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Optical properties and electroluminescence characteristics of polyacetylene derivatives dependent on substituent and layer structure
ELSEVIER
Synthetic
Metals 91 i 1997) 283-287
Optical properties and electroluminescence characteristics of polyacetylene derivatives dependent on substituent and layer structure Katsumi Yoshino ‘,*, Masaharu Hirohata ‘, Rahmat Hidayat ‘, Kazuya Tada a, Tomoaki Sada b, Masahiro Teraguchi b, Toshio Masuda ‘, Sergey V. Frolov ‘, Maxim Shkunov ‘, Z. Valy Vardeny ‘, Maki Hamaguchi ‘JI (I Depc~rrtncn~ uj Elecmmic Etlgixcermg, Fmxlry uf Eqimvring. Omka Uniwniy, Yomndn-Oka, Suifn, Osnkn, Jnpm ” Division of Po/wer Clwtnistry, K~oto Vniversiiy. Yusilicin-Honlii~nclii. Snhyoku, Kyoto, Japan ’ Lkppartrnenr of Ph,vsics, Cl~~iversif~ of Urnh, Snli Lake Ciiy. VT 84112, USA ii I;& Steel Ltd., T~ikutsukndai. Nishiku. Kobe, Hyogo, Japan
Abstract Intensephotoluminescence ( PL 1 is observedin di-subatitutedpolyacetylenederivativeseven thoughsolitonicmid-gapabsorptionis observedupondoping.contraryto non-subhtitutrd rrcirls-polyacetylene andmono-substituted polyacetylenein whichstrongPL isnotobserved. Intensegreenandblueelectroluminescence (EL) isrealizedutilizing poly( diphenylacetylene) derivativesandpoly( I-alkyl-2-phenylacetylene) derivatives,respectively.Greenish-blue emissionisalsoobservedin poly( 1-chloro-2-phenylacetylene) derivatives.Thedependence of wavelengthandintensityot’PL andEL on themolecularstructureof substituents is clarifiedin detail.Theeffectsof molecularalignmentand layer structureon the EL characteristics arealsodiscussed. Uponintenselight excitation,remarkablespectralnarrowingdueto stimulated emission is alsoobservedin thesedi-substituted polyacetylenederivatives. 0 1997 ElsevierScienceS.A. Keyrwrds;
Electrolumineaccnce:
Layer structure:
Polyacetylene
derivatives:
1. Introduction Among various conductingpolymers, aromaticconducting polymers such as polythiophene. poly(p-phenylenevinylene). poly(p-phenylene) and their derivatives have been usedas the emissionlayer in polymer electroluminescence (EL) devices [ l-91. However, polyacetylene (PA) and its derivatives have not beenstudiedin detail regardingtheir use in EL devices. This may be partly due to the early stageof experimental resultsshowing that in mrizs-polyacetylene (t-PA) photoluminescence( PL) is not observed in the visible range but in cis-polyacetylene (c-PA) weak PL is observed [ IO]. That is, in conducting polymers having a degenerateground-state structure, suchast-PA. the photoexcited statewasconsidered to relax to soliton states,resulting in the total suppressionof PL. However. it was found that even in t-PA, weak PL can beobservedin the infrared region [ I I]. Therefore, theunique * Corresponding
author.
0379.6779/97/$17.00 PllSO379-6779(97)01033-2
0 1997 Elhrvier
Science S.A. All rights reserved
Substituents
PL characteristicsof PA can be interpreted not by a simple soliton model but also by the relative energy of the excited 1‘B, and 2’A, states [ 121. When the metastable2’A, state is lower in energy than the 1‘B, state, only weak PL can be observed. On the contrary, in PA derivatives in which the 1‘B, stateis lower in energy than the 2’A, state,strong PL is expected. We have previously reported the electrochemicalandoptical properties of poly(o-trimethylsilylphenylacetylene) (PTMSiPA) [ 131, which is a solublePA derivative with one substitutedphenyl ring per acetylene unit. However, strong PL was not observedin PTMSiPA. More recently, we reported that strong PL was observed in poly( diphenylacetylene) (PDPA) modified to soluble form by perfluoroalkylation [ 61. We also reported that EL devices emitting green and blue light can be realized by utilizing di-substituted PA derivatives which are solublein the polymerized state [ 7-91. On the other hand, lasingof highly fluorescentconducting polymer has also attracted much attention, and several ex-
254
K. Yoshino et 01. /S~n~iwric
p&mental results on spectral narrowing by stimulated emission in poly(p-phenylenevinylene) derivatives have been reported [ 14j. In this paper, we discuss the detailed dependence of PL and EL of PA derivatives on substituents and also device configurations. The spectral narrowing in PA derivatives upon intense optical excitation is also discussed.
