Broken π-conjugated thiophene systems

Synthetic Metals 113 Ž2000. 161–166 www.elsevier.comrlocatersynmet

Broken p-conjugated thiophene systems 1. Synthesis and polymerization of 2,2X-di žalkylthienyl/ methanes Kenneth J. Hoffmann a , Emil J. Samuelsen b, Per H.J. Carlsen a,) a

Department of Organic Chemistry, Norwegian UniÕersity of Science and Technology, N-7491 Trondheim, Norway b Department of Physics, Norwegian UniÕersity of Science and Technology, N-7491 Trondheim, Norway Accepted 1 January 2000

Abstract A series of 2,2X-diŽalkylthienyl.methanes has been synthesized by chloroalkylation of 3-alkylthiophenes in the presence of zinc chloride. These monomers were oxidatively polymerized with ironŽIII. chloride to yield the respective partially broken p-conjugated polyŽ2,2X-diŽalkylthienyl.methane.s. Solubility and processability properties of the polymers were dependent on the alkyl groups attached to the thiophene ring. The new polymers contain quinoidal segments due to dehydrogenation of the bridge carbon during polymerization, which can be seen by UV–Vis and 1 H NMR spectroscopy. Due to the quinoidal segments, these polymers show conductivity in the range from 10y6 to 10y4 Srcm. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Conjugated polymers; Heterocycle synthesis; Polythiophene and derivatives

1. Introduction Conjugated polymers have a broad range of interesting electronic and optical properties that are dependent upon the nature of the polymeric backbone structure as well as the attached side chains w1–4x. The extent of p-electron conjugation in the polymers is assumed to further enhance these properties. Current research in the field of polythiophenes and other conducting polymers is aiming for an understanding of the role of conjugation on the conductivity and the ability to tune this conductivity, based on substitution with various side groups andror breaks in conjugation w5–8x. Other studies are being conducted to investigate how compound structure can affect the conductivity mechanism, including p-stacking w9x. Earlier investigations in our group have shown that dehydrogenation occurs during the polymerization of 2,2X-dithienyl-1,1X-nonane w10x. This paper reports the synthesis and characterization of a series of 2,2X-diŽalkylthienyl. methanes, shown in Fig. 1,

)

Corresponding author.

and the synthesis and characterization of their corresponding polymers.

2. Experimental 2.1. General Thiophene was purified by vacuum distillation. 3-Methylthiophene, zinc chloride, hydrochloric acid Ž37%., formaldehyde Ž37%., and chloroform were used with no further purification. 3-Butylthiophene and 3-octylthiophene were synthesized using the method reported by Tamao et al. w11x, and purified by vacuum distillation. 2.2. Synthesis of monomers: general procedure Monomers Ž1. were synthesized in accordance to the method of Gold’faarb and Danyushevsky w12x depicted in Scheme 1. To a stirred solution of zinc chloride in hydrochloric acid Ž37%. between y88C and y158C, 3-alkylthiophene was added over several minutes. Formaldehyde Ž37%. was added dropwise over 60 min while maintaining the temperature between y88C and y158C. The reaction mixture

0379-6779r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 Ž 0 0 . 0 0 2 0 9 - 5

K.J. Hoffmann et al.r Synthetic Metals 113 (2000) 161–166

162

Fig. 1.

