Optical and Electro-Optical Properties of Silicon ... - Springer Link

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2013, Vol. 86, No. 5, pp. 747−755. © Pleiades Publishing, Ltd., 2013. Original Russian Text © N.P. Yevlampieva, A.P. Khurchak, Yu.N. Luponosov, E.A. Kleimyuk, S.A. Ponomarenko, E.I. Ryumtsev, 2013, published in Zhurnal Prikladnoi Khimii, 2013, Vol. 86, No. 5, pp. 800−808.

MACROMOLECULAR COMPOUNDS AND POLYMERIC MATERIALS

Optical and Electro-Optical Properties of Silicon-Contaning Thiophene Derivatives of Star-Shaped and Dendritic Structure N. P. Yevlampievaa, A. P. Khurchaka, Yu. N. Luponosovb, E. A. Kleimyukb, S. A. Ponomarenkob, and E. I. Ryumtseva bInstitute

aSt.

Petersburg State University, St. Petersburg, Russia of Synthetic Polymeric Materials, Russian Academy of Sciences, Moscow, Russia e-mail: [email protected] Received April 8, 2013

Abstract—Star-thiophene derivatives with a silicon atom as the branching center were investigated by absorption spectroscopy and electro-optical Kerr effect in solutions at variations in a number and chemical structure of branches. The star-shaped oligomers were compared with dendritic analogues containing silicon atoms at the points of branching. It is shown that thiophene-containing moieties determine both spectral and electrooptical properties of the molecules. Molecular parameters of the star-shaped oligomers of various structure vary identically with increasing the number of branches. The absorption of star-shaped oligomers is additive due to the autonomy of the absorption of radiation by the separate branches. For dendritic molecules the additive nature of the absorption is kept, but their electro-optical properties are independent of a generation number. It was shown that the latter is a consequence of the manifestation by dendrimers of deformation flexibility, which is not peculiar to the starshaped derivatives. DOI: 10.1134/S1070427213050224

it may be noted: (a) synthesis of polythiophenes of starshaped [9, 10], dendritic [11, 12], and branched architectures [13, 14]; (b) the use of oligomers self-organizing in solution [15–17]; (c) synthesis of hybrid macromolecules with inclusions of metal atoms [18]. The study is related to above trends and aimed at examination of regularly branched thiophene oligomeric derivatives with covalently coupled silicon atoms and of dendrimers on their basis. Electronic properties of polymers containing chromophores (including thiophene) depend on a way of production, chemical structure and spatial organization, so the definition of the relation between structure and properties is an important step in the study of new derivatives. A task of this research was to study the optical absorption and electro-optical molecular properties of the star-shaped oligomers of thiophene with silicon atom as the center of branching depending on the chemical structure of a branch and the number of branches; and

Thiophene derivatives are among the most sought-after compounds in organic electronics [1]. Polythiophenes of linear structure are applied to photovoltaics [2–5], in the manufacture of transistors and compact batteries, used as light and photosensitive materials in lithography, as fluorescent and conductive materials in the manufacture of OLEDs [1]. A disadvantage of many polymers with conjugated bonds used in organic electronics, including linear polythiophenes, is a low degree of crystallinity, which adversely affects the film parameters such as conductivity and the energy conversion factor [6, 7]. Due to the relevance and practical significance of improving the characteristics of materials based on polythiophenes, the search for and study of new derivatives of thiophene, capable, for example, of self-organization in solutions continues. Constructing macromolecules of the dendritic topology, which is proven in self-assembly technologies in the formation of an ordered structure films [8], is one of the possible alternatives. As a general trends in recent years in the development of new thiophene derivatives 747

748

YEVLAMPIEVA et al.

