Triphenylarsane Oxide Complexes of Lanthanide Nitrates: Polymorphs and Photophysics

trans‐[Ln(NO3)2(Ph3AsO)4](NO3)2 (1) and mer‐[Ln(NO3)3(Ph3AsO)3] (2) complexes were prepared from Ln(NO3)3·xH2O and Ph3AsO in chloroform (Ln = Y, Sm, Eu, Tb, and Dy). Production of complexes 1 vs. 2 and solvent content was found to be highly dependent on crystallization solvent choice. Tb and Eu produced only 1, while the other Ln metals produced both 1 and 2. Solvent‐free, acetone‐, and methanol‐containing polymorph series were identified for complexes 1. Acetone/ether‐ and CH2Cl2‐containing polymorph series were identified for complexes 2. Luminescence measurements were performed on solvent‐free 1 (Ln = Y, Eu, Tb, and Dy) and 2 (Ln = Sm) at 78 K. Sensitized lanthanide emission bands via resonance energy transfer were observed in all cases, except the control (Ln = Y). The efficiency of this energy transfer process varies amongst the lanthanide metals studied and was rationalized using Latva's empirical rule and Density Functional Theory calculations.