Luminescence Detection of Transition and Heavy Metals by Inversion of Excited States:  Synthesis, Spectroscopy, and X-ray Crystallography of Ca, Mn, Pb, and Zn Complexes of 1,8-Anthraquinone-18-Crown-5

A lumophore composed of anthraquinone attached to a macrocyclic polyether ring containing an intraannular carbonyl is capable of selectively detecting Pb²⁺ ion in solution using an alternative photophysical detection mechanism. Of the various methods available for detection of ions in solution, a mechanism involving inversion of excited states has not been previously employed for the detection of transition and heavy metals. In this mechanism, nonradiative n−π* transitions are replaced by radiative π−π* transitions upon complexation by a suitable guest cation. Optimum fluorescence enhancement is achieved using cations of high charge, large cations that form long bonds within the host, and cations which do not coordinate solvent or the counteranion, all of which are necessary for inversion of excited states to occur. Photophysical properties and binding constants of this new class of luminescence sensors are provided, as well as the X-ray crystallographic results for Pb²⁺, Mn²⁺, and Zn²⁺ complexes of 1,8-oxybis(ethylene-oxyethyleneoxy)anthracene-9,10-dione (1), referred to as 1,8-anthraquinone-18-crown-5. ([1·Pb](ClO₄)₂ (2) (monoclinic, P21/n, a = 8.0303(6) Å, b = 25.976(2) Å, c = 12.1616(9) Å, β = 94.956(1)°, Z = 4, 4980 reflections [I ≥ 2σ(I)], R1 = 0.0266, wR2, 0.0500, 173(2) K). [1·Mn(H₂O)(NCCH₃)](ClO₄)₂ (5) (monoclinic, P21/c, a = 10.132(2) Å, b = 11.8030(4) Å, c = 23.999(7) Å, β = 95.75(2)°, Z = 4, 3000 reflections [I ≥ 2σ(I)], R1 = 0.0488, wR2, 0.0938, 203(2) K). [1·Zn(H₂O)(NCCH₃)](ClO₄)₂ (6) (monoclinic, P21/c, a = 10.177(10) Å, b = 11.977(6) Å, c = 24.166(4) Å, β = 95.83(4)°, Z = 4, 3061 reflections [I ≥ 2σ(I)], R1 = 0.0673, wR2, 0.1729, 295(2) K).)