Grinding-Triggered Single Crystal-to-Single Crystal Transformation of

As is known, crystal structure data are the most direct and accurate evidence for understanding the corresponding properties. Herein, we report single...
0 downloads 0 Views 3MB Size
Download PDF
Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

pubs.acs.org/IC

Grinding-Triggered Single Crystal-to-Single Crystal Transformation of a Zinc(II) Complex: Mechanochromic Luminescence and Aggregation-Induced Emission Properties Sai Li, Min Wu, Yang Kang, Han-Wen Zheng, Xiang-Jun Zheng,* De-Cai Fang, and Lin-Pei Jin Beijing Key Laboratory of Energy Conversion and Storage Materials, College of Chemistry, Beijing Normal University, Beijing 100875, China

Inorg. Chem. Downloaded from pubs.acs.org by EAST CAROLINA UNIV on 03/14/19. For personal use only.

S Supporting Information *

ABSTRACT: We first report single crystal X-ray analysis of ground crystals of mechanochromic luminescence (MCL) that shows single crystal-to-single crystal transformation (SCSCT). Single crystals of [ZnL2] (1-SG, HL = 2[[[4-(2-benzoxazolyl)phenyl]imino]-methyl]-5-(diethylamino)-phenol) were obtained upon slight grinding of single crystals of [ZnL2]·0.5CH3OH (1), both of which were characterized by single crystal X-ray diffraction. Crystals of 1 showed emission centered at 647 nm (red color), while crystals of 1-SG showed emission band at 624 nm (orange-red color) under UV light, indicating MCL property of the Zn(II) complex. Reversible MCL property with emission color change between red and yellow for 1 was observed upon high grinding and recrystallization. Single crystal X-ray analysis suggested that it is due to the alteration of molecular conformation of ligands in ZnL2 instead of weak intermolecular interaction that 1 exhibits MCL. Investigation of the control Zn(II) complexes (2−4) indicated that flexible substituents and rotated aromatic rings are desirable to generate the MCL-active complexes. In addition, 1 was highly fluorescent in THF solution, but its fluorescence quenched upon addition of water. DFT calculations suggested that this is due to the formation of the excited hydrated ZnL2 species via Zn−O coordination bond, which results in electron-driven proton transfer (EDPT). Aggregates formed as water fraction (f w) in THF/H2O (v/v) reached 70%, and fluorescence emission was enhanced. This phenomenon continued until f w was 90%, indicating aggregation-induced emission (AIE) property. The mechanism of AIE of ZnL2 in THF/H2O is the restriction of intramolecular rotation (RIR).

■

INTRODUCTION In past decades, organic compounds and metal complexes with dual AIE and MCL properties have attracted intense research and a number of AIE/MCL materials, especially organic compounds have been constructed for applications in diverse scientific fields, such as chemosensors,1,2 optical data storage,3,4 light-emitting diodes,5 and biomedical imaging.6 Generally, AIE properties of organic compounds are attributed to the restriction of intramolecular rotations (RIR),7,8 formation of Jor H-aggregates,9−11 intramolecular planarization,12,13 cis- and trans-isomerization,14 twisted intramolecular charge transfer (TICT),15,16 and formation of excimer and exciplex.17 The recognized MCL of luminogens is due to mechanic stimuli which leads to the structural alteration resulting from variation of molecular conformation and intermolecular weak interactions including hydrogen bonds, π−π stacking, and van der Waals forces. A number of MCL-active organic compounds were reported.18−22 For metal complexes, their AIE/or MCL behaviors are fairly attractive because metal complexes are composed of metal ions and organic ligands which are expected to exhibit more intriguing luminescent behaviors. Recently, many AIE/or MCL metal complexes were covered, such as Au(I), Ag(I), Cu(I), © XXXX American Chemical Society

Re(I), Pt(II), Zn(II), Pd(II), Ru(II), and Ir(III) complexes.23−25 Their AIE/or MCL properties are considered to arise from intra- and intermolecular metallophilic interaction,26−28 crystal-to-crystal phase transformation,29−32 variation of intermolecular hydrogen bonds and π−π interactions,33−35 change of molecular conformation, and packing mode.36,37 Compared to AIE-active MCL organoluminogens, AIE-active MCL metal complexes are rare. To our knowledge, a few examples of AIE/MCL metal complexes were reported.38−40 Thus, a comprehensive understanding of the molecular conformation, crystal packing, and structural changes of AIE-active MCL metal complexes is less demonstrated. Especially, the mechanism of MCL-active compounds were generally speculated through crystal structural analysis of samples before grinding or cultivate single crystals of the ground samples based on crystal-to-crystal phase transition.29,30 All these methods provide indirect structure information to explore MCL property. As is known, crystal structure data are the most direct and accurate evidence for understanding the corresponding properties. Herein, we report Received: January 20, 2019

A

DOI: 10.1021/acs.inorgchem.9b00195 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

were added and the mixture was heated to reflux at 80 °C for 4 h. Then, 0.1 mmol (0.0372 g) of zinc perchlorate hexahydrate in ethanol was added dropwise to the above mixture, and the mixture was cooled to room temperature after refluxing for another 4 h. After the solvent evaporated, orange crystals of [Zn(HL3)2L2]ClO4 could be obtained and filtered, then washed with ethanol and air-dried. Yield: 68.4% (0.0664g). Anal. Calcd for ZnC51H59N6O7Cl: C, 63.22; N, 8.67; H, 6.14. Found: C, 63.01; N, 8.57; H, 6.13. 1H NMR (400 MHz, DMSO-d6): δ = 13.61 (s, 2H), 8.68 (s, 2H), 8.47 (s, 1H), 7.44−7.37 (m, 4H), 7.36−7.25 (m, 8H), 7.24−7.13 (m, 5H), 7.11 (t, J = 7.0 Hz, 1H), 6.33 (d, J = 2.5 Hz, 1H), 6.31 (d, J = 2.5 Hz, 1H), 6.18 (d, J = 7.3 Hz, 1H), 6.07 (d, J = 2.4 Hz, 2H), 5.81 (d, J = 2.4 Hz, 1H), 3.38 (dt, J = 7.1, 5.6 Hz, 12H), 1.15−1.08 (m, 18H) ppm. UV−vis (1 × 10−5 M, THF): 379 nm, ε: 1.44 × 105 L·mol−1·cm−1. IR (KBr pellet, cm−1): 3427 vs, 2974 s, 1614 vs, 1517 s, 1487 w, 1415 w, 1342 s, 1292 w, 1242 m, 1190 m, 1143 w, 1082 s, 1002 w, 792 w, 696 m, 619 m, 514 w. Synthesis of Zn(II) Complex 4 ([Zn(HL3)2Cl2]). A mixture of 0.2 mmol (0.0186 g) of aniline, 0.2 mmol (0.0386 g) of 4(diethylamino)-2-hydroxybenzaldehyde, 0.1 mmol (0.0136 g) of zinc chloride, and 4 mL of methanol in a closed 25 mL Teflonlined autoclave was heated at 80 °C for 24 h and cooled to room temperature. Orange-yellow rod crystals of [Zn(HL3)2Cl2] formed and were filtered, then washed with methanol and air-dried. Yield: 78.3% (0.0525g). Anal. Calcd for ZnC34H40Cl2N4O2: C, 60.86; N, 8.35; H, 5.71. Found: C, 60.53; N, 8.28; H, 6.05. 1H NMR (400 MHz, DMSO-d6): δ = 8.39 (s, 2H), 7.63 (s, 2H), 7.31 (s, 4H), 7.20 (s, 4H), 7.12 (s, 4H), 6.19 (s, 2H), 6.07 (s, 2H), 3.35 (d, J = 7.1 Hz, 8H), 1.14 (s, 12H) ppm. UV−vis (1 × 10−5 M, THF): 371 nm, ε: 9.12 × 104 L·mol−1·cm−1. IR (KBr pellet, cm−1): 3149 vs, 2970 w, 1622 vs, 1523 s, 1481 w, 1415 m, 1340 s, 1296 w, 1238 m, 1184 w, 1145 m, 1078 m, 1002 w, 877 w, 821 w, 785 w, 696 w, 611 w.

