Facile Synthesis of Polycyclic Pentalenes with Enhanced Hückel Antiaromaticity

Pentalenes represent highly reactive Hückel antiaromatics with 8π electrons. Usually, pentalenes are stabilized by incorporation of two benzene rings in a fused fashion. In dibenzo[a ,e ]pentalenes, however, the high aromaticity of the fused benzene rings compromises the inherent antiaromaticity of the pentalene core. Herein, we disclose that this forfeited antiaromaticity can be restored by fusing four additional aromatic rings onto the peripheral positions of dibenzo[a,e]pentalenes. Such polycyclic pentalenes were prepared by successive transannular cyclizations via in situ‐generated tetrakisdehydro[16]annulenes. The thus obtained compounds showed intriguing properties, for example, characteristic absorptions in the visible‐to‐near‐infrared (NIR) region and low reduction potentials. These results hence afford a design principle to produce highly antiaromatic yet stable pentalenes. The antiaromaticity of the pentalene core can be widely tuned via the degree of aromaticity of the peripherally fused rings.     

An Electron-Accepting aza-BODIPY-Based Donor–Acceptor–Donor Architecture for Bright NIR Emission

A bright near-infrared (NIR) fluorescent molecule was developed based on the donor–acceptor–donor (D–A–D) approach using an aza-BODIPY analog called pyrrolopyrrole aza-BODIPY (PPAB) as an electron-accepting chromophore. Directly introducing electron-donating triphenylamine (TPA) to develop a D–A–D structure caused redshifts of absorption and emission of PPAB into the NIR region with an enhanced fluorescence brightness of up to 5.2×10⁴ m ⁻¹ cm⁻¹, whereas inserting a phenylene linker between the TPA donor and the PPAB acceptor induced solvatochromic behavior in emission. Transient absorption spectra and theoretical calculations revealed the presence of a highly emissive hybridized locally excited and charge-transfer state in the former case and the contribution of the dark charge-separated state to the excited state in the latter case. The bright D–A–D PPAB as a novel emitter resulted in a NIR electroluminescence with a high external quantum efficiency of 3.7 % and a low amplified spontaneous emission threshold of ca. 80 μJ cm⁻², indicating the high potential for NIR optoelectronic applications.     

Boron Difluoride Curcuminoid Fluorophores with Enhanced Two‐Photon Excited Fluorescence Emission and Versatile Living‐Cell Imaging Properties

The synthesis of boron difluoride complexes of a series of curcuminoid derivatives containing various donor end groups is described. Time‐dependent (TD)‐DFT calculations confirm the charge‐transfer character of the second lowest‐energy transition band and ascribe the lowest energy band to a “cyanine‐like” transition. Photophysical studies reveal that tuning the donor strength of the end groups allows covering a broad spectral range, from the visible to the NIR region, of the UV–visible absorption and fluorescence spectra. Two‐photon‐excited fluorescence and Z‐scan techniques prove that an increase in the donor strength or in the rigidity of the backbone results in a considerable increase in the two‐photon cross section, reaching 5000 GM, with predominant two‐photon absorption from the S₀–S₂ charge‐transfer transition. Direct comparisons with the hemicurcuminoid derivatives show that the two‐photon active band for the curcuminoid derivatives has the same intramolecular charge‐transfer character and therefore arises from a dipolar structure. Overall, this structure–relationship study allows the optimization of the two‐photon brightness (i.e., 400–900 GM) with one dye that emits in the NIR region of the spectrum. In addition, these dyes demonstrate high intracellular uptake efficiency in Cos7 cells with emission in the visible region, which is further improved by using porous silica nanoparticles as dye vehicles for the imaging of two mammalian carcinoma cells type based on NIR fluorescence emission.     

The multicomponent reaction of imidazo[1,5-a]pyridine carbenes with phthalaldehydes and dimethyl acetylenedicarboxylate: a facile construction of benzo[d]furo[3,2-b]azepines

A study on the multicomponent reaction comprising both N-heterocyclic carbenes and substituted phthalaldehydes is reported. The imidazo[1,5-a]pyridine carbenes, namely imidazo[1,5-a]pyridine-3-ylidenes, reacted with phthalaldehydes and DMAD under very mild conditions to produce novel fused tricyclic benzo[d]furo[3,2-b]azepine derivatives. The resulting fused heterocyclic compounds are fluorescent and they give an emission around 500 nm with quantum yields (Φ(F)) being up to 0.81. This study provides not only a unique approach to fused azepine derivatives that are not easily accessible by other methods, but also opens a new avenue to complicated molecular skeletons. The fluorescence properties of long emission wavelength and high fluorescence quantum yields of some products predict their potential applications as optical sensors.     

