Photoresponsive Dithienylethene-Containing Tris(8-hydroxyquinolinato)aluminum(III) Complexes with Photocontrollable Electron-Transporting Properties for Solution-Processable Optical and Organic Resistive Memory Devices

Recent Advances in Functional Materials for Optical Data Storage

In the current data age, the fundamental research related to optical applications has been rapidly developed. Countless new-born materials equipped with distinct optical properties have been widely explored, exhibiting tremendous values in practical applications. The optical data storage technique is one of the most significant topics of the optical applications, which is considered as the prominent solution for conquering the challenge of the explosive increase in mass data, to achieve the long-life, low-energy, and super high-capacity data storage. On this basis, our review outlines the representative reports for mainly introducing the functional systems based on the newly established materials applied in the optical storage field. According to the material categories, the representative functional systems are divided into rare-earth doped nanoparticles, graphene, and diarylethene. In terms of the difference of structural features and delicate properties among the three materials, the application in optical storage is comprehensively illustrated in the review. Meanwhile, the potential opportunities and critical challenges of optical storage are also discussed in detail.

Non-Conjugated Bis-(Dithienylethene)-Naphthalenediimide as a Dynamic Anti-Counterfeiting Agent: Driving the Wheel of Photoswitching Enactment

Photochromic fluorescent molecules dramatically extend their fields of applications ranging from optical memories, bioimaging, photoswitches, photonic devices, anti-counterfeiting technology and many more. Here, we have logically designed and synthesized a triazole appended bis-(dithienylethene)-naphthalenediimide based photo-responsive material, 5, which demonstrated fluorescence enhancement property upon photocyclization (ΦF =0.42), with high photocyclization (44 s, ksolution =0.0355 s-1 , ksolid =0.0135 s-1 ) and photocycloreversion (160 s, ksolution =0.0181 s-1 , ksolid =0.0085 s-1 ) rate and decent photoreaction quantum yield (Φo→c =0.93 and Φc→o =0.11). The open isomer almost converted to the closed isomer at photo-stationary state (PSS) with distinct color change from colorless to blue with 92.85 % conversion yield. A reversible noninvasive modulation of fluorescence through efficient photoinduced electron transfer (PET) process was observed both in solution as well as in solid state. The fluorescence modulation through PET process was further corroborated with thermodynamic calculations using the Rehm-Weller equation and quantum chemical studies (DFT). The thermally stable compound 5 exhibits high fatigue resistance property (up to 50 cycles) both in solution and solid state. Furthermore, the compound 5 was successfully applied as erasable ink and in deciphering secret codes (Quick Response/bar code) portending potential promising application in anti-counterfeiting.

Photochromic dinuclear iridium(III) complexes having phenoxyl-imidazolyl radical complex derivatives

We demonstrate that the phenoxyl-imidazolyl radical complex (PIC), which is a rate-tunable fast photoswitch, can be used as a ligand that directly coordinates with iridium (III) ions. The iridium complexes show the characteristic photochromic reactions originating from the PIC moiety, whereas the behaviour of transient species is substantially different from that of the PIC.

Photochromic Single-Component Organic Fulgide Ferroelectric with Photo-Triggered Polarization Response

Organic photochromic compounds have been widely investigated for optical memory storage and switches. Very recently, we pioneeringly discovered optical control of ferroelectric polarization switching in organic photochromic salicylaldehyde Schiff base and diarylethene derivatives, differently from the traditional ferroelectrics. However, the study of such intriguing photo-triggered ferroelectrics is still in its infancy and relatively scarce. In this manuscript, we synthesized a pair of new organic single-component fulgide isomers, (E and Z)-3-(1-(4-(tert-butyl)phenyl)ethylidene)-4-(propan-2-ylidene)dihydrofuran-2,5-dione (1E and 1Z). They undergo prominent photochromism from yellow to red. Interestingly, only polar 1E has been proven to be ferroelectric, while the centrosymmetric 1Z does not meet the basic requirement for ferroelectricity. Besides, experimental evidence shows that the Z-form can be converted to the E-form by light irradiation. More importantly, the ferroelectric domains of 1E can be manipulated by light in the absence of an electric field, benefiting from the remarkable photoisomerization. 1E also adopts good fatigue resistance to the photocyclization reaction. As far as we know, this is the first example of organic fulgide ferroelectric reported with photo-triggered ferroelectric polarization response. This work has developed a new system for studying photo-triggered ferroelectrics and would also provide an expected perspective on developing ferroelectrics for optical applications in trap future.

