Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
Phototriggered Mechanical Movement in A Bipyridinium-based Coordination Polymer Powered by Electron Transfer Xiao-Dong Yang, Rui Zhu, Li Sun, Rui-Yun Guo, and Jie Zhang* MOE Key Laboratory of Cluster Science, Beijing Key Laboratory of Photoelectronic/Electrophotonic Conversion Materials, School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 102488, People’s Republic of China S Supporting Information *
ABSTRACT: Photomechanical movement with a morphological deformation has been achieved in the crystalline state of a bipyridinium-based coordination polymer. The engine “heart” is the electron-deficient bipyridinium core sandwiched by electron-rich phenylcarboxylate components. The decorrelation of the CT state triggered by PET reaction in the engine “heart” promotes the occurrence of this mechanical movement.
bipyridinium parts through sunlight-induced electron transfer.11 However, the current viologen-based photomechanical devices are mainly performed in the solution state and some even require an oxygen-free environment, working against the popularization of their applications in real life. Therefore, the rational design and assembly of viologen-/bipyridinium-based photoresponsive mechanical devices that can function in a generic environment, especially in the solid state, are urgently needed. Coordination polymers (CPs), as an innovative material platform, have been well studied in the past two decades, motivated by not only their intriguing topology architectures but also their potential applications in several important fields such as gas storage/separation, catalysis, biomedicine, and so on.12−14 One of the most advantageous features of CPs is the excellent flexibility of their frameworks that is dependent on molecular design,15−17 which provides the possibility of effectuating the mechanical movement at the molecular level. Thus, incorporating viologen/bipyridinium derivatives into the backbone of CPs can be a promising scheme to fulfill the target. Recognizing the huge spaces of study and development in this field, we have invested a significant amount of effort to exploit solid photomechanical movements in viologen-/bipyridiniumbased CPs. Herein, we report the novel photomechanical CP [Zn(Bpyen)0.5(mip)(Hmip)]·3.5H2O·0.5DMF (1; BpyenBr2 = 1,2-bis(4,4′-bipyridinium)ethane dibromine; H2mip = 5-methylisophthalic acid), in which structure transformation and motion amplification are facilitated by introducing a bipyridinium core sandwiched by two kinds of electron-rich phenylcarboxylate components as the “heart” of the engine.
1. INTRODUCTION Smart materials with mechanical effects are of particular interest from both scientific and technological perspectives for their wide applications such as nanoactuators, flexible electronics, artificial muscles, soft microfluidic devices, etc.1−3 Such materials can convert molecular-scale motion to macroscopic deformation under the control of appropriate external stimuli. In comparison with other stimuli such as heat, chemical energy, magnetic field, and so on, light is an extremely attractive stimulus for triggering a mechanical device work due to its lowcost, resource-rich, high spatiotemporal precision, and ecofriendly properties. Moreover, phototriggered mechanical devices can be remotely controlled without any wired connections, which can simplify the structure and reduce the weight to a large degree. Thus, the design and synthesis of photomechanical materials are very desirable and have been viewed as a promising research hotspot.4−6 To ensure that a mechanical device runs well, a powerful “motor” has to be supplied. It is well-known that viologen/ bipyridinium derivatives are famous for their electron-deficient nature. They can exhibit photodriven electron transfer (PET) behavior to produce intensely colored free radicals and usually conduct charge transfer (CT) interactions with electron-rich units due to their low-energy LUMO.7,8 As crucial kinds of “motor” blocks, viologen/4,4′-bipyridinium derivatives have been utilized to fabricate photomechanical device since the 1990s.9−11 Mechanical movements in these devices are mainly driven by altering the intermolecular interaction (e.g., donor− acceptor interaction and radical−radical interaction) between correlated components at the molecular level under light irradiation. For example, in 2006, Stoddart and co-workers reported a molecular shuttle, in which the shuttling can be driven by perturbing the interaction between the rotaxane and © XXXX American Chemical Society
Received: December 11, 2017
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DOI: 10.1021/acs.inorgchem.7b03108 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
3. RESULTS AND DISCUSSION 3.1. Crystal Structure. Single-crystal X-ray crystallography shows that 1 is a 2D-layer-like coordination polymer crystallizing in the monoclinic space group C2/c. Its asymmetric unit contains one crystallographically independent Zn(II) ion, a half Bpyen2+ ligand, one mip2− ligand, one partially deprotonated ligand Hmip−, and some dissociated solvent molecules (H2O, DMF). The central Zn(II) ion is fourcoordinated with three oxygen atoms (O1, O5 and O3#1: #1, x, −y, 0.5 + z) and one nitrogen (N1) atoms from two different Hmip− ligands and one mip2− ligand, completing a distortedtetrahedral coordination sphere (Figure S2). Each Zn(II) ion coordinates with two mip2− ligands and one Hmip− ligand to form a 1D zigzag {[Zn(mip)(Hmip)]−}n chain, and the partially deprotonated Hmip− ligands act as lateral arms alternately distributed on both sides of these 1D chains. Such chains are further connected by Bpyen2+ ligands to form a 2D wavelike network (the blue part of Figure 1a). If Zn(II) ions are
The synergetic effect of photoinduced intermolecular ET reaction and CT interaction offers the original driving force for this photomechanical movement. The current system not only works in the atmosphere but also renders the first example involving the photoinduced transformation of the CT state.
