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Ultraviolet Photoelectron Spectroscopy of n-Type Conducting Polymers
ELSEVIER
Synthetic
Ultraviolet
Photoelectron
Metals
84 (1997)
939-940
Spectroscopy of n-Type Conducting Polymers
T. Miyamaeasf, D. Yoshimurab, H. Ishii a,b, Y. Ouchib, T. Miyazaki’, T. Koiked, T. Yamamotoe, Y.Muramatsue, H.Etorie, T.Maruyamae, and K. Sekib %stitute for Molecular Science, Okazaki 444, Japan bDept. of Chemistry, Fat. of Science, Nagoya University, Nagoya 464-01, Japan ?Dept. of Chemistry, Fat. of Science, Toyama University, Toyama 930, Japan dChemical Synthesis Laboratories, Mitsui Petrochemical Industries, LTD., Chiba 299-02, Japan eResearch Laboratory of Resources Utilization, Tokyo Institute of Technology, Yokohama 227, Japan fGraduate School of Science and Technology, Chiba University, Chiba 263, Japan Abstract Ultraviolet photoelectron spectra were measured using synchrotron radiation for four kinds of x-conjugated polymers, poly(pyridine-2,5-diyl)[Ppy], poly(2,2’-bipyridine-5,5’-diyl)pbpy], poly(2-methylantraquinone-1,4-diyl)p(2Me-I ,4-AQ)], and poly(antraquinone-1 $diyl)[P( 1,5-AQ)], which exhibit n-type electrically conducting properties. Upon K-doping of Pbpy, two new states appeared in the originally empty energy gap. This finding indicates that bipolaron bands are formed in K-doped Pbpy. While Kdoped Ppy also shows similar gap states, it requires higher dopant concentration to create bipolaron bands than in the case of K-doped Pbpy. The UPS spectra of K-doped P(2Me-1,4-AQ) also show similar gap states, while it is almost absent ii-K-doped P( 1,5-AQ). The changes in electronic structure of these polymers are discussed based on the conformational difference between these polymers, Keywords: Photoelectron
spectroscopy, n-ppe conducting polymers, donor doping
1. INTRODUCTION zconjugated conducting polymers have been the subject of steadily growing interest in the past 15 years because of their unique and interesting structural and electronic properties [l]. Recently we have found that poly(pyridine-2,5-diyl) [ppy, Fig. la], poly(2,2’-bipyridine-5,5’-diyl) pbpy, Fig. lb], poly(2methylantraquinone-I ,4-diyl)[P(2Me-1,4-AQ), Fig. lc], and poly (antraquinone-1,5-diyl)p( 1,5-AQ), Fig. 1d], exhibit n-type electrically conducting properties[2,3]. These polymers can be easily reduced due to the w-deficient nature of these monomers, In spite of their unique characters, a detailed experimental study is still lacking for the uppermost z-valence levels of these polymers, which are most important in understanding their electric properties. In this work, we will report an experimental study on the electronic structures of n-type conducting polymers, The valence electronic structures of these polymers are studied by ultraviolet photoelectron spectroscopy (UPS). We also studied the changes in electronic structure of these polymers upon potassium doping by UPS. The full account at the present work is reported elsewhere I41
Fig. 1 The molecular structure of Ppy(i), Pbpy(b), P(2Me1,4-AQ), and P( 1,5-AQ). 2. EXPERIMENTAL The samples were prepared as described in refs. 2 and 3. UPS measurements were carried out on an angle-resolving UPS spectrometer at BL8B2 of UVSOR Facility, Institute for 0379-6779/97/%17.00 0 1997 Elsevier S&XX~ S.A All rights reserved PII 90379-6779(96)04220-S
Molecular Science. Thin films of Ppy and Pbpy for UPS mcasurcments lvere prepared on a gold-coated substrate by vacuum evaporation. In the case of P(2Me-1,4-AQ) and P(l,5AQ), the films were made by solution-casting from CHCla. Potassium doping was carried out in situ at 125 “C + 5 “C using a SAES K-getter source in the preparation chamber. 3. RESULTS
AND DISCUSSION
3.1. UPS Spectra of Ppy and Pbpy. Figure 2 shows the UPS spectra of K-doped Pbpy thin films for increasing doping levels in the most intriguing energy region
7
K-doped Pbpy
(K perpytidine ring) c2.5 e’o5
Cl.0 d10.2 I . . I. . . .I o=Em, I0 me?
