Research article Received: 31 October 2014,

Revised: 1 February 2015,

Accepted: 1 February 2015

Published online in Wiley Online Library: 5 April 2015

(wileyonlinelibrary.com) DOI 10.1002/bio.2895

Multi-functional ion-sensor based on a photochromic diarylethene with a 1H-imidazo [4,5-f][1,10] phenanthroline unit Renjie Wang, Xiaorong Dong, Gang Liu, Panpan Ren and Shouzhi Pu* ABSTRACT: A new asymmetrical diarylethene containing a 1H-imidazo [4,5-f][1,10] phenanthroline unit was synthesized. The compound showed typical photochromism and functioned as a notable fluorescence switch upon alternating irradiation with ultraviolet (UV) and visible light. Its closed-ring isomer could be used as a selective ‘naked-eye’ colorimetric sensor for Cu2+, accompanied by a notable color change from blue to colorless. Furthermore, the compound was found to be selective towards Ca2+, Mg2+, and Sr2+ with significant fluorescence changes. On the basis of this characteristic, a logic circuit was constructed by utilizing both light and chemical stimuli as inputs and fluorescence intensity at 487 nm as output. Copyright © 2015 John Wiley & Sons, Ltd. Additional supporting information may be found in the online version of this article at publisher’s web site. Keywords: diarylethene; photochromism; imidazole-containing substituent; cation recognition; logic circuit

Introduction

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Copper plays an important role in the fields of biology, environmental sciences, and chemistry (1). It is an essential trace element for organisms, as its ionic form Cu2+ is found in most animal and plant tissues and in natural waters. Cu2+ is ranked third of the essential heavy metals in the human body below zinc and iron (2), However, Cu2+ exhibits toxicity at high concentrations and can cause serious neurodegenerative diseases such as Wilson disease (3), Alzheimer’s disease (4), and familial amyotrophic lateral sclerosis (5,6). Therefore, the design of a suitable colorimetric sensor with specific response to Cu2+ has been studied extensively (7–12). In addition, the calcium ion is one of the most important second messengers in cell signaling pathways. It plays an important role in cellular processes such as fertilization, development, learning, and memory (13). To date, a wide range of tools has been developed to monitor Ca2+ in live cells and tissues, including synthetic organic dyes and engineered fluorescent proteins (14). The term ‘diarylethene derivatives’ describes the reversible transformation of the chemical species by absorption of photoirradiation between two isomers that have distinguishable absorption spectra. These derivatives are promising materials for the manufacture of advanced materials due to their excellent thermal stability, remarkable fatigue resistance, rapid response and high photocyclization quantum yields (15–20). Additionally, they are also ideal candidates for use in optical functional molecular materials, such as molecular switches, sensors, actuators and biomaterials (21–23). Recently, large numbers of photochromic diarylethenes for use as versatile ion sensors have been reported. For example, Tian et al. reported a multistate dithienylethene molecule containing 1,8-naphthalimidepiperazine units, which its fluorescence can be tuned by Cu2+, protons, and light (24). Previously, our group also reported that some diarylethene derivatives could selectively respond to Cu2+ (7,25). 1H-imidazo [4,5-f][1,10] phenanthroline is an intriguing ligand,

