- Email: [email protected]
Photoluminescence properties of new PPV derivatives
Journal of Luminescence 131 (2011) 1541–1544
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Photoluminescence properties of new PPV derivatives F. Massuyeau a,n, E. Faulques a, S. Lefrant a, M. Majdoub b, M. Ghedira b, K. Alimi b, J. We´ry a a b
Institut des Mate´riaux Jean Rouxel, Universite´ de Nantes, CNRS, UMR 6502, 2 rue de la Houssinie re, 44322 Nantes, France Unite´ de Recherche, Mate´riaux Nouveaux et Dispositifs Electroniques Organiques, Faculte´ des Sciences de Monastir, 5000 Monastir, Tunisie
a r t i c l e i n f o
abstract
Article history: Received 10 September 2010 Received in revised form 16 December 2010 Accepted 3 February 2011 Available online 25 February 2011
We investigate the emission properties of two copolymers by steady-state and time-resolved photoluminescence measurements. The copolymers are derived from poly-phenylene-vinylene (PPV) to which a non-conjugated ether group was added. The difference between the two copolymers is the group attached on the phenyl rings. Their fluorescence spectra present bathochromic and hypsochromic shift with respect to pristine PPV, well explained by their chemical properties. Additionally, we observe an enhancement of their quantum yield compared to that of PPV. Photoluminescence decays are bi-exponential for the two copolymers and their lifetime are longer than that of PPV. Finally, differences in emission properties between the two copolymers put in evidence the importance of their conformational structures in the luminescence processes. & 2011 Elsevier B.V. All rights reserved.
Keywords: Poly-phenylene-vinylene derivatives Time-resolved spectroscopy Photoluminescence Excitons Polymer conformation
1. Introduction Over the past few decades, conjugated polymers have been extensively studied because they possess both the electronic properties of semi-conductors and the mechanical properties of plastics. Nowadays, their potential applications in electronics and optoelectronics cover various fields like light-emitting diodes [1,2], photovoltaic cells [3,4], field-effect transistors [5], and optically pumped lasers [6,7]. The emission wavelength behavior depends on the conjugation extension over the whole supra-molecule and can be controlled by the attachment of functional groups or by the introduction of non-p-conjugated sequences to obtain copolymers. These modifications can increase [8] or reduce the optical gap [9]. Since the first report on polymer light emitting diodes [1] (PLED), different classes of conjugated polymers have been synthesized and characterized in order to tune the electronic properties such as the bandgap or to allow their solubilization in common solvents [10,11]. Among them, Poly para-phenylenevinylene (PPV) is still one of the most useful polymers as emitting layer in PLEDs. Many PPV derivatives have been utilized in such PLEDs to overcome solubility difficulties like MEH-PPV [12], or to enhance the electron affinity like CN-PPV [8,13]. In this context, Alimi et al. have reported the synthesis of two conjugated copolymers [14,15] containing a repetition of PPV and ether units.
n
Corresponding author. E-mail address: fl[email protected] (F. Massuyeau).
0022-2313/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2011.02.010
Then, the same group has extensively studied these two compounds in order to give a description of their physical and optical properties [16–19]. In this paper, we report additional transient photoluminescence (PL) study on these copolymers in order to complete a broader comparative view of optical properties of these two systems and particularly, to investigate the nature of the excited states, their lifetime and decay processes. We also determined the Quantum Yield (QY) of the PL emission for both copolymers. The decay behaviors provide an understanding on how the conformational changes and the chemical nature of the substituent groups can tune the emission properties in such structures and change their QY.
2. Experimental details In this work, we have investigated the two copolymers derived from PPV referred to as PPV-ether and C1–4PPV-ether (scheme 1). More details for their synthesis and characterization have been described elsewhere [14–19]. The main difference between these two copolymers is, for the C1–4PPV-ether, the presence of methoxy and butoxy groups grafted in positions 2 and 5 on the phenyl rings. These two products are amorphous in powder form, like most polymers and insoluble in usual solvents. The PPV-ether powder presents a yellow color and the C1–4-PPV-ether powder is dark red. To compare the two copolymers with pristine PPV, we have prepared PPV films. The poly (p-xylene tetrahydrothiophenium chloride), a PPV precursor, is synthesized in methanol in our
1542
F. Massuyeau et al. / Journal of Luminescence 131 (2011) 1541–1544
H OH
H
C
CH
CH
C x
H
OR'
H
CH
CH
RO
C y
Cl
H
n
OR'
C x
H
C H
H
C
H
OR'
H OH
O
H
H
O
H
H
C
C
H y n
RO
Cl
H
RO
Scheme 1. The chemical structure of: (a) PPV-ether and (b) C1–4PPV-ether.
3. Results and discussion To obtain a better understanding of the excited state dynamics and determine the influence of the sample chemical structure, we performed time-resolved photoluminescence (TRPL) spectroscopy on the C1–4 PPV-ether and PPV-ether with a sweep range of 1 ns. We prepared standard PPV films converted at 300 1C for comparison. Fig. 1 depicts the PL transient spectra of PPV-ether (a), C1–4PPV-ether (b) and PPV (c) with an excitation wavelength lexc ¼400 nm at room temperature (RT). These spectra are similar to those recorded in steady state PL (not shown here). Compared to the two PL spectra of copolymers obtained at 10 K [17], those obtained at RT are structureless. At low temperature, the migration of excitons becomes inactive and thus favors the radiative recombination of the photogenerated species [22]. Furthermore, the PL profiles are enlarged at room temperature due to enhanced electron-phonon coupling.