was stirred for 60 min while maintaining this temperature and then quenched with water and extracted with ether. The organic extract was washed with 5% sodium bicarbonate solution, dried over magnesium sulfate, and the solvent evaporated. Purification by distillation afforded the monomer Ž1.. 2.2.1. 2,2X-Dithienyl methane (1a) Purification by distillation Žbp. 133–1358C at 15 mm Hg. followed by recrystallization in hexane provided the title compound Ž32.02 g, 39.5% yield. as a white crystalline powder: mp s 52–538C; 1 H NMR ŽCDCl 3 . d 7.19 Ždd, 2 H, J s 5.1, 1.1., 6.96 Ždd, 2 H, J s 5.1, 3.4., 6.91 Ždd, 2 H, J s 3.4, 1.0., 4.38 Žs, 2 H.; 13 C NMR ŽCDCl 3 . d 143.6, 127.3, 125.7, 124.6, 30.6; IR ŽKBr. 2699Žw., 1437Žw., 1423Žw., 1361Žw., 1294Žm., 1256Žw., 1220Žw., 1185Žw., 1163Žw., 1080Žw., 1034Žw., 850Žm., 822Žm., 711Žs., 699Žs.; UV–Vis ŽCHCl 3 . l max s 234 nm, ´ s 13 500 l moly1 cmy1 . Anal. calcd. for C 9 H 8 S 2 : C, 59.96; H, 4.47; S, 35.57. Found: C, 62.04; H, 4.85; S, 33.10. 2.2.2. 2,2X-di(methylthienyl)methane (1b) Purification by distillation Žbp. 94–968C at 1.6 = 10y2 mbar. gave the title compound Ž3.967 g, 74.5%. as a colorless liquid: 1 H NMR ŽCDCl 3 . d 7.08 Žd, 2 H, J s 5.1., 6.83 Žd, 2 H, J s 5.1., 4.16 Žs, 2 H., 2.25 Žs, 6 H.; 13 C NMR ŽCDCl 3 . d 137.1, 133.6, 130.5, 122.5, 26.7, 14.2; IR Žneat. 3102Žw., 3060Žw., 2919Žs., 2863Žm., 2727Žw., 1746Žw., 1553Žw., 1435Žs., 1382Žm., 1366Žw., 1301Žm., 1232Žm., 1168Žw., 1149Žw., 1078Žw., 1034Žw., 954Žw., 875Žw., 854Žs., 834Žm., 707Žs., 596Žs.; UV–Vis ŽCHCl 3 . lmax s 239 nm, ´ s 13 800 l moly1 cmy1. Anal. calcd. for C 11 H 12 S 2 : C, 63.42; H, 5.81; S, 30.78. Found: C, 63.98; H, 6.06; S, 30.72. X

2.2.3. 2,2 -di(butylthienyl)methane (1c) Purification by distillation Žbp. 139–1428C at 1.0 = y2 10 mbar. provided the title compound Ž6.17 g, 42.8% . yield as a colorless liquid: 1 H NMR ŽCDCl 3 . d 7.05 Žd, 2 H, J s 5.2., 6.82 Žd, 2 H, J s 5.2., 4.15 Žs, 2 H., 2.57 Žt, 4 H, J s 7.8., 1.56 Žquintet, 4 H, J s 7.8., 1.35 Žsextet, 4 H., 0.92 Žt, 6 H, J s 7.3.; 13 C NMR ŽCDCl 3 . d 138.9, 137.1, 129.1, 122.7, 33.2, 28.5, 26.4, 23.0, 14.4; IR Žneat. 2954Žs., 2924Žs., 2872Žs., 2849Žs., 1464Žm., 1367Žm., 1262Žw., 1218Žw., 1062Žw., 853Žw., 704Žm.; UV–Vis ŽCHCl 3 . lmax s 240 nm, ´ s 8900 l moly1 cmy1. Anal. calcd. for C 19 H 32 S 2 : C, 69.81; H, 8.27; S, 21.92. Found: C, 71.47; H, 8.62; S, 19.91.

