Photocontrolled Reversible Luminescent Lanthanide Molecular Switch

Jul 22, 2016 - A new europium complex coordinated between a Eu(III) ion and an unsymmetrical diarylperfluorocyclopentene yields a light-controlled dia...
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Photocontrolled Reversible Luminescent Lanthanide Molecular Switch Based on a Diarylethene−Europium Dyad Hong-Bo Cheng,† Guo-Fei Hu,‡ Zhan-Hui Zhang,‡ Liang Gao,† Xingfa Gao,† and Hai-Chen Wu*,† †

Key Laboratory for Biomedical Effects of Nanomaterials & Nanosafety, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China ‡ College of Chemistry & Material Science, Hebei Normal University, Shijiazhuang 050024, China S Supporting Information *

ABSTRACT: A new europium complex coordinated between a Eu(III) ion and an unsymmetrical diarylperfluorocyclopentene yields a light-controlled diarylethene−europium dyad, DAE@TpyEu(tta)3, whose photophysical properties can be reversibly switched by optical stimuli. When DAE@TpyEu(tta) 3 is exposed to 365 nm UV light, an efficient intramolecular photochromic fluorescence resonance energy transfer (pc-FRET) occurs between the emission of the Eu3+ donor (D) and the absorption of the diarylethene acceptor (A) in closed-form DAE@TpyEu(tta)3 accompanied by luminescence quenching. However, the pc-FRET process could be effectively inhibited by visible light (λ > 600 nm) irradiation, and the lanthanide emission of DAE@TpyEu(tta)3 is rapidly recovered. Furthermore, this luminescent lanthanide molecular switch could serve as a highly reliable and sensitive “turn on” fluorescent marker in living cells irradiated by red light without any optical interference.

1. INTRODUCTION

PBI dye generates a dual-color system that can work as a photoswitch for fluorescent green or red color.8 Using light to induce fluorescence changes in molecular systems has great potentials in biochemistry and medicine. Yi and co-workers developed an amphiphilic diarylethene that can form stable vesicle nanostructures in aqueous solution and enter living cells as a cellular marker with high-ratio signal change and outstanding resistance to fatigue.9 Moreover, Branda and co-workers reported a novel photoresponsive dithienylethene that can reversibly switch between biologically active and inactive forms when irradiated with different wavelengths of light.10 This molecular switch can “on−off” control biological functions in a living organism. Most of the fluorescence photoswitching mechanisms are based on molecular dyads consisting of a fluorescent dye unit and a photochromic unit in the molecule. The fluorescence behavior of these molecules can be regulated with optical stimulations through intramolecular electron/energy transfer.11 However, most of those works relied on the emission of the conjugated structure of diarylethene molecules, which is normally in the short wavelength range. For living organism imaging, long wavelength excitation is highly desirable. Sensitized luminescence from lanthanide complexes has attracted increasing attention because of their unique optical properties, such as visible light emission, long-lived excited states, narrow emission

Responsive materials are playing a leading role in a wide range of applications, such as biosensors, drug delivery, and “smart” optical systems.1 Light is one of the most extensively used triggers among different external stimuli, owing to its accessibility, instant action, and cleanness.2 Photochromic materials can be reversibly interconverted by light between two states with distinctly different spectroscopic responses, which makes them ideal candidates for constructing optically responsive materials.3 In particular, diarylethene derivatives (DAEs) are one type of the most promising optically responsive compounds owing to their notable thermally irreversible photochromic behavior and prominent fatigue resistance.4 Photoswitchable DAE molecules can undergo light-induced cyclization and decyclization reactions which result in isomers with different absorption profiles and usually different colors. Also, their luminescence emissions can alternate between “ON” and “OFF” states in response to external light stimulation.5 So far, the integration of photochromic units with fluorescent groups has found remarkable applications in optical memories and fluorescent biological markers.6 For instance, the combination of DAE photoswitches and perylenebisimide (PBI) units forms single-molecule fluorescence photoswitches, offering a new molecular design principle for photoswitching molecules with nondestructive readout capability.7 A method for pairing a single-color photochromic spiropyran connected to a fluorescent donor © XXXX American Chemical Society

