Recent advances in tunable solid-state emission based on α-cyanodiarylethenes: from molecular packing regulation to functional development.

The design and development of organic solid-state luminescent materials stand as crucial pillars within the realm of contemporary photofunctional materials. Overcoming challenges such as concentration quenching and achieving tailored luminescent properties necessitates a judicious approach to molecular structure design and the strategic utilization of diverse stimuli to modulate molecular packing patterns. Among the myriad candidates, α-cyanodiarylethenes (CAEs) emerge with distinctive solid-state luminescent attributes, capable of forming self-assembled packing structures with varying degrees of π-π stacking. This characteristic endows them with potential in the field of intelligent molecular responsive materials and optoelectronic devices. This tutorial review embarks on an exploration of design strategies geared towards attaining tunable solid-state emission through customized packing of CAEs. It explores the utilization of stimuli responses, including such as mechanical forces, light irradiation, solvent interactions, thermal influences, as well as the utilization of co-assembly methodologies. The overarching aim of this review is to provide a widely applicable platform fostering the flourishing development of modern organic photofunctional materials through integrating principles of molecular engineering, organic optoelectronics, and materials science.

Hierarchical Supramolecular Self-Assembly: Fabrication and Visualization of Multiblock Microstructures.

Due to the fast dynamics and re-equilibration of supramolecular self-assembly, bottom-up molecular strategies to fabricate well-defined and controllable multiblock structures are rare. Herein, we propose a new concept for fabrication of fluorescent multiblock microcolumns containing 1 to 7 blocks via hierarchical supramolecular self-assembly based on cucurbit[8]uril (CB[8]), NaBr and an AIEgen guest. Through the complexation between CB[8] and different numbers of AIEgen guests (2, 1, 0), the competitive displacement caused by the binding of the sodium cation to the CB[8] portal, and the reversible assembly of positively charged guests in salt solutions, one-pot hierarchical supramolecular self-assembly is realized. The molecular structure of each block is analyzed by single-crystal X-ray diffraction. The AIEgen enables the self-assembly of multiblocks to be visualized, understood, and regulated.

Cyclization-Promoted Ultralong Low-Temperature Phosphorescence via Boosting Intersystem Crossing.

Ultralong organic phosphorescence holds great promise as an important approach for optical materials and devices. Most of phosphorescent organic molecules with long lifetimes are substituted with heavy atoms or carbonyl groups to enhance the intersystem crossing (ISC), which requires complicated design and synthesis. Here, we report a cyclization-promoted phosphorescence phenomenon by boosting ISC. N-butyl carbazole exhibits a phosphorescence lifetime (τp) of only 1.45 ms and a low phosphorescence efficiency in the solution state at 77 K due to the lack of efficient ISC. In order to promote its phosphorescence behavior, we explored the influence of conjugation. By linear conjugation of four carbazole units, possible ISC channels are increased so that a longer τp of 2.24 s is observed. Moreover, by cyclization, the energy gap between the singlet and triplet states is dramatically decreased to 0.04 eV for excellent ISC efficiency accompanied by increased rigidification to synergistically suppress the nonradiative decay, resulting in satisfactory phosphorescence efficiency and a prolonged τp to 3.41 s in the absence of any heavy atom or carbonyl group, which may act as a strategy to prepare ultralong phosphorescent organic materials by enhancing the ISC and rigidification.

An Air-Stable Organic Radical from a Controllable Photoinduced Domino Reaction of a Hexa-aryl Substituted Anthracene.

Air-stable organic radicals and radical ions have attracted great attention for their far-reaching application ranging from bioimaging to organic electronics. However, because of the highly reactive nature of organic radicals, the design and synthesis of air-stable organic radicals still remains a challenge. Herein, an air-stable organic radical from a controllable photoinduced domino reaction of a hexa-aryl substituted anthracene is described. The domino reaction involves a photoinduced [4 + 2] cycloaddition reaction, rearrangement, photolysis, and an elimination reaction; 1H/13C NMR spectroscopy, high resolution mass spectrometry, single-crystal X-ray diffraction, and EPR spectroscopy were exploited for characterization. Furthermore, a photoinduced domino reaction mechanism is proposed according to the experimental and theoretical studies. In addition, the effects of employing push and pull electronic groups on the controllable photoinduced domino reaction were investigated. This article not only offers a new blue emitter and novel air-stable organic radical compound for potential application in organic semiconductor applications, but also provides a perspective for understanding the fundamentals of the reaction mechanism on going from anthracene to semiquinone in such anthracene systems.

