A Molecular Platform for Multistate Near‐Infrared Electrochromism and Flip‐Flop,Flip‐Flap‐Flop,and Ternary Memory

A diruthenium complex with a redox‐active amine bridge has been designed, synthesized, and studied by single‐crystal X‐ray analysis and DFT and TDDFT calculations. It shows three well‐separated redox processes with exclusive near‐infrared (NIR) absorbance at each redox state. The electropolymerized film of a related vinyl‐functionalized complex displays multistate NIR electrochromism with low operational potential, good contrast ratio, and long retention time. Flip‐flop, flip‐flap‐flop, and ternary memories have been realized by using the obtained film (ca. 15–20 nm thick) with three electrochemical inputs and three NIR optical outputs that each displays three levels of signal intensity.     

Ruthenium‐Amine Conjugated Organometallic Materials for Multistate Near‐IR Electrochromism and Information Storage

Monometallic and dimetallic complexes with the ruthenium‐amine conjugated structural unit have been prepared. These complexes display consecutive redox waves with low potentials and rich and intense absorptions in the near‐infrared region. The electrochemical and spectroscopic properties can be modulated using substituents or auxiliary ligands with different electronic natures. Through simple functionalization, electropolymerized or monolayer thin films of these complexes have been prepared. These films display multistate near‐infrared electrochromism with good contrast ratios and long optical retention times. In addition, flip‐flop and flip‐flap‐flop memories have been demonstrated on the basis of these thin films.

     

Electrochromism and Nonvolatile Memory Device Derived from Triphenylamine‐Based Polyimides with Pendant Viologen Units

Three novel solution‐processable polyimides containing triphenylamine and pendant viologen moieties are prepared from the newly synthesized diamine and three commercially available dianhydrides. The thermally stable polyimide with strong donor–acceptor charge‐transfer possesses write‐once read‐many‐times memory behavior with excellent operation stability. The obtained multicolored electrochromic polymer films reveal ambipolar electrochemical behavior with high optical transmittance contrast of coloration changed from transmissive neutral state to the cyan/magenta/yellow redox states, implying great potential for application in smart window and displays.

     

Near‐Infrared Fluorescent Molecular Probe for Sensitive Imaging of Keloid

Early detection of skin diseases is imperative for their effective treatment. However, fluorescence molecular probes that allow this are rare. The first activatable near‐infrared (NIR) fluorescent molecular probe is reported for sensitive imaging of keloid cells, skin cells from abnormal scar fibrous lesions. As keloid cells have high expression levels of fibroblast activation protein‐alpha (FAPα), the probe (FNP1) is designed to have a caged NIR dye and a FAPα‐cleavable peptide substrate linked by a self‐immolative segment. FNP1 can quickly and specifically turn on its fluorescence at 710 nm by 45‐fold in the presence of FAPα, allowing it to effectively recognize keloid cells from normal skin cells. Integration of FNP1 with a simple microneedle‐assisted topical application enables sensitive detection of keloid cells in metabolically‐active human skin tissue with a theoretical limit of detection down to 20 000 cells.     

Ternary Flexible Electro‐resistive Memory Device based on Small Molecules

Flexible memory devices have continued to attract more attention due to the increasing requirement for miniaturization, flexibility, and portability for further electronic applications. However, all reported flexible memory devices have binary memory characteristics, which cannot meet the demand of ever‐growing information explosion. Organic resistive switching random access memory (RRAM) has plenty of advantages such as simple structure, facile processing, low power consumption, high packaging density, as well as the ability to store multiple states per bit (multilevel). In this study, we report a small molecule‐based flexible ternary memory device for the first time. The flexible device maintains its ternary memory behavior under different bending conditions and within 500 bending cycles. The length of the alkyl chains in the molecular backbone play a significant role in molecular stacking, thus guaranteeing satisfactory memory and mechanical properties.     

