Carborane‐Induced Excimer Emission of Severely Twisted Bis‐o‐Carboranyl Chrysene

The synthesis of a highly twisted chrysene derivative incorporating two electron deficient o‐carboranyl groups is reported. The molecule exhibits a complex, excitation‐dependent photoluminescence, including aggregation‐induced emission (AIE) with good quantum efficiency and an exceptionally long singlet excited state lifetime. Through a combination of detailed optical studies and theoretical calculations, the excited state species are identified, including an unusual excimer induced by the presence of o‐carborane. This is the first time that o‐carborane has been shown to induce excimer formation ab initio, as well as the first observation of excimer emission by a chrysene‐based small molecule in solution. Bis‐o‐carboranyl chrysene is thus an initial member of a new family of o‐carboranyl phenacenes exhibiting a novel architecture for highly‐efficient multi‐luminescent fluorophores.     

Dual‐Polarization Imaging of a Dual‐Fluorophore Ion Sensor: A Single‐Molecule Study

The fluorescence emission of the dual‐fluorophore Ca²⁺ ion sensor molecule, calcium‐green 2 (CG‐2), has been characterized using dual‐polarization imaging at the single‐molecule level. By comparing the fluorescence intensity of individual CG‐2 molecules in two mutually orthogonal polarization image channels, information about the relative orientation of the two constituent fluorophores in the molecule is obtained. Experimental results from polarization measurements are compared with those predicted from a geometric model based on coupled‐fluorophores that are randomly distributed in space. The results confirm previous optical spectroscopy‐based predictions of the orientation of CG‐2′s fluorophores, and the general applications of this dual‐polarization imaging approach for characterizing the optical properties of molecules containing multiple fluorophores is discussed.     

Circularly‐Polarized Electrochemiluminescence from a Chiral Bispyrene Organic Macrocycle

The first observation of circular polarization of electrochemiluminescence (ECL) from a purely organic derivative is reported. A bispyrene scaffold mounted on a constrained polyether macrocycle displaying intense excimer fluorescence and highly circularly‐polarized (CP) photoluminescence has been selected for this purpose. The compound displays an ECL dissymmetry factor of about |8×10^?3|, which is in good agreement with the corresponding photoluminescence value. This observation is the first step towards the molecular engineering of tailored dyes that can act as both ECL and CP‐ECL reporters for (bio)analysis by bringing a new level of information when dealing with chiral environments. Additionally, it provides an extra dimension to the ECL phenomenon and opens the way to chiral detection and discrimination.     

Bright,Multi‐responsive,Sky‐Blue Platinum(II) Phosphors Based on a Tetradentate Chelating Framework

A new class of highly efficient and stable, blue‐phosphorescent Pt^II complexes based on a tetradentate chelating framework has been found to exhibit highly sensitive and reversible responses to multiple external stimuli including temperature, pressure, and UV irradiation with distinct phosphorescent color switching—from blue to red or white. Intermolecular excimer formation is the main origin of this intriguing multi‐response phenomenon. Highly efficient singlet‐oxygen sensitization by the Pt^II compounds yields UV‐light‐induced phosphorescence enhancement and color switching.     

Molecular‐Barrier‐Enhanced Aromatic Fluorophores in Cocrystals with Unity Quantum Efficiency

Singlet–triplet conversion in organic light‐emitting materials introduces non‐emissive (dark) and long‐lived triplet states, which represents a significant challenge in constraining the optical properties. There have been considerable attempts at separating singlets and triplets in long‐chain polymers, scavenging triplets, and quenching triplets with heavy metals; nonetheless, such triplet‐induced loss cannot be fully eliminated. Herein, a new strategy of crafting a periodic molecular barrier into the π‐conjugated matrices of organic aromatic fluorophores is reported. The molecular barriers effectively block the singlet‐to‐triplet pathway, resulting in near‐unity photoluminescence quantum efficiency (PLQE) of the organic fluorophores. The transient optical spectroscopy measurements confirm the absence of the triplet absorption. These studies provide a general approach to preventing the formation of dark triplet states in organic semiconductors and bring new opportunities for the development of advanced organic optics and photonics.     

