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Photochromic dithienylethene-branched triptycene hybrids
Dyes and Pigments 121 (2015) 227e234
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Photochromic dithienylethene-branched triptycene hybrids Xunzhi Cai, Lan Zhu, Shumin Bao, Qianfu Luo* Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science and Technology, Shanghai 200237, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 February 2015 Received in revised form 19 May 2015 Accepted 21 May 2015 Available online 30 May 2015
Dithienylethene and triptycene both have controllable structure and versatile performances. In this study, a series of triptycene hybrids decorated different number of dithienylethene units have been developed for photochromic switch. The synthetic process is facile and efficient and all new compounds were clearly characterized by 1H NMR, 13C NMR and HRMS. Their photochromism and spectroscopic properties have been investigated in dichloromethane and tetrahydrofuran. The result revealed that the multi-dithienylethene units grafted to triptycene behaved independently and showed distinctive solvent-dependent photochromism in chloride solvents, which probably due to the introduction of controlled spatial arrangement of triptycene core. These distinctive characteristics might be potential used for high-density multicomponent switches and detection of chloride solvents. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Triptycene Multi-dithienylethene Photochromic Solvent-dependent Switch Synthesis
1. Introduction Dithienylethene-type (DTE) photochromic materials have been extensively investigated because of their excellent properties and versatile capacity to function as various applications such as photo-switchable molecular device, multi-controllable optoelectronic units, optical memory storage materials, photocontrollable gas generator, biological imaging and intelligent materials [1e9]. However, sole dithienylethene system is often difficult to meet the various needs of practical application and manufacturing process [2,10]. In order to store more data and cater to different functions, a variety of multi-component molecular systems based on the combination of dithienylethene and other functional molecules have attracted great interesting [11e16]. On the other hand, triptycene and its derivatives possess rigid three-dimensional frameworks and 120 C orientation which constitute a useful linker group for multichromophore assemblies, and thus have been developed more and more applications as diverse as host-guest chemistry [17], supramolecular chemistry [18], materials science and so on [19e21]. They are always appealing partners for multi-functional design. In view of these, we became interested in constructing different dithienylethene-triptycene hybrids for photochromic
* Corresponding author. E-mail address: [email protected] (Q. Luo). http://dx.doi.org/10.1016/j.dyepig.2015.05.023 0143-7208/© 2015 Elsevier Ltd. All rights reserved.
switches and then synthesized a series of dithienylethene-linked triptycene complexes, in which a three-dimensional triptycene framework was chosen as central template and different number of dithienylethene units as branches, as shown in Scheme 1. The structures of the new hybrids were clearly confirmed and their photochromic performances were thoroughly investigated. Results show these multicomponent photochromic hybrids display distinctive performances.
2. Experiment 2.1. General information All chemicals were obtained commercially and used as received unless otherwise mentioned. 1H NMR spectra were recorded at 400 MHz and 13C NMR spectra were recorded at 101 MHz using CDCl3 as a solvent and tetramethylsilane as an internal standard. The UV light for irradiation was generated by LED sources. All mass spectrometric analyses were performed on ThermoStar™ mass spectrometer. Absorption spectra were recorded on a Varian Cary 500 using quartz cuvette at room temperature. For the absorption spectral measurements, optical cells with 1 cm light path lengths were used for the absorption spectral measurement of the solutions. The quantum yields of photochromic ring-cyclizaiton of compounds 1e6 were determined by means of literature [22].
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Scheme 1. The synthetic routes of the target compounds.
2.2. Synthetic procedures of the target compounds 2.2.1. Synthesis of 2-{3-[2-(5-phenyl-2-methylthien-3-yl)cyclopenteneyl]-2-methylthien-5-yl} triptycene (2) To a stirred solution of compound 10 (91 mg, 0.24 m mol) in anhydrous THF (5 mL), Pd(PPh3)4 (40 mg, 0.03 m mol) was added, and the resulting solution was stirred for 15 min at room temperature. Then anhydrous K2CO3 (0.38 g, 2.7 m mol) and distilled water (2.5 mL) were added. This mixture was heated in an oil bath just below reflux at a temperature of 60 C and the mixed solution of compound 9 was added dropwise via a syringe in a short time period of approximately 5 min. The reaction solution was then stirred at 110 C for 4 h and cooled to room temperature, after which H2O (10 mL) were added and extracted with CH2Cl2 (3 15 mL). The organic layer was separated and subsequently dried over anhydrous sodium sulfate. The sodium sulfate was filtered off, and the solvent was removed by evaporation under vacuum. The residue was purified by silica gel chromatography with dichloromethane/petroleum ether (1/4) as eluent to afford pure white solid 54 mg, yield 38.2%; 1H NMR (400 MHz, CDCl3, ppm): d ¼ 7.52-7.50 (m, 2H), 7.48 (s, 1H), 7.38-7.36 (m, 4H), 7.357.31 (m, 3H), 7.22 (t, J ¼ 8.0 Hz, 1H), 7.11 (dd, J ¼ 7.6, 1.6 Hz, 1H), 7.02 (s, 1H), 7.00-6.98 (m, 4H), 6.94 (s, 1H), 5.40 (d, J ¼ 3.2 Hz, 2H), 2.842.80 (m, 4H), 2.08-2.05 (m, 2H), 1.95 (s, 3H), 1.94 (s, 3H). 13C NMR (101 MHz, CDCl3, ppm): d ¼ 145.96, 145.02, 144.98, 144.16, 139.76, 139.62, 136.72, 136.51, 134.72, 134.55, 134.53, 134.09, 131.72, 128.83,
127.00, 125.31, 125.26, 124.01, 123.88, 123.83, 123.69, 123.60, 122.25, 121.00, 54.10, 53.69, 38.46, 34.71, 31.61, 23.01, 14.49, 14.41. HRMS (ESI): m/z [C41H32S2] calcd for: [MþH]þ: 589.2024, found 589.2022. 2.2.2. Synthesis of 2,6-bis{3-[ 2-(5-chloro-2-methylthien-3-yl)cyclopenteneyl]-2-methylthien-5-yl} triptycene (3) To a stirred solution of compound 11 (0.21 g, 0.40 m mol) in anhydrous THF (5 mL), Pd(PPh3)4 (60 mg, 0.05 m mol) was added, and the resulting solution was stirred for 15 min at room temperature. Then anhydrous K2CO3 (0.38 g, 2.7 m mol) and distilled water (2.5 mL) were added. This two-phase system was heated in an oil bath just below reflux at a temperature of 60 C and the mixed solution of compound 8 was added dropwise via a syringe in a short time period of approximately 3 min. The reaction was then stirred at 110 C overnight and cooled to room temperature, after which H2O (10 mL) were added and extracted with CH2Cl2 (3 15 mL), The organic layer was separated and subsequently dried over anhydrous sodium sulfate. The sodium sulfate was filtered off, and the solvent was removed by evaporation under vacuum. The residue was purified by silica gel chromatography with dichloromethane/petroleum ether (1/4) as eluent to afford pure white solid 0.22 g, yield 66%; 1H NMR (400 MHz, CDCl3, ppm): d ¼ 7.55 (d, J ¼ 1.5 Hz, 2H), 7.40 (m, 2H), 7.34 (d, J ¼ 7.7 Hz, 2H), 7.13 (dd, J ¼ 7.6, 1.7 Hz, 2H), 7.01 (dd, J ¼ 5.1, 3.5 Hz, 2H), 6.91 (s, 2H), 6.61 (s, 2H), 5.44 (s, 1H), 5.41 (s, 1H), 2.80-2.71 (m, 8H), 2.08-1.99 (m, 4H), 1.95 (s, 6H), 1.82 (s, 6H). 13C NMR (101 MHz, CDCl3, ppm): d ¼ 145.70,
