No abstract

Nickel-porphyrins, with their rigid quadratic planar coordination framework, provide an excellent model to study the coordination-induced spin crossover (CISCO) effect because bonding of one or two axial ligands to the metal center leads to a spin transition from S=0 to S=1. Herein, both equilibrium constants K(1S) and K(2), and for the first time also the corresponding thermodynamic parameters DeltaH(1S), DeltaH(2), DeltaS(1S), and DeltaS(2), are determined for the reaction of a nickel-porphyrin (Ni-tetrakis(pentafluorophenyl)porphyrin) with different 4-substituted pyridines by temperature-dependent NMR spectroscopy. The association constants K(1S) and K(2) are correlated with the basicity of the 4-substituted pyridines (R: OMe>H>CO(2)Et>NO(2)) whereas the DeltaH(1S) values exhibit a completely different order (OMeCO(2)Et>NO(2)). 4-Nitropyridine exhibits the largest binding enthalpy, which, however, is overcompensated by a large negative binding entropy. We attribute the large association enthalpy of nitropyridine with porphyrin to the back donation of electrons from the Ni d(xz) and d(yz) orbitals into the pi orbitals of pyridine, and the negative association entropy to a decrease in vibrational and internal rotation entropy of the more rigid porphyrin-pyridine complex. Back donation for the nitro- and cyanopyridine complexes is also confirmed by IR spectroscopy, and shows a shift of the N-O and C-N vibrations, respectively, to lower wave numbers. X-ray structures of 2:1 complexes with nitro-, cyano-, and dimethylaminopyridine provide further indication of a back donation. A further trend has been observed: the more basic the pyridine the larger is K(1S) relative to K(2). For nitropyridine K(2) is 17 times larger than K(1S) and in the case of methoxypyridine K(2) and K(1S) are almost equal.

Heilbronner in 1964 predicted that annulenes with ".. a planar perimeter of N=4r AO's, which would yield an open shell configuration when occupied by 4r electrons, can be twisted into a closed shell Möbius strip perimeter without loss in pi electron energy". We have been able to synthesize the first [4n]annulene with such a Möbius topology and now present further Möbius isomers and the details of their preparation as stable compounds. To address the question whether the twist in the pi system has an effect on the properties we systematically investigate energy, geometry and magnetic parameters of a large number isomers of [16]annulenes. The Möbius twisted annulenes are consistently more aromatic than the non-twisted isomers. This is true for the parent as well as our benzoannelated systems. Our results are in contrast to those published recently by C. Castro, W. L. Karney, P. von R. Schleyer et al.

Growing crystals form a cavity when placed against a wall. The birth of the cavity is observed both by optical microscopy of sodium chlorate crystals (NaClO_{3}) growing in the vicinity of a glass surface, and in simulations with a thin film model. The cavity appears when growth cannot be maintained in the center of the contact region due to an insufficient supply of growth units through the liquid film between the crystal and the wall. We obtain a nonequilibrium morphology diagram characterizing the conditions under which a cavity appears. Cavity formation is a generic phenomenon at the origin of the formation of growth rims observed in many experiments, and is a source of complexity for the morphology of growing crystals in natural environments. Our results also provide restrictions for the conditions under which compact crystals can grow in confinement.

Electronic self-decoupling of an organic chromophore from a metal substrate is achieved using a naphtalenediimide cyclophane to spatially separate one chromophore unit of the cyclophane from the substrate. Observations of vibronic excitations in scanning tunneling spectra demonstrate the success of this approach. These excitations contribute a significant part of the tunneling current and give rise to clear structure in scanning tunneling microscope images. We suggest that this approach may be extended to implement molecular functions at metal surfaces.electron transport | functional molecules at surfaces | scanning tunneling microscopy | vibronic states P redictions are difficult, especially about the future. We hope that this truism will be valid for the future of surface science, too. Nevertheless, it appears to be safe to predict that functional molecules at surfaces will be one of the foci of surface science over the next decade. The opportunities in this field are virtually unlimited and range from technological issues such as controlling interface effects in organic devices to adventurous endeavors such as building molecular machines at surfaces. Although these topics are most challenging, the present state of the experimental and theoretical methods of surface science is good reason for optimism that significant scientific progress will be made in this field.This article focuses on the electronic coupling between molecules and a metal substrate. At the interface, charge transfer and hybridization affect the levels of an adsorbed molecule, which may significantly modify its properties. To recover and use the intrinsic molecular properties, which may be taylored over a wide range, a degree of decoupling from the metallic surface may be desirable. Effective molecular decoupling has been achieved using multilayers of molecules (1) or ultrathin insulating layers (2-5). An alternative approach is to chemically modify a molecule using spacer groups in order to lift a particular subunit from the substrate. With this aim, for instance, bulky groups have been used to preserve switching capability of an azobenzene derivative (6-11). Previously, this approach was used to obtain an electrical insulation of a model molecular wire (12). However, these molecules turned out to be too flexible and thus deformed upon adsorption at metal substrates (13).Here we use designed cyclophanes to achieve decoupling of one chromophore from a metal surface. These cyclophanes consist of two rigidly separated parallel π-systems from which only one adsorbs to the surface, whereas the second one is expected to remain separated from the metal. Thus, cyclophanes represent a class of molecules that are particularly interesting for investigating columnar π-stacking and through-space or through-bond electronic conductance (14-16). A naphthalenediimide (NDI) cyclophane (Fig. 1) was chosen as a model compound for the present study. It is demonstrated that this organic molecule, with its height of a few angstroms on a metallic substrate, provide...

