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Poly(3-alkylthiophenes): Optimising conductivity as a function of regioregularity, dopant and casting solvent
ELSEVIER
Synthetic Metals 101 (1999) 446
Poly(3-alkylthiophenes):
Optimising Conductivity as a Function of Regioregularity, Dopant and Casting Solvent
RonaldL. Meline, RameshK. Kasim,Wei-KangLu andRonaldL. Elsenbaumer*
The University
Department of Chemistry and Biochemistry, Materials Science and Engineering Program, of Texas at Arlington, Arlington, Texas 760194065,
U. S. A.
Abstract Thiopheneand 3-methylthiopheneas castingsolventsare shownto improve the conductivity of cast films of regioregular and regiorandompoly(3-octylthiophenes).The trend is observedfrom samebatchsamplesusingdifferent dopants. Keywords:
Polythiophene,Electrical conductivity, Castingsolvent
In the quest for stable processablepolythiophene, chemically synthesized 3-alkyl substituted polythiophenes (PATS) were initially synthesizedin the mid 1980’s[1,2]. By the early 1990’s regioregular Head to Tail coupled poly(3alkythiophenes) (HT-PATS) were realized and resultant acceptordopedconductivities were shownto be much greater than their regiorandomcousins. RegiorandomPATS contain any number of unfavorable Head to Head (HH) couplings producing sterically driven twisting of thiophene rings resultingin a loss of conjugation. In comparisonsof solution UV-vis spectra of PAT’s, a red shift of hmax is found in regioregularHT-PATS comparedto regiorandomPATS. The regioregularHT-PATS have lower energy rc to n* transitions, indicating longer conjugation length [3]. The origin of thermochromismand solvatochromismof PATS has been consideredto be a result of the interruption of conjugation caused by the aforementioned twisting [4]. A poly(3hexylthiophene) solution in chloroform displays a color changefrom yellow to magentawith the addition of a poor solvent. It hasbeennoted that the optical absorptionof good solvents is similar to that of thin solid films at higher temperatures[S]. Along with regioregularity it has been demonstrated that in generalconductivitiesof PATS increaseas the size of the sidechain decreases[6]; therefore, there is a price for increasedsolubility. Recently, novel dopantshave been utilized to enhancethe stability and conductivities of PATS [7,8]. Along with regio-orientation and dopant, the importanceof casting solvent is addressedherein. We have found that casting regiorandompoly(3-octylthiophene)(POT) and regioregular poly(3-octylthiophene) (HT-POT) from thiopheneor 3-methyl thiopheneleadsto films with enhanced conductivities after doping as compared to casting from xylenes (table). It is very important here to note that such a study requiresdoping and castingfrom singlesamplebatches as POTS are quite batch dependant. Although better conductivitiesfor POTShavebeenreported,a definitetrendcan be observedfor the test samplesusedhere. The regiorandom POT was synthesizedby coupling thiophenefrom anhydrous FeC13giving a batch with a Mw = 100842and a Mn = 35394. The regioregularPAT waspurchasedfromAldrich andshoweda
Mw = 31281anda Mn = 9685. A smallredshift in hmax from 433 to 437 from a xylene solutionto a thiopheneor S-methyl thiophenesolution was exhibited for the regiorandomPOT, anda small red shift in Xmax from 449 to 452 from a xylene solutionto a thiopheneor 3-methyl solution was exhibited for the regioregularPOT. As an indicationof the compatabilityof the thiophenesolventsfor both poly(3-octylthiophenes), the POTS were more soluble in both solvent systems than in xylenes. This is an indication that PATS with shorter side chain lengthsshould be even more applicableto a thiophene basedcasting solvent system. Average Conductivity (S/cm)z SolventDopant POT H-l--POT xylenesI FeC13 1 17 5 115 xylenes/ AuCl3 3 thiophene/ FeC13 42 8 187 thiophene/ AuC13 3-Me thiophene/ FeC13 3 37 3-Me thiophene/ AuC13 9 20s *All Dopantsolutionsare acetonitrile. Upon doping, someAu(O)is platedaccordingto ESCA whenAuC13 is used.Noteref. 7. 1. R. L. Elsenbaumer, K.-Y. Jenand R. Oboodi,Synth. Met.
1986,I5,169. 2. G.G. Miller, R. L. Elsenbaumer, J. Chem. Sot., Chem. Commun. 1986, 1346. 3. R.D. McCullough,Adv. Mater. 1998, 10, 93. refs cited. 4. K.C. Park andK. Levon, Macromolecules, 1997,30, 317s. 5. 0. InganBs,W.R. Salaneck,J.-E. &terholm and J. Laakso,Synth. Met. 1988,22, 395. 6. Y. Wang and M.F. Rubner,Synth. Met. 1990,39, 153. 7. M.S.A. Abdou and S. Holdcroft, Synth. Met. 1993, 60, 93. 8. J.-L. Ciprelli, C. ClarisseandD. Delabouglise,Synth.
0379~6779/99/$ - see front matter 0 1999 Elsevier Science S.A. All rights reserved. PII: SO379-6779(98)01 147-3
Met. 1995, 74, 217.
