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Electrosynthesis and study of phenylene-carbazolylene copolymers
Synthetic Metals, 63 (1994) 89-99
89
Electrosynthesis and study of phenylene-carbazolylene copolymers Karim Faid*, Dominique
A d 6 s , A l a i n Siove a n d C l a u d e C h e v r o t
Laboratoire de Recherches sur les Macromol~cules, Universitd Paris Nord, 93430 l,Tlletaneuse (France)
(Received July 12, 1993; accepted December 10, 1993)
Abstract Phenylene-carbazolylene copolymers with variable composition and properties were prepared by electrocatalyzed dehalogenative polycondensation of 4,4'-dihalobiphenyl and N-alkyl-3,6-dibromocarbazole mixtures in the presence of a zero valent nickel catalyst. The polymers are partly soluble in polar solvents and this solubility depends on the proportion of carbazolylene units in the materials. For a given composition, solubility increases with length of the aliphatic substituent linked to the nitrogen. The conductivity upon doping varies between those of the corresponding homopolymers and is a function of the length of the alkyl substituents. Thin films of these materials can be prepared either by solvent casting or by direct electrodeposition onto various supports. The electrochemical behavior is strongly dependent on the copolymer composition and reveals the existence of two distinct electronic states (two quantum wells): the first one is related to the presence of the carbazolylene units, while the second shows the presence of phenylene moieties.
Introduction
Conducting polymers have induced a wide variety of research activities during the past decade [1]. Various structures have been investigated, among which are polyacetylene, polythiophene, polypyrrole and poly(para-phenylene). These polymers exhibit interesting electrical properties and a large number of potential applications have been envisaged, such as electrochemical sensors [2], electrochromic devices [3, 4], batteries [5], memory storage devices [6, 7] or nonlinear circuit elements [8]. Unfortunately, and due to their conjugated structures, these materials are insoluble and infusible, limiting drastically both their characterizations and their potential applications. Various approaches have been investigated to modify the physical properties of the conducting polymers. Among the most significant achievements in this field were the preparations of soluble conducting polymers through the introduction of long side groups on different positions of the starting monomers, such as in poly(3alkylthiophenes) [9], poly(N-substituted pyrroles) [10] or poly(2,5-alkyl(or alkoxy)-l,4-phenylenes) [11]. The addition of long side chains not only renders the conjugated polymers soluble and fusible, but also results in new interesting properties, such as thermochromism *Author to whom correspondence should be addressed. Present address: Department of Chemistry, University of Montreal, Montreal, Que., H3C 3J7, Canada.
0379-6779/94/$07.00 © 1994 Elsevier Sequoia. All rights reserved SSDI 0379-6779(93)02049-Q
and solvatochromism [12]. However, the steric hindrance due to the presence of such bulky substituents induces some twistings and distortions of the backbones, resulting generally in the decrease of conjugation length and conductivity of these polymers. The preparation of soluble conducting polymers has been sought also through the formation of composites with conventional polymers [13] and through copolymerization with either conventional polymers [14] or other conducting polymers [15]. Families of all-electroactive copolymers have recently received great attention because of the high potential for modifying the electronic and physical properties of these materials. Ingan~is et al. [16] reported the electrochemical copolymerization of terthiophene and pyrrole and found the electronic transport properties of the resulting copolymers to be intermediate between those of the homopolymers. Kuwabata et al. [17] found that the electrochemical behavior of thiophene-pyrrole copolymers was gradually changed from that of polypyrrole to that of polythiophene with the increase of thiophene rings in the copolymer. Peters and Van Dyke [15g] reported the tuning of the molecular composition of pyrrole-bithiophene copolymers through the control of the applied potential. Increasing this potential led to an increase in the proportion of bithiophene in the material. This latter approach has been extensively used by Iyoda et al. [15a] and Torres et al. [15b], whereby using potential-programmed electropolymerization of
90 mixtures of pyrrole and 3-methylthiophene or thiophene, random, alternate or graded-composition copolymers have been obtained. Other polymerization methods and monomers have also been used to produce copolymers of different structures and nature. Czerwinski et al. [15I] reported the synthesis and study of copolymers based on phenylene and thiophene, obtained by the Yamamoto method, and found that copolymers with a continuous molar ratio of monomers could be prepared. More recently, copolymers consisting of pyridine and thiophene units have been reported by Yamamoto and coworkers [15c], the unusual electrochemical behavior of which suggests the occurrence of intramolecular charge transfer. We reported recently some preliminary results on the study of (phenylene-N-ethylcarbazolylene) random copolymers [18] which were obtained by an electrocatalyzed polycondensation of mixtures of 4,4'-dibromobiphenyl and 3,6-dibromo-N-ethylcarbazole. These materials were found to be partly soluble and to have conductivities lying between those of the homopolymers, that were a function of the material composition. In this paper we present investigations on the copolymerization of various 3,6-dibromo-N-alkylcarbazoles with 4,4'-dihalobiphenyls. We have been concerned with the control of the copolymer composition, through the adequate choice of the copolymerization conditions, and the determination of its relationships with the physical, electrical and electrochemical properties.
Experimental Reagents 3, 6-Dibromo-N-alkylcarbazoles These compounds have been prepared according to the method of Lindeman et al. [19]. 3,6-Dibromocarbazole (0.02 mol, Aldrich) was dissolved in a solution of 100 ml of ethanol and 0.035 mol of potassium hydroxide, and 0.03 mol of alkyl halide (1-bromobutane or 1-bromooctane, Aldrich) was added dropwise over a period of 1 h. The solution was heated at 70 °C for 24 h. The crude product was purified by gel chromatography with hexane-ether mixture (90-10) as eluent. 3,6-Dibromo-N-butylcarbazole. 13C NMR (CDCI3, ppm): 14.8, 20.5, 31.2, 42.9, 110.1, 112.3, 123.5, 123.6, 129.7, 139.8. Anal. Calc. for C16H15NBr2: C, 50.39; N, 3.68; Br, 41.99. Found: C, 50.76; N, 3.77; Br, 41.72%. 3,6-Dibromo-N-octylcarbazole. 13C NMR (CDC13, ppm): 15.2, 24.1, 26.8, 30.1, 30.3, 32.5, 45.3, 110.2, 112.3,
123.4, 123.6, 130.0, 140.2. Anal. Calc. for C2oH23NBr2: C, 54.94; N, 3.21; Br, 36.61; H, 5.26. Found: C, 55.26; N, 3.20; Br, 36.08; H, 5.30%.
Other reagents 4,4'-Dibromobiphenyl (Lancaster) and 4,4'-dichlorobiphenyl (Jansen) were used without further purification. The catalytic precursor, NiBr2-Bipy (Bipy= 2,2'-bipyridine) complex, was prepared by adding 20 mmol of NiBr2-3H20 in a solution of 100 ml of ethanol and 20 mmol of 2,2'-bipyridine. The solution was heated at 60 °C for 24 h. A yellowish-green precipitate was recovered and was washed three times with ethanol. Anal. Calc. for CloHaN2NiBr2: C, 32.04; H, 2.14; N, 7.46; Br, 42.67; Ni, 15.67. Found: C, 31.99; H, 2.48; N, 7.35; Br, 42.83; Ni, 15.76%. Lithium perchlorate, LiCIO4 (Fluka), and tetrabutylammonium tetrafluoroborate, NBu4BF4 (Jansen), were dried under vacuum at 120 °C for 24 h. N,NDimethylacetamide (DMA) and acetonitrile were distilled under reduced pressure over CartE just prior to use. Tetrahydrofuran, chloroform, ethanol, hexane and diethyl ether (Merck) were used as received. Polymerizations Polymerizations were conducted in a two-compartment electrochemical cell with mercury as a working electrode and a magnesium rod as a counter electrode. All the potentials were referenced to a saturated calomel electrode (SCE). The electrosynthesis reaction involved the cathodic reduction of a NiBr2-Bipy complex into a Ni(0)-based catalytic system which, in the presence of a dihalomonomer (or a mixture of dihalomonomers), led to the coupling product through a dehalogenative step polymerization. In a typical experiment, 10 mmol of NiBr2-Bipy were dissolved in 100 ml of DMA at room temperature in the cathodic compartment. Two main experimental procedures were followed to add the different monomers to the solution. In the first one, mixtures of 1,4-dihalobiphenyl and 3,6-dibromoN-alkylcarbazole (with the alkyl being either a butyl or an octyl group) in various proportions were prepared and poured into the solution (see Table 1). After complete dissolution, the cathode was polarized at a working potential of -1.3 V versus SCE. The current intensity was monitored during the electrosynthesis and the reaction was stopped when it reached a value below 5 mA. In the second experimental procedure and after solubilizing the catalyst, a given amount of a first monomer (either a dihalobiphenyl or a 3,6-dibromoN-alkylcarbazole) was added to the solution (see Table 2) and the cathode was polarized at the working potential. After a determined time, the second monomer, already dissolved in 20 ml of DMA, was added dropwise
91 T A B L E 1. Phenylene-carbazolylene copolymers; electrolysis of mixtures of 4,4'-dihalobiphenyl and 3,6-dibromo-N-alkylcarbazole DBuC/DBB or DBoC/DBB
[M]/[C]
SPBul SPBu2 SPBu3 SPBu4 SPBu5 SPBu6 a
1 0.3 3 1 1 1
1 1 1 1 0.5 2
SOP1 SOP2 SOP3 SOP4 SOP5 SOP6 SOP7 SOP8
0.8 1 1 0.3 1 1 1 1
0.9 2 1 1 2 1 1 0.5
Solubility in T H F (%)
Cz/BP
Ea,
Ea2
Cz/BP
soluble fraction in T H F
(V)
(V)
non-soluble fraction in THF
0 0 0 2 2 2
15 15 78 48 45 70
1.03 1.62 14.31 4.24 3.41 2.76
0.82 0.85 0.87 0.84 0.85 0.83
1.11 1.15 1.14 1.12 1.16 1.18
1.33 0.40 0.27 0.98 0.99
0 1 0 0 0 2 1 2
70 61 62 60 49 67 49 36
1.71 2.02 2.28 1.80 1.77 3.97 2.96 3.95
0.97 0.94 0.85
1.34 1.25 1.20 1.20 1.20 1.15 1.12 1.20
0.69 0.32 0.84 0.44
L
aUse of 4,4'-dichlorobiphenyl instead of 4,4'-dibromobiphenyl.