Fig. 1. The chemical structure of the star-shaped oligomeric derivatives of bithiophene (R1), terthiophene (R2), and phenylene-thiophene (R3).

in the case of dendrimers, depending on the generation number. In the first part of the report we demonstrated results of a study of three types of the star-shaped oligomers of thiophene distinguished by structure of the moiety Ri (Fig. 1): bithiophenes BT-1, BT-2, BT-3; terthiophenes TT-1, TT-2, TT-3; phenylene-thiophenes PT-1, PT-2, PT-3, respectively. The second part shows the results of the study of three generations of dendrimers based on bithiophene (Fig. 2). In this paper we used the following experimental methods: electro-optical Kerr effect [19] and absorption spectrophotometry in the visible and near UV range, as well as the quantum-chemical semiempirical PM3 S

C6H13 S HB

simulation [20]. EXPERIMENTAL Ways of synthesis of the thiophene derivatives with silicon atom as the center of branching were developed at the N.S.Enikolopova Institute of Synthetic Polymeric Materials, Russian Academy of Sciences. Oligomers and dendrimers under study were synthesized according to techniques published in [21] for BT-1-3, [12] for D-13), and [22] for TT-1-3 and PT-1-3. Two model samples: 5-hexyl-2,2'-bithiophene (HB), and silicon-containing 5-hexyl-5'-trimethylsilyl-2,2'-bithiophene (HTMSB) were synthesized according to the technique described in [21]. (CH3)3Si

S

HТМSB

The HB sample may be assumed as a model chromophore for oligomeric samples BT-1–3 and dendrimers D-1–3. Experimental studies both spectral and electrooptical were conducted in the same solvent toluene. Freshly distilled toluene (analytical grade, manufactured by Vekton, St. Petersburg, Russia) was used with the following characteristics at 294 K: density of 0.8669 g cm–1, refractive index 1.4969, dielectric constant 2.379. The absorption spectra of compounds (excluding a solvent spectrum) were recorded on an automated spectrophotometer SF-2000, governed by the software SF-Scan (OKB “Spectrum”, St. Petersburg, Russia). The spectral range of the SF-2000 is 190–1100 nm, resolution 1 nm. Measurements were carried out in quartz cell (1 cm) at a temperature of 294 K. The molar extinction coefficient ε

C6H13 S

is determined from the slope of the concentration dependence of the optical density of the solutions corresponding to the by to the absorption spectrum maximum of the test compound. The error of ε determining did not exceed 10%. Equilibrium, i.e. independent of the frequency and duration of electric pulses, electro-optical Kerr effect was measured in a pulsed electric field with 1 ms duration of a rectangular pulse. The compensation method have used for registering birefringence Δn, which occurs in solution under the influence of an electric field [19]. The effect registration was conducted by photoelectric technique. The light source was a solid-state laser module (wavelength 650 nm, power 5 mW). A quartz flow cell of 2 cm long with soldered semi-cylindrical titanium electrodes was used. The gap between the electrodes was 0.05 cm.

RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 86 No. 5 2013

OPTICAL AND ELECTRO-OPTICAL PROPERTIES

The electro-optic properties of the samples were characterized by a specific Kerr constant, which was determined according to the following relationship K= limc→0[Δn – (Δn)0]/E2c, E→0

(1)

where Δn – (Δn)0 is a difference between the birefringence values of the solution at concentration c and the solvent, E is an electric field intensity, с is the solution concentration, g cm–3. Quantum-chemical simulation was carried out in the framework of HyperChem software [23]. The simulation was used to calculate the dipole moment value and tensor of optical polarizability of optimized molecules of oligomers and dendrimers. The value of anisotropy of optical polarizability was calculated as Δb = {[(b1 – b2)2 + (b2 – b3)2 + (b3 – b1)2]/2}1/2, where b1, b2, b3 are principal values of optical polarizability tensor. An effect of silicon atom as a center of branching of the star-shaped oligomeric thiophene derivatives on optical and electro-optical properties of the compounds in the solutions were discussed at successively increasing the degree of substitution of the central atom by Ri and varying the branch structure (Fig. 1), and then the same molecular properties of the dendritic topology were studied (Fig. 2). The data obtained were used to analyze changes in the electronic properties of thiophene derivatives of different topologies at purposive variation of their molecular structure. STAR-SHAPED OLIGOMERS Thiophene and its derivatives as a rule have a characteristic absorption band in the near-UV or the adjacent visible range of wavelengths with a well-defined maximum λmax of spectrum (Fig. 3). By analyzing the absorption spectra we can predict the change in the electronic properties of thiophene derivatives of nearby structures. The absorption spectra of model compound HTMSB and oligomers of bithiophene BT-1-3 show a slight shift in λmax to longer wavelengths compared to the HB model compound thereby indicating an effect of d-electrons of silicon on π-electron system of the nearest to silicon chromophore (Table 1 ). The value of the shift Δλmax is maximal in transition from linear HB to two branch BT-1, where silicon is the molecular symmetry center. Further increase in bithiophene branches covalently coupled