Robust Supramolecular Nano-Tunnels Built from Molecular Bricks*.

Herein we report a linear ionic molecule that assembles into a supramolecular nano-tunnel structure through synergy of trident-type ionic interactions and π-π stacking interactions. The nano-tunnel crystal exhibits anisotropic guest adsorption behavior. The material shows good thermal stability and undergoes multi-stage single-crystal-to-single-crystal phase transformations to a nonporous structure on heating. The material exhibits a remarkable chemical stability under both acidic and basic conditions, which is rarely observed in supramolecular organic frameworks and is often related to structures with designed hydrogen-bonding interactions. Because of the high polarity of the tunnels, this molecular crystal also shows a large CO2 -adsorption capacity while excluding other gases at ambient temperature, leading to high CO2 /CH4 selectivity. Aggregation-induced emission of the molecules gives the bulk crystals vapochromic properties.

New Wine in Old Bottles: Prolonging Room-Temperature Phosphorescence of Crown Ethers by Supramolecular Interactions.

Supramolecular macrocyclic hosts have long been used in smart materials. However, their triplet emission and regulation at crystal level is rarely studied. Herein, ultralong and universal room-temperature phosphorescence (RTP) is reported for traditional crown ethers. A supramolecular strategy involving chain length adjustment and morphological locking through complexation with K+ was explored as a general method to tune the phosphorescence lifetime in the solid state. A maximum 10-fold increase of lifetime after complex formation accompanied with by invisible to visible phosphorescence was achieved. A deep encryption based on this activated RTP strategy was also facilely fabricated. This work thus opens a new world for supramolecular macrocycles and their intrinsic guest responsiveness offers a new avenue for versatile smart luminescent materials.

Endo- and Exo-Functionalized Tetraphenylethylene M12L24 Nanospheres: Fluorescence Emission inside a Confined Space.

The intrinsic relationship between the properties of green fluorescent protein (GFP) and its encapsulated small molecular light machine has spurred many biomimicking studies, aiming at revealing the detailed mechanism and further promoting its wide applications in different disciplines. However, how to build a similar confined microenvironment to mimic the cavity of a ß-barrel and the fluorescence turn-on process is a fundamental challenge for both chemists and biologists. Herein, two distinct exo- and endo-functionalized tetraphenylethylene (TPE)-based M12L24 nanospheres with precise distribution of anchored TPE moieties and unique photophysical properties were constructed by means of a coordination-driven self-assembly strategy. Under dilute conditions, the nanospheres fluoresce more strongly than the corresponding TPE subcomponents. Meanwhile, the endo-functionalized sphere is able to induce a higher local concentration and more restrained motion of the enclosed 24 TPE units compared with exo-functionalized counterpart and thus induces much stronger emission due to the restriction of the rotation of the pendant TPE units. The biomimetic methodology developed here represents a promising way to understand and construct artificial GFP materials on the platforms of supramolecular coordination complexes.

[2]Pseudorotaxane-Based Supramolecular Optical Indicator for the Visual Detection of Cellular Cyanide Excretion.

Cyanide is extremely hazardous to living organisms and the environment. Owing to its wide range of applications and high toxicity, the development of functional materials for cyanide detection and sensing is highly desirable. Host-guest complexation between bis(p-phenylene)-34-crown-10 H and N-methylacridinium salt G remarkably decreases the detection limit for cyanide anions compared with that of the guest itself. The [2]pseudorotaxane selectively recognizes the cyanide anion with high optical sensitivity as a result of the nucleophilic addition of the cyanide anion at the 9-position of G. The host-guest complexation is further incorporated into supramolecular materials for the visual detection of cyanide anions, especially the detection of cellular cyanide excretion with a detection limit of 0.6 µm. This supramolecular method provides an extremely distinct strategy for the visual detection of cyanide anions.

