Ulrich-W. Grummt

Fluorescent ligands for optical sensing of alcohols: synthesis and characterisation of p-N,N-dialkylamino-trifluoroacetylstilbenes

Abstract Novel fluorescent ligands have been synthesised which interact with specific alcohols resulting in a change of absorbance as well as fluorescence. 4-trifluoracetyl-4′-(di-N-butylamino)stilbene (ETHT 4003) exhibits an absorption at around 450 nm in organic solvents, whereas, upon reaction with primary alcohols, the maximum shifts to around 360 nm. The fluorescence emission of the positively solvatochromic dye ranges from 500 to 700 nm and, upon reaction with alcohols, is also blue-shifted to around 450 nm. The significant shift of both absorbance and fluorescence is based on the chemical conversion of the trifluoroacetyl group of the dye into the hemiacetal, thus causing a change in the electron distribution of the dye and a spectral shift of around 100 nm. The reaction is selective for alcohols, which are not sterically hindered to approach the trifluoroacetyl group, i.e. for primary alcohols. An example for the analytical application of the ligands is given. An optical sensor is prepared by embedding 4-trifluoracetyl-4′-(di-N-butylamino)stilbene in plasticised PVC and coating the layer with white PTFE. The microporous PTFE coating provides an optical isolation and prevents interferences of ions. The sensor exhibits a sensitive range from 5–50 vol% ethanol with maximum signal changes to be in the range of 40%. The detection limit is 2 vol% ethanol.

Synthesis of amphiphilic styrylpyridinium and styrylquinolinium hemicyanines and merocyanines

Amphiphilic polymethine dyes are synthesized by alkylation of 4-methylpyridine and 4-methylquinoline and consequent condensation with hydroxy- or amino-substituted aromatic aldehydes. The resulting dyes are characterized with respect to their absorption and emission spectra. The synthetic approach is generic in that it allows adjustment of the physical properties of the respective dyes (absorption, fluorescence, charge transfer, lipophilicity) and provides chromophores useful for the preparation of Langmuir-Blodgett films with non-linear optical properties and as potential sensitive dyes in analytical chemistry.

From Byproducts to Efficient Fluorophores: A Route to the Synthesis of Fluorubines

The reaction of bis(imidoyl) chlorides of oxalic acid with monoalkyl hydrazines leads to substituted Delta(2)-1,2-diazetines, which are versatile building blocks for ring-transformation reactions. One remarkable product originating from side reactions featured by a strong orange/red fluorescence was confirmed as a novel fluorubine derivative. In continuing our studies to substituted oligoazaacenes, we developed several synthetic entries to build up novel fluorubine derivatives, in which particularly aminobridged bis(quinoxaline)s are the key products for cationic hexaazapentacenes. We would like to discuss the possible formation pathways of these fluorubine derivatives, which exhibit interesting photophysical and chemical properties. The structures of all new derivatives were confirmed by common analytical methods (NMR spectroscopy, CV, UV/Vis, mass spectrometry, elemental analysis, and X-ray structural analysis) and will be discussed on selected examples in more detail.

Novel photoswitchable rotaxanes

A photoresponsive rotaxane based on the photoheterolysis of an acridane unit which is at the same time a bulky end group has been developed.

Photophysical Properties of Fluorescent Polymers Containing 1,10-Phenanthroline-3,8-ylene and Diethinylenebenzene Units and Their Model Compounds: A Comparison to 2,2′-Bipyridine Analogues

Investigation of the absorption and emission spectra (stationary and time-resolved) of phenanthroline-based polymers shows that the enforcement of a cisoid geometry in the bipyridine unit of the main chain via an ethenylene bridge does not significantly change the spectral and the deactivation behaviour. In close analogy to similar polymers, containing bipyridine units, the conjugation length in these polymers does not exceed ca. two chromophore units. At low temperatures, the model compounds show an additional, bathochromically shifted, intense luminescence with an increased lifetime which can be interpreted as an excimer (or aggregate) fluorescence. In addition, broad, superimposed emission bands arising from various aggregated species are observed. The relative intensity of these bands depends on the temperature processing occurring during cooling. Intersystem crossing is an insignificant deactivation channel.

Second order hyperpolarizability of hydroxystilbazolium salts and their betaines-relationship to chemical structure

Amphiphilic hydroxystilbazolium cations and their corresponding neutral betaines exhibit large off-resonant positive and negative, respectively, second order hyperpolarizabilities which were measured by DFWM and Z-scan experiments. Quantitative structure-property relations were established from semi-empirical calculations.

Rotational Relaxation of Linear π-Systems with Flexible Side Chains in Solution

The classical hydrodynamic theory for Brownian rotational motion is applied to model compounds of conjugated polymers with alkoxy side chains of variable length. Theory predicts two rotational relaxation times for these types of molecules with the dipole transition moment parallel to the longest axis whereas experiments reveal only one. The rotational relaxation times and their relative amplitudes were calculated for a wide span of axial ratios of a general ellipsoid. In this way, the range in the axial ratios is obtained such that there is a chance to detect both rates experimentally. Rotational relaxation times of five particular molecules were measured in liquid n-butane. Theoretical calculations using ellipsoid parameters obtained from molecular dynamics calculations compare well with experimental results. Calculation of the rotational relaxation times from the autocorrelation function of the transition dipole moment vector requires significantly greater computational effort.

On the Fluorescence Depolarization and the Calculation of Rotational Relaxation Times of Rigid Rod Molecules

The rotational relaxation times of rod like molecules like poly[arylene-ethynylene]s and their low molecular weight model compounds calculated from a simple model agree well with the experimental ones as long as the axial ratio of the corresponding rotational ellipsoid is less than 8. For the polymer (axial ratio >10) the fluorescence depolarization cannot be described by rotational motion perpendicularly to the long molecular axis. One has to take into consideration bending motions in connection with energy transfer along the bent backbone.