A new fluorene derived Schiff-base as a dual selective fluorescent probe for Cu2+ and CN.

Abstract

A new fluorene based fluorogenic chemosensor, 2-[(9H-Fluoren-2-ylmethylene)-amino]-phenol (L), has been designed, synthesized, and characterized by CHN analyses and different spectroscopic methods. This turn-on fluorogenic chemosensor shows high selectivity and sensitivity toward Cu2+ and CN- with low detection limits of 1.54 × 10-9 M and 1.83 × 10-7 M, respectively. The stoichiometry ratio of L-Cu2+ in solution is 1:1, by the method of Job's plot and ESI-MS. The microcrystalline solid product of the chemosensor reaction with copper is characterized as CuL2. The χT value for CuL2 is temperature independent at a value of 0.403 cm3 K mol-1, which is in agreement with a mononuclear copper(II) complex with an isotropic g-value of 2.075. The fluorescence turn-on recognition process for detection of Cu2+ is attributed to the restricted imine isomerization and blocking of intramolecular charge transfer (ICT) quenching process in the analyte-bound sensor. The selectivity of L for Cu2+ is based on the chelation-enhanced fluorescence effect (CHEF) mechanism. Other interfering ions such as Na+, K+, Ca2+, Mg2+, Ag+, Fe2+, Fe3+, Co2+, Ni2+, Zn2+, Cd2+, Hg2+, Mn2+, Pb2+ and Al3+, show no change in the fluorescence intensity of L in the presence of Cu2+. Furthermore, the compound L can be used as a fluorescence and colorimetric sensor for selective detection of CN- over a number of other anions based on the nucleophilic addition to the imine CN bond, with consequent hydrogen bond formation and electrostatic interaction of the resulting product with K+. The sensing mechanism for CN- was theoretically supported by DFT calculations.