Upconverting photons at the molecular scale with lanthanide complexes

Abstract

In this perspective, we summarise the major milestones to date in the field of molecular upconversion (UC) with lanthanide based coordination complexes. This begins from the leap firstly from solid-state to nanoparticular regimes, and further down the scale to the molecular domain. We explain the mechanistic intricacies of each differing way of generating upconverted photons, critiquing them and outlining our views on the benefits and limitations of each process, also offering our perspective and opinion on where these new molecular UC edifices will take us. This nascent area is already rapidly expanding and improving, having increased in luminance efficiency by more than four orders of magnitude in the last decade: we conclude that the future is bright for molecular UC.

Fig. 1. (a) Calculated energy levels of Ln(III) and (b) ranges of radiative lifetimes τ_rad for different Ln(III) ions (in ms).

Fig. 2. (a) Schematic representation of the ESA mechanism for Ln luminescence UC. (b) ESA mechanism for Er(iii).

Fig. 3. Examples of Er(iii) complexes showing ESA (ion colors: Er = teal).

Fig. 4. (a) Increase of ESA efficiency of [ErL(H₂O)] upon addition of NaF (D₂O, c = 200 μM, r.t., λ_ex = 980 nm, P = 5 W). (b) UC intensity at 541 nm as a function of the [NaF]/[ErL(H₂O)] ratio in correlation with the concentration of [ErL(H₂O)] (in blue), [F⊂(ErL)₂]⁺ (in green) and [ErLF] (in red). (c) Emitted intensity at 541 nm as a function of LASER power for a solution of [ErL(H₂O)] containing 0.5 eq. of NaF in D₂O (c = 200 μM, r.t.).

Fig. 5. Illustration of the ETU mechanism.

Fig. 6. (a) Heterotritopic ligand L1 used in the preparation of the heterotrinuclear [Cr₂Er(L1)₃]⁹⁺ complex displaying NIR to visible ETU UC. (b) NIR dye containing tridentate ligand L2 displaying ligand to Er ETU UC. with (c) single-crystal X-ray structure (ion colors: Cr = brown, Er = teal).

Fig. 7. (a) Cooperative photosensitization (CP) mechanism. (b) bipyridine-based Yb₂Tb assembly. (c) Stepwise formation of TACN-based Yb₂Tb assembly.

Fig. 8. Single-crystal X-ray structure of (a) [Gd₇Tb₂Eu₁₁] cluster, (b) Yb₇Eu₁ metal–organic cage (ion colors: Eu = purple, Tb = green, Gd = teal, Yb = red).

Fig. 9. CP with d–f systems: (a) solid-state crystals of Cr^III/Yb^III. (b) supramolecularly assembled metastable RuYb₃.

Fig. 10. (a) Energy-level diagram for cooperative luminescence of Yb ions. (b) A [Yb₉(acac)₁₆(OD)₁₀][OD] system exhibiting CL (ion colors: Yb = red)

Fig. 11. (a) Structure of [Yb₂L₁(NO₃)][NO₃]. (b) CL spectra in red (λ_ex = 980 nm, p = 6.9 W cm⁻¹, (CD₃)₂SO), convoluted downshifted spectra in black.

Loïc J. Charbonnière

Aline M. Nonat

Richard C. Knighton

Léna Godec