The Role of Physical Environment on Molecular Electromechanical Switching

Abstract

The influences of different physical environments on the thermodynamics associated with one key step in the switching mechanism for a pair of bistable catenanes and a pair of bistable rotaxanes have been investigated systematically. The two bistable catenanes are comprised of a cyclobis(paraquat-p-phenylene) (CBPQT4+) 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-dioxynaphthalene (DNP) donor unit. The two bistable rotaxanes are comprised of a CBPQT4+ ring, interlocked with a dumbbell component in which one incorporates TTF and DNP units, whereas the other incorporates a monopyrrolotetrathiafulvalene (MPTTF) donor and a DNP unit. Two consecutive cycles of a variable scan rate cyclic voltammogram (10-1500 mV s(-1)) performed on all of the bistable switches (approximately 1 mM) in MeCN electrolyte solutions (0.1 M tetrabutylammonium hexafluorophosphate) across a range of temperatures (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 intensity, concomitant with a decrease in the intensity of the second oxidation peak. This variation changed systematically with scan rate and temperature and has been assigned to the molecular mechanical movements within the catenanes and rotaxanes of the CBPQT4+ ring from the DNP to the TTF unit. The intensities of each peak were assigned to the populations of each co-conformation, and the scan-rate variation of each population was analyzed to obtain kinetic and thermodynamic data for the movement of the CBPQT4+ 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 bipyridinium- and diazapyrenium-based bistable catenanes, respectively. These values differ from those obtained for the shuttling and circumrotational motions of degenerate rotaxanes and catenanes, 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 barriers by more than 2 kcal mol(-1). Irrespective 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 mechanism is observed for all bistable molecules.

Evaluation of synthetic linear motor-molecule actuation energetics

“…(a) The CBPQT 4ϩ ring, which carries a tether terminated by a thioctic acid ester for attachment to a gold-coated AFM tip, displays a stronger interaction with TTF than with DNP and thus resides selectively (16) on the former. (b) Chemical oxidation of TTF to TTF 2ϩ results in a strong charge-charge repulsion between the CBPQT 4ϩ ring and TTF 2ϩ , a situation that causes the CBPQT 4ϩ ring to shuttle to DNP (9,(12)(13)(14)(15)(16) in the oxidized [2]rotaxane R 6ϩ . The mechanical movement of the CBPQT 4ϩ ring from TTF 2ϩ to DNP resembles the power stroke of a linear motor.…”

“…Toward the goal of device applications, switching has been shown to operate in condensed phases such as in a polymer electrolyte gel (12), on a self-assembled monolayer (SAM) (13), on the solid supports of engineered systems (14), and in molecular switch tunnel junctions (15). Bistable rotaxanes benefit from their synthesis being highly modular, a virtue that allows for a considerable degree of flexibility in their design.…”

Mechanism of Oxidative Shuttling for [2]Rotaxane in a Stoddart−Heath Molecular Switch: Density Functional Theory Study with Continuum-Solvation Model

“…These two reduction potentials are closer to each other (0.81 and 0.90 V) than those of the dumbbell (0.32 and 0.90 V), in agreement with the experimental results. 13,30,31 …”

Possible performance improvement in [2]catenane molecular electronic switches

“…It was demonstrated that the electromechanical switching occurs within a variety of physical environments. [3][4][5][6]26 However, it was also observed that an intrinsically molecular switching cannot be observed in solid-state experiments based on purely metallic electrodes, 8,9 which signifies that the molecule-electrode barriers are required in the tunnel junction experiments. One possible reason is the formation of metallic filaments that would mask any molecular signature when the molecules are directly contacted to the electrodes.…”