Received: April 22, 2016

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DOI: 10.1021/acs.inorgchem.6b01009 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry bandwidths, and large Stokes shifts.12 Modulating luminescence intensity is one of the most useful features in the photoswitching technique.13 However, luminescent photoswitches based on lanthanide complexes, especially those that can be reversibly modulated by light, remain a challenge.14 In this work, we report the synthesis, characterization, and application of a new europium complex coordinated between a Eu(III) ion and a thermally stable unsymmetrical perfluorocyclopentene-based DAE@Tpy. The Eu(III) luminescence can be reversibly modulated by the photocyclization and photocyclo-reversion reactions of the DAE moiety caused by UV and visible light irradiation. In order to lower the excitation energy to facilitate FRET from the excited Eu(III) ion to the closed isomer of the photochromic switch, we introduced an indole group in the FRET acceptor DAE@Tpy (Scheme 1). We also

The optical modulation of the complex DAE@TpyEu(tta)3 was investigated in detail. DAE@Tpy and DAE@TpyEu(tta)3 both exhibited reversible and bistable photochromism (Figure S8 and Figure 1). Irradiation of a solution of open-form (OF)

Scheme 1. Synthetic Route for the Preparation of the Complex DAE@TpyEu(tta)3

Figure 1. (a) Absorption spectra of DAE@TpyEu(tta)3 (2.0 × 10−5 M) upon exposure to 365 nm light for 1.5 min. The complex was converted from the initial open form to the closed form after UV irradiation. Inset: changes in the color of DAE@TpyEu(tta)3 solution upon alternating UV (365 nm, 1.5 min) and visible light (λ > 600 nm, 3.0 min) irradiation in CHCl3. (b) Changes in the color of DAE@ TpyEu(tta)3 in PMMA film upon alternating UV (365 nm, 1.5 min) and visible light (λ > 600 nm, 6.0 min) irradiation.

DAE@Tpy in chloroform with 365 nm light resulted in the generation of two new peaks appearing at 395−500 nm and 500−800 nm with an absorption maximum at ∼625 nm. These new peaks were caused by the absorption of closed-form (CF) DAE@Tpy (Figure S8). Similarly, irradiation of OF-DAE@ TpyEu(tta)3 in chloroform with 365 nm light also generated two new peaks with absorption maxima at 445 and 650 nm, respectively (Figure 1a). However, the major absorption maximum of CF-DAE@TpyEu(tta)3 (∼650 nm) underwent a red shift of 25 nm compared with CF-DAE@Tpy. Meanwhile, the color of the solution changed from colorless to dark blue (Figure 1a). Interestingly, upon visible light (λ > 600 nm) irradiation for 3.0 min, the dark-blue solution of CF-DAE@ TpyEu(tta)3 gradually turned colorless, resulting from the photocyclo-reversion of DAE@TpyEu(tta)3. The signal of thermal ring opening of CF-DAE@TpyEu(tta)3 was barely observed for at least 300 min upon excitation at 650, 500, and 447 nm (Figure S9). To further investigate the kinetics of the transformation from the closed form to the open form, we measured the A−t (absorption−time) decay curves and performed the dynamic fitting. As shown in Figure S10, the absorption intensity shows a nonlinear relationship with the irradiation time. The absorption intensity follows a biexponential attenuation law, which had a fast process, followed by a slow process. The fatigue resistance of DAE@TpyEu(tta)3 was also examined after 10-cycle irradiation of UV (365 nm, for 3.0 min) and visible light (λ > 600 nm, for 3.0 min), and the results are

used perfluorocyclopentene as the central ethene linker to enhance its fatigue resistance. The structure of the complex DAE@TpyEu(tta)3 has been characterized by 1H NMR, 13C NMR, and high-resolution mass spectroscopy. Furthermore, we have applied the DAE@TpyEu(tta)3 in cell imaging studies harnessing the luminescence recovery of the dyad upon visible light (λ > 600 nm) irradiation.