single crystals of [ZnL2] (1-SG) resulting from single crystals of [ZnL2].0.5CH3OH (1) after grinding and provide a powerful method for detailed studies on its crystal structure. It is the first example of grinding-induced single crystal-tosingle crystal transformation for metal complexes. The crystal structural analysis of 1 before and after grinding demonstrated that it is alteration of the molecular conformation of the ligands; sequentially, the molecular structure of the Zn(II) complex instead of the weak intermolecular interaction between the Zn(II) complex molecules plays a key role in the MCL property of the Zn(II) complex. Further investigations on control Zn(II) complexes indicate that flexible substituents and rotated aromatic rings are desirable to produce MCL-active metal complexes.

■

EXPERIMENTAL SECTION

General Procedures. All solvents and reagents (analytical-grade) were used without further purification. Elemental analyses were conducted using a Vario EL elemental analyzer. UV−vis absorption spectra were recorded by a spectrophotometer UV-2450 and fluorescence spectra were measured on a FS5 fluorescence spectrophotometer, with a quartz cuvette (path length = 1 cm). The morphology of products was characterized by a field-emission scanning electron microscopy (FESEM, S-8010, Hitachi). Powder Xray diffraction (PXRD) patterns were obtained on a PANaytical X’Pert PRO MPD diffractometer with Cu Kα radiation (λ = 1.5406 Å), with a step size of 0.017° in 2θ. Fluorescence quantum yield was measured with the HAMAMATSU Quantaurus-QY. Fluorescence microscopic photographs of crystals and ground samples were taken under an OLYMPUS IXTI fluorescence microscope. Fourier transform infrared spectra were obtained on a IR Affinity-1 FT-IR spectrometer as KBr pellet. 1H NMR spectrum was measured on a Bruker Avance III 400 MHz spectrometer with TMS as internal standard. Syntheses of Zn(II) Complexes. Synthesis of Zn(II) Complex 1 (ZnL2·0.5CH3OH). A mixture of 0.2 mmol (0.0420 g) of 4(benzo[d]oxazol-2-yl)aniline, 0.2 mmol (0.0386 g) of 4-(diethylamino)-2-hydroxybenzaldehyde, 0.1 mmol (0.0220 g) of zinc acetate, and 4 mL of methanol in a closed 25 mL Teflon-lined autoclave was heated at 80 °C for 24 h and cooled to room temperature. Purple crystals of ZnL2·0.5CH3OH were obtained and filtered, then washed with methanol and air-dried. Yield: 87.3% (0.0742 g). Anal. Calcd for ZnC48.5H46O4.5N6: C, 68.51; N, 9.88; H, 5.45. Found: C, 68.79; N, 9.94; H, 5.27. 1H NMR (400 MHz, CHCl3-d1): δ = 8.19 (s, 2H), 8.06 (d, J = 8.7 Hz, 4H), 7.78−7.65 (m, 2H), 7.58−7.43 (m, 2H), 7.38− 7.26 (m, 4H), 7.21 (d, J = 8.7 Hz, 4H), 7.02 (d, J = 9.1 Hz, 2H), 6.20−6.09 (m, 4H), 3.39 (dt, J = 14.0, 7.1 Hz, 8H), 1.22 (t, J = 7.1 Hz, 12H) ppm. UV−vis (1 × 10−5 M, THF): 426 nm, ε: 1.14 × 105 L·mol−1·cm−1. IR (KBr pellet, cm−1): 3435 vs, 2974 w, 1614 s, 1566 s, 1487 s, 1427 w, 1381 w, 1356 w, 1315 w, 1236 m, 1188 s, 1136 m, 1074 w, 922 w, 750 m, 698 w, 590 w, 524 w. Synthesis of Zn(II) Complex 2 (Zn(L1)2). A mixture of 0.2 mmol (0.0420 g) of 4-(benzo[d]oxazol-2-yl)aniline, 0.2 mmol (0.0244 g) of 2-hydroxybenzaldehyde, 0.1 mmol (0.0220 g) of zinc acetate, and 4 mL of methanol in a closed 25 mL Teflon-lined autoclave was heated at 80 °C for 24 h and cooled to room temperature. Orange crystals of Zn(L1)2 formed and were filtered, then washed with methanol and air-dried. Yield: 70.2% (0.0485g). Anal. Calcd for ZnC40H26N4O4: C, 69.42; N, 8.10; H, 3.79. Found: C, 68.22; N, 7.90; H, 3.86. 1H NMR (400 MHz, CHCl3-d1): δ = 12.97 (s, 1H), 8.35−8.28 (m, 2H), 7.86− 7.66 (m, 2H), 7.65−7.50 (m, 2H), 7.40−7.33 (m, 4H), 7.05 (d, J = 8.1 Hz, 1H), 6.98 (dd, J = 7.4, 0.8 Hz, 1H) ppm. UV−vis (1 × 10−5 M, THF): 339 nm, ε: 6.14 × 104 L·mol−1·cm−1. IR (KBr pellet, cm−1): 3408 vs, 3057 w, 1610 s, 1527 w, 1490 w, 1433 s, 1390 m, 1319 w, 1242 m, 1147 s, 1056 w, 997 w, 858 w, 744 s, 690 w, 588 w. Synthesis of Zn(II) Complex 3 ([Zn(HL3)2L2]ClO4). In a roundbottomed flask, 0.2 mmol (0.0186 g) of aniline, 0.2 mmol (0.0386 g) of 4-(diethylamino)-2-hydroxybenzaldehyde, and 5 mL of ethanol

