- Email: [email protected]
Study of thermal conversion and patterning of a new soluble poly (p-phenylenevinylene) (PPV) precursor
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Materials Science in Semiconductor Processing 10 (2007) 77–89
Study of thermal conversion and patterning of a new soluble poly (p-phenylenevinylene) (PPV) precursor Marius Prelipceanua,, Otilia-Sanda Prelipceanua, Ovidiu-Gelu Tudosea, Liviu Leontieb, Bernd Grimma, Sigurd Schradera a
Department of Physics Engineering, Faculty of Engineering, University of Applied Sciences Wildau, Friedrich-Engels-Strasse 63, D-15475 Wildau, Germany b Faculty of Physics, ‘‘Al.I. Cuza’’ University, 11 Carol I Boulevard, 700506 Iasi, Romania Available online 16 July 2007
Abstract The investigation of conversion of new soluble poly(p-phenylenevinylene) (PPV) precursor and PPV patterns by irradiation with UV light of a film is reported. We obtained patterns by this method with well-defined edges and channel width up to 10 mm. Also, it was found that the PPV precursor polymer is a photoresist material. This allows the fabrication of PPV patterns, which can directly act as luminescent structures in organic light-emitting diodes (OLEDs). Using the atomic force microscopy (AFM) technique, investigations on the PPV thin films show that at 200 1C well-defined crystalline domains of PPV are detected, thus indicating the complete transformation of the precursor into the final polymer. The current–voltage characteristics of single and double PPV layer devices indicate that turn-on voltages around 9 and 8 V were recorded for double and single PPV layer devices, respectively. From the electroluminescence–voltage plot, an onset voltage at 10 V is detected for two PPV layers OLED. In addition, we investigate the optical, electrical and EL characteristics of pyrrolo[1,2-a][1,10]phenanthroline derivatives [Leontie L, Druta I, Danac R, Rusu GI. Synth Met 2005;155/1:138; Zugravescu I, Petrovanu M. 3+2 Dipolar cycloaddition. Bucharest: Roman Academic Publishing House; 1987 [in Romanian]; Druta I, Andrei M, Aburel P. Tetrahedron 1998;54:2107; Druta I, Dinica R, Bacu E, Humelnicu I. Tetrahedron 1998;54:10811; Dinica R, Druta I, Pettinari C. Synlett 2000;7:1013; Danac R, Rotaru A, Drochioiu G, Druta I. J Heterocyclic Chem 2003;40:283; Druta I, Danac R, Barbieru R, Tapu D, Andrei M. Sci Ann Al I Cuza Univ Iasi S Chem 2001;IX:149] as potential candidates for OLED applications. In this case, PPV was used as the hole-transport layer (HTL) and pyrrolo[1,2-a][1,10]phenanthroline derivatives, which were spin coated onto PPV, as the emissive layer. The structure of all PPV devices reported in this paper was fabricated using UV light for patterning. r 2007 Elsevier Ltd. All rights reserved. Keywords: Poly(p-phenylenevinylene); Pyrrolo[1,2-a][1,10]phenanthroline derivatives; Patterning process; Organic light-emitting diode
1. Introduction
Corresponding author. Tel.: +49 3375 508 524; fax: +49 3375 508 503. E-mail address: [email protected] (M. Prelipceanu).
1369-8001/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.mssp.2007.05.004
Polymers have attracted large interest due to their potential use as active material in electronic, optical and optoelectronic applications, such as lightemitting diodes, photodiodes, photovoltaic cells
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[1–7], field effect transistors and optically pumped lasers [8,9]. Their application, however, is inhibited at present by limitations in controlling luminescent spectra, sensitivity, efficiency and lifetime of devices. The main challenges posed by these polymers are the understanding of their electronic structure, the interplay between their chemical composition and morphology and their implementation in technological processes of device fabrication. Poly(p-phenylenevinylene) (PPV) is the luminescent polymer used for the first polymer-based organic light-emitting diode (OLED), which operated successfully at low driving voltage [10,11]. Because PPV itself is not soluble, film preparation starts from a soluble precursor polymer, which is then transformed to PPV by heat treatment [12–15]. The most simple OLED structure consists of a thin organic sandwiched between two metallic or semiconducting electrodes. Such single-layer devices consist e.g. of PPV sandwiched between a transparent indium–tin-oxide (ITO) anode and a cathode made from evaporated aluminium, magnesium or calcium. The performance of PPV single-layer OLEDs is rather poor [16–23]. The reason for this is on the one hand unbalanced charge transport, which reduces the probability of exciton formation, and on the other hand exciton quenching near the electrode. To improve the performances of PPVbased OLEDs, the main idea is to use a host of electrically and/or optically active materials, usually either blended into the polymer film (blend structure) [23] or as an additional layer (heterostructure) [24–28]. Our work is focused on the direct photolithographic patterning of PPV by using a new precursor after the Brabec [36] and Vanderzende [12] route and on the investigation of single- and two-layer devices made of this type of PPV. 2. Results and discussions 2.1. Synthesis and thermal stability of PPV 2.1.1. Synthesis of PPV The route of chemical synthesis of the PPV precursor according to Van Bremen et al. [12] and Brabec et al. [36] and the process of thermal conversion to PPV are schematically represented in Scheme 1. The synthesis was carried out by the method described by Vanderzende et al. [12] and Brabec et al. [36]. The PPV alkylsulphinyl precursor polymer was synthesised by polymerisation of 1-(chloromethyl)-
4-[(n-octylsulphinyl)methyl]benzene [8]. Step 1: 1,4Dichloroxylene reacts with tetrahydrothiophene to give 1,4-bis(tetrahydrothiopheniomethyl)xylene dichloride (I). Step 2: Compound I reacts with n-octanethiol to give 1-(chloromethyl)-4-[(n-octylsulphanyl)methyl]benzene (II). Step 3: The oxidation of II by H2O2/TeO2 yields 1-(chloromethyl)4-[(n-octylsulphinyl)methyl]benzene (III). Step 4: The polymerisation reaction is carried out with NaOtBu as a base to give the precursor polymer poly{[1,4-phenylene]-[1-(n-octylsulphinyl)ethylene]}(IV). Step 5: This polymer is dissolved in toluene and chloroform (1:1, v/v) and spin coated onto a quartz glass substrate, and then converted to PPV (V) by heating up to high temperatures in high vacuum for a certain time. 2.1.2. Thermal stability The decomposition temperature of the PPV precursor was determined by thermogravimetric analysis (TGA). These results were generated by heating the PPV sample at a constant rate of 10 1C/min. Fig. 1 shows that first weight loss starts at 87 1C and is due to the evaporation or volatilisation of the solvent. At temperatures higher than 87 1C, it is shown in the curve that there is a severe overlap in weight losses, about 9.17%. The weight loss occurring at about 174 1C is due to thermal decomposition of the material. 2.2. Preparation and characterization of PPV thin films 2.2.1. Spin coating PPV precursor films have been prepared by spin coating onto quartz substrates. The quartz substrates were previously cleaned according to a standard process and then dried in streaming N2 [9]. Spin coating was carried out at a spinning speed of 2000 rpm and a spinning time of 60 s from a solution of 50 mg/ml in analytical grade toluene and chloroform (1:1 v/v). 2.2.2. Ellipsometry The mean thickness and refractive index (n) of the films were determined by means of ellipsometry using a Plasmos SD2000 Automatic Ellipsometer operating at a wavelength of 632.8 nm. The measured different values of thickness and refractive index are shown in Table 1.