2. RESULTS AND DISCUSSION The synthesis of DAE@TpyEu(tta)3 is shown in Scheme 1 (experimental details are described in the Experimental Section and Figures S1−S7). First, the ligand DAE@Tpy was synthesized by coupling 4-(2-(1,2-dimethyl-1H-indol-3-yl)3,3,4,4,5,5-hexafluorocyclopent-1-enyl)-5-methylthiophene-2carbaldehyde with 1-(pyridin-2-yl)ethanone and ammonia under basic conditions (yield: 43%). Then, the target Eu3+ complex DAE@TpyEu(tta)3 was prepared by the complexation of the resulting ligand DAE@Tpy with europium thenoyltrifluoroacetonate (Eu(tta)3·3H2O) at room temperature. B

DOI: 10.1021/acs.inorgchem.6b01009 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry shown in Figure S11. After 10-cycle irradiation, the absorption at 650 nm was kept by 96.3%. Therefore, DAE@TpyEu(tta)3 exhibits excellent fatigue resistance, which is of utmost importance to their practical applications. The quantum yields corrected for the active conformer for the photocyclization (Φo−c) were determined to be 0.20 for DAE@Tpy and 0.19 for DAE@TpyEu(tta)3. Similarly, the quantum yields corrected for the active conformer for the photocyclo-reversion (Φc−o) were determined to be 0.12 for DAE@Tpy and 0.15 for DAE@TpyEu(tta)3. Moreover, DAE@ TpyEu(tta)3 also displayed excellent photochromic performance in poly(methyl methacrylate) (PMMA) film, as was the case in solution (Figure 1b). Upon irradiation with 365 nm light, the color of PMMA that contains DAE@TpyEu(tta)3 changed from colorless to green, resulting from the photocyclization of OF-DAE@TpyEu(tta)3. On the other hand, after visible light irradiation (λ > 600 nm) for 6.0 min, the green PMMA containing CF-DAE@TpyEu(tta)3 turned colorless (Figure 1b). In order to verify the photoreactivity of OF-DAE@ TpyEu(tta)3, a density functional theory (DFT) calculation has been performed on the structure and electronic structure of OF-DAE@TpyEu(tta)3 using the M062X DFT method.15 The 6-31G(d,p) basis set was used for all nonmetal atoms, and the SDD basis set and pseudopotential were used for Eu.16 Figure 2

Figure 3. Changes in 1H NMR spectra of DAE@TpyEu(tta)3 (2.0 × 10−3 M) in CDCl3 solution: (a) before irradiation; (b) photostationary state after 365 nm light irradiation.

shifted upfield to 2.89 ppm. Additionally, the protons located on the pyridine moiety (Hd, He, and Hf) in OF-DAE@ TpyEu(tta)3 shifted downfield (Δδ = −0.07, −0.10, and −0.34 ppm, respectively) after photocyclization. These shifts in NMR signals could be ascribed to the alterations of the chemical environment in the structure after the photocyclization reaction. The yield of the open-form to the closed-form conversion was determined to be 53.5% for DAE@TpyEu(tta)3 by 1H NMR. It is well-known that the luminescence modulation relies on good spectral overlap between the emission of the metal complex moiety and the absorption of the photochromic unit.19 In the present OF-DAE@TpyEu(tta)3, the luminescence emission occurs in the range of 605−640 nm, while the absorption of OF-DAE@TpyEu(tta)3 takes place below 400 nm (Figure 4). Therefore, no intramolecular FRET from the Eu3+ moiety of OF-DAE@TpyEu(tta)3 to the diheteroaryle-

Figure 2. Calculated equilibrium structure of OF-DAE@TpyEu(tta)3.

