ARTICLE pubs.acs.org/JPCA
Controlling Bandgap Energy and Multivibronic Modes of a Poly(2,5-thiophene-1,4-dialkoxyphenylene) Derivative by Gamma Photons H. Santos Silva,*,† S. L. Nogueira,† J. E. Manzoli,‡ N. M. Barbosa Neto,† A. Marletta,† F. Serein-Spirau,§ J.-P Lere-Porte,§ Sandrine Lois,§ and R. A. Silva†,|| †
Instituto de Física, Universidade Federal de Uberl^andia, CP 593, 38400-902, Uberl^andia - MG, Brazil Instituto de Pesquisas Energeticas e Nucleares, IPEN/CNEN-SP, Av. Prof. Lineu Prestes 2242, Cidade Universitaria, 05508-000, S~ao Paulo - SP, Brazil § AM2N-Architectures Moleculaires et Materiaux Nanostructures, Institut Charles Gerhardt-Ecole Nationale Superieure de Chimie de Montpellier (ENSCM) 34296, Montpellier, Cedex 5, France Divis~ao de Metrologia de Materiais, Instituto Nacional de Metrologia, Normalizac-~ao e Qualidade Industrial, 25250-020, Duque de Caxias - RJ, Brazil
)
‡
ABSTRACT: In this work, the influence of γ radiation on electronic, structural, and vibrational properties of a poly(2,5-thiophene-1,4-dialkoxyphenylene) derivative is studied by optical absorption and photoluminescence. A Gaussian fit of emission spectra within FranckCondon vertical transitions formalism was carried out in order to understand how vibronic coupling is affected by the dose, because an unexpected luminescence behavior was observed. Aiming to understand the ionizing radiation matter interaction processes, we employed a molecular modeling procedure, through the use of a semiempirical method (AM1) applied to conjugated oligomers’ conformational structure and equilibrium geometries, to clarify the defects induction for the used doses. From AM1 optimized structures, electronic transitions were calculated by ZINDO/SCI semiempirical method to measure the chain scission degree. Moreover, with the results presented in this work, it is possible to come up with a new physicalchemical route to treat and increase conjugated polymers’ efficiency. Finally, we believe that the present paper contributes to the literature about defects on conjugated polymers.
1. INTRODUCTION The applicability of conjugated polymers is only possible due to two charge carriers process of dissociation and recombination: the photovoltaic effect1 and electroluminescent effect,2,3 respectively. The former is characterized by the electrode capture of bound charge carriers produced during light absorption, while the last is the opposite effect, that is, an injection of charge carriers that allows a recombination among them, followed by light emission. Recent studies have shown that the quantum yield of these processes is highly dependent not only on the electronic structure of the polymer, but also on its conformational structure,4 supramolecular5 effects like inter- and intrachain interactions, substituents presence, and mainly, structural defects.611 Defects have also been studied in the literature, attempting to elucidate how the material synthesis, manipulation, characterization, and device manufacturing6,7 can be crucial and, in most of the times, harmful to polymeric material yields through undesirable doping processes,11 electronic structure degradation, or alteration. However, some authors believe that some noncharged, wellcontrolled structural defects are able to, instead of what is expected, increase electrical and maybe optical properties of π-conjugated polymers.6 For instance, we can cite Liang et al.6 r 2011 American Chemical Society
who have shown that poly(3-hexyl-thiophene) (P3HT) films rich in structural defects induced by Me2SO4 and LiAlH4 present a smaller photobleaching than pristine P3HT films. In addition, these defects can increase charge carriers concentration and mobility. This can be attributed to the inducted deformations on the trigonal planar sp2 polymeric chain structure, a fact that can create electronic states inside the band gap that not always can hinder other electronic processes.11,12 Moreover, the insertion of some defects can relieve the chain from inevitable stress suffered by film formation in optoelectronic devices’ production. Among the methods used to create and study defects reported in literature, we can highlight ionic doping, photo- and thermodegradation, saturation, and isomerization.6 However, the use of γ radiation has been lately reported as an efficient way to induce structural and electronic modifications in conjugated macromolecules, such as carbon nanotubes,1317 quantum dotsconjugated polymers composites,18 porphyrins,19,20 and so on. Received: April 7, 2011 Revised: May 31, 2011 Published: June 21, 2011 8288
