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Dynamic Orientation of Conjugated Oligomers in Nematic Liquid Crystalline Matrices
ELSEVIER
Synthtic
Metals
84 (1997)
609610
Dynamic Orientation of Conjugated Oligomers in Nematic Liquid Crystaltine M&t-ices N. Serdar Sariciftci Physikalische Chemie, Johannes Kepler Univ&t Linz Altenberg~strasse 69, A-4040 Linz, Aush’a U. Lemmer Sektion Physik Ludwig Maximilian UniversZit Miinchen, 80799 Germany D. Vacar and A. J. Heeger Institute for Polymers & Organic Solids, University of California, Santa Barbara, CA 93106, U.S.A. R. A. J. Janssen Eindhoven University of Technology, 5600 Eindhoven, The Netherlands Abstract
Polarizedphotoluminescence is observedfrom a host/guestsystemconsistingof conjugatedoligomersdissolvedin nematic liquid crystallinematrices.In this system conjugatedoligomers(guest)exhibit an emissionspectrumwell separatedTom the emissionof the liquid crystallinematrix {host)howeverpolarizedparallelto the orientationof the hostmatrix. The resultsclearly demonstrate the spontaneous orientationof the oligothiophenes alongthe director of the nematicmatrix. Uponapplyingan electric field perpendicularto the direction of the spontaneous orientation,the oligomersare dynamicallyreoriented(switched)with the hostresultingin switchingof thepolarizedphotoluminescence. Keywords: Self-organizationin macromolecules, optical absorption and emissionspectroscopy,switches,polythiopheneand dsivatives 1. Introduction
Physical properties of conjugatedpolymer tilms are heavily influenced by the intrinsic disorder phenomenawithin the structure. With material quality getting better through high degreeof uniformity of chemicalcoupling as well as through controlled recrystallization from solutions in the doped conductingstate andfurther refining of the synthesisconditions, conductingpolymerscan now be preparedwell on the metallic sideof the disorderinducedM-I transition[l-4]. Thus, creating macroscopic order within the conjugatedpolymericstructureshas alwaysbeena continuing,important,scientificchallenge. The ideaof usingliquid crystallinehostmatices for macroscopic orientation of guest moleculesis quite well known (see for example [5] and referencestherein). Araya et al. showedthat synthesisof polyacetylene’inliquid crystallinematricesresultsin highly orientedmaterials [6]. Furthermore,synthetic efforts of modifjkrg the side chainsof a conjugatedpolymer with liquid crystalline side groupshave been reported in detail (see for example [7-151 and referencestherein). Recently, polarized photoluminescence has been reported from liquid crystalline polymers with phenylenevinylene segments on the main chain[l6]. Taliani et al. reportedon a liquid crystallinephaseof the a-sexithienyl(T6 ) aboveT=312 C [17]. Moleculessuchas thiophenesand bithiophenesembeddedin liquid crystalline matriceshas beeninvestigatedwith nuclearmagneticresonance techniquesto obtainaccuratestructuralinformation[ 18,191. In this contribution we report the macroscopicorientation of conjugatedoligomers(six memberedoligothiophenes;e.g. asexithienyl, (T6)) dissolvedwithin a nematic liquid crystalline matrix (Fig.l). The orientation of the oligothiophenescan be dynamically switched with the host nematic matrix upon applicationof an electric field as observedthroughswitchingof the polarizedphotoluminescence of T6 . 0379-6779/97/$17.00
0 1997
PII SO379-fiT79(96)04074-X
Ekvia Scienm S.A.All rightsreserved
Fig. 1: Schematicillustration of the orderingof oligothiophenes (T6) within a homogeneously alignednematicliquid crystalline matrix. 2. Results & Discussion
The six memberedoligothiopheneusedin the presentstudy carry dodecyl substituents on different thiophene rings and is designatedas T6 d(2,5). The synthesishas been described previously [20,21]. Details of the experimentsare published elsewhere[22]. The liquid crystal cells filled with the solutions of T6 in E7 show uniform brightnessunder cross polarized microscopesuggestinga homogeneous alignmentof the liquid crystals. To estimate the achieved degree of homogeneous
N. Serdar
610
Sariciftci
et al. /Synthetic
alignment of T6 in nematic matrix we performed linear dichroism experiments using polarized linearly optical absorption and emission. Fig. 2 shows the polarization dependence of the emission spectra. The emission spectrum of T6 obtained by exciting and probing parallel to the rubbing direction (parallel to the director of the nematic matrix) is an order of magnitude stronger compared to the case where excitation and emission is perpendicular to the director. Since the active emission originates liom the guestmolecules,T6, this resultconfirmsthe orientation of the guestwithin the nematichost.
