Photochromism of new isomeric pyridine-containing diarylethenes with a benzofuran moiety

Dyes and Pigments 99 (2013) 812e821

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Photochromism of new isomeric pyridine-containing diarylethenes with a benzofuran moiety Hui Li, Gang Liu, Shouzhi Pu*, Bing Chen Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science & Technology Normal University, Nanchang 330013, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 10 May 2013 Received in revised form 27 June 2013 Accepted 29 June 2013 Available online 16 July 2013

Three new isomeric pyridine-containing diarylethenes with a benzofuran moiety were synthesized, and the structures of two diarylethenes were determined by single X-ray diffraction analysis. Their properties such as photochromism, acidchromism, fluorescence, as well as electrochemical properties, were investigated in detailed. The diarylethenes exhibited multi-addressable switching behavior by light and acid/base stimuli in solution and functioned as a remarkable fluorescent switch in both solution and poly(methyl methacrylate) films. Among these isomeric derivatives, the example with the nitrogen atom at the ortho-position of pyridine showed the largest absorption maxima, molar absorption coefficients, and cyclization quantum yield. The nitrogen atom position in the terminal pyridine unit could efficiently modulate the optical and electrochemical properties of the diarylethenes with a benzofuran moiety. The results suggested that the benzofuran moiety and the nitrogen atom position in the terminal pyridine unit played a vital role during the process of photoisomerization reaction for these isomeric diarylethenes. Ó 2013 Elsevier Ltd. All rights reserved.

Keywords: Photochromism Diarylethene Acidchromism Nitrogen atom position effect Pyridine Benzofuran

1. Introduction During the past several decades, reports on photochromic compounds have received increasing attention due to the widespread potential application in optical memories, fluorescence switches, as well as multi-color displays [1e5]. So far, various types of photochromic compounds have been developed in an attempt to satisfy the requirements of optoelectronic devices. Among such compounds, diarylethenes are one of the most promising candidates for the application because of their remarkable fatigue resistance, excellent thermal stability, and rapid response by stimulation of light and chemicals [1,2,6]. So far, related research has been mainly focused on the synthesis of new photochromic diarylethenes with different aryl moieties and substituents. Among the diarylethenes hitherto reported, most of the heteroaryl moieties have been thiophene or benzothiophene rings [1,2,5], with a few reports concerning other heteroaryl moieties, such as furan [7,8], thiazole [9,10], isoxazole [11], indole [12], indene [13], pyrazole [14], naphthalene [15], oxazole [16], pyrrole [17], etc. Benzofuran is an attractive aryl unit with low aromatic stabilization energy and its molecular structure is similar to that of benzothiophene, suggesting that introduction of the benzofuran

* Corresponding author. Tel./fax: þ86 791 83831996. E-mail address: [email protected] (S. Pu). 0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.dyepig.2013.06.036

ring into the diarylethene skeleton can be expected to undergo thermally irreversible photochromic reactions [18]. However, only few research on the symmetrical diarylethenes with benzofuran rings have been hitherto reported [18e21], reports on unsymmetrical diarylethenes with a benzofuran moiety are still rare [13,22]. In addition, some functional substituents were introduced into diarylethene molecules as side groups in order to further modify their photochromic properties [23,24]. Recently, the pyridinecontaining diarylethenes have attracted much attention because the pyridine ring has good aromaticity and biological activities [25e30]. The nitrogen atom of pyridine can be easily protonated or alkylated by external stimuli [31,32]. For example, Branda et al. reported a photocontrolled molecular switch based on a symmetrical dithienylethene with two terminal pyridine rings and found that it could be applied in living cells [27]. Yi et al developed a multi-responsive fluorescence switch based on terpyridinecontaining diarylethene and applied as a detector for the biological process of metal ion transmembrane transport [33]. The results reveal that pyridine-containing diarylethenes have unique physicochemical properties and can be potentially applied as the media of optoelectronic devices. Apart from the fact that the heteroaryl moieties and the substituent effects have an important role to play in modifying the properties of diarylethenes, the position of the substituent can also affect their properties significantly. To date, although many reports concerning the substituent position effects on the photochromic properties have

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been reported, the results mainly emphasis on effects of a certain same substituent linked with the different position of the terminal benzene ring on their photochromic properties [14,34,35]. To the best of our knowledge, the nitrogen atom position effect of pyridine ring on the properties of diarylethenes has been rarely reported [36,37]. In this work, we have synthesized a new class of isomeric pyridinecontaining diarylethenes with both benzofuran and thiophene moieties and mainly discussed the nitrogen atom position effect of pyridine ring on their physicochemical properties. The synthesized diarylethenes are 1-(2-ethyl-3-benzofuranyl)-2-[2-methyl-5-(2-pyri dyl)-3-thienyl]perfluorocyclopentene (1o), 1-(2-ethyl-3-benzofuran yl)-2-[2-methyl-5-(3-pyridyl)-3-thienyl]perfluorocyclopentene (2o), and 1-(2-ethyl-3-benzofuranyl)-2-[2-methyl-5-(4-pyridyl)-3-thien yl]perfluorocyclopentene (3o). The derivatives with nitrogen atom at ortho-, meta-, and para-position of the pyridine unit exhibited excellent photochromism and observable acidchromism in solution, and the photochromic scheme of diarylethenes 13 is shown in Fig. 1.