Studies on a Vinyl Ruthenium‐Modified Squaraine Dye: Multiple Visible/Near‐Infrared Absorbance Switching through Dye‐ and Substituent‐Based Redox Processes

The bis(vinyl ruthenium)‐modified squaraine dye 1 was synthesized by treatment of [RuHCl(CO)(PiPr₃)₂] with bis(ethynyl)‐substituted squaraine 8 . Spectroscopic and electrochemical measurements on 1 and its organic precursors 6 – 8 were performed to study the effect of the vinyl ruthenium “substituents,” particularly with respect to (poly)electrochromism. Attachment of the vinyl ruthenium moieties endows metal–organic squaraine 1 with two additional oxidation waves and lowers the first two oxidation potentials by approximately 300 mV with respect to its organic precursors. Squaraines 6 , 7 , 8 , and 1 strongly absorb at 648, 663, 656, or 709 nm. Although organic dyes 6 , 7 , and 8 fluoresce, no room‐temperature emission is observed for 1 . The radical cations and anions of 6 , 7 , 8 , and 1 as well as the doubly oxidized dications have been studied by IR and UV/Vis/NIR spectroelectrochemistry, and the ?/0/+/2+ redox sequences were found to be reversible in each case. Our results indicate that the 1^2?/?/0/+/2+ redox system constitutes a polyelectrochromic switch in which absorption in the visible or the near‐infrared range is reversibly turned off or shifted deep into the NIR. They also show that radical cation 1^.+ is an intrinsically delocalized system with only little contribution from the outer vinyl ruthenium tags to the oxidation process. Dication 1²⁺ constitutes a class‐II mixed‐valent system with two electronically different vinyl ruthenium moieties and has an open‐shell singlet electronic ground‐state structure. ESR and NMR spectra of chemically prepared 1^.+ and 1²⁺ corroborate these results. It has also emerged that reduction involves an orbital that is strongly delocalized across the entire squaraine π system and strongly affects the peripheral vinyl ruthenium sites.     

RuII Multinuclear Metallosupramolecular Rack‐Type Architectures of Polytopic Hydrazone‐Based Ligands: Synthesis,Structural Features,Absorption Spectra,Redox Behavior,and Near‐Infrared Luminescence

A novel class of polytopic hydrazone‐based ligands was synthesized. They gave heteroleptic Ru^II polynuclear rack‐like complexes of formula [Ru_nterpy_n(bridging molecular strand)]²ⁿ⁺ (terpy=2,2′:6′,2′′‐terpyridine). The new rack‐like systems can be viewed as being made of two identical or roughly identical peripheral subunits separated by several similar metal‐containing spacer subunits. The presence of pyrazine or pyrimidine units within the molecular multitopic strands introduces additional chemical diversity: whereas a pyrimidine unit leads to appended orthogonal subunits that are on the same side with regard to the main molecular strand, a pyrazine unit leads to orthogonal subunits that lie on different sides. Mixing pyrazine and pyrimidine units within the same (bridging) molecular strand also allows peculiar and topographically controlled geometries to be obtained. Redox studies provided evidence that each species undergoes reversible redox processes at mild potentials, which can be assigned to specific subunits of the multicomponent arrays. Non‐negligible electronic coupling takes place among the various subunits, and some electron delocalization extending over the overall bridging molecular strand takes place. In particular, oxidation data suggest that the systems can behave as p‐type “molecular wires” and reduction data indicate that n‐type electron conduction can occur within the multimetallic framework. All the multinuclear racks exhibit ³MLCT emission, both at 77 K in rigid matrix and at 298 K in fluid solution, which takes place in the near‐infrared region (emission maxima in the 1000–1100 nm region), and is quite structured. Rigidity of the molecular structures and delocalization within the large bridging ligands are proposed to contribute to the occurrence of the rather uncommon MLCT infrared emission, which is potentially interesting for optical communication devices.     

Ratiometric and Near‐Infrared Molecular Probes for the Detection and Imaging of Zinc Ions

The detection and imaging of Zn²⁺ in biological samples are of paramount interest owing to the role of this cation in physiological functions. This is possible only with molecular probes that specifically bind to Zn²⁺ and result in changes in emission properties. A “turn‐on” emission or shift in the emission color upon binding to Zn²⁺ should be ideal for in vivo imaging. In this context, ratiometric and near‐IR probes are of particular interest. Therefore, in the area of chemosensors or molecular probes, the design of fluorophores that allow ratiometric sensing or imaging in the near‐IR region is attracting the attention of chemists. The purpose of this Focus Review is to highlight recent developments in this area and stress the importance of further research for future applications.     

