Design-specific mechanistic regulation of the sensing phenomena of two Schiff bases towards Al3+

Abstract

We report herein two optical probes (R1 and R2) for the fluorogenic detection of Al³⁺ at the level of 10⁻⁸ M. R1 and R2 were synthesized by simple Schiff base condensation of 4-amino-3-hydroxy-1-naphthalene sulfonic acid with 5-bromosalicaldehyde and 2-hydroxy-1-naphthaldehyde, respectively. The same were characterized by various spectroscopic techniques. R1 and R2 both underwent fluorescence emission upon their respective interactions with Al³⁺ in an ethanol : water mixture (4 : 1, v/v). The binding modes of the receptors with Al³⁺ were studied through ¹H NMR spectroscopy, Job plots, and HR-MS, as well as through binding constant determination involving fluorescence titration data. The quenching of –CN isomerization and of photoinduced electron transfer (PET) seem to be responsible for the fluorogenic switch-on situation of R1 and R2 with Al³⁺. At the same time, excited state intramolecular proton transfer (ESIPT) also plays an important role in the ratiometric fluorescence response of R2, which is a consequence of a minor structural variation in R1 where the bromophenyl moiety is replaced with a naphthalene moiety. The mechanistic aspects of the sensing phenomenon are discussed in terms of ¹H NMR titration as well as theoretical calculations at the density functional level.