Substituent Effect on the Reactive Carbon Atoms - ACS Publications

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Photochromism of 1,2-Bis(2-thienyl)perfluorocyclopentene Derivatives: Substituent Effect on the Reactive Carbon Atoms Yuto Tatsumi,† Jun-ichiro Kitai,† Waka Uchida,‡ Koji Ogata,§ Shinichiro Nakamura,*,§ and Kingo Uchida*,† †

Department of Materials Chemistry, Faculty of Science and Technology, Ryukoku University, Seta, Otsu, Shiga 520-2194, Japan Department of Biomolecular Engineering, Tokyo Institute of Technology, Nagatsuta, Midori-ku, Yokohama 226-8503, Japan § Nakamura Laboratory, RIKEN Research Cluster for Innovation, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan ‡

S Supporting Information *

ABSTRACT: In this work, we prepared a new 1,2-bis(3-cyanothiophen-2-yl)perfluorocyclopentene with electro-withdrawing cyano groups at both reactive carbon atoms. Furthermore, we studied the substituent effects of the reactive carbon atoms on the photochromic properties of 1,2-bis(3-R-substituted thiophen-2-yl)perfluorocyclopentene derivatives by comparing the absorption wavelengths and quantum yields of the derivatives having R = cyano, methyl, and methoxy groups. The absorption bands of the closed-ring isomers generated by UV irradiation shifted to longer wavelengths with an increase in the electron-donating characteristic of the substituents. The closed-ring isomer having cyano groups at both reactive carbon atoms has an absorption band at 427 nm (λmax), whereas those of methyl and methoxy derivatives have bands at 432 and 481 nm, respectively. The derivative with cyano groups shows the largest cycloreversion quantum yield (0.45), and this yield decreased with an increase in the substituents’ donating characteristic. Theoretical calculation explains that the excited state of the closed-ring isomer with cyano groups has the highest energy, because there is no barrier to ring-opening on the excited potential surface.

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INTRODUCTION Diarylethene derivatives have been synthesized as candidates for use in optical memory devices, due to the thermal stability of both isomers and their fatigue-resistant properties.1−3 They have been studied from the viewpoint of developing molecular switches and devices in the key structure of the phototrigger for supramolecular systems.4−8 Diarylethenes with 3-thienyl groups as the aryl groups are classified as normal-type and have been extensively studied,1−7 whereas the derivatives with 2-thienyl aryl groups show very different photochromic properties from those of normal derivatives.9−14 The molecular structure of 1,2bis(2-thienyl)perfluorocyclopentenes in which thiophene rings are substituted into the ethene moiety at the 2-position, is reversed to normal 1,2-bis(3-thienyl)perfluorocyclopentene type. The π-conjugation of reverse-type is extended throughout the molecule in the open-ring isomer, whereas in the closedring isomer, two aryl rings are separated by the sp3 carbon and the sulfur atom. In the normal-type, π-conjugation is not extended over the open ring isomers but over the closed-ring isomers.2 Reflecting the differences in these structures, the absorption wavelengths and quantum yields of the cyclization and cycloreversion reactions can be discerned. For the photochromic reaction, the substituents on the reactive carbon atoms have a great influence on the photochromic properties, and the effects of 1,2-bis(3-thienyl)perfluorocyclopentene derivatives have been systematically studied.12−14 For example, © 2012 American Chemical Society

the introduction of cyano groups on both reactive carbon atoms greatly enhances the cycloreversion quantum yield in 1,2-bis(3-thienyl)perfluorocyclopentene and 1,2-bis(3benzothienyl)perfluorocyclopentene derivatives.12 In contrast, not so many derivatives of reverse-type have been synthesized compared to the number of synthesized normal-type derivatives. Due to difficulties in synthesis, the preparation of derivatives having methoxy groups at both reactive carbon atoms has required the use of a microreactor. Although theoretical study to explain the unique photochromic properties of the reverse-type has been carried out, the preparation of the derivatives was inadequate and thus these properties were not sufficiently clarified.15−20 Looking toward future molecular electronics, the elucidation of both types of diarylethenes, normal- and reverse-types, has become critically important (Scheme 1). Therefore, we synthesized a new reverse-type derivative with electronwithdrawing cyano groups at the reactive carbon atoms and compared the photochromic properties with those of other reverse-type derivatives. Furthermore, we used theoretical calculations to explain the observed differences. Received: September 19, 2012 Revised: October 18, 2012 Published: October 19, 2012 10973

dx.doi.org/10.1021/jp3092978 | J. Phys. Chem. A 2012, 116, 10973−10979

The Journal of Physical Chemistry A

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Scheme 1. Normal and Reverse Switches of Diarylethene Derivatives

