Consequences of cooperativity in supramolecular polymers

Abstract

N,N',N-Trialkylbenzene-1,3,5-tricarboxamides (BTAs) self-assemble by means of strong, threefold intermolecular hydrogen bonding into well-defined, helical, one-dimensional columnar aggregates.1 When a stereogenic centre is introduced into the alkyl side chains of these BTA molecules, strong Cotton effects are observed in dilute apolar solutions, indicating the preference for one helical conformation over the other.2 A helical sense preference can also be created by the introduction of isotope chirality into the a-position of the alkyl side chains in BTA molecules. Quantitative analysis on the observed process in dodecane demonstrates that a stereoselectively deuterated BTA forms right- (P) and left-handed (M) helical supramolecular polymers with a slight difference in stability upon cooling and the diastereomeric excess (d.e. = ([M]-[P])/([M]+[P])) is 38% at room temperature3. The subtle character of these deuterated-BTA-based supramolecular polymers makes it a highly sensitive probe to investigate the effect of the molecular structure of the alkane solvent in the self-assembly processes4. Furthermore, the effect of adding a racemization reaction into a self-assembly process by using a BTA molecule with a labile stereocenter was investigated. Racemization reaction in a self-assembling BTA system follows nonfirst-order reaction kinetics. Enantioenriched BTA molecules are obtained from a racemizing solution in the presence of a sergeant molecule which is unreactive towards base. The results are further analyzed by using a theoretical model which is capable of precise quantitative description of the experimental data obtained. The calculations are fully confirmed by experimental data with even the smallest details being fully explained by the methodology employed4. This dissertation focuses on the self-assembly process of BTA molecules and investigates the effect of isotope substitution, alkane solvent employed for self-assembly process and the introduction of racemization reaction into the self-assembly process. 1. A. R. A. Palmans, E. W. Meijer, Angew. Chem. Int. Ed. 2007, 46, 8948. 2. P. J. M. Stals, M. M. J. Smulders, R. Martin-Rapun, A. R. A. Palmans, E. W. Meijer, Chem. Eur. J. 2009, 15, 2071. 3. S. Cantekin, D. W. R. Balkenende, M. M. J. Smulders, A. R. A. Palmans, E. W. Meijer, Nat. Chem. 2011, 3, 42. 4. S. Cantekin, Y. Nakano, J. C. Everts, P. van der Schoot, E. W. Meijer, A. R. A. Palmans, Chem. Commun. 2012, 48, 3803. 5. S. Cantekin, H. M. M. ten Eikelder, A. J. Markvoort, M. A. J. Veld, P. A. Korevaar, M. M. Green, A. R. A. Palmans and E. W. Meijer, Angew. Chem. Int. Ed. 2012 DOI 10.1002/anie.201201701.