ESIPT-inspired fluorescent turn-on sensitivity towards aluminium(III) detection by derivatives of O- and S-bridged bis-(phenol-imine) molecules

Abstract

The need to explore possible benefits of molecular substituent derivatization on fluorescent chemosensor properties motivated the syntheses of six potentially pentadentate bis-(phenol-imine) fluorophores, which consist of five S-bridged O^N^S^N^O scaffolds 1–5 and an O-bridged O^N^O^N^O scaffold 6. The deliberate incorporation of multiple intramolecular N…HO donor–acceptor functions into the molecules helped to achieve Excited State Intramolecular Proton-electron Transfer (ESIPT) capability as well as the possession of low quantum yields in their ligand-only solutions. Interestingly, while the low quantum yield molecules 1, 2, 3 and 6 lacked the expected large Stokes shifted ESIPT fluorescence in their ligand-only solutions, they utilize the same ESIPT-oriented process to achieve varying extents of substituent-dependent fluorescent turn-on that produced sensitive and selective Al3+ detection. In particular, while molecules 1 and 3 gave dual-band fluorescence turn-on from both primary and secondary photoexcited states, the O-bridged ligand 6 is strongly turned on only at the large Stokes shifted ESIPT emission wavelength. Thus, with a 528-fold F/Fo turn-on ratio and a detection limit of 5.48 × 10−9 M, the efficiency of molecule 6 as chemosensor towards Al3+ detection can be considered to be outstanding. Based on results of Job plot experiment, fluorescent intensities during Al3+ titration, single crystal structural data and 1H NMR spectral analyses, we conclude that a 1:1 sensor-to-analyte stoichiometry is at play in the Al3+ sensing mechanism and that the sensor molecules form an unusual bidentate O^O coordination with the analyte as neutral iminium zwitterion. It could be concluded that synthetic designs leading to systematic substituent variation is a beneficial tool for tuning and isolating applicable molecular chemosensor species.