X. Cai et al. / Dyes and Pigments 121 (2015) 227e234
144.80, 144.71, 144.03, 140.02, 136.34, 135.41, 135.22, 134.20, 133.76, 133.36, 131.80, 126.90, 125.58, 125.47, 125.05, 123.99, 123.85, 123.72, 122.49, 121.13, 54.21, 53.43, 38.53, 23.02, 14.50, 14.28. HRMS (ESI): m/z [C50H40Cl2S4]: calcd for [MþH]þ: 839.1468, found 839.1480. 2.2.3. Synthesis of 2,6-bis{3-[ 2-(5-phenyl-2-methylthien-3-yl)cyclopenteneyl]-2-methylthien-5-yl} triptycene (4) Compound 11 (0.21 g, 0.40 m mol) and Pd(PPh3)4 (60 mg, 0.05 m mol) were dissolved in anhydrous THF (5 mL). Anhydrous K2CO3 (0.38 g, 2.7 m mol) and distilled water (2.5 mL) were added, and the mixture was stirred at rt for 10 min. After heated to 60 C, the THF solution of compound 9 was added dropwise. The reaction was then stirred at 110 C overnight and cooled to room temperature, after which H2O (10 mL) were added and extracted with CH2Cl2 (3 15 mL), The organic layer was separated and subsequently dried over anhydrous sodium sulfate, and the solvent was removed by evaporation under vacuum. Purification by silica gel chromatography with dichloromethane/petroleum ether (1/4) as eluent to afford pure white solid 74 mg, yield 40.1%; 1H NMR (400 MHz, CDCl3, ppm): d ¼ 7.52-7.50 (m, 4H), 7.48 (s, 2H), 7.38-7.36 (m, 2H), 7.33 (t, J ¼ 7.2 Hz, 6H), 7.22 (t, J ¼ 7.6 Hz, 2H), 7.12 (dd, J ¼ 8.0, 2.0 Hz, 2H), 7.02 (s, 2H), 7.01-6.99 (m, 2H), 6.94 (s, 2H), 5.38 (s, 2H), 2.84-2.81 (m, 8H), 2.09-2.05 (m, 4H), 1.95 (s, 6H), 1.94 (s, 6H). 13C NMR (101 MHz, CDCl3, ppm): d ¼ 145.69, 144.82, 143.88, 139.78, 139.59, 136.78, 136.50, 134.70, 134.57, 134.53, 134.12, 131.85, 128.81, 126.94, 125.38, 125.29, 123.99, 123.91, 123.86, 123.64, 122.32, 120.95, 53.66, 38.38, 31.58, 29.68, 27.00, 25.68, 23.10, 22.64, 14.40, 14.15. HRMS (ESI): m/z [C62H50S4] calcd for [MþH]þ: 923.2874, found 923.2877. 2.2.4. Synthesis of 2,6,14-tris{3-[ 2-(5-chloro-2-methylthien-3-yl)cyclopenteneyl]-2-methylthien-5-yl} triptycene (5) To a stirred solution of compound 12 (0.21 g, 0.33 m mol) in anhydrous THF (5 mL), Pd(PPh3)4 (80 mg, 0.07 m mol) was added, and the resulting solution was stirred for 15 min at room temperature. Then anhydrous K2CO3 (0.38 g, 2.7 m mol) and distilled water (2.5 mL) were added. This two-phase system was heated in an oil bath just below reflux at a temperature of 60 C and the mixed solution of compound 8 was added dropwise via a syringe in a short time period of approximately 3 min. The reaction was then stirred at 110 C for 48 h and cooled to room temperature, after which H2O (10 mL) were added and extracted with CH2Cl2 (3 10 mL), The organic layer was separated and subsequently dried over anhydrous sodium sulfate. The sodium sulfate was filtered off, and the solvent was removed by evaporation under vacuum. The residue was purified by silica gel chromatography with dichloromethane/ petroleum ether (1/6) as eluent to afford pure white solid 0.10 g, yield 27%; 1H NMR (400 MHz, CDCl3, ppm): d ¼ 7.54 (t, J ¼ 7.0 Hz, 3H), 7.36-7.32 (m, 3H), 7.18 -7.10 (m, 3H), 6.90 (s, 3H), 6.60 (s, 3H), 5.42 (d, J ¼ 6.9 Hz, 2H), 2.75 (m, 12H), 2.09-1.98 (m, 6H), 1.95 (s, 9H), 1.81 (s, 9H). 13C NMR (101 MHz, CDCl3, ppm): d ¼ 145.43, 143.69, 139.96, 136.36, 135.41, 135.22, 134.26, 133.79, 133.37, 131.98, 126.90, 125.06, 124.05, 123.79, 122.60, 121.12, 53.58, 38.57, 38.49, 31.74, 29.85, 23.04, 22.81, 21.22, 14.51, 14.35, 14.28. HRMS (ESI): m/z [C65H53Cl3S6] calcd for [MþH]þ: 1131.1615, found 1131.1611. 2.2.5. Synthesis of 2,6,14-tris{3-[ 2-(5-phenyl-2-methylthien-3-yl)cyclopenteneyl]-2-methylthien-5-yl} triptycene (6) Compound 12 (0.21 g, 0.33 m mol) and Pd(PPh3)4 (80 mg, 0.07 m mol) were dissolved in anhydrous THF (5 mL). Then anhydrous K2CO3 (0.38 g, 2.7 m mol) and distilled water (2.5 mL) were added, and the mixture was stirred at rt for 10 min. The solution was heated to 60 C, then the THF solution of compound 9 was added dropwise. The reaction was then stirred at 110 C for 48 h and cooled to room temperature, after which H2O (10 mL) were
229
added and extracted with CH2Cl2 (3 15 mL), The organic layer was separated and subsequently dried over anhydrous sodium sulfate, and the solvent was removed by evaporation under vacuum. Purification by silica gel chromatography with dichloromethane/petroleum ether (1/5) as eluent to afford pure white solid 96 mg, yield 23.2%; 1H NMR (400 MHz, CDCl3, ppm): d ¼ 7.51-7.50 (m, 6H), 7.48 (s, 3H), 7.33 (t, J ¼ 7.6 Hz, 9H), 7.22 (t, J ¼ 3.2 Hz, 3H),7.13 (dd, J ¼ 7.6, 1.6 Hz, 3H), 7.02 (s, 3H), 6.94 (s, 3H), 5.35 (d, J ¼ 6.4 Hz, 2H), 2.842.81 (m, 12H), 2.09-2.05 (m, 6H), 1.95 (s, 9H), 1.94 (s, 9H). 13C NMR (101 MHz, CDCl3, ppm): d ¼ 144.26, 142.47, 138.63, 138.55, 135.65, 135.46, 133.66, 133.54, 133.49, 133.11, 130.91, 127.91, 125.84, 124.40, 122.95, 122.86, 121.41, 119.95, 52.40, 37.40, 37.39, 31.07, 30.48, 28.57, 23.97, 22.10, 21.67, 13.35, 13.23, 13.11. HRMS (ESI): m/z [C83H68S6] calcd for [MþH]þ: 1257.3724, found 1257.3718. 3. Results and discussion 3.1. Synthesis The synthetic routes of the title compounds were shown in Scheme 1. Starting material 7a, 7b and borate ester 9 were prepared according to modified literature procedures [23,24]. Intermediate 11 and 12 were prepared according to the methods reported by Swager [25] and Chen [26], respectively. Target compound 1 and intermediate 8, 10 were prepared according to our previous work [21]. With them in hand, dithienylethene-triptycene hybrid 2 was prepared via Suzuki coupling reaction of the borate ester 9 and iodotriptycene 10 in the presence of a palladium catalyst under reflux. Likewise, other dithienylethene-triptycene compound 3, 4, 5 and 6 were prepared by Suzuki coupling reaction of 2,6diiodotriptycene (11) and 2,6,14-triiodotriptycene (12) with dithienylethene borate esters (8 and 9) in moderate yields, respectively. The chemical structures of new compounds were characterized by 1H NMR, 13C NMR, HRMS and UV-vis spectroscopy. 3.2. Spectroscopic characteristics and photochromism With these dithienylethene-triptycene hybrids in hand, we firstly study their optical properties. Fig. 1 show all the UV-visible absorption spectra of compound 1e6 in THF solution (~2.0 105 M) and their changes upon irradiation at the light of 254 nm at 298 K. Taking the three hybrids (compound 2, 4 and 6), which substituted by phenyl groups in side chains, as examples, their THF solutions of the open-ring isomers are colorless and there is not any absorption in the visible light region (l > 400 nm). Upon photoirradiation at 254 nm, the colorless solutions turned to purplish red dramatically. Meanwhile, a new band appeared at about 535 nm and the original absorption peak at around 290 nm gradually decreased, which is because of the photoisomerization from open-ring form to closed-ring form with large p electron delocalization [1]. Similarly, the solutions of compound 1, 3 and 5, whose dithienylethene possess substituent Cl at the thiophene ring, all generated a new absorption band at about 494 nm and the colorless solutions turn to red when irradiated by UV light. Their maximum absorption of the closed-ring forms blue-shifted about 40 nm compared to compound 2, 4 and 6. The phenyl group attached to the side chain of dithienylethene in compound 2, 4 and 6 is considered to increase the electron density on the thiophene ring and extend the length of conjugated chain in the closed-ring isomer [15]. The back processes of their closed-ring forms to open-ring forms took place under exposure to visible light (l > 500 nm), and the colored solutions were almost bleached. The existence of closed-ring isomer during photocyclization reaction is evident in the 1H NMR spectra. Hybrid 2, 4 and 6 were
