Las excepcionales propiedades de los [n]CPPs: ¿por qué importa tanto el tamaño y la estructura molecular?

Tesis inédita de la Universidad Complutense de Madrid, Facultad de Ciencias Químicas, Departamento de Química Física I, leída el 15/04/2016. Tesis formato europeo (compendio de artículos)Cycloparaphenylenes ([n]CPPs from now on), were firstly synthesized in 2008, and promptly joined the fascinating world of carbon nano-materials in their own right. These systems are formed by cyclic molecules with n phenyl units connected each other in their para-position, a configuration that resembles an ultra-short armchair single-wall carbon nanotube (SWCNT). In this Ph. D. Thesis we put forward a systematic study on the electronic properties of [n]CPPs, providing a thoroughly link between [n]CPPs, SWCNTs and their linear analogues ([n]LPPs). Curvature, strain and electronic π‒conjugation are the main characteristics of [n]CPPs that we will systematically study to examine their unexplored properties, and eventually their potential applications. In addition, we shall use [n]CPPs as models to realize the concept of radial cyclic electronic π‒conjugation, which has been only theoretically considered so far. Let us emphasize that we shall take advantage of the success of Raman spectroscopy for characterizing carbon-based materials. Raman spectroscopy is also among the most suitable tools to probe the electronic properties of such materials due to the high susceptibility of the π-electrons polarizability. In this Ph. D. Thesis we present a systematic study of a series of [n]CPPs considering all the geometrical parameters involved in their vibrational modes. We shall examine the influence of size on the Raman spectra of [n]CPPs, together with the effect of oxidation and mechanical compression on their molecular structures. Finally, we will explore the hosting capabilities of [n]CPPs as well as their mechanical properties relative to those of SWCNTs...Esta tesis doctoral tiene como eje fundamental el análisis exhaustivo de diversas propiedades de los compuestos denominados [n]ciclo-parafenilenos (desde ahora [n]CPPs). Estas moléculas cíclicas están formadas por fenilenos unidos en posición para mediante enlaces sencillos. Como consecuencia de la configuración cíclica, los [n]CPPs poseen un alto grado de tensión interna, dado que sus unidades fenilénicas se ven forzadas a doblarse parcialmente para poder cerrar el ciclo. Asimismo, esta configuración fenilénica cíclica hace que los [n]CPPs se hayan considerado como la mínima unidad de nanotubos de carbono con quiralidad única de tipo armchair. El aspecto esencial de la tesis ha sido el estudio experimental y teórico de la espectroscopía Raman de los [n]CPPs, con un análisis de la dependencia de la misma con el tamaño, curvatura, tensión y conjugación electrónica. Por otro lado, también se ha estudiado la influencia de la temperatura, presión y la oxidación de los [n]CPPs en las variables espectroscópicas. Esto nos ha permitido explorar otras propiedades de los [n]CPPs como son: la conjugación electrónica, su deformabilidad y otras propiedades mecánicas, comparando el comportamiento observado en las mismas para los oligoparafenilenos lineales y los nanotubos de carbono. Por último, también se ha comprobado que los [n]CPPs y los nanotubos de carbono comparten propiedades relacionadas con su configuración tubular como son su habilidad para hospedar sistemas del tamaño apropiado...Depto. de Química FísicaFac. de Ciencias QuímicasTRUEunpu

Investigating Piezochromic Properties of πconjugated Materials: a combined Raman and DFT Study

