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Electrogenerated poly(thiophenes) with extremely low bandgap
Synthetic
Metals
101 (1999)
149
Electrogenerated poly(thiophenes) with extremely low bandgap S. Akoudad, J. Roncali* A&fO.
CNRS bMR 6.501, Universitb dL4ngew
-19015 Angers (France)
Abstract New low bandgap poly(thiophenes)have been sJnthesiz.edby electropolymerizationof precursors combining 3,4et.hylenedioxTthiophcne with thieno[3,4-b]plTazine or benzo[1,2-c:4$c’]bis[1,2,5]thiadiazole. Keywords: poly(thiophenc),lowbandgap conjugated polymers,electrochemical polymerization
Narrow bandgap(Eg) conjugatedpolymersare subjectto a considerablecurrent interestmotivated by their high visible transparency,their ability to be p- and n-dopedand by the ultimate aim of synthesizingzero bandgappolymerswith metallicconduction.’Tanakaet al. have synthesizedpoiymers with the smallest bandgaps known to date by electropolymerization of precursorsbasedon three-ringsystems with a medianquinoidacceptorsuchas thieno[3,4&]pyrazine (TP) or benzothiadiazole(TD) and two thiopheneor pyrrole rings.’ We showherethat clectropolymcrizationof precursors basedon a combinationof 3,4-ethylenedioqtiiophene(EDOT) and with TP and TD (1, 2) leadsto new polymersshowing extremely low bandgapsand improvedstability underredos cycling. n-Hexyl
definedpolymerstructurewith a narrowdistributionof conjugated chainlengths. 0.d 1 .oo
0.00 ’ ”
rrHexyl
2.0
1.0
3.0
EieV
Fig. 1 Opticalspectrumof undopedpoly(1) on IT0 Extrapolationto the baselineof the low enera absorptionedge crosses the IT0 absorptionwall andleadsto an opticalbandgapof 0.36 eV, whichis to our knowledgethe lowestever reportedfor a conjugatedpolynler.3Poly(1) showsnegligiblesolubilityin CH$& and solubility in chlorobenzene is limrted to a few percents.The resulting solutionsshow Ima, at 800 and 1000 nm.. The improvementof solubili@by attachmentof longeralhyl chainson bothcomponents is presentlyunderinvestigation. Preliminarytestsof the stability underredox cycling showed that poly(1) retains80 oioof its eleclroaclivilyafler 1000reduction cyclesbetween-0.20 and-1.50 V at a scanrate of 500 mV s*’ or after 250 full cyclesof both oxidationandreductionat 100mV s-’ between+ 0.60and-1.50V.
Compounds1 and 2 osidize at 0.70 and 0.92VKE respectively. Electropolqmerization was carried out in potentiostatic or potentiodynamicconditions in MeCN and CH& respectivelyat very low substrateconcentrations (5x 10J M). The CV of the polymersshowstwo well definedredos systemscorresponding to the p- and n-dopingprocesses with E” valuesat 0.00 and-1.20 V for the p andn dopingof poly(l) and +0.20 and -1.33 V for poly(2). The potentialdifferencebetween the onset for oxidation and reductionleadsto clcctrochemical References bandgaps of ca. 0.60and1.30eV for poly(1) andpoly(2). [1] J. Roncali,Chem.Rev.97 (1997) 173. While undopedpoly(2) showsanabsorptionmaximumat 665 [2] S.Tanaka.Y. Yamashita,Chem.Mater. 8 (1996)570. nm andan optical bandgapof 1.30 eV, the optical spectrumof [3) S.Akoudad,J. Roncali,Chem.Commun.(1998)2081. neutralpoly(1) on IT0 (Fig. 1) showsa singleabsorptionband with a masniiuriiBt0.86eV ( 1430 nm)andi wctik shoulderat ca. 1.25 eV. Unlike the spectra of most of the small bandgap polymersreportedso far, this spectrumexhibits smallwidth at half maximum of the absorptionband (0.72 eV) and low absorbance in thewholevisibleraneeconsistentwith a well0379-6779/99/S - see front matter 0 1999 Elsevier PII: SO379-6779(98)0 1042-X
Science S.A. All rights reserved.