2.2.4. 2,2X-di(octylthienyl)methane (1d) The reaction was allowed to warm to room temperature while stirring for 22 h before quenching the reaction. The viscous reaction mixture was dissolved in chloroform and then washed with water. The organic extract was washed with a 5% sodium bicarbonate solution, dried over magnesium sulfate, and the solvent evaporated. Purification by Kugelrohr ¨ distillation Žbp. ; 2008C at 1.2 = 10y2 mbar. provided the title compound Ž1.045 g, 20.66% yield. as a colorless liquid: 1 H NMR ŽCDCl 3 . d 7.08 Žd, 2 H, J s 5.2., 6.85 Žd, 2 H, J s 5.1., 4.17 Žs, 2 H., 2.59 Žt, 4 H, J s 7.5., 1.57 Ž4 H., 1.28 Žbroad, 20 H., 0.90 Žt, 6 H, J s 6.7.; 13 C NMR ŽCDCl 3 . d 138.9, 137.1, 129.1, 122.7, 32.3, 31.0, 30.0, 29.9, 29.7, 28.8, 26.4, 23.1, 14.5; IR Žneat. 2854Žs., 2924Žs., 2853Žs., 1438Žm., 1405Žmw., 1377Žw., 1299Žw., 1237Žw., 1151Žw., 1087Žw., 854Žw., 836Žw.; UV–Vis ŽCHCl 3 . lmax s 237 nm, ´ s 13 400 l moly1 cmy1 . Anal. calcd. for C 25 H 40 S 2 : C, 74.19; H, 9.96; S, 15.84. Found: C, 74.50; H, 9.95; S, 15.94. 2.3. Chemical polymerization and doping: general procedure Chemical polymerization was carried out in accordance to the method of Andersson et al. w19x. To a stirred solution of monomer in chloroform, a slurry of ironŽIII. chloride and chloroform was added through a 2-mm tube over 2 h. The polymer solution was stirred for 23 h at room temperature. The polymer mixture was poured into methanol, which was then filtered to collect the insoluble polymer. The polymer was washed with methanol and then mixed with chloroform. The polymer mixture was refluxed with aqueous ammonia, EDTA solution, and then washed with water. The polymer mixture was filtered to remove insoluble polymer and then poured into methanol to crystallize the soluble polymer. Purification by Soxhlet extraction and subsequent in vacuo drying afforded the polymers Ž3.. Doping was effected by placing polymer pellets in open vials within a small desiccator charged with iodine crystals. 2.3.1. Poly(2,2X-ithienylmethane) (3a) Yields: 71.9% overall yield Ž5.67% soluble, 94.33% insoluble.. 1 H NMR ŽCDCl 3 . d 7.45–7.35 Žsharp., 7.2–7.1 Žsharp., 7.0–6.8 Žsharp., 6.75–6.65 Žsharp., 4.35–4.20 Žsharp, three peaks.; IR ŽKBr. 1663 Žbroad., 1617 Žbroad.,

Scheme 1. Monomer synthesis.

K.J. Hoffmann et al.r Synthetic Metals 113 (2000) 161–166 Table 1 X Results of synthesis of 2,2 -diŽalkylthienyl.methanes Monomer

Name

Yield Ž%.

Regioisomers Ž x: y: z .