1.0
Normalized Intensity PL
laboratory via a standard procedure as described elsewhere [20]. The solution is drop-casted on a silica substrate. After the evaporation of methanol, the polymer film is introduced in an oven and converted at Tc¼300 1C under dynamic secondary vacuum during 6 h to obtain a PPV film with a thickness of about 200 nm. Time-resolved photoluminescence (TRPL) experiments were carried out with a regenerative amplified femtosecond Ti:Sapphire laser system (Spectra Physics Hurricane X), generating 100 fs pulses at 800 nm with a repetitive rate of 1 kHz and a power of 1 W. The laser line is frequency-doubled with a thin BBO crystal to obtain an excitation line lexc ¼400 nm (3.1 eV). The pump energy pulse is controlled to ensure that the excitation density in the sample does not exceed 1017 cm 3, to avoid annihilation process and sample degradation. The transient signals were spectrally dispersed into an Oriel MS260i imaging spectrograph (150 grooves/mm, f¼ 1/4) designed to minimize stray light with high spectral resolution. The emission spectra were temporally resolved with a high dynamic range Hamamatsu C7700 streak camera with a temporal resolutiono 20 ps. The PL quantum yields (QYs) were measured with an integrating sphere on the steady-state PL with a Jobin-Yvon Fluorolog spectrometer using a xenon lamp (500 W) as the excitation source according to the method described in [21]. Measurements were performed before and after the time resolved PL studies to make sure that no degradation occurred. We estimated the errors for QY to be 20% of the measured value. QYs were measured at 3.1 eV (lexc ¼400 nm).
(a)
(c)
(b)
0.8 0.6 0.4 0.2 0.0 450
500
550 600 650 Wavelength (nm)
700
750
Fig. 1. Time-integrated transient photoluminescence of the different samples: (a) PPV-ether, (b) C1–4PPV-ether and (c) PPV film. Excitation : 400 nm.
In Table 1 the maximum PL peak are reported for the three polymers. The maximum for PPV-ether and C1–4PPV-ether are located at 525 nm and 609 nm respectively and surround the maximum PL peak of PPV standard at 550 nm [23]. The photoemission of these two copolymers is much broadened. In addition, the spectrum of PPV-ether covers entirely that of standard PPV. The PL spectrum of PPV-ether is blue shifted by 25 nm compared to that of PPV. This result is well explained by the presence of ether groups in the backbone of the copolymer which limits the conjugation and so increases the bandgap, whereas methoxy and butoxy groups grafted in 2 and 5 positions of phenyl rings confer to the C1–4PPV-ether a narrower energy gap with a redshift of 41 nm with respect to PPV. These results corroborate the optical absorption and vibrational experimental measurements coupled to theoretical calculations using semi-empirical, ab initio and density functional theory obtained previously showing the improved conjugated character in C1–4PPV-ether [24]. The redshift of the PL spectrum is due to the electron-donor alkoxy substituents laterally attached on the phenyl rings, as in the case of MEH-PPV [25]. These lateral groups increase the degree of delocalization of the p electrons on the carbon atoms of the copolymer backbone which are responsible for the conjugation in the copolymer [24]. In Fig. 2 we show the PL decay dynamics (normalized) on a logarithmic scale in the range of 0–1 ns. They correspond to the following samples: (a) PPV-ether, and (b) C1–4PPV-ether, (c) PPV
F. Massuyeau et al. / Journal of Luminescence 131 (2011) 1541–1544
1543
Table 1 PL quantum yield (QY), decay times (t1, t2, tmean), radiative and non-radiative lifetimes (tr and tnr), pre-exponential factor (A1, A2) and percentage contributions P1, P2 to emission of levels 1 and 2 and PL maximum peak listed for the transient PL of the different samples investigated. Samples
QY
s1(ps) (A1) (P1)
s2(ps) (A2) (P2)
smean (ps)
sr (ps)
snr (ps)
PL maximum Peak
PPV film converted at 300 1C C1–4PPV-ether PPV-ether
0.19 0.32 0.41
49 (1.13) (93%) 62 (0.90) (80%) 64 (0.87) (76%)
333 (0.08) (7%) 348 (0.23) (20%) 434 (0.25) (24%)
149 260 347
784 812 846
183 382 588
550 nm (2.254 eV) 609 nm (2.036 eV) 525 nm (2.362 eV)
(ti) is calculated by the following formula:
1
Normalized intensity PL
Pi ð%Þ ¼
(a) (b)
tr ¼
(c)
0.0
0.2
0.4
0.6
0.8
1.0
Decay time (ns) Fig. 2. Integrated transient photoluminescence decays of the different samples: (a) PPV-ether, (b) C1–4PPV-ether and (c) PPV film. Excitation : 400 nm. The solid lines are fits to the curves.