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Fig. 1. The UV-visible absorption spectra of 1e6 in THF solution (~2.0 105 M) and their changes under different irradiation time by light of 254 nm.
monitored by 1H NMR spectroscopy before photoirradiation and at the photostationary state (PSS) (Fig. 2). Here we choose compound 2 as an example to describe the differences before and after photochromism. Before photoirradiation, the proton signals of Ha and Hb on thiophene rings appeared at 6.93 and 7.02 ppm, as were shown in Fig. 2. At the PSS, the two signals obviously shifted up to 6.32 and 6.44 ppm, respectively. This change is a result of the shielding effect of the closed-ring form after irradiation [27,28]. In the high-field, two sets of new signal located at 1.96 ppm are ascribed to the methyl protons, while they lie in 1.94 and 1.95 ppm in the open-ring form. These differences clearly indicate the existence of the closed isomer for compound 2 after the cyclization reaction [29]. In addition, as the proton signals changed at dithienylethene, the signals of the triptycene bridge protons (He) changed accordingly. Two sets of new signals appeared at 5.42 ppm and 5.34 ppm were attributed to the triptycene bridge protons after photoirradiation (d ¼ 5.39 and 5.40 ppm before photoirradiation). Similar characteristics in the 1 H NMR spectra resulted from the existence of closed isomer
were also found for other hybrids, which indicates that the photochromic reaction happened and closed-ring isomers existed after photoirradiation. 3.3. Independent photochromic switches Generally, the photochromic process of dithienylethene involves a photoisomerization between the two configurations of the closed-ring isomer and open-ring isomer with different lengths of conjugated chain. The interconversions of the colors and the maximum absorbance between the two isomers are due to the change of the p-conjugation after photocyclization[15]. From Fig. 3 (left), we can see that compound 2, 4 and 6, which contain the same dithienylethene unit but different amount, exhibit similar absorption maxima at open form (~290 nm) and closed form (~535 nm), respectively. Moreover, their red-shift is nearly identical after photocyclization (~245 nm). This spectroscopic characteristics indicates the lengths of the p-conjugation in the three hybrids 2, 4 and 6 are almost the same and there are little or no interaction
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Fig. 2. Comparison of partial 1H NMR spectra of compound 2 before irradiation and at the PSS.
among the different dithienylethene branches and the triptycene core. So is the case with the other three hybrids 1, 3 and 5. This independent performance might be owing to the introduction of the rigid three-dimensional frameworks of triptycene core. On the other hand, if the multiply dithienylethene moieties in the hybrid system does not interfere with each other, the molar extinction coefficient of these hybrids cherishing different number of dithienylethene units should show a corresponding multiple relationships in theory. Fig. 4 and Table 1 summarized the molar extinction coefficient of compounds 1e6 at the open-ring and closed-ring isomers in THF. From which we can see that the molar extinction coefficient of compound 5, which contains three dithienylethene units, is 3 times as that of compound 1 which connects only one dithienylethene moiety, and the molar extinction coefficient of compound 3 linked two dithienylethene units is about two times as that of the compound 1. The same tendencies were found for the compound 2, 4 and 6. The result of the molar extinction coefficient are consistent with the theoretical expectation indicates that each dithienylethene unit in the multidithienylethene-triptycene hybrids behaves independently [30]. Furthermore, we increased the duration of photoirritation to more than 1 h at the PSS, there were not any new absorption bands appearance except those bands shown in Fig. 1. And we did not find any deviation of the isobestic points in their UV-Vis absorption spectra till the photo-damage of these compounds. Together with the performance of the consistent multiple relationship among these hybrids bearing different dithienylethene units as mentioned before, we conclude the photochromism of these hybrids all achieve their final stationary states independently. The photodynamic processes of compound 1e6 monitored by absorption spectra have been investigated in THF (supporting
information). Result shows that the rates of photocyclization for 1, 3 and 5 are essentially identical, which further indicates that each dithienylethene units attached to triptycene behaves independently [30]. Likewise, compound 2, 4 and 6 show the same result. Spectroscopic properties of compounds 1e6 before and after the photochemical reaction (in THF) are summarized in Table 1. The compound 2, 4 and 6 display larger molar extinction coefficient and maximum absorption at closed-ring form than that of compound 1, 3 and 5. It is attributed to the attachment of the substitute benzenes increasing the length of p-conjugation for the closed-ring isomers. 3.4. Solvent-dependent photochromism It has long been known that UV-vis absorption spectra of some compounds might be influenced by the surrounding medium or solvents and brings about changes in the position, intensity and shape of absorption bands [31]. Inspired by it, we further investigated the UV-vis absorption of these dithienylethene-triptycene hybrids in different solvents. Interestingly, solvent-dependent phenomena were found in chloride solvents such as dichloromethane, chloroform, tetrachloromethane and 1,2-dichloroethane. Fig. 5 shows the changes of UV-vis absorption spectra of the hybrids in dichloromethane solution irradiated by 254 nm light with different times. Just taking compound 2 as an example, upon exposure to UV light, the color of its DCM solution became purple red gradually with a new broad absorption band at around 533 nm appearing. It finally reached photostationary state (PSS) after irradiation for about 6 min, which is the same case as in THF solution
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Fig. 3. The comparison of UV-vis absorption spectroscopy of compound 2, 4, 6 (left) and 1, 3, 5 (right) at the open- and closed-ring isomers in THF, respectively.