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The Journal of Physical Chemistry A The interaction of γ rays with polymeric materials has been widely studied, focused mainly on mechanical applications.13,16,21 However, such pacific use of ionizing radiation in potential materials for electronics, to intentionally tune their properties,22 is quite recent. The interaction mechanism is well-known by the literature and involves three basic processes: photoelectric effect, Compton scatter, and pair formation.23 For carbon materials interacting with γ radiation of energy lower than 2 MeV, the pair formation effect occurs with less than 2% frequency, even though elastic and inelastic scatter are the predominant processes and are responsible for24 ionization, bond break, hydrogenation, crosslinks, degradation, and so on. Bond break processes in a poly(2,5-thiophene-1,4-dialkoxyphenylene) derivative2531 were specially studied in this work, using 60Co as the gamma source, whose emission happens in two specific energy lines (avg. 1.25 MeV). In the dose range used in this work, the radiation induces basically only chain scissions and cross-links due to the ionization of the reactive atoms.24 Defects controllability can be guaranteed by the fact that, in this dose range and dose rate, the interaction of radiation with matter is approximately linear, as it can be deduced throughout this work. Such polythiophene derivatives, poly(3-alkylthiophene) (P3AT) and poly(thiophene-co-phenylene), for instance, are a representative class of conjugated polymers that form environmentally and thermally stable materials.32,33 These compounds exhibit a range of interesting features associated with the redox activity, a reversible transition between the doped and the neutral states, and this results in conductivity, electronic, and electrochemical properties.3436 Copolymers containing both phenylene and thiophene units have also proved to be of interest in combining the properties associated with the two different conjugated rings.31,37 The presence of di(ethyleneoxide) polar side-chains was shown to give good solubility, excellent adhesion properties, and promote ion solvation and mobility to phenylenevinylene copolymers.38 This approach has proved to be efficient in increasing emission quantum yield of such structures by confining unidimensionally charged carriers, avoiding its diffusion and, consequently, nonradiative decays. Ionizing radiation interaction, especially the gamma with conjugated polymers, was first reported in the early 80s,39,40 in an attempt to develop low-cost polymeric dosimeters with measurement patterns more easily controllable than the thermoluminescent inorganics used until then. The pioneers in this field are Burroughes et al., studying gamma interaction in poly (p-phenylene-vinylene) (PPV)4143 and poly(2-methoxy-5-[ethyl]hexyloxy-p-phenylene-vinylene (MEH-PPV)44 by Graeff et al., and UV/X-rays interaction by Bianchi et al.,45,46 aiming to develop neonatal therapy dosimeters. There are few examples using wittingly ionizing radiation as a way to alter conjugated polymeric material electronic properties for direct application in devices.22 The interaction mechanism in such material is yet not totally described and most authors suggest that they are caused and intermediated by photogenerated solvent radicals.44 In halide organic solvent solutions, such as carbon tetrachloride, dibromomethane, and diiodomethane, for instance, the MEH-PPV is severely damaged by halide radicals and suffers both a chain and a conjugation break, with hypsochromic shifts in optical absorption and emission spectra, which presents a strongly reduced intensity after irradiation. When in benzene solution, no change in electronic properties is observed, leading to the assumption that the radiationmatter interaction takes place only when
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Figure 1. Conjugated polymers used for experimental (1) and theoretical (2) analysis.