Donor–acceptor–acceptor-type near-infrared fluorophores that contain dithienophosphole oxide and boryl groups: effect of the boryl group on the nonradiative decay

The use of donor–π–acceptor (D–π–A) skeletons is an effective strategy for the design of fluorophores with red-shifted emission. In particular, the use of amino and boryl moieties as the electron-donating and -accepting groups, respectively, can produce dyes that exhibit high fluorescence and solvatochromism. Herein, we introduce a dithienophosphole P-oxide scaffold as an acceptor–spacer to produce a boryl- and amino-substituted donor–acceptor–acceptor (D–A–A) π-system. The thus obtained fluorophores exhibit emission in the near-infrared (NIR) region, while maintaining high fluorescence quantum yields even in polar solvents (e.g. λ_em = 704 nm and Φ_F = 0.69 in CH₃CN). A comparison of these compounds with their formyl- or cyano-substituted counterparts demonstrated the importance of the boryl group for generating intense emission. The differences among these electron-accepting substituents were examined in detail using theoretical calculations, which revealed the crucial role of the boryl group in lowering the nonradiative decay rate constant by decreasing the non-adiabatic coupling in the internal conversion process. The D–A–A framework was further fine-tuned to improve the photostability. One of these D–A–A dyes was successfully used in bioimaging to visualize the blood vessels of Japanese medaka larvae and mouse brain.

Combination of electron-accepting diarylboryl terminal groups and dithienophosphole oxide spacers with electron-donating triarylamine moieties produces donor–acceptor–acceptor type π-systems, which exhibit emissions in the near-infrared region.     

meso-C6F5 substituted BODIPYs with distinctive spectroscopic properties and their application for bioimaging in living cells

A series of meso-C₆F₅ BODIYs have been successfully synthesized and characterized. Some of them displayed excellent spectroscopic properties, such as relatively large Stokes shift, high fluorescence quantum yield, far-red or near infrared region (NIR) emission, and good photostability. In particular, the dye functionalized with oligo(ethylene glycol) ether-phenyl groups at the 3,5-position of BODIPY core became water-soluble and its emission located in the NIR region with large Stokes shift. Time-dependent density functional theory calculations were conducted to understand the structure–optical properties relationship. Furthermore, cell staining tests demonstrated that the meso-C₆F₅ BODPY derivative with oligo(ethylene glycol) ether-phenyl group was membrane permeable and selectively stained cytosol in living cells.     

Controlling Energy Gaps of π-Conjugated Polymers by Multi-Fluorinated Boron-Fused Azobenzene Acceptors for Highly Efficient Near-Infrared Emission

We demonstrate that multi-fluorinated boron-fused azobenzene (BAz) complexes can work as a strong electron acceptor in electron donor-acceptor (D-A) type π-conjugated polymers. Position-dependent substitution effects were revealed, and the energy level of the lowest unoccupied molecular orbital (LUMO) was critically decreased by fluorination. As a result, the obtained polymers showed near-infrared (NIR) emission (λ_PL=758–847 nm) with high absolute photoluminescence quantum yield (Φ_PL=7–23%) originating from low-lying LUMO energy levels of the BAz moieties (−3.94 to −4.25 eV). Owing to inherent solid-state emissive properties of the BAz units, deeper NIR emission (λ_PL=852980 nm) was detected in film state. Clear solvent effects prove that the NIR emission is from a charge transfer state originating from a strong D-A interaction. The effects of fluorination on the frontier orbitals are well understandable and predictable by theoretical calculation with density functional theory. This study demonstrates the effectiveness of fluorination to the BAz units for producing a strong electron-accepting unit through fine-tuning of energy gaps, which can be the promising strategy for designing NIR absorptive and emissive materials.     

