English

• 1. INTRODUCTION

  Luminescence mechanochromic materials that exhibit reversible color and luminescence changes triggered by external mechanical grinding have received considerable interest due to their potential applications in the fields such as structural flaw detection materials, optical recording/storage equipment, erasable writing devices, and pressure-sensitive sensors^[1-3]. In the past two decades, great progress has been made in the design and application of such materials and many kinds of compounds including inorganic salts, organic compounds, transition-metal complexes, macro-molecules, polymers, and nanoparticles have been developed^[4-11]. It is well known that the electronic structure of Pt(II) complexes can be perturbed by changing the intermolecular interactions such as Pt-Pt contact, hydrogen bonding or aromatic π-π stacking, resulting in dramatic changes in spectroscopic properties^[12-20]. Meanwhile, the unique square-planar structure makes the molecular arrangement of Pt(II) complex very sensitive to external forces, so platinum(II) complexes are good candidate for studying luminescent mechanochromic materials. However, previous studies were mostly focused on the vapoluminescence of Pt(II) complexes, and studies on the luminescent mechanochromic property are still very limited.

  Our research interest focuses on the stimulus-responsive phosphorescence switches of squareplanar diimine platinum(II) complexes. In our previous work, we have reported a series of platinum(II) luminescence switches based on bipyridine derivatives that exhibited remarkable luminescent vapochromic, thermochromic, and mechanochromic properties^[17-20]. As the continuation of our researching work, we report herein a new platinum(II) complex with 3-trimethylsilylethynyl-1, 10-phenanthroline (Me₃SiC≡Cphen) ligand, [Pt(Me₃SiC≡Cphen)(C≡CC₆H₄Cl-3)₂] (1). This complex stacks in the columnar structure and exhibits interesting reversible luminescent mechanochromic property. On this basis, a simple device was developed and used for rewritable data storage.

  2. EXPERIMENTAL

  2.1 Materials and methods

  All operations were carried out under a dry argon atmosphere using Schlenk techniques and vacuumline systems unless otherwise specified. Ligand 3-trimethylsilylethynyl-1, 10-phenanthroline and intermediate Pt(Me₃SiC≡Cphen)Cl₂ were synthesized according to a similar method in literature^[19]. All the other chemicals were purchased commercially and used without further purification. ¹H NMR spectra were measured using a Bruker Avance II (400 MHz) spectrometer with SiMe₄ as the internal reference. Infrared spectra (IR) were recorded on a Nicolet-20DXB spectrometer as KBr pellets in the range of 4000~400 cm^-1. Elemental analyses of C, H, and N were determined on a Vario EL III Elemental Analyzer. A Finnigan LCQ mass spectrometer was used to acquire electrospray ionization mass spectra (ESI-MS) and dichloromethane was used as the mobile phases. Powder X-ray diffraction patterns (XRD) data were collected on a Rigaku SmartLab XRD instrument with a sealed Cu tube (λ = 1.54178 Å). Emission spectra were acquired at ambient temperature by using a Perkin-Elmer LS55 luminescence spectrometer. The solid UV-visible absorption spectra were obtained by a V-550 ultraviolet spectrophotometer, while the liquid absorption data were recorded using Cary-300 UV/Vis spectrometer. Emission decay lifetimes and emission quantum yield at room temperature were measured on an Edinburgh FLS 920 fluorescence spectrometer with an integrating sphere accessory. Luminescence photographs were taken by a Canon camera under ambient light and UV lamp irradiation.

