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Low band-gap pyrrole-based conducting polymers
ELSEVIER
Synthetic
Metals
84 (1997)
451-452
Low band-gap pyrrole-based conducting polymers A Berlin*a, A Canavesia, G. Pagan?, G. Schiavonb, S. Zecchinb, G. Zottib aDipatimento di Chimica Organica e Industriale dellUniversitd e Centro CNR, via C. Golgi 19, 20133 Milano, Italy bIstituto di Polarografa ed Elettrochimica Preparativa de1 CNR, c.o Stati Uniti 4, 35020 Padova, Italy
Abstract Low band-gap pyrrole-based polymers showing narrow potential windows electron rich and electron poor moieties.
of conductivity have been obtained by alternation of
Keywords: heterocycle synthesis, electrochemical polymerization, low-bandgap conjugated polymers.
1. Introduction
2. Results
Among suggested strategies for the obtainment of narrow bandgap conducting polymers, one involves the alternation of donor (D) and acceptor (A) groups in the conjugated chain [ 11. Alternation of D and A units in a polyconjugated polymer may give charge localization upon oxidation (doping) and affect also the conductive behavior. Following this approach we synthesized the monomers shown below. They are constituted of two pyrrole units joined through the position 2,2’ by different moieties properIy designed. to modulate the electronic properties of the system. In the corresponding polymers dipyrrole units @ groups) will be intercalated with the conjugative spacers (A groups).
Compounds 1, 2, 5 have been prepared as described in the literature [2-31; the synthesis of 3, 4, 6, 7 and 8 witl be described elsewhere [4]. Electrochemical experiments were performed in acetonitrile + 0.1 M tetrabutylammonium perchlorate (IBAP) under nitrogen in three electrode cells, at 25°C. The counter electrode was platinum; reference electrode was a silver/O.1 M silver perchlorate in acetonitrile (0.34V vs SCE). The cyclic voltammogram (CV) of all the monomers displays a single irreversible oxidation peak; continuous cycling of the potential over the peak develops the redox cycle of the growing electroactive polymer. While for poly(2), poly(4) and poly(6) the CV has the usual shape of a flat featureless sigmoidale curve (Fig. 2a), the CV picture of the polymers derived from the others shows two redox cycles (Fig. lb), the first of which is splitted into two peaks. The redox potentials are reported in the Table. -EQCM analysis allowed the determination of the charges involved in the redox cycles, which are 1 and 0.5 eleztrons per dipyrrolic unit. The energy gaps (Eg) of the relevant polymers are reported in Table 1. The Eg of poly(1) is close to the theoretical value of 2.5 eV arising from the weighted average of the optical gaps of polypyrrole and polyacetylene. Analogously for poly(3) and poly(s) the optical gaps are closed to the values extrapolated from those of the corresponding homopolymers. Poly(Z), poly(7) and poly(8) display even lower gaps. The weight averaged value of Eg calculated for these polymers from the corresponding homopolymers is higher than the experimental one. The reason must be sought in the conjugation of the CN group with the ethylene group and thereby with the pyrrole ends,
3
2
1
6
5
4
7
8
* Correspondingrurthor. 0379-6779/97/$17.00
0 1997 Ekevier
PII SO3794779(96)03982-3
Science SA
All rights
rez&rved
452
A. Berlin
et al. /Synthetic
Metals
84 (1997)
451-452
Table Redox potentials, UV-vis maximum absorption, optical gap and maximum conductivity of the polymers monomer E”/V wmn EgleV 1 -0.45/0.3/0.7 480 2.6 2 -0.05 560 2.2 3 -0.3/O.l/O.6 470 2.65 4 -0.3 460 2.7 5 -0.3fO.2lO.65 440 2.8 6 0.2 465 2.7 7 -0.2/0.2/0.85 630/770sh 1.6 8 0.1/o.51540/630sh 2.0
with a consequent stabilizationof quinoid structures. In-situ conductivity measurements show that poly(1) goes from the insulatingneutralform to a low conductivity oxidized statethrough a maximum locatedwithin the redox cycle (Fig. la).
zs-
(6) a
2.0k s 7.5-
o/S cm-l 15 2
2 0.15 0.3 0.1 0.5 0.5
The conductivebehaviorof poly(3) is evenmore surprising. In this case,in which the two redox voltammetric cycles are stable, the conductivity shows two windows in their correspondence (Fig. lb). A similar behavior is displayedby poly(5), althoughat high potential the polymer is affected by degradation.Both poly(7) and poly(8) show one window of conductivity centered at the redox cycle. Poly(2) (Fig. 2a), poly(4) andpoly(6) behavelike polypyrrole. It has beenobservedthat (i) the width of the conductivity windowsW and the potential rangeof the cyclic voltammetry redox cycles AE” are almostthe same;(ii) u decreases asW is decreased. 3. Conclusion
i.O-
0.5-
-1.0
4.5
0.0
0.0-r -1.0
0.5
-0.5
0.0
0.5
1.0
E/v
EN
Fig. 1In-situ o vs potentialof (a) poly(1) andof(b) poly(3).
