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Letter Cite This: Org. Lett. 2017, 19, 5728-5731

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Near-Infrared Emissive Discrete Platinum(II) Metallacycles: Synthesis and Application in Ammonia Detection Zhengtao Li,† Xuzhou Yan,‡ Feihe Huang,*,† Hajar Sepehrpour,‡ and Peter J. Stang*,‡ †

State Key Laboratory of Chemical Engineering, Center for Chemistry of High-Performance & Novel Materials, Department of Chemistry, Zhejiang University, Hangzhou 310027, P. R. China ‡ Department of Chemistry, University of Utah, 315 South 1400 East, Room 2020, Salt Lake City, Utah 84112, United States S Supporting Information *

ABSTRACT: Two novel discrete organoplatinum(II) metallacycles are prepared by means of coordination-driven self-assembly of a 90° organoplatinum(II) acceptor, cis-(PEt3)2Pt(OTf)2, with two donors, a pyridyl donor, 9,10-di(4-pyridylvinyl)anthracene, and one of two dicarboxylate ligands. Both metallacycles display aggregation-induced emission as well as solvatochromism. More interestingly, both metallacycles exhibit near-infrared fluorescent emission in the solid state and can be used to detect ammonia gas.

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integrating these two fascinating aspects into one material, we are interested in exploring metal-coordination chemistry to design and synthesize SCCs with special light-emitting properties in both states as well as attractive applications via fluorescent detection. 9,10-Distyrylanthracene (DSA) derivatives, as a class of luminogens with AIE characteristics, have been widely used in bioimaging, cancer therapy, fluorescent probe, sensors, and so on.7 By replacing the phenyl groups with pyridyl groups in DSA, we obtained the organic pyridyl donor 1 (Scheme 1), 9,10-di(4-

he continued development of luminescent materials is of great importance for both biology and materials science, which demand constantly updated molecular architectures, synthetic methodologies, and research.1 Among these materials, near-infrared (NIR)-emitting species have attracted increasing attention in optical imaging studies2 because light in the NIR region (650−900 nm)3h,i does less harm to living cells, causes less absorption and scattering interference, and has stronger penetration into deep tissues with lowest autofluorescene background. A number of near-infrared fluorescent dyes, for example, perylenediimide (PDI),3a cyanine,3b,c and BODIPY,3d−f have been reported.3 However, many of these require laborious covalent synthetic procedures and usually suffer from aggregation-caused quenching (ACQ) in concentrated solutions or in the solid state. Although aggregation-induced emission (AIE) molecules with NIR emission have attracted increasing attention, only a few studies have been reported4 and the corresponding AIE luminogens are often complicated and require tedious chemical syntheses. It is well-known that coordination-driven self-assembly via spontaneous formation of metal−ligand bonds is a powerful tool in constructing supramolecular coordination complexes (SCCs) with predictable and well-defined molecular structures.5 In particular, by making use of phosphine-capped Pt(II) as the square-planar metal acceptor and different kinds of organic ligands as donors, a series of 1D, 2D, and 3D topological architectures have been reported in nearly quantitative yields.5h By decorating the periphery and core of the precursors with various functional groups, discrete SCCs may acquire novel properties, such as fluorescence, amphiphilic behavior, thermoresponsiveness, and so on. 6 Recently, we incorporated tetraphenylethylene (TPE) cores into the discrete SCCs and thereby obtained novel highly emissive metallacages both in dilute solutions and in the aggregated state, different from both ACQ and AIE phenomena.6c Although these systems exhibited interesting photophysical properties in various solutions, their performance in the solid state is underdeveloped. With the aim of © 2017 American Chemical Society

Scheme 1. Representation of the Formation of Metallacycles 2 and 3

pyridylvinyl)anthracene (DPA), a well-designed and AIE-active ligand with suitable angle (180°) to coordinate with an organoplatinum(II) acceptor.8 Herein, we report the construction of two new metallacycles, 2 and 3 (Scheme 1), by means of coordination-driven self-assembly of 90° organoplatinum(II) acceptor 4 with organic donors, ligand 1, and dicarboxylates 5 and 6, respectively. Both metallacycles emit Received: August 8, 2017 Published: October 13, 2017 5728