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Scheme 1. Synthesis of PPV precursor (I–IV) and thermal conversion to PPV (V).
2.2.3. Profilometry A Profilometer (DEKTAK 3 from Veeco Instruments) device was used to determine the sample thickness via a mechanical method. The profilometer has the capability of measuring a step height down to a few nanometres. For the investigated PPV precursor thin film (sample no. 1), a thickness value of 87 nm (before conversion) is measured (see Fig. 2). Using the loss mass calculation method, and density of PPV precursor and PPV, we found that the thickness value after conversion decreased
to 61%. The experimental results show that the layer thickness after conversion amounts to about 51% of the original layer thickness. A big advantage of the PPV precursor polymer used here is the possibility to act as a photoresist itself. Using UV light, fabrication of PPV patterns is performed [37]. The method of patterning and converting the PPV precursor polymer consists of (1) formation of a layer of the soluble precursor polymer by spin coating, (2) irradiation of selected parts of the layer through a mask with a desired
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Fig. 1. Thermogravimetric analysis (TGA) of PPV.
Table 1 Thickness and refractive index values measured for PPV films obtained from spin coating Sample
Thickness Refractive (nm) index (n)
PPV precursor on quartz (before annealing) PPV on quartz (annealed in vacuum) PPV on quartz (annealed in normal atmosphere) PPV on quartz (after second annealing in vacuum)
8775
1.370.05
4575 5875
1.59070.05 1.56770.05
4475
1.57870.05
All measurements were performed at room temperature.
pattern with UV light (Hg lamp, 365 nm) to partially convert the irradiated portions of the layer towards the conjugated structure, (3) removal of the non-illuminated area of the layer with an organic solvent such as chloroform and (4) conversion of the remaining pattern into the conjugated polymer by heating, preferably under high vacuum (106 mbar) for 2 h at 200 1C. Fig. 3a and b shows PPV patterns created by this method. One can see well-defined edges and channel width up to 10 mm. The PPV structures obtained with this method can be used in OLEDs. Some preliminary investiga-
tions show that PPV thin films should have a thickness range between 10 and 100 nm in order to obtain PPV patterns suitable for electronic applications. This new structuring method can not only be limited to OLEDs but can also be modified and optimised for further applications. 2.2.4. Atomic force microscopy The surface topology of the films was characterized using an Atomic Force Microscope (Autoprobe VP 2 Park Scientific Instruments), operating in noncontact mode in air at room temperature. Atomic force microscopy (AFM) images of the topology of PPV before and after the conversion process are shown in Figs. 4 and 5, respectively. Fig. 5 shows the surface morphology of PPV thin films after conversion in vacuum for 2 h (Fig. 5a) and PPV thin films converted at normal atmosphere at 200 1C for 2 h (Fig. 5b). In Fig. 5, differences in the morphology of PPV thin films, converted at normal atmosphere and in vacuum, can be seen. The thickness of the thin films, which were converted under different conditions (vacuum or air), has roughly the same value (see Table 1) [28]. Also, an average surface roughness RMS of 2 nm (for the layer annealed in vacuum (Fig. 5a)) and
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Fig. 2. Dektak profile of a PPV precursor thin film (before conversion).
Fig. 3. PPV patterns created by irradiation with UV light (Hg lamp, 365 nm).
5 nm (for the layer annealed in air (Fig. 5b) is measured, respectively. In order to find optimal conversion conditions, PPV layers have been converted in vacuum (106 mbar) at different temperatures. The PPV layers were deposited and kept under the same conditions. The samples were investigated immediately after the conversion process in order to minimize factors, which can have an influence on the experiment, e.g. by changing the structure and other properties of the PPV layer due to degradation, oxidation or other chemical reactions.
Lenz et al. [29] studied the thermal conversion of a precursor polymer to PPV using real-time X-ray scattering. For temperatures lower than 87 1C, no modification of the material is observed, indicating a significant amount of conversion. At temperatures around 130 1C, characteristic features of an imperfect PPV crystalline structure are already present. As we have shown previously, at 200 1C well-defined crystalline domains of PPV are detected, thus indicating the complete transformation of the precursor into the final polymer.
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the thickness of the PPV layer and on the decomposition per cent of the material as can be seen in Fig. 1. Fig. 7 shows the optical spectra after complete conversion for single and double PPV layers. For double PPV layers, the maxima peak has greater intensity probably due to the interface between PPV layers. The main peak observed at about 425 nm is due to the p–p* transition of PPV. The spectra for single and double PPV layers after annealing at 250 1C and 200 min have smaller-intensity peaks due to the degradation of polymer. From these spectra, the peaks at 240 nm are not present due to the complete loss of the solvent. 2.3. OLED device performances Fig. 4. Atomic force microscope image (1 1 mm2) of PPV precursor with an average thickness of 87 nm deposited by spin coating on quartz substrate.
Fig. 5. (a) AFM scan of PPV thin film after conversion in vacuum at 200 1C for 2 h and (b) AFM image of PPV films after conversion in normal atmosphere at 200 1C for 2 h.