intermediated by the solvent. Bianchi et al. showed that harmful effects on MEH-PPV irradiated with UV45,46 are dependent on O2/N2 proportion and attributes the decrease of optical absorption and emission efficiency to the induced photodegradation by the O2 presence. In these studies, electronic modifications are only caused by conjugated length diminishing by the photoinduced defects and chain scission; however, how other modifications on structure can change electronic/vibrational structure has not been explored by literature. Such an approach is able to open up a range of new possible applications of organic semiconductors in electronic industry, developing more efficient devices beyond, solely, developing the dosimetry field. In the work reported here, an anomalous photoluminescence behavior for γ-irradiated samples of a poly(2,5-thiophene-1,4dialkoxyphenylene) (Figure 1) conjugated polymer derivative is observed and explained by an experimentaltheoretical approach. Optical absorption and photoluminescence for increasing doses of γ radiation in THF solutions were studied and semiempirical AM147,48 calculations were performed to determine the most reactive sites in the chain, as well as where the interaction is most likely to happen. Furthermore, ZINDO/SCI,4955 an electronic transitions prevision, was carried out for the increasing number of rings in the conjugated chain. Such data allow us to estimate how the defects are distributed and how they modulate the gap energy and multivibronic couplings, using the FranckCondon transitions within the Fermi’s Golden rule scope.56 It is also discussed how these defects can be intentionally used to develop more efficient organics-based opto-electronic devices.
2. METHODOLOGY 2.1. Experimental Section. The conjugated polymer 1 depicted in Figure 1 was synthesized by the organometallic palladium route.29 The other structure (2) in the same figure was also synthesized, but it was only used as a theoretical procedure to reduce computational costs because, for alkoxy compounds, the length of the lateral chain bonded to oxygen does not interfere in the electronic properties of the conjugated backbone, as reported elsewhere,57 and was confirmed by optical absorption (OA) measurements.58 But this length does interfere with the semicrystalline solid state structure given by the lamellar phases induced by the interdigitation of the lateral branch chains. By varying this chain, only the polymer with m = 2 and Z = butyl 8289
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was found to produce lamellar structures given by the butyl butyl interactions of neighboring backbones. Polymer 1 was diluted in tetrahydrofuran (THF) solution with no previous treatment by sonication for a couple of minutes until complete homogenization. A total of 3 mL of the 0.04 mg/mL solution was hermetically closed in a vial within a controlled atmosphere of argon to avoid oxygen presence. Then, the samples were irradiated by γ photons of 60Co (avg energy of 1.25 MeV) with a dose rate of 1.98 kGy 3 h1, containing a total dose of 20 and 40 kGy of total dose, in darkness and at room temperature. It was used a 60Co γ irradiator “Gammacell” model 220. Photoluminescence (PL) measurements for polymeric solutions were performed using a 2 mm path quartz cuvette and a CW Ar+ ion laser operating at 457 nm as excitation source. The emitted light was acquired by a USB Ocean Optics portable spectrophotometer positioned at the right angle configuration. Data were analyzed within the Fermi’s golden rule and FranckCondon approximation with multivibronic modes, so that the HuangRhys parameters could be determined. The multivibronic integrals were calculated via a Fortran 90 routine based on the formalism developed in ref 56. OA measurements were carried out in a Femto 800XI spectrophotometer using a D-W lamp for excitation. 2.2. Theoretical. To better understand the experimental data, a computational modeling procedure was performed. The optimization of the geometries of oligomers was carried out in MOPAC200959 software by AM147,48 semiempirical Hartree Fock parametrization, with a tight-convergence criteria (grandient norm lower than 0.05). The UVvis spectra were obtained by enveloping electronic transitions calculated within ZINDO/CI4955 parametrization in ArgusLab60 software by Lorentzians of 35 nm at FWHM. AM1 parametrization was chosen once it presented highly satisfactory results61,62 in geometry optimization of conjugated polymers, with the advantage of its low cost, especially when compared with other very time-consuming methods such as the Density Functional Theory (DFT). From the optimized geometries obtained, a foresight of the sites where the γ radiation might interact with the polymer chain was carried out, employing the formalism of Fukui’s Condensed Indices calculations.63 These indexes are able to measure the sensibility of the chemical potential of any atom to an external perturbation. They can be calculated for electrophilic, nucleophilic, and radical attack, giving, respectively, fk+
¼ ½qk ðN + 1Þ qk ðNÞ
fk+ ¼ ½qk ðNÞ qk ðN 1Þ fk0 ¼
fk+ + fk 2
ð1aÞ ð1bÞ ð1cÞ
The qk parameters are the Mulliken64 charges on the k-th atom for cation (N + 1), neutral (N), and anion (N 1) states, where N is the number of electrons. The charged states were calculated within the Restricted Open-Shell HartreeFock/AM1 theory level to avoid spin contamination if the unrestricted method (UHF) were used. For the UVvis spectra simulation, several chain lengths (from 2 to 16 conjugated rings) had their geometry optimized by the AM1 method. The task of optimizing structures with more
Figure 2. OA spectra for nonirradiated and (20 and 40 kGy) irradiated samples.