DOI: 10.1021/acs.orglett.7b02456 Org. Lett. 2017, 19, 5728−5731

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Organic Letters fluorescence not only in dilute solutions but also in the aggregated state. Moreover, near-infrared fluorescent emission behavior was observed for both 2 and 3 in the solid state. Furthermore, thin films made of 2 and 3 were found to be convenient for detecting ammonia gas. DPA ligand 1 was synthesized according to a reported method.9b Metallacycle 2 was prepared in one pot by the 2:2:4 stoichiometric combination of DPA 1, p-dicarboxylate ligand 5, and cis-(PEt3)2Pt(OTf)2 4 (OTf, OSO2CF3) in D2O/acetone-d6 (v/v = 8:1) at 50 °C for 12 h and then acetone-d6 at room temperature for 8 h. Similarly, by changing 5 to the mdicarboxylate donor 6, we obtained the analogous rectangle 3. The 31P{1H} and 1H NMR indicated the formation of sole, discrete, and highly symmetrical compounds. The 31P{1H} spectra of both 2 and 3 showed two coupled doublets of approximately equal intensity with concomitant 195Pt satellites due to the two distinct phosphorus environments, indicating that the Pt(II) centers had a heteroligated N,O-coordination motif with one pyridyl and one carboxylate moiety per metal center10 (Figure 1b,c). On the other hand, in the 1H NMR spectra of 2

and its emission spectrum was centered at 576 nm. When the hexane content increased gradually from 0 to 50%, its emission maximum showed a small blue shift of about 6 nm (Figure 2a). As

Figure 2. Dependence of maximum emission intensity and wavelength of metallacycles 2 (a) and 3 (b) on the composition of hexane in acetone/hexane mixtures (λex = 425 nm, c = 10.0 μM). The corresponding images of 2 (c) and 3 (d) solutions under UV lamp at 365 nm.

the fraction of hexane continued to increase to 70%, the emission maximum of 2 underwent a red shift (16 nm) (Figure 2a), accompanying the emission color change from yellow to orange red (Figure 2c). Meanwhile, during the process of hexane addition, the emission intensity of 2 gradually increased (Figure 2a), as did the corresponding quantum yields (from 6.7% to 19.4%) (Figure S9a). Furthermore, transmission electron microscopy (TEM) and dynamic light scattering (DLS) were employed to study the aggregate formation of 2 and 3 in mixed acetone/hexane solvents with different hexane contents. With the hexane fraction increasing from 10% to 30% to 40%, the morphology of the aggregates formed by 2 underwent a change from minute nanoparticles to regular nanospheres and to bigger nanoshperes tending to cluster together (Figure S17a−c), whose average diameters were in good agreement with the DLS results (Figure S10a) of 12, 103, and 230 nm, respectively, indicating a process of gradual aggregation. Thus, metallacycle 2 was still AIE-active just like the free ligand 1. Likewise, rectangle 3 in acetone/hexane solutions exhibited similar properties, such as the increase of fluorescent intensities (Figure 2b) and ΦF values (Figure S9b) and aggregation behavior (Figures S10b and S17d− f) with the gradual addition of hexanes. With these two AIE assemblies in hand, we then investigated their solvatochromism properties in different solvents. Due to the difference of the solvent polarities, the UV−vis absorption spectra of the two metallacycles underwent distinct changes in the intensities of the absorption bands and related wavelengths (Figure S11). Furthermore, the light-emitting properties of both metallacycles in different solvents were recorded by emission spectroscopy. Upon increasing the solvent polarity, irregular and colorful changes occurred. Likewise, the emission intensity of 2 in solvents, such as toluene, THF, and dioxane, was high, while in some polar solvents, like ethyl acetate (EA), methanol, and water, its intensity decreased (Figure 3a). Assembly 3 displayed similar optical properties except that the emission intensity of 3 in CCl4 solution was higher than that of 2 but lower in EA solution (Figure 3b), probably originating from the slight difference of the molecular structures of 2 and 3 (Figure S16). Under UV irradiation at 365 nm, both rectangles showed long-wavelength emission with varied colors in different solvents (Figure 3c,d). The unusual photophysics of 2 and 3 in solutions made us curious about their optical properties in the solid state or as films.

Figure 1. Partial 31P{1H} (a−c) and 1H NMR (d−f) spectra (acetoned6, 293 K) of building blocks 1 (d) and 4 (a) and of metallacycles 2 (b, e) and 3 (c, f).

and 3, signals corresponding to protons H3a and H3d on the pyridyl groups shifted downfield with respect to those of the free ligand 1, in good agreement with the coordination of N-atoms to the platinum centers (Figure 1e,f). Moreover, the formation of 2 and 3 was further demonstrated by electrospray ionization timeof-flight mass spectrometry (ESI-TOF-MS). The peak at m/z = 990.29 for metallacycle 2 corresponding to the fragment of [M − 3OTf]3+ was found to support the intact and discrete [2 + 2 + 4] assembly (Figure S3). Likewise, almost the same peak was observed to support the formation of the isomeric 3 (Figure S6). Both peaks were isotopically resolved and matched well with their calculated theoretical distributions. As AIE-active DPA 1 was successfully locked into the rigid, discrete metallacycles, we next investigated if the self-assembled SCCs were still AIE active. Fluorescence spectra of both metallacycles in acetone with varying contents of hexane were recorded. When the hexane fraction reached more than 70%, compounds 2 and 3 started to precipitate from the mixed solvents. Thus, we chose the hexane fractions from 0 to 70% of the acetone/hexane solutions of 2 or 3 to study the light-emitting properties. In pure acetone, 2 showed a pale orange fluorescence, 5729

