International Journal of Photoenergy

International Journal of Photoenergy / 2009 / Article
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Solar Energy and Nanomaterials for Clean Energy Development

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Research Article | Open Access

Volume 2009 |Article ID 238615 |

Olga G. Nikolaeva, Alexey V. Tsukanov, Evgenii N. Shepelenko, Boris S. Lukyanov, Anatoly V. Metelitsa, Olga Yu. Kostyrina, Alexander D. Dubonosov, Vladimir A. Bren, Vladimir I. Minkin, "Synthesis of Novel Iono- and Photochromic Spiropyrans Derived from 6,7-Dihydroxy-8-Formyl-4-Methyl-2H-Chromene-2-One", International Journal of Photoenergy, vol. 2009, Article ID 238615, 6 pages, 2009.

Synthesis of Novel Iono- and Photochromic Spiropyrans Derived from 6,7-Dihydroxy-8-Formyl-4-Methyl-2H-Chromene-2-One

Academic Editor: Mohamed Sabry Abdel-Mottaleb
Received31 Mar 2009
Accepted29 May 2009
Published18 Aug 2009


Novel photochromic spiropyrans (SPPs) containing -hydroxy group were synthesized and their spectral properties as well as abilities for complexation with metal ions studied. In solutions they exist as equilibrium mixture of spirocyclic (A) and merocyanine (B) isomers. The largest content of merocyanine form was found for the derivative with an electron-donating methyl group in position 5 of hetaryl fragment. The irradiation of SPPs in acetonitrile shifts the equilibrium to the B form. Similar effect causes the addition of metal cations due to formation of colored complexes with merocyanine isomers.

1. Introduction

Design and synthesis of novel photochromic spiropyrans and spirooxazines (SPOs) displaying ionochromic properties are of significant interest due to their application as magnetic [1, 2], in devices of molecular electronics [3] and as effective photoswitchable chemosensors for metal cations [46]. The structure of the latter type compounds usually contains chelate [714], calixarene [15], or crown-ether ionophore groups [16, 17]. The coordination of metal cations occurring via interaction with ligating centers located in the chromene part of a spiropyran (or spirooxazine) can lead to thermally or photochemically driven cleavage of the –O bond of the cyclic isomer followed by Z/E-isomerization to give the metastable colored merocyanine form. By contrast, the complexation with ionophore conjugated with the hetaryl fragment of a spiropyran (or spirooxazine) results in destabilization of the merocyanine form caused by electrostatic repulsion between a metal cation and the positively charged nitrogen atom in the hetaryl moiety [18, 19]. We synthesized a series of novel SPPs containing -hydroxy group in order to study their structure in solutions, photochromic properties, and ionochromic ability towards the alkali-earth and d-metal cations.

2. Experimental

NMR spectra were recorded on Bruker WM-250 (300 MHz) spectrometer using CDC as solvent. The chemical shifts were calibrated using the signals of solvent residual protons ( 7.25 ppm). IR spectra were measured in Nujol using a Specord 75IR spectrometer. UV-Vis absorption spectra were obtained in acetonitrile with a Cary-100 spectrophotometer. Irradiation of solutions was carried out by filtered light of a high-pressure mercury lamp DRSh (250 W) supplied with glass filter (  nm).

3. Results and Discussion

3.1. Synthesis

The compound 6,7-Dihydroxy-8-formyl-4-methyl-2H-chromene-2-one (2) was prepared by coupling 6,7-dihydroxy-4-methyl-2H-chromene-2-one with ethyl N-p-methoxyphenylformimidate formed in situ from 4-methoxyphenylamine and triethyl orthoformate followed by the hydrolysis of Schiff base (1) with diluted hydrochloric acid (Scheme 1). The spyropyrans (4) were synthesized by condensation of aldehyde (2) with corresponding 3H-indolium perchlorates (3) in presence of triethylamine.


