Effect of alkoxy side chains on intra and interchain exciton coupling in PPE-PPV copolymers solution

Synthetic Metals 224 (2017) 72–79

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Effect of alkoxy side chains on intra and interchain exciton coupling in PPE-PPV copolymers solution M. Guesmia,d,* , A. Ben Fredja , S. Romdhanea,b , N. Bouguerrac,d, D.A.M. Egbed,e , R.W. Lange , M. Havlicekd , N.S. Sariciftcid, H. Bouchrihaa a Laboratoire Matériaux Avancés et Phénomènes Quantiques, Faculté des Sciences de Tunis, Université Tunis El Manar, 2092 Campus Universitaire, Tunis, Tunisia b Faculté des Sciences de Bizerte, Université de Carthage, Zarzouna, 7021 Bizerte, Tunisia c Department of Chemical Engineering, Electrochemistry, Corrosion and Energetic Valorization Laboratory, A. MIRA University, Targa Ouzemmour, 06000 Bejaia, Algeria d Linz Institute for Organic Solar Cells (LIOS), Johannes Kepler University Linz, Altenbergerstr. 69, 4040 Linz, Austria e Institute of Polymeric Materials and Testing (IPMT), Johannes Kepler University Linz, Altenbergerstr. 69, 4040 Linz, Austria

A R T I C L E I N F O

Article history: Received 27 October 2016 Received in revised form 9 December 2016 Accepted 24 December 2016 Available online xxx Keywords: Conjugated polymer Side chains Photoluminescence Timeresolved photoluminescence Aggregates

A B S T R A C T

The effects of alkoxy side chains on the optical and morphological properties of five poly (p-phenyleneethylene)-alt-poly (p-phenylene-vinylene)s (PPE-PPVs) solutions are investigated. By steady-state and time-resolved photoluminescence spectroscopy combined with Franck-Condon analyses, we show that the introduction of symmetrical side chains onto the PPE-PPV backbone increases the p–p stacking between the polymer chains leading to interchain interactions hence H-aggregate domains. Total dissymmetrical side chains lead to J-aggregate domains through intrachain interactions. On the other hand partial dissymmetrical side chain configurations lead to H and HJ-aggregate domains. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Conjugated polymers have emerged as promising materials in optoelectronic device applications such as organic photovoltaic cells (OPVs) [1] because of their photophysical and electronic properties in combination with their flexibility and solution processability [2–6]. In the production of these devices, these materials are cast from a highly concentrated solution into a thin film [7]. In solution, the conjugated macromolecules tend to be rather flexible due to the torsional motion of their monomer units [8]. They can adopt then different arrangements and packing leading to a diverse range of microstructure and morphology such as aggregates. Aggregates exhibit two classes of intermolecular interactions; inter and intrachain interactions leading to H and J-aggregate

* Corresponding author at: Laboratoire Matériaux Avancés et Phénomènes Quantiques, Faculté des Sciences de Tunis, Université Tunis El Manar, 2092 Campus Universitaire, Tunis, Tunisia. E-mail address: [email protected] (M. Guesmi). http://dx.doi.org/10.1016/j.synthmet.2016.12.026 0379-6779/© 2016 Elsevier B.V. All rights reserved.

types, respectively. These two types of fundamental electronic interactions are distinguished by the intensity difference between the 0-0 electronic transition and the 0-1 -vibronic one. When the interchain interaction is dominant, the 0-0 electronic transition is forbidden by symmetry and less intense than the 0-1 transition. In contrast, when the intrachain interaction is dominant, the 0-0 transition is allowed and more intense than the 0-1- [9–11]. This classification is a key development in understanding the relationship between morphology and photophysical properties [12]. Because the degree of aggregation in solution can be preserved through casting process and affect the morphology of the thin film, studies of aggregates in solution are directly relevant to understanding thin film characteristics [7,13]. Aggregation in solution is greatly dependent on the concentration, the aromaticity of solvent and the temperature. Panzer et al. have shown, through the temperature-dependent photoluminescence measurements in poly (3-hexythiophene) (P3HT) solution, the presence at 5 K of two H- aggregates with planar polymer backbones yet different degree of order regarding their side chains [14]. The effect of solvent on the formation of aggregation has been

