Novel fluorophores: efficient synthesis and photophysical properties

The synthesis of new donor-acceptor acetylene-bridged compounds was realized via organophosphorus reactions and Sonogashira couplings. These systems exhibit high fluorescence quantum yields, and excitation induces very efficient charge redistribution in the molecules. [structure: see text]     

New fluorophores with rod-shaped polycyano pi-conjugated structures: synthesis and photophysical properties

[reaction: see text] Novel rod-shaped polycyano-oligo(phenyleneethynylene)s were synthesized by Pd cross-coupling reaction. Polycyano groups were found to greatly improve the emission efficiency (Phi(f)) of OPEs. By the end donor modification, we achieved the creation of very intense blue light-emitting fluorophore with the SMe group (Phi(f) = 0.972, log epsilon 4.89, lambda(em) 455 nm) and very intense yellow light-emitting fluorophore with the NMe(2) group (Phi(f) = 0.999, log epsilon 4.75, lambda(em) 555 nm). Contrasting Phi(f) solvent dependency of 6 and 7 and a linear relationship between Phi(f) and sigma(p)-X over the whole region of sigma(p)-X were also found.     

Dibenzosuberenylidene-ended fluorophores: rapid and efficient synthesis, characterization, and aggregation-induced emissions

A series of pi-conjugated compounds ending with dibenzosuberenylidene were synthesized efficiently. Their luminescence efficiencies were well-tuned by structural modification. Moreover, relative emission intensities were strongly affected by their existing appearances and exhibited aggregation-induced emission (AIE) behavior. Thus, emissions from nanoparticles, films, or powders were found to be more efficient than those from solutions. It demonstrated that these synthesized compounds might be practically used as fluorescent materials for potential optoelectronic applications.     

Synthesis and photophysical properties of nonbenzoid ended fluorophores

A series of pi-conjugated compounds (1-23) were successfully synthesized by the Sonogashira, Suzuki, and Wittig-Horner reactions. The structure-property relationship about photoluminescence based on these synthesized compounds was systematically discussed in detail. In addition, energy gaps between ground and excited states of these compounds, which were in good agreement with the experimental observation, were calculated by the semiempirical parametrized PM3 method. It was revealed that emission efficiency largely depends on molecular structure, aggregation type, and size of nanoaggregates.     

Novel pyrazoline and pyrazole porphyrin derivatives: synthesis and photophysical properties

2-Vinyl-5,10,15,20-tetraphenylporphyrinatozinc(II) reacts with nitrile imines, generated in situ from ethyl hydrazono-α-bromoglyoxylates, affording the corresponding pyrazolines in good to excellent yields. Treatment of pyrazoline derivatives with DDQ affords the corresponding pyrazole derivatives with moderate to excellent yields. When the hydrolysis of ester group in the pyrazoline derivatives was considered, it was observed the concomitant oxidation of the heterocyclic unit, which allowed directly obtaining porphyrin-pyrazole derivatives with a carboxylic group, in very good yields.The photophysical properties of the pyrazoline and pyrazole porphyrin derivatives show that the influence of the heterocyclic substituents is limited by the tendency of these molecules to aggregate. All other properties and especially the triplet kinetics remain unaffected. The adducts with low tendency to aggregate showed very high singlet oxygen yield, which makes these compounds interesting for their use as photosensitizers for PDT.     

Synthesis and photophysical studies of bis-enediynes as tunable fluorophores

We have synthesized a family of bis-enediynes by two complementary Pd/Cu-catalyzed Sonogashira cross-coupling methods. One is a modified Sonogashira reaction between the TMS-protected tetraalkyne 20 (or 21) and various aromatic bromides to afford bis-enediynes 22a-d and 23a-d bearing different peripheral aryl units. The other, the reaction of bifunctional 1,1-dibromo-1-alkenes with phenylacetylene, afforded a series of bis-enediynes 24-32 bearing various core aryl groups. These chemical modifications to the core and periphery of bis-enediynes induce dramatic changes in absorption and emission spectra. Bis-enediynes 22 and 23 show a large Stokes shift of about 50-110 nm when compared to the less-conjugated bis-enediynes 20 and 21. Absorptions and emissions of bis-enediynes 25, 27-29, and 31 were red-shifted relative to those of enediyne 35. Substantial increases in fluorescence quantum yields are observed as a result of extending the pi-conjugation. The emission wavelength of bis-enediynes was tailored from indigo blue to reddish-orange, suggesting that the color of emission can be tunable by modification of the core and/or peripheral units.     

