Human skin is a remarkable organ. It consists of an integrated, stretchable network of sensors that relay information about tactile and thermal stimuli to the brain, allowing us to maneuver within our environment safely and effectively. Interest in large-area networks of electronic devices inspired by human skin is motivated by the promise of creating autonomous intelligent robots and biomimetic prosthetics, among other applications. The development of electronic networks comprised of flexible, stretchable, and robust devices that are compatible with large-area implementation and integrated with multiple functionalities is a testament to the progress in developing an electronic skin (e-skin) akin to human skin. E-skins are already capable of providing augmented performance over their organic counterpart, both in superior spatial resolution and thermal sensitivity. They could be further improved through the incorporation of additional functionalities (e.g., chemical and biological sensing) and desired properties (e.g., biodegradability and self-powering). Continued rapid progress in this area is promising for the development of a fully integrated e-skin in the near future.

25th Anniversary Article: The Evolution of Electronic Skin (E-Skin): A Brief History, Design Considerations, and Recent Progress

Supramolecular materials, dynamic materials by nature, are defined as materials whose components are bridged via reversible connections and undergo spontaneous and continuous assembly/disassembly processes under specific conditions. On account of the dynamic and reversible nature of noncovalent interactions, supramolecular polymers have the ability to adapt to their environment and possess a wide range of intriguing properties, such as degradability, shape-memory, and self-healing, making them unique candidates for supramolecular materials. In this critical review, we address recent developments in supramolecular polymeric materials, which can respond to appropriate external stimuli at the fundamental level due to the existence of noncovalent interactions of the building blocks.

Stimuli-responsive supramolecular polymeric materials.

2.4.2. Interpenetrated Ladders 711 2.4.3. Unusual Motifs of Ladders 713 2.4.4. Properties of Ladderlike Chains 713 2.5. Rotaxane Polymers 714 2.5.1. 1D Polyrotaxanes 714 2.5.2. 2D Polyrotaxanes 715 2.5.3. 3D Polyrotaxanes 716 2.5.4. Hydrogen-Bonded Polyrotaxanes 716 2.6. Ribbon/Tape Polymers 717 2.7. Metal Cluster As Building Blocks for 1D CP 718 2.7.1. Metal Carboxylate Clusters 718 2.7.2. Metal Halide Clusters 719 2.7.3. Metal Chalcogenide Clusters 720 2.7.4. Polyoxometalate Clusters 721 2.7.5. Single Molecular Magnets as Building Blocks 722

One-Dimensional Coordination Polymers: Complexity and Diversity in Structures, Properties, and Applications

A stretchable polymer LED is fabricated that is capable of emitting light when subjected to strains as large as 120%. A prototype 5 × 5 pixel monochrome display based on an array of these LEDs is demonstrated.

Elastomeric polymer light-emitting devices and displays

Supramolecular Amphiphiles Based on Host-Guest Molecular Recognition Motifs.

Remarkable progress in stretchable electronics over the past few years has produced some amazing devices: Devices that can be laminated onto skin, [ 1 ] hemispherical arrays of photodetectors that mimic the human retina, [ 2 ] and arrays of light-emitting devices for use in fldisplays [ 3 ] are just a few examples. The methodology used to impart stretchability connects rigid circuit elements with stretchable conductive interconnects that absorb applied strain. [ 4 ] A different approach to stretchability is now emerging, in which the devices themselves are fabricated to be intrinsically stretchable. Here, we describe the use of a stretchable electroluminescent material to produce a light-emitting device that exhibits intrinsic stretchability at room temperature. These devices are well-suited to new applications in stretchable and conformable lighting that require uniform, diffuse light emission over large areas. We demonstrate devices with large emission areas ( ∼ 20‐175 mm 2 ) that tolerate linear strains up to 27% and repetitive cycles of 15% strain with minimal performance degradation relative to devices fabricated using a conventional electroluminescent material. There are few examples of intrinsically stretchable devices‐ that is, systems in which the device itself operates under tensile strain. The two strategies that have been used to impart stretchability to these devices are identical to those used to fabricate stretchable device interconnects: Device materials can be either dispersed into an elastomeric matrix or deposited onto a prestretched elastomeric substrate. Kaltenbrunner et al. used the fi rst strategy to produce a stretchable battery of gel materials encased in an elastomer that can withstand uniaxial strains up

Stretchable light-emitting electrochemical cells using an elastomeric emissive material.

Channels and Cavities Lined with Interlocked Components: Metal‐Based Polyrotaxanes That Utilize Pyridinium Axles and Crown Ether Wheels as Ligands

