English

• 1. INTRODUCTION

  The development of functional organic-inorganic hybrids has attracted great interest due to the potential of combining the distinct properties of organic and inorganic components into a single molecular composite^{[1, 2]}. In this field, more attention was paid to the organic functional materials because their functions can be easily modified by introducing functional groups, and consequently, fascinating physical properties such as optical signal processing, THz generation and luminescence chromisms can be achieved^[3-5]. Among various organic functional molecules, particular interest was paid to push-pull π-delocalized stilbazolium-type dyes^[6]. Their unique D-π-A structures contain an electron acceptor linked to an electron donor group via a π-conjugated bridge. Consequently, fascinating characters including the large molecular hyperpolarizability (β) and low dielectric constants will be generated^[7-9]. So far, by changing electron-donating/withdrawing groups on D/A terminals, many stilbazolium derivatives have been investigated^[10-12]. The aggregation modes of stilbazolium-type dyes can determine their optical properties (for example, electronic intramolecular charge transfer, ICT) which can also be modulated by introducing different substituents on the amine groups^[13]. Inorganic moieties are also important in achieving function promotion. To our knowledge, the following types of inorganic components have been used to incorporate with stilbazolium-type dyes: (a) metal complexes, for example, Au(CN)^2- and MPS₃ (M = Mn, Cd, Zn)^{[10, 14, 15]}, (b) polymeric metal halides^{[2, 16]} and (c) polyoxometalates^{[11, 17]}. Among them, d¹⁰ metal halides have captured special attention due to the stimulus-responsive performances like the X-ray-induced photochromism of [ZnLBr₂]^[18]. To the best of our knowledge, the study about the combination of stilbazolium-type dyes with zinc halides/pseudohalide is rare^[19]. In this work, D-π-A stilbazolium-type dye chromophore DANP (DANP = trans-4-(4΄-(N, N-diethylaminostyryl))-N-methyl-pyridinium) was hybridized with zinc chloride to give a new hybrid (DANP-H)(ZnCl₄) (1), and its unquenched photoluminescence was induced by X-aggregation mode of (DANP-H)²⁺ cations. Furthermore, dual stimuli-responsive performance and reversible photo/thermochromic behaviors can be found, which were explained by theoretical calculation.

  2. EXPERIMENTAL

  2.1 Materials and method

  All the reactants were of reagent grade and used as purchased. Elemental analyses (C, H, N) were performed on a Perkin-Elemer 240C instrument. IR spectrum was recorded as KBr discs on a Shimadzu IR-408 infrared spectrophotometer in the 400~4000 cm^-1 region. The electronic spectrum was taken on a Shimadzu UV-2101 PC UV-Vis scanning spectrophotomete equipped with an integrating sphere at ambient temperature. ¹H NMR spectrum was recorded at a Bruker AVIII 400 spectrometer, whose chemical shifts were reported in parts per million (ppm) downfield from tetramethylsilane. The photoluminescence measurement was carried out on an FLS920 fluorescence spectrophotometer.

  2.2 Synthesis

  2.2.1   Synthesis of (DANP)I

  Trans-4-(4΄-(N, N-diethylaminostyryl))-N-methyl-pyridinium) (DANP)I was synthesized by Knoevenagel reaction (Scheme 1)^[20]. 4-Methyl-pyridine (0.9300 g, 10 mmol) and methyl iodide (1.4200 g, 10 mmol) were refluxed in 30 mL methanol for 5 hours to give 1, 4-dimethylpyridin-1-ium iodide. 4-N, N-diethylamino-benzaldehyde (0.1490 g, 10 mmol) was added into the above solution and then refluxed for 5 hours using piperidine (5 drops) as catalyst. The product was washed with methanol for three times. ¹H NMR (400 MHz, DMSO, Fig. 1) δ 8.66 (d, J = 6.4 Hz, 2H), 8.03 (d, J = 6.4 Hz, 2H), 7.89 (d, J = 16.1 Hz, 1H), 7.57 (d, J = 8.5 Hz, 2H), 7.12 (d, J = 16.0 Hz, 1H), 6.75 (d, J = 8.6 Hz, 2H), 4.16 (s, 3H), 3.43 (q, J = 6.8 Hz, 4H), 1.13 (t, J = 6.9 Hz, 6H).

