Bithiophenes as starting monomers for polythiophene syntheses

Synthetic Metals, 58 (1993) 295-307

295

Bithiophenes as starting monomers for polythiophene syntheses B e r n d Krische*, Malgorzata Zagorska** and J o n a s Hellberg Department of Organic Chemistry, Royal Institute of Technology, S-100 44 Stockholm (Sweden)

(Received and accepted March 1990)

Abstract Bithiophenes, when used as starting material for polythiophene synthesis, offer advantages over corresponding thiophenes. Polymerization proceeds at lower oxidation potentials and lower monomer concentration and gives polymers with higher yield and better regularity as shown by cyclic voltammetry and ~H NMR spectroscopy.

Introduction The i n t r o d u c t i o n o f s u b s t i t u t e d t h i o p h e n e s was a g r e a t i m p r o v e m e n t in the p r o c e s s i n g t e c h n o l o g y o f c o n d u c t i n g p o l y m e r s , since poly(3-alkylthiop h e n e s ) ( f r o m butyl u p w a r d s ) are soluble in c o m m o n organic solvents, s o m e e v e n in the o x i d i z e d state [1 ]. Alkoxy-substituted p o l y t h i o p h e n e s are also soluble in the oxidized state [2] and have, f u r t h e r m o r e , the a d v a n t a g e of low oxidation p o t e n t i a l o f t h e m o n o m e r . This last p r o p e r t y is not only i m p o r t a n t for e n e r g y c o n s i d e r a t i o n s , but also b e c a u s e c o n d u c t i n g p o l y m e r s suffer irreversible o x i d a t i o n at h i g h e r potentials ( > 1.3 V). This o v e r o x i d a t i o n already o c c u r s d u r i n g e l e c t r o s y n t h e s i s , thus lowering the yield of active material and i n t r o d u c i n g irregularities into the material [3]. In spite o f the low oxidation p o t e n t i a l o f 3 - m e t h o x y t h i o p h e n e , s t e r e o r e g u l a r i t y is still n o t a m a t t e r of c o u r s e [2 ]. In the c a s e o f p o l y a c e t y l e n e , it has b e e n d e m o n s t r a t e d t h a t material p r o p e r t i e s (e.g. conductivity) can be vastly i m p r o v e d b y b e t t e r regularity [ 4 ].

Experimental 3-Alkylthiophenes w e r e s y n t h e s i z e d by Grignard coupling o f 3-bromot h i o p h e n e with alkyl G r i g n a r d c o m p o u n d s [5 ] and t h e n purified b y distillation. 4 , 4 ' - D i a l k y l - 2 , 2 ' - b i t h i o p h e n e s w e r e synthesized f r o m the c o r r e s p o n d i n g thio*Author to whom correspondence should be addressed. **On leave from: Institute of Inorganic Technology, University of Technology of Warsaw, P-00664 Warsaw, Poland.