Figure 2. Absorption spectral changes of diarylethene 2 in a hexane solution (5.64 × 10−5 mol/L): 2o (solid line), 2c (broken line), and a photostationary state (2o:2c = 23:77) (dotted line) under UV light (main band: 313 nm).

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RESULTS AND DISCUSSION Substituent Effect of Photochromic Reactions on 1,2Bis(2-thienyl)perfluorocyclopentene Derivatives. The synthesis and 1H NMR and MS characterization of compound 1 are described in the Experimental Section. The other derivatives 2 and 3 were prepared according to previous works.14,15 The absorption spectral changes of 1−3 in a hexane solution are shown in Figures 1−3. For 1, the solution was


cycloreversion reactions of 1c, 2c, and 3c are strongly dependent on the substituents. The derivative 1c having strong electron-withdrawing cyano groups showed the largest cycloreversion quantum yield (0.45), whereas the yield of 2c having weak electron donating methyl groups was 0.37, and 3c having strongest electron-donating methoxy substituents showed the smallest cycloreversion yield of 0.25. The substituent effects on the cycloreversion reaction were similar to those observed for normal 1,2-bis(3-thienyl)perfluorocyclopentenes.12−14 In these studies, however, the substituent effect observed in normal-type diarylethenes on quantum yield is 24 000 times,13 whereas the present effect is only 1.8 times. As reported previously, openring isomers of diarylethenes have two types of rotamers, where one is a photoreactive antiparallel conformer and the other is a photoinactive parallel conformer.21−23 Therefore, the ratio of photoreactive antiparallel conformation to photoinactive parallel conformation is important for estimating the cyclization quantum yields.21−23 To find the ratio of the conformers, we measured the 1H NMR spectroscopy of open-ring isomers;24 however, the proton signals of the two conformers were indistinguishable (see Supporting Information), as were those


colorless and the spectrum showed three absorption maxima at 258 (ε: 1.27 × 104 M−1 cm−1), 289 (ε: 1.30 × 104 M−1 cm−1), and 336 (ε: 1.24 × 104 M−1 cm−1) nm. Upon UV irradiation, a new band appeared at 427 (ε: 9.1 × 103 M−1 cm−1) nm, and the color of the solution changed to yellow. By visible-light irradiation to the solution, the yellow color disappeared and the original spectrum was regenerated. The quantum yields of the cyclization and cycloreversion reactions of 1 were measured, and the results are summarized in Table 1 along with those of 2 and 3 (see Scheme 2). Cyclization quantum yields of 1o, 2o, and 3o were 0.17, 0.25, and 0.22, respectively. These are similar to each other in spite of the different electronic properties of the substituents on the reactive carbon atoms. In contrast, the quantum yields of the 10974



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Table 1. Spectroscopic Properties of Bis(2-thienyl)perfluorocyclopentenes in Hexane Solution λmax/nm (ε/M−1 cm−1) 1o

2o 3o

258 289 336 319 327

(1.27 × 10 ) (1.30 × 104) (1.24 × 104) (1.55 × 104) (1.8 × 104) 4

Φoc

λmax/nm (ε/M−1 cm−1)

Φco

0.17 (366 nm)

1c

427 (9.1 × 103)

0.45 (435 nm)

0.25 (313 nm) 0.22 (366 nm)

2c 3c

432 (8.8 × 103) 481 (5.8 × 103)

