Towards General Guidelines for Aligned,Nanoscale Assemblies of Hairy‐Rod Polyfluorene

This account highlights recent progress towards understanding the complex hierarchical levels of solid‐state structure in a prototypical helical hairy‐rod polyfluorene, poly[9,9‐bis(2‐ethylhexyl)fluorene‐2,7‐diyl] (or PF2/6). This branched‐side‐chain containing polyfluorene undergoes a systematic intermolecular self‐assembly and liquid‐crystalline phase behavior in combination with uniaxial and biaxial alignment. The latter processes yield full three‐dimensional orientation of the crystallites and polymer chains. Also reviewed are the impact of the molecular structure and phase behavior on surface morphology, anisotropic film formation, and, ultimately, the overall impact of these physical attributes on optical constants. This particular polyfluorene also represents a model system for demonstrating the applicability of mean‐field theory in detailing the self‐organization of aligned hairy‐rod block‐copolymer systems. These results of PF2/6 are compared to those of other archetypical π‐conjugated hairy‐rod polymers. General guidelines of how molecular weight influences nanostructure, phase behavior, alignment, and surface morphology are given.     

The Fabrication and Characterization of Single‐Component Polymeric White‐Light‐Emitting Diodes

By using Ni⁰‐mediated polymerization, we have systematically synthesized a series of fluorene‐based copolymers composed of blue‐, green‐, and red‐light‐emitting comonomers with a view to producing polymers with white‐light emission. 2,7‐Dibromo‐9,9‐dihexylfluorene, {4‐(2‐[2,5‐dibromo‐4‐{2‐(4‐diphenylamino‐phenyl)‐vinyl}‐phenyl]‐vinyl)‐phenyl}‐diphenylamine (DTPA), and 2‐{2‐(2‐[4‐{bis(4‐bromo‐phenyl)amino}‐phenyl]‐vinyl)‐6‐tert‐butyl‐pyran‐4‐ylidene}‐malononitrile (TPDCM) were used as the blue‐, green‐, and red‐light‐emitting comonomers, respectively. It was found that the emission spectra of the resulting copolymers could easily be tuned by varying their DTPA and TPDCM content. Thus with the appropriate red/green/blue (RGB) unit ratio, we were able to obtain white‐light emission from these copolymers. A white‐light‐emitting diode using the polyfluorene copolymer containing 3 % green‐emitting DTPA and 2 % red‐emitting TPDCM (PG3R2) with a structure of indium tin oxide/poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonic acid)/PG3R2/Ca/Al was found to exhibit a maximum brightness of 820 cd m^–2 at 11 V with Commission Internationale de L'Eclairage (CIE) coordinates of (0.33,0.35), which are close to the standard CIE coordinates for white‐light emission (0.33,0.33).     

Highly Efficient Pure‐White‐Light‐Emitting Diodes from a Single Polymer: Polyfluorene with Naphthalimide Moieties

Light‐emitting diodes exhibiting efficient pure‐white‐light electroluminescence have been successfully developed by using a single polymer: polyfluorene derivatives with 1,8‐naphthalimide chromophores chemically doped onto the polyfluorene backbones. By adjusting the emission wavelength of the 1,8‐naphthalimide components and optimizing the relative content of 1,8‐naphthalimide derivatives in the resulting polymers, white‐light electroluminescence from a single polymer, as opposed to a polymer blend, has been obtained in a device with a configuration of indium tin oxide/poly(3,4‐ethylenedioxythiophene)(50 nm)/polymer(80 nm)/Ca(10 nm)/Al(100 nm). The device exhibits Commission Internationale de l'Eclairage coordinates of (0.32,0.36), a maximum brightness of 11 900 cd m^–2, a current efficiency of 3.8 cd A^–1, a power efficiency of 2.0 lm W^–1, an external quantum efficiency of 1.50 %, and quite stable color coordinates at different driving voltages, even at high luminances of over 5000 cd m^–2.     

Insight into the Role of Oxidation in the Thermally Induced Green Band in Fluorene‐Based Systems

The causes of the spectral instability of poly[9,9‐dioctylfluoren‐2,7‐diyl‐co‐2′,7′‐spiro(cyclohexane‐1,9′‐fluorene)] during thermal annealing in air, which leads to a green photoluminescence (PL) emission band, are investigated. The I_green/I_blue ratio evolution (I = intensity) is found to be independent of the amount of monoalkylfluorene defects, despite the fact that their presence might be regarded as a trigger for the radical process leading to polymer degradation in the presence of a trace amount of metal catalyst. Furthermore, the absence of a correlation between the degree of oxidation of the material and the I_green/I_blue ratio indicates that the spatial disposition of fluorenones formed during the thermal degradation of the material, rather than their amount, is to be strictly related to the I_green/I_blue ratio. The evidenced formation of fluorenone agglomerates, which could be considered the cause for the consistent increase in the I_green/I_blue ratio during a thermal oxidation of a polyfluorene, confirms that the radical mechanism can also involve dialkylfluorene systems. Finally, the higher resistance to thermal degradation shown by spirocyclohexane fluorene units with respect to dioctylfluorene ones allows the synthesis of new, spectrally stable, fluorene‐based copolymers.     

