Filled Polar Liquid Crystals from Umbrella-Shaped Mesogens

• Applicants: Professor Dr. Alexey Eremin; Professor Dr. Matthias Lehmann
• Subject Area: Preparatory and Physical Chemistry of Polymers ; Organic Molecular Chemistry - Synthesis and Characterisation
• Term: since 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 454000148

In this collaborative project between the workgroups (WG) Lehmann (Würzburg) and Eremin (Magdeburg), we propose to design and explore novel semiconducting liquid crystalline materials distinguished by their intrinsic polar properties. The design conducted by the WG Lehmann is based on the synthesis of star-shaped mesogens with a subphthalocyanine core and conjugated arms (oligothiophenes, benzothienobenzothiophenes, thienylpyrrolopyrrolthiophenes) decorated with aliphatic chains. The latter induce columnar liquid-crystalline (LC) phases. Such semiconducting mesogens form polar phases, for which an anomalous photovoltaic effect in thin, aligned films is expected. In this project, we intend to explore the structural, optical, and electronic and polar properties of these materials as well as to optimise them for perspective device applications. After synthesis and purification, the photophysical properties will be studied in solutions and thin films. The thermotropic behaviour and the structure of the mesophases will be investigated using polarised optical microscopy, differential scanning calorimetry, X-ray scattering (WAXS, SAXS, GISAXS) and the modelling with the Materials Studio software. The alignment will be optimised by surface treatment, annealing and usage of magnetic and electric fields. Using broadband dielectric spectroscopy and optical second-harmonic generation, we shall explore the polar properties of the new materials. The anomalous photovoltaic effect will be tested in thin oriented, polar films. Such materials should exhibit a photocurrent without the need of a donor-acceptor (p/n) junction. The output of these structure-property studies will be used for further optimisation of the molecular design. The WG Lehmann will prepare star mesogen derivatives, for which the conjugated arms are tethered to fullerenes (C60) via flexible spacers with different spacer lengths. These molecules are sterically overcrowded and do not form LC phases. The original star mesogens without fullerenes possess between their arms intrinsic free space, which can host C60. Consequently, the mixture between these molecules with the sterically overcrowded fullerene derivatives results in new, polar, highly ordered, columnar donor-acceptor LC phases. These "fullerene-filled" liquid crystals are novel donor-acceptor materials, which allow the control of the morphology and the alignment between electrodes. The polar properties will facilitate charge separation. The "filled" mesophases will be investigated for their structure-property relationships, i.e. the self-assembly in soft matter (LCs and soft crystals), photophysical and polar properties, alignment, charge-carrier mobilities, and photovoltaic properties.Thus, the joint, multidisciplinary project of the WGs Lehmann and Eremin will result in a new generation of liquid-crystalline, polar semiconductor materials, which can be homeotropically aligned and thus, allow the application in organic photovoltaics.

• DFG Programme: Research Grants