Novel 8-arylidenetetrahydroquinazoline N-oxides: synthesis,photophysical properties and biological evaluation

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Convenient and efficient synthesis of functionalized oligopyridine ligands bearing accessory pyrromethene-BF2 fluorophores

The synthesis of stable and highly luminescent pyridine-, bipyridine-, phenanthroline-, bipyrimidine-, and terpyridine-based ligands bearing one or two 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (bodipy) modules has demonstrated the advantages of three different protocols which have been adapted in light of the chemical stability of the alkyne-grafted starting building blocks and the chemical reactivity of the bromo-substituted starting materials. A classical method of condensation of aldehydes or acid chlorides with Kryptopyrrole has been used for direct linkage of a bodipy to the oligopyridinic platform. For the phenylethynyl-linked molecules, direct coupling between the bodipy-phenyliodo and the stable 4'-ethynyl-2,2':6',2' '-terpyridine, 6,6' '-diethynyl-2,2':6',2' '-terpyridine, 5-ethynyl-2,2'-bipyridine, 5,5'-diethynyl-2,2'-bipyridine, 6,6'-diethynyl-2,2'-bipyridine, and 5,5'-diethynyl-2,2'-bipyrimidine substrates is feasible and is promoted by Pd catalysts and sonication. This procedure provides the advantages of efficiency, versatility, and rapidity. A second set of experimental conditions is required to produce the 4-substituted pyridine, 3,8-disubstituted-1,10-phenanthroline, and 5,5' '-disubstituted-2,2':6',2' '-terpyridine derivatives. Cross coupling of a bodipy-phenylethynyl molecule with the bromo-substituted partners takes place smoothly with the pyridine but with low yields in the other cases due to the efficient formation of the homocoupled diphenylbutadiyne bodipy compounds. A third convenient protocol enabled the preparation of these target molecules in a one-pot reaction where the deprotection of the alkyne was conducted in situ by a phase-transfer process with aqueous NaOH and with Et(3)BnN(+)Cl(-) as mediator and the cross-coupling reaction realized in the benzene phase with [Pd(PPh(3))(4)] as catalyst and CuI as co-reagent. The nascent acid was quenched in the aqueous phase. This method is much more efficient when a trimethylsilyl protecting group is used instead of a propargylic alcohol. The pyridino-bodipy bases were alkylated smoothly in good yields in the presence of methyl iodide. The photophysical and electrochemical properties for these new molecules have been investigated and are discussed in terms of substitution patterns of the bipyridine and terpyridine platforms. In the pyridinium salt 18, the fluorescence from the first singlet excited state at lambda(em) = 547 nm is totally quenched at the expense of a low-lying charge-transfer emitting state located at lambda(em) = 660 nm.     

Novel silicon-based alternating copolymers: synthesis,photophysical properties,and tunable EL colors

We synthesized novel silicon-based alternating copolymers for tunable electroluminescent (EL) colors by Heck synthetic method. Their thermal, photophysical and electroluminescent properties were studied. Most of them exhibited a blue-green EL color at the operating voltage of lower than 12 V. Unusually, we observed the white EL color from a EL device based on SiPhPVK. From photophysical studies and the time-resolved PL spectroscopies, it might be attributed to the formation of stabilized excited state in SiPhPVK. Furthermore, in order to reduce the operating voltage of their LED with increasing the electron affinity of the main chain in silicon-based alternating copolymers, we synthesized the silicon-based copolymers containing electron transporting oxadiazole units in main chain. We also studied their photophysical and electroluminescent properties.     

