Thin films of 10-[3,5-di(terbutyl)phenoxy]decyl-2-({2-[(9-anthrylcarbonyl)amino]acetyl}amino) acetate (ANTPEP), the thread of an anthracene-based rotaxane, have been processed by the spin coating technique in a polycarbonate (PC) matrix. A single layer organic light emitting diode (OLED) has been demonstrated with an external emission of 2 cd/m2. The light emission, both photo- and electrostimulated, originates from species formed in the excited state and not existing in the ground state. We assign the origin of photo- and electro-emission to molecular excimers.

Biscarini, F.,

Cavallini, M.,

Cocchi, M.,

Fattori, V.,

Gadret, G.,

Giro, G.,

Leigh, D.A.,

Loontjens, T.,

Morales, A.F.,

Murgia, M.,

Ruani, G.,

Thies, J.,

Zamboni, R.,

Giro, G.

Cocchi, M.

Fattori, V.

Gadret, G.

Ruani, G.

Murgia, M.

Cavallini, M.

Biscarini, F.

Zamboni, R.

Loontjens, T.

Thies, J.

Leigh, D.A.

Morales, A.F.

University of Groningen

   University of GroningenExcimer-like electroluminescence from thin films of switchable supermolecular anthracene-based rotaxanesGiro, G.; Cocchi, M.; Fattori, V.; Gadret, G.; Ruani, G.; Murgia, M.; Cavallini, M.; Biscarini, F.;Zamboni, R.; Loontjens, T.Published in:Synthetic MetalsDOI:10.1016/S0379-6779(00)01329-1IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2001Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Giro, G., Cocchi, M., Fattori, V., Gadret, G., Ruani, G., Murgia, M., ... Morales, A. F. (2001). Excimer-likeelectroluminescence from thin films of switchable supermolecular anthracene-based rotaxanes. SyntheticMetals, 122(1), 27-29. https://doi.org/10.1016/S0379-6779(00)01329-1CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 12-11-2019Excimer-like electroluminescence from thin films of switchablesupermolecular anthracene-based rotaxanesG. Giroa, M. Cocchia, V. Fattoria, G. Gadretb, G. Ruanib, M. Murgiab, M. Cavallinib,F. Biscarinib, R. Zambonib,*, T. Loontjensc, J. Thiesc, D.A. Leighd, A.F. MoralesdaFRAE-CNR, via P. Gobetti 101, 40129 Bologna, ItalybISM-CNR, via P. Gobetti 101, 40129 Bologna, ItalycDSM Research, P.O. Box 18, 6160 MD, Geleen, The NetherlandsdCSMC, Department of Chemistry, University of Warwick, Coventry CV4 7AL, UKAbstractThin films of 10-[3,5-di(terbutyl)phenoxy]decyl-2-({2-[(9-anthrylcarbonyl)amino]acetyl}amino) acetate (ANTPEP), the thread of ananthracene-based rotaxane, have been processed by the spin coating technique in a polycarbonate (PC) matrix. A single layer organic lightemitting diode (OLED) has been demonstrated with an external emission of 2 cd/m2. The light emission, both photo- and electro-stimulated, originates from species formed in the excited state and not existing in the ground state. We assign the origin of photo- andelectro-emission to molecular excimers. # 2001 Elsevier Science B.V. All rights reserved.Keywords: Electro-emission; Rotaxanes; Thin film; OLED; Excimers1. IntroductionOrganic light emitting diodes (OLED) are most promisingnew optoelectronic devices for practical industrial applica-tions. Both molecular- [1] and polymeric-based OLEDs [2]have been successfully developed during the last decade.Recently, exciplex electro-emission at the interface of Alq3-based hetero-multilayer OLEDs, which are used as voltagedependent colour tuning devices, has been claimed as apossible mechanism [3–5]. In this work we report onexcimer formation and electro-emission in thin films of anovel molecular system, the 10-[3,5-di(terbutyl)phenoxy]-decyl-2-({2-[(9-anthrylcarbonyl)amino]acetyl}amino) acet-ate (ANTPEP). The chemical structure is shown inFig. 