New fluorescent "off-on" behavior of 9-anthryl aromatic amides through controlling the twisted intramolecular charge transfer relaxation process by complexation with metal ions

New fluorophores based on linear polyether N,N '-[oxybis(3-oxapentamethylenoxy)-2-phenyl]bis(9-anthracene- carbonamide) (3) and its analogues (2 and 4) have been synthesized, and their complexation properties with Various alkali-metal and alkaline-earth-metal ions were investigated by fluorescence, UV, and H-1 NMR spectroscopies. in the absence of metal ion, 2-4 showed almost no fluorescence emission (fluorescence quantum yield Phi = 0.0003, fluorescence "off" State) since twisted intramolecular charge transfer (TICT) occurred through the amide bond. However, 2-4 demonstrated a significant increase in fluorescence intensity around 430 nm upon complexation with alkaline-earth-metal ions. In the case of 4.Ca2+, a large enhancement effect on the fluorescence quantum yield (Phi = 0.014, [Ca2+]/[4] = 5, Phi (2+)(4.Ca)/Phi (free4) = 42, fluorescence "on" state) was observed. This "off-on" fluorescence characteristic was originated from the cooperative strong binding mode between the carbonyl group and the ethylenoxy moiety for alkaline-earth-metal ions, resulting in effective inhibition of photoinduced TICT relaxation. Fluorophores 2-4 formed a 1:1 complex (the order of the complex formation constants was Ca2+ > Sr2+ approximate to Ba2+) and showed no considerable spectral changes upon complexation with alkali-metal ions and Mg2+. The H-1 NMR study on 2-4 and their complexes indicated that free 2-4 formed a helical structure. After complexation, the conformational change of 2 from a helical structure to a semicircular structure was observed. The pseudocyclic form was supported as the complex structure of 3, whereas the large conformational change of 4 was not observed after the addition of metal ions.