- Email: [email protected]
Studies on the efficient synthesis of poly(phenylenevinylene) (PPV) and poly (dimethoxy phenylenevinylene) (dimethoxy-PPV)
Synthetic Metals, 41--43 (1991) 261-264
261
STUDIES ON THE EFFICIENT SYNTHESIS OF POLYtPHENYLENEVINYLENE) (PPV) AND POLY(DIMETHOXY PHENYLENEVINYLENE) (DIMETHOXY-PPV)
P. L. BURN*, D. D. C. BRADLEY**, A. R. BROWN**, R. H. FRIEND**, and A. B. HOLMES*. * University Chemical Laboratory, Lensfield Road, Cambridge CB2 1EW, UK. ** Cavendish Laboratory, Madingley Road, Cambridge CB3 0HE, UK.
ABSTRACT The syntheses of poly(phenylene vinylene) (PPV) and poly(2,5-dimethoxy-phenylene vinylene) (dimethoxy-PPV) have been investigated. Precursor polymers which were soluble and processible were prepared and converted to the conjugated polymers. It was found that the conditions needed to prepare precursor polymers were sensitive to the substrate and the conditions required to form the conjugated polymers were dependent on the leaving group present. The spectroscopic and structural properties of the conjugated polymers were found to be determined by the precursor polymer route followed. INTRODUCTION Conjugated polymers have attracted considerable interest as materials for applications such as electronic and non-linear optic devices, and are potentially conducting due to the extensive n-bond delocalisation. However, most conjugated polymers are unprocessible. This problem has been circumvented by preparing soluble precursor polymers which gives flexibility for materials processing and can easily be converted into the conjugated form. By this precursor method, high quality samples of controlled morphology, crystallinity, orientation and conjugation length of uninterrupted sequences can be obtained. Both PPV and dimethoxy-PPV have been synthesised via soluble precursor polymer routes [1, 2, 3]. We present the syntheses of PPV, and dimethoxy-PPV comparing by spectrophotometric techniques the products produced by the various routes. RESULTS Synthesis of PPV PPV has been prepared by a sulfonium leaving group route, whereby the precursor polymer is a polyelectrolyte and soluble in protic polar solvents such as methanol and water, and a methoxy leaving group route, whereby the precursor polymer is soluble in polar aprotic solvents such as chloroform. The synthetic mutes and reaction conditions are shown in Figure 1. 0379-6779/91/$3.50
© Elsevier Sequoia/Printed in The Netherlands
262
-O-
XHzC
+c,CN,X
X =
(0.4 M / MeOH)
Or
+
X = S(CH3) 2
+
x= +,,2+or
MoO
Cf +
0.4 M in H20 1. O H (0.4 M / H20)
1. "OH (0.4 M / H20), lh, 0 °C, N2 2. H + (0.4 M / H20), to pH = 5-7
2. H * (0.4 M ]H20)
3. Dialysis, 3 days (dist. H20 ), r.t., N2
C~e
300 °C, 1 h H2C
(IV) X = (0H3)2S
"(--H ~ - - ~ H
CH
--~n
M~
CI +
( v ) X = ~S"~
x
x
Cf +
X= " S " m) x. CI-+ Solution in MeOH
X=S(CH3) 2 ~ / ~ 200'~C,2 h /
~so~"
~ -
- ~ n ~
PPV
=pTS, 3 eq
OMe
om ou.
),~)(j
S(CH=),
~.~--~-~.-):.
(vl]) MeOH, r.t., Ns, 48 h
Solution in CHCl3
1. Sodium p-toluenesulfonate, 3 eq, 16 min, r.t. 2. Methanol, 52 °C, N2, 24 h
pT$"
,..~MMeO H, r.t., N2. J
o~
°
..o,
overnight
o,.
°x=O Cf+
Figure 1
Figure 5
a) Sulfonium Leavin~ Grouo Route. The choice of the sulfonium group affects the structure of the prepared PPV. This is best illustrated by comparing the electronic spectra of PPV prepared by two different routes [4]. The electronic spectra (Figure 2) show that PPV prepared via the tetrahydrothiophenium (THT) precursor polymer (I) is more conjugated (red shift of the band edge) and crystalline (has phonon structure) compared with PPV prepared via the dimethylsulfonium (DMS) precursor polymer (II). The increase in crystallinity is confirmed by X-ray analysis of films prepared by the two methods [4].