2.2.5. UV/vis measurements A UV/vis Spectrometer (Lambda 16 PerkinElmer) was used to determine UV–vis absorption spectra of the prepared PPV thin films on quartz substrates. Fig. 6 shows absorption spectra of PPV layers converted in vacuum at different temperatures and time ranges [30–32]. Obviously, there are differences in the intensity of absorption maxima of the PPV films prepared under different annealing conditions. The peaks obtained at about 240 nm are from chloroform, which has been used for the preparation of PPV thin layers. The main peak observed at about 425 nm is due to the p–p* transition of PPV. The maximum of absorbance peaks is dependent on
To compare the properties of the double layer of PPV devices and typical double, different materials, we used a PPV precursor as the hole-transport layer (HTL) material and pyrrolo[1,2-a][1,10]phenanthroline derivatives, which were spin coated onto PPV, as the emissive layer. In Fig. 8, the chemical structure of pyrrolo[1,2-a][1,10]phenanthroline derivatives is illustrated. Their synthesis is described below. The non-stable monosubstituted heteroaromatic N-ylides obtained in situ by deprotonation of the corresponding cycloimmonium salts in the presence of base are 1,3-dipoles that undergo cycloaddition with activated symmetrical or non-symmetrical olefins and alkynes, resulting in the formation of a fused fivemember nitrogen heterocycle [1–5]. We performed some reactions of 1,10-phenanthrolinium-ylides with dimethyl acetylenedicarboxylate (DMAD) and ethyl propiolate to yield novel pyrrolo [1,2-a][1,10]phenanthroline derivatives [9]. Initially, for obtaining 1,10-phenanthrolinium salts 1–5, reaction of 1,10-phenanthroline with reactive halide derivatives was considered [6]. Reaction between salts 1–6 suspended in different mixtures of organic solvents and an aqueous solution of 0.2 N of sodium hydroxide (NaOH) gave in situ 1,10-phenanthrolinium-ylides 6–10 (Scheme 2), which on stereoselective 3+2 dipolar cycloaddition with DMAD or ethyl propiolate furnished the following compounds: 1-(40 -R-benzoyl)-2,3-dimethoxycarbonyl-pyrrolo[1,2-a] [1,10]-phenanthroline) 17–20 and 1-(40 -R-benzoyl)3-ethoxycarbonyl-pyrrolo[1,2-a][1,10]-phenanthroline) 21–22, respectively (Scheme 3). As shown in Scheme 3, the cycloaddition reaction firstly generated non-isolating intermediate
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Fig. 6. UV–vis spectra of PPV converted in vacuum at different temperatures and time ranges (each curve is characterised by the parameters given in the legend in the succession indicated by the arrow).
Fig. 7. UV–vis spectra of PPV converted in vacuum at 200 1C and 150 min for single and double layers. Also shown are the degradation spectra curves for single- and double-layer PPV at 250 1C and 200 min annealing in vacuum.
cycloadducts of type 11–16, which due to the tendency of stabilisation suffered an oxidative dehydrogenation process because the reactions were carried out in ambient conditions.
After separation and purification by recrystallisation from appropriate solvents or flash chromatography, the crystalline slowly fluorescent cycloadducts 17–22 were obtained (Table 2). Their
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structures were proved by elemental and spectral methods (infrared spectroscopy, IR, and nuclear magnetic resonance, 1H-NMR) [7]. The device structures under study are as follows: (1)
ITO (130 nm)/PPV (100 nm). ITO (130 nm)/PPV (100 nm). ITO (130 nm)/PPV (100 nm). ITO (130 nm)/PPV (100 nm). ITO (130 nm)/PPV (100 nm). ITO (130 nm)/PPV (100 nm).
(2) (3) (4) (5) (6)
N
N O
1
3 2
R1
R
R2
(50 nm)/RA1 (50 nm)/Al (50 nm)/RA2 (55 nm)/Al (50 nm)/RA3 (53 nm)/Al (50 nm)/RA4 (60 nm)/Al (50 nm)/RA5 (50 nm)/Al (50 nm)/RA6 (57 nm)/Al
RA1: R = NO2, R1 = R2= COOCH3 RA2: R = Cl, R1= R2 = COOCH3 RA3: R = Br, R1 = R2 = COOCH3 RA4: R = CH3, R1 = R2 = COOCH3 RA5: R = Br, R1 = H, R2 = COOC2H5 RA6: R = R1= H, R2 = COOC2H5
4'
Fig. 8. Chemical structure of pyrrolo[1,2-a][1,10] phenanthroline derivatives.
The current density versus electric field characteristics for the device structures 1–4 are shown in Fig. 9. All examined structures show a clear rectification. It is an indication of good injection efficiencies. The shape of respective curves can be influenced by the use of aluminium as the cathode electrode. The current–voltage characteristics for the positive region are shown in Fig. 10. Devices 2 and 4 have a turn-on voltage of around 4 V, which is lower than that of device 3 (around 7 V). We could only measure light emission from devices 3 and 4 (Fig. 11). They were seen to emit in the yellow-green with a luminance of 0.20 and 0.26 cd m2, respectively. Devices 1 and 2 also showed yellowish-green light emission, but their luminances were too weak to be detected. As a reference, we will note the 0.06 cd m2 luminance of device 2. In the examined OLED structures, PPV constitutes the HTL. Secondly, it can improve electron injection as the holes can accumulate at the PPV/RA interface if energy is upset. This could decrease the turn-on voltage, and consequently improve the power efficiency (Fig. 12). The soluble pyrrolo[1,2-a][1,10]phenanthroline derivatives and PPV are very promising candidates for use in organic light-emitting devices, despite the fact that several parameters still need to be optimised to obtain efficient devices. The characteristics of single-layer (ITO/PPV/organic-RA
Scheme 2.
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Scheme 3.
Table 2 Substituents R, R1 and R2 (positions 40 , 2 and 3, respectively) in the molecular structure of present compounds No.