Metals
84 (1997)
609-610
ii.) high resolutionnuclearmagneticresonancespectroscopyto get anisotropicinteractions, iii.) electron spin resonanceto get detailed parameterof the radical ions (polarons) and triplet states by doping and photoexcitation,respectively. Furthermore,the combinationof the liquid crystalline display technologies with the emergingnew areaof conjugatedpolymer light emittingdiodesis interesting, Acknowledgements
We acknowledgeJim Bourassa,Mark Urish and G&her Ahlers for valuablehelp. The oligothiophenesare generously providedby PhilipsResearch Laboratories. References
[l] M. Reghu,Y. Cao,D. Moses,andA. J. Heeger,Phys.Rev. B 47, (1993) 1758. [2] C. 0. Yoon, R. Menon, D. Moses,andA. J. Heeger,Whys. Rev. B 49, (1994) 10851. [31 R. D. McCullough, R. D. Lowe, M. Jayaraman,andD. L. Anderson,J. Org. Chem. 58, (1993)904.
Wavelength [rim]
Fig. 2: Photoluminescence dichroismof T6 in E7 (10 pm cell) obtained through a polarized excitation and emission,both parallelandboth perpendicularto the rubbingdirection(nematic director),respectively. As shownin Fig. 3, the T6 moleculesembeddedinto the E7 nematichostmatrix canbe dynamicallyreorientedby applyingan electric field 18V per 1Oprnthicknessto the liquid crystal cells filled with the solutions, The quenching of the photoluminescence by the field is attributedto the reorientation of the T6 moleculesparallelto the electricfield.
Fig. 3; Switching of the photoluminescence of T6 in E7 by switchinganelectricfield (18V/ 10pm) on andoff. The possibility that an extended conjugatedoligomercan be macroscopicallyaligned by simply embedding in a liquid crystallinematrix and utilizing the propertiesof the host opens up areasfor detailed studiesof the propertiesof conjugated oligomersandpolymersusing,for example, i.) optical dichroism to get polarization information on the excitedstates,
[4] R. D. McCullough,S.Tristan-Nagle,S. P. Williams, R. D. Lowe, and M. Jayamman,J. Am. Chem. Sot. 115, (1993) 4910. [5] G. Meier, E. Sackmann,andJ. G. Grabmaier,Applications of Liquid Cqstals (SpringerVerlag, Berlin, 1975). [6] K. Araya, A. Mukoh, T. Narahara,and H. Shirakawa, Synth. Met. 14, (1986) 199. [7] H. Goto, K. Akagi, H. Shirakawa,S-Y. Oh, and K. Araya, Synth. Met. 71, (1995) 1899. [S] K. Agaki, H. Goto, H. Shirakawa,T. Nishizawa, and K. Masuda,Synth. Met. 69, (1995) 33. [9] F. Vicentini, M. Mauzac,R. Laversanne,P. Pochat,andJ. P. Parneix,Liq. Cyst. 16, (1994)721. [lo] L. Yu andZ. Bao,Adv. Mater. 6, (1994) 156. [l l] K. Yoshino, X. H. Yin, S. Morita, M. Nakazono,T. Kawai,M. Ozaki, S.-H. Jin, andS.-K. Choi, Jpn. J. Appt. Phys. 32, (1993)L1673. [12] G. H. W. Milbum, C. Campbell,A. J. Shand,andA. R. Werninck,Liq. Cyvst. 8, (1990)623. [ 131 Y. 0zcayi.r and A. Blumstein,J. Polym. Sci. Part A (Polymer Chemishyl 24, (1986) 1217. [14] Y. Ozcayir, J. Asrar, S. B. Clough, and A. Blumstein, Mol. Cry.% Liq. Cryst. 138, (1986)167. [15] A. F. Garito, in Proc. SPIE, Advances in Materials for Active Optics, San Diego, CA, USA, SPlE Proceedings 567 (SPlE, 1985)51. [16] G. Lussem,R. Festag, A. Greiner, C. Schmidt, C. Unterlechner,W. Heitz, J. H. Wendorff, M. Hopmeier,and J. Feldmann,Adv. Mats: 7, (1995) 923. [17] C. Taliani, R. Zamboni,G. Ruani, S. Rossini,and R. Lazzaroni,J; Mol. Elec. 6, (1990)225. [ 181P. B. Liescheskiand D. W. H. Rankin,J. Mol. Structure 178, (1988)227. [I91 L. C. T. Beek,D. S. Zimmerman,andE. E. Bumell, Mol. Phys. 74, (1991) 1027. 1201 W. t. Hoeve, H. Wynberg, E. E. Havinga, and E. W. Meijer, J. Am. Chem. Sot. II3, (1991) 5887. [21] E. E. Havinga,I. Rotte,E. W. Meijer, W. t. Hoeve,andH. Wynberg,Synth. Met. 41-43, (1991) 473. [22] N. S. Sariciftci, U. Lemmer,D. Vacar, A. J. Heeger,R. A. J. Janssen, Adv. Mater., inpress (1996).
