Dual role of arsenite in hydrolysis and post-hydrolysis fluorescence sensing of selective pH-dependent probes

Abstract

In comparison to the sensing activity, the reactivity of arsenite (AsO₂⁻) is less explored. Herein, we focused on AsO₂⁻ reactivity studies based on its pK_a and compared the study with other common anions. All three pK_a values of arsenite are >9.0, affording a flexible working pH range to design a probe for reactivity studies. We designed and synthesized six pH dependent benzothiazole-based Schiff bases, namely, 1-(((6-nitrobenzo[d]thiazol-2-yl)imino)methyl)naphthalen-2-ol (1), 5-(diethylamino)-2-(((6-nitrobenzo[d]thiazol-2-yl)imino)methyl)phenol (2), 9-(((6-nitrobenzo[d]thiazol-2-yl)imino)methyl)-2,3,6,7-tetrahydro-1H,5H-pyrido[3,2,1-ij]quinolin-8-ol (3), 5-methoxy-2-(((6-nitrobenzo[d]thiazol-2-yl)imino)methyl)phenol (4), 4-(((6-nitrobenzo[d]thiazol-2-yl)imino)methyl)benzene-1,3-diol (5), and 2-(((6-nitrobenzo[d]thiazol-2-yl)imino)methyl)phenol (6), as probes for hydrolysis studies containing 5% water in acetonitrile. In spite of the presence of water in the solution, no hydrolysis was observed for all the probes in the absence of a salt. In the presence of selected sodium salts of various anions in solution, intramolecular charge transfer (ICT) was observed after the deprotonation of an aromatic hydroxy group at the ortho position with respect to the imine groups within the probes. Among the studied anions, selective AsO₂⁻ induced imine hydrolysis was observed for probes 1 and 4. In the case of 5 with both o- and p-hydroxy groups, no hydrolysis was observed in the presence of AsO₂⁻. Probe 6 with only the o-hydroxy group showed very fast hydrolysis with poor selectivity. The p-hydroxy group in 4-(((6-nitrobenzo[d]thiazol-2-yl)imino)methyl)phenol (7) resulted in poor AsO₂⁻ induced hydrolysis. The aldehyde, which was generated after hydrolysis of probe 1, showed selective emission at 450 nm in the presence of AsO₂⁻. The time dependent hydrolysis reaction of probe 1 controls the emission intensity enhancement.