0.37 (435 nm) 0.25 (435 nm)

reactive carbon atoms (C2-, C2′-positions).30 They showed that the cycloreversion quantum yield with various substitutes depends on the height of TS on the excited state potential energy surface (at left of Figure 4). In reverse-type diarylethene, the quantum yield and the potential energy surface of one of the derivatives have been reported.24,27,29 However, no report has yet covered the effect of substitutes. In other words, on the downhill potential surface of the excited state as shown at right in Figure 4, it is not yet clear whether the substitutes influence the quantum yield. Consequently, this paper intends to study the experimental results for the dependence of quantum yield on the substitutes of three representative derivatives of reversetype diarylethenes (1o, 2o, and 3o). The obtained potential energy surfaces for these three molecules are shown in Figure 5. At the ground state of the closed form, the calculated distances of reactive carbon atoms (C2, C2′) are 1.560 Å for 1c, 1.559 Å for 2c, and 1.551 Å for 3c. The longer the bond length is, the larger the quantum yield of the cycloreversion reaction. The energy of the excited state (S1) of closed-ring relative to the ground state of open-ring is 80.1 kcal/mol in 1c, 71.1 kcal/mol in 2c, and 66.2 kcal/mol in 3c (Figure 5). These results suggest that the S1 of 1c is the most unstable conformer of the three whereas that of 3c is the most stable one. The potential energy surface of the excited state (S1) from closed- to open-ring cycloreversion shows a clearer downhill profile in 1c than in 2c or 3c. Considering these data, the cycloreversion reaction would be smoother in 1c than in 2c or 3c. These findings are consistent with the experimental cycloreversion quantum yield Φco: 0.45 for 1c, 0.37 for 2c, and 0.25 for 3c (Table 1). On the other hand, as shown in Figure 5, the potential energy surface of the excited state (S1) from open- to closedring cyclization reaction shows a large uphill profile in 1o, whereas the profile shows a smaller uphill shape in 2o and 3o than in 1o. Those profiles in 2o and 3o are nearly the same, except for the small barrier from open to close in S1 of 3o. Therefore, the calculations suggest that cyclization reaction may occur in 1o, but with difficulty compared to 2o or 3o. Indeed,

Scheme 2. 1,2-Bis(2-thienyl)perfluorocyclopentene Derivatives

of some derivatives such as 1,2-bis(2,5-dimethylthiophen-3yl)perfluorocyclopentene25 and 1,2-bis(2-methyl-5-phenylthiophen-3-yl)perfluorocyclopentene.26 The closed-ring isomers were nonfluorescent, whereas the open-ring isomers showed weak fluorescence. Although the Raman and supersonic free jet measurements showed the antiparallel conformer is the major one for reverse-type diarylethene, the cyclization quantum yield is lower than those of normal one.27,28 Thus, it is not clear if the fluorescence is due to the parallel or antiparallel conformers. The fluorescence quantum yields of 1o, 2o, and 3o were measured in hexane at 320 nm excitation and found to be 0.042, 6.7 × 10−3, and 0.012, respectively, in comparison with that of anthracene as a standard (Φf = 0.29). These fluorescence quantum yields were in accord with to the cyclization quantum yields, but no large difference was observed in those values of the open-ring isomers. Theoretical Explanation of the Photochromic Properties. Previous theoretical study showed that normal- and reverse-type diarylethenes have different profiles on potential energy surfaces as shown in Figure 4. For the normal-type, there is a barrier on the S1 surface from closed- to open-ring isomer, whereas for the reverse-type, no barrier or only a very small barrier exists on S1 with a simple downhill profile.29 Asano et al. reported that the quantum yield of normal-type diarylethene depends on substituents, especially on those at the

Figure 4. Potential energy surface of normal- and reverse-type diarylethenes. 10975