(i) 6,7-dihydroxy-8-[(4-methoxyphenylimino)methyl]-4-methylchromen-2-one (1). A mixture of 10 g (45 mmol) 6,7-dihydroxy-4-methyl-2H-chromene-2-one, 11 g (90 mmol) 4-methoxyphenylamine, and 14 mL triethyl orthoformate was kept for 5 hours at 170°C. The precipitate obtained after cooling was filtered and washed with ethanol. The crude product was purified by recrystallization from DMFA. Yield 25%, mp 257–258°C. NMR (CDC ): 2.35 (3H, s, C ), 3.88 (3H, s, OMe), 6.06–7.43 (7H, m, Ar-H  + OH), 9.02 (1H, s, CH=), 15.70 (1H, s, OH). Anal.Calcd for : C, 66.46; H, 4.65; N, 4.31. Found: C, 66.36; H, 4.60; N, 4.41.

(ii) 6,7-dihydroxy-8-formyl-4-methyl-2H-chromene-2-one (2). 6,7-Dihydroxy-8-[(4-methoxyphenylimino)methyl]-4-methylchromen-2-one (1) (2.5 g, 7.7 mmol) was dissolved under heating in dioxane (300 mL) and 18% HCl (150 mL) was added. The solution obtained was allowed to stand 10–15 minutes at 60–70°C and the reaction mixture was poured into the water (500 mL). The crude product was filtered, dried, and purified by recrystallization from 2-propanol. Yield 32%, mp 234–235°C. NMR (CDC ): 2.40 (3H, s, CH3), 5.64–7.35 (3H, m, Ar-H  + OH), 10.59 (1H, s, CHO), 12.42 (1H, s, OH). Anal.Calcd for : C, 60.01; H, 3.66. Found: C, 60.15; H, 3.58.

(iii) spiropyrans (4af) (general procedure). A mixture of corresponding perchlorate (3) (1 mmol), aldehyde (2) (1.1 mmol) and 0.1 mL (0.7 mmol) triethylamine was refluxed in 2-propanol (20 mL) for 5 hours. The solvent was removed, the residue was purified by column chromatography on Si with chloroform as an eluent and recrystallized from 2-propanol.

(iv) -hydroxy-1,3,3, -tetramethylspiro[indoline-2, -pyrano[2,3-f]benzopyran]- ( H)-one (4a). Yield 24%, mp 208-209°C; IR ( /c , nujol): 3120, 1685, 1580, 1560, 920. NMR (CDC ): isomer A 1.20, 1.30 (6H, , 2 gem-(C ), 2.34 (3H, s, C ), 2.77 (3H, s, NC ), 5.35 (1H, br. s, OH), 5.84 (1H, d, , J 10.5), 6.16 (1H, s, ), 6.58–7.22 (5H, m, Ar-H), 7.47 (1H, d, , J 10.5). Isomer B: 1.84 (6H, s, 2 gem-(C ), 2.31 (3H, s, C ), 3.78 (3H, s, NC ), 5.98 (1H, s, ), 6.16–7.22 (5H, m, Ar-H), 8.47 (1H, d, CH=, J 14.8), 8.92 (1H, d, CH=, J 14.8). Ratio of isomers (see Table 1). Anal.Calcd for : C, 73.58; H, 5.64; N, 3.73. Found: C, 73.41; H, 5.65; N, 3.82.

Comp.Absorption of (4), , (nm), ( , 1 mo  c )Content of form B,% = [B]/[A]

4a448 (2.31), 595 (1.72)28.60.401
4b447 (2.35), 595 (1.78)56.51.299
4c447 (2.40), 596 (1.82)23.50.307
4d452 (3.07), 605 (2.04)9.00.099
4e448 (1.85), 593 (1.63)63.01.702

(v) -hydroxy-1-octyl-3,3, -trimethylspiro[indoline-2, -pyrano[2,3-f]benzopyran]- ( H)-one (4b). Yield 27%, mp 145-146°C; IR ( /c , nujol): 3180, 1685, 1610, 1560, 920. NMR (CDC ): isomer A: 1.19–1.93 (21H, m, 3C   + 6C ), 2.34 (3H, s, Me), 3.17 (2H, m, NC ), 5.30 (1H, br. s, OH), 5.81 (1H, d, , J 10.5), 6.17 (1H, s, ), 6.58–7.45 (6H, m, Ar-H  + ). Isomer B: 1.19–1.93 (21H, m, 3C   + 6C ), 2.30 (3H, s, Me), 4.18 (2H, t, NC ), 5.96 (1H, s, ), 6.58–7.45 (5H, m, Ar-H), 8.70 (1H, br. s, CH=), 8.92 (1H, d, CH=, J 15.3). Ratio of isomers is (see Table 1). Anal.Calcd for : C, 76.08; H, 7.45; N, 2.96. Found: C, 76.18; H, 7.48; N, 2.87.

