Dieter Weiß

Studies on the Intramolecular Electron Transfer Catalyzed Thermolysis of 1,2-Dioxetanes

Abstract This work reports the synthesis and the chemiluminescence properties of the dioxetanes: 4-ethyl-4-methyl-3-(3-methoxyphenyl)-1,2-dioxetane (I), 4-ethyl-4-methyl-3-(3-tert-butyldimethylsilyloxyphenyl)-1,2-dioxetane (II), 4,4-dimethyl-3-(3-methoxybenzyl)-1,2-dioxetane (III) and 4,4-dimethyl-3-(3-tert-butyldimethylsilyloxybenzyl)-1,2-dioxetane (IV). While in the thermal decomposition of I–IV preferential formation of triplet excited states is observed, in the presence of fluoride ions the decomposition rate constants of II and IV increase drastically and singlet excited states are formed with high quantum yields. These results are discussed based on the CIEEL (‘Chemically Initiated Electron Exchange Luminescence’) mechanism where the decomposition of the dioxetanes should be initiated by an intramolecular electron transfer from the phenolate ion (generated by fluoride catalyzed deprotection of the silyloxy group), either directly bound to the peroxidic ring (II) or separated from it by a methylene bridge (IV).

Mesoionic bora-tetraazapentalenes – fully reversible two step redox systems

Bora-tetraazapentalenes have been synthesised and characterized; the radical anion shows an unusual thermodynamic stability (semiquinone formation constants K(SEM) of ca. 10(14)).

Simple procedure for reductive coupling of steroids with a cross conjugated dienone system

Abstract A simple procedure was found to dimerize cross conjugated dienones to the corresponding olefins. In contrast to the McMurry reaction this variant works without titanium halides. Bis-steroids which may be of pharmacological importance are prepared in high yields.

Tuning the polarity and surface activity of hydroxythiazoles – extending the applicability of highly fluorescent self-assembling chromophores to supra-molecular photonic structures

A small library of N,N-diethyl- and -diethanolsulfonamides of differently substituted 2-(2-pyridyl)- and 2-pyrazinyl-4-alkoxythiazoles has been synthesized and investigated in terms of their photophysical properties, electronic structure (quantum chemical calculations at the CAM-B3LYP/6-31+G(d,p) level of theory) and thin film morphology (Langmuir–Blodgett- and spin-cast films). By using the Langmuir–Blodgett technique we show how to exert direct control over the degree of aggregation in thin films made from different thiazole type dyes. In combination with spectroscopic investigations, we gained an in-depth understanding of the influence of aggregation on the electro-optical properties of thin films made of the here-investigated substances.

Syntheses and Properties of Cycloamidines Based on 4H-Imidazoles*

Employing three different syntheses a broad spectrum of 4H-imidazoles 3a - 3s has been synthesized. In the course of the two-fold aminolysis reaction leading to derivatives 3q - 3s, deeply colored byproducts could be isolated and structural characterized.These novel donor-acceptor derivatives of type 7 consist of an 1H- and 4H-imidazole which are connected by a nitrogen bridge and rearrange via rapid 1,3-/1,5-hydride shifts. Using 1H NMR experiments the aminolysis product 3p shows prototropic isomers which could be detected in equilibrium for the first time. Cyclovoltammetric measurements of a series of substituted 2-aryl derivatives 3d - 3i displayed two reversible single electron transfer steps with relatively small semiquinone formation constants between 102 and 4×103. The 4H-imidazole 3d was successfully converted into boratetraaza-pentalene 8a, which showed two well separated reduction potentials. The value of semiquinone formation constant of 8a (1.8×1015) is even higher than those reported for similar derivatives. 4H-imidazoles can also be employed for the efficient complexation of catalytically important metals as exemplified by copper complexes 11 and 12. Derivative 3m, which possesses an additional chelating pyridine substructure, formed a stable complex of structural composition Zn(3m)2 with diethyl zinc

And yet they glow: thiazole based push–pull fluorophores containing nitro groups and the influence of regioisomerism

Reported is a study on the influence of regioisomerism on the photophysical properties in 4-hydroxy-1,3-thiazole-based push-pull-chromophores/fluorophores to evaluate the molecular structure-property relationship as a basic foundation for future design strategies concerning this class of dyes. Surprisingly, the nitro groups used as acceptors do not act as a fluorescence quencher, instead the derivatives synthesized exhibit quantum yields of 37-40%. Two 4-ethoxy-1,3-thiazole derivatives which differ only in the positioning of their electron donating (methoxy) and electron withdrawing (nitro) groups have been synthesized and examined in terms of their photophysical properties, i.e. UV/Vis absorption and fluorescence emission spectra. Additionally, quantum chemical calculations have been performed to unravel the underlying fundamental transitions and to explain the experimental results.

