Synthesis and Characterization of a New Triphenylamine-Substituted PFO/PPV Copolymer

Abstract

The spiro-type bulky architecture in the polymer is intriguing since it can suppress the interchain interactions and enhance the photoluminescence efficiency. A new triphenylamine-substituted PFO/PPV alternating copolymer was synthesized by using a typical Heck-coupling reaction. This highly soluble polymer exhibits strong photoluminescence with maximum emission peaks centered at 417 and 441 nm in THF; 425 and 460 nm in benzonitrile. In deoxygenated dry benzonitrile, the fluorescence time profile of copolymer exhibited a bi-exponential decay with lifetime of 0.12 ns (98%) and 1.13 ns (2%). Upon excitation with nanosecond laser pulse at 355 nm, the transient absorption band at 610 nm was observed.KEY WORDS: Polyfluorene / Triphenylamine Functionality / Heck-Coupling Reaction / As one of the useful hole transporting materials, triphenylamine (TPA) and its organic and polymeric derivatives have been widely used in organic and polymeric light-emitting diodes (LEDs) due to their low ionization potentials, tridimensional steric, and good UV-light harvesting properties.1 The biggest disadvantage of TPA lies in their crystallizaion, surface diffusion, relatively low thermal stability, and difficulties in fabricating devices as well. These drawbacks that restrict its applications in LEDs can be overcomed by covalently incorporating TPA into polymers by a variety of reactions as part of the main chain, at the end of side chain, as end groups, as branch points of star and as junctions of networks. For example, Giovanella et al.2 prepared a copolymer containing alternating fluorene and 9,9-bis-triphenylaminofluorene residues, in which TPA moiety can increase hole injection and transport. Using FeCl 3 as an oxidant, Liou and his coworkers 0 -spirobifluorene-cored donor-acceptor bichromophore system in which TPA and carbazole (CBZ) moieties are used as the electron donor, and 1,3,4-oxadiazole (OXD) moieties as electron acceptor. The introduction of OXD units suppresses the delocalization of TPA radical cations effectively, allowing efficient electropolymerization through feasible TPA dimerization. The resulting polymer film exhibited reversible electrochemcial oxidation, accompanied by strong color changes with high coloration efficiency and contrast ratio, which could be switched through potential modulation. Two new conjugated polyfluorene derivatives (PDPF and PBPF) bearing triphenylamine moiety through a vinylene bridge have also been prepared. 5 The maximum electroluminescence efficiency of the double-layer polymer light-emitting diodes (PLEDs) based on these two polymers was both about 2.08 cd/A. A thermally stabe polymer prepared from the polycodensation of 4,4 0 -dialdehyde-4 00 -n-butyl-triphenylamine with 1,4-bis(triphenylphosphonion-methyl)benzene dibromide 6 exhibits a high fluorescence quantum efficiency of 94.3%. Fang 7 et al. reported an alternative copolymer comprising of fluorene and triphenylamine units with aldehyde group in the side chain, which showed good solubility in common organic solvents. Poly [4-ph...