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with the replacement of the nitrogen atoms at positions 1 and 10 of the phenanthroline moiety, the imidazole NH group has been used as a hydrogen-bond donor in anion sensors (26–28). For example, Wang et al. developed a Eu(III) complex containing 1H-imidazo [4,5-f][1,10] phenanthroline, which exhibited selectivity toward F–, HSO–4 and AcO– with significant changes in the absorption and fluorescence emission spectra (29). Zheng et al. explored a chemosensor containing terpyridine/phenyl imidazo [4,5-f]-phenanthroline hybrid units, which functioned as a luminescence sensor for H2PO–4 and was a highly selective colorimetric sensor for Fe2+ (30). Therefore, the design of diarylethene molecular photoswitches to incorporate a 1H-imidazo [4,5-f][1,10] phenanthroline unit as a part of a metal ion complex site is of interest, because the coordination of a diarylethene molecule into the metal complex can not only cause a shift in the absorption and photochromic properties, but also the photochromic behavior can be sensitized through excitation into the relevant excited state of the metal complex chromophore. In this work, we designed and synthesized a new hybrid photochromic diarylethene, which incorporated 1H-imidazo [4,5-f][1,10] phenanthroline as the ionic signal responsive group (1O, 1-[5-(4-methoxylphenyl)-2-methylthien-3-yl]-2-[5(4-(1H-imidazo [4,5-f][1,10] phenanthroline-2-yl)phenyl)-2methylthien-3-yl]perfluorocyclopentene). Its photophysical behaviors induced by light and metal ions were investigated systematically. The photochromic scheme for 1O is shown in Fig. 1.

* Correspondence to: Shouzhi Pu, Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science & Technology Normal University, Nanchang 330013, People’s Republic of China. E-mail: [email protected] Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science & Technology Normal University, Nanchang 330013, People’s Republic of China

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Ion-sensor based on a photochromic diarylethene

Figure 1. Photochromism of diarylethene 1O.

Experimental

Results and discussion

General methods

Photochromism

All solvents used were of spectro-grade and purified by distillation prior to use. Except for Mn2+, K+, and Ba2+ (their counter ions were chloride ions), metal ions were obtained by dissolving their respective metal nitrates (0.1 mmol) in distilled water (10 mL). NMR spectra were recorded on a Bruker AV400 (400 MHz) spectrometer with MeOD as the solvent and tetramethylsilane (TMS) as an internal standard. Infrared spectra (IR) were collected on a Bruker Vertex70 spectrometer. Mass spectra were performed with a LTQ Orbitrap XL mass spectrometer. Melting point was measured on a WRS-1B melting point apparatus. Absorption spectra were collected with an Agilent 8453 UV/vis spectrophotometer. Fluorescence spectra were recorded with a Hitachi F-4600 fluorescence spectrophotometer. Elemental analysis was carried out with a PE CHN 2400 analyzer. Photoirradiation experiments were performed on an SHG-200 UV lamp, CX-21 ultraviolet fluorescence analysis cabinet, and a BMH-250 visible lamp.

The photochromic behavior of 1O induced by photoirradiation in methanol solution was measured at room temperature. The absorption spectral and color changes of 1 upon photoirradiation are shown in Fig. 2. In methanol solution, the colorless solution containing the open-ring isomer 1O exhibited a sharp absorption peak at 288 nm (ε, 3.08 × 104 L mol–1 cm–1) and at 342 nm (ε, 2.78 × 104 L mol–1 cm–1), corresponding to the mixture of π→π* and n→π* transitions of the imidazole and thiophene rings (33,34). Upon irradiation with 297 nm UV light, the colorless 1O solution turned blue and a new visible absorption band was observed at 603 nm (ε, 1.94 × 104 L mol–1 cm–1) due to the formation of the closed-ring isomer 1C. The optical changes stopped after 580 sec of irradiation when the photostationary state (PSS) was reached, the PSS contained ~53% closed-ring isomer 1C, as estimated by comparing the signals in the 1H NMR spectra (Fig. 3). Exposing this blue colored solution to visible light (λ > 500 nm) triggered the reverse open-ring reaction, regenerating the original spectrum corresponding to 1O. In the photostationary state, a clear isosbestic point was observed at 364 nm, which supported the reversible two component photochromic reaction schemes (35). Additionally, using 1,2-bis(2-methyl-5-phenyl-3thienyl)perfluorocyclopentene as a reference (36), the cyclization/ cycloreversion quantum yields of 1 in methanol were calculated to be 0.37 and 0.01, respectively. Compared with the reported symmetrical compounds (37), the cycloreversion quantum yield

