Opto-electronic properties of fluorene-based derivatives as precursors for light-emitting diodes (original) (raw)
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Polymer, 2005
Poly(fluorene)-type materials are widely used in polymer-based emitting devices. During operation there appears, however, an additional emission peak at around 2.3 eV, leading to both a color instability and reduced efficiency. The incorporation of the carbazole units has been proven to efficiently suppress the keto defect emission. In this contribution, we apply quantum-chemical techniques to investigate two series of alternating fluorene/carbazole oligomers and copolymers poly[2,7-(N-(2-methyl)-carbazole)-co-alt-2,7-m(9,9-dimethylfluorene)], namely, PFmCz (m ϭ 1,2) and gain a detailed understanding of the influence of carbazole units on the electronic and optical properties of fluorene derivatives. The electronic properties of the neutral molecules, HOMO-LUMO gaps (⌬ H-L ), in addition to the positive and negative ions, are studied using B3LYP functional. The lowest excitation energies (E g s) and the maximal absorption wavelength abs of PFmCz (m ϭ 1,2) are studied, employing the time-dependent density functional theory (TD-DFT). The properties of the two copolymers, such as ⌬ H-L , E g , IPs, and EAs were obtained by extrapolating those of the oligomers to the inverse chain length equal to zero (1/n ϭ 0). The outcomes showed that the carbazole unit is a good electron-donating moiety for electronic materials, and the incorporation of carbazole into the polyfluorene (PF) backbone resulted in a broadened energy gap and a blue shift of both the absorption and photoluminescence emission peaks. Most importantly, the HOMO energies of PF1Cz and PF2Cz are both a higher average (0.4 eV) than polyfluorene (PF), which directly results in the decreasing of IPs of about 0.2 eV more than PF, indicating that the carbazole units have significantly improved the hole injection properties of the copolymers. In addition, the energy gap tends to broaden and the absorption and emission peaks are gradually blue-shifted to shorter wavelengths with an increase in the carbazole content in the copolymers. This is due to the interruption of the longer conjugation length of the backbone in the (F1Cz) n series.
Tuning the optoelectronic properties of polyfluorenes by copolymerisation with thiophene moieties
Synthetic Metals, 2002
A series of alternating copolymers of the type A-alt-B, where A is 9,9-bis(2 H-ethylhexyl)¯uorene and B is a thiophene-based moiety, was synthesised via the palladium-catalysed Suzuki coupling. These copolymers were characterised by optical and electrochemical methods and used in the fabrication of light-emitting diodes. We ®nd that upon increasing the conjugation length of the B moiety, from a thiophene unit to 2,5-bis(2 H-vinyl thienyl)thiophene, the¯uorescence quantum yields are reduced both in solution (from 51 to 11%) and in solid state. Both absorption and emission are red-shifted, with the solid state emission maximum increasing from 476 nm (green) to 600 nm (orange). When the B unit is changed from thiophene to thiophene-S,S-dioxide, there is a signi®cant increase in both the ionisation potential and electron af®nity, and a concurrent reduction of the solution¯uorescence ef®ciency (down to 16%). These polymers appear promising materials for optoelectronic applications.
Molecular Crystals and Liquid Crystals, 2011
Two well defined, blue luminescent homopolymers containing 9,9-diethylfluorene core with electron withdrawing pendant pyridine, Suzuki-coupled with the corresponding borolane of decoxy phenyl (PFPA) and naphthalene (PFNA) have been designed and synthesized efficiently. The obtained rod-coil polymers have the average molecular weight of 15212 (PFPA) and 15130 (PFNA) with polydispersity indices (PDIs) 1.07 and 1.3, respectively. The polymers have good solubility and high thermal stability with the decomposition temperature of 362°C (PFPA) and 367°C (PFNA) correspondingly. Furthermore the optical and electrochemical properties of the polymers have been investigated. The polymers exhibited photoluminescence (PL) maxima at 410 nm (PFPA) and 414 nm (PFNA) excited at 340 nm, as stable blue luminescence polymers at low concentration of 10 −6 M with a quantum yield of 0.64 and 0.62, respectively. In addition the annealed (150°C) polymer films showed better stability of its photoluminescence spectra. Absorbance and fluorescence emission spectra of PFPA and PFNA were compared in order to evaluate the effects of substituent, phenyl and naphthalene in pyridine pendant fluorene moieties.
Journal of Materials Chemistry, 2003
A new copolymer, constituted by the regular alternation of a fluorene derivative with a 1,1-dioxothiophene moiety, has been synthesised via Suzuki coupling. The molecular, thermal, structural, and photophysical characterisations have been performed. The material showed two dimensional order, enhanced by annealing; molecular modelling calculations afforded a consistent structural model accounting for the optical data. In comparison with both polyfluorenes and 1,1-dioxothiophene oligomers the unusual photoluminescence quantum yields observed in solution and in the solid state can be related to a significant contribution of internal conversion to the deactivation process in solution, and the formation of intra-chain hydrogen-bonds, due to a peculiar chain conformation, in the solid state. LED devices based on this new copolymer showed the highest efficiency compared with similar copolymers containing both fluorene and 1,1-dioxothiophene functionalities.
