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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Self-Assembly of a Tetraphenylethylene-Based Capsule Showing Both Aggregation- and Encapsulation-Induced Emission Properties Ting Zhang,†,‡,§ Guang-Lu Zhang,‡,§,⊥ Qian-Qian Yan,†,‡ Li-Peng Zhou,‡ Li-Xuan Cai,‡ Xiao-Qing Guo,‡ and Qing-Fu Sun*,‡ †
College of Chemistry, Fuzhou University, Fuzhou 350108, P. R. China State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, P. R. China
‡
S Supporting Information *
ABSTRACT: Functional molecular capsules have attracted a lot of attention in recent years because of their potential applications as chemosensors, catalysis, drug carriers, and so on. We report here the coordination-directed self-assembly of a fluorescent-lantern-type molecular capsule from four tetraphenylethylene-based ditopic ligands and two square-planar palladium(II) ions. The capsule has been thoroughly characterized by UV−vis, 1D/2D NMR, electrospray ionization time-offlight mass spectrometry, and single-crystal X-ray diffraction studies. The aggregation-induced emission performance of the capsule has been studied by tuning the ratio of mixed solvents. Moreover, with an open cavity, the fluorescence of the capsule also displays anion sensitivity, with the best turn-on responsiveness observed for HCO3−, demonstrating for the first time an encapsulation-induced emission property.
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INTRODUCTION Coordination-directed self-assembled molecular capsules have shown great power in chemosensing, stabilization of reactive species, drug delivery, catalysis, etc.1−14 In contrast to multicomponent polyhedral complexes with huge dimensions, small molecular capsules assembled from a fewer components (≤6 components) often hold well-defined cavities and thus more controllable host−guest properties. For example, a lantern-type M2L4 capsule, easily accessible from the selfassembly of two square-planar metals (M) and four rigid ditopic ligands (L), have been extensively studied.15−27 Recently, increasing attention has been paid to the aggregation-induced emission (AIE) materials because of their widespread applications in chemo/biosensors, light-emitting devices, and medical diagnostics.28−35 Tetraphenylethylene (TPE) is the iconic and most commonly used AIE fluorophore,36−38 which exhibits almost no fluorescence in a dilute solution because of fast nonradiative decay via the rotation of phenyl rings around the CC bond and is strongly emissive in the aggregate state, where the intramolecular motion is restricted. Several luminescent metal−organic frameworks39−42 and supramolecular coordination complexes43−47 have been constructed through the fixation of TPE building blocks via coordination. However, studies along this line are predominantly limited to their AIE properties, and encapsulation-induced emission (EIE) phenomena on the caged complexes have rarely been discussed. Herein, we report the coordination self-assembly of a lantern-type Pd2L4 capsule (Scheme 1) with both AIE and EIE properties. © XXXX American Chemical Society
RESULTS Capsule Pd2L4 was obtained by treating the ditopic TPE-based pyridinyl ligand L with 0.5 equiv of Pd(NO3)2 in dimethyl sulfoxide (DMSO) at 70 °C for 2 h. 1H NMR first confirmed the quantitative formation of a highly symmetrical complex with the observation of a single set of ligand signals, which could be fully assigned based on a 1H−1H COSY experiment (Figures 1A,B and S6). Compared to the free ligand, the peaks of Ha−d on the pyridine groups of the assembly displayed obviously downfield shifts, suggesting a loss of the electron density after coordination to metal centers. However, the peak of He,f on the phenyl rings of the assembly was noticed to be upfield-shifted, in clear contrast to the downfield shifting observed from the metallocyclic structures reported by Stang et al.48−50 Such a difference was attributed to the interligand shielding effects of the capsule or the shielding effect imposed by the encapsulated nitrate anions (see the Discussion section). A diffusion-ordered (DOSY) NMR spectrum (Figure 1C) also confirmed the formation of a sole species with a single diffusion coefficient of 1.48 × 10−10 m2/s, from which an estimated diameter of 14.7 Å was calculated based on the Stokes−Einstein equation.51 The composition of the product formulated as Pd2L4(NO3)4 was then unambiguously provided by highresolution electrospray ionization time-of-flight mass spectrometry (ESI-TOF-MS), where prominent peaks were observed at Special Issue: Self-Assembled Cages and Macrocycles Received: September 30, 2017
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DOI: 10.1021/acs.inorgchem.7b02502 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Scheme 1. Coordination-Directed Self-Assembly of a Lantern-Type Pd2L4 Capsule from a TPE-Based Ligand L and Palladium(II)
TPE-based assemblies are probably due to the lack of planarity on the TPE backbone. In contrast to the widely used rigid ligands, four phenyl rings on TPE are so close to each other that they need to be twisted. A careful screen of the determined structures from the CCDC database reveals that TPE is inclined to adopt two opposite chiral four-blade propellershaped conformations in the solid state, which should undergo fast conformational isomerization in solution.44 Fortunately, Xray-quality single crystals of Pd2L4(NO3)4 were obtained by the slow vapor diffusion of ethyl acetate (EtOAc) into a DMSO solution of the complex after 3 weeks. The compound crystallized in the P21/c space group, with an asymmetric unit containing half of the Pd2L4 capsule. In the solid state, the capsule is severely distorted from the ideal D4d molecular symmetry, where two pairs of the TPE units with opposite propeller chirality are closely compressed to each other possibly because of the packing effect (Figures 2A,B). As a result, the
Figure 1. 1H NMR (400 MHz, DMSO-d6, 298 K) spectra of ligand L (A) and capsule Pd2L4(NO3)4 (B). (C) 1H DOSY NMR spectrum and (D) ESI-TOF-MS of capsule Pd2L4(NO3)4, with insets showing the observed (Obs.) and simulated (Sim.) isotopic patterns of the 3+ peak.
