Solvent and Guest Redox State Dependence on the Host-Guest Equilibrium between Pillar[5]arene Hosts and Viologen Guests

Host-guest chemistry has garnered much attention in recent years due to its growing application in the fields of drug delivery, catalysis, molecular signaling and recognition, environmental remediation, electrochromic devices, and more.1 Pillar[n]arenes (PAs) show particular promise as hosts for a number of applications due to their high-yielding synthesis, ease of functionality, and solubility in a variety of solvents. While several electrochemical devices have been developed that take advantage of PAs' host properties, little is known about how threading (i.e. guest insertion into the cavity of the host) might influence the redox properties of guests or, conversely, how the redox properties of guests might influence their binding to hosts. This is especially important for most host-guest sensors that use the electroactivity of the guest as the critical component of their sensing mechanism. Herein we show that in solution, carboxylic acid-modified pillar[5]arene (CAP[5]) and its water-soluble, ammonium salt (WP[5]) exhibit different affinity trends for dicationic methyl viologen (MV2+), its reduced radical cation form and its doubly reduced neutral form, owing to the unique electron-rich and aromatic cavity of PA. In organic solution, there was an observed 58 mV cathodic shift in Epc for the MV2+/MV+• reduction event, while a 20 mV anodic shift for the same transition was observed under aqueous buffered conditions. This contrasts with previously reported cucurbituril and cyclodextrin systems where exclusively a cathodic shift and anodic shift were observed, respectively. This difference in affinity may allow tuning of the host-guest inclusion complex by altering the solution composition. The flexibility of PAs to differentially stabilize distinct redox states of guests is an exciting discovery that paves the way for further research into the use of host-guest chemistry in electrochemical sensing, among other applications.