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.

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.

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.

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.

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.

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.

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.

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.

[Primary angioplasty in acute inferior myocardial infarct with anterior ST-segment depression: the long-term follow-up]. / Angioplastica primaria nell'infarto miocardico acuto inferiore con sottoslivellamento anteriore del tratto ST: follow-up a lungo termine.

BACKGROUND: Concomitant anterior ST-segment depression is a marker of severe prognosis in inferior myocardial infarction. PATIENTS AND METHODS: Prospective observational study in patients with inferior acute myocardial infarction and ST-segment depression > or = 4 mm in the anterior leads, who were treated with primary angioplasty. Angiography was performed at hospital discharge and at six months, and a clinical follow-up was obtained at one year after the infarction. RESULTS: Sixty-three patients were included in the study. Pre-hospital and in-hospital delay were 147 +/- 70 minutes (20-355) and 54 +/- 11 minutes (18-80), respectively. Angioplasty was successful in all patients and 48 stents were implanted in 36 patients (57%). Angiography was performed at hospital discharge in 55 patients (87%) and showed a TIMI grade 3 coronary flow in the infarct-related artery in all cases. The left ventricular ejection fraction was 0.55 +/- 0.09 (0.4-0.8). One patient (1.6%) died before discharge, two (3.2%) had ischemic complications (one had non-fatal reinfarction, another had recurrent angina at rest), and three (4.9%) had local vascular complications. At the six-month follow-up, none of the patients had died. One had suffered reinfarction (1.6%) and another had been readmitted for recurrence of angina at rest (1.6%); none had symptoms of stable angina. The ejection fraction was 0.56 +/- 0.12 and eight patients (14%) showed angiographic restenosis. At twelve months, two patients had died (1.6%) and five (8%) had required readmission to hospital. CONCLUSIONS: Primary angioplasty yielded favorable results in this group of patients. Our data confirm the efficacy of primary angioplasty for the treatment of acute myocardial infarction, with a low rate of clinical (3.2%) and angiographic (14%) restenosis at six months, and a high rate (87%) of event-free survival at one year follow-up.

Early aggressive treatment of unstable angina without on-site cardiac surgical facilities: a prospective study of acute and long-term outcome.

BACKGROUND: The early invasive diagnostic approach with extensive use of myocardial revascularization in patients with unstable angina is a matter of debate. Both the advantages of this strategy and the choice of the best candidates are controversial. The widespread applicability of this approach in Italian hospitals is also questionable, due to limited availability of facilities for interventional cardiology. METHODS: A prospective, observational study was done on a cohort of consecutive patients, who were admitted with a diagnosis of unstable angina and treated with an early aggressive approach at a center with interventional cardiology facilities without cardiac surgery. The aim of the study was to evaluate both the immediate and long-term clinical outcome of patients and the efficiency of our therapeutic approach. RESULTS: Two-hundred and two patients were enrolled and 85% were in Braunwald class III. Coronary angiography was performed in 171 patients (85%) at 2.1 +/- 2.4 days after admission: it showed one-, two- and three-vessel disease in 40, 29 and 22% of cases, respectively; 9% of patients had no severe coronary lesion. Left ventricular ejection fraction was 0.58 +/- 0.13. Medical treatment, coronary by-pass surgery and percutaneous myocardial revascularization were chosen in 36, 24 and 40% of cases, respectively. Coronary angioplasty was performed in our center in 58 (73%) of 80 patients at 6.8 +/- 5.6 days after admission and stents were used in 42 cases (74%). Overall hospital stay was 10.4 +/- 4 days. Cumulated adverse events (death and non-fatal myocardial infarction) occurred in 2.5 and 7% of patients during the initial admission and in the following year, respectively. CONCLUSIONS: An early aggressive approach to patients with unstable angina is feasible in a hospital with interventional cardiology in the absence of cardiac surgical facilities. The immediate favorable clinical results of this strategy in an intermediate-risk cohort seem to persist at one-year follow-up.

[The use of a check list for quality assurance of the treatment of acute myocardial infarction in the coronary care unit]. / Uso di un registro per il controllo di qualità del trattamento dell'infarto miocardico acuto in unità coronarica.

UNLABELLED: The implementation of Quality Assurance programs for the treatment of acute myocardial infarction in the Cardiac Intensive Care Unit may be specially important. In fact several therapeutic options are available in these patients, and delay in treatment must be as short as possible. A Quality Assurance program has been started in our center with a registry of all patients admitted within 24 hours of onset of acute myocardial infarction. PATIENTS AND METHODS: The following data were recorded: 1) indicators of Organization: pathway to admission, pre-hospital and in-hospital delay; 2) Process Indicators: duration of hospital stay, initial choice of therapy (conservative, intravenous lysis, primary angioplasty), and further diagnostic and interventional procedures; 3) Outcome Indicators: mortality and complications during admission, and 6-12 months follow-up. RESULTS: Since february 1994 to August 1995, 211 consecutive patients were included in the registry; 156 were male, mean age 66 years. Mean pre-hospital delay was 286 minutes. Admission was decided by a physician in 99 cases and by the patient him/herself in 112 cases; pre-hospital delay was 390 min. In the former group, and 194 min. In the latter (p < .001). Mean in-hospital delay was 61 minutes. Conservative treatment, intravenous lysis, and primary angioplasty were chosen by the attending cardiologist in 89 patients (group A), 69 patients (group B), and 53 patients (group C) respectively. The latter group included patients with highest risk on the basis of clinical and electrocardiographic characteristics. In-hospital mortality was 17, 7 and 9% In the 3 groups, respectively. An echocardiogram and coronary angiography were performed before discharge in 81% and 57% of patients, respectively. The mean duration of hospital stay was 11 days, irrespective of the initial therapeutic choice. CONCLUSIONS: A registry for patients with acute myocardial infarction provides information which is essential in the evaluation of therapeutic protocols; it may also help in improving the cooperation between the Emergency Department, the attending cardiologists, and the family physicians.

[Emergency angioplasty in high-risk acute infarct]. / Angioplastica d'emergenza nell'infarto acuto ad alto rischio.

BACKGROUND: Primary coronary angioplasty in acute myocardial infarction yields superior results in terms of effective vessel patency, recurrent ischemia and acute morbidity, as compared to intravenous thrombolysis. Despite obvious logistic and economic limitations, this early invasive approach could be strictly indicated in selected groups of patients. AIM: A prospective study to test the immediate and short-term results of an early invasive strategy with angioplasty in patients with high-risk acute myocardial infarction. METHODS: Forty-one consecutive patients with high-risk acute myocardial infarction were submitted to coronary angiography and angioplasty of the involved vessel within 12 hours of onset of symptoms. Twenty-eight had anterior myocardial infarction and 6 were in cardiogenic shock. RESULTS: Primary success was achieved in 38 patients (93%): 30 of these were discharged without complications or further revascularization procedures, and none has symptoms or re-infarction at one-month follow-up. CONCLUSIONS: Immediate invasive treatment with angioplasty in high-risk acute myocardial infarction seems to achieve good results, due to both effective infarct vessel recanalization, and early identification of candidates to urgent complete surgical revascularization.