Electropolymerization of silylthiophene monomers: FT-IR and Raman spectroscopy characterization of polythiophene films
J e a n - L o u i s S a u v a j o l * Groupe de Dynamique des Phases Condens#es, UA CNRS No. 233, Universit# Montpellier II, Sciences et Techniques du Languedoc, place EugOne Bataillon, 34095 Montpellier Cedex 5 (France)
C l a u d e C h o r r o a n d J e a n - P i e r r e L 6 r e - P o r t e * Laboratoire de Chimie Gdn~rale, Universit~ Montpellier II, Sciences et Techniques du Languedoc, place Eugdne Bataillon, 34095 Montpellier Cedex 5 (France)
R o b e r t J .P . C o r r i u , J061 J . E . M o r e a u * , P h i l i p p e T h 6 p o t a n d M i c h e l W o n g Ch i M a n Unit~ Mixte CNRS/Rh6ne-Poulenc/USTL, Universitd Montpellier II, Sciences et Techniques du Languedoc, place EugOne Bataillon, 34095 Montpellier Cedex 5 (France)
(Received June 1, 1993; in revised form September 26, 1993; accepted October 5, 1993)
Abstract
Polythiophene films were obtained upon anodic oxidation of silylthiophene monomers with various structures: 2,5-bis(trimethylsilyl)thiophene (1), 5,5'-bis(trimethylsilyl)bithiophene (2), 5,5"-bis(trimethylsilyl)terthiophene (3), 2,4-bis(trimethylsilyl)thiophene (4) and 3-trimethylsilylthiophene (5). Polymerization occurred through a complete electrodesilylation for monomers 1-3 having silyl substituents at the s-position of the thiophene ring. Only partial desilylation occurred upon electropolymerization of monomers 4 and 5 with silyl substituents at the /3-position. The structural properties of the polythiophene films obtained were studied by FT-1R and Raman spectroscopy. On the basis of Raman and photoluminescence studies, the polymers obtained from 1-3 appeared highly structured, with higher mean conjugation lengths and lower amounts of defects, when compared to polymers obtained from the corresponding non-silylated monomers. Therefore, in agreement with a previous report based on electrochemical studies, the presence of ~-silyl substituents at the thiophene ring favours the formation of highly structured polymers exclusively linked 2,5 throughout. Conversely, the polymers obtained from monomers 4 and 5 revealed the presence of some 2,4-linkages in the polymer chain and a lower mean conjugation length.
Introduction
Electrically conductive organic polymers are easily obtained upon oxidative polymerization of aromatic compounds [1]. The electropolymerization reaction of thiophene monomers has been shown to form thin polymer films, the electroactive and electrochromic properties of which make them of interest for various applications [1, 2]. The high conductivities of poly- thiophenes have been related to highly structured poly- mers exclusively linked through the 2 and 5 positions of the thiophene ring. Over the last few years, efforts have been made to improve the stereoregularity of polythiophenes, in order to obtain polymers with longer mean conjugated chain lengths and exhibiting high electrical conductivity values. Thus, various polymers were prepared from monomers substituted at the 3 or 4 position of the thiophene ring to prevent branching
*Authors to whom correspondence should be addressed.
or to facilitate oxidative polymerization via a stabilization of the intermediate radical cation [24] .
Silicon-directed reactions have been widely used in organic synthesis to improve the selectivity of car- bon-carbon bond formation [5]. However, little attention has been paid to the use of this methodology for selective polymerization reaction. Since the original report of Lemaire et al. who showed that trimethylsilyl substituents allow specific activation at the 2 position of a thiophene unit [6], several other groups have reported the synthesis of polythiophene films from silylated precursors [7-9]. Our current interest in silicon-containing conjugated organic polymers [10] led us to examine the electroactive properties of silicon-containing polymers and oligomers. Electrodesilylation reactions would lead to higher reac- tivities and selectivities in the coupling reactions. We, for example, showed that polythiophenes formed in a silicate matrix upon anodic oxidation of thienylidene silsesquioxane gels [10c]. We became interested in characterizing such polythiophenes in order to correlate
the structural features of the polymers for improvement in electrochemical and electrical properties.
Raman and IR spectroscopies are powerful tech- niques for characterizing polythiophene (PT) films [11-14]. The 'quality' of the polymer film, in terms of mean conjugation length and amount of defects, was both qualitatively and quantitatively established by one of us upon analysing the Raman and photoluminescence spectra of polythiophene films [15-17]. We report here the characterization of polythiophene films prepared from various silylated 2,5-mono-, di- and terthiophene monomers as well as from 3- and 2,4-silylated thio- phenes.
Experimental
Preparation of the silylated monomers All reactions were carried out under nitrogen by use
of a vacuum line and Schlenk tube techniques. Solvents were dried and distilled before use. The reported yields refer to pure isolated compounds. Melting points were determined with a Gallemkamp apparatus and are uncorrected. IR spectra were recorded on a Perkin- Elmer 1600 FT-IR spectrophotometer. ~H NMR spectra were recorded on a Bruker AW-80 spectrometer, ~3C and 298i NMR spectra on a Bruker WP 250 SY and FT AM 300 apparatus. Solvents and chemical shifts (3 relative to Me4Si) are indicated. Elemental analyses were carried out by the Service Central de Microanalyse du CNRS.
2,5-Bis(ttimethylsilyl)thiophene (1) To a mixture of 19.7 g (0.18 mol) of chlorotrime-
thylsilane and 10 g (0.41 tool) of magnesium chips in 50 ml of THF was added dropwise about 1 ml of a solution containing 20 g (0.083 mol) of 2,5-dibromo- thiophene in 50 mt of THF. The mixture was stirred until a mild exothermic reaction occurred. It was then cooled to 0 °C and the rest of the dibromide solution was added dropwise. The mixture was then stirred at room temperature for 15 h. Hydrolysis with diluted aqueous HC1 solution, extraction with ether, drying over MgSO4 and elimination of the solvents in vacuo gave the crude silylthiophene 1. Distillation then af- forded 12.16 g (65% yield) of 2,5-bis(trimethylsilyl)- thiophene, the characteristics of which are identical to those reported in the literature [7]: b.p. 109 °C/0.35 mmHg; z9si NMR (CDC13, 3, ppm): -6.75.
