35 research outputs found
Abordagem contextual nos capĂtulos de estequiometria e de soluçÔes dos livros didĂĄticos de QuĂmica aprovados pelo PNLD (Programa Nacional do Livro DidĂĄtico - Brasil) /2012
A contextualização tem sido proposta no currĂculo escolar com o propĂłsito de promover mudanças de comportamentos, atitudes e valores. Nessa abordagem, entende-se que o ensino de quĂmica deve se desenvolver de forma ampla, envolvendo tambĂ©m aspectos sociais, polĂticos, econĂŽmicos,ambientais e culturais, e nĂŁo somente aspectos cientĂficos. Tendo em vista a grande importĂąncia dada ao livro didĂĄtico (LD) na prĂĄtica docente, buscamos analisar a abordagem contextual nos capĂtulos de estequiometria e soluçÔes nos LD de quĂmica aprovados pelo PNLD/2012. Os resultados mostraram que os autores dos LD analisados reconhecem a contextualização como elemento central para a formação da cidadania, porĂ©m a abordagem Ă© diferente em cada obra. As leituras dos LD possibilitaram a identificação de alguns "nĂveis" de contextualização que percorreram quatro categorias de anĂĄlise
Exciton Superposition States in CdSe Nanocrystals Measured Using Broadband Two-Dimensional Electronic Spectroscopy
Coherent superpositions among eigenstates are of interest
in fields
as diverse as photosynthesis and quantum computation. In this report,
we used two-dimensional electronic spectroscopy (2D ES) to measure
the decoherence time of a superposition of the two lowest-energy excitons
in colloidal CdSe nanocrystals (cubic phase) in solution at room temperature.
In the electronâhole representation, the quantum coherence
is, remarkably, a twelve-particle correlation. By comparing the measured
2D ES to simulations, we also explored the effects of inhomogeneous
broadening and examined the spectroscopic signatures of biexcitons
Coherent Energy Transfer under Incoherent Light Conditions
Recent two-dimensional electronic spectroscopy (2DES)
experiments
have reported evidence of coherent dynamics of electronic excitations
in several light-harvesting antennae. However, 2DES uses ultrafast
coherent laser pulses as an excitation source; therefore, there is
a current debate on whether coherent excitation dynamics is present
under natural sunlight â incoherent â illumination conditions.
In this letter, we show that even if incoherent light excites an electronic
state with no initial quantum superpositions among excitonic states,
energy transfer can proceed quantum coherently if nonequilibrium dynamics
of the phonon environment takes place. Such nonequilibrium behavior
manifests itself in non-Markovian evolution of electronic excitations
and is typical of many photosynthetic systems. We therefore argue
that light-harvesting antennae have mechanisms that could support
coherent evolution under incoherent illumination
Observing Vibrational Wavepackets during an Ultrafast Electron Transfer Reaction
Recent
work has proposed that coherent effects impact ultrafast
electron transfer reactions. Here we report studies using broadband
pumpâprobe and two-dimensional electronic spectroscopy of intramolecular
nuclear motion on the time scale of the electron transfer between
oxazine 1 (Ox1) and dimethylaniline (DMA). We performed timeâfrequency
analysis on the time domain data to assign signal amplitude modulations
to ground or excited electronic states in the reactive system (Ox1
in DMA) relative to the control system (Ox1 in chloronaphthalene).
It was found that our ability to detect vibrational coherence via
the excited electronic state of Ox1 diminishes on the time scale that
population is lost by electron transfer. However, the vibrational
wavepacket is not damped by the electron transfer process and has
been observed previously by detecting the Ox1 radical transient absorption.
