Advances in modelling switchable mechanically interlocked molecular architectures

Molecular-sized outward-swinging gate: Experiment and theoretical analysis of a locally nonchaotic barrier

We investigate the concept of molecular-sized outward-swinging gate, which allows for entropy decrease in an isolated system. The theoretical analysis, the Monte Carlo simulation, and the direct solution of governing equations all suggest that under the condition of local nonchaoticity, the probability of particle crossing is asymmetric. It is demonstrated by an experiment on a nanoporous membrane one-sidedly surface-grafted with bendable organic chains. Remarkably, through the membrane, gas spontaneously and repeatedly flows from the low-pressure side to the high-pressure side. While this phenomenon seems counterintuitive, it is compatible with the principle of maximum entropy. The locally nonchaotic gate interrupts the probability distribution of the local microstates, and imposes additional constraints on the global microstates, so that entropy reaches a nonequilibrium maximum. Such a mechanism is fundamentally different from Maxwell's demon and Feynman's ratchet, and is consistent with microscopic reversibility. It implies that useful work may be produced in a cycle from a single thermal reservoir. A generalized form of the second law of thermodynamics is proposed.

Ultrafast dynamics of an azobenzene-containing molecular shuttle based on a rotaxane

An ultrafast spectroscopic study was carried out for a photoisomerizable, rotaxane-based molecular shuttle, in which photoisomerization of the azobenzene moiety of the thread-like guest drives a shuttling motion of a cyclodextrin host. Femtosecond upconversion and time-resolved absorption measurements revealed distinct S₁ dynamics with time constants of 1.2 and 17 ps. Both time constants are smaller when the cyclodextrin host is absent, implying that, within the S₁ state, there are mutiple barriers to the isomerization and subsequent shuttling, due to steric interference from the cyclodextrin.

Detection of Single Molecules Using Stochastic Resonance of Bistable Oligomers

Ultra-sensitive elements for nanoscale devices capable of detecting single molecules are in demand for many important applications. It is generally accepted that the inevitable stochastic disturbance of a sensing element by its surroundings will limit detection at the molecular level. However, a phenomenon exists (stochastic resonance) in which the environmental noise acts abnormally: it amplifies, rather than distorts, a weak signal. Stochastic resonance is inherent in non-linear bistable systems with criticality at which the bistability emerges. Our computer simulations have shown that the large-scale conformational dynamics of a short oligomeric fragment of thermosrespective polymer, poly-N-isopropylmethacrylamid, resemble the mechanical movement of nonlinear bistable systems. The oligomers we have studied demonstrate spontaneous vibrations and stochastic resonance activated by conventional thermal noise. We have observed reasonable shifts of the spontaneous vibrations and stochastic resonance modes when attaching an analyte molecule to the oligomer. Our simulations have shown that spontaneous vibrations and stochastic resonance of the bistable thermoresponsive oligomers are sensitive to both the analyte molecular mass and the binding affinity. All these effects indicate that the oligomers with mechanic-like bistability may be utilized as ultrasensitive operational units capable of detecting single molecules.

Giant thermal expansion of a two-dimensional supramolecular network triggered by alkyl chain motion

Thermal expansion, the response in shape, area or volume of a solid with heat, is usually large in molecular materials compared to their inorganic counterparts. Resulting from the intrinsic molecule flexibility, conformational changes or variable intermolecular interactions, the exact interplay between these mechanisms is however poorly understood down to the molecular level. Here, we investigate the structural variations of a two-dimensional supramolecular network on Au(111) consisting of shape persistent polyphenylene molecules equipped with peripheral dodecyl chains. By comparing high-resolution scanning probe microscopy and molecular dynamics simulations obtained at 5 and 300 K, we determine the thermal expansion coefficient of the assembly of 980 ± 110 × 10^-6 K^-1, twice larger than other molecular systems hitherto reported in the literature, and two orders of magnitude larger than conventional materials. This giant positive expansion originates from the increased mobility of the dodecyl chains with temperature that determine the intermolecular interactions and the network spacing.

Investigation of net unidirectional ring shuttling in a chemically fueled [2]catenane

Switchable rotaxanes and catenanes are environmentally responsive mechanically interlocked molecular architectures (MIMAs). Because of their ability to exhibit reversible and controllable motion in response to environmental stimuli, switchable rotaxanes and catenanes show promise for the advancement of nanoscale devices. Herein we present a study of the first 'autonomous' catenane-based motor (Wilson et al. in Nature 534(7606):235-240, 2016) through a domestically developed simulation tool designed to capture the basic physics/chemistry of the ring shuttling process. The results of the simulation are consistent with the experimentally inferred unidirectional motion in the catenane motor. The factors that affect ring shuttling are explored, and the features of the system that could potentially be modified to influence the rate and directional preference of ring shuttling are reported.