Synthesis The synthesis route for diarylethene 1O is shown in Fig. S1. First, compounds 2 and 3 were synthesized according to previously reported methods (31,32). Then, 1,10-phenanthroline-5,6-dione (3) was separately cyclized with compound 2 to give the asymmetric diarylethene 1O. Finally, the raw products were obtained by filtration, and then recrystallized from methanol to give the pure 1O. The structure of 1 was confirmed by elemental analysis, nuclear magnetic resonance (NMR), infra-red (IR) spectroscopy and high resolution molecular spectroscopy (HRMS). 1-[5-(4-Methoxylphenyl)-2-methylthien-3-yl]-2-[5-(4-(1H-imidazo [4,5-f][1,10] phenanthroline-2-yl)phenyl)-2-methylthien-3-yl] perfluorocyclopentene (1O). 1,10-Phenanthroline-5,6-dione (3)

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Figure 2. Absorption spectral and color changes of diarylethene 1O by –5 –1 photoirradiation at room temperature in methanol (C = 2.0 × 10 mol L ).

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(0.14 g, 0.67 mmol) and ammonium acetate (0.81 g, 10.51 mmol) were added to an acetic acid solution (30 mL) containing compound 2 (0.30 g, 0.60 mmol), and stirred at 353 K for 3 h. The crude product was obtained by filtration and recrystallized from methanol as a blue powder to give a 79% yield. Calculated for C41H26F6N4OS2 (%): C, 64.05; H, 3.41; N, 7.29. Found: C, 64.09; H, 3.49; N, 7.21; m.p. 483–484 K; 1H NMR (400 MHz, MeOD, ppm): δ 2.05 (s, 3H, –CH3), 2.08 (s, 3H, –CH3), 3.84 (s, 3H, –OCH3), 6.98 (d, 2H, J = 8.0 Hz), 7.27 (s, 1H), 7.54 (t, 3H, J = 8.0 Hz), 7.85–7.88 (m, 5H), 8.29 (d, 2H, J = 8.0 Hz), 9.00 (d, 2H, J = 8.0 Hz ), 9.07 (s, 2H); IR (ν, KBr, cm–1): 646, 740, 740, 805, 987, 1054, 1112, 1178, 1250, 1272, 1337, 1441, 1555, 1638, 2985, 3157, 3283, 3414; HRMS-ESI (m/z): [M+H]+ calculated for (C41H26F6N4OS2), 768.1452, found: 769.1509.

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1

Figure 3. Partial H NMR spectral changes of diarylethene 1 in the photostationary state (PSS) in deuterated methanol.

of 1 was decreased notably, whereas the cyclization quantum yield was enhanced to some extent. The thermal stability of the open-ring and closed-ring isomers for 1 was tested in acetonitrile at room temperature and at 353 K. The acetonitrile solution was stored at room temperature in the dark and then exposed to air for more than a week, no changes were detected in the UV/vis spectra for 1. At 353 K, diarylethene 1 also showed good thermal stability for more than 8 h. The results indicated that diarylethene containing a 1Himidazo [4,5-f][1,10] phenanthroline unit is thermally stable. Furthermore, the coloration and decoloration cycles of 1 by alternating irradiation with UV and visible light could be repeated 100 times with only 7% degradation of 1C (Fig. S1), indicating that diarylethene 1 has good fatigue resistance in the air at room temperature.