/, .~.~~~_~ /
-~
x
~
/ ' (b)
(a)
3
~
4
•
5
Energy (eV)
Figure 2 (a) Absorbance spectra of PPV prepared via the precursor polymer (I) ; (b) Absorbance spectrum of PPV prepared via the precursor polymer (II) [41
263
(b) Methoxy Leaving Group Route. The route proposed by Tsutsui involves the exchange of the counteranion of the DMS precursor polymer (II), with p-toluenesulfonate, and this polymer is reacted with methanol to give polymer (III) [5]. We chose to use the THT sulfonium precursor polymer (1) which was prepared in aqueous media with the initial monomer concentration 0.2 M to avoid gelling. The result of exchanging the chloride anion with p-toluenesulfonate was a pale yellow sticky gum. We found that this polymer reacted with methanol at 52 °C to give a whitish solid (III). The DMS precursor polymer (II) was reported to react at room temperature with methanol to give (III) [5 I. Conversion of the sulfonium leaving group to the methoxy leaving group of polymer (III) was evidenced by IH n.m.r and I.R. spectroscopy (Figure 3a). The I.R. stretch at 1097 cm -1 is due to the methoxy leaving group and its disappearance was used to monitor the conversion of (II1) to PPV. Full conversion of the methoxy leaving group precursor polymer to PPV requires heating the sample at 200 o C in the presence of hydrogen chloride for 2 h. Acid is required because of the thermal stability of the leaving group. The I.R. spectrum of PPV prepared by this method (Figure 3b) is identical to PPV prepared via heat treatment of the THT precursor polymer (Figure 3c). The electronic spectra show that PPV prepared via the precursor polymer (Ill) (Figure 4a) has the same conjugation length (band edge at the same energy) but is less crystalline (has no phonon structure) than PPV prepared via the THT precursor polymer (I) (Figure 4b).
(a)
I,,~J,tlll I
~j~
(d)
i|11.
,...y.
= ~i
,~,
,
tl
,
"-'a
I
(e)
,, ~
jv,,
I
<= 4000
I
I
I
2700
1400
100
wavenumber
4000
I
I
I
2700
1400
100
wavenumber
Figure 3 I.R. Spectra of (a) Methoxy leaving group precursor polymer (III), (b) PPV prepared via (III), (c) PPV prepared via (I), (d) Methoxy leaving group precursor polymer (VI), (e) Hydroxy subsituted precursor polymer, (f) Dimethoxy-PPV via (VI)
Synthesis of 4im¢th0xy-PPV A sulfonium precursor polymer cannot be used directly as the processible polymer because during purification (dialysis against water) a gel forms. The precursor polymer utilised is one with a
264
,.o
<
1.24 1.74 2.24 2.74 3.24 3.74 Energy (eV) Figure 4 U.V. Spectra of (a) PPV prepared by heat/acid treatment of polymer (II1), (b) PPV i prepared via heat treatment of polymer (I) methoxy leaving group (VI) [31. We have further investigated the synthesis of dimethoxy-PPV and the origin of the gel formed during dialysis of the precursor polymer. The reaction sequence and conditions used are shown in Figure 5. The first step was to prepare a sulfonium precursor polymer (IV) or (V)) in water. We initially followed the route proposed by Tsutsui [3] in which the precursor polymer (IV) was precipitated out of solution by addition of the p-toluenesulfonate anion and the resultant whitish-yellow gum was dissolved in methanol and allowed to react to give a whitish precipitate (VI). Both I.R. (Figure 3d) and IH n.m.r, indicated conversion of the sulfonium to the methoxy leaving group was complete. Our discovery that methanol would substitute the sulfonium group of (V) without anion exchange (Figure 5) suggested that the gel formed during dialysis was due to substitution of the sulfonium group by water. We dialysed the DMS precursor polymer (IV) against distilled water to give a gel which was thoroughly dried. An I.R. spectrum (Figure 3e) of the resultant material had an OH stretch at 3400 cm-1 indicating, at least in part, the sulfonium leaving group had been substituted by water, giving the polymer gel (VII). The precursor polymer (VI) was converted into dimethoxy-PPV by heating at 200 °C for 2 hours in the presence of hydrogen chloride and the extent of conversion can be conveniently followed by the disappearance of the methoxy leaving group stretch at 1094 cm -1. The I.R. spectrum of the fully conjugated dimethoxy-PPV is shown in Figure 3f. REFERENCES 1. J. D. Capistran, D. R. Gagnon, $. Antoun, R. W. Lenz and F. H. Karasz, ACS Polvm. Preorints. 25 (1984) 282. 2. R. W.
nz, c - c . Han, J. Stenger-Smith and F. R. Karasz, ,I. Polym. Sci.: Part A; pQlym.
Chem. 26 (1988) 3241. 3. T. Momii, S. Tokito, T. Tsutsui, and S. Saito, Chem. Lett. (1988) 1201. 4. J. Martens, N. F. Colaneri, P. Bum, D. D. C. Bradley, E. A. Marseglia and R. H. Friend, Proc. NATO ASI. $Detses (Greece,. June, 1989. 5. S. Tokito, T. Momii, H. Murata, T. Tsutsui and S. Saito, Polymer. 31 (1990) 1137.
