Intramolecular C-H/C-D exchange in cofacially stacked polyfluorenes via electron-induced bond activation

EPR studies in hexamethylphosphoramide, along with DFT studies, reveal that the one-electron reduction of pi-stacked polyfluorenes containing two, three, and four fluorene units leads to the corresponding anion radicals where the odd electron is located exclusively on an outside fluorene moiety. These anion radicals disproportionate to form small concentrations of diamagnetic dianions, wherein there is a nonclassical pz overlap that crosses the C2 axis. The presence of the two extra electrons activates the C-H bonds in the ortho positions, and concomitant overlap of the p-orbitals involving adjacent fluorene moieties results in intramolecular hydrogen exchange at temperatures as low as 90 K.     

Stacking effect of polyfluorene on the chemical shift and electron transport

We investigate the structures, NMR chemical shifts, absorption spectra, frontier molecular orbitals, and transition density matrices of pi-stacked polyfluorenes by ab initio calculations. For F1-F4, we consider two different conformations, syn and anti. The simulated 1H NMR chemical shifts are in good agreement with the previous experiment, and the significantly upfielded chemical shifts explain that the fluorene moieties are stacked on each other. It is found that the relative stability for syn and anti conformers is almost equivalent in B3LYP calculations; however, the syn conformer becomes much more stable than the anti conformer in MP2 calculations, which is consistent with the experimental finding that only the syn conformers are relevant. The vertical detachment energy, which is linearly proportional to the ionization potential, shows the same size dependence as the previous experiment. The electron attachment energy decreases exponentially as the size increases, which implies that the electron transport would be possible even for long chains such as F3 and F4. This was evident from the frontier molecular orbitals (HOMO and LUMO). Also, it is found that the syn conformers are very favorable for electron transport through the pi-stacked fluorene moieties.     

Synthesis, structure, and photophysical and electrochemical properties of a pi-stacked polymer

Dibenzofulvene (DBF) was polymerized using anionic initiators to afford a vinyl polymer. Oligo(DBF)s having from two to eight side-chain fluorene moieties bearing different chain-terminal groups were isolated by preparative size-exclusion chromatography. The structures of the isolated oligomers were revealed by single-crystal X-ray and (1)H NMR analyses. Both in solution and in crystal, the in-chain fluorene moieties stacked on top of each other, while the terminal conformation varied depending on the terminal group. These conformational characteristics were supported by molecular mechanics and dynamics calculations. The oligomers and polymers indicated hypochromism and red shift in UV absorption spectra and exclusive excimer emission in fluorescence spectra. In addition, reduced oxidation potentials were observed for the oligomers in electrochemical analyses, which suggests charge delocalization over the pi-stacked electron systems. The photophysical and electrochemical effects increased with the chain length of the oligomers and leveled off around the chain length of an oligomer consisting of five fluorene units.     

Probing charge transport in pi-stacked fluorene-based systems

The molecular parameters governing charge transport along a pi-stacked fluorene chain in poly(dibenzofulvene) are studied by a joint experimental and theoretical approach involving high-resolution gas-phase photoelectron spectroscopy and quantum-mechanical methods. We specifically investigate the electronic couplings between fluorene moieties as well as the intramolecular reorganization energies, for both holes and electrons. Our results indicate that a pi-stacked fluorene chain favors hole transport over electron transport. The values for electronic couplings and reorganization energies estimated here are compared with those derived recently for pentacene.     

Intramolecular electron transfer in cofacially pi-stacked fluorenes: evidence of tunneling

The one-electron reduction of neutral pi-stacked di- and trifluorenes (F-2 and F-3) in HMPA, where ion association is absent, results in the formation of anion radicals in which the odd electron resides predominantly on just one of the external fluorene moieties, as established by EPR spectroscopy. However, in the case of tetrafluorene, introduction of a single electron leads to a kinetically controlled anion radical F-4(int)*- in which the odd electron undergoes rapid exchange between two central fluorene rings, where the anionic charge is partially shielded from solvation due to the presence of external fluorene rings. On a time scale of minutes, anion radical F-4(int)*- converts to a thermodynamically stabilized anion radical F-4(ext)*-, with the electron exhibiting coupling from the protons on an external fluorene moiety. The charge and spin residing on an external moiety allow efficient solvation of the anionic charge. A similar fast exchange of a single electron (probably with the involvement of quantum mechanical tunneling) among three and four internal fluorene moieties is initially observed via EPR spectroscopy in the penta- and hexafluorene derivatives, F-5 and F-6, respectively.     