The analysis presented here indicates that the âadditionâ
of an electron to the photoexcited electron acceptor does not significantly
perturb the vibrational coherence, suggesting its presence as a spectator,
consistent with the BornâOppenheimer separation of electronic
and nuclear degrees of freedom
Exploring Ultrafast Electronic Processes of Quasi-Type II Nanocrystals by Two-Dimensional Electronic Spectroscopy
Colloidal CdTe/CdSe heteronanostructures
are model systems for quasi-type II nanocrystals (NCs) and have been
examined extensively. However, the complex spectra in these heteronanostructures
often make it difficult to reveal details of their optical properties
by conventional techniques such as transient absorption spectroscopy.
In the present study, two-dimensional electronic spectroscopy (2DES)
is used to study colloidal CdTe, CdTe/CdSe, and CdTe/ZnS NCs revealing
the nature of absorption bands and ultrafast dynamics in a quasi-type
II system. We observe the electronic coupling between the lowest two
transitions, oscillations in the population time due to the longitudinal
optical (LO) phonon mode, and the high-frequency impulsive Raman modes
of the solvent. We observed an excited state absorption near at the
band edge only in CdTe/CdSe NCs and established that it is related
to the quasi-type II features: the redistribution of excitons among
the fine-structured states or the biexciton level shift at the ultrafast
time scale
Charge Separation and Recombination in CdTe/CdSe Core/Shell Nanocrystals as a Function of Shell Coverage: Probing the Onset of the Quasi Type-II Regime
Femtosecond transient absorption (TA) is used to investigate population dynamics of electronic states in CdTe/CdSe core/shell nanocrystals (NCs) with varying CdSe shell coverage. Upon CdSe shell growth, the CdTe/CdSe NCs show a gradual evolution of surface passivation toward a quasi-type-II charge separation regime. Our results indicate that ultrafast electron transfer (ET) through the interface of CdTe/CdSe NCs becomes measurable as the CdSe shell approaches the quasi-type-II regime which we experimentally determined to be at 0.4 nm (shell) for a 3.4 nm CdTe core. This work reports the first measurement using femtosecond TA to study the electronic states in CdTe/CdSe NCs and spectroscopically observe the evolution of an ultrafast charge transfer (CT) in such a core/shell system. Time-resolved photoluminescence (PL) spectroscopy is used to study the radiative lifetimes of the CdTe/CdSe NCs. One monolayer CdSe coverage leads to longer-lived PL, which is red-shifted by 140 nm compared to the CdTe core PL. It is concluded that this is due to greater electronâhole separation providing a longer-lived CT state
Methylene Blue Exciton States Steer Nonradiative Relaxation: Ultrafast Spectroscopy of Methylene Blue Dimer
The
photochemistry and aggregation properties of methylene blue
(MB) lead to its popular use in photodynamic therapy. The facile formation
of strongly coupled âface-to-faceâ H-aggregates in concentrated
aqueous solution, however, significantly changes its spectroscopic
properties and photophysics. The photoinitiated dynamics of the simplest
MB aggregate, MB<sub>2</sub>, was investigated over femtosecond to
nanosecond time scales revealing sequential internal conversion events
that fully relax the excited population. MB monomer dynamics were
analyzed in tandem for a direct comparison. First, ultrafast internal
conversion from the electric-dipole allowed upper exciton state to
the lower forbidden exciton state was evaluated by use of broadband
transient absorption (BBTA) and two-dimensional electronic spectroscopy
(2DES) with a time resolution of âŒ10 fs. Lineshape analysis
of MB and MB<sub>2</sub> 2DES bands at 298 and 77 K show effectively
no difference in the diagonal/antidiagonal line width ratio for the
dimer, in marked contrast to the distinct reduction of the homogeneous
line width for MB. This result is interpreted as ultrafast population
relaxation imposing a limitation to the homogeneous line width, instead
of pure dephasing as in the case of the monomer. Narrowband transient
absorption was performed with the aid of target analysis, to model
the dynamics at longer times. The MB dynamics were described by a
sequential model featuring vibrational relaxation (1â10 ps)
followed by intersystem crossing and internal conversion (Ï
⌠370 ps) leaving behind MB triplet species. Alternatively,
the dimer dynamics were entirely quenched within âŒ10 ps, yielding
a ground state recovery time of 3â4 ps. Such fast and complete
relaxation to the ground state demonstrates the effect of concentration
quenching when monomers are brought into close proximity. The formation
of exciton states introduces an initial energy funnel that eventually
leads to population relaxation to the ground state, preventing even
the dissociation of dimers despite having internal energies well above
its binding energy
Long-Lived Charge-Transfer States of Nickel(II) Aryl Halide Complexes Facilitate Bimolecular Photoinduced Electron Transfer
Here
we investigate the photophysics and photochemistry of NiÂ(II)
aryl halide complexes common to cross-coupling and Ni/photoredox reactions.
