Lanthanide Clusters with Internal Ln Ions:  Highly Emissive Molecules with Solid-State Cores

“Er(SePh)_2.5I_0.5” reacts with elemental S to give (THF)₁₀Er₆S₆I₆, a double cubane cluster with one face of the Er₄S₄ cube capped by an additional Er₂S₂. Reactions with a mixture of elemental S/Se results in the formation of (THF)₁₄Er₁₀S₆(Se₂)₆I₆, a cluster composed of an Er₆S₆ double cubane core, with two “Er₂(Se₂)₃” units condensed onto opposing rectangular sides of the Er₆S₆ fragment. This deposition of Er₂Se₆ totally encapsulates the two central Er with chalcogen atoms (4 S, 4 Se) and excludes neutral THF donors or iodides from the two primary coordination spheres. The Er₁₀ compound is the first lanthanide cluster to contain internal, chalcogen encapsulated Ln. This cluster shows strong fluorescence at 1544 nm with a measured decay time of 3 ms and an estimated quantum efficiency of 78%, which is comparable to Er doped solid-state materials. The unusual fluorescence spectral properties of (THF)₁₄Er₁₀S₆Se₁₂I₆ are unprecedented for a molecular Er complex and are attributed to the low phonon energy host environment provided by the I^-, S^2-, and Se₂^2- ligands.