937444 for 1o and 937445 for 3o. Copies of the data can be obtained, free of charge, on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: þ44 0 1223 336033 or e-mail: [email protected]). Electrochemical examinations were performed in a onecompartment cell by using a Model 263 potentiostatgalvanostat (EG&G Princeton Applied Research) under computer control at room temperature. Platinum-electrodes (diameter 0.5 mm) served as working electrode and counter electrode. Platinum wire served as a quasi reference electrode. It was calibrated using the ferrocene (Fc/Fcþ) redox couple which has a formal potential E1/2 ¼ þ0.35 V versus platinum wire. The typical electrolyte was acetonitrile (5 mL) containing 0.1 mol/L tetrabutylammonium tetrafuoroborate ((TBA)BF4) and 4.0  103mol L1 diarylethene sample. All solutions were deaerated by bubbling with a dry argon stream and maintained at a slight argon overpressure during electrochemical experiments.

2. Experimental

2.2. Synthesis of diarylethene derivatives

2.1. General methods

The synthesis route for diarylethenes 1oe3o is shown in Fig. 2. 1(2-Ethyl-3-benzofuranyl)perfluorocyclopentene (4) was synthesized by the reported synthetic method [18]. Suzuki coupling brominated pyridine derivatives with thiophene boronic acid give pyridylthiophene derivatives (5aec) [37]. Finally, compounds 5aec were separately lithiated and then coupled with compound 4 to give diarylethenes 1oe3o, respectively. The structures of diarylethenes 1oe3o were confirmed by elemental analysis, NMR, and IR.

Absorption spectra were measured using an Agilent 8453 UV/vis spectrophotometer. NMR spectra were recorded on a Bruker AV400 (400 MHz) spectrometer with CDCl3 as the solvent and tetramethylsilane as an internal standard. Elemental analysis was carried out with a PE CHN 2400 analyzer. IR spectra were recorded on a Bruker Vertex-70 spectrometer. Fluorescence spectra were measured on an HITACHI 4600 fluorescence spectrophotometer. Photoirradiation was carried out with an SHG-200 UV lamp, a CX21 ultraviolet fluorescence analysis cabinet, and a BMH-250 visible lamp. Lights of appropriate wavelengths were isolated by different light filters. All solvents used were of spectro-grade and purified by distillation prior to use. Suitable crystals of 1o and 3o were obtained by slow evaporation of hexane solutions of the two diarylethenes. The size of the crystal 1o was 0.45  0.38  0.35 mm, and that of 3o was 0.33  0.31  0.25 mm. All the measurements were collected by a Bruker SMART APEX II CCD diffractometer using a MULTI scan technique at room temperature using Mo Ka radiation. The structures were solved by direct methods and refined by full-matrix least-squares procedures on F2 by full-matrix least-squares techniques using SHELXTL-97 program. Further details on the crystal structure investigation have been deposited with The Cambridge Crystallographic Data Centre as supplementary publication CCDC

F

1o : R =

F

F

N

F F F

F

2o : R = N

S

O

OH

UV

F

HN

F

R

NH

O

O

S

F

UV

S

R'

R

3c : R =

N

H

F

NH

N 2c : R = N

F

2o' : R' = 3o' : R' =

F

OH

F F

1c : R =

F F

Vis

H

F

1o' : R' =

F

F

N 3o : R =

2.2.1. 1-(2-Ethyl-3-benzofuranyl)-2-[2-methyl-5-(2-pyridyl)-3thienyl]perfluorocyclopentene (1o) To a stirred anhydrous THF (50 mL) of compound 5a (0.44 g, 2.0 mmol) was added dropwise a 2.5 mol/L n-BuLi/hexane solution (0.8 mL) at 195 K under argon atmosphere. After 30 min, THF (15 mL) containing compound 4 (0.71 g, 2.1 mmol) was added and the reaction mixture was stirred for 2 h at this temperature. The reaction was allowed to slowly warm to the room temperature and quenched by addition of water. The product was extracted with ether, dried with MgSO4, and evaporated in vacuo. The crude product was purified by column chromatography (petroleum ether/ acetyl acetate, v/v ¼ 5:1) to give 0.33 g diarylethene 1o as a light yellow solid in 33.5% yield. Calcd for C25H17F6NOS: Calcd C, 60.85; H, 3.47; N, 2.84. Found C, 60.83; H, 3.48; N, 2.82; Mp: 370e371 K; 1H NMR (400 MHz, CDCl3, ppm): d 1.00 (t, 3H, J ¼ 7.5 Hz, eCH3), 1.91 (s, 3H, eCH3), 2.41e2.47 (m, 2H, eCH2e), 7.17 (t, 1H, J ¼ 6.1 Hz,

F

F

1c' : R' =

F F

HN 2c' : R' = NH

Vis O

Fig. 1. Photochromism of diarylethenes 1e3.

