Supramolecular Recognition: Protonmotive‐Driven Switches or Motors?

A dicationic molecular receptor bearing two cofacially disposed terpyridyl-Pd-Cl units forms stable 1:1 host-guest complexes with planar, neutral platinum(II) complexes. When the guest is modified to incorporate a pyridine group, the now basic guest is protonated by trifluoroacetic acid in acetonitrile solutions. The basic yellow guest forms a stable, deep red 1:1 host-guest complex with the yellow palladium receptor. Addition of trifluoroacetic acid to this host-guest complex leads to the displacement of the guest from the receptor. It is proposed that the dissociation of the guest is caused by electrostatic repulsion between the dicationic receptor and the positively charged protonated guest. Addition of base restores the host-guest complex. This protonmotive translocation of the guest from the host to the solution is discussed in terms of the mechanisms that drive molecular motors, the power stroke and the Brownian ratchet. It is concluded that the system is best described as a molecular switch that operates by the same mechanism as one stroke of a molecular motor     

How Can Theory Predict the Selectivity of Palladium‐Catalyzed Cross‐Coupling of Pristine Aromatic Molecules?

The new approach for palladium‐catalyzed cross‐coupling of two non‐activated aromatic compounds (D. R. Stuart, K. Fagnou, Science 2007 , 316, 1172) was studied theoretically. The energetic span model (S. Kozuch, S. Shaik, Acc. Chem. Res. 2011 , 44, 101, and references therein) was employed to analyze the kinetic behavior of the catalytic cycle. The computed energy profile, combined with the energetic span model, accounts for the experimental selectivity, which favors the hetero‐coupling of benzene with indole. This selectivity is driven by a fine balance of the entropic contributions and the high ratio of concentrations used for benzene over indole. This analysis may allow future theoretical predictions of how different aromatic compounds can be effectively coupled.     

Rhodium(III)‐Catalyzed ortho‐Heteroarylation of Phenols through Internal Oxidative C?H Activation: Rapid Screening of Single‐Molecular White‐Light‐Emitting Materials

Reported herein is the first example of a transition‐metal‐catalyzed internal oxidative C H/C H cross‐coupling between two (hetero)arenes through a traceless oxidation directing strategy. Without the requirement of an external metal oxidant, a wide range of phenols, including phenol‐containing natural products, can undergo the coupling with azoles to assemble a large library of highly functionalized 2‐(2‐hydroxyphenyl)azoles. The route provides an opportunity to rapidly screen white‐light‐emitting materials. As illustrative examples, two bis(triphenylamine)‐bearing 2‐(2‐hydroxyphenyl)oxazoles, which are difficult to access otherwise, exhibit bright white‐light emission, high quantum yield, and thermal stability. Also presented is the first example of the white‐light emission, in a single excited‐state intramolecular proton transfer system, of 2‐(2‐hydroxyphenyl)azoles, thus highlighting the charm of C H activation in the discovery of new organic optoelectronic materials.     

Direct sp3 C?H Amination of Nitrogen‐Containing Benzoheterocycles Mediated by Visible‐Light‐Photoredox Catalysts

Visible‐light‐mediated direct sp³ C? H amination of benzocyclic amines via α‐aminoalkyl radicals by using photoredox catalysts is described here. The obtained N,N‐acetals were also successfully applied for carbon–carbon bond forming reactions with carbon nucleophiles. The procedure is suitable for a late‐stage modification of C? H bonds to C? C bonds.     

Electronic Tuning of Iron–Oxo‐Mediated C?H Activation: Effect of Electron‐Donating Ligand on Selectivity

We have reported previously that an iron(III) complex supported by an anionic pentadentate monoamido ligand, dpaq^H (dpaq^H=2‐[bis(pyridin‐2‐ylmethyl)]amino‐N‐quinolin‐8‐yl‐acetamido), promotes selective C? H hydroxylation with H₂O₂ with high regioselectivity. Herein, we report on the preparation of Fe^III–dpaq derivatives that have a series of substituent groups at the 5‐position of a quinoline moiety in the parent ligand dpaq^H (dpaq^R, R: OMe, H, Cl, and NO₂), and examine them with respect to their catalytic activity in C? H hydroxylation with H₂O₂. As the substituent group becomes more electron‐withdrawing, both the selectivity and the turnover number increase, but the selectivity of epoxidation shows the opposite trend.     

A Reevaluation of the Photolytic Properties of 2‐Hydroxybenzophenone‐Based UV Sunscreens: Are Chemical Sunscreens Inoffensive?

The excited states of a set of popular sunscreen agents (2‐hydroxybenzophenone, oxybenzone, and sulisobenzone) are studied by using femto‐ and nanosecond time‐resolved spectroscopy. Upon excitation, the compounds undergo an ultrafast excited‐state intramolecular proton transfer (ESIPT) reaction as the major energy‐wasting process and the rate constant of this reaction is k=2×10¹² s^?1. The ESIPT yields a keto conformer that undergoes a fast, picosecond internal conversion decay. However, a photodegradative pathway is a monophotonic H?O bond breakage that subsequently leads to trace yields of phenoxyl radicals. Because potentially harmful phenoxyl radicals are formed upon irradiation of sunscreen agents, care should be taken about their reactivity towards biologically relevant compounds.     

Can Coordination‐Driven Supramolecular Self‐Assembly Reactions Be Conducted from Fully Aliphatic Linkers?