Elsevier Sequoia

296 p h e n e s b y lithiation with BuLi/TMEDA [6], f o l l o w e d b y oxidative c o u p l i n g with c o p p e r c h l o r i d e [7]. T h e y w e r e purified b y flash c h r o m a t o g r a p h y a n d recrystallization. A c e t o n i t r i l e (Aldrich, a n h y d r o u s ) w a s u s e d as p u r c h a s e d , a n d t e t r a - n - b u t y l a m m o n i u m p e r c h l o r a t e w a s s y n t h e s i z e d a n d recrystallized a c c o r d i n g to ref. 8 a n d s t o r e d in a n e x s i c c a t o r o v e r P205. E l e c t r o l y t e solutions (0.1 M NBu4C104 in a c e t o n i t r i l e ) w e r e p r e p a r e d a n d s t o r e d u n d e r argon. S y n t h e s i s a n d e l e c t r o c h e m i c a l e v a l u a t i o n o f p o l y m e r films w e r e p e r f o r m e d in a n e l e c t r o c h e m i c a l t w o - c o m p a r t m e n t cell in u n s t i r r e d solution. T h e a n o d e w a s usually a p l a t i n u m n e t o f 1 c m 2 a r e a ( g e o m e t r i c a l a n d overall s u r f a c e ) a n d the c o u n t e r e l e c t r o d e w a s a p l a t i n u m p l a t e o f t h e s a m e size. T h e p o t e n t i a l o f t h e w o r k i n g e l e c t r o d e w a s m o n i t o r e d v e r s u s a Ag/AgCl r e f e r e n c e e l e c t r o d e . S y n t h e s i s o f p o l y t h i o p h e n e films w a s p e r f o r m e d g a l v a n o s t a t i c a l l y with 1 - 2 m A / c m 2 at 0 °C. M o n o m e r c o n c e n t r a t i o n s w e r e 50 m M for b i t h i o p h e n e * a n d 200 mM for thiophene. Films used for electrochemical evaluation were s y n t h e s i z e d with 1 0 0 m C f o r b i t h i o p h e n e s a n d 2 0 0 m C for t h i o p h e n e s . After synthesis, t h e p o l y m e r s w e r e w a s h e d in a S o x h l e t e x t r a c t o r (15 ml) with a c e t o n i t r i l e u n d e r r e d u c e d p r e s s u r e for 1 h a n d dried u n d e r d y n a m i c v a c u u m . P o l y m e r s f o r o t h e r s t u d i e s w e r e r e d u c e d e l e c t r o c h e m i c a l l y in f r e s h e l e c t r o l y t e a n d t r e a t e d with a m m o n i a b e f o r e w a s h i n g . T h e y w e r e dissolved in c h l o r o f o r m ( s t a n d i n g o v e r n i g h t , s o m e t i m e s reflux w a s n e c e s s a r y ) * * . F o r NMR a n d GPC, materials from several syntheses were combined. Films were spun at 2000 r p m o n quartz p l a t e s o r c a s t f r o m s o l u t i o n ( c h l o r o f o r m o r t e t r a c h l o r o e t h e n e ) . Cyclic v o l t a m m o g r a m s w e r e r u n on a n E E & G 173 g a l v a n o s t a t / p o t e n t i o s t a t with p r o g r a m m e r a n d c o u l o m e t e r at 20 mV/s at r o o m t e m p e r a t u r e a n d r e c o r d e d o n a H o u s t o n XY plotter. U V - V i s s p e c t r a w e r e r e c o r d e d on an H P 8 4 1 5 A d i o d a r r a y s p e c t r o p h o t o m e t e r ( s o l u t i o n s in 1 c m cuvettes). NMR s p e c t r a w e r e r e c o r d e d on a B r u k e r AM 4 0 0 s p e c t r o m e t e r a n d v r - I R s p e c t r a o n a P e r k i n - E l m e r PE 1 7 1 0 s p e c t r o m e t e r .

Structure o f s u b s t i t u t e d p o l y t h i o p h e n e s In principle, t h e 3 - s u b s t i t u e n t in p o l y t h i o p h e n e s can be a r r a n g e d in different w a y s , n a m e l y , in r e g u l a r f a s h i o n s as in 1 A a n d 1C or at r a n d o m as in 1B ( S c h e m e 1). F o r the s t r u c t u r e o f 3 - a l k y l t h i o p h e n e s , a r a n d o m p a t t e r n as in 1B s e e m s t o b e t h e m o s t likely one, since e x c l u s i v e head-to-tail c o u p l i n g ( 1 A ) w o u l d r e q u i r e m o r e p r o n o u n c e d s p i n / c h a r g e d e n s i t y d i f f e r e n c e s f o r the 2- a n d 5p o s i t i o n s o f the i n t e r m e d i a t e radical c a t i o n t h a n p r o b a b l y is the c a s e t. T h e sterical h i n d r a n c e is, in o u r opinion, n o t decisive either since the a t t a c k h a s *Because of its poor solubility in the electrolyte, the concentration of 4,4'-dioctylbithiophene was only 1 mM and the solution was stirred during electrolysis. **PDABTs are more soluble than corresponding PATs; they dissolve more readily and in less solvent. *Simple HMO calculations [9] give orbital coefficients of the SOMO of 3-methylthiophene for the 2- and 5-positions of 0.6093 and 0.5911, and charges of 0.3311 and 0.3168, respectively.

