- Email: [email protected]
Photoluminescence of thiophene-based copolymers with tunable energy gap
ELSEVIER
Synthetic
Metals
84 (1997)
551-552
Photoluminescence of tbiophene-based copolymers with tunable energy gap Silvia Luzzati, Istituto
di Chimica
delle
Elisabetta Prevosti, Marinella
Macromolecole, C.N.R., via Bassini e-mail : vib @ icm.mi.cnr.it
Catellani IS, 20133
Milano,
Italy.
Abstract The photoluminescence spectra and the quantum yields of a series of conjugated polymers obtained by the copolymerisation of 3butylthiophene and 3,4-dibutyhhiophene in dit%erent ratio are reported. The emission maxima, like the absorption maxima, are tunable in the visible by varying the fractionfof disubstituted monomers. By increasingfthere is a blue shit% of the spectra while the Stokes shift exhibits a considerable increase both in the solid state and in solution. The role of ring rotations in the structural relaxation of the excited state and in the non radiative recombination channels is discussed. Kgwora!s:
Poly-thiophene
and derivatives, optical absorption and emission spectroscopy, photoluminescence
1. Introduction The luminescence properties of conjugated polymers are of considerable interest because of their potential application as emissive materials in light emitting diodes. Thiophene-based polymers are particularly interesting because of their great chemical versatility, through substitution in the 3- and 4- ring positions, which can be used to modify the conjugation length and therefore to obtain light emitting devices of controlled colour [l]. The emission efficiency is anyhow rather poor because of the occurrence of relevant non-radiative decay processes. The relaxation pathway of the excited state is strongly affected by interchain coupling and by the electronic and the conformational structure of the isolated chain. Recent studies have pointed out that in thiophene-based materials, both radiative [2] and non radiative [3,4] recombination processes are a&zted by the rotational motion of thienyl rings. Interring torsions favour internal conversion due to the dissipation of the excited energy through rotational modes [3]. Triplet formation is also favoured by torsional motions [4]. In poly-3alkylthiophenes the chain stifhress, varied through the regioregularity of the monomer enchainment, at&& the emission properties [3]. In this work we report the cw photoluminescence of a series of alkylsubstituted polymers obtained by the random copolymerization of 3,4-butylthiophene and 3-butylthiophene in different ratio. The presence of disubstituted monomers in the chain increases the content of head-to-head enchainments and the steric hindrance between alkyl chains on neighbouring thienyl rings induces a twisting of the backbone with a 0379-5779/97/S17.00
PII
0 1997
SO379-6779(96)04046-5
Ekevier
Science
S.A
All rights
reserved
reduction of the conjugation length. By increasing the fractionf of 3,4-alkylthiophene in the copolymer, the absorption maximum is tuned continuously from 500 nm for the poly-3butylthiophene homopolymer to 320 run for the poly-3,4dibutylthiophene homopolymer [5]. Preliminary results have shown that it is possible to obtain corn these copolymers blue, green and red electroluminescence [6] . 2. ExperimentalThe samples were prepared by chemical oxidative coupling using FeC13. The synthesis and the characterisation are reported elsewhere [5]. The absorption and emission spectra were performed on polymers dissolved in tetrahydrofuran (T’HF) at concentrations lower than 10m4 M. Films were obtained by spin coating on glass substrates. Quantum yields in solution were measured with a Jasco FP777 spectrofluorimeter using quinine sulfate solutions as a standard [7J The emission spectra were obtained keeping the films under vacuum, using a single photon counting spectrometer and were corrected for its spectral response. The 363.5 mn line of an Argon-ion laser was used as excitation. 2. Results and discussion Fig. 1 shows that the energy position of the absorption and emission peaks in solution and in the solid state are blue shitted by increasmgJ: A solvatochromic effect is observed for fcO.5 and disappears at higher content of disubstituted monomer. This indicates that at higherA the conjugated chains
552
S. Luzzati et al. /Synthetic Metals 84 (1997) 551-552
Fig. 1. Emissionand absorptionmaxima of solutions(circles) and tihns (triangles)as a function of the fraction f of the disubstitutedmonomer. arefar from a planar conformation.The Stokesshift increases with f both in solid and in solution. From the mono to the disubstitutedhomopolymerthe Stokesshift is almostdoubled. This observationsuggests that a largerelaxation is taking place in the fist singletexcited state,possiblywith a planarizationof the relaxed chain through electron-ringtorsion coupling.The cw photoinducedabsorptionspectrumof 3,4 butylthiophenes [S], aswell aspumpand probefemtosecondspectroscopyof a nonplanarsexythiophenes [9], bring supportfor a planarization of the excited state. The quantumyield of the solutions(seefig. 2) showsa smooth decreasewith f while an abrupt de-crease is observedfor the disubstitutedhomopolymer.This indicatesthat by increasing the inter-ring torsionsthe efficiency of the radiative emission stronglydecreases. An increaseof the radiative lifetime with ring torsion may possiblyaccountfor this resultbut it seemsquite unlikely asa rough estimateof the radiative lifetimesfrom the absorption spectra [lo] indicates an oppositebehaviour. This type of behaviouris alsoproposedby other studies[2]. On the other hand in a much more viscous solvent as decalin, we have observedfor poly-3,4-butylthiophenethe samespectralfeatures of the THF solutionsbut a quantum yield which is nine times higher. This indicatesthat by inhibitingring torsionalmotionsthere is a reductionof the non radiativerecombinationchannels. In conclusionthe study of the cw photoluminescence of a seriesof alkylsubstitutedpolymers obtained by the random copolymerizationof 3,4-butylthiopheneand 3-butylthiophene in differentratio, showsthat it is possibleto tailor the intensity and the *wiY of the luminescence spectra of polyalkylthiophenes.Rotational disorder appearsto have an important role in the disexcitation pathway of this classof polymers.Data suggestthat rotational modesget involved in the stn&ural relaxation of the excited statewhenf increases. Ring rotations enhancesthe non radiative recombination channels.Time resolvedspectroscopywill help in clarifying the photoexcitationscenarioof thesepolymers.