On the Origin of Green Emission Bands in Fluorene‐Based Conjugated Polymers

Blue‐light‐emitting diodes made of polyfluorenes have low stability and, under operation, rapidly degrade and produce undesirable low‐energy emission bands (green or g‐bands). A spectroelectrochemical study of the degradation process suffered by polyfluorenes is reported here. These polymers lose their electronic properties by electrochemical oxidation and reduction through σ‐bond breaking. In addition, upon electrochemical reduction, the development of a structured green emission band at 485 nm is observed. The position and shape of this band is different from the usual featureless band at 535 nm assigned to fluorenone defects. The green‐light‐emitting product is isolated and analyzed by Fourier‐transform IR spectroscopy; fluorenone formation is excluded. The isolated product is crosslinked; its green emission is probably related to the formation of an intramolecular excimer.     

White Electroluminescence from a Single‐Polymer System with Simultaneous Two‐Color Emission: Polyfluorene Blue Host and Side‐Chain‐Located Orange Dopant

Four single polymers with two kinds of attachment of orange chromophore to blue polymer host for white electroluminescence (EL) were designed. The effect of the side‐chain attachment and main‐chain attachment on the EL efficiencies of the resulting polymers was compared. The side‐chain‐type single polymers are found to exhibit more efficient white EL than that of the main‐chain‐type single polymers. Based on the side‐chain‐type white single polymer with 4‐(4‐alkyloxy‐phenyl)‐7‐(4‐diphenylamino‐phenyl)‐2,1,3‐benzothiadiazoles as the orange‐dopant unit and polyfluorene as the blue polymer host, white EL with simultaneous orange (λ_max = 545 nm) and blue emission (λ_max = 432 nm/460 nm) is realised. A single‐layer device (indium tin oxide/poly(3,4‐ethylenedioxythiophene)/polymer/Ca/Al) made of these polymers emits white light with the Commission Internationale de l'Éclairage coordinates of (0.30,0.40), possesses a turn‐on voltage of 3.5 V, luminous efficiency of 10.66 cd A^–1, power efficiency of 6.68 lm W^–1, and a maximum brightness of 21 240 cd m^–2.     

A New Donor–Acceptor–Donor Polyfluorene Copolymer with Balanced Electron and Hole Mobility

A new alternating polyfluorene copolymer poly[2,7‐(9,9‐dioctylfluoren)‐alt‐5,5‐(5′,8′‐di‐2‐thienyl‐(2′,3′‐bis‐(3′′‐octyloxyphenyl)‐quinoxaline))] (APFO‐15), which has electron donor–acceptor–donor units in between the fluorene units, is synthesized and characterized. This polymer has a strong absorption and emission in the visible range of the solar spectrum. Its electroluminescence and photoluminescence emissions extend from about 560 to 900 nm. Moreover, solar cells with efficiencies in excess of 3.5 % have been realized from blends of APFO‐15 and an electron acceptor molecule, a methanofullerene [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM). It has also been observed that electron and hole transport is balanced both in the pure polymer phase and in polymer/PCBM bulk heterojunction films, which makes this material quite attractive for applications in opto‐electronic devices.     

Novel blue light-emitting hyperbranched polyfluorenes incorporating carbazole kinked structure

A novel series of soluble hyperbranched polyfluorenes P1−P6 with various branching degrees and contents of kinked carbazole units were successfully synthesized with good yields and high molecular weight via a facile “A2 + B2 + C3 + D2” approach. The thermal, optical, and electrochemical properties as well as thermal spectral stability of the resulting hypberbranched polymers were investigated. All polymers exhibited good thermal stabilities and bright blue emission in both solutions and solid-states. Hyperbranched polyfluorenes (P3 and P6) exhibited improved spectral stability upon annealing at 200 °C in air, in sharp contrast to the linear poly(9,9-dihexylfluorene) (PDHF) that showed significant additional green emission at ca. 530 nm within minutes. In particular, outstanding spectral stability was observed with carbazole-incorporating hyperbranched polyfluorene P6. Electrochemical characterization indicated that the presence of carbazole also effectively raised the HOMO level with respect to that of polyfluorene homopolymer, suggesting better hole-injection properties. Hence, the incorporation of kinked carbazole unit into hyperbranched polyfluorenes could provide a new methodology for preparing blue light-emitting polymers with improved optoelectronic characteristics.