Luminescent Radical‐Excimer: Excited‐State Dynamics of Luminescent Radicals in Doped Host Crystals

The excited‐state dynamics of the photostable luminescent organic radical (3,5‐dichloro‐4‐pyridyl)bis(2,4,6‐trichlorophenyl)methyl (PyBTM) doped in a host crystal was investigated by using optically detected magnetic resonance (ODMR) and time‐resolved emission spectroscopies. In the radical system, the unpaired electron can be used as the probe for studying the electronic state and its dynamics. The mixed crystal with a high concentration of the radical showed excimer emission, together with the monomer emission. The ODMR signals were observed with opposite signs for monitoring the monomer and the excimer emissions. Based on their temperature and concentration dependencies, the excited‐state dynamics on the doped crystal and the mechanism of the excimer formation and the ODMR signal generation are discussed with the help of the quantum mechanical simulation of the excited‐state spin dynamics. The initial process of excimer formation has been clarified for the first time from the viewpoint of the spin‐dynamics.     

DCDHF Fluorophores for Single‐Molecule Imaging in Cells

There is a persistent need for small‐molecule fluorescent labels optimized for single‐molecule imaging in the cellular environment. Application of these labels comes with a set of strict requirements: strong absorption, efficient and stable emission, water solubility and membrane permeability, low background emission, and red‐shifted absorption to avoid cell autofluorescence. We have designed and characterized several fluorophores, termed “DCDHF” fluorophores, for use in live‐cell imaging based on the push–pull design: an amine donor group and a 2‐dicyanomethylene‐3‐cyano‐2,5‐dihydrofuran (DCDHF) acceptor group, separated by a π‐rich conjugated network. In general, the DCDHF fluorophores are comparatively photostable, sensitive to local environment, and their chemistries and photophysics are tunable to optimize absorption wavelength, membrane affinity, and solubility. Especially valuable are fluorophores with sophisticated photophysics for applications requiring additional facets of control, such as photoactivation. For example, we have reengineered a red‐emitting DCDHF fluorophore so that it is dark until photoactivated with a short burst of low‐intensity violet light. This molecule and its relatives provide a new class of bright photoactivatable small‐molecule fluorophores, which are needed for super‐resolution imaging schemes that require active control (here turning‐on) of single‐molecule emission.     

Artificial Iono‐ and Photosensitive Membranes Based on an Amphiphilic Aza‐Crown‐Substituted Hemicyanine

Artificial iono‐ and photosensitive membranes based on an amphiphilic aza‐crown‐substituted hemicyanine are assembled on liquid and solid supports and their aggregation behaviour, which is influenced by the binding of metal cations and surface density, is studied. The photoinduced charge‐transfer properties of an analogous non‐amphiphilic hemicyanine in solution are also demonstrated. An asymmetric sandwich dimer model is proposed and existence of such dimers in solution is evidenced by transient absorption and fluorescence anisotropy experiments. Changes in absorption and emission spectra, as well as compression isotherms of the amphiphile observed in the presence of cations, are discussed in terms of 2D molecular reorganisation. Surface‐pressure‐controlled reversible excimer formation at the air–water interphase and excimer‐type emission of Langmuir–Blodgett films in the presence of cations are demonstrated and are discussed on the basis of fibre‐optic fluorimetry and fluorescence microscopy results.     

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.     

Porous Peptide Complexes by a Folding‐and‐Assembly Strategy

Concerted folding and assembly processes are necessary for protein self‐assembly, yet such a concerted strategy has rarely been attempted by synthetic chemists. In this work, we have created a new porous peptide structure through a coordination‐driven folding‐and‐assembly strategy. A porous framework with 1.5 nm‐sized pores and a P_II helical peptide scaffold was successfully obtained by complexation of AgNTf₂ and tripeptide ligands containing the Gly‐Pro‐Pro sequence. The pores were modified in various ways with retention of the latent P_II helical conformation of the peptide ligand.     

