Fourteen-electron ring model and the anomalous magnetic circular dichroism of meso-triarylsubporphyrins.

The MCD spectra of meso-triarylsubporphyrins show a sign anomaly which is correlated with the acceptor properties of the aryl substituent. From the spectra, magnetic moments of the excited states are determined. In the context of a simplified orbital model, the sign change is attributed to the quenching of the magnetic moment of the LUMO by acceptor orbitals of the substituent.

Narrow Au(111) terraces decorated by self-organized Co nanowires: a low-temperature STM/STS investigation.

Deposition of Co atoms on Au(111) surfaces leads to the formation of self-organized bilayer Co nanowires at the step edges between adjacent narrow Au(111) terraces. Scanning tunneling microscopy and spectroscopy at low temperatures is used to probe the influence of the finite dimensions on the local density of states for both the Co wires and the narrow Au terraces. Confinement of Au surface state electrons to narrow Au terraces causes a shift of the Au surface state onset energy to higher energies.

Encapsulation of small base molecules and tetrahedral/cubane-like clusters of group V atoms in the boron buckyball: a density functional theory study.

A density functional theory study of small base molecules and tetrahedral and cubane-like group V clusters encapsulated in B(80) shows that the boron buckyball is a hard acid and prefers hard bases like NH(3) or N(2)H(4) to form stable off-centered complexes. In contrast, tetrahedral and cubane-like clusters of this family are metastable in the cage. The most favorable clusters are the mixed tetrahedral and cubane clusters formed by nitrogen and phosphorus atoms such as P(2)N(2)@B(80), P(3)N@B(80), and P(4)N(4)@B(80).

Energy transfer pathways in a rylene-based Triad.

Herein, a study on the excitation energy migration is reported for a newly synthesized Triad with a well-defined architecture consisting of a central terrylenediimide decorated with four perylenediimide and sixteen naphthalenemonoimide chromophores. Steady-state femto- and picosecond time-resolved spectroscopy were employed to unveil the excited states dynamics in solution. Compared with the results obtained from the corresponding model compounds, the Triad is an efficient light collector over the entire visible spectral range and the fluorescence occurs mostly from the core with a quantum yield as high as 60%.

Probing quantized image-potential states at supported carbon nanotubes.

Discrete image-potential states (ISs) are revealed at double-walled carbon nanotubes by means of scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) in the distance-voltage z(V) spectroscopy mode. The nanotubes are supported by flat Au(111) substrates. Due to the high sensitivity of the hot IS electrons to local variations of the surface potential, they can be considered as a sensitive probe to investigate interactions with the supporting substrate and impurities or defects at the nanotube surface.

The aromatic 8-electron cubic silicon clusters Be@Si(8), B@Si(8)(+), and C@Si(8)(2+).

The geometrical and electronic structures of the Si(8)(2-) dianion and isovalent silicon clusters doped by main second-row elements including Li@Si(8)(-), Be@Si(8), B@Si(8)(+), C@Si(8)(2+), N@Si(8)(3+), and O@Si(8)(4+), are investigated using quantum chemical methods. The analyses of phenomenological shell model (PSM) combined with partial electron localizability indicators (pELI-D) rationalize the existence of cubic silicon clusters. A cubic cluster can be formed, in the cases of Be@Si(8), B@Si(8)(+), and C@Si(8)(2+), when three conditions are satisfied, namely, a full occupancy of electronic shells (34 electrons), a presence of positive charge at the center, and a type of spherical aromaticity.

Far-infrared spectra of yttrium-doped gold clusters Au(n)Y (n=1-9).

The geometric, spectroscopic, and electronic properties of neutral yttrium-doped gold clusters Au(n)Y (n=1-9) are studied by far-infrared multiple photon dissociation (FIR-MPD) spectroscopy and quantum chemical calculations. Comparison of the observed and calculated vibrational spectra allows the structures of the isomers present in the molecular beam to be determined. Most of the isomers for which the IR spectra agree best with experiment are calculated to be the energetically most stable ones.

Axial ligand control over monolayer and bilayer formation of metal-salophens at the liquid-solid interface.

Nickel salophens exclusively form monolayers at a liquid-solid interface, while in contrast zinc salophens mainly self-assemble into bilayers via axial ligand self-coordination which can be disrupted by the addition of pyridine axial ligands.

Quantum confinement of hot image-potential state electrons.

Discrete image-potential state (IS) resonances at Co nanoislands on Au(111) are probed using scanning tunneling microscopy and spectroscopy. We observe particle-in-box-type standing wave patterns, which is surprising in view of the high energy of the IS electrons when compared to the confining potential imposed by the island edges. The weak confining potential experienced by the IS electrons results in electronic interaction effects between closely spaced islands.

Unraveling excited-state dynamics in a polyfluorene-perylenediimide copolymer.

