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Spectroscopic study of polyazopyrroles (a narrow band gap system)
ELSEVIER
SyntheticMetals 101(1999) 440-441
SPECTROSCOPIC STUDY OF POLYAZOPYRROLES (A NARROW BAND GAP SYSTEM) Jordan Del Nero , Bernard0 Laks Institute de Fisica, Universidade Estadual de Campinas, 13083-970
Campinas, Sdo Paula, Brazil
Abstract
Polyazopynolesaresystemsthat arederivedfrom the dipyrrolesby insertingan azo group amongthe pyrrolesrings.This newpolymer presenta significantdecrease in energygaploweringfrom 2.5 eV to 1.0 eV. In this work we presenta theoreticalabsorptionspectrumobtainedby the ZINDOlS methodusingthe geometries optimizedfrom the AM1 andPM3 semiempirical methods.As thereareexperimentalevidencesthat this systempresentspolarondefects,we alsoincludethe effect of the polarontypedefectson theelectronicstructure.Theresultsarein goodagreement with the experimental ones. keywords: Polyazopynole,Polaron, TheoreticalAbsorptionSpectrum.
1. Introduction
Films of the Polyazopyrrole (PAzo) polymer was experimentally investigated through techniques of cyclic voltanommetry(CV), UV-vis-NIR spectroscopy[l] with the objectiveof studyingmaterialswith high intrinsicconductivity. This High conductivity was obtainedthrough the structural defectspresenceof polarontype. Another material of the polyazopyrroles class is the polyazoarylenesthat differ from PAzo by just possessing a pyrrole ring in the monomer and the polyazothiophenes substitutionof thenitrogenatomof the ringsfor sulfur atoms. In the figure 1 we can see esquematicallythe monomer structure for thepolyazopyrrole,wherethe atoms1 -> 4, 8, lo>12 are carbonsand the atoms5->7, 9 are nitrogensand i3>20 correspondto hydrogen. Figure1.PAzomonomer.
In this work we presentthe calcu&ions of the electronic structure for the polyazopyrrole through semiempiricals methodsof AM1 andPM3 type.We treated the systemwith 1, 2 and3 oligomericsunits.Thesecalculationsweremadeusing the MOPAC [2] program,includingthe moleculeswith +1,+2,1,-2 charges. The ZINDOlSpectroscopic[3] programwas usedto simulate the absorptionspectrumcoupledwith the geometricstructures obtainedthrough AM1 and PM3, what allowed compatible
resultswith the experimentalabsorptionsfor thesepolymeric ones. The method INDO/l was chosen for being parameti? specificallyto describethe ultraviolet-visibleoptic transitions in organicmaterials. For the calculationsof optimizationgeometricand absorption we used a rigorous criterious for the convergence,when comparedwhich originalreferences [4], a step-sizeof 0.05 tid the bond-ordersmatrix an approachof 1u5, respectivelyfor MOPAC andZINDO. 2. Results and Discussion
In table 1we canseethe monomerchargedistribution.The first columnrepresentsliquid chargesin its respectivesite for the neutral molecule,that will be the comparisonbase for the chargedsystems. Column2 represents the modificationby a positive chargeon the whole molecule.When the sign of this “comparative charge”is positivethat meansthe atomunderthe effect-if the chargeloseelectronandit receiveelectronwhenthe signgoes negative.In the columns3,4 and5 arestripedthe comparative charge,for respectively,N-(+2), N-(-l), N-(-2). Analyzing columns2 and3 of table 1 showusa concentration of the chargein the nitrogens,mainly in NS and N9 that correspondto the ring. The carbonsCl, C3, C4, ClO, C12, C8 exhibit anaccentuated lossof charge.In the caseof the charged systemsnegatively(columns4 and 5) the comparativecharge showusa chargegain in the carbonsCl, C3, ClO, Cl2 andin the nitrogenN6 andN7 with highconcentration. Theabsorptionspectraof neutralpolyazopyrrolecanbe seenin the figure 2 for the monomer,dimerand timer, and the main absorptions correspondthe transitionH->L with the oscillator strenghtof 1.0656,1.8489,2.5980,respectively.
0379-6779/99/$ - seefrontmatter0 1999ElsevierScience S.A. All rightsreserved. PII: SO379-6779(98)01 134-5
8. L&s,
J. Del
Nero
/ Synthetic
Metals
IO1 (1999)
441
440441
Table 1. Liquid chargcs(column 1) and Comparative charge for monomx of PAzo.
