A novel amphiphilic aggregation-induced emission (AIE) copolymer, that is, poly(NIPAM-co-TPE-SP), consisting of N-isopropylacrylamide (NIPAM) as a hydrophilic unit and a tetraphenylethylene-spiropyran monomer (TPE-SP) as a bifluorophoric unit is reported. Upon UV exposure, the close form of non-emissive spiropyran (SP) in poly(NIPAM-co-TPE-SP) can be photo-switched to the open form of emissive merocyanine (MC) in poly(NIPAM-co-TPE-MC) in an aqueous solution, leading to ratiometric fluorescence of AIEgens between green TPE and red MC emissions at 517 and 627 nm, respectively, via Förster resonance energy transfer (FRET). Distinct FRET processes of poly(NIPAM-co-TPE-MC) can be observed under various UV and visible light irradiations, acid-base conditions, thermal treatments, and cyanide ion interactions, which are also confirmed by theoretical studies. The subtle perturbations of environmental factors, such as UV exposure, pH value, temperature, and cyanide ion, can be detected in aqueous media by distinct ratiometric fluorescence changes of the FRET behavior in the amphiphilic poly(NIPAM-co-TPE-MC). Moreover, the first FRET sensor polymer poly(NIPAM-co-TPE-MC) based on dual AIEgens of TPE and MC units is developed to show a very high selectivity and sensitivity with a low detection limit (LOD = 0.26 μM) toward the cyanide ion in water, which only contain an approximately 1% molar ratio of the bifluorophoric content and can be utilized in cellular bioimaging applications for cyanide detections.

A series of novel photo-switchable [2]­rotaxanes (i.e., Rot-A-SP and Rot-B-SP before and after shuttling controlled by acid–base, respectively) containing one spiropyran (SP) unit (as a photochromic stopper) on the axle and two tetraphenylethylene (TPE) units on the macrocycle were synthesized via click reaction. Upon UV/visible light exposure, both mono-fluorophoric rotaxanes Rot-A-SP and Rot-B-SP with the closed form (i.e., non-emissive SP unit) could be transformed into the open form (i.e., red-emissive merocyanine (MC) unit) to acquire their respective bi-fluorophoric Rot-A-MC and Rot-B-MC reversibly. The aggregation-induced emission (AIE) properties of bi-fluorophoric TPE combined with MC AIEgens of these designed rotaxanes and mixtures in semi-aqueous solutions induced interesting ratiometric photoluminescence (PL) and Förster resonance energy transfer (FRET) behaviors, which were further investigated and verified by dynamic light scattering (DLS), X-ray diffraction (XRD), and time-resolved photoluminescence (TRPL) measurements along with theoretical studies. Accordingly, in contrast to the model axle (Axle-MC) and the analogous mixture (Mixture-MC, containing the axle and macrocycle components in a 1:1 molar ratio), more efficient FRET behaviors and stronger red PL emissions were obtained from dual-AIEgens between a blue-emissive TPE donor (PL emission at 468 nm) and a red-emissive MC acceptor (PL emission at 668 nm) in both novel photo-switchable [2]­rotaxanes Rot-A-MC and Rot-B-MC under various external modulations, including water content, UV/Vis irradiation, pH value, and temperature. Furthermore, the reversible fluorescent photo-patterning applications of Rot-A-SP in a powder form and a solid film with excellent photochromic and fluorescent behaviors are first investigated in this report.

