Molecular design at the calamitic/discotic cross-over point. Mononuclear ortho-metallated mesogens based on the combination of rod-like phenylpyrimidines and -pyridines with bent or half-disc-shaped diketonates

Abstract

Six novel series of mononuclear ortho-palladated and ortho-platinated metallomesogens have been obtained by combination of rod-like heteroaromatic liquid crystals with bent or half-disc-shaped 1,3-diketonate units. The synthesis of these molecules was achieved by ortho-metallation of rod-like 2-phenylpyrimidines and 2-phenylpyridines, with formation of the corresponding chloro-bridged dinuclear metallomesogens. Ligand exchange with alkoxy-substituted thallium 1,3-diphenyl-1,3-diketonates led to the desired mononuclear metallomesogens, incorporating a rod-like ortho-metallated heteroaromatic core and bent or half-disc-shaped 1,3-diketonate ligands. The liquid crystalline properties of these molecules were investigated using polarized light optical microscopy, differential scanning calorimetry and X-ray scattering. By increasing the number of alkyl chains attached to the 1,3-diketonate units, a transition from lamellar (SmA, SmC) and nematic to two distinct types of hexagonal columnar phases (Col_h) was found, but no cubic phase could be detected. In the Col_h phases, depending on the number of attached chains, either only one or two molecules are organized side-by-side within the columns. However, apart from the number of alkyl chains, their distribution on the 1,3-diketonate units is of importance to the mesophase behavior. Elongation of the calamitic subunit (2,5-diphenylpyrimidines) stabilizes smectic phases and Col_h phases having two molecules in their cross-section (edge-to-edge dimers). The ortho-metallated 2-phenylpyridine unit has, for the first time, been introduced into metallomesogens. However, ortho-palladated and ortho-platinated 2-phenylpyridines are either non-mesogenic or have significantly lower mesophase stabilities than the corresponding pyrimidine derivatives. Replacement of palladium by platinum does not significantly influence the mesophase type, but it can have a mesophase-stabilizing effect, due to stronger d⁸–d⁸ closed-shell interactions between the Pt atoms.