Metal complexes and nanoparticles for energy upconversion

It is with great pleasure that we welcome you to this Dalton Transactions themed issue devoted to “Metal Complexes and Nanoparticles for Energy Upconversion”.

Energy upconversion is the process in which a high energy excited state is populated upon the absorption of two or more photons of lower energy. The electronic excited state can then undergo its own chemical and physical deactivation processes, including emission of photons that have higher energies than each of the absorbed photons (anti-Stokes emission).

Well-established approaches to upconversion rely on simultaneous multiphoton (normally biphotonic and more recently triphotonic) absorption, in which the first photon excites the molecule to a virtual state, and the excited state is only populated if the second photon arrives within the duration of the first interaction (∼10⁻¹⁶ s). On the other hand, this issue is focused on sequential multiphoton absorption mechanisms in which upconversion is obtained via energy transfer or sensitised triplet–triplet annihilation.

Before the 1960s, the known anti-Stokes emissions were linked to the thermal population of electronic states only a few kT above the excitation energy. In the 1960s, two pieces of research changed the paradigm, also thanks to the advent of high energy density laser sources:

- In 1966, Auzel suggested that energy transfer could take place between two ions (lanthanide ions), both of them being in an excited state.^1,2 Until then, energy transfer was always considered to take place between an excited state and a ground state.

- In 1962, Parker reported the first example of anti-Stokes emission based on sensitised triplet–triplet annihilation.³

In the last fifty years much progress has been made in the field, and with this themed issue we intend to gather the most recent and significant advances in energy upconversion materials and processes, and to explore possible applications.

The present issue includes reviews regarding upconversion processes via triplet–triplet annihilation based on (i) a Pt(II) porphyrin and tetraphenylpyrene in media of increasing complexity, from solution to nanostructured silica matrices and liquid filled microcapsules (Ortica, DOI: 10.1039/C8DT00020D); (ii) lead sulfide (PbS) nanocrystals for near-infrared-to-visible upconversion in solution and in solid-state devices (Bawendi, DOI: 10.1039/C8DT00419F); and (iii) selected metalloporphyrins included in metal–organic framework (MOF) or polymeric structures as dual absorber-upconverters in solar photovoltaics (Steer, DOI: 10.1039/C7DT04343K). Recent advances are reported in molecular and supramolecular lanthanide complexes for energy transfer upconversion in solution (Charbonnière, DOI: 10.1039/C7DT04737A) and the use of lanthanide-based upconversion nanoparticles for the conversion of near-IR light into visible light, where dye-sensitized solar cells typically have high efficiency (Chen, DOI: 10.1039/C7DT04461E). Biomedical applications, particularly drug-delivery, imaging (Li, DOI: 10.1039/C8DT00258D) and photodynamic therapy (Hamblin, DOI: 10.1039/C8DT00087E), are discussed with particular emphasis on ultrasmall (<10 nm) lanthanide-based nanoparticles.

Articles in this issue discuss the most important factors related to upconversion systems, namely efficiency, the energy difference between excitation and emission, photostability and applications in a variety of fields ranging from photovoltaics to bioimaging. In particular, large anti-Stokes shifts in the visible and near-infrared regions have been demonstrated (i) in hybrid materials consisting of semiconductor nanocrystals and MOFs (Kimizuka, DOI: 10.1039/C7DT04794K) and (ii) via S₀ → T₁ excitation of an Os(II) complex derivatised with a Bodipy dye to increase the lowest triplet state lifetime and thus the upconversion efficiency (Zhao, DOI: 10.1039/C7DT04803C). Strong visible absorption has been obtained in 1,10-phenanthroline Ir(III) complexes featuring coumarin-6 as ancillary ligand (Draper, DOI: 10.1039/C8DT00231B). The photostability of an upconversion system has been enhanced by a temperature-dependent sacrificial singlet oxygen scavenger (Baluschev, DOI: 10.1039/C7DT03698A). Enhancement of triplet–triplet annihilation upconversion by localized surface plasmon resonance of silver nanoparticles has been shown (Zhang, DOI: 10.1039/C8DT00269J). The use of upconversion systems to drive photochemical reactions in polymeric structures is elegantly demonstrated by visible light driven intramolecular dimerization of pendant anthracene groups on a methacrylic polymer to induce the formation of single-chain nanoparticles (Simon, DOI: 10.1039/C8DT01392F).

In the case of lanthanide-based materials, the luminescence properties have been evaluated (i) as a function of calcination temperature and Mn(II) doping in NaYF₄:Yb³⁺/Er³⁺ particles (Zhu, DOI: 10.1039/C8DT00792F); (ii) as a function of Li(I) doping, demonstrating promising properties for bioprobes and optical thermometers (Yuhua Wang, DOI: 10.1039/C8DT00928G); and (iii) in Yb³⁺ and Er³⁺ co-doped AWO₄ (A = Ca, Sr, Ba) materials combined with carbon dots (Hao, DOI: 10.1039/C7DT04756H). A core–shell–shell nanostructure composed of NaGdF₄:Yb/Tm@NaGdF₄:Nd@NaYF₄ is reported to realize Yb³⁺-sensitized upconversion and downshifting luminescence in Nd³⁺ ions (Feng Wang, DOI: 10.1039/C8DT00218E). A mathematical model is a general strategy to enhance the NIR and blue luminescence of NaY_1−(x+y)Yb_xF₄:Tm_y by manipulating the relative concentrations of the lanthanide dopants (Lim, DOI: 10.1039/C7DT04768A). In view of biomedical applications, NaYF₄-based upconverting nanoparticles were surface-modified to make them water-dispersible, with great colloidal stability and a near-neutral surface at physiological pH (Stephan, DOI: 10.1039/C8DT00241J).

The articles and reviews reported in this issue demonstrate that energy upconversion is a maturing technique with potential applications across a broad range of fields including photovoltaic devices, photocatalysis, sensing, bioimaging and photodynamic therapy.

At the moment, the main challenges and the expected foreground applications are the development of solid-state devices based on sensitised triplet–triplet annihilation upconversion for increasing the efficiency of photovoltaic cells and water compatibility, as well as the absence of toxicity for bioimaging and biosensing applications of lanthanide-based nanoparticles.

All of the authors as well as the reviewers are acknowledged for their efforts and relevant contributions. We hope that this themed issue will not only provide a timely overview of current developments, but will also encourage future cross-disciplinary developments in this exciting field of research.

References

1. F. Auzel, C. R. Acad. Sci., 1966, 262, 1016.
2. F. Auzel, C. R. Acad. Sci., 1966, 263, 819.
3. C. A. Parker and C. G. Hatchard, Sensitised anti-Stokes delayed fluorescence, Proc. Chem. Soc., 1962, 373–401.

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