M. Guesmi et al. / Synthetic Metals 224 (2017) 72–79

studied by Guo et al. in poly(4,8-bis alkyloxybenzo[1,2-b:4,5-b’])dithiophene-2,6-diyl-alt-(alkylthieno[3,4-b]thiophene2carboxylate])-2,6-diyl (PBDTTT) solution. They compared the PBDTTT/ chlorobenzene to the PBDTTT/chloroform solutions and they concluded that the easier aggregation of polymer chains is observed in aromatic solvent. In fact, the open chain conformation adopted in aromatic solvent leads to increased overlapping of the p- orbitals of adjacent chains. Furthermore, they concluded that the highly concentrated solution exhibits more aggregation [15]. On the other hand, the nature, the position and the size of the side chains attached to the conjugated polymer backbone can affect the intermolecular packing then the aggregation. Shin et al. have studied this effect in two polymers with an asymmetric alkoxythienyl side chains and a symmetric alkoxyphenyl side chains. They found that the introduction of the symmetric side chains to the conjugated backbone increases the intermolecular packing between chain, which improves both the light-harvesting of the polymer and its charge carrier mobility [16]. Moreover, Tinti et al. have shown, through studying the charge carrier mobility in a series of anthracene-containing poly (p-phenylene-ethylene)-altpoly (p-phenylene-vinylene)s (AnE-PVs) differing by the alkoxy side chains, that the polymer with a long linear dodecyl side chains has a detrimental effect on mobility due to a less compact molecular packing between chains [17]. Recently Chakraborty et al. have shown, using time-resolved measurements, that the long lived emission in poly (2-methoxy-5(2-ethylhexyloxy)-1, 4-phenylenevinylene) (MEH-PPV), is from Haggregates [18]. In the present work, we combine steady-state and timeresolved optical spectroscopy in order to study the intra- and interchain interactions in a series of five PPE-PPVs concentrated solutions with an aromatic solvent chlorobenzene. The PPE-PPV polymers named P18/8, P8/18, P18/18-8, P 8/18-8 and P18-8/18-8, whose chemical structures are given in Fig. 1, are an interesting class of materials [6,19,20]. They combine the properties of poly (pphenylene-ethylene) (PPE) and poly (p-phenylene-vinylene) (PPV) in the same conjugated backbone, but they differ by the position and nature of grafted alkoxy side chains. Due to the importance of the molecular vibration in the conjugated polymers, which plays a crucial role in exciton migration [21], FC analysis is usually involved in modelling the PL spectra. We use a single intrachain (or a modified) FC progression, to reproduce the experimental PL spectra. In what follows, we find that the aggregate type is related to the nature and the position of the side chains. Finally, from the timeresolved PL measurements, we have determined the PL lifetimes for the different substituted PPE-PPVs, in order to confirm the influence of the side chains on the aggregate type.

Fig. 1. Molecular structure of the PPE-PPV copolymers [6].

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2. Experiments The material was synthesized as described elsewhere [6]. In spectroscopic studies, we have used a chlorobenzene solution prepared at 10 mg/1.5 ml. PL spectra were recorded with a PTI Quanta Master 400 Spectrofluorimeter with a continuous Xenon arc lamp (75 W) light source in the emission range 185–680 nm. Time resolved PL measurements were performed using timecorrelated single photon counting (TCSPC) based on picoseconds diode laser PDL 800-B with 404 nm laser head running at 40 MHz repetition rate. The driver box PDL is physically separate from the actual laser head, which is attached via flexible lead. The laser head is also fixed at the FluoTime 100. The light is directed at the sample, and then filtered out from scattered excitation light by means of an optical cut off filter. After, it is detected and amplified by photomultiplier tubes. All of those optical instruments are localized in the FluoTime 100. The electrical signal obtained from the detector is led to a pre-amplifier, then to the TCSPC electronics, called Time Harp 200, via a standard 50 V coax cable. The TCSPC

Fig. 2. (a) PL spectra from P18/8, P8/18, P18/18-8, P8/18-8 and P18-8/18-8 solutions recorded at room temperature and obtained using excitation energy equal to 3.06 eV. (b) Normalized PL spectra.

