On the Stability of Metastable Monomers to Bias the Supramolecular Polymerization of Naphthalendiimides

Herein, we report the synthesis of the naphthalendiimides (NDIs) 1-3 endowed with peripheral 3,4,5-trialkoxybenzamide units and a variable number of 1,2,3-triazole rings. Both the benzamide units and the triazole rings are able to form six- or seven-membered intramolecularly H-bonded pseudocycles that behave as metastable monomeric units. Whilst freshly prepared solutions of 1-3 afford H-type aggregates, the presence or lack of the 1,2,3-triazole rings strongly conditions the kinetics and stability of the resulting aggregated species. These structural features result in highly stable metastable monomeric species M* for the symmetric 2 that can be trapped for long periods of time when the sample is subject to a heating/cooling cycle. Contrary to NDI 2, the M* species formed by 1 and 3 evolve to the final supramolecular polymers in shorter times. A detailed experimental and theoretical study display the different non-covalent supramolecular forces operating in the stabilization of such M* species. In all cases, but especially in those NDIs endowed with the triazoles rings (NDIs 2 and 3), a number of conformers for the metastable monomeric units can be modelled. The high stability of such monomeric species justifies the delay in the formation of the H-type aggregates.