Regiospecific N-Heteroarylation of Amidines for Full-Color-Tunable Boron Difluoride Dyes with Mechanochromic Luminescence

Stimuli-fluorochromic smart organic materials

Smart materials based on stimuli-fluorochromic π-conjugated solids (SFCSs) have aroused significant interest due to their versatile and exciting properties, leading to advanced applications. In this review, we highlight the recent developments in SFCS-based smart materials, expanding beyond organometallic compounds and light-responsive organic luminescent materials, with a discussion on the design strategies, exciting properties and stimuli-fluorochromic mechanisms along with their potential applications in the exciting fields of encryption, sensors, data storage, display, green printing, etc. The review comprehensively covers single-component and multi-component SFCSs as well as their stimuli-fluorochromic behaviors under external stimuli. We also provide insights into current achievements, limitations, and major challenges as well as future opportunities, aiming to inspire further investigation in this field in the near future. We expect this review to inspire more innovative research on SFCSs and their advanced applications so as to promote further development of smart materials and devices.

Recent advances in the construction of tetracoordinate boron compounds

Tetracoordinate boron compounds are a highly important class of molecules, which are the key intermediates in many organoboron-related chemical transformations and have unique luminescence properties. However, the synthesis of tetracoordinate boron compounds has never been reviewed. In this highlight, we summarize recent progress on the construction of racemic and chiral tetracoordinate borons, and hope to provide ideas for the assembly of them in more efficient ways, especially for the construction of boron-stereogenic compounds.

Thiazole Boron Difluoride Dyes with Large Stokes Shift, Solid State Emission and Room-Temperature Phosphorescence

The small Stokes shift and weak emission in the solid state are two main shortcomings associated with the boron-dipyrromethene (BODIPY) family of dyes. This study presents the design, synthesis and luminescent properties of boron difluoro complexes of 2-aryl-5-alkylamino-4-alkylaminocarbonylthiazoles. These dyes display Stokes shifts (Δλ, 77-101 nm) with quantum yields (ϕ_FL ) up to 64.9 and 34.7 % in toluene solution and in solid state, respectively. Some of these compounds exhibit dual fluorescence and room-temperature phosphorescence (RTP) emission properties with modulable phosphorescence quantum yields (ϕ_PL ) and lifetime (τ_p up to 251 μs). The presence of intramolecular H-bonds and negligible π-π stacking revealed by X-ray crystal structure might account for the observed large Stokes shift and significant solid-state emission of these fluorophores, while the enhanced spin-orbit coupling (SOC) of iodine and the self-assembly driven by halogen bonding, π-π and C-H^… π interactions could be responsible for the observed RTP of iodine containing phosphors.

Printable Off-On Thermoswitchable Fluorescent Materials for Programmable Thermally Controlled Full-Color Displays and Multiple Encryption

Thermoswitchable fluorescent materials (TFMs) have received special attention due to their unique fluorescent colorimetric responses to temperature. Conventional TFMs generally display unicolor with switching from one color to another, showing unprintable and unsatisfied performances. These limitations greatly hinder their development and expansion toward advanced applications. Herein, the superior integration of full-color, off-on switching mode, printability, and high performance to TFMs is achieved successfully. The success is due to a thermally induced synchronous "dual/multichannel" stimulus-response mode regulated by a self-crystalline phase-change material; that is, synergistic changes of the molecular existence states and subsequent colors/spectra of the fluorescent modifier and fluorophores, accompanied by corresponding high-efficiency on-off switching of Förster resonance energy transfer. These TFMs are simple to prepare and show good performance, such as high fluorescence emission contrast (>100), great reversibility (>200 cycles), and easy-to-adjust response temperature. Particularly, these R/G/B TFMs can be prepared as tricolor fluorescent inks, and thus full-color emissions on flexible substrate can be easily obtained by printing. Finally, their great potential in switchable dynamic interior decoration, programmatic temperature-control information display, and senior information encryption are illustrated. This successful exploration offers a new perspective for designing and optimizing various other switchable materials with higher comprehensive performances.