Exploiting Co‐solubilization of Warfarin,Curcumin, and Rhodamine B for Modulation of Energy Transfer: A Micelle FRET On/Off Switch

Two new FRET pairs, warfarin (WF)–curcumin (CUR) and curcumin–rhodamine B (RhB), are explored by using surfactant‐based self‐assembled soft systems as scaffolds. The study is extended to design a two‐step concurrent FRET system based on these three fluorophores, which is an important mechanism to devise artificial light‐harvesting/antenna systems. Surfactant systems of varying nature (cationic, anionic, nonionic, and zwitterionic) are exploited to modulate the energy transfer in different FRET systems. Interestingly, micelle/water interfacial‐charge‐responsive FRET is observed owing to selective solubilization of the fluorophores during co‐solubilization. The step‐one FRET (WF→CUR) is switched on in cationic and zwitterionic media but switched off in anionic/nonionic media, whereas the step‐two FRET from CUR to RhB is switched on in anionic/nonionic and zwitterionic media. However, both the FRET steps (WF→CUR→RhB) are observed to be active only in zwitterionic medium. Co‐solubilized, appropriately mixed fluorophores having multistep FRET possibilities can be switched on/off selectively as and when required and energy efficiency can be tuned to an optimal level by varying the nature and geometry of the micellar scaffold. Thus, the two FRET pairs selectively acknowledge all types of media for their anticipated applications in biological systems, as structural tools, and for the development of artificial light‐harvesting/antenna systems and lasers.     

Naphthyl End‐Capped Terthiophene‐Based Chemiresistive Sensors for Biogenic Amine Detection and Meat Spoilage Monitoring

A reliable and sensitive detection of biogenic amines (BAs) is essential to ensure food safety and maintain public health. In this study, two naphthyl end‐capped terthiophene derivatives, namely, 5‐(naphthalen‐1‐yl)‐2,2′:5′,2′′‐terthiophene ( NA‐3T ) and 5,5′′‐di(naphthalen‐1‐yl)‐2,2′:5′,2′′‐terthiophene ( NA‐3T‐NA ), were employed to develop chemiresistive sensors for detecting gaseous BAs. In contrast to NA‐3T , the NA‐3T‐NA ‐based sensor showed a higher sensitivity for trimethylamine (TMA) with an experimental detection limit lower than 22 ppm, and for aromatic BAs, including dopamine, histamine, tryptamine, and tyramine. Additionally, the recovery time for TMA was found to be shorter than 23 s. In addition, both sensors were successfully used for an in situ evaluation of meat freshness by monitoring the concentration of relevant volatile BAs. The difference in the sensing performances of the two chemiresistive sensors was tentatively ascribed to different packing structures of the derivatives and the adlayer structures of the films developed with the compounds.     

Intercalating Nucleic Acids with Pyrene Nucleotide Analogues as Next‐Nearest Neighbors for Excimer Fluorescence Detection of Single‐Point Mutations under Nonstringent Hybridization Conditions

Fluorescence and hybridization specificity is reported for intercalating nucleic acids (INAs), which are here oligodeoxynucleotides (ODNs) synthesized with insertions using (S)‐1‐[bis(4‐methoxyphenyl)(phenyl)methoxy]‐3‐[(pyren‐1‐yl)methoxy]propan‐2‐ol phosphoramidites. It is shown that an INA with two insertions placed as next‐nearest neighbors can be used for discrimination between nucleic acids and their single‐point mutants. Quenching of an excimer band at 480 nm is observed upon hybridizing to a complementary sequence, whereas the excimer band is present when the nucleobase pair between the two pyrene moieties is mismatched. It is the first example of a solution based on fluorescence detection of single‐point mutants that uses excimer formation and does not rely on stringent hybridization conditions. Furthermore, it is shown that INAs with pyrene insertions retain their sequence specificity in thermal melting.     