Mr~nls
91 (1997)
283-287
PDPA-Ad
PDPA-OPh
PDPA-tBu
PDPA-nBu
2. Experimental Substituted acetylene polymers such as poly ( 1-phenyl-2I.‘-phenoxyphenylacetylene) (PDPA-OPh), poly( l-phenyl2-p-adamantylphenylacetylene) (PDPA-Ad), poly( l-phenyl-2-p-r-butylphenylacetylene) (PDPA-tBu), poly( l-phenyl-2-p-n-butylphenylacetylene) (PDPA-nBu), poly( lphenyl-2-m-( trimethylsilyl)phenylacetylene) (PDPAmSiMe,), poly( I-phenyl-2-p-( trimethylsilyl)phenylacetylene) (PDPA-SiMe,), poly( I-phenyl-2-p-( triisopropylsilyl) phenylacetylene) (PDPA-SiiPr>) , and poly ( 1-phenyl2-p-(triphenylsilyl)phenylacetylene) (PDPA-SiPh,), poly(1-methyl-2-phenylacetylene) (PMePA), poly( l-ethyl2-phenylacetylene) (PEtPA), poly( I-hexyl-2-phenylacetylene) ( PHxPA), poly( 1-methyl-2-naphthylacetylene) (PMeNA), poly (o-trimethylsilylphenylacetylene) (PTMSiPA), poly(o-trifluoromethylphenylacetylene) (PTFMPA), poly (o-( dimethylphenylsilyl)phenylacetylene) (PDMPSiPA), poly( indolylmethylacetylene) (PIMA), pol y (diphenylaminoacetylene) (PDPAA) , pal y ( 1-chloro2-phenylacetylene) (PClPA) and poly ( 1-chloro-2-naphthylacetylene) (PClNA), the molecular structures of which are shown in Fig. 1, were studied. The synthesis of these polyacetylene derivatives is described elsewhere [ 15,161. These polyacetylene derivatives are soluble in common organic solvents such as chloroform. Absorption and PL spectra of films spin-coated from chloroform solution on quartz plates were measured under vacuum using a spectrophotometer (HP8452 or Hitachi 330) and a fluorescence spectrophotometer (Hitachi F-2000). respectively. Electrochemical measurements such as cyclic voltammetry were carried out utilizing a potentiostat (Hokuto-Denko HA501) and a programmable function generator (HokutoDenko HB-105) in a dry box filled with argon. A three-electrode electrochemical cell with Ag wire, a sample film on ITO-coated glass and a Pt plate were used as reference electrode, working electrode and counter electrode, respectively. The electrochemical cell was filled with purified acetonitrile containing dried tetrabutylammonium tetrafluoroborate as a supporting salt. In situ absorption spectrum measurement during electrochemical doping was carried out by putting the electrochemical cell with the sample film on IT0 in the sample chamber of a spectrophotometer (Hitachi 330). For preparation of the EL device, a conducting polymer film was formed on an ITO-coated glass plate by the spin-
PDPA-SiMe,
PDPA-mSiMe3
PDPA-SiPh3
PDPA-SiiPr,
PMePA
PEtPA
PHxPA
‘“=a “&$(~H3)~‘=&kCF3 ‘B=$@3)2 7’+3
PMeNA
PTMSiPA
PTFMPA
PDPAA
PCIPA
PIMA
Fig. 1. Molecular
structures
of polyacetylene
derivatives
PDMPSiPA
PCINA
used in this study.
coating method utilizing chloroform solution (0.0 1 mol l- ’ ) . Then an Mg-In alloy was deposited by vacuum evaporation on the top of the film. Multi-layered devices were prepared by forming second and third conducting polymer films on the first layer by the spin-coating method, followed by the deposition of aluminium by evaporation on them. The active area of this device was approximately 6 mm’. EL characteristics were studied either under vacuum (at room temperature (RT) ) or in liquid nitrogen by a method reported previously [2-91. For the study of PL spectral narrowing, a laser beam of 355 nm with 100 ps pulse width at 100 Hz repetition rate produced by third-harmonic generation of a Nd-YAG laser was used. The emission spectra were measured utilizing a scanning 0.25 m spectrometer with 2 nm resolution.
3. Results and discussion As shown in Fig. 2, the absorption edge of PA derivatives and the band gap evaluated from it depend on the substituents. Among various mono-substituted polyacetylenes, the band gaps of PTMSiPA ( 1.9 eV), PTFMPA (2.2 eV) and PDMPSiPA (2.0 eV) are much smaller than those of PIMA
K. Yoshirio
et d./Syriwtic
4 PDPA-“S” . . _
**’
,
I
,
b) -
PMePA
.....
PHxPA
-
-
,’ ;.. ,’ :
PMN4
-:’ “.,!,.. I
.‘\
.’