Tunable Photochromism of Spirooxazine in the Solid State: A New Design Strategy Based on the Hypochromic Effect

As an important organic photofunctional material, spirooxazine (SO) usually does not exhibit photochromism in the solid state since the intermolecular π-π stacking impedes photoisomerization. Developing photochromic SO in the solid state is crucial for practical applications but is still full of challenges. Here, a series of spirooxazine derivatives (SO1-SO4) with bulky aromatic substituents at the 4- and 7-positions of the skeleton, which provide them with a large volume with which to undergo solid-state photochromism under mild conditions, is designed and synthesized. All the compounds SO1-SO4 exhibit tunable solid photochromism without ground colors, excellent fatigue resistance, and high thermal stability. Notably, it takes only 15 s for SO4 to reach the saturation of absorption intensity, thought to represent the fastest solid-state photoresponse of spirooxazines. X-ray crystal structures of the intermediate compound SO0 and the products SO1-SO2 as well as computational studies suggest that the bulky aromatic groups can lead to a hypochromic effect, allowing for the photochromism of SO in the solid state. The ideal photochromic properties of these spirooxazines open a new avenue for their applications in UV printing, quick response code, and related fields.

Future-Oriented Advanced Diarylethene Photoswitches: From Molecular Design to Spontaneous Assembly Systems

Diarylethene (DAE) photoswitch is a new and promising family of photochromic molecules and has shown superior performance as a smart trigger in stimulus-responsive materials. During the past few decades, the DAE family has achieved a leap from simple molecules to functional molecules and developed toward validity as a universal switching building block. In recent years, the introduction of DAE into an assembly system has been an attractive strategy that enables the photochromic behavior of the building blocks to be manifested at the level of the entire system, beyond the DAE unit itself. This assembly-based strategy will bring many unexpected results that promote the design and manufacture of a new generation of advanced materials. Here, recent advances in the design and fabrication of diarylethene as a trigger in materials science, chemistry, and biomedicine are reviewed.

Metal-containing organic compounds for memory and data storage applications

With the upcoming trend of Big Data era, some new types of memory technologies have emerged as substitutes for the traditional Si-based semiconductor memory devices, which are encountering severe scaling down technical obstacles. In particular, the resistance random access memory (RRAM) and magnetic random access memory (MRAM) hold great promise for the in-memory computing, which are regarded as the optimal strategy and pathway to solve the von Neumann bottleneck by high-throughput in situ data processing. As far as the active materials in RRAM and MRAM are concerned, organic semiconducting materials have shown increasing application perspectives in memory devices due to their rich structural diversity and solution processability. With the introduction of metal elements into the backbone of molecules, some new properties and phenomena will emerge accordingly. Consequently, the RRAM and MRAM devices based on metal-containing organic compounds (including the small molecular metal complexes, metallopolymers, metal-organic frameworks (MOFs) and organic-inorganic-hybrid perovskites (OIHPs)) have been widely explored and attracted intense attention. In this review, we highlight the fundamentals of RRAM and MRAM, as well as the research progress of the applications of metal-containing organic compounds in both RRAM and MRAM. Finally, we discuss the challenges and future directions for the research of organic RRAM and MRAM.