mentioned above. After that, keep increasing the irradiation time, the solution became brown green gradually and the band at around 533 nm dramatically decreased accompanying two new absorption bands appeared in the area of 400e450 and 600e700 nm. The UV-vis absorption of other dithienylethenelinked triptycenes showed the same characteristics as the compound 2. However, these hybrids did not show this phenomenon in other common solvents (such as THF, ethyl acetate, toluene and DMF), even prolonged the photoirradiation time to more than 60 min at the PSS. It is worth mentioning that dithienylethene monomer substituted with chlorine (7a) or benzene (7b) in the thiophene ring did not show any solvent-dependent phenomena whether in chloride solvents or other general solvents such as THF and hexane (Supporting information), which demonstrates that this phenomenon is not affected by the
Fig. 4. Comparison of molar extinction coefficient of compounds 1, 3, 5 (left) and 2, 4, 6 (right) at the open- and closed-ring isomers in THF (note the molar absorptivity (ε) of 1, 2 have been multiplied by 3 and 3, 4 have been multiplied by 1.5, respectively, to allow for comparison of spectral shape).
substituent Cl or benzene in dithienylethene side chain, but might be caused by the incorporation of triptycene core. In order to gain further insight into the reason of these solventdependent phenomena, the photochromism of the dithienylethene-triptycene hybrids (i.e. compound 2, 4 and 6） was monitored by 1H NMR spectroscopy in chloride solvent CDCl3 (SI). After increasing the photoirradiation times to more than 60 min, there are not any other new peaks appearing except the signals assigned to the dithienylethene-triptycene hybrids in PSS, which further shows that these solvent-dependent phenomena are not caused by side reaction or by-products. However, the exact mechanism of this phenomenon is still not well understood.
Table 1 Optical properties of the dithienylethene-linked triptycenes in THF at 298 K.
1 2 3 4 5 6
Open-ring isomer
Closed-ring isomer
lmax/nm ε/103 M1cm1
lmax/nm ε/103 M1cm1
319 290 319 291 318 290
494 533 494 535 494 535
(17.4) (24.0) (32.5) (45.3) (55.3) (65.1)
(9.7) (13.3) (16.1) (24.7) (30.3) (36.7)
⊿lmax/nm
Quantum yields
175 243 175 244 176 245
0.22 0.25 0.21 0.25 0.21 0.22
Фo-c
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Fig. 5. Absorption spectral changes of 2e6 in DCM solution (2.0 105 M) upon irradiation at the light of 254 nm. Inserts: the absorption spectral changes of compound 2e6 after PSS upon extending irradiation with UV light.
4. Conclusions
Appendix A. Supplementary data
We have presented an effective approach for the construction of a series of photochromic multicomponent hybrids. Their spectroscopic properties and photochromism have been investigated in detail. Results show that these dithienylethene-triptycene hybrids undergo distinguishing photochromic performances and solventdependent photochromism when dissolved in tetrahydrofuran and dichloromethane. There is little or no interaction among the dithienylethene units in the multi-dithienylethene-triptycene system, and each dithienylethene switches in a molecule behaves separately. These distinctive characteristics might be potential used for high-density multicomponent switches and detection of chloride solvents.
Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.dyepig.2015.05.023.
Acknowledgments We thanks for the Nature Science Foundation of Shanghai (K100-2-13041) for financial support. We are grateful to Prof. He Tian from East China University of Science and Technology for his valuable support on this project.
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[7] Chan JCH, Lam WH, Yam VWW. A highly efficient silole-containing dithienylethene with excellent thermal stability and fatigue resistance: a promising candidate for optical memory storage materials. J Am Chem Soc 2014;136: 16994e7. [8] Kodama R, Sumaru K, Morishita K, Kanamori T, Hyodo K, Kamitanaka T, et al. A diarylethene as the SO2 gas generator upon UV irradiation. Chem Comm 2015;51:1736e8. [9] Park J, Feng DW, Yuan S, Zhou HC. Photochromic metaleorganic frameworks: reversible control of singlet oxygen generation. Angew Chem Int Ed 2015;54: 430e5. [10] Perrier A, Maurel F, Jacquemin D. Single molecule multiphotochromism with dithienylethenes. Acc Chem Res 2011;45:1173e82. [11] Peter A, Branda NR. Limited photochromism in covalently linked double 1, 2- dithienylethenes. Adv Mater Opt Electron 2000;10:245e9. [12] Kaieda T, Kobatake S, Miyasaka H, Marukami M, Iwai N, Nagata Y, et al. Efficient photocyclization of dithienylethene dimer, trimer, and tetramer: quantum yield and reaction dynamics. J Am Chem Soc 2002;124:2015e24. [13] Yagi K, Irie M. Photochromic and fluorescent properties of a dithienylethene dimer. Chem Lett 2003;32:848e9. [14] Matsuda K, Irie M. Photoswitching of intramolecular magnetic interaction using a dithienylethene dimer. J Am Chem Soc 2001;123:9896e7. [15] Kobatake S, Irie M. Synthesis and photochromic reactivity of a dithienylethene dimer linked by a phenyl group. Tetrahedron 2003;59:8359e64. [16] Areephong J, Browne WR, Feringa BL. Three- state photochromic switching in a silyl bridged dithienylethene dimer. Org Biomol Chem 2007;5:1170e4. [17] Kang SO, Llinares JM, Day VW, James KB. Cryptand- like anion receptors. Chem Soc Rev 2010;39:3980e4003. [18] Nikitin K, Helge MB, Ortin Y, Muldoon J, McGlinchey MJ. Molecular dials: hindered rotations in mono- and diferrocenyl anthracenes and triptycenes. J Am Chem Soc 2010;132:17617e22. [19] Frantz DK, Linden A, Baldridge KK, Siegel JS. Molecular spur gears comprising triptycene rotators and bibenzimidazole- based stators. J Am Chem Soc 2012;134:1528e35.
