Highly Fluorescent Crystalline and Liquid Crystalline Columnar Phases of Pyrene-Based Structures
Anna Hayer, Véronique de Halleux, Anna Ko1hler, Abdel El-Garoughy, E. W. Meijer, Joaquín Barberá, Julien Tant, Jeremy Levin, Matthias Lehmann, Johannes Gierschner, Jérôme Cornil, and Yves Henri Geerts
J. Phys. Chem. B 110 (2006) , 7653-7659
A concept for highly ordered solid-state structures with bright fluorescence is proposed: liquid crystals based on tetraethynylpyrene chromophores, where the rigid core is functionalized with flexible, promesogenic alkoxy chains. The synthesis of this novel material is presented. The thermotropic properties are studied by means of differential scanning calorimetry (DSC), cross-polarized optical microscopy (POM), and X-ray diffraction. The mesogen possesses an enantiotropic Colh phase over a large temperature range before clearing. The material is highly fluorescent in solution and, most remarkably, in the condensed state, with a broad, strongly red shifted emission. Fluorescence quantum yields (¼F) have been determined to be 70% in dichloromethane solution and 62% in the solid state. Concentration- and temperature-dependent absorption and emission studies as well as quantum-chemical calculations on isolated molecules and dimers are used to clarify the type of intermolecular interactions present as well as their influence on the fluorescence quantum yield and spectral properties of the material. The high luminescence efficiency in the solid state is ascribed to rotated chromophores, leading to an optically allowed lowest optical transition.