Single Benzene Green Fluorophore: Solid‐State Emissive,Water‐Soluble,and Solvent‐ and pH‐Independent Fluorescence with Large Stokes Shifts

Benzene is the simplest aromatic hydrocarbon with a six‐membered ring. It is one of the most basic structural units for the construction of π conjugated systems, which are widely used as fluorescent dyes and other luminescent materials for imaging applications and displays because of their enhanced spectroscopic signal. Presented herein is 2,5‐bis(methylsulfonyl)‐1,4‐diaminobenzene as a novel architecture for green fluorophores, established based on an effective push–pull system supported by intramolecular hydrogen bonding. This compound demonstrates high fluorescence emission and photostability and is solid‐state emissive, water‐soluble, and solvent‐ and pH‐independent with quantum yields of Φ=0.67 and Stokes shift of 140 nm (in water). This architecture is a significant departure from conventional extended π‐conjugated systems based on a flat and rigid molecular design and provides a minimum requirement for green fluorophores comprising a single benzene ring.     

Systematic Investigation of Molecular Arrangements and Solid‐State Fluorescence Properties on Salts of Anthracene‐2,6‐disulfonic Acid with Aliphatic Primary Amines

Organic salts of anthracene‐2,6‐disulfonic acid (ADS) with a wide variety of primary amines have been fabricated, and their arrangements of anthracene molecules and solid‐state fluorescence properties investigated. Single‐crystal X‐ray studies reveal that the salts show seven types of crystal forms and corresponding molecular arrangements of anthracene moieties depending on the amine, while anthracene shows only one form and arrangement in the solid state. Depending on the molecular arrangements, the ADS salts exhibit various solid‐state fluorescence properties: spectral shift (30 nm) and suppression and enhancement of the fluorescence intensity. Especially the ADS salt with n‐heptylamine (nHepA), which shows discrete anthracene moieties in the crystal, exhibits the highest quantum yield (Φ_F=46.1±0.2 %) in the series of ADS salts, which exceeds that of anthracene crystal (Φ_F=42.9±0.2 %). From these systematic investigations on the arrangements and the solid‐state properties, the following factors are essential for high fluorescence quantum yield in the solid state: prevention of contact between π planes of anthracene moieties and immobilization of anthracene rings. In addition, such organic salts have potential as a system for modulating the molecular arrangements of fluorophores and the concomitant solid‐state properties. Thus, systematic investigation of this system constructs a library of arrangements and properties, and the library leads to remarkable strategies for the development of organic solid materials.     

Tailoring the Solid‐State Fluorescence Emission of BODIPY Dyes by meso Substitution

4,4‐Difluoro‐4‐bora‐3a,4a‐diaza‐s‐indacene (BODIPY) derivatives bearing varied substituents at the meso position (i.e., CF₃, CH₃, COOR, CHO, CN, Cl, iPr) were synthesized to elucidate the structure–property relationships that give rise to emissive J‐aggregates. Several new BODIPY derivatives can be added to the previously reported 1,3,5,7‐tetramethyl‐8‐trifluoromethyl derivative to the list of those forming J‐aggregates, in addition to other dyes that are emissive in the solid state without forming J‐aggregates.     

Color Polymorphism: Understanding the Diverse Solid‐State Packing and Color in Dimethyl‐3,6‐dichloro‐2,5‐dihydroxyterephthalate

Dimethyl‐3,6‐dichloro‐2,5‐dihydroxyterephthalate (MCHT) is known to exist in three differently packed crystals having three different colors, namely yellow (Y), light yellow (LY), and white (W). Apart from the difference in their color, the molecules in the crystals also differ in their intramolecular O?H???O and O?H???Cl hydrogen bonds. Time‐dependent DFT calculations reveal the role of the various types of hydrogen bonds in controlling the color of the polymorphs. Mechanistic pathways that lead to such transformations in the crystal are elucidated by solid‐state dispersion‐corrected DFT studies. Relative stabilities of the various polymorphs rationalize the experimentally observed transformations between them. Calculations reveal that the minimum‐energy pathway for the conversion of the Y form to a W form is through stepwise disrotatory motion of the two ?OH groups through a hybrid intermediate having one intramolecular O?H???O and one O?H???Cl bond. The LY form is shown to exist on the higher‐energy pathway involving a concerted Y→W transformation.     