Synthtic
Metals
84 (1997)
609610
Dynamic Orientation of Conjugated Oligomers in Nematic Liquid Crystaltine M&t-ices N. Serdar Sariciftci Physikalische Chemie, Johannes Kepler Univ&t Linz Altenberg~strasse 69, A-4040 Linz, Aush’a U. Lemmer Sektion Physik Ludwig Maximilian UniversZit Miinchen, 80799 Germany D. Vacar and A. J. Heeger Institute for Polymers & Organic Solids, University of California, Santa Barbara, CA 93106, U.S.A. R. A. J. Janssen Eindhoven University of Technology, 5600 Eindhoven, The Netherlands Abstract
Polarizedphotoluminescence is observedfrom a host/guestsystemconsistingof conjugatedoligomersdissolvedin nematic liquid crystallinematrices.In this system conjugatedoligomers(guest)exhibit an emissionspectrumwell separatedTom the emissionof the liquid crystallinematrix {host)howeverpolarizedparallelto the orientationof the hostmatrix. The resultsclearly demonstrate the spontaneous orientationof the oligothiophenes alongthe director of the nematicmatrix. Uponapplyingan electric field perpendicularto the direction of the spontaneous orientation,the oligomersare dynamicallyreoriented(switched)with the hostresultingin switchingof thepolarizedphotoluminescence. Keywords: Self-organizationin macromolecules, optical absorption and emissionspectroscopy,switches,polythiopheneand dsivatives 1. Introduction
Physical properties of conjugatedpolymer tilms are heavily influenced by the intrinsic disorder phenomenawithin the structure. With material quality getting better through high degreeof uniformity of chemicalcoupling as well as through controlled recrystallization from solutions in the doped conductingstate andfurther refining of the synthesisconditions, conductingpolymerscan now be preparedwell on the metallic sideof the disorderinducedM-I transition[l-4]. Thus, creating macroscopic order within the conjugatedpolymericstructureshas alwaysbeena continuing,important,scientificchallenge. The ideaof usingliquid crystallinehostmatices for macroscopic orientation of guest moleculesis quite well known (see for example [5] and referencestherein). Araya et al. showedthat synthesisof polyacetylene’inliquid crystallinematricesresultsin highly orientedmaterials [6]. Furthermore,synthetic efforts of modifjkrg the side chainsof a conjugatedpolymer with liquid crystalline side groupshave been reported in detail (see for example [7-151 and referencestherein). Recently, polarized photoluminescence has been reported from liquid crystalline polymers with phenylenevinylene segments on the main chain[l6]. Taliani et al. reportedon a liquid crystallinephaseof the a-sexithienyl(T6 ) aboveT=312 C [17]. Moleculessuchas thiophenesand bithiophenesembeddedin liquid crystalline matriceshas beeninvestigatedwith nuclearmagneticresonance techniquesto obtainaccuratestructuralinformation[ 18,191. In this contribution we report the macroscopicorientation of conjugatedoligomers(six memberedoligothiophenes;e.g. asexithienyl, (T6)) dissolvedwithin a nematic liquid crystalline matrix (Fig.l). The orientation of the oligothiophenescan be dynamically switched with the host nematic matrix upon applicationof an electric field as observedthroughswitchingof the polarizedphotoluminescence of T6 . 0379-6779/97/$17.00
0 1997
PII SO379-fiT79(96)04074-X
Ekvia Scienm S.A.All rightsreserved
Fig. 1: Schematicillustration of the orderingof oligothiophenes (T6) within a homogeneously alignednematicliquid crystalline matrix. 2. Results & Discussion
The six memberedoligothiopheneusedin the presentstudy carry dodecyl substituents on different thiophene rings and is designatedas T6 d(2,5). The synthesishas been described previously [20,21]. Details of the experimentsare published elsewhere[22]. The liquid crystal cells filled with the solutions of T6 in E7 show uniform brightnessunder cross polarized microscopesuggestinga homogeneous alignmentof the liquid crystals. To estimate the achieved degree of homogeneous
N. Serdar
610
Sariciftci
et al. /Synthetic
alignment of T6 in nematic matrix we performed linear dichroism experiments using polarized linearly optical absorption and emission. Fig. 2 shows the polarization dependence of the emission spectra. The emission spectrum of T6 obtained by exciting and probing parallel to the rubbing direction (parallel to the director of the nematic matrix) is an order of magnitude stronger compared to the case where excitation and emission is perpendicular to the director. Since the active emission originates liom the guestmolecules,T6, this resultconfirmsthe orientation of the guestwithin the nematichost.