■

RESULTS AND DISCUSSION Mechanochromic Luminescence Properties of Zn(II) Complex 1. Zinc(II) complex 1 was prepared via one-pot reaction of zinc(II) salt with corresponding amine and aldehyde as shown in Scheme 1. The as-prepared crystals of Scheme 1. One-Pot Synthetic Route of Schiff Base-Based Zn(II) Complexes

1 showed emission centered at 647 nm with red color under UV light (Figure 1a), while ground samples exhibited emission centered at 620 nm after slight grinding in an agate mortar with pestle (Figure S1 and Movie S1). However, when the slightly ground samples were observed under a microscope, crystals with orange-red and orange color together with nontransparent mini-crystals could be seen. The results of single crystal X-ray diffraction indicate the orange-red crystals are pristine crystals of 1, while the orange crystals are new crystals, named, 1-SG. The crystals of 1-SG showed fluorescence emission at 624 nm (Figure S1). After high grinding in an agate mortar with pestle, samples with yellow emission at 608 nm were seen under UV light (Figure S1 and B

DOI: 10.1021/acs.inorgchem.9b00195 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 1. (a) Top: naked eye visualization under room light and UV light; bottom: the corresponding SEM. (b) Fluorescence spectra of crystals of 1, 1-SG, and 1 before and after slight and high grinding.

Movie S1). No suitable crystals for single crystal X-ray diffraction could be obtained after high grinding (Figure 1a). Simultaneously, the fluorescence intensity also changed significantly as shown in Figure 1b. Compared with crystals of 1 before grinding, the fluorescence intensity of samples greatly increased upon slight grinding, while upon high grinding the fluorescence intensity of samples was more greatly enhanced, as shown in Figure 1b. Also, the quantum yield of highly ground samples was 3.9%, higher than that of the crystals of 1 (1.4%). Zn(II) complex 1 exhibited the MCL property with high contrast. The highly ground samples with yellow emission reverted to the original red emission after recrystallization via diffusion method in THF/methanol at room temperature. The processes could be repeated for several cycles without obvious fatigue, suggesting good reversibility (Figure S2). To further understand the MCL properties of Zn(II) complex 1, the crystal structures of 1 and 1-SG were investigated in detail. It can be seen from their structures that the solvent molecule CH3OH was released from the lattice after slight grinding, and that 1 and 1-SG have the same crystal system (triclinic) and space group (P1̅ ), but the cell parameters changed obviously after slight grinding (Table S1). There is a change up to 1.87° for the angle O4−Zn1−N2 (Table S2). Also, further structural analyses showed that for 1 and 1-SG the zinc(II) ion is in a four-coordinated tetrahedral geometry completed by O2, N2, N5, and O4 from two chelated bidentate H2L ligands (Figure 2). In ligand L−, there are three aromatic rings which are defined as A, B, and C as shown in Figure 2. The dihedral angles of A/B, B/C, and A/C were examined to investigate the conformation variation during grinding (Table 1). In both 1 and 1-SG, the large dihedral angles for N5-containing L− reveal it adopts a more twisted conformation than N2-containing L−. The N2containing L− shows slight change after grinding. However, for N5-containing L− ligand, the dihedral angle between planes A and C increased by 3.24° after grinding. Ultimately the molecular conformation of L− changed. Compared with 1, a blue-shift of the fluorescence spectrum for 1-SG may be attributed to the weakened π-conjugation of the ligand deriving from its worse coplanarity.41 For 1 and 1-SG, their N2-containing ligands are in antiparallel arrangement. They are overlapped in different degree as shown in Figure 3. To further describe the ligand

Figure 2. Molecular structures of 1 (a), 1-SG (b), 2 (c), 3 (d), and 4 (e).

Table 1. Dihedral Angles between Different Planes in 1, 1SG, 2, and HL

1 1-SG 2 HL

N2-containing N5-containing N2-containing N5-containing

L L L− L−

A/B

B/C

A/C

3.99 18.91 3.68 16.73 10.69 7.43

11.38 43.76 11.69 44.88 13.70 3.33

14.39 25.35 14.46 28.59 6.28 4.89

packing, the distance between the B rings of N2-containing ligands is adopted as the interlayer distance. In 1 and 1-SG, the distances between adjacent molecules which are almost overlapped are 2.980 and 3.020 Å, respectively. While those between partly overlapped ones are 4.446 and 4.395 Å, respectively. The distances between adjacent ligands with same orientation are 7.426 and 7.415 Å for 1 and 1-SG, with a Zn··· Zn distance of 8.922 and 8.773 Å, respectively. This indicates a negligible interlayer distance change but a significant sliding. Therefore, grinding not only leads to the variation of molecular conformation of the ligands in the zinc(II) complex, but also the change of its crystal packing. As is reported, weak intermolecular interactions play an important role in MCL.36,37,42−44 As shown in Figure S3, there exist a large number of C−H···π, C−H···O, and C−H···N interactions in both of 1 and 1-SG. However, it can be found from Table S3 that no significant change for weak intermolecular interactions was observed. It can be suggested that slight grinding does not result in obvious change of the interactions between ZnL2 molecules. Therefore, it can be concluded that for the grinding-triggered SCSCT of ZnL2, the alteration of the molecular conformation is the key factor for its MCL. C

DOI: 10.1021/acs.inorgchem.9b00195 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

