Walter Grahn

Two-photon states in squaraine monomers and oligomers

Abstract The excited states of novel monomeric and oligomeric squaraines of the indole series were studied by two-photon fluorescence (TPF). Two-photon excitation scans have been performed in an energy range from 1.75 to 3.2 eV by linearly and circularly polarized light. In addition, results from linear absorption and fluorescence measurements are presented. For the monomers a two-photon transition into the first excited state (Bu-symmetry) allowed by vibronic coupling was found. The interpretation of this transition is different to results of other groups. A second transition to an electronic state in an energy range of 2.5–3.5 eV was observed experimentally for the first time. The optical properties of this 2Ag state strongly depend on the molecular structure. The experimental results are compared to a four-state model. It is shown theoretically and experimentally that both, the linear absorption and the two-photon spectra of squaraine oligomers can be explained by a state interaction between excitons localized at the monomeric subunits (exciton coupling theory).

Luminescence enhancement in microcavity organic multilayer structures

Abstract We have studied the photoluminescence properties of aluminum-tris(8-hydroxychinoline) (Alq 3 ) and tris (4,4,4-trifluoro-1-(2-thienyl)-1,3-butanediono)-1,10-phenanthroline europium(III) (Eu(TTFA) 3 Phen) sandwiched between a planar Fabry-Perot microcavity structure. A strong influence of the position of the emission thin film on the luminescence spectra has been observed. The emission intensity normal to the substrate surface is drastically enhanced by an order of a magnitude if the active layer is placed at the antinode of the standing wave in the cavity. In comparison to non-resonant Alq 3 structures, the full width at half-maximum (FWHM) of the spectrum is reduced from 100 to about 20 nm. We also show that the Eu(TTFA) 3 Phen luminescence is spatially directed due to the inherent linewidth of only 5 nm.

Donor-acceptor oligoenes with a locked all-trans conformation: synthesis and linear and nonlinear optical properties.

A general synthetic approach to variously polarised merocyanines and a cyanine with enhanced thermal and (photo)chemical stability by a locked all-trans conformation (derived from a rigidified hexatriene unit and a variety of common donor and acceptor groups) is presented as well as a systematic study of their (non)linear optical properties. Apart from the UV/Vis absorption and fluorescence behaviour, the ground- and excited-state dipoles, the first-, second- and third-order molecular polarisabilities were determined by electro-optical absorption measurements (EOAM) and degenerate four-wave mixing (DFWM) techniques in solution. Large values for the second- and third-order polarisability up to beta(0)=461x10(-50) C V(-2) m(3) (1242x10(-30) esu) and [gamma(LL)]=183x10(-60) C V(-3) m(4) (15x10(-34) esu) were found. The linear and nonlinear optical properties were related to the ground-state polarisation and the resonance structure of the chromophores. In order to reveal the influence of the length of the polymethinic chain (number of pi electrons within the chromophore), some lower homologues shortened by one C=C (double) bond were also taken into account. The unexpectedly high gamma values of some of the merocyanines cannot be explained by a two-level model. Molecular vibrational third- order polarisabilities (calculated from absolute Raman intensities in solution) were qualitatively correlated to the DFWM results. Furthermore, the dependence of the (13)C NMR chemical shifts of the polymethinic carbons within the merocyanines upon ground-state polarisation was investigated and compared to those within a corresponding cyanine.

First hyperpolarizability measurements via hyper-Rayleigh scattering at 1500 nm

Abstract A hyper-Rayleigh scattering, HRS, system constructed around an optical parametric power oscillator and allowing wide fundamental wavelength tunability is described in detail. The design of the HRS cell, which may overcome problems inherent with other cell designs, is discussed. The potential of this HRS system for non-resonant first hyperpolarizability, β, determination is highlighted via measurement of a dipolar chromophore at 1500 nm. The magnitude of the non-linearity may be explained by either increased conjugation in the charge-transfer state or by increased magnitude in non-diagonal β-tensor elements.

A novel entry into a new class of cyclophane derivatives: synthesis of (±)-[2.2]paracyclophane-4-thiol ☆

Abstract Two syntheses of (±)-[2.2]paracyclophane-4-thiol have been accomplished in two and five steps, with overall yields of 46 and 31%, respectively, from [2.2]paracyclophane. The strategy involves two key reactions: (a) a lithium aluminium hydride reduction of the disulphide of [2.2]paracyclophane and (b) the use of a Newman–Kwart reaction for the conversion of [2.2]paracyclophane-4-ol to [2.2]paracyclophane-4-thiol.

