Rajadurai Chandrasekar

Reversibly Shape‐Shifting Organic Optical Waveguides: Formation of Organic Nanorings, Nanotubes, and Nanosheets

because the functions of thenanoobjects are dependent on their shape and dimensions. Inthe area of organic nanophotonics in particular, the confine-ment of optical waves in shape-shifting organic nanostruc-tures is interesting because of their potential use in “smart”nanooptical devices, such as optical waveguides, photonicdetectors, and optical sensors.Until now, no methods for the fabrication of reversiblyshape-shifting organic nanostructures that have dimension-ally dependent optical waveguiding properties have beenavailable. Most of the reported organic waveguides havedefined shapes

Ordering and Stabilization of Metal-Organic Coordination Chains by Hierarchical Assembly through Hydrogen Bonding at a Surface

Keywords: copper ; scanning probe microscopy ; self-assembly ; supramolecular chemistry ; surface chemistry ; Cu(100) ; Networks ; Chemistry ; Template ; Polymers ; Acid ; Architectures ; Confinement ; Interface ; Molecules Reference EPFL-ARTICLE-160497doi:10.1002/anie.200803124View record in Web of Science Record created on 2010-11-30, modified on 2017-05-12

Tuning the spin-transition properties of pyrene-decorated 2,6-bispyrazolylpyridine based Fe(II) complexes.

Two 2,6-bispyrazolylpyridine ligands (bpp) were functionalized with pyrene moieties through linkers of different lengths. In the ligand 2,6-di(1H-pyrazol-1-yl)-4-(pyren-1-yl)pyridine (L1) the pyrene group is directly connected to the bpp moiety via a C-C single bond, while in the ligand 4-(2,6-di(1H-pyrazol-1-yl)pyridin-4-yl)benzyl-4-(pyren-1-yl)butanoate (L2) it is separated by a benzyl ester group involving a flexible butanoic chain. Subsequent complexation of Fe(II) salts revealed dramatic the influence of the nature of the pyrene substitution on the spin-transition behaviour of the resulting complexes. Thus, compound [Fe(L1)(2)](ClO(4))(2) (1) is blocked in its high spin state due to constraints caused by a strong intermolecular π-π stacking in its structure. On the other hand, the flexible chain of ligand L2 in compounds [Fe(L2)(2)](ClO(4))(2) (2) and [Fe(L2)(2)](BF(4))(2)·CH(3)CN·H(2)O (3) prevents structural constraints allowing for reversible spin transitions. Temperature-dependent studies of the photophysical properties of compound 3 do not reveal any obvious correlation between the fluorescence of the pyrene group and the spin state of the spin transition core.

Switching of a coupled spin pair in a single-molecule junction

Single-molecule spintronics investigates electron transport through magnetic molecules that have an internal spin degree of freedom. To understand and control these individual molecules it is important to read their spin state. For unpaired spins, the Kondo effect has been observed as a low-temperature anomaly at small voltages. Here, we show that a coupled spin pair in a single magnetic molecule can be detected and that a bias voltage can be used to switch between two states of the molecule. In particular, we use the mechanically controlled break-junction technique to measure electronic transport through a single-molecule junction containing two coupled spin centres that are confined on two Co(2+) ions. Spin-orbit configuration interaction methods are used to calculate the combined spin system, where the ground state is found to be a pseudo-singlet and the first excitations behave as a pseudo-triplet. Experimentally, these states can be assigned to the absence and occurrence of a Kondo-like zero-bias anomaly in the low-temperature conductance data, respectively. By applying finite bias, we can repeatedly switch between the pseudo-singlet state and the pseudo-triplet state.

Assembling Isostructural Metal‐Organic Coordination Architectures on Cu(100), Ag(100) and Ag(111) Substrates

Keywords: bipyrimidine ligands ; nanostructures ; scanning probe microscopy ; self-assembly ; supramolecular chemistry ; Scanning-Tunneling-Microscopy ; Oriented Pyrolytic-Graphite ; Weak Hydrogen-Bonds ; Terephthalic Acid ; Surface ; Networks ; Coadsorption ; Organization ; Monolayers ; Interface Reference EPFL-ARTICLE-160376doi:10.1002/cphc.200800575View record in Web of Science Record created on 2010-11-30, modified on 2017-05-12