A new route to substituted 2-phenyl-2H-indazoles through the cyclization of (2-ethynylphenyl)phenyldiazenes is presented. A coarctate reaction pathway forms the isoindazole carbene under neutral conditions, at moderate temperatures, and without the requirement of a carbene stabilizer. A wide variety of previously unknown diazene precursors was synthesized and cyclized. Trapping of the carbene with a silyl alcohol followed by deprotection affords the 3-hydroxymethyl-2-phenyl-2H-indazoles in good overall yield. The free carbene could also be trapped as a [2 + 1] cycloadduct with 2,3-dimethyl-2-butene.

The bistability of spin states (e.g., spin crossover) in bulk materials is well investigated and understood. We recently extended spin-state switching to isolated molecules at room temperature (light-driven coordination-induced spin-state switching, or LD-CISSS). Whereas bistability and hysteresis in conventional spin-crossover materials are caused by cooperative effects in the crystal lattice, spin switching in LD-CISSS is achieved by reversibly changing the coordination number of a metal complex by means of a photochromic ligand that binds in one configuration but dissociates in the other form. We present mathematical proof that the maximum efficiency in property switching by such a photodissociable ligand (PDL) is only dependent on the ratio of the association constants of both configurations. Rational design by using DFT calculations was applied to develop a photoswitchable ligand with a high switching efficiency. The starting point was a nickel-porphyrin as the transition-metal complex and 3-phenylazopyridine as the photodissociable ligand. Calculations and experiments were performed in two iterative steps to find a substitution pattern at the phenylazopyridine ligand that provided optimum performance. Following this strategy, we synthesized an improved photodissociable ligand that binds to the Ni-porphyrin with an association constant that is 5.36 times higher in its trans form than in the cis form. The switching efficiency between the diamagnetic and paramagnetic state is efficient as well (72% paramagnetic Ni-porphyrin after irradiation at 365 nm, 32% paramagnetic species after irradiation at 440 nm). Potential applications arise from the fact that the LD-CISSS approach for the first time allows reversible switching of the magnetic susceptibility of a homogeneous solution. Photoswitchable contrast agents for magnetic resonance imaging and light-controlled magnetic levitation are conceivable applications.

The molecular processes of particle binding and endocytosis are influenced by the locally changing mobility of the particle nearby the plasma membrane of a living cell. However, it is unclear how the particle's hydrodynamic drag and momentum vary locally and how they are mechanically transferred to the cell. We have measured the thermal fluctuations of a 1 μm-sized polystyrene sphere, which was placed in defined distances to plasma membranes of various cell types by using an optical trap and fast three-dimensional (3D) interferometric particle tracking. From the particle position fluctuations on a 30 μs timescale, we determined the distance-dependent change of the viscous drag in directions perpendicular and parallel to the cell membrane. Measurements on macrophages, adenocarcinoma cells, and epithelial cells revealed a significantly longer hydrodynamic coupling length of the particle to the membrane than those measured at giant unilamellar vesicles (GUVs) or a plane glass interface. In contrast to GUVs, there is also a strong increase in friction and in mean first passage time normal to the cell membrane. This hydrodynamic coupling transfers a different amount of momentum to the interior of living cells and might serve as an ultra-soft stimulus triggering further reactions.