Metals
101 (1999)
149
Electrogenerated poly(thiophenes) with extremely low bandgap S. Akoudad, J. Roncali* A&fO.
CNRS bMR 6.501, Universitb dL4ngew
-19015 Angers (France)
Abstract New low bandgap poly(thiophenes)have been sJnthesiz.edby electropolymerizationof precursors combining 3,4et.hylenedioxTthiophcne with thieno[3,4-b]plTazine or benzo[1,2-c:4$c’]bis[1,2,5]thiadiazole. Keywords: poly(thiophenc),lowbandgap conjugated polymers,electrochemical polymerization
Narrow bandgap(Eg) conjugatedpolymersare subjectto a considerablecurrent interestmotivated by their high visible transparency,their ability to be p- and n-dopedand by the ultimate aim of synthesizingzero bandgappolymerswith metallicconduction.’Tanakaet al. have synthesizedpoiymers with the smallest bandgaps known to date by electropolymerization of precursorsbasedon three-ringsystems with a medianquinoidacceptorsuchas thieno[3,4&]pyrazine (TP) or benzothiadiazole(TD) and two thiopheneor pyrrole rings.’ We showherethat clectropolymcrizationof precursors basedon a combinationof 3,4-ethylenedioqtiiophene(EDOT) and with TP and TD (1, 2) leadsto new polymersshowing extremely low bandgapsand improvedstability underredos cycling. n-Hexyl
definedpolymerstructurewith a narrowdistributionof conjugated chainlengths. 0.d 1 .oo
0.00 ’ ”
rrHexyl
2.0
1.0
3.0
EieV
Fig. 1 Opticalspectrumof undopedpoly(1) on IT0 Extrapolationto the baselineof the low enera absorptionedge crosses the IT0 absorptionwall andleadsto an opticalbandgapof 0.36 eV, whichis to our knowledgethe lowestever reportedfor a conjugatedpolynler.3Poly(1) showsnegligiblesolubilityin CH$& and solubility in chlorobenzene is limrted to a few percents.The resulting solutionsshow Ima, at 800 and 1000 nm.. The improvementof solubili@by attachmentof longeralhyl chainson bothcomponents is presentlyunderinvestigation. Preliminarytestsof the stability underredox cycling showed that poly(1) retains80 oioof its eleclroaclivilyafler 1000reduction cyclesbetween-0.20 and-1.50 V at a scanrate of 500 mV s*’ or after 250 full cyclesof both oxidationandreductionat 100mV s-’ between+ 0.60and-1.50V.
Compounds1 and 2 osidize at 0.70 and 0.92VKE respectively. Electropolqmerization was carried out in potentiostatic or potentiodynamicconditions in MeCN and CH& respectivelyat very low substrateconcentrations (5x 10J M). The CV of the polymersshowstwo well definedredos systemscorresponding to the p- and n-dopingprocesses with E” valuesat 0.00 and-1.20 V for the p andn dopingof poly(l) and +0.20 and -1.33 V for poly(2). The potentialdifferencebetween the onset for oxidation and reductionleadsto clcctrochemical References bandgaps of ca. 0.60and1.30eV for poly(1) andpoly(2). [1] J. Roncali,Chem.Rev.97 (1997) 173. While undopedpoly(2) showsanabsorptionmaximumat 665 [2] S.Tanaka.Y. Yamashita,Chem.Mater. 8 (1996)570. nm andan optical bandgapof 1.30 eV, the optical spectrumof [3) S.Akoudad,J. Roncali,Chem.Commun.(1998)2081. neutralpoly(1) on IT0 (Fig. 1) showsa singleabsorptionband with a masniiuriiBt0.86eV ( 1430 nm)andi wctik shoulderat ca. 1.25 eV. Unlike the spectra of most of the small bandgap polymersreportedso far, this spectrumexhibits smallwidth at half maximum of the absorptionband (0.72 eV) and low absorbance in thewholevisibleraneeconsistentwith a well0379-6779/99/S - see front matter 0 1999 Elsevier PII: SO379-6779(98)0 1042-X
Science S.A. All rights reserved.