Insight into the exciton dynamics occurring in a polyfluorene-perylenediimide (PF-PDI) copolymer with a reaction mixture ratio of 100 fluorene units to 1 N,N'-bis(phenyl)-1,6,7,12-tetra(p-tert-octylphenoxy)-perylene-3,4,9,10-tetracarboxylic acid diimide (PDI) is presented here. Time-correlated single photon counting and femtosecond transient absorption spectroscopy measurements on the PF-PDI copolymer have been employed to investigate the excited-state properties of the polyfluorene subunit where the exciton is localized (PF) and the incorporated PDI chromophore. The experimental results were compared with those obtained from a polyfluorene polymer (model PF) and a N,N'-bis(2,6-diisopropylphenyl)-1,6,7,12-tetra(p-tert-octylphenoxy)-perylene-3,4,9,10-tetracarboxylic acid diimide (model PDI) which were used as reference compounds.

Low-temperature scanning tunneling microscopy and spectroscopy investigation of the electronic surface state of self-organized Cr islands on Au(111).

We have investigated the morphologic and electronic properties of self-organized nanosized Cr islands grown on atomically flat Au(111) surfaces by means of low-temperature scanning tunneling microscopy and spectroscopy under ultra-high vacuum conditions. We observe the existence of an electronic surface state on the Cr islands, which manifests itself through the formation of standing wave patterns within the interior of the atomically flat islands that gain in complexity for increasing bias voltages. The patterns are only weakly observable when compared to similar standing waves formed within self-organized Co islands on Au(111).

A study of the electronic properties of Au nanowires and Au nanoislands on Au(111) surfaces.

By means of ion bombardment of clean Au(111) films, atomically flat nanoparticles of various shapes and sizes were created, ranging from several tens of nm(2) down to only a few nm(2). Both two-dimensional Au islands as well as one-dimensional Au nanowire-like structures have been investigated by means of low-temperature scanning tunneling microscopy and spectroscopy. We were able to probe their local electronic structure in a broad energy range, which was found to be dominated by pronounced size-dependent confinement effects.

A multivalent hexapod: conformational dynamics of six-legged molecules in self-assembled monolayers at a solid-liquid interface.

A molecular hexapod having a benzene core and six oligo(p-phenylene vinylene) (OPV) legs is an ideal system to probe various types of (intramolecular) dynamics of individual molecules in physisorbed self-assembled monolayers at a solid-liquid interface. Scanning tunneling microscopy reveals that molecules adsorb in 2D crystalline as well as disordered domains. Strikingly, not all molecules have the six OPV units in contact with the graphite substrate: 4% of the molecules in the 2D crystalline domains and up to 80% of the molecules in the disordered domains have one or two OPV units desorbed.

Two-dimensional oligo(phenylene-ethynylene-butadiynylene)s: all-covalent nanoscale spoked wheels.

Round and round: Covalently bound spokes induce an efficient template-directed cyclization towards a rigid molecular wheel (see figure) and afford dramatically increased shape-persistence properties compared with non-strutted macrocycles.The synthesis and characterization of a shape-persistent two-dimensional (2D) organic compound is described in detail. In a rational modular synthesis of a dodecaacetylene precursor and its subsequent template-aided cyclization, we obtained a molecularly defined, stable, C(6)-symmetric, rigid, spoked wheel.

Electronic structure of germanium monohydrides Ge(n)H, n = 1-3.

Quantum chemical calculations were applied to investigate the electronic structure of germanium hydrides, Ge(n)H (n = 1, 2, 3), their cations, and anions. Computations using a multiconfigurational quasi-degenerate perturbation approach (MCQDPT2) based on complete active space wave functions (CASSCF), multireference perturbation theory (MRMP2), and density functional theory reveal that Ge(2)H has a (2)B(1) ground state with a doublet-quartet gap of approximately 39 kcal/mol. A quasidegenerate (2)A(1) state has been derived to be 2 kcal/mol above the ground state (MCQDPT2/aug-cc-pVTZ).

Second-harmonic generation in GFP-like proteins.

The second-order nonlinear optical properties of green fluorescent proteins (GFPs), such as the photoswitchable Dronpa and enhanced GFP (EGFP), have been studied at both the theoretical and experimental levels. In the case of Dronpa, both approaches are consistent in showing the rather counterintuitive result of a larger second-order nonlinear polarizability (or first hyperpolarizability, beta) for the protonated state, which has a higher transition energy, than for the deprotonated, fluorescent state with its absorption at lower energy. Moreover, the value of beta for the protonated form of Dronpa is among the highest reported for proteins.

Guiding the self-assembly of a second-generation polyphenylene dendrimer into well-defined patterns.

A second-generation polyphenylene dendrimer 1 is shown to self-assemble into nanofibers. To guide the formation of the dendrimer fibers into well-defined patterns, 1H,1H,2H,2H-perfluorodecyltrichlorosilane is grafted in the gas phase onto a silicon substrate. De-wetting of the solution on the nanopatterned surface results in the formation of a nanostructured template, into which fiber growth subsequently occurs under the constraints set by the de-wetted morphology.