Tuning the Solid‐State Luminescence of BODIPY Derivatives with Bulky Arylsilyl Groups: Synthesis and Spectroscopic Properties

Boron dipyrromethenes (BODIPYs) with bulky triphenylsilylphenyl(ethynyl) and triphenylsilylphenyl substituents on pyrrole sites were prepared via Hagihara–Sonogashira and Suzuki–Miyaura cross‐coupling with ethynyl‐terminated tetraphenylsilane and boronic acid‐terminated tetraphenylsilane. The chromophores are designed to prevent intermolecular π–π stacking interaction and enhance fluorescence in the solid state. Single crystals of 1 a and 2 b for X‐ray structural analysis were obtained, and weak π–π stacking interactions of the neighboring BODIPY molecules were observed. Spectroscopic properties of all of the dyes in various solvents and in films were investigated. Triphenylsilylphenyl‐substituted BODIPYs generally show more pronounced increases in solid‐state emission than triphenylsilylphenyl(ethynyl)‐substituted BODIPYs. Although the simple BODIPYs do not exhibit any fluorescence in the solid state (Φ=0), arylsilyl‐substituted BODIPYs exhibit weak to moderate solid‐state fluorescence with quantum yields of 0.03, 0.07, 0.10, and 0.25. The structure–property relationships were analyzed on the basis of X‐ray crystallography, optical spectroscopy, cyclic voltammetry, and theoretical calculations.     

A Novel Water‐Soluble Fluorescence Probe with Wash‐Free Cellular Imaging Capacity Based on AIE Characteristics

A potential real‐time imaging water‐soluble fluorescent polymer ( P3 ) is facilely prepared via one‐pot method. For P3 , tetraphenylethene unit serves as the fluorescent unit, poly(acryloyl ethylene diamine) (a kind of polyelectrolyte) with specific degree of polymerization acts as water‐soluble part. ¹H‐NMR, gel permeation chromatography (GPC), UV‐vis spectroscopy, photoluminescence (PL), and confocal laser scanning microscopy are undertaken to characterize the structure and property of P3 . The results of wash‐free cellular imaging show that the signal‐to‐noise ratio is high as the concentration of P3 is 50 μg mL⁻¹. In addition, the pH‐responsive and Cd²⁺‐responsive are also investigated in this paper. The results coming from pH‐responsive show that P3 solution displays significant fluorescence under near neutral. And the result from the cellular imaging shows that intracellular fluorescence intensity enhances with the augment of concentration of Cd²⁺, which reveals that P3 can give a hint to resolve the dilemma of traditional fluorescent dyes used as living cellular fluorescent probe.

     

Polymorph‐Dependent Solid‐State Fluorescence and Selective Metal‐Ion‐Sensor Properties of 2‐(2‐Hydroxyphenyl)‐4(3H)‐quinazolinone

2‐(2‐Hydroxy‐phenyl)‐4(3H)‐quinazolinone (HPQ), an organic fluorescent material that exhibits fluorescence by the excited‐state intramolecular proton‐transfer (ESIPT) mechanism, forms two different polymorphs in tetrahydrofuran. The conformational twist between the phenyl and quinazolinone rings of HPQ leads to different molecular packing in the solid state, giving structures that show solid‐state fluorescence at 497 and 511 nm. HPQ also shows intense fluorescence in dimethyl formamide (DMF) solution and selectively detects Zn²⁺ and Cd²⁺ ions at micromolar concentrations in DMF. Importantly, HPQ not only detects Zn²⁺ and Cd²⁺ ions selectively, but it also distinguishes between the metal ions with a fluorescence λ_max that is blue‐shifted from 497 to 420 and 426 nm for Zn²⁺ and Cd²⁺ ions, respectively. Hence, tunable solid‐state fluorescence and selective metal‐ion‐sensor properties were demonstrated in a single organic material.     

Solid‐State Emissive Aroyl‐S,N‐Ketene Acetals with Tunable Aggregation‐Induced Emission Characteristics

N‐Benzyl aroyl‐S,N‐ketene acetals can be readily synthesized by condensation of aroyl chlorides and N‐benzyl 2‐methyl benzothiazolium salts in good to excellent yields, yielding a library of 35 chromophores with bright solid‐state emission and aggregation‐induced emission characteristics. Varying the substituent from electron‐donating to electron‐withdrawing enables the tuning of the solid‐state emission color from deep blue to red.     

Synthesis and Characterization of Far‐Red/NIR‐Fluorescent BODIPY Dyes,Solid‐State Fluorescence,and Application as Fluorescent Tags Attached to Carbon Nano‐onions

A series of π‐extended distyryl‐substituted boron dipyrromethene (BODIPY) derivatives with intense far‐red/near‐infrared (NIR) fluorescence was synthesized and characterized, with a view to enhance the dye’s performance for fluorescence labeling. An enhanced brightness was achieved by the introduction of two methyl substituents in the meso positions on the phenyl group of the BODIPY molecule; these substituents resulted in increased structural rigidity. Solid‐state fluorescence was observed for one of the distyryl‐substituted BODIPY derivatives. The introduction of a terminal bromo substituent allows for the subsequent immobilization of the BODIPY fluorophore on the surface of carbon nano‐onions (CNOs), which leads to potential imaging agents for biological and biomedical applications. The far‐red/NIR‐fluorescent CNO nanoparticles were characterized by absorption, fluorescence, and Raman spectroscopies, as well as by thermogravimetric analysis, dynamic light scattering, high‐resolution transmission electron microscopy, and confocal microscopy.     