,’ ,
.>’ /
44-- PDPAPi PDPAPi . -
PTMSPA
-
PTFMPA
2
3 4 Photon energy (eV)
5
Meids
91 (19971253-257
285
(3.3-3.45 eV) compared with those of poly(diphenylacetylene) (PDPA) derivatives such as PDPA-nBu (2.55-2.75 eV). Thesedi-substitutedpolyacetylenesareall highly PLemissive, in contrast to mono-substituted polyacetylenes. For example, the PL quantumefficiency of PDPA-nBu wasestimatedto reach 60%. Fig. 3 showsnormalized PL spectraof typical di-substituted polyacetylenes. As is evident in this Figure, poly( 1-alkyl-2-phenylacetylene) derivatives and poly( diphenylacetylene) derivatives exhibit blue and green emission,respectively. It shouldalsobe mentionedthat even in the samegroup of derivatives, the PL peak energy also dependsslightly on the molecularstructure ofthe substituent. PL intensity was also confirmed to be strongly dependent on the molecular structure of substituentseven in the same group of PA derivatives. For example, as shown in Fig. 4, among PDPA-mSiMe,, PDPA-SiMe, and PDPA-SiiPr,, PDPA-SiiPr, exhibited the strongestPL and PDPA-mSiMe, weakestPL. These di-substitutedpolyacetylenes also exhibited strong EL. For example, the EL intensity of PDPA-nBu was comparable to that of RO-PPV (poly( 2,5-dialkoxy-p-phenylenevinylene) derivatives at the samecondition. As shown in Fig. 5, poly( diphenylacetylene) derivatives alsoshow green EL. The relative intensitiesamong various polymers of this serieswere similar to thoseof PL, as shown in Fig. 4 with a dashedline, for example. It should also be noted from Fig. 6 that poly( I-alkyl-2phenylacetylene) s show intenseblue EL. In this seriesof PA
zE1
Fig. 2. Optical absorption spectra of (a) poly( diphenyincetylene) derivntives. (h) polyi I-alkyl-2-phenylacerylene)a and (c) mono-substituted polyacetylene derivatives. 1-
PDPA-SiiPr,
2
P m
.20.5 2 E c 2
0' 0 300
400
500
Wavelength Fig. 3. Photoluminescence
'0 PDPAmSiMe3
600 (nm)
spectra of PDPA-SiiPr;,
700
PHxPA
and PCIPA.
(3.0 eV) and PDPAA (3.0 eV). However. it shouldbe mentioned that in thesemono-substitutedPA derivatives, strong PL wasnot observed,asalready reportedby us [ 131.Though PL and EL of red colour can be observed in these monosubstituted small-band-gap PA derivatives such as PTMSiPA, PTFMPA and PDMPSiPA. they are extremely weak. Among di-substituted polyacetylenes. poly( I-alkyl-2phenylacetylene) ( PAPA) derivatives such as PMePA. PEtPA. PHxPA and also PMeNA exhibit a large band gap
PDPASiMe,
PDPASiiPr,
Fig. 4. Dependencies of PL peak intensity (diphenylacetylene) derivatives on molecular
and EL efficiency of polystructure of side group.
-
PDPA-tBu
/
.._ PDPA-SiMe3
300 Fig. 5. Electroluminescence
400
500 Wavelength
600 (nm)
700
spectra of poly(diphenylacetylene)
800
derivatives.
K. Soshim
286
300
400
500 Wavelength
Fig. 6. Electroluminescence
-
PMePA
-.-.
PHxPA
600
700
et al. /Swtlletic
Metals
91 (1997) 283-287
t
PLl
(PHxPA)
800
(nm)
spectra of poly( 1-alkpl-2-phenylacetylene)~. 0
derivatives, polymers with longer alkyl chains exhibited stronger EL. On the other hand, greenish-blue PL and EL were observed in poly( I-chloro-2-phenylacetylene) derivatives such as PUPA and PClNA, as shown in Fig. 3. It should be mentioned that in these di-substituted polyacetylenes, a drastic absorption spectrum change was commonly observed upon electrochemical doping due to the formation of mid-gap states (solitons), but still strong PL was observed [ 91, These experimental results suggest that the relative energy of the 2’A, and 1‘B, excited states depends on the substituent. The magnitude of relaxation of the main chain configuration after photoexcitation may also depend on the substituent. We have already reported that in the three-layered structure of EL devices, colour-variable EL depending on the polarity of the applied voltage can be realized utilizing the middle layer as an electron-blocking layer [ 3-51. We have also fabricated multi-layered EL cells utilizing combinations of conducting polymers based on PA derivatives and cyano-substituted poly(p-phenylenevinylene) derivative, CNPPV. However, polarity-dependent EL was not realized at this stage of experimentation. In most cases so far studied, EL from a polymer with smaller band gap was obtained. That is, the emission from a conducting polymer with larger band gap may be suppressed by the transfer of either charge or energy to the polymer of smaller band gap. In polymer films of mixtures of poly( I-alkyl-2-phenylacetylene) derivative ( PHxPA) and poly( diphenylacetylene) derivative (PDPA-nBu), the PL spectrum and intensity depend strongly on the concentration, as shown in Fig. 7. As also shown in the inset of Fig. 8, the spectrum of the mixture is composed of overlapped peaks originated from
40 Con2E)entration
100 of PD!knBu
$9~)
Fig. 8. Dependence of peakenergies of photoluminescence originating from PHxPA and PDPA-nBu in the photoiuminescence spectra of PDPA-nBu/ PHxPA composites on the PDPA-nBu concentration. The photoluminescence spectrum of PDPA-nBu/PHxPA composites is composed of overlapping peaks originating fromPDPA-nBu and PHxPA, as shown in the inset.