recover the original absorption and color (Fig. 4, inset), suggesting that the sensing process of 1C in response to Cu2+ was irreversible. Therefore, the change to Cu2+ could detected by a ‘naked-eye’ sensor based on 1C. In addition, the colorless solution of 1O–Cu2+ could not return to a blue solution when irradiated with 297 nm UV light, indicating that the 1O–Cu2+ chelate was not sensitive to UV light. The result is consistent with the symmetrical diarylethenes that contain two imidazole bridge units (27). The reason could be ascribed to the high binding affinity of 1O with Cu2+ (38), which may be strongly suppressed by the interconversion efficiency from the parallel to the anti-parallel configuration. Subsequently, the selectivity of 1C toward metal ions including alkali, alkaline earth, and transition-metal ions were investigated in acetonitrile (C = 2.0 × 10–5 mol L–1) containing 1C under the same experimental conditions. As depicted in Fig. 5, the colored solution of 1C changed from blue to colorless with a significant decrease in the absorption intensity at 603 nm after the addition of 1.0 equiv. Cu2+ (Fig. 5a). When the 1.0 equivalents of other ions, such as Fe3+, Al3+, Cr3+, Sn2+, Co2+, Mn2+, Ni2+, Pb2+, Zn2+, Cd2+, Ca2+, Mg2+, Ba2+, Sr2+, Hg2+, and K+, were added into the solution, no obvious changes in the absorption spectrum and color of 1C were observed (Fig. 5b). The results indicated that 1O has a strong binding affinity toward Cu2+ with high selectivity. This affinity can form the basis of selective ‘naked-eye’ colorimetric sensors for recognition of Cu2+. Fluorescence property

Colorimetric sensor for Cu2+ It was very interesting to observe that the closed-ring isomer 1C performed ‘naked-eye’ detection of Cu2+; the interaction between 1C and Cu2+ was investigated by UV/vis absorption spectroscopy in acetonitrile solution. Figure 4 shows the absorption spectrum and color changes of 1C as a function of Cu2+ concentration at room temperature. When Cu(NO3)2 solution was added to 1C dropwise, the absorption intensity at 603 nm decreased remarkably and leveled off after the addition of 7.5 equiv. Cu2+. The color changed from blue to colorless due to the formation of chelate 1O–Cu2+, accompanied with a 94% decrease in the absorption intensity detected by UV/vis absorption. Subsequent addition of an excess of ethylene diamine tetraacetic acid (EDTA) could not

Fluorescence properties can be utilized not only in molecular scale optoelectronics (39), but also in digital fluorescence photoswitches (40). Herein, the fluorescence property of 1O was measured in methanol (C = 2.0 × 10–5 mol L–1) at room temperature. As shown in Fig. 6, an emission peak of diarylethene 1O was observed at 454 nm when excited at 363 nm. Using anthracene as the reference, the fluorescence quantum yield of compound 1O was determined to be approximately 0.014. As with most of the reported diarylethenes (41–44), diarylethene 1 exhibited remarkable fluorescence switching properties upon photoisomerization from an open-ring isomer to a closed-ring isomer. The emission intensity of 1O was decreased to around 13% in the photostationary state. The residual fluorescence may be attributed to the incomplete cyclization reaction as well as to the existence of a parallel conformation (45). Back irradiation of the appropriate wavelength of visible light (λ > 500 nm) regenerated the open-ring isomer 1O and recovered the original emission spectra. Fluorescence changes of diarylethene 1O induced by ions

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Figure 4. Absorption spectral and color changes of 1C in acetonitrile (C = 2.0 × 5 1 2+ 10 mol L ) upon addition of Cu at room temperature: (Inset shows curve of 2+ absorption spectral of 1C at 553 nm upon addition of different equivalents Cu 2+ and color changes for 1C induced by Cu ).

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To further investigate the fluorescence sensing ability of 1 for metal ions, fluorescence titration experiments were performed. From Fig. 7(a), the emission intensity of 1 was seen to be markedly increased when the Ca2+ concentration increased from 0 to 10 equiv. Compared with 1O (λem = 454 nm), the emission peak of 1O–Ca2+ (λem = 487 nm) was red shifted by 33 nm and its intensity increased by 14-fold, accompanied with a notable fluorescence color change from dark to bright cyan (Fig. 7b, inset). In other words, the fluorescence of 1O can be turned from ‘off’ to ‘on’ by binding 1O to Ca2+. After addition of 7.5 equiv. EDTA solution to the acetonitrile solution containing 1O–Ca2+, the fluorescence intensity of 1O–Ca2+ was sharply decreased due to the complexation reaction between Ca2+ and EDTA (Fig. S1). Additionally, upon irradiation with 297 nm UV light, the emission intensity of 1O–Ca2+