Blue light-emitting diodes based on novel polyfluorene copolymers
Journal of Luminescence, 2007
This study presents the synthesis and characterisation of a series of fluorene-based conjugated copolymers, together with the preparation and characterisation of the corresponding light-emitting devices. The polymers consist of alkoxyphenyl-substituted fluorene units together with different amounts of a hole-transporting triphenylamine-substituted fluorene unit: 0%, 10%, 25% and 50%. All polymers (P0, P1, P2, and P3) show high photoluminescence efficiency (Z PL) and light emission (both PL and EL) in the blue spectral region. Electrochemical studies show improved hole injection as the ratio of the triphenylamine-substituted segment is increased. The electroluminescence quantum efficiencies (EQEs) of the devices increase six times going from P0 to P1. Compared with P1, polymers P2 and P3 show lower efficiencies in devices. These findings indicate the presence of an optimal polymer composition, where balance between the charge-carrier mobilities has been reached.
Novel Fluorene-Thiophene Copolymers: Synthesis, Photophysical and Electrochemical Properties
Macromolecular Symposia, 2008
Novel alternating fluorene-thiophene copolymers, F3HT, FIPT, and FPyT, consisting of 9,9-dihexylfluorene and 3-substituted (with hexyl, 3-methylbutyl and 2-(pyren-1-yl)vinyl, respectively) thiophene units were synthesized. Photoluminescence (PL) and electrochemical behavior of these polymers were studied. PL behavior of FPyT polymer differed from that of the other copolymers. Thin films of F3HT and FIPT showed an intense green PL emission whereas the PL emission of FPyT thin films was orange. The PL emission spectra in thin films differ from those measured in THF solutions. Aggregate formation played an important role in the solid state. The aggregation was more pronounced with FPyT than with F3HT and FIPT thin films. In all polymers reversible oxidation and reduction were observed. Similar values of ionization potentials and electron affinities were estimated for F3HT and FIPT, but a higher value of electron affinity for FPyT.
A new copolymer, constituted by the regular alternation of a fluorene derivative with a 1,1-dioxothiophene moiety, has been synthesised via Suzuki coupling. The molecular, thermal, structural, and photophysical characterisations have been performed. The material showed two dimensional order, enhanced by annealing; molecular modelling calculations afforded a consistent structural model accounting for the optical data. In comparison with both polyfluorenes and 1,1-dioxothiophene oligomers the unusual photoluminescence quantum yields observed in solution and in the solid state can be related to a significant contribution of internal conversion to the deactivation process in solution, and the formation of intra-chain hydrogen-bonds, due to a peculiar chain conformation, in the solid state. LED devices based on this new copolymer showed the highest efficiency compared with similar copolymers containing both fluorene and 1,1-dioxothiophene functionalities. JMC: b208742a.3d 25/2/03 17:54:41 Rev 6.06e/W (Aug 31 2000) (gamma) The Charlesworth Group, Hudds 01484 517077
Organic Electronics, 2007
A series of new fluorene-based alternating polymers (PF-CZ50, PF-DPA50, PF-PXZ50, PF-PZB50) composed of comonomers containing well-known hole-transporting moieties, namely carbazole, diphenylamine, phenoxazine, and phenothiazine, respectively, were synthesized via Suzuki coupling reactions. The molecular structures of the polymers were characterized with 1 H NMR spectroscopy and elemental analysis, and their photophysical, electrochemical, and electroluminescence properties were investigated. The absorption and photoluminescence (PL) emission maxima of the copolymers in the solid state varied depending on which hole-transporting unit was present. Although the polymers' LUMO levels are similar, the differences between their HOMO levels mean they have different hole-transportation properties. Light-emitting devices using the polymers as emitting layers were fabricated with ITO/PEDOT:PSS/polymer/Ca/Al configurations. The emitted light of these polymers ranged from yellowish green to red for the sequence PF-CZ50, PF-DPA50, PF-PXZ50, and PF-PZB50. Especially, PF-PXZ50 and PF-PZB50 emitted almost pure red with good brightness. The planarity of the hole-transporting moieties in the polymers is the main factor affecting the efficiency of these electroluminescence (EL) devices. The planarity of each hole-transporting moiety was calculated by computational analysis using the ab initio Hartree-Fock (HF) with the split-valence 6-31G * basis set. As the magnitude of the non-planarity of the hole-transporting moieties increased, it was found that the EL quantum efficiency of the polymers was enhanced. Compared to PF-CZ50 containing planar carbazole moieties, PF-DPA50 containing a non-planar diphenylamine moiety showed better EL performances without a significant change of the emitting color. In the case of PF-PXZ50 and PF-PZB50, their emission color was red-shifted, and the EL quantum efficiency of their LED devices was enhanced, with respect to the result for PF-CZ50, because of their extra heteroatoms (oxygen and sulfur respectively), which bend the molecules and strengthen their activities as electron donors.
Synthesis and luminescence properties of three novel polyfluorene copolymers
Polymer, 2003
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.