0.82 0.85 0.80 b
0.59 1.49
bWeak and fugitive anodic signal.
TABLE 2. Phenylene-carbazolylene copolymers; electrolysis of successively added monomers DBoC/DBB or DBuC/DBB
[M]/[C]
BOP1 BOP2 BOP3
0.9 2.9 8.6
3.94 3.91 3.87
BPO1 BPO2 BPO3 BPO4
1 1 1 1
BUP1 BUP2 a PBul PBu2 PBu3 PBu4 PBu5 PBu6 PBu7 a
Cz/BP
Eat
E,2
Eaa
Cz/BP
soluble fraction in T H F
(V)
(V)
(V)
non-soluble fraction in T H F
59 83 95
0.76 2.83 6.61
0.82 0.80 0.80
1.15 1.15 1.15
1.45 b 1.50b
0.55 0.63
30 70 30 30
51 54 22 37
9.57 18.40 1.29 36
0.75 0.80 0.85 b 0.75
1.18 1.18 1.16 1.13
1.60¢
0.47 0.32 0.36 0.27
0 2
34 20
20 59
0.46 3.43
0.79 0.90
1.20 1.25
1.66 c 1.82 c
0.22
0 1 1 1 1 1 1
48 20 25 20 20 10 20
39 22 29 48 36 23 14
0.97 0.43 0.73 1.42 3.83 0.52 1.62
0.85
1.15 1.20 1.20 1.18 1.20 1.20 1.20
1.43 b 1.45 1.50 1.73 ¢ 1.50 b 1.54 1.45
0.23 0.15 0.75 0.28 0.96 0.20
L
Q (%)
Solubility in T H F (%)
2 2 2
67 48 48
2 2 2 0.5
0 0 2 2
1 1
4 4
1 1 1 1 1 1 1
1 4 4 2 1 4 4
aUse of 4,4'-dichlorobiphenyl instead of 4,4'-dibromobiphenyl.
t h e s e c o n d m o n o m e r is r e p r e s e n t e d b y t h e r a t i o o f t h e q u a n t i t y o f e l e c t r i c i t y a l r e a d y c o n s u m e d o v e r t h a t req u i r e d f o r t h e c o m p l e t e e l e c t r o l y s i s o f t h e first m o n o m e r . At the end of the electrolysis, the reaction product into aqueous
sulfuric acid (5%)
and
0.85
bWeak and fugitive anodic signal.
to t h e s o l u t i o n . I n this c a s e , t h e i n s t a n t o f a d d i t i o n o f
was poured
0.85 0.85
a
s u s p e n s i o n o f m a t e r i a l w a s r e c o v e r e d by f i l t r a t i o n . T h i s filtrate was washed with water and ether and dried
1.60¢ 1.65c
CDeactivation of the film.
u n d e r v a c u u m f o r 24 h. T h e r e s u l t i n g m a t e r i a l w a s e x t r a c t e d by T H F in a S o x h l e t a p p a r a t u s f o r 24 h, r e s u l t i n g in a s o l u b l e f r a c t i o n a n d an i n s o l u b l e o n e .
Physical measurements T h e e l e m e n t a l a n a l y s e s w e r e p e r f o r m e d by t h e S e r v i c e of Central Analysis of CNRS, Villeurbanne, France. The nitrogen and carbon proportions allowed the de-
92 termination of the copolymer compositions through the use of the following formula:
NiBr2(Bipy ) + 2e
Bipy ---~
Ni(0)(Bipy)2 + 2Br
1 3 V vs S C E
n
Ar
l + Ni(0)(Bipy):
Ar)o+ NiBr~(Bipy)~
CkHi2.~l. ~
with A F :
0
O-
, ~ I R
R = C:H,, C4H~, C,H,7
% Carbazole = Cz =N(165 + 14k)/14 % Biphenyl =BP= 19[(7C) - (72N) - (6kN)]/126 where N is the nitrogen content, C the carbon content of the sample, and k represents the number of carbons of the N-substituted aliphatic chain. The ratio of Cz over BP gives information on the overall composition of the different soluble and nonsoluble fractions of the copolymers (Tables 1 and 2). NMR spectra were obtained in deuterated chloroform at 50 °C with a 200 MHz Bruker apparatus. The molar mass determination was performed by GPC in THF at 20 °C with a Water GPC apparatus, calibrated with standard polystyrene samples. UV-Vis spectroscopy was carried out using a Varian DMS 100. Infrared spectra were recorded on a Perkin-Elmer 580 spectrometer with KBr pellets. AsF5 doping experiments together with the electrical conductivity measurements (fourpoint) of copolymers were performed at CNET (Lannion, France). The electrochemical behavior of the different copolymers was investigated in acetonitrile with 0.1 M LiC104. Films from the soluble fractions of the copolymers were cast from solvent onto a glassy carbon electrode or a conducting ITO glass electrode. Results and discussion
General features The synthetic route used throughout this study is derived from the one set up in our laboratory for the synthesis of poly(para-phenylene) [20], poly(N-alkyl-3,6carbazolylenes) [21] and phenylene--carbazolylene copolymers [18]. The basic outline (Scheme 1) involved the electroreduction of a NiBr2-bipyridine complex generating a Ni(0)-based catalytic system which, in the presence of dihalomonomers, allowed the formation of the coupling product through a dehalogenative step polymerization [22]. We have recently conducted a detailed investigation on the mechanism of the dehalogenative step polymerization of 3,6-dibromo-N-alkylcarbazoles [23] (with the alkyl being either butyl or octyl). The influence of
Scheme 1.
X~X
.~.
B r ~ B r R
X : Br, C1 R : C4H9, C8H,7
Scheme 2.
Ni(0)
R
various parameters, such as the monomer-to-catalyst ratio or the ligand concentration, on the distribution of the molar mass population during electrolysis has been studied. It has been found that this step polymerization proceeds through the coupling of Ni(0)activated species, with the essential part of the chain growth occurring at the very beginning of the electrolysis (less than 1 h reaction). Evidence of debromination reactions has been found by GPC and deduced from elemental analysis of the materials. Because of the occurrence of these debromination reactions at nearly the end of the coupling process, very few sites are still available for further reactions after this point. The results of this study showed principally that, in view of carrying out copolymerization reactions with two dihalomonomers, the addition of the second monomer should be realized before the occurrence of debromination reactions. To minimize this aspect, two experimental procedures have been established and are described in the Experimental Section.
Copolymer composition Electrolysis of mixtures of two monomers The first experimental procedure we have used consisted in carrying out the electrolysis of mixtures of two monomers (Scheme 2). The experimental parameters used are presented in Table 1. SPBul to SPBu5 represent copolymers of 1,4-dibromobiphenyl and 3,6-
93 dibromo-N-butylcarbazole, while 1,4-dichlorobiphenyl has been used in SPBu6. SOP1 to SOP8 are copolymers of 1,4-dibromobiphenyl and 3,6-dibromo-N-octylcarbazole. The effect of the [DBuC]/[DBB] ratio on the composition of phenylene-N-butylcarbazolylene copolymer has been studied. Three ratios of 3,6-dibromoN-butylcarbazole (DBuC) and 4,4'-dibromobiphenyl (DBB) have been used (0.3, 1 and 3) while keeping the overall ratio of monomers over catalyst ([M]/[C]) equal to unity and using no excess of ligand in the feed solution. When the proportion of DBuC is low (SPBul and SPBu2) about only 15% of soluble fractions have been recovered. On the contrary, with a mixture rich in DBuC (SPBu3) more than 78% of soluble materials have been obtained. The elemental analysis of the different fractions showed that their compositions are a function of feed composition (Fig. 1). Increasing the proportion of the carbazolic monomers led to an increase of the proportion of carbazolylene units in the soluble fractions. The composition of the insoluble fraction appears to go through a maximum for the carbazolylene unit around [DBuC]/[DBB] = 1.5 and then to decrease again. The effect of the concentration of the ligand on the composition of the different fractions is illustrated in experiment SPBul and SPBu4, the ratio of the concentration of ligand over that of catalyst (L) being respectively 0 and 2. The proportion of the soluble fraction in SPBu4 was found to be at least three times greater than in SPBul, while its composition is four times richer in carbazolylene units. The use of an excess of ligand in the feed composition led to an increase in both the solubility of the copolymer and the proportion of carbazolylene units in the soluble fraction. The compositions of the insoluble fractions depend also on the initial ligand concentration and the carbazolylene content was decreased by about 30% when using an
15
......
