Wim Wenseleers

Role of Dimensionality on the Two‐Photon Absorption Response of Conjugated Molecules: The Case of Octupolar Compounds

A comparative study of the two-photon absorption (TPA) properties of octupolar compounds and their dipolar one-dimensional counterparts is presented on the basis of correlated quantum-chemical calculations. The roles of dimensionality and symmetry are first discussed on the basis of a simple exciton picture where the ground-state and excited-state wavefunctions of three-arm octupolar systems are built from a linear combination of the corresponding single-arm wavefunctions. This model predicts a factor of 3 increase in the TPA cross section in the limiting case of three independent charge-transfer pathways. When taking into account the full chemical structures of representative octupolar molecules, the results of the calculations indicate that a much larger enhancement associated with an increase in dimensionality and delocalization can be achieved when the core of the chromophore allows significant electronic coupling among the individual arms. These theoretical predictions are in agreement with the experimental determination of the TPA cross sections for crystal violet and the related compound, brilliant green, and suggest new strategies for the design of conjugated materials with large TPA cross sections.

Silver Nanoparticles with Broad Multiband Linear Optical Absorption

Keywords: thiols ; cluster compounds ; luminescence ; quantum dots ; surface plasmon resonance Reference EPFL-ARTICLE-166610doi:10.1002/anie.200900298 Record created on 2011-06-06, modified on 2017-05-10

Laser and Electron‐Beam Induced Growth of Nanoparticles for 2D and 3D Metal Patterning

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Photoresponsive Hydrogel Microstructure Fabricated by Two-Photon Initiated Polymerization

A photoresponsive polymeric hydrogel cantilever that deflects under illumination has been fabricated by using two-photon three-dimensional lithography. The hydrogel was prepared from a comonomer solution containing acryloylacetone, acrylamide, and N,N′-methylene bisacrylamide. The photoresponse of the cantilever was activated by photoexcitation of acetylacetone groups at 244 nm. Deflection of the cantilever by ∼ 45° was effected upon UV irradiation for 20 min.

Bis(dioxaborine) compounds with large two-photon cross sections, and their use in the photodeposition of silver

Compounds in which two dioxaborines are linked by a conjugated bridge exhibit high two-photon cross sections and can be used as sensitisers for the photodeposition of metallic silver lines.

Hyper-Rayleigh scattering study of η5-monocyclopentadienyl–metal complexes for second order non-linear optical materials

A series of ionic η5-monocyclopentadienyl–metal compounds possessing p-substituted benzonitrile ligands has been studied by hyper-Rayleigh scattering at the fundamental wavelength of 1.064 µm. Upon systematic variation of the metal ion, the first hyperpolarizability β was found to increase along the sequence Co, Ni, Ru, Fe, with about a three-fold increase from Ru to Fe. This yields very high values for the iron complexes, e.g., β=410×10–30 esu for [Fe(η5-C5H5)(dppe)( p-NCC6H4NO2)]+ [PF6 ]– dissolved in methanol. The high β values are attributed to π back-donation resulting in an extension of the conjugated π-system from the FeII organometallic fragment, acting as a good donor group, via the nitrile to the acceptor group NO2 . Complexes with single phenyl rings as conjugated chains perform better than their biphenyl analogues, which is explained in terms of non-planarity of the coordinated biphenyl ligands in solution. By comparing complexes with electron donor and acceptor substituted ligands it is demonstrated that the organometallic moiety can be used as an extremely effective donor group but not as a good acceptor group.

First hyperpolarizability dispersion of the octupolar molecule crystal violet: multiple resonances and vibrational and solvation effects.

The first hyperpolarizability (β) dispersion curve is measured for the first time for an octupolar nonlinear optical (NLO) molecule (crystal violet, CV) and modeled theoretically, yielding an in-depth understanding of the electronic structure and vibronic and solvation effects on such octupolar conjugated systems. Tunable wavelength hyper-Rayleigh scattering (HRS) measurements were performed on this prototypical octupolar molecule in the broad fundamental wavelength range of 620-1580 nm, showing significant shortcomings of the commonly used β dispersion models. Three well-separated β resonances involving the lowest-energy state and several higher excited states are clearly observed, including a significant contribution from a nominally one-photon forbidden transition. The experimental results for second-harmonic wavelengths above 330 nm are successfully modeled by means of a vibronically coupled essential-state description for octupolar chromophores, developed by Terenziani et al. (J. Phys. Chem. B 2008, 112, 5079), which takes into account polar solvation effects. The relative intensities of the various resonances, including the one below 330 nm, are also quantified by quantum chemical calculations. Furthermore, interesting effects of inhomogeneous broadening due to polar solvation of the two-dimensional chromophore are recognized in both linear and nonlinear spectra, allowing us to quantitatively address the long-standing problem of the band shape of the linear absorption spectrum of CV. This clearly demonstrates that extensive wavelength-dependent HRS measurements, as presented in this work, are essential to the characterization and design of NLO materials and represent a powerful tool to gain valuable information on molecular excitations and environmental effects in general.