  2.2 Synthesis of [Pt(Me₃SiC≡Cphen)(C≡CC₆H₄Cl-3)₂] (1)

  A mixture containing Pt(Me₃SiC≡Cphen)Cl₂ (271.2 mg, 0.5 mmol), 3-chlorophenylacetylene (170.7 mg, 1.25 mmol), CuI (1 mg) and diisopropylamine (2 mL) in 50 mL of CH₂Cl₂ was stirred overnight at room temperature. The mixture was dried under vacuum and then subjected to column chromatography over silica gel (300~400 mesh) and eluted with dichloromethane to afford the pure product. Yield: 83% (308.2 mg). IR (KBr, cm⁻¹): 2954 (w, C–H), 2164 (w, C≡CTMS), 2121 (s, C≡CPt), 1628(w, C=N), 1587 (s, C=C), 1548 (m, benzene ring), 1470 (s, benzene ring), 1434 (m, benzene ring), 1244 (m, C–Si), 1073 (m, C–Cl), 794 (s, Si-CH₃). Anal. Calcd. (%) for C₃₃H₂₄Cl₂N₂PtSi: C, 53.37; H, 3.26; N, 3.77. Found (%): C, 53.35; H, 3.28; N, 3.75. ESI-MS(m/z): calcd. for C₃₃H₂₄Cl₂N₂PtSi, 742.6; found 742.6 [M]⁺. ¹H NMR (d₆-DMSO, 400MHz) δ: 0.315 (s, 9H), 7.274 (m, 8H), 8.238 (t, J = 6.0 Hz, 2H), 8.331 (d, J = 7.2 Hz, 1H), 9.036 (d, J = 7.2 Hz, 1H), 9.223 (s, 1H), 9.736 (s, 2H).

  2.3 Structure determination

  A yellow single crystal of 1 with dimensions of 0.10mm × 0.10mm × 0.08mm was mounted on the top of a glass fiber for single-crystal X-ray diffraction analysis. All measurements were performed on a Bruker SMART APEX II CCD area detector system equipped with graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å) at 210(2) K. A total of 7953 reflections were collected, of which 5150 were independent (R_int = 0.0328) and 4377 were observed with I > 2σ(I). The intensity data set was collected with an ω scan mode in the range of 2.36 < θ < 25.05° at 210(2) K. The structure was solved by direct methods using SHELXS-97^[21] and refined by full-matrix least-squares procedure on F² with SHELXL-97^[22]. All non-hydrogen atoms were refined anisotropically, and hydrogen atoms were positioned geometrically. The final refinement gave R = 0.0369, wR = 0.0786 (w = 1/[σ²(F_o²) + (0.0333P)²], where P = (F_o² + 2F_c²)/3), S = 1.023, (Δρ)_max = 1.854 and (Δρ)_min = −0.976 e/ų.

  3. RESULTS AND DISCUSSION

  3.1 Crystal structure of complex 1

  Complex 1 crystallizes in a triclinic space group P$ \overline 1 $ with two formula units per unit cell. The selected bond lengths and bond angles are listed in Table 1. As shown in Fig. 1a, the platinum center in the molecule exhibits a distorted square-planar coordination geometry coordinated by two N atoms from phenanthroline and two C atoms from two 3-chlorophenylacetylene ligands. The bond distances of Pt–N and Pt–C are in the ranges of 2.062(9)~2.075(7) and 1.945(12)~1.974(8) Å (Table 1), which are all within the normal ranges of diimine platinum-alkynyl complexes^[14-17]. Adjacent molecules with the shortest Pt···Pt distance being 4.417 Å are arranged in an antiparallel pattern and form a dimer through π(C(18)≡C(19))···π(phen) and aromatic π(benzene ring containing C(23) atom)···π(phen) stacking interactions with the center-to-center distances being 3.612 and 4.071 Å, respectively (Fig. 1b)^{[18-20, 23]}. Neighboring dimers along the a axis are also connected by π(C(18)≡C(19))···π(phen) stacking interactions to construct the columnar structure (Fig. 2a). Furthermore, the molecules of adjacent dimers also adopt the antiparallel packing pattern with the Pt···Pt distance being 4.450 Å. The large Pt···Pt separation indicates the absence of intermolecular metal-metal contact in the stacking structure of complex 1. The C(23)–H(23)···π(benzene ring containing Cl(3) atom) hydrogen bonds connect the molecular column structure into a 2D supramolecular structure (Fig. 2b).