Multiple yet Controllable Photoswitching in a Single AIEgen System.

Seeking new methods to obtain elaborate artificial on-demand photoswitching with multiple functionalities remains challenging. Most of the systems reported so far possess only one specific function and their nonemissive nature in the aggregated state inevitably limit their applications. Herein, a tailored cyanostilbene-based molecule with aggregation-induced emission characteristic was synthesized and was found to exhibit efficient, multiple and controllable photoresponsive behaviors under different conditions. Specifically, three different reactions were involved: (i) reversible Z/E isomerization under room light and thermal treatment in CH3CN, (ii) UV-induced photocyclization with a concomitant dramatic fluorescence enhancement, and (iii) regio- and stereoselective photodimerization in aqueous medium with microcrystal formation. Experimental and theoretical analyses gave visible insights and detailed mechanisms of the photoreaction processes. Fluorescent 2D photopattern with enhanced signal-to-background ratio was fabricated based on the controllable "turn-on" and "turn-off" photobehaviors in different states. The present study thus paves an easy yet efficient way to construct smart multiphotochromes for unique applications.

Supramolecular polymers constructed by orthogonal self-assembly based on host-guest and metal-ligand interactions.

Supramolecular polymers constructed by orthogonal self-assembly based on host-guest and metal-ligand interactions are attracting increasing attention currently because of their interesting properties and potential applications. Host-guest interactions impart these polymers with good selectivity and convenient enviro-responsiveness, and metal-ligand interactions endow them with various coordination geometries, strong yet tunable coordination binding abilities, as well as magnetic, redox, photophysical, and electrochromic properties. Therefore, supramolecular polymers constructed by orthogonal host-guest and metal-ligand interactions have wide applications in the fields of soft matter, fluorescence sensing, heterocatalysis, electronics, gas storage, etc. In this critical review, we will address the recent development of supramolecular polymeric systems involving metal-ligand interactions and host-guest molecular recognition. Specifically, we classify the related supramolecular polymers depending on the types of macrocyclic hosts, and highlight their intriguing properties originating from the elegant combination of host-guest complexation and metal centers.

A discrete amphiphilic organoplatinum(II) metallacycle with tunable lower critical solution temperature behavior.

Oligo(ethylene glycol) (OEG)-decorated supramolecular assemblies are distinguished by their neutral character and macroscopic temperature-sensitive phase transition behavior. OEG functionalization is an emerging strategy to obtain thermoresponsive macrocyclic amphiphiles, although known methods organize the hydrophilic and hydrophobic segments by covalent bonding. Coordination-driven self-assembly offers an alternative route for organizing OEG-functionalized precursors into nanoscopic architectures, resulting in well-defined metallacycle cores surrounded by hydrophilic scaffolds to impart overall amphiphilic character. Here a tri(ethylene glycol)-functionalized thermosensitive amphiphilic metallacycle was prepared with high efficiency by means of the directional-bonding approach. The ensembles thus formed showed good lower critical solution temperature behavior with a highly sensitive phase separation and excellent reversibility. Moreover, the clouding point decreased with increasing metallacycle concentration and addition of K(+).

Responsive supramolecular polymer metallogel constructed by orthogonal coordination-driven self-assembly and host/guest interactions.

An emerging strategy for the fabrication of advanced supramolecular materials is the use of hierarchical self-assembly techniques wherein multiple orthogonal interactions between molecular precursors can produce new species with attractive properties. Herein, we unify the spontaneous formation of metal-ligand bonds with the host/guest chemistry of crown ethers to deliver a 3D supramolecular polymer network (SPN). Specifically, we have prepared a highly directional dipyridyl donor decorated with a benzo-21-crown-7 moiety that undergoes coordination-driven self-assembly with a complementary organoplatinum acceptor to furnish hexagonal metallacycles. These hexagons subsequently polymerize into a supramolecular network upon the addition of a bisammonium salt due to the formation of [2]pseudorotaxane linkages between the crown ether and ammonium moieties. At high concentrations, the resulting 3D SPN becomes a gel comprising many cross-linked metallohexagons. Notably, thermo- and cation-induced gel-sol transitions are found to be completely reversible, reflecting the dynamic and tunable nature of such supramolecular materials. As such, these results demonstrate the structural complexity that can be obtained when carefully controlling multiple interactions in a hierarchical fashion, in this case coordination and host/guest chemistry, and the interesting dynamic properties associated with the materials thus obtained.

