English

• 0.   Introduction

  It is well known that anthracene and its derivatives have fascinating physical and chemical properties, such as luminescence^[1-3], [4π+4π] photodimerization^[4], and photo-oxidation (Scheme S1, Supporting information)^[5]. Interestingly, some metal complexes incorporate anthracene units, thus leading to various functionalities, for example, anthracene-based luminescence with an application in the detections of UO₂²⁺ and PO₄^3-[6], photodimerization-induced emission switching^[7], and photo-oxidation-induced turn-on phosphorescence^[8]. Several anthracene-based Au(Ⅰ) complexes have been reported, exhibiting attractive structures and/or properties, such as charge-transfer between anthracene unit and Au(Ⅰ) ion^[9], aggregation-caused quenching (ACQ) activity^[10], and tetranuclear gold(Ⅰ)-cyclophane with intra-molecular π…π interaction between anthracene rings^[11]. These reports indicate that the structure and the related functionality of an Au(Ⅰ) complex can be effectively modulated by incorporating suitable anthracene-based ligands. On the other hand, it was found that counter anion can significantly modulate the luminescence of an Au(Ⅰ) complex^[12-13]. For example, Zhang et al. reported a gold(Ⅰ) complex 1⁺·NTf₂^- with an N-heterocyclic-carbene-based ligand (Scheme S2)^[12]. The weakly orange-emitting solid state of this complex was prepared by cooling its melted liquid to 90 ℃. Upon applying a weak pinpoint stimulus with a needle, this solid state transformed into an intensively violet-blue-emitting state. In contrast, analogs 1⁺ with different counter anions, such as BF₄^-, PF₆^-, and SbF₆^-, cannot show similar behavior to 1⁺·NTf₂^-, due to the much higher melting points. These reports about Au(Ⅰ) complexes^[9-13] indicate that the luminescence of an anthracene-based Au(Ⅰ) coordination cation can be modulated by designing ligand as well as choosing counter anion.

  So far, only a small number of anthracene-based Au(Ⅰ) complexes have been reported. To well understand the structure-luminescence relationship, more cases need to be designed and synthesized. In this work, we prepared two anthracene-based Au(Ⅰ) complexes (Scheme 1), namely [Au(anbdtim)₂]PF₆ (1) and [Au(anbdtim)₂][Au(CN)₂] (2) (anbdtim=2-(anthracenyl)-4, 5-bis(2, 5-dimethyl(3-thienyl))-1-methyl-imidazole), with the following three aims. Firstly, the different counter anions (PF₆^- in 1, and [Au(CN)₂]^- in 2) would result in distinct luminescence behaviors between 1 and 2, due to possible aurophilicity between [Au(anbdtim)₂]⁺ cation and [Au(CN)₂]^- anion in complex 2^[14]. Secondly, it is expected that the anthracene units in 1 and 2 not only can act as luminescence groups, but also can connect with aromatic solvents (e.g., benzene and toluene) through π…π stacking interaction, leading to luminescence modulation/switching. Thirdly, complexes 1 and 2 probably display photochromism, due to the incorporation of bisthienylethene (BTE) ligand anbdtim^[15]. Herein, we report the syntheses, structures, and luminescence of 1 and 2, and discuss the influence of counter anion (e.g., PF₆^- and [Au(CN)₂]^-), solvent (e.g., benzene) and heating treatment on the luminescence behaviors of 1 and 2.

  Scheme 1

  

  Scheme 1.  Molecular structures of ligand anbdtim, and complexes 1 and 2

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  1.   Experimental

  1.1   Materials and methods

  Compounds 2-(anthracenyl)-4, 5-bis(2, 5-dimethyl(3-thienyl))-1H-imidazole (anbdtiH) and [Au(SMe₂)Cl] were prepared according to the literatures^{[4, 16]}. All the other starting materials were obtained from commercial sources without further purification. Elemental analyses were performed on a Perkin Elmer 240C elemental analyzer. IR spectra were obtained as KBr disks on a VECTOR 22 spectrometer. The ¹H NMR spectra were recorded at room temperature with a 400 MHz BRUKER spectrometer. UV-Vis absorption spectra were measured on a Cary 100 spectrophotometer. The luminescence spectra at room temperature were measured on a Hitachi F-4600 fluorescence spectrometer. The luminescence lifetimes and the solid-state luminescence quantum yields were measured at room temperature on a HORIBA FL-3 Spectrofluorometer with a 374 nm LED pulsed from a NanoLED resource. The luminescence quantum yields of 1, 2, and anbdtim in CH₂Cl₂ were measured on a Hitachi F-4600 fluorescence spectrometer by using a relative method by comparing with a standard, a solution of quinine sulfate in 0.5 mol·L^-1 H₂SO₄ (Φ=54.6%, λ_ex=366 nm)^[17].

