Binding studies between triethylene glycol-substituted monopyrrolotetrathiafulvalene derivatives and cyclobis(paraquat-p-phenylene)

The synthesis of several pi-electron-donating monopyrrolotetrathiafulvalene (MPTTF) derivatives, which conceptually can be divided into three classes containing none, one, or two triethylene glycol (TEG) substituents, is described. In all cases, the complexation between the pi-electron donating MPTTF unit and the pi-electron-deficient tetracationic cyclophane cyclobis(paraquat-p-phenylene) (CBPQT(4+)) has been investigated using UV-vis dilution techniques. The results reveal that the strength of the binding between MPTTF derivatives and CBPQT(4+) is directly correlated to the pi-electron donating properties of the MPTTF derivatives. However, the pi-electron-donating properties of the MPTTF derivatives is not the only factor of importance. The results enclosed in the present studies demonstrate that the TEG substituents assist the complexation process most likely on account of their capacity to participate in [C-H center dot center dot center dot O] hydrogen bonding interactions with some of the alpha-CH protons in the bipyridinium units of CBPQT(4+) and the stabilizing effect that attachment of one or two TEG substituents to the MPTTF unit exerts upon complexation with CBPQT(4+) has been quantified to approximately 0.3 and 0.5 kcal mol(-1), respectively. These results serve to lay an extended foundation for the understanding of which buttons to push when it comes to improve the design of bistable molecular switches based on (MP)TTF and CBPQT(4+).