4 2 0 Synthetic Metals, 55-57 (1993) 420--425

STRUCTURAL STUDIES OF POLY(DIMETHYL-TETRATHIOpHENE, AN INTER-

MEDIATE B E T W E E N POLY(THIOPHENE) AND POLY(3-METHYLTHIOPHENE)

H.J. FELL, J. M/~.R.DALEN *,E.J. SAMUELSEN

Institutt for Fysikk, Universitetet i Trondheim NTH, N-7034 Trondheim, Norway

*Present adress: ESRF, BP 220, F-380043 Grenoble Cedex, France

N.U. HOFSLOKKEN and P.H.J. CARLSEN

Organisk Kjemi, Universitetet i Trondheim NTH, N-7034 Trondheim, Norway

ABSTRACT

Grignard coupling of 2-bromo-3-methylthiophene and 5,5'-dibromo-2,2'-dithiophene, and

chemical polymerization of the resulting tetramer using FeC13 as coupling agent gave a new type

of substituted polythiophene, namely Poly(3,3'"-dimethyl-(2,2':5',2":5",2'")tetrathiophene))

(PDMTT), with two new and interesting features compared to conventional polythiophenes.

Firstly, we have a polythiophene with exact methyl side group positioning, eliminating the pos-

sibility of side group position disorder. Secondly, and especially interesting from a structural

point of view, we obtained a polymer which is intermediate between the unsubstituted poly-

thiophene (PT) and poly(3-methylthiophene) (P3MT). The polymer was obtained as a black

insoluble powder which turned out to be amorphous.

X-ray diffraction profiles were measured and compared to the profiles of PT and P3MT. We

were able to follow the intensity and position variation of the diffraction peaks as a function of

the methyl substitution.

INTRODUCTION

In recent years numerous works have focussed on the synthesis and characterization of

chemically coupled polythiophenes (PT) and poly(3-alkylthiophenes) (P3AT's). Unlike PT,

P3AT's are processable if the alkyl side chains are sufficiently long; i.e., longer than butyl

[1]. This processability makes the P3AT's particular interesting for various application. A

0379-6779/93/$6.00 © 1993 - Elsevier Sequoia. All rights reserved

421

thorough understanding of physical properties of the polythiophenes requires knowledge about

their structure.

Unsubstituted PT is reported to be partially crystalline with straight polymer chains packed

in an orthorombic unit cell [2, 3]. The projection of the structure along the polymer chain

axis (c-axis) is described by a p2gg rectangular space group symmetry. An important feature

concerning the packing of the chains is that neighbouring chains are tilted relative to each other

implying no p~ overlap between neighbouring polymer chains.

Substitution with sufficiently long alkyl side chains (alkyl _> butyl) into the polymer also

results in partially crystalline polymers with straight polymer backbones. An important dif-

ference from PT is that thiophene rings from neighbouring chains tend to stack on top of each

other in a parallel [4, 5] or nearly parallel manner [6]. The substitution of hydrogen in the thio-

phene rings by an alkyl side chain change.s in other words one of the main features concerning

the crystalline packing of the polymer chains. In both of these crystalline structures are the

polymer chains found to be straight with an alternating up-down orientation of the thiophene

rings [2-6].

Between these two crystalline phases there exist polythiophenes with an intermediate length

of the alkyl side chains (alkyl = methyl - propyl), among which poly(3-methylthiophene) is

most studied. The broadness of the x-ray diffraction maxima of both doped and undoped P3MT

indicate a material lacking long range order [4,7-9]. We find no indication of a helical structure

earlier proposed for P3MT [7, 10], and find the evidence for a helical structure questionable.

The aim of this work was to synthesize and to study the strncture of poly(a,3'"-dimethyl-

(2,2':5',2":5",2'")-tetrathiophene (poly(dimethyl-tetrathiophene), PDMTT, in order to elucidate

structural changes induced by the choice of the alkyl side group. Two properties of PDMTT

are of special interest in this context: Firstly, the regular positioning of the methyl groups

eliminating the possibility head-to-tail type of side group disorder. Secondly, PDMTT may

behave as an intermediate between the crystalline PT and the amorphous PaMT.

EXPERIMENTAL

The synthesis of PDMTT is done in four different steps as indicated in figure 1. In the first

step 2,2'-dithiophene was synthesized by a Grignard coupling reaction of thiophenemagnesium-

bromide and 2-bromothiophene using 1,3-bis(diphenylphosphine)propannickel(II)chloride as a

catalyst according to the procedure described by Tamao et al. [11]. The Grignard species was

synthesized using a method described by Kumada et al. [12]. An excess of 3% of magnesium

was used in order to guarantee a complete conversion of 2-bromothiophene into thienylmagne-

siumbromide. The concentration of the Ni-catalyst was 0.4 mol%. The product was obtained

in 93.5% yield as a light green solid material of 99% purity.

Subsequent bromation with N-bromosuccinimid yielded 5,5'-dibromo 2,2' dithiophene [13]

as a white crystalline material of basically 100% purity The yield in this reaction was 69.7%.

4 2 2

S S NBS w Br Br

S S . S S S S " - c ' ' "l- r , ID

FeCI 3

CHCI 3

S S S S

~ n ---3

Figure 1: Synthesis of PDMTT via a Grignard coupling reaction and subsequent chemical polymerisation.

Synthesis of the monomer, DMTT, was achieved by a Ni-catalysed coupling reaction be-

tween 5,5'-dibromo 2,2' dithiophene and the Grignard reagent derived from 2-bromo-3-methyl-

thiophene. DMTT was obtained in 44% yield as a red-yellow crystalline material of high purity.