Q
~/Insoluble
t0
'
2
", ,, > /
fraction
N / / []
,
/s•" ~
z-"
/
0
Soluble fraction
0 2 3 DBuC/DBB Fig. 1. Effect of [DBuC]/[DBB] ratio on the composition of phenylene-N-butylcarbazolylene copolymers: electrolysis of mixtures of 4,4'-dibromobiphenyl and 3,6-dibromo-N-butylcarbazole. 0
1
excess of ligand. The effect of monomer over catalytic precursor ratio ([M]/[C]) has been investigated in experiments SPBu4 and SPBu5, carried out using a ratio [M]/[C] of, respectively, 1 and 0.5. The proportions of soluble material were found to be similar, although the composition of the SPBu5 soluble fraction was found to be slightly poorer in carbazolylene units than that of SPBu4, whereas the insoluble fractions were of almost identical composition. The effect of [DBoC]/[DBB] ratio on the composition of phenylene-N-octylcarbazolylene copolymers was considered in two experiments (SOP3 and SOP4), carried out using a [DBoC]/[DBB] ratio of 1 and 0.3 while keeping [M]/[C] = 1 and L = 0. The proportions of soluble fraction were similar in both materials, whereas their compositions varied with the [DBoC]/[DBB] ratio. Increasing the proportion of carbazolic monomer in the feed composition resulted in an increase of the proportion of carbazolylene units both in the soluble fraction and the insoluble one. However, compared to the copolymers SPBul and SPBu2, which were prepared under the same conditions, the soluble fractions of the SOP3 and SOP4 copolymers were found to be much richer in carbazolylene units. The effect of the monomer over catalytic precursor ratio ([M]/[C]) has been studied in experiments SOP2 and SOP7, with [M]/[C] ratios equal, respectively, to 2 and 1, while [DBoC]/[DBB] = 1 and L = 1. The proportion of soluble fraction was found to be higher with increasing [M]/[C], while the proportion of carbazolylene unit decreased by almost 30%. When using a higher ligand concentration (L = 2) and varying the [M]/[C] from 0.5 to 1 (SOP8 and SOP6), the proportion of soluble fraction almost doubled while the soluble fraction composition remained quite similar. The effect of excess ligand in the feed solution has been investigated in three experiments (SOP3, SOP7 and SOP6) with different proportions of ligand in the feed composition while keeping [DBoC]/[DBB] = 1 and [M]/[C] = 1. Evolution of the composition of the soluble and insoluble fractions is represented in Fig. 2. In the soluble fraction, increasing the proportion of ligand induced a linear increase of the proportion of carbazolylene units, whereas, in the insoluble fraction, a maximum has been observed for L = 1. A similar behavior is also found in experiments SOP2 and SOP5 where L = 1 and 0, while [DBoC]/[DBB] = 1 and [M]/ [Cl=2. The length of the alkyl group, N-substituted on the carbazole monomers, affected the solubility and composition of the copolymers. The use of octyl substituents in place of butyl gave rise to materials with higher proportions of soluble fractions which, in general, were richer in carbazolylene units than the insoluble fractions.
94
Soluble fraction
Br~B~ r)
Ni(0)
R
t~-
0
Insoluble fraction 1
-.~
2
Excess ligand Fig. 2. Effect of ligand concentration on the composition of phenylene-N-octylcarbazolylene copolymers: electrolysis of mixtures of 4,4'-dibromobiphenyl and 3,6-dibromo-N-octylcarbazole.
X : Br, R
CI
R
C8H]7
: C4H9,
Scheme 3.
100
Copo!ymers obtained by the electro,sis of successively added monomers The copolymers obtained by the electrolysis of monomer mixtures led to the formation of materials with random distribution of the two monomeric moieties. Moreover, and owing to the higher reactivity of 4,4'dibromobiphenyl compared to that of 3,6-dibromo-Nalkylcarbazoles [24], homocoupling reactions might be favored. In a previous investigation on the step polymerization of 3,6-dibromo-N-alkylcarbazoles [23], we found that the chain growth is limited by the occurrence of debromination reactions during electrolysis, the loss of terminal bromine reducing the number of available reaction sites. It has been found that an optimum is reached when the reaction is between 20 and 40% conversion, giving rise to long enough polymeric chains with the minimum of terminal bromine being removed. Therefore phenylene-carbazolylene copolymers have been synthesized by the electrolysis of an initial monomer and, after a determined period of time, the addition of a second monomer (Scheme 3) followed by an exhaustive electrolysis. The effect of various parameters (such as the nature of the first and second monomers, the relative proportion of the different constituents ([M]/[C] ratio), ligand concentration and the moment of addition of the second monomer) has been investigated and the results summarized in Table 2. Three copolymers (BOP1, BOP2 and BOP3) have been synthesized by starting the electrolysis with 3,6dibromo-N-octylcarbazole and adding 4,4'-dibromobiphenyl after various electrolysis times, while keeping constant the [M]/[C] ratio and the ligand concentration. By increasing the [DBoC]/[DBB] ratio, the proportion of carbazolylene units increases together with the proportion of soluble fraction (Fig. 3). Two experiments (BPO1 and BPO2) have been conducted using a [DBoC]/ [DBB] overall ratio of unity, [M]/[C] = 2 and L = 0. The electrolysis was started with DBB first and then DBoC
10
~
90
~
°
8
80
6
70
4
E
2
o
= ~e
_= O r~
60 z~
50
0 0
2
4
6 8 10 [DBoC]/[DBB]
Fig. 3. Effect of [DBoC]/[DBB] ratio on the solubility (left) and the composition (right) of phenylene-N-octylcarbazolylene copolymers: electrolysis of successively added 4,4'-dibromobiphenyl and 3,6-dibromo-N-octylcarbazole.
was added after 30 and 70% conversion. Although the proportions of soluble fraction were similar (51% versus 54%), their compositions were quite different. The proportion of carbazolylene units almost doubled in the soluble fraction in the case of the late addition of DBoC, while it decreased significantly in the insoluble fraction. By comparing experiments BPO1 and BPO3, where the excess ligand concentration (L) was respectively 0 and 2, it appears that the proportion of soluble fraction decreased by more than half when using L = 2. The composition of the soluble fractions was greatly affected by the ligand concentration as indicated by the Cz/BP ratio (9.57 versus 1.29), while the proportion of carbazolylene units in the insoluble fractions was found to decrease with the augmentation of the ligand concentration. This feature is in good agreement with the known stabilization effect of the increasing ligand concentration [23, 25]. The effect of [M]/[C] ratio on the copolymer compositions was studied in experiments BPO3 and BPO4. Increasing the relative proportion of catalyst to that of the monomers resulted in the obtention of a soluble fraction composed almost exclusively of carbazolylene units, while the insoluble
95
fraction was found to be very rich in phenylene units. Despite the presence of excess ligand in the feed solution, the homocoupling of DBB has been favored under these conditions (low [M]/[C]), indicating unsuitable copolymerization conditions. Other copolymers have been synthesized using 3,6dibromo-N-butylcarbazole and either 4,4'-dibromo- or 4,4'-dichlorobiphenyl. Experiment BUP1 (carried out with [DBuC]/[DBB] -- 1, [M]/[C] = 4 and L = 0) resulted in only 20% of soluble material with composition poor in carbazolylene units, while the insoluble fraction was found to be extremely rich in phenylenic units. Increasing the ligand concentration and using 4,4'-dichlorobiphenyl (experiment BUP2) resulted in an increase of the proportion of the soluble fraction with composition considerably richer in carbazolylene units than in the previous case. The utilization of 4,4'-dichlorobiphenyl, the reactivity of which is known to be lower than that of 4,4'-dibromobiphenyl [26], in the presence of excess ligand, resulted in a more soluble material with composition adjustable to a large extent through the adequate choice of the experimental conditions. The control of the phenylene-carbazolylene copolymer compositions is further illustrated in experiments PBul to PBu7. The electrolysis was conducted with 3,6-dibromo-N-butylcarbazole to which, after various periods of time, was added 4,4'-dibromobiphenyl or 4,4'-dichlorobiphenyl. It appeared that the nature and compositions of the copolymers are greatly affected by the instant of addition of the second monomer. The extent of reaction is directly a function of the experimental parameters, such as the [M]/[C] ratio or the excess ligand proportion. Experiment PBu2 was carried out with 1 mmol of catalyst and 2 mmol of DBB and, after the consumption of 115 C, 2 mmol of DBuC were added, resulting in [DBuC]/[DBB] = 1, [M]/[C] = 4 and L = 1. Experiment PBu5 was carried out with 2 mmol of catalyst and 1 mmol of DBB to which was added 1 mmol of DBuC after 115 C. Knowing the kinetics of the various reactions occurring during this electrocatalyzed step polymerization [23, 25] (electroreduction of Ni(II) to Ni(0), Ni(0) insertion in C-Br bonds and couplings of these latter species), it is evident that, at the moment of DBuC addition, the quantity of Ni(II) already transformed in Ni(0) is far from being similar in the two cases. This feature resulted, at the moment of addition, in phenylenic chains of various length which gave rise to copolymers of different compositions and hence properties. In fact, a higher proportion of soluble fraction has been recovered in PBu5 than in PBu2, while the proportion of carbazolylene units increased by a factor nine. The PBu4 experiment, carried out with [M]/[C]=2, showed the highest proportion of soluble fraction whereas its composition was found to be intermediate between those of PBu2 and PBu5. The
proportion of carbazolylene in the insoluble fractions increases with the increase of [M]/[C] ratio. The comparison of experiments PBu2, PBu3 and PBu6, carded out under exactly the same experimental conditions, except the moment of addition of DBuC, revealed that by allowing the electrolysis of DBB to go further resulted in a higher proportion of soluble material, richer in carbazolylene units. Surprisingly, the proportion of carbazolylene unit was found to be also higher in PBu3 than in PBu2. The utilization of 4,4'-dichlorobiphenyl resulted in a smaller soluble fraction than in PBu2, while the proportion of carbazolylene units was found to be much higher. The tuning of the copolymer composition can be realized in a various number of ways, such as the control of the feed composition or the copolymerization procedure. The copolymer composition and the nature of the carbazolic monomer affect to a large extent the solubility of these materials (Fig. 4).
Copolymer characterization and properties Various techniques have been used to characterize the different copolymers. The elemental analysis gave a knowledge on the compositions of the different fractions. Infrared spectroscopy has been used to identify the components present through the characteristic absorption bands of the carbazolylene and phenylene units. The in-plane and out-of-plane 6(C-H) vibrations of variously substituted benzene rings have been used to characterize the different copolymer fractions. In a previous study [18], we found that random phenyleneN-ethylcarbazolylene copolymers presented bands at 795 and 874 cm -1 (corresponding to 8(C-H) out-ofplane deformations of 1,2,4-trisubstituted benzene), 810-808 and 1000 cm -1 (characterizing respectively out-of-plane and in-plane deformations of para-disubstituted phenylene units). A continuous shifting of the intensity of these various bands has been observed and found to be a function of the copolymer composition. 100 ()
80 f3fz jj
60
/
40
O
j
~4"
//,tg/jjj
//
/ 2~
20
0 ~// 0
1
2
3
4
Cz/BP ratio Fig. 4. Effect of the composition and length of the N-substituted alkyl group on the solubility of phenylene-N-alkylcarbazolylene copolymers: + , butyl; O, octyl.
