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

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

Shape Tuning of Type II CdTe-CdSe Colloidal Nanocrystal Heterostructures through Seeded Growth

Shape Tuning of Type II CdTe-CdSe Colloidal Nanocrystal Heterostructures through Seeded Growt

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 electron–hole 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 – incoherent – 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 pump–probe 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 time–frequency 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 “addition” of an electron to the photoexcited electron acceptor does not significantly perturb the vibrational coherence, suggesting its presence as a spectator, consistent with the Born–Oppenheimer separation of electronic and nuclear degrees of freedom

Dynamics within the Exciton Fine Structure of Colloidal CdSe Quantum Dots

Evidence for an interaction between the quantum dot exciton fine structure states F = ±1 is obtained by measuring the dynamics of transitions among those states, exciton spin relaxation or flipping. An ultrafast transient grating experiment based on a crossed-linear polarization grating is reported. By using the quantum dot selection rules for absorption of circularly polarized light, it is demonstrated that it is possible to detect transitions between nominally degenerate fine structure states, even in a rotationally isotropic system. The results for colloidal CdSe quantum dots reveal a strong size dependence for the exciton spin relaxation rate from one bright exciton state (F = ±1) to the other in CdSe colloidal quantum dots at 293 K, on a time scale ranging from femtoseconds to picoseconds, depending on the quantum dot size. The results are consistent with an interaction between those states attributed to a long-range contribution to the electron−hole exchange interaction

The Mechanism of Energy Transfer in the Bacterial Photosynthetic Reaction Center

In the accompanying paper (Scholes, G. D.; Jordanides, X. J.; Fleming, G. R. J. Phys. Chem. 2001, 105, 1640), a generalization of Förster theory is developed to calculate electronic energy transfer (EET) in molecular aggregates. Here we apply the theory to wild-type and mutant photosynthetic reaction centers (RCs) from Rb. sphaeroides, as well as to the wild-type RC from Rps. viridis. Experimental information from the X-ray crystallographic structure, resonance Raman excitation profiles, and hole-burning measurements are integrated with calculated electronic couplings to model the EET dynamics within the RC complex. Optical absorption and circular dichroism spectra are calculated at various temperatures between 10 K and room temperature, and compare well with the experimentally observed spectra. The calculated rise time of the population of the lower exciton state of P, P-, as a result of energy transfer from the accessory bacteriochlorophyll, B, to the special pair, P, in Rb. sphaeroides (Rps. viridis) wild-type at 298 K is 193 fs (239 fs), and is in satisfactory agreement with experimental results. Our calculations, which employ a weak-coupling mechanism suggest that the upper exciton state of P, P+ plays a central role in trapping excitation from B. Our ability to predict the experimental rates is partly attributed to a proper calculation of the spectral overlap Jδα(ε) using the vibronic progressions. The main advance we have made, however, is to calculate the electronic couplings Vδα in terms of the molecular composition of donor and/or acceptor aggregates, rather than treating the acceptors P+ and P- as point dipoles associated with each spectroscopic band. Thus, we believe our electronic couplings capture the essence of the many-body interactions within the RC. Calculations for EET in two mutants, (M)L214H (the beta mutant) and (M)H202L (the heterodimer), are in reasonable agreement with experimental results. In the case of the heterodimer the agreement depends on a decrease in the electronic couplings between DM and the rest of the pigments

Controllable Phycobilin Modification: An Alternative Photoacclimation Response in Cryptophyte Algae

Cryptophyte algae are well-known for their ability to survive under low light conditions using their auxiliary light harvesting antennas, phycobiliproteins. Mainly acting to absorb light where chlorophyll cannot (500–650 nm), phycobiliproteins also play an instrumental role in helping cryptophyte algae respond to changes in light intensity through the process of photoacclimation. Until recently, photoacclimation in cryptophyte algae was only observed as a change in the cellular concentration of phycobiliproteins; however, an additional photoacclimation response was recently discovered that causes shifts in the phycobiliprotein absorbance peaks following growth under red, blue, or green light. Here, we reproduce this newly identified photoacclimation response in two species of cryptophyte algae and elucidate the origin of the response on the protein level. We compare isolated native and photoacclimated phycobiliproteins for these two species using spectroscopy and mass spectrometry, and we report the X-ray structures of each phycobiliprotein and the corresponding photoacclimated complex. We find that neither the protein sequences nor the protein structures are modified by photoacclimation. We conclude that cryptophyte algae change one chromophore in the phycobiliprotein β subunits in response to changes in the spectral quality of light. Ultrafast pump–probe spectroscopy shows that the energy transfer is weakly affected by photoacclimation

How Solvent Controls Electronic Energy Transfer and Light Harvesting: Toward a Quantum-Mechanical Description of Reaction Field and Screening Effects

This paper presents a quantum-mechanical study of electronic energy transfer (EET) coupling on over 100 pairs of chromophores taken from photosynthetic light-harvesting antenna proteins. Solvation effects due to the protein, intrinsic waters, and surrounding medium are analyzed in terms of screening and reaction field contributions using a model developed recently that combines a linear response approach with the polarizable continuum model (PCM). We find that the screening of EET interactions is quite insensitive to the quantum-mechanical treatment adopted. In contrast, it is greatly dependent on the geometrical details (distance, shape, and orientation) of the chromophore pair considered. We demonstrate that implicit (reaction field) as well as screening effects are dictated mainly by the optical dielectric properties of the host medium, while the effect of the static properties is substantially less important. The empirical distance-dependent screening function we proposed in a recent letter (Scholes, G. D.; Curutchet, C.; Mennucci, B.; Cammi, R.; Tomasi, J. J. Phys. Chem. B 2007, 111, 6978−6982) is analyzed and compared to other commonly used screening factors. In addition, we show that implicit medium effects on the coupling, resulting from changes in the transition densities upon solvation, are strongly dependent on the particular system considered, thus preventing the possibility of defining a general empirical expression for such an effect

Examining Förster Energy Transfer for Semiconductor Nanocrystalline Quantum Dot Donors and Acceptors

Excitation energy transfer involving semiconductor quantum dots (QDs) has received increased attention in recent years because their properties, such as high photostability and size-tunable optical properties, have made QDs attractive as Förster resonant energy transfer (FRET) probes or sensors. An intriguing question in FRET studies involving QDs has been whether the dipole approximation, commonly used to predict the electronic coupling, is sufficiently accurate. Accurate estimates of electronic couplings between two 3.9 nm CdSe QDs and between a QD and a chlorophyll molecule are reported. These calculations are based on transition densities obtained from atomistic semiempirical calculations and time-dependent density functional theory for the QD and the chlorophyll, respectively. In contrast to the case of donor−acceptor molecules, where the dipole approximation breaks down at length scales comparable to the molecular dimensions, we find that the dipole approximation works surprisingly well when donor and/or acceptor is a spherical QD, even at contact donor−acceptor separations. Our conclusions provide support for the use of QDs as FRET probes for accurate distance measurements