- Email: [email protected]
Optical properties of polyparaphenyl thin films from oligomers to polymers
ELSEVIER
Synthdic
Metals
84 (1997)
287-288
Optical properties of polyparaphenyl thin films fi-om oligomers to polymers L. AtJ~ouel%~, J. WCrya,B. Duliet?, J. Bullo?, J.P. Buisson’andG. Froyerqb aLaboratoire de Physique Cristalline, Institut desMat&iaux de Nantes, Universitb de Nantes 2 rue de la Houssinikre, 44072 Nantes CIdex 03 (France) bDt+partement Science et Genie desMat+iaux, Institut Universitaire de Technologie de Nantes La Chantrerie, Rue Christian Pauc, C.P. 3023, 44087 Nantes cldex 03 (France) Abstract
Parasexiphenyland paraoctiphenylwere recentlysynthesizedby an electrochemical methodusingmonobrominated compoundsand purified by sublimation.Moreover theseoligomerscan be processedby vacuum sublimationand high purity films were obtained whateverthe substrate.Various characterizationmethodsare allowedwhich facilited the understandingof the electronicand optical propertiesof the polyparaphenyls. Keywords: Infrared andRamanspectroscopy; Photoluminescence, UV-Vis-NIR absorption;Other conjugatedand/orconductingpolymers
1.
762.8cm-’ : out-of-plane vibrations of C-H on monosubstituted rings 810cm-’ : out-of-plane vibrations of C-H on parasubstituted rings.
Introduction
The studyof oligomersis a promisingway to understandat best conductingpolymer propertiesand may help to enhance performancesof electronic devices such as light emitting diodes[I]. PPP oligomers such as parasexiphenyl and paraoctiphenylhave high enoughmolecularweight to present properties close to those of PPP. This paper presentsthe experimentalresultsaboutthe spectroscopic characterizationsof PPPoligomersfrom biphenyl to principally paraoctiphenyland polymer : IN-visible and JR absorptionspectroscopies, Raman scatteringspectroscopy andphotoluminescence at 77 K. 2.
Experimental
Paraoctiphenyl,08, wassynthesizedby electroreductionof monobromoquaterphenyl in the presenceof a zero-valentnickel catalyst,similar electrochemical methodusedfor parasexiphenyl synthesis[2]. Purification of the paraoctiphenylpowder was carried out by sublimationat 690K and condensationat about 570K in a presettemperaturegradientundera residualpressure of IF5 Pa. @a thin &IS were then obtained on various substrates(glass,silicon dioxyde, monocrystallinesilicon) by sublimationat about 620K under a secondaryvacuumsystem [3]. The used depositionrate was 3.1U2nm.sS1, the substrate temperature420 K andthe film thickness250 nm. The oligomer thin films were then characterizedby various spectroscopic methods and the results are compared with those of parasexiphenylandpolyparaphenylene. 3.
Results and discussion
3.1,
IR absorption
spectrosco~
Figure la. showsthe main 1Rabsorptionsof pure @8 thin films which are assignedasfollows : 0379-6779/97/$17.00 SO379-6779(96)04010-6
PII
0 1997
Elsevier
Science
%A.