5, 5 '-Bis ( trimethylsilyl ) b ithiophene (2) To 7.5 g (0.045 mol) of bithiophene dissolved in 100
ml of hexane at room temperature was added dropwise 37 ml of n-BuLi (2.6 M solution in hexane). The mixture was then stirred at 60 °C for 1 h and cooled to 0 °C. Then 12 g of Me3SiCI (0.011 mol) in 20 ml of hexane
was added dropwise, the mixture allowed to warm to room temperature and stirred for 15 h. After hydrolysis, as described for compound 1, the above extraction procedure yielded the crude product 2. It was purified by chromatography on a silica gel column using pentane as eluent. Recrystallization in hexane afforded 11.2 g (80% yield) of compound 2: m.p. 91 °C. aH NMR (CC14, ~, ppm): 0.17 (18H, s), 6.97 (4H, m). 13C NMR (CDC13, 6, ppm): -0.10, 125.08, 134.72, 139.78, 142.48. 29Si NMR (CDCI3 ~, ppm): -6.56. Anal. Calc. for C14H22S2Siz: C, 54.13; H, 7.14; S, 20.65; Si, 18.08. Found: C, 54.24; H, 7.18; S, 20.36; Si, 17.45%.
5,5"-Bis(trimethylsilyl)terthiophene (3) This compound was obtained as described above for
compound 2, from 5 g (0.02 tool) of terthiophene, 20 ml of n-BuLi (2.5 M solution in hexane) and 6.5 g (0.06 mol) of chlorotrimethylsilane. Green crystals (6.3 g, 80% yield) were collected after purification: m.p. 112 °C. 1H NMR (CC14, ~5, ppm): 0.13 (18H, s), 6.80-7.00 (6H, m). 13C NMR (CDC13, 6, ppm): -6.44, 124.41, 124.90, 134.78, 136.29, 139.97, 142.19.29Si NMR (CDC13, 6, ppm): -6.44. Anal. Calc. for C18H24S3Si2: C, 55.05; H, 6.16; S, 24.49; Si, 14.30. Found: C, 55.11; H, 5.99; S, 25.03; Si, 14.18%.
2, 4-Bis(trimethylsilyl)thiophene (4) This compound was prepared as described for 1,
using 12.1 g (0.05 tool) of 2,4-dibromothiophene, 11.0 g (0.11 tool) of chlorotrimethylsilane and 6 g (0.25 mol) of magnesium. After work-up, distillation of the residue afforded 6.8 g (60% yield) of compound 4: b.p. 90 °C/ 10 mmHg. IH NMR (CC14, 6, ppm): 0.28 (9H, s), 0.33 (9H, s), 7.24 (1H, s), 7.61 (1H, s). '3C NMR (CDCI3, 5, ppm): -0.06, 0.67, 137.31, 138.99, 141.05, 142.68. 298i NMR (CDC13, 5, ppm): -6.72, -8.31. Anal. Calc. for C10H2oSSi2: C, 52.63; H, 8.77. Found: C, 52.67; H, 8.38%.
5-Deutero,2, 4-bis(trimethylsilyl)thiophene (4(D) ) To a solution of 4.56 g (0.02 mol) of 4 in 20 ml of
THF was added dropwise, at room temperature, 10 ml (0.025 mol) of n-BuLi (2.5 M solution in hexane). After the addition was complete, the solution was stirred for 1 h at 50 °C. The reaction mixture was then cooled to 0 °C and 1 g (0.03 mol) of CH3OD was added slowly. The solution was then allowed to warm to room temperature and was stirred for 2 h. After evaporation of the solvent, the residue was extracted with pentane. Elimination of pentane and distillation of the residue afforded 3.46 g (76% yield) of pure compound 4(D): b.p. 86-90 °C/10 mmHg. IR ~/C-D: 2200 cm-1; T C-D: 569 cm -~. 1H NMR (CC14, 6, ppm): 0.24 (3H, s), 0.29 (3H, s), 7.17 (1H, s). 13C NMR (CDC13, ~, ppm): -0.08, 0.51, 137.35, 138.98, 140.94, 142.72. 298i NMR (CDCI3,
3-(Trimethylsilyl)thiophene (5) Isopropyl Grignard reagent was first prepared by
adding dropwise a solution containing 3.92 g (0.05 mol) of isopropyl chloride in 20 ml of THF over 3.4 g of magnesium chips (0.17 mol) in 40 ml of THF. After stirring for 3 h at room temperature the mixture was cooled to - 3 0 °C and 16.3 g of 3-bromothiophene (0.1 mol) was added. The solution was stirred at -30 °C for 2 h and 16 g of trimethylchlorosilane (0.15 mol) in 20 ml of THF was then added at -40 °C. After warming to room temperature and stirring for 15 h, the reaction mixture was hydrolysed and extracted as above. Distillation afforded 10.06 g (70% yield) of compound 5: b.p. 145-148 °C/760 mmHg. The spec- troscopic characteristics were identical to those de- scribed [7].
Electrochemical polymerizations The electrochemical synthesis was performed gal-
vanostatically (2 mA cm -2) at T=5 °C in a one- compartment cell using an EG&G Model 363 poten- tiostat under computer control. The working and counter electrodes were platinum (Pt) sheets (2× 2.5 mm 2) at a distance of 18 mm and the anodic potential was measured versus an Ag/AgNO3 (10 -2 M) electrode. The Pt electrodes were carefully polished with A1203 before each electrochemical synthesis. Oxygen was ex- cluded from the electrochemical cell by using a nitrogen flow. The electrolytic medium was prepared by dissolving the monomer (0.1 tool 1-1) in a solution of nitrobenzene containing N(Et)4BF4 (0.02 tool 1-I). The doped film obtained according to this procedure was initially elec- trochemically reduced at 0.0 V (versus Ag/AgNO3 (10 z M) electrode) until the current reached a negligible value and then fully undoped by immersion in methanol for 48 h. As monitored by infrared spectroscopy, a negligible amount of dopant and no chemical degra- dation were observed after this treatment.