Photocontrolled multiple-state photochromic benzo[b]phosphole thieno[3,2-b]phosphole-containing alkynylgold(I) complex via selective light irradiation

Photochromic materials have drawn growing attention because using light as a stimulus has been regarded as a convenient and environmental-friendly way to control properties of smart materials. While photoresponsive systems that are capable of showing multiple-state photochromism are attractive, the development of materials with such capabilities has remained a challenging task. Here we show that a benzo[b]phosphole thieno[3,2‑b]phosphole-containing alkynylgold(I) complex features multiple photoinduced color changes, in which the gold(I) metal center plays an important role in separating two photoactive units that leads to the suppression of intramolecular quenching processes of the excited states. More importantly, the exclusive photochemical reactivity of the thieno[3,2‑b]phosphole moiety of the gold(I) complex can be initiated upon photoirradiation of visible light. Stepwise photochromism of the gold(I) complex has been made possible, offering an effective strategy for the construction of multiple-state photochromic materials with multiple photocontrolled states to enhance the storage capacity of potential optical memory devices.

Catalyst-free synthesis of quinoline-enols through coupling between heterocyclic N-oxides and CF3-ynones under mild conditions

The coupling between heterocyclic N-oxides and CF3-ynones has been demonstrated via the formal C–H and C–C bond cleavage under catalyst-free and mild conditions.

Aggregation-Induced Emission Luminogens with Photoresponsive Behaviors for Biomedical Applications

Fluorescent biomedical materials can visualize subcellular structures and therapy processes in vivo. The aggregation-induced emission (AIE) phenomenon helps suppress the quenching effect in the aggregated state suffered by conventional fluorescent materials, thereby contributing to design strategies for fluorescent biomedical materials. Photoresponsive biomedical materials have attracted attention because of the inherent advantages of light; i.e., remote control, high spatial and temporal resolution, and environmentally friendly characteristics, and their combination with AIE facilitates development of fluorescent molecules with efficient photochemical reactions upon light irradiation. In this review, organic compounds with AIE features for biomedical applications and design strategies for photoresponsive AIE luminogens (AIEgens) are first summarized briefly. Applications are then reviewed, with the employment of photoresponsive and AIE-active molecules for photoactivation imaging, super-resolution imaging, light-induced drug delivery, photodynamic therapy with photochromic behavior, and bacterial targeting and killing being discussed at length. Finally, the future outlook for AIEgens is considered with the aim of stimulating innovative work for further development of this field.

On-Demand Regulation of Photoreversible Color Switching for Rewritable Paper and Transient Information Encryption

The achievement of photoreversible color switching systems (PCSS) has offered great opportunities for fundamental studies and practical applications. However, the development of PCSS that possessing highly reversible cyclability and on-demand regulation of recoloration process remains a grand challenge. Herein, we report a hydrazine-mediated self-doping strategy for the synthesis of alkaline Ti³⁺ self-doped TiO_2-x nanoparticles, enabling the TiO_2-x nanoparticles/methylene blue based PCSS with long photoreversible cyclability and rapid color switching rate. The Ti³⁺ species as internal sacrificial electron donors significantly improve the photoreductive activity of TiO_2-x nanoparticles, which results in fast decoloration rate and long cycling number of the PCSS. Simultaneously, the alkaline property of TiO_2-x nanoparticles enhances the oxidation kinetics of the PCSS to dramatically accelerate the recoloration rate. Moreover, the PCSS can be integrated elaborately with biodegradable agarose to form flexible color switching films, which exhibit long-waited on-demand regulation of recoloration rate in a wide range. By taking advantage of photoreversible color switching and time-resolved color changing process, we demonstrate their potential application in self-erasing rewritable paper and transient optical information encryption. This work represents a new strategy for the future development of PCSS and their advanced applications.