1.The investigated moiety is a basic constituent of a super-molecular system, namely the methyl-exopyridine-anthra-cene rotaxane (EPAR-Me), a member of a class of syntheticpeptide rotaxanes [6]. Rotaxanes are a novel class of super-molecules where intrinsic functions can be designed forspecific uses. Rotaxanes consist of a macrocycle constrainedon a thread by two stoppers. The macrocycle could, inprinciple, move from one end of the thread to the other,rotate around it or stay at one particular point on the thread.Hydrogen bonding also plays a major role both in synthesisand in determining the location of the macrocycle on thethread of the supermolecule. The ANTPEP is made of ananthracene chromophore and a bis-terbutyl benzyl system(the stoppers) covalently bonded to a peptide unit attached toa (CH2) chain (the thread). The investigated system(ANTPEP) corresponds to the stoppers and thread of a classof peptide-based rotaxanes.2. Experimental results and discussionAn OLED has been prepared by the spin coating techni-que from a suitably prepared solution of ANTPEP mixedwith polycarbonate (PC) in dichloromethane. A glass sub-strate coated with optically transparent indium tin oxide(ITO) has been used as hole injector electrode. A highoptical quality thin film, 120 nm thick, of PC/ANTPEP indifferent concentrations (1/10 and 1/3 in weight) has beendeposited on the ITO by spin coating. This was followed byvacuum deposition of aluminium to act as an electroninjector contact. Spectroscopic investigations, namely opti-cal absorption, photoluminescence excitation profile (PLE),photoluminescence (PL) and electroluminescence (EL) havebeen performed in order to characterise the optical- andSynthetic Metals 122 (2001) 27–29* Corresponding author. Tel.: 39-051-639-8514;fax: 39-051-639-8540.E-mail address: zamboni@astbo1.bo.cnr.it (R. Zamboni).0379-6779/01/$ – see front matter # 2001 Elsevier Science B.V. All rights reserved.PII: S 0 3 7 9 - 6 7 7 9 ( 0 0 ) 0 1 3 2 9 - 1electro-emission performances and to identify the nature ofthe emitting species. In Fig. 1 the PLE and PL spectrameasured at room temperature are reported (Fig. 2).All the measured PLE spectra show the same features.This indicates that the emission processes have a commonorigin. The PLE spectra are characterised by three well-resolved peaks. This characteristic shape is reflected in thespectral behaviour of the anthracene moiety. The absorptionspectrum of the PC/ANTPEP thin film gives further evi-dence for this behaviour as we can see in Fig. 3. The PLemission spectrum is structureless and considerably spec-trally broadened (over 200 nm). A weak shoulder at around400 nm is spectrally consistent with the PL emission of theanthracene moiety, whereas the major part of the PL spec-trum is strongly red-shifted. In Fig. 3 the absorption spec-trum, together with the PL spectrum and EL spectrameasured at two different applied voltages, are shown.The EL shows different spectral behaviour with respect toPL.Two different peaks appear in the EL spectra. The first iscentred at the same energy as the maximum signal in the PLemission, whereas the second is at lower energy and isbroader than the former. This second peak seems to be moresensitive, both in intensity and in energy position, to theapplied voltage. The maximum intensity of the spectrallyintegrated external EL is 2 cd/m2. It is worthwhile to notethat the absorption, PL and PLE spectra of a 10ÿ5 dilutedsolution of ANTPEP in dichloromethane show spectralbehaviour corresponding to that of the anthracene molecule.The anthracene-like character is also still evident whenworking with diluted thin films, namely the 1/10 concentra-tion in PC.Considering the full set of experimental results, we candraw some conclusions on the origin of the emission of theANTPEP in the thin solid film. First of all, the most‘‘electronically active’’ moiety present in