Raman and FT-1R experiments Raman spectra were recorded on a Coderg T 800
triple monochromator spectrometer. The 514.5 nm (2.41 eV) wavelength of an argon ion laser was used as the light source. In order to avoid local heating and deg- radation of the samples the incident light power was kept below 10 mW and the incident beam was defocused. A back-scattering configuration was used and the in- strumental resolution was about 6 cm -1. In order to obtain the best resolution for the Raman and pho- toluminescence spectra, all the experiments were per- formed at low temperature (T= 10 K), and the samples
were mounted in a cryostat and cooled by cold helium gas. The photoluminescence and Raman spectra were recorded under the same experimental conditions. The sample volumes excited by the laser beam were identical in the two experiments. This is of importance if one wants to correlate unambiguously Raman and photo- luminescence data recorded from non-homogeneous samples, such as polythiophene films synthesized by electrochemical polymerization.
IR spectra were recorded using a Bomem DA-8 spectrometer. The free-standing films were analysed in transmission. Powders were examined as KBr pellets.
EDAX measurements Energy dispersive X-ray spectroscopy (EDAX) mea-
surements were carried out using a Cambridge 515 SEM with a PV 9800 EDAX attachment. The data were acquired by a standard analytical procedure and corrected for ZAF. We checked that the Si/S ratios measured according to this technique were in agreement with those obtained upon elemental analysis of the same sample.
Characterization of PT films by Raman spectroscopy and photoluminescence
The sample dependence of the Raman spectrum of polythiophene was studied by several authors [11-14]. A few years ago, from a detailed analysis of the sample dependence of the Raman and photoluminescence spec- tra of several polythiophene films, we completed the previous analysis [11-14] by establishing 'qualitative' and 'quantitative' criteria in order to describe the 'quality' of polythiophene films, (quality being defined in terms of conjugation length and amount of defects) [15-17]. The polythiophene films prepared by electro- chemical oxidation of silylated monomers were char- acterized by Raman and photoluminescence spectros- copy. However, before giving a detailed analysis of our results, we summarize the defined criteria.
'Raman' criteria Figure 1 shows the Raman spectrum obtained from
a highly conjugated film of polythiophene at T= 10 K. The changes of the Raman spectra with conjugation length Nc can be summarized as follows:
(a) In the wave-number region 600-1400 cm-1, the Raman spectrum exhibits three strong lines at 700, 1047 and 1222 cm-~ assigned, respectively, to the ring symmetric bending vibrational mode, the in-plane C-H bending mode and the C-C inter-ring stretching mode. The intensity of other lines at 740, 790 and 1000 cm-1 significantly increases with conjugation length. Con- versely, the intensities of the Raman lines at 652, 682, 723 and 1155 cm-~ decrease more or less strongly when
236
A
v
, i
t~ t -
r - e
r -
E re
(a)
I I I I
6 0 0 8 0 0 1 0 0 0 1 2 0 0 1 4 0 0
Raman Shift (cm ~) 1 6 0 0
"-" =." EL= 2.41 eV , ~ .
. t \ / \ 2 u) C 1800 IS00
( ) Av l cm -I
.J O.
1.5 1.75 2.0 (b) ( E / eV ) Fig. 1. Polythiophene film electrochemically prepared from the H - ( C 4 H 2 S ) 2 - H m o n o m e r : (a) Raman spectrum; (b) photoluminescence.
the conjugation length increases. These lines were as- signed to vibrational motions in a distorted conformation of the chain [11, 12, 15-17]. In agreement with Furukawa and co-workers [11, 12], we observed a strong correlation between the value of the conjugation length Arc and the ratio R of the intensities of the Raman lines pointed at 682 and 700 cm- 1. R =1682/17o o. This parameter relates the value of the conjugation length to the distortion of the chain and gives an evaluation of the amount of coplanar segments along the chain [11, 12, 16, 17]. It follows that all results will be plotted as a function of R in order to describe the behaviour of the Raman and photoluminescence spectra with the value of the conjugation length. According to this picture, the poly- mer is represented as a chain of alternating planar segments (highly conjugated segments) and non-planar segments (poorly conjugated segments). This simple description proved to be useful, although some more sophisticated models emphasized the role of confor- mational disorder in determining the electronic prop-
erties of conjugated polymers [18]. In these latter models, a 'conformational conjugation length' is defined in terms of effective torsional potential between adjacent units [191.
(b) We reported a strong temperature dependence of the intensity of the 700 cm- 1 Raman line with respect to the intensity of the other mode. At T=10 K, we observed a significant R (Arc) dependence of the ratio p=I7oo/I1o47 (the 1047 cm -1 Raman line is not tem- perature dependent and was chosen as intensity internal reference): p is about 60% when R = 70% and about 140% when R = 10% (Fig. 2, symbol O).
Owing to the strong dependence of p on No, the value of O constituted another pertinent criterion for defining the conjugation length of polythiophene sam- ples.
(c) Other Raman criteria were derived from the analysis of the wave-number region 1400-1500 cm -1 When Nc increases we showed that: (i) the 1462 cm-1 C = C stretching mode narrows [16, 17]; (ii) a Raman
237
180
160 -
~- 1 4 0 - ,~r
o 1 2 0 -
o 100-
80-
60-
40
0
. . . . I . . . . I . . . . I . . . . I . . . . I . . . . . . . . I . . ,
Fig. 2. p(]7oo/Ito47) VS. R(1682/lvoo) dependence for electrochemically prepared polythiophene films (see Table 1): O, prepared from hydrogenated monomers; O, prepared from silylated monomers.
line at 1476 cm-- ~ emerges as a shoulder to the previous line.
On the other hand, the profile of the 1498 cm-i Raman line drastically changes with R: for low values of R it appears as a narrow peak; when R increases a large broadening of this line occurs. We suggested that the normal mode of the chain overlaps with a defect mode, the latter dominatingwhen the conjugation length decreases. Finally, a line at 1400 cm-1 appears in the low-temperature spectrum. It was assigned to the first harmonic of the 700 cm-~ line. The detection of this line is related to the strong intensity of the 700 cm-~ line, in other words, according to the previous discussion, to a high value of the conjugation length.
