Chem. Mater. 1993,5, 430-436
430
Thiophene Oligomers as Polythiophene Models. 1. Anodic Coupling of Thiophene Oligomers to Dimers: A Kinetic Investigation Gianni Zotti' and Gilbert0 Schiavon Istituto di Polarografia ed Elettrochimica Preparativa, Consiglio Nazionale delle Ricerche, c.0 Stati Uniti 4, 35020 Padova, Italy
Anna Berlin* and Giorgio Pagani Dipartimento di Chimica Organica e Industriale dell'llniversith e Centro CNR, Speciali, Sintesi Organiche, via C. Golgi, 19, 20133 Milano, Italy Received July 24, 1992. Revised Manuscript Received January 18, 1993
The kinetics of anodic coupling to dimers of thiophene oligomers ( n = 3-5), methyl protected a t one a-terminal position, has been investigated in 1,2-dichloroethane by cyclic voltammetry and chronoamperometry. The process, second-order in oligomer concentration, is characterized by relatively high activation enthalpies (25/70 k J mol-l) and negative activation entropies (-loo/ -4 J K-l mol-'). Though activation free energies are linearly related t o the inverse of the oligomer length n (the dimerization rate decreases as n is increased), the activation enthalpy and entropy for the pentameric radical cation are peculiarly high, reflecting strong electron delocalization and ring coplanarization.
Introduction Most of conducting polyconjugated polymers are obtained electrochemically by anodic coupling of the corresponding monomers. Though known in its general features,lP2the mechanism of anodic coupling of aromatics to conducting polymers has not been completely elucidated, particularly in the initial (homogeneous) steps of dimerization. Kinetic investigations of this type have been performed in the past on aromatic amines, which we may consider as polyaniline precursors, such as triphenilamine^,^^^ t ~ l u i d i n enaftilamine,6 ,~ and pyrenamine.' The results indicate as the rate-determining step the reaction of the initially formed radical cation with itself, the deprotonated radical or even the neutral monomer in every possible combination. Recent investigations on substituted pyrroles8 and anilines?JOperformed with the use of suitable proton scavengers, have demonstrated that coupling proceeds according to a second-order rate law directly between initially formed cation radicals, ruling out both the intervention of the parent monomer and the previous deprotonation of the radical cation. Instead, kinetic investigations on oligomeric series are lacking in spite of their importance in explaining the polymerization pro-
cesses in which dimers are further oxidized to radical cations and consequently sustain coupling. The major problem in these investigations is right the continuation of coupling over dimerization which complicates the analysis. Since the limitation of coupling to the dimerization may be accomplished by the use of a suitable capping of the reactive positions of the monomer, we have considered the thiophene series with 3-5 units, methyl substituted at one terminal a-position where coupling preferentially occurs. This series has been preferred to the series of pyrrole, aniline, and benzene, besides the stability to the atmosphere of the starting oligomers, to avoid the proton scavenger action of the monomer occurring in pyrrole," the acid-base complications in aniline and the difficult oxidation in benzene. This investigation has been performed with cyclic voltammetry and chronoamperometry on a-methyl-substituted trimer, tetramer, and pentamer (MeT,, n = 3-5, Scheme I) in 1,Zdichloroethane. Furthermore MeT4has been compared with the cu,cu'-dimethyl-substituted tetramer (Me2T4). Experimental Section
(1) Diaz, A. F.; Bargon, J. Handbook of Conducting Polymers; Skotheim, T. A., Ed.; Marcel Dekker: New York, 1986; Vol. 1, p 85. (2) Nyberg, K. Organic Electrochemistry; Baizer, M., Ed.; Marcel Dekker: New York, 1973; p 705. (3) Nelson, R. F.; Feldberg, S. W. J.Phys. Chem. 1969, 73, 2623. (4) Creason, S.; Wheeler, J.; Nelson, R. F. J . Org. Chem. 1972, 37, 4440. (5) Yasucouchi, K.; Taniguchi, I.; Yamaguchi, H.; Tanoue, T. Bull. Chem. Soc. Jpn. 1979,52, 1573. (6) Daniele, S.; Ugo, P.; Mazzocchin, G. A.; Bontempelli, G. J. Electrounul. Chem. 1989, 267, 129. (7) Ohsaka, T., Hirabayashi, K.; Oyama, N. Bull. Chem. Soc. Jpn. 1986,59, 3423. (8) Andrieux, C. P.; Audebert, P.; Hapiot, P.; Saveant, J. M. J.Phys. Chem. 1991, 95, 10158. (9)D. Larumbe, D.; Gallardo, I.; Andrieux, C. P. J . Electrounul. Chem. 1991, 30, 4241. (10) Yang, H.; Bard. A. J. J. Electrounul. Chem. 1991, 306, 87.
