Synthesis and Structure-Property Relationship of Star Dyades and Triades as Novel, Filled Liquid Crystals

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Shape-persistent three-armed oligo(phenylenevinylene) star mesogens form columnar liquid crystalline mesophases despite the large free volume between their linear conjugated arms. The interconnection of the oligo(phenylenevinylene) arms of these star mesogens, which are donors, with fullerene building blocks via flexible spacers results in donor-acceptor dyads. Here the void is occupied by covalently bound fullerene guest molecules. The self-assembly of these molecules leads to novel filled columnar mesophases with nanosegregated donor-acceptor structures. The structural control is achieved by variation of the spacer length, the connection position, the length of the conjugated scaffold and the peripheral flexible chains. Clearing temperatures below 200 °C are realized by introduction of oligo(ethyleneoxy)- or branched chains. Star triades, based on Porphyrin and Phthalocyanine cores, to which fullerene containing stilbenoid arms are attached, are an extension of this concept. The materials structures are investigated by X-ray scattering techniques, UV-Vis- and fluorescence spectroscopy in solution and liquid crystal state. The results allow the set-up of models with the program package Materials Studio and lead to the in-depth knowledge of the structure-property relationships. The dense packing of donor and acceptor chromophores in a columnar structure should allow a fast charge transport to the electrodes in a device, if a homeotropic alignment can be achieved. The materials systems are therefore of special interest with respect to their photovoltaic properties. Charge separation and transport will be optimized not only by the tailor-made donor-acceptor mesogens but also by the preparation of mixtures between star triades with four ´bound fullerene guests, stars without fullerenes and suitable, functionalized fullerenes. The introduction of oligo(ethyleneoxy) chains are promising to promote charge separation owing to the increase of the materials dielectric constant. The liquid crystal state will be used for the correct alignment of the columns in thin solid films. The photovoltaic liquid crystal cells will be fabricated subsequently and their photovoltaic properties will be studied in collaboration with Prof. Grelet (Bordeaux).