Modular Synthesis of Pentagonal and Hexagonal Ring-Fused NBN-Phenalenes Leading to an Excited-State Aromatization-Induced Structural Planarization Molecular Library

Although polycyclic aromatic hydrocarbons (PAHs) with a nitrogen-boron-nitrogen (NBN) moiety have recently attracted tremendous interest due to their intriguing electronic and optoelectronic properties, all of the NBN-fused π-systems reported to date are called NBN-dibenzophenalenes and were synthesized by electrophilic aromatic substitution. The synthesis of NBN-phenalenes remains challenging, and transition-metal catalysis has never been utilized to construct NBN-embedded π-scaffolds. Herein, a palladium-catalyzed cyclization/bicyclization strategy was developed for the synthesis of diverse pentagonal and hexagonal ring-fused NBN-phenalenes and half-NBN-phenalenes. All of the NBN-embedded π-scaffolds presented in our paper are fluorescent in both solution and the solid state. Further investigations showed that the five-membered NBN rings exhibit the properties of traditional luminogens, while those with a six-membered NBN ring generally undergo photoinduced structural planarization (PISP) and exhibit different colors and quantum yields of fluorescence with different concentrations in solution. Time-resolved spectroscopy and TD-DFT calculations revealed that excited-state aromatization is the driving force for PISP in hexagonal ring-fused NBN-π systems, leading to the formation of excimers. Notably, the scope of PISP compounds is still quite limited, and PISP has never been observed in NBN-π systems before. These hexagonal ring-fused NBN-π systems constitute a novel PISP molecular library and appear to be a new class of aggregation-induced excimer emission (AIEE) materials. Finally, the AIEE behavior of these six-membered NBN rings was applied to the detection of nitro explosives, achieving excellent sensitivity. In general, this work provides a new viewpoint for synthesizing NBN-fused π-systems and understanding the excited-state motion of luminogens.

Labelling primary immune cells using bright blue fluorescent nanoparticles

Tracking cell movements is an important aspect of many biological studies. Reagents for cell tracking must not alter the biological state of the cell and must be bright enough to be visualized above background autofluorescence, a particular concern when imaging in tissue. Currently there are few reagents compatible with standard UV excitation filter sets (e.g. DAPI) that fulfill those requirements, despite the development of many dyes optimized for violet excitation (405 nm). A family of boron-based fluorescent dyes, difluoroboron β-diketonates, has previously served as bio-imaging reagents with UV excitation, offering high quantum yields and wide excitation peaks. In this study, we investigated the use of one such dye as a potential cell tracking reagent. A library of difluoroboron dibenzoylmethane (BF2dbm) conjugates were synthesized with biocompatible polymers including: poly(l-lactic acid) (PLLA), poly(ε-caprolactone) (PCL), and block copolymers with poly(ethylene glycol) (PEG). Dye-polymer conjugates were fabricated into nanoparticles, which were stable for a week at 37 °C in water and cell culture media, but quickly aggregated in saline. Nanoparticles were used to label primary splenocytes; phagocytic cell types were more effectively labelled. Labelling with nanoparticles did not affect cellular viability, nor basic immune responses. Labelled cells were more easily distinguished when imaged on a live tissue background than those labelled with a commercially available UV-excitable cytoplasmic labelling reagent. The high efficiency in terms of both fluorescence and cellular labelling may allow these nanoparticles to act as a short-term cell labelling strategy while wide excitation peaks offer utility across imaging and analysis platforms.

Density Functional Studies on Photophysical Properties of Boron-Pyridyl-Imino-Isoindoline Dyes: Effect of the Fusion

In this work, to make out the aryl-fusion effect on the photophysical properties of boron-pyridyl-imino-isoindoline dyes, compounds 1-5 were theoretically studied through analyses of their geometric and electronic structures, optical properties, transport abilities, and radiative (k r) and non-radiative decay rate (k nr) constants. The highest occupied molecular orbitals of aryl-fused compounds 2-5 are higher owing to the extended conjugation. Interestingly, aryl fusion in pyridyl increases the lowest unoccupied molecular orbital (LUMO) level, while isoindoline decreases the LUMO level; thus, 4 and 5 with aryl fusion both in pyridyl and isoindoline exhibit a similar LUMO to 1. Compounds 4 and 5 show relatively low ionization potentials and high electron affinities, suggesting a better ability to inject holes and electrons. Importantly, the aryl fusion is conducive to the decrease of k IC. The designed compound 5 exhibits a red-shifted emission maximum, low λh, and low k IC, which endow it with great potential for applications in organic electronics. Our investigation provides an in-depth understanding of the aryl-fusion effect on boron-pyridyl-imino-isoindoline dyes at molecular levels and demonstrates that it is achievable.

Characterization and Photophysical Properties of a Luminescent Aluminum Hydride Complex Supported by a β-Diketiminate Ligand

Aluminum hydrides are versatile compounds utilized as reducing agents, precursors of aluminum complexes, and as catalysts for polymerization reactions. However, their photophysical properties have been overlooked, although several luminescent aluminum complexes have been utilized conventionally for emitting layers in modern light-emitting devices. Herein, we report the synthesis and photophysical properties of a luminescent β-diketiminate dihydride complex through the reaction between lithium aluminum hydride and the corresponding ligand. The obtained compound exhibits crystallization-induced emission (CIE) properties at room temperature and long-lifetime phosphorescence at 80 K. Our experimental and theoretical investigations suggest that low-energy molecular vibration could play an important role in the realization of the CIE property.