Synthesis and photophysical properties of new conjugated fluorophores designed for two-photon-excited fluorescence

[structure: see text] Novel elongated push-push fluorophores (e.g., 9) were synthesized by 2-fold Sonogashira or Wittigminus signHorner reactions. Modulation of the length and topology of the conjugated connectors allows tuning of their photophysical properties. In addition, their photoluminescence can be adjusted by playing on polarity. Derivatives combining enhanced two-photon absorption cross section (sigma2) in the visible red and high fluorescence quantum yield (Phi) have been obtained. Such fluorophores hold promise for nonlinear imaging of biological systems.     

Novel fluorophores for single-molecule imaging

Nonlinear optical chromophores based on dicyanodihydrofuran acceptors paired with amine donors have been found to exhibit sufficiently large fluorescence quantum yields and stability to enable single-molecule detection in polymeric hosts. To illustrate the breadth of this class, six fluorophores are presented, spanning the emission range from 505 to 646 nm. In contrast to conventional single-molecule fluorophores, the new molecules feature sensitivity to local rigidity, large ground-state dipole moments, and large polarizability anisotropies, properties that can be used to design new reporter experiments at the single-molecule level.     

Extended phenothiazines: synthesis,photophysical and redox properties,and efficient photocatalytic oxidative coupling of amines

Herein we successfully developed a ring-fusion approach to extend the conjugation length of phenothiazines and synthesized a series of novel extended phenothiazines 1–5. The intriguing π-conjugation length-dependent photophysical and redox properties of 1–5, and their photocatalytic performance towards visible-light-driven oxidative coupling reactions of amines were systematically investigated. The results indicated that this series of extended phenothiazines exhibited continuous red shifts of light absorption with increasing numbers of fused rings. As compared with the conventional phenothiazine (PTZ), all the extended phenothiazines displayed reversible redox behavior and maintained a strong excited-state reduction potential as well. Consequently, 3, 4 and 5 with longer effective conjugation lengths could efficiently catalyze the oxidative coupling of amines to imines under visible-light irradiation; by comparison, the shorter 1, 2 and PTZ could only catalyze such reactions in the presence of UV light. Moreover, 3 showed superior catalytic performance which can result in better yields within a shorter reaction time, and in a broad substrate scope. Finally, a direct and efficient conversion of amines to imines under sunlight in an air atmosphere was successfully realized. We believe that our study including the new phenothiazine modification methodology and the newly developed extended phenothiazine-based photocatalysts will open up a new way to develop novel phenothiazine-based materials for optoelectronic and catalytic applications.

Herein, we successfully developed a ring-fusion approach to extend the conjugation length of phenothiazines that were demonstrated to be efficient photocatalysts for visible-light-driven oxidative coupling reactions of amines under an air atmosphere.     

Synthesis, structures, redox and photophysical properties of benzodifuran-functionalised pyrene and anthracene fluorophores

Benzodifuran-functionalised pyrene and anthracene fluorophores 1 and 2 were obtained in reasonable yields. Their single crystal structures, electrochemical, optical absorption, and fluorescence characteristics have been described. They show strong luminescence with high quantum yields of 0.53 for 1 and 0.48 for 2.     

Novel core-expanded rylenebis(dicarboximide) dyes bearing pentacene units: facile synthesis and photophysical properties

Two synthetic routes for the benzannulation in the "bay"-region of rylenebis(dicarboximide)s leading to new pi-system-expanded chromophores are described. The first route follows a two-step approach: Suzuki coupling of bromo-substituted perylenebis(dicarboximide) with 2-bromophenylboronic acid, followed by palladium-catalysed dehydrobromination. The second route is best described as a palladium-assisted cycloaddition of benzyne, formed in situ, to the bay-region of the bromo-substituted rylene core. Two new types of core-expanded rylene dyes were synthesised: yellow dibenzocoronenebis(dicarboximide)s, absorbing at 490 nm, and a green dinaphthoquaterrylenebis(dicarboximide), which absorbs at 700 nm. These new chromophores are characterised by significant hypsochromic shifts of absorption, compared to their parent rylenebis(dicarboximide)s, excellent photostabilities and high fluorescence quantum yields.     