Computational and ultrafast spectroscopic studies reveal that these
complexes feature long-lived <sup>3</sup>MLCT excited states, implicating
Ni as an underexplored alternative to precious metal photocatalysts.
Moreover, we show that <sup>3</sup>MLCT NiÂ(II) engages in bimolecular
electron transfer with ground-state NiÂ(II), which enables access to
NiÂ(III) in the absence of external oxidants or photoredox catalysts.
As such, it is possible to facilitate Ni-catalyzed CâO bond
formation solely by visible light irradiation, thus representing an
alternative strategy for catalyst activation in Ni cross-coupling
reactions
Vibronic Wavepackets and Energy Transfer in Cryptophyte Light-Harvesting Complexes
Determining the key features of high-efficiency
photosynthetic energy transfer remains an ongoing task. Recently,
there has been evidence for the role of vibronic coherence in linking
donor and acceptor states to redistribute oscillator strength for
enhanced energy transfer. To gain further insights into the interplay
between vibronic wavepackets and energy-transfer dynamics, we systematically
compare four structurally related phycobiliproteins from cryptophyte
algae by broad-band pumpâprobe spectroscopy and extend a parametric
model based on global analysis to include vibrational wavepacket characterization.
The four phycobiliproteins isolated from cryptophyte algae are two
âopenâ
structures and two âclosedâ structures. The closed structures
exhibit strong exciton coupling in the central dimer. The dominant
energy-transfer pathway occurs on the subpicosecond timescale across
the largest energy gap in each of the proteins, from central to peripheral
chromophores. All proteins exhibit a strong 1585 cm<sup>â1</sup> coherent oscillation whose relative amplitude, a measure of vibronic
intensity borrowing from resonance between donor and acceptor states,
scales with both energy-transfer rates and damping rates. Central
exciton splitting may aid in bringing the vibronically linked donor
and acceptor states into better resonance resulting in the observed
doubled rate in the closed structures. Several excited-state vibrational
wavepackets persist on timescales relevant to energy transfer, highlighting
the importance of further investigation of the interplay between electronic
coupling and nuclear degrees of freedom in studies on high-efficiency
photosynthesis
CarbeneâMetalâAmide Bond Deformation, Rather Than Ligand Rotation, Drives Delayed Fluorescence
We report three characteristics of
ideal thermally activated delayed
fluorescence molecular systems apparent in carbeneâmetalâamides:
(a) an exceptionally small singletâtriplet gap that effectively
eliminates the thermal activation barrier to reverse intersystem crossing;
(b) significant singlet oscillator strength promoting fluorescence
in the region of this small barrier; and (c) enlarged spinâorbit
coupling driving reverse intersystem crossing in this region. We carry
out highly correlated quantum-chemical calculations to detail the
relative energies of and spinâorbit couplings between the singlet
and triplet states, finding that they fall closer together in energy
and couple more strongly in going from the singlet ground-state to
the triplet optimized geometry. This structural reorganization is
defined not by rotation of the ligands but by a nontrivial bending
of the carbeneâmetalâamide bond angle. This bending
reduces carbeneâmetalâamide symmetry and enhances singletâtriplet
interaction strength. We clarify that the reverse intersystem crossing
triggering delayed fluorescence occurs around the coplanar triplet
geometric optimum