'Photoluminescence' criteria A typical photoluminescence (PL) band observed for
a film of highly conjugated polythiophene is shown in Fig. l(b) (the corresponding Raman spectrum is dis- played in Fig. l(a)). It exhibits a series of discrete peaks separated by an optic phonon frequency. From a detailed analysis of polytbiophene films prepared by electropolymerization we observed that the profile of the photoluminescence band is strongly sample de- pendent. In particular, the observed red shift of the photoluminescence band was attributed to an increase of the structural disorder (amorphization) of the sample [20]. We found that the number of peaks and also the position of each peak of the 'ideal' profile (see Fig. l(b)) are related to the average value of the conjugation length. The width of each peak was found to be connected to the distribution of conjugation lengths in the sample [15]. Finally, additional peaks (with respect to the ideal profile of the PL band) which appear in the photoluminescence spectrum have been assigned to luminescent centres due to 'defects' (extrinsic part of the photoluminescence).
R e s u l t s a n d d i s c u s s i o n
Preparation and electropolymerization of silylated thiophene monomers
Silylated thiophene derivatives were prepared upon silylation of the parent thiophene organobimetallic re- agent. Thus, the reaction of 2,5-dibromothiophene with magnesium or the reaction of bi- or terthiophene with n-butyllithium led to quantitative metallation. Silylation with Me3SiC1 then afforded the disilylated mono-, di- and terthiophenes 1-3 (Scheme 1). Electropolymeri- zations to give, respectively, the polythiophenes PT~-PT3 were then performed under galvanostatic reaction con- ditions (see Experimental). The latter allowed formation of homogeneous self-supported films. During the elec- trochemical reactions, the potential of the working electrode stabilized at 1.7, 0.76 and 0.68 V (versus Ag/ Ag + 0.01 mol l-J), respectively, for polymers PT~, PT2 and PT3. These values are close to the values of the oxidation potential of the monomers [9].
In the case of the oxidation of monomers 2 and 3, a thin film formed at the surface of the electrode. Some powder also deposited and the solution near the electrode turned blue, indicating that some soluble oligomers were also produced. From 2 and 3 no self- supported film could be detached from the support electrode (Pt or ITO). The examination of polymers PT1-PT3 using scanning electron microscopy (SEM) revealed a dense and homogeneous morphology for PT1, whereas a porous and somewhat fibrous morphology was identified for PT2 and PT3. The observed cyclic voltammograms of polymers PT1-PT 3 are similar to those reported. A blue-red electrochromism of the material was associated with its redox behaviour.
The IR absorption spectra of all samples exhibit vibration bands characteristic of polythiophenes: 3064 (UC_H), 1490, 1440 (UC=C), 780, 690 (t3C_H) cm -~. Ac- cording to the correlation proposed by Furukawa et al. [12] the relative intensities 11490/11440 and I78o/I65o are consistent with a mean conjugation length greater than six thiophene units and apparent degrees of polymer- ization of 30 (PT~), 16 (PT2) and 10 (PT3). Absorption
(x= 1; Y=Br)
Y Y n-BuLi / rHF (x=2,3; Y= H)
C1SiMe 3
M e 3 S i ~ ~ x SiMe 3 -e ~ ~ ~ n '
1, x=l P T 1, x=l 2, x=2 P T 2, x=2 3, x=3 PT 3, x=3
Scheme 1. Preparation and electropolymerization of a-silylated thiophene monomers 1-3.
238
bands, with very weak intensities, characteristic of the presence of a few Si(CH3)3 groups (2950-2800, 1250, 840 and 750 cm -I) were also observed (Fig. 3(a)). This was further confirmed by X-ray fluorescence spectros- copy (EDAX). Values between 0.04 and 0.06 were determined for the Si/S ratio.
The preparation and polymerization of silylated mon- omers with silyl substituents at the /3-position of the thiophene ring were also achieved. In the same way as for the above a-substituted derivative 1, monomer 4 was obtained from 2,4-dibromothiophene in 60% yield upon silylation with Me3SiC1/Mg (Scheme 2). The re- action of 4 with n-BuLi in THF allowed quantitative metallation at the 5-position of the thiophene ring and subsequent deuterolysis with D20 gave the deuterated monomer 4(D) isolated in a 76% yield. We also ex- amined, for comparison, the polymerization of the 3- substituted derivative 5 [7]. Monomer 5 was obtained
B r
I CISiMe Mg/THF
SiMe3
M e 3 S i ~ s ~
4 [ 1) BuLi/THF
2) D20
M e 3 S i ~ Me3
4 (D)
-e v
n Me3)
PT4
-e- r
M o 3 ,
PT 4 (D)
Scheme 2. Preparation and electropolymerization of fl-silylated thiophene monomers.
0 . 4 1 ' I ' I ' I ' I
0 . 3 4
g
t l . l 0.27 !
o
<
O . l g
0 . 0 5 I i i i
4 0 0 . 1200 . 2 0 0 0 . 2BOO. 3 6 0 0 . CM-%
(a)
i o 0 8 J
0 . 8 3
m . <
o . 3 3
[ I I
4o0. xzdo. 2odo. 2sdo. 36d0. :~-~
(b) • : Si(Me) 3 absorptaons
Fig. 3. IR spectra of (a) PT 2 and (b) PT 5 (4000--400 cm-1).
Br M g C I (CH3)2CHMgC1 CISiMe 3 / THF
THF
SiMe3 (SiMe3)
e
5. 70% pT 5
Scheme 3. Preparation and electropolymerization of 3-tri- methylsilylthiophene 5.
in 70% yield via the formation and silylation of a Grignard intermediate (Scheme 3).
Monomers 4, 4(D) and 5 were then polymerized by electrochemical oxidation under galvanostatic condi- tions. The working electrode was polarized at potentials around 1.5 and 1.8 V (versus Ag/Ag +, 0.01 mol 1-]), respectively, for monomers 5 and 4. In all cases a homogeneous dense film deposited on the electrode without formation of significant amounts of soluble oligomers. Cyclic voltammetry shows electroactive be- haviour with well-resolved oxidation and reduction peaks (Ep"=0.9 V, EpC=0.7 V) and with blue-red electro- chromism of the material (Fig. 4).
