9 research outputs found

    Abordagem contextual nos capítulos de estequiometria e de soluções dos livros didáticos de Química aprovados pelo PNLD (Programa Nacional do Livro Didático - Brasil) /2012

    Get PDF
    A contextualização tem sido proposta no currículo escolar com o propósito de promover mudanças de comportamentos, atitudes e valores. Nessa abordagem, entende-se que o ensino de química deve se desenvolver de forma ampla, envolvendo também aspectos sociais, políticos, econômicos,ambientais e culturais, e não somente aspectos científicos. Tendo em vista a grande importância dada ao livro didático (LD) na prática docente, buscamos analisar a abordagem contextual nos capítulos de estequiometria e soluções nos LD de química aprovados pelo PNLD/2012. Os resultados mostraram que os autores dos LD analisados reconhecem a contextualização como elemento central para a formação da cidadania, porém a abordagem é diferente em cada obra. As leituras dos LD possibilitaram a identificação de alguns "níveis" de contextualização que percorreram quatro categorias de análise

    Resolving the Impact of Hydrogen Bonding on the Phylloquinone Cofactor through Two-Dimensional Infrared Spectroscopy

    No full text
    Two-dimensional infrared spectroscopy (2DIR) was applied to phylloquinone (PhQ), an important biological cofactor, to elucidate the impact of hydrogen bonding on the ultrafast dynamics and energetics of the carbonyl stretching modes. 2DIR measurements were performed on PhQ dissolved in hexanol, which served as the hydrogen bonding solvent, and hexane, which served as a non-hydrogen bonding control. Molecular dynamics simulations and quantum chemical calculations were performed to aid in spectral assignment and interpretation. From the position of the peaks in the 2DIR spectra, we extracted the transition frequencies for the fundamental, overtone, and combination bands of hydrogen bonded and non-hydrogen bonded carbonyl groups of PhQ in the 1635–1680 cm–1 region. We find that hydrogen bonding to a single carbonyl group acts to decouple the two carbonyl units of PhQ. Through analysis of the time-resolved 2DIR data, we find that hydrogen bonding leads to faster vibrational relaxation as well as an increase in the inhomogeneous broadening of the carbonyl groups. Overall, this work demonstrates how hydrogen bonding to the carbonyl groups of PhQ presents in the 2DIR spectra, laying the groundwork to use PhQ as a 2DIR probe to characterize the ultrafast fluctuations in the local environment of natural photosynthetic complexes

    Luminescent Ce(III) Complexes as Stoichiometric and Catalytic Photoreductants for Halogen Atom Abstraction Reactions

    No full text
    Luminescent Ce­(III) complexes, Ce­[N­(SiMe<sub>3</sub>)<sub>2</sub>]<sub>3</sub> (<b>1</b>) and [(Me<sub>3</sub>Si)<sub>2</sub>NC­(RN)<sub>2</sub>]­Ce­[N­(SiMe<sub>3</sub>)<sub>2</sub>]<sub>2</sub> (R = <sup><i>i</i></sup>Pr, <b>1-</b><sup><i><b>i</b></i></sup><b>Pr</b>; R = Cy, <b>1-Cy</b>), with <i>C</i><sub>3<i>v</i></sub> and <i>C</i><sub>2<i>v</i></sub> solution symmetries display absorptive 4f → 5d electronic transitions in the visible region. Emission bands are observed at 553, 518, and 523 nm for <b>1</b>, <b>1-</b><sup><i><b>i</b></i></sup><b>Pr</b>, and <b>1-Cy</b> with lifetimes of 24, 67, and 61 ns, respectively. Time-dependent density functional theory (TD-DFT) studies on <b>1</b> and <b>1-</b><sup><i><b>i</b></i></sup><b>Pr</b> revealed the <sup>2</sup>A<sub>1</sub> excited states corresponded to singly occupied 5d<sub><i>z</i><sup>2</sup></sub> orbitals. The strongly reducing metalloradical character of <b>1</b>, <b>1-</b><sup><i><b>i</b></i></sup><b>Pr</b>, and <b>1-Cy</b> in their <sup>2</sup>A<sub>1</sub> excited states afforded photochemical halogen atom abstraction reactions from sp<sup>3</sup> and sp<sup>2</sup> C–X (X = Cl, Br, I) bonds for the first time with a lanthanide cation. The dehalogenation reactions could be turned over with catalytic amounts of photosensitizers by coupling salt metathesis and reduction to the photopromoted atom abstraction reactions

