A novel multimode sensor showing cation-dependent fluorescence colour

A novel sensor, 4-[2-(9-anthryl)ethynyl]-1,10-phenanthroline (1), exhibits highly intense fluorescent in the wavelength region of 440-600 nm (maximum wavelength (λ_f) = 470 nm) with the quantum yield (Φ_f) and lifetime (τ_f) being 0.90 and 4.2 ns, respectively, in CH₃CN at 298 K. In the presence of a divalent cation (M²⁺ = Ba²⁺, Ca²⁺, Mg²⁺, or Zn²⁺) in CH₃CN, sensor 1 can tightly bind M²⁺ and shows intense fluorescent (Φ_f = 0.90-0.19, τ_f = 2.1-6.9 ns) with the color being dependent on the nature of M²⁺ (λ_f = 514-584 nm). The results demonstrate that a single fluorescent sensor 1 is capable of simultaneous identification and quantitation of M²⁺ based on λ_f and the fluorescent intensity (Φ_f), respectively. The fluorescence maximum energy of the [1-M²⁺] complex is shown to correlate linearly with the pK_a value of M²⁺. The spectroscopic and photophysical properties of sensor 1 in the absence and presence of M²⁺ are also discussed.