  Table 1

  

  Table 1.  Selected Bond Lengths (Å) and Bond Angles (°) for 1

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  Bond	Dist.		Bond	Dist.		Bond	Dist.
  Pt(1)–C(18)	1.949(5)		Pt(1)–C(26)	1.947(5)		Pt(1)–N(1)	2.059(3)
  Pt(1)–N(2)	2.079(4)
  Angle	(°)		Angle	(°)		Angle	(°)
  N(1)–Pt(1)–N(2)	80.0(2)		N(1)–Pt(1)–C(18)	93.1(2)		N(2)–Pt(1)–C(26)	95.5(2)
  C(18)–Pt(1)–C(26)	91.4(2)		N(1)–Pt(1)–C(26)	175.5(2)		N(2)–Pt(1)–C(18)	173.0(2)

  Figure 1

  

  Figure 1.  Structure of 1. (a) An ORTEP drawing of 1 with 30% thermal ellipsoids. (b) A dimer formed by adjacent molecules with antiparallel packing pattern. π-π stacking interactions are represented as red dotted line. Symmetry code: a: –x+2, –y+2, –z+1 (The hydrogen atoms are omitted for clarity)

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  Figure 2

  

  Figure 2.  Stacking structure of 1. (a) Columnar structure along the a axis. (b) Packing of columnar structures. The π-π stacking interactions and C–H···π hydrogen bonds are represented as red dotted line. Symmetry codes: a: –x+1, –y+2, –z+1; b: –x+2, –y+1, –z+1 (All hydrogen atoms not participated in hydrogen bonds are omitted for clarity)

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  3.2 Photophysical properties of complex 1

  The UV-vis absorption spectra of 1 in dichloromethane exhibit high-energy bands at 230~335 nm and low-energy bands at about 414 and 440 nm (Fig. 3a). The high-energy bands can be mainly ascribed to intraligand charge transfer (¹ILCT) state and low-energy broad bands, resulting mostly likely from a mixture of metal-to-ligand charge transfer (¹MLCT) and ligand-to-ligand charge transfer (¹LLCT) transitions^[23-25]. In degassed dichloromethane solution at room temperature, 1 displays a bright orange-yellow luminescence peak at 570 nm with the lifetime of 0.27 μs and the quantum yield of 12.71% (Fig. 3b). This emission is mostly likely attributed to a mixture of ³MLCT and ³LLCT states^[23-25].

  Figure 3

  

  Figure 3.  (a) UV-vis absorption and (b) emission spectra of 1 in CH₂Cl₂ solution (black line), unground solid state (red line) and ground solid state (blue line)

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  In the solid state, the yellow-green crystalline species of 1 exhibits the lowest energy absorption band at 412~506 nm (Fig. 3a), arising from both ¹LLCT and ¹MLCT transitions. Under irradiation at 350 < λ_ex < 500 nm, crystalline state 1 exhibits a bright green luminescence at ca. 518 and 554 nm (598 nm, sh) with lifetime of 0.65 μs and quantum yield of 10.36% (Fig. 3b). This emission may also result from a mixture of ³MLCT and ³LLCT states according to the complexes with similar structures^[23-25].

  3.3 Luminescent mechanochromic property of complex 1

  Complex 1 displays interesting reversible luminescent mechanochromic property. Once mechanical grinding the solid species 1 in an agate mortar, the emission of 1 exhibits a drastic red shift from 518 and 554 nm to a broad unstructured emission centered at 700 nm with lifetime of 0.13 μs and quantum yield of 0.49%, affording a drastic mechanoluminescence red shift at about 146-182 nm (Fig. 3b). Meanwhile, the color of the sample also changed from yellow-green to red with the red-shift of solid absorption spectrum (Fig. 3 and Fig. 4a). The PXRD measurement reveals that the crystalline species of 1 changed to the amorphous phase after grounding (Fig. 4c). The significant red shifts of UV-vis and emission spectra demonstrate that luminescence mechanochromic property of complex 1 is most probably due to the conversion of emissive state from ³MLCT/³LLCT to ³MMLCT/³LLCT character^[15-20]. Furthermore, luminescent mechanochromic property of complex 1 displays excellent reversibility. As shown in Fig. 4, once exposed to C₂H₅OH vapor, the color and luminescence of the ground sample could be restored thoroughly to the original crystalline state of 1.