The reaction between a preassembled Cu^I bimetallic molecular clip with a short intermetallic distance and a series of fully aliphatic cyano‐capped ditopic linkers with increasing lengths was investigated. It is shown that, depending on the length of the ditopic linkers, the rational design of unprecedented supramolecular compact metallacycles containing fully aliphatic walls is possible. The specific preorganized molecular arrangement of the molecular clip used favors stabilizing interlinker London dispersion interactions, which allow, as the length of the linkers increases, the selective formation of discrete compact metallacycles at the expense of 1D coordination polymers. The generalizability of this approach was demonstrated by the reaction of fully aliphatic cyano‐capped linkers with two other types of preassembled Cu^I bimetallic molecular clips that also had short intermetallic distances.     

Non‐mammalian Hosts and Photobiomodulation: Do All Life‐forms Respond to Light?

Photobiomodulation (PBM), also known as low‐level laser (light) therapy, was discovered over 50 years ago, but only recently has it been making progress toward wide acceptance. PBM originally used red and near‐infrared (NIR) lasers, but now other wavelengths and non‐coherent light‐emitting diodes (LEDs) are being explored. The almost complete lack of side effects makes the conduction of controlled clinical trials relatively easy. Laboratory research has mainly concentrated on mammalian cells (normal or cancer) in culture, and small rodents (mice and rats) as models of different diseases. A sizeable body of work was carried out in the 1970s and 1980s in Russia looking at various bacterial and fungal cells. The present review covers some of these studies and a recent number of papers that have applied PBM to so‐called “model organisms.” These models include flies (Drosophila), worms (Caenorhabditis elegans), fish (zebrafish) and caterpillars (Galleria). Much knowledge about the genomics and proteomics, and many reagents for these organisms already exist. They are inexpensive to work with and have lower regulatory barriers compared to vertebrate animals. Other researchers have studied different models (snails, sea urchins, Paramecium, toads, frogs and chickens). Plants may respond to NIR light differently from visible light (photosynthesis and photomorphogenesis) but PBM in plants has not been much studied. Veterinarians routinely use PBM to treat non‐mammalian patients. The conclusion is that red or NIR light does indeed have significant biologic effects conserved over many different kingdoms, and perhaps it is true that “all life‐forms respond to light.”     

An Alternative Mechanistic Concept for Homogeneous Selective Ethylene Oligomerization of Chromium‐Based Catalysts: Binuclear Metallacycles as a Reason for 1‐Octene Selectivity?

An alternative concept for the selective catalytic formation of 1‐octene from ethylene via dimeric catalytic centers is proposed. The selectivity of the tetramerization systems depends on the capability of ligands to form binuclear complexes that subsequently build up and couple two separate metallacyclopentanes to form 1‐octene selectively. Comparison of existing catalytic processes, the ability of the bis(diarylphosphino)amine (PNP) ligand to bridge two metal centers, and the experimental background support the proposed binuclear mechanism for ethylene tetramerization.     

Lanthanide‐to‐Lanthanide Energy‐Transfer Processes Operating in Discrete Polynuclear Complexes: Can Trivalent Europium Be Used as a Local Structural Probe?

This work, based on the synthesis and analysis of chemical compounds, describes a kinetic approach for identifying intramolecular intermetallic energy‐transfer processes operating in discrete polynuclear lanthanide complexes, with a special emphasis on europium‐containing entities. When all coordination sites are identical in a (supra)molecular complex, only heterometallic communications are experimentally accessible and a Tb→Eu energy transfer could be evidenced in [TbEu( L5 )(hfac)₆] (hfac=hexafluoroacetylacetonate), in which the intermetallic separation amounts to 12.6 Å. In the presence of different coordination sites, as found in the trinuclear complex [Eu₃( L2 )(hfac)₉], homometallic communication can be induced by selective laser excitation and monitored with the help of high‐resolution emission spectroscopy. The narrow and non‐degenerated character of the Eu(⁵D₀?⁷F₀) transition excludes significant spectral overlap between donor and acceptor europium cations. Intramolecular energy‐transfer processes in discrete polynuclear europium complexes are therefore limited to short distances, in agreement with the Fermi golden rule and with the kinetic data collected for [Eu₃( L2 )(hfac)₉] in the solid state and in solution. Consequently, trivalent europium can be considered as a valuable local structural probe in discrete polynuclear complexes displaying intermetallic separation in the sub‐nanometric domain, a useful property for probing lanthanido‐polymers.     

Photosensitized Structural Modifications of the Lens Protein α‐Crystallin: Do All Modifications Impair Chaperone‐like Activity?¶

Among chaperone-like functioning proteins, the lens alpha-crystallins are of particular interest because they are not renewed, and even minor alterations can hurt their function of maintaining the proper refractive index and avoiding cataract formation in the lens. Several reports have suggested the occurrence of remarkable structural modifications in lens proteins in the presence of endogenous and exogenous sensitizers upon exposure to light. In particular, it has been shown in vitro that hypericin, the active ingredient of Hypericum, can bind to and, in the presence of light, cause the photopolymerization of alpha-crystallin. On the basis of these results it has also been suggested that a subsequent significant impairment of the protein function can occur. Using absorption and emission spectroscopic techniques, as well as circular dichroism, we have studied the structural modifications of alpha-crystallin resulting from its interaction with hypericin after irradiation with visible light. To investigate the chaperone-like function of alpha-crystallin, the heat-induced aggregation kinetics of another lens protein, betaLow-crystallin, was monitored by measuring the apparent absorption due to scattering at 360 nm as a function of time, and no apparent damage to its functional role was observed. Spectroscopic results, on the contrary, show a prominent reduction in both tryptophan and hypericin fluorescence emission intensity after light irradiation, suggesting an alteration in the tryptophan microenvironment and a high degree of packing of the chromophore due to photoinduced modification of the molecular framework. Control experiments on alpha-crystallin structurally modified by light in the presence of hypericin indicated that the protein still retains its ability to chaperone both lens crystallins and insulin.