Radically Enhanced Dual Recognition

Complexation between a viologen radical cation (V^.+) and cyclobis(paraquat-p-phenylene) diradical dication (CBPQT^2(.+)) has been investigated and utilized extensively in the construction of mechanically interlocked molecules (MIMs) and artificial molecular machines (AMMs). The selective recognition of a pair of V^.+ using radical-pairing interactions, however, remains a formidable challenge. Herein, we report the efficient encapsulation of two methyl viologen radical cations (MV^.+) in a size-matched bisradical dicationic host — namely, cyclobis(paraquat-2,6-naphthalene)^2(.+), i.e., CBPQN^2(.+). Central to this dual recognition process was the choice of 2,6-bismethylenenaphthalene linkers for incorporation into the bisradical dicationic host. They provide the space between the two bipyridinium radical cations in CBPQN^2(.+) suitable for binding two MV^.+ with relatively short (3.05–3.25 Å) radical-pairing distances. The size-matched bisradical dicationic host was found to exhibit highly selective and cooperative association with the two MV^.+ in MeCN at room temperature. The formation of the tetrakisradical tetracationic inclusion complex — namely, [(MV)₂⊂CBPQN]^4(.+) – in MeCN was confirmed by VT ¹H NMR, as well as by EPR spectroscopy. The solid-state superstructure of [(MV)₂⊂CBPQN]^4(.+) reveals an uneven distribution of the binding distances (3.05, 3.24, 3.05 Å) between the three different V^.+, suggesting that localization of the radical-pairing interactions has a strong influence on the packing of the two MV^.+ inside the bisradical dicationic host. Our findings constitute a rare example of binding two radical guests with high affinity and cooperativity using host-guest radical-pairing interactions. Moreover, they open up possibilities of harnessing the tetrakisradical tetracationic inclusion complex as a new, orthogonal and redox-switchable recognition motif for the construction of MIMs and AMMs.