Heterobimetallic Ln–Zn (Ln = Eu, Gd) complexes with interesting second order nonlinear optical properties

Second-order nonlinear optical chromophores based on coordination compounds offer an interesting route to enhance molecular hyperpolarizability through low-energy charge-transfer excited states and the tunable donor/acceptor character of metal centers. While lanthanide complexes are widely explored for luminescence and magnetism, their second-order NLO properties remain comparatively underinvestigated, and cationic lanthanide systems have, to the best of our knowledge, not been explored. In this context, we designed and synthesized in a simple and rapid way two new lanthanide complexes based on a europium cation or gadolinium cation and the same zinc anion: [Eu(hfa)2(tetrag)]+·[Zn(hfa)3]− and [Gd(hfa)2(tetrag)]+·[Zn(hfa)3]−. Structural and spectroscopic characterization confirms the formation of well-defined cation–anion assemblies in which the lanthanide β-diketonate cation is counterbalanced by a zinc anion counterpart. The second-order nonlinear optical response was evaluated in solution by means of the Electric-Field Induced Second Harmonic generation (EFISH) technique which affords μβEFISH values. Both compounds present values much higher than that of the benchmark Disperse Red One, rendering them of particular interest for future applications such as in electro-optic modulators.