Cooperativity Scale: A Structure-Mechanism Correlation in the Self-Assembly of Benzene-1,3,5-tricarboxamides

The self-assembly of small and well-defined molecules using noncovalent interactions to generate various nano- and microarchitectures has been extensively studied. Among various architectures, one-dimensional (1-D) nano-objects have garnered significant attention. It has become increasingly evident that a cooperative or nucleation-elongation mechanism of polymerization leads to highly ordered 1-D supramolecular polymers, analogous to shape-persistent biopolymers such as actin. With this in mind, achieving cooperativity in self-assembled structures has been actively pursued with significant success. Only recently, researchers are focusing on the origin of the mechanism at the molecular level in different synthetic systems. Taking a step further, a thorough quantitative structure-mechanism correlation is crucial to control the size, shape, and functions of supramolecular polymers, and this is currently lacking in the literature. Among a plethora of molecules, benzene-1,3,5-tricarboxamides (BTAs) provide a unique combination of important noncovalent interactions such as hydrogen bonding, π-stacking, and hydrophobic interactions, for self-assembly and synthetic ease. Due to the latter, a diverse range of BTA derivatives with all possible structural mutations have been synthesized and studied during the past decade, mainly from our group. With such a large body of experimental results on BTA self-assembly, it is time to embark on a structure-mechanism correlation in this family of molecules, and a first step toward this will form the main focus of this Account. The origin of the cooperative mechanism of self-assembly in BTAs has been ascribed to 3-fold intermolecular hydrogen bonding (HB) between monomers based on density-functional theory (DFT) calculations. The intermolecular hydrogen-bonding interaction forms the central premise of this work, in which we evaluate the effect of different moieties such as alkyl chains, and amino acids, attached to the core amides on the strength of intermolecular HB, which consequently governs the extent of cooperativity (quantified by the cooperativity factor, σ). In addition to this, we evaluate the effect of amide connectivity (C- vs N-centered), the role of solvents, amides vs thioamides, and finally the influence of the benzene vs cyclohexane core on the σ. Remarkably, every subtle structural change in the BTA monomer seems to affect the cooperativity factor in a systematic and rationalizable way. The take home message will be that the cooperativity factor (σ) in the BTA family forms a continuous spectrum from 1 (isodesmic) to <10^-6 (highly cooperative) and it can be tuned based on the appropriate modification of the BTA monomer. We anticipate that these correlations drawn from the BTA series will be applicable to other systems in which HB is the main driving force for cooperativity. Thus, the understanding gained from such correlations on a prototypical self-assembling motif such as BTA will aid in designing more complex systems with distinct functions.