297

,/

1A

1B

1C Scheme 1. Schematic orientation of substituents in substituted polythiophenes. The formulae do not represent actual molecular structures. The all-c/s orientation of the thiophene rings is chosen for reasons of clarity. to p r o c e e d p e r p e n d i c u l a r to the ring p l a n e b y interaction o f the rr-orbitals. F u r t h e r m o r e , e x p e r i m e n t a l e v i d e n c e for n o n r e g u l a r s t r u c t u r e has b e e n b r o u g h t a b o u t for c h e m i c a l l y p r e p a r e d p o l y ( 3 - b u t y l t h i o p h e n e ) b y 1 H NMR s p e c t r o s c o p y

ll0l. The m o r e o r less r a n d o m coupling o f 3-alkylthiophenes inevitably leads to a m i x t u r e of chains with i n d e t e r m i n a b l e r a n d o m structure. In principle, the r a n d o m c o u p l i n g o f n 3-substituted t h i o p h e n e s gives rise to 4 n- 1 combinations o f w h i c h m o s t are identical b e c a u s e of s y m m e t r y . Nevertheless, only t h r e e c o u p l i n g s t e p s a h e a d y give ten different q u a t e r t h i o p h e n e s . F o r a chain o f t w e n t y rings, the n u m b e r o f distinguishable c o m b i n a t i o n s is at least several h u n d r e d s ( p r o b a b l y t h o u s a n d s ) , all of which are r e p r e s e n t e d in a statistical w a y d e p e n d i n g o n the probability o f head-to-head, head-totail, taft-to-head and taft-to-tail c o u p l i n g o f the involved entities. F u r t h e r m o r e , one has, of c o u r s e , distribution in chain length. With this p i c t u r e in mind, it is r a t h e r vain to base the i n t e r p r e t a t i o n of e x p e r i m e n t a l results g a i n e d f r o m p o l y m e r s synthesized f r o m 3-substituted t h i o p h e n e s o n a m o d e l w h i c h a s s u m e s a regular structure. F u r t h e r m o r e , it is r a t h e r likely t h a t the variation of s u b s t i t u e n t orientation has a p r o n o u n c e d influence o n the p o l y m e r p r o p e r t i e s . F o r c o m m o n p o l y m e r s this is impressively d e m o n s t r a t e d b y t h e role o f tacticity. On the o t h e r hand, s y m m e t r i c a l l y s u b s t i t u t e d b i t h i o p h e n e s can give only o n e p o l y m e r s t r u c t u r e (e.g. 1C for disubstitution) [11, 12]. Additionally, b i t h i o p h e n e s offer the a d v a n t a g e o f o x i d a t i o n / p o l y m e r i z a t i o n potentials o f

298 about 600 mV less anodic than the corresponding thiophenes and, probably as a result, the yields are much better and the concentrations necessary for polymerization are much lower [ 13]. For the insoluble unsubstituted polythiophene and polybithiophene, structural differences are not easily demonstrated and, of course, geometrical isomerism of substituents does not occur. The solubility of the alkyl derivatives facilitates analysis insofar as NMR and UV-Vis spectroscopy can be done in solution. Furthermore, solution processibility defuses morphological differences imposed during polymerization (i.e. it does not matter if the product is already a film with good mechanical properties directly after synthesis or not). Comparison of polymers from 3-alkylthiophenes and the corresponding 4,4'-dialkyl-2,2'-bithiophenes clearly shows the above-mentioned advantages.

Results

Experimental results axe combined in the Table 1 and discussed in detail in the following paragraphs.

Cyclic voltammetry of polythiophenes Table 1 shows that differences in electrochemical parameters are rather small between the polymers from thiophenes and the corresponding bithiophenes. But the actual cyclic voltammograms look quite different, because the first oxidation wave is much sharper for the polybithiophenes. This fact, together with more active material (or higher activity of the polymer), results in bigger currents. Figures 1-5 show voltammograms and spectra of the butyl-substituted polymers. They are representative for alkyl-substituted polythiophenes.

