Molecular Pumps that drives a system away from equilibrium

Abstract

A pressing goal for contemporary science is to build artificial mol. systems that can perform useful work. Here we report a 'mol. pump' system that processes chem. energy (redox) to drive a macrocycle away from equil., from a low energy to a high energy state. A series of mol. rods were synthesized contg. 3,5- dimethylpyridinium (Py +) and 4,4'-bipyridinium (BIPY 2+) units connected to a bulky steric stopper by chains of different lengths. Cyclobis(paraquat-p-phenylene) (CBPQT 4+) is repelled by the Py + and BIPY 2+ initially; however, upon redn. with activated zinc dust, Coulombic repulsion between Py + and CBPQT 2(+•) dramatically decreases and a thermodynamically stable trisradical complex BIPY +•I CBPQT 2(+•) is formed within seconds. Tris(4-bromophenyl)aminium hexachloridoantimonate (Magic Blue) was used to oxidize the radical cation rapidly to the fully oxidized state. The electrostatic Py + barrier is recovered, causing the CBPQT 4+ ring to shuttle on to the glycol/alkyl chain, for which it has little affinity. Dethreading from this high energy, metastable [2]rotaxane state occurs slowly due to Coulombic repulsion between CBPQT 4+ and Py +/BIPY 2+. Exptl. results and DFT calcns. show that the mol. pump can raise the potential energy of CBPQT 4+ up to 13 kcal/mol with an efficiency up to 8%.