Theoretical studies on the electronic and optical properties of two new alternating fluorene/carbazole copolymers.
ABSTRACT 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 = 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 (Delta(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 lambda(abs) of PFmCz (m = 1,2) are studied, employing the time-dependent density functional theory (TD-DFT). The properties of the two copolymers, such as Delta(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 = 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.
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ABSTRACT: A versatile synthesis of pi-stacked polyfluorenes is described. These polyfluorenes retain their cofacial conformations both in solution and in the solid state as was judged by NMR spectroscopy and X-ray crystallography. The experimental electron-detachment energies of F1-F4 showed linear correlations with the quantity 1/n, where n is the number of fluorene moieties. These correlations allowed the estimation of the vertical ionization potential (IP) of 7.10 eV and the oxidation potential (Eox) of 0.97 V versus SCE for the multiply stacked polyfluorene donor with an infinite number of fluorene moieties. These observations with pi-stacked polyfluorenes may prove to be highly relevant to the electron-transport phenomenon observed in DNA through pi-stacked bases.Journal of the American Chemical Society 08/2003; 125(29):8712-3. · 10.68 Impact Factor
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ABSTRACT: [reaction: see text] Highly soluble and strongly blue fluorescent oligo(9,9-dihexyl-2,7-fluorene ethynylene)s (OFEs) were synthesized by a Pd/Cu-catalyzed Sonogashira coupling reaction. An organic light-emitting diode using pentamer 15 as the emitting material was successfully fabricated and emitted a bright blue light.Organic Letters 07/2001; 3(13):2005-7. · 6.14 Impact Factor
Article: Organic Electroluminescent Devices[show abstract] [hide abstract]
ABSTRACT: Electroluminescence from organic materials has the potential to enable low-cost, full-color flat-panel displays, as well as other emissive products. Some materials have now demonstrated adequate efficiencies (1 to 15 lumens/watt) and lifetimes (>5000 hours) for practical use; however, the factors that govern lifetime remain poorly understood. This article provides a brief review of device principles and applications requirements and focuses on the understanding of reliability issues.Science 09/1996; 273(5277):884-8. · 31.20 Impact Factor