This behavior is completely different from that of polypyrrole that is highly conductingin a longpotentialplateau (Fig. 2b).
In accord with theory we have shownthat alternation of donor and acceptor groups is a successfulmethodologyfor inducinga decreaseof bandgapin conjugatedmixed polymers. In particularthe decrease, which isnormally additive, is strongly enhancedin the presenceof specificcharge-transferinteractions betweenthe two groups. The polymersderivedfrom the asymmetricmonomers(2, 4, 6), behaveaspolypyrrole, while thosefrom the symmetricones shownarrow windows of conductivity. As for the latter their structure may be consideredas intermediatebetweenthat of polypyrrole, which is oxidized in a long cyclic voltammetry plateau,andthat of isolateddipyrrole unitswhich would undergo a one-electron oxidation [5] in a normally narrow redox bell-shaped process. References
[l] E.E. Havinga,PolymerBull., 29 (1992) 119. [2] G. Pagani,A. Berlin, A. Canavesi,G. Schiavon,S. Zecchin andG. Zotti, Adv. Muter, in press. [3] J.R. Reynolds,AR Katritzky, J. Soloducho,S. Belyakov, G.A. Sotzing and M. Pyo, Macromolecules, 27 (1994) 7225. [4] A. Berlin, A. Canavesi,G. Pagani,G. Schiavon,S. Zecchin -1.0
-0.5
0.0 EN
Fig. 2 In-situ
0.5
1.0
-1.0
-0.5
0.0
0.5
E/v
o vs potentialof (a) poly(2) and of(b) polypyrrole.
andG. Zotti, manuscriptin preparation. [5] G. Zotti, S. Martina, G. Wegner and A.D. Schluter,Adv. Mater, 4 (1992)798.
Synthetic
Metals
84 (1997)
451-452
Low band-gap pyrrole-based conducting polymers A Berlin*a, A Canavesia, G. Pagan?, G. Schiavonb, S. Zecchinb, G. Zottib aDipatimento di Chimica Organica e Industriale dellUniversitd e Centro CNR, via C. Golgi 19, 20133 Milano, Italy bIstituto di Polarografa ed Elettrochimica Preparativa de1 CNR, c.o Stati Uniti 4, 35020 Padova, Italy
Abstract Low band-gap pyrrole-based polymers showing narrow potential windows electron rich and electron poor moieties.
of conductivity have been obtained by alternation of
Keywords: heterocycle synthesis, electrochemical polymerization, low-bandgap conjugated polymers.
1. Introduction
2. Results
Among suggested strategies for the obtainment of narrow bandgap conducting polymers, one involves the alternation of donor (D) and acceptor (A) groups in the conjugated chain [ 11. Alternation of D and A units in a polyconjugated polymer may give charge localization upon oxidation (doping) and affect also the conductive behavior. Following this approach we synthesized the monomers shown below. They are constituted of two pyrrole units joined through the position 2,2’ by different moieties properIy designed. to modulate the electronic properties of the system. In the corresponding polymers dipyrrole units @ groups) will be intercalated with the conjugative spacers (A groups).
