L. Alcácer

A novel class of luminescent polymers obtained by the sol–gel approach

A series of amorphous Eu3+-doped hybrid organic/inorganic materials prepared by the sol–gel process is presented. The host matrix of the so-called ureasils is a silica network to which oligopolyoxyethylene chains are grafted by means of urea bonds. Trivalent europium is present as a triflate salt, Eu(CF3SO3)3. Four different compositions of salt have been considered: n=184, 92, 55 and 37, where n represents the number of (OCH2CH2) polymer repeat units per ion group of Eu(CF3SO3)3. Infrared spectroscopic studies suggest that the doped ureasils contain both free and coordinated triflate ions. The lowest decomposition temperature (about 250°C) occurs with composition n=92. This composition also exhibits the highest conductivity at room temperature (approximately 10−5 Ω−1 cm−1). The photoluminescence spectra of these materials present a series of narrow lines assigned to 5D0→7F0,1,2,3,4 transitions. The high emission intensity observed is a promising property for application of the ureasils in technological luminescent devices.

Synthesis and luminescence properties of three novel polyfluorene copolymers

We report on the synthesis and characterization (including structural, optical, electrochemical and electroluminescence properties) of three alternating F-alt-X copolymers, where F is 9,9-bis(2 0 -ethylhexyl)fluorene unit and the X comonomer varies from a phenylene, to a thiophene and to a thiophene-S,S-dioxide unit. Among these X comonomers, the phenylene group is at the origin of a blue-emitting copolymer with unitary luminescence efficiency in solution, while thiophene-S,S-dioxide promotes the highest electron affinity. These copolymers are also used in the fabrication of light-emitting diodes. q 2003 Elsevier Science Ltd. All rights reserved.

Synthesis and characterization of novel urethane cross-linked ormolytes for solid-state lithium batteries

Abstract A novel family of Li + -based organic/inorganic materials obtained by the sol–gel process is proposed. The compounds, named urethanesils , are obtained as thin, transparent, elastomeric and amorphous monolithic films. They incorporate solvating pendant methyl end-capped short poly(oxyethylene) chains which are covalently bonded to the silica backbone by means of urethane cross-links. The urethane linkages are formed by reacting 3-isocyanatepropyltriethoxysilane with hepta(ethylene glycol) methyl ether (HEGME). Li + has been introduced in the urethanesils as lithium triflate (LiCF 3 SO 3 ). Two compositions of salt have been considered: n =100 and 8, where n represents the molar ratio of (OCH 2 CH 2 ) units per lithium ion. Infrared spectroscopy provides conclusive evidence that, although the oligopolyether chains of HEGME become less disordered upon formation of the inorganic network, the addition of salt induces disorder. The FTIR spectrum of the most concentrated urethanesil strongly suggests that the triflate ions are essentially coordinated in the material. The thermal and mechanical properties of the undoped and doped urethanesils have been investigated by DSC and DMTA. At 90°C, the highest ionic conductivity (approximately 10 −6 Ω −1 cm −1 ) is observed for composition n =8. The electrochemical stability domain of the least concentrated urethanesil spans 5 V.

Self-assembly surface modified indium-tin oxide anodes for single-layer light-emitting diodes

We study the effect of indium-tin oxide surface modification by self assembling of highly polar molecules on the performance of single-layer light-emitting diodes (LEDs) fabricated with polyfluorene blends and aluminium cathodes. We find that the efficiency and light-output of such LEDs is comparable to, and sometimes better than, the values obtained for LEDs incorporating a hole injection layer of poly(3,4-ethylene dioxythiophene) doped with polystyrene sulphonic acid. This effect is attributed to the dipole-induced work function modification of indium-tin oxide.