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example, the closed-ring isomer 4c and 5c having cyano groups returned to the appropriate open-ring isomers 4o and 5o in 3.3 and 186 min at 60 °C, respectively (Scheme 3).2 The thermal stability of the closed-ring isomers of 1,2-bis(2-cyano-1benzothien-3-yl)perfluorocyclopentene 6o and 1,2-bis(2cyano-5-phenyl-3-thienyl)perfluorocyclopentene 7o having cyano groups at the reactive carbon atoms were also studied by Kobatake et al., who reported that the thermal cycloreversion (ring-opening) reactions of both isomers could not be observed even after 1 day at room temperature (Scheme 4).13 We observed the thermal stability of the closed-ring isomers of 1c−3c at room temperature and found no decoloration due to the cycloreversion reaction. Then we observed the stability at 70 °C in a hexane solution. The solution was kept at this temperature in the dark, and the absorbance at λmax of the closed-ring isomers was monitored. Figure 6 shows the decay curves of the absorbance of the closed-ring isomers. After 100 h heating, 10% of 3c reverted to 3o, and 20% of 1c reverted to 1o. The methyl-substituted 2c was the most thermally stable, due to the combination of the electron-donating characteristic and the smallest volume of substituents. In spite of introducing cyano groups, the closed-ring isomer 1c was relatively thermally stable compared to those of 4c and 5c. The absorption spectra of the thermally bleached solutions were converted to solutions of appropriate open-ring isomers upon visible-light irradiation, and then they reverted to the initial photostationary states upon UV-light irradiation to show the remaining photochromic reactivity. These results indicate that the thermal fading of the color is due to the thermal cycloreversion to the open-ring isomers. The thermal stability of the closed-ring isomers of diarylethenes is well explained by theoretical calculation, in which the stability of the closed-ring isomer is strongly dependent on the ground-state energy difference between the open-ring isomer and the closed-ring one, and the closed-ring isomer having a smaller energy difference than the open-ring isomer shows more thermal stability.31,32 Therefore, the order of calculated energy-difference 1 > 3 > 2 is in agreement with the order of thermal stability of the closed ring isomers 1c > 3c > 2c.

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CONCLUSIONS The photochromic properties of three reversed-type diarylethenes that have cyano, methyl, and methoxy groups at both reactive carbon atoms were compared. The absorption bands of the closed-ring isomers shifted to longer wavelengths with an increase in the electron-donating characteristic of the substituents on the reactive carbon atoms. The derivative having cyano groups shows the largest cycloreversion quantum yield (0.45), and this yield decreased with the increase in the donating characteristic of the substituents. These results reflect theoretical calculations showing that the closed-ring isomer with cyano groups has the highest energy level of the excited states. The substituent effects on the cycloreversion reaction were very similar to those observed for normal 1,2-bis(3thienyl)perfluorocyclopentenes.

Figure 5. Potential energy surface of ground state (solid line with dots) and excited state (S1) (dots): (a) molecule 1, (b) molecule 2, and (c) molecule 3. These values are obtained by optimization of the S0 and S1 states by fixing the distance between reactive carbon atoms. Geometries around 2.0 Å (near TS of S0) are not calculated because the TDDFT method is not appropriate (see also Figure S7 in Supporting Information for Franck−Condon states).

the experimental cyclization quantum yield Φoc is the smallest (0.17) for 1o, whereas those of 2o and 3o are similar and relatively larger (0.25 and 0.22, respectively). The profiles of potential energy surface from open- to closed-ring cyclization reaction thus provide an interpretation for the experimental Φoc. Although the state responsible for reaction and/or fluorescence is not straightforward,15 it remains consistent with additional experimental fluorescence quantum yield from the excited state of open-ring; 1o (4.2 × 10−2), 2o (6.7 × 10−3) and 3o (1.2 × 10−2). Thermal Stabilities of Closed Ring Isomers 1c−3c in Hexane. Some diarylethenes having strong electron-withdrawing groups destabilized the closed-ring isomers. For

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EXPERIMENTAL SECTION General Information. All chemicals were purchased from commercial sources (TCI, Wako) and used as received without further purification unless stated otherwise. All solvents used were reagent grade and were distilled before use. 1H NMR spectra were recorded at ambient temperature. Chemical shifts 10976