96 T A B L E 3. 13C chemical shifts of poly(N-butyl-3,6-carbazolylene), copolymer in CDCI 3 at 50 °C Carbon identification
a, d
3, 6
PBuC
140.1
133.4
140.1
133.3
3', 6'
poly(para-phenylene) and
b, c
125.5
phenylene-N-butyl-3,6-carbazolylene
2, 7
4, 5
1, 8
123.8
118.9
109.0
123.8
118.9
109.0
9
10
11
140.6
128 127.2
139 138.2
Poly(para-phenylene) Copolymer
132.8
1255
In the case of copolymers prepared with either 3,6dibromo-N-butylcarbazole or 3,6-dibromo-N-octylcarbazole and 4,4'-dihalobiphenyl, a similar behavior was found, and it was noticed that the position of the band between 795 and 810 cm-1 is almost directly a function of the copolymer composition, being close to 795-798 cm -1 for carbazolylene-rich fractions and around 808-810 cm -~ for phenylene-rich fractions. For copolymers with intermediate composition, the signal was found to lie between 800 and 804 cm -1. UV-Vis spectroscopy has been used to characterize the soluble fractions of the different copolymers. A characteristic absorption of the carbazolylene units, between 312 and 320 nm, has been found together with a more or less pronounced shoulder between 340 and 350 nm which can be attributed to the presence of phenylene units in the material. The relative intensities of these signals depend strongly on the material composition. 13C NMR spectroscopy undoubtedly allowed the more pertinent characterization of the copolymer soluble fractions. ~3C NMR chemical shifts of poly(para-phenylene) [26] consist of two signals at 128 and 139 ppm, corresponding respectively to tertiary and quaternary carbons. ~3C NMR chemical shifts of poly(N-butyl-3,6carbazolylene) [21c, 27] and poly(N-octyl-3,6-carbazolylene) [28] were determined in our laboratory. The chemical shifts of poly(N-butyl-3,6-carbazolylene), poly(para-phenylene) and of phenylene-N-butylcarbazolylene copolymer are presented in Table 3. All the chemical shifts of the homopolymers were found in the copolymers in which supplementary signals (131, 133 and 140.5 ppm) appeared, disclosing the presence of heterocouplings between phenylene and carbazolylene units. The electrochemical study of the soluble fractions of phenylene-carbazolylene copolymers revealed different redox behaviors. As a reminder, and under the same experimental conditions, the electrochemical study of poly(N-alkylcarbazolylenes) showed the occurrence of two anodic processes at + 0.85 and + 1.2 V versus SCE, associated with two cathodic waves (Fig. 5). The first anodic signal has been attributed to the formation of a radical cation and the second to the formation of a dication of a carbazolic diade [21c, 29]. The redox
c)
0.0 0.5 1.0 1.5 Potential (V vs.SCE) Fig. 5. Cyclic voltammetry of poly(N-butyl-3,6-carbazolylene); sweep rate 100 m V s -I.
20 IJA }
0.0 0.5 1.0 1.5 Potential (V vs. SCE) Fig. 6. Cyclic voltammetry of poly(para-phenylene); sweep rate 100 m V s - k
behavior of poly(para-phenylene) presents only one anodic process at + 1.5 V versus SCE which was attributed to the occurrence of a dication [30, 31] (Fig. 6). Three main types of redox behavior have been identiffed in the case of phenylene-carbazolylene copolymers. The first type is characterized by the presence of three anodic signals at +0.85, + 1.15/+ 1.2 and + 1.4/ + 1.5 V versus SCE (Fig. 7). The first two anodic waves can be attributed to the occurrence of a radical cation and a dication of the carbazolylene diades [32], while the third anodic process may be due to the oxidation of phenylenic units, of variable lengths, disubstituted
97
L;
100
0.0
0.5 1.0 1.5 Potential (V vs°SCE) Fig. 7. Cyclic voltammetry of phenylene-N-butylcarbazolylene copolymers with three redox processes.
(a)
I
(b)
(c) Fig. 8. Various possible chemical structures of phenylenecarbazolylene copolymers (see Table 3).
by carbazolylene units, as shown in Fig. 8(a). This three-step redox behavior has only been observed in copolymers prepared by the electrolysis of successively added monomers with a composition generally much richer in phenylene units than in carbazolylene ones (Cz = 0.43-0.97). The only exception was the experiment PBu7, for which Cz/BP was approx. 1.6. The use of 4,4'-dichlorobiphenyl instead of 4,4'-dibromobiphenyl led to a reduced phenylic reactivity and allowed the chain growth to occur more smoothly, giving rise to long enough chains of both moieties. The second redox behavior observed in these copolymers consisted in the occurrence of two redox processes at + 1.2 and + 1.45/+ 1.55 V versus SCE (Fig. 9). In this case, the first signal can be attributed to the oxidation of isolated and disubstituted carbazolic
A
0.0 0.5 1.0 1.5 Potential (V vs.SCE) Fig. 9. Cyclic voltammetry of phenylene-N-butylcarbazolylene copolymers with two redox processes.
units [30], while the second process can be due to the oxidation of disubstituted phenylene units of various lengths. The signal at + 0.85 V versus SCE, which is attributed to the oxidation of a carbazolylene diade into a radical cation, was not detectable and supports the idea of obtaining only isolated and disubstituted carbazole units (Fig. 8(b)). This type of electrochemical behavior has been observed for both types of copolymers: those obtained by the electrolysis of monomer mixtures and those produced by the electrolysis of successively added monomers. The third type of redox behavior observed in phenylene-carbazolylene copolymers displays two redox processes at + 0.8/+ 0.9 and + 1.1/1.2 V versus SCE, which are those related to the presence of carbazolylene units, and a third process between + 1.55 and + 1.80 V versus SCE. This latter anodic peak may be due either to the oxidation of very short terminal phenylenic units which are attached to the carbazolylene main chain (a similar behavior has been observed in the case of polydiphenylamine [33]), or to the oxidation of soluble oligo(para-phenylenes) that have been carried away during the extraction procedure [27]. This third redox behavior was essentially found in copolymers which are rich in carbazolylene and which did not display clear heterocoupling signals in the 13C NMR spectrum. The electrochemical response of phenylenecarbazolylene copolymers is strongly dependent on the composition and nature of the materials. The intensity of the different anodic processes is a function of the relative proportions of the phenylene and carbazolylene units and of their arrangements. Moreover, distinct signals can be attributed to each of the components even if they are physically bonded. The electrochemical response of these phenylene--carbazolylene copolymers shows that these materials form a novel class of electroactive polymers with two distinct electronic states (two quantum wells) the intensity of which can be tuned by varying the material composition and structure.
98
Acknowledgements 10 c O
E !02 >.,10
3 '\
~C -~ "u o= ~,2",-~ rj
1 2 3 4 5 Cz/BP ratio Fig. 10. Conductivity upon doping with AsF: as a function of copolymer composition: O, phenylene-N-butylcarbazolylene copolymer; V, phenylene-N-octylcarbazolylene copolymer; +, poly(para-phenylene).
This work was supported by the Centre National d't~tudes des T616communications (CNET), Lannion, France. K.F. is grateful to CNET and to the French-Algerian Cooperation Program for a research fellowship. The authors thank G. Froyer (CNET, Lannion) for AsF5 doping experiments and for electrical conductivity measurements.
0
Upon chemical doping with AsFs, these neutral copolymers become conducting. This conductivity varies considerably (from 10 - 6 to 1) as a function of the nature of the starting materials, copolymerization conditions and copolymer composition (Fig. 10). Copolymers containing N-butylcarbazole were found to be two to three orders of magnitude more conductive than those containing N-octylcarbazole. The increase of the alkyl chain length reduces the material conductivity and this result is in good agreement with those obtained in poly(N-alkyl-3,6-carbazolylenes), the N-methyl derivative [34] exhibiting a conductivity four orders of magnitude higher than that of the N-ethyl derivative [21a]. Moreover, the increase of the proportion of carbazolylene units in the copolymer reduces the conductivity achieved upon doping.
Conclusions A novel class of electroactive materials has been prepared and investigated. The composition of the phenylene-carbazolylene copolymers can be tuned by the adjustment of the feed composition and by the control of the experimental procedures. The physical properties of these copolymers, such as solubility and conductivity upon doping, were found to be a function of the nature of the starting monomers and of their relative proportions in the final materials. Three different types of redox behavior have been found which are related to the occurrence of different copolymer structures and compositions. The copolymerization of phenylene and various N-alkylcarbazoles allowed the preparation of materials with tunable properties.
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99 15 (a) T. Iyoda, H. Toyoda, M. Fujitsuka, R. Nakahara, H. Tsichiya, K. Honda and T. Shimidzu, J. Phys. Chem., 95 (1991) 5215; (b) W. Torres and M.A. Fox, Chem. Mater., 4 (1992) 146; (c) Z.H. Zhou, T. Maruyama, T. Kanbara, T. Ikeda, I. Kunihiro, T. Yamamoto and K. Tokuda, J. Chem. Commun., (1991) 1210; (d) Q. Pei and O. Ingan/is, Synth. Met., 46 (1992) 353; (e) M.S. Kiani and G.R. Mitchell, Synth. Met., 46 (1992) 293; (f) W. Czerwinski, N. Nucker and J. Fink, Synth. Met., 25 (1988) 71; (g) E.M. Peters and J.D. Van Dyke, J. Po!ym. Sci., Po~ym. Chem. Ed., 29 (1991) 1379. 16 O. Ingan/is, B. Liedberg and W. Chang-m, Synth. Met., 11 (1985) 239. 17 S. Kuwabata, S. Ito and H. Yoneyama, Z Electrochem. Soc., 135 (1988) 1691. 18 A. Aboulkassim, A. Siove, D. Ad6s and C. Chevrot, Synth. Met., 33 (1989) 57. 19 H. Lindeman and F. Mulhauss, Ber. Dtsch. Chem. Ges. B, 58 (1925) 2371. 20 (a) J.F. Fauvarque, M.A. Petit, F. Pfulger, A. Jutand and C. Chevrot, Makromol. Chem., Rapid Commun., 4 (1983) 455; (b) J.F. Fauvarque, A. Digua, M.A. Petit and J. Savard, Makromol. Chem., 186 (1985) 2415. 21 (a) A. Siove, D. Adds, C. Chevrot and G. Froyer, Makromol. Chem., 190 (1989) 1361; (b) E. N'gbilo, D. Ad6s, C. Chevrot and A. Siove, Potym. Bull., 24 (1990) 17; (c) A. Siove, D. Ad6s, E. N'gbilo and C. Chevrot, Synth. Met., 38 (1990) 331.