j
1:s
Ener&’
(eV)
3.5
Fig. 2. Absorptionand em%sionspectraof the copolymers,at variousf, in THF solution( c: 5x10m5M). @arethe measured quantumyields. The photoluminescence intensity is arbitrarly scaled References [l] M. R Andersson,M. Berggren,0. Inganas,G. Gut&son, J.C. Guftasson-Carlberg, D. Selse,T. Hjertberg, 0. Wennerstrom,Macromolecules,28 (1995)7525 [2] Y. Kanemitsu,K. Suzuki, Y. Masumoto,Y. Tomiuchi, Y. Shiraisbi,M. Kuroda,Phys.Rev.B, 50 (1994)2301 [3] B. Xu, S. Holdcroft, Macromolecuks,26 (1993)4457 [4] D. Oelkrug,HJ. EgeIhaaf,J. Gierschner,A Tompert, Synt.Met., 76 (1996)249 [5] M Catelhmi,S.Luzzati, R Mendichi, A Giacometti Schieroni,Polymer,37 (1996)1059 [6] S.Taschet als.,to bepubZished [7J J.N. Demas,GA. Crosby,J. Phys. Chem.,75(1971)991 [S] S. Luzzati, E. Prevosti,W. Viera, M. Catellani, to be published [9] G. Lam@ M. Nisoli, M. Magni, S. De Silvestri, G. Barbarella,G. Zambianchi,R Tubino, Phys.Rev.B, 51(1995) 13770 [lo] C.N.R Rao,Ultra-violet and VisibleSpectroscogv, Butterworths,London 1975
Synthetic
Metals
84 (1997)
551-552
Photoluminescence of tbiophene-based copolymers with tunable energy gap Silvia Luzzati, Istituto
di Chimica
delle
Elisabetta Prevosti, Marinella
Macromolecole, C.N.R., via Bassini e-mail : vib @ icm.mi.cnr.it
Catellani IS, 20133
Milano,
Italy.
Abstract The photoluminescence spectra and the quantum yields of a series of conjugated polymers obtained by the copolymerisation of 3butylthiophene and 3,4-dibutyhhiophene in dit%erent ratio are reported. The emission maxima, like the absorption maxima, are tunable in the visible by varying the fractionfof disubstituted monomers. By increasingfthere is a blue shit% of the spectra while the Stokes shift exhibits a considerable increase both in the solid state and in solution. The role of ring rotations in the structural relaxation of the excited state and in the non radiative recombination channels is discussed. Kgwora!s:
Poly-thiophene
and derivatives, optical absorption and emission spectroscopy, photoluminescence
1. Introduction The luminescence properties of conjugated polymers are of considerable interest because of their potential application as emissive materials in light emitting diodes. Thiophene-based polymers are particularly interesting because of their great chemical versatility, through substitution in the 3- and 4- ring positions, which can be used to modify the conjugation length and therefore to obtain light emitting devices of controlled colour [l]. The emission efficiency is anyhow rather poor because of the occurrence of relevant non-radiative decay processes. The relaxation pathway of the excited state is strongly affected by interchain coupling and by the electronic and the conformational structure of the isolated chain. Recent studies have pointed out that in thiophene-based materials, both radiative [2] and non radiative [3,4] recombination processes are a&zted by the rotational motion of thienyl rings. Interring torsions favour internal conversion due to the dissipation of the excited energy through rotational modes [3]. Triplet formation is also favoured by torsional motions [4]. In poly-3alkylthiophenes the chain stifhress, varied through the regioregularity of the monomer enchainment, at&& the emission properties [3]. In this work we report the cw photoluminescence of a series of alkylsubstituted polymers obtained by the random copolymerization of 3,4-butylthiophene and 3-butylthiophene in different ratio. The presence of disubstituted monomers in the chain increases the content of head-to-head enchainments and the steric hindrance between alkyl chains on neighbouring thienyl rings induces a twisting of the backbone with a 0379-5779/97/S17.00
PII
0 1997
SO379-6779(96)04046-5
Ekevier
Science
S.A
All rights
reserved