Memristors Based on an Iridium(III) Complex Containing Viologen for Advanced Synaptic Bionics

Memristors with nonvolatile memory properties are expected to open the era of neuromorphic computing. However, it remains a huge challenge to develop memristors with high uniformity, high stability, and low power consumption for advanced synaptic bionics. Herein, an electroactive iridium(III) complex Ir-vio was designed and synthesized by incorporating a viologen moiety into its N^∧N ligand. Complex Ir-vio showed multiple redox states and high sensitivity to an electrical stimulus. Importantly, two-terminal memristors with Ag/Ir-vio/W structure were successfully fabricated by the solution-processable method, which exhibited multilevel storage characteristics with a low switching threshold voltage of 0.5 V and high ON1/ON2/ON3/OFF current ratio of 10⁵/10³/10²/1 at a low reading bias of 0.05 V. Moreover, the memristors can mimic synaptic plasticity, indicating that they can act as artificial synapses to construct brain-inspired neural networks. The memristive mechanisms can be ascribed to the interconversion among different charge-transfer and redox states under various electrical stimulus. To the best of our knowledge, this work is the first experimental demonstration of memristors based on iridium(III) complexes, opening a new era for the development of synaptic bionic devices based on organometallic compounds.

Zinc-catalyzed C-H alkenylation of quinoline N-oxides with ynones: a new strategy towards quinoline-enol scaffolds

A zinc-catalyzed C-H alkenylation of quinoline N-oxides with ynones has been developed to rapidly assemble a broad collection of valuable quinoline-enol organic architectures. Uncommonly, this novel reaction involves C-H functionalization, and N-O, C-C and C[triple bond, length as m-dash]C bond cleavage in one operation, and leads exclusively to the formation of an enol rather than a keto product. Application of the enols generated was highlighted by further derivative transformation and preparation of a series of "BODIPY" analogues with high quantum yields (up to 86%).

Phosphorescent Soft Salt Based on Platinum(II) Complexes: Photophysics, Self-Assembly, Thermochromism, and Anti-counterfeiting Application

A new platinum(II) complex-based soft salt S1, ([Pt(tpp)(ed)]⁺[Pt(pba) (CN)₂]^-) (tpp = 2-(4-(trifluoromethyl)phenyl)pyridine, ed = ethane-1,2-diamine, pba = 4-(2-pyridyl)benzaldehyde), was designed and synthesized. UV-visible absorption and photoluminescence (PL) spectra were studied to elucidate the nature of ground and excited states. The soft salt complex was found to show self-assembly properties with the assistance of electrostatic, π-π stacking, and Pt···Pt interactions, resulting in the remarkable emergence of low-energy absorption and PL bands. Morphological transformation of S1 from undefined nanosized aggregates to nanofibers with different solvent compositions has been demonstrated. Interestingly, a luminescent polymer film was prepared by doping S1 into a polyethylene glycol matrix. The film displayed distinctive emission color change from yellow to red upon heating. Eventually, a high-level anti-counterfeiting application was accomplished using a time-resolved imaging technique based on the thermochromic luminescence property and long emission decay time displayed by S1. It is anticipated that this work can provide deep insights into the control of intermolecular interactions between cationic and anionic complexes of soft salt upon exposure to different external stimuli, resulting in the development of smart luminescent materials for various applications.

Synthesis, Structures, and Fluorescence Properties of Dimeric Aluminum Oxo Clusters

Aluminum is an important component for luminescence. However, the fluorescent aluminum complex with unambiguous structural information is still limited. Herein, we report a series of fluorescence aluminum oxo clusters (AlOCs). By introducing an additional coordination site to the aromatic conjugation ligand, cluster nuclearity increment and fluorescence variation are observed. Al₈(OH)₂(μ₄-O)₂(1-NA)₂(OEt)₁₆ (AlOC-41, 1-NA = 1-naphthoic acid, OEt = ethanol) is made up of two tetrahedral subunits. By introducing an additional coordination site to the aromatic conjugation ligand, we isolate a high nuclearity compound Al₁₀(μ₃-O)₂(3-HNA)₂(OEt)₂₂ (AlOC-47, 3-HNA = 3-hydroxy-2-naphthoic acid). Correspondingly, their luminescence performance is different (blue fluorescence in AlOC-41 and green in AlOC-47). Present herein is a platform to illustrate the relationship between synthesis, structure, and fluorescence properties.

