Does Enhanced Solar UV‐B Radiation Affect Marine Primary Producers in Their Natural Habitats?

This article is a highlight of the paper by Li et al. in this issue of Photochemistry and Photobiology as well as a short summary of the research on the effects of solar UV-B radiation on primary production in the oceans. Laboratory experiments under controlled conditions using artificial light sources indicate species-specific damage of many phytoplankton groups. Mesocosm studies in enclosures of limited volume allow analyzing UV effects in multigeneration monitoring of natural assemblages. Field studies to determine the effects of short-wavelength solar radiation require sensitive instrumentation and measurements over extended areas of the open ocean to yield significant results. Results from a cruise described in the paper by Li et al. indicate clear effects of UV-B and UV-A on the photosynthetic carbon fixation of phytoplankton communities with spatial differences between coastal and open-ocean waters. Increasing temperatures and acidification in the ocean due to global climate change may exacerbate the detrimental effects of solar UV-B radiation.     

How Does Electrostatic Activation Control Iminium‐Catalyzed Cyclopropanations?

Prediction of lg k = s ( N + E ) verified : In contrast to previous statements, ordinary α,β‐unsaturated iminium ions do react with sulfur ylides. Electrostatic interactions accelerate the reactions by a factor of more than 10⁵ and are responsible for the high stereoselectivity.

     

How Does CuII Convert into CuI? An Unexpected Ring‐Mediated Single‐Electron Reduction

Cu(x)O (x=1,2) nanomaterials with tailored composition and properties-a hot topic in sustainable technologies-may be fabricated from molecular sources through bottom-up processes that involve unexpected changes in the metal oxidation state and open intriguing challenges on the copper redox chemistry. How copper(II) sources may lead to copper(I) species in spite of the absence of any explicit reducing agent, and even in the presence of oxygen, is one such question-to date unanswered. Herein, we study copper "reduction without reductants" within one molecule and reveal that the actual reducing agent is abstracted atomic hydrogen. By investigating the fragmentation of a copper(II) precursor for copper oxide nanostructures by combined ESI-MS with multiple collisional experiments (ESI/MS(n)) and theoretical calculations, we highlight a copper-promoted C-H bond activation, leading to reduction of the metal center and formation of a Cu(I)-C-NCCN six-membered ring. Such a novel ring system is the structural motif for a new family of cyclic copper(I) adducts, which show a bonding scheme, herein reported for the first time, that may shed unprecedented light on copper chemistry. Beyond the relevance for the preparation of copper oxide nanostructures, the hydrogen-abstraction/proton-delivery/electron-gain mechanism of copper(II) reduction disclosed herein appears to be a general property of copper and might help to understand its redox reactivity.     

Does a Stabilising Interaction Favouring the Z,Z Configuration of ?S‐NSN‐S? Systems Exist?

The existence of the orbital interaction presented in the literature as being the cause for the stabilisation of the Z,Z configuration of Ph-S-N=S=N-S-Ph (1) and its derivatives in the crystal phase, has been investigated. The results of theoretical calculations at the DFT/B3LYP/6-311+G* level of theory suggest that such a stabilising interaction might not exist or be extremely weak and that packing forces must be the main cause of the observed Z,Z configuration in the solid. To reach this conclusion structural and energetic parameters were combined to study the bonding in these -S-N=S=N-S- systems. For the analogous Ph-Se-N=S=N-Se-Ph (2) in particular the isomeric equilibrium in solution found in the variable-temperature 77Se NMR spectrum indicates that, in the gas phase or in solution, the observed Z,Z configuration is not stabilised to a greater extent than the Z,E configuration.     

Visible‐Light Photocatalysis: Does It Make a Difference in Organic Synthesis?

Visible‐light photocatalysis has evolved over the last decade into a widely used method in organic synthesis. Photocatalytic variants have been reported for many important transformations, such as cross‐coupling reactions, α‐amino functionalizations, cycloadditions, ATRA reactions, or fluorinations. To help chemists select photocatalytic methods for their synthesis, we compare in this Review classical and photocatalytic procedures for selected classes of reactions and highlight their advantages and limitations. In many cases, the photocatalytic reactions proceed under milder reaction conditions, typically at room temperature, and stoichiometric reagents are replaced by simple oxidants or reductants, such as air, oxygen, or amines. Does visible‐light photocatalysis make a difference in organic synthesis? The prospect of shuttling electrons back and forth to substrates and intermediates or to selectively transfer energy through a visible‐light‐absorbing photocatalyst holds the promise to improve current procedures in radical chemistry and to open up new avenues by accessing reactive species hitherto unknown, especially by merging photocatalysis with organo‐ or metal catalysis.     