749

Table 1. Optic spectral properties of the star-shaped oligomers and mjdel samples in toluene Sample

Δλmaxa

λmax nm

ε

ε/nb

L mol–1 cm–1

Δe, eV

HB HTМSB

312 ± 1 327

– 15

19400 20000

19400 20000

– –

BТ-1 BТ-2 BТ-3

328 332 334

16c 4 2

40000 59500 78000

20000 19800 19500

3.33 3.27 3.21

ТТ-1 ТТ-2 ТТ-3

374 378 380

4 2

–

62000 95100 124000

31000 31700 31000

2.91 2.82 2.78

PТ-1 PТ-2 PТ-3

307 309 313

2 4

–

48400 72600 96400

24200 24200 24100

3.16 3.11 3.05

Shift of maximum of absorption spectrum relative to the previous sample from the series. b n is the number of chromophores in molecule. c Shift relative to position of maximum of spectrum of the model sample HB. a

to the central silicon atom in the compounds BT-2-3, as seen from Table 1, leads to a shift of the absorption maximum in the same direction but less in its value. The relatively small value of the shift of Δλmax towards longer wavelengths, and the dependence Δλmax on the number of branches of the star-shaped oligomers BT-1-3 suggest that the observed changes in their spectra are caused by the change in environment of the central silicon atom rather than increasing conjugation length in these compounds. This suggestion was confirmed by comparing the values of the extinction coefficient ε per one chromophore moiety in the compounds of HTMSB, BT-1-3. As can be seen from Table 1, the ratio ε/n, where n is the number of chromophores in molecule, remains constant for HTMSB and BT-1-3 and equal to the extinction coefficient of the model HB. This means that the main absorbing moiety in these compounds is the chromophore bithiophene. Spectral properties of terthiophenes TT-1-3 and phenylene-thiophenes FT-1-3, as follows from the data in Table 1, vary similarly bithiophene derivatives BT-1-3. For both oligomer series TT-1-3 and PT-1-3 the shift of λmax of absorption spectra occurs in the case of variation in the number of branches. The absorption maximum of oligomers TT is shifted by about 50 nm to the long wavelength region relative to BT with the same number of branches since their branches contain more conjugated thiophene cycles. For samples PT the absorption maxi-


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YEVLAMPIEVA et al.

Fig. 2. The chemical structure of the silicon-containing dendrimers of bithiophene of first (D-1), second (D-2), and third (D-3) generations. RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 86 No. 5 2013


mum is shifted to the opposite direction with respect to the BT region in accordance with the change of the chemical structure of their branches. Additivity of absorbing chromophore in molecules TT-1-3 and PT-1-3 is kept, as the ratio of extinction coefficient to the total number of chromophores ε/n in both rows is constant (Table 1). Thus, the analysis of absorption spectra of the starshaped oligomeric thiophene derivatives showed that, first, chromophore in the branch is the main absorbing moiety of these molecules, and secondly, the chromophore moieties covalently coupled to silicon do not optically interact. In other words, the silicon atom not supporting conjugation in the polythiophene chain, substantially isolates chromophores from each other in the oligomers. Connecting role of the silicon atom in the star-shaped oligomers manifests itself only in a small shift of the absorption maximum to the long wavelength region. The value of the λmax shift of oligomer compared to the position of the maximum of the absorption spectrum of an initial chromophore depends on the number of its branches. Table 1 shows the estimates of a band gap Δe of the samples obtained using Eq. 2 and value λе, which is corresponds to a long wavelength edge of the absorption band: Δe = hv/λе,

(2)

where v is the light speed, h is Plank constant. As can be seen from Table 1 Δe diminishes with increasing the number of chromophore, substituents at silicon atom into the oligomer structure. The value of Δe is minimal for TT thiophene derivatives possessing the most uniform and long conjugation chain in branches compared to other samples. It is known that thiophene derivatives with the width of the band gap of about 2 eV are of high electrical conductivity [24]. It is therefore evident that the star-shaped oligomers and similar oligomers, whose branches contain more than three thiophene cycles can be the most promising semiconductors among investigated.