Highly sensitive setup for tunable wavelength hyper-Rayleigh scattering with parallel detection and calibration data for various solvents.

A very sensitive experimental setup for accurate wavelength-dependent hyper-Rayleigh scattering (HRS) measurements of the molecular first hyperpolarizability beta in the broad fundamental wavelength range of 600 to 1800 nm is presented. The setup makes use of a stable continuously tunable picosecond optical parametric amplifier with kilohertz repetition rate. To correct for multi-photon fluorescence, a small spectral range around the second harmonic wavelength is detected in parallel using a spectrograph coupled to an intensified charge-coupled device. Reliable calibration against the pure solvent is possible over the full accessible spectral range. An extensive set of wavelength-dependent HRS calibration data for a wide range of solvents is presented, and very accurate measurements of the beta dispersion of the well-known nonlinear optical chromophore Disperse Red 1 are demonstrated.

Separation and Diameter‐Sorting of Empty (End‐Capped) and Water‐Filled (Open) Carbon Nanotubes by Density Gradient Ultracentrifugation

Ever since the discovery of single-wall carbon nanotubes (SWCNTs), their unique and remarkably diverse electronic and optical properties, which depend critically on their exact diameter and chiral structure, have proven to be both a blessing and a curse, as synthetic methods invariably produce mixtures of structures, while applications demand more uniform properties. The sorting of SWCNTs by buoyant density through solubilization as individually isolated tubes using bile salt surfactants, followed by ultracentrifugation to equilibrium in a density gradient (DGU), is definitely the most promising technique for separating SWCNTs that have different diameters and chiral structures. However, the diameter-sorting is still not well understood, and the isolation of few or even individual SWCNT species has been limited to SWCNTs with relatively small diameters. In the present work, we have obtained an unprecedented insight into the sorting mechanism by showing that each individual SWCNT chirality actually does not concentrate at one, but rather at two different densities, which correspond to empty and waterfilled nanotubes (Figure 1): the intact (end-capped, and therefore empty) SWCNTs, which are present mainly in carefully solubilized samples, can be separated from waterfilled SWCNTs by DGU. Remarkably, these empty tubes possess far superior properties than the water-filled tubes used in previous nanotube research, and the overall reversed trend of buoyant density with increasing diameter enhances sorting of large-diameter tubes. In recent years, important progress has been made toward the preparation of monodisperse SWCNT samples, at least for small-diameter tubes, both by more selective synthesis and by post-synthesis purification methods. The sorting of SWCNTs by DGU was pioneered by Arnold et al. , 6] and has been further developed and emerged as the most widely used and most versatile technique for sorting different SWCNT species by length, diameter, electronic type, and handedness. 7] In DGU, particles sediment in a solution of

Luminescence Properties of Individual Empty and Water-Filled Single-Walled Carbon Nanotubes

The influence of water filling on the photoluminescence (PL) properties of SWCNTs is studied by ensemble and single-molecule PL spectroscopy. Red-shifted PL and PL excitation spectra are observed upon water filling for 16 chiralities and can be used to unambiguously distinguish empty SWCNTs from filled ones. The effect of water filling on the optical transitions is well-reproduced by a continuum dielectric constant model previously developed to describe the influence of the nanotube outer environment. Empty nanotubes display narrower luminescence lines and lower inhomogeneous broadening, signatures of reduced extrinsic perturbations. The radial breathing mode phonon sideband is clearly observed in the PL spectrum of small diameter empty tubes, and a strong exciton-phonon coupling is measured for this vibration. Biexponential PL decays are observed for empty and water-filled tubes, and only the short-living component is influenced by the water filling. This may be attributed to a shortening of the radiative lifetime of the bright state by the inner dielectric environment.