A molecular switch that is derived from a mechanically interlocked molecule (MIM) can exist in two distinct molecular arrangements: the ground-state co-conformation (GSCC) and a metastable co-conformation (MSCC), which are in equilibrium. Ideally, these co-conformers are easily identifiable by using a spectroscopic technique, their relative populations are quantifiable, and the position of the equilibrium can be controlled by some external perturbation. 2] Probably the most well understood MIM switches are the bistable [2]rotaxane, or molecular shuttle, and the bistable [2]catenane, both pioneered by Stoddart and co-workers. In these systems, two different recognition sites are present on one component for the binding of a single macrocycle. The two states of the switch are co-conformers that are related by the relative positioning of the two interlocked components. In a related set of MIMs, we recently reported the first examples of a molecular “flip switch” that operates at a single recognition site on a simple [2]rotaxane. In this system, the stability of the GSCC and MSCC are related to the degree of p stacking between the axle and the wheel, and the position of the co-conformational equilibrium was shown to be sensitive to the structure of the pyridinium component or the nature of the solvent. The flip-switch [2]rotaxane is built around the templating motif of [24]crown-8 macrocycles and 1,2-bis(pyridinium)ethane axles. This motif has been successfully used in creating a variety of unique rotaxanes and catenanes, and has been incorporated into metal–organic frameworks (MOFs). The concept of a flip-switch [2]rotaxane is shown in Scheme 1. In the [2]rotaxanes [(1)(N24C8)] and [(1)(BN24C8)], although the axle 1 is symmetrical, the two ends of the molecules are different because the crown ethers contain two different aromatic rings. This result is clearly shown by low-temperature H NMR spectra in CD2Cl2, which show eight distinct sets of pyridinium protons for [(1)(N24C8)] and six sets for [(1)(BN24C8)] because the pyridinium protons experience different amounts of shielding from the presence or absence of a naphtho or benzo ring. Each pair of exchanging resonances is separated by approximately 0.35 ppm, which is consistent with the effects of shielding that arise from p stacking. The rate of end-to-end exchange or “flipping” was determined to be on the order of 10 s 1 at room temperature. Since the flip-switch equilibrium is dictated by the degree of p stacking and thus the structural composition of the pyridinium component, it was of interest to use this principle to control the switching process. Herein, we report that the relative co-conformational stabilities can be controlled by manipulating the coordination geometry of an appended chelating group. The pioneering work of Sauvage and coworkers on Cu/Cu rotaxane systems, and Leigh and coworkers on Cu and Pd molecular shuttle systems are examples in which co-conformational switching of interlocked molecules has been shown to be dependent on metal– ligand coordination. This new [2]rotaxane flip switch system incorporates a terpyridine (terpy) group on one end of the axle (see Scheme 2). The [2]rotaxane ligands [(3)(BN24C8)] and [(3)(N24C8)] were prepared by stoppering the 1,2-bis(pyridinium) axle 2 with tert-butylbenzylbromide in the presence of an excess of the appropriate crown ether. The products were purified by column chromatography on silica and the anion was exchanged to the triflate (CF3SO3 ; OTf) salt to maintain solubility in organic solvents. The Pt and Ru complexes of these ostensibly terpy ligands were then prepared using standard conditions and appropriate starting materials (Scheme 2). We chose to prepare the square-planar complexes [PtMe(3)(BN24C8)] and [PtMe(3)(N24C8)] as well as Scheme 1. [2]Rotaxanes such as [(1)(N24C8)] or [(1)(BN24C8)] (shown) with a single recognition site but containing a crown ether with two different aromatic groups (BN24C8= [24]crown-8 bearing naphtho and benzo units; N24C8= [24]crown-8 bearing only naphtho units). The crown ether undergoes a dynamic reorientation, reminiscent of a mechanical flip switch.

A [2]Rotaxane Flip Switch Driven by Coordination Geometry

Host–guest interactions template: the synthesis of a [3]catenane

[2]Rotaxanes based on the 1,2-bis(pyridinium)ethane⊂[24]crown-8 ether motif were prepared that contain a terminal terpyridine group for coordination to a transition-metal ion. These rotaxane ligands were utilized in the preparation of a series of heteroleptic [Ru(terpy)(terpy–rotaxane)]2+ complexes. The compounds were characterized by 1D and 2D 1H NMR spectroscopy, X-ray crystallography, and high-resolution electrospray ionization mass spectrometry. The effect of using a rotaxane as a ligand was probed by UV/Vis/NIR absorption and emission spectroscopy of the RuII complexes. In contrast with the parent [Ru(terpy)2]2+ complex, at room temperature the examined complexes exhibit a luminescence band in the near infrared region and a relatively long lived triplet metal-to-ligand charge-transfer (3MLCT) excited state, owing to the presence of strong-electron-acceptor pyridinium substituents on one of the two terpy ligands. Visible-light excitation of the Ru-based chromophore in acetonitrile at room temperature causes an electron transfer to the covalently linked 4,4′-bipyridinium unit and the quenching of the MLCT luminescence. The 3MLCT excited state, however, is not quenched at all in rigid matrix at 77 K. The rotaxane structure was found to affect the absorption and luminescence properties of the complexes. In particular, when a crown ether surrounds the cationic axle, the photoinduced electron-transfer process is slowed down by a factor from 2 to 3. Such features, together with the synthetic and structural advantages offered by [Ru(terpy)2]2+-type complexes compared to, for example, [Ru(bpy)3]2+-type compounds, render these rotaxane–metal complexes promising candidates for the construction of photochemical molecular devices with a wire-type structure.

Gregory J. E. Davidson

Papers

Stretchable light-emitting electrochemical cells using an elastomeric emissive material.

Channels and Cavities Lined with Interlocked Components: Metal‐Based Polyrotaxanes That Utilize Pyridinium Axles and Crown Ether Wheels as Ligands

A [2]Rotaxane Flip Switch Driven by Coordination Geometry

Host–guest interactions template: the synthesis of a [3]catenane

Wire‐Type Ruthenium(II) Complexes with Terpyridine‐Containing [2]Rotaxanes as Ligands: Synthesis, Characterization, and Photophysical Properties