the ANTPEP isthe anthracene unit. The absorption spectrum in fact reflectsthe spectral properties of the anthracene. In the PL emissionspectra, features consistent with those observed in anthra-cene are only evident for diluted samples. The solid state,namely the thin films, is responsible for the new behaviouraffecting the radiative emission of the system. The constantPLE spectrum is independent of the wavelength (from 400 to500 nm), proving that the emission originates from a com-mon electronic origin and not from defects, low-lyingspecies or impurities. The anthracene absorption featuresare reproduced in the PLE spectra. Features related toimpurities, molecular aggregates, or defects in the groundstate are excluded. Analysing the PL spectra, we can assignthe first broad feature to the prompt-fluorescence from theanthracene unit in the ANTPEP system. In fact, the spectralposition is in agreement with the expected fluorescence fromanthracene crystals. The overall broad character of theFig. 1. Chemical structure of 10-[3,5-di(terbutyl)phenoxy]decyl-2-({2-[(9-anthrylcarbonyl)amino]acetyl}amino) acetate (ANTPEP).Fig. 2. PLE and PL spectra of the PC/ANTPEP 1/3 thin film. PLE spectra were obtained at 470 nm (dash-dot), 400 nm (dash-bold) and 500 nm (dot). The PLspectrum (line) has been obtained by exciting at 330 nm.28 G. Giro et al. / Synthetic Metals 122 (2001) 27–29emission spectrum is assigned to excimer emission, i.e. fromspecies occurring in the excited states, but not in the groundstate, of the solid phase of ANTPEP. At present, we do nothave structural data in order to further support this assign-ment. Nevertheless, the spectroscopic behaviour is consis-tent with our conclusions [7]. Comparing PL and EL spectra,we note the correspondence between the first broad peak inthe EL spectra and the PL peak (see Fig. 3). This is anindication that the nature of the emission is common for thetwo processes. The important conclusion to be drawn fromthis evidence is that both the PL and the EL of the ANTPEP-based OLED devices may be assigned to the same excimericorigin. The field dependent low-energy EL spectral feature isnot completely understood, but we tentatively assign thisfeature to trapped charged species in the thin film. Furtherstudies are necessary in order to more fully understand thisaspect.We should note that in our single layer OLED device thereis a rather large mismatch between the ITO work function(typically 4.5–4.7 eV) and the ionisation potential of theinvestigated molecular system (around 7–8 eV). Neverthe-less, the external light-intensity of 2 cd/m2 obtained in ourOLED device is an encouraging result for further studies onthis new supermolecular system.AcknowledgementsWe thank EU-TMR Project ‘‘DRUM’’ contract numberCT97-0097 and CNR-PF MSTA II Project DEMO forfinancial support.References[1] C.W. Tang, S.A. van Slyke, App. Phys. Lett. 51 (1987) 913.[2] J.H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K.Mackay, R.H. Friend, P.L. Burn, A.B. Holmes, Nature 347 (1990)539.[3] K. Okumoto, Y. Shirota, J. Luminesc. 87–89 (2000) 1171.[4] H. Ogawa, R. Okuda, Y. Shirota, Appl. Phys. A 67 (1998) 599.[5] K. Itano, H. Ogawa, Y. Shirota, Appl. Phys. Lett. 72 (1998) 636.[6] A.D. Leigh, A. Murphy, J.P. Smart, A.M.Z. Slawin, Angew. Chem. Int.Ed. 36 (1997) 728.[7] M. Pope, C.E. Swenberg, Electronic Processes in Organic Crystals,Clarendon Press, New York, 1982.Fig. 3. Absorption spectrum (dot), photoluminescence (PL) spectrum (dashed) and electro-emission (EL) spectra at different working voltages (dash-dot,V0  28 V and dash-dot-dot, V0  24 V) of the PC/ANTPEP 1/3 thin film. All the spectra have been measured at room temperature.G. Giro et al. / Synthetic Metals 122 (2001) 27–29 29