Benzo[c,d]indole‐Containing Aza‐BODIPY Dyes: Asymmetrization‐Induced Solid‐State Emission and Aggregation‐Induced Emission Enhancement as New Properties of a Well‐Known Chromophore

A series of symmetric and asymmetric benzo[c,d]indole‐containing aza boron dipyrromethene (aza‐BODIPY) compounds was synthesized by a titanium tetrachloride‐mediated Schiff‐base formation reaction of commercially available benzo[c,d]indole‐2(1H)‐one and heteroaromatic amines. These aza‐BODIPY analogues show different electronic structures from those of regular aza‐BODIPYs, with hypsochromic shifts of the main absorption compared to their BODIPY counterparts. In addition to the intense fluorescence in solution, asymmetric compounds exhibited solid‐state fluorescence due to significant contribution of the vibronic bands to both absorption and fluorescence as well as reduced fluorescence quenching in the aggregates. Finally, aggregation‐induced emission enhancement, which is rare in BODIPY chromophores, was achieved by introducing a nonconjugated moiety into the core structure.     

Ring‐Shaped Silafluorene Derivatives as Efficient Solid‐State UV‐Fluorophores: Synthesis,Characterization, and Photoluminescent Properties

Four ring‐shaped silafluorene‐containing compounds ( 1 – 4 ) were synthesized and characterized as potentially promising monomers for fluorescent polymers. Their optical properties in solution and solid state (thin film and powder) were studied. These compounds have low quantum yields in solution (Φ_fl=0.13‐0.15) with fluorescence maxima at about 355 nm, but high quantum yields in the solid state (powder, Φ_fl=0.35‐0.54) with fluorescence maxima at about 377 and 488 nm. Influence of the substituents and the number of silafluorene units in 1 – 4 on their optical properties was investigated. Extensive study of the X‐ray crystal structures of 1 – 4 was undertaken to analyze and qualitatively estimate the role, extent, and influence of silafluorene moieties’ interactions on solid‐state fluorescent properties. Excited state UV/Vis and theoretical molecular orbital (MO) calculations were performed to explore possible fluorescence mechanisms and differences in quantum yields among these compounds.     

Bright White‐Light Emission from a Single Organic Compound in the Solid State

White‐light‐emitting materials and devices have attracted enormous interest because of their great potential for various lighting applications. We herein describe the light‐emitting properties of a series of new difunctional organic molecules of remarkably simple structure consisting of two terminal 4‐pyridone push–pull subunits separated by a polymethylene chain. They were found to emit almost “pure” white light as a single organic compound in the solid state, as well as when incorporated in a polymer film. To the best of our knowledge, they are the simplest white‐light‐emitting organic molecules reported to date.     

Probing the Local Structure of Pure Ionic Liquid Salts with Solid‐ and Liquid‐State NMR

Room‐temperature ionic liquids (RTILs) are gaining increasing interest and are considered part of the green chemistry paradigm due to their negligible vapour pressure and ease of recycling. Evidence of liquid‐state order, observed by IR and Raman spectroscopy, diffraction studies, and simulated by ab initio methods, has been reported in the literature. Here, quadrupolar nuclei are used as NMR probes to extract information about the solid and possible residual order in the liquid state of RTILs. To this end, the anisotropic nature and field dependence of quadrupolar and chemical shift interactions are exploited. Relaxation time measurements and a search for residual second‐order quadrupolar coupling were employed to investigate the molecular motions present in the liquid state and infer what kind of order is present. The results obtained indicate that on a timescale of ～10^?8 sec or longer, RTILs behave as isotropic liquids without residual order.     

Solid‐State Structures and Solution Analyses of a Phenylpropylpyridine N‐Oxide and an N‐Methyl Phenylpropylpyridine

The crystal structures of phenylpropylpyridine‐N‐oxide and N‐methyl‐phenylpropylpyridinium iodide are compared, revealing that hydrogen bonding with the solvent molecule plays an important role in the N‐oxide compound, whilst electrostatic interactions are predominant in controlling the solid‐state orientation of the N‐methylated compound. Fluorescence spectroscopy and NOESY indicate that in contrast to the previously reported pyridinium iodide, the N‐oxide is not subject to intramolecular π‐stacking, as judged by excimer emission and a lack of corresponding cross peaks, respectively.     