Chemicals and Reagents. All melting points are uncorrected. All reactions of air- and water-sensitive materials were performed in flame-dried glassware under argon. Air- and water-sensitive solutions were transferred with double-ended needles. The solvents used in the reactions were dried by conventional methods and freshly distilled under argon. The supporting electrolyte tetrabutylammonium perchlorate (TBAP) was previously dried under vacuum at 70 "C. 2-Methylthiophene, 2,5-dibromothiophene, N-bromosuccinimide (NBS), and all other chemicals were reagent grade and used as received. (11) Zotti, G.; Schiavon, G.; Berlin, A.; Pagani, G. Electrochim. Acta 1989, 34, 881.
0897-4756/93/2805-0430$04.00/0 0 1993 American Chemical Society
Chem. Mater., Vol. 5, No. 4, 1993 431
Thiophene Oligomers as Polythiophene Models Scheme I R
(1
- 3)
MeT,:
n=l-3;
R=Me;R’=H
4
Me2T4:
n=2;
R=R’=Me
5-Me~hyl~2,2’:5’,2‘’:5’’,2’’’-quaterthiophene (2, MeT4). Compound 2 was prepared as described for 1 from 6 (0.22 g, 0.63 mmol), 2-thienylmagnesium bromide (1.26 mmol) and PdClpcdppf) (0.01 g, 0.013 mmol). After treatment with aqueous NH4C1,the reaction mixture was extracted with CHCl3. The organic phase was washed with aqueous NaHC03 and with water and finally dried, and the solvent evaporated. The residue was crystallized from CHC13/ligroin(volumeratio 1:l)to give the title compound 2 as a yellow solid (0.19 g, 87% yeild). mp 182-184 “C. Anal. Calcd for C17H12S4:C, 59.28; H, 3.48%. Found: C, 59.19; H, 3.48%. lH NMR (CDC13,300 MHz), d 2.47 (3 H, br s, M e ) , 6.66 (1H, br d,J4*3 = 3.7 Hz, H4),6.95 (1H, d, =,3.7 Hz, H3),6.98 (1H, d, P ‘ s 4 ’ = 3.7 Hz, H3’), 7.00 (1H, dd, J4”‘s3= 3.7 Hz, = 5.1 Hz, H4”’),7.03-7.35 (3 H, m, H4‘,H3”, H4“),7.16 (1H, dd, J3“‘*4”‘ = 3.7 Hz, P”‘35”’ = 1.2 Hz), 7.21 (1H, dd, 55”‘34”‘ = 5.1 Hz, 55“‘,3”’= 1.2 Hz, H5”’),MS, m / e 344 (Ma+),311, 172. 5-Methyl-2,2”,2”:5”,2’”:5”’,2””-quinquethiophene(3, MeT5). A solution of 2,2’-bithien-5-ylmagnesiumbromide (8.74 mmol) in THF (90 mL) was added dropwise in 2 h to a mixture of 6 (2.5 g, 7.28 mmol) and NiC12(dppp)(0.3 g, 0.55 mmol) in Et20 (150 mL), kept under gentle reflux. After stirring under reflux for 2 h and at room temperature overnight, the reaction mixture was treated with 1 M HC1. The solid precipitated was filtered off, washed with water, and dried under vacuum. The crude product was extracted in Kumagawa apparatus with n-hexane for 24 h and then with toluene for further 24 h. The n-hexane solution contained the starting material 6 and the 2,2’:5’,2’’””,2’’’quaterthiophene, while the solid precipitated from toluene resulted to be a mixture of the title compound 3 and of a product of MS m / e 522, most probably corresponding to 5,5””’-dimethyl2,2’~5’,2’’:~~~~~’”:5’’‘,2’”’:5‘‘‘‘,2’‘‘’‘-sexithiophene (from MS analysis). Sublimation at 230 O C and mmHg of the above mentioned mixture afforded pure 3 (0.6 g, 20% yield), mp 255 “C. Anal. Calcd for CZ1Hl4S5:C, 59.13; H, 3.28%. Found: C, 59.02; H, 3.32%. 