(vi) -hydroxy-1-isopropyl-3,3, -trimethylspiro[indoline-2, -pyrano[2,3-f]benzopyran]- ( H)-one (4c). Yield 68%, mp 214-215°C; IR ( /c , nujol): 3160, 1685, 1660, 1615, 1580, 920. NMR (CDC ): isomer A: 1.18–1.83 (12H, m, 4Me), 2.34 (3H, s, C ), 3.84 (1H, m, CH), 5.42 (1H, br. s, OH), 5.81 (1H, d, , J 10.6), 6.16 (1H, s, ), 6.77–7.41 (5H, m, Ar-H), 7.44 (1H, d, , J 10.6). Isomer B: 1.18–1.83 (12H, m, 4C ), 2.31 (3H, s, Me), 4.02 (1H, m, CH), 5.01 (1H, br. s, OH), 5.96 (1H, s, ), 6.77–7.41 (5H, m, Ar-H), 8.80 (1H, br. s, CH=), 8.92 (1H, d, CH=, J 14.3). Ratio of isomers (see Table 1). Anal.Calcd for : C, 74.42; H, 6.25; N, 3.47. Found: C, 74.35; H, 6.39; N, 3.40.

(vii) -hydroxy-1-benzyl-3,3, -trimethylspiro[indoline-2, -pyrano[2,3-f]benzopyran]- ( H)-one (4d). Yield 31%, mp 216-217°C; IR ( /c , nujol): 3150, 1670, 1630, 1570, 928. NMR (CDC ): isomer A: 1.31, 1.34 (6H, , 2 gem-(C ), 2.35 (3H, s, C ), 4.21 (1H, d, C , J 16.7), 4.63 (1H, d, C , J 16.7), 5.40 (1H, br. s, OH), 5.89 (1H, d, , J 10.7), 6.16 (1H, s, ), 6.37–7.55 (11H, m, Ar-H + ). Isomer B: 1.25 (6H, s, 2 gem-(C ),), 1.89 (2H, s, NC ), 5.53 (1H, br. s, OH), 6.16 (1H, s, ), 6.37–7.55 (10H, m, Ar-H), 8.49 (1H, br. s, CH=), 8.93 (1H, d, CH=, J 15.3). Ratio of isomers (see Table 1). Anal.Calcd for : C, 77.14; H, 5.58; N, 3.10. Found: C, 77.07; H, 5.52; N, 3.19.

(viii) -hydroxy-1,3,3,5, -pentamethylspiro[indoline-2, -pyrano[2,3-f]benzopyran]- ( H)-one (4e). Yield 29%, mp 276-277°C; IR ( /c , nujol): 3170, 1685, 1610, 1570, 920. NMR (CDC ): isomer A: 1.20, 1.28 (6H, , 2 gem-(C ), 2.35 (3H, s, C ), 2.48 (3H, s, C ), 2.73 (3H, s, NC ), 5.38 (1H, br. s, OH), 5.84 (1H, d, , J 10.7), 6.15 (1H, s, ), 6.44–7.31 (4H, m, Ar-H), 7.46 (1H, d, , J 10.7). Isomer B: 1.25 (6H, s, 2 gem (C ), 1.83 (6H, s, Me), 2.33 (3H, s, Me), 3.85 (3H, s, NMe), 5.99 (1H, s, ), 6.44–7.31 (4H, m, Ar-H), 8.43 (1H, d, CH=, J 15.1), 8.85 (1H, d, CH=, J 15.1). Ratio of isomers (see Table 1). Anal.Calcd for : C, 74.02; H, 5.95; N, 3.60. Found: C, 74.19; H, 5.91; N, 3.55.

(ix) -hydroxy-5-nitro-1,3,3, -tetramethylspiro[indoline-2, -pyrano[2,3-f]benzopyran]- ( H)-one (4f). Yield 25%, mp 281-282°C; IR ( /c , nujol): 3170, 1700, 1600, 1560, 970. NMR (CDC ): isomer A: 1.24, 1.37 (6H, , 2 gem-(C ), 2.36 (3H, s, C ), 2.90 (3H, s, NC ), 5.36 (1H, s, OH), 5.82 (1H, d, , J 10.5), 6.19 (1H, s, ), 6.55–8.18 (6H, m, Ar-H + ). Anal.Calcd for : C, 65.71; H, 4.79; N, 6.66. Found: C, 65.70; H, 4.68; N, 6.78.

