(734f) Through-Space Activation of Mechanophores By Rotaxane Molecular Machines

Mechanophores are functional molecular units that exhibit selective chemical responses to mechanical perturbations. The outcome involves various forms: color changes, fluorescence, chemi-luminescence, activation of latent catalyst, and generation of reactive species such as free radicals, ylides, acids, and small organic molecules. Polymeric materials embedded with mechanophores have demonstrated a variety of stress-responsive properties including stress sensing, damage reporting, remote light generation, and self-healing. Among these applications, releasing small molecules from polymer backbone is one of the most challenging tasks. In order to transduce the mechanical force experienced by the polymer matrix to the mechanophores, usually two opposite end groups of the mechanophores are attached to the polymer backbone. Therefore, after mechanophore activation the residual moieties remain bonded to the polymer backbone with no molecule released.

So far, small-molecule releasing is based on two general approaches: (1) Activating a rearrangement reaction that release molecules; and (2) molecular fragmentation. Due to the nature of most rearrangement reactions, the former approach is usually limited to releasing simple species (e.g. HCl or SO₂). The latter fragmentation approach is an example of releasing more complex molecules; however, it generally requires specialized molecular design with limited varieties, and also demands significantly higher mechanical stress for the activation process.

Herein, we report a new platform for mechanophore design with molecule-releasing capability by introducing the concept of mechanically interlocked molecular machines. Molecular machines are molecular constructs that are controlled to perform programmed tasks from the molecular level. Most molecular machines are designed with mechanically interlocked structures, such as a rotaxane that compose of a mobile “wheel” threaded by an “axle” and trapped by bulky stopper groups. In a rotaxane molecular machine, tasks are accomplished by controlling the movement of the wheel along the axle. Therefore, we hypothesize that by guiding the directional sliding of the wheel along the axle, mechanical forces are applied to mechanophores via topological hindrance, allowing for activation of mechanophores on the axle and “extrude out” the generated small molecules.

To test our hypothesis, we synthesized a [2]rotaxane molecule with a mechanophore incorporated at one end of the axle as the stopper. Polymer chains are attached to the rotaxane on the wheel and the other end of the axle. After applying stress to the rotaxane mechanophore under sonication, the Diels-Alder (DA) mechanophore was activated through space, releasing the retro-DA products that are detected by NMR spectroscopy. The molecular weight of the polymer was halved, which is due to the disassembly of rotaxane structure after the removal of mechanophore-based stopper groups. This new concept of mechanophore activation generalizes the use of a variety of mechanophores and opens a pathway towards single-end functionalized mechanophores.