Oxazinoindolines as fluorescent H+ turn-on chromoionophores for optical and electrochemical ion sensors

Abstract

We propose here a new class of fluorescent H+ turn-on oxazinoindoline (Ox) dyes as chromoionophores suitable for the development of polymeric ion-selective membranes. Through rational design, oxazinoindolines with varying basicities and tunable absorption bands that extend to the near-infrared region were prepared. The basicities of the compounds were evaluated by potentiometry and fluorescence spectroscopy. By comparing the potentiometric lower detection limit with chromoionophore I (pKa = 14.80 ± 0.03 in PVC-NPOE), pKa values were determined for Ox Y, Ox R, and Ox B to be 9.80 ± 0.03, 12.85 ± 0.03, and 12.95 ± 0.03, respectively. Unlike conventional pH indicators that involve only a protonated and deprotonated form, a third species, arising from the thermal isomerization of the Ox compounds in polar solvents, may influence the overall protonation equilibrium. Interestingly, this results in an apparent pK a increase in the fluorescence mode when more polar PVC-NPOE membranes were utilized. The isomerization is suppressed in nonpolar PVC-DOS membranes. Na+-selective optical sensors were successfully prepared using the Ox dyes as an early application of the compounds in ion-selective optical sensors. The compounds were also successfully evaluated in a dynamic electrochemistry (chronopotentiometry) sensing mode for the detection of total alkalinity. © 2013 American Chemical Society.