3.2. Spectral Properties and Complexation with Metal Ions

Electronic absorption spectra of SPP (4f) in toluene, chloroform, and acetonitrile show broad absorption bands at 295 and 365 nm. For compounds (4a–e), the spectra contain also additional long-wavelength bands with maxima centered around 430 and 580 nm (Tables 1 and 2, Figure 1). For SPPs (4a–e), the appearance of long-wavelength bands is accompanied by the additional proton signals of merocyanine isomers B in the NMR spectra. The long-wavelength bands in absorption spectra of cyclic ( 370-371 nm) and merocyanine ( 580–650 nm) forms correspond to transitions while the absorption bands of the cyclic form ( 294–299 nm) and merocyanine form ( 456–471 nm) correspond to transitions (Table 2). According to NMR and absorption spectroscopy data only SPP (4f) with an electron-withdrawing N -group exists in solution of chloroform entirely as the cyclic form A, while the solutions of (4a–e) contain both spirocyclic A and merocyanine B isomers (Scheme 2). The percentage of the merocyanine form in the equilibrium mixture was estimated using NMR data (Table 1). The largest content of form B is observed for the derivative (4e) with an electron-donating methyl group in position 5.

Comp. , (nm) , (nm) , s

4a295, 354, 370458, 614161
4b298, 355, 371459, 617205.7
4c294, 354sh, 370456, 619170.9
4d295, 354, 370461, 61744.1
4e299, 356, 370459, 614257.7
4f299, 355sh, 371471, 6536.5


The irradiation of SPPs (4a–f) in acetonitrile ( 365 nm) leads to increase in intensity of the long-wavelength absorption band in the regions of 430–471 and 580–653 nm due to the formation of merocyanine form which is thermally converted to the ring-closed isomer A (Figure 1).

Lifetimes of the photoinduced absorption bands of forms B allow to evaluate their thermal stability because content of merocyanine isomers in the equilibrium state in low-polarity solvent is substantially less than that of the cyclic isomers. Therefore, relaxation process rate constant is determined by the rate constant for the reaction. The lowest lifetime of the photoinduced absorption was observed for compound (4f) with an electron-withdrawing group in 5th position and the highest lifetime for the compound (4e) with an electron donating group (Table 2). Analogous results were previously obtained for the spirooxazines [20].

Under dark conditions, addition of metal cations: (as perchlorate), , , , , (as acetates) shifts the equilibrium to the open form B with increase in the maximum of absorption band in the visible region due to the formation of complexes (5) (Figure 2). Position of the absorption maxima of these bands depends on the nature of the metal ion. For example, complex (4a) with is characterized by absorption bands at 369, 406, and 524 nm; with at 387, 440, and 550 nm; with at (372, 455, and 586 nm). The maxima of these absorption bands are hypsochromically shifted with respect to the free merocyanine absorption. The content of the complexes (5) is greatly raised under exposure of the solution to UV light (  nm).

The structure of complexes (5) was estimated by IR spectroscopy. The IR-spectra of initial compounds contain broad absorption band of the hydroxyl group at 3100–3200 c which slightly changes after complex formation, so the substitution of hydroxyl group protons was excluded.

NMR, IR spectrometry and the spectrophotometric titration method were employed for the analysis of the complexation processes. The NMR spectral changes for (4a) upon the addition of ions in C CN/nitrobenzene- ( v/v) solution are shown in Figure 3.

In a metal-free solution, signals of both spiro- and merocyanine forms are present in the spectrum. This spectrum is characterized by a doublet centered at 5.80 ppm for the proton (4A)- , singlet at 6.1 ppm for the proton (4A)- and two doublets at 8.4 ppm and 8.7 ppm for the protons (4B)- , (4B)- . The content of merocyanine isomer B was estimated as 16%. In the presence of metal sault chemical shifts of protons did not change significantly. After addition of Mg(Cl in darkness the appearance of a resonance signal at 6.00 ppm for the proton (4B)- and substantial increase in the intensity of multiplet at 8.70 ppm for the protons (4B)- , (4B)- was observed (Figure 3). At the same time the intensity of the doublet at 5.80 ppm of the proton (4A)- and singlet at 6.1 ppm for the proton (4A)- was decreased. The data obtained indicate that the equilibrium is shifted to the merocyanine form due to the formation of complex (5) (Scheme 2). The content of the merocyanine form involved into the complexation with ion was estimated as 83% (1 hour after salt addition).