UV-Vis spectra UV-Vis spectra in solution (CHCla), as well as of films on quartz, show a marked difference in the position of the main absorption peak. The poly(dialkylbithiophenes) (PDABTs) have their maximum absorption about 40 nm shorter (390 nm) than the poly(alkylthiophenes) (PATs). The films and solutions of the PDABTs look bright yellow, whereas the PATs are more red and rather dark (Fig. 2). The spectra of PDABTs are not influenced by the form of the polymer, e.g. solid film or solution, nor by the solvent (e.g. CHCla or C2C14). This is not the case for the PATs [14]. For example, we found that poly(3-butylthiophene) (PBuT) displays a substantial bathochromic shift from 432 to 444 nm together with line broadening on going from solution to film (Fig. 3). Furthermore, the spectra of polyalkylthiophenes are solvent dependent [14] (e.g. POcT: CHCla 430 nm, C2C14 444 nm). The short wavelength of the polybithiophene absorption is an intriguing problem. From 1H NMR and GPC, it is obvious that the average chain is built up of at least 24 rings and that there is no big difference between the

R

1.79

1.85

1.78

1.79

Methyl

Butyl

0ctyl

3,4-Dimethyl

238

232

232

234

226

1.01

0.91

0.81

0.59

0.91

0.91 (0.74)

0.85

0.74

0.64

0.80

(v)

Spc c

1.64

1.58

1.80

1.90 (0.36)

1.67

(V)

Eoo d

90

70

110

170 (0.26)

90

Qoo (mC)

Yield e

Amaxg

7.300 (2.25)

19.700 (1.63) 430 (260)

432 (260)

(217/w/217/~)(nm)

J~fnf

1.16

1.11

1.11

1.15

1.27

(-V)

Epa a

252 (280s)

310 (252)

310 (252)

310 (252)

302 (246)

(nm)

Amaxb

Monomer

R

0.97

0.95

0.93

0.71

0.97

Epaa (V)

0.91 (0.65)

0.87

0.78

0.64 0.23

0.59 (0.81)

Ep ¢ (V)

Polymer

R

1.73

1.70

1.72

1.79

1.81

Eood (V)

150

150

160

250

180

Qoo (mC)

Yield e

~tm~g

390 (260) 388 (260)

11.300 (2.03) 8.300 (2.30)

(Mw/)lT/~ (nm)

/•nf

aPeak potential of the first oxidation wave. bAbsorption maximum of the m o n o m e r s in chloroform, the bithiophenes exhibit a third band at 210 to 216 n m which is strong in the case of octyl substitution. Measurements in acetonitrile give the same values. CPeak potentials of the reversible reduction wave. dpeak potential of the overoxidation wave (see Fig. 1). eYield of electroactive polymer m e a s u r e d as the total overoxidation charge. For thiophene polymerization double synthesis charge was used. fMolecular weight from GPC in chloroform vs. polystyrene standard. gAbsorption maximum of the polymers in chloroform (resolution 2 nm).

2.04

Spa a

Amaxb

(um) (V)

(V)

Polymer

Epa a

Monomer

H

Substituent

Comparison of some properties of electrochemically synthesized polymers from thiophenes and c o r r e s p o n d i n g bithiophenes

TABLE 1

¢,D

300 [mA]

(b)

(a) _ .~ ~3.6

(a) "

-1

~,,

~ 2.0 V

"x../

Fig. 1. Cyclic voltammograms of (a) PBuT and (b) PDBuBT for reversal potentials 1.0 and 2.0 V. Acetonitrile, NBu4C104, 20 mV/s, 25 °C, vs. Ag/AgC1.

abs

I

I

[

i

I

I

1

[

0.5

/~ [

200

....... ] ..........

400

600

I

(nml

Fig. 2. UV-Vis spectra of (a) PBuT and (b) PDBuBT in chloroform solution. For comparison the spectrum of 4,4'-dibutylbithlophenene (c) is included.

abs_

I

I

I

I

I

]

[

i

I

I.

0.5

200

I

400

600

[him]i

Fig. 3. UV-Vis spectra of (a) PBuT and Co) PDBuBT films on quartz plates.