261
STUDIES ON THE EFFICIENT SYNTHESIS OF POLYtPHENYLENEVINYLENE) (PPV) AND POLY(DIMETHOXY PHENYLENEVINYLENE) (DIMETHOXY-PPV)
P. L. BURN*, D. D. C. BRADLEY**, A. R. BROWN**, R. H. FRIEND**, and A. B. HOLMES*. * University Chemical Laboratory, Lensfield Road, Cambridge CB2 1EW, UK. ** Cavendish Laboratory, Madingley Road, Cambridge CB3 0HE, UK.
ABSTRACT The syntheses of poly(phenylene vinylene) (PPV) and poly(2,5-dimethoxy-phenylene vinylene) (dimethoxy-PPV) have been investigated. Precursor polymers which were soluble and processible were prepared and converted to the conjugated polymers. It was found that the conditions needed to prepare precursor polymers were sensitive to the substrate and the conditions required to form the conjugated polymers were dependent on the leaving group present. The spectroscopic and structural properties of the conjugated polymers were found to be determined by the precursor polymer route followed. INTRODUCTION Conjugated polymers have attracted considerable interest as materials for applications such as electronic and non-linear optic devices, and are potentially conducting due to the extensive n-bond delocalisation. However, most conjugated polymers are unprocessible. This problem has been circumvented by preparing soluble precursor polymers which gives flexibility for materials processing and can easily be converted into the conjugated form. By this precursor method, high quality samples of controlled morphology, crystallinity, orientation and conjugation length of uninterrupted sequences can be obtained. Both PPV and dimethoxy-PPV have been synthesised via soluble precursor polymer routes [1, 2, 3]. We present the syntheses of PPV, and dimethoxy-PPV comparing by spectrophotometric techniques the products produced by the various routes. RESULTS Synthesis of PPV PPV has been prepared by a sulfonium leaving group route, whereby the precursor polymer is a polyelectrolyte and soluble in protic polar solvents such as methanol and water, and a methoxy leaving group route, whereby the precursor polymer is soluble in polar aprotic solvents such as chloroform. The synthetic mutes and reaction conditions are shown in Figure 1. 0379-6779/91/$3.50
© Elsevier Sequoia/Printed in The Netherlands
262
-O-
XHzC
+c,CN,X
X =
(0.4 M / MeOH)
Or
+
X = S(CH3) 2
+
x= +,,2+or
MoO
Cf +
0.4 M in H20 1. O H (0.4 M / H20)
1. "OH (0.4 M / H20), lh, 0 °C, N2 2. H + (0.4 M / H20), to pH = 5-7
2. H * (0.4 M ]H20)
3. Dialysis, 3 days (dist. H20 ), r.t., N2
C~e
300 °C, 1 h H2C
(IV) X = (0H3)2S
"(--H ~ - - ~ H
CH
--~n
M~
CI +
( v ) X = ~S"~
x
x
Cf +
X= " S " m) x. CI-+ Solution in MeOH
X=S(CH3) 2 ~ / ~ 200'~C,2 h /
~so~"
~ -
- ~ n ~
PPV
=pTS, 3 eq
OMe
om ou.
),~)(j
S(CH=),
~.~--~-~.-):.
(vl]) MeOH, r.t., Ns, 48 h
Solution in CHCl3
1. Sodium p-toluenesulfonate, 3 eq, 16 min, r.t. 2. Methanol, 52 °C, N2, 24 h
pT$"
,..~MMeO H, r.t., N2. J
o~
°
..o,
overnight
o,.
°x=O Cf+
Figure 1
Figure 5
a) Sulfonium Leavin~ Grouo Route. The choice of the sulfonium group affects the structure of the prepared PPV. This is best illustrated by comparing the electronic spectra of PPV prepared by two different routes [4]. The electronic spectra (Figure 2) show that PPV prepared via the tetrahydrothiophenium (THT) precursor polymer (I) is more conjugated (red shift of the band edge) and crystalline (has phonon structure) compared with PPV prepared via the dimethylsulfonium (DMS) precursor polymer (II). The increase in crystallinity is confirmed by X-ray analysis of films prepared by the two methods [4].