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electronics are contained on a single PC board. The laser driver also provides the electric sync signal needed for the photon arrival time measurement. This signal is fed to the TCSPC electronics. 3. Steady state PL: results and discussion Fig. 2(a) displays the PL spectra of the different PPE-PPVs solutions: P18/8, P8/18, P18/18-8, P8/18-8, and P18-8/18-8, recorded at room temperature. All PPE-PPVs PL spectra exhibit two bands. The first one, near 2.5 eV, originated from the 0-0 pure electronic transition. The band intensity of this peak looks higher only for P18-8/18-8. The second one, near 2.3 eV, related to 0-1 vibronic transition. However, this band looks broader on the side of low energies, which suggests the presence of other bands. 3.1. Franck-Condon analysis For an appropriate analysis of the PL spectra in Fig. 2, Franck Condon fits were carried out by modeling these spectra, as a sum of FC transitions based on several intramolecular vibrational modes [22]. The intensity I0!ni of the vibronic transitions 0 ! ni for the i mode is given by [23,24]: I0!ni að
hvÞ3 n3f Sni i

eSi ni !

ð1Þ

where nf is the real part of the refractive index at photon energy hv, ni is the vibrational level and Si is the Huang-Rhys (HR) factor
which corresponds to the average number of phonons involved in the emission process. This factor is proportional to the squared normal coordinate displacement DQ 2i between the ground and the excited states [24,25]:
1 Mi v2i Si ¼ DQ i 2 2
hvi

ð2Þ

where Mi is the reduced ionic mass for the i mode. Moreover, the HR factor is related to the relaxation energy Erel which gives a measure of the strength of the electron phonon coupling [24,26]: Erel ¼ Si Si
hvi

ð3Þ

3.2. Intrachain exciton coupling The PL spectrum of the P18-8/18-8 red curve in Fig. 2 has a vibronic structure, where the intensities of peaks decrease on going from the highest energy band down the lowest energy band. 00

The ratio 0-0 to 0-1 peak intensity is larger than 1 (II01  1:21), this suggests that the luminescence originates from an intrachain singlet exciton [27]. Recently, Spano et al. have shown, through studying the H and Jaggregate behavior in conjugated polymers, that the PL ratio

I00 I01

responds to the exciton coherence number due to the competition between the interchain and intrachain exciton coupling. This ratio is larger (less) than 1 for the intrachain (interchain) excitonic coupling [28–30]. In this sense, the intrachain exciton coupling is the most predominant in the P18-8/18-8, thus the polymer chains self-organize in the solution to form J-aggregate domains. In fact, in J-aggregates the 0-0 emission is strongly allowed, leading to a supperadiance. This is due to the total dissymmetrical side chain position along the backbone polymer. From these results, the PL spectra can be modeled as a single intrachain FC progression coupled to a single phonon mode, which

Fig. 3. PL spectrum of a P18-8/18-8 solution (black dashed line) and its best FranckCondon fit (red solid line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

can be expressed by the following formula [22,24]: Ið
hvÞað
hvÞ3 eSi S

Sni i

ni ni !

ð
hv  E0 þ ni h
vi Þ

ð4Þ

hvi is the energy of the vibrational mode and E0 is the 0-0 where
transition energy. G is the Gaussian Line shape function with constant width s , which represents the energetic disorder in the material. Fig. 3 shows the best fitting with the experimental data. The involved hvi ¼ 0:153eV assigned to the alkoxy C  OR stretching mode is
mode. This phonon mode has recently been detected by infrared (IR) and Raman spectroscopy [6]. From this analysis, we have also obtained the PL 0-0 transition energy ðE0 Þ, the relaxation energy ðErel Þ, the HR factor (S) and the energetic disorder (s ). The derived fitting parameters are summarized in Table 1. 3.3. Interchain exciton coupling PL spectra from P18/8, P8/18, P18/18-8 and P8/18-8 solutions, are structureless as compared to the P18-8/18-8 one (Fig. 2). This can be due to a close interchain packing and an efficient p overlap, which induces an emission from an interchain excitation [27].

