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The first integration of dual slide-ring motion of [c2] daisy chain molecules (as backbones) and α-cyclodextrin (α-CD)-based polyrotaxanes (as cross-linkers) into mechanofluorophoric polyurethane (PU) frameworks was successfully proceeded through step-growth polymerization to yield PU films with distinctive mechanical and optical properties under stress. The intrinsic elastic and stretchable capabilities of the PU films consisting of both mechanically interlocked molecules (MIMs) via the molecular design of daisy chain backbones and polyrotaxane cross-linkers with long gliding movements were evidently enhanced, where the contributions of both extended/contracted forms in daisy chains and different α-CD numbers in polyrotaxane cross-linkers were also investigated. Moreover, reversible ratiometric fluorescence and energy transfer features between green-emitting naphthalimide donors and yellow-orange-emitting mechanofluorophoric rhodamine acceptors could be attained during stretching and relaxation processes. In addition, the stretching deformation of PU films was studied using X-ray diffraction (XRD) techniques for confirming the correlated morphological properties of stretching states in the oriented PU films. Appealingly, notable shape recovery and reversible ratiometric mechanofluorescence behavior of PU films could be achieved upon heating, signifying the potential of PU films with artificial molecular muscle functions of daisy chains accompanied by pulley effects of polyrotaxane cross-linkers, offering excellent mechanical and optical characteristics for various applications in advanced functional materials. [ABSTRACT FROM AUTHOR]

The self-trapping nano-loop structures of [1]rotaxanes exhibited multiple Förster resonance energy transfer (FRET) patterns via dual and sequential locking/unlocking of pH-gated and UV exposure processes. As a tightened and constrained nano-loop in the acidic condition, dithienylethene (DTE) unit was locked in the highly bending open form to forbid ring closure upon UV irradiation. [ABSTRACT FROM AUTHOR]

Visualizing color changes upon mechanical force in elastomers is attractive to understand stress transfer and failure mechanisms, which is valuable for applications ranging from tamper-proof packaging to structural integrity monitoring. Herein, we synthesize mechanochromic polyurethane films with an optimum stretchability (5500% strain) and a strongest toughness (324 MJ m−3) by using the components of bis-functionalized rhodamine, Diels–Alder (DA) adduct, 2-hydroxyethyl disulfide (HEDS), polyethylene glycol, triethanolamine and hexamethylene diisocyanate via condensation polymerization. The additions of HEDS and DA moieties enhance the healing efficiency up to 87% after 24 h of curing induced by disulfide bond exchange and Diels–Alder and retro-Diels–Alder reactions. The polyurethane film exhibits a green emission (450–500 nm) before stretching, but it shows an orange-red emission (588 nm) by force stretching because of the open form of the rhodamine moieties. A high shape recovery of 90.9% and reversible mechano-fluorescence switching by heating (immersed in 60 °C hot water) after structural deformations make it useful for shape memory and soft robotic applications. Finally, a high dielectric permittivity (160 at 1 Hz) and a good capacitance value (0.28 nF cm−2 at 1 Hz) of the film exhibit its potential applications as a flexible capacitor for energy storage in modern flexible devices. [ABSTRACT FROM AUTHOR]

A series of bistable photo-switchable [c2] daisy chain rotaxanes containing photochromic spiropyran (SP) and triphenylamine-chalcone (TPA-C) chromophores were synthesized via a self-sorting strategy and click reactions, which could be controlled by acid–base shuttling processes to achieve the extended and contracted forms of [c2]-SP-Ext and [c2]-SP-Con , respectively. The molecular structures of these analogues were characterized and confirmed by nuclear magnetic resonance (NMR) and high resolution mass spectroscopy (HRMS). Both [c2] daisy chain rotaxanes ([c2]-SP-Ext and [c2]-SP-Con) and on-interlocked analogue (Cy-SP) could undergo reversible photo-isomerization to form the corresponding [c2]-MC-Ext , [c2]-MC-Con and Cy-MC upon UV/Vis exposure and their aggregation-induced emission enhancement (AIEE) properties were examined in semi-aqueous solutions (>60% H2O). Besides, these [c2] daisy chain and non-interlocked analogues consisting of yellow emissive TPA-C donors and orange-red emissive merocyanine (MC) acceptors induced interesting ratiometric photoluminescence (PL) and reversible Förster resonance energy transfer (FRET) characteristics upon UV/Vis irradiation, which were investigated by PL measurements and further confirmed by theoretical studies. Accordingly, [c2] daisy chain rotaxanes ([c2]-MC-Con and [c2]-MC-Ext) exhibited better energy transfer processes than those of the non-interlocked analogue (Cy-MC) with respective FRET efficiencies of 78.2, 73.1 and 63.9%. Various responsive effects of external stimuli on the FRET-ON/OFF behaviours for [c2] daisy chain and non-interlocked analogues, including water fractions, UV/Vis exposures, temperature controls and acid/base treatments were studied in this report. Furthermore, the interesting reversible FRET ON-OFF switching behavior and fluorescence photo-patterning applications of photo-switchable [c2]-SP-Ext -based coated papers under different environmental conditions were also explored and demonstrated. [ABSTRACT FROM AUTHOR]