[20] Goldsmith RH, Weis JV, Scott AM, Borkar S, Sen A, Ratner MA, et al. Unexpectedly similar charge transfer rates through benzo- annulated bicyclo[2.2.2] octanes. J Am Chem Soc 2008;130:7659e69. [21] Luo QF, Bao SM, Wang Q, Qu DH, Liu MQ. Preparation and solvent- dependent photochromism of triptycene- bonded dithienylethenes. Chem Lett 2014;43: 1584e6. [22] Giordano L, Jovin TM, Irie M, Jares-Erijman EA. Diheteroarylethenes as thermally stable photoswitchable acceptors in photochromic fluorescence resonance energy transfer (pcFRET). J Am Chem Soc 2002;124:7481e9. [23] Lucas LN, Jong JDD, Esch JHV, Kellogg RM, Feringa BL. Syntheses of dithienylcyclopentene optical molecular switches. Eur J Org Chem 2003: 155e66. [24] Kudernac T, Molen SJVD, Weesb BJV, Feringa BL. Uni- and bi-directional lightinduced switching of dithienylethenes on gold nanoparticles. Chem Comm 2006:3597e9. [25] Chen ZH, Swager TM. Synthesis and characterization of poly(2, 6- triptycene). Macromolecules 2008;41:6880e5. [26] Zhang C, Chen CF. Synthesis and structure of 2, 6, 14- and 2, 7, 14- trisubstituted triptycene derivatives. J Org Chem 2006;71:6626e9. [27] Li WL, Jiao CH, Li X, Xie YS, Nakatani K, Tian H, et al. Separation of photoactive conformers based on hindered diarylethenes: efficient modulation in photocyclization quantumyields. Angew Chem Int Ed 2014;53:4603e7. [28] Li B, Wang JY, Wen HM, Shi LX, Chen ZN. Redox-modulated stepwise photochromism in a ruthenium complex with dual dithienylethene-acetylides. J Am Chem Soc 2012;134:16059e67. [29] Hyunbong C, Jung Il, Kyu HS, Kihyung S, Dong SS, Sang OK, et al. Synthesis and photochromic reactivity of dithienylethene trimmers bridged by ethenyl and ethynyl unit. Tetrahedron 2006;62:9059e65. [30] Areephong J, Logtenberg H, Browne WR, Feringa BL. Symmetric six-fold arrays of photo and electrochromic dithienylethene switches. Org Let 2010;12: 2132e5. [31] Reichardt C. Solvatochromic dyes as solvent polarity indicators. Chem Rev 1994;94:2319e58.
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Dyes and Pigments journal homepage: www.elsevier.com/locate/dyepig
Photochromic dithienylethene-branched triptycene hybrids Xunzhi Cai, Lan Zhu, Shumin Bao, Qianfu Luo* Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science and Technology, Shanghai 200237, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 February 2015 Received in revised form 19 May 2015 Accepted 21 May 2015 Available online 30 May 2015
Dithienylethene and triptycene both have controllable structure and versatile performances. In this study, a series of triptycene hybrids decorated different number of dithienylethene units have been developed for photochromic switch. The synthetic process is facile and efficient and all new compounds were clearly characterized by 1H NMR, 13C NMR and HRMS. Their photochromism and spectroscopic properties have been investigated in dichloromethane and tetrahydrofuran. The result revealed that the multi-dithienylethene units grafted to triptycene behaved independently and showed distinctive solvent-dependent photochromism in chloride solvents, which probably due to the introduction of controlled spatial arrangement of triptycene core. These distinctive characteristics might be potential used for high-density multicomponent switches and detection of chloride solvents. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Triptycene Multi-dithienylethene Photochromic Solvent-dependent Switch Synthesis
1. Introduction Dithienylethene-type (DTE) photochromic materials have been extensively investigated because of their excellent properties and versatile capacity to function as various applications such as photo-switchable molecular device, multi-controllable optoelectronic units, optical memory storage materials, photocontrollable gas generator, biological imaging and intelligent materials [1e9]. However, sole dithienylethene system is often difficult to meet the various needs of practical application and manufacturing process [2,10]. In order to store more data and cater to different functions, a variety of multi-component molecular systems based on the combination of dithienylethene and other functional molecules have attracted great interesting [11e16]. On the other hand, triptycene and its derivatives possess rigid three-dimensional frameworks and 120 C orientation which constitute a useful linker group for multichromophore assemblies, and thus have been developed more and more applications as diverse as host-guest chemistry [17], supramolecular chemistry [18], materials science and so on [19e21]. They are always appealing partners for multi-functional design. In view of these, we became interested in constructing different dithienylethene-triptycene hybrids for photochromic
* Corresponding author. E-mail address: [email protected] (Q. Luo). http://dx.doi.org/10.1016/j.dyepig.2015.05.023 0143-7208/© 2015 Elsevier Ltd. All rights reserved.
switches and then synthesized a series of dithienylethene-linked triptycene complexes, in which a three-dimensional triptycene framework was chosen as central template and different number of dithienylethene units as branches, as shown in Scheme 1. The structures of the new hybrids were clearly confirmed and their photochromic performances were thoroughly investigated. Results show these multicomponent photochromic hybrids display distinctive performances.
2. Experiment 2.1. General information All chemicals were obtained commercially and used as received unless otherwise mentioned. 1H NMR spectra were recorded at 400 MHz and 13C NMR spectra were recorded at 101 MHz using CDCl3 as a solvent and tetramethylsilane as an internal standard. The UV light for irradiation was generated by LED sources. All mass spectrometric analyses were performed on ThermoStar™ mass spectrometer. Absorption spectra were recorded on a Varian Cary 500 using quartz cuvette at room temperature. For the absorption spectral measurements, optical cells with 1 cm light path lengths were used for the absorption spectral measurement of the solutions. The quantum yields of photochromic ring-cyclizaiton of compounds 1e6 were determined by means of literature [22].
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Scheme 1. The synthetic routes of the target compounds.
