Fluoride-induced intermolecular excimer formation of bispyrenyl thioureas linked by polyethylene glycol chains

New bispyrenyl thioureas linked by polyethylene glycol (PEG) chains, L1-L3, and methoxy benzene pyrene thiourea, L4, were synthesized. Upon binding with F− in CHCl3, L1-L3 exhibited strong excimer emission bands (IE) and weak monomer emission bands (IM), while L4 displayed the same intensity of both bands. However, little or no change was observed in fluorescence spectra of L1 upon adding OH−, AcO−, BzO−, H2PO4−, Cl−, Br−, and I−. Therefore, only F− induced the pyrene excimer formation. Job's plots showed 1/1 or 2/2 complexation of L1 with F−. Ratios of IE/IM of L1·F− complex were dependent on the concentration of L1, implying that the dimerization of L1 proceeded via the intermolecular excimer formation. Among L1-L4, L1 possessed the highest binding constant and sensitivity toward F− implying the importance of the linking PEG chain. L1 was demonstrated to be an excellent probe for F− in CHCl3 with the detection limit as low as 46.2 μg/L.