reduction of the conjugation length. By increasing the fractionf of 3,4-alkylthiophene in the copolymer, the absorption maximum is tuned continuously from 500 nm for the poly-3butylthiophene homopolymer to 320 run for the poly-3,4dibutylthiophene homopolymer [5]. Preliminary results have shown that it is possible to obtain corn these copolymers blue, green and red electroluminescence [6] . 2. ExperimentalThe samples were prepared by chemical oxidative coupling using FeC13. The synthesis and the characterisation are reported elsewhere [5]. The absorption and emission spectra were performed on polymers dissolved in tetrahydrofuran (T’HF) at concentrations lower than 10m4 M. Films were obtained by spin coating on glass substrates. Quantum yields in solution were measured with a Jasco FP777 spectrofluorimeter using quinine sulfate solutions as a standard [7J The emission spectra were obtained keeping the films under vacuum, using a single photon counting spectrometer and were corrected for its spectral response. The 363.5 mn line of an Argon-ion laser was used as excitation. 2. Results and discussion Fig. 1 shows that the energy position of the absorption and emission peaks in solution and in the solid state are blue shitted by increasmgJ: A solvatochromic effect is observed for fcO.5 and disappears at higher content of disubstituted monomer. This indicates that at higherA the conjugated chains
552
S. Luzzati et al. /Synthetic Metals 84 (1997) 551-552
Fig. 1. Emissionand absorptionmaxima of solutions(circles) and tihns (triangles)as a function of the fraction f of the disubstitutedmonomer. arefar from a planar conformation.The Stokesshift increases with f both in solid and in solution. From the mono to the disubstitutedhomopolymerthe Stokesshift is almostdoubled. This observationsuggests that a largerelaxation is taking place in the fist singletexcited state,possiblywith a planarizationof the relaxed chain through electron-ringtorsion coupling.The cw photoinducedabsorptionspectrumof 3,4 butylthiophenes [S], aswell aspumpand probefemtosecondspectroscopyof a nonplanarsexythiophenes [9], bring supportfor a planarization of the excited state. The quantumyield of the solutions(seefig. 2) showsa smooth decreasewith f while an abrupt de-crease is observedfor the disubstitutedhomopolymer.This indicatesthat by increasing the inter-ring torsionsthe efficiency of the radiative emission stronglydecreases. An increaseof the radiative lifetime with ring torsion may possiblyaccountfor this resultbut it seemsquite unlikely asa rough estimateof the radiative lifetimesfrom the absorption spectra [lo] indicates an oppositebehaviour. This type of behaviouris alsoproposedby other studies[2]. On the other hand in a much more viscous solvent as decalin, we have observedfor poly-3,4-butylthiophenethe samespectralfeatures of the THF solutionsbut a quantum yield which is nine times higher. This indicatesthat by inhibitingring torsionalmotionsthere is a reductionof the non radiativerecombinationchannels. In conclusionthe study of the cw photoluminescence of a seriesof alkylsubstitutedpolymers obtained by the random copolymerizationof 3,4-butylthiopheneand 3-butylthiophene in differentratio, showsthat it is possibleto tailor the intensity and the *wiY of the luminescence spectra of polyalkylthiophenes.Rotational disorder appearsto have an important role in the disexcitation pathway of this classof polymers.Data suggestthat rotational modesget involved in the stn&ural relaxation of the excited statewhenf increases. Ring rotations enhancesthe non radiative recombination channels.Time resolvedspectroscopywill help in clarifying the photoexcitationscenarioof thesepolymers.
j
1:s
Ener&’
(eV)
3.5
Fig. 2. Absorptionand em%sionspectraof the copolymers,at variousf, in THF solution( c: 5x10m5M). @arethe measured quantumyields. The photoluminescence intensity is arbitrarly scaled References [l] M. R Andersson,M. Berggren,0. Inganas,G. Gut&son, J.C. Guftasson-Carlberg, D. Selse,T. Hjertberg, 0. Wennerstrom,Macromolecules,28 (1995)7525 [2] Y. Kanemitsu,K. Suzuki, Y. Masumoto,Y. Tomiuchi, Y. Shiraisbi,M. Kuroda,Phys.Rev.B, 50 (1994)2301 [3] B. Xu, S. Holdcroft, Macromolecuks,26 (1993)4457 [4] D. Oelkrug,HJ. EgeIhaaf,J. Gierschner,A Tompert, Synt.Met., 76 (1996)249 [5] M Catelhmi,S.Luzzati, R Mendichi, A Giacometti Schieroni,Polymer,37 (1996)1059 [6] S.Taschet als.,to bepubZished [7J J.N. Demas,GA. Crosby,J. Phys. Chem.,75(1971)991 [S] S. Luzzati, E. Prevosti,W. Viera, M. Catellani, to be published [9] G. Lam@ M. Nisoli, M. Magni, S. De Silvestri, G. Barbarella,G. Zambianchi,R Tubino, Phys.Rev.B, 51(1995) 13770 [lo] C.N.R Rao,Ultra-violet and VisibleSpectroscogv, Butterworths,London 1975