Π-conjugated materials have been studied for decades due to their great interest in organic electronics. Among them, piezochromic materials, which exhibit color changes resulting from external pressure or mechanical grinding, become very attractive from a scientific viewpoint owing to their potential applications as pressure-sensing and opticalrecording systems1. The main target of the present work is to develop a joint experimental and theoretical approach able to deliver crucial information for the understanding of the structural effects causing piezochromic changes. To this end, we will focus our attention on two families of -conjugated materials with potential application as sensors. The first one is a family of 9,10-Bis((E)-n(pyrid-2-yl)vinyl) anthracene, BPnVA (n=2 orto-,3 metha- and 4 para-) compounds, see Figure 1. Interestingly, these three compounds with varying position of the nitrogen on the external pyridyl group exhibit different molecular packing modes. Grinding and the application of external pressure on the powder also led to a strong change in their photoluminescence color.2 The second familiy is based on the N,N’-Bis-Boc-3,3’-di(pyren-1yl)-2,2’-biindole compound (compound 1c in Figure 1) which also shows photoluminescence under pressure application. However, the molecular packing induced by this phenomenon is still under study 3. In both cases, it will be also very interesting to determine how temperature affects the vibrational structure of these systems.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Piezochromic properties of pyridil divinyl anthracene derivatives: a joint Raman and DFT study

Piezochromic materials, which show color changes resulting from external pressure or mechanical grinding, are of great interest from a scientific viewpoint owing to their potential applications as pressure-sensing and optical-recording systems.[1] It has been recently discovered solid-state fluorescence properties and piezochromic luminescence in a family of 9,10-Bis((E)-n-(pyrid-2-yl)vinyl)anthracene, BPnVA (n=2 orto-,3 metha- and 4 para-) compounds, see Figure 1.[2] The crystal structures indicate that the three compounds with varying position of the nitrogen on the external pyridyl exhibit different molecular packing modes. In this work, we aim at providing further insight into the interplay played by the molecular and supramolecular structural changes and the luminescence changes observed under pressure by using a combined experimental-theoretical approach that links Raman spectroscopy with DFT calculations.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tec

Piezochromic properties of a D-A-D platform: A joint experimental and theoretical perspective

Organic π-conjugated molecules with mechanochromic luminescence properties have attracted great interest in the last two decades due to their numerous applications in the optoelectronic field, such as sensors, probes and security inks. Materials that modify their colour under pressure are known as piezochromic materials. Usually, this variation is provoked by changes in the molecular structure, for example, crystal-to-amorphous phase transitions, modifications in dihedral angles or bond distances, and intermolecular interactions. The molecule proposed in this study is a TADF (Thermally activated delayed fluorescence) U-shaped molecule composed by two donors and one acceptor (D-A-D) units with a π-conjugated skeleton [1]. It was synthetized as a powder which under different crystallization methods gives rise to different conformers varying the dihedral angle of the bond that links the D and A units. The donors are two phenothiazine units and the acceptor is a dibenzo[a,j]phenazine unit located in the central core. Two different conformers have been analysed: the quasi equatorial - quasi equatorial (denoted as 1R) and the quasi axial - quasi axial (denoted as 1Y). In this project, we study the configurational changes triggering the piezochromic effects combining density functional theory (DFT) calculations with Raman spectroscopy experiments of the 1R and 1Y conformers during heating or in compression via a sapphire anvil cell [2]. Both show pressure and temperature dependence properties. Besides, these changes are reversible meaning that when the stimuli stop they revert to its original conformation. When these molecules are exposed to different ambient (like pressure or temperature variations) they evolved to a third conformer with an intermediate dihedral angle that results in different Raman, emission and absorption behaviour.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

High-pressure polymorphism in pyridine

Single crystals of the high-pressure phases II and III of pyridine have been obtained by in situ crystallization at 1.09 and 1.69 GPa, revealing the crystal structure of phase III for the first time using X-ray diffraction. Phase II crystallizes in P212121 with Z' = 1 and phase III in P41212 with Z' = ½. Neutron powder diffraction experiments using pyridine-d5 establish approximate equations of state of both phases. The space group and unit-cell dimensions of phase III are similar to the structures of other simple compounds with C 2v molecular symmetry, and the phase becomes stable at high pressure because it is topologically close-packed, resulting in a lower molar volume than the topologically body-centred cubic phase II. Phases II and III have been observed previously by Raman spectroscopy, but have been mis-identified or inconsistently named. Raman spectra collected on the same samples as used in the X-ray experiments establish the vibrational characteristics of both phases unambiguously. The pyridine molecules interact in both phases through CH⋯π and CH⋯N interactions. The nature of individual contacts is preserved through the phase transition between phases III and II, which occurs on decompression. A combination of rigid-body symmetry mode analysis and density functional theory calculations enables the soft vibrational lattice mode which governs the transformation to be identified