Synthetic Metals 101 (1999) 446
Poly(3-alkylthiophenes):
Optimising Conductivity as a Function of Regioregularity, Dopant and Casting Solvent
RonaldL. Meline, RameshK. Kasim,Wei-KangLu andRonaldL. Elsenbaumer*
The University
Department of Chemistry and Biochemistry, Materials Science and Engineering Program, of Texas at Arlington, Arlington, Texas 760194065,
U. S. A.
Abstract Thiopheneand 3-methylthiopheneas castingsolventsare shownto improve the conductivity of cast films of regioregular and regiorandompoly(3-octylthiophenes).The trend is observedfrom samebatchsamplesusingdifferent dopants. Keywords:
Polythiophene,Electrical conductivity, Castingsolvent
In the quest for stable processablepolythiophene, chemically synthesized 3-alkyl substituted polythiophenes (PATS) were initially synthesizedin the mid 1980’s[1,2]. By the early 1990’s regioregular Head to Tail coupled poly(3alkythiophenes) (HT-PATS) were realized and resultant acceptordopedconductivities were shownto be much greater than their regiorandomcousins. RegiorandomPATS contain any number of unfavorable Head to Head (HH) couplings producing sterically driven twisting of thiophene rings resultingin a loss of conjugation. In comparisonsof solution UV-vis spectra of PAT’s, a red shift of hmax is found in regioregularHT-PATS comparedto regiorandomPATS. The regioregularHT-PATS have lower energy rc to n* transitions, indicating longer conjugation length [3]. The origin of thermochromismand solvatochromismof PATS has been consideredto be a result of the interruption of conjugation caused by the aforementioned twisting [4]. A poly(3hexylthiophene) solution in chloroform displays a color changefrom yellow to magentawith the addition of a poor solvent. It hasbeennoted that the optical absorptionof good solvents is similar to that of thin solid films at higher temperatures[S]. Along with regioregularity it has been demonstrated that in generalconductivitiesof PATS increaseas the size of the sidechain decreases[6]; therefore, there is a price for increasedsolubility. Recently, novel dopantshave been utilized to enhancethe stability and conductivities of PATS [7,8]. Along with regio-orientation and dopant, the importanceof casting solvent is addressedherein. We have found that casting regiorandompoly(3-octylthiophene)(POT) and regioregular poly(3-octylthiophene) (HT-POT) from thiopheneor 3-methyl thiopheneleadsto films with enhanced conductivities after doping as compared to casting from xylenes (table). It is very important here to note that such a study requiresdoping and castingfrom singlesamplebatches as POTS are quite batch dependant. Although better conductivitiesfor POTShavebeenreported,a definitetrendcan be observedfor the test samplesusedhere. The regiorandom POT was synthesizedby coupling thiophenefrom anhydrous FeC13giving a batch with a Mw = 100842and a Mn = 35394. The regioregularPAT waspurchasedfromAldrich andshoweda
Mw = 31281anda Mn = 9685. A smallredshift in hmax from 433 to 437 from a xylene solutionto a thiopheneor S-methyl thiophenesolution was exhibited for the regiorandomPOT, anda small red shift in Xmax from 449 to 452 from a xylene solutionto a thiopheneor 3-methyl solution was exhibited for the regioregularPOT. As an indicationof the compatabilityof the thiophenesolventsfor both poly(3-octylthiophenes), the POTS were more soluble in both solvent systems than in xylenes. This is an indication that PATS with shorter side chain lengthsshould be even more applicableto a thiophene basedcasting solvent system. Average Conductivity (S/cm)z SolventDopant POT H-l--POT xylenesI FeC13 1 17 5 115 xylenes/ AuCl3 3 thiophene/ FeC13 42 8 187 thiophene/ AuC13 3-Me thiophene/ FeC13 3 37 3-Me thiophene/ AuC13 9 20s *All Dopantsolutionsare acetonitrile. Upon doping, someAu(O)is platedaccordingto ESCA whenAuC13 is used.Noteref. 7. 1. R. L. Elsenbaumer, K.-Y. Jenand R. Oboodi,Synth. Met.
1986,I5,169. 2. G.G. Miller, R. L. Elsenbaumer, J. Chem. Sot., Chem. Commun. 1986, 1346. 3. R.D. McCullough,Adv. Mater. 1998, 10, 93. refs cited. 4. K.C. Park andK. Levon, Macromolecules, 1997,30, 317s. 5. 0. InganBs,W.R. Salaneck,J.-E. &terholm and J. Laakso,Synth. Met. 1988,22, 395. 6. Y. Wang and M.F. Rubner,Synth. Met. 1990,39, 153. 7. M.S.A. Abdou and S. Holdcroft, Synth. Met. 1993, 60, 93. 8. J.-L. Ciprelli, C. ClarisseandD. Delabouglise,Synth.
0379~6779/99/$ - see front matter 0 1999 Elsevier Science S.A. All rights reserved. PII: SO379-6779(98)01 147-3
Met. 1995, 74, 217.