751

A

Fig. 3. Absorbance A (rel. units) of oligomers ТТ-1-3 in toluene. (λ) Wavelength (nm). Oligomer concentration (g cm–3): (1) 2.1 × 10–5 ТТ-1, (2) 2.9 ×10–5 ТТ-2, (3) 2.5× 10–5 ТТ-3.

The spectral properties of the star-shaped oligomers discussed above clearly demonstrate the role of the central silicon atom in the formation of their optical characteristics. On the one hand, silicon atom does not affect absorption, chromophores comprising oligomers behave independently and autonomously absorb; on the other hand, the position of the maximum of the absorption spectrum of oligomers depends on the number of chromophores bonded to silicon. The study of electro-optical Kerr effect in a pulsed field of oligomers in solution has allowed to find out a degree of independence of behavior of their chromophore-contaning branches. Common property of the oligomer molecules is that they have an axis of symmetry passing through the silicon atom which is one of the main molecular axes. The electro-optical Kerr effect in the solution of axially symmetric molecules depends on the magnitude and direction of the permanent dipole moment μ and anisotropy of optical polarizability Δb. The theoretical value of the specific Kerr constant K of such molecules in solution is expressed as [19] (3) (3a)

where k is Bolzmann constant, NA is Avogadro’s number, M is molecule mass, β is the angle between the direction

of the dipole moment and the primary axis of optical polarizability of molecule, n0 is refractive index of the


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YEVLAMPIEVA et al. (b)

(a)

K × 1010

Δn × 108

TT-1

TT-1 BT-1 D-1 c × 102

E × 10–4

Fig. 4. (a) The birefringence Δn as a function of the square of the electric field E2 [(300 V)2 cm–2] for the solutions of the sample TT-1 in toluene; (b) the concentration c (g cm–3) vs. the specific Kerr constant K for oligomers BT-2, TT-1, PT-1, and D-3 dendrimer in toluene. The concentration of TT-1 (g cm–3): (1) 3.372, (2) 2.045, (3) 1.057 × 10–2; (4) solvent.

solvent, and ε0 is dielectric constant of the solvent. The solution of oligomers in toluene are featured by positive sign and small electro-optic effect. The birefringence Δn, which was observed in an electric field in the solutions of oligomers, was proportional to the square of the electric field intensity E2 (Fig. 4a). The specific Kerr constants Kexp of the samples (Table 2) were determined by the concentration dependence [Δn – (Δn)0]/E2c for c = 0 (Fig. 4b). Simulation of oligomer molecules by quantumchemically semi-empirical method PM3 in vacuum allowed a general conclusions about the nature of their electro-optical properties in solution. To fully optimized oligomer molecules the tensor of optical polarizability

was calculated as well as components of the permanent dipole moment in the coordinate system associated with the primary axis of optical polarizability. This made it possible to estimate β, μ and Δb, and the value of the two components of the specific Kerr constant K, the first of which K1 depends only on the anisotropy of the optical polarizability Δb, and the second K2, on Δb, the permanent dipole moment μ, and the angle β [Eqs. (3), (3a)]. Calculated values of K1 and K2 of all oligomers are shown in Table 2. It is easy to conclude (Table 2), the electro-optical properties of the compounds are mainly determined by the contribution of component K1, and, consequently, the specific Kerr constant of oligomers will depend on