How Does Your MOF Grow?

Metal–organic frameworks (MOFs) are one of the most important classes of material in current chemistry. One open question is what is the mechanism of their crystal growth? In situ atomic force microscopy (see image) can be used to look at the surface of crystals as they grow, revealing a number of interesting features and giving clues to the molecular species that are important in the growth mechanism.

     

Does the Surface Matter? Hydrogen‐Bonded Chain Formation of an Oxalic Amide Derivative in a Two‐ and Three‐Dimensional Environment

We report on a multi‐technique investigation of the supramolecular organisation of N,N‐diphenyl oxalic amide under differently dimensioned environments, namely three‐dimensional (3D) in the bulk crystal, and in two dimensions on the Ag(111) surface as well as on the reconstructed Au(111) surface. With the help of X‐ray structure analysis and scanning tunneling microscopy (STM) we find that the molecules organize in hydrogen‐bonded chains with the bonding motif qualitatively changed by the surface confinement. In two dimensions, the chains exhibit enantiomorphic order even though they consist of a racemic mixture of chiral entities. By a combination of the STM data with near‐edge X‐ray absorption fine‐structure spectroscopy, we show that the conformation of the molecule adapts such that the local registry of the functional group with the substrate is optimized while avoiding steric hindrance of the phenyl groups. In the low coverage case, the length of the chains is limited by the Au(111) reconstruction lines restricting the molecules into fcc stacked areas. A kinetic Monte Carlo simulated annealing is used to explain the selective assembly in the fcc stacked regions.     

How Does a Membrane Protein Achieve a Vectorial Proton Transfer Via Water Molecules?

We present a detailed mechanism for the proton transfer from a protein‐bound protonated water cluster to the bulk water directed by protein side chains in the membrane protein bacteriorhodopsin. We use a combined approach of time‐resolved Fourier transform infrared spectroscopy, molecular dynamics simulations, and X‐ray structure analysis to elucidate the functional role of a hydrogen bond between Ser193 and Glu204. These two residues seal the internal protonated water cluster from the bulk water and the protein surface. During the photocycle of bacteriorhodopsin, a transient protonation of Glu204 leads to a breaking of this hydrogen bond. This breaking opens the gate to the extracellular bulk water, leading to a subsequent proton release from the protonated water cluster. We show in detail how the protein achieves vectorial proton transfer via protonated water clusters in contrast to random proton transfer in liquid water.     

In Search of Organic Zeolites – Does Modern Information Retrieval Inevitably Become a ‘Sieving‐the‐Desert' Exercise?

Information retrieval for planning and executing research projects and for publishing results is considered a routine task that is usually neither mentioned explicitly in a scientific publication nor described in any detail. In the information searches for the preceding publication (‘Building an Organic Zeolite from a Macrocyclic TADDOL Derivative or How to Teach an Old Dog New Tricks'), we were confronted with so many problems during retrieval of the desired information about related work that we decided to deviate from this tradition. We had to use the Cambridge Structural Database, the Chemical Abstracts structure and literature databases, and the Beilstein database to the full extent of their contents, indexing, and search facilities to retrieve the necessary information about ‘organic zeolites'. In the process, we found important limitations and deficiencies in any one of these databases, and we had to conceive search procedures that we considered rather unusual even after more than 20 years of experience in searching chemistry databases. The results and, particularly, the problems encountered underline the necessity for enhanced integration of individual compound and property databases and improved standardization as a prerequisite for this.     

Does a Thermoneutral Electrocatalyst Correspond to the Apex of a Volcano Plot for a Simple Two‐Electron Process?

Volcano analyses have been established as a standard tool in the field of electrocatalysis for assessing the performance of electrodes in a class of materials. The apex of the volcano curve, where the most active electrocatalysts are situated, is commonly defined by a hypothetical ideal material that binds its reaction intermediates thermoneutrally at zero overpotential, in accordance with Sabatier's principle. However, recent studies report a right shift of the apex in a volcano curve, in which the most active electrocatalysts bind their reaction intermediates endergonically rather than thermoneutrally at zero overpotential. Focusing on two‐electron process, this Viewpoint addresses the question of how the definition of an optimum catalyst needs to be modified with respect to the requirements of Sabatier's principle when kinetic effects and the applied overpotential are included in the analysis.     

How Strong Are Hydrogen Bonds in Metalla‐β‐diketones?