Highly selective electrochemical hydrogenation of alkynes: Rapid construction of mechanochromic materials

Electrochemical hydrogenation has emerged as an environmentally benign and operationally simple alternative to traditional catalytic reduction of organic compounds. Here, we have disclosed for the first time the electrochemical hydrogenation of alkynes to a library of synthetically important Z-alkenes under mild conditions with great selectivity and efficiency. The deuterium and control experiments of electrochemical hydrogenation suggest that the hydrogen source comes from the solvent, supporting electrolyte, and base. The scanning electron microscopy and x-ray diffraction experiments demonstrate that palladium nanoparticles generated in the electrochemical reaction act as a chemisorbed hydrogen carrier. Moreover, complete reduction of alkynes to saturated alkanes can be achieved through slightly modified conditions. Furthermore, a series of novel mechanofluorochromic materials have been efficiently constructed with this protocol that showed blue-shifted mechanochromism. This discovery represents the first example of cis-olefins-based organic mechanochromic materials.

Amphiphilic Gemini Iridium(III) Complex as a Mitochondria-Targeted Theranostic Agent for Tumor Imaging and Photodynamic Therapy

Clinical diagnostics and therapeutics of tumors are significantly benefitted by the development of multifunctional theranostic agents, which integrate tumor targeting, imaging, and therapeutics. However, the integration of imaging and therapy functionalities to a unimolecular framework remains a great challenge. Herein, a family of amphiphilic gemini iridium(III) complexes (GIC), Ir1-Ir6, are synthesized and characterized. The presence of quaternary ammonium (QA) groups endows GIC with adjustable water solubility and excellent self-assembly properties. Spectroscopic and computational results reveal that introducing QA groups into cyclometalating ligands (ĈN ligands) can overcome the drawback of aggregation-caused emission quenching and ensure Ir1-Ir3 with high emission intensity and excellent singlet oxygen (¹O₂) generation ability in aqueous media. Cell-based assays indicate that Ir3 shows higher cellular uptake efficiency and localizes specifically in the mitochondria, as well as exhibits outstanding photostability and an impressive phototoxicity index with satisfactory performance in mitochondria-targeted imaging and photodynamic therapy (PDT) of tumor cells. Furthermore, in vivo studies further prove that Ir3 possesses excellent antitumor activity and remarkably inhibits the growth of the HepG2 cells under PDT treatment. Consequently, this study presents a promising strategy for designing clinical application potential multifunctional iridium complex theranostic agents for mitochondria-targeted imaging and PDT in a single molecular framework.

Boron-based stimuli responsive materials

Representative types of boron-based molecular systems that respond to external stimuli such as temperature, pressure, light, or chemicals (oxygen, acid, base etc.) are described in this review article. The boron molecules are classified according to their operating mechanisms, with emphasis on systems, which are based on switchable boron-donor bonds and switchable excited states.

Multicolor Emission from Non-conjugated Polymers Based on a Single Switchable Boron Chromophore

Multicolor emissive and responsive materials are highly attractive owing to their potential applications in various fields, and polymers are preferred for their good processability and high stability. Herein, we report a series of new polymers based on a methacrylate monomer containing a switchable boron chromophore. In spite of their unconjugated nature, interestingly, the homopolymers from this monomer display rare multicolor fluorescence in solution that is highly dependent on the degree of polymerization (DP). With an increasing DP, the local concentration of the chromophore increases, leading to a higher propensity for switching the blue-emitting tricoordinate boron chromophore to the red-emitting tetracoordinate one. The homopolymers also display temperature- and solvent-dependent emission color change. Furthermore, pure white-light emission could be achieved in various solvents by precisely tuning the homopolymer molecular weight, or in films/solid state by copolymerizing the emissive boron monomer with non-emissive monomers in an appropriate ratio.