The second largest peak for the monomer represents a transition composed mainly by states H-3->L+l and the third H->L+2 with the oscillator strenght 1.0074 and 0.3475, respectively. For the dimer we have the second largest peak in an area of energy around 5.7 eV and oscillator strenght of 1.2046, it
number
presents competition among the transitions H-4->L, H-6->L and H->L+4, and this last transition also appears among the more strongs for the trimer, but with value of different energy, around 4.6 eV and with oscillator strenght 0.6367. The transitions H->L+2 and H-2->L also appear with oscillator strenght 0.188 and 0.1804 (3 and 4 peaks). In the figure 3 we have the absorption spectrum of PAzo with charge +I for the monomer, dimer and trimer and the strongest transitions continue being the transition composed mainly by H->L with oscillator strenght 0.5379, 1.3885 and 1.1911. We have that the first optically activate transition between 1000 nm and 1800 nm, where we can see more clearly in the figure 4. In this figure we have the first optically activate transition in function of ring numbers present in the pyrrole molecule. Clearly we have the distinction among the two systems, neutral and under the effect of the polaron type defect. The neutral system show us a minimum energy gap of 2.5 eV. The other
of rings
curve show us that the polaron defect provokes a considerable red shift characterizing a narrow band gap materials. 3. Conclusion 1. The methodology utilized for the absorption spectrum simulation was shown in agreement when compared with the experimental results of the literature [I]. 2. The comparative charge showed us that the polaron is defined in the area of the molecule. 3. The inclusion of the azo group in polypyrrole provoked significant decrease of the gap characterizing it as low band material gap. 4. The methodology was shown efficient for the design of new materials. 5. References Ul G.Zotti, S.Zechin, GSchiavon, A.Berlin, G.Pagani, A Canavesi,G.Casalbore-Miceli,Synth.Met. 78 (1996)51. PI MOPAC program,version6.0 (QCEP455). 131 J.Ridley,M.C.Zemer,Theoret.Chim.Acta, 32, 111(1973); J.Ridley,M.C.Zemer,Theoret.Chim.Acta, 42, 223 (1976); A.D.Bacon, M.C.Zemer, Theoret. Chim. Acta, 53 (1979); J.D.Head,MCZemer, Chem.Phys.Lett., 122,264 (1985); J.D.Head,M.C.Zemer,Chem.Phys.Lett.. 131,359 (1986); W.P.Anderson,W.D.Edwards,M.C. Zemer, Inorg. Chem., (1986); W.D. Edwards, M.C.Zemer, 25, 2728 Theoret.Chim.Acta,72, 347 (1987). [41 M.J.S.Dewar,E.G.Zoebisch,E.F.HeaIy,J.P.Stewart,J. Am. Chem. Sot., 107 (1985) 3902; M.J.S.Dewar, W.Thiel, J. Am. Chem.Sot., 99, 4899, 4907 (1977); J.J.P.Stewart,J. Comp.Chem.,10,209 (1989).
SyntheticMetals 101(1999) 440-441
SPECTROSCOPIC STUDY OF POLYAZOPYRROLES (A NARROW BAND GAP SYSTEM) Jordan Del Nero , Bernard0 Laks Institute de Fisica, Universidade Estadual de Campinas, 13083-970
Campinas, Sdo Paula, Brazil
Abstract
Polyazopynolesaresystemsthat arederivedfrom the dipyrrolesby insertingan azo group amongthe pyrrolesrings.This newpolymer presenta significantdecrease in energygaploweringfrom 2.5 eV to 1.0 eV. In this work we presenta theoreticalabsorptionspectrumobtainedby the ZINDOlS methodusingthe geometries optimizedfrom the AM1 andPM3 semiempirical methods.As thereareexperimentalevidencesthat this systempresentspolarondefects,we alsoincludethe effect of the polarontypedefectson theelectronicstructure.Theresultsarein goodagreement with the experimental ones. keywords: Polyazopynole,Polaron, TheoreticalAbsorptionSpectrum.
1. Introduction
Films of the Polyazopyrrole (PAzo) polymer was experimentally investigated through techniques of cyclic voltanommetry(CV), UV-vis-NIR spectroscopy[l] with the objectiveof studyingmaterialswith high intrinsicconductivity. This High conductivity was obtainedthrough the structural defectspresenceof polarontype. Another material of the polyazopyrroles class is the polyazoarylenesthat differ from PAzo by just possessing a pyrrole ring in the monomer and the polyazothiophenes substitutionof thenitrogenatomof the ringsfor sulfur atoms. In the figure 1 we can see esquematicallythe monomer structure for thepolyazopyrrole,wherethe atoms1 -> 4, 8, lo>12 are carbonsand the atoms5->7, 9 are nitrogensand i3>20 correspondto hydrogen. Figure1.PAzomonomer.