1H NMR (dioxane, 70 “C, 300 MHz), d 2.50 (3 h, br s, M e ) , 6.67 (1H, br d, J 4 9 3 = 3.5 Hz, H4),7.00 (1H, d, Pl4 = 3.3 Hz, H3), 7.01-7.04 (2 H, m, H4””,H3’),7.10-7.15 (5 H, m, H4’, H3”, H4”, H3“‘,H4“’),7.21 (1H, dd, 3 ” ” , 4 ” “ = 3.6 Hz, 53““,5““ = 1.2 Hz, H3““),7.30 (1 H, dd, = 5.7 Hz, J5””!3””= 1.2 Hz, H5””). MS, m / e 426 (M’+), 393, 261. 5,5’-Dibromo-2,2’-bithiophene (7). NBS (3.20g, 19.97mmol) was added portiowise to a solution of 2,2’-bithiophene (1.50 g, 9.02 mmol) in CHC13/AcOH (200 mL, volume ratio 1:l). The reaction mixture was kept under stirring for 24 h. The solid precipitate was filtered off and the filtrate treated with water and extracted with Et20. The organic phase was washed with aqueous NaHC03, then with water, dried and evaporated to dryness. Flash chromatography of the residue (silica gel, petrol ether) afforded the title compound 7 (1.28 g, 55% yield), mp 143-144 “C (lit.15 145-146 “C). Anal. Calcd for C8H8Br2S2: C, 29.65; H, 1.23%. Found: C, 41.62; H, 2.65%. lH NMR (CDCl3, 80 MHz) d 6.82 (2 H, d, J4s3= J4‘$3‘ = 3.9 Hz, H4,H4’),6.94 (2 H, d, 3 ’ 4 = Z . 4 ’ = 3.9 Hz, H3, H3’). 5,5”’-Dimet~~l-2f”5’,2”:5’’,2’’‘-quaterthiop hene (4, MeZT4). A solution of 5-methyl-2-thienylmagnesium bromide (4.60 mmol) in EtzO (50 mL) was added dropwise at room temperature to a stirred mixture of 7 (0.50 g, 1.92 mmol) and NiCMdppp) in Et20 (25mL). The resulting suspensionwas refluxedfor 2 h and cooled at room temperature. 1 M HC1 was added, and the obtained precipitate was filtered off and crystallized from ligroin to give the title compound 4 (0.28 g, 41% yield), mp 192-193 “C (lit.18 184-185 “C). Anal. Calcd for C18H14S4: C, 60.26; H, 3.90%. Found C, 60.05; H, 3.76%. ‘H NMR (CDC13,200 MHz) d 2.46 = 3.7 Hz, H4, H4“’), (3 H, br s, M e ) , 6.65 (1H, br d, J4s3= J4“‘33”’ 6.93 (2 H, d, = J3”‘*4”‘ = 3.7 Hz, H3, H3”’),6.95 (2 H, d, Zs4‘ = JVJ” = 3.8 Hz, H3’,4”), 7-01(2 H, d, 54’3’= J3”34“ = 3.8 Hz,H4’3”). Apparatus and Procedures. Experiments were performed in 1,2-dichloroethane (DCE) 0.1 M TBAP under nitrogen in three electrode cells. Unless otherwise stated the temperature was 25 “C. The counterelectrode was platinum; reference electrode was a silver/O.l M silver perchlorate in acetonitrile (0.34 V vs SCE). 3
1
4
54“‘95”’
The followingcompoundswere prepared according to literature 2,2’-bithien-5-ylprescriptions: 2-bromo-5-methylthiophene,lZ magnesium bromide,132,2’-bithiophene,14palladium(I1)[1,l-bis(diphenylphosphino)ferrocene]chloride (PdClz(dppf)),15nickel(II) [1,3-bi(diphenylphosphino)propane] chloride (NiClz(dppp)).16 5-Bromo-5’-methyl-2,2’-bithiophene(5). A solution of 5-methyl-2-thienylmagnesium bromide (62 mmol) in Et20 (100 mL) was added dropwise in 3.5 h to a stirred mixture of 2,5dibromothiophene (14.9 g, 62 mmol) and PdClz(dppf) (0.3 g, 0.41 mmol) in EtzO (100 mL), keeping the temperature at 0 “C. The reaction mixture was stirred for 4 h at 0 “C, hydrolyzed with aqueous NH4C1, and extracted with