m/z 1141.3117 and 740.2119, corresponding to the multiplecharged [Pd2L4(NO3)2]2+ and [Pd2L4(NO3)]3+ molecular-ion signals, respectively. Moreover, the isotopic patterns of each resolved peak agreed well with the simulated values (Figure 1D), which further ratified the composition of the complex. Similarly, capsules with different counterions were also successfully synthesized using Pd(BF4)2, Pd(CF3SO3)2, and Pd(PF6)2 as the metal sources, all of which displayed similar spectroscopic data to their nitrate counterpart (Figures S8− S10). There are several TPE-based discrete supramolecular caged structures that are elucidated by X-ray crystallography.43−47,52−54 However, most of them are constructed by using the four-armed TPE ligands, while capsule complexes based on a two-armed TPE ligand have never been established by X-ray crystallography. Difficulties for the crystallization of
Figure 2. X-ray crystal structures of capsule Pd2L4(NO3)4: side views highlighting (A) the hydrogen-bonded nitrate anions and (B) TPE units with opposite propeller chirality and the furthest Hi−Hi′ distance; crystal-packing views along the b (C) and a (D) axes. Color scheme: Pd, yellow; C, sky blue; N, dark blue; O, red; H, white; hydrogen bonds, red dashes; CH−π interactions, purple dashes. B
DOI: 10.1021/acs.inorgchem.7b02502 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Figure 3. (A) Emission spectra of Pd2L4(NO3)4 with increasing toluene fractions in toluene/DMSO mixtures (λex = 395 nm; c = 1.00 × 10−4 M). (B) Emission spectra of Pd2L4(OTf)4 with different equivalents of the HCO3− anion (TBA salt) in DMSO (λex = 380 nm; c = 1.00 × 10−4 M). (C) Change in the 1H NMR spectra of Pd2L4(OTf)4 after the addition of 1−4 equiv of HCO3−.
(Figure S17), indicating its strong AIE behavior. Similarly, capsule Pd2L4(NO3)4 also exhibited a similar emission pattern with lower fluorescence intensity and quantum yield compared to those of the free L, possibly because of the common quenching effect imposed by palladium and nitrate (Figures S16 and S18). To examine whether the capsule is still AIEactive in solution, emission spectra of Pd2L4(NO3)4 by the addition of toluene, EtOAc, 1,4-dioxane, and water into DMSO (0−99%, v/v) were recorded (Figures 3A and S20−S23). The fluorescence intensities became approximately 10-fold higher in 99/1 (v/v) toluene/DMSO than in pure DMSO, which suggests that capsule Pd2L4(NO3)4 is also AIE-active. In a high content of toluene fractions, the emission peak shows a red shift of the maximal emission wavelength, which indicates that the ligand conformation has changed. According to the TPE-based AIE mechanism, a given emission wavelength shift is usually correlated with a specific conformation of the peripheral phenyl rings: a coplanar conformation promotes πelectron conjugation, resulting in a red shift, and a perpendicular conformation weakens π-electron conjugation, thus leading to a blue shift.55 Moreover, dynamic light scattering also confirmed aggregation of both the free ligand and complexes into nanoparticles with sizes of up to 80 and 370 nm, respectively, when poor solvents were added (Figures S24 and S25). Inspired by the encapsulation of nitrates of Pd2L4(NO3)4 in the solid state, we then turned our attention to the EIE properties of the capsule. 56−59 The responsiveness of Pd2L4(OTf)4, which is regarded as a “reference” because of the bulky size of OTf−, toward different anions was assessed in solution. Amounts of 4 equiv of tetrabutylammonium (TBA) salts of various anions, including F−, Cl−, Br−, I−, HCO3−,
capsule is more like a macrocycle than a lantern in shape because there is a bigger opening along the b axis. The size of the capsule defined by the Pd−Pd distance amounts to ca. 12.43 Å and the furthest H−H distance (Hi−Hi′) ca. 24.249 Å (Figure 2B). Interestingly, two nitrate ions are found to be encapsulated inside the cavity by hydrogen-bonding and electrostatic interactions. Although disordered into four possible positions, they are forced to adopt a perpendicular geometry restrained by the limited cavity. External to the capsule are two additional nitrates, which are located close to the palladium centers (Figure 2A). Along the b axis, complexes Pd 2L 4 are packed coaxially in an end-to-end fashion. Neighboring capsules are held together by intermolecular CH−π interactions (3.58 Å, Figure 2D) along with multiple hydrogen-bonding interactions bridged by external nitrates. Such packing results in two types of 1D channels along the b axis, one of which is determined by the macrocycles of Pd2L4 and the other encircled by four neighboring molecules of Pd2L4 (Figure 2C).
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DISCUSSION In the UV−vis spectra (Figure S14), an intense absorption band for the π−π* transitions of the ligand was found from 275 to 375 nm (λA,max at 301 nm). In contrast, the Pd2L4(NO3)4 complex exhibits an intense absorption band from 280 to 380 nm (λA,max at 326 nm), which is clearly red-shifted because of coordination to palladium(II). Irradiation of a dilute methanol solution of ligand L (