    Accelerated 2D-IR Using Compressed Sensing

    No full text
    A fundamental aspect of Fourier transform (FT) spectroscopy is the inverse relationship between frequency resolution and the maximum scanned time delay. In situations where essential chemical information is contained in spectral peak amplitudes rather than in their detailed shapes, it is possible to dramatically reduce the experimental acquisition time of time domain methods such as two-dimensional infrared (2D-IR) spectroscopy. By introducing compressed sensing to the analysis and experimental design of 2D-IR spectroscopy, we show that waiting-time-dependent 2D peak amplitudes reproduce conventional FT acquisition and analysis but can be recorded in a fraction of the time. Peak amplitude data are often sufficient for measuring intramolecular vibrational redistribution, vibrational coherence, chemical exchange, population, and orientational relaxation, as well as spectral diffusion

    Ultrafast Solvation Dynamics and Vibrational Coherences of Halogenated Boron-Dipyrromethene Derivatives Revealed through Two-Dimensional Electronic Spectroscopy

    No full text
    Boron-dipyrromethene (BODIPY) chromophores have a wide range of applications, spanning areas from biological imaging to solar energy conversion. Understanding the ultrafast dynamics of electronically excited BODIPY chromophores could lead to further advances in these areas. In this work, we characterize and compare the ultrafast dynamics of halogenated BODIPY chromophores through applying two-dimensional electronic spectroscopy (2DES). Through our studies, we demonstrate a new data analysis procedure for extracting the dynamic Stokes shift from 2DES spectra revealing an ultrafast solvent relaxation. In addition, we extract the frequency of the vibrational modes that are strongly coupled to the electronic excitation, and compare the results of structurally different BODIPY chromophores. We interpret our results with the aid of DFT calculations, finding that structural modifications lead to changes in the frequency, identity, and magnitude of Franck–Condon active vibrational modes. We attribute these changes to differences in the electron density of the electronic states of the structurally different BODIPY chromophores

    Cerium Photosensitizers: Structure–Function Relationships and Applications in Photocatalytic Aryl Coupling Reactions

    No full text
    Two complete mixed-ligand series of luminescent Ce<sup>III</sup> complexes with the general formulas [(Me<sub>3</sub>Si)<sub>2</sub>NC­(N<sup><i>i</i></sup>Pr)<sub>2</sub>]<sub><i>x</i></sub>Ce<sup>III</sup>[N­(SiMe<sub>3</sub>)<sub>2</sub>]<sub>3–<i>x</i></sub> (<i>x</i> = 0, <b>1-N</b>; <i>x</i> = 1, <b>2-N</b>, <i>x</i> = 2, <b>3-N</b>; <i>x</i> = 3, <b>4</b>) and [(Me<sub>3</sub>Si)<sub>2</sub>NC­(N<sup><i>i</i></sup>Pr)<sub>2</sub>]<sub><i>x</i></sub>Ce<sup>III</sup>(OAr)<sub>3–<i>x</i></sub> (<i>x</i> = 0, <b>1-OAr</b>; <i>x</i> = 1, <b>2-OAr</b>, <i>x</i> = 2, <b>3-OAr</b>; <i>x</i> = 3, <b>4</b>) were developed, featuring photoluminescence quantum yields up to 0.81(2) and lifetimes to 117(1) ns. Although the 4f → 5d absorptive transitions for these complexes were all found at ca. 420 nm, their emission bands exhibited large Stokes shifts with maxima occurring at 553 nm for <b>1-N</b>, 518 nm for <b>2-N</b>, 508 nm for <b>3-N</b>, and 459 nm for <b>4</b>, featuring yellow, lime-green, green, and blue light, respectively. Combined time-dependent density functional theory (TD-DFT) calculations and spectroscopic studies suggested that the long-lived <sup>2</sup>D excited states of these complexes corresponded to singly occupied 5d<sub><i>z</i><sup>2</sup></sub> orbitals. The observed difference in the Stokes shifts was attributed to the relaxation of excited states through vibrational processes facilitated by the ligands. The photochemistry of the sterically congested complex <b>4</b> was demonstrated by C–C bond forming reaction between 4-fluoroiodobenzene and benzene through an outer sphere electron transfer pathway, which expands the capabilities of cerium photosensitizers beyond our previous results that demonstrated inner sphere halogen atom abstraction reactivity by <b>1-N</b>

