Carbazole-functionalized dipicolinato LnIII complexes show two-photon excitation and viscosity-sensitive metal-centered emission

Abstract

EuIII and YbIII complexes with the carbazole-dipicolinato ligand dpaCbz2−, namely K3[Eu(dpaCbz)3] and K3[Yb(dpaCbz)3], were isolated. The EuIII complex displayed metal-centered emission upon one-photon excitation with a sensitized emission efficiency ΦLLn of 1.8±0.3%, corresponding to an intrinsic emission efficiency ΦLnLn of 46% and a sensitization efficiency of ηsens 3.9%, with an emission lifetime of the emissive state τ of 1.087±0.005 ms. The YbIII complex displayed ΦLLn of 0.010±0.001%, and a τ of 2.32±0.06 μs The EuIII-centered emission was sensitized as well upon two-photon excitation and a two-photon absorption cross-section σ2PA of 63 GM at 750 nm was determined for the complex. The one- or two-photon sensitized emission intensity of the EuIII 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 YbIII 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.