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-oxapentamethylen oxy)-2-phenyl]bis(9-anthracene- carbonamide) (3) and its analogues (2 and 4 ) have been synthesized, and their complexation properties with Various alk ali-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 alm ost no fluorescence emission (fluorescence quantum yield Phi = 0.0003, fluo rescence "off" State) since twisted intramolecular charge transfer (TICT) o ccurred through the amide bond. However, 2-4 demonstrated a significant inc rease in fluorescence intensity around 430 nm upon complexation with alkali ne-earth-metal ions. In the case of 4.Ca2+, a large enhancement effect on t he fluorescence quantum yield (Phi = 0.014, [Ca2+]/[4] = 5, Phi (2+)(4.Ca)/ Phi (free4) = 42, fluorescence "on" state) was observed. This "off-on" fluo rescence characteristic was originated from the cooperative strong binding mode between the carbonyl group and the ethylenoxy moiety for alkaline-eart h-metal ions, resulting in effective inhibition of photoinduced TICT relaxa tion. Fluorophores 2-4 formed a 1:1 complex (the order of the complex forma tion constants was Ca2+ > Sr2+ approximate to Ba2+) and showed no considera ble 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 pseudocyc lic form was supported as the complex structure of 3, whereas the large con formational change of 4 was not observed after the addition of metal ions.