than 20 conjugated rings became very time and resource consuming due to the increase in computational processing time, which increases with ∼M3, where M is the number of atoms. From these simulations of the electronic transitions, for the increasing chain, it is possible to estimate how γ radiation induces chain scissions between adjacent conjugated rings of the backbone, as it can be deduced from photoluminescence measurements. It is well-known that theoretically simulated transitions do not totally agree with experimental ones, since they are calculated in a vacuum, at 0K and in a gas phase. Although, it is expected that the same error for one specific number of the conjugated rings will be the same for the increased.65 Based on this fact, if we have such a molecule with a well-defined length and if we calculate the theoretical UVvis spectrum and compare them, we can obtain the value for the disagreement between the theoretical and experimental spectra. Then, we can estimate the conjugation length of any molecule based on its UVvis experimental spectrum.
3. RESULTS 3.1. UVVis Optical Absorption. Figure 2 shows absorbance spectra for nonirradiated and 20 and 40 kGy irradiated samples. One can note a blue-shift of the maximum due to the diminish of conjugated length,66 responsible for most of visible optical absorption. It is also noted that the band associated to π f π* transition in thiophene ring (∼250 nm) is blue-shifted as well as the spectral enlargement of main band.67 Earlier studies for this class of polymer with variable lateral branch length showed that OA spectra from those, with shorter branches, are wider than those with larger lateral branches.68,69 This indicates that the polymer may have undergone changes in solubilizers lateral alkoxy branches, and this will be discussed ahead in this paper. Another factor that can also justify the main band enlargement is the conjugated segments heterogenic scission, inducing the creation of several conjugation lengths, each one absorbing in preferential range. Moreover, no polaronic band is observed, indicating no evidence of extrinsic doping process caused by the irradiation. 8290
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Figure 4. Gap behavior for increasing number of rings.
Table 2. Number of Conjugated Rings in Function of the Dose Figure 3. (a) Five-ring oligomer used to parametrize optical absorption behavior. (b) Theoreticalexperimental disagreement to five-ring structure.
Table 1. Energy Gap for Different Numbers of Chain Rings number of chain
theoretically estimated
experimentally measured
rings (p)
gap (eV)
gap (eV)
2
3.93
4
3.46
5
3.43
6
3.24
8 10
3.27 3.15
12
3.15
14
3.03
16
3.09
3.07
To estimate the degree of scission in polymeric chain induced by γ radiation, the oligomers geometries with crescent number of rings were optimized within the AM1 theory level, and all the starting structures are planar and sindiotatic. After optimization, the geometries present quasi-planar structures for the short-length (25 rings) oligomers26 and a helical chain torsion caused by a noncovalent S 3 3 3 O interaction between alkoxy branch and thiophene ring for long-length oligomers (>5 rings).70 These interactions induce high dihedral angles; however, they are not able to break conjugation with the increase in chain length. For most of the conjugated polymers, it is expected that simulated spectra for increasing number of rings could be described by a bathocromic shift due to higher electronic delocalization. To do so, the disagreement between theoretical and experimental electronic transitions must be determined, and this is obtained by comparing experimental and theoretical spectra (Figure 3b) for the fivering oligomer28,29 (3 alkoxy-phenylenes and 2 thiophenes), depicted in Figure 3a. This molecule was synthesized by the same
