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Electronic properties of polyparaphenylene prepared by a precursor route
Synthetic Metals, 41-43 (1991) 279-282
279
ELECTRONIC PROPERTIES OF POLYPARAPHENYLENE PREPARED BY A PRECURSOR ROUTE
G. LEISINGa , T. VERDONb, G. LOUARNb and S. LEFRANT b a) Institut f'tir Festk6rperphysik, TU Graz, A-80110 Graz (Austria) b) Laboratoire de Physique Cristalline, IPCM, Universit6 de Nantes, 44072 Nantes cedex 03 (France)
ABSTRACT In this work, we present a characterization study of PPP prepared in the form of polycristalline thin films, via a precursor polymer route. U.V. visible absorption spectra exhibit a main peak at 3.65 eV. Photoluminescence studies carried out with an excitation wavelength at 406 nm show different components assigned to intrinsic photoluminescence superposed to an extrinsic contribution due to structural defects. In Resonance Raman Scattering (RRS), by excitation in the near U.V. range (351.1 nm), the bands peaked at 1220, 1280 and 1600 cm -1 are similar to those observed in standard PPP synthetized by other methods.
INTRODUCTION In the field of conducting polymers, the search for materials with improved environmental stability compared to polyacetylene has been really fruitful these last years. In particular, the so-called "precursor route" has led to polymeric compounds which are stretchable, processable, highly conductive after doping, and present in some cases significant non-linear properties. Then, they constitute good candidates in a number of applications such as electro-optical, non-linear optical and electronic devices. In this paper, we present experimental results on polyparaphenylene samples obtained by a precursor route. This method of synthesis of PPP give rise to a more compact compound compared to those prepared by chemical or electrochemical reactions which exhibit a fibrillar morphology. Recent studies [1] have shown that metal/polymer/metal structures can be made using these materials.
RESULTS PPP has been synthesized by a presursor route according to a method proposed by Ballard et al [2]. The precu~or polymerss used to produce the studied samples contained naphtalene, methyl and
0379-6779/91/$3.50
© Elsevier Sequoia/Printed in The Netherlands
280
biphenyl species as sidegroups. Thin films were cast from solutions for the precursor polymer onto appropriate substrates for spectroscopic studies. The thermal conversion of the precursor to polyparaphenylene film occurred at temperatures up to 400"C for several hours under high vacuum. Details on the procedure have been published elsewhere [3]. The conversion was followed by infrared spectroscopy as shown in Ref. [3] in which a clear IR spectrum similar to that of a KovacicPPP sample is presented. In Figure 1, we show the optical absorption spectrum of the "precursor route" PPP compared to electropolymerized PPP. The maximum of the absorption band is peaked at ~ 326 nm (3.65 eV), a value slightly shifted to the high energy compared to that of the electropolymerized PPP thin films [4] which show in addition a clear shoulder at 425 nm. A similar shift in the position of the absorption maximum was already observed in desordered (CH)x compared to oriented (CH)x [5].
, i.,..i
g
.~,,i
,---
\
©
---2." 200
I 260
I 320
I 380
I 440
(nm)
Fig. 1.
500 ,,-
Optical absorption spectra of PPP recorded at room temperature :
a) prepared by the precursor route b) prepared by electroreducfion of dibromo-diphenyl (Ref. 4)
Figure 2 shows the photoluminescence spectrum of PPP prepared by the precursor route compared to that of PPP prepared by electrochemistry. Apart from the well defined bands due to the intrinsic part of the photoluminescence, the spectrum clearly exhibits an additional component whose maximum is at - 570 nm, usually associated to structural defects which should be different from those observed in Kovacic samples[6].
281
a) ..__/'" -
II
""-..,,
,"J
H
\
*s"
I t
L
450
/
%% %
~
500
550 nm
600
650 700
Fig. 2. Photoluminescence spectra of PPP, kexc = 406 nm, T=77K a) PPP precursor route ; b) PPP electropolymerized (Ref. 5)
The Raman spectrum recorded with an excitation wavelength in the near U.V. range, kexc = 351.1 nm, exhibit the usual characteristics of PPP. It is composed of three main bands peaked at 1220, 1280 and 1600 cm -1. Notice that the strong photoluminescence signal does not allow to record the Raman spectrum with any of the excitation line in the visible range. From previous experiments carried out in oligomers [7], it has been shown that the length of the polymer, or more precisely the number of phenyl rings constituting the polymeric chain, can be estimated from the ratio of the intensity of the Raman bands peaked at 1280 cm -1 and 1220 cm-1 respectively. In Fig.3, we represent the behavior of the value of 11280/11220 measured for different oligomers with texc=676.4 nm. In addition, we have indicated the values measured for PPP "Yamamoto" [8] and electropolymerized PPP. It turns out that the chains of PPP obtained by electropolymerization appears much longer than those found in other compounds although the saturation of the curve does not allow to give a precise value for the number of phenyl rings constituting the polymeric chain. In the present case, we could not record the Rarnan spectrum at Xexc=676.4 nm, but by comparing spectra taken with an excitation in the near UV, kexc=351.1 nm, one can deduce that the chains of PPP prepared by the precursor route has a reasonable length (more than 10 phenyl rings) in consistence with results reported in Ref.3.
