Luminescence properties of PPV-based copolymers with crown ether substituents

doi:10.1016/S0379-6779(99)00397-5

Synthetic Metals

Volumes 111–112, 1 June 2000, Pages 449-452

https://doi.org/10.1016/S0379-6779(99)00397-5 Get rights and content

Abstract

We present the optical properties of four poly(p-phenylenevinylene) (PPV)-based statistical copolymers, which combine monomers of highly luminescent polymers and monomers carrying crown ether side groups. The photoluminescence efficiency for films on spectrosil ranges from 28% to 52%, whereas the electroluminescence (EL) efficiency of single-layer diodes with Al cathodes reaches 0.6 cd/A (internal quantum efficiency of 1.4%). Surprisingly, the efficiency for Ca cathodes diodes is lower than for Al ones. Slight variation of the EL spectra from pixel to pixel affects all the copolymers of the family, and we consider that it is indicative of chain aggregation induced by the crown ether moieties. Studies of light-emitting electrochemical cells, LECs, based on one of these polymers show that the ion mobility induced by the presence of the crown ethers is relatively small. Typical LEC behaviour was observed upon addition of poly(ethylene oxide), with EL internal quantum efficiency reaching 2.3%.

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)

B.S. Chuah et al.
Synth. Met.
(1997)
H. Rost et al.
P. Bröms et al.
Synth. Met.
(1995)
J.H. Burroughes et al.
Nature
(1990)
Q. Pei et al.
Science
(1995)
Q. Pei et al.
J. Am. Chem. Soc.
(1996)
J.C. deMello et al.
Phys. Rev. B
(1998)
Q. Pei et al.
J. Am. Chem. Soc.
(1996)
D.-H. Hwang et al.
Macromol. Symp.
(1997)
J. Morgado et al.
Appl. Phys.
(1999)

C.J. Pedersen et al.

Angew. Chem., Int. Ed.

(1972)

Cited by (26)