All
rights
#served
700
800
900
1000
1100
200
250
300
350
400
450
Wavenumber(cm-l) Wavelength(run) Fig. 1. IR and W-visible absorptionof paraoctiphenylthin film Thesevibrationsare the only onesto be sensitiveto the polymerization degree variations. The energiesand relative intensitiesof the @aabsorptionsare in good agreementwith thoseof the oligophenylenefamily. The different values from biphenyl to PPP (Yamamoto and Kovacic) are reported in Table 1 : Table 1 Out-of-planevibrationsof C-H for the oligophenylenefamily CD2
parasubstituted rings(cm-‘) monosubstituted728 rings(cm-‘) ImonosudIparasub
* Reference[4]
-
03
a4
@5
Q6
Q3
PPP*
839
826.5
818.7
815
810
805(K) 807(Y)
745
754.1
759.1
761
762.8
766
1.57
1.19
1.08
0.98
0.63
0.18(K) 0.35(Y)
K : Kovacic
Y : Yamamoto
L. Afhout;[ et al. /SyntheticMetals 84 (1997) 287-288
288
The plot of the wavenumbersas a function of the polymerization degree shows that the out-of-plane C-H vibrationsenergiesstronglyevoluteup to n=8 andthen remains almostconstantup to PPPKovacic. Moreoverthe polymerization degreeof the oligomerscan be determinedby the relative absorptionpeaksintensities.The increaseof the aromaticrings number from terphenyl to PPP reveals a decreaseof the IIllOllOS” dpmub ratio. This is in agreement with the increaseof the parasubstitutedrings number comparedwith those of the monosubstituted rings. Figure 1.a. also showsthat the 1000cm-’ band is almost missingcontrary to the 08 powderspectrum[I]. This behaviour is significantof a notable0s chainsorientationas previously observedin parasexiphenylthin films [5]. 3.2. Raman diffusion spectroscopy
On the figure 1.b. the W-visible absorptions of paraoctiphenyland parasexiphenylare shown.Experimentally (Table3) the 0s R+ n* transition(about3.8-3.9eV) seemsto be closeto the 0.6interbandtransition,whereasits opticalgapof 2.97 eV comesin between those of 06 (3.15 eV) and PPP (2.80 eV). 3.4. Photoluminescence
The photoluminescence of @6,08 andPPP(Figure3.) showsan emitting blue light with maxima at : 3.09, 2.93, 2.76 and 2.60eV for 06, 2.94, 2.76, 2.61 and 2.43eV for m.8and 2.85, 2.68, 2.51 and 2.34 eV for PPP. Moreover a red shift is confiied by the @sspectrumwhenthe aromaticringsnumber increases,
Figure2 showsthat a light dispersionappearsin frequency with the conjugaison lengthandwith the excitationwaveenergy. Moreover the relative intensitiesratio I&Ilz80 increaseswith the sameparameters,asshownonthe table2.
-08:422
449.5 416 510
450 500 550 600 650 Wavelength(nm) Fig. 3. Photoluminescence of parasexiphenyl,paraoctiphenyl andPPPat low temperature(77 K) andhexc= 335nm 350
800
1300
1800
Wavenumber(cm-l )
4. Conclusion 800
1300
1800
Wavenumber(cm-l)
Fig. 2. Ramanspectra of parasexiphenyl,paraoctiphenyland PPPat 514.5nm and676.4m-n Table2 Ramanintensitiesratio of the 1220cm-’and 1280cm” peaks _ e 1122d112s0
0’6 08
PPP
h=1064nm 0.81 1.03 1.93
400
h=676.4nm 31=514,5mnh=363nm 0.88 1.06 fluorescence 1.17 1.36 -1.75 2.55 2.60 -2.20
The paraoctiphenyl spectroscopicproperties are in good agreementwith the oligophenylenepropertiesand get more closer to PPP’s than those of parasexiphenyl.Paraoctiphenyl seemsthen to be a good candidatefor a blue-light emitting diode.Moreoverthe resultsobtainedwith the paraoctiphenylare inducedto realizeorientedparaoctiphenylthin films. References [ll
PI 3.3. UY-visible absorption spectroscopy Table3 Interbandtransitionandopticalgapof oligophenyls Absorption Interband Absorption Optical maximum transition bottom gap (eV) (eV) ( 1 ( ) 06 3; 3.875 3E 3.15 08 closeto a)6 3X-3.9 417 2.97 3.2 443 2.8 PPP* 380-400 * references[4] and[G]
131 t41
PI Fl
G. L&sing,
S. Tascb, F. Meghdadi,
Met., (in print) G. Grem, G. Leditiy,
L. AthouBl and G. Froyer,
Synth.