22 K. Faid, D. Ad6s, C. Chevrot and A. Siove, Makromol. Chem., Macromol. Syrup., 47 (1991) 377. 23 K. Fa'id, A. Siove, D. Ad6s and C. Chevrot, Polymer, 34 (1993) 3911. 24 A. Aboulkassim, K. Faid and A. Siove, Mal~romol. Chem., 194 (1993) 29. 25 M. Troupel, Y. Rollin, O. Sock, G. Meyer and J. Perichon, Nouv. J. Chim., 10 (1986) 11. 26 G. Froyer, F. Maurice, P. Bernier and P. McAndrew, Po~:m. Commun., 13 (1982) 9. 27 A. Siove and D. Adds, Eur. Polym. £, 28 (1992) 1583. 28 K. Faid, Ph.D. Thes/s, Universit6 Paris Nord, 1991. 29 (a) J.F. Ambrose and R.F. Nelson, J. Electroanal. Soc., 115 (1968) 11 and 1159; (b) J.F. Ambrose, L.L. Carpenter and R.J. Nelson, J. Electroanal. Soc., 129 (1975) 229. 30 A. Aboulkassim, Ph.D. Thesis, Universit6 Paris Nord, 1991. 31 K. Merholz and J. Heinze, J. Angew. Chem. Int. Ed. Engl, 29 (1990) 6. 32 Y. Pelous, G. Froyer, D. Ad6s, C. Chevrot and A. Siove, Potym. Commun., 31 (1990) 341. 33 J. Guay, R. Paytner and L.H. Dao, Macromolecules, 23 (1990) 3598. 34 S.A. Jenekhe, S.T. Wellinghoff and J.F. Reed, Mol. Cryst., Liq. Cryst., 105 (1985) 175.
89
Electrosynthesis and study of phenylene-carbazolylene copolymers Karim Faid*, Dominique
A d 6 s , A l a i n Siove a n d C l a u d e C h e v r o t
Laboratoire de Recherches sur les Macromol~cules, Universitd Paris Nord, 93430 l,Tlletaneuse (France)
(Received July 12, 1993; accepted December 10, 1993)
Abstract Phenylene-carbazolylene copolymers with variable composition and properties were prepared by electrocatalyzed dehalogenative polycondensation of 4,4'-dihalobiphenyl and N-alkyl-3,6-dibromocarbazole mixtures in the presence of a zero valent nickel catalyst. The polymers are partly soluble in polar solvents and this solubility depends on the proportion of carbazolylene units in the materials. For a given composition, solubility increases with length of the aliphatic substituent linked to the nitrogen. The conductivity upon doping varies between those of the corresponding homopolymers and is a function of the length of the alkyl substituents. Thin films of these materials can be prepared either by solvent casting or by direct electrodeposition onto various supports. The electrochemical behavior is strongly dependent on the copolymer composition and reveals the existence of two distinct electronic states (two quantum wells): the first one is related to the presence of the carbazolylene units, while the second shows the presence of phenylene moieties.
Introduction
Conducting polymers have induced a wide variety of research activities during the past decade [1]. Various structures have been investigated, among which are polyacetylene, polythiophene, polypyrrole and poly(para-phenylene). These polymers exhibit interesting electrical properties and a large number of potential applications have been envisaged, such as electrochemical sensors [2], electrochromic devices [3, 4], batteries [5], memory storage devices [6, 7] or nonlinear circuit elements [8]. Unfortunately, and due to their conjugated structures, these materials are insoluble and infusible, limiting drastically both their characterizations and their potential applications. Various approaches have been investigated to modify the physical properties of the conducting polymers. Among the most significant achievements in this field were the preparations of soluble conducting polymers through the introduction of long side groups on different positions of the starting monomers, such as in poly(3alkylthiophenes) [9], poly(N-substituted pyrroles) [10] or poly(2,5-alkyl(or alkoxy)-l,4-phenylenes) [11]. The addition of long side chains not only renders the conjugated polymers soluble and fusible, but also results in new interesting properties, such as thermochromism *Author to whom correspondence should be addressed. Present address: Department of Chemistry, University of Montreal, Montreal, Que., H3C 3J7, Canada.
0379-6779/94/$07.00 © 1994 Elsevier Sequoia. All rights reserved SSDI 0379-6779(93)02049-Q
and solvatochromism [12]. However, the steric hindrance due to the presence of such bulky substituents induces some twistings and distortions of the backbones, resulting generally in the decrease of conjugation length and conductivity of these polymers. The preparation of soluble conducting polymers has been sought also through the formation of composites with conventional polymers [13] and through copolymerization with either conventional polymers [14] or other conducting polymers [15]. Families of all-electroactive copolymers have recently received great attention because of the high potential for modifying the electronic and physical properties of these materials. Ingan~is et al. [16] reported the electrochemical copolymerization of terthiophene and pyrrole and found the electronic transport properties of the resulting copolymers to be intermediate between those of the homopolymers. Kuwabata et al. [17] found that the electrochemical behavior of thiophene-pyrrole copolymers was gradually changed from that of polypyrrole to that of polythiophene with the increase of thiophene rings in the copolymer. Peters and Van Dyke [15g] reported the tuning of the molecular composition of pyrrole-bithiophene copolymers through the control of the applied potential. Increasing this potential led to an increase in the proportion of bithiophene in the material. This latter approach has been extensively used by Iyoda et al. [15a] and Torres et al. [15b], whereby using potential-programmed electropolymerization of
90 mixtures of pyrrole and 3-methylthiophene or thiophene, random, alternate or graded-composition copolymers have been obtained. Other polymerization methods and monomers have also been used to produce copolymers of different structures and nature. Czerwinski et al. [15I] reported the synthesis and study of copolymers based on phenylene and thiophene, obtained by the Yamamoto method, and found that copolymers with a continuous molar ratio of monomers could be prepared. More recently, copolymers consisting of pyridine and thiophene units have been reported by Yamamoto and coworkers [15c], the unusual electrochemical behavior of which suggests the occurrence of intramolecular charge transfer. We reported recently some preliminary results on the study of (phenylene-N-ethylcarbazolylene) random copolymers [18] which were obtained by an electrocatalyzed polycondensation of mixtures of 4,4'-dibromobiphenyl and 3,6-dibromo-N-ethylcarbazole. These materials were found to be partly soluble and to have conductivities lying between those of the homopolymers, that were a function of the material composition. In this paper we present investigations on the copolymerization of various 3,6-dibromo-N-alkylcarbazoles with 4,4'-dihalobiphenyls. We have been concerned with the control of the copolymer composition, through the adequate choice of the copolymerization conditions, and the determination of its relationships with the physical, electrical and electrochemical properties.
Experimental Reagents 3, 6-Dibromo-N-alkylcarbazoles These compounds have been prepared according to the method of Lindeman et al. [19]. 3,6-Dibromocarbazole (0.02 mol, Aldrich) was dissolved in a solution of 100 ml of ethanol and 0.035 mol of potassium hydroxide, and 0.03 mol of alkyl halide (1-bromobutane or 1-bromooctane, Aldrich) was added dropwise over a period of 1 h. The solution was heated at 70 °C for 24 h. The crude product was purified by gel chromatography with hexane-ether mixture (90-10) as eluent. 3,6-Dibromo-N-butylcarbazole. 13C NMR (CDCI3, ppm): 14.8, 20.5, 31.2, 42.9, 110.1, 112.3, 123.5, 123.6, 129.7, 139.8. Anal. Calc. for C16H15NBr2: C, 50.39; N, 3.68; Br, 41.99. Found: C, 50.76; N, 3.77; Br, 41.72%. 3,6-Dibromo-N-octylcarbazole. 13C NMR (CDC13, ppm): 15.2, 24.1, 26.8, 30.1, 30.3, 32.5, 45.3, 110.2, 112.3,
123.4, 123.6, 130.0, 140.2. Anal. Calc. for C2oH23NBr2: C, 54.94; N, 3.21; Br, 36.61; H, 5.26. Found: C, 55.26; N, 3.20; Br, 36.08; H, 5.30%.
