COMPLEXES OF 8-HYDROXYQUINOLINE DERIVATIVES AS TURN-ON FLUORESCENT CHEMOSENSOR

Abstract

Nowadays, fluorescent chemosensors play an important role in detection method in chemical, biological, and environmental fields, including detection of metal ions. In this work, turn-on fluorescent sensors for metal ions are developed from 8-hydroxyquinoline (8HQ). Three new 8-hydroxyquinoline derivatives (Q1, Q2 and Q3) are synthesized. For Q1, 8HO is O-substituted with acetate ester of diethylene glycol. The O-substitution of 8HQ with diethylene glycol followed by the extension of the pi-conjugation at 5-position with 4-ethynyl N,N-dimethyl aniline moiety gives Q2 and the acetylation of Q2 gives Q3. In CH3CN, the absorption spectra of 8HQ and Q1 are similar showing two absorption maxima around 240 and 300 nm, while those of Q2 and Q3 are around 300 and 370 nm. The emission maximum of 8HQ and Q1 is observed at 400 nm whereas that of Q2 and Q3 is red-shifted to 560 nm. In polar aprotic solvent, the fluorescence of Q2 and Q3 are visible to naked-eye under black light that become invisible in protic solvents such as CH3OH and H2O. The fluorescence quenching in protic solvents is probably associated with the hydrogen bonding between the solvents and fluorophores. Therefore, 8HQ and its derivatives are investigated as turn-on fluorescent sensors for metal ions in mixed solvents. In CH3CN/H2O (90/10 v/v), only Q1 shows selective turn-on fluorescence with trivalent ions such as Al3+ Cr3+ and Fe3+, while 8HQ, Q2 and Q3 show non-selective and low fluorescence response to metal ions. In CH3OH/H2O (30/70 v/v), 8HQ show known green fluorescence enhancement with Al3+ while Q3 shows interestingly strong green fluorescence (510 nm) enhancement selectively with Hg2+. The detection limit of Hg2+by Q3 is 64 nM or 13 ppb. The Tyndall effect observed along with the increase of fluorescence intensity of Q3 upon the addition of Hg2+ suggests that the fluorescence enhancement of Q3 with Hg2+ is due to the aggregation induced emission (AIE). The AIE of Q3 is also observed without Hg2+ at higher fraction of H2O (80-90%).