radiation
experimentally estimated
number of conjugated
dose (kGy)
gap (eV)
rings (p, avg)
0
2.72
66.61
20 40
2.77 3.04
21.07 5.32
group responsible for the synthesis of the molecules presented in Figure 1. Table 1 presents the most intense calculated electronic transitions possible to occur in oligomers with the increase in the number of conjugated rings. Figure 4 shows the energy gap behavior (taken as the most intense transition) and the hyperbolic fit, assuming that the “electron in a box” model is the most appropriate to describe the gap behavior for conjugation length. So, from the fit, the following can be obtained (Figure 4): ! Egap ðpÞ ¼
1:89 2:69 + p 0:06
( 0:10 eV
ð2Þ
where the Egap(p) = 2.69 agrees well with the experimental extrapolation to the infinite chain (p f ∞) and the correction factor 0.06 was kept for inhomogenity adjustment. The parameter 2.69 was obtained after fitting data from Table 1 and applying the equation to the structure depicted in Figure 3, determining, in this way, the theoreticalexperimental discrepancy. Using eq 2 for the irradiated polymers’ experimental gap value, it is possible to note that the number of conjugated rings in chain is strongly reduced with the increase of the dose, as shown in Table 2. Considering this, the irradiation of the solution definitively induces a conjugation break, as it was already reported in literature.44 At this point, the conjugation break was assumed to be induced by the breaking of the main chain, which is not always true, because it is not possible to reach such a conclusion solely based on the absorption spectra. However, the further analysis presented herein will strongly indicate the existence of the chain scission. 8291
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Table 3. Huang-Rhys Parameters and Data Obtained from Emission Spectra Fit radiation dose (kGy) v1 = 1100 cm1
0 S2 = 0.42
20 S2 = 0.42
40 S2 = 0.42
v2 = 700 cm1
S3 = 0.45
S3 = 0.35
S3 = 0.25
d (cm1)
370
390
390
λab (nm)
533
524
518
parameters Sj, such that " Gj ðtÞ
¼ exp
n
∑ Sjfðn̅ j + 1Þ expðitωjÞn̅ jðexpðitωjÞ 1Þg
#
j¼1
ð4Þ
Figure 5. Irradiated samples’ photoluminescence spectra.
3.2. Photoluminescence. Emission spectra measured in solution are narrow, with relative spectral resolution and present progressive shifts of the main peak of 6 nm to each 20 kGy of dose. This fact is highly attributed to conjugation scission, and possibly due to chain scission. Figure 5 presents the emission spectra for nonirradiated and 20 and 40 kGy irradiated samples. The emission spectra of the five-ring structure (Figure 3a) is also shown in Figure 5, evidencing the irradiated polymer tendency to decrease in chain length. Some aspects in Figure 5 should be highlighted: (1) in this dose range, the emission spectra shifting seems to be linear with the dose, indicating that probably ionizing radiation sensors for high doses may be developed, using such polymeric light-emitting films; (2) unlike the optical absorption spectra, the emission spectra are narrower and well-defined, with a smaller spectral line width; (3) the 0 f 1/0 f 0 transition intensity ratio decreases with the dose, probably indicating a smaller electronphonon coupling than expected: the smaller the conjugation length the greater the electronic wave function localization and the greater the coupling in vibronic modes. This fact can be explained by a Gaussian fit of spectra, from the vertical transition probability given by the Fermi’s Golden Rule scope applied to the FranckCondon vibronic structure. Thus, the emission coefficient for the ω-th photon frequency emitted by a vertical transition between b f a electronic levels can be found by Iab ðωÞ ¼
2am πω3 jμ~ab j2 3cp
Z
∞
∞
exp itðωab ωÞ
d2 t 2 2
!