Cationic conjugated polymers for homogeneous and sensitive fluorescence detection of hyaluronidase

The cationic charged water-soluble polyfluorenes containing 2,1,3-benzothiadiazole (BT) units (P1–3) have been synthesized and characterized. These polymers demonstrate intramolecular energy transfer from the fluorene units to the BT sites when oppositely charged hyaluronan is added due to the formation of electrostatic complexes, followed by a shift in emission color from blue to green or brown. Upon adding hyaluronidase, the hyaluronan is cleaved into fragments. The relatively weak electrostatic interactions of hyaluronan fragments with polyfluorenes keep their main chains separated and energy transfer from the fluorene units to the BT sites is inefficient, and the polyfluorenes recover their blue emissions. The complexes of conjugated polymers/hyaluronan can be utilized as probes for sensitive and facile fluorescence assays for hyaluronidase. The new assay method interfaces with the aggregation and light harvesting properties of conjugated polymers. Supported by the “100 Talents” Program of Chinese Academy of Sciences, the National Natural Science Foundation of China (Grant Nos. 20725308 & 20721061), and 973 Project (Grant Nos. 2006CB806200 & 2006CB932100)     

SYNTHESIS AND PROPERTIES OF POLY(AROMATIC DIACETYLENE)S CONTAINING FLUORENE

Novel fluorene-based poly(aromatic diacetylene)s have been synthesized by CuCl-catalyzed oxidative coupling of aromatic diynes.New aromatic diynes 2,7-diethynyl-9,9-bis(triphenylamine)fluorene(M_1)is synthesized by multistep reactions.The structures and properties of the polymers are characterized and evaluated by IR,NMR,TGA,UV, photoluminescence(PL),and cyclic voltammetry analyses.These polymers possess good thermal stability.All the polymers are completely soluble in coammon solvents such as toluene,TH...     

Monodisperse fluorene oligomers exhibiting strong dipolar coupling interactions

Well-defined fluorene oligomers (n = 1 to 6) were prepared step by step using Suzuki and Yamamoto couplings, while absorption and photoluminescence properties evidenced very large dipolar coupling interactions between fluorene moieties.     

Synthesis and photophysical properties of polyfluorenes bearing silicon‐based functional groups

Five polyfluorenes bearing bulky trimethylsilyl (PTMS1 and PTMS2), tris(trimethylsilyl)silyl (PTTMS1), and silsesquioxane groups (PPOSS1 and PPOSS2) were synthesized through palladium‐catalyzed Suzuki coupling reactions. In the solution state, every polymer showed comparable ultraviolet–visible spectra, and they emitted blue light with high quantum efficiency. In the solid state, however, three trimethylsilyl‐functionalized polyfluorenes indicated redshifts of the fluorescence peak. In particular, PTMS1 and PTTMS1, having a hydrogen at the C‐9 position of fluorene, also showed green‐light emissions. After the annealing of the spin‐coated films, the blue‐emissive peak decreased and the green‐emissive peak became stronger in the photoluminescence spectra of three trimethylsilyl‐functionalized polyfluorenes. In contrast, PPOSS2 showed a pure blue‐light emission in the film state and even after the thermal treatment, which could be accomplished by the encapsulation of the polymer chains by the large polyhedral oligomeric silsesquioxane molecule. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 2119–2127, 2005     

Water‐Soluble Fluorescent Probes Based on Dendronized Polyfluorenes for Cell Imaging

Novel water‐soluble dendronized fluorescent polyfluorenes (DFPFs) are prepared from hydrophilic monomers and hydrophobic comonomers. Incomplete energy transfer is found to result in a two‐color emission of the DFPFs at around 410 and 650 nm. The incomplete energy transfer can be attributed to the poor compatibility between the fluorene and benzothiadiazole units. Polyethylene oxide (PEO) encapsulation of the DFPFs shows over 90% cell viability, indicating good biocompatibility. These DFPFs show differential cellular uptake. P1 with fewer PEO chains exhibits limited cellular membrane uptake and low brightness in cells. By contrast, P3 with more PEO chains is efficiently internalized by cells and accumulated in the cytoplasm. A strong fluorescence from whole cells is also observed.     

9位芳香基取代超支化聚芴的合成与表征

以2,7-二溴芴酮为起始原料,在芴的9位引入芳香取代基代替传统的烷基取代基,增加9位的稳定性,并用AB+AB2的方法,用新单体合成了不同支化度的超支化聚合物,新合成的超支化聚合物具有良好的热稳定性和发光稳定性.聚合物的热分解温度都在400℃以上;聚合物的薄膜在空气中于200℃加热2 h后没有出现绿光发射带,发光稳定性和烷基聚芴相比得到明显的提高.DSC结果显示,线性聚合物在170℃时有玻璃化转变,超支化聚合物在300℃以内没有明显的相转变,保持一种稳定的无定形态,更有利于提高材料的发光效率.     