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Ion-sensor based on a photochromic diarylethene

–5

–1

Figure 5. Changes in the absorption spectral of 1C at 603 nm by the addition of various metal ions in acetonitrile (C = 2.0 × 10 mol L ). (a) The absorption spectral changes. (b) Color change of 1C after addition of 1.0 equiv. of different metal ions.

Figure 6. Emission spectral changes and photographs of diarylethene 1O by –5 –1 photoirradiation in methanol (C = 2.0 × 10 mol L ) at room temperature when excited at 363 nm.

was nearly completely quenched because of the formation of the non-fluorescent closing-ring isomer of 1C–Ca2+ (Fig. 7b). Therefore, these characteristics of 1 could be potentially applied as a fluorescent switch and chemical senor for certain metal ions (46). To explore the selectivity of diarylethene 1O for Ca2+, comparative tests were extended to other metal cations in acetonitrile using fluorescence spectroscopy. Metal ions including alkali, alkaline earth, and transition-metal ions were separately added into the acetonitrile solution containing diarylethene 1O under the same experimental conditions. Figure 8 shows the emission intensity changes of 1O at 454 nm in acetonitrile solutions in the presence of respective metal cations at room temperature (excited at 363 nm). The results revealed that 1O responded to various metal ions with different selectivity and possessed excellent selectivity for Ca2+, Sr2+, and Mg2+. As depicted in Fig. 8, it can be seen that the emission intensity of 1O was not obviously influenced by the addition of 10 equiv. of Cd2+, Ba2+, K+, Cu2+, Zn2+, Pb2+, Ni2+, Cr3+, Fe3+, Sn2+, Hg2+, Mn2+, Al3+, Co2+, and Cu2+. The results

2+

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Figure 7. Emission spectral and photos changes of diarylethene 1O induced by Ca /EDTA and light stimuli at room temperature, excited at 363 nm in acetonitrile solution (C = –5 –1 2+ 2.0 × 10 mol L ). (a) Emission spectral changes for 1O. Inset shows the effects of Ca concentration on emission intensity of 1O at 454 nm. F0: initial emission intensity of 1O; F: 2+ 2+ emission intensity of 1O in the presence of Ca . (b) Emission spectral change of 1O–Ca . Inset shows the color changes in fluorescence.

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Figure 8. Emission intensity of 1O at 454 nm when excited at 363 nm in acetonitrile -5 -1 (C = 2.0 × 10 mol L ) in the presence of respective metal cations (10 equiv.).

indicated that compound 1 was only selective toward Ca2+ with the exceptions of Sr2+, and Mg2+. The fluorescence enhancement of 1O induced by Ca2+, Sr2+, and Mg2+ may be attributed to the photo-induced electron transfer effect (24). Therefore,

diarylethene 1 could be potentially used as a fluorescence sensor for detection of Ca2+, Sr2+, and Mg2+ in acetonitrile. In order to confirm the coordination between 1O and Ca2+ at the imidazole part or phenanthroline moiety, a Job’s plot analysis of 1O and Ca2+ was carried out by fluorescence spectroscopy in acetonitrile, and the results are depicted in Fig. S1. It could be easily calculated that the stoichiometry between Ca2+ and 1O was 1:1. Similarly, the complex ratios of 1O–Sr2+ and 1O–Mg2+ were determined to be 1:1. Additionally, UV absorption spectrum interference titration experiments were performed to clarify the coordination at the imidazole or phenanthroline moiety. The absorption intensity of the closed-ring isomer exhibited only a minor change when 5.0 equiv. Ca2+ was added to the solution containing 1C in the PSS state. Subsequently, 7.0 equiv. Cu2+ was added dropwise to the 1C–Ca2+, resulting in a marked decrease in its absorption intensity (Fig. S1). The result indicated that the coordinate sites of compound 1 for Ca2+ and Cu2+ different. In our previous work, we demonstrated that Cu2+ was coordinated at the imidazole part (9). Therefore, we deduced that the coordinate of diarylethene 1 with Ca2+ should be situated at the two nitrogen atoms of the phenanthroline moiety.