Fig. 2 UPS spectra of neutral and increasingly K-doped The K content (K-atom per p>Tidine ring) was estimated the exposure time to K vapor.
Pbpy. from
940
T. Miyamae
et al. /SyntheticMetals
from the vacuum level Em, to bindingenergy& = 14 eV. The K concentrations for intermediatestageswere estimatedfrom the exposuretime to K vapor. At the initial stageof K doping(Fig. 2b), two new statesX and Y appearin the energygap region. Also the stateB showsdecreaseof intensity with increasingK content.The two gapstatesX and Y are at 3.7 and5.5 eV from the vacuumlevel, beingseparated by 1.8 eV. The presentresults are in goodcorrespondence with the caseof bipolaronformation in thistheoreticalapproach[51. Next we will discussthe changeof the intensityof peakB. Its intensity decreases continuouslywith increasingK-doping.This observation may indicate that the state B, with large contributionsfrom the N atomof pyridine ring, stronglyinteract with the K cations. This suggeststhat the K cations are preferentiallylocatedcloseto the two N atomsof bipyridineunit. Figure 3 showsthe UPS spectraof neutral and increasingly K-doped Ppy. The doping stepsand conditionsare similar to thosefor Pbpy in Fig. 2. At the initial stageof doping,the UPS spectraof Ppy showsmallerchangethan in the caseof Pbpy.At intermediatestagesof doping,the relative intensityof the peakB in the UPS spectrashowslittle decrease.Also two gapstatesX andY appearat the samepositionswith thoseof Pbpy, andthey canbe assigned to the bipolaronbands.The peakB still appears with significant intensity, while in Pbpy this peak almost disappears at this dopingstage.Thuswe cansummerizethat Ppy is slow in changingby K-doping, althoughthe heavily doped stateis similarto that of Pbpy. As for the reason why Ppy requires higher dopant concentrationto createbipolaronsthanPbpy,it is suggestivethat Yamamotoet al. found that Pbpyformselectrochemicallyactive transition metal complex [6] due to the chelating ability accompanied by the head-to-head structureof Pbpy.ln contrastto Pbpy,Ppy hasonly a weak coordinatingability towardtransition metals,presumablybecauseof the mainly head-to-tailstructure of Ppy.Thuswe canconcludethat moreeffective dopingto Pbpy comesfrom its strongability to coordinatewith potassiumin lightly-dopedregime,while it is almostabsentin Ppy.
84 (1997)
939-940
3.2. UPS Spectra of P(2Me-1,4-AQ)
and P(l,S-AQ).
In Fig. 4, we show the UPS spectra of neutral and increasinglyK-dopedP(2Me-1,4-AQ).From the UPS spectraof P(2Me-1,4-AQ)andP(1,5-AQ), the thresholdionizationenergies (Iti) were foundto be 6.11 and 7.0 eV, respectively,The values of Ith of the polymersindicatethat the acceptordopingto these polymersshouldbe difficult dueto the largeItt,. Upon K-doping of P(2Me-1,4-AQ), broad two statesappearin the energy gap region.Thesestatescanbe assignedto bipolaronbands,asseen in precedingsection.In contrastto the caseof P(2Me-1,4-AQ), bipolaron stateswere not observedin the caseof lightly and moderately K-doped P(l,S-AQ), while the work function determinedfrom the low-energycutoff of the UPS spectrais slightly decreased. The differencebetweenK-dopedP(2Me-1,4AQ)and K-dopedP(1,5-AQ) maybe causedtheir conformational differences.P(2Me-1,4-AQ),which hasanalogous structurewith poly@-phenylene),canbe formedbipolaronstates,In contrastto P(2Me-1,4-AQ), P(l,S-AQ) hasonly a weak ability to coordinate with potassiumpresumablybecauseof its geometry. 1 1.8 I.5 K-doped P(2Me-1 A-AQ), I
hv=40eV
14 12 10
, .u
4
kb /et
Fig. 4 UPS spectra of neutral and increasingly
, 2
0=&w
K-doped
P(2hie-1,4-AQ).