Excimer-like electroluminescence from thin films of switchable supermolecular anthracene-based rotaxanes

Thin films of 10-[3,5-di(terbutyl)phenoxy]decyl-2-({2-[(9-anthrylcarbonyl)amino]acetyl}amino) acetate (ANTPEP), the thread of an anthracene-based rotaxane, have been processed by the spin coating technique in a polycarbonate (PC) matrix. A single layer organic light emitting diode (OLED) has been demonstrated with an external emission of 2 cd/m2. The light emission, both photo- and electrostimulated, originates from species formed in the excited state and not existing in the ground state. We assign the origin of photo- and electro-emission to molecular excimers

ARTS repository - University of Groningen

   University of GroningenExcimer-like electroluminescence from thin films of switchable supermolecular anthracene-based rotaxanesGiro, G.; Cocchi, M.; Fattori, V.; Gadret, G.; Ruani, G.; Murgia, M.; Cavallini, M.; Biscarini, F.;Zamboni, R.; Loontjens, T.Published in:Synthetic MetalsDOI:10.1016/S0379-6779(00)01329-1IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2001Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Giro, G., Cocchi, M., Fattori, V., Gadret, G., Ruani, G., Murgia, M., Cavallini, M., Biscarini, F., Zamboni, R.,Loontjens, T., Thies, J., Leigh, D. A., & Morales, A. F. (2001). Excimer-like electroluminescence from thinfilms of switchable supermolecular anthracene-based rotaxanes. Synthetic Metals, 122(1), 27-29.https://doi.org/10.1016/S0379-6779(00)01329-1CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Excimer-like electroluminescence from thin ®lms of switchablesupermolecular anthracene-based rotaxanesG. Giroa, M. Cocchia, V. Fattoria, G. Gadretb, G. Ruanib, M. Murgiab, M. Cavallinib,F. Biscarinib, R. Zambonib,*, T. Loontjensc, J. Thiesc, D.A. Leighd, A.F. MoralesdaFRAE-CNR, via P. Gobetti 101, 40129 Bologna, ItalybISM-CNR, via P. Gobetti 101, 40129 Bologna, ItalycDSM Research, P.O. Box 18, 6160 MD, Geleen, The NetherlandsdCSMC, Department of Chemistry, University of Warwick, Coventry CV4 7AL, UKAbstractThin ®lms of 10-[3,5-di(terbutyl)phenoxy]decyl-2-({2-[(9-anthrylcarbonyl)amino]acetyl}amino) acetate (ANTPEP), the thread of ananthracene-based rotaxane, have been processed by the spin coating technique in a polycarbonate (PC) matrix. A single layer organic lightemitting diode (OLED) has been demonstrated with an external emission of 2 cd/m2. The light emission, both photo- and electro-stimulated, originates from species formed in the excited state and not existing in the ground state. We assign the origin of photo- andelectro-emission to molecular excimers. # 2001 Elsevier Science B.V. All rights reserved.Keywords: Electro-emission; Rotaxanes; Thin ®lm; OLED; Excimers1. IntroductionOrganic light emitting diodes (OLED) are most promisingnew optoelectronic devices for practical industrial applica-tions. Both molecular- [1] and polymeric-based OLEDs [2]have been successfully developed during the last decade.Recently, exciplex electro-emission at the interface of Alq3-based hetero-multilayer OLEDs, which are used as voltagedependent colour tuning devices, has been claimed as apossible mechanism [3±5]. In this work we report onexcimer formation and electro-emission in thin ®lms of anovel molecular system, the 10-[3,5-di(terbutyl)phenoxy]-decyl-2-({2-[(9-anthrylcarbonyl)amino]acetyl}amino) acet-ate (ANTPEP). The chemical structure is shown inFig. 1.The investigated moiety is a basic constituent of a super-molecular system, namely the methyl-exopyridine-anthra-cene rotaxane (EPAR-Me), a member of a class of syntheticpeptide rotaxanes [6]. Rotaxanes are a novel class of super-molecules where intrinsic functions can be designed forspeci®c uses. Rotaxanes consist of a macrocycle constrainedon a thread by two stoppers. The macrocycle could, inprinciple, move from one end of the thread to the other,rotate around it or stay at one particular point on the thread.Hydrogen bonding also plays a major role both in synthesisand in determining the location of the macrocycle on thethread of the supermolecule. The ANTPEP is made of ananthracene chromophore and a bis-terbutyl benzyl system(the stoppers) covalently bonded to a peptide unit attached toa (CH2) chain (the thread). The investigated system(ANTPEP) corresponds to the stoppers and thread of a classof peptide-based rotaxanes.2. Experimental results and discussionAn OLED has been prepared by the spin coating techni-que from a suitably prepared solution of ANTPEP mixedwith polycarbonate (PC) in dichloromethane. A glass sub-strate coated with optically transparent indium tin oxide(ITO) has been used as hole injector electrode. A highoptical quality thin ®lm, 120 nm thick, of PC/ANTPEP indifferent concentrations (1/10 and 1/3 in weight) has beendeposited on the ITO by spin coating. This was followed byvacuum deposition of aluminium to act as an electroninjector contact. Spectroscopic investigations, namely opti-cal absorption, photoluminescence excitation pro®le (PLE),photoluminescence (PL) and electroluminescence (EL) havebeen performed in order to characterise the optical- andSynthetic Metals 122 (2001) 27±29* Corresponding author. Tel.: 39-051-639-8514;fax: 39-051-639-8540.E-mail address: zamboni@astbo1.bo.cnr.it (R. Zamboni).0379-6779/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved.PII: S 0 3 7 9 - 6 7 7 9 ( 0 0 ) 0 1 3 2 9 - 1electro-emission performances and to identify the nature ofthe emitting species. In Fig. 1 the PLE and PL spectrameasured at