Engineering "Meso-Atom" bonding : honeycomb-network transitions in reticular liquid crystals

A library of rod-like bolapolyphiles with sticky hydrogen-bonded glycerol groups at their ends and having highly branched side chains with a carbosilane-based four-way branching point, all based on the same oligo(phenylene ethynylene) core, has been synthesized and investigated. For these compounds, a A15-type Frank–Kasper phase is formed upon side-chain elongation in the steric frustration range at the transition from the triangular to the much larger square honeycombs. In contrast to the previously known tetrahedral sphere packings the A15 phase is in this case formed by tetrahedral networks of aggregates of parallelly organized π-conjugated rods. This allows the design of compounds with wide ranges of the A15 network down to room temperature. However, its formation becomes strongly disfavored by core fluorination that is attributed to a changing mode of core–core interaction that also modifies the square honeycombs by deformation of the squares into rectangular or rhombic cells, either with or without emergence of tilt of the rods.