysical properties of the compounds synthesized were
determined, including singlet oxygen quantum yields. Of the molecules examined, the most efficient
singlet oxygen sensitizers had triethylene glycol units as the functional group that imparted water
solubility. Molecules containing the ionic moieties did not make singlet oxygen in appreciable yield
nor did they efficiently fluoresce. Rather, for these latter molecules, rapid charge-transfer-mediated
nonradiative processes appear to dominate excited state deactivation.