Y
Gj ðtÞdt
j
ð3Þ where μBab is the electric dipole matrix element, am describes the medium effect, c is the speed of light, pωab = Eb Ea is the energy difference between the localized electronic states LUMO (Ea) and HOMO (Ea) of the copolymer segments with largest conjugation length, and d is the inhomogeneous spectral line for a Gaussian distribution, that is, the mean width of segments distribution, and the multivibronic coupling given by the FranckCondon integral Gj*(t) depends on the HuangRhys
where Sj is the HuangRhys factor for the j-th vibrational mode, n j is the thermal occupaωj is the vibrational mode energy, and B tion probability in BoseEinstein statistics. This adjustment of the spectra can provide information about electronphonon couplings given by HuangRhys parameter for the j-th vibronic mode. Table 3 presents these data obtained from emission spectra within this formalism, with the assignments of the active vibronic modes for this kind of polymer, as obtained in ref 71. Table 3 shows that the only change noted is the decrease of the electronphonon coupling with the 700 cm1 phonon, indicating a higher delocalization of the electronic wave function in the -O-CH2- and -O-CH371 group. It means that this group has new degrees of freedom, indicating a vibrational relaxation new path from the stress suffered by the chain due to π-conjugation. However, this relaxation path is noncompetitive with excited states that decay in radiative way, that is, these new degrees of freedom do not induce the creation of harmful phonons that are deleterious for quantum efficiency, which can be deduced by the unaltered spectral line shape of the irradiated polymers. Moreover, the ionizing radiationmatter interaction with the vibronic structure is very selective and does not induce random defects, which might lead to an increase of spectral line width. This selectiveness of the modes can be used for a precise control of polymer’s electronic properties, such as the narrow-band electrooptical emission. Furthermore, conjugated length diminishing and easy and precise gap modulation are intense sought-after procedures to choose the emission wavelength, which is not so well controlled in the material synthesis. By extrapolation, it is possible to note that with an approximate dose of about 90 kGy, under the same conditions, samples of unique electronphonon coupling, with only 1100 cm1 phonon, can be obtained. This is probably due to the nonalteration of correspondent structure associated with this phonon, which is responsible for the j-alkoxy stretch.72 To elucidate in which sites the interaction occurs and how it is responsible for the PL electronphonon behavior, Fukui’s Reactivity Indexes were calculated from optimized geometries for neutral, cation, and anion forms, considering the Mulliken charges in each case. When optimizing radical geometries, the use a restrict open-shell method (ROHF: Restrict Open-Shell HartreeFock) is recommended in order to avoid spin contamination by an unrestricted method (UHF: Unrestricted HartreeFock). Thus, due to the computational cost involved, we calculated these indexes for two different structures: (a) a monomer of structure 1, focusing on lateral branches reactivity, and (b) a trimer of structure 2, focusing on backbone reactivity. 8292
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4. DISCUSSION AND CONCLUSION In this work, a new methodology for understanding a defect inducing process, the gap energy modulation and vibronic couplings in a conjugated polymer by γ radiation was reported. It was noted that the semiempirical simulations approach can deduct how γ radiation interacts with the light-emitting polymer. Results indicate that electronic properties of conjugated matrices with such defects can probably provide devices with higher quantum yields, based on the multivibronic coupling. In addition, the gap energy can be modulated by a mere variation of the dose. According to the data presented in this work, γ irradiation is considered a fast and viable method to introduce defects in lightemitting conjugated polymeric matrices, due to their highly energetic photons. Moreover, in a special conjugated matrix such as poly(2,5-dialkoxy-p-phenylene-thienylene), γ photons induce the chain scission in the alkoxy branch and among conjugated rings, hindering some vibronic levels in a very specific way. Finally, the approach presented here proved to be an efficient method to understand, simulate, and improve the properties of light-emitting conjugated polymers. ’ AUTHOR INFORMATION Corresponding Author Figure 6. Graphical representation of the Fukui indices for each site for both structures (reduced trimer and expanded monomer). The darker the site indicates the more reactive the site is. Geometries do not correspond to the optimized ones and are forced to be planar for illustrative reasons.
