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Photoexcitation spectroscopy of pernigraniline
Synthetic Metals, 41-43 (1991) 1311-1314
PHOTOEXCITATION SPECTROSCOPY
1311
O F PERNIGRANILINE
J.M. LENG, J.M. GINDER, R.P. MCCALL, H.J. YE*, and A.J. EPSTEIN Department of Physics, The Ohio State University, Columbus, OH 43210 (U.S.A.) Y. SUN, S.K. MANOHAR, and A.G. MACDIARMID Department of Chemistry, University of Pennsylvania, Philadelpia, PA 19104 (U.S.A.)
ABSTRACT Absorption and photoinduced absorption spectroscopies are reported for pernigrauiline base (PNB). Direct absorption of PNB shows the band gap energy at 2.3 eV and ~" - lr* transition energies at 3.8 and 4.3 eV. The photoindueed absorption (PA) spectrum at room temperature shows three induced absorption peaks at 1.0 (LE), 1.5 (ME) and 3.0 eV (HE). At a temperature of 10 K, the LE peak becomes two peaks at 1.0 eV (LE1) and 1.3 eV (LE2). The ME peak is very long-lived; in addition, there are very long-lived relatively weak infrared vibrations. Light-induced ESR studies show the long-llved 1.5 eV peak has spin-I/2. We suggest that the LE peak originates from A-B type soliton pairs, while the ME peak originates from massive (~300 me) ring rotational polarons in amorphous regions.
INTRODUCTION Pernigr~nillne, the fully oxidized state of polyaniline, has attracted substantial interest recently [1-4]. Unlike leucoemeraldine base, for which the band gap arises from the C8 rings, pernigrAn~|;ue is proposed to exhibit a Pelerls gap due to eleetron-phonon interactions [2]. A model based on effective bond-length-bond-order parameters was proposed to study the nonlinear excitations in pernigrAn;ilne [2]. Optical absorption spectra of undoped and doped pernigraniline have been reported [1,3,4]. Recently, the central role of ring-torsion-angle freedom in ring-containing polymers was proposed [5]. It is expected that pernigran~l;ne supports solitous and polarons due to ring angle and/or bond length dimerization in this co~ugated polymer. There are three order parameters operative in this system: the average ring angle and its phase (~), the dimerization of the ring angles and its phase (6), and the distortion in the bond lengths and its phase (u). *Present address, Shanghai Institute of Technical Physics, Academia Sinica, Shanghai , P.R. China. 0379-6779/91/$3.50
© Elsevier Sequoia/Printed in The Netherlands
1312
In this paper, we report absorption and photoinduced absorption studies of pernigraniline base. Our results show that the long-llved defect is predomlnsntly polaronic, while the shortlived (msec time scale) defect is likely of so]iron origin involving ~ and u. Amplitude mode analysis of the photolnduced IRAV modes and the photoinduced electronic transitions shows the effective mass of the polarons is very large (,-,300 me), indicating the importance of ring angle dimerizatlon in pernigranillnc.
EXPERIMENT Samples of pernigraniline base were prepared using the method described elsewhere [I]. Direct absorption spectra were measured with a Perkin-Elmer Lambda 9 uv/vis/nlr spectrophotometer in the range of 900-190 nm (1.4-6.0 eV). The near-steady-state photolnduced absorption experiments were carried out using either a tungsten or deuterium lamp as a probe source, which was focused onto the sample, filtered through a grating monochroma{or. The sample transmission T was detected by the appropriate photodiodes. The sample was photoexcited by the output of either an argon-ion laser or a dye laser. The pump beam was mechanically chopped at frequencies between 4 and 400 Hz, causing an induced change in the sample transmission, AT. The fractional change in transmission, - ~ T / T , which is independent of the system response, was then computed. RESULTS Fig. 1 shows chemical structures of polyanlllne at dli~erent oxidation states: leucoemeraldine base (LEB), emeraldinc base (EB), and pernigranillne base (PNB). The uv/vls/nir direct absorption spectra of PNB and EB in N-methyl pyrrolidinone (NMP) solution are shown in Fig. 2. The spectrum of PNB shows one peak at 2.3 eV, and two stronger peaks at 3.8 eV and 4.3 eV. For EB, only two peaks are observed at 1.9 and 3.7 eV, although the broad tail of the 3.7 eV peak could be the result of another small peak at higher energies.
