70 research outputs found
Circular spectropolarimetric sensing of chiral photosystems in decaying leaves
Circular polarization spectroscopy has proven to be an indispensable tool in
photosynthesis research and (bio)-molecular research in general. Oxygenic
photosystems typically display an asymmetric Cotton effect around the
chlorophyll absorbance maximum with a signal . In vegetation, these
signals are the direct result of the chirality of the supramolecular
aggregates. The circular polarization is thus directly influenced by the
composition and architecture of the photosynthetic macrodomains, and is thereby
linked to photosynthetic functioning. Although ordinarily measured only on a
molecular level, we have developed a new spectropolarimetric instrument,
TreePol, that allows for both laboratory and in-the-field measurements. Through
spectral multiplexing, TreePol is capable of fast measurements with a
sensitivity of and is therefore suitable of non-destructively
probing the molecular architecture of whole plant leaves. We have measured the
chiroptical evolution of \textit{Hedera helix} leaves for a period of 22 days.
Spectrally resolved circular polarization measurements (450-900 nm) on whole
leaves in transmission exhibit a strong decrease in the polarization signal
over time after plucking, which we accredit to the deterioration of chiral
macro-aggregates. Chlorophyll \textit{a} levels measured over the same period
by means of UV-Vis absorption and fluorescence spectroscopy showed a much
smaller decrease. With these results we are able to distinguish healthy from
deteriorating leaves. Hereby we indicate the potency of circular polarization
spectroscopy on whole and intact leaves as a nondestructive tool for structural
and plant stress assessment. Additionally, we underline the establishment of
circular polarization signals as remotely accessible means of detecting the
presence of extraterrestrial life.Comment: 29 pages, 6 figure
Circular spectropolarimetric sensing of higher plant and algal chloroplast structural variations
Photosynthetic eukaryotes show a remarkable variability in photosynthesis,
including large differences in light harvesting proteins and pigment
composition. In vivo circular spectropolarimetry enables us to probe the
molecular architecture of photosynthesis in a non-invasive and non-destructive
way and, as such, can offer a wealth of physiological and structural
information. In the present study we have measured the circular polarizance of
several multicellular green, red and brown algae and higher plants, which show
large variations in circular spectropolarimetric signals with differences in
both spectral shape and magnitude. Many of the algae display spectral
characteristics not previously reported, indicating a larger variation in
molecular organization than previously assumed. As the strengths of these
signals vary by three orders of magnitude, these results also have important
implications in terms of detectability for the use of circular polarization as
a signature of life.Comment: 25 pages, 9 figure
Direct Observation of a Dark State in the Photocycle of a Light-Driven Molecular Motor
Controlling the excited-state properties
of light driven molecular
machines is crucial to achieving high efficiency and directed functionality.
A key challenge in achieving control lies in unravelling the complex
photodynamics and especially in identifying the role played by dark
states. Here we use the structure sensitivity and high time resolution
of UV-pump/IR-probe spectroscopy to build a detailed and comprehensive
model of the structural evolution of light driven molecular rotors.