those (7.426 and 7.415 Å) in 1 and 1-SG. As is known, diethylamino group could lead to the loose packing of molecules because of its steric effect.45 Therefore, the small dihedral angle and the dense molecular arrangement of 2 make it difficult to change the molecular conformation and crystal packing to produce MCL. In Zn(II) complexes 3 and 4, all the ligands lack benzoxazolyl groups as compared with the ligand in 1. In 3, Zn(II) is coordinated with three ligands. One is chelated bidentate enol-form (L2), while the other two are monodentate keto-form (HL3). The dihedral angles between the two aromatic rings for L2, N1- and N3-containing HL3 are 19.55, 10.25, and 1.42°, respectively. The N3-containing ligands are in an antiparrallel arrangement (Figure 3d) with a distance of 3.331 Å. Along the a-axis, L2 ligand from another pair of Zn(II) complexes exhibits C−H···π interaction with a distance of 2.538 Å from hydrogen atom to the center of B ring of N3containing ligand. For Zn(II) complex 4, there are two crystallographically independent Zn(II) ions. Each Zn(II) ion is coordinated with two Cl− anion and two HL3. The dihedral angles between the two aromatic rings are 5.78, 7.22, 1.85, and 12.84° for N1-, N4-, N6-, and N7-containing HL3, respectively. The N4- and N6-containing ligands are antiparallel (Figure 3e). There is C−H···π interaction with a distance of 2.666 Å, but the existence of C−H···π interaction in Zn(II) complexes 3 and 4 actually gives no contribution to MCL property. However, the bidentate chelated mode inhibits the rotation of the aromatic ring. While the monodentate mode is beneficial to the rotation of aromatic rings to change the molecular conformation, giving rise to the slight change of luminescence property. The above results showed that Zn(II) complexes 2− 4 exhibit no significant MCL property, which indicates flexible −N(CH2CH3)2 group and rotated benzoxazolyl moiety are desirable to construct compounds with changeable conformation and play an important role in MCL of 1. In addition, the molecule HL has good coplanarity with the dihedral angles between the aromatic rings 7.43, 3.33, and 4.89°, as shown in Table 1. It exhibits no MCL property. It is coordination of H2L with Zn(II) ion that leads to the twisted molecular structure of ZnL2. Grinding can change the structure of the Zn(II) complex to some extent with alteration of molecular conformation. Aggregation-Induced Emission of Zn(II) Complex 1. Tetrahydrofuran (THF) is good solvent for Zn(II) complex 1, while 1 is insoluble in water. Complex 1 in THF exhibited green fluorescence emission at 492 nm when excited at 430 nm, as shown in Figure 4a, but the fluorescence properties of 1 in THF changed when water was added to THF. At the beginning fluorescence emission of 1 was quenched if the water fraction (f w) in THF/H2O reached 10% (Figure 4b).

Figure 3. Molecular packing modes of 1 (a), 1-SG (b), 2 (c), 3 (d), and 4 (e).

In order to investigate whether MCL of 1 is accompanied by the crystal phase change, powder X-ray diffraction (PXRD) measurement was conducted. Figure S4 showed that diffraction peaks of the ground samples are relatively broaden compared with those of as-prepared crystals, but both samples upon slight grinding and high grinding exhibited intense and sharp diffraction peaks like those of the as-prepared (1), indicative of their crystalline nature. The slightly ground samples are probably in a middle stage of crystals with different molecular conformation. The diffraction peaks of highly ground samples further broaden. Thus, it can be inferred that high grinding leads to the further conformational variation of the ligands, sequentially continuous blue-shift of fluorescence emission undergoes from 620 to 608 nm. To investigate effect of substituent groups of diethylamino and benzoxazolyl on MCL property of the Zn(II) complex, three new control Zn(II) complexes (Zn(L1)2 (2), [Zn(HL3)2L2]ClO4 (3), and [Zn(HL3)2Cl2] (4)) based on analogues of HL were prepared according to the synthetic route in Scheme 1, and their crystal structures were determined. Their fluorescence spectra indicate that there are slight changes for fluorescence intensity of crystals 2−4 after grinding (Figure S5), and the fluorescence spectra for 2−4 have blue shifts of 3, 3, and 9 nm after grinding only, respectively. In Zn(II) complex 2, the Zn(II) ion is coordinated with two ligands of the same conformation (Figure 2), in which the ligand L1 is an analogue of L in 1 without a diethylamino group. The three rings in each ligand of 2 are not coplanar with a dihedral angle of 10.69, 13.70, and 6.28°, respectively, as listed in Table 1. The interlayer distances are 3.314 and 3.226 Å, respectively (Figure 3c). The distance between adjacent two ligands with same orientation is 6.540 Å, much shorter than

Figure 4. Emission spectra of Zn(II) complex 1 (50 μM) in THF/ H2O (v/v) with different fractions of water and (b) emission intensity of complex 1 (50 μM) vs f w in THF/H2O (v/v), λex = 430 nm. D

DOI: 10.1021/acs.inorgchem.9b00195 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry When f w was above 65%, the fluorescence intensity of 1 was obviously enhanced with the increasing of f w and accompanied by red-shift of its fluorescence spectra. Quenching of fluorescence emission upon addition of H2O to complex 1 in THF is indicative of the formation of the hydrogen-bonded species.46 This could be due to the formation of hydrogen bonds (O−H···O) between OH from water and the oxygen atom from Zn−O in ZnL2, which results in fluorescence quenching because the S1 excited state of ZnL2 could be easily quenched via electron-driven proton transfer (EDPT) from the solvent (H2O) to the fluorophore (ZnL2).47,48 To further understanding, the electronic energy profiles for the groundand excited-state potential energy surfaces were explored with B3LYP+IDSCRF/6-31G(d) and TD-B3LYP+ IDSCRF/631G(d) methods, in which the PCM model49,50 with our IDSCRF radii51,52 have been employed to simulate the real THF solvent condition. All of calculations have been performed with Gaussian 09 package.53 As shown in Figure 5, we assumed that on the ground-state surface, ZnL2 could

INT1 and P become 7.2 and 10.3 kcal/mol, lower than those of initial state of ZnL2 (excited) + H2O, suggesting that the reaction processes of the combination of H2O with Zn(II) center and hydrogen shift from oxygen atom in H2O to the nitrogen atom in opened Zn−N bond are energy favorable. This is why fluorescence of ZnL2 would disappear when H2O molecule was added to the THF solution of ZnL2. The very weak fluorescence intensity of Zn(II) complex 1 in THF/H2O remained nearly unchanged until f w reached 65%. Then, the intensity continuously enhanced until f w = 90%. Shultz and Vu suggested that as f w increased the intermolecular interactions between THF and H2O molecules weakened in THF/H2O mixture.54 It is favorable for ZnL2 to form aggregates in THF/H2O medium since water is poor solvent for ZnL2. Thus, aggregates for ZnL2 could be found when H2O % reached 70%. As the aggregates formed, the mixture apparently became a suspension. From Figure 4b, it can be seen that the fluorescence intensity of ZnL2 in THF/H2O increased with f w increasing. The quantum yields of ZnL2 in THF/H2O (Table 2) increased from 0.9 to 10.5% as f w increased from 10 to 90%, and the solution became turbid when f w was 70%, which showed the formation of strong fluorescent aggregates. Table 2. Fluorescence Quantum Yield (ΦF) of Zn(II) Complex 1 in THF/H2Oa fractions of water (f w) quantum yield a

10% 0.9%

60% 1.5%

80% 8.5%

90% 10.5%

95% 9.9%

λex = 430 nm.

