In aqueous and alcohol solutions, colorless and non-fluorescent 9-aryl-9H-xanthen-9-ols equilibrate with brightly colored and fluorescent 9-arylxanthylium cations. The position of this equilibrium depends only on hydronium ion concentration and is not affected by general acids or other ions. The pH of the equivalence point can be adjusted by substitution in the xanthenol core. 3,6-dialkoxy-9-(o-tolyl)-9-xanthenol undergoes dehydroxylation to form corresponding cation at biologically relevant pH = 6.5. 9-aryl-9H-xanthen-9-ol – based pH indicators permit following acidity changes by both fluorescent and absorption spectroscopies.
Non-fluorescent 3,6-bisbenzyloxy-9-aryl-9-xanthenol undergoes photo-dehydroxylation with simultaneous photo-Claisen rearrangement upon irradiation in aqueous methanol to give fluorescent isomeric products. Alcohol substrates are simply caged by refluxing the parent 3,6-bisbenzyloxy-9-aryl-9-xanthenol with the desired alcohol in acetonitrile and catalytic amounts of acetic acid or using the alcohol as neat solvent for less expensive substrates. Formation of fluorescent fluorescein derivatives can be used to monitor substrate release.
Photolabile protecting groups can also be used to weaken or totally eradicate the fluorescence of a fluorophore. Irradiation of the caged dye removes the protection thereby revealing the fluorophore. We have effectively used 3-(hydroxymethyl)napthalen-2-ol as a photoremovable protecting group for fluorescein. The caged fluorophore is weakly fluorescent. Photolysis of the caged dye releases the fluorophore with a fluorescent quantum yield FF= 0.75 -0.93.
We have also developed a strategy for photochemical activation of anomeric position using tetrahydropyran as a model for glycosides. Irradiation of a glycosyl donor bearing a suitably designed 9-aryl-9-fluorenol leaving group generates a carbocation intermediate which is trapped intramolecularly. This reaction works well in highly polar weakly nucleophilic solvents like 1,1,1,3,3,3-hexafluoropropan-2-ol and trifluoroethanol.
INDEX WORDS: fluorescence, pH probe, fluorogenic, photoactivatable fluorophore, photo-SN1, Photo-Claisen rearrangement, photodehydroxylation, photolabile, quinone methide.