1a 1b 1c 1d

DTM DMeTM DBuTM DOctTM

39.5 74.3 42.8 20.7

– 87:11:2 88:11:1 90:9:1

1509Žw., 1410Žw., 1350Žw., 1265Žw., 1225Žw., 1142Žw., 1111Žw., 1034Žw., 881Žw., 790Žs., 723Žw., 696Žm.; UV– Vis ŽCHCl 3 . lmax s 241 nm, a s 360 l gy1 cmy1, l s 308 nm, a s 220 l gy1 cmy1 , l s 460 nm, a s 260 l gy1 cmy1 . Anal. calcd. for C 9 H 6 S 2 : C, 60.63; H, 3.40; S, 35.97. Found: C, 63.36; H, 3.51; S, 33.13. 2.3.2. Poly(2,2X-di(methylthienyl)methane) (3b) Yields: 103.4% overall yield Ž14.99% soluble, 85.01% insoluble.. 1 H NMR ŽCDCl 3 . d 7.1 Žbroad., 7.0–6.7 Žbroad., 5.7 Žbroad., 3.5–3.3 Žbroad., 2.6–1.8 Žbroad.; IR ŽKBr. 1653 Žbroad., 1359 Žbroad., 1269 Žbroad., 1143 Žbroad., 995Žm., 925Žw., 827Žm., 710Žw., 818Žw., 804Žw., 504Žm.; UV–Vis ŽCHCl 3 . l max s 239 nm, a s 58.0 l gy1 cmy1 , l s 337 nm, a s 49.0 l gy1 cmy1 , l s 486 nm, a s 56.3 l gy1 cmy1 . Anal. calcd. for C 11 H 10 S 2 : C, 64.03; H, 4.90; S, 31.07. Found: Žinsoluble polymer. C, 66.05; H, 4.61; S, 29.34; Žsoluble polymer. C, 65.59; H, 4.99; S, 29.42. 2.3.3. Poly(2,2X-di(butylthienyl)methane) (3c) Yield: 75.0% overall yield Ž60.6% soluble, 39.4% insoluble.. 1 H NMR ŽCDCl 3 . d 7.1–6.6 Žbroad., 5.85–5.58 Žbroad., 3.54–3.31 Žbroad., 2.96–2.64 Žbroad., 2.63–2.10 Žbroad., 1.44–1.16 Žbroad., 1.03–0.61 Žbroad.; IR ŽKBr. 3427 Žbroad., 2952Žm., 2923Žs., 2853Žs., 2362Žw., 1961 Žbroad., 1455Žm., 1376Žm., 1297Žw., 1248Žw., 1162Žw., 1102Žw., 832Žm., 780Žw., 747Žw., 726Žw.; UV–Vis ŽCHCl 3 . lmax s 240 nm, a s 26.6 l gy1 cmy1, l s 334 nm, a s 22.4 l gy1 cmy1 , l s 494 nm, a s 25.6 l gy1 cmy1 . Anal. calcd. for C 17 H 24 S 2 : C, 71.27; H, 6.34; S, 22.38. Found: Žsoluble polymer. C, 71.52; H, 7.29; S, 21.18. 2.3.4. Poly(2,2X-di(octylthienyl)methane) (3d) Yield: 76.3% overall yield Ž100% soluble.. 1 H NMR ŽCDCl 3 . d 7.1–6.7 Žbroad., 5.8–6.0 Žbroad., 4.3–4.0 Žbroad., 3.8–3.6, 3.6–3.4, 2.7–2.2 Žbroad., 1.4–1.0 Žbroad., 0.9–0.7 Žbroad.; UV–Vis ŽCHCl 3 . lmax s 238 nm, a s 20.00 l gy1 cmy1 , l s 318 nm, a s 9.54 l gy1 cmy1 , l s 468 nm, a s 10.14 l gy1 cmy1 , l s 497 nm, a s 10.21 l gy1 cmy1 . Anal. calcd. for C 25 H 38 S 2 : C, 74.19; H, 9.96; S, 15.84. Found: C, 74.08; H, 9.20; S, 16.72.

163

2.4. Measurements 1

H NMR and 13 C NMR spectra were recorded on Bruker Avance DPX 300 and 400 MHz spectrometers. UV–Vis spectra were obtained on a Perkin-Elmer 552 spectrophotometer. IR spectra were recorded on a Nicolet 20SXC FT-IR spectrometer. Electrical conductivity measurements were effected on a four-point probe instrument under atmospheric conditions at room temperature.

3. Results and discussion 3.1. Monomer synthesis Previous research by our group on the polymerization of 2,2X-dithienyl methane w10x has indicated that the polymer is chloroform-insoluble, thus making characterization difficult. The objective of the following syntheses was to increase the solubility of the polymer as well as to determine the effect of alkyl chain length on polymer structure and properties, including conductivity. This was accomplished by introducing alkyl chains into the monomer by substituting thiophene with various 3-alkylthiophenes. This substitution is desired to enhance the solubility of the polymers w13–15x. Though the synthesis of dithienyl alkanes has previously been demonstrated w12,16–18x, alkylgroup substitution at the 3-position of the thiophene ring in the starting material has not been attempted. The monomers were synthesized by chloroalkylation of various 3-alkylthiophenes with formaldehyde, based on a method by Gold’faarb and Danyushevskyw12x. The reaction is shown in Scheme 1. As a result of the various alkyl-group substitutions, the Gold’faarb reaction performed differently for each alkylsubstituted thiophene. With thiophene as the reactant, the reaction was very sensitive to oligomerization and required constant temperature monitoring and slow, controlled addition of the formaldehyde solution due to the exothermic properties of the reaction. On the other hand, when, e.g. 3-octylthiophene was the reactant, reaction was slow Ž20 h. and also required a special work-up procedure due to the viscosity of the product. The reasons for this are as yet not understood, but we believe that, due to steric factors, the longer alkyl chain inhibits the binding of the formaldehyde to the 2-position on the thiophene ring. The yield

Fig. 2. Regioisomers obtained in the chloroalkylation reaction.