The mild thiolation: a useful tool for the synthesis of activated building blocks

The “mild thiolation” is a term coined specifically for the oxidation of suitable CH-acidic substrates with elemental sulfur in the presence of a base and a dipolar aprotic solvent. Dithiocarboxylates are formed under mild reaction conditions as products which, due to their increased reactivity toward nucleophiles, allow various transformations. The focus of this work is particularly the class of thioamides and their great benefits for the synthesis of heterocycles.

Synthesis and Modification of Substituted 2-Azaanthraquinones

The cycloaddition-ring transformation reaction sequence of pyrido[1,2-a]pyrazines with substituted naphthoquinones furnished a series of new highly substituted azaanthraquinones. Whereas monosubstituted naphthoquinones were normally leading to two regioisomeric products, in some cases a preference for only one regioisomer was observed. The amino derivative 3b which was isolated as the main product proved to be suitable for further modifications at the primary amino group. The derivatives obtained possess groups capable of connecting the molecule with other substructures for applications as functional dyes. The newly synthesized azaquinones show strong and very broad absorptions between 400 and 600 nm in their UV/Vis spectra Graphical Abstract Synthesis and Modification of Substituted 2-Azaanthraquinones

On the Control of Chromophore Orientation, Supramolecular Structure, and Thermodynamic Stability of an Amphiphilic Pyridyl-Thiazol upon Lateral Compression and Spacer Length Variation

The supramolecular structure essentially determines the properties of organic thin films. Therefore, it is of utmost importance to understand the influence of molecular structure modifications on supramolecular structure formation. In this article, we demonstrate how to tune molecular orientations of amphiphilic 4-hydroxy thiazole derivatives by means of the Langmuir-Blodgett (LB) technique and how this depends on the length of an alkylic spacer between the thiazole chromophore and the polar anchor group. Therefore, we characterize their corresponding supramolecular structures, thermodynamic, absorption, and fluorescence properties. Particularly, the polarization-dependence of the fluorescence is analyzed to deduce molecular orientations and their possible changes after annealing, i.e., to characterize the thermodynamic stability of the individual solid state phases. Because the investigated thiazoles are amphiphilic, the different solid state phases can be formed and be controlled by means of the Langmuir-Blodgett (LB) technique. This technique also allows to deduce atomistic supramolecular structure motives of the individual solid phases and to characterize their thermodynamic stabilities. Utilizing the LB technique, we demonstrate that subtle molecular changes, like the variation in spacer length, can yield entirely different solid state phases with distinct supramolecular structures and properties.

Introducing double polar heads to highly fluorescent Thiazoles: Influence on supramolecular structures and photonic properties

HYPOTHESIS Supramolecular structures determine properties of optoelectronically active materials and can be tailored via the Langmuir-Blodgett (LB) technique. Interactions between dyes can cause high crystallinities of Langmuir monolayers, thus rendering retaining their integrity during the LB-deposition challenging. However, increasing degrees of freedom exclusively at the polar anchoring moieties of dyes might improve processability without perturbing the dyes optoelectronic properties nor the function-determining crystallinity of the layer. EXPERIMENTS (Amphiphilic) thiazole dyes without, with a mono-polar, and with a double-polar anchor were synthesized, whereas the two constituting polar moieties of the latter derivate are separated by a flexible alkyl chain. The supramolecular structures and crystallinities of Langmuir and LB monolayers were characterized by means of LB isotherms, atomic force microscopy and polarization-resolved fluorescence spectroscopy. FINDINGS As compared to the mono-polar reference the introduction of a flexible double-polar head did not deteriorate UV-vis absorption, emission or electrochemical properties of the thiazole but significantly extended the range of constant compressibility modulus, thus indicating improved processability of the Langmuir monolayers. Indeed, AFM studies revealed that the integrity of the monolayers could be retained during LB-deposition. Additionally, also the underlying supramolecular structure of the chromophore moieties is largely identical to those obtained from the mono-polar reference thiazoles.