    Energy Transfer Pathways in Light-Harvesting Complexes of Purple Bacteria as Revealed by Global Kinetic Analysis of Two-Dimensional Transient Spectra

    No full text
    Excited state dynamics in LH2 complexes of two purple bacterial species were studied by broad-band two-dimensional electronic spectroscopy. The optical response was measured in the 500–600 nm spectral region on the 0–400 fs time scale. Global target analysis of two-dimensional (2D) transient spectra revealed the main energy transfer pathways between carotenoid S<sub>2</sub>, 1B<sub>u</sub><sup>–</sup> and S<sub>1</sub> states and bacteriochlorophyll Q<sub><i>x</i></sub> state. Global analysis ascertained the evolutionary and vibration-associated spectra, which also indicated the presence of a higher-lying vibrational level in the carotenoid S<sub>1</sub> state. The estimation of the spectral overlap between the 1B<sub>u</sub><sup>–</sup> state and the Q<sub><i>x</i></sub> state indicated a significant contribution of the 1B<sub>u</sub><sup>–</sup> state to the overall S<sub>2</sub>-to-Q<sub><i>x</i></sub> excitation energy transfer

    The Hexachlorocerate(III) Anion: A Potent, Benchtop Stable, and Readily Available Ultraviolet A Photosensitizer for Aryl Chlorides

    No full text
    The hexachlorocerate­(III) anion, [Ce<sup>III</sup>Cl<sub>6</sub>]<sup>3–</sup>, was found to be a potent photoreductant in acetonitrile solution with an estimated excited-state reduction potential of −3.45 V versus Cp<sub>2</sub>Fe<sup>0/+</sup>. Despite a short lifetime of 22.1(1) ns, the anion exhibited a photoluminescence quantum yield of 0.61(4) and fast quenching kinetics toward organohalogens allowing for its application in the photocatalytic reduction of aryl chloride substrates

    Understanding and Controlling the Emission Brightness and Color of Molecular Cerium Luminophores

    No full text
    Molecular cerium complexes are a new class of tunable and energy-efficient visible- and UV-luminophores. Understanding and controlling the emission brightness and color are important for tailoring them for new and specialized applications. Herein, we describe the experimental and computational analyses for series of <i>tris</i>(guanidinate) (<b>1</b>–<b>8</b>, Ce­{(R<sub>2</sub>N)­C­(N<sup><i>i</i></sup>Pr)<sub>2</sub>}<sub>3</sub>, R = alkyl, silyl, or phenyl groups), guanidinate-amide [<b>GA</b>, <b>A</b> = N­(SiMe<sub>3</sub>)<sub>2</sub>, <b>G</b> = (Me<sub>3</sub>Si)<sub>2</sub>NC­(N<sup><i>i</i></sup>Pr)<sub>2</sub>], and guanidinate-aryloxide (<b>GOAr</b>, <b>OAr</b> = 2,6-di-<i>tert</i>-butylphenoxide) cerium­(III) complexes to understand and develop predictive capabilities for their optical properties. Structural studies performed on complexes <b>1</b>–<b>8</b> revealed marked differences in the steric encumbrance around the cerium center induced by various guanidinate ligand backbone substituents, a property that was correlated to photoluminescent quantum yield. Computational studies revealed that consecutive replacements of the amide and aryloxide ligands by guanidinate ligand led to less nonradiative relaxation of bright excited states and smaller Stokes shifts. The results establish a comprehensive structure–luminescence model for molecular cerium­(III) luminophores in terms of both quantum yields and colors. The results provide a clear basis for the design of tunable, molecular, cerium-based, luminescent materials
    corecore