282
I X
-
~,exc.= 676.4nm o
20
t'q
10
-
0
~
PPP Yamamoto
\ . I
.
. I
2
.
. I
x---o I
4
I
i
. i
6
I
8
.
. I
PPP electropolymerized
. I
10
.
. I
. |
12
Number of phenyl rings
.
~ I
I
i
14 ~-
Fig. 3. Ratio 11280/11220 versus the number of phenyl rings in phenyl oligomers and polymers. Measurements with ~.exc=676.4 nm. In conclusion, we have reported results obtained on PPP prepared by a precursor route. The properties of such samples present strong similarities with those of PPP prepared with other methods but the compact morphology of the compound offers advantages which can readily be used in applications. REFERENCES 1 T.P. Nguyen, S. Lefrant, G. Leising and F. Stelzer, Synth. Met., 10 (1990) 69. 2 D.G.M. Ballard, A. Courtis, I.M. Shriley and S.C. Taylor, J. Chem. Sot., Chem. Commun., (1983) 954. 3 G. Leising, K. Pichler and F. Stelzer, in H. Kuzmany, M. Mehring and S. Roth (eds), Electronic. Properties of Conjugated Polymers, Springer Series in Solid State Sciences, 91 (1990) 100. 4 Y. Pelous, G. Froyer, C. Herold and S. Lefrant, Synth. Met., 29 (1989) El7. 5 G.Leising, Synth. Met., 28 (1989) D215. 6 E. Rzepka, C. Jin, S. Lefrant, Y. Pelous, G. Froyer, A. Siove, Synth. Met., 29 (1989) E23. 7 S. Krichene, J.P. Buisson and S. Lefrant, Synth. Met., 17 (1987) 589. 8 T. Yamamoto and A. Yamamoto, Chem.Lett. (1977) 353.
279
ELECTRONIC PROPERTIES OF POLYPARAPHENYLENE PREPARED BY A PRECURSOR ROUTE
G. LEISINGa , T. VERDONb, G. LOUARNb and S. LEFRANT b a) Institut f'tir Festk6rperphysik, TU Graz, A-80110 Graz (Austria) b) Laboratoire de Physique Cristalline, IPCM, Universit6 de Nantes, 44072 Nantes cedex 03 (France)
ABSTRACT In this work, we present a characterization study of PPP prepared in the form of polycristalline thin films, via a precursor polymer route. U.V. visible absorption spectra exhibit a main peak at 3.65 eV. Photoluminescence studies carried out with an excitation wavelength at 406 nm show different components assigned to intrinsic photoluminescence superposed to an extrinsic contribution due to structural defects. In Resonance Raman Scattering (RRS), by excitation in the near U.V. range (351.1 nm), the bands peaked at 1220, 1280 and 1600 cm -1 are similar to those observed in standard PPP synthetized by other methods.
INTRODUCTION In the field of conducting polymers, the search for materials with improved environmental stability compared to polyacetylene has been really fruitful these last years. In particular, the so-called "precursor route" has led to polymeric compounds which are stretchable, processable, highly conductive after doping, and present in some cases significant non-linear properties. Then, they constitute good candidates in a number of applications such as electro-optical, non-linear optical and electronic devices. In this paper, we present experimental results on polyparaphenylene samples obtained by a precursor route. This method of synthesis of PPP give rise to a more compact compound compared to those prepared by chemical or electrochemical reactions which exhibit a fibrillar morphology. Recent studies [1] have shown that metal/polymer/metal structures can be made using these materials.
RESULTS PPP has been synthesized by a presursor route according to a method proposed by Ballard et al [2]. The precu~or polymerss used to produce the studied samples contained naphtalene, methyl and
0379-6779/91/$3.50
© Elsevier Sequoia/Printed in The Netherlands
280
biphenyl species as sidegroups. Thin films were cast from solutions for the precursor polymer onto appropriate substrates for spectroscopic studies. The thermal conversion of the precursor to polyparaphenylene film occurred at temperatures up to 400"C for several hours under high vacuum. Details on the procedure have been published elsewhere [3]. The conversion was followed by infrared spectroscopy as shown in Ref. [3] in which a clear IR spectrum similar to that of a KovacicPPP sample is presented. In Figure 1, we show the optical absorption spectrum of the "precursor route" PPP compared to electropolymerized PPP. The maximum of the absorption band is peaked at ~ 326 nm (3.65 eV), a value slightly shifted to the high energy compared to that of the electropolymerized PPP thin films [4] which show in addition a clear shoulder at 425 nm. A similar shift in the position of the absorption maximum was already observed in desordered (CH)x compared to oriented (CH)x [5].
