Luminescence properties of PPV-based copolymers with crown ether substituents
Introduction
After the first report of electroluminescence, EL, in poly (p-phenylenevinylene) (PPV)-based diodes [1], much effort has been devoted to the preparation of novel polymers in order to change the emission colour and/or to increase the EL efficiency. Another approach to improve the EL efficiency of light-emitting devices was proposed by Pei et al. [2] in 1995. They showed that blending of an electroluminescent polymer with an ion-transporting medium and a salt could lead to an improved charge injection balance. These devices were named light-emitting electrochemical cells, LECs, after the operation mechanism proposed by Pei et al. [2], [3]. It should be mentioned that an alternative operation mechanism, based on the effects of ionic space charge accumulation at the polymer/electrodes interface, was proposed by deMello et al. [4]. In the following, we will maintain the LEC designation of these devices, without intentional reference to their operating mechanism.
In the first LECs prepared by Pei, mixtures of poly(ethylene oxide), PEO, and lithium triflate, Li(CF3SO3), were blended with a luminescent polymer, such as PPV, or a soluble derivative, poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylenevinylene], MEH-PPV. These systems, however, showed a strong tendency to phase separate, owing to the different chemical structure between the conjugated polymer and PEO. The introduction of ethylene oxide oligomers as side groups of luminescent polymers such as polyfluorenes was then proposed by Pei and Yang [5] as a method to reduce the tendency to phase separation. Other similar polymers, with a PPV-based backbone, have since been reported [6], [7], [8], [9]. Similarly, the ion-solvating capability of crown ethers (cyclic ethylene oxide oligomers) or crown ether-containing polymers is well documented in the literature [10], [11], and their use in LECs has been reported in either blends [12], or as side groups of luminescent polymers [13].
In this communication, we report the luminescence properties of four novel statistical copolymers, which combine moieties carrying a crown ether side group (15-crown-5 or 15C5) with monomers of highly luminescent polymers (see Fig. 1). Although the PL efficiencies are lower than those of the highly luminescent parent copolymers, the EL efficiencies are relatively high, reaching ≈0.6 cd/A for single-layer LEDs. Interestingly, we find that devices with Al cathodes exhibit similar or higher EL efficiencies than those with Ca cathodes. The colour of LEDs emission may vary from device to device ranging from yellow to green, depending on the polymer used, on the area considered and also on the history of the device. We propose that this peculiar behaviour is due to aggregation induced by the presence of the crown ether side groups.
Section snippets
Experimental
The molecular structure of the four statistical copolymers and their composition estimated from 1H NMR spectroscopy is shown in Fig. 1. The preparation of 15C5-DB-PPV and 15C5-DMOS-PPV was previously reported [14]. The other two copolymers were prepared similarly.
Absorption and PL spectra were obtained for thin films on spectrosil and for diluted chloroform solutions. The absorption spectra were recorded with a Hewlett Packard 8453 UV–Vis spectrophotometer. The PL quantum efficiencies were
Optical properties
We found that the thin film optical absorption and PL emission spectra of all the copolymers are similar, and we report in Fig. 2 the spectra obtained for 15C5-DB-PPV on spectrosil, as an example. We also wish to emphasise that the optical properties are strongly dependent on the deposition procedure, and that drop casting does not yield uniform, optically transparent films. Furthermore, drop cast films of the crown ether-based parent homopolymer, 15C5-PPV, are very rough with evident signs of
Conclusions
We have reported the luminescence properties of four statistical PPV copolymers combining monomers of highly luminescent polymers and monomers carrying crown ether side groups. These exhibit very high PL efficiencies with respect to the crown ether parent homopolymer(15C5-PPV). Interestingly, single-layer devices with aluminium cathodes, showed higher EL efficiencies than those with calcium cathodes. The EL emission colour was also observed to change from device to device, ranging from yellow
Acknowledgements
We thank Fundação para a Ciência e a Tecnologia (PRAXIS/3/3.1/MMA/1792/95 and post-doctoral grant to JM), the EPSRC, The Royal Society (University Research Fellowship to FC), The Cambridge Commonwealth Trust and ORS (to BSC) for financial support.
References (23)
- et al.
Synth. Met.
(1997) - et al.
- et al.
Synth. Met.
(1995) - et al.
Nature
(1990) - et al.
Science
(1995) - et al.
J. Am. Chem. Soc.
(1996) - et al.
Phys. Rev. B
(1998) - et al.
J. Am. Chem. Soc.
(1996) - et al.
Macromol. Symp.
(1997) - et al.
Appl. Phys.
(1999)
Angew. Chem., Int. Ed.
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