This was needed because the presence of expanded lateral branches induces isoenergetic degrees of freedom, making the trimer structure with expanded branches highly time-consuming and unreliable calculations. Figure 6 shows graphically the absolute Fukui indexes f0, indicating that the most reactive interaction sites are the bonding carbon atoms of the thiophene ring, for the main backbone, and the second and third oxygen atoms in the dialkoxy group. This approach allows us to predict that the radiation induces effectively the main backbone scission in the thiophene ring (what corroborates to OA results), and the same occurs to the lateral branch at the second and third oxygen atom, which keeps intact the main conjugated backbone electronic structure, agreeing with UVvis optical absorption and photoluminescence results. In addition, the high reactivity of these sites, mainly in the trimer structure, strongly indicates the occurrence of a chain scission in these sites, which leads to a conjugation break. The highly energetic gamma photons may induce variations in charge density over these sites, what destabilizes the structure, turning the broken structure more stable than the bonded. Still, the lateral branch scission partially explains the enlargement of the UVvis spectra and the narrowing of PL spectra. These results can be used for the development of more efficient conjugated-polymer based devices, such as photovoltaic cells and light-emitting diodes. The absorption spectra overlapping in irradiated solutions with solar emission is greater than in nonirradiated ones, resulting in a larger amount of absorbed energy that can be efficiently used. Moreover, the emission spectra in OLEDs can be obtained for very-well-specified designed devices. Still, such defects can relieve the polymeric chain from the inevitable stress that it undergoes during film formation inducing,6 thus, bringing higher yields.
*E-mail: hugoss@fis.ufu.br; raigna@fafis.ufu.br.
’ ACKNOWLEDGMENT The authors would like to thank the Brazilian funding agencies: INCT/nanomateriais de carbono, INCT/INEO, INCT/INFO, CNPq, CAPES, FAPEMIG, and FAPESP, for the financial support and the CTR-IPEN Irradiation Laboratory staff. H.S.S. is deeply thankful to D. P. Andrade for the discussions. ’ REFERENCES (1) Coakley, K. M.; McGehee, M. D. Chem. Mater. 2004, 16, 4533. (2) Friend, R. H.; Gymer, R. W.; Holmes, A. B.; Burroughes, J. H.; Marks, R. N.; Taliani, C.; Bradley, D. D. C.; Santos, D. A. D.; Bredas, J. L.; Logdlund, M.; Salaneck, W. R. Nature 1999, 397, 121. (3) Bredas, J. L.; Beljonne, D.; Cornil, J.; dos Santos, D. A.; Shuai, Z. Philos. Trans. R. Soc., A 1997, 355, 735. (4) Salaneck, W. R.; Friend, R. H.; Bredas, J. L. Phys. Rep. 1999, 319, 231. (5) Brunsveld, L.; Folmer, B. J. B.; Meijer, E. W.; Sijbesma, R. P. Chem. Rev. 2001, 101, 4071. (6) Liang, Z.; Nardes, A.; Wang, D.; Berry, J. J.; Gregg, B. A. Chem. Mater. 2009, 21, 4914. (7) Yan, M.; Rothberg, L. J.; Papadimitrakopoulos, F.; Galvin, M. E.; Miller, T. M. Phys. Rev. Lett. 1994, 73, 744. (8) Wang, D.; M. R., Kopidakis, N.; Gregg, B.A. In 33rd IEEE Photovoltaic Specialists Conference; National Renewable Energy Laboratory: San Diego, CA, 2008. (9) Wong, K. F.; Skaf, M. S.; Yang, C.-Y.; Rossky, P. J.; Bagchi, B.; Hu, D.; Yu, J.; Barbara, P. F. J. Phys. Chem. B 2001, 105, 6103. (10) Yurtsever, E.; Yurtsever, M. Synth. Met. 1999, 101, 335. (11) Gregg, B. A. J. Phys. Chem. C 2009, 113, 5899. (12) Gregg, B. A. Soft Matter 2009, 5, 2985. (13) Lee, K.-Y.; Kim, K.-Y. Polym. Degrad. Stab. 2008, 93, 1290. (14) Crespi, V. H.; Chopra, N. G.; Cohen, M. L.; Zettl, A.; Louie, S. G. Phys. Rev. B 1996, 54, 5927. (15) Smith, B. W.; Luzzi, D. E. J. Appl. Phys. 2001, 90, 3509. (16) Chen, S.; Wu, G.; Liu, Y.; Long, D. Macromolecules 2005, 39, 330. 8293
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