film converted at 300 1C. It is clear that the PL lifetime of the two copolymers is longer compared with the PL lifetime of standard PPV. Due to different processes like the excitonic migration on conjugated segments with different length, a single monoexponential fit cannot reproduce these data for a temporal window of 0–1 ns. These decays can be reproduced with two coupled exponential decays convoluted with the contribution of the apparatus function given by the Gaussian temporal dependence G (t) of the laser pulse, according to the following rate equations: dn1 ¼ PðtÞn1 k1 , dt
dn2 ¼ n1 k1 n2 k2 dt
ð1Þ
In Eq. (1), n1 and n2 are the populations of the excited state levels 1 and 2, respectively, and k1 and k2 are the inverse of the lifetimes t1 and t2. In this simple model [26], the populations of levels 1 and 2 are coupled in order to account indirectly for a migration process from short to long segments [27]. The photogenerated charges populating the higher energy level 1 migrate toward defects and relax quickly on the lower energy state 2 with lifetime t1. At longer time, the photogenerated charges on level 2 are less mobile and consequently survive longer with a slower time constant t2. Furthermore, n1 and n2 represent the total populations of photogenerated charges in the energy states 1 and 2. These populations include photogenerated charges recombining radiatively and non-radiatively. The decaying population is n¼ A1n1 + A2n2, where A1 and A2 are proportional to the PL intensity from levels 1 and 2, respectively. We define below (Eq. (2)) an average decay time called tmean in order to show the average trend of the photogenerated charge migration time:
tmean ¼
A1 t21 þ A2 t22 A1 t1 þ A2 t2
ð3Þ
Pi is reported in parentheses for each decay component. Results are summarized in Table 1. Furthermore, we report in this table the value of the QY, determined from steady-state measurements, for each sample as well as tr and tnr, the radiative and non radiative lifetime, respectively, obtained from QY and tmean by the following relations :
0.1
0.01
Ai ti
SAi ti
ð2Þ
For clarity, the weight corresponding to the relative population of photogenerated charges contributing to each of the decay times
tmean QY
and tnr ¼
tmean 1QY
ð4Þ
All these compounds present the usual features to yield a PL lifetime tmean of several hundred ps and a radiative time tr of the order of one nanosecond, consistent with a fully allowed transition of a strongly absorbing molecule. This is characteristic of an emission process resulting from singlet intrachain excitons [28]. The origin of the emissive species is of crucial importance to understand the mechanism of PL in these copolymers. As in PPV films, this is the generation of singlet intrachain excited states that is the dominant product of the photoexcitation in these copolymers. In addition, the two copolymers have an improved PL QY with respect to standard PPV. PL QY of PPV-ether copolymers is higher to that of MEH-PPV [28]. From Table 1, the PPV film parameters tr ¼784 ps, tnr ¼ 183 ps, tmean ¼149 ps and QY ¼0.19 are in agreement with the values found in the literature [26,27]. The standard PPV film is characterized by long conjugated segments organized in well packed regions (formed in aggregated area) where polymer chains are close to each other [29]. It was demonstrated in a previous publication that this aggregation play a role in the decay rates in PPV film [30]. Thus interchain interactions favor a fast migration of excitons providing a fast decay channel. The exciton can easily migrate on non-radiative defects like hydroxide groups or C-C single bonds. In fact, in PPV, tnr is very low compared to tr. The non-radiative decay channel is the preferred pathway. On the contrary, PPV-ether and C1–4PPV-ether are amorphous and this state disadvantages the exciton migration, involving a decay time slower in both copolymers (260 and 347 ps) compared to that of PPV (149 ps). This freezes the exciton migration to nonradiative defects and explains the higher QY for both polymers. This experimental result agrees with the values characterizing the decay time of each product. As we can see from Table 1, tr varies slightly going from PPV (784 ps) to both copolymers (812 and 846 ps) while tnr increases. The value of tnr is doubled from PPV (183 ps) to C1–4PPV-ether (382 ps) and tripled from PPV to PPV-ether (588 ps). The non-radiative pathway for both copolymers is not as favorable as in the case of PPV. The most important result comes from the comparison between both copolymers. The decay time of C1–4PPV-ether (260 ps) is slightly faster compared to that of PPV-ether (347 ps). Also, the QY of PPV-ether (41%) is higher than that of C1–4PPV-ether (32%). This behavior can be explained by the structural differences provided by Raman and infrared absorption spectroscopies [17]. Indeed, previous studies with these techniques showed that PPV-ether presents a more planar conformation
1544
F. Massuyeau et al. / Journal of Luminescence 131 (2011) 1541–1544
in the ground-state. The graft groups on the phenyl rings in the C1–4-PPV-ether are therefore expected to affect the planarity of the structure. Similar observations have been addressed by other authors [29,30]. They compared series of 3,6 carbazolenevinylene derivatives [31] and series of triphenylamine derivatives [32] with different groups and observed a faster PL decay for derivatives presenting a more non-planar conformation. They explain that after photoexcitation all derivatives present near-planar conformation in the excited state, as in the PPV case. In the derivative with a more non-planar conformational ground-state, the twisting process during the excitation turns out to be more important. This internal twisting quenches excitons on chains and favors the non radiative pathway. In our studies, this explains why the C1–4PPV-ether presents a faster decay and a lower QY. The non-planar structure in the ground state of this copolymer favors the internal twisting during the photoexcitation and enhances the efficiency of the non-radiative pathway since tnr for C1–4-PPV-ether (382 ps) is lower than that of PPV-ether (588 ps).
4. Conclusion Transient PL results show that the combination of QY and PL lifetime measurements is a powerful tool to understand the nature of the photoexcitations generated in conjugated polymers. These studies have enabled us to show that the generation of emissive singlet intrachain excitons is predominant in both copolymers, as in standard PPV. We have explained how the copolymers can be used to tune the emission properties of pristine PPV by changing the nature of attached groups. On one hand, in PPV-ether, the shortening of the conjugation length by adding non-p-conjugated sequences ether groups leads to a blueshift of the emission compared to pristine PPV. On the other hand, lateral alkoxy substituents on the phenyl rings of C1–4PPV-ether enhance the delocalization of the p electrons and allow a red-shift of the emission compared to pristine PPV. Additionally, we observed an enhancement of the QY for both polymers due to the slower exciton migration which prevents drastically the desexcitation of photogenerated species on defects. By comparing the two copolymers, we finally observed that the attached group can change the conformation of the chains and consequently alter the emission properties by increasing the nonradiative pathway.