  1.2   Synthesis of anbdtim

  A mixture of anbdtiH (0.5 mmol, 0.232 3 g), K₂CO₃ (2 mmol, 0.276 0 g), and DMF (7 mL) was stirred at room temperature for 45 min. The solution of CH₃I (0.5 mmol, 32 μL) in DMF (4 mL) was added dropwise, and then the resultant mixture was stirred for 20 h. Finally, water (100 mL) was added, and the resultant yellow precipitate was filtrated, washed with H₂O, and dried under vacuum, obtaining a yellow solid with a yield of 214 mg (89% based on anbdtiH).¹H NMR (400 MHz, CDCl₃): δ 2.18 (s, 3H), 2.33 (s, 3H), 2.36 (s, 3H), 2.48 (s, 3H) (2.18-2.48, total 12H from four —CH₃ groups at two thiophene rings in anbdtim), 3.09 (s, 3H from a —CH₃ group attached to the imidazole unit in anbdtim), 6.68 (s, 1H), 6.72 (s, 1H) (6.68-6.72, 2H from two thiophene rings in anbdtim), 7.47-7.51 (m, 4H), 7.66 (broad, 1H), 7.74 (broad, 1H), 8.06 (d, J=8.5 Hz, 2H), 8.59 (s, 1H) (7.47-8.59, total 9H from the anthracene unit in anbdtim). IR (KBr, cm^-1): 3 432(b, s), 2 919(s), 2 853(w), 1 626(m), 1 442(s), 1 384(s), 1 326(w), 1 265(w), 1 223(w), 1 141(m), 1 016(w), 890(w), 842(w), 796(w), 736(m), 674(w). Anal. Calcd. for C₃₀H₂₆N₂S₂(%): C, 75.28; H, 5.47; N, 5.85. Found(%): C, 75.34; H, 5.68; N, 5.92.

  1.3   Synthesis of complex 1

  Under the protection of Ar, a mixture of [Au(SMe₂)Cl] (0.1 mmol, 0.029 4 g), CH₃CN (20 mL), and AgPF₆ (0.12 mmol, 0.030 4 g) was stirred at room temperature for 10 min. After the addition of anbdtim (0.2 mmol, 0.095 6 g), the mixture was heated at 80 ℃ for 24 h. The resultant AgCl solid was removed by centrifugation, and the solution was evaporated under a vacuum, leading to a white solid. This solid was washed with CH₃COOC₂H₅ (10 mL×3), and dried under vacuum, obtaining pure complex 1 with a yield of 95 mg (73% based on [Au(SMe₂)Cl]). ¹H NMR (400 MHz, CDCl₃): δ 1.66 (s, 6H), 1.89 (s, 6H), 2.21 (s, 6H), 2.38 (s, 6H) (1.66-2.38, 24H from eight —CH₃ groups at four thiophene rings in complex 1), 2.88 (s, 6H from two —CH₃ groups attached to two imidazole units in complex 1), 6.33 (s, 2H), 6.52 (s, 2H) (6.33-6.52, 4H from four thiophene rings in complex 1), 6.85 (broad, 4H), 7.31 (broad, 4H), 7.52 (t, J=7.2 Hz, 4H), 8.08 (d, J=8.4 Hz, 4H), 8.54 (s, 2H) (6.85 and 7.31-8.54, total 18H from two anthracene units in complex 1). IR (KBr, cm^-1): 3 435(b, m), 3 053(w), 2 916(m), 2 855(w), 1 732(w), 1 624(m), 1 525(w), 1 444(s), 1 403(m), 1 326(w), 1 272(w), 1 231(w), 1 141(m), 1 016(w), 839(s), 739(s), 558(m). Anal. Calcd. for C₆₀H₅₂N₄F₆PS₄Au(%): C, 55.47; H, 4.03; N, 4.31. Found(%): C, 55.58; H, 4.35; N, 4.42. Electrospray (ES) MS: m/z=1 153.27 for [Au(anbdtim)₂]⁺.