301

7:O

6:0

51.0

4:0

31.0

2:O

llO

0.0

Fig. 4. ~H NMR spectra of 3-butylthiophene (upper), poly(3-butylthiophene) (middle) and poly(4,4'-dibutyl-2,2'-bithiophene (lower). 400 MHz, CDCI3, 25 °C. t w o kinds o f p o l y m e r (with r e s p e c t to m o l e c u l a r weight). But since 3 9 0 n m indicates only a quaterthienyl unit by c o m p a r i s o n with t h i o p h e n e o l i g o m e r s [15], c o n j u g a t i o n in the neutral p o l y m e r has to be diminished. A n o t h e r e x p l a n a t i o n c o u l d be t h a t the r e g u l a r p o l y b i t h i o p h e n e f o r m s a g g r e g a t e s as q u i n q u e t h i e n y l d o e s [16] a n d that w o u l d lower Ama~*- If that is the c a s e for PDABTs, the a b s o r p t i o n s h o u l d be t e m p e r a t u r e d e p e n d e n t , o c c u r r i n g at l o n g e r w a v e l e n g t h at e n h a n c e d t e m p e r a t u r e * . The values o f Am~ o f PDABTs do n o t shift to l o n g e r w a v e l e n g t h with i n c r e a s i n g t e m p e r a t u r e in solutior/ n o r as solid films. F o r example, Ama~ of P D B u B T in t e t r a c h l o r o e t h y l e n e shifts only slightly f r o m 3 9 2 to 3 8 8 nm** o n h e a t i n g f r o m 25 to 100 °C. As a film, /~max w a s shifted to 3 8 6 n m after 2 min at 2 0 0 °C. The h y p s o c h r o m i c shifts on h e a t i n g are n o t in a g r e e m e n t with a g g r e g a t e dissolution a c c o r d i n g to ref. 16'. NMR

spectra

The 4 0 0 MHz 1H NMR s p e c t r a are s h o w n in Fig. 4; for c o m p a r i s o n the s p e c t r u m o f 3 - b u t y l t h i o p h e n e is also given. The s p e c t r u m o f 4,4'-dibutyl*Rughooputh et al. [14] reported that an extra absorption band at longer wavelength occurred upon cooling for solutions of PATs. These bands at 560 and 607 nm were interpreted as stemming from aggregates, but originate fundamentally from changes in chain conformation. **hm,x of a solution of POcT shifted from 444 to 430 nm on heating. tPDABTs are more soluble than corresponding PATs; they dissolve more readily and in less solvent.

302

bithiophene is almost identical to that of 3-butylthiophene except for the aromatic protons (doublets at 7.01 and 6.78 ppm) and a small (0.05 ppm) high field shift to 2.60 ppm of the a methylene group. It is striking that the chemical shifts of the butyl group are quite different compared to the monomers (see Table 2) and that two sets of signals are observed for the polymers from alkylthiophenes 13C NMR of PBuT does not show such a big shift for the carbons of the butyl group but those of the methylene groups are again doubled. The integration of the corresponding high/low field signals gives a ratio of 1:3. The 13C shifts of PBuT, POcT, PDBuBT and PDOcBT are listed in Table 3. Because of their low intensity the aromatic carbons could not usually be detected.

IR spectra The IR spectrum of free-standing films of PDBuBT (Fig. 5) is almost identical to one of PBuT, as has already been shown [14]. This means that the differences which can be observed in the UV-Vis and IH NMR spectra do not manifest themselves in the IR spectra. TABLE 2 Chemical shifts of the hydrogens of butyl-substituted thiophene, bithiophene and polymers t h e r e o f (400 MHz, CDC13, 25 °C) H2 4-BUT~

H3

7.24(dd)

H5

6.95-6.92(m) 6.78(d)

4,4'-DBuBt PBuT b PDBuBT

7.12(d)

a-CH2

~-CH 2

T-CH2

2.65

1.64

1.40

CH3 0.965

2.60

1.64

1.40

6.98 (7.02)

2.81 (2.58)

1.69 (1.59)

1.46 (1.35)

0.98 (0.91)

0.965

7.06

2.55

1.59

1.35

0.90

aFor reasons of consistency 3-butylthiophene is counted as 4-butylthiophene. bThe values in brackets are from the low intensity set.

