The optical properties and electronic structure of a homologous series of CdSe cluster molecules covering a size range between 0.7 and 2 nm are investigated. CdSe cluster molecules with 4, 8 10, 17, and 32 Cd atoms, capped by selenophenol ligands, were crystallized from solution and their structures determined by single-crystal X-ray diffraction. The cluster molecules are composed of a combination of adamanthane and barylene-like cages, the building blocks of the zinc blende and the wurtzite structures of the bulk CdSe. The onset of the room temperature absorption and low-temperature photoluminescence excitation spectra exhibit a systematic blue shift with reduced cluster size manifesting the quantum confinement effect down to the molecular limit of the bulk semiconductor. Blue-green emission, shifted substantially to lower energy from the absorption onset, is observed only at low temperature and its position is nearly independent of cluster size. The wavelength dependence of both photoluminescence and photoluminescence excitation was measured. The emission is assigned to forbidden transitions involving the cluster-molecule surface-capping ligands. This assignment is supported by the emission decay which exhibits distributed kinetics with microsecond time scale. The temperature dependence of the emission intensity is quantitatively explained by multiphonon-induced nonradiative relaxation mediated by low-frequency vibrations of the selenophenol capping ligands. Upon irradiation, the emission of all cluster molecules is quenched. Warming up and recooling leads to recovery of the emission (partial or complete) for all but the cluster molecule with 10 Cd atoms. This temporary darkening is assigned to the photoinduced charging of the cluster-molecule surface ligands, resembling the reversible on-off blinking of the emission observed for larger CdSe nanocrystals.

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Alternating current magnetic investigations on the trigonal-planar high-spin Co(2+) complexes [Li(15-crown-5)] [Co{N(SiMe3)2}3], [Co{N(SiMe3)2}2(THF)] (THF = tetrahydrofuran), and [Co{N(SiMe3)2}2(PCy3)] (Cy = -C6H13 = cyclohexyl) reveal that all three complexes display slow magnetic relaxation at temperatures below 8 K under applied dc (direct current) fields. The parameters characteristic for their respective relaxation processes such as effective energy barriers Ueff (16.1(2), 17.1(3), and 19.1(7) cm(-1)) and relaxation times τ0 (3.5(3) × 10(-7), 9.3(8) × 10(-8), and 3.0(8) × 10(-7) s) are almost the same, despite distinct differences in the ligand properties. In contrast, the isostructural high-spin Fe(2+) complexes [Li(15-crown-5)] [Fe{N(SiMe3)2}3] and [Fe{N(SiMe3)2}2(THF)] do not show slow relaxation of the magnetization under similar conditions, whereas the phosphine complex [Fe{N(SiMe3)2}2(PCy3)] does, as recently reported by Lin et al. (Lin, P.-H.; Smythe, N. C.; Gorelsky, S. I.; Maguire, S.; Henson, N. J.; Korobkov, I.; Scott, B. L.; Gordon, J. C.; Baker, R. T.; Murugesu, M. J. Am. Chem. Soc. 2011, 135, 15806.) Distinctly differing axial anisotropy D parameters were obtained from fits of the dc magnetic data for both sets of complexes. According to density functional theory (DFT) calculations, all complexes possess spatially nondegenerate ground states. Thus distinct spin-orbit coupling effects, as a main source of magnetic anisotropy, can only be generated by mixing with excited states. This is in line with significant contributions of excited determinants for some of the compounds in complete active space self-consistent field (CASSCF) calculations done for model complexes. Furthermore, the calculated energetic sequence of d orbitals for the cobalt compounds as well as for [Fe{N(SiMe3)2}2(PCy3)] differs significantly from the prediction by crystal field theory. Experimental and calculated (time-dependent DFT) optical spectra display characteristic d-d transitions in the visible to near-infrared region. Energies for lowest transitions range from 0.19 to 0.35 eV; whereas, for [Li(15-crown-5)][Fe{N(SiMe3)2}3] a higher value is found (0.66 eV). Zero-field (57)Fe Mößbauer spectra of the three high-spin iron complexes exhibit a doublet at 3 K with small and similar values of the isomer shifts (δ), ranging between 0.57 and 0.59 mm/s, as well as an unusual small quadrupole splitting (ΔEQ = 0.60 mm/s) in [Li(15-crown-5)][Fe{N(SiMe3)2}3].