The PT4 and PTs films were analysed using IR absorption spectroscopy. The IR spectra exhibit the expected bands for polythiophene. However, the ob- served intensities 1490 (strong) and 1440 (weak) are consistent with poorly conjugated polymers. In addition, the PT4 and PT 5 polymer samples show strong absorption bands which can be assigned to (CH3)3Si groups. Char- acteristic vibrations are clearly observed at 630, 750, 840, 1250, 2897 and 2956 cm -] (Fig. 3(b)). The polythiophenes obtained from monomers 4 and 5 thus contained numerous non-desilylated thiophene units in the polymer chain. IR spectroscopy does not allow a
i ( mA/cm 2)
,0,1 I ~ V / I -
Fig. 4. Successive cyclic vol tammograms of 3-(tr imethyl- s i lyl) thiophene 5.
ready quantitative estimate of the number of remaining silylated units in the 3200-2800 cm -1 region. It is, however, worth comparing the relative intensities of the VC-H absorption (UCsp2-H > 3000 cm- ~ versus VCs,3_H<3000 cm -1) both in the monomer and in the polymer. The ratio of the relative intensities in monomer 4 (0.14) is close to the ratio measured in the IR spectrum of polymer PT, (0.11). In the case of the monosilylated thiophene 5, the ratio varies from 0.1 in monomer 5 to 0.25 in polymer PT 5 indicating that some cleavage of Si-C bonds occurs during the polymerization reaction (Fig. 5).
A quantitative estimate of the extent of desilylation was obtained from X-ray fluorescence spectroscopy (EDAX). The values of the Si/S ratio, measured for polymers PT4 and PT5 are 0.7 and 0.5, respectively. It appears that 50 to 70% of the silyl substituents were retained during polymerization. Extensive desilylation was previously observed for related thiophene deriv- atives [7]. The partial desilylation in polymers PT4 and PT5 does not seem to be associated with the formation of 2,4-linkages in the polymer chain. The IR absorption spectra of both polymers do not show a Uc_H vibration at 3100 cm -1, characteristic of the presence of a hy- drogen atom on the a-carbon of the thiophene ring. This was further confirmed by an examination of the IR spectrum of PT4(D) obtained by electropolymeri- zation of the deuterated monomer 4(D). No absorption associated with the vc_o vibration (2200 cm -1) was observed. This, together with the absence of absorption at 569 cm-1 due to the 7c-D vibration, is consistent with extensive electropolymerization through the 2,5- positions for monomers 4 and 5.
Raman and photoluminescence characterization of PT obtained from monomers 1-3
According to the criteria previously defined (see Experimental), we attempted to characterize the
.s499 ~GNIEM, ~MICHELSON SERIES
.ao7g I
239
.2660-
,12tC
-.1600 ' ' ' l l 3ZOO. 3100. 3000.
(a)
o.oogc 3GHEN ~ICHELSON SERIES
0.0070-
O.OOSO-
O.OO3O-
0 . 0 0 1 0
-O.OOtO
3200. 3100. 3000.
(b)
I ' I I 2900. 2800 2700. CX-I
\
, I I ~ r 2gO0. 2800. 2700. CM-I
Fig. 5. IR spect ra of (a) m o n o m e r 5 and (b) PT s.
polythiophene films PT,, PT2 and PT3 prepared by electrochemical oxidation of silylated monomers 1, 2 and 3. Their Rarnan spectra are displayed in Fig. 6 and analysed as follows:
(a) The spectrum in the 600-720 cm-1 wave-number region was fitted to a Lorentzian profile for each Raman line. R values were obtained with good accuracy (see Table 1). As previously discussed, a decrease of R value means an increase of the conjugation length (increased amount of coplanar segments). With respect to the R criterion, the film synthesized from monomer 2 (PT2) appears as the more conjugated polythiophene of this series of samples.
(b) The observed p values for the various polythio- phene films are given in Table 1. According to this criterion, the PT2 film also appears as the best poly- thiophene of this series. The dependence of p on R is reported in Fig. 2 (symbol @). This plot is consistent with that for p versus R observed for polythiophene films synthesized by electropolymerization of hydro- genated monomers.
Comparison of the Raman characteristics of polymer films prepared from silylated versus hydrogenated thio- phene monomers allows us to deduce that the use of
240
A . . . . I .... I .... I .... I .... 1
iv"
600 800 1000 1200 1400 1600
R a m a n s h i f t ( c m "1) (a)
=I
e.
e=
E CC
(b)
. . . . I . . . . I . . . . I . . . . I . . . .
4
t 1
600 800 1000 1200 1400 1600
R a m a n s h i f t ( c m "1)
. . . . I . . . . I . . . . I . . . . I . . . . t
m e,,
= e,- @1
' ' ' 1 . . . . I . . . . I . . . . I . . . .
600 800 1000 1200 1400 1600
(c) R a m a n shift (cm "1) Fig. 6. R a m a n spec t ra of e lec t rochemical ly p repa red films ob ta ined from silylated m o n o m e r s : (a) l r r j ; (b) PT2; (c) PT 3.
silylated thiophene monomers results in an increase in the mean conjugation length in the polythiophene chain.
This is also supported by the analysis of the Raman spectra in the 1350-1550 cm -] wave-number region:
T A B L E 1. R and p va lues for po ly th iophenes P T r P T 4 electro- chemical ly p repared f rom silylated m o n o m e r s (O, in Fig. 2)
Fi lm R p
PTI 40 75 PT 2 10 160 PT 3 20 140 PT 4 71 55
(a) As expected, a narrowing of the 1458 cm - 1 Raman line is observed when R decreases. The dependence of this linewidth on R is in agreement with the observed Raman behaviour of polythiophene films synthesized from non-silylated monomers (Fig. 7). In addition, a slight shift of this line with increasing R is shown; this shift is usually assigned to a decrease in the conjugation length [16].
