RESUMO
A series of shape-persistent star-shaped oligo(phenylene vinylene) liquid crystals and derivatives with fullerene tethered through spacers of different lengths to one arm were successfully synthesized. Solution studies by NMR, UV/Vis and fluorescence spectroscopy revealed the preferential location of the C60 acceptor in the vicinity of the peripheral donor unit, which affects the quenching of the emission of the conjugated stilbenoid scaffold in the donor-acceptor dyad. The fluorescence was completely absent in the liquid crystal phase due to the extraordinary hierarchical self-assembly. In this family of mesogens, the intrinsic free space has to be filled either by an extremely tight packing of the stars or by the fullerenes as guests. The spacer lengths control the nanosegregation of the C60 units in the cavities of the stars in either independent triple helices or unprecedented 3D networks, in which fullerenes are positioned at the interface of neighboring columns. Eventually, the clearing temperature of such large complex donor-acceptor systems can be tuned by an entropy effect defined by the length of the spacer. The accessibility of the isotropic phase without decomposition is an important prerequisite for future alignment studies associated with potential material applications in organic electronics.
RESUMO
The molecular design of crowded hexasubstituted star mesogens based on a benzene core and alternating substitution with oligo(phenylenevinylene) arms and aryl units generates free space between the conjugated arm scaffolds. Various arylcarboxy building blocks, decorated with alkoxy chains, have been incorporated in the void by mixing, hydrogen bonding or covalent bonds to the aryl groups. The mesogens assemble in columnar stacks ranging from soft crystals to rectangular and hexagonal columnar liquid crystals, revealed by polarized optical microscopy, differential scanning calorimetry, X-ray scattering and modelling. The stability of the mesophases is crucially influenced by the binding mode of the arylcarboxy guest building blocks. The origin of the variation in clearing temperature is unravelled by modelling, cohesive energy density considerations and solid-state NMR spectroscopy. The control over the transition temperature is important for the formation of aligned thin films and thus for potential applications.
RESUMO
Star-shaped oligophenylenevinylene (OPV) mesogens are shape-persistent and possess formally large void space. A mesogen with three styrene repeating units packs densely in a columnar helical arrangement. Attachment of one fullerene through a short spacer results in an exceptional increase of the mesophase stability. X-ray scattering and modeling evidence a triple-helical arrangement in which the fullerene perfectly fills the void space between the arms of the star mesogen.