References [1] 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. [2] R.H. Friend, R.W. Gymer, A.B. Holmes, J.H. Burroughes, R.N. Markes, C. Taliani, D.D.C. Bradley, D.A.D. Santos, J.L. Bredas, M. Logdlund, W.R. Salaneck, Nature 397 (1999) 121. [3] J. Hou, C. Yang, J. Qiao, Y. Li, Synth. Met. 150 (2005) 297. [4] C. Yang, H. Li, Q. Sun, J. Qiao, Y. Li, Y. Li, D. Zhu, Solar Energy Mater. Solar Cells 85 (2005) 241. [5] G. Horowitz, Adv. Mater. 10 (1998) 365. [6] D. Ghosh, G.S. Samal, A.K. Biswas, Y.N. Mohapatra, Thin Sol. Films 477 (2005) 162. [7] S. Schrader, A. Penzkofer, W. Holzer, R. Velagapudi, B. Grimm, J. Lumin. 110 (2004) 303. [8] N.C. Greenham, S.C. Moratti, D.D.C. Bradley, R.H. Friend, A.B. Holmes, Nature 365 (1993) 628. [9] H. Horhold, Z. Chem 12 (1972) 41. [10] J.L. Segura, Acta Polymerica 49 (7) (1998) 319. [11] A. Kraft, A.C. Grimsdale, A.B. Holmes, Ange. Chem (Eds.), Intern, 37, 1998, p. 402. [12] D. Braun, A.J. Heeger, Appl. Phys. Lett. 58 (1991) 1982. [13] S.A. Chen, E.C. Chang, Macromol. 31 (1998) 4899. [14] K. Alimi, P. Molinie´, M. Majdoub, J.C. Bernede, J.L. Fave, H. Bouchriha, M. Ghedira, Eur. Polym. J. 37 (2001) 781. [15] K. Alimi, P. Molinie´, N. Blel, J.L. Fave, J.C. Bernede, M. Ghedira, Synth. Met. 126 (2002) 19. [16] A. Mabrouk, K. Alimi, M. Hamidi, M. Bouachrine, P. Molinie´, Polymer 46 (2005) 9928. [17] A. Mabrouk, S. Ayachi, B. Zaidi, J.P. Buisson, P. Molinie, K. Alimi, Euro, Polym. J. 39 (2003) 2121. [18] B. Zaı¨di, S. Ayachi, A. Mabrouk, P. Molinie, K. Alimi, Polymer Degradation and Stability 79 (2003) 183. [19] A. Mabrouk, K. Alimi, P. Molinie´, P. Nguyen, J. Phys. Chem. B 110 (2006) 1141. [20] D. Stenger-Smith, R.W. Lenz, G. Wegner, Polymer 30 (1989) 1048. [21] N.C. Greenham, I.D.C. Samuel, W. Hayes, R.T. Philips, Y.A.R. Kessener, C. Moratti, A.B. Holmes, R.H. Friend, Chem. Phys. Lett. 241 (1995) 89. [22] E. Mulazzi, R. Perego, J. We´ry, L. Mihut, S. Lefrant, E. Faulques, J. Chem. Phys. 125 (2006) 014706. [23] E. Mulazzi, A. Ripamonti, J. We´ry, B. Dulieu, S. Lefrant, Phys. Rev. B 60 (1999) 16519. [24] A. Pogantsch, A.K. Mahler, G. Hayn, R. Saf, F. Stelzer, E.J.W. List, J.L. Bredas, E. Zojer, J. Chem. Phys. 297 (2004) 143. [25] M.K. Ram, N. Sarkar, P. Bertoncello, A. Sarkar, R. Narizzano, C. Nicolini, Synth. Met 122 (2001) 369. [26] F. Massuyeau, E. Faulques, H. Athalin, S. Lefrant, J.L. Duvail, J. We´ry, E. Mulazzi, R. Perego, J. Chem. Phys. 130 (2009) 124706. ¨ [27] S. Heun, R.F. Mahrt, A. Greiner, U. Lemmer, H. Bassler, D.A. Halliday, D; D.C. Bradley, P.L. Burn, A.B. Holmes, J. Phys.: Condens Matter 5 (1993) 247. [28] I.D.W. Samuel, G. Rumbles, R.H. Friend, Primary photoexcitations in conjugated polymers : molecular excitons versus semiconductor band model, in: N.S. Saricifti (Ed.), world scientific, Singapore, 1997, p. 140. [29] J. We´ry, B. Dulieu, J. Bullot, M. Baı¨toul, P. De´niard, J.P. Buisson, Polymer 40 (1999) 519. [30] F. Massuyeau, H. Aarab, L. Mihut, S. Lefrant, E. Faulques, J. We´ry, E. Mulazzi, R. Perego, J. Phys. Chem. C 111 (41) (2007) 15111. [31] X.J. Zhao, Y.L. Liu, K. Meng, Q. Zeng, S.F. Wang, Q.H. Gong, Chem. Phys. Lett. 459 (2008) 146. [32] Q. Zeng, Y.L. Liu, K. Meng, X.J. Zhao, S.F. Wang, Q.H. Gong, Chin. Phys. Lett. 26 (7) (2009) 077810.
