Angelo Monguzzi

Fast and long-range triplet exciton diffusion in metal–organic frameworks for photon upconversion at ultralow excitation power

The conversion of low-energy light into photons of higher energy based on sensitized triplet-triplet annihilation upconversion (TTA-UC) has emerged as a promising wavelength-shifting methodology because it permits UC at excitation powers as low as the solar irradiance. However, its application has been significantly hampered by the slow diffusion of excited molecules in solid matrices. Here, we introduce metal-organic frameworks (MOFs) that promote TTA-UC by taking advantage of triplet exciton migration among fluorophores that are regularly aligned with spatially controlled chromophore orientations. We synthesized anthracene-containing MOFs with different molecular orientations, and the analysis of TTA-UC emission kinetics unveiled a high triplet diffusion rate with a micrometre-scale diffusion length. Surface modification of MOF nanocrystals with donor molecules and their encapsulation in glassy poly(methyl methacrylate) (PMMA) allowed the construction of molecular-diffusion-free solid-state upconverters, which lead to an unprecedented maximization of overall UC quantum yield at excitation powers comparable to or well below the solar irradiance.

Highly Efficient Photon Upconversion in Self-Assembled Light-Harvesting Molecular Systems

To meet the world’s demands on the development of sunlight-powered renewable energy production, triplet–triplet annihilation-based photon upconversion (TTA–UC) has raised great expectations. However, an ideal highly efficient, low-power, and in-air TTA–UC has not been achieved. Here, we report a novel self-assembly approach to achieve this, which enabled highly efficient TTA–UC even in the presence of oxygen. A newly developed lipophilic 9,10-diphenylanthracene-based emitter molecule functionalized with multiple hydrogen-bonding moieties spontaneously coassembled with a triplet sensitizer in organic media, showing efficient triplet sensitization and subsequent triplet energy migration among the preorganized chromophores. This supramolecular light-harvesting system shows a high UC quantum yield of 30% optimized at low excitation power in deaerated conditions. Significantly, the UC emission largely remains even in an air-saturated solution, and this approach is facilely applicable to organogel and solid-film systems.

DFB laser action in a flexible fully plastic multilayer

An all-plastic distributed-feedback flexible multilayer laser (FML), able to keep its lasing characteristics even upon strong bending from planarity, is proposed and the possibility of using this system to make solid-state dye lasers is suggested.

NIR emitting ytterbium chelates for colourless luminescent solar concentrators

A new oxyiminopyrazole-based ytterbium chelate enables NIR emission upon UV excitation in colorless single layer luminescent solar concentrators for building integrated photovoltaics.

Cascade sensitization of triplet–triplet annihilation based photon upconversion at sub-solar irradiance

In triplet-triplet annihilation based upconversion, high-energy photons are generated through the annihilation of fluorophore triplets, populated via energy transfer from a light-harvesting sensitizer. However, the absorption band of common sensitizers is narrow, limiting the fraction of recoverable photons. We overcome this issue using a third chromophore as an additional light-harvester in the transparency window between the upconverted luminescence and the sensitizer absorption. The third component transfers the extra-collected energy to sensitizers, realizing a cascade-sensitized upconversion that shows a 20% increment of the high-energy photon output and a conversion yield of 10% at solar irradiance.

Highly Fluorescent Metal–Organic-Framework Nanocomposites for Photonic Applications

Metal-organic frameworks (MOFs) are porous hybrid materials built up from organic ligands coordinated to metal ions or clusters by means of self-assembly strategies. The peculiarity of these materials is the possibility, according to specific synthetic routes, to manipulate both the composition and ligands arrangement in order to control their optical and energy-transport properties. Therefore, optimized MOFs nanocrystals (nano-MOFs) can potentially represent the next generation of luminescent materials with features similar to those of their inorganic predecessors, that is, the colloidal semiconductor quantum dots. The luminescence of fluorescent nano-MOFs is generated through the radiative recombination of ligand molecular excitons. The uniqueness of these nanocrystals is the possibility to pack the ligand chromophores close enough to allow a fast exciton diffusion but sufficiently far from each other preventing the aggregation-induced effects of the organic crystals. In particular, the formation of strongly coupled dimers or excimers is avoided, thus preserving the optical features of the isolated molecule. However, nano-MOFs have a very small fluorescence quantum yield (QY). In order to overcome this limitation and achieve highly emitting systems, we analyzed the fluorescence process in blue emitting nano-MOFs and modeled the diffusion and quenching mechanism of photogenerated singlet excitons. Our results demonstrate that the excitons quenching in nano-MOFs is mainly due to the presence of surface-located, nonradiative recombination centers. In analogy with their inorganic counterparts, we found that the passivation of the nano-MOF surfaces is a straightforward method to enhance the emission efficiency. By embedding the nanocrystals in an inert polymeric host, we observed a +200% increment of the fluorescence QY, thus recovering the emission properties of the isolated ligand in solution.

Retraction Note to: Retraction: Fast and long-range triplet exciton diffusion in metal-organic frameworks for photon upconversion at ultralow excitation power

Nature Materials 14, 924–930 (2015); published online 3 August 2015; retracted after print 24 November 2016. We wish to retract this Article due to concerns with some data related to upconversion in the solid-state samples presented in Fig. 4d,e, and to the reproducibility check of the triplet diffusion constant provided in the Supplementary Information. In this Article, we reported fast triplet energy migration and efficient photon upconversion at low excitation intensity in metal–organic frameworks (MOFs). We have since been able to observe the upconverted emission from MOFs both in benzene dispersions and in polymeric films; hence, the concept of photon upconversion in MOFs based on triplet energy migration remains valid. However, we are now unable to observe solid-state upconversion emission at the low excitation intensity reported in Fig. 4d,e, and to quantitatively reproduce the triplet diffusion constants in MOFs reported in Supplementary Figs 8–13 and Supplementary Tables 1–3. Since these are key parameters of this paper, all authors wish to retract this Article. We deeply regret these circumstances and sincerely apologize to the scientific community for the inconvenience this publication has caused to others.