  1.4   Synthesis of complex 2

  To a solution of complex 1 (0.015 mmol, 0.020 0 g) in CH₂Cl₂ (4 mL), was added the solution of K[Au(CN)₂] (0.046 mmol, 0.013 3 g) in deionized water (4 mL). The resultant mixture was stirred for 3.5 h in an ice water bath. Finally, the CH₂Cl₂ layer was separated, and evaporated under a vacuum, obtaining a yellow solid with a yield of 21 mg (98% based on complex 1). ¹H NMR (400 MHz, CDCl₃): δ 2.10 (s, 12H), 2.43 (s, 6H), 2.46 (s, 6H) (2.10-2.46, total 24H from eight —CH₃ groups at four thiophene rings in complex 2), 3.16 (s, 6H from two —CH₃ groups attached to two imidazole units in complex 2), 6.65 (s, 2H), 6.94 (s, 2H) (6.65-6.94, total 4H from four thiophene rings in complex 2), 7.39 (broad, 4H), 7.62 (m, 8H), 8.15 (d, 4H, J=8.0 Hz), 8.75 (s, 2H) (7.39-8.75, total 18H from two anthracene units in complex 2). IR (KBr, cm^-1): 3 435(b, s), 3 051(w), 2 917(s), 2 853(m), 2 151(m), 1 623(m), 1 522(w), 1 469(s), 1 402(m), 1 324(w), 1 278(w), 1 227(w), 1 142(m), 1 016(w), 895(w), 834(m), 792(w), 740(s), 706(w), 609(w). Anal. Calcd. for C₆₂H₅₂N₆S₄Au₂(%): C, 53.07; H, 3.74; N, 5.99. Found(%): C, 53.26; H, 3.95; N, 6.12. ES-MS: m/z=1 153.27 for [Au(anbdtim)₂]⁺, 248.96 for [Au(CN)₂]^-.

  1.5   X-ray crystallography

  Single crystals of 1·CH₃OH were grown in a CH₃OH-CH₂Cl₂ solution. A single crystal with a size of 0.14 mm×0.11 mm×0.08 mm was measured on a Bruker SMART APEX CCD diffractometer (Mo Kα, λ=0.071 073 nm) at room temperature. A hemisphere of data was collected in a θ range of 1.62° to 29.25°, using a narrow-frame method with scan width ω=0.50°, and an exposure time of 10 s per frame. The numbers of observed and unique reflections were 23 343 and 8 148 (R_int=0.050 6), respectively. The data were integrated using the Siemens SAINT program. Multi-scan absorption corrections were applied. The structure was solved by direct methods and refined on F² by full matrix least squares using SHELXTL. All the non-hydrogen atoms were located from the Fourier maps and were refined anisotropically. All H atoms were refined isotropically. In the structural refinement of 1·CH₃OH, a region of disordered electron density, probably a disordered CH₃OH molecule, was treated using the SQUEEZE routine in PLATON. The crystallographic data and selected bond lengths and bond angles of 1·CH₃OH are listed in Tables S1 and S2.

  2.   Results and discussion

  2.1   Syntheses and structural characterizations

  Complex 1 was synthesized through the reaction of anbdtim, [Au(SMe₂)Cl], and AgPF₆ in CH₃CN at 80 ℃ for 24 h. Based on 1, complex 2 was obtained by anion exchange of PF₆^- with [Au(CN)₂]^-. The powders of 1 and 2 showed different colors, almost white for 1, while yellow for 2 (Fig.S1). The structures of 1 and 2 were characterized by ¹H NMR, ES-MS, and even crystal structure. It should be noted that the ¹H NMR spectrum of complex 2 was different from that of complex 1 (Fig. 1, S3, and S4), although the two complexes have the same [Au(anbdtim)₂]⁺ cation. This suggests the aurophillic interaction between neighbouring [Au(anbdtim)₂]⁺ cation and [Au(CN)₂]^- anion in complex 2, forming a {[Au(anbdtim)₂]⁺…[Au(CN)₂]^-} unit. This aurophilicity was further confirmed by the significantly different luminescence between 1 and 2.