Dynamic Coordination of Eu-Iminodiacetate to Control Fluorochromic Response of Polymer Hydrogels to Multistimuli

New fluorochromic materials that reversibly change their emission properties in response to their environment are of interest for the development of sensors and light-emitting materials. A new design of Eu-containing polymer hydrogels showing fast self-healing and tunable fluorochromic properties in response to five different stimuli, including pH, temperature, metal ions, sonication, and force, is reported. The polymer hydrogels are fabricated using Eu-iminodiacetate (IDA) coordination in a hydrophilic poly(N,N-dimethylacrylamide) matrix. Dynamic metal-ligand coordination allows reversible formation and disruption of hydrogel networks under various stimuli which makes hydrogels self-healable and injectable. Such hydrogels show interesting switchable ON/OFF luminescence along with the sol-gel transition through the reversible formation and dissociation of Eu-IDA complexes upon various stimuli. It is demonstrated that Eu-containing hydrogels display fast and reversible mechanochromic response as well in hydrogels having interpenetrating polymer network. Those multistimuli responsive fluorochromic hydrogels illustrate a new pathway to make smart optical materials, particularly for biological sensors where multistimuli response is required.

AIE-active boron complexes based on benzothiazole–hydrazone chelates

A new family of benzothiazole–hydrazone-based monoboron and bisboron complexes with aggregation-induced emission (AIE) was developed.

Smart Luminescent Coordination Polymers toward Multimode Logic Gates: Time-Resolved, Tribochromic and Excitation-Dependent Fluorescence/Phosphorescence Emission

In this work, we propose that lanthanide cations (such as Eu³⁺ and Tb³⁺)-doped long-afterglow coordination polymers (CPs) can be an effective tool for designing multimode optical logic gates based on their tunable fluorescence/phosphorescence transformation and state-dependent emission. First, multicolor and white-light luminescence across the blue/green/yellow/red visible regions can be obtained by balancing the co-doping ratio of Eu³⁺/Tb³⁺ cations and suitable excitations. Additionally, a new tribochromic Eu-Cd-CP was developed based on the mechanism of a change in structural symmetry. Benefitting from long-afterglow, tribochromism, and excitation-dependent emission on the same luminescent CP, a new three-input and three-output logic gate was obtained. Therefore, this work not only provides detailed insights into the interesting fields of tribochromism and tunable photoemission, but also confirms that long-afterglow CPs can serve as a new platform for the construction of smart luminescent systems and multimode optical logic gates.

Optical properties and mechanofluorochromism of new BODIPY dyes based on the pyridine–pyrimidine hybrid structure

New solution/solid-state emissive boron complexes were prepared to demonstrate that high contrast and reversible mechanoresponsive luminescence behavior could be realized through mechanical stimuli triggered dimer dissolution/formation.

Copper(ii)-promoted oxidative C–H/C–H cross-coupling for rapid access to aza-BODIPY-indole derivatives with broad optical absorption

The absorption band of aza-BODIPY is remarkably broadened by coupling with indoles via a concise and inexpensive approach.

Synthesis, computational, and spectroscopic analysis of tunable highly fluorescent BN-1,2-azaborine derivatives containing the N-BOH moiety

Methods to tune the luminescence wavelength and the quantum yields by controlling the power and location of electron-donor and acceptor substituents on the ring system.

Synthesis and fluorescence properties of novel squarylium–boron complexes

Novel squarylium–boron complexes showed a significantly enhanced fluorescence quantum yield compared to the corresponding uncomplexed squarylium dyes.

Applications of Palladium-Catalyzed C-N Cross-Coupling Reactions

Pd-catalyzed cross-coupling reactions that form C–N bonds have become useful methods to synthesize anilines and aniline derivatives, an important class of compounds throughout chemical research. A key factor in the widespread adoption of these methods has been the continued development of reliable and versatile catalysts that function under operationally simple, user-friendly conditions. This review provides an overview of Pd-catalyzed N-arylation reactions found in both basic and applied chemical research from 2008 to the present. Selected examples of C–N cross-coupling reactions between nine classes of nitrogen-based coupling partners and (pseudo)aryl halides are described for the synthesis of heterocycles, medicinally relevant compounds, natural products, organic materials, and catalysts.

Highly emissive organic solids with remarkably broad color tunability based on N,C-chelate, four-coordinate organoborons

Molecular fluorophores based on N,C-chelate, four-coordinate organoborons exhibit tunable solid-state emission colors that cover the whole visible region from blue to red. The emission color can be tuned through the substituents on either quinolines or the boron center.

