Complexation of bis(aza-18-crown-6)-containing dibenzylidenecyclopentanone with alkali and alkaline-earth metal cations: specific features and photoinduced recoordination in the complexes

An analysis of the results of time-resolved transient S₁→S_n absorption spectroscopy studies and quantum chemical calculations of the dye (2E,5E)-2,5-bis[4-(1,4,7,10,13-pentaoxa-16-azacyclooctadecane-16-yl)benzylidene]cyclopentanone (1) and its metal complexes confirm the generality of the phenomenon of photoinduced recoordination of metal cations in the complexes of bis(aza-18-crown-6)-containing derivatives of the dibenzylidenecyclobutanone (dibenzylidenecyclopentanone) series. The results obtained confirmed the existence of the first stage of photoinduced recoordination of metal cations in the 1 • (Mⁿ⁺)₂ (M = Ba²⁺, Ca²⁺, K⁺) complexes which completes within a few hundreds of femtoseconds. The process involves cleavage of the N—M bond followed by displacement of the metal cation from its equilibrium position in the azacrown ether cavity and by transformation of the “axial” conformation of the complex to “equatorial” one. It was demonstrated that the barrier photoinduced recoordination of the cation in the 1 • (Ba²⁺)₂ complex is accompanied by the change of the type of the solvation shell of the crowned Ba²⁺ cation in the following order: (2+1)MeCN, (3+1)MeCN, 4MeCN. In a low-temperature butyronitrile glassy matrix at 77 K, the photoinduced recoordination is completely suppressed. The Mg²⁺, Li⁺, and Na⁺ cations can form not only the inclusion complexes, but also the 1: 3 complexes ((Mⁿ⁺) • 1 • (Mⁿ⁺)₂) of moderate stability through additional coordination to the carbonyl group of the dye. The radii, r_S, of solvates of different-stoichiometry complexes containing the same metal cation determined from the data of anisotropy decay kinetics of S₁→S_n absorption are about 9.2 Å for 1 • (Mg²⁺)₂ and nearly 11.5 Å for Mg²⁺ • 1 • (Mg²⁺)₂.