450
500
550 Wavelength
Fig. 9. Spectral narrowing
600
650
(nm)
of photoluminescence
in PDPA-nBu.
PDPA-nBu and PHxPA. It should also be noted in Fig. 8 that in the mixture the emission peaks originating from PDPAnBu and PHxPA shift to higher energy compared with those of pure sample, which may be due to the change of main chain conformation in the mixture film. We have also been interested in the laser emission from conducting polymers upon intense excitation. We have already reported remarkable spectral narrowing in RO-PPV ( poly( 2,5-dialkoxy-p-phenylenevinylene) ) upon intense photoexcitation [ 141. This spectral narrowing seems to be a common phenomenon in highly luminescent conductingpolymers. Therefore. we have also studied PA derivatives under intense light excitation. Indeed, we have found spectral narrowing upon intense photoexcitation as shown in Fig. 9 in PDPA-nBu, for example. We are now systematically studying spectral narrowing due to stimulated emission in di-substituted PA derivatives.
4. Summary and conclusions
Fig. 7. Photoluminescence spectra of PDPA-nBu/PHxPA various PDPA-nBu concentrations.
composites
at
The present experimental study can be summarized as follows. 1. In contrast to non-substituted and mono-substituted polyacetylenes, di-substituted polyacetylenes exhibit strong PL.
K. Yoshino
el d. /S~rzrhrtic
Among various di-substituted polyacetylenes, poly( diphenylacetylene) (PDPA) derivatives such as PDPA-nBu, PDPA-tBu and PDPA-SiiPr,, etc., exhibit green PL and poly( 1-alkyl-2-phenylacetylene) (PAPA) derivatives such as PMePA and PHxPA show blue PL. 3. In these di-substituted pnlyacetylenes. a mid-gap absorption is observed upon doping, which has been interpreted as being related to soliton states. Intense green and blue EL was realized utilizing PDPA and PAPA as the emissive layer of EL diodes. In poly( 1-chloro-2-phenylacetylene ) derivatives, greenish-blue PL and EL were observed. PL and EL characteristics of diodes made ofPDPA-PAPA mixtures and also multi-layered devices utilizing PDPA and PAPA layers were also studied. 7. Spectral narrowing of emission upon intense optical excitation was also observed in these di-substituted polyacetylenes due to stimulated emission. 3-.
Acknowledgements Part of this work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports and Culture and also by the Research for the Future Program of the Japan Society for the Promotion of Science (Project No. JSPS96POO206).