Synthesis and relationships between the structures and properties of monodisperse star-shaped oligofluorenes

[structure: see text] A convenient approach to a series of star-shaped pi-conjugated organic materials with oligofluorenes (OF) as the branches and a benzene ring as the core has been developed. The optical properties of these materials are liable to be tuned by introduction of more fluorene rings. The investigation of the structure-property relationship of these materials has indicated differences between star-shaped oligofluorenes and linear oligofluorenes, as well as polyfluorenes.     

Synthesis and characterization of star polyfluorenes with a C60 core

C60‐cored star polyfluorenes were synthesized and fully characterized. A high yield (81%) hexakisaddition of C60 was developed by using Prato reaction and bulky fluorene addends. Suzuki polycondensation of the hexakisadducts of C60 carrying six 2‐bromofluorene addends and AB‐type monomer (2‐bromo‐9,9‐dioctylfluorenyl‐4,4,5,5‐tetramethyl‐ [1,3,2]‐dioxaborane) with Pd(PPh₃)₄ as the catalyst precursor afforded the desired C60‐cored star polyfluorenes. Their three‐dimensional structure can effectively reduce the aggregation of the polyfluorene chains. Annealing studies indicated that the C60‐cored star polyfluorenes are of good color stability. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4696–4706, 2007     

Novel Efficient Light-Emitting Polyfluorene Derivatives Modified by Electron-Deficient Moieties with Nonlinear Structure

Two novel fluorene-based copolymers (PFSD and PFMD) containing squaric acid or maleimide unit in the main chain were synthesized in good yields by Suzuki coupling reaction. The resulting polymers possess excellent thermal stability, high electron affinity and high photolurninescence (PL) quantum yields. They can fluoresce in yellow-light range due to either the charge transfer between a fluorene segment and an electron-deficient containing squaric acid/maleimide segment of the polymers or the Forrster energy transfer between different polymer chains.The results from PL measurements of the isothermally heated polymer thin films show that the commonly observed aggregate excimer formation in polyfluorenes is very effectively suppressed in these two polymers due to the nonlinear structures of maleimide and squaric acid moieties. Double-layer polymer light-emitting diodes (PLED) were fabricated using the resulting polymers as the emitting layers and Ba or Mg：Ag(V：V=10：1) as cathodes.All the devices show bright yellow emission (562-579nm) with different maximum external quantum efficiencies (0.006%-1.13%). Compared with the other devices, indium-tin oxide (ITO)/polyethylenedioxythiophene (PEDOT):polystyrene sulfonic acid (PSS)/PFMD/Mg:Ag has the higher maximum external quantum efficiency of 1.13% at 564cd/m^2 with a bias of 8.4V.     

Scope and limitations of the SCS-MP2 method for stacking and hydrogen bonding interactions

Fluorobenzenes are pi-acceptor synthons that form pi-stacked structures in molecular crystals as well as in artificial DNAs. We investigate the competition between hydrogen bonding and pi-stacking in dimers consisting of the nucleobase mimic 2-pyridone (2PY) and all fluorobenzenes from 1-fluorobenzene to hexafluorobenzene (n-FB, with n = 1-6). We contrast the results of high level ab initio calculations with those obtained using ultraviolet (UV) and infrared (IR) laser spectroscopy of isolated and supersonically cooled dimers. The 2PY.n-FB complexes with n = 1-5 prefer double hydrogen bonding over pi-stacking, as diagnosed from the UV absorption and IR laser depletion spectra, which both show features characteristic of doubly H-bonded complexes. The 2-pyridone.hexafluorobenzene dimer is the only pi-stacked dimer, exhibiting a homogeneously broadened UV spectrum and no IR bands characteristic for H-bonded species. MP2 (second-order M?ller-Plesset perturbation theory) calculations overestimate the pi-stacked dimer binding energies by about 10 kJ/mol and disagree with the experimental observations. In contrast, the MP2 treatment of the H-bonded dimers appears to be quite accurate. Grimme's spin-component-scaled MP2 approach (SCS-MP2) is an improvement over MP2 for the pi-stacked dimers, reducing the binding energy by approximately 10 kJ/mol. When applied to explicitly correlated MP2 theory (SCS-MP2-R12 approach), agreement with the corresponding coupled-cluster binding energies [at the CCSD(T) level] is very good for the pi-stacked dimers, within +/- 1 kJ/mol for the 2PY complexes with 1-fluorobenzene, 1,2-difluorobenzene, 1,2,4,5-tetrafluorobenzene, pentafluorobenzene and hexafluorobenzene. Unfortunately, the SCS-MP2 approach also reduces the binding energy of the H-bonded species, leading to disagreement with both coupled-cluster theory and experiment. The SCS-MP2-R12 binding energies follow the SCS-MP2 binding energies closely, being about 0.5 and 0.7 kJ/mol larger for the H-bonded and pi-stacked forms, respectively, in an augmented correlation-consistent polarized valence quadruple-zeta basis. It seems that the SCS-MP2 and SCS-MP2-R12 methods cannot provide sufficient accuracy to replace the CCSD(T) method for intermolecular interactions where H-bonding and pi-stacking are competitive.     