shows the equilibrium structure of OF-DAE@TpyEu(tta)3 calculated at the nonet state. The angle between the pyrrole and thiophene planes (i.e., the dihedral angle of atoms 1−2− 5−6) is 37.14°. The atomic distance between atoms 1 and 6 is 4.128 Å, which is within the critical distance for the cyclization reaction to take place.17 Figure S12 shows the HOMO (up: −6.44 eV, down: −6.50 eV) and LUMO (up: −1.40 eV, down: −3.03 eV) of OF-DAE@TpyEu(tta)3, in which α and β denote the orbitals for spin-up and spin-down electrons, respectively. The DFT calculation suggested that the energy level gap of spin up HOMO−LUMO (5.04 eV) is higher than that of spin down HOMO−LUMO (3.47 eV) and the latter is more prone to photocyclization. It is well-documented that NMR signals can afford distinct differences between the open-form and closed-form isomers of DAEs at both high and low fields.18 As shown in Figure 3, the methyl hydrogens (Hb and Hc) on the reactive carbon in OFDAE@TpyEu(tta)3 give two single peaks at δ 1.99 ppm (Hb) and 1.91 ppm (Hc), respectively. Upon irradiation with 365 nm light, two new peaks appear at δ 1.65 (Hb*) and 1.60 (Hc*) ppm, corresponding to the methyl protons of CF-DAE@ TpyEu(tta)3. Compared with the Ha signal (3.41 ppm) in OFDAE@TpyEu(tta)3, the signal of Ha* in CF-DAE@TpyEu(tta)3

Figure 4. Normalized absorption spectra (left axis) and luminescence emission (right axis) of the photochromic complex DAE@TpyEu(tta)3. OF-DAE@TpyEu(tta)3: green line; CF-DAE@TpyEu(tta)3: red line; luminescence emission: blue line. C

DOI: 10.1021/acs.inorgchem.6b01009 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

complex solution was irradiated with UV light (365 nm) for 3.0 min, the characteristic luminescence emission of the Eu3+ complex at 615 nm was drastically quenched. However, after irradiation with visible light for 3.0 min, the luminescence intensity could be totally recovered. Similarly, the Eu(III) emission of DAE@TpyEu(tta)3 could also be efficiently and reversibly switched “on−off” in PMMA films upon alternating UV and visible light irradiation (Figure 5b). For many applications, especially those facilitating device integration, it is desirable that such photoswitching constructs could be effectively immobilized on a solid support.22 Thus, we anticipate that DAE@TpyEu(tta)3 can be used as a luminescent molecular switch integrated in novel and smart materials for use in solid-state optoelectronics. Because of its high sensitivity, the fluorescence signal offers a noninvasive means to investigate biomolecular mechanisms and biological pathways in living cells.23 However, application of fluorescence technologies for real-world samples faces many challenges and difficulties, including optical interferences from the intrinsic heterogeneity of biological samples and high background resulting from autofluorescence. The design and synthesis of novel fluorescent probes that can modulate the fluorescence output are the key to resolving these problems.24 Conventional fluorescent probes such as rhodamine and cyanine dyes have been widely employed for cell imaging studies, but they could only respond irreversibly to one event. In this work, the smart complex DAE@TpyEu(tta)3 can be used as a photoswitchable probe for imaging in living cells. First, we carried out the absorption measurements to investigate the photochromic performance of DAE@TpyEu(tta)3 in DMEM/DMSO (400:1, v/v). As shown in Figure S16, upon irradiation with 365 nm light for 1.5 min, the color of the solution that contains DAE@TpyEu(tta)3 changed from colorless to green, resulting from the photocyclization of OFDAE@TpyEu(tta)3. On the other hand, after visible light irradiation (λ > 600 nm), the new absorption peaks appearing at 500−800 nm disappeared entirely (Figure S16b) and the green solution containing CF-DAE@TpyEu(tta)3 eventually turned colorless (Figure S16c). These results demonstrate that DAE@TpyEu(tta)3 also displayed excellent photochromic performance in DMEM/DMSO (400:1, v/v). To substantiate the sensing mechanism of DAE@TpyEu(tta)3 proposed in Scheme 2, we performed several imaging experiments in living cells to interrogate the properties of this photoswitchable probe in response to external light stimulation. First, irradiation of the colorless OF-DAE@TpyEu(tta)3 with 365 nm light led to the generation of CF-DAE@TpyEu(tta)3 and the quenching of most of the luminescence because of the FRET from the excited Eu(III) ion to the closed-form isomer of the photochromic complex. Next, NIH 3T3 cells (mouse embryonic fibroblast cells) were incubated with a serum-free DMEM/DMSO (400:1, v/v) solution of CF-DAE@TpyEu(tta)3 (1 × 10−5 M) at 37 °C for 20 min. We hardly observed any luminescence in the cytoplasm of the cells by taking confocal laser scanning microscopy (CLSM) images (Figure 6a). Finally, when the NIH 3T3 cells were exposed to 635 nm light, the endocytosized CF-DAE@TpyEu(tta)3 was converted to OF-DAE@TpyEu(tta)3 and the luminescence of the openform isomer was recovered in the cells (Figure 6b). This imaging process would effectively eliminate optical interferences resulting from UV irradiation and autofluorescence. Our results have indicated that this diarylethene−europium dyad