2.2. Synthetic procedures of the target compounds 2.2.1. Synthesis of 2-{3-[2-(5-phenyl-2-methylthien-3-yl)cyclopenteneyl]-2-methylthien-5-yl} triptycene (2) To a stirred solution of compound 10 (91 mg, 0.24 m mol) in anhydrous THF (5 mL), Pd(PPh3)4 (40 mg, 0.03 m mol) was added, and the resulting solution was stirred for 15 min at room temperature. Then anhydrous K2CO3 (0.38 g, 2.7 m mol) and distilled water (2.5 mL) were added. This mixture was heated in an oil bath just below reflux at a temperature of 60 C and the mixed solution of compound 9 was added dropwise via a syringe in a short time period of approximately 5 min. The reaction solution was then stirred at 110 C for 4 h and cooled to room temperature, after which H2O (10 mL) were added and extracted with CH2Cl2 (3 15 mL). The organic layer was separated and subsequently dried over anhydrous sodium sulfate. The sodium sulfate was filtered off, and the solvent was removed by evaporation under vacuum. The residue was purified by silica gel chromatography with dichloromethane/petroleum ether (1/4) as eluent to afford pure white solid 54 mg, yield 38.2%; 1H NMR (400 MHz, CDCl3, ppm): d ¼ 7.52-7.50 (m, 2H), 7.48 (s, 1H), 7.38-7.36 (m, 4H), 7.357.31 (m, 3H), 7.22 (t, J ¼ 8.0 Hz, 1H), 7.11 (dd, J ¼ 7.6, 1.6 Hz, 1H), 7.02 (s, 1H), 7.00-6.98 (m, 4H), 6.94 (s, 1H), 5.40 (d, J ¼ 3.2 Hz, 2H), 2.842.80 (m, 4H), 2.08-2.05 (m, 2H), 1.95 (s, 3H), 1.94 (s, 3H). 13C NMR (101 MHz, CDCl3, ppm): d ¼ 145.96, 145.02, 144.98, 144.16, 139.76, 139.62, 136.72, 136.51, 134.72, 134.55, 134.53, 134.09, 131.72, 128.83,
127.00, 125.31, 125.26, 124.01, 123.88, 123.83, 123.69, 123.60, 122.25, 121.00, 54.10, 53.69, 38.46, 34.71, 31.61, 23.01, 14.49, 14.41. HRMS (ESI): m/z [C41H32S2] calcd for: [MþH]þ: 589.2024, found 589.2022. 2.2.2. Synthesis of 2,6-bis{3-[ 2-(5-chloro-2-methylthien-3-yl)cyclopenteneyl]-2-methylthien-5-yl} triptycene (3) To a stirred solution of compound 11 (0.21 g, 0.40 m mol) in anhydrous THF (5 mL), Pd(PPh3)4 (60 mg, 0.05 m mol) was added, and the resulting solution was stirred for 15 min at room temperature. Then anhydrous K2CO3 (0.38 g, 2.7 m mol) and distilled water (2.5 mL) were added. This two-phase system was heated in an oil bath just below reflux at a temperature of 60 C and the mixed solution of compound 8 was added dropwise via a syringe in a short time period of approximately 3 min. The reaction was then stirred at 110 C overnight and cooled to room temperature, after which H2O (10 mL) were added and extracted with CH2Cl2 (3 15 mL), The organic layer was separated and subsequently dried over anhydrous sodium sulfate. The sodium sulfate was filtered off, and the solvent was removed by evaporation under vacuum. The residue was purified by silica gel chromatography with dichloromethane/petroleum ether (1/4) as eluent to afford pure white solid 0.22 g, yield 66%; 1H NMR (400 MHz, CDCl3, ppm): d ¼ 7.55 (d, J ¼ 1.5 Hz, 2H), 7.40 (m, 2H), 7.34 (d, J ¼ 7.7 Hz, 2H), 7.13 (dd, J ¼ 7.6, 1.7 Hz, 2H), 7.01 (dd, J ¼ 5.1, 3.5 Hz, 2H), 6.91 (s, 2H), 6.61 (s, 2H), 5.44 (s, 1H), 5.41 (s, 1H), 2.80-2.71 (m, 8H), 2.08-1.99 (m, 4H), 1.95 (s, 6H), 1.82 (s, 6H). 13C NMR (101 MHz, CDCl3, ppm): d ¼ 145.70,
X. Cai et al. / Dyes and Pigments 121 (2015) 227e234
144.80, 144.71, 144.03, 140.02, 136.34, 135.41, 135.22, 134.20, 133.76, 133.36, 131.80, 126.90, 125.58, 125.47, 125.05, 123.99, 123.85, 123.72, 122.49, 121.13, 54.21, 53.43, 38.53, 23.02, 14.50, 14.28. HRMS (ESI): m/z [C50H40Cl2S4]: calcd for [MþH]þ: 839.1468, found 839.1480. 2.2.3. Synthesis of 2,6-bis{3-[ 2-(5-phenyl-2-methylthien-3-yl)cyclopenteneyl]-2-methylthien-5-yl} triptycene (4) Compound 11 (0.21 g, 0.40 m mol) and Pd(PPh3)4 (60 mg, 0.05 m mol) were dissolved in anhydrous THF (5 mL). Anhydrous K2CO3 (0.38 g, 2.7 m mol) and distilled water (2.5 mL) were added, and the mixture was stirred at rt for 10 min. After heated to 60 C, the THF solution of compound 9 was added dropwise. The reaction was then stirred at 110 C overnight and cooled to room temperature, after which H2O (10 mL) were added and extracted with CH2Cl2 (3 15 mL), The organic layer was separated and subsequently dried over anhydrous sodium sulfate, and the solvent was removed by evaporation under vacuum. Purification by silica gel chromatography with dichloromethane/petroleum ether (1/4) as eluent to afford pure white solid 74 mg, yield 40.1%; 1H NMR (400 MHz, CDCl3, ppm): d ¼ 7.52-7.50 (m, 4H), 7.48 (s, 2H), 7.38-7.36 (m, 2H), 7.33 (t, J ¼ 7.2 Hz, 6H), 7.22 (t, J ¼ 7.6 Hz, 2H), 7.12 (dd, J ¼ 8.0, 2.0 Hz, 2H), 7.02 (s, 2H), 7.01-6.99 (m, 2H), 6.94 (s, 2H), 5.38 (s, 2H), 2.84-2.81 (m, 8H), 2.09-2.05 (m, 4H), 1.95 (s, 6H), 1.94 (s, 6H). 13C NMR (101 MHz, CDCl3, ppm): d ¼ 145.69, 144.82, 143.88, 139.78, 139.59, 136.78, 136.50, 134.70, 134.57, 134.53, 134.12, 131.85, 128.81, 126.94, 125.38, 125.29, 123.99, 123.91, 123.86, 123.64, 122.32, 120.95, 53.66, 38.38, 31.58, 29.68, 27.00, 25.68, 23.10, 22.64, 14.40, 14.15. HRMS (ESI): m/z [C62H50S4] calcd for [MþH]þ: 923.2874, found 923.2877. 2.2.4. Synthesis of 2,6,14-tris{3-[ 2-(5-chloro-2-methylthien-3-yl)cyclopenteneyl]-2-methylthien-5-yl} triptycene (5) To a stirred solution of compound 12 (0.21 g, 0.33 m mol) in anhydrous THF (5 mL), Pd(PPh3)4 (80 mg, 0.07 m mol) was added, and the resulting solution was stirred for 15 min at room temperature. Then anhydrous K2CO3 (0.38 g, 2.7 m mol) and distilled water (2.5 mL) were added. This two-phase system was heated in an oil bath just below reflux at a temperature of 60 C and the mixed solution of compound 8 was added dropwise via a syringe in a short time period of approximately 3 min. The reaction was then stirred at 110 C for 48 h and cooled to room temperature, after which H2O (10 mL) were added and extracted with CH2Cl2 (3 10 mL), The organic layer was separated and subsequently dried over anhydrous sodium sulfate. The sodium sulfate was filtered off, and the solvent was removed by evaporation under vacuum. The residue was purified by silica gel chromatography with dichloromethane/ petroleum ether (1/6) as eluent to afford pure white solid 0.10 g, yield 27%; 1H NMR (400 MHz, CDCl3, ppm): d ¼ 7.54 (t, J ¼ 7.0 Hz, 3H), 7.36-7.32 (m, 3H), 7.18 -7.10 (m, 3H), 6.90 (s, 3H), 6.60 (s, 3H), 5.42 (d, J ¼ 6.9 Hz, 2H), 2.75 (m, 12H), 2.09-1.98 (m, 6H), 1.95 (s, 9H), 1.81 (s, 9H). 13C NMR (101 MHz, CDCl3, ppm): d ¼ 145.43, 143.69, 139.96, 136.36, 135.41, 135.22, 134.26, 133.79, 133.37, 131.98, 126.90, 125.06, 124.05, 123.79, 122.60, 121.12, 53.58, 38.57, 38.49, 31.74, 29.85, 23.04, 22.81, 21.22, 14.51, 14.35, 14.28. HRMS (ESI): m/z [C65H53Cl3S6] calcd for [MþH]þ: 1131.1615, found 1131.1611. 2.2.5. Synthesis of 2,6,14-tris{3-[ 2-(5-phenyl-2-methylthien-3-yl)cyclopenteneyl]-2-methylthien-5-yl} triptycene (6) Compound 12 (0.21 g, 0.33 m mol) and Pd(PPh3)4 (80 mg, 0.07 m mol) were dissolved in anhydrous THF (5 mL). Then anhydrous K2CO3 (0.38 g, 2.7 m mol) and distilled water (2.5 mL) were added, and the mixture was stirred at rt for 10 min. The solution was heated to 60 C, then the THF solution of compound 9 was added dropwise. The reaction was then stirred at 110 C for 48 h and cooled to room temperature, after which H2O (10 mL) were
