Photoswitching of arylazopyrazoles upon S1 (nπ*) excitation studied by transient absorption spectroscopy and ab initio molecular dynamics

Arylazopyrazoles (AAPs) are an important class of molecular photoswitches with high photostationary states (PSS) and long thermal lifetimes. The ultrafast photoisomerization of four water-soluble arylazopyrazoles, all of them featuring an ortho-dimethylated pyrazole ring, is studied by narrowband femtosecond transient absorption spectroscopy and ab initio molecular dynamics simulations. Upon S1 (nπ*) photoexcitation of the planar E-isomers (E-AAPs), excited-state bi-exponential decays with time constants τ1 in the 220-440 fs range and τ2 in the 1.4-1.8 ps range are observed, comparable to those reported for azobenzene (AB). This is indicative of the same photoisomerization mechanism as has been reported for ABs. In contrast to the planar E-AAPs, a twisted E-AAP with two methyl groups in ortho-position of the phenyl ring displays faster initial photoswitching with τ1 = 170 ± 10 fs and τ2 = 1.6 ± 0.1 ps. Our static DFT calculations and ab initio molecular dynamics simulations of E-AAPs on the S0 and S1 potential energy surfaces suggest that twisted E-isomer azo photoswitches exhibit faster initial photoisomerization dynamics out of the Franck-Condon region due to a weaker π-coordination of the central CNNC unit to the aromatic ligands.

Understanding the formation of surface relief gratings in azopolymers: A combined molecular dynamics and experimental study

The formation of surface relief gratings in thin azopolymeric films is investigated using atomistic molecular dynamics simulations and compared to experimental results for the specific case of poly-disperse-orange3-methyl-methacrylate. For this purpose, the film is illuminated with a light pattern of alternating bright and dark stripes in both cases. The simulations use a molecular mechanics switching potential to explicitly describe the photoisomerization dynamics between the E and Z isomers of the azo-units and take into account the orientation of the transition dipole moment with respect to the light polarization. Local heating and elevation of the illuminated regions with the subsequent movement of molecules into the neighboring dark regions are observed. This leads to the formation of valleys in the bright areas after re-cooling and is independent of the polarization direction. To verify these observations experimentally, the azopolymer film is illuminated with bright stripes of varying width using a spatial light modulator. Atomic force microscopy images confirm that the elevated areas correspond to the previously dark areas. In the experiment, the polarization of the incident light makes only a small difference since tiny grain-like structures form in the valleys only when the polarization is parallel to the stripes.

Reversible, Red-Shifted Photoisomerization in Protonated Azobenzenes

Azobenzenes are among the best-studied molecular photoswitches and play a key role in the search for red-shifted photoresponsive materials for extended applications. Currently, most approaches deal with aromatic substitution patterns to achieve visible light application, on occasion paired with protonation to yield red-shifted absorption of the azonium species. Appropriate substitution patterns are essential to stabilize the latter approach, as conventional acids are known to induce a fast Z- to E-conversion. Here, we show that steady-state protonation of the azo-bridge instead is possible in simple azobenzenes when the pK_a of the acid is low enough, yielding both the Z- and E-azonium as supported by UV–vis- and ¹H NMR spectroscopy as well as density functional theory calculations. Moreover, the steady-state protonation of para-methoxyazobenzene, specifically, yields photoisomerizable azonium ions in which the direction of switching is essentially reversed, that is, visible light produces the out-of-equilibrium Z-azonium. Although the current conditions render the visible light photoswitch unsuitable for in vivo and material application, the demonstrated understanding of simple azobenzenes paves the way for a great range of further work on this already widely studied photoswitch.

Self-Assembled Monolayers of Arylazopyrazoles on Glass and Silicon Oxide: Photoisomerization and Photoresponsive Wettability

Surface coatings that respond to external influences and change their physical properties upon application of external stimuli are of great interest, with light being a particularly desirable choice. Photoswitches such as azobenzenes have been employed in a range of photoresponsive coatings. One striking change in physical property of many photoresponsive coatings is their responsive wettability upon illumination. In this work, we present photoswitchable self-assembled monolayers based on arylazopyrazoles (AAPs). In solution, AAPs offer significant improvements in terms of the photostationary state, thermal stability, and fatigue resistance. The AAP photoswitch is coupled to triethoxysilanes for an easy, one-step functionalization of glass and silicon oxide surfaces. We show the synthesis of AAP-based silanes and the successful surface functionalization, and we confirm the excellent photoswitchability of the AAPs in a self-assembled monolayer upon alternating irradiation with UV (365 nm) and green (520 nm) light. The self-assembled monolayers are investigated by UV/vis spectroscopy, X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (ToF-SIMS), and contact angle goniometry. We furthermore investigate the effect of substitution of the AAPs on the photoresponsive wetting behavior and compare this with density functional theory (DFT) calculations of the dipole moments of the AAPs.

