Metal ion fluorescent sensors from fluorene derivatives

Abstract

Four novel salicylyl derivatives of fluorene are successfully synthesized by palladium-catalyzed coupling reactions. The compounds are characterized by ¹H-,¹³C-NMR, MALDI-TOF MS, UV-Vis, and fluorescence spectroscopy. Photophysical investigation of these water soluble compounds in pH 8.0 phosphate buffer show maximum absorption wavelengths in the range of 304-330 nm. For compounds with 9,9-dioctylfluorene group (F1-F2), the emission maxima appear around 420 and 430 nm with the quantum yields of 2.7 and 4.4%, respectively. However, the compounds with 9-fluorenone (F3) and 9-dicyanomthylenfluorene group (F4) are non-emissive due to the combination of the internal charge transfer caused by the salicylate group and the photo-induced electron transfer from the non-bonding electron of the carbonyl group and the pi-electron of the fluorene. Without the surfactant, the solution of compound with the alkynyl salicylate group (F2) exhibits a selective fluorescence quenching by Cu²⁺ and Fe²⁺. The selectivity towards Cu²⁺ is enhanced by the addition of Triton X-100 at 30 µM, the concentration in which lower than the critical micelle concentration. The optimal efficiency for detection of Cu²⁺ in solution phase can be calculated to 1.47 ppb. Preliminary tests of paper-based suggest that a 50 µM solution of F2 in THF can be used as a naked- eye sensor for Cu²⁺ and Fe²⁺ ions of 40 picomole.