Redox-Responsive H‑Bonding: Amplifying the Effect of Electron Transfer Using Proton-Coupled Electron Transfer

A new strategy to create highly redox-responsive H-bond dimers based on proton-coupled electron transfer is proposed that capitalizes on the importance of secondary H-bonds in determining overall binding strength in H-bond dimers. Electron transfer induced proton transfer across a H-bond can be used to significantly strengthen the overall binding by both creating strong ionic H-bonds and changing the secondary H-bonds from unfavorable to favorable. The viability and potency of this approach are demonstrated with an electroactive DAD (A = H-acceptor, D = H-donor) array, H­(MQ+)­H, paired with an electroinactive ADA array, O­(NH)­O. NMR titration of H­(MQ+)H with O­(NH)­O in 0.1 M NBu4PF6/CD2Cl2 gives a K assoc of 500 M–1, typical of DAD–ADA dimers. However, upon two-electron reduction in 0.1 M NBu4PF6/CH2Cl2, cyclic voltammetry studies indicate a 1.8 × 105 increase in binding strength, corresponding to a very large K assoc of 9 × 107 M–1. The latter value is typical of DDD–AAA H-bond dimers, consistent with proton transfer across the central H-bond upon reduction.