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(NO(3))(3) and B(2)18C6 acetonitrile solution. The coordination sphere of the Eu(III) complexes in acetonitrile solutions were also discussed by the degenerate (7)F(0) --> (5)D(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 (5)D(0) --> (7)F(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.