Table 1 The parameters determined from FC analysis of the PL spectra to Eqs. (4) and (5); the scaling factorðaÞ, the Huang Rhys factors (S), the relaxation energy ðErel Þ, the PL 0-0 transition energy ðE0 Þ and the energetic disorder ðs Þ. Sample

Aggregation type

a

S

Erel ðmeV Þ

E0 ðeV Þ

s ðmeV Þ

P18-8/18-8 P18/8 P8/18 P18/18-8 P8/18-8

J H H H H J

– 0.010 0.018 0.036 – –

0.79 0.82 0.77 0.87 1.1 0.18

120.87 125.46 117.81 131.58 168.30 27.54

2.522 2.504 2.512 2.510 2.50 2.365

62 67 70 74 67 67

M. Guesmi et al. / Synthetic Metals 224 (2017) 72–79

00

Fig. 4. The PL ratio I01 features of the S1 ! S0 transition for all PPE-PPVs I polymers (P18/8, P8/18, P18/18-8, P8/18-8, P18-8/18-8).

Moreover, we cannot fit the PL spectra using a single intrachain FC progression since

I00 I01

< 1, as we can see in Fig. 4.

This suggests the presence of domains wherein the polymers p-stack as H–aggregates. The PL spectra can then be modeled, using a modified FC progression, taking into account the decrease of the 0-0 peak intensity, according to the following expression [9,14,31,32]: Sni i ð
hv  ðE0  ni
hvi ÞÞ ni ¼1 ni !

Ið
hvÞ ¼ ð
hvÞ3 n3f eSi ½að
hv  E0 Þ þ S

ð5Þ

where the scaling factor a is a function of the disorder. This parameter quantifies the relative intensity of the 0-0 transition which is decoupled from the rest of the FC progression. For a

Fig. 5. PL spectra of a P18/8, P8/18, P18/18-8 and P8/18-8 (black, magenta, green and blue dashed lines, respectively) solutions and their best modified Franck-Condon fit (red solid lines). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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perfectly ordered H-aggregate, a would be zero hence the electronic transitions between the ground state and the lowest energy level of the vibronic excited state are symmetry forbidden. However, the structural and thermal disorder lead to relaxation of the selection rule making the 0-0 transition more allowed [33]. Fit to Eq. (5), using the alkoxy C  OR stretching mode with phonon hvi ¼ 0:153eV, are shown in Fig. 5 and the derived energy
parameters are displayed in Table 1. Moreover, the PL spectra from P18/8, P8/18 and P18/18-8 polymers exhibit a slight redshift as compared to that of P18-8/188 (see Fig. 2). This redshift confirms our assumption concerning the emission from an interchain singlet exciton, like in thin films as compared to solutions. From Table 1 we can see that when the scaling factor a increases, the energetic disorders increases, which means that the P18/18-8 polymer is the most disordered one. The disorder breaks the symmetry, making the 0-0 transition more allowed. Many parameters affect the order such as the temperature and the molecular weight [9,31]. In our case, the disorder depends on the position and nature of the grafted alkoxy side chain along the polymer backbone. In fact, the presence of three long alkoxy side chains along the P18/18-8 polymer backbone (partial dissymmetrical side chains) favours the molecular backbone torsion, which increases the energetic disorder. For the two polymers bearing symmetric alkoxy side chains, two long alkoxy side chains are grafted on the PPV section of the P8/18 polymer, while for the P18/8 they are grafted on the PPE. The presence of the C¼C bond in the PPV favors the p overlapping hence H-aggregates formation [34]. Due to enhanced torsional motion of P8/18 backbone, this polymer is more disordered than the P18/8 (aðP8=18Þ > aðP18=8Þ). Taking this into account, we conclude that the disorder depends on the side chain nature and position, and that the P18/8 polymer presents highly ordered Haggregate domains. 3.4. Inter and intrachain exciton coupling We have used a modified FC progression, according to Eq. (5), to fit the PL spectrum from the P8/18-8 polymer (Fig. 5, blue dashed line), but there is no good agreement between experimental and theoretical data. This suggests the contribution of more than one emissive state. Panzer et al. propose two distinct interchain emissive states in the poly (3-hexylthiophene) solution [14] to model the PL spectra. Baghgar et al. propose an interchain and intrachain emissive states to model the PL spectra from poly (3hexylthiophene) nanofiber [35]. We have used the superposition of two distinct FC progressions, corresponding to emission from two different H-type aggregate species labeled H1 and H2, to fit the P8/ 18-8 PL spectrum (Fig. 6(a)) but there is no improvement of the fitting. As an alternative approach we have employed two distinct FC progressions, corresponding to emission from H and J-type aggregate species. Fig. 6(b) shows a comparison of experimental P8/18-8 PL spectrum (black dashed line) with a theoretical spectrum composed of H and J-type aggregate species, with the same vibrational mode and 0-0 origins at 2.5 and 2.365 eV, respectively (solid red line). An improvement of the fit has been obtained; the derived parameters are summarized in Table 1. The results reveal that, for the P8/18-8 polymer, emission arises from two different energy states related to H and J-type aggregates and separated by  135 meV. In addition, the HR factor corresponding to the interchain interaction is higher than the one related to intrachain interaction (see Table 1). This indicates that the emission from H-type aggregate domains prevails and explains 00