Photoactivatable Aggregation‐Induced Emission Fluorophores with Multiple‐Color Fluorescence and Wavelength‐Selective Activation

Photoactivatable (caged) fluorophores are widely used in chemistry, materials, and biology. However, the development of such molecules exhibiting photoactivable solid‐state fluorescence is still challenging due to the aggregation‐caused quenching (ACQ) effect of most fluorophores in their aggregate or solid states. In this work, we developed caged salicylaldehyde hydrazone derivatives, which are of aggregation‐induced emission (AIE) characteristics upon light irradiation, as efficient photoactivatable solid‐state fluorophores. These compounds displayed multiple‐color emissions and ratiometric (photochromic) fluorescence switches upon wavelength‐selective photoactivation, and were successfully applied for photopatterning and photoactivatable cell imaging in a multiple‐color and stepwise manner.     

High‐Performance Quinoline‐Malononitrile Core as a Building Block for the Diversity‐Oriented Synthesis of AIEgens

In vivo fluorescent monitoring of physiological processes with high‐fidelity is essential in disease diagnosis and biological research, but faces extreme challenges due to aggregation‐caused quenching (ACQ) and short‐wavelength fluorescence. The development of high‐performance and long‐wavelength aggregation‐induced emission (AIE) fluorophores is in high demand for precise optical bioimaging. The chromophore quinoline‐malononitrile (QM) has recently emerged as a new class of AIE building block that possesses several notable features, such as red to near‐infrared (NIR) emission, high brightness, marked photostability, and good biocompatibility. In this minireview, we summarize some recent advances of our established AIE building block of QM, focusing on the AIE mechanism, regulation of emission wavelength and morphology, the facile scale‐up and fast preparation for AIE nanoparticles, as well as potential biomedical imaging applications.     

Palladium(II)‐Catalyzed ortho‐Arylation of Aromatic Alcohols with a Readily Attachable and Cleavable Molecular Scaffold

A palladium(II)‐catalyzed C?H arylation process of alcohols has been developed. The strategy utilizes a novel quinoline‐based hemiacetal scaffold that can direct the selective C?H bond functionalization. This reaction provides a useful method to construct biaryl compounds of benzyl alcohols in good to excellent yields. The new molecular scaffold can be readily attached, removed, and recovered.     

Ionic Self‐Assembly and Red‐Phosphorescence Properties of a Charged Platinum(II) 8‐Quinolinol Complex Associated with Ammonium‐Based Amphiphiles

A series of ionic associates based on the platinum(II) chelate of 5‐sulfo‐8‐quinolinol, [Pt(qS)₂]^2?, and ammonium‐based amphiphiles is described. At variance with the prototypical neutral complex Pt(q)₂ (q=8‐quinolinol), these dianionic fluorophores, functionalized at the periphery with sulfonate groups, can be associated by the ionic self‐assembly approach with various ammonium cations, such as (H_{2 n+1}C_n)₂Me₂N⁺ (n=12, 16, 18) or complex ammonium cations carrying three C_n carbon chains (n=12, 14, 16) and an additional amide group. Investigations of their luminescence properties in solution, in the solid state, and, when possible, in thin films revealed that the phosphorescence properties in condensed phases are directly correlated to intermolecular interactions between the luminescent [Pt(qS)₂]^2? centers. Of particular interest is also the formation of a columnar liquid‐crystalline phase around room temperature (between ?25 and +180 °C), as well as the very good film‐forming ability of some of these fluorophores from organic solvents.     