Selective Coordination Bonding in Metallo‐Supramolecular Systems on Surfaces

The confinement of molecular species in nanoscale environments strongly modifies the interaction pathways compared to homogenous, three-dimensional (bulk) conditions. A new field of chemistry featuring weak interactions, coordination bonding, and covalent chemistry at solid surfaces has recently emerged. In particular, the combination of surface-confined chemistry and scanning probe techniques with subnanometer resolution allows immediate insights into molecular self-organization processes on the nanometer level. Extended monolayers of open, two-dimensional (2D) coordination networks with high organizational periodicity, controlled symmetries, and modular dimensionality have been achieved by using designed, self-instructedmolecular building blocks. The deposition of mixtures of precursor molecules has led to more sophisticated architectures, mainly built on weak intermolecular interactions or weak interactions in combination with coordination bonding, that is, hierarchical motifs. The cooperative assembly of instructed mixtures of molecular bricks enables a high degree of structural control and functionality, for example, the stability and ordering of primary structures can be increased, or the dimensionality and geometry of supramolecular structures can be steered. Observations of molecular-level self-recognition and error correction have demonstrated collective dynamics in surface-confined supramolecular systems. A grand challenge in materials chemistry is the capability to design adaptive materials, that is, to develop systemic methods for tailored structure and function. To exploit the opportunities of systemic chemistry, a detailed understanding of the selectivity in the interaction mechanisms of molecular mixtures, if possible by direct studies at the single-molecule level, is of pivotal interest. Herein, we report on the observation of supramolecular selectivity in the simultaneous coordinative interaction of two different molecular ligands, aromatic bipyrimidines and dicarboxylic acids, with Cu and Fe atoms resulting in a selfsegregation into two distinct, surface-confined coordination network domains. The random mixture of ligands and metals separates into subdomains of pure bipyrimidine–Cu and carboxylate–Fe networks, while heteroleptic ligand combinations, though feasible, are not observed. Each 2D coordination network exhibits a tetragonal geometry with metal atom coordination nodes, but expresses unique molecular composition and spatial organization. The molecular components PBP (5,5’-bis(4-pyridyl)(2,2’bipyrimidine)) and BDA (1,4’-biphenyl-dicarboxylic acid, see Scheme 1) are co-evaporated in a 1:1 number ratio onto a Cu(100) substrate at room temperature under ultra-high vacuum (UHV) conditions. At this temperature, a diffusing copper adatom gas is present at the Cu(100) surface, which has been shown to be available for the formation of extended

Planar Active Organic Waveguide and Wavelength Filter: Self-Assembled meso-Tetratolylporphyrin Hexagonal Nanosheet

We have fabricated nearly monodispersed nanocrystalline sheet waveguides from a well-known red emitting meso-tetratolylporphyrin molecule (1) by following a bottom-up solvent assisted self-assembly technique. The nano-sheets thickness is in the range of 110-180 nm. Localized laser illumination showed excitation position dependent exciton polariton (653 and 719 nm) propagation behavior of the sheets. The spatially resolved fluorescence spectra of the sheets showed optical modes at the input and output points, indicating cavity effect. Additionally, because of the reabsorption of the 653 nm emission, the nanosheets also act as wave length filter by cutting off the 653 nm photons from reaching the output end.

Two- to one-dimensional transition of self-assembled coordination networks at surfaces by organic ligand addition

Two-dimensional metal-organic coordination networks at a Cu(100) surface are transformed to a new supramolecular structure with one-dimensional coordination character by the addition of a second organic ligand.

Visible-Near-Infrared Range Whispering Gallery Resonance from Photonic μ-Sphere Cavities Self-Assembled from a Blend of Polystyrene and Poly[4,7-bis(3-octylthiophene-2-yl)benzothiadiazole-co-2,6-bis(pyrazolyl)pyridine] Coordinated to Tb(acac)3.

A novel red emitting copolymer (P1) was prepared (Mn ∼ 10.7 kDa) by copolymerizing tridentate ligand, namely 2,6-bis(pyrazolyl)pyridine (BPP) with 4,7-bis(2-ethynyl-5-thienyl)-2,1,3-benzothiadiazole. This copolymer readily formed an orange yellow emitting metal containing conjugated polymer (P1.Tb) with Tb(acac)3. Further, a judicial blend of P1.Tb with polystyrene and its subsequent self-assembly in THF/water produced microspheres with smooth surface area. Interestingly, continuous wave laser excitation of a single microsphere displayed whispering-gallery-mode (WGM) resonance modes over a broad wavelength range covering visible (Vis) and near-infrared (NIR) regions (0.550-0.875 μm). The estimated Q factor was up to 700, which is very high for a metal containing conjugated polymer (MCCP)-based optical gain medium.

Passive optical waveguiding organic rectangular tubes: tube cutting, controlling light propagation distance and multiple optical out-puts

We propose a technique to precisely control the longitudinal dimension of self-assembled organic nanotubes to obtain tailor-made lengths using laser ablation or cutting. Since these organic nanotubes show passive optical waveguiding tendency, the presented cutting technique is quite useful to exactly control the light propagation distance and to create multiple optical outputs. The guiding efficiency of a tube is dependent on its wall thickness, lengths and defect density. A tube not in contact with the substrate also guides the optical wave indicating its true waveguiding nature.