An excess of 40% bromomethylthiophene relative to dibromodithiophene was used to ensure

complete conversion to the product. The concentration of the Ni-catalyst was 0.4mo1% relative

to dibromothiophene.

PDMTT was characterized by IR, 1H-NMR and mass spectroscopy. The NMR-measurements

were performed in solutions of chloroform at resonance frequencies 100 MHz and 400 MHz.

Chemical polymerization was performed following a method described by Sugimoto et al.

[1]. Iron(III)chloride was added to the solution of the monomer in chloroform. The polymeri-

zation yielded a black powder with was extensively washed with methanol in order to remove

impurities. Finally a black insoluble powder was obtained. The powder was pressed to 300 #m

thick pellets suitable for x-ray diffraction measurements.

X-ray diffraction profiles were measured in transmission mode with the computer controlled

diffractometer LOFTE using graphite monochromized and slightly focused CuK~radiation from

a 1500 W tube (~ = 1.5418/~). The measurements were performed as 0 - 20 scans at room

temperature using a scintillation point detector.

RESULTS AND DISCUSSION

Analyses of the monomer by means of chromatographic and spectroscopic methods revealed

the product to be of high purity. The result of the aH-NMR spectoscropy is shown in figure 2.

The doublets at 5 = 7.15 ppm, 5 = 7.13 ppm, 5 = 7.05 ppm and ~ = 6.89 ppm with two

protons each, correspond to the four non-equivalent thiophene protons. The singlets at 5 =

2.43 ppm correspond to the six methyl protons.

, [ , , , , l , t 1 , l , i ~ , l , , , , l , , , , l , [ Ii,i,~ I

423

Figure 2: thiophene.

8 7 6 5 4 3 2 1

p p m

]H-NMR (CDCI3, 100 MHz) spectrum of 3,3'"-dimethyl-(2,2':5',2":5",2'")-tetra-

0

80

70

60

50

40

30

20

10

0

P D M T T

/

I

0 1

T i 3 ~ m I i i

10 5 6

0

"= 5 0

4 0

3 0

2 0

4 5 7

I I I 1 I

2 3 4 5 6 7

Q / A i

Figure 3: X-ray diffraction pattern obtained from PDMTT. The inserts show earlier measure- ments on PT and on P3MT for comparison. Peaks marked * are due to impurities.

424

The x-ray diffraction pattern shown in figure 3 indicates that PDMTT is amorphous. The

profile basically consists of two overlapping peaks at Q = 1 A and Q = 1.24 A and a third

major peak situated at Q = 1.79 ~. For larger scattering vectors the intensity decreases with

an unsymmetric peak at Q = 2.7 A and two small smeared out peaks at Q = 4.3 ~ and

Q = 5 A. Peaks marked * are due to impurities. The peak positions with the respective real

space distances are summarized in table 1. For better comparision we listed also earlier x-ray

diffraction data of electropolymerized PT and P3MT [9]. The latter are mainly amorphous as

can be seen from the diffraction patterns shown in the inserts of figure 3. All three patterns

show essentially a similar behaviour.

P D M T T P T P 3 M T

Q/A-' d/A Q/A -1 d/A Q/A -x d/A

1 1.02 6.16 2 1.24 5.07 3 1.78 3.53 4 2.69 2.34 5 4.33 1.45 6 5.02 1.25

1.34 4.69

1.70 3.70 2.70 2.33 4.33 1.45 5.10 1.23

1.08 5.8

1.85 3.39 2.80 2.24 4.30 1.46 5.10 1.23

Table h X-ray diffraction data of PDMTT compared with previous measurements on PT and P3MT [9].

In amorphous polymers, diffraction with Q-values larger than 1.5 - 2 A -1 is believed to be

mainly due to intrachain structure [14]. This is clearly the case for the diffraction profiles of

figure 3 where the peaks numbered 4, 5 and 6 correspond to reflections I=3, 5 and 6 respectively

when compared to crystalline poly(3-alkylthiophenes)[5]. The diffraction profile for Q > 2/~-1

is hardly affected by the methyl groups. Comparision with the diffraction patterns of liquid

thiophene, bithiophene and methyl thiophene [15] clearly shows that the peaks 4, 5 and 6 are

not related to the internal structure of these molecules. We thus conclude that the diffraction

pattern for large Q-values indicate relatively straight polymer chains in these amorphously

packed polymers.

Diffraction maxima with Q < 2/~-1 are mainly related to packing of the polymer chains

[14]. Peak no. 3 in figure 3 changes slightly towards higher Q-values when more methyl groups

are attached to the thiophene rings. We think that this peak is related to the 200 peak (Q =

1.59/~-x) in crystalline PT [2, 3], and most probably develops to what is indexed as 010 peak

of crystalline P3HT (Q = 1.68 A -a) and P3OT (Q = 1.66 A -x) [5].

The broader maximum no. 1 is shifted to lower Q-values, meaning that the corresponding

d-distance increases, as methyl groups are attached to the thiophene backbone. However, the

Q-values are about the same for PDMTT and P3MT. This increment of the d-distance and

also its value indicate a relation to the 100 chain packing peak of P3HT and P3OT [5].

425

Measurements of various PDMTT samples indicate that peak no. 2 is due to impurities, but

at present we can not be totally sure about this. Supposing this peak to be a polymer peak,

it might be related to the 110 peak of crystalline PT [2,3] indicating the presence of a more

ordered phase of the polymer.

Further work is needed to develop a complete model for the structurM behaviour of tile

polyalkylthiophenes.

ACKNOWLEDGEMENTS

Financial support from Deminex for H.J. Fell and from Norges Allmennvitenskaplige Forskn-

ingsr~d, NAVF for J. M~rdalen is gratefully acknowledged.

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