Design of rapidly assembling supramolecular systems responsive to synchronized stimuli

This review treats several aspects of supramolecular self-assembling systems based on rapidly responsive hydrogels from polymeric hosts and low molecular weight guests. A series of cyclodextrin-conjugated poly(ε-lysine)s (CDPLs) have been designed as host molecules and their complexation behavior has been investigated with various functional guest moieties containing hydrophobic and anionic groups at their termini. Systematic studies on the inclusion complexation provide evidence that dual cooperative interactions, i.e. specific host-guest and intermolecular ionic interactions, play a dominant role in the fast aggregation process; the induction time for supramolecular aggregation between β-CDPL and 3-trimethylsilylpropionic acid (TPA) was found to be very short (within 100 ms). This rapidly assembling system shows a clear dependence on pH and temperature, which significantly affect the dramatic aggregation into the supramolecular structure. Comparison of the geometries of β-CDPL and TPA analogs reveals that the smaller radius and bond length of carbon as compared to silicon give smaller host cross-sectional areas and allow penetration closer to the smaller rim of β-CD (H-5); therefore, the binding constants are larger than those of trimethylsilyl groups. The length and penetration depth of the guest molecules strongly affect the supramolecular aggregation with β-CDPL, the guests acting as noncovalent crosslinkers through intermolecular ionic interactions. This rapid and delicate supramolecular self-assembling system is promising as a stimuli-responsive material and should find a broad range of applications because the functionality is variable and switchable in response to simultaneous multiple stimuli.