Other reagents 4,4'-Dibromobiphenyl (Lancaster) and 4,4'-dichlorobiphenyl (Jansen) were used without further purification. The catalytic precursor, NiBr2-Bipy (Bipy= 2,2'-bipyridine) complex, was prepared by adding 20 mmol of NiBr2-3H20 in a solution of 100 ml of ethanol and 20 mmol of 2,2'-bipyridine. The solution was heated at 60 °C for 24 h. A yellowish-green precipitate was recovered and was washed three times with ethanol. Anal. Calc. for CloHaN2NiBr2: C, 32.04; H, 2.14; N, 7.46; Br, 42.67; Ni, 15.67. Found: C, 31.99; H, 2.48; N, 7.35; Br, 42.83; Ni, 15.76%. Lithium perchlorate, LiCIO4 (Fluka), and tetrabutylammonium tetrafluoroborate, NBu4BF4 (Jansen), were dried under vacuum at 120 °C for 24 h. N,NDimethylacetamide (DMA) and acetonitrile were distilled under reduced pressure over CartE just prior to use. Tetrahydrofuran, chloroform, ethanol, hexane and diethyl ether (Merck) were used as received. Polymerizations Polymerizations were conducted in a two-compartment electrochemical cell with mercury as a working electrode and a magnesium rod as a counter electrode. All the potentials were referenced to a saturated calomel electrode (SCE). The electrosynthesis reaction involved the cathodic reduction of a NiBr2-Bipy complex into a Ni(0)-based catalytic system which, in the presence of a dihalomonomer (or a mixture of dihalomonomers), led to the coupling product through a dehalogenative step polymerization. In a typical experiment, 10 mmol of NiBr2-Bipy were dissolved in 100 ml of DMA at room temperature in the cathodic compartment. Two main experimental procedures were followed to add the different monomers to the solution. In the first one, mixtures of 1,4-dihalobiphenyl and 3,6-dibromoN-alkylcarbazole (with the alkyl being either a butyl or an octyl group) in various proportions were prepared and poured into the solution (see Table 1). After complete dissolution, the cathode was polarized at a working potential of -1.3 V versus SCE. The current intensity was monitored during the electrosynthesis and the reaction was stopped when it reached a value below 5 mA. In the second experimental procedure and after solubilizing the catalyst, a given amount of a first monomer (either a dihalobiphenyl or a 3,6-dibromoN-alkylcarbazole) was added to the solution (see Table 2) and the cathode was polarized at the working potential. After a determined time, the second monomer, already dissolved in 20 ml of DMA, was added dropwise
91 T A B L E 1. Phenylene-carbazolylene copolymers; electrolysis of mixtures of 4,4'-dihalobiphenyl and 3,6-dibromo-N-alkylcarbazole DBuC/DBB or DBoC/DBB
[M]/[C]
SPBul SPBu2 SPBu3 SPBu4 SPBu5 SPBu6 a
1 0.3 3 1 1 1
1 1 1 1 0.5 2
SOP1 SOP2 SOP3 SOP4 SOP5 SOP6 SOP7 SOP8
0.8 1 1 0.3 1 1 1 1
0.9 2 1 1 2 1 1 0.5
Solubility in T H F (%)
Cz/BP
Ea,
Ea2
Cz/BP
soluble fraction in T H F
(V)
(V)
non-soluble fraction in THF
0 0 0 2 2 2
15 15 78 48 45 70
1.03 1.62 14.31 4.24 3.41 2.76
0.82 0.85 0.87 0.84 0.85 0.83
1.11 1.15 1.14 1.12 1.16 1.18
1.33 0.40 0.27 0.98 0.99
0 1 0 0 0 2 1 2
70 61 62 60 49 67 49 36
1.71 2.02 2.28 1.80 1.77 3.97 2.96 3.95
0.97 0.94 0.85
1.34 1.25 1.20 1.20 1.20 1.15 1.12 1.20
0.69 0.32 0.84 0.44
L
aUse of 4,4'-dichlorobiphenyl instead of 4,4'-dibromobiphenyl.
0.82 0.85 0.80 b
0.59 1.49
bWeak and fugitive anodic signal.
TABLE 2. Phenylene-carbazolylene copolymers; electrolysis of successively added monomers DBoC/DBB or DBuC/DBB
[M]/[C]
BOP1 BOP2 BOP3
0.9 2.9 8.6
3.94 3.91 3.87
BPO1 BPO2 BPO3 BPO4
1 1 1 1
BUP1 BUP2 a PBul PBu2 PBu3 PBu4 PBu5 PBu6 PBu7 a
Cz/BP
Eat
E,2
Eaa
Cz/BP
soluble fraction in T H F
(V)
(V)
(V)
non-soluble fraction in T H F
59 83 95
0.76 2.83 6.61
0.82 0.80 0.80
1.15 1.15 1.15
1.45 b 1.50b
0.55 0.63
30 70 30 30
51 54 22 37
9.57 18.40 1.29 36
0.75 0.80 0.85 b 0.75
1.18 1.18 1.16 1.13
1.60¢
0.47 0.32 0.36 0.27
0 2
34 20
20 59
0.46 3.43
0.79 0.90
1.20 1.25
1.66 c 1.82 c
0.22
0 1 1 1 1 1 1
48 20 25 20 20 10 20
39 22 29 48 36 23 14
0.97 0.43 0.73 1.42 3.83 0.52 1.62
0.85
1.15 1.20 1.20 1.18 1.20 1.20 1.20
1.43 b 1.45 1.50 1.73 ¢ 1.50 b 1.54 1.45
0.23 0.15 0.75 0.28 0.96 0.20
L
Q (%)
Solubility in T H F (%)
2 2 2
67 48 48
2 2 2 0.5
0 0 2 2
1 1
4 4
1 1 1 1 1 1 1
1 4 4 2 1 4 4
aUse of 4,4'-dichlorobiphenyl instead of 4,4'-dibromobiphenyl.
t h e s e c o n d m o n o m e r is r e p r e s e n t e d b y t h e r a t i o o f t h e q u a n t i t y o f e l e c t r i c i t y a l r e a d y c o n s u m e d o v e r t h a t req u i r e d f o r t h e c o m p l e t e e l e c t r o l y s i s o f t h e first m o n o m e r . At the end of the electrolysis, the reaction product into aqueous
sulfuric acid (5%)
and
0.85
bWeak and fugitive anodic signal.
to t h e s o l u t i o n . I n this c a s e , t h e i n s t a n t o f a d d i t i o n o f
was poured
0.85 0.85
a
s u s p e n s i o n o f m a t e r i a l w a s r e c o v e r e d by f i l t r a t i o n . T h i s filtrate was washed with water and ether and dried
1.60¢ 1.65c
CDeactivation of the film.
u n d e r v a c u u m f o r 24 h. T h e r e s u l t i n g m a t e r i a l w a s e x t r a c t e d by T H F in a S o x h l e t a p p a r a t u s f o r 24 h, r e s u l t i n g in a s o l u b l e f r a c t i o n a n d an i n s o l u b l e o n e .
Physical measurements T h e e l e m e n t a l a n a l y s e s w e r e p e r f o r m e d by t h e S e r v i c e of Central Analysis of CNRS, Villeurbanne, France. The nitrogen and carbon proportions allowed the de-
92 termination of the copolymer compositions through the use of the following formula:
NiBr2(Bipy ) + 2e
Bipy ---~
Ni(0)(Bipy)2 + 2Br
1 3 V vs S C E
n
Ar
l + Ni(0)(Bipy):
Ar)o+ NiBr~(Bipy)~
CkHi2.~l. ~
with A F :
0
O-
, ~ I R
R = C:H,, C4H~, C,H,7
% Carbazole = Cz =N(165 + 14k)/14 % Biphenyl =BP= 19[(7C) - (72N) - (6kN)]/126 where N is the nitrogen content, C the carbon content of the sample, and k represents the number of carbons of the N-substituted aliphatic chain. The ratio of Cz over BP gives information on the overall composition of the different soluble and nonsoluble fractions of the copolymers (Tables 1 and 2). NMR spectra were obtained in deuterated chloroform at 50 °C with a 200 MHz Bruker apparatus. The molar mass determination was performed by GPC in THF at 20 °C with a Water GPC apparatus, calibrated with standard polystyrene samples. UV-Vis spectroscopy was carried out using a Varian DMS 100. Infrared spectra were recorded on a Perkin-Elmer 580 spectrometer with KBr pellets. AsF5 doping experiments together with the electrical conductivity measurements (fourpoint) of copolymers were performed at CNET (Lannion, France). The electrochemical behavior of the different copolymers was investigated in acetonitrile with 0.1 M LiC104. Films from the soluble fractions of the copolymers were cast from solvent onto a glassy carbon electrode or a conducting ITO glass electrode. Results and discussion
General features The synthetic route used throughout this study is derived from the one set up in our laboratory for the synthesis of poly(para-phenylene) [20], poly(N-alkyl-3,6carbazolylenes) [21] and phenylene--carbazolylene copolymers [18]. The basic outline (Scheme 1) involved the electroreduction of a NiBr2-bipyridine complex generating a Ni(0)-based catalytic system which, in the presence of dihalomonomers, allowed the formation of the coupling product through a dehalogenative step polymerization [22]. We have recently conducted a detailed investigation on the mechanism of the dehalogenative step polymerization of 3,6-dibromo-N-alkylcarbazoles [23] (with the alkyl being either butyl or octyl). The influence of
Scheme 1.
X~X
.~.
B r ~ B r R
X : Br, C1 R : C4H9, C8H,7
Scheme 2.
Ni(0)
R
various parameters, such as the monomer-to-catalyst ratio or the ligand concentration, on the distribution of the molar mass population during electrolysis has been studied. It has been found that this step polymerization proceeds through the coupling of Ni(0)activated species, with the essential part of the chain growth occurring at the very beginning of the electrolysis (less than 1 h reaction). Evidence of debromination reactions has been found by GPC and deduced from elemental analysis of the materials. Because of the occurrence of these debromination reactions at nearly the end of the coupling process, very few sites are still available for further reactions after this point. The results of this study showed principally that, in view of carrying out copolymerization reactions with two dihalomonomers, the addition of the second monomer should be realized before the occurrence of debromination reactions. To minimize this aspect, two experimental procedures have been established and are described in the Experimental Section.
Copolymer composition Electrolysis of mixtures of two monomers The first experimental procedure we have used consisted in carrying out the electrolysis of mixtures of two monomers (Scheme 2). The experimental parameters used are presented in Table 1. SPBul to SPBu5 represent copolymers of 1,4-dibromobiphenyl and 3,6-
93 dibromo-N-butylcarbazole, while 1,4-dichlorobiphenyl has been used in SPBu6. SOP1 to SOP8 are copolymers of 1,4-dibromobiphenyl and 3,6-dibromo-N-octylcarbazole. The effect of the [DBuC]/[DBB] ratio on the composition of phenylene-N-butylcarbazolylene copolymer has been studied. Three ratios of 3,6-dibromoN-butylcarbazole (DBuC) and 4,4'-dibromobiphenyl (DBB) have been used (0.3, 1 and 3) while keeping the overall ratio of monomers over catalyst ([M]/[C]) equal to unity and using no excess of ligand in the feed solution. When the proportion of DBuC is low (SPBul and SPBu2) about only 15% of soluble fractions have been recovered. On the contrary, with a mixture rich in DBuC (SPBu3) more than 78% of soluble materials have been obtained. The elemental analysis of the different fractions showed that their compositions are a function of feed composition (Fig. 1). Increasing the proportion of the carbazolic monomers led to an increase of the proportion of carbazolylene units in the soluble fractions. The composition of the insoluble fraction appears to go through a maximum for the carbazolylene unit around [DBuC]/[DBB] = 1.5 and then to decrease again. The effect of the concentration of the ligand on the composition of the different fractions is illustrated in experiment SPBul and SPBu4, the ratio of the concentration of ligand over that of catalyst (L) being respectively 0 and 2. The proportion of the soluble fraction in SPBu4 was found to be at least three times greater than in SPBul, while its composition is four times richer in carbazolylene units. The use of an excess of ligand in the feed composition led to an increase in both the solubility of the copolymer and the proportion of carbazolylene units in the soluble fraction. The compositions of the insoluble fractions depend also on the initial ligand concentration and the carbazolylene content was decreased by about 30% when using an
15
......