Increase and saturation of the third order hyperpolarizabilities in homologous series of symmetric cyanines

Abstract The chain length dependencies of the static third order hyperpolarizabilities γ STAT for the homologous series of benzthiacyanine dyes and of simple bis(dimethylamino)methine dyes were extrapolated from nondegenerate four wave mixing dispersion measurements and compared with theoretical values. Up to the heptamethine the π-electron contributions γ STAT π of both homologous series show a similar increase with the growing number of π-electrons ( N ) of the chain ( γ STAT π ∼ − N 8 ± 2 ). However, the absolute values of the benzthiacyanines are considerable higher than of the corresponding bis(dimethylamino)methines. Negative valued hyperpolarizabilities γ STAT π in the homologous series increasing up to γ STAT π = − 850 × 10 −36 esu were determined. For the first time a saturation of the nonlinearity could be observed experimentally in the series of benzthiacyanines for the longest chain (benzthicyanine nonamethine).

Chalcogens as electron donors for selected nonlinear optic phores

Three different heteroaromatic charge transfer chromophores, e.g. donor(D)–acceptor(A)-substituted thieno [3,2-b] thiophenes 3 and 4, D-substituted thieno [2,3-c] pyridinium salts 7 and 2-(2′-thienyl)pyridinium salts 10 have been prepared. Alkylchalcogenic ethers were used as D-groups. The molecular first hyperpolarizabilities β were determined by the EFISH method in the case of 3 and 4 and by the HRS method in the case of the ionic compounds 7 and 10. Substitution of the ether oxygen atom by sulfur increases markedly the β value which is not further increased by the higher chalcogens selenium and tellurium.

Fiber optic sensors for an in-situ monitoring of moisture and pH value in reinforced concrete

Concrete structures such as social buildings and bridges are important economic goods. Thus, maintenance and preservation of these structures are of major interest. Buildings of reinforced concrete are exposed to a variety of damaging influences. In particular, moisture has an important influence on the lifetime of concrete structures. This is caused by the involvement of free water in corrosion of the steel, and the fact that water acts as transport medium for damaging ions such as chloride, sulfate, carbonate and ammonium. Thus, we designed and developed an integrated fiberoptical sensor system, which allows in-situ non- destructive long-term monitoring of concrete structures. As moisture indicator we use a pyridinium-N-phenolat betainital dye, which shows a strong solvatochromic behavior in the ultraviolet-visible spectral range (UV-VIS). The dye is embedded in a polymer matrix, whose moderate polarity is enhanced by free water diffusing into the sensor. This leads to a continuous hypsochromic shift of the absorption spectrum according to the water concetration. Another appropriate dye is 4-amino-N-methylphthalimid, which shows a similar behavior in its fluorescent spectra, and presently we are developing its derivatives and suitable polymer matrices. The determination of the pH-value of concrete is of major importance for the assessment of acidic attacks which may lead to serious damage in reinforced concrete, as the embedded steel structures exhibit long-term stability (i.e. resistance to corrosion) only at pH-values of 9 or higher. Therefore we have developed a fiberoptical sensor system for the measurement of pH-values in concrete consisting of pH- indicator dyes immobilized in a highly immobilized in a highly hydrophilic polymer matrix. Any change in pH-value of the wet concrete material is indicated by a color change of the dye/polymer system. The sensor system displays long term stability even in aggressive media of pH12 - 13.

Ethynylated, vinylated, and hetarylated indodicarbocyanines by palladium-catalyzed cross-coupling reactions

Abstract Palladium-catalyzed cross-coupling reactions of chain halogenated indodicarbocyanines 2b,c with arylethynes, styrenes and hetarenes afford the ethynyl, vinyl, and hetaryl derivatives 4 . The structure of 4b is confirmed by X-ray structure determination.

2,4-Bis{[6-(2,2-dimethylpropyl)-3-ethyl-1,3-benzothiazol-2(3H)-yl­idene]methyl}cyclobutenediylium-1,3-­diolate

The title compound, C34H40N2O2S2, adopts a trans conformation. The four conjugated Csp2-Csp2 single and double bonds of the polymethinic moiety, which bridges both heterocyclic end groups and the central four-membered ring, display nearly equal bond lengths. The molecule is nearly planar, with interplanar angles between the benzothiazole end groups and the central four-membered ring of 6.9 (1) and 7.7 (1) degrees; the angle between the heterocyclic systems is 1.8 (1) degrees. The crystal packing involves pi-stacking effects, with intermolecular C...C distances varying from 3.755 (3) to 3.991 (3) A.