room temperature are reported (Fig. 2).All the measured PLE spectra show the same features.This indicates that the emission processes have a commonorigin. The PLE spectra are characterised by three well-resolved peaks. This characteristic shape is re¯ected in thespectral behaviour of the anthracene moiety. The absorptionspectrum of the PC/ANTPEP thin ®lm gives further evi-dence for this behaviour as we can see in Fig. 3. The PLemission spectrum is structureless and considerably spec-trally broadened (over 200 nm). A weak shoulder at around400 nm is spectrally consistent with the PL emission of theanthracene moiety, whereas the major part of the PL spec-trum is strongly red-shifted. In Fig. 3 the absorption spec-trum, together with the PL spectrum and EL spectrameasured at two different applied voltages, are shown.The EL shows different spectral behaviour with respect toPL.Two different peaks appear in the EL spectra. The ®rst iscentred at the same energy as the maximum signal in the PLemission, whereas the second is at lower energy and isbroader than the former. This second peak seems to be moresensitive, both in intensity and in energy position, to theapplied voltage. The maximum intensity of the spectrallyintegrated external EL is 2 cd/m2. It is worthwhile to notethat the absorption, PL and PLE spectra of a 10ÿ5 dilutedsolution of ANTPEP in dichloromethane show spectralbehaviour corresponding to that of the anthracene molecule.The anthracene-like character is also still evident whenworking with diluted thin ®lms, namely the 1/10 concentra-tion in PC.Considering the full set of experimental results, we candraw some conclusions on the origin of the emission of theANTPEP in the thin solid ®lm. First of all, the most`̀ electronically active'' moiety present in the ANTPEP isthe anthracene unit. The absorption spectrum in fact re¯ectsthe spectral properties of the anthracene. In the PL emissionspectra, features consistent with those observed in anthra-cene are only evident for diluted samples. The solid state,namely the thin ®lms, is responsible for the new behaviouraffecting the radiative emission of the system. The constantPLE spectrum is independent of the wavelength (from 400 to500 nm), proving that the emission originates from a com-mon electronic origin and not from defects, low-lyingspecies or impurities. The anthracene absorption featuresare reproduced in the PLE spectra. Features related toimpurities, molecular aggregates, or defects in the groundstate are excluded. Analysing the PL spectra, we can assignthe ®rst broad feature to the prompt-¯uorescence from theanthracene unit in the ANTPEP system. In fact, the spectralposition is in agreement with the expected ¯uorescence fromanthracene crystals. The overall broad character of theFig. 1. Chemical structure of 10-[3,5-di(terbutyl)phenoxy]decyl-2-({2-[(9-anthrylcarbonyl)amino]acetyl}amino) acetate (ANTPEP).Fig. 2. PLE and PL spectra of the PC/ANTPEP 1/3 thin film. PLE spectra were obtained at 470 nm (dash-dot), 400 nm (dash-bold) and 500 nm (dot). The PLspectrum (line) has been obtained by exciting at 330 nm.28 G. Giro et al. / Synthetic Metals 122 (2001) 27±29emission spectrum is assigned to excimer emission, i.e. fromspecies occurring in the excited states, but not in the groundstate, of the solid phase of ANTPEP. At present, we do nothave structural data in order to further support this assign-ment. Nevertheless, the spectroscopic behaviour is consis-tent with our conclusions [7]. Comparing PL and EL spectra,we note the correspondence between the ®rst broad peak inthe EL spectra and the PL peak (see Fig. 3). This is anindication that the nature of the emission is common for thetwo processes. The important conclusion to be drawn fromthis evidence is that both the PL and the EL of the ANTPEP-based OLED devices may be assigned to the same excimericorigin. The ®eld dependent low-energy EL spectral feature isnot completely understood, but we tentatively assign thisfeature to trapped charged species in the thin ®lm. Furtherstudies are necessary in order to more fully understand thisaspect.We should note that in our single layer OLED device thereis a rather large mismatch between the ITO work function(typically 4.5±4.7 eV) and the ionisation potential of theinvestigated molecular system (around 7±8 eV). Neverthe-less, the external light-intensity of 2 cd/m2 obtained in ourOLED device is an encouraging result for further studies onthis new supermolecular system.AcknowledgementsWe thank EU-TMR Project `̀ DRUM'' contract numberCT97-0097 and CNR-PF MSTA II Project DEMO for®nancial support.References[1] C.W. Tang, S.A. van Slyke, App. Phys. Lett. 51 (1987) 913.[2] J.H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K.Mackay, R.H. Friend, P.L. Burn, A.B. Holmes, Nature 347 (1990)539.[3] K. Okumoto, Y. Shirota, J. Luminesc. 87±89 (2000) 1171.[4] H. Ogawa, R. Okuda, Y. Shirota, Appl. Phys. A 67 (1998) 599.[5] K. Itano, H. Ogawa, Y. Shirota, Appl. Phys. Lett. 72 (1998) 636.[6] A.D. Leigh, A. Murphy, J.P. Smart, A.M.Z. Slawin, Angew. Chem. Int.Ed. 36 (1997) 728.[7] M. Pope, C.E. Swenberg, Electronic Processes in Organic Crystals,Clarendon Press, New York, 1982.Fig. 3. Absorption spectrum (dot), photoluminescence (PL) spectrum (dashed) and electro-emission (EL) spectra at different working voltages (dash-dot,V0  28 V and dash-dot-dot, V0  24 V) of the PC/ANTPEP 1/3 thin film. All the spectra have been measured at room temperature.G. Giro et al. / Synthetic Metals 122 (2001) 27±29 29