Metals
91 (1997)
283-287
287
References [ 1] J.H. Burrougha,D.D.C.Bradley, A.R.Brown,R.N.Marks,K.Mackey, R.H. Friend, P.L. Burns and A.B. Holmes, Nature, 317 ( 1990) 539. [ 21 Y. Ohmori. M. Uchida. K. Muro and K. Yoshino, Jpn. J. Appl. Phys., 30 (1991) Ll938. [ 31 M. Yoshida, A. Fujii, Y. Ohmori and K. Yoshino. Jpn. J. Appl. Phys.. 35 ( 1996) L397. [4] M. Hamaguchi and K. Yoshino, Appl. Phys. Lett., 69 (1996) 143. [5] M. Yoshida, A. Fujii, Y. Ohmori and K. Yoshino, Appl. Phys. Len., 69 (1996) 734. [6] K. Tada, H. Sawada, J. Kyokane and K. Yoshino, Jpn. J. Appl. Phya., 34 (1995) L1083. [ 71 K. Tada. R. Hidayat, M. Hirohata, M. Teraguchi, T. Masuda and K. Yoshino, Jpn. J. Appl. Phys., 35 (1996) Ll138. [ 81 M. Hirohata, K. Tada. R. Hidayat, T. Masuda and K. Yoshino, Jpn. J. Appl. Phys., 36 (1997) L302. [ 9 J R. Hidayat, M. Hirohata. K. Tada. M. Teraguchi, T. Masuda and K. Yoshino, Jpn. J. Appl. Phys., 36 (1997) in press. [ IO] A.J. Heeger, S. Kivelson, J.R. Schriefer and W.P. Su. Rev. Mod. Phys.. 60 (1988) 781. [ 1 I] K. Yoshino, S. Hayashi, Y. Inuishi, K. Hattori and Y. Watanabe, Solid State Commun., 46 ( 1983) 583. [ 121 D.B. Fitchen, Synth. Met., 9 (1984) 341. [ 131 K. Yoshino, H. Takahashi, S. Morita, T. Kawai and R. Sugimoto, Jpn. J. Appl. Phys., 33 (1994) L254. [ 141 S. Frolov, M. Ozaki, W. Gellermann, Z.V. Vardeny and K. Yoshino, Jpn. J. Appl. Phys., 35 (1996) L1371. [ 151 T. Masuda, T. Hamano, K. Tsuchihara and T. Higashimura, Macromol., 23 ( 1990) 1374. [ 161 H. Shirnkawa, T. Masuda and T. Takeda, in S. Patei (ed.), The Chemistry of Triple-Bonded Functional Groups, John Wiley, New York, 1994. Ch. 17, p, 945.
Synthetic
Metals 91 i 1997) 283-287
Optical properties and electroluminescence characteristics of polyacetylene derivatives dependent on substituent and layer structure Katsumi Yoshino ‘,*, Masaharu Hirohata ‘, Rahmat Hidayat ‘, Kazuya Tada a, Tomoaki Sada b, Masahiro Teraguchi b, Toshio Masuda ‘, Sergey V. Frolov ‘, Maxim Shkunov ‘, Z. Valy Vardeny ‘, Maki Hamaguchi ‘JI (I Depc~rrtncn~ uj Elecmmic Etlgixcermg, Fmxlry uf Eqimvring. Omka Uniwniy, Yomndn-Oka, Suifn, Osnkn, Jnpm ” Division of Po/wer Clwtnistry, K~oto Vniversiiy. Yusilicin-Honlii~nclii. Snhyoku, Kyoto, Japan ’ Lkppartrnenr of Ph,vsics, Cl~~iversif~ of Urnh, Snli Lake Ciiy. VT 84112, USA ii I;& Steel Ltd., T~ikutsukndai. Nishiku. Kobe, Hyogo, Japan
Abstract Intensephotoluminescence ( PL 1 is observedin di-subatitutedpolyacetylenederivativeseven thoughsolitonicmid-gapabsorptionis observedupondoping.contraryto non-subhtitutrd rrcirls-polyacetylene andmono-substituted polyacetylenein whichstrongPL isnotobserved. Intensegreenandblueelectroluminescence (EL) isrealizedutilizing poly( diphenylacetylene) derivativesandpoly( I-alkyl-2-phenylacetylene) derivatives,respectively.Greenish-blue emissionisalsoobservedin poly( 1-chloro-2-phenylacetylene) derivatives.Thedependence of wavelengthandintensityot’PL andEL on themolecularstructureof substituents is clarifiedin detail.Theeffectsof molecularalignmentand layer structureon the EL characteristics arealsodiscussed. Uponintenselight excitation,remarkablespectralnarrowingdueto stimulated emission is alsoobservedin thesedi-substituted polyacetylenederivatives. 0 1997 ElsevierScienceS.A. Keyrwrds;
Electrolumineaccnce:
Layer structure:
Polyacetylene
derivatives:
1. Introduction Among various conductingpolymers, aromaticconducting polymers such as polythiophene. poly(p-phenylenevinylene). poly(p-phenylene) and their derivatives have been usedas the emissionlayer in polymer electroluminescence (EL) devices [ l-91. However, polyacetylene (PA) and its derivatives have not beenstudiedin detail regardingtheir use in EL devices. This may be partly due to the early stageof experimental resultsshowing that in mrizs-polyacetylene (t-PA) photoluminescence( PL) is not observed in the visible range but in cis-polyacetylene (c-PA) weak PL is observed [ IO]. That is, in conducting polymers having a degenerateground-state structure, suchast-PA. the photoexcited statewasconsidered to relax to soliton states,resulting in the total suppressionof PL. However. it was found that even in t-PA, weak PL can beobservedin the infrared region [ I I]. Therefore, theunique * Corresponding
author.
