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

Abstract

Developing controlled and reproducible molecular assemblies incorporating lanthanide centers is a crucial step for 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 also been prepared and photophysically studied. Efficient sensitization via the two β-diketones employed as main ligands was probed for both Nd³⁺ and Er³⁺ ions, resulting in highly resolved emission spectra and sufficiently long excited state lifetimes, which allowed further assessment of 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 of 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 antenna to Er³⁺, the sensitization of the latter results in back-transfer to Nd³⁺ into a non-emissive, thus silent, state.

Fig. 1. Ligands 2,6-bis-((3-oxo-3-naphthalene-2-yl)propionyl)-pyridine (H₂LA) and 6-(3-(naphthalene-2-yl)-3-oxopropanoyl)-picolinic acid (H₂LB).

Fig. 2. Representation of the cation [Er₂Nd(LA)₂(LB)₂(H₂O)₂(py)]⁺ of 3 (also representing those of 1 and 2). The carbon atoms of ligands LA²⁻ and LB²⁻ are darker and lighter gray, respectively. Er is orange, Nd is green, O is red, N is purple and C is gray. H atoms are not shown. The inset shows the coordination polyhedrons of Nd³⁺ and Er³⁺, using the same color codes (both Er³⁺centers have very similar polyhedrons, despite not being crystallographically equivalent, thus, only one is shown).

Fig. 3. Excitation (λ_em = 1058 nm, black trace) and emission (λ_exc = 400 nm, red trace) spectra of complex 1 in MeOH-d₄ : DMSO-d₆ (1 : 1) at 77 K.

Fig. 4. Excitation (λ_em = 1530 nm, black trace) and emission (λ_exc = 400 nm, red trace) spectra of complex 2 in MeOH-d₄ : DMSO-d₆ (1 : 1) at 77 K.

Fig. 5. Emission (λ_exc = 400 nm, red trace) spectrum of complex 3 in MeOH-d₄ : DMSO-d₆ (1 : 1) at 77 K.

Fig. 6. Emission spectra (λ_exc = 400 nm) of iso-absorbant solutions of complexes (1) (black trace), (2) (blue trace), and (3) (red trace) in MeOD-d₄ : DMSO-d₆ (1 : 1) solution at room temperature (a) with the corresponding Er³⁺ emission zoom (b).

Fig. 7. Energy diagram levels and terms involved in the ET phenomena occurring within complex 3 [ErNdEr].

Fig. 8. Excitation spectra of complexes (1) at λ_em = 1058 nm (black trace), (2) at λ_em = 1530 nm (red trace) and (3) at λ_em = 1058 nm (blue trace) and at λ_em = 1530 nm (orange trace) in MeOH-d₄/DMSO-d₆ (1 : 1) at 77 K.