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Photoluminescence properties of new PPV derivatives F. Massuyeau a,n, E. Faulques a, S. Lefrant a, M. Majdoub b, M. Ghedira b, K. Alimi b, J. We´ry a a b
Institut des Mate´riaux Jean Rouxel, Universite´ de Nantes, CNRS, UMR 6502, 2 rue de la Houssinie re, 44322 Nantes, France Unite´ de Recherche, Mate´riaux Nouveaux et Dispositifs Electroniques Organiques, Faculte´ des Sciences de Monastir, 5000 Monastir, Tunisie
a r t i c l e i n f o
abstract
Article history: Received 10 September 2010 Received in revised form 16 December 2010 Accepted 3 February 2011 Available online 25 February 2011
We investigate the emission properties of two copolymers by steady-state and time-resolved photoluminescence measurements. The copolymers are derived from poly-phenylene-vinylene (PPV) to which a non-conjugated ether group was added. The difference between the two copolymers is the group attached on the phenyl rings. Their fluorescence spectra present bathochromic and hypsochromic shift with respect to pristine PPV, well explained by their chemical properties. Additionally, we observe an enhancement of their quantum yield compared to that of PPV. Photoluminescence decays are bi-exponential for the two copolymers and their lifetime are longer than that of PPV. Finally, differences in emission properties between the two copolymers put in evidence the importance of their conformational structures in the luminescence processes. & 2011 Elsevier B.V. All rights reserved.
Keywords: Poly-phenylene-vinylene derivatives Time-resolved spectroscopy Photoluminescence Excitons Polymer conformation
1. Introduction Over the past few decades, conjugated polymers have been extensively studied because they possess both the electronic properties of semi-conductors and the mechanical properties of plastics. Nowadays, their potential applications in electronics and optoelectronics cover various fields like light-emitting diodes [1,2], photovoltaic cells [3,4], field-effect transistors [5], and optically pumped lasers [6,7]. The emission wavelength behavior depends on the conjugation extension over the whole supra-molecule and can be controlled by the attachment of functional groups or by the introduction of non-p-conjugated sequences to obtain copolymers. These modifications can increase [8] or reduce the optical gap [9]. Since the first report on polymer light emitting diodes [1] (PLED), different classes of conjugated polymers have been synthesized and characterized in order to tune the electronic properties such as the bandgap or to allow their solubilization in common solvents [10,11]. Among them, Poly para-phenylenevinylene (PPV) is still one of the most useful polymers as emitting layer in PLEDs. Many PPV derivatives have been utilized in such PLEDs to overcome solubility difficulties like MEH-PPV [12], or to enhance the electron affinity like CN-PPV [8,13]. In this context, Alimi et al. have reported the synthesis of two conjugated copolymers [14,15] containing a repetition of PPV and ether units.
n
Corresponding author. E-mail address: fl[email protected] (F. Massuyeau).
0022-2313/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2011.02.010
Then, the same group has extensively studied these two compounds in order to give a description of their physical and optical properties [16–19]. In this paper, we report additional transient photoluminescence (PL) study on these copolymers in order to complete a broader comparative view of optical properties of these two systems and particularly, to investigate the nature of the excited states, their lifetime and decay processes. We also determined the Quantum Yield (QY) of the PL emission for both copolymers. The decay behaviors provide an understanding on how the conformational changes and the chemical nature of the substituent groups can tune the emission properties in such structures and change their QY.
2. Experimental details In this work, we have investigated the two copolymers derived from PPV referred to as PPV-ether and C1–4PPV-ether (scheme 1). More details for their synthesis and characterization have been described elsewhere [14–19]. The main difference between these two copolymers is, for the C1–4PPV-ether, the presence of methoxy and butoxy groups grafted in positions 2 and 5 on the phenyl rings. These two products are amorphous in powder form, like most polymers and insoluble in usual solvents. The PPV-ether powder presents a yellow color and the C1–4-PPV-ether powder is dark red. To compare the two copolymers with pristine PPV, we have prepared PPV films. The poly (p-xylene tetrahydrothiophenium chloride), a PPV precursor, is synthesized in methanol in our
1542
F. Massuyeau et al. / Journal of Luminescence 131 (2011) 1541–1544
H OH
H
C
CH
CH
C x
H
OR'
H
CH
CH
RO
C y
Cl
H
n
OR'
C x
H
C H
H
C
H
OR'
H OH
O
H
H
O
H
H
C
C
H y n
RO
Cl
H
RO
Scheme 1. The chemical structure of: (a) PPV-ether and (b) C1–4PPV-ether.
3. Results and discussion To obtain a better understanding of the excited state dynamics and determine the influence of the sample chemical structure, we performed time-resolved photoluminescence (TRPL) spectroscopy on the C1–4 PPV-ether and PPV-ether with a sweep range of 1 ns. We prepared standard PPV films converted at 300 1C for comparison. Fig. 1 depicts the PL transient spectra of PPV-ether (a), C1–4PPV-ether (b) and PPV (c) with an excitation wavelength lexc ¼400 nm at room temperature (RT). These spectra are similar to those recorded in steady state PL (not shown here). Compared to the two PL spectra of copolymers obtained at 10 K [17], those obtained at RT are structureless. At low temperature, the migration of excitons becomes inactive and thus favors the radiative recombination of the photogenerated species [22]. Furthermore, the PL profiles are enlarged at room temperature due to enhanced electron-phonon coupling.
