Consequences of hidden kinetic pathways on supramolecular polymerization

Abstract

In recent years, the development of sophisticated analytical tools, kinetic models and sample preparation methods has significantly advanced the field of supramolecular polymerization, where the competition of kinetic vs. thermodynamic processes has become commonplace for a wide range of building blocks. Typically, the kinetic pathways are identified in thermally controlled assembly experiments before they ultimately evolve to the thermodynamic minimum. However, there might be cases where the identification and thus the assessment of the influence of kinetic aggregates is not trivial, making the analysis of the self-assembly processes a hard task. Herein, we demonstrate that “hidden” kinetic pathways can have drastic consequences on supramolecular polymerization processes, to the point that they can even overrule thermodynamic implications. To this end, we analyzed in detail the supramolecular polymerization of a chiral Pd^II complex 1 that forms two competing aggregates (Agg I and Agg II) of which kinetic Agg II is formed through a “hidden” pathway, i.e. this pathway is not accessible by common thermal polymerization protocols. The hidden pathway exhibits two consecutive steps: first, Agg II is formed in a cooperative process, which subsequently evolves to clustered superstructures driven by rapid kinetics. At standard conditions, Agg II displays an extraordinary kinetic stability (>6 months), which could be correlated to its cooperative mechanism suppressing nucleation of thermodynamic Agg I. Furthermore, the fast kinetics of cluster formation sequester monomers from the equilibria in solution and prevents the system from relaxing into the thermodynamic minimum, thus highlighting the key implications of hidden pathways in governing supramolecular polymerization processes.