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Scheme 3. Thermally Unstable Photochromic Diarylethenes

carboaldehyde (8) (6.45 g, 57.6 mmol) and toluene (300 mL), pinacol (7.48 g, 63.3 mmol), and p-toluenesulfonic acid (300 mg) were added and refluxed for 3 h. After the reaction was over, the solvent was removed in vacuo. The residue was purified by silicagel chromatography (hexane:ethyl acetate = 97:3) to obtain 11.11 g of 9 in 91% yield as a yellow oil. 1 H NMR (400 MHz, CDCl3): δ 1.27 (6H, s), 1.30 (6H, s), 6.04 (1H, s), 7.17 (1H, dd, J = 5.2, 1.2 Hz), 7.29 (1H, dd, J = 5.2, 2.8 Hz), 7.39 ppm (1H, dd, J = 2.8, 1.2 Hz). 1,2-Bis[3-(4,4,5,5-tetramethyl-1,3-dioxo-2-yl)thiophen-2-yl]perfluorocyclopentene (10). In the argon gas atmosphere, compound 9 (3.41 g, 16.5 mmol) was dissolved into 150 mL of anhydrous ether and the flask was cooled in the ice bath at 5 °C. To the mixture was gradually added 11.3 mL (18.2 mmol) of 1.6 M n-BuLi hexane solution while the temperature was maintained. After addition was over, the mixture was allowed to warm to room temperature, followed by refluxing for 1.5 h. Then the mixture was cooled to 5 °C on an ice bath, and 0.4 mL of perfluorocyclopentene was added. After stirring for 30 min at the temperature, another 0.2 mL of perfluorocyclopentene was added. The procedure was repeated an additional three times every 30 min, and a total amount 1.2 mL of perfluorocyclopentene was added. After the reaction was monitored by TLC, water (10 mL) was added to the mixture at the temperature. Then 100 mL of ether and 100 mL of brine were added to the mixture, and the organic layer was separated. The water layer was extracted with ether (70 mL × 3), and the combined organic layer was dried over sodium sulfate anhydrous. After the filtration, ether was removed in vacuo. And the residue was purified by silica gel chromatography (hexane:ethyl acetate = 97:3) to obtain 2.15 g (3.6 mmol) of 10 in 45% yield as a yellow oil. 1 H NMR (400 MHz, CDCl3): δ1.05 (6H, s), 1.14 (6H, s), 5.05 (1H, s), 7.09 (1H, d, J = 5.6 Hz), 7.40 ppm (1H, d, J = 5.6 Hz). 1,2-Bis(3-formylthiophen-2-yl)perfluorocyclopentene (11). In a 50 mL flask, 10 (180 mg, 0.302 mmol) was dissolved in 20 mL of ethanol, and 12 M HCl (4 mL, 48 mmol) was added. The mixture was refluxed for 2.5 h. After the reaction was over, the mixture was allowed to cool to the ambient temperature and ethanol was evaporated in vacuo. To the mixture were added 40 mL of ether and 40 mL of 3 M HCl, and the organic layer was separated. The water layer was extracted with ether (2 × 40 mL). The organic layer was combined and was dried over sodium sulfate anhydrous. After

Scheme 4. Thermally Stable Photochromic Diarylethenes

Figure 6. Thermal decay of closed-ring isomers 1c (open circles), 2c (open triangles), and 3c (filled squares) at 70 °C in hexane solution.

are denoted in δ units (ppm) relative to the solvent signals CHCl3 (1H NMR: δ = 7.26 ppm. Mass spectra (FAB-MS) were obtained by using 3-nitrobenzyl alcohol as the matrix. UV light was irradiated by a hand lamp SPECTROLINE Model EB280C/J (λ: 313 nm). Visible light was irradiated by a 500 W Xe-arc lamp (USHIO SX-Ul501XQ). For the absorption spectral measurements, optical cells with 1 cm light path lengths were used for the absorption spectral measurement of the solutions. Synthetic procedure of compound 1 is shown in Scheme 5 and details are described below. 2-(3-Thienyl)-4,4,5,5-tetrametyl-1,3-dioxolane (9). In a 1000 mL flask attached with a Dean−Stark trap, thiphen-310977