To further confirm the formation of aggregates, dynamic light scattering (DLS) and scanning electron microscopy (SEM) were conducted. The DLS data and SEM images are shown in Figure 6. The DLS data indicated the size Figure 5. Possible reaction processes between ZnL2 and H2O on ground-state and excited-state potential energy surface, along with the relative energies for each stationary points.

combine with H2O molecule via hydrogen-bonding to the oxygen atom in one of Zn−O bonds to form complex 1(COM1). In transition state 1 (TS1), the oxygen atom in H2O attacks the Zn(II) center, and one Zn−N bond opens, with the Zn···O(H2O) and Zn···N bond distances being 2.078 and 2.831 Å, respectively. In intermediate 1 (INT1), the opened Zn−N bond is further apart to form typical O−H···N hydrogen bond, and then hydrogen transfers via transition state 2(TS2) to form P with O···H−N hydrogen bond. In fact, such reaction processes might not take place on the groundstate surface since the relative energies of INT1 and product (P) are 1.1 and 0.4 kcal/mol, higher than that of ZnL2 + H2O. The reaction might be stopped at COM1 since it is the most stable structure on the ground-state surface. The fluorescence calculations suggested that fluorescence emission of COM1 takes place at 470 nm with an oscillator strength of 1.908, which is almost the same as those of ZnL2 (477 nm and 1.967). The reaction processes for the excited-state energy surface are similar to those of the ground-state one (detailed structural parameters are given in Tables S8 and S9). However, the relative energies for each intermediate species and final product are quite different from those of the ground-state surface. It should be emphasized that the relative energies of

Figure 6. DLS data of Zn(II) complex 1 (50 μM) in THF/H2O: (a) 80% H2O, (b) 90% H2O, and (c) 95% H2O. SEM images of Zn(II) complex 1 (50 μM) in THF/H2O: (d) 80% H2O, (e) 90% H2O, and (f) 95% H2O.

distributions of the aggregates and the SEM images displayed the sizes and the shape of aggregates in THF/H2O. They support the formation of aggregates of ZnL2 in THF/H2O. Therefore, the emission enhancement of ZnL2 in THF/H2O is attributed to the formation of the aggregates. Clearly, ZnL2 in THF/H2O is a typical AIE fluorogen with yellow fluorescence emission. As can be seen from Figure 6, when f w reached 90%, the particle size was in the range of 200−400 nm. When f w was 95%, the nanoparticles became smaller and the fluorescence intensity dropped (Figure 4). The decrease of fluorescence E

DOI: 10.1021/acs.inorgchem.9b00195 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry intensity can be considered “concentration quenching” because smaller aggregates (f w = 95%) were formed. The weakening of interactions between THF and H2O molecules in THF/H2O makes the H2O molecules “free”;,54 nanoaggregates of ZnL2 were rapidly formed, indicating the increase of “concentration” of ZnL2 nanoparticles in THF/H2O and the increase of particle collisions. This leads to decrease of the fluorescence intensity of ZnL2 in THF/H2O. Also, the PXRD pattern of the aggregates (Figure S4) showed no resolved diffraction peaks, indicative of an amorphous phase. This implies that ZnL2 molecules are not in an orderly arrangement in the aggregates. This is attributed to the THF/H2O environment, which strongly affects the intermolecular interactions of ZnL2. To further explore the mechanism of AIE phenomenon of Zn(II) complex 1 in THF/H2O, the fluorescence properties of two control compounds 2 and 3 in THF/H2O were investigated. AIE behavior of 2 could be found in THF/H2O (Figure 7a). DLS data and SEM photographs of 2 support its

materials in different phase is a key characteristic for its potential application in the density of optical data storage.

■

CONCLUSIONS In summary, a Zn(II) complex (1) with grinding-triggered single crystal-to-single crystal transformation (SCSCT) was obtained. The Zn(II) complex exhibits a fluorescence color change from red to yellow under high grinding. The relationship between structure and mechanochromic luminescence (MCL) property of the Zn(II) complex was first accurately illustrated by single crystal structures before and after slight grinding. The MCL property of the Zn(II) complex was confirmed to be related with the molecular conformation variation of ligands and its crystal packing. Flexible substituents and rotatable aromatic rings are key factors to obtain MCLactive Zn(II) complex. For Zn(II) complex 1 in THF/H2O medium, a low fraction of water will quench fluorescence emission via electron-driven proton transfer (EDPT), while an appropriate amount of water in THF/H2O would produce aggregation-induced emission (AIE), resulting in fluorescence enhancement. The AIE phenomenon of the Zn(II) complex in THF/H2O arises from the restriction of intramolecular rotations. This work perhaps opens up a different path to develop metal complexes with MCL and suggests that the metal complexes may be potential candidates for applications as MCL materials.

■

Figure 7. Emission spectra of (a) Zn(L1)2 (2) (50 μM) in THF/H2O (v/v), λex= 405 nm and (b) [Zn(HL3)2L2]ClO4 (3) (50 μM) in THF/H2O (v/v). λex= 410 nm.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b00195. Normalized emission spectra of crystals of 1-SG and 1 before and after slight and high grinding; changes of the fluorescence intensities at 647 and 608 nm for crystals (1) and highly ground samples (1-HG) upon repeated grinding-recrystallization cycles of 1; molecular packing of 1 and 1-SG in crystals with C−H···π and multiple C− H···O, C−H···N intermolecular interactions; XRD patterns of crystals as-prepared, slight grinding, high grinding and aggregates samples of 1; emission spectra of crystals and ground samples of 2−4; DLS data and SEM images of Zn(II) complex 2 (50 μM) in THF/ H2O; fluorescence spectra, photoimages, and schematic arrangements of 1 in the THF solution, the aggregates of 1 in THF/H2O, and the slight grinding, the high grinding samples and the single crystals of 1, respectively; crystal data and structure refinement of Zn(II) complexes 1, 1-SG, and 2−4; selected bond distances (Å) and angles (deg) for 1, 1-SG, and 2−4; types and distances of hydrogen bonds present in 1 and 1-SG, respectively; optimized Cartesian coordinates (Å) of species studied; total electronic energy (a.u.) and relative electronic energy (kcal/mol, in parentheses) for the species along possible reaction pathways between ZnL2 and H2O in ground state and excited state (PDF) Movie S1: as-prepared crystals of 1 under slight grinding and high grinding in an agate mortar with pestle under UV light (AVI)

