Lattice Stiffening and Delayed Fluorescence in Columnar Liquid-Crystalline Glasses

Threefold symmetric ester derivatives of tris(biphenylyl)triazine T2T were made via coupling of tribromo-triphenyltriazines with boronic acids. With ester substituents only on the outer benzene rings, isotropic glasses were obtained, and ester groups on the inner benzenes were found to be essential for the formation hexagonal columnar mesophases and mesomorphic glasses. Methyl esters form mesomorphic and isotropic glasses with high T_g, whereas ethyl esters have a much lower T_g. At T_g, the column lattice contracts on cooling and becomes fairly resistant to shrinkage on further cooling, whereas the stacking distance within the columns is unaffected by the glass transition. Thus, the glass transition affects the plane of the periodic column lattice of the structure, but not the stacking along the columns. The T₁ energies are close to that of the parent arene T2T, a known host for phosphorescent emitters. Delayed fluorescence is observed at low temperature, that is, without thermal activation, which indicates radiative triplet-triplet annihilation (TTA-DF). This TTA-DF is prominent in the derivatives that form mesomorphic glasses and weak in those that do not. Thus, a direct correlation between liquid-crystalline stacking and fluorescence by TTA is observed.