The Role of Physical Environment on Molecular Electromechanical Switching

The influences of different physical environments on the thermo- dynamics associated with one key step in the switching mechanism for a pair of bistable catenanes and a pair of bi- stable rotaxanes have been investigated systematically. The two bistable cate- nanes are comprised of a cyclobis(para- quat-p-phenylene) (CBPQT 4 + ) ring, or its diazapyrenium-containing analogue, that are interlocked with a macrocyclic polyether component that incorporates the strong tetrathiafulvalene (TTF) donor unit and the weaker 1,5-dioxy- naphthalene (DNP) donor unit. The two bistable rotaxanes are comprised of a CBPQT 4 + ring, interlocked with a dumbbell component in which one in- corporates TTF and DNP units, where- as the other incorporates a monopyr- rolotetrathiafulvalene (MPTTF) donor and a DNP unit. Two consecutive cycles of a variable scan rate cyclic vol- tammogram (10-1500 mV s � 1 ) per- formed on all of the bistable switches (~ 1m m) in MeCN electrolyte solutions (0.1 m tetrabutylammonium hexafluoro- phosphate) across a range of tempera- tures (258-303 K) were recorded in a temperature-controlled electrochemical cell. The second cycle showed different intensities of the two features that were observed in the first cycle when the cyclic voltammetry was recorded at fast scan rates and low temperatures. The first oxidation peak increases in in- tensity, concomitant with a decrease in the intensity of the second oxidation peak. This variation changed systemati- cally with scan rate and temperature and has been assigned to the molecular mechanical movements within the cate- nanes and rotaxanes of the CBPQT 4 + ring from the DNP to the TTF unit. The intensities of each peak were as- signed to the populations of each co- conformation, and the scan-rate varia- tion of each population was analyzed to obtain kinetic and thermodynamic data for the movement of the CBPQT 4 + ring. The Gibbs free energy of activation at 298 K for the thermally activated movement was calculated to be 16.2 kcal mol � 1 for the rotaxane, and 16.7 and 19.2 kcal mol � 1 for the bipyri- dinium- and diazapyrenium-based bi- stable catenanes, respectively. These values differ from those obtained for the shuttling and circumrotational mo- tions of degenerate rotaxanes and cate- nanes, respectively, indicating that the detailed chemical structure influences the rates of movement. In all cases, when the same bistable compounds were characterized in an electrolyte gel, the molecular mechanical motion slowed down significantly, concomitant with an increase in the activation barri- ers by more than 2 kcal mol � 1 . Irrespec- tive of the environment—solution, self- assembled monolayer or solid-state polymer gel—and of the molecular structure—rotaxane or catenane—a single and generic switching mecha- nism is observed for all bistable mole- cules.