AIE property (Figure S6). This is attributed to restriction of rotations of the benzoxazolyl groups. Fluorescence emission spectra of 3 in THF/H2O are shown in Figure 7b. Fluorescence emission is much intensive in THF. However, fluorescence intensity of 3 dropped as water was added to THF. Very weak fluorescence was found in THF/H2O and no aggregates were formed. Clearly, the results of three Zn(II) complexes, 1−3, showed that AIE properties of 1 and 2 are due to the existence of benzoxazolyl group, in which the benzoxazolyl acts as a rotor. Restriction of intramolecular rotation (RIR) is the reason for AIE properties of 1 and 2 in THF/H2O.55 In the formation of the aggregates, the intramolecular rotation is restricted, resulting in decrease of nonradiation transition and enhancement of the emission efficiency. This could be confirmed by the quantum yield of ZnL2 from 1.5% for f w = 60% and 10.5% for f w = 90%, as shown in Table 2. Crystallization-Induced Red-Shift Characteristics of ZnL2. Figure S7 shows that ZnL2 exhibits fluorescence emission at 492 nm in THF, and the aggregates, the highly ground samples, the slightly ground samples, and the crystals of ZnL2 exhibit fluorescence emissions at 580, 608, 624, and 647 nm, displaying green, yellow, orange, and red colors, respectively (Figure S7b). From free molecule of ZnL2 in THF to orderly arranged molecules in crystals, ZnL2 gives four-color fluorescence between 492 and 647 nm. As the results mentioned above, we suggest that the molecular conformation and crystal packing of ZnL 2 strongly impact on its luminescence properties (Figure S7c), i.e., crystallizationinduced red-shift emission. The multicolor emission of

Accession Codes

CCDC 1874380−1874384 contain the supplementary crystallographic data for this paper. These data can be obtained F

DOI: 10.1021/acs.inorgchem.9b00195 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

(11) Wang, L. K.; Shen, Y. F.; Yang, M. D.; Zhang, X. Z.; Xu, W. N.; Zhu, Q. J.; Wu, J. Y.; Tian, Y. P.; Zhou, H. P. Novel Highly Emission H-Aggregates with Aggregate Fluorenscence Change in a Phenylbenzoxazole-Based System. Chem. Commun. 2014, 50, 8723−8726. (12) Levitus, M.; Schmieder, K.; Ricks, H.; Shimizu, K. D.; Bunz, U. H. F.; Garcia-Garibay, M. A. Steps To Demarcate the Effects of Chromophore Aggregation and Planarization in Poly(phenyleneethynylene)s. 1. Rotationally Interrupted Conjugation in the Excited States of 1,4-Bis(phenylethynyl)benzene. J. Am. Chem. Soc. 2001, 123, 4259−4265. (13) Lim, S. J.; An, B. K.; Jung, S. D.; Chung, M. A.; Park, S. Y. Photoswitchable Organic Nanoparticles and a Polymer Film Employing Multifunctional Molecules with Enhanced Fluorescence Emission and Bistable Photochromism. Angew. Chem., Int. Ed. 2004, 43, 6346− 6350. (14) Zhang, C. J.; Feng, G. X.; Xu, S. D.; Zhu, Z. S.; Lu, X. M.; Wu, J.; Liu, B. Structure-Dependent cis/trans Isomerization of Tetraphenylethene Derivatives: Consequences for Aggregation-Induced Emission. Angew. Chem., Int. Ed. 2016, 55, 6192−6196. (15) Feng, Q.; Li, Y. Y.; Wang, L. L.; Li, C.; Wang, J. M.; Liu, Y. Y.; Li, K.; Hou, H. W. Multiple-Color Aggregation-Induced Emission (AIE) Molecules as Chemodosimeters for pH Sensing. Chem. Commun. 2016, 52, 3123−3126. (16) Sasaki, S.; Drummen, G. P. C.; Konishi, G. I. Recent Advances in Twisted Intramolecular Charge Transfer (TICT) Fluorescence and Related Phenomena in Materials Chemistry. J. Mater. Chem. C 2016, 4, 2731−2743. (17) Zhao, Q.; Li, L.; Li, F. Y.; Yu, M. X.; Liu, Z. P.; Yi, T.; Huang, C. H. Aggregation-Induced Phosphorescent Emission (AIPE) of Iridium(III) Complexes. Chem. Commun. 2008, 685−687. (18) Tsujimoto, H.; Ha, D. G.; Markopoulos, G.; Chae, H. S.; Baldo, M. A.; Swager, T. M. Thermally Activated Delayed Fluorescence and Aggregation Induced Emission with Through-Space Charge Transfer. J. Am. Chem. Soc. 2017, 139, 4894−4900. (19) Yang, J.; Ren, Z. C.; Xie, Z. L.; Liu, Y. J.; Wang, C.; Xie, Y. J.; Peng, Q.; Xu, B.; Tian, W. J.; Zhang, F.; Chi, Z. G.; Li, Q. Q.; Li, Z. AIEgen with Fluorescence-Phosphorescence Dual Mechanoluminescence at Room Temperature. Angew. Chem., Int. Ed. 2017, 56, 880− 884. (20) Zhang, J. N.; Kang, H.; Li, N.; Zhou, S. M.; Sun, H. M.; Yin, S. W.; Zhao, N.; Tang, B. Z. Organic Solid Fluorophores Regulated by Subtle Structure Modification: Color-Tunable and AggregationInduced Emission. Chem. Sci. 2017, 8, 577−582. (21) Zhou, Y. B.; Qian, L. B.; Liu, M. C.; Huang, X. B.; Wang, Y. X.; Cheng, Y. X.; Gao, W. X.; Wu, G.; Wu, H. Y. 5-(2,6-Bis((E)-4(Dimethylamino)styryl)-1-Ethylpyridin-4(1H)-Ylidene)-2,2-Dimethyl-1,3-Dioxane-4,6-Dione: Aggregation-Induced Emission, Polymorphism, Mechanochromism, and Thermochromism. J. Mater. Chem. C 2017, 5, 9264−9272. (22) Yoon, S. J.; Park, S. J. Polymorphic and Mechanochromic Luminescence Modulation in the Highly Emissive Dicyanodistyrylbenzene Crystal: Secondary Bonding Interaction in Molecular Stacking Assembly. J. Mater. Chem. 2011, 21, 8338−8346. (23) Ito, H.; Muromoto, M.; Kurenuma, S.; Ishizaka, S.; Kitamura, N.; Sato, H.; Seki, T. Mechanical Stimulation and Solid Seeding Triggered Single-Crystal-to- Single-Crystal Molecular Domino Transformation. Nat. Commun. 2013, 4, 2009. (24) Zhang, X. Q.; Chi, Z. G.; Zhang, Y.; Liu, S. W.; Xu, J. R. Recent Advances in Mechanochromic Luminescent Metal Complexes. J. Mater. Chem. C 2013, 1, 3376−3390. (25) Mei, J.; Leung, N. L. C.; Kwok, R. T. K.; Lam, J. W. Y.; Tang, B. Z. Aggregation-Induced Emission: Together We Shine, United We Soar! Chem. Rev. 2015, 115, 11718−11940. (26) Wen, T.; Zhou, X. P.; Zhang, D. X.; Li, D. Luminescent Mechanochromic Porous Coordination Polymers. Chem. - Eur. J. 2014, 20, 644−648. (27) Wen, T.; Zhang, D. X.; Liu, J.; Zhang, H. X.; Zhang, J. Facile Synthesis of Bimetal Au-Ag Nanoparticles in a Cu (I) Boron

free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

■

AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-10-58805522. Fax: +86-10-58802075. E-mail: [email protected]. ORCID

Xiang-Jun Zheng: 0000-0002-7720-9000 De-Cai Fang: 0000-0003-3922-7221 Author Contributions

S.L., L.P.J., and X.J.Z. designed the experiments. S.L., M.W. and Y.K. performed the experiments. S.L., M.W., Y.K., H.W.Z., L.P.J., and X.J.Z. performed the data analysis, D.C.F. performed theoretical calculation. S.L. and X.J.Z. wrote the manuscript. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We thank the financial supports from the National Natural Science Foundation of China (21671022).