thene of the complex would be expected. However, after DAE@TpyEu(tta)3 was exposed to 365 nm light, the diheteroarylethene of OF-DAE@TpyEu(tta)3 switched from open form to closed form, accompanied by the generation of a new absorption peak in the range of 500−850 nm. Then, intramolecular FRET from the Eu3+ moiety of CF-DAE@ TpyEu(tta)3 to the diheteroarylethene of the complex could be observed. As shown in Figure 5, DAE@TpyEu(tta)3 displayed strong red luminescence in solution. When excited with 385 nm light,

Figure 5. (a) Luminescence emission spectra of the DAE@ TpyEu(tta)3 (1.0 × 10−5 M). The upper inset shows emission intensity changes at 615 nm upon UV irradiation (λex = 385 nm) in CHCl3. The lower inset shows the solution color changes upon alternating UV and visible light irradiation in CHCl3. (b) The emission color changes upon alternating UV (365 nm) and visible light (λ > 600 nm) irradiation in PMMA.

the complex exhibited multiple emission peaks at 579 nm (5D0 → 7F0), 592 nm (5D0 → 7F1), 615 nm (5D0 → 7F2), and 653 nm (5D0 → 7F3).20 The luminescence of DAE@TpyEu(tta)3 might be ascribed to intramolecular energy transfer (ET) from the excited Tpy moiety and β-diketonates to Eu3+.21 Upon irradiation with 365 nm light, the luminescence intensity of DAE@TpyEu(tta)3 at 615 nm was quenched by 83% with a concomitant decrease of the quantum yield from 0.105 to 0.017 (Figure 5a). To correlate the luminescence intensity with the irradiation time, we further measured the I−t (intensity−time) decay curve of DAE@TpyEu(tta)3 (1.0 × 10−5 M) upon UV irradiation (λex = 385 nm) in CHCl3 and performed the dynamic fitting. As shown in Figure S13, we found that the luminescence quenching follows a biexponential attenuation law, which had a fast process, followed by a slow process. In addition to the steady-state luminescence quenching, the luminescence lifetimes of Eu3+ in OF-DAE@TpyEu(tta)3 (τ0 = 102 μs; Figure S14) also decreased in CF-DAE@TpyEu(tta)3 (τ0 = 17 μs; Figure S15). These observations jointly indicated that there was a good energy match between the emission of the Eu3+ donor (D) and the absorption of the diheteroarylethene acceptor (A) in CF-DAE@TpyEu(tta)3. After the D