In this work we presentthe calcu&ions of the electronic structure for the polyazopyrrole through semiempiricals methodsof AM1 andPM3 type.We treated the systemwith 1, 2 and3 oligomericsunits.Thesecalculationsweremadeusing the MOPAC [2] program,includingthe moleculeswith +1,+2,1,-2 charges. The ZINDOlSpectroscopic[3] programwas usedto simulate the absorptionspectrumcoupledwith the geometricstructures obtainedthrough AM1 and PM3, what allowed compatible
resultswith the experimentalabsorptionsfor thesepolymeric ones. The method INDO/l was chosen for being parameti? specificallyto describethe ultraviolet-visibleoptic transitions in organicmaterials. For the calculationsof optimizationgeometricand absorption we used a rigorous criterious for the convergence,when comparedwhich originalreferences [4], a step-sizeof 0.05 tid the bond-ordersmatrix an approachof 1u5, respectivelyfor MOPAC andZINDO. 2. Results and Discussion
In table 1we canseethe monomerchargedistribution.The first columnrepresentsliquid chargesin its respectivesite for the neutral molecule,that will be the comparisonbase for the chargedsystems. Column2 represents the modificationby a positive chargeon the whole molecule.When the sign of this “comparative charge”is positivethat meansthe atomunderthe effect-if the chargeloseelectronandit receiveelectronwhenthe signgoes negative.In the columns3,4 and5 arestripedthe comparative charge,for respectively,N-(+2), N-(-l), N-(-2). Analyzing columns2 and3 of table 1 showusa concentration of the chargein the nitrogens,mainly in NS and N9 that correspondto the ring. The carbonsCl, C3, C4, ClO, C12, C8 exhibit anaccentuated lossof charge.In the caseof the charged systemsnegatively(columns4 and 5) the comparativecharge showusa chargegain in the carbonsCl, C3, ClO, Cl2 andin the nitrogenN6 andN7 with highconcentration. Theabsorptionspectraof neutralpolyazopyrrolecanbe seenin the figure 2 for the monomer,dimerand timer, and the main absorptions correspondthe transitionH->L with the oscillator strenghtof 1.0656,1.8489,2.5980,respectively.
0379-6779/99/$ - seefrontmatter0 1999ElsevierScience S.A. All rightsreserved. PII: SO379-6779(98)01 134-5
8. L&s,
J. Del
Nero
/ Synthetic
Metals
IO1 (1999)
441
440441
Table 1. Liquid chargcs(column 1) and Comparative charge for monomx of PAzo.
The second largest peak for the monomer represents a transition composed mainly by states H-3->L+l and the third H->L+2 with the oscillator strenght 1.0074 and 0.3475, respectively. For the dimer we have the second largest peak in an area of energy around 5.7 eV and oscillator strenght of 1.2046, it
number
presents competition among the transitions H-4->L, H-6->L and H->L+4, and this last transition also appears among the more strongs for the trimer, but with value of different energy, around 4.6 eV and with oscillator strenght 0.6367. The transitions H->L+2 and H-2->L also appear with oscillator strenght 0.188 and 0.1804 (3 and 4 peaks). In the figure 3 we have the absorption spectrum of PAzo with charge +I for the monomer, dimer and trimer and the strongest transitions continue being the transition composed mainly by H->L with oscillator strenght 0.5379, 1.3885 and 1.1911. We have that the first optically activate transition between 1000 nm and 1800 nm, where we can see more clearly in the figure 4. In this figure we have the first optically activate transition in function of ring numbers present in the pyrrole molecule. Clearly we have the distinction among the two systems, neutral and under the effect of the polaron type defect. The neutral system show us a minimum energy gap of 2.5 eV. The other
of rings
curve show us that the polaron defect provokes a considerable red shift characterizing a narrow band gap materials. 3. Conclusion 1. The methodology utilized for the absorption spectrum simulation was shown in agreement when compared with the experimental results of the literature [I]. 2. The comparative charge showed us that the polaron is defined in the area of the molecule. 3. The inclusion of the azo group in polypyrrole provoked significant decrease of the gap characterizing it as low band material gap. 4. The methodology was shown efficient for the design of new materials. 5. References Ul G.Zotti, S.Zechin, GSchiavon, A.Berlin, G.Pagani, A Canavesi,G.Casalbore-Miceli,Synth.Met. 78 (1996)51. PI MOPAC program,version6.0 (QCEP455). 131 J.Ridley,M.C.Zemer,Theoret.Chim.Acta, 32, 111(1973); J.Ridley,M.C.Zemer,Theoret.Chim.Acta, 42, 223 (1976); A.D.Bacon, M.C.Zemer, Theoret. Chim. Acta, 53 (1979); J.D.Head,MCZemer, Chem.Phys.Lett., 122,264 (1985); J.D.Head,M.C.Zemer,Chem.Phys.Lett.. 131,359 (1986); W.P.Anderson,W.D.Edwards,M.C. Zemer, Inorg. Chem., (1986); W.D. Edwards, M.C.Zemer, 25, 2728 Theoret.Chim.Acta,72, 347 (1987). [41 M.J.S.Dewar,E.G.Zoebisch,E.F.HeaIy,J.P.Stewart,J. Am. Chem. Sot., 107 (1985) 3902; M.J.S.Dewar, W.Thiel, J. Am. Chem.Sot., 99, 4899, 4907 (1977); J.J.P.Stewart,J. Comp.Chem.,10,209 (1989).