DOI: 10.1021/acs.orglett.7b02456 Org. Lett. 2017, 19, 5728−5731

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Organic Letters

Figure 3. Fluorescence spectra of metallacycles 2 (a) and 3 (b) in different solvents (λex = 425 nm, c = 10.0 μM). Photographs of 2 (c) and 3 (d) under UV lamp at 365 nm in different solvents. Tol, toluene; DCM, dichloromethane; THF, tetrahydrofuran; EA, ethyl acetate; Dio, dioxane; Ace, acetone.

Figure 5. Photographs of films of 2 when exposed to or removed from ammonia at room temperature: (a) under ambient light; (b) under a UV lamp at 365 nm.

After complete drying, products 2 and 3 became red solids, quite different from the free ligand 1 which was yellow. Under a UV lamp at 365 nm, the building block 1 displayed green emission (Figure S15), while metallacycles 2 and 3 showed a bright red fluorescence (inset of Figure 4). For solid-state measurements,

ammonia, the emission maximum of 2 showed a large blue shift (75 nm) (Figure S13a). After removal of ammonia, the original emission of the film recovered. The recovered thin film was still viable for ammonia detection and could be used again. We supposed that when exposed to ammonia gas, the coordination bonds of the N atoms to platinum centers in the self-assembled metallacycles were broken, causing the change of color and fluorescence. Subsequently, after removal of the ammonia vapor, the broken coordination bonds were almost recovered, resulting in the return of both color and emission. However, the selfassembled metallacycles slowly decompose during the ammonia detection. After several cycles, both the response time to ammonia and the recovery time of the film from yellow to red increased. Finally, the film would lose its ability as an indicator. A thin film of 3 had similar performance when exposed to or removed from ammonia, whereas a film of free ligand 1 had no such functional feature (Figure S15). In conclusion, two multicomponent rectangular metallacycles were constructed by means of coordination-driven self-assembly. By introducing the AIE-active ligand, DPA 1, into the systems, the resulting SCCs, 2 and 3, exhibited both AIE and solvatochromism. Interestingly, they can also emit near-infrared fluorescence in the solid state. Furthermore, upon exposure to ammonia, thin films of 2 and 3 functioned as indicators for ammonia by changing their colors from red to yellow and vice versa upon removal of the ammonia. Exploration of biological applications of these materials is now ongoing in our laboratory.

Figure 4. Fluorescence spectra of 2 and 3 in the solid state at room temperature (λex = 425 nm). Inset: photos of 2 and 3 under a UV lamp at 365 nm.

thin films of 2 and 3 on glass were made. As shown in the emission spectra (Figure 4), both emitted in the near-infrared region with emission centers at 672 nm for 2 and 667 nm for 3. We speculated that the source of the solid state red emission of both metallacycles can be attributed to three factors. First, compared to the free ligand DPA, the formation of the metallacycles make the locked DPA planar, resulting in a red shift of the emission. Second, while in the solid state or as films, the stacking of the metallacycles induce a further planarization acting on DPA, leading to a red shift of the emission again. Last but not least, the charge-transfer interaction between the peripheral pyridyl groups of DPA and organoplatinum(II) also has a positive effect on the bathochromic shift of the metallacycles’ fluorescence. To sumarize, we proposed a facile coordination-driven self-assembly strategy to design and construct NIR-emitting assemblies which provide an excellent complement to traditional NIR-emitting materials that generally need a complicated molecular design and cumbersome organic synthesis. When a thin film of 2 was exposed to ammonia gas, an obvious color change from red to yellow was observed either under a UV lamp or under ambient light (Figure 5), where the color change occurred in just a few seconds. Fluorescence spectra of the films were recorded to investigate this process. Upon exposure to

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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.orglett.7b02456. Experimental details and additional data (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Xuzhou Yan: 0000-0002-6114-5743 Feihe Huang: 0000-0003-3177-6744 Hajar Sepehrpour: 0000-0003-1638-2755 Peter J. Stang: 0000-0002-2307-0576 5730

DOI: 10.1021/acs.orglett.7b02456 Org. Lett. 2017, 19, 5728−5731

Letter

Organic Letters Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS F.H. thanks the NSFC/China (21620102006), the State Key Laboratory of Chemical Engineering, and the Fundamental Research Funds for the Central Universities for financial support. P.J.S. thanks the NIH (RO1-CA215157) for financial support.

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DOI: 10.1021/acs.orglett.7b02456 Org. Lett. 2017, 19, 5728−5731