0379.6779/97/$17.00 PllSO379-6779(97)01033-2
0 1997 Elhrvier
Science S.A. All rights reserved
Substituents
PL characteristicsof PA can be interpreted not by a simple soliton model but also by the relative energy of the excited 1‘B, and 2’A, states [ 121. When the metastable2’A, state is lower in energy than the 1‘B, state, only weak PL can be observed. On the contrary, in PA derivatives in which the 1‘B, stateis lower in energy than the 2’A, state,strong PL is expected. We have previously reported the electrochemicalandoptical properties of poly(o-trimethylsilylphenylacetylene) (PTMSiPA) [ 131, which is a solublePA derivative with one substitutedphenyl ring per acetylene unit. However, strong PL was not observedin PTMSiPA. More recently, we reported that strong PL was observed in poly( diphenylacetylene) (PDPA) modified to soluble form by perfluoroalkylation [ 61. We also reported that EL devices emitting green and blue light can be realized by utilizing di-substituted PA derivatives which are solublein the polymerized state [ 7-91. On the other hand, lasingof highly fluorescentconducting polymer has also attracted much attention, and several ex-
254
K. Yoshino et 01. /S~n~iwric
p&mental results on spectral narrowing by stimulated emission in poly(p-phenylenevinylene) derivatives have been reported [ 14j. In this paper, we discuss the detailed dependence of PL and EL of PA derivatives on substituents and also device configurations. The spectral narrowing in PA derivatives upon intense optical excitation is also discussed.
Mr~nls
91 (1997)
283-287
PDPA-Ad
PDPA-OPh
PDPA-tBu
PDPA-nBu
2. Experimental Substituted acetylene polymers such as poly ( 1-phenyl-2I.‘-phenoxyphenylacetylene) (PDPA-OPh), poly( l-phenyl2-p-adamantylphenylacetylene) (PDPA-Ad), poly( l-phenyl-2-p-r-butylphenylacetylene) (PDPA-tBu), poly( l-phenyl-2-p-n-butylphenylacetylene) (PDPA-nBu), poly( lphenyl-2-m-( trimethylsilyl)phenylacetylene) (PDPAmSiMe,), poly( I-phenyl-2-p-( trimethylsilyl)phenylacetylene) (PDPA-SiMe,), poly( I-phenyl-2-p-( triisopropylsilyl) phenylacetylene) (PDPA-SiiPr>) , and poly ( 1-phenyl2-p-(triphenylsilyl)phenylacetylene) (PDPA-SiPh,), poly(1-methyl-2-phenylacetylene) (PMePA), poly( l-ethyl2-phenylacetylene) (PEtPA), poly( I-hexyl-2-phenylacetylene) ( PHxPA), poly( 1-methyl-2-naphthylacetylene) (PMeNA), poly (o-trimethylsilylphenylacetylene) (PTMSiPA), poly(o-trifluoromethylphenylacetylene) (PTFMPA), poly (o-( dimethylphenylsilyl)phenylacetylene) (PDMPSiPA), poly( indolylmethylacetylene) (PIMA), pol y (diphenylaminoacetylene) (PDPAA) , pal y ( 1-chloro2-phenylacetylene) (PClPA) and poly ( 1-chloro-2-naphthylacetylene) (PClNA), the molecular structures of which are shown in Fig. 1, were studied. The synthesis of these polyacetylene derivatives is described elsewhere [ 15,161. These polyacetylene derivatives are soluble in common organic solvents such as chloroform. Absorption and PL spectra of films spin-coated from chloroform solution on quartz plates were measured under vacuum using a spectrophotometer (HP8452 or Hitachi 330) and a fluorescence spectrophotometer (Hitachi F-2000). respectively. Electrochemical measurements such as cyclic voltammetry were carried out utilizing a potentiostat (Hokuto-Denko HA501) and a programmable function generator (HokutoDenko HB-105) in a dry box filled with argon. A three-electrode electrochemical cell with Ag wire, a sample film on ITO-coated glass and a Pt plate were used as reference electrode, working electrode and counter electrode, respectively. The electrochemical cell was filled with purified acetonitrile containing dried tetrabutylammonium tetrafluoroborate as a supporting salt. In situ absorption spectrum measurement during electrochemical doping was carried out by putting the electrochemical cell with the sample film on IT0 in the sample chamber of a spectrophotometer (Hitachi 330). For preparation of the EL device, a conducting polymer film was formed on an ITO-coated glass plate by the spin-
PDPA-SiMe,
PDPA-mSiMe3
PDPA-SiPh3
PDPA-SiiPr,
PMePA
PEtPA
PHxPA
‘“=a “&$(~H3)~‘=&kCF3 ‘B=$@3)2 7’+3
PMeNA
PTMSiPA
PTFMPA
PDPAA
PCIPA
PIMA
Fig. 1. Molecular
structures
of polyacetylene
derivatives
PDMPSiPA
PCINA
used in this study.
coating method utilizing chloroform solution (0.0 1 mol l- ’ ) . Then an Mg-In alloy was deposited by vacuum evaporation on the top of the film. Multi-layered devices were prepared by forming second and third conducting polymer films on the first layer by the spin-coating method, followed by the deposition of aluminium by evaporation on them. The active area of this device was approximately 6 mm’. EL characteristics were studied either under vacuum (at room temperature (RT) ) or in liquid nitrogen by a method reported previously [2-91. For the study of PL spectral narrowing, a laser beam of 355 nm with 100 ps pulse width at 100 Hz repetition rate produced by third-harmonic generation of a Nd-YAG laser was used. The emission spectra were measured utilizing a scanning 0.25 m spectrometer with 2 nm resolution.