Metals
84 (1997)
609-610
ii.) high resolutionnuclearmagneticresonancespectroscopyto get anisotropicinteractions, iii.) electron spin resonanceto get detailed parameterof the radical ions (polarons) and triplet states by doping and photoexcitation,respectively. Furthermore,the combinationof the liquid crystalline display technologies with the emergingnew areaof conjugatedpolymer light emittingdiodesis interesting, Acknowledgements
We acknowledgeJim Bourassa,Mark Urish and G&her Ahlers for valuablehelp. The oligothiophenesare generously providedby PhilipsResearch Laboratories. References
[l] M. Reghu,Y. Cao,D. Moses,andA. J. Heeger,Phys.Rev. B 47, (1993) 1758. [2] C. 0. Yoon, R. Menon, D. Moses,andA. J. Heeger,Whys. Rev. B 49, (1994) 10851. [31 R. D. McCullough, R. D. Lowe, M. Jayaraman,andD. L. Anderson,J. Org. Chem. 58, (1993)904.
Wavelength [rim]
Fig. 2: Photoluminescence dichroismof T6 in E7 (10 pm cell) obtained through a polarized excitation and emission,both parallelandboth perpendicularto the rubbingdirection(nematic director),respectively. As shownin Fig. 3, the T6 moleculesembeddedinto the E7 nematichostmatrix canbe dynamicallyreorientedby applyingan electric field 18V per 1Oprnthicknessto the liquid crystal cells filled with the solutions, The quenching of the photoluminescence by the field is attributedto the reorientation of the T6 moleculesparallelto the electricfield.
Fig. 3; Switching of the photoluminescence of T6 in E7 by switchinganelectricfield (18V/ 10pm) on andoff. The possibility that an extended conjugatedoligomercan be macroscopicallyaligned by simply embedding in a liquid crystallinematrix and utilizing the propertiesof the host opens up areasfor detailed studiesof the propertiesof conjugated oligomersandpolymersusing,for example, i.) optical dichroism to get polarization information on the excitedstates,
[4] R. D. McCullough,S.Tristan-Nagle,S. P. Williams, R. D. Lowe, and M. Jayamman,J. Am. Chem. Sot. 115, (1993) 4910. [5] G. Meier, E. Sackmann,andJ. G. Grabmaier,Applications of Liquid Cqstals (SpringerVerlag, Berlin, 1975). [6] K. Araya, A. Mukoh, T. Narahara,and H. Shirakawa, Synth. Met. 14, (1986) 199. [7] H. Goto, K. Akagi, H. Shirakawa,S-Y. Oh, and K. Araya, Synth. Met. 71, (1995) 1899. [S] K. Agaki, H. Goto, H. Shirakawa,T. Nishizawa, and K. Masuda,Synth. Met. 69, (1995) 33. [9] F. Vicentini, M. Mauzac,R. Laversanne,P. Pochat,andJ. P. Parneix,Liq. Cyst. 16, (1994)721. [lo] L. Yu andZ. Bao,Adv. Mater. 6, (1994) 156. [l l] K. Yoshino, X. H. Yin, S. Morita, M. Nakazono,T. Kawai,M. Ozaki, S.-H. Jin, andS.-K. Choi, Jpn. J. Appt. Phys. 32, (1993)L1673. [12] G. H. W. Milbum, C. Campbell,A. J. Shand,andA. R. Werninck,Liq. Cyvst. 8, (1990)623. [ 131 Y. 0zcayi.r and A. Blumstein,J. Polym. Sci. Part A (Polymer Chemishyl 24, (1986) 1217. [14] Y. Ozcayir, J. Asrar, S. B. Clough, and A. Blumstein, Mol. Cry.% Liq. Cryst. 138, (1986)167. [15] A. F. Garito, in Proc. SPIE, Advances in Materials for Active Optics, San Diego, CA, USA, SPlE Proceedings 567 (SPlE, 1985)51. [16] G. Lussem,R. Festag, A. Greiner, C. Schmidt, C. Unterlechner,W. Heitz, J. H. Wendorff, M. Hopmeier,and J. Feldmann,Adv. Mats: 7, (1995) 923. [17] C. Taliani, R. Zamboni,G. Ruani, S. Rossini,and R. Lazzaroni,J; Mol. Elec. 6, (1990)225. [ 181P. B. Liescheskiand D. W. H. Rankin,J. Mol. Structure 178, (1988)227. [I91 L. C. T. Beek,D. S. Zimmerman,andE. E. Bumell, Mol. Phys. 74, (1991) 1027. 1201 W. t. Hoeve, H. Wynberg, E. E. Havinga, and E. W. Meijer, J. Am. Chem. Sot. II3, (1991) 5887. [21] E. E. Havinga,I. Rotte,E. W. Meijer, W. t. Hoeve,andH. Wynberg,Synth. Met. 41-43, (1991) 473. [22] N. S. Sariciftci, U. Lemmer,D. Vacar, A. J. Heeger,R. A. J. Janssen, Adv. Mater., inpress (1996).