(b) In connection with the narrowing of the 1458 cm -1 line, the 1476 cm -] Raman line clearly emerges as a shoulder to the previous one for the PT2 and PT3 samples.
(c) As expected, the shape and position of the maximum of the band around 1500 cm -1 are sample dependent. However, a precise evaluation of the pa- rameters of this band is not easy, especially for the 'poorly' conjugated samples, because this low intensity line appears as a shoulder to the photoluminescence band. For the sample with the highest conjugated length (PT2 film), a well-defined peak appears at 1502 cm-].
(d) Finally, a well-defined peak at 1401 cm -] appears in the PT2 and PT 3 samples, i.e., for samples in which the intensity of the 700 cm-~ Raman line is strong.
The photoluminescence spectra measured for PT], PT2 and PT3 films are displayed in Fig. 8. Similar conclusions about the 'quality' of the films can be reached, even more clearly, from the analysis of the photoluminescence band.
As expected from the Raman results, the PL band measured on the PT2 film shows three narrow and well- defined peaks with an energy separation of 0.175 eV. Other additional peaks (fine structure) appear, their origin still being under discussion. However, the same PL profile has been measured for different highly conjugated samples. The analysis of the PL changes will be analysed in detail elsewhere [21]. We report here the characterization of PT], PT2 and PT3 films according to the photoluminescence criteria: (a) the intensity of the PL normalized with respect to the intensity of the 1462 cm-~ Raman line; (b) the relative intensity of the three peaks; (c) the energy of the first (zero-phonon line) and second (one-phonon line) peaks.
The PL profiles measured on the PT~, PT2 and PT3 samples are close to the 'ideal' structure [22], so the PL parameters can be compared:
241
A - t
in e -
e -
r -
E t ~
( a )
--!
¢) C
e'-
t , -
E ca 12:
(b)
t- O _= e=
E t~ n"
. . . . I . . . . I . . . . I . . . . I . . . .
+
+
+
+
+: +
+~++ +
+ +
+ +
+ \
t . . . . I . . . . I . . . . : . . . . : . . . . :
Fig . 7. E x p a n s i o n o f t h e 1 3 5 0 - 1 5 5 0 c m -~ f r e q u e n c y r a n g e o f t h e
R a m a n s p e c t r a o f e l e c t r o c h e m i c a l l y p r e p a r e d p o l y t h i o p h e n e f i lms:
( a ) P T , ; (b ) PT2; (c) PT3.
(a) The PL intensity (with respect to the 1462 c m - R a m a n line intensity) is s t ronger for the PT2 and PT 3 samples than for the PT~ film (Fig. 8).
(b) On the o the r hand, a significant change m the relative intensity o f the th ree peaks occurs for PT,; in
A
¢) e"
. d O.
(a)
=.
t -
--I IX
(b)
A - I
t,.,
1 .2
I I I 1 1
1 . 4 1 . 6 1 . 8 2 2 . 2
P h o t o n E n e r g y ( e V ) 2 . 4
1.2 1 . 4 1 . 6 1 . 8 2
P h o t o n E n e r g y ( e V )
t .
2.2 2 .4
o e- r a
--I a .
1 .2
I l
/ 1.4 1.6 1.8 2 2 .2 2 . 4
(c) P h o t o n E n e r g y ( e V )
Fig . 8. P h o t o l u m i n e s c e n c e b a n d s o f e l e c t r o c h e m i c a l l y p r e p a r e d
p o l y t h i o p h e n e f i lms: ( a ) PT1; (b ) PT2; (c) P T 3.
the la t ter one- and two-phonon lines b e c o m e dominan t with respect to the first peak (ze ro -phonon line).
(c) Fol lowing the progression, PT2, PT3 and PT~, the energy of the peaks shows a blue shift; at the same t ime a large b roaden ing of the peaks occurs. The energy
2 4 2
of the zero-phonon line (and one-phonon line) as a function of R is plotted in Fig. 9. We see a strong correlation between these two parameters. As expected, the lowest energy is observed for PTz, i.e., the sample with the highest mean conjugation length (lowest R value).
Characterization of PT films obtained from monomers 4 and 5
The Raman spectrum of PT4 film is shown in Fig. 10. The p/R value measured for the PT 4 film indicates a low conjugation length together with a large number of distorted segments in this sample (Fig. 2). On the other hand, no Raman spectrum from PTs film could be measured because a strong photoluminescence struc- ture masks the Raman signal. As discussed below (see photoluminescence results), this feature indicates low conjugation length in the PTs film.
For the PT4 and PTs samples, the PL profiles (Fig. 11) are quite different from those observed for PT~, PT2 and PT 3 (Fig. 8). For PT 4 (Fig. ll(a)), the second peak at 1.90 eV separated by c. 0.12 eV from the first peak cannot be assigned to a phonon replica. The
2.1 >
>, 2 .06
~ 2.02 o c
- - 1 . 9 8 - c -
o c o 1.94 Q.
o 1.9
0
, , , t . . . . I . . . . I . . . . I . . . . I . . . . I . . . . I . . . .
0 • o
o
0
0 •
" ' : . . . . : . . . . F . . . . I . . . . I . . . . I . . . . I ' " 10 20 30 40 50 60 70
Fig. 11. Photolurninescence bands of (a) PT 4 and (b) PT 5.
Raman data obtained on the same film reveal a strong distortion of the chain. We attribute the 1.90 eV peak to a luminescent centre related to the distorted seg- ments.
For the PTs sample, the PL emission appears as a broad and unresolved structure (Fig. ll(b)). In con- ducting polymers (such as poly(para-phenylene) [23]), this PL profile is generally assigned to 'disorder' (defined in terms of low conjugation length, strong amorphous character of the sample and a large number of structural defects). In this type of sample, the extrinsic part of the photoluminescence dominates.
The low conjugation lengths in PT4 and Iris can be explained as follows:
(a) The presence of a Me3Si group at the B-position of the thiophene ring induces a large number of non- planar segments, due to the large steric hindrance between neighbouring thiophene units, which gives rise to a decrease of conjugation length.
(b) In addition, the presence of a Me3Si group at the B-position leads to competition between the a-/3 and a-a polymerization processes.