/ (0)
(b)
H
~.N/~
(
\
H
I~\Nz. 0 H
x
i'~'/N " " ~
0\] x
)
I~/~N"T~
vo
H
X
~N~
(C)
\N ~
0°""0 0 0 )]( \N ~
//
~
\\
c .Q8
J'
--- EB
\
2
3
I 4
Energy(eV)
Fig.1 Chemical structure of (a) LEB, (b) EB, and (c) PNB.
Fig.2 Direct absorption spectra of EB and PNB.
1313
The near-steady-state photolnduced absorption spectra of PNB in the range 0.5-3.3 eV at temperatures of 300 K and 10 K are shown in Fig. 3 for Epamp=2.41 eV, near the band gap of PNB (see Discussion). The spectrum at 300 K shows PA features at 1.0 (LE), 1.5 (ME), and 3.0 eV (HE), and a photoinduced bleaching (PB) feature at 2.0 eV. At a temperature of 10 K, the original LE peak is replaced by two peaks at 1.0 (LE1) and 1.3 eV (LE2). 10
' ' ' 1
....
I ....
..%
\
E) =
I
I ' ' ' ' l ' ' ' ' l
2.41
eV
~ =
300
=
10
K K
/
_-
P~'PlOO mW/cm' --5 f
-10
''~' 0.5
=
22.4
I
1.0
,
Hz
,
,
,
[
1.5
,
,
,
,
I
,
,
2.0
,
,
I
,
2.5
,
,
,
I
. . . .
3,0
3.5
Energy(eV)
Fig. 3 Photoinduced absorption spectra of PNB.
DISCUSSION We associate the observed ,~2.3 eV absorption peak with the absorption across the predicted Peierls gap. Boudreaux et al. calculated that the valence band of LEB has a bandwidth of ,,,2.9 eV and becomes half filled for PNB, with a gap of ~1.4 cV opening at the Brillonln zone edge, when two electrons and two protons are taken from each tworing repeat unit of LEB [6]. Theoretical calculations by SjSgren and Stafustr~m [7] of the optical absorption spectrum of pernigranillne in tetramer form demonstrate that the 2.3 eV transition involves charge transfer from benzenoid to quinoid rings. This transition evolves to the Peierls gap in PNB polymer calcnlated by dos Santos and Br~das [2]. The decay of the photoinduced bleaching at 2.0 eV is mainly bimolecular, in contrast to the monomolecnlar dyv-mics of the "excitonlc" bleaching peak at 1.9 eV of emeraldine base [8]. The photolnduced LE and ME peaks are suggested to have different origins. Dos Santos and Br6das found that nonlinear excitations Like solltons and polarons are stable in pernigraniline [2]. We propose that the LE1 and LE2 peaks may originate from a spinless A-B type soliton pair upon photoexcltation. The photolnduced F T I R spectrum demonstrates that the 1.5 eV PA is much longer lived than the 1.0 eV peak [9]. Long-lived ME photoinduced absorptions have been reported in EB and LEB, which are similar to the corresponding ME feature observed in PNB [9]. Light-induced ESR studies show this long-llved defect has s p i n - l / 2 for LEB, EB, and PNB, demonstrating the polaronic nature of the ME feature [10], X-ray diffraction studies show that pernlgraniline has a crystal]inity of -,~40% and a coherence
1314