  Scheme S1

  

  Scheme S1.  Synthesis route of (DANP)I

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  Figure 1

  

  Figure 1.  NMR spectrum of (DANP)I

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  2.2.1   Synthesis of (DANP-H)(ZnCl₄) (1)

  1 was prepared in a solution method. ZnCl₂ (0.0681 g, 0.5 mmol) and (DANP)I (0.1996 g, 0.5 mmol) were dissolved in 10 mL methanol, and the solution was stirred for 2 h. Afterwards, the pH of the mixture was adjusted as 2.0 by adding HCl, which was further filtered and kept at room temperature for slow evaporation. Although the I^- has stronger coordinated ability than Cl^-, the amount of Cl^- in the solution is much larger than that of I^-. As a result, the chloride was obtained in the final product. Yellow block crystals were obtained after one week (0.1020 g, yield 49.2% based on ZnCl₂). Anal. Calcd. for C₃₆H₄₇Cl₈N₄Zn₂ (950.18): C, 45.51; H, 4.98; N, 5.90 %. Found: C, 46.04; H, 4.82; N, 5.81%. IR(cm^-1): 3056(w), 2978(m), 2909(w), 1630(s), 1516(s), 1473(s), 1387(m), 1331(m), 1187(s), 1153(s), 1015(m), 984(s), 837(s), 566(s), 517(s).

  2.3 Computational method

  The electronic structure calculation based on density function theory (DFT) was conducted without further optimization^[21]. During the calculation, wave functions were explained in a plane wave basis set and the spin polarized version of PBE GGA was employed for the exchangecorrelation functional in the CASTEP code^[22]. In this case, a 3×3×2 Monkhorst-Pack grid with a total number of 6 k points in the irreducible Brillouin zone was used for the simulation of primitive cell, and the separation is set to 0.05 Å^-1 in the calculations. The number of plane waves included in the basis was determined by a cutoff energy E_c of 435.4 eV. The pseudoatomic calculations on Zn-3d¹⁰4s², Cl-3s²3p⁵, C-2s²2p² and N-2s²2p³ were conducted. The convergent criterion of total energy is set to 2.0 × 10^-5 eV/atom.

  2.4 X-ray crystallography

  A yellow crystal with dimensions of 0.40 × 0.30 × 0.20 mm³ was placed on an APEX II CCD area detector equipped with a graphite-monochromatic MoKα radiation (λ = 0.71073 Å) at 296(2) K. A total of 14874 reflections were collected by a ϕ-ω scan mode at room temperature in the range of 1.71≤θ≤27.51º with index ranges of –30≤h≤23, –18≤ k≤17 and –18≤l≤17 including 8272 independent ones (R_int = 0.0252), of which 5552 were observed with I > 2σ(I). The empirical absorption corrections by SADABS were carried out. The structure was solved by direct methods using SHELXS-97 program^[23] and refined with SHELXL-97^[24] by full-matrix least-squares techniques on F². All non-hydrogen atoms were refined anisotropically, while the hydrogen atoms were located geometrically and refined isotropically. The final R = 0.0408 and wR = 0.1061 (w = 1/[σ²(F_o²) + (0.0646P)² + 0.7674P], where P = (F_o² + 2F_c²)/3), S = 1.016, (Δ/σ)_max = 0.000, (Δρ)_max = 0.583 and (Δρ)_min = –0.337 e/ų. Selected bond lengths and bond angles are given in Table 1.

  Table 1

  

  Table 1.  Selected Bond Lengths (Å) and Bond Angles (º)

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  Bond	Dist.		Bond	Dist.		Bond	Dist.
  Zn(1)–Cl(1)	2.230(3)		Zn(1)–Cl(2)	2.316(3)		Zn(1)–Cl(3)	2.258(3)
  Zn(1)–Cl(4)	2.263(3)		Zn(2)–Cl(5)	2.316(3)		Zn(2)–Cl(6)	2.288(3)
  Zn(2)–Cl(7)	2.251(3)		Zn(2)–Cl(8)	2.259(3)
  Angle	(°)		Angle	(°)		Angle	(°)
  Cl(1)–Zn(1)–Cl(3)	110.89(12)		Cl(1)–Zn(1)–Cl(4)	112.77(15)		Cl(3)–Zn(1)–Cl(4)	106.44(13)
  Cl(1)–Zn(1)–Cl(2)	108.20(17)		Cl(3)–Zn(1)–Cl(2)	111.71(13)		Cl(4)–Zn(1)–Cl(2)	106.80(13)
  Cl(7)–Zn(2)–Cl(8)	110.15(13)		Cl(7)–Zn(2)–Cl(6)	109.15(14)		Cl(8)–Zn(2)–Cl(6)	106.20(13)
  Cl(7)–Zn(2)–Cl(5)	112.38(14)		Cl(8)–Zn(2)–Cl(5)	111.76(13)		Cl(6)–Zn(2)–Cl(5)	106.93(12)

