Utilization of evaporation during the crystallization process: self-templation of organic parallelogrammatic pipes

Analogues of 4-dodecyloxy-2-trifluoromethylbenzamide (12FH2) consisting of a hydrophobic alkyl chain, a trifluoromethylated aromatic ring, and a self-complementary hydrogen-bonding amido group were synthesized, and the structural effect of each component on the formation of parallelogrammatic pipes was investigated. Differential scanning calorimetry and powder XRD analyses revealed that all-trans L and gauche-rich S polymorphic forms appeared for the analogues with more than eight carbon atoms in the alkyl chain, that is, the polymorphism originates in the conformation of the alkyl groups and hydrogen-bonding patterns of the benzamide group. Also, the trifluoromethyl substituent is crucial in that it provides an appropriate molecular balance between the benzamide and alkyl groups. Scanning electron microscopy and powder XRD analyses of solids obtained by a drying-mediated assembly process revealed that production of the L polymorph by polymorphic transition from the S polymorph resulted in evolution of a three-dimensional structure when the alkyl group has more than 12 carbon atoms. Among the series of compounds, 12FH2 and 4-tetradecyloxy-2-trifluoromethylbenzamide (14FH2) formed parallelogrammatic pipes with micrometer dimensions. An atomic force microscopy study of 12FH2 suggested that a single pipe may be composed of platelike crystallites of L polymorph. From a mercury-intrusion porosimetry study, it was determined that macroporous materials with average pore diameters of about 40 microm and porosity of about 80% were obtained. The previously proposed self-templation mechanism by polymorphic transition from S to L polymorph was further discussed in view of polymorphism and the crystallization rate. An appropriate molecular balance between the benzamide and alkyl groups is necessary to induce a proper polymorphic transition for the development of a three-dimensional hollow structure in the evaporation process.