Mechanoresponsive Luminescent Molecular Assemblies: An Emerging Class of Materials

The possibility to change the molecular assembled structures of organic and organometallic materials through mechanical stimulation is emerging as a general and powerful concept for the design of functional materials. In particular, the photophysical properties such as photoluminescence color, quantum yield, and emission lifetime of organic and organometallic fluorophores can significantly depend on the molecular packing, enabling the development of molecular materials with mechanoresponsive luminescence characteristics. Indeed, an increasing number of studies have shown in recent years that mechanical force can be utilized to change the molecular arrangement, and thereby the optical response, of luminescent molecular assemblies of π-conjugated organic or organometallic molecules. Here, the development of such mechanoresponsive luminescent (MRL) molecular assemblies consisting of organic or organometallic molecules is reviewed and emerging trends in this research field are summarized. After a brief introduction of mechanoresponsive luminescence observed in molecular assemblies, the concept of "luminescent molecular domino" is introduced, before molecular materials that show turn-on/off of photoluminescence in response to mechanical stimulation are reviewed. Mechanically stimulated multicolor changes and water-soluble MRL materials are also highlighted and approaches that combine the concept of MRL molecular assemblies with other materials types are presented in the last part of this progress report.

A boron difluoride dye showing the aggregation-induced emission feature and high sensitivity to intra- and extra-cellular pH changes

A novel AIE-active boron difluoride fluorescent probe P3T was designed and synthesized. P3T exhibited high sensitivity to intra- and extra-cellular pH changes. Furthermore, a Förster resonance energy transfer (FRET) system was constructed.

Blue highly fluorescent boron difluoride complexes based on phthalazine–pyridine

Synthesis and characterization of three blue fluorescent phthalazine–pyridine boron complexes with quantum yields up to 79%.

Strategy to enhance solid-state fluorescence and aggregation-induced emission enhancement effect in pyrimidine boron complexes

The synthesis and the solution/solid-state fluorescence properties of pyrimidine-based monoboron complexes differing in terms of the substituents [either two fluorine atoms (BF2 complex) or two phenyl groups (BPh2 complex)] on the boron atom are reported herein. Unrestricted C-Ar intramolecular rotation in the non-, trifluoromethyl-, and cyano-substituted derivatives resulted in negligible fluorescence in solution. On the other hand, methoxy- and dimethylamino-substituted analogues caused the restriction of the C-Ar intramolecular rotation and consequently resulted in relatively strong fluorescence in solution. The non-, trifluoromethyl-, and cyano-substituted derivatives showed a pronounced aggregation-induced emission enhancement effect. Dimethylamino-substituted derivatives exhibited solvatochromism in the fluorescence spectra. Substitution with BPh2 effectively enhanced the fluorescence quantum yield compared to the corresponding BF2 complexes in the solid-state.

Linearly tunable emission colors obtained from a fluorescent-phosphorescent dual-emission compound by mechanical stimuli

Organic mechanoluminochromic materials are mechano/piezo-responsive and promising for applications in sensors, displays, and data storage devices. However, their switching range of emission is seriously impeded by only one kind of emission (either a fluorescent or phosphorescent peak) in the spectrum of single organic compounds. This study presents a design strategy for pure organic compounds with excellent room-temperature fluorescent-phosphorescent dual-emission (rFPDE) properties, which combines the effective factors of dipenylsulfone group, crystalline state, and heavy atom effect. Following the principle of color mixing, myriad emission colors with a wide range from orange to purple and across white zone in a straight line in the chromaticity diagram of the Commission Internationale de l'Eclairage (CIE) can be obtained by simply mechanical grinding the compound. The unique properties could be concentrated on a pure organic compound through this design strategy, which provides a new efficient channel for the discovery of efficient mechano-responsive organic materials.

High solid-state luminescence in propeller-shaped AIE-active pyridine–ketoiminate–boron complexes

Two pyridine-ketoiminate-based organoboron complexes were demonstrated to possess aggregation-induced emission, large Stokes shift and high quantum yield in the solid-state, which were rationalized through X-ray crystal analysis and electronic structure calculations.

Dipyrrolylquinoxaline difluoroborates with intense red solid-state fluorescence

Dipyrrolylquinoxalines and their BF₂ complexes were synthesized and characterized by their X-ray structural analysis, optical and electrochemical properties. These dyes exhibit strong broad absorption in the visible region and intensely tunable solid-state fluorescence.

Cu-catalysed direct C–H (hetero)arylation of [1,2,4]triazolo[4,3-a]pyridine to construct deep-blue-emitting luminophores

An efficient protocol for the synthesis of 3-aryl-[1,2,4]triazolo[4,3-a]pyridines has been developed via Cu-catalysed direct C–H (hetero)arylation. The resulting compounds exhibit deep-blue emission with high quantum yields, photostability, and thermal stability.

A BODIPY/pyrene-based chemodosimetric fluorescent chemosensor for selective sensing of hydrazine in the gas and aqueous solution state and its imaging in living cells

A BODIPY-based pyrenebutyrate-linked (BPB) chromogenic and fluorogenic probe was synthesized and characterized for the specific detection of hydrazine.