Table 2. Electro-optic and molecular parameters of the star shaped oligomers

a

K1 × 1010

K2 × 1010

K exp × 1010

Образец

М*

Δb × 1024, см3

μ, D

β, deg

BТ-1 BТ-2 BТ-3

557 791 1025

35 30 10

0.6 0.5 0.4

54 66 85

0.39 0.20 0.02

~0 –0.026 –0.003

0.14 ± 0.01 0.12 0.04

ТТ-1 ТТ-2 ТТ-3

721 1037 1354

67 60 58

0.6 0.9 1.1

70 72 69

1.09 0.59 0.47

–0.04 –0.13 –0.12

0.35 0.29 0. 10

PТ-1 PТ-2 PТ-3

765 1104 1442

65 62 51

1.1 1.0 1.2

79 63 65

0.63 0.60 0.51

–0.14 –0.05 –0.06

0.16 0.15 0.11

cm5 g–1(300 V)–2

Molecular mass М corresponds chemical formula of the samples. RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 86 No. 5 2013


the value of their anisotropy of polarizability Δb. The component of the specific Kerr constant K2, associated with the fact that oligomer molecules possess permanent dipole moment, is small in comparison with K1 and of negative sign (with the exception of the BT-1) due to the large angle β. It may be noted that at β = 54.4° K2 is zero and with a further increase in the angle β the contribution of component K2 is of negative sign. Both the calculation and experiment (Table 2, Kexp) show that the Kerr constant in each of a series of oligomers diminishes with increasing the number of chromophores-substituents at the silicon atom. Similarly, the anisotropy of optical polarizability of oligomers Δb changes (Table 2), which is most clearly seen for a series of oligomers BT-1-3 with the shortest branches. This change in electro-optical parameters and Δb is only possible in the case of the rigid binding between the silicon and chromophores, which is the cause of growing molecular symmetry of oligomers with increasing the number of chromophore substituents at the central atom. As can be seen from Table 2 the four branches oligomers in each of the series are characterized by the lowest values of Kexp. Noteworthy a mismatch of calculated K 1 and experimental K exp electro-optical characteristics of oligomers: the last are several times smaller than the corresponding calculated (Table 2). This is caused by the further polarization of conjugated molecules in solution in an electric field, and acquisition by them of the induced dipole moment that can not be taken into account by PM3 modeling in vacuo. In the case of accounting for the results of the calculation of component K2, whose absolute value will increase due to the induced dipole moment and sign will remain the same, it is clear that the additional polarization of the star-shaped thiophene will lead to a decrease in the absolute value of the electro-optic effect of positive sign tnat is observed in the solutions of oligomers under study. Thus, the mismatch of theory and experiment in this case is understandable. Moreover,

753

distinguishing Kexp and K1 allows concluding that the total dipole moment of the oligomer molecules in toluene due to additional polarization in the electric field should be increased by 2–3 times assuming that the dipole induced by field is directed in the same way as a permanent . Thus, the study of the Kerr effect in the solution of thiophene oligomer derivatives despite the structural differences revealed similarity of their electro-optic properties. Features of changes in the electro-optical properties of oligomers suggest that their molecules have a spatially rigid structure of the central part, whose symmetry increases with growing the degree of chromophore substitution of silicon atom. DENDRIMERS Patterns of change in the properties discovered for the star-shaped thiophene oligomers were taken into account in the analysis of the properties of thiophene derivatives of dendritic topology depending on the generation number. A structure of dendrimer under study was shown in Fig. 2. A series of three generations of bithiophene dendrimers (D-1-3) was studied with a three-functional centers of branching. The number of chromophores coupled to a silicon atom in the structure of the dendrimers was the same and equal to three. Oligomer BT-2 can be considered as a zero generation of the series D-1-3. Data of Table 3, which shows the spectral and electro-optical characteristics of D-1-3 dendrimers, allow concluding: (a) the position of the maximum of the absorption spectrum of dendrimer is independent of the dendrimer generation number, as that was previously observed for the absorption spectra of dendrimers of bithiophene silane in different solvents [25, 26], (b) the value of ε/n is retained and corresponds to the extinction coefficient of the model compounds (Table 1), and (c) the specific Kerr constant K is independent of the dendrimer generation number.

Table 3. Spectral and electro-optical parameters of silicon-containing dendrimers of bithiophene

a

ε

ε/n

K × 1010, g–1(300 V)–2

Sample

Generation no.

na

λmax, nm

D-1

1

9

333 ± 1

180000

20000

0.12 ± 0.01

D-2

2

21

333

420200

20010

0.12

D-3

3

45

333

890300

19800

0.12

n is the number of bithiophene moiety in molecule. RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 86 No. 5 2013

L mol–1 cm–1

cm5

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YEVLAMPIEVA et al.