A crown ether embedded responsive π-gelator for transition from a one-component gel to a two-component gel.

A novel fluorescent π-gelator, incorporating a crown ether with host-guest recognition capability and a photoactive cyanostilbene unit, was designed. This unique structure enables the successful transition from a one-component gel to a two-component gel and exhibits gel-sol transition behaviors under heat, ions, and light stimuli.

Molecular and Aggregate Synergistic Engineering of Aggregation-Induced Emission Luminogens to Manipulate Optical/Electronic Properties for Efficient and Diversified Functions.

The optical/electronic properties of organic luminescent materials can be regulated by molecular structure modification, which not only requires sophisticated and time-consuming synthesis but also is unable to accurately afford the optical properties of materials in the aggregate state. Herein, a facile strategy of molecular and aggregate synergistic engineering is proposed to manipulate the optical/electronic properties of a luminogen, ACIK, in the solid state for efficient and diversified functions. ACIK is facilely synthesized and exhibits three polymorphic states (ACIK-Y, ACIK-R, and ACIK-N) with a large emission difference of 102 nm from yellow to near-infrared (NIR). Their structure-property relationships were investigated by crystallographic analyses and computational studies. ACIK-Y, with the most twisted structure, exhibits an intriguing color-tuned fluorescence between yellow and NIR in the solid state in response to multiple stimuli. Shuttle-like ACIK-R microcrystals exhibit an optical waveguide property with a low optical loss coefficient of 19 dB mm-1. ACIK dots display bright NIR-I emission, large Stokes shift, and strong NIR-II two-photon absorption. ACIK dots show specific lipid droplets-targeting capability and can be successfully applied for two-photon fluorescence imaging of mouse brain vasculature with deep penetration and high spatial resolution. This study will inspire more insights in developing advanced optical/electronic materials based on a single chromophore for practical applications.

Solvent-Triggered Fast and Visible Switching between Cage- and Channel-Type Hydrogen-Bonded Organic Frameworks.

The inherent weak bonding nature of hydrogen-bonded organic frameworks (HOFs) performs like a double-edged sword in that it endows HOFs with superiority in processability and dynamicity but deactivates its on-demand controllability in the crystalline phase. Herein, based on the synergy of dynamic H-bonding interactions and the tailored low solubility in common organic solvents, reversible and fast topological transitions between cage- and channel-type HOFs were achieved upon immersing in the solution state. The aggregation-induced-emission character of the tecton facilitates the visualization of the elusive initial transition process with high sensitivity. In addition, the visible transition from cage- and channel-type HOFs to thermally stable crystalline phases is also achieved under thermal induction.

Advances in nanosensors for cardiovascular disease detection.

Cardiovascular diseases (CVDs) are a leading cause of morbidity and mortality around the world. The physiological or pathological processes of CVDs can be well indicated by timely and accurate diagnosis of relevant biomarkers and function parameters. Nanosensors integrating the advantages of nanomaterials and sensing platforms have shown good potential for rapid diagnosis of CVDs, especially for early prediction. In this review, recent advances in nanosensors for the detection of CVDs are summarized, including electrochemical, optical, pressure, and paper-based nanosensors. Design strategies for different nanosensors and the corresponding sensing nanomaterials, mechanisms, and properties are briefly discussed. This review also offered a preliminary analysis of the obstacles and prospects for using nanosensors to diagnose CVDs.

Intra- and Intermolecular Synergistic Engineering of Aggregation-Induced Emission Luminogens to Boost Three-Photon Absorption for Through-Skull Brain Imaging.