  Figure 1

  

  Figure 1.  ¹H NMR spectra of complexes 1 and 2 in CDCl₃

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  2.2   Crystal structure of 1·CH₃OH

  Complex 1·CH₃OH crystallizes in the orthorhombic space group C222₁. In its molecular structure (Fig. 2), two equivalent anbdtim molecules are bridged through an Au(Ⅰ) ion, forming a [Au(anbdtim)₂]⁺ cation with bond length Au—N1 of 0.201 1(4) nm and bond angle N1—Au—N1A of 177.9(3)°. In this cation, each anbdtim molecule exhibits a distorted structure with a dihedral angle of 71.1(1)° between the imidazole unit and anthracene moiety. Additionally, the two thiophene groups in an anbdtim molecule adopt a parallel conformation.

  Figure 2

  

  Figure 2.  Molecular structure of 1·CH₃OH

  All H atoms attached to carbon atoms are omitted for clarity; Symmetr code: A: 1-x, y, 0.5-z

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  In the packing structure of complex 1·CH₃OH, no aurophilicity is observed between neighboring [Au(anbdtim)₂]⁺ cations, mainly due to the big steric hindrance of ligand anbdtim. These cations are held together only by van der Waals interactions, and the inter-cation space is filled with PF₆^- anions (Fig.S7).

  2.3   Electronic absorption spectra

  The electronic absorption spectra of 1, 2, and anbdtim were measured in CH₂Cl₂ at room temperature (Fig.S8 and Table S3). Complexes 1 and 2 showed different high-energy absorption bands, two bands at 249 and 256 nm for 1, while only one band at 256 nm for 2. These bands are mainly assigned to the π→π* transitions of imidazole units and thiophene units in the two complexes. This difference is due to distinct counter anions in the two complexes (PF₆^- in 1, while [Au(CN)₂]^- in 2), indicating the aurophilicity between neighboring [Au(anbdtim)₂]⁺ cation and [Au(CN)₂]^- anion in complex 2. Moreover, both 1 and 2 revealed the π→π* transitions of anthracene unit, at 336, 352, 373, and 392 nm for 1 and 336, 352, 370, and 390 nm for 2^[10]. These bands showed a slight red shift compared to those in free ligand anbdtim (330, 349, 366, and 386 nm).

  2.4   Luminescence properties

  At room temperature, both 1 and 2 in CH₂Cl₂ showed significantly blue-shifted emission (465 nm for 1 and 445 nm for 2, Fig. 3) with respect to free ligand anbdtim (505 nm). This is due to the coordination of anbdtim toward Au(Ⅰ) ion. Compared to 1, complex 2 revealed a shorter emission wavelength, probably due to the aurophilicity in the {[Au(anbdtim)₂]⁺…[Au(CN)₂]^-} unit, which was confirmed by the different ¹H NMR spectra between 1 and 2 (Fig. 1).

  Figure 3

  

  Figure 3.  Luminescence spectra of 1, 2, and anbdtim in CH₂Cl₂

  c=0.1 mmol·L^-1, λ_ex=380 nm for 1, 361 nm for 2, and 367 nm for anbdtim; Inset: photographs of solutions of the compounds

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  The luminescence quantum yields (Φ) and lifetimes (τ) of 1, 2, and anbdtim were measured in CH₂Cl₂ at room temperature (Table S4), showing Φ=19%, τ₁=3 ns (44.0%), τ₂=8 ns (51.2%) and τ₃=45 ns (4.8%) for 1, Φ=23%, τ₁=1 ns (41.5%), τ₂=8 ns (30.1%) and τ₃=3 ns (28.4%) for 2, and Φ=32%, τ₁=11 ns (84.1%), τ₂=23 ns (15.9%) for anbdtim. The multi-exponential decay for 1, 2, and anbdtim can be attributed to intermolecular interaction in solution^[8b]. The emissions of 1, 2, and anbdtim showed fluorescence character, which is confirmed by their luminescence lifetimes in a range of 3-45 ns^[18-19]. The poly exponential lifetimes for complexes 1 and 2 and free ligand anbdtim can be attributed to inter-molecular aggregation and/or the interaction between the complex cation and counter anion^[20].