K.J. Hoffmann et al.r Synthetic Metals 113 (2000) 161–166

164

Scheme 2. Oxidative polymerization and expected polymer structure.

differences may also be influenced by the chain length, and thus, the reactivity of the 3-alkylthiophene. As shown in Table 1, DTM gave a modest yield of 39.5%, while DMeTM was obtained in a 74.3% yield. Oligomerization took place during the DTM reaction due to the higher reactivity of thiophene, thus decreasing the yield of the desired monomer. As the alkyl chain length increases, the reactivity decreases, thus decreasing the reaction yield. This again may be due to the steric factors of the longer chains. In the reaction of the 3-alkylthiophenes, three regioisomers are possible, as shown in Fig. 2. Isomer 2x is most prevalent due to the increased electron density at the 2-position on the thiophene ring. However, the reaction can take place at the 5-position, but to a much lower extent, to give isomer 2y. Chloroalkylation at the 5-position occurring twice on the same molecule is infrequent, as seen for isomer 2z. By 1 H NMR analysis, we found that the isomers were generally present in the ratio of 90:9:1 Ž2x:2y:2z.. This can be interpreted as indicating that the 2-position is roughly 10 times more reactive to chloroalkylation than the 5-position. The regioisomers were not separated or isolated. Thus, the ratios were determined by comparing the hydrogen integration ratios of the bridgecarbon hydrogens for each regioisomer. Isomer 2x gave a singlet at roughly d 4.15 ppm while isomer 2y gave a singlet that was shifted upfield roughly 0.02 ppm. Isomer 2z gave a singlet that was shifted upfield 0.01 ppm. These were verified as the correct hydrogens by COSY NMR analysis and comparison with the ring hydrogen peaks, which give different splitting patterns due to the isomer structure differences. By comparing the ratios, we were able to obtain the ratios indicated in Table 1, which were generally comparable to ratios obtained by GC analysis. It should be noted that the NMR data presented in the experimental section represent the major isomer only.

Fig. 3. Optical absorbance spectra of PDTM Ž3a., PDMeTM Ž3b., PDBuTM Ž3c., and PDOctTM Ž3d. in chloroform.

3.2. Polymer synthesis and structure The monomers, 1, were oxidatively polymerized with ironŽIII. chloride w19x. Scheme 2 shows the expected linear polymer structure, 3. This structure is anticipated, as coupling at the 5-position of thiophenes is electronically preferred to the 4-position and the method of polymerization eliminates mislinkages at the 4-position w19x. The coupling reactions yielded polymers in good yields, as shown in Table 2. The solubility of the polymer was a function of the length of the alkyl side chain. As seen in previous polythiophene research w13–15x, polymer solubility in chloroform increases with an increase in the alkyl chain length. With this increase in solubility, the polymers were readily characterized using 1 H NMR and UV spectroscopy. The yield of PDMeTM was found to be greater than 100%. This can be accounted for by the observance in the elemental analysis that the polymer presumably 0contained small amounts of inorganic material assumed to be iron salts, or other contaminating organics, which, due to the low solubility of 3b were difficult to remove by extraction. 3.3. 1H NMR and elemental analysis characterization Analysis of soluble polymers 3 by 1 H NMR indicated that a portion of the bridge hydrogens was missing. This was evident in the presence of a peak between d 5.7 and

Table 2 X Results of polymerization of 2,2 -diŽalkylthienyl.methanes Polymer

Name

Chloroform Solubility Ž%.

Yield Ž%.

3a 3b 3c 3d

PDTM PDMeTM PDBuTM PDOctTM

5.7 15 60.6 )95a

71.9 103 75 76.3

a

Polymer is miscible with chloroform.

Fig. 4. Example of Jenekhe’s polymer.

K.J. Hoffmann et al.r Synthetic Metals 113 (2000) 161–166

Scheme 3. Polymer structure containing quinoidal and broken p-conjugated segments.