The absorption at 480 nm (Figure 4) was chosen to determine the composition of the complex by the method of molar ratios [21]. The molar ratios plot shown in Figure 4 makes it evident that the composition of the (4a)- complex is .

4. Conclusions

Novel photochromic spiropyrans containing -hydroxy group in solutions exist as equilibrium mixture of spirocyclic and merocyanine isomers. The addition of metal salts leads to formation of complexes with merocyanine isomers and shifts the equilibrium to the ring-opened forms. The results obtained indicate that binding of cations is an important factor affecting position of the spyropyran-merocyanine equilibrium. These ionochromic compounds can be used as the basis for design of chemosensors for metal ions.


This work was supported by Russian Foundation for Basic Research (grants N 09-03-00052, 07-03-00234, and 09-03-00813), Ministry of Education and Science of Russian Federation (grant N RNP2348), “Russian Science Support Foundation” and and President's of RF grant (NSh-363.2008.3).


  1. A. O. Bulanov, L. D. Popov, G. I. Bondarenko, and V. A. Kogan, “New binuclear copper(II) complexes with bishydrazones derived from spiropyran of the benzoxazinonium series,” Russian Journal of General Chemistry, vol. 76, no. 8, pp. 1272–1274, 2006. View at: Publisher Site | Google Scholar
  2. A. O. Bulanov, B. S. Luk'yanov, V. A. Kogan, and V. V. Lukov, “Binuclear copper(II) complexes with hydrazones containing spiropyran fragment,” Russian Journal of Coordination Chemistry, vol. 29, no. 9, pp. 658–659, 2003. View at: Publisher Site | Google Scholar
  3. X. Guo, D. Zhang, and D. Zhu, “Photocontrolled electron transfer reaction between a new dyad, tetrathiafulvalene-photochromic spiropyran, and ferric ion,” Journal of Physical Chemistry B, vol. 108, no. 1, pp. 212–217, 2004. View at: Google Scholar
  4. M. V. Alfimov, O. A. Fedorova, and S. P. Gromov, “Photoswitchable molecular receptors,” Journal of Photochemistry and Photobiology A, vol. 158, no. 2-3, pp. 183–198, 2003. View at: Publisher Site | Google Scholar
  5. K. Kimura, H. Sakamoto, and M. Nakamura, “Molecular design and applications of photochromic crown compounds—how can we manipulate metal ions photochemically?” Bulletin of the Chemical Society of Japan, vol. 76, no. 2, pp. 225–245, 2003. View at: Publisher Site | Google Scholar
  6. A. M. A. Salhin, M. Tanaka, K. Kamada et al., “Decisive factors in the photoisomerization behavior of crowned spirobenzopyrans: metal ion interaction with crown ether and phenolate anion moieties,” European Journal of Organic Chemistry, no. 4, pp. 655–662, 2002. View at: Google Scholar
  7. N. Shao, Y. Zhang, S. Cheung et al., “Copper ion-selective fluorescent sensor based on the inner filter effect using a spiropyran derivative,” Analytical Chemistry, vol. 77, no. 22, pp. 7294–7303, 2005. View at: Publisher Site | Google Scholar
  8. B. G. Jeliazkova, S. Minkovska, and T. Deligeorgiev, “Effect of complexation on the photochromism of 5-(benzothiazol-2-yl)spiroindolinonaphthooxazines in polar solvents,” Journal of Photochemistry and Photobiology A, vol. 171, no. 2, pp. 153–160, 2005. View at: Publisher Site | Google Scholar
  9. A. V. Chernyshev, N. A. Voloshin, I. M. Raskita, A. V. Metelitsa, and V. I. Minkin, “Photo- and ionochromism of 5-(4,5-diphenyl-1,3-oxazol-2-yl) substituted spiro[indoline-naphthopyrans],” Journal of Photochemistry and Photobiology A, vol. 184, no. 3, pp. 289–297, 2006. View at: Publisher Site | Google Scholar