B. Ullrich and G. Leising, Adver. Mater., 4 (1992) 607 G. Froyer, Y. Pelous, E. Dall’arche, C. Chevrot and k Siove, E.P. PatentNo. 467 762 (1992); K. Faid, k Siove, C. Chevrot, M.T. Riou and G. Froyer, J. Chim. Phys.,89(5) (1992) 1305 L. AthouBl, G. Froyer, M.T. Riou and M. Schott, Thin SolidfYlms, 274 (1996) 35 G. Froyer, J.Y. Goblof J.L. Guilbert, F. Maurice and Y. Pelous, J. Phys., Coil. C3, Suppl. No. 6, 44 (1983) 745 S. KrichBne, S. Lefimt, Y. Pelous, G. Froyer, M. Petit, A Digua and J.F. Fauvarque, Synrh.Met,, 17 (1987) 607 L. Athouel, M.T. Riou and G. Froyer, J. Chim. Phys., 89(5) (1992) 1271 J.F. Fauvarque, MA Petit, k Digua, G. Froyer, MakromoKhem., 188 (1987) 1833
Synthdic
Metals
84 (1997)
287-288
Optical properties of polyparaphenyl thin films fi-om oligomers to polymers L. AtJ~ouel%~, J. WCrya,B. Duliet?, J. Bullo?, J.P. Buisson’andG. Froyerqb aLaboratoire de Physique Cristalline, Institut desMat&iaux de Nantes, Universitb de Nantes 2 rue de la Houssinikre, 44072 Nantes CIdex 03 (France) bDt+partement Science et Genie desMat+iaux, Institut Universitaire de Technologie de Nantes La Chantrerie, Rue Christian Pauc, C.P. 3023, 44087 Nantes cldex 03 (France) Abstract
Parasexiphenyland paraoctiphenylwere recentlysynthesizedby an electrochemical methodusingmonobrominated compoundsand purified by sublimation.Moreover theseoligomerscan be processedby vacuum sublimationand high purity films were obtained whateverthe substrate.Various characterizationmethodsare allowedwhich facilited the understandingof the electronicand optical propertiesof the polyparaphenyls. Keywords: Infrared andRamanspectroscopy; Photoluminescence, UV-Vis-NIR absorption;Other conjugatedand/orconductingpolymers
1.
762.8cm-’ : out-of-plane vibrations of C-H on monosubstituted rings 810cm-’ : out-of-plane vibrations of C-H on parasubstituted rings.
Introduction
The studyof oligomersis a promisingway to understandat best conductingpolymer propertiesand may help to enhance performancesof electronic devices such as light emitting diodes[I]. PPP oligomers such as parasexiphenyl and paraoctiphenylhave high enoughmolecularweight to present properties close to those of PPP. This paper presentsthe experimentalresultsaboutthe spectroscopic characterizationsof PPPoligomersfrom biphenyl to principally paraoctiphenyland polymer : IN-visible and JR absorptionspectroscopies, Raman scatteringspectroscopy andphotoluminescence at 77 K. 2.
Experimental
Paraoctiphenyl,08, wassynthesizedby electroreductionof monobromoquaterphenyl in the presenceof a zero-valentnickel catalyst,similar electrochemical methodusedfor parasexiphenyl synthesis[2]. Purification of the paraoctiphenylpowder was carried out by sublimationat 690K and condensationat about 570K in a presettemperaturegradientundera residualpressure of IF5 Pa. @a thin &IS were then obtained on various substrates(glass,silicon dioxyde, monocrystallinesilicon) by sublimationat about 620K under a secondaryvacuumsystem [3]. The used depositionrate was 3.1U2nm.sS1, the substrate temperature420 K andthe film thickness250 nm. The oligomer thin films were then characterizedby various spectroscopic methods and the results are compared with those of parasexiphenylandpolyparaphenylene. 3.
Results and discussion
3.1,
IR absorption
spectrosco~
Figure la. showsthe main 1Rabsorptionsof pure @8 thin films which are assignedasfollows : 0379-6779/97/$17.00 SO379-6779(96)04010-6
PII
0 1997
Elsevier
Science
%A.
All
rights
#served
700
800
900
1000
1100
200
250
300
350
400
450
Wavenumber(cm-l) Wavelength(run) Fig. 1. IR and W-visible absorptionof paraoctiphenylthin film Thesevibrationsare the only onesto be sensitiveto the polymerization degree variations. The energiesand relative intensitiesof the @aabsorptionsare in good agreementwith thoseof the oligophenylenefamily. The different values from biphenyl to PPP (Yamamoto and Kovacic) are reported in Table 1 : Table 1 Out-of-planevibrationsof C-H for the oligophenylenefamily CD2