Excitation energy transfer and spatial exciton confinement in polyfluorene blends for application in light-emitting diodes

We present picosecond time-resolved studies of thin films of two polyfluorene blends, combining a relatively high energy-gap poly(9,9-dioctylfluorene), PFO, and a polyfluorene derivative with a smaller gap. These studies show both efficient energy transfer from the donor to the acceptor, as well as spatial confinement of the excitons in the acceptor polymers. The optoelectronic characteristics of light-emitting diodes fabricated with such blends reflect the contribution of these two processes. The electroluminescence is almost exclusively from the acceptor polyfluorenes and we observe a significant improvement of the electroluminescence efficiency of the light-emitting diodes fabricated with such blends in comparison to those based on neat films of the polyfluorenes.

Crystalline structure/transport properties relationship in the (perylene)2M(mnt)2 family M=Au, Pd, Pt, Ni)

Abstract We present the crystalline structures of (Perylene)2M(mnt)2 for M=Au and Pd (mnt=maleonitriledithiolate or cis-2,3-dimercapto-2-butenedinitrile, S2C2(CN)2). Both are monoclinic, space group P 2 1 c and isostructral with the Pt analog. We discuss the correlations between structure and transport properties of the above compounds.

Use of cross-linkable polyfluorene in the fabrication of multilayer polyfluorene-based light-emitting diodes with improved efficiency

The authors report the use of a cross-linkable polyfluorene to fabricate multilayer light-emitting diodes (LEDs), thereby avoiding the restriction to combine polymeric solutions in different solvents. In particular, we find that for LEDs fabricated with a hole-injection layer of poly(3,4-ethylenedioxythiophene) doped with polystyrene sulfonic acid (PEDOT), with magnesium cathodes and with poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT), as emissive layer, its electroluminescence efficiency increases from 2to5.5cd∕A upon insertion of the cross-linked polyfluorene between PEDOT and F8BT. This efficiency increase is attributed to an improvement of charge carrier balance within the F8BT emissive layer and a reduction of exciton quenching at PEDOT interface.

Synthesis and optical properties of poly(fluorene)-based alternating copolymers

Abstract The novel poly(9,9-bis(2-ethylhexyl)fluorene- alt -2′,5′-thiophene), and the similar poly(9,9-bis(2-ethylhexyl)fluorene- alt -2′,5′-thiophene-1,1-dioxide) and poly(9,9-bis(2-ethylhexyl)fluorene- alt -1′,4′-phenylene) luminescent copolymers were synthesised by the Suzuki coupling reaction. Within this series, we show that an adequate choice of the unit combined with the fluorene monomer, effectively controls the energetic position of the frontier levels of the copolymer. This effect is evidenced by the different optical energy band gaps and different emission colours. Furthermore, the results of cyclic voltammetry (CV) studies show that the S , S -dioxide substitution on the thiophene ring increases both the ionisation potential (IP) and the electron affinity (EA) in relation to the similar thiophene-based copolymer.

Poly(9,9-dioctylfluorene)-based light-emitting diodes with pure β-phase emission

The authors report on poly(9,9-dioctylfluorene) (PFO)-based light-emitting diodes exhibiting an emission spectrum which was previously attributed to the so-called β phase. These devices were prepared by drop cast from dilute toluene solutions. Photoluminescence and electroluminescence spectra, recorded at room temperature, show no evidence of the usual amorphous phase emission. Devices with pure β-phase emission exhibit a higher color stability, upon increase of the driving voltage, than those based on amorphous PFO.

Luminescence properties of composites made of a europium(III) complex and electroluminescent polymers with different energy gaps

We present the optoelectronic properties of composites made of a europium(III) complex, Eu(NTA)3 phen (where NTA=1-(2-naphthoyl)-3,3,3-trifluoroacetonate; phen=1,10-phenantroline), dispersed in three electroluminescent polymers, namely, poly(N-vinylcarbazole), poly(9,9-dioctylfluorene) and poly(9,9-dioctylfluorene-alt-benzothiadiazole). We find that the photo- and electroluminescence (EL) properties of these composites are well rationalized in terms of the relative position of the frontier levels of the host polymers and of the europium complex. We find also that charge recombination at the europium complex sites plays a key role on the EL properties of the composites.