Compound
17 18 19 20 21 22
RA1: RA2: RA3: RA4: RA5: RA6:
l-(40 -nitro-benzoyl)-2,3-dimethoxycarbonyl-pyrrolo[l,2-a] [1,10]-phenanthroline 1-(40 - chloro-benzoyl)-2,3-dimethoxy carbonyl-pyrrolo[1,2-a [1,10]-phenanthroline 1-(40 -bromo-benzoyl)-2,3-dimethoxycarbonyl-pyrrolo[1,2-a][1,10]-phenanthroline 1-(40 -methyl-benzoyl)-2,3-dimethoxycarbonyl-pyrrolo[1,2-a][l,10]-phenanthroline l-(40 -bromo-benzoyl)-3-ethoxycarbonyl-pyrrolo[l,2-a][l,10]-phenanthroline l-benzoyl-3-ethoxycarbonyl-pyrrolo(l,2-a][l,10]-phenanthroline
compounds/Al) organic light-emitting devices have been studied. All devices show clear rectification behaviour, with a turn-on voltage between 4 and 7 V. The most successful OLED emits in the yellowgreen with a luminance of 0.26 cd m2 for a singlelayer device. In addition, double-layer devices have been studied with PPV sandwiched between transparent ITO and aluminium. The insoluble PPV is spin coated as a precursor polymer and thermally
R
R1
R2
–NO2 –CI –Br –CH3 –Br –H
–COOCH3 –COOCH3 –COOCH3 –COOCH3 –H –H
–COOCH3 –COOCH3 –COOCH3 –COOCH3 –COOC2H5 –COOC2H5
converted under vacuum prior to vacuum evaporation of the final electrode. The performance of an OLED depends on the injection conditions for holes and electrons. Usually, in a single-layer structure, electrons and holes are unbalanced, unless a situation occurs where the anode and cathode are very well matched to the molecular levels of the organic compound. In the unbalanced case, a dominant carrier can cross the whole structure without finding a carrier of
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Fig. 9. OLED structure double layer with different materials: (a) top view and (b) side view.
Fig. 10. Current–voltage characteristics of RA/PPV devices (positive region).
opposite sign to recombine, raising an energy waste and leading to low efficiency in the conversion of electrical into optical power. To achieve a better balance, two (or more) organic layers must be added: one matching the anode, optimised for transport of holes, and a second one matching the cathode, optimised for electron transport. In this way the charge injection can be balanced. Since a barrier occurs at the organic/organic interface, carriers tend to accumulate there, improving the probability of finding an available partner for
recombination. The subsequent process is the relaxation process to the ground state accompanied by the emission of light. For electrical investigations, the OLED geometry shown in Fig. 13 is used. The devices are prepared in such a way that on a pre-treated ITO/glass substrate, one or two PPV layers were deposited subsequently by spin coating followed by annealing. Aluminium electrodes were evaporated on top of these films. The thicknesses of ITO and aluminium layers were 150 and 100 nm, respectively. On one
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Fig. 11. EL emission of RA/PPV devices.
Fig. 12. Schematic band diagrams of (a) a single-layer OLED device under forward biasing and (b) a double-layer OLED device: 1—cathode, 2—anode, 3—recombination centre.
substrate, six individual OLED samples were prepared simultaneously. The active area of the light-emitting diode was 2 2 mm2 [24]. The current–voltage and electroluminescence– voltage characteristics were measured using a Keithley 2400 source meter and a Keithley 617 multimeter. EL measurements were made under forward bias (ITO positive) and the emission was measured in the forward direction through the transparent ITO.
The current–voltage characteristics of single and double PPV layer devices are plotted in Fig. 13. A turn-on voltage of about 9 and 8 V was recorded for double PPV layer and for single PPV layer devices, respectively. The behaviour of the double PPV layer devices and the promising values of efficiency for these devices can be a starting point for further investigations using a matrix of PPV doped with different organic materials. The double PPV layer device shows a strange behaviour in the
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Fig. 13. Top (a) and cross-section (b) views of OLED structure ITO/PPV/PPV/Al.
Fig. 14. Current–voltage characteristic of a light-emitting diode with single/double layers of PPV. Inset is the presented log (abs (I)) and electroluminescence function of applied voltage.
current–voltage characteristic between 5 and 0 V, probably due to different traps that are generated in the fabrication process of OLEDs at the interface between the PPV layers. As expected, these OLED structures emit green light caused by recombination of excitons in PPV bulk material. A strong increase of the electroluminescence signal is observed above the onset voltage determined at 10 V, as can be seen in the inset of Fig. 14. Wong et al. [32] investigated multilayer system devices deposited by spin-coating solutions of the different oligo(phenylenevinylenes) onto insoluble PPV layers. Using a two-layer device, electrons and holes can accumulate at the polymer/
oligomer interface. This could decrease the turn-on voltage, and consequently improve the power efficiency. The efficiency of ITO/PPV/PPV/Al devices is comparable to other published results on green light-emitting devices [33–35]. 3. Conclusions The synthesis of the PPV precursor, thermal conversion to PPV, structural, optical and electrical investigations of PPV layers are presented. Spectral characteristics of PPV films are strongly dependent on the conversion conditions. The optimal conditions of annealing are 200 1C for 2.5 h in vacuum.
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The maximum absorption in the absorption spectra is measured at 420 nm. For a conversion time less than 2.5 h, the maxima of the intensity of absorption spectra are smaller. A comparison of the PPV layers converted in vacuum and in air is made. The roughness of the layers converted in air is higher than for layers converted in vacuum. In these conditions, the PPV layers are not stable and the interface between substrate and polymer is affected. The layers obtained at optimum conditions were used successfully in OLED structures. Future work will focus on PPV used as a host system for new organic materials, e.g. chromophors, which can be used as doping materials. This will increase the efficiency and lifetime of organic lightemitting devices. Acknowledgements The authors would like to thank Dr. Peter W. Schmidt (University of Applied Sciences Wildau), Dr. R. Danac (University Al.I. Cuza Iasi) for a critical reading of the manuscript, providing of RA compounds and Monika Ehlert (University of Potsdam) for help with DSC measurements. Financial support of the European Commission under contract number HPRN-CT-2002-00327-RTN EUROFET Project and FP6–505478-1 ODEON Project is gratefully acknowledged. References [1] Leontie L, Druta I, Danac R, Rusu GI. Synth Met 2005;155/ 1:138. [2] Zugravescu I, Petrovanu M. 3+2 Dipolar cycloaddition. Bucharest: Roman Academic Publishing House; 1987 [in Romanian]. [3] Druta I, Andrei M, Aburel P. Tetrahedron 1998;54:2107. [4] Druta I, Dinica R, Bacu E, Humelnicu I. Tetrahedron 1998;54:10811. [5] Dinica R, Druta I, Pettinari C. Synlett 2000;7:1013. [6] Danac R, Rotaru A, Drochioiu G, Druta I. J Heterocyclic Chem 2003;40:283. [7] Druta I, Danac R, Barbieru R, Tapu D, Andrei M. Sci Ann Al I Cuza Univ Iasi S Chem 2001;IX:149.