length of ~50 ~ [11]. We suggest that this lower crystallinity and shorter coherence length may stabilize a long lived polaron, since in the amorphous regions the Pelerls gap would not be operative due to disorder, and the conduction band would become very narrow, similar to the band structure of EB. An analysis of the photoinduced IRAV modes and the photoinduced electronic transitions shows that the mass of the defect is .~300 me in accord with significant dimerization of the ring torsion angle to the Peierls gap. It is noted that the mass of a ring rotation defect is dependent on which of the 23 possible combinations of the order parameters are involved in the kink. Interchaln and Coulomb effects will modify these schemes. ACKNOWLEDGEMENT This work is supported in part by DARPA through a contract monitored by US ONR. REFERENCES 1 Y. Sun, A.G. MacDiarmid, and A.J. Epstein, J. Chem. Soc., Chem. Commun, (1990) 529. 2 M.C. dos Santos and J.L. Br4das, Synth. Met., 29 (1989) E321; Phys. Rev. Lett, 62 (1989) 2499; ibid., 64 (1990) 1185. 3 Y. Cao, Synth. Met., 35 (1990) 319. 4 J.M. Leng, J.M. Ginder, R.P. McCall, H.J. Ye, A.J. Epstein, Y.Sun, S.K. Manohar, A.G. MacDiarmid, Bull. Am. Phys. Soc, 35 (1990) 524. 5 J.M. Ginder and A.J. Epstein, Phys. Rev. Lett., 64 (1990) 1184; Phys. Rev. B, 41 (1990)
10674. 6 D.S. Boudreaux, R.R. Chance, J.F. Wolf, L.W. Shacklette, J.L. Br~das, R. Th4mans, J.M. Andr4, and 11. Sibey, J. Chem. Phys., 85 (1986) 4584. 7 B. Sj6gren and S. Stafstr6m, J. Chem. Phys, 88 (1988) 3840. 8 M.G. Roe, J.M. Ginder, P.E. Wlgen, A.J. Epstein, M. Angelopoulos, and A.G. MacDiarm;d, Phys. Rev. Lett., 60 (1988) 2789. 9 R.P McCall, J.M. Ginder, J.M. Leng, K.A. Coplin, H.J. Ye, A.J. Epstein, G.E. Asturias, S.K. Manohar, J.G. Masters, E.M. Scherr, Y. Sun, and A.G. MacDiarmid, Svnth. Met.. 41-43 (1991) 1329 (these Proceedings). 10 K.R. Cromack, A.J. Epstein, J.M. Masters, Y. Sun, and A.G. MacDiarmid, these proceedlngs. 11 M.E. J6zefowicz, A.J. Epstein, J.-P. Pouget, J.G. Masters, A. Ray , Y. Sun, X. Tang and A.G. MacDiarmid, Synth, M~t.. 41-43 (1991) 723 (these Proceedings).
PHOTOEXCITATION SPECTROSCOPY
1311
O F PERNIGRANILINE
J.M. LENG, J.M. GINDER, R.P. MCCALL, H.J. YE*, and A.J. EPSTEIN Department of Physics, The Ohio State University, Columbus, OH 43210 (U.S.A.) Y. SUN, S.K. MANOHAR, and A.G. MACDIARMID Department of Chemistry, University of Pennsylvania, Philadelpia, PA 19104 (U.S.A.)
ABSTRACT Absorption and photoinduced absorption spectroscopies are reported for pernigrauiline base (PNB). Direct absorption of PNB shows the band gap energy at 2.3 eV and ~" - lr* transition energies at 3.8 and 4.3 eV. The photoindueed absorption (PA) spectrum at room temperature shows three induced absorption peaks at 1.0 (LE), 1.5 (ME) and 3.0 eV (HE). At a temperature of 10 K, the LE peak becomes two peaks at 1.0 eV (LE1) and 1.3 eV (LE2). The ME peak is very long-lived; in addition, there are very long-lived relatively weak infrared vibrations. Light-induced ESR studies show the long-llved 1.5 eV peak has spin-I/2. We suggest that the LE peak originates from A-B type soliton pairs, while the ME peak originates from massive (~300 me) ring rotational polarons in amorphous regions.
