Excitation and deexcitation processes of Eu(III) benzo-15-crown-5 complex studied on comparing with its first coordination sphere

The coordination sphere and the deexcitation mechanism of the Eu(III) benzo-15-crown-5 complex, Eu(B15C5), were studied with references of the Eu(III) complexes with a similar coordination sphere; the dibenzo-18-crown-6 complex, Eu-3(B(2)18C6)(2), and the cryptand[2.2.1] complex, Eu([2.2.1]). NMR spectroscopy reveals that the Eu(B15C5) complex is quite stable in acetonitrile solution whereas only 40% of the Eu(III) ion forms the complex in the equimolar Eu(NO3)(3) and B(2)18C6 acetonitrile solution. The coordination sphere of the Eu(III) complexes in acetonitrile solutions were also discussed by the degenerate F-7(0) --> D-5(0) transition energy levels. The Eu(B15C5) have a negative shift compared with the europium(Ill) nitrate in acetonitrile and it is explained by the coordination of both nitrate ions and the crown ether ligand. Energy transfer from the n-pi* excited state located in the catechol structure to the central europium ion was first observed as the sensitized luminescence of D-5(0) --> F-7(J). The excited state lifetime of the Eu(B15C5) complex was first determined as 201 mus in the present study. (C) 2001 Elsevier Science B.V. All rights reserved.