  We further measured the luminescence behaviours of 1, 2, and anbdtim at room temperature in the other solvents, such as toluene, DCM, THF, and acetonitrile (Fig. 4, S9, and S10). Surprisingly, although toluene has a relatively small solvent polarity, complex 2 in toluene exhibited a longer emission wavelength (473 nm) than this complex in CH₂Cl₂ (448 nm), THF (452 nm), and CH₃CN (459 nm), as shown in Fig. 4. The luminescence quantum yield (Φ) of 2 in toluene (Φ=66%) was significantly higher than that in CH₂Cl₂ (Φ=23%). Moreover, a similar red-shifted emission was also observed for a benzene solution of 2 (emission at 475 nm, and Φ=67%, Fig.S11). Therefore, complex 2 reveals luminescence enhancement and red-shift in toluene/benzene. This is assigned to the interaction between the toluene/benzene molecule and the anthracene moiety from a {[Au(anbdtim)₂]⁺…[Au(CN)₂]^-} unit. Compared to 2, both 1 and ligand anbdtim have not shown luminescence enhancement and red-shift in toluene/benzene (Fig.S9, S10, and S12). Complex 1 revealed a gradual increase in emission wavelength upon increasing solvent polarity (451 nm in toluene, 465 nm in CH₂Cl₂, 471 nm in THF, and 486 nm in CH₃CN). Ligand anbdtim showed a similar change to complex 1, with an emission at 460 nm in n-hexane, 472 nm in toluene, 494 nm in THF, and 522 nm in CH₃CN.

  Figure 4

  

  Figure 4.  Luminescence spectra of 2 in different solvents

  c=0.1 mmol·L^-1, λ_ex=361 nm

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  Complexes 1 and 2 also exhibited significantly different solid-state luminescence (Fig. 5 and Table S5), with an emission at 450 nm for 1 and 478 nm for 2. The quantum yields and luminescence lifetimes were measured: Φ=10%, τ₁=1 ns (65.6%), τ₂=3 ns (34.4%) for 1, and Φ=7%, τ₁=3 ns (45.8%), τ₂=8 ns (54.2%) for 2.

  Figure 5

  

  Figure 5.  Luminescence spectra of 1 and 2 in solid state

  Inset: photographs of 1 and 2 under 365 nm light

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  Clearly, the luminescence of 2 was quite different from that of 1 both in solution and in the solid state, due to the incorporation of [Au(CN)₂]^- anion in complex 2 and the resultant aurophilicity between neighboring [Au(CN)₂]^- anion and Au(anbdtim)₂]⁺ cation. The similar aurophilicity between cation and anion was reported for complex [Au(NHC)₂][Au(CN)₂] (NHC=1, 3-dimethyl-4, 5-diphenyl-4, 5-dihydro-imidazolin-2-ylidene, Scheme S3)^[15].

  Considering the luminescence enhancement and red-shift of complex 2 in toluene/benzene, and also the stronger volatility of benzene than toluene, we explored luminescence switching upon alternately incorporating and removing benzene molecules in the solid-state structure of complex 2 (Fig. 6). A benzene solution of 2 (c=1 mmol·L^-1) was allowed to evaporate on a quartz plate, obtaining solid 2-benzene with blue-green emission at 491 nm. After heating in an oven at 100 ℃ for 5 min, the resultant solid exhibited steel-blue emission at 460 nm. This change in emission color from blue-green to steel-blue is due to the heating-induced loss of benzene molecules. Upon adding some benzene, the initial blue-green emission could be recovered (Fig. 6). Therefore, complex 2 showed reversible solid-state luminescence switching between blue-green and steel-blue upon alternately incorporating and removing benzene molecules. After ten cycles, complex 2 had not shown an obvious decrease in luminescence-switching capability, i.e. no clear change in emission wavelengths (491 and 460 nm) and the related emission intensities (Fig.S13 and S14).

