Visualization of the Necking Initiation and Propagation Processes during Uniaxial Tensile Deformation of Crystalline Polymer Films via the Generation of Fluorescent Radicals

Luminescent Radicals

Organic radicals are attracting increasing interest as a new class of molecular emitters. They demonstrate electronic excitation and relaxation dynamics based on their doublet or higher multiplet spin states, which are different from those based on singlet-triplet manifolds of conventional closed-shell molecules. Recent studies have disclosed luminescence properties and excited state dynamics unique to radicals, such as highly efficient electron-photon conversion in OLEDs, NIR emission, magnetoluminescence, an absence of heavy atom effect, and spin-dependent and spin-selective dynamics. These are difficult or sometimes impossible to achieve with closed-shell luminophores. This review focuses on luminescent organic radicals as an emerging photofunctional molecular system, and introduces the material developments, fundamental properties including luminescence, and photofunctions. Materials covered in this review range from monoradicals, radical oligomers, and radical polymers to metal complexes with radical ligands demonstrating radical-involved emission. In addition to stable radicals, transiently formed radicals generated in situ by external stimuli are introduced. This review shows that luminescent organic radicals have great potential to expand the chemical and spin spaces of luminescent molecular materials and thus broaden their applicability to photofunctional systems.

Imaging of Accumulated Mechanical Stresses Using Self-Assembled Layered Conjugated Polymer

When mechanical stresses, such as tensile, compressive, and frictional stresses, are applied to objects by various motions, they are accumulated in materials. Conventional mechanoresponsive materials and sensors detect one-time applied stress. However, the accumulated stresses are not visualized or measured in previous works. The present study demonstrated imaging and sensing of not only one-time but also accumulated tensile, compressive, and frictional stresses. Polyurethane (PU) film was combined with 2D layered polydiacetylene (PDA), a stimuli-responsive color-changing polymer. PDA generally exhibits no color changes with the application of tensile and compression stresses because the molecular motion leading to the color change is not induced by such mechanical stresses. Here the versatile mechanoresponsiveness was achieved using a block copolymer guest partially intercalated in the layered PDA. As the interlayer and outerlayer segments interact with PDA and PU, respectively, the applied stresses to the film are transferred from PU to PDA via the block copolymer guest. The color changes of the film imaged and quantified the accumulated work depending on the number and strength of the applied multiple stresses such as tensile, compressive, and frictional stresses. The design strategy of materials and methodology of sensing can be applied to the development of new sensors for accumulated mechanical stresses in a wide range of length and strength scales.

Mechanochromism and Strain-Induced Crystallization in Thiol-yne-Derived Stereoelastomers

Most elastomers undergo strain-induced crystallization (SIC) under tension; as individual chains are held rigidly in a fixed position by an applied strain, their alignment along the strain field results in a shift from strain-hardening (SH) to SIC. A similar degree of stretching is associated with the tension necessary to accelerate mechanically coupled, covalent chemical responses of mechanophores in overstretched chains, raising the possibility of an interplay between the macroscopic response of SIC and the molecular response of mechanophore activation. Here, thiol-yne-derived stereoelastomers doped covalently with a dipropiolate-derivatized spiropyran (SP) mechanophore (0.25-0.38 mol%) are reported. The material properties of SP-containing films are consistent with undoped controls, indicating that the SP is a reporter of the mechanical state of the polymer. Uniaxial tensile tests reveal correlations between mechanochromism and SIC, which are strain-rate-dependent. When mechanochromic films are stretched slowly to the point of mechanophore activation, the covalently tethered mechanophore remains trapped in a force-activated state, even after the applied stress is removed. Mechanophore reversion kinetics correlate with the applied strain rate, resulting in highly tunable decoloration rates. Because these polymers are not covalently crosslinked, they are recyclable by melt-pressing into new films, increasing their potential range of strain-sensing, morphology-sensing, and shape-memory applications.

Configuration-Induced Multichromism of Phenanthridine Derivatives: A Type of Versatile Fluorescent Probe for Microenvironmental Monitoring

Fluorescent probes are attractive in diagnosis and sensing. However, most reported fluorophores can only detect one or few analytes/parameters, notably limiting their applications. Here we have designed three phenanthridine-based fluorophores (i.e., B1, F1, and T1 with 1D, 2D, and 3D molecular configuration, respectively) capable of monitoring various microenvironments. In rigidifying media, all fluorophores show bathochromic emissions but with different wavelength and intensity changes. Under compression, F1 shows a bathochromic emission of over 163 nm, which results in organic fluorophore-based full-color piezochromism. Moreover, both B1 and F1 exhibit an aggregation-caused quenching (ACQ) behavior, while T1 is an aggregation-induced emission (AIE) fluorophore. Further, F1 and T1 selectively concentrate in cell nucleus, whereas B1 mainly stains the cytoplasm in live cell imaging. This work provides a general design strategy of versatile fluorophores for microenvironmental monitoring.

Swelling-induced Mechanochromism in Multinetwork Polymers

We report a novel and versatile approach to achieving swelling-induced mechanochemistry using a multinetwork (MN) strategy that enables polymer networks to repeatedly swell with monomers and solvents. The isotropic expansion of the first network (FN) provides sufficient force to drive the mechanochemical scission of a radical-based mechanophore, difluorenylsuccinonitrile (DFSN). Although prompt recombination generally occurs in such highly mobile environments, the resulting pink radicals are kinetically stabilized in the gels, probably due to limited diffusion in the extended polymer chains. Moreover, the DFSN embedded in the isotropically strained chain exhibits increased thermal reactivity, which can be reasonably explained by an entropic contribution of the FN to the dissociation. The utility of the MN polymers is demonstrated not only in terms of swelling-force-induced network modification, but also in the context of tunable reactivity of the dissociative unit through proper design of the hierarchical network architecture.

Mechanochromophore-linked Polymeric Materials with Visible Color Changes

Mechanical force as a type of stimuli for smart materials has obtained much attention in the past decade. Color-changing materials in response to mechanical stimuli have shown great potential in the applications such as sensors and displays. Mechanochromophore-linked polymeric materials, which are a growing sub-class of these materials, are discussed in detail in this review. Two main types of mechanochromophores which exhibit visible color change, summarized herein, involve either isomerization or radical generation mechanisms. This review focuses on their synthesis and incorporation into polymer matrices, the type of mechanical force used, factors affecting the mechanochromic properties, and their applications. This article is protected by copyright. All rights reserved.

Photo- and triboluminescent pyridinophane Cu complexes: New organometallic tools for mechanoresponsive materials

The development of mechanoresponsive polymers has emerged as a new, attractive area of research in which changes at the molecular level exert macrolevel effects in the bulk material, and vice...