Quasi-thresholdless Photon Upconversion in Metal-Organic Framework Nanocrystals.

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  Ithbox required to reach the maximum conversion yield. For materials based on thresholdless annihilators, Ithbox 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.

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.

Achieving the photon up-conversion thermodynamic yield upper limit by sensitized triplet-triplet annihilation.

Triplet-triplet annihilation (TTA) based up-conversion is a promising strategy for light harvesting the low-energy tail of the solar spectrum with photovoltaic technologies. Here, we present a bi-component system for photon managing via TTA that allows bypassing the classical statistic limit of 2/5 in the singlet generation, achieving a near unitary conversion efficiency. This result is obtained because of the peculiar relative position of the triplet and singlet energy levels of perylene, used as up-converter and emitter. The system shows a record red-to-blue external up-conversion yield of ∼10% under an irradiance of 1 sun.

Broadband up-conversion at subsolar irradiance: triplet-triplet annihilation boosted by fluorescent semiconductor nanocrystals.

Conventional solar cells exhibit limited efficiencies in part due to their inability to absorb the entire solar spectrum. Sub-band-gap photons are typically lost but could be captured if a material that performs up-conversion, which shifts photon energies higher, is coupled to the device. Recently, molecular chromophores that undergo triplet-triplet annihilation (TTA) have shown promise for efficient up-conversion at low irradiance, suitable for some types of solar cells. However, the molecular systems that have shown the highest up-conversion efficiency to date are ill suited to broadband light harvesting, reducing their applicability. Here we overcome this limitation by combining an organic TTA system with highly fluorescent CdSe semiconductor nanocrystals. Because of their broadband absorption and spectrally narrow, size-tunable fluorescence, the nanocrystals absorb the radiation lost by the TTA chromophores, returning this energy to the up-converter. The resulting nanocrystal-boosted system shows a doubled light-harvesting ability, which allows a green-to-blue conversion efficiency of ∼12.5% under 0.5 suns of incoherent excitation. This record efficiency at subsolar irradiance demonstrates that boosting the TTA by light-emitting nanocrystals can potentially provide a general route for up-conversion for different photovoltaic and photocatalytic applications.

Nickel-assisted growth and selective doping of spinel-like gallium oxide nanocrystals in germano-silicate glasses for infrared broadband light emission.

The target of taking advantage of the near-infrared light-emission properties of nickel ions in crystals for the design of novel broadband optical amplifiers requires the identification of suitable nanostructured glasses able to embed Ni-doped nanocrystals and to preserve the workability of a glass. Here we show that Ni doping of Li(2)O-Na(2)O-Ga(2)O(3)-GeO(2)-SiO(2) glass (with composition 7.5:2.5:20:35:35 and melting temperature 1480 °C, sensibly lower than in Ge-free silicates) enables the selective embedding of nickel ions in thermally grown nanocrystals of spinel-like gallium oxide. The analysis of transmission electron microscopy and x-ray diffraction data as a function of Ni-content (from 0.01 to 1 mol%) indicates that Ni ions promote the nanophase crystallization without affecting nanoparticle size (~6 nm) and concentration (~4 × 10(18) cm(-3)). Importantly, as shown by optical absorption spectra, all nickel ions enter into the nanophase, with a number of ions per nanocrystal that depends on the nanocrystal concentration and ranges from 1 to 10(2). Photoluminescence data indicate that fast non-radiative decay processes become relevant only at mean ion-ion distances shorter than 1.4 nm, which enables the incorporation of a few Ni ions per nanoparticle without too large a worsening of the light-emission efficiency. Indeed, at 0.1 mol% nickel, the room temperature quantum yield is 9%, with an effective bandwidth of 320 nm.

Highly luminescent scintillating hetero-ligand MOF nanocrystals with engineered Stokes shift for photonic applications.

Large Stokes shift fast emitters show a negligible reabsorption of their luminescence, a feature highly desirable for several applications such as fluorescence imaging, solar-light managing, and fabricating sensitive scintillating detectors for medical imaging and high-rate high-energy physics experiments. Here we obtain high efficiency luminescence with significant Stokes shift by exploiting fluorescent conjugated acene building blocks arranged in nanocrystals. Two ligands of equal molecular length and connectivity, yet complementary electronic properties, are co-assembled by zirconium oxy-hydroxy clusters, generating crystalline hetero-ligand metal-organic framework (MOF) nanocrystals. The diffusion of singlet excitons within the MOF and the matching of ligands absorption and emission properties enables an ultrafast activation of the low energy emission in the 100 ps time scale. The hybrid nanocrystals show a fluorescence quantum efficiency of ~60% and a Stokes shift as large as 750 meV (~6000 cm-1), which suppresses the emission reabsorption also in bulk devices. The fabricated prototypal nanocomposite fast scintillator shows benchmark performances which compete with those of some inorganic and organic commercial systems.