Constructing Donor-Resonance-Donor Molecules for Acceptor-Free Bipolar Organic Semiconductors

Organic semiconductors with bipolar transporting character are highly attractive as they offer the possibility to achieve high optoelectronic performance in simple device structures. However, the continual efforts in preparing bipolar materials are focusing on donor-acceptor (D-A) architectures by introducing both electron-donating and electron-withdrawing units into one molecule in static molecular design principles. Here, we report a dynamic approach to construct bipolar materials using only electron-donating carbazoles connected by N-P=X resonance linkages in a donor-resonance-donor (D-r-D) structure. By facilitating the stimuli-responsive resonance variation, these D-r-D molecules exhibit extraordinary bipolar properties by positively charging one donor of carbazole in enantiotropic N+=P-X- canonical forms for electron transport without the involvement of any acceptors. With thus realized efficient and balanced charge transport, blue and deep-blue phosphorescent organic light emitting diodes hosted by these D-r-D molecules show high external quantum efficiencies up to 16.2% and 18.3% in vacuum-deposited and spin-coated devices, respectively. These results via the D-r-D molecular design strategy represent an important concept advance in constructing bipolar organic optoelectronic semiconductors dynamically for high-performance device applications.

Photo-modulated wide-spectrum chromism in Eu3+ and Eu3+/Tb3+ photochromic coordination polymer gels: application in decoding secret information

Photo-switching emission of photochromic materials has paramount importance in the field of optoelectronics. Here, we report synthesis and characterization of a dithienylethene (DTE) based photochromic low molecular weight gelator (LMWG) and self-assembly with lanthanide (Eu3+ and Tb3+) ions to form a photochromic coordination polymer gel (pcCPG). Based on DTE ring opening and closing, the TPY-DTE gel shuttles from pale-yellow coloured TPY-DTE-O to dark blue coloured TPY-DTE-C and vice versa upon irradiating with UV and visible light, respectively, and both the photoisomers show distinct optical properties. Furthermore, integration of Eu3+ and Tb3+ lanthanides with TPY-DTE resulted in red and green emissive Eu-pcCPG (Q.Y. = 18.7% for the open state) and Tb-pcCPG (Q.Y. = 23.4% for the open state), respectively. The photoisomers of Eu-pcCPG exhibit photo-switchable spherical to fibrous reversible morphology transformation. Importantly, an excellent spectral overlap of the Eu3+ centred emission and absorption of DTE in the closed form offered photo-switchable emission properties in Eu-pcCPG based on pcFRET (energy transfer efficiency >94%). Further, owing to the high processability and photo-switchable emission, the Eu-pcCPG has been utilized as invisible security ink for protecting confidential information. Interestingly, mixed Eu3+/Tb3+ pcCPG exhibited photo-modulated multi-spectrum chromism reversibly where the colour changes from yellow, blue, and red to green and vice versa under suitable light irradiation.

Synthesis, Photophysical, Photochromic, and Photomodulated Resistive Memory Studies of Dithienylethene-Containing Copper(I) Diimine Complexes

A series of dithienylethene-containing copper(I) diimine complexes have been synthesized and structurally characterized. Systematic studies on their photophysics, electrochemistry, and photochromism have been carried out. The photoinduced color changes of the copper(I) complexes have been achieved by photoexcitation into the metal-to-ligand charge-transfer (MLCT) absorption bands, indicating the photosensitization of light-induced cyclization by the ³MLCT excited state. In addition, by an increase in either the steric bulkiness around the copper(I) center or the structural rigidity of the complexes, the quantum efficiencies of photoluminescence and photocyclization can be effectively enhanced because of suppression of the flattening distortion of the complexes at the MLCT excited state. Furthermore, one of the complexes has been employed as an active component in the fabrication of solution-processed resistive memory devices. Notable lowering of the switching threshold voltage of the binary memory devices has been realized through photocyclization of the dithienylethene-containing copper(I) system.