Q
~/Insoluble
t0
'
2
", ,, > /
fraction
N / / []
,
/s•" ~
z-"
/
0
Soluble fraction
0 2 3 DBuC/DBB Fig. 1. Effect of [DBuC]/[DBB] ratio on the composition of phenylene-N-butylcarbazolylene copolymers: electrolysis of mixtures of 4,4'-dibromobiphenyl and 3,6-dibromo-N-butylcarbazole. 0
1
excess of ligand. The effect of monomer over catalytic precursor ratio ([M]/[C]) has been investigated in experiments SPBu4 and SPBu5, carried out using a ratio [M]/[C] of, respectively, 1 and 0.5. The proportions of soluble material were found to be similar, although the composition of the SPBu5 soluble fraction was found to be slightly poorer in carbazolylene units than that of SPBu4, whereas the insoluble fractions were of almost identical composition. The effect of [DBoC]/[DBB] ratio on the composition of phenylene-N-octylcarbazolylene copolymers was considered in two experiments (SOP3 and SOP4), carried out using a [DBoC]/[DBB] ratio of 1 and 0.3 while keeping [M]/[C] = 1 and L = 0. The proportions of soluble fraction were similar in both materials, whereas their compositions varied with the [DBoC]/[DBB] ratio. Increasing the proportion of carbazolic monomer in the feed composition resulted in an increase of the proportion of carbazolylene units both in the soluble fraction and the insoluble one. However, compared to the copolymers SPBul and SPBu2, which were prepared under the same conditions, the soluble fractions of the SOP3 and SOP4 copolymers were found to be much richer in carbazolylene units. The effect of the monomer over catalytic precursor ratio ([M]/[C]) has been studied in experiments SOP2 and SOP7, with [M]/[C] ratios equal, respectively, to 2 and 1, while [DBoC]/[DBB] = 1 and L = 1. The proportion of soluble fraction was found to be higher with increasing [M]/[C], while the proportion of carbazolylene unit decreased by almost 30%. When using a higher ligand concentration (L = 2) and varying the [M]/[C] from 0.5 to 1 (SOP8 and SOP6), the proportion of soluble fraction almost doubled while the soluble fraction composition remained quite similar. The effect of excess ligand in the feed solution has been investigated in three experiments (SOP3, SOP7 and SOP6) with different proportions of ligand in the feed composition while keeping [DBoC]/[DBB] = 1 and [M]/[C] = 1. Evolution of the composition of the soluble and insoluble fractions is represented in Fig. 2. In the soluble fraction, increasing the proportion of ligand induced a linear increase of the proportion of carbazolylene units, whereas, in the insoluble fraction, a maximum has been observed for L = 1. A similar behavior is also found in experiments SOP2 and SOP5 where L = 1 and 0, while [DBoC]/[DBB] = 1 and [M]/ [Cl=2. The length of the alkyl group, N-substituted on the carbazole monomers, affected the solubility and composition of the copolymers. The use of octyl substituents in place of butyl gave rise to materials with higher proportions of soluble fractions which, in general, were richer in carbazolylene units than the insoluble fractions.
94
Soluble fraction
Br~B~ r)
Ni(0)
R
t~-
0
Insoluble fraction 1
-.~
2
Excess ligand Fig. 2. Effect of ligand concentration on the composition of phenylene-N-octylcarbazolylene copolymers: electrolysis of mixtures of 4,4'-dibromobiphenyl and 3,6-dibromo-N-octylcarbazole.
X : Br, R
CI
R
C8H]7
: C4H9,
Scheme 3.
100
Copo!ymers obtained by the electro,sis of successively added monomers The copolymers obtained by the electrolysis of monomer mixtures led to the formation of materials with random distribution of the two monomeric moieties. Moreover, and owing to the higher reactivity of 4,4'dibromobiphenyl compared to that of 3,6-dibromo-Nalkylcarbazoles [24], homocoupling reactions might be favored. In a previous investigation on the step polymerization of 3,6-dibromo-N-alkylcarbazoles [23], we found that the chain growth is limited by the occurrence of debromination reactions during electrolysis, the loss of terminal bromine reducing the number of available reaction sites. It has been found that an optimum is reached when the reaction is between 20 and 40% conversion, giving rise to long enough polymeric chains with the minimum of terminal bromine being removed. Therefore phenylene-carbazolylene copolymers have been synthesized by the electrolysis of an initial monomer and, after a determined period of time, the addition of a second monomer (Scheme 3) followed by an exhaustive electrolysis. The effect of various parameters (such as the nature of the first and second monomers, the relative proportion of the different constituents ([M]/[C] ratio), ligand concentration and the moment of addition of the second monomer) has been investigated and the results summarized in Table 2. Three copolymers (BOP1, BOP2 and BOP3) have been synthesized by starting the electrolysis with 3,6dibromo-N-octylcarbazole and adding 4,4'-dibromobiphenyl after various electrolysis times, while keeping constant the [M]/[C] ratio and the ligand concentration. By increasing the [DBoC]/[DBB] ratio, the proportion of carbazolylene units increases together with the proportion of soluble fraction (Fig. 3). Two experiments (BPO1 and BPO2) have been conducted using a [DBoC]/ [DBB] overall ratio of unity, [M]/[C] = 2 and L = 0. The electrolysis was started with DBB first and then DBoC
10
~
90
~
°
8
80
6
70
4
E
2
o
= ~e
_= O r~
60 z~
50
0 0
2
4
6 8 10 [DBoC]/[DBB]
Fig. 3. Effect of [DBoC]/[DBB] ratio on the solubility (left) and the composition (right) of phenylene-N-octylcarbazolylene copolymers: electrolysis of successively added 4,4'-dibromobiphenyl and 3,6-dibromo-N-octylcarbazole.
was added after 30 and 70% conversion. Although the proportions of soluble fraction were similar (51% versus 54%), their compositions were quite different. The proportion of carbazolylene units almost doubled in the soluble fraction in the case of the late addition of DBoC, while it decreased significantly in the insoluble fraction. By comparing experiments BPO1 and BPO3, where the excess ligand concentration (L) was respectively 0 and 2, it appears that the proportion of soluble fraction decreased by more than half when using L = 2. The composition of the soluble fractions was greatly affected by the ligand concentration as indicated by the Cz/BP ratio (9.57 versus 1.29), while the proportion of carbazolylene units in the insoluble fractions was found to decrease with the augmentation of the ligand concentration. This feature is in good agreement with the known stabilization effect of the increasing ligand concentration [23, 25]. The effect of [M]/[C] ratio on the copolymer compositions was studied in experiments BPO3 and BPO4. Increasing the relative proportion of catalyst to that of the monomers resulted in the obtention of a soluble fraction composed almost exclusively of carbazolylene units, while the insoluble
95
fraction was found to be very rich in phenylene units. Despite the presence of excess ligand in the feed solution, the homocoupling of DBB has been favored under these conditions (low [M]/[C]), indicating unsuitable copolymerization conditions. Other copolymers have been synthesized using 3,6dibromo-N-butylcarbazole and either 4,4'-dibromo- or 4,4'-dichlorobiphenyl. Experiment BUP1 (carried out with [DBuC]/[DBB] -- 1, [M]/[C] = 4 and L = 0) resulted in only 20% of soluble material with composition poor in carbazolylene units, while the insoluble fraction was found to be extremely rich in phenylenic units. Increasing the ligand concentration and using 4,4'-dichlorobiphenyl (experiment BUP2) resulted in an increase of the proportion of the soluble fraction with composition considerably richer in carbazolylene units than in the previous case. The utilization of 4,4'-dichlorobiphenyl, the reactivity of which is known to be lower than that of 4,4'-dibromobiphenyl [26], in the presence of excess ligand, resulted in a more soluble material with composition adjustable to a large extent through the adequate choice of the experimental conditions. The control of the phenylene-carbazolylene copolymer compositions is further illustrated in experiments PBul to PBu7. The electrolysis was conducted with 3,6-dibromo-N-butylcarbazole to which, after various periods of time, was added 4,4'-dibromobiphenyl or 4,4'-dichlorobiphenyl. It appeared that the nature and compositions of the copolymers are greatly affected by the instant of addition of the second monomer. The extent of reaction is directly a function of the experimental parameters, such as the [M]/[C] ratio or the excess ligand proportion. Experiment PBu2 was carried out with 1 mmol of catalyst and 2 mmol of DBB and, after the consumption of 115 C, 2 mmol of DBuC were added, resulting in [DBuC]/[DBB] = 1, [M]/[C] = 4 and L = 1. Experiment PBu5 was carried out with 2 mmol of catalyst and 1 mmol of DBB to which was added 1 mmol of DBuC after 115 C. Knowing the kinetics of the various reactions occurring during this electrocatalyzed step polymerization [23, 25] (electroreduction of Ni(II) to Ni(0), Ni(0) insertion in C-Br bonds and couplings of these latter species), it is evident that, at the moment of DBuC addition, the quantity of Ni(II) already transformed in Ni(0) is far from being similar in the two cases. This feature resulted, at the moment of addition, in phenylenic chains of various length which gave rise to copolymers of different compositions and hence properties. In fact, a higher proportion of soluble fraction has been recovered in PBu5 than in PBu2, while the proportion of carbazolylene units increased by a factor nine. The PBu4 experiment, carried out with [M]/[C]=2, showed the highest proportion of soluble fraction whereas its composition was found to be intermediate between those of PBu2 and PBu5. The
proportion of carbazolylene in the insoluble fractions increases with the increase of [M]/[C] ratio. The comparison of experiments PBu2, PBu3 and PBu6, carded out under exactly the same experimental conditions, except the moment of addition of DBuC, revealed that by allowing the electrolysis of DBB to go further resulted in a higher proportion of soluble material, richer in carbazolylene units. Surprisingly, the proportion of carbazolylene unit was found to be also higher in PBu3 than in PBu2. The utilization of 4,4'-dichlorobiphenyl resulted in a smaller soluble fraction than in PBu2, while the proportion of carbazolylene units was found to be much higher. The tuning of the copolymer composition can be realized in a various number of ways, such as the control of the feed composition or the copolymerization procedure. The copolymer composition and the nature of the carbazolic monomer affect to a large extent the solubility of these materials (Fig. 4).
