Synthesis of a 2,3';6',3''-terpyridine scaffold as an alpha-helix mimetic

[reaction: see text] A terpyridine scaffold has been designed as an alpha-helix mimetic. A facile synthesis of the ortho-functionalized 2,3'-oligopyridine has been accomplished using sequential Bohlmann-Rahtz heteroannulation reactions.     

Generation of nanospikes via laser ablation of metals in liquid environment and their activity in surface-enhanced Raman scattering of organic molecules

The formation of dense arrays of nanospikes occurs under laser ablation of bulk targets (Ag, Au, Ta, Ti) immersed in liquids such as water or ethanol. The average height of spikes is 50 nm and their density on the target amounts to 10¹⁰ cm⁻². The effect is observed with sufficiently short laser pulses. In particular, either a 350 ps or a 90 ps Nd:YAG lasers are used in their fundamental harmonics. The nanospikes are characterized by UV-Visible reflection spectrometry and atomic force microscopy. The oscillations of electrons within nanospikes result in a permanent coloration of the surface and a modification of the optical reflection spectra of the metal. Scanning the laser beam along the metal surface allows its nanostructuring over extended areas (∼1 cm²). The nanostructured Ag surface shows enhanced Raman scattering of acridine molecules at a concentration of 10⁻⁵ M/l, whereas the initial Ag targets do not show any signal within the accuracy of measurements.     

Early dynamics of guest-host interaction in dye-doped liquid crystalline materials

We have studied in detail the early dynamics of laser-induced molecular reorientation in a dye-doped liquid crystalline (LC) medium that exhibits a significant enhancement of the optical Kerr nonlinearity due to guest-host interaction. Experimental results agree quantitatively with theory based on a model in which the anisotropic dye excitation helps reorient the LC molecules through a mean-field intermolecular interaction.     

Role of square root threshold singularity in current algebra and chiral symmetry breaking calculations of Kℓ3, Kℓ4 and τ →πKν decays

It is shown that discrepancies between soft pion current algebra (or chiral symmetry) calculations in K?₄ and experiments are mostly due to the square root threshold singularity of the pion-pion interaction. For the same reason, chiral symmetry breaking calculations of the scalar K?₃ form factor cannot be extended to the threshold of τ → Kπν decay.     

Improvement of optical properties of silica colloidal crystals by sintering

The optical properties of silica colloidal crystals have been improved considerably by sintering. This can be explained by the decrease of defects and the improvement of crystalline quality due to the structure compacting under thermal treatment. A drastic blue shift of the optical attenuation band was also observed. Received: 25 January 2001 / Accepted: 26 January 2001 / Published online: 26 April 2001     

Kinetics of the formation of butanethiol monolayers on gold substrates, as studied by infrared spectroscopy

We have studied the formation of butanethiol self-assembled monolayers (C₄SAMs) and sub-monolayers chemisorbed on Au(111) surfaces using infrared reflection-absorption spectroscopy. Our IR study follows the evolution of the adsorbates from a disordered phase of sub-monolayer C₄ at short deposition times, to the more ordered arrangement of butanethiol films following extended exposure to the thiol solution. From the variation of the absorbances of CH₃ vibrations with the deposition time, we find that the initial deposition of the imperfect monolayer from solution is rapid at high concentrations, and can last many hours in very dilute solutions; this rapid absorption is followed by a much slower process of additional absorption and self-organisation. From the evolution of the frequency and intensity of the CH₂ related absorption bands, we observed that the angle of butanethiol molecular axis with respect to the surface normal is significantly greater in the initial state than for the final well-formed film; the intensities of CH₂ modes for fully equilibrated monolayers and sub-monolayers are extremely weak due to the nearly orthogonal orientation of the transition dipole moment with respect to the surface normal and to the polarisation of the incident radiation. Our data suggests that the initial deposition sites for individual butanethiol adsorbates are randomly distributed on the gold substrate.     

Screening in a one dimensional plasma

We investigate the screening behavior of a neutral one dimensional plasma made up by two types of charges not necessarily of equal values. The thermodynamic potential is given by the lowest eigenvalue of a generalized Mathieu equation. If the latter is interpreted as a Schrödinger equation, the dielectric constant is related in a simple manner to the band mass of the lowest energy band. We calculate this mass with the W.K.B. method and study the correlation function for a pair of test charges.     

The free exciton in KI and RbI: Emissions related to the triplet B exciton

We put in evidence the existence of the triplet exciton B by its emissions as associated with LO phonons in KI and RbI. We explain the possibility of such emissions by the process of interband transition coupling the B exciton with the C polariton (low branch). The exchange interaction and spin-orbit interaction values are calculated.