F. Diederich

SCALING LAW FOR SECOND-ORDER HYPERPOLARIZABILITY IN POLY(TRIACETYLENE) MOLECULAR WIRES

Poly(triacetylenes) are rodlike molecules with electrons delocalized over a one-dimensional path. We show that they exhibit a power-law dependence of the second-order hyperpolarizability gamma on the monomer unit n for short molecules and a smooth saturation toward a linear increase in longer molecules. The power law of gamma?n(a) with a approximately 2.5 from dengenerate four-wave mixing and third-harmonic generation measurements is in good agreement with quantum-chemical calculations. The critical conjugation length for saturation in the three cases is shown to be approximately 60 carbon-carbon bonds, which indicates the upper boundary for the electron delocalization in such a one-dimensional molecular wire.

Two-dimensionally conjugated molecules: The importance of low molecular symmetry for large third-order nonlinear optical effects

Low molecular symmetry with regard to the conjugation path of the delocalized electrons was found to be a good guideline towards linearly conjugated molecules with large second-order hyperpolarizabilities γ. We show that this guideline is also valid for two-dimensionally conjugated systems. We experimentally demonstrate this by third-harmonic generation measurements of two-dimensionally conjugated organic molecules, and show that the observed effects can be explained by the symmetry of the electronic wave functions. In addition, the positive impact of the substitution of phenyl rings by thienyl rings on γ is discussed.

Third-order nonlinear optical properties of in-backbone substituted conjugated polymers

We present an alternative approach to tune the third-order nonlinear optical properties of organic molecules by inserting a functional group into the path of the π-electron conjugation instead of at its ends. We show that this scheme has a beneficial influence on the second-order hyperpolarizability for short molecules, but that in two instances where such molecules were polymerized into longer molecules the overall hyperpolarizability is reduced.

Reversible optical structuring of polymer waveguides doped with photochromic molecules

We show that polymeric films doped with the photochromic molecule 1,8a-dihydro-2(4-iodophenyl)-1,1-azulenedicarbonitrile can be reversibly structured by light. We discuss the relevant material properties of the photochromic molecule in solution as well as in polymer films and demonstrate light-induced waveguides at the telecommunication wavelength of 1.313 μm.

Photoinduced reversible optical gratings in photochromic diarylethene-doped polymeric thin films

We report on our investigation of the potential for all-optical reversible photostructuring of the photochromic molecule 1,2-bis(2-methylbenzo[b]thiophen-3-yl)perfluorocyclopentene (BFCP). The light-induced isomerization of BFCP was studied in solution as well as in polymeric thin films. The absorption change that is due to the photochromic reaction of BFCP was determined by conventional spectroscopic measurements. The absolute values of the refractive indices of doped polymer films were measured by the waveguide grating coupling technique for the two states of the photochromic molecule. The dynamics of the reactions and the light-induced refractive-index changes Δn of doped polymer samples were investigated in holographic grating experiments. Changes of as much as Δn=(3.5±0.5)×10-4 were obtained at 806 nm for 41.16-wt.u2009% doping. Finally, a modified two-beam coupling experiment permitted the separation of absorption and refractive-index contributions to the grating buildup and the determination of the phase shifts of these gratings with respect to the intensity grating.

Third-order nonlinear optical properties of in-backbone substituted oligo(triacetylene) chromophores

A new approach to tuning the nonlinear optical properties of hybrid oligo(triacetylene) compounds is studied. The method is based on the insertion of a central heterospacer group between two (E)-hex-3-ene-1,5-diyne moieties. A significant increase in the second hyperpolarizability γ is expected if the central spacer fragment is an extended conjugated chromophore. We present a series of molecules with enhanced second hyperpolarizability caused by the presence of highly conjugated spacer groups, which increase the overall π-electron delocalization. Some metal complexes obtained from the coordination of these hybrid oligomers to transition-metal centers have also been investigated and revealed substantial differences in the capacities of the metal centers to act as electronic bridges. Finally, we show that theoretical predictions of the relative differences in the second hyperpolarizabilities of the new spacer compounds are in good agreement with the experimental results.

Scaling laws of second-order hyperpolarizabilities in molecular wires

Summary form only given. Whereas principles for the optimization of first-order hyperpolarizabilities /spl beta/ are well established, rules for the second-order hyperpolarizability /spl gamma/ are less well documented, owing to the significantly more complex physics. We have shown by DFWM, THG measurements, and by quantum-chemical calculations that the chain-length dependence of the second-order hyperpolarizability /spl gamma/ of poly(triacetylene) molecular wires follows a power law for short oligomers. This power law saturates smoothly around 60 carbon-carbon bonds, which corresponds to an effective conjugation length of 7.5 nm. A comparison with various other systems with electrons delocalized over a one-dimensional path shows that the exponent a tends to be around 2.5 for various polymers in the transparency range. In the presence of two- or three-photon resonances, the measured exponent a increases. Moreover, the values presented here are of similar magnitude as for other organic material systems.

One- and two-dimensionally conjugated tetrathynylethenes: Structure versus second-order optical polarizabilities

Donor−acceptor substituted molecules, based on the two-dimensionally conjugated tetraethynylethene (TEE) framework, have been investigated for the first time for their first-order hyperpolarizabilities β by electric-field-induced second-harmonic generation (EFISH). We measured high values of up to βEFISH = (510 ± 100) × 10-40 m4/V at λ =1.907 μm. We derived structure−property relationships by performing semiempirical calculations for all compounds. We show that such calculations give the correct trend in the hyperpolarizability β, and more importantly, give information as to the tensorial structure of β for molecules which are two-dimensionally donor−acceptor conjugated.

Studies Toward Computer Liquid Phase Simulations of the Solvent-Dependency of Apolar Association Strength: Conformational Analysis of a Cyclophane-Pyrene Complex by Pseudo Monte Carlo and Molecular Dynamics Methods

Previous studies showed a strong solvent dependency for the inclusion complexation of pyrene with the macrobicyclic cyclophane host 1. Upon changing from apolar to dipolar aprotic, to polar prouc solvents, and to water, the association strength of the complex (2) increases steadily. Calorimetric investigations of thermodynamic properties showed that, in all the solvents studied, the formation of 2 is enthalpically driven. For a better understanding of these solvent effects on an individual molecular level and in order to complement the experimental data, we decided to carry out computer liquid phase simulations on this system. In the absence of an X-ray crystallographic structure of 2, a detailed conformational analysis was required. In this study, we used Pseudo Monte Carlo (PMC) and Molecular Dynamics (MD) methods for the search of conformational space. Results of both techniques are described and compared. PMC-simulations show a complex with a quite rigid region around the cavity binding site of 1, which is shaped by three diphenylmethane units bridged in a cryptand-type array, and greater flexibility in the external piperidine rings. The obtained conformations prefer an arrangement of the bridges to the ‘cryptand-N-atoms’ with the protons of the aryl ether methylene groups (ArOCH 2 ) inside the cavity, in good agreement with the large upfield complexation-induced changes in the 1H-NMR chemical shifts of these protons. In the MD-simulation the pyrene guest rotates in the cavity; jumping between two favored orientations, which the PMC-simulations found to be isoenergetic. MD-simulations of the empty cyclophane 1 were carried out at 900 K. Even though distorted conformations were generated, the structure calculated by averaging the atom positions presents an open cavity, which clearly demonstrates the preorganization of the host.