pH-Controlled selection between one of three guests from a mixture using a coordination cage host.

Area of Science:

• Supramolecular Chemistry
• Host-Guest Chemistry
• Chemical Sensing

Background:

• Coordination cages are supramolecular hosts capable of encapsulating guest molecules.
• Controlling guest selectivity in host-guest systems is crucial for applications like molecular recognition and sensing.
• pH-responsive systems offer a simple and reversible method for modulating binding interactions.

Purpose of the Study:

• To demonstrate pH-controlled selective guest binding using a single coordination cage.
• To investigate the reversible switching between three different guests by manipulating pH.
• To explore the influence of guest hydrophobicity and charge on binding affinity within a coordination cage.

Main Methods:

• Utilizing a coordination cage host capable of binding guests from aqueous solution.
• Employing a pH swing strategy to alter the protonation state and thus the hydrophilicity/hydrophobicity of guests.
• Investigating the binding of three distinct guests: adamantane-1,3-dicarboxylic acid (neutral at low pH), 1-amino-adamantane (neutral at high pH), and cyclononanone (pH-independent binding).

Main Results:

• The coordination cage selectively binds adamantane-1,3-dicarboxylic acid at low pH.
• The cage binds cyclononanone at neutral pH due to its pH-independent binding.
• The cage binds 1-amino-adamantane at high pH, demonstrating reversible switching between all three guests.
• Binding is governed by the hydrophobic effect for neutral guests and repulsion for charged guests.

Conclusions:

• A simple pH swing effectively controls the selective binding of three different guests by a coordination cage.
• The system demonstrates reversible switching between bound states (cage·H2A, cage·C, cage·B) by sequential pH adjustments.
• This pH-responsive host-guest system offers a versatile platform for molecular recognition and separation applications.