Quinoacridine derivatives with one-dimensional aggregation-induced red emission property

A new series of acceptor-donor-acceptor (A-D-A) type quinoacridine derivatives (1-3) with aggregation-induced red emission properties were designed and synthesized. In these compounds, the electron-withdrawing 2-(3,5-bis(trifluoromethyl)phenyl)acetonitrile groups act as electron-accepting units, while the alkyl-substituted conjugated core acts as electron-donating units. The restriction of intramolecular rotation was responsible for the AIE behavior of compounds 1-3. All compounds were employed as building blocks to fabricate one-dimensional (1-D) organic luminescent nano- or microwires based on reprecipitation or slow evaporation approaches. Morphological transition from zero-dimensional (0-D) hollow nanospheres to 1-D nanotubes has been observed by recording SEM and TEM images of aggregated sates of compound 2 in THF/H(2)O mixtures at different aging time. It was demonstrated that the synthesized compounds with different lengths of alkyl chains displayed different wire formation properties. The single-crystal X-ray analysis of compound 2 provided reasonable explanation for the formation of 1-D nano- or microstructures.     

A Switchable Near-Infrared-Absorbing Dye Based on Redox-Bistable Benzitetraazaporphyrin

Activatable near-infrared (NIR) dyes responsive to external stimuli are used in medical and other applications. Here, we describe the design and synthesis of bench-stable 18π- and 20π-electron benzitetraazaporphyrins (BzTAPs) possessing redox-switchable NIR properties. X-Ray, NMR, and UV/Visible-NIR analyses revealed that 20π-electron BzTAP 1 exhibits NIR absorption and antiaromaticity with a paratropic ring-current, while 18π-electron BzTAP 2 shows weakly aromatic character with NIR inertness. Notably, the NIR-silent BzTAP 2 was readily converted to the NIR-active BzTAP 1 in the presence of mild reducing agents such as amine. The intense NIR absorption band of BzTAP 1 is in sharp contrast to the very weak absorption bands of previously reported antiaromatic porphyrinoids. Molecular orbital analysis revealed that symmetry-lowering perturbation of the 20π-electron porphyrinoid skeleton enables the HOMO–LUMO transition of 1 to be electric-dipole-allowed. BzTAPs are expected to be useful for constructing activatable NIR probes working in reductive environments.     

1,2,5-Triphenylpyrrole Derivatives with Dual Intense Photoluminescence in Both Solution and the Solid State: Solvatochromism and Polymorphic Luminescence Properties

Five organic luminophores, 1,2,5-triphenylpyrrole (TPP) derivatives 3 a – e bearing electron-withdrawing or electron-donating groups, have been synthesized by Pd-catalyzed Suzuki coupling of 1-phenyl-2,5-di(4′-bromophenyl)pyrrole and para-substituted phenylboronic acid derivatives. They possess good thermal stabilities with high decomposition temperatures above 310 °C. Investigation of the photophysical properties of the luminogens 3 a – e indicated that they exhibited dual intense photoluminescence in both solution and the solid state due to their twisted conformations, and their fluorescence quantum yields (Φ_F) were determined as 68.7–94.9 % in THF solution and 19.1–52.0 % in solid powder form. Compounds 3 a – c bearing electron-accepting groups exhibited remarkable solvatochromism with large Stokes shifts, attributable to their D-π-A structure and intramolecular charge-transfer effect. In particular, 3 a , bearing aldehyde groups, displayed an obvious red-shift of the emission band from 445 to 564 nm with increasing solvent polarity. However, no obvious solvatochromic behavior was observed for compounds 3 d , e bearing electron-donating groups. The luminophore 3 a exhibited polymorphic luminescence properties and crystallization-induced emission enhancement.     

Perylene-fused BODIPY dye with near-IR absorption/emission and high photostability

A N-annulated perylene unit was successfully fused to the meso- and β-positions of a boron dipyrromethene (BODIPY) core. The newly synthesized BODIPY dye 1b exhibits intensified near-infrared (NIR) absorption and the longest emission maximum ever observed for all BODIPY derivatives. In addition, this dye possesses excellent solubility and photostability, beneficial to practical applications.     