3. Results and discussion As shown in Fig. 2, the absorption edge of PA derivatives and the band gap evaluated from it depend on the substituents. Among various mono-substituted polyacetylenes, the band gaps of PTMSiPA ( 1.9 eV), PTFMPA (2.2 eV) and PDMPSiPA (2.0 eV) are much smaller than those of PIMA
K. Yoshirio
et d./Syriwtic
4 PDPA-“S” . . _
**’
,
I
,
b) -
PMePA
.....
PHxPA
-
-
,’ ;.. ,’ :
PMN4
-:’ “.,!,.. I
.‘\
.’
,’ ,
.>’ /
44-- PDPAPi PDPAPi . -
PTMSPA
-
PTFMPA
2
3 4 Photon energy (eV)
5
Meids
91 (19971253-257
285
(3.3-3.45 eV) compared with those of poly(diphenylacetylene) (PDPA) derivatives such as PDPA-nBu (2.55-2.75 eV). Thesedi-substitutedpolyacetylenesareall highly PLemissive, in contrast to mono-substituted polyacetylenes. For example, the PL quantumefficiency of PDPA-nBu wasestimatedto reach 60%. Fig. 3 showsnormalized PL spectraof typical di-substituted polyacetylenes. As is evident in this Figure, poly( 1-alkyl-2-phenylacetylene) derivatives and poly( diphenylacetylene) derivatives exhibit blue and green emission,respectively. It shouldalsobe mentionedthat even in the samegroup of derivatives, the PL peak energy also dependsslightly on the molecularstructure ofthe substituent. PL intensity was also confirmed to be strongly dependent on the molecular structure of substituentseven in the same group of PA derivatives. For example, as shown in Fig. 4, among PDPA-mSiMe,, PDPA-SiMe, and PDPA-SiiPr,, PDPA-SiiPr, exhibited the strongestPL and PDPA-mSiMe, weakestPL. These di-substitutedpolyacetylenes also exhibited strong EL. For example, the EL intensity of PDPA-nBu was comparable to that of RO-PPV (poly( 2,5-dialkoxy-p-phenylenevinylene) derivatives at the samecondition. As shown in Fig. 5, poly( diphenylacetylene) derivatives alsoshow green EL. The relative intensitiesamong various polymers of this serieswere similar to thoseof PL, as shown in Fig. 4 with a dashedline, for example. It should also be noted from Fig. 6 that poly( I-alkyl-2phenylacetylene) s show intenseblue EL. In this seriesof PA
zE1
Fig. 2. Optical absorption spectra of (a) poly( diphenyincetylene) derivntives. (h) polyi I-alkyl-2-phenylacerylene)a and (c) mono-substituted polyacetylene derivatives. 1-
PDPA-SiiPr,
2
P m
.20.5 2 E c 2
0' 0 300
400
500
Wavelength Fig. 3. Photoluminescence
'0 PDPAmSiMe3
600 (nm)
spectra of PDPA-SiiPr;,
700
PHxPA
and PCIPA.
(3.0 eV) and PDPAA (3.0 eV). However. it shouldbe mentioned that in thesemono-substitutedPA derivatives, strong PL wasnot observed,asalready reportedby us [ 131.Though PL and EL of red colour can be observed in these monosubstituted small-band-gap PA derivatives such as PTMSiPA, PTFMPA and PDMPSiPA. they are extremely weak. Among di-substituted polyacetylenes. poly( I-alkyl-2phenylacetylene) ( PAPA) derivatives such as PMePA. PEtPA. PHxPA and also PMeNA exhibit a large band gap
PDPASiMe,
PDPASiiPr,
Fig. 4. Dependencies of PL peak intensity (diphenylacetylene) derivatives on molecular
and EL efficiency of polystructure of side group.
-
PDPA-tBu
/
.._ PDPA-SiMe3
300 Fig. 5. Electroluminescence
400
500 Wavelength
600 (nm)
700
spectra of poly(diphenylacetylene)
800
derivatives.
K. Soshim
286
300
400
500 Wavelength
Fig. 6. Electroluminescence
-
PMePA
-.-.