The photodynamics of these chiral overcrowded alkene derivatives are
determined by two close-lying excited electronic states. The potential
energy landscape of these âbrightâ and âdarkâ
states gives rise to a broad excited-state electronic absorption band
over the entire mid-IR range that is probed for the first time and
modeled by quantum mechanical calculations. The transient IR vibrational
fingerprints observed in our studies allow for an unambiguous identification
of the identity of the âdarkâ electronic excited state
from which the photonâs energy is converted into motion, and
thereby pave the way for tuning the quantum yield of future molecular
rotors based on this structural motif
Bimodal dynamics of mechanically constrained hydrogen bonds revealed by vibrational photon echoes
We have investigated the dynamics of the hydrogen bonds that connect the components of a [2]rotaxane in solution. In this rotaxane, the amide groups in the benzylic-amide macrocycle and the succinamide thread are connected by four equivalent NâHâ
â
â
O=C hydrogen bonds. The fluctuations of these hydrogen bonds are mirrored by the frequency fluctuations of the NH-stretch modes, which are probed by means of three-pulse photon-echo peak shift spectroscopy. The hydrogen-bond fluctuations occur on three different time scales, with time constants of 0.1, 0.6, and >=200 ps. Comparing these three time scales to the ones found in liquid formamide, which contains the same hydrogen-bonded amide motif but without mechanical constraints, we find that the faster two components, which are associated with small-amplitude fluctuations in the strength of the NâHâ
â
â
O=C hydrogen bonds, are very similar in the liquid and the rotaxane. However, the third component, which is associated with the breaking and subsequent reformation of hydrogen bonds, is found to be much slower in the rotaxane than in the liquid. It can be concluded that the mechanical bonding in a rotaxane does not influence the amplitude and time scale of the small-amplitude fluctuations of the hydrogen bonds, but strongly slows down the complete dissociation of these hydrogen bonds. This is probably because in a rotaxane breaking of the macrocycle-axle contacts is severely hindered by the mechanical constraints. The hydrogen-bond dynamics in rotaxane-based molecular machines can therefore be regarded as liquidlike on a time scale 1 ps and less, but structurally frozen on longer (up to at least 200 ps) time scales
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Excited-state electronic asymmetry prevents photoswitching in terthiophene compounds
The diarylethene moiety is one of the most extensively used switches in the field of molecular electronics. Here we report on spectroscopic and quantum chemical studies of two diarylethene-based compounds with a non-C3-symmetric triethynyl terthiophene core symmetrically substituted with RuCp*(dppe) or trimethylsilyl termini. The ethynyl linkers are strong IR markers that we use in time-resolved vibrational spectroscopic studies to get insight into the character and dynamics of the electronically excited states of these compounds on the picosecond to nanosecond time scale. In combination with electronic transient absorption studies and DFT calculations, our studies show that the conjugation of the non-C3-symmetric triethynyl terthiophene system in the excited state strongly affects one of the thiophene rings involved in the ring closure. As a result, cyclization of the otherwise photochromic 3,3âł-dimethyl-2,2âČ:3âČ,2âł-terthiophene core is inhibited. Instead, the photoexcited compounds undergo intersystem crossing to a long-lived triplet excited state from which they convert back to the ground state
Critical Landau Velocity in Helium Nanodroplets
The best-known property of superfluid helium is the vanishing viscosity that objects experience while moving through the liquid with speeds below the so-called critical Landau velocity. This critical velocity is generally considered a macroscopic property as it is related to the collective excitations of the helium atoms in the liquid. In the present work we determine to what extent this concept can still be applied to nanometer-scale, finite size helium systems. To this end, atoms and molecules embedded in helium nanodroplets of various sizes are accelerated out of the droplets by means of optical excitation, and the speed distributions of the ejected particles are determined. The measurements reveal the existence of a critical velocity in these systems, even for nanodroplets consisting of only a thousand helium atoms. Accompanying theoretical simulations based on a time-dependent density functional description of the helium confirm and further elucidate this experimental finding
Analytical chemistry on many-center chiral compounds based on vibrational circular dichroism:Absolute configuration assignments and determination of contaminant levels
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Enantiospecific Brook Rearrangement of Tertiary Benzylic alpha-Hydroxysilanes
The Brook rearrangement of simple, chiral tertiary benzylic -hydroxysilanes is presented. The rearrangement followed by proton trapping is enantiospecific and proceeds with inversion of the configuration at the carbon center. Importantly, the [1,2]-Brook rearrangement can be followed by trapping of methyl or allyl electrophiles even in the protic environment, although with minimal retention of chirality
Amplification of the linear and nonlinear optical response of a chiral molecular crystal
We have observed large second-order nonlinear optical and vibrational circular dichroism (VCD) responses in a charge-transfer-type L-Histidinium salt. Using X-ray Diffraction, VCD spectroscopy, and time-dependent density functional theory to characterize the compound, we employ a two-level model to explain and quantify the strongly enhanced optical signals. We find that both linear and nonlinear optical responses are greatly enhanced by a single low-lying charge-transfer stat
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