Jochen Campo

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.

Practical Model for First Hyperpolarizability Dispersion Accounting for Both Homogeneous and Inhomogeneous Broadening Effects.

A practical yet accurate dispersion model for the molecular first hyperpolarizability β is presented, incorporating both homogeneous and inhomogeneous line broadening because these affect the β dispersion differently, even if they are indistinguishable in linear absorption. Consequently, combining the absorption spectrum with one free shape-determining parameter Ginhom, the inhomogeneous line width, turns out to be necessary and sufficient to obtain a reliable description of the β dispersion, requiring no information on the homogeneous (including vibronic) and inhomogeneous line broadening mechanisms involved, providing an ideal model for practical use in extrapolating experimental nonlinear optical (NLO) data. The model is applied to the efficient NLO chromophore picolinium quinodimethane, yielding an excellent fit of the two-photon resonant wavelength-dependent data and a dependable static value β0 = 316 × 10(-30) esu. Furthermore, we show that including a second electronic excited state in the model does yield an improved description of the NLO data at shorter wavelengths but has only limited influence on β0.

Diameter-Dependent Optical Absorption and Excitation Energy Transfer from Encapsulated Dye Molecules toward Single-Walled Carbon Nanotubes

The hollow cores and well-defined diameters of single-walled carbon nanotubes (SWCNTs) allow for creation of one-dimensional hybrid structures by encapsulation of various molecules. Absorption and near-infrared photoluminescence-excitation (PLE) spectroscopy reveal that the absorption spectrum of encapsulated 1,3-bis[4-(dimethylamino)phenyl]-squaraine dye molecules inside SWCNTs is modulated by the SWCNT diameter, as observed through excitation energy transfer (EET) from the encapsulated molecules to the SWCNTs, implying a strongly diameter-dependent stacking of the molecules inside the SWCNTs. Transient absorption spectroscopy, simultaneously probing the encapsulated dyes and the host SWCNTs, demonstrates this EET, which can be used as a route to diameter-dependent photosensitization, to be fast (sub-picosecond). A wide series of SWCNT samples is systematically characterized by absorption, PLE, and resonant Raman scattering (RRS), also identifying the critical diameter for squaraine filling. In addition, we find that SWCNT filling does not limit the selectivity of subsequent separation protocols (including polyfluorene polymers for isolating only semiconducting SWCNTs and aqueous two-phase separation for enrichment of specific SWCNT chiralities). The design of these functional hybrid systems, with tunable dye absorption, fast and efficient EET, and the ability to remove all metallic SWCNTs by subsequent separation, demonstrates potential for implementation in photoconversion devices.

Divergent route to the preparation of hybrid PtFe 2,4,6-tris(4-ethynyl)phenyl-1,3,5-triazine metallodendrimers for nonlinear optics

The synthesis strategy for the preparation of novel platinum acetylide homometallic and heterobimetallic dendrimers (containing Fe as the other metal fragment) based on a 2,4,6-tris(4-ethynyl)phenyl-1,3,5-triazine core (3) is reported. All the dendrimer generations (G0–G2) were synthesized under copper-free conditions following a divergent route. The G0-Pt dendrimer (4) was synthesized using the 1,3,5-triazine core (3) and cis-[Pt(PEt3)2Cl2] with a molar ratio of 1/4. The advantage of the current method is that different dendrimers can be prepared by following the same procedure with only changes in the molar ratios of the reactants involved. For instance, when 3 reacts with 4 in a 4/1 molar ratio, the G1 dendrimer 7 is afforded without the peripheral Pt moiety, but the G1 dendrimer with the peripheral Pt moiety (8) is formed when 3 reacts with 4 in a 1/3 molar ratio. On the other hand, the G2 dendrimer with a peripheral Pt moiety (9) is synthesized when 7 reacts with 4 in a 1/6 molar ratio. The heterobim...

Accurate and Practically Implementable Model for First Hyperpolarizability Dispersion

We present a practical yet accurate dispersion model for the molecular first hyperpolarizability β, incorporating both homogeneous and inhomogeneous line-broadening. With a single shape-determining parameter, a reliable description of the wavelength-dependence of β is obtained.

First Hyperpolarizability Dispersion of the Octupolar Molecule Crystal Violet

The first hyperpolarizability dispersion curve is measured for the first time for an octupolar nonlinear optical molecule (crystal violet), using highly sensitive tunable wavelength hyper-Rayleigh scattering, and the results are succesfully modeled theoretically.

Highly Accurate Wavelength-Dependent Characterization of Second-Order Nonlinear Optical Molecules

We present a very sensitive experimental setup for accurate tunable wavelength hyper-Rayleigh scattering (HRS) measurements of the molecular first hyperpolarizability β in the broad fundamental wavelength range of 600 to 1800 nm, allowing the wavelength-dependence of β, which is still not well-understood, to be studied in detail throughout and beyond resonance. An extensive set of wavelength-dependent HRS calibration data for various solvents is presented, which has also allowed us to successfully measure the β dispersion of prototypical dipolar and octupolar organic molecules (e.g. Disperse Red 1 (DR1), Crystal Violet (CV)) and organometallic complexes. This has resulted in extensive theoretical modeling enabling reliable derivation of β at other wavelengths and in particular extrapolation to the static limit, which in turn makes it possible to compare β between different molecules and to evaluate their potential for electro-optical (EO) applications.

Synthesis, characterisation and molecular hyperpolarisabilities of pseudo-octahedral hydrido(nitrile)iron(II) complexes for nonlinear optics: X-ray structure of

A series of ionic pseudo-octahedral trans-hydrido(nitrile)iron(II) complexes with the general formula [Fe(H)(dppe)2(4NCR)][PF6] [dppe = 1,2-bis(diphenylphosphanyl)ethane; R = acceptor-substituted conjugated ligand] have been synthesised by chloride abstraction from the starting compound trans-[FeHCl(dppe)2] and fully characterised. First hyperpolarisabilities (β) have been determined by hyper-Rayleigh scattering (HRS) at the fundamental wavelength of 1072 nm and the high near-resonant values obtained (up to 1130 × 10 – 30 esu) are interpreted in terms of the two-level model (TLM) and are correlated with IR and NMR spectroscopic data. Wavelength-dependent HRS has been performed in the