9, 10-Bis(8-Quinolinoxymethyl)Anthracene--A Fluorescent Sensor for Nanomolar Detection of Cu(2+) with Unusual Acid Stability of Cu(2+)-Complex.

In this work, the dipod 9,10-bis(8-quinolinoxymethyl)anthracene (1) and for comparison, monopod 9-(8-quinolinoxymethyl)anthracene (2) have been synthesized. The fluoroionophore 1 in pH 7.1 HEPES buffered CH3CN:H2O (4:1 v/v) solution shows quenching only with Cu(2+) with lowest limit of detection 150 nM, amongst various metal ions. Fluoroionophore 1 could also be applied to sense Co(2+) with lowest limit of detection 600 nM. By modulating the pH of the solution and concentration of Cu(2+), 1 shows respective "On-Off-On" and "On-Off" fluorescent switching. The self-assembly of two Cu(2+) ions and two molecules of fluoroionophore 1 to form closed structure [Cu2(L)2](4+) seems to be responsible for nanomolar sensitivity towards Cu(2+). The combination of delayed second protonation of 1 (pK(a2) = 2.6) and stepwise protonation of [Cu2 L2](4+) causes unusual stability of [Cu(LH)2](4+) even at pH < 2.