DOI: 10.1021/acs.inorgchem.6b01009 Inorg. Chem. XXXX, XXX, XXX−XXX

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spectrophotometer. Fluorescence spectra of photochromic studies and FRET measurements were recorded on a Shimadzu RF-5301PC fluorescence spectrophotometer with an excitation wavelength of 385 nm. FT-IR spectra were obtained on a Thermo Fisher iN10. The method for the preparation of the PMMA film is as follows: photochromic materials DAE@Tpy, DAE@TpyEu(tta)3 were dissolved in chloroform (3 mL). The solution was filtered, and then PMMA was added to the solution with stirring until it completely dissolved. The solution was coated on a glass substrate at room temperature. The coated film was dried in a vacuum oven at 25 °C to give a transparent and homogeneous film. Confocal fluorescence imaging experiments were performed on an Olympus FV-1000 laser scanning microscopy system, based on an IX81 (Olympus, Japan) inverted microscope. The microscope was equipped with multiple visible laser lines (405, 458, 488, 515, 543, 635 nm) and a UPLSAPO 60×/N.A 1.42 objective. NIH 3T3 cells were cultured in Dulbecco’s modified Eagle’s medium (DMEM, Gibco) supplemented with 10% fetal calf serum (FCS, Gibco), 50 g/mL penicillin/streptomycin (Hyclone) at 37 °C, and the cells were cultured for 2 days. Then, the cells were incubated with 5 μL of the probe CF-DAE@TpyEu(tta)3 (4 mM in DMSO) in 2 mL of serumfree DMEM for 20 min at 37 °C under 5% CO2, and then washed with PBS three times and bathed in PBS (1 mL) before imaging. Synthesis. Preparation of DAE@Tpy. 2-Acetylpyridine (0.27 g, 2.23 mmol) was added into a solution of compound 1 (0.33 g, 0.75 mmol) in EtOH (30 mL). KOH pellets (0.25 g, 4.46 mmol) and NH3· H2O (15.0 mL, 29.3%) were added to the solution. The solution was stirred at 35 °C for 3 days. The mixture was cooled to 20 °C, and then the off-white precipitate was collected by filtration and washed with ice-cold EtOH (10 mL). Recrystallization from EtOH obtained white needle solid DAE@Tpy (0.21 g), yield: 43%. 1H NMR (500 MHz, CDCl3) δ 8.74 (d, J = 4.4 Hz, 2H), 8.63 (d, J = 7.9 Hz, 2H), 8.60 (s, 2H), 7.88 (t, J = 7.6 Hz, 2H), 7.82 (s, 1H), 7.64 (d, J = 7.9 Hz, 1H), 7.36 (dd, J = 6.7, 5.4 Hz, 2H), 7.26 (d, J = 6.9 Hz, 1H), 7.22 (t, J = 7.5 Hz, 1H), 7.15 (t, J = 7.4 Hz, 1H), 3.61 (d, J = 7.3 Hz, 3H), 2.00 (s, 3H), 1.80 (s, 3H). 13C NMR (125 MHz, CDCl3) δ 156.1, 155.8, 149.1, 143.1, 142.6, 139.1, 138.0, 137.0, 127.2, 125.7, 125.5, 124.1, 122.0, 121.4, 121.1, 119.6, 116.8, 109.2, 101.0, 30.0, 14.9, 11.6. HRMS m/z [M + H]+ calcd for C35H25F6N4S+ 647.1704, found 647.1698. Preparation of DAE@TpyEu(tta)3. Eu(tta)3·3H2O (0.03 g, 0.04 mmol) was added into a solution of compound DAE@Tpy (1.00 g, 1.0 mmol) in EtOH (30.0 mL) to give instantaneously a yellow solution

Scheme 2. Schematic Illustration of the Luminescence Recovery of CF-DAE@TpyEu(tta)3 Irradiated by 635 nm Light in NIH 3T3 Cells

can be further exploited as a visible-light-activated fluorescent probe for organelle imaging in living cells.

