Supramolecular order of stilbenoid dendrons: importance of weak interactions

Abstract

Stilbenoid dendrons with various donor and acceptor groups on the focal unit were synthesised by a Wittig–Horner reaction, starting from an aldehyde functionalised dendron and various substituted phosphonic acid esters. The target molecules are composed of meta-branched arms, two of them with extended conjugation (distyrylbenzene) and three flexible dodecyloxy chains; the focal group consists of a donor or acceptor substituted styryl unit. The cross-conjugation of the arms prevents the strong electronic influence of substituents on the two extended oligophenylenevinylene chromophores. However, intermolecular interactions mediated by the focal unit allow control of the supramolecular stacking into liquid crystal phases. Simple weak acceptors stabilise the formation of columnar phases, whereas the additional propensity to build hydrogen bonds leads to a cubic mesophase. All acceptor substituted materials freeze at low temperature into a glassy state. Soft crystals are then formed upon heating the glassy material. Stilbenoid dendrons are photosensitive and degradation of the supramolecular order proceeds even in the glassy liquid crystal state.