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Scheme 5. Synthetic Procedure for a Reverse Switch of 1,2-Bis(3-cyanothiophen-2-yl)perfluorocyclopentene (1o)

the filtration, ether was removed in vacuo. And the residue was purified by silica gel chromatography (hexane:ethyl acetate = 4:1) to obtain 40 mg (0.1 mmol) of crude product of 11 in 34% yield as a yellow powder. Recrystallization from a mixture of hexane and dichloromethane gave 34.8 mg of 11 as pale yellow plates in 29% yield. Mp: 125 °C (sublimation). 1H NMR (400 MHz, CDCl3): δ 7.47 (d, 2H, J = 5.2 Hz), 7.56 (d, 2H, J = 5.2 Hz), 9.70 ppm (s, 2H). IR (KBr): 1686 cm−1 (CHO). MS (m/z): 396 (M+). Oxime Compound (12). 11 (61 mg, 0.154 mmol) was added to the mixture of ethanol (10 mL), NH2OH·HCl (0.021 g, 0.308 mmol), and 10% aqueous NaHCO3 (0.2 mL), and the resulting mixture was stirred for 2 h at room temperature. After the reaction was over, solvent was removed in vacuo. To the residure was added ether (40 mL) and water (40 mL), and the organic layer was separated. The water layer was extracted with ether (2 × 40 mL), and the organic layer was combined and was dried over sodium sulfate anhydrous. After the filtration, ether was removed in vacuo to obtain 60 mg of 12 in 91% yield as a pale yellow powder. The crude product was used for the next reaction witout purification. Mp: 153.3−154.0 °C. 1H NMR (400 MHz, CDCl3): δ 4.38− 4.64 (broad, 2H), 7.31 (d, 2H, J = 5.2 Hz), 7.53 (d, 2H, J = 5.2 Hz), 7.71 ppm (s, 2H). IR (KBr): 3341 cm−1 (OH), 1621 cm−1 (CN). MS (m/z): 425 ([M − H]+). 1,2-Bis(3-cyanothiophen-2-yl)perfluorocyclopentene (1o). 12 (116 mg, 0.272 mmol) was dissolved in 5 mL of acetic anhydride, and the solution was heated for 19 h at 120 °C. After the reaction was over, the reaction mixture was allowed to cool to room temperature. To the mixture was added 10 mL of water; then sodium bicarbonate was added for the neutralization. To the mixture were added ether (40 mL) and saturated aqueous sodium bicarbonate (100 mL), and the organic layer was separated. The water layer was extracted with ether (2 × 40 mL), and the organic layer was combined and was dried over sodium sulfate anhydrous. The residue was purified by silica gel chromatography (hexane:ethyl acetate = 4:1) to obtain 90 mg (0.23 mmol) of crude product of 1o in 85% yield as a yellow powder. Recrystallization from hexane gave 40 mg of 1o in 38% yield as pale yellow needles. 1o: mp 167.1−167.8 °C; 1H NMR (400 MHz, CDCl3) δ 7.32 (d, 2H, J = 5.2 Hz), 7.79 ppm (d, 2H, J = 5.2 Hz); IR (KBr) 2230 cm−1 (CN); MS (m/z) 390 (M+). Anal. Calcd for

C15H4F6N2S2: C, 46.16; H, 1.03; N, 7.18. Found: C, 46.22; H, 1.04; N, 7.26. Computational Details. All calculations were performed using density functional theory with the B3LYP33−35 exchange−correlation functional and the basis set was 631G(d)36−38 in Gaussian 09.39 The excitation energies were calculated by TD-B3LYP. The effect of hexane solvent was considered by the polarizable continuum model (PCM).40,41

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ASSOCIATED CONTENT

S Supporting Information *

1

H NMR and IR spectra of diarylethene 1o and the precursors 11 and 12, and the potential energy surface of ground state and excited state (Franck−Condon state) (S1); (a) molecule 1, (b) molecule 2, (c) molecule 3. This material is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +81-77-543-7462. Fax: +81-77-543-7483. E-mail: K.U., [email protected]; S.N., [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by Ryukoku University Science and Technology Fund. The authors are also grateful to Zeon Co., Ltd., for providing the perfluorocyclopentene.

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REFERENCES

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