Exciton-exciton annihilation in organic lanthanide complexes.

We have investigated the mechanism of exciton-exciton annihilation in organic lanthanide complexes by measuring the excitation density dependence of photoluminescence yield in neat films of a model europium chelate. The observed luminescence efficiency reduction at high excitation intensities has been attributed to the operation of the mutual annihilation of the ligand singlet excitons. The second order interaction rate constant, the exciton diffusivity, and the singlet exciton diffusion length in this material have been determined. These results were used to evaluate the role of exciton annihilation in organic light emitting diodes, lanthanide-based organic lasers, and light up-converters. Indications concerning the optimization of the performances of such devices have also been given.

Diffusion-mediated resonant energy transfer in lanthanide-based polymer white-light-emitting diodes.

We present a single layer, polymer white light emitting diode (WOLED) based on novel organic lanthanide complexes. The device is solution processed and possesses CIE coordinates of (0.33, 0.38), corresponding to a high color purity white light emission. The electroluminescence in this diode is found to originate from both the direct charge trapping on the lanthanide complexes and from the efficient diffusion-assisted long-range energy transfer between the conductive matrix and these emissive dopants. An expression for the steady-state transfer efficiency is given, the role of excitation diffusion in the system under study is quantitatively evaluated, and the values for the singlet exciton diffusion coefficients and diffusion lengths in the host are determined.

Metal Nanoclusters with Synergistically Engineered Optical and Buffering Activity of Intracellular Reactive Oxygen Species by Compositional and Supramolecular Design.

Metal nanoclusters featuring tunable luminescence and high biocompatibility are receiving attention as fluorescent markers for cellular imaging. The recently discovered ability of gold clusters to scavenge cytotoxic reactive oxygen species (ROS) from the intracellular environment extends their applicability to biomedical theranostics and provides a novel platform for realizing multifunctional luminescent probes with engineered anti-cytotoxic activity for applications in bio-diagnostics and conceivably cellular therapy. This goal could be achieved by using clusters of strongly reactive metals such as silver, provided that strategies are found to enhance their luminescence while simultaneously enabling direct interaction between the metal atoms and the chemical surroundings. In this work, we demonstrate a synergic approach for realizing multifunctional metal clusters combining enhanced luminescence with strong and lasting ROS scavenging activity, based on the fabrication and in situ protection of Ag nanoclusters with a supramolecular mantle of thiolated-Au atoms (Ag/Au-t). Confocal imaging and viability measurements highlight the biocompatibility of Ag/Au-t and their suitability as fluorescent bio-markers. ROS concentration tests reveal the remarkable scavenging activity of Ag-based clusters. Proliferation tests of cells in artificially stressed culture conditions point out their prolonged anti-cytotoxic effect with respect to gold systems, ensuring positive cell proliferation rates even for long incubation time.

[Work capacity evaluation in patients with valvular prostheses]. / Valutazione della capaciptà lavorativa nei portatori di protesi valvolare.

The possibility of evaluating the capacity for work of subjects wearing heart valve prostheses is analysed. The many factors affecting the return to normal life after operations for valvulopathies are not considered. In strictly medical terms cardiac terms cardiac function can be adequately evaluated by anamnestic and clinical surveys, supplemented by standard non-surgical instrumental examination with particular emphasis on echography, computerised radioisotope angiocardiography and physical exertion tests.

[Treatment using radiofrequency stimulation of supraventricular paroxysmal tachycardia in W.P.W. syndrome]. / Trattamento mediante stimolazione a radiofrequenza di tachicardia parossistica sopraventricolare in sindrome di W.P.W.

The case of woman patient suffering from W.P.W. syndrome and recurrent attacks of supraventricular paroxysmal tachycardia refractory to medical treatment is reported. After electrophysiological study, a radiofrequency coil is implanted. This is activated by the patient during attacks and leads to rapid interruption. The modalities under which paroxysmal tachycardias are interrupted by means of electrostimulating units are discussed and indications for radiofrequency stimulation assessed.

Myocardial infarction with acute thrombosis of multiple major coronary arteries: a clinical and angiographic observation in four patients.

Four patients are reported in whom concomitant obstructive thrombosis of two major coronary vessels was observed at coronary angiography during evolving myocardial infarction. In all cases the simultaneous involvement of both vessels as the cause of acute ischemia was confirmed by the results of sequential treatment of the lesions with emergency angioplasty.

