Exploring Broad Molecular Derivatization as Tool in Selective Fluorescent Detection of Mercury(II) by a Series of Large Stokes Shift 1,4-Bis(5-phenyl-1 H-imidazol-4-yl)benzenes

Abstract

Synthetic design of new functional molecules remains the main route to obtaining the next generation of sensor materials, and the undesirable consequences of environmental heavy metal burdens call for research undertakings toward discovery of highly sensitive and selective molecular sensors. In this work, 11 new 1,4-bis(5-phenyl-1H-imidazol-4-yl)benzene fluorophores 1-11, which were designed to have substituent electronic variations as well as diversity of π-conjugation, were synthesized, characterized, and experimentally investigated for their mercury(II) sensing potentials. Attractive sensitivity and selectivity results with more than 99% fluorescent turn-off efficiencies wererecorded for ligands 3, 4, 5, 10, and 11 at F/Fo values of 140-fold for 3, 174-fold for 4, 116-fold for 5, >302-fold for 10, and 423-fold for 11. Applicable mercury(II) detection profiles devoid of competitions or interferences were found for ligands 3, 7, and 10. The obtained sigmoidal-shaped Job plots suggest that the sensing mechanism by the fluorophores depends on more than one elementary reaction process, while results from mercury titration as well as X-ray structural data indicate an overall stoichiometry of 1:2 for the ligand to Hg2+ interactions. Structural data analyses also revealed that the mechanism of fluorescence turn-off is traceable to aromatic ring twists along the central imidazole-phenyl-imidazole moieties and the consequent loss of coplanarity, which interrupts the core π-conjugation of the sensors. This experience shows that broad synthetic derivatization is a recommendable strategy for increasing access to appropriately tuned sensor molecules, which are capable of overcoming the often-encountered interferences.