A unique approach to development of near-infrared fluorescent sensors for in vivo imaging

Near-infrared (NIR) fluorescent sensors have emerged as promising molecular tools for imaging biomolecules in living systems. However, NIR fluorescent sensors are very challenging to be developed. Herein, we describe the discovery of a new class of NIR fluorescent dyes represented by 1a/1c/1e, which are superior to the traditional 7-hydroxycoumarin and fluorescein with both absorption and emission in the NIR region while retaining an optically tunable hydroxyl group. Quantum chemical calculations with the B3LYP exchange functional employing 6-31G(d) basis sets provide insights into the optical property distinctions between 1a/1c/1e and their alkoxy derivatives. The unique optical properties of the new type of fluorescent dyes can be exploited as a useful strategy for development of NIR fluorescent sensors. Employing this strategy, two different types of NIR fluorescent sensors, NIR-H(2)O(2) and NIR-thiol, for H(2)O(2) and thiols, respectively, were constructed. These novel sensors respond to H(2)O(2) or thiols with a large turn-on NIR fluorescence signal upon excitation in the NIR region. Furthermore, NIR-H(2)O(2) and NIR-thiol are capable of imaging endogenously produced H(2)O(2) and thiols, respectively, not only in living cells but also in living mice, demonstrating the value of the new NIR fluorescent sensor design strategy. The new type of NIR dyes presented herein may open up new opportunities for the development of NIR fluorescent sensors based on the hydroxyl functionalized reactive sites for biological imaging applications in living animals.     

Exhaustive syntheses of naphthofluoresceins and their functions

Naphthofluorescein and/or seminaphthofluorescein derivatives possessing the additional benzene units to one or both sides of fluorescein were exhaustively constructed through Friedel-Crafts type reactions between corresponding aroylbenzoic acids and dihydroxynaphthalenes. Compound 4 works as a one-dye pH indicator, which shows red in strong acid condition and blue in basic solution. Compound 23 (diacetate of compound 4) shows good transitivity to the HEK 293 cells and acts as a fluorescent pigment for the living cell imaging. Compounds 5, 6, and 9 show fluorescent emission in the NIR region (>700 nm) and imply the potentialities of NIR fluorescent probes.     

Synthesis, electrochemical, and spectroscopic properties of soluble perylene monoimide diesters

Perylene derivatives are an important class of n-type semiconductors. With the exceptions of bay-substituted and N-swallow-tail alkyl/tert-butyl substituted derivatives, most perylene diimides are insoluble. The recently reported perylene tetraesters are highly soluble, but are less electron-deficient. In this paper, a series of perylene monoimide diesters have been synthesized, which have good solubility and acceptable electron-accepting property due to the introducing of imide and ester groups in one perylene molecule. Their electrochemical properties and structural ordering were examined by cyclovoltammetry and X-ray diffraction technique, respectively. Their absorption and emission spectra are reported and discussed.     

运用密度泛函活性理论的信息论方法研究苯并富烯衍生物的芳香性

文献中已有越来越多的芳香性体系被发现,同时也有越来越多的芳香性指标被提出来,但是如何解释芳香化合物稳定性的起源以及理解芳香性的本质仍然是当今理论化学中一个悬而未决的难题。运用我们新近提出的密度泛函活性理论信息论方法,不久前我们曾对一系列富烯衍生物进行了系统研究并得到了一个全新的认识。本文进一步探讨苯并富烯衍生物的芳香性行为,目的在于考察一个或多个苯环与富烯连接之后其芳香性发生变化的情况。运用香农熵,费舍尔信息,Ghosh-Berkowitz-Parr熵,Onicescu信息能,信息增益,以及相对Rényi熵六个信息量,和四种芳香指标,ASE,HOMA,FLU和NICS,我们系统地研究了信息量和芳香性指标在单、双、三苯并富烯衍生物中的相关性。我们发现,不管是否有苯环与富烯相连,芳香指标和信息量的交叉相关性都是一样的。这表明,虽然苯环本身具有芳香性,但苯环与富烯相连并不能改变富烯的芳香性与反芳香性本质。苯并富烯衍生物与富烯衍生物的芳香性和反芳香性一致。苯并富烯衍生物的芳香性和反芳香性完全取决于富烯本身的芳香性和反芳香性。这些结果为认识和理解复杂体系芳香性和反芳香性起源和本质将提供有益的启示。     

The Aggregation Regularity Effect of Multiarylpyrroles on Their Near-Infrared Aggregation-Enhanced Emission Property