DNA as a supramolecular scaffold for the helical arrangement of a stack of 1-ethynylpyrene chromophores

A highly organized helical pi-stacked arrangement of 1-ethynylpyrene moieties along the major groove of duplex DNA can only be achieved if more than three chromophores have been synthetically incorporated adjacent to each other.     

Hybridization of emitting polyfluorene with silicone

Polyfluorenes with pendant allyl groups were prepared by terpolymerization of 9,9‐dihexylfluorene‐2,7‐bis(trimethyleneborate), 2,7‐dibromo‐9,9‐bis(2‐allyloxyethyl)fluorene, and 4,7‐dibromo‐2,1,3‐benzothiazole, or 4,7‐dibromo‐2,1,3‐naphthoselenadiazole using Suzuki coupling reaction. The subsequent hydrosilylation reaction of these precursor polymers with ethoxydimethylsilane quantitatively converted the allyl groups to ethoxysilyl groups. Hybridization of the emitting polyfluorenes with silicone was successfully achieved by the solvent‐free sol–gel reaction of tetraethoxysilane and silanol‐terminated polydimethylsiloxane in the presence of the ethoxysilyl‐functionalized polyfluorenes. Fluorescence spectra of these luminescent silicones revealed that emitting polyfluorenes were dispersed homogeneously in the matrix of silicone without aggregation. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 622–628     

Synthesis,Structure, Photoluminescence,and Electroluminescence of Siloles that Contain Planar Fluorescent Chromophores

Herein, a new series of siloles that were 2,5‐substituted with planar fluorescent chromophores (PFCs), including fluorene, fluoranthene, naphthalene, pyrene, and anthracene, were synthesized and characterized. These compounds showed weak emission in the solution state, owing to active intramolecular rotation (IMR), but the synergistic effect from electronic coupling between the PFC and the silole ring compensated for the emission quenching by the IMR process to some extent, thereby affording higher emission efficiencies than those of 2,3,4,5‐tetraphenylsiloles in solution. These new siloles showed enhanced emission efficiencies in the aggregated state. The electroluminescence (EL) color and efficiency of new siloles were sensitive towards the PFC. Siloles containing naphthalene moieties showed green EL emission, whilst those containing anthracene moieties showed orange EL emission. The siloles containing pyrene moieties exhibited yellow EL emission at 546 nm, with a peak luminance of 49000 cd cm^?2 and a high current efficiency of 9.1 cd A^?1.     

Friedel–Crafts polymerization of fluorene with methylene chloride,methoxyacetyl chloride,and chloromethyl methyl ether

The polymerization of fluorene with various chloromethylating reagents (methylene chloride, methoxyacetyl chloride, and chloromethyl methyl ether) in the presence of aluminum chloride was studied. The products that were obtained were insoluble, infusible, colored powders which contain a low concentration of aromatic radicals. Each of the polymers is composed predominantly of fluorene nuclei bridged at the 2 and 7 positions by methylene groups. In addition, the materials possess varying amounts of crosslinked and ring-fused moieties. Based on physical and spectral properties and reaction stoichiometry, the fluorene/methylene chloride polymer is the most highly crosslinked of the three. The fluorene/methoxyacetyl chloride and fluorene/chloromethyl methyl ether polymers are similar to each other in most respects.     

Polyfluorenes with polyphenylene dendron side chains: toward non-aggregating, light-emitting polymers.

A polyfluorene 12 has been prepared in which bulky polyphenylene dendrimer substituents suppress formation of long wavelength emitting aggregates, thus giving a polymer with pure blue emission. Absorption- and emission spectra and molecular modeling confirm that the bulky dendrimer side chains do not cause extra torsion between the fluorene units. New polyfluorenes with 9,9-diaryl substituents have been prepared to determine the minimum size of substituent necessary for aggregation suppression. An LED using 12 has been demonstrated to produce blue emission with onset voltages below 4 V.