English

G. GIRO

M. COCCHI

V. FATTORI

G. GADRET

G. RUANI

M. MURGIA

M. CAVALLINI

R. ZAMBONI

T. LOONTJENS

J. THIES

D. A. LEIGH

A. F. MORALES

BISCARINI, FABIO

Archivio istituzionale della ricerca - Università di Modena e Reggio Emilia

Excimer-like Electroluminescence from Thin Films of Switchable Supermolecular Anthracene-based Rotaxanes

Proceedings - University of Groningen

Dissertations of the University of Groningen

G. Giro

M. Cocchi

V. Fattori

G. Gadret

G. Ruani

M. Murgia

M. Cavallini

F. Biscarini

R. Zamboni

T. Loontjens

J. Thies

D.A. Leigh

A.F. Morales

Crossref

Synthetic Metals

University of Groningen Research Database

  
 University of Groningen
Excimer-like electroluminescence from thin films of switchable supermolecular anthracene-
based rotaxanes
Giro, G.; Cocchi, M.; Fattori, V.; Gadret, G.; Ruani, G.; Murgia, M.; Cavallini, M.; Biscarini, F.;
Zamboni, R.; Loontjens, T.
Published in:
Synthetic Metals
DOI:
10.1016/S0379-6779(00)01329-1
IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite from
it. Please check the document version below.
Document Version
Publisher's PDF, also known as Version of record
Publication date:
2001
Link to publication in University of Groningen/UMCG research database
Citation for published version (APA):
Giro, G., Cocchi, M., Fattori, V., Gadret, G., Ruani, G., Murgia, M., ... Morales, A. F. (2001). Excimer-like
electroluminescence from thin films of switchable supermolecular anthracene-based rotaxanes. Synthetic
Metals, 122(1), 27-29. DOI: 10.1016/S0379-6779(00)01329-1
Copyright
Other than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the
author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).
Take-down policy
If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately
and investigate your claim.
Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the
number of authors shown on this cover page is limited to 10 maximum.
Download date: 11-02-2018
Excimer-like electroluminescence from thin films of switchable
supermolecular anthracene-based rotaxanes
G. Giroa, M. Cocchia, V. Fattoria, G. Gadretb, G. Ruanib, M. Murgiab, M. Cavallinib,
F. Biscarinib, R. Zambonib,*, T. Loontjensc, J. Thiesc, D.A. Leighd, A.F. Moralesd
aFRAE-CNR, via P. Gobetti 101, 40129 Bologna, Italy
bISM-CNR, via P. Gobetti 101, 40129 Bologna, Italy
cDSM Research, P.O. Box 18, 6160 MD, Geleen, The Netherlands
dCSMC, Department of Chemistry, University of Warwick, Coventry CV4 7AL, UK
Abstract
Thin films of 10-[3,5-di(terbutyl)phenoxy]decyl-2-({2-[(9-anthrylcarbonyl)amino]acetyl}amino) acetate (ANTPEP), the thread of an
anthracene-based rotaxane, have been processed by the spin coating technique in a polycarbonate (PC) matrix. A single layer organic light
emitting diode (OLED) has been demonstrated with an external emission of 2 cd/m2. The light emission, both photo- and electro-
stimulated, originates from species formed in the excited state and not existing in the ground state. We assign the origin of photo- and
electro-emission to molecular excimers. # 2001 Elsevier Science B.V. All rights reserved.
Keywords: Electro-emission; Rotaxanes; Thin film; OLED; Excimers
1. Introduction
Organic light emitting diodes (OLED) are most promising
new optoelectronic devices for practical industrial applica-
tions. Both molecular- [1] and polymeric-based OLEDs [2]
have been successfully developed during the last decade.
Recently, exciplex electro-emission at the interface of Alq3-
based hetero-multilayer OLEDs, which are used as voltage
dependent colour tuning devices, has been claimed as a
possible mechanism [3–5]. In this work we report on
excimer formation and electro-emission in thin films of a
novel molecular system, the 10-[3,5-di(terbutyl)phenoxy]-
decyl-2-({2-[(9-anthrylcarbonyl)amino]acetyl}amino) acet-
ate (ANTPEP). The chemical structure is shown in