Acid-catalysed liquid-to-solid transitioning of arylazoisoxazole photoswitches

Molecular photoswitches play a vital role in the development of responsive materials. These molecular building blocks are particularly attractive when multiple stimuli can be combined to bring about physical changes, sometimes leading to unexpected properties and functions. The arylazoisoxazole molecular switch was recently shown to be capable of efficient photoreversible solid-to-liquid phase transitions with application in photoswitchable surface adhesion. Here, we show that the arylazoisoxazole forms thermally stable and photoisomerisable protonated Z- and E-isomers in an apolar aprotic solvent when the pK _a of the applied acid is sufficiently low. The tuning of isomerisation kinetics from days to seconds by the pK _a of the acid not only opens up new reactivity in solution, but also the solid-state photoswitching of azoisoxazoles can be efficiently reversed with selected acid vapours, enabling acid-gated photoswitchable surface adhesion.

Extreme heat exposure in pregnancy and risk for preterm birth, low birth weight and stillbirths

Background

Exposure to high ambient temperatures during pregnancy may increase the risk of adverse pregnancy outcomes due to physiological and anatomical changes in pregnancy compromising the ability to thermoregulate.

Methods

A systematic review and meta-analysis of studies that assessed associations between ambient heat, and preterm birth, birth weight and stillbirths. Articles were identified through a search of Medline (PubMed) and Web of Science. Titles and abstracts were screened, eligible studies extracted and risk of bias assessed using the Joanna Briggs Institute appraisal tools.

Results

14,880 records were screened and a total of 70 studies included. In 40 of 47 studies, preterm births were more frequent at higher than lower temperatures. In random effects meta-analysis, the odds of a preterm birth rose 1.05-fold per 1 °C increase in temperature (95%CI=1.03, 1.07), and 1.16 fold during heat waves (95%CI=1.10, 1.23). Reduced birth weight associated with higher temperature was found in in 18 of 28 studies, with considerable heterogeneity. Studies on stillbirths (n = 8) all showed temperature-stillbirth associations, with stillbirths increasing 1.05-fold per 1 °C rise in temperature (95%CI=1.01, 1.08). Associations between temperature and outcomes were largest among women in lower socioeconomic groups and at age extremes. The multiple temperature metrics and lag analyses limited comparison between studies and settings.

Conclusions

As frequency of heat exposures is increasing, prevention for pregnant women during heat waves should become a standard aspect of heat adaptation.

Key messages

Increase in extreme temperature events may have major implications for child health unless much more is done to provide heat adaptation services for pregnant women. Links between socio-economic status and study outcomes suggest higher vulnerability for already disadvantaged groups.

An Arylazopyrazole-Based N-Heterocyclic Carbene as a Photoswitch on Gold Surfaces: Light-Switchable Wettability, Work Function, and Conductance

A novel photoresponsive and fully conjugated N-heterocyclic carbene (NHC) has been synthesized that combines the excellent photophysical properties of arylazopyrazoles (AAPs) with an NHC that acts as a robust surface anchor (AAP-BIMe). The formation of self-assembled monolayers (SAMs) on gold was proven by ToF-SIMS and XPS, and the organic film displayed a very high stability at elevated temperatures. This stability was also reflected in a high desorption energy, which was determined by temperature-programmed SIMS measurements. E-/Z-AAP-BIMe@Au photoisomerization resulted in reversible alterations of the surface energy (i.e. wettability), the surface potential (i.e. work function), and the conductance (i.e. resistance). The effects could be explained by the difference in the dipole moment of the isomers. Furthermore, sequential application of a dummy ligand by microcontact printing and subsequent backfilling with AAP-BIMe allowed its patterning on gold. To the best of our knowledge, this is the first example of a photoswitchable NHC on a gold surface. These properties of AAP-BIMe@Au illustrate its suitability as a molecular switch for electronic devices.