1.0
Normalized Intensity PL
laboratory via a standard procedure as described elsewhere [20]. The solution is drop-casted on a silica substrate. After the evaporation of methanol, the polymer film is introduced in an oven and converted at Tc¼300 1C under dynamic secondary vacuum during 6 h to obtain a PPV film with a thickness of about 200 nm. Time-resolved photoluminescence (TRPL) experiments were carried out with a regenerative amplified femtosecond Ti:Sapphire laser system (Spectra Physics Hurricane X), generating 100 fs pulses at 800 nm with a repetitive rate of 1 kHz and a power of 1 W. The laser line is frequency-doubled with a thin BBO crystal to obtain an excitation line lexc ¼400 nm (3.1 eV). The pump energy pulse is controlled to ensure that the excitation density in the sample does not exceed 1017 cm 3, to avoid annihilation process and sample degradation. The transient signals were spectrally dispersed into an Oriel MS260i imaging spectrograph (150 grooves/mm, f¼ 1/4) designed to minimize stray light with high spectral resolution. The emission spectra were temporally resolved with a high dynamic range Hamamatsu C7700 streak camera with a temporal resolutiono 20 ps. The PL quantum yields (QYs) were measured with an integrating sphere on the steady-state PL with a Jobin-Yvon Fluorolog spectrometer using a xenon lamp (500 W) as the excitation source according to the method described in [21]. Measurements were performed before and after the time resolved PL studies to make sure that no degradation occurred. We estimated the errors for QY to be 20% of the measured value. QYs were measured at 3.1 eV (lexc ¼400 nm).
(a)
(c)
(b)
0.8 0.6 0.4 0.2 0.0 450
500
550 600 650 Wavelength (nm)
700
750
Fig. 1. Time-integrated transient photoluminescence of the different samples: (a) PPV-ether, (b) C1–4PPV-ether and (c) PPV film. Excitation : 400 nm.
In Table 1 the maximum PL peak are reported for the three polymers. The maximum for PPV-ether and C1–4PPV-ether are located at 525 nm and 609 nm respectively and surround the maximum PL peak of PPV standard at 550 nm [23]. The photoemission of these two copolymers is much broadened. In addition, the spectrum of PPV-ether covers entirely that of standard PPV. The PL spectrum of PPV-ether is blue shifted by 25 nm compared to that of PPV. This result is well explained by the presence of ether groups in the backbone of the copolymer which limits the conjugation and so increases the bandgap, whereas methoxy and butoxy groups grafted in 2 and 5 positions of phenyl rings confer to the C1–4PPV-ether a narrower energy gap with a redshift of 41 nm with respect to PPV. These results corroborate the optical absorption and vibrational experimental measurements coupled to theoretical calculations using semi-empirical, ab initio and density functional theory obtained previously showing the improved conjugated character in C1–4PPV-ether [24]. The redshift of the PL spectrum is due to the electron-donor alkoxy substituents laterally attached on the phenyl rings, as in the case of MEH-PPV [25]. These lateral groups increase the degree of delocalization of the p electrons on the carbon atoms of the copolymer backbone which are responsible for the conjugation in the copolymer [24]. In Fig. 2 we show the PL decay dynamics (normalized) on a logarithmic scale in the range of 0–1 ns. They correspond to the following samples: (a) PPV-ether, and (b) C1–4PPV-ether, (c) PPV
F. Massuyeau et al. / Journal of Luminescence 131 (2011) 1541–1544
1543
Table 1 PL quantum yield (QY), decay times (t1, t2, tmean), radiative and non-radiative lifetimes (tr and tnr), pre-exponential factor (A1, A2) and percentage contributions P1, P2 to emission of levels 1 and 2 and PL maximum peak listed for the transient PL of the different samples investigated. Samples
QY
s1(ps) (A1) (P1)
s2(ps) (A2) (P2)
smean (ps)
sr (ps)
snr (ps)
PL maximum Peak
PPV film converted at 300 1C C1–4PPV-ether PPV-ether
0.19 0.32 0.41
49 (1.13) (93%) 62 (0.90) (80%) 64 (0.87) (76%)
333 (0.08) (7%) 348 (0.23) (20%) 434 (0.25) (24%)
149 260 347
784 812 846
183 382 588
550 nm (2.254 eV) 609 nm (2.036 eV) 525 nm (2.362 eV)
(ti) is calculated by the following formula:
1
Normalized intensity PL
Pi ð%Þ ¼
(a) (b)
tr ¼
(c)
0.0
0.2
0.4
0.6
0.8
1.0
Decay time (ns) Fig. 2. Integrated transient photoluminescence decays of the different samples: (a) PPV-ether, (b) C1–4PPV-ether and (c) PPV film. Excitation : 400 nm. The solid lines are fits to the curves.
