Mixed Phenanthroline–Bipyridine Europium and Ytterbium Cryptates─Bright Luminophores with Small Radiative Lifetimes

New phenanthroline-based tris(biaryl) europium(III) and ytterbium(III) cryptates with great luminescence efficiencies in solution are reported. 1,10-Phenantroline, in combination with 2,2′-bipyridine-N,N′-dioxide, delivers cryptates with high rigidity, efficient lanthanoid(III) sensitization (η$_{sens}$) and small radiative lifetimes τrad for Eu and Yb. The powerful synergy of both bidentate chelating units results in excellent absolute quantum yields of up to 20% for the europium complexes in aqueous solution, which indicates an impressive improvement compared to other well-known tris(bipyridyl)-based Eu(III) cryptates.