The energies of the kinetically inert, electronically saturated Lukehart-type metalla-beta-diketone [Re{(COMe)2H}(CO)4] (9 a) and of the kinetically labile, electronically unsaturated platina-beta-diketones [Pt{(COMe)2H}Cl2]- (10 a), [Pt2{(COMe)2H}2(micro-Cl)2] (11 a), and [Pt{(COMe)2H}(bpy)]+ (12 a) have been calculated by DFT at the B3LYP/6-311++G(d,p) level using effective core potentials with consideration of relativistic effects for the transition metals. Analogously, energies of the requisite open (non-hydrogen-bonded) equilibrium conformers (9 b, 10 c, 11 b, 12 b) and energies which were obtained from the hydrogen-bonded conformers by rigid rotation of the OH group around the C--O bond by 180 degrees followed by relaxation of all bond lengths and angles (9 c, 10 d, 11 c, 12 d) have been calculated. These energies were found to be higher by 14.7/27.2 (9 b/9 c), 20.7/27.2 (10 c/10 d), 19.2/25.7 (11 b/11 c), and 9.4/19.6 kcal mol(-1) (12 b/12 d) than those of the intramolecularly O--HO hydrogen-bonded metalla-beta-diketones 9 a, 10 a, 11 a, and 12 a, respectively. In acetylacetone (Hacac), the generic organic analogue of metalla-beta-diketones, the energies of the most stable non-hydrogen-bonded enol isomer (6 b) and of the conformer derived from the H-bonded form by rigid rotation of the OH group by 180 degrees followed by subsequent relaxation of all bond lengths and angles (6 k) were found to be 10.9/16.1 kcal mol(-1) (6 b/6 k) higher compared to the intramolecularly O--HO bonded isomer 6 a. Thus, the hydrogen bonds in metalla-beta- diketones must be regarded as strong and were found to be up to twice as strong as that in acetylacetone. A linear relationship was found between the hydrogen-bond energies based on the rigidly rotated structures and the OO separation in the hydrogen-bonded structures. Furthermore, these energies were also found to be correlated with the electron densities at the OH bond critical points (rhobcp) in the O--HO bonds of metalla-beta-diketones 9 a, 10 a, 11 a, and 12 a (calculated using the AIM theory). The comparison of the energies of the doubly intermolecularly hydrogen-bonded dinuclear platina-beta-diketone [{Pt{(COMe)2H}(bpy)}2]2+ (14) with that of the mononuclear intramolecularly hydrogen-bonded cation [Pt{(COMe)2H}(bpy)]+ (12 a) showed that the intermolecular hydrogen bonds in 14 are weaker than the intramolecular hydrogen bond in 12.     

Upconverting‐Nanoparticle‐Assisted Photochemistry Induced by Low‐Intensity Near‐Infrared Light: How Low Can We Go?

Upconverting nanoparticles (UCNPs) convert near‐infrared (NIR) light into UV or visible light that can trigger photoreactions of photosensitive compounds. In this paper, we demonstrate how to reduce the intensity of NIR light for UCNP‐assisted photochemistry. We synthesized two types of UCNPs with different emission bands and five photosensitive compounds with different absorption bands. A λ=974 nm laser was used to induce photoreactions in all of the investigated photosensitive compounds in the presence of the UCNPs. The excitation thresholds of the photoreactions induced by λ=974 nm light were measured. The lowest threshold was 0.5 W cm^?2, which is lower than the maximum permissible exposure of skin (0.726 W cm^?2). We demonstrate that low‐intensity NIR light can induce photoreactions after passing through a piece of tissue without damaging the tissue. Our results indicate that the threshold for UCNP‐ assisted photochemistry can be reduced by using highly photosensitive compounds that absorb upconverted visible light. Low excitation intensity in UCNP‐assisted photochemistry is important for biomedical applications because it minimizes the overheating problems of NIR light and causes less photodamage to biomaterials.     

How to identify promising metal scavengers? d‐penicillamine with copper as a study case

Therapeutic chelating agents are used to prevent the effects of the metal accumulation. These are molecules that form complexes with transition metals and they are referred here as metal scavengers. The main idea of this investigation is to recognize the most relevant chemical features to identify potential metal scavengers. d ‐penicillamine with copper (DPEN‐Cu) is used for this purpose. The first requirement that must be fulfilled by a good metal scavenger is the exergonicity of the chelating reaction. In the DPEN‐Cu case the most likely complexation pathway was found to have ΔG equal to −24.3 kcal/mol. It is desirable that the chelating molecule could also be a free radical scavenger. d ‐penicillamine is a good free radical scavenger following the hydrogen atom transfer reaction. An additional advantage is that the DPEN‐Cu may act as •OH‐inactivating ligand. It is proposed that chelating agents fulfilling these requirements may be a promising candidate to be used in metal chelation therapies as metal scavengers.