Fluorenyl dendrimer porphyrins: synthesis and photophysical properties

New symmetrical dendrimeric type super-porphyrin bearing sixteen fluorenyl donor groups sixteen fluorenylporphyrin SOFP (1) have been synthesized and characterized. Preliminary photophysical properties are reported; in comparison to the references first generation dendrimer tetrafluorenylporphyrin TOFP (2) bearing four peripheral fluorenyl and second generation dendrimer octafluorenylporphyrin OOFP (3) bearing eight peripheral fluorenyl, the luminescence properties are slightly improved. It is found that the excitation energy transfer occurs from the sixteen fluorenyl units to the porphyrin core, following what the porphyrin emits intense red light.     

An efficient one-pot solvent-free synthesis and photophysical properties of 9-aryl/alkyl-octahydroxanthene-1,8-diones

A facile and efficient protocol for the synthesis of 9-aryl/alkyl-3,3,6,6-tetramethyl-1,2,3,4,5,6,7,8-octahydroxanthene-1,8-diones has been developed by one-pot reaction of dimedone with aldehydes in the presence of InCl₃ or P₂O₅ under solvent-free conditions. The optical behaviors of the xanthenediones have been investigated by UV-vis and fluorescence spectroscopy. The present approach offers the advantages of simple methodology, clean and mild reaction, high atom-economy, short reaction time, low environmental impact, wide substrate scope, high yield, and excellent purity.     

Novel and efficient synthesis of 4-sulfonyl-2-pyridones

Heterocyclic 4-sulfonyl-2-pyridones represent useful scaffolds for drug discovery, and are also versatile synthetic building blocks. Herein, we describe a novel and efficient synthesis of this heterocyclic ring system utilizing an acid-mediated cyclo-condensation reaction. This synthetic method affords convenient access to structurally diverse N-substituted 4-sulfonyl-2-pyridones in moderate to good yields.     

Novel and efficient copper-catalysed synthesis of nitrogen-linked medium-ring biaryls

Herein, a new copper-catalysed strategy for the synthesis of rare nitrogen-linked seven-, eight- and nine-membered biaryl ring systems is described. It is proposed that the reaction proceeds through a highly activated intramolecularly co-ordinated copper catalyst. The process is technically simple, proceeds under relatively mild conditions, displays a broad substrate scope and forms biologically valuable products that are difficult to synthesise by other methods. We envisage that this methodology will prove useful in a wide synthetic context, with possible applications in both target-oriented and diversity-oriented synthesis.     

Design,photophysical properties,and applications of fluorene-based fluorophores in two-photon fluorescence bioimaging: A review

Two-photon fluorescence microscopy (2PFM) emerged as a powerful alternative to conventional one-photon microscopy. 2PFM typically uses two near-infrared (NIR) photons to excite fluorescent dyes, which minimizes light scattering in biological samples. Multiphoton absorption also suppresses background signal and autofluorescence from tissues and allows to achieve higher 3D resolution images with low photodamage and photobleaching. Fluorene dyes possess distinct properties that meet the strict criteria of probes used for 2PFM such as enhanced solubility, photostability, and two-photon absorption cross-section. The fluorene molecule also includes many active positions that allow versatile synthesis, selective functionalization, bioconjugation, and tuning solubility. These properties have led to reporting several fluorene probes including monomers, polymers, and dendrimers with important uses in understanding molecular dynamics and bioimaging. The current review presents a compact summary of fluorene-based fluorophores for 2PFM bioimaging applications, shedding light on structure-photophysical property relationships in fluorenes and polyaromatic probe designs.