For a long period, metal-based linear light-upconversion, that is the conversion of near-infrared (NIR) light into emitted visible light by successive linear absorption of NIR photons, was limited to the statistical doping of low phonon bulk solids and garnets. The design of closely related upconversion processes operating in nanoparticles allowed to reach the nanoscale.^[1] The next miniaturizing step aims at inducing linear light upconversion at the molecular level, which opens challenging perspectives in terms of improved reproducibility, chemical control and optical programming. Achieving this goal requires either the piling up of low-energy near-infrared photons onto a single short-lived lanthanide center via excited state absorption (ESA), or the implementation of the alternative energy transfer upconversion (ETU) mechanism, which combines a short-lived lanthanide activator surrounded with long-lived sensitizers.^[2] However, only a very few numbers of sophisticated multicenter devices which exploit the indirect ETU mechanism have been evidenced in molecular complexes and the ESA mechanism in isolated coordination complexes is essentially unknown for isolated coordination complexes except under massive incident excitation powers. The judicious wrapping of three polyaromatic ligand strands around trivalent erbium,^[3] in [Er(Lk)₃]³⁺[GaErGa(L1)₃]⁹⁺ or [CrErCr(L1)₃]⁹⁺ indeed brought a small revolution with the demonstration that low-power room temperature near-infrared to green light upconversion can be programmed in mononuclear coordination complexes.

In this contribution, the erbium-centered downshifted emission and opposite NIR to visible upconversion and their quantifying in molecular complexes will be discussed as well as their mechanisms. The associated quantum yields measured for ESA appear to be 1–3 orders of magnitude larger than those predicted by the accepted mechanism. An even larger discrepancy by 4–6 orders of magnitude occurs between theoretical models and experimental data for ETU.^[4] An unexpected boosted ESA process is proposed for reconciling predictions and measurements.

Figure 1. Erbium-based coordination complexes exhibiting linear upconversion processes and associated kinetic model for the single- ion ESA process.

[1] Jing Zhou, Qian Liu, Wei Feng, Yun Sun, Fuyou Li, Chem. Rev, 2015, 115, 395-465.
[2] Yan Suffren, Davood Zare, Svetlana V. Eliseeva, Laure Guénée, Homayoun Nozary, Timothée Lathion, Lilit Aboshyan-Sorgho, Stéphane Petoud, Claude Piguet, J. Phys. Chem. C, 2013, 117, 26957-26963.
[3] Bahman Golesorkhi, Alexandre Fürstenberg, Homayoun Nozary, Claude Piguet, Chem. Sci, 2019, 10, 6876-6685.
[4] Bahman Golesorkhi, Inès Taarit, Hélène Bolvin, Homayoun Nozary, Juan-Ramón Jiménez, Céline Besnard, Laure Guénée, Alexandre Fürstenberg, Claude Piguet, Dalton Trans, 2021, Advance Article.