  Figure 4

  

  Figure 4.  Reversible luminescent mechanochromic property of 1. (a) Photographic images of color and luminescence changes of 1 during reversible process. (b) Solid-state emission spectral changes of ground species 1 in response to ethanol vapor. (c) PXRD diagrams of 1 in different states. All above confirm the reversibility of luminescent mechanochromic property of 1

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  Based on this property, complex 1 was successfully used to develop a simple rewritable data recording device. As shown in Fig. 5, on a piece of 1-coated filter paper, a metal cone was used as the "pen" to write the message (the chemical structure of 1). The message will be recorded on the paper because color and luminescence of message changes from yellow to red and the background is unchanged. The recorded message can be easily erased upon exposure to C₂H₅OH vapor and the paper can be used to record other message (a cartoon image of sun).

  Figure 5

  

  Figure 5.  Practical application of 1 for rewritable data recording device. All photographs were taken in the UV light (365 nm) with the digital camera

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  4. CONCLUSION

  In summary, a new planar-square diimine platinum(II) complex was synthesized and characterized. It exhibits interesting reversible luminescent mechanochromic property with emission response shift of 146~182 nm. On this basis, a simple rewritable data recording device was developed and successfully used to record the messages.

  
  
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• 

  Figure 1  Structure of 1. (a) An ORTEP drawing of 1 with 30% thermal ellipsoids. (b) A dimer formed by adjacent molecules with antiparallel packing pattern. π-π stacking interactions are represented as red dotted line. Symmetry code: a: –x+2, –y+2, –z+1 (The hydrogen atoms are omitted for clarity)

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  Figure 2  Stacking structure of 1. (a) Columnar structure along the a axis. (b) Packing of columnar structures. The π-π stacking interactions and C–H···π hydrogen bonds are represented as red dotted line. Symmetry codes: a: –x+1, –y+2, –z+1; b: –x+2, –y+1, –z+1 (All hydrogen atoms not participated in hydrogen bonds are omitted for clarity)

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  Figure 3  (a) UV-vis absorption and (b) emission spectra of 1 in CH₂Cl₂ solution (black line), unground solid state (red line) and ground solid state (blue line)

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  Figure 4  Reversible luminescent mechanochromic property of 1. (a) Photographic images of color and luminescence changes of 1 during reversible process. (b) Solid-state emission spectral changes of ground species 1 in response to ethanol vapor. (c) PXRD diagrams of 1 in different states. All above confirm the reversibility of luminescent mechanochromic property of 1

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  Figure 5  Practical application of 1 for rewritable data recording device. All photographs were taken in the UV light (365 nm) with the digital camera

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  Table 1.  Selected Bond Lengths (Å) and Bond Angles (°) for 1

  Bond	Dist.		Bond	Dist.		Bond	Dist.
  Pt(1)–C(18)	1.949(5)		Pt(1)–C(26)	1.947(5)		Pt(1)–N(1)	2.059(3)
  Pt(1)–N(2)	2.079(4)
  Angle	(°)		Angle	(°)		Angle	(°)
  N(1)–Pt(1)–N(2)	80.0(2)		N(1)–Pt(1)–C(18)	93.1(2)		N(2)–Pt(1)–C(26)	95.5(2)
  C(18)–Pt(1)–C(26)	91.4(2)		N(1)–Pt(1)–C(26)	175.5(2)		N(2)–Pt(1)–C(18)	173.0(2)

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