  3. RESULTS AND DISCUSSION

  3.1 Description of the structure

  1 contains ZnCl₄^2- mononuclear anions and stilbazoliumtype dye chromophore DANP-H²⁺ cations with centrosymmetric space group of C_c. C–H···Cl hydrogen bonds and C–H···π interactions contribute to the formation of a quasi-3D network. The ZnCl₄^2- mononuclear anion was commonly observed in zinc halide system, in which Zn(II) was in a tetrahedronal environment (Fig. 2a)^{[25, 26]}. The Zn–Cl lengths of 2.288(3)~2.316(3) Ǻ and Cl–Zn–Cl angles of 106.20(13)~112.38(14)° hint a slightly distorted ZnCl₄ tetrahedron (Table 1). Two independent DANP-H²⁺ cations exhibit different configurations. In the first DANP (defined by N(1)), the distance of C=C double bond is 1.233 Å. And those around C=C bond are 1.471 and 1.512 Å, which are consistent with the sp³-hybrid C–C bond lengths (Fig. 2b). These values indicate that the conjugated system has been broken. The C–C and C–N bonds of another dependent DANP (defined by N(3)) are in normal ranges (Fig. 2c), which are consistent with those in literatures^[7-9]. The dihedral angles between pyridine and benzene rings is 7.90°, indicating that two planes (pyridine and benzene rings) are approximately coplanar. The distances around C=C double bonds (1.461, 1.330, 1.471 Å) hint the typical conjugated configuration, which is favourable to molecular hyperpolarizability according to the idea of Marder^[27]. In the pyridine and amine groups, the sum of the three C–N–C angles with N atom as centers is all close to 360.0°, suggesting that the outstretched methyl and carbon atoms of N-substituents are generally coplanar^[2]. Besides, the trans E-configuration with respect to the ethenyl double bonds has the torsion angles of 173.86 and 179.80°. The protonization of amine is deduced from the charge balance. Since there are two crystallographically independent ZnCl₄^2- and DANP moieties, for the sake of keeping charge balance, each of DANP must have two positive charges, so another positive charge should locate on the amine. The independent DANP-H²⁺ cation (defined by N(1)) is linked into a 1D chain along the b-axis via C(3)– H(3)···Cl(3) and C(17)–H(17B)···Cl(4) hydrogen bonds (Fig. 3a), and the second DANP-H²⁺ (defined by N(3)) is connected into a 1D chain along b-axis via C(21)–H(21)···Cl(8) and C(35)–H(35A)···Cl(6) hydrogen bonds. These two independent [(DANP-H)(ZnCl₄)]_n chains are connected into a double chain via C(19)–H(19B)···π (benzene ring) (H···π distance: 2.89 Ǻ, Fig. 3b). Furthermore, neighboring doublechains are linked into a 2-D layer via C(1)–H(1C)···Cl(6) and C(19)–H(19C)···Cl(4) hydrogen bonds. Finally, adjacent layers are extended into a 3D network via C(4)–H(4)···Cl(4) hydrogen bond (Fig. 3c). The shortest centroid distance among benzene/pyridine rings is 4.624(6) Ǻ, hinting the absence of π-π stacking interaction. But the distance of neighboring ethenyl double bonds is 3.60 Ǻ, and the crossing angle of two DANP-H²⁺ cations is 58.57°, suggesting the presence of X-aggregation (Fig. 2d)^[28]. In all, the hydrogenbonding interactions, C–H···π interactions and static electricity effect in the lattice solidify the crystal packing of 1.

  Figure 2

  

  Figure 2.  (a) Structure of ZnCl₄^2- tetrahedron; (b, c) Structures of two independent DANP; (d) X-aggregation of DANP (H atoms were omitted for clarity)

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  Figure 3

  

  Figure 3.  (a) 1D [(DANP-H)(ZnCl₄)]_n chain based on C–H···Cl hydrogen bonds among b-axis; (b) Double chain C–H···π interaction; (c) 3D network constructed from C–H···Cl hydrogen bonds

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  Table 2

  

  Table 2.  Hydrogen Bridging Details of 1 (Lengths in Å and Angles in º)

