Article pubs.acs.org/crystal
Gelation and Structural Transformation Study of Some 1,3,5Benzenetricarboxamide Derivatives V. Nagarajan*,†,‡ and Venkateswara Rao Pedireddi† †
School of Basic Sciences, Indian Institute of Technology Bhubaneswar, Toshali Bhavan, Bhubaneswar 751 007, India Department of Colloids and Materials Chemistry, CSIR-Institute of Minerals and Materials Technology, Bhubaneswar 751 013, India
‡
S Supporting Information *
ABSTRACT: Short-chain analogues of N,N′,N″-tris-(4-alkylphenyl)-1,3,5-benzenetricarboxamide derivatives (methyl to butyl) are prepared, and their gelation study renders a positive result for all, except the butyl derivative, in both DMSO and DMSO/H2O. The crystal structure analysis of the parent compound (1a) and its methyl (1b) and butyl (1e) derivatives shows columnar structures. However, a notable difference has been observed within the columnar structures of 1a or 1b and 1e. Detailed structural analysis was carried out to reason such varied molecular assemblies, based on the deviation angle of amide groups, along with the aid of some reported examples that reveal the structural transformation of 1,3,5-benzenetricarboxamide derivatives from sheets to different columnar structures. In addition, similarity in the simulated powder pattern of the DMSO adducts of 1a and 1b and the experimental powder pattern of the xerogels of 1a and 1b indicates the possibility of similar packing in both crystals and the fibers of the gel.
■
varied viscosities of the molecules in solvents are observed.6 However, structural evidence for such variation in molecular assemblies due to peripheral substituents in hydrogen-bonded gelators is rare, and such a structure−property relationship would provide valuable information for the design of a molecular gel. Thus, herein, we prepared some of 1,3,5-benzenetricarboxamide derivatives, as shown in Chart 1, to demonstrate the varied molecular self-assembly effected by the peripheral substituents through single-crystal X-ray diffraction analysis.
INTRODUCTION In a strategic approach of design of a molecular gel, various factors are considered to be influencing the formation of a gel, such as fiber formation, entanglement of fibers, solvent of gelation, etc.1 All of these factors are governed by various intermolecular interactions with varied energies and directionality. Control over this spectrum of intermolecular interactions is arduous; as a result, designing a molecular gel is a daunting task. However, considering the recent developments of intermolecular interactions in the formation of supramolecular assemblies,2 gelators based on hydrogen-bonding groups, such as amide and urea, have perhaps attained preliminarily success in the systematic design of molecular gels.3 This is due to the facile unidirectional self-assembly of these gelator molecules utilizing the hydrogen bonds to yield a one-dimensional molecular array, which is considered to be one of the prime requirements in the formation of gel. For instance, 1,3,5-benzenetricarboxamide based derivatives are recognized to form a columnar structure due to the cooperative triple helical hydrogen bonds among the consecutive amide groups in a column.4 Peripheral substituents in hydrogen-bonded gelators, especially alkyl groups, play a significant role in the formation of gel by influencing factors, such as molecular packing, rate of gelation, entanglement of fibers, etc.5 For instance, hydrogen-bonded gelators possessing linear long-chain alkyl groups form a gel with ease as compared to their branched chain analogues.5a In addition, the lower rate of aggregation of such branched chain analogues results into thin fibers.5b Further, within linear alkyl chains as peripheral substituents, the ability of formation of gel varies as the length of the alkyl chains varies, and in some cases, © 2014 American Chemical Society
■
RESULTS AND DISCUSSION All the derivatives of 1,3,5-benzenetricarboxamide, as shown in the Chart 1, were prepared by careful addition of the respective aniline derivatives, in excess, to trimesoyl chloride in THF solution with constant stirring. The products were isolated as colorless solids and characterized by 1H NMR, 13C NMR, and IR spectroscopic techniques (see the Supporting Information). All the amides 1a−1e are insoluble in most of the organic solvents and also in water. However, they are soluble in DMSO and DMF and partially in methanol. To understand the molecular packing of the obtained 1,3,5-benzenetricarboxamide derivatives, 1a−1e, with the help of single-crystal X-ray diffraction analysis, crystallization experiments using the slow evaporation method were carried out from DMF and methanol. Thin crystalline fibers were obtained from DMF and methanol instead of good-quality single crystals. However, gel formation Received: January 7, 2014 Revised: February 26, 2014 Published: March 3, 2014 1895
dx.doi.org/10.1021/cg500026t | Cryst. Growth Des. 2014, 14, 1895−1901
Crystal Growth & Design
Article
saturation, branching of fibers occurs, which could lead to a network structure with entrapping of the solvent to yield gel. However, below saturation, the probability of such branching is less and it could give single crystals. On the basis of this model, attempts were made for crystallization of the gelators 1a−1d at lower concentration (
