Christoph A. Briehn

CC Cross-coupling reactions for the combinatorial synthesis of novel organic materials

Abstract To accelerate the development process to organic materials combinatorial strategies and screening methodologies have been representatively elaborated for coumarin dyes and oligothiophenes. Pd-catalyzed cross-couplings of Suzuki, Sonogashira–Hagihara and Heck type were utilized to substitute the coumarin scaffold. Optimized synthetic protocols were applied to construct in a parallel manner in solution diverse libraries of more than 150 coumarin derivatives. To evaluate the fluorophore ensemble, the coumarins were screened for optical properties and several library members with high fluorescence quantum yields were identified. Using solid-phase synthesis, a 256-membered library of quater(3-arylthiophene)s was generated by using both the parallel and the ‘mix-and-split’ technique. Suzuki type couplings were employed for the stepwise oligomer growth using diverse aryl substituted thiophene boronic esters as building blocks. The rapid screening for electrochemical properties was facilitated by using an automated screening device. The data analysis led to the development of structure–property relationships on which to base future material design.

Electrochromics by intramolecular redox switching of single bonds.

Two moieties of mono- and trimethincyanines as well as those of styryl dyes were connected by a saturated alkyl tether made from compounds 3a-c, 5, 7, and 9a,b. In most cases, cyclic voltammetry and spectroelectrochemistry for these dyes together with the data for their monomeric models 4, 6, 8, and 10 reveal electrochemically irreversible transfer of two electrons but chemically reversible reaction and discoloration both on reduction and oxidation. Discoloration is interpreted as intramolecular formation of a single bond, which on redox breaking regenerates the starting colored species. Therefore, the investigated dyes exemplify a new general principle for electrochromics.

Electrochromics by intramolecular redox switching of single bonds

Compounds of the general structure presented in Scheme 1 are proposed as electrochromic systems (ESs) in which reversible electron transfer is associated with breaking or forming of single bonds. In contrast to classical ESs, both redox states represent closed-shell systems. Whereas type OL/CS (Open form − Long-wavelength absorption/Closed form − Short-wavelength absorption) has already been demonstrated, this paper presents examples of the OS/CL type, in which the long-wavelength absorption is displayed by the ring-closed form. Bis(azines) RED-8, RED-13, RED-16, and RED-17, as well as the bis(amines) RED-22, RED-23, and RED-24 were prepared and characterized by their cyclic voltammograms (CV) and spectroelectrograms (SE). Except for the case of RED-24, the reversible overall transfer of two electrons is connected to switching of a single C−C or N−N bond. This reaction is not restricted to an entropically favorable transannular reaction (OX-82+, OX-162+, and OX-172+) but also occurs with the formation of five- (OX-132+ and OX-232+) and certain six-membered rings (OX-222+). These results suggest that the OS/CL general structure type should allow broad variation both in redox potentials and in ranges of color, similarly to the OL/CS type. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

Efficient solid-phase synthesis of regioregular head-to-tail-coupled oligo(3-alkylthiophene)s up to a dodecamer

Solid-phase synthesis of isomerically pure head-to-tail-coupled (HT) oligo(3-hexylthiophene)s on chloromethylated polystyrene resin has been developed. Using novel sequences of iodination and Suzuki cross-coupling reaction, a series up to a dodecamer has been synthesized in high yield and purity. Removal of the conjugated oligomers from solid support as methyl esters, saponification and decarboxylation to the HT-coupled oligo(3-alkylthiophene)s could be effectively achieved.

Solid-phase synthesis and characterization of regioregular head-to-tail coupled oligo(3-p-tolylthiophene)s

By using a traceless silyl ether linkage the polymer-supported synthesis of regioregular head-to-tail coupled oligo(3-p-tolylthiophene)s s has been achieved in high yield and purity. Iterative sequence of iodination and Suzuki coupling reactions were used to generate the conjugated oligomcrs. UV-visible, fluorescence and electrochemical studies were conducted on the regioregular oligothiophenes.

From solid-phase synthesis of π-conjugated oligomers to combinatorial library construction and screening

A feature article describing concepts for the solid-phase synthesis of pi-conjugated oligomers as material-related structures and the translation of the synthetic routes into combinatorial protocols.