9-Aryl-9-xanthenols: a convenient platform for the design of fluorimetric and colorimetric pH indicators

Abstract

In aqueous and alcohol solutions, colorless and non-fluorescent derivatives of 9-aryl-9H-xanthen-9-ol equilibrate with brightly colored and fluorescent 9-arylxanthylium cations, thus offering a convenient platform for the design of dual-mode indicators for emission and absorption-based pH measurements. The position of the prototropic equilibrium depends only on the hydronium ion concentration and is not affected by general acids or other ions. Furthermore, the equilibrium equivalence point can be readily adjusted by introducing substituents in the xanthenol core. As dehydroxylation of 3,6-dialkoxy-9-(o-tolyl)-9-xanthenol occurs at pH = 6.5, indicators of this type are well suited for biological applications as illustrated by in vitro cell culture studies with NIH 3T3 cells.

Design Strategies for Water-Soluble Small Molecular Chromogenic and Fluorogenic Probes

“…In a recent study, 9-aryl-9H-xanthen-9-ol has been reported as an interesting platform for the development of fluorescent pH probes 5−7. 45 These probes offer a number of advantages, such as high selectivity of spectroscopic response to pH, and facile adjustment of pK a by introducing appropriate substituents (Figure 3). Furthermore, the conversion of neutral leuco-forms into xanthylium ions yields more than 1300-fold fluorescence enhancement.…”

“…In a recent study, 9-aryl-9 H -xanthen-9-ol has been reported as an interesting platform for the development of fluorescent pH probes 5 – 7 . These probes offer a number of advantages, such as high selectivity of spectroscopic response to pH, and facile adjustment of p K a by introducing appropriate substituents (Figure ).…”

Synthesis and Supramolecular Functional Assemblies of Ratiometric pH Probes

“…[65] The hydronium-ion-mediated reversible dehydroxylation of 9-aryl-9H-xanthen-9-ol has been reported as an efficient fluorimetric and colorimetricp Hi ndicator. [66] The pH-sensitive piperazine switches, operating by PET,h ave allowed the imaging of vesicular dynamics in living cells, with good intra-vesicular acidic activation,h igh staining specificity and good photostability. [67] Recently,i th as been reported that substitution of the oxygen atom of the xanthene probes by as ilicon or phosphorous atom strongly shifted the fluorescencee missiono ft hese ratiometric probes to the red.…”