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Photon upconversion based on sensitized triplet-triplet annihilation (sTTA) is considered as a promising strategy for the development of light-managing materials aimed to enhance the performance of solar devices by recovering unused low-energy photons. Here, we demonstrate that, thanks to the fast diffusion of excitons, the creation of triplet pairs in metal-organic framework nanocrystals (nMOFs) with size smaller than the exciton diffusion length implies a 100% TTA yield regardless of the illumination condition. This makes each nMOF a thresholdless, single-unit annihilator. We develop a kinetic model for describing the upconversion dynamics in a nanocrystals ensemble, which allows us to define the threshold excitation intensity Ith box required to reach the maximum conversion yield. For materials based on thresholdless annihilators, Ith box is determined by the statistical distribution of the excitation energy among nanocrystals. The model is validated by fabricating a nanocomposite material based on nMOFs, which shows efficient upconversion under a few percent of solar irradiance, matching the requirements of real life solar technologies. The statistical analysis reproduces the experimental findings, and represents a general tool for predicting the optimal compromise between dimensions and concentration of nMOFs with a given crystalline structure that minimizes the irradiance at which the system starts to fully operate.

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.

A novel chromophoric ionic liquid (IL) with two-dimensional (2D) nanostructural order is developed, and its structure-property relationship is investigated by harnessing photon upconversion based on triplet energy migration. An ion pair of 9,10-diphenylanthracene-2-sulphonate (DPAS) and asymmetric quaternary phosphonium ion exhibited both ionic crystal (IC) and supercooled IL phases at room temperature. Single crystal X-ray analysis of the IC phase showed an alternate alignment of polar (ionic) and non-polar (non-ionic) layers, and this layered structure was basically maintained even in the IL phase. The diffusion length of triplet excitons in the IL phase, obtained by the analysis of upconverted emission in succession to triplet-triplet annihilation (TTA), is larger than the domain size estimated from powder X-ray analysis. This suggests that triplet excitons in chromophoric ILs can diffuse over the nanostructured domains.

On page 531, column 2, there is a typo in eq 2. The correct exponent for the term between parentheses is-6 not6.

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.

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.

Faceted polymer microcapsules are prepared from metal-organic framework (MOF) templates. The MOF templates are removable under mild aqueous conditions. The obtained microcapsules are stiffer than their spherical counterparts, reflecting the near-incompressibility of the facet edges, and indicating that the faceting might be a useful strategy for controlling the mechanical properties of polymer microcapsules.

A quasielastic neutron scattering and solid-state H-2 NMR spectroscopy study of the polymeric spin-crossover compound {Fe(pyrazine)[Pt(CN)(4)]} shows that the switching of the rotation of a molecular fragment- the pyrazine ligand-occurs in association with the change of spin state. The rotation switching was examined on a wide time scale (10(-13)-10(-3) s) by both techniques, which clearly demonstrated the combination between molecular rotation and spin-crossover transition under external stimuli (temperature and chemical). The pyrazine rings are seen to perform a 4-fold jump motion about the coordinating nitrogen axis in the high-spin state. In the low-spin state, however, the motion is suppressed, while when the system incorporates benzene guest molecules, the movements of the system are even more restricted.

The utility of basic sites of porous coordination polymers (PCPs) for the end-functionalization of oligo(vinylidene fluoride) (OVDF) is demonstrated. Densely arranged carboxylate moieties on the PCP pore walls act as basic sites and allow the efficient elimination reaction of the terminal group of OVDF to proceed. The progress of the reaction of an OVDF precursor in the PCP nanochannels was evidenced using solid state 19F NMR spectroscopy. From 19F and 1H NMR and IR measurements of the product isolated from the composite obtained, the terminal -CH2CF 2I moiety of OVDF was shown to be successfully converted to a -CHCF2 group. Because of the large surface area of the PCP materials, the OVDF precursor easily accessed the basic sites, resulting in a large increase in the rate of the elimination reaction. In addition, a notable effect of the porous structure of PCPs upon the elimination reaction was observed. © 2011 The Royal Society of Chemistry.