Complexation and photoinduced recoordination of bis(aza-18-crown-6)-containing dienones with alkali and alkaline earth metal cations, structure—property correlations

Abstract

Analysis of absorption and fluorescence spectra at 293 and 77 K, as well as fluorescence excitation spectra of bis(aza-18-crown-6)-containing dibenzylidenecyclohexanone (CD6) and its complexes confirms the presence of photorecoordination in CD6•(Mⁿ⁺)₂ (M = Ba²⁺, Sr²⁺, Ca²⁺, K⁺). The process is completely blocked in low-temperature butyronitrile glass. Coordination of the metal cation to the carbonyl group of the CD6 molecule is accompanied by fluorescence enhancement, while coordination of the cations to the aza-18-crown-6 ether residues, on the contrary, leads to its quenching. In the case of the complexes CD6•(Mⁿ⁺)₂, correlations were observed between the stability constants, the values of the blue shifts in the absorption spectra, the red shifts in the fluorescence spectra, and the diameters of the cations. The CD6 dienone forms complexes with the compositions CD6•Mⁿ⁺, CD6•(Mⁿ⁺)₂, and Mⁿ⁺•CD6•(Mⁿ⁺)₂ with the cations Mg²⁺, Li⁺, and Na⁺. A correlation of the above-mentioned parameters with the cation surface charge density was observed for the complexes Mⁿ⁺•CD6•(Mⁿ⁺)₂. The possibility of influencing the efficiency of complexation and the degree of photorecoordination by varying the nature of the central acceptor fragment in the dye series CD4, CD5, CD6 (CD4 and CD5 are dyes similar to CD6 with central cyclobutanone (CD4) and cyclopentanone (CD5) fragments, respectively) was determined.