EtzO. The dried organic phase was evaporated. Flash chromatography of the residue (silica gel, petrol ether) afforded the compound 5 (6.7 g, 42% yield), mp 80-82 “C. Anal. Calcd for C9H7BrSz: C, 41.70; H, 2.70%. Found: C, 41.62; H, 2.65%. ‘H NMR (CDC13,80MHz) d 2.45 (3 H, d, M e ) , 6.62 (1H, m, H4’), 6.78 (1H, d, H3),6.86 (1 H, d, H4),6.90 (1H, d, H?), 7.03 (1H, d, J4‘3= 3.7 Hz, H4’),7.13 (1H, dd, = 3.7 Hz, 5 3 ” ~=~1.2 ‘ ~Hz, H3”),7.19 (1H, dd, Pl4” = 5.1 Hz, J3“*3”= 1.2 Hz, H5“). 5-Me”y1-2,2’:5’,2’’-terthiophene (1, MeT3). A solution of 2-thienylmagnesium bromide in Et20 (50mL) was added dropwise in 1.5 h to a stirred mixture of 6 (5.0 g, 19.3 mmol) and PdC12(dppf)(0.1g,0.13 mmol) inEt2O (70mL),keeping the temperature at 0 “C. Stirring was continued for 1h at 0 “C and overnight at room temperature. Aqueous NH4Clwas added and the reaction mixture was extracted with EtzO. The organic phase was washed with water and dried (NazSOd), and the solvent evaporated at reduced pressure. The residue was crystallized from methanol to give 1 (4.7 g, 93% yield), mp 94-95 “C (lit.” 93-94.5 “C). Anal. Calcd for C13H1~S3:C, 59.52; H, 3.81%. Found: C, 59.43; H, 3.75%. lH NMR (CDC13, 300 MHz) d 2.46 (3 H, br s, M e ) , 6.65 (1H, br d, = 3.6 Hz, H4),6.95 (1H, d, = 3.6 Hz, H3),6.98 (1H, d, #,4‘ = 3.7 Hz, H3’),7.0 (1H, dd, J4“v3” = 3.7 Hz, J4”,5“ = 5.1 Hz, H4“),7.03 (1H, d, J4‘v3‘ = 3.7 Hz, H4’),7.13 (1H, dd, 5 3 “ 3 4 ” = 3.7 Hz, P - 5 ” = 1.2 Hz, H3”),7.19 (1H, dd, 5 5 “ ~ ~ =”5.1 Hz, Ps3“ = 1.2 Hz, H5”). I-Bromo-5”-methyL2,2’:5’,2’’-terthiophene(6). A solution of NBS (0.27 g, 1.52 mmol) in CHC13/AcOH(28 mL, volume ratio 1:1)was added dropwiseat room temperature in 1.5 h to a stirred solution of 1 (0.49 g, 1.52 mmol) and p-hydroquinone (catalytic amount) in CHC13/AcOH (14mL, volume ratio 1:l).The reaction mixture was kept under stirring for 1 h, poured into water and extracted withEt20. Theorganic phase was washed withaqueous NaHC03and then with water, dried, and evaporated to dryness. Crystallization of the residue from ligroin afforded the title compound 6 (0.5 g, 92% yield), mp 145 “C. Anal. Calcd for CI3H9BrS3:C, 45.76; H, 2.63%. Found C, 45.65; H, 2.70%. ‘H NMR (CDC13,300 MHz), d 2.45 (3 H, br s, M e ) , 6.66 (1H, br d, J4”*3” = 3.5 Hz, H4”),6.88 (1H, d, 3’4 = 3.9 Hz, H3),6.94-6.99 (4 H, m, H4, HS’,H4‘,H3”). Pi4’‘
J413
3
1
4
(12) Campbell, T. W.;Kaeding, W. W. J. Am. Chem. SOC.1951,73, 4019. (13) Rossi, R.;Carpita, A.; Ciofalo, M.; Houben, J. L. Gazz. Chim. Ital. 1990,120,793. (14) Pham, C. V.;Burklardt, A.; Shabane, R.; Cunningham, D.; Mark, H. B.; Zimmer, H.Phosphorus, Sulfur, Silicon 1989,46,153. (15) Hayashi, T.; Koniahi, M.; Kobori, Y.; Kumada, M.; Hifuchi, T.; Hirotsu, K. J. Am. Chem. SOC.1982,106,158. (16) Kumada, M.; Tamao, K.; Sumitani, K. Org. Synth. 1978,58,127. (17) Nakayama, J.; Nakamura, Y.; Tajiri, T. Heterocycles 1986,24, 637.
J””’94””
3 3 4
+
(18) Steinkopf, W.;Letamann, R.; Hofmann, K. H. Justus Liebigs Ann. Chem. 1941,546,180.