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Materials Science in Semiconductor Processing 10 (2007) 77–89
Study of thermal conversion and patterning of a new soluble poly (p-phenylenevinylene) (PPV) precursor Marius Prelipceanua,, Otilia-Sanda Prelipceanua, Ovidiu-Gelu Tudosea, Liviu Leontieb, Bernd Grimma, Sigurd Schradera a
Department of Physics Engineering, Faculty of Engineering, University of Applied Sciences Wildau, Friedrich-Engels-Strasse 63, D-15475 Wildau, Germany b Faculty of Physics, ‘‘Al.I. Cuza’’ University, 11 Carol I Boulevard, 700506 Iasi, Romania Available online 16 July 2007
Abstract The investigation of conversion of new soluble poly(p-phenylenevinylene) (PPV) precursor and PPV patterns by irradiation with UV light of a film is reported. We obtained patterns by this method with well-defined edges and channel width up to 10 mm. Also, it was found that the PPV precursor polymer is a photoresist material. This allows the fabrication of PPV patterns, which can directly act as luminescent structures in organic light-emitting diodes (OLEDs). Using the atomic force microscopy (AFM) technique, investigations on the PPV thin films show that at 200 1C well-defined crystalline domains of PPV are detected, thus indicating the complete transformation of the precursor into the final polymer. The current–voltage characteristics of single and double PPV layer devices indicate that turn-on voltages around 9 and 8 V were recorded for double and single PPV layer devices, respectively. From the electroluminescence–voltage plot, an onset voltage at 10 V is detected for two PPV layers OLED. In addition, we investigate the optical, electrical and EL characteristics of pyrrolo[1,2-a][1,10]phenanthroline derivatives [Leontie L, Druta I, Danac R, Rusu GI. Synth Met 2005;155/1:138; Zugravescu I, Petrovanu M. 3+2 Dipolar cycloaddition. Bucharest: Roman Academic Publishing House; 1987 [in Romanian]; Druta I, Andrei M, Aburel P. Tetrahedron 1998;54:2107; Druta I, Dinica R, Bacu E, Humelnicu I. Tetrahedron 1998;54:10811; Dinica R, Druta I, Pettinari C. Synlett 2000;7:1013; Danac R, Rotaru A, Drochioiu G, Druta I. J Heterocyclic Chem 2003;40:283; Druta I, Danac R, Barbieru R, Tapu D, Andrei M. Sci Ann Al I Cuza Univ Iasi S Chem 2001;IX:149] as potential candidates for OLED applications. In this case, PPV was used as the hole-transport layer (HTL) and pyrrolo[1,2-a][1,10]phenanthroline derivatives, which were spin coated onto PPV, as the emissive layer. The structure of all PPV devices reported in this paper was fabricated using UV light for patterning. r 2007 Elsevier Ltd. All rights reserved. Keywords: Poly(p-phenylenevinylene); Pyrrolo[1,2-a][1,10]phenanthroline derivatives; Patterning process; Organic light-emitting diode
1. Introduction
Corresponding author. Tel.: +49 3375 508 524; fax: +49 3375 508 503. E-mail address: [email protected] (M. Prelipceanu).
1369-8001/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.mssp.2007.05.004
Polymers have attracted large interest due to their potential use as active material in electronic, optical and optoelectronic applications, such as lightemitting diodes, photodiodes, photovoltaic cells
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[1–7], field effect transistors and optically pumped lasers [8,9]. Their application, however, is inhibited at present by limitations in controlling luminescent spectra, sensitivity, efficiency and lifetime of devices. The main challenges posed by these polymers are the understanding of their electronic structure, the interplay between their chemical composition and morphology and their implementation in technological processes of device fabrication. Poly(p-phenylenevinylene) (PPV) is the luminescent polymer used for the first polymer-based organic light-emitting diode (OLED), which operated successfully at low driving voltage [10,11]. Because PPV itself is not soluble, film preparation starts from a soluble precursor polymer, which is then transformed to PPV by heat treatment [12–15]. The most simple OLED structure consists of a thin organic sandwiched between two metallic or semiconducting electrodes. Such single-layer devices consist e.g. of PPV sandwiched between a transparent indium–tin-oxide (ITO) anode and a cathode made from evaporated aluminium, magnesium or calcium. The performance of PPV single-layer OLEDs is rather poor [16–23]. The reason for this is on the one hand unbalanced charge transport, which reduces the probability of exciton formation, and on the other hand exciton quenching near the electrode. To improve the performances of PPVbased OLEDs, the main idea is to use a host of electrically and/or optically active materials, usually either blended into the polymer film (blend structure) [23] or as an additional layer (heterostructure) [24–28]. Our work is focused on the direct photolithographic patterning of PPV by using a new precursor after the Brabec [36] and Vanderzende [12] route and on the investigation of single- and two-layer devices made of this type of PPV. 2. Results and discussions 2.1. Synthesis and thermal stability of PPV 2.1.1. Synthesis of PPV The route of chemical synthesis of the PPV precursor according to Van Bremen et al. [12] and Brabec et al. [36] and the process of thermal conversion to PPV are schematically represented in Scheme 1. The synthesis was carried out by the method described by Vanderzende et al. [12] and Brabec et al. [36]. The PPV alkylsulphinyl precursor polymer was synthesised by polymerisation of 1-(chloromethyl)-
4-[(n-octylsulphinyl)methyl]benzene [8]. Step 1: 1,4Dichloroxylene reacts with tetrahydrothiophene to give 1,4-bis(tetrahydrothiopheniomethyl)xylene dichloride (I). Step 2: Compound I reacts with n-octanethiol to give 1-(chloromethyl)-4-[(n-octylsulphanyl)methyl]benzene (II). Step 3: The oxidation of II by H2O2/TeO2 yields 1-(chloromethyl)4-[(n-octylsulphinyl)methyl]benzene (III). Step 4: The polymerisation reaction is carried out with NaOtBu as a base to give the precursor polymer poly{[1,4-phenylene]-[1-(n-octylsulphinyl)ethylene]}(IV). Step 5: This polymer is dissolved in toluene and chloroform (1:1, v/v) and spin coated onto a quartz glass substrate, and then converted to PPV (V) by heating up to high temperatures in high vacuum for a certain time. 2.1.2. Thermal stability The decomposition temperature of the PPV precursor was determined by thermogravimetric analysis (TGA). These results were generated by heating the PPV sample at a constant rate of 10 1C/min. Fig. 1 shows that first weight loss starts at 87 1C and is due to the evaporation or volatilisation of the solvent. At temperatures higher than 87 1C, it is shown in the curve that there is a severe overlap in weight losses, about 9.17%. The weight loss occurring at about 174 1C is due to thermal decomposition of the material. 2.2. Preparation and characterization of PPV thin films 2.2.1. Spin coating PPV precursor films have been prepared by spin coating onto quartz substrates. The quartz substrates were previously cleaned according to a standard process and then dried in streaming N2 [9]. Spin coating was carried out at a spinning speed of 2000 rpm and a spinning time of 60 s from a solution of 50 mg/ml in analytical grade toluene and chloroform (1:1 v/v). 2.2.2. Ellipsometry The mean thickness and refractive index (n) of the films were determined by means of ellipsometry using a Plasmos SD2000 Automatic Ellipsometer operating at a wavelength of 632.8 nm. The measured different values of thickness and refractive index are shown in Table 1.