S

R'

3c' : R' =

NH

814

H. Li et al. / Dyes and Pigments 99 (2013) 812e821

Br F

F

F

S 5a

N

n-BuLi, 195 K

S

O

1o

Br F

F

F

F

F F

F

F F

F

S

F

5b

F

N

F

N

F F F

n-BuLi, 195 K O

O

4

S

2o

N

Br

F

F

F

S 5c

F F

F

N

n-BuLi, 195 K

S

O

3o

N

Fig. 2. Synthetic route for diarylethenes 1e3.

phenyleH), 7.23 (t, 1H, J ¼ 7.4 Hz, phenyleH), 7.29 (t, 1H, J ¼ 7.3 Hz, pyridyleH), 7.43 (d, 1H, J ¼ 8.0 Hz, phenyleH), 7.50 (d, 1H, J ¼ 7.7 Hz, phenyleH); 7.57 (s, 1H, thienyleH), 7.60 (d, 1H, J ¼ 8.0 Hz, pyridyle H), 7.70 (t, 1H, J ¼ 7.6 Hz, pyridyleH), 8.54 (d, 1H, J ¼ 4.5 Hz, pyridyleH); 13C NMR (100 MHz, CDCl3, ppm): d 11.39, 15.00, 20.84, 104.34, 111.14, 118.48, 120.10, 122.27, 123.62, 123.81, 124.55, 125.50, 126.37, 136.75, 142.86, 144.21, 149.61, 151.57, 154.27, 160.40; IR (v, KBr, cm1): 752, 778, 803, 838, 892, 984, 1042, 1075, 1124, 1196, 1271, 1337, 1433, 1456, 1554, 1587, 1651, 2878, 2937, 2975.

1H, J ¼ 8.1 Hz, phenyleH), 7.47 (d, 1H, J ¼ 8.0 Hz, phenyleH), 7.48 (s, 1H, thienyleH), 8.59 (d, 2H, J ¼ 6.2 Hz, pyridyleH); 13C NMR (100 MHz, CDCl3, ppm): d 11.84, 15.32, 21.30, 104.63, 111.64, 120.00, 120.45, 124.12, 125.12, 125.38, 126.47, 126.74, 139.57, 140.72, 144.16, 150.98, 154.75, 160.81; IR (v, KBr, cm1): 750, 800, 814, 838, 894, 985, 1046, 1073, 1102, 1123, 1194, 1278, 1340, 1454, 1596, 1644, 2880, 2941, 2976.

2.2.2. 1-(2-Ethyl-3-benzofuranyl)-2-[2-methyl-5-(3-pyridyl)-3thienyl]perfluorocyclopentene (2o) Diarylethene 2o was prepared by a method similar to that used for 1o and 0.24 g of 2o obtained as a pale yellow solid in 24.3% yield. Mp: 349e350 K; Calcd for C25H17F6NOS: Calcd C, 60.85; H, 3.47; N, 2.84. Found C, 60.88; H, 3.46; N, 2.86; 1H NMR (400 MHz, CDCl3, ppm): d 0.99 (t, 3H, J ¼ 7.6 Hz, eCH3), 1.93 (s, 3H, eCH3), 2.41e2.48 (m, 2H, eCH2e), 7.23 (t, 1H, J ¼ 8.0 Hz, phenyleH), 7.30 (t, 1H, J ¼ 8.0 Hz, phenyleH), 7.32 (t, 1H, J ¼ 7.6 Hz, pyridyleH), 7.34 (s, 1H, thienyleH), 7.44 (d, 1H, J ¼ 8.0 Hz, phenyleH), 7.49 (d, 1H, J ¼ 8.0 Hz, phenyleH), 7.77 (d, 1H, J ¼ 8.0 Hz, pyridyleH), 8.51 (d, 1H, J ¼ 4.9 Hz, pyridyleH), 8.78 (s, 1H, pyridyleH); 13C NMR (100 MHz, CDCl3, ppm): d 11.40, 14.77, 20.85, 104.24, 111.18, 120.04, 123.65, 123.76, 124.63, 125.78, 126.51, 126.74, 129.40, 132.75, 138.41, 142.59, 146.74, 148.94, 154.28, 160.36; IR (v, KBr, cm1): 749, 802, 838, 893, 988, 1042, 1074, 1125, 1191, 1277, 1339, 1401, 1456, 1594, 1637, 1728, 2876, 2962.