Thiophene Oligomers as Polythiophene Models
Chem. Mater., Vol. 5, No. 4, 1993 433
electron oxidation which appears completely reversible at u > 1V/s (E" = 0.84 V). Therefore, oxidation produces the radical cation, stable at least for some seconds, and the dication, the latter reactive with nucleophiles in the medium. Controlled-potential electrolysis at the first oxidation step requires initially one electron/molecule, as it appears from the corresponding linear part of the current vs charge plot, but as electrolysis continues the current tends to level off and extra charge passes. Partial electrolysis M solution) produces (typically at 25% on a 1 X quantitatively the radical cation which, within some hours, decays regenerating the neutral monomer in a 50% amount. This result indicates the occurrence of a disproportionation reaction and in accord with this the kinetics are second order (kdisp = 1.3 M-l s-1 at 20 "C). The disproportionation may be due to proton dissociation of the radical cation MH*+(eq 1);the resulting neutral radical M' is easily oxidized by MH'+ (eq 2) to the fully oxidized species M+, which undergoes attack by nucleophiles D in the medium (eq 3).
+ H+
(1)
products
(3)
MHO+-,M
M+ + D
-
This hypothesis has been demonstrated by increasing the acidity of the medium with anhydrous CF3COOH. Thus a solution of the radical cation, produced by electrolysis at -30 "C and warmed to 25 "C after addition of CF3COOH 1:lO v/v, is perfectly stable for several hours. In any case the disproportionation reaction in the absence of acid is so slow that it may be considered as noncompetitive with the coupling reactions herein investigated; for the same reason an easy spectroscopic characterization of the radical cation has been possible. The electronic spectrum of Me2T4*+(Figure 2) displays bands at 550, 670, and 870 nm, the central of which increases relatively to the others as the concentration is decreased or the temperature increased. Therefore the side bands are attributable to a species arising from the association of the species responsible for the central band. In fact the radical cation of tetrathiafulvalene is reported to dimerize, and the spectrum of the dimer displays analogous CT bands.21 Furthermore the radical cation of tetrathiophene at low concentration in CH2Clz displays a single band at 650 nm.22 Thus the radical cation is partly associated in dimers. While this paper was in preparation, the radical cation of the homologous Me2T3 was reportedz3. Ita behavior, similar to ours, has been similarly explained. The ESR spectrum (Figure 3) displays a signal at g = 2.002 with hyperfine splitting in seven triplets (a1 = 3.3 G, a2 = 1.0 G),due to coupling with methyl and aromatic protons. ESR spectra of radical cations of thiophene oligomers are rep0rted,~~l2~9~5 but our case is, together with (21) Torrance, J. B.; Scott, B. A.; Welber, B.; Kaufman, F. B.; Seiden, P. E. Phys. Reu. B. 1979,19, 730. (22) Fichou, D.; Horowitz, G.; Xu, B.; Garnier, F. Synth. Met. 1990, 39, 243. (23) Hill, M. G.; Mann, K. R.; Miller, L. L.; Penneau, J. F. J. Am. Chem. SOC.1992,114,2728. (24) Alberti, A.; Favaretto, L.; Seconi, G.;Pedulli, G. F. J. Chem. Soc., Perkin Trans. 2 1990, 931. (25) Caspar, J. V.; Ramamurthy, V.;Corbin, D. R. J.Am. Chem. SOC. 1991,113, 600.