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Scheme 1. Synthesis of PPV precursor (I–IV) and thermal conversion to PPV (V).
2.2.3. Profilometry A Profilometer (DEKTAK 3 from Veeco Instruments) device was used to determine the sample thickness via a mechanical method. The profilometer has the capability of measuring a step height down to a few nanometres. For the investigated PPV precursor thin film (sample no. 1), a thickness value of 87 nm (before conversion) is measured (see Fig. 2). Using the loss mass calculation method, and density of PPV precursor and PPV, we found that the thickness value after conversion decreased
to 61%. The experimental results show that the layer thickness after conversion amounts to about 51% of the original layer thickness. A big advantage of the PPV precursor polymer used here is the possibility to act as a photoresist itself. Using UV light, fabrication of PPV patterns is performed [37]. The method of patterning and converting the PPV precursor polymer consists of (1) formation of a layer of the soluble precursor polymer by spin coating, (2) irradiation of selected parts of the layer through a mask with a desired
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Fig. 1. Thermogravimetric analysis (TGA) of PPV.
Table 1 Thickness and refractive index values measured for PPV films obtained from spin coating Sample
Thickness Refractive (nm) index (n)
PPV precursor on quartz (before annealing) PPV on quartz (annealed in vacuum) PPV on quartz (annealed in normal atmosphere) PPV on quartz (after second annealing in vacuum)
8775
1.370.05
4575 5875
1.59070.05 1.56770.05
4475
1.57870.05
All measurements were performed at room temperature.
pattern with UV light (Hg lamp, 365 nm) to partially convert the irradiated portions of the layer towards the conjugated structure, (3) removal of the non-illuminated area of the layer with an organic solvent such as chloroform and (4) conversion of the remaining pattern into the conjugated polymer by heating, preferably under high vacuum (106 mbar) for 2 h at 200 1C. Fig. 3a and b shows PPV patterns created by this method. One can see well-defined edges and channel width up to 10 mm. The PPV structures obtained with this method can be used in OLEDs. Some preliminary investiga-
tions show that PPV thin films should have a thickness range between 10 and 100 nm in order to obtain PPV patterns suitable for electronic applications. This new structuring method can not only be limited to OLEDs but can also be modified and optimised for further applications. 2.2.4. Atomic force microscopy The surface topology of the films was characterized using an Atomic Force Microscope (Autoprobe VP 2 Park Scientific Instruments), operating in noncontact mode in air at room temperature. Atomic force microscopy (AFM) images of the topology of PPV before and after the conversion process are shown in Figs. 4 and 5, respectively. Fig. 5 shows the surface morphology of PPV thin films after conversion in vacuum for 2 h (Fig. 5a) and PPV thin films converted at normal atmosphere at 200 1C for 2 h (Fig. 5b). In Fig. 5, differences in the morphology of PPV thin films, converted at normal atmosphere and in vacuum, can be seen. The thickness of the thin films, which were converted under different conditions (vacuum or air), has roughly the same value (see Table 1) [28]. Also, an average surface roughness RMS of 2 nm (for the layer annealed in vacuum (Fig. 5a)) and
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Fig. 2. Dektak profile of a PPV precursor thin film (before conversion).
Fig. 3. PPV patterns created by irradiation with UV light (Hg lamp, 365 nm).
5 nm (for the layer annealed in air (Fig. 5b) is measured, respectively. In order to find optimal conversion conditions, PPV layers have been converted in vacuum (106 mbar) at different temperatures. The PPV layers were deposited and kept under the same conditions. The samples were investigated immediately after the conversion process in order to minimize factors, which can have an influence on the experiment, e.g. by changing the structure and other properties of the PPV layer due to degradation, oxidation or other chemical reactions.
Lenz et al. [29] studied the thermal conversion of a precursor polymer to PPV using real-time X-ray scattering. For temperatures lower than 87 1C, no modification of the material is observed, indicating a significant amount of conversion. At temperatures around 130 1C, characteristic features of an imperfect PPV crystalline structure are already present. As we have shown previously, at 200 1C well-defined crystalline domains of PPV are detected, thus indicating the complete transformation of the precursor into the final polymer.
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the thickness of the PPV layer and on the decomposition per cent of the material as can be seen in Fig. 1. Fig. 7 shows the optical spectra after complete conversion for single and double PPV layers. For double PPV layers, the maxima peak has greater intensity probably due to the interface between PPV layers. The main peak observed at about 425 nm is due to the p–p* transition of PPV. The spectra for single and double PPV layers after annealing at 250 1C and 200 min have smaller-intensity peaks due to the degradation of polymer. From these spectra, the peaks at 240 nm are not present due to the complete loss of the solvent. 2.3. OLED device performances Fig. 4. Atomic force microscope image (1 1 mm2) of PPV precursor with an average thickness of 87 nm deposited by spin coating on quartz substrate.
Fig. 5. (a) AFM scan of PPV thin film after conversion in vacuum at 200 1C for 2 h and (b) AFM image of PPV films after conversion in normal atmosphere at 200 1C for 2 h.
