9 research outputs found
Dark-Field Microscopy Studies Of Single Metal Nanoparticles: Understanding The Factors That Influence The Linewidth Of The Localized Surface Plasmon Resonance
This article provides a review of our recent Rayleigh scattering measurements on single metal nanoparticles. Two different systems will be discussed in detail: gold nanorods with lengths between 30 and 80 nm, and widths between 8 and 30 nm; and hollow gold-silver nanocubes (termed nanoboxes or nanocages depending on their exact morphology) with edge lengths between 100 and 160 nm, and wall thicknesses of the order of 10 nm. The goal of this work is to understand how the linewidth of the localized surface plasmon resonance depends on the size, shape, and environment of the nanoparticles. Specifically, the relative contributions from bulk dephasing, electron-surface scattering, and radiation damping (energy loss via coupling to the radiation field) have been determined by examining particles with different dimensions. This separation is possible because the magnitude of the radiation damping effect is proportional to the particle volume, whereas, the electron-surface scattering contribution is inversely proportional to the dimensions. For the nanorods, radiation damping is the dominant effect for thick rods (widths greater than 20 nm), while electron-surface scattering is dominant for thin rods (widths less than 10 nm). Rods with widths in between these limits have narrow resonances - approaching the value determined by the bulk contribution. For nanoboxes and nanocages, both radiation damping and electron-surface scattering are significant at all sizes. This is because these materials have thin walls, but large edge lengths and, therefore, relatively large volumes. The effect of the environment on the localized surface plasmon resonance has also been studied for nanoboxes. Increasing the dielectric constant of the surroundings causes a red-shift and an increase in the linewidth of the plasmon band. The increase in linewidth is attributed to enhanced radiation damping. © 2008 The Royal Society of Chemistry
Electron transfer between carotenoid and chlorophyll contributes to quenching in the LHCSR1 protein from Physcomitrella patens
Plants harvest photons for photosynthesis using light-harvesting complexes (LHCs)-an array of chlorophyll proteins that can reversibly switch from harvesting to energy-dissipation mode to prevent over-excitation and damage of the photosynthetic apparatus. In unicellular algae and lower plants this process requires the LHCSR proteins which senses over-acidification of the lumen trough protonatable residues exposed to the thylakoid lumen to activate quenching reactions. Further activation is provided by replacement of the violaxanthin ligand with its de-epoxidized product, zeaxanthin, also induced by excess light. We have produced the ppLHCSR1 protein from Physcomitrella patens by over-expression in tobacco and purified it in either its violaxanthin- or the zeaxanthin-binding form with the aim of analyzing their spectroscopic properties at either neutral or acidic pH. Using femtosecond spectroscopy, we demonstrated that the energy dissipation is achieved by two distinct quenching mechanism which are both activated by low pH. The first is present in both ppLHCSR1-Vio and ppLHCSR1-Zea and is characterized by 30-40ps time constant. The spectrum of the quenching product is reminiscent of a carotenoid radical cation, suggesting that the pH-induced quenching mechanism is likely electron transfer from the carotenoid to the excited Chl a. In addition, a second quenching channel populating the S1 state of carotenoid via energy transfer from Chl is found exclusively in the ppLHCSR1-Zea at pH5. These results provide proof of principle that more than one quenching mechanism may operate in the LHC superfamily and also help understanding the photoprotective role of LHCSR proteins and the evolution of LHC antennae
Ultrafast Dynamics of Long Homologues of Carotenoid Zeaxanthin
Three zeaxanthin homologues with
conjugation lengths <i>N</i> of 15, 19, and 23 denoted as
Z15, Z19, and Z23 were studied by femtosecond
transient absorption spectroscopy, and the results were compared to
those obtained for zeaxanthin (Z11). The energies of S<sub>2</sub> decrease from 20 450 cm<sup>–1</sup> (Z11) to 18 280
cm<sup>–1</sup> (Z15), 17 095 cm<sup>–1</sup> (Z19), and 16 560 cm<sup>–1</sup> (Z23). Fitting the <i>N</i> dependence of the S<sub>2</sub> energies allowed the estimation
of E∞, the S<sub>2</sub> energy of a hypothetical
infinite zeaxanthin, to be ∼14 000 cm<sup>–1</sup>. Exciting the 0–0 band of the S<sub>2</sub> state produces