New 3‐(Heteroaryl)‐2‐iminocoumarin‐based Borate Complexes: Synthesis,Photophysical Properties,and Rational Functionalization for Biosensing/Biolabeling Applications

Members of a series of boron difluoride complexes with 3‐(heteroaryl)‐2‐iminocoumarin ligands bearing both a phenolic hydroxyl group (acting as a fluorogenic center) and an N‐aryl substituent (acting as a stabilizing moiety) have been synthesized in good yields by applying a straightforward two‐step method. These novel fluorogenic dyes belong to the family of “Boricos” (D. Frath et al., Chem. Commun.­ 2013 , 49, 4908–4910) and are the first examples of phenol‐based fluorophores of which the photophysical properties in the green‐yellow spectral range are dramatically improved by N,N‐chelation of a boron atom. Modulation of their fluorescence properties through reversible chemical modification of their phenol moieties has been demonstrated by the preparation of the corresponding 2,4‐dinitrophenyl (DNP) ethers, which led to a dramatic “OFF‐ON” fluorescence response upon reaction with thiols. Additionally, to expand the scope of these “7‐hydroxy‐Borico” derivatives, particularly in biolabeling, amine or carboxylic acid functionalities amenable to (bio)conjugation have been introduced within their scaffold. Their utility has been demonstrated in the preparation of fluorescent bovine serum albumin (BSA) conjugates and “Borico”‐DOTA‐like scaffolds in an effort to design novel monomolecular multimodal fluorescence‐ radioisotope imaging agents.     

Redox‐Active Corannulene Buckybowls in a Crystalline Hybrid Scaffold

A porous crystalline corannulene‐containing scaffold, which combines the periodicity, dimensionality, and structural modularity of hybrid frameworks with the intrinsic properties of redox‐active π‐bowls, has been prepared. Single‐crystal and powder X‐ray diffraction, ab initio density functional theory computations, gas sorption analysis, fluorescence spectroscopy, and cyclic voltammetry were employed to study the properties of the novel corannulene derivatives and the buckybowl‐based hybrid materials. X‐ray diffraction studies revealed the preservation of the corannulene bowl inside the prepared rigid matrix, which offers the unique opportunity to extend the scaffold dimensionality through the buckybowl curvature. Merging the inherent properties of hybrid frameworks with the intrinsic properties of π‐bowls opens a new avenue for preparing redox‐active materials and potentially improving charge transport in the scaffold.     

Regulation of π‐Stacked Anthracene Arrangement for Fluorescence Modulation of Organic Solid from Monomer to Excited Oligomer Emission

The construction and precise control of the face‐to‐face π‐stacked arrangements of anthracene fluorophores in the crystalline state led to a remarkable red shift in the fluorescence spectrum due to unprecedented excited oligomer formation. The arrangements were regulated by using organic salts including anthracene‐1,5‐disulfonic acid (1,5‐ADS) and a variety of aliphatic amines. Because of the smaller number of hydrogen atoms at the edge positions and the steric effect of the sulfonate groups, 1,5‐ADS should prefer face‐to‐face π‐stacked arrangements over the usual edge‐to‐face herringbone arrangement. Indeed, as the alkyl substituents were lengthened, the organic salts altered their anthracene arrangement to give two‐dimensional (2D) edge‐to‐face and end‐to‐face herringbone arrangements, one‐dimensional (1D) face‐to‐face zigzag and slipped stacking arrangements, a lateral 1D face‐to‐face arrangement like part of a brick wall, and a discrete monomer arrangement. The monomer arrangement behaved as a dilute solution even in the close‐packed solid state to emit deep blue light. The 1D face‐to‐face zigzag and slipped stacking of the anthracene fluorophores caused a red shift of 30–40 nm in the fluorescence emission with respect to the discrete arrangement, probably owing to ground‐state associations. On the other hand, the 2D end‐to‐face stacking induced a larger red shift of 60 nm, which is attributed to the excimer fluorescence. Surprisingly, the brick‐like lateral face‐to‐face arrangement afforded a remarkable red shift of 150 nm to give yellow fluorescence. This anomalous red shift is probably due to excited oligomer formation in such a lateral 1D arrangement according to the long fluorescence lifetime and little shift in the excitation spectrum. The regulation of the π‐stacked arrangement of anthracene fluorophores enabled the wide modulation of the fluorescence and a detailed investigation of the relationships between the photophysical properties and the arrangements.