Compounds 1, 2, 5 have been prepared as described in the literature [2-31; the synthesis of 3, 4, 6, 7 and 8 witl be described elsewhere [4]. Electrochemical experiments were performed in acetonitrile + 0.1 M tetrabutylammonium perchlorate (IBAP) under nitrogen in three electrode cells, at 25°C. The counter electrode was platinum; reference electrode was a silver/O.1 M silver perchlorate in acetonitrile (0.34V vs SCE). The cyclic voltammogram (CV) of all the monomers displays a single irreversible oxidation peak; continuous cycling of the potential over the peak develops the redox cycle of the growing electroactive polymer. While for poly(2), poly(4) and poly(6) the CV has the usual shape of a flat featureless sigmoidale curve (Fig. 2a), the CV picture of the polymers derived from the others shows two redox cycles (Fig. lb), the first of which is splitted into two peaks. The redox potentials are reported in the Table. -EQCM analysis allowed the determination of the charges involved in the redox cycles, which are 1 and 0.5 eleztrons per dipyrrolic unit. The energy gaps (Eg) of the relevant polymers are reported in Table 1. The Eg of poly(1) is close to the theoretical value of 2.5 eV arising from the weighted average of the optical gaps of polypyrrole and polyacetylene. Analogously for poly(3) and poly(s) the optical gaps are closed to the values extrapolated from those of the corresponding homopolymers. Poly(Z), poly(7) and poly(8) display even lower gaps. The weight averaged value of Eg calculated for these polymers from the corresponding homopolymers is higher than the experimental one. The reason must be sought in the conjugation of the CN group with the ethylene group and thereby with the pyrrole ends,
3
2
1
6
5
4
7
8
* Correspondingrurthor. 0379-6779/97/$17.00
0 1997 Ekevier
PII SO3794779(96)03982-3
Science SA
All rights
rez&rved
452
A. Berlin
et al. /Synthetic
Metals
84 (1997)
451-452
Table Redox potentials, UV-vis maximum absorption, optical gap and maximum conductivity of the polymers monomer E”/V wmn EgleV 1 -0.45/0.3/0.7 480 2.6 2 -0.05 560 2.2 3 -0.3/O.l/O.6 470 2.65 4 -0.3 460 2.7 5 -0.3fO.2lO.65 440 2.8 6 0.2 465 2.7 7 -0.2/0.2/0.85 630/770sh 1.6 8 0.1/o.51540/630sh 2.0
with a consequent stabilizationof quinoid structures. In-situ conductivity measurements show that poly(1) goes from the insulatingneutralform to a low conductivity oxidized statethrough a maximum locatedwithin the redox cycle (Fig. la).
zs-
(6) a
2.0k s 7.5-
o/S cm-l 15 2
2 0.15 0.3 0.1 0.5 0.5
The conductivebehaviorof poly(3) is evenmore surprising. In this case,in which the two redox voltammetric cycles are stable, the conductivity shows two windows in their correspondence (Fig. lb). A similar behavior is displayedby poly(5), althoughat high potential the polymer is affected by degradation.Both poly(7) and poly(8) show one window of conductivity centered at the redox cycle. Poly(2) (Fig. 2a), poly(4) andpoly(6) behavelike polypyrrole. It has beenobservedthat (i) the width of the conductivity windowsW and the potential rangeof the cyclic voltammetry redox cycles AE” are almostthe same;(ii) u decreases asW is decreased. 3. Conclusion
i.O-
0.5-
-1.0
4.5
0.0
0.0-r -1.0
0.5
-0.5
0.0
0.5
1.0
E/v
EN
Fig. 1In-situ o vs potentialof (a) poly(1) andof(b) poly(3).
This behavior is completely different from that of polypyrrole that is highly conductingin a longpotentialplateau (Fig. 2b).
In accord with theory we have shownthat alternation of donor and acceptor groups is a successfulmethodologyfor inducinga decreaseof bandgapin conjugatedmixed polymers. In particularthe decrease, which isnormally additive, is strongly enhancedin the presenceof specificcharge-transferinteractions betweenthe two groups. The polymersderivedfrom the asymmetricmonomers(2, 4, 6), behaveaspolypyrrole, while thosefrom the symmetricones shownarrow windows of conductivity. As for the latter their structure may be consideredas intermediatebetweenthat of polypyrrole, which is oxidized in a long cyclic voltammetry plateau,andthat of isolateddipyrrole unitswhich would undergo a one-electron oxidation [5] in a normally narrow redox bell-shaped process. References
[l] E.E. Havinga,PolymerBull., 29 (1992) 119. [2] G. Pagani,A. Berlin, A. Canavesi,G. Schiavon,S. Zecchin andG. Zotti, Adv. Muter, in press. [3] J.R. Reynolds,AR Katritzky, J. Soloducho,S. Belyakov, G.A. Sotzing and M. Pyo, Macromolecules, 27 (1994) 7225. [4] A. Berlin, A. Canavesi,G. Pagani,G. Schiavon,S. Zecchin -1.0
-0.5
0.0 EN
Fig. 2 In-situ
0.5
1.0
-1.0
-0.5
0.0
0.5
E/v
o vs potentialof (a) poly(2) and of(b) polypyrrole.
andG. Zotti, manuscriptin preparation. [5] G. Zotti, S. Martina, G. Wegner and A.D. Schluter,Adv. Mater, 4 (1992)798.