3.1. Photochromism of diarylethenes 1e3

2.2.3. 1-(2-Ethyl-3-benzofuranyl)-2-[2-methyl-5-(4-pyridyl)-3thienyl]perfluorocyclopentene (3o) Diarylethene 3o was prepared by a method similar to that used for 1o and 0.35 g of 3o obtained as a brown solid in 35.5% yield. Calcd for C25H17F6NOS: Calcd C, 60.85; H, 3.47; N, 2.84. Found C, 60.86; H, 3.49; N, 2.81; Mp: 401e402 K; 1H NMR (400 MHz, CDCl3, ppm): d 1.00 (t, 3H, J ¼ 7.6 Hz, eCH3), 1.94 (s, 3H, eCH3), 2.42e2.47 (m, 2H, eCH2e), 7.23 (t, 1H, J ¼ 8.0 Hz, phenyleH), 7.30 (t, 1H, J ¼ 7.5 Hz, phenyleH), 7.38 (d, 2H, J ¼ 6.1 Hz, pyridyleH), 7.44 (d,

3. Results and discussion

Diarylethenes 1e3 were observed to undergo reversible photochromic reactions in both hexane (2.0  105 mol L1) and PMMA films (10% w/w) at room temperature. In hexane, the absorption spectral and color changes of 1e3 are shown in Fig. 3. Upon irradiation with 297 nm light, the colorless solution of 1o turned purple and a new visible absorption band centered at 544 nm (ε, 1.95  104 L mol1 cm1) emerged due to the formation of the closed-ring isomer 1c, while the original peak formed by the pep* transition at 306 nm (ε, 3.00  104 L mol1 cm1) decreased. The absorbance of the new absorption band increased with the increase of irradiation time till the photostationary state was reached. Alternatively, the purple solution was bleached completely back to colorless solution with visible light (l > 450 nm) irradiation and the original absorption spectrum was recovered. Just like 1, diarylethenes 2 and 3 showed similar photochromism with the absorption maxima at 531 and 533 nm, respectively. When arrived at the photostationary state, the isosbestic points of diarylethenes 1e3 in hexane were observed at 324 nm, 312 nm, and 316 nm, respectively. The absorption spectral and color changes of 1e3 in PMMA films are shown in Fig. 4. Upon irradiation with 297 nm light, the colors of diarylethene/PMMA films changed from colorless to purple for 1 and to magenta for both 2 and 3. Correspondingly, it could be easily seen that the absorption maxima of 1ce3c in PMMA films were observed at 551 nm, 542 nm, and 544 nm, respectively. As has been observed for most of the reported

H. Li et al. / Dyes and Pigments 99 (2013) 812e821

815

0.6

0.6

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Wavelength (nm)

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(B)

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UV

Vis

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0.30

Vis 0.15

1o

2o

3o

1c

2c

3c

0.00 300

400

500

600

700

Wavelength (nm)

(C)

(D)

Fig. 3. Absorption spectral and color changes of diarylethenes 13 upon alternating irradiation with UV and visible light in hexane (2.0  105 mol L1) at room temperature: (A) spectral changes for 1; (B) spectral changes for 2; (C) spectral changes for 3; (D) color changes for 1e3.

diarylethenes [34,38], the absorption maxima of 1e3 in PMMA films exhibited an evidently bathochromic shift, as compared with those in hexane. The red shifts of the absorption maxima of 1ce3c were 7 nm for 1c, 11 nm for 2c, and 11 nm for 3c. The red shift phenomenon may be attributed to more planar structure and the stabilization of molecular arrangement in solid polymer matrix [39,40]. The spectral properties of diarylethenes 1e3 are summarized in Table 1. The data revealed that the pyridyl and its substituted position had a significant effect on the photochromic properties of 1e3. For 1e3, the absorption maxima and molar absorption coefficients of the open-ring isomers increased in the order of para< meta- < ortho-substitution by the nitrogen atom in pyridine ring; However, the absorption maxima and molar absorption coefficients of the closed-ring isomers increased in the order of meta- < para- < ortho-substitution by the nitrogen atom in pyridine ring. As a result, the ortho-substituted derivative 1 has the longest absorption maximum in both hexane and a PMMA film and the largest molar absorption coefficients. The result is well consistent with that of the analogs with a benzothiophene moiety [37]. For diarylethenes 1e3, the cyclization quantum yields decreased in the order of 1 > 3 > 2, whereas the cycloreversion quantum yields decreased in the order of 2 > 3 > 1. Consequently, the ortho-substituted derivative 1 has the largest cyclization quantum yield (Fo-c, 1 ¼ 0.43) and the smallest cycloreversion quantum yield (Fc-o, 1 ¼ 0.046), and the meta-substituted derivative 2 has the smallest cyclization quantum yield (Fo-c, 2 ¼ 0.24) and the largest cycloreversion quantum yield (Fo-c, 2 ¼ 0.071). The result is quite different from that of the reported pyridinecontaining diarylethenes with an isoxazole or a benzothiophene moiety [36,37]. In addition, the photoconversion ratios from open-