film converted at 300 1C. It is clear that the PL lifetime of the two copolymers is longer compared with the PL lifetime of standard PPV. Due to different processes like the excitonic migration on conjugated segments with different length, a single monoexponential fit cannot reproduce these data for a temporal window of 0–1 ns. These decays can be reproduced with two coupled exponential decays convoluted with the contribution of the apparatus function given by the Gaussian temporal dependence G (t) of the laser pulse, according to the following rate equations: dn1 ¼ PðtÞn1 k1 , dt
dn2 ¼ n1 k1 n2 k2 dt
ð1Þ
In Eq. (1), n1 and n2 are the populations of the excited state levels 1 and 2, respectively, and k1 and k2 are the inverse of the lifetimes t1 and t2. In this simple model [26], the populations of levels 1 and 2 are coupled in order to account indirectly for a migration process from short to long segments [27]. The photogenerated charges populating the higher energy level 1 migrate toward defects and relax quickly on the lower energy state 2 with lifetime t1. At longer time, the photogenerated charges on level 2 are less mobile and consequently survive longer with a slower time constant t2. Furthermore, n1 and n2 represent the total populations of photogenerated charges in the energy states 1 and 2. These populations include photogenerated charges recombining radiatively and non-radiatively. The decaying population is n¼ A1n1 + A2n2, where A1 and A2 are proportional to the PL intensity from levels 1 and 2, respectively. We define below (Eq. (2)) an average decay time called tmean in order to show the average trend of the photogenerated charge migration time:
tmean ¼
A1 t21 þ A2 t22 A1 t1 þ A2 t2
ð3Þ
Pi is reported in parentheses for each decay component. Results are summarized in Table 1. Furthermore, we report in this table the value of the QY, determined from steady-state measurements, for each sample as well as tr and tnr, the radiative and non radiative lifetime, respectively, obtained from QY and tmean by the following relations :
0.1
0.01
Ai ti
SAi ti
ð2Þ
For clarity, the weight corresponding to the relative population of photogenerated charges contributing to each of the decay times
tmean QY
and tnr ¼
tmean 1QY
ð4Þ
All these compounds present the usual features to yield a PL lifetime tmean of several hundred ps and a radiative time tr of the order of one nanosecond, consistent with a fully allowed transition of a strongly absorbing molecule. This is characteristic of an emission process resulting from singlet intrachain excitons [28]. The origin of the emissive species is of crucial importance to understand the mechanism of PL in these copolymers. As in PPV films, this is the generation of singlet intrachain excited states that is the dominant product of the photoexcitation in these copolymers. In addition, the two copolymers have an improved PL QY with respect to standard PPV. PL QY of PPV-ether copolymers is higher to that of MEH-PPV [28]. From Table 1, the PPV film parameters tr ¼784 ps, tnr ¼ 183 ps, tmean ¼149 ps and QY ¼0.19 are in agreement with the values found in the literature [26,27]. The standard PPV film is characterized by long conjugated segments organized in well packed regions (formed in aggregated area) where polymer chains are close to each other [29]. It was demonstrated in a previous publication that this aggregation play a role in the decay rates in PPV film [30]. Thus interchain interactions favor a fast migration of excitons providing a fast decay channel. The exciton can easily migrate on non-radiative defects like hydroxide groups or C-C single bonds. In fact, in PPV, tnr is very low compared to tr. The non-radiative decay channel is the preferred pathway. On the contrary, PPV-ether and C1–4PPV-ether are amorphous and this state disadvantages the exciton migration, involving a decay time slower in both copolymers (260 and 347 ps) compared to that of PPV (149 ps). This freezes the exciton migration to nonradiative defects and explains the higher QY for both polymers. This experimental result agrees with the values characterizing the decay time of each product. As we can see from Table 1, tr varies slightly going from PPV (784 ps) to both copolymers (812 and 846 ps) while tnr increases. The value of tnr is doubled from PPV (183 ps) to C1–4PPV-ether (382 ps) and tripled from PPV to PPV-ether (588 ps). The non-radiative pathway for both copolymers is not as favorable as in the case of PPV. The most important result comes from the comparison between both copolymers. The decay time of C1–4PPV-ether (260 ps) is slightly faster compared to that of PPV-ether (347 ps). Also, the QY of PPV-ether (41%) is higher than that of C1–4PPV-ether (32%). This behavior can be explained by the structural differences provided by Raman and infrared absorption spectroscopies [17]. Indeed, previous studies with these techniques showed that PPV-ether presents a more planar conformation
1544
F. Massuyeau et al. / Journal of Luminescence 131 (2011) 1541–1544
in the ground-state. The graft groups on the phenyl rings in the C1–4-PPV-ether are therefore expected to affect the planarity of the structure. Similar observations have been addressed by other authors [29,30]. They compared series of 3,6 carbazolenevinylene derivatives [31] and series of triphenylamine derivatives [32] with different groups and observed a faster PL decay for derivatives presenting a more non-planar conformation. They explain that after photoexcitation all derivatives present near-planar conformation in the excited state, as in the PPV case. In the derivative with a more non-planar conformational ground-state, the twisting process during the excitation turns out to be more important. This internal twisting quenches excitons on chains and favors the non radiative pathway. In our studies, this explains why the C1–4PPV-ether presents a faster decay and a lower QY. The non-planar structure in the ground state of this copolymer favors the internal twisting during the photoexcitation and enhances the efficiency of the non-radiative pathway since tnr for C1–4-PPV-ether (382 ps) is lower than that of PPV-ether (588 ps).
4. Conclusion Transient PL results show that the combination of QY and PL lifetime measurements is a powerful tool to understand the nature of the photoexcitations generated in conjugated polymers. These studies have enabled us to show that the generation of emissive singlet intrachain excitons is predominant in both copolymers, as in standard PPV. We have explained how the copolymers can be used to tune the emission properties of pristine PPV by changing the nature of attached groups. On one hand, in PPV-ether, the shortening of the conjugation length by adding non-p-conjugated sequences ether groups leads to a blueshift of the emission compared to pristine PPV. On the other hand, lateral alkoxy substituents on the phenyl rings of C1–4PPV-ether enhance the delocalization of the p electrons and allow a red-shift of the emission compared to pristine PPV. Additionally, we observed an enhancement of the QY for both polymers due to the slower exciton migration which prevents drastically the desexcitation of photogenerated species on defects. By comparing the two copolymers, we finally observed that the attached group can change the conformation of the chains and consequently alter the emission properties by increasing the nonradiative pathway.
