Hydrogen-Bonding-Induced Melamine-Core Supramolecular Discotic Liquid Crystals

Supramolecular liquid crystals?LCs?,combining both the fluidity,anisotropic molecular orders of LC and the dynamic assembly,structural complexity of supramolecular chemistry,are considered as novel exciting functional materials and attract intense scientific attention recently.Among them,hydrogen-bonding induced supramolecular discotic LCs that are usually formed from hydrogen bonding complementary molecules containing a ?-conjugated aromatic core and several flexible long-chain alkyl tails,are particularly attractive due to their predominant columnar arrangements of mesogens which have great potential for applications ranging from ion-conductive materials to organic photovoltaic devices.In particular,nanoporous cross-linked polymers with columnar channels constructed from hydrogen-bonded supramolecular LCs,could be used to effectively recognize and isolate the original heterocyclic cores,such as benzotri?imidazole?,etc.This interesting supramolecular imprinting function reminds us of melamine.Melamine?Mel?,a common chemical used primarily in melamine resin productions,is however extremely famous for food contamination scandal.Thereafter,detection of melamine is of food industry importance.As a C₃-symmetric compound,each melamine molecule has 6 H-donors?-NH?and 3 H-acceptors?=N-?,and can form maximally 9 H-bonds per molecule.Taking advantage of melamine's hydrogen-bonding capability,several works about self-assembly behaviours of the mixtures comprised of melamine-derivatives and cyanuric acid-derivatives have been published.However,to the best of our knowledge,there is no any previous literature reporting liquid crystallinity appearing in these hydrogen-bonded supramolecular complex systems.The objective of this work is to screen suitable hydrogen-bonding complementary compounds for the construction of melamine-core supramolecular discotic liquid crystal?LC?materials which might provide a LC perspective for detection and separation of melamine in the future.We design and synthesize five categories of long-alkyl-tailed compounds bearing different hydrogen-bonding complementary functional groups,including benzoic acid,cyanuric acid,homophthalimide,succinimide and thymine derivatives,which is further combined with melamine in either a 3:1 or 2:1 stoichiometry,and investigate the self-assembly behaviours of the corresponding supramolecular complexes.The experimental results demonstrate that the imidodicarbonyl unit is the best hydrogen-bonding complementary functional group of melamine.Furthermore,the heterocyclic ring size markedly influences the hydrogen-bonding strength of melamine-imidodicarbonyl unit complex.Most importantly,liquid crystallinity can be generated from these hydrogen-bonding supramolecular complexes.In particular,a variety of columnar mesomorphic orders?such as hexagonal columnar,rectangular columnar,square columnar phases?can be effectively achieved by functionalization of homophthalimide or succinimide units and modulation of the molar ratios of melamine and these hydrogen-bonding complementary compounds.