(1,3)Pyrenophanes containing crown ether moieties as fluorescence sensors for metal and ammonium ions

Abstract

Crown ether containing (1,3)pyrenophanes 1–6 were synthesized, and UV absorption and fluorescence spectroscopic studies were carried out to determine their abilities to form complexes with metal and ammonium ions. The fluorescence spectra of 1.0 × 10⁻⁵ M solutions of 1, 2, 4 and 6 in 1 : 1 v/v CH₂Cl₂ : CH₃CN were comprised of both monomer and intramolecular excimer emission bands, while only monomer emission bands were present in the fluorescence spectra of 3 and 5. The intensities of the intramolecular excimer emission bands of 1, 2, 4 and 6 in 1 : 1 v/v CH₂Cl₂ : CH₃CN decreased and those of the monomer emission increased in conjunction with the existence of isoemissive points upon the addition of increasing concentrations of various metal perchlorates. The fluorescence spectral changes were dependent on the sizes of crown ether rings and metal ions and, as such, they reflected equilibrium constants for the formation of metal-crown ether complexes. Addition of n-Bu₂NH₂⁺PF₆⁻ or (PhCH₂)₂NH₂⁺PF₆⁻ to the solutions of the (1,3)pyrenophane linked crown ethers, which brought about similar fluorescence spectral changes, led to the formation of pseudo-rotaxanes as was evidenced by an analysis of ¹H NMR spectra and Job's plots. The fluorescence changes of 1 occurred during 5 cycles of repetitive addition and removal of Ba²⁺. The ratio of intensities of the monomer to the intramolecular excimer emission bands of 1, 2, 4 and 6 increased as the temperature decreased. Based on the experimental observations and the results of DFT calculations, it is concluded that the (1,3)pyrenophanes exist in solution as equilibrium mixtures of anti monomer emitting and syn intramolecular excimer emitting conformers and the equilibrium favors the anti form when the crown ether moieties form complexes with metal or ammonium ions.