Fig. 1.
The investigated moiety is a basic constituent of a super-
molecular system, namely the methyl-exopyridine-anthra-
cene rotaxane (EPAR-Me), a member of a class of synthetic
peptide rotaxanes [6]. Rotaxanes are a novel class of super-
molecules where intrinsic functions can be designed for
specific uses. Rotaxanes consist of a macrocycle constrained
on a thread by two stoppers. The macrocycle could, in
principle, move from one end of the thread to the other,
rotate around it or stay at one particular point on the thread.
Hydrogen bonding also plays a major role both in synthesis
and in determining the location of the macrocycle on the
thread of the supermolecule. The ANTPEP is made of an
anthracene chromophore and a bis-terbutyl benzyl system
(the stoppers) covalently bonded to a peptide unit attached to
a (CH2) chain (the thread). The investigated system
(ANTPEP) corresponds to the stoppers and thread of a class
of peptide-based rotaxanes.
2. Experimental results and discussion
An OLED has been prepared by the spin coating techni-
que from a suitably prepared solution of ANTPEP mixed
with polycarbonate (PC) in dichloromethane. A glass sub-
strate coated with optically transparent indium tin oxide
(ITO) has been used as hole injector electrode. A high
optical quality thin film, 120 nm thick, of PC/ANTPEP in
different concentrations (1/10 and 1/3 in weight) has been
deposited on the ITO by spin coating. This was followed by
vacuum deposition of aluminium to act as an electron
injector contact. Spectroscopic investigations, namely opti-
cal absorption, photoluminescence excitation profile (PLE),
photoluminescence (PL) and electroluminescence (EL) have
been performed in order to characterise the optical- and
Synthetic Metals 122 (2001) 27–29
* Corresponding author. Tel.: 39-051-639-8514;
fax: 39-051-639-8540.
E-mail address: zamboni@astbo1.bo.cnr.it (R. Zamboni).
0379-6779/01/$ – see front matter # 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 3 7 9 - 6 7 7 9 ( 0 0 ) 0 1 3 2 9 - 1
electro-emission performances and to identify the nature of
the emitting species. In Fig. 1 the PLE and PL spectra
measured at room temperature are reported (Fig. 2).
All the measured PLE spectra show the same features.
This indicates that the emission processes have a common
origin. The PLE spectra are characterised by three well-
resolved peaks. This characteristic shape is reflected in the
spectral behaviour of the anthracene moiety. The absorption
spectrum of the PC/ANTPEP thin film gives further evi-
dence for this behaviour as we can see in Fig. 3. The PL
emission spectrum is structureless and considerably spec-
trally broadened (over 200 nm). A weak shoulder at around
400 nm is spectrally consistent with the PL emission of the
anthracene moiety, whereas the major part of the PL spec-
trum is strongly red-shifted. In Fig. 3 the absorption spec-
trum, together with the PL spectrum and EL spectra
measured at two different applied voltages, are shown.
The EL shows different spectral behaviour with respect to
PL.
Two different peaks appear in the EL spectra. The first is
centred at the same energy as the maximum signal in the PL
emission, whereas the second is at lower energy and is
broader than the former. This second peak seems to be more
sensitive, both in intensity and in energy position, to the
applied voltage. The maximum intensity of the spectrally
integrated external EL is 2 cd/m2. It is worthwhile to note
that the absorption, PL and PLE spectra of a 10ÿ5 diluted
solution of ANTPEP in dichloromethane show spectral
behaviour corresponding to that of the anthracene molecule.
The anthracene-like character is also still evident when
working with diluted thin films, namely the 1/10 concentra-
tion in PC.
Considering the full set of experimental results, we can
draw some conclusions on the origin of the emission of the