Synthesis,Solid‐State NMR Characterization,and Application for Hydrogenation Reactions of a Novel Wilkinson’s‐Type Immobilized Catalyst

Silica nanoparticles (SiNPs) were chosen as a solid support material for the immobilization of a new Wilkinson’s‐type catalyst. In a first step, polymer molecules (poly(triphenylphosphine)ethylene (PTPPE); 4‐diphenylphosphine styrene as monomer) were grafted onto the silica nanoparticles by surface‐initiated photoinferter‐mediated polymerization (SI‐PIMP). The catalyst was then created by binding rhodium (Rh) to the polymer side chains, with RhCl₃ ? x H₂O as a precursor. The triphenylphosphine units and rhodium as Rh^I provide an environment to form Wilkinson’s catalyst‐like structures. Employing multinuclear (³¹P, ²⁹Si, and ¹³C) solid‐state NMR spectroscopy (SSNMR), the structure of the catalyst bound to the polymer and the intermediates of the grafting reaction have been characterized. Finally, first applications of this catalyst in hydrogenation reactions employing para‐enriched hydrogen gas (PHIP experiments) and an assessment of its leaching properties are presented.     

4‐Trifluoromethyl‐Substituted Coumarins with Large Stokes Shifts: Synthesis,Bioconjugates, and Their Use in Super‐Resolution Fluorescence Microscopy

Bright and photostable fluorescent dyes with large Stokes shifts are widely used as sensors, molecular probes, and light‐emitting markers in chemistry, life sciences, and optical microscopy. In this study, new 7‐dialkylamino‐4‐trifluoromethylcoumarins have been designed for use in bioconjugation reactions and optical microscopy. Their synthesis was based on the Stille reaction of 3‐chloro‐4‐trifluoromethylcoumarins and available (hetero)aryl‐ or (hetero)arylethenyltin derivatives. Alternatively, the acylation of 2‐trifluoroacetyl‐5‐dialkylaminophenols with available (hetero)aryl‐ or (hetero)arylethenylacetic acids followed by intramolecular condensation afforded coumarins with 3‐(hetero)aryl or 3‐[2‐(hetero)aryl]ethenyl groups. Hydrophilic properties were provided by the introduction of a sulfonic acid residue or by phosphorylation of a primary hydroxy group attached at C‐4 of the 2,2,4‐trimethyl‐1,2‐dihydroquinoline fragment fused to the coumarin fluorophore. For use in immunolabeling procedures, the dyes were decorated with an (activated) carboxy group. The positions of the absorption and emission maxima vary in the ranges 413–480 and 527–668 nm, respectively. The phosphorylated dye, 9 ,CH?CH‐2‐py,H, with the 1‐(3‐carboxypropyl)‐4‐hydroxymethyl‐2,2‐dimethyl‐1,2‐dihydroquinoline fragment fused to the coumarin fluorophore bearing the 3‐[2‐(2‐pyridyl)ethenyl] residue (absorption and emission maxima at 472 and 623 nm, respectively) was used in super‐resolution light microscopy with stimulated emission depletion and provided an optical resolution better than 70 nm with a low background signal. As a result of their large Stokes shifts, good fluorescence quantum yields, and adequate photostabilities, phosphorylated coumarins enable two‐color imaging (using several excitation sources and a single depletion laser) to be combined with subdiffractional optical resolution.     

Synthesis of N,N‐Dialkylamino‐nor‐Dihydroxanthene‐Hemicyanine Fused Near‐Infrared Fluorophores and Their First Water‐Soluble and/or Bioconjugatable Analogues

The effective synthesis of extended conjugated N ,N ‐dialkylamino‐nor ‐dihydroxanthene‐based fluorophores is described from diversely functionalized salicylic aldehydes. The access to these original fluorescent derivatives proceeded in two steps through a one‐pot construction of the unusual nor ‐dihydroxanthene (nor ‐DHX) scaffold followed by a diversification step providing a wide variety of nor ‐DHX‐hemicyanine fused dyes emitting in the range of 730–790 nm. The versatility of our approach has enabled a further extension to the late‐stage introduction of negatively/positively charged polar groups onto their terminal nitrogen heterocyclic subunit, thereby giving access to the first water‐soluble and/or bioconjugatable members of this emerging class of NIR fluorophores. Our water‐solubilizing method is easily implementable, and the nor ‐DHX‐hemicyanine skeleton maintains satisfying fluorescence quantum yields (5–20 %) under physiological conditions. Finally, the bioconjugation ability of fluorescent derivatives bearing a free carboxylic acid was demonstrated through the covalent labeling of a model protein, namely, bovine serum albumin.