289 research outputs found
Energia per l’astronave Terra
No full textLe coordinate indispensabili per orientarsi nel labirinto delle fonti di energi
Fonti di energia e modello di sviluppo
No full textIl punto sulle conoscenze tecnologiche, i costi e l'impatto ambientale delle fonti di energi
Excited-State Engineering in Heteroleptic Ionic Iridium(III) Complexes
No full textIridium(III) complexes have assumed a prominent role in the areas of photochemistry and photophysics due to the peculiar properties of both the metal itself and the ligand environment that can be assembled around it. Ir(III) is larger, heavier, and bears a higher ionic charge than its analogue and widely used d6 ions such as Fe(II) and Ru(II). Accordingly, its complexes exhibit wider ligand-field d-d orbital splitting with electronic levels centered on the metal, typically nonemissive and photodissociative, not playing a relevant role in excited-state deactivations. In other words, iridium complexes are typically more stable and/or more emissive than Fe(II) and Ru(II) systems. Additionally, the particularly strong heavy-atom effect of iridium promotes singlet-triplet transitions, with characteristic absorption features in the UV-vis and relatively short excited-state lifetimes of emissive triplet levels. Ir(III) is also a platform for anchoring ligands of rather different sorts. Its versatile chemistry includes not only coordination with classic N∧N neutral ligands but also the binding of negatively charged chelators, typically having a cyclometalating C∧N anchor. The carbon-metal bond in these systems has some degree of covalent character, but this does not preclude a localized description of the excited states of the related complexes, which can be designated as metal-centered (MC), ligand-centered (LC), or charge transfer (CT), allowing a simplified description of electronic and photophysical properties. The possibility of binding different types of ligands and making heteroleptic complexes is a formidable tool for finely tuning the nature and energy of the lowest electronic excited state of cationic Ir(III) complexes by ligand design. Herein we give an account of our work on several families of iridium complexes typically equipped with two cyclometalating bidentate ligands (C∧N), in combination with mono or bidentate "ancillary"ligands with N∧N, C∧N, and C∧C motifs. We have explored new synthesis routes for both cyclometalating and ancillary ligands, obtaining primarily cationic complexes but also some neutral or even negatively charged systems. In the domain of the ancillary ligands, we have explored isocyanides, carbenes, mesoionic triazolylidenes, and bis-tetrazolic ligands. For the cyclometalating moiety, we have investigated carbene, mesoionic triazolylidene, and tetrazolic systems. Key results of our work include new strategies to modify both cyclometalating and ancillary ligands by relocating ionic charges, the determination of new factors affecting the stability of complexes, a demonstration of subtle structural effects that strongly modify the photophysical properties, new options to get blue-greenish emitters for optoelectronic devices, and a set of ligand modifications allowing the optimization of electrochemical and excited-state properties to obtain new promising Ir(III) complexes for photoredox catalysis. These results constitute a step forward in the preparation of custom iridium-based materials crafted by excited-state engineering, which is achieved through the concerted effort of computational and synthetic chemistry along with electrochemistry and photochemistr
Photochemistry and photophysics of coordination compounds: Copper
No full textCu(I) complexes and clusters are the largest class of compounds of relevant photochemical and photophysical interest based on a relatively abundant metal element. Interestingly, Nature has given an essential role to copper compounds in some biological systems, relying on their kinetic lability and versatile coordination environment. Some basic properties of Cu(I) and Cu(II) such as their coordination geometries and electronic levels are compared, pointing out the limited significance of Cu(II) compounds (d 9 configuration) in terms of photophysical properties. Well-established synthetic protocols are available to build up a variety of molecular and supramolecular architectures (e.g. catenanes, rotaxanes, knots, helices, dendrimers, cages, grids, racks, etc.) containing Cu(I)-based centers and exhibiting photo- and electroluminescence as well as light-induced intercomponent processes. By far the largest class of copper complexes investigated to date is that of Cu(I)-bisphenanthrolines ([Cu(NN)2]+) and recent progress in the rationalization of their metal-to-ligand charge-transfer (MLCT) absorption and luminescence properties are critically reviewed, pointing out the criteria by which it is now possible to successfully design highly emissive [Cu(NN)2]+ compounds, a rather elusive goal for a long time. To this end the development of spectroscopic techniques such as light-initiated time-resolved X-ray absorption spectroscopy (LITR-XAS) and femtosecond transient absorption have been rather fruitful since they have allowed us to firmly ground the indirect proofs of the molecular rearrangements following light absorption that had accumulated in the past 20 years. A substantial breakthrough towards highly emissive Cu(I) coordination compounds is constituted by heteroleptic Cu(I) complexes containing both N- and P-coordinating ligands ([Cu(NN)(PP)]+) which may exhibit luminescence quantum yields close to 30% in deaerated CH2Cl2 solution and have been successfully employed as active materials in OLED and LEC optoelectronic devices. Also copper clusters may exhibit luminescence bands of halide-to-metal charge transfer (XMCT) and/or cluster centered (CC) character and they are briefly reviewed along with miscellaneous Cu(I) compounds that recently appeared in the literature, which show luminescence bands ranging from the blue to the red spectral region. © 2007 Springer-Verlag Berlin Heidelberg
When zero is a result: how to demonstrate the absence of an infection
No full textDemonstrating the absence of an infection from a population can be problematic since it would theoretically require the testing of all individuals with a test that had both 100% sensitivity and specificity. However, this is hardly realistic since every diagnostic test retains some inherent measure of uncertainty and disease surveys are typically carried out on a subset of individuals sampled from the target population.
This problem can be overcome by estimating the confidence of freedom from a disease, which represents the probability that an infection is truly absent given that a determinate number of individuals from that population are all negative to a specific diagnostic test. Confidence of freedom extends the concept of negative predictive value to the whole population by taking into account sample size, expected prevalence of the disease and population-level specificity and sensitivity of the diagnostic test of choice.
This approach can be most useful in the context of biological invasions, when there is a concrete risk of alien hosts introducing new pathogens that may threaten public health or native wildlife but, at the same time, there is no previous epidemiological history about either the host or the disease in the introduction area.
To illustrate this method, we will apply confidence of freedom on two case studies involving the potential introduction of a disease by alien hosts. Although in both surveys we did not find any evidence of the target pathogen, an accurate interpretation of negative results was crucial for either public health (case study I) or conservation (case study II) purposes.
Case study I – Raccoons (Procyon lotor) and Baylisascaris procyonis: we applied confidence of freedom to preliminary results obtained during a survey aimed at detecting whether introduced raccoons had carried along to northern Italy the zoonotic nematode B. procyonis.
Case study II – Grey squirrels (Sciurus carolinensis) and squirrelpoxvirus: we applied confidence of freedom to data obtained through two different diagnostic methods to determine whether alien grey squirrels had introduced to Italy a poxvirus which is known to severely threaten native squirrels in the UK.
Through these examples, we will show how assuming different prevalences and choosing different diagnostic tests (i.e. with different specificity and sensitivity) and sample sizes will affect confidence of freedom estimation
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