INTRODUCTION Pernigr~nillne, the fully oxidized state of polyaniline, has attracted substantial interest recently [1-4]. Unlike leucoemeraldine base, for which the band gap arises from the C8 rings, pernigrAn~|;ue is proposed to exhibit a Pelerls gap due to eleetron-phonon interactions [2]. A model based on effective bond-length-bond-order parameters was proposed to study the nonlinear excitations in pernigrAn;ilne [2]. Optical absorption spectra of undoped and doped pernigraniline have been reported [1,3,4]. Recently, the central role of ring-torsion-angle freedom in ring-containing polymers was proposed [5]. It is expected that pernigran~l;ne supports solitous and polarons due to ring angle and/or bond length dimerization in this co~ugated polymer. There are three order parameters operative in this system: the average ring angle and its phase (~), the dimerization of the ring angles and its phase (6), and the distortion in the bond lengths and its phase (u). *Present address, Shanghai Institute of Technical Physics, Academia Sinica, Shanghai , P.R. China. 0379-6779/91/$3.50
© Elsevier Sequoia/Printed in The Netherlands
1312
In this paper, we report absorption and photoinduced absorption studies of pernigraniline base. Our results show that the long-llved defect is predomlnsntly polaronic, while the shortlived (msec time scale) defect is likely of so]iron origin involving ~ and u. Amplitude mode analysis of the photolnduced IRAV modes and the photoinduced electronic transitions shows the effective mass of the polarons is very large (,-,300 me), indicating the importance of ring angle dimerizatlon in pernigranillnc.
EXPERIMENT Samples of pernigraniline base were prepared using the method described elsewhere [I]. Direct absorption spectra were measured with a Perkin-Elmer Lambda 9 uv/vis/nlr spectrophotometer in the range of 900-190 nm (1.4-6.0 eV). The near-steady-state photolnduced absorption experiments were carried out using either a tungsten or deuterium lamp as a probe source, which was focused onto the sample, filtered through a grating monochroma{or. The sample transmission T was detected by the appropriate photodiodes. The sample was photoexcited by the output of either an argon-ion laser or a dye laser. The pump beam was mechanically chopped at frequencies between 4 and 400 Hz, causing an induced change in the sample transmission, AT. The fractional change in transmission, - ~ T / T , which is independent of the system response, was then computed. RESULTS Fig. 1 shows chemical structures of polyanlllne at dli~erent oxidation states: leucoemeraldine base (LEB), emeraldinc base (EB), and pernigranillne base (PNB). The uv/vls/nir direct absorption spectra of PNB and EB in N-methyl pyrrolidinone (NMP) solution are shown in Fig. 2. The spectrum of PNB shows one peak at 2.3 eV, and two stronger peaks at 3.8 eV and 4.3 eV. For EB, only two peaks are observed at 1.9 and 3.7 eV, although the broad tail of the 3.7 eV peak could be the result of another small peak at higher energies.
/ (0)
(b)
H
~.N/~
(
\
H
I~\Nz. 0 H
x
i'~'/N " " ~
0\] x
)
I~/~N"T~
vo
H
X
~N~
(C)
\N ~
0°""0 0 0 )]( \N ~
//
~
\\
c .Q8
J'
--- EB
\
2
3
I 4
Energy(eV)
Fig.1 Chemical structure of (a) LEB, (b) EB, and (c) PNB.
Fig.2 Direct absorption spectra of EB and PNB.
1313
The near-steady-state photolnduced absorption spectra of PNB in the range 0.5-3.3 eV at temperatures of 300 K and 10 K are shown in Fig. 3 for Epamp=2.41 eV, near the band gap of PNB (see Discussion). The spectrum at 300 K shows PA features at 1.0 (LE), 1.5 (ME), and 3.0 eV (HE), and a photoinduced bleaching (PB) feature at 2.0 eV. At a temperature of 10 K, the original LE peak is replaced by two peaks at 1.0 (LE1) and 1.3 eV (LE2). 10
' ' ' 1
....
I ....
..%
\
E) =
I
I ' ' ' ' l ' ' ' ' l
2.41
eV
~ =
300
=
10
K K
/
_-
P~'PlOO mW/cm' --5 f
-10
''~' 0.5
=
22.4
I
1.0
,
Hz
,
,
,
[
1.5
,
,
,
,
I
,
,
2.0
,
,
I
,
2.5
,
,
,
I
. . . .
3,0
3.5
Energy(eV)
Fig. 3 Photoinduced absorption spectra of PNB.
