New fluorophores based on linear polyether N,N′-[oxybis(3-oxapentamethylenoxy)-2-phenyl]bis(9-anthracenecarbonamide) (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 1H NMR spectroscopies. In the absence of metal ion, 2-4 showed almost no fluorescence emission (fluorescence quantum yield φ = 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 (φ = 0.014, [Ca2+]/ = 5, φ4-ca2+/φfree4 -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+ ≈ Ba2+) and showed no considerable spectral changes upon complexation with alkali-metal ions and Mg2+. The 1H 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.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Surfaces, Coatings and Films
- Materials Chemistry