PHxPA
600
700
et al. /Swtlletic
Metals
91 (1997) 283-287
t
PLl
(PHxPA)
800
(nm)
spectra of poly( 1-alkpl-2-phenylacetylene)~. 0
derivatives, polymers with longer alkyl chains exhibited stronger EL. On the other hand, greenish-blue PL and EL were observed in poly( I-chloro-2-phenylacetylene) derivatives such as PUPA and PClNA, as shown in Fig. 3. It should be mentioned that in these di-substituted polyacetylenes, a drastic absorption spectrum change was commonly observed upon electrochemical doping due to the formation of mid-gap states (solitons), but still strong PL was observed [ 91, These experimental results suggest that the relative energy of the 2’A, and 1‘B, excited states depends on the substituent. The magnitude of relaxation of the main chain configuration after photoexcitation may also depend on the substituent. We have already reported that in the three-layered structure of EL devices, colour-variable EL depending on the polarity of the applied voltage can be realized utilizing the middle layer as an electron-blocking layer [ 3-51. We have also fabricated multi-layered EL cells utilizing combinations of conducting polymers based on PA derivatives and cyano-substituted poly(p-phenylenevinylene) derivative, CNPPV. However, polarity-dependent EL was not realized at this stage of experimentation. In most cases so far studied, EL from a polymer with smaller band gap was obtained. That is, the emission from a conducting polymer with larger band gap may be suppressed by the transfer of either charge or energy to the polymer of smaller band gap. In polymer films of mixtures of poly( I-alkyl-2-phenylacetylene) derivative ( PHxPA) and poly( diphenylacetylene) derivative (PDPA-nBu), the PL spectrum and intensity depend strongly on the concentration, as shown in Fig. 7. As also shown in the inset of Fig. 8, the spectrum of the mixture is composed of overlapped peaks originated from
40 Con2E)entration
100 of PD!knBu
$9~)
Fig. 8. Dependence of peakenergies of photoluminescence originating from PHxPA and PDPA-nBu in the photoiuminescence spectra of PDPA-nBu/ PHxPA composites on the PDPA-nBu concentration. The photoluminescence spectrum of PDPA-nBu/PHxPA composites is composed of overlapping peaks originating fromPDPA-nBu and PHxPA, as shown in the inset.
450
500
550 Wavelength
Fig. 9. Spectral narrowing
600
650
(nm)
of photoluminescence
in PDPA-nBu.
PDPA-nBu and PHxPA. It should also be noted in Fig. 8 that in the mixture the emission peaks originating from PDPAnBu and PHxPA shift to higher energy compared with those of pure sample, which may be due to the change of main chain conformation in the mixture film. We have also been interested in the laser emission from conducting polymers upon intense excitation. We have already reported remarkable spectral narrowing in RO-PPV ( poly( 2,5-dialkoxy-p-phenylenevinylene) ) upon intense photoexcitation [ 141. This spectral narrowing seems to be a common phenomenon in highly luminescent conductingpolymers. Therefore. we have also studied PA derivatives under intense light excitation. Indeed, we have found spectral narrowing upon intense photoexcitation as shown in Fig. 9 in PDPA-nBu, for example. We are now systematically studying spectral narrowing due to stimulated emission in di-substituted PA derivatives.
4. Summary and conclusions
Fig. 7. Photoluminescence spectra of PDPA-nBu/PHxPA various PDPA-nBu concentrations.
composites
at
The present experimental study can be summarized as follows. 1. In contrast to non-substituted and mono-substituted polyacetylenes, di-substituted polyacetylenes exhibit strong PL.
K. Yoshino
el d. /S~rzrhrtic
Among various di-substituted polyacetylenes, poly( diphenylacetylene) (PDPA) derivatives such as PDPA-nBu, PDPA-tBu and PDPA-SiiPr,, etc., exhibit green PL and poly( 1-alkyl-2-phenylacetylene) (PAPA) derivatives such as PMePA and PHxPA show blue PL. 3. In these di-substituted pnlyacetylenes. a mid-gap absorption is observed upon doping, which has been interpreted as being related to soliton states. Intense green and blue EL was realized utilizing PDPA and PAPA as the emissive layer of EL diodes. In poly( 1-chloro-2-phenylacetylene ) derivatives, greenish-blue PL and EL were observed. PL and EL characteristics of diodes made ofPDPA-PAPA mixtures and also multi-layered devices utilizing PDPA and PAPA layers were also studied. 7. Spectral narrowing of emission upon intense optical excitation was also observed in these di-substituted polyacetylenes due to stimulated emission. 3-.
Acknowledgements Part of this work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports and Culture and also by the Research for the Future Program of the Japan Society for the Promotion of Science (Project No. JSPS96POO206).
Metals
91 (1997)
283-287
287
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