ACKNOWLEDGEMENTS
One of the authors(T.M.) expresses thanksthe JapanSociety for the Promotionof Sciencefor the Fellowshipfor Japanese JuniorScientists. REFERENCES
[l]W.R.Salaneck,L.LundstrSm, B.Rtiby ed.,Conjrigated Polymers and Related Materials, The Interconnection of Chemical and Electronic &u&we. (OxfordUniv. Press,1993).
f : 1.0
d:0.2
0 =&a,
10 Eb
/ ev5
Fig. 3 UPS spectra of neutral and increasingly K-doped Ppy. The K content (K-atom per pyridine ring) was estimated from tie exposure time to K vapor.
[2]T,Yamamoto,Z-H.Zhou, T.KinbaraandT. Maruyama,Chem. Lett. 223(1990). [3]T,Yamamoto,andH.Etori, Macromolecules, 28, 3371(1995). [4]T.Miyamae, D.Yoshimura, H.Ishii, Y.Ouchi, K.Seki, TMiyazaki, T.Koike, andT.Yamamoto,J. Chem.Phys., 103, 2738 (1995). [S]J.L.Bredas, B.ThCmans, J.G.Fripiat, J.M.AndrC, and R.R.Chance,Phys.Rev. B 29,676l (1984). [6]T,Yamamoto, T.Maruyama, Z.-H.Zhou, T.Ito, T.Fukuda, Y.Yoneda,F.Begum,T.lkeda,SSasaki,H.Takezoe,A.Fukuda, andK.Kubota,J. Am. Chem.Sot. 116,4832(1994).
Synthetic
Ultraviolet
Photoelectron
Metals
84 (1997)
939-940
Spectroscopy of n-Type Conducting Polymers
T. Miyamaeasf, D. Yoshimurab, H. Ishii a,b, Y. Ouchib, T. Miyazaki’, T. Koiked, T. Yamamotoe, Y.Muramatsue, H.Etorie, T.Maruyamae, and K. Sekib %stitute for Molecular Science, Okazaki 444, Japan bDept. of Chemistry, Fat. of Science, Nagoya University, Nagoya 464-01, Japan ?Dept. of Chemistry, Fat. of Science, Toyama University, Toyama 930, Japan dChemical Synthesis Laboratories, Mitsui Petrochemical Industries, LTD., Chiba 299-02, Japan eResearch Laboratory of Resources Utilization, Tokyo Institute of Technology, Yokohama 227, Japan fGraduate School of Science and Technology, Chiba University, Chiba 263, Japan Abstract Ultraviolet photoelectron spectra were measured using synchrotron radiation for four kinds of x-conjugated polymers, poly(pyridine-2,5-diyl)[Ppy], poly(2,2’-bipyridine-5,5’-diyl)pbpy], poly(2-methylantraquinone-1,4-diyl)p(2Me-I ,4-AQ)], and poly(antraquinone-1 $diyl)[P( 1,5-AQ)], which exhibit n-type electrically conducting properties. Upon K-doping of Pbpy, two new states appeared in the originally empty energy gap. This finding indicates that bipolaron bands are formed in K-doped Pbpy. While Kdoped Ppy also shows similar gap states, it requires higher dopant concentration to create bipolaron bands than in the case of K-doped Pbpy. The UPS spectra of K-doped P(2Me-1,4-AQ) also show similar gap states, while it is almost absent ii-K-doped P( 1,5-AQ). The changes in electronic structure of these polymers are discussed based on the conformational difference between these polymers, Keywords: Photoelectron
spectroscopy, n-ppe conducting polymers, donor doping
1. INTRODUCTION zconjugated conducting polymers have been the subject of steadily growing interest in the past 15 years because of their unique and interesting structural and electronic properties [l]. Recently we have found that poly(pyridine-2,5-diyl) [ppy, Fig. la], poly(2,2’-bipyridine-5,5’-diyl) pbpy, Fig. lb], poly(2methylantraquinone-I ,4-diyl)[P(2Me-1,4-AQ), Fig. lc], and poly (antraquinone-1,5-diyl)p( 1,5-AQ), Fig. 1d], exhibit n-type electrically conducting properties[2,3]. These polymers can be easily reduced due to the w-deficient nature of these monomers, In spite of their unique characters, a detailed experimental study is still lacking for the uppermost z-valence levels of these polymers, which are most important in understanding their electric properties. In this work, we will report an experimental study on the electronic structures of n-type conducting polymers, The valence electronic structures of these polymers are studied by ultraviolet photoelectron spectroscopy (UPS). We also studied the changes in electronic structure of these polymers upon potassium doping by UPS. The full account at the present work is reported elsewhere I41
