Ernst Horkel

Synthesis, spectroscopy, and computational analysis of photoluminescent bis(aminophenyl)-substituted thiophene derivatives.

Substituted oligothiophenes have a long history in the field of organic electronics, as they often combine outstanding electro-optical properties with the ease of synthesis. To assist the rational selection of the most promising structures to be synthesized, there is the demand for tools that allow prediction of the properties of the materials. In this study, we present strategies for synthesis and computational characterization, with respect to the fluorescence behavior of oligothiophene-based materials for organoelectronic applications. In a combined approach, sophisticated computational methodologies are directly compared to experimental results. The M06-2X functional in combination with the polarizable continuum model in a state-specific formulation for excited-state solvation proved to be particularly reliable. In addition, a semiclassical approach for describing the vibrational broadening of the spectra is employed. As a result, a robust procedure for the prediction of the fluorescence spectra of oligothiophene derivatives is presented.

Structure and tautomerism of tenuazonic acid – A synergetic computational and spectroscopic approach

All reasonable tautomers and rotamers of tenuazonic acid, which is considered to be of the highest toxicity amongst the Alternaria mycotoxins, were investigated by DFT calculations at different levels of theory in gas phase and in solution to obtain optimized geometries for further examinations. Calculated NMR spectra of tautomeric structures are being presented and compared to experimental data to finally achieve a synergetic computational and spectroscopic approach for structure elucidation of 3-acetyltetramic acids, affording the predominant tautomer of tenuazonic acid in aqueous solution. Furthermore we were able to simulate the less hindered rotation of the exocyclic acetyl group, which occurs after dissociation of tenuazonic acid in protic solvents.

Synthesis and application of monodisperse oligo(oxyethylene)-grafted polystyrene resins for solid-phase organic synthesis.

In a preliminary investigation by our group, we found that poly(styrene-oxyethylene) graft copolymers (PS-PEG), for example, TentaGel resins, are advantageous for gel-phase (13)C NMR spectroscopy. Because of the solution-like environment provided by the PS-PEG resins, good spectral quality of the attached moiety can be achieved, which is useful for nondestructive on-resin analysis. The general drawbacks of such resins are low loading capacities and the intense signal in the spectra resulting from the PEG linker (>50 units). Here, we describe the characterization of solvent-dependent swelling and reaction kinetics on a new type of resin for solid-phase organic synthesis (SPOS) that allows an accurate monitoring by gel-phase NMR without the above disadvantages. A series of polystyrene-oligo(oxyethylene) graft copolymers containing monodisperse PEG units (n = 2-12) was synthesized. A strong correlation between the linker (PEG) length and the line widths in the (13)C gel-phase spectra was observed, with a grafted PEG chain of 8 units giving similar results in terms of reactivity and gel-phase NMR monitoring to TentaGel resin. Multistep on-resin reaction sequences were performed to prove the applicability of the resins in solid-phase organic synthesis.

Thieno[3,4-c]pyrrole-4,6-dione as novel building block for host materials for red PhOLEDs

In the presented work the electron accepting thieno[3,4-c]pyrrole-4,6-dione is introduced as a novel building block for donor–acceptor based host materials for Phosphorescent Organic Light Emitting Diodes (PhOLEDs). A series of three regioisomers consisting of the thieno[3,4-c]pyrrole-4,6-dione acceptor and carbazole donors linked via a phenylene linker was prepared and the impact of the phenylene substitution pattern on the molecular properties was analyzed. Regarding their applicability as host materials, the newly developed materials were investigated in red PhOLED devices achieving a high current efficiency of 30.6 cd A−1 corresponding to an external quantum efficiency of 17.7% and a high power efficiency of 23.8 lm W−1. Thus, we present the first successful application of the thieno[3,4-c]pyrrole-4,6-dione building block in host materials for PhOLEDs.

Substituted triphenylamines as building blocks for star shaped organic electronic materials

A versatile synthetic protocol toward a series of various substituted triphenylamine derivatives serving as building blocks for organic electronic materials was developed. Key steps during synthesis were either Ullmann condensations or nucleophilic aromatic substitutions giving rise to structural modification of triphenylamines and their electronic nature. In turn, these scaffolds were exemplarily attached to a dendritic tris(2-thienyl)benzene core affording star shaped organic semiconducting materials which were characterized regarding their photo-physical, electro-chemical and thermal properties. A strong influence of the substituents nature on both photo-physical and morphological thin film characteristic of star shaped target compounds was observed. The applicability of these materials in organic electronic devices was demonstrated in an organic field effect transistor configuration yielding a hole mobility of nearly 10−3 cm2 V−1 s−1. The performance of the materials can be correlated to the substituents applied.