Reversible Dimerization/Polymerization of a Janus Diradical Producing Labile CC Bonds and Giant Chromism

En esta comunicación se presenta el estudio espectroscópico y téorico de la dimerización/oligomerización reversible de un birradical derivado de naftoditiofeno el cual en su forma monomérica presenta una intenso color azul mientras que al agregar se muestra incoloro, siendo éstas formas intercambiables gracias a la aplicación de estímulos débiles como presión o temperatura.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Assessing aids for the elaboration of argumentative synthesis in the University: Guide or Rubric?

Although most of the interventions to foster the integration of multiple pieces of information has focused on teaching “how” to integrate (Barzilai et al., 2018) providing different kinds of instruction on the processes implied, instruction focusing on the criteria to evaluate integration are scarce and mainly centered on integrating complementary, but not contradictory, sources. In this work we designed and contrasted the effectiveness of two aids to improve the quality of the syntheses written by undergraduates from texts that defended opposing views on a topic: a Guide to the process of preparing a synthesis vs a Rubric with the evaluation criteria of written products. The number of arguments and the degree of integration were examined. Also, the students' perception was assessed regarding the usefulness of the aids to plan, to write and to review their synthesis. Seen that we found that offering a guide accompanied by explicit instruction is more effective than providing it without explicit instruction Mateos et al. (2018), the two aids were offered here under two conditions: Explicit Instruction vs. Traditional Instruction. Thus, we carried out an experimental study with 122 Psychology students, distributed randomly into four intervention programs. In the first and fourth session they performed a synthesis task without support. In the second session, after receiving the instruction (explicit vs. traditional), each student made a new synthesis using the specific aid provided (practice 1, with Guide/Rubric). In the third session the participants carried out another synthesis under analogous conditions (practice 2, with aids). Our results pointed out that, regardless the instruction, both aids (Guide and Rubric) improved the degree of integration of the students’ syntheses. However, the guide contributed better to improve the selection of arguments. Students who received Explicit Instruction found both aids to be equally useful for planning and writing their syntheses. In contrast, after Traditional Instruction the Guide was perceived as more useful for planning and writing syntheses. The Guide was more useful for reviewing their texts. We will discuss the implications of these findings for synthesis writing training and conclude with recommendations and suggestions for future studies.2021-2

Post-tilleyite, a dense calcium silicate-carbonate phase

Calcium carbonate is a relevant constituent of the Earth´s crust that is transferred into the deep Earth through the subduction process. Its chemical interaction with calcium-rich silicates at high temperatures give rise to the formation of mixed silicate-carbonate minerals, but the structural behavior of these phases under compression is not known. Here we report the existence of a dense polymorph of Ca5(Si2O7)(CO3)2 tilleyite above 8 GPa. We have structurally characterized the two phases at high pressures and temperatures, determined their equations of state and analyzed the evolution of the polyhedral units under compression. This has been possible thanks to the agreement between our powder and single-crystal XRD experiments, Raman spectroscopy measurements and ab-initio simulations. The presence of multiple cation sites, with variable volume and coordination number (6-9) and different polyhedral compressibilities, together with the observation of significant amounts of alumina in compositions of some natural tilleyite assemblages, suggests that post-tilleyite structure has the potential to accommodate cations with different sizes and valencies.We are thankful for the financial support received from the Spanish Ministry of Economy and Competitiveness through national projects PGC2018-097520-A-100, MAT2016-75586-4-3-P and MAT2015-71070-REDC (MALTA Consolider)