229
added and extracted with CH2Cl2 (3 15 mL), The organic layer was separated and subsequently dried over anhydrous sodium sulfate, and the solvent was removed by evaporation under vacuum. Purification by silica gel chromatography with dichloromethane/petroleum ether (1/5) as eluent to afford pure white solid 96 mg, yield 23.2%; 1H NMR (400 MHz, CDCl3, ppm): d ¼ 7.51-7.50 (m, 6H), 7.48 (s, 3H), 7.33 (t, J ¼ 7.6 Hz, 9H), 7.22 (t, J ¼ 3.2 Hz, 3H),7.13 (dd, J ¼ 7.6, 1.6 Hz, 3H), 7.02 (s, 3H), 6.94 (s, 3H), 5.35 (d, J ¼ 6.4 Hz, 2H), 2.842.81 (m, 12H), 2.09-2.05 (m, 6H), 1.95 (s, 9H), 1.94 (s, 9H). 13C NMR (101 MHz, CDCl3, ppm): d ¼ 144.26, 142.47, 138.63, 138.55, 135.65, 135.46, 133.66, 133.54, 133.49, 133.11, 130.91, 127.91, 125.84, 124.40, 122.95, 122.86, 121.41, 119.95, 52.40, 37.40, 37.39, 31.07, 30.48, 28.57, 23.97, 22.10, 21.67, 13.35, 13.23, 13.11. HRMS (ESI): m/z [C83H68S6] calcd for [MþH]þ: 1257.3724, found 1257.3718. 3. Results and discussion 3.1. Synthesis The synthetic routes of the title compounds were shown in Scheme 1. Starting material 7a, 7b and borate ester 9 were prepared according to modified literature procedures [23,24]. Intermediate 11 and 12 were prepared according to the methods reported by Swager [25] and Chen [26], respectively. Target compound 1 and intermediate 8, 10 were prepared according to our previous work [21]. With them in hand, dithienylethene-triptycene hybrid 2 was prepared via Suzuki coupling reaction of the borate ester 9 and iodotriptycene 10 in the presence of a palladium catalyst under reflux. Likewise, other dithienylethene-triptycene compound 3, 4, 5 and 6 were prepared by Suzuki coupling reaction of 2,6diiodotriptycene (11) and 2,6,14-triiodotriptycene (12) with dithienylethene borate esters (8 and 9) in moderate yields, respectively. The chemical structures of new compounds were characterized by 1H NMR, 13C NMR, HRMS and UV-vis spectroscopy. 3.2. Spectroscopic characteristics and photochromism With these dithienylethene-triptycene hybrids in hand, we firstly study their optical properties. Fig. 1 show all the UV-visible absorption spectra of compound 1e6 in THF solution (~2.0 105 M) and their changes upon irradiation at the light of 254 nm at 298 K. Taking the three hybrids (compound 2, 4 and 6), which substituted by phenyl groups in side chains, as examples, their THF solutions of the open-ring isomers are colorless and there is not any absorption in the visible light region (l > 400 nm). Upon photoirradiation at 254 nm, the colorless solutions turned to purplish red dramatically. Meanwhile, a new band appeared at about 535 nm and the original absorption peak at around 290 nm gradually decreased, which is because of the photoisomerization from open-ring form to closed-ring form with large p electron delocalization [1]. Similarly, the solutions of compound 1, 3 and 5, whose dithienylethene possess substituent Cl at the thiophene ring, all generated a new absorption band at about 494 nm and the colorless solutions turn to red when irradiated by UV light. Their maximum absorption of the closed-ring forms blue-shifted about 40 nm compared to compound 2, 4 and 6. The phenyl group attached to the side chain of dithienylethene in compound 2, 4 and 6 is considered to increase the electron density on the thiophene ring and extend the length of conjugated chain in the closed-ring isomer [15]. The back processes of their closed-ring forms to open-ring forms took place under exposure to visible light (l > 500 nm), and the colored solutions were almost bleached. The existence of closed-ring isomer during photocyclization reaction is evident in the 1H NMR spectra. Hybrid 2, 4 and 6 were
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Fig. 1. The UV-visible absorption spectra of 1e6 in THF solution (~2.0 105 M) and their changes under different irradiation time by light of 254 nm.
monitored by 1H NMR spectroscopy before photoirradiation and at the photostationary state (PSS) (Fig. 2). Here we choose compound 2 as an example to describe the differences before and after photochromism. Before photoirradiation, the proton signals of Ha and Hb on thiophene rings appeared at 6.93 and 7.02 ppm, as were shown in Fig. 2. At the PSS, the two signals obviously shifted up to 6.32 and 6.44 ppm, respectively. This change is a result of the shielding effect of the closed-ring form after irradiation [27,28]. In the high-field, two sets of new signal located at 1.96 ppm are ascribed to the methyl protons, while they lie in 1.94 and 1.95 ppm in the open-ring form. These differences clearly indicate the existence of the closed isomer for compound 2 after the cyclization reaction [29]. In addition, as the proton signals changed at dithienylethene, the signals of the triptycene bridge protons (He) changed accordingly. Two sets of new signals appeared at 5.42 ppm and 5.34 ppm were attributed to the triptycene bridge protons after photoirradiation (d ¼ 5.39 and 5.40 ppm before photoirradiation). Similar characteristics in the 1 H NMR spectra resulted from the existence of closed isomer
were also found for other hybrids, which indicates that the photochromic reaction happened and closed-ring isomers existed after photoirradiation. 3.3. Independent photochromic switches Generally, the photochromic process of dithienylethene involves a photoisomerization between the two configurations of the closed-ring isomer and open-ring isomer with different lengths of conjugated chain. The interconversions of the colors and the maximum absorbance between the two isomers are due to the change of the p-conjugation after photocyclization[15]. From Fig. 3 (left), we can see that compound 2, 4 and 6, which contain the same dithienylethene unit but different amount, exhibit similar absorption maxima at open form (~290 nm) and closed form (~535 nm), respectively. Moreover, their red-shift is nearly identical after photocyclization (~245 nm). This spectroscopic characteristics indicates the lengths of the p-conjugation in the three hybrids 2, 4 and 6 are almost the same and there are little or no interaction
X. Cai et al. / Dyes and Pigments 121 (2015) 227e234
231
Fig. 2. Comparison of partial 1H NMR spectra of compound 2 before irradiation and at the PSS.