Three-photon fluorescence microscopic (3PFM) bioimaging is a promising imaging technique for visualizing the brain in its native environment thanks to its advantages of high spatial resolution and large imaging depth. However, developing fluorophores with strong three-photon absorption (3PA) and bright emission that meets the requirements for efficient three-photon fluorescence microscopic (3PFM) bioimaging is still challenging. Herein, four bright fluorophores with aggregation-induced emission features are facilely synthesized, and their powders exhibit high quantum yields of up to 56.4%. The intramolecular engineering of luminogens endows (E)-2-(benzo[d]thiazol-2-yl)-3-(7-(diphenylamino)-9-ethyl-9H-carbazol-2-yl)acrylonitrile (DCBT) molecules with bright near-infrared emission and large 3PA cross sections of up to 1.57 × 10-78 cm6 s2 photon-2 at 1550 nm, which is boosted by 3.6-fold to 5.61 × 10-78 cm6 s2 photon-2 in DCBT dots benefiting from the extensive intermolecular interactions in molecular stacking. DCBT dots are successfully applied for 3PFM imaging of brain vasculature on mice with a removed or intact skull, providing images with high spatial resolution, and even small capillaries can be recognized below the skull. This study will inspire more insights for developing advanced multiphoton absorbing materials for biomedical applications.

Multiple yet switchable hydrogen-bonded organic frameworks with white-light emission.

The development of new strategies to construct on-demand porous lattice frameworks from simple motifs is desirable. However, mitigating complexity while combing multiplicity and reversibility in the porous architectures is a challenging task. Herein, based on the synergy of dynamic intermolecular interactions and flexible molecular conformation of a simple cyano-modified tetraphenylethylene tecton, eleven kinetic-stable hydrogen-bonded organic frameworks (HOFs) with various shapes and two thermo-stable non-porous structures with rare perpendicular conformation are obtained. Multimode reversible structural transformations along with visible fluorescence output between porous and non-porous or between different porous forms is realized under different external stimuli. Furthermore, the collaborative of flexible framework and soft long-chain guests facilitate the relaxation from intrinsic blue emission to yellow emission in the excited state, which represents a strategy for generating white-light emission. The dynamic intermolecular interactions, facilitated by flexible molecular conformation and soft guests, diversifies the strategies of construction of versatile smart molecular frameworks.

An unexpected non-conjugated AIEgen with a discrete dimer for pure intermolecular through-space charge transfer emission.

Manipulation of the charge transfer in donor-acceptor-type molecules is essential for the design of controllable aggregate luminescent materials. Apart from the traditional through-bond charge transfer (TBCT) systems which suffer from complicated structural design, poor tunability and low quantum efficiency, through-space charge transfer (TSCT) has been proved as an alternative yet facile strategy in tuning photophysical processes. In this work, by simply changing nucleophilic reaction bases, a traditional conjugated acrylonitrile AP1 and an unexpected non-conjugated AP2 with a carboxamide-functionalized oxirane linker could be obtained. The long-range π-π stacking in conjugated AP1 results in mixed intramolecular TBCT plus intermolecular TSCT emission. However, facilitated by the steric hindrance effect of the big oxirane connector and the unique discrete dimer packing, non-conjugated AP2 exhibits pure and efficient intermolecular TSCT emission in both aggregate and crystalline states. The flexibility of the non-conjugated character further leads to better reversible stimuli-responsiveness to mechanical force for AP2 than for the rigid AP1.

Biologically Excretable Aggregation-Induced Emission Dots for Visualizing Through the Marmosets Intravitally: Horizons in Future Clinical Nanomedicine.

Superb reliability and biocompatibility equip aggregation-induced emission (AIE) dots with tremendous potential for fluorescence bioimaging. However, there is still a chronic lack of design instructions of excretable and bright AIE emitters. Here, a kind of PEGylated AIE (OTPA-BBT) dots with strong absorption and extremely high second near-infrared region (NIR-II) PLQY of 13.6% is designed, and a long-aliphatic-chain design blueprint contributing to their excretion from an animal's body is proposed. Assisted by the OTPA-BBT dots with bright fluorescence beyond 1100 nm and even 1500 nm (NIR-IIb), large-depth cerebral vasculature (beyond 600 µm) as well as real-time blood flow are monitored through a thinned skull, and noninvasive NIR-IIb imaging with rich high-spatial-frequency information gives a precise presentation of gastrointestinal tract in marmosets. Importantly, after intravenous or oral administration, the definite excretion of OTPA-BBT dots from the body is demonstrated, which provides influential evidence of biosafety.