parasubstituted rings(cm-‘) monosubstituted728 rings(cm-‘) ImonosudIparasub
* Reference[4]
-
03
a4
@5
Q6
Q3
PPP*
839
826.5
818.7
815
810
805(K) 807(Y)
745
754.1
759.1
761
762.8
766
1.57
1.19
1.08
0.98
0.63
0.18(K) 0.35(Y)
K : Kovacic
Y : Yamamoto
L. Afhout;[ et al. /SyntheticMetals 84 (1997) 287-288
288
The plot of the wavenumbersas a function of the polymerization degree shows that the out-of-plane C-H vibrationsenergiesstronglyevoluteup to n=8 andthen remains almostconstantup to PPPKovacic. Moreoverthe polymerization degreeof the oligomerscan be determinedby the relative absorptionpeaksintensities.The increaseof the aromaticrings number from terphenyl to PPP reveals a decreaseof the IIllOllOS” dpmub ratio. This is in agreement with the increaseof the parasubstitutedrings number comparedwith those of the monosubstituted rings. Figure 1.a. also showsthat the 1000cm-’ band is almost missingcontrary to the 08 powderspectrum[I]. This behaviour is significantof a notable0s chainsorientationas previously observedin parasexiphenylthin films [5]. 3.2. Raman diffusion spectroscopy
On the figure 1.b. the W-visible absorptions of paraoctiphenyland parasexiphenylare shown.Experimentally (Table3) the 0s R+ n* transition(about3.8-3.9eV) seemsto be closeto the 0.6interbandtransition,whereasits opticalgapof 2.97 eV comesin between those of 06 (3.15 eV) and PPP (2.80 eV). 3.4. Photoluminescence
The photoluminescence of @6,08 andPPP(Figure3.) showsan emitting blue light with maxima at : 3.09, 2.93, 2.76 and 2.60eV for 06, 2.94, 2.76, 2.61 and 2.43eV for m.8and 2.85, 2.68, 2.51 and 2.34 eV for PPP. Moreover a red shift is confiied by the @sspectrumwhenthe aromaticringsnumber increases,
Figure2 showsthat a light dispersionappearsin frequency with the conjugaison lengthandwith the excitationwaveenergy. Moreover the relative intensitiesratio I&Ilz80 increaseswith the sameparameters,asshownonthe table2.
-08:422
449.5 416 510
450 500 550 600 650 Wavelength(nm) Fig. 3. Photoluminescence of parasexiphenyl,paraoctiphenyl andPPPat low temperature(77 K) andhexc= 335nm 350
800
1300
1800
Wavenumber(cm-l )
4. Conclusion 800
1300
1800
Wavenumber(cm-l)
Fig. 2. Ramanspectra of parasexiphenyl,paraoctiphenyland PPPat 514.5nm and676.4m-n Table2 Ramanintensitiesratio of the 1220cm-’and 1280cm” peaks _ e 1122d112s0
0’6 08
PPP
h=1064nm 0.81 1.03 1.93
400
h=676.4nm 31=514,5mnh=363nm 0.88 1.06 fluorescence 1.17 1.36 -1.75 2.55 2.60 -2.20
The paraoctiphenyl spectroscopicproperties are in good agreementwith the oligophenylenepropertiesand get more closer to PPP’s than those of parasexiphenyl.Paraoctiphenyl seemsthen to be a good candidatefor a blue-light emitting diode.Moreoverthe resultsobtainedwith the paraoctiphenylare inducedto realizeorientedparaoctiphenylthin films. References [ll
PI 3.3. UY-visible absorption spectroscopy Table3 Interbandtransitionandopticalgapof oligophenyls Absorption Interband Absorption Optical maximum transition bottom gap (eV) (eV) ( 1 ( ) 06 3; 3.875 3E 3.15 08 closeto a)6 3X-3.9 417 2.97 3.2 443 2.8 PPP* 380-400 * references[4] and[G]
131 t41
PI Fl
G. L&sing,
S. Tascb, F. Meghdadi,
Met., (in print) G. Grem, G. Leditiy,
L. AthouBl and G. Froyer,
Synth.
B. Ullrich and G. Leising, Adver. Mater., 4 (1992) 607 G. Froyer, Y. Pelous, E. Dall’arche, C. Chevrot and k Siove, E.P. PatentNo. 467 762 (1992); K. Faid, k Siove, C. Chevrot, M.T. Riou and G. Froyer, J. Chim. Phys.,89(5) (1992) 1305 L. AthouBl, G. Froyer, M.T. Riou and M. Schott, Thin SolidfYlms, 274 (1996) 35 G. Froyer, J.Y. Goblof J.L. Guilbert, F. Maurice and Y. Pelous, J. Phys., Coil. C3, Suppl. No. 6, 44 (1983) 745 S. KrichBne, S. Lefimt, Y. Pelous, G. Froyer, M. Petit, A Digua and J.F. Fauvarque, Synrh.Met,, 17 (1987) 607 L. Athouel, M.T. Riou and G. Froyer, J. Chim. Phys., 89(5) (1992) 1271 J.F. Fauvarque, MA Petit, k Digua, G. Froyer, MakromoKhem., 188 (1987) 1833