New type of tin(iv) complex based turn-on fluorescent chemosensor for fluoride ion recognition: elucidating the effect of molecular structure on sensing property

Abstract

A novel type of chemosensor based on tin(IV) complexes incorporating hydroxyquinoline derivatives has been designed and investigated for selectively detecting fluoride ions. Sn(meq)₂Cl₂ (meq = 2-methyl-8-quinolinol) (complex 1) exhibits a significant enhancement in luminescence upon the introduction of fluoride ions. This enhancement greatly surpasses that observed with Snq₂Cl₂ and Sn(dmqo)₂Cl₂ (q = 8-hydroxyquinnoline; dmqo = 5,7-dimethyl-8-quinolinol). Furthermore, complex 1 displays excellent sensitivity and selectivity for fluoride detection in comparison to halides and other anions. As a result, complex 1 serves as an outstanding turn-on fluorescent chemosensor, effectively sensing fluoride ions. The Benesi–Hilderbrand method and Job's plot confirmed that complex 1 associates with F⁻ in a 1 : 2 binding stoichiometry. Also, complex 1 exhibited a large binding constant (pK_b = 10.4 M⁻²) and a low detection limit (100 nM). To gain a deeper insight into the photophysical properties and the underlying mechanism governing the formation of the tin(IV) fluoride complex via halide exchange, we successfully synthesized partially fluorinated Sn(meq)₂F_0.67Cl_1.33 (2) and fully fluorinated Sn(meq)₂F₂ (3), all of which were characterized through computational studies, thereby elucidating their photophysical properties. DFT studies reveal that converting Sn(meq)₂Cl₂ to Sn(meq)₂F₂, an endergonic process, leads to greater stability due to reducing steric hindrance about the metal center. Furthermore, the fluorinated complex significantly increases dipole moment, resulting in high affinity toward the F⁻ ion.