2.2.5. UV/vis measurements A UV/vis Spectrometer (Lambda 16 PerkinElmer) was used to determine UV–vis absorption spectra of the prepared PPV thin films on quartz substrates. Fig. 6 shows absorption spectra of PPV layers converted in vacuum at different temperatures and time ranges [30–32]. Obviously, there are differences in the intensity of absorption maxima of the PPV films prepared under different annealing conditions. The peaks obtained at about 240 nm are from chloroform, which has been used for the preparation of PPV thin layers. The main peak observed at about 425 nm is due to the p–p* transition of PPV. The maximum of absorbance peaks is dependent on
To compare the properties of the double layer of PPV devices and typical double, different materials, we used a PPV precursor as the hole-transport layer (HTL) material and pyrrolo[1,2-a][1,10]phenanthroline derivatives, which were spin coated onto PPV, as the emissive layer. In Fig. 8, the chemical structure of pyrrolo[1,2-a][1,10]phenanthroline derivatives is illustrated. Their synthesis is described below. The non-stable monosubstituted heteroaromatic N-ylides obtained in situ by deprotonation of the corresponding cycloimmonium salts in the presence of base are 1,3-dipoles that undergo cycloaddition with activated symmetrical or non-symmetrical olefins and alkynes, resulting in the formation of a fused fivemember nitrogen heterocycle [1–5]. We performed some reactions of 1,10-phenanthrolinium-ylides with dimethyl acetylenedicarboxylate (DMAD) and ethyl propiolate to yield novel pyrrolo [1,2-a][1,10]phenanthroline derivatives [9]. Initially, for obtaining 1,10-phenanthrolinium salts 1–5, reaction of 1,10-phenanthroline with reactive halide derivatives was considered [6]. Reaction between salts 1–6 suspended in different mixtures of organic solvents and an aqueous solution of 0.2 N of sodium hydroxide (NaOH) gave in situ 1,10-phenanthrolinium-ylides 6–10 (Scheme 2), which on stereoselective 3+2 dipolar cycloaddition with DMAD or ethyl propiolate furnished the following compounds: 1-(40 -R-benzoyl)-2,3-dimethoxycarbonyl-pyrrolo[1,2-a] [1,10]-phenanthroline) 17–20 and 1-(40 -R-benzoyl)3-ethoxycarbonyl-pyrrolo[1,2-a][1,10]-phenanthroline) 21–22, respectively (Scheme 3). As shown in Scheme 3, the cycloaddition reaction firstly generated non-isolating intermediate
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Fig. 6. UV–vis spectra of PPV converted in vacuum at different temperatures and time ranges (each curve is characterised by the parameters given in the legend in the succession indicated by the arrow).
Fig. 7. UV–vis spectra of PPV converted in vacuum at 200 1C and 150 min for single and double layers. Also shown are the degradation spectra curves for single- and double-layer PPV at 250 1C and 200 min annealing in vacuum.
cycloadducts of type 11–16, which due to the tendency of stabilisation suffered an oxidative dehydrogenation process because the reactions were carried out in ambient conditions.
After separation and purification by recrystallisation from appropriate solvents or flash chromatography, the crystalline slowly fluorescent cycloadducts 17–22 were obtained (Table 2). Their
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structures were proved by elemental and spectral methods (infrared spectroscopy, IR, and nuclear magnetic resonance, 1H-NMR) [7]. The device structures under study are as follows: (1)
ITO (130 nm)/PPV (100 nm). ITO (130 nm)/PPV (100 nm). ITO (130 nm)/PPV (100 nm). ITO (130 nm)/PPV (100 nm). ITO (130 nm)/PPV (100 nm). ITO (130 nm)/PPV (100 nm).
(2) (3) (4) (5) (6)
N
N O
1
3 2
R1
R
R2
(50 nm)/RA1 (50 nm)/Al (50 nm)/RA2 (55 nm)/Al (50 nm)/RA3 (53 nm)/Al (50 nm)/RA4 (60 nm)/Al (50 nm)/RA5 (50 nm)/Al (50 nm)/RA6 (57 nm)/Al
RA1: R = NO2, R1 = R2= COOCH3 RA2: R = Cl, R1= R2 = COOCH3 RA3: R = Br, R1 = R2 = COOCH3 RA4: R = CH3, R1 = R2 = COOCH3 RA5: R = Br, R1 = H, R2 = COOC2H5 RA6: R = R1= H, R2 = COOC2H5
4'
Fig. 8. Chemical structure of pyrrolo[1,2-a][1,10] phenanthroline derivatives.
The current density versus electric field characteristics for the device structures 1–4 are shown in Fig. 9. All examined structures show a clear rectification. It is an indication of good injection efficiencies. The shape of respective curves can be influenced by the use of aluminium as the cathode electrode. The current–voltage characteristics for the positive region are shown in Fig. 10. Devices 2 and 4 have a turn-on voltage of around 4 V, which is lower than that of device 3 (around 7 V). We could only measure light emission from devices 3 and 4 (Fig. 11). They were seen to emit in the yellow-green with a luminance of 0.20 and 0.26 cd m2, respectively. Devices 1 and 2 also showed yellowish-green light emission, but their luminances were too weak to be detected. As a reference, we will note the 0.06 cd m2 luminance of device 2. In the examined OLED structures, PPV constitutes the HTL. Secondly, it can improve electron injection as the holes can accumulate at the PPV/RA interface if energy is upset. This could decrease the turn-on voltage, and consequently improve the power efficiency (Fig. 12). The soluble pyrrolo[1,2-a][1,10]phenanthroline derivatives and PPV are very promising candidates for use in organic light-emitting devices, despite the fact that several parameters still need to be optimised to obtain efficient devices. The characteristics of single-layer (ITO/PPV/organic-RA
Scheme 2.
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Scheme 3.
Table 2 Substituents R, R1 and R2 (positions 40 , 2 and 3, respectively) in the molecular structure of present compounds No.