TABLE 3 Chemical shiftsa of the carbons of butyl- and octyl-substituted polythiophenes and Dolybit h i o p h e n e s (400 MHz, CDC13, 25 °C) C3" PBuT PDBuBT POcT PDOcBT

128.44 (125.12)

a-CH 2

~3-CH2

32.44 (32.64) 32.79 31.52 31.72

28.95 (28.50) 28.74 30.18 30.45

~The italic values are higher in intensity.

~/-CH2,~-CH2, e-CH 2

29.19, 29.07 29.22, 29.06

~/-CH2

28.92 28.85

eo-CH2

CH 2

22.46

13.77 (22.31) 13.93 13.73 13.93

22.49 22.30 22.50

303

40()(}''

'3(i(}0 ....

20'00 . . . .

1.500 ....

10'00'

'

' [cmJ]

Fig. 5. FT-IR s p e c t r u m of a free-standing film of P D B u B T .

A

B

CD

A

111 Scheme 2. Different magnetic environments of substituents in a randomly coupled polythiophene chain according to Scheme 1.

Discussion It is tempting to assign the two ~H NMR signal sets of PATs to polymer regions with different substitution orientation (i.e. head-to-head versus headto-tail coupling) [17]. That would mean about 25% of the polythiophene chain is coupled in the regular fashion 1C of the bithiophene (see Fig. 1). The head-to-tail coupled chain 1A would then count for the missing 75% [17]. However, in this model, it is very odd that there are no signals in between, since substituents should be in at least four different magnetic environments (see Scheme 2). The ab s en ce of further substituent signals implies within this model a block p o l y m e r with blocks of different substituent orientation or a mixture of two kinds of polymer*. Both of these seem rather unlikely. *The spectra were obtained from combined material of several syntheses, but we regard it as unlikely that synthesis under the same conditions gives different polymers. Furthermore, the NMR spectra of chemically prepared polymers also shows the same two sets of signals [10, 181.

304

Shift differences for the entire side chain cannot be the result of throughb o n d inductive or m e s om e r i c effects as demonstrated by comparison of the spectra o f 3-butylthiophene and 4,4'-dibutylbithiophene. Therefore, they must be the result of steric factors, either different interaction between side groups or different orientation in an anisotropic field. The low field shift of substituent peaks of the major signal set of PATs is m o s t likely due to the fact that the substituents ext end into the deshielding region of neighbouring t hi ophene rings. The reason for the high field shift of PDABTs and the minor signal set of PATs, however, is not that obvious. If PBuT a d o p t e d a molecular structure as sketched in Scheme 2, interaction between substituents C and D would clearly be different from all other interactions. But such a conformation is highly unlikely from steric and energetic considerations. A transoid structure, as shown in Scheme 3, would be expected. If interaction between side groups caused the shift difference, it would be substituents B and D that would exhibit such an effect and one could indeed e x p e c t a s p e c t r u m with two signal sets. But it should be the major signal that is identical to that found for PDBuBT (3C in Scheme 3). That is not the case and therefore this explanation is ruled out. It seems more likely that the observed shift difference is not the result of a direct difference between the magnetic environments of substituents A - D o f a hypothetically planar polymer, as shown in Scheme 3. In the planar case, all substituents are in the same magnetic environment, interaction

B

3B

S

D

S

A

S

C

A

3C

S c h e m e 3. F o r m u l a e o f P B u T (B) and P D B u B T ( 3 C ) in the a l l - t r a n s orientation of t h e t h i o p h e n e rings. Letters indicate s u b s t i t u e n t s in different e n v i r o n m e n t s as i~ S c h e m e 2.