  10. N. A. Voloshin, A. V. Chernyshev, A. V. Metelitsa, I. M. Raskita, E. N. Voloshina, and V. I. Minkin, “Synthesis of photochromic 5-(4,5-diphenyl-1,3-oxazol-2-yl)-spiro[indoline-2,3-naphtho[2,3-b] pyran],” Russian Chemical Bulletin, vol. 54, no. 3, pp. 705–710, 2005. View at: Publisher Site | Google Scholar
  11. R. A. Kopelman, S. M. Snyder, and N. L. Frank, “Tunable photochromism of spirooxazines via metal coordination,” Journal of the American Chemical Society, vol. 125, no. 45, pp. 13684–13685, 2003. View at: Publisher Site | Google Scholar
  12. J. D. Winkler, C. M. Bowen, and V. Michelet, “Photodynamic fluorescent metal ion sensors with parts per billion sensitivity,” Journal of the American Chemical Society, vol. 120, no. 13, pp. 3237–3242, 1998. View at: Publisher Site | Google Scholar
  13. N. A. Voloshin, A. V. Chernyshev, A. V. Metelitsa et al., “Photochromic spiro[indoline-pyridobenzopyrans]: fluorescent metal-ion sensors,” Archive for Organic Chemistry, vol. 11, pp. 16–24, 2004. View at: Google Scholar
  14. M. S. Attia, M. M. H. Khalil, M. S. A. Abdel-Mottaleb, M. B. Lukyanova, Yu. A. Alekseenko, and B. Lukyanov, “Effect of complexation with lanthanide metal ions on the photochromism of (1,3,3-trimethyl-5-hydroxy-6-formyl-indoline-spiro2,2-[2h] chromene) in different media,” International Journal of Photoenergy, vol. 2006, Article ID 42846, 9 pages, 2006. View at: Publisher Site | Google Scholar
  15. Z. Liu, L. Jiang, Z. Liang, and Y. Gao, “Photo-switchable molecular devices based on metal-ionic recognition,” Tetrahedron Letters, vol. 46, no. 5, pp. 885–887, 2005. View at: Publisher Site | Google Scholar
  16. A. Abdullah, C. J. Roxburgh, and P. G. Sammes, “Photochromic crowned spirobenzopyrans: quantitative metal-ion chelation by UV, competitive selective ion-extraction and metal-ion transportation demonstration studies,” Dyes and Pigments, vol. 76, no. 2, pp. 319–326, 2008. View at: Publisher Site | Google Scholar
  17. M. Tanaka, K. Kamada, H. Ando, T. Kitagaki, Y. Shibutani, and K. Kimura, “Synthesis and photochromism of crowned spirobenzothiapyran: facilitated photoisomerization by cooperative complexation of crown ether and thiophenolate moieties with metal ions,” Journal of Organic Chemistry, vol. 65, no. 14, pp. 4342–4347, 2000. View at: Publisher Site | Google Scholar
  18. R. T. F. Jukes, B. Bozic, F. Hartl, P. Belser, and L. de Cola, “Synthesis, photophysical, photochemical, and redox properties of nitrospiropyrans substituted with Ru or Os tris(bipyridine) complexes,” Inorganic Chemistry, vol. 45, no. 20, pp. 8326–8341, 2006. View at: Publisher Site | Google Scholar
  19. V. V. Korolev, D. Yu. Vorobyev, E. M. Glebov et al., “Spironaphtoxazines produced from crown-containing dihydroisoquinolines: synthesis and spectroscopic study of cation-dependent photochromism,” Journal of Photochemistry and Photobiology A, vol. 192, no. 2-3, pp. 75–83, 2007. View at: Publisher Site | Google Scholar
  20. V. Lokshin, A. Samat, and A. V. Metelitsa, “Spirooxazines: synthesis, structure, spectral and photochromic properties,” Russian Chemical Reviews, vol. 71, no. 11, pp. 893–916, 2002. View at: Publisher Site | Google Scholar
  21. M. Beck and I. Nagypal, Chemistry of Complex Equilibria, Academiai Kiado, Budapest, Hungary, 1989.

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