Fig. 1 The molecular structure of Ppy(i), Pbpy(b), P(2Me1,4-AQ), and P( 1,5-AQ). 2. EXPERIMENTAL The samples were prepared as described in refs. 2 and 3. UPS measurements were carried out on an angle-resolving UPS spectrometer at BL8B2 of UVSOR Facility, Institute for 0379-6779/97/%17.00 0 1997 Elsevier S&XX~ S.A All rights reserved PII 90379-6779(96)04220-S
Molecular Science. Thin films of Ppy and Pbpy for UPS mcasurcments lvere prepared on a gold-coated substrate by vacuum evaporation. In the case of P(2Me-1,4-AQ) and P(l,5AQ), the films were made by solution-casting from CHCla. Potassium doping was carried out in situ at 125 “C + 5 “C using a SAES K-getter source in the preparation chamber. 3. RESULTS
AND DISCUSSION
3.1. UPS Spectra of Ppy and Pbpy. Figure 2 shows the UPS spectra of K-doped Pbpy thin films for increasing doping levels in the most intriguing energy region
7
K-doped Pbpy
(K perpytidine ring) c2.5 e’o5
Cl.0 d10.2 I . . I. . . .I o=Em, I0 me?
Fig. 2 UPS spectra of neutral and increasingly K-doped The K content (K-atom per p>Tidine ring) was estimated the exposure time to K vapor.
Pbpy. from
940
T. Miyamae
et al. /SyntheticMetals
from the vacuum level Em, to bindingenergy& = 14 eV. The K concentrations for intermediatestageswere estimatedfrom the exposuretime to K vapor. At the initial stageof K doping(Fig. 2b), two new statesX and Y appearin the energygap region. Also the stateB showsdecreaseof intensity with increasingK content.The two gapstatesX and Y are at 3.7 and5.5 eV from the vacuumlevel, beingseparated by 1.8 eV. The presentresults are in goodcorrespondence with the caseof bipolaronformation in thistheoreticalapproach[51. Next we will discussthe changeof the intensityof peakB. Its intensity decreases continuouslywith increasingK-doping.This observation may indicate that the state B, with large contributionsfrom the N atomof pyridine ring, stronglyinteract with the K cations. This suggeststhat the K cations are preferentiallylocatedcloseto the two N atomsof bipyridineunit. Figure 3 showsthe UPS spectraof neutral and increasingly K-doped Ppy. The doping stepsand conditionsare similar to thosefor Pbpy in Fig. 2. At the initial stageof doping,the UPS spectraof Ppy showsmallerchangethan in the caseof Pbpy.At intermediatestagesof doping,the relative intensityof the peakB in the UPS spectrashowslittle decrease.Also two gapstatesX andY appearat the samepositionswith thoseof Pbpy, andthey canbe assigned to the bipolaronbands.The peakB still appears with significant intensity, while in Pbpy this peak almost disappears at this dopingstage.Thuswe cansummerizethat Ppy is slow in changingby K-doping, althoughthe heavily doped stateis similarto that of Pbpy. As for the reason why Ppy requires higher dopant concentrationto createbipolaronsthanPbpy,it is suggestivethat Yamamotoet al. found that Pbpyformselectrochemicallyactive transition metal complex [6] due to the chelating ability accompanied by the head-to-head structureof Pbpy.ln contrastto Pbpy,Ppy hasonly a weak coordinatingability towardtransition metals,presumablybecauseof the mainly head-to-tailstructure of Ppy.Thuswe canconcludethat moreeffective dopingto Pbpy comesfrom its strongability to coordinatewith potassiumin lightly-dopedregime,while it is almostabsentin Ppy.