3. EXPERIMENTAL SECTION Materials. All the commercial reagents were purchased from Sigma-Aldrich and Alfa Aesar unless otherwise annotated and were used without further purification. 4-(2-(1,2-Dimethyl-1H-indol-3-yl)3,3,4,4,5,5-hexafluorocyclopent-1-enyl)-5-methylthiophene-2-carbaldehyde (1) was prepared according to a literature procedure.18b Measurements. 1H NMR spectra were recorded on a Bruker AV500 spectrometer at 25 °C. The optical switch experiments were carried out using a photochemical reaction apparatus with a 500 W medium-pressure mercury lamp (CEL-M500) at room temperature. Fluorescence lifetimes and quantum yield measurements were recorded in a conventional quartz cell (10 × 10 × 45 mm) at 25 °C (λ ex = 370 nm) on a HORIBA Scientific Fluorolog-3 spectrofluorometer. UV/vis spectra of photochromic studies and FRET measurements were recorded using a HITACHI U-3900

Figure 6. CLSM images of NIH 3T3 cells incubated with CF-DAE@TpyEu(tta)3 for 20 min at 37 °C (1 × 10−5 M in serum-free DMEM/DMSO, 400:1, v/v). (a) In original state; (b) irradiated with 635 nm light, (λex = 405 nm); (c, d) bright-field transmission image of NIH 3T3 cells; (e) overlay image of (a) and (c); (f) overlay image of (b) and (d). E

DOI: 10.1021/acs.inorgchem.6b01009 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

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with bright red luminescence under daylight. After evaporation of the solvent, the residue was redissolved in a small amount of diethyl ether. Addition of n-hexane to the solution led to the precipitation of the target complex of DAE@TpyEu(tta)3 as an orange powder, yield: 74%. 1 H NMR (500 MHz, CDCl3) δ 12.98 (s, 2H), 10.80 (s, 2H), 8.33 (s, 2H), 7.59 (d, J = 7.4 Hz, 1H), 7.38 (s, 1H), 7.24−7.20 (m, 1H), 7.14 (t, J = 9.6 Hz, 2H), 7.01 (s, 4H), 6.95 (s, 2H), 6.83 (s, 1H), 6.27 (s, 4H), 5.67 (s, 3H), 3.41 (s, 3H), 1.99 (s, 3H), 1.91 (s, 3H). 13C NMR (125 MHz, CDCl3) δ 159.3, 153.2, 153.0, 150.5, 150.0, 149.1, 145.5, 137.8, 137.0, 133.7, 132.9, 127.9, 127.8, 124.3, 123.7, 122.2, 121.2, 120.0, 119.3, 115.4, 109.3, 100.7, 99.6, 70.2, 64.0, 29.8, 22.7, 14.9, 14.7, 11.4, 10.5. HRMS m/z [M − TTA]+ calcd for C51H32EuF12N4O4S3+ 1241.0606, found 1241.0613.

4. CONCLUSIONS In summary, we have designed and synthesized a new luminescent lanthanide molecular switch DAE@TpyEu(tta)3, which can be reversibly controlled “on−off” both in solution and in PMMA films. This dyad shows excellent photochromic properties and efficient luminescence emission modulation as well as notable thermal stability and fatigue resistance. The luminescence of the complex can be reversibly switched “on− off” by external visible light stimulation, which represents a useful development in the exploitation of complex and functional optoelectronic systems. Furthermore, DAE@TpyEu(tta)3 can enter living cells and act as a sensitive fluorescent probe for biological imaging without optical interferences. We anticipate that the diarylethene−europium dyad could be further improved for more sophisticated imaging studies and integrated in practical optoelectronic systems.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01009. Experimental procedures and additional data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Natural Science Foundation of China (nos. 21502195, 21175135, 21375130), the National Basic Research Program of China (973 program, no. 2013CB932800), and the “100 Talents” program of the Chinese Academy of Sciences.

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DOI: 10.1021/acs.inorgchem.6b01009 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.6b01009 Inorg. Chem. XXXX, XXX, XXX−XXX