Increasing the quantum yield of near-infrared (NIR) emissive dyes is critical for biological applications because these fluorescent dyes generally show decreased emission efficiency under aqueous conditions. In this work, we designed and synthesized several multiarylpyrrole (MAP) derivatives, in which a furanylidene (FE) group at the 3-position of the pyrrole forms donor-π-acceptor molecules, MAP-FE, with a NIR emissive wavelength and aggregation-enhanced emission (AEE) features. Different alkyl chains of MAP-FEs linked to phenyl groups at the 2,5-position of the pyrrole ring resulted in different emissive wavelengths and quantum yields in aggregated states, such as powders or single crystals. Powder XRD data and single crystal analysis elucidated that the different lengths of alkyl chains had a significant impact on the regularity of MAP-FEs when they were forced to aggregate or precipitate, which affected the intermolecular interaction and the restriction degree of the rotating parts, which are essential components. Therefore, an increasing number of NIR dyes could be developed by this design strategy to produce efficient NIR dyes with AEE. Moreover, this method can provide general guidance for other related fields, such as organic solar cells and organic light-emitting materials, because they are all applied in the aggregated state.     

Luminescence Color Tuning from Blue to Near Infrared of Stable Luminescent Solid Materials Based on Bis‐o‐Carborane‐Substituted Oligoacenes

Aryl‐substituted o ‐carboranes have shown highly efficient solid‐state emission in previous studies. To demonstrate color tuning of the solid‐state emission in an aryl‐o ‐carborane‐based system, bis‐o ‐carborane‐substituted oligoacenes were synthesized and their properties were systematically investigated. Optical and electrochemical measurements revealed efficient decreases in energy band gaps and lowest unoccupied molecular orbital (LUMO) levels by adding a number of fused benzene rings for the extension of π‐conjugation. As a consequence, bright solid‐state emission was observed in the region from blue to near infrared (NIR). Furthermore, various useful features were obtained from the modified o ‐carboranes as an optical material. The naphthalene derivatives exhibited aggregation‐induced emission (AIE) and almost 100 % quantum efficiency in the crystalline state. Furthermore, it was shown that the tetracene derivative with NIR‐emissive properties had high durability toward photo‐bleaching under UV irradiation.     

Multicolor tuning of aggregation-induced emission through substituent variation of diphenyl-o-carborane

We demonstrate multicolor tuning of aggregation-induced emission (AIE) derived from o-carborane. Both electron-donating and electron-accepting arylacetylenes underwent efficient palladium coupling reaction with bis(4-bromophenyl)-o-carborane, resulting in moderate yields. The emission spectra of these compounds span almost the entire visible spectrum (λ(max) = 452-662 nm). Study on AIE mechanism indicated that CT-based emission of o-carborane derivatives was enhanced by the restriction of molecular motions. The computational study also suggests the possibility of precise color-control of AIE through substituent variation.     

Novel antennae for the sensitization of near infrared luminescent lanthanide cations

Near Infrared (NIR) luminescence is useful for many applications ranging from lasers, telecommunication to biological imaging. We have a special interest for applications in biological media since NIR photons have less interference with such samples. NIR photons can penetrate relatively deeply in tissues and cause less damage to biological samples. The use of NIR luminescence also results in improved detection sensitivity due to low background emission. The lower scattering of NIR photons results in improved image resolution. NIR emitting lanthanide compounds are promising for imaging because of their unique properties such as sharp emission bands, long luminescence lifetimes and photostability. Here, we review our efforts to develop novel sensitizers for NIR emitting lanthanides. We have employed two global strategies: (1) monometallic lanthanide complexes based on derivatives of salophen, tropolonate, azulene and pyridine; and (2) polymetallic lanthanide compounds based on nanocrystals, metal-organic frameworks and dendrimers complexes.     

Synthesis of aza-BODIPY dyes bearing the naphthyl groups at 1,7-positions and application for singlet oxygen generation

Near-infrared absorbing aza-BODIPYs with the naphthyl groups at 1,7-positions were prepared for the first time. The singlet oxygen generation of aza-BODIPY with the naphthyl groups at 1,7-positions was more effective than that of the corresponding aza-BODIPY with the phenyl groups at 1,7-positions.