5.5 ppm, which indicates the presence of quinoidal segments in the polymer w7x, while the peak at d 3.3 to 3.5 ppm was smaller than required for a non-dehydrogenated polymer. The peak integrations varied depending upon the polymer. For example, polymer 3b had an integration ratio of the peaks 3.5–3.3 ppm:5.7–5.5 ppm equal to 6:1, while polymer 3c had a ratio of 2:1. This indicates that the degree of quinoidality was greater for PDBuTM than for PDMeTM. Elemental analysis further supported the existence of a dehydrogenation mechanism toward quinoidal structures during polymerization. A decrease in the hydrogen content of the polymers was found for all polymers 3. 3.4. UV–Vis spectroscopy Analysis of the polymers by UV spectroscopy indicated that the polymers may contain not only segments with broken p-conjugation, but also quinoidal segments. Fig. 3 shows the chloroform solution spectra of polymers 3. It should be noted that the absorption intensities in Fig. 3 are arbitrary, while the intensities indicated in the experimental section represent the specific extinction coefficient, a. The chloroform solutions of the polymers were red-brown, and their spectra contained three separate absorption bands. The band between 237 and 240 nm is assigned to the thiophene moieties, while the absorptions between 308 and

165

337 nm may be assigned to p–p ) transition of bithiophene segments w7x. These absorptions were expected when compared to previously synthesized bithiophene molecules, i.e. bithiophene has lmax at 305–310 nm. However, a broad absorption band between 400 and 900 nm with maxima between 460 and 494 nm was unexpected for polymers containing isolated bithiophene moieties. Chen and Jenekhe w8x have published a study on the extent of dehydrogenation of similar polymers containing a mono-substituted bridge carbon, as shown in Fig. 4. Their work has shown that as the extent of dehydrogenation increases to 100%, the intensities of the lowest energy absorption peak Ž580 nm. in UV increases to become more intense than the corresponding bithiophene absorption band at 325 nm. In accordance with these results, the spectra of polymers 3 show the lowest energy band is more intense than the bithiophene band. This therefore indicates that dehydrogenation of the bridge carbons took place to give quinoidal segments. Thus, the structure of the polymer is more correctly described by the formula shown in Scheme 3, where the quinoidal and the broken p-conjugated segments vary based on the degree of dehydrogenation in the respective polymer. Structure 5 takes into account the UV–Vis, 1 H NMR, and elemental analysis data, which indicate the described bridge carbon dehydrogenation during oxidative polymerization. 3.5. Electrical conductiÕity of the polymers Conductivity measurements were done on the polymers to determine the influence of structural features on conductivity. The conductivities were measured using pressed powder pellets applying the four-point probe method. Table 3 includes data obtained from these measurements. The only polymer to exhibit conductivity prior to iodine-doping was the insoluble PDMeTM. This inherent conductivity is possibly due to the existence of p-stacking. All of the polymers showed a significant increase in conductivity after doping with iodine. We attribute the existence of conductivity to the partial quinoidal nature of the polymers. It is evident that in a purely broken p-conjugated system, the conjugation length

Table 3 X Conductivity measurements of polyw2,2 -diŽalkylthienyl.-methanexs Polymer

Chloroform solubility

Color

s ŽSrcm., undoped

s ŽSrcm., doped

PDTM PDMeTM

Insoluble Insoluble Soluble Insoluble Soluble Soluble

Black Black Purple-brown Black Black Purple wax

- 1e–10 4.0e–6 - 1e–10 - 1e–10 - 1e–10

1.9e–6 6.5e–5

PDBuTM PDOctTM a b

Became too brittle to measure conductivity. Waxy consistency did not allow for conductivity measurement.

UUU b

UUU a

9.1e–6 6.8e–6

UUU b

166

K.J. Hoffmann et al.r Synthetic Metals 113 (2000) 161–166

would be a bithiophene moiety. However, due to the dehydrogenation and existence of quinoidal segments, the p-conjugation length is longer than a bithiophene segment. This extended p-conjugation, therefore, induces the ability to conduct electricity upon doping. Other conduction mechanisms, which we believe may be having an effect, include intermolecular p-stacking w9x. Future work will investigate the abilities of related broken p-conjugated polymers to form p-stacks.

4. Conclusion The polymers of 2,2X-diŽalkylthienyl.methanes exhibit an increasing solubility as the alkyl chain length increases, allowing characterization using standard analytical methods. Our objective was to produce broken p-conjugated polymers. However, the polymers included quinoidal segments due to oxidative dehydrogenation at the bridge carbon during polymerization. Future work will involve blocking the bridge carbon to avoid dehydrogenation so as to obtain broken p-conjugated systems.

Acknowledgements The authors wish to express their gratitude to the Norwegian National Research Council for financial support.

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