among the different dithienylethene branches and the triptycene core. So is the case with the other three hybrids 1, 3 and 5. This independent performance might be owing to the introduction of the rigid three-dimensional frameworks of triptycene core. On the other hand, if the multiply dithienylethene moieties in the hybrid system does not interfere with each other, the molar extinction coefficient of these hybrids cherishing different number of dithienylethene units should show a corresponding multiple relationships in theory. Fig. 4 and Table 1 summarized the molar extinction coefficient of compounds 1e6 at the open-ring and closed-ring isomers in THF. From which we can see that the molar extinction coefficient of compound 5, which contains three dithienylethene units, is 3 times as that of compound 1 which connects only one dithienylethene moiety, and the molar extinction coefficient of compound 3 linked two dithienylethene units is about two times as that of the compound 1. The same tendencies were found for the compound 2, 4 and 6. The result of the molar extinction coefficient are consistent with the theoretical expectation indicates that each dithienylethene unit in the multidithienylethene-triptycene hybrids behaves independently [30]. Furthermore, we increased the duration of photoirritation to more than 1 h at the PSS, there were not any new absorption bands appearance except those bands shown in Fig. 1. And we did not find any deviation of the isobestic points in their UV-Vis absorption spectra till the photo-damage of these compounds. Together with the performance of the consistent multiple relationship among these hybrids bearing different dithienylethene units as mentioned before, we conclude the photochromism of these hybrids all achieve their final stationary states independently. The photodynamic processes of compound 1e6 monitored by absorption spectra have been investigated in THF (supporting
information). Result shows that the rates of photocyclization for 1, 3 and 5 are essentially identical, which further indicates that each dithienylethene units attached to triptycene behaves independently [30]. Likewise, compound 2, 4 and 6 show the same result. Spectroscopic properties of compounds 1e6 before and after the photochemical reaction (in THF) are summarized in Table 1. The compound 2, 4 and 6 display larger molar extinction coefficient and maximum absorption at closed-ring form than that of compound 1, 3 and 5. It is attributed to the attachment of the substitute benzenes increasing the length of p-conjugation for the closed-ring isomers. 3.4. Solvent-dependent photochromism It has long been known that UV-vis absorption spectra of some compounds might be influenced by the surrounding medium or solvents and brings about changes in the position, intensity and shape of absorption bands [31]. Inspired by it, we further investigated the UV-vis absorption of these dithienylethene-triptycene hybrids in different solvents. Interestingly, solvent-dependent phenomena were found in chloride solvents such as dichloromethane, chloroform, tetrachloromethane and 1,2-dichloroethane. Fig. 5 shows the changes of UV-vis absorption spectra of the hybrids in dichloromethane solution irradiated by 254 nm light with different times. Just taking compound 2 as an example, upon exposure to UV light, the color of its DCM solution became purple red gradually with a new broad absorption band at around 533 nm appearing. It finally reached photostationary state (PSS) after irradiation for about 6 min, which is the same case as in THF solution
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Fig. 3. The comparison of UV-vis absorption spectroscopy of compound 2, 4, 6 (left) and 1, 3, 5 (right) at the open- and closed-ring isomers in THF, respectively.
mentioned above. After that, keep increasing the irradiation time, the solution became brown green gradually and the band at around 533 nm dramatically decreased accompanying two new absorption bands appeared in the area of 400e450 and 600e700 nm. The UV-vis absorption of other dithienylethenelinked triptycenes showed the same characteristics as the compound 2. However, these hybrids did not show this phenomenon in other common solvents (such as THF, ethyl acetate, toluene and DMF), even prolonged the photoirradiation time to more than 60 min at the PSS. It is worth mentioning that dithienylethene monomer substituted with chlorine (7a) or benzene (7b) in the thiophene ring did not show any solvent-dependent phenomena whether in chloride solvents or other general solvents such as THF and hexane (Supporting information), which demonstrates that this phenomenon is not affected by the
Fig. 4. Comparison of molar extinction coefficient of compounds 1, 3, 5 (left) and 2, 4, 6 (right) at the open- and closed-ring isomers in THF (note the molar absorptivity (ε) of 1, 2 have been multiplied by 3 and 3, 4 have been multiplied by 1.5, respectively, to allow for comparison of spectral shape).
substituent Cl or benzene in dithienylethene side chain, but might be caused by the incorporation of triptycene core. In order to gain further insight into the reason of these solventdependent phenomena, the photochromism of the dithienylethene-triptycene hybrids (i.e. compound 2, 4 and 6） was monitored by 1H NMR spectroscopy in chloride solvent CDCl3 (SI). After increasing the photoirradiation times to more than 60 min, there are not any other new peaks appearing except the signals assigned to the dithienylethene-triptycene hybrids in PSS, which further shows that these solvent-dependent phenomena are not caused by side reaction or by-products. However, the exact mechanism of this phenomenon is still not well understood.
Table 1 Optical properties of the dithienylethene-linked triptycenes in THF at 298 K.
1 2 3 4 5 6
Open-ring isomer
Closed-ring isomer
lmax/nm ε/103 M1cm1
lmax/nm ε/103 M1cm1
319 290 319 291 318 290
494 533 494 535 494 535
(17.4) (24.0) (32.5) (45.3) (55.3) (65.1)
(9.7) (13.3) (16.1) (24.7) (30.3) (36.7)
⊿lmax/nm
Quantum yields
175 243 175 244 176 245
0.22 0.25 0.21 0.25 0.21 0.22
Фo-c
X. Cai et al. / Dyes and Pigments 121 (2015) 227e234
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Fig. 5. Absorption spectral changes of 2e6 in DCM solution (2.0 105 M) upon irradiation at the light of 254 nm. Inserts: the absorption spectral changes of compound 2e6 after PSS upon extending irradiation with UV light.
4. Conclusions
Appendix A. Supplementary data
We have presented an effective approach for the construction of a series of photochromic multicomponent hybrids. Their spectroscopic properties and photochromism have been investigated in detail. Results show that these dithienylethene-triptycene hybrids undergo distinguishing photochromic performances and solventdependent photochromism when dissolved in tetrahydrofuran and dichloromethane. There is little or no interaction among the dithienylethene units in the multi-dithienylethene-triptycene system, and each dithienylethene switches in a molecule behaves separately. These distinctive characteristics might be potential used for high-density multicomponent switches and detection of chloride solvents.
Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.dyepig.2015.05.023.
Acknowledgments We thanks for the Nature Science Foundation of Shanghai (K100-2-13041) for financial support. We are grateful to Prof. He Tian from East China University of Science and Technology for his valuable support on this project.
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