  Figure 6

  

  Figure 6.  Luminescence switching of solid 2-benzene

  Inset: photographs of 2-benzene under 365 nm light upon alternating heating treatment and crystallization from benzene

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  It should be noted that neither 1 nor ligand anbdtim has shown a similar luminescence switching to complex 2 (Fig.S15 and S16). This indicates that the [Au(CN)₂]^- anion in complex 2 plays a key role in the observed luminescence switching. However, it is difficult to clearly clarify this luminescence switching because of the absence of the crystal structure of complex 2. Based on our experimental results, this switching behavior is probably related to aurophilic interaction between {[Au(anbdtim)₂]⁺ and [Au(CN)₂]^-}, and/or π…π stacking interaction between benzene molecule and the anthracene moiety from a {[Au(anbdtim)₂]⁺ unit. Considering the luminescence switching of complex 2 upon alternately incorporating and removing benzene molecules, this complex has promising applications in the detection of aromatic solvents (e.g., benzene and toluene).

  3.   Conclusions

  In summary, we synthesized two anthracene-based Au(Ⅰ) complexes [Au(anbdtim)₂]PF₆ (1) and [Au(anbdtim)₂][Au(CN)₂] (2), which have the same [Au(anbdtim)₂]⁺ cations but the different counter anions. The structure of [Au(anbdtim)₂]⁺ cation was clarified by the crystal structure of 1, in which two equivalent anbdtim molecules are bridged by an Au(Ⅰ) ion. The clearly different ¹H NMR spectra between 1 and 2 suggest the aurophilicity in the {[Au(anbdtim)₂]⁺… [Au(CN)₂]^-} unit in 2. This aurophilicity leads to significantly different luminescence behaviours between 1 and 2. The CH₂Cl₂ solutions of 1 and 2 showed emission at 465 and 445 nm, respectively. The solid-state luminescence revealed an emission at 450 nm for 1 while at 478 nm for 2. It is interesting that the luminescence of 2 was sensitive to benzene molecules, with longer emission wavelength and higher quantum yield (λ_em=475 nm, Φ=66.5%) in benzene than those in CH₂Cl₂ (λ_em=445 nm, Φ=22.9%), due to π…π interactions between benzene molecule and the anthracene moiety from a {[Au(anbdtim)₂]⁺…[Au(CN)₂]^-} unit in complex 2. In contrast, similar luminescence behavior has not been observed in both 1 and anbdtim, indicating that the {[Au(anbdtim)₂]⁺…[Au(CN)₂]^-} unit in complex 2 plays a key role in catching benzene molecule. The solid 2-benzene, from the evaporation of a benzene solution of 2 (namely crystallization), showed reversible luminescence switching between blue-green (491 nm) and steel-blue (460 nm) upon alternately removing benzene molecules through heating treatment and incorporating benzene molecules through crystallization in benzene. The loss/incorporation of benzene molecules results in the corresponding destruction/ formation of π…π stacking interaction between benzene molecule and {[Au(anbdtim)₂]⁺…[Au(CN)₂]^-} unit, and the related luminescence switching. Our experimental results demonstrate that the aurophilicity between neighboring cation and anion in an Au(Ⅰ) complex not only can significantly modulate the luminescence of this Au(Ⅰ) complex but also can construct a supramolecular functional unit (e.g., {[Au(anbdtim)₂]⁺…[Au(CN)₂]^-} unit in 2) which is sensitive to some small molecules (e.g., benzene and toluene molecules for 2). Therefore, our work provides a new approach to preparing Au(Ⅰ) complexes with potential applications in host-guest molecular recognition.

  Supporting information is available at http://www.wjhxxb.cn

  
  
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  Scheme 1  Molecular structures of ligand anbdtim, and complexes 1 and 2

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  Figure 1  ¹H NMR spectra of complexes 1 and 2 in CDCl₃

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  Figure 2  Molecular structure of 1·CH₃OH

  All H atoms attached to carbon atoms are omitted for clarity; Symmetr code: A: 1-x, y, 0.5-z

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  Figure 3  Luminescence spectra of 1, 2, and anbdtim in CH₂Cl₂

  c=0.1 mmol·L^-1, λ_ex=380 nm for 1, 361 nm for 2, and 367 nm for anbdtim; Inset: photographs of solutions of the compounds

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  Figure 4  Luminescence spectra of 2 in different solvents

  c=0.1 mmol·L^-1, λ_ex=361 nm

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  Figure 5  Luminescence spectra of 1 and 2 in solid state

  Inset: photographs of 1 and 2 under 365 nm light

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  Figure 6  Luminescence switching of solid 2-benzene

  Inset: photographs of 2-benzene under 365 nm light upon alternating heating treatment and crystallization from benzene

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