Suchergebnisse

The file associated with this record is under embargo until 12 months after publication, in accordance with the publisher's self-archiving policy. The full text may be available through the publisher links provided above. ; A time-variable 1D photochemical model is used to study the distribution of stratospheric hydrocarbons as a function of altitude, latitude, and season on Uranus and Neptune. The results for Neptune indicate that in the absence of stratospheric circulation or other meridional transport processes, the hydrocarbon abundances exhibit strong seasonal and meridional variations in the upper stratosphere, but that these variations become increasingly damped with depth due to increasing dynamical and chemical time scales. At high altitudes, hydrocarbon mixing ratios are typically largest where the solar insolation is the greatest, leading to strong hemispheric dichotomies between the summer-to-fall hemisphere and winter-to-spring hemisphere. At mbar pressures and deeper, slower chemistry and diffusion lead to latitude variations that become more symmetric about the equator. On Uranus, the stagnant, poorly mixed stratosphere confines methane and its photochemical products to higher pressures, where chemistry and diffusion time scales remain large. Seasonal variations in hydrocarbons are therefore predicted to be more muted on Uranus, despite the planet's very large obliquity. Radiative-transfer simulations demonstrate that latitude variations in hydrocarbons on both planets are potentially observable with future JWST mid-infrared spectral imaging. Our seasonal model predictions for Neptune compare well with retrieved C 2 H 2 and C 2 H 6 abundances from spatially resolved ground-based observations (no such observations currently exist for Uranus), suggesting that stratospheric circulation — which was not included in these models — may have little influence on the large-scale meridional hydrocarbon distributions on Neptune, unlike the situation on Jupiter and Saturn. ; This material is based on research supported by the National Aeronautics and Space Administration (NASA) Science Mission Directorate under grant NNX13AH81G from the Planetary Atmospheres Research Program. The oxygen chemistry portion was supported by NASA grant NNX13AG55G. Fletcher was supported by a Royal Society Research Fellowship and European Research Council Consolidator Grant (under the European Union's Horizon 2020 research and innovation programme, grant agreement No. 723890) at the University of Leicester. Orton acknowledges support from NASA to the Jet Propulsion Laboratory, California Institute of Technology. ; Peer-reviewed ; Post-print

Cryptochromes compose the widespread class of blue-light sensory receptors that in plants regulate processes of development and circadian rhythm. These photoreceptors can also function as magnetoreceptors. Cryptochrome proteins bind flavin adenine dinucleotide (FAD) as a chromophore in the photolyase homology region (PHR) domain and contain the C-terminal extension (CCE) which is joined to PHR near the FAD-binding site. The cryptochrome activation is initiated by photochemical FAD conversions involving electron/proton transfer and the formation of redox forms. In plants, cryptochrome protein with photoreduced FAD undergoes conformational changes causing disengagement of the PHR domain and CCE that is accompanied by the formation of functionally active oligomers of cryptochrome molecules. Photooligomerization is considered as a key process necessary for cryptochrome signaling activity.

We gratefully acknowledge nancial support from CONACyT/Mexico (grant 253776 and PhD scholarship 436154 for AFA), PAPIIT/UNAM (project IN205118), the Spanish MICINN (grant PGC2018-095953-B-I00), and the Principado de Asturias Government (grant FCGRUPIN- IDI/2018/000117). We are also grateful to DGTIC/UNAM (grant LANCAD-UNAMDGTIC- 250) for computer time.

The review is dedicated to the photochemical reactions of radical cations (RC) of various organic oxygen-containing compounds stabilized in low-temperature matrices. The nature of RC and the products of their photochemical reactions has been established via quantum chemistry, electron paramagnetic resonance (EPR) and low-temperature optical spectroscopy. Depending on the structure of the precursor molecule, various mechanisms of photoconversion arise for these RC: charge transfer to matrix molecules, hydrogen atom and proton transfer, isomerization, dissociation. This study allowed us to posit that there is no correlation between the structure of the molecule of the precursor molecule and the variety of available phototransformation channels for the corresponding RC in frozen matrices.

Covalently tethered bichromophores provide an ideal proving ground to develop strategies for controlling excited state behavior in chromophore assemblies. In this work, optical spectroscopy and electronic structure theory are combined to demonstrate that the oxidation state of a sulfur linker between anthracene chromophores gives control over not only the photophysics but also the photochemistry of the molecules. Altering the oxidation state of the sulfur linker does not change the geometry between chromophores, allowing electronic effects between chromophores to be isolated. Previously, we showed that excitonic states in sulfur-bridged terthiophene dimers were modulated by electronic screening of the sulfur lone pairs, but that the sulfur orbitals were not directly involved in these states. In the bridged anthracene dimers that are the subject of the current paper, the atomic orbitals of the unoxidized S linker can actively mix with the anthracene molecular orbitals to form new electronic states with enhanced charge transfer character, different excitonic coupling, and rapid (sub-nanosecond) intersystem crossing that depends on solvent polarity. However, the fully oxidized SO2 bridge restores purely through-space electronic coupling between anthracene chromophores and inhibits intersystem crossing. Photoexcitation leads to either internal conversion on a sub-20 picosecond timescale, or to the creation of a long-lived emissive state that is the likely precursor of the intramolecular [4 + 4] photodimerization. These results illustrate how chemical modification of a single atom in the covalent bridge can dramatically alter not only the photophysics but also the photochemistry of molecules ; This work was supported by the Basque Government (IT588-13) and the Spanish Government MINECO/FEDER (CT2016-80955-P). C. C. is indebted to the European Research Council (ERC-2016-STG-714870) for a postdoctoral contract. M. O. W. acknowledges support from the Natural Sciences and Engineering Research Council. C. J. B. acknowledges support from the National Science Foundation grant CHE-1800187