305 between B and D is not critical and therefore it is not obvious why they should exhibit a different shift. The observed two sets of signals are, most probably, the result of differences in chain conformation of the two polymers, imposed by the substituent pattern. We believe that the shift differences are an indirect result of the coupling pattern which influences the spacial conformation of the molecule. The substituent pattern has a strong influence on conjugation length, as reflected in the UV-Vis spectra. In the regular PDBuBT, rather small units (probably bithiophene) are tilted in a regular fashion. In this way, all of the substituents are located in the shielding region above the aromatic thiophene rings, which explains the high field shift compared to the monomers. For the nonregular PBuT, the tilts also occur at locations where substituents are in the 3,3'-position; however, these links account for only 25% of all couplings. This situation results in longer conjugation length, as seen in the UV-Vis spectra, and only two different magnetic environments for most of the side chains (i.e. substituents located above neighbouring rings (C and D) and those that are not). A rise in temperature should, in that case, change the IH NMR spectrum of PBuT in the same way as it changes the UV-Vis spectrum of PATs [14]. This thermochromism is interpreted by temperature-induced conformeric twists (so-called conformons) which influence the conjugation length [19]. Variable temperature NMR experiments were performed on polyoctylthiophene (POcT) because of its better solubility compared to PBuT and the large hypsochromic shift of its main absorption (UV-Vis) at 100 °C [20]. The IH NMR spectrum of POcT is solvent dependent, e.g. replacing CDCI3 by C2C14/C~D6 shifted all signals by about 0.06 ppm to higher field. This is accompanied by a bathochromic shift of Amaxof 14 nm; a temperature increase of 75 °C shifted the absorption peak back to the blue by 14 nm. If both effects have the same origin (i.e. change in conjugation length), temperature increase should also shift the NMR absorption. The ~H NMR spectrum of POcT did not change at all up to 100 °C, which was the limit imposed by the solvent (C2C14 + C8D6). The octyl signals neither shifted nor changed intensity, they only became better resolved. Hence, the hypothesis of twists in the chain as the reason for the differences in the UV-Vis and ~H NMR spectra is not confirmed by variable temperature NMR, at least not in this temperature range. For the doped (oxidized with FeC13) polymers, the differences seem to be unimportant. Films of both have conductivities in the range 1 to 10 S/cm and their UV-Vis spectra are quite similar.

Conclusions

Comparison of polymers synthesized by electropolymerization of thiophenes and corresponding symmetrical bithiophenes shows clearly that the

306 p o l y m e r s are quite different. The u n d e r l y i n g r e a s o n is p r o b a b l y the irregular m o l e c u l a r s t r u c t u r e o f the p o l y m e r s p r e p a r e d f r o m t h i o p h e n e s . The following is a list of similarities a n d dissimilarities b e t w e e n the two kinds o f p o l y m e r s : - m o l e c u l a r w e i g h t s are similar -- IR s p e c t r a are similar - b e t t e r solubility o f p o l y b i t h i o p h e n e s - m u c h s h o r t e r Am~ o f p o l y b i t h i o p h e n e s - UV-Vis s p e c t r a o f p o l y t h i o p h e n e s are m u c h m o r e t e m p e r a t u r e a n d solvent dependent than those of polybithiophenes -- cyclic v o l t a m m e t r y exhibits s h a r p e r a n d b e t t e r r e s o l v e d p e a k s for polybithiophenes 1H NMR s p e c t r a are different, s u b s t i t u t e d p o l y t h i o p h e n e s s h o w i n g t w o sets o f lines - d.c. conductivities o f oxidized ( d o p e d ) p o l y t h i o p h e n e s a n d b i t h i o p h e n e s are similar An i m p o r t a n t result is, in o u r opinion, the fact t h a t the ~H NMR s p e c t r u m o f p o l y o c t y l t h i o p h e n e is n o t t e m p e r a t u r e d e p e n d e n t in the r a n g e 25 to 100 °C. To c o n c l u d e , we r e p e a t t h e p o i n t m a d e in ref. 2. B a s e d o n the results f o u n d b y c o m p a r i n g p o l y m e r s f r o m t h i o p h e n e a n d bithiophene, it s e e m s very s p e c u l a t i v e to interpret results f r o m p o l y m e r s m a d e f r o m t h i o p h e n e s o n the basis o f m o d e l s w h i c h a s s u m e a r e g u l a r structure.

Acknowledgements

This w o r k w a s s u p p o r t e d by a g r a n t f r o m the Swedish National B o a r d for Technical D e v e l o p m e n t (STU). W e t h a n k the Swedish Institute for g r a n t i n g a s c h o l a r s h i p to M.Z. a n d Dr S. L u n d m a r k f o r m a k i n g the m o l e c u l a r w e i g h t determinations.

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