ANTPEP in the thin solid film. First of all, the most
‘‘electronically active’’ moiety present in the ANTPEP is
the anthracene unit. The absorption spectrum in fact reflects
the spectral properties of the anthracene. In the PL emission
spectra, features consistent with those observed in anthra-
cene are only evident for diluted samples. The solid state,
namely the thin films, is responsible for the new behaviour
affecting the radiative emission of the system. The constant
PLE spectrum is independent of the wavelength (from 400 to
500 nm), proving that the emission originates from a com-
mon electronic origin and not from defects, low-lying
species or impurities. The anthracene absorption features
are reproduced in the PLE spectra. Features related to
impurities, molecular aggregates, or defects in the ground
state are excluded. Analysing the PL spectra, we can assign
the first broad feature to the prompt-fluorescence from the
anthracene unit in the ANTPEP system. In fact, the spectral
position is in agreement with the expected fluorescence from
anthracene crystals. The overall broad character of the
Fig. 1. Chemical structure of 10-[3,5-di(terbutyl)phenoxy]decyl-2-({2-[(9-
anthrylcarbonyl)amino]acetyl}amino) acetate (ANTPEP).
Fig. 2. PLE and PL spectra of the PC/ANTPEP 1/3 thin film. PLE spectra were obtained at 470 nm (dash-dot), 400 nm (dash-bold) and 500 nm (dot). The PL
spectrum (line) has been obtained by exciting at 330 nm.
28 G. Giro et al. / Synthetic Metals 122 (2001) 27–29
emission spectrum is assigned to excimer emission, i.e. from
species occurring in the excited states, but not in the ground
state, of the solid phase of ANTPEP. At present, we do not
have structural data in order to further support this assign-
ment. Nevertheless, the spectroscopic behaviour is consis-
tent with our conclusions [7]. Comparing PL and EL spectra,
we note the correspondence between the first broad peak in
the EL spectra and the PL peak (see Fig. 3). This is an
indication that the nature of the emission is common for the
two processes. The important conclusion to be drawn from
this evidence is that both the PL and the EL of the ANTPEP-
based OLED devices may be assigned to the same excimeric
origin. The field dependent low-energy EL spectral feature is
not completely understood, but we tentatively assign this
feature to trapped charged species in the thin film. Further
studies are necessary in order to more fully understand this
aspect.
We should note that in our single layer OLED device there
is a rather large mismatch between the ITO work function
(typically 4.5–4.7 eV) and the ionisation potential of the
investigated molecular system (around 7–8 eV). Neverthe-
less, the external light-intensity of 2 cd/m2 obtained in our
OLED device is an encouraging result for further studies on
this new supermolecular system.
Acknowledgements
We thank EU-TMR Project ‘‘DRUM’’ contract number
CT97-0097 and CNR-PF MSTA II Project DEMO for
financial support.
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[3] K. Okumoto, Y. Shirota, J. Luminesc. 87–89 (2000) 1171.
[4] H. Ogawa, R. Okuda, Y. Shirota, Appl. Phys. A 67 (1998) 599.
[5] K. Itano, H. Ogawa, Y. Shirota, Appl. Phys. Lett. 72 (1998) 636.
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Fig. 3. Absorption spectrum (dot), photoluminescence (PL) spectrum (dashed) and electro-emission (EL) spectra at different working voltages (dash-dot,
V0  28 V and dash-dot-dot, V0  24 V) of the PC/ANTPEP 1/3 thin film. All the spectra have been measured at room temperature.
G. Giro et al. / Synthetic Metals 122 (2001) 27–29 29


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Excimer-like electroluminescence from thin films of switchable supermolecular anthracene-based rotaxanes

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