2+

Figure 9. Schematics of molecular structures and fluorescence changes of 1O induced by Ca /EDTA and light stimuli.

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2+

Figure 10. The combinational logic circuits equivalent to the truth table given in Table 1 In1 (297 nm UV light), In2 (>500 nm visible light), In3 (Ca ), In4 (EDTA) and O1.

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Ion-sensor based on a photochromic diarylethene Table 1. Truth table for all possible strings of four binary-input data and the corresponding output digit Input In1 (UV) 0 0 0 0 1 0 0 1 1 1 0 0 1 1 1 1

In2 (Vis)

In3 (Ca2+)

In4 (EDTA)

0 0 0 1 0 0 1 1 0 0 1 1 0 1 1 1

0 0 1 0 0 1 1 0 0 1 0 1 1 0 1 1

0 1 0 0 0 1 0 0 1 0 1 1 1 1 0 1

Outputa λem = 487 nm

0 0 1 0 0 0 1 0 0 1 0 0 0 0 1 0

a

At 487 nm, the emission intensity above the original value of 1O (454 nm) was defined as 1, at other intensities it was defined as 0.

Application in logic circuit The emission intensity of 1O could be modulated by either light or chemical stimuli in acetonitrile. Therefore, a logic circuit could be constructed based on the interconversion among the different states of 1 (Fig. 9). As shown in Fig. 10, In1 (297 nm UV light), In2 (>500 nm visible light), In3 (Ca2+), and In4 (EDTA) were selected as the four inputs, while the emission intensity at 487 nm was described as the output. The emission intensity at 454 nm of 1O is regarded as the original value. When the relative emission intensity at 487 nm was 13 times larger than the original value, the output signal could be regarded as ‘on’, at other intensities it was determined to be ‘off.’ Hence, we can use the binary digits (’1’ or ’0’) instead of the two levels (‘on’ and ‘off’). As a result, diarylethene 1 can read as a string of four inputs and one output. For example, when the input string is ‘0’, ‘0’, ‘1’ and ‘0’. the corresponding input signals In1, In2, In3, and In4 are ‘off’, ‘off’, ‘on’, and ‘off’, respectively. Under these conditions, diarylethene 1O was converted to 1O–Ca2+ by stimulation of Ca2+ and its emission intensity enhanced significantly. Therefore, the output signal was ‘on’ and the output digit was ‘1’. All the possible strings of the four inputs are listed in Table 1.

Conclusion

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Acknowledgements The authors are grateful for the financial support from the National Natural Science Foundation of China ( grant nos. 21262015, 21362013, and 51373072), the Project of Jiangxi Advantage SciTech Innovative Team ( grant no. 20142BCB24012), the Science Funds of Natural Science Foundation of Jiangxi Province ( grant nos. 20122BAB213004 and 20132BAB203005), and the Project of the Science Funds of Jiangxi Education Office ( grant nos. KJLD13069, KJLD12035, and GJJ12587).

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A multiple responsive asymmetrical diarylethene with a 1Himidazo [4,5-f][1,10] phenanthroline unit was successfully synthesized and fully characterized. Its photochromic behavior and selective interaction with Cu2+ were investigated in detail. Cu2+ leads to the cycloreversion reaction of the photochromic unit and bleaches the color of the closed-ring isomer in the photostationary state. The results indicated that diarylethene could be potentially applied to the visual detection of Cu2+ in solution. Furthermore,

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