■

REFERENCES

(1) Umar, S.; Jha, A. K.; Purohit, D.; Goel, A. A TetraphenyletheneNaphthyridine-Based AIEgen TPEN with Dual Mechanochromic and Chemosensing Properties. J. Org. Chem. 2017, 82, 4766−4773. (2) Liu, G.; Yang, M. D.; Wang, L. K.; Zheng, J.; Zhou, H. P.; Wu, J. Y.; Tian, Y. P. Schiff Base Derivatives Containing Heterocycles with Aggregation-Induced Emission and Recognition Ability. J. Mater. Chem. C 2014, 2, 2684−2691. (3) Yoon, S. J.; Chung, J. W.; Gierschner, J.; Kim, K. S.; Choi, M.-G.; Kim, D.; Park, S. Y. Multistimuli Two-Color Luminescence Switching via Different Slip-Stacking of Highly Fluorescent Molecular sheets. J. Am. Chem. Soc. 2010, 132, 13675−13683. (4) Sun, H. B.; Liu, S. J.; Lin, W. P.; Zhang, K. Y.; Lv, W.; Huang, X.; Huo, F. W.; Yang, H. R.; Jenkins, G.; Zhao, Q.; Huang, W. Smart Responsive Phosphorescent Materials for Data Recording and Security Protection. Nat. Commun. 2014, 5, 3601. (5) Huang, J.; Nie, H.; Zeng, J. J.; Zhuang, Z. Y.; Gan, S. F.; Cai, Y. J.; Guo, J. J.; Su, S. J.; Zhao, Z. J.; Tang, B. Z. Highly Efficient Nondoped OLEDs with Negligible Efficiency Roll-Off Fabricated from Aggregation-Induced Delayed Fluorescence Luminogens. Angew. Chem., Int. Ed. 2017, 56, 12971−12976. (6) Mei, J.; Hong, Y. N.; Lam, J. W. Y.; Qin, A. N.; Tang, Y. H.; Tang, B. Z. Aggregation-Induced Emission: the Whole is More Brilliant than the Parts. Adv. Mater. 2014, 26, 5429−5479. (7) Luo, J. D.; Xie, Z. L.; Lam, J. W. Y.; Cheng, L.; Chen, H. Y.; Qiu, C. F.; Kwok, H. S.; Zhan, X. W.; Liu, Y.; Zhu, D. B.; Tang, B. Z. Aggregation-Induced Emission of 1-methyl-1, 2, 3, 4, 5-pentaphenylsilole. Chem. Commun. 2001, 1740−1741. (8) Huang, M.; Yu, R. N.; Xu, K.; Ye, S. X.; Kuang, S.; Zhu, X. H.; Wan, Y. Q. An Arch-Bridge-Type Fluorophore for Bridging the Gap Between Aggregation-Caused Quenching (ACQ) and AggregationInduced Emission (AIE). Chem. Sci. 2016, 7, 4485−4491. (9) Würthner, F.; Kaiser, T. E.; Saha-Möller, C. R. J-Aggregates: from Serendipitous Discovery to Supramolecular Engineering of Functional Dye Materials. Angew. Chem., Int. Ed. 2011, 50, 3376− 3410. (10) Choi, S.; Bouffard, J.; Kim, Y. Aggregation-Induced Emission Enhancement of a Meso-Trifluoromethyl BODIPY via J-Aggregation. Chem. Sci. 2014, 5, 751−755. G