ring to closed-ring isomers of 1e3 in the photostationary state were analyzed by HPLC in hexane, and the result is shown in Fig. 5. The photoconversion ratios of diarylethenes 1e3 were calculated with the value of 91% for 1, 81% for 2, and 85% for 3 in the photostationary sate. The thermal stability of the open-ring and closed-ring isomers for diarylethenes 1e3 and their N-protonated derivatives 10 e30 (Fig. 1) was tested in hexane at both room temperature and 341 K. At room temperature, exposure of the solutions of 1e3 and 10 e30 in hexane to air for more than 30 days in darkness caused no changes in color and absorption spectra. When these hexane solutions were heated under reflux (341 K) for more than 4 h in darkness, no decomposition was detected by UV/vis absorption and NMR spectroscopy. The result indicated that the pyridine-containing diarylethene derivatives exhibited good thermally irreversible photochromism. Fatigue-resistant property is a crucial factor for practical applications in optical devices [1,41]. The fatigue resistance of diarylethenes 1e3 was examined in both hexane (2.0  105 mol L1) and PMMA films (10%, w/w) by alternating irradiating with 297 nm UV (radiation energy: 25 mW/cm2) and visible light (l > 450 nm) in air at room temperature, and the result is depicted in Fig. 6. In hexane, the coloration and decoloration cycles of 1e3 could repeat 100 times with only ca. 2% degradation of 1c, 8% degradation of 2c, and 4% degradation of 3c. The degradation may be ascribed to the formation of an epoxide [42]. Compared to that in solution, the fatigue resistance of the three compounds is much stronger in PMMA films. After 200 repeat cycles, these diarylethenes still showed favorable photochromism with only ca. 4% degradation of 1c, 8% degradation of 2c, and 5% degradation of 3c. The remarkable fatigue resistance of diarylethenes 1e3 is very useful for rewritable

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H. Li et al. / Dyes and Pigments 99 (2013) 812e821

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700

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Fig. 4. Absorption spectral and color changes of diarylethenes 13 by photoirradiation in PMMA films (10%, w/w) at room temperature: (A) spectral changes for 1; (B) spectral changes for 2; (C) spectral changes for 3; (D) color changes for 1e3.

optical memories or photo-switches [1e3]. As mentioned above, it could be easily concluded that the fatigue resistance of 1 and 3 were much stronger than that of 2 in both solution and PMMA films. The result indicated that the nitrogen atom position of the terminal pyridine ring had a notable effect on the fatigue resistance of these isomeric diarylethenes. To know better the relation between the conformation and the photochromic behaviors of the pyridine-containing diarylethenes in the crystalline phase, the final structural confirmations of diarylethenes 1o and 3o were provided by X-ray crystallographic analysis. The ORTEP drawings of the two crystals and the photochromic process in the crystalline phase are shown in Fig. 7, and the X-ray crystallographic analysis data are listed in Table 2. Both of the two molecules crystallize with an appropriate C2 symmetry with photoactive anti-parallel conformation, which can be expected to undergo photocyclization in the crystalline phase [43e45]. For diarylethene 1o, the dihedral angles between the hexafluorocyclopentene ring and the two adjacent heteroaryl rings are

62.1 for O1/C16eC23 and 43.4 for S1/C6eC9, and that between the thiophene ring and the linked pyridine ring is 10.5 . The distance between the two reactive C atoms (C9 and C23) is 3.742 Å. For diarylethene 3o, the dihedral angles between the hexafluorocyclopentene ring and the two adjacent heteroaryl rings are 52.2 for O1/C16eC23 and 50.7 for S1/C6eC9, and that between the thiophene ring and the linked pyridine ring is 30.8 . The distance between the two reactive C atoms (C9 and C23) is 3.681 Å. In fact, crystals of 1o and 3o showed good photochromism, in accordance with the expected ring closure, to form 1c and 3c upon irradiation with UV light in the crystalline phase (Fig. 7C). The two colorless crystals turned magenta upon irradiation with 297 nm UV light. When the colored crystals were dissolved separately in hexane, an intense absorption band was observed at the same wavelength as that of their respective closed-ring isomer in solution. Alternatively, the magenta-colored crystals returned to colorless upon irradiation with the appropriate visible light (l > 450 nm). In the crystalline phase, the two diarylethenes also exhibited

