Structuring Semi-Conducting (Ultra)thin Films
Final Report Abstract
Conjugated oligomers with solubilizing end groups are highly attractive for electronic applications like organic field effect transistors. While the direct impact of the single structural components on the electronic properties of the molecules is well understood the relation between molecular structure and the arrangement in the confinement of two-dimensional (ultra)thin films has to be elucidated. For a better understanding at Ulm University we have systematically varied the topology of branched alkyl chains with silicon branching units as end groups in b-unsubstituted oligothiophenes with seven and more thiophene moieties. The end groups were synthesized by hydrosilylation reaction and the oligomers were built-up by common Stille or Suzuki coupling reactions. Through combination with a synthon allowing both ring closure and coupling, the oligomers could be obtained in high purity and 100 mg amounts. The bulky end groups partially caused surprisingly low transition temperatures in the bulk with extremely low transition enthalpies and entropies. In solution, the molecules displayed a highly expressed aggregation behavior in one-dimensional fiber-like structures as revealed by extensive fluorescence emission studies. Further modification of the oligomers in the core by introducing pyridine units surprisingly showed a decrease in the interaction despite potential (weak) hydrogen bonding. Apparently, the disturbance of the p-p interactions by the heteroaromatic moieties is prevailing. Additionally, shorter oligomers as quaterthiophene analoga with pyridine and pyridinium end groups were also prepared. The operation of organic electronic devices relies on the mobility of electric charges that are created by application of a voltage. At DWI we investigated the assembly and structural properties of two-dimensional molecular layers that can potentially be used for this purpose. The organization of molecules in contact with a solid (as in thin film transistors) is complex. Due to geometrical constraints (symmetry breaking at the interface) and chemical interactions (between the molecules and the substrate), the organization of the system in the vicinity of a wall is more complex than in the bulk. Understanding the impact of these interfacial interactions on the structure of hetero-junctions is crucial for the functionality of devices based on mono and multilayer films. The end substituted thiophenes are suitable materials because the thiophene cores prefer to pack parallel to optimize their π-π interactions. This leads to an increased electron mobility perpendicular to the long molecular axes. This transverse π-π coupling between neighbouring conjugated cores will be influenced by any end substituents. Especially in thin uniform films the electric properties are favoured by the occurrence of liquid-crystalline (LC) behaviour. Various substituents have been introduced at the α, ω positions of the thiophene core in order to reduce the melting point and promote LC phases (see above). The transition temperatures determining the LC window originate from a delicate balance between length and structure of both core and substituents. We investigated the library of different LC compounds with a central thiophene core (7T and up 13T) and branched alkylated end groups of increasing bulkiness (generation). In principle, the π-π interactions become sterically more difficult for the bulkier end groups. Using x-ray diffraction in combination with scanning force microscopy, we find that a way out of this packing dilemma is toward liquid-crystal phases of higher dimensionality in the order smectic→columnar↔bicontinuous cubic. For the smectic phase, packing in a monolayer is no problem, for the other ones packing considerations in films become more stringent. We proposed a basic structure of columns parallel to the substrate that provides a compromise between preserving some π-π interactions and packing the bulky alkyl groups. Scanning force microscopy and surface x-ray techniques indicate an appreciable difference between monolayer and three-layer films, in which the monolayers appear to escape from packing frustration by generating superstructures. Rationally designed oligothiophenes were demonstrated to be taken up by A549 cells and specifically target, locate, dynamically self-report their supramolecular behaviour within the cell organelles. We showed that the cell provides different complementary pathways for self-assembly that can be traced easily with fluorescence microscopy as their molecular organization emits in distinct fluorescent bands. Thus, sensing of cell environment and impact on and of aggregation depending on the subtle molecular structure of various oligomers was studied with the means of fluorescence spectroscopy and microscopy. These results were published on the homepage of Ulm University (https://www.uniulm.de/nawi/nawi-home/nawi-detailseiten/news-detail/article/reportermolekuele-berichten-aus-demzellinnerenmultifunktionale-synthetische-molekuele-in-lebende-z/) and will be presented in a review article in “Nachrichten aus der Chemie”.
Publications
- "Monolayer properties of asymmetrically substituted sexithiophene“, Langmuir 2014, 30(10), 2752-2760
J. Wang, W. de Jeu, U. Ziener, M. Polinskaya, S. Ponomarenko, U. Ruecker, M. Ruderer, E. Herzig, P. Müller-Buschbaum, M. Moeller, A. Mourran
(See online at https://doi.org/10.1021/la404918b) - „Substituted septithiophenes with end groups of different size: Packing and frustration in bulk and thin films“, Langmuir 2016, 32(6), 1533-1541
W. H. de Jeu, K. Rahimi, U. Ziener, R. Vill, E. M. Herzig, P. Müller-Buschbaum, M. Möller, A. Mourran
(See online at https://doi.org/10.1021/acs.langmuir.5b04371) - “Directing Intracellular Supramolecular Assembly with N-heteroaromatic Quaterthiophene Analogues”, Nat. Commun. 2017, 8, 1850
D. Y. W. Ng, R. Vill, Y. Wu, K. Koynov, Y. Tokura, W. Liu, S. Sihler, A. Kreyes, S. Ritz, H. Barth, U. Ziener, T. Weil
(See online at https://doi.org/10.1007/s11740-012-0369-7) - “Effect of Double Branching in a,w-Substituted Oligothiophenes on Thermal Solid-State Properties”, Eur. J. Org. Chem. 2017, 1727-1735
J. Gülcher, R. Vill, M. Braumüller, K. Rahimi, W. H. de Jeu, A. Mourran, U. Ziener
(See online at https://doi.org/10.1002/ejoc.201700046)