Metallo‐Supramolecular Gels that are Photocleavable with Visible and Near‐Infrared Irradiation

A photolabile ruthenium‐based complex, [Ru(bpy)₂(4AMP)₂](PF₆)₂, (4AMP=4‐(aminomethyl)pyridine) is incorporated into polyurea organo‐ and hydrogels via the reactive amine moieties on the photocleavable 4AMP ligands. While showing long‐term stability in the dark, cleavage of the pyridine–ruthenium bond upon irradiation with visible or near‐infrared irradiation (in a two‐photon process) leads to rapid de‐gelation of the supramolecular gels, thus enabling spatiotemporal micropatterning by photomasking or pulsed NIR‐laser irradiation     

Mussel‐Inspired Polydopamine Coating for Flexible Ternary Resistive Memory

In recent years, numerous organic molecules and polymers carrying various functional groups were synthesized and used in fabrication of wearable electronic devices. Compared to previous materials that suffer from poisonousness, stiffness and complex film fabrication, we circumvent above matters by taking advantage of mussel‐inspired polydopamine as our active material to realize resistive random access memories (RRAMs). Polydopamine thin films were grown on indium tin oxide glass catalyzed by Cu₂SO₄/H₂O₂ and characterized by Fourier infrared spectroscopy (FT‐IR), UV/Vis spectroscopy and scanning electron microscopy. The Al/Polydopamine film/ITO devices possess ternary memory behavior with good ternary device yield with two threshold voltages around 1.50 V and 3.50 V, long data retention over 10⁴ s of continuous reading or 10⁴ pulse reading. The two resistance switchings are attributed to defects functioning as charge traps and the formation of conductive filaments. A flexible device based on Al/polydopamine film/ITO/polyethylene terephthalate retains its ternary memory behavior after being bent with a bending radius of 1.54 cm and bending cycles up to 5000, demonstrating good compatibility and flexibility of polydopamine.     

Photoresponsive Molecular Memory Films Composed of Sequentially Assembled Heterolayers Containing Ruthenium Complexes

Photoresponsive molecular memory films were fabricated by a layer‐by‐layer (LbL) assembling of two dinuclear Ru complexes with tetrapodal phosphonate anchors, containing either 2,3,5,6‐tetra(2‐pyridyl)pyrazine or 1,2,4,5‐tetra(2‐pyridyl)benzene as a bridging ligand (Ru‐NP and Ru‐CP, respectively), using zirconium phosphonate to link the layers. Various types of multilayer homo‐ and heterostructures were constructed. In the multilayer heterofilms such as ITO||(Ru‐NP)_m|(Ru‐CP)_n, the difference in redox potentials between Ru‐NP and Ru‐CP layers was approximately 0.7 V, which induced a potential gradient determined by the sequence of the layers. In the ITO||(Ru‐NP)_m|(Ru‐CP)_n multilayer heterofilms, the direct electron transfer (ET) from the outer Ru‐CP layers to the ITO were observed to be blocked for m>2, and charge trapping in the outer Ru‐CP layers became evident from the appearance of an intervalence charge transfer (IVCT) band at 1140 nm from the formation of the mixed‐valent state of Ru‐CP units, resulting from the reductive ET mediation of the inner Ru‐NP layers. Therefore, the charging/discharging (“1”and “0”) states in the outer Ru‐CP layers could be addressed and interconverted by applying potential pulses between ?0.5 and +0.7 V. The two states could be read out by the direction of the photocurrent (anodic or cathodic). The molecular heterolayer films thus represent a typical example of a photoresponsive memory device; that is, the writing process may be achieved by the applied potential (?0.5 or +0.7 V), while the readout process is achieved by measuring the direction of the photocurrent (anodic or cathodic). Sequence‐sensitive multilayer heterofilms, using redox‐active complexes as building blocks, thus demonstrate great potential for the design of molecular functional devices.     