2. EXPERIMENTAL SECTION 2.1. Materials and Instruments. Zn(NO3)2·6H2O, H2mip, 4,4′bipyridine, CH2BrCH2Br, and DMF were obtained from commercial sources in AR/GR grade and used without further purification. Infrared spectra (400−4000 cm−1 region) were measured from KBr pellets on a Nicolet iS10 FT-IR spectrometer. Powder X-ray diffraction (PXRD) patterns were collected with a Bruker D8 Advance X-ray diffractometer with Cu Kα radiation (λ = 1.54056 Å). Thermogravimetric analysis (TGA) was recorded on a Mettler Toledo TGA/DSC 1/1100 analyzer in a flowing air atmosphere at a heating rate of 10 °C min−1 from 30 to 800 °C. UV−vis spectral measurement was carried out at room temperature by using a PE Lambda 900 spectrometer. 1H NMR spectrum was recorded on a Bruker AV-400 NMR spectrometer. Elemental analysis (C, H, N) was performed using a PE2400 II elemental analyzer. Electron spin resonance (ESR) signals were recorded with a Bruker A300 spectrometer. 2.2. Synthesis. 2.2.1. BpyenBr2 Ligand. BpyenBr2 was synthesized according to our previously reported literature.18 1H NMR (400 MHz, D2O, Figure S1): δ 9.069 (d, J = 6.4 Hz, 4 H), δ 8.855 (d, J = 5.2 Hz, 4 H), δ 8.564 (d, J = 6 Hz, 3H), δ 7.992 (d, J = 5.2 Hz, 2H), δ 5.482 (s, 4H). 2.2.2. Complex 1. Zn(NO3)2·6H2O (0.035 g, 0.12 mmol), H2mip (0.018 g, 0.10 mmol), and BpyenBr2 (0.025 g, 0.05 mmol) were added to the mixed solvent DMF/H2O (3 mL/3 mL), and the mixture was then sealed in a 25 mL glass bottle after stirring for 10 min. The glass bottle was heated at 70 °C for 3 days and then slowly cooled to room temperature. A brownish red solution was obtained and then filtered. The filtrate was allowed to stand at room temperature for slow evaporation. Brownish rod-shaped crystals were obtained within a few days. The yield is about 60% based on Zn. Anal. Calcd for ZnO12N2.5C30.5H33.5 (692.47): C, 52.90; H, 4.88; N, 5.06. Found: C, 52.56; H, 4.56; N, 5.34. 2.3. Structural Determination. Single-crystal X-ray analysis of 1 was performed on an Agilent dual source Super Nova diffractometer by using graphite-monochromated Cu Kα radiation (λ= 1.54178 Å) at room temperature. The absorption correction was performed by using the Multiscan program, and the structure was solved by direct methods and refined on F2 with full-matrix least-squares methods using the SHELXL-2016 program package. All non-hydrogen atoms were refined anisotropically. The hydrogen atoms except those attached to solvent molecules were calculated in ideal positions and refined by riding on their respective carbon atoms. The solvent molecules (H2O, DMF) were highly disordered and could not be modeled properly. The diffused electron densities resulting from solvent molecules were removed by the SQUEEZE routine in PLATON. The final formula of 1 was derived from crystallographic data combined with elemental and thermogravimetric analysis data. Crystal data for 1: C30.5H33.5ZnN2.5O12; Mr = 692.47; monoclinic C2/c; a = 21.2932(8) Å, b = 20.9091(6) Å, c = 16.9902(6) Å, β = 111.641(4)°, V = 7031.2 (5) Å3; T = 293 K; Z = 8; Dcalcd = 1.308 g cm−3; μ = 1.494 mm−1; F(000) = 2880; 14390 reflections collected, of which 6959 are unique (Rint = 0.0209); GOF = 1.041; R1 = 0.0476; wR2 = 0.1383 (I > 2σ(I)); CCDC 1572241. 2.4. Computation Description. The calculations of Mulliken charge population were carried out by using the Dmol3 program,19,20 in which the GGA calculations were performed by the PBE exchange correlation. Mulliken charges were calculated by projecting the occupied one-electron eigenstates onto the localized atomic basis sets.21
Figure 1. (a) 2-fold interpenetrated structure between adjacent networks in 1. (b) 3D stacking structure in 1 viewed along the c axis. (c) Orientation diagram of the donor and acceptor in 1. (d) CT interaction between neighboring 2D layers.