Table 1 Absorption spectral properties of diarylethenes 13 in hexane (2.0  105 mol L1) and in PMMA films (10%, w/w) at room temperature. Compd

1 2 3 a b c

lo,max/nma (ε/L mol1 cm1)

lc,max/nmb (ε/L mol1 cm1)

Fc

Hexane

PMMA film

Hexane

PMMA film

Fo-c

Fc-o

306 (3.00  104) 296 (2.75  104) 295 (2.49  104)

308 300 299

544 (1.95  104) 531 (1.54  104) 533 (1.93  104)

551 542 544

0.43 0.24 0.40

0.046 0.071 0.064

Absorption maxima of open-ring isomers. Absorption maxima of closed-ring isomers. Quantum yields of open-ring (Fo-c) and closed-ring isomers (Fc-o), respectively.

Conversion at PSS in hexane (%) 91 81 85

H. Li et al. / Dyes and Pigments 99 (2013) 812e821

Initial state PSS state

817

(A) 1.0

1o A/A0 (%)

0.9

1c

4

6

8

10

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0.8

0.7

2o 0.6

2c

0

20

40

60

80

100

Repeat Cycles

6

7

8

9

10

11

(B) 1.0

3o

6

7

8

9

10

11

12

T/min Fig. 5. The photoconversion ratios of diarylethenes 13 in the photostationary state in hexane by HPLC analysis upon irradiation with 297 nm light.

remarkable fatigue resistance greater than 200 cyclization/cycloreversion repeat cycles and their closed-ring isomers remained stable for more than 150 days in darkness at room temperature. Therefore, they could be potentially used for the construction of certain optoelectronic devices [46]. 3.2. Acidchromism of diarylethenes The multiple switching characteristic of 1e3 was discussed by acid/base and light stimuli. In acetonitrile, the acidchromism was observed by the addition of trifluoroacetic acid to the solution of 1oe3o, which produced N-protonated diarylethenes 1o0 e3o0 (Fig. 1). Diarylethenes 1o0 e3o0 also performed photoisomerization upon alternating irradiation with UV and visible light. The color changes among 1e3 and their N-protonated forms 10 e30 induce by acid/base are illustrated in Fig. 8, and the absorption spectral changes of their closed-ring isomers in the photostationary state are presented in Fig. 9. Addition of trifluoroacetic acid in acetonitrile to the solution of 1o, the absorption maximum bathochromically shifted from 306 nm to 343 nm due to the formation of 1o0 (ε, 2.77  104 L mol1 cm1). 1o0 could be converted back into 1o by neutralization with triethylamine. For diarylethene 1o0 , upon irradiation of with 313 nm UV light, the absorption band at 343 nm decreased and a new visible absorption band centered at 568 nm (ε, 1.75  104 L mol1 cm1) emerged due to the formation of the closed-ring isomer 1c0 . This process was accompanied by a color change from colorless to violet. The violet-colored solution was bleached completely upon irradiation with visible light (l > 450 nm) and the original absorption spectrum was recovered. It should be noted here that a reversible transformation between the colored diarylethenes 1c and 1c0 could be performed by the stimulation of acid/base. Addition of trifluoroacetic acid to the solution of 1c produced the protonated 1c0 whose absorption maximum shifted from 544 to 568 nm (Fig. 9), and this process was accompanied by a notable color change of solution from

A/A0 (%)

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0

40

80

120

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200

Repeat Cycles Fig. 6. Fatigue resistance of diarylethenes 1e3 in air atmosphere at room temperature: (A) in hexane; (B) in PMMA films. Initial absorbance of the sample was fixed to 1.0.

purple to violet (Fig. 8). Diarylethene 1c0 could be converted back into 1c by neutralization with triethylamine. As with 1, diarylethenes 2 and 3 showed similar acidchromism as observed in acetonitrile by acid/base stimuli. The absorption maxima of the protonated diarylethenes 2o0 and 3o0 were observed at 308 nm (ε, 2.73  104 L mol1 cm1) and 341 nm (ε, 3.24  104 L mol1 cm1), respectively. Upon irradiation with UV light, the colorless solutions turned magenta for 2o0 and blue for 3o0 due to the formation of Nprotonated closed-ring isomers 2c0 and 3c0 , which the absorption maxima were observed at 543 nm for 2c0 and 583 nm for 3c0 . Compare to 2c and 3c, the absorption maxima of 2c0 and 3c0 exhibited an evident red shift with the value of 12 nm for 2c0 and 50 nm for 3c0 . The result is well consistent with that of the reported pyridine-containing diarylethene derivatives [36,37,47,48]. In addition, the effect of intermediate acid concentration on the absorption spectra was investigated by the reported method [49]. The result showed that the absorbance of these diarylethenes had no obvious change with intermediate acid concentration, but their absorption maxima showed minor red shift. 3.3. Fluorescence of diarylethenes So far, the fluorescence properties of many diarylethenes have been extensively reported [29,46,48]. Herein, the fluorescence properties of 1e3 were evaluated in both hexane and PMMA films at room temperature. The fluorescence spectra of diarylethenes 1oe3o (2.0  105 mol L1) in hexane and PMMA films (10%, w/w) at room temperature are shown in Fig. 10. The emission peaks of diarylethenes 1oe3o in hexane were observed at 399 nm for 1o,