  DownLoad: CSV

  D–H···A	d(D–H)	d(H···A)	d(D···A)	∠DHA	Symmetry codes
  C(1)–H(1C)···Cl(6)	0.96	2.66	3.567(13)	157
  C(3)–H(3)···Cl(3)	0.93	2.63	3.543(12)	165
  C(4)–H(4)···Cl(4)	0.93	2.81	3.649(12)	150	–1/2 + x, 1/2 – y, –1/2 + z
  C(17)–H(17B)···Cl(4)	0.97	2.69	3.477(12)	139	x, 1 + y, z
  C(19)–H(19C)···Cl(4)	0.96	2.75	3.657(12)	158
  C(21)–H(21)···Cl(8)	0.93	2.61	3.520(12)	164
  C(35)–H(35A)···Cl(6)	0.97	2.68	3.486(10)	141	x, 1 + y, z

  3.2 Absorption and solid-state luminescent spectra

  Solid-state unpolarized UV-Vis absorption spectra of 1 and (DANP)I were recorded from powder samples at room temperature (Fig. 4a). (DANP)I exhibits broad adsorption zones from ultra-violet (250~600 nm) to near-infrared (650~800 nm), which is in agreement with other stilbazolium-type dyes and can be attributed to CT (charge transfer)^{[11, 29]}. But after hybridization with zinc halide, clear blue-shift has occurred, while the near-infrared (600~800 nm) adsorption has vanished. The sharp peaks at 288 and 362 nm correspond to the π-π* transitions of benzene/pyridine rings, which is consistent with the structural analysis that the conjugated system in one DANP has broken. The adsorption zone in 400~600 nm stems from another conjugationmaintained DANP. Besides, a great blue-shift compared with (DANP)I has happened, which might also be led by its X-aggregation mode^[28]. One-photon photoluminescence spectra of 1 and (DANP)I measured at room temperature in the solid state are shown in Fig. 4b. Upon excitation, (DANP)I presents a broad red emission at 620 nm (λ_ex = 570 nm), which is in agreement with literatures^[2]. After the formation of (DANP-H)(ZnCl₄) hybrid, red-shifted emission to 667 nm can be observed, which has also been documented in other stilbazolium-type dyes and their metal complexes^[30]. The maintenance of red emission can be ascribed to the X-aggregation of DANP in lattice, which could not quench the fluorescence. The clear red-shifted emission might be led by the hydrogen-bonding formation^[31].

  Figure 4

  

  Figure 4.  (a) Optical diffuse-reflection spectra and (b) solid-state emission spectra of 1 (λ_ex = 625 nm) and (DANP)I (λ_ex = 570 nm)

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  3.3 Reversible photo/thermochromic behavior

  Due to the presence of mixed configurations of DANP in lattice, dual stimuli-responsive performance will be expected. To our interest, 1 responses to both photo and thermal stimuli. When it is irradiated by UV light (250 nm) for 3 h, its color changes from yellow to red, and then retunes to the initial color after removing the UV light. Similarly, it becomes dark red when heating to 130 ℃. After cooling to room temperature, it again retunes to the original color (Fig. 5). Therefore, 1 exhibits reversible photo/thermochromisms. The relative low thermochromic temperature of 130 ℃ is lower than those of viologen-based chromic materials (for example, 150 ℃ for {(MV)₂[Pb₇Br₁₈]}_n)^[32], which might be led by the presence of non-covalent interactions (C–H···Cl H-bonds and C–H···π interactions) in the lattice. The photochromism could be induced by the conjugation-broken or conjugationmaintained DANPs. In order to disclose which DANP dominates the photochromism behavior, theoretical calculations were executed. The chromic behaviors of viologen-based halometallates have been extensively studied, and the mechanism can be explained as the electrons transfer from p orbital of halides in metal halides to π* antibonding orbitals of organic molecules^[33]. The chromic behavior of 1 could also be explained by this mechanism, because DANP is also electronically poor. DFT calculation was conducted to verify this process. The band structure and partial density of states (PDOS) of 1 were given (Fig. 6). The band structure shows a direct band gap at the Brillouin zone center with an energy value of 1.72 eV. PDOS indicates that the valence-band maximum mainly results from the nonbonding states of Cl-3p and π bonding orbitals of DANP, while the π* anti-bonding orbitals of DANP (conjugation-maintained one) account for the in-plane dispersion of the conduction-band minimum. Comparably, the π* anti-bonding orbital conjugation-broken DANP does not appear in the frontier orbitals (right of Fig. 5). Consequently, the photo/thermochromisms can be attributed to the electron transfer from Cl-3p/DANP-π components to π* antibonding orbitals of the conjugation-maintained DANP cations, which is similar to that in viologen-based halometallate systems^[34].