taken as nodes and Bpyen2+ and mip2− ligands as connecting rods, this network can be considered to be a (6,3) topological structure with hexagonal holes (Figure S3a). Adjacent networks interlock with each other to give a 2-fold interpenetrating layer with lateral Hmip− ligands (Figure 1a and Figure S3b). The centroid···centroid distance and interplanar angle between the pyridinium ring (electron-deficient unit) and dangling Hmip− ligand (electron-rich unit) of adjacent layers are 3.79 Å and 4.02°, respectively (Figure 1c,d). Such a specific supramolecular arrangement has significant consequences for the formation of intermolecular charge transfer (CT) interaction.22−24 The calculation of charge population reveals that the natural charge on a single pyridinium ring (N2→C28) is +0.22, while the B
DOI: 10.1021/acs.inorgchem.7b03108 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
Figure 2. (a) ESR spectra of 1 (black), 1a (red), and 1b (blue). (b) UV−vis diffuse reflectance spectral changes of 1 (black), 1a (red), and 1b (blue). (c) UV−vis diffuse reflectance spectral changes of 1 at 600−700 nm after the irradiation of 365 nm light. (d) Time-dependent reflectance at 650 nm of 1 after the irradiation of 365 nm light.
natural charges on the carboxylate groups (O5−C10−O6, O7− C16−O8) are −0.55 and −0.39 (Figure S4 and Table S1). Both values are much lower than their apparent charges of +1 and −1, respectively. In addition, the UV−vis diffuse reflectance spectrum of 1 displays three intense absorption bands centered at 220, 280, and 480 nm (Figure 2b). Except for the bands at 220 and 280 nm that correspond to the π−π* transition of the bipyridinum moiety of the Bpyen2+ ligand and the benzene moiety of the Hmip− or mip2− ligand, respectively, the appearance of a broad band at 480 nm indicates the existence of a CT interaction.25,26 These results suggest that the ground state of 1 is indeed associated with intermolecular CT interaction, which also shows up in the brownish color of the original sample. Thus, these neighboring 2D layers are further packed in −ABAB− fashion mainly via CT interactions to form the 3D structure (Figure 1b). 3.2. Photomechanical Movement and Deformation. On exposure to the irradiation of 365 nm light in air at room temperature, 1 quickly turns from brownish to dark green (1a) within 1 min. More interestingly, when it is further illuminated for another 4 h, 1a gradually turns yellow and shows a bending (1b) (Figure 3a and Figure S5). The dark green color of 1a is similar to the radical-related photochromic phenomena of other reported viologen-/bipyridinium-based compounds, and the existence of bipyridinium radicals in 1a can also be verified by
Figure 3. (a) Photographs showing the evolution from crystal 1 to 1a and 1b under 365 nm light irradiation. (b) Morphological changes of crystal 1 under the irradiation of 450 nm light.
electron spin resonance (ESR) and UV−vis diffuse reflectance spectra. As shown in Figure 2a, the strong ESR signal (g = C
DOI: 10.1021/acs.inorgchem.7b03108 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry 2.0030) in 1a resembles those for the photogenerated viologen/bipyridinium radicals.27−32 The small ESR signal in 1 may be due to the ambient-light-induced generation of bipyridinium radicals. For 1b, its ESR intensity almost reverts to the state equivalent to 1, suggesting the occurrence of free radical quenching effects.27−32 Although the sample loading pattern in diffuse reflectance measurements increases the contact area with oxygen in air and may thus facilitate the quenching of bipyridinium radicals, two weak characteristic bands of bipyridinium radicals centered at 420 and 650 nm can still be detected in 1a (the inset of Figure 2b).27−32 However, the UV−vis diffuse reflectance spectral changes of 1b seem slightly unconventional. The measurements show a decreasing absorption of the CT interaction band at about 480 nm and an increasing absorption at about 380 nm (Figure 2b). By monitoring the entire irradiation process of 1, we can clearly see that the absorption intensity of bipyridinium radicals at 650 nm first increased and reached a maximum within 1 min and then decreased rapidly, while the absorption intensity of the CT transition at 480 nm decreased directly (Figure 2c,d and Figure S6). Such observations reveal that a process different from the conventional generation and quenching of radicals, namely a unique photoinduced transformation of the CT state, may be involved. If the activation wavelength is shifted to 450 nm, 1 can exhibit a slightly different color evolution from brownish to green at first and then changes to pink (1c) after irradiation for 60 min (Figure S7). The UV−vis absorption intensity of 1c in the 400−600 nm region displays a more significant descending trend in comparison with 1b (Figure S8), which agrees well with its more obvious color change. More interestingly, this color transformation process is accompanied by a deformation more obvious than that observed under 365 nm light irradiation. As shown in Figure 3b, a single striplike crystal 1 opens a crack in the middle at first and then gradually splits into two parts just like a tweezer. Simultaneously, a color fading to pink-yellow occurs. The macroscopic deformation occurs in the crystal with a thickness of about 200 μm that is larger than the majority (thickness
