Elsevier

Polymer

Volume 44, Issue 6, March 2003, Pages 1843-1850
Polymer

Synthesis and luminescence properties of three novel polyfluorene copolymers

https://doi.org/10.1016/S0032-3861(03)00028-4Get rights and content

Abstract

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′-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.

Introduction

The huge development of polymer-based light-emitting diodes, LEDs, since the report of electroluminescence, EL, observation in poly(p-phenylene vinylene), PPV [1], is the result of a successful combination of efforts in various areas, such as polymer chemistry, materials science and device physics. The driving force behind this evolution is the preparation of efficient devices, operating at low voltages and emitting in the full visible range of the spectrum (though, for full color displays, only the primary red, green and blue (RGB) colors are required). In order to optimize these parameters, different materials and combinations have been investigated. The ability to chemically modify the polymers (both the main chain and the pendant side groups) in a predictable way, when targeting color tuning, the control of the energetic position of the frontier levels (highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO)) and, to some extent, the efficiency of light emission, is at the heart of the most significant progresses in this area [2]. Among the known luminescent polymers, polyfluorenes are presently the most attractive class of polymers for LEDs [3], and they are very promising also for fabrication of efficient photovoltaic diodes [4]. Advantages of these polymers include their capability to emit in the blue part of the visible spectrum, their chemical and photochemical stability, the high purity with which they can be synthesized, liquid crystalline properties and durability under operation in LEDs [5], [6], [7].

Various fluorene-based copolymers have been prepared, aiming at achieving color tunability over the visible spectrum and tuning of the frontier levels energetic position in order to minimize (possibly eliminate) the anodic and cathodic injection barriers of LED structures [3]. More specifically, the synthesis and properties of various fluorene–thiophene copolymers were reported by various groups [3], [8], [9], [10], [11], [12], [13]. Scherf et al. [14], have recently reported on the synthesis of a series of fluorene–oligothiophene alternating copolymers, and related their optical properties with the size of the oligothiophene unit. However, systematic studies on the effect of the comonomers combined with the fluorene units are still lacking. We have also been studying similar fluorene-based copolymers [15], [16], [17], combining thiophene-based units and 9,9-bis(2′-ethylhexyl)fluorene, looking for the systematic variation of the copolymers optical and electrochemical properties with the type of thiophene-based comonomer used. We chose 9,9-bis(2′-ethylhexyl)fluorene, instead of the isomer with linear side groups, in an attempt to reduce solid state packing and the formation of low energy interchain states, such as aggregates or excimers, which have been suggested as responsible for the existence of a low energy emission peak in polyfluorenes [18], [19], [20]. Note that, recently, the existence of keto defects (fluorenone) was also suggested to be at the origin of such low energy emission [21].

The copolymerization of thiophene-S,S-dioxide with 9,9-bis(2′-ethylhexyl)fluorene was shown to increase both the ionization potential (IP) and the electron affinity (EA) of the copolymer in comparison with the use of thiophene [15]. The choice of this comonomer follows the report, by Barbarella et al., [22], [23] showing that this thiophene functionalization was very effective at increasing EA of oligothiophenes. Note that, more recently, Beaupré and Leclerc [24] reported on the preparation and characterization of two new alternating copolymers combining 9,9-dihexylfluorene and oligothiophenes with the central thiophene unit functionalized to S,S-dioxide.

Here, we present a detailed report on the synthesis, characterization and properties of three F-alt-X alternating copolymers, where F is the 9,9-bis(2′-ethylhexyl)fluorene unit and X is either a phenylene, a thiophene or a thiophene-S,S-dioxide unit (see Scheme 1). These copolymers are members of a larger family of copolymers we have been studying [15], [16], [17]. They were prepared using Suzuki coupling [25] of the appropriate dibrominated and boron ester monomers (Scheme 1).

Section snippets

Measurements

1H and 13C NMR spectra were recorded on a Varian spectrometer (300 and 75 MHz, respectively), in CDCl3 with tetramethylsilane as internal reference. FT-IR spectra were recorded using a Mattson 1000 spectrophotometer, dispersing the samples in KBr. Number-average (Mn) and weight-average (Mw) molecular weights were estimated by Gel Permeation Chromatography (GPC) in a Waters 51 chromatograph equipped with two Waters UltraStyragel columns (1000–500 Å) in series and with two detectors (a Water 410

Synthesis and characterization

The copolymers were obtained in reasonably good yields (after purification) and with moderate average molecular weights (Table 1), as typically obtained by the Suzuki type of polymerization reaction. The copolymers show good solubility in common organic solvents, such as chloroform, tetrahydrofuran, toluene or xylene.

Optical and electrochemical properties

Fig. 1 shows the absorption and fluorescence spectra of the three copolymers in dilute cyclohexane solutions and as solid films. On going from PFP to the copolymers incorporating

Conclusion

Three novel, well defined, alternating copolymers F-alt-X, derivatives of the poly[2,7-(9,9-bis(2′-ethylhexyl)fluorene)], were prepared by the Suzuki-coupling type of polymerization. We show that the unit X, copolymerized with the fluorene moiety, effectively controls the optical, luminescence and electrochemical properties of the copolymers. In the design of new materials for full-color displays, the copolymerization is a very good strategy to realize color tuning, but its influence on other

Acknowledgements

We thank Fundação para a Ciência e a Tecnologia (contract no PRAXIS/PCEX/C/QUI/82/96 and for a PhD grant to AC) and The British Council/CRUP (Project no B-20/00) for financial support. FC is a Royal Society Research Fellow.

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