Synthesis and Structure-Property Relationship of Star Dyades and Triades as Novel, Filled Liquid Crystals
Final Report Abstract
During project a library of star mesogens with benzene, porphyrin and phthalocyanine cores, different conjugated oligomer arms consisting of phenyleneethenylene or thienyl units and with a peripheral 3,4,5-trisubstituted phenyl ring decorated with either alkyl or oligo(ethyleneoxy) chains were successfully prepared. Between the conjugated arms intrinsic free space is generated, which has to be compensated during the self-assembly process in the liquid-crystalline (LC) state. That means that this space has to be filled under the condition that nanosegregation and mobility is still possible in the LC phase. Most of these star compounds form columnar mesophases. While the benzene core derivatives with three arms are flexible enough to densely pack in helical columns, the tetra-arm compounds with a porphyrin or a phthalocyanine core assemble first in propellershaped dimers and subsequently form double helical columns with short correlation lengths. When a single fullerene guest is covalently attached via spacers of different lengths to the three arm mesogens, the molecule is transformed to a shape-amphiphile and the fullerenes nanosegregate in a triple helix along the column when the spacer is short. Longer spacers allow the “communication” of the fullerene triple helices between the columns. That means that the fullerene helices can form periodically contact sites at which fullerenes of different columns nanosegregate and consequently form 3D fullerene networks. For the four-arm stars, the synthesis of mesogens symmetrical substituted with four fullerenes has been developed. Here we followed the strategy to fill the liquid crystal by physical mixing of the “empty” stars providing intrinsic free space with the sterically overcrowded fullerene derivatives. During this study a new supramolecular reversible process was discovered, which is called the CLICK procedure. 1 : 1 mixtures of the two components starts to organise in highly ordered donor-acceptor-light collector triple nanosegregated columnar LC phases. Thereby, fullerenes nanosegregate in the intrinsic void of the star mesogens and the phthalocyanines are locked in the centre of the LC columns. This process could be widely confirmed for different molecular structures. The helical fullerene structure uncovered by a comprehensive X-ray scattering study distinguishes fundamentally from previously published dyads claimed to be helical columnar. In this project the literature compounds were found to arrange in columnar-lamellar phases. This is a consequence of the molecular design which allows a more complete nanosegregation. In contrast to these fullerene dyads, the fullerenes of the star molecules cannot completely escape from the column and nanosegregation is mainly limited to occur along the columns resulting in fullerene helices. Photophysical investigations revealed that the conjugated arms transfer energy to either the phthalocyanine, porphyrin core or the fullerene unit and that HOMO, LUMO energy levels fit well to typical electrode work functions. Furthermore, femto second spectroscopy confirmed the generation of donor radical cations and acceptor radical anions. These results suggest that these materials are appropriate for photovoltaic applications. In the device, the triad materials could be only partially aligned owing to the high clearing temperatures and high viscosity of the complex compounds. Nevertheless, after a correct thermal treatment of the thin films a partial alignment afforded a 300-fold increase in photocurrent. However, the photocurrent was still too low for application. The project highlighted an outstanding nanoscale self-assembly in new complex, functional liquid crystals based on a donor-acceptor structure by employing rationally the filling of the intrinsic free space in shape-persistent star mesogens. Future projects must now focus on lower clearing materials and their correct alignment to achieve technical applications.
Publications
- Free Space in Liquid Crystals - Molecular Design, Generation, and Usage. Acc. Chem. Res. 2019, 52, 1653-1664
M. Lehmann, M. Dechant, M. Lambov, T. Gosh
(See online at https://doi.org/10.1021/acs.accounts.9b00078) - Fullerene-Filled Liquid-Crystal Stars: A Supramolecular Click Mechanism for the Generation of Tailored Donor–Acceptor Assemblies. Angew. Chem. 2019, 131, 3649–3654; Angew. Chem. Int. Ed. 20
M. Lehmann, M. Dechant, M. Holzapfel, A. Schmiedel, C. Lambert
(See online at https://doi.org/10.1002/anie.201812465) - Fullerene-Filled Stilbene Stars: The Balance between Isolated C60 Helices and 3D Networks in Liquid-Crystal Self-Assemblies. Chem. Eur. J. 2019, 58, 3352-3361
M. Lehmann, M. Dechant, M. Hügel, N. Scheuring, T. Ghosh
(See online at https://doi.org/10.1002/chem.201805606) - Supramolecular click procedures in liquid crystals. Liq. Cryst. 2019, 46, 1985-1994
M. Lehmann, M. Dechant, L. Gerbig, M. Baumann
(See online at https://doi.org/10.1080/02678292.2019.1618936) - Click procedure of phthalocyanine star-shaped mesogens – the effect of size and spacer length. Liq. Cryst. 2020, 47, 1214-1222
M. Lehmann, M. Dechant
(See online at https://doi.org/10.1080/02678292.2020.1720329) - Liquid crystals from shape-persistent porphyrin stars with intrinsic free space. J. Mater. Chem. C, 2020, 8, 5562 – 5571
T. Ghosh, L. Gerbig, M. Lambov, M. Dechant, M. Lehmann
(See online at https://doi.org/10.1039/C9TC07086A) - Metal Phthalocyanine-Fullerene Dyads: Promising Lamellar Columnar Donor-Acceptor Liquid Crystal Phases. ChemPlusChem 2020, 85, 1934-1938
M. Lehmann, M. Dechant, D. Weh, E. Freytag
(See online at https://doi.org/10.1002/cplu.202000540) - The liquid crystal Click procedure for oligothiophene-tethered phthalocyanines – self-assembly, alignment and photocurrent. J. Mater. Chem. C, 2021, 9, 5689–5698
M. Dechant, M. Lehmann, G Uzurano, A. Fujii, M. Ozaki
(See online at https://doi.org/10.1039/D1TC00710F)