References [1] 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. [2] R.H. Friend, R.W. Gymer, A.B. Holmes, J.H. Burroughes, R.N. Markes, C. Taliani, D.D.C. Bradley, D.A.D. Santos, J.L. Bredas, M. Logdlund, W.R. Salaneck, Nature 397 (1999) 121. [3] J. Hou, C. Yang, J. Qiao, Y. Li, Synth. Met. 150 (2005) 297. [4] C. Yang, H. Li, Q. Sun, J. Qiao, Y. Li, Y. Li, D. Zhu, Solar Energy Mater. Solar Cells 85 (2005) 241. [5] G. Horowitz, Adv. Mater. 10 (1998) 365. [6] D. Ghosh, G.S. Samal, A.K. Biswas, Y.N. Mohapatra, Thin Sol. Films 477 (2005) 162. [7] S. Schrader, A. Penzkofer, W. Holzer, R. Velagapudi, B. Grimm, J. Lumin. 110 (2004) 303. [8] N.C. Greenham, S.C. Moratti, D.D.C. Bradley, R.H. Friend, A.B. Holmes, Nature 365 (1993) 628. [9] H. Horhold, Z. Chem 12 (1972) 41. [10] J.L. Segura, Acta Polymerica 49 (7) (1998) 319. [11] A. Kraft, A.C. Grimsdale, A.B. Holmes, Ange. Chem (Eds.), Intern, 37, 1998, p. 402. [12] D. Braun, A.J. Heeger, Appl. Phys. Lett. 58 (1991) 1982. [13] S.A. Chen, E.C. Chang, Macromol. 31 (1998) 4899. [14] K. Alimi, P. Molinie´, M. Majdoub, J.C. Bernede, J.L. Fave, H. Bouchriha, M. Ghedira, Eur. Polym. J. 37 (2001) 781. [15] K. Alimi, P. Molinie´, N. Blel, J.L. Fave, J.C. Bernede, M. Ghedira, Synth. Met. 126 (2002) 19. [16] A. Mabrouk, K. Alimi, M. Hamidi, M. Bouachrine, P. Molinie´, Polymer 46 (2005) 9928. [17] A. Mabrouk, S. Ayachi, B. Zaidi, J.P. Buisson, P. Molinie, K. Alimi, Euro, Polym. J. 39 (2003) 2121. [18] B. Zaı¨di, S. Ayachi, A. Mabrouk, P. Molinie, K. Alimi, Polymer Degradation and Stability 79 (2003) 183. [19] A. Mabrouk, K. Alimi, P. Molinie´, P. Nguyen, J. Phys. Chem. B 110 (2006) 1141. [20] D. Stenger-Smith, R.W. Lenz, G. Wegner, Polymer 30 (1989) 1048. [21] N.C. Greenham, I.D.C. Samuel, W. Hayes, R.T. Philips, Y.A.R. Kessener, C. Moratti, A.B. Holmes, R.H. Friend, Chem. Phys. Lett. 241 (1995) 89. [22] E. Mulazzi, R. Perego, J. We´ry, L. Mihut, S. Lefrant, E. Faulques, J. Chem. Phys. 125 (2006) 014706. [23] E. Mulazzi, A. Ripamonti, J. We´ry, B. Dulieu, S. Lefrant, Phys. Rev. B 60 (1999) 16519. [24] A. Pogantsch, A.K. Mahler, G. Hayn, R. Saf, F. Stelzer, E.J.W. List, J.L. Bredas, E. Zojer, J. Chem. Phys. 297 (2004) 143. [25] M.K. Ram, N. Sarkar, P. Bertoncello, A. Sarkar, R. Narizzano, C. Nicolini, Synth. Met 122 (2001) 369. [26] F. Massuyeau, E. Faulques, H. Athalin, S. Lefrant, J.L. Duvail, J. We´ry, E. Mulazzi, R. Perego, J. Chem. Phys. 130 (2009) 124706. ¨ [27] S. Heun, R.F. Mahrt, A. Greiner, U. Lemmer, H. Bassler, D.A. Halliday, D; D.C. Bradley, P.L. Burn, A.B. Holmes, J. Phys.: Condens Matter 5 (1993) 247. [28] I.D.W. Samuel, G. Rumbles, R.H. Friend, Primary photoexcitations in conjugated polymers : molecular excitons versus semiconductor band model, in: N.S. Saricifti (Ed.), world scientific, Singapore, 1997, p. 140. [29] J. We´ry, B. Dulieu, J. Bullot, M. Baı¨toul, P. De´niard, J.P. Buisson, Polymer 40 (1999) 519. [30] F. Massuyeau, H. Aarab, L. Mihut, S. Lefrant, E. Faulques, J. We´ry, E. Mulazzi, R. Perego, J. Phys. Chem. C 111 (41) (2007) 15111. [31] X.J. Zhao, Y.L. Liu, K. Meng, Q. Zeng, S.F. Wang, Q.H. Gong, Chem. Phys. Lett. 459 (2008) 146. [32] Q. Zeng, Y.L. Liu, K. Meng, X.J. Zhao, S.F. Wang, Q.H. Gong, Chin. Phys. Lett. 26 (7) (2009) 077810.