Mechanistic considerations Polythiophenes PT~-PT3 arising from the polymer-
ization of monomers 1-3 with Me3Si substituents at the 2- and 5-positions appear highly structured. In particular, on the basis of Raman and photolumines- cence studies, the polymers show high mean conjugation length and very low number of defects. Conversely, polythiophenes PT4 and PT 5 arising from monomers 4 and 5 with Me3Si substituents at the 3- or 4-positions of the thiophene ring exhibit low mean conjugation lengths. The latter may result from the presence of some 2,4-linkages in the polymer chains. It can also be attributed to non-polar segments owing to the pres- ence of some bulky Me3Si groups which remain in the polymer.
In agreement with previous reports [6], the presence of a-silyl substituents activates the oxidative coupling of the thiophene monomers. It appeared that the pres- ence of Me3Si groups increased the selectivity of the coupling reaction leading to polythiophene linked 2,5 throughout. The role of the Me3Si substituent may be interpreted in terms of an increased stabilization of an intermediate /3-silyl cationic species. A mechanism, shown in Scheme 4, similar to the one originally proposed for the electropolymerization of thiophene [1, 2, 4, 24], can account for the results obtained.
The first step involves oxidation of the monomer to its radical cation. C-C bond formation then occurs by coupling of two radicals owing to their high concen- tration which is maintained near the electrode surface. The radical coupling leads to a dimeric dication. The latter is stabilized by hyperconjugative interaction be- tween the electron deficient 7r molecular orbital of the thiophene ring and the filled ~r molecular orbital of the Si-C bond, as has been proposed to explain the
_ ~ M e 3 S i ~ S i M e 3
Me3S i SiMe 3 ~ Me3Si,.,..~S,,~,~SiMe 3 lb. Oxidation Coupling of
radical cations
Me3S i ~ . ~ 7 ~ - " " ~ " - SiMe3 SiMe 3
Et4N*BF4
-2 Me3SiF ~ Me3Si SiMe3
Me3Si SiMe3~
M e 3 S i ~ - ~ SiMe3 1P.
1{+~ I/ +~ ] ] / ~ - - ~ e 3 S i ~ ~ t -2 Me3SiF ~ n n • M e3Si " ~ _ , , / ] ' " ~ , ' J , , ~ " ~ - ~ " S i Me 3 ~ M e3Si S iM e 3 5 I "S -S Polymer
SiMe3 growth
243
well-known stabilization of carbocations /3 to a silyl substituent [5]. Evidence for the stabilization of the related cationic intermediate has been given in silicon- directed electrophilic aromatic substitution [25, 26]. The stabilization of the dimeric dication which is prob- ably formed in the rate-determining step [4, 27] is thought to be responsible for the observed activation and increased selectivity in the 2,5-coupling reaction. The next step is the re-aromatization which occurs with cleavage of the Si-C bond. Since the elimination of a Me3Si + silylcation [28] is highly unfavourable, the Si-C bond cleavage is more likely to occur via nucleophilic attack at the silicon. Reaction with the large excess of supporting electrolyte (Et4N + BF4-) might lead to the formation of a thermodynamically highly stable Si-F bond. However, no evidence for the formation of Me3SiF and BF3 nor for the cathodic reduction of Et4N + was obtained. The cleavage of the Si-C bond via nucleophilic attack at the silicon is, nevertheless, supported by the observation of SiO4 units produced upon oxidation of related thiophenylene-bridged silsesquioxanes [10c] (eqn. (1)). Electropolymerization then proceeds through oxidation and coupling of the bithiophene intermediate.
[ O l ' 5 S i ~ S i O l ' 5 ] X ~- Si02
.Ix
(1)
Besides the mechanism presented in Scheme 4, which consists of a series of radical coupling reactions, one cannot exclude a different mechanism [29]. The elec- trochemical polymerization of thiophenes was recently proposed to occur via the reaction of radical cation species with neutral thiophene monomers [30]. The initially formed thiophene radical cation was suggested to undergo an electrophilic aromatic substitution re- action with a thiophene molecule followed by oxidation and deprotonation. As shown in Scheme 5, a directing role similar to that in Scheme 4 may be assigned to a silyl substituent which would provide stabilization for a related /3-silyl cationic intermediate.
M e 3 S i ~ s ~ S i M e 3
-e- ] r ~ Me3Si~s~SiMe3
Me3 Si --"~ S ..~,,'~- S iMe 3 Oxidation Electrophilic Substitution
e
Me3 S i ~ S f ] ' ' ~ ' ¢ ~ ' ~ SiMe30xidatio~ Me 3Si ~ / ~ - " ' ~ ' f f j / '~ SiMe3 SiMe 3 SiMe 3
Scheme 4. Electrochemical polymerization via coupling of radical Scheme 5. Electrochemical polymerization via electrophilic sub- cations, stitution of the neutral monomer by a radical cation.
244
According to Scheme 4 or 5, the silicon plays a directing role in the electrocoupling reaction. Similarly to electrophilic aromatic substitution [25, 26], silicon exhibits an ipso-directing effect. Both the silicon and sulfur atoms thus favour 2,5-coupling in the electro- polymerization of monomers 1-3. In the case of the polymerization of 4 and 5, with a silyl substituent at the 3- or 4-position of the thiophene ring, the ipso- directing effect of silicon should favour some 2,4-cou- pling of the thiophene ring and leads to conjugation defects in the polymer. However, a large number of silyl substituents remained in the polymers PT4 and PTs, and very few deuterium substituents were left in the electropolymerization of 4(D). We can conclude that the sulfur a-directing effect in the coupling is, thus, more important than the ipso-directing effect of the silyl substituent. This is, however, consistent with the formation of polythiophene with low mean con- jugation length in this case.
Conclusions
Polythiophene samples have been prepared from various trimethylsilyl-substituted mono-, bi- or terthio- phenes. The analysis of Raman scattering and pho- toluminescence spectra appears to be a powerful tech- nique in characterizing the structural and electronic properties of the polymeric material. The results ob- tained are consistent with a previous report based on electrochemical studies [6]. The electropolymerization of silylated thiophene monomers can lead to polythio- phenes with higher or lower conjugation properties according to the position of the silyl substituent. The directing effects of silyl groups, widely used for C-C bond formation in organic synthesis, are also of great interest for increasing the selectivity in polymerization reactions. We are currently investigating the electro- chemical and chemical oxidative polymerization of var- ious silyl-substituted aromatic or heteroaromatic mon- omers.