  Figure 5

  

  Figure 5.  Photo/thermochromisms of 1

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  Figure 6

  

  Figure 6.  Band structure (left) and projected density of states (pDOS, right) for 1

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  4. CONCLUSION

  In summary, combination of D-π-A stilbazolium-type dye with zinc chloride results in a new hybrid (DANP-H)(ZnCl₄) (1), in which conjugation-broken and conjugation-maintained DANP-H²⁺ co-exist in the lattice. Furthermore, strong C–H···Cl hydrogen bonds and C–H···π interactions contribute to the formation of a 3D network, in which X-aggregation mode of (DANP-H)²⁺ can be observed. Consequently, its adsorption spectrum exhibits a blue-shift, and unquenched red photoluminescence can be monitored. Interestingly, dual stimuli-responsive performance and reversible photo/thermochromic behaviors are found, with the mechanism disclosed by theoretical calculation.

  
  
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  Scheme S1  Synthesis route of (DANP)I

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  Figure 1  NMR spectrum of (DANP)I

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  Figure 2  (a) Structure of ZnCl₄^2- tetrahedron; (b, c) Structures of two independent DANP; (d) X-aggregation of DANP (H atoms were omitted for clarity)

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  Figure 3  (a) 1D [(DANP-H)(ZnCl₄)]_n chain based on C–H···Cl hydrogen bonds among b-axis; (b) Double chain C–H···π interaction; (c) 3D network constructed from C–H···Cl hydrogen bonds

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  Figure 4  (a) Optical diffuse-reflection spectra and (b) solid-state emission spectra of 1 (λ_ex = 625 nm) and (DANP)I (λ_ex = 570 nm)

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  Figure 5  Photo/thermochromisms of 1

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  Figure 6  Band structure (left) and projected density of states (pDOS, right) for 1

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  Table 1.  Selected Bond Lengths (Å) and Bond Angles (º)

  Bond	Dist.		Bond	Dist.		Bond	Dist.
  Zn(1)–Cl(1)	2.230(3)		Zn(1)–Cl(2)	2.316(3)		Zn(1)–Cl(3)	2.258(3)
  Zn(1)–Cl(4)	2.263(3)		Zn(2)–Cl(5)	2.316(3)		Zn(2)–Cl(6)	2.288(3)
  Zn(2)–Cl(7)	2.251(3)		Zn(2)–Cl(8)	2.259(3)
  Angle	(°)		Angle	(°)		Angle	(°)
  Cl(1)–Zn(1)–Cl(3)	110.89(12)		Cl(1)–Zn(1)–Cl(4)	112.77(15)		Cl(3)–Zn(1)–Cl(4)	106.44(13)
  Cl(1)–Zn(1)–Cl(2)	108.20(17)		Cl(3)–Zn(1)–Cl(2)	111.71(13)		Cl(4)–Zn(1)–Cl(2)	106.80(13)
  Cl(7)–Zn(2)–Cl(8)	110.15(13)		Cl(7)–Zn(2)–Cl(6)	109.15(14)		Cl(8)–Zn(2)–Cl(6)	106.20(13)
  Cl(7)–Zn(2)–Cl(5)	112.38(14)		Cl(8)–Zn(2)–Cl(5)	111.76(13)		Cl(6)–Zn(2)–Cl(5)	106.93(12)

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  Table 2.  Hydrogen Bridging Details of 1 (Lengths in Å and Angles in º)

  D–H···A	d(D–H)	d(H···A)	d(D···A)	∠DHA	Symmetry codes
  C(1)–H(1C)···Cl(6)	0.96	2.66	3.567(13)	157
  C(3)–H(3)···Cl(3)	0.93	2.63	3.543(12)	165
  C(4)–H(4)···Cl(4)	0.93	2.81	3.649(12)	150	–1/2 + x, 1/2 – y, –1/2 + z
  C(17)–H(17B)···Cl(4)	0.97	2.69	3.477(12)	139	x, 1 + y, z
  C(19)–H(19C)···Cl(4)	0.96	2.75	3.657(12)	158
  C(21)–H(21)···Cl(8)	0.93	2.61	3.520(12)	164
  C(35)–H(35A)···Cl(6)	0.97	2.68	3.486(10)	141	x, 1 + y, z

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