, i.,..i
g
.~,,i
,---
\
©
---2." 200
I 260
I 320
I 380
I 440
(nm)
Fig. 1.
500 ,,-
Optical absorption spectra of PPP recorded at room temperature :
a) prepared by the precursor route b) prepared by electroreducfion of dibromo-diphenyl (Ref. 4)
Figure 2 shows the photoluminescence spectrum of PPP prepared by the precursor route compared to that of PPP prepared by electrochemistry. Apart from the well defined bands due to the intrinsic part of the photoluminescence, the spectrum clearly exhibits an additional component whose maximum is at - 570 nm, usually associated to structural defects which should be different from those observed in Kovacic samples[6].
281
a) ..__/'" -
II
""-..,,
,"J
H
\
*s"
I t
L
450
/
%% %
~
500
550 nm
600
650 700
Fig. 2. Photoluminescence spectra of PPP, kexc = 406 nm, T=77K a) PPP precursor route ; b) PPP electropolymerized (Ref. 5)
The Raman spectrum recorded with an excitation wavelength in the near U.V. range, kexc = 351.1 nm, exhibit the usual characteristics of PPP. It is composed of three main bands peaked at 1220, 1280 and 1600 cm -1. Notice that the strong photoluminescence signal does not allow to record the Raman spectrum with any of the excitation line in the visible range. From previous experiments carried out in oligomers [7], it has been shown that the length of the polymer, or more precisely the number of phenyl rings constituting the polymeric chain, can be estimated from the ratio of the intensity of the Raman bands peaked at 1280 cm -1 and 1220 cm-1 respectively. In Fig.3, we represent the behavior of the value of 11280/11220 measured for different oligomers with texc=676.4 nm. In addition, we have indicated the values measured for PPP "Yamamoto" [8] and electropolymerized PPP. It turns out that the chains of PPP obtained by electropolymerization appears much longer than those found in other compounds although the saturation of the curve does not allow to give a precise value for the number of phenyl rings constituting the polymeric chain. In the present case, we could not record the Rarnan spectrum at Xexc=676.4 nm, but by comparing spectra taken with an excitation in the near UV, kexc=351.1 nm, one can deduce that the chains of PPP prepared by the precursor route has a reasonable length (more than 10 phenyl rings) in consistence with results reported in Ref.3.
282
I X
-
~,exc.= 676.4nm o
20
t'q
10
-
0
~
PPP Yamamoto
\ . I
.
. I
2
.
. I
x---o I
4
I
i
. i
6
I
8
.
. I
PPP electropolymerized
. I
10
.
. I
. |
12
Number of phenyl rings
.
~ I
I
i
14 ~-
Fig. 3. Ratio 11280/11220 versus the number of phenyl rings in phenyl oligomers and polymers. Measurements with ~.exc=676.4 nm. In conclusion, we have reported results obtained on PPP prepared by a precursor route. The properties of such samples present strong similarities with those of PPP prepared with other methods but the compact morphology of the compound offers advantages which can readily be used in applications. REFERENCES 1 T.P. Nguyen, S. Lefrant, G. Leising and F. Stelzer, Synth. Met., 10 (1990) 69. 2 D.G.M. Ballard, A. Courtis, I.M. Shriley and S.C. Taylor, J. Chem. Sot., Chem. Commun., (1983) 954. 3 G. Leising, K. Pichler and F. Stelzer, in H. Kuzmany, M. Mehring and S. Roth (eds), Electronic. Properties of Conjugated Polymers, Springer Series in Solid State Sciences, 91 (1990) 100. 4 Y. Pelous, G. Froyer, C. Herold and S. Lefrant, Synth. Met., 29 (1989) El7. 5 G.Leising, Synth. Met., 28 (1989) D215. 6 E. Rzepka, C. Jin, S. Lefrant, Y. Pelous, G. Froyer, A. Siove, Synth. Met., 29 (1989) E23. 7 S. Krichene, J.P. Buisson and S. Lefrant, Synth. Met., 17 (1987) 589. 8 T. Yamamoto and A. Yamamoto, Chem.Lett. (1977) 353.