Force-Induced Shuttling of Rotaxanes Controls Fluorescence Resonance Energy Transfer in Polymer Hydrogels

Abstract

The molecular shuttling function of rotaxanes can be exploited to design mechanoresponsive reporter molecules. Here, we report a new approach to such rotaxane-based mechanophores, in which the fluorescence resonance energy transfer (FRET) between a donor–acceptor pair is mechanically controlled. A cyclic molecule containing a green-light-emitting FRET donor connected to a red-light-emitting FRET acceptor was threaded onto an axle equipped with a quencher at its center and two stoppers in the peripheral positions. In the force-free state, the green emitter is located near the quencher so that charge transfer interactions or photo-induced electron transfer between the two moieties suppress green emission and prevent the FRET from the green to the red emitter. The mechanophore was covalently incorporated into a linear polyurethane-urea (PUU), and stretchable hydrogels were prepared by swelling this polymer with water. Upon deformation of the PUU hydrogels and under an excitation light that selectively excites the donor, the intensity of the red fluorescence increases, as a result of a force-induced separation of the green emitter from the quencher, which enables the FRET. The switching contrast is much more pronounced in the gels than in dry films, which is due to increased molecular mobility and hydrophobic effects in the hydrogel, which both promote the formation of inclusion complexes between the ring containing the green emitter and the quencher.

Mechanochromic luminescence of phase-separated hydrogels that contain cyclophane mechanophores

“…8 The introduction of mechanochromic mechanophores is another promising method for studying the force applied to polymer chains in hydrogels. [9][10][11][12][13][14][15][16] For instance, Zheng et al introduced spiropyran (SP) groups into the hydrogels by utilizing surfactants. 9 The SP moieties in the hydrogels were converted to merocyanine (MC) by stretching to 300% strain, heating, or irradiating the hydrogels with ultraviolet (UV) light.…”

“…Therefore, mechanophores that are activated without any cleavage of covalent bonds and exhibit instantly reversible changes in photophysical properties are preferable. [16][17][18][19][20][21][22][23][24][25][26][27][28] For instance, a flapping molecule with a cyclooctatetraene moiety and two pyreneimide wings functions as a force probe in the polyurethane organogel and visualises the force distribution using a hyperspectral camera. 24 Supramolecular mechanophores [16][17][18][19][20][21][22] that change their photophysical properties based on the alteration of the assembly states a Department of Materials Science and Engineering, Tokyo Institute of Technology, of luminophores and/or quenchers are also promising candidates for in situ investigations because of their instantly reversible nature.…”

“…[16][17][18][19][20][21][22][23][24][25][26][27][28] For instance, a flapping molecule with a cyclooctatetraene moiety and two pyreneimide wings functions as a force probe in the polyurethane organogel and visualises the force distribution using a hyperspectral camera. 24 Supramolecular mechanophores [16][17][18][19][20][21][22] that change their photophysical properties based on the alteration of the assembly states a Department of Materials Science and Engineering, Tokyo Institute of Technology, of luminophores and/or quenchers are also promising candidates for in situ investigations because of their instantly reversible nature. Supramolecular mechanophores with rotaxane, [16][17][18] cyclophane, 19,20 or loop 21,22 motifs have been developed to show significant enhancement of the fluorescence intensity or ratiometric fluorescence colour changes in polymer films.…”

“…24 Supramolecular mechanophores [16][17][18][19][20][21][22] that change their photophysical properties based on the alteration of the assembly states a Department of Materials Science and Engineering, Tokyo Institute of Technology, of luminophores and/or quenchers are also promising candidates for in situ investigations because of their instantly reversible nature. Supramolecular mechanophores with rotaxane, [16][17][18] cyclophane, 19,20 or loop 21,22 motifs have been developed to show significant enhancement of the fluorescence intensity or ratiometric fluorescence colour changes in polymer films. Such supramolecular mechanophores are useful for detecting small forces arising in hydrogels.…”

Programming Positive Mechanofluorescence in Liquid Crystalline Elastomers

“…Thanks to this phenomenon, carbazole-containing LSCEs can be exploited as optical mechanotransducers, i.e., materials capable of detecting mechanical stimuli and converting them into processable optical signals . As a whole, materials that modulate their emission of light upon the application of forces and other external stimuli have sparked a great interest recently because of their widespread applications. − …”