818

H. Li et al. / Dyes and Pigments 99 (2013) 812e821

Fig. 7. ORTEP drawings of crystals 1o and 3o and their color changes by photoirradiation in the single crystalline phase: (A) ORTEP drawing of 1o, (B) ORTEP drawing of 3o, (C) color changes for crystals 1 and 3.

400 nm for 2o, and 400 nm for 3o when excited at 312 nm, while those in PMMA films were observed at 415 nm for 1o, 409 nm for 2o, and 408 nm for 3o when excited at 271 nm. Compared to those in hexane, the emission peaks of 1oe3o in PMMA films showed an evident bathochromic shift with values of 16 nm for 1o, 9 nm for 2o, and 8 nm for 3o. By using anthracene as a reference, the fluorescence quantum yields of 1oe3o were determined to be 0.0075, 0.0084, and 0.0086, respectively. The para-substituted derivative 3o has the largest fluorescence quantum yield, as compared with other two derivatives. The results suggested that the nitrogen atom

position of the terminal pyridine ring had a significant effect on the fluorescence properties of these isomeric diarylethene derivatives. As has been observed for most of diarylethenes [50e52], the emission intensity of the isomeric pyridine-containing diarylethene derivatives can reversibly change during the process of photoisomerization in both hexane and PMMA films. When irradiated by 297 nm UV light, the emission intensity of 1oe3o was decreased evidently in both hexane and PMMA films due to the formation of the weak fluorescent closed-ring isomers 1ce3c. The back irradiation by appropriate visible light regenerated the

Table 2 Crystal data for diarylethenes 1o and 3o.

Formula Formula weight Temperature Crystal system Space group Unit cell dimensions a (Å) b (Å) c (Å) a (o) b (o) g (o) Volume (Å3) Z Reflections collected Reflections observed Number of parameters m (mm1) Density (Calcd.) (g/cm3) Goodness-of-fit on F2 Radiation (Å) Final R1[I > 2s(I)] wR2[I > 2s(I)] R1 (all data) wR2 (all data)

1o

3o

C25H17F6NOS 493.46 296(2) Triclinic P-1

C25H17F6NOS 493.46 296(2) Triclinic P-1

10.1497(14) 19.038(3) 22.847(3) 90.00 90.00 90.00 4414.8(11) 8 32,036 4104 309 0.216 1.485 0.984 0.71073 0.0713 0.1335 0.1058 0.1515

9.8230(18) 10.827(2) 11.108(2) 99.584(2) 104.421(2) 95.696(2) 1115.8(4) 2 8625 4117 309 0.214 1.469 1.045 0.71073 0.0709 0.1777 0.0994 0.1985

Fig. 8. Color changes of diarylethenes 13 and 10 e30 in acetonitrile by light and acid/ base stimuli.

H. Li et al. / Dyes and Pigments 99 (2013) 812e821

0.2 0.0

Absorbance

300

400

500

600

700

800

0.4

Emission Intensity (a.u.)

(A) 4500

1c 1c'

0.4

819

Vis UV

3600 2700 1800 900 0

0.2

360

300

400

500

600

700

800

0.6 3c 3c'

0.4 0.2 0.0

300

400 500 600 Wavelength (nm)

700

800

Fig. 9. The absorption changes between 1c3c and 1c0 e3c0 by acid/base stimuli in the photostationary state.

400

440

480

Wavelength (nm)

(B) Emission Intensity (a.u.)

0.0

6000

Vis UV

5000 4000 3000 2000 1000 0

350

400

450

500

Wavelength (nm)

(A)

4500

Emission Intensity (a.u.)

Fig. 11. Emission intensity changes of diarylethene 1 by photoirradiation at room temperature: (A) in hexane (2.0  105 mol L1); (B) in a PMMA film (10%, w/w).

3600

1o 2o 3o

2700 1800 900 0 360

400

440

480

520

Wavelength (nm)

Emission Intensity (a.u.)