1.811
3 8, 450 L 4
. 7
548
fib
A/",,
6 1 8 80
9Bb Bil
Figure 2. Electronic spectrum of Me2T4*+1 X lo4 M in DCE
+ 0.1 M TBAP; optical path = 0.2 cm. a,
Figure 3. ESR spectrum of Me2T4*+5 X 10-4M in DCE + 0.1 M TBAP. the recently reported thiophene oligomersend-capped with cyclohexane,26p-ben~oquinone,2~ and trimethylsilyP moieties, a rare case of a stable oligothiophene radical cation. Electrochemical Analysis of MeT,. MeT4 is reversibly oxidized at scan rates u L 1V/s at E" = 0.58 V. Under these conditions ipu-1/2 is independent of u and napp= 1 electron/molecule. At lower scan rates nappincreases (1.3 electron/molecule at 0.05 V/s) with contemporary appearance of the reduction peak of the dimer (Figure 4). Electrolysis produces a dark suspension of the oxidized product which, after undoping with ammonia, yields a bright red precipitate. After separation by centrifugation and washing with DCE, the solid displays an IR spectrum analogous to that of Me2T4, but with the intensity ratio of the band for methyl CH stretching at 2900 cm-l to that for CH out-of-plane bending in the thiophene rings at 800 cm-l, A2900/Agw, 50% lower, in agreement with the formulation Me2Tg. The dominant molecular peak (mlz = 522)in the mass spectrum at 400"C definitively confirms the octamer. The cyclicvoltammogram of the oligomer MeT3 displays an oxidation peak at 0.73 V and on the reverse scan a reduction peak at more negative potentials, attributable to the oxidized dimer. Reversibility is instead observed at scan rates u > 100 V/s (E" = 0.7 V), thus indicating a higher coupling rate in respect with MeT4. Potential (26) Bauerle, P. Adu. Mater. 1992, 4, 102. (27) Takahashi, K.; Suzuki, T. J. Am. Chem. SOC.1989, 111, 5483. (28) Guay, J.; Diaz, A. Chem. Muter. 1992, 4 , 254.
434 Chem. Mater., Vol. 5, No. 4, 1993 3.0
I
2.0
,
0.0 -2.0
Zotti et al. / /
/ /
/ / /
-1.0
0.0 log t
Figure 4. Cyclic voltammogram for MeT4 1 X 0.1 M TBAP. u = 0.2 VIS.
+
M in DCE
cycling produces a red deposit constituted by the dimer MezTs, as results from the IR analysis of the A2900/A800 ratio and the molecular peak (mlz = 686) in the mass spectrum at 500 "C. The monomer MeT5 is oxidized reversibly with one electron at E" = 0.52 V with appearance of the dimeric peak only with repetitive potential cycling at higher temperatures, indicating a lower coupling rate in respect with MeT4. Oxidation produces a green suspension which turns to red upon ammonia addition. The material displays the A2m/Amratio corresponding to that expected for the dimer MezTlo. Kinetics of Dimerization. The electrochemical process of dimerization may be explained as follows. The radical cation MHO+,produced by oxidation of the monomer MH (eq 4), couples to dimer Mz with proton release (eq 5). The dimer, at the potential at which the
MH 2 MH*+ 2 MH"
- + M,
2H+
(4)
(5)
-2e
M,
M2'+
(6)
+2e
Mt+
M,
(7)
monomer is oxidized, is oxidized in its turn to the dication Mz2+(eq 6).29 The latter may be reduced to the neutral state in the backward scan in cyclic voltammetry (eq 7). Considering the cyclic voltammogram of MeT4 (Figure 41, processes 4 and 6 occur at peak A and process 7 at peak C. The kinetics of the dimerization process may be followed considering the time dependence of the oxidation current, i.e., analyzing either the normalized peak current (iPu-ll2) as a function of the scan rate u in cyclic voltammetry or the normalized current (itllz) as a function of time t in chronoamperometry. The number of apparent exchanged (29) Zotti, G.; Schiavon, G.; Berlin, A.; Pagani, G., to be published as part 2.
13
Figure 5. Chronoamperometric nappvs log t plot for MeT4 1 X 10-3 M in DCE + 0.1 M TBAP a t 40 "C. Calculated curve as dashed line.
electrons nappgoes from 1to 2 electrons/molecule as time goes from values lower to higher than those of the reaction. The theoretical analysis for a second-order kinetics of this type (ECE with second-order dimerization) has been developed for chron~amperometry.~~ In fact the chronoamperometric curve for MeT4, analyzed as it'12 vs log t , gives an nappvalue which goes from 1electron/molecule at short times to a value of 2 electron/molecule at long times (Figure 5). but for times higher than some seconds the current increases further, probably because the dimer deposits heavily and, as a conductor, produces capacitivelike currents3' and/or increase of the electroactive area. Thus the analysis must be limited to the lower time domain, which in its turn increases the uncertainty in the analysis itself. Therefore we have developed a cyclic voltammetric method based on the detection of the product of the reaction (oxidized dimer), i.e., on the reduction peak current for process 7 as a function of u. This approach, which would allow a more precise determination at the early stages of the reaction, i.e., before complications due to massive precipitative of the product on the electrode surface set in, is given in detail in Appendix. Considering once more MeT4 as example, the ratio of peak currents A over C, Ri (see Figure 4) is with good confidence linear with the scan rate u (Figure 6) and the inverse slope is linear with monomer concentration (Figure 7), in accordance with eq 16 in Appendix, which proves the validity of the method. A final confirmation has been obtained by the chronoamperometric analysis at CO= 1 X 10-3 M and 40 "C (Figure 5) which yields for t112(time at napp= 1.5 electrons/molecule) a value of 1.0 s, from which the theoretical treatment30 allows to evaluate kd = 660 M-l s-I. Our cyclic voltammetric analysis under the same conditions provides k d = 690 M-' s-1 in very good agreement with the chronoamperometric value. Kinetic analysis at different temperatures (10/40 "C) according to Eyring (Figure 8) gives 25 kJ mol-' for the activation enthalpy and -109 J K-I mol-' for the activation entropy. Kinetic analysis for MeT3, given the higher dimerization rate, has been performed at low temperature (-30/0"C). (30) Feldberg, S. J . Phys. Chem. 1969, 73, 1238. (31) Zotti, G.; Schiavon, G.; Berlin, A.; Pagani, G., to be published as part 3.