A series of multistimuli-responsive [2]­rotaxanes with a naphthalimide-functionalized macrocycle threaded through an axle containing a photochromic spiropyran (SP) or merocyanine (MC) stopper were fabricated and studied for their distinct photophysical properties in semiaqueous solutions. By different combinations of chemical and photochemical switchable stimuli, these [2]­rotaxanes could be interconverted between multiple states, including monofluorophoric [2]­rotaxanes with the close form of the SP unit, i.e., Rot-H-SP and Rot-SP before and after shuttling upon acid–base controls, respectively, along with their corresponding bifluorophoric [2]­rotaxanes possessing the open form of the MC unit, i.e., Rot-H-MC and Rot-MC after UV exposure. Interestingly, the photoinduced electron-transfer (PET) effects appeared between nonemissive SP and aggregation-caused quenching (ACQ) naphthalimide units in [2]­rotaxanes Rot-H-SP and Rot-SP, whereas the Förster resonance energy transfer (FRET)/Dexter energy transfer occurred in [2]­rotaxanes Rot-H-MC and Rot-MC between green-emissive naphthalimide donor (λem = 528 nm) and red-emissive MC acceptor (λem = 648 nm) moieties after UV exposure. Moreover, the PET as well as FRET/Dexter energy-transfer processes and the speculated molecular arrangements (with/without macrocyclic tilting) of all [2]­rotaxanes and related derivatives were verified by time-resolved photoluminescence (TRPL) measurements and theoretical studies. Among all [2]­rotaxanes and derivatives, Rot-MC in tetrahydrofuran (THF)/H2O (2:8, v/v) revealed the strongest red MC emission with the most effective FRET process that possessed attractive ratiometric photoluminescence (PL) due to the ideal shortest donor–acceptor distance. Regardless of pH and temperature effects, the highest red MC emissions with the optimum FRET processes of all compounds were maintained at room temperature in near-neutral conditions (i.e., pH = 5–9). Importantly, the red-emissive [2]­rotaxane Rot-MC showed high selectivity and sensitivity toward sulfite-ion sensing to recover green donor emission via FRET-OFF behavior owing to the Michael reaction of the MC moiety with the sulfite ion, which had an excellent limit of detection (LOD) value of 0.76 μM to be further utilized for the cellular imaging of sulfite detection in living cells. Accordingly, the novel ratiometric sensor approaches of bifluorophoric [2]­rotaxane systems have been well developed in this study as the first rotaxane application of FRET processes toward sulfite detection with higher sensitivities than those of their monofluorophoric analogues. Various LOD values could be evaluated to realize the sulfite-sensing mechanisms of well-designed [2]­rotaxane systems by the shuttling of macrocyclic fluorophores along with photoswitchable FRET behaviors via alternative UV–vis exposures.

Multistimuli-responsive polymers containing mechanophoric motifs are highlighted as promising mechano-luminescent materials upon stretching via optical fluorescence signals.Herein, we report a distinct ratiometric force-induced fluorescence changes from green-emissive napthalimide stopper (donor) embedded in polyrotaxane (PR) cross-linkers to red-emissive rhodamine mechanophore (acceptor) incorporated in polyurethane (PU) backbones. The utilization of PR (1 wt %) can significantly enhance both toughness and stretchability in our targeted PU elastomers, where the mechanical work is largely dissipated by the ring-sliding motion and pulley effects of PR crosslinkers during stretching. Therefore, the simple soft robotic behavior with ultrafast shape memory and reversible ratiometric mechanochromic fluorescence switching by heating (60 °C) in PR-based PU films provide a distinctive strategy for the construction of molecular machines in the high-performance elastomers, which also offer pathways for practical applications of stimuli-responsive polymers in highly stretchable artificial muscles featuring both outstanding pulley effects on enhanced mechanical properties and novel signal variations of mechanochromic fluorescence simultaneously.

Bistable [c2] daisy chain rotaxanes with respective extended and contracted forms of [ c2]A and [ c2]B containing a blue-emissive anthracene (AN) donor and orange-emissive indandione-carbazole (IC) acceptor were successfully synthesized via click reaction. Tunable-emission bistable [c2] daisy chain rotaxanes with fluorescence changes from blue to orange, including bright-white-light emissions, could be modulated by the aggregation-induced emission (AIE) characteristics and Förster resonance energy transfer (FRET) processes through altering water fractions and shuttling processes (i.e., acid/base controls). Accordingly, as a result of excellent fine-tuning AIE (at 60% water content of H2O/THF) and FRET (with a compatible energy transfer of E FRET = 33.2%) behaviors after the shuttling process (by adding base), the brightest white-light emission at CIE (0.31, 0.37) with a quantum yield of Φ = 15.64% was obtained in contracted [ c2]B with good control of molecular shuttling to possess higher photoluminescence (PL) quantum yields and better energy transfer efficiencies (i.e., the manipulation of reduced PET and enhanced FRET processes) due to their intramolecular aggregations of blue AN donors and orange IC acceptors with a proper water content of 60% H2O. Furthermore, dynamic light-scattering (DLS) and time-resolved photoluminescence (TRPL) measurements, along with theoretical calculations, were utilized to investigate and confirm AIE and FRET phenomena of bistable [c2] daisy chain rotaxanes. Especially, both bistable [c2] daisy chain rotaxanes [ c2]A and [ c2]B and noninterlocked monomer M could be exploited for the applications of ratiometric fluorescence temperature sensing due to the temperature effects on the AIE and FRET features. Based on these desirable bistable [c2] daisy chain rotaxane structures, this work provides a potential strategy for the future applications of tunable multicolor emission and ratiometric fluorescence temperature-sensing materials.