characteristic S<sub>1</sub>–S<sub><i>n</i></sub> spectral profiles in transient absorption spectra with maxima at
556 nm (Z11), 630 nm (Z15), 690 nm (Z19), and 740 nm (Z23). The red
shift of the S<sub>1</sub>–S<sub><i>n</i></sub> transition
with increasing conjugation length is caused by a decrease in the
S<sub>1</sub> state energy, resulting in S<sub>1</sub> lifetimes of
9 ps (Z11), 0.9 ps (Z15), 0.35 ps (Z19), and 0.19 ps (Z23). Essentially
the same lifetimes were obtained after excess energy excitation at
400 nm, but S<sub>1</sub>–S<sub><i>n</i></sub> becomes
broader, indicating a larger conformation disorder in the S<sub>1</sub> state after 400 nm excitation compared to excitation into the 0–0
band of the S<sub>2</sub> state. An S* signal was observed in all
samples, but only for Z15, Z19, and Z23 does the S* signal decay with
a lifetime different from that of the S<sub>1</sub> state. The S*
lifetimes are 2.9 and 1.6 ps for Z15 and Z19, respectively. In Z23
the S* signal needs two decay components yielding lifetimes of 0.24
and 2.3 ps. The S* signal is more pronounced after 400 nm excitation
Equilibration Dependence of Fucoxanthin S<sub>1</sub> and ICT Signatures on Polarity, Proticity, and Temperature by Multipulse Femtosecond Absorption Spectroscopy
To
demonstrate the value of the multipulse method in
revealing the nature of coupling between excited states and explore
the environmental dependencies of lowest excited singlet state (S<sub>1</sub>) and intramolecular charge transfer (ICT) state equilibration,
we performed ultrafast transient absorption pump–dump–probe
and pump–repump–probe spectroscopies on fucoxanthin
in various solvent conditions. The effects of polarity, proticity,
and temperature were tested in solvents methanol at 293 and 190 K,
acetonitrile, and isopropanol. We show that manipulation of the kinetic
traces can produce one trace reflecting the equilibration kinetics
of the states, which reveals that lower polarity, proticity, and temperature
delay S<sub>1</sub>/ICT equilibration. On the basis of a two-state
model representing the S<sub>1</sub> and ICT states on the same S<sub>1</sub>/ICT potential energy surface, we were able to show that the
kinetics are strictly dependent on the initial relative populations
of the states as well as the decay of the ICT state to the ground
state. Informed by global analysis, a systematic method for target
analysis based on this model allowed us to quantify the population
transfer rates throughout the life of the S<sub>1</sub>/ICT state
as well as separate the S<sub>1</sub> and ICT spectral signatures.
The results are consistent with the concept that the S<sub>1</sub> and ICT states are part of one potential energy surface
Nonconjugated Acyloxy Group Deactivates the Intramolecular Charge-Transfer State in the Carotenoid Fucoxanthin
We
used ultrafast transient absorption spectroscopy to study excited-state
dynamics of the keto-carotenoid fucoxanthin (Fx) and its two derivatives:
19′-butanoyloxyfucoxanthin (bFx) and 19′-hexanoyloxyfucoxanthin
(hFx). These derivatives occur in some light-harvesting systems of
photosynthetic microorganisms, and their presence is typically related
to stress conditions. Even though the hexanoyl (butanoyl) moiety is
not a part of the conjugated system of hFx (bFx), their absorption
spectra in polar solvents exhibit more pronounced vibrational bands
of the S<sub>2</sub> state than for Fx. The effect of the nonconjugated
acyloxy moiety is further observed in transient absorption spectra,
which for Fx exhibit characteristic features of an intramolecular
charge transfer (ICT) state in all polar solvents. For bFx and hFx,
however, much weaker ICT features are detected in methanol, and the
spectral markers of the ICT state disappear completely in polar, but
aprotic acetonitrile. The presence of the acyloxy moiety also alters
the lifetimes of the S<sub>1</sub>/ICT state. For Fx, the lifetimes
are 60, 30, and 20 ps in <i>n</i>-hexane, acetonitrile,
and methanol, whereas for bFx and hFx, these lifetimes yield 60, 60,
and 40 ps, respectively. Testing the S<sub>1</sub>/ICT state lifetimes
of hFx in other solvents revealed that some ICT features can be induced
only in polar, protic solvents (methanol, ethanol, and ethylene glycol).
Thus, bFx and hFx represent a rather rare example of a system in which
a nonconjugated functional group significantly alters excited-state
dynamics. By comparison with other carotenoids, we show that a keto
group at the acyloxy tail, even though it is not in conjugation, affects
the electron distribution along the conjugated backbone, resulting
in the observed decrease of the ICT character of the S<sub>1</sub>/ICT state of bFx and hFx