Carbazole-Functionalized Dipicolinato Ln Complexes Show Two-Photon Excitation and Viscosity-Sensitive Metal-Centered Emission.

Eu and Yb complexes with the carbazole-dipicolinato ligand dpaCbz, namely K[Eu(dpaCbz)] and K[Yb(dpaCbz)], were isolated. The Eu complex displayed metal-centred emission upon one-photon excitation with a sensitized emission efficiency of 1.8±0.3 %, corresponding to an intrinsic emission efficiency of 46% and a sensitization efficiency of 3.9%, with an emission lifetime of the emissive state τ of 1.087±0.005 ms. The Yb complex displayed of 0.010±0.001 %, and a τ of 2.32±0.06 μs. The Eu-centred emission was sensitized as well upon two-photon excitation and a two-photon absorption cross-section σ of 63 GM at 750 nm was determined for the complex. The one- or two-photon sensitized emission intensity of the Eu complex changes by more than two-fold when the solvent viscosity is varied in the range 0.5 - 200 cP and the emission is independent of dissolved oxygen. The Yb complex displays a change in emission intensity as well. However, in this case, a dependence of the emission intensity on dissolved oxygen content was observed.