Chem. Mater., Vol. 5, No. 4, 1993 435
Thiophene Oligomers as Polythiophene Models 1.5 1
2ol
n
d
0 0.0
2.0
v
4.0
/ v
s-'
M in DCE
Figure 6. Ri vs u plot for MeT4 1 X TBAP at 60 "C. 0.8
1
0.3 2.9
6.0
+ 0.1 M
1
I
3.4
3.2
,'
I/T ~o-~K-' Figure 8. ln(kd/T) vs 1/T (Eyring plot) for kinetics of dimerization of MeT4 in DCE + 0.1 M TBAP. 70
/ A
-
l
60
m
\
\ m
\
0.4
0.0 Y
0.0
I
I
0.5
1.0
I
1.5
I
2.0
40 0.15
1
co / ~ o - ~ M Figure 7. Relationship of inverse slope l/sl (from Ri vs u plot) vs monomer concentration Co for MeT4in DCE + 0.1 M TBAP a t 60 O C .
At the opposite, kinetic measurements for MeTb have been performed a t high temperatures (50/70 "C).The kinetic parameters are summarized in Table I. From a comparison of the methyl-protected oligomers it results that the free energies of activation are linearly related to the inverse of the degree of oligomerization n (Figure 91, as usually found for the electronic and electrochemical parameters in oligomeric series of this type.32 Conclusions To summarize the results of this kinetic investigation, (i) the kinetics of anodic coupling of thiophene oligomers is second order, (ii) activation enthalpies are noticeably high (25/70 kJ mol-l) for a coupling reaction, (iii) activation entropies are negative (-llO/-4 J K-' mol-'), and (iv) the coupling rate decreases as the oligomer length is increased. On the basis of recent results,&l0obtained with changes in the basic properties of the medium, proton dissociation prior the coupling rate determining step should not be implied. Eventually, oxidation of the resulting neutral
I
~
2.5
0.20
I
0.25
I
0.30
\ 0.35
I
0.40
1/n
Figure 9. AG* (at 25 "C) vs l l n for kinetics of dimerization of thiophene oligomers in DCE + 0.1 M TBAP. Table I. Kinetic Parameters for the Dimerization of Thiophene Oligomers in DCE + 0.1 M TBAP AH*/kJ AS'IJK-' AG*25/kJ kdZ5/M-' oligomer mol-' mol-l mol-ln 8-lo MeT3 MeT4 MeTb a
31(f2) 25(*2) 64(*2)
- 46(*6) -109(*7) - 4(f4)
45(&4) 58(&4) 66(13)
110000 440
19
At 25 "C.
radical by the radical cation (disproportionation), responsible for the decay of the fully end-protected radical cation, would be mostly favoured. Moreover we found that the kinetics for, e.g., MeT3 in acetonitrile, i.e., in a medium with a much higher donor number than DCE (14.1, intermediate between 0.0 for DCE and 33.1for pyridine33), proceeds essentially with the same rate. Also, the formation of a dimeric species by the radical cation (ITdimerization), observed for the bisprotected tetramer, preceeding the coupling step cannot be ruled out completely. We believe however it is not likely as association (32) Bredas, J. L.; Silbey, R.; Boudreaux, D. S.; Chance, R. R. J . Am. Chem. SOC.1983, 105, 6555. (33) Gutmann, V . The Donor-Acceptor Approach to Molecular Interactions; Plenum Press: New Yrok, 1920; p 59.