DOI: 10.1021/acs.inorgchem.9b00195 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Imidazolate Framework with Mechanochromic Properties. Chem. Commun. 2015, 51, 1353−1355. (28) Seki, T.; Tokodai, N.; Omagari, S.; Nakanishi, T.; Hasegawa, Y.; Iwasa, T.; Taketsugu, T.; Ito, H. Luminescent Mechanochromic 9Anthryl Gold (I) Isocyanide Complex with an Emission Maximum at 900 nm after Mechanical Stimulation. J. Am. Chem. Soc. 2017, 139, 6514−6517. (29) Sun, J.; Dai, F. N.; Yuan, W. B.; Bi, W. H.; Zhao, X. L.; Sun, W. M.; Sun, D. F. Dimerization of a Metal Complex through Thermally Induced Single-Crystal-to-Single-Crystal Transformation or Mechanochemical Reaction. Angew. Chem., Int. Ed. 2011, 50, 7061−7064. (30) Zhang, Q.; Su, J.; Feng, D. W.; Wei, Z. W.; Zou, X. D.; Zhou, H. C. Piezofluorochromic Metal-Organic Framework: A Microscissor Lift. J. Am. Chem. Soc. 2015, 137, 10064−10067. (31) Jin, M. G.; Sumitani, T.; Sato, H.; Seki, T.; Ito, H. MechanicalStimulation-Triggered and Solvent-Vapor-Induced Reverse SingleCrystal-to-Single-Crystal Phase Transitions with Alterations of the Luminescence Color. J. Am. Chem. Soc. 2018, 140, 2875−2879. (32) Seki, T.; Sakurada, K.; Ito, H. Controlling Mechano- and Seeding- Triggered Single-Crystal-to- Single-Crystal Phase Transition: Molecular Domino with a Disconnection of Aurophilic Bonds. Angew. Chem., Int. Ed. 2013, 52, 12828−12832. (33) Mizukami, A.; Houjou, H.; Sugaya, K.; Koyama, E.; Tokuhisa, H.; Sasaki, T.; Kanesato, M. Fluorescence Color Modulation by Intramolecular and Intermolecular π-π Interactions in a Helical Zinc (II) Complex. Chem. Mater. 2005, 17, 50−56. (34) Yan, Y.; Chen, J.; Zhang, N. N.; Wang, M. S.; Sun, C.; Xing, X. S.; Li, R.; Xu, J. G.; Zheng, F. K.; Guo, G. C. Grinding SizeDependent Mechanoresponsive Luminescent Cd (II) Coordination Polymer. Dalton Trans. 2016, 45, 18074−18078. (35) Yang, M. D.; Zhang, Y.; Zhu, W. J.; Wang, H. Z.; Huang, J.; Cheng, L. H.; Zhou, H. P.; Wu, J. Y.; Tian, Y. P. Difunctional Chemosensor for Cu(I) and Zn(II) Based on Schiff Base Modified Anthryl Derivative with Aggregation-Induced Emission Enhancement and Piezochromic Characteristics. J. Mater. Chem. C 2015, 3, 1994− 2002. (36) Zhao, S. S.; Chen, L.; Wang, L.; Xie, Z. G. Two Tetraphenylethene-Containing Coordination Polymers for Reversible Mechanochromism. Chem. Commun. 2017, 53, 7048−7051. (37) Tzeng, B. C.; Chang, T. Y.; Wei, S. L.; Sheu, H. S. Reversible Phase Transformation and Mechanochromic Luminescence of ZnIIDipyridylamide-Based Coordination Frameworks. Chem. - Eur. J. 2012, 18, 5105−5112. (38) Xu, B. J.; Chi, Z. G.; Zhang, X. Q.; Li, H. Y.; Chen, C. J.; Liu, S. W.; Zhang, Y.; Xu, J. R. A New Ligand and its Complex with MultiStimuli-Responsive and Aggregation-Induced Emission Effects. Chem. Commun. 2011, 47, 11080−11082. (39) Wang, A.; Fan, R. Q.; Wang, P.; Fang, R.; Hao, S.; Zhou, X. S.; Zheng, X. B.; Yang, Y. L. Research on the Mechanism of AggregationInduced Emission through Supramolecular Metal−Organic Frameworks with Mechanoluminescent Properties and Application in PressJet Printing. Inorg. Chem. 2017, 56, 12881−12892. (40) Chen, Z.; Zhang, J.; Song, M.; Yin, J.; Yu, G. A.; Liu, S. H. A Novel Fluorene-Based Aggregation-Induced Emission (AIE)-Active Gold(I) Complex with Crystallization-Induced Emission Enhancement (CIEE) and Reversible Mechanochromism Characteristics. Chem. Commun. 2015, 51, 326−329. (41) Pawle, R. H.; Haas, T. E.; Muller, P.; Thomas, S. W., III Twisting and Piezochromism of Phenylene-Ethynylenes with Aromatic Interactions Between Side Chains and Main Chains. Chem. Sci. 2014, 5, 4184−4188. (42) Mizukami, S.; Houjou, H.; Sugaya, K.; Koyama, E.; Tokuhisa, H.; Sasaki, T.; Kanesato, M. Fluorescence Color Modulation by Intramolecular and Intermolecular π-π Interactions in a Helical Zinc (II) Complex. Chem. Mater. 2005, 17, 50−56. (43) Hu, L. X.; Gao, M. Y.; Wen, T.; Kang, Y.; Chen, S. M. Synthesis of Halide-Modulated Cuprous (I) Coordination Polymers with Mechanochromic and Photocatalytic Properties. Inorg. Chem. 2017, 56, 6507−6511.

(44) Yan, Y.; Chen, J.; Zhang, N. N.; Wang, M. S.; Sun, C.; Xing, X. S.; Li, R.; Xu, J. G.; Zheng, F. K.; Guo, G. G. Grinding SizeDependent Mechanoresponsive Luminescent Cd(II) Coordination Polymer. Dalt. Trans. 2016, 45, 18074−18078. (45) Cheng, J. H.; Li, Y. X.; Sun, R.; Liu, J. Y.; Gou, F.; Zhou, X. G.; Xiang, H. F.; Liu, J. Functionalized Salen Ligands Linking with Lonconjugated Bridges: Unique and Colorful Aggregation-Induced Emission, Mechanism, and Applications. J. Mater. Chem. C 2015, 3, 11099−11110. (46) Roberts, E. L.; Dey, J.; Warner, I. M. Excited-State Intramolecular Proton Transfer of 2-(2 ′-Hydroxyphenyl) Benzimidazole in Cyclodextrins and Binary Solvent Mixtures. J. Phys. Chem. A 1997, 101, 5296−5301. (47) Maekawa, S.; Sato, R.; Hirao, K.; Shigeta, Y. Solvent Effects on Excited-State Electron-Transfer Rate of Pyrene-Labeled Deoxyuridine: A Theoretical Study. Chem. Phys. Lett. 2016, 644, 25−30. (48) Staniforth, M.; Quan, W. D.; Karsili, T. N. V.; Baker, L. A.; O’Reilly, R. K.; Stavros, V. G. First Step toward a Universal Fluorescent Probe: Unravelling the Photodynamics of an Amino− Maleimide Fluorophore. J. Phys. Chem. A 2017, 121, 6357−6365. (49) Miertus, S.; Scrocco, E.; Tomasi, J. Electrostatic interaction of a solute with a continuum. A Direct Utilizaion of AB Initio Molecular Potentials for the Prevision of Solvent Effects. Chem. Phys. 1981, 55, 117−129. (50) Jacquemin, D.; Perpète, E. A.; Scalmani, G.; Ciofini, I.; Peltier, C.; Adamo, C. Absorption and Emission Spectra of 1, 8Naphthalimide Fluorophores: A PCM-TD-DFT Investigation. Chem. Phys. 2010, 372, 61−66. (51) Tao, J. Y.; Mu, W. H.; Chass, G. A.; Tang, T. H.; Fang, D. C. Balancing the Atomic Waistline: Isodensity-Based Scrf radii for MainGroup Elements and Transition Metals. Int. J. Quantum Chem. 2013, 113, 975−984. (52) Fang, D. C. SCRF-RADII; Beijing Normal University, Beijing, China, 2013. (53) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, revision A.02; Gaussian, Inc.: Wallingford, CT, 2009. (54) Shultz, M. J.; Vu, T. H. Hydrogen Bonding Between Water and Tetrahydrofuran Relevant to Clathrate Formation. J. Phys. Chem. B 2015, 119, 9167−9172. (55) Zeng, Q.; Li, Z.; Dong, Y. Q.; Di, C. A.; Qin, A. J.; Hong, Y. N.; Ji, L.; Zhu, Z. C.; Jim, C. K. W.; Yu, G.; et al. Fluorescence Enhancements of Benzene-Cored Luminophors by Restricted Intramolecular Rotations: AIE and AIEE Effects. Chem. Commun. 2007, 70−72.

H

DOI: 10.1021/acs.inorgchem.9b00195 Inorg. Chem. XXXX, XXX, XXX−XXX