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7 pages; International audience; An engineering approach towards combined photonic band gap properties and magnetic functionalities, based on independent nanoscale engineering of two different materials at different length scales, is conceptually presented, backed by simulations, and experimentally confirmed. Large (>200 nm)monodisperse nanospheres of transparent silica self-assemble into a photonic crystal with a visible band gap, which is retained upon infiltration of small (

Coating gold colloids with a well controllable silica shell is finding many applications in the fields of bioconjugation and photonic crystals. In this work, we report on ravine-structured surface morphology changes on silica-coated gold (Au@SiO 2 ) colloids as observed using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and atomic force microscopy (AFM) characterizations. These Au@SiO 2 colloids were prepared in the direct Stober synthetic route and the silica-shell thickness was varied between 20 to 100 nm. An aggregation mechanism is proposed to successfully model the silica-shell growth on the gold nanoparticles (NPs) and explain the formation of the ravine-structured surface of the Au@SiO 2 colloids. Furthermore, it is found that the ravine-structured surface strongly influences the formation of the Au@SiO 2 colloidal crystal films. The improved understanding of the surface morphology changes provides a suitable methodology for tailoring the growth of the core–shell structured colloids and assembling them into highly ordered colloidal crystal films.

We report the synthesis and the characterizations of a novel dyad composed of a zinc porphyrin (ZnP) linked to a gold porphyrin (AuP) through an ethynyl spacer. The UV/Vis absorption spectrum and the electrochemical properties clearly reveal that this dyad exhibits a strong electronic coupling in the ground state as evidenced by shifted redox potentials and the appearance of an intense charge-transfer band localized at lambda = 739 nm in dichloromethane. A spectroelectrochemical study of the dyad along with the parent homometallic system (i.e., ZnP-ZnP and AuP-AuP) was undertaken to determine the spectra of the reduced and oxidized porphyrin units. Femtosecond transient absorption spectroscopic analysis showed that the photoexcitation of the heterometallic dyad leads to an ultrafast formation of a charge-separated state (+ZnP-AuP center dot) that displays a particularly long lifetime (tau=4 ns in toluene) for such a short separation distance. The molecular orbitals of the dyad were determined by DFT quantum-chemical calculations. This theoretical study confirms that the observed intense band at lambda = 739 nm corresponds to an interporphyrin charge-transfer transition from the HOMO orbital localized on the zinc porphyrin to LUMO orbitals localized on the gold porphyrin. Finally, a Hyper-Rayleigh scattering study shows that the dyad possesses a large first molecular hyperpolarizability coefficient (beta = 2100 x 10(-30) esu at lambda = 1064 nm), thus highlighting the valuable nonlinear optical properties of this new type of push-pull porphyrin system.

12 pages, 11 figures, 9 tables.-- et al., A series of chromophoric salts has been prepared in which electron-rich 4-(dimethylamino)phenyl groups are connected via polyenyl chains to electron-accepting N-methylpyridinium or 3-methylbenzothiazolium units. These compounds have been characterized by using various techniques, including electronic absorption spectroscopy and cyclic voltammetry. Single-crystal X-ray structures have been determined for several salts, all of which crystallize centrosymmetrically. Molecular quadratic nonlinear optical (NLO) responses have been determined using femtosecond hyper-Rayleigh scattering (HRS) at 1300 and 800 nm and via Stark (electroabsorption) spectroscopic studies on the intense, visible π → π* intramolecular charge-transfer (ICT) bands. Large red shifts in the ICT transitions on replacing a pyridinium with a benzothiazolium unit indicate that the latter acts as a more effective electron acceptor. Both HRS and Stark measurements show that the static first hyperpolarizability β0 increases with polyene chain extension in both types of chromophore, and the benzothiazolium salts have larger NLO responses than their pyridinium analogues. The results of time-dependent density functional theory calculations using a polarizable solvent continuum model agree with the observation that β0 increases with chain lengthening, but the observed superiority of the benzothiazolium acceptor is not predicted either in the ICT energies or β0 values. Coupled perturbed Hartree−Fock and semiempirical INDO/S calculations similarly fail to reproduce this principal conclusion from the experimental studies., We thank the EPSRC for support (Grant GR/M93864) and the Fund for Scientific Research-Flanders (FWO-V, G.0297.04), the University of Leuven (GOA/2006/3), the Belgian Government (IUAP P5/3)), MCyT-FEDER (BQU2005-01368) and Gobierno de Aragon-Fondo Social Europeo (E39).