# Ratiometric Mechano-Fluorescent Elastomers Dually Promoted via Effective Force-Triggered Radicals and Preeminent Toughnesses/Stretchabilities by Unconventional Shuttling Dimensions of Tetraphenylethylene-Suspended [c2] Daisy Chain Rotaxanes

## TL;DR

Scientists created a new type of stretchy material that changes color when stretched, using special molecular structures to make it tough and responsive.

## Contribution

The paper introduces a novel method of enhancing polymer toughness and stretchability using daisy chain rotaxanes with unconventional shuttling dimensions.

## Key findings

- PU films with daisy chain structures showed a record toughness of 1363 MJ/m³, 6.3 times tougher than standard PU films.
- The material exhibited ratiometric fluorescent responses under stretching, enabling reversible dual fluorescent switching.
- Thermal treatments enabled shape memory and reversible fluorescence behaviors in the material.

## Abstract

Innovative radical-type mechano-fluorescent polyurethane
(PU) elastomers
were developed by integrating axle- and macrocycle-exerted force modes
of tetraphenylethylene (TPE)-functionalized daisy chain rotaxanes
with expanded/contracted conformations into polymer matrices, revealing
distinct mechanical and optical performances under external tensile
forces. Surprisingly, the designed PU films containing negligible
amounts (0.02% molar ratio of all monomers) of daisy chain structures
with unconventional shuttling dimensions (as artificial molecular
muscle tougheners) exhibited ultrastretchable capabilities and preeminent
toughnesses, possessing a record-high toughness of 1363 MJ/m3 at the strain rate of 20 mm/s, approximately 6.3 times tougher than
the standard PU films, along with a significant loading weight ratio
of 150,000. Additionally, appealing ratiometric fluorescent responses
between blue-emissive TPE stoppers and yellow-emissive diarylacetonitrile
radical species could be detected in PU films by stretching due to
the introduction of TPE-based daisy chain rotaxanes into mechano-fluorophoric
PU skeletons, enabling reversible dual fluorescent switching during
tensile loading and unloading processes. Remarkably, notable shape
memory and reversible ratiometric fluorescence behaviors of synthetic
daisy chain-grafted PU films could be accessible by thermal treatments,
indicating probable applications of mechanically interlocked molecule-functionalized
PU films with splendid mechanical and optical features for designing
stimuli-responsive smart materials.