Novel ene-yne compounds as quadratic nonlinear optical materials

We present a reliable strategy towards title compounds as a novel scaffold for highly nonlinear optical chromophores, using the selective ring fragmentation of thiophenes as a key step. This methodology selectively leads to the Z-isomer of ene-yne substrates, which are subsequently modified by a Huisgen-type cycloaddition yielding the target compounds. To prove the reliability of the developed synthetic route, one target compound was successfully prepared. This material crystallizes in an enantiomorphic space group, despite of being a flexible non-chiral molecule. A detailed discussion regarding the synthetic approach towards title compounds, a determination of material characteristics, including the investigation of the second order nonlinear susceptibility as well as a systematic variation of substituents in order to examine the resulting effects on NLO properties are subject of this presentation.

Functional organic click-materials: application in phosphorescent organic light emitting diodes

In the presented work click chemistry is utilized to introduce 1,2,3-triazoles as a functional linker in organic donor–acceptor materials. A systematic series of materials was prepared and characterized to investigate the effect of the linkage mode on the molecular properties. The 1,2,3-triazole linker allowed control of the degree of intramolecular charge transfer over a wide range depending on the substitution pattern of the triazole moiety. The prepared materials were successfully employed as host materials for green and red dopants in phosphorescent organic light emitting diodes. Thus, this work presents the first application of this novel linkage mode in the design and synthesis of functional π-conjugated organic donor–acceptor materials and their application in organic light emitting diodes.

Color fine-tuning of optical materials through rational design

We report on the feasibility for color fine-tuning of optical materials using rational design principles based on chemical reasoning. For this purpose, a modular framework for the construction of symmetrical cap-linker-cap compounds, using triarylamine caps and oligothiophene linkers, is applied. The chosen structural scaffolds are heavily used in recent industrial applications and provide five possibilities for altering their electronic and steric properties: electron donor/acceptor groups, planarization/deplanarization, and modulation of the π-conjugation length. Permutation of the used building blocks leads to a set of 54 different molecules, out of which 32 are synthesized and characterized in solution as well as in example fabricated OLED devices. This setup allows for color fine-tuning in the range of 412 nm to 540 nm with typical steps of 4 nm. In addition, to further benefit from the large experimental data set the spectroscopic results are used to benchmark quantum chemical computations, which show excellent agreement thus highlighting the potential of these calculations to guide future syntheses.

Metalation of Pyridazine, Cinnoline, and Phthalazine

This chapter reviews the metalation of pyridazine, cinnoline, and phthalazine derivatives as well as the usage of the hereby formed organometallic compounds in subsequent reactions. The main topics under discussion are lithiation, magnesation, and zincation, the focus being on deprotonative metalation by properly chosen strong bases, e.g. lithiumdiisopropylamide or lithium 2,2,6,6-tetramethylpiperidide. A short discussion upon boronyl and stannyl derivatives of the three diazines under investigation is enclosed at the end of the chapter.

DFT study of the Lewis acid mediated synthesis of 3-acyltetramic acids

AbstractThe synthesis of 3-acyltetramic acids by C-acylation of pyrrolidine-2,4-diones was studied by density functional theory (DFT). DFT was applied to the mycotoxin tenuazonic acid (TeA), an important representative of these bioactive natural compounds. Lewis acid mediated C-acylation in combination with previous pH-neutral domino N-acylation–Wittig cyclization can be used for the efficient preparation of 3-acyltetramic acids. Nevertheless, quite harsh conditions are still required to carry out this synthetic step, leading to unwanted isomerization of stereogenic centers in some cases. In the presented study, the reaction pathway for the C-acetylation of (5S,6S-5-s-butylpyrrolidine-2,4-dione was studied in terms of mechanism, solvent effects, and Lewis acid activation, in order to obtain an appropriate theoretical model for further investigations. Crucial steps were identified that showed rather high activation barriers and rationalized previously reported experimental discoveries. After in silico optimization, aluminum chlorides were found to be promising Lewis acids that promote the C-acylation of pyrrolidine-2,4-diones, whereas calculations performed in various organic solvents showed that the solvent had only a minor effect on the energy profiles of the considered mechanisms. This clearly indicates that further synthetic studies should focus on the Lewis-acidic mediator rather than other reaction parameters. Additionally, given the results obtained for different reaction routes, the stereochemistry of this C-acylation is discussed. It is assumed that the formation of Z-configured TeA is favored, in good agreement with our previous studies. FigureLewis acid mediated synthesis of tetramic acids starting from pyrrolidine-2,4-diones (shown for tenuazonic acid).