the value of the PL ratio (II01  0:35). Due to the presence of only

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Fig. 7. PL decays of different PPE-PPVs solutions measured at room temperature and obtained at emission energy 2.4 eV (515 nm), using 3.06 eV excitation energy. The PL decays are normalized to their maximum value.

exponential fit is given by:   t IPL ðtÞ ¼ Aexp 

t

ð6Þ

where IPL is the PL intensity, A and t are a fitting parameters. A represents the fraction of molecules decaying and t the PL lifetime. Fig. 6. Experimental PL spectrum of P8/18-8 solution (black dashed line) and theoretical spectra (red solid line). (a) Fit using sum of two H-type vibronic progressions (H1: blue dashed line and H2: magenta dashed line) separated by  115 meV. (b) Fit using sum of H and J-type progression, with 0-0 origins at 2.5 eV (blue dashed line) and 2.365 eV (green dashed line), respectively, and separated by  135 meV. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

one long alkoxy side chain along the P8/18-8 polymer backbone (partial dissymmetrical side chains), the molecular torsion is reduced and the p–p overlap is enhanced. 4. Time-resolved PL To obtain a better understanding about how the nature and position of the side chains affect the excitonic characterin the excited states of aggregates, we measured the PL decay profiles of the different PPE-PPVs solutions, at room temperature, with an excitation energy of 3.06 eV (404 nm). Fig. 7 shows the PL time profiles normalized on a logarithmic scale and measured at 2.4 eV emission energy (515 nm emission wavelength). From this figure we remark that the PL from the P18-8/18-8 is the shorter lived than the other polymers. The linear response of the slopes indicates that all of the signals decay exponentially with a single lifetime. Then, the mono

4.1. PL decays of H- aggregates Time-resolved PL spectra of P18/8, P8/18 and P18/18-8 (Haggregate) are shown in Fig. 8 with their fitting based on a single exponential function. The long lived emission from these polymers comes from the formally forbidden 0-0 bands in the H-aggregate types [18]. From Fig. 8, we can see that the emission from the P18/18-8 polymer decay slightly fast (1.412 ns) compared to the others (1.450 and 1.539 ns). This effect comes from the strong nonradiative recombination process in the relaxation of its excited states. Vibrational and torsional energy relaxations are known to non-radiatively deactivate the excited species [35]. In the case of the single-exponential PL decay, the lifetime is determined by radiative and non-radiative photophysical processes and can be expressed with the following expression [25,35,36]:

t¼

1 kr þ knr

ð7Þ

where kr and knr are the radiative and the non-radiative decay rate constants. As mentioned previously, the P18/18-8 polymer is the most disordered one (indicated by the highest values of the scaling factor a and the relaxation energy Erel ) as compared to P18/8, P8/ 18. Consequently, the excited states of molecules are easily annihilated and therefore the non-radiative decay rate increase due to torsional and vibrational motion. While, for the P18/8, due