84 (1997)
939-940
3.2. UPS Spectra of P(2Me-1,4-AQ)
and P(l,S-AQ).
In Fig. 4, we show the UPS spectra of neutral and increasinglyK-dopedP(2Me-1,4-AQ).From the UPS spectraof P(2Me-1,4-AQ)andP(1,5-AQ), the thresholdionizationenergies (Iti) were foundto be 6.11 and 7.0 eV, respectively,The values of Ith of the polymersindicatethat the acceptordopingto these polymersshouldbe difficult dueto the largeItt,. Upon K-doping of P(2Me-1,4-AQ), broad two statesappearin the energy gap region.Thesestatescanbe assignedto bipolaronbands,asseen in precedingsection.In contrastto the caseof P(2Me-1,4-AQ), bipolaron stateswere not observedin the caseof lightly and moderately K-doped P(l,S-AQ), while the work function determinedfrom the low-energycutoff of the UPS spectrais slightly decreased. The differencebetweenK-dopedP(2Me-1,4AQ)and K-dopedP(1,5-AQ) maybe causedtheir conformational differences.P(2Me-1,4-AQ),which hasanalogous structurewith poly@-phenylene),canbe formedbipolaronstates,In contrastto P(2Me-1,4-AQ), P(l,S-AQ) hasonly a weak ability to coordinate with potassiumpresumablybecauseof its geometry. 1 1.8 I.5 K-doped P(2Me-1 A-AQ), I
hv=40eV
14 12 10
, .u
4
kb /et
Fig. 4 UPS spectra of neutral and increasingly
, 2
0=&w
K-doped
P(2hie-1,4-AQ).
ACKNOWLEDGEMENTS
One of the authors(T.M.) expresses thanksthe JapanSociety for the Promotionof Sciencefor the Fellowshipfor Japanese JuniorScientists. REFERENCES
[l]W.R.Salaneck,L.LundstrSm, B.Rtiby ed.,Conjrigated Polymers and Related Materials, The Interconnection of Chemical and Electronic &u&we. (OxfordUniv. Press,1993).
f : 1.0
d:0.2
0 =&a,
10 Eb
/ ev5
Fig. 3 UPS spectra of neutral and increasingly K-doped Ppy. The K content (K-atom per pyridine ring) was estimated from tie exposure time to K vapor.
[2]T,Yamamoto,Z-H.Zhou, T.KinbaraandT. Maruyama,Chem. Lett. 223(1990). [3]T,Yamamoto,andH.Etori, Macromolecules, 28, 3371(1995). [4]T.Miyamae, D.Yoshimura, H.Ishii, Y.Ouchi, K.Seki, TMiyazaki, T.Koike, andT.Yamamoto,J. Chem.Phys., 103, 2738 (1995). [S]J.L.Bredas, B.ThCmans, J.G.Fripiat, J.M.AndrC, and R.R.Chance,Phys.Rev. B 29,676l (1984). [6]T,Yamamoto, T.Maruyama, Z.-H.Zhou, T.Ito, T.Fukuda, Y.Yoneda,F.Begum,T.lkeda,SSasaki,H.Takezoe,A.Fukuda, andK.Kubota,J. Am. Chem.Sot. 116,4832(1994).