(B)

3o

6000

1o 2o

4500

open-ring isomers 1oe3o and recovered the original emission intensity. This characteristic can be potentially applied in molecularscale optoelectronics and photoswitchable probe for imaging living cells [29]. As shown in Fig. 11, the emission intensity of 1 was quenched to ca. 31% in hexane and 29% in a PMMA film in the photostationary state. Similarly, the emission intensity of 2 and 3 in the photostationary state was quenched to ca. 27% for 2o and 17% for 3o in hexane, and that in PMMA films was quenched to ca. 33% for 2o and 29% for 3o. The residues of fluorescence for diarylethenes 1e3 in the photostationary state may be attributed to the incomplete cyclization reaction and the existence of parallel conformations [35]. The results indicate that diarylethenes 1e3 exhibited relatively high fluorescence modulation efficiency in both solution and solid media by photoirradiation. Therefore, these diarylethenes can be potentially suitable for using as optical memory with fluorescence readout method or a fluorescence photoswitch [53,54]. 3.4. Electrochemical properties of diarylethenes

3000

1500

350

400

450

500

Wavelength (nm) Fig. 10. Emission spectra of diarylethenes 1e3 in both hexane solution (2.0  105 mol L1) and PMMA films (10%, w/w) at room temperature: (A) in hexane, excited at 312 nm; (B) in PMMA films, excited at 271 nm.

The photochromic reaction of diarylethene compounds can be initiated not only by light irradiation but also by an electrochemical or chemical redox process [55,56]. In the past decades, the electric properties of photochromic molecules have been extensively reported [54,57e62]. In order to investigate the nitrogen atom position effect of pyridine ring on the electrochemical properties of diarylethenes 1e3, the electrochemical tests were carried out by the linear sweep method under the same experimental conditions at a scanning rate of 50 mV/s. The anodic polarization curves of diarylethenes 1e3 are shown in Fig. 12. The oxidation onsets of the open-ring isomers 1oe3o were initiated at 1.29 V, 1.30 V, and

820

H. Li et al. / Dyes and Pigments 99 (2013) 812e821

240

30

3o 3c

20

Current (μΑ)

180

Advantage Sci-Tech Innovative Team (20113BCB24023), the Youth Science Funds of Natural Science Foundation of Jiangxi Province (20122BAB213004), and the Project of the Science Funds of Jiangxi Education Office (KJLD12035, GJJ11026, GJJ12587).

10

120

0 1.0

1.2

1.4

1.6

1o 1c

60

2o 2c

0 0.8

1.2

1.6

2.0

Potential (V) Fig. 12. The anodic polarization curves of diarylethenes 13.

1.52 V, and those of the closed-ring isomers 1ce3c were initiated at 1.02 V, 1.03 V, and 1.14 V, respectively. The differences of the oxidation potentials between the open-ring and closed-ring isomers of diarylethenes 1e3 (DVo-c) were 0.27 V for 1, 0.27 V for 2 and 0.38 V for 3. The results showed that the oxidation process of the open-ring isomers at a higher potential, as compared with that of the corresponding closed-ring isomers. The result is entirely contrary to that of diarylethenes with an indole moiety [63]. The possible reason may be ascribed to the effect of different aromatic heteroaryl moiety on the electrochemical properties. This is in accordance with the theory that the longer conjugation length of the closed-ring isomer generally leads to a less positive potentials [64,65]. For the closed-ring isomers, the p-electrons delocalize throughout the two condensed aryl moieties and extend to the substituents causing a lower oxidation onset. On the other hand, the oxidation onsets increased in order of ortho- < meta- < parasubstitution by the nitrogen atom in pyridine ring. Two oxidation processes can be observed during anodic oxidation of the three diarylethene derivatives. This phenomenon may be attributed to the different electron mobility resulted from the different replace position of the nitrogen atoms in the pyridine ring. The results indicated that the nitrogen atom position in the terminal pyridine ring had a great effect on the electrochemical behaviors of these isomeric diarylethenes with a benzofuran moiety. 4. Conclusions In summary, a new class of unsymmetrical isomeric diarylethenes containing a benzofuran and a pyridylthiophene were synthesized to investigate the effects of nitrogen atom position on their multiple switching characteristics by light and acid/base stimuli. The isomeric compounds exhibited favorable photochromism with good thermal stability and fatigue resistance and functioned as a notable fluorescence switch in both solution and PMMA films. Upon alternating stimulation with acid/base, the absorption spectra of these isomeric derivatives showed remarkable bathochromic shifts accompanied with distinguished color change in acetonitrile. The results demonstrated that a multi-addressable photoswitch with simplicity and efficiency could be achieved based on the tunable behaviors of isomeric pyridine-containing diarylethenes with a benzofuran moiety. Acknowledgements This work was supported by the National Natural Science Foundation of China (21162011, 21262015), the Project of Jiangxi

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