Ratiometric, fluorescent BODIPY dye with aza crown ether functionality: synthesis, solvatochromism, and metal ion complex formation.

A new pH and metal ion-responsive BODIPY-based fluorescent probe with an aza crown ether subunit has been synthesized via condensation of 4-(1,4,7,10-tetraoxa-13-aza-cyclopentadec-13-yl)-benzaldehyde with the appropriate 1,3,5,7-tetramethyl substituted boron dipyrromethene moiety. Steady-state and time-resolved fluorometries have been used to study the spectroscopic and photophysical characteristics of this probe in various solvents. The fluorescence properties of the dye are strongly solvent dependent: increasing the solvent polarity leads to lower fluorescence quantum yields and lifetimes, and the wavelength of maximum fluorescence emission shifts to the red.

The Cu7Sc cluster is a stable sigma-aromatic seven-membered ring.

Density functional theory calculations demonstrate that the global minimum of the Cu(7)Sc potential energy surface is a seven-membered ring of copper atoms with scandium in its center, yielding a planar D(7) (h) structure. Nucleus-independent chemical shifts [NICS(1)(zz) and NICS(2)(zz)] show that this cluster has aromatic character, which is consistent with the number of 4s electrons of copper and scandium plus the 3d electrons of scandium satisfying Hückel's rule. According to a canonical MO decomposition of NICS(1)(zz) and NICS(2)(zz), the MOs consisting of the 4s atomic orbitals are mainly responsible for the aromatic behavior of the cluster.

Chromium-doped germanium clusters CrGen (n = 1-5): geometry, electronic structure, and topology of chemical bonding.

The structure and properties of small neutral and cationic CrGen(0,+) clusters, with n from 1 to 5, were investigated using quantum chemical calculations at the CASSCF/CASPT2 and DFT/B3LYP levels. Smaller clusters prefer planar geometries, whereas the lowest-lying electronic states of the neutral CrGe4, CrGe5, and cationic CrGe5+ forms exhibit nonplanar geometries. Most of the clusters considered prefer structures with high-spin ground state and large magnetic moments.

A stroboscopic approach for fast photoactivation-localization microscopy with Dronpa mutants.

The photophysical properties and photoswitching scheme of the reversible photoswitchable green fluorescent protein-like fluorescent proteins Dronpa-2 and Dronpa-3 were investigated by means of ensemble and single-molecule fluorescence spectroscopy and compared to those of the precursor protein Dronpa. The faster response to light and the faster dark recovery of the new mutants observed in bulk also hold at the single-molecule level. Analysis of the single-molecule traces allows us to extract the efficiencies and rate constants of the pathways involved in the forward and backward switching, and we find important differences when comparing the mutants to Dronpa.

Origin of the training effect and asymmetry of the magnetization in polycrystalline exchange bias systems.

The training effect and asymmetry in exchange-coupled polycrystalline CoO/Co bilayers with in-plane magnetization has been investigated. This system is selected for its large training effect and initial asymmetry of the magnetic hysteresis after field cooling, which is removed after training. Applying an in-plane magnetic field perpendicular to the cooling field largely restores the untrained state with its pronounced asymmetry.

Photophysics of 3,5-diphenoxy substituted BODIPY dyes in solution.

We have prepared two fluorescent dyes derived from 8-(4-tolyl)-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene with phenoxy and (o-bromo)phenoxy substituents at the 3,5-positions by a novel nucleophilic substitution reaction of the corresponding 3,5-dichloroBODIPY analogue. UV-vis absorption, steady-state and time-resolved fluorimetry have been used to investigate their solvent-dependent photophysical properties. The two BODIPY derivatives show narrow absorption and emission bands and display small Stokes shifts.

Single molecule fluorescence spectroscopy of pH sensitive oligonucleotide switches.

Several authors demonstrated that an oligonucleotide based pH-sensitive construct can act as a switch between an open and a closed state by changing the pH. To validate this process, specially designed fluorescence dye-quencher substituted oligonucleotide constructs were developed to probe the switching between these two states. This paper reports on bulk and single molecule fluorescence investigations of a duplex-triplex pH sensitive oligonucleotide switch.

Compartmental analysis in photophysics: kinetics and identifiability of models for quenching of fluorescent probes in micelles.

The parameters describing the kinetics of excited-state processes can possibly be recovered by analysis of the fluorescence decay surface measured as a function of the experimental variables. The identifiability analysis of a photophysical model assuming errorless time-resolved fluorescence data can verify whether the model parameters can be determined. In this work, we have used the methods of similarity transformation and Taylor series to investigate the identifiability of two models utilized to describe the time-resolved fluorescence quenching of stationary probes in micelles.