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Fig. 9. Time- resolved PL spectra of the P8/18-8 (a) and P18-8/18-8 (b) solutions (black squares lines) obtained at emission energy 2.4 eV with excitation energy at 3.06 eV. The red solid lines are the fitting curves based on single exponential functions. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 8. Time- resolved PL spectra of the P18/8 (a), P8/18 (b) and P18/18-8 (c) solutions (black squares lines) obtained at emission energy 2.4 eV with excitation energy at 3.06 eV. The red solid lines are the fitting curves based on single exponential functions. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

to lower values of a and Erel , torsional and vibrational motion are reduced, as a result the radiative recombination is the dominant decay channel which is confirmed by the highest PL intensity (black curve, Fig. 2(a)). Our observations can therefore be regarded as an increase of knr (decrease of the PL lifetime) concomitant with an increase of disorder in the H-aggregate type.

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deactivation channel dominates the excited state relaxation process of the J-aggregate type hence a short emission lifetime. In organic photovoltaic devices, the active layer is generally a thin film spin cast from highly concentrated solutions. The ordered H-aggregate domains are transferred to the thin film, wherein they promote a planar chain which induces a high p-p stacking, improving then the performance of these devices. Acknowledgements M. Guesmi and N. Bouguerra are grateful to ANSOLE e.V. for financial support through the ANSOLE ANEX programme. N. Bouguerra also acknowledges the financial support from the University of Bejaia. N. Bouguerra and D. A.M. Egbe thank FWF for research funding through grant No: I 1703-N20. D. A. M. Egbe also acknowledges funding from the SOLPOL project (www.solpol.at). References Fig. 10. PL lifetimes of the different PPE-PPVs solutions obtained using 3.06 eV excitation energy, at emission energy is ranging between 1.78 and 2.4 eV.

4.2. PL decays of HJ and J-aggregates The reduced values of the PL lifetime in the P8/18-8 (HJaggregate) and P18-8/18-8 (J-aggregate) 1.325 ns and 1.073 ns, respectively (see Fig. 9), as compared to those of P18/8 and P8/18 and P18/18-8 (H-aggregate), come from the presence of intrachain interactions in the P18-8/18-8 and competition between inter- and intrachain interactions in the P8/18-8. The presence of intrachain interactions in the P8/18-8 (HJaggregate) in competition with the interchain interaction breaks the symmetry, hence making the 0-0 transition allowed. Furthermore, the highest value of the relaxation energy is attributed to this polymer, which suggests that the major part of the excitation energy is consumed in vibration through electron-phonon coupling (see Table 1). This result confirms that the lowest excited state lifetime, as compared to polymers with H-aggregate type, is due to non-radiative process mediated by vibrational/torsional motions. For the P18-8/18-8 (J-aggregate), this polymer rules out the interchain interaction. The 0-0 emission is strongly allowed; hence the decay time will be very fast. Fig. 10 shows the PL life times of all PPE-PPVs (P18/8, P8/18, P18/ 18-8, P8/18-8, P18-8/18-8) solutions with excitation energy at 3.06 eV and emission ranging between 1.78 and 2.4 eV. In this figure, it manifests that the PL lifetimes decrease with the increase of the emission energy. The mentioned decrease is attributed to an increase of knr , mediated by vibrational and torsional motion of the polymer backbone. 5. Conclusion An investigation of aggregation in a series of five PPE-PPV copolymers solutions with symmetrical and dissymmetrical (total or partial) configurations of alkoxy side chains is presented. We have shown that the configuration of these side chains attached to the polymer backbone has a great impact on the formation of H, J or HJ aggregate domains in solutions. In fact, symmetrical and total dissymmetrical configurations lead to H and J-aggregate domains, respectively. While partial dissymmetrical configurations, give rise to H or HJ-aggregate domains. The time-resolved photoluminescence reveals that the long lived emission is from H-aggregates, while the non-radiative

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