Effect of an external magnetic field on the up-conversion photoluminescence of organic films: the role of disorder in triplet-triplet annihilation.

We have investigated the influence of the magnetic field on the triplet-triplet annihilation process in organic films using a model multicomponent system for blue delayed up-conversion photoluminescence. In such a way, we have derived simple analytical expressions to estimate the overall annihilation probability, outlining the peculiar role played by the disorder and demonstrating that the triplet-triplet fusion in solid films is a diffusion limited process.

Exciton quenching in emitter blends for organic light emitting devices probed by electric field-dependent time-resolved luminescence.

We investigate quenching mechanisms of excited states in emitter layers for organic light emitting diodes (LEDs). An extensive study of a strong electric field-induced modulation (over 50%) of the time-resolved luminescence in a diamine derivative (TPD): polycarbonate blend films doped with an organic complex of europium are presented as a typical example of an important class of emitters for organic monochromatic LEDs. Using this method allowed us to identify the quenched species as the excited ligand precursors of the emissive europium ion states. Manipulating the electrode materials and their electrical bias, the electric field-enhanced dissociation, and interaction with injected charge could be separated and found as principal quenching mechanisms. We show the first one to follow the three-dimensional Onsager theory of geminate recombination, and the second one raised by their interaction with the TPD-transported holes. The interaction rate constant is found to be underlain by the three-dimensional diffusion of excited ligand singlets, combining the exciton diffusion coefficient and long-range (Forster type) energy transfer parameters. The dynamic parameters of the hole-precursor excitons interactions, extracted from the experimental data, allow us to establish the criteria for identifying useful ligands and matrices in the optimized design of electrophosphorescent, linelike emitting molecules, and device structure for organic LEDs.

Superradiance in molecular H aggregates.

Direct evidence of superradiance from an organic semiconductor (quaterthiophene) whose molecules are arranged in a H aggregate fashion, is reported. Time resolved photoluminescence measurements show a linear correlation between the radiative lifetime (tau(rad)) of the purely electronic exciton recombination and the inverse of the number (N(C)) of the coherently emitting dipoles, i.e., tau(rad) proportional, variant 1/N(C). These data support the recently developed theoretical models describing the optical properties of H aggregates of rodlike molecules with a nonvanishing component of the perpendicular molecular transition dipole moment.

[Risk stratification after uncomplicated myocardial infarct: the usefulness of the echocardiographic-dipyridamole test]. / Stratificazione del rischio dopo infarto miocardico non complicato: utilità del test eco-dipiridamolo.

To assess the value and safety of echo-dipyridamole test in risk stratification soon after an uncomplicated acute myocardial infarction, 56 consecutive patients were enrolled in a prospective study with a 1-year follow-up period for new coronary events. Echo-dipyridamole and symptom-limited ECG stress test were performed respectively 14 to 20 days and 4 to 5 weeks after acute event. Echo-dipyridamole test was performed administering 0.84 mg/kg iv of the drug in 10 min: any worsening of left ventricular regional wall motion was considered as a positive test. Up to December 1989, 43 out of 56 patients had their follow-up period completed: the infarction was anterior in 13 (30%), inferior in 22 (51%), non-Q wave in 8 (19%); mean age was 55 +/- 10; basal echocardiographic ejection fraction was 52 +/- 6%. There were no major complications during echo-dipyridamole test. Coronary events occurred in 7 patients (16%): reinfarction in 3, angina in 4; there were no cardiac deaths. A positive echo-dypiridamole test was observed in 12/43 patients (28%); sensitivity versus coronary events was 43%, specificity 75%, negative predictive value 87%. Ten out of 43 patients (23%) had positive and 9/43 (21%) non valuable ECG stress test: sensitivity versus coronary events was 50%, specificity 75%, predictive negative value 88%. The 2 tests showed no significant difference in detecting patients at risk of future coronary events.(ABSTRACT TRUNCATED AT 250 WORDS)

Intrinsic excitonic luminescence in odd and even numbered oligothiophenes.

The emission properties of crystalline oligothiophenes (OTs) are investigated and related to the different selection rules arising from the number of rings. In odd OT crystals the purely electronic emission is absent since the molecular A1-1B(1) transition dipoles cancel at the bottom of the excitonic band. On the contrary, in crystals of even OTs this transition is allowed due to the constructive interference of the off-axis components of the molecular transition dipole. It possesses a polarization in the herringbone plane and exhibits superradiant behavior.