Copolymer characterization and properties Various techniques have been used to characterize the different copolymers. The elemental analysis gave a knowledge on the compositions of the different fractions. Infrared spectroscopy has been used to identify the components present through the characteristic absorption bands of the carbazolylene and phenylene units. The in-plane and out-of-plane 6(C-H) vibrations of variously substituted benzene rings have been used to characterize the different copolymer fractions. In a previous study [18], we found that random phenyleneN-ethylcarbazolylene copolymers presented bands at 795 and 874 cm -1 (corresponding to 8(C-H) out-ofplane deformations of 1,2,4-trisubstituted benzene), 810-808 and 1000 cm -1 (characterizing respectively out-of-plane and in-plane deformations of para-disubstituted phenylene units). A continuous shifting of the intensity of these various bands has been observed and found to be a function of the copolymer composition. 100 ()
80 f3fz jj
60
/
40
O
j
~4"
//,tg/jjj
//
/ 2~
20
0 ~// 0
1
2
3
4
Cz/BP ratio Fig. 4. Effect of the composition and length of the N-substituted alkyl group on the solubility of phenylene-N-alkylcarbazolylene copolymers: + , butyl; O, octyl.
96 T A B L E 3. 13C chemical shifts of poly(N-butyl-3,6-carbazolylene), copolymer in CDCI 3 at 50 °C Carbon identification
a, d
3, 6
PBuC
140.1
133.4
140.1
133.3
3', 6'
poly(para-phenylene) and
b, c
125.5
phenylene-N-butyl-3,6-carbazolylene
2, 7
4, 5
1, 8
123.8
118.9
109.0
123.8
118.9
109.0
9
10
11
140.6
128 127.2
139 138.2
Poly(para-phenylene) Copolymer
132.8
1255
In the case of copolymers prepared with either 3,6dibromo-N-butylcarbazole or 3,6-dibromo-N-octylcarbazole and 4,4'-dihalobiphenyl, a similar behavior was found, and it was noticed that the position of the band between 795 and 810 cm-1 is almost directly a function of the copolymer composition, being close to 795-798 cm -1 for carbazolylene-rich fractions and around 808-810 cm -~ for phenylene-rich fractions. For copolymers with intermediate composition, the signal was found to lie between 800 and 804 cm -1. UV-Vis spectroscopy has been used to characterize the soluble fractions of the different copolymers. A characteristic absorption of the carbazolylene units, between 312 and 320 nm, has been found together with a more or less pronounced shoulder between 340 and 350 nm which can be attributed to the presence of phenylene units in the material. The relative intensities of these signals depend strongly on the material composition. 13C NMR spectroscopy undoubtedly allowed the more pertinent characterization of the copolymer soluble fractions. ~3C NMR chemical shifts of poly(para-phenylene) [26] consist of two signals at 128 and 139 ppm, corresponding respectively to tertiary and quaternary carbons. ~3C NMR chemical shifts of poly(N-butyl-3,6carbazolylene) [21c, 27] and poly(N-octyl-3,6-carbazolylene) [28] were determined in our laboratory. The chemical shifts of poly(N-butyl-3,6-carbazolylene), poly(para-phenylene) and of phenylene-N-butylcarbazolylene copolymer are presented in Table 3. All the chemical shifts of the homopolymers were found in the copolymers in which supplementary signals (131, 133 and 140.5 ppm) appeared, disclosing the presence of heterocouplings between phenylene and carbazolylene units. The electrochemical study of the soluble fractions of phenylene-carbazolylene copolymers revealed different redox behaviors. As a reminder, and under the same experimental conditions, the electrochemical study of poly(N-alkylcarbazolylenes) showed the occurrence of two anodic processes at + 0.85 and + 1.2 V versus SCE, associated with two cathodic waves (Fig. 5). The first anodic signal has been attributed to the formation of a radical cation and the second to the formation of a dication of a carbazolic diade [21c, 29]. The redox
c)
0.0 0.5 1.0 1.5 Potential (V vs.SCE) Fig. 5. Cyclic voltammetry of poly(N-butyl-3,6-carbazolylene); sweep rate 100 m V s -I.
20 IJA }
0.0 0.5 1.0 1.5 Potential (V vs. SCE) Fig. 6. Cyclic voltammetry of poly(para-phenylene); sweep rate 100 m V s - k
behavior of poly(para-phenylene) presents only one anodic process at + 1.5 V versus SCE which was attributed to the occurrence of a dication [30, 31] (Fig. 6). Three main types of redox behavior have been identiffed in the case of phenylene-carbazolylene copolymers. The first type is characterized by the presence of three anodic signals at +0.85, + 1.15/+ 1.2 and + 1.4/ + 1.5 V versus SCE (Fig. 7). The first two anodic waves can be attributed to the occurrence of a radical cation and a dication of the carbazolylene diades [32], while the third anodic process may be due to the oxidation of phenylenic units, of variable lengths, disubstituted
97
L;
100
0.0
0.5 1.0 1.5 Potential (V vs°SCE) Fig. 7. Cyclic voltammetry of phenylene-N-butylcarbazolylene copolymers with three redox processes.
(a)
I
(b)
(c) Fig. 8. Various possible chemical structures of phenylenecarbazolylene copolymers (see Table 3).
by carbazolylene units, as shown in Fig. 8(a). This three-step redox behavior has only been observed in copolymers prepared by the electrolysis of successively added monomers with a composition generally much richer in phenylene units than in carbazolylene ones (Cz = 0.43-0.97). The only exception was the experiment PBu7, for which Cz/BP was approx. 1.6. The use of 4,4'-dichlorobiphenyl instead of 4,4'-dibromobiphenyl led to a reduced phenylic reactivity and allowed the chain growth to occur more smoothly, giving rise to long enough chains of both moieties. The second redox behavior observed in these copolymers consisted in the occurrence of two redox processes at + 1.2 and + 1.45/+ 1.55 V versus SCE (Fig. 9). In this case, the first signal can be attributed to the oxidation of isolated and disubstituted carbazolic
A
0.0 0.5 1.0 1.5 Potential (V vs.SCE) Fig. 9. Cyclic voltammetry of phenylene-N-butylcarbazolylene copolymers with two redox processes.
units [30], while the second process can be due to the oxidation of disubstituted phenylene units of various lengths. The signal at + 0.85 V versus SCE, which is attributed to the oxidation of a carbazolylene diade into a radical cation, was not detectable and supports the idea of obtaining only isolated and disubstituted carbazole units (Fig. 8(b)). This type of electrochemical behavior has been observed for both types of copolymers: those obtained by the electrolysis of monomer mixtures and those produced by the electrolysis of successively added monomers. The third type of redox behavior observed in phenylene-carbazolylene copolymers displays two redox processes at + 0.8/+ 0.9 and + 1.1/1.2 V versus SCE, which are those related to the presence of carbazolylene units, and a third process between + 1.55 and + 1.80 V versus SCE. This latter anodic peak may be due either to the oxidation of very short terminal phenylenic units which are attached to the carbazolylene main chain (a similar behavior has been observed in the case of polydiphenylamine [33]), or to the oxidation of soluble oligo(para-phenylenes) that have been carried away during the extraction procedure [27]. This third redox behavior was essentially found in copolymers which are rich in carbazolylene and which did not display clear heterocoupling signals in the 13C NMR spectrum. The electrochemical response of phenylenecarbazolylene copolymers is strongly dependent on the composition and nature of the materials. The intensity of the different anodic processes is a function of the relative proportions of the phenylene and carbazolylene units and of their arrangements. Moreover, distinct signals can be attributed to each of the components even if they are physically bonded. The electrochemical response of these phenylene--carbazolylene copolymers shows that these materials form a novel class of electroactive polymers with two distinct electronic states (two quantum wells) the intensity of which can be tuned by varying the material composition and structure.
98
Acknowledgements 10 c O
E !02 >.,10
3 '\
~C -~ "u o= ~,2",-~ rj
1 2 3 4 5 Cz/BP ratio Fig. 10. Conductivity upon doping with AsF: as a function of copolymer composition: O, phenylene-N-butylcarbazolylene copolymer; V, phenylene-N-octylcarbazolylene copolymer; +, poly(para-phenylene).
This work was supported by the Centre National d't~tudes des T616communications (CNET), Lannion, France. K.F. is grateful to CNET and to the French-Algerian Cooperation Program for a research fellowship. The authors thank G. Froyer (CNET, Lannion) for AsF5 doping experiments and for electrical conductivity measurements.
0
Upon chemical doping with AsFs, these neutral copolymers become conducting. This conductivity varies considerably (from 10 - 6 to 1) as a function of the nature of the starting materials, copolymerization conditions and copolymer composition (Fig. 10). Copolymers containing N-butylcarbazole were found to be two to three orders of magnitude more conductive than those containing N-octylcarbazole. The increase of the alkyl chain length reduces the material conductivity and this result is in good agreement with those obtained in poly(N-alkyl-3,6-carbazolylenes), the N-methyl derivative [34] exhibiting a conductivity four orders of magnitude higher than that of the N-ethyl derivative [21a]. Moreover, the increase of the proportion of carbazolylene units in the copolymer reduces the conductivity achieved upon doping.
Conclusions A novel class of electroactive materials has been prepared and investigated. The composition of the phenylene-carbazolylene copolymers can be tuned by the adjustment of the feed composition and by the control of the experimental procedures. The physical properties of these copolymers, such as solubility and conductivity upon doping, were found to be a function of the nature of the starting monomers and of their relative proportions in the final materials. Three different types of redox behavior have been found which are related to the occurrence of different copolymer structures and compositions. The copolymerization of phenylene and various N-alkylcarbazoles allowed the preparation of materials with tunable properties.
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