9 research outputs found
Abordagem contextual nos capÃtulos de estequiometria e de soluções dos livros didáticos de QuÃmica aprovados pelo PNLD (Programa Nacional do Livro Didático - Brasil) /2012
A contextualização tem sido proposta no currÃculo escolar com o propósito de promover mudanças de comportamentos, atitudes e valores. Nessa abordagem, entende-se que o ensino de quÃmica deve se desenvolver de forma ampla, envolvendo também aspectos sociais, polÃticos, econômicos,ambientais e culturais, e não somente aspectos cientÃficos. Tendo em vista a grande importância dada ao livro didático (LD) na prática docente, buscamos analisar a abordagem contextual nos capÃtulos de estequiometria e soluções nos LD de quÃmica 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 "nÃveis" de contextualização que percorreram quatro categorias de análise
Resolving the Impact of Hydrogen Bonding on the Phylloquinone Cofactor through Two-Dimensional Infrared Spectroscopy
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
Luminescent CeÂ(III)
complexes, CeÂ[NÂ(SiMe<sub>3</sub>)<sub>2</sub>]<sub>3</sub> (<b>1</b>) and [(Me<sub>3</sub>Si)<sub>2</sub>NCÂ(RN)<sub>2</sub>]ÂCeÂ[NÂ(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
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
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
Two complete mixed-ligand
series of luminescent Ce<sup>III</sup> complexes with the general
formulas [(Me<sub>3</sub>Si)<sub>2</sub>NCÂ(N<sup><i>i</i></sup>Pr)<sub>2</sub>]<sub><i>x</i></sub>Ce<sup>III</sup>[NÂ(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>NCÂ(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
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
The hexachlorocerateÂ(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
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>, CeÂ{(R<sub>2</sub>N)ÂCÂ(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> = NÂ(SiMe<sub>3</sub>)<sub>2</sub>, <b>G</b> = (Me<sub>3</sub>Si)<sub>2</sub>NCÂ(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) ceriumÂ(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 ceriumÂ(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