Compound
17 18 19 20 21 22
RA1: RA2: RA3: RA4: RA5: RA6:
l-(40 -nitro-benzoyl)-2,3-dimethoxycarbonyl-pyrrolo[l,2-a] [1,10]-phenanthroline 1-(40 - chloro-benzoyl)-2,3-dimethoxy carbonyl-pyrrolo[1,2-a [1,10]-phenanthroline 1-(40 -bromo-benzoyl)-2,3-dimethoxycarbonyl-pyrrolo[1,2-a][1,10]-phenanthroline 1-(40 -methyl-benzoyl)-2,3-dimethoxycarbonyl-pyrrolo[1,2-a][l,10]-phenanthroline l-(40 -bromo-benzoyl)-3-ethoxycarbonyl-pyrrolo[l,2-a][l,10]-phenanthroline l-benzoyl-3-ethoxycarbonyl-pyrrolo(l,2-a][l,10]-phenanthroline
compounds/Al) organic light-emitting devices have been studied. All devices show clear rectification behaviour, with a turn-on voltage between 4 and 7 V. The most successful OLED emits in the yellowgreen with a luminance of 0.26 cd m2 for a singlelayer device. In addition, double-layer devices have been studied with PPV sandwiched between transparent ITO and aluminium. The insoluble PPV is spin coated as a precursor polymer and thermally
R
R1
R2
–NO2 –CI –Br –CH3 –Br –H
–COOCH3 –COOCH3 –COOCH3 –COOCH3 –H –H
–COOCH3 –COOCH3 –COOCH3 –COOCH3 –COOC2H5 –COOC2H5
converted under vacuum prior to vacuum evaporation of the final electrode. The performance of an OLED depends on the injection conditions for holes and electrons. Usually, in a single-layer structure, electrons and holes are unbalanced, unless a situation occurs where the anode and cathode are very well matched to the molecular levels of the organic compound. In the unbalanced case, a dominant carrier can cross the whole structure without finding a carrier of
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Fig. 9. OLED structure double layer with different materials: (a) top view and (b) side view.
Fig. 10. Current–voltage characteristics of RA/PPV devices (positive region).
opposite sign to recombine, raising an energy waste and leading to low efficiency in the conversion of electrical into optical power. To achieve a better balance, two (or more) organic layers must be added: one matching the anode, optimised for transport of holes, and a second one matching the cathode, optimised for electron transport. In this way the charge injection can be balanced. Since a barrier occurs at the organic/organic interface, carriers tend to accumulate there, improving the probability of finding an available partner for
recombination. The subsequent process is the relaxation process to the ground state accompanied by the emission of light. For electrical investigations, the OLED geometry shown in Fig. 13 is used. The devices are prepared in such a way that on a pre-treated ITO/glass substrate, one or two PPV layers were deposited subsequently by spin coating followed by annealing. Aluminium electrodes were evaporated on top of these films. The thicknesses of ITO and aluminium layers were 150 and 100 nm, respectively. On one
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Fig. 11. EL emission of RA/PPV devices.
Fig. 12. Schematic band diagrams of (a) a single-layer OLED device under forward biasing and (b) a double-layer OLED device: 1—cathode, 2—anode, 3—recombination centre.
substrate, six individual OLED samples were prepared simultaneously. The active area of the light-emitting diode was 2 2 mm2 [24]. The current–voltage and electroluminescence– voltage characteristics were measured using a Keithley 2400 source meter and a Keithley 617 multimeter. EL measurements were made under forward bias (ITO positive) and the emission was measured in the forward direction through the transparent ITO.
The current–voltage characteristics of single and double PPV layer devices are plotted in Fig. 13. A turn-on voltage of about 9 and 8 V was recorded for double PPV layer and for single PPV layer devices, respectively. The behaviour of the double PPV layer devices and the promising values of efficiency for these devices can be a starting point for further investigations using a matrix of PPV doped with different organic materials. The double PPV layer device shows a strange behaviour in the
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Fig. 13. Top (a) and cross-section (b) views of OLED structure ITO/PPV/PPV/Al.
Fig. 14. Current–voltage characteristic of a light-emitting diode with single/double layers of PPV. Inset is the presented log (abs (I)) and electroluminescence function of applied voltage.
current–voltage characteristic between 5 and 0 V, probably due to different traps that are generated in the fabrication process of OLEDs at the interface between the PPV layers. As expected, these OLED structures emit green light caused by recombination of excitons in PPV bulk material. A strong increase of the electroluminescence signal is observed above the onset voltage determined at 10 V, as can be seen in the inset of Fig. 14. Wong et al. [32] investigated multilayer system devices deposited by spin-coating solutions of the different oligo(phenylenevinylenes) onto insoluble PPV layers. Using a two-layer device, electrons and holes can accumulate at the polymer/
oligomer interface. This could decrease the turn-on voltage, and consequently improve the power efficiency. The efficiency of ITO/PPV/PPV/Al devices is comparable to other published results on green light-emitting devices [33–35]. 3. Conclusions The synthesis of the PPV precursor, thermal conversion to PPV, structural, optical and electrical investigations of PPV layers are presented. Spectral characteristics of PPV films are strongly dependent on the conversion conditions. The optimal conditions of annealing are 200 1C for 2.5 h in vacuum.
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The maximum absorption in the absorption spectra is measured at 420 nm. For a conversion time less than 2.5 h, the maxima of the intensity of absorption spectra are smaller. A comparison of the PPV layers converted in vacuum and in air is made. The roughness of the layers converted in air is higher than for layers converted in vacuum. In these conditions, the PPV layers are not stable and the interface between substrate and polymer is affected. The layers obtained at optimum conditions were used successfully in OLED structures. Future work will focus on PPV used as a host system for new organic materials, e.g. chromophors, which can be used as doping materials. This will increase the efficiency and lifetime of organic lightemitting devices. Acknowledgements The authors would like to thank Dr. Peter W. Schmidt (University of Applied Sciences Wildau), Dr. R. Danac (University Al.I. Cuza Iasi) for a critical reading of the manuscript, providing of RA compounds and Monika Ehlert (University of Potsdam) for help with DSC measurements. Financial support of the European Commission under contract number HPRN-CT-2002-00327-RTN EUROFET Project and FP6–505478-1 ODEON Project is gratefully acknowledged. References [1] Leontie L, Druta I, Danac R, Rusu GI. Synth Met 2005;155/ 1:138. [2] Zugravescu I, Petrovanu M. 3+2 Dipolar cycloaddition. Bucharest: Roman Academic Publishing House; 1987 [in Romanian]. [3] Druta I, Andrei M, Aburel P. Tetrahedron 1998;54:2107. [4] Druta I, Dinica R, Bacu E, Humelnicu I. Tetrahedron 1998;54:10811. [5] Dinica R, Druta I, Pettinari C. Synlett 2000;7:1013. [6] Danac R, Rotaru A, Drochioiu G, Druta I. J Heterocyclic Chem 2003;40:283. [7] Druta I, Danac R, Barbieru R, Tapu D, Andrei M. Sci Ann Al I Cuza Univ Iasi S Chem 2001;IX:149.
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