Energy exchange between Nd3+ and Er3+ centers within molecular complexes

The controlled and reproducible molecular assemblies incorporating lanthanide centers represents a crucial step in driving forward up- and down-conversion processes. This challenge calls for the development of strategies to facilitate the efficient in-situ segregation of different Ln metal ions into distinct positions within the molecule. The unique family of pure [LnLn′Ln] heterometallic coordination compounds previously developed by us represents an ideal platform for studying the desired Ln-to-Ln′ energy transfer (ET). In this context, we report here the new pure one-step synthetically produced [ErNdEr] (3) complex, which allows for the first time at the molecular level to study the mechanisms behind Nd-to-Er energy transfer. To further assess the photophysical properties of this complex, the analogous [LuNdLu] (1) and [ErLaEr] (2) complexes have been also prepared and photophysically studied. Efficient sensitization via the two β-diketones employed as main ligands was proben for both Nd3+ and Er3+ ions, resulting in highly resolved emission spectra and sufficiently long excited state lifetimes, which allowed to further assess the Ln-to-Ln′ ET. This intermetallic transfer was first detected by comparing the emission spectra of iso-absorbant solutions and demonstrated by comparing the lifetime values with or without the lanthanide quencher (Er3+), as well as with a deep analysis of the excitation spectrum of the three complexes. Thus, a very unique phenomenon was discovered, consisting in a mutual Nd-to-Er and Er-to-Nd ET with no net increase of brightness by any metal ; while Nd3+ transfers the energy received from the antena to Er3+, the sensitization of the latter results into back-transfer to Nd3+ to a non-emissive, thus silent state.