Overview
This lecture reviews supramolecular systems chemistry, comparing thermodynamically controlled, kinetically trapped, and far-from-equilibrium supramolecular assemblies, with examples illustrating their functions and emergent properties.
Introduction to Supramolecular Chemistry
- Supramolecular chemistry studies non-covalent interactions that lead to molecular recognition and self-assembly.
- Biological systems inspire this field, as complex assemblies like membranes and ribosomes rely on supramolecular organization.
- Biological supramolecular systems often operate far from equilibrium, continuously consuming energy.
Thermodynamic Regimes in Supramolecular Systems
- Equilibrium assemblies persist due to thermodynamic stability; examples include Borromean rings and supramolecular polymers.
- Kinetically trapped assemblies are metastable, trapped in a local energy minimum and can convert to more stable forms over time.
- Far-from-equilibrium assemblies require constant energy input and fall apart if energy supply stops, enabling emergent and unpredictable functions.
Equilibrium Assemblies
- Structures are determined by molecular design and non-covalent interactions, remaining at the lowest free-energy state.
- Examples: template-directed Borromean rings, hydrogen-bonded supramolecular polymers, and dynamic combinatorial libraries forming stable host-guest complexes.
- External changes (e.g., pH) can shift the equilibrium to a new state (e.g., muscle-like polymers switching length).
Kinetically Trapped Assemblies
- Properties stem from a specific structure, but multiple metastable states are possible from the same building blocks.
- Formation conditions and catalysis can access or stabilize different states.
- Examples: lipid vesicles, helicity inversion in supramolecular polymers, enzyme-catalyzed gels, and autocatalytic self-replicating fibers.
Far-from-Equilibrium Assemblies
- Maintain their properties only with continuous energy supply (e.g., light, chemical fuel, electric current).
- Enable unique emergent properties such as oscillations, gradients, or unidirectional molecular motion.
- Examples: light-driven nanoparticle assembly, fuel-driven gelation, reaction-diffusion systems generating chemical gradients, electrochemically maintained gradients, and nanocars powered by STM excitation.
Conclusions and Outlook
- The field is moving towards creating far-from-equilibrium supramolecular systems for richer functional behavior.
- These systems require continuous energy dissipation and can potentially exhibit life-like properties, such as Darwinian evolution.
- The main challenge is to identify compatible chemical formation and destruction reaction pairs for sustainable operation.
Key Terms & Definitions
- Supramolecular assembly — Structure formed via non-covalent interactions between molecules.
- Equilibrium assembly — Stable structure at the lowest free energy, no net energy consumption.
- Kinetic trap — Metastable state persisting due to energy barriers, not at thermodynamic minimum.
- Far-from-equilibrium system — Assembly sustained by a persistent energy input, exhibiting new properties.
- Dynamic combinatorial chemistry — Technique creating a mixture of interconverting molecules to find stable or functional assemblies.
- Autocatalysis — Reaction in which the product catalyzes its own formation.
- Emergent property — System property not predictable from its individual components.
Action Items / Next Steps
- Review and compare examples of each thermodynamic regime in supramolecular chemistry.
- Read about dynamic combinatorial chemistry and its role in system evolution.
- Prepare questions on far-from-equilibrium systems for next session.