References
1 T.A. Skotheim (ed.),Handbook of Conducting Polymers, Marcel Dekker, New York, 1986, and refs. therein.
2 (a) A.F. Diaz, Chem. Scr., 17 (1981) 142; (b) G. Tourillon and F. Garnier, J. ElectroanaL Chem., 135 (1982) 173; (e) R.J. Waltman and J. Bargon, Can. J. Chem., 64 (1986) 76; (d) S. Hotta, Synth. Met., 22 (1987) 103; (e) A.F. Diaz and J.C. Lacroix, New J. Chem., 12 (1988) 171; (f) J. Heinze, Synth. Met., 41-43 (1991) 2805; (g) C.W. Spangler and K.O. Havelka, New J. Chem., 15 (1991) 125.
3 A.O. Patil, A.J. Heeger and F. Wudl, Chem. Rev., 88 (1988) 183, and refs. therein.
4 J. Roncali, Chem. Rev., 92 (1992) 711, and refs. therein.
5 (a) E.W. Colvin, Silicon in Organic Synthesis, Butterworth, Guildford, UK, 1981; (b) W.P. Weber, Silicon Reagents for Organic Synthesis, Springer, Berlin, 1983, and refs. therein.
6 (a) M. Lemaire, W. Buchner, R. Garreau, H.A. Hoa, A. Guy and J. Roncali, J. ElectroanaL Chem., 281 (1990) 293; (b) J. Roncali, A. Guy, M. Lemaire, R. Garreau and H.A. Hoa, J. Electroanal. Chem., 312 (1991) 277.
7 S.K. Ritter and R.E. Noftle, Chem. Mater., 4 (1992) 872. 8 H. Matsuda, 5(. Taniki and K. Kaeriyama, J. Polym. Sci., Part
A, 30 (1992) 1667. 9 J. Guay, A. Diaz, R. Wu, J.M. Tour and L.H. Dao, Chem.
Mater., 4 (1992) 254. 10 (a) P. Chicart, R.J.P. Corriu, J.J.E. Moreau, F. Garnier and
A. Yassar, Chem. Mater., 3 (1991) 8; (b) J.L. Brefort, R.J.P. Corriu, P. Gerbier, C. Guerin, B. Henner and A. Jean, Organometallics, 11 (1992) 2500; (c) R.J.P. Corriu, J.J.E. Moreau, P. Th6pot, M. Wong Cbi Man, C. Chorro, J.-P. I_kre-Porte and J.-L. Sauvajol, to be published.
11 M. Akimoto, Y. Furukawa, H. Takeuchi, I. Harada, Y. Soma and M. Soma, Synth. Met., 15 (1986) 353.
12 Y. Furukawa, M. Akimoto and I. Harada, Synth. Met., 18 (1987) 151.
13 Y. Soma, M. Soma, Y. Furukawa and I. Harada, Clays Clay Miner, 35 (1987) 53.
14 G. Louarn, J.Y. Mevellec, J.P. Buisson and S. Lefrant, J. Chim. Phys., 89 (1992) 987.
15 J.-L. Sauvajol, D. Chenouni, J.-P. L6re-Porte, C. Chorro, B. Moukala and J. Petrissans, Synth. Met., 38 (1990) 1.
16 G. Poussigue, C. Benoit, J . -k Sauvajol, J.-P. L6re-Porte and C. Chorro, J. Phys.: Condens. Matter, 3 (1991) 8803.
17 J.-L. Sauvajol, G. Poussigue, C. Benoit, J.-L. L~re-Porte and C. Chorro, Synth. Met., 41-43 (1991) 1237.
18 J.L. Fave and M. Schott, J. Chim. Phys., 89 (1992) 931. 19 G. Rossi, R.R. Chance and R. Silbey, J. Chem. Phys., 90
(1989) 7594. 20 B.C. Hess, J. Shinar, Q.X. Ni, Z. Vardeny and F. Wudl,
23 E. Rzepka, C. Jin, S. Lefrant, Y. Pelous, G. Froyer and A. Siove, Synth. Met., 29 (1989) E23.
24 (a) E.M. Geni6s, G. Bidan and A.F. Diaz, J. Electroanal. Chem., 149 (1983) 101; (b) A.F. Diaz, J. Crowley, J. Bargon, G.P. Gardini and J.B. Torrance, J. Electroanal. Chem., 121 (1981) 335; (c) R.J. Waltman, J. Bargon and A.F. Diaz, J. Phys. Chem., 87 (1983) 1459.
25 (a) C. Eaborn, PureAppl. Chem., 19 (1969) 375; (b) C. Eaborn, J. Organomet. Chem., 100 (1975)43.
26 (a) J.P. Dunogu6s, Chem. Techn., 12 (1982) 373; (b) J. Dunogu6s, Ann. Chim. (Paris), 8 (1983) 135, and refs. therein.
27 (a) E. Geni~s, G. Bidan and A.F. Diaz, J. Electroanal. Chem., 149 (1983) 113; (b) C.P. Andrieux, P. Audebert, P. Hapiot and J.M. Sav6ant, J. Am. Chem. Soc., 112 (1990) 2439.
28 (a) R.J.P. Corriu and M. Henner-Leard,J. Organomet. Chem., 74 (1974) 1; (b) R.J.P. Corriu, C. Guerin and J.J.E. Moreau, Top. Stereochem., 15 (1984) 43.
29 (a) T. Inoue and T. Yamase, Bull. Chem. Soc. Jpn., 56 (1983) 985; (b) S. Asavapiriyanont, G.K. Chandler, G.A. Guna- wardena and D. Pletcher, J. Electroanal. Chem., 177 (1984) 229.
30 Y. Wei, C.-C. Chan, J. Tian, G.-W. Jang and K.F. Hsueh, Chem. Mater., 3 (1991) 888.