Perylene Diimide-Embedded Double [8]Helicenes

The rational design and modification of the helix is of significance for fully promoting properties of configurationally stable materials for various applications in chiral science. Herein, a straightforward, sterically less demanding synthetic approach involving hybridization between two [6]helicene subunits and a perylene diimide (PDI) scaffold are presented, affording perylene diimide-embedded double [8]helicenes (PD8Hs) which represent the highest double carbohelicenes reported thus far. Due to the structural features of PDI and [6]helicene, the PD8Hs have six stereoisomers including two pairs of enantiomers and two mesomers. Such structural diversity is unprecedented in the realm of double helicenes. The absolute configuration of these PD8Hs was unambiguously confirmed by single-crystal X-ray diffraction analyses, revealing that the subtle configurational differences lead to great variation in the superhelical structure and molecular packing arrangement. Due to the embedding of the PDI chromophore, the PD8Hs possess outstanding fluorescence quantum yields of approximately 30%. Two pairs of enantiomers were resolved by chiral HPLC, and the chiroptical properties were evaluated using circular dichroism and circularly polarized luminescence spectroscopy, of which PD8H-6R exhibited excellent chiroptical performances in both the absorption and emission ranges with dissymmetry factors |g_abs| of 0.012 and |g_lum| of 0.002.

Arylazopyrazole Photoswitches in Aqueous Solution: Substituent Effects, Photophysical Properties, and Host-Guest Chemistry

Getting the green light! Substituted arylazopyrazoles (AAPs) have been investigated as supramolecular photoswitches in aqueous solution. Selective photostationary states (PSSs) and improved binding affinities to β-cyclodextrin have been determined. The experimental findings are supported by results from DFT calculations.

Adaptive switching of interaction potentials in the time domain: an extended Lagrangian approach tailored to transmute force field to QM/MM simulations and back

An extended Lagrangian formalism that allows for a smooth transition between two different descriptions of interactions during a molecular dynamics simulation is presented. This time-adaptive method is particularly useful in the context of multiscale simulation as it provides a sound recipe to switch on demand between different hierarchical levels of theory, for instance between ab initio ("QM") and force field ("MM") descriptions of a given (sub)system in the course of a molecular dynamics simulation. The equations of motion can be integrated straightforwardly using the usual propagators, such as the Verlet algorithm. First test cases include a bath of harmonic oscillators, of which a subset is switched to a different force constant and/or equilibrium position, as well as an all-MM to QM/MM transition in a hydrogen-bonded water dimer. The method is then applied to a smectic 8AB8 liquid crystal and is shown to be able to switch dynamically a preselected 8AB8 molecule from an all-MM to a QM/MM description which involves partition boundaries through covalent bonds. These examples show that the extended Lagrangian approach is not only easy to implement into existing code but that it is also efficient and robust. The technique moreover provides easy access to a conserved energy quantity, also in cases when Nosé-Hoover chain thermostatting is used throughout dynamical switching. A simple quadratic driving potential proves to be sufficient to guarantee a smooth transition whose time scale can be easily tuned by varying the fictitious mass parameter associated with the auxiliary variable used to extend the Lagrangian. The method is general and can be applied to time-adaptive switching on demand between two different levels of theory within the framework of hybrid scale-bridging simulations.

P3HT:DiPBI bulk heterojunction solar cells: morphology and electronic structure probed by multiscale simulation and UV/vis spectroscopy

The effect of different annealing protocols on the morphology and light absorption of a P3HT:DiPBI mixture is studied by theory and experiment.

Structure of P3HT crystals, thin films, and solutions by UV/Vis spectral analysis

The shift in the absorption spectrum of P3HT when comparing solution, spin-coated thin films, and the bulk crystal can be reproduced by multiscale simulation and explained in terms of the degree of intramolecular torsion.

Biogenesis of the Pyoverdin Chromophore

After growing Pseudomonas aeruginosa in the presence of 2 ,4-[4-15N]-diaminobutyric acid (Dab) its incorporation into the quinoline chromophore of the pyoverdin produced by this bacterium could be shown by mass and NMR-spectroscopic techniques. In combination with earlier results it can thus be stated that the precursor of the chromophore is a condensation product of ʟ-Dab and ᴅ-Phe or ᴅ-Tyr. A synthesis for [4 -15N ]-Dab is described.

Azobenzene photoswitches in bulk materials

A nonadiabatic two-scale quantum-classical dynamics method is developed to allow for dynamical simulation of photoinduced processes and reactions in chemically complex condensed-matter systems such as photosensitive liquid crystals. A first application to photoswitching of azobenzene in the bulk reveals not only valuable insights into the excited-state dynamics underlying the cis to trans transformation, but also into the influence of the condensed phase environment on mechanism and relaxation time scales compared to vacuum.