/, .~.~~~_~ /
-~
x
~
/ ' (b)
(a)
3
~
4
•
5
Energy (eV)
Figure 2 (a) Absorbance spectra of PPV prepared via the precursor polymer (I) ; (b) Absorbance spectrum of PPV prepared via the precursor polymer (II) [41
263
(b) Methoxy Leaving Group Route. The route proposed by Tsutsui involves the exchange of the counteranion of the DMS precursor polymer (II), with p-toluenesulfonate, and this polymer is reacted with methanol to give polymer (III) [5]. We chose to use the THT sulfonium precursor polymer (1) which was prepared in aqueous media with the initial monomer concentration 0.2 M to avoid gelling. The result of exchanging the chloride anion with p-toluenesulfonate was a pale yellow sticky gum. We found that this polymer reacted with methanol at 52 °C to give a whitish solid (III). The DMS precursor polymer (II) was reported to react at room temperature with methanol to give (III) [5 I. Conversion of the sulfonium leaving group to the methoxy leaving group of polymer (III) was evidenced by IH n.m.r and I.R. spectroscopy (Figure 3a). The I.R. stretch at 1097 cm -1 is due to the methoxy leaving group and its disappearance was used to monitor the conversion of (II1) to PPV. Full conversion of the methoxy leaving group precursor polymer to PPV requires heating the sample at 200 o C in the presence of hydrogen chloride for 2 h. Acid is required because of the thermal stability of the leaving group. The I.R. spectrum of PPV prepared by this method (Figure 3b) is identical to PPV prepared via heat treatment of the THT precursor polymer (Figure 3c). The electronic spectra show that PPV prepared via the precursor polymer (Ill) (Figure 4a) has the same conjugation length (band edge at the same energy) but is less crystalline (has no phonon structure) than PPV prepared via the THT precursor polymer (I) (Figure 4b).
(a)
I,,~J,tlll I
~j~
(d)
i|11.
,...y.
= ~i
,~,
,
tl
,
"-'a
I
(e)
,, ~
jv,,
I
<= 4000
I
I
I
2700
1400
100
wavenumber
4000
I
I
I
2700
1400
100
wavenumber
Figure 3 I.R. Spectra of (a) Methoxy leaving group precursor polymer (III), (b) PPV prepared via (III), (c) PPV prepared via (I), (d) Methoxy leaving group precursor polymer (VI), (e) Hydroxy subsituted precursor polymer, (f) Dimethoxy-PPV via (VI)
Synthesis of 4im¢th0xy-PPV A sulfonium precursor polymer cannot be used directly as the processible polymer because during purification (dialysis against water) a gel forms. The precursor polymer utilised is one with a
264
,.o
<
1.24 1.74 2.24 2.74 3.24 3.74 Energy (eV) Figure 4 U.V. Spectra of (a) PPV prepared by heat/acid treatment of polymer (II1), (b) PPV i prepared via heat treatment of polymer (I) methoxy leaving group (VI) [31. We have further investigated the synthesis of dimethoxy-PPV and the origin of the gel formed during dialysis of the precursor polymer. The reaction sequence and conditions used are shown in Figure 5. The first step was to prepare a sulfonium precursor polymer (IV) or (V)) in water. We initially followed the route proposed by Tsutsui [3] in which the precursor polymer (IV) was precipitated out of solution by addition of the p-toluenesulfonate anion and the resultant whitish-yellow gum was dissolved in methanol and allowed to react to give a whitish precipitate (VI). Both I.R. (Figure 3d) and IH n.m.r, indicated conversion of the sulfonium to the methoxy leaving group was complete. Our discovery that methanol would substitute the sulfonium group of (V) without anion exchange (Figure 5) suggested that the gel formed during dialysis was due to substitution of the sulfonium group by water. We dialysed the DMS precursor polymer (IV) against distilled water to give a gel which was thoroughly dried. An I.R. spectrum (Figure 3e) of the resultant material had an OH stretch at 3400 cm-1 indicating, at least in part, the sulfonium leaving group had been substituted by water, giving the polymer gel (VII). The precursor polymer (VI) was converted into dimethoxy-PPV by heating at 200 °C for 2 hours in the presence of hydrogen chloride and the extent of conversion can be conveniently followed by the disappearance of the methoxy leaving group stretch at 1094 cm -1. The I.R. spectrum of the fully conjugated dimethoxy-PPV is shown in Figure 3f. REFERENCES 1. J. D. Capistran, D. R. Gagnon, $. Antoun, R. W. Lenz and F. H. Karasz, ACS Polvm. Preorints. 25 (1984) 282. 2. R. W.
nz, c - c . Han, J. Stenger-Smith and F. R. Karasz, ,I. Polym. Sci.: Part A; pQlym.
Chem. 26 (1988) 3241. 3. T. Momii, S. Tokito, T. Tsutsui, and S. Saito, Chem. Lett. (1988) 1201. 4. J. Martens, N. F. Colaneri, P. Bum, D. D. C. Bradley, E. A. Marseglia and R. H. Friend, Proc. NATO ASI. $Detses (Greece,. June, 1989. 5. S. Tokito, T. Momii, H. Murata, T. Tsutsui and S. Saito, Polymer. 31 (1990) 1137.