Metal‐Chelating N,N′‐Bis(4‐dimethylaminophenyl)acetamidinyl Radical: A New Chromophore for the Near‐Infrared Region

A new non‐innocent ligand redox system, N,N′‐bis(4‐dimethylaminophenyl) substituted acetamidinato/acetamidinyl, has been designed and described by example of structurally and spectroscopically characterized ruthenium complexes. The hitherto unreported ligand is responsible for rather intense and narrow absorptions in the near‐infrared region of the one‐ and two‐electron oxidized forms. The spectroscopic, computational, and first structural characterization of an amidinyl radical complex adds to the list of established N‐based radical ligands.     

Activatable Molecular Probes for Second Near‐Infrared Fluorescence,Chemiluminescence, and Photoacoustic Imaging

Optical imaging plays a crucial role in biomedicine. However, due to strong light scattering and autofluorescence in biological tissue between 650–900 nm, conventional optical imaging often has a poor signal‐to‐background ratio and shallow penetration depth, which limits its ability in deep‐tissue in vivo imaging. Second near‐infrared fluorescence, chemiluminescence, and photoacoustic imaging modalities mitigate these issues by their respective advantages of minimized light scattering, eliminated external excitation, and ultrasound detection. To enable disease detection, activatable molecular probes (AMPs) with the ability to change their second near‐infrared fluorescence, chemiluminescence, or photoacoustic signals in response to a biomarker have been developed. This Minireview summarizes the molecular design strategies, sensing mechanisms, and imaging applications of AMPs. The potential challenges and perspectives of AMPs in deep‐tissue imaging are also discussed.     

Molecular Design Principles for Near‐Infrared Absorbing and Emitting Indolizine Dyes

Desirable components for dye‐sensitzed solar cell (DSC) sensitizers and fluorescent imaging dyes include strong donating building blocks coupled with well‐balanced acceptor functionalities for absorption beyond the visible range. We have evaluated the effects of increasing acceptor strengths and incorporation of dye morphology controlling groups on molar absorptivity and absorption breadth with indolizine donor‐based dyes. Indolizine‐based D –A and D –π–A sensitizers incorporating bis‐rhodanine, tricyanofuran (TCF), and cyanoacrylic acid functionalities were analyzed for performance in DSC devices. The TCF derivatives were also evaluated as near‐infrared (NIR)‐emissive materials with the AH25 emissions extending past 1000 nm.     

Narrow‐Bandgap Chalcogenoviologens for Electrochromism and Visible‐Light‐Driven Hydrogen Evolution

A series of electron‐accepting chalcogen‐bridged viologens with narrow HOMO–LUMO bandgaps and low LUMO levels is reported. The optoelectronic properties of chalcogenoviologens can be readily tuned through heavy atom substitution (S, Se and Te). Herein, in situ electrochemical spectroscopy was performed on the proof‐of‐concept electrochromic devices (ECD). E‐BnV²⁺ (E=Se, Te; BnV²⁺=benzyl viologen) was used for the visible‐light‐driven hydrogen evolution due to the strong visible‐light absorption. Remarkably, E‐BnV²⁺ was not only used as a photosensitizer, but also as an electron mediator, providing a new strategy to explore photocatalysts. The higher apparent quantum yield of Se‐BnV²⁺ could be interpreted in terms of different energy levels, faster electron‐transfer rates and faster formation of radical species.     

A Multi‐Stimuli‐Responsive Oxazine Molecular Switch: A Strategy for the Design of Electrochromic Materials

A multi‐state and multi‐stimuli‐responsive oxazine molecular switch that combines an electro‐base property and sensitive base/acid‐responsive properties was designed and synthesized. The multi‐state structures of the molecular switch, with different colors, were predicted by comparing the optical properties with reference molecules and confirmed by using NMR spectroscopy. The color‐switching mechanism under stimulation with acids and bases was investigated by using DFT calculations. Three single states can be obtained and the switching is unidirectional under acid and base stimulation. The electrochromic phenomenon of the molecular switch, which combines its electro‐base and base‐sensitive properties, was demonstrated. An electrochromic device that exhibited good electrochromic properties with excellent reversibility (2000 cycles) and high coloration efficiency (804 cm² C^?1) was successfully constructed.