For the last few years we have been working on the synthesis and characterization of metal-chalcogenide clusters. For most of the transition metals, one observes the formation of relatively low-nuclearity cluster complexes, [1] such as [Co 6 E 8 -(PR 3 ) 6 ] (E = S, Se, Te; R = organic groups) und [Ni 34 Se 22 -(PPh 3 ) 20 ]. In contrast, for clusters of copper and silver one can find a rich variety of structures. [2] Recently, we reported the synthesis of metal-rich silverchalcogenide clusters, such as [Ag 70 S 20 (SPh) 96 ] (dppm = bis(diphenylphosphanyl)methane; dppb = 1,4-bis(diphenylphosphanyl)butane), particles with diameters in the nanometer range. The surfaces of these clusters are protected by ligands, thus preventing further reaction to form the thermodynamically stable binary silver chalcogenide salts.[3] Perhaps surprisingly, these cluster complexes could be prepared reproducibly and in high yield by the reaction at room temperature of, for example, silver carboxylates with S(tBu)SiMe 3 in the presence of tertiary phosphanes. In contrast, at higher temperatures amorphous Ag 2 S was formed. We therefore propose that these metal-rich clusters represent intermediates in the formation of solid Ag 2 S. When the progress of the reaction is monitored by dynamic light scattering, initially no particles are formed that are large enough to be observed by this technique. However, after several days particles form in the size range 2-4 nm and crystallize out of solution. The structural determinations of these large clusters proved problematic. With nuclearities of up to 100 metal atoms, crystals generally diffract well to up high 2q values (50-608 with Mo Ka ); the atoms have low temperature factors, and no high residual electron density is observed within the clusters. However, this situation changes for larger clusters with nuclearities greater than around 120 metal atoms. For such clusters, the intensities of the reflections drop off rather sharply above 2q % 408, and the structure refinement results in unsatisfactorily high R factors, with high residual electron density within the cluster molecule. Satisfactory R factors can only be obtained if this electron density can be modeled during the refinement. As this electron density generally lies close to the heavy atoms, it can be difficult to interpret and thus complicates efforts to give precise estimates of the molecular formulae. These effects may result from a range of factors: 1) There is no perfect translational order in the lattice. 2) With the silver-chalcogenide clusters there is a tendency towards nonstoichiometry, as is seen for the binary phases. [4] This behavior could be a consequence of the rather similar electronegativities of silver and the chalcogenides. There is no clear distinction between Ag + and E 2À(E = S, Se, Te), and the clusters behave rather like alloys. [5] 3) The surface tension of the spherical molecules generates a Laplace pressure within the molecule, which can result in a disorder or even a phase transition. 4) Interacti...

The thermal and photochemical E/Z isomerization of camphorquinone-derived imines was studied by a combination of kinetic, structural, and computational methods. The thermal isomerization proceeds by linear N inversion, whereas the photoinduced process occurs through C=N bond rotation with preferred directionality as a result of diastereoisomerism. Thereby, these imines are arguably the simplest example of synthetic molecular motors. The generality of the orthogonal trajectories of the thermal and photochemical pathways allows for the postulation that every suitable chiral imine qualifies, in principle, as a molecular motor driven by light or heat.

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Keywords: Copper / Sulfur / Selenium / Tellurium / ClustersThe investigation of coinage metal molecular clusters bridged by chalcogen atoms represents an area of ever increasing activity in recent chemical and material science research. This is largely due to the relatively high ionic and even higher electric conductivity of binary coinage metal chalcogenides, which leads to properties intermediate between those of semiconducting and metallic phases. In addition, the size-dependency of the chemical, physical, and structural properties of substances on going from small molecules to bulk materials is of general interest. Approaches towards the synthesis and investigation of such clusters have included the study of colloidal nanoparticles with a narrow size distribution, as well as the formation and isolation of [a] ϩ49 (0)7247/82-6368 Andreas Eichhöfer (left) was born in Hünfeld, Germany, in 1964. He received his diploma in 1991, and subsequently graduated in 1993 in the group of Prof. D. Fenske at the university of Karlsruhe with a doctoral degree on metal-phosphorous cluster complexes. He then moved to the group of Prof. G. Fritz at the same institute to work on carbosilanes as precursor compounds for the fabrication of SiC fibers. After that he returned to his previous group and became a research scientist at the Institute of Nanotechnology at the Forschungszentrum Karlsruhe in 1999, where he is currently investigating the syntheses and properties of semiconductor cluster compounds. Stefanie Dehnen (center) was born in Gelnhausen, Germany, in 1969. She obtained her diploma from the University of Karlsruhe in 1993 and her doctoral degree in 1996 under the supervision of Prof. D. Fenske on experimental and theoretical investigations of copper sulfide and copper selenide clusters. After a postdoctoral stay with Prof. R. Ahlrichs (1997) she returned to the inorganic chemistry department at the University of Karlsruhe where she is presently preparing her Habilitation. In 1997, she was awarded a Feodor-Lynen-Stipendium of the Alexander-von-Humboldt-Stiftung, and in 1998, she received a Margarete-von-Wrangell-Habilitations-Stipendium of the state of Baden-Württemberg. Her current research interests comprise the synthesis, structural elucidation and chemical reactivity of binary or ternary polyanions of main group elements. Dieter Fenske (right) was born in Dortmund, Germany, in 1942. He studied chemistry at the University of Münster where he also received his doctoral degree in 1973 with Prof. H. J. Becher. After the completion MICROREVIEWS: This feature introduces the readers to the authors' research through a concise overview of the selected topic. Reference to important work from others in the field is included.

Porphyrins bearing undecyl side chains, which confer solubility, and 4‐(methoxycarbonyl)phenyl substituents in the meso positions have been prepared. The 4‐(methoxycarbonyl) substituent could be transformed into groups tailored for self‐assembly. The crystal structure of the dihydroxymethyleneporphyrin, bis[4‐(hydroxymethyl)phenyl]porphyrin 7, shows strong hydrogen bonds between the hydroxy group and pyrrole nitrogen atoms in neighboring molecules, resulting in a two‐dimensional network of hydrogen‐bonded porphyrins. π−π Interactions are also encountered, bringing porphyrins into laddered stacks with edge‐to‐edge contacts of 3.5 Å. The isomeric diesters Zn‐3 and Zn‐4 show different tendencies in π−π stacking. Models of bacteriochlorophyll c Zn‐8 and Zn‐11, possessing a formyl group, a hydroxymethyl group, and a central zinc atom, have been synthesized. In contrast to bacteriochlorophyll c, which self‐assembles both in the natural antenna system of green photosynthetic bacteria and in nonpolar solvents, the self‐assembly Zn‐8 and Zn‐11 could not be observed in solution. The results provide information for the design of better mimics of natural light‐harvesting arrays formed by self‐assembling porphyrins.