The comparison of the λmax position of the absorption spectra of dendrimers (Table 3) and oligomer BT-2 (Table 1) shows that all four members of the series are of the same spectral characteristics and comparison of values Kexp of dendrimers D-1-3 with that of BT-2 (Table 2) leads to conclusion that the electro-optical properties of all generations are also similar. The data result in two fundamental conclusions: (1) the degree of substitution of silicon valence bonds by chromophores determines spectral and electro-optical properties of dendrimers regardless of generation number, (2) dendrimer macromolecule unlike oligomers, whose properties are described above, are not rigid. The latter conclusion follows from the fact that the specific Kerr constant of dendrimer is independent of the number of generation and coincidence of values K of dendrimers and those of oligomer BT-2. Moreover, this fact indicates that there is no correlation interaction of separate moieties of the dendrimer under the influence of electric field. Thus, in view of a strong coordinating role of silicon atom, which was shown for oligomers, a silicon atom with its immediate environment is the moiety that forms electro-optical properties of the investigated dendrimers. Small-scale nature of a response of the bithiophenesilane dendrimers to an impact of external electric field can be considered as a manifestation of their kinetic flexibility. Macromolecules are considered as kinetically flexible when their deformational speed at the external impact is much greater than the speed of their orientation as a whole. As shown, the electro-optic effect in the solution of the thiophene oligomers is determined, primarily, by the optical polarizability, specifically, by its anisotropy (K1 >> │K2│, Table 2), i.e. Kerr effect for the thiophene derivatives is of deformation type, since all the processes related to electron mobility, including polarization/polarizability, are always superior in speed the dipole-orientation processes. Furthermore, it is necessary to take into account the hybrid organic-silicon type of the compounds. The electrons of the outer shell of silicon can be different (faster) polarized by external fields, thereby accelerating the reaction of π-electrons of thiophene due to changing the intramolecular field. All these facts explain why organic-silicon thiophene-containing dendrimers in the electric field behave as kinetically flexible macromolecule. The number of modes (including vibration) of such kinetically flexible macromolecules will increase with increasing the number of generation.

This fact, apparently, explains the independence of the luminescence yield on the number of generation, which was earlier found for bithiophene silane dendrimers in THF [25]. CONCLUSIONS (1) The study of the molecular spectral and electrooptical properties of silicon-containing star-shaped and dendritic thiophene derivatives showed that organo-silicon nature of these compounds should take into account for their purposive modification. (2) Silicon atom featured by a different type of electronic system compared with conjugated system of π-electrons of chromophores retains autonomy in hybrid compounds under study. Silicon does not support conjugation chains that results in the additivity of spectral parameters of chromophores comprising of the considered oligomers and dendrimers. At the same time, the silicon exerts strong coordinating effect on chromophores surrounding it that is confirmed by a gradual shift of the absorption spectrum with increasing degree of substitution of silicon in oligomers, and also by the fact that namely silicon moiety of dendrimers determines their electrooptical properties. (3) Thiophene derivatives of dendritic topology with silicon atoms as centers of branching are kinetically flexible macromolecules, their electro-optical and fluorescent spectral properties do not depend on the number of generation. REFERENCES 1. Organic Electronics: Materials Manufacturing and Application, Klauk, N., Ed., Weinheim: Wiley–VCH, 2006. 2. Green, M.A., J. Mater. Sci. Mater. Electron., 2007, vol. 18, no. 1, pp. 15. 3. Holcombe, T.W., Woo, C.H., Kavulak, D.F., et al., J. Am. Chem. Soc., 2009, vol. 131, no. 40, pp. 14160. 4. Van, Bavel, S., Sourty, E., de With, G., et al., Macromolecules, 2009, vol. 42, no. 19, pp. 7396. 5. Troshin, P.A., Ponomarenko, S.A., Luponosov, Y.N., et al., Solar Energy, Materials a. Solar Cells, 2010, vol. 94, no. 12. P., 2064. 6. Zhao, K., Xue, L., Liu, J., et al., Langmuir, 2010, vol. 26, no. 1, pp. 471. 7. Kim, J.S., Park, Yu., Dong, Yun, Lee, et, al., Adv. Funct. Mater., 2010. V., 20, p. 540.


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