DISCUSSION We associate the observed ,~2.3 eV absorption peak with the absorption across the predicted Peierls gap. Boudreaux et al. calculated that the valence band of LEB has a bandwidth of ,,,2.9 eV and becomes half filled for PNB, with a gap of ~1.4 cV opening at the Brillonln zone edge, when two electrons and two protons are taken from each tworing repeat unit of LEB [6]. Theoretical calculations by SjSgren and Stafustr~m [7] of the optical absorption spectrum of pernigranillne in tetramer form demonstrate that the 2.3 eV transition involves charge transfer from benzenoid to quinoid rings. This transition evolves to the Peierls gap in PNB polymer calcnlated by dos Santos and Br~das [2]. The decay of the photoinduced bleaching at 2.0 eV is mainly bimolecular, in contrast to the monomolecnlar dyv-mics of the "excitonlc" bleaching peak at 1.9 eV of emeraldine base [8]. The photolnduced LE and ME peaks are suggested to have different origins. Dos Santos and Br6das found that nonlinear excitations Like solltons and polarons are stable in pernigraniline [2]. We propose that the LE1 and LE2 peaks may originate from a spinless A-B type soliton pair upon photoexcltation. The photolnduced F T I R spectrum demonstrates that the 1.5 eV PA is much longer lived than the 1.0 eV peak [9]. Long-lived ME photoinduced absorptions have been reported in EB and LEB, which are similar to the corresponding ME feature observed in PNB [9]. Light-induced ESR studies show this long-llved defect has s p i n - l / 2 for LEB, EB, and PNB, demonstrating the polaronic nature of the ME feature [10], X-ray diffraction studies show that pernlgraniline has a crystal]inity of -,~40% and a coherence
1314
length of ~50 ~ [11]. We suggest that this lower crystallinity and shorter coherence length may stabilize a long lived polaron, since in the amorphous regions the Pelerls gap would not be operative due to disorder, and the conduction band would become very narrow, similar to the band structure of EB. An analysis of the photoinduced IRAV modes and the photoinduced electronic transitions shows that the mass of the defect is .~300 me in accord with significant dimerization of the ring torsion angle to the Peierls gap. It is noted that the mass of a ring rotation defect is dependent on which of the 23 possible combinations of the order parameters are involved in the kink. Interchaln and Coulomb effects will modify these schemes. ACKNOWLEDGEMENT This work is supported in part by DARPA through a contract monitored by US ONR. REFERENCES 1 Y. Sun, A.G. MacDiarmid, and A.J. Epstein, J. Chem. Soc., Chem. Commun, (1990) 529. 2 M.C. dos Santos and J.L. Br4das, Synth. Met., 29 (1989) E321; Phys. Rev. Lett, 62 (1989) 2499; ibid., 64 (1990) 1185. 3 Y. Cao, Synth. Met., 35 (1990) 319. 4 J.M. Leng, J.M. Ginder, R.P. McCall, H.J. Ye, A.J. Epstein, Y.Sun, S.K. Manohar, A.G. MacDiarmid, Bull. Am. Phys. Soc, 35 (1990) 524. 5 J.M. Ginder and A.J. Epstein, Phys. Rev. Lett., 64 (1990) 1184; Phys. Rev. B, 41 (1990)
10674. 6 D.S. Boudreaux, R.R. Chance, J.F. Wolf, L.W. Shacklette, J.L. Br~das, R. Th4mans, J.M. Andr4, and 11. Sibey, J. Chem. Phys., 85 (1986) 4584. 7 B. Sj6gren and S. Stafstr6m, J. Chem. Phys, 88 (1988) 3840. 8 M.G. Roe, J.M. Ginder, P.E. Wlgen, A.J. Epstein, M. Angelopoulos, and A.G. MacDiarm;d, Phys. Rev. Lett., 60 (1988) 2789. 9 R.P McCall, J.M. Ginder, J.M. Leng, K.A. Coplin, H.J. Ye, A.J. Epstein, G.E. Asturias, S.K. Manohar, J.G. Masters, E.M. Scherr, Y. Sun, and A.G. MacDiarmid, Svnth. Met.. 41-43 (1991) 1329 (these Proceedings). 10 K.R. Cromack, A.J. Epstein, J.M. Masters, Y. Sun, and A.G. MacDiarmid, these proceedlngs. 11 M.E. J6zefowicz, A.J. Epstein, J.-P. Pouget, J.G. Masters, A. Ray , Y. Sun, X. Tang and A.G. MacDiarmid, Synth, M~t.. 41-43 (1991) 723 (these Proceedings).