Synthesis of tetraazacalix[2]arene[2]triazine-containing poly(dimethylsiloxane) with elastic property induced by pinning effect of the calixarene ring
2022, European Polymer Journal
Citation Excerpt :
Crown ethers [1–6], cyclodextrins [7], and calixarenes [8] have been the focus of research for applications such as ion recognition, separation, batteries, and supramolecular chemistry.
Tetraazacalix[2]arene[2]triazine dichloride was polymerized with telechelic poly(dimethylsiloxane)s having NH₂ terminals (PDMS_x; degree of polymerization x = 8, 18, 38) via solution polycondensation to give the corresponding azacalixarene-containing polymers (ACAT-PDMS_x) in good yield. The fraction of the hard ACAT moiety (f_HS) of the ACAT-PDMS polymers was calculated to be 33.0%, 20.4%, and 11.6% for x of 8, 18, and 38, respectively. The number-average molecular weight (M_n) of the polymers ranged from 20 to 32 kDa, and freestanding tough films were successfully fabricated via solvent cast method. As a control sample, a polymer not containing azacalixarene (AnDCT-PDMS) with M_n and f_HS values of 21 kDa and 19.8%, respectively, was prepared via the solution polymerization of 2-anilino-4,6-dichloro-1,3,5-triazine (AnDCT) with PDMS₈. Although the f_HS values of AnDCT-PDMS₈ and ACAT-PDMS₁₈ were similar, the film obtained from the former polymer was much softer and sticky at room temperature than that from the latter. Interestingly, the ACAT-PDMS₁₈ film exhibited a tensile strength of 2.7 MPa, a modulus of 2.0 MPa, and an elongation at break of 200%. The glass transition temperature of the ACAT-PDMS_x films ranged from 77 °C to 105 °C according to thermomechanical and dynamic mechanical analyses. Wide-angle X-ray diffraction and polarized optical microscopy studies indicated that the azacalixarene units were first locally aggregated via multiple hydrogen bonds within the ACAT-PDMS_x films and then were well dispersed in the film on heating. The ACAT-PDMS polymer showed no cytotoxicity to HeLa and IMR-90 cells in the experimental concentration of below 100 μg/mL.
Structure-properties correlation of copolymers derived from poly(phenylene vinylene) (PPV)
2012, Synthetic Metals
Bond lengths and Mullikan charges distribution as well as optoelectronic properties including electronic structure and simulated optical and emission spectra on ground and excited states of modified PPV containing a random repetition of PV and ether's units, model structure, are examined by the use of density functional theory (DFT). Accordingly, the effect of ether inserting on the phenylene vinylene units at single or both sides and those of terminal groups are carried out. Findings are correlated with time transient photoluminescence decays (TR-PL) of standard PPV structure. Then, introduction of ether's segment on PPV backbone induces exciton confinement ant trapping process.
Harnessing supramolecular interactions in organic solid-state devices: Current status and future potential
2010, Coordination Chemistry Reviews
The performance attained by electronic devices incorporating more than a single molecule is in part limited by weak electronic coupling between molecules. By implementing designed supramolecular interactions, chemists have begun taking control of the nanoscale ordering of the active layer in an effort to move beyond the trial and error tuning of the device morphology. This review describes current progress in solid-state devices in which the molecular components possess designed supramolecular interactions – as opposed to non-specific cohesive forces – used to instill or modify functionality. Supramolecular organic devices for applications in solar energy conversion, light-emitting diodes, field-effect transistors, storage and logic functions are covered.
Photo- and electroluminescence from hyperbranched phenylene vinylenes
2003, Synthetic Metals
We report photophysical and electroluminescence (EL) properties of three novel hyperbranched phenylene vinylenes (HPVs) with benzene as the core, 2,5-dimethyloxyl substituted phenylene vinylene as the connecting unit, and benzene, dimethylaniline or pyridine as the terminal group, respectively. The absorption maxima of the HPVs blue-shift from solution to the films due to their twisted molecular conformation. The emission spectra in the films red-shift compared to those in solution, which may originate from interchain excimer species. The thin films of the HPVs exhibit pretty high photoluminescence (PL) quantum yields around 50.5–87.8%. Blue–green light-emitting electrochemical cells (LECs) based on the HPVs have been demonstrated with turn-on voltages of 2.5–2.6 V. The ac impedance measurements indicate the operation of the LECs corresponds to an electrochemical doping model.
Light emitting devices based on a bifunctional block copolymer containing crown ether spacers
2003, Synthetic Metals
A novel rigid-rod alternating poly(p-phenylenevinylene) derivative with oligo(ethylene oxide) side chains
2001, Polymer
A novel yellow-green light-emitting rigid-rod copolymer, poly[(2,5-bis(triethoxy)-1,4-phenylenevinylene)-alt-(1,4-phenylenevinylene)] (BTEOPPV-alt-PPV) with an alternating structure containing oligo(ethylene oxide) side chains was synthesized by the Wittig reaction of the corresponding triphenylphosphonium chloride and dialdehyde compounds. This soluble and processable copolymer with both electronic and ionic conduction showed good thermal stability as weight loss was less than 5% on heating to about 365°C under nitrogen atmosphere. The DSC thermogram and other relevant measurements showed the liquid crystalline properties and self-ordering in the copolymer. BTEOPPV-alt-PPV complexed with metal ions shows an ionochromic effect in the absorption spectrum and also in the electroluminescence spectrum due to the covalent linkage of the oligoethyleneoxy side chains as molecular recognition sites to the PPV backbone, which represents an approach toward chemical sensors. The single-layer light-emitting diode (LED) (ITO/polymer/Al) exhibits an emission maximum at ca 552 nm corresponding to yellow-green emission with a turn-on voltage of 7.5 V and efficiency of 0.038 lm/W (120 cd/m², 10 V, 0.1 A/cm²) while the light-emitting electrochemical cell (LEC) (ITO/polymer+LiOTf/Al) without any extra supporting polymer electrolytes for ionic conduction shows a lower turn-on voltage of 2.5 V, enhanced efficiency of 0.185 lm/W (210 cd/m², 4 V, 0.09 A/cm²) and low response times in the submillisecond range.

View all citing articles on Scopus

View full text

Luminescence properties of PPV-based copolymers with crown ether substituents

Abstract

Introduction

Section snippets

Experimental

Optical properties

Conclusions

Acknowledgements

Synth. Met.

Synth. Met.

Nature

Science

J. Am. Chem. Soc.

Phys. Rev. B

J. Am. Chem. Soc.

Macromol. Symp.

Appl. Phys.

Angew. Chem., Int. Ed.