436 Chem. Mater., Vol. 5, No. 4,1993 is enthalpically favoured and this would lead to low or even null activation energies, as already observed for the coupling of 9-substituted anthracene^.^^ It is therefore suggested that radical cations coupled directly. Though the coupling process is quite highly activated considering coupling reactions in general, similar high energy values are not uncommon in aromatic coupling reactions.35 This fact cannot be attributed to electrostatic interactions, since, as reported above, changing to a medium with a much higher dielectric constant than DCE such as acetonitrile does not change the coupling rate. Instead it is likely that the relative high activation enthalpies are attributable to a decreased delocalization of the conjugated radical cation as the activated state is attained.35 This energy was found to be particularly high for the pentamer. Unless this discontinuity is due to a change in the mechanism by one of the abovementioned reasons or by others, this result may indicate that we are close to the maximum stabilization of the radical cation predicted for the localization of the polaron over 5-6 monomer units in the p ~ l y m e rand ~ ~supported ?~~ by the observed high stability of the hexameric unprotected radical cation.22 The negative values of the activation entropies indicate that a high obstacle to dimerize is constituted by the difficulty encourtered by the radical cations to orientate correctly and in agreement with this they decrease going from MeT3 to MeT4. Instead the pentamer displays a peculiarity also in the entropic term, which is virtually null instead of being even more negative. It is probably that the same reason which causes the enthalpic increase causes the entropic one and it may be individuated in a high reduction of degrees of freedom in the radical cation MeTS'+ compared with the others: a strong electron delocalization is expectedly accompanied by a particularly rigid coplanarization of the thiophene rings. Lastly we remark that the second-order reaction accounts for the fact that polymerization is favored by high concentration of radical cations, i.e., by high current densities in electropolymerizations,38 and that the decreased rate for longer oligomers accounts for the observation that higher molecular weight polymers are obtained when lower oligomers are used.39
Acknowledgment. The authors would like to thank Prof. P. Beltrame (University of Milano) for helpful (34) Hammerich, 0.; Parker, V. D. Acta Chem. Scand. B 1982,36, 519. Parker, V. D. Adu. Phys. Org. Chem. 1984,20, (35)Hammerich, 0.; 59. (36) Bredas, J. L.; Chance, R. R.; R. Silbey, R. Phys. Reu. B 1982,26, 5843. (37)And& J. M.; Delhalle, J.;Bredas, J. L. Quantum Chemistry Aided Design of Organic Polymers; Lin, S . H., Ed.; World Scientific: Singapore, 1991;p 245. (38)Roncali, J.; Garreau, R.; Yassar, A.; Marque, P.; Garnier, F.; Lemaire, M. J. Phys. Chem. 1987,91, 6706. (39)Roncali, J.; Garnier, F.; Lemaire, M.; Garreau, R. Synth. Met. 1986,15, 323.
Zotti et al. discussion, Dr. R. Seraglia for running mass spectra, Mrs. N. Comisso, Mr. S. Sitran, and Mr. R. Salmaso of CNR for technical assistance.
Appendix The oxidation charge Qoxpassed in the forward oxidation is a measure of the sum of the monomeric radical cation and the dimeric dication formed at the electrode during the scan time, while the charge involved in the reduction peak of the dimeric dication Qred measures the second term of the sum. If we indicate as CO the analytical concentration of the monomer, CR the concentration of the reacting species (monomeric radical cation), Cp that of the produced species (dimeric dication), and V the volume of the reaction layer at the electrode (where the monomer is assumed to be completely oxidized at potentials higher than the redox potential), the conditions of stoichiometric balance are
c, = C R + 2cp
(8)
and the charge relationships are Qox/FV= C,
+ 2Cp
(9)
QredIFV= 2Cp We define the charge ratio
(10)
RQ = Qox/Qred= (Co + 2Cp)/2Cp For a second-order process
(11)
1/cR - 1/cO kdt where kd is the kinetic dimerization constant. Inserting eq 8 and then eq 12 into eq 11:
(12)
RQ = l/kdCot + 2 (13) If we consider that peak currents i, are proportional to the relevant charges, then where K is constant for a given value of the switching potential EA,eq 13 turns to Ri = K/kdCot + 2K (15) Last, as the reaction time t is to a first approximation that which is spent during the oxidation and therefore given EA= switching potential by A E / u (where A E = EA - E"), and u = AE/At), we have Ri = KU/kdCoU + w( (16) Using slope s1 and intercept i from the linear Ri vs u relationship, monomer concentration CO,switching potential EAand redox potential E O , one would obtain the dimerization rate constant kd: kd
i/2S,C@
(17)