Basement membrane component laminin-5 is a target of the tumor suppressor Smad4

The tumor suppressor Smad4 is involved in carcinogenesis mainly of the pancreas and colon. Functional inactivation of Smad4 is a genetically late event that occurs upon transition from premalignant stages to invasive and metastatic growth. Smad4 encodes an intracellular messenger common to all signalling cascades induced by members of the transforming growth factor-beta (TGF-beta) superfamily of cytokines. Despite extensive knowledge about the mechanisms of TGF-beta/Smad signal transduction, little is known about Smad4 targets involved in the transition to malignancy. The hallmark of invasive growth is a breakdown of the basement membrane (BM), a specialized sheet of extracellular matrix produced through cooperation of epithelial and stromal cells. Laminin-5, a heterotrimeric epithelial-derived BM component, is commonly lost in carcinomas but not in premalignant tumors. Herein, we report that in human colon and pancreatic tumor cells, Smad4 functions as a positive transcriptional regulator of all three genes encoding laminin-5. Coordinate re-expression of the three laminin-5 chains induced by reconstitution of Smad4 leads to secretion and deposition of the heterotrimeric molecule in BM-like structures. These data define the expression control of an essential BM component as a novel function for the tumor suppressor Smad4.

Analysis of adenovirus gene transfer into adult neural stem cells

Adult neural stem cells (aNSCs) represent an attractive source for the production of specific types of neurons in degenerative CNS diseases and for the development of new regenerative gene therapies. However, the use of adult NSCs for transplantation and gene replacement strategies requires efficient gene expression in the cells. Due to the low pathogenicity of adenovirus (Ad) for humans, its large delivery capacity, and long-term transgene expression, Ad vectors are widely used. Here, we tested the potential of the Ad vector system to transduce adult NSCs. Analysis of Ad receptor expression in primary aNSCs revealed a complete lack of the coxsackie-adenovirus receptor and no or low expression of alphanu- and beta5-integrins, respectively, on mRNA and protein level. Consistently, transduction at different multiplicities of infection using an Ad vector expressing the enhanced green fluorescent protein (GFP) showed that adult NSCs are particularly resistant to Ad infection even at highest MOI (1000) in contrast to differentiated types of neural cells.

Buoyancy-driven instability of an autocatalytic reaction front in a Hele-Shaw cell

An autocatalytic reaction-diffusion front between two reacting species may propagate as a solitary wave, namely, at constant velocity and with a stationary concentration profile. Recent experiments on such reactions have been reported to be buoyancy unstable, under certain conditions. We calculate the linear dispersion relation of the resulting instability, by applying our recent analysis of the Rayleigh-Taylor instability of two miscible fluids in a Hele-Shaw cell. The computed dispersion relation as well as our three-dimensional lattice Bhatnagar-Gross-Krook (BGK) simulations fit reasonably well experimental growth rates reported previously.

Morphological evidence for a close interaction of chromaffin cells with cortical cells within the adrenal gland

The adrenal medulla appears to exert a regulatory influence on adrenocortical steroidogenesis. We have therefore studied the morphology of rat, porcine and bovine adrenals in order to characterize the contact zones of adrenomedullary and adrenocortical tissues. The distribution of chromaffin cells located within the adrenal cortex and of cortical cells located within the adrenal medulla was investigated. Chromaffin cells were characterized by immunostaining for synaptophysin and chromogranin A, both being considered specific for neuroendocrine cells. Cortical cells were characterized by immunostaining for 17 alpha-hydroxylase, an enzyme of the steroid pathway. Cellular contacts of chromaffin cells and cortical cells were examined at the electron microscopical level. In rat and porcine adrenals, rays of chromaffin cells, small cell clusters and single chromaffin cells or small invaginations from the medulla could be detected in all three zones of the cortex. Chromaffin cells often spread in the subcapsular space of the zona glomerulosa. In porcine and bovine adrenals, 17 alpha-hydroxylase immunoreactive cells were localized within the medulla. Single cortical cells and small accumulations of cells were spread throughout this region. At the ultrastructural level, the chromaffin cells located within the cortex in pig and rat adrenals formed close cellular contacts with cortical cells in all three zones. Our morphological data provide evidence for a possible paracrine role of chromaffin cells; this may be important for the neuroregulation of the adrenal cortex.